Fundamental Investigation of Oxygen Reduction Reaction on Rhodium Sulfide-Based Chalcogenides

Synchrotron-based X-ray absorption spectroscopy (XAS), including the surface-specific Delta XANES technique, is used to investigate the active reaction site for water activation and the oxygen reduction reaction (ORR) on the novel, mixed-phase chalcogenide electrocatalyst RhxSy/C (De Nora). The specific adsorption of water, OH, and O as a function of overpotential is reported. This study builds on a prior communication based solely on interpreting the XAS spectra of RhxSy with respect to the metallic Rh3S4 phase. Here, a more extensive overview of the electrocatalysis is provided on RhxSy/C, the thermally grown Rh2S3/C and Rh3S4/C preferential phases and a standard 30 wt% Rh/C electrocatalyst, including results obtained by X-ray diffraction (XRD), XAS, high-resolution transmission electron imaging, microanalysis, and electrochemical investigations. Heating of the RhxSy catalysts to prepare the two preferential phases causes Rh segregation and the formation of Rh metal particles, and immersion in TFMSA causes S dissolution and the formation of a Rh skin on the RhxSy samples. It is shown that some Rh-Rh interactions are needed to carry out the ORR. This is present on the Rh-6 moieties in both the Rh3S4 and RhxSy catalysts, but a partial Rh skin (present from acid dissolution) is also contributing to the ORR observed on RhxSy. This to our knowledge is the first time a reaction site in a multiphase inorganic framework structure has been investigated in terms of electrocatalytic pathway for oxygen reduction.