Enhancement on proton conductivity of inorganic–organic composite electrolyte membrane by addition of sulfonic acid group

Solid State Ionics 176 (2005) 2445 – 2450 www.elsevier.com/locate/ssi

Enhancement on proton conductivity of inorganic–organic composite electrolyte membrane by addition of sulfonic acid group Hirokazu Munakata, Hiroto Chiba, Kiyoshi Kanamura* Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan CREST of Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Received 2 December 2004; received in revised form 25 February 2005; accepted 1 March 2005

Abstract A proton-conducting porous silica matrix for composite membranes was prepared by introduction of sulfonic acid groups on the surface. The surface modification of pores in the porous silica membrane was performed by using 3-mercaptopropyltrimethoxysilane (SH oxidation method) or 1,3-propanesultone (direct reaction method). The sulfonated silica matrix exhibited high proton conductivity of 6.0  10 3 S cm 1 at 60 -C under 90% relative humidity. This value was about 400 times higher than that of unmodified silica matrix. The proton conductivity of the composite membrane filled by a proton-conducting gel polymer, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), was considerably enhanced by using the sulfonated silica matrix. D 2005 Elsevier B.V. All rights reserved. PACS: Fuel cells (energy conversion); 84.60.D Keywords: Fuel cell; Proton conductivity; Surface sulfonation; Three-dimensionally ordered macroporous silica; Composite membrane

1. Introduction The direct methanol fuel cell (DMFC) has been expected as a portable power source for various kinds of mobile tools because it can utilize methanol without reforming equipment of methanol to hydrogen gas [1]. However, the DMFC has two major problems for practical applications. One is a low electrochemical activity of Pt catalysts, which can be solved by using either Pt –Ru alloy catalyst or a higher operation temperature [2,3]. Another one is a methanol permeation through an ion-exchange membrane from anode to cathode [4,5]. This phenomenon leads to a voltage drop of fuel cell and lowers fuel utilization. Nafion\, one of promising perfluorinated polymers, has been widely used as a proton-conducting electrolyte membrane due to its high proton conductivity and chemical stability [6,7]. However, Nafion\ easily expands or shrinks depending on atmos* Corresponding author. Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1 MinamiOhsawa, Hachioji, Tokyo 192-0397, Japan. Tel./fax: +81 426 77 2828. E-mail address: [email protected] (K. Kanamura). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.03.027

phere [8,9]. Methanol permeation is usually enhanced by an expansion of polymer due to absorption of methanol by polymer itself [10,11]. Therefore, alternative polymer materials have been investigated by many research groups. For example, polyimide type polymers have been especially studied as desirable new proton-conducting electrolyte membranes because of their good mechanical and chemical properties [12,13]. On the other hand, inorganic– organic composite electrolyte membranes have been also investigated as another approach to realize low methanol permeability [14,15]. Recently, we developed a composite electrolyte membrane consisting of a gel polymer electrolyte and an ordered porous silica membrane [16]. It exhibited both high proton conductivity and low methanol cross-over due to suppression of polymer expansion by the hard silica matrix. However, the silica matrix has very low proton conductivity, so that only the gel polymer electrolyte contributes to the proton conductivity, leading to a lower proton conductivity of the membrane per volume. It is well known that silanol groups (Si – OH) easily react with various organosilane compounds. So, one can introduce desired properties on glass surfaces. Therefore, if

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acidic groups, especially, sulfonic acid groups are introduced into the porous silica matrix, the proton conductivity of the porous silica matrix is enhanced by this surface treatment. In this paper, a new proton-conducting path for the porous silica matrix was added to improve the proton conductivity of the composite membrane composed of the porous silica matrix and a proton-conducting gel polymer.

