Efficient electricity production and simultaneously wastewater treatment via a high-performance photocatalytic fuel cell.

Abstract

A great quantity of wastewater were discharged into water body, causing serious environmental pollution. Meanwhile, the organic compounds in wastewater are important sources of energy. In this work, a high-performance short TiO(2) nanotube array (STNA) electrode was applied as photoanode material in a novel photocatalytic fuel cell (PFC) system for electricity production and simultaneously wastewater treatment. The results of current work demonstrate that various model compounds as well as real wastewater samples can be used as substrates for the PFC system. As a representative of model compounds, the acetic acid solution produces the highest cell performance with short-circuit current density 1.42 mA cm(-2), open-circuit voltage 1.48 V and maximum power density output 0.67 mW cm(-2). The STNA photoanode reveals obviously enhanced cell performance compared with TiO(2) nanoparticulate film electrode or other long nanotubes electrode. Moreover, the photoanode material, electrolyte concentration, pH of the initial solution, and cathode material were found to be important factors influencing the system performance of PFC. Therefore, the proposed fuel cell system provides a novel way of energy conversion and effective disposal mode of organics and serves well as a promising technology for wastewater treatment.

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Efficient electricity production and simultaneously
wastewater treatment via a high-performance photocatalytic
fuel cell
Yanbiao Liua, Jinhua Lia, Baoxue Zhoua,*, Xuejin Lia, Hongchong Chena,
Quanpeng Chena, Zhongsheng Wangb, Lei Lic, Jiulin Wangc, Weimin Caia
aSchool of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
bLaboratory of Advanced Materials, Fudan University, 2005 Songhu Road, Shanghai 200438, China
cSchool of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e i n f o
Article history:
Received 1 March 2011
Received in revised form
3 May 2011
Accepted 5 May 2011
Available online 12 May 2011
Keywords:
Photocatalytic fuel cell
TiO2nanotube array
Electricity production
Wastewater treatment
a b s t r a c t
A great quantity of wastewater were discharged into water body, causing serious envi-
ronmental pollution. Meanwhile, the organic compounds in wastewater are important
sources of energy. In this work, a high-performance short TiO2nanotube array (STNA)
electrode was applied as photoanode material in a novel photocatalytic fuel cell (PFC)
system for electricity production and simultaneously wastewater treatment. The results of
current work demonstrate that various model compounds as well as real wastewater
samples can be used as substrates for the PFC system. As a representative of model
compounds, the acetic acid solution produces the highest cell performance with short-
circuit current density 1.42 mA cm?2, open-circuit voltage 1.48 V and maximum power
density output 0.67 mW cm?2. The STNA photoanode reveals obviously enhanced cell
performance compared with TiO2nanoparticulate film electrode or other long nanotubes
electrode. Moreover, the photoanode material, electrolyte concentration, pH of the initial
solution, and cathode material were found to be important factors influencing the system
performance of PFC. Therefore, the proposed fuel cell system provides a novel way of
energy conversion and effective disposal mode of organics and serves well as a promising
technology for wastewater treatment.
ª 2011 Elsevier Ltd. All rights reserved.
1.Introduction
With the rapid increase in population and fast development of
industries in recent years, large amounts of organic waste dis-
charged into water bodies have caused serious environmental
pollution. In 2009, the total discharge amount of wastewater
in China was w58.92 billion tons and the relevant amount
of chemical oxygen demand (COD) reached w12.77 million
tons. Meanwhile, the organic compounds in wastewater are
important sources of energy (Feng et al., 2010; Kaneko et al.,
2010; Liu et al., 2010; Strataki et al., 2010). According to the
statistics(JapanEnergySociety,2002),theenergyofthebiowaste
in the environment has reached 130 EJy?1, corresponding to
a third of the global energy demand of 450 EJy?1. Therefore,
finding methods for efficient recovery of the chemical energy
and rapid decomposition of the organic matters in wastewater
comprise the highest priority for future wastewater treatment
technologies.
* Corresponding author. Tel./fax: þ86 21 5474 7351.
E-mail address: zhoubaoxue@sjtu.edu.cn (B. Zhou).
0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.watres.2011.05.004
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
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Microbial fuel cell (MFC) was once considered the best
proposal for wastewater treatment, which can use bacteria as
the catalysts to produce electricity and oxidize organic
matters (Logan et al., 2006; Wang et al., 2009). The greatest
potential of MFC lies in the use of wastewater as a substrate
(fuel), which breaks the traditional concept of sewage treat-
ment and allows combining wastewater treatment with
power generation. However, the electron transfer process
within MFC devices involves complicated mechanism among
different cells or cell systems, which directly lead to poor cell
performance (Qian et al., 2010). Furthermore, MFC also
possess disadvantages such as complex operation, bacteria
cultivation, long start-up time, and stringent working condi-
tions (Logan et al., 2006).
