Geometry-engineered copper redox interfaces drive highly selective CO2 reduction to C2 products

Abstract

Nature converts CO₂ into energy-rich molecules through tightly coordinated catalytic networks that operate in water under mild conditions, relying on spatial organization, redox matching, and efficient transfer of reactive intermediates. Replicating these features in artificial systems remains a central challenge for electrocatalytic CO₂ reduction, which often suffers from slow activation and poor C₂ selectivity in neutral aqueous media. Here, we report a hybrid homogeneous–heterogeneous catalytic interface that emulates key elements of natural carbon-fixation pathways to achieve highly selective electrochemical conversion of CO₂ to ethanol. Bispidine-based Cu(II) complexes with either a flexible [Cu^L1_Flex] or a rigid, enzyme-mimetic [Cu^L2_Rigid] tetradentate ligand framework function as molecular active sites in solution and cooperate with a robust Cu₁.₅Mn₁.₅O₄ (CMO) spinel electrode that serves as a solid-state reaction platform. The rigid [Cu^L2_Rigid] complex undergoes reduction at a more positive potential closely aligned with the Cu⁺/Cu⁰ redox couple of the oxide surface, creating an energetically synchronized interface. This redox matching enables rapid interfacial electron transfer, efficient molecular activation of CO₂ to CO, and relay-style delivery of CO intermediates to the heterogeneous surface, closely resembling substrate channelling between active sites in enzymatic assemblies. This tandem catalyst achieves a faradaic efficiency of ∼68% for ethanol at −0.5 V vs. RHE, with a 1.5-fold enhancement in turnover frequency relative to the flexible analogue. Electrochemical impedance spectroscopy and distribution-of-relaxation-time analysis reveal diminished charge-transfer resistance and improved ion transport, consistent with an integrated, cooperative catalytic environment. In situ infrared spectroscopy identifies key surface-bound intermediates (*CO_bridge and *OC₂H₅) that map the ethanol formation pathway. First-principles calculations comparing CuO and Cu₁.₅Mn₁.₅O₄ demonstrate that Mn incorporation strengthens hydrogen adsorption and lowers hydrogen activation barriers, increasing *H_ad coverage and promoting C–C coupling. This function parallels the role of secondary metal centers in metalloenzymes, which modulate local electronic structure to enable complex bond formation. Together, these findings show how integration of molecular and solid-state catalysts can reproduce essential features of natural carbon-conversion systems, providing a design strategy for artificial photosynthesis and sustainable fuel synthesis.