Microbial electrosynthesis of butyrate from carbon dioxide: Production and extraction

Microbial electrosynthesis from CO2 reaches productivity of syngas and chain elongation fermentations

Carbon-based products are essential to society, yet producing them from fossil fuels is unsustainable. Microorganisms have the ability to take up electrons from solid electrodes and convert carbon dioxide (CO2) to valuable carbon-based chemicals. However, higher productivities and energy efficiencies are needed to reach a viability that can make the technology transformative. Here, we show how a biofilm-based microbial porous cathode in a directed flow-through electrochemical system can continuously reduce CO2 to even-chain C2-C6 carboxylic acids over 248 days. We demonstrate a threefold higher biofilm concentration, volumetric current density, and productivity compared with the state of the art. Most notably, the volumetric productivity (VP) resembles those achieved in laboratory-scale and industrial syngas (CO-H2-CO2) fermentation and chain elongation fermentation. This work highlights key design parameters for efficient electricity-driven microbial CO2 reduction. There is need and room to improve the rates of electrode colonization and microbe-specific kinetics to scale up the technology.

Integration of biocompatible hydrogen evolution catalyst developed from metal-mix solutions with microbial electrosynthesis

Microbial conversion of CO2 to multi-carbon compounds such as acetate and butyrate is a promising valorisation technique. For those reactions, the electrochemical supply of hydrogen to the biocatalyst is a viable approach. Earlier we have shown that trace metals from microbial growth media spontaneously form in situ electro-catalysts for hydrogen evolution. Here, we show biocompatibility with the successful integration of such metal mix-based HER catalyst for immediate start-up of microbial acetogenesis (CO2 to acetate). Also, n-butyrate formation started fast (after twenty days). Hydrogen was always produced in excess, although productivity decreased over the 36 to 50 days, possibly due to metal leaching from the cathode. The HER catalyst boosted microbial productivity in a two-step microbial community bioprocess: acetogenesis by a BRH-c20a strain and acetate elongation to n-butyrate by Clostridium sensu stricto 12 (related) species. These findings provide new routes to integrate electro-catalysts and micro-organisms showing respectively bio and electrochemical compatibility.

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.

Simultaneous Biogas Upgrading and Valuable Chemical Production Using Homoacetogens in a Membrane Biofilm Reactor

Biogas produced from anaerobic digestion usually contains impurities, particularly with a high content of CO2 (15-60%), thus decreasing its caloric value and limiting its application as an energy source. H2-driven biogas upgrading using homoacetogens is a promising approach for upgrading biogas to biomethane and converting CO2 to acetate simultaneously. Herein, we developed a novel membrane biofilm reactor (MBfR) with H2 and biogas separately supplied via bubbleless hollow fiber membranes. The gas-permeable hollow fibers of the MBfR enabled high H2 and CO2 utilization efficiencies (∼98% and ∼97%, respectively) and achieved concurrent biomethane (∼94%) and acetate (∼450 mg/L/d) production. High-throughput 16S rRNA gene amplicon sequencing suggested that enriched microbial communities were dominated by Acetobacterium (38-48% relative abundance). In addition, reverse transcription quantitative PCR of the functional marker gene formyltetrahydrofolate synthetase showed that its expression level increased with increasing H2 and CO2 utilization efficiencies. These results indicate that Acetobacterium plays a key role in CO2 to acetate conversion. These findings are expected to facilitate energy-positive wastewater treatment and contribute to the development of a new solution to biogas upgrading.

Methanol as a co-substrate with CO2 enhances butyrate production in microbial electrosynthesis

Methanol is a promising feedstock for the bio-based economy as it can be derived from organic waste streams or produced electrochemically from CO2. Acetate production from CO2 in microbial electrosynthesis (MES) has been widely studied, while more valuable compounds such as butyrate are currently attracting attention. In this study, methanol was used as a co-substrate with CO2 to enhance butyrate production in MES. Feeding with CO2 and methanol resulted in the highest butyrate production rates and titres of 0.36 ± 0.01 g L-1 d-1 and 8.6 ± 0.2 g L-1, respectively, outperforming reactors with only CO2 feeding (0.20 ± 0.03 g L-1 d-1 and 5.2 ± 0.1 g L-1, respectively). Methanol acted as electron donor and as carbon source, both of which contributed ca. 50% of the carbon in the products. Eubacterium was the dominant genus with 52.6 ± 2.5% relative abundance. Thus, we demonstrate attractive route for the use of the C1 substrates, CO2 and methanol, to produce mainly butyrate. KEY POINTS: • Butyrate was the main product from methanol and CO2 in MES • Methanol acted as both carbon and electron source in MES • Eubacterium dominating microbial culture was enriched in MES.

Enhancing microbial CO2 electrocatalysis for multicarbon reduction in a wet amine-based catholyte

Microbial CO2 electroreduction (mCO2ER) offers a promising approach for producing high-value multicarbon reductants from CO2 by combining CO2 fixing microorganisms with conducting materials (i. e., cathodes). However, the solubility and availability of CO2 in an aqueous electrolyte pose significant limitations in this system. This study demonstrates the efficient production of long-chain multicarbon reductants, specifically carotenoids (~C40), within a wet amine-based catholyte medium during mCO2ER. Optimizing the concentration of the biocompatible CO2 absorbent, monoethanolamine (MEA), led to enhanced CO2 fixation in the electroautotroph bacteria. Molecular biological analyses revealed that MEA in the catholyte medium redirected the carbon flux towards carotenoid biosynthesis during mCO2ER. The faradaic efficiency of mCO2ER with MEA for carotenoid production was 4.5-fold higher than that of the control condition. These results suggest the mass transport bottleneck in bioelectrochemical systems could be effectively addressed by MEA-assissted mCO2ER, enabling highly efficient production of valuable products from CO2.

