Tertiary sulfonium as a cationic functional group for hydroxide exchange membranes

Abstract

Tertiary sulfonium is introduced as the cationic functional group for hydroxide exchange membranes (HEMs). The methoxyl-substituted triarylsulfonium functionalized HEM (i.e., PSf-MeOTASOH) exhibits excellent thermal stability (T_OD: 242 °C), acceptable hydroxide conductivity (15.4 mS cm⁻¹ at 20 °C), and good chemical stability. Our work shows that, similar to nitrogen and phosphorus, a sulfur element with designed side groups can also be used to construct HEM cationic functional groups.

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Polymer electrolyte fuel cells are considered as efficient and environmentally friendly power sources.¹ Significant progress has been made in developing proton exchange membrane fuel cells (PEMFCs), however, their commercialization has been chiefly hindered by the high cost and low durability of their precious metal catalysts.² By switching the working ion from a proton (H⁺) to a hydroxide (OH⁻), hydroxide exchange membrane fuel cells (HEMFCs) enable non-precious metals, such as silver³ and nickel⁴ as electrocatalysts and thus have the potential to solve the catalyst cost and durability problems.⁵ Equally important, HEMFCs are not substantially limited by CO₂ contamination, which is the major barrier for traditional liquid alkaline fuel cells.⁶ In addition, HEMFCs can increase fuel diversity and reduce fuel crossover at the same time.^5c All these advantages make HEMFCs promising in many energy conversion and storage applications, ranging from portable power to vehicle propulsion to distributed power generation.^5e

To advance HEMFC technology, it is essential to develop high-performance HEMs with high hydroxide conductivity, controlled solubility, and high chemical and thermal stability.^5d These key HEM properties are fundamentally influenced by the cationic functional groups. Currently, nitrogen atom-based cationic groups (e.g., ammonium,^5a,7 pyridinium,⁸ guanidinium,⁹ and imidazolium¹⁰) have been extensively studied for HEMs. These HEMs show acceptable hydroxide conductivity but their overall performance is often limited by either poor solubility or insufficient chemical and thermal stability. Recently, a phosphorus atom-based cationic functional group (i.e., tris(2,4,6-trimethoxyphenyl)-constructed phosphonium) was designed and its HEMs were demonstrated to have controlled solubility, increased hydroxide conductivity, and improved chemical stability.^6,11 This simple and successful switch from nitrogen to phosphorus suggests that new cationic functional groups based on elements beyond nitrogen and phosphorus could be explored.

Similar to quaternary nitrogen or phosphorus, tertiary sulfur (i.e., sulfonium) possesses one unit of positive charge, and thus can be considered as a candidate for the cationic functional group in HEMs. Trialkylsulfonium-based molten salts have already been used as electrolytes,¹² showing sulfonium as a feasible cationic functional group. However, ordinary sulfoniums (e.g., trialkyl or arylalkyl constructed) are reported to have lower chemical and thermal stability than quaternary ammonium,^5d,e,13 limiting their applications in HEMs. By contrast, triaryl-constructed sulfonium (i.e., triarylsulfonium) have much improved chemical stability¹⁴ as well as enhanced thermal stability (over 300 °C),¹⁵ largely because of the pπ–dπ conjugation from the p and d orbitals of the sulfur and the π electrons of the benzene rings.^15a,16 Furthermore, adding strong electron-donating substituents in aryls has been confirmed to improve the stability of sulfonium as the strong electron-donation to sulfur helps delocalize the positive charge. For example, a phenylthio-substituted triarylsulfonium was found to have a much higher thermal stability (up to 408 °C);^15b At the same time, the basicity of triarylsulfonium hydroxide is also expected to be enhanced. For example, a methoxyl-substituted triarylsulfonium was observed to have enhanced ion-exchange ability, equivalent to ammonium hydroxides.¹⁷

In this work, a new triarylsulfonium (i.e., diphenyl(3-methyl-4-methoxyphenyl) tertiary sulfonium, MeOTAS) cationic group containing one methoxyl substituent and one methyl substituent is designed. The methoxyl substituent is expected to provide the strong electron-donation and the methyl substituent is used to offer a simple synthetic pathway to link to the polymer backbone. The MeOTAS functionalized polysulfone HEM (PSf-MeOTASOH) shows excellent thermal stability, acceptable hydroxide conductivity, and good chemical stability.

