Recent progress and perspectives in microbial fuel cells for bioenergy generation and wastewater treatment

Fuel Processing Technology 138 (2015) 284–297

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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Review

Recent progress and perspectives in microbial fuel cells for bioenergy generation and wastewater treatment☆ F.J. Hernández-Fernández a, A. Pérez de los Ríos b, M.J. Salar-García a,⁎, V.M. Ortiz-Martínez a, L.J. Lozano-Blanco a, C. Godínez a, F. Tomás-Alonso b, J. Quesada-Medina b a Department of Chemical and Environmental Engineering, Regional Campus of International Excellence “Campus Mare Nostrum”, Technical University of Cartagena, Campus La Muralla, E-30202 Cartagena, Murcia, Spain b Department of Chemical Engineering, Regional Campus of International Excellence “Campus Mare Nostrum”, University of Murcia, Campus de Espinardo, E-30100 Murcia, Spain

a r t i c l e

i n f o

Article history: Received 19 January 2015 Received in revised form 7 May 2015 Accepted 21 May 2015 Available online xxxx Keywords: Microbial fuel cells Bioenergy Electrode material Proton exchange membrane Bioreactor configuration MFC modeling

a b s t r a c t Microbial fuel cells (MFCs) use bacteria to convert the chemical energy of a particular substrate contained in wastewater into electrical energy. This is achieved when bacteria transfer electrons to an electrode rather than directly to an electron acceptor. Their technical feasibility has recently been proven and there is great enthusiasm in the scientific community that MFCs could provide a source of “green electricity” by exploiting domestic and industrial waste to generate power. By using organic matter in wastewater as a fuel, contaminants are removed from water while generating electricity. The design of new materials has led to increased levels of power being generated, particularly when compared with the levels possible using common materials. Moreover, the use of inexpensive materials, such as ceramic membranes or non-platinum catalysts, makes it possible to obtain a feasible device to produce electricity. However, it is necessary to improve the performance of MFCs before they can be scaled up since, to date, their practical implementation is not feasible. Therefore, the global objective pursued by researchers is the development and evaluation of low cost catalysts (non-precious metals) for improving electron acceptor reduction (new cathodes), new biocompatible anodes and membranes, and novel configurations which improve the power and the wastewater treatment efficiency of MFCs, while reducing their cost. This review is intended to provide a critical and global vision of recent advances in microbial fuel cells and the potential applications of this technology. In this article, an overview over all aspects concerning MFC technology is provided, including issues such as new anode and cathode materials, types of membranes, MFC configurations, their application in the treatment of different types of wastewaters, bioenergy production, modeling and future perspectives. © 2015 Elsevier B.V. All rights reserved.

Contents 1. Introduction . . . . . . . . . . . . . . . . 2. Anodes used in microbial fuel cell . . . . . . 3. Cathodes used in microbial fuel cells . . . . . 4. Membranes used in microbial fuel cell . . . . 5. Microbial fuel cell configurations . . . . . . . 6. Bioenergy production using microbial fuel cells 7. Microbial fuel cells for wastewater treatment . 8. Modeling of microbial fuel cells . . . . . . . 9. Conclusions and remarks . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .

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1. Introduction ☆ Dedicated to Antonio Fernández González, a master of life. ⁎ Corresponding author. E-mail address: [email protected] (M.J. Salar-García).

http://dx.doi.org/10.1016/j.fuproc.2015.05.022 0378-3820/© 2015 Elsevier B.V. All rights reserved.

Microbial fuel cells (MFCs) (see Fig. 1) form part of an emerging technology that makes it possible to deal with two of the major

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Fig. 1. Scheme of a double-chamber MFC using glucose as substrate.

problems facing society today: water availability and energy. Since Potter [115] confirmed that electrical energy can be generated through the degradation of organic matter by microorganisms, this process has generated much interest among the scientific community. From the mid-late 80s and early-mid 90s, microbial fuel cells started to be the focus of many research works. Many subjects covered in such studies related to MFC performance and operating conditions are still hot issues. Many authors have discussed the effect of the use of immobilized bacteria on the electrode surfaces, materials, design, fuel cell operation, reliability and applications, among other related topics [5,8,11,142]. By the late 1990s, a series of discoveries had shown that certain bacteria can be used directly to produce electricity in fuel cells not involving a hydrogen-mediated process [23]. In a microbial fuel cell, organic matter oxidation is carried out by microorganisms, while electrons resulting from their metabolism are transferred to an electrode (anode). This transfer can occur in two ways, (i) by direct contact with the electrode through conductive proteins in the cell membrane of the microorganism (e.g. cytochrome c) or (ii) through mediators — substances with redox properties, which act as intermediaries between the cell membrane and the anode. They can be added externally or excreted as a result of microbial metabolism. Electrons pass through an external electric circuit from the anode towards the cathode, where they are transferred to an electron acceptor with a high potential, such as oxygen or metals [73], oxygen being the most widely used acceptor. Once the electron acceptor is reduced, it is combined with the protons from the anodic compartment, which cross a semi-permeable membrane to form water [133]. Although the current can be produced from simple substrates such as acetate, lactate or glucose, one of the main findings was the possibility of generating electricity from more complex substrates such as industrial or domestic wastewaters [85]. The potential of MFCs is enormous since this new technology has important operational and functional advantages over the current technologies used for generating energy from organic matter: • MFCs allow the direct conversion of substrate energy to electricity with a high conversion efficiency. • MFCs operate efficiently at ambient conditions, and even at low temperatures, which distinguishes this technology from all current bio-energy processes. • MFCs do not require gas treatment because the off-gasses of these devices are enriched in carbon dioxide and normally have no useful energy content.

• MFCs do not need energy input for aeration provided that the cathode is passively aerated [78]. • MFCs have the potential for widespread applications in locations lacking electrical infrastructures and also for expanding the diversity of fuels we use to satisfy our energy requirements. • Finally, compared with conventional wastewater treatments (e.g. anaerobic digestion), microbial fuel cells would generate lower amounts of sludge, thus reducing sludge dehydration costs [107,119].

Apart from wastewater treatments based on indexes of chemical oxygen demand (COD) removal, several studies have demonstrated the potential application of microbial fuel cells to reduce some kinds of azo dyes such as methyl orange, a common pollutant in dye wastewater. Zhang et al. [163] achieved a color reduction of a contaminated sample of up to 90.4% in a short period of time (360 min) using a singlechamber MFC and this phenomenon was accompanied by bioelectricity generation (700 mV). These results show an alternative and promising method for the electrochemical degradation of dyes in wastewater. Other works have pointed to the benefits of using microbial fuel cells for removing metals such as copper, chromium, vanadium and mercury, all of which cause serious pollution problems that affect the environment. Abourached et al. [1] achieved 90% and 97% cadmium and zinc removal, respectively, in an air-cathode single-chamber system and a power production of 3.6 W·m−2. Finally, many works show that the integration of microbial fuel cells with other processes extends the application range of these devices. For example, the combination with geochemical sorption seems to be a suitable way to treat waste with a high content of nitrogenous compounds such as wastes from mines or quarries [57]. If MFCs are combined with an electrodialysis process (microbial desalination cell, MDC), desalinized water can be obtained. This system improves the performance of a desalination plant, since it reduces power consumption (bioenergy production) and improves the desalinated water when included as a pre-desalination unit [132]. All of these recent works show some of the potential applications of microbial fuel cells and the interest of many researchers in their optimization. Over the past years, the performance of MFCs has improved almost exponentially [82]. In fact, at laboratory-scale, the current densities of MFCs already approach values that would be suitable for practical implementation. Laboratory MFCs have already reached current densities of 10 A·m−2 anode surface area [34,149]. Assuming a minimum cell thickness of 1 cm to allow enough space for wastewater pumping, a

