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

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 approximate to 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 approximate to 0.425 mA cm(Rh)(-2) and approximate to 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 (OHads) 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.