Bioelectrocatalytic Interconversion Between Formate and Carbon Dioxide By Tungsten-Containing Formate Dehydrogenase
Authors: Sakai Kento , Suzuki Arato , Kitazumi Yuki , Shirai Osamu , Kano Kenji 
Published: February 2020

The reduction of carbon dioxide (CO2) to generate reduced carbon compounds for use as fuels and chemical feedstocks is an essential requirement for carbon-based sustainable energy economy. An interconversion system of formate (HCOO-)/CO2 is one of the answers for the purpose. HCOO- is the first stable intermediate during the reduction of CO2 to methanol or methane and is increasingly recognized as a new energy source. In addition, it can be easily be handled, stored, and transported. 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.
Here, we have 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 CO2 reduction with several artificial redox partners (mediators). Mediators enable the enzymatic reaction to couple with an electrode reaction by shuttling electrons between enzymes and electrodes. This reaction is called mediated electron transfer (MET)-type bioelectrocatalysis. We have evaluated the bi-molecular reaction rate constants between FoDH1 and mediators and NAD+. They show the property called a linear free energy relationship (LFER), indicating that FoDH1 would have no specificity to NAD+. Similar LFER is also observed for the catalytic reduction of CO2. 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 (Fig.1). Potentiometric measurements have revealed that the three redox couples, MV2+/MV·
+, HCOO-/ CO2, FoDH1 (ox/red), reach an equilibrium in the bulk solution when the two-way bioelectrocatalysis proceeds in the presence of FoDH1 and MV. The steady-state voltammograms with two-way bioelectrocatalytic properties are interpreted on a simple model by considering the solution equilibrium1. 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 overpotentials2. FoDH1 was used as a catalyst and immobilized on a Ketjen Black-modified glassy carbon electrode (FoDH1/KB/GCE). 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 MV as a mediator. When the solution temperature was increased up to 60 °C to improve the enzymatic kinetics, j
lim showed a clear dependence on the rotation rate (ω) and was enhanced to 145 ± 6 mA cm-
2 at ω = 6,000 rpm (Fig.2). The characteristics can be expressed as Koutecký-Levich equation and the equation provided a result that the enzymatic kinetic-controlled current density was 290 ± 50 mA cm-
2. The large value indicate that this FoDH1-based system is very useful for the HCOO- oxidation. 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.
Reference
(1) K. Sakai, et al., Sens. Biosens. Res., 5, 90 (2015)
(2) K. Sakai, et al., Electrochem. Commun., 65, 31 (2016)

Figure 1