Study On The Electrochemical Preparation And Kinetics Process Of Nickel And Its Alloys In Low Temperature Molten Salts

In recent years,Ni and its alloys have been widely used for their corrosion resistance,magnetic and electrocatalytic properties.Among them,Ni-Al alloy is a potential high-temperature structural material,Ni-Mg-Al alloy and Ni-La alloy is considered to be a promising material for hydrogen storage.In this paper,the electrochemical preparation and kinetic processes of Ni,Ni-Al,Ni-Mg-Al and Ni-La alloys were studied.The electrochemical window of low temperature molten salt of urea(67 wt%)-acetamide(21 wt%)-NaBr(12 wt%)was studied by cyclic voltammetry method.The results show that the molten salt is stable in the potential range of-1 V to 1.2 V(vs.Pt),which means its electrochemical window is 2.2 V.The kinetic process of Ni2+ in urea-acetamide-NaBr molten salt system were studied using cyclic voltammetry,chronoamperometry and chronopotentiometry methods,while the electrodeposition products were characterized by scanning electron microscopy(SEM)and X-ray diffraction(XRD).The results show that the reaction of Ni2+ on tungsten electrode is a one-step two-electron irreversible reaction in the urea-acetamide-NaBr-NiCl2 molten salt.The diffusion coefficient of Ni2+ in this electrolyte is about 3×10-6cm2·s-1,and it increases with the increase of applied potential.The nucleation of Ni in this molten salt follows a three-dimensional progressive nucleation model.More over,the amount of Ni particles deposited at-0.8 V gradually increases with the deposition time increasing.The kinetic process of Ni2+-Al3+ in molten salt were studied using cyclic voltammetry,chronoamperometry and chronopotentiometry methods,while the electrodeposition products were characterized by scanning electron microscopy(SEM)and X-ray diffraction(XRD).The results show that Al can be codeposited with Ni in the urea-acetamide-NaBr-NiCl2-AlCl3 molten salt forming Ni-Al alloy.This alloy shows a uniform and compact structure which is composed of spherical particles.The increase of the cathode potential results an increase of the particles size and arougher surface.The nucleation of Ni-Al follows a three-dimensional instantaneous nucleation model and the diffusion coefficient of metal ions increases with the increase of cathode potential.The kinetic process of Ni2+-Al3+-Mg2+ in molten salt were studied using cyclic voltammetry,chronoamperometry and chronopotentiometry methods,while the electrodeposition products were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).The results show that Ni-Mg-Al alloy is electrodeposited in the urea-acetamide-NaBr-NiCl2-AlCl3-MgCl2 molten salt at 393 K,and this alloy is composed of Ni,MgAl2O4 and AlNi3.The reduction of Ni2+occurs first on the tungsten electrode,followed by the co-deposition of Al3+ and Mg2+.The nucleation process for the alloy follows three-dimensional instantaneous nucleation model.The diffusion coefficient of metal ions in molten salt is about 10-5 cm2·s-1,and it increases with the increase of cathode potential.The kinetic process of Ni2+-La3+ in molten salt were studied using cyclic voltammetry,chronoamperometry and chronopotentiometry methods,while the electrodeposition products were characterized by SEM and XRD.The results show that Ni-La alloy is electrodeposited successfully in the urea-acetamide-NaBr-NiCl2-LaCl3 molten salt at 393 K,and it consisted of Ni,La2O3 and LaNi5 with a cauliflower-like nodules structure.La3+ can not be reduced to La metal alone in the molten salt,but it can be co-deposited with nickel forming Ni-La alloy.The nucleation process of the alloy at 393 K follows the three-dimensional instantaneous nucleation model.The diffusion coefficient of metal ions is in the order of 10-7 cm2·s-1.