ELECTRICAL MEASURMENT
26
0 -0,1
-0,05
0
0,05
0,1
0,15
0,2
0,25
0,3
-0,002 Overpotential, V
Figure 20. Current density vs overpotential for a system with i0=0.0001 A/m2, n=1, αa = αc =0.5, T=298 K
In order to study the exponential regime we may plot log |i| versus overpotential in a so-called Tafel plot. A Tafel plot will have a curved region at low current densities (linear regime) and straight lines at the anodic and cathodic sides at high current densities (Tafel regime). By assuming that the anodic or cathodic current dominate, we can rearrange the Butler-Volmer equation into log i a = log i 0 +
α a ne η 2.3kT a
(22.) log(−ic ) = log i0 +
α c ne 2.3kT (−η c ) (23.) The slopes of the plots in the anodic and cathodic directions are thus a function of the transfer coefficient and the number of electrons in the rate-limiting charge transfer reaction, and may thus be used to obtain one or both of these. -0,3
-0,2
-0,1
0,1 0
0,1
0,2
0,3
|i|, A/m2
0,01
0,001
0,0001
0,00001
0,000001
0,0000001 Ove rvoltage , V
Figure 21. Tafel plot for the same parameters as in the previous plot (i0 = 0.0001 A/m2)
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