Development of novel polymer electrolyte membranes based on a benzothiadiazole unit

Abstract

We developed novel hydrocarbon proton exchange membranes based on benzothiadiazole (BT) units. The introduction of even a few BT units effectively suppresses swelling of BT-based membranes. Furthermore, we revealed that the BT unit can provide high proton conductivity along with low activation energy compared to the parent SPES membrane under a wide range of water content conditions.

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Polymer electrolyte fuel cells (PEFCs) have attracted attention owing to their high efficiency, low environmental load and compact size.¹ For PEFCs to become widely used, it is necessary that they operate at high temperature and low humidity because operating under these conditions has several advantages, such as enhanced catalysis, easy water management and increased exhaust heat recovery.^2,3 Perfluorosulphonic acid (PFSA) membranes (e.g. Nafion®) have been used as proton exchange membranes (PEMs) for PEFCs.^4,5 However, PFSA membranes have low thermal stability and high cost and environmental load.^6,7 Sulphonated aromatic copolymers such as polyphenylenes,^8–10 polyimide,^11–13 [poly(arylene ether ether ketone)s]^14,15 and [poly(arylene ether sulphone)] (SPES)^16–19 have been investigated as alternatives to conventional PEMs owing to their low cost and environmental load and excellent thermal stability.³ Their proton conductivity is comparable to those of conventional PEMs at high humidity. On the contrary, these membranes composed of hydrocarbon polymers exhibit low performance under low humidity. Their proton conductivities drastically decrease with decreasing amount of water, which is the principal proton carrier in those conditions. Some research groups have suggested solutions to improve the proton conductivity under low humidity.^20,21 According to previous studies, it is important to achieve efficient proton transfer between water molecules and ion exchange groups such as sulphonic acid groups. In addition, recent investigations based on quantum chemical calculations reported the effect of the concentration of ion exchange groups on proton conductivity.^22,23 Specifically, high density of ion-exchange groups in PEMs leads to high proton conductivity and low activation energy when the water content is low due to an increase in the proton transfer by the hopping mechanism through ion-exchange groups. Normally, increasing the density of ion exchange groups in PEMs leads to an increase in their ion exchange capacity (IEC). However, the swelling resistance of PEMs decreases when the IEC increases because of the enhancement of hydrophilicity of polymer chains. This leads to serious problems such as decrease in proton conductivity and fuel crossover. Therefore, development of novel PEMs with high swelling resistance is strongly desired. Some research groups have attempted the improvement of swelling resistance of PEMs, mostly on the basis of the cross-link method, in which one polymer chain is linked to another by covalent or ionic bonds.^24,25

In contrast, our design concept is based on intermolecular interaction to enhance the swelling resistance and provide effective proton conduction for PEMs (Scheme 1). We focused on benzothiadiazole (BT) units,^26,27 which provide a highly hydrophobic π-stacked structure with strong intermolecular interactions (S–N interactions) and planar conformation. Several studies have reported that BT units affect the performance of functional materials.^28–30 Our current study designs novel aromatic polymers based on BT units with the expectation that the BT units would increase the hydrophobicity of the membranes and hence suppress membrane swelling. In this communication, we report the synthesis of novel aromatic polymers and the physicochemical characteristics (e.g. water content and proton conductivities) of membranes based on the polymers. We synthesized the designed copolymers based on the BT unit and prepared membranes from these copolymers. We found that the physical properties for PEMs, such as swelling resistance and proton conductivities, are significantly influenced by the introduction of just a few BT units, allowing effective proton transfer even under low water content conditions.

Scheme 1 (a) Schematic of illustration and (b) molecular structure of designed random polymer based on benzothiadiazole unit with S–N interaction (x = 0.1, 1).

The designed polymers were synthesized by a typical polycondensation reaction between 4,4′-dichlorodiphenyl sulphone, 4,4′-biphenol, 3,3′-disulphonated-4,4′-dichlorodiphenyl sulphone, BT-based monomer (4,7-bis(4-fluorophenyl)-2,1,3-benzothiadiazol) (Fig. S2 and S3†) and excess K₂CO₃ in N-methylpyrrolidone (Fig. S1†). Polymers with different mol% of BT units were prepared by changing the molar ratio of the BT monomer. Polymers with 0, 0.1 and 1 mol% of BT were labelled as SPES, 0.1% BT and 1% BT, respectively. The obtained polymers were characterized by ¹H-NMR (Fig. S4†), ¹³C-NMR (Fig. S5†), thermogravimetric analysis (Fig. S6†) and gel permeation chromatography (GPC). Their IEC was almost 2.0 meq. g⁻¹, as calculated from the intensity of the ¹H NMR signal related to the sulphonic acid groups.³¹ The polymers' molecular weight was determined by GPC with polystyrene as the standard; the number-averaged molecular weight was at least 20 kDa for a polydispersity (M_w/M_n) value of 2.5 (Table 1). The above results confirmed that the products are the target polymers based on the BT unit. Membranes were prepared by casting the polymer solution (20 wt%) onto dust-free glass plates and controlling the thickness with an applicator. The membranes were dried in a vacuum oven at 80 °C for 12 h. Subsequently, the membranes were treated for 1 h with 1 M H₂SO₄ solution at 90 °C for protonation and then washed with deionized water.

