Immobilized Cobalt Bis(benzenedithiolate) Complexes: Exceptionally Active Heterogeneous Electrocatalysts for Dihydrogen Production from Mildly Acidic Aqueous Solutions

A series of cobalt bis­(benzenedithiolate) complexes with varying benzenedithiolate (general abbreviation: bdt^2–) ring substitutions (S₂C₆X₄^2–) were prepared and adsorbed on inexpensive electrodes composed of (a) reduced graphene oxide (RGO) electrodeposited on fluorine-doped tin oxide (FTO) and (b) highly ordered pyrolytic graphite (HOPG). The catalyst-adsorbed electrodes are characterized by X-ray photoelectron spectroscopy. Catalyst loading across the ligand series improved notably with increasing halide substitution [from 2.7 × 10^–11 mol cm^–2 for TBA­[Co­(S₂C₆H₄)₂] (1) to 6.22 × 10^–10 mol cm^–2 for TBA­[Co­(S₂C₆Cl₄)₂] (3)] and increasing ring size of the benzenedithiolate ligand [up to 3.10 × 10^–9 mol cm^–2 for TBA­[Co­(S₂C₁₀H₆)₂] (6)]. Electrocatalytic analysis of the complexes immobilized on HOPG elicits a reductive current response indicative of dihydrogen generation in the presence of mildly acidic aqueous solutions (pH 2–4) of trifluoroacetic acid, with overpotentials of around 0.5 V versus SHE (measured vs platinum). Rate constant (k_obs) estimates resulting from cyclic voltammetry analysis range from 24 to 230 s^–1 with the maximum k_obs for TBA­[Co­(S₂C₆H₂Cl₂)₂] (2) at an overpotential of 0.59 V versus platinum. Controlled-potential electrolysis studies performed in 0.5 M H₂SO₄ at −0.5 V versus SHE show impressive initial rate constants of over 500 s^–1 under bulk electrolysis conditions; however, steady catalyst deactivation over an 8 h period is observed, with turnover numbers reaching 9.1 × 10⁶. Electrolysis studies reveal that halide substitution is a central factor in improving the turnover stability, whereas the ring size is less of a factor in optimizing the long-term stability of the heterogeneous catalyst manifolds. Catalyst deactivation is likely caused by catalyst desorption from the electrode surfaces.