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Hydrogen is produced conventionally by electrolyzing water or water vapor at onset voltages greater than 1 V, providing motivation for the development of more efficient electrolysis processes for a hydrogen economy. Numerous attempts have been made to use ethanol as a fuel for hydrogen production because this process reduces the electrolysis onset voltage significantly. However, ethanol feedstock from lignocellulose requires greater amounts of energy and results in higher production costs compared to those using starch as the feedstock. The current study describes direct lignin electrolysis at an onset voltage of ca. 0.25 V, with high current efficiencies of approximately 100 % for hydrogen production at the cathode and approximately 85 % for carbon dioxide production at the anode. Addition of H3PO4-impregnated lignin to the anode of a PtFe/C|Sn0.9In0.1P2O7 (150 μm)|Pt/C cell enabled hydrogen production at a temperature of 150 °C.

Methane (CH4)-producing bioelectrochemical systems (BES) are an attractive way to store excess renewable electricity and captured CO2. Studies have suggested that methanogenesis via direct electron uptake from a biocathode is more energetically efficient than hydrogenotrophic methanogenesis. However, mechanisms and key microorganisms involved in direct electron uptake remain unclear, primarily because of H2 produced by bacteria or extracellular hydrogenases in the system. In an attempt to minimize biological H2 production and enrich for methanogens that could efficiently convert electrons from the cathode surface to CH4, cathode chambers were pretreated with antibiotics targeting bacteria. We found that antibiotics pretreatment effectively reduced the proportion of H2-producing bacteria and H2-utilizing methanogens associated with the biocathode biofilm, and significantly promoted growth of acetoclastic methanogens from the genera Methanosarcina and Methanosaeta, several of which are known to participate ...

Toward applications of bio-electrochemical systems in industrial processes and extreme environments, electromethanogenesis under high-pressure conditions was examined. Stainless-steel single-chamber reactors specifically designed to examine bio-electrochemical reactions under pressurized conditions were inoculated with thermophilic microorganisms originated from an oilfield formation water. The reactors were incubated at 5 MPa, 55°C in fed-batch operational mode with an applied voltage of 0.7 V. In the first few fed-batch cycles, hydrogen was mainly produced. After the third cycle, however, the reactors produced only methane simultaneously with current generation. The methane-production rate of the reactors showed an applied-voltage dependence and increased from 34.9 to 168.4 mmol m−2 day−1 with an increase in the applied voltage from 0.4 to 0.9 V. The efficiency of capturing electrons in the produced methane on average exceeded 70% with the applied voltage of 0.4 V or higher. Cyclic voltammetry further confirmed abilities of the bioelectrodes to catalyze electrochemical reactions at 5 MPa. Performance of the electromethanogenesis system was not altered under lower pressure conditions (1.2 and 2.5 MPa). An exoelectrogenic bacterium affiliated with the genus Thermincola and a methanogen belonging to the genus Methanothermobacter were detected as the dominant species in the bioanode and biocathode microbiotas, respectively. Thus, our results indicated that electromethanogenesis systems could be developed and operated under highly-pressurized conditions, suggesting that applications of the bio-electrochemical system in high-pressure environments (including high-temperature subsurface reservoirs) can be technically feasible.

The dispersion of granules in upflow anaerobic sludge blanket (UASB) reactor represents a critical technical issue in methanolic wastewater treatment. In this study, the potentials of coupling a microbial electrolysis cell (MEC) into an UASB reactor for improving methanolic wastewater biodegradation, long-term process stability and biomethane recovery were evaluated. The results indicated that coupling a MEC system was capable of improving the overall performance of UASB reactor for methanolic wastewater treatment. The combined system maintained the comparatively higher methane yield and COD removal efficiency over the single UASB process through the entire process, with the methane production at the steady-state conditions approaching 1504.7 ± 92.2 mL-CH4 L−1-reactor d−1, around 10.1% higher than the control UASB (i.e. 1366.4 ± 71.0 mL-CH4 L−1-reactor d−1). The further characterizations verified that the input of external power source could stimulate the metabolic activity of microbes and reinforced the EPS secretion. The produced EPS interacted with Fe2+/3+ liberated during anodic corrosion of iron electrode to create a gel-like three-dimensional [-Fe-EPS-]n matrix, which promoted cell-cell cohesion and maintained the structural integrity of granules. Further observations via SEM and FISH analysis demonstrated that the use of bioelectrochemical stimulation promoted the growth and proliferation of microorganisms, which diversified the degradation routes of methanol, convert the wasted CO2 into methane and accordingly increased the process stability and methane productivity.

Since the Industrial Revolution, the heavy reliance on the limited fossil energy has emitted huge number of CO 2 into the air and led to a severely global warming. We propose a new green technology to moderate greenhouse gas CO 2 emission and transform it into low-carbon renewable energy in this paper. Through this manner, electricity is transformed into chemical energy. CO 2 and H 2 O is directly simultaneous electrolysis under constant 0.25 A with 30 cm 2 nickel anode and 30 cm 2 iron cathode in a 600 °C alkali carbonate with LiOH electrolyte in mole ratio of n H :n C = 0.15:1, achieving the storage of electricity to chemical energy. This electrolysis provides a gaseous product comprising 45.9% methane, 53.1% H 2 , 0.92% CO and trace amounts of longer (than methane) hydrocarbons. Simultaneously, the current efficiency is still ∼51% for 60 min.