SH

(a) OH Si

OH Si

OH Si

HS

Si

MeO OMe Si OMe

O Si

O Si

O Si

reflux in toluene, 24 h SO3H

2. Experimental Si

Three-dimensionally uniformly ordered macroporous silica was synthesized by using a colloidal template method with mono-disperse polystyrene beads. Colloidal silica particles (/ = 70 –100 nm, Snowtex\ZL, Nissan Chemical Co.) and mono-disperse polystyrene beads (/ = 474 nm, OptiBind\, Seradyn Inc.) were mixed to prepare a stable suspension. The ratio between polystyrene beads and colloidal silica particles was 4:1 by volume. This ratio was estimated from a volume of close-packed polystyrene beads and void volume associated with this structure. Specifically, the volume percentages of polystyrene beads and voids were 74% and 26%, respectively. This suspension was filtered on a nitrocellurose membrane filter with a pore size of 0.1 Am. The amount of filtered suspension corresponding to a formation of 150-Am thickness of macroporous silica layer was determined. After the filtration, the deposited polystyrene/silica hybrid layer was removed from the membrane filter and placed on a flat ceramic plate and then heated at 450 -C for 1 h, and then 890 -C for 1 h at a heating rate of 2 -C min 1. During this heat treatment process, the polystyrene beads were burned away, and the colloidal silica particles were sintered slightly. Further heat treatment at 890 -C for 1 h, and then at 980 -C for 10 min at a heating rate of 20 -C min 1 was conducted to sinter the colloidal silica particles. This process is necessary in order to maximize the mechanical strength of the porous silica membrane. The surface sulfonation of the porous silica membrane was conducted by SH oxidation method or direct reaction method as shown in Fig. 1. In both cases, sulfonic acid groups are introduced on the silica surface via covalently bonds. However, the as-prepared porous silica membrane had few silanol groups which were reactive points to introduce sulfonic acid groups on the silica surface, because of high temperature treatments. Therefore, as a pretreatment, a hydrothermal treatment was conducted at 170 -C in order to increase silanol groups. In the case of SH oxidation method, the porous silica membrane was refluxed in 2.6 wt.% 3-mercaptopropyltrimethoxysilane toluene solution for 24 h in order to introduce mercapto groups (– SH) on the silica surface as precursors. Then, the mercapto groups were converted to sulfonic acid groups (– SO3H) by an oxidation with H2O2 solution at 70 -C for 2 h. In the direct reaction method, the porous silica membrane was refluxed in toluene solution con-

O Si

H2O2 oxidation

O Si

O Si

70 °C , 2 h

(b) OH Si

SO3H

OH Si

OH Si

S O

O O

OH Si

O Si

OH Si

reflux in toluene, 48 h Fig. 1. Outline of surface sulfonation procedures by (a) SH oxidation method and (b) direct reaction method.

taining 0.12 mol dm 3 1,3-propanesultone for 48 h to introduce sulfonic acid groups by one step. A proton-conducting polymer gel electrolyte used in this study was prepared by polymerization of 2-acrylamido-2methyl-1-propanesulfonic acid (AMPS) and N,NV-methylenebisacrylamide (MBA). Ammonium persulfate (APS) was used as an initiator of polymerization. An aqueous solution containing of AMPS, MBA, and APS with optimized concentrations of 4.82 mol kg 1, 6.49  10 2 mol kg 1, and 4.38  10 2 mol kg 1, respectively, was injected into the pores in the porous silica membrane, and then heated at 60 -C for 1 h to take place polymerization. The equivalent weight in terms of sulfonic acid groups was 4.8 mmol g 1. The porous silica membranes were observed using JOEL JSM-5310 scanning electron microscope (SEM). Fourier transform infrared (FT-IR) measurements were performed with JASCO FT/IR-670 Plus spectrometer to detect silanol groups on the porous silica membrane. An amount of S atom was evaluated with energy dispersive X-ray spectroscopy (EDX). From X-ray photoelectron spectroscopy analysis (XPS), it was confirmed that S atom detected by EDX was derived from sulfonic acid groups or mercapto groups. The proton conductivity of the porous silica membranes and the composite membranes were measured with an impedance analyzer (YHP 4192A) in the frequency range from 5 Hz to 1 MHz under controlled temperature and humidity conditions. From the Cole – Cole plot, the resistance of the composite membrane was estimated, and then

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Fig. 2. (a) Photograph and (b) cross-sectional SEM image of the porous silica membrane prepared by use of 474 nm polystyrene template. Scale bars = 1 Am.

the conductivity of the composite membrane was calculated using the apparent thickness and electrode area. Au electrodes were used for this measurement.

3. Results and discussion The macroporous silica membrane was successfully obtained with a large size of about 4 cm  4 cm and a thickness of 150 Am, as shown in Fig. 2(a). Its thickness was able to be easily controlled by changing the volume of suspension. The obtained membrane had an adequate mechanical strength without any cracks. Fig. 2(b) shows a cross-sectional SEM image of the macroporous silica membrane. A continuous ordered structure of the silica membrane having uniform pore size of about 500 nm was obtained. This pore size well-reflected the size of polystyrene template beads (474 nm). Smaller pores observed in holes, were connective windows between large macropores. A presence of these connecting windows means a formation of highly ordered pores in a near closed packed array.

