The electrochemical perspective of bioelectrocatalytic activities in microbial electrolysis and microbial fuel cells

Yasri, Nael; Roberts, Edward P. L.; Gunasekaran, Sundaram Article The electrochemical perspective of bioelectrocatalytic activities in microbial electrolysis and microbial fuel cells Energy Reports Provided in Cooperation with: Elsevier Suggested Citation: Yasri, Nael; Roberts, Edward P. L.; Gunasekaran, Sundaram (2019) : The electrochemical perspective of bioelectrocatalytic activities in microbial electrolysis and microbial fuel cells, Energy Reports, ISSN 2352-4847, Elsevier, Amsterdam, Vol. 5, pp. 1116-1136, https://doi.org/10.1016/j.egyr.2019.08.007 This Version is available at: https://hdl.handle.net/10419/243656 Standard-Nutzungsbedingungen: Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden. Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen. 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If the documents have been made available under an Open Content Licence (especially Creative Commons Licences), you may exercise further usage rights as specified in the indicated licence. https://creativecommons.org/licenses/by-nc-nd/4.0/ Energy Reports 5 (2019) 1116–1136 Contents lists available at ScienceDirect Energy Reports journal homepage: www.elsevier.com/locate/egyr Review article The electrochemical perspective of bioelectrocatalytic activities in microbial electrolysis and microbial fuel cells Nael Yasria,b,∗, Edward P.L. Robertsb, Sundaram Gunasekaranc aDepartment of Chemistry, Faculty of Science, University of Aleppo, Syria bDepartment of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr NW, AB, Canada cDepartment of Biological Systems Engineering 460 Henry Mall, University of Wisconsin-Madison, Madison, WI 53706, USA article info Article history: Received 28 March 2019 Received in revised form 18 July 2019 Accepted 9 August 2019 Available online xxxx Keywords: Exoelectrogens Bio-electrocatalytic MECs MFCs Bioenergy Anodic respiring bacteria abstract Transforming organic waste directly into electricity or indirectly into sources of hydrogen fuel is credible through exoelectrogen microorganisms grown on the anode or cathode that catalyze electrochemical reactions. In this review, we discuss the origin of the electrochemical kinetic in both microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) that are utilized to produce energy from waste through either directly by producing electric energy, or indirectly through hydrogen gas production, respectively. The concept of utilizing electrochemical techniques of cyclic voltammetry, chronoamperometric and derivative cyclic voltammetry to study the interfacial kinetics of exoelectrogenic bacteria and characterize biofilms are described. Additionally, we discuss the influence of various parts of electrochemical cells on bioelectrocatalytic processes, i.e, system design, electrolyte properties, anode and cathode materials. Thus, the necessity of optimizing parameters impacting the efficiency, rate, bacteria enrichment, and system implementations for improved biofilm performance are briefly discussed along with the figures of merit. ©2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Contents 1. Introduction..................................................................................................................................................................................................................... 1116 2. MFC and MEC.................................................................................................................................................................................................................. 1117 3. Exoelectrogenic microorganisms .................................................................................................................................................................................. 1119 4. Electrochemical concept of ARB ................................................................................................................................................................................... 1122 5. Anode system.................................................................................................................................................................................................................. 1124 6. Cathode system............................................................................................................................................................................................................... 1126 7. Separator and membrane.............................................................................................................................................................................................. 1127 8. Bioelectrocatalytic implementation in practical applications ................................................................................................................................... 1128 9. Figures of merit .............................................................................................................................................................................................................. 1129 10. Summary ......................................................................................................................................................................................................................... 1130 11. List of notations/abbreviations and symbols............................................................................................................................................................... 1131 References ....................................................................................................................................................................................................................... 1132 1. Introduction It is said that as much as 10 times the energy required for the treatment of municipal wastewater may be recoverable from it (Shizas and Bagley,2004;Fan et al.,2012). One of the key sustainable technologies that are useful for retrieving energy ∗Correspondence to: Nael Yasri, Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive NW, Calgary AB, T2N 1N4, Canada. E-mail address: [email protected] (N. Yasri). from wastes is via the use of exoelectrogen microorganisms (or exoelectrogens), which are characterized as extracellular electron transferors (EETs), i.e., capable of donating or accepting electrons (Hernandez and Newman,2001). Based on this principle, dual benefits are obtained: treatment of wastewater and production of energy from waste (Zhang et al.,2019). Thus, as with organisms involved in the biological remediation of waste, EETs digest soluble organic or inorganic entities from streams to produce metabolism products such as alcohol, methane, or hydrogen. The production of these metabolites from waste is of great value from the environmental and energy conservation points of https://doi.org/10.1016/j.egyr.2019.08.007 2352-4847/©2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 1117 view. Hence, due to the importance of these electron transfer reactions, electrochemists have utilized some of the natural bacterial processes for energy conservation at anode and/or cathode compartments (Rittmann,2008). A schematic of the exchange of electrons at donor and acceptor EETs is simplified in Fig. 1. Growing acceptor EETs on cathode has been used as a potential alternative to platinum for catalyzing some cathodic reactions because of its low cost and acceptable microbial catalytic activity (Jeremiasse et al.,2010;Rowe et al.,2017;Rimboud et al., 2017). Microorganisms that can produce hydrogen such as Desulfovibrio vulgaris (Guiral-Brugna et al.,2001;Lojou et al.,2002) are found in a variety of environments and contain hydrogenases that catalyze the reversible reaction (2H++ 2e−↔H2) (Schwartz et al.,2013). Methanosarcina bacteria, that are abundant in the environment, also grow on cathode and use electrons for the reduction of carbon dioxide to methane (Deppenmeier,2004). Donor EETs that grow on the anode are referred to as anodic respiring bacteria (ARB). ARBs, which include a variety of microorganisms, such as dissimilatory iron-reducing bacteria (DMRB) (e.g., Shewanella and Geobacter). ARBs oxidize soluble organics in the waste stream and donate electrons within a closed electrochemical circuit to the final electron acceptor port (Deutzmann and