WO2025253103A1

WO2025253103A1 - Conducting hydrogel

Info

Publication number: WO2025253103A1
Application number: PCT/GB2025/051203
Authority: WO
Inventors: Robert DRYFE; Andinet EJIGU; Lewis LE FEVRE; Athanasios STERGIOU
Original assignee: University of Manchester
Current assignee: University of Manchester
Priority date: 2024-06-04
Filing date: 2025-06-03
Publication date: 2025-12-11
Anticipated expiration: 2026-12-04

Abstract

This disclosure provides a method of preparing a conducting hydrogel. The method comprises providing a first electrode and a spaced apart second electrode. The second electrode comprises a species which can be oxidised to provide a multivalent cation. The method further comprises at least partially disposing the first and second electrodes in an electrolyte. The electrolyte comprises a solvent and a hydrophilic polymer. The method further comprises applying a voltage across the first and second electrodes to thereby cause a multivalent cation to be generated at the second electrode and crosslinking to occur between the multivalent cation and hydrophilic polymer, to thereby provide the conducting hydrogel. This disclosure extends to a conducting hydrogel comprising a hydrophilic polymer cross-linked by a multivalent cation. The conducting hydrogel defines a layer with a thickness of less than 250 μm. The disclosure also provides an electrode comprising said conducting hydrogel and a battery comprising said conducting hydrogel and/or said electrode.

Description

CONDUCTING HYDROGEL

The present invention relates to a method of preparing a conducting hydrogel using electrodeposition. The invention extends to a layer of the conducting hydrogel and an electrode comprising said layer.

Ion-conducting hydrogel electrolytes represent an intriguing frontier in electrochemistry, offering potential breakthroughs in various fields such as energy storage, electrochemical sensing, and biomedical devices [1][2].

These materials consist of a network of crosslinked hydrophilic polymers holding an aqueous or gel electrolyte's unique properties making them promising candidates for applications requiring high ionic conductivity, mechanical flexibility, and compatibility with multivalent ions (such as Zn²⁺, Mg²⁺ or Al³⁺)[1]. One of the key advantages of multivalent ion-conducting hydrogel electrolytes is their ability to transport ions with higher charge densities compared to traditional monovalent ions. This characteristic can lead to higher energy densities in batteries and capacitors, making them more efficient and capable of storing larger amounts of energy.

Hydrogel electrolytes provide numerous benefits for zinc-based energy storage systems. They exhibit both liquid-like ionic conductivity and solid-like mechanical properties, resulting in higher ionic conductivity, improved electrochemical performance, greater flexibility in electrolyte design, and extended cycle life for zinc- based batteries [3]. The hydrogel also plays a crucial role in mitigating zinc dendrite formation by facilitating the formation of a solid electrolyte interface (SEI) on the surface of the electrode. This SEI layer acts as a protective barrier, preventing direct contact between the zinc ions and the electrode surface. As a result, it inhibits the growth of zinc dendrites, which can lead to short circuits and premature battery failure [3][4].

Hydrogels are commonly synthesised through various processes, including polymerization reactions, crosslinking methods, and gelation techniques [5]. Incorporating nanomaterials into hydrogels is a common approach to enhance their mechanical stability and other functional properties including elasticity, toughness, and conductivity. Nanomaterials such as silica nanoparticles, carbon nanotubes, or graphene oxide reinforce the hydrogel matrix, providing additional structural support and elasticity, and durability [6]. Conventional hydrogel electrolyte preparation involves a multi-step chemical process with extensive reaction times often exceeding 12 hours [7]. Achieving consistent control over key hydrogel properties such as swelling, mechanical strength, and gelation times are major challenges. Moreover, achieving precise thickness control at the micron level and film transfer, presents a significant challenge due to chemical reaction uniformity, swelling and shrinking.

The present invention arose from the inventors' work in attempting to address the problems associated with the prior art.

In accordance with a first aspect of the invention there is provided a method for producing a conducting hydrogel, the method comprising: providing a first electrode and a spaced apart second electrode, wherein the second electrode comprises a species which can be oxidised to provide a multivalent cation; at least partially disposing the first and second electrodes in an electrolyte, wherein the electrolyte comprises a solvent and a hydrophilic polymer; and applying a voltage across the first and second electrodes, to thereby cause a multivalent cation to be generated at the second electrode and crosslinking to occur between the multivalent cation and hydrophilic polymer to thereby provide the conducting hydrogel.

Advantageously, the method can produce a conducting hydrogel much more quickly than prior art processes. Furthermore, the method enables the production of a thin layer of the hydrogel.

The term "hydrophilic polymer", as used herein, may refer to a polymer which is substantially water soluble. It may be appreciated that the hydrophilic polymer may comprise an electronegative atom. The hydrophilic polymer may comprise a plurality of electronegative atoms. The or each electronegative atom may be oxygen, fluorine, nitrogen, chlorine, bromine and/or a combination thereof. In one embodiment, the hydrophilic polymer comprises an oxygen atom.

The hydrophilic polymer may be an ionic polymer. It will be appreciated that an "ionic polymer" as used herein is a polymer which comprises at least one ionized and/or ionizable functional group. An ionized group may be understood to be a charged group. The ionic polymer may comprise a plurality of ionized and/or ionizable functional groups. For instance, the ionizable group may be a carboxylic acid group, a sulfonic acid group or a hydroxyl group, or the polymer may comprise a combination thereof. It may be appreciated that these groups are ionizable as they can lose a proton under certain pH conditions. Accordingly, the ionized group may be a carboxylate group, a sulfonate group or an alkoxide group, or the polymer may comprise a combination thereof.

In embodiments where the ionic polymer comprises an ionized functional group, the ionic polymer may be present as an ionic polymer salt.

In one embodiment, the ionic polymer is an anionic polymer or an ionizable polymer which is anionic in its ionized state. It will be appreciated that an "anionic polymer" as used herein may be a polymer comprising a functional group which comprises an anion or negative charge. Accordingly, the anionic polymer may comprise a plurality of anionic or negative charges. Similarly, the ionizable polymer which is anionic in its ionized state may comprise a plurality of ionizable groups which comprise an anionic or negative charge in their ionized state.

