Keywords

15.1 Introduction

Environment protection and energy crisis are two major concerns that need to be addressed for sustainable economic growth. Water contamination proves to be one of the most alarming human effects on the environment. Industry, urbanization, and agriculture often introduce several types of pollutants including heavy metals, agrochemicals, and drugs into water environments. Recently, bio-electrochemical systems (BES) have emerged as a preeminent technology for environmental management. In particular, Microbial Fuel Cells (MFCs) attract huge interest due to their capacity to produce bioenergy from multiple substrates, including complex sources such as urban and industrial wastewater and biomass wastes (Santoro et al. 2017).

MFC devices, usually formed by respective anode and cathode compartments, exploit microbial metabolism to oxidize organic and inorganic matters (see Fig. 15.1). Theoretically, electrons and protons are produced by inoculated or naturally occurring bacteria in a given substrate. Electrons are transferred to the anode electrode and externally led through a conductive material containing a resistor for electricity generation, while protons go through the membrane that separates anodic and cathodic compartments. In practice, other positive and negative ions are involved in the ion-exchange process through the membrane. At the cathode, oxygen is often used as oxidant because of its high reduction potential. This way oxygen, one of the main cathodic reactions consist of the reduction of oxygen into water by the combination with the electrons and protons coming from the oxidation process in the anode chamber (Hernandez-Fernandez et al. 2015).

Fig. 15.1
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Schematic representation of MFC

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MFC technology offers enormous potential for waste reuse. Moreover, MFC systems provide an electrochemical framework to perform and integrate several processes for pollution degradation, including persistent industrial contaminants such as xenobiotic compounds, processes designed to reduce carbon footprint by fixing carbon dioxide, separation processes (e.g., metal removal) and product valorization. The development of MFC technology can be approached by its up-scaling for large-scale application of wastewater effluents or by its miniaturization for portable or more-scale applications (Santoro et al. 2017). For instance, MFCs can be micro-scaled to be employed for powering portable electronic devices.

MFCs are complex systems involving bioelectrochemical reactions and physical-chemical processes. The technology has been intensively investigated through experimental works in the lasts two decades in order to enhance its efficiency in terms of energy generation and wastewater treatment capacity. The optimization and analysis of MFCs require the knowledge of different scientific and engineering backgrounds, ranging from electrochemistry and microbiology to material, environmental, and chemical engineering. Despite the numerous advantages offered by MFCs, this technology still shows some limitations that hinder its up-scaling and the spreading for practical implementation. In this sense, many efforts have been addressed to enhance MFC efficiency through the development of new materials and design and the study of biodegradability of waste effluents in the anodic chamber of these devices.

Recent advances on MFC technology include intensive research on new materials to be used in the main components of the systems (anodic/cathodic electrodes and separator) in order to overcome the bottlenecks of the systems, namely low power density levels from wastewater substrates. Among them, nanostructured materials have drawn high interest for the fabrication of advanced electrode and separators due to their enhanced properties such as high specific surface, increased transfer rates and in many cases low costs and ease of fabrication (Zhao et al. 2017). The use of nanotechnology aims at: (1) increasing electron transference between microbes and anode electrode surface, (2) improving ion exchange through the separator, and (3) enhancing oxygen reduction reaction at the cathode. The improvement of all these key factors leads to increased overall MFC performance, implying higher power generation and wastewater treatment capacity.

This chapter deals with the recent advances made on the application of nanotechnology in MFC systems as an effective strategy to improve the performance of the technology. Next sections address nanostructured materials for the three main components that form MFC systems, divided into anode electrode, separator, and cathode electrode materials.

15.2 Microbial Fuel Cells for Wastewater Treatment

Global warming and water scarcity have promoted the interest in reusing wastewater for clean water and energy production, as well as nutrients extraction in order to synthesize agricultural fertilizers. Conventional methods such as aerobic or anaerobic treatments still show some limitations. In the case of aerobic treatment, it has high energy requirements and produces large amounts of wastes. Moreover, this type of procedure is not able to benefit from the wide variety of compounds contained in wastewaters. Regarding anaerobic treatments, they are capable to producing methane from organic matter oxidation, which can be directly transformed into electricity. However, this technique is not able to benefit the whole energy potential of wastewater and the treated water still need a posttreatment stage in order to increase its quality and fit the legal requirements for reuse (He et al. 2017; Hernandez-Fernandez et al. 2015).

Microbial fuel cells have demonstrated to be a promising technology capable to address two of the most concerning environmental issues: (1) fossil fuel depletion and (2) water scarcity. MFCs allow us to produce bioenergy and treat wastewater simultaneously. This technology offers several advantages in comparison with conventional wastewater treatment (see Fig. 15.2) (Hernandez-Fernandez et al. 2015; Kim et al. 2008; Logan and Regan 2006).

Fig. 15.2
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Advantages of MFCs over conventional wastewater treatments

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Pure substrates such as glucose, fructose, or sodium acetate have been widely employed as fuels in MFCs. However, the potential of this technology lies on using complex substrates such as real wastewater. Table 15.1 contains some of the most common types of wastewater used as source of energy in MFCs and the performance of the system in terms of wastewater treatment capacity.

