Compostos de coordenação visando o desenvolvimento de eletrodos para armazenamento e conversão de energia

Abstract

In this work, two materials based on ZIF-67/Ni(OH)2 and CoFe-ZIF-67/Ni(OH)2 were obtained in order to evaluate their electrochemical performances in both energy storage and conversion. A class of compounds called MOFs are three-dimensional structure consisting of a metal center and organic ligands. One class of MOFs is called ZIFs, which are materials with a tetrahedral zeolite-type topology with a central element – typically Zn or Co – that connects to each other through oxygen atoms. Therefore, two MOFs (ZIF-67 and CoFe-ZIF-67) were synthesized through the reaction between their metal precursors (cobalt acetate and iron sulfate) with 2- methylimidazole, while Ni(OH)2 was synthesized, in-situ, using the Tower method followed by electrochemical treatment of the electrodes. The composites were obtained by deposition of MOFs on a 1 cm2 of a fluorine-doped tin oxide substrate (FTO), using the drop-casting technique, followed by the deposition of Ni(OH)2 on the modified electrodes with the MOFs using the spin-coating technique. Through the X-Ray Diffraction technique it was possible to identify that the MOFs are in their amorphous form. Nickel hydroxide is in fact a nickel glycerolate, with a single characteristic diffraction peak located in the region close to 2θ = 10°, and the composites are strongly influenced by the structure of the nickel glycerolate, since the only diffraction peak presented is in the same region of 2θ = 10°, however with less intensity. In addition, the morphology of the MOFs was studied through Transmission Electron Microscopy, demonstrating their nanoparticle structures with the presence of interference fringes with an average distance of 0,130 nm (CoFe-ZIF67) and 0,125 nm (ZIF-67). The Scanning Electron Microscopy technique was also used in morphological studies, helping to understand the way in which the material is deposited on the FTO substrate. Energy dispersive spectroscopy (EDS) also contributed to the identification of the chemical elements present in the samples, such as cobalt, nitrogen, carbon, nickel, iron and oxygen, depending on the sample analyzed.The oxidation states of the metals in the composites were defined by means of X-Ray Photoelectron Spectroscopy, and for both ZIF-67/Ni(OH)2 and CoFe-ZIF67/Ni(OH)2 there is the presence of Co2+, Co3+, Ni2+ and Ni3+ ions, in addition to Fe2+ and Fe3+ ions being found in the CoFe-ZIF-67/Ni(OH)2 composite. Through electrochemical studies, such as cyclic voltammetry processes and galvanostatic charge and discharge curves, it was possible to identify that the most promising material in the area of energy storage was ZIF-67/Ni(OH)2, since it presented the highest specific capacity (106,99 mAh.g-1 at 1 A.g-1 ) and its capacity retention was 71,63% after 5000 consecutive charge and discharge cycles at 12 A.g-1 , in addition to their energy storage processes being characterized as diffusional and faradaic. Electrochemical Impedance Spectroscopy studies were performed to investigate the charge transfer resistance values for the five materials. Based on the values found, the CoFe-ZIF-67/Ni(OH)2 composite exhibits the lowest charge transfer resistance value related to the oxygen evolution process (7,6 Ω). Focused on the area of energy conversion, the material that presented the lowest overpotential value (at a current of 10 mA) in the water oxidation process during linear voltammetry readings was CoFe-ZIF-67/Ni(OH)2, with a value of 0,43 V, in addition to exhibiting good stability x and durability over 10 consecutive hours of application of a fixed current of 10 mA. The development of these new composites aggregatesin different areas: ZIF67/Ni(OH)2 for energy storage systems and CoFe-ZIF-67/Ni(OH)2 for energy conversion systems. Both materials showed positive results in their respective areas of study.