Novel Anion Exchange Membrane Design Strategies Toward Improved Water Electrolysis Performance

Long-term energy storage is crucial for transitioning from fossil fuels to renewable energies due to the fluctuating energy generation of renewable energy sources such as solar or wind. 1,2 Water electrolysis could produce hydrogen from water and electricity from renewable energy resources, thus solving the energy storage problem. 3 Recent research aims to combine the advantages of alkaline water electrolysis (AWE) and the zero-gap architecture of proton exchange membrane water electrolysis (PEMWE). Limitations of this so-called AEMWE are the lower hydroxide conductivity compared to proton conductivity and, consequently, lower current densities. Furthermore, the chemical stability of AEMs still limits their technical applicability. 4–6 Sustainion®, a commercially available AEM, is a 4-vinylbenzyl chloride styrene copolymer quaternized with 1,2,4,5-tetramethylimidazole. 5,7 Sustainion® shows excellent performance in AEMWE and CO 2 electrolysis. 7–9 However, on other polymer systems, it was found that the conductivity and alkaline stability could be increased if the cationic group is separated from the backbone by an alkyl spacer. 10–12 Furthermore, other cationic groups or polymer backbones also offer several advantages. In this study, we demonstrate some new membrane synthesis approaches aiming to enhance the performance of AEMs in electrochemical water-splitting devices. The demonstrated approaches involve attaching different side chains, which may vary in length or the presence of heteroatoms like oxygen. Furthermore, cationic groups like sterically protected imidazoliums are attached to these side chains to study their influence on long-term stability. Finally, our synthetic approaches comprise different membrane preparation strategies such as blending, crosslinking, or applying block-copolymers. The novel polymers are characterized by state-of-the-art methods of polymer chemistry and are further investigated regarding their applicability in AEMWE. Finally, their ionic conductivity, alkaline stability, ion exchange capacity, and the respective performance in AEMWE are investigated. 1 G. A. Lindquist, Q. Xu, S. Z. Oener, S. W. Boettcher, Joule 2020 , 4 , 2549. 2 A. Kiessling, J. C. Fornaciari, G. Anderson, X. Peng, A. Gerstmayr, M. Gerhardt, S. McKinney, A. Serov, A. Z. Weber, Y. S. Kim, B. Zulevi, N. Danilovic, J. Electrochem. Soc. 2022 , 169 , 24510. 3 N. Du, C. Roy, R. Peach, M. Turnbull, S. Thiele, C. Bock, Chemical reviews 2022 , 122 , 11830. 4 S. Gottesfeld, D. R. Dekel, M. Page, C. Bae, Y. Yan, P. Zelenay, Y. S. Kim, J. Power Sources 2018 , 375 , 170. 5 D. Henkensmeier, M. Najibah, C. Harms, J. Žitka, J. Hnát, K. Bouzek, J. Electrochem. Energy Convers. Storage 2021 , 18 . 6 N. Chen, Y. M. Lee, Prog. Polym. Sci. 2021 , 113 , 101345. 7 Z. Liu, S. D. Sajjad, Y. Gao, H. Yang, J. J. Kaczur, R. I. Masel, Int. J. Hydrog. Energy 2017 , 42 , 29661. 8 J. J. Kaczur, H. Yang, Z. Liu, S. D. Sajjad, R. I. Masel, Front. Chem. 2018 , 6 , 263. 9 R. I. Masel, Z. Liu, S. Sajjad, ECS Trans. 2016 , 75 , 1143. 10 H.-S. Dang, P. Jannasch, Macromolecules 2015 , 48 , 5742. 11 S. Miyanishi, T. Yamaguchi, Polym. Chem. 2020 , 11 , 3812. 12 S. P. Ertem, E. B. Coughlin, Macromol. Rapid Commun. 2022 , 43 , e2100610.