Anion exchange membranes (AEMs) are polymer membranes of varying thickness (10 - 50 microns) with covalently bound quaternary ammonium cations have been evaluated in fuel cells (AEMFCs).^1,2 Radiation-grafted (RG) ultrathin low-density polyethylene (RG-LDPE) and high-density polyethylene (RG-HDPE) AEMs exhibit high hydroxide conductivities, promising mechanical stabilities, and high AEMFC performances.^{1, 3} Despite having a similar thickness, the RG-HDPE AEM outperformed their low-density counterpart (RG-LDPE) due to enhanced water transport properties (hypothesized to be from nano and microscale level structural and morphological changes).³ Our best performing ultrathin RG-HDPE films exhibited excellent conductivity retention over the 440 h of continuous operation.³ However, water uptakes are high (ca. 150% for the two abovementioned RG-AEMs), hence, it is important to add a controlled amount of crosslinking.^4-7

Traditionally, when involving insoluble pre-aminated radiation-grafted membranes, crosslinking + amination could be achieved by using a heated amination process involving a mixture of liquid, high boiling point mono-amines (such as N-methylpiperidine) and a crosslinking diamine (such as N,N,N',N'-tetramethyl-1,6-hexane diamine - TMHDA).⁸ However, this is not safe when using the commonly supplied aqueous solutions of dissolved trimethylamine (TMA) gas. This presentation will discuss various TMHDA-crosslinking approaches that were explored, to target controlled crosslinking incorporation and to produce TMA-based RG-AEMs with reduced water uptakes and swelling (Figure 1), and the problems encountered. The most promising approach involved the amination of the poly(VBC)-grafted HDPE intermediate membranes with a sub-stoichiometric amount of TMHDA. In a second step, the membranes were then quickly (important) reacted with aqueous TMA to complete the amination process. Raman results revealed that the trimethylamine can displace TMHDA-based head-groups after long amination times, hence, we emphasize the use of short TMA amination times in the second step.

References

[1] Wang, Lianqin, et al., J. Mater. Chem. A, 6, 15404 (2018).

[2] Ferriday, T. B., and Peter H. Middleton, Int. J Hydrogen Energy, 46, 18489 (2021).

[3] Wang, Lianqin, et al., Energy Environ. Sci., 12, 1575 (2019).

[4] T Willson, Terry R., et al., Sustainable Energy Fuels, 3, 1682 (2019).

[5] Iravaninia, Mona, et al., Int. J hydrogen energy 42, 17229 (2017).

[6] Hossain, Md Masem, et al., J Power Sources, 390, 234 (2018).

[7] Xu, Shuai, et al., Mater. Chem., 22, 13295 (2012).

[8] Bance-Soualhi, Rachida, et al., J. Mater. Chem. A, 9, 22025 (2021).

Figure 1. Schematic illustration of the main approach used to reduce water uptakes and swelling in our TMA-HDPE-based RG-AEMs.