A novel photoelectrochemical cell with self-organized TiO2...
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A novel photoelectrochemical cell with self-organized TiO2
nanotubes as photoanodes for hydrogen generation
Yongkun Li a,b, Hongmei Yu a,*, Wei Song a, Guangfu Li a,b, Baolian Yi a, Zhigang Shao a
a Laboratory of Fuel Cells, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR ChinabGraduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
Article history:
Received 28 May 2011
Received in revised form
27 July 2011
Accepted 7 August 2011
Available online 10 September 2011
Keywords:
Photoelectrochemical cell
Electrochemical anodization
TiO2 nanotubes
Water splitting
Hydrogen production
* Corresponding author. Tel.: þ86 411 843790E-mail address: [email protected] (H. Yu)
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.08.026
a b s t r a c t
A photoelectrochemical (PEC) cell with an innovative design for hydrogen generation via
photoelectrocatalytic water splitting is proposed and investigated. It consisted of a TiO2
nanotube photoanode, a Pt/C cathode and a commercial asbestos diaphragm. The PEC
could generate hydrogen under ultraviolet (UV) light-excitation with applied bias in KOH
solution. The Ti mesh was used as the substrate to synthesize the self-organized TiO2
nanotubular array layers. The effect of the morphology of the nanotubular array layers on
the photovoltaic performances was investigated. When TiO2 photocatalyst was irradiated
with UV-excitation, it prompted the water splitting under applied bias (0.6 V vs. Normal
Hydrogen Electrode, NHE.). Photocurrent generation of 0.58 mA/cm2 under UV-light irra-
diation showed good performance on hydrogen production.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction cathode are used and immersed in an aqueous solution
Photoelectrochemical (PEC) water splitting has attracted
significant attention in the past decades as a promising
renewable energy source due to their low production cost and
simple manufacturing process [1e7]. Since the early 1970s,
Fujishima and Honda reported a single-crystalline TiO2
semiconductor photoanode for photoelectrocatalytic decom-
position of water under UV-excitation and an external bias for
the first time [8], great efforts have been devoted to the
development of photocatalytic hydrogen production tech-
nologies [9e14]. However, the practical applications are still
difficult. The main difficulties can be attributed to the lack of
effective photocatalysts and efficient photoelectrocatalytic
reactors [15]. In photoelectrochemical water splitting for
hydrogen generation, the ITO (indium-tin oxide)/FTO (F-doped
SnO2) glass or metal substrate-based photoanode and the Pt
51; fax: þ86 411 84379185.2011, Hydrogen Energy P
especially in alkaline solutions [16e21]. Porous material, such
as glass frit, is used as the separator in the cell compartment.
The drawback is that such a cell design is only suitable for
small scale experiments, and the use of three-electrode or two
separated electrodes in an undivided cell, or in a one-
compartment cell, can be very problematic due to less effec-
tive separation of the generated hydrogen and oxygen. The
gaseous mixture is hazardous and brings collecting problems.
Recently, Kamat and co-workers have reported a hybrid
fuel cell based on the polymer membrane electrode assembly
(MEA) consisting of a P-25 TiO2 photoanode, a Pt cathode, and
a proton exchange membrane for the hydrogen production,
and there have other literatures about the new PECs’ structure
for hydrogen generation [9,22]. However, the photocatalyst
casted onto the carbon paper or carbon cloth would be peeled
easily after long time run. Therefore, it is not suitable for long-
.
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
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 en e r g y 3 6 ( 2 0 1 1 ) 1 4 3 7 4e1 4 3 8 0 14375
term water splitting operations. Whereas, the TiO2 nanotube
arrays incorporated with the Ti meshes will avoid this issue.
Furthermore, the ions can be transferred easily from the
anode to the cathode through the openings of the Ti meshes
while it is difficult for Ti foils. In the Ti meshes substrate
system, the ions transfer distance is short, that the ions
transfer to the counter electrode faster. Besides, the growths
of titanium oxide tubes on the Ti foil/meshes will strength the
adhesion of TiO2 nanotube arrays to the substrate. This can be
a great opportunity for their use in electrically enhanced
photo processes in which the electrical contacts are crucial to
obtain high efficiency [23].
