Electricity generation by microbail fuel cells.Introduction
A number of methods and sources are currently in use for the production of the electrical energy which includes hydro-power, solar-power, wind-power, wave-power, fossil fuels, batteries and chemical fuel cells. All these technologies play a significant role in the global issue of energy management. In addition to these technologies an attractive and novel alternative for the conversion of chemical energy into electrical energy is the development of microbial fuel cells (MFCs) (Katz et al. 2003; Lovley 2006). A microbial fuel cell is a device that converts chemical energy to the electrical energy by the catalytic reaction of microorganisms (Allen and Bennetto 1993).
Presently, there is an unbalanced energy management due to increasing energy consumption. Mostly, the energy that we utilize is produced through the thermal or combustion processes and both have damaging effect to our environment. Due to an increasing demand of the electrical energy scientists are developing various methodologies for the generation of electrical energy by the more facile and cheaper methods. In recent years, research activity in fuel cell technology has increased remarkably. Great expectations are directed to fuel cells because of the forthcoming depletion of earth's fossil fuel resources. In addition, the MFCs offer an environmentally friendly alternative to fossil fuels (Lovley 2006; Katz et al. 2003).
Initially, the concept of the generation of electricity is given by M.C. Potter (Potter 1910, 1911). He stated that "the disintegration of organic compounds by microorganisms is accompanied by the liberation of electrical energy (Potter 1911). These fuel cells based on the metabolic activities of the microorganisms on the organic substrates particularly the natural sources which contain the sugars as the main component. Research to render the microbial fuel cell technology is more economically feasible and applicable. A special focus on reactor configuration, power density and material cost is required (Pham et al. 2004). Recent studies of microbial fuel cells have greatly advanced our understanding of microbial electricity generation (Bennetto 1984, 1985, 1987, 1990). In fact, much work remains to be done to explore the chemistry and biochemistry of MFCs.
In a microbial fuel cell, different substrates such as glucose, molasses, lactose and wastewater have been used for the generation of electrical energy by their fermentation with different bacteria (Suzuki et al. 1978; Allen and Bennetto 1993). Various research groups are keen interested to improve the current density by more facile and efficient methods (Palmore and Whitesides 1994; Bond et al. 2002; Angenent et al. 2004; Pham et al. 2004). Generation of electrical energy is based on the principles of fermentation in which organic substrates undergo the biochemical reactions in the presence of microorganisms which resulted in the formation of the hydrogen fuel. The fuel so formed is finally converted into electrical energy and water through redox reaction (Scheme-1).
Types of Microbial Fuel Cell:
Mediator-less Microbial Fuel Cells:
This type of microbial fuel cells is reported by Kim and his co-workers (Kim et al. 1999a, 1999b, 2002). A mediator-less microbial fuel cell does not required a mediator but uses electrochemically active bacteria to transfer electrons to the electrode (Tanisho et al. 1989; Gradskov et al. 2001; Chaudhuri and Lovley 2003; Pham et al. 2004). Shewanella putrefaciens, Aeromonas hydrophila and some others are the electrochemically active bacteria (Kim et al. 1999a; Coung et al. 2003). In the process of the development of microbial fuel cells, the pure and mixed cultures of microorganisms have been used which are found to be active in hydrogen production under aerobic and anaerobic conditions. In addition, various bacteria such as Escherichia coli, Enterobacter aerogenes, Clostridium butyricum, Clostridium acetobutylicum, Clostridium perfringens have been found to be active in hydrogen production under anaerobic conditions (Lewis 1966; Raeburn and Rabinowitz 1971; Akiba et al. 1987; Ardeleanu et al. 1983).
The conversion of carbohydrate to hydrogen is achieved by a multi-enzyme system. In bacteria the route is believed to involve glucose conversion to 2 mole of pyruvate and 2 mole of NADH by Embden-Meyerhof pathway (Scheme-2). The pyruvate is then oxidized through a pyruvateferredoxin oxidoreductase producing acetyl-CoA, C[O.sub.2] and reduced ferredoxin. NADH-ferredoxin oxidoreductase oxidizes NADH and reduces ferredoxin. The reduced ferredoxin is reoxidized by the hydrogenase to form hydrogen. The pyruvate so produced during the biochemical pathway can be alternatively oxidized to formate through a pyruvate-formate lyase which oxidized at the anode (Karube et al. 1977; Katz 2003).