Electromethanogenesis (EM) is a reaction in a bio-electrochemical system (BES) in which microorganisms catalyse the reduction of CO 2 to methane by electric current at the cathode. We have proposed a novel application of EM combined with CO 2 Capture and Storage (CCS), in which geologically-sequestrated CO 2 is reduced to methane using electricity from variable renewables. To develop this technology, further investigation and establishment of the mathematical model are indispensable. In this study, experimental and mathematical analyses were conducted to develop mathematical model. A mathematical model for EM is developed based on BES models constructed in previous studies. As a result of mathematical analysis, it is suggested that EM reaction is dominated by the direct pathway at high cathode potential, and the methane production rate is proportional to the electrical current generated in the system. To calculate methane production rate via the indirect pathway requires measurement of microorganism concentration, and it will enable completion of mathematical model. Moreover, to examine the practical application of EM, a scale-up experiment was conducted in fed-batch mode using a 3 L volume reactor. As electrode material, carbonized coconut shells were chosen due to their large surface area, good biocompatibility, and good chemical stability with relatively low cost. Thermophilic microorganisms originated from a subsurface formation (a depleted petroleum reservoir) were inoculated in the reactor and incubated at 55 °C with applied voltages of 1.0 V or 0.7 V. The maximum CH 4 production rate was 340 mmol m -3 day -1 with current-capture efficiency of >30%. A rises of current generation were observed every time after the medium was exchanged, which indicates the microbial activity is responsible for current generation and CH 4 production. The maximum CH 4 production rates in 1st, 2nd and 3rd cycle were 108, 179 and 340 mmol m -3 day -1 respectively, which shows a growth of performance in microbial catalytic ability. These results suggest that it is feasible to scale up the EM system and, however, further studies are required to commercialize the system as a CCS technology.

It is an effective way to substitute air to methane in the anode of solid oxide electrolysis cells to reduce the electrical consumption for simultaneously producing H2 and high-quality syngas. In the methane assisted mode, the thermodynamic properties and Nernst potential exhibit one order of magnitude reduction of applied voltage to produce comparable electrolysis current. Ni catalysts are infiltrated to the SFM-SDC anode to improve the catalytic properties for methane assisted steam electrolysis. After Ni infiltration, surface oxygen exchange coefficient is effectively accelerated from 3.03×10−5 to 2.20×10−4cms−1, and the current density is significantly enhanced from −487 to −1022mAcm−2 at 850°C and 0.5V. Keywords: Steam electrolysis, Ni infiltration, Methane, Molybdenum doped strontium ferrite

Anaerobic digestion (AD) constitutes the microbial conversion of organic matter into biogas under anaerobic conditions. This technology has gained significant attention over the past few decades owing to the ever-increasing concerns over the fossil fuels and the growing need for renewable energy production. Different varieties of wastes and residues can be used for biogas production via AD. Biogas typically comprises methane and carbon dioxide. One of the disadvantages of the conventional AD process is the need for biogas upgradation to improve the purity of methane. This in turn leads to requirement of high level of investments when carried out at commercial scales. Apart from AD, another process that has provided a new horizon to the methane generation process is electromethanogenesis. In this process, CO2 is bioelectrochemically converted to methane with the help of electrotrophic methanogens. It has an added advantage of CO2 fixation along with wastewater treatment and energy generation. In spite of numerous advantages, several technical and economic barriers hinder the practical implementation of this technology at commercial scales. Combining AD and electromethanogenesis can be a possible means to overcome the drawbacks of each individual technology. This chapter briefly summarizes the working principles, bioenergetics, the interplay of microorganisms, the critical factors affecting the process, etc. along with a perspective of future research needs of the bioelectrochemically assisted anaerobic digestion technology.

Electroactive microorganisms (EAMs) can act as pseudocapacitor to store energy and discharge electrons on need, while electromethanogens acting as receptor are able to utilize electrons, protons and carbon dioxide for methanization. However, external energy is required to overcome thermodynamical barriers for electromethanogenesis. Herein, electro-driving power by solar light was established to accelerate conversion of waste organics to bioenergy. The intermittent power supply modes were elucidated for favourable performances (e.g., current density, methane production rate, energy recovery efficiencies and economic evaluation), compared with the control driven by continuous applied voltage. It was found that natural intermittent solar-powered mode was more beneficial for microorganisms involved in electron transfer and energy recovery than manual sharp on-off mode. Electrochemistry analysis unrevealed that a higher redox current and lower resistance were exhibited under the solar-powered mode. A high charge storage capacity and electron mobility were found through cytochrome c content and live cells ratio in the solar-power assisted bioreactor. The intermittent power driving modes can regulate electron transfer proteins with capacitive storage behavior in biomass, which helps to understand the responses of functional communities on the stress of intermittent electric field. These findings indicate a promising perspective of microbial biotechnology driven by solar power to boost bioenergy recovery from waste/wastewater.

There are rich renewable solar and wind sources in China, but large amounts of carbon dioxide (CO 2 ) emissions are urgent at the same time. Electromethanogenesis (EMG) can convert electricity and CO 2 into methane by using the biocatalyst of methanogen, which can provide an effective solution for electricity storage and reduction of CO 2 . The advantages are as follows. (i)It can achieve the long-term and large-capacity electricity consumption to avoid large power curtailment in the distributed generation system. (ii) CO 2 is applied as the reactant, which can mitigate the greenhouse effect caused by CO 2 . (iii) The self-propagated biocatalysts are superior to the chemical catalysts in the cost and stable performance. (iv) EMG can be considered as the junction among electricity energy, chemical energy and heat energy, which can promote research on the Integrated Energy Internet. In this paper, a well-designed electrolytic cell structure and methane-generated principle in the EMG system are introduced firstly. The author try to analyze the influence and the growth of cathode methanogens in this new area based on Nyquist diagram of the electrochemical impedance. Finally, the EMG electrical model is constructed firstly by recording working current and methane production with the terminal voltage change.

