Group assignment - Group 5- Bioenergy - TEP4270

Prof. Khanh-Quang Tran

Glucose and bacterial fuel cells

Enrico Cobelli, Simone Ghio, Catherine Lamontagne, Brytt Ingunn Nydal,

Boris Alexandre Robinet, Gunnar Thomas Vorwerk-Handing Trondheim, 2015

Abstract:

Fuel cells are becoming a more and more attractive energy source because it produces clean

energy. Among them, biofuel cells are now facing technological challenges to make the big step

between technologies of laboratory to industrial size devices.

In this paper, after a short discussion about general working principles and governing laws of

general fuel cells, the central focus is on microbial fuel cells, a particular category of biofuel cells

using several communities of microbes as catalysts. The main frame is given, applications and

performance are investigated. New ideas and prototypes are proposed during the description.

Finally, glucose fuel cells are discussed. They do not represent a particular kind of fuel cell per se, but they group all types which use glucose as principal feeding. Therefore, both biotical and abiotical fuel cell could be categorized with that label. So, the most important features of these

technologies are highlighted and developed.

To have a deeper understanding of the task considered here, some additional references are

suggested during the discussion.

Keywords: fuel cell, microbes, glucose, applications, performance.

Contents:

1. Introduction ............................................................................................................................. 2 2. Theory and background of fuel cells ....................................................................................... 3

2.1 Actual scenario around the world ........................................................................ 3

2.2 Brief history of the technology ............................................................................. 3

2.3 Expected future development ............................................................................... 4 2.4 General description .............................................................................................. 4

3. Bacterial fuel cells ................................................................................................................. 6 3.1 Introduction .......................................................................................................... 6 3.2 Technology status ................................................................................................. 7

3.2.1 Anode chamber description ..................................................................... 7 3.2.2 Cathode chamber characterization ......................................................... 7 3.2.3 Analysis of membranes ............................................................................ 8

3.3 Bioenergy production using MFCs ...................................................................... 8

3.3.1 Heterotrophic MFCs ............................................................................... 8 3.3.2 Photosynthetic MFCs .............................................................................. 9 3.3.3 Hybrid MFCs........................................................................................... 9

2

3.4 Focus on the mostly used kinds of bacteria ........................................................ 9 3.5 How to feed a MFC ............................................................................................. 10 3.6 Parameters defining MFCs performance ........................................................... 10

3.6.1 Parameters influence and optimization procedure.................................. 12

3.7 Applications of MFCs ......................................................................................... 13 3.7.1 Storage devices ....................................................................................... 14

3.7.2 Biosensors ............................................................................................... 14

3.7.3 Bio implantable MFCs ............................................................................ 14

3.7.4 Wastewater treatment ............................................................................. 14

3.7.5 Constructed wetlands ............................................................................. 14

3.7.6 Mobil fuel ................................................................................................ 15

3.8 Remaining challenges and future development .................................................. 15 4. Glucose fuel cells ................................................................................................................... 16

4.1 Introduction ......................................................................................................... 16

4.2 Enzymatic glucose fuel cells ................................................................................ 16

4.3 Abiotic glucose fuel cells ..................................................................................... 17

4.4 Microbial glucose fuel cells ................................................................................ 18

5. Conclusion ............................................................................................................................. 18

References ..................................................................................................................................... 19

1. Introduction

The power production from renewable energy sources such as biomass is fundamental to force the

heavy fuel demanding system to reduce its environmental footprint. In this challenging background,

fuel cells represent a promising technology to generate directly eco-friendly electrical power and to

replace outdated batteries in nano-applications. The great expected future improvement makes them

a desirable source for investing money for private companies, governments and local policies.

In particular, biological fuel cell (BFCs) are devices able to convert directly chemical energy to

electrical power through electrochemical reactions and biochemical paths. The concept behind this

technology is not very young, in fact the first running experiment involving biology and electricity is attributed to Galvani in the 1780s. He discovered that electricity could interact with the nervous

system of a frog. The interest on biofuel cells is related to the fact that they can operate at common

temperature levels (20-40C) and they can be cheap when the expensive inorganic metallic catalysts

are substituted by enzymes or microbes. In addition, a large variety of fuel substances and

interesting applications make them very attractive.

The following flow-sheet points out the main technologies belonging to electrochemical devices.

Microbial and enzymatic FCs are sub-categories of biologically catalysed fuel cell which are

different from organic fuel cells because they operate using more complex chemical as fuel sources, but still rely on inorganic catalysts to achieve the reactions as in, for example, the direct

methanol fuel cell (DMFC) [2]. Enzymatic fuel cells (EFCs) use specific isolated enzymes for their operation, and microbial fuel cells (MFCs) utilize whole organisms containing complete enzyme pathways [48]. Finally, glucose fuel cells are essentially linked to both two types of biologically fuel cell according to the kind of catalyst that is used to enable the reactions. In fact,

enzymatic FCs employ enzymes such as glucose oxidase and laccase in their isolated forms, whereas in microbial fuel cells the enzymatic system of a whole, electro-active micro-organism is

used [26]. Another classification of BFCs is referred to the nature of the electron transfer (ET): direct electron transfer (DET) or mediated electron transfer (MET) fuel cells.

3

Figure 1: General flow-sheet of technologies and categories.

2. Theory and background of fuel cells

2.1 Actual scenario around the world

Fuel cells are becoming more and more important every year as data shows. Between 2013 and

2014, the fuel cell industry had a great increase, for instance the US government counted an

increase of $0,9 billion in that period (from $1,3 billion to $2,2 billion) [9]. The driving factor for

this growing is certainly the absence of restriction about exportation or importation of fuel cells, the

fuel cell market is completely worldwide, for example the shipping of fuel cells around the world

reported a total of 180 MW in 2014 and this technology was mostly placed in CHP power plants

(Combined Heat and Power). Nowadays, the largest plant is in South Korea with a total installed

power of 59 MW. The concept is the association of twenty-one fuel cells of power from 2 to 8 MW

per each. The interesting way is that this plant is able to totally supply the local network.

In Europe, the development of fuel cells varies according to government's policies and financial

help for the installation and construction. For instance there is an upcoming project in Mannheim

(Germany) aiming to build a 1,4 MW power plant thanks to the cooperation between

FuelCellEnergy Incorporation and EON Connecting Energies GmbH of Germany.

2.2 Brief history of the technology

Production of energy from fuel cells has appeared in the first years of the nineteenth century, but it

took approximately one hundred years to be used for non-laboratory purpose. They were initially

designed as power system for spatial missions; in fact the NASA showed early interest to install

fuel cells systems to supply expedition in the space [40].

Moreover, the oil crisis in the seventies pushed research and development of this technology

especially in occidental countries, because of the fact they were particularly worried about looking

for good alternatives to reduce their dependence on fossil fuel. Nowadays, the fuel cells are used in

several systems. The range of application has considerably increased, the main sectors are:

transportation, power production (overall CHP), spatial applications and biomedical applications.

Figure 2: Time-line of fuel cells development.

4

2.3 Expected future development

The strong investments in research show that the growing of this technology is far from its end.

Important points should be understood to forecast the development. They are the reliability of the

technology and the possibility of getting large sharing of the economic market. Could it be able to

emancipate fossil fuels and to guarantee profits? The main sources of revenue are identified as the

selling of products (hydrogen, electricity) and services connected to operation and maintenance

(O&M).

