Understanding methane bioelectrosynthesis from carbon dioxide in a two-chamber microbial electrolysis cells (MECs) containing a carbon biocathode

Iron-based materials maintain biofilm equilibrium and function as external capacitors to minimize electron loss under intermittent power supply in MEC-AD methane production

Microbial electrolysis cell-anaerobic digestion (MEC-AD) is a cost-effective approach for methane (CH₄) recovery from food waste, but its CH₄ conversion efficiency requires improvement. To address this, a MIL-100(Fe)-modified carbon cloth anode was developed to enhance anodic biofilm formation and CH₄ bioconversion efficiency. At an applied voltage of 0.8 V, the highest daily CH₄ yield reached 141.6 mL/g COD/d, a 61 % increase, and increased further to 227.5 mL/g COD/d under intermittent power supply. By facilitating extracellular electron transfer (EET) in electrogenic bacteria, MIL-100(Fe) regulated biofilm thickness and maintained dynamic biofilm equilibrium. Additionally, as an external capacitor, MIL-100(Fe) functioned as a "temporary storage site" for electrons under intermittent power supply, reducing bioelectron loss. Metagenomic analysis revealed that MIL-100(Fe) significantly enriched Bacteroidia and Methanosarcina, promoting carbohydrate metabolism and CH₄ production. Under intermittent power supply, MIL-100(Fe) further enriched Geobacter, enhancing electron transfer efficiency. This study demonstrates that iron-based anode modification effectively enhances CH₄ production from food waste by optimizing biofilm structure and metabolic pathways, providing a promising strategy for improving MEC-AD performance.

Enhanced propionate degradation and CO2 electromethanogenesis in an up-flow dual-chamber electrocatalytic anaerobic bioreactor (UF-DC-EAB): Leveraging DIET-mediated syntrophy for microbial stability

Anaerobic digestion faces numerous challenges, including high CO2 content in biogas and volatile fatty acids (such as propionate) accumulation in digestate. To address these issues, an up-flow dual-chamber electrocatalytic anaerobic bioreactor (UF-DC-EAB) was developed to enhance propionate degradation through microbial symbiosis while improving biogas quality via CO2 electromethanogenesis. Under the extreme conditions with propionate as the primary carbon source at 6-h HRT, the UF-DC-EAB achieved a propionate removal efficiency of 72.1 ± 9.4 % and a faradaic efficiency of 25.5 ± 5.1 %. Microbial community analysis revealed an enrichment of acetoclastic methanogens (Methanosarcinales, 5.4 %) and syntrophic propionate-oxidizing bacteria (Syntrophobacterales, 13.9 %) in the anode, which facilitated propionate degradation. In the cathode, hydrogenotrophic methanogens (Methanobacterium, 13.6 %) and electroactive bacteria (Geobacter, 6.2 %) were predominant, further promoting CO2 electromethanogenesis and biogas upgrading. Co-occurrence network and structural equation modeling indicated that the electrocatalytic regulation roused the intrinsic capability of the microbial community to oxidize propionate and provoked the occurrence of direct interspecies electron transfer (DIET) among the enriched functional microorganisms, by regulating the synthesis of key molecules like F420 and cytochrome c in response to propionate-induced changes. The DIET-mediated syntropy increased the net energy output by 212.5 %. This study presents a novel electrochemical system combining CO2 electromethanogenesis with propionate-rich digestate degradation, offering an efficient approach for anaerobic post-treatment.

Electrochemical optimization of bioelectrochemically improved anaerobic digestion for agricultural digestates' valorisation to biomethane

Bioelectrochemically improved anaerobic digestion (AD-BES) represents an upgrading strategy for existing biogas plants, consisting of the integration of bioelectrodes within the AD reactor. For this study, a series of laboratory-scale AD-BES reactors were operated, valorising agricultural digestates through the production of biogas. The reactors were inoculated and started-up with three different digestates, leading to significant differences in the microbial community developed on the bioelectrodes. After the start-up was completed, the AD-BES were all fed with a unique digestate, to evaluate the stability of the bioelectrodes' biofilm performances against variations of the organic feedstock. In terms of methane (CH4) production rate, the presence of bioelectrodes allowed between 25 and 82% improvement, compared with control AD reactors. The application of an optimal voltage of 0.3 V resulted in an additional 40% improvement in CH4 production rate, but only when the biofilm was previously acclimated to the fed digestate. Comprehensive microbial characterization revealed that fed digestate significantly influences the composition and homogenization of microbial communities within AD-BES reactors, with applied voltage showing only a secondary effect. Even when reactors were transitioned to a uniform digestate feeding, resulting in closely similar microbial profiles, variations in CH4 production persisted, underscoring the lasting impact of initial microbial conditioning. A critical observation was the differentiation in archaeal colonization on bioelectrodes at 0.3 V, the voltage yielding the highest CH4 conversion. These insights suggest that while the microbial community structure depends on fed digestate, operational efficiency and methanogenic potential are intricately linked to both initial microbial establishment and the specific electrochemical conditions applied to AD-BES reactors.

