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The Pennsylvania State University
The Graduate School
College of Engineering
CHARACTERIZATION AND PERFORMANCE OF ACTIVATED CARBON
CATALYSTS AND POLYMER MEMBRANE LAYERS FOR MICROBIAL FUEL CELL
CATHODES AND AN ANALYSIS OF POWER OVERSHOOT
A Dissertation in
Environmental Engineering
by
Valerie Jo Watson
2013 Valerie Jo Watson
Submitted in Partial Fulfillment of the Requirements
for the Degree of
Doctor of Philosophy
May 2013
The dissertation of Valerie Watson was reviewed and approved* by the following:
Bruce E. Logan Evan Pugh and Kappe Professor of Environmental Engineering Dissertation Advisor Chair of Committee John M. Regan Associate Professor of Environmental Engineering
Fred Cannon Professor of Environmental Engineering Michael A. Hickner Assistant Professor of Materials Science and 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 fuel cells (MFCs) are a promising technology for treatment of wastewater
streams in combination with electricity production. Commercialization and implementation of
MFCs could eliminate the large energy consumption common in traditional wastewater treatment
and allow for the utilization of this untapped renewable energy source. Polarization curves from
microbial fuel cells (MFCs) often show unexpectedly large drops in voltage with increased
current densities, leading to a phenomenon in the power density curve referred to as “power
overshoot”. Linear sweep voltammetry (LSV, 1 mV s−1) and variable external resistances (at
fixed intervals of 20 min) over a single fed-batch cycle in an MFC both resulted in power
overshoot in power density curves due to anode potentials. Increasing the anode enrichment time
from 30 days to 100 days did not eliminate overshoot, suggesting that insufficient enrichment of
the anode biofilm was not the primary cause. Running the reactor at a fixed resistance for a full
fed-batch cycle (~1 to 2 days), however, completely eliminated the overshoot. These results show
that acclimation at low fixed resistances are needed to stabilize current generation by bacteria in
MFCs, and that even relatively slow LSV scan rates and long times between switching circuit
loads during a fed-batch cycle may produce inaccurate polarization and power density results for
these biological systems.
Membrane separators reduce oxygen flux from the cathode into the anolyte in MFCs, but
water accumulation and pH gradients between the separator and cathode reduces performance. To
avoid these problems, air cathodes were spray-coated (water-facing side) with anion exchange,
cation exchange, and neutral polymer coatings of different thicknesses to incorporate the
separator into the cathode structure. The anion exchange polymer coating resulted in greater
power density (1167 ± 135 mW m−2) than a cation exchange coating (439 ± 2 mW m−2). This
power output was similar to that produced by a Nafion-coated cathode (1114 ± 174 mW m−2), and
iv
slightly lower than the uncoated cathode (1384 ± 82 mW m−2). Thicker coatings reduced oxygen
diffusion into the electrolyte and increased coulombic efficiency (CE = 56 – 64%) relative to an
uncoated cathode (29 ± 8%), but decreased power production (255–574 mW m−2).
Electrochemical characterization of the cathodes using abiotic anodes in separate reactors showed
that the cathodes with the lowest charge transfer resistance and the highest oxygen reduction
activity produced the most power in MFC tests. The results using hydrophilic cathode separator
layers revealed a tradeoff between power and CE. Cathodes coated with a thin coating of anion
exchange polymer showed the most promise for controlling oxygen transfer while minimally
affecting power production.
Platinum is commonly used as the catalyst in MFC cathodes, but platinum is an
expensive and limited resource. Activated carbon (AC) is a promising material for the
replacement of platinum catalysts because it is inexpensive and can be made from renewable
waste sources, but its catalytic performance in neutral solutions used in MFCs in not well
understood. Commercially available AC powders made from different precursor materials (coal,
peat, coconut shell, hardwood, and phenolic resin) were evaluated as oxygen reduction catalysts,
and tested as cathode catalysts in MFCs. Carbons were characterized in terms of surface
chemistry, specific surface area, and pore volume distribution, and kinetic activities were
compared to carbon black and platinum catalysts using a rotating disk electrode (RDE). Cathodes
using the coal–derived AC had the highest maximum power densities in MFCs (1620 ± 10 mW
m–2) even though this AC had only average catalytic activity, measured by reduction onset
potential (Eonset = 0.09 V), and selectivity, based on number of electrons transferred (n = 2.4).
This coal–based AC also had the lowest specific surface area (550 m2 g–1) among the ACs tested.
Peat–based AC performed similarly in MFC tests (1610 ± 100 mW m–2) but had the best catalyst
performance (Eonset = 0.17 V, n = 3.6) in RDE tests and a lower than average specific surface area
(810 m2 g–1). Hardwood based AC had the highest number of acidic surface functional groups and
v
a higher specific surface area (1010 m2 g–1), but it had the poorest performance in MFC and
catalyst tests (630 ± 10 mW m–2, Eonset = –0.01V, n = 2.1). There was a strong inverse
relationship between onset potential and the quantity of strong acid (pKa < 8) functional groups,
and a larger fraction of microporosity was negatively correlated with power production in MFCs.
These results showed that surface area alone was a poor predictor of catalyst performance, and
that a high quantity of acidic surface functional groups was detrimental to oxygen reduction and
cathode performance.
Four of the commercially available AC powders (peat, coconut shell, coal, and
hardwood) were treated with ammonia gas at 700 °C in order to improve their performance as
oxygen reduction catalysts. Ammonia treatment resulted in a decrease in oxygen (by 29 – 58%)
and an increase in nitrogen content (total abundance up to 1.8 atomic %) on the carbon surfaces,
which also resulted in an increase in the basicity of the bituminous, peat, and hardwood ACs. The
kinetic activity and selectivity of ammonia–treated carbons were evaluated using a rotating ring
disk electrode (RRDE), and compared to untreated ACs and platinum. All of the ammonia–
treated ACs exhibited better catalytic performance than their untreated precursors, with the
bituminous (treated, Eonset = 0.12 V, n = 3.9; untreated, Eonset = 0.08 V, n = 3.6) and hardwood
(treated, Eonset = 0.03 V, n = 3.3; untreated, Eonset = –0.04 V, n =3.0) based samples showing the
most improvement. These ACs were tested in MFC cathodes made by sandwiching the AC
catalyst and polytetrafluoroethylene (PTFE) binder mixture between two current collectors, one
coated with polydimethylsiloxane (PDMS) diffusion layers on the air–side, and the second one on
the solution–side used to improve power. Cathodes made from the ammonia–treated coal-based
AC had the one of the highest maximum power densities (2450 ± 40 mW m–2). Even though the
ammonia–treated peat–based AC had the greatest ORR activity in RRDE testing, the untreated
sample had higher power production in the MFC tests (2360 ± 230 mW m–2). The treated coconut
and hardwood derived ACs outperformed the untreated precursor ACs in both electrochemical
vi
and MFC testing. These results show that reduction in oxygen abundance and increase in nitrogen
functionalities on the surface of ACs can increase the catalytic performance for oxygen reduction
in neutral media.
vii
TABLE OF CONTENTS
List of Figures .......................................................................................................................... x
List of Tables ........................................................................................................................... xii
Acknowledgements .................................................................................................................. xiii
Chapter 1 Introduction ............................................................................................................. 1
1.1 Energy Demand and the Environment ....................................................................... 1 1.2 Microbial Fuel Cells ................................................................................................... 2
1.2.1 Oxygen Reduction at the MFC Cathode ......................................................... 2 1.2.2 Activated Carbon Catalysts ............................................................................. 3 1.2.3 Membranes and Separators ............................................................................. 5
1.3 Analysis Methods ....................................................................................................... 7 1.3.1 MFC Power Density and Overshoot ............................................................... 7 1.3.2 Catalyst Performance ...................................................................................... 8
1.4 Conclusions ................................................................................................................ 9 1.5 Objectives and Clarification of Contributions ........................................................... 10 1.6 References .................................................................................................................. 11
Chapter 2 Analysis of polarization methods for elimination of power overshoot in microbial fuel cells ........................................................................................................... 15
Abstract ............................................................................................................................ 15 2.1 Introduction ................................................................................................................ 16 2.2 Experimental procedures ............................................................................................ 17
2.2.1 MFC reactor construction and operation ......................................................... 17 2.2.2 Analysis ........................................................................................................... 18
2.3 Results ........................................................................................................................ 19 2.3.1. Polarization by varied resistance, single-cycle method .................................. 19 2.3.2. Polarization by varied resistance, multiple-cycle method .............................. 21 2.3.3 Polarization by LSV ........................................................................................ 21
2.4 Discussion .................................................................................................................. 22 2.5 Acknowledgements .................................................................................................... 24 2.6 References .................................................................................................................. 24
Chapter 3 Polymer coatings as separator layers for microbial fuel cell cathodes .................... 25
Abstract ............................................................................................................................ 25 3.1 Introduction ................................................................................................................ 26 3.2 Materials and methods ............................................................................................... 28
3.2.1 Polymers .......................................................................................................... 28 3.2.2 Cathode construction ....................................................................................... 29 3.2.3 MFC reactor construction and operation ......................................................... 30 3.2.4 Analysis ........................................................................................................... 31
viii
3.3 Results ........................................................................................................................ 32 3.3.1 MFC performance ........................................................................................... 32 3.3.2 Electrochemical performance .......................................................................... 35 3.3.3 Oxygen diffusion and biofilm growth ............................................................. 37
3.4 Discussion .................................................................................................................. 38 3.5 Acknowledgements .................................................................................................... 41 3.6 References .................................................................................................................. 41
Chapter 4 Influence of Chemical and Physical Properties of Activated Carbon Powders on Oxygen Reduction Catalysis and Performance in Microbial Fuel Cells ..................... 43
Abstract ............................................................................................................................ 43 4.1 Introduction ................................................................................................................ 44 4.2 Materials and Methods ............................................................................................... 46
4.2.1 Catalyst Materials ............................................................................................ 46 4.2.2 Physical and Chemical Analyses ..................................................................... 47 4.2.3 RDE Analysis .................................................................................................. 48 4.2.4 MFC Experiments ........................................................................................... 49
4.3 Results and Discussion ............................................................................................... 50 4.3.1 MFC Performance ........................................................................................... 50 4.3.2 Catalyst Activity and Selectivity ..................................................................... 52 4.3.3 Effect of Oxygen Functional Groups on ORR Catalysis................................. 54 4.3.4 Effect of Microporosity on Power Production ................................................ 56 4.3.5 Functional Group Analysis Using XPS ........................................................... 57 4.3.6 Implications of AC properties for MFC performance ..................................... 59
4.4 Acknowledgements .................................................................................................... 59 4.5 References .................................................................................................................. 60
Chapter 5 Improvement of Oxygen Reduction Catalysis in Neutral Solutions using Ammonia Treated Activated Carbons and Performance in Microbial Fuel Cells ........... 62
Abstract ............................................................................................................................ 62 5.1 Introduction ................................................................................................................ 63 5.2 Materials and Methods ............................................................................................... 65
5.2.1 Activated Carbons and Ammonia Treatment .................................................. 65 5.2.2 Chemical Surface Analysis ............................................................................. 65 5.2.3 Rotating Ring-Disk Electrochemical Analysis ................................................ 66 5.2.4 MFC Experiments ........................................................................................... 67
5.3 Results and Discussion ............................................................................................... 69 5.3.1 MFC performance ........................................................................................... 69 5.3.2 Catalyst Activity and Selectivity ..................................................................... 71 5.3.3 Effect of Surface Chemistry ............................................................................ 73
5.4 Conclusion ................................................................................................................. 76 5.5 Acknowledgements .................................................................................................... 77 5.6 References .................................................................................................................. 77
Chapter 6 Conclusions and Future Work ................................................................................. 79
Appendix A Supplemental Information to Chapter 4 ..................................................... 81
ix
Appendix B Supplemental Information to Chapter 5 ...................................................... 86
x
LIST OF FIGURES
Figure 2-1. (A) Power density and (B) polarization curves for single-cycle (20 min) resistance changes at 30 days (◊) and 100 days (Δ) and multiple-cycle resistance changes (). ...................................................................................................................... 20
Figure 2-2. Electrode (A=anode and C=cathode) potential measurements (vs. Ag/AgCl) during cell polarization for single-cycle (20 min) resistance changes at 30 days (◊) and 100 days (Δ) and multiple-cycle resistance changes (). .......................................... 20
Figure 2-3. (A) Power density and (B) polarization curves from LSV at 1 mV s−1 for three consecutive scans. ............................................................................................................ 22
Figure 3-1. (A) Power density and (B) polarization curves for polymer-coated cathodes. ..... 34
Figure 3-2. Electrode potential measurements (vs. Ag/AgCl) during cell polarization. .......... 34
Figure 3-3. Coulombic efficiencies for cycles run at 1000 Ω. ................................................. 35
Figure 3-4. EIS of coated and uncoated cathodes at 0.1V (vs. Ag/AgCl) (200mM PBS). ...... 36
Figure 3-5. LSV of coated and uncoated cathodes (100mM PBS). ......................................... 37
Figure 3-6. Optical images of biofilm growth on cathodes (100 days). .................................. 38
Figure 3-7. Inverse relationship between Rct and power density. ............................................ 39
Figure 3-8. Inverse relationship between CE and power density. ............................................ 40
Figure 4-1. A) Power density production and B) electrode potentials from polarization of MFCs using AC cathodes compared to Pt/C (100 mM Phosphate Buffer; Open symbols represent cathode potentials, closed symbols are anode potentials). ................. 51
Figure 4-2. A) LSV current response (per disk area) of the AC catalyst at the disk electrode compared to Pt/C and carbon black (100 mM Phosphate Buffer, 2100rpm) B) Average number of electrons (n) transferred (estimated by Koutecky-Levich RDE analysis) during oxygen reduction. ......................................................................... 53
Figure 4-3. Acidic/Oxygen functional groups determined by potentiometric titration. A) Bituminous and peat based activated carbon samples have similar functional groups. B) Other activated carbons have a larger variety of acidic groups. ................................. 55
Figure 4-4. The onset potential of the oxygen reduction reaction is inversely related to the amount of strong acid functional groups present on the activated carbons tested. .......... 55
Figure 4-5. Maximum power density (per m2 projected cathode surface) of the MFCs using the activated carbon cathodes is inversely related to the A) surface area (without W1 pvalue=0.0006) and B) micropore volume (without W1 pvalue=0.0011) of the powdered carbons with the exception of sample W1. ................. 57
xi
Figure 4-6. A) Maximum power density (normalized to cathode surface area) of MFCs with activated carbon cathodes are not directly related to oxygen content of activated carbon powders determined by XPS. B) Chemical state of oxygen detected by XPS varies for activated carbon powders tested (e.g. increased signal in the adsorbed O2/H2O region for sample B1.) ........................................................................................ 58
Figure 5-1. A) Power density production and B) electrode potential during cell polarization of MFCs using AC cathodes compared to Pt/C. (100 mM Phosphate Buffer; open symbols indicate cathode potentials, closed symbols anode potentials.) .... 70
Figure 5-2. A) H2O2 detection based on oxidation current at the Pt ring during oxygen reduction at the catalyst on the disk electrode. B) Oxygen reduction current response during LSV of AC catalysts at the disk electrode compared to Pt/C (100 mM Phosphate Buffer, 2100 rpm). C) Average number of electrons transferred (measured by RRDE analysis) during oxygen reduction. ................................................ 72
Figure 5-3. Proton binding isotherms for treated and untreated A) bituminous, B) peat, C) coconut shell, and D) hardwood based activated carbons. ............................................... 74
Figure 5-4. N1s peaks on ammonia treated ACs from XPS show presence of nitrogen groups on the surface of the treated AC. .......................................................................... 75
Figure 5-5. Atomic % of oxygen and nitrogen on the surface of treated and untreated AC catalysts measured using XPS and the relationship to onset potential of the ORR measured with RRDE. ...................................................................................................... 76
Figure A-1: Example of RDE LSV data for bituminous coal based sample (B1) collected at rotation rates from 100 – 2100 RPM. ........................................................................... 81
Figure A-2: Example of K-L analysis for bituminous coal based sample (B1) where the slope of the line is used to calculate n and the y-intercept is the inverse ik. ..................... 82
Figure A-3: Potentiometric titration curves showing protons bound (positive Q) or released (negative Q). The isotherm data was then further analyzed using SAIEUS software to quantify the type (pKa) and quantity of acidic functional groups. ................ 83
Figure A-4: Inverse correlation between quantity of strong acid functional groups on the AC catalyst powder and the power production in an MFC using the AC cathode (a) with and (b) without the inclusion of the bituminous coal based sample (B1) ................ 84
Figure A-5: Cumulative pore volume distribution of AC catalyst powders measured by argon adsorption and DFT analysis .................................................................................. 85
Figure B-1. (A) Power production increases with the additional stainless steel current collector. (B) Cathodes (open symbols) with the additional current collector operate at a higher potential. Anodes (filled symbols) perform the same with both cathode configurations. (100 mM Phosphate buffer, Data of MFC cathodes without the extra current collector is from Chapter 4.) ................................................................................ 86
xii
LIST OF TABLES
Table 3-1. Properties of polymers and cathode coatings. ........................................................ 29
Table 3-2. Oxygen flux, combined solution and membrane resistance, charge transfer resistance, and maximum power density of cathodes. ..................................................... 33
xiii
ACKNOWLEDGEMENTS
I would like to whole heartedly thank everyone who made the last several years possible,
especially Dr. Bruce Logan for supporting me through this research journey. I thank him for
introducing me to the incredibly interesting concept of microbial based electricity production
from wastewater treatment. I am truly honored to have been able to work with him. I would also
like to thank the other members of my committee, Dr. Jay Regan, Dr. Fred Cannon, and Dr. Mike
Hickner for their time and support in this research effort.
I would like to thank all of the members of the Logan Lab throughout my time here,
especially Dr. Rachel Wagner, who was always there to encourage me to keep moving forward. I
also appreciate the assistance of Tim Byrne and Dr. Cesar Neito Delgado with their knowledge of
activated carbon testing.
A special thank you to the National Science Foundation (NSF) Graduate Research
Fellowship program and the King Abdullah University of Science and Technology (KAUST) for
funding this work.
Most importantly I would like to thank my family. My parents, John and Carol Prothero,
and sister, Jacqueline Bealla, for never doubting that I could do this. My in-laws, Donald and
Diana Watson, for helping feed and take care of my children when I couldn’t. My husband,
Steven Watson, for putting up with me and supporting me and taking care of the kids and dealing
with a very messy house and on and on and on. And finally, to my children, Alina and Kira
Watson, for believing I could do anything and loving me even if I couldn’t. I started this journey
wanting to be an inspiration to you, but instead, you became my inspiration. Thank you all, (and
all those that I didn’t have time and space to mention, but I am thinking about gratefully right
now) for your incredible support. Without all of you (and God), none of this would have been
possible.
