Accepted Manuscript Title: Improvement of power generation of microbial fuel cell by integrating tungsten oxide electrocatalyst with pure or mixed culture biocatalysts Author: Jhansi L. Varanasi Arpan K. Nayak Youngku Sohn Debabrata Pradhan Debabrata Das PII: DOI: Reference:
S0013-4686(16)30723-X http://dx.doi.org/doi:10.1016/j.electacta.2016.03.152 EA 26982
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
16-1-2016 9-3-2016 25-3-2016
Please cite this article as: Jhansi L.Varanasi, Arpan K.Nayak, Youngku Sohn, Debabrata Pradhan, Debabrata Das, Improvement of power generation of microbial fuel cell by integrating tungsten oxide electrocatalyst with pure or mixed culture biocatalysts, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.152 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical Abstract (for review)
Figure(s)
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Table(s)
Table 1: Comparison of power densities obtained with different anode catalysts.
Electrocatalyst
Biocatalyst
Base
Power density (W m-2)
Reference
PANI/MWCNT*
Shewanella putrefaciens E.coli E.coli
Graphite felt
0.25
[24]
Carbon felt Carbon felt
0.98 0.76
[15] [15]
Activated carbon fibre Carbon felt
1.14
[25]
1.05
[26]
Glassy carbon electrode Carbon paper
1.42
[11]
0.83
[27]
PANI/mWO3* mWO3
Ni/carbon microE.coli nanofibres 16.7 wt % E.coli Mo2C/carbonnanotubes MWCNTs/SnO2* E.coli Fe3O4/CNT* Ni/β-Mo2C Graphene Fluorinated polyanilines Carbon nanotube /polyaniline Polypyrrole/ carbon nanotubes Polyaniline/three dimensional graphene Graphene WO3 WO3 *
Pt / WO3 Pt/ WO3
E.coli Klebsiella pneumoniae E.coli
#
Carbon felt
4.67
[17]
2.66
[28]
E.coli
Stainless steel mesh Platinum sheet
ND*
[29]
E.coli
Nickel foam
0.04
[18]
E.coli
Carbon paper
0.22
[12]
Shewanella oneidensis MR-1 Mixed culture
Nickel foam
0.19
[30]
Graphite plates
0.67
[31]
Shewanella putrefaciens Mixed culture
Carbon felt
1.28
Carbon felt
0.82
Shewanella putrefaciens Mixed culture
Carbon felt
1.47
Carbon felt
1.16
*PANI:Polyaniline MWNT:Multi-walled carbon nanotube WO3: Tungsten oxide Mo2C:Molybdenum carbide SnO2: Tin oxide Fe3O4:Iron oxide Ni: Nickel Pt: Platinum ND: No data available
#Power density in W m-3
Present study
Table 2: Overall performance of MFCs (for five multiple batch cycles).
