Membranes, applications and methods of their manufacture

Abstract

The present invention relates to membranes, methods of their manufacture, and their use in a variety of electrochemical applications. In particular, the membranes may be proton selective membranes.

Description

[0001] MEMBRANES, APPLICATIONS AND METHODS OF THEIR MANUFACTURE

[0002] Field of Invention

[0003] The present invention relates to membranes, methods of their manufacture, and their use in a variety of electrochemical applications. In particular, the membranes may be proton selective membranes.

[0004] Background of the Invention

[0005] In recent years, there has been a growing demand for efficient, high-performance energy storage systems, particularly with the rapid development of electric vehicles (EVs), renewable energy integration, and portable electronic devices. Traditional battery technologies, such as lithium-ion and lead-acid batteries, have been widely utilized; however, there are still limitations in terms of energy density, cycle life, and safety. As a result, there has been a significant push toward the development of alternative energy storage technologies that can overcome these challenges and provide more efficient, cost-effective, and sustainable solutions.

[0006] One promising area of innovation is the development of proton-conductive membranes for use in nextgeneration battery technologies, particularly proton exchange membrane (PEM) batteries. These batteries, which rely on the selective transport of protons (H+) through a membrane to facilitate energy conversion, offer potential advantages over traditional battery chemistries, including high energy efficiency, rapid charge / discharge cycles, and reduced environmental impact.

[0007] A key component of PEM-based batteries is the proton-selective membrane, which serves as a separator between the anode and cathode while allowing for the selective transport of protons. The membrane's ability to selectively conduct protons while blocking electrons and other ions is crucial for maintaining the efficiency and performance of the battery. However, the development of such membranes has been hindered by several challenges, including issues with proton conductivity, membrane stability, and compatibility with other battery components.

[0008] Current proton-selective membranes often suffer from low conductivity at elevated temperatures or under varying operational conditions, leading to decreased overall battery performance. In addition, many existing materials lack long-term durability, particularly when exposed to the aggressive chemical environments found within batteries. As a result, there is a need for improved proton-selective membranes that offer higher conductivity, greater chemical stability, and enhanced mechanical properties, thereby enabling more efficient and reliable energy storage solutions.Without limitation, proton-selective membranes are useful in other environmental remediation and energy storage technologies such as: electrolysis for hydrogen generation; electrosynthesis for green agrochemical and fine chemical production; electro-oxidation for wastewater treatment and purification; electrochemical carbon capture and storage systems; and in environmental monitoring technologies such as pH sensors and electrochemical gas sensors.

[0009] Regardless of intended application, an effective proton-conducting membrane will ideally provide high conductivity and high selectivity. Unfortunately, strategies employed to increase conductivity - such as increasing pore size and decreasing membrane thickness - tend to also decrease selectivity. Similarly, strategies to increase selectivity such as pore functionalisation tend to also decrease conductivity.

[0010] Incorporation of ionic liquids has been proposed as an alternative strategy to tune membrane properties. Ionic liquids are salts that exist as liquids under ambient temperature and pressure conditions. They are typically made up of diffuse ionic species, most frequently large organic cations with smaller inorganic anions. Their high chemical and thermal stabilities and low volatility make them ideal candidate materials for energy generation and storage applications. Additionally, their physicochemical and electrochemical properties can be easily tuned by judicious selection of component ions. For example, charge transfer properties can be enhanced by using protic ionic liquids that enable proton conduction via Grotthuss-type mechanisms.

[0011] H H

[0012] I . I H

[0013] .O I H

[0014] t I H

[0015] H •’K /

[0016] xHXY

[0017] ni N- 'OHV k

[0018] I H

[0019]

[0020] H H

[0021] (a) water (b) ammonia / ammonium

[0022] O II

[0023] ,C. - O

[0024] HIXO O V - -CT H

[0025] xXT

[0026]

[0027] HAO H Ox

[0028] (c) formate / formic acid

[0029] Significant research effort has been put into trying to capitalize on the conductivity of these ionic liquids to improve the performance of ion-conducting membranes in fuel cells and batteries. However, this tends to produce brittle membranes that lack the mechanical robustness required for industrial applications.At the same time, it is known that the physical properties of ionic liquids can be tuned by adjusting the number and length of alkyl chains attached to a given ionic core. Structure-property relationships can be complex, due to the presence of both short- and long-range ordering in these materials, which can also change appreciably with temperature and surface / support interactions. However, it has been shown that increasing the degree of conformational flexibility associated with ionic liquid components tends to promote glass-forming behaviour, increasing ductility.

[0030] Unfortunately, however, increased alkylation also correlates with decreased conductivity, as it hinders both ion mobility and proton transfer. Prima facie, it appears difficult, if not impossible, to reconcile these two competing demands -- for high conductivity and mechanical plasticity - within a single ionic liquid.

[0031] It is an object of the invention to provide a proton selective membrane that provides useful conductivity and / or mechanical properties.

[0032] Alternatively, it is an object of the invention to at least provide the public with a useful choice.

[0033] Summary of the Invention

[0034] The present invention seeks to address these challenges by providing a novel (typically proton selective) membrane that advantageously enhances proton conductivity, improves chemical stability, and / or offers superior mechanical strength, making it suitable for use in a range of applications including advanced battery technologies.

[0035] In a first aspect the invention provides a membrane (which may be described as a proton selective membrane) including:

[0036] i) a porous solid support layer; and

[0037] ii) a hydrophobic matrix layer in contact with the porous solid support layer,

[0038] wherein the hydrophobic matrix layer includes a hydrophobic salt in a concentration of at least 10 wt%.

[0039] In the first aspect the hydrophobic matrix layer includes a hydrophobic salt in a concentration of at least 10 wt%. The hydrophobic matrix layer may include other components that are not the hydrophobic salt, such as components that are other products of formation of the hydrophobic salt or other additives. Examples of such other components include water, inorganic salt(s), non-ionic compounds,nanoparticles. The components of the hydrophobic matrix layer may be homogenous, or may exhibit some heterogeneity. As an example, where the hydrophobic matrix layer exhibits some heterogeneity, the hydrophobic salt may be uniformly distributed within one or more self-assembled domains, such as mono- or multilayer nanostructures, and the other component(s) of the matrix layer exist in separate domain(s). More typically the hydrophobic matrix layer will include at least 80 wt% of one or more hydrophobic salts. In some embodiments, the hydrophobic matrix layer will include substantially 100 wt% of one or more hydrophobic salts.

[0040] The hydrophobic salt will preferably be formed from:

[0041] • a cation selected from ammonium and phosphonium; and / or

[0042] • an anion selected from a carboxylate, sulfate, or phosphate.

[0043] An example of such an ammonium cation is N+R3R4R5R6and typically at least one of R3and R4and R5and R6is non-hydrogen. Preferably the ammonium cation is a quaternary ammonium cation.

[0044] An example of such a carboxylate anion is R^OCT wherein R1is a carbon containing moiety.

[0045] An example of such a sulfate is R1SOT wherein R1is a carbon containing moiety.

[0046] It has now been found that what is believed to be a densely packed hydrophobic matrix layer is able to selectively conduct protons. This is extremely surprising given the lack of any identifiable porous character in the hydrophobic matrix layer and the hydrophobic nature of the material. Not only have the proton selective membranes of the invention been shown to be conductive, but the selectivity and conductivity has been shown to be at least as good as if not better than the currently commercially viable Nation proton-transfer membrane which is composed of a perfluorosulfonic acid polymer (copolymer of polytetrafluoroethylene (PTFE) with persulfonic acid). Such Nation membranes are more expensive to form, and more persistent in the environment, than the membranes of the present invention. When introduced into batteries, the membranes of the present invention are believed to be capable of providing lower cost, higher capacity, longer lasting batteries to accelerate the renewable energy revolution, to enable storage of renewably produced power on an industrial / grid scale. In addition to underpinning the development of novel battery chemistries, the membranes of the invention can be used as low-cost, drop-in replacements for existing membranes in commercially available vanadium redox flow batteries, hence decreasing their cost and improving performance by minimizing / preventing electrolyte crossover.Without wishing to be bound by theory, it is believed that the hydrophobic salts of the present invention may be chemically stable and thermally stable because they typically share some properties with ionic liquids which are generally made up of diffuse ionic species, most frequently large organic cations with smaller inorganic anions. Such ionic liquids may exist as liquids under standard temperature and pressure (STP) conditions.