Spormann,2017). Exploiting the principle of using ARB to utilize organic wastes as feed substrates and at the same time to generate energy will promote an environmentally benign technology. The energy of the electrons can be utilized for electricity generation in a microbial fuel cell (MFC) (Paitier et al.,2017;Kim et al.,2002) or for hydrogen gas production in a microbial electrolysis cell (MEC) (Yasri and Nakhla,2017a). Both systems, collectively are referred to hereafter as MXCs, and share similarities in the anodic process as well as EETs. These systems are considered among the electrochemical technologies, and in particular, the bioelectrochemical technology for microbial energy generation. Published literature supports the effectiveness and the environmental compatibilities of the MXCs for a variety of organic remediation as well as a wide range of applications, e.g. remote monitoring for water quality (Parkhey and Mohan,2019), power productions (Xin et al.,2019), renewable source of energy (Kokabian et al.,2018;Chen et al.,2019), CO2reduction (Sánchez et al., 2019), conversion of waste streams to valuable resources via biosynthesis (Wu et al.,2019;Katuri et al.,2018;Reddy and Sun, 2019). Moreover, researchers have started working on genetically engineered biofilms (Li et al.,2018;Angelaalincy et al.,2018; Davidov et al.,2019) and hybrid systems (Tee et al.,2018), hoping for a real new breakthrough of the system. In this regard, the fundamentals and applications of MFCs (Slate et al.,2019;Santoro et al.,2017) and MECs have been reviewed recently and so are some important aspects of active biofilms and biocatalysts in MECs (Babauta et al.,2012a;Hasany et al.,2016;Rathinam et al.,2019). Other related reviews on the subject include the type of anode materials and feeding substrates used to grow biofilms (Angelaalincy et al.,2018;Ghasemi et al.,2013;Kadier et al.,2014;Azeredo et al.,2017), the type of cathodes used to reduce oxygen in MFCs and to produce hydrogen in MECs (Ben Liew et al.,2014;Kundu et al.,2013). Unfortunately, however, these reviews do not focus on the electrochemical understanding of MXCs. The electrochemical principles and concepts of MXCs systems have scarcely detailed in the literature. Therefore, in this review, we focus on the electrochemical systems that mostly depend on ARBs for energy production from waste. Taking an electrochemist’s perspective of MXCs, we have sought to establish a firm foundation for the understanding of concepts, highlight the importance of each part of the system and the future trends for application. We hope this review will provide a deeper insight into the electrochemical theory and models for MXCs. 2. MFC and MEC The basic principles of MFC and MEC systems are depicted in Fig. 2. Though there are a variety of cell designs reported in the literature, for comparison purposes we will consider MEC and MFC cells that are constructed similarly with an anode, a cathode, electrolyte, and an electrical circuit. From an electrochemical point of view, the main difference between the two systems is in how they contribute to energy by allowing hydrogen production in MECs and utilizing the reduction of dissolved oxygen to produce electric current in MFCs (Harnisch and Schroder,2010; Pandit and Das,2018). Considering the oxidation of acetate at anode surface (Eq. (1)) in neutral conditions similar to the inoculum found widely in MFC and MEC systems (i.e. pH =7 in 5 mM bicarbonate), the theoretical equilibrium potential required to oxidize 1 g/L (16.9 mM) of acetate can be calculated via the Nernst equation (Eq. (2)) as about −300 mV. Although the Nernst potential should not depend on the anode material, the presence of biofilm may change the kinetics of the reaction and hence derives an additional overpotential (which is not considered in this calculation). In addition, the concentration of species in the electrolyte media are not constant, thus the calculated value of the Nernstian potential may shift from the aforementioned theoretical value due to changes in the species concentrations in the presence of the biofilm at the electrode surface, or possibly the reaction pathway may change to a different redox potential. These various possibilities will be discussed further in Section 4. CH3COO−+4H2O→9H++8e−+2HCO− 3E0=0.187 V (1) EAn =E0 An −RT 8Fln [CH3COO−] [HCO− 3]2[H+]9 =0.187 −(8.31 j molK )(298.15 K) 8(96500 C mol )ln [0.0169] [0.005]2[10−7M]9 = −0.300 V (2) Considering, on the other hand, the reactions at the cathode side, the conditions for MFCs and MECs in this chamber are different; i.e., dissolved oxygen is available in the cathodic chamber for MFC system but not for MEC. Thus oxygen reduction reaction (Eq. (3)) will occur in MFC, whereas, hydrogen production occurs in MEC (Eq. (5)). In the MFC system, considering also a neutral pH (= 7) and an oxygen saturated catholyte media (an oxygen partial pressure of pO2 =0.21 atm), the cathodic Nernst potential is calculated to be about 805 mV (Eq. (4)). Thus, the MFC’s equilibrium cell voltage corresponds to about 1.105 mV (i.e. Ee,cell =[0.805 V] −[−0.300 V]). 1 2O2+2H++2e−→H2OE0=1.229 V (3) ECa =E0 Ca −RT nF ln 1 [O2]1/2[H+]2(4) ECa =1.229 −(8.31 j molK )(298.15 K) 2(96500 C mol )×ln 1 [0.2]1/2[10−7M]2 =0.805 V For a deoxygenated cathodic chamber in the MEC system, hydrogen will be produced. The calculated voltage from Nernst equation (Eq. (6)) at neutral pH (=7) is −414 mV. Thus, the corresponding equilibrium cell voltage is about −114 mV (Ee,cell = [−0.414 V] −[−0.300 V]). The reaction, in this case, is not spontaneous, i.e., external energy is required. Hence, biofilm formation is almost universally performed at a constant applied potential 1118 N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 Fig. 1. Schematic of exchange of electrons at EET donor (a) and acceptor (b). Fig. 2. Schematic of (A) MEC and (B) MFC systems consisting of two-compartment cells of anolyte and catholyte separated by cation exchange membrane (CEM). MEC produces H2by combining substrate utilization reaction at the anode by ARBs under small applied potential and MFC produces electric current by combining two half-cell reactions of substrate utilization at the anode and oxygen reduction at the cathode. (−0.6 V to +0.5 V vs. standard hydrogen electrode, SHE) to produce H2in MEC systems (Rimboud et al.,2014). 2H++2e−→H2E0=0.000 V (5) ECa =E0 Ca −RT nF ln H2 [H+]2(6) ECa =0.0−(8.31 j molK )(298.15 K) 2(96500 C mol )×ln 1 [10−7M]2= −0.414 V According to these calculations of the total Nernstian cell potentials, MFC is a galvanic cell capable of energy generation at a maximum cell voltage of 1.105 V [Ee,cell =(0.805 V) −(−0.300 V)]; and MEC is an electrolytic cell requiring external energy input at a minimum cell voltage of 0.114 V [Ee,cell =(−0.414 V) −(−0.300 V)], with the production of gaseous hydrogen at the cathode. Besides, it is worth noticing that in the aforementioned calculation we have not included the resistance encountered within the MXC (ohmic loss, activation overpotential and concentration overpotential, which may vary from system to another). Thus, hydrogen production will require a voltage higher than the calculated Ee,cell (ranging between 0.2–0.8 V). Thus, MECs can be considered as modified MFCs, which are known as bio-catalyzed electrolysis cells or bio-electrochemically assisted microbial reactors (Pandit and Das,2018;Roy and Pandit,2019;Liu et al., 2005a). Considering, however, hydrogen as a green and sustainable source of energy that releases no greenhouse gases upon combustion (Edwards et al.,2007), the use of MEC is considered a sustainable option for H2production (Venkata Mohan and Pandey,2019) that utilizes substrates such as wastes and/or other biomass for energy production (Wang and Ren,2013). Studying, on the other hand, substrates other than acetate, e.g., glucose (Eqs. (7)–(9), the conversion reactions are the fermentation to acetate or butyrate are favored (negative ∆G◦values) rather than complete hydrogen evolution (Eq. (7); nonspontaneous reaction). To understand the advantages of MEC system over fermentation and its variations, we compare here the energy requirement for oxidation of some simple waste products (e.g., glucose, butyrate, and acetic acids) when they are utilized as feed substrates in both MECs and fermentation processes. Theoretically, the maximum yield for complete oxidation of 1 M of glucose in an MEC is 12 M of H2(Eq. (7)), whereas, in the fermentation process it is 4 M of H2if acetate