This document discusses four types of electrode polarizations that occur in fuel cells: ohmic, concentration, and activation polarizations at both the anode and cathode. Ohmic polarization is due to the resistance of cell components to ion/electron flow. Concentration polarization occurs when the transport of reactants or products to/from the electrode-electrolyte interface is too slow. Activation polarization is associated with the reaction rate at the electrodes. Electrochemical impedance spectroscopy can be used to measure these polarizations by analyzing the different time dependencies and relaxation processes associated with each polarization type.

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