[0047] In a second aspect the invention provides a membrane (which may be described as a proton selective membrane) including:

[0048] i) a porous solid support layer; and

[0049] ii) a hydrophobic matrix layer in contact with the porous solid support layer,

[0050] wherein the hydrophobic matrix layer includes a hydrophobic salt including:

[0051] i) a quaternary ammonium cation having the formula N+R3R4R5R6wherein each of R3, R4, R5, and R6are independently selected from: optionally substituted alkyl; and optionally substituted alkenyl; and

[0052] ii) a carboxylate anion having the formula R1COO_or a sulfate anion having the formula R1SOT wherein R1is selected from: optionally substituted alkyl; and optionally substituted alkenyl.

[0053] Typically, R1will contain 4 or more, such as 6 or more, such as 8 or more, such as 10 or more, such as 12 or more carbon atoms. Typically R1will contain no more than 20 carbon atoms, such as no more than 18 carbon atoms, such as no more than 16 carbon atoms.

[0054] Typically at least one of R3, R4, R5, and R6will contain 2 or more, such as 3 or more, such as 4 or more, such as 6 or more, such as 8 or more, such as 10 or more, such as 12 or more carbon atoms. Typically at least one of R3, R4, R5, and R6will contain no more than 20 carbon atoms, such as no more than 18 carbon atoms, such as no more than 16 carbon atoms.

[0055] In a third aspect the invention provides a membrane (which may be described as a proton selective membrane) including:

[0056] i) a porous solid support layer; and

[0057] ii) a hydrophobic matrix layer in contact with the porous solid support layer,

[0058] wherein the membrane provides proton conductivity of at least 0.4 pmol / L / min.In a fourth aspect the invention provides a process for forming a membrane (which may be described as a proton selective membrane), the process including the steps of:

[0059] i) providing a hydrophobic salt;

[0060] ii) providing a porous solid support layer;

[0061] ii) applying the hydrophobic salt to the porous solid support layer so as to form a hydrophobic matrix layer that includes a hydrophobic salt in a concentration of at least 10 wt%.

[0062] In a fifth aspect the invention provides a process for forming a membrane (which may be described as a proton selective membrane), the process including the steps of:

[0063] i) providing a hydrophobic salt;

[0064] ii) providing a porous solid support layer;

[0065] ii) applying the hydrophobic salt to the porous solid support layer so as to form a hydrophobic matrix layer, wherein the hydrophobic salt includes:

[0066] i) a quaternary ammonium cation having the formula N+R3R4R5R6wherein each of R3, R4, R5, and R6are independently selected from: optionally substituted alkyl; and optionally substituted alkenyl; and

[0067] ii) a carboxylate anion having the formula R1COO_or a sulfate anion having the formula R1SO4-wherein R1is selected from: optionally substituted alkyl; and optionally substituted alkenyl.

[0068] In a sixth aspect the invention provides a an electrochemical cell including:

[0069] i) a first vessel containing a first protic solvent system;

[0070] ii) a second vessel containing a second protic solvent system;

[0071] iii) a membrane (which may be described as a proton selective membrane) separating the first protic solvent system from the second protic solvent system, wherein the membrane (which may be described as a proton selective membrane) is configured to allow the movement of protons between the first protic solvent system and the second protic solvent system.The first vessel will typically be provided with an electrode (such as an anode) in contact with the first electrolyte solution. The second vessel will typically be provided with an electrode (such as a cathode) in contact with the second electrolyte solution. When the electrochemical cell is connected to an electrical device (either separately, or together with one or more other electrochemical cells) the electrical device may be connected between the electrode in the first vessel and the electrode in the second vessel.

[0072] Further aspects of the invention, which should be considered in all its novel aspects, will become apparent to those skilled in the art upon reading of the following description which provides at least one example of a practical application of the invention.

[0073] Brief Description of the Drawings

[0074] Figure 1A shows small-angle X-ray scattering from DADMA stearate and Aliquat stearate membranes. Figure IB shows diagrams illustrating nanostructuring consistent with the observed scattering patterns.

[0075] Figure 2 shows snapshots from molecular dynamics trajectories of (a) DADMA stearate and (b) Aliquat stearate in contact with bulk water. The hydrophobic salt regions are apparent as the darker ordered / semi-ordered regions.

[0076] Figure 3 shows examples of cationic and anionic species which may be combined to form ionowaxes that comprise the functional layer of the membranes of the present invention.

[0077] Figure 4 shows a schematic diagram of experimental setups for bulk solution membrane permeability testing (left) and electrochemical impedance spectroscopy (right).

[0078] Figure 5 shows: (a) Representative data from conductivity testing, (b) Proton conduction rates (1 M formic acid to MilliQ solution), averaged over n = 4 (Fumasep, Nation, Aliquat stearate) or 5 (DADMA stearate) runs. Background rates of non-proton ion conduction (1 M ammonium formate to MilliQ) were negligible except across the Nation membrane (white bar). Error bars display the range of rates observed, (c) Representative pH monitoring data, (d) Proton conduction rates, with error bars encompassing the measured values.

[0079] Figure 6 shows electrochemical impedance spectra of DADMA stearate, Aliquat stearate, Fumasep and Nation membranes following soaking 1 M formic acid (20 minutes). Fitted models correspond to the equivalent circuits illustrated in Figure 7.Figure 7 shows equivalent circuit models used in fitting electrochemical impedance spectra for (a) Fumasep, (b) Nation and (c) hydrophobic salt membranes of the present invention (R = resistor, C = capacitor, CPE = constant-phase element, RG = Gerischer element, W = Warburg element, Ws= finite-length Warburg element) with a schematic diagram illustrating the proposed mechanism of proton conduction in the proton-selective membranes of the present invention and its mapping to the corresponding equivalent circuit model, whereby Rbuik accounts for bulk solution resistance, CPEdi is a constant-phase element which accounts for electrical double layer formation and Wsis a finite-length Warburg element which accounts for mass transport of protons across the bilayer, assuming a permeable boundary on each side.

[0080] Detailed Description of the Invention

[0081] As used herein the expression "proton selective membrane" refers to a membrane that selects for the movement of protons through the membrane and selects against the movement of non-proton species through the membrane. Ideally, such a proton selective membrane would allow little or preferably no movement of non-proton species through the membrane. The proton selective membrane would preferably provide little or preferably no inhibition of the movement of protons through the membrane. The proton selectivity of membranes is commonly quantified using the proton transference number, which is defined as the rate of proton conduction divided by the total rate of ion conduction.

[0082] Membranes with a proton conductance number in excess of 0.95 are commonly considered to be proton selective. In the context of the present invention, membranes will typically display proton conductance numbers > 0.95, preferably > 0.98.

[0083] Examples of non-proton species that may be selected against include electrolytes present in solution with the protons that are selected for. Examples of other electrolytes include the conjugate base of an acidic species used to supply the proton(s). By selecting for the movement of the protons, and against the movement of non-proton species, it is possible to create a potential difference between solutions on either side of the membrane, so that a current (the nominal movement of electrons through an external circuit) can be generated, and hence supply energy to a device powered by the potential difference generated.