1
Chapter 1
Introduction
1.1 Energy Demand and the Environment
According to the 2011 United States Energy Information Administration worldwide
report, world energy consumption is projected to grow by 53% from 2008 to 2035. In 2009, the
US was the largest total energy consumer (94.5 quadrillion Btu) and accounted for about 20% of
the world’s energy consumption, although it only contained 5% of the world’s population. Most
worldwide energy consumption is currently derived from fossil fuels, but there are many issues
associated with their use. Fossil fuels are non-renewable resources and they are mainly sourced
from countries with volatile political and economic environments. In 2011, the US was the
world’s largest importer and consumer of petroleum. Burning fossil fuels also produces the main
source of carbon dioxide emissions. Increased anthropogenic CO2 emissions have been linked to
climate change which endangers global ecosystems. The US is the second largest producer of the
world’s CO2 emissions, with China being the number one producer. However most of the
projected increase in global CO2 emissions is attributed to developing countries, with global CO2
emissions projected to increase by 43% from 2008 to 2035[1, 2].
Wastewater treatment consumes a substantial amount of energy. It is estimated that
treatment of wastewater containing mostly organic compounds consumes about 15 GW of power,
which is equivalent to about 3% of US electricity production. It is also estimated that domestic,
industrial, and agricultural (animal) wastewater combined contains about 17 GW of power, which
is mostly lost in current treatment processes [3]. Developing low cost, low energy consumption
2
methods to treat these waste sources, while utilizing or capturing the energy contained in them,
would be beneficial to both environmental and energy demand issues.
1.2 Microbial Fuel Cells
Microbial fuel cells (MFCs) are a promising technology for treatment of wastewater
streams in combination with electricity production [3, 4]. In a typical design, bacteria oxidize
organic waste in anaerobic conditions and transfer electrons to an anode. Electrons are conducted
through a circuit to the passively air-fed cathode consisting of a porous carbon structure with an
oxygen reduction catalyst, where oxygen is ideally reduced to water through a 4e– transfer
pathway, producing electrical current [4]. Commercialization and implementation of MFCs could
eliminate the large energy consumption common in traditional wastewater treatment (mostly
through the elimination of aeration of the waste stream) and allow for the utilization of this
untapped renewable energy source. Although improvements in MFC design and performance has
progressed rapidly over the past several years, more technological advances are needed for MFCs
to become commercially viable. Specifically, improvement in cathode design and low-cost
materials are needed in order to increase power production, decrease material costs, and improve
long–term stability [5].
1.2.1 Oxygen Reduction at the MFC Cathode
Oxygen reduction air cathodes are commonly used in MFCs since oxygen is readily
available, has a positive redox potential, and is environmentally innocuous. Unfortunately, the
oxygen reduction reaction (ORR) is kinetically slow in common MFC conditions (neutral pH and
ambient temperatures) [6]. Platinum particles loaded onto high surface area carbon black (e.g.
3
Vulcan XC-72) are commonly used to catalyze the ORR in MFCs and other fuel cells because of
its well-known high activity and performance, but platinum is expensive, finite in abundance, and
prone to inhibition by contaminants found in the waste streams to be treated [7, 8].
Depending on the catalyst, the ORR proceeds through either a 4e– pathway producing
water or hydroxide [9] or 2e– pathway producing hydrogen peroxides as intermediates [10, 11].
Currently cathode material costs account for 47-75% of MFC capital costs [12]. Several other
catalyst materials have been considered for use in MFCs, including other metal compounds such
as cobalt and iron tetramehoxyphenylporphyrin (TMPP) or phthalocyanine (Pc) [7] and
manganese oxides [13, 14]. Recently there have been promising studies using activated carbon
(AC) powder based air–cathodes [15-18].
1.2.2 Activated Carbon Catalysts
Activated carbon (AC) is relatively inexpensive and can be made from a variety of
materials that are readily available, including several waste sources [19-21]. Coal (Bituminous
and Lignite), wood, phenol resin, coconut shells, and silk fibroin have been used as AC
precursors. Utilization of different precursors as well as different activation processes (e.g. steam
vs. chemical activation) can lead to differences in the surface functional groups, surface area, and
pore structure (micro-, meso-, and macropore development) of the AC [19-21]. For instance, silk
fibroin was used as an AC precursor because the material contained no metal elements and
eighteen types of amino acids that contain nitrogen atoms [20].
The use of AC as an ORR catalyst has been evaluated by several researchers, but most of
the work has been done under conditions that are not applicable to MFCs, such as using acidic
and alkaline solutions commonly used in other types of fuel cells. In these other solutions,
researchers have found impacts of specific surface area, pore structure, and precursor material on
4
the AC catalyst activity, but the results are not always in agreement on which characteristics have
the most influence on the ORR [20, 22]. A silk based AC was compared to a phenolic resin–
derived AC, and the materials showed different ORR activity that did not seem to correlate with
specific surface area [20]. X-ray photoelectron spectroscopy (XPS) was used to analyze the types
of nitrogen atoms present in AC materials (e.g. pyridine-like, pyrrole-like, and quaternary) and it
was found that the presence of quaternary nitrogen atoms improved the performance of the
catalyst for oxygen reduction. Unfortunately no analysis of pore size distribution was mentioned
in this study.
Iron and nitrogen loaded ACs with different pore size distributions were tested as oxygen
reduction catalysts [22]. The pore sizes were varied for the same carbons by heating the materials
at 500°C in air for 20 to 180 minutes. The researchers found that surface area and pore size varied
with the “burn-off” time and that the pore size had an important effect on catalytic activity, with
pore sizes around 13Å showing the best performance. The relationship between specific surface
area and catalyst activity was not linear. Again, nitrogen content of the ACs was also found to
have an effect on the catalyst activity for oxygen reduction. In a similar study, researchers found
that only pore sizes greater than 15Å can be utilized for oxygen reduction in alkaline media [10].
ACs have also been tested as catalysts in MFC cathodes, but most often they are used as a
high surface area catalyst support for platinum or other metal catalysts [23]. In recent studies, AC
powder based cathodes have been successful in attaining power densities similar to those
achieved with commonly used platinum cathodes. An MFC with an AC cathode made using a
proprietary process, which contained a polytetrafluoroethylene (PTFE) binder and a nickel
current collector, produced 1220 mW m–2, compared to 1060 mW m–2 using a cathode with a Pt
catalyst and a Nafion binder [15]. An MFC incorporating a similar AC cathode structure that had
a polydimethylsiloxane (PDMS) coated cloth diffusion layer reached 1255-1310 mW m–2
compared to 1295 mW m–2 with a standard Pt/C cathode [16]. However, very little testing has
5
been done to compare the activity of ACs obtained from different precursor materials and with
different pore structures in MFCs. A cathode that was formed by rolling (rather than pressing) the
AC and PTFE catalyst layer onto a stainless steel mesh current collector produced 1086 mW m–2
in an MFC with a high surface area (1701 m2 g–1) AC, and 1355 mW m–2 with a lower surface
area (576 m2 g–1) AC powder. Power production using a standard Pt/C cathode as a baseline
reference was not reported [18]. The higher power production by the lower surface area AC
cathode was attributed to a more uniform distribution of microporosity. However, only two ACs
were compared and there was no analysis of AC surface chemistry, which could have affected the
ORR.
Several studies have shown that the number of nitrogen functional groups on carbon
surfaces can be increased by treatment with ammonia gas at elevated temperatures [11, 24-26].
During the process of incorporating the nitrogen into the carbon structure, there is a
corresponding reduction in acidic oxygen groups as the oxygen atoms are desorbed from the
carbon surface as CO/CO2. This rearrangement of surface functional groups results in an increase
in the basic properties of the carbon surface at the expense of acidic properties [24, 27-31].
Nitrogen incorporation on carbon surfaces has been shown to increase the catalytic activity and
selectivity for oxygen reduction through a four electron pathway in both acidic and alkaline
environments, but it has not been examined under neutral pH conditions [11, 32, 33].
1.2.3 Membranes and Separators
Some MFCs include an ion exchange membrane in the electrolyte compartment between
the anode and cathode. However, membranes have been shown to negatively impact the power
production of the MFC by increasing the internal resistance of the cell and inducing pH gradients
during cell operation [34, 35]. Cations such as Na+, K+, and NH4+ are preferentially transferred
6
through cation exchange membranes (CEMs) due to their higher concentrations in water, rather
than protons present at lower concentrations, in order to maintain charge balance. This results in a
decrease in performance due to pH changes [6, 34]. Anion exchange membranes (AEMs)
outperform CEMs and other types of membranes in MFCs and microbial electrolysis cells
(MECs) mostly due to lower internal resistances that result from lower charge transport resistance
[35-38]. Charge balance can be facilitated by transfer of buffer anions (such as phosphate) when
using an AEM [35]. However, both AEMs and CEMs negatively impact MFC performance due
to the formation of pH gradients at the electrodes [39].
MFC power production can be improved by removing the membrane from the system
[40] and reducing the electrode spacing to decrease ohmic losses. When the electrodes become
closely spaced, however, a separator is needed to prevent short circuiting and also to reduce
oxygen diffusion into the anode chamber which can adversely affect power production [41].
Oxygen diffusion into the anode chamber negatively affects MFC performance by serving as an
alternative electron acceptor for the facultative bacteria at the anode. If the bacteria use the
oxygen as the terminal electron acceptor instead of the anode current collector, the coulombic
efficiency (CE) will decrease, the anode potential will become more positive, and the current
density will decrease [42]. Cloth separators have been used to decrease oxygen diffusion into the
anolyte, but over time the cloth became completely degraded by the bacteria in the reactor [43,
44]. Positioning a glass fiber separator next to the cathode in an MFC with 2 cm electrode spacing
has been shown to increase CE to 80% compared to 30% without a separator [43]. However,
power production with the separator decreased from 896 mW m−2 to 791 mW m−2 as a result of
decreased cathode potential and increased ohmic resistance. To improve power production, the
electrode spacing was decreased using the separator which prevented short-circuiting. With the
decreased electrode spacing, the power density increased to 1195 mW m−2 while maintaining CE
at 80%. In the same study, growth of biofilm on the cathode was also found to improve CE over
7
time due to a decrease in oxygen diffusion into the electrolyte from the air cathode, but the
biofilm also hindered proton migration to the cathode and limited power production [43].
Currently more work needs to be done in order to develop a material that can be used to separate
the anode and cathode electrodes/chambers without decreasing power production.
1.3 Analysis Methods
1.3.1 MFC Power Density and Overshoot
Power production is one of the main measures of MFC performance, but estimates of the
amount of power that can be produced in an MFC are a function of the technique used to obtain
polarization data. Linear sweep voltammetry (LSV) is commonly used in MFC studies to obtain
polarization data, but high scan rates can overestimate power production [45]. An alternate
approach is to vary the circuit resistance at fixed time intervals, ranging from 10 s to 24 h, or even
running each resistance for a full cycle [46, 47]. A common problem often encountered when
evaluating polarization curves is “power overshoot” [45, 48-50]. Power overshoot refers to the
response of the system at high current densities (past maximum power) in a power density curve
where the cell voltage and current drop very quickly resulting in a doubling back of the power
density curve, producing lower power than previously measured at lower current densities [50].
One hypothesis on the cause of this power overshoot is that as the current resistance is decreased
the bacteria on the anode are unable to produce sufficient current at lower voltages [50]. Accurate
methods are needed to ensure that power densities reported by different researchers reflect the
performance of the MFC in stable conditions.
8
1.3.2 Catalyst Performance
In order to evaluate catalyst candidates for ORR activity, many studies have employed
the use of a rotating disk electrode (RDE) in order to isolate mass transfer effects and focus on the
kinetics of the reaction [21, 51, 52]. Catalysts are applied in a uniform thin Nafion bound film to
a glassy carbon disk electrode and evaluated by comparing current production during a scan of
the RDE potential in the range of the ORR. Reduction current free from mass transfer effects (ik)
can be determined using the Koutecky-Levich equation
1𝑖
= 1𝑖𝑘
+ 1𝑖𝑑
= 1𝑛𝐹𝐴𝑘𝐶𝑂2
− 1
0.62𝑛𝐹𝐴𝐷𝑂22 3⁄ 𝜐−1 6⁄ 𝐶𝑂2𝜔
1 2⁄ (1-1)
where i is the measured current, ik the kinetic current, id the diffusion-limiting current, F
Faraday’s constant, A the projected surface area of the disk electrode, k the rate constant, CO2 the
concentration of oxygen in solution, DO2 the diffusion coefficient of oxygen, υ the kinematic
viscosity, and ω the rotation rate of the electrode [13]. By plotting -1/i vs. ω-1/2, the number of
electrons transferred during oxygen reduction can be determined by using the slope of the linear
regression and ik can be determined by the y-intercept [13, 21, 53]. RDE analyses have been used
to study the ORR catalysis of many materials, including AC powders [18] and other materials
such as carbon–supported magnesium oxide nanoparticles [13, 14], and FeTMPP and FePc [7],
where the RDE results were well correlated with MFC performance.
Rotating Ring Disk Electrode (RRDE) experiments can be used to directly determine the
fraction of peroxide produced at the catalyst during oxygen reduction while simultaneously
evaluating the overall ORR activity of the catalyst [51]. While the potential of the glassy carbon
disk electrode is scanned, the platinum ring electrode is held at a potential where the oxidation of
peroxide is diffusion limited (1.2 V vs. RHE). The amount of current produced at the ring
electrode indicates the amount of peroxide formed at the catalyst on the disk electrode. The 4e–
9
pathway is ideal for fuel cell cathodes because it can induce higher currents for increased power
production. The number of electrons transferred measured using a rotating ring-disk electrode
(RRDE) can be influenced by the proportion of the reactions taking place including other
reactions that may occur [51], such as:
Reduction of H2O2: H2O2 + 2H+ + 2e– → 2H2O
Decomposition of H2O2: 2H2O2 → 2H2O + O2
This RRDE method was used in silk fibroin AC testing to simultaneously measure activity and
selectivity [20]. The researchers determined that 25% of the oxygen reacted followed the 2e–
pathway to H2O2 production verifying the overall 3.5e– transfer per mole of O2. The average
number of electrons transferred (n) in the ORR at the disk electrode can be calculated based on
the amount of H2O2 detected using [54]
𝑛 = 4𝑖𝑑𝑖𝑠𝑘𝑖𝑑𝑖𝑠𝑘+𝑖𝑟𝑖𝑛𝑔 𝑁⁄
(1–4)
where idisk is reduction current at the disk, iring the oxidation current at the ring, and N is the
collection efficiency of the RRDE.
1.4 Conclusions
MFC technology is a promising innovation for treating wastewater streams while
simultaneously salvaging energy that is typically wasted. In order to ensure the
commercialization of these systems, the capital costs of these systems must be reduced while
increasing performance. Many advances have been made to reduce the cost of the materials in a
short period of time. Still, more work needs to be done to find low-cost cathode materials in order
to decrease the cost and increase the power production of MFCs.
10
1.5 Objectives and Clarification of Contributions
The objective of this research was to improve the understanding and performance of
MFCs, with a special focus on cathode materials and performance. The first Chapter provided an
overview into the need for alternative technologies for wastewater treatment and energy
production, as well as an introduction to MFC technology.
A study of the use of different methods to obtain power density analysis in MFCs and
their effect on the observation of power overshoot was described in Chapter 2. I was the primary
author and contributor on this research, as I performed all the experiments and analysis. The
coauthor (Dr. Bruce Logan) contributed to the editing on the manuscript.
A method of isolating the cathode from bacterial growth was presented in Chapter 3. I
was the primary author of this paper. My co-authors selected the polymer materials to test based
on our objectives and the MFC environment and Dr. Tomonori Saito prepared the polymer
solutions and provided the material characteristics listed in Table 3-1. I constructed the cathodes,
coated them with the polymer materials, and performed the electrochemical and MFC analysis.
All of the coauthors contributed to the editing of the paper.
The comparison of several different types of powdered AC and their catalytic and MFC
performance was presented in Chapter 4. I realized there was a need to further characterize the
AC used in MFC electrodes to better understand their performance since most research about
ACs and AC electrodes are done in acidic and alkaline environments and there was little research
comparing different types of activated carbon as the ORR catalyst in MFC cathodes. I therefore
obtained commercially produced ACs made from different precursor materials and that had
different surface areas in order to study the relationship between the chemical and physical
characteristics of the ACs and their performance as ORR catalysts in neutral solutions. I
performed the experimentation and analysis with the exception of running the potentiometric
11
titrations which were done by Dr. Cesar Nieto Delgado, and the XPS operation which was done
by Vince Bojan. I was the primary author and the co-author (Dr. Bruce Logan) contributed to the
editing of the draft manuscript.
Since the results reported in Chapter 4 indicated that that strong acid oxygen groups were
detrimental to ORR catalysis in ACs, I decided to study the effect of increasing the amount of
nitrogen groups. In Chapter 5, the comparison in performance of ammonia treated activated
carbons to the non-treated samples was presented. I was the primary author and prepared the
treated activated carbons. I performed all of the electrochemical and MFC performance
experimentation and analysis. The potentiometric titrations were run by Dr. Cesar Nieto
Delgado, and the XPS was performed by Vince Bojan. The co-author (Dr. Bruce Logan)
contributed to the editing of the draft manuscript.
1.6 References
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Using Microbial Electrochemical Technologies. Science, 2012. 337: p. 686-690. 4. Logan, B.E., Microbial Fuel Cells. 2008, New York: John Wiley & Sons. 5. Rismani-Yazdi, H., et al., Cathodic limitations in microbial fuel cells: An overview.
Journal of Power Sources, 2008. 180: p. 683-694. 6. Zhao, F., et al., Challenges and constraints of using oxygen cathodes in microbial fuel
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Electrochimica Acta, 2011. 56: p. 1505-1511. 8. Harnisch, F., S. Wirth, and U. Schröder, Effects of substrate and metabolite crossover on
the cathodic oxygen reduction reaction in microbial fuel cells: Platinum vs. iron(II) phthalocyanine based electrodes. Electrochemistry Communications, 2009. 11(11): p. 2253-2256.
9. Popat, S., et al., Importance of OH- Transport from Cathodes in Microbial Fuel Cells. ChemSusChem, 2012. 5: p. 1071-1079.
10. Qu, D., Investigation of oxygen reduction on activated carbon electrodes in alkaline solution. Carbon, 2007. 45: p. 1296-1301.
12
11. Zhong, R.-S., et al., Effect of carbon nanofiber surface functional groups on oxygen reduction in alkaline solution. Journal of Power Sources, 2013. 225: p. 192-199.
12. Rozendal, R.A., et al., Towards practical implementation of bioelectrochemical wastewater treatment. Trends in Biotechnology, 2008. 26(8): p. 450-459.
13. Chen, Y., et al., Stainless steel mesh coated with MnO2/carbon nanotube and polymethlyphenyl siloxane as low-cost and high-performance microbial fuel cell cathode materials. Journal of Power Sources, 2012. 201: p. 136-141.
14. Roche, I. and K. Scott, Carbon-supported manganese oxide nanoparticles as electrocatalysts for oxygen reduction reaction (orr) in neutral solution. Journal of Applied Electrochemistry, 2009. 39: p. 197-204.
15. Zhang, F., et al., Power generation using an activated carbon and metal mesh cathode in a microbial fuel cell. Electrochemistry Communications, 2009. 11(11): p. 2177-2179.
16. Wei, B., et al., Development and evaluation of carbon and binder loading in low-cost activated carbon cathodes for air-cathode microbial fuel cells. RSC Advances, 2012. 2: p. 12751-12758.
17. Dong, H., et al., A novel structure of scalable air-cathode without Nafion and Pt by rolling activated carbon and PTFE as catalyst layer in microbial fuel cells. Water Res, 2012. 46: p. 5777-5787.