H2 mediated mixed culture microbial electrosynthesis for high titer acetate production from CO2

Microbial electrosynthesis (MES) converts CO2 into value-added products such as volatile fatty acids (VFAs) with minimal energy use, but low production titer has limited scale-up and commercialization. Mediated electron transfer via H2 on the MES cathode has shown a higher conversion rate than the direct biofilm-based approach, as it is tunable via cathode potential control and accelerates electrosynthesis from CO2. Here we report high acetate titers can be achieved via improved in situ H2 supply by nickel foam decorated carbon felt cathode in mixed community MES systems. Acetate concentration of 12.5 g L-1 was observed in 14 days with nickel-carbon cathode at a poised potential of -0.89 V (vs. standard hydrogen electrode, SHE), which was much higher than cathodes using stainless steel (5.2 g L-1) or carbon felt alone (1.7 g L-1) with the same projected surface area. A higher acetate concentration of 16.0 g L-1 in the cathode was achieved over long-term operation for 32 days, but crossover was observed in batch operation, as additional acetate (5.8 g L-1) was also found in the abiotic anode chamber. We observed the low Faradaic efficiencies in acetate production, attributed to partial H2 utilization for electrosynthesis. The selective acetate production with high titer demonstrated in this study shows the H2-mediated electron transfer with common cathode materials carries good promise in MES development.

Novel electrochemical strategies for the microbial conversion of CO2 into biomass and volatile fatty acids using a fluid-like bed electrode in a three-phase reactor

Microbial electrosynthesis (MES) constitutes a bioelectrochemical process where bacteria uptake electrons extracellularly from a polarized electrode to incorporate them into their anabolic metabolism. However, the efficiency of current MES reactor designs can be lower than expected due to limitations regarding electron transfer and mass transport. One of the most promising bioreactor configurations to overcome these bottlenecks is the Microbial Electrochemical Fluidized Bed Reactor (ME-FBR). In this study, microbial CO2 fixation is investigated for the first time in a ME-FBR operated as a 3-phase reactor (solid-liquid-gas). An electroconductive carbon bed, acting as a working electrode, was fluidized with gas and polarized at different potentials (-0.6, -0.8 and -1 V vs. Ag/AgCl) so it could act as an electron donor (biocathode). Under these potentials, CO2 fixation and electron transfer were evaluated. Autotrophic electroactive microorganisms from anaerobic wastewater were enriched in a ME-FBR in the presence of 2-bromoethanosulfonic acid (BES) to inhibit the growth of methanogens. Cyclic voltammetry analysis revealed interaction between the microorganisms and the cathode. Furthermore, volatile fatty acids like propionate, formate and acetate were detected in the culture supernatant. Acetate production had a maximum rate of ca. 1 g L-1  day-1 . Planktonic cell biomass was produced under continuous culture at values as high as ca. 0.7 g L-1 dry weight. Overall, this study demonstrates the feasibility of employing a fluidized electrode with gaseous substrates and electricity as the energy source for generating biomass and carboxylic acids.

Selective butyric acid production from CO2 and its upgrade to butanol in microbial electrosynthesis cells

Microbial electrosynthesis (MES) is a promising carbon utilization technology, but the low-value products (i.e., acetate or methane) and the high electric power demand hinder its industrial adoption. In this study, electrically efficient MES cells with a low ohmic resistance of 15.7 mΩ m2 were operated galvanostatically in fed-batch mode, alternating periods of high CO2 and H2 availability. This promoted acetic acid and ethanol production, ultimately triggering selective (78% on a carbon basis) butyric acid production via chain elongation. An average production rate of 14.5 g m-2 d-1 was obtained at an applied current of 1.0 or 1.5 mA cm-2, being Megasphaera sp. the key chain elongating player. Inoculating a second cell with the catholyte containing the enriched community resulted in butyric acid production at the same rate as the previous cell, but the lag phase was reduced by 82%. Furthermore, interrupting the CO2 feeding and setting a constant pH2 of 1.7-1.8 atm in the cathode compartment triggered solventogenic butanol production at a pH below 4.8. The efficient cell design resulted in average cell voltages of 2.6-2.8 V and a remarkably low electric energy requirement of 34.6 kWhel kg-1 of butyric acid produced, despite coulombic efficiencies being restricted to 45% due to the cross-over of O2 and H2 through the membrane. In conclusion, this study revealed the optimal operating conditions to achieve energy-efficient butyric acid production from CO2 and suggested a strategy to further upgrade it to valuable butanol.

Carbon dioxide reduction to high-value chemicals in microbial electrosynthesis system: Biological conversion and regulation strategies

Large emissions of atmospheric carbon dioxide (CO2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO2 to high-value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO2 reduction.

Sustainable circular biorefinery approach for novel building blocks and bioenergy production from algae using microbial fuel cell

The imminent need for transition to a circular biorefinery using microbial fuel cells (MFC), based on the valorization of renewable resources, will ameliorate the carbon footprint induced by industrialization. MFC catalyzed by bioelectrochemical process drew significant attention initially for its exceptional potential for integrated production of biochemicals and bioenergy. Nonetheless, the associated costly bioproduct production and slow microbial kinetics have constrained its commercialization. This review encompasses the potential and development of macroalgal biomass as a substrate in the MFC system for L-lactic acid (L-LA) and bioelectricity generation. Besides, an insight into the state-of-the-art technological advancement in the MFC system is also deliberated in detail. Investigations in recent years have shown that MFC developed with different anolyte enhances power density from several µW/m2 up to 8160 mW/m2. Further, this review provides a plausible picture of macroalgal-based L-LA and bioelectricity circular biorefinery in the MFC system for future research directions.

