Microbial Electrochemical and Fuel Cells


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Woodhead Publishing Series in Energy:
Number 88

Microbial Electrochemical
and Fuel Cells
Fundamentals and Applications

Edited by

Keith Scott and Eileen Hao Yu

AMSTERDAM • BOSTON • CAMBRIDGE • HEIDELBERG
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Contents

Contributors
Woodhead Publishing Series in Energy

ix
xi

Part One

1

1

2

3

The workings of microbial fuel cells

An introduction to microbial fuel cells
K. Scott
1.1 Introduction
1.2 Fuel cells
1.3 Biological FCs
1.4 The MFC

1.5 Biological enzyme FC
1.6 Conclusions
References
Electrochemical principles and characterization of
bioelectrochemical systems
K. Scott
2.1
Introduction
2.2
Electrochemical principles
2.3
Voltammetric electrochemical methods
2.4
Rotating disk and ring-disk electrodes
2.5
Electrochemical impedance spectroscopy
2.6
Chronoamperometry
2.7
Square wave voltammetry
2.8
Differential pulse voltammetry
2.9
Other techniques
References
Further reading
Electron transfer mechanisms in biofilms
J. Philips, K. Verbeeck, K. Rabaey and J.B.A. Arends
3.1 Introduction
3.2 Mechanisms for delivering electrons to an anode

3.3 Mechanisms for electron uptake from cathodes
3.4 EET between microorganisms
3.5 Future trends and research needs

3
3
4
9
11
24
25
26
29
29
30
41
49
52
59
61
62
62
63
66
67
67
73
82
92
96



vi

Contents

3.6 Conclusion
Acknowledgments
References
Further reading

Part Two Materials for microbial fuel cells and
reactor design
4

5

6

7

98
99
99
113

115

Anode materials for microbial fuel cells
A. Dumitru and K. Scott

4.1 Introduction
4.2 Anode materials
4.3 Surface modification of MFC anode materials
4.4 Conclusions and future perspective
References
Further reading

117

Membranes and separators for microbial fuel cells
K. Scott
5.1 Introduction
5.2 Cell separators
5.3 Transport processes in membranes and diaphragms
5.4 Membranes for microbial fuel cells
5.5 Future trends
References
Bibliography

153

Cathodes for microbial fuel cells
S. Bajracharya, A. ElMekawy, S. Srikanth and D. Pant
6.1 Introduction
6.2 Redox reactions for MFCs
6.3 The oxygen reduction mechanism
6.4 Hydrogen evolution mechanism
6.5 ORR cathode configuration in MFC
6.6 Non-precious metal cathodes
6.7 Enzymatic cathodes

6.8 Future trends
Acknowledgment
References

179

Reactor design and scale-up
G.C. Premier, I.S. Michie, H.C. Boghani, K.R. Fradler and J.R. Kim
7.1 Introduction
7.2 Performance indicators for MFCs

215

117
118
125
144
146
152

153
155
158
161
175
176
177

179
179

183
189
192
193
198
205
205
206

215
216


Contents

7.3 What governs the performance of MFCs
7.4 Determining the performance of MFCs
7.5 MFC architectures
7.6 Connectivity and control mechanisms
7.7 MFC scale-up, application, and integration
7.8 Future trends
References

Part Three Applications of microbial electrochemical
and fuel cells
8

9

Microbial fuel cells for wastewater treatment and energy

generation
V.G. Gude
8.1
Wastewater treatment
8.2
Wastewater–energy–environment nexus
8.3
Energy requirements for wastewater treatment
8.4
Energy recovery in wastewater treatment systems
8.5
Microbial fuel cells
8.6
Organic removal in MFCs
8.7
Algae biocathode for MFCs
8.8
Nitrogen removal in MFCs
8.9
Phosphorus removal in MFCs
8.10 Metals removal in MFCs
8.11 Source separation
8.12 Conclusions
Acknowledgments
References
Microbial electrolysis cells for hydrogen production
S. Cotterill, E. Heidrich and T. Curtis
9.1
Introduction
9.2

