ELECTROCHEMICAL METHODS Fundamentals and Applications - Allen.J.Bard

Chapter 1. Introduction and Overview of Electrode Processes
does not exist. Even though the open-circuit potential of the cell is not available from
thermodynamic data, we can place it within a potential range, as shown below.
Let us now consider what occurs when a power supply (e.g., a battery) and a microammeter
are connected across the cell, and the potential of the Pt electrode is made
more negative with respect to the Ag/AgBr reference electrode. The first electrode reaction
that occurs at the Pt is the reduction of protons,
2H + + 2e-*H 2
(1.1.7)
The direction of electron flow is from the electrode to protons in solution, as in Figure
1.12a, so a reduction (cathodic) current flows. In the convention used in this book, cathodic
currents are taken as positive, and negative potentials are plotted to the right. 3
As
shown in Figure 1.1.4, the onset of current flow occurs when the potential of the Pt electrode
is near E° for the H + /H 2
reaction (0 V vs. NHE or -0.07 V vs. the Ag/AgBr electrode).
While this is occurring, the reaction at the Ag/AgBr (which we consider the
reference electrode) is the oxidation of Ag in the presence of Br~ in solution to form
AgBr. The concentration of Br~ in the solution near the electrode surface is not changed
appreciably with respect to the original concentration (1 M), therefore the potential of the
Ag/AgBr electrode will be almost the same as at open circuit. The conservation of charge
requires that the rate of oxidation at the Ag electrode be equal to the rate of reduction at
the Pt electrode.
When the potential of the Pt electrode is made sufficiently positive with respect to the
reference electrode, electrons cross from the solution phase into the electrode, and the ox-
Pt/H + , ВГ(1 M)/AgBr/Ag
Cathodic
1
1.5
1
I
/ \
1 \
/ Onset of Br"
/ oxidation on Pt
Onset of H +
reduction on Pt.
\
i
L
\y
0
: /
-0.5
Cell Potential
Anodic
Figure 1.1.4 Schematic current-potential curve for the cell Pt/H + , Br~(l M)/AgBr/Ag, showing
the limiting proton reduction and bromide oxidation processes. The cell potential is given for the Pt
electrode with respect to the Ag electrode, so it is equivalent to £ Pt (V vs. AgBr). Since ^Ag/AgBr =
0.07 V vs. NHE, the potential axis could be converted to E Pt (V vs. NHE) by adding 0.07 V to each
value of potential.
3 The convention of taking / positive for a cathodic current stems from the early polarograhic studies, where
reduction reactions were usually studied. This convention has continued among many analytical chemists and
electrochemists, even though oxidation reactions are now studied with equal frequency. Other
electrochemists prefer to take an anodic current as positive. When looking over a derivation in the literature
or examining a published i-E curve, it is important to decide, first, which convention is being used (i.e.,
"Which way is up?").