Conversion of CO2 to formate in an electroenzymatic cell using Candida boidinii formate dehydrogenase

• CO2 was a substrate for formic acid formation with FDH (formate dehydrogenase).
• Cu was best material for NADH regeneration among Au, Ag, In and Cu.
• Rh complex was not only electron mediator, but also catalyst for CO2 conversion.
• HCOOH could be continuously produced from CO2 in a bioelectrochemical cell.

Electroenzymatic conversion of environmentally detrimental CO2 into useful chemicals using a NADH-dependent formate dehydrogenase has been of great interest, but the system remained at a conceptual level because of its complexity. In this study, we found that CO2 was a direct substrate for formate formation with Candida boidinii formate dehydrogenase (CbsFDH), but HCO3− was not. Enzymatic formate formation with CbsFDH was inhibited at NADH concentrations higher than 0.35 mM. Electrochemical NADH regeneration was performed at a Cu electrode using the [Cp*Rh(bpy)Cl]+ complex, which catalyzed the conversion of NAD+ into the active NADH with almost 100% selectivity. The electroenzymatic reaction for HCO2H formation was conducted at 1.0 V, 0.25 mM NADH and 0.25 mM Rh complex during electrochemical NADH regeneration. CO2 was reduced into formate by enzymatic catalysis under the NADH regeneration condition, where protons and electrons was continuously supplied into a cathode cell through Nafion® membrane from water splitting at a Pt anode. The interactions of the Cu electrode, Rh mediator, NADH, and CbsFDH were analyzed for the first time. The Rh(III) mediator was hydrolyzed and reduced reversibly into a Rh(I) intermediate (Mred1) as well as irreversibly into a Rh(I) hydride intermediate (Mred2) at the Cu electrode. Interestingly, the Rh(I) species showed activity toward the direct reduction of CO2 as well as NADH regeneration, although the primary CO2 reduction occurred through CbsFDH at 1.0 V.

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