Graphitic carbon nitride film

Abstract

A carbon nitride film, wherein the thickness of the film is from 0.3 to 40nm. A product comprising substrate and the carbon nitride film, wherein the film is in contact with the substrate.

Description

[0001] GRAPHITIC CARBON NITRIDE FILM

[0002] Field of the Invention

[0003] The present invention relates to a carbon nitride film. The film can be used as a layer on a substrate, such as a membrane. The membrane can be used as an ion exchange membrane, for example finding application in fuel cells, water electrolysers and / or batteries. The present invention also relates to a method of making a film and / or providing a layer of a carbon nitride film to a substrate.

[0004] Background of the Invention

[0005] Ion exchange membranes (IEMS) are essential components of electrochemical technologies, including fuel cells (FCs), water electrolysers (WEs), and batteries. FCs and WEs require a proton exchange membrane (PEM) to permit proton transport between electrodes whilst preventing or minimising crossover of gases (e.g. H2) from one side to another. Such crossover can reduce efficiency and promote degradation.

[0006] Providing IEMs with high proton conductivity and high proton selectivity is a current challenge for fuel cell and electrolyser devices.

[0007] To date, device output has been optimised by decreasing the PEM thickness. However, this increases susceptibility to electron and gas crossover leading to deleterious efficiency losses and accelerated degradation.

[0008] 2D materials are composed of a rigid atomic lattice that offer significant mass transport control with unmatched species selectivity and the lowest attainable thicknesses. As such, they are highly sought after for membrane applications. Single layer graphene and its derivatives with high mechanical strength and chemical stability have been shown to offer complete gas rejection, but sub Angstrom lattice pores have large Eafor proton translocation, and only a relatively small surface area of film defects or regions of accumulated lattice strain and wrinkling permit hindered proton permeability. Electron conductivity and hydrophobicity, as well as complete proton insulation with multiple stacking, limit practical use of such 2D materials. Ion (e.g. proton, hydroxide) flux and selectivity is governed by pore size and geometry, electrostatics, and functionality, therefore 2D materials with increased hollow site radius and presence of hydrogen-bonding functionality can overcome these limitations, but are non-trivial to prepare in large format.

[0009] Carbon nitrides, such as polytriazine imide (PTI), contain a regular arrangement of structural voids that are of unique size and chemical nature to permit proton and water transport. However, two dimensional (2D) particles of carbon nitrides, such as PTI, are conventionally prepared with lateral dimension of less than 100 nm. Such particles therefore have limited commercial potential unless manipulated in their assembly.

[0010] Previous attempts to provide atom-scale barrier layers have relied on chemical vapour deposition (CVD). However, high vacuum, high purity and high temperature conditions make such films costly, and still result in error-prone transfer to application substrates. Furthermore, only a limited number of 2D materials can be produced using CVD and with limited growth substrates.

[0011] Suter et al., Chem. Sci., 2019, 10, 2519-2528 describes a procedure for exfoliating PTI. WO2021074401 Al describes a procedure for exfoliating PTI and making a membrane of the exfoliated PTI. WO2021074401 Al describes the use of filtration to prepare the membrane.

[0012] US 11075395B2 discloses CVD-grown hexagonal boron nitride (hBN) as a selective layer that can be incorporated into a fuel cell membrane electrode assembly (with or without a Nafion membrane).

[0013] 2D materials are commonly dispersed in a polymer matrix to reduce crossover of gases, but this relies on tortuosity increases rather than utilisation of anisotropic properties of the 2D materials.

[0014] The present invention has been devised with the foregoing in mind. Summary of the Invention

[0015] According to a first aspect the claimed invention provides a carbon nitride film, wherein the thickness of the film is from 0.3 to 40nm.

[0016] It will be understood that the carbon nitride film should be relatively uniform. The film of the invention is typically topologically planar. The film of the invention is formed of a layer of abutting and / or overlapping carbon nitride nanosheets. The film of the invention is typically more smooth and more continuous than films of the prior art.

[0017] According to a second aspect the claimed invention provides a product comprising substrate and a carbon nitride film according to the first aspect, wherein the film is in contact with (e.g. laminated with) the substrate. The substrate may be a support membrane (e.g. a Nafion™ (sulfonated tetrafluoroethylene based fluoropolymer- copolymer) membrane), in which case the product may be an ion exchange membrane. The substrate may be an electrode body, in which case the product may be an electrode (e.g. a passivated and / or protected electrode). The substrate may be a battery separator membrane, in which case the product may be a battery separator. The substrate may be a battery current collector, in which case the product may be a battery (e.g. an anode- free battery).

[0018] The carbon nitride film has been found to enhance proton conduction of substrates, such as ion exchange membranes and electrodes.

[0019] The carbon nitride film has been found to enhance the water management, and hydration, of the substrates, thereby preventing substrates from drying out.

[0020] This has been found to facilitate electrochemical reactions, reduce flooding and increase the performance of a cell at high temperatures.

[0021] In addition, this has been found to provide a twofold reduction in gas crossover between electrodes, yielding increased durability.

[0022] The film of the first aspect can enable the lifetime of a substrate (e.g. support membrane) to increase to around three times greater than the substrate alone. In contrast to the other 2D materials, nanosheets of carbon nitrides, such as PTI (especially two-dimensional PTI), intrinsically contain repeating structural voids that can undergo facile (de-)protonation, promote through-void water transport, and provide steric and an electrostatic barrier to molecular H2 and O2. For instance, PTI has repeating structural voids of C12N12H3, with a diameter of 3.4 A.

[0023] Due to the size and characteristics of the pores in carbon nitrides, such as PTI, it is expected that the proton (hydroxonium ion) conductivity is indicative that hydroxide ions can be efficiently transported by the membrane, based on the analogous hydrogen bond rearrangement in both mechanisms. Therefore, the ion exchange membrane may be used for acidic and / or alkaline systems.

[0024] The present invention allows thinner support membranes (e.g. ionomers) to be used without deleterious consequences to the stability and crossover rate of the support membranes.

[0025] Preferably the thickness of the carbon nitride film is from 0.4 to 20nm, especially from 0.4 to 5nm. It has been found that thicknesses from 0.4 to 5nm are particularly optimal for limiting or preventing the passage of hydrogen molecules, whilst permitting the transport of ions such as protons.

[0026] Traditionally, binders have been used to hold randomly dispersed particles of carbon nitride (e.g. PTI) in a mixed-matrix membrane. However, providing the carbon nitride in a binder matrix means that the carbon nitride is not able to form a substantially continuous film, thereby reducing the effectiveness of the carbon nitride through plane pore structure.

[0027] The film of the invention has been found to be stable over a relatively wide temperature range, such as up to 200°C. The invention may therefore be used at temperatures of 200°C or less, such as from -30°C to 200°C. such as from 15°C to 200°C.

[0028] Whilst the ion exchange membrane is detailed herein primarily in relation to fuel cells, selective ion exchange membranes work on similar principles in a wide range of applications, including electrochemical devices (e.g. fuel cells, batteries such as redox flow batteries and Li-S batteries, and water electrolysers), photoelectronic components, catalysts, filtration and separation (e.g. isotope separation). Certainly, the prevention of degradation and crossover is critical to the function of water electrolysers and flow batteries. Products of the invention may have applications in photoelectronic components and / or catalysts (e.g. photocatalysts). Carbon nitride (e.g. PTI) has a bandgap that is suitable for photostimulation and / or photocatalysis.

[0029] According to a third aspect, the claimed invention provides an apparatus comprising a product of the second aspect. The apparatus may be a fuel cell, a water electrolyser, a battery (e.g. redox flow battery, proton battery and / or Li-S battery), a photoelectronic component, a catalyst, a filtration apparatus, and / or a stationary phase for chromatography. Preferably the apparatus is a fuel cell, a water electrolyser, and / or a battery. Preferably the film of the product of the second aspect (e.g. where the substrate is a proton exchange membrane) is in contact with a cathode. Where the film is in contact with the cathode, there is an increased benefit provided by hydration of the film compared to when the film is in contact with the anode, causing an increase in the proton conduction of the product.

[0030] Preferably the apparatus comprises an ion exchange membrane and an electrode, wherein (in use) ions (e.g. protons) can pass to and / or from the electrode and through the ion exchange membrane. Preferably the apparatus comprises an ion exchange membrane and at least two electrodes, wherein (in use) ions (e.g. protons) can pass from one electrode to the other through the ion exchange membrane.

[0031] According to a fourth aspect the claimed invention provides a method of preparing a product (e.g. of the second aspect), the method comprising providing a substrate and applying (e.g. coating) a carbon nitride film to the substrate (e.g. support membrane and / or electrode). The film may be according to the first aspect.

[0032] Methods of applying the carbon nitride film may include vapour and / or aerosol phase deposition, spray (e.g. ultrasonic, electrospray) or slurry coating. Preferably the method comprises applying the carbon nitride to the substrate by spray coating, especially ultrasonic spray coating. As shown by the Examples, spray coating can provide a relatively uniform film of the carbon nitride on a substrate. Spray coating has been found to be more scalable than alternative methods. The method may comprise providing a film according to the first aspect before applying the film to the substrate. The film may be a Langmuir film. A Langmuir film is a film that is self-assembled and typically suspended on a liquid surface. Preferably the carbon nitride film is applied to the substrate by Langmuir-Blodgett, Langmuir-Schaefer, and / or horizontal precipitation deposition (G.G. Roberts (Ed.), Langmuir-Blodgett Films, Plenum, New York, 1990). Horizontal precipitation deposition has been used to successfully prepare a thin film of densely tessellated carbon nitride particles of around 5 nm in thickness, and apply this onto a substrate. The applied film may be known as a Langmuir film, which has essentially been transferred by lowering the liquid surface onto the parallel substrate.

[0033] The unique structure and chemical properties, and mechanochemical and electrochemical stability of carbon nitrides, such as PTI, combined with the systematic fabrication protocol has been found to provide a membrane technology that has significant benefits over conventional ion exchange membranes.

[0034] When using Langmuir deposition, the method may comprise the steps of: providing a dispersion of (exfoliated) carbon nitride in an organic solvent, applying the dispersion to the surface of an aqueous bath, separating the organic solvent from the carbon nitride so as to leave the carbon nitride on the surface of the aqueous bath, reducing the surface area of the aqueous bath so as to densify the carbon nitride on the surface of the aqueous bath and so as to form the carbon nitride film, and contacting the substrate (e.g. support membrane) with the carbon nitride film.

[0035] Langmuir assembly has been demonstrated to uniquely provide uniform nanometre thick films from a variety of solution processable 2D materials for coating applications.

