Project Report / Essay - Cells : Introduction

Cells : Although every spontaneous redox reaction can be used as the basis of an electrochemical cell, yet every such electrochemical cell is not suitable for purpose. This is because of the following two reasons :-

(i) Electrochemical cells using salt bridges have high internal resistance. As a result, if we try to draw large current from them, their voltage drops sharply.

(ii) They lack the compactness and ruggedness for portability.

Types of cells

The various commercial cells may be classified mainly into the following three types :

(a) Primary Cells.

(b) Secondary Cells.

(c) Fuel Cells.

Primary cells are those which the redox reaction occurs only once and the cells becomes dead after some time, and cannot be used again. Two common examples of this type are dry cell and mercury cell. Secondary cells are those which can be recharged by passing an electric current through them & hence can be used over and again. Two well know example of this type are lead storage battery and nickel–cadmium cells.

Full cells are those in which the energy produced from the combination of fuels such as H₂, CO, CH₄ etc. is directly converted into electrical energy such a conversion is possible because combustion reactions are also redox reactions.

A brief description of the examples of each type is given below :

(a) Primary cells :

(i) Dry cell : the dry cell is a compact form of the leclanche cell. It is consists of cylindrical Zinc container which acts as the anode. A graphite rod paced in the center (but not touching the base) acts as the cathode. The space between the anode and the cathode is so packed that Zinc container is in touch with the paste of NH₄Cl and the graphite rod is surrounded by powdered MNO₂ and carbon as shown in fig. The graphite rod is fitted with a metal cap and the cylinder is sealed at the top with pitch. The Zinc container is covered with cardboard to protect it form the atmosphere.

The reaction taking place at the electrode are quite complex.

At anode :- Zn (s) ----> Zn⁺² (aq) + 2e^–         ---->

At Cathode :- 2 MnO₂ (s) + 2 NH₄ (aq) + 2e^– ---->

                    Mn₂O₃ (s) + 2 NH₃ (g) + H₂O

(i.e. Mn has been reduced from oxidation state +4 to +3)

The NH₃ formed is not librated as gas but combines immediately with Zn²⁺ ion & the Cl^- ions to from complex salt. {Zn (NH₃)₂ Cl₂} (Dichlorodiammine Zinc). These cells have voltage in the range 1.25 V to 1.50 V. However they do not have a long life because the acidic NH₄ Cl corrodes the Zinc container even when the cell in not in use.

(b) Secondary cells :

(i) Lead storage battery :- This is one of the most common batteries used in the automobile. A 12 V lead storage battery is generally used which consists of 6 cells. Each producing 2.0 V. Each cell consists of a lead anode and a grid of lead packed with lead dioxide as the cathode. These electrodes are arranged alternately, separately by then wooden or fibre glass sheets and suspended in dilute sulphuric acid which acts as the electrode. To increase the current output of each cell, the cathode plates are joined together i.e. the cells are connected in parallel.

The electrode reaction that occur during discharge are as follows :

At anode :- Pb (s) + SO₄^–2 (aq) ----> PbSO₄ (s) + 2e^–

At Cathode:- PbO₂(s) + SO₄^–2 (aq) +4H (aq)+ 2e^– ----> PbSO₄ + 2H₂O

During recharging the cell is operated like an electrolytic cell i.e. now electrical energy is supplied to it from an external. The electrode reactions are the reverse of those that occur during discharging.

PbSO₄ (s) + 2e^- ----> Pb (s) SO₄^–2 (aq)

PbSO₄ (s) 2H₂O + ---> PbO2(s) SO₄^–2 + 4H⁺ + 2e^–

2PbSO₄ (s) 2H₂O + ---> Pb(s) + PbO₂ + 4H⁺ + 2SO₄^–2

Such operation is possible because the PbSO₄ formed during discharge is a solid & strikes to electrodes. It is, therefore, in a position to either receive or give up electrons during electrolysis.

(C) Fuel cells :

As already explained fuel cells are the devices which convert the energy produced during the combustion of fuels directly into electrical energy. One such cell which has been very successful is the hydrogen-oxygen fuel cell. This cell was used as the primary source of electrical energy on the Appolo moon flights. This may be compared with the several tons that would have been required for the engine-generator set.

The General design of H₂ - O₂ fuel cell is shown in fig. It consists of porous carbon electrodes containing suitable catalysts incorporated in them. Concentrated KOH or NAOH solution is placed between the electrodes to act as the electrolyte. The following electrode reaction take place.

At anode 2H₂ (g) OH^– (aq) ---> 2H₂O + 4e^–

At Cathode   

Thus gaseous materials are consumed & continuously supplied.