EP4247997A1

EP4247997A1 - Membrane-less electrolyzer with porous walls for high throughput and pure hydrogen production

Info

Publication number: EP4247997A1
Application number: EP20828667.4A
Authority: EP
Inventors: Demetri Psaltis; Seyyed Mohammad HOSSEINI HASHEMI; Pooria HADIKHANI; Steven Alexandre SCHENK
Original assignee: Ecole Polytechnique Federale de Lausanne EPFL
Current assignee: Ecole Polytechnique Federale de Lausanne EPFL
Priority date: 2020-11-23
Filing date: 2020-11-23
Publication date: 2023-09-27
Also published as: WO2022106874A1; US20240102185A1

Abstract

The present disclosure concerns a membrane-less electrolyzer comprising a first electrode and a second electrode, a first fluidic channel configured to receive an electrolyte, a second fluidic channel configured to receive the electrolyte from the first fluidic channel, a third fluidic channel configured to receive the electrolyte from the first fluidic channel. The first fluidic channel being in fluidic communication with the second and third fluidic channels. The first fluidic channel is connected to the second and third fluidic channels via a plurality of inclined fluidic canals extending from the first fluidic channel to each of the second and third fluidic channels.

Description

MEMBRANE-LESS ELECTROLYZER WITH POROUS WALLS FOR HIGH THROUGHPUT AND PURE HYDROGEN PRODUCTION

FIELD OF THE INVENTION

The present disclosure relates to an electrolyzer, and in particular to a membrane-less electrolyzer.

BACKGROUND

Membrane-less electrolyzers utilize fluidic forces instead of membranes or separators for the separation of the electrolysis gas products. These electrolyzers have low ionic resistance, simple design, and ability to work with electrolytes at different pH values. However, the interelectrode distance and the flow velocity should be large at high production rates in order to prevent gas cross over. The ionic resistance is higher at larger interelectrode distances and the required pumping power increases by the flow velocity.

Renewable energies are being developed as a clean source of energy to substitute fossil fuels. However, the sources of renewable energies are intermittent. The surplus energy of renewables can be stored in the form of hydrogen using water electrolysis when the renewable sources are available. Furthermore, hydrogen produced from water electrolysis is a clean alternative to fossil fuels for the transportation which reduces the air pollutant emissions from vehicles [1, 2]. Hydrogen from water electrolysis can reach high purities [3], High purity hydrogen can be used directly in fuel cells to produce energy for the vehicles or compensation of the shortage of energy from renewable sources at the energy peak demand. Hydrogen production from water electrolysis has been considered an expensive process [4], The design, efficiency, and throughput of water electrolyzers determine the cost of hydrogen production. Therefore, it is necessary to improve the performance of the electrolyzers.

The main water electrolysis methods are alkaline electrolyzers and polymer electrolyte membrane (PEM) electrolyzers that are available commercially, and anion exchange membrane (AEM) electrolyzers, solid oxide electrolyzers (SOE), and membrane-less electrolyzers that are in the development and research stage [3, 5, 6],

Alkaline electrolyzers work with basic electrolytes [7], The electrodes of alkaline electrolyzers are separated by a diaphragm in order to prevent gas cross-contamination [8], Alkaline electrolyzers are the most commercially used technologies for hydrogen production due to their simple design and inexpensive catalyst materials [8], PEM electrolyzers use a membrane coated by catalysts on both sides [9], This membrane allows the protons migration but prevents the gas cross over [10], PEM electrolyzers can be used at high current densities, they have low gas cross over, and they are compact [9], AEM electrolyzers work with alkaline electrolytes and use an anion conductive membrane [11]. This technology can reduce the capital cost of electrolyzers due to the usage of non-precious metal electrodes [12], SOEs use a solid electrolyte with two porous electrodes on the two sides of this electrolyte [13], These electrolyzers have higher efficiency since they are operating at high temperatures [14], However, the solid electrolyte and the electrodes should be chemically stable in the highly oxidizing and reducing environments [13],

The above-mentioned electrolyzers use a membrane or a diaphragm to prevent the gas cross-contamination. Membrane-less electrolyzers have been introduced to remove the need for the membranes [15], Membraneless electrolyzers rely on the fluidic flow [15-19], buoyancy forces [20], or surface forces [21] for the gas product separation. Removing the membrane simplifies the design of the electrolyzer, reduces the electrolyzer cost, and increases the lifetime and durability of the device [5], Furthermore, a membrane-less electrolyzer is compatible with a wide range of electrolytes at different PH [15], Thus, it can be used for various electrochemical reactions such as water electrolysis for hydrogen production and brine electrolysis for chlorine production without significant modification in its design [16, 22],

The membrane-less electrolyzers geometry can be classified based on the electrode configuration into parallel [15, 16] and mesh electrodes [17-19], In the parallel electrode electrolyzer (PE electrolyzer), the electrodes are at the two opposites sides of a rectangular channel. The liquid is flowing between the electrodes. Gas products are flowing in between the electrodes until the end of the channel. The liquid flow keeps the bubbles at the channel sides to prevent cross over. The volume fraction of bubbles increases by going downstream. Therefore, the channel length and the flow rate should be decided carefully to prevent the formation of bubbles larger than half of the channel width.

The mesh electrodes geometry is made of two plane meshes that act as catalysts. The liquid flow enters the area between the meshes and flows through the pores of the mesh. The bubbles are formed on the surface of the mesh and they move to the outer side of the mesh after the detachment. The mesh electrodes electrolyzer can achieve higher production rate compared to the PE electrolyzer as the bubbles go through the mesh pores and leave the interelectrode region faster. An equal velocity in all pores of the mesh is necessary for removing the growing bubbles at all positions of the mesh. Furthermore, the bubbles growing on the inner side of the mesh should detach at sizes smaller than the mesh pore size in order to allow the bubbles to flow through the mesh. The product separation is a challenge for the membrane-less electrolyzers due to the absence of the membrane or separator. The bubble growth at the electrode and movement in the channel are being investigated in order to overcome this challenge [23-25], The bubble coalescence and large bubble detachment from the electrodes can lead to the formation of bubbles larger than half of the channel width which leads to gas cross over. Besides, the large bubble formation is more frequent at high production rates. Consequently, the gas cross over is higher at higher production rates.

Enlarging the space between the electrodes leads to higher maximum production rate, but it imposes additional ohmic losses due to the longer interelectrode distance. Moreover, bubbles moving between electrodes block ionic pathways that adds to the overpotential losses [26],

In mesh electrode electrolyzers, however, bubbles larger than the pores cannot go through the pores easily and they flow between the electrodes until the end of the channel which limits the production rate.

SUMMARY OF THE INVENTION

It is thus a goal of the present disclosure to address the above-mentioned incon veniencies.

The present invention addresses the above-mentioned limitations by providing an electrolyzer according to claim 1.

The membrane-less electrolyzer of the present disclosure increases the throughput of the electrolyzer without having to increase the interelectrode distance and flow velocity. The electrolyzer may have, for example, three channels separated by porous walls. The electrolyte enters, for example, a middle channel and flows to the outer channels through the wall pores. The gas products are, for example, produced in the outer channels. Results show Hydrogen cross over to be as low as 0.14 ± 0.06 % in this electrolyzer at Re = 109 and j = 300 mA/cm³. This cross over is 58 times lower than hydrogen cross over in an equivalent membrane-less electrolyzer with parallel electrodes under the same working conditions.

The electrolyzer of the present disclosure allows the production rate to be improved by adding pores to the wall of PE electrolyzers and generating bubbles in the outer sides of the interelectrode region. Producing bubbles outside the interelectrode region resolves the production rate limitation in known electrolyzers. Moreover, by doing so, the overpotential due to the bubble movement between electrodes decreases and, in addition, the electrodes distance can be decreased. Another aspect of the present disclosure concerns a device assembly comprising a plurality of electrolyzers.

Yet another aspect of the present disclosure concerns an electrolysis method according to claim 43 and a method for operating an electrolyzer according to claim 46.

Moreover, the addition of, for example, heptadecafluorooctancesulfonic acid potassium (PFOS) as a surfactant to the electrolyte is shown to further reduce the hydrogen cross over to 0.11 ± 0.05 % and the overpotential by 1.9 % at Re=109 and j = 300 mA/cm³.

Modification of electrolyte properties with surfactant reduces the size of bubbles. The addition of the surfactant to the electrolyte decreases the surface tension. The bubble detachment size is smaller when the surface tension is lower. Furthermore, the surfactant molecules adsorbed to the interface of the bubble prevent the bubble coalescence. Therefore, the surfactant improves the production throughput of the electrolyzer. In an electrolyte with the surfactant, the bubbles detach faster from the electrode surface. This fast bubble detachment reduces the electrode surface coverage by the bubbles and decreases the corresponding ohmic losses.

