Alkaline-Stable Anion Exchange Blend Membranes (AEBMs) with a Novel Sterically Hindered Cationic Head Group

Abstract

In this study, synthesis and characterization of Anion Exchange Blend Membranes (AEBMs) are introduced. AEBMs were prepared by mixing polymer solutions of a halomethylated polymer (Br-PPO, brominated poly(2,6-dimethyl-1,4-phenylene oxide)), polybenzimidazole (PBI, F6PBI or PBI-OO) and a sulfonated polymer (S-polymer, SAC 098 (a sulfonated poly(phenylethersulfone), nonfluorinated) or SFS 001 (a sulfonated aromatic polyether, partially fluorinated)). A halomethylated polymer was used as the anion exchange ionomer precursor. Polybenzimidazole was used as a matrix polymer in order to enhance the mechanical strength of the AEBN. A minor amount of sulfonated polymer was added, forming ionic cross-links, and covalent bonds were formed in the blend membrane by reaction of a small amount of the CH2Br groups of Br-PPO with imidazole N-H groups of the PBI, respectively, leading to an increase of the chemical and dimensional stability of the novel AEBMs [1]. Membranes were fabricated by following procedure 1) mixing of the polymer solutions to homogeneity, 2) solvent evaporation in oven, 3) quaternization of the blend membrane by soaking the membrane in amine solution, 4) Ion exchange into chloride form in sodium chloride solution. It is well known that many types of anion exchange membranes are chemically unstable under alkaline condition. S. Holdcroft published a novel polybenzimidazolium AEM with a mesitylene building block shielding the dimethylimidazolium moiety of the polybenzimidazolium from OH- attack, leading to excellent hydroxide stability of this AEM: after immersion of this AEM in 2M KOH for 10 days at 60°C, no significant change of the NMR spectrum was observed [2]. Therefore it can be concluded that introduction of bulky steric hindrance groups in the vicinity of the cationic groups of AEMs can prevent the quaternized amine in the polymer structure from nucleophilic attack of hydroxide ions. Consequently in this study several types of sterically hindered tertiary amines were introduced in the polymer blend system for quaternization with Br-PPO, among them 1,2,2,6,6-pentamethylpiperidine (pempidine), quinuclidine, and 1,2,4,5-tetramethyl-1H-imidazole (abbreviated TMIm). Among these AEBMs, the TMIm-based membranes showed the best properties in terms of conductivity and stability. We investigated different combinations of polymers for AEBMs. The membrane containing PBI-OO and SAC098 showed good weight maintenance than other material combinations in DMAc extraction test at 90oC. This membrane was prepared and post-treated with TMIm as mentioned above. The membrane exhibited a chloride conductivity of 6.5 mS/cm at 30oC and 90% RH which is higher than that of commercial membrane (Tokuyama A202) under the same condition. The change of ion exchange capacities (IECs) was also investigated in order to examine the hydroxide stability. The membrane was exposed to 1M KOH solution at 90oC for 10days, and ion-exchange capacities (IECs) were compared to each other before and after KOH treatment. The IECs before and after KOH treatment were 2.63 and 2.59 mmol/g respectively, indicating excellent hydroxide stability. Apart from Br-PPO, other halomethylated polymers such as polyvinylbenzylchloride and polyepichlorohydrine were also investigated as precursors for AEBMs in this study, yielding good Cl- conductivities (in excess of 30 mScm-1) and alkaline stabilities with the novel 1,2,4,5-tetramethyl-1H-imidazolium head group. Addition of a hydrophilic polymer phase to the novel AEBMs resulted in even higher Cl- conductivities (e. g. 131 mScm-1@90°C@90% r.h.) after 10 d of KOH treatment (1M, 90°C).