Particle Size Effects for Oxygen Reduction on Highly Dispersed Platinum in Acid Electrolytes

The particle size effect for oxygen reduction kinetics on highly dispersed Pt particles in acid electrolytes are discussed. It is suggested that the change in the fraction of surface atoms on the (100) and (111) crystal faces of Pt particles, which are assumed to be cubo‐octahedral structures, can be correlated to the mass activity (A/g Pt) and specific activity (μA/cm 2 Pt) of highly dispersed Pt electrocatalysts. The maximum in mass activity that is observed at ∼3.5 nm in several studies is attributed to the maximum in the surface fraction of Pt atoms on the (100) and (111) crystal faces, which results from the change in surface coordination number with a change in the average particle size. The reduction of oxygen on supported Pt particles in acid electrolytes is classified as a demanding or structure‐sensitive reaction; the specific activity increases with an increase in particle size.