sustainabe energy

8/10/2019 Sustainabe Energy

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Introduction

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2. INTRODUCTION

2.1. Energy needs

There are over seven billion people on the planet with 9.4 billion projected for 2050

(Lewis and Noceru 2006). Fossil fuels have supported the industrialization and economic growth

of countries during the past century, but it is clear that they cannot indefinitely sustain a global

economy. Oil will not appreciably run out for at least 100 years or more, but demand for oil is

expected to exceed production from known and anticipated oil reserves ten or twenty years from

now, or within the 2015 to 2025 time frame. This may seem distant to many consumers and

businesses that rarely plan for more than three to five years in the future, but this is a very short

time frame for society as a whole.

Planning a single section of an interstate highway in a city, for example, can take ten

years or more. The infrastructure changes needed to address our global energy needs will be far

more extensive and will likely require changes not only to our infrastructure but also to our

lifestyle. Changes will affect everything from home heating and lighting, to where we prefer to

live and work and how we get there. The costs of energy and how much energy we use will come

to dominate our economy and our lifestyle in the coming decades.

Microbial fuel cells (MFCs) have emerged in recent years as a promising yet challenging

technology. In a MFC, microorganisms interact with electrodes using electrons, which are either

removed or supplied through an electrical circuit (Rabaey et al., 2007). MFCs are the major type

of bioelectrochemical systems (BESs) which convert biomass spontaneously into electricity

through the metabolic activity of the microorganisms. MFC is considered to be a promising

sustainable technology to meet increasing energy needs, especially using wastewaters as

substrates, which can generate electricity and accomplish wastewater treatment simultaneously,

thus may offset the operational costs of wastewater treatment plant (Lu et al., 2009). The

knowledge that bacteria can generate electric current was first reported by Potter (1911).

However, the real interest in MFCs has tremendously grown in recent years, both in terms of

number of researchers as well as the applications for these systems.Fig. 1A shows that Scopus

search with keyword microbial fuel cell indicates almost 60-fold increase in the number of

articles published over the last one decade (19982008). Moreover, the reported electric current
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Introduction

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output from the MFCs has also increased tremendously over the recent years.Fig. 1B shows the

country-wise distribution of MFC researchers, the data for which was also drawn from Scopus.

It is evident that the interest in MFC research is truly global with more and more researchers

coming up from different countries.

Fig. 2.1. (A) The number of articles on MFCs. The data is based on the number of articles

mentioning MFC in the citation database Scopus in September 2009. (B) The country-wise

distribution in MFC research. The data is based on the number of articles mentioning MFC in the

citation database Scopus in September2009.
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2.1.1. Renewable energy

Energy demands for conventional water and wastewater processes are a large part of the

problem. During the last century the fossil sources resources have been the main energy source,

but its scarcity, due to the high consumption rate, and the environmental problems caused

recommend the search of alternative energy sources. One of the most appropriate options are the

renewable energy sources, which stand up as one of the ways to reach the energetic proposed

objective.

Energy demands for conventional water and wastewater processes are a large part of the

problem. During the last century the fossil sources resources have been the main energy source,

but its scarcity, due to the high consumption rate, and the environmental problems caused

recommend the search of alternative energy sources. One of the most appropriate options are the

renewable energy sources, which stand up as one of the ways to reach the energetic proposed

objective.

In this context, microbial fuel cell (MFC) technologies are a promising and yet completely

different approach to energy production, allowing to recover the energy contained in chemicals.

This technology consists of bio-electrochemical reactors which generate electricity directly from

an organic fuel using micro-organisms. In the MFC, the microorganisms present in the anodic

chamber oxidize substrates generating electrons, protons, and other metabolic products. The

electrons produced by the microorganisms can be transferred to the anode by different

mechanisms: electron shuttles, direct membrane associated electron transfer, nanowires, etc.

Then the electrons flow to the cathode through a wire. In most cathodic compartments of MFC,

the electrons combine with oxygen and protons, which diffuse from the anodic compartment

through the membrane, producing water. It is not essential to place the cathode in water or in a

separate chamber when using oxygen at the cathode, this is because the cathode can be

configured for direct contact with air. As indicated, oxygen is the most commonly used terminal

electron acceptor, however, its presence in the anode could inhibit the electricity generation. This

is because microorganism would oxidise the organic substrates with oxygen as final electron

acceptor, avoiding the external electric circuit, and therefore resulting in a lower Columbic

efficiency. Thus the MFC must be designed to keep the bacteria separated from the oxygen. This


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separation can be achieved by using a membrane that allows proton transport from the anode to

the cathode, forming two separate chambers: the anodic one and the cathodic one.

