Poly(phenylene oxide)s incorporating N-spirocyclic quaternary ammonium cation/cation strings for anion exchange membranes

Abstract

The knowledge that N-spirocyclic quaternary ammonium (QA) cations appear to have good alkaline stability has triggered the quest to incorporate these into anion exchange membranes (AEMs) with the anticipation of high operating stability in alkaline fuel cells. Herein, a series of poly(2,6-dimethyl-1,4-phenylene oxide)s (PPOs) incorporating spirocyclic 3,6-diazaspiro[5.5]undecane (DSU) cation/cation strings are synthesized by a two-step quaternization strategy. The properties of DSU-based single-cation and double-cation AEMs (PPO-SDSU and PPO-DDSU, respectively) are discussed and the effect of introducing heterocyclic quaternary piperidinium (Qpip) on spirocyclic DSU cation strings is investigated. Ionic clusters of larger size scattering are observed by atomic force microscopy for PPO-SDSU membranes, suggesting the dominant effect of polar tertiary nitrogen atom on the formation of ion cluster. The well-connected hydrophilic domains contributed to further increasing the water uptake and hydroxide conductivity of the AEMs. As a result, the PPO-SDSU membrane with ion exchange capacity (IEC) of 1.91 meq/g shows the highest hydroxide conductivity (31.9 mS/cm) with 76% water uptake and 9.6% linear dimensional swelling at 20 °C. Alkaline stability tests imply that the presence of Qpip in DSU cation strings alleviates the degradation of spirocyclic QA from SN2 substitution, due to increased steric hindrance, but other considerations must address the potentially detrimental effect of Hofmann elimination on spirocyclic QA and PPO backbone scission, arising from the electron withdrawing effect of Qpip. Although hydroxide conductivity and IEC values were better retained for PPO-DDSU membranes, the inferior alkaline stability of the benzylic attached Qpip counteracted the overall stability enhancement anticipated for incorporating spirocyclic DSU cations. These results may provide valuable insights toward more effective molecular designs and new synthetic pathways to AEMs incorporating base-stable N-spirocyclic QA.