In situ acetate separation in microbial electrosynthesis from CO2 using ion-exchange resin

ZnMo-MOF as anti-CO hydrogen electrocatalyst enhance microbial electrosynthesis for CO/CO2 conversion

Microbial electrosynthesis (MES) is an electrically driven technology that can be used for converting CO/CO2 into chemicals. The unique electronic and substrate properties of CO make it an important research target for MES. However, CO can poison the cathode and increase the overpotential of hydrogen evolution reaction (HER), thus reducing the electron transfer rate via H2. This work evaluated the effect of an anti-CO HER catalyst on the performance of MES for CO/CO2 conversion. ZnMo-metal-organic framework (MOF) materials with different calcination temperatures were synthesized. ZnMo-MOF-800 with Mo2C nanoparticles as active centers exhibited excellent resistance to CO toxicity. It also obtained the highest hydrogen evolution and enhanced electron transfer rate in CO atmosphere. MES with ZnMo-MOF-800 cathode and Clostridium ljungdahlii as biocatalyst obtained 0.31 g L-1 d-1 acetate yield, 0.1 g L-1 d-1 butyrate yield, and 0.09 g L-1 d-1 2,3-butanediol yield in CO/CO2, while Pt/C only get 0.076 g L-1 d-1 acetate yield, 0.05 g L-1 d-1 butyrate yield and 0.02 g L-1 d-1 2,3-butanediol yield. ZnMo-MOF-800 was conducive to biofilm formation, enabling it to better resist CO toxicity. This work provides new opportunities for constructing a highly efficient cathode with an anti-CO hydrogen evolution catalyst to enhance CO/CO2 conversion in MES.

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.

Selective Extraction of Medium-Chain Carboxylic Acids by Electrodialysis and Phase Separation

Carboxylic acids obtained via the microbial electrochemical conversion of waste gases containing carbon dioxide (i.e., microbial electrosynthesis) can be used in lieu of nonrenewable building-block chemicals in the manufacture of a variety of products. When targeting valuable medium-chain carboxylic acids such as caproic acid, electricity-driven fermentations can be limited by the accumulation of fermentation products in the culturing media, often resulting in low volumetric productivities and titers due to direct toxicity or inhibition of the biocatalyst. In this study, we tested the effectiveness of a simple electrodialysis system in upconcentrating carboxylic acids from a model solution mimicking the effluent of a microbial electrochemical system producing short- and medium-chain carboxylic acids. Under batch extraction conditions, the electrodialysis scheme enabled the recovery of 60% (mol mol-1) of the total carboxylic acids present in the model fermentation broth. The particular arrangement of conventional monopolar ion exchange membranes and hydraulic recirculation loops allowed the progressive acidification of the extraction solution, enabling phase separation of caproic acid as an immiscible oil with 76% purity.

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.

Microbial electrosynthesis: Towards sustainable biorefineries for production of green chemicals from CO2 emissions

Decarbonisation of the economy has become a priority at the global level, and the resulting legislative pressure is pushing the chemical and energy industries away from fossil fuels. Microbial electrosynthesis (MES) has emerged as a promising technology to promote this transition, which will further benefit from the decreasing cost of renewable energy. However, several technological challenges need to be addressed before the MES technology can reach its maturity. The aim of this review is to critically discuss the bottlenecks hampering the industrial adoption of MES, considering the whole production process (from the CO₂ source to the marketable products), and indicate future directions. A flexible stack design, with flat or tubular MES modules and direct CO₂ supply, is required for site-specific decentralised applications. The experience gained for scaling-up electrochemical cells (e.g. electrolysers) can serve as a guideline for realising pilot MES stacks to be technologically and economically evaluated in industrially relevant conditions. Maximising CO₂ abatement rate by targeting high-rate production of acetate can promote adoption of MES technology in the short term. However, the development of a replicable and robust strategy for production and in-line extraction of higher-value products (e.g. caproic acid and hexanol) at the cathode, and meaningful exploitation of the currently overlooked anodic reactions, can further boost MES cost-effectiveness. Furthermore, the use of energy storage and smart electronics can alleviate the fluctuations of renewable energy supply. Despite the unresolved challenges, the flexible MES technology can be applied to decarbonise flue gas from different sources, to upgrade industrial and wastewater treatment plants, and to produce a wide array of green and sustainable chemicals. The combination of these benefits can support the industrial adoption of MES over competing technologies.

