Ether-free polyfluorenes tethered with quinuclidinium cations as hydroxide exchange membranes

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2019, Vol.7 (47), p.27164-27174
• Main Authors: Allushi, Andrit, Pham, Thanh Huong, Olsson, Joel S, Jannasch, Patric
• Format: Article
• Language: English
• Subjects: Ammonium; Anion exchanging; Aromatic compounds; Backbone; Cation exchanging; Cationic polymerization; Cations; Chemical Engineering; Chemical reactions; Chemical Sciences; Conductors; Copolymers; Electrochemistry; Energy conversion; Energy storage; Engineering and Technology; Ion exchange; Kemi; Kemiteknik; Materialkemi; Materials Chemistry; Membranes; Moisture content; Monomers; Natural Sciences; Naturvetenskap; NMR; Nuclear magnetic resonance; Polymer Chemistry; Polymer Technologies; Polymerkemi; Polymers; Polymerteknologi; Reference materials; Ring structures; Sodium hydroxide; Stability analysis; Stiffness; Substitution reactions; Teknik; Water content
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/c9ta09213g

We report on aryl ether-free 2,7-diphenylfluorene-based copolymers tethered with quinuclidinium (Qui) cations via hexyl spacers, prepared through superacid catalyzed Friedel-Crafts polycondensation and quaternization reactions. The 2,7-diphenylfluorene monomers were synthesised by Suzuki coupling and were employed to increase polymer backbone stiffness. Corresponding copolymers and anion-exchange membranes (AEMs) tethered with piperidinium (Pip) and trimethylalkyl ammonium (TMA) cations were prepared as reference materials. At a given water content, the AEM functionalized with Qui cations was the most efficient hydroxide conductor and its OH − conductivity reached 100 mS cm −1 at 80 °C at an ion exchange capacity of 2.0 mequiv. g −1 . Moreover, this membrane showed the highest thermal and alkaline stability in the series. 1 H NMR analysis of AEMs stored in 2 M aq. NaOH at 90 °C over 672 h revealed the complete absence of any ring-opening β-elimination in the bicyclic cage-like Qui structure, and less than 2% β-elimination in the hexyl spacer. In contrast, the Pip cations were found to degrade via β-elimination in both the monocyclic ring structure and the hexyl spacer. Results on the Pip-modified AEM implied that a β-hydrogen in the linear alkyl spacer chain was approximately 4 times more vulnerable to elimination than a β-hydrogen in the 6-membered ring. In addition, all the cations degraded via substitution reactions to some degree, and the total loss of Qui, Pip and TMA cations over the period was estimated to be 4, 12 and 9%, respectively. The overall findings demonstrate that the combination of aryl ether-free backbone polymers and Qui cations results in durable and high-performance AEMs suitable for use in alkaline electrochemical energy conversion and storage devices. Quinuclidinium cations attached to ether-free aromatic copolymers show outstanding resistance against degradation via ring-opening β-elimination and facilitate high and efficient hydroxide ion conduction.