The surfactant enhances the production rate and product separation in membrane-less electrolyzers, but the production rate cannot be increased further when the space between the electrodes is filled with bubbles, a limitation which is resolved by the electrolyzer of the present disclosure. The electrolyzer of the present disclosure combined with the surfactant thus can assure a significant increase in production rate while assuring low cross over.

Other advantageous features can be found in the dependent claims.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description with reference to the attached drawings showing some preferred embodiments of the invention.

A BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figure 1A schematically shows a membrane-less electrolyzer that is a parallel electrode (PE) electrolyzer: This electrolyzer has two solid parallel catalysts. The bubbles evolve and flow between the parallel electrodes. Figure IB schematically shows a mesh electrode electrolyzer: The electrolyte enters the space between two mesh catalysts. The electrolyte flows through the meshes and removes the generated bubbles in the meshes. Figures 1C and ID schematically shows a porous wall (PW) electrolyzer according to the present disclosure: Bubbles are being generated at the electrodes in the outer channels. The porous wall and the liquid flow prevent bubble cross over. The bubbles are not flowing between the electrodes in this design. Therefore, it has smaller overpotential due to the movement of bubbles compared to mesh electrode and parallel electrode electrolyzers.

Figure 2A shows bubbles generation and flow at different locations in the PE electrolyzer working with 1 M H2SO4, where the applied current density is 300 mA/cm². The large bubbles are the primary cause of gas cross over since they are moving to the channel centerline. The bubbles become smaller by increasing the Re as the flow removes the bubbles faster from the channel.

Figure 2B shows that the bubbles are smaller at a lower current density of j = 75 mA/ cm². Thus, the gas cross-over is smaller at lower current densities. The scale bar is 400 gm.

Figure 3 shows pictures of different regions of the PE electrolyzer working with 1 M H2SO4 + 0.056 mg/ml PFOS surfactant. The bubbles detach at smaller sizes at higher Re. The surfactant reduces the bubble detachment size by reducing the surface tension. Moreover, the surfactant inhibits the bubble coalescence. For this reason, the flowing bubbles are smaller in the electrolyte with the surfactant compared to the surfactant-free electrolyte. Thus, product separation is more effective by using a surfactant. The applied current density is 300 mA/cm². The scale bar is 400 gm.

Figure 4A shows a polarization curve of the PE electrolyzer working with 1 M H2SO4 with and without the PFOS surfactant: The surfactant reduces the electrode surface coverage by bubbles. Besides, the hydrogen evolution reaction has lower overpotential due to the decrease in the hydrogen solubility in the electrolyte with PFOS. Thus, the PFOS surfactant improves the electrochemical performance of the PE electrolyzer.

Figure 4B shows Hydrogen cross over to the oxygen side at different Re: The flammability limit is shown by a dashed line. The cross over is inversely proportional to the Re. The cross over is less than the flammability limit in the surfactant-free electrolyte at Re=109 and j = 75 mA/cm². However, the cross over of the PE electrolyzer is not safe by increasing the current density to 300 mA/cm² in the surfactant- free electrolyte. The PE electrolyzer can operate with a secure cross over by adding the PFOS surfactant to the electrolyte. Figure 5A schematically shows geometry optimization of the PW electrolyzer of the present disclosure and shows the design and boundary conditions at four steps of geometry optimization. The length of wall pores/canals is 100 gm in geometry 1. The length of wall pores/canals is increased to 200 /rm in geometry

2 to reduce the gas cross over. Afterwards, the porous walls are rotated by 0.75° in geometry 3 to have equal velocities in the wall pores. The wall pores angle with the horizontal line is 45° in geometry 4 to suppress the gas cross over completely. The contours are the water volume fraction at t = 0.2 s. Hydrogen and oxygen bubbles enter the left and right channels, respectively.

Figure 5B shows the velocity distribution at wall pores: The velocity distribution is uniform in geometries

3 and 4 thanks to the inclined porous walls.

Figure 5C shows average water volume fraction at porous walls and channel centerline: Water volume fraction below one at the centerline indicates gas cross over.

Figure 6 shows pictures of the bubble generation and flow in the PW electrolyzer of the present disclosure at j = 300 mA/cm² and various Re: The porous walls keep the hydrogen and oxygen bubbles separated. This device is less vulnerable to the large bubble formation since these bubbles cannot go through the wall pores. Some small bubbles move to the middle channel and coalesce. They form a large bubble in the middle channel that cannot go through the pores. This bubble grows until the device reaches the steadystate. The final size of this bubble decreases by increasing the Re. There is no bubble in the middle channel at Re=109 since the flow does not allow bubbles migration to the middle channel. The scale bar is 400 gm.

Figure 7A shows the effect of PFOS surfactant on the bubble generation and flow in the PW electrolyzer of the present disclosure working with 1 M H2SO4 + 0.056 mg /ml PFOS, and presents images showing the bubble generation and flow at different R and j = 300 mA/cm². The addition of PFOS to the electrolyte reduces the size of bubbles. There is no bubble in the middle channel in the electrolyte with surfactant at low Re. Moreover, energy losses are smaller since the wall pores as ion-conducting pathways are free from the bubbles.

Figure 7B shows that current density of 450 mA/cm² is achieved without cross overusing PFOS surfactant and the PW electrolyzer design of the present disclosure. The scale bar is 400 gm. Figure 8 A shows a polarization curve of the PW electrolyzer of the present disclsoure: The electrode surface coverage by bubbles is smaller and the wall pores are not blocked by the bubbles in the presence of PFOS.As a result, the electrochemical performance improves by adding PFOS to the electrolyte.

Figure 8B shows that the gas cross over is less than the flammability limit in the surfactant-free electrolyte at Re > 54. The cross over decreases further by adding the PFOS surfactant to the electrolyte. A comparison of this Figure with Figure 4B shows that the cross over is significantly smaller in the PW electrolyzer of the present disclosure than the cross over in the PE electrolyzer.

Figure 8C shows that more active area is available at higher Re due to the smaller surface coverage by the bubbles. Therefore, the overpotentials decrease by increasing the Re.

Figure 9A shows an exemplary fabricated PE electrolyzer and Figure 9B shows an exemplary fabricated PW electrolyzer according to the present disclosure. The walls are indicated by solid lines. The device images are constructed by placing images of three different positions of each device which are indicated by dashed lines. The scale bars are 200 /im.

Figure 10A shows that changing the wall pores angle from 30° to 60° does not affect the velocity distribution in the pores or the water volume fraction in the middle channel.

Figure 10B shows that the velocity distribution in the pores remains uniform across the pores and the water volume fraction in the middle channel does not decrease when the pore size increases from 80 pm to 160 pm.

Figure 11 shows an exemplary fabrication process of the electrolyzer of the present disclosure.

Herein, identical reference numerals are used, where possible, to designate identical elements that are common to the Figures.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

Figures 1C and 9B shows an exemplary embodiment of the electrolyzer 1 of the present disclosure.

The electrolyzer 1 is, for example, a membrane-less electrolyzer or a porous wall PW electrolyzer.

The electrolyzer 1 may comprise at least a first electrode 3 and at least a second electrode 5 for carrying out electrolysis. The electrolyzer 1 may also comprise a first fluidic channel 7 configured to receive an electrolyte EL, a second fluidic channel 9 configured to receive the electrolyte EL from the first fluidic channel 7 and a third fluidic channel 11 configured to receive the electrolyte EL from the first fluidic channel 7.

The second fluidic channel 9 includes, for example, the first electrode 3 and the third fluidic channel 11 includes, for example, the second electrode 5. The first fluidic channel 7 may, for example, be electrode- free or electrode-less.

The first fluidic channel 7 is in fluidic communication with the second and third fluidic channels 9, 11. The first fluidic channel 7 may, for example, be located between or enclosed by the second and third fluidic channels 9, 11. The first fluidic channel 7 may, for example, be located above the second and third fluidic channels 9, 11.

The first fluidic channel 7 is connected to the second and third fluidic channels 9, 11 via a plurality of fluidic canals 15 extending from the first fluidic channel 7 to each of the second and third fluidic channels 9, 11.

The first fluidic channel 7 is connected to each of the second and third fluidic channels 9, 11 via a plurality of inclined fluidic canals 15.

The plurality of fluidic canals 15 are distributed or spaced apart along a direction of extension (or along the length (L)) of the first fluidic channel 7 to provide the electrolyte EL to each of the second and third fluidic channels 9, 11 in a distributed manner along a direction of extension (or along the length (L)) of the second and third fluidic channels 9, 11.

