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The Pennsylvania State University
The Graduate School
College of Engineering
PERFORMANCE OF MICROBIAL ELECTROLYSIS CELLS WITH SEPARATOR-
ELECTRODE ASSEMBLY
A Thesis in
Environmental Engineering
by
Arupananda Sengupta
2013 Arupananda Sengupta
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Master of Science
August 2013
ii
The thesis of Arupananda Sengupta was reviewed and approved* by the following:
John M. Regan
Professor of Environmental Engineering
Thesis Advisor
Matthew M. Mench
Professor, Department of Mechanical, Aerospace and Biomedical Engineering
University of Tennessee, Knoxville
Christopher Gorski
Assistant Professor of Environmental Engineering
Peggy Johnson
Professor of Civil Engineering
Head of the Department of Civil and Environmental Engineering
*Signatures are on file in the Graduate School
iii
ABSTRACT
Microbial electrolysis cells (MECs) use microorganisms as biocatalysts to recover energy
from organic matter in the form of hydrogen, thus providing the cleanest fuel without using a
fossil-fuel precursor. MECs, together with microbial fuel cells (MFCs) that directly produce
electricity, are speculated to offer an energy-neutral or -positive wastewater treatment technology.
In this context, MECs have advantages over MFCs since theoretically there are fewer losses in
MEC systems and the hydrogen recovered has wider uses than electricity. However, large-scale
implementation of this technology has been delayed due in part to low hydrogen production rates
compared to existing hydrogen-production technologies. Reducing the distance between the
electrodes in bioelectrochemical systems offers a strategy to increase the current in these systems
by reducing the internal resistance.
In this study, MECs with separator electrode assembly (SEA) and membrane electrode
assembly (MEA) configurations were used to test the performance of MECs with minimal
electrode separation. All MECs were constructed using carbon cloth as the anode and stainless
steel (SS) mesh of pore size 60 with Pt coating (0.5 mg/cm2) as the cathode. The electrodes were
electrically insulated from each other in the system using 46% cellulose-54% polyester textile
separators (SEA MECs) or CMI7000 cation exchange membrane (MEA MECs), with a standard
two-chamber MEC having a 2-cm electrode spacing used as a benchmark. The adjacent electrode
configurations resulted in higher current densities (187 ± 44 A m-3
for the SEA MECs with 50
mM buffer and 121 ± 16 A m-3
for the MEA MECs with 100 mM buffer) as compared to the
benchmark design (103 ± 7 A m-3
with 100 mM buffer).
A problem of hydrogen recycling was anticipated due to the electrodes being placed close
together. The reduced electrode spacing facilitates hydrogen crossover from cathode to anode,
which can support current from recycled hydrogen as well as the growth of hydrogen-consuming
iv
methanogens, thereby reducing the hydrogen gas recovery. This was observed in the SEA MECs,
which had negligible production of hydrogen in later cycles and coulombic efficiencies (CEs) of
over 100%, as well as high CH4 production. This problem was operably arrested by using the
membrane instead of the textile-type separator in the MECs. However, the use of a membrane
resulted in high pH imbalances, which necessitated a higher buffer capacity. Using 100 mM
phosphate buffer, cathodic efficiencies of 121 ± 26% and an overall energy efficiency of 92 ±
10% were achieved. The pH imbalance of the MEA MECs was still not fully addressed with this
buffer strength, resulting in reduced batch times and residual acetate in the effluent.
The MECs were further studied using cyclic voltammetry (CV) and electrochemical
impedance spectroscopy (EIS) to obtain the internal resistances of the MECs. The CV results
suggested that the anodic biofilm in both the benchmark and MEA MECs were similar. EIS
results showed that at the operational applied potential of 0.7 V the total internal resistance of the
MEA MECs was 25.2 ± 4.0 Ω as compared to 41.4 ± 6.8 Ω for the benchmark MECs. The ohmic
resistances were of the order MEA MEC (8.8 ± 4.1 Ω, 100 mM buffer) < SEA MECs (9.8 ± 0.0
Ω, 50 mM buffer) < benchmark system MECs (25.5 ± 0.3 Ω, 100 mM buffer). Collectively these
results show that the SEA and MEA configurations decrease the system ohmic resistance and can
yield higher currents than an MEC design with an electrode spacing of 2 cm, but the proximity of
cathodic hydrogen production in these alternate designs increases the potential for hydrogen
recycling to the anode and losses.
v
TABLE OF CONTENTS
List of Figures .......................................................................................................................... vi
List of Tables ........................................................................................................................... vii
Acknowledgements .................................................................................................................. viii
Chapter 1 Introduction ............................................................................................................. 1
1.1 The future of H2 supply ............................................................................................... 1 1.2 Production of H2 ......................................................................................................... 2 1.3 Microbial electrolysis ................................................................................................. 4 1.4 Objective .................................................................................................................... 6 1.5 Thesis organization .................................................................................................... 6
Chapter 2 Literature review ..................................................................................................... 7
2.1 Microbial Electrolysis Cells (MECs) ......................................................................... 7 2.2 Membrane electrode assembly (MEA) in MFCs ....................................................... 16 2.2 Reducing electrode spacing in MECs ........................................................................ 21
Chapter 3 Materials and Methods ............................................................................................ 24
3.1 Reactor setup .............................................................................................................. 24 3.2 Reactor operation ....................................................................................................... 26 3.3 Chemical analysis ...................................................................................................... 27 3.4 Gas and energy calculations ....................................................................................... 27 3.5 Electrochemical analyses ........................................................................................... 30
3.5.1 Cyclic voltammetry…………………………………………………………….30
3.5.2 Electrochemical impedance spectroscopy……………………………………...30
Chapter 4 Results and Discussion…………………………………………………………….32
4.1 Performance of SEA MECs ....................................................................................... 32 4.2 Performance of MEA MECs ...................................................................................... 35
4.2.1 Initial runs using 50 mM phosphate buffer…………………………………….35
4.2.2 Effect of increasing buffer strength…………………………………………….37
4.3 Electrochemical analyses ........................................................................................... 40
4.3.1 Cyclic voltammetry…………………………………………………………….40
4.3.2 Electrochemical impedance spectroscopy……………………………………...41
4.4 Comparison to previous studies ................................................................................. 44
Chapter 5 Conclusions ............................................................................................................. 47
Chapter 6 Future work ............................................................................................................. 48 References ........................................................................................................................ 49
vi
LIST OF FIGURES
Figure 2-1. Standard MEC setup with membrane/separator (which can be removed to
make it a single-chamber MEC). Substrate is oxidized at the anode by the microbial
biofilm which produces electrons and protons. Electrons travel through the circuit
and combine with the protons at the cathode where H2 is produced. If it is a two
chambered MEC, there would be the buffer solution sans the substrate in the cathode
chamber (modified from Logan et al26
). ........................................................................... 9
Figure 2-2. MEC reactors based on different classification criteria (adapted from Zhou et
al35
for MECs) .................................................................................................................. 11
Figure 3-1. Schematics for (a) the control reactor and (b) the SEA or MEA MECs
(replacing the separator with textile-type separator or CMI7000 membrane). Photos
of (c) the duplicates of the control, with the membrane visible on the right; (d) the
MEA assembly with the membrane hidden by the anode; and (e) the MEA MEC
showing the reference electrode in front of the anode. .................................................... 25
Figure 4-1. Current profiles from initial runs of the SEA MECs before methanogenesis
was prevalent. ................................................................................................................... 33
Figure 4-2. Cathodic recovery (Rcat) shows the emergence of methanogenesis in the SEA
MECs, and CE shows evidence of hydrogen recycling. (The Rcat for CH4 is
calculated in the same way as that for hydrogen assuming consumption of 8 e-
/molecule of CH4) ............................................................................................................ 33
Figure 4-3. (a) The final anode and cathode pH and (b) current generation of the control
and MEA MECs (Note: figure represents one of the duplicates of each system, all
batches not shown) ........................................................................................................... 36
Figure 4-4. Comparison of the current densities (on an anode volume basis) in different
batches and the anode and cathode potentials (w.r.t. SHE) [Note: data of last batch
of each buffer strength] .................................................................................................... 38
Figure 4-5. Cathodic and total H2 recovery from the control and MEA MECs at 50 and
100 mM phosphate buffer strengths. ................................................................................ 39
Figure 4-6. (a) CV and (b) DCV of the control and MEA MECs ............................................ 40
Figure 4-7 Equivalent circuit used to fit the EIS data (adapted from Lu et al.109
) ................... 42
Figure 4-8 Nyquist plots obtained from whole-cell EIS at an Eap = 0.7 V of (a) SEA
MECs and (b) control (C1 and C2) and MEA (M1 and M2) MECs. (NOTE: noise
signal at lower frequencies have been removed from the figure) .................................... 42
Figure 4-9 Components of the internal resistance as calculated from fitting the model to
the EIS spectra. ................................................................................................................ 43
vii
LIST OF TABLES
Table 2-1. A comparative study of the MEA or SEA in MFCs with acetate medium as
anolyte. ............................................................................................................................. 19
Table 2-2 . A comparative study of the MEA or SEA in MFCs.(continued)........................... 20
Table 2-3. Evolution of MECs through electrode spacing. (Note: the MEA MECs are
kept for comparison in the discussions) ........................................................................... 23
Table 4-1. Energy calculations for the control and MEA MECs in 50 and 100 mM
buffer ............................................................................................................................... 38
Table 4-2. Comparison of MECs with MEA and SEA ...................................................... 46
Table 4-3. Comparison of MECs with high cathodic pH .................................................. 46
viii
ACKNOWLEDGEMENTS
I am immensely grateful to my advisor, Dr. John M. Regan, for his patient support,
advice and encouragement throughout my master‘s study. I would also like to thank Dr. Matthew
M. Mench and Dr. Christopher Gorski for their suggestions and serving as my committee.
I cherish my labmates, presently Dr. Hengjing Yan, Hiroyuki Kashima, Yi Zhao, Nick
Keyes, Alex Roll and formerly Dr. Sokhee Jung and Nick Rose, for their generous assistance and
constant support during this time. I would also like to thank Dr. Fang Zhang, Dr. Michael Siegert,
Dr. Ivan Ivanov and former lab members Dr. Justin Tokash and Bin Wei of Logan group for their
valuable inputs and help in various aspects of this study. I also appreciate everyone else in the lab
for providing opportunity of some of the most intellectually stimulating conversations I‘ve had in
my life.
