
185Electrochemical Kinetics
The plot of η versus j, called the Tafel plot, is a straight line with inter-
section a and slope b (Tafel slope). The ratio of Tafel slope and current
density has units of resistance. In the Tafel kinetic region, the charge
transfer resistance (R
ct
) is dened by
R
j
b
j
ct
d
d
η
.
(5.90)
Thus, in the Tafel kinetic region, R
ct
decreases inversely with j or, in other
words, η increases with an increase of j for a cathodic reaction.
Thus, experimentally, j
0
and α can be determined using a Tafel plot.
The Tafel form of equation holds if the reverse reaction contributes to
less than 1% of the current density or
e
–αFη/RT
/e
βFη