Electrochemical Oxygen Reduction at Platinum/Mesoporous Carbon/Zirconia/Ionomer Thin-Film Composite Electrodes

Platinum catalysts for electrochemical oxygen reduction were prepared on mesoporous carbon supports, some of which included embedded zirconia nanoparticles, by borohydride reduction of hexachloroplatinic acid in ethylene glycol solution. The resulting materials are approximately 20 percent platinum by weight and consist of platinum particles having diameters in the 2-3 nm range, evenly distributed throughout the support structure. Electrochemical surface areas for platinum were measured by hydrogen adsorption/desorption and carbon monoxide stripping and were found to be lower for platinum on carbon containing zirconia than for carbon without zirconia. This finding is thought to reflect a slightly larger platinum particle size and also a higher degree of particle aggregation for platinum on the carbon support containing zirconia, possibly caused by localized patches of positive charge on zirconia particles under the acidic conditions used to deposit platinum. Zirconia-containing carbon supports that had been treated with platinum were subsequently treated with polymer electrolytes having terminal aryl phosphonate groups that can covalently bind to exposed zirconia sites, thereby producing composite materials having intimately integrated platinum catalysts and polymer electrolytes immobilized within a mesoporous carbon support structure. Electrochemical ORR activity of these materials as thin-film electrodes was assessed using rotating disk electrode voltammetry in aqueous sulfuric acid solutions. Activity for platinum on a mesoporous carbon support without zirconia is comparable to that of benchmark materials, e.g., platinum on Vulcan carbon XC-72, but is diminished on the mesoporous carbon support containing zirconia. Remarkably, the platinum ORR activity is fully recovered and slightly enhanced on zirconia-containing supports following treatment with polymer electrolyte. Possible reasons for the recovery of the platinum ORR activity upon polymer electrolyte treatment, and implications for possible application in polyelectrolyte membrane fuel-cell technology, are discussed.