18. Dong, H., H. Yu, and X. Wang, Catalysis kinetics and porous analysis of rolling activated carbon-PTFE air-cathode in microbial fuel cells. Environ Sci Technol, 2012. 46: p. 13009-13015.
19. Matsis, V.M. and H.P. Grigoropoulou, Kinetics and equilibrium of dissolved oxygen adsorption on activated carbon. Chemical Engineering Science, 2008. 63: p. 609-621.
20. Iwazaki, T., et al., High oxgen-reduction activity of silk-derived activated carbon. Electrochemistry Communications, 2009. 11: p. 376-378.
21. Maruyama, J. and I. Abe, Carbonized hemoglobin functioning as a cathode catalyst for polymer electrolyte fuel cells. Chemistry of Materials, 2006. 18: p. 1303-1311.
22. Yang, R., T.R. Dahn, and J.R. Dahn, Fe-N-C oxygen reduction catalysts supported on "burned-off" activated carbon. Journal of The Electrochemical Society, 2009. 156(4): p. B493-B498.
23. Aelterman, P., et al., Microbial fuel cells operated with iron chelated air chathodes. Electrochimica Acta, 2009. 54: p. 5754-5760.
24. Chen, W., F.S. Cannon, and J.R. Rangel-Mendez, Ammonia-tailoring of GAC to enhance perchlorate removal. I: Characterizationof NH3 thermally tailored GACs. Carbon, 2005. 43: p. 573-580.
25. Arrigo, R., et al., Tuning the acid/base properties of nanocarbons by functionalization via ammination Journal of the American Chemical Society, 2010. 132: p. 9616-9630.
26. Shafeeyan, M.S., et al., A review on surface modification of activated carbon for carbon dioxide adsorption. Journal of Analytical and Applied Pyrolysis, 2010. 89: p. 143-151.
27. Mangun, C.L., et al., Surface chemistry, pore sizes and adsorption properties of activated carbon fibers and precursors treated with ammonia. Carbon, 2001. 39: p. 1809-1820.
28. Biniak, S., et al., The characterization of activated carbons with oxygen and nitrogen surface groups. Carbon, 1997. 35(12): p. 1799-1810.
29. Szymanski, G.S., et al., The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon. Carbon, 2002. 40: p. 2627-2639.
30. Shen, W., Z. Li, and Y. Liu, Surface chemical functional groups modification of porous carbons. Recent Patents on Chemical Engineering, 2008. 1: p. 27-40.
13
31. Hulicova-Jurcakova, et al., Combined effect of nitrogen- and oxygen-containing functional groups of microporous activated carbon on its electrochemical performance in supercapacitors. Advanced Functional Materials, 2009. 19: p. 438-447.
32. Kruusenberg, I., et al., Non-platinum cathode catalysts fo alkaline membrane fuel cells. International Journal of Hydrogen Energy, 2012. 37: p. 4406-4412.
33. Nallathambi, V., et al., Development of High Performance Carbon Composite Catalyst for Oxygen Reduction Reaction in PEM Proton Exchange Membrane Fuel Cells. Journal of Power Sources, 2008. 183: p. 34-42.
34. Rozendal, R.A., H.V.V. Hamelers, and C.J.N. Guisman, Effects of membrane cation transport on pH and microbial fuel cell performance. Environmental Science & Technology, 2006. 40(17): p. 5206-5211.
35. Kim, J.R., et al., Power generation using different cation, anion and ultrafiltration membranes in microbial fuel cells. Environmental Science & Technology, 2007. 41(3): p. 1004-1009.
36. Rozendal, R., et al., Performance of single chamber biocatalyzed electrolysis with different types of ion exchange membranes. Water Research, 2007. 41: p. 1984-1994.
37. Zuo, Y., S. Cheng, and B.E. Logan, Ion Exchange Membrane Cathodes for Scalable Microbial Fuel Cells. Environmental Science & Technology, 2008. 42(18): p. 6967-6972.
38. Sleutels, T.H.J.A., et al., Ion transport resistance in Microbial Electrolysis Cells with anion and cation exchange membranes. International Journal of Hydrogen Energy, 2009. 34(9): p. 3612-3620.
39. Harnisch, F., U. Schröder, and F. Scholz, The Suitability of Monopolar and Bipolar Ion Exchange Membranes as Separators for Biological Fuel Cells. Environmental Science & Technology, 2008. 42(5): p. 1740-1746.
40. Liu, H. and B.E. Logan, Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ Sci Technol, 2004. 38(14): p. 4040-4046.
41. Logan, B., Scaling up microbial fuel cells and other bioelectrochemical systems. Applied Microbiology and Biotechnology, 2010. 85(6): p. 1665-1671.
42. Harnisch, F. and U. Schröder, Selectivity versus Mobility: Separation of Anode and Cathode in Microbial Bioelectrochemical Systems. ChemSusChem, 2009. 2(10): p. 921-926.
43. Zhang, X., et al., Separator Characteristics for Increasing Performance of Microbial Fuel Cells. Environmental Science & Technology, 2009. 43(21): p. 8456-8461.
44. Fan, Y., H. Hu, and H. Liu, Enhanced Coulombic efficiency and power density of air-cathode microbial fuel cells with an improved cell configuration. Journal of Power Sources, 2007. 171(2): p. 348-354.
45. Velasquez-Orta, S.B., T.P. Curtis, and B.E. Logan, Energy from algae using microbial fuel cells. Biotechnology and Bioengineering, 2009. 103(6): p. 1068-1076.
46. Zuo, Y., et al., Tubular membrane cathodes for scalable power generation in microbial fuel cells. Environmental Science & Technology, 2007. 41(9): p. 3347-3353.
47. Menicucci, J.H., et al., Procedure for determining maximum sustainable power generated by microbial fuel cells. Environmental Science & Technology, 2006. 40(3): p. 1062-1068.
48. Aelterman, P., et al., Continuous electricity generation at high voltages and currents using stacked microbial fuel cells. Environmental Science & Technology, 2006. 40: p. 3388-3394.
14
49. Kim, J.R., et al., Modular tubular microbial fuel cells for energy recovery during sucrose wastewater treatment at low organic loading rate. Bioresource Technology, 2010. 101(4): p. 1190-1198.
50. Ieropoulos, I., J. Winfield, and J. Greenman, Effects of flow-rate, inoculum and time on the internal resistance of microbial fuel cells. Bioresource Technology, 2010. 101(10): p. 3520-3525.
51. Paulus, U.A., et al., Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: a thin-film rotating ring-disk electrode study. Journal of Electroanalytical Chemistry, 2001. 495: p. 134-145.
52. Schmidt, T.J., et al., Characterization of high-surface-area electrocatalysts using a rotating disk electrode configuration. Journal of The Electrochemical Society, 1998. 145(7): p. 2354-2358.
53. Bard, A.J. and L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications. 2nd ed. 2001, New York: Wiley.
54. Kim, J.R., et al., Application of Co-naphthalocyanine (CoNPc) as alternative cathode catalyst and support structure for microbial fuel cells. Bioresource Technology, 2011. 102: p. 342-347.
55. Yu, E.H., et al., Microbial Fuel Cell Performance with non-Pt Cathode Catalysts. Journal of Power Sources, 2007. 171: p. 275-281.
56. Gojkovic, S.L., S. Gupta, and R.F. Savinell, Heat-treated iron(III) tetramethoxyphenyl porphyrin supported on high-area carbon as an electrocatalyst for oxygen reduction - I. Characterization of the electrocatalyst. Journal of the Electrochemical Society, 1998. 145: p. 3493-3499.
57. Bandosz, T.J., J. Jagiello, and C. Contescu, Characterization of the surfaces of activated carbons in terms of their acidity constant distributions. Carbon, 1993. 31(7): p. 1193-1202.
58. Seredych, M. and T.J. Bandosz, Investigation of the enhancing effects of sulfur and/or oxygen functional groups of nanoporous carbons on adsorption of dibenzothiophenes. Carbon, 2011. 49: p. 1216-1224.
15
Chapter 2
Analysis of polarization methods for elimination of power overshoot in microbial fuel cells
Abstract
Polarization curves from microbial fuel cells (MFCs) often show an unexpectedly large drop in
voltage with increased current densities, leading to a phenomenon in the power density curve
referred to as “power overshoot”. Linear sweep voltammetry (LSV, 1 mV s−1) and variable
external resistances (at fixed intervals of 20 min) over a single fed-batch cycle in an MFC both
resulted in power overshoot in power density curves due to anode potentials. Increasing the anode
enrichment time from 30 days to 100 days did not eliminate overshoot, suggesting that
insufficient enrichment of the anode biofilm was not the primary cause. Running the reactor at a
fixed resistance for a full fed-batch cycle (~1 to 2 days), however, completely eliminated the
overshoot in the power density curve. These results show that long times at a fixed resistance are
needed to stabilize current generation by bacteria in MFCs, and that even relatively slow LSV
scan rates and long times between switching circuit loads during a fed-batch cycle may produce
inaccurate polarization and power density results for these biological systems.
This chapter was published as: Watson, V. J. and Logan, B. E., Analysis of polarization methods for elimination of power overshoot in microbial fuel cells. Electrochemistry Communications 2011, 13, (1), 54-56.
16
2.1 Introduction
Much of the research involving the use of microbial fuel cells (MFC) for combined
electricity production and wastewater treatment is focused on producing the most power through
improved reactor designs [1]. However, estimates of the amount of power that can be produced in
an MFC are a function of the technique used to obtain polarization data. Linear sweep
voltammetry (LSV) is commonly used in MFC studies to obtain polarization data, but high scan
rates can overestimate power production [2]. An alternate approach is to vary the circuit
resistance at fixed time intervals, ranging from 10 s to 24 h [3,4]. There have been few studies
comparing the different techniques, but in one study it was found that power production with scan
rates higher than 0.1 mV s−1 produced higher power densities than those where the circuit
resistance was varied [2]. A common problem often encountered when evaluating polarization
curves is “power overshoot” [2,5–7]. Power overshoot refers to the response of the system at high
current densities (past the maximum power) in a power density curve where the cell voltage and
current drop very quickly resulting in a doubling back of the power density curve, producing
lower power than previously measured for the lower current densities [7]. One hypothesis on the
cause of this power overshoot is that as the current resistance is decreased the bacteria on the
anode are unable to produce sufficient current at lower voltages [7]. However, there does not
seem to be a correlation in the literature between the magnitude of current density and power
curve shape.
Accurate methods are needed to ensure that power densities reported by different
researchers reflect the true performance of the MFC. So far there has been no study on how
different polarization techniques might affect the development of power overshoot or methods to
eliminate it. We therefore examined MFCs that exhibited power overshoot when analyzed using
common LSV and fixed resistances methods, and showed that power overshoot could be
17
eliminated by allowing sufficient time for the biofilm to adjust to a change in resistance by using
a single fixed resistance for each separate fed-batch cycle.
2.2 Experimental procedures
2.2.1 MFC reactor construction and operation
Cube-shaped MFCs with a cylindrical chamber (28 mL, 7 cm2 cross section) were
constructed without a membrane as previously described [8]. The brush anode was constructed
from carbon fibers (PANEX®33 160 K, ZOLTEK) wound into a titanium wire core (2.5 cm
diameter, 2.5 cm length, 0.22 m2 surface area) which was heat treated at 450 °C [9] and placed
horizontally in the center of the chamber. Air cathodes (projected surface area of 7 cm2) were
made from carbon cloth (30 wt.% wet proofing polymer, #B1B30WP, BASF Corp.) with four
PTFE diffusion layers and 0.5 mg-Pt cm−2 [10]. The electrode spacing was 2.5 cm (center of the
anode to the face of the cathode).
Seven MFCs were inoculated using effluent from another MFC operated under similar
conditions (50% v/v inoculum and medium) [3] at 30 °C in a controlled-climate room, and were
covered to exclude light. The medium was a 100 mM phosphate buffer solution (PBS) containing
(g L−1): 9.125 Na2HPO4, 4.904 NaH2PO4·H2O, 0.31 NH4Cl, and 0.13 KCl; pH 7, vitamins and
minerals [11]; and 1 g L−1 sodium acetate. The electrodes were connected through a 1000 Ω
resistor, except as noted. Once an MFC produced ≥100 mV at 1000 Ω, no additional inoculum
was added to the medium over subsequent fed-batch cycles. MFCs were considered enriched and
ready for testing once they achieved the same maximum voltage for three consecutive batch
cycles [3].
18
2.2.2 Analysis
The voltage across the resistor was recorded every 30 min using a multimeter data
acquisition system (model 2700 Keithley Instruments, Cleveland, OH). Polarization was
performed once the voltage stabilized after the MFC was fed. Polarization curves were obtained
by three different methods. In the first method (single-cycle), conducted on days 30 and 100,
various external resistances (OCV, 1000, 500, 250, 100, 75, 50, and 25 Ω, except where noted)
were connected across the MFC, with each resistance being connected for 20 min and the voltage
recorded using a digital multimeter (Model 83 III, Fluke) over a single batch cycle [12]. For the
second method (multiple-cycle), conducted after 100 days, the maximum sustainable voltage over
the cycle (typically sustained for 7 to 30 h depending on the total length of the cycle) was
recorded using a single resistor (decreasing for each batch) over a complete fed-batch cycle [12].
Each resistance was tested for three consecutive cycles to ensure that the voltage response was
unchanged with successive cycles. The third method, linear sweep voltammetry (LSV), was run
after the multiple-cycle method. LSVs were run three times at the recommended scan rate of 1
mV s−1 over a range of 0.5 V starting from the measured open circuit voltage [13]. For the applied
resistance methods, current density was calculated from I=E/R, where I is the current, E the
measured voltage, and R the external resistance, and normalized to the projected cathode surface
area. Power densities were calculated using P=IE, and normalized by the projected cathode
surface area [13].
19
2.3 Results
2.3.1. Polarization by varied resistance, single-cycle method
Polarization curves obtained using the single-cycle method (20 min intervals) exhibited a
steep drop in voltage at higher current densities resulting in power overshoot occurrences in
power density curves. In an example power density curve (Figure 2-1), for the 30 day test the
MFC produced a maximum power of 856mWm−2 (0.22 mA cm−2) before the power rapidly
decreased. The measured electrode potentials indicate that the anode potential was responsible for
power overshoot. At the point of overshoot, the anode potential rapidly became more positive
(from −0.354 V to +0.022 V) and the current decreased while the cathode potential returned to a
value consistent with that previously measured at that current (Figure 2-2). Not all reactors tested
exhibited power overshoot, and therefore additional tests were conducted to further investigate
this phenomenon.
To rule out substrate depletion at the end of the polarization cycle as the cause of the
power overshoot, each MFC was refilled with fresh medium, left to stabilize at 1000 Ω (30 min to
1 h) and polarization testing was started at a lower resistance (500 Ω instead of OCV). The same
rapid increase in anode potential was still observed under these new starting conditions (−0.341 V
to 0.034 V) at the same resistances (from 250 Ω to 100 Ω).
In order to see if the overshoot was caused by incomplete enrichment of the anode with
biofilm, the MFCs were maintained over repeated fed-batch cycles for another 70 days. After 100
days of operation, polarization curves still exhibited power overshoot using the 20 min single-
cycle method (Figure 2-1) (1027mWm−2, 0.24 mA cm−2). As before, the anode potential dropped
off when changing resistance from 250 Ω to 100 Ω (Figure 2-2).
20
Figure 2-1. (A) Power density and (B) polarization curves for single-cycle (20 min) resistance changes at 30 days (◊) and 100 days (Δ) and multiple-cycle resistance changes ().
Figure 2-2. Electrode (A=anode and C=cathode) potential measurements (vs. Ag/AgCl) during cell polarization for single-cycle (20 min) resistance changes at 30 days (◊) and 100 days (Δ) and multiple-cycle resistance changes ().
21
2.3.2. Polarization by varied resistance, multiple-cycle method
Polarization data using the multiple-cycle method (a separate resistor for each fed-batch
cycle) conducted after 100 days, produced power density curves without overshoot. At low
current densities, the power curve followed that obtained using the single-cycle (20 min) method,
but at the point where the other curves dropped off, the multiple-cycle curve extended to a higher
power density (1296 mW m−2) with increased current production (0.61 mA cm−2) (Figure 2-1).
The anode potential did not undergo a rapid increase during the multiple-cycle method as it did
when measuring current after 20 min in the single-cycle method (Figure 2-2).
The fed-batch cycle curves resembled those reported previously [8], where the cell
voltage increases rapidly (over a few hours) after the reactor is fed, and then stabilizes for most of
the cycle. Polarization data were obtained during this stable voltage period. Only one full cycle is
needed at each different resistance for the multiple-cycle method. Power production over three
consecutive cycles at the same applied resistance did not show any noticeable change in
maximum voltage from the first cycle to the third cycle (data not shown).
2.3.3 Polarization by LSV
Power density curves obtained using LSV (following the above multiple-cycle results)
also exhibited power overshoot as shown by a doubling back of the power density curve (Figure
2-3). The maximum power recorded for cycle 1 (2530 mW m−2, 0.56 mA cm−2) was higher than
for cycle 2 (1840 mWm−2, 0.44 mA cm−2) and cycle 3 (1860 mW m−2, 0.44 mA cm−2). In
addition, there was another type of power overshoot in that the maximum power densities in three
of the LSV curves were much higher than power densities measured by the applied resistance
22
methods. Also, none of the LSV cycles measured power at current densities as high as was those
measured using the multiple-cycle method.
Figure 2-3. (A) Power density and (B) polarization curves from LSV at 1 mV s−1 for three consecutive scans.
2.4 Discussion
Power overshoot was observed in power density curves obtained using the single-cycle
(20 min intervals) and LSV (1 mV s−1) methods even with MFCs enriched for 100 days or more.
However, power density curves obtained using the multiple-cycle method did not exhibit
overshoot even at current densities up to 0.82 mA cm−2. The overshoot resulted from a rapid
increase in the anode potential as resistance to current flow was decreased, indicating electron
transfer limitation at the anode. The limitation was most likely related to a slow response from the
23
microbes to adjust to the new resistance [7]. When the biofilm was given sufficient time to adjust
to a set resistance by fixing the resistance over an entire cycle (multiple-cycle method), the
biofilm produced increased currents at lower voltages. Since the maximum cell potential did not
change during 3 cycles at the same resistance, the improved performance using the multiple-cycle
method was not a consequence of the long-term enrichment of the anode community. Instead,
these results showed that the biofilm needed much more time to adapt to the applied resistance
than could be obtained in brief intervals at fixed resistances. Additional data in our laboratory has
shown that even 1-hr intervals or slower LSV scan rates do not eliminate overshoot. For fed-batch
MFCs, longer time periods are problematic as the series of resistances needed to produce a
polarization curve cannot be obtained over the whole cycle due to depletion of the substrate.
Thus, it is recommended that the full cycle (or in other cases at least a day or more) be used at a
fixed resistance when obtaining polarization data in MFCs that exhibit overshoot.
It was also observed that the maximum power produced in a single-cycle polarization
curve was less than that produced in a multiple-cycle curve. Thus, power would be
underestimated as a result of reporting data where power overshoot occurs. A full polarization
curve should be obtained in order to see if overshoot is present. Maximum power densities
obtained by LSV were all higher than that obtained using either of the resistance methods,
consistent with previous studies [2].
