Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 4096–4103
An inorganic/organic self-humidifying composite membranes
for proton exchange membrane fuel cell application
Yu Zhang a,b , Huamin Zhang a,∗ , Cheng Bi a,b , Xiaobing Zhu a,b
a
Lab of PEMFC Key Materials and Technologies, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
457 Zhongshan Road, Dalian 116023, China
b Graduate School of Chinese Academy of Sciences, Beijing 100039, China
Received 30 September 2007; received in revised form 19 November 2007; accepted 4 December 2007
Available online 25 December 2007
Abstract
With an aim to operate the proton exchange membrane fuel cells (PEMFCs) with dry reactants, an inorganic/organic self-humidifying membrane
based on sulfonated polyether ether ketone (SPEEK) hybrid with Cs2.5 H0.5 PW12 O40 supported Pt catalyst (Pt-Cs2.5 catalyst) has been investigated.
The Pt-Cs2.5 catalysts incorporated in the SPEEK matrix provide the site for catalytic recombination of permeable H2 and O2 to form water, and
meanwhile avoid short circuit through the whole membrane due to the insulated property of Cs2.5 H0.5 PW12 O40 support. Furthermore, the Pt-Cs2.5
catalyst can adsorb the water and transfer proton inside the membrane for its hygroscopic and proton-conductive properties. The structure of the
SPEEK/Pt-Cs2.5 composite membrane was characterized by XRD, FT-IR, SEM and EDS. Comparison of the physicochemical and electrochemical
properties, such as ion exchange capacity (IEC), water uptake and proton conductivity between the plain SPEEK and SPEEK/Pt-Cs2.5 composite
membrane were investigated. Additive stability measurements indicated that the Pt-Cs2.5 catalyst showed improved stability in the SPEEK matrix
compared to the PTA particle in the SPEEK matrix. Single cell tests employing the SPEEK/Pt-Cs2.5 self-humidifying membrane and the plain
SPEEK membrane under wet or dry operation conditions and primary 100 h fuel cell stability measurement were also conducted in the present
study.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: PEMFC; Self-humidifying membrane; Pt-Cs2.5 catalyst; SPEEK; Proton conductivity
1. Introduction
During the past several decades, much attention has been
focused on the research and development of proton exchange
membrane fuel cells (PEMFCs) due to their advantages of high
power density, simplicity of operation, high energy conversion
efficiency and low harmful emissions [1–3]. However, the cost,
durability and operation flexibility of PEMFCs still remain the
hurdles to its commercialization and should be greatly improved.
Currently, the proton exchange membranes (PEMs), such as
Nafion or sulfonated poly (ether ether ketone) (SPEEK), require
water to maintain their proton conductivity. Thus, to prevent
drying out of the membrane and keep the membrane at most conductive state, the reactant gases are usually humidified through
an external humidification system before entering the fuel cells.
∗
Corresponding author. Tel.: +86 411 84379072; fax: +86 411 84665057.
E-mail address: [email protected] (H. Zhang).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.12.045
However, this method increases the weight and complexity of
the fuel cell system, and makes PEMFCs unsuitable for portable
application.
In order to realize the operation of PEMFCs without
external humidification, many composite membranes with
self-humidifying ability have been developed. Currently, the
researchers developed self-humidifying membranes mainly
focusing on the following directions: (1) incorporating Pt or
Pt/C catalysts in the membrane to combine the permeable oxygen and hydrogen to produce water and humidify the membrane
[4–6]; (2) incorporating hygroscopic metal oxides, such as SiO2 ,
or TiO2 to adsorb water and accordingly improve the proton
conductivity [4,7,8]; (3) incorporating some proton-conductive
particles, such as ZrP, HPA, ZrO2 /SO4 −2 or Cs2.5 H0.5 PW12 O40
to improve the proton conductivity of the membrane under dry
operation condition [9–12]. For the first method, how to avoid
the electron short circuit through the membrane after incorporating the Pt or Pt/C particles is an important issue. Many
researchers developed two-layered or three-layered membrane
Y. Zhang et al. / Electrochimica Acta 53 (2008) 4096–4103
structures to resolve this problem [13–15]. However, these membranes fabrication processes are too complex to spread widely.
