Highly selective electrocatalytic CO2 reduction to formate by a dinuclear nickel complex: cooperative catalysis across diverse conditions.

Achieving high selectivity is vital for the practical application of electrocatalytic CO2 reduction reaction (CO2RR). While CO and HCOOH are both possible two-electron reduction products, significantly fewer electrocatalysts are known to selectively produce HCOOH, and very few maintain this selectivity across diverse reaction conditions. Herein, we report a dinuclear nickel complex, [NiII2(tpy)2(μ-bpp)(μ-Cl)](PF6)2 (1, bpp = 3,5-bis(2-pyridyl)pyrazolato, tpy = terpyridine), designed with pre-organized dual Ni sites for highly selective CO2-to-HCOOH electrocatalysis under a broad range of conditions. The bpp bridging ligand creates a well-suited space for CO2 binding, and its balanced structural rigidity and flexibility allow the intermetallic distance to adjust, accommodating the conformational changes of CO2 during catalysis. This structural feature enables complex 1 to catalyze the CO2-to-HCOOH conversion with high selectivity in acidic, neutral, and basic environments, under both homogeneous (non-aqueous) and heterogeneous (aqueous) conditions. In homogeneous catalysis, Faradaic efficiencies > 95% for HCOOH production were achieved in dimethylformamide solutions with phenol, water, or triethylamine as proton sources. In heterogeneous catalysis, Faradaic efficiencies > 92% were obtained in CO2-saturated 0.1 mol L-1 KHCO3 aqueous solutions in H-cells. Furthermore, gas diffusion electrode-based flow cells achieved Faradaic efficiencies > 90% in 1.0 mol L-1 KOH aqueous solutions, with a large HCOOH production current density of > 150 mA cm-2 and a turnover frequency of 110 s-1. In situ infrared spectroelectrochemistry, operando X-ray absorption spectroscopy, and computational investigations demonstrate that the two Ni sites of 1 collaboratively bind CO2 and facilitate the ensuing hydrogenation step, promoting selective HCOOH formation. This work presents an unparalleled example of a molecular electrocatalyst for selective CO2-to-HCOOH conversion across diverse conditions and highlights the critical role of pre-organized, cooperative metal sites in CO2 activation.