Tetrazole-Containing Polyelectrolytes: A New Class of Ion-Solvating Membranes for Alkaline Water Electrolysis

Abstract

Alkaline ion-solvating membranes, such as those based on polybenzimidazole (PBI), offer high ion conductivity and good gas barrier properties, making them promising for next-generation alkaline water electrolyzers.1 However, the stability of commonly used PBI derivatives is limited due to gradual backbone degradation. Consequently, alternative PBI chemistries and other polymer options like poly(vinyl alcohol),2 and carboxylate functionalized styrene-ethylene-butylene copolymers are being explored to address this inherent stability issue.3 In this study, we present the synthesis and characterization of a novel type of ion-solvating membranes for alkaline water electrolyzers. These membranes are based on tetrazole functionalized polymers derived from poly(styrene-co-acrylonitrile). A one-step click-type [3+2] cycloaddition reaction was utilized to introduce tetrazole pendants by converting acrylonitrile groups in poly(styrene-co-acrylonitrile). The degree of functionalization was controlled by adjusting the content of acrylonitrile units in the polymer. Subsequent treatment in aqueous KOH solution led to the formation of negatively charged tetrazolide pendants with high ionization degrees.The alkaline stability of the tetrazolides was evaluated using both DFT simulations and model compound studies, demonstrating remarkable stability with no degradation observed for over 3000 hours in 25 wt.% KOH at 80 °C. Physicochemical properties such as electrolyte uptake, swelling behavior, thermal stability, and ion conductivity were assessed for the synthesized polymers. The electrolyte-imbibed membranes exhibited high ion conductivity, reaching up to 90 mS cm-1 in 15 wt% KOH at 80 °C. Electrolysis operation using these membranes showed stable performance for 600 hours with no signs of degradation. 1 D. Aili, M. R. Kraglund, S. C. Rajappan, D. Serhiichuk, Y. Xia, V. Deimede, J. Kallitsis, C. Bae, P. Jannasch, D. Henkensmeier, J. O. Jensen, ACS Energy Letters, 2023, 1900–1910. 2 Y. Xia, S. C. Rajappan, D. Serhiichuk, M. R. Kraglund, J. O. Jensen, D. Aili, J Memb Sci 2023, 680, 121719. 3 D. Serhiichuk, D. Tian, M. R. Almind, Z. Ma, Y. Xia, M. R. Kraglund, C. Bae, J. O. Jensen, D. Aili ACS Applied Energy Materials 2024 7 (3), 1080-1091 Figure 1 Tetrazolation of poly(styrene-co-acrylonitrile) and formation of corresponding potassium tetrazolide following equilibration in aqueous KOH. Figure 1