Recently, many Pt-based supported catalysts with the support
of nonelectron-conducting as well as hygroscopic properties
(SiO2, zeolite) were synthesized and incorporated in the polymer matrix to fabricate the self-humidifying membranes [16,17].
These electron-insulated catalysts can avoid electron circuit in
the whole membrane, and keep good membrane hydration for
in situ adsorbing water produced at Pt particles on the surface
of hygroscopic supports. However, the supports of these catalysts were not proton-conductive materials and thus limited the
further enhancement of cell performance.
To enhance the proton conductivity of the membrane operated
at low humidity conditions, inorganic/organic composite membranes based on Heteropolyacids as the additive were widely
studied [18–21]. Among them, 12-Phosphotungstic acid (PTA)
of Keggin structure was the most widely used due to its high
acid strength. However, the extreme high water solubility of
PTA is a potential problem for its detrimental effect to the
membrane structure. Cs2.5 H0.5 PW12 O40, which was insoluble
for less exothermic of hydration enthalpy, was synthesized as
an additive used in PEMFCs in the recent reports [10,22]. Furthermore, it was reported that the acidity per unit acid site of
Cs2.5 H0.5 PW12 O40 was superior to Nafion-H as well as homogeneous acids, e.g., H2 SO4 , H3 PW12 O40 , and p-toluenesulfonic
acid [23]. Recently, Pt-Cs2.5 H0.5 PW12 O40 catalyst as a supported catalyst was widely studied for the application of skeletal
isomerization of n-butane [24,25]. So far, no research of Pt-Cs2.5
catalyst was investigated as an additive in self-humidifying composite membrane.
Currently, perfluorosulfonic acid (PFSA) membranes, in particular Nafion® , are a favorable option and are commonly used
in fuel cell stacks, but they are difficult to synthesize, and their
capital cost still remains high. In recent researches, SPEEK is
considered as a promising candidate of PEMs because it possess
good thermal stability, mechanical property, proton conductivity and low cost. Several studies have been reported on SPEEK
used as a PEM material in both hydrogen and direct methanol
fuel cells [26–28].
In the present study, the SPEEK/Pt-Cs2.5 H0.5 PW12 O40
(SPEEK/Pt-Cs2.5) self-humidifying membrane was fabricated
to improve the fuel cell performance using dry reactant
gas. The structure of SPEEK/Pt-Cs2.5 self-humidifying membrane was characterized by X-ray power diffraction (XRD),
Fourier transform infrared (FT-IR) spectroscopy, scanning
electron microscopy (SEM) and energy dispersive X-ray
detector (EDS). Furthermore, the physicochemical and electrochemical properties of the membranes, e.g., ion exchange
capacity (IEC) value, water uptake and proton conductivity
were also investigated. The results of single cell evaluation
showed that the SPEEK/Pt-Cs2.5 self-humidifying membrane exhibited better performance than the plain SPEEK
membrane under both wet and dry conditions. Electrochemical impedance spectroscopy (EIS) measurements were also
carried out on the plain SPEEK and SPEEK/Pt-Cs2.5 membrane under dry operation condition to further corroborate
the better cell performance of SPEEK/Pt-Cs2.5 membrane.
4097
Furthermore, additive stability and primary 100 h fuel cell
stability measurements were also conducted in the present
work.
2. Experimental
2.1. Preparation of the Pt-Cs2.5 H0.5 PW12 O40 catalyst and
the membranes
Pt-Cs2.5 H0.5 PW12 O40 catalyst (Pt-Cs2.5) was synthesized
by a titration method [29]. An aqueous solution of H2 PtCl6
(0.03 mol dm−3 ) was added to an aqueous solution of
H3 PW12 O40 (0.08 mol dm−3 ) at room temperature to obtain
a yellow solution. Then an aqueous solution of Cs2 CO3
(0.12 mol dm−3 ) was added dropwise to the mixture with
vigorous stirring at room temperature. The resulting milky
solution was evaporated at 50 ◦ C to solid and then reduced
by H2 at 200 ◦ C for 3 h. The designing loading of Pt on
Cs2.5 H0.5 PW12 O40 was 3 wt.%.
