The principle and thermodynamics of lead-acid batteries

The principle and thermodynamics of lead-acid batteries

The total reaction formula of lead-acid battery discharge is

Pb+ PbO2 +2H2SO4=2PbSO4+2H2O

The total reaction formula for charging a lead-acid battery is

2PbSO4+2H2O=Pb+ PbO2 +2H2SO4

The positive and negative reactions of lead-acid batteries are separated, but proceed at the same time. When the external circuit is connected to discharge, the lead (Pb) on the negative electrode loses electrons and is oxidized to divalent lead (Pb2+). The reaction formula is

Pb+ H2SO4-2e =PbSO4+2H

When the positive electrode is discharged, tetravalent lead (Pb4+) gets electrons and is reduced to divalent lead (Pb2+). The reaction formula is

PbO2+H2SO4+2H+2e =PbSO4+2H2O

The lead ions on the positive and negative electrodes are very small, and it forms lead sulfate (PbSO4) with sulfate ions. As the reaction progresses, the sulfuric acid in the electrolyte and the Pb2+ in the positive and negative plates continue to form PbSO4, which crystallizes on the plates, and the concentration of sulfuric acid in the electrolyte gradually decreases.

When a lead-acid battery is connected to a DC power supply for charging, the divalent lead in the lead sulfate on the negative electrode is reduced to lead (Pb), and sulfuric acid is precipitated and enters the electrolyte. This reaction is the reverse reaction of the negative electrode discharge reaction formula. The divalent lead in the lead sulfate on the positive electrode is oxidized to tetravalent lead to form lead dioxide (PЬO2). The sulfuric acid is precipitated and enters the electrolyte. This reaction is the reverse reaction of the positive electrode discharge reaction. As the reaction progresses, the concentration of sulfuric acid in the electrolyte gradually increases.

Thermodynamics is the study of the possibility and degree of progress of chemical reactions. In an electrochemical reaction, when there is no current through the electrodes, the battery system is in equilibrium. The state of the battery system is certain, and each thermodynamic parameter has a definite value; the thermodynamic function is only related to the component and the energy state, and has nothing to do with the reaction process and pathway. Under equilibrium conditions, the performance parameters of thermodynamics reach the maximum.

The thermodynamic functions of electrochemical reactions include baking H, Gibbs free energy G, and entropy S. Using these functions to express the amount of material state has little practical application significance, but the difference before and after the reaction is commonly used to indicate change characteristics or The parameter of changing state generally uses the change of reaction enthalpy △H, which represents the energy released or absorbed by the reaction; the change of free energy △G, represents the (maximum) chemical energy that can be converted into electrical or mechanical energy; the change of reaction entropy △S , A parameter related to the loss of energy or the gain of energy in the process of chemical reaction or electrochemical reaction, the product of T and ΔS, that is, the reversal thermal effect, represents the heat exchange between the surrounding environment during the reversible process.
The important relationship of thermodynamic parameters is

△G =△H-T△S

In the formula, T-thermodynamic temperature (K).

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