Rhodium metal–rhodium oxide (Rh–Rh2O3) nanostructures with Pt-like or better activity towards hydrogen evolution and oxidation reactions (HER, HOR) in acid and base: correlating its HOR/HER activity with hydrogen binding energy and oxophilicity of the catalyst

Abstract

Design and synthesis of HER/HOR catalysts are of crucial importance for the development of polymer fuel cells and water electrolyzers. We report the synthesis of Rh–Rh₂O₃ nanoparticles/nitrogen-doped carbon composite (Rh–Rh₂O₃-NPs/C) for HER/HOR applications. The HER activity of this catalyst is ∼2.2 times and 1.43 times better than that of commercial Pt/C in base and acid, respectively. Rh–Rh₂O₃-NPs/C exhibited 10 mA cm⁻² current density at an overpotential of 63 mV and 13 mV with Tafel slopes of 70 mV dec⁻¹ and 32 mV dec⁻¹ in base and acid, respectively. The catalyst showed superior HOR activity at all pH values. The exchange current densities were ∼0.425 mA cm_Rh⁻² and ∼0.43 mA cm_Rh⁻² in base and acid, respectively. In base, the HOR and HER activities of Rh–Rh₂O₃-NPs/C are 50-fold and 10-fold higher, respectively, in comparison with those of the Rh₂O₃-free RhNPs/C catalyst, although the HER/HOR activity of both the catalysts is comparable in acid. In base, the adsorption of OH⁻ species (OH_ads) on Rh₂O₃ sites increases the reactivity of hydrogen intermediate (H_abs), which leads to the enhancement of HOR activity of Rh–Rh₂O₃-NPs/C. For HER in base, the adsorptive dissociation of water occurs on the Rh₂O₃ sites to form H_ads on the neighboring Rh sites and then, the recombination of H_ads results in the formation of hydrogen molecules. This study may provide an opportunity to develop an efficient catalyst for hydrogen-based renewable energy technologies.