Impacto do Polimorfismo das estruturas Metal-Orgânicas nas propriedades eletrocatalíticas do CeO2 na Reação de Evolução do Oxigênio

Abstract

One of the most promising alternatives for generating clean energy is the use of gaseous Hydrogen (H2) produced by water splitting electrolysis. Water electrolysis consists of two half-reactions: the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER), the latter being known by slow kinetics, making large-scale hydrogen production less viable. Therefore, it is essential to use electrocatalysts to enhance the water splitting. Thus, this work synthesized two electrocatalysts based on cerium (Ce) due to lower cost and high abundance compared to noble metal based electrocatalysts like Pt, Ru, Ir and Pd. Metal-Organic-Frameworks (MOFs) were used as template to produce the materials. Therefore, two MOFs-Ce were synthesized, one via the hydrothermal method (MOF-Ce (H)) and the other at room temperature (MOFCe (RT)) using the succinate ligand and cerium (III) precursor. Both MOFs were calcined to obtain Cerium Oxides (CeO2) characterized by X-ray diffraction with Rietveld refinement, UV-vis spectroscopy, Raman spectroscopy, infrared spectroscopy, XPS and scanning electron microscopy. The diffraction patterns showed the average crystallite size and the crystalline structure of the cerium oxides, also identified by the infrared spectra that indicated the M-O absorption bands. A surface chemical analysis was performed by X-ray photoelectron spectroscopy (XPS). The UV–vis spectra exhibited a ligand-to-metal charge transfer band. Raman spectra indicated the three signals associated with the active modes of cerium oxide. The SEM images showed irregular morphologies. Electrochemical analyzes revealed overpotentials of η= 326 mV (CeO2 (H)) and η=319 mV (CeO2 (RT)) at j= 10 mA cm-2 and 1 mol L-1 KOH electrolyte, the Tafel slopes were 58.92 mV dec-1 (CeO2 (H)) and 62.25 mV dec-1 (CeO2 (RT)) and electrical double layer capacitance of 12.28 mF (CeO2 (H)) and 16.92 mF (CeO2 (RT)). Chronopotentiometry indicated stability within 24 hours. The EIS showed lower ohmic resistance to the CeO2 (RT) electrocatalyst. The results revealed that the electrocatalysts, mainly CeO2 (RT), presented excellent performance in relation to the materials already mentioned in the literature.