Bipolar plate

Abstract

The present invention relates to a bipolar plate comprising a metal plate and at least one coating layer disposed on the metal plate, wherein the coating layer comprises a cured coating composition comprising: a) a matrix comprising at least one polymeric resin; and b) a plurality of conductive particles dispersed within the matrix; wherein a number of the conductive particles are arranged in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer.

Description

[0001] BIPOLAR PLATE

[0002] The present invention relates to bipolar plates and processes for the production thereof. The present invention further relates to energy-conversion devices comprising bipolar plates and the use of bipolar plates in the production of said devices.

[0003] Background

[0004] Bipolar plates (BPPs), also known as separators, are important components of energy-conversion devices such as electrolysers and fuel cells. In a typical protonexchange membrane fuel cell stack, the role of a BPP is to collect and conduct current and heat between single cells, separate and transport gases and fluids, and provide mechanical strength. BPPs represent up to 80% of the weight and about 30% of the cost of a typical stack assembly and are thus key components in reducing the cost and increasing the lifetime of energy-conversion devices.

[0005] The stability of BPPs in the low pH environment (e.g. as formed inside an operational fuel cell or electrolyser) is a major concern. In order to be effective in such environments, BPPs need to have good (high) corrosion resistance whilst simultaneously having good (high) electrical conductivity. These requirements restrict the range of materials suitable for making bipolar plates.

[0006] Traditionally, bipolar plates are produced from graphite or carbon-based composites. However, BPPs made of graphite generally have low mechanical strength, resulting in the need to use a relatively thick BPP design which leads to low power density. More recently, the use of metals (e.g. stainless steel) to produce bipolar plates has been suggested. Typical metals useful for making bipolar plates are however susceptible to corrosion in the harsh environment of a typical energy conversion device and must therefore be protected. Most often, but not exclusively, this protection takes the form of modifying the surface of the metal with noble metals, e.g. by coating the metal with a coating comprising one or more noble metals. Noble metals are however expensive. The application of protective corrosion-resistant coatings to metallic bipolar plates has been investigated previously.

[0007] United States Patent Application Publication No. US2011 / 0223522A1 discloses a bipolar plate for a fuel cell, comprising: a metal plate; and a coating layer disposed on the surface of the metal plate, the coating layer including a polymer of a specific oxazine-based compound and a conducting material.

[0008] European Patent Application Publication No. EP2234192A2 discloses a metallic bipolar plate for fuel cells, comprising: a metal plate; and a coating layer formed on the metal plate and having carbon particles dispersed throughout a volume of a matrix composed of a binder resin.

[0009] European Patent Application Publication No. EP1107340A2 discloses a PEM fuel cell having electrical contact elements comprising a corrosion-susceptible substrate metal coated with an electrically conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles.

[0010] United States Patent Application Publication No. US2004 / 0110058A1 discloses an electrochemical cell having a catalytic electrode and an electrically conductive contact element facing the electrode. The electrically conductive coating element is coated with an electrically conductive, corrosion resistant coating.

[0011] United States Patent Application Publication No. US2022 / 0064800 discloses a surface treatment method for a metal member involving a) imparting a charge to one region of the metal member; b) imparting, to a coating material (which is optionally a mixture of titanium particles and an epoxy resin), a charge that is opposite to the charge imparted to the one regions of the metal member; and c) forming a coating by applying the charged coating material to the one region of the metal member. The static charging of the metal member however will uniformly attract particles to the surface of the metal member which will not promote any spatial arrangement or alignment of particles, i.e. the particles are not arranged in particle assemblies (e.g. chains or strings) extending across the thickness direction of the coating. There still exists therefore a need for bipolar plates which are cost-effective yet characterised by high corrosion resistance and high electrical conductivity. In particular, the provision of a bipolar plate based on cost-effective metallic substrates (such as stainless steel) having maintained or even improved corrosion resistance and / or electrical conductivity would be highly desirable.

[0012] The present inventors have now provided a solution to the afore-mentioned problems in the form of a bipolar plate according to claim 1 . By coating the bipolar plate with a conductive particle-containing coating layer in which a number of the conductive particles are arranged so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways (e.g. chains or strings) extending across the thickness of the coating layer, the present inventors surprisingly established that the electrical conductivity of the coating is increased relative to a coating layer having an equivalent loading of conductive particles but where the particles are arranged isotropically (i.e. randomly). As a result, the present inventors established that it is possible to obtain the same or higher level of electrical conductivity with the bipolar plates of the present invention whilst using a lower loading of conductive particles. Since a large part of the cost of the bipolar plate is associated with the conductive particles used in the coating, this lower loading of conductive particles can reduce the overall cost of the bipolar plate.

[0013] Surprisingly, having the particles arranged so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer also leads to an increase in corrosion resistance. Without wishing to be bound by theory, this is the result of a decrease in the proportion of the surface area of the bipolar plate covered by the more corrosion- prone conductive particles and a corresponding increase in the proportion of the surface area of the bipolar plate covered by the less corrosion-prone matrix.

[0014] The present invention thus surprisingly enables the production of a cost-effective metallic bipolar plate having high corrosion resistance and high electrical conductivity, as well as the use of said bipolar plate in the production of energyconversion devices such as fuel cells, fuel cell stacks, electrolysers or electrolyser stacks. Summary of the Invention

[0015] In a first aspect, the present invention provides a bipolar plate comprising a metal plate and at least one coating layer disposed on the metal plate, wherein the coating layer comprises a cured coating composition comprising: a) a matrix comprising at least one polymeric resin; and b) a plurality of conductive particles dispersed within the matrix; wherein a number of the conductive particles are arranged in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer.

[0016] In a second aspect, the present invention provides an energy-conversion device, such as a fuel cell, fuel cell stack, electrolyser or an electrolyser stack, comprising at least one bipolar plate according to the first aspect of the present invention.

[0017] In a third aspect, the present invention provides the use of a bipolar plate according to the first aspect of the present invention in the production of an energy-conversion device such as a fuel cell, fuel cell stack, electrolyser or an electrolyser stack.