In embodiments where the anionic polymer is present as a salt, the anionic polymer salt may comprise one or more counter cations to balance the one or more negative charges. Accordingly, if the anionic polymer comprises a plurality of negatively charged groups, the anionic polymer salt may comprise a plurality of counter cations. The counter cation may be a metal cation. In some embodiments, the counter cation is Ca²⁺, Na⁺ or K⁺. In an embodiment, the counter cation is Na⁺.

The hydrophilic polymer may be a non-ionic polymer. It will be appreciated that a "non-ionic polymer" as used herein is a polymer which does not comprise an ionic or ionizable functional groups.

The hydrophilic polymer may be a homopolymer made from a plurality of monomers. Alternatively, the hydrophilic polymer may be a copolymer made from a plurality of monomers. In some embodiments, the hydrophilic polymer is a random copolymer made from a plurality of monomers. In some embodiments, the hydrophilic polymer is a block copolymer comprising homopolymer blocks.

The plurality of monomers may comprise a monomer comprising a carboxylic acid group or a carboxylate group, a monomer comprising a hydroxyl group or an ionised form thereof, and/or a monomer comprising a sulfonic acid group or a sulfonate group. In an embodiment, the plurality of monomers may comprise a monomer comprising carboxylic acid group or a carboxylate group, and/or a monomer comprising a sulfonic acid group or a sulfonate group. In an embodiment, the plurality of monomers may comprise a monomer comprising carboxylic acid group or a carboxylate group.

The plurality of monomers may comprise a sugar moiety or a derivative thereof. The plurality of monomers may comprise a plurality of sugar moieties. The or each sugar moiety may be a monosaccharide or a derivative thereof. The or each sugar moiety may be or comprise glucose, galactose, fructose, mannose, idose, and/or a derivative of any thereof.

A derivative of a sugar moiety may be a uronic acid monosaccharide or a derivative thereof. Accordingly, the plurality of monomers may comprise a mannuronic acid, a gluconic acid, a glucuronic acid, a galacturonic acid, an iduronic acid and/or a derivative of any thereof.

A derivative of a sugar moiety may be an amino sugar or a derivative thereof. Accordingly, the plurality of monomers may comprise glucosamine, N- acetylglucosamine, sialic acid and/or a derivative of any thereof.

A derivative of a sugar moiety may be a sulphated sugar or a derivative thereof.

The hydrophilic polymer may be or comprise a polysaccharide. Accordingly, the hydrophilic polymer may be or comprise chitosan, an alginate, carrageenan, ulvan, starch, agarose, xanthan gum, gelatin or pectin. In one embodiment, the hydrophilic polymer is or comprises chitosan, an alginate, carrageenan, xanthan gum or pectin. It may be appreciated that these polymers are ionic or ionisable polymers.

In one embodiment, the hydrophilic polymer is made from a plurality of monomers comprising guluronic acid and/or guluronate and mannuronic acid and/or mannuronate. In an embodiment, the hydrophilic polymer is alginic acid or an alginate salt. The hydrophilic polymer may be or comprise an alginate salt. The hydrophilic polymer may be or comprise sodium alginate, potassium alginate or calcium alginate. In one embodiment, the hydrophilic polymer is sodium alginate.

Alternatively, the hydrophilic polymer may be poly(acrylic acid), poly(acryl amide), poly(allyl alcohol) or poly(vinyl alcohol). It may be appreciated that these polymers are ionic or ionisable polymers.

In an embodiment, the electrolyte comprises a plurality of hydrophilic polymers. It may be appreciated that the hydrophilic polymer will comprise a plurality of mers.

It may be appreciated that a mer is a repeating unit within a polymer. It may be appreciated that each mer in the hydrophilic polymer is derived from a monomer. It may be appreciated that the total number of mers in a polymer is the total number of repeating units in the polymer. It may be appreciated that when all the mers are the same, the polymer is a homopolymer. Alternatively, when the polymer comprises two or more different mers then the polymer is a copolymer.

The hydrophilic polymer may comprise one or mers comprising an ionized and/or ionizable functional group. The ionized and/or ionizable functional group may be as defined above. In one embodiment, the ionized and/or ionizable group is a carboxylate group and/or a carboxylic acid group.

In one embodiment, at least 5 % of the mers in the polymer comprise an ionized and/or ionizable functional group. In an embodiment, at least 10 %, at least 20 %, at least 30 %, at least 40 % or at least 50 % of the mers in the polymer comprise an ionized and/or ionizable functional group. In an embodiment, at least 60 %, at least 70 %, at least 80% or at least 90 % of the mers in the polymer comprise an ionized and/or ionizable functional group. In an embodiment, substantially all of the mers in the polymer comprise an ionized and/or ionizable functional group.

In one embodiment, the mers in the polymer comprise an average of at least 0.1 ionized and/or ionizable functional groups per mer. In an embodiment, the mers in the polymer comprise an average of at least 0.2, at least 0.3, at least 0.4 or at least 0.5 ionized and/or ionizable functional groups per mer. In an embodiment, the mers in the polymer comprise an average of at least 0.6, at least 0.7, at least 0.8, at least 0.9 or at least 1 ionized and/or ionizable functional groups per mer. The average may be understood to be the mean number of ionized and/or ionizable functional groups per mer.

In embodiments where the polymer is a polysaccharide, the plurality of mers may comprise a mer comprising or consisting of at least 1 sugar moiety or a derivative thereof, at least 2 sugar moieties or a derivative thereof, at least 3 sugar moieties or a derivative thereof, at least 4 sugar moieties or a derivative thereof, or at least 5 sugar moieties or a derivative thereof. In some embodiments, each of the mers in the plurality of mers is a mer comprising or consisting of at least 1 sugar moiety or a derivative thereof, at least 2 sugar moieties or a derivative thereof, at least 3 sugar moieties or a derivative thereof, at least 4 sugar moieties or a derivative thereof, or at least 5 sugar moieties or a derivative thereof. The sugar moiety or derivative thereof may be as defined above. Each sugar moiety or derivative thereof may be the same or different to the other sugar moieties or derivative thereof.