is formed (Eq. (8)) or 2 M of H2if butyrate is formed (Eq. (9)). We should note that oxidation of glucose and its fermentation products (acetate and butyrate) (Eqs. (7),(10),(11)) are not directly converted to H2 without an external energy input (free energy values of reactions are not spontaneous under standard conditions). Therefore, the use of MEC to oxidize these chemicals will theoretically yield more H2and more energy than what can be obtained with fermentation. The increase in H2production is perhaps due to the ability of MECs to use various bacteria and wider sources of organic matter compared to fermentation (Lu et al.,2012). C6H12O6+12H2O→6HCO− 3+6H++12H2 ∆G◦= +3.2 kJ/mol (7) C6H12O6+4H2O→2CH3COO−+2HCO− 3+4H++4H2 N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 1119 Fig. 3. Illustration of EET mechanisms employed by microorganisms at the electrode–biofilm interface. (a) use of cell appendages for electron transfer (pili), (b) direct electron transfer, and (c) electron transfer mediated by electron shuttle compounds (With modification from Carmona (2012)). ∆G◦= −206.3 kJ/mol (8) C6H12O6+2H2O→2CH3CH2CH2COO − +2HCO− 3+3H++2H2 ∆G◦= −256.8 kJ/mol (9) CH3COO−+4H2O→2HCO− 3+H++4H2 ∆G◦= +104.6 kJ/mol (10) CH3CH2CH2COO−+10H2O→4HCO− 3+3H++10H2 ∆G◦= +257.3 kJ/mol (11) Fermentation is a simple process, in both reactor conditions and design; however, considering MXCs many parts and parameters should be of importance. In MXCs, the construction of all the system components (i.e., electrodes, membrane, media condition, inoculum sources, cell design) contribute to the better performance of the process. Henceforth, the role of each component in improving the bioelectrocatalytic performance of the system will be discussed. 3. Exoelectrogenic microorganisms Exoelectrogenic microorganisms (Logan,2009a) are bioelectrocatalytically active species that grow in MXCs and transfer electrons to the anode port without the need for exogenous mediators. These microorganisms are usually grown in anaerobic condition on the anode surface as electron acceptors and utilize substrates (organic and inorganic) in the effluent as electron donors. Babauta et al. (2012a) and Hasany et al. (2016) reviewed various factors impacting the activity of biofilms and biocatalysts in microbial electrochemical systems. We fund that to understand the electrochemical concept of the MXCs system, it is necessary to examine from an electrochemical perspective the main factors and methods impacting the system. In this section, we will discuss the EET mechanisms, inocula cultivation, acclimation, substrate variation, and enrichment of microorganisms that influence the electrochemical catalytic activity of biota in MXCs to help understand the concept of exoelectrogenic microorganisms. Simplified mechanisms of EET digesting acetate ions, as an example of electron donor are depicted in Fig. 3 (Carmona,2012), which are based on three proposed pathways: (1) direct electron transfer through microbial outer membrane cytochromes, (2) electron shuttle molecules, mediated by the grown biofilm, and (3) via a solid conductive protein appendages produced by bacteria and shaped like nanowires or pili (Rathinam et al.,2019; Logan,2009a;Torres et al.,2010;Lovley,2008;Amit Kumar et al., 2012;Lienemann et al.,2018). Many detailed investigations on the type and structure of the nanowires, specifically those performed by the Malvankar research group at Yale University (USA), reveal recently that the structures are assembled by micrometerlong polymerization of the hexaheme cytochrome, with hemes packed within ∼3.5–6 Å of each other (Wang et al.,2019a). This structure explains the significant capability of biofilm grown on electrode to conduct electrons toward remote collectors. Moreover, in the surrounding media, the electrochemical gradient is considered the main driving force for the diffusion of the basic cellular functions, including chemo-osmotic transport and ATP synthesis (Amit Kumar et al.,2012). EET mechanisms are not mutually exclusive within a species nor in one pathway. For example, pure Shewanella oneidensis culture can transfer electrons through the interiorly produced riboflavins that can function as electrons shuttle. Pure culture of Geobacter sulfurreducens also has an outer membrane cytochrome to grow nanowires that can conduct electrons through a 50 µm thick anodic biofilm, however, do not produce flavins or other mediators (i.e. pyocyanin, melanin, or quinones) (Nevin et al.,2008). In mixed culture biofilms, however, due to the variety of biota involved and their secretions, it is impossible to distinguish the electron transfer mechanisms involved (Zhen et al.,2017). Elucidation of the mechanism of EET requires working with model microorganisms (pure strains), under controlled conditions, or via simulation (Cereda et al., 2014). Substrates which have been exploited as feed in MXCs are varied, both in number and implementations, with special interests usually focused on waste containing organics for bioenergy production (Zhang et al.,2019;Pant et al.,2011). The major substrates that have been utilized include wastewaters, fermentable and non-fermentable organic materials (Kadier et al.,2014;Pant et al.,2010). Pant et al. (2010) and Kadier et al. (2014) reviewed various substrates used in MFCs and MECs systems, respectively. However, for benchmarking newly developed or designed operational conditions, acetate is commonly used as carbon source, that is considered as an end product of several metabolic pathways and an inertness molecule toward alternative microbial conversions (i.e. fermentations) (Biffinger et al.,2008). It has been reported, for both of MFCs (Liu et al.,2005b) and MECs (Yang et al., 2015) systems, that the power generation and hydrogen production with the acetate-fed system are higher than those produced with butyrate, propionate, and glucose. Very recently, Yang et al. (2015) compared the performance of three common fermentation products of acetate, butyrate and propionate as MEC’s substrates in terms of H2and power output. Acetate-fed MEC showed the highest hydrogen production rate (0.53 m3/m3.day), current density (6.0 A/m2) and Coulombic efficiency (87%), followed by butyrate (0.18 m3/m3.day, 2.5 A/m2, 72%), and propionate (0.072 m3/m3.day, 1.6 A/m2, 51%). They attributed the variation in efficiency to the ease of acetate utilization by ARB in MEC, while butyrate and propionate could not be oxidized to the same degree. The use of more complex substrates resulted in lower utilization rate and efficiency. For example, Montpart et al. (2015) evaluated the efficiency of a single-chamber MEC fed with synthetic wastewater containing carbon sources of different complexity, such as glycerol, milk and starch. Although their results indicated that milk-fed MEC is the highest in hydrogen production rate (0.086 m3/m3.day), current density (4.0 A/m2 , calculated from data in Montpart et al. (2015)) and Coulombic efficiency (52%), but these values are still far less than those obtained with acetate. Biofilm formation on electrode starts immediately after adding inocula to the substrate medium, that contains electron donors (Sultana et al.,2015), and closing the electrochemical circuit 1120 N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 in which this growing phase indicted as lag-phase (or startup phase). Cultivation of ARB biofilms is preferably conducted at steady state in a fed-batch system, as the flowing or movement of inocula lengthens the start-up of the acclimatization process (Yi et al.,2009). The importance of the start-up phase has been reviewed by the literature of Kumar et al. (2017). Their review revealed the relations between the start-up phase and efficient bioenergy generation and adequate long-term operation. The factors that impact the start-up phase and hence reflect on the effeminacy of the total process include inoculum selection, enrichment, operating conditions and cell architecture (Kumar et al.,2017). They summarized that to obtain as high process efficiencies as possible, cell design, taking into account electrode materials, interfacial charge-transfer resistances, and membranes must all be of primary concern to aid start-up and subsequent steady-state operation. Indeed, we proved in a recent study on the impact of interfacial charge transfer on the start-up of bioelectrochemical system inoculated from mixed culture (Yasri and Nakhla,2017b) that both anodic interfacial conductivity and natural