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What are we going to learn?
Concepts: overvoltage, polarization, overpotential; concentration/diffusion
polarization, electrochemical polarization
Method: overpotential measurement;
Rules: (1) polarization direction;
(2) Tafel equation;
Model: exchange equilibrium
Theoretical treatment: Master equation, Butler-Volmer equation
Applications: electrolysis, battery, corrosion protection
§7.11 Polarization of electrode
7.11.0 Introduction
§7.11 Polarization of electrode
7.11.0 Introduction
Electrode process: Fast equilibrium and rate-determining step
Diffusion—concentration polarization;
Electrochemical reaction – electrochemical polarization
§7.11 Polarization of electrode
7.11.0 Introduction
Electrochemical kinetic parameters:
1/ mol s ni nFr nFkcj
A A A
,0
0exp( ) exp( )cB
G nFk T nFk k
h RT RT
Reaction rate: current density
Activation energy: overpotential
7.11.1. Decomposition voltage and overvoltage
Electrolysis of waterReversible decomposition
voltage
§7.11 Polarization of electrode
Effective decomposition
voltage
The reversible electromotive force of the cell (Theoretical decomposition voltage) is
1.229 V. The effective decomposition voltage is 1.70 V. A discrepancy of ca. 0.5 V,
which is named as overvoltage, exist.
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
7.11.1. Decomposition voltage and overvoltage
§7.11 Polarization of electrode
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
E = c + a
(c, re c, ir ) = c
c,ir c,re c a,ir a,re a
§7.11 Polarization of electrode
For galvanic cell:
Ere > Eir; E = Ere Eir > 0
E = (c, re a, re)( c, ir a, ir)
= (c, rec, ir) + (a, ira,re)
E = c + a
(c, re c, ir ) = c (a, ir a, re ) = a
c,ir c,re c a,ir a,re a
Galvanic cell Electrolytic cell
c, ir = c, re c
a, ir = a, re + a
c, ir = c, re c
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.
7.11.2 Thermodynamics of irreversible cell
§7.11 Polarization of electrode
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.
Galvanic cell Electrolytic cell
c, ir = c, re c
a, ir = a, re + a
c, ir = c, re c
a, ir = a, re + a
7.11.2 Thermodynamics of irreversible cell
§7.11 Polarization of electrode
7.11.3 Origin of overpotential
1) Resistance overpotential (R)
2) Concentration overpotential (C)
3) Activation overpotential (a)
1) Resistance overpotential (R)
Electrode, electrode/solution interface, solution and separator all have
resistance.
Elimination: How can we lower the inner resistance of a cell?
R = I R
= R + D + A
§7.11 Polarization of electrode
2) Concentration/diffusion overpotential (D)
i0 = ib = if
2+
0
Culn
RTa
nF
7.11.3 Origin of overpotential
§7.11 Polarization of electrode
Exchange current
Electrochemical equilibrium
Surface concentration
Bulk concentration
2+Culn sRT
anF
2+
2+
Cu
0
Cu
ln
saRT
nF a
elimination: 1) stir the solution in electroplating and in space battery; 2)
discharge the battery with intervals
2+
2+
Cu
0
Cu
ln
saRT
nF a
2) Concentration/diffusion overpotential (D)
7.11.3 Origin of overpotential
§7.11 Polarization of electrode
3) Activation/Electrochemical overpotential (A)
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 polarization.e
e
e
e
e
e
e
Fe3+
Fe2+
Depolarizer, depolarization: chemical species that can
undergo oxidation or reduction on the electrode surface can
slow the shift of electrode potential.
7.11.3 Origin of overpotential
§7.11 Polarization of electrode
7.11.4 Measurement of overpotential
W.E.: Working electrode
R.E.: Reference electrode
C.E.: Counter/auxiliary electrode
Conventional three-electrode cell
potentiostat
C.E. W.E. R.E.
H2SO4
potentiostat
Polarization circuit
Measurement circuit
§7.11 Polarization of electrode
7.11.5. Hydrogen overpotential
If H+ acts as depolarizer
e
e
e
e
e
e
e
H+
H 2000
6000
10000
0.00.40.81.2
Black Pt
bright PtAu
Ag
HgC
/ V
j / Am-2
Polarization curve
2H+ + 2e H2
1) Hydrogen polarization and Tafel plot
§7.11 Polarization of electrode
In 1905, Tafel reported the log J ~ curves
of hydrogen evolution on different metal
surfaces.
Tafel equation
a and b are empirical constant, which can
be obtained from the Tafel plot.
jba log
At higher polarization > 118 mV, a linear
relation exists:
7.11.5. Hydrogen overpotential
§7.11 Polarization of electrode
Metal 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 0.11
Values of a and b of different metals
7.11.5. Hydrogen overpotential
§7.11 Polarization of electrode
Categories a Metals
Metal with high hydrogen
overpotential 1.0-1.5
Hg(1.41), Pb(1.56), Zn(1.24),
Sn(1.20)
Metal with medium hydrogen
overpotential 0.5-0.7 Fe(0.7), Ni(0.63), Cu(0.87)
Metal with low hydrogen
overpotential 0.1-0.3 Pt(0.05), Pd(0.24)
2) Classification of metal according to a value
7.11.5. Hydrogen overpotential
§7.11 Polarization of electrode
7.11.6. Theories of hydrogen overpotential
The discharge of protons on metal
surface comprises five steps.
1) diffusion: H+ diffuses from bulk
solution to the vicinity of the double
layer
2) Foregoing step: H+ transfers across
the double layer and undergoes
configuration changes such as
dehydration etc.
3) Electrochemical step:
H3O+ + M + e M-Had + H2O
Volmer reaction, forms adsorbed H
atom
§7.11 Polarization of electrode
The slowest step will control the overall
rate of the electrochemical reaction.
Electrochemical desorption:
M-H + H3O+ + e- H2 + M
(Heyrovsky reaction)
4) Desorption of H atom:
Combination desorption (catalytic reaction):
2 M-H 2M + H2 (Tafel reaction)
5) Succeeding step: diffusion, evolution.
The theories of hydrogen overpotential:
1) The slow discharge theory
2) the slow combination theory
7.11.6. Theories of hydrogen overpotential
§7.11 Polarization of electrode
According to Tafel equation, how can
we lower hydrogen overpotential ?
jba log
Discussion:
1) The Way to reduce hydrogen
overpotential
7.11.7. Application of hydrogen overpotential
§7.11 Polarization of electrode
(1) Use materials with low a as electrode
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.
For electrolysis of water, in laboratory,
we use Pt (a = 0.05) as cathode, while in
industry, we use iron (a = 0.7).
7.11.7. Application of hydrogen overpotential
§7.11 Polarization of electrode
(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) Powder/Porous electrode. In lead-acid
battery, porous lead electrode and porous
lead dioxide electrode is adopted.
SEM image of porous electrode. The particle is in fact
aggregate of nanoparticles.
7.11.7. Application of hydrogen overpotential
§7.11 Polarization of electrode
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.
(3) Take advantage of hydrogen overpotential
7.11.7. Application of hydrogen overpotential
§7.11 Polarization of electrode
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