A RhxSy/C Catalyst for the Hydrogen Oxidation and Hydrogen Evolution Reactions in HBr

Jahangir Masud,Bing-Joe Hwang,Keith Hohn,Nirala Singh,Eric McFarland,Myles Ikenberry,Trung Van Nguyen,Chun-Jern Pan

doi:10.1149/2.0901504jes

Abstract

Rhodium sulfide (Rh2S3) on carbon support was synthesized by refluxing rhodium chloride with ammonium thiosulfate. Thermal treatment of Rh2S3 at high temperatures (600°C to 850°C) in presence of argon resulted in the transformation of Rh2S3 into Rh3S4, Rh17S15 and Rh which were characterized by TGA/DTA, XRD, EDX, and deconvolved XPS analyses. The catalyst particle size distribution ranged from 3 to 12 nm. Cyclic voltammetry and rotating disk electrode measurements were used to evaluate the catalytic activity for hydrogen oxidation and evolution reactions in H2SO4 and HBr solutions. The thermally treated catalysts show high activity for the hydrogen reactions. The exchange current densities (io) of the synthesized RhxSy catalysts in H2-saturated 1M H2SO4 and 1M HBr for HER and HOR were 0.9 mA/cm2 to 1.0 mA/cm2 and 0.8 to 0.9 mA/cm2, respectively. The lower io values obtained in 1M HBr solution compared to in H2SO4 might be due to the adsorption of Br− on the active surface. Stable electrochemical active surface area (ECSA) of RhxSy catalyst was obtained for CV scan limits between 0 V and 0.65 V vs. RHE. Scans with upper voltage limit beyond 0.65 V led to decreased and unreproducible ECSA measurements.

Highlights

The intermittent availability of renewable electricity from solar and wind sources has increased significantly.[1]
Rhodium sulfide catalysts were synthesized on carbon supports and characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS) and Energy-dispersive X-ray spectroscopy (EDX) techniques
Adsorption of OH ions on Rh sites of the catalyst and formation of rhodium hydrous oxide species which could not be reduced during the cathodic scan

Summary

Results and Discussion

Physical characterizations.— Thermal analysis measurement of the synthesized precursor (Rh2S3/C) was carried out using thermogravimetric analysis (TGA) and differential thermal analysis (DTA). Weight loss from 570◦C to 850◦C was due to the phase transformation of the Rh2S3 precursor This phase transformation is supported by XRD results. Similar TGA/DTA results were observed by Zhang et al.[24] during the hydrothermal synthesis of rhodium sulfides. The XRD of the synthesized catalysts at different temperatures shown in Fig. 2 confirmed the presence of different phases of rhodium sulfides, such as Rh2S3 (ICSD No.15344), Rh3S4 (ICSD No 410813), Rh17S15 (ICSD No 410838) and Rh (ICSD No.426969). As the XRD spectrum of Rh3S4 is almost superimposed with the spectrum of Rh17S15 (see the reference spectrum in Fig. 2), it is difficult to index them These results indicate that the Rh2S3 material was transformed to other phases by loss of sulfur during thermal reduction. The average catalyst particle size was calculated from the XRD diffraction peak width using the Scherer equation:[25]

Precursor o

Fη RT

Conclusions