Graphene-coupled Ti₃C₂ MXenes-derived TiO₂ mesostructure: Promising sodium-ion capacitor anode with fast ion storage and long-term cycling

Sodium-ion-based capacitors and batteries are considered as a low-cost energy storage technology alternative to their lithium-ion counterparts owing to the abundance of sodium in Earth. Their widespread use is however limited by the lack of high-performance electrode materials. In this work, we report that MXenes-Ti₃C₂ can be oxidized into a Ti-peroxo complex gel at room temperature by simply adding H₂O₂, from concentrated to dilute. The highly water-soluble property of this gel allows the synthesis of a graphene-supported TiO₂ nanocomposite with highly porous nano-/meso-hybrid architecture via a more facile and environmentally friendly way. The unique hybrid architecture of the produced TiO₂-RGO nanocomposite results in pseudocapacitive behavior in Na⁺ charge storage with high reversibility, fast kinetics, long cyclability, and negligible degradation to the parent structure. By incorporating the TiO₂-RGO composite as the anode, a novel sodium-ion capacitor is constructed that is capable of operating at a high voltage of 4.0 V and delivering a maximum energy density of 94.7 W h kg^-1, which is comparable to lithium-ion based capacitors. The approach reported here could be potentially extended for fabricating a host of MXenes-derived metal oxide nanomaterials or nanocomposites for numerous applications, particularly in view of the expanding MXenes portfolio.