Table 1 Properties of BT-based polymers with various amounts of BT units

Samples	X^a [%]	IECdry^b [meq. g^−1]	Mn^c [kg mol^−1]	Mw^c [kg mol^−1]	Mw/Mn [-]
a Sulphonation degree estimated from ^1H-NMR measurements.b IEC in the dry condition calculated from the sulphonation degree.c Number-averaged molecular weight and weight-averaged molecular weight calculated from the GPC measurements using polystyrene standard samples.
SPES	44.5	2.01	30.6	60.3	2.0
0.1% BT	44.7	2.02	29.9	78.0	2.6
1% BT	45.8	2.08	20.3	45.8	2.3

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We used tapping-mode atomic force microscopy (TMAFM) to characterize membranes surface. Fig. 1 shows phase images of the membranes. The blue regions correspond to soft hydrophilic parts containing sulphonic acid groups or small amounts of water. In contrast, the yellow regions indicate hard hydrophobic phases. The BT-based membranes show a greater proportion of yellow regions compared with the membranes without BT units. The strongly hydrophobic regions are probably composed of condensation of hydrophobic domains owing to introduction of BT units. Hence, the AFM images indicate that the membrane morphology is influenced by the introduction of a small amount of BT units.

Fig. 1 TM AFM images of (a) SPES membrane; (b) 0.1% BT membrane; (c) 1% BT membrane (scan size: 500 nm).

The density of the sulfonic acid group of these membranes was investigated via FT-IR measurements. In the FT-IR spectra of the BT-based membranes, one absorption peak related to the O–S–O asymmetric stretching vibration shifted to the higher wavenumbers compared to SPES (Fig. S7†). This peak shift is caused in a structure having highly interacting sulfonic acid groups, which created weaker H-bonds.^22,23 This result suggests that sulfonic acid groups in the membranes concentrated with expression of the unique morphology due to the introduction of the BT unit, which indicated that BT unit induces to construct favourable conformation for proton conduction when the water content is low.

The water content in the BT-based membranes was measured at 80 °C (Fig. 2a). Compared with SPES membranes, the water content in the BT-based membranes decreases significantly as the BT content increases, which suggests that the membranes become more hydrophobic due to introduction of the hydrophobic unit. Furthermore, we calculated the swelling ratio from the dry and swollen volumes for these membranes (Fig. 2b). We found that the swelling ratio decreases as the BT content increases. In particular, swelling of the membranes can be suppressed to half by introducing of just 1% BT units. This result confirms that the introduction of BT units effectively enhances the swelling resistance of the PEMs.

Fig. 2 Influence of BT unit on (a) water content and (b) swelling ratio for BT-based membranes at 80 °C.

We also evaluated the proton conductivity of the BT-based membranes as a function of relative humidity (RH) at 80 °C (Fig. S8†). The BT-based membranes exhibited high values of proton conductivity, which are three times higher than that of SPES membranes at 40% RH. We calculated the activation energy at 40% RH for these membranes from the Arrhenius plot (Fig. 3a). The activation energy values for the 0.1% BT and 1% BT membranes are nearly 20 kJ mol⁻¹, which is lower than that for SPES membranes (24.8 kJ mol⁻¹), indicating that effective proton transfer occurs in the BT-based membranes. We assumed that the introduction of the BT unit enhances proton transfer with hopping mechanism due to the increase in the density of the sulphonic acid group. Therefore, proton conduction through sulphonic acid groups is accelerated in the BT-based membranes under low humidity conditions.

Fig. 3 (a) Effects of BT unit on activation energy (RH = 40%; 80%) and (b) proton conductivity at 80 °C.

Finally, proton conductivity values for these membranes at 80 °C are plotted as a function of water content (Fig. 3b). The BT-based membranes showed much higher proton conductivity values than SPES membranes under a wide range of water content. In particular, the proton conductivity increased as the BT content increased, which indicated that BT units promote proton transfer despite low water content conditions in PEMs. Therefore, the performance for PEMs can be improved significantly because the BT units suppress swelling and enhance proton conductivity at low water content.

In conclusion, we have succeeded in synthesizing the designed novel polymers based on BT units using a simple synthesis method. Compared with a pure SPES membrane, introduction of a small amount of the BT units promotes condensation of hydrophobic chains. Consequently, the BT-based membranes exhibit high swelling resistance. Furthermore, the BT-based membranes afford high proton conductivities, with lower activation energies than the parent SPES membrane, particularly at low water content conditions.

We believe that our design concept, wherein BT units are used to suppress membrane swelling and enhance favourable proton transfer, is effective for developing PEMs with high proton conductivities when the water content is low.

Acknowledgements

This study was supported by the Kanagawa Academy of Science and Technology and KAKENHI (26820355 for S. A.). The authors would like to thank Dr Motoya Suzuki (Material Analysis Suzukake-dai Center, Technical Department, Tokyo Institute of Technology) for AFM measurements and discussion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20784g

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