Bioelectrosynthesis E-Book
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Introduces basic principles and mechanisms, covers new developments, and provides a different view of the main facets of bioelectrosynthesis
Bioelectrosynthesis represents a promising approach for storing renewable energy or producing target chemicals in an energy-sustainable and low-cost way. This timely and important book systemically introduces the hot issues surrounding bioelectrosynthesis, including potential value-added products via bioelectrochemical system, reactor development of bioelectrosynthesis, and microbial biology on biofilm communities and metabolism pathways. It presents readers with unique viewpoints on basic principles and mechanisms along with new developments on reactor and microbial ecology.
Beginning with a principle and products overview of bioelectrosynthesis, Bioelectrosynthesis: Principles and Technologies for Value-Added Products goes on to offer in-depth sections on: biogas production and upgrading technology via bioelectrolysis; organic synthesis on cathodes; chemical products and nitrogen recovery; external electron transfer and electrode material promotion; and the microbiology of bioelectrosynthesis. Topics covered include: hydrogen production from waste stream with microbial electrolysis cell; microbial electrolysis cell; inorganic compound synthesis in bioelectrochemical system; microbial growth, ecological, and metabolic characteristics in bioelectrosynthesis systems; microbial metabolism kinetics and interactions in bioelectrosynthesis system; and more.
* Comprehensively covers all of the key issues of biolelectrosynthesis
* Features contributions from top experts in the field
* Examines the conversion of organic wastes to methane via electromethanogenesis; methane production at biocathodes; extracellular electron transport of electroactive biofilm; and more
Bioelectrosynthesis: Principles and Technologies for Value-Added Products will appeal to chemists, electrochemists, environmental chemists, water chemists, microbiologists, biochemists, and graduate students involved in the field.
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Table of Contents
Cover
Preface
Bioelectrosynthesis: Here to Stay!
References
Section I: Principle and Products Overview of Bioelectrosynthesis
1 Principle and Product Overview of Bioelectrosynthesis
1.1 Introduction
1.2 Evolution of Bioelectrosynthesis
1.3 Fundamental Principles of Bioelectrosynthesis
1.4 Plethora of Applications for Chemical Production
1.5 Key Factors for Improving MES Performance
1.6 Summary
References
Section II: Biogas Production and Upgrading Technology via Bioelectrolysis
2 Hydrogen Production from Waste Stream with Microbial Electrolysis Cells
2.1 Construction of MEC and Scale-up
2.2 Electrode Material of MEC
2.3 Effect of Operation Conditions on Hydrogen Production
2.4 Electroactive Biofilm Microbiome and Syntrophic Interaction in MEC
2.5 Coupled System for Biohydrogen Production
2.6 Challenges and Outlook
Acknowledgment
References
3 A Promising Strategy for Renewable Energy Recovery: Conversion of Organic Wastes to Methane via Electromethanogenesis
3.1 Introduction
3.2 Advances in Electromethanogenesis
3.3 Mechanisms of Electromethanogenesis
3.4 Applications of Electromethanogenesis
3.5 Outlook
References
Note
4 Microbial Electrolysis Cell (MEC): An Innovative Waste to Bioenergy and Value-Added By-product Technology
4.1 Introduction
4.2 Microbial Electrolysis Cell (MEC) for Hydrogen Production and Waste Treatment
4.3 Different Types of Waste Feedstocks Used in MECs
4.4 Current Applications of MEC
4.5 Existing Challenges and Bottlenecks for the Use of Wastewaters as Substrates in MECs
4.6 Conclusion and Future Outlook
Acknowledgments
References
5 Methane Production at Biocathodes: Principles and Applications
5.1 Introduction
5.2 Fundamentals of Methane-Producing Biocathode
5.3 Enhancing Methane Production Rates in AD
5.4 Upgrading of Biogas
5.5 Storage of Renewable Energy Through Methane-Producing Bioelectrochemical System
5.6 Conclusions and Outlook
References
Section III: Organic Production in Microbial Electrosynthesis System
6 Organic Synthesis on Cathodes
6.1 Carbon Reduction for Organics Synthesis at Cathode
6.2 Acetate Synthesis
6.3 Formic acid Synthesis
6.4 Alcohol Synthesis
6.5 Conclusions and Future Outlook
Acknowledgments
References
Section IV: Chemical Products and Nitrogen Recovery
7 Inorganic Compound Synthesis in Bioelectrochemical System: Generation Rate Increase and Application
7.1 Introduction
7.2 Hydrogen Peroxide Produced in BES: Optimization and Application
7.3 Metal Ion Reduction in BES: Waste Treatment and Metal Reuse
7.4 Struvite Crystallization Recovery: Principle and Application in BES Systems
7.5 Ammonia Recovery and Other Inorganics Synthesis in BES Systems
7.6 Outlook
Acknowledgments
References
8 Bioelectrochemical Ammonium Production – Nitrogen Removal and Recovery in BES
8.1 Ammonium Migration and Recovery
8.2 Anodic Ammonium Oxidation
8.3 Nitrification/Denitrification in BESs
8.4 Existing Problems and Challenges
References
9 Bioelectrochemical Systems for Heavy Metal Pollution Control and Resource Recovery
9.1 Introduction
9.2 BES and its Application in Heavy Metal Pollution Control
9.3 Outlook and Concluding Remarks
Acknowledgments
References
Section V: External Electron Transfer and Electrode Material Promotion
10 External Electron Transfer and Electrode Material Promotion
10.1 External Electron Transfer
10.2 Promotion of Material Development
10.3 Modified Electrodes for High Bioelectrosynthesis
10.4 Interspecies Electron Transfer Pathway
10.5 Future Perspectives
References
11 External Electron Transfer: Pathway, Mechanism, and Microorganisms Involved
11.1 External Electron Transfer of Cathode
11.2 Promotion of Material Development
11.3 Interspecies Electron Transfer Pathway
References
12 Extracellular Electron Transport of Electroactive Biofilm
12.1 Electroactive Bacteria
12.2 Electron Transport Across Geobacter(−Dominated) EABs
References
Section VI: The Microbiology of Bioelectrosynthesis
13 Microbial Growth and Ecological and Metabolic Characteristics in Bioelectrosynthesis Systems
13.1 Microbial Growth Kinetics and Energetics
13.2 Microbial Ecological Characterization and Biofilm-Related Aspects
13.3 Influence of Bioelectrochemistry on Microbial Community and Metabolism Pathway
References
14 An Update Perspective of Electron Transfer in Electrosyntrophic Methanogenesis: From VFAs to Methane
14.1 Introduction
14.2 Interspecies Hydrogen/Formate Electron Transfer and Transport/Flow in Methanogens
14.3 Beyond Hydrogen/Formate Electron Carriers
14.4 Power Drives Interspecies Electron Transfer (Kinetics and Energetics)
14.5 Multi-VFA Degradation Disturbance by Electrosyntrophic DIET
14.6 Overview of Application
14.7 Direct Electron Transfer in Methane Oxidation
14.8 Challenges
14.9 Conclusion
Acknowledgment
References
15 Microbial Metabolism Kinetics and Interactions in Bioelectrosynthesis System
15.1 Introduction
15.2 Microbial Metabolism Kinetics of Anode
15.3 Electrosynthesis Kinetics of Cathode
15.4 Energy Balance in Bioelectrosynthesis Systems
15.5 Microbial Community Growth on Electrode
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Assumptions regarding the theoretical production rates as well as e...
