3D Flowerlike α-Nickel Hydroxide with Enhanced Electrochemical Activity Synthesized by Microwave-Assisted Hydrothermal Method

Nickel hydroxide has received increased attention especially due to its electrochemical properties and potential applications in rechargeable Ni-base alkaline batteries, e.g., Ni/Cd, Ni/Zn, and Ni/MH. Ni(OH)2 has a hexagonal layered structure with two polymorphs, α- and β-Ni(OH)2. α-Ni(OH)2 shows superior electrochemical properties compared to those of the β-form. Nanosized flowerlike α-nickel hydroxide materials with an interlayer spacing of 7.0 Å have been prepared by a microwave-assisted hydrothermal method. The experimental results from XRD and FT-IR showed that the Ni(OH)2 sample prepared by this method had the typical α-phase. FE-SEM images showed many uniform flowerlike architectures with diameters of 700 nm−1µm which consisted of the aggregated flakes. TEM results showed the flakes were built up from many nanocrystals with 2–3 nm diameters. TGA and TPD were employed to investigate thermal stability and gas evolution during the heating process. α-Nickel hydroxide was transferred to NiO with a cubic crystalline structure after being calcined at 450 °C; the NiO still kept the morphology of α-Ni(OH)2. Cyclic voltammetry was used to determine the electrochemical properties of the Ni(OH)2 electrode in 1 M KOH. α-Ni(OH)2 prepared by MW-HT had the best electrochemical activity for the electrochemical reduction of O2 compared with α-Ni(OH)2 synthesized by conventional HT methods and β-Ni(OH)2. The effects of nickel sources and precipitators on the phase and morphology of the products were studied. Conventional hydrothermal methods were used to study the role of microwave irradiation. The possible growth mechanism is discussed here. The CV experiments showed that H2O2 can be reduced to OH− on the α-Ni(OH)2 electrode. The Levich equation was used to calculate the number of electrons transferred during the O2 redox reaction.