Carbon Nano-Fibre Supported Electrocatalysts for Fuel Cells
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This thesis covers topics related to fuel cell electrocatalyst preparation and electrochemical characterization. In particular, the influence of catalyst support material for Pt catalysts is examined in detail. The aim is to improve the understanding of the role of the support material on electrocatalytic activity for fuel cell relevant reactions like the oxygen reduction reaction (ORR), CO oxidation reaction (COR) and methanol oxidation reaction (MOR). Carbon supported Pt electrocatalysts were prepared by two different synthesis approaches on Vulcan XC72 and carbon nanofibres. The applied synthesis methods were the polyol and citrate (CA) methods. The CA method was studied in detail and optimized for production of catalysts with small particle size and narrow particle size distribution. The effect of chloride on the stability of platinum electrocatalysts was studied by rotating disk measurements in sulfuric acid electrolyte with increasing concentration of chloride anions. The activity towards oxygen reduction was found to be reduced by a factor of seven when 140 ppm chloride was present. Platinum corrosion was found to be severe at high potentials, presumably accelerated by potential cycling, and greatly enhanced by mass transport. The Pt dissolution rate increased with increasing Cl– concentration up to 20 ppm above which corrosion rate was unaffected by further increases in the Cl– content. The use of potential of zero total charge (pztc) as a descriptor of catalyst activity was explored. Electrochemical measurements on three different electrochemical reactions, which all are expected to depend on the oxidation state of the Pt metal surface, confirmed the hypothesis that catalytic activity is strongly related to pztc. The results showed that changing the catalyst supports for catalysts with Pt particles of similar size alters pztc. Changes in pztc affected the activity towards the studied electrochemical reactions, but the effect was different for the different supports. These findings are strong evidence of metal support interactions occuring between Pt and carbon support materials.