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Battery principle
- Jan 11, 2019 -

imageIn chemical batteries, the direct conversion of chemical energy into electrical energy is the result of spontaneous chemical reactions such as oxidation and reduction in the interior of the battery. This reaction is carried out on two electrodes. The negative electrode active material is composed of a reducing agent having a relatively low potential and being stable in the electrolyte, such as an active metal such as zinc, cadmium or lead, and hydrogen or a hydrocarbon. The positive active material is composed of an oxidizing agent having a positive potential and being stable in the electrolyte, such as metal oxides such as manganese dioxide, lead dioxide, and nickel oxide, oxygen or air, halogens and salts thereof, oxyacids and salts thereof, and the like. . The electrolyte is a material having good ionic conductivity, such as an aqueous solution of an acid, a base, a salt, an organic or inorganic nonaqueous solution, a molten salt or a solid electrolyte. When the external circuit is disconnected, there is a potential difference (open circuit voltage) between the two poles, but there is no current, and the chemical energy stored in the battery is not converted into electric energy. When the external circuit is closed, a 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 is inevitably accompanied by oxidation or reduction of the interface between the bipolar active material and the electrolyte, and migration of the reactants and reaction products. The transfer of charge in the electrolyte is also accomplished by the migration of ions. Therefore, the normal charge transfer and mass transfer process inside the battery is a necessary condition for ensuring normal output of electric energy. When charging, the direction of power transmission and mass transfer inside the battery is exactly opposite to the discharge; the electrode reaction must be reversible to ensure the normal mass transfer and power transmission in the opposite direction. Therefore, the reversible electrode reaction is a necessary condition for constituting a battery. G is the Gibbs reaction free energy increment (joule); F is Faraday constant = 96500 library = 26.8 ampere-hour; n is the equivalent number of battery reactions. This is the basic thermodynamic relationship between the battery electromotive force and the battery reaction, and is the basic thermodynamic equation for calculating the energy conversion efficiency of the battery. In fact, when current flows through the electrode, the electrode potential deviates from the thermodynamically balanced electrode potential, a phenomenon known as polarization. The greater the current density (the current passing through the unit electrode area), the more severe the polarization. Polarization is one of the important causes of battery energy loss.


There are three reasons for polarization:


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


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


3 The polarization caused by the slow mass transfer process in the electrode-electrolyte interface layer is called concentration polarization. The method of reducing 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.