In some embodiments, the plurality of mers comprise a mer comprising or consisting of 1 sugar moiety or a derivative thereof. In some embodiments, each of the mers in the plurality of mers consists of 1 sugar moiety or a derivative thereof.

For instance, it will be appreciated that in alginic acid or an alginate salt the plurality of mers are made up of two types of mers (e.g. (1^4)-linked p-D-mannuronate and o-L-guluronate residues). These two different types of mers both consist of 1 sugar moiety, namely mannuronic acid or mannuronate and guluronic acid or guluronate, respectively. Each of these mers comprises a carboxylate or carboxylic acid group.

The crosslinked hydrophilic polymer may form a three-dimensional lattice structure which incorporates water therein. It may be appreciated that a "hydrogel" as used herein may refer to a crosslinked polymer provided as a three-dimensional lattice structure which incorporates water molecules to form a gel. The hydrophilic polymer may be crosslinked via electrostatic interactions, hydrogen bonds and dipole-dipole interactions.

It may be appreciated that crosslinking of the hydrophilic polymer may involve the formation of a coordination complex, wherein the multivalent cation is coordinated to the hydrophilic polymer at two or more points. It may be appreciated that the multivalent cation may coordinate to the hydrophilic polymer via electrostatic interactions. It may be appreciated that electrostatic interactions include ionic interactions. Accordingly, it may be appreciated that the hydrophilic polymer chains may be considered to be crosslinked via the multivalent cation.

The conducting hydrogel may be an ion-conducting hydrogel.

Applying the voltage across the first and second electrodes may cause the second electrode to have a higher potential than the first electrode.

It will be appreciated that when the voltage is applied across the first and second electrodes an electrolysis reaction occurs. A reduction reaction occurs at the first electrode and an oxidation reaction occurs at the second electrode. The reduction reaction may generate hydrogen at the first electrode. The hydrogen may be vented to the atmosphere. It may be appreciated that the oxidation reaction generates the multivalent cation at the second electrode. The multivalent cation may diffuse into the electrolyte. Advantageously, the multivalent cation may facilitate crosslinking of the hydrophilic polymer. It may be appreciated that in embodiments where the hydrophilic polymer comprises an anionic polymer present as a salt, the multivalent cation may displace a cation from the anionic polymer salt and facilitate crosslinking of the anionic polymer. The multivalent cation may facilitate crosslinking of the hydrophilic polymer via electrostatic interactions.

In an embodiment, the crosslinking between the multivalent cation and hydrophilic polymer may occur substantially adjacent to the second electrode. It may be appreciated that the multivalent cation is generated on or adjacent to the second electrode. Accordingly, the concentration of the multivalent cation may be greatest adjacent to the second electrode. This may cause the crosslinking to occur substantially adjacent to the second electrode.

The hydrogel which is produced may be disposed on the second electrode. In particular, the hydrogel may define a layer on the second electrode.

In an embodiment, the above method provides an electrode with a layer of a conducting hydrogel disposed thereon. The method may comprise a further step of rinsing the second electrode and layer of conducting hydrogel.

The voltage may be applied for between 10 seconds and 30 minutes, between 15 seconds and 20 minutes or between 30 seconds and 15 minutes. In some embodiments, the voltage may be applied for between 45 seconds and 10 minutes, between 1 minute and 8 minutes. In some embodiments, the voltage may be applied for between 2 minutes and 5 minutes.

In one embodiment, the voltage may be applied for between 10 seconds and 10 minutes, between 15 seconds and 8 minutes, between 30 seconds and 6 minutes, or between 1 minute and 5 minutes. In some embodiments, the voltage may be applied for between 2 minutes and 4 minutes. The voltage may be applied for about 3 minutes.

Additionally, the thickness of the film can be regulated by adjusting the deposition potential. The voltage applied across the first and second electrodes may be at least 1.0 V, at least 1.5 V, at least 1.8 V or at least 2.0 V . The voltage applied across the first and second electrodes may be less than 3 V, less than 2.7 V, less than 2.5 V, less than 2.3 V, or less than 2.1 V. In an embodiment, the voltage applied across the first and second electrodes may be between 1 and 3 V, between 1.25 and 2.5 V or between 1.5 and 2.3 V.

The inventors have found that controlling the voltage at the second electrode provides improved control over hydrogel deposition. The voltage applied at the second electrode may be between -0.2 and -0.8 V, between -0.3 V and -0.7 V or between - 0.4 and -0.6 V. It will be appreciated that voltage applied at the second electrode may be measured relative to a reference electrode.

Accordingly, in addition to providing the first electrode and the spaced apart second electrode, the method may comprise providing a spaced apart reference electrode. The reference electrode may be understood to be a different electrode to both the first and second electrodes.

The method may comprise at least partially disposing the reference electrode in the electrolyte.

Accordingly, the method may comprise: providing a reference electrode; a first electrode; and a spaced apart second electrode, wherein the second electrode comprises a species which can be oxidised to provide a multivalent cation; at least partially disposing the reference, first and second electrodes in an electrolyte, wherein the electrolyte comprises a solvent and a hydrophilic polymer; and applying a voltage across the electrodes, such that the second electrode has a higher potential than the first electrode, to thereby cause a multivalent cation to be generated at the second electrode and crosslinking to occur between the multivalent cation and hydrophilic polymer to thereby provide the conducting hydrogel.

The method may further comprise applying a fixed voltage at the second electrode, relative to the reference electrode. The voltage applied at the second electrode relative to the reference electrode may be between -0.2 and -0.8 V, between -0.3 V and -0.7 V or between -0.4 and -0.6 V. The reference electrode may comprise any material capable of providing a stable reference. For instance, the reference electrode may comprise a silver/silver chloride (Ag/AgCI) electrode.

The first electrode may have a high surface area. In some embodiments the first electrode comprises a mesh. The first electrode may be a carbon-based electrode or a metal based-electrode. The carbon-based first electrode may comprise graphite. The metal-base first electrode may comprise platinum. Advantageously, graphite and platinum are stable in the electrolyte solution, and so do not cause contamination thereof.

