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Given the aggravated greenhouse effect caused by CO2 and the current energy shortage, CO2 capture and reuse has been gaining ever-increasing concerns. Microbial Electrolysis Cells (MECs) has been considered to be a promising alternative to recycle CO2 bioelectrochemically to low-carbon electrofuels such as CH4 by combining electroactive microorganisms with electrochemical stimulation, enabling both CO2 fixation and energy recovery. In spite of the numerous efforts dedicated in this field in recent years, there are still many problems that hinder CO2 bioelectroconversion technique from the scaling-up and potential industrialization. This review comprehensively summarized the working principles, extracellular electron transfers behaviors, and the critical factors limiting the wide-spread utilization of CO2 electromethanogenesis. Various characterization and electrochemical testing methods for helping to uncover the underlying mechanisms in CO2 electromethanogenesis have been introduced. In addition, future research needs for pushing forward the development of MECs technology in real-world CO2 fixation and recycling were elaborated.

Electromethanogenesis (EMG) system could efficiently convert CO2 to CH4 by using excess renewable electricity. However, the fluctuation and interruption of renewable electricity will adversely affect the biocathode and therefore the CH4 production of the EMG system. In this work, a novel biocathode acclimation strategy with intermittent step-up voltage (ISUV) was proposed to improve the stability and resilience of the EMG system against the unstable input of renewable power. Compared with the intermittent application of constant voltage (IACV), the ISUV increased the rate of CH4 production by 11.7 times with the improvement of the stability and resilience by 56% and 500%, respectively. Morphology and microflora structure analysis revealed that the biofilm enriched with ISUV exhibited a compact microflora structure with high-density cells and nanowires interconnected. This study provided a novel effective strategy to regulate the biofilm structure and enhance the performance of the EMG system.

Bioelectrochemical conversion of CO2 to CH4 is a promising way to increase the calorific value of biogas produced during anaerobic digestion. There are two groups of methanogens enriched in these systems, hydrogenotrophs and acetoclastic methanogens that can also directly accept electrons from an electrode or another microorganism. In this study, a microbial electrolysis cell (MEC) poised at −500 mV (vs. SHE) was operated for biogas upgrading. Methane content in the biogas increased from 71% to >90%, and 8.2% of the CO2 was converted to methane. Methanothrix, an acetoclastic methanogen that can participate in direct electron transfer (DET), and Azonexus, an acetate-oxidizing electrogen, were enriched on the cathode. Transcriptomics revealed that Methanothrix on the cathode were using the CO2 reduction pathway, while Methanothrix in the bulk sludge were using the acetate decarboxylation pathway for production of methane. These results show that stimulation of DET in MEC enhances biogas-upgrading processes.

Anaerobic digestion (AD) is an important energy strategy for converting organic waste to CH4. A major factor limiting the practical applicability of AD is the relatively long hydraulic retention time (HRT) which declines the treatment efficiency of digesters. A coupling process of anaerobic digestion and ‘electromethanogenesis’ was proposed to enhance anaerobic digestion at a short HRT in this study. Microorganisms analysis indicated that the electric-biological reactor enriched hydrogenotrophic methanogens in both cathodic biofilm and suspended sludge, helping achieve the high organic removal (71.0% vs 42.3% [control reactor]) and CH4 production (248.5 mL/h vs 51.3 mL/h), while the additional electric input was only accounted for 25.6% of the energy income from the increased CH4 production. This study demonstrated that a bioelectrochemical enhanced anaerobic reactor could improve the CH4 production and organic removal at a short HRT, providing an economically feasible scheme to treat wastewater.

This work investigated the electrocatalytic activity of a thermophilic methanogenic consortia (TMC) for developing a bioelectrosynthesis process to convert food and paper wastes to methane. Electroanalytical techniques were used to analyze the electrocatalytic activity of the TMC biofilm formed onto the electrodes. The developed electromethanogenesis process enhanced the yield of methane by 54.7% than control experiments. Scanning electron micrographs of the TMC bioelectrodes showed that the electrosynthesis process accelerates biofilm formation onto the electrodes leading to enhanced direct electron transfer reactions at electrode–electrolyte interface. This study will help in developing a novel approach for valorization of food and paper waste to biofuels.