attraction of material present on the electrode surface can be important factors to initiate attachment of microorganisms on the surface (Yasri and Nakhla,2017b). We evaluated the initial acclimatization behavior of ARB on various anode surfaces doped with conductive (Fe3O4), semi-conductive (FeS) or nonconductive (CaS) interface. Our work demonstrates that when dealing with common species of ARB (e.g., Fe, Ca, S), the interfacial charge transfer is not among the factors which enhance EET. During start-up, the attraction of biota to grow on the iron-containing interface was high, however, in the subsequent growth stages, the electrochemical kinetic study indicated suppression of the bacteria-produced mediators on iron-containing anodes which reduces their electrochemical activity. However, electrode interface containing nonconductive species (e.g., CaS) shows higher electrochemical efficiency than a conductive ironcontaining anode that may naturally attract bacteria to behave differently (Yasri and Nakhla,2017b). Moreover, the electrical output in MXCs is independent of the concentration of the substrate used, i.e. increasing the concentration of electron donor substrate does not increase the electric current output (Yasri and Nakhla,2017a;Liu et al.,2005b;Yang et al.,2015;Strycharz et al.,2011). For example, Liu et al. (2005b) reported that the voltages generated in an MFC system using acetate at different concentrations (from 80 mg/L to 800 mg/L) was nearly stable at around 0.45 V. Yang et al. (2015) noted that the current density in an MEC system also remained the same upon changing the influent acetate concentration from 1600 mg/L to 800 mg/L. This concentration independent behavior reflects the basic concept of the limiting catalytic activity to produce a limiting current in MXCs, i.e., the current generated by ARB is the sum of the metabolism rates of bacteria which is associated with substrate utilization, biomass synthesis, respiration, and decay. The current output depends only on the type of the cultivated bacterium, interface materials on the electrode, and the type of utilized substrate. The phenomena of limiting current density in bioelectrochemical cells is well established in the literature (Yang et al.,2015), for example, Oh and Logan (2005) noted that the limiting current density remained nearly constant by doubling the amount of propionate substrate from 0.26 mM to 0.53 mM. The limiting current is dependent on the amount of bacteria grown on the electrode and also correlated with the type of bacteria as well as the type of electronically conductive strain present in the biofilm. An important study performed by Yi et al. (2009) found that the growth of electrically conductive microbial nanowires in the biofilm (in the presence of KN400 strain) reached a higher limiting current of 7.6 A/m2than non-conductive strain (1.4 A/m2). Moreover, continuous investigation in this domain by Wang et al. (2019a) reveals that wild-type OmcS filaments show 100-fold greater conductivity than other filaments from a ∆omcS strain, emphasizing the significant of conductive strains in biofilms. On the other hand, different electrode materials will show different limiting current density (Yasri and Nakhla,2016), these mostly depend on the type of electrode interface to provide an attracting surface for biotic species to attach. For example, although Indium-Tin oxides (ITO) film shows a highly conductive surface electrode, the modification of the surface with three-dimensional conductive nanowire networks produce higher limiting current density as compared to the plain ITO which was attributed to the enhanced electron transfer via mediator molecules in the surface structures of the electrode (Zhao et al.,2010). The type of substrate also impacts the limiting current density, which has been attributed to the rate at which bacteria utilize the substrate (Yang et al.,2015). Moreover, the amount of mediator excreted by biofilms is equally important in limiting the current. For example, Marsili et al. (2008a) reported a drop of 80% in current produced via S. Oneidensis MR-1 on graphite electrode following the replacement of a spent growth medium with fresh medium. They presume that this drop in the limiting current is due to the presence of flavins in the spent medium which acts as redox mediators, impacting the current production. When the substrate is consumed, the current will reduce, hence, the current generation can be boosted again by adding or dosing the substrate (Yasri and Nakhla,2017a,2016); however, in some cases, it will be necessary to partially or completely replace the medium (Zhu et al.,2012;Chen et al.,2012). Thus, successive additions of an inoculum are necessary in the first batches, and then only the addition of electron donor substrate (Yasri and Nakhla,2017a). Following the growth of biofilms, their stability can be determined by obtaining a constant current during successive addition of the same substrate (Pasternak et al., 2018;Ramírez-Vargas et al.,2018). However, to maintain a stable current output the substrate concentration is usually kept above a threshold corresponding to the required limiting current output (Torres et al.,2007). Although mixed culture biofilms are generally cultivated from the primer inoculum containing mixed communities that coexist in natural (sludge, sediments, biological treatment plants, etc.), purifying these types of biofilm can be approached by eliminating or washing off non-active spices, e.g., non-exoelectrogens microorganisms. This can be performed by transplanting biofilm from one MXC system to another, which also helps with the enrichment of microorganisms. Secondary biofilms obtained via transplanting usually enables the growth of fewer varieties of culture in the biofilm (Mahmoud et al.,2018). On the other hand, some experimental designs require bacterial enrichment to promote specific species in the inoculum (Saratale et al.,2017). Considering the fact, however, that waste stream which may be fed to MXCs will contain a variety of complex species, efficient MXCs will require acclimation of bacterial to this media. In this regard, efforts were made to adapt organism growing on anode with various strains to enhance the efficiency. Yi et al. (2009) found that adaption of KN400 strain associated with the abundance of electrically conductive microbial nanowires was more effective in current production (7.6 A/m2) than strain DL- 1 (1.4 A/m2), which is non-motile and flagella. Furthermore, adaption of organisms at various conditions were made to crop robust exoelectrogens that are capable of enhancing the production of current density and facilitating the operation in extreme cultivating conditions of pH (Zhuang et al.,2010;Yoho et al., 2014;Liao et al.,2014), potential (Torres et al.,2009), temperature (Tkach et al.,2017) and polluted stream (oil, grease, or saline conditions) (Liao et al.,2014;Badalamenti et al.,2013; N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 1121 Table 1 Examples of recalcitrant waste treated via Bioelectrochemical systems. Substrate Culture condition Anode type/applied voltage Reactor/other conditions Influent COD (mg/L) COD Removal efficiency (%) CE (%) Ref. Dyes (methyl orange) Municipal wastewater Graphite brush/0.8 V cell Membraneless single-chamber MEC/acetate as co-substrate & A TiO2-coated photocathode 50 300 98 76 114 80 Liu et al. (2005b) Pesticides (Atrazine ) Municipal wastewater Graphite fiber brushes/−0.2 V vs SHE Membraneless single-chamber MFC or MEC/ 1.0 g/L acetate as co-substrate 50 µg/L 74/MFCs 85/MECs Pant et al. (2011) Insecticide (DEET (N,N- Diethyl-meta- toluamide)) Municipal wastewater Graphite fiber brushes/−0.2 V vs SHE Membraneless single-chamber MFC or MEC/ 1.0 g/L acetate as co-substrate 50 µg/L 28/MFCs 39/MECs Pant et al. (2011) Furanic mixturea Working MFC Porous carbon felt /0.6 V cell H-type MEC provided with cation exchange membrane/ No co-substrate 1200 mg/L 57 ±10 based on SCOD 44–69 Biffinger et al. (2008) Petroleum refinery wastewater Mixed culture with refinery wastewater Carbon cloth with carbon coating/318 mV cell (Batch) Sandwich type MFC/ No co-substrate 95% oil content 84.4 2 ±0.8 Yang et al. (2015) Produced water from oil field processing Anaerobic sludge acquired from the same treated wastewater produced from a desalination unit of oilfield Graphitevarnished stainless steel mesh/330 mV cell MXC (MEC or MFC)/salinity 40000 ppm 250–700 produced water 90% 0.2 Yi et