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Bioelectrochemical power-to-gas presents a promising technology for long-term storage of excess renewable energy in the form of methane. The transition of the technology from laboratory to applied scale is currently challenged by low volumetric production rates, energy losses at the cathode, as well as the unknown physiology of the microbes in an electrochemical reactor. Here, we introduce a stable electromethanogenesis system based on efficient in situ hydrogen production by non-precious-metal catalysts and effective hydrogen uptake by the methanogenic microorganisms. Using NiMo-cathodes and pure cultures of hydrogenotrophic Methanococcus maripaludis, our system achieved an unprecedented volumetric methane production rate from CO2 of 1.4 L methane per L per day. The system performed stably for over 4 weeks with columbic efficiencies steadily above 90%. A physiological analysis of cells in the electromethanogenic reactor revealed robustly-growing cells with nearly identical protein expression patterns to gas-fed controls. Local pH fluctuations at the surface of cathode and cation exchange membrane resulted in a small but noticeable fraction of cell lysis. Our data collectively indicate that physiologically uncompromised cells of a pure methanogenic culture can perform methanogenesis robustly at high specific rate in a biocompatible electromethanogenic reactor using inexpensive, earth-abundant cathode materials.

Electromethanogenesis (EM) process was specifically designed to increase the conversion efficiency of organic substrates towards methane production and was further evaluated towards biogas upgradation applications. Four single-chambered EM systems were operated using designed synthetic wastewater (carbon source: 10 g L−1) to comparatively analyze the effect of applied voltage (AV −0.8 V; CC 100 Ω) and electrode placement on methanogenesis. The experiments were performed in three phases: the biomethanation phase (BM) (without electrodes assembly), electrode phase (MESOC; with electrodes assembly in an open circuit operation) and EM phase (MESCC/AV; electrodes assembly in a closed circuit or applied voltage operation). Electrode placement with a submerged anode and variations in the position of the cathode [partially air-exposed cathode; AV (R1/MESAV) and CC (R2/MESCC)] and completely submerged cathode [AV (R3/IE-MESAV) and CC (R4/IE-MESCC)] were evaluated. The results for the EM phase showed higher total biogas formation in R3/IE-MESAV (1.12 L), followed by R1/MESAV (0.91 L), R4/IE-MESCC (0.88 L) and R2/MESCC (0.59 L). Higher methane content was observed in R3/IE-MESAV (0.88 L) with a conversion efficiency of 76%, followed by R1/MESAV (0.66 L; 60%), R4/IE-MESCC (0.60 L; 56%) and R2/MESCC (0.37 L; 34%). EM operation depicted higher CH4 fraction with the co-generation of lower fractions of H2 and CO2 [R3/IE-MESAV (CH4-78%; H2-5%; CO2-17%), R1/MESAV (CH4-73%; H2-3%; CO2-22%), R4/IE-MESCC (CH4-68%; H2-5%; CO2-28%), R2/MESCC (CH4-63%; H2-4%; CO2-34%)] compared to control operation (CH4-61%; H2-4%; CO2-33%). The polarized microenvironment provided during EM (CC/AV) significantly contributed to biogas upgradation with a simultaneous CO2 reduction as compared to the BM and MESOC phases. Electrochemical characterization showed a high regulatory influence of the electron flux in R3/IE-MESAV, which helped to increase CO2 conversion/utilization compared to other conditions. EM influenced direct interspecies electron transfer (DIET), creating a microbe–electrode synergy leading to neutralize the disruptors influence (from side reactions) on final product formation. This study provides a specific strategy for increasing the overall methane yields with the influence of in situ/ex situ potential, while overcoming the limitations of methanogenesis.

Electromethanogenesis, which is different from traditional hydrogenotrophic and acetoclastic methanogenesis, is a novel means of methane production involving the direct transfer of electrons to methanogens. Nano-Fe3O4 (nano-magnetite) has been found to strengthen the link between electricity-producing bacteria and methanogens; however, whether nano-Fe3O4 changes the carbon- and/or electron-flow pathways involved in the methanogenic processes in natural soil remains unknown. Experiments on wetland soil were reported in this work. Experimental results showed that nano-Fe3O4 particles significantly stimulated methane production. Combining the application of a stable isotope tracer (13CH3COOH) and an acetoclastic methanogenesis inhibitor (CH3F) and thermodynamic calculations, the results indicated that nano-Fe3O4 accelerated the CO2 reduction process rather than acetoclastic methanogenesis. Comprehensive utilization of a variety of methods (thermodynamic calculations, current density measurements, and model analysis) suggested that electrons, which came from syntrophic acetate oxidation (SAO), were the reducing agents for methane production. Nano-Fe3O4 did not appear to change the carbon flow but did promote electron transfer, and the whole process can be finished within an hour-long timescale. Furthermore, a dramatic increase in acetoclastic methanogenesis was observed within only 9 h incubation. This suggests that the methanogenic pathways are flexible and transitory in natural soil. Coupled with cDNA sequencing results for the active bacteria and archaea, it is suggested that SAO and electromethanogenesis were preferred when Geobacter and Methanosarcina were involved. These findings add to the knowledge of the role of nano-Fe3O4 in methanogenic processes in natural and artificial environments.

