14.1.1 Microbial Fuel Cells

Energy consumption across the globe has increased exponentially during the first decade of the 21st century and is continuing to do so. To meet the ever-increasing energy demand, there is a need to identify more and sustainable feasible sources of energy. Indiscriminate exploitation of fossil fuels to meet demand has posed a threat to biological life on the planet via its secondary effects of global warming and environmental pollution [11, 12]. The dire need for alternatives to fossil fuels has encouraged researchers to seek alternative sources of power which can be harnessed by utilizing modern tools of technology developed in recent years. Proper and optimised use of renewable energy resources may be an answer to this serious problem. An extensive range of energy solutions have been explored by researchers, because any one of the presently available alternatives is unlikely to replace fossil fuels. As a consequence of these efforts, one of the recently proposed alternatives is energy derived from fuel cells, utilising microbial digestion of biomass [13, 14].

An electrochemical engine, which converts the existing energy of chemical bonds into electricity, is called a fuel cell [15, 16]. Being a green source of energy, this option seems attractive, as the energy obtained thereof is both renewable and environmentally friendly. Fuel cells utilizing biological material for power generation involve enzymatic catalysis of ingredients in an electrolysis chamber. Biological fuel cells are capable of directly transforming chemical energy to electrical energy by way of electrochemical reactions. There are two types of biological fuel cells, namely Microbial Fuel Cells (MFCs) and Enzymatic Fuel Cells (EFCs). If biological fuel cells are using biomass to act as substrate for bioelectricity production, then they may be named biomass fuel cells [17].

MFCs are novel devices that use a bacterial community as the biocatalyst for the oxidation of organic (or inorganic) matter to generate current [18]. A biopotential, developed between the bacterial metabolism and the substrate, leads to the generation of bioelectricity in MFCs. Anaerobic conditions are necessary in the anode chamber as oxygen will hinder the production of electricity, so that a pragmatic arrangement must be designed, in which the bacteria are separated from oxygen [11, 17–19].

MFC is an impressive technology, with the capability to digest a wide range of substrates with bacteria to generate bioelectricity, despite the fact that power levels are low. It is mostly preferred for sustainable long-term power applications [11, 20–22]. As normal fuel cell (FC), being a conventional energy resource, energize the distributed generation (DG) units of power system. Distributed Generations (DGs), a term commonly used for small-scale generations, offer solution to many of new energy generation challenges. DG is an electric power generation source connected directly to the distribution network or on the customer side of the meter, having generation from ‘a few kilowatts up to 50MW. Similarly, with high-power generation capabilities, MFC may act as a source for distributed electricity generations. Fig. 14.1 shows a schematic of the basic components of a double-chamber MFC.

14.1.2 Mechanism of Microbial Fuel Cells

MFCs utilize microbes as the catalysts to oxidize organic matter in these bio-electrochemical devices, to generate current. An MFC unit, as shown in Fig. 14.1, is a double chamber having an anodic as well as a cathodic chamber, the two being separated by a semi-permeable membrane, generally known as a proton-exchange membrane (PEM). In the anodic chamber, the microflora results in the generation of protons and electrons via oxidation of organic matter in an anaerobic environment, generating carbon dioxide and other compounds as final products. The protons travel to the cathode chamber through the membrane and the movement of electrons generated in the process is facilitated via an external circuit, where electrons are transmitted to the cathodic chamber. In the cathode chamber, protons and electrons react, along with the parallel reduction of oxygen to water. Therefore, bioelectricity is generated in an MFC by bacterial metabolism, due to the development of biopotential. MFCs are gaining consideration due to their capability to use a variety of biodegradable substrates under mild conditions. An air−cathode MFC, shown in Fig. 14.2, is a single-chamber MFC, in which the anode is placed in the anodic chamber where organic matter is present. The cathode is pasted outside the anodic chamber, separated by the PEM and exposed to the air. The working principle of the air–cathode MFC is the same as for the double-chamber MFC.