Table 15.1 Types of wastewater sources used in MFCs
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MFCs have demonstrated to be a suitable technology for the removal of different types of compounds such as nutrients, azo dyes, or even heavy metals. Nitrogen and phosphorous-based compounds can be efficiently removed in MFCs, especially in biocathode chambers. They can be recovered as ammonia or struvite (MgNH4PO4.6H20). Other potential application of MFC is the removal of metal ions such as Cr (VI), Cu (II), Zn (II), or Fe(III), among others. The high redox potential of some of these heavy metals makes possible that they can act an electron acceptors, allowing their reduction and precipitation. Other type of pollutants such as azo dyes have also been successfully removal from wastewater by using MFCs (Gude 2016; Hernandez-Fernandez et al. 2015; Pandey et al. 2016).

Although MFC technology offers several advantages over the conventional wastewater treatment, they still need further improvement in order to facilitate their large-scale application. Nanotechnology is one of the most promising fields for the design of advanced and high efficiency materials. Their application in different research fields including MFCs has proven to be a success. In the case of MFCs, nanotechnology has been employed to improve their performance in term of power output and wastewater treatment capacity. Nanomaterials have been used to increase the activity of the catalyst for the oxygen reduction reaction, to facilitate the growth of biofilm around the anode electrode or even to improve the selectivity of the separator.

15.3 Nanomaterials for MFC Anode Electrodes

Anode performance is a key factor in MFC systems that can limit the power level achieved. Generally, the anode reaction is greatly influenced by the surface properties of the anode material employed. Thus, the improvement of anode electrode through new materials and design is critical to enhance the efficiency of this technology for practical implementation. One of the aims in using nanomaterials for anode fabrication is to enhance the electron transfer mechanisms between microorganism, which act as biocatalysts in the anode chamber, and the material forming the anode electrode itself, to increase current generation. Nanostructured materials can be used for the modification of the surface of a given electrode made of other material types or as base electrode material. Commonly used anode nanomaterials include carbon and metal compounds with high conductivity, high specific surface areas, chemical stability, and biocompatibility. This last property is fundamental since they must ensure the affinity connection between the microorganisms and the electrode for efficient electron transference. Carbonaceous materials are inexpensive and are among the most widely used materials for MFC anode fabrication. Carbon cloth, carbon fiber, carbon felt, or carbon brush is commonly used as base anode electrodes. These materials, in comparison with nanostructured electrodes, offer limited surface area (Erbay et al. 2015).

Carbon nanotubes (CNTs) have been greatly studied in MFC systems in the last years. Many works have shown that CNT can greatly improve anode performance due to their high conductivity and high specific surface area, by increasing the transference rate of electrons produced by bacterial metabolism. CNTs are usually employed as contribute materials for anode modification, both as raw CNTs and doped with other chemical elements. In the last case, doped CNTs can ever offer higher power densities in comparison with non-doped nanotubes. For instance, multiwall (MW) carbon nanotubes have been employed for the modification of electrodes made of carbon cloth and carbon paper, increasing the performance of the systems. The methods for the modification of carbon anodes with CNTs can be simple, e.g., submersion of the electrode material in a solution containing CNTs. For example, Sun et al. (2010a, b) synthesized carbon nanotubes (in the presence of polyelectrolyte polyethyleneimine, PEI) for the construction of carbon paper electrodes. When comparing the bare anode based on carbon paper and the anodes including the CNTs, the modified anodes are capable of producing up to 20% more of power output. This increase in power output have also been observed when using CNTs for the modification of carbon cloth electrodes in comparison with bare carbon cloth electrodes, both in terms of power density and coulombic efficiency (Tsai et al. 2009).

CNTs-textile composites have been studied as promising electrode material in MFCs (Xie et al. 2011). Carbon textile fibers modified with CNTs provide high performance because of the resulting porous structure, with a 3-D network formed that enhances substrate transference and efficient biofilm development due to high surface area availability. Specifically, Xie et al. (2011) observed an increase of 68% in power density as compared to conventional carbon cloth anodes. The higher performance of CNT-textile anodes also leads to significantly higher current densities (158%) and energy recovery rates. Nevertheless, when employing CNTs as anode materials, the biocompatibility of these materials with microorganism present in the anodic chamber need to be comprised, since some works have reported that CTNs can pose several toxicity issues, e.g., limiting cellular proliferation (Yu et al. 2018).

As aforementioned, the doping of CNTs with chemical elements can improve the performance provided by non-doped carbon nanotubes. Research works on nitrogen-doped CNTs (Ci et al. 2012) have shown that the addition of nitrogen greatly contributes to enhancing the efficiency of the anode electrode by creating large number of actives sites and improving biocompatibility with bacteria populations. As study case, a maximum power density of 1.04 W·m−2 was reported by using N-doped CNTs in dual-chamber MFCs by Ci et al. (2012), while the power densities for non-doped CTNs and bare carbon cloth anodes in the same systems reached only 0.71 and 0.47 W·m−2, respectively.

Conducting polymers can also be combined with carbon nanomaterials to enhance electron conductive and stability. These types of polymers include polyaniline, polyethylenimine, and polypyrrole, among others. Polyaniline or PANI has been specially studied for the modification of anode electrodes in MFC systems in combination with CNTs (PANI-CNT composites). In such case, the positive charges of PANI can interact with the negative charges of the bacterial membrane increasing the attachment of the biofilm formed to the anode electrode and thus the electron transfer rate. For example, Qiao et al. (2007) developed PANI/CNTs composites for anode modification, obtaining higher efficiencies in MFC systems versus devices equipped with plain PANI anodes. The combination of polyaniline and carbon nanotubes provides larger specific surface areas. Similarly, and more recently, polypyrrole-CNTs composites were employed in carbon felt anodes (Roh and Woo 2015), offering 38% more power (287 mW·m−2) versus non-modified carbon felt anodes. In this case, the polypyrrole-CNTs composites were further treated with acid (mixture of nitric and sulfuric acid) and organic germanium (Ge-132).

As seen, the nanomaterials commented until now have been tested on several support materials such as carbon paper, felt, or textile fibers. According to Erbay et al. (2015), the orders of magnitude of the surface areas in these types of composites are generally lower than 1 m2·g−1. In contrast, 3-D nanostructured electrodes offers high surface areas and boost microbial electron transfer. In this regard, 3-D porous CNT-based sponges have been recently developed for the direct construction of MFC anodes (Erbay et al. 2015). These materials, which are formed by the interconnection of carbon nanotubes, are capable of producing power densities of up to 2150 W·m−3 (anode volume). Moreover, the method reported in the bibliography for the fabrication of the said nanostructured sponge-type anodes is relatively simple and offer low costs.

In the last years, graphene has attracted growing attention as advanced anode material in MFC devices. In addition to the outstanding properties of graphene in terms of electrical conductivity and chemical stability, the use of graphene prevents possible toxicity issues posed by other materials such as carbon nanotubes. Graphene can be synthesized in different arrangement types (graphene sheets, graphene aerogel, composites, etc.). Among the first attempts to apply graphene as electrocatalyst in the anode electrode is the work by Zhang et al. (2011), in which a stainless steel mesh modified with graphene is analyzed as anode material. The results show that MFCs working with graphene-based anodes are capable of producing power densities of 2668 mW·m−2. This level of power output is significantly higher than the power density obtained with anode electrodes lacking graphene (18-fold higher). Other relevant works reported in the bibliography include the study of graphene aerogel doped with nitrogen (Yang et al. 2016a, b). This type of material presents a (hierarchical) porous structure facilitating the bacterial cell diffusion and electron transfers. This material provides significant power densities in milliliter-scaled MFCs, with maximum values of 750 W·m−3 (referred to anode volume). More recently, 3-D porous graphene aerogels prepared by hydrothermal reduction methods has been reported as enhanced electrodes with high specific capacitance values (>3600 F·m−2) and improved stability in terms of electricity generation. The maximum power density obtained with this anode type is in the order of 2400 mW·m−3 (anode volume).

Graphene has also been combined with other metallic materials, for instance, nickel foam. Wang et al. (2013) developed 3-D graphene oxide-nickel nanomaterials consisting of the deposition of graphene oxide onto nickel foam. In this case, the material structure can be analyzed from both macro and micro perspectives. While nanostructure provides large surface area promoting bacterial growth and electron transference, this type of anode material also offers a microporous structure that boosts mass diffusion. This anode material has shown to be effective for MFC systems, offering power densities of 660 W·m−3 (anode volume material). This power density is higher in comparison with that obtained with plan nickel foam electrodes and other common carbon electrodes such as carbon cloth .

Graphene oxide has also been recently studied for the degradation of dyes in the anode chamber of MFCs. Dyes are xenobiotic compounds frequently present in wastewater effluents that are regarded as persistent and toxic pollutants. Enhanced anode materials based on graphene oxide have shown to be effective at removing this type of contaminants with simultaneous power generation (Khalid et al. 2018). Other material options include graphene-based sponges. As in the case of CNTs, MFC anodes based on graphene sponges were developed and studied by Xie et al. (2012). Apart from the graphene sponges, the anodes comprised stainless steel as material for current collectors. This type of anode material can be considered low cost, offering great advantages such as enhanced conductivity .

Another important group of materials for anode construction are metal nanostructured materials . For example, several works have studied the influence of gold nanoparticles as an effective way to increase MFC performance (Alatraktchi et al. 2014; Guo et al. 2012; Sun et al. 2010a). Au nanoparticles are used to decorate the anode electrode, showing that higher current outputs can be obtained when employed along with other material substrate (e.g., carbon paper). Alatraktchi et al. (2014) deposited Au nanoparticles (with particle size from 60 to 100 nm), obtaining almost twofold power density comparing with carbon papers lacking Au. Although gold nanoparticles could enhance MFC performance, the high price of this noble metal make it unfeasible the spreading of its use for practical implementations, and thus alternative materials based on transition metals could be preferable to noble materials .

Other innovative and noble nanostructure materials employed in MFC anodes are bio-palladium nanoparticles reported by Quan et al. (2015). These Pd nanoparticles are synthesized by microorganisms (Shewanella oneidensis) from a solution of Palladium(II), with the advantage of less amounts of chemicals needed for the nanoparticle synthesis compared with conventional methods and improved biocompatibility materials. When decorating carbon cloth electrodes (support material) with bio-Pd nanoparticles, the obtained power output maximum power and coulombic efficiency are, respectively, improved by 14 and 31% in comparison with bare carbon cloth anodes.

Nano spinel materials such as Ni-ferrite (NiFe2O4) are an example of low cost nanostructure materials that can be employed for MFC anode decorating (Peng et al. 2017). This non-noble material acts as promotor of the transference between the bacteria and the anode electrode. By the addition of a low amount of Ni-ferrite (5%), an increase in power density of almost 30% higher is obtained in comparison with non-decorated carbon electrodes.

Other transition metal-type nanomaterials that have drawn interest for anode fabrication in MFC are TiO2 nanotubes (Feng et al. 2016; Wen et al. 2013). TiO2 is a well-known photocatalyst employed in electrochemical energy systems. Although this compound can pose some biocompatibility issues, several authors have developed methods to successfully apply these TiO2 nanotubes in MFC anodes (Feng et al. 2016). With this type of nanostructure material, high current densities can be obtained in the order of 12 A·m−2, which is up to 190-fold higher values in comparison with non-nanostructured titanium electrode and also significantly higher than in the case of using bare carbon felt-type anodes. The TiO2 nanotubes also showed to provide resistance against corrosion and high conductivity.

15.4 MFC Membrane Composites Including Nanomaterials

Proton exchange membranes (PEM) play an important role in MFC performance. They have to selectively transport protons from the anode to the cathode and simultaneously avoid both the transfer of substrate from the anode to the cathode and oxygen crossover from the cathode to the anode. Commercial membranes such as Ultrex or Nafion have been widely used as separator in MFCs, Nafion being the most common. However, these materials show some limitations such as their high cost and in the case of Nafion, both oxygen and substrate transfer, cation transport and biofouling among others. Due to these drawbacks, in recent years big efforts have been made in order to synthesize alternative materials to overcome the limitation of the commercial membranes. Polymeric/inorganic nanoparticle-based membranes have gained much attention in recent years due to their promising applications in biomedicine, environmental science, and energy production.

The presence of nanoparticles in membranes generates preferential permeation paths, which improve the separation process. Moreover, they avoid the permeation of undesired species in addition to increasing thermal and mechanical stability. Therefore, the presence of nanoparticles in the membrane structure modifies the chemical and physical properties of the separator (Jadav and Singh 2009; Mahreni et al. 2009; Pan et al. 2010).

15.4.1 PES-Based Membranes

Poly(ether)sulfone is one of the polymers reported in literature, which has been used as supporting material for nanoparticle-type membranes. In 2012, Rahimnejad et al. (2012) synthesized a novel type of nanocomposite membranes based on Fe3O4 nanoparticles. Their work focuses on the effect of different amounts of nanoparticle/polymer on the performance of a double chamber MFC using Saccharomyces cerevisiae as anodic biocatalyst. The system was fed with glucose and neutral red as mediator. Membranes with 10, 15, and 20% of Fe3O4 nanoparticles were synthesized by casting method. The iron-based nanoparticles were synthetized by using FeCl2, FeCl3, and sodium hydroxide solutions as precursors. Atomic force microscopy (AFM) images show that the higher the amount of Fe3O4 nanoparticles, the larger the porous size. However, for amounts of Fe3O4 nanoparticles higher than 15%, the porous size is not uniform and the roughness of the surface is very high due to the appearance of some aggregations. Membranes containing 15% of Fe3O4 nanoparticles allow MFC to increase the power output by 29% over the value obtained by using a Nafion 117-based membrane (15.4 mW·m−2). MFCs working with membranes prepared with the highest content of nanoparticles (20%) show lower value of power output than those with membranes containing only 10% of nanoparticles. The results demonstrate that the unique properties of ferric nanoparticles such as their conductivity, magnetism, or high catalytic activity, clearly favor the performance of MFCs.

More recently, melt extrusion technique was employed for synthesizing Fe3O4 nanoparticles-type membranes using PES as polymeric matrix. Di Palma et al. (2018) prepared Fe3O4 nanoparticles by co-precipitating Fe+2 and Fe+3 using NH4OH as precipitating agent. They used a microextruder in order to obtain nanocomposite sheets of PES and two different amounts of magnetite nanoparticles, 5 and 20%. The membranes were tested as separators in a double chamber MFC fed with a synthetic solution containing sodium acetate as carbon source. The results reported in this work are in line with those obtained by Rahimnejad et al. (2012). The roughness of the nanocomposite membranes increases as the amount of nanoparticles also increases, being maximum for PES-based membranes containing 20% of magnetite nanoparticles (1.215 μm). Regarding the MFC performance, nanocomposite membranes containing 20% of Fe3O4 allow MFCs to reach a maximum power output of 9.59 mW·m−2, which is similar to that obtained by using the commercial membrane CMI-7000 (12.58 mW·m−2). The results show that the higher the presence of magnetite nanoparticles in the membranes, the higher the MFC performance in terms of power output. Regarding the wastewater treatment capacity and coulombic efficiency, MFCs working with this type of membranes exhibit a total organic carbon (TOC) removal of 75% and a coulombic efficiency of 11.36%. Among the material synthesized, nanocomposite membranes containing 20% of magnetite seem to exhibit a balance between electrochemical and chemical performance, being suitable for both bioenergy production and wastewater treatment .

15.4.2 Sulfonated Poly Ether Ether Ketone-Based Membranes

Other polymer commonly used to prepare nanocomposite membranes is sulfonated poly ether ether ketone (SPEEK) . Prabhu and Sangeetha (2014) reported the preparation of Fe2O4 nanoparticles-type membranes using SPEEK as polymeric matrix. The sulfonated polymer was obtained by mixing sulfuric acid and PEEK, and the polymer membranes were prepared by casting method dissolving an appropriate amount of SPEEK into N-methyl pyrrolidone. On the other hand, Fe2O4 nanoparticles were synthetized from FeCl3 and FeCl2, as previously commented, and then added to the casting solution. The effect of the amount of magnetite nanoparticles on the membrane properties and the performance of MFCs was investigated. Membranes with a content of 2.5, 5, 7.5, and 10% of iron nanoparticles were used as separator in a single chamber MFCs. E. coli (DH5-α) was used as active bacteria in the anode and glucose as carbon source whereas carbon cloth loaded with platinum as catalytic cathode for the oxygen reduction reaction. The results show that the roughness of the membranes increases as the amount of magnetite nanoparticles also increase. On the other hand, the higher the amount of iron nanoparticles, the lower the oxygen transfer rate for amounts of nanoparticles fixed between 2.5 and 7.5%. However, for amounts of nanoparticles beyond 7.5%, the oxygen transfer increases due to the space in the internal structure. These results are in line with those previously described for PES-based nanocomposite membranes. Regarding the MFC performance, SPEEK-based membranes containing 7.5% of iron nanoparticles allow MFCs to reach a maximum power output of 104 mW·m−2, which is higher than SPEEK-based membranes without nanoparticles and also higher than Nafion 117 membranes. A similar trend is observed for the columbic efficiency, being maximum for membranes containing 7.5% of iron nanoparticles (78%). The roughness of the 7.5% based membranes is enough to allow a thin layer of biofilm to develop over the surface of the membrane avoiding the oxygen transfer from the cathode to the anode and increasing the MFC performance. However, despite the high roughness of the 10% membrane, the space in the internal membrane structure negatively affects the MFC performance. Regarding the ion transport, iron nanocomposite membranes exhibit a lower selectivity to cations such as sodium, calcium, potassium, magnesium, and ammonium than Nafion-based membranes, which favor the proton selectivity. These results show that SPEEK/Fe3O4 nanocomposite membranes are a suitable alternative to commercial membranes as separator in MFCs.

Besides magnetite nanoparticles, membranes based on SPEEK have been doped with montmorillonite nanoparticles (MMT). Hasani-Sadrabadi et al. (2014) studied the effect of both the sulfonation degree of SPEEK and the amount of montmorillonite nanoparticles on the performance of single chamber MFCs. Although the maximum ion exchange capacity belongs to a sulfonation degree of 89%, this sample was water soluble. Thus, it was determined that 82% of sulfonation degree is the maximum value which allows the sample to be mechanically and hydrolytically stable. Regarding the liquid uptake and the proton conductivity, both parameters increase as the sulfonated degree also increases. However, the results show that the higher the amount of sulfonate groups, the higher the oxygen transfer rate because the sulfonate groups might affect the crystallinity of the SPEEK structure. The oxygen molecules can diffuse across the ionomeric nanochannels formed, which increase as the sulfonated group also increases. Nevertheless, the oxygen permeability showed by the sample with different sulfonated degrees is lower than that exhibited by Nafion 117. The author considers the selectivity of the membranes as the ratio of proton conductivity to oxygen permeability. According to the sulfonated degree, the selectivity of the membrane increases as the number of sulfonate group also increases, being maximum for a sulfonated degree of 70%. Therefore, this was the polymeric matrix selected in order to prepare the nanocomposite membranes based on montmorillonite. In terms of selectivity, membranes with 3% of nanoclay exhibited the highest value. Therefore, SPEEK-based membranes with 70% of sulfonated degree and 3% of nanoclay were tested as separator in single chamber MFCs. In this case, E. coli Top10F was used as bacteria in the anodic chamber fed with glucose, electrodes were made of carbon cloth, and the cathode catalyst consisted of a blend of platinum and carbon black. The results show that the nanocomposite membrane with 70% of SPEEK and 3% of nanoclay allows MFCs to reach about 40% more power than the commercial membrane Nafion 117. The high power output displayed in MFCs, the low cost materials, the simple synthesis process, as well as the low oxygen transfer make SPEEK-based nanoclay membranes suitable alternatives to commercial MFC separators.

Sulfonated silica has also been used as nanomaterial in order to improve the properties of SPEEK-based membranes . Sivasankaran and Sangeetha (2015a) studied the effect of the addition of SiO2 and sulfonated SiO2 nanoparticles to SPEEK-type membranes on their proton conductivity as well as their application as separator in MFCs. They added different amounts of sulfonated silica (2.5, 5, 7.5, and 10%) into the SPEEK matrix and the results were compared to those obtained with SPEEK/SiO2-type membranes as well as with Nafion 115. The membranes synthetized were characterized before being used as separator in a single chamber air cathode MFC fed with domestic wastewater in batch mode. Nyquist plots from electrochemical impedance spectroscopy (EIS) showed that the resistance of the membranes decreases as the amount of sulfonated silica also increases, being minimum for membranes containing 7.5%. This might be due to the low conductivity of SiO2 particles, whose presence increases the resistance of the membrane and reduces the conductivity. However, the addition of a more conductive compound such as SiO2-SO3H reduces the resistance of the membrane, and consequently increases its conductivity. All membranes containing sulfonated silica exhibit higher conductivity than both SPEEK-SiO2 and SPEEK membranes, being maximum for membranes containing 7.5% of SiO2-SO3H (1.018 S·cm−1). Amounts of sulfonated silica beyond 7.5% increases the resistances of the membrane due to the agglomeration of the sulfonated silica over the SPEEK matrix, reducing the conductivity.

Regarding the use of these membranes as MFC separator, membranes prepared with 7.5% of SiO2-SO3H allow MFC to reach the maximum power output (1008 mW·m−2), even higher than Nafion 115 (320 mW·m−2). The columbic efficiency is about 90%, also higher than the rest of materials tested. This work reports that the addition of SiO2-SO3H to SPEEK-based membranes increases the conductivity, and therefore improves their performance as MFC separator. In comparison to commercial membranes such as Nafion 115, membranes containing 7.5% of SiO2-SO3H exhibit similar conductivity and water uptake; however, the oxygen mass transport is an order of magnitude lower for sulfonated silica SPEEK membranes, which enhances the power performance of MFCs.

15.4.3 Polyvinyl Derived-Based Membranes

Polyvinylidene fluoride (PVDF) has commonly been used as polymeric matrix for different kinds of membranes due to its thermal and chemical stability as well as its biocompatibility. This material has shown promising results in different research fields such as separation processes, bioenergy production in direct methanol fuel cell or for biological applications. Considering all these advantages, in 2014 Shahgaldi et al. (2014) employed electrospinning method in order to produce PVDF nanofibers. The aim of their work is to synthesize PDVF nanofiber/Nafion-based membranes to be used as separator in double chamber MFCs, which use baker’s yeast as biocatalyst in the anode and glucose as fuel. Moreover, they studied the effect of different combination of PDVF nanofibers and Nafion (10 wt%/0.2 g, 18 wt%/0.4 g and 29 wt%/0.6 g) on MFC performance. The results showed that the membranes containing 0.4 g of Nafion allow MFCs to reach the maximum power output (4.9 mW·m−2) while MFCs working with membranes prepared with 0.6 g of Nafion exhibit a similar power performance to commercial Nafion 117. This can be explained by the higher proton conductivity of membranes containing 0.4 g of Nafion. Regarding the columbic efficiency, this membrane also exhibits the highest value (12.1%). On the other hand, membranes prepared with a proportion of PVDF nanofiber/Nafion of 29 wt%/0.6 g offer both higher power output and higher columbic efficiency than membranes containing 10 wt%/0.2 g of PVDF nanofiber/Nafion. These results allow them to conclude that the amount of PVDF nanofiber is a key factor in Nafion composite membranes. In terms of wastewater treatment capacity, all system studied allow to remove more than 70% of chemical oxygen demand (COD), being all of them suitable for treating wastewater .

More recently, polyvinyl alcohol (PVA) and sulfonated styrene (SS) were crosslinked in order to be used as polymeric matrix for nanocomposite membranes . In 2017, Rudra et al. (2017) incorporated different amounts of graphite oxide (0.2 wt%, 0.4 wt% and 0.6 wt%) into a polymer matrix based on PVA and SS for synthesizing nanocomposite materials to be used as separator in single chamber MFCs. The system worked with Lysinibacillus species as active bacteria in the anode fed with synthetic wastewater. In this work, the hydrophilicity of PVA is reduced by its crosslinking with SS as well as the addition of GO. Moreover, the presence of GO reduces the oxygen transfer through the membranes. Regarding the resistance, the Nyquist plots showed that among all the materials synthesized, nanocomposite membranes containing 0.4 wt% of GO exhibit the minimum resistance (5.4 Ω), and therefore the highest conductivity. In terms of power output, MFCs working with this type of nanocomposite membrane are capable of reaching a maximum power of 194 mW·m−2, which is higher than the values achieved by the rest of materials tested. In terms of wastewater treatment capacity, all materials synthesized allow MFCs to reach COD removal rates higher than 70% after 25 day working , being maximum for those containing 0.2% of GO (88.97%). These results show the potential application of this low cost nanocomposite material as separator in MFCs for simultaneously producing bioenergy and treating wastewater .

15.4.4 Sulfonated Polystyrene-Ethylene-Butylene-Polystyrene-Based Membranes

Sulfonated polystyrene-ethylene-butylene-polystyrene (SPSEBS) has also been employed for synthesizing nanocomposite-type membranes for MFCs. Sivasankaran and Sangeetha (2015b) added nanoparticles of sulfonated TiO2 ranged between 2.5 and 10% to a SPSEBS matrix. The materials elaborated were tested in a single chamber air cathode MFCs working with wastewater as bacteria inoculum and glucose as carbon source. The ionic conductivity of the samples increases as the content of sulfonated TiO2 also increases, being maximum for membranes prepared with 7.5% of titanium nanoparticles (3.35 meq·g−1). These results might be due to the increase of the acid sites caused by the presence of sulfonated TiO2. The same trend is observed in terms of proton conductivity, being maximum for nanocomposite membranes prepared with 7.5% of sulfonated TiO2 (3.57 × 10−2 S·cm−1). However, the oxygen mass transfer decreases as the content in titanium-based nanoparticles increases, which benefits their use in MFCs. Regarding their application as MFC separator, the maximum power output of 1345 mW·m−2 was achieved by using nanocomposite membranes containing 7.5% of sulfonated TiO2, higher than using the rest of membranes prepared and four times higher than using Nafion 117. In respect of columbic efficiency, MFCs working with 7.5% sulfonated TiO2-type membranes achieve a value of 87%, higher than using the rest of materials.

Then, the same polymeric matrix was dopped with a range of sulfonated silica nanoparticles from 2.5 wt% up to 10 wt% by the same author (Sivasankaran and Sangeetha 2015b). They tested the materials synthesized as separator in single chamber air cathode MFCs inoculated with wastewater and fed with glucose as main carbon source. The work reports that the higher the amount of sulfonated silica, the higher the ion exchange capacity of the membranes. The maximum value is observed for membranes containing 7.5% of sulfonated silica (3.015 meq·g−1). For higher amounts of silica nanoparticles, this parameter decreases due to the agglomeration of the nanoparticles. Regarding the internal resistance, it shows the opposite trend, decreasing as the amount of sulfonated silica increases in the membrane structure. The minimum resistance is exhibited by the membranes containing 7.5% of sulfonated silica nanoparticle (37 Ω), and therefore the maximum conductivity (0.321 S·cm−1). In respect of their application as MFC separator, SSEBS-based membranes containing 7.5% of sulfonated silica allow MFC to reach a maximum power output of 1209 mW·m−2 and a columbic efficiency of 85%, both values higher than using the rest of membranes. Moreover, the authors report that the selected membranes offer results much better than other composite membranes achieving power densities more than 10 times higher .

15.4.5 Nafion-Based Membranes

Nafion solution has also been tested as supporting matrix for nanocomposite membranes. Ghasemi et al. (2012) compared the use of commercial membranes Nafion 112 and Nafion 117 with both carbon nanofiber/Nafion and activated carbon nanofiber/Nafion as separator in MFCs. They use Nafion solution in order to avoid the agglomeration of carbon nanofiber (CNF) and the activated carbon nanofiber (ACNF). The new membranes were tested in a double chamber MFC fed with glucose as carbon source. The roughness of the membrane decreases as the pore size decreases. For both parameters, the membranes studied show the following trend: Nafion 112 > Nafion 117 > CNF/Nafion > ACNF/Nafion. Small pore sizes reduce the oxygen transfer through the membrane, which benefits the MFC performance. Regarding the porosity, this factor increases as the pore size decreases, being maximum for ACNF/Nafion (47.6%). In terms of power generation, both materials synthesized allow MFCs to reach higher values of power output than commercial membranes Nafion 112 and Nafion 117. Among the carbon-based nanocomposite membranes, ACNF/Nafion are able to produce a maximum power output of 57.64 mW·m−2 when they are used as separator in MFCs. Concerning the wastewater treatment capacity, all membranes synthesized allow to reach a maximum COD removal higher than 70%, being maximum for ACNF/Nafion membranes. These results confirm that both types of carbon fiber-based nanocomposite membranes are suitable alternative to commercial membranes for bioenergy production and wastewater treatment in MFCs.

A few years later, Bazrgar Bajestani and Abbas Mousavi (2016) also employed Nafion solution in order to elaborate TiO2-based nanocomposite membranes. The authors studied the effect of different solvents such as dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP) on the behavior of the membranes synthesized as MFC separator. In this case, the MFC assembly selected was a double chamber system fed with sludge containing DMF and molasses. Regarding the characterization, TiO2 nanoparticles-based membranes containing DMF show the highest conductivity (12.6 mS·cm−1), up to threefold higher than commercial membrane Nafion 112. This membrane also exhibits the highest ion exchange capacity (1.32 meq·g−1). In terms of MFC performance, TiO2 nanocomposite membranes prepared with DMF as casting solvent allow these devices to reach an open circuit voltage of 330 mV, higher than the rest of materials tested. These results show that the casting solvent play an important role on the performance of TiO2/Nafion nanocomposite membranes as separator in MFCs .

15.5 Nanomaterials for MFC Cathode Electrodes

At the cathode, oxygen reduction reaction (ORR) can generally follow two different pathways, 4-electron and 2-electron (peroxide) pathways (Mustakeem 2015). The 4-electron pathway implies that oxygen is reduced directly to water and is preferred over the 2-electron pathway, which includes the production of hydrogen peroxide, since in 4-electron pathway, double quantity of electrons is transferred. Generally, the 4-electron pathway appears to be predominant on noble metal catalysts while the peroxide pathway is more common on carbon-based electrodes. In any case, specific reaction mechanisms depend on pH conditions as seen in Table 15.2 (Gajda et al. 2015; Kinoshita 1988; Sawant et al. 2016).

Table 15.2 Reaction pathways for the ORR
Full size table

As indicated in Table 15.1, the acidic 2-electron pathway involves peroxide as intermediate. In acidic conditions, for both 4- and 2- electron pathways, H2O is a final product (Santoro et al. 2017). In neutral/basic conditions, the produced OH− can accumulate at the sites of the catalysts lowering kinetic performance (Sawant et al. 2016). As indicated by Sawant et al. (2016), a combination of two- and four-electron pathways can even occur in specific types of doped carbon materials employed as catalysts (e.g., N-doped carbon) (Yue et al. 2015).

A wide range of nanostructured materials have been investigated for MFC cathodes, including nanomaterial composites based on metal and carbon compounds. As in the case of anode fabrication, carbon nanotubes (CNTs) have been widely studied as ORR catalyst in the presence and in the absence of additional materials. CNTs have also been investigated in the presence of platinum in order to minimize the amount required of this noble catalysts. In this case, CNTs act as cathode support (Ghasemi et al. 2013), showing that PT/CNTs can increase power output up to 32% in comparison with Pt-coated cathodes in the absence of CNTs, while the amount of platinum can be lowered by 25% respect to the conventional amount required (0.5 mg/cm2 cathode area) to maintain the cathode efficiency. In other works (Sanchez et al. 2010), the use of CNTs is aimed to increasing the efficiency of platinum-based cathodes by increasing the total surface area and the ratio of surface/volume.

Noble nanomaterials have been employed in cathode MFC systems. An et al. (2011) analyzed the effect of including silver nanoparticles (AgNP) to coat graphite cathodes and compared their performance with graphite cathodes covered with platinum. The comparison of the performance of respective AgNP and Pt-coated cathodes showed that the cathode based on silver nanoparticles could offer the highest currents (from to 0.04 to 0.12 mA). The different materials were assessed in membraneless systems, and in the case of the use of silver nanoparticles, the materials also posed several limitations related to biofilm growth (biofilm inhibition). This last issue could be overcome by the use of MFC separators systems. More recently, gold-palladium (Au-Pd) nanoparticles with core-shell have been investigated. The nanoparticles consisted of Au cores and thin Pd shells (Yang et al. 2016a, b). This type of cathode can achieve power outputs of generates a. 16 W m−3 (volume anode), which in turns showed to be more than double the power obtained with hollow structured-based platinum cathodes (7.1 W m−3). Materials with core-shell structures can provide high catalytic properties because of the presence of lattice strain between the shell and the core domains.

Despite the good performance of precious catalysts for the ORR , they pose limitations related to high costs and low abundance in nature (Bullock 2017). Due to their lower price, transition metal-based catalysts are of high interest for their application in bioelectrochemical systems (BES) and therefore also in microbial fuel cells. Cobalt-based catalysts have shown promising results as cathode materials to increase the rate of the oxygen reduction reaction. Cheng et al. (2006) tested cobalt tetramethylphenylporphyrin (CoTMPP) in air-cathode single-chamber MFCs obtaining comparable performance to platinum (12% lower in the case of the macro-cyclic compound CoTMPP). Other macrocyclic complexes of cobalt and iron have also proposed to catalyze the ORR. For example, the addition of cobalt oxide into iron phthalocyanine was tested by Ahmed et al. (2012), showing that these material can offer significant power outputs in MFCs (654 ± 32 mW·m−2).

Yuan et al. (2011) synthesized iron phthalocyanine (FePc) supported onto amino-functionalized multiwalled CNTs for the construction of MFCs cathodes, which showed high efficiency for the ORR. These materials even enhanced the performance of Pt-based cathodes, offering a power output of up to 601 mW·m−2.

Manganese oxide (MnO2) has drawn much attention in the last years as low cost and promising catalysts in MFC systems. In terms of nanostructured materials, several works have studied MnO2 nanoparticles prepared by relatively simple methods offering good results. For example, hybrid manganese oxide nanostructures have been reported by Haoran et al. (2014) synthesized by a hydrothermal route. This type of nanomaterial offers significant efficiency towards ORR catalysis, allowing the oxygen reduction reaction to be performed by the four-electron pathway in basic solution. Other interesting works have combined nanostructured MnO2 and graphene as cathode material. Gnana kumar et al. (2014) investigated nanotubular MnO2/GO achieving higher power densities in comparison with cathodes only based on higher neat manganese oxides and nanotubes and nano-rods, respectively .

Other transition metal-based nanostructured oxide catalysts displaying spinel structure have been successfully studied in MFC systems (Hu et al. 2015; Mahmoud et al. 2011; Ortiz-Martínez et al. 2016). Spinel oxides are compounds that have the general formula AB2O4, in which A and B are metal cations. Depending on the composition, these compounds can display normal or inverse, tetrahedral and cubic structures. The cations are placed in octahedral and tetrahedral sites, with the possibility of one metal cation type displaying multiple valences in the same structure. In terms of catalytic activity towards the ORR, cubic phase spinel oxides offer higher efficiencies in comparison with tetragonal-structured oxides due to a stronger surface binding capacity with oxygen. Cobalt-based spinel oxides haven drawn much attention due to their low cost and the presence of multiple valences of Co cation in the spinel structure. The addition of manganese into cobalt oxide spinels can even offer higher efficiencies. Several spinel compositions with formula MnxCu1-xCo2O4 supported onto carbon were studied by Hu et al. (2015). These compounds were prepared by a relatively simple hydrothermal method. Some of the phases investigated offered performances comparable to Pt (supported onto carbon) as MFC catalysts. Other authors (Ortiz-Martínez et al. 2016) analyzed several cobalt spinel oxides doped with Cu and Ni, synthesized by thermal decomposition at different metal atomic ratios. The synthesized compounds were assessed both in power output densities and removal of COD from real wastewater (industrial effluents) in air-cathode single-chamber systems. Among the phases studied, the Cu-Co cobalt oxide with chemical formula (Cu0.30Co0.70)Co2O4 achieved a significant power density of approximately 570 mW·m−2 (87% out of the power generated by platinum). In terms of wastewater treatment, spinel MFCs working with spinel oxide-coated cathodes achieved COD removal values around 56% after 240 h of operation.

15.6 Conclusions

Traditional wastewater treatment technologies still show some limitations related to the energy requirements and the yield of the process. Microbial fuel cells (MFCs) have demonstrated to overcome these drawbacks, becoming a potential alternative to conventional wastewater treatment methods. MFC is a sustainable technology, which mitigates two of the most urgent challenges for human being: global warming and water scarcity. However, despite MFCs are able to simultaneously treat wastewater and produce bioenergy, there are some limitations which difficult their scaled-up application. In recent years, nanotechnology has gained attention due to their benefits in different research fields such as membrane technology for energy or separation processes as well as environmentally friendly materials for biomedicine, among others. This chapter addresses the application of nanotechnology in order to improve the wastewater treatment capacity of MFCs. Despite further works are needed for facilitating the MFC commercialization, it has been demonstrated that the use of nanomaterials in anode, cathode, or membrane allows MFC performance to be improved.