Size-Dependent Activity of Palladium Nanoparticles for Oxygen Electroreduction in Alkaline Solutions

The objective of this study is to investigate the effect of palladium particle size on catalytic activity for oxygen reduction reactions (ORRs) in alkaline solutions. Carbon-supported palladium catalysts with average Pd particle sizes from 3 to 16.7 nm were obtained by treating a 20% Pd/C catalyst in a H-2/N-2 gas at elevated temperatures from 300 to 600 degrees C, respectively. The specific activity of the Pd/C catalysts for the ORRs, described by a kinetic ORR current (i(k)) normalized to the Pd active surface area, was found to increase continuously by a factor of about 3 with increased particle size from 3 to 16.7 nm without reaching a maximum. The mass activity increased by a factor of 1.3 when Pd particle size increased from 3 to 5 nm and then decreased with increased Pd particle size. The hydrogen peroxide yield on the Pd/C catalysts was negligible, especially in the mixed kinetic-diffusion controlled region, indicating that the four-electron transfer path is dominant for the ORR on the Pd/C in alkaline solutions. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3098478] All rights reserved.