According to the US government market report of 2014 the evolution of trades is piloted and

completely dependent on huge mergers and bankruptcies. However, the global evolution seems to

be an increasing of benefits for companies involved into the fuel cells production.

2.4 General description

A fuel cell is a completely different kind of power system in comparison to conventional

technologies because of different physics. The main function is still energy conversion but through

a different path: the chemical energy, stored in the fuel, is directly converted in the chemical

energy. However the global reaction is exactly the corresponding fuel combustion reaction, but it

doesnt mean there is a combustion process in the device because the global reaction is just the sum of all the intermediate reactions and it doesnt correspond to the real process. It has been considered this case:

(i)

(ii)

(iii)

Different fuels can be exploited in fuel cells but the most common is hydrogen which is used in

AFC, PEMFC, PAFC.

Three main components can be recognized into a fuel cell:

a) two electrodes: anode and cathode, where half reaction of fuel oxidation and oxygen reduction separately occur, generating a difference of electrical potential between poles.

They are always made of porous material to enhance the chemical reactions by increasing

the contact area;

b) the electrolyte: it is placed between the electrodes with different aims: avoiding the contact between fuel and supplied oxygen at the cathode, and then combustion, being permeable to

the protons produced at the anode, otherwise the global reaction is not satisfied and finally it

is designed to not be conductive for the electrons, forcing them flowing in the external

circuit;

c) the electrical circuit: it is the external path that the current, generated at the anode, follows and it is connected to an external load.

Figure 3: General operating scheme of a fuel cell [19].

5

In terms of performance, we may think fuel cells have much higher efficiency compared with other

kinds of power systems. In conventional energy systems a large number of steps is required for

energy conversion, instead in fuel cells the conversion is direct.

g

Figure 4: Energy conversion paths, first for conventional systems, seconds for FCs.

Each step in the conversion process leads to energy losses, both from the first and the second law of

thermodynamics. In particular, the conversion from chemical to thermal energy is one of the most

impacting losses in conventional power plants and it affects broadly the efficiency. This is true only

in principle as this analysis does not account for other losses (kinetics).

The fuel cell efficiency, simply defined as output energy by input, is affected by many factors:

a) Temperature: The Nersts law states that temperature increasing causes the reversible potential produced by the cell decreasing; for that reason traditional fuel cells work with

chemical reactions described by decreasing number of moles, which means a negative Free

Gibbs Energy;

(iv)

b) Pressure: Increasing the pressure leads to volume sloping down and this is beneficial as the generated electrical voltage increases. This is valid till the reaction occurs with a decreasing

number of moles. Furthermore pressure rising enhances the kinetics and consequently

decreases the irreversible potential losses;

(v)

c) Composition: The Nerst law shows the effect of the composition through the partial pressures. Increasing reactants consumption leads to decrease their partial pressure and to

increase products partial pressure, finally the generated electrical potential decreases.

(vi)

The polarization curve in the graph below (figure 5) illustrates the actual voltage of the cell is a

function of the current. Complex trends are related to several factors. It is possible to distinguish

four main different regions:

a) for low values of the current the voltage drops due to activation losses, in fact the reaction has to overcome energy barriers and this subtracts energy to the electrical circuit;

b) the second cause of the voltage drop is represented by ohmic losses, which depend on the material electrical resistance, therefore it is suitable to decrease the distance between the

cathode and the anode;

c) in the last part, the voltage drop is associated to mass transfer losses, as the cell is not able to provide the desired consumption and production rates;

d) finally, further losses like crossover of reactants and internal short circuit affect also the voltage.

6

Figure 5: Irreversible losses of a fuel cell [17].

All the losses discussed above affect broadly the efficiency of fuel cells, and they become less

competitive than conventional technologies as it is evident in the diagram below.

Figure 6: Systems efficiencies respect to power produce [17].

3. Bacterial fuel cells

3.1 Introduction

Microbial fuel cells (MFCs) have attracted growing attention of the scientific community in the

recent years as a promising and challenging technology. In a MFC, microbes interact with

electrodes through electrons, which are both removed and supplied by the electrical external circuit.

MFCs represent the major type of bio-electro-chemical systems (BESs) that convert biomass or other fuel directly into electricity thanks to the metabolic activity of the microorganisms. Power

produced is very low, but the technique is making large steps forward.

Bacterial fuel cells are per se a smart answer to processes which require considerable operation and

capital investment. It provides high energy and high environmental impact such as wastewater

treatment. In fact, MFCs are able to digest organic compounds producing electrical energy through

catalytic reactions of their microorganisms under anaerobic conditions.

The earliest MFC application was provided by Potter in 1910. He generated electrical energy from living cultures of Escherichia coli and Saccharomyces by using platinum electrodes [12]. After a first phase without large interest, the possibility of increasing power output with electron mediators

was discovered in 1980s. At the end of the following century, a real breakthrough was made when

they found bacteria (Shewanella putrefaciens) able to directly transfer electrons to the anode

reaching high efficiency and being more stable. According to the historical development and

mechanisms of electron transfer, three different generation of MFC can be distinguished. Gen-I

focuses on synthetic redox mediators, Gen-II, the most efficient (64%), utilizes mediating properties

of sulphide/sulphate and Gen-III, with the highest electric density output, does not rely on mediators

[24].

7

3.2 Technology status

In general bacterial fuel cells can oxidize several kinds of organic substrates by using different

kinds of bacteria as biocatalyst. The most common installation consists of two chambers which

contains an anode and a cathode respectively. This anode- and cathode-chambers are physically

separated by a proton exchange membrane. During the oxidation process under anaerobic

conditions, electrons from the bacteria metabolism are conveyed from the anode to the cathode by

an external electric circuit. The concrete transfer of the electrons to the anode can occur in three

different ways. First there could be a direct contact between the conductive proteins in the cell

membrane and the anode, second some bacteria can produce conductive nanowires, third there

could be mediators which act as intermediaries between the cell membrane and the anode.

Simultaneously the arising protons are transferred through the proton exchange membrane to the

cathode-chamber. At the cathode the electrons are transferred to an electron acceptor for example

oxygen or metals whereat oxygen is the most widely used acceptor. Afterwards the reduced electron

acceptor become combined with the protons to form water [12].

Besides the mentioned double-chamber MFCs there are also some research works on single-

chamber MFCs, for more details see [11] and [35]. Such single-chamber MFCs have only an anodic

chamber and the cathode is normally exposed to the air.

Figure 7: General scheme of a bacterial fuel cell. All three mechanisms of electrons transfer are

represented [54].

3.2.1 Anode chamber description

As mentioned above, the whole degradation process of the biomass takes place in the anode

chamber. Concretely biodegradable organics get degenerated by active bacteria under anaerobic

conditions. During this process CO2, H+ and e

- are formed. The essential components for this

degeneration process are the substrate which gets degenerated, bacteria which degenerate the

substrate and the anode which accept the produced electrons. One of the most effective factors to

increase the performance of a MFC is to enhance anodic microbial electron transfer. To reach this

goal, it is possible to add specific electron mediators or to optimize the cell design and electrode.

