Salinity-gradient energy driven microbial electrosynthesis of value-added chemicals from CO2 reduction

Genetic tools for the electrotroph Sporomusa ovata and autotrophic biosynthesis

Sporomusa ovata is a Gram-negative acetogen of the Sporomusaceae family with a unique physiology. This anerobic bacterium is a core microbial catalyst for advanced CO2-based biotechnologies including gas fermentation, microbial electrosynthesis, and hybrid photosystem. Until now, no genetic tools exist for S. ovata, which is a critical obstacle to its optimization as an autotrophic chassis and the acquisition of knowledge about its metabolic capacities. Here, we developed an electroporation protocol for S. ovata. With this procedure, it became possible to introduce replicative plasmids such as pJIR751 and its derivatives into the acetogen. This system was then employed to demonstrate the feasibility of heterologous expression by introducing a functional β-glucuronidase enzyme under the promoters of different strengths in S. ovata. Next, a recombinant S. ovata strain producing the non-native product acetone both from an organic carbon substrate and from CO2 was constructed. Finally, a replicative plasmid capable of integrating itself on the chromosome of the acetogen was developed as a tool for genome editing, and gene deletion was demonstrated. These results indicate that S. ovata can be engineered and provides a first-generation genetic toolbox for the optimization of this biotechnological workhorse.IMPORTANCES. ovata harbors unique features that make it outperform most microbes for autotrophic biotechnologies such as a capacity to acquire electrons from different solid donors, a low H2 threshold, and efficient energy conservation mechanisms. The development of the first-generation genetic instruments described in this study is a key step toward understanding the molecular mechanisms involved in these outstanding metabolic and physiological characteristics. In addition, these tools enable the construction of recombinant S. ovata strains that can synthesize a wider range of products in an efficient manner.

Copper foam supported g-C3N4-metal-organic framework bacteria biohybrid cathode catalyst for CO2 reduction in microbial electrosynthesis

Microbial electrosynthesis (MES) presents a versatile approach for efficiently converting carbon dioxide (CO2) into valuable products. However, poor electron uptake by the microorganisms from the cathode severely limits the performance of MES. In this study, a graphitic carbon nitride (g-C3N4)-metal-organic framework (MOF) i.e. HKUST-1 composite was newly designed and synthesized as the cathode catalyst for MES operations. The physiochemical analysis such as X-ray diffraction, scanning electron microscopy (SEM), and X-ray fluorescence spectroscopy showed the successful synthesis of g-C3N4-HKUST-1, whereas electrochemical assessments revealed its enhanced kinetics for redox reactions. The g-C3N4-HKUST-1 composite displayed excellent biocompatibility to develop electroactive biohybrid catalyst for CO2 reduction. The MES with g-C3N4-HKUST-1 biohybrid demonstrated an excellent current uptake of 1.7 mA/cm2, which was noted higher as compared to the MES using g-C3N4 biohybrid (1.1 mA/cm2). Both the MESs could convert CO2 into acetic and isobutyric acid with a significantly higher yield of 0.46 g/L.d and 0.14 g/L.d respectively in MES with g-C3N4-HKUST-1 biohybrid and 0.27 g/L.d and 0.06 g/L.d, respectively in MES with g-C3N4 biohybrid. The findings of this study suggest that g-C3N4-HKUST-1 is a highly efficient catalytic material for biocathodes in MESs to significantly enhance the CO2 conversion.

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.

Boosting hydrogen production from fermentation effluent of biomass wastes in cylindrical single-chamber microbial electrolysis cell

Microbial electrolysis cells (MECs) are considered as green technologies for H₂ production with simultaneously wastewater treatment. Low H₂ recovery and production rate are two key bottlenecks of MECs fed with real H₂ fermentation effluent of biomass wastes. This work established a 1 L cylindrical single chamber MEC and enriched electroactive anodic biofilms from cow dung compost to overcome the bottlenecks. These MEC components (platinum-coated cylindrical titanium mesh cathode and cylindrical graphite felt anode) were arranged in a concentric configuration. A high H₂ production rate of 6.26 ± 0.23 L L^-1 day^-1 with H₂ yield of 5.75 ± 0.16 L H₂ L^-1 fermentation effluent was achieved at 0.8 V. The degradation of acetate and butyrate (main components of H₂ fermentation effluent) could reach to 95.3 ± 2.1% and 78.4 ± 3.6%, respectively. The microbial community analysis for anode showed the abundance of Geobacter (22.6%), Syntrophomonas (8.7%), and Dysgonomonas (6.3%) which are responsible to complex substrate oxidation, electrical current generation, and H₂ production. These results prove the feasibility of cylindrical single-chamber MEC for high H₂ production rate and high acetate and butyrate removal from H₂ fermentation effluent.

