Oxygen vacancies boosting ultra-stability of mesoporous ZnO-CoO@N-doped carbon microspheres for asymmetric supercapacitors

Abstract

Long-term cycling stability of pseudocapacitive materials is pursued for high-energy supercapacitors. Herein, the mesoporous zinc-cobalt oxide heterostructure@nitrogendoped carbon (ZnO-CoO@NC) microspheres with abundant oxygen vacancies are self-assembled through a hydrothermal method combined with an annealing post-treatment. The multifunctional polyvinyl pyrrolidone (PVP) is used as a structure-directing agent, the precursor of NC and the initiator of abundant oxygen vacancies in zinc-cobalt oxide microspheres. XPS demonstrates the generation of surface oxygen vacancies resulted from the reduction effect of conductive NC, and further confirms the weaker interaction between the metal ions and oxygen atoms. As a result, the electrode based on ZnO-CoO@NC in 2 mol L−1 KOH shows enhanced capacitive performance with an excellent cycle stability of 92% retention of the initial capacitance after 40,000 charge-discharge cycles at 2 A g−1, keeping the morphology unchanged. The assembled asymmetric supercapacitor, graphene//ZnO-CoO@NC, also performs good cyclic stability with 94% capacitance retention after 10,000 cycles at 2 A g−1. The remarkable electrochemical performance of the self-assembled ZnO-CoO@NC composite is attributed to the mesoporous architecture, abundant oxygen vacancies, conductive ZnO scaffold for CoO crystals forming heterostructure of ZnO-CoO and the high conductive NC layer covering outside of the multi-metal oxide nanoparticles. Hence, the ZnO-CoO@NC holds great promise for high-performance energy storage applications.