Our results show that it is important to use a reliable and consistent method for measuring
maximum current densities in MFCs in order to obtain valid results concerning maximum power
densities. If overshoot occurs, it may not be possible to properly compare maximum power from
different MFC studies or different conditions within the same study. For fed-batch MFCs, the use
of multiple cycles of data, each at different fixed resistances, offers the best method to obtain
polarization data for producing power curves representative of performance during steady
operation conditions.
24
2.5 Acknowledgements
This research was supported under a National Science Foundation (NSF) Graduate
Research Fellowship, NSF grant CBET-0730359, and the King Abdullah University of Science
and Technology (KAUST) (Award KUS-I1-003-13).
2.6 References
1. B. Logan, Appl. Microbiol. Biotechnol. 85 (2010) 1665–1671. 2. S.B. Velasquez-Orta, T.P. Curtis, B.E. Logan, Biotechnol. Bioeng. 103 (2009)
1068–1076. 3. Y. Zuo, S. Cheng, D. Call, B.E. Logan, Environ. Sci. Technol. 41 (2007) 3347–
3353. 4. J.H. Menicucci, H. Beyenal, E. Marsili, R.A. Veluchamy, G. Demir, Z.
Lewandowski, Environ. Sci. Technol. 40 (2006) 1062–1068. 5. P. Aelterman, K. Rabaey, T.H. Pham, N. Boon, W. Verstraete, Environ. Sci.
Technol. 40 (2006) 3388–3394. 6. J.R. Kim, G.C. Premier, F.R. Hawkes, J. Rodríguez, R.M. Dinsdale, A.J. Guwy,
Bioresour. Technol. 101 (2010) 1190–1198. 7. I. Ieropoulos, J. Winfield, J. Greenman, Bioresour. Technol. 101 (2010) 3520–
3525. 8. H. Liu, B.E. Logan, Environ. Sci. Technol. 38 (2004) 4040–4046. 9. Y. Feng, Q. Yang, X. Wang, B.E. Logan, J. Power Sources 195 (2010) 1841–
1844. 10. S. Cheng, H. Liu, B.E. Logan, Electrochem. Commun. 8 (2006) 489–494. 11. O. Bretschger, A. Obraztsova, C.A. Sturm, I.S. Chang, Y.A. Gorby, S.B. Reed,
D.E. Culley, C.L. Reardon, S. Barua, M.F. Romine, J. Zhou, A.S. Beliaev, R. Bouhenni, D. Saffarini, F. Mansfeld, B.-H. Kim, J.K. Fredrickson, K.H. Nealson, Appl. Environ. Microbiol. 73 (2007) 7003–7012.
12. J. Heilmann, B.E. Logan, Water Environ. Res. 78 (2006) 531–537. 13. B.E. Logan, P. Aelterman, B. Hamelers, R. Rozendal, U. Schröder, J. Keller, S.
Freguiac, W. Verstraete, K. Rabaey, Environ. Sci. Technol. 40 (2006) 5181–5192.
25
Chapter 3
Polymer coatings as separator layers for microbial fuel cell cathodes
Abstract
Membrane separators reduce oxygen flux from the cathode into the anolyte in microbial fuel cells
(MFCs), but water accumulation and pH gradients between the separator and cathode reduces
performance. Air cathodes were spray-coated (water-facing side) with anion exchange, cation
exchange, and neutral polymer coatings of different thicknesses to incorporate the separator into
the cathode. The anion exchange polymer coating resulted in greater power density (1167 ± 135
mW m−2) than a cation exchange coating (439 ± 2 mW m−2). This power output was similar to
that produced by a Nafion-coated cathode (1114 ± 174mW m−2), and slightly lower than the
uncoated cathode (1384 ± 82 mW m−2). Thicker coatings reduced oxygen diffusion into the
electrolyte and increased coulombic efficiency (CE = 56–64%) relative to an uncoated cathode
(29 ± 8%), but decreased power production (255–574 mW m−2). Electrochemical characterization
of the cathodes ex situ to the MFC showed that the cathodes with the lowest charge transfer
resistance and the highest oxygen reduction activity produced the most power in MFC tests. The
results on hydrophilic cathode separator layers revealed a tradeoff between power and CE.
Cathodes coated with a thin coating of anion exchange polymer show promise for controlling
oxygen transfer while minimally affecting power production.
This chapter was published as: Watson, V. J.; Saito, T.; Hickner, M. A.; Logan, B. E., Polymer coatings as separator layers for microbial fuel cell cathodes. Journal of Power Sources 2011, 196, (6), 3009-3014.
26
3.1 Introduction
Microbial fuel cells (MFCs) represent one of the latest innovations for the treatment of
wastewater streams. MFC technology combines waste treatment with electricity production [1].
Typical MFCs consist of a microbe-enriched anode where organic matter is oxidized. Electrons
are conducted through a circuit to the air-fed cathode consisting of a porous carbon structure with
platinum catalyst, where oxygen is reduced to water [2]. Some MFCs include an ion exchange
membrane in the electrolyte compartment between the anode and cathode. However, membranes
have been shown to negatively impact the power production of the MFC by increasing the
internal resistance of the cell and inducing pH gradients during cell operation [3,4]. MFC power
production can be improved by removing the membrane from the system [5] and reducing the
electrode spacing to decrease ohmic losses. When the electrodes become closely spaced,
however, a separator is needed to prevent short circuiting and also to reduce oxygen diffusion into
the anode chamber which can adversely affect power production [1,6].
The performance characteristics of membrane separators have been investigated in
bioelectrochemical systems, including cation exchange (CEM), anion exchange (AEM), bipolar,
and ultrafiltration membranes [4,7–10]. It has been shown that the cations (Na+, K+, and NH4+)
are preferentially transferred through the CEM due to their high concentrations rather than
protons to maintain charge balance, and as a result there is a decrease in performance due to pH
changes [3,11]. AEMs outperform CEMs and other types of membranes in MFCs and microbial
electrolysis cells (MECs) mostly due to lower internal resistances that result from lower charge
transport resistance [4,8,9,12]. Charge balance can be facilitated by transfer of buffer anions
(such as phosphate) when using an AEM [4]. However, both AEMs and CEMs negatively impact
microbial fuel cell performance due to the formation of a pH gradient at the electrodes [7].
27
Oxygen diffusion into the anode chamber negatively affects MFC performance by
serving as an alternative electron acceptor for the facultative bacteria at the anode. If the bacteria
use the oxygen as the terminal electron acceptor instead of the anode current collector, the
coulombic efficiency (CE) will decrease, the anode potential will become more positive, and the
current density will decrease [13]. Cloth (J-cloth) separators have been used to decrease oxygen
diffusion into the anolyte, but over time the cloth became completely degraded by the bacteria in
the reactor [14,15]. Positioning a glass fiber separator next to the cathode in an MFC with 2 cm
electrode spacing has been shown to increase CE to 80% compared to 30% without a separator
[14]. Power production with the separator decreased from 896 mW m−2 to 791 mW m−2 as a result
of decreased cathode potential and increased ohmic resistance. To improve power production, the
electrode spacing was decreased using the separator which prevented short-circuiting. With the
decreased electrode spacing, the power density increased to 1195 mW m−2 while maintaining CE
at 80%. In the same study, growth of biofilm on the cathode was also found to improve CE over
time due to a decrease in oxygen diffusion into the electrolyte from the air cathode, but the
biofilm also hindered proton migration to the cathode and limited power production [14].
Zhang et al. [16] placed AEMs and CEMs in the electrolyte compartment directly
adjacent to the cathode and obtained around 90% CE. However, the membranes deformed after
several cycles due to membrane swelling during ion and water transport, and the deformation
created a void space between the membrane and electrode filled with water and gas. The water
trapped between the membrane and the cathode had a higher pH than the anode chamber and
decreased the cathode potential. The researchers used stainless steel mesh to keep the membrane
pressed against the cathode and prevent water accumulation behind the membrane. In this
configuration, the ohmic resistance of the reactor decreased from 120Ω to 15Ω with the AEM and
from 49Ω to 16Ω with the CEM, while the power density increased from 16 W m−3 to 46 W m−3
28
with the AEM and from 21 W m−3 to 32 W m−3 with the CEM [16]. In previous studies when
Nafion was hot-pressed on to carbon cloth preventing deformation, the CE increased from 9–12%
to 40–50%, but the power density decreased from 12.5 W m−3 to 6.6 W m−3 [5]. Hot pressing the
membrane to the cathode decreased the power by increasing the ohmic resistance of the
membrane, likely due to adverse effects of the bonding process on membrane permeability [4].
Therefore, it is important to incorporate the membrane into the cathode to prevent deformation
while also striving to minimize the ohmic resistance of the membrane.
In next-generation MFC systems, a separator between the anode and cathodes will be
important to facilitate minimum electrode spacing while preventing short circuiting of the
electrodes [1]. The separators must limit oxygen diffusion to the anolyte while not impeding
proton transfer to the cathode catalyst. This study explored the use of spray coating for applying
thin layers of hydrophilic cation exchange, anion exchange, and neutral polymers to the
electrolyte side of the cathode structure and measured the layers’ effect on power production and
CE with respect to polymer type, oxygen diffusivity, and biofilm growth at the cathode.
3.2 Materials and methods
3.2.1 Polymers
Bisphenol A-based poly(sulfone) (Udel P-3500 LCD, Mw 79,000 g mol−1, 1.24 g cm−3)
and poly(phenylsulfone) (Radel R-5500, Mw 63,000 g mol−1, 1.29 g cm−3) were kindly donated by
Solvay Advanced Polymers, LLC. Radel was aminated (A-Radel, ion exchange capacity of IEC =
2.64 meq g−1) or sulfonated (S-Radel, IEC = 2.54meq g−1) as previously described [17–19].
Nafion solution (Nafion® 117 solution), ∼5 wt% in a mixture of lower aliphatic alcohols and
water was purchased from Aldrich and used as received. Poly(styrene)-b-poly(ethylene oxide)
29
diblock copolymer PS156-b-PEO110 (PEO-110, Mn 21,100 g mol−1, Mw/Mn = 1.01) was
synthesized as previously described [20], where subscripted numbers denote the corresponding
number of repeat units of each block. Polymer solutions (5 wt%) were prepared by dissolving
Udel and PEO-110 in tetrahydrofuran and A-Radel and S-Radel in methanol. Properties of each
of the polymers are summarized in Table 3-1.
Table 3-1. Properties of polymers and cathode coatings.
Cathode Type IEC
(meq g−1) IECν (wet) (meq cm−3)
Water uptake
Solvent Density (g cm−3)
Weight (mg)
Thickness (dry) (µm)
Thickness (wet) (µm)
Nafion-62 CEM 0.91 1.59 20% Aliph-Alc 2.10 122.3 ± 0.8 52 ± 0.3 62 ± 0.4 A-Radel-146 AEM 2.64 1.22 180% MeOH 1.29 75.8 ± 0.1 52 ± 0.0 146 ± 0.0 A-Radel-67 AEM 2.64 1.22 180% MeOH 1.29 34.7 ± 4.5 24 ± 1.9 67 ± 2.3 S-Radel-60 CEM 2.54 1.93 70% MeOH 1.29 51.7 ± 0.7 35 ± 0.3 60 ± 0.4 S-Radel-47 CEM 2.54 1.93 70% MeOH 1.29 40.7 ± 1.0 28 ± 0.4 47 ± 0.5 PEO110-101 N-Phila 0 0 50% THF 1.10 83.8 ± 6.6 67 ± 2.8 101 ± 3.4 Udel-32 N-Phobb 0 0 0% THF 1.24 44.5 ± 2.8 32 ± 1.2 32 ± 1.4 a Neutral – hydrophilic polymer. b Neutral – hydrophobic polymer.
3.2.2 Cathode construction
Platinum-catalyzed air cathodes (projected surface area of 7cm2) were constructed from
carbon cloth containing 30 wt% wet proofing polymer (#B1B30WP, BASF Corp.) with PTFE
diffusion layers, and 0.5 mg-Pt cm−2 catalyst loading [21]. The polymer layers were applied to the
cathodes in layers using an air brush (Paache, BearAir, S. Easton, MA). The sprayed polymer
coating was allowed to dry between layers and then checked for resistivity using a handheld
digital multimeter (Model 83 III, Fluke) and weighed to determine the amount of coating applied
(Table 3-1). Once coated, all cathode surfaces produced a resistance greater than the
30
measurement range of the multimeter, effectively electrically insulating the electrolyte-facing
surface of the cathode. Two cathodes were coated for each polymer tested. The average wet and
dry thicknesses of the coatings were calculated from the measured mass of the applied polymer,
the density of the dry polymer, and the polymer’s water uptake (Table 3-1). The thicknesses of
the polymer layers applied to the solution side of the cathode structure are included in the names
of the samples as indicated by the number to the right of the dash, for instance A-Radel-146
indicates that the A-Radel coating on those cathodes averaged 146 µm in thickness.
3.2.3 MFC reactor construction and operation
Cube-shaped MFCs were constructed as previously described [5]. The anode chamber
was a 28 mL cylindrical chamber (7 cm2 cross section) bored into a Lexan block. The brush
anode was constructed from carbon fibers (PANEX®33 160K, ZOLTEK) wound into a titanium
wire core (2.5 cm diameter, 2.5 cm length, and 0.22 m2 surface area) which was heat treated at
450 °C [22] then placed horizontally in the center of the cylinder. The electrode spacing was
2.5cm (center of the anode to the face of the cathode).
Effluent from the anode chamber of an enriched MFC operated under similar conditions
to those in this study was used for the mixed culture inoculum. The medium used in MFC
performance tests was a 100 mM phosphate buffer solution (PBS) (9.125 g L−1 Na2HPO4, 4.904 g
L−1 NaH2PO4·H2O, 0.31 g L−1 NH4Cl, and 0.13 g L−1 KCl; pH 7) with vitamins and minerals [23]
and 1g L−1 sodium acetate. The PBS concentration was doubled to 200 mM in tests where
indicated. MFCs were inoculated with a 50% (v v−1) inoculum of effluent and medium and were
covered to exclude light. The electrodes were connected through a 1000 Ω resistor, except as
noted. Once an MFC produced ≥100 mV, no additional inoculum was added to the medium over
31
subsequent fed batch cycles. All MFCs were operated at 30 C in a controlled climate room. The
MFCs were considered enriched and ready for testing once they achieved the same maximum
voltage for three consecutive batch cycles. Once the MFC anodes were enriched, the uncoated
cathodes used for startup were removed and the coated cathodes and new uncoated cathodes were
placed in the reactors. All MFC tests were conducted with duplicate cathodes and averages over
two cycles were reported with standard deviations for n = 4 (two cathodes over two cycles),
except for CE which was averaged from duplicate reactors over three cycles (n = 6).
3.2.4 Analysis
The voltage across the resistor was recorded every 30 min using a multimeter (model
2700 Keithley Instruments, Cleveland, OH) with a computerized data acquisition system.
Polarization curves were obtained by applying a different external resistance to the circuit for a
complete batch cycle and the maximum sustainable voltage (typically sustained for 7–30 h
depending on the total length of the cycle) was recorded for each resistance. Current density was
calculated from I = E/R, where I is the current, E the measured voltage, and R the external
resistance, and normalized to the projected cathode surface area. Power densities were calculated
using P = IE, and normalized by the projected cathode surface area [24].
CE was calculated from the ratio of the total electrical charge produced during the
experiment (at 1000 Ω) to the theoretical amount of electrons available from the oxidation of
acetate to carbon dioxide. Therefore, CE [%] = (CEx/CTh) × 100, where 𝐶𝐸𝑥 = ∑ (𝐸𝑖𝑡𝑖)/𝑅𝑇𝑡=1 , CTh
= FbMv, F is Faraday’s constant (96,485 C mol e−1), b is the number of moles of electrons
available per mole of substrate (8 mol e (mol acetate)−1), M is the acetate concentration (mol L−1),
and v is the volume of liquid in the anode chamber (L) [24].
32
The oxygen flux into the electrolyte chamber through each cathode was calculated by
measuring the change in dissolved oxygen concentration (NeoFox, Ocean Optics Inc., FL) over
time in a stirred abiotic MFC reactor (30 mL) without an active anode as previously described
[4].
The impedance of each cathode half-cell was measured by electrochemical impedance
spectroscopy (EIS) at 0.1 V (vs. Ag/AgCl) over a frequency of 100,000–0.1 Hz with sinusoidal
perturbation of 10 mV using a potentiostat (PC 4/750, Gamry Instrument Inc.) at 30 °C. The half-
cell consisted of a 7 cm2 platinum disk counter electrode set parallel to the test cathode and
equipped with an Ag/AgCl reference electrode (+0.2 V vs. NHE) (RE-5B, BASI, IN). The test
cell was filled with 200 mM PBS (13 mL, pH 7) without substrate or other nutrients. The
combined solution and membrane resistances (Rs + Rm) were obtained from Nyquist impedance
plots at the point where Zimag was equal to zero at high frequency. The charge transfer resistance
(Rct) for each cathode was estimated from a semi-circular fit of the charge transfer impedance in
the Nyquist plot [25,26].
The oxygen reduction response of each cathode was measured by linear sweep
voltammetry (LSV) using the same experimental setup as with EIS at a scan rate of 1 mV s−1 over
the range of 0.6 to −0.3 V (vs. Ag/AgCl) with current interrupt correction. The oxygen reduction
activity of the cathodes was measured in both 200 mM and 100 mM PBS solution (pH 7).
3.3 Results
3.3.1 MFC performance
MFCs with cathodes coated with a thin layer of anion exchange polymer (A-Radel-67)
produced approximately the same maximum power (Table 3-2) as the cells with cathodes coated
33
with Nafion of similar thickness (Nafion-62) (Figure 3-1). MFC tests with uncoated cathodes
resulted in slightly higher power density than Nafion-62 or A-Radel-67 coated cathodes. MFCs
with cation exchange Radel polymer coatings on the cathode (S-Radel-47) produced much less
power than the cells with the uncoated control cathodes, as did reactors with thin layers of Udel
hydrophobic polymer, Udel-32. The A-Radel-146 and S-Radel-60 coated cathodes had thicker
coatings and produced less power than the cathodes coated with a thinner layer of the same
polymer (A-Radel-67 and S-Radel-47), most likely due to increased impedance of proton transfer.
The observed differences in power production during polarization were due to differences in
cathode potentials (Figure 3-2) since anode potentials did not vary over the current density range
tested.
Table 3-2. Oxygen flux, combined solution and membrane resistance, charge transfer resistance, and maximum power density of cathodes.
Cathode Oxygen flux
(mg cm-2 h-1) Rs + Rm (Ω) Rct (Ω) Maximum power density (mWm−2)
Uncoated 0.055 5 19 1384 ± 82
Nafion-62 0.022 7 57 1114 ± 174 A-Radel-146 0.010 7 85 574 ± 32 A-Radel-67 0.023 7 22 1167 ± 135 S-Radel-60 0.004 9 >100 255 ± 28 S-Radel-47 0.012 7 >100 439 ± 2 PEO110-101 0.002 7 >100 307 ± 9 Udel-32 0.008 18 >100 266 ± 16 a Neutral – hydrophilic polymer. b Neutral – hydrophobic polymer.
34
Figure 3-1. (A) Power density and (B) polarization curves for polymer-coated cathodes.
Figure 3-2. Electrode potential measurements (vs. Ag/AgCl) during cell polarization.
