Cycloaliphatic Quaternary Ammonium Functionalized Poly(oxindole biphenyl) Based Anion-Exchange Membranes for Water Electrolysis: Stability and Performance

Sara Gjoshi; Paraskevi Loukopoulou; Michaela Plevova; Jaromir Hnat; Karel Bouzek; Valadoula Deimede

doi:10.3390/polym16010099

Cycloaliphatic Quaternary Ammonium Functionalized Poly(oxindole biphenyl) Based Anion-Exchange Membranes for Water Electrolysis: Stability and Performance

Polymers (Basel). 2023 Dec 28;16(1):99. doi: 10.3390/polym16010099.

Authors

Sara Gjoshi¹, Paraskevi Loukopoulou¹, Michaela Plevova², Jaromir Hnat², Karel Bouzek², Valadoula Deimede¹

Affiliations

¹ Department of Chemistry, University of Patras, GR-26504 Patras, Greece.
² Department of Inorganic Technology, University of Chemistry and Technology, Prague, Technická 5, 16628 Prague, Czech Republic.

Abstract

Mechanically robust anion-exchange membranes (AEMs) with high conductivity and long-term alkali resistance are needed for water electrolysis application. In this work, aryl-ether free polyaromatics containing isatin moieties were prepared via super acid-catalyzed copolymerization, followed by functionalization with alkaline stable cyclic quaternary ammonium (QA) cationic groups, to afford high performance AEMs for application in water electrolysis. The incorporation of side functional cationic groups (pyrrolidinium and piperidinium) onto a polymer backbone via a flexible alkyl spacer aimed at conductivity and alkaline stability improvement. The effect of cation structure on the properties of prepared AEMs was thoroughly studied. Pyrrolidinium- and piperidinium-based AEMs showed similar electrolyte uptakes and no obvious phase separation, as revealed by SAXS and further supported by AFM and TEM data. In addition, these AEMs displayed high conductivity values (81. 5 and 120 mS cm^-1 for pyrrolidinium- and piperidinium-based AEM, respectively, at 80 °C) and excellent alkaline stability after 1 month aging in 2M KOH at 80 °C. Especially, a pyrrolidinium-based AEM membrane preserved 87% of its initial conductivity value, while at the same time retaining its flexibility and mechanical robustness after storage in alkaline media (2M KOH) for 1 month at 80 °C. Based on ¹H NMR data, the conductivity loss observed after the aging test is mainly related to the piperidinium degradation that took place, probably via ring-opening Hofmann elimination, alkyl spacer scission and nucleophilic substitution reactions as well. The synthesized AEMs were also tested in an alkaline water electrolysis cell. Piperidinium-based AEM showed superior performance compared to its pyrrolidinium analogue, owing to its higher conductivity as revealed by EIS data, further confirming the ex situ conductivity measurements.

Keywords: AEMs; alkaline stability; alkaline water electrolysis; ionic conductivity; piperidinium; poly(oxindole biphenyl) backbone; pyrrolidinium.

Grants and funding

862509/European Union