Direct interspecies electron transfer (DIET) between exoelectrogenic fatty acid oxidizers and electrotrophic methanogens plays an important role in keeping the overall anaerobic digestion (AD) process well-balanced. This study examined the individual and combined effects of two different DIET-promoting strategies, i.e., magnetite addition (20 mM Fe) and external voltage application (0.6 V), in continuous digesters treating dairy wastewater. Although the strategies were both effective in enhancing the process performance and stability, adding magnetite had a much greater stimulatory effect. External voltage contributed little to the methane yield, and the digester with magnetite addition alone achieved stable performance, comparable to that of the digester where both strategies were combined, at short hydraulic retention times (down to 7.5 days). Diverse (putative) electroactive microorganisms were significantly enriched under DIET-promoting conditions, particularly with magnetite addition. The overall results suggest that magnetite addition could effectively enhance AD performance and stability by promoting DIET-based electro-syntrophic microbial interactions.

A new approach to hydrogen production using an integrated pyrolysis–microbial electrolysis process is described. The aqueous stream generated during pyrolysis of switchgrass was used as a substrate for hydrogen production in a microbial electrolysis cell, achieving a maximum hydrogen production rate of 4.3 L H2/L anode-day at a loading of 10 g COD/L-anode-day. Hydrogen yields ranged from 50 ± 3.2% to 76 ± 0.5% while anode Coulombic efficiency ranged from 54 ± 6.5% to 96 ± 0.21%, respectively. Significant conversion of furfural, organic acids and phenolic molecules was observed under both batch and continuous conditions. The electrical and overall energy efficiency ranged from 149–175% and 48–63%, respectively. The results demonstrate the potential of the pyrolysis–microbial electrolysis process as a sustainable and efficient route for production of renewable hydrogen with significant implications for hydrocarbon production from biomass.

A microbial electrolysis cell (MEC) is a promising technology for enhancing biogas production from an anaerobic digestion (AD) reactor. In this study, the effects of the MEC on the rate of methane production from food waste were examined by comparing an AD reactor with an AD reactor combined with a MEC (AD+MEC). The use of the MEC accelerated methane production and stabilization via rapid organic oxidation and rapid methanogenesis. Over the total experimental period, the methane production rate and stabilization time of the AD+MEC reactor were approximately 1.7 and 4.0 times faster than those of the AD reactor. Interestingly however, at the final steady state, the methane yields of both the reactors were similar to the theoretical maximum methane yield. Based on these results, the MEC did not increase the methane yield over the theoretical value, but accelerated methane production and stabilization by bioelectrochemical reactions.

Functioning biocathodes are essential for electromethanogenesis. This study investigated the development of a biocathode from non-acclimated anaerobic sludge in an electromethanogenesis cell at a cathode potential of -0.7V (vs. standard hydrogen electrode) over four cycles of repeated batch operations. The CO2-to-CH4 conversion rate increased (to 97.7%) while the length of the lag phase decreased as the number of cycles increased, suggesting that a functioning biocathode developed during the repeated subculturing cycles. CO2-resupply test results suggested that the biocathode catalyzed the formation of CH4 via both direct and indirect (H2-mediated) electron transfer mechanisms. The biocathode archaeal community was dominated by the genus Methanobacterium, and most archaeal sequences (>89%) were affiliated with Methanobacterium palustre. The bacterial community was dominated by putative electroactive bacteria, with Arcobacter, which is rarely observed in biocathodes, forming the largest population. These electroactive bacteria were likely involved in electron transfer between the cathode and the methanogens.

Performance of methane-producing microbial electrochemical systems (MESs) is highly reliant on electron transfer efficiency from electrode to microorganisms and vice versa. In this study, magnetite nanoparticles were used as electron carriers to enhance extracellular electron transfer in single chamber MESs. The MES with magnetite exhibited the highest methane yield and current generation of 0.37 ± 0.009 LCH4/gCOD and 9.6 mA, respectively among the tested reactors. The experimental data was observed to be highly consistent with modified Gompertz model results (R2 > 0.99), which also showed 74.2% and 22.1% enhanced methane production rate in MES with magnetite as compared to control AD and MES without magnetite, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy analysis confirmed that magnetite enhanced catalytic activity of biofilm and lowered both solution and charge transfer resistance. Therefore, supplementing magnetite in MESs could be a strategy to develop an efficient syntrophic biomethanation in field scale applications.

