Electrochemical and Solid-State Letters, 7 共10兲 A315-A317 共2004兲
A315
1099-0062/2004/7共10兲/A315/3/$7.00 © The Electrochemical Society, Inc.
Pt-CÕSPEEKÕPTFE Self-Humidifying Composite Membrane
for Fuel Cells
Dan-min Xing,z Bao-lian Yi, Yong-zhu Fu, Fu-qiang Liu, and Hua-min Zhang
Fuel Cell Research and Development Center, Dalian Institute of Chemical Physics,
Chinese Academy of Science, Dalian 116023, China
A Pt-C/sulfonated poly共ether ether ketone兲/polytetrafluoroethylene 共Pt-C/SPEEK/PTFE兲 self-humidifying composite membrane
for the H2 /O2 fuel cell was developed. Compared with the performance of the cells prepared with SPEEK/PTFE composite
membranes, the performance with the self-humidifying composite membranes was appreciably improved when holding the
humidity of cathode gas constant. The Pt-C/SPEEK/PTFE composite membrane can supply water to the anode electrode side
under dry H2 condition and result in good cell performance.
© 2004 The Electrochemical Society. 关DOI: 10.1149/1.1792274兴 All rights reserved.
Manuscript submitted December 23, 2003; revised manuscript received March 5, 2004. Available electronically September 3,
2004.
Proton exchange membrane fuel cells 共PEMFCs兲 attract much
attention due to their high efficiency, high power density, zero emission, and low-temperature start-up as power sources suitable for
both stationary and mobile application. Low cost and high efficiency
are the important factors affecting its popularization.1,2 At present,
there are many investigations that focus on the sulfonated poly共ether
ether ketone兲 共SPEEK兲 PEM as the substitute for perfluorosulfonated ionmer membranes.3,4 Another deficiency of PEMFCs is
that their performance is greatly influenced by the state of humidification of the solid PEMs, therefore the fill gas must be humidified
when the fuel cell is running.
Watanabe et al.5-7 proposed a concept of self-humidifying composite membranes in which the nanometer-size Pt and/or metal oxides were highly dispersed in the Nafion resin. The Pt particles catalyze the oxidation of crossover hydrogen with oxygen to generate
water to humidify the membrane. The process of dispersing the Pt
particles is shown as the Pt nanocrystallites were dispersed in the
PEMs by the cation exchange treatment with a 关 Pt(NH3 ) 4 兴 Cl2 solution at 60°C overnight, followed by reducing the exchanged Pt2⫹
with hydrazine. This method has many disadvantages, such as nonuniform distribution of Pt particles throughout the membrane and
formation of an electron-conducting path via the network of dispersed Pt particles. Recently, Yang et al.8 and Kwak et al.9 developed a self-humidifying membrane with sandwich structure. A sputtered Pt layer was hot-pressed in the middle of two Nafion resin
layers. The prepared composite membranes are very thick and gas
permeability and water generated in the membranes are very limited.
In
our
previous
work,
we
developed
Nafion/
polytetrafluoroethylene 共PTFE兲 self-humidifying composite
membranes.10 Using the solution-cast method, the Pt-C selfhumidifying layer adhered to the anode side of the Nafion/PTFE
composite membrane. The permeating oxygen and hydrogen was
combined in the Pt-C self-humidifying layer to form water to humidify the membrane. Thus, by simply changing the composition of
the cast membrane solutions and following the fabrication procedure
for a Nafion/PTFE membrane, a self-humidifying function can be
obtained. The cells using these Pt-C-PEMs could minimize membrane conductivity loss under dry conditions.
Here, novel self-humidifying Pt-C/SPEEK/PTFE composite
membranes were developed to improve the mechanical strength of
the membrane and to increase the cell performance under dry gas
conditions. The Pt/C catalyst particles were dispersed in the SPEEK
layer that was adjacent to the anode side in the mebrane electrode
assembly 共MEA兲. EPMA revealed that the Pt/C catalyst particles
were surrounded by the SPEEK resin and could catalyze the chemical reaction of hydrogen and oxygen to produce water. The Pt-C/
SPEEK/PTFE composite membrane can supply water to the anode
electrode side under dry condition, The performance of the cells
z
E-mail: [email protected]
with the SPEEK membrane and SPEEK/PTFE composite membrane
were compared and the influence of humidification on the performance of the cell with the self-humidifying composite membrane
was investigated.