In this paper, a novel PEC cell with an innovative design is
proposed and investigated. The core of the PEC cell is an MEA
incorporated with Ti mesh as the substrates of the photo-
anode and Pt/C carbon paper as the cathode. The MEA serves
as a compact but effective reactor for water splitting as well as
an effective gas separator. A self-organized TiO2 nanotube
arrays around the whole Ti mesh with high stabilities in
alkaline environments is employed as the photoanode.
Besides, a detailed investigation of this photoanode applying
on photoelectrolysis of water under illumination of UV light is
carried out. As for this type of PEC cell, the driving force for the
water electrolysis is the applied potential and light source. In
this way, gases generated at each electrode can be separated,
and the utilization of light can be improved in comparison
with the traditional three-arm reactor.
Fig. 1 e Schematic diagram of the photoelectrolysis cell for
hydrogen generation.
2. Experimental section
2.1. Materials
All chemical reagents were commercially available, including
ethylene glycol, ammonium fluoride (NH4F, AR). The purity of
the titanium meshes was 99.8%. A commercial Pt/C electrode
(Pt/C anchored on the Toray TGP-H-60 carbon paper, Sunrise
Power Co. Ltd.) with Pt loading of 0.4 mg/cm2 was used as the
cathode, and asbestos (pore diameter 160 nm, thickness
0.5 mm, SiChuan, China) was used as the diaphragm. The
electrolyte was KOH solution (1 M, pH z 13.6).
2.2. Photoanode preparation
Ti meshes (0.2 mm thick, 99.8% metal basis, Dexmet Co. Ltd.)
with 58% porosity were used as starting material to obtain the
TiO2 photoelectrodes. Prior to anodization, the Ti mesh was
ultrasonically cleaned and degreased in acetone and ethanol
successively, followed by rinsing with deionized water and
drying in an air stream. Anodization was performed in a two-
electrode configuration with titanium mesh as the working
electrode under constant potential (30 V) at room temperature
(20 �C). A rectangle-shaped carbon electrode (thickness 2 mm,
area 4 � 4 cm2) served as the cathode. The distance between
the two electrodeswas kept at 4 cm in all experiments. A direct
current power supply (HY 1791-20S, YaGuang Electronics Co.
Ltd.) was used as the voltage source to drive the anodization.
The Ti mesh was anodized in the electrolyte (pH z 6.2) with
a mixture of ethylene glycol (97.5 wt%), H2O (2 wt%) and NH4F
(0.5 wt%). During the first 2 min of the anodization, little
bubbles were generated out of the surface of the titanium
mesh and afterward less gradually. The colour of the titanium
mesh changed fromwhite to light purple and finally brown. In
order to achieve different morphologies of nanotubular array
layers, anodization time ranged from 4 h to 10 h. After
oxidation, the anodized samples were properly rinsed with
deionized water to remove the occluded ions, and then dried
in an air oven. A subsequent annealing treatment, performed
at 450 �C (heating rate of 2 �C/min) in air, was needed to
transfer the amorphous structure into crystalline anatase.
2.3. Measurements
A field emission scanning electron microscope (FESEM; Hita-
chi, S-4800) was used to analyze the morphology of the
nanotubes and X-ray diffraction (XRD) measurements were
carried out by using an X’pert PRO (PAN-alytical) with a Cu Ka
tube. Photoelectrochemical features of TiO2 photoelectrode
were tested in a three-arm reactor, in which the TiO2 sample
worked as the anode, a platinum grid worked as the counter
electrode and a saturated calomel electrode (SCE) as the
reference electrode; all the values of potential in the text were
referred to SCE. The light source was an 8 W Deuterium lamp;
1 M KOH solution was employed as the electrolyte. The
effective photoactive area of the cell was 1.3 cm2. A computer-
controlled potentiostat (CHI760, CH Instruments Co. Ltd.,
China) was employed to control the external bias and to
record the photocurrent. The samples were anodically polar-
ized at a scan rate of 5 mV/s under the illumination, and the
photocurrent was recorded.