Table 1.2 A selection of H
2
production rates normalized to the reactor volume...
Table 1.3 Electroacetogenesis with pure cultures (maximum reported values).
Table 1.4 Electroacetogenesis with mixed cultures (maximum reported values).
Chapter 2
Table 2.1 Comparison of hydrogen production by MECs with different configurat...
Table 2.2 Hydrogen production by MECs with different cathode base materials a...
Table 2.3 Hydrogen production by MECs using different substrates.
Chapter 3
Table 3.1 The microbial communities of biocathode linked to the potential rea...
Chapter 4
Table 4.1 Performance of MEC with different feedstocks, electrodes (anode and...
Table 4.2 Performance of MEC with hydrogen production and ammonium recovery f...
Table 4.3 Performance of MEC with heavy metal/sulfate removal or recovery fro...
Chapter 5
Table 5.1 A comparison of two different electron donors: water and organic wa...
Table 5.2 BES–AD integration: conditions and performance.
Table 5.3 Comparative perspectives on methanation process.
Chapter 6
Table 6.1 Progress and milestones of microbial electrosynthesis.
Table 6.2 Bacteria along with the metabolic pathway involved for CO
2
reductio...
Chapter 7
Table 7.1 Hydrogen peroxide production efficiency and parameters in BESs with...
Chapter 9
Table 9.1 Heavy metal pollution control using abiotic cathode.
Table 9.2 Heavy metal pollution using biocathode BES.
Chapter 13
Table 13.1 Standard Gibbs energy and enthalpy of compounds associated in MES.
Table 13.2 Energetic requirements of carbon fixation for 1 mol of acetate pro...
Chapter 14
Table 14.1 Reactions for methanogenesis [1].
Chapter 15
Table 15.1 Summary of dynamic modeling in bioelectrochemical systems.
Table 15.2 Description of commonly used equations and laws in modeling.
Table 15.3 Equilibrium potentials for cathode reactions.
List of Illustrations
Chapter 1
Figure 1.1 Overview of different routes toward bioproduction from CO
2
.
Figure 1.2 Number of published journal articles on bioelectrosynthesis conta...
Figure 1.3 Basic principles of MES [12, 14, 24, 55]. A plethora of choices c...
Figure 1.4 Principles of the bioelectrosynthesis of H
2
in microbial electrol...
Figure 1.5 Schematic drawing of a bioelectrochemical system for wastewater t...
Figure 1.6 Yield (solvent g/substrate g) of 1,3-propanediol production, etha...
Figure 1.7 Sulfide-driven bioelectrosynthesis.
Figure 1.8 Ammonia recovery in BESs through ammonium transport and conversio...
Figure 1.9 Mechanisms for electron transfer from electrodes to microorganism...
Chapter 2
Figure 2.1 Schematic diagram of the hydrogen generation in microbial electro...
Figure 2.2 The CSLM micrographs of bacterial biofilms developed on the anode...
Figure 2.3 (a) Schematic diagram of MEC–MFC-coupled system. (b) Schematic di...
Chapter 3
Figure 3.1 The major achievements during the development of electromethanoge...
Figure 3.2 Potential mechanisms, participant microorganisms, and dominant ca...
Figure 3.3 Schematic diagram of the
ex situ
/
in situ
bioelectrochemical syste...
Chapter 4
Figure 4.1 Schematic representation of a typical MEC and its operation.
Figure 4.2 Mechanism of hydrogen evolution reaction (HER) in MECs.
Figure 4.3 (a) Number of publications per year and (b) in various countries ...
Figure 4.4 Variation in different substrates used in MECs. WW, wastewater....
Figure 4.5 Applications of MECs for wastewater treatment and valuable resour...
Figure 4.6 Scaling up issues for MEC field applications.
Chapter 5
Figure 5.1 The development of BES during the past 20 years in the areas of b...
Figure 5.2 Numbers of publications for methane-producing biocathode from 200...
Figure 5.3 Overview of numbers of publications addressing methane-producing ...
Figure 5.4 Schematic diagram of a dual-chamber methane-producing BESs, where...
Figure 5.5 Mechanisms of methane-producing biocathode. For direct electron t...
Figure 5.6 The effects of catholyte pH and H
2
partial pressure on theoretica...
Figure 5.7 Development of methane production rate in methane-producing BESs....
Figure 5.8 An overview of the voltage loss in a dual-chamber methane-produci...
Figure 5.9 Integration opportunities of AD and BES: “in series” configuratio...
Figure 5.10 Scheme of ionic species migration across the membrane in a BES e...
Figure 5.11 Overview of different electricity storage technologies including...
Figure 5.12 Process overview for renewable electricity storage in the form o...
Chapter 6
Figure 6.1 Illustration of various point source and nonpoint sources of gase...
Figure 6.2 Illustration of (a) direct and (b) mediated electron uptake pheno...
Figure 6.3 Schematic representation of Wood–Ljungdahl pathway for synthesis ...
Figure 6.4 Schematic representation of microbial electrochemical systems for...
Chapter 7
Figure 7.1 The application of BESs for inorganic compound synthesis.
Figure 7.2 The schematic diagram of hydrogen peroxide production in BES.
Figure 7.3 The fabrication method for a novel air-cathode fabricated with a ...
Figure 7.4 Theoretical working principle of membrane charge transport in fou...
Figure 7.5 Working mechanisms for resource recovery as well as wastewater tr...
Figure 7.6 The configuration for acid and alkaline production in a four-cham...
Chapter 8
Figure 8.1 Schematic diagram showing possible nitrogen cycle in bioelectroch...
Figure 8.2 16S rDNA analysis of the biofilm in one representative nitrifying...
Figure 8.3 The BES systems designed for complete nitrogen removal involving ...
Chapter 9
Figure 9.1 The environmental redox potential of selected heavy metal pollut...
Figure 9.2 The schematic diagram of different BESs in controlling heavy meta...
Figure 9.3 The schematic diagram of different BESs in controlling heavy meta...
Chapter 10
Figure 10.1 Electron transfer mode on the cathode in bioelectrosynthesis.
Figure 10.2 Goal and strategy of the improved cathodes for high-performance ...
Figure 10.3 SEM images of different material-modified cathodes. (a) CNT-modi...