TiO2-based photocatalytic oxidation is a promising and
efficient process that can be used to degrade various recalci-
trant organic pollutants (Fujishima et al., 2008; Kim and Choi,
2010; Zhang et al., 2008). Upon UV illumination, the electrons
are excited from the valence band to the conduction band of
TiO2, generating electron/hole pairs. The positive holes are
powerful oxidants for degrading organic compounds into CO2
and H2O, and the negative electrons are powerful reductants
(Choi et al., 2010).
By substituting the slow and complex biochemical electron
transfer process in traditional MFC with the fast and direct
transportation process of photogenerated electrons in pho-
tocatalysis (i.e. substituting the microbial anode of MFC with
the TiO2photoanode), the TiO2photocatalytic process may
degrade organic matter and produce photogenerated elec-
trons that pass through the conductive substrate to the
cathode. In turn, this forms a TiO2-based photocatalytic fuel
cell (PFC) system and the chemical energy of organics can be
transformedintoelectricity
compounds degradation at the expense of incident light.
Different from traditional MFC, the generation of electrons in
the PFC system comes from photoexcitation, which is a much
fast and direct process. It is, therefore, possible to generate
electricityandsimultaneous
compounds via the PFC system. However, the research on PFC
system was in its infancy, there are still many problems need
to be solved or improved and the most crucial factor was
remaining the photoanode material.
accompaniedwithorganic
decompositionoforganic
The properties of functional materials are highly depen-
dent on their microstructure. Recently, the highly ordered
TiO2nanotube array (TNA) fabricated by anodization of tita-
nium in HF or [F?]-based electrolyte has attracted much
attention for its peculiar architecture and remarkable prop-
erties (Grimes, 2007; Mor et al., 2006). Within nanotubular
microstructures, vertically oriented TiO2nanotubes directly
grown on the electrically conductive Ti substrate, forms
a natural Schottky-type contact and provides an unidirec-
tional channel for the rapid transport of photogenerated
electrons. Lots of work can be found regarding the photo-
catalytic applications of TNA into organic compounds degra-
dation (Liu et al., 2008; Xu et al., 2006). Nevertheless, most
studies have mainly focused on the efficiency and the extent
of mineralization. An equally important aspect of photo-
catalysis, energy recovery, has not received much attention.
Preparation by sonoelectrochemical anodization of a short,
robust, and highly ordered TiO2nanotube array (STNA) with
superior electron transfer performance and excellent mechan-
icalstabilityhasbeenreported(Liu etal., 2009).Inthiswork,the
high-performanceSTNAelectrodewasappliedasaphotoanode
material in the novel fuel cell system for electricity yield
and simultaneously wastewater treatment. Various model
compounds and actual wastewater were investigated using the
fuel cell system. The results of current work demonstrate that
Fig. 1 e Continuous cyclic voltammograms (CV) of STNA electrode (curve 1 and 10) and TiO2nanoparticulate film electrode
(curve 2 and 20) over five cycles in 0.1 molLL1Na2SO4(a) under UV illumination and (b) in the dark, respectively. The inset is
the SEM image of typical STNA (a) and conventional TiO2nanoparticulate film (b), respectively.
Fig. 2 e Schematic diagram of working principle of the
STNA-based PFC system. The photogenerated electrons
flow from the conduction band of TiO2nanotubes and the
holes move toward the surface to generate proton. CB and
VB refer to the energy levels of the conduction and valence
band of TiO2.
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the PFC system provides a novel way of energy conversion and
a much effective disposal mode of organics.
2. Materials and methods
2.1.Materials
Titanium sheets (0.25 mmthick, 99.9% purity) were supplied by
Sumitomo Chemical (Japan). Unless otherwise indicated,
reagents were obtained from the Sinopharm Chemical Reagent
Company and were used as received. The detailed information
regardingtheactualwastewaterispresentedinSupplementary
data. Natural urine was collected from male students in
Shanghai Jiao Tong University directly after release.
2.2.
cathode
Preparation of STNA photoanode and Pt-black/Pt
Details of the preparation of STNA have been published in
previous work (Liu et al., 2009). The preparation of Pt-black/Pt
is carried out by cathodic polarization in the electrolyte
solution containing 30 kgm?3H2PtCl6 and 0.2e0.5 kgm?3
Pb(CH2COOH)2at a constant current density (10e40 mAcm?2)
and at room temperature within 20 min. For comparison,
a conventional TiO2 nanoparticulate film was prepared by
screen printing technique according to the research work of
our co-worker (Wang et al., 2004).