Perspectives for Using CO2 as a Feedstock for Biomanufacturing of Fuels and Chemicals

Microbial cell factories offer an eco-friendly alternative for transforming raw materials into commercially valuable products because of their reduced carbon impact compared to conventional industrial procedures. These systems often depend on lignocellulosic feedstocks, mainly pentose and hexose sugars. One major hurdle when utilizing these sugars, especially glucose, is balancing carbon allocation to satisfy energy, cofactor, and other essential component needs for cellular proliferation while maintaining a robust yield. Nearly half or more of this carbon is inevitably lost as CO2 during the biosynthesis of regular metabolic necessities. This loss lowers the production yield and compromises the benefit of reducing greenhouse gas emissions-a fundamental advantage of biomanufacturing. This review paper posits the perspectives of using CO2 from the atmosphere, industrial wastes, or the exhausted gases generated in microbial fermentation as a feedstock for biomanufacturing. Achieving the carbon-neutral or -negative goals is addressed under two main strategies. The one-step strategy uses novel metabolic pathway design and engineering approaches to directly fix the CO2 toward the synthesis of the desired products. Due to the limitation of the yield and efficiency in one-step fixation, the two-step strategy aims to integrate firstly the electrochemical conversion of the exhausted CO2 into C1/C2 products such as formate, methanol, acetate, and ethanol, and a second fermentation process to utilize the CO2-derived C1/C2 chemicals or co-utilize C5/C6 sugars and C1/C2 chemicals for product formation. The potential and challenges of using CO2 as a feedstock for future biomanufacturing of fuels and chemicals are also discussed.

Bioelectrocatalytic Synthesis: Concepts and Applications

Bioelectrocatalytic synthesis is the conversion of electrical energy into value-added products using biocatalysts. These methods merge the specificity and selectivity of biocatalysis and energy-related electrocatalysis to address challenges in the sustainable synthesis of pharmaceuticals, commodity chemicals, fuels, feedstocks and fertilizers. However, the specialized experimental setups and domain knowledge for bioelectrocatalysis pose a significant barrier to adoption. This review introduces key concepts of bioelectrosynthetic systems. We provide a tutorial on the methods of biocatalyst utilization, the setup of bioelectrosynthetic cells, and the analytical methods for assessing bioelectrocatalysts. Key applications of bioelectrosynthesis in ammonia production and small-molecule synthesis are outlined for both enzymatic and microbial systems. This review serves as a necessary introduction and resource for the non-specialist interested in bioelectrosynthetic research.

Bioelectrochemical systems (BESs) for agro-food waste and wastewater treatment, and sustainable bioenergy-A review

Producing food by farming and subsequent food manufacturing are central to the world's food supply, accounting for more than half of all production. Production is, however, closely related to the creation of large amounts of organic wastes or byproducts (agro-food waste or wastewater) that negatively impact the environment and the climate. Global climate change mitigation is an urgent need that necessitates sustainable development. For that purpose, proper agro-food waste and wastewater management are essential, not only for waste reduction but also for resource optimization. To achieve sustainability in food production, biotechnology is considered as key factor since its continuous development and broad implementation will potentially benefit ecosystems by turning polluting waste into biodegradable materials; this will become more feasible and common as environmentally friendly industrial processes improve. Bioelectrochemical systems are a revitalized, promising biotechnology integrating microorganisms (or enzymes) with multifaceted applications. The technology can efficiently reduce waste and wastewater while recovering energy and chemicals, taking advantage of their biological elements' specific redox processes. In this review, a consolidated description of agro-food waste and wastewater and its remediation possibilities, using different bioelectrochemical-based systems is presented and discussed together with a critical view of the current and future potential applications.

Progress and perspectives on microbial electrosynthesis for valorisation of CO2 into value-added products

Microbial electrosynthesis (MES) is a neoteric technology that facilitates biocatalysed synthesis of organic compounds with the aid of homoacetogenic bacteria, while feeding CO₂ as an inorganic carbon source. Operating MES with surplus renewable electricity further enhances the sustainability of this innovative bioelectrochemical system (BES). However, several lacunae exist in the domain knowledge, stunting the widespread application of MES. Despite significant progress in this area over the past decade, the product yield efficiency is not on par with other contemporary technologies. This bottleneck can be overcome by adopting a holistic approach, i.e., applying innovative and integrated solutions to ensure a robust MES operation. Further, the widespread deployment of MES exclusively relies on its ability to mature a sessile biofilm over a biocompatible electrode, while offering minimal charge transfer resistance. Additionally, operating MES preferably at H₂-generating reduction potential and valorising industrial off-gas as carbon substrate is crucial to accomplish economic sustainability. In light of the aforementioned, this review collates the latest progress in the design and development of MES-centred systems for valorisation of CO₂ into value-added products. Specifically, it highlights the significance of inoculum pre-treatment for promoting biocatalytic activity and biofilm growth on the cathodic surface. In addition, it summarizes the diverse materials that are commonly used as electrodes in MES, with an emphasis on the importance of inexpensive, robust, and biocompatible electrode materials for the practical application of MES technology. Further, the review presents insights into media conditions, operational factors, and reactor configurations that affect the overall performance of MES process. Finally, the product range of MES, downstream processing requirements, and integration of MES with other environmental remediation technologies are also discussed.

Metatranscriptomic insights into the microbial electrosynthesis of acetate by Fe2+/Ni2+ addition

As important components of enzymes and coenzymes involved in energy transfer and Wood-Ljungdahl (WL) pathways, Fe²⁺ and Ni²⁺ supplementation may promote the acetate synthesis through CO₂ reduction by the microbial electrosynthesis (MES). However, the effect of Fe²⁺ and Ni²⁺ addition on acetate production in MES and corresponding microbial mechanisms have not been fully studied. Therefore, this study investigated the effect of Fe²⁺ and Ni²⁺ addition on acetate production in MES, and explored the underlying microbial mechanism from the metatranscriptomic perspective. Both Fe²⁺ and Ni²⁺ addition enhanced acetate production of the MES, which was 76.9% and 110.9% higher than that of control, respectively. Little effect on phylum level and small changes in genus-level microbial composition was caused by Fe²⁺ and Ni²⁺ addition. Gene expression of 'Energy metabolism', especially in 'Carbon fixation pathways in prokaryotes' was up-regulated by Fe²⁺ and Ni²⁺ addition. Hydrogenase was found as an important energy transfer mediator for CO₂ reduction and acetate synthesis. Fe²⁺ addition and Ni²⁺ addition respectively enhanced the expression of methyl branch and carboxyl branch of the WL pathway, and thus promoted acetate production. The study provided a metatranscriptomic insight into the effect of Fe²⁺ and Ni²⁺ on acetate production by CO₂ reduction in MES.

