Monometallic Palladium for Oxygen Reduction in PEM Fuel Cells: Particle-Size Effect, Reaction Mechanism, and Voltage Cycling Stability

In this experimental study, we determine the size dependent activity as well as accelerated voltage cycling stability of various carbon supported palladium electrocatalysts for the oxygen reduction in acidic medium. Furthermore, ex-situ transmission electron microscopy studies before and after accelerated voltage cycling provide a deeper understanding regarding particle stability during voltage cycling. Regarding oxygen reduction, a particle size effect on the specific activity is observed, with bulky Pd-black ( ≈ 4 m 2 g − 1 Pd ) exhibiting ≈x6 times higher activity than Pd supported on Vulcan carbon ( ≈ 190 m 2 g − 1 Pd ). Mass activities, however, exhibit a strong correlation with catalyst surface area at small electrochemically active surface area (ECSA) values, but are observed to be nearly constant between ≈ 50 − 200 m 2 g − 1 Pd . As stability tests during voltage cycling reveal a benefit for smaller surface areas, i.e. bigger particles, a limited gain in stability can be achieved by increasing catalyst particle size at a negligible cost of electrocatalytic mass-based activity. Moreover, we provide a deeper insight regarding the oxygen reduction reaction mechanism and show significant hints that a sequential two-plus-two electron reduction mechanism via intermediate hydrogen peroxide is likely to occur on carbon supported Pd catalysts.