A TiO2-nanotube-array-based photocatalytic fuel cell using refractory organic compounds as substrates for electricity generation.

Abstract

A TiO(2)-nanotube-array-based photocatalytic fuel cell system was established for generation of electricity from various refractory organic compounds and simultaneous wastewater treatment. The present system can respond to visible light and produce obviously enhanced cell performance when a narrow band-gap semiconductor (i.e. Cu(2)O and CdS) was combined with TiO(2) nanotubes.

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10314Chem. Commun., 2011, 47, 10314–10316This journal is c The Royal Society of Chemistry 2011
Citethis: Chem. Commun.,2011,47,10314–10316
A TiO2-nanotube-array-based photocatalytic fuel cell using refractory
organic compounds as substrates for electricity generationw
Yanbiao Liu,aJinhua Li,aBaoxue Zhou,*abHongchong Chen,aZhongsheng Wangcand
Weimin Caia
Received 8th June 2011, Accepted 20th July 2011
DOI: 10.1039/c1cc13388h
A TiO2-nanotube-array-based photocatalytic fuel cell system was
established for generation of electricity from various refractory
organic compounds and simultaneous wastewater treatment. The
present system can respond to visible light and produce obviously
enhanced cell performance when a narrow band-gap semiconductor
(i.e. Cu2O and CdS) was combined with TiO2nanotubes.
Since the first discovery of photocatalytic water-splitting on
a single-crystal TiO2 electrode by Fujishima and Honda,1
extensive research has shown that titania is a promising
catalyst for photocatalytic applications due to its high efficiency,
low cost, chemical inertness and photostability.2A variety of
preparation routes have been applied to fabricate nano-scaled
TiO2 of different geometric shapes and microstructures
(i.e. particulates, rods, wires, belts, or tubes), including hydro-
thermal synthesis, template synthesis, magnetic sputtering,
or sol–gel.3Of these fabrication methods, the architecture
demonstrating by far the most remarkable properties are
highly ordered TiO2nanotube arrays (TNA) synthesized by
anodization of titanium in fluoride-based baths, the dimensions
of which can be precisely controlled.4Well aligned TiO2
nanotubes with various lengths (0.5–720 mm), pore sizes
(20–250 nm) and wall thicknesses (7–40 nm) can be achieved
by finely tuning the electrochemical parameters during their
synthesis.5For applications where the use of highly ordered
TiO2 nanotubes has been studied, these advantages have
manifested themselves in an extraordinary enhancement of
the properties when compared to any other form of titania.6
Recently, a TNA-based photocatalytic oxidation technique
has been proven to be a promising process that can be used to
degrade various persistent and hazardous organic pollutants.7
Upon illumination, photo-generated electrons are excited
from the valence band to the conduction band, generating
electron–hole pairs. The positive holes are powerful oxidants
for degrading the organic compounds adsorbed on the nano-
tubes surface. Most studies on TNA-based photocatalysis
have mainly focused on the efficiency and the extent of
mineralization. However, the important application of the
TNA electrode in energy yield from refractory organic waste-
water with simultaneous wastewater treatment has not been
reported yet. Meanwhile, organic compounds in wastewater are
important sources of energy. Based on the statistics,8energy
loss in terms of organic matter caused by biowaste discharge
has reached 130 EJ per year, corresponding to almost one
third of the global yearly energy demand. Therefore, seeking
sustainable approaches that recover the energy from these
energy-rich organic wastes and degrade them to non-hazardous
products are highly desirable.
Designing a system in which organic compounds from
wastewater are degraded and the chemical energy is converted
into electrical energy simultaneously can achieve a double
benefit—organic pollutant degradation and energy reclamation.
The TiO2photocatalytic process may degrade organic matter
and constantly produce photogenerated electrons that directly
pass through the conductive substrate to the cathode. In turn,
this forms a TiO2-based photocatalytic fuel cell (PFC) system.
In 2005, Drew and co-workers9first reported a hybrid-catalyst-
based fuel cell system. In that work, the TiO2photocatalytic
technology was introduced in the Pt–Ru fuel cell system to
generate electricity by using methanol as a substrate.
