NL2036627B1

NL2036627B1 - 3D electrode for integrated battery and electrolyser application

Info

Publication number: NL2036627B1
Application number: NL2036627A
Authority: NL
Inventors: Marten Mulder Fokko; Möller-Gulland Robin
Original assignee: Univ Delft Tech
Priority date: 2023-12-21
Filing date: 2023-12-21
Publication date: 2025-07-07
Also published as: WO2025136098A1

Abstract

5 The invention provides an electrode (1000) comprising a porous electrode material (700) and a cavity (600), Wherein the electrode (1000) comprises an outer surface (1010), wherein the outer surface (1010) comprises a first side (1011) and a second side (1012); wherein: (A) the cavity (600) is arranged at the first side (1011) and conﬁgured extending towards the second side (1012); wherein the cavity (600) has a cross-sectional diameter (Doc) selected from the 10 range of 0.5-1.5 mm; and wherein the cavity (600) is deﬁned by an inner surface (1020) of the electrode (1000); (B) the porous electrode material (700) has a porosity selected from the range of 20-90%; wherein for at least 50 vol% of the porous electrode material (700) applies that a distance (d1) to a nearest electrode surface is selected from the range of £1 mm; and wherein the electrode surface is selected from the group comprising the outer surface (1010) and the 15 inner surface (1020); (C) the electrode (1000) has an electrode thickness (dE) selected from the range of Z 2 mm; and (D) the electrode (1000) is a nickel-based electrode or an iron-based electrode. Fig. 1

Description

3D electrode for integrated battery and electrolyser application

FIELD OF THE INVENTION

The invention relates to an electrode. The invention further relates to a method for producing the electrode. Further, the invention relates to an energy apparatus. Further yet, the invention relates to a method of storing electrical energy and one or more of hydrogen and oxygen.

BACKGROUND OF THE INVENTION

Electrodes are known in the art. For instance, WO2023119730A1 describes an electrode provided with: a metal porous sheet comprising a metal porous body having a skeleton of a three-dimensional mesh structure; and a plate-shaped metal support disposed on one main surface of the metal porous sheet. The one main surface has a plurality of machined holes that are formed perpendicular to the one main surface, and at least some of the plurality of machined holes are disposed at regular intervals. The metal support has a plurality of through-holes passing through both surfaces of the metal support, and at least some of the plurality of through-holes are disposed at regular intervals. The porous metal sheet and the metal support are disposed so that the center of each of the aforementioned some of the plurality of machined holes and the center of each of the aforementioned some of the plurality of through-holes are shifted in one direction in a plan view.

SUMMARY OF THE INVENTION

Electrodes may be used in batteries and electrolysis units, as is known in the art.

Electrodes for use in batteries may have a high battery capacity (expressed in Ah) during charge and discharge. Electrodes used for both battery functionality and electrolysis are also known.

Yet, when such electrodes become fully charged, or when such electrodes are used in a unit which performs electrolysis, they may start to evolve gas (bubbles), wherein the gas may especially emerge between and inside the (confined) active material. Due to the structure of the electrode, the transport of gas (bubbles) away from the electrodes may be slow, and the gas (bubbles) may accumulate between and inside the (confined) active material of the electrode, thereby replacing an electrolyte (solution) facilitating the transfer of (ions and therefore) (electrical) energy to and from the electrode. Consequently, ionic resistances may be increased, and the actively participating fraction of electrode material may be reduced, reducing the efficiency (and capacity) of the electrode. A possible solution may be to suppress the gas evolution in the electrode, to increase the charging efficiency. Yet, such a solution is undesired for electrodes used in electrolysis units. A possible solution for electrodes used in electrolysis units may therefore be to increase the distance between the electrode and an adjacent element in the electrolysis unit (e.g. a membrane). However, this may result in an increase in electrolyte resistance. Hence, electrolyser units may employ open electrode structures such as meshes or perforated metal sheets that can be placed closer to e.g. the membrane, reducing the electrolyte resistance while facilitating gas removal from between the electrodes. Yet, such open structures may not be feasible for all electrode materials (due to e.g. difficulties in preparation or lack of strength. Further, such electrodes may have a reduced battery capacity per unit volume.

Increasing the volume of the electrode may increase the (geometric) areal capacity (expressed in mAh/cm?), resulting in systems with a higher energy density, but may decrease the power density (i.e., the charge and discharge currents) by increasing the resistance within the electrode. Increasing the resistance within the electrode may also lead to electrolysis occurring before the electrode is fully charged, thereby reducing the utilization of the (active electrode) material in the electrode for battery applications.

Hence, it is an aspect of the invention to provide an alternative electrode, which preferably further at least partly obviates one or more of above-described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

According to a first aspect, the invention provides an electrode comprising a porous electrode material and a cavity. The electrode may further comprise an outer surface, wherein the outer surface may comprise a first side and a second side. In embodiments, the cavity may be arranged at the first side, and configured extending towards the second side.

Further, in embodiments, the cavity may have a cross-sectional (equivalent circular) diameter

Dc. The cross-sectional (equivalent circular) diameter Dc: may be selected from the range of 0.5-1.5 mm. In embodiments, the cavity may be defined by an inner surface of the electrode.

Additionally or alternatively, in embodiments, the porous electrode material may have a porosity selected from the range of 20-90%. In embodiments, for at least 50 vol% of the porous electrode material, it may apply that a distance (di) to a nearest electrode surface may be selected from the range of <1 mm, wherein the (nearest) electrode surface may be selected from the group comprising the outer surface and the inner surface. In embodiments, the electrode may have an electrode thickness (dg) selected from the range of > 2 mm. Further, in embodiments, the electrode may be a nickel-based electrode. Additionally or alternatively, in embodiments, the electrode may be an iron-based electrode.

Hence, in specific embodiments, the invention may provide an electrode comprising a porous electrode material and a cavity, wherein the electrode comprises an outer surface, wherein the outer surface comprises a first side and a second side, wherein: (a) the cavity is arranged at the first side and configured extending towards the second side; wherein the cavity has a cross-sectional diameter (Dc) selected from the range of 0.5-1.5 mm, and wherein the cavity is defined by an inner surface of the electrode; (b) the porous electrode material has a porosity selected from the range of 20-90%, wherein for at least 50 vol% of the porous electrode material applies that a distance (di) to a nearest electrode surface is selected from the range of <1 mm, and wherein the electrode surface is selected from the group comprising the outer surface and the inner surface; (c) the electrode has an electrode thickness (de) selected from the range of > 2 mm; and (d) the electrode is a nickel-based electrode or an iron-based electrode.

Such an electrode may provide (with the cavity) a flow cavity for an electrolyte, such that the electrolyte may permeate (essentially) a full volume of the (porous) electrode (material), thereby decreasing an (effective) ionic resistance and increasing a capacity of the electrode. Further, the (essentially) full permeation of electrolyte throughout the porous electrode material may facilitate, upon applying a current to (one side of) the electrode (during operation), a homogeneous distribution of the current over the electrode, thereby increasing a charging efficiency of the electrode. Further, the cavity may provide a flow channel for gas (bubbles), preventing an accumulation of gas by facilitating a transport of gas away from the electrode. Further yet, in embodiments wherein the electrode 1s used in an electrolysis unit, the cavity may especially provide an extraction channel for the gas produced during electrolysis.

In embodiments, the electrode may comprise an outer surface. The outer surface may comprise a first major face, a second major face, and an edge face (or “third face”) bridging the first major face and the second major face (around the perimeter of the first major face and the second major face). That is, the electrode may have a front (e.g. the first major face), a back (e.g. the second major face), and (one or more, such as especially) a plurality of sides, wherein the edge face may comprise the plurality of sides. The first major face and second major face may especially provide the largest external area of the electrode. The edge face may be curved in one or two dimensions. Alternatively, the edge face may be planar. Especially, in embodiments wherein the edge face is planar, the edge face may comprise a plurality of edge face facets, wherein the edge face facets may be configured at an angle with respect to each other to form one or more corners, especially a plurality of corners, in the edge face. The electrode may have a rectangular or circular cross-section (in a plane parallel to the first major face and/or the second major face), though other cross-sections may also be possible, like e.g. hexagonal, octagonal, etc. Hence, the electrode may have an n-gonal cross-section, wherein n is at least 3, like > 4 (square or rectangular cross-section), such as > 5 (pentagonal cross- section), especially > 6 (hexagonal cross-section), like > 8 (octagonal cross-section), such as up to 24. In embodiments, the edge face may (thus) comprise a single (curved) facet (for a circular electrode cross-section), or n facets (for an n-gonal cross-section).

Especially, the electrode may have an elongated cross-section. That is, in embodiments, the electrode may have dimensions length and width (in a cross-sectional plane parallel to the first major face and/or the second major face), wherein the length may be larger than the width. In embodiments, the electrode may especially be elongated along an electrode axis of elongation Agr (configured intersecting a geometrical center of the electrode) The electrode axis of elongation Ar may in embodiments be configured parallel to the first major face (and/or second major face) of the electrode. Perpendicular to the afore-mentioned cross- section (i.e., in a plane intersecting the edge face), may be another cross-section, which may in embodiments be circular or rectangular, such as rectangular. Hence, the electrode may e.g. have a cubic shape, a (non-cubic) cuboid shape, an n-gonal prism shape with n being at least 5 (such as pentagonal prism, hexagonal prism), and a cylindrical shape. Other shapes, however, may also be possible.

In embodiments, the outer surface may further comprise a first side and a second side (different from the sides comprised by the edge face). The first side may be different from the first major face. Similarly, the second side may be different from the second major face. In embodiments, the terms “first side” and “second side” may herein only be used to indicate a side of the electrode comprising an opening for the cavity, and a direction towards which the (opposite side of the) cavity extends. That is, the cavity may have an cavity opening in one of the first major face, second major face, and edge face, wherein the (opposite side of the) cavity may extend towards another of the first major face, second major face, and edge face. Further, in embodiments, the cavity may form a channel, wherein the channel (or cavity) may define an open fluid channel from one of the first major face, second major face, and edge face to another of the first major face, second major face, and edge face, wherein the first side and second side may indicate the electrode faces between which the cavity (especially the channel) defines an open fluid channel. Hence, in embodiments, the first side and second side may refer to a face of the electrode, yet may not indicate a (fixed) face of the electrode. The first side and the second side may in embodiments be individually selected from the group consisting of the first major face, the second major face, and the edge face. Further, as indicated above, in embodiments, the cavity (especially when forming a channel) may define an open fluid channel from the first side to the second side. Hence, in embodiments, the first major face may comprise 5 the first side, and the second major face may comprise the second side, such that the cavity (or channel) defines an open fluid channel from the first major face to the second major face.

Alternatively, in embodiments, the edge face may comprise the first side (e.g. at a top of the electrode), and the edge face may (also) comprise the second side (on an opposite side of the edge face from the first side, e.g. at a bottom of the electrode), such that the cavity (or channel) defines an open fluid channel from one side of the edge face (e.g. at the top of the electrode) to another side of the edge face (e.g. at the bottom of the electrode). In further embodiments, the first major face (or second major face) may comprise the first side, and the edge face may comprise the second side, such the cavity (or channel) defines an open fluid channel from the first major face (or second major face) to the edge face.

Further, in embodiments, the electrode may comprise an inner surface. The inner surface may especially define a perimeter (or “wall”) of the cavity. That is, in embodiments, the cavity may be defined by the inner surface of the electrode. Further, in embodiments, the inner surface may be the surface whereat the porous electrode material transitions into the cavity. That is, in embodiments, the inner surface may be the surface whereat a porosity changes from a microscale (i.e., average pore size of < 100 um) of the porous electrode material into a macroscale (i.e., average pore size of > 100 pm) of the cavity. The inner surface may (thus) be a porous surface, wherein the macroscopic pore of the cavity overlaps (or intersects) the microscopic pores of the porous electrode material. Yet, when viewed at < 30x magnification, the inner surface may appear as a (substantially) non-porous (flat) surface (defining the cavity). The inner surface may comprise one or more inner surface faces, configured defining (and surrounding) a shape of the cavity (see also below). In embodiments, the inner surface may have an (inner) surface area. Further, the outer surface may have an (outer) surface area. In embodiments, the surface area of the inner surface may be smaller than the surface area of the outer surface. Alternatively, in embodiments, the surface area of the inner surface may be larger than the surface area of the outer surface. Especially, a ratio between the surface area of the inner surface and the surface area of the outer surface may be selected from the range of > 0.25, such as from the range of > 0.5, especially from the range of > 1, such as from the range of > 2, i.e, in embodiments the surface of the inner surface may be at least twice as large as the surface area of the outer surface. Additionally or alternatively, in embodiments, a ratio between the surface area of the inner surface and the surface area of the outer surface may be selected from the range of < 10, such as from the range of < 5, especially from the range of < 4. Hence, in embodiments, a ratio between the surface area of the inner surface and the surface area of the outer surface may be selected from the range of 0.25-10, such as from the range of 0.5-5, especially from the range of 1-4. In specific embodiments, a ratio between a surface area of the inner surface and a surface area of the outer surface may be selected from the range of 0.5-5.

In embodiments, the first major face and the second major face of the electrode may define a thickness of the electrode de. Hence, the electrode may have an electrode thickness dg, defined as the (largest) distance between the first major face and the second major face. In embodiments, the electrode thickness de may be selected from the range of > 0.7 mm, such as from the range of > 0.85 mm, especially from the range of > 1 mm, like from the range of > 1.5 mm. Further, in embodiments, the electrode thickness dg may be selected from the range of > 2 mm, such as from the range of > 2.5 mm, especially from the range of > 3 mm, like from the range of > 3.5 mm. Additionally or alternatively, in embodiments, the electrode thickness de may be selected from the range of < 10 mm, such as from the range of < 7 mm, especially from the range of < 6 mm, like from the range of < 5.5 mm. In embodiments, the electrode thickness de may be (essentially) constant over at least 80% of the first major face (and the second major face), such as over at least 90%, especially over at least 99%, including 100%. Alternatively, in embodiments, the electrode thickness dg may vary over the first major face (and the second major face). For instance, the first major face (and/or the second major face) may be curved in one or two dimensions, such that the electrode thickness de at an edge of the first major face (and/or the second major face) close to the edge face may be larger or smaller than an electrode thickness de at a center of the first major face (and/or the second major face). In embodiments, the electrode thickness at any point on the first major face (and/or the second major face) may be (individually) selected from the range of 0.7-10 mm, such as from the range of 0.85-7 mm, especially from the range of 1-6 mm, like from the range of 1.5- 5.5 mm,

Further, the electrode (such as especially the first major face and/or the second major face) may have an electrode length Le in a direction parallel to the electrode axis of elongation Ag, and an electrode width We in a direction perpendicular to the electrode axis of elongation Ag. In embodiments, the electrode length Li and electrode width We may be equal, e.g. when the electrode has a square or circular shape. Additionally or alternatively, the electrode length Le may be different from the electrode width Wg. In embodiments, the electrode length Lg and electrode width We may be individually selected from the range of > 2 cm, such as from the range of > 3.5 cm, especially from the range of > 5 cm. Additionally or alternatively, in embodiments, the electrode length Lg and electrode width WE may be individually selected from the range of < 5 m (i.e. > 400 cm), such as from the range of <4 m, especially from the range of <3 m. Further (or alternatively), in embodiments, the electrode length Lg and electrode width WE may be individually selected from the range of > 2*dg, such as from the range of > 5*dg, especially from the range of > 10*de. Additionally or alternatively, in embodiments, the electrode length Le and electrode width Wg may be individually selected from the range of < 5000*dg, such as from the range of < 4000*dg, especially from the range of <3500*de. In embodiments, the electrode length Le and electrode width WE may determine an (external geometric) electrode surface area SAE of the electrode. The electrode surface area

SA: may in embodiments especially be the (external geometric) surface area of the electrode in a plane parallel to the first major face (and/or the second major face) and to the electrode axis of elongation Ag. In embodiments, the electrode surface area SAg may further be determined by the electrode shape (in a cross-section parallel to the first major face and/or second major face). For instance, the electrode may have a square or rectangular shape, wherein the electrode surface area SAg may be provided by Lg*WE. Alternatively, in embodiments, the electrode may have a circular or elliptical shape, wherein the electrode surface area SAE may be provided by 0.25*n*Lg*WE.

In embodiments, the electrode thickness dg, electrode length Lg, and electrode width WE may determine an electrode volume Ve. Hence, in embodiments, the electrode may have an electrode volume VE, selected from the range of (0.5*Le*We*de) — (Le*We*de).

Further, in embodiments, the first major face, second major face, and edge face may (together) define an electrode volume Ve. Hence, the electrode volume Ve may be determined by an exterior geometry of the electrode, and may (thus) comprise the volume of the cavity (and (the micropores of) the porous electrode material). In particular, the (external geometric) electrode volume Ve may be the volume defined by a convex hull of the electrode. In embodiments, the electrode volume Ve may be selected from the range of > 0.5 cm’, such as from the range of > 0.65 cm’, especially from the range of > 0.8 cm’, like from the range of > 1 cm’. Further, in embodiments, the electrode volume VE may be selected from the range of <2 m’, such as from the range of < 1 m’, especially from the range of <0.5 m’.

In embodiments, the electrode may have a (theoretical) capacity. Especially, the electrode may have a theoretical areal capacity Ce (or “area-normalized specific capacity”), provided in units mAh/cm?. The theoretical areal capacity Cg (herein also “capacity”) of the electrode may be determined by Cg = Can*de*pam™(1-P), wherein Cam is the (theoretical) specific capacity of the (pure) active material (see also below) in mAh/g, dg is the electrode thickness (see above), pam is the density of the (pure) active material in the electrode in g/cm’, and P is a porosity of the electrode defined as (Vp+Vc)/ VE, wherein Vp 1s the volume of the pores in the porous electrode material and Vc is the volume of the cavity (see below). In embodiments, the electrode may further have an observed areal capacity Crop, which may be the (maximum) areal battery capacity (expressed in mAh/cm?) reached by the electrode during use of the electrode. The observed areal capacity Ceop of the electrode may depend on the operating parameters during charging and discharging (such as e.g. the charging and discharging times and currents), and may therefore deviate from the theoretical areal capacity

Ce of the electrode. For instance, when (dis)charging the electrode over a short time with a current large enough to (dis)charge the theoretical capacity in that time, a relatively larger percentage of the charge provided to (and recovered from) the electrode may not be (dis)charged (due to heat), reducing the observed areal capacity Ceo. Conversely, when (dis)charging the electrode over a longer time with a current large enough to (dis)charge the theoretical capacity in that time, a relatively smaller percentage of the charge provided to (and recovered from) the electrode may be lost, such that the observed areal capacity Cop may be closer to the theoretical areal capacity Cg. In embodiments, the electrode may be (fully) charged over a charging time tc, selected from the range of 30-60 minutes, wherein the observed (areal) capacity Ceop may be selected from the range of > 0.1*Cg, such as from the range of > 0.3*CE, especially from the range of > 0.5*Cg, like from the range of > 0.8*Cg, including 1*Cg. Further, in embodiments, the electrode may be (fully) discharged over a discharging time tq. selected from the range of 30-240 minutes, wherein the observed (areal) capacity Ceo» may be selected from the range of > 0.1*Cg, such as from the range of > 0.3*Cg, especially from the range of > 0.5*%Cg, like from the range of > 0.8*Cg, including 1*Cg. Additionally or alternatively, in embodiments, the electrode may be (fully) discharged over a discharging time tac selected from the range of 240-600 minutes, wherein the observed (areal) capacity Cg ob may be selected from the range of > 0.5*Cg, such as from the range of > 0.65*Cg, especially from the range of > 0.8*Cg, like from the range of > 0.9*Cg, including 1*Cg. In embodiments, even over relatively long discharge times ta (1.e., tac > 600 minutes), not all charge provided to the electrode may be recovered. In such embodiments, the unrecovered charge may have been converted to Ha and Oz during electrolysis. In embodiments, the electrode may have a theoretical (areal) capacity C selected from the range of > 0.1 mAh/cm?, such as from the range of > 20 mAh/cm?, especially from the range of > 40 mAh/cm?, like from the range of > 50 mAh/cm?. Further, the electrode may have a theoretical (areal) capacity Ce selected from the range of > 60 mAh/cm?, such as from the range of > 80 mAh/cm?, especially from the range of > 100 mAh/cm?, like from the range of > 200 mAh/cm?. Additionally or alternatively, in embodiments, the electrode may have a theoretical (areal) capacity Cg selected from the range of < 2000 mAh/cm? such as from the range of < 1500 mAh/cm?, especially from the range of < 1000 mAh/cm?®. Further, in embodiments, the electrode may have a theoretical (areal) capacity Ck selected from the range of < 800 mAh/cm?, such as from the range of < 600 mAh/cm?, especially from the range of < 450 mAh/cm?, like from the range of < 400 mAb/em?. Hence, in specific embodiments, the electrode may have a theoretical capacity Cg selected from the range of > 100 mAh/cm?.