2.3.Apparatus and methods
The currentevoltage characteristics of the PFC system were
studied using a CHI electrochemical analyzer (CHI 660C, CH
Instruments, Inc., USA) in a two-electrode system, with a TiO2
photoanode and a Pt-black/Pt cathode. All runs were repeated
at least three times at ambient temperature, to check their
reproducibility. The circuit current was calculated from the
voltage across the external resistance (1 U) which was
continuously recordedusing
analyzer. The output voltage of the PFC was directly measured
by a high-precision digital multimeter (Victor 98A, Shenzhen
Victor Hi-tech Co., Ltd.). The COD value of the sample was
the CHIelectrochemical
determined via our previously reported thin-cell technology
(Zhang et al., 2009).
3. Results and discussion
3.1. Characterization of the photoanode materials
A low magnification SEM image of the titania nanotube array
obtained by sonoelectrochemical anodization of titanium in
Table 1 e System currentevoltage characteristics of the PFC system using various model compounds and actual
wastewater samples as substrate.
Organic compoundsVoc(V)Jsc(mAcm?2)JVmax(mWcm?2) FF
Model compoundNa2SO4(0.1 mol L?1)
Glucose (0.05 mol L?1)
Glutamic acid (0.05 mol L?1)
Nicotinic acid (0.05 mol L?1)
Acetic acid (0.05 mol L?1)
Urea (0.05 mol L?1)
Ammonia (0.05 mol L?1)
1.13
1.28
1.34
1.39
1.48
1.41
1.24
0.35
0.83
1.08
0.61
1.42
0.91
0.72
0.12
0.38
0.51
0.30
0.67
0.51
0.37
0.31
0.36
0.35
0.35
0.32
0.40
0.41
Actual wastewatera
Pharmaceutical wastewater (COD ¼ 24,572 mg L?1)
Petroleum exploiting wastewater (COD ¼ 19,087 mg L?1)
Dying wastewater (COD ¼ 10,842 mg L?1)
Chemical plant wastewater (COD ¼11,700 mg L?1)
Original urine solution (COD ¼9642 mg L?1)
0.88
1.34
1.53
1.11
0.93
1.36
0.98
1.21
0.99
0.61
0.43
0.34
0.50
0.30
0.19
0.36
0.26
0.27
0.27
0.34
a The specific information of various actual wastewater samples are given in Supplementary data.
Fig. 3 e System currentevoltage characteristic (a) and JV
product (b) of STNA-based PFC in the presence of an
electrolyte solution composed of 0.05 molLL1acetic
acidD0.1 molLL1Na2SO4solution under UV illumination.
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HFeH2O electrolyte is shown in the inset of Fig. 1a, which
reveals a regularly arranged tube structure with nanotubes
w65 nm in diameter and w280 nm in length (Liu et al., 2009).
Under ultrasonic wave irradiation, increased mass transfer
results in a layer of highly ordered and robust nanotube film
with decreased tube length and enhanced mechanical
stability. The TiO2nanoparticulate film possesses a sponge-
like structure with nanoparticles
(Fig. 1b and Supplementary data). Fig. 1a presents the
continuous cyclic voltammograms (CV) of STNA electrode and
TiO2 nanoparticulate film electrode over five cycles in
0.1 molL?1Na2SO4as a function of applied potential under UV
illumination. By comparing the CV curves, the STNA electrode
w23 nmindiameter
reveals obviously enhanced photocurrent response and the
saturated photocurrent density of STNA electrode is w1.63
times as high as that for TiO2nanoparticulate film electrode
under the same irradiation intensity. Since within the tradi-
tional TiO2nanoparticulate film electrode, most nanoparticles
are not in direct contact with glass support, resulting in easy
recombination of photogenerated charges and poor mechan-
ical stability of the electrode material (Zhu et al., 2007). While
the nanotubular microstructures are perpendicular to the
electrically conductive Ti substrate, forming a Schottky-type
contact naturally and providing an unidirectional electric
channel for the transport of photogenerated electrons (Mor
et al., 2006). Furthermore, the reproducibility of the CV
curves is much better for the nanotubular electrode. Here, the
five continuous CV curves nearly coincided with each other,
indicating excellent stability of the STNA electrode, the satu-
rated photocurrent of five continuous CV curves remains
nearly constant over the flat stage (0.5e1.8 V). In contrast,
continuous CV measurements of the traditional TiO2nanofilm
electrode indicate poor reproducibility and undesirable char-
acteristics with repeated excursions of the saturated photo-
current over time. The observed dark current for both samples
may be considered negligible (Fig. 1b).