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.

Perovskite-Based Multifunctional Cathode with Simultaneous Supplementation of Substrates and Electrons for Enhanced Microbial Electrosynthesis of Organics

Microbial electrosynthesis (MES) is an electricity-driven technology for the microbial reduction of CO₂ to organic commodities. However, the limited solubility of CO₂ in a solution and the inefficient electron transfer make it impossible for microorganisms to obtain an efficient surface for catalytic interaction, thus resulting in the low efficiency of MES. To address this, we introduce a multifunctional perovskite-based cathode material Pr_0.5(Ba_0.5Sr_0.5)_0.5Co_0.8Fe_0.2O_3-δ-carbon felt (Pr_0.5BSCF-CF), which provides a simultaneously significant increase in CO₂ absorption and hydrogen production. As a result, the volumetric acetate production rate of MES obtained by Pr_0.5BSCF-CF is 0.24 ± 0.01 g L^-1 day^-1, and it achieves a maximum acetate titer of 13.74 ± 0.20 g L^-1 within 70 days. An adequate supply of CO₂ and H₂ also provides a sufficient amount of substrates and energy for the self-replication of the biocatalysts in the MES reactor. This effect not only increases the amount of biocatalysts but also optimizes the functions of the biocatalysts; the above benefits further improve the production efficiency of the MES system. This strategy demonstrates that the development of perovskite-based multifunctional cathodes with a simultaneous supplementation of substrates and electrons is a promising approach toward improving the MES efficiency.

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.

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.

Mo2C-induced hydrogen production enhances microbial electrosynthesis of acetate from CO2 reduction

BACKGROUND

Microbial electrosynthesis (MES) is a biocathode-driven process, in which electroautotrophic microorganisms can directly uptake electrons or indirectly via H₂ from the cathode as energy sources and CO₂ as only carbon source to produce chemicals.

RESULTS

This study demonstrates that a hydrogen evolution reaction (HER) catalyst can enhance MES performance. An active HER electrocatalyst molybdenum carbide (Mo₂C)-modified electrode was constructed for MES. The volumetric acetate production rate of MES with 12 mg cm^-2 Mo₂C was 0.19 ± 0.02 g L^-1 day^-1, which was 2.1 times higher than that of the control. The final acetate concentration reached 5.72 ± 0.6 g L^-1 within 30 days, and coulombic efficiencies of 64 ± 0.7% were yielded. Furthermore, electrochemical study, scanning electron microscopy, and microbial community analyses suggested that Mo₂C can accelerate the release of hydrogen, promote the formation of biofilms and regulate the mixed microbial flora.

CONCLUSION

Coupling a HER catalyst to a cathode of MES system is a promising strategy for improving MES efficiency.

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.

Fe3O4/granular activated carbon as an efficient three-dimensional electrode to enhance the microbial electrosynthesis of acetate from CO2

Microbial electrosynthesis (MES) allows the transformation of CO₂ into value-added products by coupling with renewable energy.

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.