I am also thankful to staff members David Jones, Peggy VanOrnum and Judy Heltman
for accommodating my research and study during this time.
Finally I would like to thanks my parents who have supported my throughout my life and
are there consistently as moral pillars, and a shout to all my friends who have held me up in times
when I have been down.
Arupananda Sengupta
August 2013
1
Chapter 1
Introduction
Microbial electrolysis is a recently developed technology for H2 production from waste
streams. It combines the removal of organic material with the production of a valuable product.
However, microbial electrolysis is still in its infancy. This thesis deals with improving the
performance of microbial electrolysis cells (MECs), with the ultimate goal of developing a cost‐
effective H2 production process.
1.1 The future of H2 supply
Fossil fuels are a finite resource1, and their combustion causes air pollution through
emissions of NOx, SO22 and particulate matter
3 and seems to contribute to global warming
4
through emissions of greenhouse gasses such as CO2. Hydrogen is considered by many to be a
part of the solution to the fast depleting supply of fossil fuels as well as the threat of global
warming caused by their use4–6
. Hydrogen has the highest energy content per weight compared to
other fuels, at 142 MJ/kg, and combustion of hydrogen leads to water as the main byproduct6–8
.
Use as fuel is only one aspect of H2. With respect to the industry it sustains, H2 is
commonly divided into 1) captive H2, 2) fuel H2, and 3) merchant H29. Captive H2 is produced
and used on‐site as a chemical for industry, which is comprised mainly of ammonia production,
petroleum processing (e.g. desulfurization and hydro cracking), and methanol production9,10
. As
a fuel, H2 is used primarily in rockets, automotive applications, heating, and power production.
This application, though presently l imited , has the potential to substitute for our fossil-
2
based transportation fuels. Its low volumetric energy density demands advanced storage
technologies to reduce the required storage space and weight in hydrogen fuel cell vehicles,
which has so far hindered its wide-scale application in this field6–8
. Merchant H2 is produced at
one location and transported to another location for various applications, such as hydrogenation
of oils and fats and production of pharmaceuticals9,11
.
According to one of the scenarios of the International Energy Agency (IEA)8, total H2
use may increase to 22 EJ in 2050, of which 12 EJ is projected to be fuel H2. In this scenario, H2
would become important for transportation, fuelling 30% of the passenger cars in 2050. Although
a large share of H2 in the transportation sector is uncertain, the H2 needs of fast growing
economies (e.g. China) make it inevitable that future H2 demands will largely exceed 5 EJ.
1.2 Production of H2
Hydrogen is the most abundant element in the universe. However, there are no effective
stores of hydrogen gas on earth12
hence it need to be produced. Production of H2 requires an
energy and H source. Present ly most H2 is produced through partial oxidation of fossil fuels,
which does not solve the problems related to the use of fossil fuels. Hence there is a growing need
for alternate green avenues of producing H2 from renewable sources. Biophotolysis, water
electrolysis, the biological water-gas shift reaction, and the use of biomass feedstocks are such
processes13,14
. Biophotolysis involves using algae or other organisms to split H2O into H2 and O2.
This process is limited in part by O2 inhibiting the hydrogenases used in hydrogen evolution, and
the maximum energy production rate achieved is 0.38 MJ m-3
hr-1 15
. Water electrolysis consists of
applying a current to water to split it into H2 and O2; the focus is on using electricity from a
renewable source (e.g., solar or wind) to make it sustainable.
3
Biomass feedstock include dedicated crops and organic waste. The use of organic waste
is preferred over the cash crops, which compete with food crops for the limited amount of arable
land16
. Biomass can be converted into H2 through the following processes.
Thermal processes convert biomass through pyrolysis, partial oxidation, and s t e a m
gasification at high temperatures (600‐1000 °C) into H213,14,17. This is mainly suitable for solid
rather than liquid organic waste since high water content results in low thermal efficiency.
Thermal conversion of biomass results in a mixture of CO, H2, CO2 CH4 and N2. The H2
concentration of the gas can be increased through the water gas shift reaction:
CO + H2O → CO2 + H2 (1.1)
This requires methane concentrations in the influent stream to be kept below 3%14
. A subsequent
separation process is required to produce pure H2 and remove tars generated from thermal
processes13,17
. Efficiencies of 35‐50 % based on the lower heating value are achieved in large
reactors for the thermal conversion of biomass into H213. Production rates of 250 kg H2 d
-1
(2800 m3 d
-1) have been reported for pilot plants
18. A disadvantage of thermal processes is the
loss of valuable nutrients in the ashes, which cannot be recycled back to the agricultural
fields7.
Fermentative processes can also be used to convert biomass into H2. These are
commonly divided into dark fermentation and photo fermentation and are suitable for solid and
liquid organic wastes. Dark fermentation, in which bacteria (e.g. Clostridium species)
anaerobically convert sugars into H2, has high energy flow rates for hydrogen, at ~18 MJ m-3
hr-1
18. It requires pretreatment of the medium either by heat, acidification, or sterilization to prevent
methanogenic utilization of the hydrogen19,20
. Even under ideal circumstances, only 33 % of the
electrons are recovered as H2, leaving considerable energy in the form of acids and alcohols19–21
.
Attempts have been made to increase the yield by optimizing reactor conditions such as pH,
4
hydraulic retention time, and partial H2 pressure22
. The produced gas is a mixture of mainly H2
and CO2, but may also contain other components such as CH423. Therefore, gas purification is
required. Production rates up to 29 m3
m-3
d-1
(volume H2/volume substrate/day) for synthetic
wastewaters and up to 8.9 m3 m
-3 d
-1 for real wastewaters have been measured
19.
Photo-fermentation involves the production of hydrogen as a byproduct of the
nitrogenase enzyme in nitrogen-limiting environments. Catering this process to produce
significant amount of H2 requires modification of the nitrogenase enzyme to allow hydrogen
production under growth conditions (i.e., with ammonia present) and modification of the
hydrogenase enzyme to prevent cells from consuming hydrogen24
. The process is energy
intensive and requires significant fermentation products as well as large anaerobic photoreactors
14,16,20. H2 production rates of photo‐fermentation are generally lower than of dark fermentation,
and so far have been below 1 m3
m-3
d-1
. Photo‐fermentation can be coupled to dark fermentation
in a 2‐step process to increase the overall yield25
.
1.3 Microbial electrolysis
In 2005, two independent research groups discovered another method for hydrogen
production called microbial electrolysis cells (MECs), in which microbes assisted in the
production of hydrogen from a variety of different substrates26
. This was an adaptation of the
microbial fuel cell (MFC) that involved microbes oxidizing organic material at an anode and
electrons sent through a circuit to a cathode, where they combined with protons to form
hydrogen. Since the process is not spontaneous, a power input is required, though the required
voltage is much less than that needed for water hydrolysis. A minimum theoretical limit of 0.14
V is necessary with acetate as the electron donor as compared to 1.23 V required for water
5
hydrolysis26–28
. Even with this input energy, the efficiency of the process can have electrical
energy efficiencies over 100 %29–31
, whereas in water electrolysis energy efficiencies are in the
range of 56-73 %32
. This is due to the energy added to the system by the substrate, which is
usually wastewater.
Much progress has been made in the MEC technology, notably from the evolution of the
membrane-less MECs28
, and though microbial electrolysis is still considered to be a nascent
technology as compared to the above-described processes of biohydrogen production, it is the
fastest growing in terms of research advancements26. It is a widely accepted fact that the
internal resistance of bioelectrochemical systems such as MECs and MFCs provides the major
drawback to the process. Strategies have been used to reduce the internal resistance, including not
only finding better microbes suitable for the process but also better materials for the electrodes,
catalysts, and different reactor configurations24,27,33–35
. Reducing the electrode spacing is a major
factor in this regard, which is known to reduce the ohmic resistance of the system. Sandwich
assemblies of electrodes with a membrane or a separator is prevalently used in MFCs, and has
been tried in MECs as well36–38
, but the only comparative study that has been done in terms of the
effect of reduced electrode spacing only took the inter-electrode distance as low as 1 cm39
. Also,
the comparison was based on current generated and hydrogen production only, and not in terms
of internal resistance. To the best of our knowledge, the only analysis of the internal resistance
solely measured the ohmic resistance of the specific system (membrane electrode assembly with
cation- and anion-exchange membrane)37
. With the prevalent use of electrochemical impedance
spectroscopy (EIS) in MFCs, a more thorough analysis can be made of the advantages and
disadvantages of a membrane/separator electrode assembly in MECs. Reactors in which the
specific issues have been identified and addressed may then be scaled up further, increasing the
potential of this field of research.
6
1.4 Objective
The main objective of this study was to analyze the performance of a small-scale MEC
with a separator electrode assembly in terms of (a) current production, (b) hydrogen production
and efficiencies, and (c) the internal resistance using EIS.
1.5 Thesis organization
This thesis is organized as follows. Chapter 2 is a literature review of MECs, use of
MEAs in MFCs, and the possible outcomes of the MEA design in the MEC configuration.
Chapter 3 follows with the materials and methods used in this research. The results and
discussions constitute Chapter 4, which is followed by conclusions and future work.
