Bulky and Rigid Spiro-Adamantane-Fluorene Unit Promoted Microphase Separation in Di-Cation Side Chain Grafted Anion Exchange Membrane

Abstract

Constructing well-defined ion channels is an effective way to achieve high conductivity of anion exchange membranes (AEMs) for fuel cell applications. Nevertheless, the conventional backbones often tend to be tightly packed, which may hinder the formation of ion clusters. We herein report a novel series of poly (aryl indole) AEMs containing spiro-adamantane-fluorene (SAF) units and di-cation side chains. The integrated bulkiness and rigidity of adamantane and fluorene moieties in SAF can lower packing density of the backbone, creating higher fraction of inter-chain free volume, and thus lower the resistance of the side chain cation aggregation during microphase separation, benefiting ion channel formation. Molecular dynamic simulation indicates that the SAF unit leads to faster OH- diffusion. As a result, the AEM with 37% SAF displays a hydroxide conductivity of 161.9 mS cm-1 at 80 ºC, which is retained by 94.4% after 1000 h treatment in 1 M NaOH at 80 ºC; the membrane also shows a good dimension stability (20.7% swelling at 80 ºC); its H2/O2 fuel cell yields a peak power density of 2.07 W cm-2 at 80 ºC, and can operate for over 400 h with a good voltage retention. This work provides a novel AEM design based on the synergy between SAF bulkiness and di-cation side chain, which can construct efficient ion channels and achieve better conductivity-stability balance; it may contribute to more practical advancement of alkali fuel cell technology.

Keywords: anion exchange membrane, ion channel, spiro-adamantane-fluorene, free volume, microphase separation