Microbiome for the Electrosynthesis of Chemicals from Carbon Dioxide

Recent advances in electrochemical 1,2-difunctionalization of alkenes: mechanisms and perspectives

In recent years, significant achievements have been made in the field of electroorganic chemistry regarding the difunctionalization of alkenes. Researchers have developed innovative strategies utilizing the unique reactivity of electrochemical processes to synthesize complex molecules with high regioselectivity and stereoselectivity. This technology is widely applied in the total synthesis of natural products and in the pharmaceutical industry. This article reviews the research progress in the electrochemical difunctionalization of alkenes through three different radical-mediated pathways over the past five years. It includes discussions on 1,2-stereoselective and non-diastereoselective difunctionalization reactions, rearrangements, intramolecular migrations, and cyclization processes. The summary emphasizes innovative electrode designs, reaction mechanisms, and the integration with other emerging technologies, highlighting the potential of this method in modern organic chemistry. Additionally, it aims to address current challenges and propose possible solutions, providing a promising direction for electrochemically mediated difunctionalization reactions of alkenes.

Improved reactor design enables productivity of microbial electrosynthesis on par with classical biotechnology

Microbial electrosynthesis (MES) converts (renewable) electrical energy into CO2-derived chemicals including fuels. To achieve commercial viability of this process, improvements in production rate, energy efficiency, and product titer are imperative. Employing a compact plate reactor with zero gap anode configuration and NiMo-plated reticulated vitreous carbon cathodes substantially improved electrosynthesis rates of methane and acetic acid. Electromethanogenesis rates exceeded 10 L L-1catholyte d-1 using an undefined mixed culture. Continuous thermophilic MES by Thermoanaerobacter kivui produced acetic acid at a rate of up to 3.5 g L-1catholyte h-1 at a titer of 14 g/L, surpassing continuous gas fermentation without biomass retention and on par with glucose fermentation by T. kivui in chemostats. Coulombic efficiencies reached 80 %-90 % and energy efficiencies up to 30 % for acetate and methane production. The performance of this plate reactor demonstrates that MES can deliver production rates that are competitive with those of established biotechnologies.

Potential and challenge in accelerating high-value conversion of CO2 in microbial electrosynthesis system via data-driven approach

Microbial electrosynthesis for CO2 utilization (MESCU) producing valuable chemicals with high energy density has garnered attention due to its long-term stability and high coulombic efficiency. The data-driven approaches offer a promising avenue by leveraging existing data to uncover the underlying patterns. This comprehensive review firstly uncovered the potentials of utilizing data-driven approaches to enhance high-value conversion of CO2 via MESCU. Firstly, critical challenges of MESCU advancing have been identified, including reactor configuration, cathode design, and microbial analysis. Subsequently, the potential of data-driven approaches to tackle the corresponding challenges, encompassing the identification of pivotal parameters governing reactor setup and cathode design, alongside the decipheration of omics data derived from microbial communities, have been discussed. Correspondingly, the future direction of data-driven approaches in assisting the application of MESCU has been addressed. This review offers guidance and theoretical support for future data-driven applications to accelerate MESCU research and potential industrialization.

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.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

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.

Electrosynthesis of Spiro-indolenines via Dearomative Arylation of Indoles in Batch and Continuous Flow

An electrosynthesis of spiro-indolenines in batch and continuous flow was achieved through dearomative arylation of indoles with good functional group compatibility. User-friendly undivided cells were used under catalyst- and oxidant-free conditions. Moreover, the use of a flow electrolysis cell gave high daily productivity and excellent scale-up potential under less supporting electrolyte and higher substrate concentration conditions.

