Sulfur-Modified Low-Crystalline NiFe MOF-74 Precursors for Enhanced Alkaline Oxygen Evolution

Sulfur incorporation modulates the electronic structure of NiFe-MOF-74. • Enhanced OER kinetics with low overpotential and small Tafel slope. • Operando EIS and ATR-FTIR reveal facilitated intermediate evolution. • DFT reveals optimized adsorption energetics via d-band modulation. • Electronic inheritance effect persists after electrochemical reconstruction. Regulating the local chemical environment of transition metal-based electrocatalysts is important for improving oxygen evolution reaction (OER) kinetics, but the relationship among heteroatom modification, electrochemical reconstruction, and the resulting active phase remains difficult to establish unambiguously. Herein, a sulfur-modified low-crystalline NiFe MOF-74 precursor is developed to examine how sulfur-containing species influence the precursor coordination environment and subsequent OER behavior. Structural characterizations support the introduction of sulfur-containing species into the MOF-74-related precursor, accompanied by reduced crystallinity and a lower surface area. The optimized catalyst exhibits enhanced OER activity, delivering an overpotential of 257 mV at 10 mA cm⁻² with a Tafel slope of 63.4 mV dec⁻¹. Operando electrochemical impedance spectroscopy, methanol oxidation reaction probing, and operando ATR-FTIR measurements indicate that sulfur modification affects the electrochemical evolution process from precursor activation to oxygen-containing intermediate conversion. Post-OER XRD, XPS, and TEM mapping support reconstruction into a NiFe (oxy)hydroxide-like phase in which sulfur-containing species remain detectable after reconstruction. Density functional theory calculations based on representative reconstructed NiFeOOH/S-NiFeOOH models suggest that sulfur-associated local perturbation may induce electronic redistribution and reduce the maximum uphill free-energy change of the potential-determining step. These results are consistent with a possible sulfur-assisted evolution process in which sulfur-containing species modify the precursor and remain detectable after electrochemical reconstruction, while their exact role in the reconstructed active phase remains unresolved. Download: Download high-res image (292KB) Download: Download full-size image