Abstract

Renewable and clean forms of energy are one of society's greatest needs.  At the same time, 2 billion people in the world lack adequate sanitation and the economic means to afford it. Abundant energy, stored primarily in the carbohydrates can be found in waste biomass, agricultural, municipal and industrial sources as well as in dedicated energy crops. A microbial fuel cell is a device that directly converts the metabolic and enzyme catalytic energy to electricity by using conventional electrochemical technology. Chemical energy can be converted into electricity by coupling of biocatalytic oxidation of organic or inorganic compounds to the chemical reduction of an oxidant at the interface between cathode and anode.

Key words: biohydrogen, MFC, biocatalyst.

Introduction:

Typically, heat generated by burning of solid biomass, biogas or biofuels can be converted into electricity by conventional means. In the conventional methods two steps are involved in electricity generation, firstly heat generation by combustion of fuels and secondly the spinning of turbine with the help of that heat produced. Combustion of carbonaceous fuels contributes green house gases to the environment causes adverse change in climate. Also the combustion efficiency restricts the overall efficiency of electricity generation. The spinning of turbine is the mechanical process which converts the heat energy to electrical energy again limited by its conversion efficiency. These drawbacks of the conventional methods shift the focuses of electricity generation in a direct way, which is microbial fuel cell.

Microbial Fuel Cells (MFC) are capable of converting energy, available in bio-convertible substrate, directly into electricity with the help of bacteria. These bacteria switch from the natural electron acceptor, such as oxygen or nitrogen, to an insoluble acceptor, such as MFC anode. This transfer can either occur by membrane-associated components or by soluble electron shuttles. The electron then flow through a resistor to a cathode, at which the electron acceptor is reduced. In contrast to anaerobic digestion, a MFC creates electrical current and an off-gas containing mainly carbon dioxide instead of an energy- reach gas such as methane or hydrogen. The concept of using microorganism as catalyst in fuel cell was explored starting in the 1970s.

Microbial fuel cells treating domestic waste-water were presented in 1992 [1-2]. A schematic diagram of MFC is as follows.

Fig.1. Schematic diagram of typical microbial fuel cell

When bacteria are placed in the anode chamber of a specially-designed fuel cell that is free of oxygen, they attach to an electrode.  Because they do not have oxygen, they must transfer the electrons that they obtain from consumption (oxidation) of their food somewhere else than to oxygen- they transfer them to the electrode. In a MFC these electrons therefore go to the anode, while the counter electrode (the cathode) is exposed to oxygen. At the cathode the electrons, oxygen and protons combine to form only water.  The two electrodes are at different potentials (about 0.5 V), creating a bio-batter (if the system is not refilled) or a fuel cell (if we constantly put in new food or "fuel" for the bacteria).

Mainly the generation of electricity with the help of microorganisms involves three steps.

•         Breaking of carbonaceous source by Microorganisms.

•         Selective movement of ions into cathode and anode.

•         Neutralization of ions in electrodes within the cell.

The reactions occurred in the microbial fuel cell are as follows,

Anode reaction,
Glucose + 2NAD+ (Embden-Meyerhof pathway)= 2 Pyruvate + 2NADH.

Pyruvate + Ferredoxin (ox) (Pyruvate-ferredoxin Oxidoreductase) = Acetyl-CoA + CO2 + Ferredoxin( red).

NADH + Ferredoxin (ox) (NADH-Ferredoxin Oxidoreductase) = NAD+ + Ferredoxin( red.)
Ferredoxin(red) (Hydrogenase)Ferredoxin (ox) + 2H+ + 2e-.

Cathode reaction,
4 H+  + 4e- + O2 = H2O

The maximum electrical work obtainable from microbial fuel cell is equal to the Gibb's free energy change. The overall reaction of a MFC is given by,

C₆H₁₂O₆ + 6O₂ = 6 C O₂ + 6 H₂O                      DG° = - 2870 kJ

Where DG° is Gibb's free energy change.

Similar to that of a galvanic cell, the change in energy and entropy, the heat energy dispersed or absorbed and the useful energy produced or consumed in a MFC system is subject to the laws of thermodynamics (Rossini 1950). For this reason, limiting a thermodynamic analysis to known reversible chemical reactions that take place within the MFC simplifies calculations. However, this limits the thermodynamic analysis to that of the second law efficiency calculations instead of including first law efficiency calculations. Given a known reversible chemical reaction, a calculation of the Gibbs free energy can be expressed as

ΔG_r = ΔG⁰_r + RT ln(Π)

where

ΔG_r = Gibbs free energy

ΔG⁰_r = Gibbs free energy under standard conditions

R = universal gas constant

T = absolute temperature

Π = reaction quotient of the products divided by the reactants

According to researchers of the Logan Group, the Gibbs free energy under standard conditions is calculated from the tabulated energies associated with the formation for organic compounds in aqueous solutions. The negative value of the Gibbs free energy is known as the maximum work of the system and can be deduced to terms of the overall cell electromotive force (emf) as follows,

− ΔG_r = Wmax = Eemf · (Q) = Eemf · (n · F)

where

Wmax = maximum theoretical work

Eemf = potential difference between the cathode and anode

Q = charge

n = number of electrons per reaction

F = Faraday's constant

Rearranging the above equation yields,

Eemf = − ΔG_r /(n · F)