Microbial cells producing [H.sub.2] gas during fermentation have been immobilized directly in the anodic compartment of a [H.sub.2]/[O.sub.2] fuel cell (Karube et al. 1977; Tanisho et al. 1989). A rolled Pt-electrode was introduced into a suspension of Clostridium butyricum and the suspension was polymerized with acrylamide to form a gel (Karube et al. 1977). The fermentation was conducted directly at the anode surface, supplying the anode with the hydrogen fuel. In this case some additional by-products of the fermentation process (hydrogen = 0.60 mole, formic acid = 0.20 mole, acetic acid = 0.60 mole, lactic acid = 0.15 mole) could also be utilized as additional fuel components (Karube et al. 1977).
Mediator Microbial Fuel Cells:
Most of the microorganisms are electrochemically inactive. The electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, methyl viologen, humic acid etc (Lithgow et al. 1986; Vega and Fernandez 1987; Kreysa and Kramer 1989; Kim et al. 1999a-c; Yamazaki et al. 2002; Jang et al. 2004). Once the fuel is produced, the electrons obtained from the oxidized fuel are not immediately transported to the anode since the electron transfer rate would be too slow. To improve the electron transfer rate, an initially oxidized redox mediator is used to extract electrons. The electrons are then transferred to the anode and the mediator is once again oxidized (Kim et al. 2000).
Mediator microbial fuel cells are based on the metabolic activities of the microorganisms on the substrates such as glucose, sucrose and wastewater in the presence of electron transfer mediators (Davis and Yarbrough 1962; Bennetto et al. 1985; Lithgow et al. 1986; Vega and Fernandez 1987; Park et al. 1997; Park and Zeikus 2000; Park et al. 2000; Rabaey et al. 2003). It is because the contact of the microbial cells with an electrode usually results in a very minute electron transfer across the membrane of the microbes. These electron carriers are able to generate anodic current in the presence of terminal electron acceptors (under anaerobic conditions) which is the exceptional example (Kim et al. 1999a, 1999b).
A number of organic and organometallic compounds have been tested in combination with bacteria to test the efficiency of mediated electron transport from the internal bacterial metabolites to the anode of a biofuel cell. Thionine has been used extensively as a mediator of electron transport from Proteus vulgaris and from Escherichia coli (Davis and Yarbrough 1962; Ardeleanu et al. 1983; Bennetto et al. 1983, 1984; Roller et al. 1984; Vega and Fernandez 1987; Kim et al. 2000; Park and Zeikus 2000). Monitoring and control of bacterial fuel cell system is investigated by using color analysis of the biofilm reactor which converts substrate to electroactive substances in the presence of mediators (Halme et al. 1998).
Connecting several microbial fuel cell units in series or parallel can increase voltage and current. Six individual continous MFC units in a stacked configuration produced a maximum hourly average power output of 258 W [m.sup.-3] by using a hexacyanoferrate cathode. The connection of the six MFC units in series and parallel enabled an increase of voltage (2.02 V at 228 W [m.sup.-3]) and the current (255 Am at 248 W [m.sup.-3]) (Aelterman, et al. 2006). The electricity generation and energy conversion rate depends upon the size and the structure of the microbial fuel cells (Zhang and Halme 1997).
There are many microorganisms producing metabolically reduced sulfur-containing compounds (e.g. [S.sup.2]-, [H.sup.S]-, S[O.sub.4.sp.2-]). Sulfate reducing bacteria (e.g. Desulfovibrio desulfuricans) form a specialized group of anaerobic microbes that use sulfate (S[O.sub.4.sup.2-]) as a terminal electron acceptor for respiration. These microorganisms yield S2-while using a substrate (e. g. lactate) as a source of electrons (Scheme-3). This microbiological oxidation of lactate with the formation of sulfide has been used to derive an anodic process in biofuel cells. The metabolically produced sulfide was oxidized directly at electrode, providing an anodic reaction that produces sulfate or thiosulfate. Sulphate reducers produce sulphide which can abiotically react with the anode yielding two electron and sulphur (Habermann and Pommer 1991; Cooney et al. 1996; Lovley 2006).
