An ultra-high energy density flexible asymmetric supercapacitor based on hierarchical fabric decorated with 2D bimetallic oxide nanosheets and MOF-derived porous carbon polyhedra

Flexible supercapacitors (SCs) are an emergent and promising technology for next-generation energy storage devices. However, low energy densities hindered their practical applications. Two-dimensional (2D) nanosheets can exhibit excellent electrochemical charge storage properties due to their short ion-diffusion distance and rich electroactive sites with multiple valence states. Herein, we report the direct growth of mesoporous 2D zinc cobaltite nanosheets on a flexible carbon cloth substrate (Zn–Co–O@CC) with an average thickness of ∼45 nm by a facile hydrothermal method at low temperature. The Zn–Co–O@CC electrode displays a high capacitance of 1750, 1573.65 and 1434.37 F g −1 at a current density of 1.5 A g −1 in LiCl, NaCl and KCl neutral aqueous electrolytes, respectively, with excellent rate capabilities at high current densities and demonstrates good cycling stability (>94%) for up to 5000 cycles. Moreover, highly flexible asymmetric supercapacitor (ASC) devices have been fabricated using Zn–Co–O@CC as a positive electrode and bimetallic organic framework (MOF)-derived nanoporous carbon polyhedra (NPC@CC) as a negative electrode (Zn–Co–O@CC//NPC@CC). The as-fabricated ASC can operate at a large potential window of 0.0–2.0 V and shows outstanding energy storage performance by delivering an ultra-high energy density of 117.92 W h kg −1 at a power density of 1490.4 W kg −1 with a cycling stability of 94% after 5000 charge/discharge cycles. To the best of our knowledge, the achieved energy storage performance of the ASC device is very competitive and the highest among all binary metal oxides, carbonaceous materials, and MXene-based SCs and ASCs to date. The applied strategy to fabricate SCs is capable of enhancing both electrochemical activity and cycling stability, and can be readily applied to other metal oxide-based SCs.