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

A binder-free cobalt phosphate hydrate (Co 3 (PO 4 ) 2 ·8H 2 O) 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 Co 2+ and PO 4 –3 were used to obtain different mass loading of cobalt phosphate on the nickel foam. The Co 3 (PO 4 ) 2 ·8H 2 O 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 Co 3 (PO 4 ) 2 ·8H 2 O/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 ).