Chapter 11
Figure 11.1 Mechanisms of external electron transfer.
Figure 11.2 The method of material promotion. (a) Unmodified electrode, (b) ...
Figure 11.3 Mechanisms of microbial interspecies electron transfer. (a) Exch...
Chapter 12
Figure 12.1 Illustration of microbial anode and cathode, respectively. At th...
Figure 12.2 Schematic illustration of different proposed extracellular elect...
Figure 12.3 Illustration of redox conduction via electron hopping across an ...
Figure 12.4 Simplified illustration of long-distance EET through a multicell...
Figure 12.5 (a) Principle of a cyclic voltammetry. (b) Recorded data with a ...
Figure 12.6 Principle of double-potential step chronoamperometry for an elec...
Chapter 13
Figure 13.1 The main processes during microbial growth.
Figure 13.2 Electroactive biofilm potential loss.
Figure 13.3 The Wood–Ljungdahl (WL) pathway from autotrophic acetogens.
Figure 13.4 Microbial community taxonomic plots based on relative abundance ...
Chapter 14
Figure 14.1 Conventionally metabolic pathway for methanogenesis.
Figure 14.2 DIET mode of
Geobacter
and
Methanoseata
.
Figure 14.3 Potentially metabolic pathway for electrosyntrophic methanogenes...
Figure 14.4 Working mode of bioelectrochemistry.
Figure 14.5 The three-stage methanogenesis of anaerobic digestion [14].
Figure 14.6 Conversion of propionate and butyrate to methane.
Chapter 15
Figure 15.1 Electron production by anode respiring bacteria.
ATP
,
adenosine
...
Figure 15.2 Factors influencing the anode kinetics.
Figure 15.3 An example of LSCV for anode biofilm.
Figure 15.4 Energy gradients in the electron transfer from electron donor to...
Figure 15.5 Typical types for electrosynthesis kinetics of cathode.
Figure 15.6 Irreversible potential losses at the different set anode potenti...
Figure 15.7 Cyclic voltammograms for abiotic cathodes made by stainless stee...
Figure 15.8 Energy transfer and dissipation in bioelectrosynthesis systems....
Figure 15.9 Simulated polarization across a Nafion-117 membrane as a functio...
Figure 15.10 Bioelectrosynthesis system and its equivalent circuit.
Figure 15.11 The three-dimensional metabolic structure of pure culture (a) a...
Figure 15.12 The three-dimensional metabolic structure of anodic biofilms gr...
Figure 15.13 Functional community distribution in test group and control gro...
Figure 15.14 Electron balance analysis for glucose digestion with (without) ...
Guide
Cover
Table of Contents
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Bioelectrosynthesis
Principles and Technologies for Value-Added Products
Edited by
Aijie Wang
Wenzong Liu
Bo Zhang
Weiwei Cai
Editors
Prof. Aijie WangRes. Ctr. for Eco-Environmental SciencesKey Laboratory of EnvironmentalBiotechnology18 Shuangqing RoadHaidian District100085 BeijingChina
Dr. Wenzong LiuRes. Ctr. for Eco-Environmental SciencesKey Laboratory of EnvironmentalBiotechnology18 Shuangqing RoadHaidian District100085 BeijingChina
Dr. Bo ZhangRes. Ctr. for Eco-Environmental SciencesKey Laboratory of EnvironmentalBiotechnology18 Shuangqing RoadHaidian District100085 BeijingChina
Dr. Weiwei CaiHarbin Institute of TechnologyState Key Lab. of Urban Water ResourcesNo 83 Huanghe Road150090 HarbinChina
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Preface
Bioelectrosynthesis: Here to Stay!
Many things in science move in waves until they finally land. In the 1960s, with the advent of the fuel cell, it was found that microbial decomposition of organic matter could also be directly linked to power production in microbial fuel cells (MFC) [1]. The power production was modest, and the top ic lead a latent existence until the petroleum crisis of the late 1970s when novel routes for chemical production were explored. Visionary scientists then coupled this microbial electron transfer for the production of organic molecules such as glutamic acid and butanol [2–4]. At that time, technology did not follow and titers and rates were limited, not competitive to existing bioproduction approaches. Then, in the late 1990s, the old topic of the MFC resurfaced, with new discoveries on electron flow [5–7]. This time, technology had evolved as well, enabling higher production rates [8, 9] and also enabling different process outcomes such as hydrogen [10] or caustic soda [11] production. Unfortunately, for MFCs, even by then also, an alternative technology generating power from biomass had matured: anaerobic digestion. In this process, biogas is produced. Nowadays, anaerobic digesters can deal from small to large scale with complex waste streams, they are very robust, and most importantly they can deal with high loading rates. Top systems now convert over 50 kg organics/m3 reactor per day to methane. In terms of electron flow, this implies a current of almost 7000 A, going to methane. To my opinion, it will be extremely difficult for MFCs to become an alternative to this, certainly considering the higher complexity of the systems and the presently lower rate.
No, besides the niches for MFC in sensing, the major promise lies in the return of the second wave: bioelectrosynthesis. Although there were some isolated reports on production of methane at cathodes in the late 1990s as well [6], around 2010, the topic truly resurfaced in the context of production of modest amounts of acetate from CO2 and electricity [12] in so-called microbial electrosynthesis (MES). Since then, titers and rates rapidly increased because of the availability of better technology, to now reach gram per liter levels [13]. Simultaneously, it is possible to also extract the product and thus obtain concentrates [14]. The product portfolio has expanded, from acetate to butyrate, caproate, and caprylate [15], toward alcohols such as ethanol [16] and isopropanol [17], even toward esters such as ethylacetate [18]. It appears that electricity-driven CO2 reduction is here to stay, and there are multiple good reasons for this: society is electrifying, which means that new applications are shifting to the use of electricity as energy source. The electricity is ubiquitously available, can be produced from renewable sources, and when used in the context of production leaves no traces such as salts in the water or the product. The coupling of electricity to CO2 conversion in the so-called carbon capture and utilization is rapidly emerging and MES will find its place within this portfolio.
Already in 2010, we made the point that MES is more than reducing CO2 [19]. Many existing production processes are imbalanced in terms of electrons, requiring, e.g., the supply of very well-controlled amounts of oxygen, which complicates many production processes. Flynn et al. [20] showed elegantly that an anode could solve the electron imbalance, enabling production of ethanol from glycerol with an engineered Shewanella oneidensis strain. Later, Lai et al. [21] showed anode-associated conversion of glucose to α-ketogluconic acid at efficiencies over 90% by coupling the metabolism of Pseudomonas putida, remarkably a strict aerobe, to an anode. The use of this organism opens up an enormous array of novel production routes, multiple of the most attractive routes have recently been identified by Kracke and Krömer [22].
