EQCM contributions to the reactions of the nickel oxide electrode

Abstract

Films of nickel hydroxide (thickness 0.35 μm) were electrochemically deposited on smooth and rough Au electrodes of an electrochemical quartz crystal microbalance. The films consisted of α-Ni(OH)₂ (diffractogram) and contained traces of NO₃⁻ and CO₃²⁻ (IR). They were cycled (10 mV s⁻¹) in KOH solutions (1, 0.1, 0.01 M) between 0 and 0.6 V as well as between − 0.3 and 0.6 V s. Hg/HgO iss. Cyclovoltammograms and mass flux curves were used to study the redox processes. As the mass flux peaks of the redox reactions of Bode's scheme (α^II⇌γ^III/IV and β^II⇌β^III) have different signs and different redox potentials (E_α/γ<E_β/β), they can easily be recognized. A new form of β^III (β^III*) was discovered, which forms on oxidation of α^II, when not enough KOH is present to be intercalated into γ^III/IV, i.e. at low KOH concentrations and near the Au/film interface whereas near the film/solution interface α^II transforms into β^II (ageing). β^III* is supposed to contain O²⁻ defects that form from the foreign anion sites (substitutional defects) in α^II. This explains its low oxidation potential (E_α/β<E_α/γ) and its very slow reduction to β^II. The reduction process was studied by mass controlled dissolution of the films in H₂SO₄ and by extending the lower vertex potential of the scan range from 0 to − 0.3 V, so that reduction peaks of β^III* (−0.2 V) could be observed. This second reduction region, which lies about 0.55 V below the main one, provides an explanation of the second discharge potential of Ni-accumulators.