Lead acid battery chemical reaction
Working principles of & reactions of a lead-acid battery
All Batteries are electrochemical systems which function as a source of electrical power and energy. Each system has 2 electrodes (Positive and Negative), electrolyte and separator. Most electrochemical systems have a metal oxide or oxygen itself as Positive and a metal as Negative. The systems can be further classified as Primary and secondary batteries. The primary batteries are for one-time use; while the secondary batteries can be discharged and recharged several times.
Some of the commercially established and successful secondary batteries are given in the following table:
|Electrochemical system||Positive Electrode||Negative||Electrolyte||Remarks|
|Lead Acid Battery||Lead Peroxide PBO2||Lead metal in spongy form||Dilute Sulphuric acid||Electrolyte used in reactions + conducting electronic ions|
|Lithium ion battery||Lithium with oxide of Cobalt, Nickel, Manganese, Iron||Graphite, Silicon with (intercalated) bound Lithium||Organic solvent mixture for lithium salts||Electrolyte to conduct lithium ions between 2 electrodes - No chemical reactions|
|Nickel Cadmium||Nickel oxyhydroxide Ni(O) OH||Cadmium Metal||Dilute Potassium Hydroxide||Electrolyte only to conduct electronic ions|
|Nickel Metal Hydride||Nickel oxyhydroxide Ni(O) OH||Hydrogen absorbed in a metal alloy||Dilute Potassium Hydroxide||Electrolyte only to conduct electronic ions|
In the lead acid battery chemical reactions
Lead Acid battery has 3 main working components:
- Lead Dioxide (PbO₂) forms the Porous Positive Electrode.
- Lead in Spongy condition forms the porous Negative electrode.
- Dilute Sulphuric acid of density varying from 1.200 to 1.280 specific gravity is the electrolyte. In VRLA batteries the volume of acid is low. Therefore, a higher specific gravity of acid like 1.300 -1.320 is commonly used to achieve the designed capacity.
The electrodes are made porous using special additives during the manufacture, to ensure reactions occur throughout the bulk of the battery plate. The battery separator (a non-conductor) helps in isolating the 2 electrodes from shorting, but allows the electronic ions to pass through with minimum electrical resistance.
When the battery is connected to a load (discharge), the lead atom on the negative plate splits into lead ion (Pb²⁺) and 2 electrons. The electrons which form the fundamental unit of current originates at the negative plate and flows through the negative terminal into the external circuit.
After passing through the load the electrons arrive at the positive terminal. The electrons convert (reduces) the lead dioxide to lead ions.
In both positive and negative electrodes, the lead ions (Pb²⁺) react with sulphuric acid to form LEAD SULPHATE. (Double Sulphate theory of Gladstone). In other electrochemical systems like Nickel-Cadmium batteries, Lithium-ion batteries, the electrolytes do not take part in the reactions. Their role is only to conduct the ions between the two electrodes.
Reactions during Discharge (Which is the Main Function of a Battery)
Pb (Negative) → Pb²⁺ + 2 e⁻ ——————————1
PbO₂( Positive) Pb⁴⁺ + 2 e⁻ → Pb²⁺ —————————–2
Pb²⁺ + SO₄²⁻ (from acid)→ PbSO₄ ( in both electrodes)——–3
During the charging of a Discharged lead acid battery, all the 3 reactions take place in the reverse direction, The above is the simplified chemical and electrochemical reactions taking place in lead acid battery making it the most dependable RECHARGEABLE battery system or SECONDARY Battery system.
What is the difference between primary & secondary battery? While primary batteries are use & throw & cannot be recharged; secondary batteries, on charging, all the 3 components – positive, negative and acid are regenerated.
Thus a rechargeable or secondary cell/battery is created. Hence the name secondary battery
Internal Oxygen cycle
During the charging of VRLA battery:
At the positive plate, O2 gas is evolved and protons and electrons are produced.
2H2O → 4H+ + O2 ↑ + 4e- ……… Eq. 1
2Pb + O2 → 2PbO
2PbO + 2H2SO4 → 2PbSO4 + 2H2O
2Pb + O2 + 2H2SO4 → 2PbSO4 + 2H2O + Heat ……… Eq. 2
But, this being a charging process, the lead sulphate thus produced again has to be converted to lead; sulphuric acid is generated by an electrochemical route by reacting with the protons (hydrogen ions) and electrons resulting from the decomposition of water at the positive plates when they are charged.
2PbSO4 + 4H+ + 4e− → 2Pb + 2H2SO4 ……… Eq. 3
Discharge and charge reactions
The reactions of a galvanic cell or battery are specific to the system or the chemistry:
For example, the lead acid cell:
Pb + PbO2 + 2H2SO4 Discharge ↔ Charge 2PbSO4 + 2H2O E° = 2.04 V
In a Ni-Cd cell
Cd + 2NiOOH + 2H2O Discharge ↔ Charge Cd(OH)2 + 2Ni(OH)2 E° = 1.32 V
In a Zn-Cl2 cell:
Zn + Cl2 Discharge ↔ Charge ZnCl2 E° = 2.12 V
In a Daniel cell (This is a primary cell; here note the absence of reversible arrows)
Zn + Cu2+ Discharge ↔ Charge Zn2+ + Cu(s) E° = 1.1 V
What happens during a discharge and charge reactions inside a cell?
Electrolyte: 2H2SO4 = 2H+ + 2HSO4‾
Negative plate: Pb° = Pb2+ HSO4 + 2e
Pb2+ + HSO4‾ = PbSO4 ↓ + H+
Positive plate: PbO2 = Pb4+ + 2O2-
Pb4+ + 2e = Pb2+
Pb2++ 3H+ + HSO4‾ +2O2- =PbSO4 ¯ ↓+ 2H2O
Sulphuric acid being a strong electrolyte, it is dissociated as hydrogen ions and bisulphate ions (also called hydrogen sulphate ion).