Advances in bioelectrochemical systems for bio-products recovery

Bioelectrochemical systems (BES) have emerged as a sustainable and highly promising technology that has garnered significant attention from researchers worldwide. These systems provide an efficient platform for the removal and recovery of valuable products from wastewater, with minimal or no net energy loss. Among the various types of BES, microbial fuel cells (MFCs) are a notable example, utilizing microbial biocatalytic activities to generate electrical energy through the degradation of organic matter. Other BES variants include microbial desalination cells (MDCs), microbial electrolysis cells (MECs), microbial electrosynthesis cells (MXCs), microbial solar cells (MSCs), and more. BESs have demonstrated remarkable potential in the recovery of diverse products such as hydrogen, methane, volatile fatty acids, precious nutrients, and metals. Recent advancements in scaling up BESs have facilitated a more realistic assessment of their net energy recovery and resource yield in real-world applications. This comprehensive review focuses on the practical applications of BESs, from laboratory-scale developments to their potential for industrial commercialization. Specifically, it highlights successful examples of value-added product recovery achieved through various BES configurations. Additionally, this review critically evaluates the limitations of BESs and provides suggestions to enhance their performance at a larger scale, enabling effective implementation in real-world scenarios. By providing a thorough analysis of the current state of BES technology, this review aims to emphasize the tremendous potential of these systems for sustainable wastewater treatment and resource recovery. It underscores the significance of bridging the gap between laboratory-scale achievements and industrial implementation, paving the way for a more sustainable and resource-efficient future.

Relevance of extracellular electron uptake mechanisms for electromethanogenesis applications

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO2 into methane. Unlike biomethanation processes where CO2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology.

Optimizing bioelectromethanosynthesis of CO2 and membrane fouling mitigation in MECs via in-situ biogas recirculation

The CO2 bioelectromethanosynthesis via two-chamber microbial electrolysis cell (MEC) holds tremendous potential to solve the energy crisis and mitigate the greenhouse gas emissions. However, the membrane fouling is still a big challenge for CO2 bioelectromethanosynthesis owing to the poor proton diffusion across membrane and high inter-resistance. In this study, a new MEC bioreactor with biogas recirculation unit was designed in the cathode chamber to enhance secondary-dissolution of CO2 while mitigating the contaminant adhesion on membrane surface. Biogas recirculation improved CO2 re-dissolution, reduced concentration polarization, and facilitated the proton transmembrane diffusion. This resulted in a remarkable increase in the cathodic methane production rate from 0.4 mL/L·d to 8.5 mL/L·d. A robust syntrophic relationship between anodic organic-degrading bacteria (Firmicutes 5.29%, Bacteroidetes 25.90%, and Proteobacteria 6.08%) and cathodic methane-producing archaea (Methanobacterium 65.58%) enabled simultaneous organic degradation, high CO2 bioelectromethanosynthesis, and renewable energy storage.

The Effect of Bismuth and Tin on Methane and Acetate Production in a Microbial Electrosynthesis Cell Fed with Carbon Dioxide

This study investigates the impacts of bismuth and tin on the production of CH4 and volatile fatty acids in a microbial electrosynthesis cell with a continuous CO2 supply. First, the impact of several transition metal ions (Ni2+, Fe2+, Cu2+, Sn2+, Mn2+, MoO42-, and Bi3+) on hydrogenotrophic and acetoclastic methanogenic microbial activity was evaluated in a series of batch bottle tests incubated with anaerobic sludge and a pre-defined concentration of dissolved transition metals. While Cu is considered a promising catalyst for the electrocatalytic conversion of CO2 to short chain fatty acids such as acetate, its presence as a Cu2+ ion was demonstrated to significantly inhibit the microbial production of CH4 and acetate. At the same time, CH4 production increased in the presence of Bi3+ (0.1 g L-1) and remained unchanged at the same concentration of Sn2+. Since Sn is of interest due to its catalytic properties in the electrochemical CO2 conversion, Bi and Sn were added to the cathode compartment of a laboratory-scale microbial electrosynthesis cell (MESC) to achieve an initial concentration of 0.1 g L-1. While an initial increase in CH4 (and acetate for Sn2+) production was observed after the first injection of the metal ions, after the second injection, CH4 production declined. Acetate accumulation was indicative of the reduced activity of acetoclastic methanogens, likely due to the high partial pressure of H2. The modification of a carbon-felt electrode by the electrodeposition of Sn metal on its surface prior to cathode inoculation with anaerobic sludge showed a doubling of CH4 production in the MESC and a lower concentration of acetate, while the electrodeposition of Bi resulted in a decreased CH4 production.

