Journal of Energy Chemistry 23(2014)331–337
Preparation and characterization of Ti0.7Sn0.3O2 as catalyst
support for oxygen reduction reaction
Yuan Gaoa,b , Ming Houa∗ ,
Zhigang Shaoa∗ ,
Changkun Zhanga,b , Xiaoping Qina , Baolian Yia
a. Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China;
b. Graduate School of Chinese Academy of Sciences, Beijing 100049, China
[ Manuscript received November 29, 2013; revised March 3, 2014 ]
Abstract
Sn-doped TiO2 nanoparticles with high surface area of 125.7 m2 ·g−1 are synthesized via a simple one-step hydrothermal method and explored
as the cathode catalyst support for proton exchange membrane fuel cells. The synthesized support materials are studied by X-ray diffraction
analysis, energy dispersive X-ray spectroscopy and transmission electron microscopy. It is found that the conductivity has been greatly improved by the addition of 30 mol% Sn and Pt nanoparticles are well dispersed on Ti0.7 Sn0.3 O2 support with an average size of 2.44 nm.
Electrochemical studies show that the Ti0.7 Sn0.3 O2 nanoparticles have excellent electrochemical stability under a high potential compared to
Vulcan XC-72. The as-synthesized Pt/Ti0.7Sn0.3 O2 exhibits high and stable electrocatalytic activity for the oxygen reduction reaction. The
Pt/Ti0.7 Sn0.3 O2 catalyst reserves most of its electrochemically active surface area (ECA), and its half wave potential difference is 11 mV,
which is lower than that of Pt/XC-72 (36 mV) under 10 h potential hold at 1.4 V vs. NHE. In addition, the ECA degradation of Pt/Ti0.7 Sn0.3 O2
is 1.9 times lower than commercial Pt/XC-72 under 500 potential cycles between 0.6 V and 1.2 V vs. NHE. Therefore, the as synthesized
Pt/Ti0.7 Sn0.3 O2 can be considered as a promising alternative cathode catalyst for proton exchange membrane fuel cells.
Key words
tin; titanium oxide; support; durability; proton exchange membrane fuel cells
1. Introduction
Proton exchange membrane fuel cells (PEMFCs) are
gaining great attention as clean and energy efficient power
sources due to their high efficiency, and high energy density as
well as low to zero emissions [1−4]. Unfortunately, there are
several factors that affect the durability of PEMFCs, such as
Pt particle dissolution and sintering, carbon support corrosion,
and membrane thinning [5−10]. One of the major challenges
lies in the degradation caused by the corrosion (or oxidation)
of carbon support [10,11].
Regarding the carbon supported Pt catalyst, the most
common support material at present is Vulcan XC-72. However, this carbon support material can be oxidized at high potentials during fuel cell operation [12−16]. Carbon corrosion
could weaken the attachment of Pt particles to the support and
decrease the electronic continuity of the catalyst layer. Moreover, the existence of Pt particles could also accelerate car-
bon corrosion [17,18]. These effects would result in a rapid
degradation of the Pt catalyst and thus shorten the lifetime of
the PEMFCs. Therefore, it would be desirable to use more
stable carbon-free support to improve the durability of the
catalyst. Recently, many researches have been conducted to
investigate the conducting metal oxides with high corrosionresistant properties as potential supports [19−24]. However,
compared to carbon materials, these metal oxides couldn’t
meet the requirements of support electron conductivity, chemical/electrochemical stability, and catalyst activity/stability.
Among carbon-free support materials, titanium oxide
(TiO2 ) has been tested as a promising catalyst support material due to its stability in PEMFCs as well as the features
of inexpensiveness, no toxicity and commercial availability
[25,26]. However, the conventional TiO2 material is not
conductive enough to be a suitable catalyst support because
the Ti is in its highest oxidation state. Nevertheless, TiO2
can be partially reduced [27,28] or doped with other metal
cations [29−31], resulting in metallic or semiconducting
∗ Corresponding authors. Tel: +86-411-84379051; Fax: +86-411-84379185; E-mail: [email protected] (Ming Hou); Tel: +86-411-84379153; Fax:
+86-411-84379185; E-mail: [email protected] (Zhigang Shao)
This work was supported by the National High Technology Research and Development Program of China (863 Program, Grant No. 2013AA110201),
the National Basic Research Program of China (973 Program, Grant No. 2012CB215500), and the National Natural Science Foundations of China (Grant No.