al. (2009) aFuranic compounds mixture are: furfural, 5-hydroxymethyl furfural, syringic acid, 4-hydroxybenzoic acid and Vanillic acid. Cheng et al.,2010). In this respect, exoelectrogens acclimatize in extreme conditions are useful to treat recalcitrant and persistent wastes such as pesticides (Mu et al.,2009;Werner et al., 2015), insecticide (Werner et al.,2015), heavy metals (Luo et al., 2014), furanic (Zeng et al.,2015), dyes (Hou et al.,2017), phenolic (Zeng et al.,2015) compounds, and wastewater products from petroleum (Srikanth et al.,2016) and natural gas (Stoll et al.,2015;Jain et al.,2017;Ghasemi Naraghi et al.,2015; Fakhru’l-Razi et al.,2009) operations. Recalcitrant wastes are usually incompatible with typical MXCs systems (Nevin et al., 2008;Zhen et al.,2017;Zhuang et al.,2010;Yoho et al.,2014; Liao et al.,2014;Torres et al.,2009;Tkach et al.,2017;Badalamenti et al.,2013;Cheng et al.,2010;Huang et al.,2011) thus acclimatizing bacteria in extreme conditions that simulate the natural existence of the treated effluent will produce biofilms that can bear with the acclimatization conditions and capable of utilizing recalcitrant wastes as substrates (Mohanakrishna et al., 2018). Examples of recalcitrant waste that have been treated via bioelectrochemical systems grown at extreme conditions are presented in Table 1. Products such as water from oil and natural gas processing [250–700 mg COD/L, 40000 ppm salinity], furanic mixture [1200 mg COD/L], pesticides or insecticide [50 µg/L] and textile dyeing [50–300 mg COD/L] have been treated at reasonable rates when biofilms pre-acclimatized with the same pollutants as substrates in the mixed cultures of bacteria. Moreover, thermophilic and halophilic exoelectrogens have been cultivated from extreme natural conditions of pH, temperature and saline conditions such as seawater (Zhang et al.,2011). The enrichment processes can be classified into three main categories: (1) substrate, (2) physical or (3) chemical. Substrates are feed sources for ARB, existing in the effluent and are converted or oxidized within the MXCs system. Substrate-related enrichment can be performed by feeding the inoculum with specific nutrients (Cercado-Quezada et al.,2010) or substrates (Yang et al.,2015) that enhance the growth of some species but not others. For examples, Zhang et al. (2013) found that adding iron salts to mixed culture inoculum enrich the dominant archaebacterium in the anode biofilm. Kadier et al. (2014) discussed substrates used in MECs, including simple to complex sources such as methanol-rich industrial molasses and waste streams from refinery, food processing, winery, dairy, swine, and domestic operations. Among these, use of complex substrates helps in obtaining a stable electrochemically active microbial community; whereas, use of simple substrates (e.g. acetate, or glucose) typically leads to microbial communities that degrade easily but improve the H2production rate. Altering the physical conditions such as temperature (Patil et al.,2010), sonication (More and Ghangrekar,2010), dissolved oxygen concentration (i.e. partial aerobic or anaerobic conditions) (Cercado-Quezada et al.,2010), electrode potential (Cereda et al.,2014;Torres et al.,2009;Nam et al.,2011), the application of intermittent energy input (Cho et al.,2019), electrode type (Yong et al.,2012), material doped on anode (Yasri and Nakhla,2016), may promote species to tolerate the applied conditions. On the other hand, inhibiting some microorganisms can enable biological enrichment in MXCs by adding chemicals (Theivasanthi and Alagar,2011) or modifying the chemical conditions of the anolyte chamber, e.g., pH (Patil et al., 2011), alkalinity (Yasri and Nakhla,2017a,2016), adding antibody or fungicides (Fenner et al.,2006). For instance, Babauta et al. (2012b) noted that the pH inside the anolyte compartment is an important factor affecting the intensity of current output, type of bacterial growth, and the total electron transfer process, which is also tied to proton transfer within the biofilm. They concluded: (1) pH varied within the biofilm during different growth phases, (2) pH is not always a limiting factor for a biofilm, and (3) biofilms respire in a unique internal environment, so that variation of pH and redox potential are associated only with the biofilm. The pH dependency of the microbial community is due to the complex biological processes within bioelectrochemical systems (Patil et al.,2011), which has not been fully explained. Studies of ARB grown from municipal waste stream indicates 1122 N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 that the bioelectrocatalytic activity was suppressed at pH lower than 3.0 and higher than 11.0 (Patil et al.,2011). However, in principle the anode-biofilm generates hydronium cations; thus, the access of hydroxyl anion around biofilm will kinetically favor withdrawal of hydronium cations generated from the biofilm to accelerate the metabolic processes. Similarly, the withdrawal of hydronium cations explains the limitation of performance associated with proton transport out of the biofilm at lower pH (Zhao et al.,2014). This interpretation can justify the lower activity of biofilm at low pH (due to lack of OH−) but not at high pH ( > 11.0), which may be due to the low tolerance of the biofilm to high pH environment. To the best of our knowledge, real-time monitoring of the pH gradient at the biotic/abiotic interface, and pH within the biofilm have not been investigated; however, such an approach using pH indicator during metabolism could be a subject of future studies, which may provide insight into these pH effects (Jin and Kirk,2018). 4. Electrochemical concept of ARB Understanding the principles and behavior of electrochemical interface reactions between biotic entities in biofilm, substrate utilization, and the surface of the electrode are important to continue advancements in MXC systems. Electrochemical studies for MXCs include voltammogram analysis of the interface reactions (Strycharz et al.,2011;Richter et al.,2009), the performance of the biofilm on electrodes during start-up (Paitier et al.,2017;Yasri and Nakhla,2017b), growth (Choi and Sang, 2016;Rodrigues and Rosenbaum,2014;Yoho et al.,2015) and starvation (Zhu et al.,2014), as well as studying of their kinetic parameters (Torres et al.,2010,2008). The relationship between current, ARB, and the electrode has been well established, indicating that the electron generation is a result of biocatalytic utilization of substrates and electron transfer via various EETs to the electrode (Lovley,2008;Richter et al., 2009;Reguera et al.,2005). However, electrons flowing within MXCs containing both of biotic and abiotic phases will encounter resistance, leading to voltage losses within the cell. The total voltage loss is the sum of losses at the interface between the biofilm and the electrode (anode), the ohmic losses in the electrode, the biofilm, the electrolyte medium, the membrane, and the cathode overpotential (for H2production or O2reduction) (Zhen et al., 2017). Hence, an efficient electrochemical system would require minimization of potential losses; whereas, tuning of the potential losses at the biofilm/electrode interface is imperative. Voltammetric studies of the anodic electrode–biofilm interface are not very common. Generally, the behavior of local current that is generated as a result of electron exchange at an electrode– solution interface is characterized via the Butler–Volmer equation, which describes the current–voltage response in electrochemical systems (Torres et al.,2008). However, in the case of biofilm grown on an anode, both biotic and abiotic processes must be considered on the electrode surface, including the metabolism associated with both conductive and nonconductive microorganisms (Torres et al.,2008). Thus, the kinetic behavior of this type of anodes, which is attributed to substrate utilization via both metabolism and the electron transfer process, will deviate from the abiotic system and hence will also deviate from the conventional Butler–Volmer equation. Various kinetic models have been proposed to evaluate the kinetic responses of substrate utilization (biotic) and the electrochemical electron transfer (abiotic) processes. Among these, two important modified versions of Monod model will be discussed hereafter; (1) the Butler–Volmer–Monod model that describes the relationship between overpotentials of the metabolized substrate (Hamelers et al.,2011), and (2) Monod–Nernst model that describes the rate of substrate metabolism and its link with species concentration and the redox potential (Torres et al.,2008). In developing Butler–Volmer–Monod model, Hamelers et al. (2011) proposed a two-step process: (1) the first step involves intercellular biochemical utilization reaction that promotes conversion of organic and electron generation, where the rate depends on enzyme kinetics that is described via the Monod equation; and (2) the second step involves the extracellular electron transfer from biofilm to the electrode and is described via the conventional electrochemical electron transfer kinetics of the Butler– Volmer equation. The efficiency of process conversion is typically lower than 100% due to losses at the bio-anode and can be quantified via the anode polarization curve (i.e. potential as a function of the current density). Based on this argument, Hamelers et al. (2011) developed a basic Butler–Volmer–Monod model (Eq. (12)) that described the kinetics of microbial metabolism based on a combination of enzyme kinetics (Monod) and electrochemical kinetics (Butler–Volmer). This polarization model was found to fit the measured anode-biofilm polarization curves, and it was found that the apparent Monod constant of an anode-biofilm is dependent on the anode overpotential (Hamelers et al.,2011): j=jlim (1−eF RT ηanode K1.e−(1−α)F RT ηanode +K2.eF RT ηanode +(KM S+1))(12) where Fis Faraday’s constant, Ris the universal gas constant, Tis the operating temperature, ηanode the anode overpotential, jis current density (A m−2), jlim is the limiting current density (Am−2), αis the charge transfer coefficient of the anode reaction, KMis the substrate affinity constant (describing the effect of substrate on the biochemical conversion), and Sis the substrate concentration (Zhao et al.,2014;Hamelers et al.,2011). Based on the Butler–Volmer–Monod model, Zhao et al. (2014) developed a mathematical model for an MFC system with air cathode at various period of growth levels of the anode-biofilm. They were able to model the measured polarization curves and provide insights into the limiting physical, chemical and electrochemical phenomena and their effects on cell performance. For example, the MFC data demonstrated that the performance of the cell is primarily limited by cathode electrochemical kinetics, and that anode biofilms with longer growth time will lead to higher limiting current and less polarization losses under the same operating current. They were also able to simulate the overpotentials developed from ohmic, cathodic, and anodic losses. They noted that the overall ohmic overpotentials were relatively small, and that the cathode overpotential is dominant at both low and high current density region, and for short (110 h) and long (160.5 h) growth periods of anode-biofilm. Compared to the electrode overpotential, the overall ohmic overpotentials are relatively small. Thus, the limitations of performance are bounded to cathodic electrocatalytic performance and proton transport from the biofilm. Thus, they concluded that the limitation can be mitigated by using a high buffer concentration (Zhao et al.,2014). The Nernst–Monod model considers the biofilm as an electron generator via substrate utilization and the anode is the final electron acceptor (Torres et al.,2008). The Nernst equation is commonly utilized in electrochemistry to describe the relationship between species concentration and the redox potential at the electrode–solution interface. In biology, however, the Monod model is used to describe the kinetics of biological growth and its dependence on the concentration of substrates (Wang et al.,2010a). Kato Marcus et al. (2007) combined the two to propose the Nernst–Monod model (Eq. (13)), which describes the kinetic behavior of bacteria (current production) under an electrochemical potential. j=jlim (1 1+exp⌈− F RT (E−EKA)⌉)(13) N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 1123 where: j=current density (Am−2); jlim =limiting current density (Am−2), which is the maximum current density reached at the applied conditions; R=ideal gas constant; F=Faraday constant; T=absolute temperature (303.15 K); and EKA =catalytic potential at which j=1 2jlim. Ideally, the voltammetric response of ARB grown at anode interface should follow the sigmoidal Nernst–Monod model (Fig. 4A) (Torres et al.,2008;Kato Marcus et al.,2007). However, the experimental responses of biofilm anode vary from ideality (Fig. 4B) (Torres et al.,2010), which will be discussed later after describing some of the experimental setups in the following section. Generally, to obtain an ideal voltammogram, a steady state condition has to be reached and a slow potential variation with a slow scan rate is usually applied. At this slow variation in potential condition at the interface, the electrocatalytic activity is independent of the scan direction and the ARB reach a stable metabolic status (Marsili et al.,2008b). Hence, in a bioelectrocatalytic investigation, a linear-scan cyclic voltammetry (LSCV) with a slow scan rate of about 1 mVs−1, is considered a powerful technique. At this LSCV condition, the catalytic response (current) can be measured at a steady-state condition of ARB as a function of the anode potential (Torres et al.,2010). The prime postulation of the Nernst–Monod model is that the biofilm anode catalyzes the interfacial electron exchange process and hence controls the kinetics as well as the current generation. Thus, the lower the potential loss at the biofilm/electrolyte interface during current flow, the higher the efficiency of the electrochemical system. The potential loss at both the biofilm/electrolyte and the electrode/biofilm interfaces are anode potential losses (APL). Thus, from an electrochemical point of view, the evaluation of APL is of prime importance for the development of MXC systems. However, the nature of APL, its relationship to anode biofilm performance and the method of calculation are not clearly defined in the literature. The APL (ηanode) is associated with the electrocatalytic features of bio-active sites, and ideally, the APL should be small so that the electrode potential is as close as possible to the redox potential of the substrate being oxidized. That is, the amount of potential (energy) losses by bacteria during utilization of substrate and EET as well as electron transfer to anode surface should be as low as possible for the process to be efficient. Torres et al. (2010) defined the APL as the losses in potential due to extracellular electron transfer, and electron transfer to the electrode surface, indicating that the lower the APL the higher the transfer of energy, and consequently a more efficient process. Thus, from an electrochemical point of view, the main goal during process optimization of MXCs is to reduce potential loss (i.e. close to the redox potential of the substrate being utilized) and to increase the biologically active surface area to obtain a maximum current output. Whereas, the practical goal in MXCs is to find a bioelectrocatalytic system that can utilize specific organic (or waste) to produce high current density at low cell potential (Torres et al.,2010;Grattieri and Minteer,2018). Fig. 4B represents an ideal experimental LSCV curve (scan rate of 1 mVs−1) (Torres et al.,2010) for a system containing ARB biofilm that utilizes acetate as substrate and produces a limiting current density (jlim)of about 0.15 A.m−2. The obtained steadystate conditions can be distinguished from the similarity in the forward and backward scan, whereas, the APL ηanode can be measured by calculating Edonor : for acetate as electrons donor in this case, vs. SHE. When the anode potential is equal to Edonor the biofilm will not have enough energy to transfer electrons to the electrode, hence no catalytic current will be observed, i.e. j= 0. Increasing the anode potential to a more positive value, the metabolic activity of ARB will increase, and the electron-exchange between biofilm and electrode will take place, hence the utilization process of the substrate will catalyze the current to reach a maximum value (limiting current). The interfacial impedance may restrict the current flow and APL will take place. Thus, the potential window between the Edonor and the point where the current starts to increase (onset potential) can be considered the value of APL (mV). The Nernst–Monod equation is a kinetic description of a bioelectrocatalytic system that