Nickel(II) Benzil Bis(Thiosemicarbazonato) Complexes as Electrocatalysts for Hydrogen Evolution Reaction

Abstract

We report the electrocatalytic performance in hydrogen evolution reaction (HER) of three Ni(II) complexes based on benzil bis(thiosemicarbazone) ligands derived from 1,2-diphenylethanedione and 4-isopropyl-3-thiosemicarbazide (L¹H₂), 4-methyl-3-thiosemicarbazide (L²H₂) or 3-thiosemicarbazide (L³H₂). Cyclic voltammetry in dimethylformamide (DMF) exhibit two diffusion-controlled reversible processes at negative potentials, corresponding to [Ni^IIL³]/[Ni^IIL^3·]⁻ and Ni^II/Ni^I reduction processes. The catalytic activity in DMF was evaluated using acetic acid as proton source, showing the appearance of a catalytic cathodic current associated with the Ni^II/I redox couple. Analysis of the gas evolved from controlled potential coulometry (CPC) confirmed that the three complexes can effectively catalyse hydrogen evolution and that the catalytic activity depends on the substituent attached to the terminal amine. Electrolysis induces electrodeposition of the complexes on the electrode surface forming a film, which were also tested as HER catalysts. The results showed that the total charge produced by the films is, in all cases, lower than with the dissolved complex, but they exhibit higher faradaic efficiencies. DFT calculations were made to establish the catalytic route, and the results support an ECEC mechanism, in which E represents an electrochemical step and C is a chemical one (protonation), centred in the metal assisted by the ligand.