Carbon Supports on the Catalytic Layers in PEMFCs

6 INFLUENCE OF CARBON SUPPORTS ON THE CATALYTIC LAYERS IN PEMFCs 

The critical components in PEMFCs are the CLs, where the electrochemical reactions take place that transform oxygen molecules into water at the cathode and hydrogen molecules into protons at the anode. Optimization of the CLs is a multiparameter problem, since they must meet numerous requirements simultaneously, ensuring optimum rates of electrocatalytic reactions as well as providing pathways for electrons, protons, reagents, and products. The strongest constraints are imposed on the cathode, where water molecules produced electrochemically must be transferred to the gas phase and removed from the catalytic layer to avoid its flooding. The latter leads to a drastic decrease in cell performance. 

Carbon supports strongly affect fuel cell performance. They may influence the intrinsic catalytic activity and catalyst utilization, but also affect mass transport and ohmic losses. This makes analyses of the role of carbon materials rather complicated. Although numerous studies have been devoted to the carbon support improvement, only a few have attempted to establish relationships between the substructural characteristics of carbon support materials and cell performance. The influence of carbon supports on the intrinsic catalytic activity is the subject of Section 12.6.1. In Section 12.6.2 we consider the influence of support on macrokinetic parameters such as the catalyst utilization, mass transport, and ohmic losses. In Section 12.6.3 we review briefly recent data obtained upon utilization of novel carbon materials as supports for fuel cell electrocatalysts. 

Dependence of the intrinsic catalytic activity of metal nanoparticles on the support material may have various origins. First, the support structure and morphology 

CARBON MATERIALS AS SUPPORTS FOR FUEL CELL ELECTROCATALYSTS 

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Catalytic layers in the anode and cathode are also critical components in a PEMFC. Platinum (Pt) or a Pt-Ru alloy supported by nano-carbon particles is often used as the catalyst in the anode [1-6]. Favorable electronic properties have been suggested by density-functional theory studies, which have shown fliat flie CO adsorption energy is the lowest on the Pt monolayer located above Ru compared with the adsorption energies on pure Pt, pure Ru, and a Pt-Ru mixed surface layer over Pt [7, 8], The bifunctional mechanism [4] suggests that Ru provides an active surface for oxidative removal of adsorbed CO at the neighboring Pt sites. Thus, a... 

Electrochemical energy conversion for technical applications relies on a high catalytic reactivity. The electro catalytic reactivity is strongly influenced by the structure and composition of the surface of the catalysts. In PEMFC the catalysts consist of Pt or Pt alloys, which are of nanometer size and often supported on carbon in order to optimize the surface area and the costs of the noble metals. However, the important relation between the reactivity and the structure is obscured in technical electro catalysts by a variety of parameters, like, e.g. the properties of the carbon support, the preconditioning of the catalyst, and the structure of the interface between the electrolyte and the active layer (e.g. the Nafion content). An excellent review on the PEM fuel cell electrodes is given by Litster and McLean (2004)... 

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