21203191).
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
doi: 10.1016/S2095-4956(14)60155-8
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Yuan Gao et al./ Journal of Energy Chemistry Vol. 23 No. 3 2014
behavior. In addition, as a catalyst support, TiO2 has low surface area. Recently, Subban et al. [29] used a facile sol-gel
method to synthesize Ti0.7 W0.3 O2 nanoparticles with a high
surface area of 230 m2 ·g−1 . The as-prepared Ti0.7 W0.3 O2
showed no evidence of decomposition in a Nafion solution
for 3 weeks at 80 ◦ C. Importantly, the rates of hydrogen
oxidation and oxygen reduction by Pt/Ti0.7 W0.3 O2 are comparable to those of commercial Pt/C. Ho et al. [30] also
investigated the electrochemical stability of nanostructured
Ti0.7 Mo0.3 O2 as the PEMFC cathode catalyst support, and
found that Pt/Ti0.7 Mo0.3O2 had much higher corrosion resistance than commercial Pt/C and PtCo/C. Nb-doped TiO2 with
different structures was synthesized by Wang’s group [31],
and they found that the support conductivity plays an important role in enhancing catalyst mass activity. Hence, metaldoped TiO2 has the possibility of being both electrically conducting and kinetically stable under fuel cell conditions, assuming that the doping does not radically alter the chemical
stability of the host lattice.
In this study, we report a simple synthesis of Ti0.7 Sn0.3 O2
nanoparticles with large surface area and use Ti0.7 Sn0.3 O2 as
the cathode catalyst support for oxygen reduction reaction in
PEMFCs. Compared with the Pt/XC-72, the Pt/Ti0.7 Sn0.3 O2
catalyst shows excellent stability under both high potential
hold and potential cycling, indicating its promising application for PEMFCs.
2. Experimental
2.1. Materials
The materials used in the present work included tin (IV)
tetrachloride penthydrated (SnCl4 ·5H2 O, AR), titanium (IV)
tetrachloride (TiCl4 , AR), concentrated HCl (37 wt%), deionized water, hexachloroplatinic (IV) acid (H2 PtCl6 ·6H2 O, AR),
ethylene glycol, sodium hydroxide, nitric acid (65 wt%), carbon black (Vulcan XC-72R), isopropyl alcohol and Nafionr
solution (5 wt%, Alfa Aesar).
2.2. Synthesis of Ti0.7 Sn0.3 O2 support
The TiO2 nanoparticles were synthesized by a simple onestep hydrothermal process [32]. The Sn-doped TiO2 nanoparticles were prepared by adding the stannic chloride (SnCl4 ) in
the reaction solution. Typically, 0.425 g SnCl4 was dissolved
in 100 mL of deionized water, then 0.311 mL TiCl4 and 2 mL
37 wt% HCl were added sequentially. The precursor solution
was transferred to an autoclave and kept at 200 ◦ C for 2 h in
an oven. The sample was collected by repeated centrifugation
and washed with deionized water, then dried at 100 ◦ C for the
electrochemical and textural analyses.
2.3. Synthesis of Pt/Ti0.7 Sn0.3 O2 catalyst
Pt/Ti0.7 Sn0.3 O2 catalyst was prepared by a polyol-assisted
reduction method. Typically, 50 mg Ti0.7 Sn0.3 O2 nanoparticles were suspended in 50 mL ethylene glycol solution, and an
appropriate amount of H2 PtCl6 solution was added. Then the
mixture was ultrasonicated for 30 min, followed by the addition of sodium hydroxide (0.8 mol/L) to adjust the pH to 11.0.
The suspension was heated at 160 ◦ C for 1 h. When the reaction was complete, the sample was cooled and the pH of the
final mixture was about 2.0 by the addition of nitric acid. After 24 h stirring, the precipitate was collected by repeated centrifugation and washed with ethanol and deionized water. The
resulting Pt/Ti0.7 Sn0.3 O2 was dried at 60 ◦ C in a vacuum oven
overnight. The Pt loading of the prepared Pt/Ti0.7 Sn0.3 O2 catalyst was 47.1 wt% according to the measurement of inductively coupled plasma atomic emission spectrometry (ICPAES). For comparison, Pt/XC-72 was also prepared by the
same method.
