Direct Electron Transfer-Type Gas Diffusion Electrode for Efficient CO₂‑to-Formate Bioconversion across Broad and Low...

Bioelectrocatalytic CO₂ reduction to value-added chemicals offers a sustainable route for carbon recycling; however, achieving high activity under low CO₂ conditions remains a major challenge. Herein, we report a direct electron transfer (DET)-type gas diffusion electrode (GDE) utilizing formate dehydrogenase 1 (FoDH1) from Methylorubrum extorquens AM1 for efficient CO₂-to-formate conversion across a broad range of low CO₂ levels. The electrode surface was modified with pyrene derivatives using multiwalled carbon nanotubes (MWCNTs) to tailor the enzyme-electrode interface, enabling favorable enzyme orientation for rapid electron transfer. A 1-pyrenemethylamine (PyNH₂)-modified GDE with FoDH1 exhibited a current density of -1.2 mA cm^-2 at a reduction potential of -0.9 V, which was twice that of the pristine electrode, and generated formate with a Faradaic efficiency of 82 ± 5%, outperforming previously reported GDE systems. Kinetic analyses under controlled CO₂ conditions (0.04-100%) revealed a notably low apparent Michaelis constant (K _M,app) of 2.6 ± 0.3%, which is less than one-third of that of the solution system, indicating high CO₂ affinity. This enhancement is attributed to the direct supply of gaseous CO₂ to the catalytic layer, avoiding equilibrium limitations in the electrolyte solution. These results lead to a promising tool for enzymatic CO₂ reduction using emission gases from industrial sources and provide key design principles for practical CO₂-recycling technologies.