No doubt, hydrogen shows the highest overvoltage on a mercury surface. This is because mercury is a liquid and so there is always a smooth surface with almost no "hot spots" for the development of hydrogen. Keep in mind that hydrogen on a very smooth platinum surface shows also overvoltage, that's the reason why platinum electrodes (e.g. for measuring differences in potentials) are "platinized" with platinum black. But this phenomeneon is not only dedicated to mercury and platinum, many other other metals show this effect too.
To name a few in increasing order of overvoltage:
Ag < Ni < Cu < Pb < Hg
Of particular interest here are lead and copper. Copper is well known from the famous zinc/copper couple already used in some reductions, but what about an aluminium/copper or an aluminium/lead couple?
In galvanic applications the effect of overvoltage is increased by forming a colloidal layer around the electrodes (blocking of "hot spots"). Colloids used are gelatinous organic substances or chromium(III)oxyhydrates or the corresponding oxyhydrates of iron(II).
Fertile field for the experimenter...
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