Moreover, MFC not only allows to recover the energy of the chemical bonds, but also to

oxidise chemical compounds. In 2004, it was demonstrated that domestic wastewater could be

treated to practical levels while simultaneously generating electricity . A few years earlier Reimers

et al., (2001) had demonstrated that organic and inorganic matter in marine sediments could be used

in a novel type of MFC, making it that a wide variety of substrates, materials, and system

architectures could be used to capture electricity from organic and inorganic matter with bacteria.

Because of that, MFC can also be considered as an environmental technology for the wastes

treatment.

The potential use of wastes as energy sources in MFC seems a very interesting option.

The organic substrates contained in the wastes represent a significant share of the wastes

generated daily; however, its energy cannot be recovered by traditional processes because of its

complex composition and because in most of the cases is highly diluted.

The development of processes that can use microorganisms to produce electricity from

wastes represents a fantastic way for bioenergy production as the microorganisms are not as

selective and sensitive as the chemical catalysts, increasing the spectrum and qualities of the

potential fuels to be used, and because microorganisms are self-replicating, and thus the catalysts

for the substrate oxidation are self-sustaining. In literature it can be found experiences with MFC

operating with pure or mixed cultures. It has been reported that MFC operated using mixed

cultures achieved substantially greater power densities than those with pure cultures. Mixed

cultures employ and undefined consortia of organisms that grow based on ecological selection

principles. This approach is very interesting not only because of the greater power but also

because reduce the operating costs and allows to deal with wastes.


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2.2. Fuel Cells

2.2.1. How Fuel Cell works

Fuel cells are electrochemical devices that directly convert chemical energy to electrical

energy. They consist of an electrolyte medium sandwiched between two electrodes (Fig. 1). One

electrode (called the anode) facilitates electrochemical oxidation of fuel, while the other (called

the cathode) promotes electrochemical reduction of oxidant. Ions generated during oxidation or

reduction are transported from one electrode to the other through the ionically conductive but

electronically insulating electrolyte. The electrolyte also serves as a barrier between the fuel and

oxidant. Electrons generated at the anode during oxidation pass through the external circuit

(hence generating electricity) on their way to the cathode, where they complete the reduction

reaction.

On the cathode, reduction takes place.

Oxidant + n e- Reductant

On the anode, oxidation takes place.

Reductant n e-+ Oxidant

From the thermodynamic point of view it can be stated that the fuel cells therefore

convert the free energy of a spontaneous reaction to electricity. The chemical energy is related to

the supply via the following equation:

emfG n F E [1]

where n is the number of exchanged electrons, F the Faraday constant (96,485 cal / mol)

and E0 the potential of the cell in thermodynamic equilibrium. Ideally, the value of the

equilibrium potential is the potential difference between the anode reaction (Ea, 0) and cathode(Ec, 0). However, the actual cell potential is always lower than the ideal value. This potential

drop on the value of great potential is called overpotential () and is calculated by the following

equation (Reel et al. 2001; Mennola 2000, Mikkola 2001):

= E0E [2]


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Ideally, to start a sufficient electrochemical reaction with the applied potential is higher

than the equilibrium. But in reality, since the kinetic point of view, the reaction rate is

proportional to the difference between the applied potential and the thermodynamics. The

dependence between the overpotential and the resulting current intensity is not equal in all

systems, but depends on the electrode surface (for electrocatalytic phenomena) of the concrete

and reaction temperature. Overpotential is related to the electrochemical kinetics, the electrical

resistances (ohmic drops) and mass transfer.

i ) Activation overpotential (act)

The relationship between the intensity of resultant current (i) and the overpotential (act)

is given by the Butler-Volmer equation:

0 exp expa act a act

F Fi i

R T R T

[3]

where i0is the exchange current density, aaand acare the load factors for the anodic and

cathodic reaction, respectively, F the Faraday constant, R the ideal gas constant and T the

temperature. Represents the absolute value of the current density at the anode and cathode for the

electrochemical reaction equilibrium considered. Relates to the particular reaction, the surface

and the concentration of oxidizing and reducing species according to the following expression:

[4]

where n is the number of exchanged electrons, A is the active area of reaction, k0is the

rate constant for reaction under standard conditions and is the charge-transfer coefficient for this

reaction.

ii) Ohmic overpotential (Ohm)

In any electrochemical cell energy losses occur related to the resistance to flow of ions

generated by the electrolyte and the resistance to electron flow occurs at the electrodes, current

collectors, cables, connections and other mechanical elements of the cell. As a consequence,

causes a voltage drop whose value is related to the electrical current which is generated or


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consumed in the electrochemical reactor and this is a function of the materials used, the

geometry of the cell and the temperature.