[0084] One application of the proton selective membranes of the invention is for use in vanadium redox flow batteries where it is believed that the proton selective membranes of the invention will minimize / prevent electrolyte crossover. As used herein, crossover is a term of the art that refers to theundesired movement of species from one side of the membrane to the other. In the case of proton selective membranes, crossover will refer to the undesired movement of non-proton species from one side of the membrane to the other. Other possible applications for the proton selective membranes of the invention include their use in: aqueous organic redox-flow batteries; all-ionic liquid batteries; electrolysers for hydrogen generation and wastewater treatment; fuel cells; electrosynthesis systems; and electrochemical sensors.

[0085] As used herein, the term "hydrophobic" in the context of "hydrophobic matrix layer" refers to a matrix layer that has a strong tendency to repel or resist water. The layer may include one or more materials, such as the hydrophobic salt neat or together with one or more other components. In the event that the hydrophobic matrix layer includes the hydrophobic salt and one or more other components, such one or more other components may each independently be hydrophobic or not. For instance, the hydrophobic matrix layer may include an inorganic salt such as sodium chloride, and / or the hydrophobic matrix layer may include water. In such cases where at least one of the one or more other components are not hydrophobic it will nonetheless be understood that the hydrophobic matrix layer as a whole has a strong tendency to repel or resist water. Such a hydrophobic matrix layer will typically provide a contact angle that is greater than 90°, greater than 100°, greater than 110°, greater than 120°, greater than 130°, greater than 140°, greater than 150°, greater than 160°, or greater than 170°.

[0086] As used herein, the term "hydrophobic" in the context of "hydrophobic salt" refers to a salt (especially when provided neat) that has a strong tendency to repel or resist water. Such salts (when provided neat) will typically provide a contact angle that is greater than 90°, greater than 100°, greater than 110°, greater than 120°, greater than 130°, greater than 140°, greater than 150°, greater than 160°, or greater than 170°.

[0087] Static water contact angle is measured (sessile drop, 5 pL) on a goniometer after 60 s stabilization; values are reported as the mean of at least five drops on one or more (such as five) independently prepared membranes.

[0088] As used herein, the term "salt" takes its standard meaning and refers to a compound that is an assembly of positively charged ions (cations) and negatively charged ions (anions), resulting in a compound with no net electric charge.

[0089] In the context of the present invention, the "hydrophobic salt" will typically include an anion selected from a carboxylate, sulfate, sulfonate, or phosphate. While in its simplest form the carboxylate anioncould be formate (HCOO j, preferred examples of carboxylates for use as anions in the present invention will be substituted with a carbon containing moiety and may be represented as f COO-wherein R1is a carbon containing moiety (and hence not hydrogen). Further any such sulfate, sulfonate, or phosphate will typically be substituted with a carbon-containing moiety, rather than merely being selected from the following allowable, but less preferred forms being: unsubstituted sulfate (SO42); bisulfate (hydrogensulfate; HSO4 ); unsubstituted phosphate (PO43), hydrogenphosphate (HPO42), or dihydrogenphosphate (H2PO4 ). Preferably the anion is selected from a carboxylate or substituted sulfate. In other terms, anions of the formulae: R1S04-(alternatively depicted as (R^jSOa ; R1SO3_; R1PO42-(alternatively depicted as (R^jPOa ; and R1R2PO4-(alternatively depicted as (R1O)(R2O)PO2_) are preferred anions of the present invention, wherein R1and R2are each independently selected carbon containing moieties. The carbon containing moiety, where present, may be saturated, partially saturated, or aromatic, each of which may be optionally substituted. Preferably, the carbon containing moiety, where present, contains 4 or more, such as 6 or more, such as 8 or more, such as 10 or more, such as 12 or more carbon atoms, each of which may be optionally substituted. Preferably, the carbon containing moiety, where present, contains no more than 20 carbon atoms, such as no more than 18 carbon atoms, such as no more than 16 carbon atoms, each of which may be optionally substituted. Preferably the carbon containing moiety is selected from: optionally substituted alkyl; optionally substituted alkenyl; and optionally substituted aryl. Such carbon containing moieties are especially preferred when they are provided as substituents on carboxylate, although this should not be seen as limiting. Examples of preferred carbon-containing anions include: octanoate, butyrate, laurate (dodecanoate), palmitate (CH3(CH2)I3CH2COO ), stearate (CHa Cl^isCI- COO ), oleate (cis;

[0090] CH3(CH2)7CH=CH(CH2)7COO ), elaidate (trans; CH3(CH2)7CH=CH(CH2)7COO ), linoleate (CH3(CH2)4CH=CH(CH2)CH=CH(CH2)7COO ), dodecyl sulfate (CH3(CH2)I0CH2O-SO3), trans-cinnamate (PhCH=CHCOO ), hydro-cinnamate (PhCH2CH2COO ), 2-amino-3-phenylpropionate (PhCH2(CHNH2)COO ), benzoate (PhCOO ), and dodecylsulfate, p-toluenesulfonate, dodecylbenzenesulfonate, tridecylphosphate, dimethylphosphate. Preferably the carboxylate is stearate.

[0091] In the context of the present invention, the "hydrophobic salt" will typically include an ammonium cation (N+R3R4R5R6) or a phosphonium cation (P+R3R4R5R6). Preferably the cation will be an ammonium cation (N+R3R4R5R6). Where present, such ammonium cation will typically be a substituted ammonium cation, such that at least one of R3and R4and R5and R6is non-hydrogen. Such a substituted ammonium cation may be a substituted primary (N+R3H3), secondary (N+R3R4H2), tertiary (N+R3R4R5H), or quaternary (N+R3R4R5R6) ammonium cation. Examples of primary, secondary, and tertiary ammonium cationsinclude: decylammonium, dodecylammonium, hexadecylammonium, didecylammonium, triethanolammonium, triethylammonium, and 1,12-diammoniumdodecane. The last example 1,12-diammoniumdodecane will be understood to be an ammonium cation which contains a further ammonium group. Such ammonium cations, having a further charged group (such as a further ammonium group or a phosphonium group) are contemplated for all ammonium cations (N+R3R4R5R6) or phosphonium cations (P+R3R4R5R6) according to the present invention. Preferably the cation will be a quaternary ammonium cation. The R3and R4and R5and R6groups may be each independently selected carbon containing moieties. The carbon containing moiety, where present, may be saturated, partially saturated, or aromatic. That said, it is believed that aromatic substituents are less preferred. Preferably, at least one of the carbon containing moieties, where present, contains 2 or more, such as contains 3 or more, such as contains 4 or more, such as 6 or more, such as 8 or more, such as 10 or more, such as 12 or more carbon atoms. Generally at least one of the carbon containing moieties, where present will contain no more than 20 carbon atom, such as no more than 18 carbon atoms, such as no more than 16 carbon atoms. Preferably the carbon containing moiety is selected from: optionally substituted alkyl; optionally substituted alkenyl.

[0092] Especially preferred carbon containing moieties that may be independent selected for use in each of R3, R4, R5, and R6include optionally substituted Ci.2oalkyl (such as Ci-isalkyl, such as Ci-igal kyl ) and optionally substituted Ci.2oalkenyl (such as Ci-ioalkenyl, such as Ci^alkenyl). Still further preferred examples are optionally substituted: methyl, butyl, allyl, octyl, decyl, hexadecyl.

[0093] More particular examples of such cations for use in the present invention include: diallyldimethylammonium (DADMA); the cation of Aliquat 336 (a quaternary ammonium salt containing a methyl substituent and a mixture of C8 (octyl) and CIO (decyl) alkyl substituents (predominating in C8); available as the chloride salt form known as Stark's Catalyst); the cation of cetyltrimethylammonium (CTA; providing a cation having C16 (hexadecyl) and three methyl substituents; available from the bromide salt CTAB); didodecyldimethylammonium (DDDMA); and tetrabutylammonium (TBA).

