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, [Ni^II₂(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.