Advantages
9.3
Disadvantages
9.4
Role in the hydrogen economy
9.5
How to characterize an MEC
9.6
Rhetoric to reality?
9.7
Problems
9.8
Beyond hydrogen
9.9
Prospects for deployment of MEC
9.10 Conclusions: How to make MECs happen?
Further reading
References

vii

222
224
228
230
235
237
239

245

247
247
247
249
251
254
260
266
268
271
272
274
275
276
276
287
287
289
290
290
291
295
312
314
314
315
316
316



viii

10

11

12

Contents

Resource recovery with microbial electrochemical systems
E.H. Yu
10.1 Introduction
10.2 Metal recovery
10.3 Nutrients removal and recovery
10.4 Converting CO2 to valuable chemicals
10.5 Prospective
References

321

Use of microbial fuel cells in sensors
M. Di Lorenzo
11.1 An introduction to biosensors
11.2 Microbial biosensors
11.3 The use of microbial fuel cells as electrochemical sensor
11.4 Operation of the MFC sensor
11.5 MFC sensor design
11.6 MFCs as BOD sensors
11.7 Detection of toxicants in water by MFCs

11.8 Conclusions
References

341

The practical implementation of microbial fuel cell technology
I. Ieropoulos, J. Winfield, I. Gajda, A. Walter, G. Papaharalabos,
I.M. Jimenez, G. Pasternak, J. You, A. Tremouli, A. Stinchcombe,
S. Forbes and J. Greenman
12.1 Introduction
12.2 Direct use of microbial fuel cells
12.3 Implementing energy harvesting
12.4 Field trials
12.5 Conclusions
References

357

Index

321
322
328
332
334
335

341
341
342

343
346
347
351
353
353

357
358
363
372
378
378
381


Contributors

J.B.A. Arends Ghent University, Gent, Belgium
S. Bajracharya VITO - Flemish Institute for Technological Research, Mol, Belgium,
and Wageningen University, Wageningen, The Netherlands
H.C. Boghani University of South Wales, Pontypridd, UK
S. Cotterill Newcastle University, Newcastle upon Tyne, UK
T. Curtis Newcastle University, Newcastle upon Tyne, UK
M. Di Lorenzo University of Bath, Bath, UK
A. Dumitru University of Bucharest, Magurele, Romania
A. ElMekawy VITO - Flemish Institute for Technological Research, Mol, Belgium,
and University of Sadat City, Sadat City, Egypt
S. Forbes University of the West of England, Bristol, UK
K.R. Fradler University of South Wales, Pontypridd, UK

I. Gajda University of the West of England, Bristol, UK
J. Greenman University of the West of England, Bristol, UK
V.G. Gude Mississippi State University, Mississippi, MS, USA
E. Heidrich Newcastle University, Newcastle upon Tyne, UK
I. Ieropoulos University of the West of England, Bristol, UK
I.M. Jimenez University of the West of England, Bristol, UK
J.R. Kim Pusan National University (PNU), Busan, Republic of Korea
I.S. Michie University of South Wales, Pontypridd, UK


x

Contributors

D. Pant VITO - Flemish Institute for Technological Research, Mol, Belgium
G. Papaharalabos University of the West of England, Bristol, UK
G. Pasternak University of the West of England, Bristol, UK
J. Philips Ghent University, Gent, Belgium
G.C. Premier University of South Wales, Pontypridd, UK
K. Rabaey Ghent University, Gent, Belgium
K. Scott Newcastle University, Newcastle upon Tyne, UK
S. Srikanth VITO - Flemish Institute for Technological Research, Mol, Belgium
A. Stinchcombe University of the West of England, Bristol, UK
A. Tremouli University of the West of England, Bristol, UK
K. Verbeeck Ghent University, Gent, Belgium
A. Walter University of the West of England, Bristol, UK
J. Winfield University of the West of England, Bristol, UK
J. You University of the West of England, Bristol, UK
E.H. Yu School of Chemical Engineering and Advanced Materials, Newcastle
University, Newcastle upon Tyne, UK



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Part One
The workings of microbial
fuel cells


