Chapter 7 Electrochemistry

§7.11 Polarization of electrode

Reversible cell correlates electrochemistry with

thermodynamics and has great theoretical importance.

However, either electrolytic cell or galvanic cell

always works in an irreversible way.

Why do we concern the irreversible electrochemical processes

7.11.1. Decomposition voltage and overvoltage

Electrolysis of water

H2O

O2

H2

pH

/ V2 4 6 8 10 12 14

0.401

-0.828

0.000

1.229

0

At what voltage can water undergo decomposition?

decomposition voltage does not depend on pH.

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 V1.229 V

1.0 2.0 0.0 E / V

I / A

Onset potential

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 > 0For 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

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.

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

a, re c, re

Ere

Eir

a, ir c, ir

ca

/ V

I / A

a, rec, re

Ere

Eira, ir c, ir

ca

/ VI

/ A

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: lower the inner resistance

R = I R

= r + d + a = r + d + a

2) Concentration overpotential (C)

i0 = ib = if 2+Culn

RTa

nF y

Cu = Cu2+ + 2e

elimination: 1) stir the solution in electroplating and in space battery; 2) discharge the battery with intervals

c

cb

if > ib Cu Cu2+ + 2e

ir > re

c

d

c

cb

if < ib Cu2+ + 2e Cu

ir < re

c

d

if

ib

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+ Cu2+

Cu2+ Cu2+

Cu2+ Cu2+

Cu2+ Cu2+

Cu2+ Cu2+

Cu2+ Cu2+

3) Activation 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 polarizaiton.

e

e

e

e

e

e

e

Fe3+

Fe2+

Chemical species that can undergo oxidation or reduction

on the electrode surface can slow the shift of electrode

potential. depolarizer, depolarization

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

Measurement circuit

Polarization circuit

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

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

log

j / A

m-2

E / V0.0

Tafel plot

At higher polarization > 118 mV, a linear relation exists:

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

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

Mechanism of electrode process

H+

Surface region

Bulk solution

H+

Mass transfer

H+Chem. rxn

H+ Desorption/adsorption

H

EC rxn

H2

Desorption/ adsorption

H2H2 Mass transferChem. rxn

electrode

Interfacial reaction

Heterogeneous reaction

7.11.6. Theories of hydrogen overpotential

The discharge of hydrogen ions 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-H + H2O, Volmer reaction, forms adsorbed H atom

4) Desorption of H atom:

Electrochemical desorption:

M-H + H3O+ + e- H2 + M

(Heyrovsky reaction)

Combination desorption (catalytic reaction):

2 M-H 2M + H2 (Tafel 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

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

How can we reduce overpotential of an 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).

(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.

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