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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 8: 1200 - 1216

DOI: https://doi.org/10.7536/PC240104

Review

Principle and Application of Electro-Fermentation Technology for Enhancing the Resource Utilization of Organic Waste

  • Xiaoyan Sun 1, 2 ,
  • Yanan Yin , 1, * ,
  • Hui Chen 2 ,
  • Lei Zhao 2 ,
  • Cheng Wang 1 ,
  • Jianlong Wang 1
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  • 1 Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
  • 2 The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science ＆ Technology, Wuhan 430081, China
*e-mail:

Received date: 2024-01-08

  Revised date: 2024-06-05

  Online published: 2024-07-01

Supported by

National Natural Science Foundation of China(22206103)

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Abstract

the increasing production of organic waste poses a challenge for waste treatment and disposal.Due to the richness of nutrients such as polysaccharides,proteins and minerals,the Bio-resourcing of organic waste has attracted much attention.Currently,the traditional anaerobic fermentation system for organic waste treatment has problems such as long fermentation period,low product yield,poor product selectivity,and low degradation rate of organic waste.the bio-electro-fermentation technology formed by applying electrochemical system to the traditional anaerobic fermentation system can regulate the redox balance of microbial reaction by electrochemical methods,overcome the thermodynamic limitations of traditional fermentation,strengthen the microbial electron transfer,promote the degradation of organic wastes and the generation of resource products,and achieve the high efficiency of organic waste resourcing.in this paper,we systematically investigate the basic principles of electro-fermentation technology to promote organic waste resources,review the electrode materials,microorganisms,reactor structure,and methods to enhance the operation of the system,introduce the current research status of electro-fermentation technology to strengthen the generation of organic waste resource products(including methane,hydrogen,alcohols,short-chain fatty acids,medium-chain fatty acids,polyhydroxybutyrate,polyhydroxyalkanoate,etc.),summarize and look forward to the prospects and challenges for the development of electro-fermentation technology In the application of organic waste resource utilization。

Contents

1 Introduction

2 Principle of electro-fermentation

2.1 Principle of cathodic electro-fermentation

2.2 Principle of anodic electro-fermentation

2.3 Extracellular electron transfer

3 Electrode materials used in electro-fermentation

3.1 Carbon-based materials

3.2 Metal-based materials

3.3 Composite-based materials

4 Microorganisms in electro-fermentation systems

5 Single and double chamber electro-fermentation systems

6 Methods for improving the operation of electro-fermentation systems

6.1 Condition optimization

6.2 Conductive medium

6.3 Bioaugmentation

7 Products of electro-fermentation

7.1 Methane

7.2 Hydrogen

7.3 Alcohols

7.4 Volatile fatty acids

7.5 High-value chemicals

8 Conclusions and Perspectives

Key words： electro-fermentation technology; organic waste; electrode material; electroactive bacteria; electro-fermentation products; extracellular electron transfer; high value chemicals

Cite this article

Xiaoyan Sun , Yanan Yin , Hui Chen , Lei Zhao , Cheng Wang , Jianlong Wang . Principle and Application of Electro-Fermentation Technology for Enhancing the Resource Utilization of Organic Waste[J]. Progress in Chemistry, 2024 , 36(8) : 1200 -1216 . DOI: 10.7536/PC240104

1 Introduction

With The rapid improvement of human living standards,the rapid development of economy and the continuous acceleration of urbanization,the amount of global organic waste has increased significantly.According to statistics,the global output of solid waste is 2.24 billion tons/year in 2020,and it is expected to reach 3.88 billion tons/year in 2050.the traditional treatment processes of organic waste mainly include landfill,composting and incineration,which require high treatment conditions and will cause new environmental problems if not properly treated[1]。 organic waste is rich in organic matter,and the use of organic waste to produce biofuels and chemicals can achieve the dual benefits of waste disposal and resource and energy generation at the same time。
Anaerobic fermentation technology is an effective biological means of organic waste recycling.in the process of Anaerobic fermentation,the organic matter In the organic waste is converted into Volatile fatty acids(VFAs),hydrogen,methane and other value-added substances through a variety of microbial metabolic reactions.Anaerobic fermentation has the advantages of mild reaction conditions,low cost and easy operation[2,3]。 Therefore,anaerobic fermentation of organic wastes plays an important role in the development of renewable resources[4]。 However,there are still some problems In the utilization of organic waste by anaerobic fermentation technology,such as low efficiency of fermentation products,poor product selectivity and low utilization rate of organic waste.in order to improve the reaction effect of anaerobic fermentation,a lot of explorations have been made from the aspects of substrate treatment,microbial domestication and reaction condition optimization,and certain results have been achieved[5,6]。 Recent studies have found that electron transfer plays a key role in the anaerobic metabolism of microorganisms,and the effect of fermentation can be effectively improved by regulating the electron transfer of the reaction system[7⇓~9]。
in 2010,Rabaey and Rozendal first proposed the concept of Electric fermentation technology,which is an organic combination of anaerobic fermentation technology and Bio-electrochemical system(BES),that is,electrodes are placed In the reactor as an electron bridge for long-distance electron transfer,and the process of microbial metabolic reaction is promoted or regulated by the mediation of electrodes,so as to improve the conversion rate and efficiency of products[10⇓⇓~13]。 Park et al.Showed that the application of micro-voltage in anaerobic fermentation technology can accelerate the biological oxidation process and promote the production of methane[14]。 Selembo et al.Used glycerol fermentation to produce hydrogen,and found that the hydrogen yield obtained by electric fermentation was 56%higher than that obtained by traditional anaerobic fermentation[15]。 Mathew et al.Applied voltage to the Saccharomyces cerevisiae culture system and found that the ethanol production was increased by 14%and the production efficiency was increased by 2 to 3 times compared with the traditional anaerobic fermentation[16]。 In addition,the synthesis of butanol,1,3-propanediol,VFAs,Medium-chain fatty acids(MCFAs)and so on by microorganisms has been enhanced by electrofermentation technology[17⇓⇓~20]。 Electrofermentation alleviates the thermodynamic hindrance of anaerobic fermentation process,which provides a new direction for the realization of efficient recycling of organic waste,and has attracted wide attention of scholars at home and abroad[21,22]。
in this paper,the application of bioelectric fermentation technology in organic waste recycling was reviewed.Firstly,the electrofermentation technology was comprehensively introduced from four aspects:reaction principle,electrode materials,microorganisms and reactor structure;Secondly,the methods to improve the operation effect of electric fermentation are introduced in detail from three aspects of operation conditions,additives and bioaugmentation;the application of electrofermentation in the production of different biochemicals,including methane,hydrogen,alcohols,VFAs,MCFAs,Polyhydroxyalkanoates(PHA)and Polyhydroxybutyrate(PHB),was further introduced;Finally,the development and application of electrofermentation technology were summarized and prospected,aiming to provide reference and suggestions for further research and application of electrofermentation。

2 Principle of electric fermentation

In the electrofermentation system,a pair of strong electron sources through the electrode can significantly change the extracellular electron transfer process of the relevant microorganisms,thus affecting the metabolism of microorganisms and the interaction between microbial populations,thereby affecting the ability of microorganisms to produce target products[21]。 When the target product is mainly oxidation products,the anode is used as the working electrode to dissipate excess electrons,which is called Anodic electro-fermentation[23]。 in contrast,In Cathodic electro-fermentation,the cathode acts as a working electrode and promotes the formation of reduction products by donating electrons to the microbial fermentation[24]。 In addition,the current regulates the whole biological reaction by changing the balance of NADH/NAD+,thus affecting the generation of final products[25]。 The effect of current is the same for both anodic and cathodic fermentation,that is,under the condition of redox imbalance,the process of microbial fermentation is stimulated and maintained,and even a small current density will affect the extracellular and intracellular redox potential,regulate the dynamic balance of NADH/NAD+,and then change the production of fermentation products 。

