Síntese fitoassistida de óxidos metálicos para aplicação na reação de evolução de oxigênio (OER)

Abstract

Global ecological demands reinforce the need for the use of clean and renewable energy sources, such as hydrogen (H2), which is a promising energy source that can be obtained through water electrolysis. Although this process is environmentally friendly, it requires high energy input, and electrocatalysts are needed to make it viable. Therefore, efforts have been made to develop high-performance electrocatalysts based on earth-abundant transition metals. Greener synthesis routes, such as using plant extracts that stabilize nanoparticles, are also on the rise as they help reduce environmental impacts. Thus, this study reports a green synthetic route using aqueous (H) and ethanolic (E) extracts of the wild parsley plant (Ipomoea asarifolia) as a stabilizing agent to obtain nickel oxide (NiO) nanoparticles at different calcination temperatures (300 °C, 500 °C, and 700 °C). The plant’s phytochemistry was investigated by HPLC, and the materials were characterized by X-ray diffraction (XRD), vibrational spectroscopy in the infrared region (FTIR), Raman spectroscopy, solid-state UV-VIS spectroscopy, and scanning electron microscopy with energy- dispersive X-ray spectroscopy (SEM-EDS). The techniques confirmed the cubic structure of NiO and Co3O4, with residual organic groups on the material surfaces. Raman spectroscopy signals also confirmed the presence of structural defects, which positively contribute to the electrocatalytic process. The synthesis using aqueous extracts led to the formation of materials with diverse morphology, while ethanolic extracts resulted in more uniform particles. Electrocatalytic performances in alkaline solution (KOH, 1.0 mol L-1) were investigated. The experimental overpotentials at J = 10 mA cm-2 and Tafel slope values were promising, starting from 307 mV and 60.35 mV dec-1 for NiO samples, and 290 mV and 47.11 mV dec-1 for Co3O4 samples. Therefore, this green synthesis route proves to be economically viable, producing high- performance electrocatalysts compared to similar results in the literature.