Zinc: A promising material for electrocatalyst-assisted microbial electrosynthesis of carboxylic acids from carbon dioxide

Using a non-precious metal catalyst for long-term enhancement of methane production in a zero-gap microbial electrosynthesis cell

Microbial electrosynthesis (MES) cells exploit the ability of microbes to convert CO2 into valuable chemical products such as methane and acetate, but high rates of chemical production may need to be mediated by hydrogen and thus require a catalyst for the hydrogen evolution reaction (HER). To avoid the usage of precious metal catalysts and examine the impact of the catalyst on the rate of methane generation by microbes on the electrode, we used a carbon felt cathode coated with NiMo/C and compared performance to a bare carbon felt or a Pt/C-deposited cathode. A zero-gap configuration containing a cation exchange membrane was developed to produce a low internal resistance, limit pH changes, and enhance direct transport of H2 to microorganisms on the biocathode. At a fixed cathode potential of -1 V vs Ag/AgCl, the NiMo/C biocathode enabled a current density of 23 ± 4 A/m2 and a high methane production rate of 4.7 ± 1.0 L/L-d. This performance was comparable to that using a precious metal catalyst (Pt/C, 23 ± 6 A/m2, 5.4 ± 2.8 L/L-d), and 3-5 times higher than plain carbon cathodes (8 ± 3 A/m2, 1.0 ± 0.4 L/L-d). The NiMo/C biocathode was operated for over 120 days without observable decay or severe cathode catalyst leaching, reaching an average columbic efficiency of 53 ± 9 % based on methane production under steady state conditions. Analysis of microbial community on the biocathode revealed the dominance of the hydrogenotrophic genus Methanobacterium (∼40 %), with no significant difference found for biocathodes with different materials. These results demonstrated that HER catalysts improved rates of methane generation through facilitating hydrogen gas evolution to an attached biofilm, and that the long-term enhancement of methane production in MES was feasible using a non-precious metal catalyst and a zero-gap cell design.

Redox mediators stimulated chain elongation process in fluidized cathode electro-fermentation systems for caproate production

Medium chain fatty acids (MCFAs), the secondary products of traditional anaerobic fermentation, can be produced via chain elongation (CE), a process often retarded due to the difficulty during interspecies electron transfer (IET). This study employed redox mediators, neutral red (NR), methyl viologen (MV), and methylene blue (MB) as electron shuttles to expedite the electro-fermentation for caproate production by improving IET. Results showed that MV increased the MCFAs production by promoting acetate to ethanol conversion, leading to the highest MCFAs selectivity of 68.73%. While NR was indicated to improve CE by encouraging H2 production, and the biocathode had the highest electrical activity due to the smallest internal resistance and largest capacitance increase of 96% than the control. A higher proportion of Sutterella, Prevotella, and Hydrogenophaga, linked with the H2 mediated interspecies electron transfer (MIET) during CE process, was observed across redox mediators supplied groups compared to the control. The presence of mediators led to an elevated abundance of key enzymes for enhanced CE process and electron transfer. This study provided the perspective of the stimulated electron transfer for improved MCFAs production in electro-fermentation systems.

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.

Engineered nanomaterials for carbon capture and bioenergy production in microbial electrochemical technologies: A review

The mounting threat of global warming, fuelled by industrialization and anthropogenic activities, is undeniable. In 2017, atmospheric carbon dioxide (CO2), the primary greenhouse gas, exceeded 410 ppm for the first time. Shockingly, on April 28, 2023, this figure surged even higher, reaching an alarming 425 ppm. Even though extensive research has been conducted on developing efficient carbon capture and storage technologies, most suffer from high costs, short lifespans, and significant environmental impacts. Recently, the use of engineered nanomaterials (ENM), particularly in microbial electrochemical technologies (METs), has gained momentum owing to their appropriate physicochemical properties and catalytic activity. By implementing ENM, the MET variants like microbial electrosynthesis (MES) and photosynthetic microbial fuel cells (pMFC) can enhance carbon capture efficiency with simultaneous bioenergy production and wastewater treatment. This review provides an overview of ENMs' role in carbon capture within MES and pMFC, highlighting advancements and charting 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.