3.2. Working principle of the PFC system
The process involving TiO2-based photocatalytic degradation
of organic compounds in aqueous media concerns energy
yield and energy conversion routes. Upon UV irradiation, the
STNA electrode produces electron/hole pairs in the conduc-
tion band and valence band, respectively. The photogenerated
holes oxidize the organic substance R to R0. Photogenerated
electrons move through an external circuit to the cathode.
Hydrogen ions generated from photooxidation move toward
the cathode by diffusing through the electrolyte solution.
These are either reduced by externally arriving electrons
producing molecular hydrogen under anaerobic conditions, or
interact with oxygen and produce water under aerobic
conditions. In both cases, an electric current flows between
the anode and the cathode. This reaction system is very
attractive, because it leads to energy yield and water cleaning.
Under O2atmosphere, the theoretical maximum voltage of
the present fuel cell system can be obtained according to the
calculated value based on the redox potential of organic
Fig. 4 e Four STNA-based PFC systems in series to light
a LED indicative lamp under UV illumination using
0.05 mol LL1acetic acidD0.1 molLL1Na2SO4solution as
substrate.
Fig. 5 e (a) The variation of current of the PFC system by applying 0.05 molLL1acetic acid as substrate under UV
illumination. Inset is the COD removal performance of acetic acid over 4 h. (b) The variation of the output voltage of the PFC
system.
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compounds (R0/R) and the redox potential for O2/H2O. The net
effect of the system is degradation of organics through the
STNA photocatalytic process to produce electrical energy. The
main working principle of the PFC system is presented in Eqs.
(1)e(3) and Fig. 2.
At the photoanode,
TiO2þ hn/hþþ e?
(1)
R þ nhþ/R0þ nHþ
At the cathode,
(2)
nHþþn
4O2þ ne?/n
2H2O
?4q¼ 1:23 V?
(3)
3.3.Electricity generation from organic wastewaters
Table 1 lists the system currentevoltage (JeV) characteristics
of STNA-based PFC using various model compounds and
actual wastewater samples as substrate under aerobic
conditions. Fill factor (FF) is the ratio of a real maximum
electric power output per theoretical maximum power output
and is calculated by the following equation (O’Regan and
Gra ¨tzel, 1991):
FF ¼JVmax
JscVoc
(4)
where JVmaxis the maximum power density (mWcm?2) yield
by the fuel cell system as obtained from the JV vs. V plot. Jsc
and Voc are short-circuit current density and open-circuit
voltage of the PFC system, respectively. The FF then gives
the extent of diversion between the actual maximum power
density that can be produced by the fuel cell system and the
product of Jsc$Voc.
The pure electrolyte with no organic additive presents the
smallest Voc, Jsc, and JVmax. In this case, oxygen was produced
at the photoanode (Eq. (5)), and a concentration cell of oxygen
was formed in the PFC system.
2H2O ? 4e?/4?Hþ?þ O2[
However, the presence of organic materials resulted in the
production of much higher photocurrents as compared with
their absence. Among the group of model compounds, acetic
acid(pH¼2.65)producesthebestperformance(seeFig.3),with
Jsc, Voc, and JVmaxof 1.42 mAcm?2, 1.48 V and 0.67 mWcm?2,
respectively. This may be partly due to the simple molecular
structure of acetic acids, which can be easily degraded by the
photocatalytic process of TiO2. Other complex organics might
necessitate more oxidative steps to achieve complete decom-
position and mineralization. Furthermore, from the viewpoint
of standard electrodepotential,the theoretical redox potential
of acetate (CH3COOH/CO2) is ca. 4q¼?0.4 V (see Eq. (2)), which
is much lower that the theoretical redox potential of O2/H2O
(4q¼ 1.23 V). Therefore, the acetic acid molecule can be readily
oxidized by photogenerated holes and produce desirable cell
performance. Human wastes, such as ammonia, can also be
successfully photodecomposed by the TiO2nanotubular elec-
trode and present JeV characteristics. Although urea is
a refractory organic, it still produces comparable Jscand JVmax,
indicatingthatureamoleculescanbefurtherdecomposedinto
NH3or [NO3]?.
To examine the applicability of the PFC system to actual
wastewater produced in everyday life, five different types of
real wastewater were investigated under UV illumination and
aerobic conditions. Various samples of actual wastewater
generated electrical power even under low UV intensity
(3.8 mWcm?2), demonstrating that the chemicals within the
wastewater samples can be removed by the PFC system.
However, there is no specific regularity between the cell
performance of actual wastewater samples and their corre-
sponding COD (or salinity, etc.), which might be ascribed to
the complex composition of the actual wastewater. An orig-
inal urine sample (without the addition of electrolyte) also
generated electric power, although the Jsc, Vocand JVmaxwere
much lower than those of other samples. The experimental
data in Table 1 provides compelling support for the suitability
of STNA-based PFC for producing electrical energy while
?4q¼ 1:23 V?
