Transition Metal Sulfide Hydrogen Evolution Catalysts for Hydrobromic Acid Electrolysis.

Mixed metal sulfides contg. combinations of W, Fe, Mo, Ni, and Ru were synthesized and screened for activity and stability for the H evolution reaction (HER) in aq. HBr. Co- and Ni-substituted RuS2 were identified as potentially active HER electrocatalysts by high-throughput screening (HTS), and the specific compns. Co0.4Ru0.6S2 and Ni0.6Ru0.4S2 were identified by optimization. H evolution activity of Co0.4Ru0.6S2 in HBr is greater than RuS2 or CoS2 and comparable to Pt and com. RhxSy. Structural and morphol. characterizations of the Co-substituted RuS2 suggest that the nanoparticulate solids are a homogeneous solid soln. with a pyrite crystal structure. No phase sepn. is detected for Co substitutions <30% by x-ray diffraction. In 0.5 M HBr electrolyte, the Co-Ru electrode material synthesized with 30% Co rapidly lost ∼34% of the initial loading of Co; thereafter, it was obsd. to exhibit stable activity for HER with no further loss of Co. D. functional theory calcns. indicate that the S22- sites are the most important for HER and the presence of Co influences the S22- sites such that the H binding energy at sufficiently high H coverage is decreased compared to Ru sulfide. Although showing high HER activity in a flow cell, the reverse reaction of H oxidn. is slow on the RuS2 catalysts tested when compared to Pt and Rh sulfide, leaving Rh sulfide as the only suitable tested material for a regenerative HBr cell due its stability compared to Pt. [on SciFinder(R)]