Particle size and alloying effects of Pt-based alloy catalysts for fuel cell applications

Carbon-supported Pt-based binary alloy electrocalalysts (Pt-Co, Pt-Cr and Pt-Ni) were prepared by incipient wetness method to investigate the origin of the enhanced activity of the oxygen reduction reaction in fuel cells. The composition of these catalysts was adjusted to 3:1 (Pt:M, atomic%, M = Co, Cr and Ni). Prepared catalysts were characterized by various physical and electrochemical techniques, that is, energy dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and cyclic voltammetry (CV). XRD analysis showed that all prepared catalysts exhibited face-centered cubic structures and had smaller lattice parameters than Pt-alone catalyst. Pt-alone and Pt-based alloy catalysts showed increasing specific activities with decreasing surface area. This indicates that oxygen reduction on platinum surface is a structure-sensitive reaction. According to CV and X-ray absorption near-edge structure (XANES) results, the structure-sensitivity of Pt and Pt-based alloy catalysts for oxygen reduction seems associated with the adsorption strength of oxygen intermediates on the Pt surface. In addition, the Pt-based alloy catalysts showed significantly higher specific activities than Pt-alone catalysts with the same surface area. This phenomenon comes from the seduced Pt-Pt neighboring distance as the catalysts were alloyed. The reduced Pt-Pt neighboring distance is favorable for the adsorption of oxygen. (C) 2000 Elsevier Science Ltd. All rights reserved.