Core@shell CoO@Co3O4 nanocrystals assembling mesoporous microspheres for high performance asymmetric supercapacitors

Cobalt oxides (denoted as CoOx) mesoporous microspheres assembled by core@shell CoO@Co3O4 nanocrystals are successfully prepared via a solvothermal method followed by a special post annealing treatment. Benefiting from its unique structure, high porosity, and the synergistic effect between CoO and Co3O4, when employed as electrode materials for supercapacitors, the CoOx microspheres exhibit a high specific capacitance of 3377.8 F g(1) at a current density of 2 A g(1), which are far superior to those of individual CoO or Co3O4. In addition, the CoOx electrode shows an excellent rate property of 66.5% capacity retention at 20 A g(1) and can still remain a high specific capacitance of 1331.1 F g(1) after 4000 cycles at a high current density of 10 A g(1). Besides, an asymmetric supercapacitor (ASC) is constructed by using the CoOx microspheres as the positive electrode and graphene as the negative electrode. The ASC delivers a high specific capacitance of 123.9 F g(1) at 1 A g(1) and an energy density of 44.06 Wh kg(1) at a power density of 800 W kg(1). Furthermore, the CoOx//graphene ASC shows a good cycling stability with 74.9% specific capacitance retention after 6000 cycles at a high current density of 3 A g(1). This work demonstrates a facile synthesis strategy of CoOx mesoporous microspheres with outstanding electrochemical properties, which can be used in supercapacitors and other energy storage devices. (c) 2017 Published by Elsevier B.V.