The second electrode may have a substantially smooth surface or have a substantially textured or porous surface. Accordingly, the second electrode comprise a foil, a rod, a mesh or a felt. The second electrode may comprise a carbon-based electrode or a metal-based electrode. The carbon-based second electrode may comprise graphite. The metal-based second electrode may comprise zinc or titanium.

In embodiments where the second electrode comprises a carbon-based electrode, the second electrode may further comprise the species which can be oxidised to provide a multivalent cation. In embodiments where the second electrode comprises a metalbased electrode, the second electrode may further comprise the species which can be oxidised to provide a multivalent cation or the metal may be the species which can be oxidised to provide a multivalent cation. The second electrode may comprise at least a layer of the species which can be oxidised to a multivalent cation. The layer of the species which can be oxidised to a multivalent cation may be an outer or external layer of the second electrode. The layer of the species which can be oxidised to a multivalent cation may define a thickness of at least 0.001 mm, at least 0.01 mm, at least 0.05 mm, at least 0.10 mm, at least 0.15 mm or at least 0.20 mm. The layer of the species which can be oxidised to a multivalent cation may define a thickness of less than 5 mm, less than 2.5 mm, less than 1 mm, less than 0.5 mm, less than 0.4 mm or less than 0.3 mm. The layer of the species which can be oxidised to a multivalent cation may define a thickness of between 0.001 and 5 mm, between 0.01 and 2.5 mm, between 0.05 and 1 mm, between 0.1 and 0.5 mm, between 0.15 and 0.4 mm or between 0.2 and 0.3 mm.

Alternatively, in some embodiments, the second electrode may comprise substantially of or consist of the species which can be oxidised to produce a multivalent cation. The species which can be oxidised to a multivalent cation may be a metal. The metal may be zinc, aluminium, calcium or magnesium. In an embodiment, the metal is zinc or aluminium. In an embodiment, the metal is zinc. It will be appreciated that a multivalent cation is a cation which has a formal charge which is greater than one. The multivalent cation may be Zn²⁺, Al³⁺, Ca²⁺ or Mg²⁺. In an embodiment, the multivalent cation is Zn²⁺.

Accordingly, the second electrode may comprise or consist of zinc. In some embodiments, the second electrode is a zinc electrode, i.e. it consists substantially of zinc. In alternative embodiments, the second electrode comprises a coating of zinc. Accordingly, in some embodiments, the second electrode is a zinc coated-carbon or zinc coated titanium electrode.

The hydrophilic polymer may be dissolved or dispersed in the solvent. In an embodiment, the hydrophilic polymer is dissolved in the solvent.

The electrolyte may comprise the hydrophilic polymer at a concentration of at least 0.01 wt%, at least 0.1 wt%, at least 0.2 wt%, at least 0.3 wt%, at least 0.4 wt% or at least 0.45 wt%. The electrolyte may comprise the hydrophilic polymer at a concentration of less than 25 wt%, less than 10 wt%, less than 5 wt%, less than 3 wt%, less than 1 wt%, or less than 0.6 wt%. The electrolyte may comprise the hydrophilic polymer at a concentration of between 0.01 and 25 wt%, between 0.1 and 10 wt%, between 0.2 and 5 wt%, between 0.3 and 1 wt%, or between 0.4 and 0.6 wt%.

The solvent may be or comprise water, an organic solvent and/or an ionic liquid. In an embodiment, the solvent may be or comprise water. The water may be ultrapure water.

The electrolyte may further comprise an electrolyte salt. The electrolyte salt may be dissolved in the solvent. In embodiments where the hydrophilic polymer is provided as a hydrophilic polymer salt, the electrolyte salt may be a different salt to the hydrophilic polymer salt. The electrolyte salt may be understood to comprise a cation and an anion. In one embodiment, the salt does not cause significant corrosion of the first and second electrodes. In one embodiment, the cation is not Zn²⁺. In one embodiment the cation is a monovalent cation. The monovalent cation may be a metal cation. The metal cation may be an alkali metal cation. In some embodiments, the monovalent cation is NH4⁺, Na⁺ or K⁺. In an embodiment, the monovalent cation is Na⁺. In one embodiment the anion is a monovalent anion. The anion may be sulphate, citrate, nitrate or acetate. Accordingly, the electrolyte salt may be or comprise sodium acetate.

The concentration of the electrolyte salt in the electrolyte may be between 1 mM and 1 M, between 50 mM and 800 mM, between 100 mM and 600 mM, between 150 mM and 400 mM or between 200 mM and 300 mM. In an embodiment, the concentration of the electrolyte salt in the electrolyte may be between 225 mM and 275 mM or between 240 mM and 260 mM.

The electrolyte may comprise one or more additives dissolved or suspended therein. Accordingly, the one or more additives may be incorporated into the conducting hydrogel. Advantageously, the inventors have found that such additives may be alter the physical, chemical and electrochemical properties of the hydrogel.

Accordingly, in an embodiment, the electrolyte further comprises one or more additives.

The additive may be a two-dimensional (2D) material and/or a nanomaterial.

A nanomaterial may be understood to be a material where at least one of the dimensions of the material is in the nanoscale range. A dimension may be understood to be in the nanoscale range if it is less than 200 nm or less than 100 nm. In some embodiments, a dimension may be understood to be in the nanoscale range if it is less than 10 nm. A dimension may be understood to be in the nanoscale range if it is between 0.01 and 200 nm, between 0.1 and 100 nm or between 1 and 10 nm. One or two dimensions of the nanomaterial may be above the nanoscale range. In particular, one or two dimensions of the nanomaterial may be at least 200 nm.

The 2D material may comprise of a single layer of atoms. Accordingly, the 2D material may be selected from graphene, graphene oxide (GO) or hexagonal boron nitride (hBN).

In some embodiments, the nanomaterial may comprise or consist of platelets of the 2D material.

Advantageously, the short reaction time enables the use of an additive which is not soluble in the solvent, but can be dispersed in the electrolyte. Accordingly, in one embodiment the additive is not soluble in the solvent. For instance, the additive may not be water soluble. The electrolyte may comprise the additive dissolved therein.

In another, embodiment, the additive is not soluble in the solvent. For instance, the additive may be water soluble. The electrolyte may comprise the additive dispersed therein.