7
Chapter 2
Literature review
2.1 Microbial Electrolysis Cells (MECs)
Microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) are
bioelectrochemical systems utilizing bacteria to release the electrons from the organic matter. In
both systems, an organic or inorganic substrate is oxidized by microbes on the anode, releasing
electrons to a circuit and protons into solution. Acetate is shown as an example of an anode
reaction below:
CH3COOˉ + 3H2O → HCO3ˉ + CO2 + 8H+ + 8eˉ (2.1)
In an MFC, an air-cathode is often used to join these protons and electrons with outside
oxygen to form water:
2O2 + 8H+ + 8eˉ → 4H2O (2.2)
Microbial electrolysis cell, formerly known as a biocatalyzed electrolysis cell (BEC) or
bioelectrochemically assisted microbial reactor (BEAMR), functions by electrohydrogenesis,
where a completely anaerobic cathode is employed with a small electrical input to combine the
protons and electrons, usually at a catalytic site on the cathode, to form hydrogen gas (Figure
2.1):
8H+ + 8eˉ → 4H2 (2.3)
Gibbs‘ free energy values are given at standard biological conditions (303.15K, 1 atm,
pH = 7) In aqueous solution, the standard condition of all solutes is 1 mol kg-1
activity, and that of
water is the pure liquid 40
. For acetate to hydrogen conversion through combining equations (2.1)
and (2.3):
8
CH3COOˉ + 4H2O → 2HCO3ˉ + 4H2 + H+ ΔGrº´ = +104.6 kJ (2.4)
An electrical input is required because the conversion of acetate into hydrogen is not
spontaneous as shown in equation (2.4). This positive Gibbs‘ free energy can be overcome with
the addition of a small electrical input. The minimum voltage input that must be applied is equal
to the equilibrium voltage which is calculated from the Gibbs free energy of the reaction, using
equation (2.5) , where n is the number of moles exchanged in the reaction and F is Faraday‘s
constant (96,485 C eq-1
). Calculation for acetate is shown in equation (2.6)26
:
nF
GE
o
req
' (2.5)
VeqCeqmole
molekJEeq 14.0
)/(485,96)/(8
)/(6.104
(2.6)
Thus a minimum of 0.14 V must be applied to an acetate-fed MEC to produce hydrogen
from the reactor. However, over-potentials on the electrodes and bacterial energy usage require
higher applied potentials to account for these losses. It was found that in MECs a minimum
applied voltage of at least 0.2 V is required in practice to produce hydrogen28
.
9
Figure 2-1. Standard MEC setup with membrane/separator (which can be removed to make it a
single-chamber MEC). Substrate is oxidized at the anode by the microbial biofilm which
produces electrons and protons. Electrons travel through the circuit and combine with the protons
at the cathode where H2 is produced. If it is a two chambered MEC, there would be the buffer
solution sans the substrate in the cathode chamber (modified from Logan et al26
).
MEC has many design and operational differences compared to an MFC. Air (i.e.,
oxygen) is not required in an MEC, hence simplifying the cathode design. However since the
product is a gas rather than electricity, the reactor must be modified to collect the gas. While
oxygen diffusion into an anode chamber can substantially reduce recovery of electrons from
substrate as current in an MFC, the lack of oxygen in an MEC results, on average, in greater
electron recoveries. Additionally MFCs generally have air-cathodes causing oxygen diffusion
into the anode chamber whereas MECs operate under completely anaerobic conditions and
therefore promote the growth of obligate anaerobic bacteria such as Geobacter sp., as well as non-
exoelectrogenic fermentative or methanogenic microorganisms. Thus, microbial communities in
10
MECs may be dissimilar from those in MFCs. Still, the common practice for enrichment of the
bacterial community in an MEC is to operate an MFC and then transfer the anode into an MEC to
ensure biofilm formation. Other strategies such as inoculation using MFC/MEC effluents and
scraped biofilm from a working anode are also used26
.
Various substrates have been tested in MECs, including glucose, cellulose, acetic acid,
and lactic acid. Lower hydrogen recovery was obtained from fermentable substrates (glucose and
cellulose) as also lower overall energy efficiencies than non fermentable substrates (acetic acid
and lactic acid) in a two chamber system with an anion exchange membrane (AEM) and graphite
granule anode 41
. Hydrogen recoveries from fermentable substrates are typically around 70 %
while recoveries from non fermentable substrates are 91 %. Overall energy efficiencies have been
reported to be 64 % for fermentable substrates and 82 % for non fermentable substrates41
.
Hydrogen production rates for acetate are reported in the range of 1.1 m3m
-3d
-1 to 6.32 m
3m
-3d
-1
and for cellulose 0.11 -1.11 at 0.11 m3m
-3d
-1
27. For glucose volumetric hydrogen production,
values have been reported to reach a maximum rate of 1.87 m3 m
-3 d
-1 31
.
Various MEC reactor designs have been reviewed by Logan et al26
, Liu et al27
and Zhou
et al35
. Figure 2.2 shows a summary of reactor types based on different classification criteria.
11
Figure 2-2. MEC reactors based on different classification criteria (adapted from Zhou et al
35 for
MECs)
MEC
rea
cto
r d
esig
n
MEC configuration
single chamber
dual-chamber
reactor chamber structure
flat plate
disc
concentric cylinders
separator
anion exchange membrane
cation exchange membrane
bipolar and charge mosaic membrane
j-cloth
membrane-less
flow type
batch/fed-batch
continuous flow
serpentine flow
upflow
convective flow
integrated systems
multiple electrodes
integration with MFCs
12
The anode materials used in MECs are mostly carbon based, since they are chemically
stable under the anaerobic anodic processes. However, the carbon anodes do tend to corrode at
the high potentials on the oxygen side during electrolysis of water. Such anodes include graphite
granules, graphite felt, graphite brushes, carbon cloth and carbon paper. Graphite granules and
graphite brushes provide high surface area for the biofilm to adhere to and proliferate, but they
also add a large electrode resistance to the reactor. The graphite granules which get disconnected
with the flow of the electrolyte and/or growth of biofilm and the brush would have higher
resistance than woven-fiber carbon cloth or felt as well as have the possibility of an increased
average inter-electrode spacing27
.
The cathodes on an MEC usually have a catalyst to reduce the activation losses and
improve kinetics of the hydrogen formation at the surface. Pt is the best known catalyst, but NiO2,
Ni-alloys, tungsten carbide and stainless steel have been found to perform comparable to Pt,
while reducing costs by one or two orders of magnitude29,42,43
. Carbon is usually used as the
support for the catalyst loading with a binder (i.e., Nafion®), or other processes are used to
deposit the catalyst on the cathode like electrodeposition. A check needs to be kept on the
thickness of these materials to facilitate reduced electrode spacing on scale-up27
. Biocathodes
have also been investigated in MECs44,45
with promising results.
Many MEC reactors contain membranes or separators to separate the anode and
cathode36,41,46,47
. The membrane is used to minimize hydrogen transfer to the anode where it could
be consumed. The membrane also separates the gas produced at the anode from the gas produced
at the cathode, making a purer product gas. A membrane causes a pH gradient across the
membrane which can lead to lower pHs at the anode and higher pHs at the cathode, which can
cause performance losses36
. Membranes are expensive and add a significant cost to the MEC
system28,29,48
. Most MECs use a cation exchange membrane (CEM) such as Nafion 117, or
13
Fumasep FKE. Other types of membranes examined include anion exchange membranes (AEM),
a bipolar membrane (BPM) and a charge mosaic membrane (CMM). Based on the transport
numbers for protons and/or hydroxyl ions and the ability to prevent increase of pH in the cathode
chamber, it has been found that the membranes can be ranked in the order of BPM, AEM, CMM,
CEM27
The search for the perfect anode respiring consortia is still a very active research. Amongst the
group of electrode respiring microorganisms, bacteria from the phylum Proteobacteria seem to be
prevalent in the anode communities. Aelterman et al49
observed that 64% of the anode population
belonged to the class of α-, β-, γ-, or δ-Proteobacteria, the most studied of these belonging to the
families of Shewanella and Geobacteraceae. Geobacter sulfurreducens produced 10-times-higher
current levels in lactate-fed microbial electrolysis cells than Shewanella oneidensis50
. Both the
organisms are capable of acting as brilliant models to further explain the mechanisms of electron
transfer between microorganism and electrode51
. G. sulfurreducens (hydrogen utilizing) was
compared to both a Geobacter metallireducens (non-hydrogen oxidizer) and a mixed consortium
in order to compare the hydrogen production rates and hydrogen recoveries of pure and mixed
cultures in microbial electrolysis cells (MECs)52
. At an applied voltage of 0.7 V, both G.
sulfurreducens and the mixed culture generated similar current densities (ca.160 Am-3
), resulting
in hydrogen production rates of 1.9 m3 m
-3 d
-1, whereas G. metallireducens exhibited lower
current densities and production rates of 110 Am-3
and 1.3 m3 m
-3 d
-1, respectively. The mixed
consortium achieved the highest overall energy recovery, before methane production, of 82% as
compared to G. sulfurreducens (77%) and G. metallireducens (78%), due to the higher coulombic
efficiency (CE) of the mixed consortium. At an applied voltage of 0.4 V, CH4 production
increased in the mixed-culture MEC leading to decreased hydrogen recovery and the overall
energy recovery compared to G. sulfurreducens and G. metallireducens. Advantages of using
14
mixed cultures within MECs include: i) increased versatility with respect to substrate utilization,
ii) increased system robustness due to biological diversity, and iii) practicality e wastewater
treatment, for example, will never be a pure process. However, the major disadvantage to using
mixed consortia is the undesired selection for methanogenic bacteria27
.
Factors affecting performance of MECs
1. Energy loss: Production of hydrogen in an MEC is not a spontaneous reaction.
Supplementary energy is needed to overcome the thermodynamic limits fixed by the chemical
reactions at the electrodes and the potential losses within the system. This translates into the
minimum amount of energy required to generate H2.
2. Electrode overpotentials: Overpotentials in MECs are categorized into: i) activation
losses, ii) coulombic losses, and iii) concentration losses51,53
. The transfer of electrons to the
anode from the microbe and from the cathode to the electron acceptor needs to overcome an
energy barrier to do so, which results in the activation losses. Also the loss in potential during the
extracellular electron transfer (EET) to the anode has additional contribution24
, though this
depends mainly on the resistance of the conductive biofilm matrix, which is normally less than
2000Ωcm54
, hence cumulating to a loss of around 0.01V for a 100µm-thick biofilm producing
5Am-2
. CE is the quantity of electrons recovered as current against the amount of electrons
available in the substrate for H2 generation. This gets reduced depending on the intracellular
microbial metabolism, which in turn depends on the terminal electron carrier. G. sulfurreducens
has a terminal cytochrome with an average mid-point potential of -0.18V vs SHE, which
cumulates to an intracellular energy loss of -0.18-(-0.28V, Eo
acetate) i.e., 0.1V at pH 7, whereas the
midpoint potential of cytochrome c3 of Shewanella. putrefaciens is -0.23V which translates to a
0.05V loss24
. When the current flows, the surface concentration of the substances involved in the
reaction vary, relative to the bulk concentration in solution. Concentration loss results when the
15
supply or elimination of each substance is restricted by poor mass transfer kinetics33,51,53,55
. For
example proton gradient between the anode and cathode chamber cause a potential loss of 0.06V
per pH unit56
. Concentration gradients of reactants and products in the biofilm and the
intercellular gradient in potential of the electrons through the microbial metabolism also
contribute to these losses.