Efficient butyrate production from rice straw in an optimized cathodic electro-fermentation process

Butyrate production from renewable biomass shows great potential against climate change and over-consumption of fossil fuels. Herein, key operational parameters of a cathodic electro-fermentation (CEF) process were optimized for efficient butyrate production from rice straw by mixed culture. The cathode potential, controlled pH and initial substrate dosage were optimized at -1.0 V (vs Ag/AgCl), 7.0 and 30 g/L, respectively. Under the optimal conditions, 12.50 g/L butyrate with yield of 0.51 g/g-rice straw were obtained in batch-operated CEF system. In fed-batch mode, butyrate production significantly increased to 19.66 g/L with the yield of 0.33 g/g-rice straw, but 45.99% butyrate selectivity still needs to be improved in future. Enriched butyrate producing bacteria (Clostridium cluster XIVa and IV) with proportion of 58.75% on the 21st day of the fed-batch fermentation, contributed to the high-level butyrate production. The study provides a promising approach for efficient butyrate production from lignocellulosic biomass.

New microbial electrosynthesis system for methane production from carbon dioxide coupled with oxidation of sulfide to sulfate

Microbial electrosynthesis system (MES) is a promising method that can use carbon dioxide, which is a greenhouse gas, to produce methane which acts as an energy source, without using organic substances. However, this bioelectrical reduction reaction can proceed at a certain high applied voltage when coupled with water oxidation in the anode coated with metallic catalyst. When coupled with the oxidation of HS^- to SO₄^2-, methane production is thermodynamically more feasible, thus implying its production at a considerably lower applied voltage. In this study, we demonstrated the possibility of electrotrophic methane production coupled with HS^- oxidation in a cost-effective bioanode chamber in the MES without organic substrates at a low applied voltage of 0.2 V. In addition, microbial community analyses of biomass enriched in the bioanode and biocathode were used to reveal the most probable pathway for methane production from HS^- oxidation. In the bioanode, electroautotrophic SO₄^2- production accompanied with electron donation to the electrode is performed mainly by the following two steps: first, incomplete sulfide oxidation to sulfur cycle intermediates (SCI) is performed; then the produced SCI are disproportionated to HS^- and SO₄^2-. In the biocathode, methane is produced mainly via H₂ and acetate by electron-accepting syntrophic bacteria, homoacetogens, and acetoclastic archaea. Here, a new eco-friendly MES with biological H₂S removal is established.

Carbon Fibers for Bioelectrochemical: Precursors, Bioelectrochemical System, and Biosensors

Abstract

Carbon fibers (CFs) demonstrate a range of excellent properties including (but not limited to) microscale diameter, high hardness, high strength, light weight, high chemical resistance, and high temperature resistance. Therefore, it is necessary to summarize the application market of CFs. CFs with good physical and chemical properties stand out among many materials. It is believed that highly fibrotic CFs will play a crucial role. This review first introduces the precursors of CFs, such as polyacrylonitrile, bitumen, and lignin. Then this review introduces CFs used in BESs, such as electrode materials and modification strategies of MFC, MEC, MDC, and other cells in a large space. Then, CFs in biosensors including enzyme sensor, DNA sensor, immune sensor and implantable sensor are summarized. Finally, we discuss briefly the challenges and research directions of CFs application in BESs, biosensors and more fields.

Highlights

CF is a new-generation reinforced fiber with high hardness and strength.Summary precursors from different sources of CFs and their preparation processes.Introduction of the application and modification methods of CFs in BESs and biosensor.Suggest the challenges in the application of CFs in the field of bio-electrochemistry.Propose the prospective research directions for CFs.

Graphical abstract

Microbial fingerprints of methanation in a hybrid electric-biological anaerobic digestion

Biomethane as a sustainable, alternative, and carbon-neutral renewable energy source to fossil fuels is highly needed to alleviate the global energy crisis and climate change. The conventional anaerobic digestion (AD) process for biomethane production from waste(water) streams has been widely employed while struggling with a low production rate, low biogas qualities, and frequent instability. The electric-biologically hybrid microbial electrochemical anaerobic digestion system (MEC-AD) prospects more stable and robust biomethane generation, which facilitates complex organic substrates degradation and mediates functional microbial populations by giving a small input power (commonly voltages < 1.0 V), mainly enhancing the communication between electroactive microorganisms and (electro)methanogens. Despite numerous bioreactor tests and studies that have been conducted, based on the MEC-AD systems, the integrated microbial fingerprints, and cooperation, accelerating substrate degradation, and biomethane production, have not been fully summarized. Herein, we present a comprehensive review of this novel developing biotechnology, beginning with the principles of MEC-AD. First, we examine the fundamentals, configurations, classifications, and influential factors of the whole system's performances (reactor types, applied voltages, temperatures, conductive materials, etc.,). Second, extracellular electron transfer either between diverse microbes or between microbes and electrodes for enhanced biomethane production are analyzed. Third, we further conclude (electro)methanogenesis, and microbial interactions, and construct ecological networks of microbial consortia in MEC-AD. Finally, future development and perspectives on MEC-AD for biomethane production are proposed.