[0094] Preferably the ammonium cation is DADMA or Aliquat 336.

[0095] As used herein, "alkyl" typically refers to a saturated hydrocarbon substituent derived from an alkane, being a group of carbon and hydrogen atoms that can be straight, branched, or cyclic. Examples of alkyl include: methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl. The alkyl group(s) may be optionally substituted.As used herein, "alkenyl" typically refers to an unsaturated hydrocarbon substituent derived from an alkene, being a group of carbon and hydrogen atoms that can be straight, branched, or cyclic with one or more double carbon-carbon bonds. The or each double bond may be independent selected from the cis or trans configuration. Examples of alkenyl include: ethenyl, propenyl (such as allyl), butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl, octadecenyl, nonadecenyl. The alkenyl group(s) may be optionally substituted.

[0096] As used herein, "aryl" typically refers to an unsaturated mono- or poly-cyclic system having at least some degree of aromaticity. Typically the aryl group will have from 6 to 14 contiguous carbon atoms. Examples of aryl groups are phenyl, and naphthyl. The aryl group(s) may be optionally substituted.

[0097] As used herein, "optionally substituted" means substituted with one or more substituents selected from alkyl, alkenyl, alkynyl, halo, hydroxy, alkoxy, amino (or protonated amino, namely ammonium), nitro, cyano, carboxyl, sulfonyl, phosphonyl, silyl, siloxy, epoxy, azido, aryl, heteroaryl, heterocyclyl, fluoroalkyl, and polymer- or surface-reactive groups including (meth)acrylates, vinyl, maleimide, trialkoxysilyl, and thiol. Unless otherwise indicated, the following optional substituents are examples of those contemplated:

[0098] i) alkyl, alkenyl, alkynyl and alkoxy substituents each independently selected to contain 1-20 carbon atoms (preferably 1-12, more preferably 1-8);

[0099] ii) fluoroalkyl substituents containing 1-12 carbon atoms (preferably 1-8);

[0100] iii) aryl substituents containing 6-20 carbon atoms (preferably 6-12). For instance, where R3is a methyl group substituted with a phenyl group, the resultant moiety may be described as a benzyl group;

[0101] iv) heteroaryl substituents containing 4-20 carbon atoms (preferably 5-12);

[0102] v) heterocyclyl substituents containing 4-20 carbon atoms (preferably 5-12); and

[0103] vi) silyl or siloxy substituents comprising -Ce alkyl groups attached to silicon.

[0104] Where a moiety may include a plurality of optional substituents, each optional substituent is independently selected from those described above.The carboxylate anion RiCOO-may be a residue of an amino acid, whether naturally occurring or non-naturally occurring, in its deprotonated carboxylate form. In many cases, such amino acids may fit within the definition of RiCOO-wherein R1 is a substituted alkyl group (such as Ci), wherein the alkyl group (such as Ci) is substituted with: a) an amine group; and b) the relevant side chain of the amino acid - such as a benzyl group for phenylalanine, or a methyl group for alanine. Suitable amino acids include, but are not limited to, glycine, alanine, valine, leucine, isoleucine, serine, threonine, tyrosine, phenylalanine, tryptophan, lysine, arginine, histidine, aspartic acid, glutamic acid, cysteine, methionine, proline and hydroxyproline. In one embodiment, the carboxylate is the carboxylate of phenylalanine (i.e. phenylalaninate, otherwise known as 2-amino-3-phenylpropanoate). Without wishing to be bound by theory is it believed that where the carboxylate anion RiCOO-is a residue of an amino acid, then the use of a relatively "bulky" and / or non-polar amino acid (such as phenylalanine, tyrosine, or tryptophan) is preferred.

[0105] Preferred hydrophobic salts for use in the hydrophobic matrix layer of the present invention include: diallyldimethylammonium (DADMA) stearate; DADMA oleate; DADMA palmitate; DADMA linoleate; DADMA p-toluenesulfonate; DADMA dodecylsulfate; DADMA dodecylsulfonate; DADMA transcinnamate; DADMA hydrocinnamate; DADMA tridecylphosphate; Aliquat 336 (A336) stearate; Aliquat 336 oleate; Aliquat 336 palmitate; Aliquat 336 linoleate; Aliquat 336 myristate; Aliquat 336 dodecylsulfate; Aliquat 336 dodecylsulfonate; Aliquat 336 trans-cinnamate; Aliquat 336 hydrocinnamate; Aliquat 336 p-toluenesulfonate; Aliquat 336 tridecylphosphate; benzyltrimethylammonium (BzTMA) stearate; BzTMA oleate; BzTMA myristate; BzTMA palmitate;

[0106] BzTMA linoleate; BzTMA p-toluenesulfonate; BzTMA dodecylsulfate; BzTMA dodecylsulfonate; BzTMA trans-cinnamate; BzTMA hydrocinnamate; BzTMA tridecylphosphate; cetyltrimethylammonium (CTA) stearate; CTA oleate; CTA myristate; CTA palmitate; CTA linoleate; CTA p-toluenesulfonate; CTA dodecylsulfate; CTA dodecylsulfonate; CTA trans-cinnamate; CTA hydrocinnamate; CTA tridecylphosphate; cetrimide stearate; cetrimide oleate; cetrimide myristate; cetrimide palmitate; cetrimide linoleate; cetrimide p-toluenesulfonate; cetrimide dodecylsulfate; cetrimide dodecylsulfonate; cetrimide trans-cinnamate; cetrimide hydrocinnamate; cetrimide tridecylphosphate; didodecyldimethylammonium oleate; tetrabutylammonium (TBA) palmitate; TBA stearate; TBA oleate; cetyltrimethylammonium dodecylsulfate. More preferred hydrophobic salts are: DADMA stearate, DADMA palmitate, DADMA trans-cinnamate, A336 stearate, A336 palmitate, A336 trans-cinnamate, A336 oleate, A336 dodecylsulfate. Most preferred hydrophobic salts are DADMA stearate and Aliquat 336 stearate. Most preferred is DADMA stearate. Other examples of hydrophobic salts for use in thehydrophobic matrix layer of the present invention include: decylammonium octanoate, dodecylammonium and hexadecylammonium butyrate, octanoate and laurate, didecylammonium octanoate and laurate, triethanolammonium laurate, triethylammonium laurate, and 1,12-diammoniumdodecane octanoate and oleate.

[0107] As used herein, the compounds, moieties, and optional substituents of the invention include all stereochemical forms thereof. Accordingly, the invention encompasses any and all stereoisomers, including enantiomers, and diastereomers and mixtures thereof, such as racemic and scalemic mixtures. Where a compound or moiety may exist in multiple stereochemical configurations due to the presence of one or more stereocentres, double bonds, or conformational constraints, all such configurations, whether isolated or in any combination, are contemplated as falling within the scope of the invention.

[0108] It is known that increasing the degree of conformational flexibility associated with ionic liquid components tends to promote glass forming behaviour, increasing ductility. And yet, increasing conformational flexibility is compromised by it being known that increased alkylation of ionic liquids also correlates with decreased conductivity, as it hinders both ion mobility and proton transfer.

[0109] Advantageously, the present invention realises that the properties of the hydrophobic salt may be tuned by judicious choice of anion and / or cation, in order to generate a membrane that provides both suitable mechanical properties and conductivity. The present inventors have identified that it may be possible to exploit the phenomenon of proton conductivity through lipid bilayer membranes by choosing ionic liquid components that chemically and structurally resemble compounds known to spontaneously selfassembly into nanolayered structures, such as lipid bilayers.

[0110] The hydrophobic salt of the present invention may be present as the only hydrophobic salt, or may be provided with one or more other compounds in the hydrophobic matrix layer, such as one or more other hydrophobic salt(s).

