Page 187 - Handbook of Battery Materials

156 5 Nickel Hydroxides

(<3%) of nitrate ions [43]. TGA indicates that the adsorbed water is removed
◦
between 50 and 90 C, whereas the intercalated water is removed between 90 and
◦
180 C [30].
Pandya et al. have used extended X-ray ascription fine structure (EXAFS) to study
both cathodically deposited α-Ni(OH) 2 and chemically prepared β-Ni(OH) 2 [44].
Measurements were done at both 77 and 297 K. The results for β-Ni(OH) 2 are
in agreement with the neutron diffraction data [22]. In the case of α-Ni(OH) 2 ,
they found a contraction in the first Ni–Ni bond distance in the basal plane.
The value was 3.13 ˚ A for β-Ni(OH) 2 and 3.08 ˚ A for α-Ni(OH) 2 . The fact that a
similar significant contraction of 0.05 ˚ A was seen at both 77 and 297 K when using
two reference compounds (NiO and β-Ni(OH) 2 ) led them to conclude that the
contraction was a real effect and not an artifact due to structural disorder. They
speculate that the contraction may be due to hydrogen bonding of OH groups in the
brucite planes with intercalated water molecules. These ex situ results on α-Ni(OH) 2
were compared with in situ results in 1 mol L −1 KOH. In the ex situ experiments,
the α-Ni(OH) 2 was prepared electrochemically, washed with water, and dried in
vacuum. In the in situ experiment, the hydroxide film, after preparation, was simply
rinsed without drying and immediately immersed in the cell containing 1 mol L −1
KOH. The coordination numbers for Ni–O for the in situ samples were consistently
higher. The significance of this is not clear: it might suggest some water association
with nickel, as was postulated by Kober [24, 27].
Raman spectroscopy results indicate that the structure of α-Ni(OH) 2 is very
dependent on how it is prepared [32, 33, 45]. Data on chemically prepared
[32, 33], cathodically deposited [32, 33, 45], and electrochemically reduced γ -NiOOH
have been obtained [32, 33, 45]. There are differences in all the spectra, both in the
lattice modes and the O–H stretching modes. The shifts in the lattice modes for
the reduced γ -NiOOH may be due to this material having an oxidation state higher
than two. The changes in the O–H stretching modes may be due to changes in
water content and hydrogen bonding.
Figlarz and his co-workers have suggested that the formula Ni(OH) 2 ·nH 2 Ois
not the correct one for α-Ni(OH) 2 [46, 47]. They studied α-Ni(OH) 2 materials
made by precipitation of the hydroxide by the addition of NH 4 OH to solutions
of various nickel salts. In addition to Ni(NO 3 ) 2 and NiCO 3 ,theyusednickelsalts
with carboxylic anions of various sizes. They found that the interlaminar distance
in the α-Ni(OH) 2 depended on the nickel salt anion size. For instance, when
nickel adipate was used, the interlaminar distance was 13.2 ˚ A. Infrared studies
of α-Ni(OH) 2 precipitated from Ni(NO 3 ) 2 indicated that NO 3 − was incorporated
into the hydroxide and was bonded to Ni. They suggested a model based on
hydroxide vacancies and proposed a formula Ni(OH) 2−x A y B z ·nH 2 O, where A and
B are mono- or divalent anions and x = y + 2z. Chemical analysis of α-Ni(OH) 2
precipitated from Ni(NO 3 ) 2 indicates OH vacancies in the range of 20–30%.
α-Ni(OH) 2 is unstable in water and is slowly converted to β-Ni(OH) 2 . Transmis-
sion electron micrographs of the reactants and products indicate that the reaction
proceeds via the solution [42, 45]. In concentrated KOH, the reaction is much more
rapid and the product has a smaller particle size. For instance, the α-Ni(OH) 2