Further, in embodiments, the electrode may have a charge rate (or “C-rate”) with units C.

Especially, in embodiments, the electrode may have the charge rate at a temperature selected from the range of 10-90 °C, such as at 40 °C. In embodiments, the charge rate indicates the time needed to (fully) charge the electrode (to the theoretical capacity Cg). That is, a charge rate of IC indicates the electrode may be (theoretically) fully charged in 1 hour, a charge rate of 0.5C indicates the electrode may be fully charged in 2 hours, a charge rate of 0.2C indicates the electrode may be fully charged in 5 hours, etc. In embodiments, the electrode may have a charge rate selected from the range of < 3C, such as from the range of < 2C, especially from the range of < 1C. Additionally or alternatively, in embodiments, the electrode may have a charge rate selected from the range of > 0.1C, such as from the range of > 0.2C, especially from the range of > 0.5C.

The electrode may in embodiments comprise (the) porous electrode material.

The term “porous material”, such as in “porous electrode material”, may herein refer to a (three- dimensional) material comprising pores (such as cavities, (micro)channels, interstices, and voids) which are accessible and/or permeable to liquids and gases. Further, a (block of) porous (electrode) material may have a volume V (note that this is a different volume from the electrode volume Vg) and a specific surface area A, wherein the specific surface area A is the area of the porous material accessible to liquid molecules. In embodiments, the specific surface area A may be determined using the pore volume (determined from the uptake of isopropanol by the electrode (see below)), and the average pore size (see below). Alternatively (or additionally), the specific surface area A may be determined by measuring the Electrochemical surface area (ECSA). Further, in embodiments, the volume V (of the porous material) may be determined by the volume of space contained within an imaginary box (or “convex hull”) that has been exactly fitted around the outside of the porous material’s exterior geometry. In embodiments, for a volume of 1 cm’, it may apply that A/V > 100 cm’, such as A/V > 1000 cm’! especially A/V > 10000 cm™. Additionally or alternatively, in embodiments, for a volume of 1 cm’, it may apply that A/V < 100000 cm'!, such as A/V < 75000 cm, especially A/V < 50000 cm. Further, in embodiments, the porous electrode material may have a porosity, defined as the fraction of the volume V occupied by the pores, i.e, Vp/V, wherein Vp is the (total) volume of the pores (or “pore volume”). The pore volume may in embodiments be determined by the uptake of isopropanol by the porous electrode material. That 1s, the pore volume may be determined using a method comprising the following steps: (a) drying the porous electrode material in a vacuum (at a pressure of < 0.02 bar) to remove ambient moisture (and optionally present liquids) from the pores, (b) weighing the porous electrode material to obtain a first weight, (c) immersing the porous electrode material in isopropanol to saturate the pores with isopropanol, wherein the porous electrode material is immersed until no more gas is observed coming out of the porous electrode material, (d) weighing the porous electrode material saturated with isopropanol (within 1 minute of removing the porous electrode material from the isopropanol) to obtain a second weight, and (e) calculating a volume of isopropanol contained in the porous electrode material by subtracting the first weight from the second weight.

In embodiments, the porosity may further be expressed as a percentage, i.e., by (Vp/V)*100%. In embodiments, the electrode may be a nickel-based electrode, wherein the porous electrode material may have a porosity selected from the range of > 5%, such as from the range of > 10%, especially from the range of > 15%, like from the range of > 20.

Additionally or alternatively, the electrode may be a nickel-based electrode, wherein the porous electrode material may have a porosity selected from the range of < 70%, such as from the range of < 60%, especially from the range of < 45%. Further, in embodiments, the electrode may be a iron-based electrode, wherein the porous electrode material may have a porosity selected from the range of < 95%, such as from the range of < 85%, especially from the range of < 80%, like from the range of < 75%. Additionally or alternatively, the electrode may be a iron-based electrode, wherein the porous electrode material may have a porosity selected from the range of > 40%, such as from the range of > 50%, especially from the range of > 60%, like from the range of > 65%. Further, in embodiments, the porous electrode material (of the nickel- based and/or iron-based electrode) may have a porosity selected from the range of 5-95%, such as from the range of 15-75%, especially from the range of 20-70%, like from the range of 20- 65%. In (other) embodiments, the porous electrode material may have a porosity selected from the range of 20-90%, such as from the range of 40-80%, like from the range of 50-75%, especially from the range of 60-70%. Alternatively, in embodiments, the porous electrode material may have a porosity selected from the range of 15-50%, such as from the range of 20- 45%, like from the range of 25-40%, especially from the range of 28-35%. In embodiments, the pores in the porous electrode material may have an average pore size. The average pore size may in embodiments be determined using scanning electron microscopy (SEM), wherein the (equivalent circular) diameter of (e.g., at least 100) pores of the porous electrode material may be determined and averaged, to provide the average pore size. In embodiments, the average pore size may be selected from the range of > 0.01 um, such as from the range of > 0.05 um, especially from the range of > 0.1 um, like from the range of > 0.5 um. Additionally or alternatively, in embodiments, the average pore size may be selected from the range of < 60 um, such as from the range of < 40 um, especially from the range of < 20 um, like from the range of < 15 um. In embodiments, the average pore size may (thus) be selected from the range of 0.01-60 um, such as from the range of 0.05-40 um, especially from the range of 0.1-20 um, like from the range of 0.5-15 um. Hence, in specific embodiments, the porous electrode material may have an average pore size selected from the range of 0.1 - 20 um. Further, in embodiments, each of the pores in the porous electrode material may have a pore size individually selected from the range of 0.005-80 um, such as from the range of 0.01-60 um, especially from the range of 0.05-40 um, like from the range of 0.1-20 um. Such an (average) pore size may provide large enough pores for easy access (to the pores) by an electrolyte (upon installation of the electrode into a battery and/or electrolyser unit). Additionally or alternatively, such an (average) pore size may be small enough that contaminants (e.g. in the electrolyte) may not enter the porous electrode material structure (due to their size), and/or that loose electrode particles may not leave the pores. Further, with such a pore size and different pore sizes, a high porosity may be achieved.

In embodiments, the porous electrode material may have a porous electrode material volume Vpem defined by the outer surface and inner surface of the electrode.

Essentially, Vpem = VE — Vc. From any point within the porous electrode material volume, the (local) porous electrode material may be configured at a distance from a nearest electrode surface, wherein the (nearest) electrode surface may be selected from the group comprising the outer surface and the inner surface (of the electrode). Especially, in embodiments, for at least vol% of the porous electrode material, the distance to the nearest electrode surface may be provided by a distance of at most di. That is, for at least 50 vol% of the porous electrode material, it may apply that, from any point within said (50 vol% of the) porous electrode material, the shortest distance to an electrode surface (selected from the group comprising the outer surface and the inner surface) may be provided by at most the distance di, i.e by a distance < di. In embodiments, the distance di may be selected from the range of <2 mm, such as from the range of < 1.5 mm, especially from the range of < 1.2 mm, like from the range of < 1 mm, such as from the range of < 0.7 mm. Additionally or alternatively, in embodiments, the distance di may be selected from the range of > 0.15 mm, such as from the range of > 0.3 mm, especially from the range of > 0.5 mm, like from the range of > 0.6 mm, such as from the range of > 0.7 mm. A distance d; selected from the range of <2 mm may provide the benefit that a (liquid and/or gaseous) electrolyte (or gas) may (relatively easily) flow into (or out of) the at least 50 vol% of the porous electrode material (compared to an electrode having a higher distance d; for the porous electrode material, e.g. by not comprising a cavity), thereby (substantially) decreasing the electrolyte resistance and/or (substantially) increasing the actively participating fraction of the porous electrode material, facilitating a higher efficiency of the electrode. In embodiments, at least 60 vol®%, such as at least 70 vol%, especially at least 80 vol%, like at least 90 vol%, including (essentially) 100 vol%, of the porous electrode material may have at most the distance d; to a nearest electrode surface.

In embodiments, the porous electrode material may comprise a material capable of (i) storing (electrical and/or chemical) energy and/or (11) generating a gaseous product (e.g. one or more of O2, Ha, etc.). Such a material may herein be referred to as an active material, 1e, a material facilitating the battery and/or electrolyser function of the electrode (and participating in the reactions occurring at and/or within the electrode). In specific embodiments, the porous electrode material (excluding the pore volume V;) may consist of the active material.

Alternatively, in embodiments, the porous electrode material may (further) comprise a scaffold material. A scaffold material may refer to a material (i) not (actively) participating in the reactions occurring at and/or within the electrode to facilitate the battery and/or electrolyser function of the electrode, and/or (ii) providing strength, rigidity and/or structure to the porous electrode material. Hence, in embodiments, the scaffold material may provide a framework for the porous electrode material, whereas the active material may (also) provide the (operational) function of the porous electrode material. In embodiments, the scaffold material may (further) be electrically conductive, i.e, the scaffold material may comprise an electrically conductive material. In embodiments, the porous electrode material may comprise the active material and the scaffold material, wherein the active material may especially be configured (deposited) on (and/or attached to) the surface of the scaffold material. That is, the scaffold material may provide a framework (or “skeleton”) of the porous electrode material, and the active material may be configured coating the scaffold material framework. In such embodiments, the scaffold material framework may especially have one or more of a higher porosity and a larger pore size, which may be reduced upon coating the scaffold material framework with (and/or depositing and/or attaching) the active material (onto the scaffold material framework).

Alternatively, as indicated above, the porous electrode material may consist of the active material. In such embodiments, the active material may in embodiments comprise an active material scaffold and an active material coating. The active material scaffold may in embodiments comprise the (active compound in the) active material in a lower purity than the active material coating. Additionally or alternatively, the active material scaffold may comprise a mixture of the (active compound in the) active material and an (electrically conductive) scaffold material. In embodiments, the specific surface of the porous electrode material, i.e., the surface of the porous electrode material accessible to liquid molecules, may comprise the active material (coating) over at least part of the specific surface area (or “area of the specific surface”). Especially, in embodiments, the specific surface of the porous electrode material may comprise the active material (coating) over at least 60%, such as at least 70%, especially at least 80%, like at least 90%, including (essentially) 100%, of the specific surface area.

In embodiments, the active material (coating) may, especially when the electrode is configured to be used as a positive electrode, comprise one or more of Ni(OH),,

NiosFeo2 hydroxide, and NijxTmx hydroxide, wherein Tm is a transition metal. In embodiments, the active material (for the positive electrode) may be doped with one or more of Y, Ca and Ba. Further, in embodiments, when the electrode is configured to be used as a negative electrode, the active material may contain one or more of Fe, Fe(OH), Fe30,, platinum (Pt), NiMo, NiFey, FeMoy, NiCoFe, LaNis and LaNis type materials such as MmNis. ‘yCoxAl: where Mm stands for a mix of two or more lanthanides, and molybdenum sulfide (MoS). MmNis.xCoxAly is a LaNis type compound. Mm may especially comprise at least one or more of Ce, La, Pr, and other rare earth elements (including Y). Further, x and y are chosen, as known in the art, to be equal or larger than zero. Additionally or alternatively, in embodiments, the active material (coating) may comprise tungsten sulfide (WS) or selenide (W Sey), iron sulfide, and molybdenum sulfide (MoSx). Here, x is especially in the range of 1- 3, more especially in the range of 1.9-2.1. These materials may especially be used as catalyst (for addition to e.g. iron-based electrodes). In embodiments, the active material may especially comprise nickel(-based active compounds) when the electrode is used as a positive electrode, and iron(-based active compounds) when the electrode is used as a negative electrode. That is, in specific embodiments, the electrode may be a nickel-based electrode or an iron-based electrode.

In embodiments, a nickel-based (positive) electrode may comprise a nickel- based active compound, such as (at least) Ni(OH)2. Hence, in specific embodiments, the electrode may be a nickel-based electrode, wherein the nickel-based electrode may comprise

Ni(OH)2. A nickel-based electrode may facilitate a lower production cost of the electrode compared to electrodes comprising precious metals such as e.g. iridium (in the form of iridium oxide) and/or ruthenium (RuQz). Further, a nickel-based electrode may provide the benefit of being applicable in both a battery and an electrolyser unit, thereby providing a multi-functional electrode.

In (other) embodiments, the electrode may be an iron-based electrode, comprising an iron-based active compound such as one or more of Fe and Fe(OH):. Hence, in specific embodiments, the electrode may be an iron-based electrode, wherein the iron-based electrode comprises one or more of Fe and Fe(OH).. An iron-based electrode may be relatively less expensive to produce than an electrode comprising precious metals such as e.g. platinum.

In embodiments, the electrode, especially the iron-based electrode, may comprise the active material in an active material concentration Cr. The active material may in such embodiments especially be selected from the group comprising one or more of Fe,

Fe(OH), Fe;04, platinum (Pt), NiMo, NiFe,, FeMo,, NiCoFe, LaNis, and LaNis type materials such as MmNis+CoxAls, where Mm stands for a mix of two or more lanthanides, and molybdenum sulfide (MoSy) (see also above). In embodiments, the active material concentration Cg, may be selected from the range of > 50 wt%, such as from the range of > 60 wt%, especially from the range of > 70 wt%, like from the range of > 80 wt%, wherein the weight percentage (wt%) may especially be provided by (WE/WET)*100%, wherein Wg, is the weight (or mass) of the active material in the electrode, and WET is the (total) mass of the electrode (including WE). In embodiments, the active material concentration Cg, may (further) be selected from the range of < 99 wt%, such as from the range of < 98 wt%, especially from the range of < 95 wt%, like from the range of < 90 wt%. Hence, the active material concentration Cg, may be selected from the range of 50-99 wt%, such as from the range of 60- 98 wt%, especially from the range of 70-95 wt%, like from the range of 80-90 wt%.

The phrase ‘A comprises > X wt% of B’ and similar phrases may herein refer to atleast X % of the weight of A consisting of B. Similarly, the phrase ‘A comprises > X vol% of B’ and similar phrases may herein refer to at least X % of the volume of A consisting of B.

Further, the (iron-based) electrode may comprise carbon (C) in a carbon concentration Cgc. In embodiments, the carbon concentration Cec may be selected from the range of > 0.008 wt%, such as from the range of > 0.04 wt%, especially from the range of >

0.08 wt%. Further, the carbon concentration Cgc may be selected from the range of > 0.2 wt%, such as from the range of > 0.4 wt%, especially from the range of > 0.8 wt%. Additionally or alternatively, in embodiments, the carbon concentration Cec may be selected from the range of < 8 wt%, such as from the range of < 6.5 wt%, especially from the range of < 5 wt%, like from the range of < 4 wt%. Further, in embodiments, the carbon concentration Cgc may be selected from the range of 0.008-8 wt%, such as from the range of 0.04-6.5 wt%, especially from the range of 0.08-5 wt%, like from the range of 0.8-4 wt%. Such a carbon concentration

Cec may provide an electrode with an improved (electrical) conductivity (and strength), compared to electrodes not comprising carbon. Further, such a carbon concentration Cgc may facilitate a higher discharge capacity of the (iron-based) electrode.

In embodiments, the (iron-based) electrode may (further) comprise a crosslinker. In embodiments, the crosslinker may be selected from the group comprising sodium borate, sodium tetraborate, trimethylolpropane triglycidyl ether (TMPGDE), diisocyanates (e.g. 4,4-diphenyl diisocyanate (DDI) and 1,6-hexamethylene diisocyanate (HDD), though other options may be possible, as is known to a person skilled in the art.

Especially the (iron-based) electrode may comprise the crosslinker in a crosslinker concentration Cgc. In embodiments, the crosslinker concentration Cgc may be selected from the range of > 0.001 wt%, such as from the range of > 0.005 wt%, especially from the range of > 0.01 wt%. Further, the crosslinker concentration Cg may be selected from the range of > 0.025 wt%, such as from the range of > 0.05 wt%, especially from the range of > 0.1 wt%.

Additionally or alternatively, in embodiments, the crosslinker concentration Cea may be selected from the range of < 5 wt%, such as from the range of < 4 wt%, especially from the range of < 3 wt%, like from the range of < 1 wt%. Hence, the (iron-based) electrode may comprise the crosslinker in a crosslinker concentration Cg. selected from the range of 0.001-5 wt%, such as from the range of 0.005-4 wt%, especially from the range of 0.01-3 wt%, like from the range of 0.025-1 wt%. Hence, in specific embodiments, the iron-based electrode may comprise an active material, carbon, and a crosslinker, wherein: (A) the electrode may comprise the active material in an active material concentration Cg, selected from the range of 50-99 wt%; (B) the electrode may comprise the carbon in a carbon concentration Cgc selected from the range of 0.008-8 wt%; and (C) the electrode may comprise the crosslinker in a crosslinker concentration Ce selected from the range of 0.001-5 wt%. Such a composition of the (iron- based) electrode may provide a stronger electrode, due to the presence of the carbon (and crosslinker). Hence, the electrode may be more durably, and may have a longer lifetime compared to electrodes not comprising carbon (and a crosslinker material).

Returning to the cavity, in embodiments, the cavity may be elongated.

Especially, the cavity may be elongated in a direction (extending) from the first side to the second side. Hence, in embodiments, the cavity may have a cavity axis of elongation Ac (intersecting the first side and the second side), along which the cavity may be elongated.

Further, the cavity may be configured arranged at the first side (with a cavity first end) and configured extending towards the second side (with a cavity second end). That is, the cavity may have a cavity opening at the first side of the electrode, wherein the cavity opening may indicate a transition of the inner surface to the outer surface of the electrode. Further, the cavity may extend towards the second side (with the cavity second end), yet may not form a cavity opening at the second side (however, see also below). That is, the cavity may form a “blind hole”, wherein the cavity extends over a part of the electrode thickness dg, such that the cavity second end is configured in an interior of the electrode. In embodiments, the cavity may extend over < 0.9*dg, such as over < 0.8*dg, especially over <0.7*de. Additionally or alternatively, in embodiments, the cavity may extend over > 0.2*dg, such as over > 0.3*dg, especially over > 0.4*de. Alternatively, the cavity may extend over the full electrode thickness de. In such embodiments, the cavity may form a channel, wherein the channel may define an open fluid channel from the first side to the second side. That is, in such embodiments, the cavity may form a cavity opening at the cavity first end (and the first side) and at the cavity second end (and the second side). Hence, in specific embodiments, the cavity may define an open fluid channel from the first side to the second side. In further embodiments, the cavity (forming a blind hole or a channel) may be configured arranged at one or more of (i) the first major face, (i1) the second major face, and (iii) the edge face with a side of the cavity (i.e., with a side of the cavity bridging the cavity first end and cavity second end). In such embodiments, the cavity may form a (partial) groove in the electrode surface.

