Eletrodeposição de óxidos de Ni-Co por corrente continua e pulsada: produção, caracterização e avaliação eletrocatalítica

Abstract

The depletion of fossil fuel reserves and the concerns about climate change and global warming have made sustainable clean energydemandsessential. The O2 detachment reactions are interesting in this aspect, because they involve many conversion processes and energy storage. The Ni insertion into the Co3O4structure leads to the formation of a mixed oxide,the nickel cobaltite (NiCo2O4), which presents electrocatalytic activity for the O2 evolution reaction (OER) superior than those of Ni or Co monometallic oxides. In this thesis, Ni-Comixed oxides filmswere produced on carbon steel substrate using electrochemical techniques, such as linear voltammetry and/or chronopotentiometry, by direct (DC) and pulsed (PC) current. Solutions contained Ni(II) and Co(II) ions in 1:2 molar ratio, in the presence of sodium citrate as the ligand, at different pH values (5.0, 7.5 and 10.5). The produced films were characterized morphological, microstructural and electrocatalytically. It was observed that the films produced using DC showed distinct morphologies for each experimental condition and solution used. The chemical and microstructural analyses performed in acid medium (pH = 5.0) and using potential sweep procedures have not indicated the presence of Ni or Co in their composition. However, when chronopotentiometry was used, the produced coating contained mostly Co and or cobalt oxides. At pH 7.5, under the conditions of potential sweep, the weight ratio of Ni:Co was nearly to 1:2. On the other hand, films with a higher Co content were formed when other experimental conditions were used,which may be related to the presence of cobalt oxyhydroxide phase (CoOOH) in these coatings. Polarization curves for the Ni-Co oxide films, they didn t present a potential defined as oxygen depletion reactions (OER).However, these Ni-Co oxide films have not shown feasibility to accelerate the OER process. Studies at pH more alkaline (10.5) showed filmswith homogeneous morphology and small grains. The microstructural results indicated the formation of the mixed Co-Ni oxide in the ratio of 1: 2 for all tested production conditions. All the films produced using the more alkaline pH solution (10.5) presented homogeneous morphology with small grains. Microstructural evaluations indicated the formation of mixed Ni-Co oxide in a 1:2 ratio for all tested production conditions. In turn, the coatings obtained by PC presented even lower grains than those produced by DC, using the solution at pH 10.5. However, the microstructural analysis confirmed that only a few conditions produced Ni-Co oxides. The electrocatalytic evaluations of the Ni-Co oxide films produced by DC,in solution at pH 10.5, and by PC, in solution at pH 7.5, indicated that the films produced are suitable to be used as OER electrocatalysts, although they are not suitable to act as capacitors