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

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Published online 24 June 2020 | https://doi.org/10.1007/s40843-020-1357-9

Oxygen vacancies boosting ultra-stability of mesoporous ZnO-CoO@N-doped carbon microspheres for asymmetric supercapacitors 1

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Di Yao , Fulei Wang , Wu Lei , Yan Hua , Xifeng Xia , Jinping Liu and Qingli Hao 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 −1 electrode based on ZnO-CoO@NC in 2 mol L KOH shows enhanced capacitive performance with an excellent cycle stability of 92% retention of the initial capacitance after 40,000 −1 charge-discharge cycles at 2 A g , keeping the morphology unchanged. The assembled asymmetric supercapacitor, graphene//ZnO-CoO@NC, also performs good cyclic stability −1 with 94% capacitance retention after 10,000 cycles at 2 A g . 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 ZnOCoO@NC holds great promise for high-performance energy storage applications. Keywords: supercapacitor, zinc oxide, cobaltous oxide, doped carbon, cycling stability, heterostructure

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INTRODUCTION Supercapacitors (SCs) are attractive alternatives to batteries with a great application potential and attracting ever-growing attention recently, due to their intriguing features such as high power capabilities, long cycle life and rapid charge/discharge processes [1–5]. But the low energy density of SCs, especially the electric double layer capacitors (EDLCs) based on carbonous materials, must be boosted further for the demand of next generation electronic devices. Pseudocapacitors as another kind of SCs, based on pseudocapacitive materials storing charges through Faradaic reactions, may reach several times higher specific capacitance and energy density than normal EDLCs. With unique characteristics like environmental benignity and high specific capacitance, transition metal oxides are intensively explored as redox oxides with high pseudocapacitance for SCs. Cobalt oxides, like Co3O4 [6,7] and CoO, also have high theoretical specific capacitance [8]. However, they suffer from the inferior rate capability and cycling stability because of their poor electrical co