Overpotential is the extra voltage beyond the thermodynamic minimum needed to drive electrode reactions at an appreciable rate. It varies with electrode material, current density, temperature, and the nature of the reaction. Gas evolution reactions (H2, O2) have high overpotentials on most metals. O2 has particularly high overpotential, which is why Cl2 is produced at the anode during electrolysis of concentrated NaCl (even though E for O2 evolution is more favourable thermodynamically). On platinum (platinised), overpotential is low. On mercury, H2 overpotential is very high — this is exploited in the mercury cell process for NaOH production.