Ab Initio Molecular Dynamics Study of Hydroxide Diffusion Mechanisms in Nanoconfined Structural Mimics of Anion Exchange Membranes

The development of reliable, cost-effective polymer architectures for use as anion exchange membranes (AEMs) is an important challenge facing emerging electrochemical device technologies. Elucidation of key design principles underlying these electrolytes requires a fundamental understanding of the hydroxide ion transport mechanism in the aqueous region of an AEM. To this end, we have carried out a series of atomistic ab initio molecular dynamics calculations. To mimic the complex AEM nanoconfined environment, we employ graphane bilayers or carbon nanotubes to which selected cationic groups are attached and which are subsequently filled with water and hydroxide ions to achieve target water-to-cation ratios and overall electrical neutrality. The complex structure of water under nanoconfinement differs from the bulk and is controlled by the shape and size of the confining volume. Consequently, the local hydroxide ion diffusion mechanisms in different chemical and geometric environments is also seen to differ from that in bulk aqueous solution and depends on a number of design parameters, including hydration level, cation spacing, and cell geometry. An exploration of this large parameter space will be presented in a series of reports; in this first one, we introduce analysis tools to characterize the system, elucidate hydroxide transport mechanisms, and present our first set of case studies.