2.1 Principle of cathodic electrofermentation

When the target product is reducing substances such as hydrogen,butyric acid,butanol,MCFAs,etc.,cathodic electrofermentation technology can be used.the intensification mechanism of the cathodic electrofermentation system is shown in Fig.1。
图1 阴极电发酵的潜在机制：（a）强化电子传递，（b）氧化还原电位调控，（c）互营协作（黄色为微生物，橙色为电活性微生物，MedOx为氧化介质，MedRed为还原介质）

Fig. 1 Potential mechanism of cathodic electro-fermentation (a) strengthening of electron transfer, (b) regulation of redox potential, (c) mutual cooperation (yellow represents microorganisms, orange represents electroactive microorganisms, MedOx is the oxidizing medium, and MedRed is the reducing medium)

[26]; (2)Redox potential regulation:Electrons supplied by the cathode can affect the intracellular NADH and NADH/NAD+levels of microorganisms(as shown in Figure 1b),thereby regulating the production of products during microbial fermentation[21]。 Compared with the direct supply of electrons by the cathode,the redox potential-regulated cathodic electrofermentation requires less electrons,usually less than 1%of the reduction products.For example,in the study of enhancing microbial fermentation of glycerol to 1,3-propanediol by cathodic electrofermentation,the cathode provided only 0.23 mmol of electrons to the fermentation substrate,and the production of 1,3-propanediol increased by 1.5 times[27]; (3)Mutual cooperation of microorganisms:As shown in Figure 1C,for the mixed flora fermentation process,cathodic electrofermentation can accelerate the utilization of substrates,improve the yield and yield of target products,and reduce the formation of by-products by enriching microorganisms with specific functions and improving the metabolic cooperation among microorganisms.in the process of butyric acid production by rice straw fermentation,the relative abundance of butyric acid fermentation microorganism Clostridium clusterⅣin the mixed microorganisms was increased by 31.34%compared with the open circuit control group,and the butyric acid production by cathodic electric fermentation reached 5.54 G/L in cooperation with Bacteroides,which was increased by 17.37%compared with the open circuit system[28]。

2.2 Principle of anodic electrofermentation

the performance of anodic electrofermentation is driven by the anode as the final electron acceptor.Similar to cathodic electrofermentation,there are mainly three strengthening mechanisms in anodic electrofermentation(Fig.2).(1)Enhanced electron transfer:the substrate is directly converted to the product,and the anode is the main receiver of electrons,as shown in Figure 2a,and the excess electrons are completely dissipated on the anode through the extracellular electron transfer mechanism.Flynn et al.Inoculated S.oneidensis into an electrode chamber at an oxidizing potential to ferment glycerol,and observed that a current flux appeared at the anode accompanied by glycerol consumption.the results showed that 78%of the carbon flux was directly converted into ethanol and electrons were released,indicating that the biotransformation of glycerol was achieved by the direct transfer of electrons to the oxidizing electrode[29]; (2)Redox potential regulation:As shown in Figure 2b,redox potential is used to regulate intracellular redox homeostatic（NADH/NAD+）,thereby realizing cellular functional metabolism,such as gene expression and enzyme synthesis,regulating product types,reducing by-product generation,and dissipating redundant electrons to the anode[30]。 In the study of 3-hydroxypropionic acid production from glycerol,Kim et al.Increased the production of 3-hydroxypropionic acid by controlling the internal redox state of Klebsiella pneumoniae L17 cells[23]。 The results showed that when Klebsiella pneumoniae L17 was grown at applied potential,the proportion of intracellular NADH/NAD+was significantly reduced,and the production of 3-hydroxypropionic acid was increased by 7 times compared with the control group without applied potential;(3)Mutual cooperation of microorganisms:As shown in Fig.2C,the electrons generated by fermentation bacteria after oxidizing the substrate cannot be directly transferred to the cathode,and the electroactive microorganisms act as intermediaries between the fermentation microorganisms and the anode to realize interspecies electron transfer,consuming the by-products of substrate fermentation,and more electrons are beneficial to the whole fermentation process[31]。 In the process of glycerol fermentation to ethanol,the anodic electrofermentation is driven by the synergistic metabolism of electrochemically active bacteria Geobacter sulfurreducens and fermentative bacteria Clostridium cellobioparum,which promotes the increase of glycerol consumption(up to 50 G/L)and ethanol production(up to 10 G/L)[32]。
图2 阳极电发酵的潜在机制：（a）强化电子传递，（b）氧化还原电位调控，（c）互营协作（黄色为微生物，橙色为电活性微生物，MedOx为氧化介质，MedRed为还原介质）

Fig. 2 Potential mechanisms of anodic electro-fermentation. (a) strengthening of electron transfer, (b) regulation of oxidation-reduction potential, (c) mutual collaboration (Yellow represents microorganisms, orange represents electroactive microorganisms, MedOx is the Oxidizing Medium, and MedRed is the reducing medium)

2.3 Extracellular electron transfer

According to the principle of anode and cathode electrofermentation,it can be seen that the mechanism of Extracellular electron transfer(EET)includes the process of electron transfer between microorganisms and the process of electron transfer between microorganisms and electrodes.Both electron transfer processes include two main electron transfer pathways:Direct electron transfer(DET)and Indirect electron transfer(IET)[33,34]。
As shown in Table 1,direct electron transfer between microorganisms mainly includes direct contact pathway and nanowire pathway;Indirect electron transfer mainly includes electron shuttle pathway and conductive material mediated pathway[35⇓⇓~38]。
表1 Extracellular Electron Transfer Pathway between Microorganisms

Table 1 Extracellular electron transfer pathways between microorganisms

Electronic transfer type Electron transfer mechanism Notes
Direct electron transfer Direct contact route Microorganisms form aggregates or directly contact solid surfaces, and electron transfer between microorganisms or between microorganisms and electrodes is carried out through the pigment proteins on the surface of microorganisms
Nanowire pathway Microbial conductive pili mediate electron transfer between microorganisms
Indirect electron transfer Electron shuttle mediated Redox substances present in the environment or secreted by microorganisms themselves, such as H2, formic acid, phenazine, humic acid, riboflavin, and anthraquinone, mediate electron transfer
Conductive materials mediate Electron transfer mediated by non-biological conductive materials such as carbon, metal elements, metal oxides, and alloys
As shown in Fig.3,it is currently believed that both direct electron transfer and indirect electron transfer exist between the microorganism and the electrode.(1)direct electron transfer is mainly performed by direct contact between microorganisms and solid electrodes(Fig.3A)or mediated by microbial nanowires(Fig.3B).Geobacter biofilm can receive electrons directly from the solid graphite electrode to reduce nitrate to nitrite[39]。 the Shewanella oneidensis MR 1 and the G.sulfurreducens can realize direct electron transfer between microorganisms and an electrode surface by utilizing pili or nanowires with high electric conductivity on the outer part of the cells[40,41]; (2)Indirect electron transfer is mediated by artificially added electron mediators(such as iron oxides,activated carbon,conductive biochar,and trace elements)(Figure 3C)or primary metabolites produced by microorganisms(such as H2,formic acid,etc.)(Figure 3D).For example,Deutzmann et al.Reported that M.maripaludis secretes hydrogenase and formate dehydrogenase at the cathode,catalyzing the generation of M.Maripaludis and formate,respectively[42]。 These two low molecular weight products can act as electron donors in the electrofermentation system and support the indirect electron transfer between the M.maripaludis and the cathode.Cai et al.Transferred electrons into the soil by applying an external voltage to the electrode,and demonstrated for the first time the possibility of realizing long-range electron transfer based on an exogenous electron medium(biochar)in a large soil matrix,which provided an idea for further promoting electron transfer in soil[43]。
图3 微生物与电极之间的（a,b）直接电子传递和（c,d）间接电子传递过程

