Continued study of the effects of heat treatment and microstructure on electrocatalyst performance. Final report, July 29, 1991--December 20, 1991

Platinum and platinum-chromium bulk electrodes were analyzed in 100 wt.% H{sub 3}PO{sub 4} at 200{degrees} C to evaluate the kinetics for the oxygen reduction reaction (ORR) in the phosphoric acid fuel cell (PAFC) operating environment. Dispersed platinum and platinum-chromium catalyst electrodes were operated potentiostatically under simulated PAFC operating conditions (0.7 V vs. RHE in 100 wt.% H{sub 3}PO{sub 4} at 200{degrees} C) using the submerged mode to evaluate the degradation mechanism. Differences observed between the hanging meniscus rotating disk (HMRD) electrode and the rotating disk electrode (RDE) method were related to the different hydrodynamics present in the hanging meniscus. The boundary conditions applicable to an infinite rotating disk are no longer present. The most probable cause for the observed differences is that the surface of the HMRD electrode is no longer uniformly accessible with respect to the diffusing species. Therefore, the kinetic data extracted from the mixed control conditions are altered sufficiently so that precise potential-current curves cannot be obtained. However, the data obtained are highly reproducible. Therefore, the method appears suitable for determining comparative kinetic data for platinum and platinum-chromium at 200 a C in 100 wt.% H{sub 3}PO{sub 4}, where RDE designs will not function. Cyclic voltammetry conducted on platinum and platinum-chromium electrodes in 100 wt.% H{sub 3}PO{sub 4} at 200{degrees} C were similar qualitatively to those obtained at room temperature. Comparison of the double layer currents demonstrated that an approximately two-fold increase in surface area, due to dissolution of chromium from the electrode surface, occurred during typical cycling durations; extensive chromium dissolution was observed during long-term cycling.