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Plasma enhanced chemical vapor deposition (PECVD) technique has been employed to deposit nano-scale lms of silica (10, 32,
68 nm) on Naon membrane. Ion conductivity, methanol permeability and single cell performance of the resultant nano-silica/Naf-
most attractive power sources for wide applications
proton conductivity and chemical stability, in spite of
ability for methanol, stability against radical attack and
membranes [1318]. The third approach is to employ
surface modied Naon membranes exposed to lowdose electron beam (EB) irradiation, radiation-modied
Naon membranes with vinylphosphonic acid (VPA),
plasma polymerized barrier lms on Naon membranes,
.
* Corresponding author. Tel.: +82-2-958-5275; fax: +82-2-958-
5199.
E-mail address: [email protected] (H.Y. Ha).
Electrochemistry Communications1388-2481/$ - see front matter 2004 Elsevier B.V. All rights reservedfrom portable power sources to vehicular applications,
due to the simplicity of the system and the adaptability
of liquid fuel, methanol. One of the main impediments
to practical realization of the DMFC as a power sourceis crossover of methanol through the polymer electrolyte
membrane (PEM). This results in poor cell performance.
It is widely accepted that commercially available Naf-
ion membrane is an advanced material in the present
days as solid electrolyte for low temperature fuel cells.
Naon membrane has attractive properties like good
low cost.
Intensive research eorts are focused mainly on
decreasing the crossover of methanol through the poly-
mer electrolyte membrane while maintaining good ionconductivity [13]. These attempts on the polymer elec-
trolyte membrane in DMFCs can be broadly classied
into three categories. First one is to manufacture new
membrane that can be synthesized from polyhydrocar-
bon materials or peruorinated materials [412]. Second
one is to employ inorganicorganic polymer compositeion composite membranes were measured to ascertain its suitability as a candidate membrane for direct methanol fuel cell (DMFC)
applications. Experimental results revealed that ion conductivity of the composite membrane containing silica lm with 10 nm thick-
ness was similar to the unmodied Naon membrane, but its methanol permeability was reduced to an extent of 40%. Cell perform-
ance of the composite membrane with 10 nm silica was higher than that of the bare Naon membrane by about 20%. The open
circuit voltage (OCV) was increased and the cell temperature at OCV was decreased with an increase in the thickness of the silica
lm. Physical and electrochemical analyses were conducted to investigate the properties of silica-layered membrane and the DMFC
employing the membrane.
2004 Elsevier B.V. All rights reserved.
Keywords: Composite membrane; DMFC; Ion conductivity; Methanol permeability; Silica/Naon; PECVD
1. Introduction
A direct methanol fuel cell (DMFC) is one of the
its limitation including high cost and methanol permea-
tion. Thus, requirements for a good polymer electrolyte
membrane include high proton conductivity, low perme-Nano-silica layered composite mfor direct met
Daejin Kim, M. Aulice Scibioh, Soonjo
Fuel Cell Research Center, Korea Institute of Science and Tec
Received 16 June 2004; received in rev
Abstractdoi:10.1016/j.elecom.2004.07.006mbranes prepared by PECVDnol fuel cells
Kwak, In-Hwan Oh, Heung Yong Ha *
gy, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea
orm 6 July 2004; accepted 7 July 2004
www.elsevier.com/locate/elecom
6 (2004) 10691074
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wt%) decreased methanol uptake from the liquid phase.
Dimitrova et al. [25,26] noted a rise in conductivity with
2.2. Measurement of ion conductivity and methanol
permeability
Ion conductivity of the Naon/silica composite mem-
branes was measured by using a complex impedance
analyzer (ZAHNER IM-6) with a four-point probeAC electrochemical impedance spectroscopy [36,37]. A
conductivity cell was made up of two platinum foils car-
rying the current and two platinum wires sensing the
potential drop. The membrane was held in between
two platinum electrodes and the IM-6 impedance ana-
lyzer was functioning in galvanostatic mode with AC
current amplitude of 0.01 mA at frequency range from
8 MHz to 10 mHz.Methanol permeability was measured using an in-
house built permeation cell which consisted of two
compartments that were separated by the membrane
[4]. A 5% methanol solution was fed into one com-
partment of the cell and deionized water was circu-
lated through the other compartment. Each
compartment was stirred continuously during permea-
bility measurement. A dierential refractive indexdetector was employed to monitor the methanol
permeability.
