A simple voltaic cell has two major defects. They are

1. Local action and
2. Polarisation

1. Local Action : This is caused by the impurities present in zinc rod. When the zinc rod isimmersed in acid, the zinc atoms and the impurity atoms form a large number of local cells and the zinc rod gets consumed even when the cell is not in use. This defect can be avoided by using amalgamated zinc rods. Mercury dissolves zinc and allows only zinc atoms to come into contact with acid and thus avoids formation of local cells.
2. Polarisation: The hydrogen ions reaching the negative electrode get accumulated preventing further neutralization of hydrogen ions. This results in decrease of emf of the cell and produces a back emf. This can be eliminated by :

(i) Mechanical method. In this method, the hydrogen layer from the positive electrode is removed, using a brush. But this method is not so satisfactory.

(ii) Electrochemical method. In this method, a liquid called depolariser is used. The hydrogen ions react with this and some other gas is produced which does not – cause polarisation.

(iii) Chemical method. In this method, hydrogen is converted to water with an oxidising agent like manganese dioxide, potassium dichromate etc.

courtesy : chemistry.2luv.in