Lead Acid Battery Electrochemistry Explained

 


A battery is a collection of one or more electrochemical cells that converts chemical energy contained within its active materials directly into electric energy. Many types of batteries are commonly used in modern day applications, and they are named by the elements that make up the electrochemical cell of the battery they include 


1) Nickel Cadmium (Ni-Cd)

2) Lithium Ion (Li-Ion)

3) Nickel-Metal Hydride (NiMH)

4) Lead Acid

The lead-acid battery was the first rechargeable battery created by Gaston Planté in 1859 it uses lead and sulphuric acid to function. The lead is submerged into the sulphuric acid and controlled chemical reaction creates voltage potential.

Components of an Electrochemical cell

Anode: Oxidation occurs at the anode

Cathode: Reduction occurs at the cathode.

Electrodes: In order to hook up an external circuit you need have something to physically connect the wire to. There are two types of electrodes, active and inert. An Inert electrode is made of a substance that does not undergo oxidation or reduction.

Electrolyte: An electrolyte is an aqueous solution of ionic compounds that functions as a conducting medium for ions to move between the cathode to the anode.


How a Lead Acid Electrochemical Cell works.

A lead acid electrochemical cell uses 

Lead dioxide (PbO2) as the cathode or positive electrode (+)

Spongy Lead (Pb)as the anode or negative electrode (-)

Sulphuric acid(H2SO4) and H2O) as the electrolyte where 1/3 of the total volume is Sulphuric acid and 2/3 is water.

Discharging the Cell

As you connect a load across the electrodes of the electrochemical cell, current flows and the electrodes dipped into the dilute sulphuric acid, begin to react. The acid molecules split into positive hydrogen ions (H+) and negative sulfate ions (SO4 − −). A reaction between the cathode (PbO2) and Sulphate (SO4) inside the electrolyte, this reaction causes a layer of Lead sulphate (PbSO4) to begin to form on the cathode. During this reaction, Oxygen(O2) is ejected from the cathode into the electrolyte. Once oxygen(O2) enters the electrolyte, the Oxygen ion combines with H2 in the electrolyte(H2SO4) to form water (H2O). Making the electrolyte more dilute and therefore weaker.

At the same time, the lead atoms (Pb) on the anode reacts with negative sulfate ions (SO4 − −) to form a layer of Lead sulphate (PbSO4) on the anode too. 

Pb + H2SO4 -> PbSO4 + H2O + 2e-

The reaction at the anode releases electrons, causing an excess of electrons at the anode.

Over time the electrolyte becomes more diluted and weaker due to formation of H2O and the reaction also slows down due to buildup of lead sulphates on both electrodes.

Charging The Cell

If we supply the electrochemical cell with electricity from a battery charger, electrons enter the negative terminal(anode) and are pulled away from the positive terminal.

Electrons supplied to the negative terminal (the anode), recombine with the lead sulfate (PbSO4) formed during discharge causing the lead sulfate (PbSO4) to decompose to form sulfuric acid (H2SO4) and spongy lead (Pb).

The lead regains electrons and becomes spongy lead (Pb) again which is a restoration of the anode's active material while the sulfate ions (SO₄²⁻) from the decomposed lead sulfate is released into the electrolyte to participate in other reactions within the battery.

PbSO4 + 2H2O + 2e- → Pb + 2H2SO4
Anode Reaction

This reaction also absorbs electrons, which are supplied by the external power source.

 The released sulfate ions (SO₄²⁻) combine with hydrogen ions from the water (H2O) in the electrolyte to form sulphuric acid (H2SO4), this replenishes the sulfuric acid concentration on the electrolyte. Oxygen ions released from the reaction combine with lead from the lead sulfate (PbSO4) at the cathode to create lead dioxide (PbO2) at the cathode. this also releases the sulphate ions (SO₄²⁻) trapped in the cathode back into the electrolyte.

PbSO4 + 2H2O + 2e- → PbO2 + 2H2SO4
Cathode Reaction

Over Discharging
If we were to leave the battery to discharge for too long, the layer of lead sulphate formed on the electrodes could break away from the electrode and accumulate at the bottom of the battery. This means they will no longer participate in the chemical reaction that occurs during charging and the battery needs to be repaired or replaced.

Over Charging
Initially, the charging current primarily drives the conversion of lead sulfate (PbSO4) back into lead (Pb) and lead dioxide (PbO2) at the positive and negative plates, respectively. If we were to leave the battery plugged in after the charging is complete and the lead sulfate (PbSO4) on the plates has fully decomposed the battery, electrolysis of water in the electrolyte solution would begin resulting in the decomposition of water into its constituent elements, hydrogen (H2) and oxygen (O2).

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