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.
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:
Cathodic
reaction:
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 (C),
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
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 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 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%).
·
Effect of Electrode Spacing. The
effect of electrode spacing on Microbial Fuel Cell performance was investigated
by reducing the distance between the anode and cathode from 4 to 2 cm. The
maximum power density increased from 720 to 1210 mW/m2 when the electrode distance
was decreased to 2 cm (ionic strength 100 mM). This increase in power density
corresponded to a decrease of internal resistance from 161 to 77 Ω when the
electrode spacing was reduced from 4 to 2 cm. No further improvement in power
generation was observed if the medium ionic strength was increased to 400mM,
because there was little change in internal resistance. The internal resistance
was 71Ω for an electrode spacing of 2 cm (ionic strength ) 400 mM), which is
only 10% lower than that obtained under the same conditions but with a 4-cm
electrode spacing(79 Ω). Improvements on both the cathode and anode potentials
were seen with a decrease in the electrode spacing with the low ionic strength
solution (ionic strength 100mM),while no improvement was observed when the
solution ionic strength was increased to 400mM. Coulombic efficiency and energy
recovery were also both improved by decreasing the electrode spacing when the
low ionic strength solution was used (ionic strength ) 100 mM;). However, the
coulombic efficiency and energy recovery were not affected when using the
higher ionic strength solution (ionic strength 400 mM).
Metabolism In Microbial Fuel Cells:
To
assess bacterial electricity generation, metabolic pathways governing microbial
electron and proton flows must be determined. In addition to the influence of the
substrate the potential of the anode will also determine the bacterial
metabolism. Increasing MFC current will decrease the potential of the anode,
forcing the bacteria to deliver the electrons through more-reduced complexes.
The potential of the anode will therefore determine the redox potential of the
final bacterial electron shuttle, and therefore, the metabolism. Several
different metabolism routes can be distinguished based on the anode potential:
high redox oxidative metabolism; medium to low redox oxidative metabolism; and
fermentation. Hence, the organisms reported to date in MFCs vary from aerobes
and facultative anaerobes towards strict anaerobes. At high anodic potentials, bacteria can use the respiratory chain
in an oxidative metabolism. Electrons and, concomitantly, protons can be
transported through the NADH dehydrogenase, ubiquinone, coenzyme Q or
cytochrome. The use of this pathway was investigated. They observed that the
generation of electrical current from an MFC was inhibited by various
inhibitors of the respiratory chain. The electron transport system in their MFC
used NADH dehydrogenase, Fe/S (iron/sulphur) proteins and quinines as electron
carriers, but does not use site 2 of the electron transport chain or the
terminal oxidase. Processes using oxidative phosphorylation have regularly been
observed in MFCs, yielding high energy efficiencies of up to 65%. Examples are
consortia containing Pseudomonas aeruginosa, Enterococcus faecium and
Rhodoferax ferrireducens. An overview of different bacterial species and their
(putative) electron transport pathway is given in. If the anode potential
decreases in the presence of alternative electron acceptors such as sulphate,
the electrons are likely to be deposited onto these components. Methane
production has repeatedly been observed when the inoculum was anaerobic sludge
[, indicating that the bacteria do not use the anode. If no sulphate, nitrate
or other electron acceptors are present, fermentation will be the main process when
the anode potential remains low. For example, during fermentation of glucose,
possible reactions can be:
This
shows that a maximum of one-third of a hexose substrate electrons can theoretically
be used to generate current, whereas two thirds remain in the produced fermentation
products such as acetate and butyrate.The one-third of the total electrons are
possibly available for electricity generation because the hydrogenases, which
generally use the electrons to produce hydrogen gas, are often situated at
places on the membrane surface that are accessible from outside by mobile electron
shuttles or that connect directly to the electrode. As repeatedly observed,
this metabolic type can imply a high acetate or butyrate production. This
pathway is further substantiated by the significant hydrogen production observed
when MFC enriched cultures are incubated anaerobically in a separate
fermentation test.
Micro-Organisms
·
Axenic bacterial cultures
Some bacterial species in MFCs, of which metal-reducing bacterial
are the most important, have recently been reported to directly transfer
electrons to the anode. Metal-reducing bacteria are commonly found in
sediments, where they use insoluble electron acceptors such as Fe (III) and Mn
(IV). Specific cytochromes at the outside of the cell membrane make Shewanella
putrefaciens electrochemically active in case it is grown under anaerobic
conditions. The same holds true for bacteria of the family Geobacteraceae,
which have been reported to form a biofilm on the anode surface in MFCs and to
transfer the electrons from acetate with high efficiency.
