Trimetallic synergy in Ni_2-x-y Co _x Fe _y P precatalysts enables robust oxygen evolution

Developing earth-abundant and stable oxygen evolution reaction (OER) catalysts remains important for practical utilization of alkaline water electrolysis. Previous efforts revealed that bimetallic, Mn-containing phosphides, including Co_2-x Mn _x P, Fe_2-x Mn _x P, and Ni_2-x Mn _x P, are promising OER precatalysts forming relatively stable oxyhydroxide catalysts upon oxidation of phosphide to solubilized phosphate under alkaline conditions. Notably, the addition of Mn produces more active catalysts than monometallic Co₂P, Fe₂P or Ni₂P, with optimal activity (minimized overpotential) found near 1 : 1 M : Mn ratio. In the present work, the role of Mn in catalysis is addressed indirectly by probing trimetallic phosphides comprising Co, Fe, and Ni without Mn. Ni_0.67Co_0.67Fe_0.67P nanocrystals exhibit enhanced catalytic activity relative to their mono- and bimetallic counterparts, achieving an overpotential (η ₁₀) of 276.5 ± 3.7 mV at 10 mA cm^-2. The composition was further varied to create Ni-rich (NiCo_0.5Fe_0.5P), Co-rich (Ni_0.5CoFe_0.5P) and Fe-rich (Ni_0.5Co_0.5FeP) phases, with the Ni-rich phase exhibiting a lower overpotential (η ₁₀ = 257.0 ± 5.0 mV) than the 1 : 1 : 1 trimetallic phase, while the Co- and Fe-rich phases proved less active. This non-monotonic trend indicates that activity is not governed simply by the concentration of an individual metal, but instead arises from an optimized Ni-Co-Fe environment in which Ni enrichment most effectively promotes the catalytically active state. The stability of Ni_0.67Co_0.67Fe_0.67P was evaluated through chronoamperometry (15 h bulk electrolysis at 1.7 V vs. RHE) and X-ray photoelectron spectroscopy (XPS) before and after catalysis. The current density showed minimal decay (∼6%, from ∼32 to ∼30 mA cm^-2), indicating robust performance. Notably, relevant to the bimetallic Mn-containing systems, the trimetallic (Co, Fe, Ni) phases are both more active and more stable under active OER conditions. This study demonstrates that trimetallic synergy between Co, Fe, and Ni results in enhanced alkaline OER activity and stability relative to bimetallic systems, including those containing Mn.