Sacrificial MoO42− containing Ni/Co-(pre)catalysts: understanding of active structure and Fe-dynamics under steady-state conditions in alkaline oxygen electrocatalysis

Abstract

Incorporation of trace iron (Fe) into nickel (Ni)- and cobalt (Co)-based catalysts is widely recognized to enhance oxygen evolution reaction (OER) activity in alkaline media. In this regard, the economic approach involves utilizing as little Fe as possible while maintaining a sustained catalysis. Here, we probe the minimum Fe content required to sustain long-term steady-state water oxidation by employing Ni/CoMoO₄ as a (pre)catalyst that undergoes a two-stage reconstruction: spontaneous conversion to amorphous Ni³⁺-oxo (Ni(O)OH) and crystalline Co^3/4+-oxo (Co(O)OH) phases in alkaline electrolyte. By alternately cycling in Fe-containing and Fe-free electrolytes, we uncover distinct Fe dynamics in the reconstructed phases—Fe incorporation is minimal and surface-localized in Ni(O)OH but more stable within Co(O)OH. The Fe species undergo continuous dissolution–redeposition cycles that directly govern the catalytic activity. In situ Raman spectroscopy reveals the emergence of Ni³⁺ and Co⁴⁺ redox centers at the Fe-operating potentials, exhibiting distinct vibrational signatures upon Fe uptake and loss. These findings establish that a trace yet replenishable Fe³⁺ supply is essential for maintaining high OER performance and long-term catalyst durability.