Síntese e estudo do óxido de alta entropia (Fe0,2,Ni0,2Co0,2Al0,2Zn0,2)3O4 pelo método sol-gel assistido por polivinilpirrolidona (PVP) e sua aplicação como eletrocatalisador para a reação de evolução de oxigênio

Abstract

High-entropy oxides (HEOs) have emerged as a promising class of functional materials due to their tunable electrochemical and magnetic properties, with potential applications in electrocatalysis, energy storage, and electronic devices. In this context, the present study aims to synthesize and thoroughly characterize the high-entropy oxide (Fe₀.₂Ni₀.₂Co₀.₂Al₀.₂Zn₀.₂)₃O₄, referred to as HEO-600, synthesized at 600 °C via the sol-gel method assisted by polyvinylpyrrolidone (PVP). Additional samples were produced at higher temperatures, revealing the formation of distinct crystalline phases, including spinel-type and rock-salt structures. Structural and compositional characterization was carried out using various techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FEG-SEM) with energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). XPS analysis confirmed the equimolar composition of the elements in the HEO-600 sample, whereas the EDS analysis revealed a homogeneous distribution of cations, corroborating the formation of a uniform solid solution. TEM micrographs showed well-dispersed, non-agglomerated particles with an average size of 9.2 nm. The magnetic properties of the HEO-600 sample were investigated using Mössbauer spectroscopy, direct current (DC) magnetometry, and alternating current (AC) magnetometry. The Mössbauer spectrum indicated that iron ions were distributed between tetrahedral (A) and octahedral (B) sites in a 43% and 57% ratio, respectively, confirming a partially inverted spinel structure. Magnetic susceptibility measurements (DC and AC) revealed superparamagnetic behavior at room temperature, as well as the presence of a spin glass-like phase, with a transition temperature at 206 K. This magnetic phase was suppressed under an external field of approximately 4300 Oe, highlighting the complexity of magnetic interactions in the system. In addition to structural characterization, spectroscopic and electrochemical analyses were performed to evaluate the catalytic potential of the HEO-600 sample for the oxygen evolution reaction (OER). Fourier-transform infrared (FTIR) spectroscopy provided additional information on the vibrational configuration of the system, and the chemical bonds present in the oxide. Electrochemical experiments demonstrated that the HEO-600 sample exhibits promising performance for OER, with an overpotential of 358 mV at a current density of 10 mA cm⁻². This value indicates competitive catalytic activity compared to other transition metal-based high-entropy oxides. Thus, this work significantly contributes to the advance in the understanding of the structural, magnetic, and electrochemical properties of high-entropy oxides. The results obtained not only expand the knowledge of the correlation between synthesis, structure, and properties of these materials but also highlight their potential for technological applications in electrochemical catalysis.