The electrolyte may comprise the additive at a concentration of between 0.1 and 10 mg/mL, between 0.5 and 8 mg/mL, between 0.6 and 5 mg/mL, between 0.7 and 5 mg/mL, between 0.8 and 4 mg/mL, between 0.9 and 3 mg/mL or between 1 and 2 mg/mL.

The electrolyte may further comprise a pH adjusting reagent. The pH adjusting reagent may be configured to alter the pH of the electrolyte such that the percentage of mers comprising at least one ionized group is changed. The change may be measured relative to the percentage of mers comprising at least one ionized group at a neutral pH (i.e. a pH of 7 at 20°C). In one embodiment, the pH adjusting reagent alters the pH of the electrolyte, such that the percentage of mers comprising at least one ionized group is increased.

In a second aspect of the invention, there is provided a conducting hydrogel wherein the conducting hydrogel defines a layer with a thickness of less than 250 pm and comprises a hydrophilic polymer cross-linked by a multivalent cation.

The conducting hydrogel of the second aspect may be produced by the method of the first aspect. Accordingly, the hydrophilic polymer and multivalent cation may be as defined in relation to the first aspect.

The conducting hydrogel may have a thickness of less than 225 pm, less than 200 pm, less than 150 pm, less than 100 pm, less than 80 pm or less than 60 pm.

The conducting hydrogel may have a thickness of at least 10 pm, at least 50 pm, at least 100 pm, at least 150 pm or at least 200 pm.

In one embodiment, the conducting hydrogel has a thickness of between 10 and 250 pm, between 50 and 240 pm, between 100 and 230 pm, between 150 and 220 pm, between 160 and 210 m, between 170 and 200 pirn, between 185 and 195 pirn, or between 190 and 194 pirn.

The hydrogel may comprise water. The water may be ultrapure water.

The hydrogel may comprise water in an amount of between 20 and 90 wt%. In one embodiment, the hydrogel comprises water in an amount between 50 and 90 wt%, between 60 and 88 wt%, between, 70 and 86 wt%, between 75 and 84 wt%, between 76 and 83 wt%, or between 78 and 80 wt%.

The hydrogel may comprise the hydrophilic polymer in an amount of at least 0.01 wt%, at least 0.1 wt%, at least 0.2 wt%, at least 0.3 wt%, at least 0.4 wt% or at least 0.45 wt%. The hydrogel may comprise the hydrophilic polymer in an amount of less than 25 wt%, less than 10 wt%, less than 5 wt%, less than 3 wt%, less than 1 wt%, or less than 0.6 wt%. The hydrogel may comprise the hydrophilic polymer at a concentration of between 0.01 and 25 wt%, between 0.1 and 10 wt%, between 0.2 and 5 wt%, between 0.3 and 1 wt%, or between 0.4 and 0.6 wt%.

The hydrogel may further comprise an additive. The additive may be as defined in relation to the first aspect. The additive may be physically crosslinked with the hydrophilic polymer.

The hydrogel may comprise the additive is an amount of at least 0.01 wt%, at least 0.1 wt%, at least 0.5 wt%, at least 1 wt%, at least 2 wt%, at least 5wt%, at least 7 wt% or at least 9 wt%. The hydrogel may comprise the additive in an amount less than 20 wt%, less than 17 wt%, less than 15 wt%, less than 13 wt%, or less than 11 wt%.

Advantageously, the hydrogel is a conducting hydrogel. The conducting hydrogel may be understood to be an ion-conducting hydrogel. Without wishing to be bound by theory, the inventors note that the multivalent cations conduct through the crosslinked polymer network via a hopping mechanism, wherein ions "hop" or jump between available sites in the hydrogel matrix. This movement is facilitated by the water content within the hydrogel, which acts as a medium for ionic transport.

The conducting hydrogel may have a conductivity of at least 1 mS/cm, at least 1.5 mS/cm, at least 2 mS/cm, at least 2.5 mS/cm, at least 3 mS/cm, at least 3.5 mS/cm, at least 4 mS/cm or at least 4.5 mS/cm. In an embodiment, the conducting hydrogel may have a conductivity of at least 5 mS/cm, at least 5.5 mS/cm or at least 6 mS/cm.

The conducting hydrogel may have a conductivity of between 1 and 15 mS/cm, between 1.5 and 12.5 mS/cm, between 2 and 10 mS/cm, between 2.5 and 8 mS/cm, between 3 and 6 mS/cm, between 3.5 and 5.5 mS/cm, between 4 and 5 mS/cm or between 4.5 and 4.8 mS/cm. In one embodiment, the conducting hydrogel may have a conductivity of between 4.5 and 7.5 mS/cm, between 5.0 and 7.2 mS/cm, between 5.2 and 7.0 mS/cm, between 5.4 and 6.8 mS/cm, between 5.6 and 6.6 mS/cm, between 5.8 and 6.4 mS/cm or between 6.0 and 6.2 mS/cm.

The conductivity may be measured as described under the heading "electrochemical measurements" in the methodology section of the examples below.

Advantageously, the inventors have found that the conducting properties of the layer of the conducting hydrogel mean that it is useful as an electrolyte.

Accordingly, in a third aspect of the invention, there is provided a use of the hydrogel of the second aspect as an electrolyte.

In a fourth aspect of the invention, there is provided a coated electrode comprising an electrode comprising a layer of the conducting hydrogel of the second aspect disposed thereon.

The coated electrode may be produced by the method of the first aspect. Accordingly, the electrode may be defined in relation to the second electrode of the first aspect. In an embodiment, the electrode comprises or consists of zinc.

The layer of the hydrogel may be understood to define a coating on the electrode. Accordingly, the hydrogel may protect the electrode from corrosion.

In accordance with a fifth aspect, there is provided a battery comprising the coated electrode of the fourth aspect.