Experimental
The SPEEK/PTFE composite membranes were prepared by
pouring 5-10% SPEEK solution in 1-methyl-2-pyrrolidinone 共NMP兲
and acetone on a porous PTFE film 共15 ␮m thick兲 extended over a
flat glass plate. The glass plate was first dried at 50-60°C, and then
dried in a vacuum oven at 100°C for 24 h. The membranes were
30-70 ␮m thick.
The Pt-C/SPEEK/PTFE self-humidifying composite membranes
were obtained by dispersing the Pt/C catalyst 共20% Pt兲 in the
SPEEK solution. The solution was then cast on the SPEEK/PTFE
composite membrane to form the Pt/C catalyst layer. The amount of
Pt loading in the composite membranes was controlled at
0.05 mgPt/cm2 . The thickness of the composite membranes was
controlled by the amount of SPEEK solution.
The MEA was assembled by a hot-pressing process. Two electrodes with active area 5 cm2 were hot-pressed onto one piece of
membrane at 140°C and 2.0 MPa for 1-2 min to form the MEA. The
loading of Pt/C catalyst on the electrode was 0.3 mgPt/cm2 .
The MEA was positioned in a single cell with stainless steel end
plates and stainless steel mesh as current collectors. First, the cells
were operated with humidified reactant gases. The cell was operated
at 80°C and the humidifier temperatures for H2 and O2 were 80°C.
The pressures of H2 and O2 were each set at 0.20 MPa. The fluxes of
H2 and O2 were controlled at 10 and 15 mL/min, respectively. After
stable performance was obtained, the cells were then operated with
either low temperature humidifier or dry gas. Before the performance was obtained, the cell was maintained at each experimental
condition for 8 h.
The elemental distribution analysis of the composite membranes
was tested with a EPMA-1600 共electron probe microanalyzer, Shimadzu兲.
The oxygen permeability of composite membranes was measured by a gas chromatographic method.12
Mechanical strength 共maximum strength and break strength兲 of
the membranes was measured with a tension tester, AG2000A 共Shimadzu, AUTO graph兲, at room temperature. The programmed elongation rate was 50 mm/min.
The conductivity of the membranes was measured with electrical
impedance spectroscopy 共EIS兲. A frequency-response detector
共EG&G model 1025兲 and a potentiostat/galvanostat 共EG&G model
373A兲 were used.
Results and Discussion
Self-humidifying composite membrane.—As we know, the humidity is more important for the anode than for the cathode because
Electrochemical and Solid-State Letters, 7 共10兲 A315-A317 共2004兲
A316
Table I. Oxygen permeability, mechanical strength, and conductivity of the SPEEK membrane and composite membranes.
Membrane
SPEE
K
SPEEK/PT
FE
Pt-C/SPEEK/PT
FE
Thickness
共mm兲
Oxygen permeability ⫻
109
(m3 "m/m2 "s"MPa at
80°C兲
Maximum strength
(kgf/mm2 )
Break strength
(kgf/mm2 )
Dimensional change
共in water at 80°C, %兲
Conductivity
共s/cm兲
0.065
0.065
0.065
0.405
1.44
2.22
4.69
6.48
6.00
3.84
4.93
4.11
3.5
3.2
0.048
0.076
59.0
0.096
the water is formed in the cathode during the cell operation. Gases
can permeate through PEMs. That means that the hydrogen and
oxygen have a crossover through the membranes and they can combine to form water in the PEMs and cause humidification. The permeability of oxygen in the SPEEK/PTFE composite membrane is
larger than that of the SPEEK membrane as shown in Table I. It is
believed that oxygen diffuses through the hydrophobic part of the
membrane and hydrogen diffuses through the ion-exchange functional groups in the membrane.12 Because the SPEEK/PTFE composite membranes have larger mechanical strength and better dimensional stability than SPEEK membranes as shown in Table I, we can
prepare thinner composite membranes to allow more oxygen to diffuse from cathode to anode. We prepared a multilayer composite
membrane by adding a Pt-C/SPEEK self-humidifying layer to the
SPEEK/PTFE composite membrane. It was used in a fuel cell and
the structure is shown in Fig. 1. The Pt/C catalyst particles in the
self-humidifying layer can cause combination of the oxygen that
diffused from cathode with hydrogen to form water at the anode side
of the PEM, which could contribute to humidification of the PEM.
Figure 2. EPMA image of a cross section of the self-humidifying composite
membrane 共image 5 is the absorption electron bean image兲.
are surrounded by the SPEEK resin and are not on the surface of the
membrane. This means that the Pt/C particles can catalyze the
chemical reaction of hydrogen and oxygen but not transmit electrons, which will reduce the efficiency of the cell. The loading of Pt
in the membrane was too low to analyze quantificationally.
