CO2 in Ionene–Ionic Liquid Composite Membranes

Ionene – ionic liquid (IL) composites are promising materials for CO₂ separation, yet a molecular-level understanding of their structure and its impact on CO₂ speciation, solubility, rotation, and diffusivity remains unclear. Herein, using multimodal nuclear magnetic resonance (NMR), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM), and molecular dynamics (MD) simulations, we reveal that the composites contain IL-rich domains extending across hundreds of nanometres within the ionene matrix, and these bicontinuous domains span the entire membrane depth. CO₂ also absorbs into the ionene matrix, with the distribution between two CO₂ species varying with temperature and time. The rotational correlation times of these two species are on the timescale of 0.1 and 1 ns, respectively. As IL content increases, the ionic domains expand, resulting in higher CO₂ solubility due to enhanced molecular dynamics and increased free volume in both ionene backbones and IL-rich regions. Although CO₂ diffusion in the membranes is an order of magnitude slower than in bulk IL, the activation energy for CO₂ diffusion remains comparable. Ionene-IL composites represent a promising platform for designing CO₂ separation membranes, offering enhanced CO₂diffusion and selectivity through IL-rich domains, and increased CO₂ solubility and mechanical integrity from the ionene matrix.