Ch # 9 Electrode Polarization.pptx

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 η ohm =( ρ e I e + ρ c I c + ρ a I a + R contact )i

6. Main contribution to η ohm 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 ρ e = electrolyte resistivity ; I e = electrolyte thickness ρ c = cathode resistivity ; I c = electrolyte thickness ρ a = anode resistivity; I a = electrolyte thickness R contact = Possible contact resistance.

7. Concentration Polarization In fuel cells, the reacting species are gaseous; H 2 (or H 2 + CO) at anode and O 2 at the cathode. At the anode, H 2 (or H 2 + CO) must be transported from the fuel stream (through the porous anode) to the anode/eIectrolyte interface. Hydrogen (or H 2 + CO) then reacts with oxide ions transported through the electrolyte, at anode/electrolyte interface, to form H 2 O (or H 2 O + CO 2 ), and release electrons to the anode, for subsequent transport to the external circuit. The H 2 O (or H 2 O + CO 2 ) formed must be transported away from the electrolyte/anode interface. This transport of H 2 (or H 2 + CO) and H 2 O (or H 2 O + CO 2 ) 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 D H2-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. η a conc is a function of several parameters, given as η a conc = f (D H2-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. Cathodic Activation Polarization Anodic Activation Polarization

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

11. η a 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.

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