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full-scale MFC can be expected to exhibit a volumetric current of density around 1000 A·m− 3 reactor volume. This represents a volumetric wastewater treatment capacity of 7.1 kg chemical oxygen demand (COD)·m− 3 reactor volume·day− 1, which is in the same range as conventional wastewater treatment systems, such as activated sludge systems (0.5–2 kg COD·m− 3 reactor volume·day− 1) and high-rate anaerobic systems (8–20 kg COD·m−3 reactor volume·day−1) [83]. While these results are encouraging, a deeper knowledge of this technology is required for the practical implementation of MFCs for wastewater treatment, especially to minimize costs and create architectures that are inherently scalable. Until now, pilot-scale tests have offered unsatisfactory results. There are some bottlenecks in MFCs that need to be overcome, such as, charge transfer, concentration overpotential or the cathodic reaction, which usually requires costly catalysts [171]. In this sense, inexpensive materials need to be researched in order to make this technology efficient. As regards new materials, many recent works have pointed to the benefits, in terms of power output, of using microporous cathodes or modified anodes [80,105,106]. In addition, the use of ceramic membranes or the replacement of platinum catalysts by other less expensive materials, such as manganese oxide, metal macrocycle compounds or enzymes, has led to reductions in the cost of MFCs and a wider application range [75,156]. Also, there have been promising results concerning the use of nanosilver/iron oxide composites based on graphite carbon (AgNPs/Fe3O4/GC), a low cost source, offering higher power density and durability than platinum/carbon [89]. Much effort has also been directed at designing new configurations in order to make the scaling-up of MFCs viable [26,27,39,65–67,71,80,102,103,147,152,153,172]. This review shows how strong the interest of many researchers is in microbial fuel cells due to their potential applications and the great advantage they offer for simultaneous bioenergy production and wastewater treatment. They could reduce the cost of many industrial processes since a part of the energy requirement is generated from the wastes of the process itself. This work covers the most important areas related to MFCs, from optimization to new applications, focusing on recent advances in cathode and anode materials, membranes, bioreactor configurations, the application of different types of wastewater treatments, bioenergy production and MFC modeling. Furthermore, the new challenges posed by this technology for its practical implementation are discussed.

2. Anodes used in microbial fuel cell New materials for MFCs are being developed to improve their economic feasibility and performance. In this sense, a wide variety of materials and configurations have been studied for the anode, such as carbonaceous, metal and metal oxides, and composite materials [69,83]. Carbonaceous materials show good biocompatibility, chemical stability and conductivity; they are also relatively cheap and have therefore become the most widely used materials for MFC anodes. There are three types of anode arrangement: plane, packed and brush [153]. For example, a packed configuration consisting of 2 cm of graphite granules in a volume of 0.1 dm3 was used as anode in single and double-chamber MFCs with carbon cloth covered by platinum as cathode and catalyst, respectively. The results were analyzed in terms of efficiency based on soluble organic matter removal and energy generation. A mixture of brewery and domestic wastewater (initial soluble chemical oxygen demand of 1200 and 492 mg·L−1 of volatile suspended solids) was the source of carbon for the experiments. COD removal and electricity production are directly related to the temperature of the process and therefore this factor was a crucial factor for the yield of the MFCs. The results ranged from 58% final COD removal and a maximum power of 15.1 mW·m−3reactor during polarization (at 4 °C) to 94% final COD removal and a maximum power of 174.0 mW·m−3 (at 35 °C) for singlechamber MFCs [68].

A recent work by Liu et al. [80] showed that electron transfer could increase if carbon cloth anodes modified with formic acid are used. This simple modification method improved the performance of an aircathode single-chamber microbial fuel cell by 38.1%, enhancing the large-scale application and the commercial viability of these devices. Another possible option for improving the power generation of an MFC would be the electrodeposition of manganese dioxide (MnO2) onto the surface of carbon felt. Through this method, Zhang et al. [164] obtained better performance (up to 24.5%) compared with bare carbon felt anodes. This finding could be explained by the synergistic effect of the material properties (biocompatibility, high specific surface area and pseudocapacitive behavior), which would facilitate electron transfer. Metals are much more conductive than carbon-based materials, but their application in MFCs is not so widespread. Only stainless steel and titanium have been used as common base materials for anodes since many other metals have corrosive properties. The main drawback of these materials is their smooth surface that hinders the adhesion of the bacteria. This is why several non-corrosive metals, such as stainless steel [30], titanium [3,47] or gold [121], failed to achieve higher power densities than carbon materials [153]. Some modifications based on increasing the anode surface have been developed to solve this problem, facilitating the adhesion of the bacteria and electron transfer to the anode, thereby improving power production. This option involves the treatment of the surface with physical or chemical methods or the use of composite electrodes [153,173]. Regarding surface treatments, Tang et al. [146] carried out the oxidation of graphite felts and their anodes produced a 39.5% higher current than an untreated anode. This is because the treatment generates carboxylcontaining functional groups on the anode surface that encourage the formation of strong hydrogen bonds with a peptide chain in bacterial cytochromes, facilitating the electron transfer from the bacteria to the electrode. The combination of different materials, such as the inclusion of conductive polymers, has given rise to composite electrodes. Chen et al. [18] enhanced the electrical performance of MFC systems using an aluminum-alloy mesh composite carbon cloth as an anode compared with a carbon cloth and an aluminum alloy mesh. Luckarift et al. [88] used cellular biocompatibility (3-hydroxybutyrateco-3-hydroxyvalerate) scaffolds doped with carbon materials that allowed the effective immobilization of Shewanella oneidensis. Using lactate as substrate, the functionalized electrodes provided stable and reproducible anodic open circuit potentials of −320 ± 20 mV (vs Ag/AgCl) and a maximum power density (5 mW·cm−3) higher than the densities previously obtained with graphite felt anodes. New composite anodes based on graphene, whose special properties include outstanding electrical conductivity, very high specific surface area, biocompatibility, etc., provide new applications for MFCs. However, 2D structures derived from this material pose some problems such as a reduced bacteria loading capacity and high initial specific area caused by their stacking [37]. On the other hand, 3D structures of graphene enhanced the performance of MFCs and the inclusion of some conductive polymers, such as polyaniline, led to a maximum power density (768 mW·m−2) considerably higher than the results obtained using carbon cloth anodes [159]. Regardless of the structure, the synthesis process is a key factor due to the incorporation of impurities that reduces the conductivity of these materials. Polymeric-metals such as polyaniline (PANI)/Pt composites are another type of new material used in MFCs. These electrodes are biocompatible and the presence of the polymer avoids the deactivation of the anode caused by the metabolic products resulting from bacterial and electro-catalytic oxidation [86,99,141]. Also PANI is particularly important because it has unique properties, including stability at room temperature and reversible conductivity at different pH values; it is also easily prepared in the form of thin films, fibers and particles. Moreover, these properties can be improved by doping PANI with different compounds such as camphorsulfonic acid [166]. Park and Zeikus [108]

