NEW PROTON CONDUCTIVE COMPOSITE MEMBRANES
BASED ON MF-4SC
E.Yu. Safronova, I.A. Stenina, A.B. Yaroslavtsev
Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of
Sciences, Moscow, Russia
Composite materials based on perfluorinated cation-exchange membrane
MF-4SC incorporating nanoparticles of silica and phosphotungstic acid were
synthesized. Transport properties of obtained membranes were studied. Composite
membranes have the higher proton conductivity than the polymer-only system. Ion
conductivity was studied at different relative humidity. Membrane modification by
silica and phosphotungstic acid results in the increase in water content and
conductivity, especially at low relative humidity.
INTRODUCTION
Perfluorosulfonic cation-exchange membranes, such as Nafion (Du Pont, USA),
Dow (Dow, USA), and MF-4SC (Plastpolymer, Russia) are widely used for
electrochemical synthesis, water purification and fuel cell applications (1,2).
However, all these membranes are expensive and have some disadvantages e.g. low
conductivity at low humidity, relatively low mechanical strength at high temperatures.
In order to improve their properties many approaches have been developed.
Incorporation of inorganic species can assist in improving membrane conductivity,
mechanical properties, water management, structure of pores and channels. Finely
dispersed inorganic particles contribute to ion sorption at the phase boundary which
results in defect concentration and conductivity increase (3-7). From this point of
view, silica with high sorption ability can be considered as perspective additive (3, 8).
Phosphotungstic acid (PWA) is a strong acid with anion size about 10Å and high
proton conductivity of about 2.10-1 S/cm at 25°C (9). Incorporation of PWA into
membrane matrix can lead to increase in electrical carrier concentration and water
content (10).
In this paper we report transport properties of composite membranes based on
MF-4SC membranes incorporating nanoparticles of silica and PWA.
EXPERIMANTAL
Composite membranes were obtained using “in situ” method. Commercial
MF-4SC membranes (thickness 50-60 m) were pre-treated by tetraethoxysilane
(≥98 %, Fluka). Each membrane was treated with diluted NH4OH, NaOH, HCl
solutions or boiling water to cause silica formation in membrane pores. PWA was
incorporated into membrane matrix by membrane treatment with diluted PWA
solution. For prevention of PWA washing out two methods were used: PWA sorption
by silica and its conversion into insoluble cesium form (CsxH3-xPW12O40). All
membranes were conditioned by means of standard techniques (11).
Ion exchange capacity (IEC) was determined by a standard procedure (12). A
weighed membrane sample was exposed to 0.5M NaCl for 24 h under continuous
stirring, then the membrane was decanted and alkali titrated with 0.10 M HCl.
The membrane samples were equilibrated at desired relative humidity inside a
desiccator containing saturated solutions of inorganic salts up to a constant weight
was achieved. The water content of membranes was determined as a difference
between the starting membrane weight and the weight of the membrane dried at
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110°C. The water content (n) was found as the number of water molecules per one
sulfo-group, n= [H2O]/[SO3]:
n=W/(Mr IEC),
(1)
where W is the water weight fraction and Mr is the water molecular weight (18 g/mol).
Proton conductivity was measured in water in the temperature range 20–100°C
and at different relative humidity at 25°C. An Elins Z-350m impedance analyzer (0.1
Hz to 1 MHz) in carbon/membrane/carbon symmetrical cells, with the active surface
area varied from 0.2 to 0.5 cm2. Conductivity values were obtained by semicircle
extrapolation to the resistance axis.
X-ray diffraction patterns were obtained using a Rigaku D/MAX-2000
diffractometer, Cu-Kradiation. Thermal analysis was performed on a Netzsch-TG
209 F1 instrument in aluminum crucibles, heating rate 5o/min in argon. The
morphology of membranes was analyzed using a transmission electron microscope
(Jeol, TEM-101) at an accelerating voltage of 100 kV. Before the TEM measurements
the specimens were ultrasonically dispersed in methanol and supported onto carboncoated Cu grids.
RESULTS AND DISCUSIONS
All the composite membranes are visually homogeneous. The incorporation of
inorganic solids results in a slight turbidity of membranes and fine dispersed oxide
formation after the membrane calcination at 900ºC. The transmission electron
micrograph of the MF-4SC/SiO2 composite membrane is shown in Fig. 1. SiO2
nanoparticles (dark spots) fill MF-4SC membrane pores.
According to the Gierke model (13) self-organization takes place in a membrane
matrix. It is presumed that there are clusters (about some nanometers in size) of
sulfonated perfluoroalkyl ether groups that are organized as the inverted micelles,
arranged in a lattice and filled by water molecules. These micelles form pores, which
are connected by channels. Inorganic ions or hydrophilic fragments of organometallic
compounds can be sorbed in these -SO3H-coated pores. Thus, it is possible to expect
oxide or hydroxide nanoparticles formation in these pores after the hydrolysis of the
organometallic precursor. The polymer matrix (membrane pores) acts as a template
for the direct growth of the inorganic phase within the ‘‘nanoreactors’’ of the ionic
clusters. According to thermal analysis data, silica content does not exceed 2–5% for
all composite membranes.
