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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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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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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67. Huang, Y., He, Z. & Mansfeld, F. Performance of microbial fuel cells with and without

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69. Yuan, Y., Zhou, S. & Zhuang, L. A new approach to in situ sediment remediation based on

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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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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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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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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

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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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105. Patil, S. A., Harnisch, F., Kapadnis, B. & Schröder, U. Electroactive mixed culture

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107. Dominguez-Benetton, X., Sevda, S., Vanbroekhoven, K. & Pant, D. The accurate use of

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108. Manohar, A. K., Bretschger, O., Nealson, K. H. & Mansfeld, F. The use of

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109. Lu, L., Xing, D., Liu, B. & Ren, N. Enhanced hydrogen production from waste activated

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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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