Tuning aerogel morphology by water - ionic liquid binary mixtures: a mechanistic journey from the synthesis to the carbon properties

Abstract

Ionic liquids (ILs) and their mixtures with water are often used as templates and porogens in the preparation of porous polymers. The IL 1-ethyl-3-methylimidazolium ethyl sulfate in the precursor solution catalyzes the polymerization of resorcinol by formaldehyde, while the IL/water ratio is able to tune the pore size of the resulting resorcinol-formaldehyde polymer and carbon aerogels. Although the unique solution behavior of this IL and the interactions in the IL/water mixtures are known, its complex effect on the aerogel morphology needs further investigations. In this study a comprehensive molecular-level study was carried out by NMR and MD simulations throughout the preparation process at two water/IL ratios, at which the aerogels showed anomalous properties. IL ions strongly interact with resorcinol and the oligomer intermediates as well, altering the conformation of the latter. In the resulting cavities, the resorcinol is replaced by IL cations, which further influences the polymerization process. The resulted polymer and carbon aerogels were characterized by SEM, N₂ porosimetry and NMR cryoporometry and relaxometry in aqueous environment. The results revealed that the specific IL-resorcinol interactions, the catalytic effect, and the solution structure of the mixture resulted in diverse aerogel morphologies with considerable micro- and ultramicroporosity, which improved the hydration properties of the derived carbon aerogels. These findings contribute to the design of carbon aerogel-based electrode materials, where the pore morphology and the surface wettability play a crucial role.