[0018] In a fourth aspect, the present invention provides a process for the production of a bipolar plate according to the first aspect of the present invention, the process comprising the steps of: i) applying to a metal plate a coating layer of a curable coating composition comprising: a) a matrix comprising at least one polymeric resin; and b) a plurality of conductive particles dispersed within the matrix; ii) applying an electric field between two alignment electrodes across the coating layer so as to cause a number of the conductive particles to arrange themselves in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer; and iii) curing the coating layer whilst the electric field is still being applied, e.g. by heating the coating layer and / or exposing the coating layer to UV light. Brief Description of the Figures

[0019] Figure 1 : A schematic diagram of apparatus used to measure the ex-situ ICR, with: a) Gold coated copper plates, b) GDL, c) Sample, d) Spring loaded pin.

[0020] Figure 2: Schematic diagram of 4-wire resistance circuit used in resistance measurement method.

[0021] Figure 3: Graphical representation of ICR as a function of particle loading (aligned vs non-aligned) for the samples shown in Table 1.

[0022] Figure 4: Graphical representation of ICR as a function of coating thickness for the samples shown in Table 2.

[0023] Figure 5: Current density as a function of time for samples in T able 3.

[0024] Figure 6: Electrical impedance spectroscopy (EIS) results for sample CE6.

[0025] Figure 7: EIS results for sample IE6.

[0026] Figure 8: EIS results for sample CE8.

[0027] Figure 9: EIS results for sample IE8.

[0028] Detailed Description of the Invention

[0029] The present invention relates to a bipolar plate, a process for the production of a bipolar plate, an energy-conversion device comprising a bipolar plate, and the use of a bipolar plate in the production of an energy-conversion device.

[0030] Definitions

[0031] As used herein, the term “particle assembly” refers to a structure comprising a plurality of conductive particles (i.e. 2 or more), wherein each particle in the structure is typically directly or indirectly connected (e.g. electrically connected) to each other particle in the structure. For example, the particle assemblies may be linear e.g. comprising or consisting of a chain or string of conductive particles. Alternatively, the particle assemblies may have a branched structure and / or form loops. Preferably, the aligned particle assemblies are chains or strings.

[0032] As applied to the plurality of particle assemblies in the coating layer, the term “aligned” refers to the fact that the particle assemblies typically have a principal direction (e.g. having one dimension which is substantially greater in length than the others, such as at least twice as long, such as at least 5 times as long, such as at least ten times as long, in some cases up to 100 times as long, as any other direction) and are arranged in the coating layer so that these principal directions are approximately co-linear. In a preferred embodiment, the plurality of particle assemblies in the coating layer are substantially parallel. The coating layer may be regarded as anisotropic.

[0033] For the avoidance of doubt, not all conductive particles in the coating composition need be part of a particle assembly; nor is it necessary that all particle assemblies be aligned particle assemblies. Rather, it is sufficient that a number (i.e. a proportion) of the conductive particles are arranged in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer. In a preferred embodiment however, substantially all (e.g. >95%, such as >99%) of the conductive particles dispersed within the matrix are arranged in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer.

[0034] Unless otherwise stated, all percentages (%) are weight percentages (wt%).

[0035] Metal Plate

[0036] The bipolar plate of the present invention comprises a metal plate. The metal plate provides structural support to the bipolar plate and typically makes up a majority of the weight of the bipolar plate. In one embodiment, the metal plate comprises, preferably consists of, a metal or metal alloy. In general, the choice of metal or metal alloy is not particularly limited provided that it has suitable mechanical strength, electrical conductivity and corrosion resistance. For example, any metal plate conventionally used in bipolar plates can be used, such as metal plates formed of steel (especially stainless- steel), Inconel®, nickel, titanium, aluminium, gold, silver or platinum, or a metal plate coated or clad with any one of these metals or a combination thereof.

[0037] The skilled person will understand that when certain metals or metal alloys are used, the metal plate will comprise a metal oxide surface layer. This is typically the result of natural oxidation e.g. caused when the metal plate is exposed to air. In one embodiment therefore, the metal plate comprises a body portion comprising a metal or a metal alloy and a surface portion comprising one or more metal oxides. In this case, the skilled person will understand that the metal of the one or more metal oxides in the surface portion will typically be the same as the metal(s) in the body portion.

[0038] In a preferred embodiment, the metal plate comprises, preferably consists of, a metal or metal alloy other than a precious metal or precious metal alloy, e.g. a metal or metal alloy other than gold, silver, platinum or palladium or alloys thereof. In a preferred embodiment, the metal plate is a steel plate, more preferably a stainless-steel plate, especially an austenitic stainless-steel plate. Steel plates have the advantage that they are relatively cheap and easy to mass-produce.

[0039] The metal plate is preferably formed of a single part but may alternatively be formed of multiple parts joined or otherwise connected together. The metal plate is generally flat but may comprise a flow field, e.g. a set of channels or grooves machined or stamped into the plate which are configured to allow fluids to flow over the face of the cell when the bipolar plate is used in an energy-conversion device.

[0040] The bipolar plate typically has two major faces - a first face configured to contact the anode-side of one cell in a stack of cells of an energy-conversion device; and a second face configured to contact the cathode-side of an adjacent cell. The average thickness of the metal plate is not particularly limited but is typically in the range of 10 to 1000 pm, preferably 20 to 500 pm, more preferably 50 to 250 pm. Coating Layer

[0041] In addition to the metal plate, the bipolar plate of the present invention comprises at least one coating layer which is disposed on the metal plate. The function of the coating layer is typically to protect the metal plate from corrosion whilst at the same time having a high enough electrical conductivity to allow current to flow between cells when the bipolar plate is used in an energy-conversion device.

[0042] In one embodiment, the coating layer covers substantially the entirety of the surface of the metal plate. For example, in one embodiment the coating layer completely covers the surface of the metal plate. In an alternative embodiment, the coating layer partially covers the surface of the metal plate. Preferably the coating layer covers (e.g. completely covers) at least one side of the metal plate (i.e. at least one of the two major faces of the metal plate), more preferably covers (e.g. completely covers) both sides of the metal plate. When the metal plate is coated on both sides, each coating layer can be the same or different but is preferably the same.