(5)
Table 2 e System currentevoltage characteristics of 0.1 molLL1Na2SO4and different model compound solutions as
affected by the photoanode materials.
Model compoundsPhotoanode materialVoc(V)Jsc(mAcm?2)Jsc/Jsc
JVmax(mWcm?2) JVmax/JVmax
Acetic acidSTNA
TiO2film
1.48
1.36
1.42
0.45
3.150.67
0.47
1.43
Glucose STNA
TiO2film
1.28
1.32
0.83
0.052
16.0 0.38
0.06
6.23
Urea STNA
TiO2film
1.41
1.44
0.91
0.11
7.970.51
0.13
3.92
Table 3 e System currentevoltage characteristics of 0.05 molLL1acetic acidD0.1 molLL1Na2SO4as affected by the
morphological structure of TNA.
L (nm)D (nm)W (nm)GVoc(V)Jsc(mAcm?2)JVmax(mWcm?2)
STNA
Medium TNA
Long TNA
280
1500
19,400
65526.27
75.02
500.47
1.48
1.56
1.63
1.42
1.04
0.65
0.67
0.31
0.28
100
220
15
20
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simultaneously photodecomposition of a large variety of
organic waste.
To demonstrate visually the electrical energy generation,
an example of 0.05 molL?1acetic acid could light a LED
indicative lamp by applying four PFC devices in series under
UV illumination, as shown in Fig. 4, since the excitation
voltage of the LED indicative lamp requires w4 V. It can be
predicted that the present PFC system can respond to visible
light and give enhanced cell performance by modifying the
STNA photoanode with existing modification technologies
(e.g. by depositing noble metal on its surface, sensitizing it
with dyes, or doping it with transition metal elements or
complexes with matching semiconductors). This work is
currently under study in our group and the results will be
presented in a future publication.
In order to estimate the COD removal performance of the
substrates in the PFC system, acetic acid (0.05 molL?1) was
selectedas a typical organic compound. Fig. 5a (inset)presents
the COD removal curve via the novel PFC system. It is evident
that the COD removal of acetic acid followed the L-H model
satisfactorily and w35% was mineralized with 4 h. The
Coulombic efficiency, EC, is defined as the ratio of total
Coulombs actually transferred to the anode from the
substrate, to maximum possible Coulombs if all substrate
removal produced current. The total Coulombs obtained is
determined by integrating the current over time, so that the
Coulombic efficiency for the proposed PFC evaluated over
a period of t can be calculated as (Logan et al., 2006):
EC¼
M
Zt
0
Idt
FbvDCOD
(6)
where M¼32, the molecular weight of oxygen, F is Faraday’s
constant, b ¼4 is the number of electrons exchanged per mole
of oxygen, v is the volume of liquid in the reaction system, and
DCOD is the change in COD over time t. In the above case,
w35% of the acetic acid was degraded over 4 h reactions and
the total Coulombs actually transferred to the anode from the
substrate was measured to be 17.2 C by integrating the cur-
rentetime curve shown in Fig. 5a, which leading to ECequals to
w15%. The ECof the employed PFC system was relatively low,
and the main reason can be ascribed to the high COD load in
the substrate solution. Moreover, the EC would have been
better if, for instance, the distance between the two electrodes
was shorter, the higher light intensity adopted, larger photo-
anode areaexposedto illumination, and lower CODloadin the
substrate. All these factors affect cell performance and the
organic pollutants removal.
The change of output voltage of the composite system with
UV illumination is shown in Fig. 5b. The output voltage
increased sharply initially, and then decreased slowly. The
initial output voltage increment was attributed to the appli-
cation of UV illumination contributed positively to the output
voltage. During the following voltage decay, this can be
ascribed to the generation of photogenerated electrons and
the removal of substrates in the reaction system. Moreover,
with the aim to examine the stability of the PFC system, the
output voltage was measured over 3 repeated PFC cycles
(Supplementary data). It is evident that the reproducibility of
the output voltage curves is excellent for the PFC system.
3.4. Parameters affecting the PFC performance
3.4.1.
The effect on PFC performance of different photoanode mate-
rials was tested under an O2atmosphere with the same initial
concentration of aceticacid, glucose,and urea(0.05 molL?1), as
shown inTable 2. The results ofcurrent work demonstrate that
the STNA electrode evidently enhanced cell performance
compared with TiO2nanoparticulate film. The Jscand JVmax
produced by STNA electrode are found to be 3.15e16.0 (Jsc/Jsc)
and 1.43e6.23 JVmax/JVmax times higher than those of the
respective TiO2nanoparticulate electrode in various organic
compounds solutions. This result is line with the enhanced
photoelectrochemical performance of the nanotubular elec-
trode given in Fig. 1 and the reason can be ascribed to the poor
mechanical stability and high recombination rate of the TiO2
Effect of photoanode materials
Table 4 e System currentevoltage characteristics of STNA-based PFC in 0.05 molLL1acetic acid as affected by the
concentration of Na2SO4.