Bioelectrocatalysis for CO2 reduction: recent advances and challenges to develop a sustainable system for CO2 utilization

Activation and turning CO₂ into value added products is a promising orientation to address environmental issues caused by CO₂ emission. Currently, electrocatalysis has a potent well-established role for CO₂ reduction with fast electron transfer rate; but it is challenged by the poor selectivity and low faradic efficiency. On the other side, biocatalysis, including enzymes and microbes, has been also employed for CO₂ conversion to target C_n products with remarkably high selectivity; however, low solubility of CO₂ in the liquid reaction phase seriously affects the catalytic efficiency. Therefore, a new synergistic role in bioelectrocatalysis for CO₂ reduction is emerging thanks to its outstanding selectivity, high faradic efficiency, and desirable valuable C_n products under mild condition that are surveyed in this review. Herein, we comprehensively discuss the results already obtained for the integration craft of enzymatic-electrocatalysis and microbial-electrocatalysis technologies. In addition, the intrinsic nature of the combination is highly dependent on the electron transfer. Thus, both direct electron transfer and mediated electron transfer routes are modeled and concluded. We also explore the biocompatibility and synergistic effects of electrode materials, which emerge in combination with tuned enzymes and microbes to improve catalytic performance. The system by integrating solar energy driven photo-electrochemical technics with bio-catalysis is further discussed. We finally highlight the significant findings and perspectives that have provided strong foundations for the remarkable development of green and sustainable bioelectrocatalysis for CO₂ reduction, and that offer a blueprint for C_n valuable products originate from CO₂ under efficient and mild conditions.

Metabolic Engineering of Microorganisms to Produce Pyruvate and Derived Compounds

Pyruvate is a hub of various endogenous metabolic pathways, including glycolysis, TCA cycle, amino acid, and fatty acid biosynthesis. It has also been used as a precursor for pyruvate-derived compounds such as acetoin, 2,3-butanediol (2,3-BD), butanol, butyrate, and L-alanine biosynthesis. Pyruvate and derivatives are widely utilized in food, pharmaceuticals, pesticides, feed additives, and bioenergy industries. However, compounds such as pyruvate, acetoin, and butanol are often chemically synthesized from fossil feedstocks, resulting in declining fossil fuels and increasing environmental pollution. Metabolic engineering is a powerful tool for producing eco-friendly chemicals from renewable biomass resources through microbial fermentation. Here, we review and systematically summarize recent advances in the biosynthesis pathways, regulatory mechanisms, and metabolic engineering strategies for pyruvate and derivatives. Furthermore, the establishment of sustainable industrial synthesis platforms based on alternative substrates and new tools to produce these compounds is elaborated. Finally, we discuss the potential difficulties in the current metabolic engineering of pyruvate and derivatives and promising strategies for constructing efficient producers.

Boosting ethanol production rates from carbon dioxide in MES cells under optimal solventogenic conditions

Microbial Electrosynthesis (MES) has been widely applied for acetic acid (HA) production from CO₂ and electricity. Ethanol (EtOH) has a higher market value than HA, and wide application in industry and as a biofuel. However, it has only been obtained sporadically and at low concentrations, probably due to sub-optimal operating conditions. This study aimed at enhancing EtOH productivity in MES cells by jointly optimising key operation parameters, including pH, H₂ and CO₂ partial pressure (pH₂ and pCO₂), and HA concentration, to promote solventogenesis. Two H-type cells were operated in fed-batch mode at -0.8 V vs. SHE with CO₂ as the sole carbon source. A mixed culture, enriched with Clostridium ljungdahlii was used as the biocatalyst. The combination of low pH (<4.5) and pCO₂ (<0.3 atm), along with high HA concentration (about 6 g L^-1) and pH₂ (>3 atm), were mandatory conditions for maintaining an efficient solventogenic culture, dominated by Clostridium sp., capable of high-rate EtOH production. The maximum EtOH production rate was 10.95 g m^-2 d^-1, and a concentration of 5.28 g L^-1 was achieved. Up to 30 % of the electrons and 15.2 % of the carbon provided were directed towards EtOH production, and 28.1 kWh were required for the synthesis of 1 kg of EtOH from CO₂. These results highlight that strict conditions are required for a continuous, reliable, EtOH production in MES cells. Future investigation should focus on improving cell configuration to achieve EtOH production at higher current densities while minimizing the electric energy input.

CO2 in indoor environments: From environmental and health risk to potential renewable carbon source

In the developed world, individuals spend most of their time indoors. Poor Indoor Air Quality (IAQ) has a wide range of effects on human health. The burden of disease associated with indoor air accounts for millions of premature deaths related to exposure to Indoor Air Pollutants (IAPs). Among them, CO₂ is the most common one, and is commonly used as a metric of IAQ. Indoor CO₂ concentrations can be significantly higher than outdoors due to human metabolism and activities. Even in presence of ventilation, controlling the CO₂ concentration below the Indoor Air Guideline Values (IAGVs) is a challenge, and many indoor environments including schools, offices and transportation exceed the recommended value of 1000 ppm_v. This is often accompanied by high concentration of other pollutants, including bio-effluents such as viruses, and the importance of mitigating the transmission of airborne diseases has been highlighted by the COVID-19 pandemic. On the other hand, the relatively high CO₂ concentration of indoor environments presents a thermodynamic advantage for direct air capture (DAC) in comparison to atmospheric CO₂ concentration. This review aims to describe the issues associated with poor IAQ, and to demonstrate the potential of indoor CO₂ DAC to purify indoor air while generating a renewable carbon stream that can replace conventional carbon sources as a building block for chemical production, contributing to the circular economy.