SPEEK polymers were prepared following the procedure reported in the literature [30]. The SPEEK/PtCs2.5 H0.5 PW12 O40 (SPEEK/Pt-Cs2.5) membrane was prepared
by solution cast method. First, the SPEEK was dissolved in N,N
dimethylacetamide (DMAc) at room temperature to prepare a
10 wt% solution. Then required quantity of 15.0 wt.% Pt-Cs2.5
catalyst was added to the polymer solution and stirred with a
magnetic stirrer for 4 h. The resulting solution was cast onto
a clean flat glass and then removed at 60 ◦ C for 12 h followed
by further drying at 120 ◦ C under vacuum. The loading of the
platinum in the membranes was 1.2 × 10−2 mg/cm2 . The thickness of the composite membrane was controlled to 24 ␮m. For
comparison, the plain SPEEK membrane was fabricated with
the same method and the thickness was also 24 ␮m.
2.2. Membrane characterizations
2.2.1. XRD measurement of the Pt-Cs2.5 catalyst
The X-ray power diffraction (XRD) analysis on the
Cs2.5 H0.5 PW12 O40 particles and Pt-Cs2.5 catalysts was performed using a Panalytical X’pert PRO diffractmeter (Philps
X’pert PRO) with Cu K␣ radiation source. The X-ray diffractogram was obtained for 2θ varying between 20 and 90◦ .
2.2.2. SEM-EDS measurement of the self-humidifying
membrane
The morphology of the cross-sectional SPEEK/Pt-Cs2.5
self-humidifying membrane was investigated by SEM (JEOL
6360LV, Japan) measurement. To determine the Pt-Cs2.5 catalyst distribution along the membrane cross-section, the Cs/S
elemental profiles across the sample thicknesses was carried out
by EDS (Oxford Instruments Microanalysis 1350).
2.2.3. FT-IR measurement of the membranes
FTIR spectrums of the Pt-Cs2.5 catalyst, plain SPEEK membrane and SPEEK/Pt-Cs2.5 self-humidifying membrane were
recorded on a JASCO FT-IR 4100 spectrometer. The KBr pellet
method was used to measure the spectrum of Pt-Cs2.5 catalyst.
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Y. Zhang et al. / Electrochimica Acta 53 (2008) 4096–4103
The Pt-Cs2.5 powder was mixed with KBr at ratio of 1 wt% and
the IR spectrum was measured. For the membranes measurement, the samples were dried at 100 ◦ C for 4 h and subsequently
measured using ATR mode.
2.2.4. Additive stability measurement
To determine the stability of Pt-Cs2.5 catalyst in the SPEEK
matrix, the samples were immersed in water and H2 SO4
(0.5 mol/L) at 60 ◦ C for 100 h, respectively. During this period,
the samples were taken out several times and dried to constant
weight and subsequently weighed. Thus, the weight changes
of the membranes as a function of the immersion time were
recorded. The SPEEK/PTA and SPEEK/Pt-Cs2.5 composite
membranes were measured in this test.
2.4. Single cell evaluation
Firstly, the single cells were operated at 60 ◦ C with fully
humidified H2 /O2 . The operation pressure was set at 0.20 MPa
and the gas utilizations were fixed at 90% for H2 and 50% for
O2 (40% when air was used). After stable performances were
obtained, the cells were then operated with dry gases. Before
operation with dry reactants, the cells were dried overnight
by flowing dry N2 . The fuel cell stability test was performed
by an intermittent process. The single cell was operated at
800 mA/cm2 with dry H2 /O2 during the day and left off during
the night.
3. Results and discussion
3.1. XRD measurement of the Pt-Cs2.5 catalyst
2.2.5. Ion exchange capacity (IEC) of the membranes
The IEC values of plain the SPEEK membrane and
SPEEK/Pt-Cs2.5 self-humidifying membrane were determined
by titration method: 2–3 g of the samples was placed in 1 M
aqueous NaOH and kept for 24 h. The solution was then back
titrated with 0.1 M HCl using phenolphthalein as an indicator.
