The Influence of the Anion Structure of Ionic Liquids on the Physical and Electrochemical Properties of Sustainable Cellulose Acetate Propionate-Based Membranes

Today, the development of ion exchange membranes has increased considerably in various applications, such as water treatment, energy conversion and storage, as well as environmental applications. In this study, several ion exchange membranes based on cellulose acetate propionate (CAP) and ionic liquids (ILs) were fabricated using the phase inversion method, aiming to develop more sustainable membrane materials for environmental and energy applications. Three different ILs with a similar cation and different anions (1-(4-sulfobutyl)-3-methylimidazolium trifluoromethanesulfonate [SMIM][TFS], 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate [SMIM][HS], and 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [SMIM][TFSI]) were used in three concentrations (i.e., 9, 17, and 23 wt.%). The main objective of this work is to evaluate the influence of the anion structure on the membrane’s physical, morphological, hydrophilic, thermal, mechanical, and electrochemical properties. Water contact angle measurements demonstrated the weaker hydrophilicity of composite membranes containing [SMIM][TFS] (81–106°) and [SMIM][TFSI] (87–94°) in comparison with pure CAP (~79°) and CAP/[SMIM][HS] (79–83°) membranes. The CAP/[SMIM][HS] membrane showed higher elongation at break (~36%) compared with the pure CAP membrane (~24%), confirming the plasticization behavior of [SMIM][HS]. The CAP/[SMIM][TFS] membrane containing 23 wt.% of IL showed promising membrane potential, permselectivity, transport number and ion flux ratio values of 53.2 mV, 74.7%, 0.85, and 5.5, respectively, indicating its potential as a candidate for further evaluation in electrochemical membrane processes such as electrodialysis and fuel cells.