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developed new electrodes based on a graphite anode linked to an electron mediator such as Mn4 + and a graphite cathode linked to Fe3+, reaching a power density of 788 mW·m−2 with sludge as substrate, which is 1000 times higher than that obtained with conventional graphite. In the case of the integration of nanoparticles in composite materials, it is crucial to assess their biocompatibility with the microorganisms in the anodic chamber since nanomaterials may have an adverse effect on microbial activity. Due to their unique electrical and structural properties, carbon nanotubes (CNTs) show the most promising results as catalyst support in fuel cells. Qiao et al. [117] obtained a maximum power density of 42 mW·m−2 through the use of a nickel foam anode with a polyaniline-carbon nanotube (20 wt.%), while Higgins et al. [53] achieved 4.75 W·m−3 in a stack reactor with a cylindrical glassy carbon modified with a mixture of chitosan and carbon nanotubes. One of the key factors in this type of composite material is the ratio of the polymer to nanomaterial in the final blend to avoid damage to the biological growth of the cell. 3. Cathodes used in microbial fuel cells Air- and aqueous-cathodes are the most common configurations used in MFCs. The main difference between them is that in the case of air-cathodes one side is directly in contact with oxygen (exposed to the air). This type of cathode is the most widely used due to its simple design that does not require aeration and which offers high power densities. It typically consists of three layers: a diffusion layer exposed to the air, a conductive supporting material, which sometimes works as a diffusion layer, and a blend of catalyst and binder in contact with the water. However, in the case of an aqueous-cathode, the electrode often is made up of conductive supporting material, such as carbon cloth or platinum mesh, coated with a binder/catalyst layer, which is immersed in an aqueous phase with a limited oxygen concentration [84,139, 161]. The most common supporting material for air-cathodes is carbon cloth [20,21,81] and the binder used to fix the catalyst onto the electrode is often perfluorosulfonic acid (Nafion) or poly(tetrafluoroethylene) (PTFE). Cheng et al. [21] obtained a 14% higher maximum power density, around 480 mW·m−2, using Nafion rather than PTFE as binder at the same platinum loading. Another advantage of using Nafion is that the biofilm developed on the cathode is thicker than that obtained using PTFE, thus avoiding losses. However, the main disadvantage of Nafion is its price, nearly 500 times higher than the price of poly(tetrafluoroethylene). To avoid the diffusion of oxygen into the reactor and water losses, a hydrophobic diffusion layer must be placed on the air side of the cathode. With four diffusion layers of PTFE, Cheng et al. [20] achieved an increase of 42% in the maximum power density compared with a cathode without diffusion layers. Because of this, some companies include a hydrophobic coating layer in their commercial conductive supporting material, such as carbon cloth. One of the major limiting factors for the performance of singlechamber MFCs is the biofouling of the cathode. As previously mentioned, Ma et al. [89] demonstrated that a catalyst based on graphitic carbon (GC) and silver/iron oxide composite (AgNPs/Fe3O4/GC), which has an antibacterial effect on the cathode surface, can increase the conductivity and the catalytic activity of the system. They obtained a maximum power density of 1712 mW·m−2, and consequently this material seems to be a low cost alternative to Pt/C, being both more efficient and durable. The cathode and the membrane are often manufactured separately and then assembled to form the air cathode, ensuring a good contact between both elements with a consequent reduction in the cell internal resistance. To minimize this resistance in air cathode MFCs, a new configuration, called membrane cathode assembly (MACA), has been developed. These can be prepared in many ways, for example, pressing the separator on the air cathode [77,96,109] or bringing both the membrane and the cathode into contact with hydrogel to improve

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membrane hydration [60,62]. As regards the catalyst, most materials used to prepare both the anode and cathode can be used to catalyze the oxygen reduction reaction, but, due to its low overpotential the most commonly used is platinum (Pt). However, the high price of this precious metal limits its application and many researchers have reduced the amount of this catalyst necessary to cover the cathode to around 0.1 mg·cm− 2 [21]. Because of its low overpotential for oxygen reduction, gold (Au) is considered a good alternative, although, once again, it is very expensive [13,58]. To overcome this problem, the first row of transition metals seems to be the best alternative as they are relatively cheap, offer good stability and do not affect the microbial activity of the cell. Some composite materials such as molybdenum disulfide or tungsten carbide present good results, although nickel alloys and stainless steel offer the highest performances [136]. Alternatively, some researchers have proposed the use of noble metals such as phthalocyanine (FePc), pyrolyzed iron(II) and cobalt tetramethoxy-phenyl-porphyrin (CoTMPP) to prepare new catalyst-free cathodes for MFCs [21,170]. New nanocomposite materials are much less expensive and have improved the performance of these devices in recent years. Some examples of these electrocatalysts are palladium nanoparticles [114] and Ni-based-nanomodified materials [97]. These enhance their surface area, electrochemical catalytic activity and mechanical stability compared with common carbon materials covered with platinum. However, as mentioned above, the main drawback of these nanostructured materials is their harmful effect on the biofilm, reducing the electrical performance of the MFCs. Another option is to use nanoparticles on a metal-coated cathode. An et al. [7] prepared and tested plain graphite cathodes and others coated with platinum/carbon (Pt/C), silver nanoparticles (AgNPs) or Pt/C + AgNPs. Among the MFCs studied, those incorporating a Pt/ C + AgNP electrode presented the worst performance probably due to the negative impact of silver ions (Ag+) on the microorganisms in the anodic chamber. The Pt/C-coated cathode had a higher overpotential than the AgNP-coated cathode since the biofilm built up on the Pt/Ccoated cathode may compete with the cathode for oxygen by hindering the abiotic oxygen reduction reaction (ORR). The AgNP-coated cathode presented a much lower kinetic activity than the Pt/C-coated cathode but produced the highest current. Ghasemi et al. [40] compared the performance of microbial fuel cells using platinum electrodes and platinum deposited onto carbon nanotubes as catalyst. The tests were carried out using a substrate with several COD concentrations (100, 500, 1000 and 2000 mg/L) and their results showed that CNTs improved the ORR because of the large surface and therefore improved the catalytic activity of platinum. Furthermore, the addition of nanoparticles decreases the amount of Pt needed for its application in MFCs. Thus CNT/Pt could be an alternative cathode for Pt in MFCs. Also, several methods for improving the performance of air-cathode MFCs using activated carbon have been developed [28]. Pu et al. [151] achieved a 69% improvement in performance with silver electrodeposition compared with bare activated carbon (1080 mW·m− 2 ). This method improves the oxygen reduction reaction (four-electron pathway) and reduces electrode resistance, avoiding biofilm growth over the cathode surface. To reduce the material costs and improve the ORR kinetic, other non-metal cathodes have been developed such as nitrogen-doped granules [33], carbon powder [138], carbon nanotubes [35] and carbon nanofibers [19]. Among these novel cathodic materials, carbon nanofibers (CNFs) are regarded as the most promising because of their properties: high ORR, outstanding electron transfer ability due to their nanostructure, large specific area, good stability and, above all, their reasonable price [59]. As can be seen, many studies have made important advances in developing new catalysts such as nanomaterials, transition metal oxide nanoparticles or macrocyclic compounds, carbon nanotubes, graphene or nitrogen-doped graphene. However, their application in

