Abstract

This dissertation is focused on the investigation of the oxygen reduction reaction (ORR) utilizing both existing and newly discovered electrocatalysts. Interest in the ORR is motivated by its application in the cathodes of most fuel cells. The slow kinetics of the ORR are a major barrier to the widespread usage of fuel cells, thus motivating research aimed at increasing understanding of existing electrocatalysts and driving demand for the development of improved electrocatalysts for the ORR.

The first section focuses on laccase, a multicopper oxidase that catalyzes the four-electron reduction of oxygen to water. Upon adsorption to an electrode surface, laccase is known to reduce oxygen at overpotentials lower than the best noble metal electrocatalysts usually employed. Whereas the electrocatalytic activity of laccase is well established on carbon electrodes, laccase does not typically adsorb to better defined noble metal surfaces in an orientation that allows for efficient electrocatalysis. In this work, anthracene-2-methanethiol (AMT) was employed to modify the surface of Au electrodes and the electrocatalytic activity of adsorbed laccase was examined. AMT facilitated the adsorption of laccase, and the onset of electrocatalytic oxygen reduction was observed as high as 1.13 V_RHE. Linear Tafel behavior was observed with a 144 mV/dec slope, consistent with an outer-sphere single-electron transfer from the electrode to a Cu site in the enzyme as the rate-determining step of the oxygen reduction mechanism.

Inspired by the multicopper active site of laccase, the second section focuses on the precipitation of an insoluble complex of copper(II) with 3,5-diamino-1,2,4-triazole onto a carbon black support that leads to the formation of an efficient catalyst for the ORR referred to as CuDAT. The oxygen-reduction activity is reported over a wide pH range from 1 to 13 and the onset of the ORR occurs at potentials as high as 0.86 V_RHE, making CuDAT the most active synthetic copper-based electrocatalyst for the ORR reported to date. For the first time, ex situ magnetic susceptibility measurements were used to demonstrate the presence of multicopper sites on the electrode.

The final section addresses the question of whether or not the active sites for the ORR in electrocatalysts based on carbon-supported transition-metal complexes are metal-centered as this has become controversial, especially for heat-treated materials. Some have proposed that the transition metal only serves to form highly active sites based on nitrogen and carbon. Here, we examine the oxygen reduction activity of carbon-supported iron(II) phthalocyanine (FePc) before and after pyrolysis at 800 °C and CuDAT in the presence of several anions and small-molecule poisons, including fluoride, azide, thiocyanate, ethanethiol, and cyanide. CuDAT is poisoned in a manner consistent with a copper-based active site. Although FePc and pyrolyzed FePc are remarkably resilient to most poisons they are poisoned by cyanide indicative of iron-based active sites.