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MICROBIAL FUEL CELLS
Introduction:
In an era of climate change, alternate energy sources are desired to replace oil and carbon resources. Subsequently, climate change effects in some areas and the increasing production of biofuels are also putting pressure on available water resources. Microbial Fuel Cells have the potential to simultaneously treat wastewater for reuse and to generate electricity; thereby producing two increasingly scarce resources. While the Microbial Fuel Cell has generated interest in the wastewater treatment field, knowledge is still limited and many fundamental and technical problems remain to be solved Microbial fuel cell technology represents a new form of renewable energy by generating electricity from what would otherwise be considered waste, such as industrial wastes or waste water etc. A microbial fuel cell [Microbial Fuel Cell] is a biological reactor that turns chemical energy present in the bonds of organic compounds into electric energy, through the reactions of microorganism in aerobic conditions.
Construction and Working of Microbial Fuel Cells:
A schematic diagram representing a two chamber microbial fuel cell.
Microbial fuel cell consists of anode and cathode, connected by an external circuit and separated by Proton Exchange Membrane.
Anodic material must be conductive, bio compatible, and chemically stable with substrate. Metal anodes consisting of noncorrosive stainless steel mesh can be utilized, but copper is not useful due to the toxicity of even trace copper ions to bacteria. The simplest materials for anode electrodes are graphite plates or rods as they are relatively inexpensive, easy to handle, and have a defined surface area. Much larger surface areas are achieved with graphite felt electrodes
The most versatile electrode material is carbon, available as compact graphite plates, rods, or granules, as fibrous material (felt, cloth, paper, fibers, foam), and as glassy carbon
Proton Exchange Membrane is usually made up of NAFION or ULTREX.
Microbial Fuel Cells utilise microbial communities to degrade organics found within wastewater and theoretically in any organic waste product; converting stored chemical energy to electrical energy in a single step.
Oxygen is most suitable electron acceptor for an microbial fuel cell due to its high oxidation potential, availability, sustainability and lack of chemical waste product, as the only end product is water.
If acetate is used as substrate, following reaction takes place:
Anodic reaction:
CH3COO- + H2O 2CO2 + 2H+ +8e-
Cathodic reaction:
O2 + 4e- + 4 H+ 2 H2O
Electrons produced by bacteria from these substrates are transferred to anode (negative terminal) and flow to the cathode ( positive terminal) linked by a conductive material.
Protons move to cathodic compartment through Proton Exchange Membrane and complete the circuit. Microbial fuel cells use inorganic mediators to tap into the electron transport chain of cells and steal the electrons that are produced. The mediator crosses the outer cell lipid membranes and plasma wall; it then begins to liberate electrons from the electron transport chain that would normally be taken up by oxygen or other intermediates. The now-reduced mediator exits the cell laden with electrons that it shuttles to an electrode where it deposits them; this electrode becomes the electro-generic anode (negatively charged electrode). The release of the electrons means that the mediator returns to its original oxidised state ready to repeat the process. It is important to note that this can only happen under anaerobic conditions, if oxygen is present then it will collect all the electrons as it has a greater electronegativity than the mediator.
Organic substrates are utilized by microbes as their energies are transferred to electron acceptor( molecular oxygen) in absence of such electron acceptors micro-organisms shuttle electron into anode surface with help of mediators. However few micro-organisms are able to transfer electrons directly to electrode. This type of system is called as Mediator Less Microbial Fuel Cell. Examples of such micro-organisms which are currently available are : shwanella, geobacter etc. Mediator Less Microbial Fuel Cell have more commercial potential as mediators are expensive and sometimes toxic to microorganisms.
Thermodynamics and the Electromotive Force.
Electricity is generated in an Microbial Fuel Cell only if the overall reaction is thermodynamically favorable. The reaction can be evaluated in terms of Gibbs free energy expressed in units of Joules (J), which is a measure of the maximal work that can be derived from the reaction calculated as,
G r = Gr0 + RT(lnπ)
Where, Gr (J) is the Gibbs free energy for the specific conditions, G0r (J) is the Gibbs free energy under standard conditions usually defined as 298.15 K, 1 bar pressure, and 1 M concentration for all species, R (8.31447 J mol-1 K-1) is the universal gas constant, T (K) is the absolute temperature, and π is the reaction quotient calculated as the activities of the products divided by those of reactants. The standard Gibbs free energy is calculated from tabulated energies of formation for organic compounds in water.
