Power generation and organics removal from wastewater using

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 5 8 8 e1 5 9 7
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Power generation and organics removal from wastewater
using activated carbon nanofiber (ACNF) microbial fuel cells
(MFCs)
Udayarka Karra a, Seetha S. Manickam b, Jeffrey R. McCutcheon b, Nirav Patel a,
Baikun Li a,*
a
b
Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269, United States
Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269, United States
article info
abstract
Article history:
Carbon-based materials are the most commonly used electrode material for anodes in
Received 8 June 2012
microbial fuel cell (MFC), but are often limited by their surface areas available for biofilm
Received in revised form
growth and subsequent electron transfer process. This study investigated the use of
31 October 2012
activated carbon nanofibers (ACNF) as the anode material to enhance bacterial biofilm
Accepted 1 November 2012
growth, and improve MFC performance. Qualitative and quantitative biofilm adhesion
Available online 12 December 2012
analysis indicated that ACNF exhibited better performance over the other commonly used
carbon anodes (granular activated carbon (GAC), carbon cloth (CC)). Batch-scale MFC tests
Keywords:
showed that MFCs with ACNF and GAC as anodes achieved power densities of
Microbial fuel cell (MFC)
3.50 0.46 W/m3 and 3.09 0.33 W/m3 respectively, while MFCs with CC had a lower power
Activated carbon nanofibers (ACNF)
density of 1.10 0.21 W/m3 In addition, the MFCs with ACNF achieved higher contaminant
Wastewater treatment
removal efficiency (85 4%) than those of GAC (75 5%) and CC (70 2%). This study
Biofilm
demonstrated the distinct advantages of ACNF in terms of biofilm growth and electron
Bioavailable surface area
transport. ACNF has a potential for higher power generation of MFCs to treat wastewaters.
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1.
Introduction
Bioenergy is a renewable energy derived from biological
sources. Production usually involves various biological processing strategies, primarily from biomass and organic
contaminants associated with wastewaters. The most extensively studied bio-electrochemical system (BES) for generating
electricity is the microbial fuel cells (MFCs), which is an
innovative technology exploiting anaerobic bacteria to
generate electricity from wastewater [1e7] and the organic
matters stored in marine sediments [1,8]. MFCs hold a great
capability to address the escalating concerns of energy
demands and sustenance of current water infrastructure. The
power generation in an MFC system is dependent on both
Abbreviations: ACNF, Activated Carbon Nanofiber; BES, Bio-Electrochemical System; BET, BrunauereEmmetteTeller; CC, Carbon
Cloth; CFU, Colony Forming Unit; CE, Coulombic Efficiency; COD, Chemical Oxygen Demand; EIS, Electrochemical Impedance
Spectroscopy; FE-SEM, Field-Emission Scanning Electron Microscope; GAC, Granular Activated Carbon; LSV, Linear Sweep Voltammetry;
MFC, Microbial Fuel Cell; OCP, Open Circuit Potential; OD, Optical Density; PAN, Polyacrylonitrile; PTFE, Polytetrafluoroethylene; Rext,
External Resistance; Rin, Internal Resistance; SEM, Scanning Electron Microscope; SCMFC, Single Chamber Microbial Fuel Cell; SSA,
Specific Surface Area.
* Corresponding author. Tel.: þ1 860 486 2339.
E-mail address: [email protected] (B. Li).
0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijhydene.2012.11.005
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 5 8 8 e1 5 9 7
biological and electrochemical processes. In the anode
compartment, anaerobic bacteria degrade organic substances
in wastewater and transfer the electrons to the anode surface.
The electrons then flow through an external electrical circuit
and combine with an electron acceptor (e.g. oxygen) at the
cathode, and electricity is generated [6e9]. The driving force
for the power generation process in MFCs is extracellular
electron transport from the anode to the cathode conducted
by anaerobic exoelectrogenic bacteria [10,11]. The MFC technology has multiple advantages of power production and
simultaneous treatment of wastewaters along with sludge
minimization [5e8]. Prior to the application of MFCs in
wastewater treatment plants, the critical issues of low power
density and the high cost of electrode materials need to be
resolved.