Further, in embodiments, the cavity may have a cavity shape, such as especially in a cross-section perpendicular to the cavity axis of elongation Ac. In embodiments, the cavity shape may be selected from the group comprising a circular and an n-gonal shape, wherein n may be selected from the range of > 2, such as from the range of > 3, especially from the range of > 4, such as at most 6. In embodiments, the sides of the (n-gonal) shape may be curved and/or planar. Yet, in embodiments, the cavity shape may comprise at least one corner. That is, the cavity shape may be defined by a plurality of inner surface facets (forming an n-gonal cavity shape), wherein at least two of the inner surface facets may be configured planar and at an angle with respect to each other (to form the at least one corner). In embodiments, the at least one corner (comprised by the cavity shape) may have an angle selected from the range of < 110°,

such as from the range of < 100°, especially from the range of < 90°, like from the range of < 80°, wherein the angle may especially be determined from an interior of the cavity shape.

Further, in embodiments, the at least one corner (comprised by the cavity shape) may have an angle selected from the range of > 15°, such as from the range of > 25°, especially from the range of > 35°, like from the range of > 45°. In embodiments, the cavity shape may comprise at least 2 corners (having the angle), such as at least 3 corners, especially at least 4 corners. In embodiments, the cavity shape may (thus) e.g. be selected from the group comprising a navette (or “pointed oval”), a triangle, a square, a rectangle, a teardrop, a partial circle (e.g. a semicircle), a pentagon, a trapezoid, a cross, a parallelogram, an L-shape, an irregular shape (comprising at least one corner), etc.. A person skilled in the art may know and be able to select a suitable cavity shape comprising at least one corner having the angle as defined above. In embodiments, the cavity shape may be (essentially) the same along at least 80% of a cavity length along the cavity (axis of elongation Ac), such as along at least 90%, especially at (essentially) any point along the cavity (axis of elongation Ac). Alternatively, the cavity shape may change (such as for each point be individually selected) along the cavity (axis of elongation

Ac). Yet, in embodiments, at any point along the cavity (axis of elongation Ac), the cavity shape may have the at least one corner. Hence, in specific embodiments, the cavity may have a cavity shape, wherein the cavity shape may comprise at least one corner having an angle of < 90°. A cavity shape comprising at least one corner may have as advantage that gas bubbles (formed during use of the electrode in e.g. a battery and/or an electrolyser) may not fill the (entire) cavity shape, and may especially be excluded from the at least one corner. Hence, a cavity shape comprising at least one corner may prevent blocking of the cavity by a gas bubble, thereby facilitating a continuous (ionically conductive) electrolyte stream (or pathway) through the cavity. In specific embodiments, during use of the electrode, the at least one corner may be configured in the upwards direction, i.e, the at least one corner may be configured at the top of the cavity shape. Such an upwards direction may facilitate that, when a gas bubble moves along the cavity (in the upwards direction), the contact surface between the gas bubble and the inner surface is reduced (compared to e.g. a circular cavity shape), facilitating an earlier detachment of the gas bubble from the cavity.

Further, the cavity may have a second shape in a cross-section perpendicular to the first side (and/or the second side), and parallel to the cavity axis of elongation Ac. In embodiments, the second shape may be selected from the group comprising a triangle and a rectangle, i.e., the cavity may have a triangular second shape or a rectangular second shape.

Alternatively, the second shape may be selected from the group comprising (a circular and) an n-gonal shape, wherein n may be selected from the range of > 2, such as from the range of > 3, especially from the range of > 4, such as at most 6. Especially, in embodiments wherein the cavity spans over part of the electrode width dg (i.e., wherein the cavity forms a “blind hole”), the cavity may have a triangular second shape, with the base of the triangle arranged at the cavity first end, and the tip of the triangle arranged at the cavity second end. Further, in embodiments wherein the cavity defines an (open fluid) channel, the cavity may have a rectangular shape. Hence, in specific embodiments, the cavity may have a second shape in a cross-section perpendicular to the first side, wherein the second shape may be selected from the group comprising a triangle and a rectangle. A triangular shape may have as advantage that at least one of the inner surface faces (of the cavity) may be tilted in a (slightly) upwards direction (when looking from the cavity second end to the cavity first end) during use of the electrode. As such, a gas bubble may be directed in an upwards direction by the cavity (second shape).

In embodiments, the cavity, especially the cavity shape, may further have a cross-sectional diameter Dce. The cross-sectional diameter Dce may (thus) be determined in a cross-section of the cavity taken perpendicular to the cavity axis of elongation Ac. In embodiments, the cross-sectional diameter Dcc may especially be an equivalent circular diameter Dc. of the cavity. The equivalent circular diameter (or ECD) (or “circular equivalent diameter”) of an (irregularly shaped) two-dimensional shape is the diameter of a circle of equivalent area. For instance, the equivalent circular diameter of a square with side a is 2a/SQRT(x). For a circle, the diameter D is the same as the equivalent circular diameter D.

Would a circle in an xy-plane with a diameter D be distorted to any other shape (in the xy- plane), without changing the area size, then the equivalent circular diameter of that shape would be D. In embodiments, the cross-sectional diameter Dc. of the cavity may be selected from the range of > 0.25 mm, such as from the range of > 0.4 mm, especially from the range of > 0.5 mm, like from the range of > 0.7 mm. Additionally or alternatively, in embodiments, the cross- sectional diameter Dc: of the cavity may be selected from the range of < 1.5 mm, such as from the range of < 1.2 mm, especially from the range of < 0.9 mm, like from the range of <0.7 mm.

A cavity having a cross-sectional (equivalent circular) diameter Dc. selected from the range of 0.25-1.5 mm may facilitate the flow of electrolyte and gas (bubbles) through the electrode (especially through the cavity), thereby providing (1) access to the porous electrode material for the electrolyte, and (ii) gas transport into and/or out of the porous electrode material, increasing the efficiency and capacity per unit volume (and/or area). In embodiments, the (equivalent circular) cross-sectional diameter Dc: may be equal throughout the cavity. Alternatively, the

(equivalent circular) cross-sectional diameter Dc: may alter across the cavity. Especially, in embodiments wherein the cavity extends over part of the electrode thickness dg, the cross- sectional diameter Dc may decrease upon moving from the cavity first end to the cavity second end. That is, in embodiments, the cavity may taper towards the cavity second end. In such embodiments, the cavity may especially have a cross-sectional diameter Dc: selected from the range of 0.25-1.5 mm at the cavity first end. Further, in such embodiments, the second shape of the cavity may especially be a triangle. Yet, alternatively, the cavity may form a channel (having e.g. a rectangular second shape), wherein the channel may have an (essentially) equal (equivalent circular) cross-sectional diameter Dcc may be equal throughout the cavity.

In embodiments, the cross-sectional diameter Dce may at least partially determine a volume of the cavity. Hence, in embodiments, the cavity may have a cavity volume

Vc. The cavity volume Vc may in embodiments especially be the volume enclosed by the inner surface of the electrode. In embodiments, the cavity volume Vc may be selected to be a fraction of the electrode volume VE (see above). That is, in embodiments, the cavity volume Vc may be selected from the range of 0.01*VEg < Vc < 0.8*%VE, such as from the range of 0.05% Vg < Vc < 0.65*VE, especially from the range of 0.1*Vg < Vc < 0.5% Vg, like from the range of 0.25% Vg < Vc £ 0.35*Ve. Hence, in specific embodiments, the electrode may comprise an electrode volume Ve, the cavity may comprise a cavity volume Vc, and 0. IVE < Ve <0.5%Ve. Such a ratio between the cavity volume Vc and the electrode volume Ve may facilitate an increase in the electrode surface area (comprising the surface area of the outer surface and of the inner surface), thereby providing a larger surface area from which an electrolyte (solution) may penetrate the porous electrode material and produced gas can flow out. Further, such a ratio between the cavity volume Vc and the electrode volume Vg may facilitate an increase in the electrode surface area over which the electrolyte (solution) may flow, thereby facilitating improved heat dissipation for the electrode (during use). In embodiments, an improved heat dissipation capability may facilitate operating the electrode at higher current densities, improving the efficiency of the electrode for e.g. gas production by electrolysis. In embodiments, the electrode may further comprise a plurality of cavities (see below). In such embodiments, the cavity volume Vc may especially be a combined cavity volume Vc (for all of the plurality of cavities), i.e., the plurality of cavities may have a combined cavity volume

Ve, especially wherein Vc is selected from the range of 0.05%Ve < Vc < 0.65*VE (see also above).

As indicated above, in embodiments, the electrode may have an electrode axis of elongation Ag, and the cavity may have a cavity axis of elongation Ac. In embodiments, the cavity axis of elongation Ac may be configured at an angle a with the electrode axis of elongation Ag.

Additionally or alternatively, the cavity axis of elongation Ac may be configured at an angle a with a plane parallel to the first (and/or second) major face of the electrode.

The angle a may in embodiments be selected from the range of 0-90°, that is, in embodiments, the cavity may in embodiments have any possible configuration within the electrode (and may be arranged at the first side). Further, in embodiments, the angle a may be selected from the range of 0-80°, such as from the range of 0-70°, especially from the range of 0-60°, like from the range of 0-50°. In embodiments, the cavity axis of elongation Ac and the electrode axis of elongation Ag may be oriented (roughly) parallel with respect to each other.

Hence, in embodiments, the angle a may be selected from the range of 0-45°, such as from the range of 0-30°, especially from the range of 0-20°, like from the range of 0-10°. Alternatively, the cavity axis of elongation Ac and the electrode axis of elongation Ag may be oriented (roughly) perpendicular with respect to each other.

Hence, in embodiments, the angle a may be selected from the range of 45-90°, such as from the range of 50-90°, especially from the range of 65-

90°, like from the range of 80-90°. Further, in embodiments, the angle a may be selected from the range of 10-80°, such as from the range of 20-70°, especially from the range of 25-65°, like from the range of 30-60°. Yet, in specific embodiments, the electrode may have an electrode axis of elongation Ag, and the cavity may have a cavity axis of elongation Ac, wherein the cavity axis of elongation Ac may be configured at an angle a with the electrode axis of elongation Ag, and wherein the angle a may be selected from the range of 0-60°. Such an angle a between the cavity axis of elongation Ac and the electrode axis of elongation Ag may facilitate that the cavity may be oriented (roughly) parallel to the first major face (and/or second major face) of the electrode.

In such embodiments, at least one of the first side and the second side may comprise the edge face.

In embodiments, during use of the electrode in e.g. a battery and/or an electrolyser, the first major face (or second major face) may be configured facing a membrane (or diaphragm, or separator, etc.), while an electrolyte flow may be applied to the (cell comprising the) electrode from the direction of the edge face.

Hence, by configuring the cavity axis of elongation Ac and the electrode axis of elongation Ag at an angle a selected from the range of 0-60°, the electrolyte flow (during operation) may be at least partially directed into the cavity (especially when the cavity defines an open fluid channel), facilitating heat dissipation and electrolyte penetration, and reducing electrolyte resistance.

In particular, the angle o may be selected such that during use of the electrode (an upper wall of) the cavity is not in a level configuration.

For instance, for an electrode for which the electrode axis of elongation may be vertically arranged during operation, the angle a may be selected from the range of 0-60° to provide an upwards direction of the cavity, facilitating the removal of gas from (inside) the electrode.

As indicated above, in embodiments, the electrode may comprise a plurality of cavities, each having a respective cavity axis of elongation Ac. In embodiments, the angle a may be individually selected for each of the plurality of cavities (axes of elongation Ac).

Alternatively, each of the plurality of cavities (axes of elongation Ac) may have the same angle a with the electrode axis of elongation Ag. In specific embodiments, the angle o may be selected such for each cavity, that (upon installation of the electrode in a system) the cavity first end of each cavity may be located above the respective cavity second end. That 1s, in embodiments, the plurality of cavities may be oriented with the cavity opening facing upwards (with respect to the cavity second end), such that the plurality of cavities may be oriented (from the cavity second end to the cavity first end) in a direction of gas flow from the electrode, thereby facilitating the removal of gas bubbles from the electrode and in the direction away from a counter electrode (of the system). Yet, in embodiments, the angle o may be individually selected from the range of 0-90° for each of the plurality of cavities.

In embodiments, each of the plurality of cavities may have an individually selected cross-sectional (equivalent circular) diameter Dc, selected from the range of 0.25-1.5 mm (see also above). Additionally, in embodiments, each of the plurality of cavities may have an individually selected (cross-sectional) cavity shape, comprising at least one corner having an angle of < 110°, such as especially having an angle of < 90°. Further, in embodiments, the electrode may comprise a plurality of cavities, wherein the first side and the second side of the electrode (between which a respective cavity may be oriented with its axis of elongation Ac) may be individually selected for each cavity. Especially, in embodiments, the electrode may comprise 4-60 cavities per cm? of electrode surface area SAg, such as 7-50 cavities per cm?, especially 10-40 cavities per cm’, like 15-35 cavities per cm’. In embodiments, (the longitudinal axes Ac of) at least two of the plurality of cavities may be configured at an angle

B with respect to each other. In embodiments, the angle B may be selected from the range of 10-90°, such as from the range of 20-90°, especially from the range of 30-90°, like from the range of 45-90°. That is, the at least two of the plurality of cavities may in embodiments be configured (roughly) perpendicular to each other. Hence, in specific embodiments, the electrode may comprise a plurality of cavities, wherein at least two of the plurality of cavities are configured at an angle selected from the range of 20-90°. Such a configuration of cavities may facilitate an electrolyte (and/or gas) flow in at least two directions through the electrode.

Hence, the distribution of electrolyte over the electrode, and/or the transport of gas (bubbles)

out of the electrode may be improved, compared to an electrode comprising one cavity (and especially compared to an electrode not comprising cavities).

Additionally or alternatively, (the axes of elongation Ac of) at least two of the plurality of cavities may be configured (essentially) in parallel. That is, in embodiments, the angle B (for the at least two of the plurality of cavities configured in parallel) may be selected from the range of 0-20°, such as from the range of 0-10°, especially from the range of 0-5°.

Especially, (the axes of elongation Ac of) the at least two of the plurality of cavities configured in parallel may be configured at an angle (with respect to each other) of 0°. In embodiments, an average distance de. between the at least two of the plurality of cavities (configured in parallel) may be selected from the range of < 4 mm, such as from the range of <3 mm, especially from the range of < 2.4 mm, like from the range of < 2 mm, such as from the range of < 1.6 mm. In embodiments, the average distance dc. may especially be an average of the (local) distance between the at least two of the plurality of cavities over the (full) length of the respective cavities (parallel to the axes of elongation Ac of the at least two of the plurality of cavities). Further, in embodiments, the average distance dc. may especially be the (average) distance between a (closest) side of a first cavity (of the at least two cavities) and a (closest) side of a second cavity (of the at least two cavities). That is, the average distance dc. may be the (average) distance between a (closest) side of a first inner surface of the electrode (defining the first cavity) and a (closest) side of a second inner surface of the electrode (defining the second cavity). Hence, in specific embodiments, the electrode may comprise a plurality of cavities, wherein at least two of the plurality of cavities may be configured in parallel. An electrode comprising at least two cavities configured in parallel may provide the benefit that the cavities may be more evenly distributed over the electrode volume Ve in (at least) one direction, without hampering and/or decreasing the flow of electrolyte and/or gas (bubbles).

Hence, the size of each individual cavity may be reduced, and porous electrode material may be configured in between the (parallel) cavities, increasing the strength of the overall electrode.

Further, in embodiments wherein the plurality of cavities partially extend over the electrode thickness de, and have a tapered shape towards the (respective) cavity second ends, a configuration with parallel cavities may provide an interdigitated array of cavities, wherein a first subset of the plurality of cavities may have their cavity first ends at an opposite face of the electrode from a second subset of the plurality of cavities, and wherein the cavity second ends of the first subset may be interspersed with the cavity second ends of the second subset.

In embodiments, a combination of parallel and angled cavities may (also) be possible. That is, in embodiments, the electrode, especially the plurality of cavities, may comprise a first subset of first cavities, wherein the first cavities may (all) be arranged in parallel. Further, in embodiments, the plurality of cavities may comprise a second subset of second cavities, wherein the second cavities may (all) be arranged in parallel. In embodiments, the first cavities may be arranged orthogonal relative to the second cavities. That is, an axis of elongation Ac of a first cavity and an axis of elongation Ac of a second cavity may be configured at an angle of 90°. Alternatively, in embodiments, the first cavities may be arranged at an angle selected from the range of 20-90°, such as from the range of 35-90°, especially from the range of 50-90°, relative to the second cavities. Yet, in specific embodiments, the first cavities may be arranged orthogonal relative to the second cavities (i.e., at an angle of 90°). In embodiments, the first subset may comprise selected from the range of > 2, such as from the range of > 4, especially from the range of > 6, first cavities. Additionally or alternatively, in embodiments, the second subset may comprise selected from the range of > 2, such as from the range of > 4, especially from the range of > 6, second cavities. In embodiments, the (first subset of) first cavities may be configured in an array. The array may be regular, random, or quasi random. In embodiments, the array may be a 1D array (i.e., the first cavities may be configured in a (single) row). Alternatively, in embodiments the array may be a regular 2D array. However, other arrays, like a phyllotaxis tessellation or a sunflower tessellation, may also be possible.

The term “tessellation” may herein especially refer to a pattern of (repeated) shapes, especially polygons, that fit together closely without gaps or overlapping, and wherein the (first) cavities may be configured on the edges of the (repeated) shapes. Yet, especially, the array may be a regular array, such as an n*m array, wherein n and m are each individually selected from the range of at least 1. In specific embodiments, per cm? of the electrode surface area SAg, n and m (of the array) may each be individually selected from the range of 1-10, such as from the range of 2-8, especially from the range of 3-7. Hence, in embodiments there may be one or two constant pitches. Hence, the (first subset of) first cavities may be configured arranged in a (regular) 1D array or a (regular) 2D array. Further, in embodiments, the (second subset of) second cavities may be configured in an array, such as especially in a (regular) 1D array or a (regular) 2D array. Hence, configuring the first subset of first cavities at an angle (of especially 90°) with respect to the second subset of second cavities may result in the formation of a (regular) 2D or 3D array comprising the first cavities and the second cavities. That is, in embodiments, the plurality of cavities (comprising the first subset and the second subset) may be configured in a 2D or 3D array. In (such) embodiments, (and especially in embodiments wherein the cavities form open fluid channels,) each first cavity from the first subset may intersect at least one second cavity from the second subset. Especially, in embodiments, the axis of elongation Ac of each first cavity from the first subset may intersect (the axis of elongation Ac of) at least one second cavity from the second subset. Hence, in specific embodiments, the plurality of cavities may comprise a first subset of first cavities arranged in parallel and a second subset of second cavities arranged in parallel, wherein the first cavities may be arranged orthogonal relative to the second cavities, and wherein each first cavity from the first subset may intersect at least one second cavity from the second subset. Such a configuration of cavities may provide a homogeneous distribution of cavities over the electrode, such that an electrolyte (solution) and/or gas bubbles may flow evenly throughout the electrode (in at least two directions). Further, by (at least partially) intersecting the first subset and the second subset (of cavities), a 2D or 3D grid of cavities may be provided, facilitating a smaller distance di, thereby reducing the ionic resistance and increasing the actively participating fraction of the porous electrode material, facilitating a higher efficiency and utilization of the electrode.