3. Ohmic loss: In MECs, the ohmic voltage loss is regulated by the resistance to electron
flow through electrical conductors (electrodes and the conducting wires) and the resistance to ion
flow through ionic conductors (liquid medium and membrane/separator, if any). This depends on
the distance between the electrode, resistivity of the electrolytes and current density.
4. H2 recycling and/or methanogenesis: Even with the use of membranes, there is some
leaching of the hydrogen produced to the anode compartment, where some of the microbes can
utilize that as electron donors for further metabolism, which reduces the yield. Call et al52
reported evidence of H2 recycling by G. sulfurreducens which was not present when the non-H2
utilizing G. metallireducens was used. Using mixed consortia, hydrogenotrophic methanogens
pose a looming problem in using up the hydrogen produced, especially in single chambered
membrane less MECs45,51,57,58
. This leads to deteriorating energy efficiencies as extra current
from hydrogen oxidation increases energy losses and applied voltage without increasing
hydrogen production59
.
To overcome the above factors various attempts have been made. Apart from the search for
‗super bugs‘35
and the varieties of electrode materials and membranes discussed earlier, a number
of design optimization attempts have been made throughout the years which have increased the
performance of the MEC technology by over two order of magnitudes in the short period of its
existence. Some of these optimizations are direct manifestations of the same in MFC technology.
16
Pretreatment of the carbon anode with ammonia gas lead to a faster start-up and increased current
densities which is thought to be caused by the more favorable adhesion of microorganisms to the
positively charged anode and to improved electron transfer to the chemically modified surface26
.
Heat treatment of the same was later on found to be similarly effective60
. Au and Pd nanoparticles
decorated graphite anodes were also tried in multi electrode MEC61
. The anodes decorated with
Au nanoparticles produced current densities up to 20-fold higher than plain graphite anodes,
while those of Pd-decorated anodes produced 50–150% higher than plain graphite anodes. Use of
multiple anode systems in MECs in parallel with the cathode reduced the solution biofilm and
polarization resistance of the MEC and resulted in an enhancement of the current and hydrogen
production rate (HPR) of MEC62
. The use of non buffered salt solution instead of the non-
sustainable phosphate buffer as the catholyte provided an inexpensive method of increasing the
performance of the MECs63
.
The liquid medium has high resistance to the flow of protons. This becomes more and
more pronounced when the switch is made from single carbon source medium used in the lab
scale experiments to wastewaters from various treatment plants which have very low
conductivity. Apart from adding materials to increase the ionic conductivity of the same, a simple
way to reduce the ohmic losses is to reduce the inter-electrode distance of the MEC. The attempts
made in this regard are discussed in details in the following sections.
2.2 Membrane electrode assembly (MEA) in MFCs
Min et al64
was one of the earliest adopters of the configuration which theoretically is
supposed to provide with the least ohmic resistance associated with the inter-electrode distance.
This configuration of the membrane sandwiched between the two electrodes is commonly known
17
as the membrane electrode assembly (MEA). This ―serpentine-flow reactor‖ achieved a power
density of 309 mW m-2
as the maximum power density with 286 mW m-2
being produced in the
continuous mode.
Following this, a separator electrode assembly (SEA) was implemented65
, where a
separator material act as a physical barrier to the two electrode to prevent short circuiting, with no
preference towards diffusion in any direction, hence further reducing the internal resistance as
well as reducing the cost of the materials for the MFCs.
Over the years researchers have adopted these configurations in different reactor designs.
consisting of upflow66
, tubular67–69
, U-tube MFCs70
, baffled MFCs71
, solid phase MFCs72
, sensor
type MFCs73
, submersible MFCs74
and various modes ranging from single to multiple MEA
assemblies in a cubic shaped MFC65,75–79
. In the initial phases, most of these designs were tested
with the so called ―synthetic‖ wastewater, which is a misnomer80
since they comprise of mostly
single carbon source as the e- donor (viz., acetate, glucose, dextran, butyrate and others), but this
was necessary to provide proof for the effectiveness and sustainability studies of the same. This is
not to say that simple organic feed have lost their prevalence in the present era, but now they are
more appropriate to test pure cultures81,82
or niche purposes like testing out new materials or
understanding the underlying principles of existing design concepts83,84
. But acetate is useful to
aid us in the comparison of various reactor types. From Table 1-1, we see that higher power
densities were achieved using a separator other than a membrane. The highest power densities
are in those reactors with multiple assemblies of separators and electrodes65,79,85
.
Subsequently, as is required in the real world scenarios, these MFCs have been
acclimatized to various complex feed types, ranging from domestic wastewater64,68,74,76,77,86
, starch
processing wastewater87,88
, cattle manure89
, human feces wastewater90
, paddy field soil91
, lake
sediment92
saline soil contaminated by petroleum hydrocarbon93
and so forth with various
18
outlooks ranging from their use as a dissolved oxygen probe, a provider of recycled water in
space research stations, cleanup of petroleum contamination as well as the preliminary aim to
gain a cheap and sustainable source of green energy. Till date the highest power density achieved
using a non-acetate medium is 832 mW m-2
(value normalized to cathode surface is reported here
from the publication since anode chamber volume couldn‘t be determined) using a submergible-
type MFC with multiple MEA with Nafion® 117 as the membrane. This MFC was acclimatized
from domestic wastewater and run on a basic anaerobic medium74
. For MFCs using actual
wastewater, an MEA MFCs running on molasses mixed sewage wastewater produced a
maximum power density of 5.06 Wm-3
with a 0.93Ωcm-2
internal resistance and 59% COD
removal94
.
19
Table 2-1. A comparative study of the MEA or SEA in MFCs with acetate medium as anolyte.
Reference
Reactor type
(anode volume) Separator
Anode (an)
material
(area - cm2)
Cathode (cat)
material (area -
cm2)
Internal
resistance
(Ω)
Power
density
(W m-3
) CE (%)
Huang et
al67
tubular (0.15L) CMI-7000
carbon
granules
layer of a
carbon/Pt
mixture on Ni
coated carbon
fibers (211.11) 117 0.18 N.A.
Choi et
al83
Two chambered
rectangular
(0.34L)
non-woven
paper fabric
filter
graphite felt
(36)
graphite felt with
0.5 mg Pt cm-2
(36) 51 1.207 20
Lefebvre
et al95
single chamber
Cylindrical
(0.028L)
Selemion ion
exchange
membrane
non wet
proof carbon
cloth (7)
wet proof carbon
cloth with 0.5
mgPt cm-2
(7) 1082 4.3
Min and
Logan64
serpentine flow
(0.022L) Nafion
carbon paper
(100)
Carbon cloth
with 10% Pt 0.5
mg/cm2 N.A. 14.04* 65 (acetate)
Ahn and
Logan75
single chamber
multi anode
2 layers textile
separator 46%
cellulose, 54%
polyester
3 carbon
fiber brush
single air
cathode with 0.5
mg Pt cm-2
(35)
33 (2 ohmic,
12 cathode
& 4 anode
charge
transfer, 16
diffusion) 30 53
* calculated from information in reference
20
Table 2-2 . A comparative study of the MEA or SEA in MFCs.(continued)
Reference
Reactor type
(anode volume) Separator
Anode (an)
material
(area - cm2)
Cathode (cat)
material (area
- cm2)
Internal
resistance
(Ω)
Power
density
(W m-3
) CE (%)
Neburchilov
et al96
single chamber
cubic (0.1L) J-cloth graphite felt
Co/FeTMPP
(3:1) 44 (acetate)
33.2
(acetate) N.A.
Hays et al77
single chamber
cubic (0.028L)
glass fiber
mat/pulp
laminated
glass fiber
carbon mesh,
carbon fiber
brush (7)
activated
carbon (7) N.A.
76 (brush
&
acetate)
7.2 (brush
reactor), 4.6
mesh
Zhang et al84
single chamber
cubic (0.028L)
CEM:
CMI-7000,
AEM
:AMI-7001
carbon cloth
carbon
brushes
carbon cloth
having
platinum with
4 PTFE (7) N.A.
98 ± 14
(Double
cthode
MFC
with
AEM)
94 ± 2% for
double
cathode AEM
MFCs
Fan et al65
cubic single
chamber (double
CEA) (0.028L) J-cloth
non wet proof
carbon cloth
(7)
carbon cloth
0.5 mg cm−2
Pt
and PTFE 3.9 (ohmic)
627 (fed
batch),
1010
(continuo
us) 63
Zhang et al79
cubic single
chamber (double
SEA) (0.028L)
Nonbiodeg
radable
glass fiber
separators
Ammonia gas
treated gas
cloth (7)
carbon cloth
with 0.5 mg Pt
cm-2
and 4
PTFE layers 2.2
696
(double
SEA) 81
Fan et al85
Single chamber
double CEA
(0.03L)
non-woven
fabric layer
(Armo
Style #600)
carbon cloth
(100x2)
carbon
cloth/PTFE
20% Pt/C
(100x2) 2.34 2080 83.5
21
2.2 Reducing electrode spacing in MECs
It has been widely speculated from the performance of the MFCs that the reduced
electrode spacing would enhance the performance of the MECs as well. From the start of the
technology with the two chambered bottle reactor21
which had an electrode spacing of 15 cm,
attempt was made by Rozendal et al36
to use the MEA assembly in the MECs. That system, with
an applied potential of 0.5V, generated only 0.02 m3
m-3
d-1
of hydrogen, being plagued with
hydrogen recycling. Subsequent to that another attempt was made by the same group to run a
similar configuration in continuous mode, this time with a better result of 0.3 m3
m-3
d-1
, and it
was found that there was the double problem of pH imbalance across the membrane as well as
methanogenesis 37
, though the ohmic resistance of 2Ω was found to be comparable with some of
the best of the MFC systems. Following that the focus was more based on increasing the
hydrogen yield based on the other parameters. Evolution of membrane-less MECs have greatly
improved the performance28
, but such systems suffer from the contamination of the gas produced
with CO2 and CH4 as well, in case of the inevitable methanogen contamination. Using high
surface area anode as well as cathode with a greatly reduced electrode spacing resulted in a
decently high current density of 188 Am-3
with a H2 production rate of 1.7 m3 m
-3 d
-1 29
.