MFC performance depends mainly on several important factors, such as the system configuration, the nature of the organic matter, the bacterial species, the electrode material and surface area, type of catholyte, operating conditions, rate of oxidation in the anodic chamber, electron shuttle from the anodic chamber to the surface of the anode, the way of supply organic matter into MFC, consumption rate in the cathode chamber, and the permeability of the PEM [11, 22]. Anaerobic conditions are essential for the anodic compartment, as the configurations are designed for an oxygen-free region [11, 17, 18]. A continuous supply of biological raw material at regular intervals is necessary to ensure a steady generation of electrical energy [23, 24]. The chemical reactions taking place in anode and cathode chambers for an organic substrate are as follows:

In addition to the generation of bioelectricity, the end products are carbon dioxide and water. About 24 electrons participate in the flow of current, with bacteria acting as the catalysts to activate the chemical reactions.

14.1.3 Microbial Fuel Cell Technology and Advances

Potter (1911) [15] introduced the concept of MFC and reported that any physiological process, accompanied by chemical changes, involves a related electrical change. The breakdown of organic compounds by micro-organisms is accompanied by the release of electrical energy. With the action of microorganisms, the electrical effects are introduced and are influenced by temperature, the number of active bacteria, and the concentration of the nutrient medium. These effects are limited by the temperature favourable for the microorganisms and for protoplasmic activity. The maximum recorded voltage from an MFC was 0.3–0.5 V.

Based on this concept, Davis et al. (1962) [19] experimented to determine the role of microbes and hydrocarbons in the generation of electrical energy. The addition of glucose oxidase or microbes to a solution of glucose resulted in electrical output. In the absence of oxygen, biological dehydrogenation took place and it was considered that, with a hydrogen ionisation reaction, a wire could link oxygen with microbial dehydrogenations. The electrons transferred through the semi-permeable membrane produced hydroxyl ions at the oxygen electrode and reacted with hydrogen ions to complete the cyclic process. Experimental findings have shown that addition of methylene blue increased the open circuit voltage (OCV) from 80 to 180 mV and from 50 to 100 mV, maintained under 1000 ohms load. Similarly, addition of the gut bacterium Escherichia coli increased OCV from 150 to 625 mV and to 500 mV under a load of 1000 ohms. Addition of potassium ferricyanide resulted in only a slight increase in current.

Berk (1964) [20] reported a study of the interaction between electrode material and photosynthetic microorganisms. A sandblasted platinum electrode, on which marine algae were growing, generated a current density of 4.3 μA/cm² with a 0.6 V potential. Appropriate combinations of bioelectrodes, using Rhodospirillum rubrum (a bacterium which is photosynthetic under anaerobic conditions) with malate, have shown the capability for light-dependent production of electrical energy.

In early2000, rigorous research had started to increase the generation capabilities of MFCs and Steele et al. (2001) [16] presented data that fuel cells operate at high efficiency with low levels of pollutants in the production of electrical energy. The vital issues relating to fuel cell technology, such as alternative materials for the stacking of fuel cells and optimal selection of fuels for MFCs, were discussed. Present cells use traditional materials but commercialization studies and cost analysis have uncovered the limitations of these materials.

Logan et al (2006) [18] reported that research into MFCs was developing swiftly but lacked the methods of evaluation for system performance. Researchers were facing technical problems in comparing the performances of MFCs on an appropriate basis with conventional electricity generation systems. MFC construction and performance studies require information on microbiology, materials electrochemistry, and fundamentals of engineering. Performances of MFCs constructed in different configurations and from different materials were being analyzed by standard polarization curves.

You et al. (2006) [25] reported that MFC is a novel bioprocess, producing electrical energy from organic matter. A peak value of power density of 115.60 mW/m² was obtained in two-chamber MFCs, with permanganate as the cathodic electron acceptor as compared to hexa-cyanoferrate and oxygen, with power densities of 25.62 mW/m² and 10.2 mW/m², respectively. In comparison to double-chambered MFC, a bushing MFC (a different MFC reactor design), using permanganate, achieved an unparalleled maximum power output of 3986.72 mW/m². This study has presented permanganate as an effective electron acceptor to augment MFC efficiency.