In general, an ideal electrode material should have good electrical conductivity and low resistance,

strong biocompatibility, chemical stability (anti-corrosion), large surface area and appropriate

mechanical strength and toughness [58]. Currently, a lot of different materials, such as

carbonaceous, metal and composite, are studied for use as an anode [21]. Each of them has specific

advantages. For example carbonaceous materials show a good biocompatibility, chemical stability

and conductivity and they are also relatively cheap. At this moment, these materials are most widely

spread for anodes in bacterial fuel cells (see also [43]).

3.2.2 Cathode chamber characterization

There are two common configurations of cathodes used in MFCs: air-and aqueous-cathodes. Air-

cathodes typically consists of three layers: a diffusion layer exposed to the air, a conductive supporting material, which sometimes works as a diffusion layer, and a blend of catalyst and binder

in contact with the water [21]. One side of the cathode is directly in contact with oxygen, normally

8

exposed to the air. Because of their very simple design, this configuration is the most widely used.

However, aqueous-cathodes which are immersed in aqueous phase with a limited oxygen

concentration can be used in MFCs. In this case the electrode often is made up of conductive supporting material, such as carbon cloth or platinum mesh, coated with a binder/catalyst layer [21] (see [70] for special kind of based reduction catalysts used as cathode).

Beside the mentioned cathodes there is also another kind of cathodes which can be used in MFCs: it

is called bio-cathode-MFC. Instead of a solid air-or aqueous-cathode (chemical catalyst), a bio-

cathode in form of microorganisms which promotes the cathodic reaction is used. In comprehension

with conventional cathodes, this bacterial bio-cathodes are able to decrease the construction costs

while increasing the operational sustainability of the cathode. By using this type of cathodes it is

also possible to remove unwanted compounds, for example, nitrate compounds, which are familiar pollutants in agricultural, industrial and domestic wastewater [67].

3.2.3 Analysis of membranes

As mentioned above the anode- and cathode-chambers of typical double chamber MFCs are

physically separated by a proton exchange membrane. This proton exchange membrane (PEM) has

several functions. First of all, it separates the two chambers and it allows the proton transfer from

the anode to the cathode. Furthermore it prevents short-circuit and the permeation of substrate and

oxygen. These two basic points are important factors influencing the performance of the whole

MFC because if oxygen penetrates substrate or the other way around, the columbic efficiency and

the activity of microorganism will drop [58]. Faced with these approaches, other options have been suggested, including the use of other types of membrane: cation-exchange membranes (CEM),

anion-exchange membranes (AEM), bipolar membranes (BPM) and ultracentrifugation membranes

(UCM) [21; 69]. Furthermore, there are some researches in membrane-less MFC, which seem to be a very breakthrough technology able to increase performance reducing costs (see [15]).

3.3 Bioenergy production using MFCs

MFCs harvest electricity thanks to the interaction between microorganisms and electronic devices.

There are two kinds of microorganisms which give satisfactory results for bioelectricity production:

heterotrophic and photosynthetic bacteria. They can work in different systems or be combined

together into a single system.

3.3.1 Heterotrophic MFCs

In heterotrophic MFCs the power output comes from the microbial respiration (Krebs Cycle and

ETC, Electron Transport Chain) which needs continuous supply of organic carbon. Their typical

configuration is anodic and cathodic chambers separated by a proton exchange membrane (PEM) allowing only H

+ or other cations to pass from the anode to the cathode [5]. The microbes are

localized into the anodic chamber and their task is to oxidize organic matter by transferring

electrons to the anode and then in the external circuit (EET, extracellular electron transfer). Some

enzymes and coenzymes are used to enhance the reaction, as the NAD+ (nicotinamide adenine

dinucleotide).

Challenging points in MFCs are to improve metabolic efficiency (respiration phase) and to optimize

biofilm electron transfer activity to the electrode. To enhance current density and energy efficiency,

electron transfer is supposed to be enabled. For this purpose, electrochemically active

microorganisms living in the electro biofilm should work in optimal operational conditions.

Despite the great potential microbial heterotrophic fuel cell represents, the lack of complete

knowledge about mechanisms for electron harvesting from the microorganism, and fundamental factors that maximize MFCs power-generating capabilities [5] is a huge obstacle in developing this technology.

9

Figure 8: Functional scheme of heterotrophic MFCs.

3.3.2 Photosynthetic MFCs

The bioelectric power production in photosynthetic MFCs, also called bio-solar fuel cells, relies on

biocatalytic reactions that occur in specific photosynthetic microorganisms like algae or

cyanobacteria. The electric current in the external circuit comes from electrons released during the

regeneration of carbon dioxide and water in the respiratory processes. In fact, during photosynthesis, the microorganisms capture solar energy to convert carbon dioxide and water into

oxygen and carbohydrates [5], which are fundamental ingredients for the following metabolism. In the latter process, protons are even relisted and they diffuse through the PEM from the anodic

chamber to the cathode to form water by reacting with electrons and oxygen.

Because photosynthetic MFCs require only water, carbon dioxide and sunlight to operate, they offer

advantages in terms of simplicity, absence of specific refueling, reliability in comparison to the

other main kind of fuel cell. Nevertheless these promising aspects, performance and scaling

limitation force them to the status of laboratory technology.

Figure 9: Functional scheme of photosynthetic MFCs.

3.3.3 Hybrid MFCs

Systems which integrate both heterotrophic and photosynthetic microorganisms are called hybrid

microbial fuel cells. Bioelectricity production is raised by heterotrophic bacterial respiration at the

anode while the right amount of oxygen is efficiently supplied by photosynthetic bacteria.

3.4 Focus on the mostly used kinds of bacteria

At the state-of-the-art, it has not been identified the most suitable bacteria for bioelectricity

production. In fact, different kinds of microorganisms work better under certain boundary

conditions and with determined system architectures. For instance, bacterial reactions could be

carried over a large interval of temperature, ranging from moderate or room-level temperature (13-35C) to both high temperatures (50-60C) tolerated by thermophiles and low temperatures

(

10

electrochemical performance, while mixed cultures are better suited for practical applications, especially when mixed fuels are used, as in wastewater treatment applications [48]. In general, they allow to get more power output.

Biofilms are usually made by various communities of microorganisms with specific characteristics.

For example, Pseudomonas aeruginosa shows a particular electron transfer mechanism between

cell and surfaces which can increase power output and Shewanella putrefaciens and Geobacter sulfurreducens species produce nanowires that are highly conductive [37]. These two microorganisms couple their metabolism with a reduction of available external electron acceptors such as Fe(III) or Mn(IV) oxides [22] in absence of oxygen when they are in their natural habitats. In particular, it has been demonstrated that the anodic Geobacteraceae communities play a

fundamental role in electricity generation in MFCs and gas production in MECs (microbial

electrolysis cells) (see also [28]).

Yeast Saccharomyces cerevisae and bacteria such as Escherichia coli were shown to produce a voltage, resulting in electricity generation [60]. Furthermore, D. desulfuricans 27774 bacteria are able to direct exchange electrons with different electrode materials, namely stainless steel and graphite [7], producing a stable cathodic current. Where to look for bioelectricity producing bacteria? A convenient and versatile source of bacterial mixed cultures for energy generation in microbial fuel cell microorganisms [45] is represented by the soil: in some cases, after heat treatment, microbes are ready to produce protons and electrons. In

addition, marine sediment, wastewater, fresh water sediment and activated sludge are all rich sources for these microorganisms [12].