(b)

Absorbance / a.u.

24 hours 12 hours

3 hours

Peak area ratio

(a)

However, an apparent porosity of the porous silica membrane measured with a pycnometer was 70%. An ideal value of the porosity was 74%, so that the obtained value was slightly smaller than the ideal one. This means that some of the pores were not ideally connected with each other. Fig. 3(a) shows FT-IR spectra of the porous silica membranes before and after the hydrothermal treatment at 170 -C. Each spectrum shows a broad peak at 3200 – 3700 cm 1 due to silanol groups. The peak intensity of silanol groups was increased with increasing duration for hydrothermal treatment. The areas of these peaks were estimated, and the ratios of peak areas between modified membranes and unmodified one were plotted in Fig. 3(b). After the treatment for longer than 24 h, the amount of silanol groups was saturated. Therefore, the hydrothermal treatment of the silica membrane was conducted for at least 24 h before the surface modification. Fig. 4 shows EDX spectra of the sulfonated porous silica membranes modified by SH oxidation method or direct reaction method. In both cases, two peaks at 1.7 eV

3

2.5 2 1.5 1 0

5

10 15 20 Time / hour

25

30

unmodified

4000 3000 2000 Wavenumber / cm-1 Fig. 3. (a) FT-IR spectra of the porous silica membranes before and after the hydrothermal treatment at 170 -C. (b) The ratio of peak area between the modified membranes and unmodified one in 3200 – 3700 cm 1.

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(a)

(b) 104

Intensity / cps

Intensity / cps

104

103

102

101

103

102

101 1

1.5

2 2.5 Energy / eV

3

1

1.5

2 2.5 Energy / eV

3

Fig. 4. EDX spectra of the sulfonated porous silica membranes prepared by (a) SH oxidation method and (b) direct reaction method.

and 2.3 eV due to Si atom and S atom were observed, respectively. The large amount of S atom, about 9.9 at.%, was detected in the sulfonated silica prepared by the direct reaction method. In the case of the direct reaction method, each silanol group gives a sulfonic acid group as shown in Fig. 1(b). However, in SH oxidation method, a sulfonic acid group has to be introduced on the surface by consuming three silanol groups according to the silane coupling reaction as shown in Fig. 1(a). The small S amount of 2.6 at.% obtained in the case of SH oxidation method may be due to such a scheme. Fig. 5 shows XPS spectra for S 2p of the sulfonated silica membranes. Each spectrum had a peak at 168 eV which was attributed to S atom in sulfonic acid groups. A peak around 163 eV derived from mercapto groups was not detected in both cases. Therefore, it can be said that mercapto groups formed in the course of SH oxidation method are perfectly converted to sulfonic acid groups. In addition, almost the same spectra were obtained regardless of etching time. This result suggests that sulfonic acid groups distribute in the porous silica membrane uniformly.

(a)

Both methods were successfully performed to obtain sulfonic acid groups on the porous silica matrix. In comparison of two methods, the direct reaction process provided a larger amount of sulfonic acid groups onto the silica surface. In the following discussion, the silica membrane sulfonated by the direct reaction was employed. Fig. 6 shows the Arrhenius plots for proton conductivity of the porous silica membranes before and after the surface modification by the direct reaction method. The conductivity of the sulfonated membrane was about 400 times higher than that before the surface sulfonation. With an increase in temperature, the proton conductivity of the sulfonated silica membrane increased from 3.4  10 3 S cm 1 at 30 -C up to 6.0  10 3 S cm 1 at 60 -C under 90% relative humidity. Activation energy for the proton conduction in the sulfonated silica membrane was estimated to be 0.18 eV, which was smaller than that observed for the unmodified one of 0.33 eV. The smaller activation energy permits the easier proton transfer. Proton migration in polymer electrolyte membrane such as Nafion\ is well studied by many research groups and discussed in the view

(b)

0 sec

1 sec

Etching Time

Intensity / a. u.

Intensity / a. u.