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.

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

Sporomusa ovata as Catalyst for Bioelectrochemical Carbon Dioxide Reduction: A Review Across Disciplines From Microbiology to Process Engineering

Sporomusa ovata is a bacterium that can accept electrons from cathodes to drive microbial electrosynthesis (MES) of acetate from carbon dioxide. It is the biocatalyst with the highest acetate production rate described. Here we review the research on S. ovata across different disciplines, including microbiology, biochemistry, engineering, and materials science, to summarize and assess the state-of-the-art. The improvement of the biocatalytic capacity of S. ovata in the last 10 years, using different optimization strategies is described and discussed. In addition, we propose possible electron uptake routes derived from genetic and experimental data described in the literature and point out the possibilities to understand and improve the performance of S. ovata through genetic engineering. Finally, we identify current knowledge gaps guiding further research efforts to explore this promising organism for the MES field.

Advances in cathode designs and reactor configurations of microbial electrosynthesis systems to facilitate gas electro-fermentation

In gas fermentation, a range of chemolithoautotrophs fix single-carbon (C1) gases (CO₂ and CO) when H₂ or other reductants are available. Microbial electrosynthesis (MES) enables CO₂ reduction by generating H₂ or reducing equivalents with the sole input of renewable electricity. A combined approach as gas electro-fermentation is attractive for the sustainable production of biofuels and biochemicals utilizing C1 gases. Various platform compounds such as acetate, butyrate, caproate, ethanol, butanol and bioplastics can be produced. However, technological challenges pertaining to the microbe-material interactions such as poor gas-liquid mass transfer, low biomass and biofilm coverage on cathode, low productivities still exist. We are presenting a review on latest developments in MES focusing on the configuration and design of cathodes that can address the challenges and support the gas electro-fermentation. Overall, the opportunities for advancing CO and CO₂-based biochemicals and biofuels production in MES with suitable cathode/reactor design are prospected.

Nanofluidic osmotic power generators - advanced nanoporous membranes and nanochannels for blue energy harvesting

The increase of energy demand added to the concern for environmental pollution linked to energy generation based on the combustion of fossil fuels has motivated the study and development of new sustainable ways for energy harvesting. Among the different alternatives, the opportunity to generate energy by exploiting the osmotic pressure difference between water sources of different salinities has attracted considerable attention. It is well-known that this objective can be accomplished by employing ion-selective dense membranes. However, so far, the current state of this technology has shown limited performance which hinders its real application. In this context, advanced nanostructured membranes (nanoporous membranes) with high ion flux and selectivity enabling the enhancement of the output power are perceived as a promising strategy to overcome the existing barriers in this technology. While the utilization of nanoporous membranes for osmotic power generation is a relatively new field and therefore, its application for large-scale production is still uncertain, there have been major developments at the laboratory scale in recent years that demonstrate its huge potential. In this review, we introduce a comprehensive analysis of the main fundamental concepts behind osmotic energy generation and how the utilization of nanoporous membranes with tailored ion transport can be a key to the development of high-efficiency blue energy harvesting systems. Also, the document discusses experimental issues related to the different ways to fabricate this new generation of membranes and the different experimental set-ups for the energy-conversion measurements. We highlight the importance of optimizing the experimental variables through the detailed analysis of the influence on the energy capability of geometrical features related to the nanoporous membranes, surface charge density, concentration gradient, temperature, building block integration, and others. Finally, we summarize some representative studies in up-scaled membranes and discuss the main challenges and perspectives of this emerging field.

Bio-electrochemically extracted nitrogen from residual resources for microbial protein production

Upcycling of nutrients from residual resources for producing microbial protein (MP) is an attractive method to valorize residues. In this study, we investigated bio-electrochemical methods to recover ammonia-N, for further production of MP. Reject water and digestate were used for ammonia-N recovery in microbial fuel cell (MFC) system. In one-stage process, ammonia-N recovery was 32 - 42% with 57 - 154 kJ/m³ waste stream of electricity generation. For further enhancing recovery efficiency, a two-stage process was developed, achieving efficiency of 53 - 61%. Subsequently, MP was grown with the extracted ammonia-N, and amino acid concentration was 421 and 272 mg/L under 25 °C and 35 °C, respectively. Similar essential amino acid content of MP (especially under 25 °C) with the one from fish demonstrated the attractiveness of upcycling residues to proteins. Based on simplified economic evaluation, the produced energy performed the potential to catch 1.63 - 6.54 €/m³ waste stream.