Fig. 3 (a,b) Direct electron transfer and (c,d) Indirect electron transfer processes between microorganisms and electrodes

3 Electrode materials used in electrofermentation

As the electron donor(cathode)and electron acceptor(anode)of microorganisms,the selection of electrode materials plays a vital role in the electrofermentation reaction and the formation of the target product[44,45]。 In order to improve the product selectivity and the overall current efficiency of the electrofermentation system,it is generally believed that the electrode materials should have the characteristics of biocompatibility,high conductivity,large specific surface area,chemical stability and low cost[46⇓⇓⇓⇓~51]。 Common electrode materials include carbon-based materials,metal-based materials and composite materials。

3.1 Carbon-based material

carbon-based electrodes are the most commonly used electrode materials in electrofermentation due to their excellent corrosion resistance,low cost,and biocompatibility.Porous Carbon-based electrodes can increase the specific surface area for microbial attachment,thereby enhancing the current density[52]。 Commonly used carbon-based electrodes include graphite,carbon rod,carbon block,carbon cloth,carbon felt,activated carbon,etc.Since Nevin et al.Used graphite cathode to successfully reduce CO2in 2010,researchers have found that the good conductivity and stability of carbon-based materials have a positive impact on microbial electrofermentation system,and more carbon-based materials have been used in electrofermentation system 。
graphite,carbon cloth and carbon felt are the most commonly used electrode materials in electrofermentation systems.For example,in the process of methane production by electrofermentation,the introduction of conductive Graphite electrode increased the methane production by 54.3%[53]。 The production of lactic acid and hydrogen can be effectively promoted by constructing Thermotoga neapolitana electroactive biofilm on carbon cloth[54]。 the introduction of carbon felt as a cathode in the electric fermentation can increase the yield of butyric acid in rice straw,and the yield of butyric acid in the electric fermentation is increased by 28.13%compared with the open circuit control group[28]。
In addition,emerging carbon materials such as carbon nanotubes and graphene have also been used to modify and strengthen carbon-based electrodes.For example,taking advantage of the unique conductive and porous structure of carbon nanotubes,researchers used carbon nanotubes to construct porous electrode structures on the surfaces of carbon paper,carbon cloth,and graphite electrodes,and found that the carbon nanotube network structure can fully enhance the electron transfer between the electrode and cytochrome C,and enhance the attachment of microorganisms on the surface[55]。 Cui Yi et al.Designed and prepared three-dimensional microporous electrodes by using carbon nanotubes for interface modification[56]; Carbon nanotubes provide more active sites for the attachment of microorganisms and accelerate the electron transfer from electrochemically active microorganisms to the anode.Graphene has good conductivity and high specific surface area[57]。 the researchers constructed a composite mesh electrode structure on the surface of the carbon cloth anode by deposition to promote the growth of microorganisms on the electrode surface[58]。 the graphene structure can be formed in situ on the surface of the graphite electrode by an electrochemical exfoliation method,and the internal resistance of electron transfer of the electrode with the graphene structure is reduced by one third compared with that of a common graphite electrode,so that more current is generated in the microbial fuel cell[59]。
Although carbon-based electrodes are widely used in electrofermentation systems,there are still some defects to be broken through in different carbon-based electrodes.For example,the smooth surface of graphene is not conducive to the adhesion of electrochemically active bacteria;the diffusion and mass transfer efficiency of substances in the carbon felt electrode system need to be further improved,and the mechanical strength and plasticity of carbon cloth are low.Therefore,it is necessary to develop electrode materials with both excellent electrochemical properties and plasticity to enhance the reaction efficiency of the electrofermentation system[60,61]。

3.2 Metal based material

in addition to carbon-based materials,highly conductive metal materials(such as stainless steel,nickel,iron and copper)have also been used to construct electrodes In electric fermentation systems[62⇓⇓~65]。 Among them,stainless steel is the most promising electrode material because of its high conductivity,low cost and great potential for expansion,which can effectively solve the problem of electrode selection for large-scale applications.the stainless steel electrode has proved to be of great potential in the treatment of real wastewater by electrofermentation[66,67]。 Xie et al.Compared different electrode materials,including carbon electrode,stainless steel mesh electrode and Cu/Ni rod electrode,in the study of biological hydrogen production from organic wastewater,and found that the large specific surface area of stainless steel mesh effectively promoted the electron transfer in the process of organic matter degradation and reduced the internal resistance of the electrofermentation system[68]。 the configuration of these electrodes effectively increases the specific surface area of the electrode,providing a favorable environment for the enrichment of electrochemically active bacteria.In addition,stainless steel fiber felt is a commercially available and inexpensive 3D porous material with good conductivity,high specific surface area,and flexibility,which has been used as an efficient cathode material.the performance of the microbial electrolysis cell with stainless steel fiber felt cathode was comparable to that of the cell with Pt/C cathode[69]。
nickel(Ni)as well as different forms of Nickel alloys(NiMo,NiW,NiFeMo,NiFe,NiFeP,NiFeCoP and NiCr)are also widely used as cathode materials for microbial electrolyzers[70]。 This is due to the lower onset potential(−84 mV),lower overpotential(140∼250 mV),smaller electron transfer resistance,and higher current density（10~20 mA·cm−2）of the Ni-based electrode[71]。 Bachvarov et al.Studied NiFeCo and NiFeCoP alloys as cathodes in microbial electrolyzers,and found that the presence of Fe and Co in nickel alloys significantly reduced the overpotential of hydrogen evolution reaction,and the internal resistance of electron transfer was reduced to about 1/1000 of the original compared with pure Ni electrode[72]。 Cai et al.Fabricated a self-assembled 3D NiFe graphene cathode using a simple hydrothermal method,obtaining better electrochemical activity and efficient electron transfer[73]。
Similar to stainless steel and nickel-based electrodes,the ductile metal Cu also has good thermal and electrical conductivity.Therefore,copper was also selected as the cathode material for microbial electrolyzers to explore the feasibility in real industrial wastewater[62]。 In addition to this,molybdenum sulfide（MoSX）is a promising material for hydrogen evolution reaction with excellent chemical stability,and is inexpensive.Molybdenum sulfide can be detailed as an ionic compound containing cations of Mo4+and sulfide（S2-）and disulfide anions.For molybdenum sulfide,Kokko et al.Showed that hydrogen atoms can easily combine with sulfur anions at the edge of the MoSXlayer,which is helpful to improve the hydrogen evolution rate of the microbial electrolyzer[74]。
Metal-based electrodes have increasingly become common electrodes in electrofermentation systems[62,64,65,75]。 Although metal-based electrode materials have excellent electrochemical characteristics,most metal electrodes are corroded and passivated under reaction conditions,and their biocompatibility is much lower than that of carbon-based electrodes,which limits their application in electrofermentation systems[76]。