Rhodoferax species
isolated from an anoxic sediment were able to efficiently transfer electrons to
a graphite anode using glucose as a sole carbon source. Remarkably, this bacterium is the first reported strain that can
completely mineralize glucose to CO2 while concomitantly generating
electricity at 90% efficiency.
In terms of performance, current densities in the order of 0.2-0.6mA and a total power density of 1-17 mW/m2 graphite
surface have been reported for Shewanella putrefaciens, Geobacter
sulfurreducens and Rhodoferax ferrireducens at conventional (woven)
graphite electrodes (Bond and Lovley 2003, Chaudhuri and Lovley 2003, Kim et
al. 2002) (Table 20.4). However, in case woven graphite in the Rhodoferax
study was replaced by highly porous graphite electrodes, the current and
power output was increased up to 74 mA/m2 and 33 mW/m2,
respectively.
Although these bacteria generally show high electron transfer
efficiency, they have a slow growth rate, a high substrate specificity (mostly
acetate or lactate) and relatively low energy transfer efficiency compared to
mixed cultures. Furthermore, the use of a pure culture implies a continuous
risk of contamination of the MFCs with undesired bacteria.
·
Mixed
bacterial cultures
MFCs that make use of mixed bacterial cultures have some important
advantages over MFCs driven by axenic cultures: higher resistance against
process disturbances, higher substrate consumption rates, smaller substrate
specificity and higher power output. Mostly, the electrochemically active mixed
cultures are enriched either from sediment (both marine and lake sediment) or
activated sludge from wastewater treatment plants. By means of molecular
analysis, electrochemically active species of Geobacteraceae, Desulfuromonas,
Alcaligenes faecalis, Enterococcus faecium, Pseudomonas
aeruginosa, Proteobacteria, Clostridia, Bacteroides and
Aeromonas species were detected in the before-mentioned studies. Most
remarkably, the study of Kim et al. (2004) also showed the presence of
nitrogen fixing bacteria (e.g., Azoarcus and Azospirillum)
amongst the electrochemically active bacterial populations. The study of Rabaey
et al. (2004a) showed that by starting from methanogenic sludge and by
continuously harvesting the anodic populations over a 5-month period using
glucose as carbon source, an electrochemically active consortium can be
obtained that mainly consists of facultative anaerobic bacteria (e.g. Alcaligenes,
Enterococcus and Pseudomonas species). In this particular study,
very high glucose-to-power efficiencies could be reached in the order of 80% .
.
To estimate the power
per unit surface to putative power output per unit reactor volume, one can take
into account that at present some 100-500 m2 of anode surface can be
installed per m3 anodic reactor volume. Hence, the state of the art power
supply ranges from approximately 1 to 1800 W per m3 anode reactor volume
installed.
To render the anode more susceptible for receiving electrons from
the bacteria, electrochemically active compounds can be incorporated in the
electrode material. This approach has been investigated by Park and Zeikus
(2003), who incorporated dyes such as neutral red and metals such as Mn4+ into
Fe3+ containing
graphite anodes. In this way, the main disadvantages of mediators in solution,
namely toxicity and degradation, can thus be circumvented since the mediator is
not released from the electrode material and thus has a longer life time.
Moreover, bacteria are still able to form a biofilm on the modified anode
surface.
Microbial
Fuel Cell Designs
Many different
configurations are possible for Microbial Fuel Fells. A widely used and
inexpensive design is a two chamber Microbial Fuel Fell built in a traditional
“H” shape, consisting usually of two bottles connected by a tube containing a separator
which is usually a cation exchange membrane (CEM) such as Nafion or Ultrex, or
a plain salt bridge. The key to this design is to choose a membrane that allows
protons to pass between the chambers (the CEM is also called a proton exchange membrane,
PEM), but optimally not the substrate or electron acceptor in the cathode
chamber (typically oxygen). In the H-configuration, the membrane is clamped in
the middle of the tubes connecting the bottle . However, the tube itself is not
needed. As long as the two chambers are kept separated, they can be pressed up
onto either side of the membrane and clamped together to form a large surface.
An inexpensive way to join the bottles is to use a glass tube that is heated
and bent into a U-shape, filled with agar and salt (to serve the same function
as a cation exchange membrane), and inserted through the lid of each bottle .
