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Microbial bioelectrocatalysis has become important for storage and energy conversion, synthesis of valuable products such as hydrogen and methane, and waste treatment among others.

The production of methane from CO₂ reduction in microbial biocathodes has been proposed as a frontier technology. The first study on CO₂ conversion to CH₄ was reported by Cheng et al. in 2008 [178]. The interest of using CO₂ as gaseous substrate lies on their availability as atmospheric gas and waste gas; it is also land-independent and ease to handle (Figure 1.13).

The MECs conjugate characteristics of an electrochemical process and an enzymatic-type process for CO₂ reduction [179]. The difference is that bacteria are utilized as catalyst either at the cathode or both at the anode and cathode. The diverse possible pathways for CO₂ reduction are still uncertain. However, at least two mechanisms are envisaged for this reaction:

Figure 1.13 Integration of bioprocesses with microbial electrolysis cells (MEC).

indirect production when methane is formed in the bulk and direct when methane is formed near the biocathode (Figure 1.14). The mechanisms for biocatalyzed methane formation are diverse since the sources of reactants are multiple; moreover, the metabolic pathways of electroactive microorganisms receiving electrons are still poorly understood.

Rosenbaum et al. stated various hypothesis on the molecular phenomena [180]:

1 • Direct electron transfer involves c-type cytochrome and electron transfer chains

2 • Direct electron transfer includes cytochrome linked to hydrogenase partnerships

3 • A mediated electron transfer to a periplasmic hydrogenase takes place.

At a bioelectrochemical process level, one mechanism for CO₂ reduction is explained by hydrogen production at the cathode, which is then utilized by bacteria to reduce CO₂. The biochemical pathway covers homoacetogenic fermentation by chemolithotrophic species. Examples of chemolithotrophic acetogens are Clostridium aceticum and Acetobacterium woodii [173].

Jiang et al. reported that formation of CH₄ from CO₂ can follow two pathways, through the direct use of electrical current for hydrogen formation or via biohydrogen production and then CO₂ bioelectrochemical reduction. Hydrogen produced through water electrolysis can provide the substrate for methanogens to produce methane. However, abiotic hydrogen production requires the use of catalyst whereas biohydrogen does not [171].

Figure 1.14 Overview of hypothetic mechanism for CH₄ production from CO₂. (a) Indirect mechanism, (b) Direct mechanism, (c) Alternative direct mechanism.

Blanchet et al. agree that hydrogen is produced on the cathode as a reactant for the microbial reduction of CO₂. They propose that two consecutive steps occurs, hydrogen production by water electrolysis and then reduction of CO₂ by microbial species that utilize that hydrogen [79].

Two pathways are described by Villano et al., hydrogenotrophic methanogenesis and direct extracellular electron transfer. The contribution of each pathway depends on the set cathode potential. The author points that the reactants for CH₄ formation, electrons and CO₂ are produced by the bioanode and then utilized on the biocathode. The influence of hydrogen produced abiotically on indirect electron transfer is also discussed [181].

Siegert et al. propose that methane production is performed via direct electron transfer from the electrode to the microorganisms, which produce methane by using the electrical current [172]. The authors investigated the stoichiometry 4:1 of H₂:CH₄, where hydrogen production was considered abiotic and the methane as a biological production. The formed methane could be explained by hydrogen formation on Pt, but the production on other materials did not correspond to the amount of methane harvested. Hydrogen produced abiotically was insufficient for the amount of methane measured. Since hydrogen produced in absence of a metallic catalyst was insufficient for the methane harvested, a direct electron transfer very likely controlled the process.

Coulombic efficiencies higher than 100% have been reported for processes using biocathodes; this suggests a corrosion process is also present. Corrosion may be an issue to overcome when using microbial biocathodes since it is known that Archea group have a significant effect on metallic materials. Previous research focused on microbial-influenced corrosion by methanogens [182]. Therefore, alternative semiconductor minerals like magnetite have been proposed as cathode material; moreover, magnetite promotes interspecies electron transfer [183].