[0043] The coating layer can be disposed directly or indirectly on the metal plate. For example, in one embodiment there may be at least one intermediate layer in between the metal plate and the coating layer, e.g. a metal oxide layer. Preferably however the coating layer is disposed directly on the metal plate, i.e. without any intermediate layers between the metal plate and the coating layer.

[0044] In a preferred embodiment, the bipolar plate comprises only one coating layer. In an alternative embodiment however, the bipolar plate comprises more than one (i.e. two or more) coating layers, such as two or three coating layers. When there are multiple coating layers, each coating layer can be the same as or different than any other coating layer. Any additional coating layers generally should have sufficiently high conductivity and corrosion resistance to allow the bipolar plate to function in an energy-conversion device. It is therefore preferred in the case that there are multiple (i.e. more than one) coating layers that each coating layer is a coating layer as defined in any preceding aspect of the present invention, i.e. a coating layer comprising a cured coating composition comprising: a) a matrix comprising at least one polymeric resin; and b) a plurality of conductive particles dispersed within the matrix; wherein a number of the conductive particles are arranged in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer.

[0045] In one embodiment, the coating layer has an average thickness in the range of 1 to 500 pm. In a preferred embodiment, the coating layer has an average thickness in the range of 10 to 400 pm, more preferably 25 to 300 pm. In a most preferred embodiment, the coating layer has an average thickness in the range of 50 to 250 pm.

[0046] When there is more than one coating layer disposed on the metal plate, the above ranges for the average thickness of the coating layer refer to the average total thickness of the coating layers, i.e. the sum of the average thicknesses of each of the coating layers disposed on the metal plate.

[0047] In one embodiment, the coating layer has an electrical conductivity greater than 10 S / cm, preferably greater than 50 S / cm, more preferably greater than 100 S / cm. Electrical conductivity is measured according to the method described under the heading “Determination Methods”. In one embodiment, the coating layer has an electrical conductivity of up to 650000 S / cm.

[0048] In one embodiment, the coating layer has an interfacial contact resistance (ICR) less than 150 mQ / cm2, preferably less than 100 mQ / cm2, more preferably less than 50 mQ / cm2. In one embodiment, the coating layer may have an interfacial contact resistance as low as 1 mQ / cm2, e.g. as low as 5 mQ / cm2or as low as 10 mQ / cm2. Interfacial contact resistance is measured according to the method described under the heading “Determination Methods”.

[0049] Cured Coating Composition

[0050] The coating layer disposed on the metal plate comprises, preferably consists of, a cured coating composition. As used herein, the term “cured” refers to the fact that the coating composition has been transformed from a first state wherein the coating composition has a first viscosity to a second state wherein the coating composition has a second viscosity, wherein the second viscosity is higher than the first viscosity. The first viscosity is preferably such that the uncured coating composition can be applied to the metal plate so as to form a coating having a uniform thickness. Curing the coating composition ensures that the conductive particles, especially the aligned assemblies of conductive particles, remain fixed in the final coating composition. Curing is sometimes referred to as hardening.

[0051] Curing of the coating composition may occur when solvent, which may be comprised within the coating composition before curing, evaporates. Additionally or alternatively, curing may occur via the formation of crosslinks, e.g. covalent bonds, between components within the coating composition. In a preferred embodiment, the cured coating composition is crosslinked, e.g. physically and / or chemically crosslinked.

[0052] In one embodiment, the cured coating composition is formed by exposing the uncured coating composition to heat and / or UV light. In one embodiment therefore, before curing, the (uncured) coating composition is a thermally curable and / or UV curable coating composition. Preferably the (uncured) coating composition is thermally curable.

[0053] Matrix

[0054] The cured coating composition comprises a matrix comprising at least one polymeric resin. The matrix is typically the main structural and film-forming component of the coating composition and serves to protect the metal plate from corrosion and hold (fix) the conductive particles in place. The matrix is preferably electrically non-conductive, i.e. it is preferably an insulator.

[0055] The polymeric resin can suitably be any one of many film-forming polymeric resins known in the art. For example, in one embodiment, the polymeric resin is selected from the group consisting of epoxy-resins, acrylic resins, phenolic resins, urethane resins, melanin resins, fluorine resins, silicone resins, and combinations thereof. In one embodiment, the cured coating composition comprises a mixture of different polymeric resins. In a preferred embodiment, the matrix comprises a single polymeric resin, preferably an epoxy-resin.

[0056] In a preferred embodiment, the at least one polymeric resin is an epoxy-resin, i.e. the matrix comprises, or consists of, at least one epoxy-resin. Both UV-curable and thermally-curable epoxy-resins or mixtures thereof may be used. Examples of suitable epoxy-resins include the Duralco series of epoxy-resins commercially available from Cotronics (such as Cotronics Duralco 4460 and Cotronics Duralco 4538) and the Vitralit series of epoxy-resins available from Panacol (e.g. Panacol Vitralit UD1405, Panacol Vitralit UD1406, Panacol Vitralit 2009F and Panacol Vitralit 1600LV).

[0057] Before curing, the coating composition preferably has a viscosity in the range of 10 to 10,000 mPas, preferably 50 to 8000 mPas, more preferably 100 to 5000 mPas. Viscosity is measured according to the method described under the heading “Determination Methods”.

[0058] In one embodiment, the polymeric resin has a glass transition temperature (Tg) in the range of -100 to 500 °C, preferably in the range -50 to 250 °C, more preferably in the range of -20 to 200 °C.

[0059] Generally, the matrix forms 20 to 80 wt% of the cured coating composition, preferably 30 to 70 wt%, more preferably 40 to 60 wt%, relative to the total weight of the cured coating composition.

[0060] Conductive Particles

[0061] In addition to the matrix, the cured coating composition comprises a plurality of conductive particles. The plurality of conductive particles is dispersed within the matrix.