[0036] Use of Langmuir deposition permits use with 2D materials that cannot be grown using CVD and can be applied directly to any desired substrate. The Langmuir approach has a transfer efficiency of >100 m2g1, with scope for increase using optimised deposition parameters, making the already cheap synthesis of carbon nitrides, including PTI, an economical technology when deposited as a thin film. The unique chemical and structural properties of PTI result in selective proton transport and improved water management. The deposition of nanomaterials in large area film was disclosed by US 2021 / 0060603 Al, but this document is primarily focussed on the deposition of graphene oxide on mica or polyethylene terephthalate, and of molybdenum disulfide on glass. This document does not disclose ion exchange membranes, and does not mention carbon nitrides such as PTI, or the specific benefits the membranes of the invention can provide.

[0037] Traditionally, vacuum deposition has been used to deposit carbon nitrides such as PTI on membranes. However, the resulting films typically have a thickness of lOOnm or more. For example, the procedure disclosed by WO 2021 / 074401A1 is not similar to that defined herein, using vacuum filtration to prepare the membrane. The sheets of PTI stack and overlap with one another in an uncontrollable organisation, rather than forming a continuous sheet of parallel particles. Furthermore, the PTI is included as a gas separation material and application relevant performance required a 6: 1 ratio of polymer to PTI in a composite polymer membrane. The total PTI containing film used was over lOOnm in thickness. Therefore, the membrane does not comprise a carbon nitride film having a thickness of from 0.3 to 40nm, as required by the claimed invention.

[0038] Exfoliated Graphene Composite Membrane for the All-Vanadium Redox Flow Battery. ACS Appl. Energy Mater. 2023, 6, 12, 6505-6517 describes the use of Langmuir Blodgett deposition to produce an ultrathin graphene layer used in redox flow batteries to increase performance. However, there was no indication that carbon nitrides could be used with Langmuir Blodgett deposition, or description of any benefits that could be attained by that. The use of this method with carbon nitrides, such as PTI, seems to be particularly beneficial due to the small and regular shape of each carbon nitride particle that can close pack. By comparison, graphene-related materials generally have very little shape or size consistency, which can lead to less surface coverage and / or lower density.

[0039] Mohamed et al., Journal of Alloys and Compounds (2019) 818, page 152916 and Xu et al., Journal of the American Chemical Society (2017), 139, pages 6026-6029 describe the polymerisation of melamine by calcination. As shown in the characterisation presented by these papers, they provides amorphous particles containing heptazine molecular structures that would form more of a rough agglomeration rather than a thin film. There is no disclosure of the upper limit of film thickness (especially within the claimed range), of films made of PTI, or of triazine structures (as produced by PTI). The film of the present invention is significantly smoother and more continuous than the rough amorphous graphitic carbon nitride agglomerations described by Mohamed and Xu. The film of the present invention is formed of crystalline flakes, which have a high aspect ratio, and lead to the surprising beneficial effects described herein.

[0040] The present application includes the subject-matter of the following clauses:

[0041] 1. A carbon nitride film, wherein the thickness of the film is from 0.3 to 40nm.

[0042] 2. The carbon nitride film of clause 1, wherein the smallest dimension of the film in the plane of the film is 100pm or more.

[0043] 3. The carbon nitride film of clause 2, wherein the smallest dimension of the film in the plane of the film is from 1mm to Im.

[0044] 4. The carbon nitride film of any preceding clause, wherein the thickness of the film is from 0.4 to 20nm.

[0045] 5. The carbon nitride film of clause 4, wherein the thickness of the film is from 0.4 to 5nm.

[0046] 6. The carbon nitride film of any preceding clause, wherein the carbon nitride is a graphitic carbon nitride (g-C3N4).

[0047] 7. The carbon nitride film of clause 6, wherein the graphitic carbon nitride comprises triazine and / or heptazine sub-units.

[0048] 8. The carbon nitride film of clause 7, wherein the carbon nitride is poly(triazine imide).

[0049] 9. A product comprising substrate and a carbon nitride film according to any preceding clause, wherein the film is in contact with the substrate.

[0050] 10. The product of clause 9, wherein: a) the substrate is a support membrane, and the product is an ion exchange membrane; b) the substrate is an electrode body, and the product is an electrode; and / or c) the substrate is a battery separator membrane, and the product is a battery separator, d) the substrate is a battery current collector, and the product is a battery.

[0051] 11. The product of clause 10, wherein the substrate is a support membrane that comprises a sulfonated tetrafluoroethylene based fluoropolymer-copolymer. 12. The product of clause 10, wherein the substrate is an electrode body that comprises an electrically conductive form of carbon, a metal, and / or an electrically conductive metal oxide.

[0052] 13. The product of clause 10, wherein the substrate is a battery separator membrane that comprises a polyolefin.

[0053] 14. The product of any one of clauses 9 to 13, wherein the substrate has a thickness of 500pm or less.

[0054] 15. The product of any one of clauses 9 to 14, wherein the carbon nitride film covers 90% or more of the surface area of an active region of at least one side of the substrate.

[0055] 16. An apparatus comprising a product of any one of clauses 9 to 15.

[0056] 17. The apparatus of clause 16, wherein the apparatus is a fuel cell, a water electrolyser, a battery, a photoelectronic component, a catalyst, a filtration apparatus, and / or a stationary phase for chromatography.

[0057] 18. The apparatus of clause 17, wherein the apparatus is a fuel cell, a water electrolyser, and / or a battery.

[0058] 19. A method of preparing a product, wherein the method comprises providing a substrate and applying a carbon nitride film to the substrate.

[0059] 20. The method of clause 19, wherein the carbon nitride film is according to the first aspect.

[0060] 21. The method of clause 19 or clause 20, wherein the carbon nitride film is a Langmuir film and is applied to the substrate by Langmuir-Blodgett, Langmuir- Schaefer, or horizontal precipitation deposition.

[0061] 22. The method of any one of clauses 19 to 21, wherein the method comprises the steps of: - providing a dispersion of carbon nitride in an organic solvent, - applying the dispersion to the surface of an aqueous bath, - separating the organic solvent from the carbon nitride so as to leave the carbon nitride on the surface of the aqueous bath, - reducing the surface area of the aqueous bath so as to densify the carbon nitride on the surface of the aqueous bath and so as to form the carbon nitride film, and - contacting the substrate with the carbon nitride film.

[0062] 23. The method of clause 22, wherein the organic solvent comprises a solvent selected from the list consisting of: DCE, DMF, DMAc, methylene chloride, chloroform, ethyl acetate and isopropyl acetate. Detailed Description of the Invention

[0063] Carbon Nitride Film

[0064] Carbon nitrides contain differing amounts of carbon and nitrogen. Some carbon nitrides, such as PTI, also include hydrogen in their structure. The skilled person will understand that the exact composition of the carbon nitride may be modified to control the size of the pores in the carbon nitride.

[0065] The film should be composed of ordered (i.e. crystalline) carbon nitride, for example abutting and / or overlapping crystalline carbon nitride nanosheets. The carbon nitride may be referred to as a covalent organic framework. The (ordered) carbon nitride preferably comprises linked triazine (TGCN) and / or heptazine (HGCN) sub-units. The carbon nitride may be a graphitic carbon nitride (g-C3N4, C3N5, MCN-12 (C3N6), and / or MCN-13 (C3N7). The carbon nitride may include C, N, H and O atoms; preferably comprising, consisting essentially of or consisting of C, N and H atoms. The carbon nitride may have the formula of approximately C3N4, such as heptazine or poly(triazine imide) (PTI), but may have a small quantity of additional atomic constituents due to the synthetic protocol used, such as H, Li, and / or Cl, and / or O. The additional atomic constituents may be present in an amount of 40 At% (atomic percentage) or less. Intercalant free PTI has been observed with atomic compositions from C6N9H3to C6N9H6. Preferably the carbon nitride is a graphitic carbon nitride, such as PTI.

[0066] The carbon nitride film may be described as exfoliated. The carbon nitride may have ions (e.g. lithium, chloride and / or bromide) intercalated within its structure. The carbon nitride may be ion-free. For example, the carbon nitride film may include an amount of lithium, chloride and / or bromide ions of 15 At% or less, such as 10 At% or less, or 5 At% or less, such as 2 At% or less, or 0.2 At% or less, for example 0.1 At% or less, or 0.001 At% or less, such as no lithium, chloride and / or bromide ions.

[0067] PTI is an attractive candidate for many technological applications and with only small changes to the assembly protocol this technology could be applied in a diverse range of fields. Based on its specific band gap, PTI has been shown to be a promising photocatalyst in feedstock conversion reactions. By controlled deposition in an ultrathin film its use in this application could be achieved at low cost. For quantum sieving of hydrogen and deuterium, a membrane with molecular structure possessing voids of 3 - 3.4 A is believed to achieve maximum separation. In addition, the steric barrier for reactant molecules, such as H2, and the high thermal and (electro-)chemical stability at both high and low pH of PTI permits the use of PTI at the electrode-electrolyte interface in fuel cells and other ionomer-related applications.

[0068] The tendency of PTI and similar carbon nitrides to form relatively small lateral sheets of up to several microns is an obstacle, but can be overcome by precise processing to fabricate a dense film as described herein.

[0069] Compared to other 2D materials, such as graphene, carbon nitrides such as PTI have beneficial hydrogen bonding sites on the inside of pores in its structure that facilitate ion transport.

[0070] The thickness of the carbon nitride film is from 0.3 to 40nm. Preferably the thickness is from 0.3 to 30nm, such as from 0.3 to 20nm, preferably from 0.3 to lOnm, or from 0.3 to 5nm, such as from 0.3 to 2nm, or from 0.3 to l .Onm. The thickness may be from 0.4 to 40nm, such as from 0.4 to lOnm, or from 0.5 to 40nm, such as from 0.5 to lOnm. The carbon nitride film should be substantially continuous, but may have some apertures in it. The film may be termed a sheet. The film is relatively uniform. As such, the upper endpoint of the range (e.g. 40nm) is preferably a maximum for the thickness of the film. The lower endpoint of the range (e.g. 0.3nm) may be a minimum for the thickness of the film. As such, the endpoints of the ranges may be absolute limits for the thickness of the film.

[0071] A smallest dimension of the film in the plane of the film (e.g. from one edge of the film to another edge of the film through the centre of the film) may be 100 pm or more, for example 1mm or more, or 2mm or more, or 5mm or more, such as 10mm or more, or 20mm or more, or 50mm or more, for example 100mm or more. The smallest dimension of the film may be 10m or less, such as 2m or less, or Im or less. The smallest dimension of the film may be from 100pm to 10m, such as from 10mm to Im.

