Fabrication of chitosan/zwitterion functionalized titania-silica hybrid membranes with improved proton conductivity

Based on the discovery that acid-base pairs can construct efficient proton conduction channels, a binary titania-silica inorganic dopant functionalized with carboxyl groups and amino groups (denoted as TiC-SiN) is introduced into chitosan (CS) to fabricate novel hybrid membranes with proton-conducting and methanol-rejecting properties. The titania precursor and the silica precursor are prehydrolyzed, and the mixed sol is functionalized with -COOH and -NH2 groups successively by the facile chelation method to obtain TiC-SiN sol. The hybrid membranes are prepared by mixing the TiC-SiN sol with CS followed by in situ so-gel process. The membranes are characterized in terms of thermal property, water uptake, proton conductivity, etc. The results show that incorporation of hygroscopic inorganic phase increases the water uptake ability of the membranes. Moreover, the zwitterionic groups provided by the TiC-SiN dopants construct new proton pathways, which can enhance the proton conductivity of the membranes. Particularly, incorporating 7 wt% TiC-SiN affords the hybrid membrane a proton conductivity of 0.0408 S cm−1 at room temperature, which is 4 times higher than that of the pure CS membrane. Overall, the highest selectivity of the hybrid membranes is 4.85×104 S s cm−3, which is nearly 3 times higher than that of the pure CS membrane.