The salt bridge Microbial Fuel Fell, however, produces little power due the
high internal resistance observed. H-shape systems are acceptable for basic
parameter research, such as examining power production using new materials, or
types of microbial communities that arise during the degradation of specific
compounds, but they typically produce low power densities. The amount of power
that is generated in these systems is affected by the surface area of the
cathode relative to that of the anode
and the surface of the membrane . The power density (P) produced
by these systems is typically limited by high internal resistance and electrode-based
losses. When comparing power produced by these systems, it makes the most sense
to compare them on the basis of equally sized anodes, cathodes, and membranes. Using
ferricyanide as the electron acceptor in the cathode chamber increases the
power density due to the availability of a good electron acceptor at high
concentrations. Ferricyanide increased power by 1.5 to 1.8 times compared to a Pt-catalyst
cathode and dissolved oxygen (H-design reactor with a Nafion CEM) . The highest
power densities so far reported for MFC systems have been low internal
resistance systems with ferricyanide at the cathode . While ferricyanide is an
excellent catholyte in terms of system performance, it must be chemically
regenerated and its use is not sustainable in practice. Thus, the use of
ferricyanide is restricted to fundamental laboratory studies.
MODIFICATIONS IN MICROBIAL FUEL CELL
·
Mediator Less
Microbial Fuel Cell It has recently been shown that certain
metal-reducing bacteria, belonging primarily to the family Geobacteraceae can
directly transfer electrons to electrodes using electrochemically active redox
enzymes, such as cytochromes on their outer membrane12,13. These microbial fuel
cells does not need mediator for electron transfer to electrodes and are called
as mediator less Microbial Fuel Cells. Mediator less Microbial Fuel Cells are
considered to have more commercial application potential, because mediators
used in Biofuel cells are expensive and can be toxic to the microorganisms. In
a Microbial Fuel Cell, two electrodes (anode and cathode) are placed in water
in two compartments separated by a proton exchange membrane (PEM). Most studies
have used electrodes of solid graphite, graphite-felt, carbon cloth and
platinum coated graphite cathode electrode. Microbes in the anode compartment
oxidize fuel (electron donor) generating electrons and protons. Electrons are
transferred to the cathode compartment through the external circuit, and the
protons through the membrane. Electrons and protons are consumed in the cathode
compartment reducing oxygen to water.
In addition to microorganisms that can transfer
electrons to the anode, the presence of other organisms appears to benefit Microbial
Fuel Cell performance. It is reported that, a mixed culture generated a current
that was six fold higher that that generated by a pure culture. Hence, the
microbial communities that develop in the anode chamber may have a similar
function as those found in methanogenic anaerobic digesters, except that
microorganisms that can transfer electrons to the electrode surface replace
methanogens. Rabaey referred to such microbial communities as adapted
anodophilic consortia. Anodophilic bacteria from different evolutionary
lineages from the families of Geobacteraceae, Desulfuromonaceae,
Alteromonadaceae, Enterobacteriaceae, Pasteurellaceae, Clostridiaceae,
Aeromonadaceae, and Comamonadaceae were able to transfer electrons
to electrodes. Methanogens also reported to have a capacity to transfer
electrons. Because the power output of Microbial Fuel Cells is low relative to
other types of fuel cells, reducing their cost is essential, if power
generation using this technology is to be an economical method of energy
production. Further research is required to enhance the power production by
overcoming these limitations. The main disadvantage of a two chamber Microbial
Fuel Cell is that the solution cathode must be aerated to provide oxygen to the
cathode. The power output of an Microbial Fuel Cell can be improved by
increasing the efficiency of the cathode, e.g. power is increased by adding
ferricyanide to the cathode chamber. The effects of operational conditions of a
microbial fuel cell were investigated and optimized for the best performance of
a mediator-less microbial fuel cell. The optimal pH reported was 7. The
resistance higher than 500 Ω was the rate determining factor by limiting
electron flow from anode to cathode. At the resistance lower than 200 Ω, proton
and oxygen supplies to the cathode were limited. For the construction of an
efficient microbial fuel cell, a non-compartmentalized fuel cell with an
electrode having a high oxygen reducing activity should be developed. Since the
concentration of fuel determines the amount of electricity generation from the
fuel cell, the device can be used as a BOD sensor. It is possible to design a Microbial
Fuel Cell that does not require the cathodes to be placed in water. In hydrogen
fuel cells, the cathode is bonded directly to the PEM so that oxygen in air can
directly react at the electrode. This technique was successfully used to
produce electricity from wastewater in a single chamber Microbial Fuel Cell.
However, a maximum of 788 mW/m2 power density was reported by Park
and Zeikus with a Mn4+ graphite anode and a direct air Fe3+
graphite cathode.
