CO₂ in Ionene–Ionic Liquid Composite Membranes

Abstract 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.