Produção de materiais com gradiente funcional para células a combustível de óxido sólido

Abstract

Fuel cells principle is the conversion of chemical energy into electrical energy. These cells have the advantage of being a clean energy source. There are various types of fuel cells, and the main differences between them are the electrolyte used and the operating temperature. Among these cells, there is the solid oxide fuel cell (SOFC), in which the electrolyte is produced by solid oxides. Fuel cells are composed by two porous electrodes, separated by a dense electrolyte. To produce electrolytes of SOFC, cerates and zirconates have been studied for their good protonic conductivity properties, especially when doped. For the electrode, the addition of nickel to other oxides is widely used to improve electrical conductivity. The metallic phase facilitates the reduction of fuel and creates a path for the H+ ion to the electrode-electrolyte boundary. One of the problems presented in the fuel cells production is the interface between the electrolyte and the electrodes, as due to different properties between the materials, tensions can cause detachment between them. One possible way to avoid this problem is to use of materials with a composition gradient. The gradual variation allows a better adhesion between the electrode and the electrolyte and can avoid, or at least mitigate, thermal expansion problems. In this work, the production, by the co-pressing process, of a half-fuel cell composed of a BCZY electrolyte (BaCe0.2Zr0.7Y0.1O3–δ) and a BCZY-NiO electrode, with NiO added in the form of functional composition gradient was studied. Three layers of the BCZY-NiO electrode were made, with NiO amounts of 50, 30 and 10% by weight (compositions 50-50, 70-30 and 90-10, respectively). The initial composition was made with NiO, and after sintering, the sample was heated at 900°C for 6 hours, in a reducing atmosphere of 96% Ar and 4% H2, to reduce of NiO to metallic nickel. The sintering temperature (1350°C) was determined after the physical characterization of the compositions individually. By X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive spectroscopy, the formation of metallic Ni and the increase in porosity were confirmed in the compositions 50-50 and 70-30. By SEM, it was possible to observe that the adhesion between the functional gradient layers was without detachment or cracks. Cells made without a functional gradient, with only the 50-50 layer and BCZY, showed numerous cracks after sintering, indicating the efficiency of the layers to maintain cell integrity. By impedance spectroscopy, the conductivity for the 50-50 composition was 0.1 S/cm and its activation energy 0.04 eV, indicating predominantly electronic conductivity. The conductivity of 70-30 and 90-10 compositions were 7.22x10-5 S/cm and 5.05x10-5 S/cm and their activation energies of 0.52 eV and 0.51 eV, both results indicating predominantly proton conductivity. The cell showed conductivity of 2.14x10-3 S/cm at 800°C, a value comparable to works using similar configurations of composition.