Gas Diffusion Electrode

Abstract

A gas diffusion electrode 1 comprises a gas diffusion layer 2 on which a catalyst layer 8 is applied wherein the catalyst layer includes platinum or a platinum alloy and a separate noble metal or noble metal alloy. The noble metal may be gold or palladium, most preferably gold. The platinum or alloy thereof may be a nanowire 12, preferably within a nanowire array. The noble metal or alloy thereof may comprise nanoparticles 10. The amount of platinum or alloy thereof may be between 0.01-2 mg / cm2 while the amount of noble metal or allow thereof may be between 0.05-50 µg / cm2. The noble metal or alloy thereof in the catalyst layer is preferably between 0.1% and 4%. The gas diffusion layer may comprise carbon and the gas diffusion electrode may be for an anode and / or cathode in a membrane electrode assembly for a fuel cell. A method of making the electrode is also provided, preferably wherein the noble metal or alloy thereof is deposited on the gas diffusion layer by sputtering and the platinum or alloy thereof is deposited on the gas diffusion layer by growing nanowires.

Description

Technical Field of the Invention The present invention relates to a gas diffusion electrode, a membrane electrode assembly comprising a gas diffusion electrode, a fuel cell and a method of manufacturing a gas diffusion electrode. Background to the Invention Gas diffusion electrodes are key elements of fuel cells and the development of a stable, durable and highly efficient gas diffusion electrode is key for areas requiring high power, low cost fuel cells, for example heavy duty transportation. Gas diffusion electrodes comprise a gas diffusion layer and a catalyst layer. Commonly used platinum / carbon catalyst layers (Pt / C) suffer from poor stability resulting in catalyst dissolution, Ostwald ripening, agglomeration, detaching from their supports and carbon corrosion. It would be advantageous to provide a gas diffusion electrode which is stable and durable over time. It would also be advantageous to provide a gas diffusion electrode which provides high power density and mass activity. It would furthermore be advantageous to provide a gas diffusion electrode which maintains high power density over time and does not suffer from power density decrease. Known methods of manufacturing gas diffusion electrodes may comprise several complex steps such as catalyst preparation using organic solvents at high temperature, catalyst separation, drying, annealing, re-dispersing in organic solvent to make catalyst ink and also surface coating to fabricate electrodes that are required for a conventional gas diffusion electrode. Known methods of manufacturing gas diffusion electrodes may comprise many reagents and high energy conditions which can be expensive and inefficient. It would be advantageous to provide a facile method of manufacturing a gas diffusion electrode comprising a reduced number of steps. It would be advantageous to provide a method of manufacturing a gas diffusion electrode wherein the method does not comprise a significant number of reagents or solvents. It is an aim of embodiments of the invention to overcome one or more problems of the prior art, whether expressly disclosed herein or not. Summary of the Invention According to a first aspect of the invention there is provided a gas diffusion electrode comprising a gas diffusion layer on which a catalyst layer is applied wherein the catalyst layer comprises platinum or an alloy thereof and a separate noble metal or an alloy thereof. The gas diffusion electrode of the invention may be advantageous as it may have high stability and durability. The gas diffusion electrode of the invention may be advantageous because it may have a high permeability and porosity and thereby have reduced mass transfer resistance. The gas diffusion electrode of the invention may be advantageous because the catalyst layer comprising gold and platinum may have a high tolerance to catalyst degradation mechanisms such as dissolution, Ostwald ripening and / or aggregation. By “separate” it is meant that the catalyst may comprise both a distinct platinum or an alloy thereof and a distinct noble metal or an alloy thereof, as opposed to solely a single alloy of platinum and a noble metal. The platinum alloy may comprise platinum and a second element selected from the group consisting of: iridium, rhodium, ruthenium, gold, cobalt and palladium and any combination thereof. The platinum or an alloy thereof may preferably comprise native (non-aHoycd) platinum. In some embodiments the platinum alloy may preferably comprise platinum and gold. In some embodiments the platinum alloy may preferably comprise a platinum and gold alloy. “Platinum or an alloy thereof” will hereinafter be referred to as “platinum”. The noble metal may be selected from the group consisting of: rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, and gold. In preferred embodiments the noble metal may be gold or palladium. In further preferred embodiments the noble metal may be gold. The noble metal alloy may comprise a noble metal and a second, different element wherein the noble metal may be selected from the group consisting of: rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, and gold. The noble metal alloy may comprise a noble metal and a second, different metal wherein the noble metal may be selected from the group consisting of: rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, and gold. The noble metal alloy may comprise a noble metal and platinum wherein the noble metal may be selected from the group consisting of: rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, and gold. The noble metal alloy may comprise gold and platinum. The noble metal alloy may comprise a gold and platinum alloy. “Noble metal or an alloy thereof’ will hereinafter be referred to as “noble metal”. In some embodiments the noble metal comprises noble metal particles. In some embodiments the noble metal comprises noble metal nanostructures. In some preferred embodiments the noble metal comprises noble metal nanoparticles. Such embodiments may be advantageous because noble metal particles or nanoparticles may act as an anchoring seeding particle which may allow the platinum to be securely anchored to the gas diffusion layer which in turn may result in a gas diffusion electrode with improved power density, outstanding durability and less power performance drop over time. The diameter of the noble metal nanoparticles may be between 1 nm and 8 nm. The diameter of the noble metal nanoparticles may be between 2 nm and 6 nm, 2 nm and 4 nm, or between 2.5 nm and 4 nm, or 2 nm and 3.5 nm or between 2.5 nm and 3.5 nm. This embodiment may be preferred because it may allow for more of the