[0062] The meaning of the term “particles” as used herein will be understood by the skilled person but generally covers any type of particle conventionally added to bipolar plate coating layers in the art. In one embodiment, the particles are nanoparticles or microparticles, e.g. having a maximum transverse dimension in the range of 0.1 to 500 pm, such as 1 to 200 pm, such as 2 to 100 pm, such as 2 to 10 pm or 10 to 50 pm or 50 to 100 pm. The particles may be spherical or non-spherical. The particles may have an aspect ratio in the range of 1 to 100, such as 1 to 50, such as 1 to 20 or 1 to 10 or 1 to 5.

[0063] As used herein, the term “conductive particles” refers to (typically solid) particles which are electrically conductive. Preferably the conductive particles are also thermally conductive. The role of the conductive particles is to conduct electricity and preferably also heat across the coating layer of the bipolar plate.

[0064] In one embodiment, the conductive particles are metal particles or metal-coated particles. The metal particles or metal-coated particles can comprise a single metal or a mixture of metals, i.e. an alloy.

[0065] Metals or metal alloys having high electrical conductivity and high corrosion resistance are preferred for the conductive particles. In a preferred embodiment therefore, the conductive particles comprise at least one element selected from the group consisting of Au, Ag, Ni, Ti, Pt and Pd. In an especially preferred embodiment, the conductive particles are gold or gold-coated particles. The present inventors have established that gold or gold-coated particles have an excellent balance of high electrical and thermal conductivity and high corrosion resistance that make them ideally suited for use in the coating layer of the bipolar plate of the present invention. Gold-coated particles are especially preferred due to their relatively lower cost.

[0066] In another embodiment, the conductive particles are ceramic particles. The nature of the ceramic particles is not particularly limited and examples of suitable ceramics include oxides, carbides or borides of metals or semi-metals. In one embodiment, the ceramic particles comprise one or more transition metal oxides, transition metal carbides, transition metal borides, or mixtures thereof, especially oxides, carbides and borides of Group IV-VI transition metals such as Ti, Ta or W. In one embodiment, the conductive particles comprise or consist of titanium nitride, titanium carbide, titanium oxide, titanium boride, tantalum boride, tungsten carbide, or mixtures thereof. Titanium nitride (TiN) is especially preferred. Other suitable ceramic materials include silicon carbide, tantalum carbide, antimony tin oxide, and indium tin oxide. Conductive particles comprising mixed metal oxides such as lanthanum-doped strontium cobalt oxides may also be used. In one embodiment, the conductive particles comprise a mixed metal oxide having the following formula (I):

[0067] (Lai-xSrx)yCo03+5(I) wherein x is 0.0 to 1.0; y is 0.9 to 1.0; and

[0068] 5 is a number so that formula (I) is uncharged.

[0069] In a preferred embodiment, the conductive particles are non-carbon based particles. For example, in one embodiment the conductive particles are particles other than carbon nanotubes, carbon nanofibres, or carbon fibres. The use of noncarbon based particles generally leads to higher electrical conductivity. It also allows for a greater range of coating layers to be produced (e.g. due to the diverse range of non-carbon based particles available).

[0070] In one embodiment, the particles have a core-shell type structure. In this embodiment, the conductive particles comprise a metal, metal alloy, or non-metal core surrounded (partially or completely) by at least one shell layer comprising a metal, metal alloy, or conductive ceramic.

[0071] The nature of the core in the optional core-shell type structure is not particularly limited. In one embodiment, the core comprises an inorganic material such as glass. In another embodiment, the core comprises an organic material such as graphite. The core may be electrically conductive or electrically non-conductive.

[0072] The shell layer is electrically conductive and typically comprises or consists of a single metal, a mixture of metals, e.g. an alloy, or an electrically conductive ceramic. The shell layer preferably comprises at least one element selected from the group consisting of Au, Ag, Ni, Ti, Pt and Pd. In an especially preferred embodiment, the shell layer comprises, preferably consists of, Au. In another embodiment, the shell layer comprises or consists of titanium nitride (TiN). The conductive particles are typically microparticles. In one embodiment, the particles have a median particle size (D50VOi) in the range of 0.1 to 500 pm, preferably 1 to 200 pm, more preferably 2 to 100 pm. In one embodiment, the median particle size (D50VOi) is in the range of 2 to 10 pm or 10 to 50 pm or 50 to 100 pm. Preferably the particles are approximately spherical.

[0073] In the embodiment where the conductive particles have a core-shell type structure, the shell layer generally has an average (mean) thickness in the range of 1 to 500 nm, preferably 10 to 250 nm, more preferably 20 to 200 nm, most preferably 50 to 150 nm. The shell layer preferably has a uniform thickness, e.g. wherein the thickness of the shell at any given point does not extend beyond ±10% of the average (mean) value of the shell thickness.

[0074] Examples of suitable conductive particles include the E-fill series of particles available from Oerlikon, the Conduct-O-Fil series of particles available from Potters Industries (PA, USA) and Monel particles available from NanoChemazone.

[0075] In one embodiment, the conductive particles are present in the cured coating composition in an amount of 20 to 80 wt%, preferably 30 to 70 wt%, e.g. 35 to 65 wt%, more preferably 40 to 60 wt%, relative to the total weight of the cured coating composition. The present inventors have surprisingly established that this amount of conductive particles (especially 40 to 60wt%) gives the best balance of conductivity and adhesion to the substrate. Generally, a too low loading of conductive particles results in a low conductivity whilst a too high loading results in poor adhesion of the coating layer to the metal plate.

[0076] In a preferred embodiment, the conductive particles are present in the cured coating composition in an amount below the percolation threshold of the cured coating composition. As is well known to the person skilled in the art, the percolation threshold is the lowest concentration of conductive particles necessary to achieve long-range conductivity in the random system. By having a loading of conductive particles below the percolation threshold but arranged in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer, the overall loading of conductive particles can be minimised resulting in lower costs whilst maintaining good (high) electrical conductivity and corrosion resistance.

[0077] The conductive particles are arranged in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer. By extending across the thickness of the coating layer, the particle assemblies thereby (electrically) connect the outer surface of the coating layer with the interface between the coating layer and the metal plate. In one embodiment, the particle assemblies extend in a direction which is substantially perpendicular to the plane of the coating layer (i.e. substantially parallel to the normal direction of the coating layer) or are inclined at an angle between 0 and 90°, e.g. between 0 and 45°, such as between 1 and 30°, relative to the normal direction of the coating layer. Preferably the particle assemblies extend in a direction which is substantially parallel to the normal direction of the coating layer.

