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

Tin oxide (SnO2) loaded with a variable concentrations of metallic rhodium (Rh) was explored for the overall splitting of water (H2O) in an alkaline medium. Three different catalysts, viz. Rh-SnO2-I, Rh-SnO2-II and Rh-SnO2-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-SnO2-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-SnO2-II catalyst, with Rh loading slightly higher than that of Rh-SnO2-I, showed OER activity with η = 250 mV and HER activity with η = 226 mV. The Rh-SnO2-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 SnO2 substantially altered the OER and HER activity. The lowest Tafel slope of 200 mVdec–1 in OER was found in the case of Rh-SnO2-I. Rh-SnO2-III catalyst had the lowest Tafel slope value of 128 mVdec–1 in HER. The current study implied that the introduction of Rh in SnO2 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 (H2) at the Rh center. The formation of the Sn–H bond during the second H2 molecule liberation was detected to be the rate-determining step.