2.4. Material characterizations
X-ray diffraction (XRD) measurements were carried out
using a Cu Kα source (PANalytical X’Pert PRO X-ray
diffractometer) operated at 40 kV and 40 mA. The BrunauerEmmet-Teller (BET) area and pore size distribution were estimated using a QuadraSorb SI4 system. Transmission electron
microscope (TEM) characterization was performed on a JEOL
JEM-2000EX microscope. Elemental analysis was carried out
on a JEOL 6360LV scanning electron microscopy equipped
with an energy dispersive X-ray spectrometer (EDX). The
conductivities of support materials were tested using fourpoint probe measuring system (Suzhou Jingge Electronic Co.,
China).
2.5. Electrochemical measurements
All electrochemical measurements were conducted using a CHI730 electrochemical station. Pt foil and saturated
calomel electrode (SCE) were employed as the counter and
reference electrodes, respectively. All the potentials, however,
are given versus the normal hydrogen electrode (NHE). Working electrode was prepared by coating appropriate amount of
electrocatalyst and Nafionr on the glassy carbon electrode
(d = 4 mm) according to the literature [4,33]. Catalyst ink
was obtained by sonicating 5 mg catalyst, 10 µL of Nafionr
solution (5 wt%, Alfa Aesar), 0.5 mL of isopropanol, and
1.99 mL of deionized water into homogeneous slurry. Then,
6.4 µL of the ink was transferred onto the glassy carbon
electrode. All cyclic voltammetry (CV) measurements were
profiled in 0.5 mol/L H2 SO4 solution deaerated with high purity N2 in the potential range of 0 V to 1.2 V at a scan rate
of 50 mV·s−1 . The oxygen reduction curve was measured
in oxygen saturated 0.5 mol/L H2 SO4 from 1.1 V to 0.2 V at
10 mV·s−1 with a rotating speed of 1600 rpm. A constant
potential of 1.4 V and a potential cycling test from 0.6 V to
1.2 V were conducted to examine the electrochemical stability of the catalysts, respectively. The electrochemical active
surface area (ECA) of catalysts was estimated according to
the charge of hydrogen desorption after double-layer correc-
Journal of Energy Chemistry Vol. 23 No. 3 2014
tion, assuming monolayer hydrogen adsorption on Pt surface
(0.21 mC·cm−2 ).
333
peak at 39.9o and 46.2o attributed to the Pt [111] and [200]
reflection, respectively. The broadening of two peaks indicates the small particle size of Pt.
3. Results and discussion
3.1. Structural characteristics
Figure 1 shows the XRD patterns of Ti0.7 Sn0.3 O2
support material and Pt/Ti0.7 Sn0.3 O2 catalyst.
All the
diffraction peaks of the support material match well with
the standard XRD pattern of Ti0.7 Sn0.3 O2 (JCPDS card
No. 01−070−4405), and without peaks arising from impurity, such as TiO2 and SnO2 . The observed characteristic
diffraction patterns at 2θ = 27.1o, 35.4o , 53.5o and 68.0o correspond to the planes of (100), (101), (211) and (301), respectively. EDX measurements indicated a Ti : Sn atomic ratio of
0.72 : 0.28, which agrees closely with the expected atomic ratio of 0.70 : 0.30 for Ti0.7 Sn0.3 O2 . The presence of metallic
Pt for Pt/Ti0.7 Sn0.3 O2 catalyst is also clearly revealed by the
Figure 1. XRD patterns of Ti0.7 Sn0.3 O2 support (1) and Pt/Ti0.7 Sn0.3 O2
catalyst (2)
Figure 2. TEM images of Ti0.7 Sn0.3 O2 (a) and Pt/Ti0.7 Sn0.3 O2 (b) and corresponding particle size distribution histograms
Figure 2 shows TEM images of the Ti0.7 Sn0.3 O2 and
Pt/Ti0.7 Sn0.3 O2 sample.
The TEM image (Figure 2a)
shows that the Ti0.7 Sn0.3 O2 support material is composed of
nanoparticles with an average diameter of around 5.67 nm. It
can be seen in Figure 2(b) that Pt nanoparticles are well dispersed on the surface of Ti0.7 Sn0.3 O2 supports. The mean size
of Pt particles is 2.44 nm according to statistical measurement
of random chosen areas.