The ohmic overpotential can be expressed as:

ohm

i R

[5]

The total resistance (R) considers all elements of the cell: resistance to electron flow (Re),

ion flow resistance (Ri) and contact resistance arising from the different constituents of the cell

(Rc)

R = Re + Ri+ Rc[6]

iii) Diffusion overpotential (dif) .

Appears as a consequence of the lack of reagents to reach the catalyst active sites where the

electrochemical reaction occurs. It is calculated with the following equation:

1difL

R T iLn

n F i

[7]

where iLis limiting current density.

In a polarization curve will be appreciated three characteristic regions of voltage

decrease in the MFC: (1) a rapid voltage drop as current flows through the circuit (at high

external resistance); (2) a nearly linear decrease in voltage; ( 3 ) a second rapid voltage decrease

at high current densities.


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Fig.2.1. Characteristics of a polarization curve, showing regions where different types of

losses reduce the useful current. The region of the constant voltage drop is shown by the solid

red line.

Ohmic losses

The flow of electrons is hampered by the resistance of the electrode material, which

introduces an ohmic voltage loss. The higher the conductivity of the electrode material and the

lower the contact losses and travel distance of the electrons within the electrode, the moreefficient are the electron conduction and the lower the ohmic loss.

Activation losses

In order to start the transfer of electrons from the electrochemical active microorganisms

(EAM) towards the electrode or to transfer electrons towards a final electron acceptor, an energy

barrier needs to be overcome, which results in a voltage loss or activation overpotential (Logan

et al., 2006). Activation losses are characterized by an initial steep decrease of the cell voltage at

the onset of the electricity generation As the current steadily increases, the other losses e.g.

ohmic and mass transfer losses become proportionally more important. Low activation losses can

be achieved by increasing the electrode surface area, improving the electrode catalysis,

increasing the operating temperature, and in case of the microbial catalysis, through the

establishment of an enriched biofilm on the electrode(s). It is hypothesized that microorganisms


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can lower the activation overpotential and thus increase their metabolic energy gain by

optimizing their electron transferring strategies.

Electron quenching reactions and energy efficiency

Substrate competing processes, such as fermentation or methanogenesis and respiration

(if oxygen intrudes), result in a loss of electrons (He et al., 2005, Liu and Logan, 2004). Also,

part of the substrate is inherently converted into anodophilic biomass. Moreover, a leakage of

substrate towards the cathode results in a potential electron loss. All these processes lower the

conversion of substrate into current which is expressed by the coulombic efficiency (CE). The

CE is defined as the ratio of the amount of substrate administered and the amount of electrons

recovered.

2.2.2.

Types of Fuel Cells

Fuel cells are usually classified by their operating temperature and the type of electrolyte

they use. Some types of fuel cells work well for use in stationary power generation plants. Others

may be useful for small portable applications or for powering cars. The main types of fuel cells

include:

Polymer exchange membrane fuel cell (PEMFC)The Department of Energy (DOE) is focusing on the PEMFC as the most likely candidate

for transportation applications. The PEMFC has a high power density and a relatively low

operating temperature (ranging from 60 to 80 degrees Celsius, or 140 to 176 degrees Fahrenheit).

The low operating temperature means that it doesn't take very long for the fuel cell to warm up

and begin generating electricity. Well take a closer look at the PEMFC in the next section.

Solid oxide fuel cell (SOFC)

These fuel cells are best suited for large-scale stationary power generators that could

provide electricity for factories or towns. This type of fuel cell operates at very high temperatures

(between 700 and 1,000 degrees Celsius). This high temperature makes reliability a problem,

because parts of the fuel cell can break down after cycling on and off repeatedly. However, solid

oxide fuel cells are very stable when in continuous use. In fact, the SOFC has demonstrated the


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longest operating life of any fuel cell under certain operating conditions. The high temperature

also has an advantage: the steam produced by the fuel cell can be channeled into turbines to

generate more electricity. This process is called co-generation of heat and power (CHP)and it

improves the overall efficiency of the system.

