3.2.3. Ammonia Production

Ammonia (NH3) is an industrially beneficial compound used as a chemical raw material, fuel, hydrogen storage, and so on [155,156]. Nitrogenase (N2ase) is an enzyme that catalyzes a nitrogen fixation from dinitrogen (N2) to NH<sup>3</sup> using adenosine triphosphate (ATP) hydrolysis energy in microorganisms under an anaerobic condition [155,156]. Particularly, molybdenum-dependent N2ase, which comprises a catalytic MoFe protein and a homodimeric MgATP-binding Fe protein, has been mainly investigated in view of bioelectrocatalysis [156,157]. N2ase reduces not only N<sup>2</sup> but also nitrite (NO<sup>2</sup> <sup>−</sup>) and azide (N<sup>3</sup> <sup>−</sup>) to NH<sup>3</sup> on electrodes, and the MoFe protein disassociated with the Fe protein shows the DET-type bioelectrocatalytic activity without ATP [154]. Furthermore, ATP-independent NH<sup>3</sup> bioelectrosynthesis was improved in the MET-type system using N2ase and a cobaltocene-functionalized polymeric mediator [51]. On the other hand, Milton et al. reported a NH3-producing H2/N<sup>2</sup> biofuel cell using MET-type reactions of N2ase and H2ase [148]. If the fatal weakness of N2ase, a lack of oxygen tolerance, is improved, N2ase will be expected to have further industrial applications.
