Hierarchically Porous 3D Freestanding Holey-MXene Framework via Mild Oxidation of Self-Assembled MXene Hydrogel for Ultrafast Pseudocapacitive Energy Storage

The true promise of MXene as a practical supercapacitor electrode hinges on the simultaneous advancement of its three-dimensional (3D) assembly and the engineering of its nanoscopic architecture, two critical factors for facilitating mass transport and enhancing an electrode’s charge-storage performance. Herein, we present a straightforward strategy to engineer robust 3D freestanding MXene (Ti₃C₂T_x) hydrogels with hierarchically porous structures. The tetraamminezinc(II) complex cation ([Zn(NH₃)₄]²⁺) is selected to electrostatically assemble colloidal MXene nanosheets into a 3D interconnected hydrogel framework, followed by a mild oxidative acid-etching process to create nanoholes on the MXene surface. These hierarchically porous, conductive holey-MXene frameworks facilitate 3D transport of both electrons and electrolyte ions to deliver an excellent specific capacitance of 359.2 F g^-1 at 10 mV s^-1 and superb capacitance retention of 79% at 5000 mV s^-1, representing a 42.2% and 15.3% improvement over pristine MXene hydrogel, respectively. Even at a commercial-standard mass loading of 10.1 mg cm^-2, it maintains an impressive capacitance retention of 52% at 1000 mV s^-1. This rational design of an electrode by engineering nanoholes on MXene nanosheets within a 3D porous framework dictates a significant step forward toward the practical use of MXene and other 2D materials in electrochemical energy storage systems.