High-performance aqueous zinc-ion hybrid micro-supercapacitors enabled by oxygen-rich functionalised MXene nanofibres

Aqueous zinc-ion hybrid micro-supercapacitors (AZIHMSCs) with high power density, moderate energy density, good cycle life and excellent safety are promising candidates for micro-energy storage. Among them, AZIHMSCs based on Ti₃C₂T_x MXene anodes and battery-type cathodes can provide superior performance. However, two-dimensional (2D) Ti₃C₂T_x MXene electrodes have an inherent restacking issue and -F surface terminations that hinder ion diffusion and ultimately reduce the energy storage capacity of the corresponding AZIHMSCs. Herein, a deep alkalisation strategy was developed to synthesise oxygen-rich, functionalised MXene (O-MXene) nanofibres to solve these problems. Compared with the traditional 2D few-layered Ti₃C₂T_x MXene electrode, O-MXene electrodes exhibit an interconnected, three-dimensional (3D) microstructure and ample oxygen functional groups, enhancing Zn²⁺ affinity and improving capacitance and rate performance. First-principles calculations further reveal the enhanced interactions between O-MXene electrodes and Zn²⁺ supported by atomic interaction, electronic behaviour and orbital hybridization. The AZIHMSCs fabricated with an O-MXene film anode and a MnO₂-multiwalled carbon nanotubes (MnO₂-MWCNTs) film cathode exhibit excellent energy density (130.6 μWh cm^-2), power density (9.5 mW cm^-2), cycling stability (93.29 % after 5000 cycles) and flexibility (98.43 % capacitance retained at 120° bending). This study will open new avenues for modifying MXene materials and next-generation high-performance AZIHMSCs.