Synthesis and characterization of MXene based electrodes for energy storage.

Mn+1AXn MAX phase ceramics are a class of nanolaminated solids comprising early transition metals (M), A-group elements (A) and carbon/nitrogen (X). Exhibiting highly unusual metallic and ceramic properties alike, their layered atomic stacking allows for exfoliation via selective etching and subsequent delamination. The forthcoming 2D-materials are known as MXenes and benefit from a large surface area, metal-like conductivity, tuneable chemistry and a functionalized surface. This dissertation encompasses a broad fundamental assessment of the processing cycle of MXenes: starting from the sintering of precursor MAX phases, their subsequent exfoliation into MXenes, the processing of MXene dispersions, and, finally, their consolidation and functional performance in dedicated nanocomposites. Taking a holistic approach, the implications of synthesis imperfections on the final MXene functionality and, vice versa, application-driven materials requirements are addressed and investigated from a fundamental perspective.

Enhancing the phase purity of MAX phases is a crucial, yet challenging, task within the context of high-end structural (nuclear) applications and within the context of MXene research. Two complementary, but synergetic methodologies are presented to inhibit secondary phase formation in MAX phase ceramics. The first approach entails the reactive powder sintering of transition metal hydride precursors, which thermally decompose into fine, reactive metallic granules that facilitate the diffusion-limited formation of antecedent phases at lower temperatures, thereby inhibiting the competing formation of carbides. Secondly, the powerful strategy of solid solution engineering is elucidated, which encompasses the alloying of multiple elements on the M- and A-sublattices of the MAX phase unit cell that have meticulously balanced mismatches in their atomic radii. By relieving the destabilizing lattice distortions, disintegration of the MAX phase is avoided and virtually MAX phase pure ceramics can be produced in highly complex systems. Harmonizing both approaches in an attempt to overcome the challenging diffusion kinetics, phase pure complex solid solution MAX phases have been synthesized in the (Ti,V,Zr,Nb,Hf,Ta)-(Al,Sn)-C and (Ti,Zr)-(Al,Zn,Sn,Pb,Bi)-C systems. Furthermore, a fundamental study on MAX phase formation in the Hf-Al-C system revealed a topotactic transformation between Hf2AlC and Hf3AlC2 that does not follow the conventional MAX phase formation sequence. Demonstrating the occurrence of peculiar stacking inconsistencies and the formation of an intermediate Hf5Al2C3 superstructure, an alternative reaction sequence is hypothesized that explains some of the more unusual empirical observations.

Following up an extensive review on the multitude of MAX phase exfoliation routes, and unravelling the fundamental chemistry of the prominent LiF/HCl MILD etching route, its severe limitations have been unearthed and remediated. The pervasive Li3AlF6 etching-induced impurity is characterized and selectively removed with a simple sulphuric acid posttreatment. Additional processing adaptations that aimed for high-throughput MXene handling are proposed that allow convenient long-term dry storage of MXene clay powders in ambient conditions. The practical use of MXenes is showcased in a couple of applications in the mechanochemical and electronic realms. MXene-based carbon fibre sizings demonstrate a strengthening of the interphase between the fibre and the polymer matrix, exploiting the active surface chemistry of the MXenes. The same surface groups exhibit excellent compatibility with DMF and PVDF, resulting in well-dispersed and highly aligned MXene/PVDF nanocomposites with promising dielectric performance, improving the permittivity by 4 orders of magnitude at low frequencies, whilst keeping the loss currents at an unprecedented low. Furthermore, a brief proof of concept envisaging electromagnetic interference shielding by the same composites is exemplified. In summary, this dissertation attempts to serve as a strong fundamental materials scientific base to understand and master the production and processing of layered ceramics and to establish a synthesis framework that aids the forthcoming generation of researchers to maximally exploit the full potential of these enticing compounds.