Why can impurities interfere with zinc deposition during electrolysis?

In the electrolytic production of zinc, the deposition of metal onto the cathode relies on a phenomenon known as hydrogen overvoltage. Theoretically, the voltage required to deposit zinc from a zinc sulfate solution is high enough that water would decompose to produce hydrogen instead of zinc. However, the use of a zinc cathode creates an overvoltage that prevents this hydrogen generation, allowing zinc to deposit successfully.1

The presence of certain impurities in the zinc sulfate electrolyte, even at very low concentrations, causes a drastic lowering of this hydrogen overvoltage. When the overvoltage is reduced, the system favors the generation of hydrogen gas rather than the deposition of zinc, which interferes with the efficiency and success of the process. Because of this, extreme purification of the electrolyte—involving the removal of iron and cementation with zinc dust to eliminate metallic impurities like copper, cobalt, and nickel—is a critical necessity in zinc metallurgy.1