Rising levels of carbon dioxide (CO­₂) in the atmosphere is a leading cause for global climate change. To reduce the level of CO₂ in the atmosphere, the reduction of carbon dioxide into useful organic compounds and fuels is attractive. However, due to the inertness and stability of carbon dioxide molecule, the electro-reduction of carbon dioxide is not facile. Formate is the first stable intermediate in the electrochemical reduction of carbon dioxide. Further, formic acid is useful as a chemical feedstock, a fuel, and an efficient carrier of hydrogen.^1-2 The production of formate is expected to be facile compared to that of other organic compounds as it involves the formation of just a C-H bond without dissociation of the strong C-O bond.

We have studied the interconversion of CO₂ and formate using metal-independent type enzyme, Formate Dehydrogenase (FDH) derived from the yeast Candida boidinii. This enzyme relies on cofactors such as nicotinamide adenine dinucleotide phosphate (NAD(P)+/ NAD(P)H). To achieve steady-state electrolysis, we have examined the continuous electrochemical regeneration of the reduced form of NAD(P) cofactor. However, we find that formation of an inactive-stable dimer of NAD is a major obstacle to continuous electrolysis. Thus, we have focused on using methyl viologen (MV) as an artificial co-enzyme in conjunction with FDH. MV has the advantage of not being de-activated upon re-generation and also prevents the re-oxidation of formate to carbon dioxide.³

MV in its reduced form was synthesized in a three-electrode system using Toray paper working electrode, platinum counter electrode and Ag/AgCl reference electrode at 0.45 V vs NHE. The concentration of MV in its redox states was followed by scanning voltammetry using a carbon fiber microelectrode (Figure 1). CO₂reduction to formate by the reduced form of the MV in the presence of FDH was investigated in this study. Formate production was confirmed by NMR. The presentation will discuss effect of enzyme concentration, carbon dioxide source and pH on the rate of production of formate.

Acknowledgement:

Authors acknowledge Loker Hydrocarbon Research Institute and University of Southern California for financial support of this work.

Refernces:

1.Olah, G. A.; Goeppert, A.; Prakash, G. S., Beyond oil and gas: The Methanol Economy. John Wiley & Sons: 2011.

2.Narayanan, S.; Haines, B.; Soler, J.; Valdez, T., Electrochemical conversion of carbon dioxide to formate in alkaline polymer electrolyte membrane cells. Journal of The Electrochemical Society 2011, 158(2), A167-A173.

3.Noji, T.; Jin, T.; Nango, M.; Kamiya, N.; Amao, Y., CO2 Photoreduction by Formate Dehydrogenase and a Ru-Complex in a Nanoporous Glass Reactor. ACS Applied Materials & Interfaces 2017, 9 (4), 3260-3265.