Electron transfer from the biofilms to the anode is critical
for contaminant removal and electricity generation in MFCs
[12]. Electron transfer can occur directly via extracellular
redox cofactors (e.g. c type cytochromes) and nanowires
[13,14], or through soluble redox mediators [9]. Biofilm
formation on the anode surfaces is governed by different
factors including: substratum conditions, hydrodynamic
characteristics of the aqueous medium and cell surface
properties [15]. Previous studies have indicated the importance of large anode surface area to achieve high power
generation [16,17]. Ideally, a greater extent of bacterial adhesion should assist in a higher transfer of electrons to the
anode. However, due to the presence of diverse microbial
consortia in wastewater, uncontrolled microbial growth could
result in competition for the available organic substrates by
non-electrogenic bacteria, which divert the MFC systems
away from useful power generation [6,18]. Therefore, it is
important to achieve optimal biofilm growth on anode
surfaces for effective contaminant degradation, high bacterial
electron transfer capacity, and power generation.
Extensive studies have been conducted to explore novel
anode materials to enhance electron transfer and biofilm
formation [5,15]. The anode materials need to be highly
conductive, chemically stable, biocompatibility, with high
ratios of surface area to volume, and a porous structure to
allow internal colonization of microbial biofilms for a stronger
interaction with the anode material [6e8,19,20]. Diverse
carbon-based anode materials have been investigated for
application in MFC systems, including plain graphite, carbon
paper, carbon cloth (CC), reticulated vitreous carbon (RVC),
granular activated carbon (GAC), and graphite granules
[18,21e24]. CC type woven materials have been limited by
their thick structures and small surface areas, which result in
high electron transport resistance. GAC has higher surface
areas than CC, but has the problem of poor granular interconnectivity and inefficient usage of their porous structure for
biofilm growth. This caused the problems of diffusional limitations that severely affected the biofilm kinetics [5,23,25].
Most importantly, these carbon-based anode materials are
normally expensive, and have high resistances for mass
transfer and electron transfer, which hinder the scale up
applications of MFCs [18].
Carbon-based nanofiber material could be the next stage of
anode materials with enhanced electron transfer capabilities,
owing to their high porosities and specific surface areas, high-
1589
temperature tolerance, and good electrical and thermal
conductivity. Currently, nanofiber materials are being used in
drug delivery systems, high-temperature filters and catalysts,
nanoelectronics, and supercapcitors/batteries [19,26,27].
Polyacrylonitrile (PAN) based electrospinning technique is
a simple, effective and inexpensive technique to produce the
electrospun nonwoven carbon nanofibers with a high carbon
yield and thermally stable structure [26,27]. The production of
electrospun nanofibers also involves the stabilization and
carbonization with heat treatment to activate the nanofibers
for better surface properties [26,28].
The objective of this study was to employ activated carbon
nanofibers (ACNF) as the novel anode material in MFC
systems. Compared with other activated carbon materials, the
unique features of ACNF are its mesoporous structure,
excellent porous interconnectivity, and high bioavailable
surface area for biofilm growth and electron transfer. The
performance of ACNF was compared with two commonlyused anodes: granular activated carbon (GAC) and carbon
cloth (CC). Three aspects were analyzed in this study: the
extent of bacterial adhesion on each anode, their performance
in MFC systems and finally, the contaminant removal and
Coulombic efficiency (CE) of the three anodes were compared.
2.
Materials and methods
2.1.
Anode materials
Three anode materials with different morphologies were
examined in this study. The first anode was carbon cloth (CC;
non-wet proofed, Fuel Cell Earth; geometric area: 16 cm2),
which has been widely used in MFCs [6e8]. The second was
granular activated carbon (GAC, mesh size: 8 30, General
Carbon, Paterson; anode working volume: 30 mL), which has
been used in MFCs due to its high specific and interfacial
surface areas [18]. GAC particles (diameter: 0.5e2 mm) were
packed into a polypropylene mesh bag (length: 2.5 cm; width:
2.25 cm) to maintain a consistent distance to cathode. The
third anode was activated carbon nanofibers (ACNF;
geometric area: 16 cm2). This novel material was fabricated by
heat-treatment of electrospun polyacrylonitrile precursor and
steam activation to produce carbon nanofibers in a nonwoven
architecture [26,28,29]. Due to the light-weight of ACNF, it was
sandwiched in a polypropylene mesh bag with the dimensions proportional to their geometric area in order to maintain
its mechanic integrity and keep it fully submerged in MFC
solution. Supporting information on the anode material
construction is shown in Table 1.