Efficient recovery of dissolved Fe(II) from near neutral pH Fenton via microbial electrolysis

Fe(II) regeneration from ferric sludge via a biocathode and citrate system has recently been proposed to avoid iron-sludge accumulation and iron consumption in homogeneous Fenton treatments. However, poor regeneration rate of Fe(II) from ferric sludge at a near-neutral pH, without an iron-complexing agent, limited its wider practical application. Here, a biocathode augmented with Geobacter sulfurreducens hosted by a microbial electrolysis cell was developed to efficiently regenerate dissolved Fe(II) from ferric sludge at near-neutral pH levels, without using iron-complexing agents. In the Geobacter sulfurreducens-rich biocathode without complexing agents, the regeneration rate of dissolved Fe(II) increased three-fold compared with the biocathode before inoculating Geobacter sulfurreducens. The highest concentration of dissolved Fe(II) increased from 45 mg Fe/L to 199 mg Fe/L at pH 6 when 0.5 V of voltage was applied. Furthermore, 84 mg Fe/L of dissolved Fe(II) was successfully regenerated from ferric sludge during the 123 days' operation of flow-through biocathode. Finally, the regenerated Fe(II) solution without organic matters was successfully applied in a near-neutral pH Fenton treatment to remove recalcitrant pollutants. This Geobacter sulfurreducens-rich biocathode, with its low chemical consumption, high regeneration rate and feasibility for continuous flow operation, offers a more efficient method to realize iron-free in homogeneous Fenton treatments.

Reduced overpotential of methane-producing biocathodes: Effect of current and electrode storage capacity

Cathode overpotential is a key factor in the energy efficiency of bioelectrochemical systems. In this study the aim is to demonstrate the role of applied current density and electrode storage capacity on cathode overpotential. To do so, eight reactors using capacitive granular activated carbon as cathode material were operated. Four reactors were controlled at -5 A m^-2 and four at -10 A m^-2. Additionally, to evaluate the electrode storage capacity, weekly charge/discharge tests were conducted for half of the reactors at each applied current density. Results show that cathode potential as high as -0.50 V vs. Ag/AgCl can be reached. Furthermore, the resulting low cathode overpotential is both dependent on applied current density and employment (or not) of charge/discharge tests: reactors at -10 A m^-2 without charge/discharge regimes did not result in increasing cathode potential whereas reactors at -5 A m^-2 and at -10 A m^-2 with charge/discharge regimes did.

Response of Methanogen Communities to the Elevation of Cathode Potentials in Bioelectrochemical Reactors Amended with Magnetite

Electromethanogenesis refers to the process whereby methanogens utilize current for the reduction of CO₂ to CH₄. Setting low cathode potentials is essential for this process. In this study, we tested if magnetite, an iron oxide mineral widespread in the environment, can facilitate the adaptation of methanogen communities to the elevation of cathode potentials in electrochemical reactors. Two-chamber electrochemical reactors were constructed with inoculants obtained from paddy field soil. We elevated cathode potentials stepwise from the initial -0.6 V versus the standard hydrogen electrode (SHE) to -0.5 V and then to -0.4 V over the 130 days of acclimation. Only weak current consumption and CH₄ production were observed in the bioreactors without magnetite. However, significant current consumption and CH₄ production were recorded in the magnetite bioreactors. The robustness of electroactivity of the magnetite bioreactors was not affected by the elevation of cathode potentials from -0.6 V to -0.4 V. However, the current consumption and CH₄ production were halted in the bioreactors without magnetite when the cathode potentials were elevated to -0.4 V. Methanogens related to Methanospirillum were enriched on the cathode surfaces of magnetite bioreactors at -0.4 V, while Methanosarcina relatively dominated in the bioreactors without magnetite. Methanobacterium also increased in the magnetite bioreactors but stayed off electrodes at -0.4 V. Apparently, the magnetite greatly facilitates the development of biocathodes, and it appears that with the aid of magnetite, Methanospirillum spp. can adapt to the high cathode potentials, performing efficient electromethanogenesis. IMPORTANCE Converting CO₂ to CH₄ through bioelectrochemistry is a promising approach to the development of green energy biotechnology. This process, however, requires low cathode potentials, which entails a cost. In this study, we tested if magnetite, a conductive iron mineral, can facilitate the adaptation of methanogens to the elevation of cathode potentials. In two-chamber reactors constructed by using inoculants obtained from paddy field soil, biocathodes developed robustly in the presence of magnetite, whereas only weak activities in CH₄ production and current consumption were observed in the bioreactors without magnetite. The elevation of cathode potentials did not affect the robustness of electroactivity of the magnetite bioreactors over the 130 days of acclimation. Methanospirillum strains were identified as the key methanogens associated with the cathode surfaces during the operation at high potentials. The findings reported in this study shed new light on the adaptation of methanogen communities to the elevated cathode potentials in the presence of magnetite.

