Question

Methanogenesis suppression in microbial fuel cells review

Answer 1

The external resistance (R ext) of microbial fuel cells (MFCs) regulates both the anode availability as an electron acceptor and the electron flux through the circuit. We evaluated the effects of Rext on MFCs using acetate or glucose. The average current densities (I) ranged from 40.5 mA/m2(9,800 Ω) to 284.5 mA/m2 (150 Ω) for acetate-fed MFCs (acetate-fed reactors [ARs]), with a corresponding anode potential (E an) range of −188 to −4 mV (versus a standard hydrogen electrode [SHE]). For glucose-fed MFCs (glucose-fed reactors [GRs]), I ranged from 40.0 mA/m2 (9,800 Ω) to 273.0 mA/m2 (150 Ω), with a corresponding Ean range of −189 to −7 mV. ARs produced higher Coulombic efficiencies and energy efficiencies than GRs over all tested R ext levels because of electron and potential losses from glucose fermentation. Biogas production accounted for 14 to 18% of electron flux in GRs but only 0 to 6% of that in ARs. GRs produced similar levels of methane, regardless of the R ext. However, total methane production in ARs increased as R extincreased, suggesting that E an might influence the competition for substrates between exoelectrogens and methanogens in ARs. An increase of R ext to 9,800 Ω significantly changed the anode bacterial communities for both ARs and GRs, while operating at 970 Ω and 150 Ω had little effect. Deltaproteobacteria and Bacteroidetes were the major groups found in anode communities in ARs and GRs. Betaproteobacteria and Gammaproteobacteria were found only in ARs. Bacilli were abundant only in GRs. The anode-methanogenic communities were dominated by Methanosaetaceae, with significantly lower numbers of Methanomicrobiales. These results show that R ext affects not only the E an and current generation but also the anode biofilm community and methanogenesis.

The bioanode, a crucial component in bioelectrochemical systems (BESs), is composed of an anode biofilm and a conductive electrode. The main catalytic components of interest in anode biofilms are exoelectrogens, microorganisms that are capable of exocellular electron transfer (31). In mixed-culture systems, exoelectrogens compete for electron donors with other functional groups such as fermenters, acetogens, and methanogens. The complexity of anode biofilms makes it hard to elucidate electrochemical mechanisms at the bioanode, but a precise understanding of exoelectrogenesis and competition in anode biofilms will aid in improving the performance of BESs. Several reviews provide insightful summaries and perspectives regarding bioanodes