Transition Metal Sulfide Hydrogen Evolution Catalysts for Hydrobrornic Acid Electrolysis

Mixed metal sulfides containing combinations of W, Fe, Mo, Ni, and Ru were synthesized and screened for activity and stability for the hydrogen evolution reaction (HER) in aqueous hydrobromic acid (HBr). Co- and Ni-substituted RuS2 were identified as potentially active HER electrocatalysts by high-throughput screening (HTS), and the specific compositions Co0.4Ru0.6S2 and Ni0.6Ru0.4S2 were identified by optimization. Hydrogen evolution activity of Co0.4Ru0.6S2 in HBr is greater than RuS2 or CoS2 and comparable to Pt and commercial RhxSy. Structural and morphological characterizations of the Co-substituted RuS2 suggest that the nanoparticulate solids are a homogeneous solid solution with a pyrite crystal structure. No phase separation is detected for Co substitutions below 30% by X-ray diffraction. In 0.5 M HBr electrolyte, the Co-Ru electrode material synthesized with 30% Co rapidly lost approximately 34% of the initial loading of Co; thereafter, it was observed to exhibit stable activity for HER with no further loss of Co. Density functional theory calculations indicate that the S-2(2-) sites are the most important for HER and the presence of Co influences the S-2(2-) sites such that the hydrogen binding energy at sufficiently high hydrogen coverage is decreased compared to ruthenium sulfide. Although showing high HER activity in a flow cell, the reverse reaction of hydrogen oxidation is slow on the RuS2 catalysts tested when compared to platinum and rhodium sulfide, leaving rhodium sulfide as the only suitable tested material for a regenerative HBr cell due its stability compared to platinum.