[0072] The surface area of the film (in the plane of the film) may be 0.1cm2or more, such as 0.2 cm2or more, or 0.5 cm2or more, for example 1 cm2or more, or 5 cm2or more, for example 10 cm2or more. The surface area of the film may be 10m2or less, such as 2 m2 or less, or 1 m2or less, for example 0.1 m2or less, or 100 cm2or less. The surface area of the film may be from 0.1cm2to 10m2, such as from 1cm2to Im2.

[0073] The carbon nitride film should normally substantially cover at least one side of the substrate (e.g. membrane). The skilled person will understand that a product (e.g. membrane) may have an “active” region that is, for example, exposed to the electrolyte in use. The carbon nitride film may cover 90% or more of the active region of the surface area of at least one side of the substrate (e.g. membrane), such as 95% or more, or 98% or more, preferably 99% or more, or 99.5% or more, or 99.9% or more, such as 99.99% or more. The carbon nitride film may cover 99.99999% or less of the active region of the surface area of at least one side of the substrate (e.g. membrane, e.g. from 90 to 99.99999%), such as 99.99% or less (e.g. from 90% to 99.99%). The carbon nitride film may cover 90% or more of the surface area of at least one side of the substrate (e.g. membrane), such as 95% or more, or 98% or more, preferably 99% or more, or 99.5% or more, or 99.9% or more, such as 99.99% or more. The carbon nitride film may cover 99.99999% or less of the surface area of at least one side of the substrate (e.g. membrane, e.g. from 90 to 99.99999%), such as 99.99% or less (e.g. from 90% to 99.99%). This may be determined by AFM, TEM and / or XPS. Preferably this is determined by AFM.

[0074] In contrast to amorphous carbon nitride compositions of the prior art, for example prepared by the calcination of melamine, the film of the present invention has been found to achieve a ratio of C-N C units to N-C3 units close to 1 :0.33, as measured by X-ray diffraction and / or XPS. The carbon nitride film may have a proportion of N-C3 units (e.g. by moles), relative to C-N=C units (e.g. as measured by X-ray diffraction), of from 20% to 45%, such as from 25% to 40%, preferably from 30% to 36%, such as from 32% to 35%, or from 33% to 34%.

[0075] The carbon nitride film preferably comprises pores encircled by from 12 to 30 atoms. The number of atoms bordering the pore may be from 20 to 28, such as from 22 to 26, preferably 24. More preferably, the number of atoms bordering the pore may be from 14 to 22, yet more preferably from 16 to 20, most preferably 18. The number of atoms bordering the pores may be determined by X-ray diffraction. Substrate

[0076] The substrate may, for example, be a support membrane (e.g. a Nafion membrane), in which case the product may be an ion exchange membrane. It will be appreciated that the substrate is normally different from the carbon nitride film. The substrate may not comprise PTI, or another carbon nitride.

[0077] A wide variety of ion exchange membranes could benefit from a carbon nitride film as disclosed in the present invention. Suitable support membranes include hydrocarbonbased PEMs that are typically cheaper and more prone to radical degradation, or AEMs that are prone to dehumidification and CO2 induced degradation.

[0078] The support membrane may be an ionomer, such as polystyrene sulfonate, Nafion and Hycar (acrylic resin, Lubrizol). The support membrane may be an anion and / or cation exchange membrane.

[0079] Anion (e.g. hydroxide) exchange membranes may comprise poly(fluorenyl-co-aryl piperidinium) (PFAP), and / or polysulfone (PS), polyethylene (PE) and / or polytetrafluoroethylene (PTFE) backbones with cationic head-groups for example cationic head-groups including; benzyltrialkylammonium, quaternary ammonium (QA) and crosslinking diammonium groups (where the link chain is >C4 in length), DABCO- based QA groups, quinuclidinium-based QA groups, imidazolium groups (where R = Me or H and R', R" = alkyl or aryl groups, pyridinium groups, pentamethylguanidinium groups, and / or alkali stabilised quaternary phosphonium groups.

[0080] Cation (e.g. proton) exchange membranes may comprise a polymer selected from the group consisting of: (acid-based) polyether ether ketone (US PolyFuel Inc.), (acidbased) polyether ether sulfone (Toshiba), OH-modified fullerene-based membranes (Sony), polystyrene sulfonic acid-based compounds (JSR Corporation, Japan), sulfonation sulfone polymer; PVDF-g-SPS (Los Alamos National Laboratory), Sulfonated-F-styrene (Ballard), Nafion (DuPont), PFS / PTFE fibrils (Japan’s Asahi Glass Co., Ltd. (Asahi Glass)), short side chain copolymers of tetrafluoroethylene and sulfonyl fluoride vinyl ether, polyamide, acid doped polymers (including polybenzimidazole (PBI), Polyvinylpyrrolidone (PVP) and / or polymers of intrinsic microporosity (PIMs)), polyethylene oxide and / or ionic liquid doped polymers (such as polyimide and / or polyethyleneimine).

[0081] Preferably the support membrane is Nafion. Nafion is a commercially available sulfonated tetrafluoroethylene based fluoropolymer-copolymer, which has the following general structure:

[0082] Nafion may be the sodium salt of this copolymer. Nafion exhibits excellent chemical and mechanical stability.

[0083] The substrate (e.g. support membrane) may have a thickness of 500pm or less, such as 200 pm or less, or 150 pm or less, or 100 pm or less, such as 80 pm or less or 60 pm or less, or 50 pm or less, such as 40 pm or less, or 30 pm or less. The substrate (e.g. support membrane) may have a thickness of 0.1 pm or more, such as 1 pm or more, or 2 pm or more, such as 3 pm or more, or 5 pm or more, such as 10 pm or more, or 20 pm or more, or 40 pm or more, or 50 pm or more. The thickness of the substrate (e.g. support membrane) may be from 0.1 to 500 pm, such as from 10 to 150 pm. Support membranes for fuel cells preferably have a thickness of from 10 to 30 pm. Support membranes for water electrolysers preferably have a thickness of from 50 to 100 pm.

[0084] The substrate may be an electrode body, in which case the product may be an electrode. It will be understood that the electrode body should be electrically conductive. The electrode body may comprise an electrically conductive material, such as an electrically conductive form of carbon (e.g. graphite, carbon cloth and / or carbon fibre paper, and / or carbon black), a metal (e.g. platinum, palladium, rhodium, ruthenium, osmium, iridium, gold and / or nickel, and / or sodium and / or lithium), and / or an electrically conductive metal oxide (e.g. perovskite (such as LaCoO3and / or LaMnO3) and / or lanthanum cobaltite and / or LiCoO2and / or lanthanum manganite (e.g. La0.8Sr0.2MnO3and / or La0.6Sr0.4Co0.2Fe0.8O3)). The electrode body may comprise a binder (e.g. a polymeric binder). The substrate may be a battery separator membrane, such as a polyolefin (e.g. polyethylene, polypropylene, polyvinylchloride, or blends thereof, which may be poly(methyl methacrylate) -grafted, siloxane-grafted and / or modified with polytriphenylamine) .

[0085] The substrate may be a battery current collector, such as a metal (e.g. aluminium, copper, nickel, titanium, steel (e.g. stainless steel)) or a conductive carbon-based material (e.g. graphite).

[0086] Method

[0087] One means of forming the membrane of the invention is to deposit the carbon nitride film onto the substrate (e.g. support membrane) using Langmuir deposition.

[0088] Langmuir deposition uses a volatile, water-immiscible solvent to spread a molecule / material, at the air-water interface. After removal / evaporation of the solvent, movable barriers compress the floating insoluble material to a desired density while measuring the surface tension, γ, or surface pressure, π = γ0- γ, where γ0is the surface tension of pure water. As the molecules on the surface are squeezed closer together, surface pressure increases with surface area decrease. The Wilhelmy plate technique is commonly used to measure the surface pressure by placing a thin plate of filter paper or platinum at the interface and the surface tension force exerted on the plate determined by a tensiometer. During surface film densification, 2D phase transitions can occur from gaseous to liquid to solid phases where particles transition from being well-spaced and free flowing, to densely free-flowing, to densely packed and rigid on the surface, respectively. To achieve transfer of dense films, the solid phase is reached, and the surface pressure maintained by the barriers while a substrate is dip-coated.

[0089] Whilst the Langmuir technique is commonly employed to achieve monolayers of amphiphilic molecules that are stabilised at the interface due to polar interaction with water and non-polar groups repelled by the water, high surface energy / tension at the water surface can create floating layers of lightweight materials. To date the Langmuir technique has allowed nanomaterials, such as metal nanoparticles, carbon nanotubes, and GO, to be dispersed at the air-water interface for fabrication of monolayer films. Deposition from aqueous solutions, however, can result in materials being lost in the subphase, such as the case for GO where over 99% of sheets are lost, leaving only the largest sheets at the surface.

[0090] When using Langmuir deposition, the method may comprise the steps of: providing a dispersion of the carbon nitride in an organic solvent, applying the dispersion to the surface of an aqueous bath (e.g. in a liquid (e.g. suspension), spray and / or aerosol), separating the organic solvent from the carbon nitride so as to leave the carbon nitride on the surface of the aqueous bath, reducing the surface area of the aqueous bath so as to densify the carbon nitride on the surface of the aqueous bath, and contacting the substrate (e.g. support membrane) with the carbon nitride.

[0091] The step of providing a dispersion of the carbon nitride in an organic solvent may comprise preparing the carbon nitride using a molten salt synthesis procedure. Ions (e.g. lithium, chloride and / or bromide ions) may be removed from the carbon nitride produced by this method by extraction, such as Soxhlet extraction, with water.

[0092] The step of providing a dispersion of the carbon nitride in an organic solvent may comprise exfoliating the carbon nitride in the organic solvent. Preferably the carbon nitride is exfoliated in the organic solvent under sonication. The carbon nitride may be exfoliated under sonication for a period of time of 1 minute or more, such as 5 minutes or more, or 10 minutes or more, such as 12 minutes or more, or 14 minutes or more. The period of time may be 2 hours or less, such as 1 hour or less, or 30 minutes or less, such as 20 minutes or less, or 16 minutes or less. The period of time may be from 1 minute to 2 hours, such as from 5 minutes to 30 minutes. The skilled person will appreciate that sonication must be performed at a sufficient amplitude / power to achieve exfoliation. The dispersion of the carbon nitride may be milled (e.g. ball milled), for example to reduce the particle size and / or to homogenise the dispersion. Milling (e.g. ball milling) may be used as an additional and / or alternative means of exfoliating the carbon nitride.

