(a) Cite the major differences between activation and concentration polarizations. (b) Under what conditions is activation polarization rate controlling? (c) Under what conditions is concentration polarization rate controlling?

Activation polarization refers to the potential difference at an electrode due to the energy required to start and maintain an electrochemical reaction at its surface. On the other hand, concentration polarization refers to the potential difference at an electrode due to the variations in the concentration of electroactive species at the electrode-solution interface.

There are several key differences between activation and concentration polarizations: (i) Activation polarization is mainly related to the energy required for electrochemical reactions to occur, whereas concentration polarization is mainly related to the changes in the concentration of the reacting species at the electrode-solution interface. (ii) Activation polarization is mostly affected by the kinetics of the electrochemical reaction, while concentration polarization is highly influenced by mass transport (diffusion and convection) of the reacting species. (iii) Catalysts and other surface treatment techniques can significantly impact activation polarization by altering the electrode surface activity, while the effect on concentration polarization is minimal. (iv) Activation polarization usually occurs at relatively low current densities, while concentration polarization becomes more significant at higher current densities.

Activation polarization is usually rate controlling under the following conditions: (i) When the electrochemical reaction happens at the electrode surface is slow compared to the mass transport of the reacting species. (ii) Low current densities, where the energy barrier for the electrochemical reaction has a more significant effect on the overall rate of the reaction. (iii) For less active electrode materials, where the electrode surface does not efficiently facilitate the electrochemical reaction.

Concentration polarization becomes rate controlling under the following conditions: (i) When the mass transport of the reacting species (e.g., ions) to and from the electrode surface occurs at a slower rate than the rate of the electrochemical reaction. (ii) High current densities, where the rapid rate of the electrochemical reaction leads to a significant decrease in the concentration of the reactants near the electrode surface. (iii) When the electrolyte solution is highly resistive, leading to a slow diffusion of the ions involved in the electrochemical reaction.