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MFCs is still under study since the power density obtained is lower than that obtained using common materials. An attractive and economic alternative is to use biocathodes [75,79], in which the precious metal is replaced by an enzyme as catalyst coated onto a relatively inexpensive electrode material, thus reducing the final cost of the fuel cells. Studies on biocathodes point to the possibility of using microorganisms as electrocatalysts or mediators, in cathodes known as microbial biocathodes [12,24,36,128]. When a seawater biofilm on a stainless steel cathode was tested by Bergel et al. [12] at laboratory scale, the biological layer improved the reduction of the oxygen in the electrode and better hydrodynamic control was achieved. This suggests new possibilities for easily producing cheap and corrosion resistant biocathodes. Rozendal et al. (2008) [128] developed a biocathode for hydrogen production with a current density of about 1.2 A/m2 at a potential of 0.7 V. While this value obviously needs to be improved, this performance represented a significant increase in comparison with their previous studies using a Pt-coated titanium electrode as cathode. Other researchers like Clauwaert et al. [24] combined the anode of an acetate oxidizing tubular microbial fuel cell with an open air biocathode, achieving around 83 W·m−3 operating in fed-batch mode and around 65 W·m−3 in continuous mode (1.5 kg COD·m−3·day−1 of acetate). Their results show that biocathodes obviate the need for noble or non-noble catalysts for reducing oxygen, which is one of the main bottlenecks in MFCs and increase the viability and sustainability of MFCs. 4. Membranes used in microbial fuel cell Another important factor in MFC design is the ion exchange membrane, which directly affects the final cost of the process and the performance. Internal resistance, oxygen diffusion, substrate loss or biofouling are some important considerations that must be taken into account when selecting a membrane. Some researchers have shown some advantages of membraneless MFCs, which include the lack of membrane biofouling, lower internal resistances due to the absence of a membrane and a reduction in operational costs. Nevertheless, in the long term, problems associated with high substrate and oxygen crossover rates can reduce the performance of this type of MFC [72,77]. As seen above, two possible MFC configurations are separator electrode assembly (SEA) and spaced electrode assembly (SPA) design [165]. The use of a membrane or separator between the anode and the cathode in an MFC keeps them physically separated, preventing a short-circuit. Additionally, the use of a membrane limits oxygen diffusion from the cathodic to the anodic compartment, and therefore boosts the columbic efficiency and fuel transfer from the anode to the cathode. The SPA design, which excludes the use of a membrane, hinders the output power even if the electrodes are separated by less than 2 cm [45]. A further consideration is that solutions of low-conductivity, like household wastewater, need very small distances between electrodes to reduce ohmic losses [45]. Zhang et al. [165] compared SPA and SEA set-ups for MFCs fed with refinery wastewater. The distance between the cathode and anode in the SPA cell was 1.5 cm, while in the SEA MFC it was 0.5 cm. In both cases, the anode was made out of fiber graphite brush and the separator used in the SEA cell consisted of porous cloth. The output power (maximum value) for the SEA design (≈ 280 mW·m− 2) was higher than for the SPA (≈ 255 mW·m− 2), which can be explained in terms of internal resistance. Proton exchange membranes (PEM) are often used in MFCs. Perfluorinated ionomer membranes such as DuPont™ Nafion® membranes have been widely used as proton exchange membranes in MFCs due to their high proton conductivity when fully hydrated. Nevertheless, this type of membrane remains very expensive (around $1400 per m2), making their large scale application unfeasible. In addition, not only protons can pass through Nafion® membranes, but also cation + + species such as Na+, K+, NH+ 4 or Mg2 at similar rates to those for H . As

a result, these species may build up in the cathode with typical concentrations that are 105 times higher than for protons. As protons are involved in the cathode reaction, they do not accumulate, unlike cation species which accumulate in the cathode chamber, increasing the pH and disfavoring the overall MFC efficiency due to the drop in the cathode potential [130]. Consequently, a decrease in the pH at the anode can severely affect the biological activity of the cell, even leading to deactivation if it disrupts the bacteria, causing cells to die. On the other hand, pH gradients undermine the thermodynamic potential of the cell [41], which is crucial for MFC performance. Several approaches have been suggested to avoid problems with pH variations, which disturb cell performance. In this regard, pH static control [130] or configurations lacking a separator [13,77] have been put forward as possible solutions. The first strategy implies a continuous control by adding bases and acids to the anodic and the cathodic chambers, respectively. This may be technically demanding and thus not very convenient from an economic viewpoint. The second solution is detrimental to Coulombic efficiency since oxygen can enter the anode, as mentioned above. Faced with these two approaches, other options have been suggested, including the use of other types of membrane: cation-exchange membranes (CEM), anion-exchange membranes (AEM), bipolar membranes (BPM) and ultracentrifugation membranes (UCM). When tested in MFCs [48,61,129], they confirmed their potential application, particularly in the case of anion-exchange membranes, which can improve cell efficiency. The main drawback is that the operating conditions could not be changed easily. Kim et al. [61] assessed acetated-fed air-cathode microbial fuel cells assembled with different separators, in particular, Nafion-based protonexchange, cation-exchange, anion-exchange and ultrafiltration (UF) membranes (with molecular weights of 0.5, 1 and 3 K for the last one). The anion-exchange membrane yielded the highest power density, 610 mW·m−2, with a Coulombic efficiency of around 70%. Some of them presented very high internal resistance values (0.5 K in the case of the UF membrane). The anion-exchange membrane also proved to be the best option for oxygen and fuel (acetate) impermeability. It is clear that pore size is a key factor for MFC performance and a balance must be found in this respect. If the pore size is too big, then fuel will easily diffuse to the cathodic compartment and, in the same way, oxygen will pass through the membrane to the anode. If this happens, Coulombic efficiency and output power density will be greatly reduced, because electrons will be used for oxygen reduction instead of being transferred to the cathode to produce an external current. By contrast, if the pore size is too small, protons will not be able to cross through the membrane to reach the cathode, which will directly affect the efficiency of the cell (current and power densities). Separators based on cloth and glass fibers have been shown to be very effective at maintaining high levels of power densities and limiting oxygen diffusion, although cloth can degrade within time [168]. Different materials were compared as separators in MFCs: glass fiber (1 and 0.4 mm thickness), cation-exchange membrane (CEM) and J-Cloth (JC). It was seen that glass fiber (1 mm) and J-Cloth permitted the highest power density (46 W·m−3) when compared to CEM and glass fiber (0.4 mm) in an MFC where the electrodes were placed 2 cm apart and the separator was put against the cathode (S configuration). The high Coulombic efficiency obtained with glass fiber (1 mm) can be explained by the low oxygen transfer rate. The difference between glass fiber (1 mm) and J-cloth is the extent to which biomass builds up on them and their biodegradability, which harms their long term MFC performance [168]. Many recent works have focused on reducing the cost of MFCs by using ceramic materials as support to prepare different types of membranes. For example, Behera et al. [10] obtained a maximum power of 16.8 W·m−3 using a clayware-based separator as an effective proton exchange membrane. Later, Ghadge and Ghangrekar [38] designed a cylindrical air-cathode MFC made of baked clayware as a cation exchange

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Fig. 2. Examples of ions involved in ionic liquids.