Enhancing the selective synthesis of butyrate in microbial electrosynthesis system by gas diffusion membrane composite biocathode

The reduction of carbon dioxide (CO₂) to high value-added multi-carbon compounds at the cathode is an emerging application of microbial electrosynthesis system (MES). In this study, a composite cathode consisting of hollow fiber membrane (HFM) and the carbon felt is designed to enhance the CO₂ mass transfer of the cathode. The result shows that the main products are acetate and butyrate without other substances. The electrochemical performance of the electrode is significantly improved after biofilm becomes matures. The composite cathode significantly reduces the "threshold" for the synthesis of butyrate. Moreover, CO₂ is dissolved and protons are consumed by synthesizing volatile fatty acids (VFAs) to maintain a stable pH inside the composite electrode. The synthesis mechanism of butyrate is that CO₂ is converted sequentially into acetate and butyrate. The microenvironment of the composite electrode enriches Firmicute. This composite electrode provides a novel strategy for regulating the microenvironment.

Transition roadmap for thermophilic carbon dioxide microbial electrosynthesis: Testing with real exhaust gases and operational control for a scalable design

Human activities release more carbon dioxide (CO₂) into the atmosphere than the natural process can remove. This study attempts to address the main challenges for the thermophilic (50 °C) bioelectrochemical conversion of CO₂ into acetate. First, real gaseous emissions were tested with mixed microbial consortia, which had no substantial influence on production rates (difference of 2.5%). Subsequently, a bench-scale system (TRL 4-5) was designed and launched to control key operational variables. Fixing the current at 1.3 A m^-2, CO₂ was reduced at a rate of 2.21 kg CO₂ kg^-1 acetate, while the electricity consumption was 2.07 kWh kg^-1, the most efficient value so far. The results suggest that the operation with real effluents is feasible and the proposed design is energy efficient, but the right balance between maximising current densities without compromising the biocompatibility with catalysts will determine the transition from laboratory scale towards its implementation in the market.

Bioelectrochemical methanation by utilization of steel mill off-gas in a two-chamber microbial electrolysis cell

Carbon capture and utilization has been proposed as one strategy to combat global warming. Microbial electrolysis cells (MECs) combine the biological conversion of carbon dioxide (CO₂) with the formation of valuable products such as methane. This study was motivated by the surprising gap in current knowledge about the utilization of real exhaust gas as a CO₂ source for methane production in a fully biocatalyzed MEC. Therefore, two steel mill off-gases differing in composition were tested in a two-chamber MEC, consisting of an organic substrate-oxidizing bioanode and a methane-producing biocathode, by applying a constant anode potential. The methane production rate in the MEC decreased immediately when steel mill off-gas was tested, which likely inhibited anaerobic methanogens in the presence of oxygen. However, methanogenesis was still ongoing even though at lower methane production rates than with pure CO₂. Subsequently, pure CO₂ was studied for methanation, and the cathodic biofilm successfully recovered from inhibition reaching a methane production rate of 10.8 L m⁻²d⁻¹. Metagenomic analysis revealed Geobacter as the dominant genus forming the anodic organic substrate-oxidizing biofilms, whereas Methanobacterium was most abundant at the cathodic methane-producing biofilms.

The oxygen dilemma: The challenge of the anode reaction for microbial electrosynthesis from CO2

Microbial electrosynthesis (MES) from CO2 provides chemicals and fuels by driving the metabolism of microorganisms with electrons from cathodes in bioelectrochemical systems. These microorganisms are usually strictly anaerobic. At the same time, the anode reaction of bioelectrochemical systems is almost exclusively water splitting through the oxygen evolution reaction (OER). This creates a dilemma for MES development and engineering. Oxygen penetration to the cathode has to be excluded to avoid toxicity and efficiency losses while assuring low resistance. We show that this dilemma derives a strong need to identify novel reactor designs when using the OER as an anode reaction or to fully replace OER with alternative oxidation reactions.

Microbial electrosynthesis of methane and acetate—comparison of pure and mixed cultures

Abstract

The electrochemical process of microbial electrosynthesis (MES) is used to drive the metabolism of electroactive microorganisms for the production of valuable chemicals and fuels. MES combines the advantages of electrochemistry, engineering, and microbiology and offers alternative production processes based on renewable raw materials and regenerative energies. In addition to the reactor concept and electrode design, the biocatalysts used have a significant influence on the performance of MES. Thus, pure and mixed cultures can be used as biocatalysts. By using mixed cultures, interactions between organisms, such as the direct interspecies electron transfer (DIET) or syntrophic interactions, influence the performance in terms of productivity and the product range of MES. This review focuses on the comparison of pure and mixed cultures in microbial electrosynthesis. The performance indicators, such as productivities and coulombic efficiencies (CEs), for both procedural methods are discussed. Typical products in MES are methane and acetate, therefore these processes are the focus of this review. In general, most studies used mixed cultures as biocatalyst, as more advanced performance of mixed cultures has been seen for both products. When comparing pure and mixed cultures in equivalent experimental setups a 3-fold higher methane and a nearly 2-fold higher acetate production rate can be achieved in mixed cultures. However, studies of pure culture MES for methane production have shown some improvement through reactor optimization and operational mode reaching similar performance indicators as mixed culture MES. Overall, the review gives an overview of the advantages and disadvantages of using pure or mixed cultures in MES.

Key points

• Undefined mixed cultures dominate as inoculums for the MES of methane and acetate, which comprise a high potential of improvement

• Under similar conditions, mixed cultures outperform pure cultures in MES

• Understanding the role of single species in mixed culture MES is essential for future industrial applications

Review on microbial fuel cells applications, developments and costs

The microbial fuel cell (MFC) technology has attracted significant attention in the last years due to its potential to recover energy in a wastewater treatment. The idea of using an MFC in industry is very attractive as the organic wastes can be converted into energy, reducing the waste disposal costs and the energy needs while increasing the company profit. However, taking aside these promising prospects, the attempts to apply MFCs in large-scale have not been succeeded so far since their lower performance and high costs remains challenging. This review intends to present the main applications of the MFC systems and its developments, particularly the advances on configuration and operating conditions. The diagnostic techniques used to evaluate the MFC performance as well as the different modeling approaches are described. Towards the introduction of the MFC in the market, a cost analysis is also included. The development of low-cost materials and more efficient systems, with high higher power outputs and durability, are crucial towards the application of MFCs in industrial/large scale. This work is a helpful tool for discovering new operation and design regimes.