3.5 How to feed a MFC

It has been proved that electricity can be produced from any biodegradable material, ranging from pure compounds such as acetate, glucose, cysteine, bovine serum albumin and ethanol to complex

mixtures of organic matter including domestic (human), animal, food-processing and meat-packing

wastewaters [37]. However, it is important to underline that the efficiency and economic capability of converting organic wastes to bioenergy depend on the characteristics and components of the waste material [49]. The availability and renewability of lingo-cellulosic material from agricultural residues is a low-

cost promising feedstock, also if it cannot be directly utilized by microorganisms in MFCs for electricity generation. It has to be converted to monosaccharides or other low-molecular-weight

compounds [49]. In the case of cellulose substrate, biofilms with both cellulolytic and exoelectrogenic activities are required. MFCs fed by synthetic or chemical wastewater with well-

defined composition show generally high electricity generation efficiency because of the production

of particular intermediates. In addition, starch processing wastewater is an important energy-rich

resource, which could be converted into a great variety of useful products. It has been presented a

fuel cell that is fed by starch and generates electricity at high current and power output combining

highly productive and versatile biocatalysts Clostridium butyricum and Clostridium beijerinckii, and a novel anode design consisting of a layered conductive polymer/platinum composite material [44].

Among practical MFCs sources examples are anaerobic sludge, landfill leachates, which are heavily

polluted landfill effluents, industrial and municipal wastewaters, which need particulate substrates

like cellulose and chitin. Finally, solar energy can be used as energy source for MFC: the deriving

concept of living solar cell refers to the green alga Chlamydomonas reinhardtii which is able to produce hydrogen that turns in oxidized in situ producing current.

3.6 Parameters defining MFCs performance

The power output from a MFC is dependent on both biological and electrochemical processes. The

most important parameters involved are firstly the substrate conversion rate, which is linked to

several other factors like the amount of bacterial cells, the bacterial kinetics, the efficiency of the

PEM, and secondly the over-potential at the anode, the over circuit potential (OCP) which is

11

between 0.75 V and 0.798 V. Other relevant parameters are the PEM performance, which is its

sensitivity, and the internal resistance of the MFCs [57].

In the literature, the cell growth rate represents an important value to be evaluated: it is usually

expressed as function of the substrate concentration (for more detail see [30]).

However, in the ideal case, the performance of a microbial fuel cell is strictly related to

electrochemical reactions which develop between the low potential organic substrate and the high

potential final electron acceptor (e.g. oxygen).

The current I (Ampere) measured at an electrode is evaluated summing all the local current

densities related to the electrode surface.

(vii)

The charge Q (Coulombs) is obtained from the integration of the current over time.

(viii)

The Coulombic yield (Yq, CE) represents the ratio between the actual charge produced Q and the

maximum theoretical one Qmax which takes in account all the electrons available for a redox

reaction in those conditions (for more detail see [50]).

(ix)

Other formulas have been proposed for evaluating the Coulombic efficiency (see [64]).

The real potential of the cell is always lower than its equilibrium potential (the maximum imposed

by thermodynamic) because of the presence of irreversibility. The first one can be measured over

the external resistance Rext (load resistance). The second one is simply expressed as the difference

between cathodic and anodic voltage, it represents the electromotive force of the cell. To move

from the ideal case to the real case, losses should be taken into account. The most affecting ones are

activation (energy barrier) and concentration (inability to maintain the initial substrate concentration in the bulk fluid [12]) polarization of both anodic and cathodic chambers and ohmic losses, which are linked to the resistance to ions and electrons flows.

(x)

Ohmic losses are dominant in electrolytes and could be reduced decreasing the distance between

electrodes; they are lightly affected by the rising of cathodic surface. Concentration polarization

losses could be minimized by increasing mass transfer.

The internal resistance Rint (Ohm) is one of the most limiting factors in MFCs performance. In fact,

high internal resistance limits the performance of the MFC by limiting current supply within the system [30]. It could be computed considering the ohmic losses, the real voltage and the open circuit potential (OVC).

The power output P (W) is simply obtained knowing the voltage across the external resistor. In

general the reactor liquid volume (specifically, the volume of the anode) is used as a normalizing

factor for evaluating the power density of the cell (W/m3). The power density is sometimes limited

by the cathode, so the cathode area is the normalization factor in this case.

Until now the MFCs are reported to produce power density in the range of 20 to 6860 mW/m2 [22]. For example, by using an air-cathode MFC exploiting domestic wastewater, it is possible to

reach as much 146 mW/m2, which is ten times larger than usage of other materials like anaerobic

sediments (16-28 mW/m2) or high-starch content wastewater (19-20 mW/m

2) [35].

12

The power density is also related to the cell design, for instance a single-chamber MFC produced more power from cellulose than a two-chamber MFC [4], because of internal resistance reduction and mass of cellulose increasing.

3.6.1 Parameters influence and optimization procedure

Biological optimization means selection of the most appropriate colonies for the given working

conditions. However, other kind of specific modification could be done in order to obtain better

performance. So, both bio-factors (anode communities) and abio-factors (cathode electron acceptor,

PEM) have a great impact on the power output. For example, acetate substrate is most preferable

than glucose-fed-MFCs which generate lower CE as a result of electron loss by competing bacteria,

[3]. This behaviour is linked to its fermentable nature implying its layer reduction by other bacteria

which do not produce electricity. Talking about abio-factors, for instance, replacing the PEM with

salt bridge decreases the electricity production, and the replacing of oxygen-saturated cathode solution with Fe(III)NTA or K3Fe(CN)6 solution remarkably improves the performance of MFC [36].

In short, improving MFC performance means enhancing the recovery of electrons from the

substrate (Coulombic efficiency), rising power production, reducing the cost of materials. Many

times these interests conflict each other, for instance the use of cation (CEMs) and anion exchange membranes (AEMs) in MFCs increases CE but also increases internal resistance, creates pH

gradients, and reduces the power densities compared to systems that lack membranes [69]. The most impacting factors in terms of power production could be expressed as fuel oxidation at the anode, presence of electrochemically active redox enzymes for efficient electrons transfer to the

anode, external resistance of the circuit, proton transfer through the membrane to the cathode, and

oxygen reduction at the cathode [15].

Nowadays, there is a still large gap between ideal and laboratory performances mostly because of

microbe type, fuel biomass type and concentration, ionic strength, pH, temperature and reactor configuration [12]. The electrochemical mediator oxidation rate depends overall on pH and the H+ concentration comes from a local charge balance applied in each part of the biofilm.

Recent research studies have verified that the direct contact between bacterial cells and electrodes

enhances electrons transfer and the existence of a direct correlation between the biofilm growth on surfaces and the electro-activity of the modified electrode [7]. However, the physical contact of the involved cells to the electrode limits the achievable density of active cells and thus the

achievable power density [45]. In any cases, increasing the thickness of the biofilm causes indirect electron transfer mechanism and requires mediators. As recently discovered, microorganisms even produce their own electron mediators that can be exploited to enhance transfer in microbial fuel

cells [45]. A study about how to implement very efficient microbial starting from a non-compartmentalized

fuel cell with electrode having a high oxygen reducing activity is proposed in [16]. In fact, it has

been validated that the concentration of fuel is strictly connected to the amount of electricity

generated. So, the right amount of oxygen might be guarantee through a BOD (biochemical oxygen

demand) sensor. Another approach has been proposed in order to minimize the effects of oxygen

diffusion into the other chamber in [41]. Using mixed-cultures might reduce oxygen diffusion; in

fact aerobic bacteria do not contribute to electronic generation.

