Multicationic Anion-Exchange Membranes from Pyridine-Bridged Polybenzimidazole and Polymer-Ionic-Liquid-Grafted Graphene Oxide

Abstract

In this study, we functionalized the surface of graphene oxide (GO) nanosheets by grafting polymer ionic liquid (PIL) chains of varying structures and then utilized PIL-g-GO materials as nanofillers with poly-(butylated pyridinium benzimidazolium)iodide (PyPBI-BuI-OBA) in order to prepare hydroxide-ion-conducting mixed-matrix alkaline anion-exchange membranes with multiple ion-exchange sites. Three different ionic-liquidbased monomers, namely, [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MATMA), 3-butyl-1-vinylimidazolium iodide [VImBu][I], and 1butyl-2-methyl-1-(4-vinylbenzyl)imidazolium iodide [VBImBu][I], were polymerized on the activated GO surface via a surface-initiated reversible addition− fragmentation chain-transfer process to obtain three different types of PILgrafted GOs, namely, pMATMA-g-GO (GOPIL1), pVImBu-g-GO (GOPIL2), and pVBImBu-g-GO (GOPIL3). Exfoliation of GO sheets upon grafting of PIL was observed and verified by electron microscopic studies. The grafted PIL chains were found to have a narrow polydispersity index (Đ) and precise control over the molecular weight, confirming polymerization occuring through SI-RAFT technique. The amount of polymer grafted on the GO surface was determined by TGA analysis, and a strong dependence on the polymer structure was found. The mixed-matrix membranes (MMMs) were prepared by solution blending of PyPBI-BuI-OBA with varying loadings of GOPIL materials, and then MMMs were immersed in KOH to prepare alkaline anion-exchange membranes (AAEMs) consisting of imidazolium and pyridinium quaternary ammonium ion-exchange sites. Among all AAEMs, those with 2 wt % GOPIL2 and GOPIL3 exhibited superior hydroxide ion conductivities of 137 and 127 mS/cm, respectively, at 80 °C. The structural, chemical, thermal, mechanical, and conductive properties of the membranes remained unaltered after alkaline treatment at elevated temperatures. These AAEMs exhibited ∼90 and ∼80% retention of ion-exchange capacity (IEC) and ∼90 and ∼70% retention of OH − conductivity after alkaline treatment in 1 M KOH, at 60 °C for 500 h and in 5 M KOH, at 60 °C for 375 h, respectively. To the best of our knowledge, this will be the first report on the utilization of surface PIL-grafted GO nanosheets in AAEMs.