Platinum Monolayer Electrocatalysts for O2 Reduction: Pt Monolayer on Carbon-Supported PdIr Nanoparticles

The kinetics of oxygen reduction was investigated in acid solutions on Pt monolayers deposited on modified carbon-supported PdIr nanoparticles using the rotating disk-electrode technique. Iridium is introduced into the Pd substrate in order to fine-tune the Pt–Pd interactions and to improve Pd stability under operating conditions of the fuel cell. The kinetics of the oxygen reduction reaction shows enhancement with the Pt monolayer on the PdIr nanoparticle surfaces in comparison with the reaction on Pt/C and Pt monolayer on Pd/C nanoparticles. The electrochemical measurements suggest that reduced oxidation of Pt monolayer on PdIr/C compared to Pt/C and Pt monolayer on Pd/C is the cause of enhanced activity. Besides a ligand effect induced to the Pt surface by the presence of PdIr in the second sublayer of the nanoparticle, as suggested by our density functional theory analysis, Ir also leads to a Pd skin contraction, so the Pt monolayer on PdIr/C is compressed more than on Pd/C. Both effects lead to further weakening of the Pt–OH interaction, thus increasing the ORR activity. The Pt-specific activity for PtMLPdIr/C is three times and 25% higher than that of Pt/C and PtMLPd/C respectively; the Pt-mass activity of PtMLPdIr/C is more than 20 times and 25% higher than that of Pt/C and PtMLPd/C, respectively.