And under standard conditions,

E⁰_emf = − ΔG⁰_r/(n · F)

Using the afore mentioned equations, an expression for the overall electromotive force of a particular reaction at any condition can then be calculated as,

Eemf = E⁰_emf - (RT/nF).ln(Π )

The previous research indicates that generic equation for the electromotive force could be used for each half-reaction that takes place at the cathode and at the anode. The amount of research needed to evaluate every half-reaction that takes place would be beyond the scope of this project. In general, the electromotive force of the MFC, under specific conditions, can then be calculated as,

Eemf = Ecathode − Eanode

where

Ecathode = electromotive force of a specific reaction taking place at the cathode

Eanode = electromotive force of a specific reaction taking place at the anode

The MFC second law efficiency can be evaluated by relating the theoretical electromotive force to the measured cell potential based on the assumption that the simple reactions evaluated at the anode and cathode are similar to that of the more complicated reactions involved with the bio-degradation of wastewater,

η_MFC =Wactual/Wmax =Vmeasured · (n · F)/Eemf · (n · F) = Vmeasured/Eemf

Where

η_MFC = MFC second law efficiency

Wactual = actual work output

Vmeasured = measure voltage potential[3].

Material and Methods:

Microorganisms and growth

The sample, collected from the dustbin soil was inoculated in distilled water and sedimentation of soil particle was allowed. Then the water above the settling was spread on the MacConkey agar. Then the plate is incubated for 16-20 hrs to find the colonies grew from the sample, which gave colony of pink color, was isolated from plate. These cultures were taken for microbial identification test, like Indole test, Methyl red test, Vogus- Prosceur test and citrate test. These tests reveals that the pink colony probably contain Enterobacter cloacae. Further confirmation is strongly recommended and will be carried out during the project.

The colony was then inoculated in 500 ml of growth media (i.e. nutrient broth) and then incubated for 20 hrs in 37°C and 200 rpm. After each 2 hrs 50 ml sample was collected and OD was taken against blank (water) in 560 nm. Then the sample's weight was taken by centrifuging in 4000 rpm in 27°C, and further discarding supernatant, only keeping pellet. Then a curve was constructed i.e. Time Vs. OD and weight. From this graph the log phase was determined (i.e. 8 Hrs).

Next, the microbes were transferred to production media MYG (1% Malt Extract, 0.4% Yeast Extract and 1% Glucose) from 8 hrs culture of growth media.  The produced gas was collected by passing through KOH and replacing H₂O in a gas collector. The produced gas was taken for Gas chromatography, where presence of Hydrogen was observed. Then the production media was optimized for different percentage of compositions. After confirmation of hydrogen production by the isolated microorganisms it was transferred into a microbial fuel cell.

Analytical method

The composition of the gas was analyzed by thermal conductivity detector (GC) using a 80/100 Porapak-Q (3.2 mm diam X 2 m) column. The oven, injector and detector were held at 80⁰C, 150⁰C and 200⁰C respectively. N2 was used as the carrier gas at 20 ml/min. For each batch the gas samples were analyzed twice. Biomass was measured turbid metrically at 600 nm. Reducing sugar (glucose) content of the medium was estimated spectrophotometrically using DNS dinitosalicylic acid method.[4-6]

Results and Discussion.

Studies reveal that the electricity generation in MFC solely depends on the movement of cations (H⁺) to the cathode and neutralization of cations (H⁺) in the cathode. Production of cations (H⁺) as well as hydrogen by microorganisms is the important part of the overall process. Maximum production of hydrogen may maximize the number of cation (H⁺) as well as its movement. Offering the suitable conditions to the fermentation process, maximization of hydrogen production is possible. Present experiments are focusing on this hydrogen production.

Table1. reveals that the lag phase from the growth characteristics was 1 h. The growth phase was 4-14 h. The above growth characteristic shows that the organisms should transfer into the production media after overnight culture. The temperature of the production media was optimized at 37±1^oC. The composition of production media was optimized as Glucose 1%, Malt extract 1% and Yeast Extract 0.4%. The glucose above 1% shows substrate inhibition as the production of hydrogen decreased. 25ml of 12 hrs culture of microorganisms was inoculated with 250 ml of above production media produces 77 ml of hydrogen in overnight production.

Conclusion:

Among various processes of electricity generation, microbial technique is one of the promising avenues as it does not emit any green house gases. MFC does not contain any rotating parts so it gives continuous electricity without mechanical damage of the equipment and its maintenance cost is minimal. Practical application of the MFC requires extensive research work and awareness among the research groups all over the world.

Reference:

1.Fuel cell handbook -07.

2. Application of Microbial Fuel Cell technology for a Waste Water Treatment Alternative. Eric A. Zielke, February 15, 2006.

3. Thermodynamic Analysis of a single chamber Microbial Fuel Cell,Eric A. Zielke,May 5, 2006.

4.Improvement of biohydrogen production under decreased partial pressure of H2 by Enterobacter cloacae by Biswajit Mandal , Kaushik Nath , Debabrata Das.

5. Microbial fuel cell - Logan.

6. http//www.engr.psu.edu/ce/enve/