smaller noble metal nanoparticles to be present in the catalyst layer. This embodiment may be preferred because smaller noble metal particle sizes reduce the amount of platinum agglomeration, creating more triple phase boundaries. The amount of noble metal in the catalyst layer may be at least 0.1 wt. % of the catalyst layer. The amount of noble metal in the catalyst layer may be at least 0.2 wt. %, 0.3 wt. %, 0.4 wt. % or at least 0.5 wt. % of the catalyst layer. The amount of noble metal in the catalyst layer may be no more than 4 wt. % of the catalyst layer. The amount of noble metal in the catalyst layer may be no more than 3.5 wt. %, 3 wt. %, 2.5 wt. %, 2 wt. % or no more than 1.5 wt. % of the catalyst layer. The amount of noble metal in the catalyst layer may be between 0.1 wt. % and 4 wt. % of the catalyst layer. The amount of noble metal in the catalyst layer may be between 0.2 wt. % and 4 wt. %, 0.3 wt. % and 3 wt. %, 0.4 wt. % and 2 wt. % or between 0.5 wt. % and 1 wt. % of the catalyst layer. Such embodiments may be advantageous because it may improve the power performance and increase the peak power density of the gas diffusion electrode whilst not overloading the gas diffusion electrode. Overloading the gas diffusion electrode may have the negative effect of forming nanoparticles which are larger than desired. Larger nanoparticles are not desired because they may inhibit platinum nanowire growth. In some embodiments the noble metal may be facing the gas diffusion layer. In some embodiments the noble metal may be indirectly contacting gas diffusion layer. In some embodiments the noble metal may be directly contacting the gas diffusion layer. In some embodiments at least 80 % of the noble metal is directly contacting the gas diffusion layer. In some embodiments at least 85 %, 90 %, 95 %, 96 %, 97 %, 98 % or at least 99 % of the noble metal is directly contacting the gas diffusion layer. In preferred embodiments approximately 100 % of the noble metal is directly contacting the gas diffusion layer. This embodiment may be advantageous because the noble metal may reduce the hydrophobicity of the gas diffusion layer and / or may increase the number of sites available for crystal nuclei on the gas diffusion layer thereby allowing the platinum to be securely anchored to the gas diffusion layer. This embodiment may be advantageous because the noble metal may provide an anchoring effect on the gas diffusion layer thereby improving the uniformity of the catalyst layer and providing a gas diffusion electrode with improved power density, outstanding durability and less power performance drop over time. The “equivalent thickness” is a theoretical value that represents the thickness of a layer that would be achieved if deposited metals were spread uniformly over a specified area. It is calculated based on the deposition rate and the deposition time. In some embodiments the noble metal may form a layer on the gas diffusion layer. In some embodiments the equivalent thickness of the noble metal layer is between 0.1 nm and 6 nm. In some embodiments the equivalent thickness of the noble metal layer is between 0.1 nm and 5 nm, 0.1 nm and 4 nm, 0.1 nm and 3 nm, 0.2 nm and 3 nm, 0.2 nm and 2.5 nm, 0.2 nm and 2 nm, 0.2 nm and 1.5 nm, 0.4 nm and 1.5 nm, 0.4 nm and 1 nm or between 0.5 nm and 1 nm. This embodiment may be advantageous because it may improve the power performance and increase the peak power density of the gas diffusion electrode whilst not overloading the gas diffusion electrode which may have the negative effect of forming nanoparticles with are larger than desired. The amount of noble metal may be between 0.05 pg / cm2 and 50 pg / cm2. The amount of noble metal may be between 0.1 pg / cm2 and 50 pg / cm2, 0.5 pg / cm2 and 40 pg / cm2, 0.5 pg / cm2 and 30 pg / cm2, 0.5 pg / cm2 and 20 pg / cm2, 0.5 pg / cm2 and 18 pg / cm2, 1 pg / cm2 and 18 pg / cm2, 0.5 pg / cm2 and 16 pg / cm2, 1 pg / cm2 and 16 pg / cm2, 0.5 pg / cm2 and 14 pg / cm2, 1 pg / cm2 and 14 pg / cm2, 0.5 pg / cm2 and 12 pg / cm2, 1 pg / cm2 and 12 pg / cm2, 0.5 pg / cm2 and 10 pg / cm2, 1 pg / cm2 and 10 pg / cm2, 2 pg / cm2 and 10 pg / cm2, 4 pg / cm2 and 10 pg / cm2, or between 5 pg / cm2 and 10 pg / cm2. In some embodiments the platinum of the catalyst layer comprises platinum particles. In some embodiments the platinum of the catalyst layer comprises platinum nanoparticles. In some embodiments the platinum of the catalyst layer comprises platinum nanorods. In some embodiments the platinum of the catalyst layer comprises platinum nanorods comprising an aspect ratio (length / width) of between 2 and 50. In preferred embodiments the catalyst layer comprises at least one platinum nanowire. In preferred embodiments the catalyst layer comprises a platinum nanowire array. The platinum nanowire array may be a vertically aligned nanowire array. The platinum nano wire array may be an organised array. The platinum nano wire array may be a random array. The platinum nano wire array may be a uniform array. A uniform array may be advantageous because it may result in a gas diffusion electrode with improved power density, outstanding durability and less power performance drop over time. Each platinum nanowire may comprise a diameter of between 1 nm and 5 nm. Each platinum nanowire may comprise a diameter of between 2 nm and 5 nm, 3 nm and 4.5 nm, 3.5 nm and 5 nm or between 3.5 nm and 4.5 nm. This embodiment may be advantageous because it may provide a nanowire with a larger specific area, or a larger electrochemical surface area compared to nano wires with larger diameters. Each platinum nanowire may comprise a length of at least 5 nm. Each platinum nano wire may comprise a length of at least 5 nm, 10 nm, 20 nm, 30 nm, 40 nm or at least 50 nm. Each platinum nanowire may comprise a length of no more than 200 nm. Each platinum nanowire may comprise a length of no more than 150 nm, 100 nm, 80 nm, 75 nm, 70 nm, 65 nm, 60 nm, 55 nm or no more than 50 nm. The length of the platinum nanowire may be between 5 nm and 200 nm. The length of the platinum nanowire may be between 5 nm and 150 nm, 10 nm and 100 nm, 10 nm and 90 nm, 10 nm and 80 nm, 10 nm and 75 nm, 10 nm and 70 nm, 10 nm and 60 nm or between 10 nm and 50 nm. The amount of platinum may be between 0.01 mg / cm2 and 2 mg / cm2. The amount of platinum may be between 0.01 mg / cm2 and 1 mg / cm2 0.05 mg / cm2 and 0.9 mg / cm2, 0.05 mg / cm2 and 0.8 mg / cm2, 0.05 mg / cm2 and 0.7 mg / cm2, 0.05 mg / cm2 and 0.6 mg / cm2, 0.05 mg / cm2 and 0.5 mg / cm2, 0.1 mg / cm2 and 0.5 mg / cm2, 0.1 mg / cm2 and 0.4 mg / cm2,0.1 mg / cm2 and 0.3 mg / cm2 or between 0.15 mg / cm2 and 0.25 mg / cm2. The amount of platinum may be approximately 0.2 mg / cm2. In some embodiments the noble metal comprises