According to a second aspect, the invention may further provide a method for producing the electrode as defined herein. In embodiments, the method may comprise providing a first mixture. In embodiments, the first mixture may comprise an electrode material precursor. Additionally, the first mixture may comprise a carbon source. Further, the first mixture may comprise metal fibers and/or metal powder. Further yet, the first mixture may (also) comprise a polymeric binding agent comprising a crosslinker. In embodiments, the polymeric binding agent may especially comprise the crosslinker in a crosslinker concentration

Ca. The crosslinker concentration C« may in embodiments be selected from the range of 0.01- 5 wt%. In embodiments, the first mixture may comprise the carbon source in a carbon source concentration Cc. The carbon source concentration Cc may be selected from the range of 0.01- 10 wt%. The method may comprise transferring the first mixture to a mold, especially at a molding temperature Ty. In embodiments, the molding temperature Tm may be selected from the range of > 50 °C. The mold may comprise a molding chamber. Especially, the molding chamber may in embodiments comprise a first side and a second side (opposite the first side).

In embodiments, the mold may (further) comprise a cavity placeholder arranged in the molding chamber (and (partially) extending from the first side to the second side). The cavity placeholder may have a cross-sectional (equivalent circular) diameter Dp selected from the range of 0.5-1.5 mm. In embodiments, the method may comprise a curing stage. The curing stage may especially comprise hardening the first mixture into a solid, to provide the mold containing an electrode precursor structure. Additionally, the method may comprise removing the electrode precursor structure from the mold. In embodiments, the method may further comprise removing the cavity placeholder from the electrode precursor structure (at the same time as removing the electrode precursor structure from the mold). Further, in embodiments, the method may comprise a sintering stage. The sintering stage may comprise heating the electrode precursor structure at a sintering temperature Ts to provide the electrode. In embodiments, the sintering temperature Ts may be selected from the range of > 650°C, especially from the range of > 700 °C. Further, in embodiments, the polymeric binding agent may be (at least partially) removed from the electrode precursor structure in the sintering stage.

Hence, in specific embodiments, the invention may provide a method for producing the electrode as defined herein, the method comprising: (A) providing a first mixture comprising (i) an electrode material precursor, (ii) a carbon source, (iii) metal fibers and/or metal powder, and (iv) a polymeric binding agent comprising a crosslinker, wherein the polymeric binding agent comprises the crosslinker in a crosslinker concentration Cc selected from the range of 0.01-5 wt%; wherein the first mixture comprises the carbon source in a carbon source concentration Cc selected from the range of 0.01-10 wt%; (B) transferring the first mixture to a mold at a molding temperature Tm, wherein the molding temperature Tm is selected from the range of > 50 °C, wherein the mold comprises a molding chamber, wherein the molding chamber comprises a first side and a second side, wherein the mold comprises a cavity placeholder arranged in the molding chamber, wherein the cavity placeholder has a cross- sectional diameter Dp selected from the range of 0.5-1.5 mm; (C) a curing stage, wherein the curing stage comprises hardening the first mixture into a solid, to provide the mold containing an electrode precursor structure; (D) removing the electrode precursor structure from the mold; and (E) a sintering stage, wherein the sintering stage comprises heating the electrode precursor structure at a sintering temperature Ts to provide the electrode, wherein the sintering temperature Ts is selected from the range of > 700 °C, and wherein the polymeric binding agent is removed from the electrode precursor structure in the sintering stage.

Such a method may especially provide an electrode comprising at least one cavity. Further, such a method may facilitate combining the steps of sintering the electrode and removing the binder into one heating step (in the sintering stage), thereby reducing the number of steps during production and thus providing a less labor-intensive method.

In embodiments, the first mixture may comprise an electrode material precursor.

For a nickel-based electrode and/or an iron-based electrode, the electrode material precursor may especially be selected from the group comprising (carbonyl) nickel powder, nickel metal particles, stainless steel particles, sponge iron (optionally comprising impurities such as Mn and Cr), Fe:0s, FeSO:.xH;0, Fe;0s4, FeO, FeS, and FeS,, though other options may be possible,

as 1s known to a person skilled in the art. In embodiments, for the nickel-based electrode, the electrode material precursor may (after the sintering step) provide either the active material (or active compound) of the electrode (e.g. when using nickel powder or nickel metal particles) or the (conducting) scaffold material (see also below) onto which the active material may be deposited. In specific embodiments, the electrode material precursor (for the (negative) iron- based electrode) may comprise one or more of carbonyl iron, Fe; 0s, FeSO:.xH;0, Fe;04, FeO, sponge iron, FeS, and FeS:. An electrode material precursor selected from this group may provide (additional) porosity to the electrode, as the oxygen and sulfur in these electrode material precursors may be (fully) removed from the electrode precursor structure during the sintering step when these materials are reduced to Fe, thereby providing (additional) (nano- and/or microscale) pores in the active material of the porous electrode material, and/or in the scaffold material (for the nickel-based electrode). Further, in embodiments, the electrode material precursor may comprise one or more additional compounds, configured to enhance the performance of the electrode (especially for a combined battery and electrolyser application). For instance, for the iron-based electrode, a small mass percentage (relative to the electrode material precursor) of one or more of Ni-Mo-Zn (e.g. codeposited with Fe), or alternatively Ni-S-Co, Ti2Ni, nitrogen doped graphene, Ni-Mo-N, Ni (as a dopant of e.g. carbonyl iron), Ni(OH)» nanoparticles, Ni-Cr, MmNis intermetallic hydride former, nanocrystalline NisP4, Ru, Ru0O:, AgNi, MoS,, WS, WSe; or the noble elements Pd, Pt, etc, may be added to the electrode material precursor, e.g. in order to reduce the hydrogen generation overpotential of the iron-based electrode. Additionally or alternatively, for the nickel-based electrode, a small mass percentage (relative to the electrode material precursor) of one or more of spinel type Co304, spinel type NiCo20:, Ni- and La-doped Co30., Li-doped

C0304, LagsSrosCo0s, Nip2CooslaOs, (ProsBaos)Co0s.s, Ni-Fe hydroxides such as Nii. «Fe(OH);, and NiO/NiFe;O4 may be added to the electrode material precursor, e.g. in order to reduce the overpotential for oxygen evolution of the nickel-based electrode. Alternatively, also up to 25% of Ni in Ni(OH): may be substituted by Al to yield a higher capacity and electrochemical activity. Additionally or alternatively, the Ni electrode can have Fe substituting

Ni to form Nij.xFex(OH): or NiosFeo2 layered double hydroxide.

In embodiments, the first mixture may comprise the electrode material precursor in an electrode precursor concentration Cgp. The electrode precursor concentration Cgp may in embodiments be selected from the range of > 30 wt%, such as from the range of > 40 wt%, especially from the range of > 50 wt%, like from the range of > 55 wt%, wherein the weight percentage (wt%) may especially be provided by (Wep/Wt)*100%, wherein Wx is the weight

(or mass) of the electrode material precursor, and Wr is the (total) mass of the first mixture (including Wg). In embodiments, the electrode precursor concentration Cgp may (further) be selected from the range of < 90 wt%, such as from the range of < 85 wt%, especially from the range of < 80 wt%, like from the range of < 75 wt%.

The first mixture may in embodiments further comprise a carbon source (such as especially when the electrode may be an iron-based electrode). The carbon source may be converted into carbon during the sintering stage, facilitating an increase in the (electrical) conductivity of the electrode, thereby facilitating a higher discharge capacity of the (iron based) electrode. Herein, the discharge capacity (for a certain “discharge rate”) may especially be a measure for the discharge capacity (in units Ah) reached in a time needed to (fully) discharge the electrode (in units C, see also above). The carbon source may be selected from the group comprising graphite, carbon nanostructures (e.g. carbon nanotubes (CNT), carbon nano fibers (CNF)), carbon black, graphene, activated carbon, SuperP, Vulcan, and carboxymethyl cellulose (CMC), though other options may be possible, as is known to a person skilled in the art. In embodiments, the first mixture may comprise a (dedicated) carbon source, which is specifically added for the addition of carbon. Alternatively, in embodiments, another constituent of the first mixture may comprise (sufficient) carbon. In further embodiments, the first mixture may comprise the carbon source in a carbon source concentration Cc selected from the range of > 0.01 wt%, such as from the range of > 0.05 wt%, especially from the range of > 0.1 wt%. Further, the first mixture may comprise the carbon source in a carbon source concentration Cc selected from the range of > 0.25 wt%, such as from the range of > 0.5 wt%, especially from the range of > 1 wt%. Additionally or alternatively, in embodiments, the first mixture may comprise the carbon source in a carbon source concentration Cc selected from the range of < 10 wt%, such as from the range of < 8 wt%o, especially from the range of < 6 wt, like from the range of < 5 wt%. Further, in embodiments, the first mixture may comprise the carbon source in a carbon source concentration Cc selected from the range of 0.01-10 wt%, such as from the range of 0.05-8 wt%, especially from the range of 0.1-6 wt%, like from the range of 1-5 wt%.

In embodiments, the first mixture may further comprise the polymeric binding agent. The polymeric binding agent may in embodiments comprise a solution (and/or suspension) of a polymeric material in a solvent. In embodiments, the solvent may be selected from the group comprising water, ethanol, (iso)propanol, acetone, acetonitrile, xylene, N- methyl-2-pyrrolidone (NMP), and mixtures thereof. Further, in embodiments, the polymeric material may be selected from the group comprising agar, gelatine, pectin, xanthan gum, starch

(e.g. cornstarch, arrowroot starch, tapioca starch, etc.), sodium alginate, guar gum, gum arabic, polypropylene (especially using xylene as the solvent), and carrageenan.

In specific embodiments, the polymeric material may especially comprise agar.

Using a polymeric binding agent (comprising a polymeric material such as agar) may provide the benefit that the polymeric binding agent may (non-covalently) bind the solvent (e.g. water) and may facilitate pore formation, as a result of decomposition of the (gel-forming) polymeric material during the (debinding and) sintering stage, thereby leaving behind voids (i.e., pores) in the electrode (precursor structure). In embodiments, the polymeric binding agent may comprise the polymeric material in a polymeric material concentration Cpm.

The polymeric material concentration Cpm may in embodiments be selected from the range of > 0.01 wt%, such as from the range of > 0.05 wt%, especially from the range of > 0.1 wt%. Further, the polymeric material concentration Cpm may be selected from the range of > 0.25 wt%, such as from the range of > 0.5 wt%, especially from the range of > 1 wt%. Additionally or alternatively, in embodiments, the polymeric material concentration Cm may be selected from the range of <

10 wt%, such as from the range of < 8 wt%, especially from the range of < 6 wt%, like from the range of < 5 wt%. Further, in embodiments, the polymeric binding agent may comprise a crosslinker.

The crosslinker may provide structural stabilisation for the electrode precursor structure (after hardening of the first mixture). In embodiments, the crosslinker may be selected from the group comprising sodium borate, sodium tetraborate, trimethylolpropane triglycidyl ether (TMPGDE), diisocyanates (e.g. 4,4-diphenyl diisocyanate (DDI) and 1,6-hexamethylene diisocyanate (HDI)), though other options may be possible, as is known to a person skilled in the art.

In embodiments, the polymeric binding agent may comprise the crosslinker in a crosslinker concentration Ce.

The crosslinker concentration C« may in embodiments be selected from the range of > 0.001 wt%, such as from the range of > 0.005 wt%, especially from the range of > 0.01 wt%. Further, the crosslinker concentration C«1 may be selected from the range of > 0.025 wt%, such as from the range of > 0.05 wt%, especially from the range of

> 0.1 wt%. Additionally or alternatively, in embodiments, the crosslinker concentration Ca may be selected from the range of < 7 wt%, such as from the range of < 6 wt%, especially from the range of < 5 wt%, like from the range of < 1 wt%. Hence, in embodiments, the polymeric binding agent may comprise the crosslinker in a crosslinker concentration Cc; selected from the range of 0.001-7 wt%, such as from the range of 0.005-6 wt%, especially from the range of 0.01-5 wt%, like from the range of 0.05-1 wt%. Further, the first mixture may comprise the polymeric binding agent in a polymeric binder concentration Cp.

In embodiments, the polymeric binder concentration Cp may be selected from the range of > 10 wt%, such as from the range of > 15 wt%, especially from the range of > 20 wt%, like from the range of > 25 wt%.

In embodiments, the polymeric binder concentration Cp may (further) be selected from the range of < 70 wt%, such as from the range of < 65 wt%, especially from the range of < 60 wt%, like from the range of < 55 wt%.

In embodiments, the first mixture may further comprise metal fibers and/or metal powder. The metal fibers and/or metal powder may provide the benefit of increasing the (electrical) conductivity and the mechanical strength of the (final) electrode. In embodiments, the metal fibers and/or metal powder may thus comprise an electrically conductive material.

Especially, in embodiments, the metal fibers and/or metal powder may comprise the same metal as the active material. In embodiments, in the (negative) electrode the metal fibers and/or metal powder may comprise one or more of iron fibers and/or powder (and/or flakes), nickel fibers and/or powder (and/or flakes), and copper fibers and/or powder (and/or flakes). Yet, in embodiments, other metal fibers and/or metal powder, such as e.g. stainless steel fibers and/or powder, may also be possible. In the (positive) electrode, the metal fibers and/or metal powder may comprise one or more of nickel fibers and/or powder (and/or flakes) and stainless steel fibers and/or powder. In embodiments, the first mixture may comprise the metal fibers and/or metal powder in a metal fiber concentration Cr. The metal fiber concentration Cr may in embodiments be selected from the range of > 0.01 wt%, such as from the range of > 0.05 wt%, especially from the range of > 0.1 wt%. Further, the metal fiber concentration Cr may be selected from the range of > 0.25 wt%, such as from the range of > 0.5 wt%, especially from the range of > 1 wt%. Additionally or alternatively, in embodiments, the metal fiber concentration Cr may be selected from the range of < 10 wt%, such as from the range of <8 wt, especially from the range of < 6 wt%, like from the range of <5 wt%.

In embodiments, the first mixture may further comprise a poreforming agent.

The poreforming agent may comprise particulate material, wherein the poreforming agent is configured to be (selectively) removable from the electrode precursor structure (during and/or after the curing stage). In embodiments, the poreforming agent may especially be selectively removable by dissolving the poreforming agent in a second solvent, wherein the second solvent may especially be different from the solvent comprised by the polymeric binding agent. For instance, the solvent comprised by the polymeric binding agent may be water, and the second solvent may be an alkaline (aqueous) solution of potassium hydroxide (KOH). Alternatively, the poreforming agent may be selectively removable by thermal decomposition and/or evaporation in the sintering stage. In embodiments, the poreforming agent may be selected from the group comprising paraffin (powder), polymer powder, such as polypropylene,

polyethylene, PVA, and PPO, CaCO3, BaCO;, MgSO:, MgO, S102, Al2(S04)3, and Al:0:. In embodiments, the first mixture may comprise the poreforming agent in a concentration selected from the range of > 10 vol%, such as from the range of > 20 vol%, especially from the range of > 40 vol%, wherein the volume percentage (vol%) may especially be provided by (Vp#/VT)*100%, wherein Vr is the volume of the poreforming agent, and Vr is the (total) volume of the first mixture (including Vr). Additionally or alternatively, the first mixture may comprise the poreforming agent in a concentration selected from the range of < 90 vol%, such as from the range of < 60 vol%, especially from the range of < 35 vol%.

In embodiments, the first mixture may be configured to form a gel or solid at room temperature and below (i.e., at temperatures < 25 °C). Hence, in embodiments, the method may comprise heating the first mixture, especially before transferring the first mixture to the mold. In embodiments, the first mixture may especially be heated to a molding temperature Tm. In embodiments, the molding temperature Tm may be selected to be higher than a melting temperature Tey and/or a glass transition temperature Ts of the first mixture (especially of the polymeric binding agent). That is, in embodiments, the molding temperature

Tm may be selected from the range of Tiel < Tm < Tmet+60 °C, such as from the range of Tien < Tm < Tmeir+40 °C, especially from the range of Tmen < Tm < Tmer+10 °C. Additionally or alternatively, in embodiments, the molding temperature Tm may be selected from the range of

Te < Tm < Tg+60 °C, such as from the range of Tg < Tm < Tg+40 °C, especially from the range of Tp < Tm < Tyt20 °C. Further, in embodiments, the molding temperature Tm may be selected from the range of > 35 °C, such as from the range of > 50 °C, especially from the range of > 70 °C, like from the range of > 90 °C. Additionally or alternatively, in embodiments, the molding temperature Tm may be selected from the range of < 200 °C, such as from the range of < 175 °C, especially from the range of < 150 °C, like from the range of <99 °C. In embodiments, the first mixture may be transferred to the mold at the molding temperature Tm. That is, while transferring the first mixture to the mold, the temperature of the first mixture may be maintained at a transfer temperature T, selected from the range of T-30 °C < Tt < Tm+30 °C, such as from the range of Tm-20 °C < Ti < Tm+20 °C, especially from the range of Tm-15 °C < Ti < Tm+15 °C. Further, in embodiments, the mold may be maintained at the transfer temperature Tt while transferring the first mixture to the mold. In embodiments, the first mixture may be poured into the mold. Alternatively, the first mixture may be transferred to the mold via metal injection molding (MIM) and/or slurry casting. In alternative embodiments, the first mixture may not be transferred to a mold. For instance, in such embodiments, the first mixture may be used as feed for an (extrusion) 3D printer, wherein the first mixture may be cured upon exiting the 3D printer nozzle (and/or after depositing the extruded first mixture) to provide a 3D-printed electrode precursor structure. In other embodiments, the mixture may be extruded in an appropriate cavity-comprising shape. Yet, in embodiments, the first mixture may be transferred to a mold.

In embodiments, the mold may comprise a molding chamber. The molding chamber may especially comprise a first side and a second side, wherein the second side may be configured opposite the first side. In embodiments, the first side and second side of the molding chamber may correspond to the first side and second side of (the outer surface of) the electrode, respectively. Further, in embodiments, the mold may comprise a cavity placeholder, wherein the cavity placeholder may be arranged in the molding chamber. In embodiments, the cavity placeholder may be an elongated structure having a placeholder axis of elongation.

Especially, the cavity placeholder may (partially) extend from (such as be elongated in a direction from) the first side to the second side of the molding chamber. In (further) embodiments, the cavity placeholder may be configured attached to the molding chamber at (at least one of) the first side and the second side. In embodiments, the cavity placeholder may determine the location and size of the cavity (of the electrode). Hence, in embodiments, the cavity placeholder may have a cross-sectional (equivalent circular) diameter Dp (in a cross- section perpendicular to the placeholder axis of elongation). The cross-sectional (equivalent circular) diameter Dp (of the cavity placeholder) may in embodiments be selected from the range of > 0.25 mm, such as from the range of > 0.4 mm, especially from the range of > 0.5 mm, like from the range of > 0.7 mm. Additionally or alternatively, in embodiments, the cross- sectional (equivalent circular) diameter Dp may be selected from the range of <2.5 mm, such as from the range of <2 mm, especially from the range of < 1.5 mm, like from the range of < 1 mm. Further, in embodiments, the cavity placeholder may have a placeholder shape, such as especially in a cross-section perpendicular to the placeholder axis of elongation. The shape of the cavity placeholder (or “placeholder shape”) may in embodiments be selected based on the desired shape of the cavity in the (final) electrode. Hence, in embodiments, the placeholder shape may comprise at least one corner having an angle selected from the range of < 110°, such as from the range of < 100°, especially from the range of < 90°, like from the range of < 80°, wherein the angle may especially be determined from an interior of the placeholder shape. In embodiments, the molding chamber may comprise a plurality of cavity placeholders, wherein each of the cavity placeholders may have a size, shape, and orientation corresponding to one of the (plurality of) cavities in the (final) electrode as defined above.

