Exploration of anion transport in a composite membrane via experimental and theoretical methods

Abstract

The conductivity of polymerized ionic liquid graft copolymers (PILGCs) is usually three orders of magnitude lower than ionic liquids (ILs); researches are being performed actively to improve ion transport in PILGCs. In the research, the composite membrane of poly(1-butyl-3-vinylimidazolium-acetate) (poly([BVIM]-[OAc])) PILGC and NPs filled with 1-butyl-3-methylimidazolium acetate ([BMIM]-[OAc]) IL was developed. Ion transport in the composite is a combination of that in PILGC and IL and mechanisms underlying anion transport in the PILGC and the IL should both be explored in order to obtain profound knowledge of ion transport (mechanism) in the composite. Researchers still have discrepancy in the mechanisms of ion transport in PILGCs. We explored the mechanisms and time scales of ion transport in the PILGC and compared the differences in ion transport phenomena and mechanisms in the PILGC and the IL. 2D-IR experiments were performed and the results show that the peak-shape of the spectra of SCN− in the PILGC changes on a much slower time scale than that in the IL, indicating that the IL completes structure decay in a way significantly faster than the PILGC. Anion diffusivities in the composite, PILGC and IL at various temperatures were measured and calculated. Subdiffusion phenomena are caused by hindrance influence of surrounding units on anion motion. Therefore exploring subdiffusive regime gives mechanisms underlying ion transport: the subdiffusion phenomena of anion transport in the IL are caused by the coupling between anion motion and the motion of ions around, while that in the PILGC are caused by the trap of anions in cages composed of the PILGC chains before they get the opportunities to hop to cages nearby. According to our analysis, van der Waals (VDW) interactions play a more significant role than electrostatic interactions, including ion association interactions, in influencing the magnitude of anion diffusivities in the PILGC. It is wrong to explore ion transport mechanism via only analyzing electrostatic interactions between ions; anion transport is along PILGC chains most probably in successive steps and between PILGC chains most probably in separate steps, depending on jumping between neighboring cages. The trap time of an anion inside a cage, instead of ion association time, gives the time scale that associates with anion transport. The anion transport mechanism in the composite is a combination of fast structural relaxation in the IL and slow hopping between cages in the PILGC.