[0111] Without wishing to be bound by theory, it is believed that the ability to combine two or more hydrophobic salts may provide the user with the ability to have greater flexibility over the proton selectivity, conductivity, and / or mechanical properties of the hydrophobic matrix layer. For instance:

[0112] DADMA stearate has been found to provide relatively higher conductivity but relatively lower mechanical stability compared to Aliquat 336 stearate which has been found to provide relatively lower conductivity and relatively higher mechanical stability. These two hydrophobic salts were combined in various ratios (eg 3:1; 1:1; and 1:3) and were shown to providemembranes with suitable ease and possessing similar proton conductivity to Aliquat 336 stearate.

[0113] • DADMA stearate has been found to provide useful conductivity and ease of production compared to DADMA palmitate which has been found to be more difficult to work with given its tendency to be brittle. These two hydrophobic salts were combined in a 1:1 ratio and that combination was shown to be mechanically robust and display higher conductivity than either hydrophobic salt by itself.

[0114] Likewise, without wishing to be bound by theory, it is believed that the ability to combine a hydrophobic salt with a compound that is not a hydrophobic salt (such as a non-ionic species) may provide the user with the ability to have greater control over the proton selectivity, conductivity, and / or mechanical properties of the hydrophobic matrix layer. For instance DADMA stearate was shown to tolerate up to about 20 wt% inclusion of non-ionic wax without loss of function. This result signals that it may be possible to tune the conductivity and / or mechanical properties and / or cost of a membrane using a non-ionic wax.

[0115] Additionally, without being bound by theory, it is believed that the incorporation of materials that allow for electronic conductivity through the membrane (such as percolating networks of conductive nanoparticles) may decrease the overall electrical resistivity of the membrane. Conductive nanoparticles (e.g., carbon black, carbon nanotubes, graphene, metallic powders) at, for example, 0.1-15 wt% (such as 10-15 wt%) can reduce through-plane resistivity without materially affecting proton selectivity. For example, incorporation of up to 15 wt % carbon black nanoparticles into an Aliquat 336 membrane was shown to decrease the bulk electrical resistivity of the membrane by up to an order of magnitude.

[0116] As indicated above, in some embodiments the hydrophobic matrix layer may include one or more other components that are not the hydrophobic salt. Examples of such one or more other components are water, inorganic salt(s), non-ionic waxes, and nanoparticles. Preferably the hydrophobic matrix layer includes at least 10 wt%, such as at least 20 wt%, such as at least 30 wt%, such as at least 40 wt%, such as at least 50 wt%, such as at least 60 wt%, such as at least 70 wt%, such as at least 80 wt%, such as at least 90 wt%, such as substantially 100 wt% (such as 100%) of one or more hydrophobic salts. Preferably the hydrophobic matrix layer includes at least 10 wt%, such as at least 20 wt%, such as at least 30 wt%, such as at least 40 wt%, such as at least 50 wt%, such as at least 60 wt%, such as at least 70 wt%, such as at least 80 wt%, such as at least 90 wt%, such as substantially 100 wt% (such as 100%) of any one hydrophobic salt. Preferably the hydrophobic matrix layer includes at least 10 wt%, such as at least 20wt%, such as at least 30 wt%, such as at least 40 wt%, such as at least 50 wt%, such as at least 60 wt%, such as at least 70 wt%, such as at least 80 wt%, such as at least 90 wt%, such as substantially 100 wt% (such as 100%) of a single hydrophobic salt.

[0117] The hydrophobic salts of the present invention may be formed by any conventional means, including: ion-exchange; and acid / base reaction. By way of example only, a general method for forming a hydrophobic salt of the present invention is as follows: an ammonium halide is combined with an equimolar amount of an alkali metal carboxylate and mixed thoroughly. The mixture is then washed with a volume of water to remove alkali metal halide, leaving an insoluble hydrophobic salt provided as the ammonium carboxylate which is partitioned from the aqueous solution of the alkali metal halide.

[0118] The proton selective membranes of the present invention may be formed by a range of methods, including roll coating, hot pressing, hot dipping, or via solvent / gel dispersal, followed by immersion in an aqueous environment.

[0119] The hydrophobic matrix layer may be applied by melt processing at, for example, 40-90 °C and 30-50 PSI (250-350 kPa) via parallel-plate hot press to a porous support (e.g., microporous polypropylene or polyethylene; thickness such as 10-50 pm; porosity such as 30-60%). The area loading of the hydrophobic salt may be 0.5-10 mg-crrr2, preferably 1-5 mg-crrr2.

[0120] Advantageously the membranes of the present invention are reusable - namely they can be dried out and rewet without loss of function. This ability is advantageous over phospholipid bilayers which may operate by similar principles but do not provide that ability.

[0121] Without wishing to be bound by theory, it is believed that the hydrophobic salts of the present invention form layered nanostructures. It is also believed that the hydrophobic salts may self-assemble into ordered structures, such as bi- or multi-layer membranes. Again, without wishing to be bound by theory it is believed that the ability of the hydrophobic salts to self-assemble may aid in the membrane formed using the hydrophobic salts having a substantially non-porous character, necessitating proton transport via proton tunnelling or Grotthuss conduction rather than bulk solution transport or diffusion. As these mechanisms are not available to other ions, this promotes proton selectivity.

[0122] In support of the belief that the hydrophobic salts of the present invention may form layered nanostructures, small-angle X-ray scattering patterns of example hydrophobic salts (DADMA stearate and aliquat stearate) deposited on Celgard membranes in aqueous solution for at least 1 hour revealedthe presence of horizontally oriented layers and randomly-oriented lamellae (Figure 1), providing evidence for self-assembly into functional nanostructures.

[0123] Small-angle X-ray scattering (SAXS) on hydrated membranes yields lamellar reflections consistent with layered nanostructures. In certain embodiments, the primary lamellar d-spacing is 2-10 nm, preferably 3-7 nm, with at least one higher-order reflection observable at twice the q ratio of the primary scattering band, to within 5 % error tolerance. Higher order reflections at higher integer q ratios may also be present to within the same error tolerance. Reflections may be isotropic or anisotropic.

[0124] In further support of the belief that the hydrophobic salts of the present invention may form layered nanostructures, molecular dynamic simulations of 2-dimensional sheets of example hydrophobic salts (DADMA stearate and Aliquat stearate) in bulk water from two different starting configurations (preorganised into a bilayer membrane and randomly packed), performed under standard thermodynamic conditions, showed that the hydrophobic salt DADMA stearate in particular resolved to tightly packed bilayers from any simulation starting point used (Figure 2(a)). On the other hand Aliquat stearate was disordered but still displayed layered regions (Figure 2(b)) with interspersed clusters of water molecules. Preferably the hydrophobic salts of the present invention are provided as oriented layered nanostructures, such as bilayer membranes.

[0125] United-atom molecular dynamics simulations using the GROMOS-54A7 force field were performed according to the following protocol: NVT equilibration to 300 K (1 ps), NPT equilibration at 300 K to a pressure of 100 atm (10 ps), repeating sequential NVT (9 ps) + NPT (1 ps) simulations until the structure of the system did not change upon addition cycling. Each simulation cell contained 200 copies of each ionowax component covering the x-y plane, with a 20 nm thick layer of water molecules in the z direction. Periodic boundary conditions were applied throughout.

[0126] Without wishing to be bound by conjecture, it is believed that the ability of the membranes of the present invention to conduct protons selectively may be explained by a Grotthus-like mechanism of transfer along water wires / webs that are believed to form transiently through lipid bilayer membranes. Alternatively, it is hypothesised that proton tunnelling could occur between clusters of water encapsulated within randomly-oriented lamellar nanostructures.

