electrolysis

Duringthe electrolysis of concentrated sodium chloride solution, sodium ions andhydroxide ions migrate to the cathode. Hydroxide ions are formed by the selfionization of water, and thus the concentration of hydroxide ions are very low.

Consider that a cathode other than mercury is used. Although the concentrationof sodium ions is much higher than that of hydroxide ions, sodium ions are NOTpreferentially discharged. This isbecause it is hardly for the sodium ions to be reduced to sodium metal, assodium metal is very reactive in the presence of water.

However, if mercury is used as the cathode, sodium ions would be preferentiallydischarged. This is because the sodium metal formed is "dissolved" inthe mercury cathode to form sodium amalgam, Na(Hg), which is rather stable.
Na^+(aq) + e^- → Na(Hg)

I've the same question too so it's great that sb already Q&A about it, thx
But is there any other special cases like this that hydrogen can't discharge?
Thx again