Nanosized Ni-Mn Mixed Oxide Electrode Materials Derived From The Citrate-gel Process And Capacitance Characteristics

Electrochemical Capacitors (ECs) due to pseudocapacitance and double layer capacitance are characterized with high specific power density, long cycle life and nontoxic and have attracted ever-increasing attention for mobile communication, portable computers, zero emission electric vehicles and so on in the past few years. These devices become one of directions for developing the advanced power sources. The ECs based on metal oxide electrode active materials due to pseudocapacitances have greater potential uses than the current double-layer capacitors based on active carbons. One of the key factors for the ECs performances is the electrode active material and nanosized Ni-Mn mixed oxides are considered to be promising electrode active materials as good electrochemical characteristics and lower price than ruthenium oxides. The conventional processes for nanosized metal oxides are precipitation, sol-gel and electrodeposition and these methods may not be commercially applied as shortcomings such as severe preparative conditions, low utility of the raw materials, limited capacitance and working potential window. Therefore, it is meaningful in reality and theory to develop a novel process for nanosized Ni-Mn mixed oxides electrode materials and to study these materials' electrochemical characteristics.The present paper studied the preparation of the nanosized Ni-Mn mixed oxide electrode materials by the citrate-gel process from nickel carbonate basic, manganese acetate and citric acid and characterized these materials' structures and electrochemical capacitance properties. The formation of Ni/Mn citrate gels was investigated via Fourier transform infrared (FTIR) spectrometer and X-ray diffraction (XRD); Thermal decompositions of the Ni/Mn citrate gel precursors were examined using Thermogravimetric and differential scanning calorimetry (TG-DSC); The structure and properties of Ni/Mn mixed oxides derived from the thermal decomposition of the citrate gel precursors under various conditions were characterized by XRD, Scanning electron microscope (SEM) and Energy dispersive X-ray(EDX) spectrum and Brunauer, Emett, Teller (BET) porosimetry. The experimental results show that the nanosized Ni-Mn oxides characterized with particlesize <10 rim, specific surface area >109 m2/g and the content of carbon approximate 37% were prepared under the optimized processing conditions (The molar stoichiometric ratio of citric acid to nickel to manganese of 8.4:6:1 and thermal decomposition temperature of 400 °C and duration of 1 hour) and formed a solid solution of nickel and manganese oxides by Mn ion entering into NiO crystal lattice..The working electrode and simulate electrochemical capacitors were made from the nanosized Ni-Mn oxides as-prepared under the optimized conditions by the citrate-gel process and the electrochemical properties in KOH solution were investigated via cyclic voltammograms (CV) and electrochemical impedance spectroscopy(EIS) and direct current charge/discharge cycling using a VMP2 Multi-Potentiostat system. The results show that capacitance of the oxide electrode consists of pseudocapacitances due to Faradaic reactions and double-layer capacitance; The electrode exhibited characteristics of metal oxides and carbon composite active materials; The specific capacitance for electrode is influenced by the concentration of electrolyte, sweeping rate and the structure or component of the active materials. When KOH concentration was 4 mol/L, potential window 0.8V, and the discharge current density was lOmA/cm2, the simulate electrochemical capacitors offering the specific capacitance was 194.8F/g, the specific energy and power densities were 54.8Wh/kg and 3.75kW/kg respectivly, with only 21.8% decrease of the specific capacitance after 1000 charge/discharge cycles.