Unraveling Water Dynamics and Ion Transport in Polymer Electrolytes via Molecular Simulation and 2D IR Spectroscopy

Anion exchange membranes (AEMs), composed of polymeric materials, play a pivotal role in energy storage and conversion technologies essential for a sustainable future. However, a comprehensive understanding of molecular transport within polymer networks remains elusive, due to the lack of a general methodology capable of capturing and connecting water dynamics, ionic transport, and polymer dynamics over multiple 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 (2D IR) spectroscopy and molecular dynamics (MD) simulations to examine how water molecules are arranged into successive solvation shells, and to elucidate how that structure influences the dynamics of bromide ion transport processes in polynorbornene-based materials. We find that the transition to a 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. To the best of our knowledge, this study represents the first use of 2D IR spectroscopy in the context of ion transport in hydrated polymers; our results serve to establish that, when coupled to semi-classical molecular models, it provides a powerful tool for the characterization of dynamics in these types of materials.