Preparation and evaluation of double perovskite and misfit structured layered cobaltites as IT-SOFC cathodes

Abstract

1. PrBaCo2-xFexO5+δ (PBCFx, 0 ≤ x ≤ 2.0) are investigated as cathode materials for intermediate temperature solid oxide fuel cells. X-ray diffraction confirms that the material retains double perovskite structure and undergoes structural change from tetragonal to cubic as the Fe doping level reaches to x = 0.4. High-temperature X-ray diffraction patterns reveal that all the structures of PBCFx (x = 0.0→0.4) are thermodynamically stable upon heating to 900 °C. The increased doping of Fe deteriorates the electrical conductivity monotonically, with an exception of a big jump in the conductivity at x = 0.4, due to the structural change. Thus, except the low temperature region of 300~400 °C, PBCF0.4 possesses maximum values in the electrical conductivity at temperature range of 500~900 °C among all the Fe doping levels in PBCFx. PBCF0.4 used as cathode in the symmetrical cells of PBCFx/ Ce0.8Sm0.2O2-γ (SDC) exhibits the lowest cathode polarization resistance: 0.07 Ω•cm2 and 0.13 Ω•cm2 at 750°C and 700°C respectively. For anode-supported button cells of PBCFx

SDC

NiO+SDC, the highest power density is also observed for PBCF0.4 with a maximal power density of 446.4 and 346.3 mW•cm-2 at 700 and 650 °C, respectively. We conclude that Fe doping does not increase the conduction properties by itself but improves the electrochemical properties indirectly by changing the structure of PBCFx from tetragonal into cubic.2. The misfit compounds Ca3-xBixCo4O9-δ (x = 0.1−0.5) were successfully synthesized via conventional solid state reaction and evaluated as cathode materials for intermediate temperature solid oxide fuel cells. The powders were characterized by X-ray diffraction, scanning emission microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and oxygen-temperature programmed desorption. The monoclinic Ca3-xBixCo4O9-δ powders exhibit good thermal stability and chemical compatibility with Ce0.8Sm0.2O2-γ electrolyte. Among the investigated single-phase samples, Ca2.9Bi0.1Co4O9-δ shows the maximal conductivity of 655.9 S•cm−1 and higher catalytic activity compared with other Ca3-xBixCo4O9-δ compositions. Ca2.9Bi0.1Co4O9-δ also shows the best cathodic performance and its cathode polarization resistance can be further decreased by incorporating 30 wt.% Ce0.8Sm0.2O2-γ. The maximal power densities of the NiO/Ce0.8Sm0.2O2-γ anode-supported button cells fabricated with the Ce0.8Sm0.2O2-γ electrolyte and Ca2.9Bi0.1Co4O9-δ+30 wt.% Ce0.8Sm0.2O2-γ cathode reach 430 and 320 mW•cm−2 at 700 and 650 °C respectively. 3. A systematic study of 10 transition metal ion (M= Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) doping in Ca2.9Bi0.1Co4O9-δ cathode for solid oxide fuel cells is performed by measuring their crystal structures, electrical conductivities and electrochemical performances. The presence of metal ion dopants in the Ca2.9Bi0.1Co4O9-δ matrix significantly influences the properties of crystal structure and electrochemical performances. The electrochemical performances of metal ion-doped Ca2.9Bi0.1Co4O9-δ cathodes are quantified in terms of the electrical conductivity, impedance and power density of button cell. Doping with small amounts of ions for cobalt has negligible effect on the structure of powder samples as all of them form single-phase solid solutions with monoclinic misfit layered structure. However, the bar type samples of doping with Ti, Cr, Mn, Fe, Co, Ni, Cu and Zn keep the structure intact while those of doping with Sc and V slightly decompose into two phases after sintering. It is proposed that the metal dopants are located at different sites of double layered Ca2.9Bi0.1Co4O9-δ matrix due to their different ion radii, which mainly accounts for the difference of conductivity of doped samples. Among them, the Cu doped Ca2.9Bi0.1Co4O9-δ sample (Ca2.9Bi0.1Co3.9Cu0.1O9-δ) shows the highest electrical conductivity in the whole temperature range and has the lowest area specific resistance at 750 °C and 800 °C. The Ca2.9Bi0.1Co3.9Cu0.1O9-δ

Ce0.8Sm0.2O2+γ

NiO+Ce0.8Sm0.2O2+γ button cell shows obvious improvement than Ca2.9Bi0.1Co4O9-δ

NiO+Ce0.8Sm0.2O2+γ button cell. The maximal power densities of the Ca2.9Bi0.1Co3.9Cu0.1O9-δ cathode-cell were 689, 465 and 331 mW•cm−2 at 800, 750 and 700 °C respectively.4. The misfit compounds Bi1.74Ca2Co1.63O6.59 was successfully synthesized via ethylene glycol-citrate sol-gel method and evaluated as cathode materials for intermediate temperature solid oxide fuel cells. The powder was characterized by X-ray diffraction, scanning emission microscopy and X-ray photoelectron spectroscopy. The Bi1.74Ca2Co1.63O6.59 sample exhibit good thermal stability and chemical compatibility with Ce0.9Gd0.1O2-γ electrolyte. Area specific polarization resistance values of 28.3, 1.8 and 0.1 Ω•cm2 at 587, 695 and 800 °C in air were achieved respectively based on symmetric cell configuration, which are comparable to literature results. It demonstrated the potential application of Bi1.74Ca2Co1.63O6.59 as a cathode for intermediate temperature-solid oxide fuel cells. The maximal power densities of the NiO/ Ce0.9Gd0.1O2-γanode-supported button cells fabricated with the Ce0.9Gd0.1O2-γelectrolyte and Bi1.74Ca2Co1.63O6.59 cathode reach 551.2 and 374.7 mW•cm−2 at 700 and 650 °C respectively.