The exemplary embodiment of Figure 1C shows fifteen fluidic canals 15 symmetrically arranged either side of the first fluidic channel 7. However, a different number of fluidic canals 15 may be used. The fluidic canals 15 may also be arranged asymmetrically and at different heights with respect to each other along the first fluidic channel 7. For example, the relative width of the fluidic canals 15 may be decreased from entrance (left-hand side in Figure 1C) to the end (right-hand side) of the first fluidic channel 7 to ensure equal velocity distribution in all fifteen fluidic canals 15. The fluidic canals 15 may, for example, also be located or distributed fully across or around the first fluidic channel 7. The first fluidic channel 7 is, for example, configured to provide the electrolyte EL directly via fluidic canals 15 to both the second and third fluidic channels 9, 11. The first fluidic channel 7 is, for example, configured to provide the electrolyte EL simultaneously, via fluidic canals 15, to both the second and third fluidic channels 9, 11.

The first fluidic channel 7 comprises or consists of a plurality of stacked or superposed (substantially) Y- shaped sections SY or partial Y-shaped sections SY (see, for example, Figure 1C). Each Y-shaped section SY comprises at least one (partial Y -shaped sections SY) or a plurality (two or more) of the inclined fluidic canals 15. The inclined fluidic canal 15 defines a wing of the Y-shaped section SY extending away from a central body of the Y-shaped section SY. When a plurality of inclined fluidic canals 15 are present, the inclined fluidic canals 15 may, for example, be located symmetrically or asymmetrically (deformed Y- shape) on the Y -shaped section SY. The inclined fluidic canals 15 extend from the central body of the Y - shaped section SY to the second and/or third fluidic channels 9, 11.

The fluidic canals 15 are, for example, inclined fluidic canals. Each fluidic canal 15 may be an inclined fluidic canal, or a subset or part of the plurality of fluidic canals 15 may be inclined fluidic canals.

The inclined fluidic canals 15 are, for example, inclined with respect to the first fluidic channel 7 in a direction non-perpendicular to an electrolyte flow direction F through the first fluidic channel 7.

The inclined fluidic canals 15 are inclined with respect to the first 7, second 9 or third 11 fluidic channels. The inclined fluidic canals 15 are preferably inclined with respect to each of the first 7, second 9 and third 11 fluidic channels.

The inclined fluidic canals 15 define, for example, an angle a with respect to the first fluidic channel 7. The angle a is, for example, not equal to 90°. The angle a is, for example, an acute angle.

The inclined fluidic canals 15 define an angle 0 with respect to the second fluidic channel 9 and an angle 8 with respect to the third fluidic channel 11. The angles 0 and 8 are, for example, not equal to 90°. The angles 0 and 8 are, for example, acute angles.

Each inclined fluidic canal 15 may define angles 0 and 8 between 10° and 80°, or between 20° and 70°, or between 30° and 60° with respect to the second fluidic channel 9 or with respect to the third fluidic channel 11. Each inclined fluidic canal 15 may define an angle a between 10° and 80°, or between 20° and 70°, or between 30° and 60° with respect to first fluidic channel 7.

The first fluidic channel 7 is interconnected to the second and third fluidic channels 9, 11 via the plurality of inclined fluidic canals 15. The plurality of inclined fluidic canals 15 may be separated by pillars 25 consisting of or comprising solid material. Alternatively, in an alternative non-limiting exemplary embodiment, the fluidic canals 15 may be, for example, suspended in air or in suspension between the first fluidic channel 7 and the second and third fluidic channels 9, 11.

The plurality of inclined fluidic canals 15, or a subset thereof, extends for example outwards and away from the first fluidic channel 7, or outwards and away from the first fluidic channel 7 and away from each other.

The inclined fluidic canals may extend outwards and away from the first fluidic channel 7 and extend in a direction of electrolyte flow F through the first fluidic channel 7 or membrane-less electrolyzer 1.

Each fluidic canal 15 extends from the first fluidic channel 7 to the second and third fluidic channels 9, 11 to each connect physically with or to a different part of the second and third fluidic channels 9, 11.

Each fluidic canal 15 includes a passage PS 15 through which the electrolyte passes from the first fluidic channel 7 to the second and/or third fluidic channels 9, 11. 10. The fluidic canal 15 includes at least one partition 17 defining the passage PS 15 through which the electrolyte flows from the first fluidic channel 7 to the second and/or third fluidic channel 9, 11. The partition 17 extends between the first fluidic channel 7 and the second 9 and/or third 11 fluidic channel.

The first, second and third fluidic channels 7, 9, 11 each include a porous wall permitting fluidic communication between the first, second and third fluidic channels. The first 7, second 9 and third 11 fluidic channels each include at least one wall SW7, SW9, SW11 each defining a plurality of apertures or pores P7, P9, Pl 1. Each fluidic canal 15 may extend between an aperture or pore P7 of the first fluidic channel 7 and an aperture or pore P9, Pl 1 of the second or third fluidic channel 9, 11. As mentioned, the first fluidic channel 7 may, for example, be located between the second and third fluidic channels 9, 11. A portion SW7A of the porous wall SW7 of the first fluidic channel 7 located and extending opposite the second fluidic channel 9 and a portion SW7B of the porous wall SW7 of the first fluidic channel 7 located and extending opposite the third fluidic channel 11 extend non-parallel with respect to each other, or extend inclined at an angle with respect to a central axis CA to distribute the electrolyte EL (substantially) equally in the fluidic canals 15 that are distributed along the first fluidic channel 7. The central axis CA can for, example, be located at the geometrical center.

The inclination with respect to the central axis CA is, for example, at an angle of less than 5, or 4, or 3, or 2 or 1 degree to distribute the electrolyte equally in the fluidic canals (15).

Each fluidic channel 7, 9, 11 includes or defines a passage PS7, PS9, PS 11 through which the electrolyte flows inside the electrolyzer 1.

In a non-limiting exemplary embodiment, the second fluidic channel 9 and/or the third fluidic channel 11 may comprise first and second side walls SW1, SW2 (see, for example, Figure 11) and a floor FL extending between the first and second side walls. A ceiling CL extending between the first and second side walls may also be included.

The first or second side wall SW1, SW2 may include the first or second electrodes 3, 5. In a non-limiting exemplary embodiment shown in Figure 11, the first electrode 3 is provided on the side wall SW2 of the second fluidic channel 9 and the second electrode 5 is provided on the side wall SW2 of the third fluidic channel 9. The first electrode 3 and the second electrode 5 may extend (substantially) parallel to each other in a direction following the flow direction F of the electrolyzer 1 or first fluidic channel 7.

The second and the third fluidic channels 9, 11 may, for example, include electrodes E3, E5 inside the second and the third fluidic channels 9, 11 and on an outer wall or an outer wall section of the second and the third fluidic channels 9, 11. The outer electrodes E3, E5 may, for example, be located on the first side walls SW1 of the second and the third fluidic channels 9, 11.

Similarly, the first fluidic channel 7 (see, for example, Figure 11) and the fluidic canals 15 may also include first and second side walls SW1, SW2, a floor FL and a ceiling CL. As previously mentioned, the first and second porous side walls SW1, SW2 of the first fluidic channel 7 may inclined at an angle with respect to the central axis CA to distribute the electrolyte EL (substantially) equally in the fluidic canals 15. The fluidic channels 7, 9, 11 and the fluidic canals 15 may, however, define different cross-sectional profiles for the passages PS7, PS9, PS 11 and PS 15 and not necessarily a rectangular profile shown in the exemplary embodiment of Figure 11.

The first electrode 3 is, for example, located on the at least one wall SW9 of the second fluidic channel 9 and the second electrode 5 is, for example, located on the at least one wall SW11 of the third fluidic channel 11.

The electrodes may, for example, be non-porous electrodes or non-mesh electrodes. The fluidic canals 15 are, for example, electrode-less.

The first and second electrodes 3, 5 may, for example, include a plurality of apertures or pores PEL3, PEL5 extending through the first and second electrodes 3, 5 and communicating with the plurality of apertures or pores P9, Pl 1 defined by the at least one wall SW9, SW11 of the second and third fluidic channels 9, 11. The plurality of apertures or pores PEL3, PEL5 in the first and second electrodes 3, 5 may, for example, be aligned or symmetrically located opposite one another, as shown for example in Figure 1C.

The apertures or each aperture or pore P7 of the first fluidic channel 7 and/or the apertures or each aperture or pore P9 the second fluidic channel 9 and/or the apertures or each aperture or pore Pl 1 of the third fluidic channel 11 may have an opening width (W) or diameter between 50pm and 200pm, or between 60pm and 180pm, or between 80pm and 160pm.

The fluidic canals 15 or each fluidic canal 15 may, for example, define an opening width (W) or diameter between 50pm and 200pm, or between 60pm and 180pm, or between 80pm and 160pm. The passages PS 15 of the fluidic canals 15 may also, for example, define opening widths or diameters of such values. The plurality of apertures or pores PEL3, PEL5 extending through the first and second electrodes 3,5 may similarly define, for example, opening widths or diameters of such values.

