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• W3107744891 abstract "In the past decade, research in the field of microbial electrosynthesis (MES) has been driven forward by the development of cathode materials, electroactive bacteria or microbiome enrichment, and productivity improvements.As the close of three complete funding cycles for the field is reached, recent reviews have sought to refocus emphasis to the eventual application of MES; a means of measurably reducing CO2 waste via the formation of valuable products.Using present knowledge of bioelectrochemistry, and by learning lessons from adjacent fields, it becomes apparent that the simplest gains in performance are likely to come from advancements in the reactor rather than the biocatalysts. Varying the reactor and operating conditions of the system, however, require adapting these biocatalysts. The valorization of CO2 to valuable products via microbial electrosynthesis (MES) is a technology transcending the disciplines of microbiology, (electro)chemistry, and engineering, bringing opportunities and challenges. As the field looks to the future, further emphasis is expected to be placed on engineering efficient reactors for biocatalysts, to thrive and overcome factors which may be limiting performance. Meanwhile, ample opportunities exist to take the lessons learned in traditional and adjacent electrochemical fields to shortcut learning curves. As the technology transitions into the next decade, research into robust and adaptable biocatalysts will then be necessary as reactors shape into larger and more efficient configurations, as well as presenting more extreme temperature, salinity, and pressure conditions. The valorization of CO2 to valuable products via microbial electrosynthesis (MES) is a technology transcending the disciplines of microbiology, (electro)chemistry, and engineering, bringing opportunities and challenges. As the field looks to the future, further emphasis is expected to be placed on engineering efficient reactors for biocatalysts, to thrive and overcome factors which may be limiting performance. Meanwhile, ample opportunities exist to take the lessons learned in traditional and adjacent electrochemical fields to shortcut learning curves. As the technology transitions into the next decade, research into robust and adaptable biocatalysts will then be necessary as reactors shape into larger and more efficient configurations, as well as presenting more extreme temperature, salinity, and pressure conditions. The production of chemicals and fuels using CO2 and renewable energy as feedstocks is a key aspect in achieving a sustainable society [1.Sharon D.A.M. K.H.G.J. A Circular Economy in the Netherlands by 2050.Publisher Name: Government of the Netherlandshttps://www.government.nl/documents/policy-notes/2016/09/14/a-circular-economy-in-the-netherlands-by-2050Date: 2016Google Scholar]. As CO2 is the most oxidized form of carbon however, substantial energy is required to convert the inert molecule into a useful product. One of the research avenues being investigated for CO2 conversion is bioelectrochemistry (see Glossary), which allows for the production of more complex chemical compounds than purely electrochemical methods. The technology is rooted in the ability for microorganisms to take up electrons from solid-state electrodes, use them within their metabolism to convert CO2, and excrete a reduced chemical as an electron sink [2.Jourdin L. Strik D.P.B.T.B. Electrodes for cathodic microbial electrosynthesis processes: key-developments and criteria for effective research and implementation.in: Flexer V. Brun N. Functional Electrodes for Enzymatic and Microbial Bioelectrochemical Systems. World Scientific, 2017: 429-473Crossref Scopus (8) Google Scholar,3.Kerzenmacher S. Engineering of microbial electrodes.in: Harnisch F. Holtmann D. Bioelectrosynthesis. Springer International Publishing, 2019: 135-180Google Scholar]. This electricity-driven microbial conversion of CO2 is called microbial electrosynthesis (MES)[4.Rabaey K. Rozendal R.A. Microbial electrosynthesis — revisiting the electrical route for microbial production.Nat. Rev. Microbiol. 