Poly(fluorenyl-indolinedione) based hydroxide conducting membrane for anion exchange membrane water electrolyzers

Abstract

Anion exchange membranes (AEMs) serve as the heart of anion exchange membrane water electrolyzers (AEMWE) for green energy conversion electrochemical devices, directly determining the ultimate performance of the electrochemical devices. Conductivity in trade-off with stability (chemical and dimensional stability) is a major target in the development of alkaline membranes. We prepared chemically stable polyisatin-aromatics without ether linkages along the backbone by superacid-catalyzed alkylation. The alkali-stable dimethylpiperidinium cations were covalently linked to the polymer backbone through the aliphatic chain to give comb-type polyelectrolytes. Incorporating the fluorene structural units along the polymer skeleton is expected to improve conductivity and stability. The effect of polymer backbone structure and conformation on AEM properties is systematically elucidated. The results indicate the PipPFTI-25 AEM with 25 mol% dimethylfluorene fragments in the backbone has lower IEC (∼1.36 mmol g−1) but achieves high ionic conductivity (84.6 mS cm−1 at 80 °C). PipPFTI-25 exhibited superior dimensional and alkali stability, mainly reflected in the swelling ratio of less than 10 % while the OH− conductivity remained at 70 % after immersion in 2 M NaOH solution at 80 °C for 1680 h. The AEMWE with PipPFTI-25 operating at 60 °C in 1 M KOH achieves a current of 838 mA cm−2 at 2.5 V. This work demonstrated the feasibility of poly(fluorenyl-indolinedione) AEMs for electrolyzer applications.