Editorial article on the research topic Microbial synthesis, Gas-Fermentation and Bioelectroconversion of CO2 and other gaseous streams

Microbial electrosynthesis (MES) has been gaining considerable interest as the next step in the evolution of microbial electrochemical technologies. Understanding the niche biocathode environment and microbial community is critical for further developing this technology as the biocathode is key to product formation and efficiency. MES is generally operated to enrich for a specific functional group (e.g., methanogens or homoacetogens) from a mixed-culture inoculum. However, due to differences in H2 and CO2 availability across the cathode surface, competition and syntrophy may lead to overall variability and significant beta-diversity within and between replicate reactors, which can affect performance reproducibility. Therefore, this study aimed to investigate the distribution and potential spatial variability of the microbial communities in MES methanogenic biocathodes. Triplicate methanogenic biocathodes were enriched in microbial electrolysis cells (MEC) for 5 months at an applied voltage of 0.7 V. They were then transferred to triplicate dual-chambered MES reactors and operated at –1.0 V vs. Ag/AgCl for six batches. At the end of the experiment, triplicate samples were taken at different positions (top, center, bottom) from each biocathode for a total of 9 samples for total biomass protein analysis and 16S rRNA gene amplicon sequencing. Microbial community analyses showed that the biocathodes were highly enriched with methanogens, especially the hydrogenotrophic methanogen family Methanobacteriaceae, Methanobacterium sp., and the mixotrophic Methanosarcina sp., with an overall core community representing > 97% of sequence reads in all samples. There was no statistically significant spatial variability (p > 0.05) observed in the distribution of these communities within and between the reactors. These results suggest deterministic community assembly and indicate the reproducibility of electromethanogenic biocathode communities, with implications for larger-scale reactors.

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.

Direct electron uptake by prokaryotes is a recently described mechanism with a potential application for energy and CO2storage into value added chemicals. Members of Methanosarcinales, an environmentally and biotechnologically relevant group of methanogens, were previously shown to retrieve electrons from an extracellular electrogenic partner performing Direct Interspecies Electron Transfer (DIET) and were therefore proposed to be electroactive. However, their intrinsic electroactivity has never been examined. In this study, we tested two methanogens belonging to the genusMethanosarcina, M. barkeriandM. horonobensis,regarding their ability to accept electrons directly from insoluble electron donors like other cells, conductive particles and electrodes. Both methanogens were able to retrieve electrons fromGeobacter metallireducensvia DIET. Furthermore, DIET was also stimulated upon addition of electrically conductive granular activated carbon (GAC) when each was co-cultured withG. metallireducens. However, when provided with a cathode poised at −400 mV (vs. SHE), onlyM. barkericould perform electromethanogenesis. In contrast, the strict hydrogenotrophic methanogen,Methanobacterium formicicum, did not produce methane regardless of the type of insoluble electron donor provided (Geobactercells, GAC or electrodes). A comparison of functional gene categories between the twoMethanosarcinashowed differences regarding energy metabolism, which could explain dissimilarities concerning electromethanogenesis at fixed potentials. We suggest that these dissimilarities are minimized in the presence of an electrogenic DIET partner (e.g.Geobacter), which can modulate its surface redox potentials by adjusting the expression of electroactive surface proteins.

This study compares microbial and bioelectrochemical methods of biogas upgrade via CO2 conversion to CH4. First, an in situ approach for CO2 conversion is investigated in an anaerobic reactor with a continuous supply of exogenous H2. This approach increased CH4 content of the biogas to 80–85%, yet also increased the effluent acetate concentration to 5,000–6,000 mg L−1 due to homoacetogenic activity. In a second approach, an anaerobic biofilter was continuously fed with biogas and an exogenous supply of H2, resulting in a CH4 content of 85–90% and a H2 consumption of 4.2–4.5 mol per mol of CO2 removed. Finally, biogas upgrade via CO2 to CH4 conversion was demonstrated in the cathode compartment of a membraneless microbial electrosynthesis cell. This test resulted in a CH4 purity of 85 – 90% and an energy consumption (per L of treated biogas) of 3.1 ± 1.6 Wh L−1.

Biogas produced from anaerobic digestion usually contains 30%-50% CO2, much of which must be removed, before utilization. Bioelectrochemical biogas upgrading approaches show promise, however, they have not yet been optimized for practical applications. In this study, a bioelectrochemical system with low energy input (applied cathode potential of -0.5 V vs. standard hydrogen electrode, SHE) was used for in-situ biogas upgrading. High efficiency CO2 conversion (318.5 mol/d/m2) was achieved when the system was operated with an organic load of 1.7 kgCOD/(m3 d). Methane content in the upgraded biogas was 97.0% and CO2 concentrations stayed below 3%, which is comparable to biogas upgraded with more expensive and less sustainable physiochemical approaches. The high efficiency of this approach could likely be attributed to a significant enrichment of Methanothrix (92.7%) species on the cathode surface that were expressing genes involved in both acetogenic methanogenesis and direct electron transfer (DET). Electromethanogenesis by these organisms also increased proton consumption and created a higher pH that increased the solubility of CO2 in the bioreactor. In addition, CO2 removal from the biogas was likely further enhanced by an enrichment of Actinobacillus species known to be capable of CO2 fixation. Artificial neural network (ANN) models were also used to estimate CH4 production under different loading conditions. The ANN architecture with 10 neurons at hidden layers fit best with a mean square error of 6.06 × 10−3 and R2 of 0.99.

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.

A microbial electrolysis cell, though considered as a promising, environmentally friendly technology for hydrogen production, suffers from concomitant production of methane. The high hydrogen/methane ratio at the initial operation stage decreases with time. Here we report for the first time the photoassisted microbial electrolysis cell (MEC) for persistent hydrogen production using polyaniline nanofibers as a cathode. Under 0.8 V external bias and laboratory fluorescent light illumination in a single-chamber MEC, continuous hydrogen production from acetate at a rate of 1.78 mH2 3 m−3 d−1 with 79.2 % overall hydrogen recovery was achieved with negligible methane formation for six months. Energy efficiencies based on input electricity as well as input electricity plus substrate were 182 and 66.2 %, respectively. This was attributed to the p-type-semiconductor characteristics of polyaniline nanofibers in which photoexcited electrons are used to reduce protons at the surface and holes are reduced with electrons originating from acetate oxidation at the anode. This method can be extended to microbial wastewater treatment for hydrogen production.

