Poulomi Pramanik and Pranab Roy, BAOJ Physics 2017, 2: 32: 017

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BAOJ Physics, an open access journal Volume 2; Issue 3; 017

*Corresponding author: Pranab Roy, Department of Biotechnology, Haldia Institute of Technology, Haldia, India, E-mail: pranabroy@redif-fmail.com

Sub Date: April 21, 2017, Acc Date: May 9, 2017, Pub Date: May 9, 2017.

Citation: Poulomi Pramanik and Pranab Roy (2017) An Unending Source of Energy: Microbial Fuel Cells. BAOJ Physics 2: 017.

Copyright: © 2017 Poulomi Pramanik and Pranab Roy. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

An Unending Source of Energy: Microbial Fuel Cells

Poulomi Pramanik1 and Pranab Roy1*

1Department of Biotechnology, Haldia Institute of Technology, Haldia, India

Abstract

Microbial Fuel Cell or MFC has created new opportunities for generation of sustainable energy from bio-degradable substrates. The conventional MFC consists of an anode and a cathode chamber. Microorganisms catabolize the substrates in the presence of active bio-catalysts and generate bioelectricity. MFCs could be applied as a power generator in small devices. Despite the advantages, there are still some limitations to commercialize the use of this MFC technology such as low power output and current density. Depending upon the operational parameters, different metabolic pathways are used. This determines the selection of specific microorganisms. Here the different parts of MFC which are anode, cathode and membrane have been discussed. Also some options have been suggested how to overcome the practical barriers. It also discusses about the improvement of MFC with the possible and future application.

Introduction

In this present era, consumption of energy is continuously increasing. The energy sources have been classified into three categories: (a) Fossil Fuels; (b) Renewable Energy; (c) Nuclear Energy [1]. However, vast proportion of energy consumption in our world could be classified into two categories: fossil fuel and nuclear energy [2]. But non-renewable sources of energy from fossil fuels emit harmful gases including CO2, SO2, SO3 etc. which have major contribution in polluting our environment and causing global warming [3]. However, miscellaneous countries all over the world have made noteworthy efforts to find solutions for energy crisis by exploring renewable energy sources like solar energy, wind energy, hydroelectricity, geo-thermal energy etc. [3]. As an upshot of these efforts, one of the proposed alternative energy sources is fuel cell. Fuel cell generates energy using metals as catalysts. One variety of the fuel cell is known as microbial fuel cell (MFC) [4].

Microbial fuel cells or MFCs are bio-electrochemical system which generates electricity by the use of microbes, mainly bacteria. MFC converts energy in bio-convertible substrates directly into electricity. This electricity is generated when bacteria change from the natural electron acceptors to insoluble acceptors. This electron transfer can occur via membrane-associated components or soluble

electron shuttles [5]. Generally, MFCs consist of an anode and a cathode chambers, separated by a proton exchange membrane (PEM) [6]. Active biocatalyst present in the anode oxidizes the bio-convertible substrate and produces electron and proton. Then these protons are carried to the cathode chamber through PEM and the electrons pass through the external circuit [7] (Figure 1).

Fig. 1: The MFC System consists of Anode & Cathode Chambers [20]

Metabolism in Microbial Fuel Cell

To understand the bacterial electricity generation, metabolic pathways concerning the microbial electron and proton must be determined first. By the influence of the bio-convertible substrate,

Citation: Poulomi Pramanik and Pranab Roy (2017) An Unending Source of Energy: Microbial Fuel Cells. BAOJ Physics 2: 017. Page 2 of 6

BAOJ Physics, an open access journal Volume 2; Issue 3; 017

the potential of the anode would determine the microbial metabolism. As the increasing MFC current decreases the potential of the anode, the bacteria is forced to deliver electrons through more reduced complex. The potential of anode will determine the redox potential of the bacterial electrons and therefore, metabolism. Different metabolism pathways are recognized based on the anode potential: high redox oxidative metabolism, medium to low redox oxidative metabolism, fermentation.