[0078] Optionally, the cured coating composition further comprises one or more additives, e.g. of the kind conventionally used in coating compositions. In one embodiment, the cured coating composition further comprises at least one additive selected from the group of wetting agents, antioxidants, defoaming agents, adhesion promotors and / or dispersants. Preferably the amount of said additives is no more than 5.0 wt%, such as no more than 2.0 wt% or no more than 1 .0 wt% relative to the total weight of the cured coating composition. For example, in one embodiment the cured coating composition further comprises at least one additive in an amount of 0.1 to 5.0 wt%. In another embodiment, the cured coating composition comprises less than 20 wt%, such as less than 10 wt%, such as less than 5 wt%, of components other than the polymeric resin and conductive particles. In an alternative embodiment, the cured coating composition consists or consists essentially of the polymeric resin and the conductive particles.

[0079] Process

[0080] In another aspect, the present invention relates to a process for the production of a bipolar plate as defined in the first aspect of the present invention, the process comprising the steps of: i) applying to a metal plate a coating layer of a curable coating composition comprising: a) a matrix comprising at least one polymeric resin; and b) a plurality of conductive particles dispersed within the matrix; ii) applying an electric field between two alignment electrodes across the coating layer so as to cause a number of the conductive particles to arrange themselves in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer; and iii) curing the coating layer, e.g. by heating the coating layer and / or exposing the coating layer to UV light.

[0081] In one embodiment, the step of applying the coating layer first comprises mixing a matrix comprising at least one polymeric resin with a plurality of conductive particles to form the curable coating composition. It will be understood by the person skilled in the art that the curable coating composition is the precursor to the cured coating composition in the first aspect of the present invention. It follows therefore that the embodiments disclosed in relation to the cured coating composition are also applicable to the curable coating composition. In particular, features relating to the matrix and conductive particles disclosed in the context of the cured coating composition also apply to the curable coating composition.

[0082] In one embodiment, the curable coating composition further comprises one or more solvents. This aids in the dispersion of particles in the matrix. In an alternative embodiment, the curable coating composition consists of the polymeric resin and the conductive particles, i.e. is free of any additional solvent.

[0083] The arrangement of the conductive particles within the uncured coating composition is preferably homogeneous and isotropic.

[0084] In one embodiment, the step of applying the curable coating composition to the metal plate comprises spreading, pouring, dipping or spraying the curable coating composition onto the metal plate. The curable coating composition may be applied so as to partially cover the surface of the metal plate (e.g. one side of the metal plate) or may alternatively be applied so as to cover essentially the entirety of the surface of the metal plate (e.g. both sides of the metal plate). In one embodiment, the step of applying the curable coating composition to the metal plate comprises the step of completely covering the metal plate with the curable coating composition.

[0085] The curable coating composition is preferably UV curable and / or thermally curable. A thermally curable coating composition is preferred. Optionally, the step of applying the curable coating composition to the metal plate further comprises pressing the curable coating composition. This ensures the formation of a coating layer having a uniform thickness.

[0086] In step ii) of the process of the present invention, an electric field is applied between two alignment electrodes across the coating layer so as to cause a number of the conductive particles to arrange themselves in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer.

[0087] Without wishing to be bound by theory, it is believed that the application of an electric field across the coating layer generates dipoles within the conductive particles within the coating layer. The generation of dipoles leads to induced dipoledipole interactions between particles which in turn generates forces which act upon the particles, causing them to arrange themselves into aligned particle assemblies. The use of an electric field to align particles within a matrix has been described in previous publications such as W02010 / 151141A1 and W02010 / 151142A1 - both of which are hereby incorporated by reference.

[0088] In one embodiment, the electric field has an electric field strength in the range of 0.01 to 20.0 kV / cm, preferably 0.05 to 5.0 kV / cm, more preferably 0.1 to 1 .0 kV / cm. This means that for an alignment distance (i.e. the distance between the two alignment electrodes) in the range of 10 pm to 1 mm the voltage applied can be in the range of 10 to 2000 V.

[0089] The electric field is preferably an alternating current (AC) field. Typically the AC field has a frequency of 1 Hz to 200 kHz. Generally, higher frequencies (e.g. >1 kHz) are optimal for smaller particle sizes (< 1 pm) whilst larger particles may be aligned at lower frequencies. Direct current (DC) electric fields or very low frequencies < 10 Hz lead to asymmetric chain formation and build up, which can nevertheless conduct current. The present inventors have surprisingly established that an AC field having a frequency of 20 kHz to 200 kHz, preferably 50 kHz to 150 kHz, results in particularly excellent arrangement of the conductive particles into aligned particle assemblies. In a further aspect therefore, the present invention relates to a process for the production of a bipolar plate as defined in any previous aspect, wherein the electric field is an alternating current (AC) electric field having a frequency in the range of 20 kHz to 200 kHz, preferably 50 kHz to 150 kHz.

[0090] The direction of the electric field may be predetermined by the electrode arrangement and thereby the orientation of the electrically conductive pathways formed by the aligned particle assemblies can be controlled. In one embodiment, one or both of the alignment electrodes are in direct contact with the coating layer. In an alternative embodiment, the alignment electrodes are not in direct contact with the coating layer during the alignment step (step ii)). For example, there may be one or more insulating layers between the alignment electrode(s) and the coating layer. In another embodiment, the alignment electrode(s) are not in contact with the coating layer at all, i.e. neither directly nor indirectly.

[0091] In a preferred embodiment, the metal plate is also one of the alignment electrodes. This arrangement advantageously simplifies the production process..

