Electrocatalytic CO₂ reduction catalyzed by nitrogenase MoFe and FeFe proteins

Nitrogenases catalyze biological dinitrogen (N₂) reduction to ammonia (NH₃), and also reduce a number of non-physiological substrates, including carbon dioxide (CO₂) to formate (HCOO^-) and methane (CH₄). Three versions of nitrogenase are known (Mo-, V-, and Fe-nitrogenase), each showing different reactivities towards various substrates. Normally, electrons for substrate reduction are delivered by the Fe protein component of nitrogenase, with energy coming from the hydrolysis of 2 ATP to 2 ADP+2 Pi for each electron transferred. Recently, it has been demonstrated that energy and electrons can be delivered from an electrode to the catalytic nitrogenase MoFe-protein without the need for Fe protein or ATP hydrolysis. Here, it is demonstrated that both the MoFe- and FeFe-protein can be immobilized as a polymer layer on an electrode and that electron transfer mediated by cobaltocene can drive CO₂ reduction to formate in this system. It was also found that the FeFe-protein diverts a greater percentage of electrons to CO₂ reduction versus proton reduction compared to the MoFe-protein. Quantification of electron flow to products exhibited Faradaic efficiencies of CO₂ conversion to formate of 9% for MoFe protein and 32% for FeFe-protein, with the remaining electrons going to proton reduction to make H₂.