Processamento, microestrutura e caracterização eletroquímica de cermets Ni-CGO obtidos por síntese em uma etapa

• Ano de defesa: 2013
• Autor(a) principal: Macedo, Daniel Araújo de
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal do Rio Grande do Norte; Brasil; UFRN; Programa de Pós-Graduação em Ciência e Engenharia de Materiais; Centro de Ciências Exatas e da Terra
• Palavras-chave em Português: Nanocompósito; Síntese em uma etapa; Polarização anódica; Espectroscopia de impedância.; CNPQ::ENGENHARIAS
• Link de acesso: https://repositorio.ufrn.br/jspui/handle/123456789/27550

Ni-Ce0.9Gd0.1O1.95 (Ni-CGO) cermets have been widely investigated as solid oxide fuel cell (SOFC) anodes. The electrochemical performance of these materials depends strongly on the so-called triple phase boundaries (CTFs) length, electrochemically active sites where gas, solid electrolyte (CGO) and metal (Ni) are in mutual contact. The microstructural optimization of Ni-CGO anodes, involving an initial step of powder preparation an additional steps of ceramic processing, has been the most used strategy to increase CTFs length. In this context, this work is focused on assess the properties of different NiO-CGO powders obtained by chemical synthesis and by mechanical mixture of powders, as anode materials for SOFC. Emphasis is given to the study of the nanocomposite NiO-CGO obtained by a chemical route developed at UFRN, called "one-step synthesis". The first part of this paper provides a comparative microstructural study between Ni-CGO cermets prepared from the nanocomposite and a mixture of oxides previously synthesized by the polymeric precursor method. The microstructure of NiO-CGO composites and Ni-CGO cermets is investigated as a function of the content of a pore-forming agent. The electrical percolation of NiO and CGO phases is analyzed by impedance spectroscopy measurements. The second part of this paper deals with a comparative study of the correlation between the microstructure and the electrical properties of NiO-CGO composites obtained by one-step synthesis and by mechanical mixing of commercial powders. The samples were sintered between 1350 and 1450 °C and then characterized by scanning electron microscopy and impedance spectroscopy. It is observed that the total electrical conductivity of the composites obtained by one-step synthesis, measured between 100 and 500 °C, is more than an order of magnitude higher than for samples prepared from commercial powders. This result is associated to the presence of a network of well-connected and homogeneously distributed NiO grains, which is responsible for the improved percolative behavior and lower activation energy (0.24 eV at 250 - 650 °C after sintering at 1450 °C) of samples derived from nanocomposite. In the third part of this work, cermets were obtained by screen printing of NiO-CGO suspensions on dense CGO substrates. Reference anodes were prepared by mixing commercial oxides. The electrochemical performance of the cermets was studied by impedance spectroscopy and anodic polarization. Measurements were carried out in the temperature range between 650 and 750 °C and wet hydrogen atmosphere. The results indicate that the overall resistance of the anodes is dominated by the low frequency impedance. The cermet derived from the one-step 9 Daniel Araújo de Macedo Tese de Doutorado PPGCEM - UFRN powder and sintered at 1450 °C exhibits an area specific resistance of 0.15 ohm.cm2 at 750 °C and a polarization of 91 mV (measured at 750 °C and current density of 322 mA/cm2), values much better than those obtained for the reference anodes, under the same experimental conditions. The enhanced electrochemical performance of one-step cermet anodes is mainly attributed to unique microstructural features, namely small grain size (submicrometric scale even after sintering at 1450 °C) and homogeneous phase distribution, which is expected to extend the triple-phase boundary length.