Microbial electrosynthesis of acetate from CO2 in three-chamber cells with gas diffusion biocathode under moderate saline conditions

The industrial adoption of microbial electrosynthesis (MES) is hindered by high overpotentials deriving from low electrolyte conductivity and inefficient cell designs. In this study, a mixed microbial consortium originating from an anaerobic digester operated under saline conditions (∼13 g L^-1 NaCl) was adapted for acetate production from bicarbonate in galvanostatic (0.25 mA cm^-2) H-type cells at 5, 10, 15, or 20 g L^-1 NaCl concentration. The acetogenic communities were successfully enriched only at 5 and 10 g L^-1 NaCl, revealing an inhibitory threshold of about 6 g L^-1 Na⁺. The enriched planktonic communities were then used as inoculum for 3D printed, three-chamber cells equipped with a gas diffusion biocathode. The cells were fed with CO₂ gas and operated galvanostatically (0.25 or 1.00 mA cm^-2). The highest production rate of 55.4 g m^-2 d^-1 (0.89 g L^-1 d^-1), with 82.4% Coulombic efficiency, was obtained at 5 g L^-1 NaCl concentration and 1 mA cm^-2 applied current, achieving an average acetate production of 44.7 kg MWh^-1. Scanning electron microscopy and 16S rRNA sequencing analysis confirmed the formation of a cathodic biofilm dominated by Acetobacterium sp. Finally, three 3D printed cells were hydraulically connected in series to simulate an MES stack, achieving three-fold production rates than with the single cell at 0.25 mA cm^-2. This confirms that three-chamber MES cells are an efficient and scalable technology for CO₂ bio-electro recycling to acetate and that moderate saline conditions (5 g L^-1 NaCl) can help reduce their power demand while preserving the activity of acetogens.

Electricity-Driven Microbial Metabolism of Carbon and Nitrogen: A Waste-to-Resource Solution

Electricity-driven microbial metabolism relies on the extracellular electron transfer (EET) process between microbes and electrodes and provides promise for resource recovery from wastewater and industrial discharges. Over the past decades, tremendous efforts have been dedicated to designing electrocatalysts and microbes, as well as hybrid systems to push this approach toward industrial adoption. This paper summarizes these advances in order to facilitate a better understanding of electricity-driven microbial metabolism as a sustainable waste-to-resource solution. Quantitative comparisons of microbial electrosynthesis and abiotic electrosynthesis are made, and the strategy of electrocatalyst-assisted microbial electrosynthesis is critically discussed. Nitrogen recovery processes including microbial electrochemical N₂ fixation, electrocatalytic N₂ reduction, dissimilatory nitrate reduction to ammonium (DNRA), and abiotic electrochemical nitrate reduction to ammonia (Abio-NRA) are systematically reviewed. Furthermore, the synchronous metabolism of carbon and nitrogen using hybrid inorganic-biological systems is discussed, including advanced physicochemical, microbial, and electrochemical characterizations involved in this field. Finally, perspectives for future trends are presented. The paper provides valuable insights on the potential contribution of electricity-driven microbial valorization of waste carbon and nitrogen toward a green and sustainable society.

Deciphering the role and mechanism of nano zero-valent iron on medium chain fatty acids production from CO2 via chain elongation in microbial electrosynthesis

The integrated system of microbial electrosynthesis (MES) coupled with chain elongation has been considered a promising platform for carboxylic acids production. However, this biotechnology is still in its infancy, and many limitations are needed to be transcended, such as low electron transfer efficiency between cathode and microbes. In this study, nano zero-valent iron (NZVI) was employed to improve carboxylic acid production in the integrated system, and the promotion mechanisms were revealed. Results suggested that the highest production concentrations of acetate, butyrate, and caproate were observed at 7.5 g/L optimized NZVI dosage, increasing the total yield and coulomb efficiency by 23.7 % and 40.3 % compared to the control. Mechanism studies indicated that the hydrogen and electron released by the anaerobic corrosion of NZVI could be used as additional reducing equivalents, thereby enhancing the electron transfer performance. Besides, NZVI was also proven to facilitate the formation of electroactive biofilms according to the results of biofilm characterization and total DNA. In functional microbes' respect, the moderate NZVI enriched the chain elongator in biofilm, like Clostridium_sensu-stricto_12, and upregulated the activities of key enzymes of homoacetogenesis and chain elongation metabolic pathways, like carbon-monoxide dehydrogenase and hydroxyacyl-CoA dehydratase. This study provided the evidence and revealed how NZVI assisted carboxylic acid production from CO₂ via chain elongation in MES.