2.2.
Physical characterization of anode materials
Specific surface area (SSA) and pore size distributions of the
anode materials were measured using ASAP 2020 Physisorption Analyzer (Micromeritics Instrument Corporation).
BrunauereEmmetteTeller (BET) model adsorption isotherms
technique [30], owing to its easy of applicability, was employed
for estimating the SSA. BarretteJoynereHalenda (BJH) model,
which had been used for estimating the broad size distributions of medium to large mesopores [31], was employed for
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Table 1 e Characteristics of three anode materials. All results presented were the average of three individual tests.
Anode
CC
GAC
ACNF
BET SSAa (m2/g)
Specific conductivityb (S/m)
Mass of anode used (g)
Volume (106 m3)
0.11
843
1159
333
748c
19
0.15 0.06
12.00 0.93
0.04 0.01
1
40
0.002
a BET specific surface area.
b Specific conductivity calculated by EIS.
c Ref. [19,24].
estimating the pore size distribution of the anode materials.
The specific conductivities of the anode materials were estimated based on the resistance measurements using electrochemical impedance spectroscopy (EIS) [24].
2.3.
Single chamber MFC (SCMFC) setup and operation
Batch-mode single chamber MFCs (SCMFCs: effective working
volume: 100 mL) made with glass bottles (Wheaton Scientific)
were used in this study. SCMFCs eliminated the proton
exchange membrane (PEM) used in two-chamber MFCs. This
lowered the internal resistance (Rin), and directly used oxygen
present in air as the electron acceptor in the cathode [7]. The
anode to be tested was place inside the bottle, and the cathode
(carbon cloth, 30% wet proofing, Fuel Cell Earth, MA; geometric
area: 3 cm2) was placed in the extension arm of the bottle. The
anodeecathode distance was maintained at 4 cm in all
SCMFCs tested. The waterside of the cathode contacting with
the solution in the SCMFCs was doped with platinum (10% by
weight on carbon black) (0.5 mg/cm2), while the airside of the
cathode was coated with 3 layers of polytetrafluoroethylene
(PTFE) to prevent water seepage and evaporation from the
anode chamber to air. The airside of the cathode only permits
limited diffusion of oxygen to complete the reduction half
reaction and generate a potential for MFCs.
The influent wastewater to the University of Connecticut
Wastewater Treatment Plant that contained diverse anaerobic
bacteria was used as inocula in the anode chamber. The
wastewater had an initial chemical oxygen demand (COD) of
250e300 mg/L and a pH of 7e8. Sodium acetate was added to
the wastewater as supplemental organic substrate to achieve
the desired initial COD concentrations (1500e3000 mg/L) and
stabilize pH (pKb 9.25) in the SCMFCs. The SCMFC reactors
were thoroughly shaken after the addition of sodium acetate
to ensure the uniform mixing of organic substrates in solution. The voltage over an external resistance (Rext) of 100 U was
recorded by a data log system (Keithley 2700) at intervals of
2 h. All analysis was conducted in duplicate SCMFCs which
were operated at 30 C in an incubator.
2.4.
washed in the buffer solution, followed by stepwise dehydration in a series of ethanol/water dilutions (volume
proportions: 25, 50, 75, 95 and 100%) for 15 min each [32].
Subsequently, the samples were dried in a drying oven at 30 C
and sputtered with gold at 2.2 kV and 10 mA for 30 s before
SEM observation.
Optical density (OD) and viable colony forming unit (CFU)
plate counting measurements were employed to quantify the
actual bacterial growth in the anode materials [33]. Two model
biofilm strains of Pseudomonas aeruginosa (ATCC-97) and Shewanella oneidensis MR-1 (ATCC-BAA1096) were selected for
anode bacterial growth tests. The anode materials to be tested
were grown in the pure culture strains in the Laurie broth (LB)
media and shaken at 35 C for 48 h. The incubated anode
materials were then transferred into the sterilized LB media
(volume: 10 mL) and sonication for 1e2 min (Branson Sonicator, Model: 3510R-MT, 40 kHz 6%) to detach the adhered
bacterial cells from the anode surfaces. The OD of the serial
diluted solution containing the detached cells was measured
at the wavelength of 600 nm (OD600) using a spectrophotometer (HACH: DR2700). Viable CFU (CFU/mL) of the solution
containing the detached cells was performed on LB agar plates
using the surface spread method through serial dilution
technique [34]. For both OD and CFU tests, sample controls
(bacterial cell suspension solution) with only the pure cultures
were analyzed to assure the consistency of the procedure. The
analysis was conducted under sterile conditions in duplicates.
