Synthesis of gemini basic ionic liquids and their application in anion exchange membranes

Abstract

A gemini-type basic morpholine ionic liquid ([Nbmd][OH]) was synthesized via a two-step method with morpholine, bromododecane and 1,4-dibromobutane as raw materials, and its structure was characterized by ¹H NMR and FT-IR spectroscopy. Meanwhile, a series of anion exchange membranes ([Nbmd][OH]_x–QCS) were prepared with quaternized chitosan (QCS) as the polymer matrix and [Nbmd][OH] as the dopant owing to its strong alkalinity and good solubility. The structures of the [Nbmd][OH]_x–QCS composite membranes were characterized in detail by FT-IR spectroscopy, the OH⁻ conductivity by AC impedance spectroscopy, and the morphological features by scanning electron microscopy (SEM), thermal gravity analysis (TGA), etc. The results show that the [Nbmd][OH]_x–QCS composite membranes have uniform surfaces and cross-section morphology. Increasing the content of [Nbmd][OH] not only enhances the thermal stability but also increases the OH⁻ conductivity; the thermal decomposition temperature of the [Nbmd][OH]₄₀–QCS membrane is nearly 20 °C higher than that of the pristine QCS membrane, and the maximum OH⁻ conductivity is approximately 1.37 × 10⁻² S cm⁻² at 70 °C. The methanol permeability of the [Nbmd][OH]₄₀–QCS membrane in 1 M methanol at room temperature is 2.21 × 10⁻⁶ cm⁻² s⁻¹, which is lower than that of Nafion®115, indicating a promising potential use in alkaline direct methanol fuel cells. Moreover, the [Nbmd][OH]₄₀–QCS membrane exhibits the best alkaline stability of all the membranes prepared in this work, retaining approximately 81% of its initial conductivity after immersion in 3 M KOH solution for 120 h at 70 °C.