Hybrid Enzyme-Electrocatalyst Cascade Modified Gas-Diffusion Electrodes for Methanol Formation from Carbon Dioxide

We propose a hybrid electrocatalytic-bioelectrocatalytic reaction cascade integrated on a gas diffusion electrode for CO₂ reduction under selective formation of methanol. Ag-Bi₂O₃ selectively reduces gaseous CO₂ to formate at neutral pH conditions. A subsequent enzymatic cascade comprising formaldehyde dehydrogenase and alcohol dehydrogenase, which are both nicotinamide adenine dinucleotide (NAD)-dependent, further reduce formate sequentially to formaldehyde and methanol. The enzymatically oxidized redox cofactor NAD⁺ is regenerated through the enzyme diaphorase which is electrochemically coupled to the electrode by embedding it into a cobaltocene-based low-potential redox polymer. Methanol formation is confirmed using a highly selective biosensor based on alcohol oxidase in combination with horseradish peroxidase integrated into a specifically adapted redox polymer. The proposed hybrid electrocatalytic-bioelectrocatalytic cascade is an example for a general strategy for enhancing the selectivity of electrocatalytic reactions and/or to replace bioelectrochemically challenging steps such as the synthesis of formate from CO₂ using highly oxygen-sensitive formate dehydrogenases by more robust inorganic electrocatalysts.