Recent Advances In Microbe-Photocatalyst Hybrid Systems for Production of Bulk Chemicals: A Review

Solar-driven biocatalysis technologies can combine inorganic photocatalytic materials with biological catalysts to convert CO₂, light, and water into chemicals, offering the promise of high energy efficiency and a broader product scope than that of natural photosynthesis. Solar energy is the most abundant renewable energy source on earth, but it cannot be directly utilized by current industrial microorganisms. Therefore, the establishment of a solar-driven bio-catalysis platform, a bridge between solar energy and heterotrophic microorganisms, can dramatically increase carbon flux in biomanufacturing systems and consequently may revolutionize the biorefinery. This review first discusses the main applications of microbe-photocatalyst hybrid (MPH) systems in biorefinery processes. Then, various strategies to improve the electron transfer by microorganisms at the inorganic photocatalytic material interface are discussed, especially biohybrid systems based on autotrophic or heterotrophic bacteria and photocatalytic materials. Finally, we discuss the current challenges and offer potential solutions for the development of MPH systems.

Microbial electrosynthesis of acetate from CO2 under hypersaline conditions

Microbial electrosynthesis (MES) enables the bioproduction of multicarbon compounds from CO₂ using electricity as the driver. Although high salinity can improve the energetic performance of bioelectrochemical systems, acetogenic processes under elevated salinity are poorly known. Here MES under 35-60 g L^-1 salinity was evaluated. Acetate production in two-chamber MES systems at 35 g L^-1 salinity (seawater composition) gradually decreased within 60 days, both under -1.2 V cathode potential (vs. Ag/AgCl) and -1.56 A m^-2 reductive current. Carbonate precipitation on cathodes (mostly CaCO₃) likely declined the production through inhibiting CO₂ supply, the direct electrode contact for acetogens and H₂ production. Upon decreasing Ca²⁺ and Mg²⁺ levels in three-chamber reactors, acetate was stably produced over 137 days along with a low cathode apparent resistance at 1.9 ± 0.6 mΩ m² and an average production rate at 3.80 ± 0.21 g m^-2 d^-1. Increasing the salinity step-wise from 35 to 60 g L^-1 gave the most efficient acetate production at 40 g L^-1 salinity with average rates of acetate production and CO₂ consumption at 4.56 ± 3.09 and 7.02 ± 4.75 g m^-2 d^-1, respectively. The instantaneous coulombic efficiency for VFA averaged 55.1 ± 31.4%. Acetate production dropped at higher salinity likely due to the inhibited CO₂ dissolution and acetogenic metabolism. Acetobacterium up to 78% was enriched on cathodes as the main acetogen at 35 g L^-1. Under high-salinity selection, 96.5% Acetobacterium dominated on the cathode along with 34.0% Sphaerochaeta in catholyte. This research provides a first proof of concept that MES starting from CO₂ reduction can be achieved at elevated salinity.

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.

Microbial Electrosynthesis of Acetate Powered by Intermittent Electricity

Microbial electrosynthesis (MES) of acetate is a process using electrical energy to reduce CO₂ to acetic acid in an integrated bioelectrochemical system. MES powered by excess renewable electricity produces carbon-neutral acetate while benefitting from inexpensive but intermittent energy sources. Interruptions in electricity supply also cause energy limitation and starvation of the microbial cells performing MES. Here, we studied the effect of intermittent electricity supply on the performance of hydrogen-mediated MES of acetate. Thermoanaerobacter kivui produced acetic acid for more than 4 months from intermittent electricity supplied in 12 h on-off cycles in a semicontinuously-fed MES system. After current interruptions, hydrogen utilization and acetate synthesis rates were severely diminished. They did not recover to the steady-state rates of continuous MES within the 12 h current-on period under most conditions. Accumulating high product (acetate) concentration exacerbated this effect and prolonged recovery. However, supply of a low background current of 1-5% of the maximum current during "off-times" reduced the impact of current interruptions on subsequent MES performance. This study presents sustained MES at a rate of up to 2 mM h^-1 acetate at an average concentration of 60-90 mM by a pure thermophilic microbial culture powered by intermittent electricity. We identified product inhibition of accumulating acetic acid as a key challenge to improving the efficiency of intermittently powered MES.

