Supercapattery Based on Binder-Free Co₃(PO₄)₂·8H₂O Multilayer Nano/Microflakes on Nickel Foam

A binder-free cobalt phosphate hydrate (Co₃(PO₄)₂·8H₂O) multilayer nano/microflake structure is synthesized on nickel foam (NF) via a facile hydrothermal process. Four different concentrations (2.5, 5, 10, and 20 mM) of Co²⁺ and PO₄^-3 were used to obtain different mass loading of cobalt phosphate on the nickel foam. The Co₃(PO₄)₂·8H₂O modified NF electrode (2.5 mM) shows a maximum specific capacity of 868.3 C g^-1 (capacitance of 1578.7 F g^-1) at a current density of 5 mA cm^-2 and remains as high as 566.3 C g^-1 (1029.5 F g^-1) at 50 mA cm^-2 in 1 M NaOH. A supercapattery assembled using Co₃(PO₄)₂·8H₂O/NF as the positive electrode and activated carbon/NF as the negative electrode delivers a gravimetric capacitance of 111.2 F g^-1 (volumetric capacitance of 4.44 F cm^-3). Furthermore, the device offers a high specific energy of 29.29 Wh kg^-1 (energy density of 1.17 mWh cm^-3) and a specific power of 4687 W kg^-1 (power density of 187.5 mW cm^-3).