2010; 8: 706-716Crossref PubMed Scopus (1057) Google Scholar]. Figure 1 depicts the six main products formed in MES to date, alongside their current main industrial production methods (depicted in red). To date, 75% of all MES studies have reported solely acetate production, with a greater diversification of the product spectrum occurring only within the past few years [5.Flexer V. Jourdin L. Purposely designed hierarchical porous electrodes for high rate microbial electrosynthesis of acetate from carbon dioxide.Acc. Chem. Res. 2020; 53: 311-321Crossref PubMed Scopus (31) Google Scholar]. Over the past decade, since the original proof-of-concept [6.Nevin K.P. et al.Microbial electrosynthesis: feeding microbes electricity to convert carbon dioxide and water to multicarbon extracellular organic compounds.mBio. 2010; 1: 1-4Crossref Scopus (627) Google Scholar], the focus of the MES research community has mainly been on developing cathode materials, enriching microbial catalysts and electroactive microorganisms, increasing productivity and selectivity, and shedding light on fundamental extracellular electron transfer (EET) mechanisms and microbial functions (with the relative research emphasis depicted visually in Figure 2A ). These steps have been vital to uncover further microorganisms and microbiomes, as well as demonstrating reasonable productivities. Together, these fundamental and applied advancements have continued to motivate the technology as a means of large-scale CO2 conversion. Looking forward to the next decade of MES, how will the field shift focus to accomplish the envisioned goal of replacing existing fossil-fuel production routes for these carbon-containing compounds? The following mainly focuses on biofilm-driven MES. Others have extensively discussed systems built around microorganisms in suspensions [7.Claassens N.J. et al.Making quantitative sense of electromicrobial production.Nat. Catal. 2019; 2: 437Crossref Scopus (79) Google Scholar]. In a recent article, Prévoteau and colleagues outlined in-depth the figures of merit envisioned to make MES a reality [8.Prévoteau A. et al.Microbial electrosynthesis from CO2: forever a promise?.Curr. Opin. Biotechnol. 2020; 62: 48-57Crossref PubMed Scopus (123) Google Scholar]. Further, Jourdin and coworkers recently provided a techno-economic analysis illustrating the combined cost and performance barriers to a profitable demonstration of MES [9.Jourdin L. et al.Techno-economic assessment of microbial electrosynthesis from CO2 and/or organics: An interdisciplinary roadmap towards future research and application.Appl. Energy. 2020; 279: 115775Crossref Scopus (25) Google Scholar]. Here, a different perspective is taken and the following question is asked: what are the barriers currently limiting MES, and how can this field shift its everyday research to overcome these limitations in the next ten years? Upon unpacking this question, it becomes apparent that many of the improvements in performance that are easily accessible are non-biological in nature, such as minimizing anode–cathode spacing and increasing salinity/temperature. These improvements have yet to be seriously considered as a way to improve the performance and commercial outlook of MES, which was the motivation for the writing of this opinion piece, providing a more in-depth perspective. Specifically, it needs to be considered that the vast majority of changes which can be made in reactor design, provide conditions that are unsuitable for current biocatalysts and cathode systems developed in the past decade. The remainder of this opinion will then discuss how the biocatalysts and reactors in MES systems will need to evolve, as there is a shift to more commercially-representative conditions. As the interaction between microbial catalysts and the electron-providing cathode is the central component of MES, discussing their relationship is essential as the field seeks to move to higher current densities and efficiencies. Importantly, how can both the structure of the biocatalyst and electrode be modified to overcome limitations in both cellular and geometric electron transfer rates. To date, both pure and mixed microbial consortium have been successfully used in MES [10.Marshall C.W. et al.Electrosynthesis of commodity chemicals by an autotrophic microbial community.Appl. Environ. Microbiol. 