The Pt particle-size effect on oxygen reduction reaction (ORR) activity of Pt/C catalyst in liquid-phase polymer electrolyte fuel cell membrane electrode assemblies (PEFC MEAs) is a topic researched widely in the field of surface science. However, the conclusion has not yet been clarified whether the size of Pt particle affects the ORR activity. In this research, to verify this effect, PEFC MEAs was systematically investigated in gas-phase systems in which the phase and physical characteristics are under similar conditions as those in practical applications and are significantly different from those in liquid-phase systems. PEFC MEAs experimented in this study were prepared with different Pt particle sizes ranging from 2–8 nm. The ORR activity was observed to be almost constant at each cell temperature in a range of 25–80 °C, irrespective of the particle size. By stripping voltammetry, the quantity of oxygen-containing species adsorbed on the Pt surface was found to be linearly dependent on the potential and the relative humidity, and this dependence was identical to all sizes. This implies that there was no particle-size effect on electronic properties of the Pt surface, resulting in no change in the ORR activity due to the size variation.

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