Alkali-Resistant Anion-Exchange Membranes for Electrochemical Hydrogen Conversions
Authors: Ponce-Gonzalez Julia , Whelligan Daniel , Wang Lianqin , Bance-Soualhi Rachida , Varcoe John 
Published: February 2020

H2 fuel cells and water electrolysers containing solid alkaline electrolytes represents a promising approach for the development of a cost-effective H2-based infrastructure. The alkaline media promises the use of cheaper non-precious metal catalysts [1]. Therefore, there is a push to fulfil the current performance requirements of such electrochemical devices. The employment of alkaline anion-exchange membrane (AAEM) in fuel cells and water electrolysers allows the reduction of the inter-electrode distance with better energy efficiencies without an increase in undesirable gas crossover and a reduction of the purity of the product H2 [2]. However, the stability of the central AAEM at operation temperatures (up to 90°C) remains the biggest challenge of this technology.

Numerous researchers around the world are seeking the most stable chemistries of AAEMs in strongly basic media. Among the cationic head group options, the use of quaternary ammoniums (QA) is common, due to the accessibility of the starting materials and their demonstrated ion-conducting properties. Numerous studies of hydroxide-derived degradation products have enabled the identification of the main degradation pathways with such tetraalkylammonium functionalities [3].

Herein, we have employed radiation-induced grafting for the preparation of conductive AAEMs (> 100 mS cm-2 at 80°C, fully hydrated) with various benzylic QA head-groups (cyclic and non-cyclic). The radiation induced grafting of commodity polymer films affords a reproducible method for the cost competitive, large lab-scale preparation of ion-exchange materials that is scalable for further applications [4]. The spectroscopic and composition analysis of the AAEMs after aqueous hydroxide (1 mol dm-3) treatment at 80°C revealed various degradation mechanisms are occurring. According to this, we have developed chemistries to introduce QA head groups that afforded AAEMs with improved alkali stabilities. Furthermore, fuel cell testing has demonstrated that some of these new chemistries significantly enhance the in situ electrochemical performance of the AAEMs compared to the radiation-grafted benzyltrimethylammonium benchmark [5] (> 800 mW cm-2 H2/O2 peak power densities at 60°C, no back-pressurization, with 50 μm thick AAEMs and Pt- benchmark catalysts).

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[4] M. Nasef, Chem.Rev, 2012, 114, 12278.

[5] J. R. Varcoe, R. C. T. Slade, E. Lam How Yee, S. D. Poynton, D. J. Driscoll and D. C. Apperley, Chem. Mater., 2007, 19, 2686–2693.

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