[0127] As used herein the "porous solid support layer" refers primarily to a mechanically supportive contiguous material (as opposed to discrete support particles for example as might be found in polystyrene resin used in chromatography) in contact with the hydrophobic matrix layer and capable of supporting thehydrophobic matrix layer so that the composite of the support layer and hydrophobic matrix layer can be used, for example, as a membrane. Such a contiguous material may be provided as a sheet of material of minimal thickness - such as less than 10 mm, such as less than 5 mm, such as less than 1 mm, such as less than 0.5 mm, such as less than 100 pm, such as less than 50 pm. Typically the contiguous material will be provided as a sheet of material with a thickness of at least 5 pm, such as at least 10 pm, such as at least 14 pm. Membrane thickness may be measured at one or more locations (and averaged where multiple location thicknesses are measured) and may be measured with a resolution of ± 0.5 pm, such as ± 0.2 pm, such as ± 0.1 pm. Examples of the types of materials that may be used as the porous solid support layer include: fibreglass or cellulose filter paper, macroporous woven polypropylene, microporous lined polypropylene, microporous polyethylene, microporous polyethylene terephthalate." Combinations of these materials may also be contemplated. It will be appreciated that these materials contain pores that would themselves provide (little or) no proton selectivity. As such, by providing the hydrophobic matrix layer in contact with the porous support layer, the hydrophobic matrix substantially covers the porous support layer, and fills the pores of the porous support layer, thus forming a membrane. In so doing, the hydrophobic matrix layer is provided as a barrier, to some extent, to the movement of all species from one side of the membrane to the other side of the membrane.

[0128] As used herein a "protic solvent system" refers to a solvent system that comprises at least one protic molecular solvent capable of donating a hydrogen bond and / or bearing a labile proton (e.g., -OH, -NH, -COOH). Where an embodiment of the invention includes a plurality of protic solvent systems (such as a first protic solvent system and a second protic solvent system), the constitution of each such protic solvent system is independently chosen. The protic molecular solvent in a protic solvent system may be water or an organic protic solvent, or a mixture thereof. The protic solvent system may further include one or more cosolvents, additives, salts, buffers, or stabilizers.

[0129] Suitable protic molecular solvents include, without limitation: water; lower alcohols (e.g., methanol, ethanol, n-propanol, isopropanol, n-butanol, tert-butanol); polyols (e.g., ethylene glycol, propylene glycol, glycerol); fluorinated alcohols (e.g., trifluoroethanol, hexafluoroisopropanol); carboxylic acids (e.g., formic acid, acetic acid, lactic acid); amines and ammonium protic media (e.g., ammonia, ammonium hydroxide, alkylammonium solvents); and protic ionic liquids (e.g., ethylammonium nitrate, diethylammonium formate).In certain embodiments, the system comprises at least 10 wt% of protic solvent(s), preferably >30 wt%, more preferably >50 wt%, based on the total weight of the solvent system. In some embodiments, aprotic cosolvents (e.g., acetonitrile, acetone, DMSO, DMF) may be present in minor amounts (e.g., <40 wt%, preferably <20 wt%) provided the overall system remains substantially protic in character.

[0130] Typically the "protic solvent system" is a "protic electrolyte system" which herein refers to a protic solvent system that comprises one or more electrolytes (acids, bases, or salts) in an amount effective to provide ionic conductivity. Where an embodiment of the invention includes a plurality of protic electrolyte systems (such as a first protic electrolyte system and a second protic electrolyte system), the constitution of each such protic electrolyte system is independently chosen. The electrolyte may be inorganic or organic, and may include monovalent, divalent, or multivalent cations and / or anions.

[0131] Suitable electrolytes include, without limitation: alkali or alkaline-earth salts (e.g., Li+, Na+, K+, Mg2+, Ca2+salts of chloride, bromide, nitrate, acetate, phosphate, sulfate, hydrogensulfate, carbonate); ammonium and organic onium salts (e.g., tetramethylammonium, tetraethylammonium, choline salts); buffers (e.g., phosphate, acetate, citrate, borate); acids (e.g., HCI, HNO3, H2SO4, H3PO4, methanesulfonic acid, p-toluenesulfonic acid); and bases (e.g., NaOH, KOH, NH40H, organic amines).

[0132] In certain embodiments, the electrolyte concentration is 0.001-5.0 M, preferably 0.01-2.0 M, more preferably 0.05-1.5 M, still more preferably 0.75-1.2 M, such as about 1 M; the system exhibits a bulk conductivity of >0.1 mS-crrr1, preferably >1 mS-crrr1at 20-25 °C; and the pH may typically range from 0-7, as appropriate for the application.

[0133] Preferably the "protic solvent system" is an "aqueous electrolyte system" which herein refers to a protic electrolyte system in which water is the primary component of the continuous phase and constitutes >50 wt% of the solvent system, preferably >70 wt%, more preferably >85 wt%. Where an embodiment of the invention includes a plurality of aqueous electrolyte systems (such as a first aqueous electrolyte system and a second aqueous electrolyte system), the constitution of each such aqueous electrolyte system is independently chosen. The system further comprises one or more electrolytes as described herein to provide ionic conductivity.

[0134] Suitable aqueous electrolytes include, without limitation: alkali metal chlorides, bromides, iodides, nitrates, acetates, phosphates, carbonates, sulfates and hydrogensulfates; ammonium and organic ammonium salts (e.g., ammonium acetate, choline chloride); and acidic or basic media (e.g., HCI, H2SO4, H3PO4, NaOH, KOH). In certain embodiments, the electrolyte comprises carboxylate or sulfatecounter-ions to match the anionic species of the complexes described herein (e.g., acetate, lactate, phenylalaninate, sulfate, hydrogensulfate).

[0135] In redox-flow battery applications (including vanadium systems), the first and / or second protic solvent system is independently selected from an aqueous electrolyte system wherein water constitutes > 60 wt% and the electrolyte concentration is 0.5-2.0 M.

[0136] Unless otherwise stated, measurements are performed at 22 ± 5 °C and 1 atm.

[0137] Examples

[0138] One or more embodiments of the invention will be described below by way of example only, and without intending to be limiting.

[0139] Hydrophobic salt synthesis & membrane fabrication

[0140] Hydrophobic salts of the present invention were synthesized by ion metathesis, as detailed below. All products were obtained in quantitative yields. Membranes were fabricated by pre-heating each hydrophobic salt to 50 °C on a hotplate for two minutes, which was then contact rolled onto a porous solid support material between two silicon sheets. Membranes were left to cure in a fume hood for 1 -24 hours; until the membrane could be peeled off cleanly. Membranes were incubated in an aqueous environment for at least one hour before salt and proton conductivity measurements were taken. Examples of hydrophobic salt components are illustrated in Figure 3.

[0141] Diallyldimethylammonium (PADMA) stearate : NaCI : H2O (1:1:62.5):

[0142] 65% aqueous DADMA chloride solution (6.436 g) was combined with an equimolar quantity of solid sodium stearate (7.943 g), producing a clumpy, grainy hydrated powder. The sample was brought to 80 °C in a water bath, and Milli-Q water was added until it formed a smooth, glossy white wax (27 mL).

[0143] The DADMA stearate was then used to form a membrane according to the general procedure described above.

[0144] When set, membranes were dry, semi-translucent and white in colour. In thin layers (< 0.5 mm), membranes retained a paper-like texture with some flexibility. In thicker layers (> 0.5 mm), membranes were brittle, and cracking could occur. When hydrated, the membrane gained increased plasticity during and following use.DADMA palmitate : KCI : H2O (1:1:0.54):

[0145] 65% aqueous DADMA chloride solution (6.259 g) was combined with an equimolar quantity of solid potassium palmitate (8.007 g), producing an heterogenous viscous mixture, comprising a soft grey wax interspersed with a small volume of translucent yellow gel.

[0146] The soft grey DADMA palmitate wax was then used to form a membrane according to the general procedure described above.

