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 (αII⇌γIII/IV and βII⇌βIII) have different signs and different redox potentials (Eα/γ<Eβ/β), they can easily be recognized. A new form of βIII (βIII*)
was discovered, which forms on oxidation of αII, when not enough KOH is present to be intercalated into γIII/IV, i.e. at low KOH concentrations and near the Au/film interface whereas near the film/solution interface αII transforms into βII (ageing). βIII* is supposed to contain O2− defects that form from the foreign anion sites (substitutional defects) in αII. This explains its low oxidation potential (Eα/β<Eα/γ) and its very slow reduction to βII. 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 βIII* (−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.
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