An important study was made in terms of studying the effect of reducing the electrode
spacing by Cheng and Logan39
. In that study it was found that decreasing the electrode spacing to
2cm provided the best performance, though for distances smaller than that they used a carbon
cloth electrode as opposed to the graphite brush used in the previous configuration as the anode
due to fear of short-circuiting. Hence it was no surprise that the reactor with 1 cm reduced
spacing failed to perform as well as the other reactor. This study resulted in a current density of
385 Am-3
with a hydrogen production rate of 18.2 m3 m
-3 d
-1 which still retains the top spot to this
22
day. Liang et al62
have achieved higher current density (621 Am-3
) but the hydrogen production
rate of 10.8m3
m-3
d-1
is still less than the previous study. Rader and Logan97
have stressed on the
fact of reducing the electrode spacing, but also expressed concerns in terms of the
methanogenesis that is usually encountered, with virtually no way to remove the methanogens
once they contaminate the system.
The H2 recycling phenomena was studied by Lee and Rittman59
who observed high
volumetric densities of 1630 Am-3
but with only 4.3 m3
m-3
d-1
hydrogen production rate, and
after detailed investigation concluded that the hydrogen recycling accounted for around 62-76%
of the current. This was manifested in observed CE of 190-310% at steady state whereas the
cathodic conversion efficiency was only 16-24%.
Cheng and Logan39
inferred that higher surface area of the anodes for growth of the
microbial biofilm is more important than the reduced spacing of the electrodes. Though this is a
very apt insight, it has been argued that this higher surface area will be shared with methanogens
and other organisms that might adversely affect the performance of the reactor27
. The minimum
distance between electrodes is possible in the electrochemical systems by sandwiching the
membrane or the separator between the electrodes. This is a very prevalent practice in MFCs and
has been tried in MEC as well36–38
, but in all of these systems the performance was nowhere as
close as that reported by Cheng and Logan39
.
23
Table 2-3. Evolution of MECs through electrode spacing. (Note: the MEA MECs are kept for comparison in the discussions)
Reference Reactor type
Anode
volume
(L)
Electrode
distance
(cm)
Applied
potential
Eap (V)
H2 rate
(m3m-3d-1)
Total H2
yield
(RH2%)
Cathodic
H2 yield
(Rcat%)
CE
(%)
current
density
(Am-3)
Internal
resistance
(Ω)
Liu et al21
two chamber
bottle 0.2 15 0.25 N.A. 54 N.A. 60 N.A. N.A.
Rader and
Logan97
multiple
electrodes 2.7 5.7 0.9 0.53 N.A. 80 147 74 N.A.
Call and
Logan28
single chamber
cubic 0.028 4 0.6 N.A. 88 N.A. 92 N.A. N.A.
Cheng and
Logan41
single chamber
cubic (acetate
data) 0.04 4 0.6 1.1 91 N.A. 88 N.A. N.A.
Hu et al42
tubular (NiMo
catalyst, CEA)
4 0.6 2 N.A. 86 75
270
(NiMo) N.A.
Wang et
al98
cuboid, 2C H-
shaped 0.34 4 0.6 N.A. 35.5 N.A. 45 N.A. 17
Cheng and
Logan 39
single chamber
cubic 0.028 2 1 17.8 N.A. N.A. 115 1830 N.A.
Hu et al48
single chamber
bottle 0.3 2 0.6
0.69
(pH5.8) 63 62 75
39.2*
(pH 5.8) N.A.
Sleutels et
al99 two chamber 0.28 1.7 1
2.1
(AEM) N.A. 83 (AEM) N.A.
5.3
(AEM)
192
(AEM)
Liang et
al62
single chamber
cubic, 2 anodes 0.064 1.5 0.5-1.0 10.88 N.A. over 70% N.A.
1355
(1.5V)
anodic
and
solution
Call et al29
single chamber
cubic 0.028 0.5 0.6 1.7 N.A. 83 N.A. 188 N.A.
Tartakovs
ky et al100
single
chamber, gas
cathode 0.05 0.03 1.2 6.3 130 N.A. N.A.
1.8
(1.15V,
1C) 19
Lee and
Rittman 59
single chamber
upflow 0.125 N.A. 0.6-1.5 4.32 45-51 23
310 (H2
recycle) 1630 N.A.
*-calculated from information in reference
24
Chapter 3
Materials and Methods
3.1 Reactor setup
Two-chambered MECs were constructed from cubes of polycarbonate drilled to contain a
cylindrical chamber 3 cm in diameter. The chamber lengths were 2 cm for the anodes (working
volume of 14 mL) and 4 cm for the cathodes with a cylindrical gas collection tube on top
(working volume of 32 mL which brings the catholyte level to the base of the gas collection
tube). The chambers were separated by 4 layers of textile separators (Amplitude Prozorb, Contec
Inc., 46% cellulose, 54% polyester) as used by Ahn and Logan75
for the separator-electrode
assembly (SEA) reactors or a CEM (CMI7000, Membranes International Inc.) for the membrane-
electrode assembly (MEA) reactors (Figure 3-1). The membrane had a thickness of 0.45 ± 0.025
mm, while the separators had a thickness of 0.3 mm each. The cathode chamber had a cylindrical
glass tube attached to the top, which was sealed using a butyl rubber stopper and an aluminum
crimp cap. The gas produced was collected in a gas bag (0.05 L capacity, Cali-5-Bond, Calibrated
Instruments Inc.). The anode was heat treated non-wet proof carbon cloth (7 cm2 surface area,
Etek). The cathode was stainless steel (SS) mesh (type 304, #60 mesh, wire diameter 0.019 cm,
pore size 0.0023 cm, McMaster-Carr) coated with a Pt catalyst layer (0.5 mg cm-2
, 10% Pt on
Vulcan XC-71 with 33.3 mL cm-2
of 5% Nafion solution as binder, projected cross section area of
7 cm2) on the side facing the cathode chamber. Sections of the SS cathode at the top and bottom
were cut and folded to allow the gas produced to freely escape to the headspace and hence
prevent accumulation between the cathode and the separator or membrane. The SEA or MEA
MECs contained the anode, separators/membrane, and cathode placed together with titanium
25
plates (3 cm × 0.5 cm) as current collectors for each electrode at the junction of the two
chambers. Thus the distance between the electrodes was 1.2 mm for the SEA MECs and 0.45 mm
for the MEA MECs. For the control system used as a performance benchmark for the SEA and
MEA configurations, the anodes were placed on the end of the anode chamber opposite the
cathode chamber with a Ti wire current collector, while the separators/membrane and the
cathodes were placed together at the junction of the two chambers with a similar Ti plate as
before, thus making the distance between the electrodes 2 cm. An Ag/AgCl reference electrode
(RE-5B, BASi) was placed in the anode chamber to record the anode potential. All reactor
treatments were tested in duplicate.
Figure 3-1. Schematics for (a) the control reactor and (b) the SEA or MEA MECs (replacing the
separator with textile-type separator or CMI7000 membrane). Photos of (c) the duplicates of the
control, with the membrane visible on the right; (d) the MEA assembly with the membrane
hidden by the anode; and (e) the MEA MEC showing the reference electrode in front of the
anode.
26
3.2 Reactor operation
The reactors were fed with buffered acetate medium containing 1 gL-1
sodium acetate and
phosphate buffer solution (50 mM – 4.58 g/L Na2HPO4, 2.45 g/L NaH2PO4·H2O, 0.31 g/L
NH4Cl, 0.13 g/L KCl, or 100 mM – 9.16 g/L Na2HPO4, 4.90 g/L NaH2PO4·H2O, 0.62 g/L NH4Cl,
0.26 g/L KCl,) with trace vitamins and minerals28
. Anodes were pre-acclimatized with
exoelectrogenic biofilm by being first run in single-chamber air-cathode MFCs. The MFCs were
started with equal volumes of buffered acetate medium (50 mM) and an inoculum of suspended
bacteria from acetate-fed MFCs that were operating for more than six months. The inoculum was
added until the MFCs reached a cell voltage of 0.1 V across a 100Ω resistor connected across the
cell. The anodes were deemed acclimatized and used in MECs once the MFCs reached a stable
voltage for three batch cycles.
Acclimatized anodes were transferred to duplicate reactors for each of the three MEC
configurations. A constant voltage of Eap = 0.7 V was applied to the circuit using a power source
(Model 3645A, Circuit Specialists Inc.) by connecting the positive lead of the power supply to a
10 Ω resistor and the anode, and the negative lead to the cathode. Voltage drop across the resistor
and the electrode potentials were measured using a multimeter (Model 2700, Keithley
Instruments Inc.).
In the SEA MEC assembly, it was assumed the separators would not arrest acetate
crossover from the anode to the cathode chamber, hence the setup was treated as a single-
chamber MEC with both chambers being filled with 50 mM buffered acetate medium at the start
of each batch. The anode chambers of the control and MEA MEC were filled with 50 mM or 100
mM buffered acetate solution, whereas the cathode chambers were filled with the corresponding
strength of phosphate buffer. After addition of the medium at the start of each batch, both the
anode and the cathode chambers were sparged with ultra-high purity (UHP) nitrogen for 20
minutes. After each batch cycle, both anode and cathode chambers were emptied and the reactors
27
were left open to air for approximately 30 minutes to inhibit growth of methanogens 28
. All batch
tests were performed in a temperature-controlled room at 30°C.
3.3 Chemical analysis
Conductivity and pH of the anolyte and catholyte solutions were tested before and after
each batch cycle using a probe (Mettler Toledo). The chemical oxygen demand (COD) of the
same was measured using Hach® COD kit and DR2700TM
portable spectrophotometer.
Gas chromatographs (GCs) were used to analyze the gas composition in both the
collection tube and the gas bag. Samples (250 μL) were taken using a gastight syringe (250 μL,
Hamilton Samplelock Syringe). The concentrations of H2, N2, and CH4 were analyzed with one
GC (carrier gas Ar; model 2610B; SRI Instruments), and the concentration of CO2 was analyzed
with a separate GC (carrier gas He; model 310; SRI Instruments). Standards were prepared with
pure H2, N2, CH4, and CO2 samples to calculate gas volumes. Because N2 served as a dilution gas,
it was removed from the calculations in order to find the concentrations of H2, CH4, and CO2 -
produced in the system. The volume of gas produced was calculated by the gasbag method as
described by Ambler and Logan101
.