Preparation by sonoelectrochemical anodization of a short,
robust, and highly ordered TiO2nanotube array (STNA, Fig. S1,
ESIw) with superior electron transfer performance and excellent
mechanical stability has been reported.10In this work, the high-
performance STNA electrode was applied as a photoanode
material in the novel fuel cell system for electricity yield using
various refractory organic compounds as ‘‘substrates’’ and
simultaneous wastewater treatment. The studied refractory
model compounds include aromatics, azo dyes, pharmaceutical
and personal care products (PPCPs), endocrine disrupting
compounds (EDCs), surfactants, etc. The present PFC system
can respond to visible light and give even better cell performance
by utilizing a composite electrode prepared by decorating
the TiO2nanotubes with a narrow band-gap semiconductor
(i.e. Cu2O and CdS). Our findings suggest that the PFC system
may provide the basis for the development of novel devices for
generation of electrical power from renewable resources with
aSchool of Environmental Science and Engineering,
Shanghai Jiao Tong University, 800 Dongchuan Rd.,
Shanghai, 200240, People’s Republic of China
bThe Key Laboratory of Thin Film and Microfabrication Technology,
Ministry of Education, Shanghai, 200240,
People’s Republic of China. E-mail: zhoubaoxue@sjtu.edu.cn
cLaboratory of Advanced Materials, Fudan University,
2005 Songhu Road, Shanghai, 200438, People’s Republic of China
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc13388h
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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 10314–1031610315
simultaneous degradation of refractory organic waste in liquid
or soluble form.
Fig. 1 shows the working principle of the STNA-based PFC
system. Upon illumination, the TiO2nanotubes photoanode
will produce electron/hole pairs in the conduction band and
the valence band, respectively. The photogenerated holes will
oxidize the organic molecules (fuel) to CO2and H+. While the
photogenerated electrons will move through an external circuit
to the Pt-black/Pt cathode. Hydrogen ions generated from
photo-oxidation move toward the cathode via diffusing
through the electrolyte solution. These will interact with
oxygen and externally arriving electrons to produce water
under aerobic conditions.
The current–voltage (J–V) characteristics of various typical
refractory organic compounds (including PPCPs, azo dyes and
aromatics etc.) are listed in Table 1 and Table S1 (ESIw).
On comparing the results, the organic acids present relatively
higher cell performance compared with other compounds
shown here, identifying the organic acids can be easily photo-
decomposed by the photocatalytic process of TiO2nanotubes.
Especially, in the case of malonic acid, the highest cell perfor-
mance with a short-circuit current density (Jsc) of 1.53 mA cm?2,
an open-circuit voltage (Voc) of 1.55 V and a maximum power
output (JVmax) of 0.64 mW cm?2was obtained. Among the
group of azo dyes, methylene blue with a simplest molecular
structure produced the best performance (Fig. S9, ESIw). This
may partly be due to smaller molecular volume of methylene
blue compared with other azo dyes. More methylene blue
molecules can be adsorbed per unit photoanode area. Other
complex organics might necessitate more oxidative steps to
achieve complete decomposition and mineralization.
Other recalcitrant organic compounds such as aromatics,
EDCs, PPCPs as well as surfactants were also successfully
photodecomposed by the TiO2 nanotubes photoanode and
gave the relevant PFC characteristics (Table S1, ESIw). The
cell efficiencies listed in Table 1 are relatively large. The reason
is that the exciting radiation has a limited spectral extent
around 254 nm and it is directly absorbed by titania with
small losses.11The data in Table 1 reveal that the above
electrochemical cell can be successfully employed to photo-
degrade a large variety of refractory water pollutants, leading
to water cleaning with simultaneous production of electricity.
One of the advantages of the PFC is that the quantum
efficiency of the system can be obtained.12Since within the
system, a forward bias was exerted between the cathode and the
photoanode of PFC, which would facilitate efficient electron
transfer within the photoanode material, most of the electrons
would flow towards the cathode via the external circuit. Under
this condition, the charges that passed through the external
circuit could be regarded as the amount of the charges used
effectively for substrate oxidation at the STNA photoanode.
The internal quantum efficiency (Z0) of the system could then
be defined as the ratio of the total decomposition number
against photon number effectively used for activating the
substrate, and the denominator can be estimated from the
charges passed in the external circuit in the PFC.13
Tetracycline was selected as a model substrate in this work.
Within 8 h, tetracycline was decreased from 0.22 to
0.012 mmol L?1(Fig. S6 and S7, ESIw), which corresponds
to conversion of 2.08 mmol tetracycline via a 78-electron process
per one tetracycline molecule. This amounts to the charge of
96485 ? 2.08 ? 10?6? 78 = 15.6 coulombs (C) for the
oxidative decomposition. The charges passed in the outer
circuit during the 8 h reaction were measured to be 7.36 C
(Fig. S8, ESIw), which leading to Z0equals to 212%. This
provides compelling support that both the incident light
energy and the chemical energy contribute to the system
performance of the PFC.
The PFC system can also respond to visible light and give
even higher performance under the irradiation of simulated
solar light illumination if a narrow band-gap semiconductor is
complexed with the highly-ordered TiO2nanotubes photo-
anode. Table 2 summarizes the J–V characteristics of the PFC
system applying different photoanode materials under solar
light irradiation. As can be seen that, the pure STNA electrode
produces comparable or even better PFC performance compared
with that under UV illumination. This can be ascribed to the
low UV irradiation intensity used, B3.6 mW cm?2, which is
nearly comparable with the intensity of the UVA part of the
solar spectrum. Moreover, compared with the traditional TiO2
nanoparticulate film (Fig. S2, ESIw),14the STNA electrode
gave much higher Jsc values in various organic solutions,
Fig. 1
based PFC system, where R and R0are the organic compound and its
oxidation product respectively.