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

Photosynthetic biohybrid coculture for tandem and tunable CO2 and N2 fixation

Solar-driven bioelectrosynthesis represents a promising approach for converting abundant resources into value-added chemicals with renewable energy. Microorganisms powered by electrochemical reducing equivalents assimilate CO2, H2O, and N2 building blocks. However, products from autotrophic whole-cell biocatalysts are limited. Furthermore, biocatalysts tasked with N2 reduction are constrained by simultaneous energy-intensive autotrophy. To overcome these challenges, we designed a biohybrid coculture for tandem and tunable CO2 and N2 fixation to value-added products, allowing the different species to distribute bioconversion steps and reduce the individual metabolic burden. This consortium involves acetogen Sporomusa ovata, which reduces CO2 to acetate, and diazotrophic Rhodopseudomonas palustris, which uses the acetate both to fuel N2 fixation and for the generation of a biopolyester. We demonstrate that the coculture platform provides a robust ecosystem for continuous CO2 and N2 fixation, and its outputs are directed by substrate gas composition. Moreover, we show the ability to support the coculture on a high-surface area silicon nanowire cathodic platform. The biohybrid coculture achieved peak faradaic efficiencies of 100, 19.1, and 6.3% for acetate, nitrogen in biomass, and ammonia, respectively, while maintaining product tunability. Finally, we established full solar to chemical conversion driven by a photovoltaic device, resulting in solar to chemical efficiencies of 1.78, 0.51, and 0.08% for acetate, nitrogenous biomass, and ammonia, correspondingly. Ultimately, our work demonstrates the ability to employ and electrochemically manipulate bacterial communities on demand to expand the suite of CO2 and N2 bioelectrosynthesis products.

Direct utilization of industrial carbon dioxide with low impurities for acetate production via microbial electrosynthesis

The present study aimed to demonstrate the utilization of unpurified industrial CO₂ with low impurities for acetate production via microbial electrosynthesis (MES) for the first time. In MES experiments with CO₂-rich brewery gas, the enriched mixed culture dominated by Acetobacterium produced 1.8 ± 0.2 g/L acetic acid at 0.26 ± 0.03 g/L_catholyte/d rate and outperformed a pure culture of Clostridium ljungdahlii (1.1 ± 0.02 g/L; 0.138 ± 0.004 g/L_catholyte/d). The electron recovery in acetic acid was also more for mixed culture (84 ± 13%) than C. ljungdahlii (42 ± 14%). Electrochemical analysis of biocathodes suggested the role of microbial biofilm in improved hydrogen electrocatalysis. In comparative gas fermentation tests, the mixed culture outperformed C. ljungdahlii and produced acetic acid at a similar level with both industrial and pure CO₂ feedstocks. These results suggest the robustness and capability of the mixed microbial community for utilizing slightly impure industrial CO₂ for bioproduction and presents a major advancement in MES technology.

Mitochondria at Work: New Insights into Regulation and Dysregulation of Cellular Energy Supply and Metabolism

Mitochondria are of great relevance to health, and their dysregulation is associated with major chronic diseases. Research on mitochondria-156 brand new publications from 2019 and 2020-have contributed to this review. Mitochondria have been fundamental for the evolution of complex organisms. As important and semi-autonomous organelles in cells, they can adapt their function to the needs of the respective organ. They can program their function to energy supply (e.g., to keep heart muscle cells going, life-long) or to metabolism (e.g., to support hepatocytes and liver function). The capacity of mitochondria to re-program between different options is important for all cell types that are capable of changing between a resting state and cell proliferation, such as stem cells and immune cells. Major chronic diseases are characterized by mitochondrial dysregulation. This will be exemplified by cardiovascular diseases, metabolic syndrome, neurodegenerative diseases, immune system disorders, and cancer. New strategies for intervention in chronic diseases will be presented. The tumor microenvironment can be considered a battlefield between cancer and immune defense, competing for energy supply and metabolism. Cancer cachexia is considered as a final stage of cancer progression. Nevertheless, the review will present an example of complete remission of cachexia via immune cell transfer. These findings should encourage studies along the lines of mitochondria, energy supply, and metabolism.