MES thus encompasses a broad range of production processes, both anodic and cathodic, both starting from CO2 and from substrate organics [23]. Similar processes emerge to produce methane or upgrade biogas and to produce inorganic products such as hydrogen peroxide or ammonia, many of which are discussed in detail in the following book and which all have the potential to evolve into mature technologies and processes.
The challenges toward this are considerable and are both technological and microbial. When reading this book, grasp the excitement of this great field of science and engineering on the verge of breakthrough. This interface between biology and electrochemistry has already taught us many things about how microorganisms and microbial communities work, and they will continue to amaze us. Think about new, creative uses of bugs and electricity or how electron flow could affect our natural environment.
Enjoy,
Korneel RabaeyJanuary 2018Ghent University, Centre for Microbial Ecology and Technology (CMET), Faculty of Bioscience Engineering, Coupure Links 653, 9000 Gent, Belgium
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Arends, J.B.A., Patil, S.A., Roume, H., Rabaey, K. (2017). Continuous long-term electricity-driven bioproduction of carboxylates and isopropanol from CO
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Rabaey, K. and Rozendal, R.A. (2010). Microbial electrosynthesis—revisiting the electrical route for microbial production.
Nat. Rev. Microbiol.
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20
Flynn, J.M., Ross, D.E., Hunt, K.A. et al. (2010). Enabling unbalanced fermentations by using engineered electrode-interfaced bacteria.
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Lai, B., Yu, S., Bernhardt, P.V. et al. (2016). Anoxic metabolism and biochemical production in
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Section IPrinciple and Products Overview of Bioelectrosynthesis
1Principle and Product Overview of Bioelectrosynthesis
Fang Zhang1, Yuquan Wei2, and Guanghe Li1
1Tsinghua University, School of Environment and State Key Joint Laboratory of Environment Simulation and Pollution Control, Haidian District, Beijing, 100084, China
2China Agricultural University, College of Resources and Environmental Sciences, Haidian District, Beijing, 100193, China
1.1 Introduction
With the pressing crisis of depletion of fossil fuels, the past decade has seen the significant growth in the use of renewable energy, which leads to the growing research efforts toward electricity production from solar, wind, wave, or biomass energy (as opposed to petroleum, coal, or gas) in a sustainable way [1]. As electricity produced based on these renewable sources is usually intermittent and off-grit, electrosynthesis has been considered as an effective strategy to store electrical energy from renewable sources in the forms of chemical compounds [2]. Adequate electrocatalysts are necessary to catalyze the electrode-driven chemical reactions, yet these chemical catalysts are usually too expensive to be scaled up for practical applications. As a result, biocatalysts, which can be an enzyme, an organelle, or even a whole cell, have drawn increasing attention in electrosynthetic processes because of their higher specificity and versatility [3]. Moreover, microbes as catalysts are inexpensive to grow and, if the microbes catalyzing the reactions gain enough energy for cell maintenance, are self-sustaining and long-lived. Therefore, bioelectrosynthesis represents a promising approach to store renewable energy or produce target chemicals in an energy-sustainable and low-cost way.
Bioelectrosynthesis has emerged that electrical energy can be combined with biosynthesis to drive CO2 fixation as a means to directly produce the target compound or lead to the formation of acetyl-CoA and its derivatives for further synthesis. There are many assumptions for bioproduction in different pathways (Figure 1.1), which require inputs of solar energy or electrical energy (as an indirect solar derivative). One could speculate that instead of the Wood–Ljungdahl pathway (which would produce acetyl-CoA), the Calvin–Benson–Bassham cycle (which yields triose phosphates) can be driven on electrical current, leading to the formation of fermentable substrate from electricity and CO2. This fermentable substrate could then further be used for bioproduction purposes. Lastly, the fermentation itself can be complemented by electrical current to provide reducing equivalents to the cell. This can be considered as a hybrid metabolism when effective charge transfer occurs toward the cell. The major assumptions are summarized in Table 1.1. The theoretically achievable bioproduction densities for bioelectrosynthesis (product–carbon per hectare per annum) appear excessive at first glance. However, it is crucial to point out that photovoltaic panels are relatively efficient in capturing solar energy and that a first study producing acetate from CO2 has indicated high electron yields. Other factors such as CO2 and nutrient supply are likely to become limiting before these theoretical values are achieved. Therefore, electrosynthesis of organic compounds via abiotic or enzymatic catalysis of carbon dioxide reduction at electrode surfaces has been evaluated as a strategy for converting electricity into useful organic products for some time [7–9].
Figure 1.1 Overview of different routes toward bioproduction from CO2.
Source: Rabaey et al. 2011 [1]. Reproduced with permission of Elsevier.
Bioelectrosynthesis relies on the use of biocatalysts on the electrode surfaces to achieve electricity-driven synthesis. For biocatalyst, it has the following advantages in the bioelectrosynthetic processes: (i) the high reaction specificity and controllability of enzymes and organelles, (ii) self-regeneration of the whole microorganisms as the catalyst, (iii) adaptation of the microbial (catalyst's) quantity to the required conversion activity, (iv) flexibility in substrate use, (v) high versatility for product formation or conversion pathways, and (vi) improving the performance by decreasing the overpotentials at both anodes and cathodes [10–12]. However, microorganisms as biocatalysts are still far from perfect. Unlike true catalyst, microbes have been shown to consume part of the substrate or donor for growth albeit possibly only intermittently and are hard to keep a steady function or phenotype in different microenvironments.
Microbial electrosynthesis (MES) is a form of microbial electrocatalysis, which is an emerging area in microbial electrochemical research and development. The concept of MES was used to describe the process when a microbial catalyst reduces CO2 into multicarbon chemical commodities with electrons derived from the cathode of a bioelectrochemical system (BES) by applying an electric current designed primarily to perform biological reductive reactions. Generally, the electric current would ideally be produced by a renewable source of power. To be corresponding to the definition of conventional electrosynthesis and the microbial versatility found in different MES-based systems, it was expanded to mean “an alternative bioenergy strategy to use electrical energy as a source of reducing/oxidizing power for biochemical production, wherein the microorganisms facilitate the transfer of electrons from the cathode of a electrochemical system and production of desirable liquid transportation fuels and value-added chemicals.” Thus, in addition to the electricity-driven reduction of CO2, bioelectrosynthesis also includes the electricity-driven reduction or oxidation of other organic feedstocks [1, 8, 12–14].
Table 1.1 Assumptions regarding the theoretical production rates as well as expected substrate requirements for bioelectrosynthesis, conventional fermentations, and algal bioproduction systems (c$ refers to dollar cent) [1, 4–6].