Etching Time

0 sec

1 sec

5 sec 5 sec 168 164 160 Binding Energy / eV

168 164 160 Binding Energy / eV

Fig. 5. XPS spectra of the sulfonated porous silica membranes prepared by (a) SH oxidation method and (b) direct reaction method.

H. Munakata et al. / Solid State Ionics 176 (2005) 2445 – 2450

10-1

100

Conductivity / S cm-1

of a vehicle mechanism and a Grotthuss (proton-jump) mechanism [17,18]. In the vehicle mechanism, the proton diffuses together with solvent molecules by forming a complex such as H3O+, H5O2+, and CH3OH2+. In the Grotthuss mechanism, however, the protons jump from one solvent molecule to the next through hydrogen bonds. This mechanism preferentially occurs in acid solutions with the activation energy ranging from 0.1 eV to 0.4 eV [19]. The activation energy estimated for both sulfonated and unmodified silica membranes are well corresponding to this range, so that the proton conduction is still explained via Grotthuss mechanism. Fig. 7 shows the Arrhenius plots for proton conductivity of the composite membranes composed of AMPS gel electrolyte and the porous silica membrane. The proton conductivity of the composite membrane was also enhanced by using the sulfonated silica matrix. At 60 -C under 90% relative humidity, the sulfonated composite exhibited 1.8  10 1 S cm 1. This value was comparable with that of Nafion\117 membrane (1.5  10 1 S cm 1) [20]. It is noted worthy that the observed enhancement on proton conductivity in the composite membrane was considerably high compared to a simple contribution observed between the sulfonated silica matrix and unmodified one. For example, the proton conductivity of the composite membrane increased 0.34  10 1 S cm 1 at 30 -C, though the increase was only 0.03  10 1 S cm 1 in the case of the silica matrix itself. This phenomenon may suggest that there is an interaction between sulfonic acid groups on the surface and those in the AMPS gel electrolyte. The values of the activation energy for the proton in the composite membranes with and without surface modification were 0.16 eV and 0.09 eV, respectively. These small values indicate that

2449

10-1

10-2 3

3.1

3.2

3.3

-1

1000 / T / K

Fig. 7. Arrhenius plots for proton conductivity of the composite membranes of AMPS gel electrolyte filled into unmodified porous silica matrix (>) and the porous silica matrix sulfonated by direct reaction method (g). The measurement was performed under 90% relative humidity.

the sulfonic acid groups are highly dissociated and proton transfer follows a similar proton jump mechanism. The interaction between the sulfonated silica surface and AMPS gel is mainly supported by the difference in the preexponential factor of conductivity. This value, the intercept in the Arrhenius plot, represents a frequency factor comprised of the orientation contribution of sulfonic acid groups in the composite membrane, i.e. the microstructure of proton conducting paths. The BET surface area of the porous silica membrane prepared in this study was determined to 19 m2 g 1. This value was roughly consistent with an ideal one of 15 m2 g 1. If smaller polystyrene beads are used as templates, the porous silica membrane having larger surface area can be prepared. Therefore, it may be possible to elucidate the interfacial interaction between the gel electrolyte and the silica matrix more clearly.

Conductivity / S cm-1

10-2

4. Conclusions 10-3

-4

10

10-5

10-6 3

3.1

3.2

3.3

-1

1000 / T / K

Fig. 6. Arrhenius plots for proton conductivity of the porous silica membrane before (>) and after (g) the surface sulfonation by direct reaction method. The measurement was performed under 90% relative humidity.

A new proton-conducting path was formed on the surface of three-dimensionally ordered silica membrane by the surface sulfonation. The sulfonic acid groups were more effectively introduced by the direct reaction method through a ring-open reaction of 1,3-propanesulton. The sulfonated silica matrix exhibited high proton conductivity of 6.0  10 3 S cm 1 at 60 -C under 90% relative humidity. This value was about 400 times higher than that of unmodified silica matrix. In the case of the composite membrane, the proton conductivity was also enhanced. In addition, an interaction between sulfonic acid groups on the matrix surface and those in the AMPS gel was observed. By using the porous silica membrane with larger surface area, it will be possible to improve the proton conductivity of

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composite membrane. Further optimizations of the pore size and electrolytes are now underway.

Acknowledgements The present work was partly supported by Grant-in-Aid for Scientific Research on Priority Areas (B) of ‘‘New Technologies of DMFC’’ (No. 13134101) from MEXT.

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