Poly(arylene alkylene)s Carrying Quaternary Ammoniums via Flexible Phenylpropyl Spacers as Hydroxide Conducting Membranes

Abstract

Anion exchange membranes (AEM)s are attracting growing research interests as central components in various alkaline energy conversion and storage devices such as AEM fuel cells and water electrolyzers.1,2 Compared to the devices operated under acidic conditions, the alkaline conditions open up for possibilities to use, e.g., nickel and iron catalysts instead of platinum. However, the insufficient hydroxide conductivity and long-term stability of AEMs at elevated temperatures are still hindering a widespread application of these devices.3 An effective strategy to improve AEM properties by structure design is to functionalize hetero-atom free polymer backbones with quaternary ammonium cations via flexible spacers.4-6 In the present work, we have synthesized a series of AEM materials by tethering three different quaternary ammonium cations to poly(arylene alkylene)s via a phenylpropyl spacer (Figure 1a). The Friedel-Crafts acylation and polyhydroxyalkylation employed in the monomer and polymer syntheses are both free of noble metals and have excellent yields. All the polymers prepared in this way formed robust membranes reaching high hydroxide conductivities – up to 175 mS/cm at 80 °C (Figure 1b). PpT-DMP, carrying dimethyl piperidinium cations, was found to have the highest alkaline stability with less than 7% ionic loss after treatment in 7 M aq. NaOH at 90 oC during 240 h. The hydroxide conductivity of PpT-DMP reached 104 mS/cm at an ion exchange capacity (IEC) of 1.79 mequiv./g., which is still quite high. Atom force microscope (AFM) phase imaging revealed a phase separated morphology of PpT-DMP, most probably involving ion-clustering (Figure 1c). In this presentation we will further discuss the synthesis of these polymers, and influence of the phenylpropyl spacer on AEM properties.[1] Xue, J.; Zhang, J.; Liu, X.; Huang, T.; Jiang, H.; Yin, Y.; Qin, Y.; Guiver, M. D. Toward alkaline-stable anion exchange membranes in fuel cells: cycloaliphatic quaternary ammonium-based anion conductors. Electrochem. Energy Rev. 2022, 5, 348-400.[2] Chatenet, M.; Pollet, B. G.; Dekel, D. R.; Dionigi, F.; Deseure, J.; Millet, P.; Braatz, R. D.; Bazant, M. Z.; Eikerling, M.; Staffell, I.; Balcombe, P.; Horn, Y. S.; Schafer, H. Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments. Chem. Soc. Rev. 2022, 51, 4583-4762.[3] Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K. N.; Scott, K.; Xu, T.; Zhuang, L. Anion-exchange membranes in electrochemical energy systems. Energy Environ. Sci. 2014, 7, 3135-3197.[4] Dang, H. S.; Jannasch, P. Exploring different cationic alkyl side chain designs for enhanced alkaline stability and hydroxide ion conductivity of anion-exchange membranes. Macromolecules 2015, 48, 5742-5751.[5] Allushi, A.; Pham, T. P.; Olsson, J. S.; Jannasch, P. Ether-free polyfluorenes tethered with quinuclidinium cations as hydroxide exchange membranes. J. Mater. Chem. A 2019, 7, 27164-27174.[6] Lee, W. H.; Kim, Y. S.; Bae, C. Robust Hydroxide Ion Conducting Poly(biphenyl alkylene)s for Alkaline Fuel Cell Membranes. ACS Macro Lett. 2015, 4, 814-818. Figure 1. a) Poly(arylene alkylene)s tethered with different quaternary ammonium cations via phenylpropyl spacers, b) hydroxide conductivity of the AEMs in the fully hydrated state, and c) an AFM phase image of PpT-DMP. Figure 1