Ligand-Mediated Electrocatalytic Activity of Pt Nanoparticles for Oxygen Reduction Reactions

High-performance electrocatalysts for oxygen reduction reactions (ORR) are crucial for the development of proton exchange membrane fuel cells (PEMFCs). In this study, a novel method was developed by which the ORR activity of Pt nanoparticles was deliberately manipulated by selective organic capping ligands. By coreduction of diazonium salts and H2PtCl4, a series of Pt nanoparticles (core size 2.0−2.5 nm) stabilized by para-substituted (R = −CH3, −F, −Cl, −OCF3, and −CF3) phenyl groups were synthesized. The experimental results demonstrated that the electron-withdrawing capability of the substituent moieties, as manifested by the Hammet substituent constant (σ), plays a key role in controlling the ORR activity, where the higher σ, the higher ORR activity. Within the present experimental context, Pt nanoparticles stabilized by trifluoromethylphenyl groups (Pt−Ar−CF3) exhibit the highest catalytic activity among the series, with an ORR specific activity 3.2 times higher than that of commercial Pt/C catalysts. The enhanced activity may be correlated with the weakened oxygen adsorption by the electronegative ligands.