Voltaic cells are the electrochemical cells that generate electricity. In common usage these cells are known as batteries (since a battery contains one or more of such cells). These cells operate on redox reactions between metals. In simple terms, one metal electrode loses electrons and the other electrode gains them. This flow of electrons generates electricity. Once the chemical reaction is done, the battery discharges.
Copper and zinc are commonly used as the electrodes, with the zinc anode undergoing oxidation (losing electrons) and the copper cathode undergoing reduction (gaining electrons), thus completing the redox reaction.
An alloy is a heterogeneous mixture of two or more metal elements. In the case of voltaic cells, we use pure metals as electrodes and not their alloys. So, we use pure zinc and not its alloy in a voltaic cell.
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