Superior performance of ZnCo2O4/ZnO@multiwall carbon nanotubes with laminated shape assembled as highly practical all-solid-state asymmetric supercapacitors

Abstract

Herein, a ZnCo₂O₄/ZnO@multiwall carbon nanotube (Zn1Co2@MWCNT) hybrid in situ grown on Ni foam was designed via a two-step method, involving a hydrothermal reaction to synthesize ZnCo₂O₄/ZnO (Zn1Co2) on Ni foam and then a dipping-drying process to cover MWCNTs on Zn1Co2 at room temperature. The sandwich structure of Zn1Co2@MWCNTs with jungle flake inside was directly used as an electrode material displaying a high specific capacitance of 2069 F g⁻¹ at 1 A g⁻¹, which is higher than those of ZnCo₂O₄-based electrode materials in most previous reports, and a great cycling stability of 80.2% retention after 3000 cycles. Meanwhile, an all-solid-state asymmetric supercapacitor (ASC) was assembled, utilizing Zn1Co2@MWCNTs as positive electrode and activated carbon (AC) as negative electrode. The ASC device had a high specific capacitance of 111.8 F g⁻¹ (at 1 A g⁻¹), a high energy density of 48.1 W h kg⁻¹ (at 900 W kg⁻¹) and excellent stability with capacitance retention of 92.9% after 3000 cycles. Two serial connected Zn1Co2@MWCNTs//AC ASCs are utilized to light four parallel connected light-emitting diode (LED) bulbs (5 mm, red color), which keep bright for 23 minutes. If three serial connected Zn1Co2@MWCNTs//AC ASCs are utilized to light one LED bulb (5 mm, yellow color), the LED bulb can keep bright for 55 minutes. The excellent electrochemical performances mean the all-solid-state Zn1Co2@MWCNTs//AC ASCs are highly practical for application in the energy storage field.