Tin Oxide with Trace Metallic Rhodium (0) Impurity for Overall Water Splitting Reaction: Impact of Rh Loading and Twin Boundaries

Abstract

Tin oxide (SnO<inf>2</inf>) loaded with a variable concentrations of metallic rhodium (Rh) was explored for the overall splitting of water (H<inf>2</inf>O) in an alkaline medium. Three different catalysts, viz. Rh-SnO<inf>2</inf>-I, Rh-SnO<inf>2</inf>-II and Rh-SnO<inf>2</inf>-III, with different Rh-to-Sn ratios, were evaluated for simultaneous oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) processes in 1 M KOH. The catalyst Rh-SnO<inf>2</inf>-I, with low Rh loading, showed better OER activity with an onset potential of 1.45 V and an overpotential (η) of 240 mV, while the same for HER was found to be 470 mV. The Rh-SnO<inf>2</inf>-II catalyst, with Rh loading slightly higher than that of Rh-SnO<inf>2</inf>-I, showed OER activity with η = 250 mV and HER activity with η = 226 mV. The Rh-SnO<inf>2</inf>-III catalyst, with the highest Rh content and twin boundaries, was found to show superior HER activity with η of 222 mV at a current density (J) of 10 mAcm-2. The OER and HER activity in all cases were found to decrease at low pH. Tafel plot analysis and other comparative studies indicated that the loading of Rh into SnO<inf>2</inf> substantially altered the OER and HER activity. The lowest Tafel slope of 200 mVdec-1 in OER was found in the case of Rh-SnO<inf>2</inf>-I. Rh-SnO<inf>2</inf>-III catalyst had the lowest Tafel slope value of 128 mVdec-1 in HER. The current study implied that the introduction of Rh in SnO<inf>2</inf> can improve the HER activity and OER activity depending on the loading of Rh content. Density Functional Theory (DFT) calculations were used to understand the mechanism of overall water-splitting reactions. The study suggested that the water-splitting reaction would be more favorable if the hydrolysis process proceeded through the abstraction of hydrogen (H<inf>2</inf>) at the Rh center. The formation of the Sn-H bond during the second H<inf>2</inf> molecule liberation was detected to be the rate-determining step. © 2025 American Chemical Society.