This page intentionally left blank


An introduction to microbial
fuel cells

1

K. Scott
Newcastle University, Newcastle upon Tyne, UK

1.1

Introduction

The increasing demand for energy worldwide and the requirement to limit and control
carbon emissions means that a major emphasis is being placed on providing sustainable
sources of energy. Thus, major efforts are being put into technologies based on renewables and in producing hydrogen as a fuel using, for example, wind or solar power to
power electrolysis. Fermentation, photobiological methods, and use of algae are alternative ways of producing hydrogen (or methane) from plant and biomass (Luzzi et al.,
2004). Many of these processes have limitations in efficiency and thus a mix of technologies will be required to satisfy the potential energy requirements or create fuels such as
hydrogen. As yet, no technology can compete cost-wise with the generation of hydrogen
from fossil fuels. Faced with this unsustainable nature of hydrogen generation, more efficient alternative methods need to be developed to operate in conjunction with other
energy supply routes. In parallel with research and technology development to produce

hydrogen, there has been a significant growth in fuel cell (FC) technologies as a means of
supplying clean and efficient power from hydrogen. However, as most FCs use hydrogen
produced from hydrocarbon sources, this presents a dilemma in balancing sustainability
in energy and carbon emissions. Attempts to use more sustainable fuels such as alcohols
(methanol, ethanol) and sugars (glucose), which can be sourced naturally or by fermentation, in FCs has had limited success due to either poor efficiency or poor performance of
catalysts to break down the fuels. However, the Earth has an abundant resource of carbonbased potential fuels occurring naturally or produced via industrial processes in the form
of wastes and by-products. Many of these carbon sources are not immediate, viable fuels
for current FCs, and technological research is underway to indirectly use FCs, for example, through purification (and reforming) of biogas. A technology that can directly recover
electrical energy from wastes is an attractive proposition and in this context biofuel cells
(BioFCs) have a potential role to play if they can capitalize on a wide range of carbon
sources (e.g., urea, waste, and sludge), at sufficiently low cost. The fact that BioFCs
can convert readily available substrates from sustainable sources into hydrogen or electrical energy could make a major contribution to energy requirements. Such a process
would also provide a means of simultaneously reducing the waste treatment costs currently associated with waste carbon sources. However, BioFCs are at an early stage of
development and significant research is needed to approach technology readiness. This
chapter provides an overview of the basic principles of biological FCs, materials, and their
applications that are described in much more detail in the remaining chapters of the book.
Microbial Electrochemical and Fuel Cells. />Copyright © 2016 Elsevier Ltd. All rights reserved.


4

1.2

Microbial Electrochemical and Fuel Cells

Fuel cells

FCs are electrochemical devices that convert the intrinsic chemical energy in fuels
into electrical energy directly. As the intermediate steps of producing heat and
mechanical work in most conventional power generation methods are avoided in

FCs, they are not affected by the thermodynamic limitations of conventional heat
engines, defined by the Carnot efficiency (EG&G Technical Services Inc., 2004).
As such, FCs promise power generation at high efficiency and low environmental
impact (i.e., minimal pollutants). FCs can, in principle, process a wide variety of fuels
and oxidants, although of most interest today are common fuels, such as natural gas
(and derivatives) or hydrogen, in which air is used as the oxidant.
In a FC, fuel is fed continuously to the anode (negative electrode) and an oxidant
(often oxygen in air) is fed continuously to the cathode (positive electrode). The electrochemical reactions take place at the electrodes to produce an electric current
through an electrolyte, while driving a complementary electric current that performs
work on the load. A schematic representation of a hydrogen and oxygen FC (based on
an acidic electrolyte) is presented in Figure 1.1.

H2 in

e–

H+
H+

e–
e–

Cathode current collector

e–

H+

O2 + 4 H+ + 4 e– →
2 H2O

e–
Cathode catalyst layer

e–

Anode catalyst layer

e–

H2 out

Anode current collector

2 H2 → 4 H+ + 4 e–

Membrane electrode
assembly
(MEA)

Figure 1.1 Principle of operation of a chemical fuel cell (FC) based on proton transfer.


