Sulfuric Acid Electrolyte Impacts Palladium Chemistry at Reductive Potentials

Abstract

We report herein that sulfuric acid electrolyte affects the kinetics of hydrogen sorption and desorption, the amount of absorbed hydrogen, and the electrocatalytic activity of palladium using X-ray photoelectron spectroscopy (XPS) and in situ temperature-programmed desorption (TPD). This study utilized a custom TPD instrument tailored to the in situ characterization of palladium films electrochemically saturated with absorbed hydrogen (PdHx for x > 0.6), and we present the first experimental data for the formation of sulfates on palladium hydride at E < 0 V (vs RHE). The surface adsorption of sulfates leads to a retardation of hydrogen sorption and desorption kinetics by increasing the desorption energy of absorbed hydrogen by 92 kJ mol–1 relative to a clean palladium surface. Although previous studies have reported that the SO42– anions of H2SO4 only adsorb on palladium under oxidative potentials E > 0.5 V (vs RHE), the differing chemistry of hydrogen-free and hydrided palladium was not considered. The sulfate adsorbates affect the reaction chemistry through both a competitive binding of surface sites and by altering the electronic structure of surface atoms. The influence of the electrolyte is evidenced by a sluggish kinetics of H absorption and desorption in H2SO4 electrolyte relative to HCl, concomitant with a lower hydrogen uptake in H2SO4 (x = 0.64 ± 0.01) versus HCl (x = 0.697 ± 0.007). The findings of this study emphasize the critical and often overlooked role of the electrolyte utilized in electrochemical studies of the palladium–hydrogen system.