Ion transport on self-assembled block copolymer electrolytes with different side chain chemistries

Abstract

Ion-exchange membranes (IEMs) are used in electrochemical systems for a wide variety of applications, including water purification, mineral recovery, and energy storage and conversion. These materials often dictate the ohmic overpotential drop in electrochemical systems and can have a profound impact on process efficiency. Central to the rationale design of ion-conducting polymers is a fundamental understanding as to how chemical composition and macromolecular architecture influence ion and water transport. Herein, we report the preparation of three microphase separated block copolymer electrolytes (BCEs) with long-range order that have different side chain chemistries in the ionic domain. The side chain variants are alkyl and alkoxy pendants and a zwitterionic group. The side chain chemistries were installed post-assembly of crosslinked block copolymers. Unexpectedly, we observed that more hydrophobic alkyl side chain yields about an order of magnitude greater ionic conductivity when compared to alkoxy and zwitterionic side chains despite similar IEC values and water uptake. Molecular dynamics simulations reveal that the hydrophilic alkoxy moieties and zwitterion structure coordinate with water making less free water available for mediating ionic conductivity. Conversely, the hydrophobic alkyl side chains give rise to large, interconnected water clusters that promote ionic conduction of the counterion.