Microbial uptake of free cathodic electrons presents a poorly understood aspect of microbial physiology. Uptake of cathodic electrons is particularly important in microbial electrosynthesis of sustainable fuel and chemical precursors using only CO2 and electricity as carbon, electron and energy source. Typically, large overpotentials (200 to 400 mV) were reported to be required for cathodic electron uptake during electrosynthesis of, for example, methane and acetate, or low electrosynthesis rates were observed. To address these limitations and to explore conceptual alternatives, we studied defined co-cultures metabolizing cathodic electrons. The Fe(0)-corroding strain IS4 was used to catalyze the electron uptake reaction from the cathode forming molecular hydrogen as intermediate, and Methanococcus maripaludis and Acetobacterium woodii were used as model microorganisms for hydrogenotrophic synthesis of methane and acetate, respectively. The IS4-M. maripaludis co-cultures achieved electromethanogenesis rates of 0.1–0.14 μmol cm−2 h−1 at −400 mV vs standard hydrogen electrode and 0.6–0.9 μmol cm−2 h−1 at −500 mV. Co-cultures of strain IS4 and A. woodii formed acetate at rates of 0.21–0.23 μmol cm−2 h−1 at −400 mV and 0.57–0.74 μmol cm−2 h−1 at −500 mV. These data show that defined co-cultures coupling cathodic electron uptake with synthesis reactions via interspecies hydrogen transfer may lay the foundation for an engineering strategy for microbial electrosynthesis.

Bioelectrochemical power-to-gas (BEP2G) is considered a potentially convenient way of storing renewable surplus electricity in the form of methane. In methane-producing bioelectrochemical systems (BESs), carbon dioxide and electrical energy are converted into methane, using electrodes that supply either electrons or hydrogen to methanogenic archaea. This review summarizes the performance of methane-producing BESs in relation to cathode potential, electrode materials, operational strategies, and inoculum. Analysis and estimation of energy input and production rates show that BEP2G may become an attractive alternative for thermochemical methanation, and biochemical methanogenesis. To determine if BEP2G can become a future power-to-gas technology, challenges relating to cathodic energy losses, choice of a suitable electron donor, efficient reactor design/operation, and experience with large reactors need to be overcome.

The use of liquid fraction of pressed municipal solid waste (LPW) for hydrogen production was evaluated via electrohydrogenesis in a single-chamber microbial electrolysis cell (MEC). The highest hydrogen production (0.38 ± 0.09 m3 m−3 d−1 and 30.94 ± 7.03 mmol g−1 CODadded) was achieved at an applied voltage of 3.0 V and pH 5.5, increasing by 2.17-fold than those done at the same voltage without pH adjustment (pH 7.0). Electrohydrogenesis was accomplished by anodic oxidation of fermentative end-products (i.e. acetate, as well as propionate and butyrate after their acetification), with overall hydrogen recovery of 49.5 ± 11.3% of CODadded. These results affirm for the first time that electrohydrogenesis can be a noteworthy alternative for hydrogen recovery from LPW and simultaneous organics removal. Electrohydrogenesis efficiency of this system has potential to improve provided that electron recycling, electromethanogenesis and deposition of non-conductive aggregates on cathode surface, etc. are effectively controlled.

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.

The importance of methane conversion, syngas selectivity, and oxidizer conversion for efficient syngas production by the partial oxidation of methane using a metal oxide redox cycle is quantified. The operating conditions which enable high conversion of methane to syngas over cerium oxide and conversion of carbon dioxide to carbon monoxide in the subsequent oxidation reaction are identified experimentally. The parametric study considers operating temperatures of 900 and 1000 °C and methane flow rates from 1 to 15 mL min−1 g−1 in a fixed bed of porous ceria particles. The reduced ceria is reoxidized in a flow of 10 mL min−1 g−1 CO2 to produce CO. A trade-off of achieving high methane conversion is observed. For example, at 1000 °C, the cycle-averaged methane conversion increases from 13% for reduction in 15 mL min−1 g−1 to 60% in 1 mL min−1 g−1. For the same change in methane flow rate, the cycle-averaged selectivities decrease from 78% to 39% (CO) and 77% to 40% (H2) and the oxidizer conversion decreases from 93% to 48%. The maximum projected solar-to-fuel thermal efficiency is 27% for cycling at 1000 °C with reduction in 5 mL min−1 g−1 methane.

Microbial electrolysis cells (MECs) have great potential as a technology for wastewater treatment in parallel to energy production. In this study we explore the feasibility of using a low-cost, membraneless MEC for domestic wastewater treatment and methane production in both batch and continuous modes. Low-strength wastewater can be successfully treated by means of an MEC, obtaining significant amounts of methane. The results also suggest that hydrogenotrophic methanogenesis reduce the incidence of homoacetogenic activity, thus improving the overall MEC performance. However, gas production rates are low and important aspects such as methane solubility in water still remain a challenge. Overall, MECs can offer competitive advantages not only for low-strength wastewater treatment but also as an aid to anaerobic methane production by improving the chemical oxygen demand (COD) removal and methane production rates.

