A novel proton-conductive membrane with reduced methanol per
A novel proton-conductive membrane with reduced methanol
permeability prepared from bromomethylated
poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) Dan Wu, Rongqiang Fu*, Tongwen Xu, Liang Wu and Weihua Yang Functional Membrane Laboratory, School of Chemistry and material Science, University of
Science and Technology of China (USTC), Hefei, Anhui 230026, P.R. China
Abstract
A series of proton-conducting membranes were developed for direct methanol fuel cell (DMFC) applications via sulfonation of bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) base membranes. Besides the low manufacture cost, the membranes exhibited an excellent control on methanol crossover and swelling, and a sound balance with high proton conductivities. An optimal membrane was obtained after investigating the effects of the bromination degree and sulfonation process on the performances of corresponding membranes, i.e., the membrane possesses the methanol permeability of 2.64×10-8 cm2/s and characteristic factor Φ value of thirty times higher than that of Nafion? 117. Keywords: Proton-conductive membrane; Methanol crossover; DMFC; Bromomethylation; PPO
1. Introduction
Direct methanol fuel cell (DMFC) is attractive as a promising power source for several applications including automotive and portable power sources in view of some advantages [1]. The current popular proton conductive membrane is Nafion?, but its high methanol crossover and cost limit its use in DMFC commercialization.
Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is a versatile and well-known thermally stable engineering plastic [2]. Bromomethylated PPO (BPPO) has excellent membrane-forming and mechanical properties as well as good resistance to a number of chemical agents [3].
In this study, the efforts were made to prepare proton conductive membranes for the potential application in DMFC by heterogeneous sulfonating BPPO base membranes.
2. Experimental
*第一作者，吴丹，女，1982-9-23生于湖北鄂州，中韩联合培养博士研究生
Present address: Dept. of Chemical & Biomolecular Eng.National University of Singapore.
To whom the correspondence should be addressed. E-mail:twxu@https://www.wendangku.net/doc/e818361975.html,. Tel.: +8605513601587.
The bromomethylation of PPO was achieved according to a method reported in a previous article [3] to prepare base membranes. The different sulfonation processes of the base membranes were conducted as shown in Table 1.
Table 1 Sulfonation processes and the resulting properties Sulfonation processes
Properties
Membran e number
Bromin
ation degree(%)
Concentrationof
ClSO 3H (%
v/v) Time
(h)
Temperat ure (o
C)
IEC
(meq/g
dry)
Sulfonation
degree (%)
Water
uptake
(%)
Proton
conductivity
(S/cm)
M-1 60 30 1 50 1.50 28.6 12.2 0.0819 M-2 80 30 1 50 0.97 19.3 10.3 - M-3 100 30 1 50 0.54 11.2
5.82
-
M-4 60 10 1 50 0.20 3.40 1.02 0.0062 M-5 60 20 1 50 0.74 13.2 2.10 0.0100 M-6 60 40 1 50 2.62 55.5 21.6 0.0714 M-7 60 50 1 r. t. 0.59 10.4 9.21 0.0385 M-8 60 50 2 r. t. 1.30 24.3 13.4 0.0756 M-9 60 50 4 r. t. 1.51 28.8 15.5 0.0894 M-10 60 50 6 r. t. 2.07 41.6 34.4 0.1052 M-11 60 50
12 r. t. 3.52 82.1
95.5 0.1753
Nafion ?
- - - - 0.93 100 22.6 0.0834 The prepared membrane was characterized by FTIR, SEM-EDXA, IEC, proton conductivity [4],
methanol permeability [5] and water uptake et al.
3. Results and discussion
3.1. Instrumentation on the prepared membranes: FTIR and SEM-EDXA
As shown in Fig. 1, in the sulfonated samples, the absorption bands at 675, 1065 and 1396 cm -1 were assigned to sulfonic acid groups [6]. The absorption band at 1158 cm -1 was the characteristic band of S=O stretching vibration of sulfone –SO 2- [7], which implied that side reaction of forming sulfone –SO 2- existed and resulted in cross-linking structure.
40080012001600
2000
T r a n
s
m i t
t a n
c e
Wavenumber (cm -1
)
M-9M-6
M-1
BPPO 675cm
-1
1065cm
-1
1158cm
-1
1396cm
-1
Fig. 1 FTIR spectra Fig. 2 SEM-EDXA images
Figs. 2a-b show the SEM images of the membranes fracture (cross-section) and the EDXA of sulfur element. It is inferred that in this heterogeneous sulfonation process, the sulfonation substitution took place from the surface to the middle of the membrane and the membrane could be sulfonated uniformly in the thickness direction. 3.2. Sulfonation processes
As listed in Table 1, it is suggested that BPPO base membranes with the bromination degree of 60% is easier to be sulfonated compared to other bromination degrees. High concentration of ClSO 3H is propitious to the sulfonation reaction but strong sulfonation condition results in degradation of the polymer at relatively high temperature such as 50 o C. In mild sulfonation conditions, the sulfonation
process could be controlled within a proper
period of time.
3.3. Properties related to DMFC
As shown in Fig. 3, the prepared membranes possess rather low permeability for the special structure. Such index is very attractive in the application in DMFC. A characteristic factor Φ used to evaluate
membrane performance for a DMFC application was calculated by the ratio of proton conductivity and methanol permeability. Obviously, all the prepared membranes showed a much higher Φ value than
(a) M-5
-2024681012141618
2022
M e t h a n o l P e r m e a b i l i t y (10-7c m 2
/s )0
24
68
10
121416
Φ (105
S*s/cm 3
)
Nafion? 117, which is mainly attributed to the extraordinary control on methanol crossover.
4. Conclusions
Oriented for application in DMFC, a series of proton-conductive membranes were developed, which have the following merits: low cost, extremely low methanol permeability and swelling, as well
as high proton conductivities. The optimal membrane (M-7) was obtained by sulfonating the BPPO base membrane with the bromination degree of 60% in a 50% v/v ClSO3H/H2SO4 solution for 1 h at room temperature.
Acknowledgements
This research was supported in part by the National Science Foundation of China (No.20636050) and the NSFC-KOSEF Scientific Cooperation Program (No. 20611140649).
References
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[4] Y Sone, P Ekdunge D Simonsson Proton conductivity of Nafion 117 as measured by a
four-electrode AC impedance method [J] J Electrochem Soc, 1996, 143 (4): 1254-1259.
[5] Y Woo, S Y Oh, Y S Kang et al Synthesis and characterization of sulfonated polyimide
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[6] R Y M Huang, J J Kim Synthesis and Transport Properties of Thin Film Composite Membranes. I.
Synthesis of Poly(phenylene Oxide) Polymer and Its sulfonation [J] J Appl Polym Sci, 1984, 29: 4017-4027.
[7] V Deimede, G A V oyiatzis, J K Kallitsis et al Miscibility behavior of
polybenzimidazole/sulfonated polysulfone blends for use in fuel cell applications [J] Macromolecules, 2000, 33 (20): 7609-7617.
“第六届全国膜与膜过程学术报告会”参会回执
姓名吴丹职称职务博士生性别女年龄２６
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中国科技大学化学化工学院功能膜研究室
论文题目 A novel proton-conductive membrane with reduced methanol permeability prepared from bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide)
(BPPO)
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