Electrode kinetics and mass transport

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• Electrode kinetics and mass transport
• Plan
• Electrode reaction as a series of multiple
  consecutive steps
• 2. Mass transport phenomena
• - diffusion
• - convection
• - migration
• 3. Reactions controlled by charge transfer step
• - Butler Volmer equation and Tafel plot
• 4. Reactions controlled by mixed charge
  transfer-mass transfer step
• - Butler Volmer equation with correction for
  mass transport
• 5. Electrical circuits for
• - ideally polarized electrode
• - non-polarized electrode
• - equilibrium potential
• - mixed potential

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• Electrode reaction as a series of multiple
  consecutive steps

A charge transfer reaction provides an additional
channel for current to flow through the
interface. The amount of electricity that flows
through this channel depends on the amount of
species being oxidized or reduced according to
the Faraday law
This expression may be rearranged to give
expression for current
n- number of electrons in a redox reaction,
N-number of moles, MA- molecular weight, F-
Faraday constant
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This equation described average current flowing
through the electrode During time t. During
infinitesimal period dt the number of
electrolyzed moles is dN and the expression for
the instantaneous current is
The rate of a chemical reaction is
Hence faradaic current is a measure of a reaction
rate
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For a multistep reaction
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2. Mass transport phenomena
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3. Reactions controlled by charge transfer step
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4.Reactions controlled by mixed charge
transfer-mass transfer step - Butler Volmer
equation with correction for mass transport
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5. Electrical circuits for - ideally polarized
electrode - non-polarized electrode
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- equilibrium potential Example Pt electrode
in Fe3/ Fe2 solution
Fe2 Fe3 e
Fe3 e Fe2
Eeq
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• mixed potential example corrosion of Fe in HCl
  cathodic reaction 2 H 2e H2
• Anodic reaction Fe Fe2 2e

Slope
Slope
2H 2e H2
ln icorr
Fe Fe2 2e
Ecorr