Synthesis of ZnO@Ni(OH)2 Nanostructures for Application in High-Performance Supercapacitors

Recently, supercapacitor has become a preferred energy storage device because it has higher power density than batteries and higher energy density than conventional capacitors and fast charge/discharge rates. Among the often used supercapacitor electrode materials, pseudocapacitive metal oxide exhibits much high capacitance, but the poor electrical conductivity limits its performance. In order to overcome this disadvantage, one dimensional ZnO nanowires (NWs) are widely employed because they can serve as a direct electrical pathway for electrons and provide a larger surface area for loading metal oxide. ^[1]

Base on above considerations, in this work, we fabricated the Ni(OH)₂ coated ZnO electrodes for use in supercapacitors. As shown in Fig. 1, the Ni(OH)₂ coated ZnO electrode consists of upstanding nanowires (NWs) and outstretched Ni(OH)₂flakes grown all over each nanowire.

First, ZnO NWs was grown by immersing the seeded FTO substrate in aqueous solutions containing 0.01 M zinc acetate dihydrate and 0.01 M hexamethylenetetramine (HMTA) at 90 ^oC for 5 h. These ZnO NWs were then used as the trunks for the growth of Ni(OH)₂ flakes. The Ni(OH)₂ flakes were synthesized by the pulse galvanostatic electrodeposition onto the surface of ZnO NWs in aqueous solution of 5 mM nickel nitrate hexahydrate for 2 min at room temperature. The parameters of pulse galvanostatic electrodeposition include current density, pulse time (t_on), and relaxation time (t_off), the optimized conditions for pulse galvanostatic electrodeposition are 1 mA/cm², 1 s, and 10 s respectively.

We have achieved a high specific capacitance 1827 F/g at a discharge current density of 2 A/g, corresponding discharge curves are shown in Fig. 2. Fig. 3 reveals the highest energy density and power density are 185.0 Wh/kg and 9.0 kW/kg, respectively. Effects of current density, t_on, t_off, and deposition time on the performance of supercapacitors were investigated.

Reference

1. Y. B. He, G. R. Li, Z. L. Wang, C. Y. Su, Y. X. Tong, Energy Environ. Sci., 2011, 4, 1288-1292.