Cation exchange poly(arylene ether sulfone) copolymers for capacitive deionisation applications

In this thesis, cation exchange poly(arylene ether sulfone) copolymers for capacitive deionisation applications were studied, with the focus of enhancing the performance of activated carbon electrodes. Activated carbon has been studied extensively as an electrode material for capacitive deionisation, but faces a number of significant drawbacks, including the requirement of a polymeric binder, unfavourable pore distribution and low conductivity. Poly(arylene ether sulfone) random and multiblock copolymers were synthesised from 2,5- diphenylhydroquinone and diphenylsulfone monomers. The 2,5-diphenylhydroquinone monomer was found to impart conductivity and ion exchange properties, while the diphenylsulfone increased the stability of the polymers when cast as membranes. Sulfonation of pendant phenyl rings using post- sulfonation resulted in a high degree of sulfonation. The synthesis of copolymers with different monomer ratios was used to examine the effects of side-chain sulfonation, monomer ratio and copolymer structure on fundamental membrane properties for capacitive deionisation. Membranes prepared from poly(arylene ether sulfone) random and multiblock copolymers were found to have favourable properties, especially ion-exchange capacity, water uptake and transport number. Importantly, their conductivities were found to be similar to commercially available membranes used for capacitive deionisation. The use of activated carbon as an electrode material and the effects of graphite as a conductive filler were examined. The graphite content, while not greatly affecting electrode wettability, was found to greatly vary electrochemical performance. Samples with too low a graphite content were observed to have poor conductivity and low mesoporous and microporous capacitance. Increased graphite content resulted in only modest capacitance increases, highlighting the need for both an appropriate conductive filler and a carbon material with a favourable pore size distribution. Through electrochemical impedance spectroscopy and equivalent circuit modelling, slow double layer formation in these electrodes was found to be caused by poor wettability and large resistance to electrolyte diffusion. When cation exchange copolymers were applied as coatings to activated carbon electrodes, the additional resistance was found to not significantly hinder the rate of double layer formation. Cyclic voltammetry revealed that a greater micropore capacitance could be achieved over a wide range of potentials due to the selective transport properties of polymers. The penetration of polymer into the carbon substrate during casting was also observed to improve electrode capacitance and kinetics. Electrochemical impedance spectroscopy results identified that both conductivity and water uptake are important properties that affect charging resistance and capacitance, and that optimal polymer design for membrane capacitive deionisation applications will require a high ion-exchange capacity without compromising polymer mechanical stability. To further improve the properties of activated carbon electrodes, random copolymers were used as novel binder materials. With a high enough concentration, good adhesion of particles comparable to commonly used hydrophobic binders was achieved. The wettability of the electrodes was greatly increased, however electrode capacitance was reduced due to polymer swelling and a loss of particle conductivity. Nonetheless, electrochemical impedance spectroscopy revealed lower charging resistances compared to electrodes prepared with hydrophobic binders, suggesting there is potential to improve the performance of activated carbon electrodes with optimised hydrophilic binders.