Since diaryl sulfides are significantly less nucleophilic than tertiary amines or tertiary phosphines, the general “chloromethylation–quaternization–alkalization” protocol could not be applied to the triarylsulfonium synthesis. Therefore, we adopted a nitrogen-bridge strategy including three major steps to synthesize PSf-MeOTAS (Scheme 1): 1) synthesis of diphenyl(3-methyl-4-methoxyphenyl) sulfonium chloride by the condensation reaction between diphenyl sulfoxide and 2-methylanisole, followed by a bromination reaction and anion exchange; 2) synthesis of butylaminated polysulfone (PSf-BA) through a chloromethylation and amination reaction to introduce a nitrogen atom bridge; 3) incorporation of the MeOTAS cation to the polysulfone backbone via the reaction between the bromomethyl group of diphenyl(3-bromomethyl-4-methoxyphenyl) sulfonium chloride and the butylamine group of PSf-BA. The high purity and chemical structure of all the synthesized compounds were confirmed by ¹H NMR spectroscopy (Fig. S1–S5, ESI†). The degree of functionalization of the polysulfone backbone by the triarylsulfonium cationic group is 46% (¹H NMR). The PSf-MeOTASOH HEMs are colorless, transparent, and flexible (Fig. 1). The small-angle X-ray scattering (SAXS) analysis shows that there existed some local order with scatters of 5 Å and 18 Å, probably arising from the incomplete functionalization of the polysulfone backbone. (Fig. S6, ESI†).

Scheme 1 Synthesis of MeOTASOH-functionalized polysulfone (PSf-MeOTASOH).

Fig. 1 Optical images of PSf-MeOTASOH HEMs.

The PSf-MeOTASOH HEM shows excellent thermal stability (Fig. S7, ESI†). Under the same test conditions (N₂ atmosphere and 10 °C min⁻¹ heating rate), PSf-MeOTASOH HEM (242 °C, Table 1) has a much higher onset decomposition temperature (T_OD) than the quaternary ammonium-based HEM (same polysulfone backbone, 140–170 °C)¹⁸ and quaternary phosphonium-based HEM (same polysulfone backbone, 178 °C).⁶ This T_OD (with ca. 1% weight loss) is also higher than those of quaternary guanidinium-based poly(arylene ether sulfone) (185 °C with 5% weight loss)^9a and imidazolium-based polyfluorene (200 °C with 4% weight loss).¹⁹

Table 1 Thermal decomposition temperature of the pristine PSf, butylaminated polysulfone (PSf-BA) and PSf-MeOTASOH

Polymer	T OD ^a (°C)	T FD ^b (°C)
a T OD, onset decomposition temperature. b T FD, fastest-weight-loss decomposition temperature.
PSf	476	523
PSf-BA	308	414
PSf-MeOTASOH	242	406

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With an ion exchange capacity (IEC) of 0.69 mmol g⁻¹, the PSf-MeOTASOH has 26.9% of a water uptake and less than 5% of a swelling ratio at 20 °C. The hydroxide conductivity of PSf-MeOTASOH is 15.4 mS cm⁻¹ at 20 °C (Table 2), which meets the basic requirement of HEMFCs (>10⁻² S cm⁻¹).^5f Generally, the conductivity increases almost linearly with the density of the ionic group (i.e., IEC), and thus an IEC-normalized hydroxide conductivity (hydroxide conductivity over IEC, Table 2) is a more objective measure of the intrinsic hydroxide conductivity for HEMs.⁶ PSf-MeOTASOH shows a high IEC-normalized hydroxide conductivity (22.3 mS g cm⁻¹ mmol⁻¹) that is very close to that of quaternary ammonium-based polysulfone HEMs (19 mS g cm⁻¹ mmol⁻¹),²⁰ indicating a similar efficiency of hydroxide conduction. This value is higher than those of imidazolium-based polysulfone HEMs (8.4 mS g cm⁻¹ mmol⁻¹),²¹ but lower than those of the quaternary phosphonium-based polysulfone HEMs (39 mS g cm⁻¹ mmol⁻¹).^11bNote, there is significant room to increase the hydroxide conductivity of PSf-MeOTASOH HEMs by increasing their IEC.

