The Dark Side of Molecular Catalysis: Diimine-Dioxime Cobalt Complexes Are Not the Actual Hydrogen Evolution Electrocatalyst in Acidic Aqueous Solutions
Résumé
The perspective of integrating molecular catalysts for hydrogen evolution into operating devices requires the benchmarking of their activity preferentially in aqueous media. Within a series of cobalt complexes assessed in that way, cobalt diimine dioxime derivatives were shown to be the most active catalysts with onset overpotential for proton reduction as low as 260 mV in phosphate buffer (pH = 2.2) (McCrory et al. J. Am. Chem. Soc. 2012, 134, 3164-3170). Combining a set of analytical techniques (electrochemistry, gas chromatography, SEM, and XPS), we demonstrate here that the electrochemical wave previously assigned to H-2 evolution catalyzed by the molecular complex actually corresponds to low levels of catalytic hydrogen production (27% faradaic yield). Instead, we assign this wave to the reductive degradation of the molecular complex and to the formation of a nanoparticulate deposit at the electrode. Actually, this coating is responsible for the high faradaic yields for hydrogen evolution observed at more cathodic potentials. The catalytic nanoparticulate material is metastable and readily redissolves, so that rinse-test experiments were insufficient here to rule out the formation of solid-state materials. This point accounts for the previous misidentification of the active species in H2 evolution mediated by a cobalt diimine dioxime complex in aqueous phosphate buffer (pH = 2.2). Our finding, exemplified on a cobalt complex, may be extended to other molecular systems and suggests that the routine use of rinse test experiments may not be sufficient to ascertain the molecular nature of active water-splitting catalytic species.