Enhanced Bio-Electrochemical Reduction of Carbon Dioxide by Using Neutral Red as a Redox Mediator

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.

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.

Renewable Hydrogen Production and Storage Via Enzymatic Interconversion of CO2 and Formate with Electrochemical Cofactor Regeneration

The urgent need to reduce CO2 emissions has motivated the development of CO2 capture and utilization technologies. An emerging application is CO2 transformation into storage chemicals for clean energy carriers. Formic acid (FA), a valuable product of CO2 reduction, is an excellent hydrogen carrier. CO2 conversion to FA, followed by H2 release from FA, are conventionally chemically catalyzed. Biocatalysts offer a highly specific and less energy-intensive alternative. CO2 conversion to formate is catalyzed by formate dehydrogenase (FDH), which usually requires a cofactor to function. Several FDHs have been incorporated in bioelectrochemical systems where formate is produced by the biocathode and the cofactor is electrochemically regenerated. H2 production from formate is also catalyzed by several microorganisms possessing either formate hydrogenlyase or hydrogen-dependent CO2 reductase complexes. Combination of these two processes can lead to a CO2 -recycling cycle for H2 production, storage, and release with potentially lower environmental impact than conventional methods.

Recent Applications and Strategies to Enhance Performance of Electrochemical Reduction of CO2 Gas into Value-Added Chemicals Catalyzed by Whole-Cell Biocatalysts

Carbon dioxide (CO2) is one of the major greenhouse gases that has been shown to cause global warming. Decreasing CO2 emissions plays an important role to minimize the impact of climate change. The utilization of CO2 gas as a cheap and sustainable source to produce higher value-added chemicals such as formic acid, methanol, methane, and acetic acid has been attracting much attention. The electrochemical reduction of CO2 catalyzed by whole-cell biocatalysts is a promising process for the production of value-added chemicals because it does not require costly enzyme purification steps and the supply of exogenous cofactors such as NADH. This study covered the recent applications of the diversity of microorganisms (pure cultures such as Shewanella oneidensis MR1, Sporomusa species, and Clostridium species and mixed cultures) as whole-cell biocatalysts to produce a wide range of value-added chemicals including methane, carboxylates (e.g., formate, acetate, butyrate, caproate), alcohols (e.g., ethanol, butanol), and bioplastics (e.g., Polyhydroxy butyrate). Remarkably, this study provided insights into the molecular levels of the proteins/enzymes (e.g., formate hydrogenases for CO2 reduction into formate and electron-transporting proteins such as c-type cytochromes) of microorganisms which are involved in the electrochemical reduction of CO2 into value-added chemicals for the suitable application of the microorganism in the chemical reduction of CO2 and enhancing the catalytic efficiency of the microorganisms toward the reaction. Moreover, this study provided some strategies to enhance the performance of the reduction of CO2 to produce value-added chemicals catalyzed by whole-cell biocatalysts.

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

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

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

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

Applications of Synthetic Biotechnology on Carbon Neutrality Research: A Review on Electrically Driven Microbial and Enzyme Engineering

With the advancement of science, technology, and productivity, the rapid development of industrial production, transportation, and the exploitation of fossil fuels has gradually led to the accumulation of greenhouse gases and deterioration of global warming. Carbon neutrality is a balance between absorption and emissions achieved by minimizing carbon dioxide (CO2) emissions from human social productive activity through a series of initiatives, including energy substitution and energy efficiency improvement. Then CO2 was offset through forest carbon sequestration and captured at last. Therefore, efficiently reducing CO2 emissions and enhancing CO2 capture are a matter of great urgency. Because many species have the natural CO2 capture properties, more and more scientists focus their attention on developing the biological carbon sequestration technique and further combine with synthetic biotechnology and electricity. In this article, the advances of the synthetic biotechnology method for the most promising organisms were reviewed, such as cyanobacteria, Escherichia coli, and yeast, in which the metabolic pathways were reconstructed to enhance the efficiency of CO2 capture and product synthesis. Furthermore, the electrically driven microbial and enzyme engineering processes are also summarized, in which the critical role and principle of electricity in the process of CO2 capture are canvassed. This review provides detailed summary and analysis of CO2 capture through synthetic biotechnology, which also pave the way for implementing electrically driven combined strategies.

