title: Hydroxide Solvation and Transport in Anion Exchange Membranes.
abstract: Understanding hydroxide solvation and transport in anion exchange membranes (AEMs) can provide important insight into the design principles of these new membranes. To accurately model hydroxide solvation and transport, we developed a new multiscale reactive molecular dynamics model for hydroxide in aqueous solution, which was then subsequently modified for an AEM material. With this model, we investigated the hydroxide solvation structure and transport mechanism in the membrane. We found that a relatively even separation of the rigid side chains produces a continuous overlapping region for hydroxide transport that is made up of the first hydration shell of the tethered cationic groups. Our results show that hydroxide has a significant preference for this overlapping region, transporting through it and between the AEM side chains with substantial contributions from both vehicular (standard diffusion) and Grotthuss (proton hopping) mechanisms. Comparison of the AEM with common proton exchange membranes (PEMs) showed that the excess charge is less delocalized in the AEM than the PEMs, which is correlated with a higher free energy barrier for proton transfer reactions. The vehicular mechanism also contributes considerably more than the Grotthuss mechanism for hydroxide transport in the AEM, while our previous studies of PEM systems showed a larger contribution from the Grotthuss mechanism than the vehicular mechanism for proton transport. The activation energy barrier for hydroxide diffusion in the AEM is greater than that for proton diffusion in PEMs, implying a more significant enhancement of ion transport in the AEM at elevated temperatures.
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.|College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University , Wuhan 430072, China.
authors.forename: Chen
authors.initials: C
authors.lastname: Chen
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.
authors.forename: Ying-Lung Steve
authors.initials: YL
authors.lastname: Tse
authors.affiliation: Department of Chemistry and Biochemistry, Northern Arizona University , Flagstaff, Arizona 86011, United States.
authors.forename: Gerrick E
authors.initials: GE
authors.lastname: Lindberg
authors.affiliation: Leadership Computing Facility, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors.forename: Chris
authors.initials: C
authors.lastname: Knight
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Voth