[0093] The organic solvent for exfoliation is preferably a polar aprotic solvent, such as N,N- dimethylformamide (DMF), N-methyl pyrrolidinone (NMP), dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), dihydrolevoglucosenone (cyrene), 1,2- dichloroethane (DCE), acetone, acetonitrile, methylene chloride, chloroform, dimethylpropyleneurea, ethyl acetate, isopropyl acetate, hexamethylphosphoramide, pyridine, sulfolane, and tetrahydrofuran. Preferably the polar aprotic solvent is relatively volatile, for example selected from the list consisting of DMF, DMAc, acetone, acetonitrile, methylene chloride, chloroform, ethyl acetate, isopropyl acetate, pyridine, and tetrahydrofuran. More preferably the polar aprotic solvent is DMF. The organic solvent may be a polar protic solvent, such as an alcohol, for example isopropanol, n-propanol or 1,3-butanediol.

[0094] DMF is preferable over other polar aprotic solvents, such as NMP and DMSO, due to its comparatively low density and boiling point that mitigate sub-phase loss of PTI and facilitate solvent removal, respectively.

[0095] Preferably the carbon nitride is present in the organic solvent at a concentration of lOpg / mL or more, such as 50pg / mL or more, or 100 pg / mL or more, or 150 pg / mL or more, such as 200 pg / mL or more. The concentration may be Img / mL or less, such as 500 pg / mL or less, or 400 pg / mL or less, such as 300 pg / mL or less, or 250 pg / mL or less. The concentration may be from 10 pg / mL to Img / mL, such as from 100 to 400 pg / mL.

[0096] When dispersed in the organic solvent, the carbon nitride is exfoliated. The carbon nitride at this stage may be present as sheets (platelets or flakes). The thickness of the sheets may be from 0.3 to 40nm. Preferably the thickness is from 0.3 to 30nm, such as from 0.3 to 20nm, preferably from 0.3 to lOnm, or from 0.3 to 5nm, such as from 0.3 to 2.0nm, or from 0.3 to l .Onm. The thickness may be from 0.4 to 40nm, such as from 0.4 to lOnm, or from 0.5 to 40nm, such as from 0.5 to lOnm. The thickness may be measured by atomic force microscopy (AFM). The thickness may be a number average. The sheets may have a substantially hexagonal shape, for example as determined by AFM. The number average lateral dimension of the sheets may be 40nm or more, such as 60nm or more, or 80nm or more, or lOOnm or more. The number average lateral dimension of the sheets may be 10pm or less, or 4pm or less, such as 1pm or less, or 500nm or less, such as 300nm or less, or 250nm or less, such as 200nm or less. The number average lateral dimension of the sheets may be from 40nm to 10pm, such as from 100 to 500nm. It is noted that some individual sheets in a given sample may have a lateral dimension of lOnm or less. The method may comprise, after exfoliation, separating unexfoliated particles of the carbon nitride from the exfoliated carbon nitride. Separation may comprise filtration and / or centrifugation.

[0097] The organic solvent in the dispersion applied to the surface of an aqueous bath during deposition (i.e. the deposition dispersant) may comprise solvents that are water soluble, that can be separated from the carbon nitride by allowing them to diffuse into the aqueous bath, and / or solvents that are volatile that can be separated from the carbon nitride by evaporation (e.g. allowing them to evaporate). The deposition dispersant may comprise one or more solvents selected from the list consisting of: an organic solvent for exfoliation, DMF, DMAc, DCE, methylene chloride, chloroform, ethyl acetate and isopropyl acetate. The deposition dispersant preferably comprises a substantially nonwater miscible volatile polar aprotic organic solvent, such as a solvent selected from the list consisting of: DCE, methylene chloride, chloroform, ethyl acetate and isopropyl acetate. Preferably the deposition dispersant comprises DCE. More preferably the deposition dispersant comprises a mixture of DMF and DCE.

[0098] Xu et al. Langmuir 35, 51-59 (2019), improved transfer efficiency using co-solvent systems employing DCE to provide monolayers of tiled graphene-based materials with area coverages upwards of 95% when reaching surface pressures of 10-15 mN m1.

[0099] DCE has two main advantages as a spreading solvent: i) its low surface tension facilitates surface spreading to form a thin layer which evenly distributes the dissolved nanoparticles over the surface, and ii) its high volatility allows DCE to evaporate rapidly, allowing rapid deposition.

[0100] The deposition dispersant may include the substantially non-water miscible volatile polar aprotic organic solvent in an amount of 20 vol% or more, such as 40 vol% or more, or 60 vol% or more, for example 70 vol% or more, or 80 vol% or more, such as 85 vol% or more compared to the total volume of the deposition dispersant. The deposition dispersant may include the substantially non-water miscible volatile polar aprotic organic solvent in an amount of 98 vol% or less, such as 95 vol% or less, such as 92 vol% or less, or 90 vol% or less, such as 88 vol% or less, or 86 vol% or less. The deposition dispersant may include the substantially non-water miscible volatile polar aprotic organic solvent in an amount of from 20 to 98 vol%, such as from 70 to 95 vol%, or from 80 to 90 vol%. About 85% is preferred to ensure good solution spreading on the surface, to minimise the amount of PTI that transfers into the subphase (aqueous bath), and to deposit an observable amount of PTI in a reasonable timescale.

[0101] The dispersion may be applied to the surface of an aqueous bath slowly to encourage the solvent to evaporate / disperse into the water before further dispersion is added. However, the skilled person will appreciate that too slow a rate may cause the solvent to evaporate before it reaches the aqueous bath. The skilled person will also appreciate that the rate of evaporation and the optimal rate of application of the dispersion will depend on a variety of factors such as the scale of the apparatus / process, the solvents used, and the environmental conditions.

[0102] Substantially removing the solvent from the bath can help to provide a substantially continuous / homogenous sheet of carbon nitride. This can, for example, be achieved by using a higher concentration of carbon nitride dispersion, leaving the solvent to evaporate for longer (for example by slowing the rate of deposition) and / or using a more volatile solvent. The skilled person will be able to balance these parameters depending on the embodiment they are performing.

[0103] Preferably, after the dispersion is applied, 20 vol% or more of the organic solvent is removed (compared to the amount of solvent applied), such as 50 vol % or more, or 80 vol % or more, such as 90 vol % or more, or 95 vol % or more, for example 99 vol % or more. The amount of solvent removed may be 99.99 vol% or less, such as 99 vol% or less, or 98 vol% or less, such as 95 vol% or less. The amount of solvent removed may be from 20 to 99.99 vol%, such as from 50 to 98 vol%. The solvent may be left to evaporate under atmospheric conditions, such as a temperature of from 15 to 30 °C for a period of time of 24 hours or less (e.g. from 10 minutes to 24 hours), such as 6 hours or less, preferably 4 hours or less, or 2 hours or less (e.g. from 10 minutes to 2 hours), for example 1 hour or less.

[0104] After application of the dispersion, and preferably after removal of the organic solvent, the surface area of the aqueous bath may be reduced, for example using a barrier, to compress (densify / concentrate) the carbon nitride on the surface of the aqueous bath. The method may comprise measuring the surface pressure of the aqueous bath, for example using a Wilhelmy plate. The surface pressure may be increased until it reaches a value of 2 mN m1or more, such as 3 mN m1, preferably 5 mN m1, such as 7 mN m1, or 8 mN m1. While optimal results are observed when the surface pressure is increased to about 8 mN m1, different surface pressure values provide different film thicknesses and allow the properties of the film to be tuned to the desired application.

[0105] The method may comprise contacting the substrate with the carbon nitride, for example by submerging the substrate in the aqueous bath, and lowering the surface of the aqueous bath (e.g. by removing the sub-phase / aqueous bath, described as the horizontal precipitation (deposition) method in Zhavnerko, G. K. et al., Physics, Chemistry and Application of Nanostructures: Reviews and Short Notes to Nanomeeting ’99; 1999; pp 218-220) and / or bringing the substrate up through the surface of the aqueous bath, such that the carbon nitride on the surface of the bath is deposited on the substrate. The substrate may be lowered onto the film to transfer the film onto the substrate.

[0106] The coated substrate may be dried, for example under atmospheric conditions, or in vacuo and / or under increased temperature (e.g. 25 to 200°C). The coated substrate may be washed (e.g. in water and / or hydrogen peroxide) and / or soaked in acid.

[0107] Other methods of applying the carbon nitride film may include vapour phase deposition, spray (e.g. ultrasonic, electrospray) or slurry coating. Aqueous solvents may be used for exfoliation when using these methods of application, for example wherein the pH of the aqueous solvent is acidic (e.g. pH 4 or less, such as 2 or less) or basic (e.g. pH 10 or more, such as 12 or more).

[0108] Preferably the method comprises applying the carbon nitride to the substrate by spray coating, especially ultrasonic spray coating. As shown by the Examples, spray coating can provide a relatively uniform film of the carbon nitride on a substrate. Spray coating may be performed onto a substrate. The concentration of the solution of carbon nitride used for spray coating may be O.Olmg / mL or higher, such as O. lmg / mL or higher, or 0.3mg / mL or higher. The concentration of the solution of carbon nitride may be lOmg / mL or less, such as 4 mg / mL or less, or l .Omg / mL or less. The concentration may be from 0.01 to 10 mg / mL, such as from 0.3 to l .Omg / mL. The solvent used to dissolve or disperse the carbon nitride may be as described above, especially organic solvents, and / or the solvent may be aqueous. The pH of the solvent (e.g. aqueous solvent) may be acidic (i.e. lower than 7.0, such as lower than 5.0), but preferably the pH of the solvent is basic (i.e. higher than 7.0, such as higher than 9.0).

[0109] A bed used to support the substrate may be at a temperature of 20°C, such as 80°C or higher, or 110°C or higher, or 130°C or higher. The temperature may be 250°C or lower, such as 200°C or lower, or 150°C or lower. The temperature may be from 20 to 250°C, such as from 110 to 150°C. The skilled person will understand that parameters such as the number of passes, the particular spray coating apparatus can be controlled, and the flow rate.

[0110] Preferred Embodiments

[0111] In a preferred embodiment: the thickness of the film is from 0.3 to 40nm (e.g. from 0.4 to 20nm); the smallest dimension of the film in the plane of the film is 100pm or more (e.g. from 1mm to Im); and the carbon nitride is a graphitic carbon nitride (e.g. comprising triazine and / or heptazine sub-units).

[0112] In a preferred embodiment: the thickness of the film is from 0.3 to 40nm; the smallest dimension of the film in the plane of the film is 100pm or more; and the carbon nitride comprises triazine and / or heptazine sub-units.

[0113] In a preferred embodiment: the thickness of the film is from 0.4 to 5nm; the smallest dimension of the film in the plane of the film is 100pm or more; and the carbon nitride is a graphitic carbon nitride.

[0114] In a preferred embodiment: the thickness of the film is from 0.4 to 5nm; the smallest dimension of the film in the plane of the film is from 1mm to Im; and the carbon nitride comprises triazine and / or heptazine sub-units (e.g. poly(triazine imide)).

