Superior performance asymmetric supercapacitors based on a directly grown three-dimensional lawn-like cobalt-zinc hydroxyfluorides nanoneedle arrays electrode

Abstract

The three-dimensional (3D) lawn-shaped Co-Zn(OH)F nanoneedle arrays have been directly grown on Ni foam by a hydrothermal method. The morphological evolution process of Co-Zn(OH)F has been investigated by scanning electron microscopy (SEM) at different time intervals in detail. Owing to the high activity of the well-ordered Co-Zn(OH)F nanoneedles with a diameter of about 50nm at the sharp tip, the 3D electrode architectures achieve an ultra-high specific capacitance of 1970Fg−1 at 1Ag−1 and excellent cycle stability (105% initial capacity retention after 5000 cyclic voltammetry test). An aqueous asymmetric supercapacitor (ASC) is developed by using Co-Zn(OH)F and activated carbon as the positive and negative electrode. The ASC delivers a specific capacitance of 275Fg−1 at 1Ag−1 with a potential window of 0–1.5V. It retains 99.9% initial specific capacitance even after 10,000 continuous charge-discharge cycles (5Ag−1). The optimized ASC cell can deliver an energy density of 86.2Whkg−1 at a power density of 285Wkg−1 and it retains 18.8Whkg−1 at a power density of 5062Wkg−1.