3.3 Composite matrix material

in order to obtain electrodes with high electron transfer efficiency,good biocompatibility,low cost and excellent electrochemical characteristics,the preparation of modified composite electrodes has become a research hotspot In the field of electrofermentation[77]。 modified composite electrodes mainly include conductive polymer composite carbon-based materials,metal/metal oxide modified carbon-based materials and carbon materials modified metal-based materials.For example,Wang et al.Successfully synthesized polyaniline-modified carbon felt as the anode of electric fermentation by electrochemical polymerization,which reduced the internal resistance of the system from 358Ωto 156Ωcompared with the unmodified carbon felt[78]。 Chen et al.Fabricated CNFs-SSM composite electrodes by directly growing carbon nanofibers(CNFs)on stainless steel(SSM)by chemical vapor deposition[79]。 stainless steel is used as a conductive network to ensure the efficient transfer of electrons and protons,and carbon nanofibers provide a place for microbial attachment.the experimental results showed that under the same experimental conditions,the current generated by CNFs-SSM electrode was 200 times higher than that of the Stainless steel electrode control group,which provided a simple and potential new method for the preparation of high-performance and low-cost electrode materials for electric fermentation system.A novel hierarchical porous silicon/nitrogen-doped carbon composite(Si/N-PC)was prepared by simple biomass fermentation using inexpensive wheat flour,and the composite was used as an anode.the results show that the N-doped layered porous structure is helpful to slow down the volume change,ensure the electrode stability,and improve the conductivity of Si/N-PC composite electrode[80]。 Compared with the traditional carbon nanotube anode,the nitrogen-doped carbon nanotube electrode generates hydrophilic defects on the surface of carbon nanotubes,which promotes the electron transfer of proteins on the cell membrane and improves the performance of the electric fermentation system[81]。 Yellappa et al.Used a polyaniline/carbon nanotube(PANi/CNT)composite electrode to treat urban leachate,and found that the modified electrode showed higher redox peak current,lower electron transfer resistance and ohmic loss,which improved the bioelectrical activity and electron transfer ability between the electrode and organisms[82]。

4 Microorganisms in Electrofermentative System

Microbial cells are generally non-conductive because their cell membranes contain non-conductive substances,such as polysaccharides,lipids,and peptidoglycans.Electrochemically active bacteria(EAB)are an important group of bacteria in electrofermentation[21]。 in the electrofermentation system,electrochemically active bacteria can complete the reaction through extracellular electron transfer and intracellular electron transfer to achieve the degradation of target pollutants and the generation of target products.As mentioned above,there are two main mechanisms of extracellular electron transfer:one is direct electron transfer using cytochrome C and"nanowires";the second is indirect electron transfer using endogenous or exogenous electron transfer mediators.In addition,corresponding to extracellular electron transfer is intracellular electron transfer,that is,electron acceptors(oxygen,nitrate,etc.)enter the cell through substrate diffusion,receive electrons generated In the substrate oxidation reaction,and then be reduced。
A variety of microorganisms have been shown to be Electrochemically active,including bacteria(e.g.,Geobacter),archaea(e.g.,Ferroplasma),and fungi(e.g.,Saccharomyces),among others.in the electro-fermentation system,electrochemically active bacteria are the most widely used.electrochemically active bacteria are widely distributed In phyla,such as Protebacteria,Firmicutes,Acidobacteria and Actinobacteria.At present,Geobacteraceae,Shewanellaceae,Petococca ceae and Desulfobulba ceae have been isolated and confirmed。
Geobacteraceae is a Gram-negative microorganism with a short rod(0.5μm×(1.2~1.5)μm)shape,which is strictly anaerobic and suitable for growth at 30~35℃.as the most common electrochemically active bacteria,Geobacteraceae has a variety of electron transfer forms,not only using soluble electron acceptors such as nitrate,fumarate and chlorine-containing compounds for intracellular electron transfer,but also using insoluble electron acceptors such as iron(manganese)oxides,electrodes and humus for extracellular electron transfer[40]。 Among them,iron(manganese)respiration is the most typical form of extracellular electron transport In Geobacteraceae.in this process,Geobacteraceae oxidizes organic matter to release electrons,which are transferred to the outside of the cell through the extracellular electron transport chain and ultimately used for the reduction of extracellular iron oxides.the results show that the electron transport chain is composed of conductive nanowires and cytochrome C[40]。 In microbial fuel cells,Geobacteraceae can use artificial electrodes as extracellular electron acceptors[83]。 The diversity of electron transport forms in Geobacteraceae determines its wide distribution.Geobacteraceae has been found in a variety of environments,including soil,groundwater,activated sludge,marine and estuarine sediments[84]。
Shewanella ceae is a gram-negative microorganism,which is straight or curved rod-shaped,flagellated,strictly anaerobic,and suitable for growth at temperature of 10~36℃and pH of 6~9.Shewanellaceae can release electrons by oxidizing organic matter,which are transferred to the iron-containing protein cytochrome A through the quinone pool on the cytoplasmic membrane,and then transferred to various metalloreductases OmcB embedded in the outer membrane through a variety of functional proteins CytC3 or PpcA distributed in the periplasmic space to carry out electron transfer and participate in the iron cycle[85]。 The genus Shewanella ceae is widely distributed in nature,including deep-cold-water marine environments,shallow Antarctic habitats,and freshwater lakes.it is of great significance for environmental governance because It can convert heavy metals and toxic substances(such as iron,sulfur and uranium)into less toxic products[86]。 Shewanella oneidensis MR-1 is one of the well-studied xenobiotic microorganisms and has shown great promise as a core biocatalyst in bioelectrochemical systems[87]。 Geobacteraceae and Shewanella ceae can not only produce electricity,hydrogen and methane through extracellular electron transfer,but also produce high-value chemicals through intracellular electron transfer[88,89]。
Peptococcaceae is a Gram-positive microorganism,which is spherical in shape,strictly anaerobic,and suitable for growth at 37℃.The cells are 0.5 to 2.5μm in diameter,without flagella and endogenous spores,and can ferment amino acids or carbohydrates to produce lower fatty acids,CO2and H2,or succinic acid or ethanol,or lactic acid by heterofermentation.Peptococcaceae is common in the oral,intestinal,and respiratory tracts of humans and animals.Thermincola ferriacetica and Thermincola potens are two thermophilic metal-reducing bacteria in the genus Peptococcaceae,which can generate current using acetic acid as a substrate in a microbial fuel cell[90]。
Desulfobulba ceae is a Gram-negative microorganism,which is filamentous,strictly anaerobic,and suitable for growth at 25~40℃.Most species of the genus Desulfobulbaceae are considered to be sulfate-reducing microorganisms,which can transfer electrons from various reducing compounds to electron acceptors other than oxygen,and are found mainly in marine sediments,hydrothermal vents,acid mine drainage,and alkaline lakes[91]。 Since the discovery of bioelectrochemical systems such as microbial fuel cells,they have often been observed in anode communities and biofilms[92]。 Examples of studies on related electrochemically active bacteria are shown in Table 2。
表2 Main electroactive microorganisms in electrofermentation system

