Electrochemistry of nickel(111) in alkaline electrolytes

The electrochemical properties of Ni(111) prepared and characterized in ultrahigh vacuum (UHV) by surface analytical techniques have been examined by cyclic voltammetry in 0.1 M KOH electrolytes using a UHV/ electrochemical cell transfer system. For most of the experiments, a protecting c(4×2)-CO overlayer was adsorbed on the Ni(111) surface prior to the transfer, to decrease the extent of contamination with adventitious gas-phase dioxygen, and later removed by electrochemical oxidation. Coulometric analyses of the cyclic voltammetry curves for both bare and CO-protected Ni(111) surfaces in the range between 1.2 and 1.5 V vs RHE indicate that the electrochemically-formed, redox-active hydrous oxide layer involves a charge equivalent to 1 (for the first few cycles) to about 1.5 (for 20 or more cycles) monolayers of Ni(111) metal atoms. In analogy with the behavior observed for nickel electrodes prepared by more conventional means, extensively cycled hydrated nickel oxide films displayed a shoulder on the negative side of the NiOOH reduction peak. Bare Ni(111) surfaces were found to yield much sharper redox peaks than their CO-protected counterparts. This effect has been ascribed to the presence of small amounts of oxygen on the unprotected Ni(111) surfaces (as detected by AES and TPD in independent experiments) that serve as nucleation sites for the growth of hydrated nickel oxide domains, which are larger than those formed on CO-protected surfaces.