MICROBIAL FUEL CELL: NEED OF CLEAN ENVIRONMENT

Abstract

Renewable and clean forms of energy are one of society's greatest needs. Microbial

fuel cells (MFCs) represent a completely new method of renewable energy recovery

and are a new aspect of environmental engineering: the direct conversion of organic

matter to electricity using bacteria and also treating wastewater. Like conventional

methods of power generation it does not produce any toxic gases (CO, SO2) or

chemicals in environment. It neither requires any chemicals nor modified organisms

for electricity generation, it is completely environment friendly. Also, it makes the

biological treatment more feasible and simpler and recovers the energy (i.e. generates

electricity). MFC in wastewater treatment are expected to generate much less excess

sludge than the conventional process, as the major part of energy stored in organic

wastes is converted to electricity, and the remaining energy is utilized for microbial

growth.

Fuel cells operate in principle similar to battery: they convert fuel to electricity by

electrochemical means. However, unlike a battery, a fuel cell needn’t be recharged. In

the long term more dilute substrates, such as domestic sewage, could be treated with

MFCs, decreasing society’s need to invest substantial amounts of energy in their

treatment. The growing pressure on our environment, and the call for renewable

energy sources will further stimulate development of this technology, leading soon we

hope to its successful implementation.

1. INTRODUCTION

Our needs have always been mother of all inventions. The clean and renewable

energy forms are our greatest needs at present. The growing concern of scientists and

environmentalists to generate an alternative source of energy via some natural sources

that is sustainable, eco-friendly and is economical opens the way for production of

Microbial Fuel Cell (MFC). Microbial fuel cell is a biobattery, capable of harvesting

electricity using various species of microbes (generally metal reducing) and is

environmentally safe. It has been described that microbial respiration can be exploited

to generate electricity, which can help prevent from several problems arising from

CO2 releasing processes.

2. ADVANTAGES OF MFC OVER OTHER CONVENTIONAL METHODS

1. Direct conversion of substrate energy to electricity.

2. MFC operate efficiently at ambient, and even at low temperature.

3. It doesn’t require the use of any toxic or heavy metals or metalloid and even

doesn’t produce any toxic substances. Favours environment.

4. It doesn’t require gas treatment. No energy input is needed for aeration.

3. INSTRUMENTATION AND MATERIALS

MFCs are being constructed using a variety of materials, and in an ever increasing

diversity of configurations. Each material shows a different efficiency rate. The basic

requirement of a MFC is mentioned below:

I) Proton Exchange Membrane:

PEM is not absolutely necessary for a single chambered microbial fuel cell

design or naturally separated systems like sediment MFCs. It has other benefits like

preventing micro organisms from crossing to the cathode. Some common materials

studied for PEM are:

Nafion-117, a PEM, I.C.E. 450, a cation exchange membrane, and Hybond™-N.

Nafion-117 transfers H+ across the PEM to the cathode, but does not allow electrons

to cross. Other important material is nanowires which are yet to discover.

II) ANODE:

Anodic materials must be conductive, biocompatible, and chemically stable in the

reactor solution. Metal anodes consisting of non corrosive stainless steel mesh can be

utilized. Copper can’t be used even in trace as it is harmful to microorganisms. Thus

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. Commercial scale-up for carbon fibers paper cells hence carbon fiber brushes

which are electrically conducting are in use.

III) CATHODE

Oxygen is the most suitable electron acceptor for an MFC due to its high

oxidation potential, availability, low cost (it is free), sustainability, and the lack of a

chemical waste product (water is formed as the only end product). Due to its good

performance ferricyanide (K3[Fe(CN)6]) is popular as an experimental electron

acceptor in microbial fuel cells. Pt is an expensive component for MFC construction,

cheaper alternatives such as Co-tetramethyl phenylporphyrin (CoTMPP) can be used

which makes only small changes in MFC performance.

IV) SHUTTLE AND MEDIATORS:

The addition of electron shuttling compounds such as neutral red, thionin, methyl

viologen, and phenazine ethosulfate are effective in electron transfer from

microorganism to electrode. These electron shuttles accept electrons from intracellular

and membrane-bound redox proteins and transfer the electrons to the electrode

surface, with the regeneration of the oxidized form of the shuttle. Microorganisms,

such as Shewanella and Geothrix species, produce their own electron shuttles and it

has been suggested that a microorganism producing an electron shuttle will have the

advantage that it can be positioned at a distance from the electrode and yet still

transfer electrons to the electrode surface. Pili produced by some bacteria have so far

been shown to be electrically conductive using scanning tunnelling electron

microscopy.