3.4 Gas and energy calculations
All energy calculations were done following Call and Logan 2008, Logan et al. 2006, and
Logan et al. 2008. Reactor performance was examined in terms of hydrogen recovery, energy
recovery, volumetric current density (based on anode chamber volume), and hydrogen production
rate. The theoretical number of moles produced based on substrate usage ( thn ) was calculated as:
28
S
LsHth
M
Svbn
/2 (3.1)
where the maximum stoichiometric production of hydrogen based on the substrate SHb /2 is 4
mol/mol acetate, the liquid volume of the reactor Lv is 14 mL, the change in substrate
concentration over the batch period is S (g sodium acetate L-1
), and the molecular weight of the
substrate (sodium acetate) SM is 82 g mol-1
. The change in substrate concentration was
calculated based on change of COD of the anode chamber converted to acetate with a factor of
0.78 g COD/g sodium acetate.
The current across the resistor ( I ) was calculated from the output voltage readings using
exRVI / , where Rex was 10 Ω. The maximum number of moles of hydrogen that could be
recovered based on the cumulative current ( CEn ) was calculated by:
F
Idt
n
t
tCE
2
0
(3.2)
where F is Faraday‘s constant (96,485 C/mol electron), dt is the voltage recording interval (10
minutes), and 2 is the ratio of moles of electrons per mole of H2.
The coulombic hydrogen recovery ( CEr ) is the same as the Coulombic efficiency (CE;
the number of electrons recovered in the system over the number of electrons theoretically
available from the substrate), and is calculated by:
CEn
nr
th
CECE (3.3)
The cathodic hydrogen recovery ( catR ) and the total hydrogen recovery (2HR ) were
calculated by:
CE
H
catn
nR 2 (3.4)
29
and
th
H
Hn
nR 2
2 (3.5)
where 2Hn is the number of moles of hydrogen recovered during the batch cycle.
The maximum volumetric hydrogen production was calculated by:
catVH TRIQ 5
2 1068.3 (3.6)
where VI is the volumetric current density and T is the temperature in Kelvin. The current
density was averaged over four hours of maximum current production for a batch cycle. The
value 51068.3 is a constant that includes Faraday‘s constant, 1 atm of pressure, and unit
conversions.
Energy added to the circuit by the applied voltage (EW ), compensating for losses across
the resistor, was calculated by:
n
exapE tRItIEW1
2 )( (3.7)
The work added to the system by the substrate ( SW ) and energy recovered from the
system as hydrogen (2HW ) were calculated by:
SSS nHW (3.8)
222 HHH nHW (3.9)
where SH is the heat of combustion of the substrate (870.28 kJ/mol acetate), Sn is the number
of moles of substrate consumed during the batch cycle, and 2HH is the heat of combustion of
hydrogen (285.83 kJ/mol H2 ).
The overall energy efficiency taking into account the energy provided by the substrate
and the electricity ( SE ) was determined by:
30
SE
H
SEWW
W
2 (3.10)
The percentages of energy contributed by the power source (eE) and substrate were
calculated as:
SE
E
EWW
We
(3.11)
SE
S
SWW
We
(3.12)
3.5 Electrochemical analyses
3.5.1 Cyclic voltammetry
Cyclic voltammetry (CV) was performed to check the activity of the biofilm in the MECs
using a potentiostat (VMP3, Bio-Logic), with the anode as the working electrode and the cathode
as the counter electrode. The anode potential was scanned from 0 V to -0.7 V at a scan rate of 1
mV/s, covering a range beyond the acclimatization potential to beyond the theoretical anode
potential of -0.52 V vs Ag/AgCl electrode according to the electron transfer reaction of the
NADH in microbes 98
.
3.5.1 Electrochemical impedance spectroscopy
Internal resistances of the MECs were characterized by whole-cell electrochemical
impedance spectroscopy (EIS) using a potentiostat (Bio-Logic, VMP3). The EIS scan was
performed over a range of 100 kHz to 10 mHz with a sinusoidal perturbation of 10 mV RMS
31
amplitude, while applying a constant potential of 0.7 V across the cell. The software EC-Lab
v10.23 (Bio-Logic SAS) was used to identify the components of the internal resistance using the
Zfit modeling tool applying the Randomize-Simplex algorithm to fit a suitable equivalent
impedance model to the Nyquist plot.
32
Chapter 4
Results and Discussion
4.1 Performance of SEA MECs
In the initial runs with acetate in both chambers (Figure 4-1), the SEA MECs had a
maximum current production of 2.63 ± 0.62 mA (187 ± 44 Am-3
), which is similar to that
observed by Call and Logan28
using a single-chamber MEC with a brush anode and an applied
voltage of 0.6 V (current density 188 Am-3
). The CE of the SEA systems (84.8 ± 5.8%) was also
comparable to the range reported for the single-chamber MEC (79 to 98%). However, the
hydrogen production rate was considerably lower for the SEA MECs (0.40 ± 0.16 m3m
-3d
-1) than
the single-chamber MECs (3.12 m3
m-3
d-1
). The cathodic hydrogen recovery (Rcat) was calculated
as 61 ± 8%, with a total H2 recovery (2HR ) of 54 ± 13%. A total energy efficiency ( SE ) of 52 ±
15% was calculated with the power supply providing 20 ± 5% of the energy, and the rest derived
from the medium. The pH at the end of the batch for the SEA MECs was 6.93 ± 2.1 and 7.71 ±
0.61 for the anolyte and the catholyte respectively, and the conductivities 7.17 ± 0.53 and 7.9 ±
0.3 mS cm-1
respectively.
This performance level was short lived, since even though an attempt was made to arrest
methanogens by exposing the anode to air after every batch run, CH4 was evolved from the
reactors and subsequently the hydrogen production almost stopped (Figure 4-2). This is similar to
what was observed by Rader and Logan52
, who had high CEs but little hydrogen production,
suggesting that the hydrogen gas that was produced was almost completely converted to CH4 or
recycled in the anode52
. It was not determined whether it was hydrogenotrophic or acetoclastic
methanogens which were responsible for the problem. The high CEs indicate H2 recycling59
and
suggest less possibility of the presence of acetoclastic methanogens because they would consume
33
acetate without contributing to the current production. The direct acceptance of electrons from the
cathode by electromethanogenic bacteria could not be ruled out57,102
. The pH and the conductivity
of the anolyte and catholyte at the end of the batch runs were similar to the earlier runs with no
significant methanogenesis.
Figure 4-1. Current profiles from initial runs of the SEA MECs before methanogenesis was
prevalent.
Figure 4-2. Cathodic recovery (Rcat) shows the emergence of methanogenesis in the SEA MECs,
and CE shows evidence of hydrogen recycling. (The Rcat for CH4 is calculated in the same way as
that for hydrogen assuming consumption of 8 e-/molecule of CH4)
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
0 20 40 60 80
cu
rr
en
t (m
A)
time(h)
SEA1
SEA2
0
20
40
60
80
100
120
140
160
180
200
1 2 3 4 5 6 7 8
eff
icie
nc
y %
batch #
Rcat(H2)
Rcat (CH4)
CE
34
Before methanogenesis, the reactors were found to have little residual acetate (ΔCOD for
the whole reactor being 0.69 ± 0.01 gL-1
), though there was a higher concentration of acetate left
over in the cathode chamber compared to the anode chamber at the end of the batch cycles. In
some batch runs attempted after those shown in Figure 4-2 with 50 mM buffered acetate medium
in the anode and 50 mM phosphate buffer in the cathode, the crossover of acetate was observed
based on the cathode effluent having a significant amount of COD (120 ± 40 g COD/L vs 30 ± 5
g COD/L for freshly prepared phosphate buffer). This showed that the textile-type separator did
not allow the reactors to behave as fully single-chambered nor fully two-chambered. Hence the
decision was made to switch to using a membrane instead of the textile-type separator to reduce
the hydrogen recycling and acetate crossover.
35
4.2 Performance of MEA MECs
4.2.1 Initial runs using 50 mM phosphate buffer
The reactors were initially setup with the acetate medium prepared with 50 mM
phosphate buffer and just the phosphate buffer as the catholyte, which were run in duplicate. The
MEA MECs produced a maximum current of 0.83 ± 0.12 mA (60.1 ± 8.6 Am-3
) as compared to
0.71 ± 0.01 mA (50.1 ± 0.8 Am-3
) by the control MECs (Figure 4-3). The current production was
expected to decrease with the replacement of the textile-type separators with the CEM since the
latter would increase the internal resistance of the system. As compared to the control MECs, the
MEA MECs showed only 14% higher CE but 25% higher total hydrogen recovery, with a
maximum hydrogen production rate of 0.43 ± 0.05 m3m
-3d
-1 as compared to 0.38 ± 0.02 m
3m
-3d
-1
for the control. There was no CO2 observed in the headspace or the gas bags, and there was very
little gas trapped in the headspace of the anode, hence the gas analysis of the anode chamber was
not possible. There was consistent production of hydrogen and very little methanogenesis (2.48 ±
0.03% cathodic recovery), and the overall performance of control and the MEA MECs were quite
similar.
The initial pH of the anolyte was 7.0 ± 0.1 with a conductivity of 7.8 mS cm-1
, while the
catholyte had the same pH and a conductivity of 6.8 mS cm-1
. The final anolyte pHs ranged from
5.36 ± 0.35 for the control MECs, whereas the MEA MECs faced a more drastic reduction in
final anolyte pH to 5.12 ± 0.27. The catholyte solutions had final pHs of 7.89 ± 0.19 for the
controls and 8.29 ± 0.31 for the MEA MECs. The conductivity of the anolytes and the catholytes
also change to 5.99 ± 0.2 and 7.8 ± 0.1 mS cm-1
respectively for the control and to 6.1 ± 0.15 and
7.91 ± 0.01 mS cm-1
for the MEA MECs. The low anode pH led to a lower activity of the
36
exoelectrogens in both reactors configurations, which was evident from residual acetate at the end
of the batch runs (ΔCOD = 0.46 ± 0.05 g/L for control and 0.37 ± 0.09 g/L for the MEA MECs).