Correction for ‘In situ electrochemical H2 production for efficient and stable power-to-gas electromethanogenesis’ by Frauke Kracke et al., Green Chem., 2020, 22, 6194–6203, DOI: 10.1039/D0GC01894E.

Efficient and direct conversion of CO2 to hydrocarbons through electrolysis is a promising approach for energy storage and CO2 utilization. In this study, high temperature co-electrolysis of H2O–CO2 and low temperature methanation processes are synergistically integrated in a micro-tubular reactor. The temperature gradient along the micro-tubular reactor provides favorable conditions for both the electrolysis and methanation reactions. Moreover, the micro-tubular reactor can provide high volumetric factor for both the electrolysis and methanation processes. When the cathode of the micro-tubular reactor is fed with a stream of 10.7% CO2, 69.3% H2 and 20.0% H2O, an electrolysis current of −0.32 A improves CH4 yield from 12.3% to 21.1% and CO2 conversion rate from 64.9% to 87.7%, compared with the operation at open circuit voltage. Furthermore, the effects of the inlet gas composition in the cathode on the CO2 conversion rate and the CH4 yield are systematically investigated. Higher ratio of H:C in the inlet results in higher CO2 conversion rate. Among all the cases studied, the highest CH4 yield of 23.1% has been achieved when the inlet gas in the cathode is consisted of 21.3% CO2, 58.7% H2 and 20.0% H2O with an electrolysis current of −0.32 A.

Biogas upgrading is an expanding field dealing with the increase in methane content of the biogas to produce biomethane. Biomethane has a high calorific content and can be used as a vehicle fuel or directly injected into the gas grid. Bioelectrochemical systems (BES) could become an alternative for biogas upgrading, by which the yield of the process in terms of carbon utilisation could be increased. The simulated effluent from a water scrubbing-like unit was used to feed a BES. The BES was operated with the biocathode poised at −800 mV vs. SHE to drive the reduction of the CO2 fraction of the biogas into methane. The BES was operated in batch mode to characterise methane production and under continuous flow to demonstrate its long-term viability. The maximum methane production rate obtained during batch tests was 5.12 ± 0.16 mmol m−2 per day with a coulombic efficiency (CE) of 75.3 ± 5.2%. The production rate increased to 15.35 mmol m−2 per day (CE of 68.9 ± 0.8%) during the continuous operation. Microbial community analyses and cyclic voltammograms showed that the main mechanism for methane production in the biocathode was hydrogenotrophic methanogenesis by Methanobacterium sp., and that electromethanogenesis occurred to a minor extent. The presence of other microorganisms in the biocathode, such as Methylocystis sp. revealed the presence of side reactions, such as oxygen diffusion from the anode compartment, which decreased the efficiency of the BES. The results of the present work offer the first experimental report on the application of BES in the field of biogas upgrading processes This research was nancially supported by the Spanish Government (CTQ 2014-53718-R). P. B-V. and A. V-P. were supported by a project grant from the Catalan Government (2014 FIB1 00119 and 2014 FI-B 00093). LEQUIA has been recognised as consolidated research group by the Catalan Government with code 2014-SGR-1168. Authors acknowledge the collaboration of Lluis Baneras from the Group of Molecular Microbial Ecology (University of Girona), who helped with the microbial analyses

Directly converting CO2 to hydrocarbons offers a potential route for carbon-neutral energy technologies. Here we report a novel design, integrating the high-temperature CO2–H2O co-electrolysis and low-temperature Fischer–Tropsch synthesis in a single tubular unit, for the direct synthesis of methane from CO2 with a substantial yield of 11.84%.

High temperature electrolysis of water and steam may provide an efficient, cost effective and environmentally friendly production of H2 using electricity produced from sustainable, non-fossil energy sources. To achieve cost competitive electrolysis cells that are both high performing i.e. minimum internal resistance of the cell, and long-term stable, it is critical to develop electrode materials that are optimal for steam electrolysis. In this article electrolysis cells for electrolysis of water or steam at temperatures above 200 °C for production of H2 are reviewed. High temperature electrolysis is favourable from a thermodynamic point of view, because a part of the required energy can be supplied as thermal heat, and the activation barrier is lowered increasing the H2 production rate. Only two types of cells operating at high temperature (above 200 °C) have been described in the literature, namely alkaline electrolysis cells (AEC) and solid oxide electrolysis cells (SOEC). In the present review emphasis is on state-of-the art electrode materials and development of new materials for SOECs. Based on the state-of-the-art performance for SOECs H2 production by high temperature steam electrolysis using SOECs is competitive to H2 production from fossil fuels at electricity prices below 0.02–0.03 € per kWh. Though promising SOEC results on H2 production have been reported a substantial R&D is still required to obtain inexpensive, high performing and long-term stable electrolysis cells.

In non-catalytic direct conversion of methane to acetylene by using direct current pulse discharge under conditions of ambient temperature and atmospheric pressure, the selectivity of acetylene from methane was >95% at methane conversion ranging from 16 to 52%; coexisting oxygen was very effective in removing deposited carbon and stabilized the state of discharge.