A Multi-Scale Structural Engineering Strategy for High-Performance MXene Hydrogel Supercapacitor Electrode

MXenes as an emerging two-dimensional (2D) material have attracted tremendous interest in electrochemical energy-storage systems such as supercapacitors. Nevertheless, 2D MXene flakes intrinsically tend to lie flat on the substrate when self-assembling as electrodes, leading to the highly tortuous ion pathways orthogonal to the current collector and hindering ion accessibility. Herein, a facile strategy toward multi-scale structural engineering is proposed to fabricate high-performance MXene hydrogel supercapacitor electrodes. By unidirectional freezing of the MXene slurry followed by a designed thawing process in the sulfuric acid electrolyte, the hydrogel electrode is endowed with a three-dimensional (3D) open macrostructure impregnated with sufficient electrolyte and H+-intercalated microstructure, which provide abundant active sites for ion storage. Meanwhile, the ordered channels bring through-electrode ion and electron transportation pathways that facilitate electrolyte infiltration and mass exchange between electrolyte and electrode. Furthermore, this strategy can also be extended to the fabrication of a 3D-printed all-MXene micro-supercapacitor (MSC), delivering an ultrahigh areal capacitance of 2.0 F cm(-2) at 1.2 mA cm(-2) and retaining 1.2 F cm(-2) at 60 mA cm(-2) together with record-high energy density (0.1 mWh cm(-2) at 0.38 mW cm(-2)).