Ion Transport in Cross-linked Nanocellulose Membranes

Sammanfattning: Ion-selective membranes, which allow ions with a certain charge and/or size to pass through while blocking other ions, have attracted much attention due to their diverse applications and outstanding roles in overcoming problems related to energy. In addition to the performance, the financial cost and renewability of materials are equally significant in the development of these membranes. Commonly, ion-selective membranes are prepared from traditional synthetic polymers that have put a heavy burden on the environment. Therefore, exploring low-cost, environment-friendly materials as the substitution of traditional polymers for ion-selective membranes will be beneficial from a sustainable perspective.Nanocellulose is a promising candidate for the next generation of ionic membranes due to its unique chemical structure and suitable physical dimensions. Furthermore, it can be produced from cellulose, which is the most abundant biopolymer on earth. Nanocellulose has many hydroxyl groups that provide many possibilities to introduce ion-functionalized groups on the cellulose chain through chemical treatment and modification. In addition, the physical entanglement of cellulose nanofibrils can generate a nanoscale porous structure that improves the ion permselectivity of membranes and provides a strong network that enhances the toughness of membranes. Among the disadvantages of nanocellulose-based products is poor wet stability due to the swelling induced by their hydrophilicity. This problem can be effectively solved using covalent cross-linking.This thesis aims to develop nanocellulose-based ionic membranes with excellent ionic transport properties as well as good wet stability and to explore their potential applications. First, the nanocellulose membranes cross-linked by 1,2,3,4-butanetetracarboxylic acid (BTCA) were developed. The relationship between the amount of cross-linker and the membranes’ pore size, charge density, and ionic transport properties was demonstrated. Based on the above fundamental understanding of the membranes’ performance, especially ion conductivity, and selectivity, their performance was then investigated in two potential applications, including osmotic power generators and redox flow batteries. Finally, the original cross-linked membrane, which is negatively charged, was combined with a corresponding membrane with positive surface charges to obtain bipolar membranes, which can be used for rectification. The properties of these bipolar membranes were investigated, with the conclusion that they can be used as an ionic diode under certain conditions.