UNIDIRECTIONALLY FREEZE-CAST MXENE AEROGELS FOR ELECTROCHEMICAL CAPACITORS

Abstract

MXenes, which are a new class of two dimensional (2D) materials, have become attractive for their use within energy storage devices, due to their distinguished surface compositions, high conductivity and rich elemental composition. Integration of MXenes into porous electrode structures is of great importance in order to fully translate these properties into practical supercapacitor applications. Some of the current fabrication approaches disturb the functional properties of MXene flakes which results in changes in capacitive performances. Unidirectional freeze-casting is an easily accessible technique that utilizes a physical segregation process to control final morphology of materials. This thesis surveys the manufacturing of MXene aerogels from aqueous MXene suspensions via unidirectional freeze-casting. The as-prepared MXene aerogels (MAs) by unidirectional freeze-casting possessed highly porous nature (porosities above 99%) and large lamellar spacings. Alterations of freeze-casting parameters such as initial suspension concentration and freezing rate widened the type of created microstructures (e.g. finer/wider pores, excessive bridging, and openings within the walls). Subsequent to the production of MAs, pressing or rolling MAs into practical electrode films provided a wide control of film thickness, volumetric density, lamellar spacing and (hence) final film properties. The investigations on MAs microstructure upon pressing and rolling, and their capacitive performances broadened our understanding of the relation between these two phenomena. These post-processing techniques directly enhanced the volumetric capacitance of MAs. However, the disintegration of pore structure during these processes led to losses in capacitive performance. Pressed MAs (P-MAs) due to inhomogeneity in pore structures and crumbled lamellae only achieved a limited capacitance. On the other hand, rolled MAs (R-MAs) produced via rolling with a calendering machine keep their integrity of lamellae structures and preserved their electrochemical behaviour. The optimized pore structures of R-MAs facilitated ion accessibility and diminished the charge-transfer resistance. The cycling tests of R-MAs up to ten thousand cycles demonstrated their stability for long-term practical applications.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ian Kinloch (Supervisor) & Suelen Barg (Supervisor)