Impregnation of Metal ions on ZIF-67 Towards the Production of Mixed-Metals Spinels for OER Electrocatalysis

Electrochemical water splitting is a key method for sustainable hydrogen (H₂) production, but its efficiency is hindered by the slow kinetics and high overpotential of the oxygen evolution reaction (OER). This study investigates the synthesis and OER electrocatalytic performance of MCo₂O₄ (M = Co, Fe, Ni, Mn, or Zn) nanoparticles derived from ZIF-67, named here Co₃O₄, Co₃O₄(Fe), Co₃O₄(Ni), Co₃O₄(Mn), and Co₃O₄(Zn). Here, a systematic detailed study was also conducted to investigate the influence of metal cation impregnation on ZIF-67 and its impact on the OER electrocatalytic activity of these transition metal cobaltites nanoparticles. The materials were obtained via direct calcination of the previously modified ZIF-67, and the impregnation process was also investigated. Structural (XRD), physicochemical (Raman spectroscopy, FT-IR and UV-VIS), and magnetic characterizations confirm the formation of pure crystalline phases for Co₃O₄, NiCo₂O₄, MnCo₂O₄, and ZnCo₂O₄, while the Fe-modified sample resulted in a nanocomposite (FeCo₂O₄/Co₃O₄). Morphological analysis revealed highly agglomerated sphere-like nanoparticles, with sizes between 12.1 nm (Co₃O₄(Fe)) and 20.8 nm (Co₃O₄(Mn)). OER performance in 1.0 M KOH showed overpotential values of 330 mV, 318 mV, 321 mV, 316 mV, and 310 mV for Co₃O₄, Co₃O₄(Fe), Co₃O₄(Ni), Co₃O₄(Mn), and Co₃O₄(Zn), respectively, at 10 mA cm^-2. Tafel slopes ranged from 64.13 mV dec^-1 (Co₃O₄(Fe)) to 97.42 mV dec^-1 (Co₃O₄(Ni)), indicating a surface adsorption-controlled kinetics. Co₃O₄(Zn) exhibited the highest electrochemical surface area (123.50 cm²) and double-layer capacitance (4.94 mF), correlating with superior electrocatalytic performance. Stability tests confirmed chemical durability for up to 15 hours. These results demonstrate the potential of ZIF-67-derived transition metal cobaltites as effective OER electrocatalysts, with Co₃O₄(Zn) showing the most promising activity.