Hydrogen is considered a clean energy carrier for the future. At present, the production of hydrogen via a solid oxide electrolysis cell is of interest because water is the only reactant used; however, hydrogen production through electrolysis technology is still costly due to high electrical energy consumption. To reduce this energy demand, an addition of methane to the anode side of the solid oxide electrolysis cell, where it behaves like the anode side of the solid oxide fuel cell and generates heat and electricity to accomplish the electrolysis process, is one interesting method. In this study, modeling of the solid oxide fuel-assisted electrolysis cell is performed based on an electrochemical model to analyze the performance of the electrolyzer with/without the addition of methane in terms of the power input and the energy efficiency. In addition, the effect on the electrolyzer cell by key operating parameters, such as current density, steam fraction, steam-to-carbon ratio, temperature, pressure, steam utilization and fuel utilization, is presented. The simulation analysis shows that the performance of the solid oxide fuel-assisted electrolysis cell is higher than that of conventional solid oxide electrolysis cell, as it requires a lower power input. Furthermore, it is possible to run the solid oxide fuel-assisted electrolysis cell without an external electrical energy input.

A novel electrochemical oxidation-methanogenesis (EO-M) system was proposed for the first time to simultaneously achieve antibiotic degradation and a bioelectrochemical conversion of CO2 to CH4 with low energy costs. A dual-chamber system was installed with an antimony-doped tin oxide anode (Ti/SnO2-Sb) for the electrocatalytic generation of hydroxyl radicals to degrade ciprofloxacin (CIP), and a CO2-reducing methanogenic biocathode was enriched based on a three-dimensional (3D) graphitized granular activated carbon (GGAC) for microbial electromethanogenesis. The anode achieved removal efficiencies as high as 99.99% and 90.53% for CIP (14 mL, 50 mg L−1) and the chemical oxygen demand (COD, 89 mg L−1), respectively. The biocathode was rapidly enriched within 15 days and exhibited a methane production rate that stabilized at 15.12 ± 1.82 m3 m−3 d−1; additionally, the cathodic coulombic efficiency reached 71.76 ± 17.24%. The energy consumption of CIP degradation was reduced by 3.03 Wh L−1 compared to that of a single electrochemical oxidation system due to the lower cathodic overpotential of CO2 bioelectrochemical reduction in the EO-M system. A detailed analysis of the biofilm evolution in the 3D biocathode during the start-up process demonstrated that the enhanced absorption of extracellular polymeric substances by the GGAC cathode accelerated the enrichment of methanogens and induced the formation of methanogens with a large number of flagella. An analysis of the microbial community showed that a high relative abundance of Methanobacterium movens could promote a flagella-mediated direct electron transfer of the biocathode, eventually reducing the cathodic overpotential and energy costs of the EO-M system.

Anaerobic digestion is a traditional method of producing methane-containing biogas by utilizing the methanogenic conversion of organic matter like agricultural waste and animal excreta. Recently, the application of microbial electrolysis cell (MECs) technology to a traditional anaerobic digestion system has been extensively studied to find new opportunities in increasing wastewater treatability and methane yield and producing valuable chemicals. The finding that both anodic and cathodic bacteria can synthesize methane has led to the efforts of optimizing multiple aspects like microbial species, formation of biofilms, substrate sources and electrode surface for higher production of the combustible compound. MECs are very fascinating because of its ability to uptake a wide variety of raw materials including untreated wastewater (and its microbial content as biocatalysts). Extensive work in this field has established different systems of MECs for hydrogen production and biodegradation of organic compounds. This review is dedicated to explaining the operating principles and mechanism of the MECs for electromethanogenesis using different biochemical pathways. Emphasis on single- and double-chambered MECs along with reactor components is provided for a comprehensive description of the technology. Methane production using hydrogen evolution reaction and nanocatalysts has also been discussed.

Electromethanogenesis is a promising strategy for bioconversion of CO2 to CH4 via direct electron transfer (DET) and/or H2-mediated indirect electron transfer (IET). However, specific contribution ratios of DET or H2-mediated IET are unclear. Here, pathway preferences were firstly calculated in the “cage” type cathode colonized by Methanosarcina barkeri. The highest CH4 production rate of 4.4 μmol cm−2•day−1 was detected from 36 to 72 h at −1.2 V. The average ratios of H2-mediated IET were calculated as 59.73 ± 8.29%, 69.45 ± 20.75%, 69.55 ± 18.30% and 82.97 ± 16.13% at −0.6, −0.8, −1.0 and −1.2 V (vs SHE), respectively. While above −0.4 V, only the DET pathway was detected. The real time polymerase-chain reaction amplification at the transcriptional level of ccdA and frhB showed the similar trend for the pathway preference in CH4 production. Results provide specific ratios and preference of CH4 production via these two pathways, which may be used for parameter reference for industrial applications.

Lately, extracellular electron transfer (EET) is widely disclosed on the surface of the bioelectrodes, and conductive (bio)carriers involved in anaerobic biodegradation/biosynthesis. By electrostimulation, microbial consortia colonize the electrodes and accelerate substrate (waste/wastewater) metabolization on the bioanode or biosynthesize value-added products (methane, acetate, etc.) on the biocathode. However, the connections and contributions of planktonic microbial communities have not been effectually understood. Herein, electromethanogenesis were comprehensively investigated in response to different driving-force modes: intermittent electric field applied by manual on-off or natural solar power and continuous electrical field. Intermittent modes implied preferable electron transfer efficiency, higher methane yield and energy recovery efficiencies from wastewater by the microbes in the bulk solutions. Microbial community analysis revealed that less electroactive microorganisms and acetotrophic methanogens in the bulk solutions were accommodated under the intermittent modes than the continuous electrical field, whereas more fermentative bacteria and hydrogenotrophic methanogens evolved in the intermittent driving modes, implying that the interspecies electron transfer both on and away from the electrodes were favorably regulated. Redundancy and network analysis proved that more complicated ecological interactions were shown in the bulk solutions with the periodic on/off of electrical field. These results hinted that the electrostimulation effectively regulated EET bacteria, even in the bulk solutions, while more efficient electron flow to methane through interspecies electron transfer was developed during the intermittent driving regulation.