2.4. Photoelectrochemical generation of hydrogen fromwater
In our early patent [24], a PEC cell was described as Fig. 1. With
the anode (TiO2 nanotube) and the cathode (Pt/C carbon paper)
on two sides of the asbestos diaphragm, the membrane elec-
trode assembly (MEA)was pressed between two stainless steel
flow-field plates. There were parallel channels flow at the
cathode side, and an open chamber with a 24 cm2 quartz
window at the anode side, which allowed the photocatalyst
surface exposed to the light. Rubber rings were positioned in
the cell as the sealing. A potentiostat (PARSTAT 2273,
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 4 3 7 4e1 4 3 8 014376
Princeton) was employed to control the external bias and to
record the photocurrent generated. A 300 W Xenon Lamp
(Beijing Perfectlight Co. Ltd.) served as the light source and
optical filters kept the UV light (365 � 15 nm) intensity at
70 mW/cm2. A peristaltic pump circulated 1 M KOH solution
from a reservoir to the reactor chamber. The gas was analyzed
by a gas chromatography (GC-14C, CHIMADZU).
3. Results and discussions
3.1. Characterization of the photoanode
Fig. 2 shows the FESEM image of the highly ordered titania
nanotube arrays prepared by the electrochemical anodization
in an electrolytic solution with 2 wt% water and 0.5 wt% NH4F
in ethylene glycol. As shown in Fig. 2a, the structure of the
nanotube arrays prepared at 30 V for 4 h is well ordered with
circular shape, whereas, the nanotubes prepared for 6 h, 8 h
and 10 h are not well ordered and not exactly circular (Fig. 2b,
c, d). It is also observed in Fig. 2 that the length of the titania
nanotubes varies with the anodization time. The length of the
titania nanotubes increases as the anodization time changes
from 4 h to 8 h, but the length limits at 4 mm which does not
Fig. 2 e FESEM images of titania nanotubes prepared by electro
ethylene glycol at 30 V for (a) 4 h (b) 6 h (c) 8 h (d) 10 h.
continue to increase with longer anodization time. On the
other hand, the pore diameter and the wall thickness of the
titania nanotubes does not completely depend on the anod-
ization time. In all the prepared titania nanotubes, most of
them are straight and highly compact, however, some of them
are slightly bent resulting from the strained environment. The
most interesting phenomenon is the nanowire/belt structure
(Fig. 2b, c, and d), that the TiO2 nanowire/belt likes nanograss
floating on the top of the nanotube arrays. The lower part of
the nanotube arrays is straight andwell ordered, while the top
is with poor uniformity and floating at random orientation.
The possible reasons of this phenomenon are as follows.
Firstly, the nanotube wall thickness of the prepared TiO2
arrays is not even that the corrosion happens on the weak
point, while the nanotubular openings that are exposed for
more time than the bottom parts are firstly corroded. Hence, it
is speculated that the corrosion firstly happens on the weak
point along the nanotubes. Secondly, when increasing the
water content to 10 wt% in the electrolyte and keeping F�
constant and other conditions unchanged, the nanowire/belt
structure does not appear, and the nanotubes is well ordered
with circular shape. Therefore, it can be inferred that the
water content is a key role on the formation of the nanowire/
belt structure. F� transfers faster in water than in ethylene
chemical anodization in 2 wt% water D 0.5 wt % NH4F in
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glycol, and there are more F� on the surface of the TiO2 arrays
in water than that in ethylene glycol which leads to the faster
corrosive speed. Compared with the effect of the nanotube
wall nonuniformity, the formation of nanograss’s structure is
more sensitive to the water content in the electrolyte. This
process is effective to produce highly ordered TiO2 nanotube
arrays with tunable morphology via changing the anodization
conditions.
The nanotube arrays prepared at 30 V for 4 h using aqueous
ethylene glycol and ammonium fluoride solution are selected
for further investigation. The XRD patterns of the as-anodized
titania nanotube arrays before annealing treatment shows
only the amorphous structure (figure not included) in nature
and are annealed to convert them into crystalline phases. A
representative XRD pattern of TiO2 nanotubes annealed at
450 �C for 2 h is shown in Fig. 3, which shows anatase phase
except substrate titanium. However, the tube wall thickness
varies a lot compared with the previous state. Obviously, it
does not vary uniformly along the nanotubes. The possible
reason is that the amorphous nanotube arrays have different
dilatabilities during the phase transition.