MFCs Based on Wastewater:
Special focus on the development of microbial fuel cells by using different wastewater such as domestic wastewater, industrial wastewater and agriculture wastewater was given by a types of number of research groups (Suzuki 1978; Wang et al. 2003; Angenent 2004; Liu et al. 2005a, 2005b; Min and Logan 2004. He et al. 2005; Logan 2005; Min et al. 2005). Microbial fuel cells can simultaneously be used to generate the electricity and for the wastewater treatment (Rabaey 2006; Angenent et al. 2004).
Tubular and Square Type MFCs:
The square type MFCs demonstrated a potential-dependent conversion of sulfide to sulfur. In the tubular system, up to 514 mg sulfide [L.sup.-1] net anodic compartment (NAC) [day.sup.-1] (241 mg [L.sup.-1] day-1 total anodic compartment, TAC) was removed. The sulfide oxidation in the anodic compartment resulted in electricity generation with power outputs upto 101 mW [L.sup.-1] NAC (47 W [m.sup.-3] TAC) (Rabaey et al. 2006). A tubular, single-chambered, continuous microbial fuel cell that generates high power outputs using a granular graphite matrix as the anode and a ferricyanide solution as the cathode. The maximal power outputs obtained were 90 and 66 W [m.sup.-3] net anodic compartment (NAC), 48 and 38 W [m.sup.-3] total anodic compartment (TAC) for feed streams based on acetate and glucose, respectively, whereas 59 and 48 W [m.sup.-3] NAC was recorded for digester effluent and domestic wastewater, respectively. For acetate and glucose the total coulombic conversion efficiencies were 75 [+ or -] and 59 [+ or -] 4%, respectively, at loading rates of 1.1kg chemical oxygen demand [m.sup.-3] NAC volume [day.sup.-1] (Rabaey et al. 2005).
Upflow Microbial Fuel Cell (UMFC):
A new type of upflow microbial fuel cell (UMFC) was devised by a group of researcher in USA (He et al. 2006; Zheng et al. 2006). The device is fed continuously and unlike most microbial fuel cell works with chambers atop each other than beside each other. Angenet has created electricity with the UMFC, which is about the size of the thermost bottle (He et al. 2006; Zheng et al. 2006).
Bioelectrically Assisted Microbial Reactor (BEAMR):
This device demonstrates the feasibility of generating hydrogen from any biodegradable organic matter. An additional voltage of 250 mV was used in this MFC to produce hydrogen at the cathode directly from the oxidized organic matter (Liu, et al, 2005 a,b). More than 90% of the protons and electrons produced by the bacteria from the oxidation of acetate were recovered as hydrogen gas with an overall coulombic efficiency (total recovery of electrons from acetate) of 60-78%. This is equivalent to an overall yield of 2.9 mole [H.sub.2]/mole acetate (assuming 78 % coulombic efficiency and 92 % recovery of electrons as hydrogen). This bioelectrochemically assisted microbial system, if combined with hydrogen fermentation that produces 2-3 mole [H.sub.2]/mole glucose, has the potential to produce ca. 8-9 mole [H.sub.2]/mole glucose at an energy cost equivalent to 1.2 mole [H.sub.2]/mole glucose. Production of hydrogen by this anaerobic process is not limited to carbohydrates, as in a fermentation process, as any biodegradable dissolved organic matter can theoretically used in this process to generate hydrogen from the complete oxidation of organic matter (Liu et al. 2005 a,b).
Biomass and Wastewater Large-scale MFCs:
Researchers are interested to develop the large scale MFCs by the fermentation of the sewage, marine algae, garbage and wastewater using the microorganisms (Lovley 2006). Upflow microbial fuel cell (UMFC) can be scaled up considerably to someday handle the 2 million gallons of wastewater needs to churn out enough power for 900 homes. Angenent said, "We have proven that we can generate the electricity on a small scale. It will take time but we will believe that the process has potential to be used for the local electricity generation. He also added that the UMFC is a promising wastewater treatment process and has a lab-scale unit, generated electricity and purified wastewater simultaneously for more than five months" (He et al. 2006; Zheng et al. 2006).