Bioelectrochemical systems performing electromethanogenesis (EMG-BES) represent an emerging technology for Power-to-gas as well as wastewater treatment. Moreover, EMG-BES can be used as a high-capacity energy storage system to absorb surplus energy in the electrical grid. This paper presents a modelling approach, which is based on building an equivalent electric circuit of the EMG-BES, which can be used to emulate static and dynamic non-linear behaviour of EMG-BES for different input voltages, which is advantageous if compared to other existing models. This model is a suitable choice for future studies in the development of the electric converters for EMG-BES plants connected to the electrical grid. The proposed model consists of practical and commercial elements, including capacitors, resistors, voltage sources, and a diode. The modelling of non-linear behaviour is achieved by adding a diode to the model. Four simple tests were performed to determine the equivalent circuit parameters in a medium-scale EMG-BES prototype. This prototype was built by stacking 45 cells together and connecting them in parallel, and it was long-term operated and tested under different electric inputs to determine the model parameters. A comparative study was finally conducted as reported in this paper in order to validate the proposed model against experimental results and values collected with other models. The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and from the Agency for Business Competitiveness of the Government of Catalonia. Also, this work has been supported by the Spanish Ministry of Economy and Competitiveness under the projects RTI2018-100921-BC21. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the host institutions or funders. The authors wish to acknowledge the Leitat collaborators who took part in Power2Biomethane project, which was financially supported by the Spanish Ministry of Economy and Competitiveness (RTC-2016- 5024-3, 2016).

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.

Microbial electrochemical system is a promising CO2-to-CH4 conversion technology that can upgrade raw biogas to high calorific content for downstream applications. The development of efficient, robust, and cost-effective biocathode is the pivotal issue for its industrial application. In this study, intact anaerobic granular sludge (AGS), which was the conventional biocatalyst in anaerobic digestion processes for high-efficiency CH4 production, was for the first time employed as biocathode in electromethanogenesis system (EM) for biogas upgrading. The applied voltage (0, 3, 4, and 5 V) and biogas flow rate (5.22–23.43 mL/h) were modulated to optimize biogas upgrading performance in the AGS-EM system. The CH4 content in treated biogas could reach as high as 97.9 ± 2.3% at an applied voltage of 4 V and a gas flow rate of 17.79 mL/h. The system showed superior stability and anti-interference ability in the continuous operation mode for 2 months. 16S rRNA sequencing results showed that Methanobacterium and Azoarcus were the dominant populations in biocathode. The AGS-EM system obtained an energy benefit of 477.3 kJ/molbiogas, and economic benefit of 446.4 EUR/m3biogas. The novel AGS-EM system showed the promising perspectives for the industrial application in the field of biogas upgrading and renewable energy storage.

The aim of this work was to enhance methane production from swine manure anaerobic digestion (AD) with in-situ formed graphene in microbial electrolysis cell (AD-MEC). The AD-MEC for in-situ graphene exfoliation incorporated a carbon felt anode and a titanium mesh cathode with an applied voltage of 2.5 V. Atomic force microscopic and Raman analyses confirmed that the graphene was in-situ formed. Voltage stimulation (1 h per day) and graphene production enhanced AD efficiency by promoting direct interspecies electron transfer between AD microorganisms. The increased biomass owing to enrichment of AD microorganisms on the carbon felt could also enhance AD efficiency. The voltage-graphene group (VGG) greatly enhanced the biogas yield (356.49 m3/t dry swine manure) and methane yield (222.17 m3/t dry swine manure), which were 41.49% and 60.89% higher than that of the control group, respectively. Microbial community analysis identified the dominant methanogens in the VGG system as Methanosarcina and Methanobrevibacter, while only Methanosarcina was dominant in the control system. Network analysis showed that the VGG system established a narrower niche than the control system, and microorganisms trended to gather more in the module.

Anaerobic digestion of sewage sludge produces large amounts of CO2 which contribute to global CO2 emissions. Capture and conversion of CO2 into valuable products is a novel way to reduce CO2 emissions and valorize it. Membrane contactors can be used for CO2 capture in liquid media, while bioelectrochemical systems (BES) can valorize dissolved CO2 converting it to CH4, through electromethanogenesis (EMG). At the same time, EMG process, which requires electricity to drive the conversion, can be utilized to store electrical energy (eventually coming from renewables surplus) as methane. The study aims integrating the two technologies at a laboratory scale, using for the first time real wastewater as CO2 capture medium. Five replicate EMG-BES cells were built and operated individually at 0.7 V. They were fed with both synthetic and real wastewater, saturated with CO2 by membrane contactors. In a subsequent experimental step, four EMG-BES cells were electrical stacked in series while one was kept as reference. CH4 production reached 4.6 L CH4 m−2 d−1, in line with available literature data, at a specific energy consumption of 16⁻18 kWh m−3 CH4 (65% energy efficiency). Organic matter was removed from wastewater at approximately 80% efficiency. CO2 conversion efficiency was limited (0.3⁻3.7%), depending on the amount of CO2 injected in wastewater. Even though achieved performances are not yet competitive with other mature methanation technologies, key knowledge was gained on the integrated operation of membrane contactors and EMG-BES cells, setting the base for upscaling and future implementation of the technology.

Biomethane is an energy product obtained biologically by anaerobic digestion (AD) as well as through bioelectrochemical systems or electromethanogenesis. The coupling of AD with microbial electrolysis cell systems aims to increase the yield and purity of methane. In this process, there are two routes to produce methane: (1) via electrochemistry in which CO2 reacts with protons and electrons to be reduced to CH4 and (2) biologically by hydrogenotrophic microorganisms that improve the production rate and yield. This chapter discusses the types of microorganisms involved in the metabolic pathways, the cell architecture, as well as the types of electrodes and materials utilized in the configuration of the cells. The impacts of operating conditions such as pH, temperature, organic load, and substrates on the process are reviewed. Finally, the challenges and future directions are discussed.

