Investigation On Cathode & Anode Reaction Mechanism Of Nickel Electrodeposition In Ammonia Complex

Compared with the traditional nickel electrodeposition techniques, nickel electrodeposition carried out in leaching solution containing ammonia and chloride has great priorities and characteristics. Based on the nickel electrodeposition process, the nickel electrodeposition mechanism, the electrochemical nucleation of nickel on vitreous carbon, the redox process of black nickel formed on anode, the nitrogen evolution on anode and electrocatalytic activity for nitrogen evolution of Ti based IrO2 anodes were investigated systematically.In the third chapter of this thesis, the nickel electrodeposition process in leaching solution containing ammonia and chloride was summarized, and the electrochemical behaviour of nickel electrodeposition in ammonia complex bath was investigated by measuring polarization curves. The effects of total nickel, ammonia, ammonium chloride concentrations in the electrolyte as well as its anion species and temperature were studied systematically. What's more, the electrodeposition mechanism of nickel in leaching solution containing ammonia and chloride was elucidated emphatically.It is demonstrated by measuring steady state polarization curves that Ni(NH3)2+ is the direct discharge ion within ammonia concentration range from 1.25 to 2.75 mol L-1. The electrochemical impedance spectroscopy of nickel electrodeposition indicates that nickel electrodeposition occurs in two steps, the medium frequency inductive loop is ascribed to the relaxation of the electrode coverage by an adsorbed intermediate such as NiOHads, the low frequency capacitive loop may be due to the inhibition of nickel electrodeposition by adsorbed hydrogen. The mechanism and equivalent circuit of nickel electrodeposition were proposed on the basis of the analysis of electrochemical impedance spectroscopy. Meanwhile, the kinetics law of nickel electrodeposition was investigated by means of steady-state polarizationcurves.In the fourth chapter of this thesis, the initial stages of electrocrystallization of nickel on vitreous carbon from a leaching solution containing ammonia and chloride were illustrated in terms of cyclic voltammetric and chronoamperometric techniques. The results show that the deposition of nickel on the substrate do not undergo UPD process, but undergoes nucleation process. In the experimental conditions, the electrocrystallization of nickel follows the mechanism of three dimensional progressive nucleation and growth. By analyzing of the potentiostatic transients, the diffusion coefficient D of the depositing nickel ions and saturated nucleus number density Nsat were estimated, the effects of applied potential on nucleation and growth ware also discussed.In the fifth chapter of this thesis, the cyclic voltammetry, and galvanostatic reduction and electrochemical impedance spectroscopy techniques were used to investigate the redox process and reaction mechanism of black nickel formed on anode. Moreover, the X-ray diffraction technique was used to examine black nickel species. The results show that the formation process of black nickel is quasi-reversible, it is involved in pre-adsorption transformation process( CE ), ie Ni2 + is dissociated firstly from ammonia complex, and then combines with OH" into Ni(OH)2 particles, which are oxidized on anode. During the anodic process, Ni(OH)2 which on further charge or overcharge forms NiOOH, and NiOOH which on self-discharge with H2O forms a-Ni(OH)2. This reaction process is probably defined as self-catalytic reaction. It is demonstrated by XRD and chemical analysis that high valent compounds such as Ni3O4, NiO2, Ni2O3 besides-NiOOH, y-NiOOH may be formed on electrode surface. The electrochemical impedance spectroscopy of anodic reaction shows that the formation of black nickel was mainly controlled by electrochemical reaction at low anodic potential, and is mainly controlled by electrochemical reaction and diffusion at high anodic potental. With the increasing of anodic potential., the formation of black nickel is still controlled by electrochemical re...