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Why is a zinc cathode used to deposit zinc from zinc sulfate solution?

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sphalerite — zinc processing Sphalerite from Baxter Springs, Kansas.

A zinc cathode is used during the electrolysis of a zinc sulfate solution to facilitate the deposition of zinc by utilizing the principle of overvoltage.

Theoretically, the voltage required to deposit zinc from a zinc sulfate solution is about twice the voltage necessary to decompose water. Under normal circumstances, this would result in the production of hydrogen gas at the cathode rather than the deposition of zinc. However, when a zinc cathode is employed, the hydrogen overvoltage—the additional potential required to evolve hydrogen gas—prevents the generation of hydrogen, thereby allowing the zinc to deposit instead.1

It is essential to note that this process relies heavily on the purity of the electrolyte. The presence of certain impurities can drastically lower the overvoltage, which would interfere with the deposition of zinc. Consequently, extreme purification of the zinc sulfate solution is a critical necessity in this metallurgical process.1