Lovely (2006) [12] reported that, though the technology for MFC has been established, there has been less development in practical usage than would have been expected. Sediment MFC has shown the practical application, for feasibility studies, of electricity generation in remote areas. MFC has the capability to treat a range of organic wastes to make MFCs a feasible self-sustaining source for electricity generation. With recent developments, power output of MFC has increased but it still needs optimization of parameters to achieve large-scale electricity production.

14.1.4 Recent Developments

Fornero et al. (2010) [21] explored the feasibility of MFC technology for wastewater treatment. They discussed the problems occurring with respect to generation of current from complex wastewater due to different types of microbes in the microbial community. This diversity led to undefined microbial communities, low coulombic efficiencies, slow kinetic rate, and non-linear power density increase during scaling. To analyze these parameters in MFCs, comparison between studies are difficult, due to the use of different electrode materials, membranes, substrates, bacterial communities, configurations, electrode conductivities, electron transfer rate, temperatures, and pH levels.

Franks and Nevin (2010) [14] reported that MFCs have the capability to treat a broad range of organic substrates for the generation of electrical energy. The intensity of research in this field has increased many fold, to assess organic waste as a sustainable energy producer. Sediment MFCs have been successful to provide current for low-power applications. For advances in MFC technology, knowledge of the limitations of the technology and of the behavior of microbes is required. Several researchers consider the greatest achievement of MFCs is the ability to treat organic waste and to degrade toxic wastes rather than electricity generation. More emphasis is required on the understanding of the microbial process in MFCs for further development of practical applications.

Gupta et al. (2011) [17] reviewed biofuel cells and their classification into either microbial fuel cells or enzymatic fuel cells. The main focus was on performance and developments made in MFCs, as here the challenge is to achieve the correct blend of biological parameters with electrodes. Researchers have confirmed the capability of MFC technology for low-power generation and the means to degrade organic waste and toxic chemicals. The authors reviewed the research on MFC in terms of electrodes, performance evaluation methods, and environmental treatments. Various approaches to overcoming existing challenges were reviewed. This potentially high-impact technology has applications mainly in the field of energy generation from organic materials, as well as in clinical research and medical sciences.

Cheng et al. (2011) [26] stated that, to meet the challenge of the scaling-up of MFCs, the importance of various significant parameters, like the surface area of the electrodes, reactor geometry, substrate conductivity and concentration, is essential. Substrate conductivity affects the cathode, whereas substrate concentration affects the anode significantly. Using wastewater as the substrate for scaling-up MFCs, the most important deciding factor is the cathode-specific surface area. In favor of power generation, volumetric power density increases linearly with cathode surface area, but substrate strength and conductivity need to be high. Higher volumetric densities are possible with reactor configurations of smaller liquid volumes and closer electrode spacings. Studies concluded that the most essential feature for scaling-up of MFCs is cathode surface area.

Rahimnejad et al. (2012) [22] carried out studies on a novel stack of four MFCs of a bio-degradable material at continuous mode for production of clean and sustainable energy. An active biocatalyst, the yeast Saccharomyces cerevisiae, was added to enhance the power generation capability. In pure glucose substrate, addition of Natural Red as a mediator in the anode chamber and potassium permanganate as an oxidizing agent in the cathode achieved a maximum current of 6447 mA/m² and power generation of 2003 mW/m². Electrical performances were evaluated using polarization techniques, and electricity generation was the prime parameter under study. Graphite surface images showed that the uniform growth of the microorganisms was the major factor contributing to high electrical performance.

Zhao et al. (2012) [27] constructed an MFC to explore the possibilities of power generation from cattle dung. A continuous operation of MFC was set up in batch mode for a period of 120 days. Stable electricity generation was obtained after 30 days of operation, with maximum power density of 0.220 W/m³. After 120 days, the removal of total chemical oxygen demand (TCOD) and coulombic efficiency (CE) were accomplished at 73.9% and 2.79%, respectively. Analysis of the microbial community confirmed that Firmicutes were central to cattle dung degradation, whereas Proteobacteria were the most plentiful phylum during the process of power generation. This study confirmed the potential of using cattle dung fuel to generate electrical power.

