Development of Cathodic Electrocatalysts for Low Temperature H2 Fuel Cell Applications: Improving Oxygen Reduction Activity through the Manipulation of Size, Shape, and Composition.

In this dissertation, the oxygen reduction activity of metal nanoparticle electrocatalysts is improved through the manipulation of their size, shape, and composition. The design of superior catalysts first requires identifying processes that limit overall performance. On both silver and platinum electrodes, the rate of oxygen reduction is limited by the initial proton/electron transfer to O2, however the limited site availability also limits ORR activity of platinum at high electrochemical potentials. Different approaches must employed to improve the activity of platinum and silver electrocatalysts. The first project in this thesis describes the development of Pt alloy electrocatalysts with enhanced ORR activity compared to Pt/C standards. The design of Pt monolayer electrocatalysts is informed by quantum chemical calculations. By understanding how alloying impacts the reactivity of surface atoms, we are able to prepare a special class of catalysts with finely tunable activity dependent upon the composition of their nanoparticle core. The best performing materials have specific activities up to four times higher than Pt/C electrocatalysts. Rigorous characterization and electrochemical testing confirm these enhancements and the observed trends in activity result from the alloy structure rather than size and shape effects. The second project focuses on controlling the morphology of Ag electrocatalysts in order to improve their alkaline ORR activity. In this study, the rates on Ag nanospheres and Ag nanocubes are compared over a wide range of metal loadings. Contrary to our original hypothesis, nanospheres are found to be slightly more active than nanocubes. Rigorous experimental work confirms the reported shape dependence effects are consistent with the relative abundance of 111 and 100 sites on Ag nanoparticles.PhDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120863/1/vancleve_1.pd