Fig. 1. Electron micrograph of MF-4SC membrane with in situ synthesized silica.
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X-ray data show that silica particles are amorphous at room temperature. Their
X-ray diffraction patterns present a broad halo, which can be attributed to highdisperse non-crystalline materials. Hydrous oxides, as a rule, crystallize during hightemperature treatment. However, X-ray data indicate that inorganic phase does not
crystallize even after membrane calcination at 900°C. This is due to the small size of
silica particles embedded in the membrane matrix, their isolation from each other, and
fluorine formation under heating (the high affinity of fluorine to silica results in
partial F/O exchange in SiO2). As a result, silica surface is highly defective and
contains considerable concentrations of fluoride ions. High defect concentration can
also inhibit silica crystallization. Sorption exchange capacity does not change
noticeably after modification and consists of 0.61 and 0.63 mmol/g for MF-4SC and
MF-4SC/SiO2 samples respectively. Probably, such difference is a result of low
dopant concentration.
Membrane modification by silica results in the increase of water content in
membrane (Fig. 2). The water loss of MF-4SC/SiO2 membranes is less than that of
initial MF-4SC membrane by about 20%. This is evidence of better water retention of
composite membranes. Thus, composite membranes can be more efficient at low
humidity.
Fig. 2. Water content as a function of relative humidity for MF-4SC (1) and
MF-4SC/SiO2 membranes (2).
Recently, it was shown that the pH of precipitation and treatment temperature
were the main factors influencing on the surface properties of hydrated zirconia (14,
15) and composite materials based on it (16). Therefore, the properties of materials
contains silica obtained by different methods were compared.
Proton conductivity as a function of temperature for some MF-4SC/SiO2
membranes is shown in Fig. 3. Composite membranes have the higher proton
conductivity than the polymer-only system measured under the same conditions. The
conductivity of MF-4SC/SiO2 (H) membranes (hydrolysis in acid solution) is higher
than that for MF-4SC/SiO2 (OH) membranes (hydrolysis in alkaline solutions) (Fig.3).
Silica structure can explain this difference. Modification of the silica surface during
formation in acid solution imparts acid properties. This can be considered as sorption
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of additional protons. It is impossible to remove these protons even after flushing. As
a result charge carrier concentration and conductivity increase. The activation energy
of conductivity for MF-4SC/SiO2 (H) is higher than that of MF-4SC/SiO2 (OH)
membranes. Change in the activation energy of the composite membranes can be
attributed to the change in the defect formation energy at the phase boundary between
the membrane matrix and nanoparticles of silica due to ion sorption. The activation
energy of conductivity can be expressed as the sum of activation energy of proton
migration and a half of defect formation energy. Sorption of protons of SO3H-groups
by SiO2 at the interface of the membrane matrix and silica leads to defect formation
energy decrease. High proton activity on the surface of the silica hydrolyzed in acid
solution hinders this process. Probably, this results in activation energy increase for
MF-4SC/SiO2 (H) membranes.
Fig. 3. Ion conductivity as a function of temperature for MF-4SC membrane (1);
MF-4SC/SiO2(H) membrane – hydrolysis by acid solution (2) and MF-4SC/SiO2(OH)
membrane – hydrolysis by alkaline solution (3).
Incorporation of PWA or its cesium salt can additionally increase the
conductivity. PWA is a high proton conductive material. Ion conductivity as a
functions
of
temperature
for
MF-4SC/SiO2(OH)/CsxH3-xPW12O40
and
MF-4SC/CsxH3-xPW12O40 is shown in Fig. 4. Simultaneous incorporation of cesium
salt and silica results in conductivity increase. But conductivity of MF-4SC/SiO2
membranes is higher (Fig. 3, 4). Modification by cesium salt of PWA results in the
decrease in conductivity in the comparison with initial membrane.
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Fig. 4. Ion conductivity as a function of temperature for MF-4SC membrane (1);
MF-4SC/SiO2(OH)/CsxH3-xPW12O40 (2) and
MF-4SC/CsxH3-xPW12O40 (3) membranes.
The relative humidity decrease leads to the conductivity decrease for all
materials, especially for initial MF-4SC membrane at RH<40% (Fig. 5). The
conductivity of MF-4SC membrane at 95% is higher than that at 9% by 3 orders of
magnitude. Membrane modification by silica results in the conductivity increase at
low RH. The conductivity of MF-4SC/SiO2 exceeds that of initial membrane by 1.5
orders of magnitude at RH=9%.
Incorporation of PWA or its cesium salt into MF-4SC/SiO2 results in the
considerable
conductivity
increase
(Fig.
5).
The
conductivity
of
(MF-4SC/SiO2/H3PW12O40 or MF-4SC/SiO2/CsxH3-xPW12O40) composite systems is
higher than that of MF-4SC/SiO2 membranes at RH<80%. Thus, composite effect
reaches 2.5 orders magnitude at RH=9%.
