1. §7.11 Polarization of electrode
Chapter 7 Electrochemistry
§7.11 Polarization of electrode

2. Why do we concern the irreversible electrochemical processes
Reversible cell correlates electrochemistry with thermodynamics and has great theoretical importance.
However, either electrolytic cell or galvanic cell always works in an irreversible way.

3. 7.11.1. Decomposition voltage and overvoltage
H2O O2 H2 pH
 / V 2 4 6 8 10 12 14
0.401
-0.828
0.000
1.229
Electrolysis of water
decomposition voltage does not depend on pH.
At what voltage can water undergo decomposition?

4. Decomposition voltage:
the minimum potential difference which must be applied between electrodes before decomposition occurs and a current flows.
1.70 V
1.229 V
1.0
2.0
0.0
E / V
I / A
Onset potential
The reversible electromotive force of the cell (Theoretical decomposition voltage) is V.
The effective decomposition voltage is 1.70 V.
A discrepancy of ca. 0.5 V, which is named as overvoltage, exist.

5. 7.11. 2 Thermodynamics of irreversible cell
For reversible cell: Wre = nFEre;
For irreversible cell: Wir = nFEir
For electrolytic cell: Ere < Eir ; E = Eir - Ere > 0
E = (a, ir-c, ir) - (a, re - c, re) = (a, ir - a, re) + (c, re - c, ir)
(a, ir  a, re ) = a
(c, re  c, ir ) = c
E = c + a

6. For galvanic cell: Ere > Eir; E = Ere  Eir > 0
E = (c, re a, re)( c, ir  a, ir) = (c, rec, ir) + (a, ira,re)
(c, re  c, ir ) = c
(a, ir  a, re ) = a
E = c + a

7. c, ir = c, re  c a, ir = a, re + a a, ir = a, re + a
galvanic cell
electrolytic cell
c, ir = c, re  c
a, ir = a, re + a
a, ir = a, re + a
Under irreversible conditions, electrode potential differs from its reversible value, this phenomenon is defined as polarization.
The discrepancy between reversible potential and irreversible potential is termed as overpotential (). By definition, overpotential always has positive value.

8. galvanic cell electrolytic cell c, ir = c, re  c
a, ir = a, re + a
a, ir = a, re + a
a, re
c, re
Ere
Eir
a, ir
c, ir c a
 / V
I / A
a, re
c, re
Ere
Eir
a, ir
c, ir c a
 / V
I / A
The irreversible potential and the irreversible electromotive force of cell depend on the current density imposed.
Polarization cause decrease in electromotive force of galvanic cell and increase in decomposition voltage of electrolytic cell.

9. 7.11.3 Origin of overpotential
1) Resistance overpotential (R)
2) Concentration overpotential (C)
3) Activation overpotential (a)
 = r + d + a
1) Resistance overpotential (R)
Electrode, electrode/solution interface, solution and separator all have resistance.
R = I R
Elimination:
lower the inner resistance

10. i0 = ib = if 2) Concentration overpotential (C) if ib Cu = Cu2+ + 2e­cb
if > ib
Cu  Cu2+ + 2e
ir > re c d c cb
if < ib
Cu2+ + 2e  Cu
ir < re c d
elimination: 1) stir the solution in electroplating and in space battery; 2) discharge the battery with intervals

11. 3) Activation overpotential (a)
Fe3+
Fe2+
If the removal of electron from the electrode is not fast enough, excess charge will accumulate on the electrode’s surface, which results in shift of electrode potential i.e., electrochemical / activation polarizaiton.
Chemical species that can undergo oxidation or reduction on the electrode surface can slow the shift of electrode potential. depolarizer, depolarization

12. 7.11.4 Measurement of overpotential
Conventional three-electrode cell
potentiostat
C.E.
W.E.
R.E.
H2SO4
potentiostat
Polarization circuit
Measurement circuit
C.E.: Counter/auxiliary electrode
W.E.: Working electrode
R.E.: Reference electrode