A preliminary experimental investigation of dry reforming of methane with carbon dioxide, that has been performed on an iron bed activated with an electric current is reported. Operating conditions for the reaction included temperature ranging from 700 to 800° C and pressure close to 1 atm. The reaction, involving an excess of pure methane and carbon dioxide, was performed with and without addition of water vapour, provided by hot water saturation of the gaseous feed. According to syngas compositions, the electron flow has a dramatic effect on the conversion of both methane and carbon dioxide. It was shown also that hot water saturation of the CO(2) and CH(4) mixture allowed very good conversion, giving a syngas with a composition very close to what was expected from equilibrium calculations.

This study describes the performance of a microbial biocathode, based on a hydrogenophilic methanogenic culture, capable of reducing carbon dioxide to methane, at high rates (up to 0.055 ± 0.002 mmol d−1 mgVSS−1) and electron capture efficiencies (over 80%). Methane was produced, at potentials more negative than −650 mV vs. SHE, both via abiotically produced hydrogen gas (i.e., via hydrogenophilic methanogenesis) and via direct extracellular electron transfer. The relative contribution of these two mechanisms was highly dependent on the set cathode potential. Both cyclic voltammetry tests and batch potentiostatic experiments indicated that the capacity for extracellular electron transfer was a constitutive trait of the hydrogenophilic methanogenic culture. In principle, both electrons and carbon dioxide required for methane production could be obtained from a bioanode carrying out the oxidation of waste organic substrates.

The functional and taxonomic microbial dynamics of duplicate electricity-consuming methanogenic communities were observed over a 6 months period to characterize the reproducibility, stability and recovery of electromethanogenic consortia. The highest rate of methanogenesis was 0.72 mg-CH4/L/day, which occurred during the third month of enrichment when multiple methanogenic phylotypes and associated Desulfovibrionaceae phylotypes were present in the electrode-associated microbial community. Results also suggest that electromethanogenic microbial communities are very sensitive to electron donor-limiting open-circuit conditions. A 45 min exposure to open-circuit conditions induced an 87% drop in volumetric methane production rates. Methanogenic performance recovered after 4 months to a maximum value of 0.30 mg-CH4/L/day under set potential operation (-700 mV vs Ag/AgCl); however, current consumption and biomass production was variable over time. Long-term functional and taxonomic analyses from experimental replicates provide new knowledge toward understanding how to enrich electromethanogenic communities and operate bioelectrochemical systems for stable and reproducible performance.

To better understand the underlying mechanisms for methane bioelectrosynthesis, a two-chamber MECs containing a carbon biocathode was developed and studied. Methane production substantially increased with increasing cathode potential. Considerable methane yield was achieved at a poised potential of -0.9 V (vs. Ag/AgCl), reaching 2.30±0.34 mL after 5 h of operation with a faradaic efficiency of 24.2±4.7%. Confirmatory tests done at 0.9 V by switching the type of flushed substrates (CO2/N2) or the electrical exposure modes (ON/OFF) demonstrated that cathode serving as an electron donor was the vital driving force for methanogenesis occurring at microbe-electrode surface. Fluorescence in situ hybridization reveled Methanobacteriaceae (particularly Methanobacterium) was the predominant methanogens, supporting the mechanisms of direct electron transfer between cell-electrode. Additionally, the analysis of scanning electron microscope confirmed that the multiple pathways of electron transfer, including direct cathode-to-cell, interspecies exchange and semi-conductive conduits all together ensured the successful electromethanogenesis process.

The anode of a two-chamber methane-producing microbial electrolysis cell (MEC) was poised at +0.200 V vs. the standard hydrogen electrode (SHE) and continuously fed (1.08 gCOD/L d) with acetate in anaerobic mineral medium. A gas mixture (carbon dioxide 30 vol.% in N2) was continuously added to the cathode for both pH control and carbonate supply. At the anode, 94% of the influent acetate was removed, mostly through anaerobic oxidation (91% coulombic efficiency); the resulting electric current was mainly recovered as methane (79% cathode capture efficiency). Low biomass growth was observed at the anode and ammonium was transferred through the cationic membrane and concentrated at the cathode. These findings suggest that the MEC can be used for the treatment of low-strength wastewater, with good energy efficiency and low sludge production.