The tungstates Ln 6 WO 12 are proton-conducting materials exhibiting sufficient electronic conductivity to consider them as potential candidates for the separation of hydrogen at high temperature. Hydrogen-permeable membranes will find application in power plants applying precombustion strategies, process intensification using high-temperature catalytic membrane reactors, and in components for electrochemical systems as proton conducting fuel cells (PCFCs) and electrolyzers. This work presents the preparation and characterization of nanocrystalline mixed conducting materials with three different nominal compositions (Nd 6 WO 12 -Eu 6 WO 12 -Er 6 WO 12 ) using a sol-gel complexation synthesis method. The evolution of the crystalline structure and crystallite size is studied as a function of the sintering temperature. Generally, the nanosized oxides show a (pseudo)-cubic crystalline fluorite structure which evolves into the most stable fluorite symmetry (tetragonal for Nd and rhombohedral for Er) with increasing sintering temperatures, i.e., crystallite sizes. Shrinkage behavior was analyzed for the three compositions in the range from 1000 to 1500 °C and the nanosized Nd-based oxide showed a very high sintering activity even at relatively low temperatures (1100-1200 °C). In addition, the total conductivity in different environments has been studied systematically for samples sintered at different temperatures and the highest total conductivity was obtained for the Eu-based compound having structure with tetragonal symmetry (0.009 S/ cm at 850 °C). Hydrogen permeation was studied for a disk-shaped Nd 6 WO 12 membrane in the range of 700-1000 °C. Finally, stability of these materials at 700 and 800 °C has been evaluated in contact with a CO 2 -rich gas stream (dry or humidified) as well as thermochemical compatibility with yttriastabilized zirconia in the range 1200-1500 °C.

Several elements were studied as potential A-site substituents in the perovskite A 0.68 Sr 0.3 Fe 0.8 Co 0.2 O 3−␦ system. The considered elements included La, Pr, Sm, Nd, Er, Eu, Gd, Dy, and Ba. The multicomponent oxides were prepared following a complexationpolymerization-pyrolysis method. The materials were characterized by X-ray diffraction, thermal dilatometry, and electrical conductivity under different oxidant atmospheres. The obtained materials were studied as solid oxide fuel cell cathodes, preparing porous films on top anode-supported cells with a yttria-stabilized zirconia electrolyte and a CGO protective layer. The complete cell was characterized by direct current voltamperometry using air and wet H 2 as fuel, whereas the porosity of the layer was studied by gas diffusion experiments after electrochemical testing. Oxygen conduction was investigated on gastight membranes prepared for La-and Pr-based materials under flow of air and helium ͑sweep͒ in the range from 650 to 1000°C. Pure perovskite structure was not obtained for the cations with the smallest ionic radii. The materials with the best electrochemical performance at 650°C contained Pr, Sm, La, and Ba. The good electrochemical performance seems to be principally related to the intrinsic electrocatalytic properties of the material ͑perovskite or small clusters of the single oxide͒ because no clear correlations of the electrochemical performance and ionic conductivity, electronic conductivity, or gas diffusivity could be found. The electrochemical performance at 650°C could be correlated with the catalytic activity for methane oxidation in a fixed bed reactor in the same temperature range. Finally, the catalytic promotion of a Pr-containing perovskite was evaluated by impregnation with Pd.Solid oxide fuel cells ͑SOFCs͒ are promising candidates ͑de-vices͒ for high-efficiency energy production in the near future. SOFCs allow directly transforming the chemical energy of a fuel into electrical energy, generating, as well, a high quality heat stream. Prior to introduction into the market, different issues must be improved: ͑i͒ fuel cell system price per produced kilowatt hour, which is still too high due to manufacturing costs, and ͑ii͒ the high operating temperature, which increases the start-up time, the requirements of construction materials, and shortens the operating life of the fuel cell stack. Furthermore, operating at intermediate temperatures offers the possibility of better thermal integration with fuel reformers, increasing, in turn, the power generation efficiency. Consequently, it is necessary to decrease the operating temperature to 500-700°C. However, the catalytic performance and ionic conductivity of both electrodes have to be strongly improved for that temperature range. Indeed, the performance of intermediate temperature SOFC is strongly dependent on the electrochemical properties of the cathode because the overpotential for oxygen reduction increases significantly at lower temperatures. The electrochemical processes going on in the ...

Thin oxygen-ion-conducting films (5−10 µm) were prepared and characterized on mixed-conducting porous substrates. When this film is made of a pure ionic conductor, such as a gadolinia-doped ceria (Ce0.8Gd0.2O1.9) electrolyte material, the assembly can be used as a cathode-supported solid oxide fuel cell (SOFC) for operation at intermediate temperatures (500−600 °C). In this case, a porous anode comprising a Ni-CGO cermet or a Pt coating is deposited on top of the highly conductive electrolyte. Another structure with promising applications is created when the supported gastight layer includes a mixed conductor such as ferrite or cobaltite perovskites acting as oxygen-permeable membrane, which can be applied in the combustion of fuel with pure oxygen or in the intensification of other industrial processes currently using aerial catalytic oxidation. The different supported films and multilayer assemblies were characterized by SEM, EDS-WDX, SIMS, helium, and oxygen permeation, showing that gastight thin films can be achieved (values >1 × 10−6 mbar L s−1 cm−2) by inexpensive coating procedures (screen-printing or slip-casting). The electrochemical properties of thin CGO electrolytes were studied by impedance spectroscopy (EIS) and DC voltammetry on fully assembled fuel cells. It is concluded that thin CGO electrolytes under hydrogen atmosphere have enough n-type electronic conductivity to reduce the cell potential by 5−20% from the Nernst potential, which causes the consequent power density decrease. A possible solution to these leakage currents is the use of a thin electron-blocking layer as stabilized zirconia (YSZ or SSZ) deposited on the fuel-exposed CGO electrolyte side. Oxygen permeation of thin LSFC layers was measured using air and helium (sweep) in the range from 700 to 1000 °C. Supported thin films exhibit much better permeation fluxes than bulk thick membranes, especially when oxygen-reduction catalytic porous coatings were applied.

Redox stability and electrochemical study of nickel doped chromites as anodes for H-2/CH4-fueled solid oxide fuel cells. Applied Catalysis B Environmental. 115:346-356.