2012; 78: 8412-8420Crossref PubMed Scopus (303) Google Scholar,11.Nevin K.P. et al.Electrosynthesis of organic compounds from carbon dioxide is catalyzed by a diversity of acetogenic microorganisms.Appl. Environ. Microbiol. 2011; 77: 2882-2886Crossref PubMed Scopus (500) Google Scholar], and a variety of electron transfer processes from the cathode surface to the biocatalyst have been demonstrated or hypothesized, including direct electron transfer [6.Nevin K.P. et al.Microbial electrosynthesis: feeding microbes electricity to convert carbon dioxide and water to multicarbon extracellular organic compounds.mBio. 2010; 1: 1-4Crossref Scopus (627) Google Scholar,11.Nevin K.P. et al.Electrosynthesis of organic compounds from carbon dioxide is catalyzed by a diversity of acetogenic microorganisms.Appl. Environ. Microbiol. 2011; 77: 2882-2886Crossref PubMed Scopus (500) Google Scholar] and mediated electron transfer mechanisms. In CO2 to acetate conversion for example, H2 has been shown to act as electron mediator, whether the H2 species originated electrochemically [12.Blanchet E. et al.Importance of the hydrogen route in up-scaling electrosynthesis for microbial CO 2 reduction.Energy Environ. Sci. 2015; 8: 3731-3744Crossref Google Scholar,13.LaBelle E.V. May H.D. Energy efficiency and productivity enhancement of microbial electrosynthesis of acetate.Front. Microbiol. 2017; 8: 756Crossref PubMed Scopus (68) Google Scholar] or was biologically-induced [14.Jourdin L. et al.Biologically-induced hydrogen production drives high rate/high efficiency microbial electrosynthesis of acetate from carbon dioxide.ChemElectroChem. 2016; 3: 581-591Crossref Scopus (99) Google Scholar]. In alcohol and longer-chain carboxylate production, both EET mechanisms and microbial functions in complex microbiomes must be investigated further [15.Batlle-Vilanova P. et al.Microbial electrosynthesis of butyrate from carbon dioxide: Production and extraction.Bioelectrochemistry. 2017; 117: 57-64Crossref PubMed Scopus (95) Google Scholar, 16.Jourdin L. et al.Critical biofilm growth throughout unmodified carbon felts allows continuous bioelectrochemical chain elongation from CO2 up to caproate at high current density.Front. Energy Res. 2018; 6: 7Crossref Scopus (91) Google Scholar, 17.Jourdin L. et al.Enhanced selectivity to butyrate and caproate above acetate in continuous bioelectrochemical chain elongation from CO2: steering with CO2 loading rate and hydraulic retention time.Bioresource Technol. Rep. 2019; 7: 100284Crossref Scopus (42) Google Scholar, 18.Marshall C.W. et al.Metabolic reconstruction and modeling microbial electrosynthesis.Sci. Rep. 2017; 7: 8391Crossref PubMed Scopus (66) Google Scholar, 19.Vassilev I. et al.Microbial electrosynthesis of isobutyric, butyric, caproic acids, and corresponding alcohols from carbon dioxide.ACS Sustain. Chem. Eng. 2018; 6: 8485-8493Crossref Scopus (103) Google Scholar, 20.Arends J.B.A. et al.Continuous long-term electricity-driven bioproduction of carboxylates and isopropanol from CO2 with a mixed microbial community.J. CO2 Util. 2017; 20: 141-149Crossref Scopus (89) Google Scholar]. Regardless of the exact method of electron transfer, it is accepted that the cathode and biocatalyst should be in close proximity to one another to facilitate this transfer, and the number of microbes should be high to increase the overall geometric rate of CO2 conversion. This combination of needs has led many researchers to pursue the formation of a thick biofilm on the surface of the cathode [5.Flexer V. Jourdin L. Purposely designed hierarchical porous electrodes for high rate microbial electrosynthesis of acetate from carbon dioxide.Acc. Chem. Res. 2020; 53: 311-321Crossref PubMed Scopus (31) Google Scholar]. A thick and thriving external biofilm alone, however, is insufficient to meet the eventual required current density for MES applications, often discussed to be above 50–100 mA cm−2 [8.Prévoteau A. et al.Microbial electrosynthesis from CO2: forever a promise?.Curr. Opin. Biotechnol. 2020; 62: 48-57Crossref PubMed Scopus (123) Google Scholar,9.Jourdin L. et al.Techno-economic assessment of microbial electrosynthesis from CO2 and/or organics: An interdisciplinary roadmap towards future research and application.Appl. Energy. 