Simulation of cathode for synthesizing organic acids by MES reduction of CO2

Microbial electrochemical system (MES) is a favorable tool for CO2 emission reduction. Microbial cathode is one of the core components of the system, and its surface energy transfer characteristics can greatly affect the yield of organic matter from MES. In order to solve the problem that the energy transfer characteristics of microbial cathode are not clear, the mathematical model of MES was constructed on the basis of the preliminary experiment with an electrode made of copper foam modified with the reduced graphene oxide, analyzing the current and substrate concentration in the biofilm with conductivity, cathode potential and porosity of the biofilm. The results show that when the cathode potential is higher than -0.8 V (VS SHE), the substrate concentration and current density in the biofilm are related to the cathode potential. However, when the cathode potential decreased to -0.8 V (vs SHE), the ability of biofilm to reduce CO2 basically reached saturation. Low conductivity (<10-3S/m) will lead to the formation of significant potential difference in the biofilm, which will reduce the substrate utilization rate and seriously affect the performance of microbial cathode. The current density is highest, when the porosity of the biofilm is about 0.35.

A comparative analysis of biopolymer production by microbial and bioelectrochemical technologies

Production of biopolymers from renewable carbon sources provides a path towards a circular economy. This review compares several existing and emerging approaches for polyhydroxyalkanoate (PHA) production from soluble organic and gaseous carbon sources and considers technologies based on pure and mixed microbial cultures. While bioplastics are most often produced from soluble sources of organic carbon, the use of carbon dioxide (CO₂) as the carbon source for PHA production is emerging as a sustainable approach that combines CO₂ sequestration with the production of a value-added product. Techno-economic analysis suggests that the emerging approach of CO₂ conversion to carboxylic acids by microbial electrosynthesis followed by microbial PHA production could lead to a novel cost-efficient technology for production of green biopolymers.

Biopolymers production from renewable carbon sources.

Wastewater-powered high-value chemical synthesis in a hybrid bioelectrochemical system

A microbial electrochemical system could potentially be applied as a biosynthesis platform by extracting wastewater energy while converting it to value-added chemicals. However, the unfavorable thermodynamics and sluggish kinetics of in vivo whole-cell cathodic catalysis largely limit product diversity and value. Herein, we convert the in vivo cathodic reaction to in vitro enzymatic catalysis and develop a microbe-enzyme hybrid bioelectrochemical system (BES), where microbes release the electricity from wastewater (anode) to power enzymatic catalysis (cathode). Three representative examples for the synthesis of pharmaceutically relevant compounds, including halofunctionalized oleic acid based on a cascade reaction, (4-chlorophenyl)-(pyridin-2-yl)-methanol based on electrochemical cofactor regeneration, and l-3,4-dihydroxyphenylalanine based on electrochemical reduction, were demonstrated. According to the techno-economic analysis, this system could deliver high system profit, opening an avenue to a potentially viable wastewater-to-profit process while shedding scientific light on hybrid BES mechanisms toward a sustainable reuse of wastewater.

Valorisation of CO2 into Value-Added Products via Microbial Electrosynthesis (MES) and Electro-Fermentation Technology

Microbial electrocatalysis reckons on microbes as catalysts for reactions occurring at electrodes. Microbial fuel cells and microbial electrolysis cells are well-known in this context; both prefer the oxidation of organic and inorganic matter for producing electricity. Notably, the synthesis of high energy-density chemicals (fuels) or their precursors by microorganisms using bio-cathode to yield electrical energy is called Microbial Electrosynthesis (MES), giving an exceptionally appealing novel way for producing beneficial products from electricity and wastewater. This review accentuates the concept, importance and opportunities of MES, as an emerging discipline at the nexus of microbiology and electrochemistry. Production of organic compounds from MES is considered as an effective technique for the generation of various beneficial reduced end-products (like acetate and butyrate) as well as in reducing the load of CO2 from the atmosphere to mitigate the harmful effect of greenhouse gases in global warming. Although MES is still an emerging technology, this method is not thoroughly known. The authors have focused on MES, as it is the next transformative, viable alternative technology to decrease the repercussions of surplus carbon dioxide in the environment along with conserving energy.

Reactor Designs and Configurations for Biological and Bioelectrochemical C1 Gas Conversion: A Review

Microbial C1 gas conversion technologies have developed into a potentially promising technology for converting waste gases (CO₂, CO) into chemicals, fuels, and other materials. However, the mass transfer constraint of these poorly soluble substrates to microorganisms is an important challenge to maximize the efficiencies of the processes. These technologies have attracted significant scientific interest in recent years, and many reactor designs have been explored. Syngas fermentation and hydrogenotrophic methanation use molecular hydrogen as an electron donor. Furthermore, the sequestration of CO₂ and the generation of valuable chemicals through the application of a biocathode in bioelectrochemical cells have been evaluated for their great potential to contribute to sustainability. Through a process termed microbial chain elongation, the product portfolio from C1 gas conversion may be expanded further by carefully driving microorganisms to perform acetogenesis, solventogenesis, and reverse β-oxidation. The purpose of this review is to provide an overview of the various kinds of bioreactors that are employed in these microbial C1 conversion processes.

Mixed-culture biocathodes for acetate production from CO2 reduction in the microbial electrosynthesis: Impact of temperature

The temperature effect on bioelectrochemical reduction of CO₂ to acetate with a mixed-culture biocathode in the microbial electrosynthesis was explored. The results showed that maximum acetate amount of 525.84 ± 1.55 mg L^-1 and fastest acetate formation of 49.21 ± 0.49 mg L^-1 d^-1 were obtained under mesophilic conditions. Electron recovery efficiency for CO₂ reduction to acetate ranged from 14.50 ± 2.20% to 64.86 ± 2.20%, due to propionate, butyrate and H₂ generation. Mesophilic conditions were demonstrated to be more favorable for biofilm formation on the cathode, resulting in a stable and dense biofilm. At phylum level, the relative abundance of Bacteroidetes phylum in the biofilm remarkably increased under mesophilic conditions, compared with that at psychrophilic and thermophilic conditions. At genus level, the Clostridium, Treponema, Acidithiobacillus, Acetobacterium and Acetoanaerobium were found to be dominated genera in the biofilm under mesophilic conditions, while genera diversity decreased under psychrophilic and thermophilic conditions.

