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Basic knowledge of electroplating - electrode and reaction mechanism

August 3, 2023

A. Electrode potential
When a metal electrode is immersed in a solution containing the metal ion, the following equilibrium exists, that is, the reaction that the metal loses electrons and dissolves in the solution and the reverse reaction that the metal ion gains electrons and precipitates the metal should exist simultaneously:

                                                                        Mn++ne M

The equilibrium potential is related to the nature of the metal and the temperature and concentration of the solution. In order to accurately compare the influence of the nature of the substance on the equilibrium potential, it is stipulated that when the solution temperature is 250C, the metal ion when the concentration is 1mol/L, the measured potential is called the standard electrode potential. Metals with larger negative standard electrode potentials are prone to lose electrons and be oxidized, while metals with larger positive standard electrode potentials are prone to gain electrons and be reduced.

B. Polarization
The so-called polarization refers to the phenomenon that the electrode potential deviates from the equilibrium electrode potential when a current passes through the electrode. Therefore, the current-potential curve is also called the polarization curve. The main causes of polarization are electrochemical polarization and concentration polarization.

(1) Electrochemical polarization
Because the electrochemical reaction speed on the cathode is lower than the speed of the electrons supplied by the external power source, the electrode potential is shifted to the negative direction and caused by the polarization.

(2) Concentration polarization
The polarization caused by the difference between the concentration of the surface liquid layer adjacent to the electrode and the concentration of the main body of the solution is called concentration polarization, which is caused by the diffusion speed of ions in the solution being slower than the movement of electrons.