[0147] When set, membranes formed a highly brittle, white material. When hydrated, the membrane appeared to disperse into the solution suggesting sub-optimal structural integrity.

[0148] Aliquat 336 stearate:

[0149] Aliquat 336 solution (12.203 g) was combined with an equimolar quantity of solid sodium stearate (9.248 g) under heating (50 °C) and sonication (30 kHz) for 2 hours. After 16 hours at room temperature, the resulting sample set to form a grainy, off-white wax, which was washed with MilliQ. and returned to a 90 °C water bath for 2 hours. This resulted in the separation of the remaining NaCI / H2O which was decanted, leaving a homogenous solid wax that had translucent off-white colour and was very firm.

[0150] The Aliquat 336 stearate was then used to form a membrane according to the general procedure described above.

[0151] When cured, the membrane was a translucent, firm wax that softened slightly when in use, though its structural integrity was retained.

[0152] Aliquat 336 palmitate:

[0153] Aliquat 336 solution (9.509 g) was combined with an equimolar quantity of solid potassium palmitate (6.929 g) under heating (50 °C) and sonication (30 kHz) for 2 hours. After 16 hours at room temperature, the product was isolated as a pale brown viscous solution which formed on top of a solid KCI layer. Over approximately 1 week, the top product fraction solidified to smooth, firm wax.

[0154] The Aliquat 336 palmitate was then used to form a membrane according to the general procedure described above.Upon curing, membranes were smooth, pale brown and slightly sticky. Aliquat 336 palmitate membranes retained a high level of level of plasticity / f lexibility, though softened further when in use.

[0155] Starting materials were sourced from a variety of suppliers:

[0156] Substance CAS Supplier Diallyldimethylammonium chloride 65 % 7398-69-8 Aldrich

[0157] aqueous solution

[0158] Aliquat 336 63393-96-4 Sigma Aldrich

[0159] Sodium chloride 7647-14-5 ECP Limited

[0160] Sodium stearate 822-16-2 Riedel-de-Haen

[0161] Potassium palmitate 2624-31-9 Hopkin & Williams Ammonium formate 540-69-2 AK Scientific

[0162] Formic acid 64-18-6 VWR Chemicals

[0163]

[0164] Ion-conducting properties of each membrane were characterized by bulk solution testing and electrochemical impedance spectroscopy, using experimental apparatus as illustrated in Figure 4.

[0165] Membrane permeability testing

[0166] During bulk solution testing, 1 M solutions of ammonium formate or formic acid were placed in a membrane-capped glass cylinder, which was then immersed in a beaker of MilliQ water. Following a pre-rinse to remove any excess surface salt, changes in conductivity and / or pH of the MilliQ were recorded every 5 minutes for 1.5 hours. As a negative control, MilliQ was used in place of the salt or acid solution.

[0167] Without wishing to be bound by theory, ammonium formate / formic acid were chosen as a challenging test system to probe the limitations of the hydrophobic matrix layer of the invention in part because it was considered that: a) since the system contains similar functional groups to the tested hydrophobic salt (ammonium, carboxylate) that they could mix with the salt and disrupt its ability to selectively conduct protons; and b) such ammonium formate / formic acid was subject to the same Grotthussconduction type mechanisms as hydronium ions. Despite this challenging set of test conditions, the results herein demonstrate the remarkable proton selective properties of the membrane.

[0168] Representative ion permeability kinetics data is illustrated in Figure 5 for a representative selection of proton selective membranes of the present invention and commercial functionalized nanoporous polymer membranes. A more complete set of ionic and proton conduction rate constants for a wider range of potential embodiments of the present invention are reported in Table 1.

[0169] Table 1. Kinetic parameters characterizing ion and proton conductivity across tested membranes. Where error bars are reported, these represent the range of observed values across at least four replicates.

[0170] Ionic Conduction Proton Conduction Membrane kion (mS cm1min1) / fprot (mM min_1) Fumasep 375 0.37 ± 0.04 1.2 ± 0.3 Nation 117 0.27 ± 0.03 0.9 ± 0.2 Diallyldimethylammonium stearate 0.32 ± 0.05 0.9 ± 0.1 Diallyldimethylammonium dodecylsulfate 0.36 0.8 Diallyldimethylammonium trans-cinnamate 0.34 1.9 Aliquat 336 stearate 0.22 ± 0.05 0.7 ± 0.1 Aliquat 336 palmitate 0.23 ± 0.04 0.6 ± 0.2 Aliquat 336 dodecylsulfate 0.28 0.5 Aliquat 336 trans-cinnamate 0.14 0.4 Didodecyldimethylammonium oleate 0.06 Not determined Tetrabutylammonium stearate 0.08 Not determined

[0171]

[0172] The DADMA-based membranes of the present invention perform comparably to commercial Fumasep 375 membranes in terms of proton selectivity and conductivity, while aliquat-based membranes display lower proton conductivities. The commercial Nation 117 membrane is not particularly selective for protons under our experimental conditions, which is surprising as it is widely considered the gold standard in proton exchange membranes for fuel cell and electrolysis applications. It is alsocounterintuitive that the Fumasep membrane does appear to conduct anions, despite its primary application as an ion-conducting membrane. However, past studies have shown that anion conductivity across Fumasep membranes is highly dependent on the nature of the anion.

[0173] In addition to the above test system, in parallel a CuCL (aq) electrolyte system was also investigated. It was considered that the inventors would be able to see whether there was bulk water transport across the membrane by looking for a change in colour on the other side of the membrane from the CuCL (aq) electrolyte solution. Conductivity was also monitored, but there was no change in the colour of the solution on the other side of the membrane. The inventors did, however, observe the membrane being stained blue which is consistent with copper ions adsorbing to the surface of the membrane, which further confirms the working hypothesis of the hydrophobic salt having layered nanostructuring, and charged head groups oriented towards the interface with the water.

[0174] The inventors also consider that CuCL (aq) could offer a low cost, easily visualisable model system for testing the ability of the proton selective membranes of the invention to prevent cross-over in aqueous metal-ion-based redox flow batteries (e.g. vanadium redox-flow batteries).

[0175] Electrochemical impedance spectroscopy

[0176] Electrical impedance spectroscopy may provide additional insight into proton conduction mechanisms, as it characterizes timescale-dependent responses to an applied alternating current at different frequencies, which may be fitted to an equivalent circuit model designed to characterize the main iontransfer pathways present in the system.

[0177] Electrochemical impedance spectroscopy was performed using a PalmSens EmStat4S potentiostat, scanning over an alternating current (AC) frequency range of 1 - 100,000 Hz, recording 10 data points per order of magnitude change, linearly spaced on a logarithmic scale. Membrane samples were prepared by first soaking the membrane in MilliQ water for 10 minutes, followed by 20 minutes in 1 M formic acid. For the negative control, membrane samples were soaked in MilliQ water for 30 minutes.

[0178] Proton conductivity (mM-min-1) across the membrane is measured using a two-compartment diffusion cell (donor: 1.0 M formic acid; receiver: deionized water), under gentle stirring in the receiver compartment. Receiver [H+] is quantified by pH electrode calibrated at 25 °C; conductivity is computed by linear regression of cumulative moles versus time over the initial linear region (typically 0-90 min).

[0179] Preferably the proton conductivity is at least 0.4 pmol / L / min.In control experiments using 1 M ammonium formate, typical ionic conduction rates are < 0.05 uS / cm / min and frequently < 0.005 uS / cm / min. These "background salt conduction" rates are at least one and frequently 2 orders of magnitude lower than the proton conduction rates in Table 1. In the case that the salt-only conduction rate is 2 orders of magnitude lower than the acid conduction rate, the proton transference number is > 0.99.

