Síntese verde e efeito da dopagem de Manganês nas propriedades de eletrodos à base de óxidos de valência mista MnXCo3-XO4 (0 ≤ X ≤ 1) para reação de evolução de oxigênio

Abstract

In recent decades, with the exponential growth of the population, the intensification of fossil fuel use has generated major impacts on the environment. In this scenario, the worldwide interest in the development and use of clean, sustainable, and renewable energy resources has become necessary. An increasing solution is the use of Hydrogen (H2) that has been considered as the most promising renewable energy source to supply the growing global demand. Thus, among the methods for producing Hydrogen that has received a lot of attention is water splitting, which occurs through two half-reactions: the Hydrogen evolution reaction (HER) and the Oxygen evolution reaction (OER). However, the OER is a slow reaction, due to the many steps involved in the process. Therefore, the development of effective electrocatalysts to accelerate the reaction and reduce the overpotential is of extreme value. Within this perspective, mixed valence oxides present themselves as efficient electrocatalysts for OER due to their multiple valence states, their abundance in nature, their low cost, and the possibility of obtaining them by several synthesis methods. Among the methods for obtaining mixed valence oxides, the green synthesis using organic precursors has stood out because it allows the preparation of single phase nanoparticles, with good homogeneity and in a sustainable way, minimizing the waste generated to the environment. In this context, this work presents the green synthesis of mixed valence oxide of the MnXCo3-XO4 (0≤X ≤1) type, using Agar-Agar as organic precursor. In view of this, the powders were calcined at 1173 K and characterized by various analyses such as X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Transmission Electron Microscopy (TEM), Raman spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Vibrating Sample Magnetometer (VSM) and were also investigated as electrocatalysts for the oxygen evolution reaction (OER) in alkaline medium. Through XRD analysis, it was observed that pure cubic phase was obtained for all samples. The presence of Co3+, Co2+, Mn2+, Mn3+ and Mn4+ was confirmed by X-ray photoelectron spectroscopy (XPS). With respect to OER, the best result was observed for the MnCo2O4 sample (X=1,0), which obtained an overpotential of 299 mV vs. RHE, thus, the good electrocatalytic performance of this material compared to electrodes in the literature is explained by the presence of Manganese doping in the MnXCo3-XO4, system especially Mn+3, which due to its electronic configuration, has a distortion in the structure, which favors charge and mass transport processes. Moreover, the same sample showed a lower Tafel value of 52 mV dec-1 and excellent electrochemical stability for 15 h. Therefore, the green synthesis method presented in this work, showed great potential for obtaining electrocatalysts used in the oxygen evolution reaction for water electrolysis.