Schematic diagram of the working principle of an STNA-
Table 1
using various refractory organic compounds as substrates
System current–voltage characteristics of the PFC system
ReactantJsc/mA cm?2
Voc/V
JVmax/
mW cm?2
Phenol (0.05 mol L?1)
Malonic acid (0.05 mol L?1)
Methylene blue (0.27 mmol L?1)
Tetracycline (0.22 mmol L?1)
Bisphenol A (0.09 mol L?1)
Sodium dodeceyl benzene
sulfonate (0.05 mol L?1)
0.62
1.53
0.94
0.43
0.72
0.33
1.29
1.55
1.22
1.39
1.30
1.28
0.26
0.64
0.30
0.29
0.25
0.15
Table 2
under solar light illumination (AM1.5) as affected by different photo-
anode materials
System current–voltage characteristics of the PFC system
Reactant
Photoanode
MaterialsJsc/mA cm?2
Voc/V
JVmax/
mW cm?2
Phenol
(0.05 mol L?1)
TiO2nano film
STNA
Cu2O/STNA
CdS/STNA
0.32
0.76
0.99
1.33
1.49
1.30
1.31
1.73
0.23
0.27
0.49
0.75

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10316Chem. Commun., 2011, 47, 10314–10316This journal is c The Royal Society of Chemistry 2011
which can be due to the different microstructures of the two
electrode materials (Fig. S4 and S5, ESIw). The excellent
transportation properties of photogenerated electrons and
desirable mechanical stability of the STNA electrode make it
a desirable photoanode material for PFC applications.
The combination of the pure STNA electrode material with
narrow band-gap semiconductors (e.g. Cu2O and CdS)15
increases greatly both the Jscas well as the JVmaxvalue under
simulated solar light illumination (AM1.5). Especially for the
CdS/STNA nanocomposite photoanode (Fig. S3, ESIw), Jsc
and JVmaxof the PFC system are found to be 1.66–1.75 and
1.38–2.41 times higher than those of the respective pure TiO2
nanotubes photoanode in three different organic compounds
solution (Table S2, ESIw). Furthermore, to demonstrate visually
the electrical energy generation, an example of 0.05 mol L?1
phenol could light a LED indicative lamp by applying four
CdS/STNA-based PFC systems in series under visible light
illumination (Fig. 2 and Fig. S10, ESIw), since the LED
requires an excitation voltage of 2–4 V. Even without applying
the light sources, the present PFC systems can work outdoors
under solar illumination and light a LED indicative lamp by
using the composite photoanode materials (Fig. S11, ESIw).
That means the PFC systems can work outdoors efficiently
under natural solar illumination by applying modified photo-
anode materials. Practical applications of the above procedure
are then feasible.
The employed PFC system was,however,not optimized, since
optimization of the system was not the main focus of the present
work. The cell performance would have been better if, for
instance, the distance between the two electrodes were shorter,
the higher light intensity adopted and larger photoanode area
exposed to illumination. All these factors affect cell performance
and organic compounds degradation. Furthermore, the present
PFC system is a small laboratory prototype, which we believe
can be further optimized and up-scaled to synthesize a large size
unit module towards practical environmental applications.
In conclusion, a photocatalytic fuel cell (PFC) system was
established for recovering energy from various refractory organic
compounds and simultaneous water cleaning. The studied
model compounds include aromatics, azo dyes, pharmaceutical
and personal care products (PPCPs), endocrine disrupting
compounds (EDCs), etc. and their cell performances were
demonstrated. The short, robust and highly-ordered TiO2
nanotubes photoanode enhanced cell performance under both
UV and solar light illumination compared with the traditional
TiO2 nanoparticulate film. Furthermore, the present PFC
system could utilize solar or visible light rather than UV light
by applying appropriate narrow band-gap semiconductor
material to produce electricity from various poorly-degradable
environmental pollutants. Such a PFC system may serve well
as a promising technology for wastewater treatment.
This study was supported by the National Nature Science
Foundation of China (No.21177xxx), the State Key Develop-
ment Program for Basic Research of China (Grant zNo.
2009CB220004), and Shanghai Tongji Gao Tingyao Environ-
mental Science and Technology Development Foundation.
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Fig. 2
LED indicative lamp under visible light irradiation in 0.05 mol L?1
phenol + 0.1 mol L?1Na2SO4solution.
Four CdS/STNA-based PFC systems in series to light a