188
4 – Electrode reactions
Table 42 – Anodic overpotential of electrode as a function of current density.
i [mA cm
−2
] 2
5
10
20
35
50
85
135 200 300 400
η [V]
0.013 0.029 0.052 0.086 0.120 0.145 0.182 0.217 0.247 0.277 0.299
a. Propose a reaction model that describes the behavior of an anode with
two electrons available for the charge-transfer reaction.
b. When the oxidation kinetics is limited by the charge-transfer step, the
characteristic current density i and overpotential η are related by the
Butler-Volmer equation:
i i e
e
0
RT
n F
RT
(1 ) n F
=
−
η
η
−
a
a
−
`
j
2 Determine the exchange-current density i 0 and the anodic transfer coefficient α.
2 Give the experimental formula for the current density as a function of
the overpotential obtained for this electrode.
c. On a single plot, graph the curves η = f (log i) representing the experimental points and the formula obtained in 2(b). Comment.
d. What type of formula can we use for high current densities? Determine
this formula.