2.5.
Electrochemical characteristics of SCMFCs with
different anode materials
The volumetric power densities (W/m3) for SCMFC systems
with different anode materials were calculated using the
voltage results across different external resistors (Rext)
(46e1500 U) applied in the external circuit during the polarization curve measurement. The voltage over each Rext was
recorded by a multimeter (RadioShack, TX) [6,35]. The polarization curves record the current (mA) as a function of voltage
[1]. The power and current densities were calculated according to Eqs. (1) and (2).
Bacterial adhesion analysis of anode material
Bacterial adhesion on the anode materials was qualitatively
observed using a field emission scanning electron microscope
(FE-SEM) (Model: Joel 6335F). After 5 weeks of SCMFC operation, a small section of the anode material (0.25 cm2) was cut
out and later fixed in a solution containing 2.5% paraformaldehyde, 1.5% glutaraldehyde and 0.1 M cocadylate
buffer solution at 4 C for 12 h. The samples were then triple
Power density ¼
V2
Rext k
Current density ¼
V
Rext k
(1)
(2)
where V is the voltage across the external resistor (mV), Rext is
the external resistance (U), and k is the operating volume
(125 mL) of the SCMFCs.
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The electrode potential in open circuit conditions, termed
as the open circuit potential (OCP) is an indicator of the
maximum possible voltage under the operated thermodynamic limits. The electrode OCP was measured using
a potentiostat (Gamry Reference 600) [7] with the target electrode (anode or cathode) as the working pole and an Ag/AgCl
reference electrode as the counter and reference pole.
The internal resistance (Rin) in an MFC system is the
primary source of voltage loss. The Rin for the whole cell was
estimated using galvano electrochemical impedance spectroscopy (EIS), with CHI-601C electrochemical workstation
(CH Instruments, USA) [24]. Data analysis was performed
using EC Lab Software (BioLogic Science Instruments, V9.56) to
obtain the Rin of SCMFCs based on impedance plots [36].
Linear sweep voltammetry (LSV) analysis is a powerful
approach to study the electron transfer kinetics in MFCs.
Specifically, LSV measured the current response at the anode
electrode as a function of an applied potential [6,37,38]. The
LSV analysis of the anode materials in SCMFCs was performed
weekly using a potentiostat (Gamry Reference 600) in a threeelectrode electrochemical cell setup, with the current density
at the working electrode (anode) being measured, while the
potential between the anode electrode and reference electrode (Ag/AgCl) was swept linearly over time under no
convection. The platinum spiral was served as the counter
pole. A scan rate of 0.1e0 mV/s, from the anode OCP to the
minimum cell potential was employed in the LSV analysis. All
the LSV measurements were performed in duplicate SCMFCs
operated under similar conditions of pH, temperature, and
solutions conductivities.
2.6.
Wastewater characterization and Coulombic
efficiency (CE) calculation
The chemical oxygen demand (COD) of the substrate was
measured using HACH COD vials for high range measurements (0e15,000 mg/L) and a DR 220 spectroscopy (HACH,
Loverland, CO, USA) according to the HACH analysis protocol.
The Coulombic efficiency (CE, ε) is defined as the ratio of
actual charge generated to the maximum theoretical charge
attainable (Eq. (3)). It is a critical parameter used to evaluate
the performance of an MFC system.
Z
t
Idt
ε¼
0
4
DCOD vF
32
(3)
where M ¼ 32 is the molecular weight of oxygen (g/mol), I is the
current (mA), F is the Faraday constant (96,485 C/mol), b ¼ 4 is the
number of electrons exchanged per mole of oxygen, v is the
reactor working volume (mL), and DCOD is the change in COD
concentrations between the influent and effluent streams (mg/L).
3.
Results and discussion
3.1.
