Chemistry ETDs

Publication Date

2-14-2014

Abstract

Efficient electrocatalysts for the oxygen reduction reaction (ORR) are needed for fuel cells and next generation metal-air batteries, as the overall efficiency of these systems is currently limited by cathode kinetics. While Pt and Pt-based alloys demonstrate excellent electrocatalytic activities, their prohibitive cost and rarity ultimately prevents their widespread utilization. Catalyst materials that are highly active, stable, and cost effective are needed. Manganese oxide (α-MnO2) catalysts doped with varying amounts of Cu or Ni were synthesized, characterized, and evaluated for their performance as catalysts for the oxygen reduction reaction in alkaline electrolyte. Cu-α-MnO2 nanowire catalysts consistently attained higher terminal current densities than those achieved with undoped α-MnO2. As the amount of Cu-dopant in the nanowire increases, the kinetic rate constants increase, and the charge transfer resistances decrease. By studying the physicochemical properties of the Cu-doped catalysts, it was discovered that an increase in Cu-content corresponds to an increase in catalyst surface area, as well as an expansion of the crystalline lattice. The observed improvement in catalytic behavior can also be attributed to an increase in surface Mn3+ character, as measured by X-ray photoelectron spectroscopy (XPS), that corresponds to an increase in Cu-dopant at the surface. Similarly, an increase in Mn3+ character is hypothesized to be a significant factor in the activities of Ni-α-MnO2 catalysts, though the observed catalytic performance of this catalyst series does not trend directly with Ni-content. In addition, a novel composite containing Ag and graphene nanoribbons (Ag-GNRs) was also studied. Catalytic studies revealed that the Ag-GNR composite outperforms a commercial Ag-Carbon catalyst as determined by comparing terminal current densities, n-values, kinetic rate constants, onset potentials, and half-wave potentials. In addition, the Ag-GNR composite is more electrocatalytically selective than the benchmark Pt/C catalyst when subjected to simulated methanol crossover effects.

Project Sponsors

This work was funded by Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energys National Nuclear Security Administration under Contract DE-AC04-94AL8500.'

Language

English

Keywords

electrocatalysts, electrochemistry, oxygen reduction, manganese oxide, graphene nanoribbons

Document Type

Thesis

Degree Name

Chemistry

Level of Degree

Masters

Department Name

Department of Chemistry and Chemical Biology

First Advisor

Kemp, Richard A.

First Committee Member (Chair)

Qin, Yang

Second Committee Member

Habteyes, Terefe

Third Committee Member

Lambert, Timothy N.

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