Tuning aerogel morphology using water–ionic liquid binary mixtures: a mechanistic journey from synthesis to 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 precursor solutions catalyzes the polymerization of resorcinol with formaldehyde, while the IL/water ratio determines 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 investigation was carried out using NMR and MD simulations throughout the preparation process at two water/IL ratios at which the aerogels showed anomalous properties. IL ions strongly interacted with resorcinol and the oligomer intermediates, altering the conformation of the latter. In the resulting cavities, resorcinol was replaced by IL cations, further influencing the polymerization process. The resulting polymer and carbon aerogels were characterized by SEM, N₂ porosimetry and NMR cryoporometry and relaxometry in an aqueous environment. Results revealed that the specific IL-resorcinol interactions, catalytic effect, and solution structure of the mixture resulted in diverse aerogel morphologies with considerable micro- and ultramicro-porosity, which improved the hydration properties of the derived carbon aerogels. These findings contribute to the design of carbon aerogel-based electrode materials, where pore morphology and surface wettability play crucial roles.