35
The CEs of the MFCs ranged between 29% and 64% (Figure 3-3; fixed external
resistance of 1000 Ω). Cathodes with thicker coatings of the same polymer type had higher CEs
(A-Radel-146, 56 ± 2%; S-Radel-60, 64 ± 5%) than the cathodes with thinner coatings (ARadel-
67, 33 ± 8%; S-Radel-47, 40 ± 10%).
Figure 3-3. Coulombic efficiencies for cycles run at 1000 Ω.
3.3.2 Electrochemical performance
The A-Radel-67 cathode had the lowest impedance (Rs + Rm =7 Ω and Rct =22 Ω) of all
the coated cathodes (Figure 3-4 and Table 3-2) and only slightly higher resistances than the
uncoated cathode (Rs + Rm =5 Ω and Rct =19 Ω). Since Rm is zero for the uncoated cathode, Rs is 5
Ω for the half-cell control geometry used in these experiments. Thus, for all coated cathodes
except Udel-32, the coating added an Rm of between 2 and 4 Ω. However, larger effects of the
coatings can be observed in the Rct, most likely due to the decrease of reactant concentration at
the catalyst or a decrease in available catalyst sites. A-Radel-67 had the smallest increase in Rct
(+3 Ω) compared to the uncoated cathode while Nafion-62 showed an Rct of 38 Ω greater than the
36
uncoated control cathode. S-Radel-60, S-Radel-47, PEO110-101, and Udel-32 coated cathodes
had Rct values of greater than 100 Ω.
Figure 3-4. EIS of coated and uncoated cathodes at 0.1V (vs. Ag/AgCl) (200mM PBS).
The effect of the coatings on the oxygen reduction performance of the cathodes can be
observed by the decrease in current density during LSV testing compared to the uncoated control
(Figure 3-5). The current densities obtained from LSV for each coated cathode showed the same
trends as power production in MFC tests. For example, A-Radel-67 produced higher current
densities in LSV and the maximum power in the MFC tests, and S-Radel-60 and Udel-32
produced the lowest current densities in LSV and the lowest power densities. LSV showed
similar trends between cathodes using either the 100 mM PBS or 200 mM PBS solution.
37
Figure 3-5. LSV of coated and uncoated cathodes (100mM PBS).
3.3.3 Oxygen diffusion and biofilm growth
The coatings applied to the cathode decreased the rate of oxygen diffusion into the anode
solution as demonstrated by the measured oxygen flux into the electrolyte compartment (Table 3-
2). The decrease in oxygen diffusion was not exclusively a function of the amount of polymer
applied (i.e., the thickness or the weight of the coating), but was a combined result of the type of
polymer, the processing of the layer (e.g. solvent used for coating deposition), and the coating
thickness on the cathode. The oxygen flux was inversely related to the CE during MFC testing.
The cathodes with the highest rate of oxygen diffusion and the lowest CE developed a significant
layer of biofilm after 100 days of operation (Figure 3-6). S-Radel-60, PEO110-101, A-Radel-60,
and Udel-32 cathodes did not develop a visible biofilm layer.
38
Figure 3-6. Optical images of biofilm growth on cathodes (100 days).
3.4 Discussion
Of the coated cathodes, the MFCs using the A-Radel-67 cathodes produced the highest
power density. In general, MFCs that had cathodes with the lowest Rct achieved the highest power
density (Figure 3-7). The A-Radel (AEM) had a lower Rct and higher power production than both
of the CEMs (S-Radel and Nafion). The better performance of the A-Radel is consistent with
results in previous studies comparing AEM and CEM separators in MFCs [4,8] which indicate
phosphate anions buffer pH changes and maintain charge balance. We therefore conclude that
positively charged quaternary ammonium groups on the AEM layer aid anion transport and result
in less accumulation of cations compared to CEM layers. The preferential anion transport in
AEMs may also decrease the pH gradient toward the catalyst moiety compared to that of CEM
layers [9]. The AEM had higher water uptake than the CEM when comparing similar IECs and
polymer backbones (i.e. A-Radel and S-Radel). The higher water uptake of the AEM likely
39
decreased its ion transport resistance which could have also contributed to the higher power
density output of AEM coating than that of the CEM coating.
Figure 3-7. Inverse relationship between Rct and power density.
Although S-Radel and Nafion are both CEMs, the S-Radel hindered power production
more than the Nafion coating of the same thickness, which is reflected in the increase in Rct. The
greater Rct can be explained by considering the IECν, the volumetric concentration of ions in the
swollen polymer. The S-Radel had a higher IECν than Nafion and as seen in previous studies, the
higher IECν can impede proton diffusion at neutral pH [19]. Sulfonate groups in the CEM layers
were most likely saturated with Na+ and K+ rather than H+ due to the high Na+ and K+
concentration in the electrolyte. The accumulated cations hindered proton diffusion through the
CEM layer on the cathode and within the electrode where the layers had penetrated the porous
structure. It is also possible that polymer seepage into the cathode pores inhibited oxygen
transport to the catalyst surface, which increased Rct. Rct of the A-Radel cathodes increased and
the corresponding maximum power density decreased as the applied layer thickness increased and
the same effect was observed for the S-Radel cathodes.
The uncharged, hydrophilic polymer coatings of PEO110 had agreater Rct than the A-
Radel coating of similar thickness (A-Radel-146 compared to PEO110-101, and A-Radel-67
40
compared to Udel-32), most likely due to the A-Radel coating having a greater water uptake
(Table 3-1) and therefore less impedance to proton transfer. The significant increase in Rct for
PEO110-101, with a reasonably high water uptake, implies that anion transport to decrease the
pH gradient in an AEM may be an important factor in the resulting Rct.
In general, there was an inverse relationship between the maximum power density and
CE (Figure 3-8), except for the S-Radel-47, which had a lower power density (440 ± 4 mW m−2)
and lower CE (40 ± 10%) than the A-Radel-146 (574 ± 32 mW m−2 and CE = 56 ± 2%). The
cathode coatings with lower oxygen permeability did not show an improvement in anode
potential resulting from a decrease in oxygen intrusion, most likely due to biofilm formation on
coatings with higher oxygen permeability, which limited oxygen diffusion to the anode. The
biofilm formation on the cathodes with higher oxygen permeability was most likely the cause of
the decrease in CE compared to the less permeable cathodes. Despite similar anode performance,
the cathodes with less oxygen permeability and higher CE produced less power due to an increase
in Rct caused by the increased resistance of the coatings to either proton or oxygen diffusion to the
catalyst surface.
Figure 3-8. Inverse relationship between CE and power density.
41
Polymer coated cathodes can be useful in MFC designs as further efforts are made to
develop polymer coatings that facilitates proton transfer to the cathode but limit oxygen diffusion
into the electrolyte and provide an electrically insulating surface. Anion exchange polymers such
as A-Radel, integrated as a thin membrane coating into MFC cathodes, have potential for
controlling oxygen diffusion into the MFC while minimally affecting power production.
3.5 Acknowledgements
This research was supported under a National Science Foundation Graduate Research
Fellowship, National Science Foundation Grant CBET-0730359, and the King Abdullah
University of Science and Technology (KAUST) (Award KUS-I1-003-13). Thanks to Solvay
Advanced Polymers for the donation of Radel® and Udel® polymer and to Justin Tokash for
insights into EIS theory and application.
3.6 References
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(2006) 5206–5211. 4. J.R. Kim, S. Cheng, S.-E. Oh, B.E. Logan, Environ. Sci. Technol. 41 (2007)
1004–1009. 5. H. Liu, B.E. Logan, Environ. Sci. Technol. 38 (2004) 4040–4046. 6. W.-W. Li, G.-P. Sheng, X.-W. Liu, H.-Q. Yu, Bioresour. Technol.,
doi:10.1016/j.biortech.2010.03.090. 7. F. Harnisch, U. Schroder, F. Scholz, Environ. Sci. Technol. 42 (2008) 1740–
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Acta 55 (2010) 3398–3403. 20. T. Saito, T.H. Roberts, T.E. Long, M. Hickner, B.E. Logan, Energy Environ.
Sci., 2010, doi:10.1039/c0ee00229a. 21. S. Cheng, H. Liu, B.E. Logan, Electrochem. Commun. 8 (2006) 489–494. 22. Y. Feng, Q. Yang, X. Wang, B.E. Logan, J. Power Sources 195 (2010) 1841–
1844. 23. O. Bretschger, A. Obraztsova, C.A. Sturm, I.S. Chang, Y.A. Gorby, S.B. Reed,
D.E. Culley, C.L. Reardon, S. Barua, M.F. Romine, J. Zhou, A.S. Beliaev, R. Bouhenni, D. Saffarini, F. Mansfeld, B.-H. Kim, J.K. Fredrickson, K.H. Nealson, Appl. Environ. Microbiol. 73 (2007) 7003–7012.
24. B.E. Logan, P. Aelterman, B. Hamelers, R. Rozendal, U. Schröder, J. Keller, S. Freguiac, W. Verstraete, K. Rabaey, Environ. Sci. Technol. 40 (2006) 5181–5192.
25. S. Cheng, H. Liu, B.E. Logan, Environ. Sci. Technol. 40 (2006) 2426–2432. 26. Z. He, N. Wagner, S.D. Minteer, L.T. Angenent, Environ. Sci. Technol. 40
(2006) 5212–5217.
43
Chapter 4
Influence of Chemical and Physical Properties of Activated Carbon Powders on Oxygen Reduction Catalysis and Performance in Microbial Fuel Cells
Abstract
Commercially available activated carbon (AC) powders made from different precursor
materials (coal, peat, coconut shell, hardwood, and phenolic resin) were evaluated as oxygen
reduction catalysts, and tested as cathode catalysts in microbial fuel cells (MFCs). Carbons were
characterized in terms of surface chemistry, specific surface area and pore volume distribution,
and kinetic activities were compared to carbon black and platinum catalysts using a rotating disk
electrode (RDE). Cathodes using the coal–derived AC had the highest maximum power densities
in MFCs (1620 ± 10 mW m–2) even though this AC had only average catalytic activity and
selectivity (Eonset = 0.09 V, n = 2.4), and the lowest specific surface area (550 m2 g–1) among these
materials. Peat-based AC performed similarly in MFC tests (1610 ± 100 mW m–2) but had the
best catalyst performance (Eonset = 0.17 V, n = 3.6) in RDE tests and a lower than average specific
surface area (810 m2 g–1). Hardwood based AC had the highest number of acidic surface
functional groups and a higher specific surface area (1010 m2 g–1), but it had the poorest
performance in MFCs and catalysis tests (630 ± 10 mW m–2, Eonset = –0.01V, n = 2.1). There was
a strong inverse relationship between onset potential and the quantity of strong acid (pKa < 8)
functional groups, and a larger fraction of microporosity was negatively correlated with power
production in MFCs. These results show that surface area alone is a poor predictor of catalyst
performance, and that a high quantity of acidic surface functional groups can be detrimental to
oxygen reduction and cathode performance.
44
4.1 Introduction
Microbial fuel cells (MFCs) are a promising technology for treatment of wastewater
streams in combination with electricity production [1]. MFCs can reduce energy consumption for
wastewater treatment through the elimination of the need for wastewater aeration, and allow for
the utilization of an untapped renewable energy source in the wastewater organic matter. MFCs
consist of a microbe–enriched anode where organic matter is oxidized, and a circuit through
which electrons are conducted to (typically) an air-fed cathode, consisting of a porous carbon
structure and an oxygen reduction catalyst where oxygen is reduced [2]. Power production from
MFCs is often limited by the overpotential of the oxygen reduction reaction (ORR) at the
cathode, and the ORR is negatively impacted by the conditions of neutral pH and ambient
temperature inherent to MFCs. Depending on the catalyst selected, the ORR proceeds through
either a 4e– pathway producing water or hydroxide [3], or a 2e– pathway producing hydrogen
peroxides as an intermediate [4].
To limit the large cathode overpotentials, platinum is often used as a catalyst for oxygen
reduction, but it is an expensive material and a limited resource. Cathode materials account for
47-75% of MFC capital costs [5], and therefore it is important to choose less expensive materials
as the cathode catalyst. Several catalysts have been considered for use in MFCs, including other
metal compounds such as cobalt and iron tetramehoxyphenylporphyrin (TMPP) or
phthalocyanine (Pc) [6, 7] and manganese oxides [8, 9]. Recently, promising results have been
obtained using activated carbon (AC) powder based air-cathodes [10-13]. ACs are especially
interesting as they can be made from many different renewable waste materials such as coconut
shells, wood chips, and sawdust, making them an inexpensive and renewable resource.
AC powder based cathodes have produced power densities by MFCs similar to or slightly
higher than those made with a platinum catalyst. An MFC with an AC cathode made using a
45
proprietary process, which contained a polytetrafluoroethylene (PTFE) binder and a nickel
current collector, produced 1220 mW m–2, compared to 1060 mW m–2 using a cathode with a Pt
catalyst and a Nafion binder [10]. An MFC incorporating a similar AC cathode structure that had
a polydimethylsiloxane (PDMS) coated cloth diffusion layer reached 1255-1310 mW m–2
compared to 1295 mW m–2 with a standard Pt/C cathode [11]. Using a rolling process to produce
a cathode that consisted of an AC/PTFE layer supported by a stainless steel mesh current
collector produced 1086 mW m–2 in an MFC with a high surface area (1701 m2 g–1) AC, and 1355
mW m–2 with a lower surface area (576 m2 g–1) AC powder. Power production using a standard
Pt/C cathode was not reported [13]. The higher power production by the lower surface area AC
cathode was attributed to a more uniform distribution of microporosity. However, only two ACs
were compared and there was no analysis of AC surface chemistry, which could have affected the
ORR.
The catalytic activity of ACs and other materials can be evaluated independently of mass
transfer limitations using a rotating disk electrode (RDE) and Koutecky-Levich modeling.
Because the method requires only a small sample of material, and it is a relatively quick
comparative analysis that allows the researcher to select promising samples for further testing,
RDE is a common tool used to evaluate catalysts before testing them as fuel cell cathode catalysts
[14]. The evaluation of the material is based on kinetic rates and reaction pathways under non-
mass transfer limited conditions. RDE analyses have been used to study the ORR catalysis of
many materials, including AC powders [13] and other materials such as carbon supported
magnesium oxide nanoparticles [8, 9], and FeTMPP and FePc [7], where the RDE results were
well correlated with MFC performance.
The ORR mechanism at the AC catalyst is not well understood, especially in the neutral
pH and phosphate or carbonate buffered solutions used in many MFC studies. Both physical and
chemical characteristics of the AC catalyst are important to its performance. The AC surface can
46
contain many different chemical functional groups, and specific surface areas and pore size
distributions vary between different materials. The most common heteroatom found in AC
functional groups is oxygen, which is present in chemical groups that can take on acidic and basic
characteristics. Acid oxygen groups are often analyzed by potentiometric titration (PT) [4, 15-17].
Basic functional groups cannot be determined by PT, and so XPS is often used to detect groups
with a pKa >10 [16].Using these methods can provide insight into the nature and quantity of the
functional groups found on the surface of the ACs and possibly their role in catalysis.
In order to better understand the factors that affect the performance of AC cathodes in
MFCs, nine different ACs made from four different precursor materials were examined as
catalysts for oxygen reduction in terms of kinetics and selectivity (based on number of electrons
transferred) using RDEs, in neutrally buffered solutions. The catalytic rates obtained under these
conditions relevant to MFC operation were then compared in terms of chemical and physical
properties that included relative abundance of acidic oxygen functional groups, specific surface
area, and pore volume distribution. The results of these kinetic and material property analyses
were compared to the power production obtained using different ACs in the cathodes of MFCs.
4.2 Materials and Methods
4.2.1 Catalyst Materials
Nine different samples were chosen to represent a range of physical and chemical
characteristics found in commercially available AC powders. The ACs used were: peat based
carbons, Norit SX1 (P1), SXPlus (P2), and SXUltra (P3) (Norit, USA); coconut shell based
carbons, YP50 (C1) (Kuraray Chemical, Japan), USP8325C (C2) (Carbon Resources, USA),
ACP1250C (C3) (Charcoal House, USA); a hardwood carbon, Nuchar SA-1500 (W1)
47
(MeadWestvaco, USA); a phenolic resin based carbon, RP20 (R1) (Kurraray Chemical, Japan);
and a bituminous coal carbon, CR325B (B1) (Carbon Resources, USA). The performance of
these carbons in terms of ORR were compared with those of carbon black XC-72 (CB), and
Pt(10%) in carbon black XC-72 (PtC) (Fuel Cell Store, USA).
4.2.2 Physical and Chemical Analyses
Detailed incremental surface area and pore volume distributions were determined for
each AC from argon adsorption isotherms (at 87.3K) determined from progressively increasing
relative pressures of 10–6 to 0.993 atm atm–1 (ASAP2003, Micrometrics Instrument Corp., GA) as
described previously [18]. Pore size distributions were calculated from the isotherms using
Density Functional Theory (DFT) modeling software (Micrometrics Instrument Corp., GA) [18].
Three pore size classifications were used based on the International Union of Pure and Applied
Chemistry (IUPAC) definitions, of micropores <2 nm, mesopores between 2–50 nm, and
macropores >50 nm [19].
The elements present on the surface of the AC powder samples were identified by X-ray
photoelectron spectroscopy (XPS) (Axis Ultra XPS, Kratos Analytical, UK, monochrome AlKα
source, 1486.6eV). CASA XPS software was used for the elemental and peak fitting analysis of
O1s (531–536 eV) and C1s (285–289 eV) signals [16].
Potentiometric titrations were performed using a DL53 automatic titrator (Mettler
Toledo, USA) in the pH range of 3–11, with NaOH (0.1 M) used as the titrant and NaCl (0.01 M)
as the electrolyte. Before titration, samples were adjusted to a ~pH 3 using HCl (0.1 M). Proton
binding isotherms were measured and deconvoluted using the SAIEUS numerical procedure to
obtain the distribution of acidity constants [15, 20, 21]. This analysis produces separate peaks that
denote a difference in type of functional group, with the area under the peak corresponding to the
48
quantity of functional groups detected in mmol g–1 based on binding/release of protons during
titration.
4.2.3 RDE Analysis
Catalyst ink was prepared by adding 30 mg of the powdered sample (except Pt/C which
was 6 mg to represent the same loading comparison used in the cathodes) to 3 ml of DMF and
homogenized with a sonifier (S-450A, Branson, country) fitted with a 1/8 inch micro tip, pulsed
at 50% for 15 minutes, in an ice bath. Nafion (5 wt% solution, 270 µl) was added and the solution
was mixed for an additional 15 minutes. The ink solution (10 µl) was drop coated onto a 5 mm
diameter glassy carbon disk (Pine Instruments, USA) and allowed to dry overnight. The disk was
prepared before coating by polishing with 5.0 and 0.05 µm alumina paste and cleaned in an
ultrasonic bath for 30 minutes.
All RDE experiments were run first in nitrogen sparged solution, before switching to an
air sparged 100 mM phosphate buffer solution. Solutions were sparged for 30 min before LSVs
were run and then the gas was streamed into the headspace for the duration of the experiment). In
order to clean the electrode surface of possible contaminants or excess oxygen trapped in the
pores of the carbon, the disk potential was cycled between 0.4 and –1.0 V at 100 mV s–1 until the
current response was the same from cycle to cycle. Then, the potential of the disk electrode was
scanned from 0.4 to 1.0 V at 10 mV s–1 and rotation rates of 100 to 2100 rpm. The current
obtained under nitrogen sparging was subtracted from that obtained under air sparging to obtain
the faradaic current attributed to oxygen reduction [14]. Catalyst activity was evaluated by the
onset potential (Eonset) and limiting current (ilim). Kinetic current (ik) and average number of
49
electrons transferred (n) in the ORR were obtained from the Koutecky-Levich (K-L) analysis
using the following equation.