EPMA study of the self-humidifying composite membrane.—The
elemental distribution of the Pt-C/SPEEK/PTFE self-humidifying
composite membrane was analyzed by EPMA, and the results are
shown in Fig. 2 and 3. It can be seen that in the membrane, sulfur
共S兲 is dispersed homogeneously along the cross section of the membrane, which means that the SPEEK resin was uniformly dispersed
in the membrane. Fluorine 共F兲 is distributed on one side of the
membrane and the extent is in agreement with the thickness of the
porous PTFE film, which means that the porous PTFE film is asymmetrically positioned in the composite membrane. The most important information is that carbon 共C兲 is asymmetrically dispersed in the
membrane because Pt/C catalyst was added in the composite layer.
The other interesting phenomenon is that the Pt/C catalyst particles
Figure 1. Schematic diagram of the MEA with Pt-C/SPEEK/PTFE selfhumidifying composite membrane: 共1兲 Anode; 共2兲 Pt-C/SPEEK selfhumidifying layer (d ⫽ 0.010-0.020 mm); 共3兲 SPEEK layer (d
⫽ 0.010-0.030 mm); 共4兲 SPEEK/PTFE (d ⫽ 0.015 mm) composite membrane layer; 共5兲 cathode.
Figure 3. EPMA elemental analysis result for the self-humidifying composite membrane 共the label of length is same as image 5 in Fig. 2兲.
Electrochemical and Solid-State Letters, 7 共10兲 A315-A317 共2004兲
Figure 4. Voltage-current performance of H2 /O2 fuel cell with SPEEK/
PTFE composite membrane operated at different gas humidifier temperatures.
Fuel cell performance.—In Fig. 4, the voltage-current polarization curves of fuel cells with SPEEK/PTFE composite membranes
are shown. It is obvious that the cell voltage rapidly drops at high
current density; this is because the PTFE in the composite membrane is hydrophobic and cannot transfer protons, so the conductivity of the composite membranes is low. This defect can be expiated
by thinning the membrane, a change which is possible due to its
excellent mechanical strength. The voltage-current performance of
the composite membrane is considerably influenced by the humidity
conditions. The current density at 0.6 V decreases 80% when the
humidifier temperature falls 40°C.
The Pt/C catalyst layer of the self-humidifying SPEEK/PTFE
composite membrane was arranged at the anode side in the MEA
shown as Fig. 1. The amount of Pt/C catalyst is small compared to
the amount of SPEEK and the catalyst particles were uniformly
dispersed in the SPEEK. The Pt-C catalyst can only chemically catalyze the reaction of the crossover gases to generate water. During the
operation of the self-humidifying PEMFC using dry gases, the water
will humidify the proton exchange membranes.
Figure 5 shows the voltage-current performance of a H2 /O2 fuel
cell with Pt-C/SPEEK/PTFE composite membrane operated at different gas humidifier temperatures. The performance of the cell with
a self-humidifying composite membrane was better than that with
the SPEEK/PTFE membrane. This is because of the conductivity of
the self-humidifying composite membrane is larger than the latter as
shown in Table I. At the same time, the performance of the fuel cell
with a self-humidifying composite membrane improved appreciably
when the degree of humidification was decreased. Compared with
the SPEEK/PTFE composite membrane with the same thickness, the
performance of the cells with the self-humidifying composite membrane is only slightly influenced by humidification. The results
prove that the Pt-C/SPEEK/PTFE composite membrane could supply water to the anode electrode side under dry condition, resulting
in better cell performance. The reason of the performance with
A317
Figure 5. Voltage-current performance of H2 /O2 fuel cell with Pt-C/
SPEEK/PTFE composite membrane operated at different gas humidifier temperatures.
self-humidifying membrane was not larger at T H2 /T cell /T O2
⫽ 80/80/80°C than at 40/80/40°C and 80/80°C/dry should be that
water flooding possibly occurred when the cell was completely humidified.
Conclusions
A Pt-C/SPEEK/PTFE self-humidifying composite membrane for
a H2 /O2 fuel cell was developed. EPMA analysis shows that the
Pt/C catalyst particles were uniformly dispersed on one side of the
composite membrane and were surrounded by the SPEEK resin.
Compared to the cells with a SPEEK/PTFE composite membrane,
the performance of the cells with the self-humidifying composite
membrane is only slightly influenced by the humidification. The
Pt-C/SPEEK/PTFE composite membrane can supply water on the
anode electrode side resulting in good cell performance under the
dry H2 condition.
The Chinese Academy of Science assisted in meeting the publication
costs of this article.
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