Oxygen overpotential

1. Oxygen overvoltage
Dr. Mithil Fal Desai
Shree Mallikarjun and Shri Chetan
Manju Desai College Canacona Goa

2. Understanding hydrogen overpotential
Electrode
H+
(aq)
e-
½H2(g)
Step 1.
Reduction of H+ ions to form H atoms
H+
(aq) + e– → H
Step 2.
Adsorption of H atoms on the cathode surface.
Step 3.
Combination of adsorbed atoms to give
hydrogen molecules
2H  H2(g)

3. The overvoltage is the difference between the
theoretical cell voltage and the actual voltage
that is necessary to cause electrolysis.
Overvoltage/Overpotential
Oxygen Overvoltage
The overvoltage is the difference between the
theoretical oxygen reduction potential (+1.23 V)
experimentally observed electrode potential.

4. H2 O(l)  ½ O2 (g) + 2H+
(aq) + 2e-
½ O2 (g) + 2H+
(aq) + 2e-  H2O(l)
Oxidation and reduction  oxygen
Oxidation
Reduction

5. Electrochemical series (E°)
½ F2(g) + e-  F-
(aq)
(MnO4)-
(aq) + 8H+
(aq) + 5e-
(g)  Mn2+
(aq) + 4H2O(l)
½ Cl2(g) + e-  Cl-
(aq)
½ O2 (g) + 2H+
(aq) + 2e-  H2 O(l)
Fe3+
(aq) + e-  Fe2+
(aq)
H+
(aq) + e-  ½ H2(s)
Zn2+
(aq) + 2e- Zn(s)
Li+
(aq) + e-
(g)  Li(s)
(E° in V)
+2.87
+1.51
+1.36
+1.23
+0.77
0
-0.76
-3.04
Redox couple

6. An Overpotential
ELECTRODE
Concentration
Distance from electrode surface

7. Overvoltage/Overpotential
Electrode
e-
H2O(l)
H+
(aq)
½ O2(g)
H+
(aq)
e-