CO2 Poisoning of CNx Catalysts for the Oxygen Reduction Reaction

Abstract

CN_x catalysts show promising activity and stability for the oxygen reduction reaction (ORR) under acidic conditions, but the nature of the active site is still under debate. ORR on CN_x has been found to be resistant to common poisons such as CO, H₂S, and CN. In this study, we demonstrate that bubbling CO₂ in the electrolyte can lead to the partial poisoning of CN_x for ORR activity. Cyclic voltammetry (CV) experiments show a partial decrease in the ORR activity for CN_x catalysts after bubbling CO₂ through a 0.1 M HClO₄ electrolyte. The relative stability of CO₂-derived species (CO₂*, H₂CO₃*, HCO₃*, and CO₃*) on 13 CN_x site models at 1.0 V-RHE was examined using density functional theory (DFT). The calculations predict that HCO₃ is favored adjacent to the N species on several CN_x site models and CO₃ is favored on pyrrolic sites. Difference spectra of the N 1s from near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) of the ex-situ poisoned CN_x and pristine CN_x shows a shift in binding energies of N species that qualitatively match the DFT N 1s binding energy shifts due to HCO₃/CO₃ on the CN_x site models. DFT predicts that these HCO₃ surface species can block and delay the initial step of the ORR on several CN_x sites.