Microbial Fuel Cell: A Source of Bioelectricity Production

Introduction

In addition to the considerable (and increasing) demand for energy in rural and urban Indian communities, the trend is gradually shifting from non-renewable energy sources to renewable ones, the latter being acknowledged as effective alternatives for generating energy for distribution to consumers. Agriculture-, forest-, and livestock- based biowastes or by-products are called biomass or bioresidues, and are available in large quantities in India [1, 2]. Biomass-based energy generation is popular in rural areas due to infrastructural constraints to delivery via conventional sources. Bioenergy has merit as it is renewable and extractable from organic matter by utilizing simple and economical techniques, and processes of anaerobic digestion (AD), yielding high levels of practically usable biogas [3-5]. Biogas is a recognized eco- friendly energy source, with the main components being methane (60-70%) and carbon dioxide (30-40%) [6-8].

Another innovative technique, developed in recent years to utilize available biomass for energy generation, is the Microbial Fuel Cell (MFC). MFCs are currently under intensive research and researchers have been able to obtain a maximum power density of 3600 mW/m2 [11] with a glucose-fed substrate, using commonly available raw biomass constituents. A typical MFC is a bioreactor which converts chemical energy, existing in bioconvertible substrates, directly into electricity by the action of specific microorganisms which facilitate the conversion of substrate directly into electrons [9-11].

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.

Mechanism of Microbial Fuel Cells

MFCs utilize microbes as the catalysts to oxidize organic matter in these bio-elec- trochemical 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

Schematic of the basic components of a double-chamber MFC

FIGURE 14.1 Schematic of the basic components of a double-chamber MFC.

Schematic of an air-cathode single-chamber MFC

FIGURE 14.2 Schematic of an air-cathode single-chamber MFC

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.

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 pA/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.

 
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