[0115] In a preferred embodiment: the thickness of the film is from 0.3 to 40nm (e.g. from 0.4 to 20nm); the smallest dimension of the film in the plane of the film is 100pm or more (e.g. from 1mm to Im); the carbon nitride is a graphitic carbon nitride (e.g. comprising comprises triazine and / or heptazine sub-units); and the substrate is a support membrane, an electrode body or a battery separator membrane.

[0116] In a preferred embodiment: the thickness of the film is from 0.3 to 40nm (e.g. from 0.4 to 20nm); the smallest dimension of the film in the plane of the film is 100pm or more (e.g. from 1mm to Im); the carbon nitride is a graphitic carbon nitride (e.g. comprising comprises triazine and / or heptazine sub-units); and the substrate comprises a sulfonated tetrafluoroethylene based fluoropolymercopolymer, an electrically conductive form of carbon, a metal, an electrically conductive metal oxide, and / or a polyolefin.

[0117] In a preferred embodiment: the thickness of the film is from 0.3 to 40nm (e.g. 0.4 to 5nm); the smallest dimension of the film in the plane of the film is 100pm or more; the carbon nitride comprises triazine and / or heptazine sub-units; the substrate is a support membrane, an electrode body or a battery separator membrane; and the carbon nitride film covers 90% or more of the surface area of at least one side of the substrate.

[0118] In a preferred embodiment: the thickness of the film is from 0.4 to 5nm; the smallest dimension of the film in the plane of the film is 100pm or more (e.g. 1mm to Im); the carbon nitride is a graphitic carbon nitride (e.g. comprising triazine and / or heptazine sub-units (e.g. poly(triazine imide))); and the carbon nitride film covers 90% or more of the surface area of at least one side of the substrate.

[0119] Examples

[0120] The use of polytriazine imide as a proton selective PEM barrier film in fuel cells was explored. Langmuir deposition was first developed to deposit an ultra-thin layer of PTI nanosheets as a dense film onto a PEM substrate. The Langmuir approach nullifies high energy vapour deposition and can be applied to a vast library of 2D materials. The performance of the 1-5 nm thick PTI film in fuel cells was evaluated based on their resistance, power density, selectivity, and durability. Activation energy analysis provided some fundamental understanding to the transport process of proton conduction and environmental factors highlighted. PTI was demonstrated to not only be transparent to proton transport but benefitted the PEM by improved water management, whilst inhibiting parasitic crossover of gases, peroxides and contaminants resulting in a threefold increase in cell lifetime. Degradation mitigation shows huge potential of this approach to provide devices with lifetimes that can overcome the challenges of hydrogen technologies for a relatively facile and cheap material addition.

[0121] Characterisation

[0122] AFM images of films held with carbon tape were produced by operating a Bruker Dimension Icon AFM in tapping mode with an SNL-A probe (nominal tip radius 2 nm, resonance frequency: 65 kHz, spring constant: 0.35 N m1) with scan rate 0.5 Hz and 512 scans per line. Ex-situ cross sectional imaging used a Keyence Digital Microscope VHX-7000. A 20 x 10 mm section of MEA from the centre of the active area was embedded in epoxy resin, dried under vacuum for 24 h and cross-sections mechanically polished by sequentially reducing grit size down to 6 pm to achieve a mirror-like surface.

[0123] Electrochemical impedance spectroscopy (EIS) measurements were conducted using a Scribner 885 Potentiostat with frequency response analyser at different temperatures and relative humidity (RH) with H2 / air (0.3 L min ’ ) and ambient pressure. Impedance spectra were obtained at constant current densities of 0.2 and 0.5 A cm'2over a frequency range of 0.1 MHz to 0.1 Hz with 10% AC signal amplitude and recording 10 points per decade. The high frequency resistance (HFR) was used to calculate proton conductivity, σ, of the PEM using the following equation: σ=PEM Thickness / (PEM areaxHFR)

[0124] The activation energy Eaof proton conduction was derived by fitting the data In σ versus T-1. The temperature dependent proton conductivity is fulfilled for a semiempirical Arrhenius equation σ=Aexp( / Ea / RT). where σ is the proton conductivity (S cm-1), A is a pre-exponential factor, Eais the activation energy of proton migration (kJ mol-1), R is the universal gas constant (8.31 J mol-1K-1), and T is the absolute temperature (K).

[0125] AST protocols were performed on cells that had been operated for ~1.5 days with beginning of life (BoL) tests. PEM AST protocol 1 was operated at 90 °C with cycles of alternating wet (45 s) and dry (30 s) gas streams of H2 / air flow at 0.2 L min1whilst OCV was constantly measured. Every 1000 cycles, CV sweeps (0.06 - 1 V at 20 mV s’1) and polarisation curves were used to regenerate catalyst poisoning and obtain cell performance before measuring hydrogen crossover with LSV (0.06 - 0.6 V at 2 mV s1) . For PEM AST protocol 2 the cell was held at 90 °C whilst gas inlets supplied H2 / air at 30% RH (62 °C) for 100 h, whilst OCV drop was monitored, after which the previous regeneration steps were used and polarisation curve and LSV obtained.

[0126] Preparation of Exfoliated PTI Solution

[0127] PTI H2O was prepared by first undertaking the molten salt synthesis of PTI LiCl and subsequent Soxhlet extraction in deionized water to remove Li+and Cl’ ions, as described in the following sections.

[0128] Synthesis of PTI LiBr and PTI LiCl

[0129] PTI LiBr and PTI LiCl were synthesised by a procedure based upon that first reported by Bojdys et al. Eur. J., 2008, 14, 8177-8182. Dicyandiamide (2 g, Sigma Aldrich) and a eutectic mix of KBr (4.8 g, Alfa Aesar) and LiBr (5.2 g, Aldrich), or KC1 (5.5 g, Aldrich) and LiCl (4.5g, Aldrich), were thoroughly ground together by hand under an N2 or Ar atmosphere. This mix was packed into an alumina crucible and placed under an inert N2 flow and heated in a tube furnace to 400 °C for 6 hours (10 °C min’1ramp rate, Carbolite, MTF 12 / 38 / 250). This process allowed ‘pre-synthesis products’ to form, with the release of NH3. The resultant solid mass was then placed in a glovebox and again ground by hand. Approximately 1 1.5 g of this powder was transferred, under Ar, to a quartz ampoule (OD 23mm, ID 20mm, Multilab Ltd) that was evacuated to 10’6mbar using a turbo pump (Leybold vacuum systems, PT70G compact). The tube was then flame-sealed to form an ampoule under static vacuum, placed in a chamber furnace (Carbolite, CWF1 1 / 13) and heated to 550°C or 600 °C (ramp rate 2 or 10 °C min1) where it was held for 12 hours before being cooled to room temperature (ramp rate 10 °C min1). The solid PTI product was recovered by washing the solidified salt block multiple times with distilled water (Millipore) to remove the soluble salt component, followed by ethanol, before it was dried using a rotary evaporator.

[0130] De-intercalation of PTI LiX to form IF-PTI

[0131] The intercalated ions were removed from the PTI LiX compounds via Soxhlet extraction (E. J. McDermott et si., J. Phys. Chem. C, 2013, 1 17, 8806-8812). In each extraction ~1.5 g of PTI was loaded into a cellulose thimble (Whatman Cellulose extraction thimbles, OD 27 mm, ID 25 mm, length 80 mm) and was typically washed for 20 days with hot distilled water. The resulting intercalant-free (IF-) PTI was filtered, then washed several times with ethanol, before being dried at 150 °C at 10’2mBar and stored under an inert atmosphere.

[0132] A high concentration solution was prepared by grinding PTI powder in pestle and mortar for 20 mins followed by ultrasonication in an amount of 0.2 mg / ml in N,N- dimethylformamide (DMF, Fisher Chemical, 99.8%) at 60% amplitude for 10-15 min. Longer timeframes were avoided to prevent damage to the PTI sheets. The solution was centrifuged at 3000 rpm for 20 min allowing unexfoliated PTI to sediment. This ultrasonic exfoliated PTI remained stable in solution for over a year.

[0133] AFM of deposited aliquots confirmed the solution to contain good quality flakes with the pseudo-hexagonal shapes of 40 - 200 nm lateral dimension and around 0.3-0.5 nm height. This indicates that single sheets of PTI were formed.

[0134] Preparation of PTI Film by Langmuir method

[0135] Due to Langmuir monolayer sensitivity to surface active impurities, all parts of the Langmuir trough were cleaned by wiping with ethanol followed by dichloroethane (DCE) and rinsed with ultrapure water (18 MQ). Approximately 120 mL of milliQ water was added to the trough and the surface was cleaned by a vacuum aspirator. Figure 1 of the accompanying drawings shows a schematic illustration of an optimised strategy for PTI deposition, and the single barrier Langmuir method-based approach to coat substrates with PTI film.

[0136] The deposition solution was prepared by adding 1,2-dichloroethane (DCE, 99.5%, for electronic use, residue free, ACROS Organics) to the PTI / DMF solution with a volumetric ratio of 1 :6 PTI / DMF:DCE.

[0137] 8 mL of the resulting dispersion was dripped onto the water surface at a flow rate of 0.05 mL min1over a period of 160 mins, controlled by a syringe pump (KD Scientific) using PTFE tubing with 0.312 mm outer diameter.

[0138] The flow rate was sufficient to encourage the solvent to evaporate / disperse into the water before further dispersion is added. 0.01 ml min1was found to be too slow as it allowed the DCE to evaporate from the forming droplet while it was emerging from the tip of the tubing, resulting in different solvent ratio deposition.

[0139] A 2 h pause between deposition and compression was thus chosen to allow complete solvent loss and surface stabilisation, without allowing detrimental PTI change (e.g. aggregation) over time.

[0140] The surface pressure was monitored using a platinum Wilhelmy plate contacting the water surface. During deposition of the exfoliated PTI / DMF solution, the surface pressure increased slowly as more material was added and accumulated on the surface. Local spikes in pressure were observed, caused by the dripping solution that rippled the water and the spreading pressure of the DCE that is capable of forcing previously deposited material to the outer edges of the trough.

[0141] After transferring the desired amount of dispersion, the film was compressed by encroaching the barrier at a speed of 15 mm min1until the surface pressure increased to about 8 mN m1. Films have successfully been made when the surface pressure was increased to values from 3 to 30 mN m1. This coincided with a ‘gas to liquid’ phase transition of the surface PTI, providing a dense film that could be deposited onto substrates (e.g. mica, Nafion). The resulting film was then deposited onto a substrate (e.g. micra or Nafion®) by the horizontal precipitation (deposition) method (Zhavnerko, G. K. et al., Physics, Chemistry and Application of Nanostructures: Reviews and Short Notes to Nanomeeting ’99; 1999; pp 218-220). Therefore, a vacuum aspirator was used to slowly remove the water from outside the deposited area and the PTI film was deposited on the substrate surface. The water was slowly removed to lower the film onto the substrate placed beneath the sub-phase. The resulting coated substrate was dried under atmospheric conditions.