membrane. Their large volume system (26 L) showed good long term stability (14 months), confirming the viability of this low cost material for practical application in MFCs. An interesting inexpensive alternative membrane was used by Winfield et al. [157], who employed natural rubber as membrane and compared it with a commercial cation exchange membrane. This natural material remained intact after almost 500 days in an MFC, showing higher long-term power than conventional materials. Despite the promising results, this low cost material requires further examination and improvements. The use of membranes based on ionic liquids (ILs) could open up new possibilities in the MFC field [6]. ILs are organic salts that remain liquid at room temperature. They are often formed by an organic cation (e.g. imidazolium and pyridinium) and a poly-atomic inorganic anion (e.g. hexafluoroborate), or, more commonly, an organic anion (e.g. trifluoromethylsulfonate) [42]. Fig. 2 shows the structure of typical cations and anions present in ionic liquids. Among the advantages of ILs, of particular note is their vapor pressure, which is near zero, and their chemical and thermal stability for a wide temperature range. Because of these properties, they are considered environment-friendly “green solvents” compared to volatile organic solvents. In addition, their properties (viscosity, solubility, etc.) can be adapted to specific purposes by varying their composition [16]. In fact, ILs have already been applied in lab-scale processes to replace conventional organic solvents in purification or separation processes [15,51,52,92,124,125], in reaction media in biochemistry [123,126, 131] and as chemical catalysts [32,137,140]. In membranes, they have been found to be of great interest due to their high ion conductivity and solvent power. In the last decade, research has shown that supported liquid membranes (SLMs) based on ionic liquids have selective transport properties for organic compounds like organic acids, amines, alcohols, ethers, ketones and aromatic hydrocarbons [92], blended gasses [98] and metallic ions [122]. All these properties has led ionic liquids to be considered as “designer solvents” [135]. In the MFC field, polymer inclusion membranes (PIMs) based on ILs have shown their potential to replace conventional proton exchange membranes (Nafion®) [49]. These membranes are prepared with PVC and IL using a casting method. When they were tested as proton exchange membranes in singlechamber MFCs assembled with carbon cloth cathodes on which platinum was sprayed and incorporating an external resistor of 1 kΩ, they showed similar or even better performance than conventional membranes such us Ultrex® and Nafion® membranes [50].

5. Microbial fuel cell configurations Different types of MFC configurations have been proposed for lab-scale studies. As mentioned above, the greatest limitation of this technology is the subsequent scaling-up. The distance between electrodes, and the specific surface of the anode, cathode or separator are key factors on which the internal resistance of the system and the power density directly depend [102]. A typical two-chamber or double-chamber MFC configuration mainly consists of an anodic and cathodic chamber physically separated by a proton exchange membrane (PEM) [12,14,94,100,101]. Bacteria in the anode chamber degrade the substrate (oxidation) and transfer the electrons released to the anode through an external circuit. Protons pass through the proton exchange membrane to the cathode, where they react with oxygen (reduction) and the electrons from the anode to form water. However, double-chamber MFCs are complex and pose practical problems when they need to be scaled up. To overcome this difficulty, a single-chamber MFC represents a real alternative, offering cost and operational savings among other advantages. A typical single-chamber MFC has only an anodic chamber, while the cathode remains exposed to the air. Liu and Logan [77] built a microbial fuel cell that consisted of a cylindrical reactor with an inner anode and the cathode outside the chamber. The anode was carbon paper with no waterproof treatment. There were two options for the cathode: (1) carbon electrode and membrane assembled by placing the PEM directly on flexible carbon cloth, or (2) rigid carbon paper without PEM ([20], [76]). A tubular MFC set-up with an external cathode and an inner anode using graphite was proposed by Rabaey et al. [118], who noted that a sustainable open-air cathode is crucial for real implementation. Another alternative consists of an up-flow chamber with a fixed biofilm, continuously fed with substrate solution through permeable anodes to the membrane [46]. Another promising configuration by Logan et al. [81] used graphite fiber brush electrodes and tubular cathodes plunged into a tank. Finally, it should be noted that higher output voltages and currents can be attained by connecting stacked MFCs in series or in parallel [4,55]. However, more light needs to be shed on the long-term impact of voltage reversal on the power generation by stacked MFCs to efficiently improve voltage production. Aelterman et al. [4] obtained enhanced

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voltages (up to 2.02 V) and currents (up to 255 mA) by connecting six MFC stacks in series and parallel, respectively. The development of new MFC configurations allows these devices to be used for novel applications. Dong et al. [29] designed a singlechamber membrane-less microbial fuel cell which simulated the colonic environment. It was used to provide bioenergy for implantable medical devices. Although the power generated by this novel continuous MFC (1.6 mW) was not sufficient to power devices with high energy requirement, these promising results open up an important research area. Regarding submersible MFC configurations, Zhang and Angelidaki [169] developed an innovative single-chamber selfstacked submersible microbial fuel cell (SSMFC). The set-up is composed of a rectangular cathode chamber (non-conductive polycarbonate plate). On each side of the cathode chamber, a sandwichtype electrode (anode/membrane) was placed, thus reducing electrode spacing. The whole self-stacked cell is then submersed in a glass reactor. This configuration was designed for harvesting electricity from lake sediment (see Fig. 3). This type of configuration allows the cell to be operated both as two single cells (PEM1 and PEM2 working individually) and as a single self-stacked cell. The SSMFC offered a maximum power density of 294 mW/m 2 and an open circuit voltage (OCV) of 1.12 V. Finally, Köroĝlu et al. [65] presented a novel up-flow multitube microbial fuel cell (UM2 FC), which mainly consisted of tubular perforated stainless steel electrodes working as cathode electrodes, carbon fiber tubes as anode electrodes and a tubular Nafion membrane, all assembled in a cylindrical glass reactor (see Fig. 4). For the experimental study of this configuration, the reactor was

Fig. 4. Self-stacked submersible microbial fuel cell.

inoculated with sediment and fed with domestic wastewater in continuous mode (COD removal up to 87% and maximum power density of 25.138 mW·m− 2).

Air

Water

Sediment

A n o d e

C a t h Cathodic o Chamber d e

PEM 1 Fig. 3. Self-stacked submersible microbial fuel cell.

