Ionic Equilibrium and Transport in the Alkaline Anion Exchange Membrane

Abstract

Several groups have demonstrated alkaline anion exchange membranes (AAEMs) with promising hydroxyl anion conductivity and stability without a supporting metal hydroxide solution. This work examines the equilibrium dissociation and solvation processes associated with the AAEM’s quaternary ammonium side chains and its impact on the membrane’s ionic conductivity. This is undertaken using an analogy to acidity functions for solutions containing a strong base of similar chemical structure to the AAEM’s functional groups. The AAEM has a lower base dissociation constant, relative to the acid dissociation constant for DuPont’s Nafion. The lower dissociation constant results in a smaller degree of dissociation and thus availability of ionic charge carriers. An ionic transport model is then used to predict membrane conductivities. The dissociation and solvation models are used to prescribe the concentration of the available charge carriers. The dissociation and solvation models permit a consistent description of the AAEM and Nafion to predict ionic conductivity. This model is validated with experimental ionic conductivity data. The dissociation and solvation processes are recognized as a prominent factor influencing membrane conductivity.