1𝑖
= 1𝑖𝑘
+ 1𝑖𝑑
= 1𝑛𝐹𝐴𝑘𝐶𝑂2
− 1
0.62𝑛𝐹𝐴𝐷𝑂22 3⁄ 𝜐−1 6⁄ 𝐶𝑂2𝜔
1 2⁄ (4-1)
where i is the measured current, ik the kinetic current, id the diffusion-limiting current, F
Faraday’s constant, A the projected surface area of the disk electrode, k the rate constant, CO2 the
concentration of oxygen in solution, DO2 the diffusion coefficient of oxygen, υ the kinematic
viscosity, and ω the rotation rate of the electrode [9].
4.2.4 MFC Experiments
AC cathodes (31 mg cm–2 loading, projected surface area of 7 cm2) were constructed as
previously described [11], except that two PDMS diffusion layers were applied to the air side of
the stainless steel mesh current collector (50×50 mesh, type 304, McMaster-Carr, OH) prior to
application of the carbons [22]. AC powder was mixed with 10 wt% PTFE binder (in a 60%
emulsion) and spread evenly onto the solution side and pressed at 4.54 metric ton-force for 20
min (Carver press, Model 4386, Carver Inc., IN) [11]. Cathodes were made with carbon black
(XC-72) following the same technique. Platinum-catalyzed air cathodes (projected surface area of
7 cm2) were constructed from carbon cloth (30 wt% wet proofing, Fuel Cell Earth LLC) with four
PTFE diffusion layers, and a catalyst loading of 0.5 mg-Pt cm–2 (on carbon black XC-72) [23].
Cube-shaped MFCs were constructed as previously described [24]. The anode chamber
was a 28 mL cylindrical chamber (7 cm2 cross section) bored into a Lexan block. The anodes
were carbon fiber brushes with a titanium wire core (2.5 cm diameter, 2.5 cm length, and 0.22 m2
surface area) which was heat treated at 450 °C [25] and then placed horizontally in the center of
the cylinder. The electrode spacing was 2.5 cm (center of the anode to the face of the cathode).
The MFCs were inoculated using effluent from an MFC operated under conditions similar to
those used here. The medium in MFC tests was a 100 mM phosphate buffer solution (PBS) (9.13
50
g L–1 Na2HPO4, 4.90 g L–1 NaH2PO4·H2O, 0.31 g L–1 NH4Cl, and 0.13 g L–1 KCl; pH 7) amended
with vitamins and minerals [26] and 1 g L–1 sodium acetate. Anodes were inoculated and
acclimated under the same conditions in MFCs containing standard Pt/C carbon cloth cathodes,
and then tested with the different cathodes. A 1000 Ω resistor was used during acclimation, and
then the resistance was changed to lower resistance (100 Ω) for several cycles before running
polarization tests to avoid power overshoot [27]. All MFCs were operated at 30 °C in a constant
temperature controlled room. Once the MFC produced a steady voltage for 3 cycles, the Pt/C
cathodes were removed and replaced with the AC cathodes, CB cathodes, or new Pt/C cathodes.
All MFC tests were conducted in duplicate.
The voltage across the resistor was recorded every 30 min using a multimeter (model
2700 Keithley Instruments, Cleveland, OH) with a computerized data acquisition system.
Polarization curves were obtained by applying a different external resistance to the circuit for a
complete batch cycle (multiple cycle method), and the average sustainable voltage was recorded
for each resistance. Current density was calculated from I = E/R, where I is the current, E the
measured voltage, and R the external resistance, and normalized to the projected cathode surface
area. Power densities were calculated using P = IE, and normalized by the projected cathode
surface area [28].
4.3 Results and Discussion
4.3.1 MFC Performance
Based on polarization results, MFCs using cathodes made from different AC powders
had quite different maximum power densities (Figure 4-1A). These ranged from 1620 ± 10 mW
m–2 (0.48 ± 0.00 mA cm–2) using a bituminous coal (B1) to 630 ± 10 mWm–2 (0.36 ± 0.00 mA
cm–2) for the hardwood (W1). One of the peat-based cathodes (P2) had a high power density of
51
1610 ± 100 mW m–2 that was similar to that obtained with cathodes using AC derived from
bituminous coal (B1). The MFCs using a standard Pt/C cathode all produce a maximum power
density of 2110 ± 0 mW m–2 (0.55 ± 0.00 mA cm–2). The lower maximum power densities
produced by the AC cathodes was due to decreased cathode potentials (increased cathode
overpotential), as the anodes in all of the MFCs maintained similar working potentials (Figure 4-
1B). The best performing MFCs had cathodes with the lowest overpotentials, as seen with the
bituminous sample (B1) operating potential at –0.07 ± 0.00 V (vs. Ag/AgCl) at peak power, while
the hardwood AC (W1) which had the lowest power operated at –0.23 ± 0.01 V (a 230% decrease
in potential). The standard Pt/C cathode potential was –0.01 ± 0.00 V.
Figure 4-1. A) Power density production and B) electrode potentials from polarization of MFCs using AC cathodes compared to Pt/C (100 mM Phosphate Buffer; Open symbols represent cathode potentials, closed symbols are anode potentials).
0
400
800
1200
1600
2000
Pow
er D
ensit
y [m
W/m
2 ]
A
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Elec
trod
e Po
tent
ial [
v]
Current Density [mA/cm2]
Pt/C B1P2 P3P1 C2CB C3C1 R1W1
B
52
4.3.2 Catalyst Activity and Selectivity
Catalyst performance evaluated using LSV and RDE produced trends in performance
with the different AC precursor materials that generally were similar to those obtained with the
MFCs (Figure 4-2A, Figure A-1). However, the peat-based AC (P2) had the greatest oxygen
reduction activity (Eonset = 0.17 V and ilim = 0.87 mA cm–2 at 2100 rpm), and the bituminous coal
(B1) sample had only average performance (Eonset = 0.09 V and ilim = 0.78 mA cm–2), compared to
MFC tests where the bituminous coal sample produced a higher peak power density than the peat
(P2) sample. It is possible that the differences in MFC and RDE performance were partially due
to the differences in the catalyst layer formation (pressing with PTFE binder versus drop–coating
with Nafion binder); however the point of using the RDE was to test the catalyst performance
without diffusion limitations found in MFC cathode testing. Also, the amount of binder used in
both cases is small, but the large amount of AC used in the MFC cathodes, compared to the thin
layer used in RDE tests, can increase diffusion and electrical conductivity limitations. The
hardwood (W1) AC activity in terms of oxygen reduction again had the worst performance (Eonset
= –0.01 V and ilim = 0.73 mA cm–2), but all AC materials had superior performance to carbon
black (Eonset = –0.06 V and ilim = 0.66 mA cm–2) or the plain glassy carbon disk (Eonset = –0.40 V
and ilim = 0.60 mA cm–2). The AC materials also had less catalytic activity than the Pt/C catalyst
(Eonset = 0.36 V and ilim = 1.11 mA cm–2).
53
Figure 4-2. A) LSV current response (per disk area) of the AC catalyst at the disk electrode compared to Pt/C and carbon black (100 mM Phosphate Buffer, 2100rpm) B) Average number of electrons (n) transferred (estimated by Koutecky-Levich RDE analysis) during oxygen reduction.
The performance of the catalysts was examined using the K-L analysis to focus on kinetic
current (ik) and the average number of electrons transferred (n) in the ORR without the effects of
diffusion limitation (Figure A-2). ACs that had larger ilim and more positive Eonset also had higher
kinetic current production, where the peat-based (P2) current (ik = 1.1 mA cm–2 at –0.2 V) was
greater than the bituminous coal (B1) (ik = 0.5 mA cm–2) and the hardwood (W1) (ik = 0.4 mA
cm–2) AC catalysts. The selectivity of the catalyst, estimated by the number of electrons
transferred in the ORR, of the peat-based (P2) activated carbon catalyst at –0.2 V (Figure 2B) was
near four electrons with n = 3.6, indicating a mixed reaction that tended toward H2O/OH–
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4
Curr
ent D
ensit
y [m
A/cm
2 ]
Disk Potential [V vs. Ag/AgCl]
PtCP2P3P1C1C2C3B1R1W1CBGC
A
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1
n
Disk Potential [V vs. Ag/AgCl]
PtC P2P1 P3C3 C1C2 B1R1 W1CB GC
B
54
formation either through a direct 4e– reduction or through a series of reductions where the
peroxide product was further reduced at the catalyst. The bituminous (B1) (n = 2.4) and
hardwood (W1) (n = 2.1) based samples were closer to a 2e– reduction, where the reduction
product was mostly through peroxide formation without further catalytic reduction.[4].
4.3.3 Effect of Oxygen Functional Groups on ORR Catalysis
The prevalence and variety of acidic functional groups on the surface of the AC samples,
determined by potentiometric titration analysis, influenced their catalytic activity for reduction of
oxygen. ACs made from the same or similar precursor materials had acidic functional groups
with similar pKa values (Figure 4-3, Figure A-3). The two best performing AC samples peat (P2)
and bituminous coal based (B1) had very similar acidic functional groups present, with pKa
values around 4.5, 6.5, 8.5 and 10 (Figure 4-3A). The quantity (mmol g–1) of acidic functional
groups present on the surface of the AC samples, estimated by the area under the curve at each
pKa , varied for the different carbons There was a greater quantity of strong acid groups (pKa < 8,
typically attributed to carboxyl goups) for the bituminous coal sample (B1, 0.17 mmol g–1) than
the three peat based samples (P1, 0.06 mmol g–1; P2, 0.05 mmol g–1; P3, 0.06 mmol g–1). The ACs
made from the other precursor materials showed a larger variety of acidic functional groups
(Figure 4-3B), with the hardwood sample (W1, 0.36 mmol g–1) having the largest quantity of
strong acid functional groups. This suggests that the AC surface chemistry, based on the quantity
of strong acid groups, had a strong influence on the activity of the AC catalyst for ORR (Figure
4-4). An increase in strong acid functional groups led to a decrease in the onset potential of
oxygen reduction (p = 0.00003). However, the presence of strong acid functional groups did not
correlate as well with the power densities produced when these carbons were used in the cathodes
of the MFCs (Figure A-4).
55
Figure 4-3. Acidic/Oxygen functional groups determined by potentiometric titration. A) Bituminous and peat based activated carbon samples have similar functional groups. B) Other activated carbons have a larger variety of acidic groups.
Figure 4-4. The onset potential of the oxygen reduction reaction is inversely related to the amount of strong acid functional groups present on the activated carbons tested.
0
0.1
0.2
0.3
0.4
0.5
F(pK
) [m
mol
/g]
P1P2P3B1
A
0
0.1
0.2
0.3
0.4
0.5
3 4 5 6 7 8 9 10 11
F(pK
) [m
mol
/g]
pKa
C1C2C3R1W1
B
P2P3P1
B1
W1
C2C1
C3R1
y = -1.7x + 0.3
0
0.1
0.2
0.3
0.4
-0.05 0 0.05 0.1 0.15 0.2
Stro
ng A
cid
Grou
ps (p
K a<8
) [m
mol
/g]
Eonset [V vs. Ag/AgCl]
P-value=3x10-5
56
4.3.4 Effect of Microporosity on Power Production
The ACs had cumulative surface areas ranging from 550 m2 g–1 (B1) to 1440 m2 g–1 (R1),
and cumulative pore volumes between 0.3 mL g–1 (B1) and 1.1 mL g–1 (W1) (Figure 4-5). A
majority of the surface area was attributed to micropores (Figure 4-5A). With the exception of the
hardwood AC (W1), there was a strong inverse relationship between the surface area of the AC
powder and the maximum power density achieved in the MFCs. The bituminous sample (B1),
which had the least surface area, produced the highest power density in MFC tests, and the
phenolic resin sample (R1), which had the most surface area, produced one of the lowest power
densities. A similar trend was observed with micropore volume (Figure 5B), where the power
density increased inversely with the micropore volume of the carbon. Most likely the micropores
hinder diffusion of the reactants to the catalytic functional sites on the activated carbon, as well as
the diffusion of the reduction product from the pores, thereby negatively impacting the
favorability of the reaction. The negative effect of increased microporosity of the AC powders on
power production explains the higher power production of the MFC using the bituminous based
cathode (B1) despite the average catalytic performance of the bituminous based AC in the RDE
analysis. In the case of the hardwood (W1) based cathode, the lower microporosity did not
compensate for the poor intrinsic catalytic performance of the hardwood based AC powder based
on RDE tests.
57
Figure 4-5. Maximum power density (per m2 projected cathode surface) of the MFCs using the activated carbon cathodes is inversely related to the A) surface area (without W1 pvalue=0.0006) and B) micropore volume (without W1 pvalue=0.0011) of the powdered carbons with the exception of sample W1.
4.3.5 Functional Group Analysis Using XPS
The quantity of acidic functional groups measured by potentiometric titration is
commonly attributed to oxygen containing groups [15]. Although the onset potential was
inversely correlated to the quantity of strong acid/oxygen containing functional groups detected,
neither the onset potential nor the power density obtained were correlated with the total atomic
percent of oxygen present on the AC surface as determined by XPS (Figure 4-6A). For instance,
the bituminous based AC (B1) had the most oxygen atoms present (9.4%), but only a moderate
0
300
600
900
1200
1500
1800
0
300
600
900
1200
1500
1800
R1 C1 C2 C3 W1 P3 P1 P2 B1
Pow
er D
ensit
y [m
W/m
2 ]
Cum
ulat
ive
Surf
ace
Area
[m2 /
g]
meso micro PowerA
0
300
600
900
1200
1500
1800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
R1 C1 C3 C2 W1 P3 P1 P2 B1
Pow
er D
ensit
y [m
W/m
2 ]
Cum
ulat
ive
Pore
Vol
ume
[ml/g
]
B
58
abundance of strong acid functional groups, and average ORR catalytic activity. The chemical
state of the oxygen present varied between the samples (Figure 4-6B), where the bituminous
based (B1) sample had a larger amount of oxygen present in the adsorbed O2/H2O region (BE =
536 – 536.6 eV) [16]. This suggests that not all oxygen functional groups (e.g. quinones) are
detrimental to oxygen reduction catalysis, in agreement with AC ORR catalysis studies using
acidic or alkaline media [29]. Trace amounts of nitrogen were found in the Peat and Bituminous
samples, with no detectable nitrogen in the remaining samples.
Figure 4-6. A) Maximum power density (normalized to cathode surface area) of MFCs with activated carbon cathodes are not directly related to oxygen content of activated carbon powders determined by XPS. B) Chemical state of oxygen detected by XPS varies for activated carbon powders tested (e.g. increased signal in the adsorbed O2/H2O region for sample B1.)
0
500
1000
1500
2000
2500
3000
3500
4000
525530535540545
CPS
Binding Energy [eV]
C3R1C1B1C2W1P1P2P3
B
0
300
600
900
1200
1500
1800
P1 P2 P3 C2 C1 R1 W1 C3 B10
2
4
6
8
10
Pow
er D
ensit
y [m
W/m
2 ]
Atom
ic %
O2
A
59
4.3.6 Implications of AC properties for MFC performance
Both the surface chemistry and the pore structure of the AC catalyst affected performance
of the catalyst in the cathode of an MFC. AC catalysts selected for neutral buffered environments,
like those in MFCs, should have less acidic surface functional groups, which can hinder the ORR
activity. The effect of basic (rather than acidic) functional groups on ORR catalysis should be
further investigated. ACs used in MFC cathodes should therefore not be chosen solely because
they have the largest surface area, but instead ACs should be selected that have a moderate
amount of micropore volume and surface area to avoid the negative impact of diffusion
limitations to the active catalyst sites. Binders, AC loading, and manufacturing methods can also
affect the diffusion characteristics of the cathodes and their performance relative to standard Pt/C
cathodes [11, 13], and therefore this aspect of cathode construction should also be taken into
consideration when constructing MFC cathodes. Longevity of AC catalysts in MFC cathodes is
also an issue for MFC applications [30], and therefore changes in surface chemistry and rates of
mass transfer to catalytic sites should also be considered in future studies.
4.4 Acknowledgements
The authors thank Dr. Cesar Nieto Delgado for assistance with the potentiometric
titration and Vince Bojan for assistance with XPS analysis. The authors acknowledge support
from the King Abdullah University of Science and Technology (KAUST) by Award KUS-I1-
003-13 and the National Science Foundation Graduate Research Fellowship Program (NSF-
GRFP).
60
4.5 References
1. Logan, B. and K. Rabaey, Conversion of Wastes into Bioelectricity and Chemicals by Using Microbial Electrochemical Technologies. Science, 2012. 337: p. 686-690.
2. Logan, B.E., Microbial Fuel Cells. 2008, New York: John Wiley & Sons. 3. Popat, S., et al., Importance of OH- Transport from Cathodes in Microbial Fuel Cells.
ChemSusChem, 2012. 5: p. 1071-1079. 4. Zhong, R.-S., et al., Effect of carbon nanofiber surface functional groups on oxygen
reduction in alkaline solution. Journal of Power Sources, 2013. 225: p. 192-199. 5. Rozendal, R.A., et al., Towards practical implementation of bioelectrochemical
wastewater treatment. Trends in Biotechnology, 2008. 26(8): p. 450-459. 6. Yu, E.H., et al., Microbial Fuel Cell Performance with non-Pt Cathode Catalysts. Journal
of Power Sources, 2007. 171: p. 275-281. 7. Birry, L., et al., Application of iron-based cathode catalysts in a microbial fuel cell.
Electrochimica Acta, 2011. 56: p. 1505-1511. 8. Roche, I. and K. Scott, Carbon-supported manganese oxide nanoparticles as
electrocatalysts for oxygen reduction reaction (orr) in neutral solution. Journal of Applied Electrochemistry, 2009. 39: p. 197-204.
9. Chen, Y., et al., Stainless steel mesh coated with MnO2/carbon nanotube and polymethlyphenyl siloxane as low-cost and high-performance microbial fuel cell cathode materials. Journal of Power Sources, 2012. 201: p. 136-141.
10. Zhang, F., et al., Power generation using an activated carbon and metal mesh cathode in a microbial fuel cell. Electrochemistry Communications, 2009. 11(11): p. 2177-2179.
11. Wei, B., et al., Development and evaluation of carbon and binder loading in low-cost activated carbon cathodes for air-cathode microbial fuel cells. RSC Advances, 2012. 2: p. 12751-12758.
12. Dong, H., et al., A novel structure of scalable air-cathode without Nafion and Pt by rolling activated carbon and PTFE as catalyst layer in microbial fuel cells. Water Res, 2012. 46: p. 5777-5787.
13. Dong, H., H. Yu, and X. Wang, Catalysis kinetics and porous analysis of rolling activated carbon-PTFE air-cathode in microbial fuel cells. Environ Sci Technol, 2012. 46: p. 13009-13015.
