Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction: An In Situ XANES and EXAFS Investigation

Abstract

The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, P tMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the pr imary phase of the type P t3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution f rom this phase) being of the type P tM (LIo structure with tetragonal lattice) as evidenced f rom x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95 ~ and 5 a tm pressure showed a two- to threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em-2 relative to Pt/C eleetrocatalyst. The P tCr /C alloy exhibited the best performance. I n s i tu EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per a tom (with the exception of P tMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances wh ich con-f i rmed the results f rom ex s i tu XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species f rom the electrolyte following a Temkin isotherm behavior, wh ich does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond