Keywords

1 Introduction

Organic wastes and wastewater treatment are necessary for environmental protection that requires high energy [41]. Also, rapidly depleting fossil fuel resources along with a recent increase in their consumption are serious global issues [49]. These problems have urged scientists globally to find renewable energy resources and economical waste treatment technologies [64]. In this regard, the use of microorganisms to produce electricity with subsequent waste treatment has emerged as a potential technology termed microbial fuel cell (MFC) [58]. In MFC, the microorganisms act as biocatalysts and help in degrading the wastes which are ultimately converted into biocurrent/bioelectricity. This eco-friendly electrochemical device provides dual benefits, i.e., waste treatment and electricity generation [40]. MFCs have been employed for various kinds of municipal as well as industrial wastewaters for simultaneous electricity generation and wastewater treatment [16, 38]. Other applications include biosensors, metal ion recovery, nutrient remediations, and the synthesis of high-value compounds.

Generally, the MFC comprises cathode and anode chambers which are separated by a proton exchange membrane (PEM) [29]. The anodic chamber contains microbes that act as a biocatalyst to decompose waste materials. As a result, electrons are produced that are transferred to the cathodic chamber via an external circuit. The protons diffuse to the cathode through PEM and combine with electrons and O2 to form water [15]. In most of the cases, current production is very small, so many advancements have been suggested by the scientists to get better performance by the MFCs and to find the most suitable feedstock [2], microbial consortia [26], catholyte/anolyte strength [34], and electrodes [44]. Electrochemical cell configuration is another very important domain that could improve the MFC performance [65].

The commercialization of MFCs requires the scaling-up of reactors. Various proposals which have been suggested include modifications in the design of electrodes, the design of membrane, using membrane-less reactors, and stacked systems. Some researchers have shown that a linear increase in power output can be achieved by expanding the size of MFC systems by stacking many units [69]. To get better power output, many researchers recommend minimizing the size of the MFCs system and sustaining a high feedstock supply, with optimization of the number of units for efficient performance. Another advantage of minimizing the MFC size is to explore MFC for various micro-level applications such as remote biosensors instead of bioenergy generation (which aims at offsetting the power requirements for wastewater treatment). In recent times, high-throughput MFC systems have been also designed for expediting the research in the MFC field by studying many parameters simultaneously for optimizing the MFC systems. In this chapter, we will discuss the advancements in configuration and design of MFCs which are economically feasible and that could be utilized to scale up the MFC reactor to get improved outputs. Future aspects and suggestions to further advance the MFC technology are also discussed.

2 Design and Configuration of MFCs Reactor

Cell design is a very important component in the successful operation of MFC. There is not a single standard size, configuration, or design for the MFC reactors. The configuration is entirely dependent on the researchers for a specific application. MFC performance based on reactors configuration could be controlled by various factors such as varying volumes, supply of oxygen, area of the membrane, and spacing of electrode [39]. Different reactor shapes, such as cylindrical, cubic, horseshoe, and H-shaped reactors have been proposed. Single and dual-chamber MFC reactors have been commonly studied in most MFC research projects [51]. Among the various shapes of MFC, H-shaped reactors are typically used in MFC due to the easy placement of PEM. The cell material could be glass or some type of plastic [20]. The size of the reactor varies from square centimeters to square meters having a volume of microliters to thousand liters [8]. MFC configurations based on different designs are discussed below.

2.1 Configuration Based on Number of MFC Chambers

2.1.1 Single-Chamber MFCs

Natural aeration of cathode for utilizing O2 as the ultimate electron acceptor leads to the construction of MFCs with one chamber and air cathode assembly (Fig. 1). In the single-chamber MFCs, the cathode is directly connected to PEM permitting a direct supply of oxygen to the electrode [67]. Several advantages associated with single-chamber MFC include simple operation, less internal resistance, small electrode spacing, better proton diffusion as well as an efficient cathode for O2 reduction [70]. Since no aeration is provided by using a compressor/pump and also catholyte is not required, this configuration makes it more easily adaptable and less expensive. In addition, more power density of single-chamber cell as compared to dual-chamber MFC has been reported in earlier studies [37]. Mainly this type of configuration comprises simple anodic chambers with no distinctive cathodic chamber and sometimes without any PEM. Cathode having pores on a side of the wall utilizes atmospheric oxygen and permits the protons to diffuse through the pores. The single-cell configuration is attracting researchers because of the above-mentioned advantages as well as ease of scaling-up of the system as compared to the dual-chamber MFCs [6, 37]. Carbon electrodes are used as anode in single-chamber MFC while the cathode is mostly PEM/carbon cloth hybrid or porous carbon electrodes [51]. However, cathode might be enclosed in graphite where electrolytes are added slowly which act as catholyte and avoided drying of the cathode and its membrane. Hence, fluid management is a limiting factor in such constructed cells. On the other hand, leakage of fluid, diffusion of oxygen, and evaporation are the flaws of this configuration and need to be addressed for efficient MFCs’ operation. The use of different diffusion layers on the cathode surface has been proved to get better power density and oxygen diffusion [5, 71]. A comparison of a few of the studies involving single-chamber MFCs has been reported in Table 1.