noble metal nanoparticles and the platinum comprises platinum nano wires. In some embodiments the noble metal comprises gold nanoparticles and the platinum comprises platinum nanowires. In some embodiments the noble metal comprises noble metal nanoparticles and the platinum comprises platinum nanowires wherein at least one platinum nanowire extends from at least one noble metal nanoparticle. In some embodiments the noble metal comprises noble metal nanoparticles and the platinum comprises platinum nanowires wherein a plurality of platinum nano wires extend from at least one or each noble metal nanoparticle. In some embodiments the noble metal comprises noble metal nanoparticles and the platinum comprises platinum nanowires wherein each platinum nanowire extends from a noble metal nanoparticle. In some embodiments the noble metal comprises noble metal nanoparticles and the platinum comprises platinum nanowires wherein at least 70 %, 80 %, 90 %, 95 % or at least 99 % of the platinum nanowires extend from a noble metal nanoparticle. In some embodiments the noble metal comprises noble metal nanoparticles and the platinum comprises platinum nanowires wherein at least one platinum nanowire extends from each noble metal nanoparticle. This embodiment may be advantageous because the noble metal nanoparticle may act as an anchoring seed particle which may allow the platinum nanowire to form on the noble metal nanoparticle and therefore form a more uniform distribution of longer nanowires than without the noble metal nanoparticle, which in turn would result in a gas diffusion electrode with improved power density, outstanding durability and less power performance drop over time. In some embodiments the ratio of the amount of platinum to noble metal in the catalyst layer is between 98:2 and 99.9:0.1. The ratio of platinum to noble metal in the catalyst layer may be between 98:2 and 99.5:0.5, 98.5:1.5 and 99.5:0.5 or between 99:1 and 99.5:0.5. In some embodiments at least part of the gas diffusion layer comprises the catalyst layer. In some embodiments at least 60 % of the gas diffusion layer comprises the catalyst layer. At least 65 %, 70 %, 75 %, 80 %, 85 %, 90 %, 95 %, 96 %, 97 %, 98 % or at least 99 % of the gas diffusion layer may comprise the catalyst layer. In some embodiments the thickness of the catalyst layer is at least 0.05 pm. The thickness of the catalyst layer may be at least 0.1 pm, 0.3 pm, 0.5 pm, 0.8 pm or at least 1 pm. In some embodiments the thickness of the catalyst layer is no more than 6 pm. The thickness of the catalyst layer may be no more than 5.5 pm, 5 pm, 4.5 pm, 4 pm or no more than 3 pm. In some embodiments the thickness of the catalyst layer is between 0.05 pm and 6 pm. The thickness of the catalyst layer may be between 0.1 pm and 5.5 pm, 0.3 pm and 5 pm, 0.5 pm and 4.5 pm, 0.8 pm and 4 pm or between 1 pm and 3 pm. This embodiment may be advantageous because the thin catalyst layer may comprise a shorter diffusion path length which in turn may result in better mass transfer behaviour and improved power performance. The gas diffusion layer may comprise a microporous layer in contact with the catalyst layer. The microporous layer may comprise carbon. The microporous layer may comprise carbon spheres. The microporous layer may comprise carbon nanospheres. The microporous layer may comprise carbon nanospheres treated with a polymer. The microporous layer may comprise carbon nanospheres treated with polytetrafluoroethylene (PTFE). The microporous layer may comprise a porosity of between 10 % and 30 %. The gas diffusion layer may comprise at least one layer. The gas diffusion layer may comprise at least two layers. The gas diffusion layer may comprise at least three layers. The gas diffusion layer may comprise a first layer comprising the microporous layer contacting the catalyst layer and a second layer contacting the first layer and not contacting the catalyst layer. The second layer may comprise a substrate layer. The substrate layer may comprise carbon. The substrate layer may comprise carbon paper. The substrate layer may comprise carbon black. The substrate layer may comprise graphite. The substrate layer may comprise carbon fibre. The substrate layer may comprise carbon nanotubes. The gas diffusion layer may comprise a first layer comprising the microporous layer contacting the catalyst layer and a second layer comprising the substrate layer comprising carbon fibre, wherein the second layer is contacting the first layer and not contacting the catalyst layer. The gas diffusion layer may comprise a first layer comprising the microporous layer contacting the catalyst layer, a second layer contacting the first layer and not contacting the catalyst layer and a third layer contacting the second layer. The third layer may comprise a substrate layer. The second layer and the third layer may comprise different substrate layers. In some embodiments the gas diffusion electrode comprises in order, a catalyst layer, a first gas diffusion layer and a second gas diffusion layer. In some embodiments the gas diffusion electrode comprises in order, a catalyst layer, a first gas diffusion layer, a second gas diffusion layer and a third gas diffusion layer. The first layer and / or the second layer and / or the third layer may comprise carbon. The first layer and / or the second layer and / or the third layer may comprise carbon paper. The first layer and / or the second layer and / or the third layer may independently comprise carbon selected from the group consisting of: carbon black, graphite, carbon fibre and carbon nanotubes or any combination thereof. In some embodiments the first layer comprises carbon particles and the second layer comprises carbon paper. In some embodiments the first layer comprises carbon nanoparticles or carbon nanospheres and the second layer comprises carbon fibre. In some embodiments the first layer comprises carbon nanospheres treated with a polymer and the second layer comprises carbon paper or carbon fibre. In preferred embodiments the first layer comprises carbon nanospheres treated with polytetrafluoroethylene (PTFE) and the second layer comprises carbon paper or carbon fibre. This embodiment may be advantageous because it may provide a gas diffusion layer with hydrophobic (or superhydrophobic) features to prevent water flooding during the fuel cell operation. The gas diffusion layer may be hydrophobic with a water contact angle of between 120° and 165°. In some embodiments the gas diffusion electrode comprises a gas diffusion layer comprising a first layer and a second layer wherein at least one of the first or the second layer comprises carbon, and a catalyst layer in contact with the first layer of the of the gas diffusion layer wherein the catalyst