Page 47 - Handbook of Battery Materials

1.3 Thermodynamics 13

1.3
Thermodynamics
1.3.1
Electrode Processes at Equilibrium

Corresponding to chemical reactions, it is possible to treat electrochemical reactions
in equilibrium with the help of thermodynamics.
As well as determining the potential at standard conditions by means of mea-
surement, it is possible to calculate this value from thermodynamic data [9]. In
addition, one can determine the influence of changing pressure, temperature,
concentration, and so on.
During the determination of standard electrode potentials an electrochemical
equilibrium must always exist at the phase boundaries, for example, electrode/
electrolyte. From a macroscopic viewpoint, no external current flows and no
reaction takes place. From a microscopic viewpoint or on a molecular scale,
however, a continuous exchange of charges occurs at the phase boundaries.
Figure 1.7 demonstrates this fact at the anode of the Daniell element.
The exchange of charge carriers in the molecular sphere at the phase boundary
zinc/electrolyte solution corresponds to an anodic and an equal cathodic current.
These compensate each other in the case of equilibrium.
Three kinds of equilibrium potentials are distinguishable:
1) A metal ion potential exists if a metal and its ions are present in balanced
phases, for example, zinc and zinc ions at the anode of the Daniell element.
2) A redox potential exists if both phases exchange electrons and the electron
exchange is in equilibrium; for example, the normal hydrogen half cell
with an electron transfer between hydrogen and protons at the platinum
electrode.
3) If two different ions are present, only one of which can cross the phase
boundary, which may exist at a semi-permeable membrane, one gets a so-called

-
+2 e
I cathodic
Zn 2+
Zn (s) (aq)
I anodic
-
−2 e
Figure 1.7 Phase boundary metal (zinc)/
electrolyte solution electrolyte solution (zinc sulfate) in equilibrium.