Aerobic fermentations
Anaerobic fermentations
Current-driven lithoautotrophy – aerobic
Current-driven lithoautotrophy – anaerobic
Algal production
Carbon source (cost c$/mol C)
a)
Glucose (c$0.6)
Glucose (c$0.6)
CO
2
($0)
CO
2
($0)
CO
2
($0)
Electron donor (cost c$/mol C)
a)
Glucose (c$0.15)
Glucose (c$0.15)
Electricity (c$0.16)
Electricity (c$0.16)
Water (c$ 0)
Growth yield (mol C/mol C)
0.57
0.14
0.13
0.015
0.04–0.10
Maximal production density per hectare
b)
<3 tonnes C (as glucose)
<3 tonnes C (as glucose)
<1121 tonnes C (as butanol)
<1121 tonnes C (as butanol)
<25 tonnes C (as biodiesel)
a) Assumptions: Cost of fermentable substrate c$20 per kilogram and at 24 electrons per glucose molecule; electrical power delivered at 1 V and 0.06 $/kWh.
b) Assumes 100% conversion of raw materials to product. For glucose, annual average production per hectare from sugarcane is assumed. For lithoautotrophy, case calculations assume photovoltaic electricity productions at 500 W/m2 averaged irradiation and 20% efficiency of the photovoltaic panel. The bioelectrochemical system is assumed to have a 1 V cell voltage. For algal production, assumed annual production 50 tonnes biomass dry weight per hectare, of which <50% biodiesel-C.
MES relies on electrical current as a driver, which can allow on-site conversion of electrical energy (current) to chemical energy (a fuel). When coupled with a renewable source of electricity, the process will not only avoid the use of fossil fuels but utilize CO2 from waste streams, which would not compete for food crops or arable land and would only use small amounts of water and nutrients compared with the agricultural production of biofuels and chemicals. Therefore, the MES has several advantages, including (i) the double benefits of carbon sequestration and organics production, (ii) the feedstock mainly coming from wastes, (iii) high electricity efficiency to chemical commodities (c. 80–90%), and (iv) the potential that it could address the harvesting, storage, and distribution problems associated with energy crops, solar and wind farms, and natural gas exploration because the electricity can be from any renewable source, no matter how it is supplied, intermittent, stranded, or curtailed, and microbes may harvest it as power, especially utilizing solar energy in a 100-fold higher efficiency than biomass-based chemical production [1, 8, 13–18].
These characteristics demonstrate that a better understanding of the processes and mechanisms of bioelectrosynthesis would highly likely help address the need of energy and carbon storage as well as chemical production. As more is learned, additional applications will probably emerge. This chapter provides an overview of the principles and products of bioelectrosynthesis. This also includes a history of bioelectrosynthesis usage together with the properties of microbial catalysts and electrochemical hardware that affect their productivity, stability, long-term efficiency, and versatility in the environment and related topics are discussed in greater detail in subsequent chapters.
1.2 Evolution of Bioelectrosynthesis
In the context of electricity-driven bioproduction, reducing power provided by means of an electrode can either redirect fermentation pathways (sometimes called electrofermentation) or drive respiration. Electrofermentation has evolved since the report in 1979 that current supply through a mediator could increase L-glutamic acid yields [19, 20]. A significant advance was the finding in 2004 that direct electron transfer happened from cathodes to an attached biofilm Geobacter sulfurreducens with reducing fumarate to succinate [21]. In the past 15 years, microbial electrochemical systems developed rapidly as a key environmental technology at the nexus of water and energy research interests, particularly in the concept of microbial electrolysis cells (MECs) to produce H2 at the cathode through the reduction of protons, which could be used as fuel in turbines, internal combustion engines, fuel cells, as well as ovens and heaters [12, 14, 22–24]. For example, it was reported that anodic biofilms can be converted to cathodic biofilms for H2 production [23]. Meanwhile, bioelectrochemical synthesis of H2 can also be achieved by cathodically generating H2 accompanied with methane production in a bioanode [22]. It should be noted that the elimination of membranes or separators converted dual-chamber MECs to single-chamber reactors and significantly increased H2 generation rate, but the increase in H2 was more likely inhibited by methanogenesis to generate CH4 [25, 26]. Moreover, other inorganic chemicals have been produced in the cathode chamber of MECs. Rozendal et al. reported that hydrogen peroxide can be produced by reducing oxygen through the two electron reduction [27].
However, the concept of MES was only introduced in 2009–2010, with the initial findings related to the conversion of electrical current into methane from carbon dioxide by the Methanobacterium palustre on the biocathode at a set cathode potential less than −0.7 V (vs. Ag/AgCl) [28]. It was recognized by 2010 that biofilms of Sporomusa ovata growing on graphite cathode surfaces using pure cultures could use electrons derived from an electrode for the reduction of carbon dioxide to acetate and small amounts of 2-oxobutyrate at a high coulombic efficiencies (CEs) of acetate production (over 85%) [13]. However, considering that hydrogen is typically produced at the low potentials that were required for active methanogenesis, subsequent studies have questioned whether hydrogen produced at the cathode was the actual electron donor [29, 30]. Studies showed that a wide diversity of microorganisms, such as Clostridium ljungdahlii, Clostridium aceticum, Sporomusa sphaeroides, and Moorella thermoacetica, are capable of reducing carbon dioxide to produce organic acids with electrons derived from an electrode as the sole electron donor without using hydrogen [14, 16]. The mixed cultures were reported to generate acetate from a biocathode poised at −590 mV (vs. standard hydrogen electrode [SHE]) with CO2 as the only carbon source over 150 days, which further demonstrated the stability, resilience, and improved performance of electrosynthetic biocathodes following long-term operation [31, 32]. It was also recognized that 13.5 mM of alcohols as well as C4 compounds can be produced by reducing acetate at the cathode, but some processes required addition of mediators, such as methyl viologen (MV) [33]. In a similar way, the use of a cathode potential at −0.9 V vs. SHE in a BES without addition of an external mediator leads to the cathodic formation of medium chain fatty acids including caproate, butyrate, and smaller fractions of caprylate as the main products from acetate [34], which can be harvested as a valuable chemical. Notably, butyrate is an industrial feedstock with many applications in the pharmaceutical and chemical industries and can be converted into fuels through esterification [35]. Recently, a study has shown the bioelectrochemical transformation of CO2 as a sole carbon source to butyrate using mixed microbial cultures for the first time, but the products were a mixture of acetate, butyrate, ethanol, and butanol, and CO2 reduction to butyrate was hydrogen driven [36].
