Thermal treatment of high-density polyethylene films to increase crystallinity for the fabrication of superior radiation-grafted anion-exchange membranes for fuel cells

Abstract

When fabricating radiation-grafted anion exchange membrane (RG-AEMs), it is important to understand the microstructure of the precursor film (such as high-density polyethylene, HDPE). This study builds on recent work showing that variations in the degree of crystallinity of the HDPE precursor led to variations in the final RG-AEM properties (up to a threshold of 81 %, above which the properties become more consistent). This study shows that the degree of crystallinity of the HDPE film can be increased to > 81 % by thermal treatment at 115 °C for 24 h (followed by natural cooling in the oven). In addition to increasing the bulk degree of crystallinity, it was shown that thermal treatment increased crystalline domain sizes and lamella width distributions, while it reduced orientational order of the lamellae. The treated HDPE films resulted in optimised RG-AEMs with increased IECs and water uptakes values at high relative humidities (RH > 70 %); there was also a small improvement in Cl¯ conductivities in water. Terahertz time domain spectroscopy (THz-TDS) showed that the proportion of bulk water present in the optimised RG-AEM was greater than its untreated counterpart (66 % vs. 58 % when fully hydrated); bulk water is essential for high ion conductivities and rapid water diffusion. In fuel cell testing, the optimised RG-AEM had improved in situ water transport compared to the untreated benchmark, as it required lower operating RHs to achieve a peak power density of 2.8 W cm^-2. Furthermore, the optimised RG-AEM maintained a higher operating current at intermediate voltages, 2.7 A cm^-2 at 0.7 V vs. 2.1 A cm^-2 for the benchmark RG-AEM