Table 2 Ion exchange capacity, hydroxide conductivity, and IEC-normalized hydroxide conductivity of PSf-MeOTASOH and PSf-TASOH HEMs

HEM	IEC^a mmol g^−1	HC^b mS cm^−1	HCIEC^c mS g cm^−1 mmol^−1
a IEC, theoretical ion exchange capacity based on the degree of functionalization. b HC, hydroxide conductivity measured in water at 20 °C. c HCIEC, IEC-normalized hydroxide conductivity. d 46% of the degree of functionalization. e 44% of the degree of functionalization.
PSf-MeOTASOH^d	0.69	15.4	22.3
PSf-TASOH^e	0.68	7.7	11.3

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In addition to the excellent thermal stability and high IEC-normalized hydroxide conductivity, PSf-MeOTASOH also has good alkaline stability, which is critical for HEMFCs. The PSf-MeOTASOH remained intact after an immersion treatment in 1 M KOH at 60 °C for 10 days, confirmed by ¹H NMR spectra (Fig. S8, ESI†), while under similar conditions the commercial quaternary ammonium-based FAA membranes became very brittle due to severe degradation. Besides, there is no obvious loss of conductivity observed for PSf-MeOTASOH HEM after immersion in 1 M KOH for 30 days at room temperature, indicating good long-term stability.

The methoxyl substituent is critical for the triarylsulfonium cationic group in terms of alkaline stability and hydroxide conductivity, as it helps delocalize the positive charge and increase the basicity, simultaneously through the strong electron donation. Similar to its membrane, the MeOTAS cation [Scheme S1(a), ESI†] remained almost intact after the degradation test in 1 M KOD/D₂O solution at 60 °C for 10 days. In contrast, the MeO-free TAS one [Scheme S1(b), ESI†] completely degraded, evidenced by the ¹H NMR spectra (Fig. S9 and Fig. S10, ESI†). On the other hand, MeO-free TAS cation-based polysulfone HEMs (PSf-TASOH) were synthesized through the same synthesis route. With almost the same degree of functionalization (44% vs. 46%), PSf-TASOH has a conductivity of only half of that of PSf-MeOTASOH (7.7 vs. 15.4 mS cm⁻¹, Table 2). Apparently, the enhanced basicity, caused by introducing a strong electron-donating methoxyl group, translates into increased hydroxide conductivity. These results also suggests that the alkaline stability and hydroxide conductivity of TASOH can be further improved by introducing more or even stronger electron-donating groups.

In conclusion, a methoxyl-substituted triarylsulfonium functionalized polysulfone (PSf-MeOTASOH) has been successfully synthesized. The PSf-MeOTASOH-based HEMs are colorless, transparent, and flexible. PSf-MeOTASOH shows excellent thermal stability (T_OD: 242 °C), higher than quaternary nitrogen-based and phosphorus-based HEMs. With an IEC of 0.69 mmol g⁻¹, it exhibits an acceptable hydroxide conductivity of 15.4 mS cm⁻¹ at room temperature, and a high IEC-normalized hydroxide conductivity (22.3 mS g cm⁻¹ mmol⁻¹). It is expected that the structure of the triarylsulfonium cationic group can be optimized through simple chemistry to provide improved chemical stability as well as hydroxide conductivity.