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.

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.

Clostridium kluyveri enhances caproate production by synergistically cooperating with acetogens in mixed microbial community of electro-fermentation system

As a chain elongation (CE) model strain, Clostridium kluyveri has been used in the studies of bioaugmentation of caproate production. However, its application in the novel electro-fermentation CE system for bioaugmentation is still unclear. In this study, the CE performances, with or without bioaugmentation and in conventional or electro-fermentation systems were compared. And the mechanism of electrochemical-bioaugmentation by constructing a co-culture of Acetobacterium woodii and Clostridium kluyveri were further verified. Results demonstrated that the bioaugmentation treatments have better CE performance, especially in electro-fermentation system, with a highest caproate concentration of 4.68 g·L^-1. Mechanism analysis revealed that C. kluyveri responded to the electric field and emerged synergy with the acetogens, which was proved by the increases of C. kluyveri colonization and the acetogens abundance in biofilm and supported by the co-culture experiment. This study provides a novel insight of microbial synergy mechanism of C. kluyveri during CE bioaugmentation in electro-fermentation system.

Unveiling the bioelectrocatalyzing behaviors and microbial ecological mechanisms behind caproate production without exogenous electron donor

Bioelectrochemical systems were proposed as a promising approach for the efficient valorization of biomass into 6-8 carbon atom medium-chain fatty acids (MCFAs), the precursors for high value-added chemicals or renewable energy, via acetyl-CoA-mediated chain elongation (CE). To achieve CE processes, exogenous electron donors (EDs), e.g., ethanol or lactic acid, were normally prerequisites. This research built a microbial electrolysis cell (MEC) for MCFAs biosynthesis from acetate without exogenous EDs addition. A wide range of applied voltages (0.6-1.2 V) was first employed to investigate the bioelectrocatalyzing response. The results show that caproate and butyrate were the main products formed from acetate under different applied voltages. Maximum caproate concentration (501 ± 12 mg COD/L) was reached at 0.8 V on day 3. Under this applied voltage, hydrogen partial pressure stabilized at about 0.1 bar, beneficial for MCFA production. Electron and carbon balances revealed that the electron-accepting capacity achieved 32% at 0.8 V, showing the highest interspecies electron transfer efficiency. Most of the carbon was recovered in the form of caproate (carbon loss was 9%). MiSeq sequencing revealed Rhodobacter and Clostridium_sensu_stricto playing the crucial role in the biosynthesis of caproate, while Acetobacterium, Acetoanaerobium, and Acetobacter represented the main ED contributors. Four available flora, i.e., homo-acetogen, anaerobic fermentation bacteria, electrode active bacteria, and nitrate-reducing bacteria, interacted and promoted caproate synthesis by molecular ecological network analysis.

Metal nanoparticles increased the lag period and shaped the microbial community in slurry-electrode microbial electrosynthesis

Concerns about energy crisis and CO₂ emission have motivated the development of microbial electrosynthesis (MES); recent studies have showed the potential of novel slurry-electrode MES. In this study, the effect of nonprecious metal nanoparticles (NPs) on the performance of slurry-electrode MES was systematically evaluated in terms of chemical production, physicochemical properties, electrochemical characterization, and microbial community. Ni and Cu NPs increased the lag period from 6 to 15 days for acetate production, while Mo NPs showed no apparent effect. However, these metal NPs slightly affected the final total acetate production (ca. 10 g L^-1), Faradic efficiency (ca. 50%), net water flux across the anion exchange membrane (ca. 6 mL d^-1), or electrochemical characterization of catholyte. BRH-c20a was enriched as the dominated microbe (>48%), and its relative abundance was largely affected by the addition of metal NPs. This study demonstrates that metal NPs affect the performance of biocathodes, mainly by shaping the microbial community.