Characterization of the anode materials
The specific surface area (SSA) of the anode materials was
analyzed using the BET model. The results showed that CC
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had the lowest SSA value (0.11 m2/g), GAC had the medium
(GAC: 830 m2/g) and ACNF had the highest value (1159 m2/g)
(Table 1). The analysis gave an estimate of the SSA available
for bacterial adhesion and biofilm growth, with ACNF having
the highest surface area as a result of steam activation step.
CC anode material pore size distribution is predominantly
macropores (>0.3 mm), but the significance of their large pore
size was diminished by their low SSA. The BJH analysis of
GAC and ACNF anode materials indicated that about 65%
of the SSA for GAC had a pore size distribution in the
mesoporous range (<0.3 mm), whereas only 35% of the SSA
for ACNF was in the mesoporous range, with the remaining
primarily being macropores. Since typical bacterial size is
about 0.5e3 mm, the pore size distribution greater than
0.3 mm would greatly contribute to the bioavailable surface
area for the bacteria to access and colonize. More than half
of GAC surface areas were not be accessible by bacterial
cells, while the architecture of ACNF could establish an
effective biofilm structure where it was easier for microorganisms to access the wastewater nutrients and better
transfer electrons.
3.2.
Bacterial adhesion analysis on anode materials
The extent of bacterial adhesion varied with the properties of
the anode material, as represented in the SEM images (Fig. 1).
CC had the lowest surface area (<0.11 m2/g, Table 1) (Fig. 1a),
while GAC (w850 m2/g) (Fig. 1b) and ACNF (w1200 m2/g)
(Fig. 1c) had much higher surface areas. The interconnected
and nonwoven structure of ACNF was expected to promote
biofilm growth and enhance electron transport. The SEM
images show the extents of biofilm formation on CC (Fig. 1d)
and GAC (Fig. 1e) were limited, due to the SSA and BET pore
size. On the contrary, a more extensive biofilm formed on
ACNF (Fig. 1f), which could be the result of relatively easy
diffusion of organic substrates through the material. The
random interconnectivity of the ACNF nanofibers enabled
more accessibility for biological interactions, which resulted
in a more widespread biofilm formation.
The biofilm thickness is often limited by the substrate
diffusion resistance inside biofilms [39,40]. These diffusional
resistances are further increased by the lower pH/redox
conditions inside biofilms and the reduced cytochromes on
the outer biofilm layers, which may not support additional
bacterial layers [39e41]. The substrate transfer resistances
could be partially solved in the continuous flow MFCs treating
wastewater, since it would enable a constant biofilm thickness and sufficient buffer capability of wastewater to prevent
pH decrease.
Microbial biofilm growth quantified on the three anode
materials well corresponded with the SEM observations.
Indirect spectroscopy measurements (OD600 results, Fig. 2a)
and direct viable dilution spread plate count technique (CFU
results, Fig. 2b) of the pure culture strains (P. aeruginosa and S.
oneidensis MR-1) indicated that ACNF had a higher amount of
bacterial biomass than GAC and CC. The OD results demonstrated that a higher amount of biomass attached on the anode
materials (ACNF) led to higher turbidity in the cell suspension
and higher absorbance values. The log10 normalized, CFU/mL,
accounted for the average number of viable bacterial cells
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Fig. 1 e Scanning electron microscopy (SEM) images of the anode materials, after 5 week operational period: carbon cloth
(CC) (10003) (a); granular activated carbon (GAC) (24003) (b); activated carbon nanofibers (ACNF) (60003), Inset (24003) (c); CC
with biofilm (24003) (d); GAC with biofilm (27003) (e); ACNF with biofilm (60003), Inset (24003) (f).
present in the anode material biofilms, indicating that they
were metabolically active. Although these two tests were
conducted using pure cell cultures to obtain the clear trend
and avoid the interference of mixed cultures and particles in
wastewater, it was expected that the same trend could be
followed in the MFC systems treating wastewater containing
diverse microbial consortia.
3.3.