An introduction to microbial fuel cells

5

At the anode of the FC, hydrogen gas ionizes, releasing electrons and creating H+
ion (protons), thereby releasing energy
2H2 ! 4H + + 4eÀ


(1.1)

At the cathode oxygen reacts with protons and electrons taken from the anode to form
water
O2 + 4H + + 4eÀ ! 2H2 O

(1.2)

The electrons (negative charge) flow from anode to cathode in the external circuit and
the H+ ions pass through the electrolyte.
Importantly, the electrolyte should only allow proton transfer (or other ions in the
case of other FC types) and not electron transfer (i.e., the electrolyte should be an electronic insulator). Otherwise the electrons would not pass around the external circuit
and thus they would “short circuit” the cell and the function of the FC would be lost.
In theory, any substance capable of chemical oxidation (the reductant) that can be
supplied continuously can be used “galvanically” as a fuel at the anode of a FC.
Similarly, the oxidant can be any fluid that can be reduced at a sufficient rate. For
practical reasons, the most common oxidant is gaseous oxygen, which is readily
available from air. Moreover, because of kinetic limitations in catalysts for fuel oxidation, the fuels typically used are ones with simple molecules such as hydrogen,
methane, and methanol. It is the kinetic limitation in classic chemical FCs that has
helped to stimulate greater interest in biological FCs to utilize a wider range of fuels.

1.2.1 Cell voltage
Performance of a FC is related to the voltage it generates in consumption of a fuel. Cell
voltage is determined from thermodynamics of the reactions, the electrochemical
kinetics, transport processes and the cell design. The thermodynamic potential (at zero
current) is determined from the Gibbs free energy change which for an aqueous hydrogen oxygen FC under standard conditions is 1.23 V. This potential will change
depending upon conditions of operation, such as an increase in pressure which
increase the voltage while a decrease in temperature decreases the voltage, but only
by a small amount in the range of 20–100 °C.
The voltage of a FC falls as power is drawn from it by the flow of current through

the load. As the current density (current normalized to the cross section area of the cell
in A/cm2) increases a series of internal “resistances,” referred to as polarizations,
reduce the voltage. These polarizations of the cell voltage include contributions from
the anode and cathode potentials and Ohmic polarization. The extent of the electrode
polarization losses in a FC are illustrated in Figure 1.2. The net result of current flow in
a FC is to increase the anode potential and to decrease the cathode potential, therefore
decreasing the cell voltage.
The cell voltage for a FC is thus written as


6

Microbial Electrochemical and Fuel Cells

1.2

Theoretical thermodynamic equilibrium potential 1.23 V
At open circuit, voltage loss due to crossover and
electrode heterogeneity

1

E (V)

0.8

Sharp intial voltage fall: kinetic
dominated
E vs j linear region: Ohmic losses


More rapid fall in E:
mass transport limits

0.6

0.4
Sharp E fall: mass transport
limited
0.2

0
0.0

Current density (A/cm2)
0.2

0.4

0.6

0.8

1.0

Figure 1.2 FC voltage losses and power density.


 

Ecell ¼ ΔEe À ηc, activation  À ηa, activation  À IR À ηmt


(1.3)

where ΔEe is the sum of the equilibrium potentials of cathode and anode reactions
(i.e., the reversible cell voltage); Ee. The terms ηa and ηc are overpotentials at the cathode and the anode; ηmt relates to mass transport overpotentials; and IRcell is the sum of
the Ohmic voltage losses in the electrolytes, the cell separator, electrodes, and in the
connections from the power supply to the electrodes.
In practice for low-temperature FCs, a lower potential than the theoretical
potential is achieved due to several factors such as fuel and oxidant crossover, and
the heterogeneous nature of the electrode (e.g., surface equilibria, oxidation of
catalyst).
Activation losses are caused by sluggish electrode kinetics and are the result of
complex, surface electrochemical reactions. In the case of most cells operating at practical (and thus with relatively high) overpotentials (ηact > 50–100 mV), Tafel relationships can approximate the voltage loss due to activation polarization:
ηact ¼

 
RT
i
ln
αnF
io

(1.4)

where α is the charge transfer coefficient and io is the exchange current density.
Note that this relationship does not consider the influence of mass transport and
thus concentration changes on the activation polarization. Also, io is a function of
the concentrations of active constituents.