Alternatively, the plurality of apertures or pores PEL3, PEL5 extending through the first and second electrodes 3,5 may define openings larger in width (W) or diameter than the openings defined by the pores or apertures P9, Pl l of the first or second fluidic channels 9, 11, for example, between 10% and 90% greater. The apertures or pores PEL3, PEL5 extending through the first and second electrodes 3, 5 may, for example, tapered or curved inwards at the extremity of the aperture or pore PEL3, PEL5, and tapered or curved inwards towards the supporting wall upon which the electrode is attached. This assures a smoother displacement of the electrolyte and bubbles and the easier detachment of bubbles.

The internal width (W) or diameter of the first fluidic channel 7 and/or second fluidic channel 9 and/or third fluidic channel 11 may, for example, be greater than the opening defined by the pores or apertures P7, P9, Pl l of the first, second, or third fluidic channels 7, 9, 11. The internal width (W) or diameter may, for example, be between 10% and 200% greater. The internal width (W) or diameter may, for example, be between 55 pm and 600 pm. The internal height (H) may, for example, also be between 10% and 200% greater. The internal height (H) may, for example, be between 55pm and 600pm.

The length (L) of the first fluidic channel 7 and/or second fluidic channel 9 and/or third fluidic channel 11 may, for example, be between 5mm and 50mm. The length (L) of the fluidic canals 15 may, for example, be between 50pm and 500pm, for example 200pm.

The fluidic channels, 7, 9, 11 may, for example, define micro-fluidic channels in cross-section. The fluidic canals 15 may define micro-fluidic canals in cross-section.

It is, however, noted that these values are provided as non-limiting and exemplary values.

An internal and/or external width (W) or diameter of the first fluidic channel 7, may, for example, be tapered, for example, tapered along the length (L) of the first fluidic channel 7. The internal and/or external width (W) or diameter reduces in the direction of extension from an inlet 21 to outlets 23A, 23B. The reduction from one end of the of the first fluidic channel 7 to an opposite end of the first fluidic channel 7 may, for example, be between 5% and 35%, for example, 25%. This assures a uniform distribution of the electrolyte to the outer fluidic channels for electrolysis.

The first fluidic channel 7 includes an input section IS configured to input the electrolyte, the input section IS including the inlet 21 through which the electrolyte EL is inserted into the electrolyzer 1. The first fluidic channel 7 or electrolyzer 1 also includes an output section OS consisting of or comprising, for example, a plurality of inclined fluidic canals 15 and a V-shaped abutment or stop 27. The second 9 and third 11 fluidic channels may, for example, also include inlets 21A and 21B (for example, a first lateral inlet 21A and a second lateral inlet 21B) to remove the bubbles faster from the second 9 and third 11 fluidic channels. The liquid velocity at inlets 21A and 21B can, for example, be different since the number of bubbles in the hydrogen side, for example, the second fluidic channel 9 is twice the number of bubbles in the oxygen side, for example, the third fluidic channel 11.

The inlets 21A and 21B are attached to and in fluid communication with the second 9 and third 11 fluidic channels.

The first 7, second 9 and third 11 fluidic channels are closed or partially closed fluidic channels. The electrolyzer 1 may, for example, comprise a lid 31 configured to close the first, second and third fluidic channels 7, 9, 11.

The electrolyzer 1 may further include a pump (not shown) for pumping the electrolyte EL into the first fluidic channel 7 and through the fluidic canals 15, as well as through the second and third fluidic channels 9, 11 and out of the electrolyzer 1 via the outlets 23A, 23B for accessing products generated by the membrane-less electrolyzer. The pump may, for example, be connected to the inlet 21 and connected thereto, for example, directly or indirectly via tubing.

The electrolyzer 1 may also include electrical connection wires or lines 33 in electrical connection with the first and second electrodes 3, 5 and configured to be connected or connected to an electrical energy source (for example, a solar cell) for providing electrical energy (current, voltage) to the first and second electrodes 3,5 to carry out the electrolysis. The first electrode 3 may for example define a cathode and the second electrode 5 may for example define an anode.

An electrolyte EL is provided to the inlet 21 and the first fluidic channel 7 is configured to distribute the electrolyte EL to the second and third fluidic channels 9, 11, via the inclined fluidic canals 15 and the second and third fluidic channels 9,11 are configured to generate products for output via the electrical energy provided to the electrodes 3, 5 of the second and third fluidic channels 9,11.

The electrolyte may, for example, comprises or consists of water and the generated products comprise or consist of Hydrogen and Oxygen. However, the electrolyzer 1 of the present disclosure is not limited to such an electrolyte and output gases and these are solely provided as a non-limiting example in the present description and in the Figures. For example, the electrolyzer 1 may be used for brine electrolysis for chlorine production.

The present disclosure also concerns a device assembly comprising a plurality of membrane-less electrolyzers 1.

The electrolyzer 1 or porous wall electrolyzer 1 (PW electrolyzer) shown in Figure 1C permits to achieve a higher production rate and lower gas cross over. The liquid electrolyte EL enters the middle channel 7 and goes to the outer channels 9, 11 through the inclined wall pores. The electrodes 3, 5 are on the outer sides of the porous walls and the bubbles are being generated in the outer channels 9, 11. The inclined walls or partitions 15 and pores ensure the sufficient flow in each electrodes pore. The flow through the wall pores prevent the migration of bubbles to the opposite side. In this design, the volume fraction of gas in the interelectrode area is low since there is no flow of bubbles in the middle channel 7. Therefore, the ohmic loss due to the presence of flowing bubbles between the electrodes3, 5 is smaller compared to the parallel electrodes design.

The performance of a PW electrolyzer 1 can be improved further by using a surfactant in the electrolyte EL.

Accordingly, the present disclosure also concerns an electrolysis method comprising the steps of providing the membrane-less electrolyzer 1 and providing an electrolyte EL and a surfactant into the membrane-less electrolyzer 1 to carry out electrolysis. The surfactant may comprise or consist solely of heptadecafluorooctancesulfonic acid potassium (PFOS). Electrolysis is, for example, carried out to produce Hydrogen and Oxygen with an electrolyte comprising or consisting solely of water.

The present disclosure also concerns a method for operating an electrolyzer comprising the steps of providing a membrane-less electrolyzer 1 and providing the electrolyte EL to the first or middle fluidic channel 7 of the membrane-less electrolyzer 1 for distribution to the outer fluidic channels 9, 11 to carry out electrolysis. The electrolyte EL is provided simultaneously to the second and third fluidic channels and circulated through the second and third fluidic channels to extract or carry the generated (gas) products to the electrolyzer 1 outlets 23 A, 23B. The electrolyte EL may be provided with a surfactant. The surfactant may comprise or consist solely of heptadecafluorooctancesulfonic acid potassium (PFOS). Electrolysis may for example be carried out to produce Hydrogen and Oxygen. The Inventors compared the product purity and the performance of the PW electrolyzer 1 with a parallelelectrode electrolyzer (PE electrolyzer). This comparison demonstrates the effectiveness of the porous walls. The present disclosure provides guidelines for the design of membrane-less electrolyzers 1 for achieving, for example, high throughput production of hydrogen with high purity.

The PE and the PW electrolyzers shown in Figure IB and 1C are used for hydrogen generation. Both electrolyzers have an equal electrode area. The interelectrode distance of the PE electrolyzer is 620 /im which is equal to the average interelectrode distance of the PW electrolyzer 1. Initially, the Inventors discuss the performance and product purity of the PE electrolyzer. Afterwards, the Inventors present the results of the PW electrolyzer 1 and compare its performance and product purity with the PE electrolyzer.

Figure 2A shows the bubble generation in the PE electrolyzer at the current density j = 300 mA/cm² at different Reynolds numbers. The electrolyte is 1 M sulfuric acid. This Figure indicates the liquid flow detaches bubbles at smaller sizes as the Re increases. However, the small bubble detachment is not enough to prevent large bubble formation in the channel. The bubble coalescence leads to the formation of large bubbles. The equilibrium position of these large bubbles is at the center of the channel [23], Bubbles moving at the centerline are a mixture of hydrogen and oxygen since these bubbles coalesce with bubbles originating from both sides. For this reason, large bubbles should be avoided in the membrane-less electrolyzers.

Decreasing the production rate is one approach to reduce the bubbles’ size and cross over. Figure 2B presents the bubbles flow at Re=109 and j = 75 mA/cm². This Figure depicts the decrease in the bubbles’ size when the current density is decreased from j = 300 mA/cm² to j = 75 mA/cm². The number of bubbles in the channel decreases with decreasing the current density. The bubble coalescence becomes less frequent at lower current densities. As a result, the bubbles become smaller, and the cross over decreases.