Table 2 Main electroactive microorganisms in the electro-fermentation system

Strain Genus Example
Geobacter sulfurreducens Geobacteraceae Acetate as an electron donor for G. sulfurreducens to produce methane[93]
Geobacter metallireducens Geobacteraceae G. metallireducens acts as an electron donor, transferring electrons to Rhodopseudomonas palustris to achieve CO2 fixation[94]
Shewanella oneidensis MR-1 Shewanellaceae In microbial fuel cells, when lactic acid is used as a carbon source, it can be metabolized by S. oneidensis MR-1 to form DADH, and the released electrons can be transferred to the anode to form a current[95]
Shewanella loihica Shewanellaceae Shewanella loihica PV-4 can reduce fumarate by using the electrode as the sole electron donor[96]
Thermincola potens Peptococcaceae In microbial fuel cells, proteins on the surface of Thermincola potens JR cells transfer electrons to the electrode to reduce Fe (Ⅲ)[97]
Thermincola ferriacetica Peptococcaceae In microbial fuel cells, T. ferricetica can directly transfer electrons from acetic acid to the anode working electrode to generate current[90]
Calditerrivibrio nitroreducens Desulfobulbaceae In microbial fuel cells, C. nitroreducens can utilize nitrate as an electron acceptor, and the system stably generates a current density of 272 mW·m-2[98]
Desulfobulbus propionicus Desulfobulbaceae Desulfobulbus propionicus uses pyruvate as an electron donor to transfer electrons to the anode working electrode and convert them into acetate[99]
In addition to electrochemically active bacteria,there is also an important flora in the electrofermentation system,which is the main player in the degradation of target pollutants or the generation of target products,called functional flora.The functional flora includes hydrolytic bacteria that hydrolyze complex organic matter into simple compounds;Acetogens that convert simple organic compounds into VFAs(such as acetic acid)while producing H2and CO2;Methane bacteria that produce methane from acetic acid or H2and CO2;And a carbon chain elongator for elongating the carbon chain of a short-chain fatty acid to synthesize MCFAs[100,101]。 In the electro-fermentation system,the efficient recycling of organic waste is often achieved through the synergistic effect of electrochemical active bacteria and specific functional bacteria。

5 Single-chamber and two-chamber electric fermentation system

the electrofermentation system is usually designed as a two-chamber structure,with The anode and cathode separated by a diaphragm,forming two relatively independent units,as shown in Figure 4A[102]。 Components that can be used for the diaphragm mainly include a proton-exchange membrane,an anion-exchange membrane,and a cation-exchange membrane,of which the proton-exchange membrane is the most commonly used[103]。 the advantage of double-chamber electrofermentation is that the membrane module divides the system into two relatively independent units,and the anode reaction and the cathode reaction are separated from each other,thus reducing the interaction of microorganisms between the cathode and the anode,which is more conducive to the optimization of microorganisms in the cathode and anode and the purification of products[61]。 However,the presence of the membrane may affect the rate of H+from the anode to the cathode,resulting in the accumulation of H+in the anode,resulting in a pH gradient between the two chambers,which is not conducive to the growth and reproduction of microorganisms in the anode,and increases the internal resistance of the system,reduces the rate of product formation,and increases energy consumption[104,105]。
图4 （a）双室电发酵反应器，（b）单室电发酵反应器

Fig. 4 (a) Dual chamber electric fermentation reactor, (b) single chamber electric fermentation reactor

In order to further reduce the internal resistance and operating cost of the reactor,a membraneless single-chamber electric fermentation system was developed(Fig.4B)[106,107]。 the greatest advantage of the single-chamber electric fermentation system is that it eliminates the large internal resistance of the system caused by the ion exchange membrane,and is conducive to the material exchange between the two cathodes and anodes,so that the current density is increased,the production rate is improved,and the equipment cost and operation cost are reduced[108]。 Wang et al.Showed that because there was no membrane in the single chamber electrofermentation,it not only reduced the internal resistance of the system,but also promoted the material exchange between the anode and cathode chambers,and improved the efficiency of biogas generation[109]。 However,the single-chamber electrofermentation system is not conducive to the optimization of cathode and anode microorganisms,which may be due to the different microorganisms needed in the cathode and anode chambers,and the interaction between cathode and anode microorganisms in the single-chamber system may reduce the stability of the system[110]。 Call et al.Used membraneless single-chamber electrofermentation to produce H2,and the hydrogen production rate was as high as（3.12±0.02）m3H2/m3per day,but the hydrogen recovery rate was unstable and showed a downward trend due to the long circulation time of the reactor[111]。
the two-chamber electric fermentation system is mostly used for the screening and enrichment of electrogenic bacteria and the performance testing of electrode materials and exchange membranes,while the single-chamber electric fermentation system is mostly used to improve the overall economy of information and provide higher power output[112⇓~114]。 Therefore,the structural differences between single-chamber and double-chamber electric fermentation systems determine their scope of application。

6 Method for improving operation of electric fermentation system

6.1 Conditional optimization

6.1.1 pH

pH is one of the main factors affecting the operation effect of electrofermentation,which will affect the growth of microorganisms,the concentration of ions,the motive force of protons,the enzyme activity of microorganisms and the selective permeability of cell membrane[115]。 At different pH values,different types of fermentation occur and different metabolites are produced due to the interaction between microorganisms and enzymes[116]。 For example,the study on the fermentation of food waste under different pH conditions showed that when pH=3.2,the product was mainly lactic acid,and Lactobacillus was the preferred flora.when pH increased to 4.5,Bifidoba cteria significantly increased and promoted the production of acetic acid,and butyric acid was observed when pH was 4.7~5.0[117]。 Hashemi et al.Showed that too high or too low pH would reduce the activity ofα-amylase,even lead to enzyme inactivation,thus inhibiting the metabolic activity of microorganisms[118]。 Therefore,controlling the pH of the fermentation system can help to avoid this series of effects[119]。 the complete utilization of substrate can be achieved by periodically adjusting the pH value of the electrofermentation system.In addition,controlling pH can also affect the distribution of products.For example,lowering the pH of the reaction system from 5.75 to 4.75 promoted the formation of alcohols(mainly ethanol),but inhibited the formation of acetic acid[120]。 When fructose was used as substrate and the pH value was controlled at 5.8,the conversion rate of fructose reached 100%,and the formation of organic acids and alcohols increased by 76.6%[121]。
Therefore,in order to maintain the stable operation of the reaction system,it is very important to maintain a relatively suitable pH value.There are two ways to keep the reaction system at a suitable pH:(1)adding a buffer solution to increase the buffer capacity of the reaction system,thereby maintaining a more stable pH[122]; and(2)monitoring And adjusting the pH in real time during the reaction to maintain the stability of the pH during the reaction。

6.1.2 Temperature

Temperature is an important factor affecting the growth and biochemical reactions of microorganisms.Temperature affects the growth of bacteria,the degradation of substrates and the distribution of metabolites.The temperature range design of the electrofermentation process includes a low temperature section(ambient temperature),a medium temperature section(30–38°C),and a high temperature section(50–57°C).In order to maintain stable performance in long-term operation,the temperature of the reaction system should conform to the optimal temperature for microbial growth and metabolism.Wang et al.Confirmed the effect of temperature on H2generation[123]。 The yields of H2at 30,35,40,45,and 50°C were 0.038,0.056,0.046,0.036,and 0.019 m3·m-3·d-1,respectively,proving that the optimum temperature for H2production in the electrofermentation system was 35°C.However,under certain circumstances,increasing the temperature of electric fermentation can reduce the activation overpotential and enhance substrate diffusion,so pyrophilic fermentation may be superior to mesophilic electric fermentation in terms of activity and durability.In this regard,Yu et al.Demonstrated the potential of pyrophilic synthesis of organic acids and achieved enhanced performance by immobilizing thermophilic microorganisms on the cathode at 55°C,significantly better than mesophilic electrofermentation(37°C )[24]。 More often,H2have been shown to be generated in the low temperature range,but the system may fail because the mesophilic community is difficult to fully adapt to lower temperatures.Xu et al.Showed that H2could be successfully produced in electrofermentation at 0°C,which is of great significance for the generation of biological resources in low temperature areas[125]。