2020; 279: 115775Crossref Scopus (25) Google Scholar]. Using a 2D electrode structure as a basis, Claassens and colleagues completed a comprehensive review of microbial growth parameters associated with different feedstock and assimilation pathways, including acetogens using H2/CO2 [7.Claassens N.J. et al.Making quantitative sense of electromicrobial production.Nat. Catal. 2019; 2: 437Crossref Scopus (79) Google Scholar]. In this work, it was calculated that with a high electron consumption rate of 100 μmol s−1 gDCW−1 (dry cell weight), a 100 μm thick biofilm, and a density of bacteria of 0.5 gDCW cm−3, a maximum current density of only circa 50 mA cm−2 could be achieved in MES. Such an analysis assesses the limitations of electron transfer rates of biocatalysts from the perspective of functional biofilm thickness/density and the rate of microbial electron consumption. It is then clear that the net quantity of the biocatalyst must be increased through other means such as using 3D or fibrous electrodes, which 70% of MES studies have now utilized (Figure 2B). Extending the back-of-the-envelope calculations from Claassens and coworkers to 3D structures (see supplemental information online for calculation details), one can start to determine what microbial-cathode structures would be required to meet specific geometric current densities and begin assessing the trade-offs that may exist from this approach. Here a 1.2 cm thick carbon felt (fibrous) electrode is taken as a representative base case, which has previously been shown experimentally to reach an MES current density of − 17.5 mA cm−2 for an estimated external biofilm thickness of 400 μm [16.Jourdin L. et al.Critical biofilm growth throughout unmodified carbon felts allows continuous bioelectrochemical chain elongation from CO2 up to caproate at high current density.Front. Energy Res. 2018; 6: 7Crossref Scopus (91) Google Scholar]. For such a 3D electrode, biofilms can exist both on the exterior planar surface, as well as on the internal fibers of the thick carbon electrode (Figure 3A ). Assuming similar activity parameters as Claassens and colleagues, Figure 3B–C shows the maximally achievable current density from a purely metabolic perspective, for different internal and external biofilm thicknesses. It can be seen already that with a 2 μm-thick inner biofilm, a current density ranging from − 750 to − 1100 mA cm−2 can in theory be reached (Figure 3B), which is far beyond that reached in the experimental results to date. These results also show that the inner biofilm thickness is more influential than the external biofilm thickness. Such findings are logical as the inner electrode surface area is orders of magnitude higher than the outer surface area (Figure 3C). Lastly, Figure 3D shows the impact of the porosity and total surface area per unit volume of 3D and fibrous electrodes, on the theoretically achievable current density. Even with a fairly open porosity which reduces the electrode area available to biofilms, sufficiently high current densities are metabolically attainable given appropriate microbial attachment and biofilm coverage. Since it is known that biofilms have been shown to be present throughout the entirety of such 3D fibrous structures [16.Jourdin L. et al.Critical biofilm growth throughout unmodified carbon felts allows continuous bioelectrochemical chain elongation from CO2 up to caproate at high current density.Front. Energy Res. 2018; 6: 7Crossref Scopus (91) Google Scholar], the results here indicate that factors other than maximum metabolic rates are limiting geometric MES rates in these systems. In our view two distinctive research avenues deserve our interest in order to understand and realize greater activity of 3D MES systems. One direction takes a more biological approach and focuses on homogeneous biofilm growth strategies in thicker 3D structures, while another seeks to improve the system from a purely electrochemical reactor design perspective, considering factors such as mass transport and current distribution. The following section expands the views on these themes. In both cases, methods to determine the kinetic rates on both a cellular (i.e., biomass-specific) and geometric level, would be a valuable metric for interpreting these advancements, and to assess whether there are intrinsic limitations of the MES microorganisms, which impact their metabolic rates from the values suggested by Claassens and