Adding a surfactant to the electrolyte is another solution for reducing the bubbles’ size. Heptadecafluorooctancesulfonic acid potassium (PFOS) is used as the surfactant in 1 M sulfuric acid. Figure 3 shows the bubble generation at j = 300 mA/cm² and different Re. This Figure shows many bubbles evolving close to each other. But the surfactant in the electrolyte prevents the coalescence of these bubbles. Therefore, the bubbles at the end of the electrodes are smaller in the electrolyte with the surfactant compared to the surfactant-free electrolyte (Figure 2A).

Figure 4A shows the polarization curve of the PE electrolyzer. The slope of the polarization curve is steeper in in the electrolyte with the surfactant. This can be attributed to the faster bubble detachment from the electrodes and smaller bubbles flowing between the electrodes. Furthermore, PFOS reduces the dissolved hydrogen concentration close to the electrode [27], This leads to a lower concentration overpotential due to the hydrogen supersaturation at the electrode surface [28], The required potential for the reactions decreases at constant current densities as shown in Figure 4A due to small bubble residence time on the surface of the electrode and lower concentration overpotential.

Figure 4B presents the hydrogen cross over to the oxygen side for the experiments shown in Figure 2 and Figure 3. The lower flammability limit of hydrogen -oxygen mixture is 4% [29], The dashed line in Figure 4b determines the flammability limit of hydrogen. The cross over is measured using a gas chromatograph (GC). The cross over is low at high Re because the liquid flow can detach bubbles faster at smaller sizes and remove them from the channel more efficiently. However, the cross over is higher than the flammability limit when the surfactant-free electrolyte is used even at high Re=109. The bubble coalescence at Re=41 and Re=54 creates large bubbles in the hydrogen outlet. Subsequently, these bubbles merge in the downstream and they block the hydrogen outlet. Hereafter, the liquid and all the bubbles flow through the oxygen outlet. The hydrogen channel remains blocked until the end of the experiment which creates an enormous hydrogen cross over. Thereby, the hydrogen cross over is not shown in Figure 4B Re=41 and Re=54 in the surfactant-free electrolyte. The cross over reduces to the values below the flammability limit at Re=109 either by adding the surfactant to the electrolyte or by decreasing the production rate.

One goal of the PW electrolyzer 1 design is to reduce the gas cross over. There are two porous walls between nucleation sites in the PW electrolyzer 1 that helps the product separation. The bubbles cannot go through the wall pores due to the opposite flow direction from the middle channel 7. Furthermore, a bubble reaches the minimum energy in the spherical shape. Consequently, the large bubbles flow in the outer channels 9, 11 rather than going through the wall pores as these bubbles deform less in the outer channels 9, 11. A confined bubble travels in the outer channel 9, 11 rather than the wall pores as it experiences smaller deformation. As a result, this design can efficiently separate the large bubbles that are forming at high current densities. However, smaller bubbles can flow through the wall pores and move to the middle channel 7. This happens especially when there is a reverse flow in the wall pores. Moreover, the ionic resistance between the electrodes 3,5 is directly proportional to the size of wall pores. Therefore, this design is preferably optimized in order to achieve good product separation and minimize the ionic resistance.

Two exemplary design criteria of the PW electrolyzer 1 are an equal distribution of liquid flow in the wall pores and minimum gas cross over. The numerical simulations are carried out using, for example, ANSYS Fluent software from ANSYS Inc to optimize the design. Figure 5A shows four steps of the design optimization for achieving the two mentioned objectives. For each geometry, two types of 2dimensional simulations are carried out. Initially, the single-phase flow conservation of mass and momentum are solved to determine the flow distribution in the wall pores. In this simulation, the working fluid is water. Water enters the middle channel 7 at the velocity of 0.4 m/s and exits through outlets 23A, 23B of the outer channels 9, 11.

Secondly, mixture equations are solved to estimate the gas cross over in each geometry. The primary phase is water and the secondary phases are hydrogen and oxygen. The water enters from the bottom of the device at the velocity of 0.4 m/s and exits through the outer channels’ outlets. Hydrogen and oxygen enter the channel through small inlets on the outer sides of posts at the velocities of 0.02 m/s and 0.01 m/s, respectively. The boundary conditions are shown in Figure 5A. The surface tension is 0.072 N/m for the hydrogen-water and oxygen-water pairs. The surface tension between hydrogen and oxygen is neglected. The diameter of hydrogen and oxygen bubbles is assumed to be 10 pm. Table 1 presents the density and viscosity of the fluids used in these simulations.

Table 1 presents the density and viscosity of the fluids used in the simulations.

The maximum velocity along the wall pores are drawn in Figure 5B based on single-phase simulations. The water volume fraction contours shown in Figure 5Aa are the results of the three-phase simulations at time=0.2 s. In this Figure, hydrogen and oxygen are entering the left and right outer channels, respectively. Figure 5C shows the water volume fraction along lines drawn in the middle of the channel and middle of porous walls based on three-phase simulations. The water volume fraction is one at the centerline if there is no cross over. The water volume fraction below one in the left and right walls determines that some pores are filled with gas.

Geometry 1 has canals/pores 15 with 100 /im length. The liquid flow velocity is not equal in the pores of this design and the water volume fraction is less than 0.8 in the centerline. The length of canal/pores 15 is increased from 100 im in Geometry 1 to 200 im in Geometry 2. This change increases the water volume fraction to 0.93 at the centerline. However, the flow distribution is not uniform in the canals/pores 15. The porous walls are rotated by 0.75° in the opposite direction to construct Geometry 3. The flow distribution is uniform and the water volume fraction is 0.99 in Geometry 3. Figure 5A shows the gas cross over at the end of the middle channel 7 of Geometry 3. In the final step, Geometry 4 is produced by tilting the wall pores/canals 15 by 45°. The water volume fraction becomes 1.0 at the centerline after this change. This modification completely suppresses the gas cross over and keeps the uniform flow distribution in the wall pores. Figures 10A and lOBshow the simulations that are carried out to investigate the effect of the wall pore/canal 15 angle and size. The water volume fraction is 1.0 and the velocity is equally distributed when the wall pore/canal 15 angle is between 30° and 60° or the wall pore/canal 15 size is between 80 /im and 160 /rm. An exemplary PW electrolyzer 1 was designed and fabricated based on Geometry 4.

Figure 6 shows bubbles generation and flow in the PW electrolyzer 1 at j = 300 mA/cm² at different Re values. In addition to the liquid flow, the porous walls and the bubble production on the outer walls are contributing to product separation. The bubbles’ size is inversely proportional to the Re and the large bubbles are more frequent at smaller Re. The large bubbles flowing in the outer walls do not cross the wall pores/canals 15 towards the main channel 7 due to the surface tension. However, these large bubbles create a pressure imbalance between the two outer channels 9, 11. Some smaller bubbles move to the middle channel 7 due to this pressure imbalance. These bubbles coalesce in the middle channel 7 and create a larger bubble that can be seen at the end of the middle channel 7 in Figure 6. The bubble in the middle channel 7 cannot go through the pores and stays in the middle channel 7. This bubble is large at Re=41 but its size decreases by increasing the Re and it disappears completely at Re=109. At moderate Re (Re=54 and Re=82), the flow reaches the steady-state when this bubble forms in the middle channel 7. Henceforth, the small bubbles do not cross the wall pores.

The bubble generation after adding the PFOS surfactant to the electrolyte EL is shown in Figures 7A and 7B. The surfactant decreases the bubble size for the same reasons discussed previously in relation to the PE electrolyzer. The pressure imbalance due to the bubbles flow decreases when the bubbles become smaller. Consequently, bubbles do not migrate to the middle channel 7. As a result, there is no bubble formation in the middle channel 7 even at low Re=41. Moreover, the production rate is further increased by adding PFOS surfactant to the electrolyte due to small bubble generation. Comparison of Figure 6 and Figure 7A shows that the bubbles are not filling the wall pores in the electrolyte with surfactant. The ions migrate through the wall pores/canals 15 between the electrodes 3, 5. The bubbles in the wall pores block the ionic path and the ions needs to migrate through another pore with larger distance to reach the opposite electrode. The absence of bubbles in the wall pores/canals 15 reduces the ohmic resistance related to the ionic transport between the electrodes 3,5. The reduction in the ohmic resistance leads to the improvement of the PW electrolyzer 1 efficiency as shown in Figure 8A. Consequently, the input power for hydrogen production is lower at constant current density when PFOS is added to the electrolyte. The PW electrolyzer 1 has a slightly better electrochemical performance than the PE electrolyzer by comparing the polarization curves of the PE electrolyzer (Figure 4A) with the PW electrolyzer (Figure 8A). This shows that putting electrodes on the outer side of the porous walls does not deteriorate the electrochemical performance of the device.

