Characterizing Coverage of Phosphoric Acid on Carbon-Supported Platinum Nanoparticles Using In Situ Extended X-Ray Absorption Fine Structure Spectroscopy and Cyclic Voltammetry

Abstract

Using extended X-ray absorption fine structure (EXAFS) spectroscopy and cyclic voltammetry (CV) in combination, the fraction of surface Pt atoms poisoned by the phosphoric acid adsorbed (θPA) on Pt/C was analyzed for various electrode potentials (0.35, 0.60, and 0.8 V). The θPA values at 0.35 V, 0.6 V, and 0.8 V were determined to be 0.32, 0.45, and 0.38, respectively. The differences in the θPA values determined using EXAFS spectroscopy in this study and those estimated from the relative number of phosphoric acid molecules adsorbed are probably attributable to an increase in the number of Pt-O bonds per adsorbed phosphoric acid molecule. This, in turn, is owing to an increase in the degree of electrochemical adsorption with an increase in the potential. That the θPA value for the Pt(111) surface at 0.35 V as determined using EXAFS spectroscopy was higher than that estimated through the CV analysis was because of the chemical adsorption and/or electrochemical adsorption of phosphoric acid on the (100) facets and edge sites of the Pt/C surfaces.