Communications 6 (2004) 10691074the addition of silicon dioxide particles (Aerosil 380) toNaon compared to the unmodied Naon 117. In
particular, the silica nano-particles retain water even at
high temperatures and this property may prevent dry-
ing of the membrane during the fuel cell operation.
Walker [27] added untreated fumed silica in a proton-
conducting polymer membrane to increase water
absorption and water retention capacity, since fumed sil-
ica is hydrophilic with an ability to adsorb moisture.Jung et al. [18] reported that the proton conductivity
in the silica hybrid membranes was lower than, or equal
to, that in unmodied Naon membranes.
Plasma enhanced chemical vapor deposition
(PECVD) is a lucrative technique for layer fabrications
since one can have easy control over factors like lm
thickness, refractive index and roughness of the layers,
in addition to low deposition temperatures and highdeposition rates associated with this system [28,29].
Hence, PECVD technique was advantageously em-
ployed for the low-temperature deposition of silica lms
[3035].
Grounded on these properties of silica and PECVD
technique, we have prepared nano-silica layered Naon
membrane by PECVD technique. The morphology of
Naon/silica composite material was studied using scan-ning electron microscopy (SEM) and their properties as
a candidate membrane for DMFC were investigated in
terms of ionic conductivity, methanol permeability and
single cell performance measurements.
2. Experimental
2.1. Preparation of Naon/silica composite membranes
Thin layer of silica was deposited on the surface of
Naon 115 membranes by using a PECVD system as
follows. Silicon ethoxide gas was pumped into the cham-
ber where electric eld from RF power source of 10500
W was applied at pressures ranging from 1 to 500 mTorr
at ambient temperature. The thickness of silica layercoated on Naon membrane has been controlled by var-the surface-modied Naon membranes using plasma
etching and palladium-sputtering and Pd-layered Naon
membranes, and so on [1923].
Among these new membranes for DMFCs, the
Naon/silica hybrid membrane exhibited advantageous
properties like higher water uptake, lower methanol up-take, higher ion conductivity and greater mechanical
strength than the bare Naon membrane though the
improvements are somewhat controversial depending
on the reports. Miyake et al. [24] reported that the hy-
brid membranes with higher silica content (10 and 21
1070 D. Kim et al. / Electrochemistryying the deposition time.2.3. Single cell performance measurement in DMFC
Electrocatalysts used in the anode and cathode
were unsupported PtRu (50:50 at.%) black and PtFig. 1. Schematic of a passive DMFC.
-
black (Johnson Matthey), respectively. The catalyst
slurry containing Naon solution (DuPont) was di-
rectly scattered on the Naon/silica composite mem-
branes using a spray gun and 8 mg/cm2 of catalyst
was coated. Teonized carbon paper (Toray) was set
on the both side of the membrane electrode assembly(MEA) that could function as diusion layers. The cell
was held together between plastic plates by means of a
set of eight retaining bolts. Single cell performance
was measured by using a passive DMFC where 4.5
M methanol solution was put into the built-in reser-
voir and methanol was diused into the anode and
air was supplied to the cathode from environment
by a kind of air-breathing action (Fig. 1) [38]. The cellperformance experiments were carried out at room
temperature (273K) and at atmospheric pressure.
Polarization curves were obtained by using an elec-
tronic load (DAE GIL 200P).
3. Results and discussion
Thin silica lms were formed on the Naon mem-
brane by PECVD technique. Thickness of silica lm
was controlled by changing the deposition time. At rst,
the morphology and microstructure of composite mem-branes were examined using SEM. The SEM images of
surface and cross sectional view of the Naon/silica
composite membranes are shown in Fig. 2. A smooth sil-
ica lm was formed on the membrane. Some cracks were
found in the surface of the coated silica layer and it has
been observed that the gaps in the cracks were increased
in proportion to an increase in the thickness of silica
layer. It is assumed that the cracks might be caused byan increase of cohesive energy according to an increase
in the thickness of silica layer and also by a dierence
in the degree of swelling between two materials. Further,
it can be seen that the silica layer with thickness of 32
D. Kim et al. / Electrochemistry Communications 6 (2004) 10691074 1071Fig. 2. SEM images of Naon/silica composite membranes for various thickness of silica lm: (a) 10 nm; (b) 32 nm; (c) 68 nm.