describes the ideal j-E trend for a pure culture and electron transfer pathways (Lee,2018). Anodebiofilms grown from mixed culture media may not obey the Nernst–Monod model. Although the j-E voltammogram may show a sigmoidal shape, the curve may not fit the Nernst–Monod model, and the interpretation of the kinetic data becomes complicated (Yasri and Nakhla,2016). The deviation of experimental data from the Nernst–Monod model can arise for several different reasons, including the composition of electrode material, the type of the culture grown on the electrode and the existence of several EET pathways, which may occur at various electrode potentials. For example, in a previous work, we observed a range of different sigmoidal j-E voltammograms grown from the same inoculum with the same substrate (acetate), but with different electrode compositions. A carbon anode doped with magnetite (Fe3O4) produced sigmoidal-shaped cyclic voltammogram with a series of convex shapes indicating multiple EET pathways (Fig. 5A). Fitting the voltammogram (red line) with the Nernst–Monod model (black line) shows a significant deviation (Yasri and Nakhla, 2016;Yoho et al.,2014,2015). However, the voltammogram can be characterized using the derivative cyclic voltammogram (DCV) which revealed several steps from the series of convex sections (red curves in Fig. 5B) (Yoho et al.,2015), indicating multiple EET pathways. These convex sections indicate a series of electron transfer processes occurring at different potentials. Multiple derivatives of the Nernst–Monod equation fit well with each convex curve in the forward section of the DCV (dashed lines, Fig. 5B) (Yasri and Nakhla,2016). The derivative of the sigmoidal curve is described by a convex shape. Thus, in the case of Nernst–Monod equation, the variation of dj/dE(A V−1) versus anode potential (V) is convex and the value of EKA can be found from the position of the peak. Applying this approach to an experimentally measured DCV by fitting the Nernst–Monod equation at each convex point will help deconvolute the curve and distinguish multiple EET pathways (Yasri and Nakhla,2016;Yoho et al.,2014,2015). The derivative of an experimental LSCV curve will generate, during the forward and backward scans, multiple convex and concave curves, that may be associated with multiple EET pathways (Yasri and Nakhla,2016), i.e. each pair of opposing convex/concave couples corresponds to an EET pathway. In our earlier investigation (Yasri and Nakhla, 2016), as well as the study performed by Yoho et al. (2015), the forward scan (oxidation) of the DCVs were deconvoluted by fitting with multi Nernst–Monod derivatives to provide an overall electron exchange response. In these two studies, the midpoint potential (EKA, V) of each convex curve in the forward DCV scan were fitted to the Nernst–Monod equation using the corresponding EKA value and adjusting the current and assuming n=1. Considering n=1, we can infer that each Nernst– Monod fit will correspond to an EET pathway with one electron transfer process. Thus, by fitting multiple curves, the existence of multiple EET pathways can be established, and the differences in the potential may be due to the variation in ARB type or responses to the surrounding conditions (Yasri and Nakhla,2016; Yoho et al.,2015). Discussion related to the variation of pathway-potentials can be derived from the aforementioned APL, that is the higher the potential windows between the redox potential of the substrate being oxidized and the associated potential (i.e. the EKA of the convex section), the higher the APL. Consider the DCV in Fig. 5B, 1130 N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 Table 2 Coulombic efficiency and current densities produced in some MXC systems. Substrate Culture condition Reactor/ Anode type/other conditions Substrate conc. (mg/L) Current density (A/m2) CE (%) Ref. Acetate Pre-acclimated bacteria from MFC Cube shaped MFC/ Graphite fiber brush anode. 1000 8.0 60 Cercado-Quezada et al. (2010) Glucose Pre-acclimated MFC in Na-acetate Cylindrical chamber MFC/ carbon cloth anode 1200 7.0 28 Chaudhary et al. (2000) Butyrate Pre-acclimated from domestic wastewater Single-chambered MFC/ Carbon paper 1000 0.77 8–15 Kundu et al. (2013) Na-Acetate Pre-acclimated from domestic wastewater Sandwich type two-chambers MEC 1600 mg COD/L 6.16 91 Harnisch and Schroder (2010) Na-Butyrate 2.57 70 Na-propionate 1.46 59 Starch processing wastewater Starch processing wastewater as inoculum Sandwiching type two-chamber MFC/ carbon paper/proton exchange membrane 9703 mg COD/L 8.9 8.0 Jadhav et al. (2019) Cassava mill wastewater Activated sludge Two-chambered MFC/ graphite anode/Glass wool separator 16000 mg-COD/L 10 20.0 Logan (2008) Typical CEs determined for various MXC systems are presented in Table 2. In general, the CEvalues and current densities obtained from conventional volatile fatty acids substrates (e.g., acetate, propionate, or butyrate) are higher than those obtained from complex substrates (such as starch). The utilization of complex substrates requires a microbial community that is capable of breaking down these types of compounds to low-molecular- weight fractions that can be utilized by ARB. For example, when starch- or cassava-containing wastewaters are utilized as substrates, although their hydrolysis products may be good sources of ARB food, electricity generation in MXC system requires a microbial community that can hydrolyze complex starch and cellulose compounds at the same time and has exoelectrogenic activity (Lu et al.,2009;Kaewkannetra et al.,2011). Power production in MFC system is usually expressed as power density (W per m2of anode surface area), or power intensity (W per m3or reactor volume) (Logan et al.,2006). Early investigations indicated that the power intensity obtained for reactor volumes larger than 1 L is typically less than 1 kW/m3, which for electric production facilities is considered as the threshold for practical industrial application for energy recovery from organic matter (Pham et al.,2009). In the case of hydrogen recovery in an MEC system, however, the output of a system is usually expressed by the hydrogen yield (mol-H2/mol-substrate) and the H2production rate (HPR) (m3m−3d−1normalized to the volume of catholyte, anode or reactor m3) (Logan,2008;Logan et al.,2006). The values of the power output and H2production in MFCs and MECs, respectively, are dependent on the surface area of both the biofilm anode as well as the cathode. Power output values reported in the literature are mostly estimates based on the normalization to the anode or cathode surface area or their respective compartment volumes, or reactor volume. The assumption when the calculation is based on the anode (volume or surface area), is that the power output is limited by the electrons flux at the anode interface, whereas, when the calculation is based on the cathode (volume or surface area), the proton flux is the limiting factor (Logan,2008). Both assumptions can be used to compare reactors of similar volumes to explore materials and understand factors that limit the reactor output (Logan,2008). However, to shift the attention to reflects the engineering interest for industrial implementation and scale-up, the calculation also should be shifted to normalize the output to the reactor volume (Logan,2008). In some examples found in literature, moderately high HPR values (>20 m3m−3d−1) are reported when the calculation is based on normalizing the production rate only to the cathode or anode compartment volumes. For example, Cheng and Logan (2011) reported an HPR of 17.8 m3m−3d−1 for a reactor using a carbon brush anode of about 6 mL volume and acetate as the substrate, operated with an applied voltage of 1 V. Although the HPR values is high, these are bench-scale reactors and their volumes do not support the feasibility of scaleup and industrial implementation. Recently, a larger reactor with a volume of 1 L has been reported by Guo et al. (2017) that is capable of continuous production of 7.1 m3m−3d−1HPR using a novel design with tubular electrodes and acetate as the electron donor, operating at an applied voltage of 1 V. This reactor yielded a promising HPR, but the design has not yet been tested for substrates other than acetate. The energy efficiency (%) from a reactor is usually measured by comparing the total energy recovered to the total energy input. The source of energy input to the system includes both the external electrical