Principles of reverse electrodialysis and development of integrated-based system for power generation and water treatment: a review

Reverse electrodialysis (RED) is among the evolving membrane-based processes available for energy harvesting by mixing water with different salinities. The chemical potential difference causes the movement of cations and anions in opposite directions that can then be transformed into the electrical current at the electrodes by redox reactions. Although several works have shown the possibilities of achieving high power densities through the RED system, the transformation to the industrial-scale stacks remains a challenge particularly in understanding the correlation between ion-exchange membranes (IEMs) and the operating conditions. This work provides an overview of the RED system including its development and modifications of IEM utilized in the RED system. The effects of modified membranes particularly on the psychochemical properties of the membranes and the effects of numerous operating variables are discussed. The prospects of combining the RED system with other technologies such as reverse osmosis, electrodialysis, membrane distillation, heat engine, microbial fuel cell), and flow battery have been summarized based on open-loop and closed-loop configurations. This review attempts to explain the development and prospect of RED technology for salinity gradient power production and further elucidate the integrated RED system as a promising way to harvest energy while reducing the impact of liquid waste disposal on the environment.

Pushing microbial desalination cells towards field application: Prevailing challenges, potential mitigation strategies, and future prospects

Microbial desalination cells (MDCs) have been experimentally proven as a versatile bioelectrochemical system (BES). They have the potential to alleviate environmental pollution, reduce water scarcity and save energy and operational costs. However, MDCs alone are inadequate to realise a complete wastewater and desalination treatment at a high-efficiency performance. The assembly of identical MDC units that hydraulically and electrically connected can improve the performance better than standalone MDCs. In the same manner, the coupling of MDCs with other BES or conventional water reclamation technology has also exhibits a promising performance. However, the scaling-up effort has been slowly progressing, leading to a lack of knowledge for guiding MDC technology into practicality. Many challenges remain unsolved and should be mitigated before MDCs can be fully implemented in real applications. Here, we aim to provide a comprehensive chronological-based review that covers technological limitations and mitigation strategies, which have been developed for standalone MDCs. We extend our discussion on how assembled, coupled and scaled-up MDCs have improved in comparison with standalone and lab-scale MDC systems. This review also outlines the prevailing challenges and potential mitigation strategies for scaling-up based on large-scale specifications and evaluates the prospects of selected MDC systems to be integrated with conventional anaerobic digestion (AD) and reverse osmosis (RO). This review offers several recommendations to promote up-scaling studies guided by the pilot scale BES and existing water reclamation technologies.

Formate production from CO2 electroreduction in a salinity-gradient energy intensified microbial electrochemical system

The electricity production of microbial electrochemical system can be substantially strengthened by coupling with a reverse electrodialysis stack which extracts energy from salinity gradient, therefore provides a possible way for value-added products in cathode without external energy input. Here, a microbial reverse-electrodialysis CO₂ reduction cell (MRECC) was developed and successfully utilized to drive CO₂-to-formate conversion on a Bi/Cu cathode. Results confirmed the optimal anodic COD load and cathodic CO₂ flow rate to be 1 g NaAc L^-1 and 10 mL min^-1. MRECC could yielded 143.5 ± 8.1 mg L^-1 of formate with total energy efficiency of 4.6 ± 0.9% and coulombic efficiency of 46.4 ± 2.4%. Increasing or decreasing anode or cathode load impaired MRECC performance from economic and environmental viabilities. MRECC provided a promising platform for simultaneous CO₂ reduction and value-added chemicals production by using sustainable energy from wastewaters.

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.

Electrolytic cell engineering and device optimization for electrosynthesis of e-biofuels via co-valorisation of bio-feedstocks and captured CO2

Utilizing CO2 in an electro-chemical process and synthesizing value-added chemicals are amongst the few viable and scalable pathways in carbon capture and utilization technologies. CO2 electro-reduction is also counted as one of the main options entailing less fossil fuel consumption and as a future electrical energy storage strategy. The current study aims at developing a new electrochemical platform to produce low-carbon e-biofuel through multifunctional electrosynthesis and integrated co-valorisation of biomass feedstocks with captured CO2. In this approach, CO2 is reduced at the cathode to produce drop-in fuels (e.g., methanol) while value-added chemicals (e.g., selective oxidation of alcohols, aldehydes, carboxylic acids and amines/amides) are produced at the anode. In this work, a numerical model of a continuous-flow design considering various anodic and cathodic reactions was built to determine the most techno-economically feasible configurations from the aspects of energy efficiency, environment impact and economical values. The reactor design was then optimized via parametric analysis.

