Effect of Heterocyclic Ring Systems on Properties of Aromatic Proton Exchange Membranes for PEFCs
Authors: Amari Shuntaro , Ando Shinji , Yamaguchi Takeo 
Published: February 2020

Polymer electrolyte fuel cells (PEFCs) have attracted attention because of their high efficiency, low environmental load and small application size. Development of novel proton exchange membranes (PEMs) with high proton conductivity under low humidity is strongly desired to enhance a performance of PEFCs. In this context, precedence research based on quantum chemical calculations shows that effective proton transfer through the counter anion (e.g. -SO3 in the case of sulphonic acid-based PEMs) occurs by increasing their density1. From this viewpoint, we focused on heterocyclic ring systems such as benzothiaziazole (BT) and thiazolothiazole (TT) units. These units promote a hydrophobic structure owing to its planar conformation and strong intermolecular interactions (S-N or S-S interactions)2,3. Therefore, we first designed novel aromatic polymers based on BT units with the expectation that the BT units would enhance to suppress membrane swelling and increase the density of the sulphonic acid groups (Figure 1-a, b).
The designed polymers which composed of sulphonated poly (ether sulphone) (SPES) and BT units were synthesized by typical polycondensation. The obtained polymers were characterized by 1H-NMR, 13C-NMR and GPC. AFM analysis was used to observe the morphology of membrane surface, which suggested that the BT-based membrane exhibit a regular structure　(Figure 2). Hence, the membrane structure is greatly influenced by the introduction of BT units. Given the different structuring described above, the density of sulphonic acid group for the BT-based membrane was investigated by FT-IR measurements. Consequently, we observed an interesting tendency that the absorption peaks arised from the sulphonic acid group are gradually shifted to the higher wavenumbers, which suggested the progression of high-density growth favorable for the efficient proton transfer. Then, the water contents of humidity dependency for these membranes were calculated from their weights. The results indicate that the introduction of BT unit is effective to prompt the swelling resistance of PEMs.
Being motivated by the above results, we evaluated the proton conductivity of the BT-based membrane as a function of RH at 80°C (Figure 2). The BT-based membrane exhibits high proton conductivity over a wide range of RH. In particular, the conductivity of the BT-based membrane is 4 times higher than that of SPES at 30% RH. Furthermore, the activation energy from the Arrhenius plot at 40% RH is 19.3 kJ/mol, which is lower than that of SPES (24.8 kJ/mol), indicating that effective proton transition occurs in the BT-based membrane4.
In conclusion, we found that the BT-based membrane afford high proton conductivities, particularly in low RH conditions along with a unique structure by the introduction of just a small amount of the BT unit. We believe that this design concept based on BT units is greatly effective for constructing a proton conduction favorable structure when developing novel PEMs.

Reference

(1) T. Ogawa, K. Kamiguchi, T. Tamaki, H. Imai, and T. Yamaguchi, Anal. Chem., 86, 9362 (2014).
(2) M. Akhtaruzzaman, N. Kamata, J. Nishida, S. Ando, H. Tada, M. Tomura, and Y. Yamashita, Chem. Commun., 25, 3183 (2005).
(3) S. Ando, J. Nishida, H. Tada, Y. Inoue, S. Tokito, and Y. Yamashita, J. Am. Chem. Soc., 127, 5336 (2005).
(4) N. Hara, H. Ohashi, T. Ito, and T. Yamaguchi, J. Phys. Chem. B., 113, 4656 (2009).

Figure 1