NADH-Free Electroenzymatic Reduction of CO₂ by Conductive Hydrogel-Conjugated Formate Dehydrogenase

The electrocatalytic reduction of CO₂ under low overpotential and mild conditions using redox enzyme is a propitious route for carbon capture and conversion. Here, we report bioelectrocatalytic CO₂ conversion to formate by conjugating a strongly CO₂-reductive, W-containing formate dehydrogenase from Clostridium ljungdahlii (ClFDH) to conductive polyaniline (PANi) hydrogel. The ClFDH in the hybrid electrode successfully gained electrons directly from PANi and exhibited high capability for electroenzymatic conversion of CO₂ to formate at low overpotential without NADH. We describe a potential electron-transfer pathway in the PANi-ClFDH bioelectrode on the basis of multiple spectroscopic analyses and a QM/MM-based computational study. The 3D-nanostructured PANi hydrogel facilitated rapid electron injection to the active site of ClFDH. In the absence of NADH, the PANi-ClFDH electrode showed stable CO₂-to-formate transformation at an overpotential as low as 40 mV, with 1.42 μmol h^-1 cm^-2 conversion rate, 92.7% faradaic efficiency, and 976 h^-1 turnover frequency.