Re-examination of the Pt Particle Size Effect on the Oxygen Reduction Reaction for Ultrathin Uniform Pt/C Catalyst Layers without Influence from Nafion

•Pt particle size effect on ORR was re-evaluated for Pt/C catalysts.•Nafion-free activity of Pt/C catalysts was evaluated using thin-film RDE methods.•Ultrathin-uniform catalyst layers were employed to obtain accurate activity values.•Specific activity increased steeply from 2 to 10nm and less steeply at over 10nm.•Re-evaluated effect agrees with a particle model assuming terrace active sites. The platinum ‘particle size effect’ on the oxygen reduction reaction (ORR) has been re-evaluated using commercial Pt/C catalysts (2–10nm Pt particle) and polycrystalline Pt (poly-Pt) in 0.1M HClO4 with a rotating disk electrode method. Nafion-free catalyst layers were employed to obtain specific activities (SA) that were not perturbed (suppressed) by sulfonate anion adsorption/blocking. By using ultrathin uniform catalyst layers, O2 diffusion limitation was minimized as confirmed from the high SAs of our supported catalysts that were comparable to unsupported sputtered Pt having controlled sizes. The specific activity (SA) steeply increased for the particle sizes in the range ∼2–10nm (0.8–1.8mA/cm2Pt at 0.9V vs. RHE) and plateaued over ∼10nm to 2.7mA/cm2Pt for bulk poly-Pt. On the basis of the activity trend for the range of particle sizes studied, it appears that the effect of carbon support on activity is negligible. The experimental results and the concomitant profile of SA vs. particle size was found to be in an agreement to a truncated octahedral particle model that assumes active terrace sites.