colleagues (100 μmol s−1 gDCW−1). The preceding section addressed the limitations of MES activity from a metabolic perspective, using the dimensions of the electrode and biofilm as primary factors in determining limiting rates. In reality, as the dimensions of the electrode and the overall quantity of bacteria are increased, so too are limitations reached, which requires the invocation of reactor design concepts to ensure productivity. One of the clearest challenges of operating 3D MES electrode structures is ensuring that ample CO2, protons, and nutrients, can be provided to all layers of microbes within the electrode, such that desired growth and reaction rates of each individual microorganism throughout the entirety of the 3D electrode can be sustained. An exterior biofilm with thicknesses on the order of 400 μm, for example, is likely to run into diffusion limitations of reactants from the bulk electrolyte to the biofilm closest to the electrode. Conversely, electrons (or electron carriers) transferred from the electrode may deplete prior to reaching the exterior biofilm surface, and product and hydroxide (OH−) build-up could result in reduced stability or intrinsic productivity. Both aspects will hurt productivity per cell due to nonideal transport. Ensuring ample transport is even more complex for biofilm on the interior surfaces of thicker fibrous structures, particularly if fluid flow is constrained to only one side of the electrode, as is common in flow-by systems (~95% of current MES studies as shown in Figure 2B). In cases where the electrolyte is forced to flow through the porous electrode matrix, higher current densities and improved biofilm coverage have been demonstrated versus H-type reactors with magnetic stirring [5.Flexer V. Jourdin L. Purposely designed hierarchical porous electrodes for high rate microbial electrosynthesis of acetate from carbon dioxide.Acc. Chem. Res. 2020; 53: 311-321Crossref PubMed Scopus (31) Google Scholar]. While MES research to date has not placed substantial emphasis on reactor design concepts to improve mass transport (Figure 2A) [16.Jourdin L. et al.Critical biofilm growth throughout unmodified carbon felts allows continuous bioelectrochemical chain elongation from CO2 up to caproate at high current density.Front. Energy Res. 2018; 6: 7Crossref Scopus (91) Google Scholar,21.Enzmann F. et al.Transferring bioelectrochemical processes from H-cells to a scalable bubble column reactor.Chem. Eng. Sci. 2019; 193: 133-143Crossref Scopus (29) Google Scholar, 22.Giddings C.G.S. et al.Simplifying microbial electrosynthesis reactor design.Front. Microbiol. 2015; 6: 1-6Crossref PubMed Scopus (88) Google Scholar, 23.Alqahtani M.F. et al.Porous hollow fiber nickel electrodes for effective supply and reduction of carbon dioxide to methane through microbial electrosynthesis.Adv. Funct. Mater. 2018; 28: 1804860Crossref Scopus (69) Google Scholar], small modifications to the reactor itself can allow for improved geometric metabolic output. A separate transport consideration in 3D electrode structures is the ionic transport between the anode and cathode. As discussed by Prévoteau and coworkers, the ohmic drop within the electrolyte will constitute a significant portion of the operating cell voltage [8.Prévoteau A. et al.Microbial electrosynthesis from CO2: forever a promise?.Curr. Opin. Biotechnol. 2020; 62: 48-57Crossref PubMed Scopus (123) Google Scholar]. For fixed voltage operation, the portion of the anode and cathode closest to one another will then have the greatest electrochemical activity, as the ohmic drop will be the lowest. A consequence of thicker electrodes, is increased ohmic drops deeper in the electrode structure, which effectively results in reduced potentials and current densities on the back of the electrode [24.Haverkort J.W. A theoretical analysis of the optimal electrode thickness and porosity.Electrochim. Acta. 2019; 295: 846-860Crossref Scopus (29) Google Scholar] independent of CO2/nutrient transport. Under extremely high metabolic rates, 2D electrodes are actually preferred. From an ionic perspective, the need for a dispersed biological system is contradicted by an ideal 2D reactor design, implying that a compromised electrode thickness must be found. As a quick reference, Figure S1 (see the supplemental