6.1.3 Working electrode potential

the working electrode potential is one of the important factors affecting the operation of the electrofermentation reactor.In the electric fermentation system,increasing the working potential can increase the electron transfer rate,accelerate the rate of electrons received by the cathode,and promote the reaction process.the working electrode potential can also change the metabolism of microorganisms,domesticate the generation of microorganisms,improve the activity of microorganisms,and promote the reaction of the working electrode.when the bioelectric fermentation reactor was used to produce hydrogen from molasses alcohol wastewater at low temperature,the hydrogen production reached the maximum When the external voltage was 0.8 V[126]。 Mukherjee et al.Controlled the metabolism of Bacillus subtilis and the production of target products by changing the equilibrium potential(-0.8 to+0.8 vs Ag/AgCl)based on the regulation of NADH/NAD+ratio,and found that among all the applied potentials,a higher negative equilibrium potential(-0.8 V)was beneficial to the production of H2and fatty acids,while a positive potential(+0.6 V)was beneficial to the production of succinate[127]。 However,the applied voltage will bring additional operating costs,and too high voltage is not suitable for the energy-saving requirements of reactor design[25]。 Therefore,providing an appropriate working electrode potential is a key point in the research of electrofermentation reactors。

6.1.4 Substrate concentration

substrate concentration is one of the important parameters affecting fermentation performance.Usually,the growth activity of microorganisms increases with the increase of substrate concentration within a certain range,but when the substrate concentration exceeds a certain threshold,the higher concentration of substrate will inhibit the growth and reproduction of microorganisms[128]。 For example,high ethanol concentration(500 mmol/L)will promote microbial growth and caproate production,while ethanol concentration exceeding 700 mmol/L will inhibit microbial growth and affect the fermentation process[129]。 Therefore,Zhao et al.Used different concentrations of liquor(20%,40%,60%)to study the formation and mechanism of MCFAs by traditional fermentation and electric fermentation[130]。 When the substrate concentration was 40%,the highest concentrations of caproate and butyrate were obtained by both electric fermentation and traditional fermentation.Compared with the traditional fermentation,the total substrate consumption and product recovery of the electric fermentation system were increased by 2.6%~43.5%and 54.0%~83%,respectively.the results of microbial analysis showed that electrofermentation effectively alleviated substrate toxicity and enriched carbon chain elongators,especially Clostridium sensustricto12,thus promoting ethanol oxidation and product formation.Therefore,in order to obtain higher electrofermentation performance,it is necessary to determine the optimal range of substrate concentration and achieve efficient production of the target product[131]。

6.2 Conductive medium

In order to enhance the electron transfer process of the electric fermentation system,a conductive mediator was added to the reaction system[38,132]。 Appropriate conductive mediators can not only use their own electron conduction function to assist electron transfer,but also use their own physical properties(specific surface area,porous structure,surface functional groups,etc.)to assist microorganisms to form polymers and promote electron transfer[133,134]。 Carbon materials and electron mediators(such as organic redox mediators and iron oxide mediators)have been reported as additives。

6.2.1 Carbon material

In recent years,the use of conductive carbon-based materials as additives has been considered as a strategy to improve the performance of electrofermentation systems due to their excellent electrical conductivity,high specific surface area,chemical stability,and excellent catalytic activity[135,136]。 the addition of conductive carbon-based materials can not only trigger electron transfer between co-trophic microorganisms through their electrochemical properties,but also act as supporting carriers for microbial attachment and biomass growth,releasing trace metals and volatile substances to support The growth of fermentative bacteria[134,137,138]。 Yun et al.Made the residue of lignocellulose into a biological carbon-based additive[139]。 These biocarbon-based materials are composed of small-sized graphitic carbon skeletons with excellent electrical conductivity,and this unique characteristic facilitates direct electron transfer between fermentation bacteria and methanogens,accelerating the metabolism of acetic acid and the production of methane.carbon black,as a common carbon-based additive,is often used as a conductivity enhancer to improve desalination performance because it can act as a conductive bridge between the current collector and activated carbon[140]。 In addition,at the dose of 0.5 G/L,activated carbon promoted the activities of protease,glucanase and lipase,and significantly increased the abundance of Proteobacteria that can degrade organic compounds into acetic acid and other products[141]。 Therefore,the carbon-based material is added as an external conductive mediator,so that the target product microorganism does not need to carry out electron transfer through microbial connection,the direct electron transfer can be induced,the microbial syntrophic metabolism rate is accelerated,the activity of enzymes and proteins is improved,and the yield of the target product is improved。

6.2.2 Electron mediator

Although the uptake of exogenous electrons In cell culture-based electrofermentation systems is very low,the enhancement of their efficacy often depends on exogenous electron mediators.some redox-active substances,such as oxides containing iron or manganese,can act as mediators or acceptors for direct electron transfer in Some metal-reducing bacteria.in this case,they extend the distance of direct electron transport and maximize transport efficiency to couple intracellular redox reactions or metabolism with extracellular electron transport[142,143]。 Among them,magnetite has been found to promote interspecies electrons from Geobacter sp.It is transmitted to methanogens to promote methane production[144]。 In addition,it has also been reported that the oxidation of artificial redox mediators such as neutral red,methyl viologen,methylene blue,and anthraquinone-2,6-disulfonate promotes electron transfer[145⇓⇓~148]。 These small molecules exhibit strong redox activity,facilitating the absorption of electrons from the electrode by microorganisms and improving the efficiency of the electrofermentation system[149]。 For example,Neutral red is usually used as an electron mediator because of its low standard reduction potential(-525 mV vs Ag/AgCl),high stability,high solubility,and low toxicity.neutral red is used in an electrofermentation system to enhance the electron transfer between microorganisms and the electrode,thereby converting CO to valuable VFAs[150][151]。 Paiano et al.Studied the conversion of glucose to butyric acid by adding neutral red and anthraquinone-2,6-disulfonate in an electrofermentation system,and found that neutral red and anthraquinone-2,6-disulfonate provided the necessary reducing power to the microorganism and improved the selectivity of ethanol and acetic acid in the formation of butyric acid[152]。 Choi et al.took advantage of the catalytic ability of Clostridium tyrobutyricum,using the cathode as the electron donor[153]。 They observed that the addition of methyl viologen and neutral red increased butyrate production by 1.4-and 1.8-fold,respectively.In the methyl viologen added electrofermentation system,acetate was used as the electron donor of the cathode,and the ethanol concentration was increased by two times compared with the control group,while hydrogen was continuously generated In the system.In the study of electrofermentation-enhanced B.fiavum No.2247,P.freudenreichii and C.tyrobutyricum BAS7 fermentation,the fermentation system was supplemented with 0.01 mmol/L neutral red,0.40 mmol/L cobalt sulfate and 0.01 mmol/L neutral red as electron mediators,respectively.the results showed that the exogenous electron mediator could not only change the original metabolic distribution of cells,but also mediate the electron transfer between microorganisms and electrodes,and enhance the promotion of electrofermentation on cell product synthesis[153]。 In addition,Engel et al.Studied biobutanol production by electrofermentation of C.acetobuutylicum without using exogenous medium[154]。 the results showed that a higher product concentration could be obtained using the yeast-supplemented synthetic medium compared to the yeast-extract-supplemented synthetic medium.It can be seen that in order to enhance the fermentation performance of products through the electric fermentation system,the electron mediator is also an important factor affecting the synthesis of products。

