Abstract
A theoretical investigation was made on the anodic formation of the higher-valence oxides on nickel from the standpoint of the electrochemical thermodynamics. On the electrode of Ni|compact NiO|H2O polarized with a constant anodic current the potential difference in NiO increases in proportion to the film thickness. This potential difference can be expressed in the form on EII=(Ni⁄NiO)μ\ominus−(NiO⁄H2O)μ\ominus, and the chemical potential of the dissolved nickel atom in NiO is given by (NiO⁄H2O)μNi=(Ni)μNi−2FEII at the phase boundary of NiO|H2O. (NiO⁄H2O)μNi decreases with the increase of EII until it reaches the critical value of (NiO⁄H2O)μNi≤(Ni)μNi+4Ac, where Ac is the affinity of the disproportionation reaction (c): NiO→\dfrac14Ni3O4+\dfrac14Ni. This reaction can proceed only when (NiO⁄H2O)μNi becomes less than the critical value shown above. Therefore, beyond the critical electrode potential which is given by E(NiO⁄Ni3O4)=Ea+4Ac⁄2F, the surface film becomes a double oxide film composed of NiO and Ni3O4.The equilibrium potential of Ni|NiO|Ni3O4|H2O electrode can be given by the equilibrium potential for the redox reaction of (f): 3NiO+H2O→Ni3O4+2H++2e, which is also equal to E(NiO⁄Ni3O4). When the potential difference in Ni3O4 exceeds the critical value at which the disproportionation reaction (d): Ni3O4→\dfrac43Ni2O3+\dfrac13Ni occurs, the surface film changes again to a triple oxide film of NiO|Ni3O4|Ni2O3.
