There are diffusible substances in 1. system, and the species in electrochemical reaction will diffuse before and after the reaction.
2. The electrochemical reaction rate is controlled by substance diffusion, that is, the reaction steps of each electrode are synchronized with each other.
3. The concentration of electrolyte in the system has little change, and the conductivity of electrolyte has no obvious change.
4. Compared with the diffusion rate of substances, the exchange rate of electrolytes can be ignored, that is, the substances between electrodes quickly reach equilibrium.
It should be noted that the kinetic equation of concentration polarization is only applicable to the electrode reaction under the condition of unsteady current, that is, the current density cannot be kept constant throughout the experiment. When the current density reaches a certain level, polarization will seriously affect the current response and the dynamic equation will become inapplicable.
Therefore, when applying the kinetic equation of concentration polarization, it is necessary to comprehensively analyze and consider species diffusion, electrode thermodynamic calculation, mass transfer and other aspects of electrode reaction system to ensure its applicability and effectiveness.
Brief introduction of concentration polarization;
Concentration polarization means that in the separation process, the solution in the feed liquid is driven by pressure to penetrate the membrane, and the solute (ions or solutes with different molecular weights) is intercepted, and the concentration is getting higher and higher at the interface between the membrane and the bulk solution or near the membrane interface.
Under the action of concentration gradient, solute will diffuse from the membrane surface to the bulk solution, forming a boundary layer, increasing fluid resistance and local osmotic pressure, thus leading to the decrease of solvent permeation flux.
The phenomenon that the electrode potential deviates from the equilibrium potential because the ion concentration of the electrode interface layer solution in the electrolytic cell is different from that of the bulk solution. This is a basic form of electrode polarization.
The concentration difference of the solution in the electrolytic cell during electrolysis is caused by the mass transfer of the liquid, that is, the diffusion speed of the solution through the interface layer can not keep up with the electrolysis speed.
Concentration polarization is not conducive to metal electrolysis and electroplating. It increases cell voltage, increases power consumption, deteriorates cathode deposition or coating quality, and even causes hydrogen precipitation and discharge of impurity metal ions. The concentration polarization can be eliminated to some extent by stirring, heating the solution or moving the electrode, but it cannot be completely avoided because of the existence of the diffusion layer on the electrode surface.