The ideal mediator should have reversible redox reactions, solubility in aqueous solution, stability,

high electrode reaction rate, non-biodegradability and non-toxicity to microbes, low formal

potential to enhance electrons transfer between microorganisms and the electrode.

Long-term MFC performances are also influenced by the different substrates used. In fact, cathode

performances degrade over time with different evolutionary paths and that causes an important

reduction in power density. In the table below it is shown which power density can be expected

after one year of operation for some substrates in the scenario with new cathode, without biofilm or

with biofilm [29].

13

Figure 10: Effect of time on performance of several MFCs [29].

The temperature is a crucial parameter in microbial fuel cell performance. It is directly related to the

system kinetics (activation energy, mass transfer coefficients, conductivity of solution), thermodynamic (free Gibbs energy, electrode potentials), and nature and distribution of the

microbial community (different species will have different optimum temperatures) [34]. In fact, MFCs involved in wastewater treatment could deliver higher power density if running at

higher temperature range. Although, membrane-base cathodes allow to overcome fouling problem

and to run a MFC at low temperature.

Figure 11: Temperature influence on MFCs performance [34].

It has been demonstrated that the application of logic based control systems of the external

resistance can increase both power and cumulative energy maintaining stable operating conditions

(see [53]). However, another research has demonstrated that if the microbial community changes

with different external resistance, diverse communities are able to produce the same amount of

power. So, in the case a certain electrodes limits MFC performance, several combination of other

microbes could reach similar power output, making the system flexible and strong (see [39]).

Finally, a numerical method has been implemented to validate the influence of internal load Rint and

Rext on the power output and long-term performance of fuel cell. The periodical adjustment of the

external load can prevent proliferation of methanogens (see [51]).

Other improvements and possible optimization acting on material features are discussed in [12],

[64], [20], [55]. In fact, doping ions in cathode, CNT or CNT/PANI nano-composite are

investigated as suitable options to reduce costs, increasing performance and power output.

3.7 Applications of MFCs

Although large scale implementation of MFCs seems to be out far away in time, some applications

in micro-scale are very promising in short term period. In fact, micro-power sources are required in

a wide variety of electronic devices, wireless sensors and military mobile tech. In addition, MFCs

are per se an excellent way to evaluate microbial behavior under predefined and very well

controlled boundary conditions in order to study and select the most suitable genes in terms of

electricity production [37].

14

3.7.1 Storage devices

Micro-scale MFCs could represent a suitable storage device of renewable electricity, thanks to their

capability of accumulating low currents and high potentials. In fact, electrons could be stored and

then released during discharge phases. Advantages of this application as current/power generator

are linked to the large surface-to-volume ratio, efficient mass transport, and short proton/electron travel distance [5]. In this way it is possible to generate current and power density of 450 A/m2 and 202.5 W/m

2, and then get an excellent cycle stability of up to 1.000.000 cycles with charging / discharging of 0.25 s / 0.25 s [5].

3.7.2 Biosensors

Another possible application of MFCs is the use as biosensors for monitoring water quality:

Biochemical oxygen demand (BOD) and toxicity are strictly linked to the bacterial metabolic

activity. For instance, if bacteria are forced to operate in toxic environment, their metabolism will

substantially decrease and consequently the current generation will drop. And also the compact and miniaturized design of the MFC sensor will make it suitable for portable and real time

monitoring of environmental parameters [5]. For example, the microorganism Shewanella guarantees a precise quantification of the biological oxygen demand in sewage [38].

3.7.3 Bio implantable MFCs

Micro-scale MFCs could also be employed as permanent source for implantable biomedical devices, perhaps directly consuming the glucose as fuel in the bloodstream and other organic fuel in

the intestines/colons [5]. For practical applications, a safe implantable MFC should exchange blood and carbon dioxide between the fuel cell and the surrounding, and prevent leakage of electron

mediator, oxidant, and microbes into the blood stream [61].

3.7.4 Wastewater treatment

The possibility to convert chemical energy which is contained in wastewater into electrical energy

using MFCs has aroused great enthusiasm in the scientific community [21]. While purifying

wastewater and producing electrical power, the power consumption and the generation of sludge is

much lower than with traditional technology. The amount of excess sludge which has to be disposed

is an important financial factor in wastewater treatment. Compared with traditional technology the

amount of sludge can be reduced more than 50 % by using MFCs [21]. There are different kinds of

wastewater, MFCs have been successfully tested with human-wastewater, industrial-wastewater and

animal-wastewater. In the case of industrial-wastewater, it is also possible to remove other

substances like nitrate or chromium or neutralize alkalinity while using suitable (bio-) catalysts. A

lot of successful experiments have been reported by literature: swine wastewater treatment in [42]

or simultaneous nitrification, denitrification and carbon in [65], or MFC based on Desulfovibrio

Desulfuricans able to remove sulphur-based pollutants and monitoring wastewater in [71] (for more

details see also [46]). However, it is necessary to improve the performance of MFCs before they can be scaled up since, to date, their practical implementation is not feasible [21].

3.7.5 Constructed wetlands

Constructed wetlands are used to treat wastewater by a combination of physical, chemical and

biological processes. Due to their comparatively low cost in term of installation, operation and maintenance has seen their population increasing in the last two decades [11]. Since both, constructed wetlands and MFCs are reliant on bacteria these technologies are compatible. The main

idea behind the combination of these two technologies is to improve the capacity of wastewater

treatment of wetlands and producing electrical energy simultaneously.

15

3.7.6 Mobil fuel

Another future development of MCFs might be linked to the production of mobile fuel from renewable biomass energy sources. With further improvements, modified MFCs could become

auto-fed practical methods for hydrogen production from biodegradable materials for fuelling

vehicles. It has been shown that hydrogen could be produced directly from biomass sources through

the BEAMR process (biohydrogen, see [12]).

3.8 Remaining challenges and future development

Cost, restricted knowledge and physical limits are the largest barriers for a real spreading of MFCs.

It is totally unrealistic to expect that the power output from a MFC could reach the performance of

a conventional chemical fuel cell like a hydrogen fuel cell [12]. In fact, MFCs are fed by dilute

biomass (as in wastewater treatment) in the anodic chamber which has a limited energy (reflected

by its BOD). Indeed, the high internal (ohmic) resistance is hardly reducible and it is the most

impacting limiting agent in the power production in MFCs. It is due to restricted proton-transport is the dominant limiting factor in most MFC designs [48]. Moreover, the loss of substrate due to diffusion of oxygen through the membrane [35] is in addition one of the most affecting impacts in reducing Coulombic efficiency in MFCs. Other competitors in CE dropping are the co-existence of

other electron acceptors in the anode solution different from the bacteria [] or bacterial growth and competence [21]. If fermentation or methanogenesis processes are activated, electrons are used instead of being produced and consequently the electrical current generated falls down.

The latter negative phenomenon could be reduced through improvements in coating on the side of the cathode exposed to chamber or perhaps through continued biofilm development [35]. Anyways, the total power generated can be rose when individual cells are connected in parallel or in

series and for overcoming not enough power generation for common applications electricity

produced could be to store in rechargeable devices before transferring it to the user.