Fig. 5. Ion conductivity as a function of relative humidity for MF-4SC (1),
MF-4SC/SiO2 (2), MF-4SC/SiO2/H3PW12O40 (3) and
MF-4SC/SiO2/CsxH(3-x)PW12O40 (4) membranes.
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The conductivity of composite membranes containing PWA or its cesium salt
exceeds that for three-component systems with SiO2 by 0.2 orders of magnitude
(Fig. 6). The conductivity of membranes with cesium salt of PWA is slightly lower
than for membranes with PWA. Conversion of PWA into insoluble cesium form can
prevent dopant washing out.
Fig. 6. Ion conductivity as a function of RH for MF-4SC/SiO2(OH)/H3PW12O40 (1);
MF-4SC/SiO2(OH)/CsxH3-xPW12O40 (2); MF-4SC/H3PW12O40 (3) and
MF-4SC/CsxH3-xPW12O40 (4) membranes.
The modification of MF-4SC membranes by silica, PWA or its cesium salt
nanoparticles results in the proton conductivity increase especially at low humidity.
This work was supported by the Program №27 of Presidium of the Russian
Academy of Sciences “Bases of fundamental investigations of nanotechnologies and
nanomaterials”.
REFERENCES
1. Mauritz K.A., Moore R.B. State of understanding of Nafion. Chem. Rev. 2004 104
4535-4585.
2. Kreuer K.-D., Paddison S.J., Spohr E., Schuster M. Transport in proton conductors
for fuel-cell applications: simulations, elementary reactions, and phenomenology.
Chem. Rev. 2004 104 4637-4678.
3. Bonnet B., Jones D., Roziere J., Tchicaya L., Alberti G., Casciola M., Massinelli
L., Bauer B., Peraio A., Ramunni E. Hybrid organic-inorganic membranes for a
medium temperature fuel cell. J. New Mater. Electrochem. Syst. 2000 3 87-92.
4. Jones D.J., Roziere J. in Handbook of Fuel Cells - Fundamentals, Technology and
Applications New York, John Wiley & Sons. 2003.
5. Yaroslavtsev A.B., Nikonenko V.V., ZabolotskyV.I. Ion transfer in ion-exchange
and membrane materials. Russ. Chem. Rev. 2003 72 393-421.
6. Xu T. Ion exchange membranes: State of their development and perspective.
J.Membrane Sci. 2005 263 1-29.
216
7. Tazi B., Saadogo O. Parameters of PEM fuel-cells based on new membranes
fabricated from Nafion, silicotungstic acid and thiophene. Electrochem. Acta 2000
45 4329-4339.
8. Adjemian K.T., Lee S.J., Srinivasan S., Benziger J., Bocarslya A. B. Silicon oxide
Nafion composite membranes for proton-exchange membrane fuel cell operation at
80-140°C. J. Electrochem. Soc. 2002 149 256-261.
9. Yaroslavtsev A. Ionic exchange on inorganic sorbents. Russian Chem. Rev. 1997
66 (7) 641-660.
10. Staiti P., Freni S., Hosevar S. Synthesis and characterization of proton-conducting
materials containing dodecatungstophosphoric and dodecatungstosilic acid
supported on silica. J. of Power Sources 1999 79 250-255.
11. Berezina N.P., Timofeev S.V., Kononenko N.A. Effect of conditioning
techniques of perfluorinated sulphocationic membranes on their hydrophylic and
electrotransport properties. J.Membr. Sci 2002 209 509-518.
12. Berezina N. P., Kononenko N. A., Dvorkina G. A., Shel’deshov N.V. Physical
and Chemical Properties of Ion-Exchange Materials. Krasnodar, Izd-vo Kubansk.
Gos. Univ., 1999 [in Russian]
13. Hsu W.Y., Gierke T.D., Molnar C.J. Morphological effects on the physical
properties of polymer composites. Macromolecules 1983, 16, 1945–1947.
14. Stenina I.A., Voropaeva E.Yu., Brueva T.R., Sinel’nikov A.A., Drozdova N.A.,
Ievlev V.M., Yaroslavtsev A.B. Heat-treatment induced evolution of the
morphology and microstructure of zirconia prepared from chloride solutions.
Russian Journal of Inorganic Chem. 2008 53 (6) 842–848 [In Russian].
15. Stenina I.A., Voropaeva E.Yu., Veresov A.G., Kapustin G.I., Yaroslavtsev A.B.
Effect of precipitation pH and heat treatment on the properties of hydrous zirconium
dioxide. Russian Journal of Inorganic Chem. 2008 53 (3) 397–403.
16. Voropaeva E.Yu., Stenina I.A., Yaroslavtsev A.B. Proton conductive of composite
materials on acid indium sulphate and hudrated zirconium dioxide. Russian Journal
of Inorganic Chem. 2007 52 (1) 5-11.
217