Kinetic Analysis of Electrochemical Oxygen Reduction and Development of Ag-alloy Catalysts for Low Temperature Fuel Cells.

Abstract

This dissertation applies insights from quantum chemical calculations and heterogeneous kinetic analysis to interpret macroscopic reactivity trends in electrochemical systems and design optimal electrocatalysts. Specifically we explore the mechanism of the electrochemical oxygen reduction reaction (ORR) on the surfaces of Pt (a near-optimal catalyst) and Ag electrodes. We have identified design criteria for improving the reaction rate in each case and developed cost-effective Ag-based alloy materials with activity approaching that of more costly Pt catalysts. We first demonstrate, using microkinetic modeling and density functional theory calculations, that deviations from ideal electrode kinetics (a linear potential vs. log current relationship) are inherent to the ORR and any multi-step heterogeneous electrocatalytic reaction. Deviations result from simultaneous changes in the rate of the rate-limiting elementary step and the number of available active sites on the electrode surface as potential is shifted. We show the ORR kinetic variations on Pt electrodes are well-reproduced by a simple description of changes in OH and H2O surface intermediate coverages, and that weaker binding materials exhibit higher rates due to higher active-site availability. In contrast, on Ag a very weak relation is found between adsorbate coverage and changes in the apparent rate law. This points to a strong role of under-coordinated active sites, which become poisoned at low potentials while the majority of the surface is still clean. Moving toward stronger binding on Ag should yield higher ORR activity by increasing turnover rates on the more predominant surface facets. Using the mechanistic insights mentioned, we illustrate the design of relatively inexpensive Ag-Co surface alloy nanoparticle electrocatalysts for ORR, with equivalent area-specific activity to commercial Pt-nanoparticles at realistic fuel cell operating conditions. The Ag-Co materials were identified with quantum chemical calculations and synthesized with a novel bimetallic precursor decomposition technique that generates a surface alloy, despite bulk immiscibility of the elements. Characterization studies show the origin of activity improvement comes from a ligand effect, in which Co perturbs Ag surface sites. We also explore bimetallic precursor decomposition to produce Ag-Ni and Ag-Fe alloys but find that the products exhibit substantial segregation and have ORR activities similar to monometallic Ag.