A slurry electrode integrated with membrane electrolysis for high-performance acetate production in microbial electrosynthesis

Microbial electrosynthesis (MES) technology employs electrotrophic microbes as biocatalysts to produce chemicals from CO₂. The application of a slurry electrode can enlarge the surface area to volume ratio, and membrane electrolysis (ME) for on-line extraction can solve the problem of product inhibition. This study constructed a novel dual-chamber ME-MES integrated system equipped with a slurry electrode, and the effect of concentration of powder-activated carbon (AC) in the catholyte on chemical production was also evaluated. The integrated system amended with 5 g L^-1 AC produced up to 13.4 g L^-1 acetate, showing a 179% increase compared with the control group without AC (4.8 g L^-1). However, further increasing the AC concentration to 10 and 20 g L^-1 resulted in decreased acetate production. A high concentration of AC showed higher antimicrobial activity to methanogens, as compared to acetogens. Amending AC exacerbated the process of electroosmosis. Also, amending AC with 0 to 10 g L^-1 decreased the electrochemical losses via both the membrane and electrolyte. The chemical production using H₂ or the electrode as electron donors showed a similar trend when amending AC. The present study provided important information for guiding future research to construct an efficient configuration of an MES bioreactor.

Parameters influencing the development of highly conductive and efficient biofilm during microbial electrosynthesis: the importance of applied potential and inorganic carbon source

Cathode-driven applications of bio-electrochemical systems (BESs) have the potential to transform CO2 into value-added chemicals using microorganisms. However, their commercialisation is limited as biocathodes in BESs are characterised by slow start-up and low efficiency. Understanding biosynthesis pathways, electron transfer mechanisms and the effect of operational variables on microbial electrosynthesis (MES) is of fundamental importance to advance these applications of a system that has the capacity to convert CO2 to organics and is potentially sustainable. In this work, we demonstrate that cathodic potential and inorganic carbon source are keys for the development of a dense and conductive biofilm that ensures high efficiency in the overall system. Applying the cathodic potential of -1.0 V vs. Ag/AgCl and providing only gaseous CO2 in our system, a dense biofilm dominated by Acetobacterium (ca. 50% of biofilm) was formed. The superior biofilm density was significantly correlated with a higher production yield of organic chemicals, particularly acetate. Together, a significant decrease in the H2 evolution overpotential (by 200 mV) and abundant nifH genes within the biofilm were observed. This can only be mechanistically explained if intracellular hydrogen production with direct electron uptake from the cathode via nitrogenase within bacterial cells is occurring in addition to the commonly observed extracellular H2 production. Indeed, the enzymatic activity within the biofilm accelerated the electron transfer. This was evidenced by an increase in the coulombic efficiency (ca. 69%) and a 10-fold decrease in the charge transfer resistance. This is the first report of such a significant decrease in the charge resistance via the development of a highly conductive biofilm during MES. The results highlight the fundamental importance of maintaining a highly active autotrophic Acetobacterium population through feeding CO2 in gaseous form, which its dominance in the biocathode leads to a higher efficiency of the system.

Microbial electrosynthesis from CO2: Challenges, opportunities and perspectives in the context of circular bioeconomy

Recycling CO₂ into organic products through microbial electrosynthesis (MES) is attractive from the perspective of circular bioeconomy. However, several challenges need to be addressed before scaling-up MES systems. In this review, recent advances in electrode materials, microbe-catalyzed CO₂ reduction and MES energy consumption are discussed in detail. Anode materials are briefly reviewed first, with several strategies proposed to reduce the energy input for electron generation and enhance MES bioeconomy. This was followed by discussions on MES cathode materials and configurations for enhanced chemolithoautotroph growth and CO₂ reduction. Various chemolithoautotrophs, effective for CO₂ reduction and diverse bioproduct formation, on MES cathode were also discussed. Finally, research efforts on developing cost-effective process for bioproduct extraction from MES are presented. Future perspectives to improve product formation and reduce energy cost are discussed to realize the application of the MES as a chemical production platform in the context of building a circular economy.