Emerging bioelectrochemical technologies for biogas production and upgrading in cascading circular bioenergy systems

Biomethane is suggested as an advanced biofuel for the hard-to-abate sectors such as heavy transport. However, future systems that optimize the resource and production of biomethane have yet to be definitively defined. This paper assesses the opportunity of integrating anaerobic digestion (AD) with three emerging bioelectrochemical technologies in a circular cascading bioeconomy, including for power-to-gas AD (P2G-AD), microbial electrolysis cell AD (MEC-AD), and AD microbial electrosynthesis (AD-MES). The mass and energy flow of the three bioelectrochemical systems are compared with the conventional AD amine scrubber system depending on the availability of renewable electricity. An energy balance assessment indicates that P2G-AD, MEC-AD, and AD-MES circular cascading bioelectrochemical systems gain positive energy outputs by using electricity that would have been curtailed or constrained (equivalent to a primary energy factor of zero). This analysis of technological innovation, aids in the design of future cascading circular biosystems to produce sustainable advanced biofuels.

Empower C1: Combination of Electrochemistry and Biology to Convert C1 Compounds

The idea to somehow combine electrical current and biological systems is not new. It was subject of research as well as of science fiction literature for decades. Nowadays, in times of limited resources and the need to capture greenhouse gases like CO₂, this combination gains increasing interest, since it might allow to use C1 compounds and highly oxidized compounds as substrate for microbial production by "activating" them with additional electrons. In this chapter, different possibilities to combine electrochemistry and biology to convert C1 compounds into useful products will be discussed. The chapter first shows electrochemical conversion of C1 compounds, allowing the use of the product as substrate for a subsequent biosynthesis in uncoupled systems, further leads to coupled systems of biology and electrochemical conversion, and finally reaches the discipline of bioelectrosynthesis, where electrical current and C1 compounds are directly converted by microorganisms or enzymes. This overview will give an idea about the potentials and challenges of combining electrochemistry and biology to convert C1 molecules.

Recent advances in methanogenesis through direct interspecies electron transfer via conductive materials: A molecular microbiological perspective

Conductive materials can serve as biocatalysts during direct interspecies electron transfer for methanogenesis in anaerobic reactors. However, the mechanism promoting direct interspecies electron transfer in anaerobic reactors, particularly under environments in which diverse substrates and microorganisms coexist, remains to be elucidated from a scientific or an engineering point of view. Currently, many molecular microbiological approaches are employed to understand the fundamentals of this phenomenon. Here, the direct interspecies electron transfer mechanisms and relevant microorganisms identified to date using molecular microbiological methods were critically reviewed. Moreover, molecular microbiological methods for direct interspecies electron transfer used in previous studies and important findings thus revealed were analyzed. This review will help us better understand the phenomena of direct interspecies electron transfer using conductive materials and offer a framework for future molecular microbiological studies.

Explore the difference between the single-chamber and dual-chamber microbial electrosynthesis for biogas production performance

Microbial electrosynthesis (MES) is an advanced technology for efficient treatment of organic wastewater and recovery of new energy, with the advantages and disadvantages of single-chamber and dual-chamber MES reactors being less understood. Therefore, we explored the effects of single-chamber and dual-chamber structures on the methane production performance and microbial community structure of MES. Results indicated that methane concentration and current density of single-chamber MES were higher than those of dual-chamber MES, and the system stability was better, while chemical oxygen demand (COD) removal rate and cumulative methane production were not significantly different. Analysis of microbial community structure showed the abundance of acidogens and H₂-producing bacteria was higher in single-chamber MES, while fermentation bacteria and methanogens was lower. The abundance of methanogens of dual-chamber MES (21.74-24.70%) was superior to the single-chamber MES (8.23-10.10%). Moreover, in dual-chamber MES, methane was produced primarily through acetoclastic methanogenic pathway, while in single-chamber MES cathode, methane production was mainly by hydrogenotrophic methanogenic pathway. Information provided will be useful to select suitable reactors and optimize reaction design.

Bioelectrical Methane Production with an Ammonium Oxidative Reaction under the No Organic Substance Condition

The present study investigated bioelectrical methane production from CO₂ without organic substances. Even though microbial methane production has been reported at relatively high electric voltages, the amount of voltage required and the organisms contributing to the process currently remain unknown. Methane production using a biocathode was investigated in a microbial electrolysis cell coupled with an NH₄⁺ oxidative reaction at an anode coated with platinum powder under a wide range of applied voltages and anaerobic conditions. A microbial community analysis revealed that methane production simultaneously occurred with biological denitrification at the biocathode. During denitrification, NO₃^– was produced by chemical NH₄⁺ oxidation at the anode and was provided to the biocathode chamber. H₂ was produced at the biocathode by the hydrogen-producing bacteria Petrimonas through the acceptance of electrons and protons. The H₂ produced was biologically consumed by hydrogenotrophic methanogens of Methanobacterium and Methanobrevibacter with CO₂ uptake and by hydrogenotrophic denitrifiers of Azonexus. This microbial community suggests that methane is indirectly produced without the use of electrons by methanogens. Furthermore, bioelectrical methane production occurred under experimental conditions even at a very low voltage of 0.05‍ ‍V coupled with NH₄⁺ oxidation, which was thermodynamically feasible.

Bioenergy recovery from wastewater accelerated by solar power: Intermittent electro-driving regulation and capacitive storage in biomass

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.