All features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which:-

Figure 1 shows an image of zinc foil covered in a layer of graphene oxide-zinc alginate composite hydrogel;

Figure 2 shows the Raman spectrum of a graphene oxide-zinc alginate composite film;

Figure 3 shows an image of titanium mesh covered in a layer of hexagonal boron nitride-zinc alginate composite hydrogel;

Figure 4 shows the Raman spectrum of a hexagonal boron nitride-zinc alginate composite film;

Figure 5 shows stress vs strain curves for a zinc alginate hydrogel film (ALG@Zn), a graphene oxide-zinc alginate composite hydrogel film (GO-ALG@Zn) and a hexagonal boron nitride-zinc alginate composite hydrogel film (hBN-ALG@Zn);

Figure 6 shows Tafel corrosion curves of pristine zinc (Bare Zn), a zinc alginate hydrogel film (ALG@Zn), a graphene oxide-zinc alginate composite hydrogel film (GO- ALG@Zn) and a hexagonal boron nitride-zinc alginate composite hydrogel film (hBN- ALG@Zn), each was in 2M H2SO4 electrolyte; and

Figure 7 shows a three-electrode electrochemical cell configuration used in a method of the invention.

Example 1 - Electrodeposition of a zinc alginate hvdroqel (ALG@Zn)

An electrochemical cell was configured as described in the Methodology section. A solution comprising 0.5 wt % sodium alginate and 250 mM sodium acetate dissolved in ultrapure water was deposited in the electrodeposition bath. Electrodeposition was carried out as described in the Methodology section.

Applying a potential of -0.5 V vs Ag/AgCI to the zinc electrode initiates an oxidation reaction, causing zinc atoms from the working (second) electrode to lose electrons and convert into zinc ions (Zn²⁺). These zinc ions then diffuse into the surrounding environment. This leads to ionic crosslinking of the alginate and transformation of the alginate solution into a hydrogel. This hydrogel formation typically occurs on the electrode surface, where the zinc ions concentrate, resulting in a crosslinked, gel-like layer. This ultimately forms a layer of hydrogel on the zinc surface. The electrodeposition experiment was repeated successfully using different working electrodes (second electrode) including a zinc coated-carbon electrode; zinc foil of 99.9% purity; and zinc-coated titanium mesh.

The inventors found that the thickness of the hydrogel layer can be controlled by adjusting the electrodeposition time or potential, allowing for fine tuning of the hydrogel's properties. Longer deposition times or higher applied potentials lead to the formation of thicker hydrogel layers due to greater zinc ion release. Advantageously, control over hydrogel thickness provides versatility in applications like bioelectronic devices, drug delivery, tissue engineering and zinc conducting solid layer interface, where precise hydrogel characteristics are crucial.

Example 2 - Electrodeoosition of a graphene oxide-zinc alginate composite hydrogel (GO-ALG@Zn)

An electrochemical cell was configured as described in the Methodology section. A solution containing 0.5 wt % of sodium alginate, 1 mg/mL of graphene oxide (GO) and 250 mM sodium acetate dissolved in ultrapure water was deposited in the electrodeposition bath. Hydrogel electrodeposition was carried out as described in the Methodology section.

The electrodeposition experiment was repeated successfully using different working electrodes including a zinc coated-carbon electrode; zinc foil of 99.9% purity; and zinc-coated titanium mesh. Figure 1 shows an image of the graphene oxide-zinc alginate composite hydrogel deposited onto zinc foil.

Raman spectroscopy was used to confirm the incorporation of graphene oxide into the zinc alginate film. The Raman spectrum of a 499 pm thick composite hydrogel film deposited using a zinc foil working electrode and a deposition time of 5 minutes is shown in Figure 2. The spectrum shows two key peaks characteristic of graphene oxide: the D-band near 1350 cm^-1, reflecting structural disorder, and the G-band around 1580 cm^-1, corresponding to the Ezg vibrational mode of sp² hybridized carbon. The high intensity of the D-band indicates significant functionalization and disruption of the carbon lattice.

Advantageously, the physical crosslinking (via hydrogen bonding) between graphene oxide and alginate facilitates the co-deposition of graphene oxide during the electrodeposition process. Graphene oxide has oxygen-containing functional groups (hydroxyl and carboxyl) capable of forming hydrogen bonds with the carboxylate and hydroxyl groups present in alginate. As zinc ions are released during electrodeposition, a crosslinked alginate network is generated, and the graphene oxide is integrated within the hydrogel network.

Example 3 - Electrodeoosition of a hexagonal boron nitride-zinc alginate composite hydrogel (hBN-ALG@Zn)

An electrochemical cell was configured as described in the Methodology section. A solution containing 0.5 wt % of sodium alginate, 2 mg/mL of boron nitride (h-BN) and 250 mM sodium acetate dissolved in ultrapure water was deposited in the electrodeposition bath, and hydrogel electrodeposition was carried out as described in Example 1.

The electrodeposition experiment was repeated successfully using different working electrodes including a zinc coated-carbon electrode; zinc foil of 99.9% purity; and zinc-coated titanium mesh. Figure 3 shows a picture of the hBN-zinc alginate composite layer deposited onto titanium mesh.

Raman spectroscopy was used to confirm the incorporation of hexagonal boron nitride and zinc alginate in the composite film. The Raman spectrum for a 200 pm thick film deposited using a zinc foil working electrode and 5 min deposition is shown in Figure 4, and a band observed with hBN can be observed at -1346cm’¹.

Thus, the inventors showed that, like graphene oxide, hexagonal boron nitride may be co-deposited with alginate during the electrodeposition process.

Example 4 - Investigation of the physical properties of zinc alginate and nanomaterial- zinc alginate composite hydrogels

The incorporation of nanomaterials like graphene oxide or hexagonal boron nitride (hBN) to an alginate hydrogel significantly improves its physical properties. It has been shown that these nanomaterials, when mixed with alginate and zinc ions, enhance the hydrogel's strength, elasticity, and durability [7].

Figure 5 shows the results of investigations into the tensile strength of the zinc alginate hydrogel and nanomaterial-zinc alginate composite hydrogels synthesised using the methods described in Examples 1 to 3. The films were synthesised using a zinc foil working electrode. The deposition time for the zinc alginate hydrogel was 200 s and the film had a thickness of 285 pm. The deposition time for the hBN-zinc alginate nanocomposite hydrogel was 90 s and the film had a thickness of 350 pm. The deposition time for the GO-zinc alginate nanocomposite hydrogel was 180 s and the film had a thickness of 250 pm. . The inventors found that incorporation of nanomaterials into the hydrogel matrix leads to stronger crosslinking and a more robust network structure.