Alkaline fuel cell (AFC)

This is one of the oldest designs for fuel cells; the United States space program has used

them since the 1960s. The AFC is very susceptible to contamination, so it requires pure

hydrogen and oxygen. It is also very expensive, so this type of fuel cell is unlikely to be

commercialized.

Molten-carbonate fuel cell (MCFC)

Like the SOFC, these fuel cells are also best suited for large stationary power generators.

They operate at 600 degrees Celsius, so they can generate steam that can be used to generate

more power. They have a lower operating temperature than solid oxide fuel cells, which means

they don't need such exotic materials. This makes the design a little less expensive.

Phosphoric-acid fuel cell (PAFC)

The phosphoric-acid fuel cell has potential for use in small stationary power-generation

systems. It operates at a higher temperature than polymer exchange membrane fuel cells, so it

has a longer warm-up time. This makes it unsuitable for use in cars.

Direct-methanol fuel cell (DMFC)

Methanol fuel cells are comparable to a PEMFC in regards to operating temperature, but

are not as efficient. Also, the DMFC requires a relatively large amount of platinum to act as a

catalyst, which makes these fuel cells expensive.

In the following section, we will take a closer look at the kind of fuel cell the DOE plans

to use to power future vehicles - the PEMFC.


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2.3. Microbial Fuel Cell

A microbial fuel cell is a device that converts chemical energy to electrical energy by the

catalytic reaction ofmicroorganisms.

A typical microbial fuel cell consists ofanode andcathode compartments separated by a

cation (positively charged ion) specific membrane.In the anodic compartment, fuel is oxidized

by microorganisms, generating electrons and protons. Electrons are transferred to the cathode

compartment through an external electric circuit, while protons are transferred to the cathode

compartment through the membrane. Electrons and protons are consumed in the cathode

compartment, combining with oxygen to form water.

Microbial fuel cell (MFC) technologies represent the newest approach for generating

electricity-bioelectricity generation from biomass using bacteria. While the first observation of

electrical current generated by bacteria is generally credited to Potter in 1911 (Potter 1911), very

few practical advances were achieved in this field even 55 years later (Lewis 1966). In the early

1990s, fuel cells became of more interest and work on MFCs began to increase (Allen and

Bennett 1993). However, experiments that were conducted required the use of chemical

mediators, or electron shuttles, which could carry electrons from inside the cell to exogenous

electrodes. The breakthrough in MFCs occurred in 1999 when it was recognized that mediators

did not need to be added (Kim et al. 1999a; Kim et al. 1999b). In an MFC, microorganisms

degrade (oxidize) organic matter, producing electrons that travel through a series of respiratory

enzymes in the cell and make energy for the cell in the form of ATP. The electrons are then

released to a terminal electron acceptor (TEA) which accepts the electrons and becomes reduced.

A schematic of an MFC system is shown in Fig. 2.2.

The anode and cathode chambers are separated by a membrane. The bacteria grow on the

anode, oxidizing organic matter and releasing electrons to the anode and protons to the solution.

The cathode is sparged with air to provide dissolved oxygen for the reactions of electrons,

protons and oxygen at the cathode, with a wire (and load) completing the circuit and producing

power. The system is shown with a resistor used as the load for the power being generated, with

the current determined based on measuring the voltage drop across the resistor using a

multimeter hooked up to a data acquisition system.
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Fig. 2.2. Schematic of the basic components of a microbial fuel cell

2.3.1.

Design of Microbial Fuel Cells (MFC)

A typical MFC consists of an anodic chamber and a cathodic chamber separated by a

PEM. A one-compartment MFC eliminates the need for the cathodic chamber by exposing the

cathode directly to the air.

i) Two-compartment MFC systems

Two-compartment MFCs are typically run in batch mode often with a chemically defined

medium such as glucose or acetate solution to generate energy. They are currently used only in

laboratories. A typical two compartment MFC has an anodic chamber and a cathodic chamber

connected by a PEM, or sometimes a salt bridge, to allow protons to move across to the cathode

while blocking the diffusion of oxygen into the anode. The compartments can take various


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Introduction

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practical shapes. The schematic diagrams of five two-compartment MFCs are shown in Fig. 3.