Figure 4-3. (a) The final anode and cathode pH and (b) current generation of the control and
MEA MECs (Note: figure represents one of the duplicates of each system, all batches not shown)
The drastic drop of pH in the anode chamber may be attributed to the transport resistance
of the CEM (discussed in detail in the section on internal resistance), which leads to proton
accumulation in the anode chamber 99
. This effect of pH imbalance between the anode and the
cathode chambers might be the reason the MEA MECs performed similar to the control MECs.
The use of membranes has been associated with pH changes in the anode and cathode chambers
63,103. A prior study of MECs with a sandwich MEA assembly by Rozendal et al.
37, in a single-
37
chamber with a gas diffusion electrode, reported a similar problem, where the pH of the leachate
through their MEA assembly on the cathode side was found to be 6.4 or 4.4 units higher than that
of the anolyte on using CEM or AEM, though the pH of the anode chamber was not that low as
compared to the present study since the reactors were run in continuous mode.
4.2.2 Effect of increasing buffer strength
Simultaneously with increasing the phosphate buffer strength, an attempt was made to
repair the damage made by the low pH to the exoelectrogenic community in the anode biofilm by
poising the anode using a potentiostat (Bio-Logic, VPM3) at 0 V vs. SHE for two 24 hour cycles.
In the subsequent batches, the current was observed to increase to a maximum of 1.69 ± 0.22 mA
(121 ± 16 Am-3
) for the MEA MECs and to 1.45 ± 0.09 mA (103.3 ± 6.5 Am-3
) for the control
MECs (Figure 4-3). The currents did not stabilize but based on a previous study by Nam and
Logan63
, which had similar pH imbalances between the anode and the cathode chamber, we
deduced that the biofilm must be developed and the fluctuations were due to the pH imbalance. In
the batch runs with the 100 mM buffer, the difference of the performance of the MEA MECs vs.
the control became more pronounced since the pH drop in the anode chamber was generally
lower (Figure 4-3). The discrepancy in the third cycle at 100 mM buffer strength (Figure 4-3)
may have been due to precipitation observed in the buffer solution in this batch.
The pH and the conductivity of the acetate medium using 100 mM buffer was 7.01 ± 0.01
and 12.7 ± 0.1 mS cm-1
(12.3 ± 0.1 mS cm-1
for the buffer). At the end of the batches, the pH of
the anolyte ranged from 6.05 ± 0.33 for the control and 6.19 ± 0.31 for the MEA MECs. The
catholyte pH was 8.41 ± 0.27 for the control and 8.45 ± 0.28 for the MEA MECs. Thus the pH
drop was not arrested fully. The anode potential was found to drop from -329 ± 8mV to -300.7 ±
38
1.8mV over the course of the 50 mM buffer runs, but increased back up to -326 ± 4 mV in the
100 mM buffer conditions.
Figure 4-4. Comparison of the current densities (on an anode volume basis) in different batches
and the anode and cathode potentials (w.r.t. SHE) [Note: data of last batch of each buffer
strength]
Rcat improved significantly both for the control and the MEA MECs, with the cathodic
recovery reaching beyond 100% (Figure 4-5). A slight increase in CH4 production was also
noticed, but unlike in the SEA MECs, the CH4 recovery only increased to 6.0 ± 0.5%. The
maximum H2 production rate increased to 1.54 ± 0.27 m3
m-3
d-1
for the MEA MECs and 1.12 ±
0.12 m3m
-3d
-1 for the control reactors.
Table 4-1. Energy calculations for the control and MEA MECs in 50 and 100 mM buffer
Reactor ηE+S(%) eE(%) eS(%) CE (%)
Control (50 mM) 31.3 ± 5.4 32 69 87.2 ± 0.1
MEA (50 mM) 34.2 ± 2.1 36 64 99.6 ± 0.2
Control (100 mM) 73 ± 5.9 22 78 79.2 ± 6.3
MEA (100 mM) 92 ± 10 20 80 94.3 ± 6.4
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0
20
40
60
80
100
120
140
160
50mM 100mM
po
ten
tial
(V
)
curr
en
t d
en
sity
(A
/m3 )
control current density
MEA current density
control anode
MEA anode
control cathode
MEA cathode
39
The CEs of both MEC configurations reduced slightly on using the higher buffer
strength, though the energy efficiencies improved with more energy being derived from the
acetate as compared to the power supply (Table 4-1). Though the acetate utilization increased
with the use of higher buffer strength to 0.54 ± 0.07 and 0.57 ± 0.8 g COD/L for the control and
the MEA MECs respectively, full utilization of acetate was still not achieved due to the problem
of lower pH not being solved using the higher buffer strength. Apart from the losses due to the
pH imbalance, the high catholyte pH was not much of concern since this has been observed not to
be detrimental for hydrogen production38,63
.
Figure 4-5. Cathodic and total H2 recovery from the control and MEA MECs at 50 and 100 mM
phosphate buffer strengths.
0
20
40
60
80
100
120
140
160
50mM 100mM
eff
icie
nc
y %
control Rcat
MEA Rcat
control total H2
MEA total H2
40
4.3 Electrochemical analyses
4.3.1 Cyclic voltammetry
The CV scans were sigmoidal in shape (Figure 4-6), indicating what is known as the
turnover condition104
. This means that when a suitable electron donor is added as a substrate, such
as acetate, the exchange of electrons between the biofilm and the electrode is commensurate with
the intracellular metabolism reaction, thus ensuring that a constant current is provided to a
working electrode poised at a suitably positive potential. The active site on the bioanode is
continuously reduced by the microbial metabolism and is subsequently oxidized by the electrode.
In presence of substrate, the cells can continuously re-reduce the active site and thus achieve an s-
shaped oxidative voltammogram. No reduction peak is observed under these conditions and, if
the scan rate is sufficiently low, the maximum current depends only on the enzyme kinetics or
substrate mass transfer. This sigmoidal shape is similar to those reported in previous studies with
wastewater-derived anodic biofilms105
.
Figure 4-6. (a) CV and (b) DCV of the control and MEA MECs
41
From the voltammograms, it was observed that the MEA MECs started producing
positive current at E = -0.545 V, while the control MECs started at a slightly higher overpotential
of -0.514 V. The peak current density of the MEA MECs was 23% higher than that of the control
MEC, though neither design showed saturated current density at the highest scanned potential.
The MEA MECs also operated at lower anodic overpotentials throughout the scan.
The first derivative analysis of the CVs (DCVs) showed two inflection points, which are
at -0.364 V and -0.335 V for the control and -0.364 V and -0.337 V for the MEA MECs. The
closeness of the inflection points as well as the shape of the peaks for the two type of reactors
indicate similar composite microbial consortia anode-reduction features106
.
4.3.2 Electrochemical impedance spectroscopy
A number of equivalent circuits were tried before choosing the one that best fit the data.
The equivalent circuit (Figure 4-7) was comprised of three parallel R-CPE units, the first two (R1-
CPE1 and R3-CPE3) representing the charge transfer resistances of the anode and the cathode. The
constant phase element (CPE) is a widely chosen replacement of the capacitor to represent the
charge double layer on the rough surface of the electrodes in bioelectrochemical systems107,108
.
The resistor (R2) in series between them represented the ohmic loss due to the electrolytes and the
membrane. A component of inductance (L) was added before the third R4-CPE4 unit, which
together represented the finite diffusion element. This circuit has been used in a previous study to
explain the whole-cell EIS data of an MEC with a membrane109
.
42
Figure 4-7 Equivalent circuit used to fit the EIS data (adapted from Lu et al.109
)
For the SEA MECs, the circle fit method was used to determine the ohmic resistance of
the system110
, since the system had already been taken over by methanogens and might hence
provide misguiding results for the other resistances.
Figure 4-8 Nyquist plots obtained from whole-cell EIS at an Eap = 0.7 V of (a) SEA MECs and
(b) control (C1 and C2) and MEA (M1 and M2) MECs. (NOTE: noise signal at lower frequencies
have been removed from the figure)
43
Figure 4-9 Components of the internal resistance as calculated from fitting the model to the EIS
spectra.
Both the SEA and MEA MECs had very similar ohmic resistances (9.80 ± 0.01 Ω and
8.78 ± 4.1 Ω respectively), though the former was in a 50 mM buffer and the latter in 100 mM
buffer, indicating that the higher conductivity might have compensated some of the increased
resistance due to the membrane. Still, these systems had 60% less ohmic resistance than that of
the control MEC (25.52 ± 0.29 Ω). This was still substantially high than the 2 Ω observed by
Rozendal et al37
, but it confirmed the reduction in the ohmic resistance due to the reduction in the
inter-electrode distance. The ohmic resistances of the two replicates of the MEA MECs varied
from each other, since the carbon cloths bulged out somewhat, thus varying the actual inter-
electrode distance. This didn‘t have much effect on the control reactors, since the carbon cloth
deflection was relatively small compared to the 2-cm inter-electrode distance. The overall
resistances of the MEA MECs are around 40% less than that of the control MECs (Figure 4-9).
It was expected that the textile-type separators would lead to lesser ohmic resistance than
the MEA, since they are just a physical barrier separating the electrodes. On the other hand 4
layers of the same do contribute to a slightly higher inter-electrode distance (1.2 mm). This, along
0
5
10
15
20
25
30
35
40
45
control MEA
re
sis
tan
ce
(Ω)
Rohm
Rct
44
with the lower strength of buffer (50 mM) used for the SEA MECs might have led to the 1 Ω
difference between the SEA MECs and the MEA MECs.
The charge transfer resistance (R1+R3) of the MEA MECs (16.4 ± 1.1 Ω) was very
similar to the control MECs (15.8 ± 8.5 Ω). This suggests that the electrodes of the two types of
reactors behaved similarly, though the large variance of the control MEC does warrant further
investigation before this can be conclusively stated.
The diffusion resistance was not taken into account since the use of the membrane led to
the aforementioned pH imbalance, which would greatly affect the diffusion resistance and
possibly lead to misguiding analysis. CEMs are known to have relatively higher transport
resistance than AEMs99
. Due to the charge of the membrane, counter ions from the electrolyte
move into it to balance the charge, which changes the concentration inside the membrane from
the surrounding solution. This gives rise to a completely different concentration gradient which
varies throughout the batch run, and along with the similarly varying pH imbalance, lead to an
inaccurate reading for the diffusion resistance99
.
