How lead-acid batteries work
Time:2024-04-02
Views:150
1. The generation of electromotive force in lead-acid batteries
After charging a lead-acid battery, a small amount of lead dioxide (PbO2) on the positive electrode plate reacts with water molecules in a sulfuric acid solution to form a dissociative unstable substance - lead hydroxide (Pb (OH) 4). The hydroxide ions remain in the solution, while the lead ions (Pb4) remain on the positive electrode plate, resulting in a lack of electrons on the positive electrode plate.
After charging a lead-acid battery, the negative electrode plate is made of lead (Pb), which reacts with sulfuric acid (H2SO4) in the electrolyte to form lead ions (Pb2). The lead ions transfer to the electrolyte, leaving two extra electrons (2e) on the negative electrode plate.
It can be seen that when the external circuit is not connected (the battery is open), due to chemical reactions, the positive electrode plate lacks electrons and the negative electrode plate has excess electrons, resulting in a certain potential difference between the two plates, which is the electromotive force of the battery.
2. The electrochemical reaction during the discharge process of lead-acid batteries. During the discharge of lead-acid batteries, under the potential difference of the battery, electrons on the negative plate enter the positive plate through the load to form a current I. Simultaneously conducting chemical reactions inside the battery.
After each lead atom on the negative electrode plate releases two electrons, the generated lead ions (Pb2) react with sulfate ions (SO4-2) in the electrolyte to generate insoluble lead sulfate (PbSO4) on the electrode plate.
After receiving two electrons (2e) from the negative electrode, the lead ion (Pb4) on the positive electrode plate becomes a divalent lead ion (Pb2), which reacts with the sulfate ion (SO4-2) in the electrolyte to generate insoluble lead sulfate (PbSO4) on the electrode plate. The oxygen ions (O-2) hydrolyzed from the positive electrode plate react with the hydrogen ions (H) in the electrolyte to form a stable substance, water.
The sulfate ions and hydrogen ions present in the electrolyte move towards the positive and negative poles of the battery under the action of the electric field, forming a current inside the battery, forming the entire circuit, and the battery continuously discharges outward.
During discharge, the concentration of H2SO4 continuously decreases, the lead sulfate (PbSO4) on the positive and negative electrodes increases, the battery resistance increases (lead sulfate is non-conductive), the electrolyte concentration decreases, and the battery electromotive force decreases.
3. Electrochemical reactions during the charging process of lead-acid batteries
When charging, an external DC power source (charging electrode or rectifier) should be connected to restore the substances generated by the positive and negative electrode plates after discharge to their original active substances, and convert external electrical energy into chemical energy for storage.
On the positive electrode plate, under the action of external current, lead sulfate is dissociated into divalent lead ions (Pb2) and sulfate anions (SO4-2). As the external power source continuously draws electrons from the positive electrode, the free divalent lead ions (Pb2) near the positive electrode plate continuously release two electrons to charge and become tetravalent lead ions (Pb4), which continue to react with water, ultimately generating lead dioxide (PbO2) on the positive electrode plate.
On the negative electrode plate, under the action of external current, lead sulfate is dissociated into divalent lead ions (Pb2) and sulfate anion (SO4-2). As the negative electrode continuously obtains electrons from the external power source, the free divalent lead ions (Pb2) near the negative electrode plate are neutralized into lead (Pb) and attached to the negative electrode plate as fluffy lead.
In the electrolyte, the positive electrode continuously produces free hydrogen ions (H) and sulfate ions (SO4-2), while the negative electrode continuously produces sulfate ions (SO4-2). Under the action of the electric field, hydrogen ions move towards the negative electrode, and sulfate ions move towards the positive electrode, forming an electric current.
In the later stage of charging, under the action of external current, water electrolysis reaction will also occur in the solution.
The electrode reaction formula of lead-acid batteries is
Charging: 2PbSO4+2H2O=PbO2+Pb+2H2SO4 (electrolytic cell)
Anode: PbSO4+2H2O-2e -==PbO2+4H++SO42-
Cathode: PbSO4+2e -==Pb+SO42-
Discharge: PbO2+Pb+2H2SO4=2PbSO4+2H2O (primary battery)
Negative electrode: Pb+SO42-2e -==PbSO4
Positive electrode: PbO2+4H++SO42-+2e -===PbSO4+2H2O
Patrick Qi
Patrick Qi
Gitenno