Chemical Leaching of Pt–Cu/C Catalysts for Electrochemical Oxygen Reduction: Activity, Particle Structure, and Relation to Electrochemical Leaching

Abstract

Carbon-supported Pt–Cu alloy nanoparticles (Cu/Pt=2.6 on average) were obtained by a liquid precursor impregnation–reduction technique, with alloying in dilute hydrogen at 800 °C. They were subjected to leaching (chemical—CL: 1 m H2SO4, 80 °C for 36 h, electrochemical—EL: 200 times potential cycling between 0.05 and 1.2 V in 0.1 m HClO4) to study the influence of leaching on their activity in the oxygen reduction reaction (ORR) and on particle structure, in particular, for the chemical route that provided sufficient material for the characterization techniques employed (X-ray diffraction, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, low-energy ion scattering). CL resulted in core–shell-structured nanoparticles, in which an external monolayer of pure Pt covered an alloy core significantly depleted in Cu. Cu/Pt ratios in the cores were distributed around Cu/Pt=0.4–1, which differs from particle architectures described in the literature as a result of EL. The alloy superstructure initially present was destroyed. A minority of pure Pt particles, probably very small in size, coexisted with the alloy particles. The specific ORR activity of these leached nanoparticles was twice that of the original Pt–Cu alloy particles (264 vs. 140 μA cm−2 Pt). Electrochemical leaching applied to the initial particles or to the chemically dealloyed ones also resulted in activation (direct EL −175 μA cm−2 Pt, EL after CL −315 μA cm−2 Pt). Particles with a Cu-depleted alloy core (EL after CL) provided higher activity after EL.