V) MICROORGANISMS USED:

Various species of bacteria have been experimented to yield higher output. Several

micro organisms are selected on the basis of the reaction or metabolic property bear.

The choice of such organism is done in such a way that it doesn’t interfere with

reactions on the other side by producing some toxic substances.

METABOLIC TYPE TRANSFER TYPE EXAMPLES OF

ORGANISM

OXIDATIVE

METABOLISM

MEMBRANE DRIVEN

MEDIATOR DRIVEN

Rhodoferax ferriducens,

Geobacter

sulfurreducens,Aeromonas

hydrophila

Escherichia coli,

Shewanella putrefaciens,

Pseudomonas aeruginosa

Desulfovibrio

desulfuricans

FERMENTATIVE

METABOLISM

MEMBRANE DRIVEN

MEDIATOR DRIVEN

Clostridium butyricum

Enterococcus faecium

POWER: The overall performance of an MFC is evaluated in many ways, but

principally through power output and Coulombic efficiency. Power is calculated as

P =IEcell

Normally the voltage is measured across a fixed external resistor (Rext), while the

current is calculated from Ohm’s law (I ) Ecell/Rext). Thus, power is usually calculated

as

P =E2cell/Rex

BASIC DESIGN OF MICROBIAL FUEL CELL

5. CLASSIFICATION OF MFC:

As mentioned above this is the technology which aims to produce energy using

materials those which would be at once flushed away as waste. Several types of

MFCs are developed till date, the common design single chambered or double

chambered.

I) DOUBLE CHAMBERED MFC:

A typical microbial fuel cell (MFC) consists of two separate chambers which can be

inoculated with any type of liquid media. An anaerobic anode is placed in one

chamber and the anaerobic cathode in other chamber these are generally separated by

a Proton Exchange Membrane (PEM) such as Nafion (Oh and Logan 2004). On this

basis, MFC can be classified into two types: Mediator using and Mediatorless MFC.

Mediator using MFC generates electricity from the addition of artificial electron

shuttles (mediators) to Microbial Fuel Cell technology (Zielke 4) accomplish electron

transfer to the electrode. Some exogenous mediators (i.e. ones external to the cell) in

use are such as potassium ferricyanide, thionine, or neutral red.

Another type of MFC is a mediatorless, which does not require these additions of

exogenous chemicals. These are more efficient, cheaper, requires no exogenous

mediator, chemicals toxic to the micro organism. Shewanella putrefaciens, Geobacter

sulfurreducens, Geobacter metallireducens and Rhodoferax ferrireducens are

organisms used in mediatorless MFC. Bacteria present in mediatorless MFCs have

electrochemically active redox enzymes on their outer membranes that transfer the

electrons to external materials.

II) SINGLE CHAMBERED MFC:

In hydrogen fuel cells, the cathode is bonded directly to a Proton Exchange

Membrane which allows for oxygen from the air to directly react at the electrode.

This same principle can be used to design a single chamber MFC (SCMFC) where the

anode chamber is separated from the air-cathode chamber by a gas diffusion layer

(GDL) allowing for a passive oxygen transfer to the cathode, eliminating the need for

energy intensive air sparging of the liquid (Lui and Logan 2004).

6. TYPES OF MFC:

MFC are of several kinds on the basis of the substrate they utilize for energy

generation. Some of them are mentioned below:

1. Wastewater MFC: MFCs using wastewater is the most important one and biggest

area of concern now because this can really utilize the organic waste, treat the waste

water, requires no other special bacteria; give immediate economic benefit to

communities and industries by reducing or even eliminating net costs for wastewater

treatment required. To treat wastewater, process is based on electricity production

directly from organic matter in wastewater.  This approach is based on the anaerobic

oxidation of organic matter in a mediator-less microbial fuel cell (MFC). We can

produce electricity from ordinary domestic wastewater and also from animal

wastewaters.

