cell Also called voltaic or galvanic cell. A device in which chemical action of electrolyte (generally an aqueous solution of chemicals) on two electrodes (generally metallic plates), produces potential difference between the electrodes. When the electrodes are connected externally by a conductor, a current flows which continues as long as the chemical reaction goes on inside the cell. The potential difference between its electrodes when external circuit is open is called electromotive force (emf) of the cell. In a primary cell the chemical reactions are irreversible, while in a secondary cell, the chemical reaction can be reversed by sending current in opposite direction. Therefore secondary cells are rechargeable. There are many types of cells. Descriptions of more commonly used cells are given below.
1. Primary cells (a) Daniel cell: Negative electrode of the cell is amalgamated zinc, which is kept in a porous pot containing dilute sulphuric acid. This pot is placed in copper sulphate solution. The positive electrode is copper. In the negative electrode zinc is dissolved producing zinc sulphate, and at the positive electrode copper is deposited. emf of the cell is 1.1 V, which is comparatively steady.
(b) Leclanche cell: Positive electrode of the cell is carbon surrounded by mixture of manganese dioxide, and powdered carbon in a porous pot. This is placed in a solution of ammonium chloride, electrolyte in a jar that has negative electrode of zinc. When external circuit is completed chlorine ions in the electrolyte move towards zinc, and ammonium ions decompose at the positive electrode giving ammonia and hydrogen. The hydrogen is oxidized by manganese dioxide. emf of the cell is 1.5 V.
(c) Dry Cell : Form of Leclanche cell. The ammonium chloride electrolyte is in the form of a paste which also contains zinc chloride. The outer container is made of zinc that acts as negative electrode.
(d) Cadmium cell: Also called Weston standard cell. The cell is constructed in a H-shaped glass container. The electrolyte is saturated solution of cadmium sulphate. The positive electrode is mercury covered with a paste of mercury sulphate and cadmium sulphate crystals. The negative electrode is amalgam of cadmium (12.5% Cd, 87.5% Hg) covered with cadmium sulphate crystals. These electrodes are in two legs of the H shaped container. The external connection is taken using platinum wire. The emf of the cell is almost constant = 1.0186 V at 20oC. It is taken as a standard.
2. Secondary cell: Also called accumulator or storage battery. a) Lead-acid accumulator : Each cell gives e.m.f. of about 2V and in motor cars 6 such cells are used giving 12 V. It produces 80-120 kJ/kg. Electrodes are made of lead alloy (7-12% antimony for increasing hardness and small amount of tin) and dilute sulphuric acid is the electrolyte. The electrodes are coated with a paste of lead monoxide (Pb0) and finely divided lead. When the forming current is passed it converts lead oxide (Pb0) into finely divided lead in the negative plate, and lead dioxide (Pb02) on the positive plate. The overall reaction is given by,
PbO2 + 2H2SO4 + Pb ® 2 PbSO4 + 2H2O
b) Nickel-Iron accumulator : (NIFE cell) e.m.f. 1.2 V and stored energy = 100 kJ / Kg. It has positive plate of nickel oxide and negative plate of iron immersed in potassium hydroxide electrolyte. The overall reaction is,
2 NiO.OH.H2O + Fe ® 2Ni (OH)2 + Fe (OH)2