·
Membrane Less MFC In a
meditor less MFC, the membrane separates the anode from the cathode as in other
MFCs, and the membrane functions as an electrolyte that plays the role of an
electric insulator and allows protons to move through. However, the use of
membrane can limit the application of MFC to wastewater treatment. Proton
transfer through the membrane can be a rate limiting factor especially with
fouling expected due to suspended solids and soluble contaminants in a large
scale wastewater treatment process. In addition, membranes are expensive and
hence may limit its application. A membrane-less microbial fuel cell (ML-MFC)
was developed and used successfully to
enrich electrochemically active microbes that converted organic contaminants to
electricity. The COD (Chemical Oxygen Demnd) removal rate of 526.67 g/m3
day was reported with maximum power production 1.3 mW/m2 and current
density 6.9 mA/m2. The design used in the study showed poor cathode
reaction allowing a large quantity of oxygen to diffuse toward the anode.
Further studies are required to improve the design of ML-MFC to improve current
yield and COD removal efficiency.
·
Sediment Microbial Fuel Cells. A
likely application of microbial fuel cell (MFC) technology is in remote bodies
of water where electric energy can be extracted from organic-rich aquatic
sediments. For this purpose, researchers have developed sediment MFCs that
consist of an anode electrode embedded in the anaerobic sediment and a cathode
electrode suspended in the aerobic water column above the anode electrode. Electricigenic
bacteria in the sediment transfer electrons produced during the oxidation of
organic or inorganic matter to the anode electrode; while oxygen is reduced in
the water column by accepting electrons from the cathode electrode. As a
result, an electric current is generated. Classically, H-type MFCs have been
used to study microbial respiration in the anode. Such MFCs contain a cation
exchange membrane to separate the anaerobic anode from the aerobic cathode. A
cation exchange membrane is not necessary in sediment MFCs, because the
decreasing oxygen gradient over the depth of water and sediment columns creates
the necessary potential difference naturally By placing one electrode
into a marine sediment rich in organic matter and sulfides, and the other in
the overlying oxic water, electricity can be generated at sufficient levels to
power some marine devices. Protons conducted by the seawater can produce a
power density of up to 28 mW/m2. Graphite disks can be used for the electrodes,
although platinum mesh electrodes have also been used. “Bottle brush” cathodes
used for seawater batteries may hold the most promise for long-term operation
of unattended systems as these electrodes provide a high surface area and are
made of noncorrosive materials. Sediments have also been placed into H-tube
configured two-chamber systems to allow investigation of the bacterial
community.
·
Presence of Nanowires. Electrically
conductive bacterial appendages known as nanowires have only recently been
discovered so their structure(s) are therefore not well studied or understood.
Pili produced by some bacteria have so far been shown to be electrically
conductive using scanning tunneling electron microscopy. There is no data at
the present time whether nanowires can be detected or can be distinguished from
adsorbed chemical shuttles via standard electrochemical methods such as CV. If
electron shuttles associate with a nonconductive pili, or if the pili are
covered with metal precipitates, they will be included in the CV measurements
as membrane associated shuttles or may appear to be nanowires using STM. If
redox shuttles are enclosed within the pilus’ tubular structure they are
unlikely to be detected using CV. Additional research will be needed to determine
the best methods for detecting nanowires and determining their importance
relative to other methods of electron transfer from cells to electrodes
·
Use Of Permanganate As The Cathodic Electron Acceptor: Permanganate has
been used as an environment-friendly oxidant in industries for many years. Its
high redox potential offers the possibility of its application in a fuel cell
system to establish a high potential difference between the anode and the
cathode. Five-fold more power density can be achieved in a permanganate
two-chamber MFC than with other electron acceptors such as hexacynoferrate and
oxygen; In a MFC, also a three-fold maximum power density can be produced when
using permanganate as the electron acceptor as compared to using hexacynoferrate
. It is the outstanding redox potential of the permanganate that enhanced the
power output of a MFC. The similar mechanism also applies to the other high
redox potential electron acceptors such as hexacynoferrate which generates
higher power by higher redox potentials than dissolved oxygen
Moreover,
it is worth pointing out that this permanganate method has no need for a
catalyst, which makes this process simple and economical. But on the other
hand, it should be noted that like the other liquid-state electron acceptors
this permanganate MFC also requires liquid replacements to compensate its
depletion.
APPLICATIONS
·
Waste
Water Treatment And Electricity Generation.
Due to unique metabolic assets of microbes, variety of microorganisms are used
in Microbial Fuel Cells either single species or consortia. Some substrates
(sanitary wastes, food processing waste water,swine waste water and com
stovers) are exceptionally loaded with organic matter that itself feed wide
range of microbes used in Microbial Fuel Cells. Microbial Fuel Cells using
certain microbes have a special ability to remove sulfides as required in waste
water treatment. Microbial Fuel Cell substrates have huge content of growth
promoters that can enhance growth of bio-electrochemically active microbes
during waste water treatment. This simultaneous operation not only reduces
energy demand on treatment plant but also reduces amount of unfeasible
sludge produced by existing anaerobic
production. Microbial Fuel Cells connected in series have high level of removal
efficiency to treat leachate with
supplementary benefit of generating electricity.
