The effect of particle size on structural and catalysts for oxygen evolution reaction of (CoFeNiMnCr)₃O₄ prepared by controlled synthesis with polyvinylpyrrolidone (PVP)

In this study, high-entropy spinel oxides (CoNiMnFeCr)₃O₄ were synthesized using a PVP-assisted sol-gel method, marking the first report of this approach for producing high-entropy oxides. This method provides new insights into morphology customization through precise temperature control during calcination. Samples were calcined at 800, 900, and 1000 °C, and structural, optical, and electrochemical characterizations were performed to evaluate the impact of synthesis conditions on the oxygen evolution reaction (OER) performance. X-ray diffraction (XRD) confirmed the formation of a single-phase spinel structure with face-centered cubic symmetry. UV-Vis spectroscopy revealed a band gap shift associated with calcination temperature, indicating subtle changes in electronic structure that can influence catalytic behavior. The S-HEO 800 sample exhibited the highest catalytic activity, achieving an overpotential of 316 mV at 10 mA cm^-2. Electrochemical tests showed excellent short-term durability, with the electrodes maintaining stable performance for 24 h at 10 mA cm^-2. Field emission gun scanning electron microscopy (FEGSEM) analysis revealed that particle size increased with calcination temperature, ranging from 96 nm (S-HEO 800) to 475 nm (S-HEO 1000). X-ray photoelectron spectroscopy (XPS) showed a higher concentration of Cr⁶⁺, Cr⁴⁺, and Ni³⁺ ions on the surface of S-HEO 800, correlating with its superior OER performance. Additionally, Raman and FT-IR spectra confirmed the formation of the spinel phase and provided insights into metal-oxygen bonding. Electrochemical impedance spectroscopy (EIS) results indicated that S-HEO 800 exhibited the lowest charge transfer resistance (R_ct), further supporting its enhanced catalytic behavior. These findings demonstrate the potential of the PVP-assisted sol-gel method to produce customized high-entropy oxides with tunable morphology, making them promising candidates for energy conversion applications, particularly in water electrolysis.