An interconversion of HCOO^-/CO₂ is one of essential requirements for carbon-based sustainable energy economy. However, when CO₂ is reduced and HCOO^- is oxidized directly on electrodes, carbon monoxide is generated and quite high overpotential is required. One of the most promising strategies for solving these issues is the utilization of enzymes as catalysts. Enzymes allow the system to function in a specific biological reaction under mild conditions. The electro-enzymatic devices can be used as energy conversion system such as HCOO^-/O₂ biofuel cells and an efficient bioelectrochemical system of the CO₂ reduction.

We focused on the catalytic properties of tungsten-containing formate dehydrogenase (FoDH1; EC 1.2.1.2) from Methylobacterium extorquens AM1 to construct a bioelectrochemical interconversion system of HCOO^-/CO₂. FoDH1 is a heterodimeric soluble enzyme and catalyzes the oxidation of HCOO^- to CO₂ in coupled reduction of NAD⁺ to NADH. In this study, we have found that FoDH1 catalyzes both of the HCOO^- oxidation and the HCO₃^- reduction with several artificial redox partners (mediators). The bi-molecular reaction rate constants between FoDH1 and mediators and show a linear free energy relationship. The reversible reaction between HCOO^- and CO₂ through FoDH1 has been realized on cyclic voltammetry by using methyl viologen (MV) as a mediator and by adjusting pH from the thermodynamic viewpoint. The steady-state voltammograms with two-way bioelectrocatalytic properties are interpreted on a simple model by considering the solution equilibrium. Furthermore, we have constructed a light driven HCOO^- production system using a spinach thylakoid membrane, MV, and FoDH1.

When the interconversion between HCOO^- and CO₂ is applied to the construction of efficient bioelectrochemical devices, a large current density should be realized at potentials close to the formal potential of the HCOO^-/CO₂ couple (E°'_CO2). We show a great possibility of MET-type bioelectrocatalysis in the HCOO^- oxidation and the CO₂ reduction at high current densities and low overpotentials. For the HCOO^- oxidation, a high limiting current density (j_lim) of about 24 mA cm^-2 was realized with a half wave potential (E_1/2) of only 0.12 V more positive than E°'_CO2 at 30 °C in the presence of MV²⁺ as a mediator. When a viologen-functionalized polymer was co-immobilized with FoDH1 on the porous electrode, j_lim of about 30 mA cm^-2 was attained at 60 °C with E_1/2 = E°'_CO2 + 0.13 V. On the other hand, the CO₂ reduction was also realized at j_lim of about 15 mA cm^-2 with E_1/2 = E°'_CO2 – 0.04 V at pH 6.6 and at 60 °C in the presence of MV. This is the first report of the enzyme-based bioelectrocatalytic CO₂ reduction at such low overpotential and a high j_lim.

The present results are very useful to construct an effective bioelectrochemical reaction for the CO₂ reduction and HCOO^-/O₂ biofuel cells as effective energy conversion systems.