Nanoparticles of Nickel Hexacyanoferrate Derivatives as the Components of Electrode Materials for Electrochemical Capacitors

Nickel-potassium hexacyanoferrate (KNiHCF) and its analogues (NaKNiHCF) with different Na : K ratio are synthesized by chemical coprecipitation. The salt structure is characterized by the methods of X-ray diffraction, energy-dispersive microanalysis, and transmission electron microscopy. The electrochemical characteristics of composite electrodes containing 20 wt % salt are studied by the methods of cyclic voltammetry, galvanostatic charge-discharge measurements, and impedance spectroscopy. The composite electrodes are shown to exhibit the higher specific capacitance (C-sp) as compared with the carbon electrode. The incorporation of sodium into the KNiHCF structure increases C-sp at the high charge-discharge rates. It is shown that electrodes containing the analogue of NaKNiHCF with the Na/K ratio 0.92 : 0.24 demonstrate the high electronic and ionic conductivity and the low equivalent series resistance and charge-transfer resistance. The latter material can be used as the cathode material in hybrid electrochemical capacitors. The ease of synthesis of these salts and the simple method of electrode fabrication makes possible their large-scale application in rechargeable batteries.