Thomas H. Madden

Direct Mechanism of OH Radicals Formation in PEM Fuel Cells

The generation of hydroxyl and/or peroxyl (·OOH) radicals on fuel cell catalysts has direct consequences for the durability of the polymer electrolyte membrane. Whether radical generation occurs from hydrogen peroxide decomposition vs. direct generation on the Pt catalyst surface is of key interest. Ab initio calculations were performed with Gaussian software employing both cluster models of a platinum surface, as well as single / double Pt atom ensembles. Although complete four-electron reduction to water on Pt is desired, hydrogen peroxide is also formed at potentials below 0.6 Vrhe. In this work, we show that the main pathway of H2O2 formation on platinum involves the ·OH free radical as an intermediate. ·OH radicals may be released from the Pt surface either by dissociation of the adsorbed OOH intermediate at high potentials or by dissociation of one-site adsorbed peroxide intermediate at low potentials. Adsorption of hydrogen peroxide transported to the Pt surface to achieve this state, though possible, is unfavored. These results indicate that the direct formation of hydroxyl radicals on Pt surfaces is possible under certain conditions and need not proceed through a peroxide intermediate.

GDL Degradation in PEFC

Performance decay in a polymer-electrolyte fuel cell can result from changes in the gas-diffusion layer, particularly in the micro-porous layer. Examples and characteristics of this decay will be presented here. Performance decay is greater with exposure to potential cycles than with exposure to constant high potential holds. Presumably, electrochemical oxidation of the carbon within the micro-porous layer is primarily responsible for this performance decay; it is known that transient potentials can accelerate the electrochemical oxidation of carbon. The implication that these results have on the development of advanced gas-diffusion layer materials is also considered. Finally, a recommendation for a standardized accelerated stress test protocol focused on the gas-diffusion layer is proposed.