In embodiments, after transferring the first mixture to the mold, the first mixture may be hardened (or “cured”). Hence, in embodiments, the method may comprise a curing stage. The curing stage may especially comprise hardening the first mixture into a (rubbery) solid (and/or a gel). Hardening of the first mixture may in embodiments be done with one or more of cooling, heating, irradiation with visible and/or (N)UV light, and irradiation with an electron beam. Especially, the curing stage may comprise cooling the (mold comprising the) first mixture, wherein the first mixture may harden into a solid (and/or gel) upon cooling, to provide the mold containing the electrode precursor structure. In embodiments, the first mixture (and the mold) may be cooled (during the curing stage) to a curing temperature Tc selected from the range of < 60 °C, such as from the range of < 40 °C, especially from the range of < 30 °C, like from the range of < 20 °C. Further, in embodiments, the first mixture (and the mold) may be cooled (during the curing stage) to a curing temperature Tc selected from the range of > -15 °C, such as from the range of > -10 °C, especially from the range of > -5 °C, like from the range of > 5 °C. In embodiments, the curing stage may provide, upon hardening the first mixture into a solid (and/or gel), the (mold containing the) electrode precursor structure. The electrode precursor structure may especially comprise the electrode material precursor, (optional) carbon source, and metal fibers and/or metal powder (and optional poreforming agent), embedded in a (solid or gel) matrix of the polymeric binding agent. In embodiments, the electrode precursor structure may not (yet) be porous (after the curing stage).

Further, in embodiments, the method may comprise removing the electrode precursor structure from the mold. In embodiments, removing the electrode precursor structure from the mold may comprise dissolving the mold (including the cavity placeholder) in a third solvent, different from the solvent (comprised by the polymeric binding agent) and (optionally) different from the second solvent (used to remove the poreforming agent). For instance, a mold made of PLA (polylactic acid) may be dissolved in a 30 wt% KOH solution (in water), and rinsed with demiwater to obtain the electrode precursor structure. Additionally or alternatively, removing the electrode precursor structure from the mold may comprise releasing the electrode precursor structure from the mold (i.e., taking the electrode precursor structure out of the mold, wherein the mold may remain intact to be used for the production of a next electrode). In such embodiments, the cavity placeholder may in embodiments be configured separate from the molding chamber at one of the first side and the second side. That is, in embodiments, the cavity placeholder may be configured attached to the molding chamber at one of the first side and the second side, and configured separate (“non-attached”) (but in physical contact with) the molding chamber at another of the first side and the second side. Hence, upon releasing the electrode precursor structure from the mold, the cavity placeholder may be (simultaneously) removed from the cavity precursor in the electrode precursor structure. Alternatively, the cavity may not form a channel, i.e., the cavity may not provide a through opening through the channel.

In such embodiments, the electrode precursor structure may be lifted out of the mold to provide the electrode precursor structure comprising the cavity precursor (structure).

In embodiments, the electrode precursor structure may be submerged in a fourth solvent, wherein the fourth solvent may be configured to displace the (first) solvent of the polymeric binder agent in the electrode precursor structure. In embodiments, the fourth solvent may be an organic solvent with a faster evaporation rate than the (first) solvent. In specific embodiments, the fourth solvent may be selected from the group comprising acetone, ethanol, dimethyl ether, and methanol. Submerging the electrode precursor structure in the fourth solvent may provide the benefit that the fourth solvent may be more easily evaporated and/or removed in the (debinding step of the) sintering stage.

Hence, the method may further comprise a sintering stage. The sintering stage may comprise heating the electrode precursor structure (to and) at a sintering temperature Ts to provide the electrode (or a sintered electrode precursor). In embodiments, the sintering temperature Ts may be selected from the range of > 500 °C, such as from the range of > 600 °C, especially from the range of > 700 °C, like from the range of > 800 °C. Additionally or alternatively, in embodiments, the sintering temperature Ts may be selected from the range of < 1200 °C, such as from the range of < 1100 °C, especially from the range of < 1000 °C, like from the range of < 900 °C. In embodiments, the sintering stage may comprise heating the electrode precursor structure to the sintering temperature Ts with a heating ramp of > 0.5 °C/min, such as a heating ramp of > 1 °C/min, especially with a heating ramp of > 2 °C/min, like with a heating ramp of > 5 °C/min. That is, the temperature may be increased from an initial temperature (e.g. room temperature or a debinding temperature Ta, (see below)) to the sintering temperature Ts with a (heating) rate of > 0.5 °C/min, such as with a rate of > 1 °C/min, especially with a rate of > 2 °C/min, like with a rate of > 5 °C/min.

Further, in embodiments, the sintering stage may comprise heating (or “sintering”) the electrode precursor structure (at the sintering temperature Ts) for a sintering duration ts. In embodiments, the sintering duration ts may be selected from the range of > 20 min, such as from the range of > 40 min, especially from the range of > 60 min. Additionally or alternatively, the sintering duration ts may be selected from the range of < 5 h, such as from the range of < 4 h, especially from the range of <3 h. In embodiments, heating the electrode precursor structure may facilitate a reduction of the electrode material precursor to (facilitate forming) the active material (of the porous electrode material). Hence, in embodiments, the sintering stage may comprise reducing the electrode material precursor to form the active material (of the electrode, especially of the porous electrode material). In embodiments, the reduction of the electrode material precursor (to the active material) may be facilitated by a reducing atmosphere, i.e, by a sintering gas (mixture) having reducing properties to facilitate and/or aid the reduction of the electrode material precursor. Hence, the sintering stage may especially comprise exposing the electrode precursor structure to a (reducing) sintering gas (during heating of the electrode precursor structure). In embodiments, the sintering gas may comprise one or more reducing gases selected from the group comprising (molecular) hydrogen, carbon monoxide, natural gas, ammonia, and hydrogen sulfide, such as especially (at least) hydrogen. In embodiments, the sintering gas may further comprise one or more carrier gases selected from the group comprising nitrogen, argon, helium, and (other) inert gases. In embodiments, the sintering gas may comprise a mixture of the reducing gas in the carrier gas, wherein the concentration of the reducing gas (in the carrier gas) may be selected from the range of > 4%, such as from the range of > 10%, especially from the range of > 20%. Further, the concentration of the reducing gas (in the carrier gas) may be selected from the range of < 50%, such as from the range of < 40%, especially from the range of < 30%. In embodiments, the sintering gas may be flown over the electrode precursor structure, such that “fresh” sintering gas may be provided (continuously) to the electrode precursor structure. Such a flow may further facilitate the protection from air, and removal of one or more of H.O, CO:2, polymer decomposition products, and pore former degradation products formed during the sintering process. Hence, in embodiments, the sintering gas (in combination with the sintering temperature Ts) may facilitate the removal of the polymeric binder agent, especially by either evaporation and/or decomposition of the (polymeric material and/or crosslinker comprised by the) polymeric binder agent. Hence, in embodiments, the method and the composition of the first mixture may have as advantage that the reduction of the electrode material precursor to the active material may be combined with the removal of the polymeric binder agent in the sintering stage. Further, the sintering stage may facilitate converting the carbon source of the first mixture into the carbon comprised by the (iron-based) electrode (see above).

In embodiments, the sintering stage may comprise a debinding step, wherein the debinding step comprises heating the electrode precursor structure at a debinding temperature

Ta, especially for a debinding duration tg. In embodiments, the debinding step may especially be performed before heating the electrode precursor structure to the sintering temperature Ts.

That is, the sintering stage may comprise heating the electrode precursor structure at a debinding temperature Tgp for a debinding duration tg, and then (further) heating the electrode precursor structure at the sintering temperature Ts for a sintering duration ts. In embodiments,

the debinding temperature Tas may be selected from the range of > 150 °C, such as from the range of > 200 °C, especially from the range of > 300 °C. Additionally or alternatively, in embodiments, the debinding temperature Ta, may be selected from the range of < 700 °C, such as from the range of < 600 °C, especially from the range of < 500 °C. Further, in embodiments, the debinding duration tg, may be selected from the range of > 10 min, such as from the range of > 25 min, especially from the range of > 40 min. Additionally or alternatively, the debinding duration ta may be selected from the range of <3 h, such as from the range of < 2 h, especially from the range of < 1 h. In embodiments, the debinding step may comprise heating the electrode precursor structure to the debinding temperature Tay with a heating ramp of > 1 °C/min, such as a heating ramp of > 2 °C/min, especially with a heating ramp of > 5 °C/min, like with a heating ramp of > 10 °C/min. That is, the temperature may be increased from an initial temperature (e.g. room temperature) to the debinding temperature Tas with a (heating) rate of > 1 °C/min, such as with a rate of > 2 °C/min, especially with a rate of > 5 °C/min, like with a rate of > 10 °C/min. Hence, the sintering stage may e.g. comprise heating the electrode precursor structure to a debinding temperature Ta; of 300 °C with a heating rate of 5 °C/min, maintaining the debinding temperature Tao for a debinding duration tm of 1 h, further heating the electrode precursor structure to a sintering temperature Ts of 800 °C with a heating rate of 1 °C/min, maintaining the sintering temperature Ts for a sintering duration ts of 2 h, and allowing the sintered electrode (precursor) to cool down naturally (e.g. to room temperature).

In embodiments, as indicated above, the sintering stage may provide a sintered electrode precursor. The sintered electrode precursor may especially comprise the cavity (or cavities) and scaffold material of the electrode, yet may not comprise the active material. Hence, the electrode material precursor may in embodiments not be reduced to the active material in the sintering stage, yet may be reduced to the scaffold material. In such embodiments, the method may further comprise a deposition stage. The deposition stage may in embodiments especially be executed after the sintering stage. Further, in embodiments, the deposition stage may comprise depositing an active material onto the sintered electrode precursor to provide the electrode. Deposition of the active material may be performed by one or more of chemical vapor deposition and submerging the sintered electrode precursor in(to) a molten precursor active material salt (e.g. molten nickel nitrate hydrate at 90 °C), or a solution of such salt (especially under vacuum). Further, the deposition stage may comprise a second heating stage, wherein the deposited precursor salt is converted to precursor material that can form the active material upon thermal decomposition (e.g. the molten nickel nitrate hydrate may decompose towards NiO or Ni(OH)NO:;, precursors of the active material Ni(OH)z). The active material may in embodiments then be formed in a KOH solution at e.g. 80 °C, wherein the precursor of the active material is converted into to Ni(OH),. The second heating stage may require a temperature of < 500 °C, such as < 400 °C, especially < 300 °C, like < 250 °C. Multiple impregnation cycles of the deposition stage (comprising immersion of the sintered electrode precursor in molten salt (under vacuum) and decomposition) may be performed to provide the (final) electrode, as 1s known in the art for e.g. sintered nickel-based electrodes. For the iron- based electrode, a second sintering stage may (also) be performed, wherein the (second) electrode material precursor may be reduced to the active material. In embodiments, the conditions for the second sintering stage may be equal to the conditions of the (first) sintering stage. Alternatively, the sintering temperature Ts, sintering duration ts, and reducing atmosphere may be individually selected for the (first) sintering stage and the second sintering stage. Hence, in specific embodiments, the sintering stage may provide a sintered electrode precursor, and the method may further comprise a deposition stage executed after the sintering stage, wherein the deposition stage may comprise depositing an active material onto the sintered electrode precursor to provide the electrode. A method comprising a deposition stage may provide the benefit that the active material loading (on the surface of the scaffold material and/or) in the electrode may be controlled separately from the structure of the electrode. Further, a separate deposition stage may provide the benefit that the sintered electrode precursor (comprising the scaffold material) may be mass-produced, after which the electrode may be tuned towards a specific application and/or active material in the deposition stage. The scaffold material for a positive (nickel-based) and/or negative (iron-based) electrode may be chosen to be stable at applied potentials in the electrolyte while the active materials may undergo oxidation and reduction reactions.

In embodiments, as indicated above, the electrode as defined herein may be used in a battery and/or electrolyser. Especially, the electrode may be a 3D electrode for an integrated battery and electrolyser application. Hence, according to a further aspect, the invention may provide an energy apparatus (“apparatus”) having (both) an electrical storage functionality and an electrolysis functionality. The energy apparatus may comprise a functional unit, wherein the functional unit may comprise the electrode as defined herein. Further, the functional unit may comprise a first cell, wherein the first cell may comprise a first cell electrode and one or more first cell openings for a (basic) first cell aqueous liquid and for a first cell gas. In embodiments, the first cell electrode may especially comprise an iron-based electrode. Further, the functional unit may comprise a second cell, wherein the second cell may comprise a second cell electrode and one or more second cell openings for a (basic) second cell aqueous liquid and for a second cell gas. In embodiments, the second cell electrode may especially comprise a nickel-based electrode. In embodiments, the functional unit may (further) comprise a separator, wherein the first cell and the second cell share the separator, and wherein the separator is configured to block transport of one or more of O2 and Hs: from one cell to another while having permeability for at least one or more of hydroxide ions (OH"), monovalent sodium (Na*), monovalent lithium (Li"), and monovalent potassium (K*). In embodiments, the functional unit may comprise a first electrical connection in electrical connection with the first cell electrode, and a second electrical connection in electrical connection with the second cell electrode. Further, in embodiments, the first cell electrode may comprise the electrode as defined herein. Additionally or alternatively, in embodiments, the second cell electrode may comprise the electrode as defined herein.

Hence, in specific embodiments, the invention may provide an energy apparatus having an electrical energy storage functionality and an electrolysis functionality, the energy apparatus comprising a functional unit, wherein the functional unit comprises the electrode as defined herein, the functional unit further comprising: (A) a first cell, comprising a first cell electrode and one or more first cell openings for a basic first cell aqueous liquid and for a first cell gas, wherein the first cell electrode comprises an iron-based electrode; (B) a second cell, comprising a second cell electrode and one or more second cell openings for a basic second cell aqueous liquid and for a second cell gas, wherein the second cell electrode comprises a nickel-based electrode; (C) a separator, wherein the first cell and the second cell share the separator, wherein the separator is configured to block transport of one or more of O2 and H: from one cell to another while having permeability for at least one or more of hydroxide ions (OH), monovalent sodium (Na"), monovalent lithium (Li*), and monovalent potassium (K*); and (D) a first electrical connection in electrical connection with the first cell electrode, and a second electrical connection in electrical connection with the second cell electrode; wherein the following applies: (a) the first cell electrode comprises the electrode (of the first aspect) as defined herein, and/or (b) the second cell electrode comprises the electrode (of the first aspect) as defined herein.

The main advantages of combining a battery and electrolyser in one device as presently claimed are numerous. For instance, normally the overpotentials applied and the water splitting is considered a loss factor in the operation of a Ni-Fe battery. Here there is made use of that energy in the electrolysis process, resulting in higher overall efficiency. The heat dissipated during charging the battery and electrolysing water is required for generating hydrogen and oxygen. This required heat is a result from the increase in entropy when splitting liquid water in gaseous H: and Oz. The increase in entropy dS corresponds with an amount of heat TdS that is required to continue the reaction, next to supplying the Gibbs free energy dG.

So in total the energy provided by the system equals dG + TdS = dH, where dG is provided as electrical power and TdS as heat. Hence, with such apparatus it is possible to discharge and generate H: at the same time. Further, with such apparatus it is possible to store electricity, when charging the apparatus, and generate hydrogen and oxygen. Further, the hydrogen production occurs during chemical reduction of the iron electrode to Fe metal and also continuing for a desired period after that. It appears that the battery function operates better reversibly and only reaches its full capacity when overcharged like this. The original Ni-Fe batteries are however not charged that fully because then the energy efficiency is low because of the overpotentials that occur and the gassing, and also the electrolyte needs refilling. Here it is intentionally done (when there is sufficient electricity supply each cycle), which actually appears to increase lifetime of the battery electrolyser. That such overcharging is allowed and intentionally realised makes the power electronics also relatively simple; for normal battery systems overcharging is prevented at the individual cell level, making the battery cell management more demanding.

Using the electrode as defined herein (i.e., comprising the cavity and the porous electrode material) in the energy apparatus as defined herein may further provide the benefit that the cavity may facilitate the removal of one or more of hydrogen and oxygen from the electrode during operation of the energy apparatus. Especially, the dimensions of the electrode may facilitate dissolved gas diffusion out of the porous electrode material and into the cavity (or cavities). Consequently, gas bubble formation may occur relatively little in the porous electrode material during the battery charging stage, as the gas evolution may not reach sufficient supersaturation, and the battery charge reactions may have a much larger exchange current. In embodiments, water splitting (to produce oxygen and hydrogen) and bubble formation may occur (only) after charging the electrode fully, as then the electrical current may be (fully) utilized for the water splitting reactions. During the water splitting phase (or electrolysis phase), the cavity may facilitate a removal of the gas bubbles from (inside) the porous electrode material, while the porosity of the porous electrode material may facilitate a larger surface area being in contact with the electrolyte (in the cavity), thereby decreasing the local current density and increasing the active mass participating in the water splitting reaction.

Hence, the cavity and porosity of the porous electrode material of the (3D) electrode may increase electrolyte accessibility, thereby reducing the effective ionic resistance of the electrode. Consequently, the current may be distributed more homogeneously over the electrode, thereby increasing the utilization of the electrode, also at higher current densities.

During charging, this may result in an increase in charging efficiency, as a larger fraction of the electrode may be charged before electrolysis may start at the front of the electrode (i.e., the major face configured facing the separator). In addition, the decrease in ionic resistance may also increase the discharge capacity. Due to the increased utilization during charging and discharging, the electrode may reach its final capacity during activation sooner (than a non- porous electrode not comprising a cavity), and may provide increased power density and energy density.

The energy apparatus may comprise one or more functional units. In embodiments, the energy apparatus may include a single functional unit; in other embodiments the apparatus may include a plurality of functional units. The functional units can be coupled parallel or in series. Each functional unit may in embodiments comprise a first cell, a second cell and a separator. Especially, a functional combination of a first cell and a second cell can be used as battery or electrolyzer.

Especially, the functional unit may comprise a first cell, comprising a first cell electrode and one or more first cell openings for a first cell aqueous liquid and for a first cell gas, wherein the first cell electrode especially comprises an iron based electrode, and a second cell, comprising a second cell electrode and one or more second cell openings for a second cell aqueous liquid and for a second cell gas, wherein the second cell electrode especially comprises a nickel based electrode. Each cell at least comprises an opening for introduction of the respective aqueous liquids. The aqueous liquid used is especially a basic aqueous liquid, such as comprising one or more of KOH, LiOH, and NaOH. Especially, the concentration of OH is at least 3 mol/l. Especially, the concentration of the hydroxide (especially one or more of KOH,

NaOH and LiOH) in water is in the range of 4.5 — 8.4 mol/L (25-47 wt% for KOH). Hence, these openings, respectively, may be configured as inlets of recycled electrolyte with water added to maintain the chosen concentration of KOH, LiOH and/or NaOH. The first cell aqueous liquid and the second cell aqueous liquid within the cells are especially alkaline, such as at least 0.1 mmol/l OH’, especially at least 1 mol/l OH’, even more especially at least 3 mol/l OH", such as at least about 6 mol/l OH". The liquid in the cells may be supplemented with liquids from an aqueous liquid control system (see below). Fresh water may not necessarily be alkaline, as the alkali in the cells may substantially be effectively not used. The “cell aqueous liquid” may also be indicated as electrolyte.

Further, each cell may also comprise a further opening, especially configured for removal of the aqueous liquid and/or for removal of gas. Both may escape from the same opening. The first cell gas especially comprises Hz gas; the second cell gas especially comprises

O:. The aqueous liquid in the cell and the cell gas may escape from the same opening.

Alternatively or additionally, two or more openings may be used, e.g. one for the removal of aqueous liquid and one for the removal of gas. As each cell may have two openings, the aqueous liquid may be flowed through each cell, where the flow aids in gas removal, cooling (or heating) when necessary and water refilling. Depending on the applied current per cm? electrode surface area the flow (in volume/area/time) may be for instance in the range of about 0.3 pl/cm*/h — 3.5 ml/cm?/h.

