31.17: Electrodes and Membrane-Electrode Assembly
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In the first fuel cells, platinum was used in relatively large quantities. This perhaps led to the false belief that most of the cost of a fuel cell is down to the platinum in it. Generally this is not the case. Platinum particles are deposited very finely onto carbon powders so that the platinum is very finely divided with a maximal surface area. With catalysts produced in this way, the raw material platinum cost is just $10 for a 1 kW cell stack.
Before the catalyst layer is applied to the electrolyte, a coating of soluble electrolyte is brushed onto it. This ensures that there is good contact between the platinum and the electrolyte to achieve the important three-phase interaction between gas, catalyst and electrolyte necessary for the reaction to proceed.
The catalyst can be applied to the membrane in one of two ways: Either the catalyst powder can be applied directly to the membrane, by rolling, spraying or printing, and then have the supporting electrode structure (often called the gas-diffusion layer) added afterwards, or the electrodes can be assembled separately and bonded to the membrane in complete form by hot pressing. The catalyst powder is sometimes mixed with PTFE to drive out product water and prevent the electrode becoming water logged. The “gas diffusion layer” is added between the catalyst and the bipolar plate to provide some rigidity to the MEA and to ensure ease of diffusion. This layer is usually composed of carbon cloth or carbon paper 0.2–0.5mm thickness, with more PTFE added to expel water.