For Microbial Fuel Cell calculations, it is more convenient to evaluate the reaction in terms of the overall cell electromotive force (emf), Eemf (V), defined as the potential difference between the cathode and anode. This is related to the work, W(J), produced by cell, or
W = EemfQ = Gr
Where, Q = nF is the charge transferred in the reaction, expressed in coulomb ©, which is determined by the number of electrons exchanged in the reaction, n is the number of electrons per reaction mole and F is Faraday’s constant(9.64853×104 C/mol). Combining these two equations, we have,
Eemf = Gr
nF
If all reactions are evaluated at standard conditions, π = 1, then
E0emf = G0r
nF
where E0emf (V) is the standard cell electromotive force. We can therefore use the above equations to express the overall reaction in terms of the potential as
Eemf = E0emf –RT ln(π)
nF
The advantage of above equation is that it is positive for a favorable reaction e , and directly produces a value of the emf for the reaction. This calculated emf provides an upper limit for the cell voltage; the actual potential derived from the Microbial Fuel Cell will be lower due to various potential losses.
Factors affecting performance of Microbial Fuel Cell
Power density, electrode potential, coulombic efficiency, and energy recovery in single-chamber microbial fuel cells were examined as a function of solution ionic strength, electrode spacing and composition, and temperature.
A series of experiments were conducted to study the individual effects of solution ionic strength, electrode spacing, temperature, and cathode materials on Microbial Fuel Cell performance. In one set of tests, the conductivity of the solution was increased by adding 100 (final ionic strength 200 mM), 200 (ionic strength 300 mM), or 300mM NaCl (ionic strength 400mM)to the medium in order to investigate the effect of ionic strength on power generation. At the highest (400 mM) and lowest (100 mM) solution ionic strength, the electrode spacing was changed from 4 to 2 cm. Temperature was reduced from 32 to 20 °C, and the cathode material was changed from the carbon paper to the carbon cloth electrode.
• Effect of Ionic Strength. A maximum power of 720 mW/ m2 was obtained at a current density of 0.26 mA/cm2 using the This increase was likely a consequence of a decrease in the operation time due to faster substrate utilization, resulting in less oxygen transfer into the chamber before exhaustion of the substrate. Coulombic efficiency also increased slightly with NaCl addition, reaching a maximum of 61% at a current density of 0.51 mA/ cm2 (IS ) 400 mM). The overall energy recovery, which represents the energy harvested as electricity from bacteria versus that lost to other processes, also increased with ionic strength from 6.9-9.6% (0.11-0.36 mA/cm2; IS ) 100 mM) to 12.9-15.1% (0.20-0.51 mA/cm2; IS ) 400 mM
Effect of ionic strength (100-400 mM) on power generation at different current densities (electrode space: 4 cm; 32C).
• Effect of Temperature. The maximum power density was reduced to only 660 mW/m2 (current density of 0.22 mA/cm2, 200Ω) when the Microbial Fuel Cell was operated at 20 °C, which was only 9% less than that for the Microbial Fuel Cell at 32 °C (720mW/m2). Bacterial activities are well-known to be affected by temperature, with biological processes often modeled as an empirical function of temperature as θ(T-20), where θ = 1.20 for microbial growth under anoxic conditions and 1.094 for heterotrophs, and T is the temperature in Celsius. The observed difference by a factor of 1.1, versus factors of 2.9 to 8.9 predicted by this equation (relative to 20 °C), suggests that either the bacteria were not growing under optimal conditions at the higher temperature or that factors other than bacterial growth, such as the diffusion of substrate or products, limited electricity generation. Decreasing the temperature did not affect the anode working potential over a current range of 0.11 to 0.36 mA/cm2. The cathode working potential of Microbial Fuel Cell operated at 20 °C was also comparable to that operated at 32 °C for current densities in the range of 0.11-0.23 mA/cm2. However, at higher current densities (>0.24 mA/cm2), the cathode potential at 20 °C was lower than that at 32 °C. Thus, this suggests that the performance of the cathode was the main factor affecting power generation at higher current density.
Effect of temperature (20 and 32 C) on power generation (A) and electrode potential (vs Ag/AgCl reference electrode; 195 mV vs NHE) (B) at different current density (IS: 100 mM; electrode space: 4 cm).
• Effect of Cathode Material. By replacing the carbon paper with a carbon cloth electrode, the maximum power density was increased from 660 mW/m2 (0.22 mA/cm2) to 1114 mW/m2 (0.33 mA/cm2), or an overall increase of 69% at 20 °C. This increase in power production was reflected by a significant increase in the cathode potential using the carbon cloth, while the anode potentials were essentially unchanged in the current density range of 0.07-0.39 mA/cm2. Coulombic efficiency increased with current density for both cathode materials, similar to that found in previous tests but under different conditions, ranging from 17 to 45% (0.10-0.36 mA/cm2) using the carbon paper cathode, and from 22 to 52% (0.09-0.50 mA/cm2) with the carbon cloth cathode. A similar energy recovery (9.2%) was observed at a current density of 0.21 mA/cm2 for both materials. However, at a higher current density of 0.27 to 0.50 mA/cm2, energy recovery was greater (6.8-9.0%) using the carbon cloth cathode than with the carbon paper cathode (4.6-8.8%).