Catalyst layer operation electrode processes

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

The apparent transfer coefficient of the cathodic reaction, ac, is a measure of the sensitivity of the transition state to the drop in electrostatic potential between electrolyte and metal [109,112]. According to Ref. 113, it is ac = 0.75 for the O2 reduction on Pt in aqueous acid electrolytes. In Ref. Ill the value ac = 1.0 was reported instead. Since the cathodic reaction is a complex multistep process, it might follow several reaction pathways, and the competition between them is affected by the operation conditions (rj, p, T). Therefore, different values of ac have been reported in different regimes of operation. Although in the simple reactions the transfer coefficient is a microscopic characteristic of the elementary act [112], for complex multistage reactions in fuel cell electrodes it is rather an empirical parameter of the model. The dependence of effective a for methanol oxidation on the catalyst layer preparation was recently studied [114]. 

The transport of energy, mass, and charge is at the heart of proton exchange membrane fuel cell (PEMFC) operation. The porous layers in modern membrane electrode assemblies (MEAs) lie at the Interface between the macroscopic phenomena occurring in the flow channels and the micro- and nanoscopic processes in the catalyst layers (see Figure 5.1). These layers must deliver the reactants and remove the products from the electrochemical reactions at the fuel cell electrodes. They must also provide connections to the current collecting plates with minimal thermal and electrical resistances. 

The process of oxide-layer growth on platinum has been thoroughly investigated for smooth platinum surfaces in aqueous electrolytes and in the gas phase (Angerstein-Kozlowska et al. 1973 Conway et al. 1990 Conway and Jerkiewicz 1992 Harrington 1997). While Conway et al. (1990) proposed rapid diffusion of oxide species followed by a slow oxide turnover process, Harrington (1997) opted for slow formation of the oxide species foUowed by rapid diffusion of oxide species across the surface. The kinetics of surface oxide formation on fuel-cell-type platinum catalysts has also been studied. Paik et al. (2004) observed that surface oxide formation on a platinum electrode occurs rapidly under realistic operating conditions of... 

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