electrolysis

In the NiI2 solution, NiI2 is dissociated to give Ni²⁺(aq) and I⁻(aq) ions, while water is slightly ionized to give H⁺(aq) and OH⁻(aq) ions.

Ni²⁺(aq) and H⁺(aq) migrate to the cathode. Ni²⁺(aq) ion is preferentially discharged to give Ni(s) deposited on the cathode. This is because the concentration of Ni²⁺(aq) ions is much higher than that of H⁺(aq) ions (the concentration of Ni²⁺(aq) ions is 1 M, while the concentration of H⁺(aq) in the neutral solution is 0.0000001 M).