MFC
Inoculum
OCV
Max
Max
Ipa
Ipc
RCT
Charge
Capaci
(V)
power
curren
(mA
(mA
(Ω)
(C)
tance
densit
t
y
densit
(mW
y
-2
cm )
-2
-2
cm )
cm )
CODr
CE
(%)
(%)
(F)
(mA cm-2)
Pure
0.57±0 0.049
0.10±
2.31±
-1.20
17.51
0.73±
1.29
70.24
15.94
culture
.013
0.05
0.75
± 0.50
±0.64
0.02
± 0.03
±2.13
±0.60
Mixed
0.51±0 0.043
0.097
0.79±
-0.82
23.31
0.36
0.56±
72.78
13.52
culture
.016
±0.01
±0.04
0.25
± 0.34
±0.81
± 0.01
0.01
±2.18
±0.56
Pure
0.69±0 0.082
0.18±
4.60±
-4.40
13.47
3.15
5.46±
69.30
32.41
culture
.019
0.09
1.51
± 1.84
±0.44
± 0.12
0.15
±2.04
±0.63
Mixed
0.62±0 0.076
0.15±
2.81±
-2.44
17.67
2.37±
3.61±
69.16
29.57
culture
.019
0.07
0.91
± 1.02
±0.51
0.08
0.10
±2.08
±0.58
Pure
0.87±0 0.15±
0.24±
8.20±
-7.81
10.92
6.54
8.35±
65.44
48.47
culture
.019
0.12
2.66
± 3.28
±0.40
± 0.24
0.23
±1.83
±0.98
Mixed
0.75±0 0.11±
0.21±
5.09±
-4.61
12.57
4.54
6.20±
66.09
45.55
culture
.015
0.10
1.65
± 1.93
±0.48
± 0.16
0.17
±1.86
±0.82
±0.02
MFCuncatalyzed
MFCwo3
±0.02
±0.04
0.03
MFCPt/WO3
0.05
Research Highlights
Highlights:
WO3 as an efficient electrocatalyst for microbial fuel cell
Effect of nanocatalyst depends on the type of biocatalyst used
Significant improvement in biocatalysis with WO3 modified electrodes
Amendment of WO3 improves capacitive properties of anodes
Pt/WO3 composites provide maximum power densities
*Revised Manuscript (including Abstract)
Improvement of power generation of microbial fuel cell by integrating tungsten oxide 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
electrocatalyst with pure or mixed culture biocatalysts
Jhansi L Varanasia, Arpan K Nayakb, Youngku Sohnc, Debabrata Pradhanb, Debabrata Dasa*
a
Department of Biotechnology, Indian Institute of Technology, Kharagpur 721302, India
b
Material Science Centre, Indian Institute of Technology, Kharagpur 721302, India
c
Department of Chemistry, Yeungnam University, Gyeongsan 712-749, Republic of Korea
*
Corresponding author: Tel: +91-3222-283758; Fax: +91-3222-255303,
E-mail:
[email protected] 1
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1
Abstract
2
The anode of microbial fuel cell was impregnated with tungsten oxide (WO3) and platinum-
3
tungsten oxide (Pt/WO3) nanocomposites to improve its power generation. The amended
4
anodes were tested against pure and mixed culture type of biocatalysts. Improved
5
performance was exhibited by the modified electrodes as compared to the uncatalyzed
6
electrodes using both biocatalysts. However, pure culture showed higher power outputs as
7
compared to the enriched mixed consortia. The maximum power density up to 0.15 mW cm-2
8
(1.46 W m-2) was obtained using pure culture which was almost 45% higher as compared to
9
uncatalyzed electrodes. The anode modification also helped in lowering the charge transfer
10
resistance and improving the coulombic efficiencies of the MFCs. High capacitance with
11
nanostructure catalysts implied their role in holding an electric charge while SEM and
12
epifluorescent images revealed enhanced bacterial adhesion. The high electrode conductivity,
13
stability, and biocompatibility of the modified anodes make them more attractive for practical
14
microbial fuel cell applications.
15 16 17 18 19 20 21 22
Keywords: tungsten oxide; electrocatalyst; biocatalyst; anode; microbial fuel cell.
23 24 2
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25
1. Introduction
26
Microbial fuel cell (MFC) is a promising technology that converts chemical energy present in
27
organic wastes directly into electricity. A major drawback of these systems is the low power
28
generation which needs to be substantially improved for commercialization of this process
29
[1]. Although several research work are focused towards the development of cathode
30
materials for MFC considering its catalytic limitations [2; 3; 4; 5], characteristics of bio-
31
anode is also considered to be significant as it directly affects the bacterial interaction,
32
electron transfer, and substrate oxidation [6]. Most of the electrogenic bacteria used in MFCs
33
form electroactive biofilm over the electrode surface to perform direct electron transfer [7].
34
These biofilm structures increase the distance of electrons that need to travel from e- donor to
35
e- acceptor [8]. At present, carbon-based anode materials (such as carbon paper, graphite,
36
carbon felt, etc.) are used widely in MFCs to act as an exogenous solid electron acceptor.
37
However, their low surface area fails to accommodate several microbial reactions occurring
38
at the anode surface [9].