Consider
a conventional Waste Water Treatment Plant designed for 30000 IE, receiving a
daily influent of 5400m3. At a biodegradable chemical oxygen demand
(bCOD) concentration 0f 500mg/L, this represents a.n influx of organic matter
of 2700kg dry weight per day. The amount of sludge formed\, at a nominal yield
of 0.4g cell dry weight per g bCOD converted will be 1080 kg per day. This
needs to be disposed off at a cost which can rise up to €5 00 per ton dry
matter. The other costs contained in the operational cost are the aeration
costs and pump costs for recirculation and processing.
If
a Microbial Fuel Cell is used with an open air cathode, no aeration is needed.
The putative energy of the input organic matter amounts to 8950kWH/day. The
costs for sludge processing will be lower, since no aerobic cell yields can be
attained . for methanogenesis, the
cell yield is about 0.05g CDW/g substrate; for Microbial Fuel Cell the yield
can be estimated somewhere in between aerobic and methanogenic conditions. At
an energetic efficiency of 35%, which should be attainable on large scale,
approximately 3150 kWh/day of useful energy will be produced. This comparison
does not take into account the capital cost of both systems. However, if the
capital cost is of same order, the comparison illustrates a significant
difference in operational costs. Hence, if large scale Microbial Fuel Cells can
be built at an acceptable price, this will be a viable technology.
Under
present investigation, the membrane less MFC was used effectively for synthetic
wastewater treatment with COD and BOD removal about 90%. The power production
of this MFC observed was 6.73 mW/m2. If power generation in these systems
can be increased, MFC technology may provide a new method to offset wastewater
treatment plant operating cost, making wastewater treatment more affordable for
developing and developed nations. The possibility of direct conversion of
organic material in wastewater to bio-electricity is exciting, but fundamental understanding
of the microbiology and further development of technology is required. With
continuous improvements in microbial fuel cell, it may be possible to increase
power generation rates and lower their production and operating cost. Thus, the
combination of wastewater treatment along with electricity production may help
in saving of millions of rupees as a cost of wastewater treatment at present
·
Secondary Fuel Production: With minor modifications, fuels
Microbial Fuel Cells can be employed to produce secondary fuels like hydrogen
(H2) as an alternative of electricity. Under standard experimental
conditions, proton and electron produced in anodic chamber get transferred to
cathode, which then combines with oxygen to form water. H2 generation
is thermodynamically not favored or it is a harsh process for a cell to convert
aproton into H2. Increase in external potential applied at cathode
can be competent to overcome thermodynamic barrierin reaction and used for H2
generation. As a result, proton and electron produced in anodic reaction
chamber combine at cathode to form H2. Microbial Fuel Cells can
probably produce extra H2 as compared to quantity that pull off from
classical glucose fermentation method. Single-chamber
membrane-free MECs were designed and successfully produced hydrogen from
organic matter using one mixed culture and one pure culture: Shewanella
oneidensis MR-1. At an applied voltage of 0.6 V, a hydrogen production rate of
0.53m3/day/m3 was obtained using a mixed bacterial
culture by the single-chamber MECs operated at pH 7.0. Higher hydrogen
production rate (0.69m3 /day/m3 ) was obtained when the
MECs were operated at pH 5.8. High current densities of 9.3 A/m2 (pH
7) and 14 A/m2 Were achieved with the mixed culture in the
single-chamber MEC system, attributing to the reduced potential losses
associated with membrane. Applied voltages exerted significant influences on MEC’s
performance. The performances at 0.6 V were more than two times higher than
those at 0.4 V in terms of hydrogen production rate, overall energy efficiency,
hydrogen yield, Coulombic efficiency and current density. While 0.3 V was the
minimum applied voltage to achieve measurable hydrogen production rate in the
MEC system. Hydrogenotrophic methanogens in the mixed culture systems adversely
affected hydrogen production. However, their activities can be effectively
suppressed by exposing cathodes to air for 15min combined with control of
retention time less than two days. Lowering solution pH (5.8) and heat
treatment (100oC) for 15min) of electrode did not effectively
inhibit the activities of methanogens.
Methanogenesis was avoided by using the pure bacterial culture S.
oneidensis in this MEC system. However, the current hydrogen production rates
were much lower than those with the mixed culture systems. The current density
and volumetric hydrogen production rate of this system have potential to
increase significantly by further reducing the electrode spacing and increasing
the ratio of electrode surface area/cell volume.
