Mechanical force-driven growth of free-standing tassels-like nickel cobalt phosphate electrode for high-performance supercapacitor

Although transition metal phosphates (TMPs) possess superb theoretical capacity and favorable crystalline structure, their practical applications are still hindered by the low capacity and inferior rate performance caused by the severely stacked layer structures. Herein, the free-standing tassels-like nickel cobalt phosphate (NCP) electrode composed of ultrathin nanosheets is firstly prepared via a facile hydrothermal stirring method. The ultrathin nanosheets can provide large surface area, tremendous exposed active sites, and effortless ion penetration path at the same time. The as-prepared Ni₂Co(PO₄)₂ (NCP-2) electrode demonstrates a maximum specific capacity of 2518 mC cm⁻² (1007 C g⁻¹) at 2 mA cm⁻² with 76.7% capacity retention at 50 mA cm⁻², outperforming the other previously reported TMP electrodes. According to the density functional theory calculation results, the bimetallic NCP-2 electrode exhibits a higher conductivity and stronger adsorption capacity of OH⁻ compared with the monometallic Ni₃(PO₄)₂ (NP) electrode. Moreover, hybrid supercapacitors (HSCs) with NCP-2 and graphene hydrogel (GH) as the positive and negative electrodes, respectively, deliver a high energy density of 46.25 W h kg⁻¹ and a high-power density of 9579 W kg⁻¹. After being fully charged, the HSC can illuminate the light-emitting diode light for more than 5 min. Furthermore, when the HSC is integrated with an insole shaped triboelectric nanogenerator, it can be used to harvest and store the mechanical energy from human walking. Finally, the HSC can also serve as the voltage source for flexible piezoresistive sensors. Thus, this hydrothermal stirring method can be extended to prepare other electroactive materials with unique nanostructures for high-performance supercapacitors.