At high anodic potential, bacteria can use their respiratory chain in oxidative metabolism. The electrons and protons are transported through NADH dehydrogenase, ubiquinone, and cytochrome [8]. This was basically investigated by Kim et al (2005). They observed that the generation of electrical energy from an MFC was inhibited by various inhibitors present in the respiratory chain. The electron transport system in their MFCs used NADH dehydrogenase, Iron/Sulphur proteins and quinones as electron carriers. Processes using oxidative phosphorylation in MFCs yield high energy efficiency, almost 65% [9]. If anode potential decreases in presence of electron acceptors like sulphate or nitrate mediator, electrons are deposited onto these components (Figure 2). Otherwise fermentation is the main process when the anode potential remains low. For example, during fermentation of glucose substrate, possible reactions are:

C6H12O6+2H2O=2CH3COOH+ 2CO2+4H2O

Or,

C6H12O6 = 2H2 + C4H8O2+ 2CO2

This shows that maximum of one-third of hexose substrate electrons can be used to generate electricity theoretically and the rest remains in the produced fermentation products, such as butyrate, acetate [10]. This one-third of electrons are available for electricity generation because of hydrogenase, which is placed at

membrane surface that are possibly accessible from outside by mobile electron shuttle or that connected through an electrode [11]. Several organisms that belong to the genus Clostridium, Alcaligenes, Enterococcus have been isolated from MFCs. An MFC, operated at low external resistance, initially generates low current due to low biomass build-up. Therefore, it contains high anode potential. Upon the growth of culture, metabolic turnover rate, the current generation will gradually increase.

Anodic Electron Transfer Potential in Microbial Fuel Cell

The electrons to be carried towards the electrode need an electron transport system for extracellular electron transfer. This electron transfer ca either happen through the use of soluble electron shuttles [12] or through the proton exchange membrane [13,14].

The oxidative, membrane-bound electron transfer is said to be occurred through the compounds which belongs to the respiratory chian. Good examples of bacteria to use this type of pathway are Geobacter metallireducens [14], Aeromonus hydrophila, Rhodoferux ferrireducens. The prime requirement for a component to act as an electron gateway is likely to be the steric accessibility, a physical contact between electron donor and acceptor [15]. The potential of the gateway to the anode will determine whether the gateway is actually used or not.

Many fermentative bacteria in MFCs possess hydrogenase, for example Clostridium butyricum [15] and Enterococcus faecium [16]. Hydrogenases can directly be involved in transportation of electron towards electrode. Recently, this possibility of electron transfer was observed by McKinley and Zeikus [11]. However, this was done with the combination of a mobile redox shuttle.

Bacteria can use soluble components which physically transport the electron from an intracellular compound, which becomes

Fig. 2: Electron Transport Chain in presence of inhibitors [8]

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BAOJ Physics, an open access journal Volume 2; Issue 3; 017

oxidized, to the electron surface. Redox mediators like neutral red, thionin, methyl viologen were added to the bioreactor to create redox potential. However, bacteria can produce redox mediators themselves which can occur in 2 ways: (1) by the production of organic reducible compounds (Shewanella putrfaciens); (2) by the production of oxidizable metabolites (Escherichia coli).

Energy Available for Electricity Generation

The amount of energy obtained from an electrochemical process may be calculated based on power output and process duration: E=P.t; where P= Power (Watt), t= Time. The power depends on both voltage (V) and current (I): P=V x I. The current and voltage factors are linked by fuel cell resistance, which can be described by Ohm’s law VI=R; where R is resistance. The voltage over resistance can be expressed by: V= E0- ηa- ηc-I x R; where, E0=Maximum Cell Voltage, ηa and ηc = Over potential losses at electrodes, IR = Loss during electrolyte resistance [17].Thus, what is measured in the fuel cell will be lower than the attainable voltage. Practically, it has been observed that the maximal open circuit potentials are in the range of 750-800 mV, when no current is running through MFC. Upon closure of the electrical loop, the voltage decreases significantly chiefly due to the over potentials, which are potential losses owing to electron transfer resistances and internal resistances (Figure 3). There are three kinds of over potentials: (1) Activation over potentials; (2) Ohmic losses; (3) Concentration Polarization. For MFCs, the activation over potentials is a major limiting factor [18]. To a great extent, the activation over potential is dependent on the current density flowing through the anode, the electrochemical properties of the cathode, the presence of mediating compounds and the operating temperature [18,19].