layer comprises noble metal particles or nanoparticles in contact with the first layer of the gas diffusion layer. In some embodiments the gas diffusion electrode comprises a gas diffusion layer comprising a first layer and a second layer wherein the first layer comprises carbon nanospheres treated with a polymer and the second layer comprises carbon paper and a catalyst layer in contact with the first layer of the of the gas diffusion layer wherein the catalyst layer comprises noble metal particles or nanoparticles in contact with the first layer of the gas diffusion layer. This embodiment may be advantageous because the gas diffusion layer may prevent water flooding during the fuel cell operation and the noble metal particles or nanoparticles may reduce the hydrophobicity of the first layer of the gas diffusion layer and also increase the number of sites available for crystal nuclei on the gas diffusion layer. Increasing the number of sites available for crystal nuclei on the gas diffusion layer may be advantageous because it may allow the catalyst layer to be deposited and grown directly onto the gas diffusion layer and may reduce the formation of large, non-uniform particles on the gas diffusion layer. In some embodiments the gas diffusion electrode is an oxygen reduction reaction gas diffusion electrode. In some embodiments the gas diffusion electrode is a cathode. In some embodiments the gas diffusion electrode is an anode. According to a second aspect of the invention there is provided a membrane electrode assembly comprising in order, a cathode gas diffusion electrode, a membrane and an anode gas diffusion electrode, wherein at least one of the cathode gas diffusion electrode or the anode diffusion electrode comprises a gas diffusion electrode according to the first aspect of the invention. In preferred embodiments the cathode gas diffusion electrode may comprise the gas diffusion electrode of the first aspect of the invention. In some alternative embodiments the anode gas diffusion electrode may comprise the gas diffusion electrode of the first aspect of the invention. The membrane may be a polymer membrane. The membrane may be a polymer electrolyte membrane. The membrane may be a composite membrane. The membrane may be selected from the group consisting of: fluorinated membranes, partially fluorinated membranes, non-fluorinated membranes and acid-based composite membranes. The membrane may be a Nafion membrane. The membrane may comprise at least one compound selected from the group consisting of: sulfonated polyimide, polybenzimidazole polycarbonate, polystyrene, polyphosphazene, sulfonated poly(arylene ether ketone) (SPAEK), sulfonated poly(ether ether ketone) (SPEEK), sulfonated poly(ether amide), sulfonated poly(arylene ether nitrile) SPEN and any composites thereof, and any combination thereof. In some embodiments the membrane may comprise sulfonated polytetrafluoroethylene, perfluorosulfonic acid (PFSA) or a PFSA / polytetrafluoroethylene copolymer, or a combination thereof. In some embodiments the anode gas diffusion electrode comprises a gas diffusion layer and a catalyst. The anode gas diffusion electrode may comprise a gas diffusion layer and a catalyst wherein the catalyst comprises platinum. In embodiments wherein the anode gas diffusion electrode is not according to the first aspect of the invention, the anode gas diffusion electrode may comprise a gas diffusion layer and a catalyst wherein the catalyst comprises platinum and carbon. The anode gas diffusion electrode may comprise a gas diffusion layer and a catalyst wherein the catalyst comprises platinum nanowires and carbon nanospheres. The anode gas diffusion layer may comprise carbon. The anode gas diffusion layer may comprise carbon paper. The anode gas diffusion layer may comprise carbon black. The anode gas diffusion layer may comprise graphite. The anode gas diffusion layer may comprise carbon fibre. The anode gas diffusion layer may comprise carbon nanotubes. In some embodiments the membrane electrode assembly comprises in order, the cathode gas diffusion electrode, the membrane and the anode gas diffusion electrode, wherein the cathode gas diffusion electrode comprises a gas diffusion electrode according to the first aspect of the invention and the anode gas diffusion electrode comprises a gas diffusion layer and a catalyst wherein the catalyst comprises platinum and carbon. In some embodiments the cathode gas diffusion electrode comprises a gas diffusion layer and a catalyst. The cathode gas diffusion electrode may comprise a gas diffusion layer and a catalyst wherein the catalyst comprises platinum. In some embodiments the membrane electrode assembly comprises in order, the cathode gas diffusion electrode, the membrane and the anode gas diffusion electrode, wherein the anode gas diffusion electrode comprises a gas diffusion electrode according to the first aspect of the invention and the cathode gas diffusion electrode comprises a gas diffusion layer and a catalyst wherein the catalyst comprises platinum. According to a third aspect of the invention there is provided a fuel cell comprising at least one membrane electrode assembly according to the second aspect of the invention and a fuel source. The fuel cell may be selected from group consisting of: proton exchange fuel cells, anion exchange membrane fuel cells, alkali anion exchange membrane fuel cells, direct alcohol fuel cells, direct formic acid fuel cells, phosphoric acid fuel cells, solid acid fuel cells, alkaline fuel cells, direct borohydride fuel cells, microbial fuel cells, regenerative fuel cells, reformed methanol fuel cells, enzymatic biofuel cells and magnesium-air fuel cells. The direct alcohol fuel cell may be a methanol fuel cell. The direct alcohol fuel cell may be an ethanol fuel cell. In preferred embodiments the fuel cell is a proton exchange membrane fuel cell. In preferred embodiments the fuel cell is a hydrogen-oxygen proton exchange membrane fuel cell. The fuel source may comprise a fuel selected from the group consisting of: hydrogen, an alcohol, an acid and an alkaline or any combination thereof. The fuel cell may comprise at least two bipolar plates. The membrane electrode assembly may be located between at least two bipolar plates. The bipolar plates may comprise metal, coated metal, graphite, flexible graphite, C-C composite or carbonpolymer composites or any combination thereof. According to a fourth aspect of the invention there is provided a method of manufacturing a gas diffusion electrode comprising: (a) Providing a gas diffusion layer (b) Depositing a noble metal on the gas diffusion layer to form a gas diffusion layer comprising the noble metal; and (c) Depositing platinum on the gas diffusion layer comprising