The nitrogen sorption (adsorption and desorption)
isotherm and the corresponding Barrett-Joyner-Halenda
(BJH) pore size distribution of Ti0.7 Sn0.3 O2 are shown in Figure 3. The Ti0.7 Sn0.3 O2 support exhibits a typical type IV
isotherm with a distinct hysteretic loop which is attributed to
the presence of mesopores in the Ti0.7 Sn0.3 O2 . This fact is
further confirmed by the pore size distribution curve showing
pore size centered at 7 nm for the Ti0.7 Sn0.3 O2 support. The
BET surface area of the Ti0.7 Sn0.3 O2 support synthesized in
this study is found to be 125.7 m2 ·g−1 .
Figure 3. Nitrogen adsorption/desorption isotherms of Ti0.7 Sn0.3 O2 support
and corresponding BJH pore size distribution
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Yuan Gao et al./ Journal of Energy Chemistry Vol. 23 No. 3 2014
3.2. Electrochemical properties
The electronic conductivity of the Ti0.7 Sn0.3 O2 nanoparticles was measured as approximately 1.07×10−4 S·cm−1 ,
which is significant higher than that of the undoped TiO2
nanoparticles (1.42×10−7 S·cm−1 ).
In order to investigate the improvement of electronic conductivity, the CV curves and polarization curves of Pt/TiO2
and Pt/Ti0.7 Sn0.3 O2 are shown in Figure 4. As can be seen
in Figure 4(a) that the Pt/Ti0.7 Sn0.3 O2 exhibits clear peaks
of the hydrogen adsorption/desorption, indicating that the
Pt/Ti0.7 Sn0.3 O2 displays a higher electrochemical Pt surface
area than Pt/TiO2 . In addition, the polarization curve results
also show that the Pt/Ti0.7 Sn0.3 O2 catalyst has a more positive onset potential (1.0 V) than Pt/TiO2 (0.82 V), and thus
Pt/Ti0.7 Sn0.3 O2 is more active than the Pt/TiO2 . These results
are mainly due to high electronic conductivity of Ti0.7 Sn0.3 O2
support material compared to pure TiO2 support.
Figure 4. (a) CV curves of Pt/TiO2 and Pt/Ti0.7 Sn0.3 O2 in nitrogen-purged 0.5 mol/L H2 SO4 at a scan rate of 50 mV·s−1 , (b) polarization curves for the ORR
on Pt/TiO2 and Pt/Ti0.7 Sn0.3 O2 catalysts in oxygen-saturated 0.5 mol/L H2 SO4 at a scan rate of 10 mV·s−1
Figure 5. CV curves of (a) XC-72, Ti0.7 Sn0.3 O2 and (b) Pt/XC-72, Pt/Ti0.7 Sn0.3 O2 before and after 1.4 V oxidation; Polarization curves for the ORR on (c)
Pt/XC-72 and (d) Pt/Ti0.7 Sn0.3 O2 before and after 1.4 V oxidation
Journal of Energy Chemistry Vol. 23 No. 3 2014
The stabilities of Vulcan XC-72, Ti0.7 Sn0.3 O2 , Pt/XC72 and Pt/Ti0.7 Sn0.3 O2 were studied by a rotating disk electrode (RDE) in 0.5 mol/L H2 SO4 at an elevated potential.