High-rate microbial electrosynthesis using a zero-gap flow cell and vapor-fed anode design

Microbial electrosynthesis (MES) cells use renewable energy to convert carbon dioxide into valuable chemical products such as methane and acetate, but chemical production rates are low and pH changes can adversely impact biocathodes. To overcome these limitations, an MES reactor was designed with a zero-gap electrode configuration with a cation exchange membrane (CEM) to achieve a low internal resistance, and a vapor-fed electrode to minimize pH changes. Liquid catholyte was pumped through a carbon felt cathode inoculated with anaerobic digester sludge, with humidified N₂ gas flowing over the abiotic anode (Ti or C with a Pt catalyst) to drive water splitting. The ohmic resistance was 2.4 ± 0.5 mΩ m², substantially lower than previous bioelectrochemical systems (20-25 mΩ m²), and the catholyte pH remained near-neutral (6.6-7.2). The MES produced a high methane production rate of 2.9 ± 1.2 L/L-d (748 mmol/m²-d, 17.4 A/m²; Ti/Pt anode) at a relatively low applied voltage of 3.1 V. In addition, acetate was produced at a rate of 940 ± 250 mmol/m²-d with 180 ± 30 mmol/m²-d for propionate. The biocathode microbial community was dominated by the methanogens of the genus Methanobrevibacter, and the acetogen of the genus Clostridium sensu stricto 1. These results demonstrate the utility of this zero-gap cell and vapor-fed anode design for increasing rates of methane and chemical production in MES.

Rechargeable microbial fuel cell based on bidirectional extracellular electron transfer

Rechargeable microbial electrochemical systems can be used as renewable energy storage systems or as potable bioelectronics devices. In this study, a bioelectrode capable of bidirectional extracellular electron transfer was firstly introduced to construct the rechargeable microbial fuel cell (MFC). The performance of rechargeable MFC was enhanced with the increase of charge/discharge cycles, and a maximum energy efficiency of 4.5 ± 0.2% and Coulombic efficiency of 29.4 ± 4.1% were obtained. H₂ was the main charge carrier, while the accumulated acetate was only about 10 mg L^-1. The charge time under constant current mode largely affected the energy recovery. A decreased abundance of Mycobacteria, Geobacter, and Azospirillum, accompanied by an increase of Azonexus and Rhodococcus was observed in the rechargeable MFC, compared to control tests fueled with acetate. This study demonstrates the potential of bioelectrode for energy storage and recovery.

Integrating microbial electrolysis cell based on electrochemical carbon dioxide reduction into anaerobic osmosis membrane reactor for biogas upgrading

It has been reported that anaerobic osmosis membrane bioreactors have the potential for energy recovery since dissolved methane was almost rejected by commercial forward osmosis membranes. Notwithstanding, upgraded biogas has to be achieved by removing as much carbon dioxide as possible. In this study, a novel anaerobic osmotic membrane bioreactor-microbial electrolysis cell (AnOMBR-MEC) system was developed for simultaneous biogas upgrading and wastewater treatment. The AnOMBR-MEC elicited an excellent and stable soluble chemical oxygen demand and phosphorus removal. As the experiment progressed, unwanted carbon dioxide produced from biogas was reduced to formate using a SnO₂ nanoparticles electrocatalytic cathode in an electrocatalytic-assisted MEC, with the highest faradic efficiency of formate being 85% at 1.2V. Compared to AnOMBR, the methane content increased from 55% to 90% at the end of operation and methane yield experienced a1.6-fold increment in the AnOMBR-MEC. Microbial community analysis revealed that hydrogenotrophic methanogens (e.g. Methanobacterium and Methanobrevibacter) converted the produced H₂ and formate to methane at saline conditions. These results have demonstrated an efficient strategy based on the integration of an electrocatalytic-assisted MEC into AnOMBR for upgrading biogas, enhancing methane yield and wastewater treatment.

Recent developments and key barriers to microbial CO2 electrobiorefinery

The electrochemical conversion of CO₂ can include renewable surplus electricity storage and CO₂ utilisation. This review focuses on the microbial CO₂ electrobiorefinery based on microbial electrosynthesis (MES) which merges electrochemical and microbial conversion to produce biofuels and higher-value chemicals. In this review, recent developments are discussed about bioelectrochemical conversion of CO₂ into biofuels and chemicals in MES via microbial CO₂-fixation and electricity utilisation reactions. In addition, this review examines technical approaches to overcome the current limitations of MES including the following: engineering of the biocathode, application of electron mediators, and reactor optimisation, among others. An in-depth discussion of strategies for the CO₂ electrobiorefinery is presented, including the integration of the biocathode with inorganic catalysts, screening of novel electroactive microorganisms, and metabolic engineering to improve target productivity from CO₂.