Fig.3: Potential Losses during Electron Transfer in MFC (1) loss during bacterial electron transfer; (2) loss due to electrolyte resistance; (3) losses at the anode; (4) loss at potential difference; (5) losses at cathode; (6) loss owing to electron acceptor reduction.

Bacterial Potential for Electricity Generation

Bacteria gain energy by transferring electrons from a reduced substrate (glucose) at a low potential to an electron acceptor of higher potential (oxygen). The gained energy can be expressed by: ∆G=−n.F.∆E; where, n= numbers of electron exchanged, F= Faraday’s Constant= 96485 coulomb/mol, ∆E=Potential difference between electron donor and acceptor. If bacteria derive reducing compounds from glucose in the form of NADH and shuttle electrons from NADH to oxygen, the potential difference is ~1.2V [∆E=(+0.840V)− (−0.320V)]. The Gibbs energy is to be obtained for 2 molecules per NADH, ∆G= 2 x 102 kJ/mol. If the electron acceptor is sulphate, the potential difference decreases to ~100mV and gives ∆G=2 x 10kJ/mol. Thus, the amount of energy generated for the bacterial growth is very low. If an anode of higher potential than the electron acceptor present in the feed stream, the energetic gain would be much higher for bacteria that can deliver to the anode. Therefore, the anode becomes the preferred electron acceptor [20].

Parameters for the Performance of MFCs

The electricity generated in a microbial fuel cell is dependent on both biological and electrochemical processes.

The Substrate Conversion Rate

The substrate conversion rate depends upon the amounts of bacterial cells, the mixing and mass transfer phenomenon of the reactor, the bacterial growth kinetics, and biomass loading rate, the efficiency of the PEM for transporting protons and overall potential of MFCs [9].

Over potential at the Anode & the Cathode

When the open circuit potential (OCP) of MFC is measured, the OCP range in between 750mV to maximum of 798mV. Parameters influencing the over potentials are electrode surface, electrode potential and the kinetics together with the mechanism of electron transport and the electricity of MFC. Cathode exhibits significant potential losses, similar to anode. To overcome this, several researchers have used hexacyanoferrate solutions [9,22]. But hexacyanoferrate is not totally reoxidized by air, so it can be considered as it can be considered as an electron acceptor rather than a mediator [23].

The Efficiency of Proton Exchange Membrane

Most number of MFC has applied NafionTM proton exchange membranes manufactured by DuPont in 1970. Howsoever, NafionTM

membranes are sensitive to bio-fouling by ammonium and other compounds [16]. Liu et al. (2004) neglected the use of membranes and used only pressed carbon paper as a separator. However, this omission significantly decreased the internal resistance of MFC but it provoked the growth at the cathode based on necessary anolytes

Citation: Poulomi Pramanik and Pranab Roy (2017) An Unending Source of Energy: Microbial Fuel Cells. BAOJ Physics 2: 017. Page 4 of 6

BAOJ Physics, an open access journal Volume 2; Issue 3; 017

and allowed poisoning of cathode catalyst [18,21].

Internal Resistance of Microbial Fuel Cell

It is dependent upon both the resistance of electrolyte between the electrodes and the membrane resistance. It is necessary that anode and cathode should be placed close to one another for optimal operation [18]. Also, proton migration through PEM significantly influences the resistance-related losses. These losses could be minimized if the adequate mixing is done [24].

