Preparation and properties of anion exchange membranes with exceptional alkaline stable polymer backbone and cation groups

Abstract

To study the effect of cation groups on the alkaline stability and other properties of anion exchange membranes (AEMs), common quaternary ammonium (QA), N-alicyclic quaternary ammonium and imidazolium cations with bulky substituents were grafted onto an aryl ether-free polyaromatic backbone synthesized via acid-catalyzed polyhydroxyalkylation. The size of cation groups is calculated by Gaussian. The size of bulky imidazolium (MIm) is much larger than QA and N-alicyclic quaternary ammonium (Py and Pi). Because of the large size of the cation groups, the chain entanglement density of AEM with bulky imidazolium (PFBA-MIm-0.4) is lower than other three membranes, resulting PFBA-MIm-0.4 displays the highest water uptake of 225.1% at 80 °C, whereas the IEC of PFBA-MIm-0.4 is the lowest (1.49 mmol g−1). Meanwhile, the toughness of the membranes decreases when the size of the cation group increases. AEM with common quaternary ammonium (PFBA-QA-0.4) exhibits the highest hydroxide conductivity of 142 mS cm−1 at 80 °C because it exhibits the highest ion exchange capacity (IEC). More interestingly, PFBA-QA-0.4 shows long term alkaline stability that is as good as that of the AEMs with alicyclic quaternary ammonium or bulky imidazolium, even in 5 mol L−1 aqueous NaOH at 80 °C; however, the small size of the substituents of common quaternary ammonium does not have steric hindrance that is as large as that of the substituents of alicyclic quaternary ammonium and bulky imidazolium. Therefore, we selected PFBA-QA-0.4 to prepare a membrane electrode assembly for the further study of alkaline H2/O2 fuel cell performance. The single H2/O2 fuel cell exhibits a peak power density as high as 559 mW cm−2 at 80 °C under 100% RH with 0.1 MPa backpressure. These results of the comparison of AEMs with different size cation groups give some insights for future directions in the development of advanced AEMs.