Electrochemical Cells

The fact that chemical reactions produce electrical effects was discovered accidentally in 1791 by Luigi Galwani, professor of anatomy at the University of Bologna, Italy. He found that an electric current flowed across two dissimilar metals between which was a moist substance. In his case, the moist substance was a frog and the passage of electrical current was detected by the twitching of its leg. Galvani thought that this was a manifestation of animal electricity. Ten years later, Alessandro Volta, a professor of natural philosophy at the nearby University of Pavia, successfully reproduced some of Galvani"s results with inanimate things. He assembled a series of silver and zinc discs in pairs separating each pair with a sheet of pasteboard soaked in conducting liquid. When the top most disc of silver was connected by a wire with the lower zinc disk, a steady current of electricity was produced. This is a voltaic pile. The basic reason for this electrochemical effect became clear rather slowly.

Electrodes: Copper (+ve) and Zinc (- ve)

Electrolyte: dil. H₂SO₄ (dilute sulphuric acid)

At cathode:

At anode:

The Zn²⁺ repel the H⁺ in the solution towards the copper electrode. They extract e^- from the copper rod and become neutral hydrogen atoms and finally escapes as gas. Due to this, the copper rod becomes positively charged and the zinc rod negatively charged. This process continues and a maximum potential difference is reached between the two electrodes dipped in the electrolyte. The emf is 1.08 volt. When connected externally through any resistor or bulb, conventional current flows from Cu to Zn. Electric current can be continuously drawn from the cell till all the zinc is dissolved or sulphuric acid is consumed.

This arises due to the presence of impurities in the zinc rod. These impurities form minute local cells all along the zinc rod; small local currents are set up resulting in the wastage of zinc even when the cell is not being used. This can be eliminated by rubbing mercury on the zinc rod (i.e., amalgamation).

A layer of neutral hydrogen is formed near the copper rod thus weakening the action of the cell by increasing its internal resistance and also producing a back emf. This is removed by using a depolariser like MnO₂ or CuSO₄, which oxidizes hydrogen into water.

In spite of the small emf and limited energy capacity, cells are used widely because of their compactness and convenience. We discuss here a few. Types of electrochemical cells, mostly those in common use.

Electrodes: Copper vessel (+ve)

Zinc rod (-ve) (amalgamated)

Electrolyte: dil. H₂SO₄

Depolarizer: CuSO₄ solution

The porous pot (of fired clay or porcelain) allows ions to pass between the two solutions, but prevents the solutions from mixing. A perforated shelf containing CuSO₄ crystals is placed near the top of the Cu vessel to maintain the CuSO₄ solution concentration.

The hydrogen ions so formed diffuse through the porous pot and interact with copper sulphate forming H₂SO₄ and Cu²⁺ ions.

Therefore, 2H⁺ + CuSO₄ ------> H₂SO₄ + Cu²⁺.

The copper ions are deposited on the vessel and there is no depolarisation. The zinc rod, which is amalgamated, avoids local action. The emf is 1.12 V.

Electrodes: Carbon rod (+ve)

Zinc rod (-ve)

Electrolyte: Saturated NH₄Cl solution

Depolariser: MnO₂

The electrons thus released move to the zinc rod via the external circuit. The ammonia gas so produced escapes. The hydrogen ions diffuse through the porous pot and interact with MnO₂. (The charcoal powder makes MnO₂ electrically conducting.) Therefore,

These positive charges are given to the carbon rod increasing its positive potential with respect to the electrolyte. The depolarisation action proceeds slowly. When current is drawn continuously, a partial polarisation therefore sets in, because of which the current falls. So the circuit is switched off till the hydrogen gas escapes. It is ready for use after sometime.

The Leclanche cell is used when intermittent currents are needed.

The emf of the cell is 1.45 V.

This is a portable form of Leclanche Cell. A moist paste of NH₄Cl and ZnCl₂ is contained in a zinc vessel acting as the negative electrode. ZnCl₂ is added to the electrolyte NH₄Cl. It keeps the paste moist because it is highly hygroscopic. The top is sealed with shellac or pitch so that paste does not dry up. The carbon rod covered with a brass cap is placed in the middle of the vessel surrounded by a paste of charcoal and MnO₂ in a muslin bag acts as the positive electrode. A small hole is provided in it to allow the gases formed by chemical action to escape.

The emf is ~ 1.5V

The dry cell is useful in circuits requiring only intermittent current because it can provide only 0.25 A continuously. If more than this is drawn continuously, polarization occurs