energy input (for MECs), and the energy input from the substrate. Thus, a higher energy input to the system will typically be reflected in a lower energy efficiency. For example, the MEC with tubular electrodes reported by Guo et al. (2017) decreased electrical energy efficiency from 209 ±5% to 149 ±1% as the voltage increased from 0.55 V to 1.0 V. The efficiency was evaluated here in terms of the electric input. In general, when evaluating the energy efficiency of the MEC in terms of the external electrical energy input only, the values show high efficiency and in most cases exceeding 100%. The efficiency, in this case, compares the combustion energy of the actual hydrogen produced in the system with the equivalent amount of hydrogen produced via water electrolysis when applying the same electric energy input. Thus this is not a true energy efficiency, since the energy of substrate oxidation is not accounted for. 10. Summary The catalytic activity of biofilm grown on the surface of electrodes in aqueous media has been exploited in electrochemical systems for simultaneous remediation of environmental pollutants and interfacial electron transfer for energy generation. In this review, we have addressed the main electrochemical aspects N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 1131 of ARBs and their utilization in MECs and MFCs systems. The aim is to draw more attention to theory and models from an electrochemistry perspective, provoking research and innovation into this nascent and synchronized approaches of sustainable power generation and waste remediation. The information provided in this manuscript is summarized and presented to provide the scientific community insight into the electrochemical theory related to these developments. Following the consideration of the concepts and the viability of both the galvanic MFC and electrolytic MEC systems, the exoelectrogenic microorganisms and their bioelectrocatalitical activity were discussed. The ability of both bioelectrochemical systems to effectively oxidize organic containing waste to extract energy has been demonstrated. Remediation processes accompanied by extracellular electron transferors processes in ARBs utilize a wider sources of organic matter compared to the nonexoelectrogenic fermentation process. Systematic electrochemical approaches to investigate the inoculum interactions at the interface during start-up, growth, or starvation of biofilms can reveal important information related to kinetic parameters and their effects on process limitation and cell performance. Among these, interpretation of Butler–Volmer–Monod, Nernst–Monod, and their derivatives are useful for the investigation of overpotentials developed in various parts of the cell, to distinguish EET pathways of various types of ARB, and the response of ARB to the environmental conditions to improve the catalytic activity of the system and reduce the energy loss. The review also highlights the importance of each part of the system from current and forthcoming trends in MXCs. Unlike other biological systems, the dominant populations of ARB biofilm usually differ from those that they are originally enriched from, and also depend on the type of substrate and the MXCs conditions. Moreover, differences are also existing between MFCs and MECs due to the intrusion of oxygen through the cathode in MFCs and the release of H2gas in MECs. Careful control of the enrichment process of biofilm e.g. substrate, physical or chemical conditions, can promote a specific stable electrochemically active microbial community that can be utilized in extreme environmental conditions for the treatment of recalcitrant and persistent wastes. Careful reactor design, choice of electrodes, and systematic control of parameters can lead also to an effective bioelectrochemical process. In MEX, the anode is considered the main component that captures the biological response. Thus, among factors impacting the immobilization of biofilms, (i.e. cell design, applied voltage, interfacial charge transfer, electrolyte conductivity, pH, substrate types, electrode material and porosity), the selection of anode surface with low ohmic loss and supported with compatible materials, most suitable for bacteria growth, are among the important factors to be investigated. In a closed circuit electrochemical MXCs system, cathodic parts are also important in terms of the system efficiency. Ohmic resistance of the electrolyte and interfacial electron transfer at the cathode contribute significantly to the potential loss in MXC systems. The use of a high cost platinum is unsuitable for MXCs, due to deactivation by constituents present in wastewater (such as sulfide), and alternative choices can be considered on a caseby-case basis. In term of membranes, although it introduces ohmic loss to the MXC systems, its presence is important to prevent the growth of reducing bacteria on an MEC cathode, and to avoid the bioanode from aerobic contact in MFCs. Moreover, the use of an AEM may be useful for stabilizing the anodic media and has been shown to exhibit better efficiency compared with CEM. Future developments in electrochemical remediation technologies should be dictated not only by the cost but also by the need to meet the public expectations and satisfying regulatory guidelines. Some moderately successful attempts have been achieved in the implementation of MXCs to generate power and for sensors, e.g. monitoring the level of BOD, inhibitors, DO, pathogenic bacteria and VFAs. Although the performance is acceptable, further improvement is required for commercial viability. The use of organic wastes as resources for cost-effective energy generation offers significant potential to reduce treatment costs and enhance the sustainability of waste treatment processes. In this regard, microbial electrocatalytic techniques show promise. However, there are challenges to overcome such as the low current density, slow rate of remediation, limited scaleup potential, poor portability etc. Thus, further investment to investigate the concept and innovate the technology for process engineering scale-up and for practical application. Furthermore, the investment in bioelectrocatalytic processes would be more promising if acclimatization of such a system can be integrating to assist remediation of substrates that are difficult to break down or sustainable in the environment e.g. oil byproducts or persistent organic pollutants. Thus dual targets can be achieved of sustainable energy production and remediation of such pollutants. 11. List of notations/abbreviations and symbols Notations/Abbreviations ACE: Average current efficiency AEM, Anion exchange membrane APL: Anode-potential losses ARB: Anodic respiring bacteria ATP: Adenosine triphosphate CEM: Cation exchange membrane COD: Chemical oxygen demand CV: Cycle voltammetry DCV: Derivative cycle voltammetry DO: dissolved oxygen EET: Extracellular electron transfer GAC: Granular activated carbon HPR, Hydrogen production rate (m3H2/m3/d) ITO: Indium-Tin oxides LSCV: low-scan cyclic voltammetry MEC: microbial electrolysis cell MFC: microbial fuel cell PBS: phosphate buffered saline PEDOT: Poly(3,4-ethylenedioxythiophene SCE: Saturated calomel electrode SCOD: Soluble chemical oxygen demand SHE: Saturated hydrogen electrode TCOD: Total chemical oxygen demand TOC: Total organic carbon VFA: volatile fatty acids 1132 N. Yasri, E.P.L. Roberts and S. Gunasekaran / Energy Reports 5 (2019) 1116–1136 Symbols α: charge transfer coefficient eo: elementary charge of electron (C) E: electric potential (V) Eo: equilibrium potential (V) EAn:anodic potential (V) EKA: potential (V) when j=1 2jmax EPS : applied voltage (V) Ee: equilibrium cell voltage (V) Eca:cathodic potential (V) F: Faraday constant (96,485 C mol−1) j: current density (Am−2) jlim: limiting current density (Am−2) jmax: maximum current density (Am−2) KM: substrate affinity constant n: ion valence nH2: Number of moles of the hydrogen (mol) nS: number of moles of the substrate utilized (mol) p: pressure (atm) Q: electric charge (C) rcat : cathodic hydrogen recovery (%) R: ideal gas constant (8.314 4621 J K−1mol−1) RH2: overall H2 recovery (%) Rex: external resistance (Ω) t: duration (s) T: absolute temperature (303.15 K): VS: solution volume (L) WH2: energy content of the hydrogen (J) WR: energy loss on the external resistor (J) WS: energy added by substrate (J) Win: energy input to the system (J). 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