Benthic Unattended Generators (BUGs):
During the last few years a number of research groups were able to harvest electricity from the organic matter in the bottom of the oceans (Bond et al. 2002; Tender et al. 2002; Lovley 2006). BUGs are designed for powering the electronic devices in remote locations, such as the bottom of the ocean where it would be expensive and technically difficult to routinely exchange traditional batteries (DeLong and Chandler 2002; Tender et al. 2002;
Lovley 2006). A complex organic matter is present in the bottom of the oceans which naturally undergo the process of fermentation by the microorganisms present in the bottom. Due to this natural fermentation chemical energy is converted into electrical energy. The artificial system is required which can accumulate the energy and use to power the monitoring devices of the oceans. This system is called Benthic Unattended Generators (BUGs) because it can run for ever without the recharging (Scheme-4).
Advantages of MFCs:
1. Development of microbial fuel cells is an attractive alternative for the production of electricity with minimal environmental interference as compared to the currently used thermal and combustion methods.
2. Most of the portable electronic devices possessing some disadvantages such as they run out too fast, recharging is inconvenient and time consuming. MFCs potentially offer the solution to all these problems.
3. Fossil fuels that have sustained civilized society for so long, have been abused and are now rapidly becoming non-existent. It is important for us to learn that whatever the energy source of the future turn out to be, we must learn to conserve and value it by the development of MFCs.
4. It is convenient to move these MFCs from one place to another place due to their compact size and weight.
Disadvantages of MFCs:
1. Certain conditions are required for the survival of the microorganisms.
2. Presently, the current density is not enough to run the heavy machinery.
3. Most of the available mediators are expensive and toxic.
Applications of MFCs:
1. Advances in the medical sciences are leading to an increasing number of implantable electrically operated devices. These items need power supplies that will operate for extremely long duration. An important potential use of MFCs is to power the pacemaker which can control the heart beat of the cardiac patient. The fuel cell in the pacemaker will be powered by taking the glucose from the blood stream of the patient (Katz 2003; Lovley 2006).
2. Similarly, the insulin pump in the diabatic patient can be powered by the glucose of the blood stream of the patient (Katz et al. 2003; Lovley 2006).
3. One of the most interesting examples of a novel device is robot named "Gastronome", uses a MFC system to directly convert carbohydrate fuel to an electrical power source without combustion (Wilkinson 2000; Katz et al. 2003).
4. Microbial fuel cells can potentially be used to power the large number of portable devices such as laptop, digital camera, cell phones, military applications, mobile devices and to maintain the telecommunication in remote areas including outer space, weather stations and rural locations (Katz et al. 2003; Lovley 2006).
Microbial fuel cells can be used to generate electrical energy by using any biowaste material that contains significant amount of carbohydrates, proteins and lipids etc. Mircobial catalyzed fuel cells could be the best alternative of fossil fuels to overcome global warming and energy crisis. They can play a significant role to power the biomedical devices such as pacemaker and insulin pump. There is an urgent need to give the special focus for the advanced research in this direction. The collective efforts of Chemists, Biochemists, Microbiologists, Environmental Enegineers and some other disciplines in this area of reseach can produce the fruitful result for the development of novel technology of bio-energy around the globe.
The author of this review article is greatly thankful to the Higher Education Commission (HEC) of Pakistan for providing the financial support (Grant No. 20-1061/R & D/07/694) to AMK for the research work that has been included in this paper. This grant was provided under the "National Research Grant Program for Universities" to the project entitled "study of the generation of electrical energy by the fermentation of domestic wastewater", conducted at Research Laboratory of Medicinal Chemistry and Bio-energy (RLMCB), Department of Chemistry, Federal Urdu University of Arts, Science and Technology (FUUAST).
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Abdul Majeed Khan
Research Laboratory of Medicinal Chemistry and Bioenergy (RLMCB) Department of Chemistry, Federal Urdu University of Arts, Science and Technology, Gulshan-e-Iqbal Campus, University Road, Karachi-75300, Pakistan
Corresponding Author: Abdul Majeed Khan, Research Laboratory of Medicinal Chemistry and Bioenergy (RLMCB) Department of Chemistry, Federal Urdu University of Arts, Science and Technology Gulshan-e-Iqbal Campus, University Road, Karachi-75300, Pakistan
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