[0142] AFM and XRR investigation revealed a densely covered surface film 2 - 5 nm thick, showing that that the Langmuir method could successfully deposit PTI on the water surface and compression could achieve reasonable surface coverage.

[0143] Due to the highly regular quasi-hexagonal shape and small lateral size of PTI sheets a dense film is achieved with >99.5% surface coverage. Edge functionality of PTI sheets with ‘acidic’ NEL and ‘basic’ triazine N groups may permit favourable acid-base interactions that allow close PTI contact upon compression. In addition, the shape is well suited to achieve a tessellated structure of sheets that form a sheet with few voids. Individual PTI flakes were hard to resolve, likely due to the extremely close association of adjacent sheets and low roughness of the compressed film.

[0144] Based on 0.3-0.5 nm AFM height of individual PTI particles, the film was likely composed of 3 - 10 sheets, which could be due to deposition of multilayer stacks or favourable restacking of single sheets on the water surface during compression. Based on aggregate overlap, the thicker film was likely a result of compression-induced multilayering. Linear features composed of PTI particles with greater height were observed that were likely the edge of these larger aggregates of PTI sheets that have constructively compressed. When an appropriate time was used to remove solvent, these aggregates compressed with close packing to achieve a dense film of several PTI layers. This alludes to a level of film thickness tuneability possible from adjusting compression parameters.

[0145] Exfoliated PTI was deposited on Nafion® using a single barrier, double the deposition volume, and larger starting area to ensure a compressed surface film of appropriate size to cover a Nafion® substrate, as shown in Figure 1. To deposit onto the membrane, Nafion-211 (N211) was equilibrated in water for 24 h, to ensure complete water uptake and swelling, before being held in the trough under the water subphase. Successful film formation and transfer to N211 with surface coverage >99% was demonstrated on three randomly selected regions of the coated membrane. The hydrophilic surface of Nafion® ensured favourable interaction with the PTI covered water surface ensuring facile coating as the water was removed.

[0146] AFM topography images of PTI coated N211 showed complete coverage of the surface due to formation of a dense PTI film on the water surface that was transferred to the Nafion® support. Due to the minute size of PTI particles, suspended on a relatively rough polymer substrate resolving individual PTI flakes was not facile. However, a surface dissimilar to pure Nafion® was revealed with material build up seen to be slightly heterogenous and out of plane formation of what appear to be raised PTI aggregates and randomly distributed -200 nm particles that could be less exfoliated sheets. Additional ‘holey’ regions were observed that represented a surface coverage on top of a subphase, i.e., PTI on Nafion®. Particles of 20 - 40 nm diameter were observed, attributed to close packed PTI sheets.

[0147] Figure 2 of the accompanying drawings shows SEM images of the PTI film coating the Nafion® substrate at three different magnifications. The images show that the PTI formed a consistent layer on the Nafion® substrate. Electron beam damage resulted in free flowing polymer substrate, which caused a slow shift of the surface PTI layer.

[0148] Preparation of PTI Film by Ultrasonic Spray Coating

[0149] A 10 mb, 0.5 mg / ml solution of PTI in DMF was prepared in accordance with the methods described above, and ball milled using 3 mm zirconia balls for 10 hours at 500 rpm, with a ten minute operating time and a ten minute dwell time. The zirconia balls were sieved out and the PTI solution centrifuged at 4000 rpm for 30 minutes (Sigma 2- 16P). The sediment was removed and the remaining solution sonicated using a sonic horn (Fisherbrand 505) at a power of 20% and pulse rate of 5 seconds for ten minutes.

[0150] To deposit the PTI particles, the PTI solution was spray deposited through an ultrasonic nozzle with flow rate of 0.1 ml min1(SonoTek Exactacoat ultrasonic spray coater). The shaping air was held at 1.5 psi and ultrasonic head power held at 5 W to optimise atomisation. The heated bed was set at 140 °C and the PTI films were sprayed onto substrates including: silicon, Nafion, Aluminium current collector, Celgard separator. 1 - 10 spray passes (each depositing 1.6x 10-3mL cm'2) were used to build up PTI layers having a total thickness of 5 - 30 nm.

[0151] Fuel Cell Assembly and Testing

[0152] To obtain clean, activated composite films, PTI-coated Nafion® N211 or commercial N211 was submerged in the following, deionised (DI) water at 80 °C for 1 h, H2SO4(IM) at 80 °C for 2h, DI water at 80 °C for 2h and allowed to cool from 80 to 25 °C over the course of several hours before removing the film and drying. The PEMs were dried in appropriate frames at room temperature. Membranes were then sandwiched between commercially available Hyplat gas diffusion electrodes (0.4 mgPt cm-2) with PTI contacting either the anode or cathode and hot pressed at 135°C and 440 PSI for 3 mins to form the MEAs. MEAs with 5 cm2active area were assembled in single-cell hardware (Scribner, USA) with a single channel serpentine flow field, using 4.3 N m torque and Teflon gaskets to achieve ~ 25% thickness GDL compression. An 850e fuel cell test station (Scribner Associates, USA) was used to control cell and gas line temperature, gas flow of H2 and air (0.3 L min-1) feeds at the anode and cathode, respectively. The MEA was activated with constant voltage holds between 0.6 - 0.5 V with retention of these cell voltages until a steady current density was observed. Beginning-of-life PEMFC performance data were collected under controlled galvanostatic discharge steps. The internal ohmic resistance was estimated using the 850e instrument’s internal current interrupt method. The hydrogen crossover was obtained from the LSV technique at 80 °C with H2 and N2 gas flow (0.2 L min-1) at anode and cathode, respectively. LSV was recorded from 0.06 to 0.6 V at a scan rate of 2 mV s-1. Cell cathode was used as the working electrode and anode was the counter and reference electrodes. The maximum current at 0.4 V was used to quantify the hydrogen crossover of PEM.

[0153] Figure 3 of the accompanying drawings shows AFM topography images obtained on silicon substrates coated in PTI following the above spray coating procedure, using 1, 2 or 5 passes. The image following 1 pass highlights effective dispersal of PTI from ultrasonic spraying. The images following 2 and 5 passes show successive layers of dispersed PTI building up to form a dense PTI film that completely coats the substrate. The skilled person will be able to control the spray coating parameters to ensure that a film is formed that completely coats the substrate.

[0154] Figure 4 of the accompanying drawings shows AFM topography images obtained on a Nafion® substrate coated in PTI following the above spray coating procedure, using 2 passes. The right-hand image is a magnified version of the left-hand image. This further highlights effective dispersal of PTI onto a substrate from ultrasonic spraying. The images show that successive layers of dispersed PTI can be built up to form a dense PTI film that completely coats the substrate. The Nafion® substrate is an example of a membrane that could be used in commercial applications of the present invention. The skilled person will be able to control the spray coating parameters to ensure that a film is formed that completely coats the substrate.

[0155] Figure 5 of the accompanying drawings shows AFM topography images obtained on an aluminium substrate coated in PTI following the above spray coating procedure, using 2 passes. The right-hand image is a magnified version of the left-hand image. This further highlights effective dispersal of PTI onto a substrate from ultrasonic spraying. The images show that successive layers of dispersed PTI can be built up to form a dense PTI film that completely coats the substrate. The aluminium substrate is an example of a membrane that could be used in commercial applications of the present invention, for example as a current collector, such as in a cell or battery. The skilled person will be able to control the spray coating parameters to ensure that a film is formed that completely coats the substrate.

[0156] The films shown in Figures 3-5 are topologically planar, showing that the carbon nitride (PTI) is present in a substantially uniform layer comprising abutting and / or overlapping carbon nitride nanosheets. The films shown in Figures 3-5 are more smooth and more continuous than films of the prior art.

[0157] PTI Coated Nafion as Fuel Cell Barrier Membrane H2 / air fuel cells (5 cm2) were assembled using Hyplat GDEs with either; a bare N211, N211 with a coating PTI film contacting the cathode (N21 I PTI Cath) or anode (N21 l_PTI_An). Figure 6 of the accompanying drawings shows a schematic representation of MEA assembly of N211 (left), N211_PTI_An (centre) and N21 I PTI Cath (right) using N211 or PTI coated N211. Beginning of Life FC Performance

[0158] Beginning of life (BoL) performance of N211 PTI Cath and N211, in a H2 / air cell, without backpressure, was investigated vs temperature from 60 - 80 °C, and reduced RH of 50 and 25% RH at 80 °C.

[0159] Figure 7 of the accompanying drawing shows single cell FC testing of (a) N211 PTI Cath and (b) N211 showing polarisation curve and power density plot with H2 / air (stoichiometric control of 1.5:3) vs T at 100% RH and vs RH at 80 °C; (c) single cell FC polarisation curve and power density plot comparing N211 and N21 I PTI Cath with H2 / air (stoichiometric control of 1.5:3) at low RH and 80 °C; and (d) LSV conducted at 80 °C and 100% RH with H2 / N2 0.2 L min1for three separate MEAs containing N211 or N211_PTI to obtain beginning of life crossover current density at 0.4 V.

[0160] N211 shows increased power density with temperature up to 70 °C, with a peak power density of 0.851 W cm'2, at which point performance plateaus before dropping for 80 °C.

[0161] In contrast, N21 I PTI Cath continues to increase to a maximum for 80 °C. At this optimal condition, performance exceeded requirements of several DOE targets with a maximum power density of 0.923 W cm"2and current draw of 0.302 A cm"2at 0.8 V. In addition to higher attainable power densities with PTI, operation at higher temperatures has several advantages, such as faster reaction kinetics and increased carbon monoxide tolerance of the cell.

[0162] This high current draw was strong evidence that proton transport was unimpeded by the PTI, confirming that the high density of pores, 1.6 x 1014pores cm"2, with electron density voids of ~3.4 A allowed the translocation of protons.

[0163] BoL H2 crossover, taken at 0.4 V on the linear sweep voltammogram (LSV), found PTI addition to reduce crossover by over half from 2.7 to 1.3 mA cm-2. Adding this 2 - 5 nm thick film of PTI had the equivalent effect of adding ~25 pm of Nafion (N212 crossover current density = 1 - 2 mA cm"2) which is evidence of significant gas blocking capability, per PTI sheet. H2 and O2 molecules with kinetic diameters similar to the electron density gap in PTI, 2.9 A and 3.5 A, respectively, likely have a steric barrier to penetration.

