electrode polarization

1. Electrode Polarizations

2. CONTENTS
Ohmic Polarization
Concentration Polarization
Activation Polarization
Measurement of Polarization (By
Electrochemical Impedance Spectroscopy)

3. Polarization
 Voltage loss or overpotential, which is a function of current density.
 Originates due to various phenomenon that occur during finite
current flow in the cell.
 Three dominant polarizations
 Ohmic polarisation or ohmic loss
 Concentration polarisation
 Activation polarisation
Nernst Potential
 Under open circuit conditions, voltage difference appears between the anode and the cathode.
 Not a function of current density

4.  Activation polarisation is usually dominant at low current densities,
 Concentration polarisation is dominant at high current densities
when the transport of reactive species to the electrolyte-electrode
interface becomes a limiting factor for the cell reaction.

5. Ohmic Polarization
 All matters offer a resistance to the motion of electrical charge.
 Resistivity describes the linear behavior between voltage drop and current density
 Transport of oxide ions through the electrolyte is governed by the ionic resistivity of the electrolyte
 Transport of electrons through the electrodes is governed by their respective electronic resistivities
 Voltage loss is given as
ηohm=(ρeIe+ ρcIc+ ρaIa+ Rcontact)i

6.  Main contribution to ηohm is from the electrolyte
 Ionic resistivity is much greater than electronic resistivities of the cathode and the anode
 Use of higher conductivity electrolyte materials such as doped ceria and lanthanum gallate lowers
the ohmic polarisation
where ρe= electrolyte resistivity ; Ie= electrolyte thickness
ρc= cathode resistivity ; Ic= electrolyte thickness
ρa= anode resistivity; Ia= electrolyte thickness
Rcontact= Possible contact resistance.

7. Concentration Polarization
 In fuel cells, the reacting species are gaseous; H2 (or H2 + CO) at anode and O2 at the cathode.
 At the anode, H2 (or H2 + CO) must be transported from the fuel stream (through the porous anode) to the
anode/eIectrolyte interface.
 Hydrogen (or H2 + CO) then reacts with oxide ions transported through the electrolyte, at anode/electrolyte
interface, to form H2O (or H2O + CO2), and release electrons to the anode, for subsequent transport to the
external circuit.
 The H2O (or H2O + CO2) formed must be transported away from the electrolyte/anode interface.
 This transport of H2 (or H2 + CO) and H2O (or H2O + CO2) must be consistent with the net current flowing
through the cell.

8.  Transport of gaseous species occurs by binary diffusion.
 Effective binary diffusivity is a function of the fundamental binary diffusivity DH2-H2O, and
microstructural parameters of the anode.
 Physical resistance to the transport of gaseous species through the anode at a given current density
causes an electrical voltage loss.This polarisation loss is known as concentration polarisation.
 ηa
conc is a function of several parameters, given as
ηa
conc = f (DH2-H2O, Microstructure, Partial Pressures, Current Density)

9. Activation Polarization
 Charge transfer as a fundamental step occurs at electrodes
 Voltage loss is associated with reaction rate, called activation polarisation.
 Activation polarization may be of two types.
 CathodicActivation Polarization
 AnodicActivation Polarization

10. (a) Cathodic Activation Polarisation
ηc
act = f (Material Properties,
Microstructure, Temperature,
Atmosphere, Current Density)

11. ηa
act = f (Material Properties, Microstructure,Temperature, Atmosphere, Current Density)
(b) Anodic Activation Polarisation

12. Measurement of Polarization (By Electrochemical Impedance Spectroscopy)
 Electrical characterization of electrochemical systems.
 Reveals both the relaxation times and relaxation amplitudes of the various processes present
in a dynamic system over a wide range of frequencies.
 Various polarizations exhibit different time dependence, due to different origins of the
kinetic processes involved.
 Response time for ohmic polarisation is essentially zero.
 Response time for concentration polarisation is related to the relevant gas phase
transport parameters (Diffusivity).
 Response time for activation polarisation is related to details of the charge transfer
process.

13. ThankYou