Structure-property relationship of phosphonium-based polymerized ionic liquids as anion conducting membranes

Membrane technology in sustainable energy conversion and storage requires the development of tailored membranes able to conjugate high performance (ionic conductivity, perm-selectivity and durability) with acceptable costs and sustainability in their production. In this perspective, polymerizable ionic liquids (PILs) are conductive materials suitable to make high-performing and green ion-conductive membranes combining the unique properties of the ionic liquids, with the advantages of a macromolecular crosslinked polymer. This work presents a deep investigation of the structure-property relationship of phosphonium-based PILs as a functional material for anion-conducting membranes produced by casting and successive photopolymerization (almost solvent-free conditions). The PIL-based membranes prepared were dense, flexible, and completely stable after prolonged contact with water, saline and alkaline solutions. The crosslinking reaction avoided the dissolution of the membrane in water. However, mechanical test highlighted the role of water uptake on mechanical properties of the membranes. Moreover, it was also validated the possibility to blend different PILs in order to combine in synergic way the specific advantages of each component. Electrochemical impedance spectroscopy and membrane potential measurements pointed out a trade-off relationship between the ionic conductivity and perm-selectivity. Moreover, Small Angle X-ray Scattering and differential scanning calorimetry findings shed light on the role of the chemical nature of the PIL on membrane microstructure and transport properties. The main outcome of this research is the possibility to balance the low ionic resistance transport through the charged PILs, with a good stability, tailoring the chemistry of these advanced functional materials.