Electrochemical kinetics - Shandong · PDF fileWhat are we going to learn? Concepts:...

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Transcript of Electrochemical kinetics - Shandong · PDF fileWhat are we going to learn? Concepts:...

  • 7.11 Polarization of electrode

    Electrochemical kinetics

  • 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

    Diffusionconcentration 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 ca, ir = a, re + a

    c, ir = c, re ca, 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 ca, ir = a, re + a

    c, ir = c, re ca, 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 s

    RTa

    nF

    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 electrodes

    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

  • 7.11.4 Measurement of overpotential

    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