Power generation with different anode materials
The comparison of power density curves showed that SCMFCs
with GAC and ACNF as anodes achieved the power density
values of 3.09 0.33 W/m3 and 3.5 0.46 W/m3, respectively,
while SCMFCs with CC had a lower power density of
1.1 0.21 W/m3 (Fig. 3a). As a biofilm-based process, the power
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Fig. 2 e Optical density (OD600) (a), and colony forming unit (CFU) (b); (standard deviation now included), measurements for
the three anode materials using the pure culture strains Pseudomonas aeruginosa (PA) and Shewanella oneidensis MR-1 (SO).
All data were duplicate mean values.
generation in MFCs is affected by the interactions between
bacterial cells and the anode surface [19,42,43]. Although
ACNF qualitatively and quantitatively showed a greater extent
of biofilm growth over GAC, which might lead to higher power
generation, GAC being more conductive over ACNF (Table 2)
served to facilitate lower Rin for the SCMFC systems and had
power outputs similar to ACNF [44]. CC had low surface areas
and less biomass growth, resulting in less electron generation
and hence lower power generation.
a
b 0.7
4
CC
GAC
ACNF
3.5
The polarization curves represent the relation between
voltage and current (Fig. 3b). The cell voltage for the current
above zero exhibited a steep drop for CC, while maintained
a gentle drop for GAC and ACNF. The likely reasons could be: (i)
activation overpotentials and the morphology of the anode
materials play a vital role in attaining higher voltages over
a long range of current values [24]. Overpotential is an essential
loss of cell voltage in the form of activation, metabolic
and mass transfer losses. In the SCMFCs with three anode
0.6
3
0.5
-3
Power Density (W*m )
CC
GAC
ACNF
Cell Voltage (V)
2.5
2
1.5
0.3
0.2
1
0.1
0.5
0
0.4
0
5
10
15
20
-3
Current Density (A*m )
25
30
0
0
5
10
15
20
25
30
-3
Current Density (A*m )
Fig. 3 e Power generation (a), and polarization curve (b) for three anode materials after 6-week operational period. Mean
values (standard deviation not included) of two independent runs, performed on duplicate MFC systems operated under
similar conditions.
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Table 2 e OCP, internal resistance (Rin), COD removal, and Coulombic efficiencies (CE) of three anodes. Mean data for
duplicate MFC systems operated under similar conditions with standard deviation, after 9 week operational period.
Anode
Anodic potential
vs. Ag/AgCl (mV)
Internal resistance
(Rin) (U)
COD removal
efficiency (%)
Coulombic efficiency
(CE, %)
Peak power
densities (W/m3)
460 8
480 13
495 10
65 13
50 17
103 8
70 2
75 5
85 4
11 1
15 4
82
1.10 0.21
3.09 0.33
3.50 0.46
CC
GAC
ACNF
materials to be tested, the determining factor was the electron
transfer between the bacteria cells and the anode surfaces,
since the proton transport resistance was minimized with pH
buffering capacity by replenishing sodium acetate and
wastewater at the end of each batch cycle, and (ii) the current
densities are a function of the electron generation and the
electron transport capacity. Even though the three anode
materials had the similar starting points (a high voltage at
a low current densities, Fig. 2b), the ACNF exhibited the slower
voltage decrease with current increase, perhaps due to a higher
electron output. The distinct advantage of ACNF over the other
anode materials is the higher bioavailable surface area, which
resulted in a more extensive biofilm formation (Fig. 2). This
benefit is critical for MFC systems, since an efficient biofilm
growth leads to the prominence of electro-active bacteria and
the electron transport to the anode electrode [24,45].
Power generation at different stages of biofilm growth on
ACNF was also studied. The results showed that the power
densities for ACNF increased with biofilm growth on anode
surfaces, with 0.9 0.32 W/m3 in the 3rd week and increasing
to 3.5 0.46 W/m3 in the 4th week (Fig. 4a). The increase in
power density with biofilm growth was caused by the
increased electron transport of the electrogenic bacteria
growing on the well-connected ANCF fibers. Previous studies
indicated that a higher bioavailable anode surface area could
a
3.4.