For example, Figure 5 shows the stress-strain curve of the GO-zinc alginate nanocomposite hydrogel. It can be seen that the maximum stress threshold (i.e. ultimate strength) for the GO-zinc alginate nanocomposite hydrogel increased fourfold compared to the non-nanomaterial containing zinc alginate hydrogel.

Figure 5 shows that the hBN-zinc alginate nanocomposite hydrogel showed improved tensile properties compared to the purely zinc alginate hydrogel. In particular, the hBN-zinc alginate nanocomposite hydrogel is more elastic and stronger.

These significant improvements in tensile strength demonstrate the reinforcing effect of nanomaterials when co-deposited with the zinc alginate hydrogel. These improvements allow the composite hydrogels to be used in applications where increased mechanical strength is crucial, such as in flexible electronics, bioelectronics, or high-stress biomedical devices.

Example 5 - Ionic Conductivity of Electrodeoosited Hydrogels

The ionic conductivity of zinc alginate and hBN/GO-zinc alginate composite hydrogels of different thicknesses were measured using AC impedance spectroscopy. The results are summarised in Table 1.

Table 1. Ionic Conductivity of Electrodeposited Zinc Alginate Hydrogels

Advantageously, the synthesised hydrogels showed ionic conductivity. Zinc ions conduct through the network of the hydrogel via a hopping mechanism, wherein ions "hop" or jump between available sites in the hydrogel matrix. This movement is facilitated by the water content within the hydrogel, which acts as a medium for ionic transport [13]. The water helps maintain a hydrated environment, allowing zinc ions to move with relative ease, leading to the ionic conductivity.

Table 1 shows that at the thickness of the GO-zinc alginate composite hydrogel increased from 372 pm to 499 pm, its conductivity decreased from 4.6 mS/cm to 2.8 mS/cm. This inverse relationship between conductivity and thickness can be attributed to the increased resistance to ion transport through the thicker hydrogel. The inventors found that reducing the thickness of the GO-ALG film to less than 300 pm does not lead to an improvement in ionic conductivity. This plateau effect indicates that there may be intrinsic properties or structural limitations that prevent further gains in conductivity with thinner films.

Hexagonal boron nitride containing zinc alginate films have shown the highest conductivity among the films investigated. Advantageously, the conductivity value of the 240 pm hBN-ALG@Zn film (6.1 mS/cm) is in the same order of magnitude and comparable to that of nonaqueous electrolytes (9.0 mS/cm) used in commercial LIBs [12]. The combination of h-BN and zinc alginate creates a network that facilitates the movement of zinc ions, thereby enhancing ionic conductivity. h-BN's layered structure might help in organizing the polymer matrix in a way that promotes efficient ion transport.

The inventors hypothesise a number of reasons why the h-BN containing films show improved conductivity compared to the GO- containing films. In particular h-BN might be better dispersed within the alginate matrix than GO, providing more uniform pathways for ion transport; h-BN's lamellar structure may facilitate better ion mobility by creating channels or enhancing the alignment of alginate chains; GO’S functional groups might interact with alginate more strongly, potentially hindering ion mobility; h-BN may interact less strongly with zinc ions, leaving more free zinc ions in the matrix to contribute to ionic conductivity; and/or GO, with its oxygen-containing functional groups, might bind zinc ions more strongly, thus reducing the number of free ions available for conduction. Example 6 - Corrosion Tests

Linear polarization tests were conducted as described in the Methodology section to assess the corrosion behaviour of different Zn samples in a highly corrosive environment (2 M H2SO4 solution). The following zinc samples were investigated: bare zinc, zinc foil coated with zinc alginate hydrogel (AIG@Zn), zinc foil coated with hexagonal boron nitride-zinc alginate composite hydrogel (hBN-ALG@Zn), and zinc foil coated with graphene oxide-zinc alginate composite hydrogel (GO-ALG@Zn). The coated zinc samples were prepared as described in Examples 1, 2 and 3. The corresponding Tafel corrosion curves are shown in Figure 6 and the results are summarised in Table 2.

Table 1. Corrosion Behaviour of Zinc and Zinc Alginate Hydrogel -Coated Zinc

Figure 6 shows that all of the zinc alginate hydrogel-coated zinc foil samples exhibited a smaller corrosion current compared to bare zinc. A lower corrosion current typically indicates a reduced corrosion rate, thus suggesting that the hydrogel layer contributes to enhancing the stability of Zn metal [8].

Notably, the hydrogel layer incorporating graphene oxide demonstrated the lowest corrosion current density. Similarly, when estimating the corrosion rate based on extrapolated anodic and cathodic curves, the corrosion rate of the GO-ALG@Zn coated sample (77.21 mm/year) was the lowest among all samples in this corrosive environment. Additionally, the GO-ALG@Zn sample exhibited the most negative corrosion potential (-0.981V vs. Ag/AgCI), indicating the highest overpotential for the hydrogen evolution reaction. This suggests that the anti-corrosion properties of the pristine zinc sample can be influenced by additives in the alginate hydrogel. These findings highlight the potential of zinc alginate hydrogel-based coatings, particularly those incorporating graphene oxide, for enhancing the corrosion resistance and stability of zinc-based electrodes in acidic environments.

Materials

Graphene oxide (4 mg/mL dispersion in water), hexagonal boron nitride (nanopowder, <150 nm), alginic acid sodium salt and sodium acetate (>99%) were obtained from Merck. Zinc foil (0.25mm thickness, 99.9 %) was obtained from Fisher Scientific.

Methodology

Configuration of the Electrochemical Cell

A three-electrode electrochemical cell was used, the experimental set up is shown in Figure 7. The working electrode was either zinc foil or a zinc-plated conducting substrate, where the conducting substrate was a titanium mesh or a form of carbon (e.g. graphite felt, porous carbon or graphite rod/foil). When a non-zinc working electrode was used, zinc was plated onto the working electrode using electrodeposition prior to use. The counter electrode was either a platinum mesh or a graphite rod. The reference electrode was Ag/AgCl.