References

1. (a) M. A. Hickner, H. Ghassemi, Y. S. Kim, B. R. Einsla and J. E. McGrath, Chem. Rev., 2004, 104, 4587–4611; (b) K. A. Mauritz and R. B. Moore, Chem. Rev., 2004, 104, 4535–4585.
2. R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. I. Kimijima and N. Iwashita, Chem. Rev., 2007, 107, 3904–3951.
3. J. R. Varcoe, R. C. T. Slade, G. L. Wright and Y. L. Chen, J. Phys. Chem. B, 2006, 110, 21041–21049.
4. S. F. Lu, J. Pan, A. B. Huang, L. Zhuang and J. T. Lu, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 20611–20614.
5. (a) E. Agel, J. Bouet and J. F. Fauvarque, J. Power Sources, 2001, 101, 267–274; (b) G. F. McLean, T. Niet, S. Prince-Richard and N. Djilali, Int. J. Hydrogen Energy, 2002, 27, 507–526; (c) J. R. Varcoe and R. C. T. Slade, Fuel Cells, 2005, 5, 187–200; (d) G. Couture, A. Alaaeddine, F. Boschet and B. Ameduri, Prog. Polym. Sci., 2011, 36, 1521–1557; (e) G. Merle, M. Wessling and K. Nijmeijer, J. Membr. Sci., 2011, 377, 1–35; (f) J. Pan, C. Chen, L. Zhuang and J. T. Lu, Acc. Chem. Res., 2012, 45, 473–481.
6. S. Gu, R. Cai and Y. S. Yan, Chem. Commun., 2011, 47, 2856–2858.
7. (a) W. Juda and W. A. McRae, J. Am. Chem. Soc., 1950, 72, 1044–1044; (b) N. Li, Q. Zhang, C. Y. Wang, Y. M. Lee and M. D. Guiver, Macromolecules, 2012, 45, 2411–2419; (c) N. W. Li, T. Z. Yan, Z. Li, T. Thurn-Albrecht and W. H. Binder, Energy Environ. Sci., 2012, 5, 7888–7892.
8. (a) T. Sata, Y. Yamane and K. Matsusaki, J. Polym. Sci., Part A: Polym. Chem., 1998, 36, 49–58; (b) A. B. Huang, C. Y. Xia, C. B. Xiao and L. Zhuang, J. Appl. Polym. Sci., 2006, 100, 2248–2251.
9. (a) J. H. Wang, S. H. Li and S. B. Zhang, Macromolecules, 2010, 43, 3890–3896; (b) D. S. Kim, A. Labouriau, M. D. Guiver and Y. S. Kim, Chem. Mater., 2011, 23, 3795–3797.
10. (a) M. L. Guo, J. Fang, H. K. Xu, W. Li, X. H. Lu, C. H. Lan and K. Y. Li, J. Membr. Sci., 2010, 362, 97–104; (b) B. C. Lin, L. H. Qiu, J. M. Lu and F. Yan, Chem. Mater., 2010, 22, 6718–6725.
11. (a) S. Gu, R. Cai, T. Luo, Z. W. Chen, M. W. Sun, Y. Liu, G. H. He and Y. S. Yan, Angew. Chem., Int. Ed., 2009, 48, 6499–6502; (b) S. Gu, R. Cai, T. Luo, K. Jensen, C. Contreras and Y. S. Yan, ChemSusChem, 2010, 3, 555–558; (c) S. Gu, J. Skovgard and Y. S. Yan, ChemSusChem, 2012, 5, 843–848.
12. (a) H. Matsumoto, T. Matsuda and Y. Miyazaki, Chem. Lett., 2000, 1430–1431; (b) O. O. Okoturo and T. J. VanderNoot, J. Electroanal. Chem., 2004, 568, 167–181; (c) L. Xiao and K. E. Johnson, Can. J. Chem., 2004, 82, 491–498; (d) H. Matsumoto, H. Sakaebe and K. Tatsumi, J. Power Sources, 2005, 146, 45–50; (e) L. Yang, Z. X. Zhang, X. H. Gao, H. Q. Zhang and K. Mashita, J. Power Sources, 2006, 162, 614–619.
13. E. B. Trostyanskaya and S. B. Makarova, Zh. Fiz. Khim., 1966, 39, 1754–1760.
14. S. Smiles and R. Le Rossignol, J. Chem. Soc. Trans., 1906, 89, 696–708.
15. (a) K. Gyoryova and B. Mohai, Thermochim. Acta, 1985, 92, 771–774; (b) J. F. Cameron, T. G. Adams, A. J. Orellana, R. M. M. Rajaratnam and R. F. Sinta, J. Proc. SPIE, 1997, 3049, 473–484.
16. J. V. Crivello, Adv. Polym. Sci., 1984, 62, 1–48.
17. S. Lindenbaum, G. E. Boyd and G. E. Myers, J. Phys. Chem., 1958, 62, 995–999.
18. (a) H. Herman, R. C. T. Slade and J. R. Varcoe, J. Membr. Sci., 2003, 218, 147–163; (b) J. Fang and P. K. Shen, J. Membr. Sci., 2006, 285, 317–322; (c) G. G. Wang, Y. M. Weng, D. Chu, D. Xie and R. R. Chen, J. Membr. Sci., 2009, 326, 4–8; (d) Y. Xiong, Q. L. Liu and Q. H. Zeng, J. Power Sources, 2009, 193, 541–546; (e) J. H. Wang, Z. Zhao, F. X. Gong, S. H. Li and S. B. Zhang, Macromolecules, 2009, 42, 8711–8717.
19. B. C. Lin, L. H. Qiu, B. Qiu, Y. Peng and F. Yan, Macromolecules, 2011, 44, 9642–9649.
20. J. Pan, S. F. Lu, Y. Li, A. B. Huang, L. Zhuang and J. T. Lu, Adv. Funct. Mater., 2010, 20, 312–319.
21. F. X. Zhang, H. M. Zhang and C. Qu, J. Mater. Chem., 2011, 21, 12744–12752.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental methods, Fig. S1–S10 and Scheme S1. See DOI: 10.1039/c2ra21402d

‡ This work is supported by the MURI program of the ARO under contract W911NF-10-1-0520.

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