Insights into electrodeposition process of nickel from ammonium chloride media with speciation analysis and in situ synchrotron radiation X-ray imaging

The electrodeposition processes of nickel in ammonium chloride media were studied in combination of speciation analysis and in situ synchrotron radiation X-ray imaging. The X-ray absorption spectra indicated that the nickel(II) species present an octahedral configuration independent on aqueous pH. The UV-vis spectra and fraction-pH diagram showed that the dominant species is Ni(H2O)62+, Ni(NH3)m(H2O)6-m2+(m = 2, 3 and 4) and Ni(NH3)62+ in the electrolytes of pH 3, 7.5 and 10, respectively, and chloride ions are not bonded to the inner coordination sphere of nickel species. The electrochemical behaviors and nucleation mechanism of nickel(II) species are dependent on the solution pH and species. The deposited nickel morphology and bubble evolution at the interface of electrode/electrolyte were clearly elucidated by the X-ray real-time images. The results indicate that the inner coordination sphere of nickel species can induce the morphology, growth and adherence processes of the deposited nickel. The competition process between nickel reduction and hydrogen evolution has also been discussed at the different applied potentials. The mixed coordination species and the higher applied potentials are beneficial for the formation of the well-adhered and dense nickel deposits.