Figure 5(a) and 5(b) show the CV curves of Vulcan XC-72,
Ti0.7 Sn0.3 O2 , Pt/XC-72 and Pt/Ti0.7 Sn0.3 O2 before and after
oxidation treatment at 1.4 V. As seen in Figure 5(a), the carbon support shows a significant increase in the oxidation current after a potential hold at 1.4 V for only 1 h, suggesting
that the severe carbon corrosion has occurred. In contrast,
for Ti0.7 Sn0.3 O2 , only a negligible change in the CV is observed after a potential hold at 1.4 V for 10 h, indicating good
335
resistance towards oxidation when subjected to high potentials. It is observed in Figure 5(b) that Pt/Ti0.7 Sn0.3 O2 shows
a similar electrochemical behavior to the Pt/XC-72, but the
Pt/XC-72 exhibits a better electrocatalytic activity. It may
be attributed to the fact that the electronic conductivity and
specific surface area of commercial XC-72 are higher than
those of Ti0.7 Sn0.3 O2 support. Generally, the difference between half wave potentials is a measure of the stabilities of
catalysts for ORR. Hence, after the oxidation treatment at
1.4 V for 10 h, the ORR of Pt/XC-72 and Pt/Ti0.7 Sn0.3 O2 catalysts were tested in an oxygen-saturated solution, as shown
Figure 6. CV curves of (a) Pt/XC-72 and (b) Pt/Ti0.7 Sn0.3 O2 under potential cycling between 0.6 V and 1.2 V; Normalized ECA (c) as a function of cycle
numbers for Pt/XC-72 and Pt/Ti0.7 Sn0.3 O2 ; Polarization curves for the ORR on (d) Pt/XC-72 and (e) Pt/Ti0.7 Sn0.3 O2 under 500 potential cycles between 0.6 V
and 1.2 V
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Yuan Gao et al./ Journal of Energy Chemistry Vol. 23 No. 3 2014
in Figure 5(c) and 5(d). It is found that the half wave potential difference is 36 mV for Pt/XC-72. While for the
Pt/Ti0.7 Sn0.3 O2 , only 11 mV difference in the polarization
curves is observed, indicating good electrochemical stability
when conducted at high potential. These results clearly indicate that the Ti0.7 Sn0.3 O2 support is more electrochemically
stable than the conventional carbon supports.
To further investigate the durability of catalysts, an accelerated corrosion method is also applied to investigate the
behavior of the catalyst materials. Figure 6(a) and 6(b)
show CV curves of the Pt/XC-72 and Pt/Ti0.7 Sn0.3 O2 during
the continuous cycling between 0.6 V and 1.2 V for 500 cycles. The initial electrochemical active surface area (ECA)
of Pt/Ti0.7 Sn0.3 O2 is 43.575 m2 ·g−1 , which is lower than that
of Pt/XC-72 (66.877 m2 ·g−1 ). As mentioned, the low electronic conductivity of Ti0.7 Sn0.3 O2 maybe one of the effect
factors on the ECA of Pt/Ti0.7 Sn0.3 O2 electrode. Both the
catalysts exhibit a decrease in ECA with increased cycles. As
can be seen from Figure 6(c), for Pt/XC-72 catalyst, the ECA
retention is found to be only 26.45% after the 500 cycles,
while that for the Pt/Ti0.7 Sn0.3 O2 catalyst maintains 62.2%,
confirming its higher electrochemical stability than that of
Pt/XC-72. Figure 6(d) and 6(e) show the polarization curves
for the ORR on Pt/XC-72 and Pt/Ti0.7 Sn0.3 O2 catalysts under
potential cycling between 0.6 V and 1.2 V for 500 cycles. As
expected, the Pt/Ti0.7 Sn0.3 O2 catalyst has a similar onset potential (1.0 V) compared to the Pt/XC-72 (0.98 V), indicating
the excellent electrocatalytic activity of the Pt/Ti0.7 Sn0.3 O2
catalyst toward the ORR. Furthermore, the half wave potential
difference of the Pt/Ti0.7 Sn0.3 O2 catalyst (69 mV) after 500
cycles is smaller than that of the Pt/XC-72 (98 mV). The excellent electrocatalytic activity and electrochemical stability
of Pt/Ti0.7 Sn0.3 O2 can be attributed to the small size and good
dispersion of the Pt catalyst on the porous, conductive and stable Ti0.7 Sn0.3 O2 support that creates a large electrochemical
Pt surface area, thereby resulting in an excellent durability.
4. Conclusions
In this study, non-carbon Ti0.7 Sn0.3 O2 nanoparticles were
successfully synthesized via a simple one-step hydrothermal
method, and used as the catalyst support for oxygen reduction reaction. The as-synthesized Ti0.7 Sn0.3 O2 support with
high specific surface area gives higher electronic conductivity
and exhibits more excellent electrochemical stability against
oxidation than Vulcan XC-72 does. Electrochemical studies
show that the Pt/Ti0.7 Sn0.3 O2 is significantly more stable than
Pt/XC-72. This work demonstrates that Sn-doped TiO2 has
a promising future as the cathode catalyst support for proton
exchange membrane fuel cells.
Acknowledgements
We gratefully acknowledge the financial support of this research
by the National High Technology Research and Development Program of China (863 Program, Grant No. 2013AA110201), the Na-
tional Basic Research Program of China (973 Program, Grant No.
2012CB215500), and the National Natural Science Foundations of
China (Grant Nos. 21203191).
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