[0092] In step iii) of the process according to the present invention, the coating layer is cured, thereby stabilising the coating layer and preserving (e.g. fixing) the electrically conductive pathways formed by the aligned particle assemblies. Curing is typically conducted whilst the electric field (e.g. as applied in step ii)) is still being applied but after allowing some time for the conductive particles to rearrange within the layer and form the aligned particle assemblies. Thus in one embodiment, step iii) of the process of the present invention is conducted concurrently or after step ii). Curing may be conducted by any conventional means such as exposing the coating layer to UV light and / or heat. For example, in one embodiment, the step of curing the coating layer comprises exposing the coating layer to an elevated temperature and optionally holding the coating layer at that temperature for an extended period of time. Typical curing temperatures are in the range of 20 to 180 °C. Typical curing times are in the range of 10 to 600 minutes. In some cases however, depending on external factors such as the mass of the system, the curing temperature and times may be outside of these ranges, as is well known by the person skilled in the art. In general, a higher curing temperature requires a shorter curing time and vice versa.

[0093] In one embodiment, the metal plate is fixed in relation to the alignment electrodes during alignment and curing. Therefore, in one embodiment, the process is a non- continuous process.

[0094] In an alternative embodiment, the metal plate is allowed to move relative to the alignment electrodes, e.g. pass between the two electrodes, during alignment and curing. Therefore, in one embodiment, the process is a continuous process. In a particularly preferred embodiment, the process is a roll-to-roll (R2R) process.

[0095] Applications

[0096] In another aspect, the present invention relates to an energy-conversion device, such as a fuel cell, fuel cell stack, electrolyser or an electrolyser stack, comprising at least one bipolar plate according to any aspect of the present invention. For the avoidance of doubt, any of the embodiments described in connection with the bipolar plate of any previous aspect of the present invention are also applicable to the bipolar plate of this aspect.

[0097] In a preferred embodiment, the energy-conversion device comprises an anode, a bipolar plate according to the first aspect of the present invention, and a cathode in that order. In one embodiment, the energy-conversion device comprises one or membrane electrode assemblies, wherein each membrane electrode assembly is separated from one another by a bipolar plate according to the first aspect of the present invention.

[0098] In another aspect, the present invention relates to the use of a bipolar plate according to the first aspect in the production of an energy-conversion device such as a fuel cell, fuel cell stack, electrolyser or an electrolyser stack. Any of the embodiments described in connection with the bipolar plate of any previous aspect are also applicable to the bipolar plate of this aspect. Determination Methods

[0099] The viscosities of the uncured coating compositions were determined according to ISO 2884-1 :1999 using a Cone and Plate viscometer set at a temperature of 23 °C, working at a shear rate of 10000 s-1.

[0100] Interfacial Contact Resistance

[0101] In order to check the performance of the BPPs' coating, ICR (interfacial contact resistance) measurement was performed using a custom-made apparatus. The samples were cut to remove all uncoated substrate, and a H26 C2 gas diffusion layer (GDL) (Freudenberg) cut to the same size. The sample was then placed between two gold coated copper plates with the GDL on top. A pneumatic piston was used to raise the sample / GDL into contact with the top plate, then provided variable pressure between 30 and 260 N cm-2. A XDL 56-4 DC power supply (Xantex) was used to provide a current of 1 .5 A between two gold coated copper plates, whilst a multimeter (Fluke 76 True RMS) was used to measure the resulting voltage between the top plate and the spring-loaded pin isolated from the bottom plate, over the range of pressures. A contact resistance value was calculated using Ohm's Law and reported at a pressure of 140 N cm-2. A schematic diagram of the apparatus is shown in Figure 1 .

[0102] Initial measurements were performed with only the GDL in place, to determine the resistance contribution from the measurement apparatus, and to isolate the sample / GDL contact resistance. All subsequent results are reported as the normalised ICR (i.e. , without the GDL / apparatus contribution). The error in the ICR measurements is estimated at around 3% of the measured value, mostly due to small changes in the contact between the GDL and sample because of uneven surface morphology. Electrical Volume Resistivity / electrical conductivity

[0103] The method that is chosen to be used for measuring the resistance of a test sample is a 4-wire resistance measurement method, also known as the Kelvin method. The 4-wire resistance measurement circuit involves the use of both an amperemeter and a voltmeter, as shown in Figure 2. Since this method measures the voltage only dropped across the subject resistance, it is very precise and hence could be effectively used for measuring the resistance of samples that have below 1 Ohm (R < 1 Q) electrical resistance.

[0104] The resistance measurements unit is an assembly module that has been designed to be used in assembly applications such as pressing in bearing dampers, shafts, etc. Here, it is used to apply a gentle force on a test sample, i.e. , a sample that lies on a copper pad. In other words, it is used for connecting the test subject to a 4- wire resistance measurement circuit. The average data is read from the test instrument software. The average data is obtained by data collection with an interval of one second and an elapsed time of one minute.

[0105] The resistivity measurements on copper foil at all the different loadings were conducted using a purpose built apparatus for 4-wire measurement of the resistance of the sample under controlled pressure towards the contact area. The measurements were done on a copper pad with a copper piston head with a contact area of 100 mm2. The force used for the measurements were 20N and pressure 200 kPa. Measurements were done over a minute with data collection interval of 1 second.

[0106] Particle size

[0107] Unless otherwise stated, the term “particle size” refers to the particle diameter at 50% cumulative volume (D50VOi), i.e. the median particle diameter.

[0108] Corrosion Resistance

[0109] The sample is placed in an electrolyte consisting of Na2SO4adjusted to pH 3 using H2SO4, heated to 80 °C and purged with nitrogen. A potential of 1 .0 VSHE is applied using a potentiostat for a duration of 1 hour, normally, or up to 24 hours, and the current density of the sample is measured using a potentiostat. thickness

[0110] Coating layer thickness is measured using a dial indicator. The measured height position of the substrate surface is subtracted from the measured height position of the coating surface.

[0111] The invention will now be further described by reference to the following nonlimiting examples.

[0112] Examples

[0113] Materials

[0114] Particles:

[0115] Conduct-O-Fil® S2429S particles (Potter Industries) - silver coated solid glass spheres of 75pm.

[0116] Conduct-O-Fil® S2429S-G2 particles (Potter Industries) - S2429S particles with a gold coated layer (100nm) on top of the silver layer.

