Example:

The exchange current density for H⁺ + e H₂ on Mercury is 10^-14 A/cm², but is several orders of magnitude higher on Platinum at 10^-2 A/cm². A large current density means a very stable potential which cannot be easily disturbed, hence, Platinized platinum is made a standard hydrogen electrode, so that the potential remains stable.

Figure below shows the exchange current density values for hydrogen reduction on Hg, Fe and Pt and overvoltage h_c Vs i (current density).