Choi and Ahn (2013) [23] examined the continuous electricity generation from air–cathode MFCs under two different conditions, ambient and mesophilic, with different organic loading rates for wastewater treatment. Examination showed that operating parameters, mode of flow, and electrode connections significantly affected the power density and process stability. In series flow, connections in parallel mode under mesophilic conditions achieved a maximum power density of 420 mW/m² and chemical oxygen demand (COD) removal of 44%. Evaluations highlighted the significance of a pre-fermentation process prior to wastewater treatment with the design of stacked MFCs.

Inoue et al. (2013) [24] demonstrated that cassette-electrode MFCs are scalable and competent for electricity production at relatively high efficiencies, using artificial wastewater as the substrate. With cattle manure as substrate, an individual CE-MFC was constructed and run in batch mode for 49 days. The highest power density achieved was on day 26, at 16.3 W/m³. Biofilms on the CE-MFC anode suggested the presence of large quantities of Chloroflexi and Geobacteraceae bacteria, identified through sequencing analysis. Results supported the findings that CE-MFCs can be used for electricity generation in scalable MFC, using suspended cattle manure.

Jia et al. (2013) [28] presented MFCs as a novel and alternate way to treat waste for energy utilization from food wastes. At a COD of 3200 mg/L, a maximum power density of 556 mW/m² was achieved, with a CE of 27% at a COD of 4900 mg/L. The total carbohydrates, maximum COD removal, and total nitrogen were 95.9%, 86.4% and 16.1% respectively. Exoelectrogenic Geobacter spp. and fermentative Bacteroides spp. were the prominent bacteria, which showed high efficiency for electricity generation and degradation of organic food wastes.

Haque et al. (2014) [29] demonstrated the performance of a sediment MFC, with a common cathode of graphite felt and marine sediment as substrate. The performance of MFC was examined with Zn, Al, Cu, Fe or graphite felt anodes in a single-chamber mediator-less set-up. To determine the most efficient anode material, cell voltage, current and power density, oxygen reduction potential, COD, and pH were measured. Maximum power densities of 913 mW/m², 646 mW/m², 387.8 mW/m², 266 mW/m² and 127 mW/m² were achieved for Zn, Fe, Cu, Al, and graphite anodes, respectively. Comparatively weaker electricity generation was observed with graphite, as a consequence of its bio-oriented mechanism. Studies concluded that selection of the most appropriate anode material could lead to superior performance of sediment MFCs.

El Chakhtoura et al. (2014) [30] demonstrated that, for MFCs, the most suitable substrate is the organic fraction of municipal solid waste (OFMSW), which normally has more than 60% of waste matter. Studies tested the two set-ups, air–cathode MFCs with wastewater or cattle manure, separately. For wastewater, the performances of the MFCs were evaluated in terms of power density, coulombic efficiency, COD removal, and carbohydrate removal, and results were 116 mW/m², 23%, 86% and 98% respectively. Similarly, performance was also evaluated for cattle manure as fuel, and the parameters power density, coulombic efficiency, COD removal, and carbohydrate removal gave results of 123 mW/m², 24%, 86% and 98%, respectively. Analysis shows that the presence of high numbers of Firmicutes played a prominent role in electricity generation.

Gopinath et al. (2014) [31] reported that decomposer bacteria are successful in disassociating the complex matter, with the discharge of energy through sequential breakdown. Production of biogas occurs in an anaerobic environment, and a consortium of microorganism degrades the organic matter and releases methane. Research was carried out to identify the groups of bacteria present in cattle dung to analyze their efficiency in biogas production. The findings of the study were that one consortium had large numbers of methanogenic bacteria and the capability to achieve 79.45% methane production.

Rodenas Motos et al. (2015) [32] concluded that bio-electrochemical systems are a promising technology for the accumulation of copper metal from a copper sulphate stream. For reduction of internal voltage losses, a novel cell configuration was set up. Electroactive microorganisms produced electrons at the anode electrode, which were shared with the cathode, where copper was reduced, with 99% purity. A cell voltage of 485 mV, current density of 23 A/m² and a power density of 5.5 W/m² were produced. At the highest current, most of the voltage drop happened at the cathode and membrane.

