Advancement of Perfluorinated Alkaline Exchange Membrane Materials Toward Improved Durability

Abstract

Perfluorinated (PF) alkaline exchange membranes have garnered attention due to the commercial success of the acidic analogue Nafion™, owing to it’s chemical stability, enhanced water mobility, and high performance in PEMFC systems. Furthermore, the incorporation non PGM catalyst materials available for AEMFC has the potential to significantly reduce cost. Current state-of-the-art PF AEM materials are derivatives of PF sulfonic acid ionomers wherein the sulfonic acid groups are modified to tether tetraalkylammonium cations. Previous research has indicated that a tether length of 6 carbon atoms from the sulfonamide linkage was sufficient to insulate the quaternary ammonium cation from the strong electron withdrawing PF backbone and was stable at 80°C for 2 weeks in 2M KOH indicating only ~10 % loss of IEC.1 Current research indicates that the most vulnerable functional group in the PF AEMs is the quaternary ammonium cation. Furthermore, there exists literature demonstrating small molecule imidazolium cations with enhanced base stability compared to tetraalkylammonium cations.2-4 The research presented will focus on furthering the base stability of PF AEMs through the incorporation of a sterically hindered imidazole ring. Presented is the synthesis of the 2-(2,6-dimethylphenyl)-1,4,5-trimethyl-imidazole (DMPTMIm) small molecule and its base stability, incorporation the DMPTMIm cation into PF materials, and membrane performance. Park, A. M.; Owczarczyk, Z. R.; Garner, L. E.; Yang-Neyerlin, A. C.; Long, H.; Antunes, C. M.; Sturgeon, M. R.; Lindell, M. J.; Hamrock, S. J.; Yandrasits, M. A.; Pivovar, B. S., Synthesis and characterization of perfluorinated anion exchange membranes. ECS Trans. 2017, 80 (8, Polymer Electrolyte Fuel Cells 17 (PEFC 17)), 957-966.Hugar, K. M.; Kostalik, H. A.; Coates, G. W., Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure-Stability Relationships. J. Am. Chem. Soc. 2015, 137 (27), 8730-8737.Price, S. C.; Williams, K. S.; Beyer, F. L., Relationships between Structure and Alkaline Stability of Imidazolium Cations for Fuel Cell Membrane Applications. ACS Macro Lett. 2014, 3 (2), 160-165.Sturgeon, M. R.; Long, H.; Park, A. M.; Pivovar, B. S., Advancements in anion exchange membrane cations. ECS Trans. 2015, 69 (17, Polymer Electrolyte Fuel Cells 15), 377-383.