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

Abstract

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%).