Dissertação de mestrado em Biotecnologia, Storage of renewable electricity in the form of methane (CH4) is a promising technology. Electrochemically assisted CH4 production from carbon dioxide (CO2) in a bioelectrochemical system (BES) allows the production of a biological fuel that can be stored and converted into electricity when necessary. In addition, CO2 is simultaneously captured contributing to mitigate climate change and global warming. However, this technology is still at an early stage with various technical and scientific problems and limitations that require further studies. Several aspects related to the biocathode performance, such as the low electron transfer rate between electrode and electrotrophs have been identified as important drawback to be overcame. The work developed in this thesis aimed to study the effect of carbon nanotubes (CNTs), as conductive material, on CH4 production in BESs. The production of CH4 was analyzed in two BESs, one working with a modified electrode (BESCNTs), in which CNTs were deposited, and another one that works as a control with a nonmodified electrode (BES-CTRL). The potential of CNTs to improve CH4 production was investigated under different electrochemical control modes, potentiostatic (at -1.0 V and -1.2 V vs. Ag/AgCl) and galvanostatic (at 2.7 mA and 7.5 mA). Parallel to the production of CH4, CO2 consumption, as well as the current-to-CH4, voltage and energy efficiencies were also evaluated. The results demonstrated that for both electrochemical control modes, the production of CH4 was higher in the presence of CNTs compared to the control assay. The current-to-methane efficiency varied between 15 % and 90 %, in both assays, while the voltage efficiency remained below 0.12 % and energy efficiency below 5.5 %. The study of the microbial community developed at the biocathode under galvanostatic control demonstrated a clear enrichment of methanogens (78 % in BES-CNTs and 86 % in BES-CTRL) compared to the initial inoculum (10 %), However, no significant differences were observed between both BES, since 98 % of the sample was classified as “unknown”. In conclusion, this work contributed with new insights on the effect of carbon nanomaterials to improve biocathode performance on BESs for CH4 production from CO2., O armazenamento de eletricidade renovável na forma de metano (CH4) é uma tecnologia promissora. A produção eletroquímica de CH4 a partir de dióxido de carbono (CO2) num sistema bioeletroquímico (BES) permite a produção de um combustível biológico que pode ser armazenado e convertido em eletricidade quando necessário. Além disso, o CO2 é simultaneamente capturado contribuindo para mitigar as mudanças climáticas e o aquecimento global. No entanto, esta tecnologia ainda está em fase inicial, apresentando vários problemas e limitações técnicas e científicas que exigem mais estudo. Vários aspetos relacionados com o desempenho do biocátodo, como a baixa taxa de transferência de eletrões entre elétrodos e eletrotrófitos, foram identificados como uma desvantagem a ser superada. O trabalho desenvolvido nesta tese teve como objetivo estudar o efeito dos nanotubos de carbono (CNTs), como material condutor, na produção de CH4 em BESs. A produção de CH4 foi analisada em dois BESs, um que trabalha com um elétrodo modificado (BES-CNTs), no qual os CNTs foram depositados, e outro que funciona como controlo com um elétrodo não modificado (BES-CTRL). O potencial dos CNTs para melhorar a produção de CH4 foi investigado diferentes modos de controlo eletroquímico potenciostático (em -1.0 V e -1.2 V vs. Ag/AgCl) e galvanostático (em 2.7 mA e 7.5 mA). Paralelamente à produção de CH4, o consumo de CO2, bem como as eficiências atuais para CH4, tensão e energia também foram avaliadas. Os resultados demonstraram para ambos os controlos, a produção de CH4 foi maior na presença de CNTs. A eficiência da corrente-para-CH4 variou entre 15 % e 90 %, enquanto a eficiência de voltagem permaneceu abaixo de 0.12 % e energética abaixo de 5.5 %. O estudo da comunidade microbiana desenvolvido no biocátodo do ensaio B, demonstrou um enriquecimento de metanógenos (78 % em BES-CNTs e 86 % no BES-CTRL) em comparação com o inóculo inicial (10 %). No entanto, nenhuma diferença significativa foi observada entre ambos o BES, uma vez que 98 % da amostra foi classificada como "desconhecida". Em conclusão, este trabalho contribuiu com novos insights sobre o efeito dos nanomateriais de carbono para melhorar o desempenho do biocátodo em BESs na produção de CH4 a partir de CO2.

Electromethanogenesis is the bioreduction of carbon dioxide (CO2) to methane (CH4) utilizing an electrode as electron donor. Some studies have reported the active participation of Methanobacterium sp. in electron capturing, although no conclusive results are available. In this study, we aimed at determining short-time changes in the expression levels of [NiFe]-hydrogenases (Eha, Ehb and Mvh), heterodisulfide reductase (Hdr), coenzyme F420-reducing [NiFe]-hydrogenase (Frh), and hydrogenase maturation protein (HypD), according to the electron flow in independently connected carbon cloth cathodes poised at– 800 mV vs. standard hydrogen electrode (SHE). Amplicon massive sequencing of cathode biofilm confirmed the presence of an enriched Methanobacterium sp. population (>70% of sequence reads), which remained in an active state (78% of cDNA reads), tagging this archaeon as the main methane producer in the system. Quantitative RT-PCR determinations of ehaB, ehbL, mvhA, hdrA, frhA, and hypD genes resulted in only slight (up to 1.5 fold) changes for four out of six genes analyzed when cells were exposed to open (disconnected) or closed (connected) electric circuit events. The presented results suggested that suspected mechanisms for electron capturing were not regulated at the transcriptional level in Methanobacterium sp. for short time exposures of the cells to connected-disconnected circuits. Additional tests are needed in order to confirm proteins that participate in electron capturing in Methanobacterium sp This work was funded by Spanish Government (MINECO, CTQ2014-53718-R) and the Universitat de Girona (MPCUdG2016/139 and MCPUdG2016/121). Sebastià Puig is a Serra Hunter Fellow (UdG-AG-575). Ramiro Blasco-Gómez is grateful for the Research Personnel Training (FPI) grant (BES-2015-074229). Elisabet Perona-Vico received a Research Training grant from the University of Girona (IFUdG2018/52). LEQUiA and IEA have been recognized as consolidated research groups by the Generalitat de Catalunya (2017SGR-1552, and 2017SGR-548)