information online) highlights how the maximum possible current density varies for different electrode thicknesses, if only the metabolic rate is limiting. From the previous transport arguments, a strong motivation for greater mass transfer, fluid dynamics, and cell geometry modeling in MES reactors can be seen, which can then be validated using experiments. Up to now, computational modeling of MES at all relevant scales (i.e., from μm to m-scale), has been underexplored, and is necessary to achieve breakthrough understanding of the process-limiting steps, and for rational design and scale-up. To our knowledge, only Gadkari, Kazemi, and colleagues, modeled the (inter)-dependence of some operating parameters [25.Gadkari S. et al.Understanding the interdependence of operating parameters in microbial electrosynthesis: a numerical investigation.Phys. Chem. Chem. Phys. 2019; 21: 10761-10772Crossref PubMed Google Scholar], and current density and biofilm thickness on substrate concentration [26.Kazemi M. et al.bio-electrosynthesis in a reverse microbial fuel cell to produce acetate from CO2 and H2O.Phys. Chem. Chem. Phys. 2015; 17: 12561-12574Crossref PubMed Google Scholar], while Enzmann and coworkers modeled some design parameters from their bubble column reactor [21.Enzmann F. et al.Transferring bioelectrochemical processes from H-cells to a scalable bubble column reactor.Chem. Eng. Sci. 2019; 193: 133-143Crossref Scopus (29) Google Scholar]. Salimijazi and colleagues also very recently modeled the theoretical interdependence between electrical-to-fuel efficiency, and biofilm resistivity and thickness [27.Salimijazi F. et al.Constraints on the efficiency of electromicrobial production.Joule. 2020; 4: P2101-P2130Abstract Full Text Full Text PDF Scopus (15) Google Scholar]. While only a 2D system was modeled, they concluded that as the biofilm resistivity increases, its thickness must decrease and its geometric area increase, in order to maintain a given efficiency. However, following their conclusion, a 3D or fibrous electrode would allow to maintain thin, low-resistivity, biofilm throughout the whole cathode, and thus may allow the maintenance of high energy efficiency at a reasonable reactor footprint. Ideally, a cheap commercial material with appropriate thickness, porosity, and other important physical–chemical properties [2.Jourdin L. Strik D.P.B.T.B. Electrodes for cathodic microbial electrosynthesis processes: key-developments and criteria for effective research and implementation.in: Flexer V. Brun N. Functional Electrodes for Enzymatic and Microbial Bioelectrochemical Systems. World Scientific, 2017: 429-473Crossref Scopus (8) Google Scholar,5.Flexer V. Jourdin L. Purposely designed hierarchical porous electrodes for high rate microbial electrosynthesis of acetate from carbon dioxide.Acc. Chem. Res. 2020; 53: 311-321Crossref PubMed Scopus (31) Google Scholar,28.Guo K. et al.Engineering electrodes for microbial electrocatalysis.Curr. Opin. Biotechnol. 2015; 33: 149-156Crossref PubMed Scopus (196) Google Scholar] can be used as a cathode. Otherwise, innovative synthesis methods could be explored such as, for example, 3D-printed materials that fulfil the characteristics discussed earlier. It should be noted that higher energy efficiency could be targeted upon replacing the energy-intensive water oxidation anodic reaction (Box 1).Box 1Alternatives to Anodic Water Oxidation May Prove FavorableTo date, the focus of microbial electrosynthesis (MES) development has been on the biocathode, with water oxidation performed at the anode largely for convenience. Attempts have been made to couple the biocathode with a biological anode [54.Xiang Y. et al.High-efficient acetate production from carbon dioxide using a bioanode microbial electrosynthesis system with bipolar membrane.Bioresour. Technol. 2017; 233: 227-235Crossref PubMed Scopus (56) Google Scholar], though additional effort is required to make this configuration technically and economically viable [9.Jourdin L. et al.Techno-economic assessment of microbial electrosynthesis from CO2 and/or organics: An interdisciplinary roadmap towards future research and application.Appl. Energy. 