Cobalt Amide Imidate Imidazolate Frameworks as Highly Active Oxygen Evolution Model Materials.

Two imidazolate-based Co-MOFs, IFP-5 and IFP-8 (imidazolate framework Potsdam), with a different peripheral group -R (-Me and -OMe, respectively) have been synthesized by a solvothermal method and tested toward the oxygen evolution reaction (OER). Remarkably, IFP-8 presents much lower overpotentials (319 mV at 10 mA/cm and 490 mV at 500 mA/cm) than IFP-5 toward OER, as confirmed by online gas chromatography measurements (Faradaic yield of O > 99%). Moreover, the system is extraordinarily stable during 120 h, even when used as a catalyst toward the overall water splitting reaction without any sign of fatigue. An integrated ex situ spectroscopic study, based on powder X-ray diffraction, extended X-ray absorption fine structure, and attenuated total reflection, allows the identification of the active species and the factors that determine the catalytic activity. Indeed, it was found that the performances are highly affected by the nature of the -R group, because this small change strongly influences the conversion of the initial metal organic framework to the active species. As a consequence, the remarkable activity of IFP-8 can be ascribed to the formation of Co(O)OH phase with a particle size of a few nanometers (3-10 nm) during the electrocatalytic oxygen evolution.