Redefining Fe-doping effects: Solution-phase redox cycling dominates pre-lattice Ni3<b>+</b> formation and 2/1.5D nanostructure for enhanced OER

Abstract

Fe doping is widely known to enhance the oxygen evolution reaction (OER) activity of Ni-based electrocatalysts; however, the underlying mechanisms remain controversial, with interpretations focused on post-incorporation effects. Here, we uncover a previously overlooked pre-incorporation mechanism: Fe3+-mediated redox cycling in solution that directly generates high-valent Ni3+ species, facilitating the formation of catalytically active Ni2O3H and nanostructures. Under hydrothermal conditions, spontaneous electron transfer occurs from Ni2+ to Fe3+, yielding Ni3+, while simultaneous regeneration of Fe2+ sustains a self-perpetuating redox cycle for continuous Ni2+ oxidation. Remarkably, at ultra-low Fe3+ concentrations, a hybrid morphology featuring both 2D and 1.5D structural characteristics emerges. Most Fe3+ precipitates as colloidal Fe(OH)3 without incorporation into the lattice, particularly near critical doping thresholds. The trace Fe-induced 2/1.5D nanostructures exhibit superior OER performance, achieving a current density of 10 mA cm−2 at an overpotential of merely 225 mV––comparable to that of heavily doped counterparts. Additionally, Fe doping synergistically activates both the lattice oxygen mechanism and the adsorbate evolution mechanism. These findings reveal a paradigm in Fe–Ni catalyst design, wherein controlled dopant-mediated redox chemistry and nanostructure play pivotal roles in optimizing electrocatalytic performance.