Direct Electron Transfer-Type Gas Diffusion Electrode for Efficient CO 2to-Formate Bioconversion across Broad and Low CO 2 Levels

Bioelectrocatalytic CO 2 reduction to value-added chemicals offers a sustainable route for carbon recycling; however, achieving high activity under low CO 2 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 CO 2 -to-formate conversion across a broad range of low CO 2 levels. The electrode surface was modified with pyrene derivatives using multi-walled carbon nanotubes (MWCNTs) to tailor the enzyme-electrode interface, enabling favorable enzyme orientation for rapid electron transfer. A 1-pyrenemethylamine (PyNH 2 )-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 CO 2 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 CO 2 affinity. This enhancement is attributed to the direct supply of gaseous CO 2 to the catalytic layer, avoiding equilibrium limitations in the electrolyte solution. These results lead to a promising tool for enzymatic CO 2 reduction using emission gases from industrial sources and provide key design principles for practical CO 2 -recycling technologies.