3.2.1. Dihydrogen Production

Dihydrogen (H2) is a typical clean energy source that emits no harmful products in combustion. H2ase, a common anodic bioelectrocatalyst for H<sup>2</sup> oxidation, can also catalyze proton (H+) reduction [135,136]. The reversible reaction is biased to favor H<sup>2</sup> oxidation, and it is difficult to realize a large cathodic current density of H<sup>+</sup> reduction in DET-type systems. H<sup>+</sup> reduction with a large current density was achieved in a H2ase MET-type system using a viologen polymer as a redox mediator [53]. On the other hand, a dual DET-type bioelectrocatalytic water–gas shift reaction using CODH and H2ase (H2O + CO → H<sup>2</sup> + CO2) was reported [146]. The system requires no external power supplies.