the noble metal. The gas diffusion electrode may be according to the gas diffusion electrode of the first aspect of the invention. The method of the invention provides a method of directly depositing a catalyst layer comprising noble metal and platinum onto a gas diffusion layer. The method of the invention may be substantially surfactant free. The surfactant free method may be advantageous because it improves the efficiency of the reaction by reducing the number of reactants and removing the requirement for a surfactant removing step after manufacturing the gas the diffusion electrode. The surfactant free method may be advantageous because it improves the cost efficiency of the reaction by reducing the number of reactants. The surfactant free method may be advantageous because it is typically difficult to remove the surfactant after manufacturing the gas diffusion electrode and residual surfactant can negatively affect the catalytic active of the electrode. In some embodiments step (b) comprises depositing the noble metal on the gas diffusion layer using physical vapour deposition. The physical vapour deposition method may be selected from the group consisting of: sputtering, ion-plating, magnetron sputtering, electron beam physical phase deposition, thermal evaporation deposition, cathodic arc physical vapour deposition and pulsed laser deposition. In preferred embodiments step (b) comprises sputtering the noble metal on the gas diffusion layer. The temperature of the sputtering may be between 20 °C and 40 °C. The temperature of the sputtering may be room temperature. This embodiment may be advantageous because it is does not require energy intensive heating or cooling. Step (b) may comprise depositing the noble metal in the form of noble metal particles. In some preferred embodiments step (b) comprises depositing the noble metal nanoparticles. The diameter of the noble metal nanoparticles may be between 1 nm and 8 nm. The diameter of the noble metal nanoparticles may be between 2 nm and 6 nm, 2 nm and 4 nm, or between 2.5 nm and 4 nm, or 2 nm and 3.5 nm or between 2.5 nm and 3.5 nm. In some embodiments step (b) comprises depositing between 0.05 pg / cm2 and 50 pg / cm2 of noble metal. In some embodiments step (b) comprises depositing 0.05 pg / cm2 and 40 pg / cm2, 0.05 pg / cm2 and 30 pg / cm2, 0.05 pg / cm2 and 20 pg / cm2, 0.05 pg / cm2and 15 pg / cm2, 0.05 pg / cm2 and 10 pg / cm2, 0.05 pg / cm2 and 8 pg / cm2, 0.05 pg / cm2 and 6 pg / cm2, 0.05 pg / cm2 and 4 pg / cm2 or between 0.05 pg / cm2 and 2 pg / cm2 of noble metal. Step (c) may comprise depositing the platinum in the form of platinum particles. Step (c) may comprise depositing the platinum in the form of platinum nanoparticles. Step (c) may comprise depositing the platinum in the form of at least one platinum nanowire. Step (c) may comprise depositing the platinum in the form of a platinum nanowire array. The platinum may be provided in a platinum precursor. The platinum precursor may comprise a chloroplatinic acid hydrate. The platinum precursor may be chloroplatinic acid (HoPtCk,), potassium hexachloroplatinate (KoPtCk,) or sodium hexachlorotplatinate (Na2PtC16). In some embodiments step (c) comprises depositing between 0.01 mg / cm2 and 1 mg / cm2 of platinum. In some embodiments step (c) comprises depositing between 0.02 mg / cm2 and 1 mg / cm2, 0.02 mg / cm2 and 0.8 mg / cm2, 0.05 mg / cm2 and 0.6 0 *2 7 7 7 7 mg / cm , 0.1 mg / cm' and 0.5 mg / cm , 0.1 mg / cm and 0.4 mg / cm', 0.1 mg / cm and 0.3 mg / cm2 or between 0.15 mg / cm2 and 0.25 mg / cm2 of platinum. In some embodiments the method comprises a growing step after step (c) wherein the growing step comprising growing at least one platinum nanowire. In some embodiments the method comprises a growing step after step (c) wherein the growing step comprising growing platinum nano wire array. The platinum nano wire array may be a vertically aligned nano wire array. The platinum nano wire array may be an organised array. The platinum nano wire array may be a random array. The platinum nano wire array may be a uniform array. A uniform array may be advantageous because it may result in a gas diffusion electrode with improved power density, outstanding durability and less power performance drop over time. Each platinum nanowire may comprise a diameter of between 1 nm and 5 nm. Each platinum nanowire may comprise a diameter of between 3 nm and 4.5 nm, 3.5 nm and 5 nm or between 3.5 nm and 4.5 nm. Each platinum nanowire may comprise a length of at least 5 nm. Each platinum nano wire may comprise a length of at least 10 nm, 20 nm, 30 nm, 40 nm or at least 50 nm. Each platinum nanowire may comprise a length of no more than 200 nm. Each platinum nano wire may comprise a length of no more than 150 nm, 100 nm, 80 nm, 75 nm, 70 nm, 65 nm, 60 n, 55 nm or no more than 50 nm. The length of the platinum nanowire may be between 5 and 200 nm. The length of the platinum nanowire may be between 5 nm and 150 nm, 10 nm and 100 nm, 10 nm and 90 nm, 10 nm and 80 nm, 10 nm and 75 nm, 10 nm and 70 nm, 10 nm and 60 nm or between 10 nm and 50 nm. The growing step may be carried out at a temperature of between 5 °C and 45 °C. The growing step may be carried out at a temperature of between 10 °C and 40 °C, 10 °C and 35 °C, 15 °C and 40 °C, 15 °C and 35 °C or between 15 °C and 30 °C. The growing step may be carried out for at least 6 hours. The growing step may be carried out for at least 12 hours, 20 hours, 30 hours, 40 hours, 50 hours, 60 hours or at least 96 hours. The growing step may be carried out for no more than 200 hours. The growing step may be carried out for no more than 160 hours, 150 hours, 120 hours, 110 hours or no more than 100 hours. The growing step may be carried out for between 6 hours and 200 hours. The growing step may be carried out for between 12 hours and 160 hours, 20 hours and 150 hours, 30 hours and 120 hours, 60 hours and 110 hours or between 70 hours and 100 hours. Detailed Description of the Invention In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 illustrates a cross sectional view of a first embodiment of a gas diffusion electrode of the first aspect of the invention. Figure 2 illustrates a cross sectional view of a first embodiment of a membrane electrode assembly of the second aspect of the invention. Figure 3 is SEM images of surfaces of (A) a gas diffusion layer without a noble metal or platinum (GDL), (B) a gas diffusion electrode comprising a catalyst layer comprising a platinum nanowire array only (Pt NWA GDE) and a gas diffusion electrode of the first aspect of the invention comprising a gas diffusion layer and a catalyst layer comprising gold nanoparticles and platinum nanowire array (Au-Pt NWA GDE) (C) at 5 400 nm scale Figure 4 is TEM images