2.2.6. Water uptake of the membranes
The water uptake of the membranes was calculated from Eq.
(1), W1 is the weights of the wet membrane after immersing in
water at 60 ◦ C for 6 h and W2 is the weight of the membrane
dried under vacuum at 100 ◦ C for 12 h.
W(wt.%) =
(W1 − W2 )
× 100%
W2
(1)
2.2.7. Proton conductivity and areal resistance measured
by EIS
Proton conductivity of the membranes was determined from
membrane resistance measured by electrochemical impedance
spectroscopy (EIS) over a frequency range of 100 mHz to
100 kHz. The membrane samples were humidified by vapor
water at 60 ◦ C in a sealed vessel described in the literature [31].
A frequency response daetector (EG&G model 1025, Princeton
Applied Institute) and a potentiostat/galvanostat (EG&G model
273A, Princeton Applied Institute) were employed for the measurements. Moreover, areal resistances of the cells operated at
the current density of 100 mAcm−2 under dry or wet conditions
were also measured by EIS.
Fig. 1 showed the results of XRD measurement employing
the Cs2.5 particle and Pt-Cs2.5 catalyst. The power XRD pattern of Cs2.5 presented the characteristic peaks corresponding to
the H3 PW12 O40 cubic phase and was consistent with the results
reported by other literatures [32,33]. For the Pt-Cs2.5 catalyst,
two obvious peaks corresponding to the Pt (1 1 1) and Pt (2 0 0)
were observed besides the peaks of pure Cs2.5 particle. According the Debye–Scherrer formula, the particle size of the Cs2.5
and Pt were about 12 and 4 nm, respectively.
3.2. SEM-EDS images of the self-humidifying membrane
It is desirable that the inorganic additive is high-uniformly
dispersed so as to increase the interface between the additive
and the polymer matrix and thus increase the possibility of their
synergism. To examine the morphology of the cross-sectional
SPEEK/Pt-Cs2.5 membrane and the distribution of Pt-Cs2.5 catalyst in the membrane, SEM attachment of EDS was conducted
and the results were shown in Fig. 2. From Fig. 2(a) it can be seen
that the cross-sectional SPEEK/Pt-Cs2.5 membrane appeared
dense and clean, with no agglomerates of Pt-Cs2.5 particles in
2.3. The membrane electrode assemblies (MEAs)
preparation
The MEAs with active area of 5 cm2 were fabricated by hotpressing method at 160 ◦ C and 10 MPa for 2 min. The anode and
the cathode were prefabricated using SGL carbon paper as the
substrate and the 46.6 wt.% Pt/C (TKK, Japan) as the catalyst.
The respective loadings of Pt and Nafion in the electrode were
0.4 mg/cm2 .
Fig. 1. XRD patterns of the Cs2.5 particle and Pt-Cs2.5 catalyst.
Y. Zhang et al. / Electrochimica Acta 53 (2008) 4096–4103
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Fig. 2. SEM and EDS images of the cross-sectional SPEEK/Pt-Cs2.5 self-humidifying membrane.
the whole membrane cross-section. This implied that Pt-Cs2.5
particles were not recrystallized into large particles after incorporating with SPEEK, but was highly dispersed throughout the
polymer matrix. Fig. 2(b) showed that the Cs element distributed
uniformly in the whole membrane cross-section. Furthermore,
as can be seen in Fig. 3, the Cs/S ratios were almost the same in
the whole membrane cross-section, indicating the good dispersive quality of Pt-Cs2.5 particles in SPEEK matrix. The uniform
distribution of additive is good for the membrane structure and
membrane performance.
3.3. FT-IR spectrum of the self-humidifying membrane
To obtain the structure information of the Pt-Cs2.5 catalyst, plain SPEEK membrane and SPEEK/Pt-Cs2.5 membrane,
FT-IR measurements were conducted and showed in Fig. 4.
