Electrochemical cells (Contd…)

Standard cell

The electrodes used in the cells discussed so far, deteriorate with the passage of current and cannot offer a constant emf indefinitely. However, there was a few cells called standard cells, which can maintain a fairly constant emf over very long periods of time compared to the other cells. The commonly used standard cell is the Weston Cell. This cell is usually in the form of an H shaped tube. One leg of the tube contains mercury in contact with a paste of mercurous sulphate (Hg₂SO₄) and is the cathode. The other leg of the tube contains an amalgam of cadmium with mercury, which acts as the anode of the cell. The electrolyte is a saturated solution of cadmium sulphate. The mercurous sulphate paste serves as depolariser. Platinum wires are sealed at the bottom of each leg to serve as terminals for connecting the cell to the external circuit. The emf of cell is 1.0813 V at 20^oC. This is independent of temperature over a considerable range.

Weston Standard cell

Lead Accumulator

This can be recharged by passing a current through it in the reverse direction. The chemical processes that occur at the electrodes during discharge are reversed by this. Thus the cell recovers its original state, except for some energy loss. Such cells are called secondary cells or accumulators.

The lead-sulphuric acid cells is a common example. It was inverted by a French physicist, Gaston plate, in 1859.

Electrodes: Alternate parallel plates of lead dioxide (+ve electrode) (oxidised from PbO)

Spongy lead (reduced from PbO) (-ve electrode)

These are kept separate by porous separators made of wood, plastic or glass fibre.

Electrolytes: Dilute sulphuric acid

Container: Glass or bakelite

Discharging Process

Here stored chemical energy is converted to electrical energy or current is drawn from the cell.

The hydrogen ions go to the +ve electrode and SO₄^2- to the -ve electrode. After giving their charges they react with the electrodes and reduce the active material to lead sulphate.

Therefore, at the -ve electrode

At the +ve electrode,

Both plates (but only half of the active materials) are converted into PbSO₄ (whitish). Water is formed thus lowering the specific gravity of H₂SO₄ (electrolyte).

The emf of the cell falls and sulphuric acid is consumed.

Recharging Process

Current is passed through the two terminals in the reverse direction to that in which the cell provided current. That is, the anode is connected to the positive terminal of the d.c. source, and the cathode to the negative terminal.

The hydrogen ions move to the -ve electrode and sulphate ions to the +ve electrode.

At -ve electrode,

At the +ve electrode,

Water is consumed and sulphuric acid is formed thus raising the specific gravity of the electrolyte. In the charging process, the +ve electrode is coated with dark brown lead peroxide and the -ve electrode with grey spongy lead. The emf of the cell rises, and the electrical energy supplied is converted into chemical energy which is stored in the cell.

The charging process is mentioned by measuring the specific gravity of the electrolyte. It varies from 1.28 when fully charted (sulphuric acid and water) to 1.12 when discharged (mostly water). The emf of a full charged cell is ~2.1V. It should not be discharged to below 1.8V. This secondary cell has a low internal resistance that is, it can deliver a high current. It can be recharged a very large number of times without any deterioration.

Solid State cells

The cells discussed above use a liquid electrolyte, one cathode and an anode. Some of the major disadvantages of these cells are leakage on long term storage, corrosion due to the use of liquid acidic/alkaline solution, short life, low weightage per kg mass of the cell and limit on miniaturization.

Recently, some cells have been developed in which the electrolyte is a solid in which ions can move (Solid state electrolytes). Such materials are available in the form of gels, polymers, composites, polycrystalline solids or thin solid films. The basic geometry of a solid state cell is given in Fig.4.13 using a solid electrolyte with mobile cation M⁺ and anion X^-. Either one of these ions or both can move.

In a lithium solid state cell, (Figure below) the basic electrochemical reaction with the electrode (say, I₂) is

A solid state cell

Some electrolyte is also mixed in the cathode or anode to decrease polarization. Many solid state materials for use as cathode, anode and electrolyte have been recently developed. Some of the Li⁺ batteries used in mobile phones are based on solid electrolytes. Some heart pacer batteries also use Li-button cell in which the electrolyte is (LiI + AI₂O₃) composite or a similar electrolyte. Polymer Li - batteries and H⁺ - batteries are in the advanced state of development for electrical cars. Some electrode materials like doped LiCoO₂ or LiMnO₂ have provided excellent rechargeability to these cells.