Page 192 - Handbook of Battery Materials
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5.4 Electrochemical Reactions  161


               the charge in the brucite layer; this part of the structure reverts to the β-Ni(OH) 2
               structure and the intercalated water and anions are expelled from the lattice. With
               this, there is a concomitant irreversible contraction of the interlayer spacing from
               7.80 to 4.65 ˚ A [72].


               5.4
               Electrochemical Reactions

               5.4.1
               Overall Reaction and Thermodynamics of the Ni(OH) 2 /NiOOH Couple

               In normal battery operation, several electrochemical reactions occur on the nickel
               hydroxide electrode. These are the redox reactions of the active material, oxygen
               evolution, and, in the case of nickel–hydrogen and nickel–metal hydride batteries,
               hydrogen oxidation. In addition, there are parasitic reactions such as the corrosion
               of nickel current collector materials and the oxidation of organic materials from
               separators. The initial reaction in the corrosion process is the conversion of Ni to
               Ni(OH) 2 .
                Because of the complexity of the redox reactions, they cannot be conveniently
               presented in a Pourbaix pH-potential diagram. For battery applications, the revised
               diagram given by Silverman [80] is more correct than that found in the Pourbaix
               Atlas [81]. The diagram is shown in Figure 5.5.
                The respective literature values for the free energy of formation of Ni(OH) 2 ,
               NiOOH, H 2 O, and HgO are −78.71, −109.58, −56.69, and −13.98 kcal mol −1  [80].
               The calculated Ni(OH) 2 /NiOOH reversible potential is 0.41 V vs Hg/HgO, and
               the reversible oxygen potential is 0.30 V vs Hg/HgO. Unlike other battery positive

                  2.5
                  2.0
                                  NiOOH
                  1.5
                EMF (V) vs. SHE at 273 K  0.5  Ni ++  Ni(OH) 2  Ni(OH) 3 -
                  1.0



                  0.0
                 -0.5

                 -1.0
                            Ni
                 -1.5

                 -2.0
                    -2  0  2  4  6  8  10  12  14  16
                                                   Figure 5.5  The modified Pourbaix dia-
                                 pH                gram for Ni [80].
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