The characteristic peaks of Pt-Cs2.5 catalyst were attributed
the peaks of Cs2.5 H0.5 PW12 O40 particles. From Fig. 4 typical characteristic peaks at 1080 cm−1 for υas (P–O), 890 cm−1
for υ(W–Oc–W) and 798 cm−1 for υ(W–Oe–W), which were
assigned to the Keggin’s structure of H3 PW12 O40 , were
observed in Pt-Cs2.5 spectrum, and the whole spectrum showed
Fig. 3. Relative intensity of Cs/S across the SPEEK/Pt-Cs2.5 self-humidifying
membrane.
to be a good accordance with those previous reported [34]. However, the typical vibration of υ(W = O) at 983 cm−1 splits into
two components at 992 and 984 cm−1 in the Pt-Cs2.5 spectrum. This splitting can be assigned as W = O associated with
Fig. 4. FT-IR spectrums of the Pt-Cs2.5 catalyst, plain SPEEK membrane and SPEEK/Pt-Cs2.5 composite membrane.
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Y. Zhang et al. / Electrochimica Acta 53 (2008) 4096–4103
3.5. IEC value measurement
The IEC values of the plain SPEEK and SPEEK/Pt-Cs2.5
membrane were listed in Table 1. It can be seen the SPEEK/PtCs2.5 composite membrane has the higher IEC value relative to
the plain SPEEK membrane, indicating more acid property than
the plain SPEEK membrane. This is attributed to the incorporation of high acidity Pt-Cs2.5 catalyst. The higher acid property
of the SPEEK/Pt-Cs2.5 membrane was beneficial to improve
water adsorbing and proton conducting abilities.
3.6. Water uptake measurement
Fig. 5. Weight changes of SPEEK/PTA and SPEEK/Pt-Cs2.5 composite membranes during the additive stability measurement.
H+ (H2 O)n species (984 cm−1 ), and W = O interacting with Cs+
ions (992 cm−1 ) [35]. For the plain SPEEK membrane, the peaks
at 1080, 1020 and 1257 cm−1 were assigned to the sulfonic acid
group in SPEEK [36]. In the case of the SPEEK/Pt-Cs2.5 selfhumidifying membrane, both characteristic peaks of Pt-Cs2.5
catalyst and plain SPEEK membrane were clearly found.
3.4. Additive stability measurement
For the heteropolyacids-based composite membranes, the
water stability of the heteropolyacids particles in the polymer
matrix was important for its close correlation to the membrane
structure stability. Fig. 5 showed the quantified weight changes
of the SPEEK/PTA and SPEEK/Pt-Cs2.5 composite membranes
as a function of immersion time. It can be seen that the weight
of the SPEEK/PTA composite membrane obviously decreased
during the 100 h immersion in water with the remaining weight
of 85.3%, which indicated the most of PTA particles leached out
from the SPEEK matrix during the measurement (original PTA
content in the membrane was 15 wt.%). In contrast, the weight
of the SPEEK/Pt-Cs2.5 membrane immersed in water slightly
decreased during the beginning 70 h with the weight loss of
2%, and was stable during the last 30 h immersion. The small
weight loss was attributed the leaching out of fine Cs2.5 particle [23]. Furthermore, the weight loss of the SPEEK/Pt-Cs2.5
membrane immersed in H2 SO4 (0.5 M) was almost the same
with that immersed in water, which indicated that the Cs+ did
not ion-exchanged by H+ during the immersion period. The good
stability of Pt-Cs2.5 catalyst in the SPEEK matrix is beneficial
to membrane structure stability.
For the PEMs, water uptake is an important property for its
direct relation to the proton conductivity. Table 1 showed that
the water uptake of SPEEK/Pt-Cs2.5 membrane was higher than
that of the plain SPEEK membrane, with the value of 30.6%
and 21.2% at 60 ◦ C, respectively. The similar trend of increasing water uptake after incorporation of Cs2.5 H0.5 PW12 O40 was
reported by Li et al. [10] and the reason was the hydrophilic property of Cs2.5 H0.5 PW12 O40 for its strong interaction with water.
When the membrane absorbs higher amount of water, the number of exchange sites available per cluster increases, this results
in the increase of the proton conductivity of the membrane.