An introduction to microbial fuel cells

7

Ohmic polarization occurs from the resistance to the flow of ions in the electrolyte
and the flow of electrons through the electrode
V ¼ IR

(1.5)

where R is the total cell resistance, which includes electronic, ionic, and contact
resistance.
The resistance of the electrolyte and associated cell separator frequently constitutes
a major component of the IR loss in a cell. For electrolytes the resistance is defined by
the electrolyte resistivity, ρ (Ω m), or alternatively through the electrolyte conductivity κ (S/m) (i.e., the inverse of resistivity) as
R ¼ ρd=A ¼ d=ðκAÞ

(1.6)

where A is the cross section and d is the interelectrode gap.
The voltage loss is thus expressed as
V ¼ iρl ¼ il=κ
(1.7)
Ideally, the electrolyte conductivity should be as high as possible, using appropriate
conducting electrolytes, although this must not prove detrimental to the stability of the
electrodes and the separator and to the performance of the reactions. To be consistent
with the previous terminology of using current density, the Ohmic resistance is often
normalized by the cross-sectional cell area (A) in an area specific resistance (ASR)
that has units of Ω cm2:
V ¼ I ðASRÞ


(1.8)

1.2.2 Mass transport and concentration effects
Mass transport-related polarization losses arise as a reactant is consumed at the electrode by electrochemical reaction. The reactant is often diluted by the products and
can only move at a finite mass transport rate. As a consequence, a concentration gradient is formed, which drives the mass transport process (Figure 1.3). At low current
densities and high bulk reactant concentrations mass transport losses are not significant, while at practical conditions (high current densities, low fuel and air concentrations), they often contribute significantly to loss of cell potential.
Mass transport limitations have an effect on the theoretical Nernst potential and on
activation overpotentials. For example the effect on activations losses can be represented by (Scott, 2015):
ηa ¼


RT
ln i
αnF

j

1À

i
iL

!

!
À ln ðio CB Þ

(1.9)


where iL is the mass transport limited current density and CB is the bulk concentration
of the active reactant species (e.g., oxygen in the oxygen reduction cathode).


8

Microbial Electrochemical and Fuel Cells
CA

Electrochemical reaction
at surface

Reactant diffusion
CAS

CAs→0

Limiting
current
density
=nFkLCA

d

Diffusion region

Figure 1.3 Concentration gradient and mass transport at electrodes.

The mass transport limiting current density are determined by diffusion processes
in electrolytes or electrode structures and may be, in the simplest form, given by

iL ¼ nFkL CB , where kL ¼ D=δ

(1.10)

where D is the diffusion coefficient in the mass transport limiting region of
thickness δ.
An additional mass transport-related factor in FCs is that associated with fuel and
oxidant crossover. Electrolytes in FCs are selected because of their inherent ion
conducting properties. However, they may also exhibit permeability to reactants. In
the case of fuel permeation through the electrolyte, from anode to cathode, this
so-called crossover of fuel reacts with the oxygen at the cathode directly wasting
the two electrons associated with it electrochemical reaction, via electron flow around
the external circuit. The crossover not only wastes fuel, but also causes a loss in
performance through an additional polarization at the cathode. This polarization is
particularly significant in low-temperature FCs near open circuit conditions, where
typically the value of cell potential is much lower than the theoretical value.
At equilibrium (or open circuit) conditions, hydrogen, which diffuses through the
electrolyte membrane to a cathode, reacts with oxygen to form water. One approach to
estimating the effect of crossover is to consider that the hydrogen and oxygen at the
cathode produce an internal current that polarizes the cathode when it is at equilibrium
(Larmanie and Dicks, 2003).

1.2.3 Figures of merit
FCs have several figures of merit that are used to define various performance capabilities and that are used as means of performance comparisons for one cell to another.
For comparison of FC electrodes the power density (W/m2 or mW/cm2) is frequently
used to indicate performance as it is linked to the ability of a system to supply power in
relation to cell size and cost. The power density, which is the product of the cell voltage and current density, increases as the current density increases and goes through a
maximum at a particular current density. The peak power density is the maximum