The crossover of hydrogen to the oxygen side in the PW electrolyzer 1 is shown in Figure 8B. In the surfactant-free electrolyte, the cross over is higher than the flammability limit only when Re=41. The liquid velocity is not enough at this Re to remove the bubbles from the device before the bubbles become large. The cross over falls below the flammability limit by increasing the Re. In the surfactant-free electrolyte, there are bubbles in the middle channel 7 at Re=54 and 82 as shown in Figure 6 but these bubbles do not contribute to cross-contamination. The addition of PFOS to the electrolyte EL decreases the cross over further due to the coalescence inhibition and faster bubble detachment which permits an increase in the current density from 300 mA/cm² to 450 mA/cm² with cross overs below the flammability limit.

A comparison of Figure 4B and Figure 8B indicates a clear improvement in the cross over by changing the design from the PE electrolyzer to the PW electrolyzer 1 design. The PE electrolyzer can produce products with safe cross overs only at high Re=109 and using the surfactant. However, the PW electrolyzer 1 achieves a better product separation at smaller Re without using the surfactant. As an example, the mean value of the hydrogen cross over in the PE electrolyzer is 8.2 + 0.4 % while it is 0.14 + 0.06 % in the PW electrolyzer 1 when 1 M H2SO4 is used at Re=109 and j = 300 mA/cm². The cross over reduces in both electrolyzers in the electrolyte with the PFOS surfactant. The product separation is still more successful in the PW electrolyzer 1 compared to the PW electrolyzer when PFOS is added to the electrolyte. The cross over in the PE electrolyzer and PW electrolyzer 1 is 0.5 + 0.3 % and 0.11 + 0.05 %, respectively in 1 M H2SO4 + 0.056 mg/ml PFOS at Re = 109 and j = 300 mA/cm².

Figure 8C illustrates the average and standard deviation of the applied potential to the PW electrolyzer 1 at different Re and constant current density of 300 mA/cm². The bubble residence time on the surface of the electrode decreases as the Re increases. There is more available active area if the bubbles leave the electrode surface faster. Consequently, the overpotential due to the electrode surface coverage by bubbles is smaller. Therefore, the applied potential and the potential oscillation decrease by increasing the Re. The present disclosure shows that successful product separation is a challenging task in the membrane-less electrolyzers at high current densities. This disclosure describes geometry modification and the addition of surfactant to the electrolyte as two strategies and solutions that facilitate product separation. A PE electrolyzer achieves good product separation at a low current density of 75 m/l/cm² and Re = 109. However, the hydrogen cross over becomes more than the flammability limit at a higher current density of 300 mA/cm². The bubble coalescence and large bubble detachment are the main reasons for the formation of large bubbles in the channel that leads to unsafe cross-contamination. The addition of PFOS surfactant to the electrolyte reduces the cross over to the values below the flammability limit at j = 300 mA/cm² as a result of smaller bubble generation and bubble coalescence prevention. Moreover, the PFOS surfactant decreases the surface screening of the electrodes by the bubbles resulting in lower overpotentials.

The geometry of membrane-less electrolyzers can be modified to suppress the cross-contamination of products. The Inventors present PW electrolyzers 1 as an innovative type of membrane-less electrolyzers with an enhanced product separation compared to PE electrolyzers. The PW electrolyzer 1 has a middle channel 7 where the electrolyte EL flows and two outer channels 9, 11 for the product generation. The hydrogen cross over is 58 times smaller in the PW electrolyzer 1 compared to the equivalent PE electrolyzer using IM H2SO4 at Re=109 and j = 300 mA/cm². A bubble forms in the middle channel 7 of the PW electrolyzer 1 at small Re due to the pressure imbalance imposed by two-phase flows. This bubble forms at the beginning of the PW electrolyzer 1 operation. This bubble grows until the flow reaches the steady-state. Although the bubble in the middle channel 7 does not lead to the cross over, it increases the ionic resistance. The size of this bubble is inversely proportional to the Re and it does not form at Re=109. The PFOS surfactant enhances the electrochemical performance of the PW electrolyzer 1 since the bubbles are not forming in the middle channel 7 and they are not blocking the wall pores/canals 15.

The large-scale membrane-less electrolyzers have a larger interelectrode distance than the microfluidic electrolyzers due to the flow of larger bubbles in the scaled-up electrolyzers. The PW electrolyzer 1 design can be scaled-up without increasing the interelectrode distance since there is no bubble flowing between the electrodes. The fabrication of inclined pores/canals 15 and deposition of catalysts only on one side of the porous walls might be challenging. However, additive manufacturing technologies can be used for the fabrication of large-scale PW electrolyzers [30],

Figure 11 shows a non-limiting and exemplary fabrication process of the electrolyzer of the present disclosure. The fabrication of the PW electrolyzer 1 starts by depositing 200 nm titanium on a silicon wafer. The electrical connections are made by doing photolithography and metal ion beam etching on the titanium layer. Afterwards, the porous walls SW2 supporting vertical electrodes 3,5 are made using the SU8 process with a height of 70 pm. Platinum is sputtered on the device followed by ion beam etching [31], The ion beam etching removes Platinum from the horizontal surfaces but platinum remains on the vertical walls SW2 of SU8. The platinum on the vertical walls SW2 is in contact with titanium on the horizontal floor. Subsequently, the fluidic channels 7, 9, 11 are fabricated in SU8. The height of the fluidic channels 7, 9, 11 is 80 /rm. The inner sides of the porous walls are covered with SU8 in this step to define fluidic channel 7. Therefore, platinum is in contact with electrolyte only in the outer channels 9, 11. The inlet 21 and outlets 23 A, 23B are punched in a Polydimethylsiloxane PDMS piece. A thin layer of SU8 is coated on this piece. Next, this PDMS piece is bonded to the device to seal the channels 7, 9, 11 [32], Figure 11 shows the detailed process flow. The minimum interelectrode distance is 550 /rm at the end of electrodes at the outlet end. The maximum interelectrode distance is 690 /rm at the beginning of electrodes at the inlet end. The electrodes active area of the PW electrolyzer is 0.347 mm².

The PE electrolyzer was fabricated with the same process. The interelectrode distance and electrodes active area of the PE electrolyzer are 620 /rm and 0.347 mm². Figures 9A and 9B show the fabricated PE and PW electrolyzers.

A Cronus Sigma 1000 Series syringe pump was used for flowing electrolytes in the channel 7. The applied current to the device 1 is controlled by a Bio-Logic SP-300 potentiostat. The images of the bubbles generation and flow is captured using a Photron FASTCAM Mini UX100 camera at the 4000 fps and 1/10000 s shutter speed. Two test tubes are filled with liquid electrolyte. These test tubes are held inversely in a larger container of the liquid electrolyte. The generated hydrogen and oxygen are collected in these test tubes. The diluted gas with air is injected to the SRI 8610C gas chromatogram with a thermal conductive detector. The current is applied to the device for 15 minutes in each experiment. The experiments are repeated three times at each current density and Re.

The Re is calculated using the following equation: pVD

Rc - (1) where p, V, D, and p are the density of the electrolyte, average velocity at the inlet of the device, hydraulic diameter of the inlet, and viscosity of the electrolyte, respectively. The density and viscosity of 1 M sulfuric acid are 1060 kg/m³ and 0.00114 kg/m/s. The dimension of inlet is 300 /zm x 80 /zm.

While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments, and equivalents thereof, are possible without departing from the sphere and scope of the invention. Accordingly, it is intended that the invention not be limited to the described embodiments and be given the broadest reasonable interpretation in accordance with the language of the appended claims. Features of one of the above described embodiments may be included in any other embodiment described herein.

REFERENCES

1. Balat, Mustafa, Potential importance of hydrogen as a future solution to environmental and transportation problems. International journal of hydrogen energy, 2008. 33(15): p. 4013-4029.

2. Cano, Zachary P, Dustin Banham, Siyu Ye, Andreas Hintennach, Jun Lu, Michael Fowler, and Zhongwei Chen, Batteries and fuel cells for emerging electric vehicle markets. Nature Energy, 2018. 3(4): p. 279-289.

3. Ursua, Alfredo, Luis M Gandia, and Pablo Sanchis, Hydrogen production from water electrolysis: current status and future trends. Proceedings of the IEEE, 2011. 100(2): p. 410-426.

4. Glenk, Gunther and Stefan Reichelstein, Economics of converting renewable power to hydrogen. Nature Energy, 2019. 4(3): p. 216-222.

5. Esposito, Daniel V, Membraneless electrolyzers for low-cost hydrogen production in a renewable energy future. Joule, 2017. 1(4): p. 651-658.