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.

Purposely Designed Hierarchical Porous Electrodes for High Rate Microbial Electrosynthesis of Acetate from Carbon Dioxide

Carbon-based products are crucial to our society, but their production from fossil-based carbon is unsustainable. Production pathways based on the reuse of CO₂ will achieve ultimate sustainability. Furthermore, the costs of renewable electricity production are decreasing at such a high rate, that electricity is expected to be the main energy carrier from 2040 onward. Electricity-driven novel processes that convert CO₂ into chemicals need to be further developed. Microbial electrosynthesis is a biocathode-driven process in which electroactive microorganisms derive electrons from solid-state electrodes to catalyze the reduction of CO₂ or organics and generate valuable extracellular multicarbon reduced products. Microorganisms can be tuned to high-rate and selective product formation. Optimization and upscaling of microbial electrosynthesis to practical, real life applications is dependent upon performance improvement while maintaining low cost. Extensive biofilm development, enhanced electron transfer rate from solid-state electrodes to microorganisms and increased chemical production rate require optimized microbial consortia, efficient reactor designs, and improved cathode materials. This Account is about the development of different electrode materials purposely designed for improved microbial electrosynthesis: NanoWeb-RVC and EPD-3D. Both types of electrodes are biocompatible, highly conductive three-dimensional hierarchical porous structures. Both chemical vapor deposition (CVD) and electrophoretic deposition were used to grow homogeneous and uniform carbon nanotube layers on the honeycomb structure of reticulated vitreous carbon. The high surface area to volume ratio of these electrodes maximizes the available surface area for biofilm development, i.e., enabling an increased catalyst loading. Simultaneously, the nanostructure makes it possible for a continuous electroactive biofilm to be formed, with increased electron transfer rate and high Coulombic efficiencies. Fully autotrophic biofilms from mixed cultures developed on both types of electrodes rely on CO₂ as the sole carbon source and the solid-state electrode as the unique energy supply. We present first the synthesis and characteristics of the bare electrodes. We then report the outstanding performance indicators of these novel biocathodes: current densities up to -200 A m^-2 and acetate production rates up to 1330 g m^-2 day^-1, with electron and CO₂ recoveries into acetate being very close to 100% for mature biofilms. The performance indicators are still among the highest reported by either purposely designed or commercially available biocathodes. Finally, we made use of the titration and off-gas analysis sensor (TOGA) to elucidate the electron transfer mechanism in these efficient biocathodes. Planktonic cells in the catholyte were found irrelevant for acetate production. We identified the electron transfer to be mediated by biologically induced H₂. H₂ is not detected in the headspace of the reactors, unless CO₂ feeding is interrupted or the cathodes sterilized. Thus, the biofilm is extremely efficient in consuming the generated H₂. Finally, we successfully demonstrated the use of a synthetic biogas mixture as a CO₂ source. We thus proved the potential of microbial electrosynthesis for the simultaneous upgrading of biogas, while fixating CO₂ via the production of acetate.

Microbial electrochemical stimulation of caproate production from ethanol and carbon dioxide

The production of value added chemicals from CO₂ is of critical importance for the practical application of microbial electrosynthesis (MES). Here, a binary electron donor (ED) design (using electrode and ethanol) was introduced to provide an efficient caproate production with the bioconversion of both CO₂ and ethanol. A maximum caproate production rate of 2.41 ± 0.69 g L^-1 d^-1, and a final concentration of 7.66 ± 1.38 g L^-1 was achieved. Caproate production selectivity based on the substrate increased to 91.47 ± 0.58% (Binary EDs) from 32.22 ± 32.58% (open circuit Electrode ED). An observed amount of 23.43 ± 0.69% of carbon within the final binary ED products originated from the CO₂. This work proves for the first time the potential of caproate production from CO₂ utilization and ethanol upgrading using solid electrodes to regulate the chain elongation process.