The present invention relates to the use of a composition of formula (I): Fe9-a-b-cNiaCobMcS8-dSed, wherein M stands for one or more elements having in the ionic state an effective ionic radius in the range of 70-92 pm, a is a number within the range of 2.5≤a≤3.5, more preferably 2.7≤a≤3.3, b is a number within the range of 1.5≤b≤5.0, more preferably 1.5≤b≤4.0, most preferably 2.5≤b≤3.5, c is a number within the range of 0.0≤c≤2.0, more preferably 0.0≤c≤1.0, d is a number within the range of 0.0≤d≤4.0, more preferably 0.0≤d≤1.0, wherein the sum of a, b and c is in the range of 5≤a+b+c≤8 and wherein ≥90 wt. % of the composition is in the pentlandite phase for electrocatalytic splitting of water, preferably for hydrogen evolution reaction.