Structural and Electronic Synergy in Mixed-Valence Metal Sulfide/Al-MOF Composites as Efficient OER Catalysts

Developing highly active electrocatalysts with sufficient stability and cost-efficiency remains a major challenge for achieving low overpotentials in the oxygen evolution reaction (OER) under alkaline conditions. In this work, we present the design and synthesis of mixed-valence, mixed-metal sulfide nanocomposites anchored on an Al-based metal–organic framework (MOF-303), known for its outstanding chemical stability, tunable porosity, and enhanced charge transport characteristics. Two nanocomposites, CoIICo0.44IIINi0.84IINiIIIS4/MOF-303 (CNS/MOF-303) and NiIINi0.44IIICo0.84IICoIIIS4/MOF-303 (NCS/MOF-303), are synthesized via an in situ strategy to ensure homogeneous dispersion of active sites within the porous MOF architecture. Comprehensive structural, morphological, and electrochemical characterizations, including HR-TEM, FE-SEM, EDX, UV–vis DRS, PXRD, XPS, FT-IR, BET surface analysis, Mott–Schottky plots, and various electrochemical techniques, confirmed the formation of crystalline, porous materials with optical band gaps of 2.09 and 2.06 eV for CNS/MOF-303 and NCS/MOF-303, respectively. Among the catalysts examined in this study, the NCS/MOF-303 and CNS/MOF-303 nanocomposites show the best OER activity, delivering the lowest overpotential at 10 mA cm–2 (η10 = 283 and 319 mV) and an acceptable Tafel slope (39 mV dec–1 and 55 mV dec–1). Remarkably, the NCS/MOF-303 electrocatalyst exhibits a low charge-transfer resistance (0.74 Ω cm2) and maintains a stable current density for 10 h under a constant applied potential, confirming its superior reversible energy efficiency and resistance, further confirming its superior OER activity. The outstanding catalytic performance of NCS/MOF-303 is linked to its low band gap, elevated carrier density, and favorable flat-band potential (−0.869 V vs Ag/AgCl), along with the inherent structural benefits of MOF-303. Such enhancements result from the synergistic interplay of mixed-valence metal centers, sulfur-induced charge-transfer improvement, and the high surface area and porosity of MOF-303.