39
Electrode modification by using nanostructures with high surface area is an attractive strategy
40
to improve the performance of MFCs [4; 5]. The catalytic mechanism occurring in the anode
41
involves a combination of biocatalysis and electrocatalysis [11]. The biocatalyst harvests
42
electrons from the organic wastes and transfers these electrons to the anode (which acts as a
43
terminal electron acceptor). These electrons are then traversed along the external circuit
44
reaching cathode where they are utilized in the reduction of O2 and H+ ions producing an
45
electric current in the process. Electrocatalysts on the other hand function at electrode
46
surfaces which catalyze the undergoing electrochemical reactions and increase the rate of the
47
reaction. The effective surface area of anode increases considerably with the help of nano-
48
electrocatalysts and this improves the contact with the biocatalyst. Thus, it is essential that the 3
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49
nanostructure chosen should have high biocompatibility with the host bacteria and help in
50
enhancing the electron transfer rate [5; 6; 7; 8].
51
Tungsten oxide (WO3) nanoparticles are known to have good biocompatibility and electric
52
conductivity due to which they are extensively used in biosensors, bio-imaging and
53
bioelectrochemical systems [16]. Recently, an anode electrocatalyst based on WO3 was
54
developed for MFC applications [15]. It was suggested that due to the rough surface of the
55
WO3 particles, bacterial colonization was favored that enhanced biofilm formation [8; 9]. So,
56
in the present study, WO3 nanoparticles were synthesized and fabricated as anode
57
electrocatalysts due to their good electrocatalytic property, low cost, biocompatibility and
58
non-toxicity while platinum-tungsten oxide (Pt/WO3) composites were used to compare the
59
performance of MFCs.
60
Most of the studies dealt with anode modification in MFCs used pure cultures such as E.coli,
61
Shewanella sp. Pseudomonas sp. etc. as biocatalyst [8; 10; 11; 12]. Use of pure cultures in
62
MFCs require stringent sterile conditions and thus increases the overall cost of the process.
63
Also, in a practical scenario of wastewater treatment, a diversity of the microbial population
64
is expected to be present that can alter the performance of modified anode [20]. This suggests
65
that anode modification studies with single species are not good enough to completely
66
understand the biocatalyst-nanocatalyst interactions in MFCs that might help in enhancing
67
electron transfer rates. Therefore, in the present study, an enriched mixed consortium was
68
used in an unsterile environment and compared with a pure culture (Shewanella sp.) in sterile
69
conditions to understand the effect of an electrocatalyst on different types of biocatalysts.
70
The overall objectives of the present study were to investigate the electrocatalytic properties
71
of WO3 and Pt/WO3 modified anodes in MFC and to compare the modified and unmodified
72
anodes with respect to biocatalyst (pure and mixed cultures). 4
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73
2. Materials and methods
74
2.1
75
Analytical grade of sodium tungstate dihydrate (Na2WO4.2H2O) (SRL, India); hydrochloric
76
acid (HCl) (Merck, India); oxalic acid (C2H2O4.2H2O) (Merck, India); ethanol (C2H5OH)
77
(ChangshuYangyuan Chemical, China), chloroplatinic acid hexahydrate (H2PtCl6.6H2O)
78
(Sigma-Aldrich, India) were used in the present studies.
79
2.2 Synthesis of WO3 nanoplates and Pt/WO3 nanoplates
80
WO3 flowers-like structures consisting of nanoplates were successfully synthesized by using
81
a hydrothermal method. 1.6 g sodium tungstate hydrate (0.12 M) was added to 40 mL
82
distilled water and stirred for few min. Then 4 mL of concentrated HCl (35% v/v) was added
83
dropwise to the above solution. The resulting solution turns yellow. Then 1 g oxalic acid (0.2
84
M) was added to above solution. The solution became colorless. The final colorless solution
85
was transferred to a 50 mL Teflon-lined stainless steel autoclave and sealed. The autoclave
86
was heated at 200 °C in a muffle furnace for 12 h and then cooled naturally to room
87
temperature. The precipitated product was collected by centrifuging. The powder was finally
88
washed with ethanol and distilled water and dried at 60 °C for 4 h. For the synthesis of Pt
89
nanoparticles decorated WO3 nanoplates (Pt/WO3), 1 mL of chloroplatinic acid hexahydrate
90
(0.05 M) was added additionally keeping other reaction parameters constant.
