Page 193 - Handbook of Battery Materials

162 5 Nickel Hydroxides

electrode materials, such as AgO or PbO, the nickel hydroxide electrode is a good
catalyst for oxygen evolution. Toward the end of charge, oxygen evolution occurs in
all nickel batteries, and during constant voltage hold in charge mode self-discharge
occurs via a couple involving the reduction of NiOOH and the oxidation of water to
oxygen.
The self-discharge process has made experimental determination of the reversible
potential of the Ni(OH) 2 /NiOOH couple very difficult. A major advance was the
realization by Bourgault and Conway that the open-circuit potential of a charged
nickel oxide electrode was a mixed potential, not a true equilibrium potential [82],
and was the result of two processes: the discharge of NiOOH and oxygen evolution.
They devised an extrapolation technique for the determination of the open-circuit
potential of Ni(OH) 2 as a function of charge state. Later, the work was expanded
by Barnard and his co-workers to include oxidation of both the α/γ and the
β/β couples [83]. The open-circuit potentials depended on pretreatment, such as
formation cycles and aging in concentrated KOH electrolytes. The β/β couples had
open-circuit potentials in the range of 0.44–0.47 V vs Hg/HgO, whereas the α/γ
couples had values in the range 0.39–0.44 V. In cyclic voltammetry experiments,
the respective anodic and cathodic peaks for the α/γ couple occur at 0.43 and 0.34
V. For the β/β couple, the peaks are at 0.50 and 0.37 V. The reversible potentials of
−
the β/β couple are essentially invariant with KOH concentration, whereas those
−
of the α/γ couple vary with OH concentration, and aging of α-NiOH) 2 reduces
−
the OH dependence of the reversible potential [84]. This is due to the conversion
to the β/β couple. The reactions for both the β/β and the α/γ couples are highly
reversible. Barnard and Randell, in a simple experiment, showed that β-NiOOH
could oxidize α-Ni(OH) to γ -NiOOH [85]. This reaction is possible during cyclic
voltammetry on α-Ni(OH) 2 thin film electrodes in KOH electrolyte, and some of
the α material gets transformed to the β form. This could account for the negative
drift that is seen in the anodic peaks in the early stages of cycling [66]. Reactions of
this type can introduce distortions and features in cyclic volammograms that are
difficult to interpret.

5.4.2
Nature of the Ni(OH) 2 /NiOOH Reaction

The Ni(OH) 2 /NiOOH reaction is a topochemical type of reaction that does not
involve soluble intermediates. Many aspects of the reaction are controlled by the
electrochemical conductivity of the reactants and products. Photoelectrochemical
measurements [86, 87] indicate that the discharged material is a p-type semi-
conductor with a bandgap of about 3.7 eV. The charged material is an n-type
semiconductor with a bandgap of about 1.75 eV. The bandgaps are estimates from
absorption spectra [87].
The simple experiments of Kuchinskii and Ershler have provided great insights
into the nature of the Ni(OH) 2 /NiOOH reaction [88, 89]. They investigated oxidation
and reduction of a single grain of Ni(OH) 2 with a platinum point contact. On
charge, the Ni(OH) 2 turned black and oxygen was evolved preferentially on the