Electroreduction of CO2 to Formate with Low Overpotential Using Cobalt Pyridine Thiolate Complexes

Electrocatalytic CO2 reduction to value-added products provides a viable alternative to the use of carbon sources derived from fossil fuels. Nevertheless, the ability to carry out these transformations at reasonable energetic costs, e.g. with low overpotential, remains a significant challenge. Molecular catalysts offer a great option in this context, as fine control of their activity and selectivity can be obtained via the tuning of their coordination sphere and ligand set. To this end, we investigated here a series of cheap cobalt(III) pyridine-thiolate complexes as electrocatalysts for CO2 reduction. The effect of the ligands and proton sources on activity was examined. We were able to identify [bipyridine-bis-(2-pyridinethiolato)-cobalt(III)-hexaflurophosphate] as a highly selective catalyst for formate production operating at a very low overpotential of 110 mV to achieve a TOF of 10 s-1. Detailed electrokinetic analysis coupled with density functional theory allowed establishing a mechanistic pathway for these catalysts, highlighting the role of key metal hydride intermediates. The catalysts deactivate via the formation of stable Co carbonyl complexes, but we demonstrated that the active species could be regenerated upon oxidation and release of coordinated CO ligands.