eha

ng

hnolo

ised f

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: hyha@kist.re.kr (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

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.

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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