Tuning the Performance of Poly(quaterphenyl piperidinium) Anion-Exchange Membranes by Monomer Configuration

Poly(arylene piperidinium)s are attractive anion exchange membranes (AEMs) for alkaline membrane fuel cells and water electrolyzers because of their high chemical stability and hydroxide conductivity. Here, ether- and fluorine-free hydroxide conducting poly(quaterphenyl piperidinium) membranes with very similar ion exchange capacity (IEC), but different ratios of meta to para connectivity in the quaterphenyl units, are synthesized and explored. Ionic clustering, water uptake, and the hydroxide conductivity of the AEMs increase gradually with increasing backbone flexibility, i.e., the fraction of meta connectivity. At 80 °C, the AEMs with para,para- and meta,meta-quaterphenyl units, respectively, reach a conductivity of 96 and 178 mS cm⁻¹, respectively, at a water uptake of 37 and 209%, respectively. Alkaline stability evaluations reveal high alkaline stability, increasing with backbone flexibility. Under very harsh conditions, the cationic loss increases with chain stiffness and occurs mainly through Hofmann β-elimination, but also increasingly via nucleophilic methyl substitution. Copolymerization of quaterphenyls provides properties in between the corresponding homopolymers. In conclusion, it is demonstrated that the configuration of the quaterphenyl monomer can be efficiently tailored to manipulate the chain flexibility of highly alkali-stable AEMs for control over the water uptake and ionic conductivity in a wide range without changing the ionic content of the membranes.