Tuning the Hydrophilicity of Polymers for Ion-Solvating Membranes for Alkaline Electrolysis

Ion-solvating membranes constitute an alternative to both porous diaphragms and anion-exchange membranes for alkaline electrolysis. [1,2] The underlying principle is to imbibe a nonporous hydrophilic membrane with concentrated aqueous KOH and thus obtain high ionic conductivity from the sorbed electrolyte. It is well-established that high ionic conductivity can be reached, [3] and the main challenge is to identify or develop polymers with the right affinity for aqueous KOH and with long-term stability in the aggressive environment. Polymers known for their alkaline stability are generally hydrophobic and will not take up KOH. One strategy to promote and control the KOH uptake is to tune the polymer hydrophilicity to the extent that enough KOH is sorbed without dissolving the polymer. [4] The paper presents recent progress with hydrophilicity adjustment of polymers by hydrophilic functionalities. Hydrophilic tetrazole moieties are particularly stable in the alkaline environment due to deprotonation at high pH and the resulting formation of a negative charge that repels the aggressive hydroxide ions. See Figure 1. It is easy to adhere to the backbone by a click reaction. Chemical stability screening, electrolyte uptake, ion conductivity, and initial electrolyzer tests are presented. - References [1] D. Aili, M. R. Kraglund, S. C. Rajappan, D. Serhiichuk, Y. Xia, V. Deimede, J. Kallitsis, C. Bae, P. Jannasch, D. Henkensmeier and J. O. Jensen. Electrode Separators for the Next-Generation Alkaline Water Electrolyzers . ACS Energy Lett. 8 1900-1910 (2023). DOI: 10.1021/acsenergylett.3c00185 [2] D. Henkensmeier, W. Cho, P. Jannasch, J. Stojadinovic, Q. Li, D. Aili and J. O. Jensen. Separators and membranes for advanced alkaline water electrolysis. In press, Chemical Reviews. DOI: 10.1021/acs.chemrev.3c00694 [3] M. R. Kraglund, M. Carmo, G. Schiller, S. A. Ansar, D. Aili, E. Christensen and J. O. Jensen. Ion-solvating membranes as a new approach towards high rate alkaline electrolyzers. Energy Environ. Sci. 12, 3313 – 3318 (2019). DOI: 10.1039/c9ee00832b [4] D. Serhiichuk, S. Rajappan, Y. Krishnan, Y. Xia, M. R. Kraglund, H. A. Hansen, J. O. Jensen and D. Aili. Tetrazole functionalization: A new strategy toward stable ion-solvating polymer electrolytes for alkaline water electrolysis. Submitted . DOI: 10.26434/chemrxiv-2024-29p7m - Figure 1. Tetrazole functionalization of poly(styrene-co-acrylonitrile) (SAN) followed by formation of the corresponding potassium tetrazolide. [4] Figure 1