In short, the most critical challenges to be overcome are essentially reaching much higher power

density and energy efficiency, finding suitable electrode materials able to maintain performance

against problem caused by corrosion, loss of enzyme activity, degradation, fouling and maximizing

cell voltage through the improvement of the cell construction. Indeed, many efforts might be

devoted to understand completely the time dependent performance due to long-term changes in

enzyme activity, membrane blockage, and to investigate deeply scale-up consequences linked to

bio-electrochemical reaction kinetics, distribution of current density. Finally yet importantly,

another crucial aspect of MFCs is the immobilization of the mediator on the electrode [] ensuring an efficient electron transfer process [48]. Precise and accurate mathematical models should be developed in order to forecast the behaviour and to model an optimized MFC.

According to the mentioned challenges, the future research should be oriented to develop (i) the metabolic description of microbial processes, (ii) the mechanisms of electron transfer to electrode

and (iii) differences in electron transfer efficiency in different organisms and consequently the

effect of anode microbial community composition [50]. In a practical case, critical issues linked to application of MFC in large scale wastewater treatment

are represented by investment cost and fouling of the membrane. According to this viewpoint,

trying to switch to membrane-less MFC means improving feasibility and acceptability.

16

4. Glucose fuel cells

4.1 Introduction

Glucose fuel cells (GFCs) are devices able to convert chemical energy to electricity using glucose

as fuel. Research on GFCs focuses essentially on developing implantable fuel cell systems capable

to run indefinitely [26]. For more details about GFCs implementation methods and metabolism see

[63] and [27].

The efficiency of the glucose fuel cell depends on the ability to catalyse the oxidation of glucose.

Glucose fuel cells can be divided into three groups, defined by the type of catalyst: enzymatic,

abiotic and microbial fuel cells.

4.2 Enzymatic glucose fuel cells

Enzymatic biofuel cells use enzymes as catalysts to oxidize the fuel. The enzymes utilize catalytic

oxidation-reduction reactions, and they are oxido-reductase enzymes. Biofuel cells which use

enzymes have many advantages over primary batteries and traditional fuel cells. The attractive

points of these biofuel cells are the ability to operate optimally at ambient temperature, the

flexibility of fuels that can be employed, including renewables, the use of non-platinum renewable

catalysts, the simplicity of the structure.

Figure 12: Representation of an enzymatic biofuel cell [52].

There are two main application areas that are being considered for enzymatic biofuel cells. These

are power supplies for small portable power devices, implantable power supplies for sensors and for

pacemakers. It has been anticipated an integration of this power-generating prototype with other

sensors and small low power electronics systems, because of its miniaturized size. This system is

expected to have military as well as civilian applications. Indeed, they must be cheap and safe fuel

cells, capable of operating using physiological flux as blood, for being really power implanted

medical devices [59]. The first successful operation of a GBFC inside an animal is reported in [6].

It was not until 1962 that the enzymatic biofuel cell was invented employing the enzyme glucose at

the anode. However, there came up some issues with this type of biofuel cells. These included short

active lifetimes, low power densities compared to conventional fuel cells, and inefficient oxidation

of fuels.

After that, many improvements have been introduced. One of the most significant advance has been

the development of biocathodes and bioanodes that employ direct electron transfer (DET) [66]. The

importance of this method is that problems associated with the use of mediators are overcome,

because electrons are transferred from the catalyst directly to the electrode [47]. A second advance

has been an increase in the active lifetime of the enzymes, which have been extended beyond 1

year.

There are still some key issues that need to be solved in developing effective enzymatic biofuel

cells. They require porous anode and cathode structures that support fuel transport to the catalyst

reaction sites. Three technical difficulties need to be solved to overcome these issues. First, the cell

anodes should be three-dimensional, as opposed to biosensors, which are less sensitive to this

requirement. Secondly, the successful immobilization of multi-enzyme systems that can completely

oxidize the fuel to carbon dioxide is needed. Third, the anode must support efficient charge transfer

mechanisms, and balance electron transfer with proton transfer.

17

Different designs and configuration are proposed in literature: enzyme electrodes made by a film of

polythiophene derivative in [32], enzyme electrodes fabricated by covalent immobilization of

glucose oxidase (GOx) and bilirubin oxidase (BOx) on the films of a thiophene derivative in [33],

GFC systems employing protein engineered PQQ glucose dehydrogenase to increase stability of

processes in [68].

4.3 Abiotic glucose fuel cells

Unlike biotic fuel cells, abiotic fuel cells utilize abiotic catalyst like inorganic or precious metal-

based catalysts. The progress on the abiotic fuel cells has been limited due to the poisoning effect,

which results in a significant decrease in catalytic activity. Because of this most of the focus has

been on developing biotic systems [26], [14]. Anyways, this kind of GFCs guarantees large life

time periods which are suitable for long-term medical implants [62].

In the case of platinum based catalysts, the most important reactions that occur are:

(xi)

(xii)

(xii)

For more details see [31].

Abiotic fuel cells can be implanted inside or on the surface of an organism where there is a large

quantity of glucose and oxygen. An example is the vascular system of an organism. This can act as

the medium to conduct the reactants and help to remove the byproducts of the fuel cell reactions.

The problem with this approach is that glucose and oxygen exist as mixtures in body fluid.

Currently there has not been found an abiotic catalyst which can selectively catalyse glucose

oxidation in the presence of oxygen.

The ability of different abiotic material to catalyse oxygen and glucose reactions has been studied.

Catalysts like silver and activated carbon can selectively catalyse oxygen reduction in the presence

of glucose, while platinum alloys can catalyse glucose oxidation. To solve this problem, some

researches have proposed the use of a structure where the cell is open to fluid only on one side.

With this method, oxygen is reduced at the cathode before reaching the anode. The concentration of

oxygen at the anode is now low enough and the mixed potential is small.

Figure 13: Simple scheme of abiotic GFCs with one opening toward the fluid.

The main application for abiotic fuel cells is an autonomous energy supply for medical implants. In

the late 1960s implantable glucose fuel cells, based on abiotic catalysts such as noble metals and

activated carbon, was developed as power supply for cardiac pacemakers. No further developments

have been reported after that, since high-capacity lithium iodine batteries became available in the

mid-1970s.

Abiotic glucose fuel cells are promising because they offer reasonable performance and higher

stability than enzymatic and microbial glucose fuel cells. It could also greatly simplify the fuel cell

design for implantable devices, but to get there we need to overcome the challenges mentioned

above.

18

Some particular applications and interesting cases study are reported in literature: microfluidic

glucose fuel cells using Au/C anode electrode have shown one of the highest performance ever

reported until now (0.5 mWcm2

) [18], functionalized carbon nanotube (Pt/f-CNT) GFCs have very

low resistance [23], Au and Au/Ag electro-catalysts GFCs are characterized by high current density

[8] and raney-platinum film electrodes by high stability and conductivity [27]. Moreover, glucose

oxidase GOx-electrode GFCs guarantee improving of performance [32], carbon riveted Pt/TiO2C catalysts are optimal solutions for increasing mechanical resistance and stability [25]. Finally, an

alkaline direct glucose fuel cell with non-platinum catalysts has been developed and, in comparison

with biotical glucose fuel cells, it can yield a higher power output (38 mWcm2

at 60 C) and longer

durability [1].