Applications

The main applications of microbial fuel cells developed in recent decades are categorized into the following forms:

Generation of Bioelectricity

Microbial fuel cell is a very fascinating technology which can utilize a wide variety of substrate (glucose, starch, acetate, and

waste water), materials and system architecture with bacteria and other microorganisms to obtain the bio-energy production despite knowing that the amount of electricity produced is relatively low. It is primarily preferred for sustainable power applications with potential health and also safety issues [25]. The main objective of MFC is to obtain a suitable current and power for the use of small electrical devices (Figure 4).

Bio Hydrogen Production

MFCs can be readily adapted to the production of bio hydrogen instead of generating electricity. The fuel cells supply renewable hydrogen source which can be donated to entire hydrogen demand. To produce hydrogen gas in a typical microbial fuel cell, anodic potential must be increased to an additional voltage of approximately 0.23V and the presence of oxygen in the cathode chamber must be eliminated [26] (Figure 5)

Fig.4: Bioelectricity Generation in MFC [33]

Fig. 5: Biological Hydrogen Production [32]

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BAOJ Physics, an open access journal Volume 2; Issue 3; 017

Waste water Treatment

As an optimal energy source, large amount of potential is hidden in wastewaters including diverse organic substrate [27, 28]. Different kinds of wastewater like sanitary wastes, food processing wastewater, corn stover which contains tremendous amount of energy in the form of biodegradable organic wastes [25]. MFC technology was at first considered to be used in waste water treatment in early 1991. It is favorable as a completely different method, harvesting energy in the form of electricity or hydrogen gas [26]. Scientists have reported that to remove nitrogen and other organic wastes from leachate, biological treatment with the help of MFCs (Figure 5) can be used as a highly cost-effective method [29]. The capability of microbial fuel cell for electricity generation and simultaneously, the removal of salinity from selenium containing waste water were observed and it was said that at higher concentration both power output and Coulombic efficiency (CE) are lower [30].

MFC could be an efficient method of power generation and odour removal, and Kim et al. described the MFC technology which accelerates the rate of removing odour when the electricity generation reaches 228 mW/m2.

A novel MFC-membrane bioreactor, used for waste water treatment has recently been reported to gain a maximum power density of 6.0 W/m3 with the average current of 1.9±0.4mA and to achieve a good pollutant removal performance, attributed to high biomass retention and solid rejection [31].

Limitations

As the MFCs generate very low amount of power, it might not be

enough to drive an electrical device continuously. It is primarily due to the use of microbial cells which can be solved by increasing the surface area of the two electrodes. The power generated could be stored in capacitors for future use.

Another limitation of MFC is that they cannot operate at very low temperatures at which the biochemical reactions are too slow [8].

Conclusion

As the non-renewable energy sources are depleting, energy crisis encouraged researchers in the world to consider alternative energy sources. Besides, the use of these fossil fuels, petroleum, coal and natural gas bring about the environmental pollution. Clean energy, significantly fuel cells and bio-fuels, as new sources of energy without causing any kind of pollution are suitable replacements of conventional or traditional fossil fuel energy. Microbial fuel cells are now evolving into a simple, robust technology. They are individual cells which use the active bio-catalysts (microorganisms and enzymes) to generate electricity. MFCs have become one of the promising technologies to produce energy from various substrates such as organic wastes, waste water etc. Due to this promise of sustainable energy generation, research has been intensified in this field for the past few years. Microbial fuel cells have different usages based on generated power. However, the generated current is still too low and to increase the power output to a stable 1 kW/m3 of reactor, many technological improvements are required [20]. But as the biological understanding increases and the electrochemical technology advances, this MFC technology might qualify as a new technology for electricity generation in future.

Fig. 6(a) Set up of an MFC for the waste water treatment [20] 6(b) Showing the details of the MFC for the waste water treatment

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BAOJ Physics, an open access journal Volume 2; Issue 3; 017

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