(525a) Pt-Free Electrocatalysts for Efficient Oxygen Reduction In Alkaline Fuel Cells: Experimental and Computational Insights
The slow rate of the oxygen reduction reaction (ORR) is the major source of overpotential loss in low-temperature hydrogen fuel cells. Expensive Pt-based materials have been found to be the most effective catalysts, and exploration of cheaper alternatives has been hampered by the requirement of operation at low pH, where most metals are susceptible to corrosion. The pH constraint has primarily been due to a lack of electrolyte membranes that are stable under basic conditions, but recent advances in membrane technology have made fuel cell operation at high pH feasible. This is a landscape-changing event which expands the phase space of potential catalytic materials. Most notably, in basic medium Ag is redox stable and shows comparable ORR activity to Pt on a power-per-dollar basis.
In this work, we couple cyclic voltammetry (CV) and rotating disk electrode (RDE) experiments with quantum chemical density functional theory (DFT) calculations to compare ORR mechanisms on Pt and Ag. The calculations account for the potential-dependent interactions of reactive intermediates and co-adsorbed species on the catalyst surface in an aqueous environment. Predicted surface structures agree well with CV adsorption measurements, and predicted overpotentials agree with the onset of catalytic activity measured with the RDE method. Our studies indicate the main tradeoff for an ORR catalyst is between the ability to activate molecular O2 and the ability to remove adsorbed hydroxyl groups in the form of water. While Ag has difficulty activating O2, Pt binds hydroxyl groups too strongly. Using these insights, we utilize DFT calculations to identify bimetallic nanostructures of silver and 3d elements (Ni,Co,Fe), which more effectively activate oxygen without binding hydroxyls too strongly. We synthesize these bimetallic structures and demonstrate their enhanced ORR activity. The improvements are discussed in a general framework designating the optimal electronic structure for ORR catalysis.