Exploring the influence of the cation type and polymer support in bis(fluorosulfonyl)imide-based plastic crystal composite membranes for CO2/N2 separation

Abstract

There is an urgent need for more efficient technologies to mitigate climate change, such as the separation and capture of CO₂ from flue gas. This work explores light gas separation based on a series of novel Organic Ionic Plastic Crystals (OIPCs) and polymer composites with the advantage of competitive and tuneable performance. Five OIPCs based on the bis(fluorosulfonyl)imide ([FSI]⁻) anion were selected and co-cast with poly(vinylidene fluoride) (PVDF) or poly(ethylene oxide) (PEO). Their gas transport properties were evaluated at temperatures where some of the OIPCs exist in different thermal phases. It was found that the nature of the polymer has a strong impact on separation performance, with PVDF demonstrating more advantageous properties than PEO. However, every OIPC has a different response to a given polymer type and ratio. A remarkable increase in permeability, from ∼100 to ∼360 Barrer, was achieved for triethyl(methyl)phosphonium bis(fluorosulfonyl)imide ([P₁₂₂₂][FSI])-based composites upon increasing the PVDF ratio, while preserving high selectivity (α ≈ 29). Additionally, large changes in separation performance were observed in OIPCs that undergo a solid–solid phase transition. A shift from phase II to phase I resulted in a dramatic increase in permeability (from ∼12 to ∼500 Barrer) for hexamethylguanidinium bis(fluorosulfonyl)imide ([HMG][FSI])-based composites. Analysis of the relationship between structure and transport properties of the different composites reveals insights into the effect of different OIPC and polymer chemistries and points to promising new directions in the development of highly permeable and selective gas separation membranes.