·
Bio Sensors: Bacteria show lower metabolic activity
when inhibited by toxic compounds. This will cause a lower electron transfer
towards an electrode. Bio-sensors could be constructed, in which bacteria are
immobilized onto an electrode and protected behind a membrane. If a toxic
component diffuses through the membrane, this can be measured by the change in
potential over the sensor. Such sensors could be extremely useful as indicators
of toxicants in rivers, at the entrance of wastewater treatment plants, to
detect pollution or illegal dumping, or to perform research on polluted site.
MFCs with replaceable anaerobic consortium could be
used as a biosensor for online monitoring of organic matter. Though diverse
conventional methods are used to calculate organic content in terms of
Biological Oxygen Demand(BOD) in waste water, most of them are unsuitable for on
line monitoring and control of biological waste water treatment process. A
linear correlation between coulombic yield and strength of organic matter in
waste water makes MFC a possible BOD sensor. Coulombic yield of MFC provides an
idea about BOD of liquid stream that proves to be an accurate method to measure
BOD value at quite wide concentration range of organic matter in waste water.
A mediator-less microbial fuel
cell was tested as a continuous BOD sensor. At a feeding rate of 0.35ml/min
(HRT = 1.05 h), BOD values of up to 100mg/l could be measured based on a linear
relation. Higher BOD values were then measured using either a model fitting
method or a lower feeding rate. About 60min was required to reach a new
steady-state current after changing the strength of the AW. When the MFC was
starved, the original current value was regained with varying recovery periods
depending on the length of the starvation. During starvation, the MFC generated
a background level current, probably through an endogenous metabolism. New
protein synthesis was not required for the recovery
Advantages Of Microbial
Fuel Cells:
Microbial fuel cells present several advantages,
both operational and functional, in comparison to the currently used technologies
for generation of energy out of organic matter or treatment of waste streams:
·
Generation
Of Energy Out Of Biowaste/Organic Matter
This feature is certainly the most ‘green’
aspect of microbial fuel cells. Electricity is being generated in a direct way
from biowastes and organic matter. This energy can be used for operation of the
waste treatment plant, or sold to the energy market. Furthermore, the generated
current can be used to produce hydrogen gas. Since waste flows are often
variable, a temporary storage of the energy in the form of hydrogen, as a
buffer, can be desirable.
·
Direct
Conversion Of Substrate Energy To Electricity
As previously reported, in anaerobic processes
the yield of high value electrical energy is only one third of the input energy
during the thermal combustion of the biogas. While recuperation of energy can
be obtained by heat exchange, the overall effective yield still remains of the
order of 30%.
A microbial fuel cell has no substantial
intermediary processes. This means that if the efficiency of the MFC equals at
best 30% conversion, it is the most efficient biological electricity producing
process at this moment. However, this power comes at potentials of
approximately 0.5 Volts per biofuel cell. Hence, significant amounts of MFCs
will be needed, either in stack or separated in series, in order to reach
acceptable voltages. If this is not possible, transformation will be needed,
entailing additional investments and an energy loss of approximately 5 %.
Another important aspect is the fact that a fuel
cell does not –as is the case for a conventional battery- need to be charged
during several hours before being operational, but can operate within a very
short time after feeding, unless the starvation period before use was too long
too sustain active biomass.
·
Sludge
production
In an aerobic bioconversion process, the growth
yield is generally estimated to be about 0.4 g Cell Dry Weight / g Chemical
Oxygen Demand removed. Due to the harvesting of electrical energy, the
bacterial growth yield in a MFC is considerably lower than the yield of an
aerobic process. The actual growth yield, however, depends on several
parameters:
o
The amount
of electrons diverted towards the anode and the energy they represent. This
energy (J) can be calculated as E = P x t = V x I x t, with E energy (J), P
power (W), t time (s), V voltage (V) and I current (A)
o
The amount
of substrate converted to volatile fatty acids that are not further converted:
often, the effluent of a MFC still contains considerable amounts of VFA that
need removal during post-treatment. These VFA represent an additional loss in
energetic efficiency, and will yield additional sludge if the effluent is
post-treated aerobically
o
The amount
of hydrogen formed: per equivalent of bio-hydrogen formed, two equivalents of
electrons are not diverted to the anode. Hydrogen formation appears to be in competition
with anodic electron transfer. Normally, bio-hydrogen formation can be
completely suppressed in microbial fuel cells, indicating that the anode is a
more energetically feasible electron acceptor than protons, due to a higher
overall redox potential.
