EQCM contributions to the reactions of the nickel oxide electrode

Films of nickel hydroxide (thickness 0.35 mum) were electrochemically depos ited on smooth and rough Au electrodes of an electrochemical quartz crystal microbalance. The films consisted of alpha -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 vs. Hg/HgO iss. Cyclovoltammograms and mass flux curves were used to study the redox processes. As the mass flux peaks of the redox reaction s of Bode's scheme (alpha (II)reversible arrow gamma (III/IV) and beta (II) reversible arrow beta (III)) have different signs and different redox poten tials (E-alpha/gamma < E-<beta>/beta), they can easily be recognized. A new form of beta (III) (beta (III)*) was discovered, which forms on oxidation of alpha (II), when not enough KOH is present to be intercalated into gamma (III/IV), i.e. at low KOH concentrations and near the Au/film interface wh ereas near the film/solution interface alpha (II) transforms into beta (II) (ageing). beta (III)* is supposed to contain O2- defects that form from th e foreign anion sites (substitutional defects) in aII. This explains its lo w oxidation potential (E-alpha/beta < E-<alpha>/gamma) and its very slow re duction to beta (II). The reduction process was studied by mass controlled dissolution of the films in H2SO4 and by extending the lower vertex potenti al of the scan range from 0 to-0.3 V, so that reduction peaks of beta (III) * (-0.2 V) could be observed. This second reduction region, which lies abou t 0.55 V below the main one, provides an explanation of the second discharg e potential of Ni-accumulators.