[0117] Polymeric resins:

[0118] Vitralit 2009F (Panacol) - a combined UV- and thermally curable coating compound, based on epoxies.

[0119] Araldite 2020 - XW 3961 XW 397 (Huntsman) - a heat curing epoxy resin.

[0120] Preparation of Samples

[0121] Epoxy part A was mixed with the conductive particles using a planetary mixer (Thinky, 1000 rpm, 8 minutes, 1 atm) before Epoxy part B was added under further mixing (1000 rpm, 1 minute, 1 atm). The final slurry was mixed under vacuum (1000 rpm, 30 seconds, 10 kPa).

[0122] Some samples were prepared on copper foil. Other samples for fuel cell testing were prepared on stainless steel plates (Grade 316, 0.5 mm, Goodfellow). The stainless steel substrates were cleaned by submersion in HCI (12.5 %, Sigma Aldrich) for 15 minutes before removal and drying. The substrate was used immediately after cleaning to avoid oxide re-formation.

[0123] After substrate cleaning and slurry preparation, the slurry containing epoxy resin and the conductive particles was deposited onto the metal substrate, which acts as a bottom electrode. The slurry is then clamped between a top electrode and the bottom electrode with a constant force. The desired sample thickness is achieved using spacers placed between the electrodes. An AC electric field (10 to 1500 V; 0.5 to 100 kHz) is then applied between the two electrodes. The electric field causes the particles dispersed in the slurry to align into chains through the thickness of the sample between the electrodes.

[0124] Once alignment is achieved the slurry is cured. In the case of the Araldite® 2020 epoxy, curing takes place in air for 2 hours at a temperature of 90 °C. For the Vitralit epoxy, UV radiation is applied (Mercury arc lamp spectrum, 90 seconds).

[0125] Example 1 - Production of coatings comprising Vitralit 2009F resin and Conduct-O-Fil® S2429S particles on copper foil

[0126] Aligned and non-aligned samples consisting of coatings containing Vitralit 2009F as epoxy resin and Conduct-O-Fil® S2429S particles as conductive particles applied to copper foil were prepared using the preparation method recited above. Measurements of ICR and electrical volume resistivity were then conducted on all samples, using the methods described above under the heading “Determination Methods”.

[0127] The various samples, their particle loading, whether they are aligned or non- aligned, and the measured ICR, electrical volume resistivity and coating thickness of the samples are shown in Table 1 below. The ICR results are shown graphically in Figure 3.

[0128] “IE” = Inventive Example. “CE” = Comparative (Reference) Example.

[0129] Table 1 :

[0130] From Table 1 and Figure 3, it is apparent that the aligned samples are more electrically conductive (lower ICR) than the non-aligned samples at equivalent particle loadings. The difference in ICR between aligned and non-aligned samples is more pronounced at lower particle loadings, i.e. at loadings considerably below the percolation threshold.

[0131] Example 2 - Comparison of silver-coated and gold-coated particles

[0132] Samples consisting of Conduct-O-Fil® S2429S particles or Conduct-O-Fil® S2429S-G2 particles dispersed in Vitralit 2009F epoxy resin at a particle loading of 50 wt% on copper foil were prepared using the same method as for the samples in Example 1 except that the particles were not aligned. Details of each sample is shown in Table 2 below (all samples are non-aligned). The ICR results as a function of coating thickness are shown in Figure 4. Table 2:

[0133] As shown in Table 2 and Figure 4, the samples comprising gold-coated particles showed good (low) ICR over a range of coating thickness. At a coating thickness of 200 pm, the samples comprising gold-coated particles and the samples comprising silver-coated particles had almost equivalent ICR.

[0134] Example 3 - Corrosion resistance

[0135] Samples IE2 from Table 1 and C from Table 2 were tested for their stability in a simulated PEMFC environment, as described in “Determination Methods”. The current density response was measured, as was the ICR before and after testing. The results are shown in Table 3 and Figure 5.

[0136] Table 3:

[0137] In this case, the gold coated glass (S2429S-G2) sample shows good stability in the simulated PEMFC environment, with a very low current density over the entire one hour test, and no notable increase in ICR after testing. Example 4 - Production of Coatings comprising Araldite 2020 resin and gold- coated particles on stainless steel plates

[0138] Coating compositions comprising Araldite 2020 as the matrix and Conduct-O-Fil® S2429S-G2 particles as the conductive particles were prepared on stainless steel plates as described under the heading “Preparation of Samples”. The samples are detailed in Table 4 below.

[0139] Table 4:

[0140] Samples IE6, IE8, CE6 and CE8 were subjected to in-situ ICR and electrochemical impedance spectroscopy (EIS).

[0141] The fuel cell set up was as follows: Current collectors (bipolar plates) were tested in situ using single PEM fuel cell with an active area of 1 *5 cm2. The cell utilised two duralumin hotplates tightened with torque of 1 .5 N m with temperature detection by thermocouple probes in both plates. Between the hotplate and endplate, expanded PTFE sealing with thickness of 500 pm was inserted (GORE). Membrane-electrode assembly (MEA) consisted of catalyst-coated membrane based on Nation N212 (Chemours), prepared by ultrasonic spray deposition (Cheersonic) at 60 °C from an ink containing Pt / C catalyst HiSPEC4000 (Johnson-Matthey), acetone and isopropanol solvents with weight ratio 1 :1 and ionomer Nation D521 5 wt.% dispersion in water-isopropanol mixture with weight ratio of 1 :1 (Ion Power). Weight ratio of solid to liquid phase in the ink was 1 :60 and the final layer contained 0.3 mgPt cm-2and ionomencarbon weight ratio of 0.4 on both sides. Before the assembly of MEA, catalyst-coated membrane was humidified in deionised water at 80 °C for 2 h. On each side of catalyst-coated membrane, gas-diffusion layer Sigracet 28BC (SGL) was pressed. Instead of flowfield, porous carbon felt with hydrophobic treatment and thickness of 3 mm (Mersen) was used, facilitating contact between MEA and current collector. MEA was sealed to current collector using expanded PTFE sealing of thickness 1000 pm (GORE).

