Platinum-based oxygen reduction electrocatalysts.

An efficient oxygen reduction reaction (ORR) offers the potential for clean energy generation in low-temperature, proton-exchange membrane fuel cells running on hydrogen fuel and air. In the past several years, researchers have developed high-performance electrocatalysts for the ORR to address the obstacles of high cost of the Pt catalyst per kilowatt of output power and of declining catalyst activity over time. Current efforts are focused on new catalyst structures that add a secondary metal to change the d-band center and the surface atomic arrangement of the catalyst, altering the chemisorption of those oxygencontaining species that have the largest impact on the ORR kinetics and improving the catalyst activity and cost effectiveness. This Account reviews recent progress in the design of Pt-based ORR electrocatalysts, including improved understanding of the reaction mechanisms and the development of synthetic methods for producing catalysts with high activity and stability. Researchers have made several types of highly active catalysts, including an extended single crystal surface of Pt and its alloy, bimetallic nanoparticles, and self-supported, low-dimensional nanostructures. We focus on the design and synthetic strategies for ORR catalysts including controlling the shape (or facet) and size of Pt and its bimetallic alloys, and controlling the surface composition and structure of core-shell, monolayer, and hollow porous structures. The strong dependence of ORR performance on facet and size suggests that synthesizing nanocrystals with large, highly reactive {111} facets could be as important, if not more important, to increasing their activity as simply making smaller nanoparticles. A newly developed carbon-monoxide (CO)-assisted reduction method produces Pt bimetallic nanoparticles with controlled facets. This CO-based approach works well to control shapes because of the selective CO binding on different, low-indexed metal surfaces. Post-treatment under different gas environments is also important in controlling the elemental distribution, especially the surface composition and the core-shell and bimetallic alloy nanostructures. Besides surface composition and facet, surface strain plays an important role in determining the ORR activity. The surface strain depends on the crystal size, the presence of an interface-lattice mismatch or twinned boundary, and between nanocrystals and extended single crystal surfaces, all of which may be factors in metal alloys. Since the common, effective reaction pathway for the ORR is a four-electron process and the surface binding of oxygen-containing species is typically the limiting step, density functional theory (DFT) calculation is useful for predicting the ORR performance over bimetallic catalysts. Finally, we have noticed there are variations among the published values for activity and durability of ORR catalysts in recent papers. The differences are often due to the data quality and protocols used for carrying out the analysis using a rotating disk electrode (RDE). Thus, we briefly discuss some practices used in such half-cell measurements, such as sample preparation and measurement, data reliability (in both kinetic current density and durability measurement) and iR correction that could lead to more consistency in measured values and in evaluating catalyst performances.