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 (H 2 ) 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 MCo 2 O 4 (M = Co, Fe, Ni, Mn, or Zn) nanoparticles derived from ZIF-67, named here Co 3 O 4 , Co 3 O 4 (Fe), Co 3 O 4 (Ni), Co 3 O 4 (Mn), and Co 3 O 4 (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 Co 3 O 4 , NiCo 2 O 4 , MnCo 2 O 4 , and ZnCo 2 O 4 , while the Fe-modified sample resulted in a nanocomposite (FeCo 2 O 4 /Co 3 O 4 ). Morphological analysis revealed highly agglomerated sphere-like nanoparticles , with sizes between 12.1 nm (Co 3 O 4 (Fe)) and 20.8 nm (Co 3 O 4 (Mn)). OER performance in 1.0 M KOH showed overpotential values of 330 mV, 318 mV, 321 mV, 316 mV, and 310 mV for Co 3 O 4 , Co 3 O 4 (Fe), Co 3 O 4 (Ni), Co 3 O 4 (Mn), and Co 3 O 4 (Zn), respectively, at 10 mA cm −2 . Tafel slopes ranged from 64.13 mV dec −1 (Co 3 O 4 (Fe)) to 97.42 mV dec −1 (Co 3 O 4 (Ni)), indicating a surface adsorption-controlled kinetics. Co 3 O 4 (Zn) exhibited the highest electrochemical surface area (123.50 cm 2 ) and double-layer capacitance (4.94 mF), correlating with superior electrocatalytic performance. Stability tests confirmed chemical durability for up to 15 h. These results demonstrate the potential of ZIF-67-derived transition metal cobaltites as effective OER electrocatalysts , with Co 3 O 4 (Zn) showing the most promising activity.