Concentration of Na2SO4(mol L?1)Voc(V)Jsc(mAcm?2)FF JVmax(mWcm?2)
0.00
0.01
0.05
0.10
0.50
1.16
1.37
1.42
1.48
1.46
0.18
0.51
0.67
1.42
1.25
0.31
0.32
0.43
0.32
0.33
0.064
0.22
0.41
0.67
0.60
Table 5 e System currentevoltage characteristics of
STNA-based PFC in 0.05 mol LL1acetic acidD0.1 molLL1
Na2SO4as affected by the pH of the initial solution.
Jsc(mAcm?2)
pHVoc(V) FF JVmax(mWcm?2)
2.65
3.96
5.12
7.13
1.48
1.38
1.27
1.22
1.42
1.24
0.92
0.90
0.32
0.30
0.34
0.33
0.67
0.51
0.40
0.37
Table 6 e System currentevoltage characteristics of
STNA-based PFC in 0.05 molLL1acetic acidD0.1 mol LL1
Na2SO4as affected by the cathode materials.
Jsc(mAcm?2)
Cathode
material
Voc(V) FFJVmax(mWcm?2)
Ti
Pt
Pt-black/Pt
1.19
0.96
1.48
0.22
0.41
1.42
0.13
0.14
0.32
0.035
0.065
0.67
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nanoparticulates electrode. As for different performances of
the same photoanode material in different substrates (organic
solutions), which may be ascribed to different molecular
structures of the organics and the relevant degradation
mechanisms still need further research.
The roughness factor, i.e. the physical surface area of the
film per unit of projected area, measures the internal surface
area of the electrode materials and is of crucial significance in
photochemical applications (Shankar et al., 2009). In a typical
TNA-based photocatalytic process, the amount of light har-
vested by the electrode film is directly related to the film
roughness factor. Herein, assuming an idealized nanotubular
structure of inner diameter D, wall thickness W and tube
length L, the purely geometric roughness factor G of the
nanotubes is calculated as:
G ¼
h
4pLfD þ Wg
.n ffiffiffi
3
p
ðD þ 2WÞ2oi
þ 1(7)
Table 3 summarizes the structural parameters of three
different kinds of TNA photoanodes (Supplementary data)
and compares their corresponding PFC characteristics in
0.05 mol L?1acetic acid and 0.1 mol L?1Na2SO4solution. The
medium TNA was prepared by anodization in a solution
containing 0.1 mol L?1KF, 1 mol L?1NaHSO4, and 0.2 mol L?1
trisodium citrate. Samples anodized at 20 V for 8 h achieved
a nanotube layer of w1.5 mm in length and w100 nm in
diameter. While samples anodized at 40 V for 50 h in dimethyl
sulfoxide (?99.8%) and 5 wt% HF mixture results much longer
nanotubes w19.4 mm in length and w220 nm in diameter. It is
evident that the Vocwas found increased with the increment
of roughness factor G (or tube length L), while the Jscand JVmax
were quite the contrary. Literatures (Liu et al., 2008; Zhang
et al., 2009) have also demonstrated that the increase in
length of the nanotubes may not contribute positively to the
performance of the electrode materials and the short nano-
tubes prepared in inorganic electrolytes (e.g. HFeH2O) were
reported to possess enhanced photochemical properties
contrarily. The increase in tube length (or tube diameter)
favors the increment in the surface area of TNA, which can
improve the light harvesting capability of the electrode
materials. However, this will also increase the transport
resistance for photogenerated electrons and the recombina-
tion rate between photogenerated charges. Moreover, the
increase in tube length will lead to decreased mechanical
stability of the electrode material and broken tubes and other
debris from the organic anodization bath will readily block
the top surface of the nanotube film (Paulose et al., 2006),
these will inevitably detrimental to the electron transfer
performance as well as the photochemical reactivity of the
electrode materials. Therefore, although the short tube
length of STNA electrode is not favorable for incident light
absorption, the superior electron transfer properties, low
recombination rate and excellent mechanical stability make
it an ideal electrode material for application as a photoanode
material in PFC.
3.4.2.
