Polarization and electromotive force of lead-acid battery electrodes

Polarization and electromotive force of lead-acid battery electrodes

The electromotive force of a lead-acid battery is the difference between the positive electrode potential and the negative electrode potential of the lead-acid battery in a balanced state. The electromotive force can be calculated using thermodynamic formulas or electrode potential.

The size of the electromotive force of the battery is determined by the nature and conditions of the reaction in the battery, and has nothing to do with the shape and size of the battery. The electromotive force is the driving force for the battery to generate electrical energy.

The reaction free energy △G describes the amount of energy that can be converted into electrical energy. The battery voltage under reversible conditions, that is, the voltage when all reactions are in equilibrium, actually means that no current flows in the battery.

When the metal becomes a cation and enters the solution and the metal ion in the solution deposits on the metal surface at the same rate, the reaction reaches dynamic equilibrium. At this time, the charge and substance in the positive and negative process of the electrode reaction have reached equilibrium, so the net reaction rate is zero. There is no current flowing, that is, the external current is equal to zero, and the electrode potential at this time is the equilibrium electrode potential. When current passes through the electrode, the electrode potential will deviate from the equilibrium value. The greater the current, the greater the deviation. This phenomenon of deviation from the equilibrium electrode potential is called electrode polarization.

If an oxidation reaction occurs on the electrode (such as the negative electrode of a lead-acid battery during discharge), the current passing through the electrode is called the anode current, and the electrode potential changes in the positive direction, which is higher than the balance electrode potential, which is called anodic polarization. If a reduction reaction occurs on the electrode (such as the positive electrode of a lead-acid battery during discharge), the current passing through the electrode is called the cathodic current, and the electrode potential changes in the negative direction, which is lower than the equilibrium electrode potential, which is called cathodic polarization. Overpotential (superelectromotive force) is the difference between the electrode potential and the equilibrium electrode potential when there is polarization.

There are three types of polarization. The first is electrochemical polarization. The electrode reacts between the solution interface and the polarization is caused by irreversibility. The second is concentration polarization. Due to the consumption of reactants or the production of products, they cannot be supplied or evacuated in time, resulting in the deviation of the electrode potential from the equilibrium electrode potential, which is called concentration polarization. The third is ohmic polarization. The ohmic resistance of the electrolyte, electrode materials, conductive materials, etc. causes the difference between the actual potential and the theoretical electrode potential, which is called ohmic polarization.

Electrochemical polarization is caused by the rate at which the electrochemical reaction on the electrode produces electrons, which lags behind the rate at which the electrons are exported from the electrode. The overpotential caused by electrochemical polarization increases with the increase of the current density. Tafel verified that there is a linear relationship between the overpotential and the logarithm of the current density, which is called the Tafel equation.

n = a+blgl

It is a commonly used relationship in electrochemistry. The constant a mainly depends on the nature of the electrode system, and is also affected by the surface treatment of the electrode, and whether there are impurities that interfere with the reaction of the electrode; the constant b is a very useful parameter for analyzing the reaction mechanism of the electrode.

When the electrode undergoes an electrochemical reaction, the ion concentration involved in the reaction in the electrolyte produces an imbalance problem. During discharge, the positive electrode (electrochemical cathode) of the lead-acid battery requires H and So42- to participate in the reaction and crystallize to On the electrode, the negative electrode is the same. During charging, the electrode reacts to release ions, so the ion concentration on the electrode surface is higher than the ion concentration in the electrolyte. The movement of ions from one position to another in the solution is called the transfer of matter in the liquid phase, or liquid phase mass transfer for short. There are three ways of liquid phase mass transfer: ion diffusion, ion electromigration and convection.

Due to the polarization of the electrodes, the lead-acid battery causes the terminal voltage to decrease when discharging, which is lower than the open circuit voltage; while it increases the terminal voltage when charging. Generally speaking, polarization is the total polarization of the three polarizations. Each polarization has different effects due to different states. When the lead-acid rose battery is discharged at room temperature, the concentration polarization of the positive electrode is dominant, so the positive electrode is called concentration polarization control, that is, the liquid phase mass transfer is the slowest, also known as the positive electrode liquid phase mass transfer control.

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