Myoung-Ki Min

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

Abstract Carbon-supported Pt-based binary alloy electrocatalysts (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 reduced Pt–Pt neighboring distance as the catalysts were alloyed. The reduced Pt–Pt neighboring distance is favorable for the adsorption of oxygen.

An XPS study on oxidation states of Pt and its alloys with Co and Cr and its relevance to electroreduction of oxygen

Cathodic reduction of oxygen in fuel cells is known to be enhanced on platinum alloys in relation to the platinum metal. The higher performance of the platinum alloys is as a result of the improved oxygen-reduction kinetics on the alloys but there is hardly any increase in the electrode platinum-surface-areas for the platinum alloys as compared to the platinum metal, and thus the higher performance is solely due to the enhanced electrocatalytic activity of the alloys as compared to the platinum metal. The present X-ray photoelectron spectroscopic (XPS) study on carbon-supported Pt, Pt–Co and Pt–Co–Cr electrocatalysts suggests the presence of a relatively lower Pt-oxide content on the alloys. The X-ray powder diffraction patterns for these electrocatalysts show that while the carbon-supported platinum electrocatalyst has a face-centered cubic (fcc) phase, carbon-supported Pt–Co and Pt–Co–Cr electrocatalysts exhibit a face-centered tetragonal (fct) phase. But, Pt electrocatalyst has a lower particle-size and, hence, a higher dispersion. Previous studies have shown higher activities on the Pt-alloys than on Pt, and have attributed it to changes in the electronic and structural characteristics of Pt. These changes can be correlated with the lower oxidation-state of Pt sites, as found in this study.