14. Gojkovic, S.L., S. Gupta, and R.F. Savinell, Heat-treated iron(III) tetramethoxyphenyl porphyrin supported on high-area carbon as an electrocatalyst for oxygen reduction - I. Characterization of the electrocatalyst. Journal of the Electrochemical Society, 1998. 145: p. 3493-3499.
15. Bandosz, T.J., J. Jagiello, and C. Contescu, Characterization of the surfaces of activated carbons in terms of their acidity constant distributions. Carbon, 1993. 31(7): p. 1193-1202.
16. Seredych, M. and T.J. Bandosz, Investigation of the enhancing effects of sulfur and/or oxygen functional groups of nanoporous carbons on adsorption of dibenzothiophenes. Carbon, 2011. 49: p. 1216-1224.
17. Szymanski, G.S., et al., The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon. Carbon, 2002. 40: p. 2627-2639.
61
18. Moore, B.C., et al., Changes in GAC pore structure during full-scale water treatment at Cincinnati: a comparison between virgin and thermally reactivated GAC. Carbon, 2001. 39: p. 789-807.
19. Rouquerol, J., et al., Recommendations for the characterization of porous solids. Pure and Applied Chemistry, 1994. 66(8): p. 1739-1758.
20. Jagiello, J., Stable numerical solution of the adsorption integral equation using splines. Langmuir, 1994. 10(8): p. 2778-2785.
21. Bandosz, T.J., et al., Efffect of surface chemisty on sorption of water and methanol on activated carbons. Langmuir, 1996. 12: p. 6480-6486.
22. Zhang, X., et al., Improved performance of single-chamber microbial fuel cells through control of membrane deformation. Biosensors and Bioelectronics, 2010. 25: p. 1825-1828.
23. Cheng, S., H. Liu, and B.E. Logan, Increased performance of single-chamber microbial fuel cells using an improved cathode structure. Electrochem Commun, 2006. 8: p. 489-494.
24. Liu, H., R. Ramnarayanan, and B.E. Logan, Production of electricity during wastewater treatment using a single chamber microbial fuel cell. Environmental Science & Technology, 2004. 38(7): p. 2281-2285.
25. Feng, Y., et al., Treatment of carbon fiber brush anodes for improving power generation in air–cathode microbial fuel cells. Journal of Power Sources, 2010. 195: p. 1841 - 1844.
26. Bretschger, O., et al., Current production and metal oxide reduction by Shewanella oneidensis MR-1 wild type and mutants. Appl Environ Microbiol, 2007. 73(21): p. 7003-7012.
27. Hong, Y., et al., Adaptation to high current using low external resistances eliminates power overshoot in microbial fuel cells. Biosens Bioelectron, 2011. 28(1): p. 71-76.
28. Logan, B.E., et al., Microbial fuel cells: methodology and technology. Environ Sci Technol, 2006. 40(17): p. 5181-5192.
29. Zhang, J., ed. PEM Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications. Vol. XXII. 2008. 489.
30. Zhang, F., D. Pant, and B.E. Logan, Long-term performance of activated carbon air cathodes with different diffusion layer porosities in microbial fuel cells. Biosens Bioelectron, 2011. 30(1): p. 49-55.
62
Chapter 5
Improvement of Oxygen Reduction Catalysis in Neutral Solutions using Ammonia Treated Activated Carbons and Performance in Microbial Fuel
Cells
Abstract
Commercially available activated carbon (AC) powders made from different precursor
materials (peat, coconut shell, coal, and hardwood) were treated with ammonia gas at 700 °C and
evaluated as oxygen reduction catalysts in neutral pH phosphate buffer for application in
microbial fuel cell (MFC) cathodes. Ammonia treatment resulted in a decrease in oxygen (by 29
– 58%) and an increase in nitrogen content (total abundance up to 1.8 atomic %) on the carbon
surfaces, which also resulted in an increase in the basicity of the bituminous, peat, and hardwood
ACs. The kinetic activity and selectivity of ammonia treated carbons were evaluated using a
rotating ring-disk electrode (RRDE) and compared to untreated ACs and platinum. All of the
ammonia treated ACs exhibited better catalytic performance than their untreated precursors, with
the bituminous (treated, Eonset = 0.12 V, n = 3.9; untreated, Eonset = 0.08 V, n = 3.6) and hardwood
(treated, Eonset = 0.03 V, n = 3.3; untreated, Eonset = –0.04 V, n = 3.0) based samples showing the
most improvement. Cathodes using the ammonia treated coal based AC had the highest maximum
power densities in MFCs (2450 ± 40 mW m–2). Even though the ammonia treated peat based AC
had the greatest ORR activity in RRDE testing, the untreated sample had higher power
production in the MFC tests (2360 ± 230 mW m–2). The treated coconut and hardwood derived
ACs outperformed the untreated precursor ACs in both electrochemical and MFC testing. These
results show that reduction in oxygen abundance and increase in nitrogen functionalities on the
surface of ACs can increase the catalytic performance for oxygen reduction in neutral media.
63
5.1 Introduction
Microbial fuel cells (MFCs) are a promising option for reduction of energy costs
associated with the treatment of wastewater sources [1]. Power production from MFCs is limited
by the overpotential of the oxygen reduction reaction (ORR) at the cathode, which is negatively
impacted by the conditions of neutral pH and ambient temperature common in MFCs. Depending
on the catalyst properties, the ORR can proceed through either a 4e– pathway producing water or
hydroxide [2], or 2e– pathway producing hydrogen peroxide as an intermediate [3]. Peroxides can
be further reduced through an additional 2e– reduction step, resulting in a mixed reduction
pathway that can approach an apparent four electron transfer to the cathode [3].
In order to achieve commercial viability, low cost materials are essential to the success of
MFC technology. Activated carbons (ACs) are inexpensive ORR catalysts that can be made from
several biomass waste streams such as coconut shells, wood chips and sawdust. They have a
complex surface chemistry that can be tailored to improve their performance for the desired
application. AC powder based cathodes have produced power densities in MFCs similar to or
slightly higher than those made with a typical platinum catalyst. An MFC with an AC cathode,
that consisted of a polytetrafluoroethylene (PTFE) binder and a nickel current collector, produced
1220 mW m–2, compared to 1060 mW m–2 using a cathode with a Pt catalyst and a Nafion binder
[4]. An MFC incorporating a similar AC cathode structure with a polydimethylsiloxane (PDMS)
coated cloth diffusion layer reached 1255-1310 mW m–2 compared to 1295 mW m–2 with a
standard Pt/C cathode [5]. Another type of AC air cathode made by rolling out an AC/PTFE
layer on a stainless steel mesh current collector produced between 1086 – 1355 mW m–2 using
two different types of AC powders. Power production using a standard Pt/C cathode was not
reported [6]. In the research reported in Chapter 4, a greater abundance of strong acid functional
64
groups of the surface of AC powders was found to be detrimental to the ORR catalytic activity in
MFC environments.
Several studies have shown that the number of nitrogen functional groups on carbon
surfaces can be increased by treatment with ammonia gas at elevated temperatures [3, 7-9].
During the process of incorporating the nitrogen into the carbon structure, there is a
corresponding reduction in acidic oxygen groups as the oxygen atoms are desorbed from the
carbon surface as CO/CO2. This rearrangement of surface functional groups results in an increase
in the basic properties of the carbon surface at the expense of acidic properties [9-14]. Nitrogen
incorporation on carbon surfaces has been shown to increase the catalytic activity and selectivity
for oxygen reduction through a four electron pathway in both acidic and alkaline environments,
but it has not been examined under neutral pH conditions [3, 15, 16].
In order to improve the performance of AC cathodes in MFCs, ACs made from four
different precursor materials were treated with ammonia gas (5%, balance He) at 700 °C and
examined as catalysts for oxygen reduction in neutrally buffered solution. Treated ACs were
evaluated in terms of activity and selectivity using a rotating ring-disk electrode (RRDE) and
linear sweep voltammetry (LSV). The catalytic activity observed, in conditions relevant to MFC
operation (pH 7, 30 °C), was then compared to the change in surface chemistry that included the
relative abundance of surface oxygen and nitrogen functional groups. The results of the kinetic
and chemical property analyses were compared to the power production produced using the
ammonia treated ACs in the cathodes of MFCs.
65
5.2 Materials and Methods
5.2.1 Activated Carbons and Ammonia Treatment
Four AC samples that were previously studied for ORR catalysis (Chapter 4) were
chosen to represent a range of physical and chemical characteristics found in commercially
available AC powders. The ACs used were: a peat based carbon, Norit SXPlus (P, Norit, USA); a
coconut shell based carbon, YP50 (C, Kuraray Chemical, Japan); a hardwood carbon, Nuchar
SA-1500 (W1, MeadWestvaco, USA); and a bituminous coal carbon, CR325B (B1, Carbon
Resources, USA). The base ACs were treated with ammonia gas (5% in helium) at 700 °C using a
vertical cylindrical glass tube reactor in a programmable furnace (model 3210, Applied Test
Systems, Inc., Butler, PA). Before heating, the furnace (including sample) was purged with ultra
pure nitrogen gas for 30 min. Gas flow was changed to dilute ammonia while temperature was
ramped at 5 °C min–1, then held at 700 °C for 1 hr. The furnace and sample were purged with
ultra pure nitrogen gas while cooling to ambient temperature. The performance of these carbons
(denoted as –N) in terms of ORR catalysis were compared to the base AC samples, as well as
carbon black XC–72 (CB), and Pt (10%) in carbon black XC–72 (PtC) (Fuel Cell Store, USA).
5.2.2 Chemical Surface Analysis
The elements present on the surface of the AC powder samples were identified by X-ray
photoelectron spectroscopy (XPS, Axis Ultra XPS, Kratos Analytical, UK, monochrome AlKα
source, 1486.6eV). A base survey scan was performed first, followed by a detailed scan of C1s
(285–289 eV), O1s (531–536 eV), and N1s (398–406 eV) signals [17]. CASA XPS software was
used for the elemental and peak fitting analysis.
66
Potentiometric titrations were performed using a DL53 automatic titrator (Mettler
Toledo, USA) in the pH range of 3–11, with NaOH (0.1 M) used as the titrant and NaCl (0.01 M)
as the electrolyte. Before titration, samples were adjusted to a ~pH 3 using HCl (0.1 M). Proton
binding isotherms were measured and deconvoluted using the SAIEUS numerical procedure to
obtain the distribution of acidity constants [18-20]. This analysis produces separate peaks that
denote a difference in type of functional group, with the area under the peak corresponding to the
quantity of functional groups detected in mmol g–1 based on binding/release of protons during
titration.
5.2.3 Rotating Ring-Disk Electrochemical Analysis
Catalyst ink was prepared by adding 30 mg of the powdered sample to 3 ml of DMF and
homogenized with a sonifier (S-450A, Branson, country) fitted with a 1/8 inch micro tip, pulsed
at 50% for 15 minutes, in an ice bath. Nafion (270 µL; 5 wt% solution) was added and the
solution was mixed for an additional 15 minutes. The ink solution (10 µL) was drop coated onto a
5 mm diameter glassy carbon disk (Pine Instruments, USA) and allowed to dry overnight. The
disk was prepared before coating by polishing with 5.0 and 0.05 µm alumina paste and cleaned in
an ultrasonic bath for 30 minutes.
All RRDE experiments were run first in nitrogen sparged solution in order to obtain the
baseline current, before switching to an air sparged 100 mM phosphate buffer solution. Solutions
were sparged for 30 min before LSVs were run, and then the gas was streamed into the headspace
for the duration of the experiment. In order to clean the electrode surface of possible
contaminants or excess oxygen trapped in the pores of the carbon, the disk potential was cycled
between 0.4 and 1.0 V at 100 mV s–1 until a consistent current response was observed from one
cycle to the next. All potentials are reported vs. 3M Ag/AgCl reference electrodes (0.197 V vs.
67
SHE). The potential of the disk electrode was then scanned from 0.4 to –1.0 V at 10 mV s–1 and
rotation rates of 100 to 2100 rpm, while the potential of the platinum ring was held constant at
0.62 V for H2O2 oxidation. The current obtained under nitrogen sparging was subtracted from that
obtained under air sparging to obtain the faradaic current attributed to oxygen reduction [21].
Catalyst activity was evaluated by the onset potential (Eonset) and limiting current (ilim) [21]. The
average number of electrons transferred (n) in the ORR at the disk electrode was calculated based
on the amount of H2O2 detected using [22]
𝑛 = 4𝑖𝑑𝑖𝑠𝑘𝑖𝑑𝑖𝑠𝑘+𝑖𝑟𝑖𝑛𝑔 𝑁⁄
(5–1)
where idisk is reduction current at the disk, iring the oxidation current at the ring, and N is the
collection efficiency of the RRDE.
5.2.4 MFC Experiments
AC cathodes (31 mg cm–2 loading, projected surface area of 7 cm2) were constructed as
previously described [5], except that the diffusion layer consisted of two PDMS layers that were
applied to the air side of the stainless steel mesh current collector (50×50 mesh, type 304,
McMaster-Carr, OH) prior to application of the carbons [23]. In addition, a second stainless steel
mesh current collector was pressed onto the solution side of the activated carbon to improve
electrical conductivity of the cathode. AC powder was mixed with 10 wt% PTFE binder (in a
60% emulsion) and spread evenly onto the solution side of the PDMS coated mesh, and pressed
at 4.54 metric ton-force for 20 min (Carver press, Model 4386, Carver Inc., IN) [5]. Pt-catalyzed
air cathodes (projected surface area of 7 cm2) were constructed from carbon cloth (30 wt% wet
proofing, Fuel Cell Earth LLC) with four PTFE diffusion layers, and a catalyst loading of 0.5 mg-
68
Pt cm–2 (on carbon black XC-72) [24] to benchmark AC cathodes against this commonly used
cathode.
Cube-shaped MFCs were constructed as previously described [25]. The anode chamber
was a 28 mL cylindrical chamber (7 cm2 cross section) bored into a Lexan block. The anodes
were carbon fiber brushes with a titanium wire core (2.5 cm diameter, 2.5 cm length, and 0.22 m2
surface area) which was heat treated at 450 °C [26] and then placed horizontally in the center of
the cylinder. The electrode spacing was 2.5 cm (from center of the anode to the cathode). The
MFCs were inoculated using effluent from an MFC operated under conditions similar to those
used here. The medium in MFC tests was a 100 mM phosphate buffer solution (PBS) (9.13 g L–1
Na2HPO4, 4.90 g L–1 NaH2PO4·H2O, 0.31 g L–1 NH4Cl, and 0.13 g L–1 KCl; pH 7) amended with
vitamins and minerals [27] and 1 g L–1 sodium acetate. Anodes were inoculated and acclimated
under the same conditions in MFCs containing standard Pt/C carbon cloth cathodes, and then
tested with the different cathodes. A 1000 Ω resistor was used during acclimation, and then the
resistance was changed to lower resistance (100 Ω) for several cycles before running polarization
tests to avoid power overshoot [28]. All MFCs were operated at 30 °C in a constant temperature
controlled room. Once the MFC produced a steady voltage for 3 cycles, the Pt/C cathodes were
removed and replaced with the AC cathodes, CB cathodes, or new Pt/C cathodes. All MFC tests
were conducted in duplicate.
The voltage across the resistor was recorded every 30 min using a multimeter (model
2700 Keithley Instruments, Cleveland, OH) with a computerized data acquisition system.
Polarization curves were obtained by applying a different external resistance to the circuit for a
complete batch cycle (multiple cycle method), and the average sustainable voltage was recorded
for each resistance. Current density was calculated from I = E/R, where I is the current, E the
measured voltage, and R the external resistance, and normalized to the projected cathode surface
69
area. Power densities were calculated using P = IE, and normalized by the projected cathode
surface area [29].
5.3 Results and Discussion
5.3.1 MFC performance
In general, cathodes made from ammonia treated AC powders had higher maximum
power densities in MFCs than the cathodes made with the corresponding untreated AC (Figure 5-
1A). The highest power was obtained using the treated bituminous coal AC (B–N) cathode (2450
± 43 mW m–2; 0.83 ± 0.01 mA cm–2), which was a 28% increase in power compared to the MFC
with the untreated bituminous AC (B) cathode (1910 ± 188 mW m–2; 0.73 ± 0.04 mA cm–2), and a
16% improvement in maximum power compared to a standard Pt/C cathode (2100 ± 1 mW m–2;
0.55 ± 0.01 mA cm–2). Treating the hardwood AC powder (W–N) produced the largest
improvement, with 53% higher power production than the untreated hardwood (W). Ammonia
treatment of the coconut AC (C–N) resulted in a 29% increase in maximum power. The peat-
based cathodes were the exception to improved power with treatment, as the untreated peat AC
(P) cathode had a higher power density of 2360 ± 230 mW m–2 (0.81 ± 0.04 mA cm–2) than the
treated AC (P–N) cathode (1860 ± 84 mW m–2; 0.72 ± 0.01 mA cm–2).
70
Figure 5-1. A) Power density production and B) electrode potential during cell polarization of MFCs using AC cathodes compared to Pt/C. (100 mM Phosphate Buffer; open symbols indicate cathode potentials, closed symbols anode potentials.)
Power and current densities of the untreated base ACs obtained here were higher than
those in the previous study (Chapter 4) due to the addition of a second stainless steel mesh current
collector on the solution side of the cathode (Figure B-1A). These increases in maximum power
densities resulted from increased operating potentials of the cathodes (decreased cathode
overpotential), since the anodes in all of the MFCs operated at similar potentials (Figure 5-1B,
0
500
1000
1500
2000
2500
Pow
er D
ensit
y [m
W m
-2]
A
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Elec
trod
e Po
tent
ial [
V]
Current Density [mA cm-2]
P-N PB-N BW-N WC-N CPtC
B
71
Figure B-1B). MFCs that produced the most power at a given current density had cathodes with
the lowest overpotentials, such as the treated bituminous sample (B–N) (–0.05 ± 0.00 V at peak
power), which had a 43% increase in working potential compared to the untreated AC.
5.3.2 Catalyst Activity and Selectivity
Ammonia treatment of the ACs improved catalytic activity for oxygen reduction in all
samples based on evaluation using LSV and RRDE (Figure 5- 2). Despite the reduced power
production observed in the MFC cathode tests, the ammonia treated peat-based AC (P–N) had the
greatest oxygen reduction activity (Eonset = 0.16 V and ilim = 0.96 mA cm–2 at 2100 rpm), and was
improved from the untreated peat AC performance (Eonset = 0.14 V and ilim = 0.83 mA cm–2). The
other treated ACs had catalytic improvements that aligned with the increased power production
observed in MFC tests. The treated bituminous coal (B–N) sample had reduced overpotential with
a similar limiting current (Eonset = 0.12 V and ilim = 0.78 mA cm–2) compared to the untreated
sample (B) (Eonset = 0.08 V and ilim = 0.76 mA cm–2). The activity of the hardwood AC sample
improved after ammonia treatment with both a reduction in overpotential and an increase in
oxygen reduction current activity (W–N, Eonset = 0.03 V and ilim = 0.82 mA cm–2; W, Eonset = –0.04
V and ilim = 0.67 mA cm–2). Even though the peat based (P) and ammonia treated bituminous (B–
N) AC achieved higher power densities than standard Pt/C cathodes in MFC tests, all of the AC
materials had less catalytic activity than the Pt/C catalyst in RDDE tests (Eonset = 0.36 V and ilim =
1.28 mA cm–2), suggesting that mass transfer to the catalyst material is important for MFC
performance.