·
Omission
Of Gas Treatment
Generally, off-gases of anaerobic processes
contain high concentrations of nitrogen gas, hydrogen sulphide and carbon
dioxide next to the desired hydrogen or methane gas. The off-gases of MFCs have
generally no economic value, since the energy contained in the substrate was
prior directed towards the anode. The separation has been done by the bacteria,
draining off the energy of the compounds towards the anode in the form of
electrons. The gas generated by the anode compartment can hence be discharged,
provided that no large quantities of H2S or other odorous compounds are
present in the gas, and no aerosols with undesired bacteria are liberated into
the environment.
·
Aeration
The cathode can be installed as a ‘membrane electrode
assembly’, in which the cathode is precipitated on top of the proton exchange
membrane or conductive support, and is exposed to the open air. This omits the
necessity for aeration, thereby largely decreasing electricity costs. However,
from a technical point of view, several aspects need additional consideration
when open air cathodes are used.
First, the cathode needs to remain sufficiently moist to ensure
electrical contact. Preliminary experiments by Rabaey et al. (unpublished
data) indicated that the water formation through oxygen reduction is
insufficient to keep the cathode moist. Therefore, a water recirculation needs
to be installed, possibly entailing extra energy costs. Secondly, the cathode
needs to contain a non-soluble redox mediator to efficiently transfer the
electrons from the electrode to oxygen. Generally, platinum is being used as a
catalyst, at concentrations up to 40% w/w, representing considerable costs.
However, new catalysts need to be developed, which would compensate their possible
lower efficiency by a significantly reduced cost and higher sustainability.
LIMITATIONS
·
Low power density: The major limitations to implementation
of MFCs for are their power density is still relatively low and the technology
is only in the laboratory phase. Based on the potential difference, ΔE, between
the electron donor and acceptor, a maximum potential of nearly 1V can be
expected in MFCs, which is not much greater than the 0.7 V that is currently
being produced. However, by linking several MFCs together, the voltage can be
increased. Current and power densities are lower than what is theoretically
possible, and system performance varies considerably. The maximum power density
reported in the literature, 3600mW/m2, was observed in a dual-chamber
fuel cell treating glucose with an adapted anaerobic consortium in the anode
chamber and a continuously aerated cathode chamber containing an electrolyte
solution that was formulated to improve oxygen transfer to cathode
·
High Initial Cost: A limiting factor to general MFC use is the high cost of
materials, such as the nafion membrane commonly used in laboratories as a
proton permeable membrane. Attempts are currently underway to produce low cost
MFCs constructed from earthen pots for use in India. By removing the proton
permeable membrane, utilizing locally produced 400 ml earthen pots, stainless
steel mesh cathodes and a graphite plate anode, each MFC unit could be produced
for US $1. The earthen pot MFCs used sewerage sludge as an initial inoculum and
experiments were conducted using acetate as a carbon source. While producing
low levels of power, these devices could potentially be incorporated in large
numbers into oxidation ponds for the treatment of concentrated wastewater while
generating power. In areas where off grid applications are required, even low
power MFC devices may prove useful. The World Bank has provided funding to a
company named Lebone (http://www.lebone.org/) to start trials with MFC
technology to provide energy to isolated communities. Initial trials will be
based in Tanzania and attempt to provide power for high efficiency LEDs and
battery powered devices. Current applications are all limited to low power
level devices. If power can be increased, or cells engineered for specific
applications, then a large range of potential applications have been speculated
to be possible
·
Upscaling
problems Scale-up of microbial fuel cells (MFCs)will require
a better understanding of the effects of reactor architecture and operation
mode on volumetric power densities. We compared the performance of a smaller
MFC (SMFC, 28mL) with a larger MFC (LMFC, 520mL) in fed-batch mode. The SMFC
produced 14Wm−3 , consistent with previous reports for this reactor
with an electrode spacing of 4 cm. The LMFC produced 16Wm−3 ,
resulting from the lower average electrode spacing (2.6 cm) and the higher
anode surface area per volume (150m2 m−3 vs. 25m2m−3
for the SMFC). The effect of the larger anode surface area on power was shown
to be relatively insignificant by adding graphite granules or using graphite
fiber brushes in the LMFC anode chamber. Although the granules and graphite
brushes increased the surface area by factors of 6 and 56, respectively, the
maximum power density in the LMFC was only increased by 8% and 4%. In contrast,
increasing the ionic strength of the LMFC from 100 to 300mM using NaCl
increased the power density by 25% to 20Wm−3 When the LMFC was
operated in continuous flow mode, a maximum . power density of 22Wm−3 was
generated at a hydraulic retention time of 11.3 h. Although a thick biofilm was
developed on the cathode surface in this reactor, the cathode potentials were
not significantly affected at current densities <1.0mAcm−2 These results
demonstrate that power output . can be maintained during reactor scale-up;
increasing the anode surface area and biofilm formation on the cathode do not
greatly affect reactor performance, and that electrode spacing is a key design
factor in maximizing power generation
Several aspects needed for an efficient MFC are
hampering upscaling:
-
The
influent needs to reach the whole anode matrix sufficiently
-
Protons
need rapid diffusion towards the membrane
-
Sufficient
electrical contact needs to be established between bacteria in suspension and
the anode
-
Sufficient
voltage needs to be reached over the MFC to have a useful power
-
Instatement
of an aeration device should be avoided
·
Activation Losses: Due
to the activation energy needed for an oxidation/reduction reaction, activation
losses (or activation polarization) occur during the transfer of electrons from
or to a compound reacting at the electrode surface. This compound can be
present at the bacterial surface, as a mediator in the solution, or as the
final electron acceptor reacting at the cathode. Activation losses often show a
strong increase at low currents and steadily increase when current density
increases. Low activation losses can be achieved by increasing the electrode
surface area, improving electrode catalysis, increasing the operating temperature,
and through the establishment of an enriched biofilm on the electrode(s).
