Non-Fluorinated High Performance Pre-Irradiation Grafted Anion-Exchange Membranes

Abstract

There have been a boom in the development of anion-exchange membranes (AEMs) for fuel cells (AEMFC) and water electrolyzers in recent years. These devices permit the use of a wider range of non-precious-metal electrocatalysts along with additional fuel flexibility (e.g. the use of N-containing fuels such as hydrazine). A significant tool for the production of AEMs is radiation induced graft polymerization. We have proved that a water-based, emulsion radiation graft polymerization of vinylbenzyl chloride (VBC) onto partially fluorinated ETFE film enhanced the degree of grafting compared to when an organic diluent was used. 1 A major challenge in handling ETFE-based radiation-grafted AEMs has always been their rather poor mechanical stability due to the radiation treatment. 2 Locally, fuel cell tests were not necessarily terminated due to chemical degradation but due to swelling stresses and membrane rupture. This study focuses on the emulsion-type radiation (peroxidation) grafting of VBC onto non-fluorinated LDPE films of 25 μm thickness using electron-beams (in air). VBC is an ideal grafting monomer as it contains both a polymerizable double bond and a reactive benzyl chloride group, the latter of which can be converted to positively charged anion-exchange groups on reaction with amines. ETFE with the same thickness, 25 μm, was grafted under identical conditions as the benchmark. An elevated electron-beam dose was required with the LDPE to obtain the desired grafting levels (cf. ETFE). The LDPE- and ETFE-based AEMs were characterized in terms of ion-exchange capacity (IEC), water uptakes and swelling, and in-plane conductivity. Raman microscopy of the AEM cross-sections was used to map the distribution of the graft component across the samples’ thicknesses (μm spacial resolution). The LDPE-AEMs exhibited enhanced mechanical stabilities (relative tensile stress–strain testing). A high-performance LDPE-AEM, with enhanced properties compared to the reference ETFE-AEM was evaluated in single H2/O2 fuel cell tests. The fuel cell performances were tested up to T = 80°C. The polarisation curves are presented in Fig. 1a. A significantly higher fuel cell performance was obtained with the LDPE-AEM. The enhancement is due to a combination of high anion conductivity and enhanced water management (facilitated water back transport from the anode to the cathode). Furthermore, the LDPE-AEM exhibited superior mechanical strength (cf. ETFE-AEM) even after alkali treatment (Fig. 1b). Figure 1. a) H2/O2 AEMFC performance at 80 °C (PtRu/C anodes and Pt/C cathodes, Pt loadings all 0.4±0.02 mg cm-2) and no gas back-pressurization. b) Tensile measurements on the LDPE- and ETFE-AEMs (before and after aqueous KOH (1 M) treatment at 80°C for 7 days). 1. L. Q. Wang, E. Magliocca, E. L. Cunningham, W. E. Mustain, S. D. Poynton, R. Escudero-Cid, M. M. Nasef, J. Ponce-González, R. Bance-Soualhi, R. C. T. Slade, D. K. Whelligan and J. R. Varcoe, Green Chemistry, 2017, 19, 831-843. 2. J. Huslage, T. Rager, B. Schnyder and A. Tsukada, Electrochim Acta, 2002, 48, 247-254. Figure 1