-
and 68 nm in composite membrane were found to be de-
tached from Naon lm. Consequently, the contact
resistance between electrode and membrane would in-
crease, causing a reduction in cell performance when
used as solid electrolyte membrane material in fuel cell
[39,40].The ion conductivity and methanol permeability of
Naon/silica composite membranes are given in Table
1. The relative ion conductivity and methanol permea-
bility in the Naon/silica composite membranes com-
pared with those in untreated Naon membrane are
given in parenthesis. The ion conductivity of the Naf-
ion/silica composite membrane with 10 nm thickness
was similar to the unmodied Naon membrane(0.091 S/cm), however, its methanol permeability
(1.68 106 cm2/s) was reduced to an extent of 40%.Naon/silica composite membranes with thicker silica
layers such as 32 and 68 nm showed lower ion conduc-
tivity by about 20% and their methanol permeability
were reduced up about 70% than those of bare Naon
membrane. Fig. 3 shows the ratio between ion conduc-
tivity and methanol permeability, a characteristic factor(U), in each Naon/silica composite membrane underthis study. It can be seen that the ratios (U) of all thecomposite membranes coated with silica are higher than
that of bare Naon. It indicates that the Naon/silica
composite membranes exhibit superior characteristics
The cell temperature decreased and OCV increased with
increasing the thickness of silica lm, indicating that the
Naon/silica composite membranes reduced methanol
crossover to the cathode side and thereby the extent of
methanol oxidation reaction and the mixed potential
generated in the cathode are decreased. That is, the
amount of permeated methanol through the polymerelectrolyte membrane was lowered according to increase
in thickness of silica layer.
Fig. 6 shows the resistance of the cells with Naon/sil-
ica composite membranes that was measured by AC mil-
liohm tester (HIOKI) at OCV. The resistance of Naon/
silica composite membrane with 10 nm silica thickness
was same as that of bare Naon, while the other Naf-
ion/silica composite membranes with higher silica thick-ness (32, 68 nm) showed cell resistance two or three
times larger than that of the Naon membrane. This re-
sult further conrmed the poor cell performance of
DMFC when Naon/silica composite membranes with
1072 D. Kim et al. / Electrochemistry Communications 6 (2004) 10691074Table 1
Ion conductivity and methanol permeability of Naon/silica composite
membranes
Membrane material Ion conductivity
(S/cm)
Methanol permeability
(cm2/s)
Naon 115 0.098 (100%) 2.77 106 (100%)Naon10 nm silica lm 0.091 (93%) 1.68 106 (61%)Naon32 nm silica lm 0.076 (78%) 0.91 106 (33%)Naon68 nm silica lm 0.077 (79%) 0.92 106 (33%)
Nafion 10 nm silica 32 nm silica 68 nm silica0
20
40
60
80
. 10
-3
(-1 cm
-3 s)
Fig. 3. Ratio (U) of ion conductivity to methanol permeability of
Naon/silica composite membranes.than Naon membrane in their properties for DMFC
applications [41].
The single cell performance of DMFC with Naon/
silica composite membranes as solid electrolytes is
shown in Fig. 4. The performance with 10-nm
Naon/silica composite membrane (36 mW/cm2) was
higher than that of the Naon membrane (30 mW/
cm2) by about 20%. However, the other thicker Naf-ion/silica membranes (32, 68 nm thick silica) showed
lower power density compared with bare Naon 115.
The decrease of cell performance in the presence of
32- and 68-nm Naon/silica composite membranes
might be caused by increased contact resistance as
can be seen from SEM image of Naon/silica compos-
ite membranes in Fig. 2.
The cell temperature and voltage of Naon/silicacomposite membranes at OCV are shown in Fig. 5.
-20 0 20 40 60 80 100 120 140 160 180100
200
300
400
500
600
700 Nafion 115silica 10 nmsilica 32 nmsilica 68 nm
Current Density (mA/cm2)
Vol
tage
(mV)
-5
0
5
10
15
20
25
30
35
40
Power D
ensity (mW/cm
2)
Fig. 4. DMFC single cell performance with Naon/silica composite
membranes.thickness of 32 and 68 nm silica were employed.
-
D. Kim et al. / Electrochemistry Communications 6 (2004) 10691074 10731600
Nafion 115 10 nm silica 32 nm silica 68 nm silica38
40
42
44
46
48
50
Temperature OCV
Tem
pera
ture
(oC
) at O
CV
0.52
0.54
0.56
0.58
0.60
0.62
0.64
0.66
Voltage (V)
Fig. 5. Cell temperature and voltage of Naon/silica composite
membranes at OCV.4. Conclusions
The performance of passive DMFC with nano-silica
layered Naon composite membranes prepared by
PECVD technique was investigated. The thickness of sil-
ica layer coated on the Naon membrane was 10, 32 and
68 nm. These composite membranes showed good prop-
erties in terms of ion conductivity and methanol perme-ability for DMFC applications. The ion conductivity of
the Naon/silica composite membranes was declined by
about 722% to the unmodied Naon membrane, but
its methanol permeability was reduced by about
4070%. The values of OCV with all the Naon/silica
composite membranes were higher than that of the bare
Naon membrane. However, only Naon/silica com-
posite membrane with thickness of 10 nm silica layershowed higher cell performance of ca. 20% compared
with Naon membrane. Though number of reports are
available in literature on Naon/silica composite by
using various methods of incorporating silica into Naf-
[31] S.K. Ray, C.K. Maiti, S.K. Lahiri, N.B. Chakrabart, J. Vac. Sci.
Technol. B 10 (1992) 1139.
Nafion115 10 nm silica 32 nm silica 68 nm silica0
200
400
600
800
1000
1200
1400
Res
istan
ce (m
. cm
2 )
Fig. 6. Resistance in the DMFC with Naon/silica composite mem-
branes at OCV.[32] K.H.A. Bogart, N.F. Dalleska, E.R. Fisher, J. Vac. Sci. Technol.
A 13 (1995) 476.
[33] A. Rhallabi, G. Turban, J. Vac. Sci. Technol. A 19 (2001) 743.
[34] Y. Inoue, H. Sugimura, O. Takai, Thin Solid Films 345 (1999)ion lm, the PECVD approach is found to be a promis-
ing one in coating very thin lm of silica on Naon
membrane.
References
[1] C.-H. Wirguin, J. Membr. Sci. 120 (1996) 1.
[2] J.A. Kerres, J. Membr. Sci. 185 (2001) 3.
[3] K.D. Kreuer, J. Membr. Sci. 185 (2001) 29.
[4] Y. Woo, S.Y. Oh, Y.S. Kang, B. Jung, J. Membr. Sci. 220 (2003)
31.
[5] J.L. Bredas, R.R. Chance, R. Silbey, Phys. Rev. B 26 (1982) 5843.
[6] H. Kobayashi, H. Tomita, H. Moriyama, J. Am. Chem. Soc. 116
(1994) 3153.
[7] F. Wang, J. Roovers, Macromolecules 26 (1993) 5295.
[8] C. Bailly, D.J. Williams, F.E. Karasz, W.J. Macknight, Polym. 28
(1987) 1009.
[9] M. Rikukawa, K. Sanui, Prog. in Polym. Sci. 25 (2000) 1463.
[10] N. Carretta, V. Tricoli, F. Picchioni, J. Membr. Sci. 166 (2000)
189.
[11] R.W. Kopitzke, C.A. Linkous, H.R. Anderson, C.L. Nelson, J.
Electrochem. Soc. 147 (5) (2000) 1677.
[12] D.H. Jung, Y.B. Myoung, S.Y. Cho, D.R. Shin, D.H. Peck, Int.
