What are irreversible cells

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Reversible galvanic cells

At the beginning of the 19th century, galvanic cells were already being used as power sources. For example, Faraday used it in his famous electrolysis experiments. In their first form these were - and - plates, between which filter papers soaked with thinner were placed. Multiple layers were used to obtain higher stresses.

Etc.

These galvanic cells had the disadvantage of a mostly short service life, since chemical reactions took place in the cell even in the absence of a connecting wire between the poles; here it is the dissolution of the zinc by the dilute sulfuric acid. These cells, in which chemical reactions take place even without current flow, are called irreversible cells.

This naming becomes more understandable if we consider the opposite case: the reversible cells.

If we apply an (external) voltage to a Daniell element, which is opposite to the EMF, no current flow occurs, although there is a conductive connection (namely via the external current source). If you now reduce the external voltage (- 1 mV), a current flows from right to left. If this current is exactly 1 mA with an increase of 1 mV, the cell is called a reversible cell. This is observed to a good approximation for the Daniell element. But the Daniell element is also not exactly reversible, since ion currents occur at the phase separation surface, which cannot be completely reversed when the voltage is reversed.