Further, each cell may comprise at least one electrode. The first cell may comprise the first cell electrode, which may especially comprise an iron-based electrode. The iron-based electrode may comprise in a charged state essentially Fe (metal) and in a discharged state essentially Fe(OH)z, as was the case in the Edison Ni-Fe battery. In embodiments, the first cell electrode may (at least) comprise the electrode (of the first aspect) as defined herein, such as especially the iron-based electrode as defined herein. Further, the first cell electrode may comprise a plurality of the electrodes (of the first aspect) as defined herein, wherein the plurality of electrodes may be arranged together to form the first cell electrode. In such embodiments, the plurality of electrodes may especially be arranged with their respective edge faces facing (each other), such that the thickness of the (combined) plurality of electrodes may be equal to the electrode thickness de, and the surface area of the (combined first major faces of the) plurality of electrodes may be equal to n*SAg, wherein n is the number of electrodes (according to the first aspect) comprised by the first cell electrode. In embodiments, the cavities (especially the cavities forming channels) in neighboring electrodes may be aligned, such that a first cell electrode channel (extending through the first cell electrode) may be provided. The term “first cell electrode” may also relate to a plurality of first cell electrodes. Further, the second cell may comprise the second cell electrode, which may especially comprise a nickel-based electrode.

The nickel-based electrode may comprise in a charged state essentially NiOOH and in a discharged state essentially Ni(OH)2. In embodiments, the second cell electrode may comprise (at least) the electrode (of the first aspect) as defined herein, such as especially the nickel-based electrode as defined herein. Similarly as for the first cell electrode, the second cell electrode may comprise a plurality of the electrodes (of the first aspect) as defined herein, wherein the plurality of electrodes may be arranged together (with their respective edge faces facing) to form the second cell electrode. The term “second cell electrode” may also relate to a plurality of second cell electrodes.

Hence, in specific embodiments, the first cell electrode may comprise the electrode as defined herein, and the second cell electrode may comprise the electrode as defined herein. As indicated above, using the electrode as defined herein for an integrated battery and electrolyser application may provide improved gas transport during the electrolysis phase, and an improved charging efficiency during the charging phase.

The first cell and the second cell may share a separator, but may be separated from each other by this separator. Hence, liquid may not flow from one cell to the other via the separator. Also, hydrogen gas and/or oxygen gas may not flow from one cell to the other via the separator. However, the separator may be permeable for specific ions, such as at least one or more of OH" ions, neutral H,O, monovalent sodium (Na*), monovalent lithium (Li*), and monovalent potassium (K*). Hence, the first cell and the second cell may share the separator, wherein the separator may be configured to block transport of one or more of O; and H: from one cell to another, especially to block transport of both O2 and Hz, while having permeability for at least one or more of OH ions, neutral HO, monovalent sodium (Na*), monovalent lithium (Li"), and monovalent potassium (K*), especially all. Hence, especially the separator may have a relative high ionic conductivity and a relatively low ionic resistance. For instance, the ionic resistance may be lower than <0.3 Q.cm? in 30 wt% KOH solution (at 30 °C). The separator may e.g. comprise a membrane, such as electrolysis membranes known in the art.

Examples of membranes may e.g. include alkaline resistant polymer membranes and polymer composite mambranes, such as e.g. a Zirfon (from Agfa) membrane. Such membrane may e.g. consist of a polymer matrix in which ceramic micro-particles (zirconium oxide) are embedded.

This body is reinforced internally with a mesh fabric made from monofilament polyphenylene sulphide (PPS) or polypropylene (PP) fabric. It has a controlled pore size of about 0.15 um and bubble point (especially defined as gas pressure against one side of the membrane required to form bubbles at the other side where there is liquid) of about 2 +/- 1 bar (overpressure). Such membrane may be permanently hydrophilic, by incorporated metal oxide particles, perfectly wettable in water and most common electrolytes. Such membrane may be stable in strong alkaline (up to 6M KOH) conditions and thermally stable up to 110°C. The pore size may e.g. be in the range of about 0.05-0.3 um, such as about 0,15um; the thickness may e.g. be in the range of about 100-1000 um, such as about 500 um. Between the separator and each electrode, a respective spacer may be configured. These spacers may include openings for transport of the aqueous liquids and providing access for these liquids to the respective electrode. Yet, in embodiments, the electrodes (comprising the electrode as defined herein, i.e, comprising cavities) may be placed directly against the separator, as the cavities (especially the cavities forming open fluid channels and having a cavity axis of elongation Ac configured perpendicular to the electrode axis of elongation Ag) may provide transport of the aqueous liquids and access for these liquids to the respective electrode. In this way, a functional unit is provided which is substantially closed, except for the herein indicated openings. For electrical connection, the electrodes may be connected with an electrical connection which is also accessible from external from the functional unit. Hence, the functional unit may further comprise a first electrical connection in electrical connection with the first cell electrode, and a second electrical connection in electrical connection with the second cell electrode.

For a good processing with the functional unit, the energy apparatus may comprise one or more of an aqueous liquid control system, a gas storage system, a pressure system, a charge control unit, a first connector unit, a second connector unit, and a control unit.

Further, additionally the apparatus may comprise a thermal management system and/or thermal insulation. Especially, the energy apparatus may comprise all these items.

Hence, in an embodiment the energy apparatus may further comprise an aqueous liquid control system configured to control introduction of one or more of the first cell aqueous liquid and the second cell aqueous liquid into the functional unit. Such aqueous liquid control system may include one or more valves. Further, such aqueous liquid control system may — during operation — functionally be connected with a service pipe for water. In combination with the pressure system (see also below), the aqueous liquid may also be provided under pressure to the functional unit (see further also below). Further, the aqueous liquid control system may include storage for caustics, such as one or more of NaOH, LiOH, and KOH, especially at least

KOH. The aqueous liquid control system may independently provide the liquid to the first cell and the second cell. Further, the aqueous liquid control system may include a return system, configured to receive the liquid downstream from the first cell and/or the second cell and reuse at least part of the first liquid and/or second liquid. The term “aqueous liquid control system” may also refer to a plurality of aqueous liquid control systems.

Further, in an embodiment the energy apparatus may further comprise a storage system configured to store one or more of the first cell gas and the second cell gas external from said functional unit. Hence, storage may be done external from the functional unit. To this end the apparatus may comprise a storage system configured to store Ha and/or a storage system configured to store O2. At least, the apparatus may comprise a storage system configured to store Ha. In combination with the pressure system (see also below), the storage system may also be configured to store the one or more of the first cell gas and the second cell gas under pressure (see further also below). The term “storage system” may also refer to a plurality of storage systems.

Hence, in an embodiment the energy apparatus may further comprise a pressure system configured to control one or more of (a) the pressure of the first cell gas in the functional unit, (b) the pressure of the first cell gas in the storage system, (c) the pressure of the second cell gas in the functional unit, and (d) the pressure of the second cell gas in the storage system.

To this end, the pressure system may comprise a pump, a valve, etc.. In an embodiment, the pressure system essentially comprises one or more valves. The term “pressure system” may also refer to a plurality of pressure systems. Especially when two or more different types of fluids have to be pressurized, two or more independent pressurize systems may be applied.

In yet a further embodiment the energy apparatus may further comprise a charge control unit configured to receive electrical power from an external electrical power source and be configured to provide said electrical power to said functional unit during at least part of a charging time at a current (sometimes also indicated as “current strength”) that results in a potential difference between the first cell electrode and the second cell electrode of more than 1.55 V at 18 °C and 1.50 V at 40 °C, i.e. in practice generally at least 1.50 V, e.g, at an operating temperature of 40°C or higher, especially at (about) 45 °C, or especially at (about) 60 °C.

Starting from the discharged state the current is first applied to mainly charge the battery; by applying this current voltages reach up to 1.65 V at 18 °C and 1.55 V at 40 °C before the battery is approximately fully charged, i.e. in practice thus at least 1.55 V. Progressively more hydrogen is produced after the battery capacity is reached and the voltage can then reach up to 1.75 V (at 18 °C) and 1.62 V at 40 °C, i.e. in practice thus at least 1.62 V. The energy efficiency of the battery functionality charging and the electrolytic gas production is calculated as the integral of the battery output current times its voltage integrated over discharge time plus the higher heating value (HHV) of the amount of hydrogen gas produced during charge and (self- )discharge over the total cycle, divided by the integral of the input current times its voltage over the charge time. It appears that very good results are obtained in terms of total energy efficiency, even when going well above the normal voltage upper limits of 1.65 (at 18 °C) or 1.55 V (at 40 °C) (i.e. in practice thus at least 1.55 V) for Ni-Fe charging for full nominal charge, and especially when charging/inserting current far beyond the nominal capacity of the

Ni and Fe battery electrodes. The charge control unit may include electronic devices to convert high voltages to the required voltage and/or to convert AC voltage to DC voltage. Especially, in an embodiment of the energy apparatus, the charge control unit configured to provide said electrical power to said functional unit during at least part of a charging time at a current that results in a potential difference between the first cell electrode and the second cell electrode selected from the range of 1.4-1.75 V. Best results in terms of battery electrochemical reversibility, gas amount production, and overall energy efficiency are obtained for applied currents that result in cell potentials in this voltage range.

For discharge best results may be obtained when discharge is continued to a level preferably not lower than 1.10V for the cell. The control system, optionally in combination with the charge control unit, may also be configured to control discharging of the functional unit. Discharging may be done to an industrial object or vehicle, etc., using electrical energy. However, alternatively or additionally, the functional unit may also be discharged to an electricity grid.

Further, the charge control unit may be configured to provide said electrical power to said functional unit during at least part of a charging time at a current corresponding to the nominal battery capacity Cm expressed in Ah divided by minimum of 2h, i.e. C/time with time > 2h. Such applied currents may lead to a potential difference between the first cell electrode and the second cell electrode of especially more than 1.37 V, but especially at maximum not more than 2.0 V

As indicated above, the apparatus may further include thermal insulation, especially configured to keep loss of thermal energy from the functional unit low. Further, the apparatus may comprise a thermal management system, configured to keep the temperature of the unit equal to or below a predetermined maximum temperature, for instance equal to or below 95 °C. Hence, in an embodiment, especially for large systems (such as 10 kW or more), the temperature of the cells is monitored and the applied charge and discharge currents may be reduced when the temperature rises above the set limit of 60 °C. The thermal management system may at least partly be comprised by the control system, i.e. with respect to the controls.

Further, the thermal isolation may be comprised by the thermal management system.

As indicated above, the energy apparatus may include a plurality of functional elements, configured electrically in series and/or parallel, such as to increase the potential difference (in series) and or the charge (parallel) that can be provided.

In an embodiment the energy apparatus may further comprise a first connector unit for functionally coupling to a receiver to be electrically powered and the electrical connection. An example of a device may be a car. Hence, especially the apparatus may include a(n electrical) plug or a socket that can be connected to such device, which may thus especially include a socket or a plug. The first connector is especially configured to transfer electrical power from the apparatus to a receiver, such as an external device, such as a battery of such device, or to an electricity grid. The term “first connector unit” may also refer to a plurality of first connector units. In an embodiment the energy apparatus may further comprise a second connector unit for functionally connecting a device to be provided with one or more of the first cell gas and the second cell gas with said storage system. Hence, especially the apparatus may include a(n hydrogen gas) plug or a socket, that can be connected to such device, which may thus especially include a socket or a plug. The second connector is especially configured to transfer hydrogen gas from the storage to a receiver, such as an external device, such as a hydrogen storage unit of such device, or to a gas grid. The term “second connector unit” may also refer to a plurality of second connector units. Note that the receiver for the gas is not necessarily the same as the receiver for the electricity.

In embodiments the energy apparatus may further comprise a control system configured to control one or more of the aqueous liquid control system (if available), the storage system (if available), the pressure system (if available), and the charge control unit (if available). The control system is especially configured to control the apparatus, and the individual elements, especially the aqueous liquid control system, the storage system, the pressure system, and the charge control unit. In this way, the charging and electrolysis process may be optimized at maximum efficiency, amongst others e.g. dependent upon the availability of electrical power from an external electrical power source and the consumption of electrical power and/or hydrogen gas. Hence, in a specific embodiment of the energy apparatus, the control system is configured to control the charge control unit as function of a charge status of the functional unit and an availability of electrical power from the external electrical power source. Yet further, the control system is configured to control the charge control unit as function of a charge status of the functional unit, the status of a gas storage (full or further fillable), and an availability of electrical power from the external electrical power source.

Optionally, the charge control unit may also be configured to feed electricity back into the electricity grid. The control system may especially be configured to control the operation conditions of the energy apparatus as a function of electricity demand and/or gas demand from one or more clients (like the devices herein indicated) and/or availability of electricity (in the grid). Hence, the control system may amongst others control one or more of temperature, liquid flow, voltage difference, voltage sign, etc., as function of the presence of external demand and/or the type of external demand (Hz and/or electricity). Hence, in specific embodiments, the energy apparatus may further comprise: (A) an aqueous liquid control system configured to control introduction of one or more of the first cell aqueous liquid and the second cell aqueous liquid into the functional unit; (B) a storage system configured to store one or more of the first cell gas and the second cell gas external from said functional unit; (C) a pressure system configured to control one or more of (a) the pressure of the first cell gas in the functional unit, (b) the pressure of the first cell gas in the storage system, (c) the pressure of the second cell gas in the functional unit, and (d) the pressure of the second cell gas in the storage system; (D) a charge control unit configured to receive electrical power from an external electrical power source and configured to provide said electrical power to said functional unit during at least part of a charging time at a potential difference between the first cell electrode and the second cell electrode of more than 1.37 V, wherein the energy apparatus is configured to operate in the range of 1.48 to 2.0 V when producing hydrogen; (E) a first connector unit for functionally coupling to a receiver to be electrically powered and the electrical connection, and a second connector unit for functionally connecting a device to be provided with one or more of the first cell gas and the second cell gas with said storage system; and (F) a control system configured to control the aqueous liquid control system, the storage system, the pressure system, and the charge control unit. An energy apparatus comprising such systems may provide the benefit of allowing more control over the conditions and operation of the energy system. Further, such systems may facilitate controlling the (current) function of the apparatus (i.e., function (mainly) as a battery, or function (mainly) as an electrolyser) over time, depending on the supply and demand of electrical energy and/or hydrogen gas.

In embodiments, as indicated above, the energy apparatus may comprise a plurality of functional units, such as at least two functional units. In such embodiments, a first cell electrode of a first functional unit and a second cell electrode of a second functional unit may be separated from each other by a bipolar plate. A bipolar plate may especially connect and separate the cells of different functional units in series to form a stack with required voltage (when discharging, charging or generating hydrogen and oxygen). The bipolar plate may conduct electrical current from the anode of one cell of a unit to the cathode of the next cell of another functional unit. Further, the bipolar plate may facilitate water management within the functional unit and may support the membrane and electrodes etc.. Hence, in specific embodiments, the energy apparatus comprises at least two functional units, wherein a first cell electrode of a first functional unit and a second cell electrode of a second functional unit are separated from each other by a bipolar plate, wherein the bipolar plate is electrically conductive.

In yet further embodiments, the bipolar plate may comprise at least two bipolar plate sections which are configured electrically separated from each other, wherein one or more first cell electrodes are associated with a first bipolar plate section, wherein one or more first cell electrodes are associated with a second bipolar plate section, wherein one or more second cell electrodes are associated with said first bipolar plate section, and wherein one or more second cell electrodes are associated with said second bipolar plate section.

Features of embodiments of the energy apparatus may be described in more detail in WO2016178564A1, which is hereby herein incorporated by reference.

Yet, in a further aspect the invention also provides a system including the energy apparatus as defined herein. Such system may further include a power source, especially an electrical power source. Hence, in embodiments, the invention may provide an energy system comprising the energy apparatus as defined herein and an external (electrical) power source.

The power source may be used to charge the functional unit (i.e. to charge the battery). The apparatus may be functionally connected to a mains. However, the apparatus may also be functionally connected to a local electrical power generator. For instance, a plant generating biomass or a site where biomass is collected, may include a device for converting biomass into electricity, which can be used for powering the apparatus. Likewise, a local wind turbine, or local wind turbines, or a local photovoltaic or local photovoltaics, or a local water turbine, or local water turbines, may be used to provide electrical power to the apparatus. Of course, such external power source may also be integrated in an electrical power infrastructure, which may include various renewable and conventional power plants. Hence, in an embodiment the external power source may comprise one or more of a photovoltaic cell, a wind turbine, and a water turbine. Hence, the energy apparatus may be comprised in one or more of an electrical energy grid, a Hz gas grid, and an O» gas grid. The term “energy apparatus” may also refer to a plurality of “energy apparatus”. Hence, in an embodiment the energy system may comprise a plurality of energy apparatus and a plurality of external power sources. These energy apparatus and external power forces may be functionally associated, such as via an electricity grid. For instance, in an embodiment the energy apparatus are arranged remote from each other along highways and roads. The energy system may further include an electricity grid.

Especially, the external power sources may be functionally coupled to this electricity grid. Also industry, houses, etc., may functionally be coupled to such electricity grid. Hence, in an embodiment the energy system may comprise a plurality of energy apparatus and a plurality of external power sources and an electricity grid.

Yet, in a further aspect the invention also provides a method of storing electrical energy and one or more of hydrogen (Hz) and oxygen (O:) with a (single) energy apparatus.

Especially, the invention also provides a method of storing electrical energy and one or more of hydrogen (H:) and oxygen (O2) with the energy apparatus as defined herein, the method comprising: providing the first cell aqueous liquid, the first cell aqueous liquid, and electrical power from an external power source to the functional unit, thereby providing an electrically charged functional unit and one or more of hydrogen (Hz) and oxygen (O2) stored in the storage system as defined herein, wherein during at least part of a charging time the functional unit is charged at a potential difference between the first cell electrode and the second cell electrode of more than 1.37 V, especially at least 1.55 V. In embodiments, the method may comprise operating in the range of 1.48 to 2.0 V when producing hydrogen. Further, in embodiments, during at least part of a discharging time, the potential difference between the first cell electrode and the second cell electrode may be selected from the range of 0.9-1.37 V. In embodiments, the method may comprise operating in the range of 1.0 to 1.37 V when discharging. Hence, in specific embodiments, the invention may provide a method of storing electrical energy and one or more of hydrogen (Hz) and oxygen (O2) with the energy apparatus as defined herein, the method comprising: providing the first cell aqueous liquid, the second cell aqueous liquid, and electrical power from an external power source to the functional unit, thereby providing an electrically charged functional unit and one or more of hydrogen (H;) and oxygen (O:) stored in the storage system as defined herein, wherein during at least part of a charging time the functional unit is charged at a potential difference between the first cell electrode and the second cell electrode of more than 1.37 V, wherein the method comprises operating in the range of 1.48 to 2.0 V when producing hydrogen, wherein during at least part of a discharging time the potential difference between the first cell electrode and the second cell electrode is selected from the range of 0.9-1.37 V, and wherein the method comprises operating in the range of 1.0 to 1.37 V when discharging.

In embodiments, during at least part of a charging time, a current may be selected resulting in a potential difference between the first cell electrode and the second cell electrode selected from the range of 1.50-2.0 V, such as from the range of 1.55-1.75 V, like at least 1.6V. Further, especially a current density (of the electrodes) may be selected from the range of 0.001-10 A/em?, or more commonly from the range of 0.1 — 2 A/dm? Hence, in embodiments, the charge control unit may be configured to provide electrical power to the functional unit during at least part of a charging time at a potential difference between the first cell electrode and the second cell electrode selected from the range of 1.6-2.0 V and at a current density selected from the range of 0.001-10 A/cm?. Here, the area refers to the external area of the electrodes, as known in the art. For instance, an electrode having an area of 1 cm? with nickel material or iron material has an external area of 1 cm?, notwithstanding the fact that the electrode may comprise the porous electrode material having a very high surface area.

Therefore, the term “external” area is used. Especially, the external area is defined by just the outside surface of the porous electrode material. Herein, instead of the term “external area” also the term “geometrical surface area” may be applied.

