FeO Nanoparticles Anchored on the TiCT MXene Paper for Flexible Supercapacitors with Ultrahigh Volumetric Capacitance.

TiCT MXene, with high conductivity and flexibility, has drawn great attention in the wearable energy storage devices. However, the easy nanoflake-restacking phenomenon greatly restricts the achievable electrochemical performance of TiCT-based supercapacitors, in particular volumetric capacitance. Herein, we report a flexible hybrid paper consisting of FeO nanoparticles (NPs) anchored on TiCT (FeO NPs@MX) electrostatic self-assembly and annealing treatments. The interlayer spacing of TiCT nanoflakes is effectively enlarged through the incorporation of FeO NPs, allowing more electrochemical active sites to store charge. Meanwhile, TiCT nanoflakes form a continuous metallic skeleton and inhibit the volume expansion of FeO NPs during the charging/discharging process, enhancing the cycling stability. The flexible, ultrathin (4.1 μm) FeO NPs@MX hybrid paper shows considerably improved electrochemical performances compared to those of pure TiCT and FeO, including a wide potential window of 1 V, an ultrahigh volumetric capacitance of ∼2607 F cm (584 F g), and excellent capacitance retention after 13,000 cycles. Besides, the as-assembled symmetric solid-state supercapacitor exhibits an energy density of 29.7 Wh L and excellent mechanical flexibility. We believe that the present nanostructure design, decorating NPs within a two-dimensional metallic network, has general applicability and could be used to fabricate highly efficient composites for advanced energy storage devices.