Rahimnejad et al. (2015) [11] reported that, in recent times, researchers had shown great curiosity toward MFCs, due to their ability to use a wide range of substrates and simple operating conditions. The authors reviewed the anode, cathode, and membrane parts of the MFC in an attempt to overcome the low current and power density barriers which hamper the practicability of this technology. They also quoted the maximum power and current densities achieved by using a wide range of anode and cathode materials. Discussions also included the advantages and near-future possible applications of MFCs. Factors responsible for decreasing the bioelectricity potential of MFCs were also addressed.

Hernandez-Fernandez et al. (2015) [33] stated that the MFC concept has boosted the technical feasibility of green electricity by utilizing household and industrial wastes for both the removal of contaminants and the generation of electricity. The advances in new materials have increased the power output of MFCs, even with inexpensive materials like ceramic membranes or non-platinum crystals. To scale up MFCs, the most important task is to improve the performance, as, to date, power generation is still not practicable. The development of low-cost catalysts, new cathodes, biocompatible anodes, membranes, and novel configurations to enhance MFC efficiency are the global objectives of MFC researchers. the Authors predicted that optimization of anode and cathode materials, membranes, configurations, applications, and modeling will also be significant for the future development of MFCs.

Baudler et al. (2015) [34] demonstrated that anti-microbial properties did not affect the electrochemically active, electrode-respiring bacteria in the case of copper and silver electrodes in MFCs. Studies showed that bacterial groups grow very rapidly on these metals and form energetic biofilms. Current densities of 1.1 mA/cm² for silver and 1.5 mA/cm² for copper were achieved, as compared with a value for graphite of 1.0 mA/cm². Other suitable metals for the anode included stainless steel, titanium, nickel, and cobalt. Copper is a highly promising material for anode electrodes in bio-electrochemical systems, such as MFCs.

Chaturvedi and Pradeep (2016) [35] reviewed the use of MFCs for the generation of bioelectricity by utilization of waste. To meet the enormous energy demand with inadequate resources, utilization of renewable energy sources is the central strategy. MFC technology has major drawbacks of low power output, whereas scaling-up leads to decreases in output. Due to these drawbacks, this technology has yet to be commercialized.

Prakash (2016) [36] indicated that greater joint understanding of both scientific and engineering fields is required for the construction and analysis of energy-efficient MFCs, with a review on the current knowledge of the role of micro-organisms in electricity production and their applications to MFC technology also needing to be highlighted.

Sonu and Das (2016) [37] presented results from research on sewage sludge, cow dung, or kitchen waste as substrates to produce electricity at ambient temperature from a single-chamber single-electrode MFC. The maximum voltage achieved was 1652 mV, with a power density of 988.32 mW/m² from sewage sludge waste. Maximum voltages of 657 mV from kitchen waste and 452 mV from cow dung were achieved during MFC operation.

Sonaware et al. (2017) [38] described the MFC as a novel one-step bio-electrochemical device, which converts the biomass-based substrate into electricity through the metabolic activities of bacteria. Unfortunately, the high construction cost and time-consuming microbial kinetics of MFCs have limited the commercial utilization of this technology. With the advent of new, more cost-effective materials, there is scope for further development in the design and utilization of MFCs. A critical review of recent advances in novel anode materials, functions of anodes, and techniques for upgrading the surface area of anodes was carried out, showing that the benefits arising from the use of nano-materials is of great significance. The barriers for the commercialization of MFC also need to considered.

14.2.1 Bioelectricity Production-: Practical Application of Microbial Fuel Cells

The chief function of an MFC is to utilize the biomass obtained from wastes of agriculture, the food industry, and municipalities for the production of bioelectricity. Another strong feature of MFC is the direct conversion of fuel energy into electricity without any intermediate step which limits the efficiency of the conversion process. At present, MFCs are not an economical method for power production, but, with time, research, and advances in this technology, the past decade has proved to be a progressive period for improvement of power production by MFCs. Therefore, MFC technology can be considered to be a potential source of sustainable source of energy for the future.