91
2.3 Characterization
92
The surface morphology of synthesized power was examined using a Carl Zeiss SUPRA 40
93
field-emission scanning electron microscope (FE-SEM). The structural property of the
94
samples was investigated with a PANalytical High-Resolution X-ray diffractometer (XRD)
95
(PW 3040/60) operated at 40 kV and 30 mA with Cu Kα X-rays (1.54 Å) in the 2 θ angle 20-
Chemicals
5
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96
80°. The detail microstructures of the samples were studied using an FEI TECNAI G2
97
transmission electron microscope (TEM).
98
2.4 NanostructuredWO3 and Pt/WO3 decorated anode preparation
99
Nanostructured WO3 and Pt/WO3 composites were used for the modification of anode in
100
MFCs. A requisite amount of nanoparticle (0.5 mg cm-2) was first dispersed in the acetone-
101
isopropyl alcohol (1:1) solution. This solution was then mixed with 1 % v/v
102
polytetrafluoroethylene (PTFE; Sigma-Aldrich) which acts as a binder. The overall mixture
103
was subsequently sprayed on a carbon felt (16 cm2; projected surface area) by a gravity spray
104
gun and dried at 70 °C in an oven for 6 h. The modified WO3 (or) Pt/WO3 composite loaded
105
carbon felts were then used as anodes in MFCs.
106
2.5 Inoculum and anolyte
107
Shewanella putrefaciens (ATCC® BAA1097TM) was used as inoculum for pure culture
108
experiments while an enriched mixed consortium was developed from fly ash leachate in the
109
laboratory [21]. This was used for mixed culture experiments. The anolyte comprised of
110
synthetic wastewater with the composition of NaHCO3 – 2.5 g L-1; KCl – 0.1 g L-1; NH4Cl –
111
1.5 g L-1; NaH2PO4 – 0.6 g L-1; vitamins and trace elements; pH adjusted to 7. Sodium acetate
112
was used as electron donor. Desirable chemical oxygen demand (COD) of 3 g L-1 was
113
maintained by altering the electron donor concentration in the medium.
114
2.6 MFC assembly construction and operation
115
Twelve identical single chambered MFCs (sMFCs) made up of polyacrylic material (working
116
volume 100 mL) were used for the experiments with an anode compartment and an air
117
cathode placed on opposite side. The anode consisted of a carbon felt of working surface area 6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
118
16 cm2 with a stainless steel wire welded to form the terminal. The membrane cathode
119
assembly was prepared by bonding the carbon ink coated anion exchange membranes
120
(RALEX™ AM-PES, Mega Inc.) onto a flexible stainless steel (SS) mesh (32 cm2). This SS
121
mesh was then attached on the air-facing side of MFC with the help of a conducting paint
122
(Siltech corp., India) serving as air cathode. Higher cathode surface area was maintained to
123
minimize cathodic losses. Based on the type of modification on the carbon felt anode, the
124
pure and mixed culture systems were divided as MFCuncatalyzed (unmodified), MFCWO3 (WO3
125
modified)
126
temperature (25±5 ºC).