4.4 Microbial glucose fuel cells

Recent studies have verified that microbial GFCs have some advantages with respect to enzymatic

GFCs. They have a better catalytic oxidation process and they are less sensitive to poisons and

reduction of activity under normal operation conditions. However, mediators are suitable if the

transport of electrons seems difficult to maintain [10].

The working principle of MGFCs is based on the transfer of electrons from the glucose substrate at

low potential to the electron acceptor at high potential such as oxygen [57]. In general, during

glucose conversion in anodes, the majority of glucose is firstly fermented to hydrogen and acetate,

then these new substrata are used for electricity generation. So, the reactions taking place are

fermentations, hydrogen consuming reactions and electricity generating reactions, they are all

exothermic. Finally, the above mentioned bio-reactions could be expressed through combined

reactions which describe the total process of conversion of glucose into electricity and carbon

dioxide, with some energy and carbon reserved for the growth of the communities [13]. The two

most important total reactions are:

(xiii)

(xiv)

For more details about glucose MFCs efficiency see [56].

5. Conclusion

As discussed above, because of the fact that biofuel cells are able to convert chemical energy from

biomass directly into electricity without the intermediate step of combustion, they are one of the

most promising new technology for generating electricity from renewable biomass. Although low

performance, few experience, expensive materials and core parts slow their exploitation in large

scale, some applications in micro-scale are interesting and very promising in next future.

Biofuel cells have been often developed through purely chemical viewpoint and far from thinking to

be really used in the common life. Problems related to stability and total power output have too

often been ignored, because of tests done in very small scale and set up with an initial load of fuel

which is allowed to deplete, adding then an intrinsic limitation on output duration. The next step

might be producing and testing devices that have real-world applications to show they are reliable

for long time and could effort several working conditions. Afterwards, scale up and

commercialization of MFCs could become a thinkable solution for the green revolution. In comparison to other common energy technologies, biofuel cells are defined by low power

production. To increase power output several improvements and prices decreasing are required.

19

Figure 14: Power range of biofuel cells in comparison to other technologies [2].

The driving forces which push MFCs to get an increasing part of the energy sources market are the

direct conversion of substrate energy to electricity with high efficiency and the efficient operation at

ambient condition. In addition, they do not need gas treatment because the off-gases of MFCs are enriched in carbon dioxide [] without useful energy content and they [] do not need energy input for aeration provided the cathode is passively aerated [66]. Moreover, they are able to use energy sources which are not or cannot be used at this moment like very diluted organic waste or

wastewater and they represent a possible alternative wherever electrical infrastructures are missing

[57].

Finally yet importantly, glucose fuel cells represent a real promising technology for implantable

devices because they offer the possibility of running for indefinite lifetime. However, microbial

glucose fuel cells do not seem to be the most suitable typology for glucose digestion in terms of

efficiency and stability. Direct transfer glucose abiotic fuel cells are definitely superior at the

current state-of-the-art.

References

[1] Basu and Basu, A study on direct glucose and fructose alkaline fuel cell, Electrochimica

Acta, 2010, 55, 5775-5779.

[2] Bullen et al., Biofuel cells and their development, Biosensors and Bioelectronics, 2006, 21,

2015-2045.

[3] Chae et al., Effect of different substrates on the performance, bacterial diversity, and

bacterial viability in microbial fuel cells, Bioresource Technology, 2009, 100, 3518-3525.

[4] Cheng et al., Pre-acclimation of a wastewater inoculum to cellulose in an aqueouscathode MEC improves power generation in aircathode MFCs, Bioresource Technology, 2011, 102, 367-371.

[5] Choi, Microscale microbial fuel cells: Advances and challenges, Biosensors and

Bioelectronics, 2015, 69, 8-25.

[6] Cinquin et al., A Glucose BioFuel Cell Implanted in Rats, PLoS ONE, 2010, 5, e10476.

[7] Cordas et al., Electroactive biofilms of sulphate reducing bacteria, Electrochimica Acta,

2008, 54, 29-34.

[8] Cuevas-Muiz et al., Performance of Au and AuAg nanoparticles supported on Vulcan in a

glucose laminar membraneless microfuel cell, Journal of Power Sources, 2011, 196, 5853-

5857.

[9] Curtin and Gangi, Fuel Cell Technologies Market Report, US Department of Energy 2014,

1-77.

[10] Davis and Higson, Biofuel cellsRecent advances and applications, Biosensors and Bioelectronics, 2007, 22, 1224-1235.

[11] Doherty et al., A review of a recently emerged technology: Constructed wetland Microbial fuel cells, Water Research, 2015, 85, 38-45.

20

[12] Du et al., A state of the art review on microbial fuel cells: A promising technology for

wastewater treatment and bioenergy, Biotechnology Advances, 2007, 25, 464-482.

[13] Freguia et al., Syntrophic Processes Drive the Conversion of Glucose in Microbial Fuel Cell

Anodes, Environmental Science & Technology, 2008, 42, 7937-7943.

[14] Fujiwara et al., Nonenzymatic glucose fuel cells with an anion exchange membrane as an

electrolyte, Electrochemistry Communications, 2009, 11, 390-392.

[15] Ghangrekar and Shinde, Performance of membrane-less microbial fuel cell treating

wastewater and effect of electrode distance and area on electricity production, Bioresource

Technology, 2007, 98, 2879-2885.

[16] Gil et al., Operational parameters affecting the performannce of a mediator-less microbial

fuel cell, Biosensors and Bioelectronics, 2003, 18, 327-334.

[17] Groppi, Politecnico di Milano,

[18] Guerra-Balczar et al., Synthesis of Au/C and Au/Pani for anode electrodes in glucose

microfluidic fuel cell, Electrochemistry Communications, 2010, 12, 864-867.

[19] Gutaz, Dans lintimit dune pile combustible, CEA service information mdia, 2014,1-2. [20] HaoYu et al., Microbial fuel cell performance with non-Pt cathode catalysts, Journal of

Power Sources, 2007, 171, 275-281.

[21] Hernndez-Fernndez et al., Recent progress and perspectives in microbial fuel cells for

bioenergy generation and wastewater treatment, Fuel Processing Technology, 2015, 138,

284-297.

[22] Hubenova and Mitov, Extracellular electron transfer in yeast-based biofuel cells: A review,

Bioelectrochemistry, 2015, 106, Part A, 177-185.

[23] Hussein et al., Functionalized-carbon nanotube supported electrocatalysts and buckypaper-

based biocathodes for glucose fuel cell applications, Electrochimica Acta, 2011, 56, 7659-

7665.

[24] Ieropoulos et al., Comparative study of three types of microbial fuel cell, Enzyme and

Microbial Technology, 2005, 37, 238-245.

[25] Jiang et al., Ultrahigh stable carbon riveted Pt/TiO2-C catalyst prepared by in situ

carbonized glucose for proton exchange membrane fuel cell, Energy & Environmental

Science, 2011, 4, 728-735.

[26] Kerzenmacher et al., Energy harvesting by implantable abiotically catalyzed glucose fuel

cells, Journal of Power Sources, 2008, 182, 1-17.

[27] Kerzenmacher et al., A potentially implantable glucose fuel cell with Raney-platinum film

electrodes for improved hydrolytic and oxidative stability, Journal of Power Sources, 2011,

196, 1264-1272.