Electrochemical characteristics of SCMFCs with
different anode materials
The electrochemical characteristics for SCMFC systems with
these three anode materials were compared (Table 2). In an
MFC system, a greater difference between the negative anodic
potential and the positive cathode potential leads to a higher
power generation. A higher anode potential implies electrons
are likely to be consumed by alternate electron acceptors,
indicating that the bacteria do not use the anode and lower
energy recovery for the MFC system [5]. Thereby, the anode
potential should be lower in conjunction with the anaerobic
environment and the generation of negative charges (electrons)
by the degradation of the organic substrates in wastewater [6,9].
b
4
Week-3
Week-6
Week-9
3.5
enhance biofilm formation and result in more electron
transfer and higher power density generation [43,44,46]. With
mature biofilms growing on anode surfaces, the SCMFCs
developed stable voltage generation and maintain a steady
and linear voltageecurrent correlation [47]. The benefits of
biofilm growth for power generation were also indicated in the
polarization curves (Fig. 4b). The voltage of the ACNF with
biofilm growth in the 9th week had more stable voltage trend
with current changes than the ACNF in the 3rd week, which is
concurrent with previous studies [37,48].
0.5
Week-3
Week-6
Week-9
0.45
0.4
0.35
-3
Power Density (W*m )
3
Cell Voltage (V)
2.5
2
1.5
0.3
0.25
0.2
0.15
1
0.1
0.5
0
0.05
0
5
10
15
20
-3
Current Density (A*m )
25
30
0
0
5
10
15
20
-3
Current Density (A*m )
Fig. 4 e Power generation (a), and polarization curves (b) for ACNF during the operational periods (Week 3eWeek 9). Mean
values (standard deviation not included) of two independent runs, performed on duplicate MFC systems operated under
similar conditions.
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The anode OCP (the maximum voltage that can be obtained at
infinite resistance) varied between 450 and 500 mV for the
three materials, with ACNF having the most negative value.
The results indicated that ACNF had the lowest anodic OCP
(495 10 mV), followed closely by GAC (480 13 mV), and CC
exhibited the highest OCP (460 8 mV). The difference in
anodic OCP was predominantly a function of the anode materials, the microbial communities inside biofilms, and the biofilm thickness. Since most anaerobic microorganisms rely on
similar electron transport chains for their metabolic requirements, reduced form of electron carriers (e.g. NADH) were
generated when these microorganisms degrade the organic
substrates [5,9,24]. The accumulation of these reductive metabolic byproducts in biofilms caused the decrease in the anodic
potential.
The Rin in an MFC can be attributed to three major
components: charge transfer activation (or kinetic losses), ion
and electron transport (or Ohmic losses), and substrate
concentration (or mass transfer losses) [8,24,36]. Given that
bacterial metabolic losses are inevitable, the activation and
mass transfer losses at the electrode can be reduced by
changing electrode morphological properties and by
enhancing substrate flux to the electrode surface. The Rin as
estimated from EIS (Table 2) for the whole SCMFCs indicated
that Rin of the SCMFC with ACNF was higher (103 8 U) (Table
2) than those of GAC (50 17 U) and CC (65 13 U), even as
ACNF demonstrated more biofilm growth and lower negative
anodic OCP. This may be explained based on the EIS specific
conductivity (S/m) measurements, where ACNF showed lower
conductivity (19 S/m) than CC (333 S/m) and GAC (748 S/m [19])
(Table 1). The low conductivity of ACNF could be ascribed to
the activation process during fabrication. Although all the
anodes tested have the same carbon-based material, the
steam activation step essentially hydrophilized the ACNF
material, which may have resulted in the addition of some
hydroxyl (OH) groups to the nanofibers and given it an overall
negative surface charge [49e51]. Other studies have indicated
the importance of surface modifications to promote biofilm
adhesion and enhance performance of MFCs [18,52], which
could be employed for further enhancing the ACNF conductivity. Despite the lower conductivity of ACNF, its performance was better than that of GAC and CC.