Fig. 1
A diagram of the MFC Chamber. The chamber includes wires at the top, an anode with microbes, P E M, and a cathode on the right.

Schematic presentation of a single-chamber MFC

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Table 1 A comparison of single-chamber MFCs
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2.1.2 Dual-Chamber MFCs

The two or dual-chambered MFC is also commonly used for energy generation along with wastewater treatment. It comprises anodic and cathodic compartments separated via a PEM (Fig. 2). PEM functions as a medium for the transfer of protons from the anode compartment to the cathode compartment [27]. The PEM also helps to prevent diffusion/contact of oxygen and other oxidizing agents to the anode [35]. Some problems associated with the dual-chamber microbial fuel cell are the large distance between the electrodes, which causes more internal resistance, the use of a batch process which requires regular maintenance, and enrichment of medium by some additives to get high current generation [52, 72]. These conditions for optimum power production are the hindrance to scaling up of the dual-chamber MFCs. Many studies have focused on the advancement of dual-chamber MFC to overcome the above-mentioned problems. For example, the internal resistance has been tried to reduce by having fewer distances between the two electrodes and placing them closer to the PEM but this will eventually reduce the power density by having more diffusion of O2 from the cathodic chamber to the anodic side [18]. The continuous mode MFC has also been proposed which provides even better power density than simple bottle types dual-chamber MFC. In continuous mode dual-chamber MFC, configuration comprised a cathode hot pressed on a PEM connected with the anode and anchored with two polycarbonate plates where both chambers can be fed with feedstock in a continuous manner. Changing the electrode material in dual-chamber MFCs has also been shown to gain high MFC performance without the use of any external additives [30, 59]. Various other designs of two-chamber MFCs have also been proposed to resolve the issues related to the lower current output.

Fig. 2
A diagram of the MFC Chambers. It includes 2 chambers connected through a pipe. The left chamber includes an anode in the absence of oxygen. The right chamber includes a cathode in the presence of oxygen. The electrodes are connected through the wires at the top of the chamber.

Main components of dual-chamber MFC: 1. Anodic chamber with anaerobic conditions; 2. Cathodic chamber with air supply; 3. Wire for connecting electrodes; 4. Proton exchange membrane (PEM)/salt bridge; 5. Substrate to feed bacteria; 6. Catholyte; 7. Bacterial culture, and 8. Anode (with bacterial attachments)

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2.1.3 Stacked MFCs

Since scaled-up MFCs are urgently required to commercialize the MFC technology, stacking arrangements of modular multiple units is an applicable solution while utilizing advanced power management systems [25]. In real applications, the multielectrode assembly from the modular designs coupled with an electrical circuit for the storage of charge has been proposed for stacked MFCs [24]. A battery-style MFC can be formed by stacking the fuel cells (Fig. 3). The cell efficacy, i.e., power density, current, and voltage is known to be improved in stacked cells. Moreover, the Columbic efficacy of individual cell also remains unaffected. The cell stacking may be in a series or parallel manner [14]. Both methods are important and could be utilized as conventional power sources and the voltage and current requirements of electronic devices could be achieved. The design of parallel or series stacked MFC circuits is very crucial, and shape modulation and horizontal and vertical directions of the staking can considerably affect the cell efficacy [43, 57]. Efforts are in progress to further improve the MFC technology with better efficiency and feasibility, and for utilizing MFC stacks to meet the practical needs of the industry and society.

Fig. 3
A schematic of stacked M F C in six columns of the granular anode, P E M, cathode, and rubber gasket arranged in a row and closed with screws.

