Synergistically Enhancing Capacitive Performance of Ti3C2Tx MXene via Building Hierarchical Structure of TiO2 Nanowire/MXene Composites and Utilizing Iron-Ion-Based Redox-Active Electrolytes

In this work, a strategy for synergistic regulation of the Ti3C2Tx surface structure and redox activity of the electrolyte has been proposed. The surface modification of MXene was achieved via KOH treatment. Meanwhile, to cooperate with the surface-modified MXene electrode materials, Fe3+/Fe2+ was introduced into its common H2SO4 electrolyte to operate as a redox-active electrolyte for the first time. The results indicate that alkali treatment not only effectively reduces the amount of fluorine-terminal groups on the MXene surface but also forms in situ TiO2 nanowires on its surface, thereby forming a unique hierarchical structure for facilitating the electrochemical reaction. Further utilization of the Fe2+/Fe3+ redox-active electrolyte introduced additional pseudocapacitive reactions at the electrode/electrolyte interface, significantly enhancing the capacitive performance of the system. This synergistic effect of both the hierarchical 1D TiO2/MXene composite electrode materials and the redox-active electrolyte resulted in a substantial increase in specific capacitance from 78.17 F g−1 to 655.54 F g−1 at a current density of 10 Ag−1. The reaction kinetics of the electrochemical systems were studied, along with their energy storage mechanism. It is revealed that there is a transition of the energy storage mechanism from being dominated almost solely by diffusion control to collaborative diffusion and surface reactions in the synergistic electrode/electrolyte system, and the corresponding equivalent circuit has evolved from the single-interface model to a dual-interface model. This work has demonstrated that the proposed synergistic strategy can effectively enhance the capacitive performance of the MXene energy storage system and can be applied to other electrochemical systems.