Intrinsic and extrinsic compositional effects in ceria/carbonate composite electrolytes for fuel cells

Composite ionic conductors for fuel cells were produced by combination of one ceria-based ceramic electrolyte and various mixtures of Na and Li carbonates, using several processing routes. These materials were characterized by impedance spectroscopy, scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy with attenuated total reflectance. The chemical composition of the mixed carbonates and the processing route were influential in the development of impressive conductivity levels (close to 0.1 S/cm) at modest temperatures (below 600 °C), suggesting a complex role of chemical composition (Na/Li ratio), phase composition (solid/liquid ratio), processing route (mechanothermal history) and microstructure. Furthermore, exposure to humidity showed formation of OH groups. Such hydrogenated species are believed to increase the complexity of the global surface processes and charge transport mechanisms where several native and foreign species may play a significant role.     

Synthesis of nano-ceria powder of high thermal stability

A new method is studied for preparing nano-ceria by acid leaching MgO from CeO₂−MgO mixed powder made by calcining a gel. The MgO in the mixed powder can be leached completely with acetic acid solution at room temperature, but the CeO₂ remains unaffected. The surface area (Sg) of the resulting CeO₂ increases with Mg/Ce ratio, but approaches constant after the ratio is larger than a given value which is higher for the higher gel calcining temperature. When Mg/Ce is 8 (molar ratio) and the calcining temperature is 1253 K, the Sg of the resulting ceria remains 25.1 m²/g even after annealed at 1253 K for 4 h. In comparison with the conventional methods, CeO₂ from the new method shows higher surface area and higher thermal stability.     

Oxygen release behavior of metastable tetragonal t′meta-(Ce0.5Zr0.5)2 phases prepared by reduction and successive oxidation of t′ phase

Tetragonal CaF₂-related-type t′-/Ce_0.5Zr_0.5)O₂ phase was reduced at 773 K ≤ T_red. ≤ 1073 K to prepare precursors with various oxygen compositions: Ce₂Zr₂O_712d /0.111 < δ < 0.379). Metastable tetragonal t^′_meta-(Ce_0.5Zr_0.5)O₂ phases were prepared by oxidizing in O₂ at 873 K the precursors, and subjected to evolved oxygen gas analysis by heating the samples at a constant rate. When the precursor was prepared at T_red: ? 773 K; i.e. δ ? 0:379; the oxygen release behavior of the tetragonal phase agreed approximately with that of the t^′. For δ < 0.29, distinct features of the t^′_meta-(Ce_0.5Zr_0.5)O₂ appeared in the XRD results and Raman spectra. The temperature exhibiting the maximum release rate of oxygen from the t^′_meta became lower with decreasing the d value. It could be concluded that the thermodynamic behavior of the t^′_meta is related closely to the precursor composition and becomes more unstable with decrease in the δ value. The t^′ and t^′_meta phases with disordered arrangement of Ce and Zr ions may belong to the same space group; nevertheless, difference in the random arrays of the Ce and Zr ions was suggested on the basis of the present experimental results.     

Structural and electrical properties of grain boundaries in Ce0.85Gd0.15O1.925 solid electrolyte modified by addition of transition metal ions

In this work we present studies on applicability of transition metal additives as sintering and electrical conductivity aids for cerium gadolinium oxide electrolyte. The nanosized Ce_0.85Gd_0.15O_1.925 powder obtained by coprecipitation method was modified with Cr³⁺, Fe³⁺, Ni²⁺ or Cu²⁺ ions. Using high-intensity high-resolution X-ray powder diffraction data we have determined that Cr, Fe and Ni ions do not incorporate into the cerium gadolinium oxide surface or bulk when sintered at 1300 °C, but react with Gd ions to form Cr_0.9Gd_0.1O, GdFeO₃ and GdNiO₃ phases, while Cu incorporates in the material up to 0.7 mol% with a significant fraction of remaining material showing poorly crystalline CuO phase. The nanosized Ce_0.85Gd_0.15O_1.925 material shows already improved sintering properties than previous reports but full sintering is not achieved below 1300 °C, however Cr, Fe and mainly Cu impregnation allows full sintering at 1300 °C. 0.5 mol% Ni impregnated material sintered at 1500 °C shows enhanced grain boundary conductivity that probably indicates that Ni incorporates into Ce_0.84Gd_0.15O_1.925 above 1300 °C. The global results indicate, however, that optimization of ceria microstructure is at least of equal importance for sinterability and grain boundary conductivity than impregnation of the material with transition metal ions.     