Furthermore, research in the area of metabolic engineering attempted to optimize the cellular metabolism of an organism to satisfy the desired process objectives mainly including significantly facilitating electron uptake and improving organic synthesis by modifying microorganisms. Typically, this is achieved by introducing exogenous metabolic pathways and manipulating native metabolic pathways or by manipulating cellular redox and energy reactions in order to overproduce desired metabolites [14, 37, 38]. Bioelectrochemical techniques are also used to manipulate the redox metabolism, such as supplying reducing power by generating reduced NADH within the cell through interactions with an electrode, which are effective to increase the synthesis or biotransformation of several products including ethanol, n-butanol, and succinate in a variety of hosts including Saccharomyces cerevisiae, Clostridium acetobutylicum, and Actinobacillus succinogenes [37, 39, 40]. It is widely known that acetyl-CoA, the central intermediate in acetate production in acetogens, is the building block for microbial synthesis of a wide diversity of desirable organic products, which should be possible with genetic engineering to divert carbon and electron flow in acetogenic microbes toward the production of butanol [41, 42].
Until now, most of the studies based on the technology of microbiology and molecular biology in the bioelectrochemical areas focused on the mechanisms for electron exchange from microbe, such as the model Shewanella oneidensis or G. sulfurreducens, to electrode, while electron transfer from electrodes to microbes may not be a simple reversal of electron transfer from cells to electrodes. For example, deletion of the genes for pili or OmcZ production essential for optimal current production of G. sulfurreducens had no impact on the capacity for current uptake [8, 43]. As discussed in several conceptual review articles [8, 12, 24, 30, 44, 45], it is likely that different pathways exist for electron uptake in microorganisms attached to cathodes, that is, either accepting electrons directly from electrodes (direct electron transfer) or alternatively using electron shuttles and cathodic H2 as electron carriers for the reduction of carbon dioxide (indirect electron transfer). Even though there is considerable progress in understanding the pathways for how electrons may be transported from the electrode to the cell, the complex interactions between microorganisms and cathode as well as interspecies for extracellular electron transfer (EET) toward microorganisms are not yet known, which is one of the key challenges to scale bioelectrosynthesis to practical applications. Besides, microbial attachment, biofilm development, electron transfer rate at the cathode surface, chemical production rate as well as biocathode materials, selective microbial consortia, and efficient reactor designs are all crucial elements to be optimized toward this objective.
To our knowledge, the study interests of bioelectrosynthesis have blossomed in the past decade, resulting in an exponential growth in the number of journal articles (Figure 1.2), but it is still in its infancy and there are also many technological and economic challenges, especially the significant engineering of the microbes and the reactors to be solved before it can be applied practically in large scale.
Figure 1.2 Number of published journal articles on bioelectrosynthesis containing the phrases “bioelectrosynthesis,” “microbial electrosynthesis,” or “microbial electrolysis.”
1.3 Fundamental Principles of Bioelectrosynthesis
Microbial fuel cells (MFCs) and MECs are examples of recent biotechnologies known as BESs that combine biological and electrochemical processes to produce electricity, hydrogen, or other useful chemicals [8, 12, 24, 46]. MFCs deliver electrical power from nearly any source of biodegradable organic or inorganic matter in waste streams by exoelectrogenic microorganisms on the anode, which have attracted extensive attentions at the early stage of BESs research [47–51]. MECs needs a small external power to make the reaction thermodynamically feasible, which can enable the generation of many different more value-added chemical products from biomass, such as hydrogen production [24, 46, 52, 53]. Energy is added into an MEC by either using an external power source or setting an electrode potential using potentiostat. Over the past decade, it has been known that MECs not only further store electricity as the desired commodities for conserving energy and reducing the dependency on fossil fuels but also capture/fix carbon dioxide while alleviating the greenhouse effect [12, 13, 16, 24, 54], which leads to the emergence of bioelectrosynthesis.
Microbial electrosynthetic processes are conducted in so-called BESs, which consist of an anode, a cathode, and, typically, a membrane separating the two chambers. An oxidation process occurs at the anode, whereas a reduction process occurs at the cathode, and the electrodes are surrounded by an electrolyte, which is generally an aqueous solution or wastewater (as a feed source) and contains the reactants and/or products. Microorganisms utilize electrons derived from the cathode to directly (via electron transfer) or indirectly (through evolved chemicals) catalyze the production of chemicals, including hydrogen, methane, short-chain organic acids, alcohols, etc., with only electricity and CO2 as feedstock (Figure 1.3) [12, 15, 36, 55]. Moreover, MES can also start from basic organic compounds, such as acetate, butyrate, and lactate, which are ubiquitously present in wastewaters and fermenter effluents, and then produce more attractive higher value end products (Figure 1.3) [12, 33]. Therefore, bioelectrosynthesis as a new platform technology that could produce the versatile fuels has gained increasing concerns [29, 31, 56, 57].
Figure 1.3 Basic principles of MES [12, 14, 24, 55]. A plethora of choices can be made regarding the membrane, the nature of the catalysts at both the anode and the cathode, and the source of the reducing power. This leads to a highly versatile technology that can carry out a diverse range of processes. MES can also be coupled to environment-friendly anodic processes.
There are several challenges in bioelectrosynthetic systems, such as the internal losses, which lead to the considerably less energy gained or more invested in reality, similar to the other systems of BESs [12, 58–60]. Firstly, the oxidation or reduction reaction at the electrode will incur the so-called activation overpotential, causing a voltage loss because of imperfect catalysis at the electrode. Secondly, when electrons flow through an electrical circuit, the resistance of electrolyte together with losses in the electrodes and the electrical circuit will lead to an ohmic loss. Thirdly, at higher current densities (or low mixing), the supply of substrate to the electrode or the discharge of protons or hydroxyl ions may cause diffusion limitations. Multiple mixed communities or pure cultures have shown the ability to catalyze CO2 reduction by using electricity as donor. Multicarbon compound production rates by MES have been increased substantially over the past five years; for example, the acetate production rate has been increased 433-fold (c. 282 mM/d/m2), whereas the electron transfer rate enhanced 521-fold (c. 475 mA/m2) [17, 18, 24, 55]. Yet the obstacles of low microbial productivity, poor stability, and low efficiency of CO2 to multicarbon compounds still stood out. Ongoing efforts conducted on the development of MES as an economically viable technology mainly include optimizing microbial catalysts and electrochemical hardware and characterizing the electron transfer mechanisms from cathode to microbes.
1.4 Plethora of Applications for Chemical Production
Currently, as the focus has shifted to microbial reductive processes at the cathode, bioelectrosynthesis, which has a better energy efficiency than MFCs alone and can couple chemical oxygen demand (COD) removal and energy recovery from waste with chemical synthesis, is being explored for a number of applications [46, 61, 62]. For example, microorganisms can catalyze electrochemical reactions such as proton reduction to molecular hydrogen or the reduction of carbon dioxide to organics such as methane, acetate, etc. (the process of MES), which holds strong promises for a new concept for biofuel generation. The following mainly focuses on the known function of bioelectrosynthesis for different valuable extracellular chemical end products because of the electrocatalyzed reduction reaction in the cathode. Individual applications are described and discussed in detail in various chapters of this book.
