Nanoscale imaging of MXenes by X-ray microscopy
The global transition towards a carbon-neutral society by 2050 necessitates not only the development of renewable energy sources but also advanced energy storage technologies to address the intermittent nature of solar, wind, and other sustainable energy systems. In this context, MXenes, a novel class of two-dimensional (2D) materials, have attracted considerable interest for their promising applications in energy storage. MXenes, with their unique structure and chemical diversity, offer a combination of metallic conductivity and tunable surface properties, making them ideal candidates for energy storage devices like batteries and supercapacitors. Despite their potential, fundamental questions remain regarding the intercalation and redox mechanisms of individual MXene flakes, the coverage of MXenes with surface terminations, and defects that influence their electrochemical performance. This thesis explores the chemical and structural properties of Ti3C2Tx, Mo2CTx, and vacancy-ordered Mo1.33CTx MXenes, focusing on their thermal stability, surface chemistry, and interaction with aqueous electrolytes. Scanning X-ray Microscopy (SXM) is a suitable technique for the investigation of the surface and core chemistry of MXenes, particularly for capturing post mortem the intercalation mechanisms of a Li-ion battery and during exposure to varying conditions (temperature, gas, and liquid). SXM provided critical insights into the local distribution and chemical nature of surface terminations, defects, and intercalated water molecules, as well as the redox behavior of MXene flakes at the nanoscale. The thesis reveals that intercalants and surface terminations such as O, OH, and F play a significant role in the thermal stability and redox reactions of MXenes, with O-terminated surfaces being active sites for redox reactions. These findings also highlight the effect of ordered vacancies and surface chemistry in Mo-based MXenes on their stability. These MXenes present signs of degradation during annealing above 400 °C, unlike the thermally stable Ti3C2Tx MXene. Additionally, the surface oxidation of Ti3C2Tx MXene flakes, induced by their exposure to alkaline electrolyte, is only partially reversible by exposure to acidic electrolyte. These findings enhance the understanding of MXenes in energy storage, revealing at the nanoscale the interplay of intercalation and surface chemistry. Monitoring surface chemistry down to the level of single MXene flake provides valuable insights into the chemical changes that can occur during oxygenation or ion intercalation, adding to the understanding of electrochemical energy storage mechanisms in MXenes.
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