A rod-coil grafts strategy for N-spirocyclic functionalized anion exchange membranes with high fuel cell power density

Abstract N-spirocyclic cations possess double-cyclic non-planar structure that exhibit the highest alkali stability among quaternary ammonium cations, however, the extremely rigidity usually causes fragile membranes and poor conductivity. In this work, a rod-coil grafts design is proposed for N-spirocyclic anion exchange membranes (AEMs), in which microphase separation of the hydrophilic N-spirocyclic rod grafts is significantly improved by the hydrophobic aggregation of the flexible alkyl coil grafts with polysulfone backbone. Molecular dynamic simulations indicate that the coil grafts contribute to microphase separation but fill in free volume to reduce water reservoir, therefore the rod-coil grafts design provides a way to evaluate the effects of microphase separation and free volume on conductivity. The increasing conductivity with the length of coil grafts suggests a greater contribution of good microphase separation to OH− conduction. With optimized n-octylamine hydrophobic coil graft length, the N-spirocyclic AEM exhibits toughness (elongation at break of about 28.7%) and high OH− conductivity (136.2 mS cm−1 at 80 °C), resulting in high power density (850.1 mW cm−2), which is far greater than that assemble with other N-spirocyclic AEMs, and also bring N-spirocyclic AEMs into the top level of the cycloaliphatic AEMs reported in literatures.