One-step synthesis of composite electrolytes of Eu-doped ceria and alkali metal carbonates

Europium-doped ceria (EDC, Ce_0.9Eu_0.1O_2−δ)/alkaline carbonate (LNC, (Li,Na)₂CO₃) composite ceramics prepared through a one-step citrate-based route were analyzed by powder X-ray diffraction, infrared and laser Raman spectroscopies as well as scanning and transmission electron microscopy. The electrochemical behavior of the electrolyte material was studied by impedance spectroscopy in air, CO₂ and N₂ + H₂ (90/10 vol%, respectively) gas mixtures, in the temperature range 300–600 °C. The sub micrometric and even nanosized ceramic particles appeared as merged inside the mixed carbonates, with modest grain to grain necking. The EDC/LNC composite electrolytes showed a conductivity of 0.27 S cm⁻¹ at 600 °C in air, amongst the best ever reported, exceeding the usual requirements for fuel cell applications.     

Partial oxidation of methane over Rh/supported-ceria catalysts: Effect of catalyst reducibility and redox cycles

Partial oxidation of methane (POM) was studied over Rh/(Ce_0.56Zr_0.44)O_2−x, Rh/(Ce_0.91Gd_0.09)O_2−x, Rh/(Ce_0.71Gd_0.29)O_2−x and Rh/(Ce_0.88La_0.12)O_2−x. The effect of catalyst reducibility and redox cycles was investigated. It was found that the type of doped-ceria support and its reducibility played an important role in catalyst activity. It was also observed that redox cycles had a positive influence on H₂ production, which was enhanced as the number of redox cycle increased. Results of carbon formation are discussed as a function of ionic conductivity. Temperature programmed reduction (TPR) profiles, BET surface area, ionic conductivity and XRD patterns were determined to characterize catalysts. Catalytic tests revealed that of the materials tested, Rh/(Ce_0.56Zr_0.44)O_2−x was the most active material for the production of syngas, which correlates with its TPR profile. It was observed that doping CeO₂ with Zr, rather than with La or Gd caused an enhanced reducibility of Rh/supported-ceria catalysts.     

Electrical properties of Mg-doped Gd0.1Ce0.9O1.95 under different sintering conditions

Ce_0.9Gd_0.1O_1.95 with various Mg doping contents was synthesized by citric acid-nitrate low temperature combustion process and sintered under different conditions. The crystal structures, microstructures and electrical properties were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and ac impedance spectroscopy. Low solubility of Mg²⁺ in Ce_0.9Gd_0.1O_1.95 lattice was evidenced by XRD and FESEM micrographs. The samples sintered at 1300 °C exhibited the higher total conductivity than those sintered at 1100 and 1500 °C, with the maximum value of 1.48 × 10⁻² S cm⁻¹ (measured at 600 °C) at the Mg doping content of 6 mol%, corresponding to the minimum total activation energy (E_tol) of 0.84 eV (150–400 °C). The effect of Mg doping on the electrical conductivity was significant particularly at higher sintering temperatures. At the sintering temperature of 1500 °C, the addition of Mg (10 mol%) enhanced the grain boundary conductivity by over 10² times comparing with that of undoped Ce_0.9Gd_0.1O_1.95, which may be explained by the optimization of space charge layer due to the segregation of Mg²⁺ to the grain boundaries.     

Selective oxidehydrogenation of ethane with CO2 over CeO2-based catalysts

Ceria catalysts were found active and selective to the oxidehydrogenation of ethane (ODE) with CO₂ and the actual contribution for C₂H₄ formation from heterogeneous catalysis was 75–55% in the range 953–993 K. The presence of calcium ions in solid solution in the ceria crystalline network increased significatively the selectivity to ethene and the efficiency of CO₂ as oxidant in the heterogeneous reaction.