Structural engineering and surface modification of MOF-derived cobalt-based hybrid nanosheets for flexible solid-state supercapacitors

Cobalt oxide (Co₃O₄) has emerged as a promising battery-type material for electrochemical energy storage devices; however, the low ionic diffusivity, sluggish charge transfer kinetics, and dramatic volume expansion that occur during cycling hamper the further improvement of its electrochemical properties. Herein, a surface and structural engineering strategy to prepare hybrid nanosheets with a metal-organic framework (MOF) as a template is employed, in which in situ phosphorus-doped Co₃O₄ nanoparticles are evenly integrated within a conducting P–N co-doped carbon matrix (denoted as P–Co₃O₄@PNC). The hybrid architecture provides a shortened ion diffusion distance, an expanded surface-to-volume ratio, newly created active sites, and enrichened structural defects. The high availability of electrochemical active sites/interfaces along with the strong intercomponent synergy of heteroatom-doped Co₃O₄ and carbon enable the fast charge/mass transfer kinetics required for superior charge-storage capabilities. P–Co₃O₄@PNC hybrid nanosheets deliver a high specific capacity of 614 ​mC ​cm⁻² ​at 1 ​mA ​cm⁻² and an extraordinary cycling stability. Flexible solid-state asymmetric supercapacitor (ASC) devices constructed with self-supported P–Co₃O₄@PNC and PNC materials exhibit a high energy density of 69.6 ​W ​h ​kg⁻¹ ​at a power density of 750 ​W ​kg⁻¹, and display excellent cycling stability with a capacitance retention of 96.8% even after 10000 cycles at 20 ​A ​g⁻¹. Moreover, the fabricated ASC devices present superior performance uniformities and high flexibilities with no significant capacitance changes under different flexing conditions.