127
2.7 Analytical measurements
128
Polarization and power density curves for MFCs were obtained by varying the external
129
resistance using a variable resistance box (range 1000 KΩ – 10 Ω) in discrete steps and
130
measuring the corresponding voltage drop. The current densities, power densities and
131
coulombic efficiencies were calculated as described previously [21]. Cyclic voltammograms
132
(CVs) of bioanode were recorded with a Potentiostat/Galvanostat system (Gamry Reference
133
600, United States of America) connected to personal computer at a scan rate of 1 mV s-1 and
134
a potential window of +0.6 V to -0.6 V. A three electrode configuration consisting of
135
bioanode (working), cathode (counter), and Ag/AgCl/ 3 M KCl (reference) was used for
136
electrochemical measurements. Electron transfer behavior of bioanode was studied by
137
electrochemical impedance spectroscopy (EIS). The EIS of bioanode was performed with a
138
three electrode configuration consisting of bioanode, platinum wire, and Ag/AgCl as
139
working, counter and reference electrode, respectively. EIS was done over a frequency range
140
of 100 kHz to 1Hz with a sinusoidal perturbation of 10 mV. The COD was measured by
141
APHA standard methods [22] using a COD measurement instrument (DRB200 & DR2800
and MFCPt/WO3 (Pt/WO3 composite modified). All the experiments were carried out at room
7
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142
Portable Spectrophotometer, HACH, USA). The percentage COD removal efficiency (CODr)
143
was calculated as
144
concentrations of COD in MFC. The standard deviations were obtained by averaging the
145
values obtained in five multiple batch experiments.
146
2.8 Microscopic study of the biofilm
147
2.8.1 Scanning Electron Microscopy (SEM)
148
To examine the surface morphology of the anode, the electrode samples of the different
149
experimental setups (MFCuncatalyzed, MFCWO3 and MFCPt/WO3) of both pure and mixed culture
150
systems were aseptically taken out and fixed with 0.02 %w/v picric acid in 0.1 M sodium
151
phosphate buffer (pH 7.2) for 5 min. The fixed samples were then subsequently dehydrated in
152
ethanol gradient of 40-100 %v/v for 5 min each. After dehydration step, the samples were
153
stored in a desiccator to remove trace amount of moisture present (if any). Gold sputtering
154
was carried out in HITACHI E-101 sputter coater maintained at 0.1 – 0.01 torr for uniform
155
coating. The SEM images of the prepared samples were obtained using JEOL JSM5800
156
scanning electron microscope with incident electron beam energy of 20 kV.
157
2.8.2 Fluorescence microscopy
158
In order to perform fluorescence microscopy study, cell fixation was performed with 4 %
159
(w/v) paraformaldehyde in 10 mM PBS at 4 ºC for overnight. The overnight fixed samples
160
were subsequently washed with PBS and stored at 10 mM PBS: ethanol (1:1) at -20 ºC. 10 µl
161
of 4,6 diamidino 2- phenylindole (DAPI, 1 µg/ml) was added to the same amount of fixed
162
sample and incubated for 15 min at room temperature in dark. Excess DAPI was removed by
163
washing the sample with 10 mM PBS. The DAPI stained samples were visualized using
, where
8
are the initial and final
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
164
Olympus model epifluorescence microscope equipped with UV filter set (excitation
165
wavelength 330-385 nm) at 40 X magnification.
166
3
167
3.1 WO3 structure and morphology
168
The morphology of prepared nano samples was investigated by using FE-SEM as shown in
169
Figure 1. FESEM image of WO3 shows flower-like spherical structures composed of
170
nanoplates (Figure 1a). A magnified FESEM image of flower-like spherical structure (inset
171
of Figure 1a) reveals the plate-like structure more clearly with 40-400 nm length/width and
172
20-60 nm thickness. Figure 1b shows FESEM images of Pt-decorated WO3 nanoplates. Pt
173
nanoparticles are found to be sparsely deposited on the WO3 nanoplates. The microstructure
174
of the prepared WO3 nanoplates and Pt/WO3 nanoplates was investigated by TEM. Figure 1c
175
represents the TEM image of WO3 nanoplates of either square or rectangle shape of size in
176
the range of 40-400 nm. Inset of Figure 1c shows the regular spot selected area diffraction
177
pattern (SAED) which confirms the single crystalline nature of WO3 nanoplates. Figure 1d
178
shows the TEM image of Pt/WO3 nanoplates. The Pt nanoparticle clusters are found to be
179
deposited on the nanoplates with individual Pt nanoparticles diameter