Electrocatalysis as an enabling technology for organic synthesis

Electrochemistry has recently gained increased attention as a versatile strategy for achieving challenging transformations at the forefront of synthetic organic chemistry. Electrochemistry's unique ability to generate highly reactive radical and radical ion intermediates in a controlled fashion under mild conditions has inspired the development of a number of new electrochemical methodologies for the preparation of valuable chemical motifs. Particularly, recent developments in electrosynthesis have featured an increased use of redox-active electrocatalysts to further enhance control over the selective formation and downstream reactivity of these reactive intermediates. Furthermore, electrocatalytic mediators enable synthetic transformations to proceed in a manner that is mechanistically distinct from purely chemical methods, allowing for the subversion of kinetic and thermodynamic obstacles encountered in conventional organic synthesis. This review highlights key innovations within the past decade in the area of synthetic electrocatalysis, with emphasis on the mechanisms and catalyst design principles underpinning these advancements. A host of oxidative and reductive electrocatalytic methodologies are discussed and are grouped according to the classification of the synthetic transformation and the nature of the electrocatalyst.

Impact of Carbon Felt Electrode Pretreatment on Anodic Biofilm Composition in Microbial Electrolysis Cells

Sustainable technologies for energy production and storage are currently in great demand. Bioelectrochemical systems (BESs) offer promising solutions for both. Several attempts have been made to improve carbon felt electrode characteristics with various pretreatments in order to enhance performance. This study was motivated by gaps in current knowledge of the impact of pretreatments on the enrichment and microbial composition of bioelectrochemical systems. Therefore, electrodes were treated with poly(neutral red), chitosan, or isopropanol in a first step and then fixed in microbial electrolysis cells (MECs). Four MECs consisting of organic substance-degrading bioanodes and methane-producing biocathodes were set up and operated in batch mode by controlling the bioanode at 400 mV vs. Ag/AgCl (3M NaCl). After 1 month of operation, Enterococcus species were dominant microorganisms attached to all bioanodes and independent of electrode pretreatment. However, electrode pretreatments led to a decrease in microbial diversity and the enrichment of specific electroactive genera, according to the type of modification used. The MEC containing isopropanol-treated electrodes achieved the highest performance due to presence of both Enterococcus and Geobacter. The obtained results might help to select suitable electrode pretreatments and support growth conditions for desired electroactive microorganisms, whereby performance of BESs and related applications, such as BES-based biosensors, could be enhanced.

Supply of proton enhances CO electrosynthesis for acetate and volatile fatty acid productions

The microbial electrosynthesis is a platform to supply protons and electrons to improve the conversion efficiency and production rate for the valorization of C1 gas. This study examined proton migration and electron transfer of the electrode and microbe by using various external parameters in the electrosynthesis of CO. The CO electrosynthesis achieved almost double of coulombic efficiency than the conventional CO₂ electrosynthesis. The maximum volumetric acetate production rate was 0.71 g/L/day in the BES, which was 2-6 times higher than reported elsewhere. These results show that the efficient proton migration and electron transfer can enhance the productivity and conversion efficiency of the biological CO conversion in a bioelectrochemical system.

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.

Bioelectrosynthetic Conversion of CO2 Using Different Redox Mediators: Electron and Carbon Balances in a Bioelectrochemical System

Microbial electrosynthesis (MES) systems can convert CO2 to acetate and other value-added chemicals using electricity as the reducing power. Several electrochemically active redox mediators can enhance interfacial electron transport between bacteria and the electrode in MES systems. In this study, different redox mediators, such as neutral red (NR), 2-hydroxy-1,4-naphthoquinone (HNQ), and hydroquinone (HQ), were compared to facilitate an MES-based CO2 reduction reaction on the cathode. The mediators, NR and HNQ, improved acetate production from CO2 (165 mM and 161 mM, respectively) compared to the control (without a mediator = 149 mM), whereas HQ showed lower acetate production (115 mM). On the other hand, when mediators were used, the electron and carbon recovery efficiency decreased because of the presence of bioelectrochemical reduction pathways other than acetate production. Cyclic voltammetry of an MES with such mediators revealed CO2 reduction to acetate on the cathode surface. These results suggest that the addition of mediators to MES can improve CO2 conversion to acetate with further optimization in an operating strategy of electrosynthesis processes.