Constructing a hierarchically structured KNi0.67Co0.33PO4·H2O-graphene hydrogel/Ni foam electrode for superior all-solid-state supercapacitor

Abstract

A monolithic supercapacitor electrode of a KNi_0.67Co_0.33PO₄·H₂O-graphene composite hydrogel supported on Ni foam (KNCP-GH/NF) is first prepared by a one-step hydrothermal method, which achieves notable improvements in the electrode surface area and mass-loading of active materials. The KNCP-GH/NF electrode enjoys a hierarchical open-porous structure, where the KNCP-GH composite hydrogel fills in the voids in NF and the porous graphene hydrogel (GH) simultaneously provides a large support surface for growing active KNCP nanoflowers. Accordingly, the KNCP-GH/NF electrode exhibits a strikingly high capacity of 3240 mC cm⁻² (876 C g⁻¹) at 2 mA cm⁻² and a satisfactory rate performance with 78.3% retention at 100 mA cm⁻². Further, an all-solid-state asymmetric supercapacitor, constituted by using KNCP-GH/NF and Fe₂P/GH/NF as the cathode and anode, respectively, and PVA-KOH as the solid-state gel electrolyte, delivers a high energy density of 69.2 W h kg⁻¹ (3.9 mW h cm⁻³) and a power density of 13 229 W kg⁻¹ (720 mW h cm⁻³) as well as notable cyclability with 81.2% capacity retention after 10 000 charge/discharge cycles. These attractive performances suggest a promising potential for a hierarchically structured KNCP-GH/NF electrode for the high-performance energy storage application.