In situ studies of the oxidation of nickel electrodes in alkaline solution

Results of X-ray scattering measurements using synchrotron radiation on a Ni(1 1 1) single crystal electrode in 1 M KOH are presented. On first immersion at the open circuit potential (OCP) the samples are shown to have a “duplex” two-layer oxide film, composed of an amorphous hydroxide outer layer separated from the Ni substrate by NiO 1–2 layers thick that display a NiO(1 1 1) orientation. The NiO layer is thinner than that for the air-formed oxide implying that a proportion of the NiO is converted to Ni(OH)2 following immersion in 1 M KOH. This duplex oxide formed at the OCP is not removed at negative potentials up to onset of hydrogen evolution implying that more extensive reduction within hydrogen evolution is necessary to reduce the oxide. Cycling the potential over the Ni(OH)2/NiOOH redox peaks increases the thickness of both the inner oxide (NiO) and outer hydroxide layers. It is proposed that the thickening of this compact inner oxide leads to the self-limiting oxide growth observed during potentiodynamic cycling through the Ni(OH)2/NiOOH redox peaks. The crystalline NiO(1 1 1) present in the native oxide is removed by potential cycling over the redox peaks. The oxide and hydroxide layers electrochemically formed from 1 M KOH show no sign of crystalline ordering.