In
concentration of the electrolyte solution is of significant
importance. In this study, sodium sulphate was selected as
Effect of electrolyte concentration
typical photocatalytica process, the selection and
the electrolyte solution and varying levels of concentration (0,
0.01, 0.05, and 0.1 mol L?1) were used to study the influence of
electrolyte concentration (Table 4). Due to the fact that acetic
acid can be easily used, this substance was chosen for the rest
of the experiments. The system performance of the fuel cell
increased with increasing electrolyte concentration from
0.0e0.1 molL?1. The maximum power densities were 0.064,
0.22, 0.41, and 0.67 mWcm?2for the electrolyte concentra-
tions of 0.0, 0.01, 0.05 and 0.1 mol L?1, respectively. This
phenomenon is explained by the increasedconductivity of the
solutions, which contributes positively to the photogenerated
electron transportbetween the TiO2/solution interface.
Further increase of the electrolyte solution to 0.5 molL?1leads
to a decrement of system performance. Therefore, there is an
optimum electrolyte concentration applied in the PFC system.
3.4.3.
The pH of a solution is one of the factors known to influence
the rate of degradation of organic compounds in the photo-
catalytic process. As such, pH is also an important operational
parameterin wastewater treatment. Table 5 demonstrates the
influence of pH on STNA-based PFC system using different pH
levels (2.65, 3.6, 5.12 and 7.13). The maximumpower density of
the present system decreased with the increase of the reac-
tion solution pH. The photocatalytic process of acetic acid
performed at pH values of 2.65 and 7.13 resulted in a reduction
of the maximum power density from 0.67 to 0.37. In strong
acid conditions, the surface of the nanotubular material was
positively charged and was favorable for the degradation of
negatively charged donors (acetic acid).
Effect of pH of initial solution
3.4.4.
To estimate the effect of cathode materials on system
performance, the present work compared JV values and
maximum power density of the fuel cell system made with
three different kinds of cathodes (Table 6). The experimental
results indicate that the cathode had a major influence on
system efficiency. Pure titanium foil and pure Pt both gave
much poorer performance than the Pt-black coated Pt elec-
trode. The marked increase of overall efficiency could be
attributed to the large active site produced by Pt-black, which
favors O2 adsorption as well as the transport of photo-
generated electrons.
Effect of cathode material
4.Conclusion
In summary, various model compounds and actual waste-
water samples were applied as “fuel” in a novel PFC system,
consisting of high-performance STNA photoanode and Pt-
black/Pt cathode, to produce electricity at the expense of UV
light. The STNA photoanode evidently enhanced cell perfor-
mance compared with TiO2nanoparticulate film electrode as
well as other long nanotubes electrode. Various parameters
affecting the PFC performance were studied. The STNA-based
PFC system serves well as a promising technology for waste-
water treatment and the present fuel cell system could utilize
visible light in the near future by combining with appropriate
low band-gap semiconductors to generate electrical power
under solar light illumination.
water research 45 (2011) 3991e3998
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Author's personal copy
Acknowledgement
The authors would like to acknowledge the Science and Tech-
nology Commission of Shanghai Municipality (0952nm01800,
08JC1411300), the State Key Development Program for Basic
Research of China(Grant No.2009CB220004),theNational High
Technology Research and Development Program of China
(Grant No. 2009AA063003), and Shanghai Tongji Gao Tingyao
Environmental Science and Technology Development Foun-
dationforfinancialsupport.TheauthorswouldliketothankDr.
Xianwei Liu and Dr. Xin wang for their valuable technical
discussions.
Appendix. Supplementary data
Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.watres.2011.05.004.
r e f e r e n c e s
Choi, W., Yeo, J., Ryu, J., Tachikawa, T., Majima, T., 2010.
Photocatalytic oxidation mechanism of As(III) on TiO2: unique
role of As(III) as a charge recombinant species. Environ. Sci.
Technol. 44, 9099e9104.
Feng, Y.J., Lee, H., Wang, X., Liu, Y., He, W., 2010. Continuous
electricity generation by a graphite granule baffled air-cathode
microbial fuel cell. Bioresour. Technol. 101, 632e638.
Fujishima, A., Zhang, X.T., Tryk, D.A., 2008. TiO2photocatalysis
and related surface phenomena. Surf. Sci. Rep. 63, 515e582.
Grimes, C.A., 2007. Synthesis and application of highly ordered
arrays of TiO2nanotubes. J. Mater. Chem. 17, 1451e1457.
Japan Energy Society, 2002. Biomass Handbook. Ohmusha, Tokyo.
Kaneko, M., Suzuki, S., Ueno, H., Nemoto, J., Fujii, Y., 2010.
Photoelectrochemical decomposition of bio-related
compounds at a nanoporous semiconductor film photoanode
and their photocurrent-photovoltage characteristics.
Electrochim. Acta 55, 3068e3074.
Kim, J., Choi, W., 2010. Hydrogen producing water treatment
throughsolarphotocatalysis.EnergyEnviron.Sci.3,1042e1045.