[0164] Whilst water (which is smaller than either gas molecule at 2.7 A) has been shown to intercalate within bulk PTI, this is permitted due to the favourable hydrogen bonding interactions formed upon reorientation of the H2O molecule. Non-polar gases of H2 and O2 will interact unfavourably with the polar N-H decorated pores and unlikely achieve energy minimisation for crossover.

[0165] H2 permeation of PTI has previously been observed with strong temperature dependence with an application relevant permeance observed only at 250 °C and an order of magnitude lower permeance at 100 °C. Crucially, the large extent of pores occupied by H2O will exclude any possible gas translocation. In addition, strongly interacting multilayer stacking of PTI sheets and overlap of adjacent sheet edges results in restricted gas diffusion occurring around individual PTI sheets. The dense packing of 2 - 3 sheets via multilayering adds an additional barrier to penetration exceeding that of the simple case of single layer sheets and the gas molecules must overcome the adsorption barrier several times to translocate the film. The small H2 crossover is therefore hypothesized to be permitted mainly by film imperfections (<1 % of surface), that could be reduced by further Langmuir deposition optimisation, or through MEA defects from hot pressing or assembly of gaskets and bipolar plates, which would have a diminishingly small effect as MEA size increased for commercially relevant cells (>100 cm2).

[0166] LSV carried out for N211 and N211_PTI showed no temperature dependent crossover, showing that in this cell environment PTI is able to block gas crossover at operational conditions.

[0167] Low RH Performance

[0168] Reduced RHs of 50 and 25% had less significant impact on performance of N211 PTI Cath compared to N211 (Figure 7c).

[0169] Figure 8A of the accompanying drawings shows EIS of FCs (H2 / air) carried out at 200 mA cm"2for N211 vs N211 PTI Cath. Figure 8B of the accompanying drawings shows the HFR obtained for each MEA at each RH. EIS revealed lower PEM ionic resistance and cathode charge transfer resistance for N21 I PTI Cath, becoming more prominent at 50 and 25% RH. The hydrophilic PTI film is therefore likely interacting with water, improving water management within the cell at low and optimal RHs. N211 performance dropped from 70 to 80 °C, a result of exceeding the membrane dehydration limit, and thus reducing conductivity, whereas a hygroscopic PTI film likely increased water retention in this region for facile proton conduction and ORR.

[0170] Excess water content may also impede reactant diffusion and block active sites, especially on the cathode, increasing mass transport losses. PTI may also control electrode flooding by restricting transport of water from electroosmotic drag into the cathode and drawing water generated from the cathode into the PEM. This has been observed previously for contact of hydrophilic SiCU with ionomer at the cathode and anode, and has been shown operate via a bifunctional water trapping and absorbing mechanism to improve FC performance operated under low and high humidity.

[0171] Utilising the PTI film at the PEM-cathode interface thus improves water management and reduces crossover resulting in a greater performing FC, especially at high current density where water management is critical.

[0172] The ORR at the cathode generates water, whilst electroosmotic drag causes a hydration gradient increasing towards the cathode and dehydrates the anode. Following a reducing HFR and cathode charge transfer (RCT), cell performance increased in the order N211 < N21 I PTI Cath. This suggests that the proton translocation mechanism through PTI was facilitated by presence of water. PTI therefore had a symbiotic relationship with the cathode, with PTI contributing to water management whilst experiencing reduced proton transport resistance in a hydrated state regardless of low or high RH operation of FC.

[0173] Activation Energy

[0174] PTI coated Nafion® samples have a large step in performance gain when increasing temperature from 60 to 70 °C. This suggests an additional conduction mechanism is being initiated or resistance removed. Resistance / conductivity of the PEM will likely be dominated by Nafion®, but changes in temperature, i.e., activation energy (Ea), may resolve contributions from the PTI component. Earefers to the energy required for protons to overcome an energy barrier and move through a material. Thermal energy of the proton increases with temperature, making it easier to overcome the energy barrier and translocate the material. A change in structure, such as defects or vacancies can affect the activation energy by introduction or removal of additional energy barriers.

[0175] EIS was performed vs temperature to resolve greater detail of the material resistances and conduction mechanism within the cell. HFR, due to the PEM resistance was converted to conductivity. N211 followed a linear relationship from 60 to 90 °C with an Eaof 2.5 kJ mol1, close to that obtained of Nafion within a hot-pressed MEA environment. When using a single linear fit N21 I PTI Cath had an Eaof 5.1 kJ mol"1. This almost exactly matches the previously found Eaof water translocation through PTI voids (5 kJ mol"1). Due to the previously observed performance and resistance step at intermediate temperatures, N211_PTI samples were also fit using two linear fits either side of the observed conductivity transition. Despite the expected extra resistance added by PTI with a previously observed conductivity of 1-5 mS cm"1at 25 °C. Such a thin film in contact with the cathode barely impacts the overall conductivity; the single layer PTI film possesses minimal proton resistance within the cell. In addition, the favourable impact of PTI on water management of the adjacent N211 may improve in-situ conductivity of N211. At 75 °C N21 I PTI Cath had a conductivity and Eaincrease of 5.6 kJ mol"1. Highly hydration dependent resistance, and an Easimilar to the water translocation process within PTI reveal a through PTI proton conduction mechanism directly linked with water dynamics inside PTI voids. DFT simulations recently revealed hydroxyl terminated hydrogen-bonding networks on graphene vacancies to facilitate proton transfer through the defect centre via a Grotthuss mechanism before release on the opposite side. In the presence of an acidic environment and electrochemical gradient, the intercalated H2O in PTI could contribute to conductivity by obtaining a proton from one side and translocating the PTI void as a hydronium ion before hydrogen bond rearrangement results in proton association with water molecules on the opposite side and overall proton transport through the void.

[0176] The observed change in Eaprovides evidence that ion transport occurs through voids in the structure of the carbon nitride, rather than around particles of the carbon nitride. Therefore, the present invention allows the benefits discussed herein to be controlled in a way that previous technologies have not achieved. FC Durability

[0177] The MEA in an operating FC is subject to a harsh environment with high temperatures and humidity variations inducing polymer deformation and creep which lead to pinholes and cell failure. PEM degradation can be compounded by chemical attack over extended operation with radical / peroxide attack. Peroxyl (OOH«) and hydroxyl (OH«) radicals are generated at the anode or cathode as a result of reactant crossover and direct (electro)chemical reaction of H2 and O2 into H2O2 at either electrode. Direct combustion of oxygen and hydrogen also releases heat that can generate hotspots that thermally degrade local regions of the Nafion® accelerating further crossover. Chemical degradation of the PEM, due to reactant crossover, is most severe under OCV as reactant partial pressures are at their maximum due to a lack of consumption of reactant gases. These degradation processes were studied using several reported accelerated stress testing (AST) protocols (Rodgers et al., Chem. Rev. 112, 6075-6103 (2012)).

[0178] A high temperature cycled wet-dry gas feed protocol (AST protocol 1) was applied to examine effects of mechano-chemical stress. As the Nafion® swells and shrinks the overall film will contract and compress against the electrodes. Degradation was characterised by hydrogen crossover after 1000 cycle intervals, with a crossover current of 15 mA cm'2understood to signify significant PEM degradation, such as pinhole formation.

[0179] Figure 9 of the accompanying drawings shows a plot of OCV vs duration of AST protocol 1, where the cell was held at 90 °C with gas lines alternating between dry (30 s) and wet (45 s), continued until crossover current density exceeded 15 mA cm'2.

[0180] Figure 10 of the accompanying drawings shows plots resulting from AST protocol 1 with (a) irreversible OCV loss and (b) crossover current density plotted against number of AST cycles. AST protocol 2 of 90 °C and 30% RH for 100 h with (c) OCV drop plotted vs time and (d) crossover current density before and after the 100 h AST protocol (N211 repeat was not plotted as crossover exceeded potentiostat capability with >400 mA cm'2at 0.4 V).

[0181] The OCV of each cell vs time is shown in Figure 9 and the OCV and crossover current density after each 1000 cycle period is shown in Figure 10a and 10b. During RH cycling, spikes are repeatedly observed in the OCV due to flooding and drying of the MEA, observed as a -0.03 V fluctuation. A plateauing OCV drop with time / cycles was a result of loss of catalyst surface area and electrode mixed potentials due to crossover. Regeneration steps and polarisation curves were run after 1000 cycles to isolate the reversible and irreversible degradation of the FC. OCV regeneration occurred due to polarisation ‘cleaning’ catalyst active sites that become oxidised or bound to contaminants and fixed RH stabilised PEM and electrode hydration.

[0182] Both reversible and irreversible degradation occurred at far greater rates in N211.

[0183] Increasing crossover was direct evidence of the detrimental effects of gas crossover and resulting active radicals on PEM with irreparable PEM damage occurring between 2000 - 3000 cycles.

[0184] As shown in Figure 10a and 10b, the regeneration steps of N211 PTI MEAs show good recovery of OCV and little increase in crossover for the first several thousand cycles, suggesting that most degradation of the electrodes is reversible.

[0185] After -6000 cycles there appeared to be an increasing amount of irreversible degradation, with irreparable damage initiated in N21 I PTI Cath at an average of 10,000 cycles: over three times longer than N211.

[0186] Due to crossover and shorting after cycling for N211, ECSA could not be reliably attained, whilst N211 PTI achieved stable ECSA for the first 4000 / 5000 cycles before a gradual decrease upon further cycles.

[0187] Durability improvements were observed when the PTI contacted either the cathode or the anode.

[0188] Durability improvements appeared to be better when PTI contacted the cathode, compared to when the PTI contacted the anode. This could be a result of H2 having a greater propensity to crossover than O2 (~2x greater) and therefore the dominant mechanism of degradation where H2 directly reacts with O2 originating at the cathode. Whilst PTI coating on either electrode limits crossover of H2, peroxides may also be blocked by the PTI and stopped from passing into the Nafion® where polymer breakdown leads to sequential degradation processes.

[0189] A second AST protocol (AST protocol 2) held the cell at 90 °C with gas feeds at 30% RH for 100 h. Low humidity causes the Nafion® to become brittle and fragile, while decreased water content lowers the pH which facilitates radical formation.

[0190] As shown by Figure 10c, whilst N21 I PTI Cath had a gradual loss in OCV, N211 experienced several step-like drops that resulted in a 3 and 7 times greater OCV loss in two separate measurements.

[0191] After cyclic voltammetry (CV) sweeps, polarisation curves and potential holds, used to regenerate the catalysts active sites and rehydrate the PEM, reducing free volume for gas transport, the OCV increased to 96% of BoL for N21 I PTI Cath.

