Guest-Matched Encapsulation in Ni-MOF-274 for Phase-Controlled FeNi Alloy Electrocatalysts toward Oxygen Evolution

The facile synthesis of metal-organic framework (MOF)-derived catalysts with precisely controlled compositions and structures for oxygen evolution reaction (OER) remains a formidable challenge. Herein, we propose a host-guest encapsulation strategy to modulate the phase evolution by leveraging the structural dimensions of the host Ni-MOF. Specifically, two iron-based guests with varying molecular sizes (e.g., ferrocene (FcFe) and iron phthalocyanine (PcFe)) are selected to enter a large-pore Ni-MOF-274. A significant host-guest compatibility effect was identified, whereby the compact FcFe molecules facilitate deep penetration and uniform distribution within the MOF channels. This leads to the formation of ultrafine and highly dispersed Ni-rich FeNi3 alloy nanoparticles (FcFe@Ni-C) during pyrolysis. In contrast, the large PcFe molecules encounter substantial steric hindrance, resulting in the formation of an Fe0.5Ni0.5 phase (PcFe@Ni-C) characterized by larger particle sizes and lower active site density. The optimized FcFe@Ni-C exhibits satisfactory OER activity in 1.0 M KOH, requiring an overpotential of only 294 mV to achieve a current density of 10 mA cm-2 with a low Tafel slope of 72.91 mV dec-1, while maintaining 93.7% current retention over 30 h. Furthermore, DFT calculations reveal that the FeNi3-derived Fe4Ni14OOH model exhibits a distinct spin-resolved Fe 3d electronic structure, which is associated with more favorable adsorption energetics of oxygen-containing intermediates and a lower free-energy requirement for the *O-*OOH step.