3.2.2. Formate Production

Formate/formic acid (HCOO−/HCOOH) is a liquid energy resource with a high energy density due to its property of being completely oxidized to CO2. FoDH interconverts HCOO<sup>−</sup> and CO<sup>2</sup> both in DETand MET-type systems [75,136–139]. Sakai et al. reported efficient bioelectrocatalytic CO<sup>2</sup> reduction using FoDH from *Methylobacterium extorquens* AM1 and a synthesized redox mediator with the low formal potential on a gas-diffusion electrode [150]. In addition, spontaneous interconversion between HCOO<sup>−</sup> and H<sup>2</sup> without any external power supplies was achieved by coupling two bioelectrocatalyses by FoDH and H2ase (H<sup>2</sup> + CO<sup>2</sup> → HCOO<sup>−</sup> + H+) [147]. The direction of the reaction is controlled by pH- and concentration-dependent equilibrium potentials of the 2H+/H<sup>2</sup> and CO2/HCOO<sup>−</sup> couples.