1.4.1 Hydrogen Production
Hydrogen (H2) has a high energy content of 121 MJ/kg, which is a clean energy carrier with zero carbon emission. Currently, 96% of commercial H2 produced today is delivered from fossil fuels via steam reforming, thermochemical conversion (pyrolysis), and gasification. However, the above-mentioned methods are not always environment-friendly. The bioelectrosynthesis of H2 in MECs, probably the cleanest and the most efficient method, provided completely new avenue for sustainable hydrogen production from renewable biomass and wastewaters [46, 63–65].
In principle, exoelectrogenic microorganisms colonized on the anode surface to form an anode-respiring biofilm and decompose the organic matter or wastes into CO2, electrons, and protons as a part of its metabolism. Meanwhile, the electrons traveled through an external circuit to a cathode, where the reduction of H+ to molecular H2 gas takes place. As this reaction is nonspontaneous (thermodynamically not favored because of the positive Gibbs free energy of the reaction), an external voltage practically at least 0.2–0.25 V must be supplemented to make it happen for the H2 production in MECs (Figure 1.4).
Crucially, there are several limits that can affect the performance of the MEC toward up-scaling and widespread applications, including low hydrogen production rate (HPR), high internal resistance, complicated architecture, and expensive materials. As for the problem that the hydrogen evolution reaction (HER) on plain carbon electrode is very slow and a high overpotential is needed to generate H2, platinum (Pt), an expensive metal catalyst, is usually used as the catalyst at the cathode in MECs, which has two major drawbacks including its high cost and poisoning by chemicals such as sulfide (a common constituent of wastewater). To resolve this problem, several attempts have been made to search for Pt-free cathode materials for HER in MECs. To date, it was found that first-row transition metals are very useful because of their stability, easy availability, low cost, low overpotentials, and low toxicity to living organisms. Considerable research efforts on cathodic material for MEC show that stainless steel(SS) and nickel alloys as well as nanostructured cathode materials represent a good compromise between cost and efficiency [63–67].
Figure 1.4 Principles of the bioelectrosynthesis of H2 in microbial electrolysis cell.
Many researchers have studied and explored several metabolic processes present in the cathode, stepping toward a possibility to develop a biocathode. For instance, Rozendal et al. firstly carried out investigation of a bioelectrode for H2 production from a naturally selected mixed culture of electrochemically active bacteria [23]. Chen et al. attempted to modify biocathodes with polyaniline(PANI)/multiwalled carbon nanotube(MWCNT) composites to improve hydrogen production in single-chamber MECs, which achieved an HPR of 0.67 m3 H2/m3 d at Eap = 0.9 V [68]. Croese et al. demonstrated that a mixed microbial consortium established on graphite felt cathodes of MEC could produce H2 at a rate of 2.4 m3/m3/d [69]. Compared with chemical catalysts, the use of electroactive microorganisms as cathode catalysts to make the biocathode MECs is superior to abiotic cathodes because H2 could be produced at a similar rate and cheaper biocatalysts can also self-generate without producing secondary pollution (Table 1.2). Therefore, biocathodes are a welcome advancement, i.e. increasing the bioelectrosynthesis of H2, in the quest to implement MECs for practical applications.
1.4.2 Methane Production
Methane is an excellent fuel and is being widely used all over the world. The production of methane has been the most common aim for respiratory bioproduction in dark conditions. Meanwhile, it is commonly detected in the MECs during hydrogen production because of the growth of methanogens, which was considered to be the result of diffusion from the anode to cathode at the early stage [53, 78]. Substantial reports in the literature have suggested that cathodic methane production in the MECs was expected by the catalytic conversion of hydrogen to methane (e.g. 4H2 + CO2 → CH4 + 2H2O) with abiotic cathode [46, 56]. Although methane is sometimes considered a nuisance by-product in hydrogen-producing MECs as it increases the energy and economical cost for purification, several studies have made the production of methane a key objective [79–81]. Cheng et al. for the first time described the production of methane from carbon dioxide reduction in a two-chamber MEC with a methanogen-attached biocathode at a methane production rate about 0.06 mmol/l/h at a voltage of 1.2 V [28]. The authors of this study suggested direct EET as the core mechanisms that the conversion of an electrical current toward methanogens was direct and did not proceed through H2 (e.g. CO2 + 8H+ + 8e− → CH4 + 2H2O). Likewise, Villano et al. utilized both the electrons and CO2 released at the anode during the microbial oxidation of the organic matter contained in a waste stream for the cathodic generation of methane (Figure 1.5) and obtained a methane production ratio of 0.055 ± 0.002 mmol/D mg volatile suspended solids (VSSs) from CO2 in a two-chamber MECs with a biocathode incubated with a hydrogenophilic methanogenic culture [29]. Nowadays, research regarding direct EET for methane bioelectrosynthesis is still highly needed to fully explore electron transfer occurring at the cathode surface.
Table 1.2 A selection of H2 production rates normalized to the reactor volume (m3/m3/d) and cathode surface area (m3/m2/d) in MEC experiments.
Source: Kitching et al. 2010 [65]. Reproduced with permission of Elsevier.
Substrate (mg/l)
Inoculum
H
2
production rate (m
3
/m
3
/d)
H
2
production rate (m
3
/m
2
/d)
Cathode,
E
vs. Ag/AgCl/V
Mode of operation
MEC configuration
Cathode
Anode
References
Acetate (60)
Mixed (enriched)
2.2
0.010
−0.7
Continuous (0.01 h)
Dual chamber with cation exchange membrane
Biocathode, graphite paper
Graphite
[
70
]
Acetate (300)
Geobacter sulfurreducens
0.31
0.005
−0.8
Continuous (0.04 h)
Single chamber
Biocathode, nanoporous graphite
Nanoporous graphite
[
71
]
Acetate (600)
Mixed (enriched)
2.4
0.024
−0.9
Continuous (0.64 h)
Dual chamber with cation exchange membrane
Biocathode, graphite felt
Graphite felt
[
69
]
Acetate (100)
Mixed (enriched)
0.02
0.002
0.5
Batch
Dual chamber with cation exchange membrane
Titanium mesh disk
Graphite felt disk
[
66
]
Acetate (2720)
Mixed (enriched from prior MEC)
50
50
1
Continuous (0.003 h)
Dual chamber with anion exchange membrane
Ni foam
Graphite felt
[
72
]
Ammonia (510)
Mixed (enriched)
0.01
0.001
0.6
Batch
Dual chamber with anion exchange membrane
Carbon felt
Carbon felt
[
73
]
Lactate (910)
Shewanella oneidensis
MR-1
0.25
3.068
0.6
Batch
Aerated H-type reactor with anion exchange membrane
Graphite (modified with 5% Pt in activated carbon powder)
Carbon fiber fabric
[
74
]
Methanol (1600)
Mixed (enriched MFC culture)
0.1
0.004
0.8
Batch
Dual-chamber reactor with anion exchange membrane
Graphite fiber cloth (coated with Pt)
Graphite fiber brush
[
75
]
Sodium bicarbonate (2500)
Mixed (enriched)
45.27
Surface area not reported
−0.8 (vs. SHE)
Batch
Dual chamber with cation exchange membrane
Biocathode, graphite granules
Graphite granules
[
76
]
Sodium bicarbonate (5000)
Mixed (enriched)
1.15
Surface area not reported
−0.85 (vs. SHE)
Batch
Dual chamber with cation exchange membrane
Biocathode, vitreous carbon (doped with CNT)
Graphite plates
[
77
]
Hydrogen production rates normalized to electrode surface area were calculated using the projected surface area or the total surface area based on the working electrode.
