The Investigation of Ti3C2Tx MXene Surface Chemistry for Electrochemical Energy Storage

Abstract

Electrochemical energy storage devices, such as batteries and supercapacitors, play a pivotal role not only for the increasing demand on renewable energy storage but also for the growing electric vehicles industry. In this context, surface redox (pseudocapacitive) active materials have shown a remarkable increase in both energy and power densities. Titanium carbide Ti3C2Tx MXene is an efficient pseudocapacitive 2D material combining high metallic conductivity with hydrophilic surfaces. The Ti3C2Tx reveals large capacitance by using sulfuric acid as an electrolyte owing to the surface redox charging mechanism. The electrochemical performance of Ti3C2Tx MXene is significantly influenced by interlayer spacing between MXene nanosheets which is altered by the amount of the nanoconfined water and/or intercalants. In this work, X-ray-based techniques were used to study the impact of the MXene surface chemistry as well as its interlayer spacing on its overall electrochemical performance. Ti3C2Tx MXene were investigated using synchrotron-based soft X-ray absorption spectroscopy (XAS) and X-ray photoemission electron microscopy (XPEEM). The XAS peaks are very sensitive to changes in the local chemical environment induced by different types of intercalants and the nanoconfined water between the MXene layers. The oxidation state of the surface Ti atoms in Ti3C2Tx has been then extensively investigated as it constitutes a key element in the electrochemical performance. Here we show that the intercalation of organic molecules, like urea, as well as mono- and multi-valent cations such as Li+, Na+, K+, and Mg2+ affects the Ti oxidation state in different environments. We show that a controlled higher oxidation state increases the MXene capacitance. In addition, spatially resolved XA spectra were implemented to study the Ti oxidation state of pristine and intercalated single multi-layered Ti3C2Tx flakes. On the other hand, the interlayer spacing between Ti3C2Tx nanosheets was monitored by X-ray diffraction (XRD). In situ XRD patterns taken at different temperatures revealed for the first time the signature of the nanoconfined water in MXene at low temperatures, which shows the coexistence of hexagonal and cubic ice structures. This work illustrates the significance of the X-ray-based techniques to probe the electronic structure of transition metal oxide surfaces and the nanoconfined water of MXenes in various environments. It paves the way to operando XAS combined with electrochemical performance (cyclic voltammogram) which would help to identify the changes in chemical bonds during a redox reaction.