PEM 2

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6. Bioenergy production using microbial fuel cells MFCs transform the chemical energy present in biomass into electrical energy through the action of microorganisms. A Carnot cycle with limited thermal efficiency can be sidestepped since chemical energy from the fuel oxidation is directly turned into electricity rather than heat, improving efficiency, as in the case of conventional chemical fuel cells (N70%) [27]. The output power of a cell, which depends on the oxidation rate, can be formulated as a function of the fuel concentration, showing a Monod-type relationship, with a half saturation constant ranging from 79 to 102 mg/L for glucose [77] and from 461 to 719 mg/L for wastewater [96]. Chaudhuri and Lovley [17] reported that Rhodoferax ferrireducens can produce electricity with a Coulombic efficiency as high as 80%, and Rabaey et al. [120] achieved an even higher yield of 89%. However, the maximum Coulombic efficiency was reported by Rosenbaum et al. [127], with a value of 97% in an MFC fed with formate and black Pt as catalyst. However, when complex substrates are used instead of pure substrates, the electron yield is likely to be lower. For instance, Larrosa-Guerrero et al. [69] reported a low Coulombic efficiency of 25% for MFC fed with wastewater and with a membrane-based cathode configuration at 4 °C. The wastewater was obtained through the dilution of brewery wastewater in domestic wastewater, with a resulting chemical charge of 1200 mg·L−1 (DQO) and 492 mg·L−1 of volatile suspended solids. Under these conditions, maximum power output was 294.6 mW·m− 3 (normalized to the anode chamber volume). When the system was run at 35 °C, these values dropped to 1.76% and 174.0 mW·m− 3 for the Coulombic efficiency and the power output, respectively. Coulombic efficiency decreases when there are other electron acceptors in the anode solution different from the bacteria. They can either be present in the media, for example in wastewater, or diffuse through the membrane, e.g., oxygen. Other causes for low Coulombic efficiency may be biological (e.g., bacterial growth and competence). When fermentation or methanogenesis occurs, electrons are used instead of being transferred to the cathode to produce an electrical current, consequently disfavoring electron yield. Despite significant advances in MFC technology, MFC power generation is still low [25,148]. A possible way to overcome this handicap would be to store electricity in rechargeable devices and then distribute it to a final application [55]. Ultimately, MFC devices could be regarded as a potential alternative for local energy production, particularly in underdeveloped regions of the world. 7. Microbial fuel cells for wastewater treatment Conventional wastewater treatments under aerobic conditions for domestic and industrial waste are both energy and resourcedemanding processes. The aeration required for these operations imposes an energy consumption of over 0.5 kW·h·m−3 (30 kW·h per capita per year). It follows that large quantities of surplus sludge are generated and need to be treated [154]. The use of microbial fuel cells for wastewater treatment offers several advantages over current technology. Cells are able to recover the chemical energy present in wastewater and turn it into electrical power, while purifying it. Moreover, the power consumption is much lower than with traditional technology and generates less sludge. It is well known that sludge disposal is expensive and greatly increases water treatment costs [61,119]. The amount of excess sludge to be disposed of can be from 50 to 90% higher when traditional technology is used, rather than using MFCs [54]. In an aerobic process microorganisms use all the energy from organic compounds (pollutants), but only a small proportion of this energy is available to them for their growth. In contrast, in MFCs, most of this energy is converted into electricity. In a further step, the power generated by MFCs could potentially be used in the same wastewater treatment process; in other words, for energy feedback. MFCs are also able to metabolize some xenobiotic compounds [56]. Finally, apart

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from those microbial species which are naturally present in wastewater, other exogenous bacteria can be inoculated into the medium to improve water treatment. MFC technology was deemed useful for wastewater treatment in early 1991 [43]. Since then, wastewaters from different origins have been used as fuel in microbial fuel cells, among them sanitary wastes, wastewater from the food industry, brewery wastewater or corn stover, which are rich in organic matter, and also chemical industrial wastewater [63,78,95,100,145,158,160,174]. Sewage and industrial wastewater have large quantities of organic compounds that can also serve as fuel in MFCs. Some studies have reported that the final COD removal efficiency can reach up to 80% [78]. Apart from using wastewater from one source, wastewater mixes have been used in MFCs. For instance, Larrosa-Guerrero et al. [70] tested brewery wastewater diluted in domestic wastewater in single and double-chamber MFCs in batch mode operating at different temperatures (from 4 to 35 °C). For temperatures of 4, 8 and 15 °C, the system presented considerable improvements in terms of COD removal when compared to the control and baseline reactors (anaerobic digestion with graphite granules and anaerobic digestion, respectively). By contrast, these differences were negligible at temperatures of 20, 25, 30 and 35 °C. In the case of a single chamber configuration with carbon cloth, the maximum values of COD removal and power density were obtained at 35 °C (94% and 8.1 mW·m−2cathode for each), while at 4 °C the results were lower (50% less COD removal and 100 times lower maximum power density). The results showed that both parameters, COD removal and power generation, are highly dependent on the operating temperature and improve at high temperatures. Microbial fuel cells were seen as promising alternatives for conventional anaerobic wastewater treatments since the results were very similar in both cases, even at the low operating temperatures tested. Brewery wastewater has been widely applied as fuel in MFCs (Katuri and Scott 2010). For example, a continuous brewery wastewater flow has been used as substrate in double chamber MFCs with an aerated cathode [155]. This MFC configuration displayed a maximum power of 23.1 mW·m−2 anode surface and a COD removal of 91.7–95.7%. Besides, the authors found that the electrochemical performance and COD removal efficiency of the microbial fuel cells could be improved by adding a phosphate buffer solution and increasing the concentration of the substrate. Power generation from food processing wastewaters was investigated in a double chamber reactor MFC without a catalyst or mediator [90]. The anodic chamber contained food industry wastewater as substrate while the cathodic chamber contained the buffer solution. With an organic load of 0.364 g COD·L−1·day−1, the maximum power density generated was 230 mW·m− 2 anode surface. However, the best Coulombic efficiency was obtained with a half-organic matter load and a final COD removal of 86%. The configuration studied presented many benefits, such as reproducibility and high efficiency in addition to cleanliness. This technology would make it possible to reduce the cost of conventional wastewater treatment through a combination of both methods. Another example concerned the reuse of wastewater from potato-processing industries [31]. The wastewater from starch processing is an important energy-rich resource as it contains a high load of carbohydrates, sugars and proteins. Lu et al. [87] used this kind of fuel (4852 mg/L of COD) in an air-cathode MFC with MACA, operating for 140 days (four batch cycles), obtaining maximum voltage and power density values in the third cycle (490.8 mV and 4 mW·m−2), which involved the minimum internal resistance (120 Ω). This process allowed almost 98% of the initial COD to be removed. These results demonstrate that MFCs are efficient devices for treating this type of wastewater. Another MFC source is domestic wastewater [2,22,116]. In this line, a single-chamber MFC was used to test domestic wastewater mixed with olive mill wastewater (14:1, w/w) as anodic substrate [134]. This medium produced 0.38 V at 1 kΩ, which is 2.9 times higher than when using

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domestic wastewater, and a power density of 124.6 mW·m−2. The process reduced chemical oxygen demand and biological oxygen demand by up to 60% and 69%, respectively. Microbial fuel cells have also been used for treating animal wastewaters and producing electricity at the same time [95]. More specifically, swine wastewater with a COD of 8320 mg·L−1 was used as fuel in a double chamber microbial fuel cell with an aqueous cathode. This configuration generated a maximum power density of 45 mW·m−2. By contrast, a single chamber with an air cathode configuration produced a maximum power density 79% greater than that obtained with the same system using domestic wastewater due to the high organic matter content in the animal wastewater. Again, these results suggest that MFCs could produce electricity using this type of substrate. In the case of wastewater from biodiesel production processes, an up-flow bio-filter microbial fuel cell with a biocatalyst was employed to reduce the COD and neutralize its alkalinity [143]. This process was preceded by two previous stages: fermentation and a procedure to adjust the influent. Operating with an organic loading rate of 30 g COD·L− 1·day− 1, a hydraulic retention time of 1.04 day, a pH range of 6.5–7.5 and 2.0 L·min−1 of aeration, a maximum efficiency in terms of COD removal (15.0 g COD·L−1·day−1) was obtained. Azo dyes are present in effluents from textile industries. These are very toxic and attempts have been made to purify them before discharge using MFCs. For example, a single-chamber air-cathode microbial fuel cell with a microfiltration membrane was studied to decolorize an azo dye with simultaneous power production. Operating in batch mode, the decolorization of active brilliant red X-3B (ABRX3) was achieved faster than with a conventional anaerobic process. Of the several co-substrates tested to improve the performance of the system, glucose presented the best results and acetate the worst. Moreover, it was confirmed that the main mechanism of dye removal involved biodegradation. Avoiding concentrations higher than 300 mg·L−1, this substrate could be an economic substrate for electricity generation in fuel cells [144]. Finally, a double chamber microbial fuel cell was tested for treating electroplating wastewater to remove chromium (VI) and produce electricity at the same time [74]. Best results were achieved using graphite paper and acetate at pH 2 as cathode and fuel, respectively, in anaerobic conditions. A maximum power density of 1600 mW·m−2 was obtained with a real electroplating wastewater that initially contained 204 ppm 2 to Cr2O3 enabled 99.5% of Cr(VI). Besides, the reduction of Cr2O− 7 removal of Cr(VI) and 66.2% of the total Cr through precipitation onto the cathode surface. These results confirm this technology as a promising device for removing Cr(VI) from electroplating wastewater. 8. Modeling of microbial fuel cells Much scientific effort has been put into optimizing MFC devices since this technology was seen to be a promising way to generate clean energy. However, modeling and mathematical optimization of MFCs have taken more time to gain the attention of researchers and only recently have works dealing with MFC modeling proliferated [27]. Modeling is a powerful tool for understanding the operation of this technology, and a robust mathematical model can describe the processes that take place in these systems. The simulation of MFCs includes the study of multiple scenarios in different conditions, with significant cost and time savings. The complexity of a model depends on the dimension selected, the level of detail when describing the system and the assumptions or simplifications considered. The phenomena that can be taken into account in a model cover a wide range of processes, such as mass transport through the cell, microbial growth, phases of matter and its boundary conditions, anode and cathode reaction kinetics or the electrochemical behavior of the cell. The robustness of the model will be confirmed by its predictive capacity and a balance between the computation time and the accuracy of the results may be needed. In addition, models in the literature deal with a wide range of