Microbial electrosynthesis feasibility evaluation at high bicarbonate concentrations with enriched homoacetogenic biocathode

An enrichment methodology was developed for a homoacetogenic biocathode that is able to function at high concentrations of bicarbonates for the microbial electrosynthesis (MES) of acetate from carbon dioxide. The study was performed in two stages; enrichment of consortia in serum bottles and the development of a biocathode in MES. A homoacetogenic consortium was sequentially grown under increasing concentrations of bicarbonate, in serum bottles, at room temperature. The acetate production rate was found to increase with the increase in the bicarbonate concentration and evidenced a maximum production rate of 260 mg/L d^-1 (15 g HCO₃^-/L). On the contrary, carbon conversion efficiency decreased with the increase in the bicarbonate concentration, which evidenced a maximum at 2.5 g HCO₃^-/L (90.16%). Following a further increase in the bicarbonate concentration up to 20 g HCO₃^-/L, a visible inhibition was registered with respect to the acetate production rate and the carbon conversion efficiency. Well adapted biomass from 15 g HCO₃^-/L was used to develop biocathodic catalyst for MES. An effective biocathode was developed after 4 cycles of operation, during which acetate production was improved gradually, evidencing a maximum production rate of 24.53 mg acetate L^-1 d^-1 (carbon conversion efficiency, 47.72%). Compared to the enrichment stage, the carbon conversion efficiency and the rate of acetate production in MES were found to be low. The production of acetate induced a change in the catholyte pH, from neutral conditions towards acidic conditions.

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.

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

Microbial electrosynthesis (MES) has been proposed as a sustainable platform to simultaneously achieve wastewater treatment, renewable energy generation and chemicals production. Currently, the CO₂ valorization via MES is restricted by the low production rate, while that via electrochemical reduction is limited by the production of C1 products with high efficiency and selectivity. The electrocatalyst-assisted MES could potentially solve these bottlenecks of both MES and electrochemical reduction technology by increasing the production rate and expanding the product range. Here, four types of metals were evaluated for mixed culture-based, electrocatalyst-assisted MES with the fabrication of electrical-biological hybrid cathodes. Cathodes based on In, Zn, Ti and Cu showed high parallelism at 30 A/m². However, no parallelism was observed at 50 A/m², and only Zn experienced a further increase of the maximum acetic acid production rate (1.23 ± 0.02 g/L/d, 313 ± 5 g/m²/d) and titer (9.2 ± 0.1 g/L), with the highest value of the production rate normalized to the project area of the fiber cathodes. Other volatile fatty acids and ethanol were below 0.5 g/L. Moreover, it was the sharp H₂ generation, which mainly caused the fluctuation of coulombic efficiency. The application of such Zn-based electrical-biological hybrid system shall provide a more efficient route for CO₂ valorization.

Carbon dioxide and organic waste valorization by microbial electrosynthesis and electro-fermentation

Carbon-rich waste materials (solid, liquid, or gaseous) are largely considered to be a burden on society due to the large capital and energy costs for their treatment and disposal. However, solid and liquid organic wastes have inherent energy and value, and similar as waste CO₂ gas they can be reused to produce value-added chemicals and materials. There has been a paradigm shift towards developing a closed loop, biorefinery approach for the valorization of these wastes into value-added products, and such an approach enables a more carbon-efficient and circular economy. This review quantitatively analyzes the state-of-the-art of the emerging microbial electrochemical technology (MET) platform and provides critical perspectives on research advancement and technology development. The review offers side-by-side comparison between microbial electrosynthesis (MES) and electro-fermentation (EF) processes in terms of principles, key performance metrics, data analysis, and microorganisms. The study also summarizes all the processes and products that have been developed using MES and EF to date for organic waste and CO₂ valorization. It finally identifies the technological and economic potentials and challenges on future system development.

Expanding the product spectrum of value added chemicals in microbial electrosynthesis through integrated process design-A review

Microbial electrosynthesis (MES) is a novel microbial electrochemical technology proposed for chemicals production with the storage of sustainable energy. However, the practical application of MES is currently restricted by the limited low market value of products in one-step conversion process, mostly acetate. A theme that is pervasive throughout this review is the challenges associated with the expanded product spectrum. Several recent research efforts to improve acetate production, using novel reactor configuration, renewable power supply, and various 3-D cathode are summarized. The importance of genetic modification, two-step hybrid process, as well as input substrates other than CO₂ are highlighted in this review as the future research paths for higher value chemicals production. At last, how to integrate MES with existing biochemicals processes is proposed. Definitely, more studies are encouraged to evaluate the overall performances and economic efficiency of these integrated process designs to make MES more competitive.