Covalent surface coordination enables efficient and stable formaldehyde-water co-electrolysis over non-stoichiometric...

The activity-stability trade-off of copper-based oxide catalysts in formaldehyde oxidation remains a fundamental challenge. Here, we resolve this trade-off by grafting N-heterocyclic carbenes (NHCs) onto non-stoichiometric Cu₂₊₁O nanocubes (denoted as Cu₂₊₁O, a Cu⁺-rich single-phase oxide) through covalent CuC bonds. This molecular surface coordination strategy delivers two concurrent benefits. First, strong σ-electron donation from NHCs shifts the Cu d-band center from -2.106 eV to -2.416 eV, weakening intermediate adsorption and lowering the reaction free-energy change of the rate-determining step (CHOOH^⁎ desorption) from 2.56 eV to 1.38 eV. Second, the covalent CuC anchors suppress copper leaching in alkaline electrolyte by a factor of 160 (from 8 mg L^-1 to 0.05 mg L^-1 over 100 h). The resulting Cu₂₊₁O-NHC catalyst drives formaldehyde oxidation at an onset potential of 0.1 V vs. RHE (Tafel slope: 590 ± 2 mV dec^-1, versus 96 ± 3 mV dec^-1 for unmodified Cu₂₊₁O; mean ± SD, n = 3), achieving near-unity Faradaic efficiency for both formate (98.65 ± 0.32%) and anodic H₂ evolution from CH bond cleavage (99.14 ± 0.28%). Isotope-labeled DEMS confirms that anodic H₂ originates predominantly from the CH bond of formaldehyde, as no HD or D₂ was detected when the reaction was performed in D₂O. After 100 h of continuous operation, the cubic morphology remains largely intact. This work establishes covalent surface coordination as a promising molecular strategy to decouple activity and stability in oxide electrocatalysts, potentially extendable beyond the specific case of Cu₂₊₁O for aldehyde oxidation.