6.3 Bioaugmentation

Recent studies have shown that bioaugmentation is an effective way to improve the performance of microbial communities,not only to obtain the desired product,but also to improve the interaction of microbial communities to better adapt to various environmental conditions[155,156]。 bioaugmentation in electrofermentation system mainly includes two aspects:(1)bioaugmentation of functional microorganisms.For example,the carbon chain elongation microorganism Clostridium kluyveri is used to enhance the production of MCFAs during the carbon chain elongation reaction.Zhang et al.Compared the carbon chain elongation performance with and without bioaugmentation,and further promoted the carbon chain elongation reaction by constructing Acetobacterium woodii and Clostridium kluyveri co-cultures[157]。 the results showed that the concentration of caproate reached 4.68 G/L in the electric fermentation system enhanced by functional microorganisms;(2)Bioaugmentation of electrochemically active microorganisms.For example,the electrofermentation process enhanced by electrochemically active microbial G.sulfurreducens increased the production of 1,3-propanediol by about 10%[18]。 Rahimi et al.promoted the growth of Clostridium butyricum and Clostridium beijerinckii in a bioelectrochemical system,which in turn Promoted butyric acid formation[158]。

7 Product of electric fermentation

An important application of electrofermentation is the generation of value-added products from organic waste.This section reviews some representative value-added products in electrofermentation,including methane,hydrogen,alcohols,volatile fatty acids(VFAs),high-value chemicals(such as medium-chain fatty acids(MCFAs),polyhydroxyalkanoates(PHA),polyhydroxybutyrate(PHB)),etc.,as shown in Fig.5。
图5 电发酵底物和产物

Fig. 5 Substrates and products of electro-fermentation

7.1 Methane

Methane is the main component of natural gas and biogas,and is an important energy resource,which can be used in gas engine combustion for power generation,boiler heating,etc.Methanogenesis by anaerobic fermentation is one of the effective ways to achieve energy recovery from waste,but traditional anaerobic fermentation is usually carried out in a sealed container under mesophilic conditions(35~37℃)or thermophilic conditions(55℃),so excessive acidification and systemic deterioration may occur[159]。 in addition,in the process of stepwise bioconversion from organic waste to biogas,organic substrates are easily decomposed into Volatile fatty acids(VFAs).When VFAs accumulate in the digester,the pH value will be significantly reduced,methanogens and some fermenting bacteria will be inhibited,and the process of conversion into methane will be quite slow[160]。 in recent years,In order to improve the stability of anaerobic fermentation and accelerate the production of methane,electrochemical methods have been proved to be an effective way to improve anaerobic digestion.In the electric fermentation system,soluble and available intermediate products are generated through the hydrolysis and primary fermentation of organic wastes,the intermediate products are used as substrates,and the electrode controls the metabolism of microorganisms in the substrates and the interaction between microbial populations,the charged electrode with powerful electron source can significantly change the extracellular electron transfer process of related microorganisms,consume a large number of hydrogen ions to slow down the decline of pH value,and ultimately achieve the purpose of accelerating the degradation rate and improving the methane production capacity of the system[10]。 the mechanism of electrofermentation promoting methane production from organic matter fermentation mainly includes four ways:(1)the cathode of electrofermentation provides additional electrons for cells to promote methane production.Cai et al.Used an electric fermentation system to promote methane production from organic waste(excess sludge),and found that hydrogenotrophic methanogens were enriched in the cathode of electric fermentation,which increased the methane production rate of the electric fermentation system by two times compared with the anaerobic fermentation system[161]; (2)Electro-fermentation can change CO2into methane through cathode reaction,which can improve the production of methane.Gerasimos et al.Studied the conversion of CO2to methane in the cathode by electrofermentation[162,163]。 The results show that the electrons from the electrode or organic matter react with the CO2to form methane,and the cathodic reaction is like：CO2+8H++8e-→CH4+2H2O;(3)Electrofermentation can produce H2at the cathode,so that the H2can react with CO2to synthesize methane through the action of methane bacteria.Gahyyn et al.Reported a similar study,which showed that the biocathode archaea community was dominated by methanobacteria,and promoted methanogenesis by participating in the direct or indirect（H2of the cathode to mediate the：2H++2e-→H2）electron transfer mechanism,such as：CO2+4H2→CH4+2H2O[164]; (4)Electrofermentation can produce H2at the cathode,so that homoacetogenic bacteria can use H2and CO2to produce acetic acid,and then decompose to produce methane.Oh et al.Showed similar results with a reaction formula of：$\mathrm{CH}_{3}\mathrm{COO}^{-}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{CH}_{4}+\mathrm{HCO}_{3}^{-}$[165]。

7.2 Hydrogen

Hydrogen,as a clean and renewable energy,is widely used as a key starting fuel/chemical in different industrial developments[166]。 Hydrogen production by biological anaerobic fermentation is one of the important hydrogen production technologies.Due to the limitation of thermodynamic conditions,traditional fermentative hydrogen production can only proceed to the carboxylic acid stage,and carboxylic acids cannot be naturally fermented to produce hydrogen,which makes it difficult to extract hydrogen energy from biomass.Electrofermentation provides a viable method for hydrogen generation by cathodic reduction.The mechanism of hydrogen production from organic matter fermentation by electrofermentation mainly includes three ways:(1)Electrofermentation can degrade the substrate by electroactive microorganisms attached to the anode of electrofermentation,releasing protons and electrons,and the electrons are directly transferred to the cathode to reduce H+to H2.Sun et al.Reported that H2was produced in the cathode of the electrofermentation system,and the hydrogen production rate reached（2.2±0.2）mL·L-1·d-1under the phosphate buffer of 10 mmol·L-1by providing electrons from the cathode[167]; (2)Electrofermentation can enhance the production of H2by inhibiting hydrogen-consuming methanogens,homoacetogens,and sulfate-reducing bacteria[168]。 Singh et al.Found that the electro-fermentation system could inhibit the production of methane and promote the production of H2[169]; (3)Electrofermentation can produce H2at the cathode through acetate.Liu et al.Demonstrated that electrofermentation can overcome the thermodynamic limitation of anaerobic fermentation to produce H2from acetate[170]。 The reaction formula is：$\mathrm{CH}_{3}\mathrm{COO}^{-}+4\mathrm{H}_{2}\mathrm{O}\rightarrow 2\mathrm{HCO}_{3}^{-}+\mathrm{H}^{+}+4\mathrm{H}_{2}$ 。

7.3 Alcohols

alcohols are hydroxyl compounds with hydroxyl groups or carbon in The side chain of benzene ring in the molecule,which have high economic value and are used in various chemicals and fuels for related industrial applications.the production of Alcohols has been demonstrated by gas fermentation and organic waste fermentation[171]。 However,gas fermentation requires not only CO2as raw material,but also H2and/or CO to provide reducing power for the reaction.Electro-fermentation can use CO2as the only carbonaceous raw material,which provides a potential opportunity for sustainable production of alcohols.At present,organic matter fermentation can be promoted to produce alcohols such as ethanol,butanol and hexanol by electric fermentation.The mechanism of electro-fermentation promoting organic matter fermentation to produce alcohols mainly includes three ways:(1)Electro-fermentation can use CO2as carbon source and directly reduce it to alcohols on the cathode.Vassilev et al.Reported a specific hybrid microbiome reactor capable of generating a mixture of C4and C6corresponding alcohols(isobutanol,n-butanol,and n-hexanol)using CO2as the sole carbon source and the reducing power provided by the electrodes[172]; [38,173]; (3)Electrofermentation can use acetic acid as an electron acceptor to produce ethanol at the cathode.Blasco et al.Found that H2accumulated in the biocathode could reduce CO2to acetate,and then molecular hydrogen acted as an electron donor to obtain ethanol[174]。 The reaction scheme is as follows：2H++2e-→H2,4H2+2CO2→Acetate-+H++2H2O,Acetate-+H++2H2→Ethanol+H2O 。