Schematic presentation of stacked MFC

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2.1.4 Configuration Based on MFC Size

MFC size is another design parameter intensively studied for commercializing the MFC technology. In this regard microliter to thousands of liter-scale MFC have been studied for various applications. The small-scale MFC manufacturing and deployment are relatively easy. In addition, they offer better prospects as a long-standing power source at distant sites for the avoidance of regular maintenance, converting waste into electricity and non-requirement of refined chemicals [4, 46]. Liter-sized MFCs have also been used for process optimizations and the development of MFC technology for practical purposes [3, 32]. Nevertheless, small-sized cells present better efficiency because of the small electrode distance which helps to avoid internal resistance. Another advantage is the small-sized electrode present provides a high surface-to-volume ratio and fast response time. Furthermore, the surface modification of small-sized electrode is far easier and more economical, and it offers better performance than large size cells [19]. Generic techniques for MFC fabrication are etching, metal deposition photolithography, and polymer molding. These could be applied for the fabrication of micro as well as macro-sized cells. Although large size MFCs provide better electrode material and microbial performance assessment, the conventional large-sized MFCs have limited performance because of large resistance and the low surface-to-volume ratio [11, 22]. High-throughput MFC systems are another domain in which investigations are being carried out for the advancement of the MFC system. Keeping in view the importance of micro-sized and high-throughput MFC systems, further discussion is provided below.

2.1.5 Micro-sized MFCs

The milliliter-scale MFCs have a huge potential for long-term power supply at remote sites where a regular change of batteries is impractical. The capability of bacteria for producing bioelectricity using indigenous resources helps in the easy deployment of micro-sized MFCs as there is no requirement for an external power source and artificial mediators [55]. With a proper supply of the carbon source, microbes’ propagation and replenishing enable self-sustainable power generation. Other than milliliter-scale MFCs representation of miniatured MFCs, such MFCs are also designed for on-chip power production and fast screening of optimum operating conditions. The micro-sized MFC offers very unique features, such as a large surface area-to-volume ratio, shorter electrode distance, faster response time, and lower Reynolds number, along with various choices of design in constructing MFCs. The fabrications of miniature MFC devices are known to have high precision and have less cost when microfabrication processes are utilized. Also, the materials utilized for fabricating this type of MFCs are usually of inert character and have suitability for microbial research. The miniatured MFCs having carbon-based anodes [56] or improved designs having cloth electrode shows high efficiency than large-scale MFCs in terms of volumetric current and power densities [68]. The current milliliter-scale MFCs still have an issue of lower volumetric power densities and coulombic efficiencies because of the higher internal resistance. Nevertheless, such MFC systems have huge potential in the fast screening of electrochemically active strains and electrode materials [23].

Current microfabrication techniques offer improved design of MFCs having submicroliter reactor volume that enables improved biofilm growth at anodic electrode [53] and a quick startup [10]. Most of the miniatured MFC devices used similar design configurations as of the conventional dual-chamber MFC, i.e., they have anodic and cathodic chambers parted by a PEM. Polydimethylsiloxane (PDMS) and silicon are extensively utilized in such MFCs owing to the flexible designs they offer in microfabrication. Because of the smaller reactor volume, miniatured MFCs are usually provided with the electrolyte replenishing system for continuous or periodic exchange, and hence allow sustained operations [9, 54]. For instance, Fig. 4a shows a micro-sized MFC system with a 4 μl chambers volume that has microfluidic flow cells for both anodes and cathodes [54]. Reproducible bioelectricity production and improved power densities were established. Further micro-sized improved MFC designs (Fig. 4b) allow the sustaining of a higher level of nutrient utilization, minimized the consumption of substrates, and reduced the response time of bioelectricity generation because of the fast mass transport [28].

Fig. 4
2 illustrations of micro-sized MFC. A 1 displays an anode chamber, cathode chamber, and P E M with anode and cathode. A 2 displays a diagram with M R - 1 cell, entry of medium from the top left and F e 3 from the bottom left. A 3 displays a photo of micro M F C. B displays a diagram with labeled parts and the photo of microfluidic M F C.

A The design and assembly of a micro-MFC (i) the single MFC parts, (ii) a cross-sectional view of the MFC, and (iii) a picture of a PDMS micro-MFC. The scale bar is 2 cm. Adopted from [54]. B Microfluidic MFC design, showing (i) schematic of the device parts for the microfluidic MFC. (ii) Photograph of a ~1 × 1 × 3 inch3 array of six microfluidic MFCs. Adopted from [28]

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Yet there are shortcomings in such MFC systems and to resolve these issues such as lower power of miniaturized MFCs, a combination of multiple MFCs in series or parallel can be utilized which help in achieving larger current and power output. Also, there is a reversal of voltage issue in these network systems which is required to be resolved for the long-term operation of MFC-based sensor networks and/or environmental toxin monitoring systems. A comparison of different micro-sized MFCs is provided in Table 2.