[28] Kiely et al., Anode microbial communities produced by changing from microbial fuel cell to

microbial electrolysis cell operation using two different wastewaters, Bioresource

Technology, 2011, 102, 388-394.

[29] Kiely et al., Long-term cathode performance and the microbial communities that develop in

microbial fuel cells fed different fermentation endproducts, Bioresource Technology, 2011,

102, 361-366.

[30] Kim et al., An analysis of the performance of an anaerobic dual anode-chambered microbial

fuel cell, Journal of Power Sources, 2011, 196, 1909-1914.

[31] Kloke et al., A Single Layer Glucose Fuel Cell Intended as Power Supplying Coating for

Medical Implants, Fuel Cells, 2011, 11, 316-326.

[32] Kuwahara et al., Properties of the enzyme electrode fabricated with a film of polythiophene

derivative and its application to a glucose fuel cell, Journal of Applied Polymer Science,

2007, 104, 2947-2953.

[33] Kuwahara et al., Fabrication of enzyme electrodes with a polythiophene derivative and

application of them to a glucose fuel cell, Synthetic Metals, 2009, 159, 1859-1864.

[34] Larrosa-Guerrero et al., Effect of temperature on the performance of microbial fuel cells,

Fuel, 2010, 89, 3985-3994.

21

[35] Liu and Logan, Electricity Generation Using an Air-Cathode Single Chamber Microbial

Fuel Cell in the Presence and Absence of a Proton Exchange Membrane, Environmental

Science & Technology, 2004, 38, 4040-4046.

[36] Liu and Li, Effects of bio- and abio-factors on electricity production in a mediatorless

microbial fuel cell, Biochemical Engineering Journal, 2007, 36, 209-214.

[37] Logan and Regan, Electricity-producing bacterial communities in microbial fuel cells,

Trends in Microbiology, 2006, 14, 512-518.

[38] Lovley, Microbial fuel cells: novel microbial physiologies and engineering approaches,

Current Opinion in Biotechnology, 2006, 17, 327-332.

[39] Lyon et al., Is resistance futile? Changing external resistance does not improve microbial

fuel cell performance, Bioelectrochemistry, 2010, 78, 2-7.

[40] Matthey, http://www.fuelcelltoday.com/.

[41] Min et al., Electricity generation using membrane and salt bridge microbial fuel cells, Water

Research, 2005, 39, 1675-1686.

[42] Min et al., Electricity generation from swine wastewater using microbial fuel cells, Water

Research, 2005, 39, 4961-4968.

[43] Niessen et al., Fluorinated polyanilines as superior materials for electrocatalytic anodes in

bacterial fuel cells, Electrochemistry Communications, 2004, 6, 571-575.

[44] Niessen et al., Exploiting complex carbohydrates for microbial electricity generation a bacterial fuel cell operating on starch, Electrochemistry Communications, 2004, 6, 955-958.

[45] Niessen et al., Heat treated soil as convenient and versatile source of bacterial communities

for microbial electricity generation, Electrochemistry Communications, 2006, 8, 869-873.

[46] Oh et al., Sustainable wastewater treatment: How might microbial fuel cells contribute,

Biotechnology Advances, 2010, 28, 871-881.

[47] Oncescu and Erickson, A microfabricated low cost enzyme-free glucose fuel cell for

powering low-power implantable devices, Journal of Power Sources, 2011, 196, 9169-9175.

[48] Osman et al., Recent progress and continuing challenges in bio-fuel cells. Part II:

Microbial, Biosensors and Bioelectronics, 2010, 26, 953-963.

[49] Pant et al., A review of the substrates used in microbial fuel cells (MFCs) for sustainable

energy production, Bioresource Technology, 2010, 101, 1533-1543.

[50] Picioreanu et al., A computational model for biofilm-based microbial fuel cells, Water

Research, 2007, 41, 2921-2940.

[51] Pinto et al., A two-population bio-electrochemical model of a microbial fuel cell,

Bioresource Technology, 2010, 101, 5256-5265.

[52] Poulpiquet et al., Carbon Nanotube-Enzyme Biohybrids in a Green Hydrogen Economy,

2013,

[53] Premier et al., Automatic control of load increases power and efficiency in a microbial fuel

cell, Journal of Power Sources, 2011, 196, 2013-2019.

[54] Qian and Morse, Miniaturizing microbial fuel cells, Trends in Biotechnology, 2011, 29,

62-69.

[55] Qiao et al., Carbon nanotube/polyaniline composite as anode material for microbial fuel

cells, Journal of Power Sources, 2007, 170, 79-84.

[56] Rabaey et al., A microbial fuel cell capable of converting glucose to electricity at high rate

and efficiency, Biotechnology Letters, 2003, 25, 1531-1535.

[57] Rabaey and Verstraete, Microbial fuel cells: novel biotechnology for energy generation,

Trends in Biotechnology, 2005, 23, 291-298.

[58] Rahimnejad et al., Microbial fuel cell as new technology for bioelectricity generation: A

review, Alexandria Engineering Journal, 2015, 54, 745-756.

[59] Sato et al., Enzyme-based glucose fuel cell using Vitamin K3-immobilized polymer as an

electron mediator, Electrochemistry Communications, 2005, 7, 643-647.

[60] Sharma and Kundu, Biocatalysts in microbial fuel cells, Enzyme and Microbial Technology,

2010, 47, 179-188.

[61] Siu and Mu, A Microfabricated PDMS Microbial Fuel Cell, Microelectromechanical

Systems, Journal of, 2008, 17, 1329-1341.

22

[62] Stetten et al., A One-Compartment, Direct Glucose Fuel Cell for Powering Long-Term

Medical Implants, 2006, last viste 2006,

[63] THURSTON, Glucose Metabolism in a Microbial Fuel Cell. Stoichiometry of Product

Formation in a Thionine-mediated Proteus vulgaris Fuel Cell and its Relation to

Coulombic Yields, Journd of General Microbiology, 1985, 131, 1393-1401.

[64] Tsai et al., Microbial fuel cell performance of multiwall carbon nanotubes on carbon cloth

as electrodes, Journal of Power Sources, 2009, 194, 199-205.

[65] Virdis et al., Simultaneous nitrification, denitrification and carbon removal in microbial fuel

cells, Water Research, 2010, 44, 2970-2980.

[66] Wu et al., A one-compartment fructose/air biological fuel cell based on direct electron

transfer, Biosensors and Bioelectronics, 2009, 25, 326-331.

[67] Xie et al., Simultaneous carbon and nitrogen removal using an oxic/anoxic-biocathode

microbial fuel cells coupled system, Bioresource Technology, 2011, 102, 348-354.

[68] Yuhashi et al., Development of a novel glucose enzyme fuel cell system employing protein

engineered PQQ glucose dehydrogenase, Biosensors and Bioelectronics, 2005, 20, 2145-

2150.

[69] Zhang et al., Improved performance of single-chamber microbial fuel cells through control

of membrane deformation, Biosensors and Bioelectronics, 2010, 25, 1825-1828.

[70] Zhao et al., Application of pyrolysed iron(II) phthalocyanine and CoTMPP based oxygen

reduction catalysts as cathode materials in microbial fuel cells, Electrochemistry

Communications, 2005, 7, 1405-1410.

[71] Zhao et al., Factors affecting the performance of microbial fuel cells for sulfur pollutants

removal, Biosensors and Bioelectronics, 2009, 24, 1931-1936.