14. 7.11.5. Hydrogen overpotential
1) Hydrogen polarization and Tafel plot
Polarization curve
If H+ acts as depolarizer
2000
6000
10000
0.0
0.4
0.8
1.2
Black Pt
bright Pt Au Ag Hg C
 / V
j / Am-2 e H+ H
2H+ + 2e  H2

15. At higher polarization > 118 mV, a linear relation exists:
In 1905, Tafel reported the log J ~  curves of hydrogen evolution on different metal surfaces.
log j / A m-2
E / V
0.0
At higher polarization > 118 mV, a linear relation exists:
Tafel equation
a and b are empirical constant, which can be obtained from the Tafel plot.
Tafel plot

17. Values of a and b of different metals
a / V
b / V
black Platinum
0.0
bright Platinum
0.1
0.03
nickel
0.63
0.11
silver
0.95
0.10
zinc
1.24
0.12
mercury
1.40

18. 2) Classification of metal according to a value
Categories a
Metals
Metal with high hydrogen overpotential
Hg(1.41), Pb(1.56), Zn(1.24), Sn(1.20)
Metal with medium hydrogen overpotential
Fe(0.7), Ni(0.63), Cu(0.87)
Metal with low hydrogen overpotential
Pt(0.05), Pd(0.24)

19. H+ H H2 Mechanism of electrode process Interfacial reaction
Heterogeneous reaction H+
Surface region
Bulk solution
Mass transfer
Chem. rxn
Desorption/adsorption H
EC rxn H2
Desorption/ adsorption
electrode

20. 7.11.6. Theories of hydrogen overpotential
The discharge of hydrogen ions on metal surface comprises five steps.
diffusion: H+ diffuses from bulk solution to the vicinity of the double layer
Foregoing step: H+ transfers across the double layer and undergoes configuration changes such as dehydration etc.
Electrochemical step: H3O+ + M + e  M-H + H2O, Volmer reaction, forms adsorbed H atom
Desorption of H atom:

21. Combination desorption (catalytic reaction):
2 M-H  2M + H2 (Tafel reaction)
Electrochemical desorption:
M-H + H3O+ + e-  H2 + M
(Heyrovsky reaction)
5) Succeeding step: diffusion, evolution.
The slowest step will control the overall rate of the electrochemical reaction.
The theories of hydrogen overpotential:
1) The slow discharge theory
2) the slow combination theory

22. 7.11.7. Application of hydrogen overpotential
1) The Way to reduce hydrogen overpotential
Discussion:
According to Tafel equation, how can we lower hydrogen overpotential ?
How can we reduce overpotential of an electrode?

23. (1) Use materials with low a as electrode
For electrolysis of water, in laboratory, we use Pt (a = 0.05) as cathode, while in industry, we use iron (a = 0.7).
Now, Ni-S alloy is used for evolution of hydrogen.
For evolution of oxygen, we now use RuO2 as anodic catalyst.
Electrocatalysis and electrocatalyst
Pt nanoparticles loaded on carbon.

24. (2) Enlarge effective surface area: porous electrode
1) Why do we use platinized platinum electrode?
Its effective area is more than 1000~3000 times larger than that of bright platinum.
2) Porous electrode. In lead-acid battery, porous lead electrode and porous lead dioxide electrode is adopted.
SEM photograph of porous electrode. The particle is in fact aggregate of nanoparticles.

25. (3) Take advantage of hydrogen overpotential
1) Electroplating of active metal from aqueous solution (Pb, Zn, Sn). Why Zn/Zn2+ is a reversible electrode?
2) Corrosion protection: zinc- or tin-plated iron
3) In battery: Pb negative electrode; amalgamated zinc negative electrode in dry-battery. (homogeneity, tension, overpotential)
4) Use lead or lead alloy as cathode materials in electrosynthesis to improve current efficiency.