configurations and operating conditions [104]. For instance, in early modeling works [167] a chemical redox mediator was used, while in more recent studies it tends to be omitted. A model of MFCs is constructed using laws which describe the physicochemical phenomena that occur in them. The application of such equations depends on the specific configuration as well as the chemical species present. The most common equations are the (i) Monod, (ii) Bulter–Volmer, (iii) Tafel, (iv) Nernst, and (v) Nernst–Planck equations, (vi) Fick's law, (vii) the Maxwell–Stefan diffusion equation, and (viii) Ohmꞌs law. Table 1 shows these expressions: The first model developed by Zhang and Halme [167] can be considered as relatively recent in light of the date when MFCs started to be studied. This first one-dimensional model was based on a double chamber configuration extended to three cathodes in parallel and three respective separators (commercial membrane, DuPont) in batch mode. The source of organic matter was fermented marine sediment with buffer solution in both compartments. A chemical mediator [2-hydroxy-1,4-naphthoquinone, HNQ] was added to the anode, which allows electrons to be transferred to the electrodes. This model considers that the limiting factor of the system is the anode and thus the anode compartment is modeled to reproduce the cell performance. The main equations used in the model are (i) mass balance according to the substrate oxidation rate and chemical mediator redox reaction in the anode, including the Monod equation, (ii) equation for calculating the current produced in the cell by Faraday's law and (iii) electrochemical behavior modeled by Nernst's equation. Once the model had been validated, the relationships between the current produced and some process variables, such as the optimal concentration of the substrate or the chemical mediator, were studied. This model allows operational and control decisions to be made to optimize the MFCs under study. In the case of a chemical mediator, its use must be minimized due to its high cost and toxicity. Picioreanu et al. [112] introduced a new and more complex model that involves two- and three-dimensional simulations. The model focused on the description of the mechanism by which electroactive microorganisms present in the anode degrade the organic matter and transfer the electrons released to the anode by the reduction of a mediator. MFCs fed with acetate and operating in batch mode were selected for this study. The model really consists of two sub-models: i) an electrochemical sub-model (Bulter–Volmer equation) to calculate the current density over the anode surface and ii) biofilm and species in solution (bulk liquid) in the anode. The voltage is calculated by balancing the cell equilibrium potential and voltage losses (polarization and overpotential losses) and combining them by means of Ohm's law. In addition, two- and three-dimensional analyses of the biofilm growth on the anode make it possible to study this phenomenon in detail. Other important parameters are also studied and their profiles over time obtained: current, biomass in suspension, substrate concentration, and reduced and oxidized mediator concentration. Picioreanu et al. [111] integrated the previous model with the international water association IWA Anaerobic Digestion Model No. 1 (IWA ADM1) [9] to introduce the competition that takes place between methanogenic and anodophilic bacteria. Methanogenic bacteria degrade organic matter by producing methane, reducing the efficiency of the MFC. Once again, two- and three-dimensional simulations are performed. Among the conclusions obtained from the simulated results, it should be noted that low external resistance values boost the growth of anodophilic bacteria and improve the current density. Finally, Picioreanu et al. [110] developed an extended version of the initial model with a double perspective at both macro and micro levels. The first level covers the mass balance for the bulk liquid, and the second level includes the ion fluxes of the chemical species in the charge balances (Nernst–Planck equation) and also considers the mass transport by convection. In this way, the approach becomes more realistic. Besides, the pH is calculated over the geometry of the cell based on the charge balance. The pH is considered as a limiting factor in the

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Table 1 Equations and laws used for MFC modeling [150]. Monod equation for microorganism growth

Bulter–Volmer equation for electrode current

Tafel equation for overpotential

μ ¼ μ max K sSþS μ: specific growth rate of microorganisms; μmax: maximum specific growth rate of microorganisms; S: concentration of the limiting substrate for growth; Ks: value of S when μ/μmax = 0.5. h i 
 Þn F
F  E−Eeq −i0  ; exp − αn i ¼ i0  ; exp ð1−α RT RT  E−Eeq i: current density; i0: exchange current density; E: electrode potential; Eeq: equilibrium potential; T: temperature; n: number of electrons involved in the electrode reaction; F: Faraday constant; R: universal gas constant; α: charge transfer coefficient. RT i E−Eeq ¼ ð1−α ÞnF  ; ln i0 E: electrode potential; Eeq: equilibrium potential; i: current density; i0: exchange current density; T: temperature; n: number of electrons involved in the electrode reaction; F: Faraday constant; R: universal gas constant; α: charge transfer coefficient.

Nernst equation for electrode potential

Nernst–Planck equation for ion motion

Fick's diffusion law Maxwell–Stefan diffusion equation

E ¼ E0 − RT nF ; ln ðQ Þ E: electrode potential under nonstandard conditions; E0: standard electrode potential; R: universal gas constant; T: temperature; n: number of electrons involved in the electrode reaction; F: Faraday constant; Q: reaction quotient (example: aA + bB → cC + dD, then Q = [C]c[D]d[A]−a[B]−b) h  i ∂c ¼ ▽  D▽c−uc þ kDze c ▽ϕ þ ∂A ∂t ∂t BT t: time; D: diffusivity of chemical species; c: concentration of the chemical species; u: velocity of the fluid; ϕ: potential; z: valence of ionic species; A: magnetic vector potential; e: elementary charge; kB: Boltzmann constant; T: temperature. j=−D⋅▽c j: diffusion flux; D: diffusion coefficient of diffusivity; c: concentration. ! !! n n χ i χ j ! !  J j ci c j J ▽μ i ∑ v j − v i ¼¼ ∑ − i RT ¼ ▽ ; ln ai ¼¼ 2 ci j ¼¼ 1 Di j j ¼¼ 1 c Di j c j j≠i

