Water-mediated ion transport in an anion exchange membrane
Zhongyang Wang, Ge Sun, Nicholas H. C. Lewis, Mrinmay Mandal, Abhishek K. Sharma, Mincheol Kim, Joan M. Montes de, Kai Wang, Aaron D. Taggart, Alex B. F. Martinson, Paul A. Kohl, Andrei Tokmakoff, Shrayesh N. Patel, Paul F. Nealey, Juan Pablo
Abstract
Water is a critical component in polyelectrolyte anion exchange membranes (AEMs). It plays a central role in ion transport in electrochemical systems. Gaining a better understanding of molecular transport and conductivity in AEMs has been challenged by the lack of a general methodology capable of capturing and connecting water dynamics, water structure, and ionic transport over time and length scales ranging from those associated with individual bond vibrations and molecular reorientations to those pertaining to macroscopic AEM performance. In this work, we use two-dimensional infrared spectroscopy and semiclassical simulations to examine how water molecules are arranged into successive solvation shells, and we explain how that structure influences the dynamics of bromide ion transport processes in polynorbornene-based materials. We find that the transition to the faster transport mechanism occurs when the reorientation of water molecules in the second solvation shell is fast, allowing a robust hydrogen bond network to form. Our findings provide molecular-level insights into AEMs with inherent transport of halide ions, and help pave the way towards a comprehensive understanding of hydroxide ion transport in AEMs.