Fig. 4 e (a) IeV characteristics of TiO2 electrode operated
with UV-illumination at different applied potentials. Scan
rate was 5 mV/s (b) Potentiostatic (Iet) curve obtained from
the photoelectrolytic cell containing TiO2 nanotube as an
anode and Pt grid as a cathode at a potential of 0.2 V vs.
SCE.
3.2. Photoelectrochemical performance of TiO2
photoanode
The titania nanotube arrays (effective photoactive
area ¼ 1.3 cm2) prepared at 30 V for 6 h in 2 wt% and 10 wt%
water electrolyte are used as the photoanodes in a typical
three-electrode system to evaluate the activity for water
photoelectrolysis, respectively.
Fig. 4a shows the photocurrent response to the light.
Photocurrent reaches 112 mA/cm2 and 90 mA/cm2 under exci-
tation, respectively, even with low external bias. The current
drops to zerowhen the light source is switched off. It indicates
that the photogenerated charges can be separated effectively
in the TiO2 anode and the current generated is from photo-
current. It is worth to note that the photocurrent boosts from
Fig. 3 e XRD pattern of the titania nanotubular arrays
prepared at 30 V for 4 h and annealed at 450 �C showing
anatase phases.
90 mA/cm2 to 112 mA/cm2 (Fig. 4b) as the water content
increases from 2 wt% to 10 wt% at the same condition. This
phenomenon indicates that the photoexcited charges of the
10wt%water sample are separatedmore effectively under the
illuminated conditions.
The FESEM images of titania nanotubes prepared by
electrochemical anodization in 2 wt% water (a) and 10 wt%
water (b) þ0.5 wt% NH4F in ethylene glycol at 30 V for 6 h are
showed in Fig. 5. As it is discussed previously, there is
nanowire/belt formed on the top of the nanotubes during the
preparation in the electrolyte of 2 wt% water. This nanowire/
belt can be easily collapsed after calcinations. The collapsed
nanowire/belt blocks the nanotubular openings and prevents
the light passing through. As a result, it cannot absorb and
utilize the sunlight efficiently. However, the openings of the
TiO2 nanotube arrays prepared in the electrolyte of 10 wt%
water are well ordered and uniform, and the generated
photocurrent is obviously higher than that in 2 wt% water
system.
Fig. 5 e FESEM images of titania nanotubes prepared by
electrochemical anodization in 2 wt% water (a) and 10 wt%
water (b) D0.5 wt% NH4F in ethylene glycol at 30 V for 6 h.
Fig. 6 e Photocurrent response of the PEC cell during oneoff
cycles to UV-illumination under applied potential (0.6 V vs.
NHE), measured in 1 M KOH solution.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 4 3 7 4e1 4 3 8 014378
The photo conversion efficiency (h) of the photoanode
which converts the light energy into chemical energy is
calculated using the following equation [10,25e27].
hð%Þ ¼ ½ðtotal power output� electrical power outputÞ=light power input� � 100
¼ JP��E0rev � Eapp
��I0�� 100
where, Jp ¼ photocurrent density (mA/cm2); Jp Erev0 ¼ total
power output; JpEapp ¼ electrical power input; I0 ¼ power
density of incident light (mW/cm2).
Erev0 is the standard reversible potential which is 1.23 V vs.
NHE, and the applied potential Eapp¼ Emeas� Eaoc, where Emeas
is the electrode potential (vs. SCE) of the working electrode at
which photocurrent is measured under illumination. Eaoc is
the electrode potential (vs. SCE) of the sameworking electrode
at open circuit conditions under the same illumination and in
the same electrolyte. With the equation, h of the photoanode
usingUV spectrum can be calculated as 2.6% (at�0.5 V vs. SCE,
Jp ¼ 0.112 mA/cm2, OCP ¼ �0.8 V vs. SCE, I0 ¼ 4 mW/cm2). The
OCP is the potential difference between the working electrode
(anode) and the reference electrode (SCE) at equilibrium.
