Natural Hydrogen

Hydrogen formation by Fe(+II) oxidation with temperature :

Initial 0.1 m Fe(+II)Cl2 aqueous solution heated from 25 to 300°C.
PHREEQC thermodynamic calculation with the Thermoddem 2020 version database; [i]: aqueous species i concentration; m: molality (mol/kgH2O)

Thermodynamics of the ferrous iron oxidation reactions at 25°C :

25°C standard Gibbs free energies of iron oxidation reactions (SUPCRT/slop07.dat)
25°C standard electrode potential values of the water and iron redox couples (Wikipedia)