The development and implementation of energy storage solutions is essential for the sustainability of renewable energy penetration in the electrical system. In this regard, power-to-gas technologies are useful for seasonal, high-capacity energy storage. Bioelectrochemical systems for electromethanogenesis (EMG-BES) represent an additional power-to-gas technology to the existing chemical and biological methanation. EMG-BES process can be retrofitted in traditional anaerobic digesters, with advantages in terms of biologic process stability and high-quality biogas production. Nowadays, there are no reported studies of scaled-up EMG-BES plants for energy storage. The present work describes the setup and operation of a medium-scale EMG-BES prototype for power-to-gas, storing energy in the form of biomethane. The prototype was built by stacking 45 EMG-BES cells, accounting for a total volume of 32¿L. It was continuously fed with 10¿L¿day-1 municipal wastewater, and it was long-term operated at different voltage and temperature ranges. A steady-state current density demand of 0.5¿A¿m-2 was achieved at 32¿°C while producing 4.4¿L¿CH4¿m-2¿d-1 and removing 70% of the initial organic matter present in wastewater. Microbial competition between electro-active bacteria and acetoclastic methanogens was observed. Energy storage efficiency was estimated around 42–47%, analyzing surplus CH4 production obtained when applying voltage to the stack. A first order electric model was calculated, based on the results of a series of electrical characterization tests. The model may be used in the future to design the converter for EMG-BES plant connection to the electrical grid. The obtained results show that energy storage based on EMG-BES technology is possible, as well as its future potential, mixing renewable power overproduction, biomethane generation and wastewater treatment under the circular economy umbrella.

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.

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

As a reliable source for methane production, the major goal of our research was to design and testify a novel microbial electrolysis assisted upflow anaerobic (Upflow-MEC) reactor for beer wastewater treatment and simultaneous methane production. Three reactors with different cathode materials were constructed and the reactor with Ni cathode had a maximum COD removal of 85%, methane yield of 142.8 mL/gCOD, TOC removal of 83%, Carbohydrate removal of 97%, Protein removal of 62% and current production of 8.6 mA, under 0.8 V applied voltage at HRT of 24 h. It was an application-oriented, membrane-free, continuous reactor and shortening the reaction time and increasing organic content conversion to methane were the key developing targets.

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.

One pathway to remove the greenhouse gas carbon dioxide to mitigate climate change is by dissolution and electrolysis in molten carbonate to produce stable, solid carbon. This study determines critical knowledge to minimize the required electrolysis energy, the reaction stoichiometry in which carbon and O2 are the principal products, and that CO2 can be electrolyzed inexpensively. Thermochemical and experimental results indicate that the principal carbon-deposition reaction in molten Li2CO3 or Li2O/Li2CO3 electrolytes at 750 °C is Li2O + 2CO2 → Li2CO3 + C + O2. The reaction occurs at high Faradaic efficiency of the 4e– reduction of CO2 to carbon and oxygen at an electrolysis voltage as low as

The present study investigated the biological treatment of produced water in a microbial electrochemical cell (MXC). The main objectives were to develop a novel spiral microbial electrochemical cell (SMXC) and test its performance for produced water treatment under highly saline conditions (salinity > 200000 ppm). The bioelectrochemical performance of the system was also evaluated in terms of power and hydrogen production over time. The comparatively inexpensive material and ease of application increased the feasibility of the SMXC configuration for produced water treatment. Optimal SMXC performance as a microbial fuel cell was achieved at a maximum open circuit potential of 330 mV, maximum power density of 0.65 mW m −3 , and internal resistance of 110 kΩ. Hydrogen gas was generated in a SMXC as a microbial electrolysis cell at a rate of 400 ml-H 2 m −3 day −1 and showed a total hydrogen recovery of 0.18%. A consequential reduction in the organic content of produced water (about 90%) indicates efficient biological treatment without costly saline acclimation.

An active carbon recycling energy system (ACRES) based on carbon recycling has been proposed as a new energy transformation system. This energy transformation system reduces the carbon dioxide (CO 2 ) emissions in the atmosphere during the iron-making process. An experimental study for electrochemical CO production by CO 2 electrolysis based on the ACRES concept was carried out using a tubular solid oxide electrolysis cell. Experimental results show that the CO and oxygen (O 2 ) production rates at 800, 850, and 900 °C were almost proportional to the current passing through the cell. Both ionic conductivity and the chemical kinetics of CO 2 decomposition increased with increasing temperature. The highest current density and CO production rate at 900 °C were 2.97 mA/cm 2 and 0.78 μmol/(min cm 2 ), respectively. On the basis of the electrolytic characteristics of the cell, the scale of the combined ACRES CO 2 electrolysis/iron-making system was estimated.

Methane is the primary end product from cathodic current in microbial electrolysis cells (MECs) in the absence of methanogenic inhibitors, but little is known about the archaeal communities that develop in these systems. MECs containing cathodes made from different materials (carbon brushes, or plain graphite blocks or blocks coated with carbon black and platinum, stainless steel, nickel, ferrihydrite, magnetite, iron sulfide, or molybdenum disulfide) were inoculated with anaerobic digester sludge and acclimated at a set potential of −600 mV (versus a standard hydrogen electrode). The archaeal communities on all cathodes, except those coated with platinum, were predominated by Methanobacterium (median 97% of archaea). Cathodes with platinum contained mainly archaea most similar to Methanobrevibacter. Neither of these methanogens were abundant (