A General Model for Biofilm-Driven Microbial Electrosynthesis of Carboxylates From CO2

Up to now, computational modeling of microbial electrosynthesis (MES) has been underexplored, but is necessary to achieve breakthrough understanding of the process-limiting steps. Here, a general framework for modeling microbial kinetics in a MES reactor is presented. A thermodynamic approach is used to link microbial metabolism to the electrochemical reduction of an intracellular mediator, allowing to predict cellular growth and current consumption. The model accounts for CO₂ reduction to acetate, and further elongation to n-butyrate and n-caproate. Simulation results were compared with experimental data obtained from different sources and proved the model is able to successfully describe microbial kinetics (growth, chain elongation, and product inhibition) and reactor performance (current density, organics titer). The capacity of the model to simulate different system configurations is also shown. Model results suggest CO₂ dissolved concentration might be limiting existing MES systems, and highlight the importance of the delivery method utilized to supply it. Simulation results also indicate that for biofilm-driven reactors, continuous mode significantly enhances microbial growth and might allow denser biofilms to be formed and higher current densities to be achieved.

Hydrogen-driven microbial biogas upgrading: Advances, challenges and solutions

As a clean and renewable energy, biogas is an important alternative to fossil fuels. However, the high carbon dioxide (CO₂) content in biogas limits its value as a fuel. 'Biogas upgrading' is an advanced process which removes CO₂ from biogas, thereby converting biogas to biomethane, which has a higher commercial value. Microbial technologies offer a sustainable and cost-effective way to upgrade biogas, removing CO₂ using hydrogen (H₂) as electron donor, generated by surplus electricity from renewable wind or solar energy. Hydrogenotrophic methanogens can be applied to convert CO₂ with H₂ to methane (CH₄), or alternatively, homoacetogens can convert both CO₂ and H₂ into value-added chemicals. Here, we comprehensively review the current state of biogas generation and utilization, and describe the advances in biological, H₂-dependent biogas upgrading technologies, with particular attention to key challenges associated with the processes, e.g., metabolic limitations, low H₂ transfer rate, and finite CO₂ conversion rate. We also highlight several new strategies for overcoming technical barriers to achieve efficient CO₂ conversion, including process optimization to eliminate metabolic limitation, novel reactor designs to improve H₂ transfer rate and utilization efficiency, and employing advanced genetic engineering tools to generate more efficient microorganisms. The insights offered in this review will promote further exploration into microbial, H₂-driven biogas upgrading, towards addressing the global energy crisis and climate change associated with use of fossil fuels.

Copper ferrite supported reduced graphene oxide as cathode materials to enhance microbial electrosynthesis of volatile fatty acids from CO2

Copper ferrite/reduced graphene oxide (CF/rGO) nanocomposites (NCs) was synthesized using the bio-combustion method and applied as a cathode catalyst in the microbial reduction of CO₂ to volatile fatty acids (VFAs) in a single chamber microbial electrosynthesis system (MES). The synthesized NCs exhibited a porous network-like structure with a high surface area of CF/rGO (158.22 m²/g), which was 2.24 folds higher than that of CF. The Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) analysis for CF/rGO/Carbon cloth (Cc) revealed a high reduction current density of -7.3 A/m² and a low charge transfer resistance of 2.8 Ω. The isobutyrate and acetate in MES-2 (Cu/rGO/Cc) were produced at 35.37 g/m²/d, which was 1.53 folds higher than that of MES-1 (bare Cc: 23.10 g/m²/d). The columbic efficiency (77.78%) and total VFA concentration (1941.13 ± 83 mg COD/L) were noted to be 1.97 and 1.6 folds higher for MES-2 than MES-1, respectively. The Tafel plot drawn from the CV curves exhibited an exchange current density value of MES-2 that was 3.46 A/m², and this value was 1.19 and 33.92 folds higher than that of MES-1 and abiotic CF/rGO/Cc, respectively. Field emission scanning electron microscopy (FESEM) observations revealed enhanced rod-shaped bacteria had grown on the cathode suggesting excellent biocompatible and multi-length scale porosity of CF/rGO catalysts for enhanced colonization of microbes. The phyla Proteobacteria (Betaproteobacteria), Bacteroidetes, and Firmicutes were highly abundant as the dominant microbial communities on the cathode, which might played a major role in bioelectrochemical CO₂ reduction to VFAs. The results from this study clearly demonstrate that the CF/rGO/Cc electrode could serve as a conductive element between microbes and bactericidal electrodes with excellent electrochemical properties to enable performance of the MES.

Enhanced product selectivity in the microbial electrosynthesis of butyrate using a nickel ferrite-coated biocathode

Microbial electrosynthesis (MES) is a potential sustainable biotechnology for the efficient conversion of carbon dioxide/bicarbonate into useful chemical commodities. To date, acetate has been the main MES product; selective electrosynthesis to produce other multi-carbon molecules, which have a higher commercial value, remains a major challenge. In this study, the conventional carbon felt (CF) was modified with inexpensive nickel ferrite (NiFe₂O₄@CF) to realize enhanced butyrate production owing to the advantages of improved electrical conductivity, charge transfer efficiency, and microbial-electrode interactions with the selective microbial enrichment. Experimental results show that the modified electrode yielded 1.2 times the butyrate production and 2.7 times the cathodic current production of the CF cathode; product selectivity was greatly improved (from 37% to 95%) in comparison with CF. Microbial community analyses suggest that selective microbial enrichment was promoted as Proteobacteria and Thermotogae (butyrate-producing phyla) were dominant in the NiFe₂O₄@CF biofilm (~78%). These results demonstrate that electrode modification with NiFe₂O₄ can help realize greater selective carboxylate production with improved MES performance. Hence, this technology is expected to be greatly useful in future reactor designs for scaled-up technologies.