Conductive magnetite nanoparticles trigger syntrophic methane production in single chamber microbial electrochemical systems

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 L_CH4/g_COD 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.

Bioelectrochemical approach for enhancing lignocellulose degradation and biofilm formation in Geobacillus strain WSUCF1

Investigations on microbial electrocatalysis as a strategy for enhancing the rates of substrate utilization leading to enhanced yield of biomass and enhanced biofilm formation are reported. A thermophilic Geobacillus sp. strain WSUCF1 (60 °C), a potential lignocellulose degrading microorganism was used as the electrocatalyst. Glucose, cellulose, and corn stover were used as the feedstocks. The results of this investigation showed that applying the oxidation potential of -0.383 mV (vs PRE) increased the glucose utilization and COD removal by 25.5% and 29.7% respectively. The bioelectrocatalysis strategy also increased the biomass yield by 81.2, 42.1, and 49.5% in the case of systems fed with glucose, cellulose, and corn stover, respectively, when compared with the systems without applied oxidation potential. This is the first work reporting the effects of applied oxidation potential on increasing the rates of degradation of lignocellulosic biomass and enhanced biofilm formation.

Microbial electron uptake in microbial electrosynthesis: a mini-review

Microbial electron uptake (EU) is the biological capacity of microbes to accept electrons from electroconductive solid materials. EU has been leveraged for sustainable bioproduction strategies via microbial electrosynthesis (MES). MES often involves the reduction of carbon dioxide to multi-carbon molecules, with electrons derived from electrodes in a bioelectrochemical system. EU can be indirect or direct. Indirect EU-based MES uses electron mediators to transfer electrons to microbes. Although an excellent initial strategy, indirect EU requires higher electrical energy. In contrast, the direct supply of cathodic electrons to microbes (direct EU) is more sustainable and energy efficient. Nonetheless, low product formation due to low electron transfer rates during direct EU remains a major challenge. Compared to indirect EU, direct EU is less well-studied perhaps due to the more recent discovery of this microbial capability. This mini-review focuses on the recent advances and challenges of direct EU in relation to MES.

Progress towards catalyzing electro-methanogenesis in anaerobic digestion process: Fundamentals, process optimization, design and scale-up considerations

Electro-methanogenesis represents an emerging bio-methane production pathway that can be achieved through integrating microbial electrolysis cell (MEC) with conventional anaerobic digester (AD). Since 2009, a significant number of publications have reported superior methane productivity and kinetics from MEC-AD integrated systems. The overall objective of this review is to communicate the recent advances towards promoting electro-methanogenesis in the anaerobic digestion process. Firstly, the electro-methanogenesis pathways and functional roles of key microbial members are summarized. Secondly, various extrinsic process parameters, such as applied voltage/potential, pH, and temperature are discussed with emphasis on process optimization. Moreover, available methods for the inoculation and start-up of MEC-AD process are critically reviewed. Finally, system design and scale-up considerations, such as the selection of electrode materials, surface area and surface chemistry of electrode materials, and electrode spacing are summarized.

Strategies for Bioelectrochemical CO2 Reduction

Atmospheric CO₂ is a cheap and abundant source of carbon for synthetic applications. However, the stability of CO₂ makes its conversion to other carbon compounds difficult and has prompted the extensive development of CO₂ reduction catalysts. Bioelectrocatalysts are generally more selective, highly efficient, can operate under mild conditions, and use electricity as the sole reducing agent. Improving the communication between an electrode and a bioelectrocatalyst remains a significant area of development. Through the examples of CO₂ reduction catalyzed by electroactive enzymes and whole cells, recent advancements in this area are compared and contrasted.

Electro-conversion of carbon dioxide (CO2) to low-carbon methane by bioelectromethanogenesis process in microbial electrolysis cells: The current status and future perspective

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.

Development of methanogens within cathodic biofilm in the single-chamber microbial electrolysis cell

The aim of this study was to investigate the development of cathodic biofilm and its effect on methane production in a single-chamber microbial electrolysis cell (MEC). The MEC with 1 g/L acetate was successfully operated within 31 cycles (∼2400 h). The maximum methane production rate and average current capture efficiency in the MEC reached 93 L/m³·d and 82%, respectively. Distinct stratification of Methanobacteriaceae within cathodic biofilm was observed after 9 cycles of operation. The relative abundance of Methanobacteriaceae in the microbial community increased from 45.3% (0-15 μm), 57.6% (15-30 μm), 66.9% (30-45 μm) to 77.2% (45-60 μm) within the cathodic biofilm. The methane production rates were positively correlated with the mcrA gene copy numbers in the cathodic biofilm. Our results should be useful to understand the mechanism of methane and hydrogen production in the MEC.