of the surface of the gas of the first aspect of the invention comprising a gas diffusion layer and a catalyst layer comprising gold nanoparticles and platinum nanowire array (Au-Pt NWA GDE) of figure 3 at (A) 100 nm scale, (B) 40 nm scale and (C) 2 10 nm scale. Figure 5 is a graph of the cell polarisation and the power density curves of the membrane electrode assemblies of the second aspect of the invention comprising gas diffusion electrodes of the first aspect of the invention (XAu-Pt NWA GDE) wherein X is the thickness (nm) of the gold 15 nanoparticles and a control gas diffusion electrode comprising platinum and not gold (Pt NWA GDE) and a second control gas diffusion electrode comprising a platinum and carbon catalyst layer (Pt / C GDE). Figure 6 is a graph of the cyclic voltammetry curves of the membrane electrode assemblies of the second aspect of the invention comprising gas 20 diffusion electrodes of the first aspect of the invention (XAu-Pt NWA GDE) wherein X is the thickness (nm) of the gold nanoparticles and a control gas diffusion electrode comprising platinum and not a noble metal (Pt NWA GDE) and a second control gas diffusion electrode comprising a platinum and carbon catalyst layer (Pt / C GDE). Figure 7 A is a graph showing the peak power density (figure 7A) and the electrochemical surface area (ECSA) (figure 7B) change during an accelerated stress test up to 40,000 cycles of membrane electrode assemblies of the second aspect of the invention comprising a gas diffusion electrode of the first aspect of the invention comprising a 0.6 nm thick layer of gold nanoparticles (Au-Pt NWA GDE), a control gas diffusion electrode comprising platinum and not a noble metal (Pt NWA GDE) and a second control gas diffusion electrode comprising a platinum and carbon catalyst layer (Pt / C GDE). Figure 7B is a graph showing the electrochemical surface area (ECSA) of the membrane electrode assemblies of figure 7A during the accelerated stress test up to 40,000 cycles. Gas diffusion electrode A first embodiment of a gas diffusion electrode of the first aspect of the invention is illustrated by figure 1. The gas diffusion electrode (1) comprises a gas diffusion layer (2) comprising a front face (4) and a back face (6) wherein at least a portion of the front face (4) comprises a catalyst layer (8) wherein the catalyst layer comprises gold nanoparticles (10) and platinum nanowires (12). The gas diffusion layer (2) comprises a substrate layer of Sigracet 39BC carbon paper (14) and a microporous layer (16) comprising carbon nanoparticles treated with polytetrafluoroethylene. The gold nanoparticles (10) are in contact with the microporous layer (16). The platinum nano wires (12) extend from the gold nanoparticles (10) to form a uniform platinum nano wire array. The overall thickness of the catalyst layer (8) is between 1 pm and 3 pm. Membrane electrode assembly A first embodiment of a membrane electrode assembly according to the second aspect of the invention is illustrated in figure 2. The membrane electrode assembly (20) comprises in order, a gas diffusion electrode (1) according to the first embodiment of gas diffusion electrode of the invention wherein the gas diffusion electrode (1) is a cathode gas diffusion electrode, a membrane (24) and an anode gas diffusion electrode (26). The cathode gas diffusion electrode (1) comprises a gas diffusion layer (2) and a catalyst layer (8). The thickness of the catalyst layer (8) is between 1 pm and 3 pm. The anode gas diffusion electrode comprises a carbon paper gas diffusion layer (28) and a catalyst layer (30) wherein the catalyst layer (30) comprises a commercial Pt / C catalyst with a loading of 0.2 mgpt / cm2. The thickness of the catalyst layer (30) is between 8 pm and 9 pm. The membrane (24) is a Nafion 212 membrane. Nafion 212 is a chemically stabilised perfluoro sulfonic acid / PTFE copolymer membrane. The first embodiment of the gas diffusion electrode (1) was manufactured according to a method of the third aspect of the invention wherein the method comprised: (a) Providing the gas diffusion layer (2) (b) Depositing gold in the form of gold nanoparticles (10) on the gas diffusion layer (2) to form a gas diffusion layer (2) comprising gold; (c) Depositing platinum on the gas diffusion layer (2) comprising gold; and (d) Growing an array of platinum nano wires (12). Step (a) comprised preparing the gas diffusion layer (2) by cleaning it under nitrogen flow. Step (b) comprised sputtering the gold on the gas diffusion layer (2) using a physical vapour deposition technique at a high vacuum of 10-6 ton-, at room temperature and with a current of 20 mA. The gold was deposited with an equivalent thickness of 0.2 nm to 2 nm. Step (c) comprised washing the gas diffusion layer (2) comprising the gold nanoparticles (10) using isopropyl alcohol (IPA) and rinsing with deionised water followed by depositing the platinum using a modified wet chemical reduction method. The gas diffusion layer was contacted with a mixture of 10.6 mL water and 265 pL of 8 % H2PtCle such that the platinum was deposited on the gas diffusion layer. Step (d) comprised growing the platinum nanowire (12) array at 25 °C for 72 hours. The resulting gas diffusion layer (2) comprising the gold nanoparticles (10) and the platinum nanowire (12) array was washed with deionised water and IPA and dried at 40 °C for 24 hours. This forms the catalyst layer (8) comprising gold nanoparticles (10) and platinum nanowires (12) on the gas diffusion layer (2). An ionomer solution comprising 200 pL IPA and 46 pL of 10 % Nafion solution was applied to the gas diffusion layer (2) comprising the catalyst layer (8). This step may increase the proton conductivity of the catalyst layer (8). This step may also improve the contact between the catalyst layer (8) and the membrane (24). Physical characterisation of the gas diffusion electrodes 5 A series of gas diffusion electrodes (1) of the first embodiment of the first aspect of the invention were tested wherein the equivalent thickness of the gold nanoparticles (10) on the gas diffusion layer (2), the concentration of gold in the measured solution for the ICP analysis, the concentration of platinum in the measured solution for the ICP analysis and the ratio of gold / platinum of the gas is shown in table 1. 