Electrodeposition Method

Hydrogel electrodeposition was carried out by applying a constant potential of -0.5 V vs. Ag/AgCl for a duration of 0.5 to 5 min. Once the electrodeposition was completed, the zinc electrode with the hydrogel film was rinsed with ultrapure water. The hydrogel film was then peeled from the electrode for characterisation.

Thickness Measurements

The thicknesses of the synthesised hydrogel films were measured a digital micrometer (digital thickness gauge).

Raman Spectroscopy

Raman spectra were obtained using a Renishaw inVia microscope with a 532 nm excitation laser operated at a power of 0.274 mW with a grating of 1800 lines/mm and 50x objective.

Electrochemical Measurements

Electrochemical measurements were performed using an Autolab potentiostat (model PGSTAT302N, Metrohm Autolab, The Netherlands). Tensile Strength

Tensile tests were conducted using MultiTest-dV Motorised force tester using a 50N load cell. A strain rate of 10 mm/min was used for all samples.

Corrosion Tests

The corrosion behaviour of zinc foil, both bare and zinc alginate hydrogel containing different nanomaterials (boron nitride and graphene oxide), was investigated in a highly acidic solution (2M H2SO4) using linear polarization in a three-electrode setup. In this configuration, a working electrode with a working area of 2 cm² consisting of either bare zinc or zinc foil coated with nanomaterial containing alginate was employed, alongside a platinum mesh counter electrode and a reference electrode of (Ag/AgCI, 3M KCI). Potentiodynamic polarization measurements were performed to generate polarization curves (both anodic and cathodic) over a potential range of ±100 mV around the Open Circuit Potential (OCP). A low scan rate (1 mV/s) was selected to ensure accurate electrochemical kinetic data, taking advantage of the enhanced accessibility of the working electrode's surface to the electrolyte, thus maximizing the utilization of the electrode surface area [8].

References

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Claims

1. A method for producing a conducting hydrogel, the method comprising: providing a first electrode and a spaced apart second electrode, wherein the second electrode comprises a species which can be oxidised to provide a multivalent cation; at least partially disposing the first and second electrodes in an electrolyte, wherein the electrolyte comprises a solvent and a hydrophilic polymer; and applying a voltage across the first and second electrodes to thereby cause a multivalent cation to be generated at the second electrode and crosslinking to occur between the multivalent cation and hydrophilic polymer to thereby provide the conducting hydrogel.

2. The method of claim 1, wherein the hydrophilic polymer is an ionic polymer comprising at least one ionized and/or ionizable functional group.

3. The method of claim 2, wherein the ionic polymer is an anionic polymer or an ionizable polymer which is anionic in its ionized state.

4. The method of any preceding claim, wherein the hydrophilic polymer is or comprises a polysaccharide.

5. The method of claim 4, wherein the polysaccharide is selected from the group consisting of chitosan, an alginate, carrageenan, ulvan, starch, agarose, xanthan gum, gelatin or pectin.

6. The method of claim 5, wherein the polysaccharide is an alginate.

7. The method of any preceding claim, wherein the voltage is applied for between 10 seconds and 10 minutes, between 15 seconds and 8 minutes, between 30 seconds and 6 minutes, between 1 minute and 5 minutes, or between 2 and 4 minutes.

8. The method of any preceding claim, wherein the voltage applied across the first and second electrodes is between 1.0 and 2.3 V.

9. The method of any preceding claim, wherein the method further comprises providing a space apart reference electrode and applying a fixed voltage at the second electrode relative to the reference electrode.

10. The method of claim 9, wherein the voltage applied at the second electrode relative to the reference electrode is between -0.2 and -0.8 V, between -0.3 V and - 0.7 V or between -0.4 and -0.6 V.

11. The method of any preceding claim, wherein the second electrode comprises at least a layer of the species which can be oxidised to a multivalent cation.

12. The method of any preceding claim, wherein the species which can be oxidised to a multivalent cation is a metal.

13. The method of claim 12, wherein the metal is zinc, aluminium, calcium or magnesium and the multivalent cation is Zn²⁺, Al³⁺, Ca²⁺ or Mg²⁺.

14. The method of claim 13, wherein the metal is zinc and the multivalent cation is Zn²⁺.

15. The method of any preceding claim, wherein the electrolyte comprises the hydrophilic polymer at a concentration of between 0.01 and 25 wt%, between 0.1 and 10 wt%, between 0.2 and 5 wt%, between 0.3 and 1 wt%, or between 0.4 and 0.6 wt%.

16. The method of any preceding claim wherein the solvent is or comprises water.

17. The method of any preceding claim, wherein the electrolyte further comprises an electrolyte salt.

18. The method of any preceding claim, wherein the electrolyte further comprises one or more additives dissolved or suspended therein.

19. The method of claim 18, wherein the additive is a 2D material, optionally wherein the additive is graphene, graphene oxide (GO) or hexagonal boron nitride (hBN).

20. The method of claim 18 or 19, wherein the electrolyte comprises the additive at a concentration of between 0.1 and 10 mg/mL, between 0.5 and 8 mg/mL, between 0.6 and 5 mg/mL, between 0.7 and 5 mg/mL, between 0.8 and 4 mg/mL, between 0.9 and 3 mg/mL or between 1 and 2 mg/mL.

21. A conducting hydrogel comprising a hydrophilic polymer cross-linked by a multivalent cation, wherein the conducting hydrogel defines a layer with a thickness of less than 250 pm.

22. The conducting hydrogel of claim 21, obtained or obtainable by the method of any of claims 1 to 20.

23. The conducting hydrogel of claim 21 or 22, wherein the hydrogel has a conductivity of at least 1 mS/cm, at least 1.5 mS/cm, at least 2 mS/cm, at least 2.5 mS/cm, at least 3 mS/cm, at least 3.5 mS/cm, at least 4 mS/cm or at least 4.5 mS/cm.

24. Use of the hydrogel of any of claims 21 to 23 as an electrolyte.

25. An electrode comprising a layer of the hydrogel of any of claims 21 to 23.

26. A battery comprising the hydrogel of any of claims 21 to 23 and/or the electrode of claim 25.