6. Bessarabov, Dmitri and Pierre Millet, PEM water electrolysis. Vol. 1. 2018: Academic Press.

7. Bodner, Merit, Astrid Hofer, and Viktor Hacker, H2 generation from alkaline electrolyzer. Wiley Interdisciplinary Reviews: Energy and Environment, 2015. 4(4): p. 365-381.

8. David, Martin, Carlos Ocampo-Martinez, and Ricardo Sanchez-Pena, Advances in alkaline water electrolyzers: A review. Journal of Energy Storage, 2019. 23: p. 392-403.

9. Carmo, Marcelo, David L Fritz, Jurgen Mergel, and Detlef Stolten, A comprehensive review on PEM water electrolysis. International journal of hydrogen energy, 2013. 38(12): p. 4901-4934.

10. Xiang, Chengxiang, Kimberly M Papadantonakis, and Nathan S Lewis, Principles and implementations of electrolysis systems for water splitting. Materials Horizons, 2016. 3(3): p. 169173.

11. Vincent, Immanuel and Dmitri Bessarabov, Low cost hydrogen production by anion exchange membrane electrolysis: A review. Renewable and Sustainable Energy Reviews, 2018. 81: p. 16901704.

12. Ayers, Katherine E, Everett B Anderson, Christopher B Capuano, Michael Niedzwiecki, Michael A Hickner, Chao-Yang Wang, Yongjun Leng, and Wei Zhao, Characterization of anion exchange membrane technology for low cost electrolysis. ECS Transactions, 2013. 45(23): p. 121.

13. Ni, Meng, Michael KH Leung, and Dennis YC Leung, Technological development of hydrogen production by solid oxide electrolyzer cell (SOEC). International Journal of Hydrogen Energy, 2008. 33(9): p. 2337-2354.

14. Mohammadi, Amin and Mehdi Mehrpooya, Techno-economic analysis of hydrogen production by solid oxide electrolyzer coupled with dish collector. Energy Conversion and Management, 2018. 173: p. 167-178. 15. Hashemi, S Mohammad H, Miguel A Modestino, and Demetri Psaltis, A membrane-less electrolyzer for hydrogen production across the pH scale. Energy & Environmental Science, 2015. 8(7): p. 2003-2009.

16. Hashemi, S Mohammad H, Petr Karnakov, Pooria Hadikhani, Enrico Chinello, Sergey Litvinov, Christophe Moser, Petros Koumoutsakos, and Demetri Psaltis, A versatile and membrane-less electrochemical reactor for the electrolysis of water and brine. Energy & Environmental Science,

2019. 12(5): p. 1592-1604.

17. Gillespie, Ml and RJ Kriek, Scalable hydrogen production from a mono-circular filter press Divergent Electrode-Flow-Through alkaline electrolysis stack. Journal of Power Sources, 2018. 397: p. 204-213.

18. Gillespie, Ml and RJ Kriek, Hydrogen production from a rectangular horizontal filter press Divergent Electrode-Flow-Through (DEFT™) alkaline electrolysis stack. Journal of Power Sources, 2017. 372: p. 252-259.

19. O'Neil, Glen D, Corey D Christian, David E Brown, and Daniel V Esposito, Hydrogen production with a simple and scalable membraneless electrolyzer. Journal of The Electrochemical Society, 2016. 163(11): p. F3012.

20. Davis, Jonathan T, Ji Qi, Xinran Fan, Justin C Bui, and Daniel V Esposito, Floating membraneless PV-electrolyzer based on buoyancy-driven product separation. International Journal of Hydrogen Energy, 2018. 43(3): p. 1224-1238.

21. Long, Zhiyun, Yuyan Zhao, Chunhui Zhang, Yuheng Zhang, Cunming Yu, Yuchen Wu, Jun Ma, Moyuan Cao, and Lei Jiang, A Multi-Bioinspired Dual-Gradient Electrode for Microbubble Manipulation toward Controllable Water Splitting. Advanced Materials, 2020. 32(17): p. 1908099.

22. Talabi, Oyinkansola O, Anna E Dorfi, Glen D O'Neil, and Daniel V Esposito, Membraneless electrolyzers for the simultaneous production of acid and base. Chemical Communications, 2017. 53(57): p. 8006-8009.

23. Hadikhani, Pooria, S Mohammad H Hashemi, Gioele Balestra, Lailai Zhu, Miguel A Modestino, Frangois Gallaire, and Demetri Psaltis, Inertial manipulation of bubbles in rectangular microfluidic channels. Lab on a Chip, 2018. 18(7): p. 1035-1046.

24. Davis, JT, DE Brown, X Pang, and DV Esposito, High Speed Video Investigation of Bubble Dynamics and Current Density Distributions in Membraneless Electrolyzers. Journal of The Electrochemical Society, 2019. 166(4): p. F312.

25. Li, Yifan, Gaoqiang Yang, Shule Yu, Zhenye Kang, Jingke Mo, Bo Han, Derrick A Talley, and FengYuan Zhang, In-situ investigation and modeling of electrochemical reactions with simultaneous oxygen and hydrogen microbubble evolutions in water electrolysis. International Journal of Hydrogen Energy, 2019. 44(52): p. 28283-28293.

26. Angulo, Andrea, Peter van der Linde, Han Gardeniers, Miguel Modestino, and David Fernandez Rivas, Influence of bubbles on the energy conversion efficiency of electrochemical reactors. Joule,

2020. 4(3): p. 555-579.

27. Zhao, Xu, Ruchiranga Ranaweera, and Long Luo, Highly efficient hydrogen evolution of platinum via tuning the interfacial dissolved-gas concentration. Chemical communications, 2019. 55(10): p. 1378-1381.

28. Dukovic, John and Charles W Tobias, The influence of attached bubbles on potential drop and current distribution at gas-evolving electrodes. Journal of the Electrochemical Society, 1987. 134(2): p. 331.

29. Miao, Haiyan, Lin Lu, and Zuohua Huang, Flammability limits of hydrogen-enriched natural gas. international journal of hydrogen energy, 2011. 36(11): p. 6937-6947. 30. Hashemi, S Mohammad H, Ugljesa Babic, Pooria Hadikhani, and Demetri Psaltis, The potentials of additive manufacturing for mass production of electrochemical energy systems. Current Opinion in Electrochemistry, 2020.

31. Kilchenmann, Samuel C, Enrica Rollo, Pietro Maoddi, and Carlotta Guiducci, Metal-coated SU-8 structures for high-density 3-D microelectrode arrays. Journal of Microelectromechanical Systems, 2016. 25(3): p. 425-431.

32. Wang, Lisen, Lisa Flanagan, and Abraham P Lee, Side-wall vertical electrodes for lateral field microfluidic applications. Journal of microelectromechanical systems, 2007. 16(2): p. 454-461.

The content of each reference being entirely incorporated by reference herein.

Claims

1. Membrane-less electrolyzer (1) comprising:

- at least a first electrode (3) and at least a second electrode (5);

- a first fluidic channel (7) configured to receive an electrolyte (EL);

- a second fluidic channel (9) configured to receive the electrolyte (EL) from the first fluidic channel (7);

- a third fluidic channel (11) configured to receive the electrolyte (EL) from the first fluidic channel (7); the first fluidic channel (7) being in fluidic communication with the second and third fluidic channels (9, i i); wherein the first fluidic channel (7) is connected to each of the second and third fluidic channels (9, 11) via a plurality of inclined fluidic canals (15) extending from the first fluidic channel (7) to each of the second and third fluidic channels (9, 11).

2. Membrane-less electrolyzer (1) according to the previous claim, wherein the inclined fluidic canals (15) are inclined with respect to each of the first, second and third fluidic channels (7, 9, 11).

3. Membrane-less electrolyzer (1) according to anyone of the previous claims, wherein the first fluidic channel (7) comprises or consists of a plurality of stacked Y -shaped sections (SY), each Y -shaped section (SY) comprising a plurality of the inclined fluidic canals (15) extending from the first fluidic channel (7) to each of the second and third fluidic channels (9, 11).

4. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the inclined fluidic canals (15) are inclined in a direction non -perpendicular to an electrolyte flow direction (F) through the first fluidic channel (7).

5. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first fluidic channel (7) is located between or enclosed by the second and third fluidic channels (9, 11).

6. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first fluidic channel (7) is interconnected to the second and third fluidic channels (9, 11) via the plurality of inclined fluidic canals (15) suspended between the first fluidic channel (7) and the second and third fluidic channels (9, 11).

7. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first fluidic channel (7) includes at least one porous wall, or a first porous wall located opposite the second fluidic channel (9) and a second porous wall located opposite the third fluidic channel (11), and wherein a portion (SW7A) of the at least one porous wall located opposite the second fluidic channel (9) and a portion (SW7B) of the at least one porous wall located opposite the third fluidic channel (11) are inclined at an angle with respect to a central axis (CA) of the membrane-less electrolyzer (1) to distribute the electrolyte (EL) equally in the fluidic canals (15), or the first porous wall and the second porous wall are inclined at

26 an angle with respect to said central axis (CA) to distribute the electrolyte (EL) equally in the fluidic canals (15).

8. Membrane-less electrolyzer (1) according to the previous claim, wherein the inclination is at an angle of less than 5, or 4, or 3, or 2, or 1 degree to distribute the electrolyte equally in the fluidic canals (15).

9. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first, second and third fluidic channels (7, 9, 11) each include a porous wall permitting fluidic communication between the first, second and third fluidic channels (7, 9, 11).

10. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the plurality of inclined fluidic canals (15), or a subset thereof, extends outwards and away from the first fluidic channel (7).

11. Membrane-less electrolyzer (1) according to anyone of the previous claims, wherein the plurality of inclined fluidic canals (15), or a subset thereof, extends outwards and away from the first fluidic channel (7) and away from each other.

12. Membrane-less electrolyzer (1) according to anyone of the previous claims, wherein the plurality of inclined fluidic canals (15), or a subset thereof, extends outwards and away from the first fluidic channel (7) and extends in a direction of electrolyte flow (F) through the first fluidic channel (7).

13. Membrane-less electrolyzer (1) according to anyone of the previous claims, wherein each inclined fluidic canal (15) includes at least one partition or wall (17) defining a passage (PS15) through which the electrolyte (EL) flows from the first fluidic channel (7) to the second (9) or third fluidic channel (11).

14. Membrane-less electrolyzer (1) according to the previous claim, wherein the at least one partition (17) extends between the first fluidic channel (7) and the second or third fluidic channel (9, 11).

15. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first, second and third fluidic channels (7, 9, 11) each include at least one wall (SW7, SW9, SW11) defining a plurality of apertures or pores (P7, P9, Pl l), and each inclined fluidic canal (15) extends between an aperture or pore (P7) of the first fluidic channel (7) and an aperture or pore (P9, Pl 1) of the second or third fluidic channel (9, 11).

16. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the second fluidic channel (9) includes the first electrode (3) and the third fluidic channel (11) includes the second electrode (5).

17. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the second and the third fluidic channels (9, 11) include electrodes (E3, E5) on outer walls of the second and the third fluidic channels (9, 11).

18. Membrane-less electrolyzer (1) according to any one of the previous claims 15 to 16, wherein the first electrode (3) is located on the at least one wall (SW9) of the second fluidic channel (9) and the second electrode (5) is located on the at least one wall (SW11) of the third fluidic channel (11).

19. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first and second electrodes (3,5) include a plurality of apertures or pores (PEL3, PEL5) extending through the first and second electrodes (3,5) and communicating with the plurality of apertures or pores (P9, Pl 1) defined by the at least one wall (SW9, SW11) of the second and third fluidic channels (9, 11).

20. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein each inclined fluidic canal (15) defines an angle between 10° and 80°, or between 20° and 70°, or between 30° and 60° with respect to the second fluidic channel (9) and/or with respect to the third fluidic channel (11).

21. Membrane-less electrolyzer (1) according to the previous claim, wherein each inclined fluidic canal (15) defines an angle between 10° and 80°, or between 20° and 70°, or between 30° and 60° also with respect to first fluidic channel (7).

22. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein each aperture or pore (P7, P9, Pl 1) of the first fluidic channel (7) and/or the second fluidic channel (9) and/or the third fluidic channel (11) has an opening width or diameter between 50pm and 200pm, or between 60pm and 180pm, or between 80pm and 160pm.

23. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein each inclined fluidic canal (15) defines an opening width or diameter between 50pm and 200pm, or between 60pm and 180pm, or between 80pm and 160pm.

24. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the plurality of apertures or pores (PEL3, PEL5) extending through the first and second electrodes (3,5) define openings larger in width or diameter than the opening defined by the pores or apertures (P9, Pl 1) of the first or second fluidic channels (9, 11).

25. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein an internal width (W) or diameter of the first fluidic channel (9) and/or second fluidic channel (9) and/or third fluidic channel (11) is greater than the opening defined by the pores or apertures (P9, Pl 1) of the first or second fluidic channels (9, 11).

26. Membrane-less electrolyzer (1) according to anyone of the previous claims, wherein an internal width (W) or diameter of the first fluidic channel (7) and/or second fluidic channel (9) and/or third fluidic channel (11) is between 10% and 200% greater than the opening defined by the pores or apertures (P9, Pl 1) of the first or second fluidic channels (9, 11).

27. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein an internal width (W) or diameter of the first fluidic channel (7) is tapered.

28. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the apertures or pores (PEL3, PEL5) extending through the first and second electrodes (3, 5) are tapered or curved inwards at the aperture or pore extremity.

29. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first fluidic channel (7) includes an input section (IS) configured to input the electrolyte (EL).

30. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first fluidic channel (7) includes an output section (OS) consisting of or comprising a plurality of inclined fluidic canals (15) and a V-shaped abutment (27).

31. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first fluidic channel (7) is configured to distribute the electrolyte (EL) to the second and third fluidic channels (9, 11), and the second and third fluidic channels (9, 11) are configured to generate products via electrolysis for output.

32. Membrane-less electrolyzer (1) according to the previous claim, wherein the electrolyte (EL) comprises or consists of water and the generated products comprise or consist of Hydrogen and Oxygen.

33. Membrane-less electrolyzer (1) according to anyone of the previous claims, further comprising an inlet (21) for providing the electrolyte (EL) into the first fluidic channel (7), and a first and second outlet (23A, 23B) for accessing products generated by electrolysis.

34. Membrane-less electrolyzer (1) according to anyone of the previous claims, further comprising inlets (21A, 21B) for removing bubbles faster from the second (9) and third (11) fluidic channels through the outlets (23 A, 23B).

35. Membrane-less electrolyzer (1) according to anyone of the previous claims, further comprising electrical connection wires or lines (33) in connection with the first and second electrodes (3, 5).

36. Membrane-less electrolyzer (1) according to anyone of the previous claims, further comprising a pump for pumping the electrolyte (EL) into the first fluidic channel (7) and/or an electrical energy source for providing electrical energy to the first and second electrodes (3,5).

37. Membrane-less electrolyzer (1) according to anyone of the previous claims, wherein the second fluidic channel (9) and/or the third fluidic channel (11) comprise first and second side walls (SW1, SW2) and a floor (FL) extending between the first and second side walls (SW1, SW2).

38. Membrane-less electrolyzer (1) according to the previous claim, wherein the first or second side wall (SW1, SW2) includes the first or second electrode (3,5).

39. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the first, second and third fluidic channels (7, 9, 11) are closed or partially closed fluidic channels.

40. Membrane-less electrolyzer (1) according to any one of the previous claims, further comprising a lid (31) configured to close the first, second and third fluidic channels (7, 9, 11).

29

41. Membrane-less electrolyzer (1) according to any one of the previous claims, wherein the plurality of inclined fluidic canals (15) are distributed or spaced apart along a direction of extension of the first fluidic channel (7) to provide the electrolyte (EL) to each of the second and third fluidic channels (9, 11) in a distributed manner along a direction of extension of the second and third fluidic channels (9, 11).

42. Device assembly comprising a plurality of membrane-less electrolyzers (1) according to any one of the previous claims.

43. Electrolysis method comprising the steps of:

- providing a membrane-less electrolyzer (1) according to any one of the previous claims; and

- providing an electrolyte and a surfactant into the membrane-less electrolyzer (1) to carry out electrolysis.

44. Electrolysis method according to the previous claim, wherein the surfactant comprises or consists solely of heptadecafluorooctancesulfonic acid potassium (PFOS).

45. Electrolysis method according to the previous claim 43 or 44, wherein electrolysis is carried out to produce Hydrogen and Oxygen.

46. Method for operating an electrolyzer comprising the steps of:

- providing an electrolyte (EL) to the first or middle fluidic channel (7) of the membrane-less electrolyzer (1) for distribution to the outer fluidic channels (9, 11) to carry out electrolysis.

47. Method for operating an electrolyzer according to the previous claim, including the step of providing an electrolyte (EL) and a surfactant into the membrane-less electrolyzer (1) to carry out electrolysis.

48. Method for operating an electrolyzer according to the previous claim, wherein the surfactant comprises or consists solely of heptadecafluorooctancesulfonic acid potassium (PFOS).

49. Method for operating an electrolyzer according to anyone of the previous claims 46 to 48, wherein electrolysis is carried out to produce Hydrogen and Oxygen.

30