In-situ Oxidizing Ni Foams Into Ni（OH）₂-based Electrode Materials And Their Electrochemical Performance

Recently, energy storage technology has emerged as one of the most essential topics to solve air pollution problems and energy crisis, besides the development of alternative renewable energy sources. Supercapacitor, a new energy storage device between traditional capacitor and lithium battery, has continuously attracted great interest because of its outstanding advantages such as lost-cost, environmental friendliness, high power density, and long cycling stability. The electrode is one of the most important factors to determine the properties of supercapacitor, therefore, achievement of high-performance electrode materials is one of the hot topics for the electrochemical energy storage devices.Ni（OH）₂ has been intensively investigated for application as electrodes for electrochemical capacitors owing to its layered structure with large interlayer spacing, low cost, high theoretical capacitance and environmental compatibility. Ni foams have drawn significant attention as substrates（current collector） and templates for constructing 3D electrodes, because of their exceptional uniformity, light weight, intrinsic strength, corrosion resistance and good electrical and thermal conductivity, which simplify electrode preparation technology and enhance the electrochemical performance.In this paper, 3D Ni foams were in-situ oxidized into Ni（OH）₂ and Zn-Ni DHs（Zn-Ni double hydroxides） by a facile chemical bath deposition with NH₄NO₃, Zn（NO₃）₂, NH₃·H₂O and commercial nickel foam as raw materials. The composition and structure of the prepared composites were analyzed using XRD（X-ray duffraction）, FESEM-EDS（Field emission scanning electron microscope-Energy dispersive spectrometer）, XPS（X-ray photoelectron spectroscope） and N₂ adsorption-desorption. Their electrochemical properties before and after the cycle were tested by cyclic voltammetry, galvanostatic charge-discharge, electrochemical impedance spectroscopy in 2M KOH aqueous electrolyte. The main contents and conclusions are as follows:1. The preparation of 3D Ni@Ni（OH）₂ composite and its electrochemical properties: Ni@Ni（OH）₂ foam electrodes were prepared by chemical bath deposition from in-suit oxidation of Ni foams in NH₄NO₃ solution at 80°C for 24 h. Electrochemical tests show that Ni@Ni（OH）₂ electrode demonstrated a high areal capacitance of 6.4 F/cm² at a current density of 2.5 m A/cm2（or 1.62 F/cm² at a high current density of 30 m A/cm²ï¼‰. Electrochemical cycle test（5000 cycles） shows that the capacitance of Ni@Ni（OH）₂ increased during the first 1200 cycles, a capacitance retention of 70.4% and 42% was achieved after 2000 cycles and 5000 cycles, respectively. Such remarkable electrochemical performance can be primarily ascribed to the structure design of combination of Ni（OH）₂ and Ni foam substrate, which provides fast and efficient diffusion of the electrolyte ions to the active material surface and good electron transport within Ni（OH）₂. The reduced porosity and specific surface area of Ni@Ni（OH）₂ after 5000 cycles decreased the contact areas and deteriorated the ion accessibility between electrolyte and active materials, resulting in a poor cycling stability.2. The facile preparation of Ni@Zn-Ni DHs composite and its capacitance perfoamance: Zn（OH）₂-Ni（OH）₂ nano-sheet arrays were grown on Ni foam substrates by chemical bath deposition method with Zn（NO₃）₂ and NH₃H₂ O, without any surfactant. When used as electrode materials for supercapacitors, the Zn-Ni DHs displayed a good specific capacitance of 746.2 F/g at 1 A/g（or an areal capacitance of 3.18 F/cm2 at 4.27 m A/cm2）. The capacitance attenuation of the Zn-Ni DHs after 3000 galvanostatic charge-discharge cycles was 29%. The good performance of Zn-Ni DHs nano-sheets arrays can be attributed to the following two aspects: firstly, the nano-sheet built network structures not only provide numerous sites for the adsorption of ions, but also contribute efficient pathways for charge transport; secondly, directly anchoring Ni-Zn DHs on Ni foam can effectively enhance the utilization efficiency of active materials by enhancing the electric contact between active materials and substrates. As revealed by the XRD and FESEM characterization of Ni@Zn-Ni DHs after cycling, the structure of Zn-Ni DHs nano-sheets maintained well, but became much more wrinkled and aggregated. This morphology evolution should be responsible for the deterioration of the capacitive performance of Zn-Ni DHs nano-sheets after cycling.