| dc.description.abstract | Hydrogen-Bromine (H2-Br2) flow battery system is an attractive option for electrochemical energy storage due to its high round-trip energy conversion efficiency, high power density and storage capacity, fast kinetics of both hydrogen and bromine electrode reactions, low cost of active materials and reliability. The highly active conventional platinum (Pt) catalyst for hydrogen oxidation and reduction reactions (HOR/HER) is not stable in the corrosive HBr/Br2 environment of the H2-Br2 fuel cell system. Hence, an alternative HOR/HER catalyst which is stable and highly active in HBr/Br2 electrolyte is required. Rhodium sulfide (RhxSy) catalysts supported on high surface area carbon (C) were synthesized by refluxing rhodium chloride with ammonium thiosulfate to replace the Pt catalyst in the H2-Br2 fuel cell system. Rhodium sulfide catalyst is a mixture of several phases i.e. Rh2S3, Rh3S4 and Rh17S15 and Rh metal with Rh2S3 reported as a semiconductor, Rh3S4 and Rh17S15 as conducting semi-metals, and Rh metal reported to be unstable in HBr/Br2 electrolyte. Thermal reduction of inactive Rh2S3 phase at high temperatures in inert gas atmosphere results in the transformation of Rh2S3 into active phases of Rh3S4, Rh17S15 and Rh metal. Comprehensive understanding of the equilibrium phase composition at various temperatures is critical to the optimization of the performance of the catalyst towards hydrogen oxidation and evolution. Hence, an effort was made to provide an equilibrium phase diagram of rhodium sulfide catalyst heat treated at 650°C, 660°C, 670°C, 680°C, 690°C and 700°C by X-Ray Diffraction (XRD) analysis and Rietveld refinement. In order to study the relationship between the equilibrium phase composition and HOR/HER activity, electrochemical active surface area (ECSA) and exchange current density (io) of the synthesized RhxSy/C catalysts were calculated in 1 M H2SO4 solution using the rotating disk electrode (RDE) approach in a three-electrode electrochemical cell. At phase equilibrium, the weight percentages (%) observed at different temperatures for Rh2S3, Rh3S4, Rh17S15 phases and Rh metal respectively were as follows, (28.4 ± 2.9, 13.5 ± 3.4, 58.1 ± 1.4, 0.0 at 650°C); (26.9 ± 2, 15.6 ± 2.9, 57.5 ± 0.9, 0.0 at 660°C), (23.6 ± 1.2, 9.0 ± 1.7, 67.4 ± 3.4, 0.0 at 670°C); (21.3 ± 1.9, 8.9 ± 0.7, 69.8 ± 1.3, 0.0 at 680°C); (2.3 ± 2.3, 1 ± 1.4, 95.9 ± 1.8, 0.8 ± 0.2 at 690°C) and (1.2 ± 1.3, 1.6 ± 1.5, 96.6 ± 1.3, 0.6 ± 0.2 at 700°C). The mass specific ECSAs (m2/mg of Rh) of RhxSy/C catalyst heat-treated (HT) for 3 h at 650°C, 660°C, 670°C, 680°C, 690°C and 700°C were observed to be 3.4 m2/mgRh , 3.3 m2/mgRh, 5.2 m2/mgRh, 5.4 m2/mgRh, 8.1 m2/mgRh and 8.7 m2/mgRh respectively. The HOR/HER exchange current densities were observed to be 0.34 ± 0.04 mA/cm2, 0.35 ± 0.04 mA/cm2, 0.41 ± 0.02 mA/cm2, 0.43 ± 0.03 mA/cm2, 0.57 ± 0.06 mA/cm2 and 0.59 ± 0.06 mA/cm2 for RhxSy/C catalysts HT at 650°C, 660°C, 670°C, 680°C, 690°C and 700°C for 3 h respectively. The electrochemical measurements showed an increase in activity with an increase in the percentage of the active phases (Rh3S4 and Rh17S15) of RhxSy/C catalysts which was in good agreement with the XRD results. | |
