Preparation Of Imidazolium-Based Anion Exchange Membranes And Study On Alkaline Stability Of Imidazolium Cations

Anion exchange membranes (AEMs) are functionalized polymer films, which consist of the polymer backbones, functional cations and removable anions. AEMs have the widely potential to be used for energy storage batteries (such as vanadium redox flow batteries or alkaline fuel cells) applications. As one of the core components in energy storage batteries, AEMs are employed to separate the positive and negative electrolytes or fuels, and complete the current circuit by transporting anions. However, compared with the commercial Nafion membranes, AEMs have inherent disadvantages, such as lower ionic conductivity and poorer chemical stability. The ideal AEMs should have high ionic conductivity, good chemical stability and low cost.For improving the properties of AEMs, we prepared novel imidazolium-based anion exchange membranes. The performance of imidazolium-based AEMs was studied, especially the alkaline stability of imidazolium cations. The alkaline degradation mechanism and pathway of imidazolium cations were proposed, and the stable working conditions of imidazolium cations were further discussed.(1) Based on the needs of vanadium redox flow batteries (VFB), a series of imidazolium-based anion exchange membranes (BIM-PPO) were synthesized by functionalization of poly(2,6-dimethyl-1,4-phenylene oxide)(PPO) using1-butylimidazole. The BIM-PPO-40membrane had the excellent ability to prevent vanadium ions permeability. The BIM-PPO-40membrane had a vanadium permeability of4.8×10-9cm min-1at25℃, and an ionic conductivity of10.8mS cm-1at60℃. Moreover, The BIM-PPO-40membrane showed high chemical stability in acidic and oxidizing vanadium solution as opposed to poor stability in alkaline solutions. Based on density functional theory (DFT) studies, this was attributed to the larger HOMO-LUMO energy gap (6.394eV) of [DMIM][HSO4] as compared to [DMIM][OH](5.387eV).(2) The fluorinated cross-linked AEM with a micro-phase separation structure (C-FPAEO-x-MIM) was synthesized by functionalization of fluorinated poly(aryl ether oxadiazole)s with1-methyl imidazole and cross-linking with N,N,N’,N’-tetramethyl-1,6-hexanediamine (TMHDA). The optimal C-FPAEO-75-MIM membrane had an ionic conductivity of17mS cm-1, a tensile strength of28.02MPa, a water uptake of51wt%, and a swelling ratio of6wt%at20℃.(3) The IEC and OH-ionic conductivity of imidazolium-based anion exchange membranes (FPAEO-2.2MIM) were obviously declined after treatment in NaOH aqueous solution. The alkaline degradation products of FPAEO-2.2MIM membranes were studied by FT-IR and13C solid state NMR. DFT studies were preformed to better understand the degradation mechanism and pathway of dimethylimidazolium cations (DMIM) on exposure to hydroxide. The C2position of DMIM was susceptible to be attacked by hydroxide ions and undergo degradation in alkaline conditions through a ring-opening mechanism. The ring-opening degradation pathway followed the following three steps:a) a nucleophilic reaction, b) a ring-opening reaction, and c) a rearrangement reaction. Based on DFT calculation, DMIM had reasonable chemical stability under lower pH conditions. However, the hydroxide-ion induced degradation of DMIM was found to be more facile in high-pH environments. Good solvation of DMIM and hydroxide ions was helpful in minimizing the degradation and enhancing the chemical stability of anion exchange membranes based on these cations.(4) In1-benzyl-3-methylimidazolium and1-benzyl-2,3-dimethyl-imidazolium, the bond energy of the C-N bond connecting the imidazole ring to the benzyl group was studied by DFT. Based on DFT calculation, the C-N bond energy was calculated to be almost2.5times higher than the activation energy barrier for the ring-opening reaction. Hence, we concluded that the imidazolium head-groups were more likely to be degraded by the ring-opening reaction mechanism than by dissociation from the polymer chain in alkaline conditions. Noteworthily, the effect of the structure of polymer chains on the fall-off of imidazolium cations was not considered. The effect of N1alkyl (methyl or butyl) or benzyl substituent group and C2methyl substituent group was discussed by DFT. The DFT calculation results indicated that the imidazolium head-group tethered to the polymer backbone via the alkyl or benzyl group had almost no significance for the alkaline stability. The activation free energy barrier for the nucleophilic reaction of the trimethylimidazolium cation (TMIM) and hydroxide ion exceeded about40percent. Moreover, TMIM also had a higher LUMO energy (-1.013eV) than other imidazolium cations. Therefore, the alkaline stability of TMIM was much better than that of C2unsubstituted imidazolium cations. Therefore, the C2methyl substituted imidazolium cations are more suitable as functional groups in alkaline anion exchange membranes.(5) C2unsubstituted imidazolium-based AEM (FPAEO-2.2MIM), C2methyl substituted imidazolium-based AEM (FPAEO-2.2TMIM) and guanidinium-based AEM (FPAEO-2.2QG) were prepared. And the alkaline stability of these AEMs was studied. After treatment in5mol L-1NaOH aqueous solution for30days, IEC of FPAEO-2.2MIM, FPAEO-2.2TMIM and FPAEO-2.2QG were decreased respectively by approximately82%,13%and8%. Based on DFT calculation, QG had a higher LUMO energy (-0.938eV) than TMIM (-1.013eV). Compared with TMIM, QG was slightly less susceptible to be nucleophilic attacked by OH". The alkaline stability of functional groups was as follows:QG> TMIM> DMIM. Therefore, C2methyl substituted imidazolium-based AEMs are not suitable for a long-term application in a high pH environment.Imidazolium-based AEMs are not suitable for use in a high pH environment, but can be applied in a low pH environment or in an acidic environment. We believe that imidazolium-based AEMs have a broader application in vanadium redox flow batteries.