Graphite-assisted electro-fermentation methanogenesis: Spectroelectrochemical and microbial community analyses of cathode biofilms

The stimulatory effect of conductive particles on anaerobic digestion has been demonstrated in recent years. However, it is yet to be determined whether and how conductive particles affect methanogenesis via electro-fermentation (electro-fermentation methanogenesis). In this study, it was demonstrated, for the first time, that conductive graphite boosted the methane production yield by 54.3% and increased the maximum methane production rate by 72.2% during electro-fermentation methanogenesis. Graphite significantly affected the composition of cathode biofilms, with more live and large aggregates being observed. Spectroelectrochemical analyses further showed that the kinds and intensities of biocatalytic active sites and redox groups on the cathode biofilms increased during graphite-assisted electro-fermentation methanogenesis. Particularly, c-type cytochromes, humic acid-like substances, and humic substances improved the long-range electron transport to methanogens such as Methanobacterium and Methanosarcina. The results have implications for the improvement of electro-fermentation process and the use of conductive materials for biofuel recovery.

Applications of Emerging Bioelectrochemical Technologies in Agricultural Systems: A Current Review

Background: Bioelectrochemical systems (BESs) are emerging energy-effective and environment-friendly technologies. Different applications of BESs are able to effectively minimize wastes and treat wastewater while simultaneously recovering electricity, biohydrogen and other value-added chemicals via specific redox reactions. Although there are many studies that have greatly advanced the performance of BESs over the last decade, research and reviews on agriculture-relevant applications of BESs are very limited. Considering the increasing demand for food, energy and water due to human population expansion, novel technologies are urgently needed to promote productivity and sustainability in agriculture. Methodology: This review study is based on an extensive literature search regarding agriculture-related BES studies mainly in the last decades (i.e., 2009–2018). The databases used in this review study include Scopus, Google Scholar and Web of Science. The current and future applications of bioelectrochemical technologies in agriculture have been discussed. Findings/Conclusions: BESs have the potential to recover considerable amounts of electric power and energy chemicals from agricultural wastes and wastewater. The recovered energy can be used to reduce the energy input into agricultural systems. Other resources and value-added chemicals such as biofuels, plant nutrients and irrigation water can also be produced in BESs. In addition, BESs may replace unsustainable batteries to power remote sensors or be designed as biosensors for agricultural monitoring. The possible applications to produce food without sunlight and remediate contaminated soils using BESs have also been discussed. At the same time, agricultural wastes can also be processed into construction materials or biochar electrodes/electrocatalysts for reducing the high costs of current BESs. Future studies should evaluate the long-term performance and stability of on-farm BES applications.

A comprehensive comparison of five different carbon-based cathode materials in CO2 electromethanogenesis: Long-term performance, cell-electrode contact behaviors and extracellular electron transfer pathways

Each carbon-based material, due to the discrepancy in critical properties, has distinct capability to enrich electroactive microbes able to electrosynthesize methane from CO₂. To optimize electromethanogenesis process, this study physically prepared and examined several carbon-based cathode materials: carbon stick (CS), CS twined by Ti wire (CS-Ti) or covered with carbon fiber (CS-CF), graphite felt (CS-GF) and carbon cloth (CS-CC). CS-GF electrode had constantly stable methane production (75.8 mL/L/d at -0.9 V vs. Ag/AgCl) while CS-CC showed a suppressed performance over time caused by the desposition of inorganic shell. Electrode material properties affected biofilms growth, cell-electrode contact behaviors and electron exchange. Methane formation with CS-CC biocathode was H₂-concnetration dependent; CS-GF cathode possessed high antifouling properties and extensive space, enriching the microorganisms capable of catalyzing electromethanogenesis through more efficient non-H₂ route. This study re-interpreted the application potentials of carbon-based materials in CO₂ electroreduction and electrofuel recovery, providing valuable guidance for materials' selection.

Bioelectrochemically assisted anaerobic digestion system for biogas upgrading and enhanced methane production

The objective of this study was to evaluate the effect of biofilm and external voltage on the performance and microbial community composition of batch-fed, combined anaerobic digestion-bioelectrochemical cell (AD-BEC) systems under different operational conditions. A dextrin/peptone mixture was fed at a range of organic loading rates (0.34 to 1.37g COD/L-d). The hybrid system with both suspended biomass and biofilm without any external potential application achieved a substantially higher initial soluble COD consumption (53.7±2.3% vs. 39.7±3.7) and methane (CH₄) production (331 vs. 225mL) within one day of feeding than the conventional AD system (suspended biomass only). Compared to the conventional AD system, the hybrid systems had higher resilience to shock organic loadings. A range of external potential (0.5 to 2.0V vs. Ag/AgCl) was applied to AD-BEC reactors, developed with two different start-up procedures. A potential of 2.0V resulted in water electrolysis leading to a higher CH₄ production rate (105 vs. 84mL/L-d) and biogas CH₄ content (88.5±1.4 vs. 64.5±1.9%) in the AD-BEC reactor (closed vs. open circuit condition, respectively). Application of external potential enriched putative exoelectrogens at the anode biofilm and hydrogenotrophic methanogens at the cathode biofilm, which may have contributed to the observed enhanced CH₄ production in the AD-BEC system. A phylotype related to Methanobacterium formicicum, a hydrogenotrophic methanogen, dominated the archaeal community in the AD-BEC cathode biofilm.

