The\nEffects of Morphology and Hydration on Anion Transport\nin Self-Assembled Nanoporous Membranes

Ordered nanoporous polymer membranes\noffer opportunities for systematically\nprobing the mechanisms of ion transport under confinement and for\nrealizing useful materials for electrochemical devices. Here, we examine\nthe impact of morphology and ion hydration on the transport of hydroxide\nand bromide anions in nanostructured polymer membranes with 1 nm scale\npores. We use aqueous lyotropic self-assembly of an amphiphilic monomer,\nwith a polymerizable surfactant to create direct hexagonal (H<sub>I</sub>) and gyroid mesophases. UV-induced cross-linking leads to\nthe formation of nanoporous polymers with water continuous channels.\nThe membranes are mechanically robust and chemically durable, resisting\ndegradation during extended exposure to 1 M NaOH solutions. We use\na combination of electrochemical impedance spectroscopy, pulsed-field\ngradient NMR spectroscopy, and molecular simulations to elucidate\nanion and water transport. The as-prepared hexagonal systems display\nhigher conductivity and lower activation energies for both anions\nrelative to the gyroid system. When compared at equivalent hydration,\nhowever, gyroid and hexagonal membranes show similar activation energies,\nwith nearly identical conductivities at ambient temperatures. Both\nionic conductivity and water diffusivity increase with increasing\nhydration. The water uptake as a function of relative humidity for\nthe hexagonal and gyroid mesophases ultimately dictates the water\ndiffusion and magnitude of the ionic conductivity, with the hexagonal\nsystem showing overall higher capacity for hydration and thus faster\nion transport. The durability of these materials under aggressive\nalkaline conditions and their relatively high hydroxide ion conductivity\nsuggest that these nanostructured polymers could be of interest as\nmembranes for alkaline fuel cells.