Non‐Stoichiometric NiFeMo Solid Solutions; Tuning the Hydrogen Adsorption Energy via Molybdenum Incorporation

Solution precursor plasma spraying is used to produce catalytic trimetallic coatings containing Ni, Fe and Mo directly onto stainless‐steel mesh, Ni foam and carbon paper. The resulting material is mostly comprised of face centered cubic FeNi3 alloy forming a highly porous coating with nanostructured features. The addition of Mo (up to ≈14 at%) generates no new crystal phases but only an increase in the lattice parameter, indicating the formation of FeNi3Mox solid solutions. The FeNi3Mox solid solutions are used as electrocatalyst for the hydrogen evolution reaction (HER) in alkaline media. The addition of Mo increases the HER activity significantly reaching an optimum performance at ≈9 at% Mo (FeNi3Mo0.40) with an overpotential at −10 mA cm−2 of 112 mV and a Tafel slope of 109 mV dec−1. The enhanced HER activity is attributed to the formation of a FeNi3Mox solid solution with an increased work function that is correlated to smaller hydrogen adsorption energies. Theoretical activity maps reveal that sites near superficial Mo atoms forms catalytic hot spots and are responsible for the observed activity. The incorporation of Mo into nanocrystalline FeNi3 is achieved via plasma spraying. The resulting material is characterized by a metallic solid solution with distinct morphology and electronic properties. The presence of Mo increases the work function, reduces the metal–hydrogen interaction, and reduces the formation of superficial Ni and Fe oxides, resulting in enhanced catalytic activity toward hydrogen production.