Comb-shaped anion exchange membrane with densely grafted short chains or loosely grafted long chains?

Abstract

Anion exchange membranes (AEMs) are crucial components for advanced energy and environment processes including alkaline fuel cells, redox flow batteries, and industrial effluent treatment; while low anionic conductivity and poor stability remain the major challenges for the widespread implementation of AEMs. Through molecular engineering, comb-shaped AEMs have been proved possessing the capability of delivering both high conductivity and good alkaline stability. However, how to precisely control the side chain topology and how the chain topology would influence the membrane properties need to be further elucidated. We hereby propose a radically novel and readily scalable route towards the controllable synthesis of comb-shaped AMEs and we were able to determine the length of side chains and the number of ionic groups along the side chains. To probe the effect of side chain topology on membrane properties, two types of AEMs of densely grafted short side chains or loosely grafted long side chains with similar ion exchange capacity (IEC, ∼1.7 mmol/g) were synthesized and compared. We found that the comb-shaped AEMs with loosely grafted long chains (LG-LS-DIm), with higher hydroxide conductivity (55 mS cm−1 at 30 °C) and better alkaline stability (∼80 % of IEC retention after soaking in 2 mol L−1 NaOH solution at 60 °C for 25 days), outperform those with densely grafted short chains (HG-SS-DIm) and the benchmark main-chain type AEMs (DIm-PPO), which is also superior to those of conventional linear AEMs with densely functionalized structure.