10 Table 1 Samples Average thickness of the layer of the gold nanoparticles (nm) Concentration of gold (pg / L) Concentration of platinum (pg / L) Au / Pt ratio 0.2Au-Pt NWA 0.2 26.807 10484.577 0.26% 0.6Au-Pt NWA 0.6 37.205 7394.199 0.50% lAu-Pt NWA 1 37.838 6695.904 0.57% 2Au-Pt NWA 2 82.753 6066.910 1.36% SEM images were taken of the gas diffusion layer and are provided in figure 3. Figure 3A illustrates a control gas diffusion layer without gold or platinum, figure 3B 15 illustrates a control gas diffusion layer comprising platinum only. Figure 3B shows the formation of large agglomerates and non-uniformly distributed platinum nanowires within the catalyst layer. Figure 3C, illustrates the surface of the gas diffusion electrode (1) of the invention comprising gold nanoparticles (10) and the platinum nano wire (12) array. TEM images (figure 4) were also taken of the inventive gas diffusion electrode (1). The SEM and TEM images of figure 4A, 4B and 4C show that the inventive gas diffusion electrode (1) comprises a monolayer of a uniformly distributed platinum nanowire (12) array across the entire surface of the gas diffusion electrode (1). The lattice spacing of 0.23 nm indicated by the HR-TEM image (figure 4C) confirms that the single crystal platinum nanowire is grown along the <111 >direction. Without being bound by theory, it is understood that the gold nanoparticles (10) act as anchoring seeds from which the long and non-agglomerated platinum nanowire (12) array grows from, which in turn provides a thin and uniform catalyst layer (8). Samples of the catalyst layer (8) were removed from the surface of the gas diffusion layer (2) and measured by TEM. The measured samples showed that the platinum nano wires (12) comprise an average diameter of 3-5 nm and a length of up to 50 nm. Fuel cell tests Each of the gas diffusion electrodes (1) of table 1 were assembled into a membrane electrode assembly (20) of the first embodiment of the second aspect of the invention and the cell polarisation and the power density curves of the membrane electrode assemblies (20) were recorded. The results are provided in figure 5. A control membrane electrode assembly comprising a cathode electrode which only comprised platinum and not gold (Pt NWA GDE) and a control membrane electrode assembly comprising a gas diffusion electrode comprising a platinum and carbon catalyst layer (Commercial Pt / C GDE) were also tested. The test was conducted in 5 cm2 FE / air PEM fuel cells at 80 °C with a backpressure of 2 bar and 100 % relative humidity. Figure 5 shows that all of the tested membrane electrode assemblies comprising the inventive gas diffusion electrodes (1) (XAu-Pt NWA) of table 1 provided increased power density and cell potential compared to the control membrane electrode assemblies. Without being bound by theory, it is understood that the improved performance of the inventive gas diffusion electrode is due to the high uniformity of the distribution of the platinum nano wire (12) array within the catalytic layer (8). The optimal thickness of the layer of the gold nanoparticles (10) was demonstrated at 0.6 nm (0.6Au-Pt NWA GDE) wherein the highest power density was achieved. The peak power density reached 1.24 Wcm'2 which is 18.7% and 22.1 % higher than that of the control Pt NWA gas diffusion electrode and commercial Pt / C gas diffusion electrode, respectively. Furthermore, compared to the commercial Pt / C GDE, all of the tested inventive gas diffusion electrodes (XAu-Pt NWA) showed a slower potential drop in the large current density region. This can be attributed to the enhanced mass transfer characteristic of the unique thin catalytic layer (8). This is advantageous because it results in a gas diffusion electrode (1) and a resulting membrane electrode assembly (20) with a high power density, high durability and stability over time. Cathode cyclic voltammetry curves were also recorded for all the inventive gas diffusion electrodes (1) of table 1 in the membrane electrode assembly (20) of the first embodiment of the second aspect of the invention. The cyclic voltammetry curves are provided in figure 6. Figure 6 shows that the inventive gas diffusion electrodes (1) exhibit a typical cyclic voltammetry curve of the platinum; however, there is a positive shift of the P-0 reduction peak at around 0.8 V for all the inventive gas diffusion electrodes comprising gold nanoparticles (10) and the platinum nano wire (12) array, compared to the Pt / C gas diffusion electrode. This indicates a better capability of the inventive gas diffusion electrodes for oxygen reduction reactions (ORR) compared to the Pt / C gas diffusion electrode. This is advantageous because it results in a gas diffusion electrode (1) with high durability and stability. To evaluate the durability of the inventive gas diffusion electrodes (I), an accelerated stress test (AST) was conducted on the first embodiments of the membrane electrode assemblies comprising the inventive gas diffusion electrodes (1), wherein the inventive gas diffusion electrode (1) comprised a 0.6 nm thick layer of gold nanoparticles (10). The AST comprised cycling the potential from 0.05 to 1.2 V for 50,000 cycles with N2 in the cathode and the hydrogen in the anode. The polarization curves were recorded every 500 cycles. The results are shown in figure 7A and figure 7B. Figure 7A shows that the inventive gas diffusion electrode (I) (Au-Pt NWA GDE) showed a drop of 33.6 % in peak power density after the first 5,000 cycles compared to the significant drop of 55.3 % shown by the control Pt / C gas diffusion electrode. After 20,000 cycles, the degradation of the peak power density becomes 92.4% and 41.1% for the Pt / C gas diffusion electrode and inventive (Au-Pt NWA) gas diffusion electrode (1) respectively. Figure 7B shows that the inventive gas diffusion electrode (1) (Au-Pt NWA GDE) showed a lower decrease of ECSA over time compared to the gas diffusion electrode comprising only platinum or platinum and carbon. The performance of the inventive gas diffusion electrode (1) becomes very stable in the following 20,000 cycles and only a 2.0% further decrease is observed, 5 showing the advanced durability of the inventive electrode. This result suggests that the platinum nano wire (12) array of the inventive gas diffusion electrode (1) has a much higher tolerance to the catalyst degradation mechanisms such as dissolution, Ostwald ripening, and aggregation. It is understood that after the introduction of gold nanoparticles (10), the 10 platinum nano wire (12) array obtains even better stability due to the strengthened anchoring effect between the gold nanoparticles (10) and the carbon particles (16) within the gas diffusion layer (2), thus further improved durability is achieved. The inventive gas diffusion electrode (1) is therefore advantageous because it provides high power density, reduced power performance over time and high durability 15 and stability. The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. A gas diffusion electrode comprising a gas diffusion layer on which a catalyst layer is applied wherein the catalyst layer comprises platinum or an alloy thereof and a separate noble metal or an alloy thereof.