Methanogens: biochemical background and biotechnological applications

Since fossil sources for fuel and platform chemicals will become limited in the near future, it is important to develop new concepts for energy supply and production of basic reagents for chemical industry. One alternative to crude oil and fossil natural gas could be the biological conversion of CO2 or small organic molecules to methane via methanogenic archaea. This process has been known from biogas plants, but recently, new insights into the methanogenic metabolism, technical optimizations and new technology combinations were gained, which would allow moving beyond the mere conversion of biomass. In biogas plants, steps have been undertaken to increase yield and purity of the biogas, such as addition of hydrogen or metal granulate. Furthermore, the integration of electrodes led to the development of microbial electrosynthesis (MES). The idea behind this technique is to use CO2 and electrical power to generate methane via the microbial metabolism. This review summarizes the biochemical and metabolic background of methanogenesis as well as the latest technical applications of methanogens. As a result, it shall give a sufficient overview over the topic to both, biologists and engineers handling biological or bioelectrochemical methanogenesis.

Combined pretreatment of electrolysis and ultra-sonication towards enhancing solubilization and methane production from mixed microalgae biomass

This study investigated the effect of combination of pretreatment methods such as ultra-sonication and electrolysis for the minimum energy input to recover the maximal carbohydrate and solubilization (in terms of sCOD) from mixed microalgae biomass. The composition of the soluble chemical oxygen demand (COD), protein, carbohydrate revealed that the hydrolysis method had showed positive impact on the increasing quantity and thus enhanced methane yields. As a result, the combination of these 2 pretreatments showed the greatest yield of soluble protein and carbohydrate as 279 and 309mg/L, which is the recovery of nearly 85 and 90% in terms of total content of them. BMP tests showed peak methane production yield of 257mL/gVS_added, for the hydrolysate of combined pretreatment as compared to the control experiment of 138mL/gVS added.

Zero-Valent Iron Enhances Biocathodic Carbon Dioxide Reduction to Methane

Methanogenic bioelectrochemical systems (BESs), which convert carbon dioxide (CO₂) directly to methane (CH₄), promise to be an innovative technology for anaerobic digester biogas upgrading. Zero-valent iron (ZVI), which has previously been used to improve CH₄ production in anaerobic digesters, has not been explored in methanogenic biocathodes. Thus, the objective of this study was to assess the effect of biocathode ZVI on BES performance at 1 and 2 g/L initial ZVI concentrations and at various cathode potentials (-0.65 to -0.80 V versus SHE). The total CH₄ produced during a 7-day feeding cycle with 1 and 2 g/L initial ZVI was 2.8- and 2.9-fold higher, respectively, than the mean CH₄ production in the four prior cycles without ZVI addition. Furthermore, CH₄ production by the ZVI-amended biocathodes remained elevated throughout three subsequent feeding cycles, despite catholyte replacement and no new ZVI addition. The fourth cycle following a single ZVI addition of 1 g/L and 2 g/L yielded 123% and 231% more total CH₄ than in the non-ZVI cycles, respectively. The higher CH₄ production could not be fully explained by complete anaerobic oxidation of the ZVI and utilization of produced H₂ by hydrogenotrophic methanogens. Microbial community analysis showed that the same phylotype, most closely related to Methanobrevibacter arboriphilus, dominated the archaeal community in the ZVI-free and ZVI-amended biocathodes. However, the bacterial community experienced substantial changes following ZVI exposure, with more Proteobacteria and fewer Bacteroidetes in the ZVI-amended biocathode. Furthermore, it is likely that a redox-active precipitate formed in the ZVI-amended biocathode, which sorbed to the electrode and/or biofilm, acted as a redox mediator, and enhanced electron transfer and CH₄ production. Thus, ZVI may be used to increase biocathode CH₄ production, assist in the start-up of an electromethanogenic biocathode, and/or maintain microbial activity during voltage interruptions.

Continuous micro-current stimulation to upgrade methanolic wastewater biodegradation and biomethane recovery in an upflow anaerobic sludge blanket (UASB) reactor

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 Fe^2+/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.

Microbiome involved in microbial electrochemical systems (MESs): A review

Microbial electrochemical systems (MESs) are an attracting technology for the disposal of wastewater treatment and simultaneous energy production. In MESs, at the anode microorganisms through the catalytic activity generates electrons that can be converted into electricity or other valuable chemical compounds. Microorganisms those having ability to donate and accept electrons to and from anode and cathode electrodes, respectively are recognized as 'exoelectrogens'. In the MESs, it renders an important function for its performance. In the present mini-review, we have discussed the role of microbiome including pure culture, enriched culture and mixed culture in different BESs application. The effects of operational and biological factors on microbiome development have been discussed. Further discussion about the molecular techniques for the evaluation of microbial community analysis is addressed. In addition different electrochemical techniques for extracellular electron transfer (EET) mechanism of electroactive biofilms have been discussed. This review highlights the importance of microbiome in the development of MESs, effective operational factors for exo-electrogens activities as well their key challenges and future technological aspects are also briefly discussed.