The anode polarization results clearly showed that anodes
had a significant influence on the SCMFC performance (Fig. 5),
while the cathode polarizations (not shown) were similar for
all the SCMFCs tested, since the cathode structure, size and
configuration were the same for all the SCMFCs tested. The
LSV polarization scans of the anode material after the 5-week
operational period indicated that ACNF superseded CC and
GAC as the anode material. The scan started at the cell
potential under open circuit conditions, and then was carried
out from left to right of the current density and potential plot,
the anode potentials were corrected for Rin obtained from EIS
measurements. The current density increased linearly with
the potential at low overpotentials, and then increased more
rapidly at higher potential and reached a maximum stationary
region before it began to drop. This current increase under
voltage sweep from its initial electrochemical kinetic equilibrium region was a function of the availability of organic
substrates and the reactivity for electron transfer. The region
of stationary current was influenced by the diffusional limitations for the substrate mass transport to the electrode
surface, as a result of the microbial biofilm growth. Thereby,
the flux of the organic substrates was not fast enough to satisfy
the Nernstian conditions, and was dominated by mass transfer [53e55]. The LSV curves showed that ACNF reached the
diffusional limited current region much later than GAC and CC
(Fig. 5), and corresponded with the results obtained at the fixed
Rext (Fig. 3b) [56]. This ascertained our hypothesis that the
novel architecture of ACNF promoted a better biofilm growth
and an efficient substrate transfer on ACNF surfaces to achieve
higher power densities. However, the power generation of
SCMFCs with ACNF as the anode was only 20e40% higher than
those with CC and GAC as anodes. The possible reason could
-0.25
CC
GAC
ACNF
Potential (V vs. Ag/AgCl)
-0.3
-0.35
-0.4
-0.45
-0.5
0
5
10
15
-
20
25
30
Current Density (A*m 3)
Fig. 5 e Linear sweep voltammetry (LSV) scans for the anode materials (ACNF, GAC, and CC) after 5-week operational period.
Mean values (standard deviation not included) of two independent runs, performed on duplicate MFC systems operated
under similar conditions.
1596
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 5 8 8 e1 5 9 7
be the heterogeneity of electronic active biofilms, which were
composed of more or less active fractions of microbial consortia and their distance from the electrode surfaces. This
reveals that novel MFC configurations should be developed to
fully bring out the advantages of ACNF as anodes.
3.5.
COD removal with different anode materials
The SCMFCs with ACNF exhibited higher COD removal efficiency (85 4%) than those of GAC (75 5%) and CC (70 2%)
(Table 2). Previous studies using carbon-based electrodes
(such as CC and GAC) had the COD removal efficiencies of
65e75% [38,41,43]. ACNF had a better performance due to the
good contact between the biofilms and the nanofiber network,
which resulted in the better access of the organic substances
to the microbial consortium and more efficient degradation.
The SCMFCs with the ACNF had lower CE values (8 2%)
than those with GAC (15 4%) and CC (11 1%) (Table 2).
CE and COD removal are inversely related (Eq. (3)). Previous
studies showed that higher COD removal efficiencies (%)
and lower CE values (%) were potentially caused by the
consumption of organic substrates by non-electrogenic
bacteria which divert the electron flow toward other metabolic processes [39,41,42,56], so that electrogenic bacteria were
unable to effectively obtain organic substrates. The high COD
removal efficiency with relatively higher power densities but
lower CE values of ACNF might indicate that even though
organic substrates were degraded in the wastewater by the
microbial communities including methanogens and fermentative microorganisms [25], the presence of electrogenic
microorganisms was constrained. This can be attributed to
the fact that domestic wastewater in the anode compartment
is heterogeneous in chemical composition and comprise
diverse microbial consortia, where numerous biochemical
reactions govern the bacterial metabolic pathways [25,40,42].
The relationship between COD removal and CE values is
of potential significance for further research, since the
degradation of organic substrates in MFCs is a biochemical
process dependent on microbial consortia, biofilm growth,
and functioning.
4.
Conclusions
This study compared the three anode materials (ACNF,
GAC and CC) with different morphologies, in terms of power
generation and COD removal in MFCs. As a good biofilm
support material, ACNF possessed high bioavailable surface
area and an interconnected network, which substantially
reduced the mass transfer resistance and in turn enhanced
electron transport. The results clearly demonstrated the
distinct advantages of ACNF for biofilm growth, power
generation, and COD removal. The significant findings include
(i) higher extents of biofilm growth on ACNF than GAC and
CC; (ii) higher power densities for ACNF and GAC than CC; and
(iii) higher COD removal efficiency for ACNF than that of
GAC and CC. Overall, the performance of ACNF as an MFC
anode in comparison to the other anode substrates has shown
a potential to develop and upscale novel MFC systems for
treating wastewaters.
Acknowledgments
The project was sponsored by the CBET program: 0933553
under the US National Science Foundation (NSF).
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