Page 47 - Lindens Handbook of Batteries

2.4 PRINCIPLES OF OPERATION

TABLE 2.2 Standard Reduction Potentials (Aqueous Solutions) of Electrode Reactions at 25°C
Electrode reaction E°/N Electrode reaction E°/N
+
Li + e  Li -3.045 CuCl + e  Cu + Cl - 0.121
-
+
K + e  K -2.925 AgCl + e  Ag + Cl 0.2223
+
-
Na + e  Na -2.714 AgCl + e  Ag + Cl
(seawater, pH 8.2) 0.2476
3
Al + + 3e  Al −1.67 Hg Cl + 2e  2Hg + 2Cl - 0.2682
2
2
-
H O + e  ½ H + OH - 0.8277 Hg Cl + 2e  2Hg + 2Cl
2
2
2
2
(satd KCl (SCE)) 0.2412
-
H O + e  ½H + OH (seawater, pH 8.2) 0.5325 O +2H O + 4e  4OH - 0.401
2
2
2
2
2+
-
Ni(OH) + 2e  Ni + 2OH - −0.72 Cu + Cl + e  CuCl 0.559
2
+
+
O + H + e  HO 2 −0.046 O + 4H + 4e  2H O
2
2
2
(pure water, pH 7) 0.815
+
+
2H + 2e  H 2 0.000 O + 4H + 4e  2H O 1.229
2
2
HgO + H O + 2e  Hg + 2OH - 0.0977 Cl + 2e  2Cl - 1.358
2 2
The change in the standard Gibbs energy ∆G° of this reaction is expressed as
∆G° = -nFE° (2.5)
-1
where F = constant known as the Faraday (96,487 coulombs equiv ) and E° = standard electromo-
tive force. Selected values of standard electrode potentials are given in Table 2.2, and additional
values can be found in Appendix B.
When conditions are other than in the standard state, the voltage E of a cell is given by the Nernst
equation,
c
E = E - o RT ln aa d (2.6)
C D
a b
nF aa
A B
-1
-1
where a = activity of relevant species, R = gas constant (8.314 J K mol ), and T is the absolute
i
temperature in Kelvin.
The change in Gibbs energy ∆G° of a cell reaction is the driving force which enables a battery
to deliver electrical energy to an external circuit. The measurement of the electromotive force, inci-
dentally, also makes available data on changes in free energy, entropies, and enthalpies together with
activity coefficients, equilibrium constants, and solubility products.
1
Direct measurement of single (absolute) electrode potentials is considered practically impossible.
To establish a scale of half-cell or standard potentials, a reference potential “zero” must be established
against which single electrode potentials can be measured. By convention, the standard potential of
+
the H /H (aq) reaction is taken as zero and all standard potentials are referred to this potential. Table
2
2.2 and Appendix B list the standard potentials of a number of anode and cathode materials.
2.3 ELECTRODE PROCESSES
Reactions at an electrode are characterized by both chemical and electrical changes and are hetero-
geneous in type. Electrode reactions may be as simple as the reduction of a metal ion and incorpora-
tion of the resultant atom onto or into the electrode structure. Despite the apparent simplicity of the
reaction, the mechanism of the overall process may be relatively complex and often involves several
steps. Electroactive species must be transported to the electrode surface by migration or diffusion
prior to the electron transfer step. Adsorption of electroactive material may be involved both prior to
and after the electron transfer step. Chemical reactions may also be involved in the overall electrode