Table 2 A comparison of micro-scaled MFCs
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2.1.6 High-Throughput MFCs

High-throughput MFCs are recently developed and used for the evaluation and microbial enrichment in different process conditions. In recent times [48], a 128-channel potentiostat is developed that connects with the printed circuit board. The entire array of channels was dipped in the anolyte medium, while a common reference electrode was used to carry out a high-throughput investigation for checking the effects of anode potentials on electroactive bacterial biofilms (Fig. 5a). Earlier, Zhou et al. developed a well-plate high-throughput colorimetry-based assay for the monitoring of bacterial respiration, which can show the presence of electroactive microbes associated with extracellular electron transfer (EET) capability [73]. Also, paper-based electrofluidic arrays having 6, 8, 64, and 96 wells via the fabrication method of wax printing have also been developed [21, 62, 63]. These designs help in eliminating the issues of small MFC devices such as large internal resistance, complicated assembling, and lower sample accessibility. One of the high-throughput-developed MFC reactors having 96-well plate showed these characteristics as well [60]. Additionally, it enables longer operational capability and reusability which helps in selective enrichments of EET-capable microbial culture. Also, such designs have better strength; therefore, they can be used for fieldwork which enables high-throughput in situ operations. The well plate connected to the potentiostat by electrical connections was similar to that described earlier [47]. Since the most widely studied application of MFCs is wastewater treatment, the microbial culture needs to be enriched for the specific wastewater treatment and to improve MFC performance. In this regard, preconditioning of consortia can be done using high-throughput systems which can help in achieving an easy scale-up process [1, 31]. For instance, a new 96-well MFC developed array (Fig. 5b) helped in the screening, selection, and sources of enriched EET culture by high-throughput [61]. The high-throughput MFC systems are still in their infancy, and continuous efforts are in progress to make them more reliable and useful for screening various optimization parameters.

Fig. 5
2 illustrations of the High-Throughput MFC. A 1. A device with a connector to power supply, P C, and an electrode array. A 2. A device with a connector to measure and reference electrode. A 3. High throughput MFC system. B displays an array with wells, a labeled well, and M F C with anode and cathode of 150 and 25 centimeters squares, respectively.

A High-throughput bioelectrochemical system (i) a potentiostat having 128 channels connected to the computer (top view), (ii) 128 gold WE (0.5 × 0.5 mm2) and platinum CE’s scheme, and (iii) a complete bioelectrochemical system having 128-electrode array which is immersed in an electrolyte having a single RE. Adopted from [48]. B 3-D view of a 96-well MFC system including (i) the complete plate, (ii) cross-sections of individual wells, and (iii) a schematic of large-scale MFC. Adopted from [61].

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3 Conclusions and Future Prospects

Various designs and configurations have been developed for MFCs to improve their performance and achieve commercialization of this emerging biotechnology. The designs and configurations include single chamber, dual chamber, stacked, micro-sized, large scale, and high throughput MFC systems. There is significant progress in the MFC research owing to various designs; however, many commercialization challenges are still being addressed and need further research. In addition to improving the power densities under real conditions, the capital and running costs linked with materials such as electrodes must be further reduced. For this, costly anodic and cathodic electrodes and catalyst layers on the cathode should not be used, as these costs add largely to the cost of MFC construction. Further scaling up is required with high surface area anodes and cathodes, which can help in achieving larger power densities. In the current scenario, because of lower bioelectricity production, expensive materials, and the continuously decreasing cost of renewable energy, it is less likely that bioelectricity produced from MFCs will outplay existing technologies. Therefore, applications other than bioelectricity production should also be considered and explored. Further, research is required in exploring the device configurations for minimizing the internal resistances for the improvement of micro-sized MFCs. Similarly, the high-throughput MFC systems for studying the impact of electrode’s potential and external load for controlling bacterial metabolism are vital for understanding MFC operations. Currently, there are limited studies about high throughput MFC systems for EET capable microbial species and communities and it has huge potential for making a breakthrough in the commercialization of MFC technology. High throughput MFCs can be effectively utilized for various applications such as genetic engineering, screening of phenotypes, and mutants development studies for both microbial communities and single cultures [61]. With the recent discovery of human pathogens having EET capability [35, 45, 50], these systems can be very useful for evaluating the current production mechanism and the importance of EET-capable pathogens in human health, which can open a spectrum of new research areas.