Thus, compared to the plain SPEEK membrane, the property
of higher water uptake for SPEEK/Pt-Cs2.5 self-humidifying
membrane is expected to possess higher proton conductivity
under dry operation condition.
3.7. Proton conductivity of the membranes
Proton conductivity is the foremost requirement for PEMs,
higher proton conductivity resulting in higher cell performance.
Here, the membrane proton conductivity was determined by
measuring the membrane resistance at 60 ◦ C at fully hydrated
state by ac impedance. From Table 1, it indicated that the proton conductivity of the SPEEK/Pt-Cs2.5 membrane was higher
than that of the plain SPEEK membrane. The incorporation of
Pt-Cs2.5 catalyst increased the acidity and the water uptake of
the membrane from above experiments and thus increased the
proton conductivity. Furthermore, the addition of HPAs-based
particles in the polymer matrix may decrease the activation
energy for proton hopping by bridging proton conducting pathway between shrunken clusters, and thus increase the proton
conductivity according to the V.Ramani et al. [37].
3.8. Single cell evaluation
To verify the self-humidification effect of SPEEK/Pt-Cs2.5
self-humidifying membrane, the single cell performances of the
Table 1
Comparison of IEC, water uptake and proton conductivity between the plain SPEEK and SPEEK/Pt-Cs2.5 membranes
Membrane
Thickness (␮m)
IEC (mmolg−1 )
Water uptake (%,60 ◦ C)
Proton conductivity (S/cm, 60 ◦ C)
SPEEK
SPEEK/Pt-Cs2.5
24
24
1.81
1.94
21.2
30.6
0.042
0.053
Y. Zhang et al. / Electrochimica Acta 53 (2008) 4096–4103
Fig. 6. Performance comparison of single cell employing the plain SPEEK,
SPEEK/Pt-Cs2.5 membranes with wet and dry H2 /O2 at 60 ◦ C.
plain SPEEK membrane and SPEEK/Pt-Cs2.5 self-humidifying
membrane with dry H2 /O2 at Tcell = 60 ◦ C and with fully humidified H2 /O2 at TH2 = Tcell = TO2 = 60 ◦ C were evaluated, as
shown in Fig. 6. The single cell employing the SPEEK/PtCs2.5 self-humidifying membrane outperformed that of the
plain SPEEK membrane under fully humidified operation conditions, with the peak power density of 1.43 and 1.24 W cm−2 ,
respectively. This was consistent with their proton conductivity
results listed in Table 1. At dry operation condition, the plain
SPEEK membrane exhibited very poor output performance. As
shown in Table 2, the areal ohmic resistance of the cell employing the plain SPEEK membrane was large with the value of
0.145 cm2 at 100 mA/cm2 . Furthermore, in the IR-corrected
I–V curves shown in Fig. 7, the large polarization at both the
anode and the cathode, which was due to the low electrocatalyst utilization at dry condition, was also a reason leading to
the poor performance. In contrast, the SPEEK/Pt-Cs2.5 selfhumidifying membrane showed much better performance than
the plain SPEEK membrane. The areal ohmic resistance of
SPEEK/Pt-Cs2.5 self-humidifying membrane was 0.107 cm2
at 100 mA/cm2 , which was close to that of the fully humidified plain SPEEK membrane (0.103 cm2 ). The existence of
Pt-Cs2.5 catalyst can in situ adsorb the water produced on Pt
particles by chemical catalytic reaction of permeable H2 and O2
to hydrate the membrane, and meanwhile provide the new acid
sites for proton transport, thus leading to the small resistance.
Furthermore, the polarization at both electrodes was reduced by
using the self-humidifying membrane.
The open circuit voltage (OCV) is a good measurement
of hydrogen or oxygen crossover through the PEMs during the operating fuel cell. The cell with less hydrogen and
4101
Fig. 7. IR-corrected I–V curves of single cells employing the plain SPEEK,
SPEEK/Pt-Cs2.5 membranes with wet and dry H2 /O2 at 60 ◦ C.