J. Hydrogen Energy. 26 (2001) 1263.
[13] I. Honma, S. Hirakawa, K. Yamada, J.M. Bae, Solid State Ionics
118 (1999) 29.
[14] L. Depre, M. Ingram, C. Poinsignon, M. Popall, Electrochim.
Acta 45 (2000) 1377.
[15] S.P. Nunes, B. Rumann, E. Rikowski, S. Vetter, K. Richau, J.
Membr. Sci. 203 (12) (2002) 215.
[16] C. Yang, S. Srinivasan, A.S. Arico, P. Creti, V. Baglio, V.
Antonucci, Electrochem. Solid-State Lett. 4 (4) (2001) A31.
[17] N. Miyake, J.S. Wainright, R.F. Savinell, J. Electrochem. Soc.
148 (8) (2001) A898.
[18] D.H. Jung, S.Y. Cho, D.H. Peck, D.R. Shin, J.S. Kim, J. Power
Sources 4683 (2002) 1.
[19] L.J. Hobson, H. Ozu, M. Yamauchi, S. Hayase, J. Electrochem.
Soc. 148 (10) (2001) A1185.
[20] Z. Florjanczyk, E. WielgusBarry, Z. Poltarzewski, Solid State
Ionics 145 (2001) 119.
[21] J. Feichtinger, R. Galm, M. Walker, K.-M. Baumgartner, A.
Schulz, E. Rauchle, U. Schumacher, Surf. Coat. Technol. 142
144 (2001) 181.
[22] W.C. Choi, J.D. Kim, S.I. Woo, J. Power Sources. 96 (2001) 411.
[23] S.R. Yoon, G.H. Hwang, W.I. Cho, I.-H. Oh, S.-A. Hong, H.Y.
Ha, J. Power Sources. 106 (2002) 215.
[24] N. Miyake, J.S. Wainright, R.F. Savinell, J. Electrochem. Soc.
148 (8) (2001) A905.
[25] P. Dimitrova, K.A. Friedrich, U. Stimming, B. Vogt, Solid State
Ionics 150 (2002) 115.
[26] P. Dimitrova, K.A. Friedrich, B. Vogt, U. Stimming, J. Electro-
anal. Chem. 532 (2002) 75.
[27] C.W. Walker Jr., J. Power Sources. 110 (2002) 144.
[28] Y.T. Kim, S.M. Cho, H.Y. Lee, H.D. Yoon, D.H. Yoon, Coat.
Technol. 174175 (2003) 166.
[29] T.-S. Larsen, S. Bouwstra, O. Leistiko, J. Electrochem. Soc. 144
(1997) 1505.
[30] J. Batey, E. Tierney, J. Appl. Phys. 60 (1986) 3136.90.
-
[35] K. Teshima, Y. Inoue, H. Sugimura, O. Takai, Surf. Coat.
Technol. 146 (2001) 451.
[36] Y. Sone, P. Ekdunge, D. Simonsson, J. Electrochem. Soc. 143 (4)
(1996) 1254.
[37] J.J. Sumner, S.E. Creager, J.J. Ma, D.D. DesMarteau, J.
Electrochem. Soc. 145 (1) (1998) 107.
[38] D. Kim, E.A. Cho, S-.A. Hong, I.-H. Oh, H.Y. Ha, J. Power
Sources 130 (2004) 172.
[39] T. Hatanaka, N. Hasegawa, A. Kamiya, M. Kawasumi, Y.
Morimoto, K. Kawahara, Fuel 81 (2002) 2173.
[40] B. Bae, D. Kim, J Membr. Sci. 220 (12) (2003) 75.
[41] V. Tricoli, J. Electrochem. Soc. 1445 (11) (1998) 3798.
1074 D. Kim et al. / Electrochemistry Communications 6 (2004) 10691074
Nano-silica layered composite membranes prepared by PECVD for direct methanol fuel cellsIntroductionExperimentalPreparation of Nafion/silica composite membranesMeasurement of ion conductivity and methanol permeabilitySingle cell performance measurement in DMFC
Results and discussionConclusionsReferences