Electrode kinetics and mass transport

Plan

1. 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

transport5. Electrical circuits for:

- ideally polarized electrode- non-polarized electrode- equilibrium potential- mixed potential

1. 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 oxidizedor reduced according to the Faraday law:

itnF

Mm A

This expression may be rearranged to give expression for current:

t

NnF

tM

mnFi

A

n- number of electrons in a redox reaction, N-number of moles, MA- molecular weight, F- Faraday constant

t

NnF

tM

mnFi

A

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:

dtnFdNi /

The rate of a chemical reaction is :

dtdNv /Hence faradaic current is a measure of a reaction rate

nFvdt

dNnFi

For a multistep reaction

2. Mass transport phenomena

3. Reactions controlled by charge transfer step

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

Example: Pt electrode in Fe3+/ Fe2+ solution

Fe3+ + e = Fe2+Fe2+ = Fe3+ + e

Eeq

-mixed potential: example corrosion of Fe in HCl: cathodic reaction: 2 H+ + 2e = H2

Anodic reaction: Fe = Fe2+ +2e

Ecorr

ln icorr

Slope =RT

nFHSlope =

RT

nFFe )1(

2H+ + 2e = H2

Fe = Fe2++ 2e