Síntese e caracterização eletroquímica de catodos à base de cobaltita de cálcio para célula a combustível de óxido sólido

Abstract

The misfit cobaltite with Ca3Co4O9-δ composition (C349), over the years investigated as thermoelectric material due to its great electrical conductivity and low thermal conductivity, has recently attracted great attention as cathodic material of solid oxide fuel cells (SOFC, Solid Oxide Fuel Cell). The present work is the first report of C349 powder synthesis by solid state reaction using mollusc shell powder as a natural source of calcium carbonate. The synthesized C349 was structurally characterized by X-ray diffraction (applying Rietveld refinement of diffraction data), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For electrochemical investigation, symmetric cells of C349 as well as C349 + PrOx composite, were prepared by screen printing, sintered and characterized by impedance spectroscopy as a function of temperature (600-800 °C) and the partial pressure of oxygen (pO2 = 1-10-2 atm.). For the Ca3Co4O9-δ + Ba0,5Sr0,5Co0,8Fe0,2O3-δ (BSCF) composite, cells were prepared presenting platinum counter-electrode and reference electrode. The results showed that the polarization resistances (Rp) for the pure C349 cathode are in agreement with the values reported in the literature (Rp of 5.73 Ω.cm2 and 0.88 Ω.cm2 at 700 and 800 °C in pure oxygen atmosphere, respectively). For the C349 + PrOx and C349 (a.e.) + BSCF composites, the results showed a significant reduction of the total polarization resistance, presenting values of 0.75 Ω.cm2 and 0.34 Ω.cm2 at 700 and 750 °C for composite C349 + PrOx and 0.40 Ω.cm2 and 0.20 Ω.cm2 at 700 and 750 °C for the composite C349 (a.e.) + BSCF, respectively, in pure oxygen atmosphere. This improvement was mainly due to a large increase in the surface Exchange processes, which dominate the total polarization resistance. The composite cathode (C349 + PrOx) was considerably more efficient (up to 87%) than the pure C349 cathode due to changes in the praseodymium oxidation state (Pr+3/Pr+4), as well as to its high surface area of contact, allowing better adsorption, dissociation and reduction of molecular oxygen in the triple phase boundaries (TPB’s). In the same way, the composite containing BSCF showed a significant improvement due to the high values of mixed conductivity (ionic and electronic) and high contact surface area of the compound BSCF, being significantly more efficient in relation to the pure C349 cathode, with up to 97.7% improvement in performance at 600 °C and 93% improvement at 700 °C.