MATERIAIS MISTOS ELETROATIVOS LIOFILIZADOS APLICADOS NA ELETROCATÁLISE DE ÁLCOOIS E UREIA

Abstract

The present work presents the results of the study of the structural behavior and the electrochemical stability of αNi(OH)2 combined with the Alrgel and Volclay clays, denominated mixed electroactive materials (MMEs). Nickel hydroxide presents two main polymorphs, the alpha phase and beta phase, which are distinguished by the organization between their lamellae and oxidation states. The alpha phase presents better electrochemical efficiency, but has lower structural stability, since it is easily converted into the beta phase. The chemical analysis by atomic absorption spectroscopy (EAA), X-ray dispersive energy (EDS) and simultaneous thermogravimetric analysis (TGA) were used to highlight that Argel clay has a higher capacity to intercalate ions, such as Ni2+ of exchangeable cations (Na+ and Mg+2), compared to Volclay clay. X-ray diffraction (XRD) profiles show an increase in the distance d(001) from the basal plane of the clays, compatible with the intercalation ability of αNi(OH)2 nanoparticles. Cyclic voltammetry (VC) studies show that MME keeps its electrochemical profile after 500 cycles, whereas electrochemical impedance spectroscopy (EIS) data corroborate with the proposed Ni(OH)2 nanoparticles being intercalated as well as dispersed on the surface of the clay. The MMEs are kept in aqueous suspension for the purpose of modifying the working electrode by drop casting, so it was necessary to dry for structural and spectroscopic characterization. Two different methods were used for drying: i) conventional method in a vacuum desiccator, and ii) lyophilization method. After drying the MMEs presented different morphologies, inducing a study about the influence of the drying method on the morphological, structural and electrochemical characteristics of the mixed electroactive materials. The dry vacuum MME has an ordering degree in its structure larger than lyophilized MME. This characteristic presupposed that dry vacuum MME could present better electrochemical behavior. However, VC results showed that MME-lyophilized has higher electrochemical activity, and electrocatalysis tests (methanol and urea) showed a significant increase of current when compared to MME-vacuum. Finally, it was verified that the mixed material can be reused in the electrocatalysis of urea when its electrolyte solution is replaced, having the ability to regenerate again presenting the Ni+2/Ni+3 redox processes responsible for the electrocatalysis of the organic molecule, showing to be an interesting, reusable and inexpensive material.