2. A gas diffusion electrode according to claim 1 wherein the platinum or alloy thereof is a platinum nanowire.

3. A gas diffusion electrode according to claim 1 or claim 2 wherein the platinum or alloy thereof is a platinum nanowire within a platinum nanowire array.

4. A gas diffusion electrode according to any one of claims 2 or 3 wherein the diameter of the platinum nanowire is between 1 nm and 5 nm.

5. A gas diffusion electrode according to any one of claims 2 to 4 wherein the length of the platinum nanowire is between 5 nm and 200 nm.

6. A gas diffusion electrode according to any preceding claim wherein the amount of platinum or alloy thereof is between 0.01-2 mg / cm27. A gas diffusion electrode according to any preceding claim wherein the noble metal is gold or palladium, preferably gold.

8. A gas diffusion electrode according to any preceding claim wherein the amount of noble metal or alloy thereof is between 0.05 and 50 pg / cm2.

9. A gas diffusion electrode according to any preceding claim wherein the amount of noble metal or alloy thereof in the catalyst layer is between 0.1% and 4 %.

10. A gas diffusion electrode according to any preceding claim wherein the noble metal or alloy thereof comprises nanoparticles.

11. A gas diffusion electrode according to any preceding claim wherein the noble metal or alloy thereof comprises gold nanoparticles.

12. A gas diffusion electrode according to claim 10 or 11 wherein the diameter of the nanoparticles is between 1 and 8 nm.

13. A gas diffusion electrode according to any preceding claim wherein the noble metal or alloy thereof comprises noble metal nanoparticles and the platinum or alloy thereof comprises platinum nano wires.

14. A gas diffusion electrode according to any preceding claim wherein the noble metal comprises gold nanoparticles and the platinum comprises platinum nano wires.

15. A gas diffusion electrode according to any preceding claim wherein the ratio of platinum or alloy thereof to noble metal or alloy thereof is between 98:2 and 99.9:0.1.

16. A gas diffusion electrode according to any preceding claim wherein thickness of the catalyst layer is between 0.5 pm and 6 pm.

17. A gas diffusion electrode according to any preceding claim wherein the gas diffusion layer comprises carbon.

18. A membrane electrode assembly comprising in order, a cathode gas diffusion electrode, a membrane and an anode gas diffusion electrode, wherein the cathode gas diffusion electrode or the anode diffusion electrode comprises a gas diffusion electrode according to any one of claims 1 to 17.

19. A membrane electrode assembly according to claim 18 wherein the membrane is a polymer electrolyte membrane.

20. A fuel cell comprising a membrane electrode assembly according to any one of claims 18 or 19 and a fuel source.

21. A fuel cell according to claim 20 wherein the fuel cell is selected from the group consisting of: proton exchange membrane fuel cells, anion exchange membrane fuel cells, direct alcohol fuel cells and direct formic acid fuel cells.

22. A fuel cell according to claim 20 or claim 21 wherein the fuel cell is a hydrogenoxygen proton exchange membrane fuel cell.

23. A method of manufacturing a gas diffusion electrode of any one of claims 1 to 17 comprising:a. Providing a gas diffusion layerb. Depositing a noble metal or alloy thereof on the gas diffusion layer to form a gas diffusion layer comprising the noble metal or alloy thereof; andc. Depositing platinum or an alloy thereof on the gas diffusion layer comprising the noble metal or alloy thereof.

24. A method according to claim 23 wherein step (c) comprises growing a platinum nano wire, preferably a platinum nano wire array.

25. A method according to claim 23 or 24 wherein step (c) comprises growing at least one platinum nanowire or the platinum nanowire array at a temperature of between 5 °C and 45 °C.

26. A method according to any one of claims 23 to 25 wherein step (a) comprises sputtering the noble metal or alloy thereof on the gas diffusion layer.

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