It is further noted that for the battery electrolyser the current densities reached are high for typical battery charging since the (dimensioning of the) electrodes (is such that they) become fully charged within ~10 or ~1 hours for a current of 0.2 and 2A/dm? respectively. For higher current densities up to 400 mA/cm? or up to 2000 mA/cm? as are used in electrolysers the integrated battery and electrolyser (battolyser) energy apparatus may have first cell and second cell electrodes that have larger thickness, i.e. a storage capacity of up to e.g. 800 or 4000 mAh/cm? of electrode surface. A duration to full charge of about 5 hours is compatible with the daytime charging of the cell with electricity from solar power, leaving still more hours for producing hydrogen. The overpotentials for electrolysis remain low at such current densities because of the large active surface area available in the battery electrodes (due to the porous electrode material and/or the cavity (or cavities)). This results in higher energy efficiency (typical electrolyser operates at 2.0V, the battery-electrolyser as defined herein operates in the range 1.48 to 2.0 V when producing hydrogen). The remaining overpotentials are required to generate the hydrogen and oxygen. Hence, in embodiments, the method may comprise operating in the range of 1.3 to 2.3 V, such as especially in the range of 1.48 to 2.0

V, when producing hydrogen.

In yet a further embodiment, the method may comprise maintaining a first pressure in the first cell and a second pressure in the second cell at a pressure of at least 200 bar, such as in the range of 200-800 bar. Further, the method may also comprise maintaining a pressure in the storage system over | bar, such as in the range of up to 800 bar, especially 200- 800 bar. As indicated above, pressures in the first cell and second cell may be controlled independently of each other. Likewise, when both storing Hz and Og, the pressure of the Hz and

O» in the storage may be controlled independently, when desired. During charging, the temperature of the functional unit is especially kept at a temperature in the range of -10 - +60 °C, even more especially at a temperature of at least 10 °C. To this end, the energy apparatus may also include a temperature control unit. Especially, the control unit may be configured to limit the temperature of the functional unit by reducing the applied current when the temperature rises above the set limits.

Further, during at least part of a discharging time, the potential difference between the first cell electrode and the second cell electrode may be selected from the range of 0.7-1.57 V, such as from the range of 0.8-1.47 V, especially from the range of 0.9-1.37 V.

Further, the method may comprise operating in the range of 0.8 to 1.57 V, such as in the range of 0.9 to 1.47 V, especially in the range of 1.0 to 1.37 V, when discharging.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Fig. 1A-B schematically depict embodiments of the electrode; Fig. 2 schematically depicts a top view of several embodiments of the electrode; Fig. 3 schematically depicts an embodiment of the method for producing the electrode; Fig. 4A-B schematically depict experimental observations obtained using an embodiment of the electrode; and Fig. 5A-B schematically depict embodiments of the energy apparatus. The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1A-B schematically depict embodiments of the electrode 1000 of the invention. In embodiments, the electrode 1000 may comprise a porous electrode material 700 and a cavity 600. Further, the electrode 1000 may comprise an outer surface 1010, wherein the outer surface 1010 may comprise a first side 1011 and a second side 1012. Further, in embodiments, the electrode 1000 may comprise a first major face 1015, a second major face 1016 (configured opposite the first major face 1015), and an edge face 1017 configured bridging the first major face 1015 and the second major face 1016. The first side 1011 may be different from the first major face 1015. Similarly, the second side 1012 may be different from the second major face 1016. In embodiments, the terms “first side 1011” and “second side 1012” may herein only be used to indicate a side of the electrode 1000 comprising an opening for the cavity 600, and an (opposite) side of the electrode 1000 towards which the cavity 600 extends. Hence, the cavity 600 may be arranged at the first side 1011, and may be configured extending towards the second side 1012. That is, in embodiments, the cavity 600 may extend over a part of the electrode thickness de (see Fig. 1B). Alternatively, the cavity 600 may define an open fluid channel from one of the first major face 1015, second major face 1016, and edge face 1017 to another of the first major face 1015, second major face 1016, and edge face 1017, wherein the first side 1011 and second side 1012 may indicate the electrode faces 1015,1016,1017 between which the cavity 600 defines an open fluid channel (see Fig. 1A).

Hence, in embodiments, the first side 1011 and second side 1012 may refer to a face 1015,1016,1017 of the electrode 1000, yet may not indicate a (fixed) face 1015,1016,1017 of

Sl the electrode. In embodiments, the first side 1011 and the second side 1012 may (each) be individually selected from the group comprising the first major face 1015, the second major face 1016, and the edge face 1017. In Fig 1A, the electrode 1000 comprises multiple (sets of) cavities 600, wherein a first (set of) cavity (or cavities) 600 defines an open fluid channel from one side of the edge face 1017, indicated by a first first side 10114, to another side of the edge face 1017, indicated by a first second side 10124. Further, a second (set of) cavity (or cavities) 600 defines an open fluid channel from the first major face 1015, indicated by a second first side 1011b, to the second major face 1016, indicated by a second second side 1012b. Hence, the cavity 600 may define an open fluid channel from the first side 1011 to the second side 1012. In embodiments wherein the electrode 1000 comprises a plurality of cavities 600 (see also below), the first side 1011 and second side 1012 may be individually selected for each cavity 600. In embodiments, (each) cavity 600 may have a cross-sectional equivalent circular diameter Dc. selected from the range of 0.5-1.5 mm, wherein the cross-sectional equivalent circular diameter Dc: indicates the diameter of a circle having an equal surface area to the cavity shape (indicated by a dashed circle in the cavity shape). The cavity 600 may especially be defined by an inner surface 1020 of the electrode 1000. Further, the porous electrode material 700 may have a porosity selected from the range of 20-90%. In the embodiments depicted in

Fig. 1A-B and Fig. 2 (and Fig. 3), the pores in the porous electrode material 700 are not depicted, as these may not be visible without magnification (e.g. with a microscope). In embodiments, for at least 50 vol% of the porous electrode material 700 it may apply that a distance di to a nearest electrode surface may be selected from the range of <1 mm, wherein the electrode surface is selected from the group comprising the outer surface 1010 and the inner surface 1020. In Fig. 1A-B and Fig. 2, the distance d; is indicated for a (randomly selected) point in the porous electrode material 700 configured at (at most) the distance di from an electrode surface 1010,1020. This may in embodiments be the point located furthest from an electrode surface 1010,1020 (see e.g. Fig. 2(I)), but this need not be the case. In general, the electrode 1000 may have an electrode thickness dz selected from the range of > 2 mm. Further, in embodiments, the electrode 1000 may especially be a nickel-based electrode or an iron-based electrode. In embodiments, the electrode 1000 may have a capacity selected from the range of > 100 mAh/cm?. Further, the electrode 1000 may have an axis of elongation Ag. Further, the cavity 600 may have a cavity axis of elongation Ac. The cavity axis of elongation Ac may be configured at an angle a with the electrode axis of elongation Ag. In embodiments, the angle a may be selected from the range of 0-60°. Further, the electrode 1000 may have an electrode volume Vg. Similarly, the cavity 600 may have a cavity volume Vc, wherein 0.1%Vg < Vc < 0.5*VE.

The electrode 1000 may comprise a plurality of cavities 600, as depicted in e.g.

Fig. 1A. (The cavity axes of elongation Ac of) at least two of the plurality of cavities 600 may be configured in parallel. Especially, an average distance de. between the at least two of the plurality of cavities 600 (configured in parallel) may be selected from the range of <1.2 mm.

Alternatively, (the cavity axes of elongation Ac of) at least two of the plurality of cavities 600 may be configured at an angle B (with respect to each other) selected from the range of 20-90°.

Especially, the plurality of cavities 600 may comprise a first subset of first cavities 610 arranged in parallel (and (each) having a first cavity axis of elongation Aci), and a second subset of second cavities 620 arranged in parallel (and (each) having a second cavity axis of elongation

Aca). The first cavities 610 may in such cases be arranged orthogonal relative to the second cavities 620. Especially, the plurality of cavities 600 may be configured in a 2D or 3D grid.

Further, in embodiments, (the cavity axis of elongation Ac; of) each first cavity 610 from the first subset may intersect at least one second cavity 620 from the second subset.

Fig. 1B schematically depicts a further embodiment of the electrode 1000. Fig. 1B(I) and Fig. 1B(II) schematically depict a top view of the electrode 1000; Fig. 1B(III) schematically depicts a side view of a cavity 600; and Fig. 1B(IV) schematically depicts a front view of the (same) electrode 1000. As depicted in Fig. 1B(I) and Fig. 1B(II), the cavities 600 may not form channels from the first side 1011 to the second side 1012. Instead, the cavities 600 may be arranged at the first side 1011, and configured extending towards the second side 1012. Especially, the cavities 600 may extend over part of the electrode thickness de. Further, the cavities 600 may not have an equal cross-sectional diameter Dc: over the full length of the cavity 600. That is, the cavities 600 may have a second shape in a cross-section perpendicular tothe first side 1011 (and/or the second side 1012), wherein the second shape may be a triangle.

Hence, in embodiments, the cavity (or cavities) 600 may taper towards the second side 1012.

In such embodiments, the cavity 600 may especially have a cross-sectional diameter Dc selected from the range of 0.5-1.5 mm at the first side 1011. Fig. 1B(I) depicts a plurality of cavities 600 arranged in parallel, wherein each cavity 600 has the same first side 1011 and second side 1012. Fig. 1B(II) depicts a plurality of cavities 600 arranged in parallel, wherein each cavity 600 has the same first side 1011 and second side 1012, and wherein the first side 1011 of a first subset of cavities 600 is the second side 1012 of a second subset of cavities 600.

Hence, the cavities 600 may be interdigitated along the electrode axis of elongation Ag, wherein the cavity second ends of the first subset and second subset are interspersed.

Fig. 1B(III) schematically depicts a gas bubble (indicated with reference 601) moving through the cavity 600. As depicted, upon growth of the bubble 601, the bubble 601 may be pushed towards the first side 1011 (and/or the cavity first end), thereby removing the gas bubble 601 from the interior of the electrode 1000 and improving the ionic conductivity within the electrode 1000. In embodiments, the at least one corner in the cavity shape may reduce the contact area between the gas bubble 601 and the cavity 600 (especially the inner surface 1020 of the electrode), thereby facilitating an earlier detachment of the gas bubble 601 from the cavity 600. Further, the at least one corner may facilitate electrolyte access to the cavity 600 (second end), as the gas bubble 601 may not fill the corner (completely), thereby preventing blocking of (electrolyte access to) the cavity 600 by the gas bubble 601.

Fig. 1B(IV) schematically depicts a front view of an embodiment of the electrode 1000 (depicted in top view in Fig. 1B(T) and/or Fig. 1B(II)). As depicted here, the at least one corner in the cavity shape may especially be placed in an upwards direction (i.e., at the top of the cavity shape). Such a configuration may, as indicated, facilitate an earlier detachment of the gas bubble 601 from the cavity 600, as the gas bubble 601 may not be able to “stick” to the at least one corner at the first side 1011, and may therefore be more easily removed from the channel 600. In embodiments, Fig. 1B(IV) further depicts an embodiments of an electrode 1000 comprising a porous body, wherein: (A) the porous body is defined by (porous) electrode material 700 and a cavity 600 in the (porous) electrode material 700; wherein the porous body comprises an outer surface 1010, wherein the outer surface 1010 comprises a first side 1011 and a second side 1012 (corresponding to the first major face 1015 and the second major face 1016), here further defining an electrode thickness dg; wherein the electrode thickness dE is selected from the range of > 2 mm; (B) the cavity 600 is arranged at the first side 1011 and configured extending towards the second side 1012; wherein the cavity has a cross-sectional diameter Dc. selected from the range of 0.5-1.5 mm; wherein the cavity 600 defines an inner surface 1020; (C) the porous body has a porosity selected from the range of 20-90%, wherein for at least 50 vol.% of the (porous) electrode material 700 applies that a distance di to a nearest electrode surface selected from the group comprising (i) the outer surface 1010 and (i1) the inner surface 1020 is selected from the range of <1 mm; and (D) the electrode 1000 is a nickel-based electrode or an iron-based electrode.

Fig. 2 schematically depicts top views of several embodiments of the electrode 1000, comprising a plurality of cavities 600. Each cavity 600 may have a cavity shape, especially in a cross-section perpendicular to the cavity axis of elongation Ac of the respective cavity 600. The cavity shape may be selected from the group comprising a circular and an n-

gonal shape, wherein n > 2, such as n > 3, especially n > 5, like at most 7, and wherein the sides of the n-gonal shape may be curved and/or planar. Further, the cavity shape may be an irregular shape. Yet, especially, the cavity shape may comprise at least one corner, wherein the corner may have an angle of < 90° (as determined from an interior of the cavity shape). Fig. 2(I) schematically depicts an electrode 1000 comprising a plurality of cavities 600 having a teardrop shape, wherein the point of the teardrop shape may comprise the corner having an angle of < 90°. Further, Fig. 2(1T) schematically depicts an electrode 1000 comprising a plurality of cavities 600 having a square shape, wherein each corner has an angle of 90°. Fig. 211) schematically depicts an electrode 1000 comprising a plurality of cavities 600 having a triangular shape, wherein each corner has an angle of < 90°. Further, Fig. 2(IV) schematically depicts an electrode 1000 comprising a plurality of cavities 600 having a semi-circular shape, wherein the transition from the flat section to the curved (semi-circular) section comprises two corners having an angle of < 90°. The depicted embodiments in Fig. 2 are only meant to demonstrate possible shapes for the cavity 600, and are not indicated to limit the invention to such shapes.

Fig. 3 schematically depicts an embodiment of the method of the invention. The invention may provide a method for producing the electrode 1000 as defined herein, wherein the method may comprise providing a first mixture 2000 comprising (1) an electrode material precursor 2100, (ii) a carbon source 2200, (iii) a polymeric binding agent 2300 comprising a crosslinker, wherein the polymeric binding agent comprises the crosslinker in a crosslinker concentration Cg selected from the range of 0.01-5 wt%, and (iv) metal fibers and/or metal powder 2400. For illustrative purposes, the components of the first mixture 2000 are indicated as circles in Fig. 3, it will be clear to the person skilled in the art that this does not reflect the actual appearance and/or state of said components. In embodiments, the first mixture 2000 may comprise the carbon source 2200 in a carbon source concentration Cc selected from the range of 0.01-10 wt%. The method may further comprise heating the first mixture 2000, to obtain a liquid first mixture 2000. Further, the method may comprise transferring the first mixture 2000 to a (heated) mold 40. The first mixture 2000 may especially be (heated to a molding temperature Tm and) transferred to the mold 40 at a molding temperature Tn. The molding temperature Tm may be selected from the range of > 50 °C. The heating and transferring step is indicated in Fig. 3 by reference 3. Returning to the mold 40, the mold 40 may comprise a molding chamber 41. The molding chamber 41 may comprise a first side 411 and a second side 412 (configured opposite the first side). Further, the mold 40 may comprise a cavity placeholder 42 arranged in the molding chamber 41. The cavity placeholder 42 may extend from the first side 411 to the second side 412, especially when the cavity 600 defines an open fluid channel.

Alternatively, the cavity placeholder 42 may be arranged at the first side 411 and be configured extending towards the second side 412. Hence, the first side 411 and second side 412 of the molding chamber 41 may correspond to the first side 1011 and second side 1012 of the electrode 1000. In embodiments, the cavity placeholder 42 may have a cross-sectional (equivalent circular) diameter Dp selected from the range of 0.5-1.5 mm.

The method may further comprise a curing stage 4. The curing stage 4 may especially comprise hardening the first mixture 2000 into a solid (e.g. through cooling), to provide the mold 40 containing an electrode precursor structure 1050. Further, the method may comprise removing the electrode precursor structure 1050 from the mold 40 (which may especially (also) include removing the cavity placeholder 42. In Fig. 3, the removing of the electrode precursor structure 1050 from the mold 40 is not depicted as a separate step, and is comprised by the step indicated by reference 4. The electrode precursor structure 1050 may, after removal of the cavity placeholder 42, comprise a cavity precursor (structure) 605. Next, the method may comprise (exposing the electrode precursor structure 1050 to) a sintering stage 5. The sintering stage 5 may comprise heating the electrode precursor structure 1050 at a sintering temperature Ts to provide the electrode 1000. In embodiments, the sintering temperature Ts may be selected from the range of > 700 °C. Further, in embodiments, the polymeric binding agent 2300 may be removed from the electrode precursor structure 1050 in the sintering stage 5, to provide a porous structure (especially the porous electrode material 700). Yet, in embodiments, the sintering stage 5 may not provide the electrode 1000. That is, the sintering stage 5 may provide a sintered electrode precursor structure. The sintered electrode precursor structure may especially comprise the scaffold material of the porous electrode material 700, but not the active material. Hence, the method may further comprise a deposition stage 6 executed after the sintering stage 5, wherein the deposition stage 6 may comprise depositing an active material onto the sintered electrode precursor structure to provide the electrode 1000. In table 1, several examples are provided of compositions of the first mixture 2000, and the properties of the resulting electrode 1000.

Table 1: embodiments of the first mixture for an iron-based electrode, and properties of the resulting electrode # | Fe PBA" | PM’ Crosslinker | CS” Ts |t de Porosity | Cg’ 2100 | 2300 | (wt%)" | (wt%)" 2200 | (°C) | (b) (mm) | (%) (mAh/cm?) (wt) | (wt% (wt%) eee

Eee

Ees Jes [ww [i [en ee EA EB Lc * PBA = polymeric binding agent 2300 (including solvent), PM = polymeric material, CS = carbon source, Cg = (areal) capacity * Wit% refers to the weight percentage in the polymeric binding agent 2300 3 electrode without cavity

Fig. 4 schematically depicts a comparison of a nickel-based electrode 1000 according to the first aspect (reference 1000) and a “conventional” nickel-based electrode not comprising a cavity, indicated by reference 4000. Graph A depicts the electrode potentials E in volts (V) as a function of the electrode utilization U during a first (left graph) and a fortieth (right graph) activation cycle of the electrodes 1000,4000, as compared to a standard Hg/HgO electrode. The utilization U of an electrode is an indication for the fraction of the theoretical capacity reached by the electrode. That is, based on the concentration, a theoretical capacity

Cu may be provided by Cu = Cam*Wam mAh, wherein Can is the specific capacity of the (pure) active material in mAh/g, and Wan is the amount of active material comprised by and/or deposited on the electrode in grams. The electrode utilization U may then be determined by measuring the (actual) capacity Cea of the electrode, and dividing this by the theoretical capacity Cum, i.e, U = Ce /Cm. Both electrodes 1000,4000 (conventional and proposed) were overcharged by 50 % at a charge rate of 1C and a discharge rate of C/2. Further, both electrodes have an electrode thickness dg of 0.52 cm, wherein the proposed electrode 1000 has a surface area (of the first major face 1015) of 6.05 cm’, and the conventional electrode 4000 has a surface area (of a first major face) of 6.34 cm?. The active material in both electrodes 1000,4000 is the redox couple Ni(OH)/NiOOH. Further, the theoretical capacity Cw of the proposed electrode 1000 is 848 mAh, and the theoretical capacity Ci of the conventional electrode 4000 is 919 mAh. As depicted, the overpotentials of the electrode 1000 according to the first aspect are reduced compared to the conventional electrode 4000. With such reduced overpotentials, the charging reaction (for battery applications) is facilitated, yet a limited amount of (oxygen)

gas 1s produced in and/or on the electrode 1000 (surface), as the exchange current for the (oxygen) gas evolution reaction is lower that that for the charging reaction.