Table 3
OCV values of single cells employing different membranes under dry and wet
operation
Membrane
SPEEK
SPEEK/Pt-Cs2.5
Open circuit voltage (V)
Wet operation
Dry operation
1.01
1.01
0.96
0.99
oxygen crossover will lead to a higher OCV value. Table 3
showed the OCV values of the plain SPEEK membrane and
SPEEK/Pt-Cs2.5 self-humidifying membrane under dry and wet
operation conditions. It was obvious that the single cells with the
SPEEK/Pt-Cs2.5 self-humidifying membrane exhibited higher
OCV values than those of the plain SPEEK membrane both
under dry or wet conditions. The Pt-Cs2.5 catalyst inside the
self-humidifying membrane could catalyze the permeable H2
and O2 and thus result in the higher OCV values.
The cell performances of a SPEEK/Pt-Cs2.5 selfhumidifying membrane at different operating temperatures
under dry operation conditions were presented in Fig. 8. It was
observed from Fig. 8 that the best performance was obtained
Table 2
Ohmic resistances of single cells operated at 0.1 A/cm2 under dry and wet
operation
Membrane
SPEEK
SPEEK/Pt-Cs2.5
Ohmic resistance (cm2 )
Wet operation
Dry operation
0.103
0.094
0.145
0.107
Fig. 8. Single cell performances employing the SPEEK/Pt-Cs2.5 membrane
with dry H2 /O2 at different operation temperatures.
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Y. Zhang et al. / Electrochimica Acta 53 (2008) 4096–4103
4. Conclusion
Fig. 9. Performance of single cell employing the SPEEK/Pt-Cs2.5 membranes
with dry H2 /Air at 60 ◦ C.
at 60 ◦ C. Increasing cell temperature bring two reverse effects
on the cell performance change, improved kinetics of the
cell reaction and better proton transport leading to increased
performance, and more water loss due to vaporization leading
to decreased performance.
Fig. 9 showed the single cell performance of the selfhumidifying membrane with dry H2 /Air at 60 ◦ C. The
performance of the plain SPEEK membrane was too unstable to
be recorded at this condition. However, it can be seen from Fig. 9
that the SPEEK/Pt-Cs2.5 self-humidifying membrane still have
peak power density of 0.54 W cm−2 , indicating that the Pt-Cs2.5
is a very effective additive for membrane self-humidification.
To determine the stability of fuel cell performance employing the SPEEK/Pt-Cs2.5 self-humidifying membrane, primary
100 h fuel cell operation test with dry H2 /O2 was conducted and
the results were shown in Fig. 10. It was observed that the performance with the SPEEK/Pt-Cs2.5 membrane does not exhibit
obvious drop on both OCV and the voltage at 800 mA/cm2 after
100 h operation at 60 ◦ C with dry reactants. However, the longterm operation of fuel cell with SPEEK/Pt-Cs2.5 membrane
would be investigated in the future work.
Fig. 10. Single cell stability measurement employing the SPEEK/Pt-Cs2.5
membrane at 60 ◦ C with dry H2 /O2 .
An inorganic/organic self-humidifying membrane SPEEK/
Pt-Cs2.5 was developed to improve the single cell performance
operating with dry H2 and O2 . The addition of supported catalyst
Pt-Cs2.5 can avoid the short circuit through the whole membrane due to the insulated property of the support. The XRD,
FTIR and SEM coupled EDS measurements were conducted to
characterize the catalyst property and the membrane structure.
The IEC, water uptake and proton conductivity measurement
indicated that the SPEEK/Pt-Cs2.5 self-humidifying membrane
has higher water adsorbing, acid and proton-conductive properties relative to the plain SPEEK membrane due to the highly
hygroscopic and acidy properties of Pt-Cs2.5 catalyst. The single cell employing the SPEEK/Pt-Cs2.5 membrane exhibited
higher cell OCV values and cell performances than those of plain
SPEEK membrane under dry or wet conditions. Furthermore, the
SPEEK/Pt-Cs2.5 membrane showed good water stability and
performance stability. Therefore, this self-humidifying membrane is very promising for application in PEM fuel cells.
Acknowledgement
This work was supported by National Natural Science
Foundation of China (Grant No. 20476104) and (Grant No.
50236010)
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