PNNL

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, we employ multimodal NMR methods, ToF-SIMS, and MD simulations to investigate these aspects. Our findings reveal that the composites contain IL-rich domains extending across hundreds of nanometers within the ionene matrix, and these bicontinuous domains span the entire membrane depth, facilitating efficient CO2 absorption and transport. CO2 also absorbs into the ionene matrix, with the distribution between two CO2 species varying with temperature and time. The rotational correlation times of the two CO2 species are on the timescale of 0.1 ns 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. The rapid desorption of CO2 from the membranes may be attributed to the membrane thinness and the additional free volume created by the extensive interfaces between ionene matrix and IL-rich domains. Therefore, ionene-IL composites represent a promising platform for designing CO2 separation membranes, offering enhanced CO2 diffusion and selectivity through IL-rich domains, increased CO2 solubility and mechanical integrity from the ionene matrix, and efficient CO2 desorption at the interfaces.