2020; 279: 115775Crossref Scopus (25) Google Scholar]. Now that biocathodes and production are better understood after a decade of research, it is worthwhile to begin pairing MES with a more energetically and economically favorable anodic reactions.The water-oxidizing electrode of the cell not only represents the main cost contribution of MES, amounting to 59% of the total capital expense (CAPEX), but requires substantial overpotentials [9.Jourdin L. et al.Techno-economic assessment of microbial electrosynthesis from CO2 and/or organics: An interdisciplinary roadmap towards future research and application.Appl. Energy. 2020; 279: 115775Crossref Scopus (25) Google Scholar]. As adjacent research fields have recently sped up the development of anodic catalysts for the oxidation of organics, these advancements can be incorporated into MES systems in the near future [55.Verma S. et al.Co-electrolysis of CO2 and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption.Nat. Energy. 2019; 4: 466Crossref Scopus (221) Google Scholar]. Table I lists a nonexhaustive list of four promising anodic reactions for the oxidation of glycerol and glucose, together with the Gibbs free energy of the overall reaction, and the resulting cell voltage under different pH conditions.Several important conclusions can be made from Table I when considering replacing water oxidation. First, oxidizing glycerol or glucose requires lower cell voltages than water oxidation. Second, the proposed organic reactions favor high pH environments, which is in contrast to the current acidic anolytes used in MES studies. And third, positive Ecell values are in theory achievable when coupling electro-oxidation of glycerol or glucose at pH 14 to a biocathode at pH 7, given a suitable means of separating the two pH electrolytes (e.g., bipolar membranes).From an economical perspective, alternative anodic reactions provide the potential to make a second valuable product. This promise, however, is not without additional constraints. For example, it needs to be ensured that the market of the anodic product pairs well with the cathode product in terms of location, global production (tons/year), feedstock availability, and cost. Further, if the goal of MES is to replace substantial portions of waste CO2, then the anodic product market size must also be substantial. Detailed life cycle and techno-economic assessments should then drive the choice of the anodic feedstock.For now, water oxidation will continue to persist due to its ease of operation for investigating biocathodes. Water is abundant, pH operation can be flexible, and current densities are easy to match with the cathode. As the field moves forward, so too does the possibility of replacing the anode as a promising development for commercializing MES technology.Table ITheoretical Gibbs Free Energy of Reaction and Cell Voltage for the Cathodic Microbial Electroreduction of CO2 to Hexanoate (6CO2 + 32H+ + 32e− → C6H12O2 + 10H2O), Coupled to Anodic O2 Evolution, or Glycerol and Glucose Electro-Oxidation, at Different pH ConditionsPossible anode reactionsPossible overall reactionsStd. conditions(pH 0, 298K)Anode pH 1 – cathode pH 7Anode pH 14 – cathode pH 7ΔG0r (kJ mol–1)E0cell (V)ΔGr (kJ mol–1)Ecell (V)ΔGr (kJ mol–1)Ecell (V)Water → oxygen2H2O → O2 + 4H+ + 4e−6CO2 + 6H2O → C6H12O2 + 8O23453.48–1.124549.36–1.412174.96–0.70Glycerol → glyceraldehydeC3H8O3 → C3H6O3 + 2H+ + 2e−6CO2 + 16C3H8O3 → C6H12O2 + 16C3H6O3 + 10H2O896.68–0.291992.56–0.65–381.840.12Glycerol → lactic acidC3H8O3 → C3H6O3 + 2H+ + 2e−6CO2 + 16C3H8O3 → C6H12O2 + 16C3H6O3 + 10H2O426.28–0.141522.16–0.49–852.240.28Glycerol → formic acidC3H8O3 + 3H2O → 3CH2O2 + 8H+ + 8e−6CO2 + 4C3H8O3 + 2H2O → C6H12O2 + 12CH2O282.36–0.031178.24–0.38–1196.160.38Glucose → gluconic acidC6H12O6 + 1H2O → C6H12O7 + 2H+ + 2e−6CO2 + 16C6H12O6 + 6H2O → C6H12O2 + 16C6H12O7–562.520.18533.36–0.17–1841.040.60 Open table in a new tab To date, the focus of microbial electrosynthesis (MES) development has been on the biocathode, with water oxi" @default.
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• W3107744891 date "2021-04-01" @default.
• W3107744891 modified "2023-10-13" @default.
• W3107744891 title "Microbial Electrosynthesis: Where Do We Go from Here?" @default.
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• W3107744891 doi "https://doi.org/10.1016/j.tibtech.2020.10.014" @default.
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