Electrode Polarization Concepts

Electrode Polarization

• Electrode polarization has three meanings:

  • The phenomenon of change in electrode potential under current flow.
  • An operation performed by an experimenter who aims at obtaining a potential change by passing a current of specific strength and direction.
  • The quantitative measure (ΔE) of the electrode potential shift (E) relative to its equilibrium value (E°) under current flow, calculated as ΔE = E_i - E_o.
  • E_i is the potential under current, E_o is the potential under zero current.

• Equation 7.1 provides polarization values at equilibrium potential and open-circuit potential.

• Anodic currents cause a positive shift (ΔE positive), while cathodic currents cause a negative shift (ΔE negative).

• In high polarization cases, the absolute value of polarization (ΔE) is implied for cathodes.

• Polarization at one electrode is independent of the other electrode and the processes occurring there.

Overall and Partial Reaction Currents

• Net current (i), anodic current density (i_a), cathodic current density (i_c), and exchange current density (i₀) are defined.
• Increased anodic current density (i_a) typically leads to decreased cathodic current density (i_c) and a more positive potential.
• Conversely, decreased anodic current density (i_a) typically leads to increased cathodic current density (i_c) and a more negative potential.
• Net current density is the difference between partial current densities (i = i_a - i_c).
• When anodic polarization is reached, cathodic partial current density becomes very low (i ≈ i_c).
• Similarly, when cathodic polarization is reached, anodic partial current density becomes very low (i ≈ i_a).
• The total potential range can be divided into three regions: low polarization (comparable reaction rates), high anodic polarization (reverse reactions negligible), and high cathodic polarization (reverse reactions negligible).

Anodic and Cathodic Polarization

• Anodic polarization is positive, while cathodic polarization is negative.
• Current-potential curves are often monotonous and have a positive slope, though this isn't a universal rule.
• No necessary correlation exists between polarization and polarity.
• During electrolysis, the anode's potential and polarization are positive when current flows, exceeding its potential without current.
• The cathode's potential and polarization are negative (in electrolysis mode).

Various Types of Polarization (Overpotential)

• Diffusional polarization (ΔE_α): influenced by the balance between electrode reaction rates and the supply/removal of substances through diffusion processes. Polarization change in equilibrium under concentration change is called concentration polarization.
• Activation polarization (overvoltage): occurs when concentration changes affect operation, but not when activation polarization is present. Polarization occurs when diffusion mode is present.
• Mixed mode: occurs when both diffusional and activation polarization mechanisms operate simultaneously. Diffusion and kinetic current modes, under mixed mode.

General Kinetic and Polarization Equations

• At high polarization, kinetic equations for partial current densities (i_a, i_c) overlap with the equation for the net current density.
• Equations for net cathodic or anodic partial current densities are given.

Measuring Polarization Under Various Cases

• Solution with Excess Foreign Electrolyte: If the system involves pure concentration polarization (absence of activation polarization), electrode potential maintains equilibrium. Surface concentration may differ from equilibrium concentration.

  • E = E° + (RT/nF)ln(C_s,j)
  • ΔE_α = E - E° + (RT/nF)ln (C_s,j/C_b,j)

• The surface concentration of components is a result of the balance between the electrode reaction rates, and the rate of supply/removal via diffusion and migration, and is given by equations Cs,j = Cb,j (1 - i/i_L),.

• i_Lim is related to the limiting current diffusion current density for the reactant j (i = i_Lj (1 - i/i_L)), and the components present in the system.

More Specific Considerations

• Half-wave potential is independent of component concentration, but the polarization value at this point is affected by concentration changes.
• Special cases include components with zero concentration and limiting current density(i) zero. In this scenario, the kinetic equation for potential should be used, not that for polarization.
• A linear relationship between polarization and the logarithm of current density exists when a component with a constant concentration is the oxidizing or reducing agent.

Additional Notes

• Figures and equations presented in the text are crucial for a complete understanding of the subject.
• The variables described here are important for understanding the concepts of electrochemistry presented in the text.

This quiz explores the concept of electrode polarization, detailing its definitions, how to measure potential changes, and the effects of anodic and cathodic currents. Understand the qualitative and quantitative aspects of polarization under current flow and their implications in electrochemical processes.