The impact of chloride ions and the catalyst loading on the reduction of H2O2 on high-surface-area platinum catalysts

Abstract

Minor electrolyte constituents that are present even in the best grade, and highly pure electrolytes can have a major impact on electrocatalytic reactions. In this work, we discuss particularly the influence of chloride adsorption on the interaction of hydrogen peroxide with carbon-supported platinum nanoparticle catalysts, as this is of high relevance for the oxygen reduction reaction (ORR). Under non-equilibrium conditions, the surface coverage of chloride at a certain time is determined by the ratio of the chloride flux within the diffusion layer to the number of surface sites. Therefore, by decreasing the catalyst loading, the same amount of chloride ions that reaches the surface under a given mass transport regime leads to a higher coverage. Similarly, by increasing the mass transport rates, chloride ions diffuse to the surface with a higher rate and thus cover a larger fraction of the surface at a given time. The increased coverage with chloride in both cases leads to an inhibition of the hydrogen peroxide reduction reaction (PRR), and concomitantly to desorption of the H2O2 intermediate during the ORR on Pt electrodes. The contribution of the electrolyte impurities to the macroscopic H2O2 release during the ORR on low-loading catalysts or under high mass transport rates is thus an important – if not the decisive – factor that needs to be taken into consideration, for instance during fuel cell operation or in the interpretation of experimental observations for mechanistic studies.