Generally, the bioelectrosynthesis of methane in MECs holds several advantages over conventional biogas, including (i) the possibility to store electricity or H2 as methane with a high content, (ii) saving energy because the process occurs at ambient temperature and heating is not required, and (iii) the limited sensitivity of the process to toxic compounds such as ammonia, which can be present in the feedstock (this relates to the sensitivity of methanogens to ammonia, which is formed at high pH values) [22, 46, 64]. Even though the disadvantages, i.e. the low value of methane as a product, the energy investment that is required to produce the methane and the cost of pressurizing such a gas for transport, need to be considered, understanding the underlying mechanisms for methane bioelectrosynthesis is highly attractive from an engineering standpoint.
Figure 1.5 Schematic drawing of a bioelectrochemical system for wastewater treatment and simultaneous CH4 bioelectrosynthesis based on CO2 reduction.
Source: Villano et al. 2010 [29]. Reproduced with permission of Elsevier.
1.4.3 Alcohol Production
In the process of the bioelectrochemical reduction of CO2 to acetate, acetyl-CoA is the key central intermediate, which could be a versatile building block for a range of useful organic chemicals and potential biofuels such as ethanol, n-butanol, and alcohols, or longer chain fatty acids.
Ethanol as a liquid fuel has been produced by microbial reduction of acetate as the main intermediate of anaerobic digestion with hydrogen as electron donor, whereas the feasibility of ethanol production by using electrode instead of hydrogen as electron donor has been demonstrated in a two-chamber MEC for biological acetate reduction by mixed cultures, which obtained 1.82 mM ethanol production and 49% of CE at best via the assistance of electron mediator such as MV [33, 82]. This suggested that BES aiming to bioelectrosynthesis provides a new way to overcome the limitation of traditional biological ethanol production. However, there are underlying challenges that need to be addressed for making the technology industrially applicable, including whether hydrogen was involved in the mechanism of acetate reduction, how to decrease the operation cost (e.g. irreversibly electron acceptors and energy for distillation), selecting electroactive microorganisms that can accept electrons directly from cathode rather than via mediator for ethanol production, improving the ethanol production rate and the final concentration, and increasing the efficiency of systems by reduction in electrode overpotential, system internal resistance, and energy losses [46, 64].
Butanol is an important chemical intermediate for the precursor of many industrial chemicals in food, chemical, and pharmaceutical industries. Bio-based butanol produced from acetone–butanol–ethanol (ABE) fermentation is preferred as its green renewable feature. Bioelectrosynthesis based on microbial BESs is another method to supply electrons for microbial metabolism, which works at a biocathode by external power input for butanol production [83]. Compared to conventional fermentation method or microbial synthesis, the production rate, efficiency, and concentration of different chemicals were enhanced by assistance of bioelectrosynthesis (Figure 1.6) [84].
Acetogenic microorganisms are an attractive catalyst for the conversion of carbon dioxide to a diversity of multicarbon organic products [86]. Some acetogens will produce high titers of ethanol rather than acetate under the appropriate conditions, and in some instances, 2,3-butanediol and butanol are also produced in wild-type cells [87, 88]. Besides, it has been reported that the final products of MES included acetate, butanol, propanol, and ethanol by use of mixed culture [44]. However, the bioelectrosynthesis starting from CO2 to alcohols has a key disadvantage that CO2 as an electron acceptor is the large electron requirement [12]. For instance, although the theoretical potentials for the reduction of butyrate to butanol ( = −0.37 vs. SHE) and the reduction of CO2 to butanol ( = −0.30 V vs. SHE) are similar, the reduction of butyrate to butanol requires only 4 electrons, whereas the reduction of CO2 to butanol requires 24 electrons, which implies a sixfold higher current demand and an equivalently large power demand for this reaction. Besides, the conversion of CO2 to butanol will also probably involve multiple synthesis steps, each with certain efficiency losses.
Figure 1.6 Yield (solvent g/substrate g) of 1,3-propanediol production, ethanol efficiency, and butanol yield in MEC [33, 83–85]. neutral red(NR); methyl viologen (MV); anthraquinone-2,6-disulfonate (AQDS); fixed electrode potential(FP); fixed-potential increased electrode surface area(FP-ISA); varying potential(VP); nonelectrochemical CO2 spared(NE-CO2); electrochemical N2 spared(NE-N2); electricity of cathodic potential(E); electricity reduced neutral red(ENR).
1.4.4 Short-chain Organic Acid Production
The production of formic acid, which is an important chemical used in pharmaceutical syntheses as well as in paper and pulp production, was achieved based on organic matter oxidation in the anode and CO2 reduction in the cathode [46]. In the electrochemical processes driven by direct-current power supplies, the reduction of CO2 to formic acid has been demonstrated by several studies on different metal cathodes, e.g. expensive Pt at a high Faraday efficiency (above 94%) [89]. To explore an environmentally friendly method, Zhao et al. utilized the electricity in situ generated from the degradation of the carbonaceous substances in the anodic chambers in a series-connected MFC stack, which also achieved the electrochemical reduction of CO2 to formic acid at 4.27 mg/l/h [90]. The Desulfovibrio and Sulfurospirillum may contribute to H2 and formate metabolism that may then support acetogenesis. Desulfovibrio in particular is well known for its ability to generate H2 off of an electrode, and it has been shown to grow while converting formate into H2 [91–93]. However, a CO2 reductase in Acetobacterium woodii can catalyze the reversible and direct conversion of H2 and CO2 to formate, and it is thermodynamically favorable for sulfate reducing bacteria to generate formate from H2 and CO2 as long as sulfate is limiting and an acetogen is available to consume the formate [94, 95].
The production of acetate (or acetyl-CoA) is central to the bioelectrosynthesis of chemicals beyond H2 and methane. Acetate can be an important end product as well as a platform for further chemical syntheses [15]. Many acetogenic microorganisms and enriched microbial communities have been tested for the ability to produce acetate with electrons supplied at a cathode [15]. Generally, mixed cultures performed better than pure cultures (Tables 1.3 and 1.4). Nevin et al. demonstrated that an acetogenic microorganism S. ovata