The mini-MFC shown in Fig. 3C having a diameter of about 2 cm, but with a high volume power

density was reported by Ringeisen et al. (2006). They can be useful in powering autonomous

sensors for long-term operations in less accessible regions. Upflow mode MFCs as shown in Fig.

3D and E is more suitable for wastewater treatment because they are relatively easy to scale-up

(He et al., 2005, 2006). On the other hand, fluid recirculation is used in both cases. The energy

costs of pumping fluid around are much greater than their power outputs. Therefore, heir primary

function is not power generation, but rather wastewater treatment. The MFC design in Fig. 3E

offers a low internal resistance of 4 because the anode and cathode are in close proximity over

a large PEM surface area. Min and Logan (2004) designed a Flat Plate MFC (FPMFC) with only

a single electrode/PEM assembly. Its compact configuration resembles that of a conventional

chemical fuel cell. A carbon-cloth cathode that was hot pressed to a Nafion PEM is in contact

with a single sheet of carbon paper that serves as an anode to form an electrode/PEM assembly.

The FPMFC with two non-conductive polycarbonate plates is bolted together.


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Fig. 2.3. Schematics of a two-compartment MFC in cylindrical shape (A), ectangular

shape (B), miniature shape (C), upflow configuration with cylindrical shape (D), cylindrical

shape with an U-shaped cathodic compartment (E).


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ii) Single-compartment MFC systems

Due to their complex designs, two-compartment MFCs are difficult to scale-up even

though they can be operated in either batch or continuous mode. One compartment MFCs offer

simpler designs and cost savings. They typically possess only an anodic chamber without the

requirement of aeration in a cathodic chamber. Park and Zeikus (2003) designed a

onecompartment MFC consisting of an anode in a rectangular anode chamber coupled with a

porous air-cathode that is exposed directly to the air as shown in Fig. 4A. Protons are transferred

from the anolyte solution to the porous air-cathode (Park and Zeikus,2003). Liu and Logan

(2004) designed an MFC consisting of an anode laced inside a plastic cylindrical chamber and a

cathode placed outside. Fig. 4B shows the schematic of a laboratory prototype of the MFC

bioreactor. The anode was made of carbon paper without wet proofing. The cathode was either a

carbon electrode/ PEM assembly fabricated by bonding the PEM directly onto a flexible carbon-

cloth electrode, or a stand alone rigid carbon paper without PEM (Liu and Logan, 2004; Liu et

al., 2005a; Cheng et al., 2006a). A tubular MFC system with an outer cathode and an inner anode

using graphite granules is shown in Fig. 4C (Rabaey et al., 2005b). In the absence of a cathodic

chamber, catholyte is supplied to the cathode by dripping an electrolyte over the outer woven

graphite mat to keep it from drying up. Rabaey et al. (2005b) pointed out that the use of

sustainable, open-air cathodes is critical to practical implementation of such MFCs.


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Fig. 2.4. An MFC with a proton permeable layer coating the inside of the window-

mounted cathode (A), an MFC consisting of an anode and cathode placed on opposite side in a

plastic cylindrical chamber (B), and a tubular MFC with outer cathode and inner anode

consisting of graphite granules (C).

iii) Up-flow mode MFC systems


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Jang et al. (2004) provided another design (Fig. 5A) of an MFC working in continuous

flow mode. A Plexiglas cylinder was partitioned into two sections by glass wool and glass bead

layers. These two sections served as anodic and cathodic chambers, respectively as shown in Fig.

5A .The disk-shaped graphite felt anode and cathode were placed at the bottom and the top of the

reactor, respectively. Fig. 5B shows anotherMFC design inspired by the same general idea

shown in Fig. 5A but with a rectangular container and without a physical eparation achieved by

using glass wool and glass beads (Tartakovsky and Guiot, 2006). The feed stream is supplied to

the bottom of the anode and the effluent passes through the cathodic chamber and exits at the top

continuously (Jang et al., 2004; Moon et al., 2005). There are no separate anolyte and catholyte.

And the diffusion barriers between the anode and cathode provide a DO gradient for proper

operation of the MFCs.