2. Marine organic waste MFC: This microbial fuel cell that can harvest electricity

from the organic matter stored in marine sediments had demonstrated the feasibility of

producing useful amounts of electricity in remote environments such as the bottom of

the ocean. These systems are now known as Benthic Unattended Generators or BUGs.

BUGs consist of an anode buried in anoxic marine sediments connected to a cathode

suspended in the overlying aerobic water.

3. Fermentation products MFC: Here fuel cells oxidize fermentation products

(hydrogen, methanol, etc.) on electro catalytic electrodes, that is, electrodes that have

been chemically modified to oxidize such metabolites. These are capable of carbon

containing substrates.

4. Metal reducing bacterial MFC: It uses metal reducing bacteria like R. ferrireducens,

Geobacteraceae. These have ability to directly transfer electrons to the surface of

electrodes. MFCs using this organism are superior to many other MFC.

7. FACTORS EFFECTNG PERFORMANCE OF MFC:

Several factors affect MFC performance with respect to power generation. The factors

to be taken care during construction are:

1. Thickness of the anode compartment (less than 2 cm).

2. Larger membrane area (larger the membrane are, higher the energy output).

3. Smaller size of the fuel cell improves the efficient.

4. Other important factors are microbial inoculums, chemical substrate, cell internal

and external resistance, solution ionic strength, electrode materials, and electrode

spacing,

8. LIMITING FACTORS:

In MFC systems, the activity of biocatalysts, electron transfer between the bacteria

and the anode, internal resistance and overpotentials at both electrodes are the main

limiting factors. In addition there are also:

I) Electron Transfer

The mechanism of bacterial electron transfer to the anodic electrode and the issue of

how to improve the electron transfer have been and are still the focus of much

controversy. As mentioned earlier, a hypothesis exists describing a direct electron

transfer in which some outer-membrane bound proteins, such as cytochromes, play

the role of transferring electrons to the electrode.

II) Internal Resistance

This is a common problem that MFC designers face. A high internal resistance causes

a considerable potential drop due to ohmic losses. With or without the PEM, the

internal resistance still remains a limiting factor. Upon increasing the MFC size, the

internal resistance will at best remain at the same level; while the current flowing

through the system increases and causes considerable potential lose

III) Cathode Reaction

The cathode reaction is considered to be one of the key factors limiting the

performance of an MFC. In many MFC systems, oxygen is the cathodic electron

acceptor but usually the poor contact between gaseous oxygen and the cathode, and

the imperfect catalysis of the reaction limits the turnover rate. In addition, oxygen

leaking to the anode can occur in such systems reducing electricity generation

efficiency.

9. AREAS OF APPLICTIONS:

Areas of application of MFC are several; it is not confined to a single field or purpose.

MFCs can also be used to treat high concentration COD substrates. A unique niche of

the MFC technology could be the combination of energy generation without any toxic

gas liberation, water production and possibly hydrogen production.

1. Waste water treatment: MFC utilizes waste water as substrate and the microbial

community used is the one present in the waste water itself for treating the water. It

decreases the treatment cost and power consumption used in treatment of water. This

can treat waste water and drive clean water. Food processing wastewaters and digester

effluents are good candidates; to examine the potential for electricity generation at

such a site, consider a food processing plant producing 7500 kg/d of waste organics in

an effluent.

2. Removal of sulphide: It can also remove sulphide. Sulfur compounds are

ubiquitously present in organic waste and wastewater. Sulfur is utilized in the MFC.

MFC with a hexacyanoferrate cathodic electrolyte is found to convert dissolved

sulfide to elemental sulfur.

3. Bioremediation: An MFC can be modified in interesting and useful ways, and this

can lead to new types of fuel-cell-based technologies. Monitoring groundwater

plumes undergoing natural attenuation or bioremediation is costly and time

consuming. Target contaminants include heavy metals and chlorinated ethenes and

other compounds that undergo reductive biotransformation.

10. CONCLUSION

MFC is an answer to all the problems caused by other methods of power generation.

MFC, which converts biochemical to electrical energy, may be part of the picture of

clean environment. MFCs could be used in biomass-based energy production.

Although MFC is an attractive technology many technical challenges must be

overcome before they will be practical for renewable energy production. Further

research is required to overcome some technical limitations and give world a safe and

pollution free energy source.