Asymmetric Fe–O–Ni Pair Sites in Two-Step Dealloyed Prussian Blue Analogue Nanocubes Enrich Linear-Adsorbed Intermediates for Efficient Oxygen Evolution

Ni-Fe (oxy)hydroxides are among the most active oxygen evolution reaction (OER) catalysts in alkaline media. However, achieving precise control over local asymmetric Fe-O-Ni active sites in Ni-Fe oxyhydroxides for key oxygenated intermediates' adsorption steric configuration regulation of the OER is still challenging. Herein, we report a two-step dealloying strategy to fabricate asymmetric Fe-O-Ni pair sites in the shell of NiOOH@FeOOH/NiOOH heterostructures from NiFe Prussian blue analogue (PBA) nanocubes, involving anion exchange and structure reconstruction. Initially, ammonium sulfide forms a Ni-S surface layer on NiFe-PBA (partial sulfidation), followed by anodic polarization to convert the sulfide shell to amorphous NiOOH while triggering Fe exsolution and redeposition. The combinations of ex/in situ characterizations and theoretical calculations reveal that, compared to the symmetric Ni-O-Ni, the asymmetric Fe-O-Ni structure shortens Ni-O bonds and upshifts the d-band center. This preferentially enriching linear-adsorbed oxygenated intermediates (LAOs) over bridging-adsorbed oxygenated intermediates (BAOs), lowering the energy barrier of the rate-determining step of OH deprotonation to O from 1.70 eV (BAO pathway) to 1.54 eV (LAO pathway). The optimized catalyst achieves an ultralow overpotential of 232 mV at 10 mA cm^-2 and a Tafel slope of 46 mV dec^-1 on a glassy carbon electrode, along with a high durability (>48 h). Furthermore, the catalyst can achieve a high current density of 1 A cm^-2 at 1.69 V in a laboratory-scale electrolyzer for 600 h, demonstrating its potential for practical application. This work elucidates the vital role of asymmetric sites in reshaping intermediate adsorption steric configurations, offering deep insights into designing high-efficiency OER catalysts.