A cathodic electro-fermentation system for enhancing butyric acid production from rice straw with a mixed culture

Bio-electrochemical system (BES) emerges as a versatile approach to handling environmental problems with the harvest of sustainable energy and value-added chemicals. To enhance the butyric acid production from rice straw, microbial fuel cell (MFC) and cathodic electro-fermentation (CEF) systems were constructed in this study. Inoculated with the same mixed culture, fermentative butyric acid production efficiency of the two BESs were evaluated with/without neutral red (NR) as electron mediator, respectively. It was found that the butyric acid fermentation efficiency in the MFC system was inefficient. While, the CEF system presented an evident positive effect on butyric acid production. The production and specific yield of butyric acid in the CEF system reached 5.54 g/L and 0.41 g/g, higher than that in the open circuit (OC) system by 17.37% and 28.13%, respectively. Mass percentage of butyric acid in the produced total volatile fatty acids (VFAs) was also increased from 44.74% to 52.76%. The addition of NR had no positive effect on the butyric acid production, due to the low contribution of electric current to the end-products. With the cathode potential of -0.80 V (vs Ag/AgCl), relative abundance of the butyric acid fermenting bacteria (Clostridium cluster IV and cluster XIVa) in the microbial mixture was increased from 20.25% in the OC system to 33.61% in the CEF system. This research work not only presents a novel method for enhancing butyric acid production by rice straw fermentation, but also aids an understanding of the fermentation mechanism in CEF systems.

Electrodeposited Hybrid Biocathode-Based CO2 Reduction via Microbial Electro-Catalysis to Biofuels

Microbial electrosynthesis is a new approach to converting C1 carbon (CO₂) to more complex carbon-based products. In the present study, CO₂, a potential greenhouse gas, was used as a sole carbon source and reduced to value-added chemicals (acetate, ethanol) with the help of bioelectrochemical reduction in microbial electrosynthesis systems (MES). The performance of MES was studied with varying electrode materials (carbon felt, stainless steel, and cobalt electrodeposited carbon felt). The MES performance was assessed in terms of acetic acid and ethanol production with the help of gas chromatography (GC). The electrochemical characterization of the system was analyzed with chronoamperometry and cyclic voltammetry. The study revealed that the MES operated with hybrid cobalt electrodeposited carbon felt electrode yielded the highest acetic acid (4.4 g/L) concentration followed by carbon felt/stainless steel (3.7 g/L), plain carbon felt (2.2 g/L), and stainless steel (1.87 g/L). The alcohol concentration was also observed to be highest for the hybrid electrode (carbon felt/stainless steel/cobalt oxide is 0.352 g/L) as compared to the bare electrodes (carbon felt is 0.22 g/L) tested, which was found to be in correspondence with the pH changes in the system. Electrochemical analysis revealed improved electrotrophy in the hybrid electrode, as confirmed by the increased redox current for the hybrid electrode as compared to plain electrodes. Cyclic voltammetry analysis also confirmed the role of the biocatalyst developed on the electrode in CO₂ sequestration.

Microbial electrosynthesis: is it sustainable for bioproduction of acetic acid?

Microbial electrosynthesis (MES) is an innovative technology for electricity driven microbial reduction of carbon dioxide (CO2) to useful multi-carbon compounds. This study assesses the cradle-to-gate environmental burdens associated with acetic acid (AA) production via MES using graphene functionalized carbon felt cathode. The analysis shows that, though the environmental impact for the production of the functionalized cathode is substantially higher when compared to carbon felt with no modification, the improved productivity of the process helps in reducing the overall impact. It is also shown that, while energy used for extraction of AA is the key environmental hotspot, ion-exchange membrane and reactor medium (catholyte & anolyte) are other important contributors. A sensitivity analysis, describing four different scenarios, considering either continuous or fed-batch operation, is also described. Results show that even if MES productivity can be theoretically increased to match the highest space time yield reported for acetogenic bacteria in a continuous gas fermenter (148 g L-1 d-1), the environmental impact of AA produced using MES systems would still be significantly higher than that produced using a fossil-based process. Use of fed-batch operation and renewable (solar) energy sources do help in reducing the impact, however, the low production rates and overall high energy requirement makes large-scale implementation of such systems impractical. The analysis suggests a minimum threshold production rate of 4100 g m-2 d-1, that needs to be achieved, before MES could be seen as a sustainable alternative to fossil-based AA production.

Magnetite nanoparticle anchored graphene cathode enhances microbial electrosynthesis of polyhydroxybutyrate by Rhodopseudomonas palustris TIE-1

Microbial electrosynthesis (MES) is an emerging technology that can convert carbon dioxide (CO₂) into value-added organic carbon compounds using electrons supplied from a cathode. However, MES is affected by low product formation due to limited extracellular electron uptake by microbes. Herein, a novel cathode was developed from chemically synthesized magnetite nanoparticles and reduced graphene oxide nanocomposite (rGO-MNPs). This nanocomposite was electrochemically deposited on carbon felt (CF/rGO-MNPs), and the modified material was used as a cathode for MES production. The bioplastic, polyhydroxybutyrate (PHB) produced by Rhodopseudomonas palustris TIE-1 (TIE-1), was measured from reactors with modified and unmodified cathodes. Results demonstrate that the magnetite nanoparticle anchored graphene cathode (CF/rGO-MNPs) exhibited higher PHB production (91.31 ± 0.9 mg l^-1). This is ∼4.2 times higher than unmodified carbon felt (CF), and 20 times higher than previously reported using graphite. This modified cathode enhanced electron uptake to -11.7 ± 0.1 μA cm^-2, ∼5 times higher than CF cathode (-2.3 ± 0.08 μA cm^-2). The faradaic efficiency of the modified cathode was ∼2 times higher than the unmodified cathode. Electrochemical analysis and scanning electron microscopy suggest that rGO-MNPs facilitated electron uptake and improved PHB production by TIE-1. Overall, the nanocomposite (rGO-MNPs) cathode modification enhances MES efficiency.