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Introduction

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Fig. 2.5. Schematics of mediator-and membrane-less MFC with cylindrical shape (A),

and with rectangular shape (B)

IV) Stacked microbial fuel cell

A stacked MFC is shown in Fig. 6 for the investigation of performances of several MFCs

connected in series and in parallel (Aelterman et al., 2006). Enhanced voltage or current output

can be achieved by connecting several MFCs in series or in parallel. No obvious adverse effect

on the maximum power output per MFC unit was observed. Coulombic efficiencies (In fact it is

not real Coulombic efficiency but Coulombic percent conversion. Coulombic efficiency

describes how much of the electrons can be abstracted from the electron-rich substrates via the

electrodes. It is not a measurement of electron transfer rate, while the authors described howmuch substrate was used for electricity generation before the stream flowed out of the MFCs or

MFC stacks) differed greatly in the two arrangements with the parallel connection giving about

an efficiency six times higher when both the series were operated at the same volumetric flow

rate. The parallel-connected stack has higher short circuit current than the series connected stack.

This means that higher maximum bioelectrochemical reaction rate is allowed in the connection


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Introduction

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of MFCs in parallel than in series. Therefore to maximize Chemical Oxygen Demand (COD)

removal, a parallel connection is preferred if the MFC units are not independently operated

(Aelterman et al., 2006).

Fig. 2.6. Stacked MFCs consisting of six individual units with granular graphite anode.

(Figure drawn to illustrate a photo in Aelterman et al.,2006)

2.3.2. Electron transfer in microbial fuel cellsdiscussion

i) Direct electron transfer (DET)

The direct electron transfer takes place via a physical contact of the bacterial cell

membrane or a membrane organelle with the fuel cell anode, with no diffusional redox species

being involved in the electron transfer from the cell to the electrode. Since living cells are

generally assumed to be electronically non-conducting, such a transfer mechanism has long been

considered impossible. The direct electron transfer requires that the microorganisms possess

membrane bound electron transport protein relays that transfer electrons from the inside of the

bacterial cell to its outside, terminating in an outermembrane redox protein that allows the

electron transfer to an external, solid electron acceptor (a metal oxide or an MFC anode). In the

focus of the discussion are c-type cytochromes, multi-heme proteins especially evolved with

sediment inhabiting metal reducing microorganisms such as, e.g., Geobacter,19,36

Rhodoferax37 and Shewanella38,39 that, in their natural environment, often have to rely on solid


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terminal electron acceptors like iron(III) oxides. In the case of these organisms the MFC anode

can conveniently resume the role of the solid electron acceptor (Fig. 2.7.A). As mentioned, the

DET via outer membrane cytochromes requires the physical contact (adherence) of the bacterial

celland of the cytochrometo the fuel cell anode, with the consequence that only bacteria in

the first monolayer at the anode surface are electrochemically active.19 The MFC performance is

thus limited by the maximum cell density in this bacterial monolayer.

Fig.2.7. Illustration of the DET via (A) membrane bound cytochromes,(B) electronically

conducting nanowires.

Recently it has been demonstrated that, e.g., some Geobacter and the Shewanella strains

can evolve electronically conducting molecular pili (nanowires) that allow the microorganism to

reach and utilize more distant solid electron acceptors.These pili also allow the organisms to use

an electrode that is not in direct cell contact as its sole electron acceptor (Fig. 2.7.b). The pili are

connected to the embranebound cytochromes, via which the electron transfer to the outside of

the cell is accomplished. The formation of such nanowires may allow the development of thicker

electroactive biofilms and thus higher anode performances. Thus, Reguera and co-workers

reported a ten-fold increase of fuel cell performance upon nano-wire formation of Geobacter

sulfurreducens. The evaluation of the energy efficiency of the DET is difficult since

unambiguous information is still very scarce. Unfortunately, many papers in which the DET

from living bacteria to an electrode has been clearly identified, no exact evaluation of the redox

potential of the involved species has been undertaken. Thus, Bond and Lovley used the open

circuit potential of a Geobacter sulfurreducens colonized electrode, growing on acetate, to


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determine the redox potential of the DET as _0.17 V. At open circuit, however, the redox

potential of a metabolizing anaerobic bacterial culture will shift considerably towards negative

potentials (up to several hundred mV), due to the strong shift of the concentration term of the

Nernst equation towards the reduced species. Thus, the reported open circuit potential may not

be equal to the formal potential of the cytochrome based electron transfer.

