Principle of battery

2021-07-27 11:13:51

Principle of battery
        In a chemical battery, the direct conversion of chemical energy into electrical energy is the result of chemical reactions such as oxidation and reduction that occur spontaneously inside the battery. This reaction is carried out on two electrodes respectively. The negative electrode active material is composed of a reducing agent that has a relatively negative potential and is stable in the electrolyte, such as active metals such as zinc, cadmium, and lead, and hydrogen or hydrocarbons. The positive electrode active material consists of an oxidant with a positive potential and stable in the electrolyte, such as manganese dioxide, lead dioxide, nickel oxide and other metal oxides, oxygen or air, halogens and their salts, oxyacids and their salts, etc. . Electrolytes are materials with good ionic conductivity, such as aqueous solutions of acids, alkalis, and salts, organic or inorganic non-aqueous solutions, molten salts, or solid electrolytes. When the external circuit is disconnected, although there is a potential difference (open circuit voltage) between the two poles, there is no current, and the chemical energy stored in the battery is not converted into electrical energy. When the external circuit is closed, current flows through the external circuit under the action of the potential difference between the two electrodes. At the same time, inside the battery, due to the absence of free electrons in the electrolyte, the transfer of charge will inevitably be accompanied by the oxidation or reduction reaction of the interface between the bipolar active material and the electrolyte, and the material migration of reactants and reaction products. The transfer of charge in the electrolyte is also completed by the migration of ions. Therefore, the normal charge transfer and material transfer process inside the battery is a necessary condition to ensure the normal output of electric energy. When charging, the direction of the electric and mass transfer process inside the battery is exactly opposite to that of the discharge; the electrode reaction must be reversible to ensure the normal progress of the mass transfer and electric process in the opposite direction. Therefore, the reversibility of the electrode reaction is a necessary condition for the formation of a battery. G is the increase in free energy of Gibbs reaction (Coke); F is Faraday's constant = 96500 ku = 26.8 A·h; n is the equivalent number of battery reaction. This is the basic thermodynamic relationship between the electromotive force of the battery and the battery reaction, and it is also the basic thermodynamic equation for calculating the energy conversion efficiency of the battery. In fact, when current flows through the electrode, the electrode potential will deviate from the thermodynamically balanced electrode potential. This phenomenon is called polarization. The greater the current density (current passing per unit electrode area), the more serious the polarization. Polarization is one of the important reasons for battery energy loss.

There are three reasons for polarization:

The polarization caused by the resistance of each part of the battery is called ohmic polarization;

The polarization caused by the retardation of the charge transfer process in the electrode-electrolyte interface layer is called activation polarization;

The polarization caused by the slow mass transfer process in the electrode-electrolyte interface layer is called concentration polarization. The method to reduce the polarization is to increase the electrode reaction area, reduce the current density, increase the reaction temperature and improve the catalytic activity of the electrode surface.
 
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