[0142] In each fuel cell, one of the plates was stainless steel (SS316L) with gold plating, and the other plate was stainless steel coated with IE6, IE8, CE6 or CE8. CE6 and IE6 were on the anode. CE8 and IE8 were on the cathode.

[0143] The ICR setup was as follows. Samples were compressed between 2 GDLs and 2 copper disks at 0.5 GDL thickness. The Cu disks had a diameter of 10 mm. Resistance was measured using a PROVA Milli-ohmmeter Model 700. Interfacial contact resistance was calculated using the following equation: Rcontact—(Rohm—Roell) ‘A, wherein:

[0144] Rcontact is; internal cell contact resistance (ICR)

[0145] Rohm is; ohmic resistance of the setup with a sample inserted Rceii is; ohmic resistance of the cell without sample and A is area of the sample.

[0146] EIS results are shown in Figures 6-9. Figure 6 shows the results for CE6, Figure 7 shows the results for IE6, Figure 8 shows the results for CE8 and Figure 9 shows the results for IE8. ICR results are shown in Table 4 below. AR-E2707-7 / 5 and AR- E2707-7 / 6 are comparative samples consisting of a coating layer formed of nickel coated spherical graphite particles (E-fill 2707 from Oerlikon) dispersed in Araldite 2020 resin, wherein the coating layer is applied to a stainless steel plate. The comparative samples are not aligned. Table 5:

[0147] Inhomogeneous sample - ICR measured at two different places

[0148] From Figures 6-9 and Table 5, it can be seen that the aligned (inventive) samples improved in terms of performance over time whereas the non-aligned (comparative) samples got worse. The alignment of particles within the coatings enables one to reduce their content in the coating, whilst keeping the electrical conductivity at an acceptable level. This has several positive impacts:

[0149] - improved mechanical stability of the protective film due to the higher content of the binding epoxy;

[0150] - increased chemical stability of the film due to the reduction of the interfacial area with the fuel cell environment;

[0151] - improvement of the protective properties due to the reduced interfacial area between the filler particles and binding epoxy, as these domains represent potential danger for the liquid penetration to the surface of the steel carrying the protective film. Such penetration would cause corrosion of the steel and degradation of the bipolar plate as such;

[0152] - aligning of the particles reduces skin film effect. Skin film typically cover surface of this type of composites, preventing effectively electrical contact to the filler particles. Aligning allows the particles at the surface to penetrate this film and to provide corresponding electrical contact.

Claims

Claims1 . A bipolar plate comprising a metal plate and at least one coating layer disposed on the metal plate, wherein the coating layer comprises a cured coating composition comprising: a) a matrix comprising at least one polymeric resin; and b) a plurality of conductive particles dispersed within the matrix; wherein a number of the conductive particles are arranged in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer.

2. A bipolar plate as claimed in claim 1 , wherein the metal plate is a steel plate, preferably a stainless-steel plate such as an austenitic stainless-steel plate.

3. A bipolar plate as claimed in claim 1 or 2, wherein the polymeric resin is an epoxy resin.

4. A bipolar plate as claimed in any preceding claim, wherein the conductive particles are metal particles or metal-coated particles, preferably gold or gold-coated particles; or wherein the conductive particles are ceramic particles, preferably titanium nitride (TiN) particles.

5. A bipolar plate as claimed in any preceding claim, wherein the conductive particles comprise at least one element selected from the group consisting of Au, Ag, Ni, Ti, Pt and Pd.

6. A bipolar plate as claimed in any preceding claim, wherein the average (mean) thickness of the coating layer is in the range of 1 to 500 pm, preferably 10 to 400 pm, more preferably 25 to 300 pm, most preferably 50 to 250 pm.

7. A bipolar plate as claimed in any preceding claim, wherein the conductive particles are present in the cured coating composition in an amount below the percolation threshold of the cured coating composition.

8. A bipolar plate as claimed in any preceding claim, wherein the conductive particles are present in the cured coating composition in an amount of 20 to 80 wt%, preferably 30 to 70 wt%, more preferably 40 to 60 wt%, relative to the total weight of the cured coating composition.

9. A bipolar plate as claimed in any preceding claim, wherein the coating layer has an electrical conductivity greater than 10 S / cm, preferably greater than 50 S / cm, more preferably greater than 100 S / cm when measured according to the method described in the description under the heading “Determination Methods”; and / or an interfacial contact resistance (ICR) less than 150 mQ / cm2, preferably less than 100 mQ / cm2, more preferably less than 50 mQ / cm2when measured according to the method described in the description under the heading “Determination Methods”.

10. A bipolar plate as claimed in any preceding claim, wherein the aligned particle assemblies are chains or strings.

11. An energy-conversion device, such as a fuel cell, fuel cell stack, electrolyser or an electrolyser stack, comprising at least one bipolar plate as claimed in any of claims 1 to 10.

12. An energy-conversion device as claimed in claim 11 , wherein the at least one bipolar plate comprises a flow field, e.g. a set of channels or grooves machined or stamped into the plate.

13. Use of a bipolar plate as claimed in any of claims 1 to 10 in the production of an energy-conversion device such as a fuel cell, fuel cell stack, electrolyser or an electrolyser stack.

14. A process for the production of a bipolar plate as claimed in any of claims 1 to 10, the process comprising the steps of:i) applying to a metal plate a coating layer of a curable coating composition comprising: a) a matrix comprising at least one polymeric resin; and b) a plurality of conductive particles dispersed within the matrix; ii) applying an electric field between two alignment electrodes across the coating layer so as to cause a number of the conductive particles to arrange themselves in the matrix so as to form a plurality of aligned particle assemblies constituting electrically conductive pathways extending across the thickness of the coating layer; and iii) curing the coating layer, e.g. by heating the coating layer and / or exposing the coating layer to UV light.

15. A process as claimed in claim 14, wherein the metal plate is one of the alignment electrodes.

16. A process as claimed in claims 14 or 15, wherein the electric field is an alternating current (AC) electric field and wherein the AC electric field preferably has a frequency in the range of 20 kHz to 200 kHz, more preferably 50 kHz to 150 kHz.

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