3.3. PEC cell performance
The principle of the PEC cell has shown in Fig. 1. Excitation of
TiO2 nanoparticles with UV-light leads to the electron-holes
separation, most of them recombine again, and the rest can
induce redox reactions. The excited electrons that promote to
the unoccupied conduction band transfer to the cathode
through an external circuit. The holes which stay at the
valence band transfer to the interface of the electrode/elec-
trolyte, and OH� is oxidized. (Equation (1)e(3)).
Excitation : TiO2 þ hv/e� þ hþ (1)
Photoanode : 2hþ þ 2OH�/1=2O2 þH2O (2)
Cathode : 2e� þ 2H2O/H2 þ 2OH� (3)
The PEC cell with the TiO2 photoanode is operated in the
alkaline electrolyte under the applied potential. The TiO2
nanotube photoanode is prepared by electrochemical anod-
ization in 2 wt% water þ 0.5 wt% NH4F in ethylene glycol at
30 V, and the obtained nanotubes length was 3 mm with
anodization time of 4 h.
Fig. 6 shows the photocurrent response of the PEC cell
with TiO2 electrode under UV irradiation during the on/off
cycles under the potentiostatic mode. During on/off cycles of
the illumination, the photocurrent reaches 0.58 mA/cm2 and
the current drops to almost zero without illumination. It
indicates that the PEC cell has good photo-electricity prop-
erties. A 4-h experiment is performed under UV-illumination
with bias potential of 0.6 V for the PEC cell for hydrogen
production. Deaerated KOH solution (1 M) is cycled by
a pump with constant rate through both the compartments
of the PEC cell. The bubbles coming out of the cell are
collected into a vessel. GC-14C gas chromatography is used to
analyze the trapped gas and confirms it to be hydrogen. The
photocurrent keeps a stable value (about 0.58 mA/cm2)
during the entire 4-h test. It indicates that the TiO2 photo-
anode is stable enough to withstand the mechanical stress
and shows no loss of photoactivity. During the 4-h operation,
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 en e r g y 3 6 ( 2 0 1 1 ) 1 4 3 7 4e1 4 3 8 0 14379
the PEC cell with photoactive area of 10.08 cm2 generates
nearly 7.2 ml of hydrogen under the steady-state condition,
so the hydrogen evolution rate of 0.178 ml/h cm2 can be
detected at the average current of 0.58 mA/cm2. If we
consider an average current of 0.58 mA/cm2 flowing through
the circuit, it would expect to observe H2 formation at
a theoretical rate of 0.25 ml/h cm2 (at 15 �C). The observed H2
evolution rate of 0.178 ml/h cm2 accounts for 71% of the
amount predicted on the basis of current flow. The discrep-
ancy between the theoretical and observed yield is likely to
originate from the H2 collecting.
4. Conclusions
A novel PEC cell was constructed by using Ti mesh as the
substrate of the photoanode and Pt/C carbon paper as the
cathode, respectively. Compared with the typical three-arm
reactor, it has many advantages. The Ti mesh was anodized
to synthesize the self-organized TiO2 nanotubular array layer.
The effect of the morphology of the nanotubular array layers
on the photovoltaic performances of the cell was investigated
and the results indicated that the well ordered nanotubular
array layer is a key factor to achieve high conversion efficiency
in the PEC cell. The highest photocurrent density of the
anodization TiO2 nanotubes was about 0.58 mA/cm2 with a H2
evolution rate up to 0.178ml/h cm2. The designed PEC cell was
effective for hydrogen production andwas able to separate the
evolved gases completely. The Pt/C carbon paper as the
cathode rather than Pt foil minimized the catalyst loading,
which was an important progress to cut down the cost of the
PEC cell. Further works are essential to optimize the material
and the structure of the cell, and the progress will be reported
in the near future.
Acknowledgments
This work was financially supported by the National Natural
Science Foundations of China (No. 20636060, No. 20876154).
We also thank Dr. Jingying Shi, the Molecular Catalysis & In-
situ Characterization Group, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences.
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