Graph B schematically depicts the utilization U of the same (conventional and proposed) electrodes 1000,4000 as a function of the number of activation cycles (indicated by reference #). Here, the utilization U is indicated (on the vertical axis) as a percentage rather than a fraction. As depicted in graph B, the reduced overpotentials in the proposed electrode 1000 (i.e., the electrode according to the first aspect) facilitate a higher utilization U of the electrode 1000. Hence, the proposed electrode 1000 may be charged further (i.e., to a higher capacity) before (oxygen) gas evolution occurs, increasing the efficiency (and storage capacity) of the electrode 1000.

Further, with respect to an electrolysis application, the cavity (or cavities) 600 may facilitate transport of the generated gas, e.g., O2, away from the active material of the electrode 1000. Hence, the electrode 1000 of the invention may have improved properties for both a battery and an electrolysis application.

Fig. 5A schematically depicts some aspects of an embodiment of a functional unit 2 (comprised by the energy apparatus 1 of the invention). More details are shown in the embodiment of Fig. 5B. Fig. SA (and 5B) schematically depicts the functional unit 2 comprising: a first cell 100, a second cell 200, and a separator 30. The first cell 100 comprises a first cell electrode 120. Especially, the first cell electrode 120 comprises an iron based electrode. The second cell 200 comprises a second cell electrode 220. The second cell electrode 220 especially comprises a nickel based electrode. Further, the first cell 100 and the second cell 200 share the separator 30. The separator is configured to block transport of one or more of O2 and H: from one cell to another while having permeability for at least one or more of OH, monovalent sodium (Na*), monovalent lithium (Li*) and monovalent potassium (K*). As indicated above, the separator 30 especially comprises a membrane. Further, the separator 30 and the electrodes 120 and 220 may be spaced apart with a spacer, indicated with reference 23.

This spacer may be configured to provide a spacing between the electrode and the separator, but also allow the water based electrolyte to come into contact with the electrode at the separator side of the electrode. Hence, first and second cell aqueous liquids 11,21 may pass at both sides of the respective electrodes 120,220. Yet, the first cell electrode 120 may comprise the electrode 1000, especially the iron-based electrode 1000. Additionally or alternatively, the second cell electrode 220 may comprise the electrode 1000, such as especially the nickel-based electrode 1000. In such embodiments, the cavities 600 in the electrodes 1000 comprised by respectively the first cell electrode 120 and the second cell electrode 220 may facilitate a flow of the first and second cell aqueous liquids 11,21 through and/or around the first and second cell electrodes 120,220. In such embodiments, the spacer 23 may be reduced in thickness or be removed completely. The separator 30 and the respective electrodes 120,220 may substantially have the same surfaces areas, i.e. external surface areas, and may thereby form a stack (with optionally the spacers 23 in between). Hence, the electrodes and the separator may substantial have the same heights (as depicted here) and the same width (here the plane perpendicular to the plane of drawing).

Especially, the functional unit 2 is an integrated unit substantially entirely enclosed by pressure containment. During charging, the following reaction may take place at the first cell electrode 120: Fe(OH): + 2¢” = Fe + 20H" (-0.877 V vs. SHE), followed by 2H:0 +2 e = Hx + OH (-0.83 vs. SHE). Hence, when the battery is charged, Fe may act as a catalyst for H: formation (comprised by the first cell gas 12). Further, during charging at the second cell electrode 220, the following reaction may take place: Ni(OH): + OH = NiOOH + HO + e (+0.49 V vs. SHE), followed by 4 OH = O; +2 H,0 +4¢” (0.40 vs. SHE). When the battery is charged, the NiOOH acts as O; evolution catalyst with some overpotential with respect to the O: evolution equilibrium potential. The O: gas generated at the second cell electrode 220 may especially be comprised by the second cell gas 22.

Fig. 5A shows electrolysis reactions. When the arrows are reversed, discharge reactions are indicated. Hence, the open cell potential (for discharging) is 1.37 V. The equilibrium potential for electrolysis is 1.23 V; however, for having significant O2 and Hz evolution, overpotentials are required with respect to the equilibrium potentials. In addition, the thermo neutral potential for splitting water is 1.48V, taking into account also heat that is required if that is to be generated only from the applied potential during electrolysis. In the present invention, however, heat is also available from the overpotentials of the battery charging, which provides some additional heat. In practice, during electrolysis, the potential rises to at least 1.55-1.75 V. Heat from overpotentials is therefore available for the electrolysis.

A remarkable fact is that the battery can be charged first although the potential energy levels are very close to the Hz and O: evolution potentials.

Fig. 5B schematically depicts an embodiment of the energy apparatus 1 having an electrical energy storage functionality and an electrolysis functionality. The system 1 may comprise the functional unit 2 (see also above). The first cell 100 comprises a first cell electrode 120 and one or more first cell openings 110 for a first cell aqueous liquid 11 and for a first cell gas 12. The second cell 200 comprises a second cell electrode 220 and one or more second cell openings 210 for a second cell aqueous liquid 21 and for a second cell gas 22, wherein the second cell electrode 220 comprises a nickel based electrode.

Further, a first electrical connection 51 in electrical connection with the first cell electrode 120, and a second electrical connection 52 in electrical connection with the second cell electrode 220, are depicted. These may be used to provide electrical contact of the electrodes 120,220 with the external of the functional unit 2.

The energy apparatus 1 further comprises an aqueous liquid control system 60 configured to control introduction of one or more of the first cell aqueous liquid 11 and the second cell aqueous liquid 21 into the functional unit 2. The liquid control system 60 by way of example comprises a first liquid control system 60a and a second liquid control system 60b.

The former is functionally connected with a first inlet 110a of the first cell 100; the latter is functionally connected with a first inlet 210a of the second cell 200. The aqueous liquid control system 60 may include recirculation of the aqueous liquid (and also supply with fresh aqueous liquid (not shown in detail)).

Yet further, the apparatus 1 comprises a storage system 70 configured to store one or more of the first cell gas 12 and the second cell gas 22 external from said functional unit 2. The storage by way of example comprises a first storage 70a and a second storage 70b. the former is functionally connected to a first outlet 110b of the first cell 100; the latter is functionally connected to a first outlet 210b of the second cell 200. Note that e.g. only the first storage 70a may be available, i.e. a storage for hydrogen gas. Separation between gas and liquid, upstream of the storage and/or downstream from the first cell 100 or the second cell 200 may be executed with a H: valve and/or a H2O dryer and an O: deoxidiser as they are known in the art, or with a O2 valve and/or a HoO/H: condenser, respectively.

The energy apparatus | further comprises a pressure system 300 configured to control one or more of (a) the pressure of the first cell gas 12 in the functional unit 2, (b) the pressure of the first cell gas 12 in the storage system 70, (c) the pressure of the second cell gas 22 in the functional unit 2, and (d) the pressure of the second cell gas 22 in the storage system 70. The pressure system may e.g. include different pressure systems, which may be independent from each other or may be connected. By way of example a first pressure system 300a is depicted, especially configured to provide one or more of the first liquid 11 and the second liquid 21 under pressure to the first cell 100 and second cell 200, respectively. Further, another pressure system 300b may be configured to control the pressure of the storage for the first gas 12. Yet, another pressure system 300c may be configured to control a pressure of the storage for the second gas 22. Further, the pressure system 300 may be configured to control the pressure in the first cell 100 and/or second cell 200. To this end, the pressure system may include one or more pumps, one or more valves, etc.

Yet, the apparatus in this embodiment also comprises a charge control unit 400 configured to receive electrical power from an external electrical power source (reference 910, see further below) and configured to provide said electrical power to said functional unit 2 during at least part of a charging time at a potential difference between the first cell electrode 120 and the second cell electrode 220 of especially more than 1.37 V during the first battery charge and larger than 1.48V and up to 2.0V during electrolysis when the battery is already fully charged.

Schematically depicted are also a first connector unit 510 for functionally coupling a device 930 to be electrically powered and the electrical connection 51,52, as well as a second connector unit 520 for functionally connecting a device to be provided with one or more of the first cell gas 12 and the second cell gas 22 with said storage system 70. Here, in fact two second connectors 520 are depicted, a first second connector 520a, functionally connected with the first storage 70a, and a second second connector 520b, functionally connected with the second storage 70b.

The apparatus may be controlled by a control system 80, which may be especially configured to control at least one of the aqueous liquid control system 60, the storage system 70, the pressure system 300, and the charge control unit 400, and especially all of these.

Fig. 5B also schematically depicts an embodiment of an energy system 5 comprising the energy apparatus 1 and an external power source 910, here by way of example comprising a wind turbine and a photovoltaic electricity generation source. The apparatus 1 or energy system 5 may be used for providing one or more of electrical power, hydrogen (Hs) to device 930, such as a motorized vehicle comprising an engine deriving its propulsion energy from one or more of a hydrogen source and an electrical power source. Alternatively or additionally, apparatus 1 or energy system 5 may be used by an industrial object 940, comprising such device 930. Here by way of example, the industrial object uses O: for e.g. a chemical process. Hence, of course alternative or additionally, first storage 70a may also be functionally coupled to a gas grid; likewise second storage 70b may functionally be coupled to a gas grid. Fig. 5B also indicates a return system for aqueous liquid (see also above).

Experimental results

Oxygen evolution reaction potential

The (over)potential (compared to a (standard) Hg/HgO electrode) during the oxygen evolution reaction (OER) is recorded for three pairs of electrodes comprising an electrode 1000 as defined herein and a reference electrode (indicated by reference 5000) not comprising cavity/cavities 600, wherein the three pairs of electrodes have different current densities. The results of the experiment are found in Table 2.

Table 2: OER (over)potential vs. a standard Hg/HgO electrode for an electrode 1000 as defined herein and an electrode 5000 not comprising cavities 600.

Current density (mA/cm?) OER potential of electrode | OER potential of electrode

EE ane te

As depicted in Table 2, the (over)potentials of the OER decrease upon the additions of cavities to the electrode. Hence, a lower potential is needed to facilitate the production of oxygen using the electrode 1000 as defined herein.

Charge rate performance of the electrode 1000 and the reference electrode 5000 not comprising cavities.

Both the electrode 1000 and the reference electrode 5000 are charged at different charge rates, subsequently discharged at a discharge rate of 0.25C (or “C/4”), and the observed discharge capacity is recorded (see Table 3).

Table 3: Observed discharge capacities for the electrode 1000 and the reference electrode 5000 not comprising cavities 600 at different charge rates.

Charge rate Observed discharge capacity | Observed discharge capacity

TT eS et

EP

As depicted in Table 3, the observed discharge capacity for an electrode 1000 comprising a cavity/cavities 600 as defined herein may be increased compared to an electrode 5000 not comprising such cavities 600. Especially, the cavities 600 may decrease the (ionic) resistance within the electrode 1000, facilitating an increase in the observed discharge capacity.

Additionally, the cavities 600 may, especially when forming channels, provide additional cooling during charging of the electrode 1000, reducing energy losses.

The term “plurality” refers to two or more. Furthermore, the terms “a plurality of” and “a number of” may be used interchangeably. The terms “substantially” or “essentially”

herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms “about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”, “about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

The term “comprise” also includes embodiments wherein the term “comprises” means “consists of”. The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of" but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species”. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment, but may also refer to an alternative embodiment. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”,

“include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which 1s functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings.

Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively.

The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.

Claims

Conclusions

1. An electrode (1000) comprising a porous electrode material (700) and a cavity (600), the electrode (1000) comprising an outer surface (1010), the outer surface (1010) comprising a first side (1011) and a second side (1012), wherein: - the cavity (600) is configured to be disposed on the first side (1011) and extending toward the second side (1012); the cavity (600) having a cross-sectional diameter (Dcs) selected from the range of 0.5-1.5 mm, and the cavity (600) being defined by an inner surface (1020) of the electrode (1000); - the porous electrode material (700) has a porosity selected from the range of 20-90%, wherein for at least 50 vol% of the porous electrode material (700) a distance (di) to a nearest electrode surface is selected from the range of <1 mm, and wherein the electrode surface is selected from the group consisting of the outer surface (1010) and the inner surface (1020); - the electrode (1000) has an electrode thickness (dr) selected from the range of > 2 mm; and - the electrode (1000) is a nickel-based electrode or an iron-based electrode.

The electrode (1000) of claim 1, wherein the cavity (600) defines an open fluid channel from the first side (1011) to the second side (1012); wherein the cavity (600) has a cavity shape, and wherein the cavity shape includes at least one corner with an angle of < 90°.

The electrode (1000) of any preceding claim, wherein the cavity (600) has a second shape in a cross-section perpendicular to the first side (1011), the second shape being selected from the group consisting of a triangle and a rectangle.

The electrode (1000) of any preceding claim, wherein the electrode (1000) has an electrode extension axis (Ag), the cavity (600) has a cavity extension axis (Ac), the cavity extension axis (Ac) being configured at an angle (α) with the electrode extension axis (Ag), and the angle (θ) being selected from the range of 0-60°.

The electrode (1000) of any preceding claim, wherein the electrode (1000) has an electrode volume Ve, wherein the cavity (600) has a cavity volume Vc, and wherein 0, I*VE < Vc < 0,5*VE.

6. The electrode (1000) according to any one of the preceding claims, wherein one or more of the following applies: - the electrode (1000) has a theoretical capacity Ce selected from the range of > 100 mAh/cm₂; and - a ratio between an inner surface area and an outer surface area 1s selected from the range of 0.5-5.

The electrode (1000) of any preceding claim, wherein the electrode (1000) comprises a plurality of cavities (600), at least two of the plurality of cavities (600) being configured at an angle β selected from the range of 20-90°.

The electrode (1000) of any of the preceding claims 1-6, wherein the electrode (1000) comprises a plurality of cavities (600), at least two of the plurality of cavities (600) being configured in parallel.

The electrode (1000) of any of the preceding claims 7-8, wherein the plurality of cavities (600) comprises a first subset of parallel-arranged first cavities (610) and a second subset of parallel-arranged second cavities (620), the first cavities (610) being arranged orthogonally to the second cavities (620), and each first cavity (610) from the first subset intersecting with at least one second cavity (620) from the second subset.

The electrode (1000) of any preceding claim, wherein the electrode (1000) is a nickel-based electrode, and wherein the nickel-based electrode comprises Ni(OH)2.

The electrode (1000) of any one of claims 1 to 9, wherein the electrode (1000) is an iron-based electrode, and wherein the iron-based electrode comprises one or more of Fe and Fe(OH);

The electrode (1000) of claim 11, wherein the electrode (1000) comprises an active material, carbon, and a crosslinker, wherein: - the electrode (1000) comprises the active material in an active material concentration Ce selected from the range of 50-99 wt%; - the electrode (1000) comprises the carbon in a carbon concentration Cgc selected from the range of 0.008-8 wt%; and - the electrode (1000) comprises the crosslinker in a crosslinker concentration Cg selected from the range of 0.001-5 wt%.

A method for producing the electrode (1000) according to any of claims 1-12, the method comprising: - providing a first mixture (2000) comprising (i) an electrode material precursor (2100), (ii) a carbon source (2200), (iii) metal fibers and/or metal powder (2400), and (iv) a polymeric binder (2300) comprising a crosslinker, the polymeric binder comprising the crosslinker in a crosslinker concentration Cc selected from the range of 0.01-5 wt%; wherein the first mixture (2000) comprises the carbon source (2200) in a carbon source concentration (Cc) selected from the range of 0.01-10 wt%; - transferring the first mixture (2000) to a mold (40) at a forming temperature Tm, the forming temperature Tm being selected from the range of > 50 °C, the mold (40) comprising a mold chamber (41), the mold chamber (41) comprising a first side (411) and a second side (412), the mold (40) comprising a cavity place holder (42) disposed in the mold chamber (41), the cavity place holder (42) having a cross-sectional diameter (Dp) selected from the range of 0.5-1.5 mm; - a curing step (4), the curing step (4) comprising curing the first mixture (2000) to a solid to provide the mold (40) containing an electrode precursor structure (1050); - removing the electrode precursor structure (1050) from the mold (40); and - a sintering step (5), wherein the sintering step (5) comprises heating the electrode precursor structure (1050) at a sintering temperature Ts to provide the electrode (1000), wherein the sintering temperature Ts is selected from the range of > 700 °C, and wherein the polymeric binder (2300) is removed from the electrode precursor structure (1050) in the sintering step (5).

14. An energy device (1) having an electrical energy storage functionality and an electrolysis functionality, the system (1) comprising a functional unit (2), the functional unit (2) comprising the electrode (1000) according to any of claims 1-12, the functional unit (2) further comprising: - a first cell (100) comprising a first cell electrode (120) and one or more first cell openings (110) for a first aqueous-liquid cell (11) and for a first gas cell (12), the first electrode (120) comprising an iron-based electrode; - a second cell (200) comprising a second cell electrode (220) and one or more second cell openings (210) for a second aqueous-liquid cell (21) and for a second gas cell (22), the second electrode (220) comprising a nickel-based electrode; - a separator (30), the first cell (100) and the second cell (200) sharing the separator (30), the separator being configured to block a transport of one or more of O; and H: from one cell to another while having permeability to at least one or more of hydroxide ions (OH), monovalent sodium (Na*), monovalent lithium (Li*), and monovalent potassium (K*);

- a first electrical connection (51) in electrical contact with the first cell electrode (120), and a second electrical connection (52) in electrical contact with the second cell electrode (220); wherein the following applies: (a) the first cell electrode (120) comprises the electrode (1000); and/or (b) the second cell electrode (220) comprises the electrode (1000).

The energy device of claim 14, wherein the first cell electrode (120) comprises the electrode (1000) of claim 11, and wherein the second cell electrode (220) comprises the electrode (1000) of claim 10.

The energy device (1) according to any of the preceding claims 14-15, wherein the energy device (1) further comprises: - an aqueous-fluid control system (60) configured to control the introduction of one or more of the first cell aqueous-fluid (11) and the second cell aqueous-fluid (21) into the functional unit (2); - a storage system (70) configured to store one or more of the first cell gas (12) and the second cell gas (22) externally of said functional unit (2); - a pressurization system (300) configured to control one or more of (a) the pressure of the first cell gas (12) in the functional unit (2), (b) the pressure of the first cell gas (12) in the storage system (70), (c) the pressure of the second cell gas (22) in the functional unit (2), and (d) the pressure of the second cell gas (22) in the storage system (70); - a charge control unit (400) configured to receive electrical energy from an external electrical energy source and configured to provide said electrical energy to said functional unit (2) for at least a portion of a charge time at a potential difference between the first cell electrode (120) and the second cell electrode (220) of more than 1.37 V, the energy device (1) being configured to operate in the range of 1.48 to 2.0 V during hydrogen production;

- a first connection unit (510) for operatively connecting an electrically actuated receiver and the electrical connection (51, 52), and a second connection unit (520) for operatively connecting a device to be supplied with one or more of the first cell gas (12) and the second cell gas (22) to said storage system (70); and - a control system (80) configured to control the aqueous-liquid control system (60), the storage system (70), the pressure system (300), and the charge control unit (400).

A method for storing electrical energy and one or more of hydrogen (Hz) and oxygen (O2) with the energy device (1) according to any of the preceding claims 14-16, the method comprising: providing the first aqueous-liquid cell (11), the second aqueous-liquid cell (21) and electrical energy from an external energy source (910) to the functional unit (2) thereby providing an electrically charged functional unit (2) and one or more of hydrogen (Hz) and oxygen (O2) stored in the storage system (70) as defined in claim 14, wherein during at least a portion of a charging time the functional unit is charged at a potential difference between the first cell electrode (120) and the second cell electrode (220) of more than 1.37 V, the method comprising operating in the range of 1.48 to 2.0 V during the production of hydrogen, wherein during at least a portion of a discharge time the potential difference between the first cell electrode (120) and the second cell electrode (220) selected from the range of 0.9-1.37 V 1s, and wherein the method comprises operating in the range of 1.0 to 1.37 V during discharging.