Page 194 - Handbook of Battery Materials

5.4 Electrochemical Reactions 163

black material and not on the platinum. This implies that NiOOH is conductive
and has a lower oxygen overvoltage than platinum oxide. They found that discharge
started at the point contact and that formation of resistive Ni(OH) 2 at this interface
could stop current flow and result in an incomplete discharge. These results provide
a good macroscopic picture of how the electrode works.
This type of mechanism has been considered by Barnard et al.[83].They
postulate the initiation of the charging reaction at the Ni(OH) 2 /current collector
interface with the formation of a solid solution of Ni 3+ ions in Ni(OH) 2 .With
3+
2+
further charging when a fixed nickel ion composition (Ni ) x ·(Ni ) 1−x is
reached, phase separation occurs with the formation of two phases, one with the
2+
3+
composition (Ni ) 1−x ·(Ni ) x in contact with the current collector and the other
2+
3+
with the composition (Ni ) x ·(Ni ) 1−x further out into the active mass. This
scheme is consistent with the observations of Briggs and Fleischman on thick
α-Ni(OH) 2 films [90]. In microscopic observations of cross-sections of partially
charged electrodes, they observed a green layer of uncharged Ni(OH) 2 in front of
the electrode. The central part of the electrode was coated with a black material,
and a thin layer in contact with the current collector had a yellowish metallic luster.
On discharge, the reverse process occurred. It is possible for some of the NiOOH
to be isolated in the poorly conducting matrix of Ni(OH) 2 and not to be discharged.
This has been confirmed in recent in Raman spectroscopy studies in situ [66].
Sometimes two discharge voltage plateaus are seen on nickel oxide electrodes.
Early observations are documented in previous reviews [2, 9]. Normally, nickel oxide
electrodes have a voltage plateau on discharge in the potential range 0.25–0.35 V
vs Hg/HgO. The second plateau, which in some cases can account for up to 50%
of the capacity, occurs at −0.1 to −0.6 V. At present, there is a general consensus
that this second plateau is not due to the presence of a new, less-active, compound
[91–94]. Five interfaces have been identified for a discharging NiOOH electrode
[93]. These are
1) the Schottky junction between the current collector and the n-type NiOOH,
polarized in the forward direction,
2) the p–n junction between Ni(OH) 2 and NiOOH, polarized in the forward
direction,
3) the NiOOH/electrolyte interface,
4) the Ni(OH) 2 /electrolyte interface, and
5) the Schottky junction between the current collector and Ni(OH) 2 , polarized in
the forward direction.

At the beginning of discharge only junctions (1) and (3) are present. As discharge
progresses, junction (5) develops. The passage of current shifts the electrode poten-
tial to more negative values. The hole conductivity of the Ni(OH) 2 increases and a
second discharge plateau appears. A quantitative modeling effort by Zimmerman
[94] supports this hypothesis.