·
Ohmic Losses. The
ohmic losses (or ohmic polarization) in an MFC include both the resistance to
the flow of electrons through the electrodes and interconnections, and the resistance
to the flow of ions through the CEM (if present) and the anodic and cathodic
electrolytes. Ohmic losses can be reduced by minimizing the electrode spacing, using
a membrane with a low resistivity, checking thoroughly all contacts, and (if
practical) increasing solution conductivity to the maximum tolerated by the
bacteria.
·
Bacterial Metabolic Losses: To generate metabolic energy,
bacteria transport electrons from a substrate at a low potential through the
electron transport chain to the final electron acceptor (such as oxygen or
nitrate) at a higher potential. In an MFC, the anode is the final electron
acceptorandits potential determines the energy gain for the bacteria. The
higher the difference between the redox potential of the substrate and the
anode potential, the higher the possible metabolic energy gain for the
bacteria, but the lower the maximum attainable MFC voltage. To maximize the MFC
voltage, therefore, the potential of the anode should be kept as low (negative)
as possible. However, if the anode potential becomes too low, electron
transport will be inhibitedandfermentation of the substrate (if possible) may
provide greater energy for the microorganisms. The impact of a low anode
potential, and its possible impact on the stability of power generation, should
be addressed in future studies.
·
Concentration Losses. Concentration
losses (or concentration polarization) occur when the rate of mass transport of
a species to or from the electrode limits current production. Concentration
losses occur mainly at high current densities due to limited mass transfer of
chemical species by diffusion to the electrode surface. At the anode
concentration losses are caused by either a limited discharge of oxidized
species from the electrode surface or a limited supply of reduced species
toward the electrode. This increases the ratio between the oxidized and the
reduced species at the electrode surface which can produce an increase in the
electrode potential. At the cathode side the reverse may occur, causing a drop
in cathode potential. In poorly mixed systems diffusional gradients may also
arise in the bulk liquid. Mass transport limitations in the bulk fluid can
limit the substrate flux to the biofilm, which is a separate type of
concentration loss. By recording polarization curves, the onset of
concentration losses can be determined.
CONCLUSION:
Development of MFCs was
triggered by USA space program in 1960s as a possible technology for a waste
disposal system for space flights that would also generate power. MFC
technology has been extensively reviewed focusing on recent improvement,
practical implementation, anode performance, cathodic limitations, different
substrates etc. MFCs have been explored as a new source of electricity
generation during operational waste water treatment. In addition, some of the
recent modification in MFCs (MEC), in which anoxic cathode is used increased
external potential at cathode. Phototropic MFCs and solar powered MFC also
represent an exceptional attempt in the progress of MFCs technology for
electricity production.
MFC is an ideal way of
generating electricity since it not only as a renewable source but also it can
be used to treat waste. It can also be used for production of secondary fuel as
well as in bioremediation of toxic compounds. However, this technology is only
in research stage and more research is required before domestic MFCs can be
made available for commercialization
Microbial fuel cells are evolving to become a
simple, robust technology. Certainly in the field of wastewater treatment, middle
term application can be foreseen at market value prices. However, to increase
the power output towards a stable 1kW per m 3 of reactor, many technological improvements
are needed. Provided the biological understanding increases, the
electrochemical technology advances and the overall electrode prices decrease,
this technology might qualify as a new core technology for conversion of
carbohydrates to electricity in years to come.
No comments:
Post a Comment