Date of Award

1-1-2011

Document Type

Campus Access Dissertation

Department

Chemical Engineering

First Advisor

John W Weidner

Abstract

Electrocatalysts are essential for the development of active and durable fuel cells and hydrogen production technologies. Generally, electrochemical processes of energy conversion and hydrogen generation in a Proton Exchange Membrane (PEM) utilize precious metals, such as platinum, iridium and ruthenium, as electrocatalysts. For the methanol oxidation and oxygen evolution reaction, a bimetallic structure can be used to enhance kinetics and increase stability. It is desired to support electrocatalysts to disperse nanoparticles on the surface and promote better catalyst utilization. Traditionally, carbon has been used as an electrochemical support because it has a high surface area and high electrical conductivity. The problem with carbon is that it is not a very stable material and can corrode at voltages more than 0.9 V, affecting performance of the electrochemical reaction. Therefore, it would be useful to support electrocatalysts in a stable material with suitable conductivity

Using titanium dioxide as a support can be advantageous due to its corrosion-resistant capability. TiO2 exhibit different crystalline structures, such as anatase and rutile, which can have an effect on catalytic activity. Unfortunately, it is not conductive; hence, it is not used in electrochemical applications. However, it can be doped with niobium to increase electronic conductivity; but, it usually come at the expense of surface area. In this work, TiO2 and Nb-TiO2 were studied as platinum/ruthenium and iridium/ruthenium nanoparticles supports for the electrochemical oxidation of methanol and oxygen evolution, respectively. Even though the conductivity of our supports was very low, adding a considerable loading of nanoparticles increased conductivity of the composite material (support + catalyst) to acceptable levels. Using cyclic voltammetry (CV) and direct methanol fuel cell tests creating a membrane electrode assembly (MEA), Pt-Ru supported on Nb-TiO2 and TiO2 showed superior activity over similar catalysts supported on carbon. Also, the supported electrocatalysts on anatase TiO2 were more active than supported on rutile for methanol electrooxidation. For the case of oxygen evolution reaction (OER), supported Ir:Ru nanoparticles had higher performance than unsupported metal and corresponding metal oxide. It is known that metal oxides are more durable than bare metals for OER. Performing durability studies in CV it was demonstrated that even the metal oxide Ir0.5Ru0.5O2 can degrade. However, when titanium was added to this metal oxide, the stability improved. Polarization experiments in a PEM water electrolyzer were tested for Ir0.45Ru0.45Ti0.10O2; and it was found a higher performance at current densities more than 1 A/cm2, compared to the Ir:Ru supported nanoparticles.

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