(Invited) Molecular Engineering of Aromatic Polymer Electrolytes for Anion Exchange Membranes

Abstract

Recently, anion exchange membrane (AEM) fuel cell has attracted attention as a clean energy technology device to convert chemical energy into electric energy. AEM fuel cells have several advantages, such as fast kinetics and non-precious metal catalysts.1 However, desired AEMs should have high hydroxide conductivity, thermochemical stability at high pH for durable operation, and convenient synthetic process.2-5 These challenges can be approached by designing of new polymer backbones, side chain functional groups and new anion conductive groups.6 In designing polymer structure, aryl ether groups are known to cause polymer backbone degradation in high pH environment.5 Several research groups including us have reported that a long side chain between polymer backbone and quaternary ammonium (QA) end group is helpful for enhancing micro-phase separation based on thermochemical stable backbone.7-9 In this study, we will present the effects of polymer structures (i.e. QA group, tether chain, polymer backbone) on AEM characteristics and fuel cell performance, including morphology results. Representative polymer structures of AEM are shown in Figure 1. The physical properties of the precursor polymers and AEM characteristics are summarized in Table 1. We observed morphological difference between m-TPN1 and p-TPN1 membranes from small-angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). The substitution pattern of the meta-structure membrane (m-TPN1) played a major role in microdomain structures, which contributed to improved conductivity and fuel cell performance. Table 1. Properties of precursor polymers and IECs, water uptake, hydration number, and hydroxide conductivity of ionic polymers Polymer Mn § Mw § PDI IEC a IEC b WU (%) c λ (OH-) d σ (mS cm-1) e 30 °C 80 °C p-TPN1 30.9 61.3 1.98 2.12 2.15 65 17 43 81 m-TPN1 62.2 126.2 2.02 2.15 2.13 70 18 54 112 BPN1-65 f 72.1 138.8 1.93 1.94 1.93 g 85 24 41 88 BPN1-100 f 70.8 110.1 1.6 2.70 2.80 g 124 26 62 122 § kg/mol of precursor polymer. a IEC values by 1H NMR analysis (meq./g). b IECs by 1H NMR analysis after alkaline test at 1M NaOH at 95 °C for 30 days (meq./g). c Water uptake was measured in OH- form at 80 °C. d Based on water uptake value at 80 °C. e hydroxide conductivity. f Reference 8 g Alkaline test at 80 °C (reference 8). 1. Y. J. Wang, J. Qiao, R. Baker, J. Zhang, J., Chem. Soc. Rev. 42 , 5768, (2013). 2. O. M. M. Page, S. D. Poynton, S. Murphy, A. Lien Ong, D. M. Hillman, C. A. Hancock, M. G. Hale, D. C. Apperley, J. R. Varcoe, RSC Adv. 3 , 579 (2013). 3. K. J. Noonan, K. M. Hugar, H. A. Kostalik, E. B. Lobkovsky, H. D. Abruna, G. W. Coates, J. Amer. Chem. Soc.134 , 18161 (2012). 4. J. Ran, L. Wu, T. Xu, Polym. Chem. 4 , 4612 (2013). 5. C. Fujimoto, D.-S. Kim, M. Hibbs, D. Wrobleski, Y. S. Kim, J. Membr. Sci. 423 −424 , 438 (2012). 6. A. D. Mohanty, C. Y. Ryu, Y. S. Kim, C. Bae, Macromolecules 48 , 7085 (2015). 7. H.-S. Dang, P. Jannasch, Macromolecules 48 , 5742 (2015). 8. W.-H. Lee, Y. S. Kim, C. Bae, ACS Macro Lett. 4, 814 (2015). 9. W.-H. Lee, A.D. Mohanty, C. Bae, ACS Macro Lett. 4, 453 (2015). Figure 1