Capacitive performance of two-dimensional metal carbides

Recently a new family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. Unlike graphene, whose chemistry is restricted to carbon, MXenes allow a variety of chemical compositions and are establishing themselves as a large new class of two-dimensional materials. MXenes combine the metallic conductivity of transition metal carbide layers with the hydrophilic nature of their mostly hydroxyl or oxygen terminated surfaces. In essence, they behave as "conductive clays" and have shown much of promise as electrode materials for Li-ion batteries. Prior to the initiation of this study, there have been no reports on the capacitive properties of MXenes. In this work the potential was explored of the new family of the two-dimensional carbides, MXenes, as electrode materials for electrochemical capacitors. This study was focused on Ti₃C₂T_x. It was established that variety of single- and multiply charged cations (such as Li⁺, Na⁺, K⁺, NH4+, Mg²⁺) can intercalate MXenes (chemically or electrochemically) and participate in energy storage. Highly reversible electrochemical insertion of the same cations has been demonstrated for Ti₃C₂T_x in aqueous electrolytes. Perfect capacitive behavior was observed for Ti₃C₂T_x MXene even at quite high charge and discharge rates, all coupled with excellent cyclability; no drop in capacitance was observed even after 10 000 cycles. Further investigation showed that surface chemistry has significant effect on the resulting capacitance, i.e. by creating predominantly oxygen-containing functionalities the capacitance can be substantially boosted in comparison to the as-received material. It was also demonstrated that Ti₃C₂T_x clay produced using LiF-HCl mixturewith predominantly oxygen-containing functionalities, showed outstanding capacitance up to 900 F/cm3 and can be manufactured in to electrodes in less than 10 min without need of binder or conductive additive. Electrochemical in-situ XAS measurements detected changes in Ti oxidation state during cycling, which matched closely the observed experimental values of the material's capacitance. Therefore it was concluded that mechanism of electrochemical storage of the Ti₃C₂T_x MXene clay is predominantly pseudocapacitive. . Also concept of all-solid-state asymmetrical supercapacitor (freestanding and current collector free) based on Ti₃C₂T_x was developed. Among other applications, using in-situ AFM the potential of the use of MXenes in electrochemical actuators was demonstrated. It was also shown that MXenes other than Ti₃C₂T_x also demonstrated a lot of promise for electrochemical capacitors: Nb2CTx/CNT paper electrodes showed high volumetric capacitance of 325 F/cm3 when tested in a Li-ion capacitor configuration.