[0180] Electrochemical impedance spectra for a representative selection of proton selective membranes of the present invention and commercial functionalized nanoporous polymer membranes are shown in Figure 6 and corresponding equivalent fitted circuit models depicted in Figure 7.

[0181] EIS spectra are fitted with a constant-phase element (CPE_dl) in parallel with a finite-length Warburg element (W_s), in series with R_bulk. The W_s time constant (T_W) lies between 0.001-0.2 s for hydrated membranes at 25 °C, consistent with proton transport via Grotthuss conduction.

[0182] It is observed that the EIS behaviour of the proton selective membranes of the present invention is qualitatively similar to one another, but very different from the commercial Fumasep and Nation membranes, which require different equivalent circuit models to fit their EIS spectra.

[0183] Dynamic charge transport across the proton-selective membranes of the present invention is best modelled using a constant-phase element in parallel with a "short" or "transmissive boundary" Warburg element. The equivalent circuit model for this system is completed by adding a constant resistance term which accounts for bulk, frequency-independent resistance across the membrane.

[0184] Without wishing to be bound by theory, this model is consistent with the inventors' hypothesis that hydrophobic salts self-assemble into bilayer-like membranes, which then respond to the applied current by forming an electrical double-layer at each electrode modelled by the constant-phase element and shuttling protons between electrodes modelled by the short Warburg element. The bulk resistance can be ascribed to electrical resistivity, which is relatively high in these systems because electrical conduction across hydrophobic membranes is expected to be significantly impeded.

[0185] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to".

[0186] The entire disclosures of all applications, patents and publications cited above and below, if any, are herein incorporated by reference.Reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in the field of endeavour in any country in the world.

[0187] The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features.

[0188] Where in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth.

[0189] It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the present invention.

[0190] The embodiments described herein illustrate various ways of making and using the invention. The inventors have developed and tested multiple approaches and consider the described configurations to represent practical and workable implementations. Variations in materials, parameters, processing conditions, or system architecture may be made without departing from the scope of the invention. Any preferred approaches presently contemplated by the inventors are reflected within the disclosed embodiments, though such preferences need not be specifically identified as being superior to others.

[0191] Interpretation of numerical ranges. Unless otherwise indicated, numerical values and ranges (e.g., '1-20') are inclusive of the end-points and support all sub-ranges and individual values therewithin (e.g., 1-12, 1-8, 2-10, 3, 4, 5 ...). Where the term 'about' , 'substantially', or 'approximately' precedes a numeric value or range, it denotes ±10% of the stated value unless the context dictates otherwise.

[0192] Measurements are, unless stated, at 25 ± 1 °C and 1 atm.

[0193] Open / closed transitional phrases. 'Comprise' and 'including' are used in an inclusive sense (i.e., 'including but not limited to'). 'Consisting essentially of' excludes features that materially affect the stated basic and novel characteristics but allows immaterial substances (e.g., processing aids, residual solvents). 'Consisting of' is closed.Independent selection. Where a list is preceded by 'independently selected from / each instance of the variable may be chosen without reference to the others.

[0194] Equivalents. References to a given material, component, or method encompass functional equivalents and known alternatives that achieve a substantially similar result under substantially similar conditions.

[0195] 1

Claims

Claims:

1. A membrane including:i) a porous solid support layer; andii) a hydrophobic matrix layer in contact with the porous solid support layer,wherein the hydrophobic matrix layer includes a hydrophobic salt in a concentration of at least 10 wt%.

2. The membrane according to claim 1 wherein the hydrophobic matrix layer includes a hydrophobic salt in a concentration of at least 80 wt%.

3. The membrane according to claim 1 wherein the hydrophobic matrix layer includes a hydrophobic salt in a concentration of 100%.

4. The membrane according to claim 1 wherein the hydrophobic salt is formed from:• a cation selected from ammonium and phosphonium; and / or• an anion selected from a carboxylate, sulfate, or phosphate.

5. The membrane according to claim 4 wherein the hydrophobic salt includes:i) a quaternary ammonium cation having the formula N+R3R4R5R6wherein each of R3, R4, R5, and R6are independently selected from: optionally substituted alkyl; and optionally substituted alkenyl; andii) a carboxylate anion having the formula R1COO_or a sulfate anion having the formula R1SOT wherein R1is selected from: optionally substituted alkyl; and optionally substituted alkenyl.

6. The membrane according to any one of claims 1 to 5 wherein the membrane is proton selective.

7. The membrane according to any one of claims 1 to 6 wherein the membrane has a proton conductance number of >0.95.

8. The membrane according to any one of claims 1 to 7 wherein the membrane has a proton conductance number of >0.99.

9. The membrane according to any one of claims 1 to 8 having proton conductivity of greater than 0.4 pmol / L / min.

10. A membrane including:a porous solid support layer; andii) a hydrophobic matrix layer in contact with the porous solid support layer,wherein the hydrophobic matrix layer includes a hydrophobic salt including:i) a quaternary ammonium cation having the formula N+R3R4R5R6wherein each of R3, R4, R5, and R6are independently selected from: optionally substituted alkyl; and optionally substituted alkenyl; andii) a carboxylate anion having the formula R1COO_or a sulfate anion having the formula R1SOT wherein R1is selected from: optionally substituted alkyl; and optionally substituted alkenyl.

11. The membrane according to claim 10 wherein the membrane has a proton conductance number of >0.95.

12. The membrane according to claim 10 or claim 11 wherein the membrane has a proton conductance number of >0.99.

13. The membrane according to any one of claims 10 to 12 having proton conductivity of greater than 0.4 pmol / L / min.

14. The membrane according to any one of claims 1 to 13 wherein the hydrophobic salt is selected from: DADMA stearate; DADMA oleate; DADMA palmitate; DADMA dodecylsulfate; DADMA transcinnamate; Aliquat 336 stearate; Aliquat 336 oleate; Aliquat 336 palmitate; Aliquat 336 dodecylsulfate; Aliquat 336 trans-cinnamate; DDDMA oleate; TBA palmitate; TBA stearate; TBA oleate; CTA dodecylsulfate.

15. The membrane according to any one of claims 1 to 14 wherein the hydrophobic salt is selected from: DADMA stearate and Aliquat 336 stearate.

16. The membrane according to any one of claims 1 to 15 wherein the hydrophobic salt is DADMA stearate.

17. A membrane including:a porous solid support layer; andii) a hydrophobic matrix layer in contact with the porous solid support layer,wherein the membrane provides proton conductivity of at least 0.4 pmol / L / min.

18. The membrane according to claim 17 wherein the membrane has a proton conductance number of >0.95.

19. The membrane according to claim 17 or claim 18 wherein the membrane has a proton conductance number of >0.99.

20. A process for forming a membrane, the process including the steps of:i) providing a hydrophobic salt;ii) providing a porous solid support layer;ii) applying the hydrophobic salt to the porous solid support layer so as to form a hydrophobic matrix layer that includes a hydrophobic salt in a concentration of at least 10 wt%.

21. A process for forming a membrane, the process including the steps of:i) providing a hydrophobic salt;ii) providing a porous solid support layer;ii) applying the hydrophobic salt to the porous solid support layer so as to form a hydrophobic matrix layer, wherein the hydrophobic salt includes:i) a quaternary ammonium cation having the formula N+R3R4R5R6wherein each of R3, R4, R5, and R6are independently selected from: optionally substituted alkyl; and optionally substituted alkenyl; andii) a carboxylate anion having the formula R1COO_or a sulfate anion having the formula R1SOT wherein R1is selected from: optionally substituted alkyl; and optionally substituted alkenyl.

22. An electrochemical cell including:i) a first vessel containing a first protic solvent system;ii) a second vessel containing a second protic solvent system;iii) a membrane separating the first protic solvent system from the second protic solvent system, wherein the membrane is configured to allow the movement of protons between the first protic solvent system and the second protic solvent system.

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