[0192] This shows that the high temperature treatment had minor long-term effects on the modified cell membrane.

[0193] In contrast, N211 had further OCV loss with regenerative steps, evidence that stepped OCV drops observed in the AST was direct observation of PEM damage.

[0194] After the 100 h AST, crossover current density increased by a factor of 58 for N211 whilst N211 PTI only had a 2.5-fold increase (Figure lOd).

[0195] The appearance of MEA cross sections of N211, N21 I PTI Cath and N211_PTI_An after the presented AST protocols were compared to a BoL MEA containing N211.

[0196] The PEM was visible as the central dark line and sandwiching CL and MPL visible as lighter surrounding. In both AST protocols, N211 thins heterogeneously compared to BoL. The 100 h AST resulted in a 50% average thickness reduction with some regions undergoing extreme thinning to 25% of BoL. In correlation with OCV and crossover, the 3000 AST cycle MEA degrades slightly less retaining 50 - 75% of the PEM thickness. High initial gas crossover for N211 results in rapid chemical degradation that thin the PEM and sequentially results in further crossover and degradation finally resulting in cell failure. Additional loss of material was visible in the CL / MPL possibly a result of chemical degradation at the source of radical formation or due to hot spots.

[0197] In contrast, N211 PTI after 100 h low RH AST had only slightly reduced PEM thickness, whilst some deviation from a flat film was observed under 1000x magnification due to the compressive forces of bipolar plate land and channel, amplified during high temperature and low RH conditions. These minor changes result in a slightly increased crossover after 100 h AST, whilst retaining a good FC performance. The suppressed gas crossover from the PTI barrier inhibits the formation of reactive oxygen radicals and thus reduced membrane thinning and CL decomposition occurs.

[0198] High crossover after 7000 and 12,000 AST cycles for N211_PTI_An and N21 I PTI Cath, respectively, in AST protocol 1 suggested that PTI slowed the inevitable degradation processes.

[0199] Despite minor thinning of the N211 (5 - 20%), the MEAs still appeared to be healthy, with homogenous PEM films and retained catalyst and ionomer in the electrodes. Closer inspection of OCV vs AST cycles showed little change until the final 1000 cycles where significant OCV loss and corresponding crossover occurred.

[0200] This suggests that PTI successfully mitigated effects of chemical degradation throughout the cell life and did not degrade in the extreme environment at the electrode - electrolyte interface. Localised processes that occur where incomplete PTI transfer leaves a bare section of N211 can permit a relatively large amount of crossover that results in a pinhole propagating through the whole membrane and large parasitic gas crossover results in sudden OCV drop. A single pinhole is unlikely to be observed in the cross section of only one slice of the active area.

[0201] This confirms the major contribution that a PTI film can have on cell durability and necessitates the further exploration of transfer efficiency to further extend cell lifetime.

[0202] AST experiments conclusively showed improved durability when employing the PTI barrier film with a 3 - 4-fold increase in cell lifetime. Figure 11 of the accompanying drawings shows BoL vs EoL FC performance at optimal condition (100% RH, 80 °C for N211_PTI and 70 °C for N211) using AST protocol 1 with loss in current density at 0.6 V highlighted in polarisation curve (left) and maximum power density decay shown in the power density plot (right).

[0203] FC performance at BoL and end of life (EoL) in Figure 11 show that even with over double the AST duration the EoL performance of N211 PTI had decreased considerably less than N211 with power density decrease from 0.923 to 0.741 W cm"2vs 0.851 to 0.513 W cm"2and current density decrease from 1.48 to 0.97 A cm"2vs 1.4 to 0.38 A

[0204] Despite similar crossover, a greater EoL performance in N22_PTI_Cath is likely a result of localised degradation that retains normal FC operation in >99% of the cell, whilst in N211 that fails far earlier, an overall thinner PEM with localised pinholes reduces overall efficiency of electrochemical reactions at each electrode.

[0205] In addition to gas crossover, the narrow channels in PTI likely suppress access of other harmful molecules and contaminants to the MEA. Hydrogen sulfide (H2S = 3.6 A) and carbon monoxide (CO = 3.8 A) are common catalyst poisoning species due to their strong affinity for metals. CO can access the cell as part of the hydrogen feed at the anode (10 ppm in the zero grade H2 used in these tests) or atmospheric air feed at the cathode and competes with reactant gases by favourably adsorbing to the platinum active sites, increasing reaction overpotential and resistance, being extremely detrimental to the cell performance and lifetime. Regardless of inlet electrode, CO content of as little as 2 ppm has been found to cross the membrane and result in additional degradation of the opposite electrode. Therefore, mitigation of crossover by PTI void rejection and isolating contaminants on either side of the MEA can half the negative impact of impurities and may contribute to the greatly retained OCV in PTI coated samples after ASTs.

[0206] Cationic impurities, such as Pt+that can dissolve in the high voltage conditions of the cathode and redeposit in the PEM, can bind with sulfonic acid groups with greater affinity than H+, considerably reducing proton transference and membrane hydration. In addition, Fe2+and Cu2+, arising from bipolar or end plate corrosion, can catalyse the reactions that result in radical formation, strongly accelerating the chemical degradation of membranes. Decreasing infiltration of metal impurities into the membrane can considerably increase long duration performance. Hydroxyl or aquo complexes of such metal ions larger than the PTI voids likely mitigate transfer of the dissolved ions into the PEM, and degradation due to these mechanisms was therefore slowed.

[0207] Conclusions

[0208] The present invention provides a cheap, low energy and facile deposition procedure to obtain a nanometre thick film composed of tessellated single layer PTI films.

[0209] A transfer efficiency of >100 m2g-1was achieved, with scope to increase using optimised deposition parameters. This is the first use of PTI as a proton conducting electrolyte material and achieved significant success in this application.

[0210] Incorporation of a 2 - 5 nm thick PTI film onto a Nafion® substrate was found to enhance cell performance with a maximum power density of 0.923 W cm"2at 80 °C surpassing commercial N211. A performance increase occurring around 70 °C suggests a temperature dependent conductivity mechanism operates through PTI.

[0211] PTI contact with the cathode benefited from greater hydration from the ORR that facilitated through void proton diffusion. Hydration facilitated conductivity supports the proposed theory of through void proton diffusion operating via a ‘Grotthuss type’ mechanism, reliant on a water bridge that transmits the proton from a hydronium ion on one side to a water molecule on the other side.

[0212] The PTI film effectively reduced reactant crossover that resulted in cell lifetime increases of over 300%, with EoL performance far outperforming EoL N211 and still exceeding BoL performance of many commercially available materials. This durability even exceeds that of 50 pm PEMs, whilst providing far greater power and current densities at beginning and end of life. With a transfer efficiency of 100 m2g-1, an equivalent durability and crossover reduction to using 1 g of PTI would require $10,000 worth of Nafion® (25 pm x 100 m2of Nafion®) costing, (-US$100 per m2).

[0213] The use of this barrier film directly facilitates the use of thinner PEMs for realistic applications. However, the potential applications for this PTI film to enhance performance and service life of a variety of devices is enormous. Cheap hydrocarbon PEMs that are more prone to radical degradation, anion exchange membranes that are prone to dehumidification, and electrolysers that have serious consequences of crossover and PEM failure could all benefit from a PTI barrier film. This tuneable approach could allow the deposition of PTI to various substrates for a multitude of applications, such as photocatalytic thin film devices. In addition, this film-layered composite fabrication procedure sets an approach for film-layered PEMs that can utilise a potentially infinite library of solution processable 2D materials for atom thick electrolytes.

Claims

CLAIMS1. A carbon nitride film, wherein the thickness of the film is from 0.3 to 40nm.

2. The carbon nitride film of claim 1, wherein the carbon nitride is crystalline.

3. The carbon nitride film of claim 1 or claim 2, wherein the graphitic carbon nitride comprises triazine sub-units.

4. The carbon nitride film of claim 3, wherein the carbon nitride is poly(triazine imide).

5. The carbon nitride film of any preceding claim, wherein the film has a proportion of N-C3 units, relative to C-N=C units of from 30% to 36%, as determined by X-ray diffraction.

6. The carbon nitride film of any preceding claim, wherein the carbon nitride film comprises pores encircled by from 12 to 30 atoms, as determined by X-ray diffraction.

7. The carbon nitride film of any preceding claim, wherein the smallest dimension of the film in the plane of the film is 100pm or more.

8. The carbon nitride film of claim 7, wherein the smallest dimension of the film in the plane of the film is from 1mm to Im.

9. The carbon nitride film of any preceding claim, wherein the thickness of the film is from 0.4 to 20nm.

10. A product comprising substrate and a carbon nitride film according to any preceding claim, wherein the film is in contact with the substrate.

11. The product of claim 10, wherein: a) the substrate is a support membrane, and the product is an ion exchange membrane; b) the substrate is an electrode body, and the product is an electrode; c) the substrate is a battery separator membrane, and the product is a battery separator; and / or d) the substrate is a battery current collector, and the product is a battery.

12. The product of claim 11, wherein the substrate is a support membrane that comprises a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.

13. The product of claim 11, wherein the substrate is an electrode body that comprises an electrically conductive form of carbon, a metal, and / or an electrically conductive metal oxide.

14. The product of claim 11, wherein the substrate is a battery separator membrane that comprises a polyolefin.

15. The product of any one of claims 10 to 14, wherein the substrate has a thickness of 500pm or less.

16. The product of any one of claims 10 to 15, wherein the carbon nitride film covers 99% or more of the surface area of an active region of at least one side of the substrate.

17. An apparatus comprising a product of any one of claims 10 to 16.

18. The apparatus of claim 17, wherein the apparatus is a fuel cell, a water electrolyser, a battery, a photoelectronic component, a catalyst, a filtration apparatus, and / or a stationary phase for chromatography.

19. A method of preparing a product, wherein the method comprises providing a substrate and applying a carbon nitride film to the substrate.

20. The method of claim 19, wherein the carbon nitride film is according to the first aspect.

21. The method of claim 19 or claim 20, wherein the carbon nitride film is applied to the substrate using spray coating.

22. The method of claim 19 or claim 20, wherein the method comprises the steps of:- providing a dispersion of carbon nitride in an organic solvent,- applying the dispersion to the surface of an aqueous bath,- separating the organic solvent from the carbon nitride so as to leave the carbon nitride on the surface of the aqueous bath,- reducing the surface area of the aqueous bath so as to densify the carbon nitride on the surface of the aqueous bath and so as to form the carbon nitride film, and- contacting the substrate with the carbon nitride film.

23. The method of claim 22, wherein the organic solvent comprises a solvent selected from the list consisting of: DCE, DMF, DMAc, methylene chloride, chloroform, ethyl acetate and isopropyl acetate.

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