Cathode catalyst Direct conversion

While the membrane represents the heart of the fuel cell, determining the type of cell and feasible operating conditions, the two catalyst layers are its pacemakers. They fix the rates of electrochemical conversion of reactants. The anode catalyst layer (ACL) separates hydrogen or hydrocarbon fuels into protons and electrons and directs them onto distinct pathways. The cathode catalyst layer (CCL) rejoins them with oxygen to form liquid water. This spatial separation of reduction and oxidation reactions enables the electrons to do work in external electrical appliances, making the Gibbs free energy of the net reaction, —AG, available to them. 

In fuel cells hydrocarbons are dehydrogenated by active catalysts. The main purpose of these cells is the direct conversion of combustion energy into electrical energy. At the fuel cell cathode the hydrocarbon is in must cases oxidized to carbon dioxide (equation 26a), because the intermediates are more easily oxidized than the starting hydrocarbon. Only in few cases are substitution, dehydrogenation or coupling products of the hydrocarbon obtained. At the fuel cell anode oxygen is reduced to water (equation 26b). 

The thickness of the cathode catalyst layer (CCL) in low-temperature fuel cells varies from ten micrometres in modern PEFCs to hundred of micrometres in direct methanol fuel cells (DMFCs). The CCL in DMFCs operates at a high level of flooding, which reduces ORR efficiency. The considerable thickness of the active layer facilitates electrochemical conversion under these conditions. 

In contrast to stationary applications, portable applications require frequent start and stop procedures. Therefore for SOFC, a robust cell design and adapted electrode-electrolyte assemblies are an important issue. Frequent thermal cycles between room temperature and an operation temperature of about 600-800 °C pose challenges to the layered system consisting of solid anode, ceranfic electrolyte and solid cathode with respect to thermal and mechanical stability. For several years, different approaches to developing tubular nficro SOFC have been undertaken but did not lead to a commercial product yet. As SOFC can be operated with pure hydrogen, reformate and hydrocarbons as fuel as well - the latter option means direct internal reforming at the anode catalyst — various investigations focused on reduced operation temperature and a parallel conversion of fuels [21]. 

On the cathode side, permeated methanol reacts directly with oxygen the presence of catalyst particles facilitates the direct combustion. This parasitic reaction consumes methanol and lowers the amount of oxygen available for useful electrochemical conversion (Gottesfeld, 2007). Thus, any realistic model of DMFC should include crossover. 

To produce a structure capable of performing the above-mentioned electrochemical task, an anode and cathode, each containing catalyst particles, are adhered to opposite sides of the PEM to form a layered composite structure. This composite structure is responsible for the electrochemical conversions and directed flow of fuel, byproducts, ions, electrolytes and electrons requisite for the electrochemical functioning of a fuel cell. This layered composite structure is also referred to as a membrane electrode assembly (MEA). 

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