Porous two-dimensional materials (MXenes) for high capacity energy storage

Abstract

Energy storage is becoming a key challenge of the 21st century as the global energy system transitions away from the use of fossil fuels, which have been linked to damaging climate change and increasing air pollution. Electrochemical energy storage devices, such as batteries and supercapacitors, offer great potential to enable the clean energy transition, but require significant improvements to their performance characteristics such as energy density, power density and lifetime. The design and study of new electrode materials is key to achieving these improvements. MXenes are a new class of two-dimensional transition metal carbides and nitrides which have shown early promise in the field of electrochemical energy storage. Particularly interesting is the discovery of pseudocapacitive intercalation as their charge storage mechanism, since this could enable high power and energy densities with long cycling lifetimes into one device. However, the performance of MXene electrodes greatly depends on the electrode architecture, with multilayered or restacked MXenes showing unsatisfactory performances. This thesis reports on the development of pillaring techniques to increase the interlayer spacing between MXene (Ti3C2 and Mo2TiC2) sheets creating porous electrode architectures. This is shown to lead to large increases in the interlayer spacings, with up to 60-fold increases in the specific surface areas, among the highest reported for MXenes to-date. The pillared MXenes are then tested in a variety of systems for metal-ion capacitor applications, including organic Li-ion and Na-ion and aqueous Zn-ion systems. It was found that the pillared materials outperformed the non-pillared materials in each system studied, with improvements in capacities, rate capabilities, cycling stabilities and coulombic efficiencies. In addition, the pillaring and electrochemical mechanisms are studied using a combination of microscopy, spectroscopy and electrochemical techniques, providing important understanding to assist with the further development of this field.