In previous articles, we have explored core electrochemical concepts such as Tafel slope, overpotential, and photocurrent (→Click to view collection). This issue will focus on electrode polarization, providing an in-depth analysis of its principles and mechanisms.
Electrode polarization refers to the phenomenon where the electrode potential deviates from its equilibrium potential, typically caused by an external current or electric field. This phenomenon reflects the combined effects of electrode reaction kinetics and interfacial mass transfer processes.
When an electrode is in equilibrium, the rate of oxidation reactions equals the rate of reduction reactions. The oxidation current ia is equal in magnitude and opposite in direction to the reduction current ic, resulting in a net current of zero. At this point, the electrode potential remains at the equilibrium potential. However, when a disturbance (such as passing a current) is applied, the equilibrium is disrupted, the rates of oxidation and reduction reactions become unequal, a net current is generated, and the electrode potential deviates from the equilibrium potential. This phenomenon is known as electrode polarization.
Theoretically, electrode polarization can be described using the Nernst equation and electrode kinetics models. According to Bard and Faulkner's research, the polarization phenomenon reflects the combined effects of kinetic limitations in electrode reactions and interfacial mass transfer processes (Bard & Faulkner, 2001).
The essence of electrode polarization lies in the mismatch between the rate of electrode reactions and the rate of electron transfer when current passes through the electrode. Changes in surface charges on the electrode adjust the double-layer structure. Only when the interface reaction rate is sufficiently fast can electrons be quickly consumed, maintaining the balance present before the current was applied. However, in practical reactions, the electrode reaction rate is typically slower than the rate of electron transfer, causing charge accumulation on the electrode surface. This increases the electric field strength in the double-layer region, resulting in the deviation of electrode potential from its equilibrium value. As the reaction rate increases, the electrode-solution interface stabilizes, allowing the double-layer structure to readjust and recover.
Specifically, when current passes through the electrode, the electrode/solution interface undergoes the following changes:
No.1 Electron Accumulation and Interface Potential Shift
During electrode polarization, electron flow causes charge accumulation on the electrode surface, leading to a deviation of the electrode potential from its equilibrium state. This phenomenon can be described using the Butler-Volmer equation:
Where:
j is the current density,
j₀ is the exchange current density,
η is the overpotential (the potential difference from equilibrium),
α is the charge transfer coefficient,
n is the number of electrons,
F is the Faraday constant,
R is the gas constant,
T is the absolute temperature.
This illustrates that as the electrode potential deviates from the equilibrium potential, the rates of redox reactions change, leading to polarization.
No.2 Dynamic Balance of Polarization and Depolarization
During electrode polarization, two opposing effects occur:
Polarization: Electron flow causes charge accumulation on the electrode surface, shifting the electrode potential away from equilibrium.
Depolarization: Electrode reactions absorb electrons, consuming accumulated charges and attempting to restore the electrode’s equilibrium state.
Since electron movement is generally much faster than electrode reaction rates, polarization phenomena are often observed when current flows through the electrode system.
No.3 Polarization Phenomena at Anodes and Cathodes
Cathodic Polarization: Electron influx exceeds reaction rates, causing negative charge accumulation on the cathode surface and shifting the electrode potential in the negative direction.
Anodic Polarization: Electron efflux exceeds reaction rates, causing positive charge accumulation on the anode surface and shifting the electrode potential in the positive direction.
Therefore, the fundamental cause of electrode polarization lies in the discrepancy between electron motion rates and electrode reaction rates.
Based on its mechanisms, electrode polarization can be divided into the following categories:
No.1 Concentration Polarization
Concentration polarization arises from differences in reactant or product concentrations between the electrode surface and the bulk solution. Its characteristics include:
▷ The consumption or generation of reactants during electrode reactions causes rapid decreases or increases in ion concentration in the electrode surface layer.
▷ A concentration gradient forms between the electrode surface and the bulk solution, causing the electrode potential to deviate from equilibrium values.
▷ This deviation is termed the concentration overpotential.
The essence of concentration polarization is the limitation of mass transfer, i.e., the slow movement of reactants from the bulk solution to the electrode surface or products from the electrode surface to the bulk solution. According to Fick’s law of diffusion, concentration polarization can be described using the following equation:
Where:
J is the diffusion flux,
D is the diffusion coefficient,
C is the concentration,
x is the diffusion distance.
No.2 Electrochemical Polarization
Electrochemical polarization results from the slow rate of electrode reactions, causing charge accumulation on the electrode surface. Its characteristics include:
▷ At high current densities, the electrochemical reaction rate cannot keep pace with charge transfer rates.
▷ Charge accumulation on the electrode surface causes the electrode potential to deviate from equilibrium values, resulting in electrochemical polarization.
▷ Electrochemical polarization is related to activation energy and is generally not completely eliminable.