Since the DET via bacterial nanowires (Fig2.7.b) is reported to proceed via the

membrane bound cytochromes18 it will, for the following considerations, be assumed that the

same standard redox potentials apply for the cytochrome and nanowire based electron transfer.

ii) Mediated electron transfer (MET)

Some scientists consider DET to be the first (and only) choice for an efficient current

generation in MFCs.18 Yet, so far the performance (in terms of current and power densities) of

pure DET systems has often been even orders of magnitude below that of systems involving/or

based on mediated electron transfer.

As illustrated in the following sections, MET mechanisms may represent an effective

means to wire the microbial metabolism to a fuel cell anode. Very different approaches have

been proposed, and they can be classified by the nature of the mediating (or linking) redox

species.

MET via exogenous (artificial) redox mediators.

In the following paragraph an approach is described that, due to a number of severedisadvantages, haswith the exception of some fundamental researchbeen generally

abandoned. The approach will be briefly outlined, without going into the discussion of the

energetic aspects. In 1930 Cohen stated that although bacterial cultures, when grown

anaerobically, may exhibit a strongly negative potential, the produced current is generally

minute.7 He ascribed this low current generation capacity to a lack of electromotively active

oxidationreduction products. As a solution to this problem he proposed the introduction of

inorganic or organic substances of the type potassium ferricyanide or benzoquinone to facilitate

the electron transfer from cultures to immersed electrodes.

MET via secondary metabolites.

Often microorganisms grow under conditions in which neither soluble electron acceptors

are available nor solid electron acceptors are in direct reach (for DET). An example is the

conditions that rule within thick biofilms, where, e.g., oxygen diffusion into the depth of the film


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is limited and the cell is not in direct contact with a solid electron acceptor. Here, the

microorganism may either use externally available (exogenous) electron shuttling compounds

like humic acids or metal chelates, or can itself even produce low-molecular, electron shuttling

compounds via secondary metabolic pathways.

MET via primary metabolites.

In contrast to the secondary metabolites the production of reduced primary metabolites is

closely associated with the oxidative substrate degradation. Naturally, the total amount of

reduction equivalents produced matches the amount of oxidized metabolites.

To be utilizable as a reductant for anodic oxidation the metabolite has to fulfil certain

requirements. Its redox potential should be as negative as possible (but within the limit imposed

by the oxidation potential of the substrate) and it must be accessible for electrochemical

oxidation under MFC conditions.

In principle, two major anaerobic metabolic pathways can lead to the formation of

reduced metabolites suitable for MFC utilization: anaerobic respiration and fermentation.

2.1. Materials

i) Anode materials

The requirements of an anode material are: highly conductive, non-corrosive, high

specific surface area (area per volume), high porosity, non-fouling (ie., the bacteria do not fill it

up), inexpensive, and easily made and scaled to larger sizes. Of these properties, the single most

important one that is different from other biofilm reactors is that the material must be electrically

conductive.

The use of carbon paper, cloth, and foam forms for the MFC anode is very common. The

main disadvantage of the material is that it is quite brittle.


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Introduction

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Fig. 2.8. Photographs of carbon materials used for MFC anodes: (A) carbon paper (E-

TEK); (B) carbon cloth (E-TEK); (C) three different types of reticulated vitreous carbon (RVC)

with different pore sizes (10, 20, and 45 pores per inch).

We can use a large variety of graphite materials to choose from for MFC electrodes

which vary greatly in price, composition, and surface area.


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Introduction

24

Fig. 2.9. Some graphite materials used for MFC anodes: (A) graphite rod; (B) thick

graphite plate, (C) thinner graphite electrode, and (D) sheet shown with square electrode cut out.

ii) Membranes and separators

The membrane are used in first of all as a method for keeping the anode and cathode

liquids separate. Secondly, membranes also serve as a barrier to the transfer of other species in

the chamber and it is used that protons produced at the anode can migrate to the cathode. For

this reason, it need to be permeable. .


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Introduction

25

Fig. 2.10. Different membranes tested in MFCs: (A) cation exchange membrane (CMI- 7000,

Membranes International, Inc.); (B) anion exchange membrane (AMI-7001, MembranesInternational, Inc.); (C) Nafion 117 membrane (Ion Power, Inc.).

iii) Cathode materials

In generally, the materials that have been described above for use as the anode have also

been used as cathodes.

Thus, studies have used carbon paper, cloth, graphite, woven, graphite, graphite granules,

brushes, etc.


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Introduction

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Fig. 2.11. Material used as cathode in the MFC

2.2.