Fluidized granular activated carbon electrode for efficient microbial electrosynthesis of acetate from carbon dioxide

The electricity-driven bioreduction of carbon dioxide to multi-carbon organic compounds, particularly acetate, has been achieved in microbial electrosynthesis (MES). MES performance can be limited by the amount of cathode surface area available for biofilm formation and slow substrate mass transfer. Here, a fluidized three-dimensional electrode, containing granular activated carbon (GAC) particles, was constructed via MES. The volumetric acetate production rate increased by 2.8 times through MES with 16 g L^-1 GAC (0.14 g L^-1 d^-1) compared with that of the control (no GAC), and the final acetate concentration reached 3.92 g L^-1 within 24 days. Electrochemical, scanning electron microscopy, and microbial community analyses suggested that GAC might improve the performance of MES by accelerating direct and indirect (via H₂) electron transfer because GAC could provide a high electrode surface and a favorable mass transport. This study attempted to improve the efficiency of MES and presented promising opportunities for MES scale-up.

Separation of Acetate Produced from C1 Gas Fermentation Using an Electrodialysis-Based Bioelectrochemical System

The conversion of C1 gas feedstock, such as carbon monoxide (CO), to useful platform chemicals has attracted considerable interest in industrial biotechnology. One conversion method is electrode-based electron transfer to microorganisms using bioelectrochemical systems (BESs). In this BES system, acetate is the predominant component of various volatile fatty acids (VFAs). To appropriately separate and concentrate the acetate produced, a BES-type electrodialysis cell with an anion exchange membrane was constructed and evaluated under various operational conditions, such as applied external current, acetate concentration, and pH. A high acetate flux of 23.9 mmol/m2∙h was observed under a −15 mA current in an electrodialysis-based bioelectrochemical system. In addition, the initial acetate concentration affected the separation efficiency and transportation rate. The maximum flux appeared at 48.6 mmol/m2∙h when the acetate concentration was 100 mM, whereas the effects of the initial pH of the anolyte were negligible. The acetate flux was 14.9 mmol/m2∙h when actual fermentation broth from BES-based CO fermentation was used as a catholyte. A comparison of the synthetic broth with the actual fermentation broth suggests that unknown substances and metabolites produced from the previous bioconversion process interfere with electrodialysis. These results provide information on the optimal conditions for the separation of VFAs produced by C1 gas fermentation through electrodialysis and a combination of a BES and electrodialysis.

Efficiently "pumping out" value-added resources from wastewater by bioelectrochemical systems: A review from energy perspectives

Bioelectrochemical systems (BES) can accomplish simultaneous wastewater treatment and resource recovery via interactions between microbes and electrodes. Often deemed as "energy efficient" technologies, BES have not been well evaluated for their energy performance, such as energy production and consumption. In this work, we have conducted a review and analysis of energy balance in BES with parameters like normalized energy recovery, specific energy consumption, and net energy production. Several BES representatives based on their functions were selected for analysis, including direct electricity generation in microbial fuel cells, hydrogen production in microbial electrolysis cells, nitrogen recovery in BES, chemical production in microbial electrosynthesis cells, and desalination in microbial desalination cells. Energy performance was normalized to water volume (kWh m^-3), organic removal (kWh kg COD^-1), nitrogen recovery (kWh kg N^-1), chemical production (kWh kg^-1), or removed salt during desalination (kWh kg^-1). The key operating factors such as pumping system (recirculation/feeding pumps) and external power supply were discussed for their effects on energy performance. This is an in-depth analysis of energy performance of various BES and expected to encourage more thinking, analysis, and presentation of energy data towards appropriate research and development of BES technology for resource recovery from wastewater.

Beyond Solar Fuels: Renewable Energy-Driven Chemistry

The future feasibility of decarbonized industrial chemical production based on the substitution of fossil feedstocks (FFs) with renewable energy (RE) sources is discussed. Indeed, the use of FFs as an energy source has the greatest impact on the greenhouse gas emissions of chemical production. This future scenario is indicated as "solar-driven" or "RE-driven" chemistry. Its possible implementation requires to go beyond the concept of solar fuels, in particular to address two key aspects: i) the use of RE-driven processes for the production of base raw materials, such as olefins, methanol, and ammonia, and ii) the development of novel RE-driven routes that simultaneously realize process and energy intensification, particularly in the direction of a significant reduction of the number of the process steps.