Abstract
We report the synthesis of unsupported and carbon-supported, mixed phase, rhodium sulfide, using both a hydrogen sulfide source and a solid sulfur source. Samples with several different distributions of rhodium sulfide phases (Rh2S3, Rh17S15, RhS2 and metallic Rh) were obtained by varying the temperature and exposure time to H2S or sulfur to rhodium ratio when using solid sulfur. Samples were characterized by X-ray diffraction (XRD), and the unsupported rhodium sulfide compounds studied using Raman spectroscopy to link Raman spectra to catalyst phases. The electrocatalytic activity of the rhodium sulfide compounds for hydrogen evolution and oxidation was measured using rotating disk electrode measurements in acidic conditions to simulate use in a flow cell. The most active phases for hydrogen evolution were found to be Rh3S4 and Rh17S15 (−0.34V vs. Ag/AgCl required for 20mA/cm2), while Rh2S3 and RhS2 phases were relatively inactive (−0.46V vs. Ag/AgCl required for 20mA/cm2 using RhS2/C). The hydrogen oxidation activity of all rhodium sulfide phases is significantly lower than the hydrogen evolution activity and is not associated with conductivity limitations.
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