4.4 Comparison to previous studies
Previous attempts were made at testing membrane-electrode assemblies in MECs
following their success in MFCs (Table 4-2). Rozendal et al36
studied this configuration first in
batch mode, and subsequently in a continuous mode using a gas diffusion cathode37
as mentioned
in the literature review. Both systems had problems of hydrogen recycling and pH imbalance. The
latter was quite substantial, leading to overpotentials of 0.199 ± 0.03 V (difference between
anolyte and catholyte pH, ΔpH = 3.33 ± 0.6) and 0.138 ± 0.04 V (ΔpH = 2.31 ± 0.74) in the MEA
MECs with 50 mM and 100 mM phosphate buffers respectively.
45
The high pH of the catholyte makes it more comparable to the study by Nam and Logan63
(Table 4-3), where a saline catholyte was used in two-chambered MECs with AEM. In that study
the catholyte pH went as high as 10.8, with a H2 production rate of 1.6 m3 m
-3 d
-1, which is similar
to what we observed. Kyazze et al38
also studied MECs with a CEM in a membrane-electrode
assembly at both high and low catholyte pH in continuous mode. In that study, with a cathodic
pH of 9, a very low rate of hydrogen production was observed, though the current drawn was
slightly higher than the present study at Eap = 0.6 V but more than twice that of the present study
at Eap = 0.85 V, indicating that a higher applied potential might increase the performance of the
system.
The present study confirms that though a membrane-electrode assembly does lead to a
high pH imbalance between the anode and the cathode chambers, the anolyte pH can be tempered
somewhat using a higher strength buffer. This along with the lower ohmic resistance offers a
potential direction for better hydrogen generation in these systems.
46
Table 4-2. Comparison of MECs with MEA and SEA
Reference Reactor
configuration
Applied
potential
Eap (V)
Current
density
(Am-3
anode)*
Ohmic
resistance
(Ω)
H2 rate
(m3m
-3d
-1)
H2 yield
(Rcat%) CE (%) ΔpH**
this study SEA 0.7 187 9.8 0.40 61 88 0.78
this study MEA, 50 mM buffer 0.7 60.1 N.A. 0.43 72.74 99 3.17
this study MEA, 100 mM buffer 0.7 121 8.8 1.54 121 94 2.26
Rozendal et al37
CEM MEA 1 28.96 2 0.33 101.4 23 6.4
Rozendal et al37
AEM MEA 1 26.06 2 0.31 101.3 23 4.4
Kyazze et al38
CEM MEA, 50 mM
buffer, catholyte pH 9) 0.6, 0.85
158.7 (0.6V),
391.1 (0.85V) N.A.
0.04 (0.6V),
0.18 (0.85V)
54 (0.6V),
57 (0.85V)
25 (0.6V),
39
(0.85V)
2.55
Kyazze et al38
CEM MEA, 50 mM
buffer, catholyte pH 5) 0.6, 0.85
333.6 (0.6V),
444 (0.85V) N.A.
0.1 (0.6V),
0.2 (0.85V)
32 (0.6V),
45 (0.85)
40(0.6V),
60 (0.85) 1.45
*recalculated from data in some reference ** difference between anolyte and catholyte pH (calculated from data)
Table 4-3. Comparison of MECs with high cathodic pH
Reference Reactor
configuration
applied
potential
Eap (V)
current
density
(Am-3
anode)*
ohmic
resistance
(Ω)
H2 rate
(m3m
-3d
-1)
H2 yield
(Rcat%) CE (%)
Anode
pH
Cathode
pH
this study SEA 0.7 187 9.8 0.40 61 88 6.93 7.71
this study MEA, 50 mM buffer 0.7 60.1 N.A. 0.43 72.74 99 5.12 8.29
this study MEA, 100 mM buffer 0.7 121 8.8 1.54 121 94 6.19 8.45
Kyazze et al38
CEM MEA, 50 mM
buffer 0.6, 0.85
158.7 (0.6V),
391.1 (0.85V) N.A.
0.04 (0.6V),
0.18 (0.85V)
54 (0.6V),
57 (0.85V)
25 (0.6V),
39 (0.85V) 6.45 9
Nam and
Logan63
AEM, brush anode,
saline catholyte (68
mM)
0.9 131 N.A. 1.6 117 83 6.5 10.7
*recalculated from data in some reference
47
Chapter 5
Conclusions
Reduction in electrode spacing led to lower ohmic resistance. This also resulted in better
hydrogen production rate, current density and efficiencies. However, the separator-
electrode assembly or a membrane-electrode assembly had their own drawbacks.
1. The separator-electrode assembly had high current production initially but was
plagued by methanogens, presumably supported by hydrogen recycling to the anode.
2. The membrane-electrode assembly resulted in high pH imbalance between the
anode and the cathode chambers, which resulted in considerable residual acetate.
3. Use of a stronger buffer did not fully arrest the pH imbalance (and hence the
overpotentials associated with it).
Compared to previous studies, the MECs in this study generally showed improvement in
terms of hydrogen production and efficiencies. The use of separator-electrode assembly
and membrane-electrode assembly both resulted in significantly reduced ohmic resistance
relative to a benchmark system with 2-cm electrode spacing. The use of higher buffer
strength enhanced acetate utilization and current production due to slightly tempering the
anode pH and increasing the solution conductivity.
48
Chapter 6
Future work
There are still several hurdles to overcome in the optimization of SEA or MEA MECs.
1. Long-term MEA MEC experiments that focus on further reducing the pH
imbalance between anode and cathode to obtain better performance.
2. Further experimentation to test the effectiveness of textile-type and/or other
separators in the SEA MEC configuration, since it provides a low-cost design, an
effective method to reduce the ohmic resistance, as well as reduce the pH imbalance.
Separators with different physical or geometric properties such as pore size or different
transport properties might lead to better performance of the system.
3. Better reactor configurations should be developed to reduce methanogenesis and
hydrogen recycling. This has been a problem with MEC systems for a long time, and is of
a large concern for scale up considerations. Possibly a higher applied potential might help
the exoelectrogens outcompete the methanogens.
4. The microbial profile of the biofilms in the separator/membrane electrode
assembly can be analyzed to find differences from the benchmark reactor to find whether
the reduced distance between the electrodes has any effect on the microbial community.
49
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42. Hu, H., Fan, Y. & Liu, H. Hydrogen production in single-chamber tubular microbial
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46. Ditzig, J., Liu, H. & Logan, B. E. Production of hydrogen from domestic wastewater using a
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47. Liu, W. et al. Electrochemically Assisted Biohydrogen Production from Acetate †. Energy &
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50. Call, D. F. & Logan, B. E. Lactate Oxidation Coupled to Iron or Electrode Reduction by
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51. Wrana, N., Sparling, R., Cicek, N. & Levin, D. B. Hydrogen gas production in a microbial
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52. Call, D. F., Wagner, R. C. & Logan, B. E. Hydrogen Production by Geobacter Species and a
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54. Kim, J. R., Jung, S. H., Regan, J. M. & Logan, B. E. Electricity generation and microbial
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55. Clauwaert, P. et al. Combining biocatalyzed electrolysis with anaerobic digestion. Water
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57. Clauwaert, P. & Verstraete, W. Methanogenesis in membraneless microbial electrolysis cells.
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58. Wagner, R. C., Regan, J. M., Oh, S.-E., Zuo, Y. & Logan, B. E. Hydrogen and methane
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59. Lee, H.-S. & Rittmann, B. E. Significance of Biological Hydrogen Oxidation in a Continuous
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63. Nam, J.-Y. & Logan, B. E. Enhanced hydrogen generation using a saline catholyte in a two
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64. Min, B. & Logan, B. E. Continuous Electricity Generation from Domestic Wastewater and
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65. Fan, Y., Hu, H. & Liu, H. Enhanced Coulombic efficiency and power density of air-cathode
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66. Fornero, J. J., Rosenbaum, M., Cotta, M. A. & Angenent, L. T. Carbon Dioxide Addition to
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75. Ahn, Y. & Logan, B. E. A multi-electrode continuous flow microbial fuel cell with separator
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76. Ayyaru, S. & Dharmalingam, S. Development of MFC using sulphonated polyether ether
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77. Hays, S., Zhang, F. & Logan, B. E. Performance of two different types of anodes in
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78. Katuri, K. P. & Scott, K. On the dynamic response of the anode in microbial fuel cells.
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79. Zhang, X., Cheng, S., Wang, X., Huang, X. & Logan, B. E. Separator Characteristics for
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80. Logan, B. E. Essential data and techniques for conducting microbial fuel cell and other types
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81. Prakash, G. K. S. et al. Inoculation procedures and characterization of membrane electrode
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83. Choi, S. et al. Enhanced power production of a membrane electrode assembly microbial fuel
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84. Zhang, X., Cheng, S., Huang, X. & Logan, B. E. Improved performance of single-chamber
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85. Fan, Y., Han, S.-K. & Liu, H. Improved performance of CEA microbial fuel cells with
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86. Zhang, Y. & Angelidaki, I. A simple and rapid method for monitoring dissolved oxygen in
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89. Lee, Y. & Nirmalakhandan, N. Electricity production in membrane-less microbial fuel cell
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96. Neburchilov, V. et al. Microbial fuel cell operation on carbon monoxide: Cathode catalyst
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97. Rader, G. K. & Logan, B. E. Multi-electrode continuous flow microbial electrolysis cell for
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98. Wang, A., Liu, W., Ren, N., Cheng, H. & Lee, D.-J. Reduced internal resistance of microbial
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99. Sleutels, T. H. J. A., Hamelers, H. V. M., Rozendal, R. A. & Buisman, C. J. N. Ion transport
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101. Ambler, J. R. & Logan, B. E. Evaluation of stainless steel cathodes and a bicarbonate
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102. Cheng, S., Xing, D., Call, D. F. & Logan, B. E. Direct Biological Conversion of
Electrical Current into Methane by Electromethanogenesis. Environmental Science &
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103. Kim, J. R., Cheng, S., Oh, S.-E. & Logan, B. E. Power Generation Using Different
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104. Harnisch, F. & Freguia, S. A Basic Tutorial on Cyclic Voltammetry for the Investigation
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110. Hutchinson, A. J., Tokash, J. C. & Logan, B. E. Analysis of carbon fiber brush loading in
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