7.4 Volatile fatty acid

Volatile fatty acids(VFAs),also known as short-chain fatty acids(SCFAs),refer to saturated carboxylic acids of Volatile fatty acids,including acetic acid（C2H4O2）,propionic acid（C3H6O2）,butyric acid（C4H8O2）,and valeric acid（C5H10O2）[175⇓~177]。 In recent years,the generation of VFAs from organic wastes by anaerobic fermentation is an effective biorefinery technology[178,179]。 However,CO2and H2have always been by-products in the anaerobic generation of VFAs,which are called acidogenic waste gases.It has been reported that the acidogenic waste gas generated during anaerobic fermentation can account for up to 30%of the substrate consumed[180]。 The acidogenic off-gas accumulates in the headspace of the acidogenic reactor,resulting in an increase in the pressure of CO2and H2,inhibiting the generation of VFAs.Electrofermentation proposed three ways to promote VFAs production from organic matter:(1)VFAs production from acid-producing waste gas(mainly CO2and H2）)at the cathode of electrofermentation.Zhou et al.Fermented organic wastewater to produce VFAs,and found that the production of VFAs could be effectively improved by using acid-producing waste gas,which was 48.70%higher than that of the control group[181]。 By utilizing the acid-producing waste gas,the inhibition of the headspace H2partial pressure is eliminated,the generation of VFAs can be accelerated,and the reaction formula is：4H2+2CO2→CH3COOH+2H2O;(2)Acetogens use electrons from the cathode to reduce CO2to produce VFAs.Nevin et al.Found that Sporomusa ovata attached to the cathode consumes electrons to reduce CO2to acetic acid,and the reaction formula is：2H2O+2CO2→CH3COOH+2O2[26]; (3)producing VFAs by using lactic acid at the cathode of electric fermentation.Similar results were reported by Isipato et al.,who fermented lactic acid as a substrate to produce propionic acid at the cathode with the reaction equation：$3\mathrm{C}_{3}\mathrm{H}_{5}\mathrm{O}_{3}^{-}\rightarrow 2\mathrm{C}_{3}\mathrm{H}_{5}\mathrm{O}_{2}^{-}+\mathrm{C}_{2}\mathrm{H}_{3}\mathrm{O}_{2}^{-}+\mathrm{CO}_{2}+\mathrm{H}_{2}\mathrm{O}$[182]。

7.5 High-value chemicals

Medium-chain fatty acids(MCFAs),Polyhydroxyalkanoates(PHA)and polyhydroxybutyrate(PHB)are high-value chemicals produced by electrofermentation。
MCFAs refers to saturated carboxylic acids of C6~C12,Include caproic acid（C6H12O2）,heptanoic acid（C7H14O2）,caprylic acid（C6H12O2）,Pelargonic acid（C8H16O2）,capric acid（C9H18O2）,undecanoic acid（C11H22O2）and dodecanoic acid（C12H24O2）,It has strong hydrophobicity and low separation cost,and is an important chemical raw material and agricultural product.Anaerobic fermentation provides a sustainable method for the generation of MCFAs.In the process of anaerobic fermentation,small molecular substances such as VFAs and CO2can produce MCFAs by carbon chain elongation through the action of microorganisms[183]。 However,the carbon chain elongation process of anaerobic fermentation is often limited by the accumulation of intermediates.As an attractive method to promote the carbon chain elongation process,electrofermentation is expected to achieve higher energy efficiency than the traditional anaerobic fermentation pathway of organic waste to produce MCFAs.Electrofermentation proposed three ways to promote the production of MCFAs from organic matter:(1)Electrofermentation provides electrons through the cathode to extend the carbon chain of VFAs to produce MCFAs.In the absence of chemical electron donors,acetic acid was reduced to ethanol by electrofermentation cathode,and then the production of MCFAs was realized[184]; [185]; (3)electrofermentation can promote the carbon chain elongation of VFAs to produce MCFAs in the presence of chemical electron donors.Van Eerten-Jansen et al.Used acetic acid and ethanol as substrates to enhance the production of caproic acid by Electrofermentation under the action of Clostridium[184,186]。
PHA is a natural polyester synthesized by microorganisms under conditions of carbon source excess and nutrient limitation.it is biodegradable in nature and is considered to be an alternative to petrochemical-based polymers.in addition,It has good application prospects in biomedicine[187,188]。 Fermentation broth rich in VFAs is an effective substrate for PHA biosynthesis under laboratory conditions[189⇓~191]。 Therefore,VFAs can be converted to PHA at the cathode by electrofermentation.Srikanth et al.Studied the production of PHA in the electric fermentation system,and observed that the concentration of VFAs in the cathode chamber was gradually converted into PHA with time,and the accumulation of PHA was realized[192]。
PHB is a bioplastic synthesized by microorganisms.It is a carbon and energy reserve in the cell when microorganisms are grown under environmental stress conditions[193]。 PHB can be produced by glycerol fermentation,but the slow growth of microorganisms,product inhibition and high operation cost of pure bacteria system limit the development and application of PHB synthesis by biological fermentation.Lai et al.Found that electrofermentation of glycerol effectively promoted microbial cell growth and substrate consumption,alleviated microbial growth inhibition,significantly promoted PHB synthesis and accumulation,and increased PHB production by 30%[194]。

8 Conclusion and prospect

Compared with anaerobic fermentation system,electrofermentation system,As a new system combining biological reaction and electrochemical reaction,has shown good application potential in promoting waste degradation and enhancing product generation.as an emerging technology,electrofermentation technology faces many scientific,technical and economic challenges。
(1)the principle of microbial metabolism enhanced by electrofermentation system and the mechanism of extracellular electron transfer are still unclear.Although a large number of studies have observed the enhancement of microbial metabolism and electron transfer by electrofermentation system,the research on the mechanism of enhancement is still in its infancy,and how electrochemical action regulates microbial metabolism,promotes extracellular electron transfer,and enhances the conversion of substrate to product remains to be further explored.the revelation of the mechanism can provide a theoretical basis for the further optimization and application of the electrofermentation system。
(2)the material and structure of the electric fermentation reactor need to be further optimized.for the diaphragm of double-chamber reactor,it is necessary to develop ion exchange membranes with stronger ion exchange capacity,higher mechanical strength and toughness,and lower cost,so as to reduce the internal resistance of the electric fermentation system and improve the efficiency of electron transfer;For working electrodes,it is also necessary to develop electrode materials with high conductivity,good biocompatibility,high chemical stability and low cost For specific scenarios to promote efficient electron transfer between microorganisms and electrodes。
(3)the research on the synthesis of high-value chemicals by electrofermentation needs to be strengthened.At present,most of the studies on electrofermentation system focus on the production of hydrogen,methane and VFAs,while There are few reports on high-value chemicals(such as MCFAs,PHA and PHB).there is still a lack of research on higher value products(such as biofuel,bioprotein,organic polymer,etc.).In the future,the research on higher value products should be strengthened,the types of higher value products should be expanded,the generation of higher value product should be realized,and the operation benefit of electric fermentation reaction system should be improved。
(4)there is a lack of large-scale practical research,and the economy of the reaction system needs to be further improved and verified.Due to the limitations of technology and economy,most of the existing studies are still In the laboratory stage,and There is still a lack of large-scale application research.in order to improve the economy of the electric fermentation system,on the one hand,it is necessary to reduce the investment cost of the electric fermentation system from the aspect of reactor materials;on the other hand,it is necessary to strengthen the research on the generation technology of high-value products,further develop the generation of higher-value products,improve the economic and environmental benefits of the electric fermentation system,and provide technical support for the industrial application of the electric fermentation system。
to sum up,by further deepening the mechanism research,improving the material research,strengthening the method research and carrying out the application research,the electrofermentation system is expected To be applied in many fields such as waste degradation and biochemical synthesis,which provides a new method with both environmental and economic benefits for the efficient recycling of organic waste。

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