Oxygen reduction on platinum-ruthenium alloy electrodes in 85% orthophosphoric acid

Abstract

There is no evidence that the platinum-ruthenium alloys are ever better than similarly prepared pure platinum surfaces for oxygen reduction in acid solutions under clean conditions at practical working potentials; however, platinum alloy electrodes containing small percentages of ruthenium are apparently more active under contaminated conditions than pure platinum. Since adsorbed impurities donate electrons to the d -bands of the electrode, thus reducing its power to adsorb electrode reaction intermediates, we would expect that the materials containing a larger number of d -orbital vacancies per atom would be less affected by the impurities. Platinum has 0.6 vacancies per atom and ruthenium 17 2.2, so that platinum alloys containing small amounts of ruthenium, with intrinsic activities close to that of platinum, are more active than platinum itself when contaminated. For the alloys, a “compensation effect” between the heat of activation and preexponential factor is noted so that rates on the alloys, pure platinum, and pure ruthenium are much the same in the 75°–100°C range. Platinum, however, has the greatest heat of activation (22.9±0.9 kcal) at the reversible potential, and for alloys the value falls with increasing ruthenium content to the value of approximately 11.7±0.5 kcal on ruthenium. Owing to the increasing Tafel slope with alloy ruthenium content, platinum-rich compositions are always more active for oxygen reduction in the 900 mV (HRE) range or below even at low temperatures where ruthenium-rich i 0 values are favorable.