72
Figure 5-2. A) H2O2 detection based on oxidation current at the Pt ring during oxygen reduction at the catalyst on the disk electrode. B) Oxygen reduction current response during LSV of AC catalysts at the disk electrode compared to Pt/C (100 mM Phosphate Buffer, 2100 rpm). C) Average number of electrons transferred (measured by RRDE analysis) during oxygen reduction.
0.000
0.002
0.004
0.006
0.008
0.010
I ring
(mA)
WW-NCC-NPP-NBB-NCBGCPtC
A
2.0
2.5
3.0
3.5
4.0
-1 -0.8 -0.6 -0.4 -0.2 0 0.2
n
Disk Potential [V vs Ag/AgCl]
PtCP-NPB-NBC-NCW-NWCBGC
C
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4
J disk
(mA
cm-2
)
Disk Potential (V vs Ag/AgCl)
B
73
The selectivity of the catalysts for a complete four electron reduction (and limited H2O2
production) was evaluated using RRDE, and the average number of electrons transferred (n) was
calculated using an empirical collection efficiency of 0.2 (Figure 5-2C). The selectivity of the
catalyst improved after ammonia treatment for all of the samples tested. The treated bituminous
(B–N) AC had the most consistent, near complete reduction of oxygen over the range of
potentials with an average n = 3.85. The treated peat-based (P–N) AC catalyst was also near four
electrons with n = 3.9 at –0.2 V, but this tapered off to n = 3.5 as the disk potential was decreased
to –1 V. Peroxide formation was detected by the platinum ring electrode for all of the ACs tested.
This indicated that there was a mixed reaction pathway with some catalytic sites possibly
reducing oxygen to H2O/OH– through a direct four electron reduction, with others sites reducing
oxygen to H2O2 through a two electron transfer route, and then a second reduction pathway where
the peroxide product was further reduced to H2O/OH– by an additional two electron reduction [3].
5.3.3 Effect of Surface Chemistry
Ammonia treatment of the AC samples increased the basicity of the carbon surface for
the bituminous, peat, and hardwood based samples (Figure 5-3) as expected based on previous
studies [9-14]. PT analysis of the coconut based sample did not show a noticeable change in basic
properties. Acidic oxygen functional groups could not be reliably quantified using this
deconvolution of the proton binding isotherm because some nitrogen functional groups have pKas
~ 4.5 and 9 [14]. However, the measured increase in the basicity is evidence that the heat
treatment increased the relative amount of basic to acidic groups on the carbon surface.
74
Figure 5-3. Proton binding isotherms for treated and untreated A) bituminous, B) peat, C) coconut shell, and D) hardwood based activated carbons.
Based on the XPS analysis, the ammonia treatment process successfully increased the
amount of nitrogen groups present on the surface of the ACs to a measureable level (Figure 5-4).
The hardwood based AC incorporated the most nitrogen, and the type of groups present were
similar among all of the treated sample types. From the quantitative analysis (Figure 5-5),
ammonia treatment reduced the relative abundance oxygen atoms by between 29 to 58% on the
surface of all ACs, with the peat and hardwood losing the largest percent and the coconut shell
AC losing the least. The corresponding gain in surface nitrogen groups resulted in nitrogen levels
between 0.9 and 1.8 atomic %. The decrease in relative abundance along with the increase in
-0.1
0.1
0.3
0.5
0.7
0.9
H+Bo
und
(Q) [
mm
ol/g
]
B-NB
A P-NP
B
3 5 7 9 11pH
W-NW
D
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
3 5 7 9 11
H+Bo
und
(Q) [
mm
ol/g
]
pH
C-NC
C
75
surface nitrogen groups resulted in the increase in catalytic activity for oxygen reduction
observed through the increase in onset potential, current response, and increased electron transfer
numbers nearing a four electron reduction pathway. These results are similar to those reported
for treated carbon nanofibers tested in an alkaline solution [3], where oxygen reduction at the
electropositive carbon sites adjacent to nitrogen groups follow the Yeager Model which leads to a
direct four electron reduction. This is in contrast to the Pauling model where end-on oxygen
adsorption to the electropositive site at the carbonyl group leads to a two electron transfer and
peroxide production, which can be further reduced through a sequential two electron pathway on
any oxygen functional group [3].
Figure 5-4. N1s peaks on ammonia treated ACs from XPS show presence of nitrogen groups on the surface of the treated AC.
0
200
400
600
800
1000
1200
1400
395400405410
CPS
Binding Energy [eV]
W-NC-NP-NB-N
76
Figure 5-5. Atomic % of oxygen and nitrogen on the surface of treated and untreated AC catalysts measured using XPS and the relationship to onset potential of the ORR measured with RRDE.
5.4 Conclusion
Ammonia treatment of AC powders resulted in an increase in ORR catalytic activity and
selectivity in a neutral phosphate buffer solution due to an increase in nitrogen and decrease in
acidic oxygen surface functional groups. Selection of an ideal AC catalyst for neutral buffered
environments, like those in MFCs, should therefore focus on the presence of surface nitrogen
groups. Stability of the treated AC catalysts should be explored in MFC applications, since
increasing the basicity of AC surfaces has been shown to increase the adsorption of dissolved
organic matter and other contaminants commonly found in wastewater. Adsorption of these
molecules may block active surface sites, which can interfere with ORR catalysis [10, 13].
-0.05
0.00
0.05
0.10
0.15
0.20
0
1
2
3
4
5
6
7
8
9
10
B B-N P P-N C C-N W W-N
E ons
et[V
vs A
g/Ag
Cl]
Atom
ic %
O 1s % N 1s % Eonset
77
5.5 Acknowledgements
The authors thank Dr. Cesar Nieto Delgado for assistance with the potentiometric
titration and Vince Bojan for assistance with XPS analysis. The authors acknowledge support
from the King Abdullah University of Science and Technology (KAUST) by Award KUS-I1-
003-13 and the National Science Foundation Graduate Research Fellowship Program (NSF-
GRFP).
5.6 References
1. Logan, B. and K. Rabaey, Conversion of Wastes into Bioelectricity and Chemicals by Using Microbial Electrochemical Technologies. Science, 2012. 337: p. 686-690.
2. Popat, S., et al., Importance of OH- Transport from Cathodes in Microbial Fuel Cells. ChemSusChem, 2012. 5: p. 1071-1079.
3. Zhong, R.-S., et al., Effect of carbon nanofiber surface functional groups on oxygen reduction in alkaline solution. Journal of Power Sources, 2013. 225: p. 192-199.
4. Zhang, F., et al., Power generation using an activated carbon and metal mesh cathode in a microbial fuel cell. Electrochemistry Communications, 2009. 11(11): p. 2177-2179.
5. Wei, B., et al., Development and evaluation of carbon and binder loading in low-cost activated carbon cathodes for air-cathode microbial fuel cells. RSC Advances, 2012. 2: p. 12751-12758.
6. Dong, H., H. Yu, and X. Wang, Catalysis kinetics and porous analysis of rolling activated carbon-PTFE air-cathode in microbial fuel cells. Environ Sci Technol, 2012. 46: p. 13009-13015.
7. Arrigo, R., et al., Tuning the acid/base properties of nanocarbons by functionalization via ammination Journal of the American Chemical Society, 2010. 132: p. 9616-9630.
8. Shafeeyan, M.S., et al., A review on surface modification of activated carbon for carbon dioxide adsorption. Journal of Analytical and Applied Pyrolysis, 2010. 89: p. 143-151.
9. Chen, W., F.S. Cannon, and J.R. Rangel-Mendez, Ammonia-tailoring of GAC to enhance perchlorate removal. I: Characterization of NH3 thermally tailored GACs. Carbon, 2005. 43: p. 573-580.
10. Mangun, C.L., et al., Surface chemistry, pore sizes and adsorption properties of activated carbon fibers and precursors treated with ammonia. Carbon, 2001. 39: p. 1809-1820.
11. Biniak, S., et al., The characterization of activated carbons with oxygen and nitrogen surface groups. Carbon, 1997. 35(12): p. 1799-1810.
12. Szymanski, G.S., et al., The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon. Carbon, 2002. 40: p. 2627-2639.
78
13. Shen, W., Z. Li, and Y. Liu, Surface chemical functional groups modification of porous carbons. Recent Patents on Chemical Engineering, 2008. 1: p. 27-40.
14. Hulicova-Jurcakova, et al., Combined effect of nitrogen- and oxygen-containing functional groups of microporous activated carbon on its electrochemical performance in supercapacitors. Advanced Functional Materials, 2009. 19: p. 438-447.
15. Kruusenberg, I., et al., Non-platinum cathode catalysts fo alkaline membrane fuel cells. International Journal of Hydrogen Energy, 2012. 37: p. 4406-4412.
16. Nallathambi, V., et al., Development of High Performance Carbon Composite Catalyst for Oxygen Reduction Reaction in PEM Proton Exchange Membrane Fuel Cells. Journal of Power Sources, 2008. 183: p. 34-42.
17. Seredych, M. and T.J. Bandosz, Investigation of the enhancing effects of sulfur and/or oxygen functional groups of nanoporous carbons on adsorption of dibenzothiophenes. Carbon, 2011. 49: p. 1216-1224.
18. Bandosz, T.J., J. Jagiello, and C. Contescu, Characterization of the surfaces of activated carbons in terms of their acidity constant distributions. Carbon, 1993. 31(7): p. 1193-1202.
19. Jagiello, J., Stable numerical solution of the adsorption integral equation using splines. Langmuir, 1994. 10(8): p. 2778-2785.
20. Bandosz, T.J., et al., Efffect of surface chemisty on sorption of water and methanol on activated carbons. Langmuir, 1996. 12: p. 6480-6486.
21. Gojkovic, S.L., S. Gupta, and R.F. Savinell, Heat-treated iron(III) tetramethoxyphenyl porphyrin supported on high-area carbon as an electrocatalyst for oxygen reduction - I. Characterization of the electrocatalyst. Journal of the Electrochemical Society, 1998. 145: p. 3493-3499.
22. Kim, J.R., et al., Application of Co-naphthalocyanine (CoNPc) as alternative cathode catalyst and support structure for microbial fuel cells. Bioresource Technology, 2011. 102: p. 342-347.
23. Zhang, X., et al., Improved performance of single-chamber microbial fuel cells through control of membrane deformation. Biosensors and Bioelectronics, 2010. 25: p. 1825-1828.
24. Cheng, S., H. Liu, and B.E. Logan, Increased performance of single-chamber microbial fuel cells using an improved cathode structure. Electrochem Commun, 2006. 8: p. 489-494.
25. Liu, H., R. Ramnarayanan, and B.E. Logan, Production of electricity during wastewater treatment using a single chamber microbial fuel cell. Environmental Science & Technology, 2004. 38(7): p. 2281-2285.
26. Feng, Y., et al., Treatment of carbon fiber brush anodes for improving power generation in air–cathode microbial fuel cells. Journal of Power Sources, 2010. 195: p. 1841 - 1844.
27. Bretschger, O., et al., Current production and metal oxide reduction by Shewanella oneidensis MR-1 wild type and mutants. Appl Environ Microbiol, 2007. 73(21): p. 7003-7012.
28. Hong, Y., et al., Adaptation to high current using low external resistances eliminates power overshoot in microbial fuel cells. Biosens Bioelectron, 2011. 28(1): p. 71-76.
29. Logan, B.E., et al., Microbial fuel cells: methodology and technology. Environ Sci Technol, 2006. 40(17): p. 5181-5192.
79
Chapter 6
Conclusions and Future Work
Through the research included in this dissertation we found that MFC polarization curves
measured by running for a full cycle at each resistance allows the anode performance to
stabilize and eliminate power overshoot. We also showed that polymer layers on the liquid
side of the cathode surface can hinder oxygen diffusion into the anode chamber, decrease
biofilm growth on the cathode and increase coulombic efficiency. Also, coating the cathode
with polymer layers increased the charge transfer resistance of the cathode, which decreased
power production, but the anion exchange polymers did not increase the resistance as much
as the other polymers studied. In the study of AC catalysts, we saw that both surface
chemistry and pore structure influenced the ORR performance and that increased surface area
or microporosity did not lead to improved performance. We also found that strong acid
oxygen functional groups (such as carboxyl groups) hindered the oxygen reduction activity of
the ACs, and that an increase in nitrogen groups and decrease in oxygen groups on the AC
surface (resulting from ammonia treatment) resulted in increased ORR activity.
Despite recent findings and improvements, further research is essential to the successful
development of MFC technology, such as:
• Improvement to the cathode structure to address diffusion and conductivity concerns
• Development of cathodes using activated carbon fiber as a way to increase
conductivity and stability of the cathode structure
• Development of ACs with higher concentrations of nitrogen groups either by pre–
oxidation or utilization of a precursor material that has higher nitrogen content (such
as silk fibroin)
80
• Investigation and improvement in the stability/longevity of activated carbon catalysts
in the presence of organic contaminants
81
Appendix A
Supplemental Information to Chapter 4
Figure A-1: Example of RDE LSV data for bituminous coal based sample (B1) collected at rotation rates from 100 – 2100 RPM.
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
-1 -0.8 -0.6 -0.4 -0.2 0 0.2
J (m
A/c
m2 )
E (V vs Ag/AgCl)
B1 100rpm
B1 350rpm
B1 600rpm
B1 1100rpm
B1 1600rpm
B1 2100rpm
82
Figure A-2: Example of K-L analysis for bituminous coal based sample (B1) where the slope of the line is used to calculate n and the y-intercept is the inverse ik.
y = -84.9x - 6.1R² = 1.00
y = -76.8x - 4.0R² = 1.00
y = -71.0x - 3.3R² = 1.00
y = -70.7x - 2.6R² = 1.00
y = -77.3x - 1.5R² = 1.00
y = -80.4x - 1.2R² = 1.00
y = -86.9x - 11.8R² = 1.00
-45
-40
-35
-30
-25
-20
-15
-10
-5
00 0.1 0.2 0.3 0.4
I-1(m
A-1
)
ω-1/2 (rad-1/2s1/2)
Koutecky-Levich Analysis
B1 -0.3VB1 -0.4VB1 -0.5VB1 -0.6VB1 -0.8VB1 -1VB1 -0.2V
83
Figure A-3: Potentiometric titration curves showing protons bound (positive Q) or released (negative Q). The isotherm data was then further analyzed using SAIEUS software to quantify the type (pKa) and quantity of acidic functional groups.
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
2 3 4 5 6 7 8 9 10 11 12
Q (m
mol
/g)
pH
Proton Binding Isotherm
P2P3P1C1C3C2B1R1W1
84
Figure A-4: Inverse correlation between quantity of strong acid functional groups on the AC catalyst powder and the power production in an MFC using the AC cathode (a) with and (b) without the inclusion of the bituminous coal based sample (B1)
P2P3P1
B1
W1
C2
C1
C3
R1
R² = 0.521
0200400600800
10001200140016001800
0 0.1 0.2 0.3 0.4
Pow
er D
ensit
y [m
W/m
2 ]
Strong Acid Groups (pKa<8) [mmol/g]
a
P2P3P1
W1
C2
C1
C3
R1
R² = 0.8169
0200400600800
10001200140016001800
0 0.1 0.2 0.3 0.4
Pow
er D
ensit
y [m
W/m
2 ]
Strong Acid Groups (pKa<8) [mmol/g]
b
85
Figure A-5: Cumulative pore volume distribution of AC catalyst powders measured by argon adsorption and DFT analysis
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1 10 100 1000 10000
Cum
ulat
ive
Por
e Vo
lum
e (m
l/g)
Pore Width (Å)
P3
P2
P1
C1
C3
C2
B1
R1
W1
micro meso macro
86
Appendix B
Supplemental Information to Chapter 5
Figure B-1. (A) Power production increases with the additional stainless steel current collector. (B) Cathodes (open symbols) with the additional current collector operate at a higher potential. Anodes (filled symbols) perform the same with both cathode configurations. (100 mM Phosphate buffer, Data of MFC cathodes without the extra current collector is from Chapter 4.)
0
500
1000
1500
2000
2500
0.0 0.5 1.0 1.5
Pow
er D
ensit
y [m
W/m
2 ]
P-ssPB-ssBC-ssCW-ssWPtC
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.0 0.5 1.0 1.5
Elec
trod
e Po
tent
ial (
V vs
Ag/
AgCl
)
I (mA/cm2)
Pt/CBPCWB-ssP-ssW-ssC-ss
B
CURRICULUM VITAE
VALERIE J. WATSON
182 Kathryn Drive, Bellefonte, PA 16823, 814-880-1810, [email protected]
EDUCATION Ph.D. Environmental Engineering, May 2013 The Pennsylvania State University, University Park, PA Advisor: Bruce E. Logan, Evan Pugh Professor and Kappe Professor of Environmental Engineering M.S. Environmental Engineering, May 2009 The Pennsylvania State University, University Park, PA Advisor: Bruce E. Logan, Kappe Professor of Environmental Engineering M.M.M. Quality and Manufacturing Management, May 1998 The Pennsylvania State University, University Park, PA B.S. Chemical Engineering, May 1997 The Pennsylvania State University, University Park, PA
EXPERIENCE Graduate Research Fellow & Assistant, Environmental Engineering, Penn State, 2006-2013 ISO Project Manager, Armstrong World Industries, Floor Products Division, Lancaster, PA, 2001 – 2002 Project Chemical Engineer, Armstrong World Industries, Floor Products Division, Lancaster, PA, 1999 2001 Quality Engineer, AMP Incorporated, Harrisburg, PA, 1998 – 1999 Quality Engineer Intern, Carpenter Technology Corporation, Reading, PA, Summer 1997
PUBLICATIONS Watson, V.J. and B.E. Logan. 2011. Analysis of polarization methods for elimination of power overshoot in microbial fuel cells. Electrochem. Commun. 13(1):54-56. Watson, V.J., T. Saito, M.A. Hickner, and B.E. Logan. 2011. Polymer coatings as separator layers for microbial fuel cell cathodes. J. Power Sources. 196(6):3015-3025. Saito, T., M.D. Merrill, V.J. Watson, B.E. Logan, and M. A. Hickner. 2010. Investigation of ionic polymer cathode binders for microbial fuel cells. Electrochim. Acta. 55(9):3398-3403. Watson, V.J., and B.E. Logan. 2010. Power production in MFCs inoculated with Shewanella oneidensis MR-1 or mixed cultures. Biotechnol. Bioengin. 105(3):489-498. Logan, B.E., Cheng, S., Watson, V., and Estadt, G. (2007) “Graphite fiber brush anodes for increased power production in air-cathode microbial fuel cells.” Environmental Science & Technology, 41(9): p. 3341-3346
HONORS AND AWARDS Presentation Award Winner, North American Meeting of the International Society for Microbial Electrochemistry and Technology, Cornell University (2012) National Science Foundation Graduate Fellowship (2008-2011) Hydrogen Day at Penn State, Poster presentation honorable mention (2006) Cecil M Pepperman Memorial Graduate Fellowship, Penn State University (2006) General Electric’s First-Year Faculty for the Future Fellowship, Penn State University (2006) Manager’s award for team excellence, Armstrong World Industries (2002) Manager’s award for product development project (2000) QMM Scholarship for Academic Excellence, Penn State University (1998) Omega Chi Epsilon, Chemical Engineering Honor Society, Penn State University (1995-1997)
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