Page 79 - Introduction to Transfer Phenomena in PEM Fuel Cells

68 Introduction to Transfer Phenomena in PEM Fuel Cells
This potential difference is referred to as the standard electrode potential
0
(E ) or the Galvani potential. The tables give the values of this measured
potential difference with respect to a reference electrode (hydrogen electrode
+
H 3O /H 2) whose potential is arbitrarily set to zero. If we have a system
where the two electrodes are in contact with the same electrolyte, the voltage
at the terminals of these electrodes, equal to the difference between the
0
potentials of two electrodes, is denoted as (ΔE ), under normal conditions of
temperature and pressure (0°C and 1 atm or 273.15 K and 101 325 Pa). This
voltage corresponds to an open circuit, therefore to a system at equilibrium.
As soon as the electrons circulate, the system is out of equilibrium and the
0
measured voltage (E) is different from the voltage (E ). This difference is
called the overpotential and it is indicated by the symbol (η) [BAR 05,
BOU 07, HOR 09, SPI 07].
2.4.2. Activation energy

The surface of the electrodes is therefore the site of electrochemical
reactions. These reactions result from the collision of the different molecules
present. The kinetic energy of each molecule is the main source of energy to
initiate a reaction. However, there is a minimum energy that must be reached
in order for two molecules to react: the activation energy (energy threshold
to cross). Part of the energy is lost because it is used to reach the activation
level. The corresponding voltage loss is the activation overpotential
mentioned (η act). After collision, there is formation of an intermediate body
(activated complex) which in turn decomposes to form the reaction products:

∗
AB ⇔ AB  C D [2.60]
+
+
*
where AB is the activated complex.
This activation energy is calculated from the binding enthalpies. For the
reaction between hydrogen and oxygen, for example, the necessary energies
are those of dissociation of a molecule of hydrogen and a half-molecule of
oxygen. From the tables of binding enthalpy values, we obtain:

⋅
H → 2H with H436kJ mol − 1 [2.61]
Δ=
2