The NH4F-induced morphology control of hierarchical CoO@MnO2 core-shell arrays for high performance supercapacitor electrodes

Hierarchical CoO@MnO2 core-shell arrays with controllable morphology were fabricated by direct growth on 3D nickel foam by a facile two-step hydrothermal method. It was found that the morphology of CoO could be effectively controlled by the amount of loaded NH4F with the assistance of ethylene glycol and urea. As a result, disordered nanowires, highly ordered nanowires, dense nanowire clusters and compact bulk CoO were obtained upon increasing the NH4F amount, which in turn were passed on to the subsequent CoO@MnO2 core-shell arrays. Electrochemical tests indicate that the micro-morphology of the CoO@MnO2 arrays significantly affected their electrochemical performance. Among all the samples, the synthesized hierarchical 1:2CoO@MnO2 core-shell arrays formed from highly ordered CoO nanowires exhibited the highest specific capacitance (1505.7 F g(-1) at a current density of 1 A g(-1)), low charge transfer resistance, and excellent rate performance and cycling stability (91.7% retention after 5000 cycles at 5 A g(-1) and the coulombic efficiency remained at over 98.6%).