Platinum and platinum-chromium as bulk and dispersed catalyst electrodes at elevated temperature

Platinum and platinum-chromium bulk electrodes were analyzed in 100 wt.% H{dollar}sb3{dollar}PO{dollar}sb4{dollar} at 200{dollar}spcirc{dollar}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{dollar}sb3{dollar}OP{dollar}sb4{dollar} at 200{dollar}spcirc{dollar}C) using the submerged mode to evaluate the degradation mechanism(s).; Following analysis and verification of the validity of the hanging meniscus rotating disk electrode method for evaluating the kinetics for the ORR on platinum in dilute phosphoric acid at room temperature, the relative ORR kinetics for platinum and platinum-chromium in 100 wt.% H{dollar}sb3{dollar}OP{dollar}sb4{dollar} at 200{dollar}spcirc{dollar}C were determined. Lower oxygen reduction current densities were obtained from platinum-chromium than from platinum held potentiostatically at 0.9 V versus RHE in these conditions. The ORR polarization scans from rotating disks did not possess a sigmoidal shape for either platinum or platinum-chromium; pseudo-ohmic behavior was observed from 0.9-0 V versus RHE. Of the possible controlling mechanisms which could be responsible for the observed potential-current response, the presence of an adsorbed layer, probably composed of polyphosphoric acid, is consistently supported by the data. Slower ORR kinetics were again measured for the platinum-chromium bulk electrodes. This is consistent with previous results for these catalysts in dilute H{dollar}sb3{dollar}OP{dollar}sb4{dollar} at room temperature, but opposite to the relationship when these compositions are operated as dispersed catalysts in gas diffusion electrodes in PAFC's.; Analysis of the platinum dispersed catalyst microstructure as a function of operation using the submerged mode revealed a linear catalyst size increase with operating time. A dissolution-deposition mechanism, in which a charged platinum-ligand complex is formed on platinum dissolution in these conditions, has been proposed. A potential gradient, present due to the noncompensated solution resistance within the length of the pore, establishes the conditions necessary for simultaneous dissolution and deposition of platinum within the electrode. Extensive dissolution of the platinum-chromium catalyst and greater catalyst growth under similar conditions indicates that the presence of chromium does not stabilize the catalyst particles.