Liu, X.W., Sun, X.F., Huang, Y.X., Sheng, G.P., Zhou, K., Zeng, R.J.,
Dong, F., Wang, S.G., Xu, A.W., Tong, Z.H., Yu, H.Q., 2010.
Nano-structured manganese oxide as a cathodic catalyst for
enhanced oxygen reduction in a microbial fuel cell fed with
a synthetic wastewater. Water Res. 44, 5298e5305.
Liu, Y.B., Zhou, B.X., Li, J.H., Gan, X.J., Bai, J., Cai, W.M., 2009.
Preparation of short, robust and highly-ordered TiO2nanotube
arrays and their applications as electrode. Appl. Catal. B:
Environ. 92, 326e332.
Liu, Z.Y., Zhang, X.T., Nishimoto, S., Jin, M., Tryk, D.A.,
Murakami, T., Fujishima, A., 2008. Highly ordered TiO2
nanotube arrays with controllable length for
photoelectrocatalytic degradation of phenol. J. Phys. Chem.
C 112, 253e259.
Logan, B.E., Hamelers, B., Rozendal, R.A., Schroder, U., Keller, J.,
Freguia, S., Aelterman, P., Verstraete, W., Rabaey, K., 2006.
Microbial fuel cells: methodology and technology. Environ.
Sci. Technol. 40, 5181e5192.
Mor, G.K., Varghese, O.K., Paulose, M., Shankar, K., Grimes, C.A.,
2006. A review on highly ordered, vertically oriented TiO2
nanotube arrays: fabrication, material properties, and solar
energy applications. Sol. Energy Mater. Sol. Cells 90,
2011e2075.
O’Regan, B., Gra ¨tzel, M., 1991. A low-cost, high-efficiency solar
cell based on dye-sensitized colloidal TiO2films. Nature 353,
737e740.
Paulose, M., Shankar, K., Yoriya, S., Prakasam, H.E., Varghese, O.
K., Mor, G.K., Latempa, T.A., Fitzgerald, A., Grimes, C.A., 2006.
Anodic growth of highly ordered TiO2nanotube arrays to
134 mm in length. J. Phys. Chem. B 110, 16179e16184.
Qian, F., Wang, G.M., Li, Y., 2010. Solar-driven microbial
photoelectrochemical cells with a nanowire photocathode.
Nano Lett. 10, 4686e4691.
Shankar, K., Basham, J.I., Allam, N.K., Varghese, O.K., Mor, G.K.,
Feng, X.J., Paulose, M., Seabold, J.A., Choi, K.S., Grimes, C.A.,
2009. Recent advances in the use of TiO2nanotube and
nanowire arrays for oxidative photoelectrochemistry. J. Phys.
Chem. C 113, 6327e6359.
Strataki, N., Antoniadou, M., Dracopoulos, V., Lianos, P., 2010.
Visible-light photocatalytic hydrogen production from
ethanolewater mixtures using a PteCdSeTiO2photocatalyst.
Catal. Today 151, 53e57.
Wang, Z.S., Kawauchi, H., Kashima, T., Kashima, T., Arakawa, H.,
2004. Significant influence of TiO2photoelectrode morphology
on the energy conversion efficiency of N719 dye-sensitized
solar cell. Coord. Chem. Rev. 248, 1381e1389.
Wang, X., Cheng, S.A., Feng, Y.J., Merrill, M.D., Saito, T., Logan, B.
E., 2009. Use of carbon mesh anodes and the effect of different
pretreatment methods on power production in microbial fuel
cells. Environ. Sci. Technol. 43, 6870e6874.
Xu, C., Killmeyer, R., Gray, M.L., Khan, S.U.M., 2006. Photocatalytic
effect of carbon-modified n-TiO2nanoparticles under visible
light illumination. Appl. Catal. B: Environ. 64, 312e317.
Zhang, J.L., Zhou, B.X., Zheng, Q., Li, J.H., Bai, J., Liu, Y.B., Cai, W.M.,
2009. Photoelectrocatalytic COD determination method using
highly ordered TiO2nanotube array. Water Res. 43, 1986e1992.
Zhang, M., Chen, C.C., Ma, W.H., Zhao, J.C., 2008. Visible-light-
induced aerobic oxidation of alcohols in a coupled
photocatalytic system of dye-sensitized TiO2and TEMPO.
Angew. Chem., Int. Ed. 47, 9730e9733.
Zhu, K., Neale, N.R., Miedaner, A., Frank, A.J., 2007. Enhanced
charge-collection efficiencies and light scattering in dye-
sensitized solar cells using oriented TiO2nanotubes arrays.
Nano Lett. 7, 69e74.
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