Applications

Power generation

Microbial fuel cells have a number of potential uses. The most readily apparent is

harvesting electricity produced for use as a power source. Virtually any organic material could

be used to feed the fuel cell, including coupling cells towastewater treatment plants.

Bacteria would consume waste material from the water and produce supplementary

power for the plant. The gains to be made from doing this are that MFCs are a very clean andefficient method of energy production. Chemical processing wastewater and designed synthetic

wastewater have been used to produce bioelectricity in dual and single chambered mediatorless

MFCs (non-coated graphite electrodes) apart from wastewater treatment.

Higher power production was observed with biofilm covered anode (graphite).A fuel

cells emissions are well below regulations. MFCs also use energy much more efficiently than

standard combustion engines which are limited by the Carnot Cycle. In theory an MFC is

capable of energy efficiency far beyond 50% (Yue & Lowther, 1986). According to new research

conducted by Ren Rozendal, using the new microbial fuel cells, conversion of the energy to

hydrogen is 8x as high as conventional hydrogen production technologies.

However MFCs do not have to be used on a large scale, as the electrodes in some cases

need only be 7 m thick by 2 cm long. The advantages to using an MFC in this situation as

opposed to a normal battery is that it uses a renewable form of energy and would not need to be
http://en.wikipedia.org/wiki/Wastewaterhttp://en.wikipedia.org/wiki/Biofilmhttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Carnot_Cyclehttp://en.wikipedia.org/wiki/Carnot_Cyclehttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Biofilmhttp://en.wikipedia.org/wiki/Wastewater


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Introduction

27

recharged like a standard battery would. In addition to this they could operate well in mild

conditions, 20 C to 40 C and also at pH of around 7. Although more powerful than metal

catalysts, they are currently too unstable for long term medical applications such as in

pacemakers (Biotech/Life Sciences Portal).

Besides wastewater power plants, as mentioned before, energy can also be derived

directly from crops. This allows the set-up of power stations based on algae platforms or other

plants incorporating a large field of aquatic plants. According to Bert Hamelers, the fields are

best set-up in synergy with existing renewable plants (e.g. offshore windturbines). This reduces

costs as the microbial fuel cell plant can then make use of the same electricity lines as the wind

turbines.

Education

Soil-based microbial fuel cells are popular educational tools, as they employ a range of

scientific disciplines (microbiology, geochemistry, electrical engineering, etc.), and can be made

using commonly available materials, such as soils and items from the refrigerator. There are also

kits available for classrooms and hobbyists, and research-grade kits for scientific laboratories and

corporations.

Biosensor

Since the current generated from a microbial fuel cell is directly proportional to the

energy content of wastewater used as the fuel, an MFC can be used to measure the solute

concentration of wastewater (i.e. as abiosensor system).

The strength of wastewater is commonly evaluated as biochemical oxygen demand

(BOD) values. BOD values are determined incubating samples for 5 days with proper source of

microbes, usually activate sludge collected from sewage works. When BOD values are used as a

real time control parameter, 5 days' incubation is too long.

An MFC-type BOD sensor can be used to measure real time BOD values. Oxygen and

nitrate are preferred electron acceptors over the electrode reducing current generation from an

MFC. MFC-type BOD sensors underestimate BOD values in the presence of these electron

acceptors. This can be avoided by inhibiting aerobic and nitrate respirations in the MFC using

terminal oxidase inhibitors such as cyanide and azide. This type of BOD sensor is commercially

available.
http://en.wikipedia.org/wiki/PHhttp://en.wikipedia.org/wiki/Artificial_pacemakerhttp://en.wikipedia.org/wiki/Biosensorhttp://en.wikipedia.org/wiki/Biosensorhttp://en.wikipedia.org/wiki/Artificial_pacemakerhttp://en.wikipedia.org/wiki/PH


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Introduction

Current research practices

Some researchers point out some undesirable practices, such as recording the maximum

current obtained by the cell when connecting it to a resistance as an indication of its

performance, instead of the steady-state current that is often a degree of magnitude lower. Often

times the data about the values of the used resistance is minimal, or even non-existent, making

much of the data non-comparable across all studies. This makes extrapolation from standardized

procedures difficult if not impossible.

Commercial applications

A number of companies have emerged to commercialize microbial fuel cells. These

companies have attempted to tap into both the remediation and electricity generating aspects of

the technologies. Some of these are companies are mentioned here.
http://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance

sustainabe energy

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