Immobilized Cobalt Bis(benzenedithiolate) Complexes: Exceptionally Active Heterogeneous Electrocatalysts for Dihydrogen Production from Mildly Acidic Aqueous Solutions EadyShawn C. MacInnesMolly M. LehnertNicolai 2017 A series of cobalt bis­(benzenedithiolate) complexes with varying benzenedithiolate (general abbreviation: bdt<sup>2–</sup>) ring substitutions (S<sub>2</sub>C<sub>6</sub>X<sub>4</sub><sup>2–</sup>) 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<sup>–11</sup> mol cm<sup>–2</sup> for TBA­[Co­(S<sub>2</sub>C<sub>6</sub>H<sub>4</sub>)<sub>2</sub>] (<b>1</b>) to 6.22 × 10<sup>–10</sup> mol cm<sup>–2</sup> for TBA­[Co­(S<sub>2</sub>C<sub>6</sub>Cl<sub>4</sub>)<sub>2</sub>] (<b>3</b>)] and increasing ring size of the benzenedithiolate ligand [up to 3.10 × 10<sup>–9</sup> mol cm<sup>–2</sup> for TBA­[Co­(S<sub>2</sub>C<sub>10</sub>H<sub>6</sub>)<sub>2</sub>] (<b>6</b>)]. 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 (<i>k</i><sub>obs</sub>) estimates resulting from cyclic voltammetry analysis range from 24 to 230 s<sup>–1</sup> with the maximum <i>k</i><sub>obs</sub> for TBA­[Co­(S<sub>2</sub>C<sub>6</sub>H<sub>2</sub>Cl<sub>2</sub>)<sub>2</sub>] (<b>2</b>) at an overpotential of 0.59 V versus platinum. Controlled-potential electrolysis studies performed in 0.5 M H<sub>2</sub>SO<sub>4</sub> at −0.5 V versus SHE show impressive initial rate constants of over 500 s<sup>–1</sup> under bulk electrolysis conditions; however, steady catalyst deactivation over an 8 h period is observed, with turnover numbers reaching 9.1 × 10<sup>6</sup>. 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.