Ohm's law

j≠i

T: temperature; R: universal gas constant; χ: mole fraction; μ: chemical potential; ai,j: activity; i,j: indexes for components i and j; n: number of components; Dij: Maxwell–Stefan diffusion coefficient; vj,i: velocity diffusion of component i,j; ci,j: molar concentration of component i,j; c: total molar concentration; J: flux of component i,j. V=I∗R V: voltage; I: current intensity; R: resistance value.

anode since its acidification can severely affect the performance of the cell as a consequence of the accumulation of protons (H+) at the anode. Marcus et al. [91] proposed a one-dimensional dynamic model based on the anode compartment. An important difference here is the absence of a mediator — electrons move directly from the substrate to the anode through the biofilm due to the action of electroactive microorganisms (Geobacter sulfurreducens bacteria). The model describes electron transfer based on both electron donor oxidation and endogenous respiration. The model focuses on the biofilm formed on the anode which is characterized by a bio-conductivity matrix-parameter (kbio) closely related to the transfer of electrons to the cathode and thus the power output. The equations and laws used are similar to those mentioned above but applied to the specific conditions. Oxidation and electron production rates are modeled by the Monod and Nernst equations, mass balances cover the active and inactive zones of the biofilm, including diffusion phenomena and, finally, the electrochemical model includes a calculation of the current density (Ohmꞌs Law) and the local potential of the anode. The model sheds much light on the electron transfer mechanism and allows study of the cell power depending on biofilm parameters such as the local potential, biofilm thickness, conductivity or concentration gradient of the species involved. However, this approach does not cover the effect of the pH variable, which is not calculated across the cell. Merkey and Chopp [93] published a new two-dimensional model based on the model described by Marcus et al. [91] with the purpose of studying the relationship between the power output of a given MFC and its geometry and operating conditions. The model considers three main regions (substrate solution, biofilm and electrode) and two interface zones (solution-biofilm and biofilm-anode), all of which represent the computational domain. The model presents high mathematical complexity and it is an example of how far MFC modeling has evolved. The main results concern the optimization of the number of electrodes in the anodic compartment and their geometric pattern and how they can affect the growth of the biofilm (growth rate, thickness, etc.). Pinto et al. [113] created a one-dimensional model for an air-cathode single-chamber configuration continuously fed with acetate and nutrients, using J-Cloth as separator. This configuration lacks a mediator to transfer the electrons released in the anode reaction. An external

recirculation loop is added to improve the mixing of the anode compartment. This model includes methanogenesis, which is disadvantageous for the electrical efficiency of the cells. The model includes (i) mass balances for the substrate, considering anodophilic and methanogenic competition and biofilm retention rate, (ii) intracellular mass balance for the oxidized and reduced bacterial species (electroactive), (iii) the kinetics described by Monod's equations and (iv) an electrochemical model based on Ohm's law, Nernst's equation and voltage loss balance. This one dimensional model provides interesting results with regard to operating conditions, more specifically, the optimal external resistance value. The set of scenarios simulated showed that the growth rate of anodophilic bacteria depends on the external resistance, and is at the maximum when the internal resistance of the cells is the same as the external resistance load. The concentration of the substrate is another key factor on which a balance must be reached. It has to be high enough to feed the anodophilic bacteria population but not too high to avoid an excessive growth of methanogenic bacteria. All the models discussed above consider that the anode is the limiting factor of MFCs. However, other approaches can be found in the literature, for instance, Zeng et al.'s model [162]. This studies both the anode and the cathode to simulate the performance of the cells without assuming that the anodic reaction is the limiting factor of the system. The model comprises a two-chamber set-up using acetate as substrate, platinum on the cathode as catalyst and a commercial Nafion®117 membrane as separator. The reactions that take place in the system are described by combining the Monod and Bulter–Volmer equations. The anode and the cathode are modeled as a CSTR (continuous stirred tank reactor). For each of them, the mass balance is formulated taking into account the chemical species present. The current density is calculated by relating it to the flow of M+ ions through the membrane. Along with the charge balance in the anode and cathode, and taking into consideration the respective overpotential, the voltage can be calculated to complete the electrical model. One of the main findings of this study is that the cathode reaction is the most important limiting factor of the system, according to a sensitivity analysis. This conclusion contrasts with that of most models, which claim that the anode reaction is the limiting factor of the system. The major drawback of this model in question is that the biofilm cannot be studied in depth. Furthermore,

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the model is applied to a system that uses artificial wastewater as substrate, in which more than one oxidation reaction takes place. However, in this case there is a higher discrepancy between simulated and experimental data. More models based on complex substrates are needed in order to study real scenarios for MFC applications (e.g. wastewater treatment). Oliveira et al. [103] created a one-dimensional model based on a double-chamber MFC that follows a similar approach to Zeng et al. [162], but introducing the study of heat transport. This approach assumes that heat transport is caused by diffusion processes (convection is negligible). The study of heat transfer includes heat balances for both the anode and cathode and takes into account the energy from electrochemical reactions, the activation energy, mass transfer overpotential and heat losses. In addition, the anode is modeled as a CSTR for the mass balance, which is related to the concentration gradient by Fick's law. Current density is calculated by applying Monod and Tafel's equations. Modeling of the cathode chamber is based on the oxygen reduction reaction. A major advantage of this model is that it allows the temperature profile across the cell to be calculated. Finally, along with those models intended to simulate the overall performance of a microbial fuel cell, there are cases in the literature devoted to studying a specific component or process. Such is the case of the work presented by Harnisch et al. [44], which models ion transport across an exchange membrane (Nafion®117), shedding more light on this mechanism and its limitations when used in a microbial fuel cell. The model is aimed at reproducing the exchange mechanism of ionic species across the separator and studying the pH and polarization around the membrane and its possible influence on the cell performance. The two-chamber configuration incorporates two more electrodes to monitor the pH value in both chambers and two Luggin capillaries connected to another two Ag/AgCl electrodes via a KCl solution, both located 1 mm from the membrane to measure membrane polarization. The model describes the ion transport across the membrane with the Nernst–Planck equation for each species, in terms of flux density. This equation has two terms, each of which describes the transport mechanisms involved: diffusion and migration. The polarization of the membrane is related to its resistance value, which contributes to hindering ion transport through the separator. The simulation of membrane polarization suggests that when very concentrated electrolytes are used in the anode and in the cathode, the pH difference around the membrane is high and the flow resistance through the membrane increases.

9. Conclusions and remarks This work presents an overview of MFC technology, including issues such as new materials for use in the main components of these devices (cathode, membrane and anode), types of configurations and MFC modeling. The results of recent studies suggest that MFCs will be of practical use in the future and will become a preferred option among sustainable bioenergy processes. Many scientific works have demonstrated that this technology offers potentially promising prospects for wastewater treatment and power generation but also for applications such as water desalination, metal recovery and dye removal. New low cost materials are being developed and applied in MFCs in order to improve their efficiency. In this respect, ceramic materials, nano-particles and non-platinum catalysts are examples of current research lines that aim to improve the efficiency of MFCs. For their practical implementation, MFCs need to be scaled-up by several orders of magnitude from the laboratory scale (10−6 to 10−3 m3) to a scale suitable for wastewater treatment (1 to 103 m3). Such implementation of microbial fuel cells also requires an increase in efficiency and the use of new low cost materials and, to date, pilot-scale attempts have proved to be unsatisfactory. Modeling and simulation of MFCs have also been seen to be useful tools for optimizing performance. An increase in the number of models is

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