One-step coaxial electrodeposition of Co_0.85Se on CoNi₂S₄ nanotube arrays for flexible solid-state asymmetric supercapacitors

A one-step electrochemical method is applied to coaxially deposit highly conductive Co_0.85Se nanosheets composed of ultrasmall Co_0.85Se nanocrystals on three-dimensional (3D) CoNi₂S₄ nanotube arrays supported on graphene foam (GF) (Co_0.85Se@ CoNi₂S₄/GF) as an electrode for flexible solid-state supercapacitors. The Co_0.85Se@CoNi₂S₄/GF electrode has an areal capacitance of 5.25 F cm^-2 at 1 mA cm^-2 and good rate capability (2.65 F cm^-2 at 20 mA cm^-2). A solid-state asymmetric supercapacitor with Co_0.85Se@NiCo₂S₄/GF as the cathode and hollow carbon spheres (HCSs) as the anode in a PVA/KOH electrolyte was assembled which shows an energy density of 46.5 W h kg^-1 at a power density of 750 W kg^-1, and 89.0% capacity retention after 10000 cycles over a potential window of up to 1.55 V. These results demonstrate that the electrodeposition method is applicable for engineering of 3D microstructured electrodes and also provides an efficient strategy for fabricating transition metal selenide based nanodevices.