Wilhelm Albert Meulenberg

Impedance Study of Alternative ( La , Sr ) FeO3 − δ and ( La , Sr ) ( Co , Fe ) O3 − δ MIEC Cathode Compositions

In this study, two mixed electronic ionic conducting (MIEC) cathode materials, La 0.68 Sr 0.3 FeO 3-δ and La 0.68 Sr 0.3 Co 0.2 Fe 0.8 O 3-δ , are characterized by electrochemical impedance spectroscopy. The chemical surface and diffusion coefficients of both cathode materials are obtained directly from impedance measurements performed on full anode-supported solid oxide fuel cells. This is done by a combined distribution of relaxation times and an equivalent circuit impedance analysis method, which allows a high resolved identification and deconvolution of each single polarization mechanisms contributing to the overall loss of the cell.

Natural Gas to Fuels and Chemicals: Improved Methane: Aromatization in an Oxygen-Permeable Membrane Reactor

Adding value with membranes: Improved methane aromatization was achieved by using an oxygen-permeable membrane. The resulting membrane reactor shows a superior methane conversion and a higher resistance towards catalyst deactivation.

Enhanced H2 Separation through Mixed Proton–Electron Conducting Membranes Based on La5.5W0.8M0.2O11.25−δ

La(5.5) WO11.25-δ is a proton-conducting oxide that shows high protonic conductivity, sufficient electronic conductivity, and stability in moist CO2 environments. However, the H2 flows achieved to date when using La(5.5) WO11.25-δ membranes are still below the threshold for practical application in industrial processes. With the aim of improving the H2 flow obtained with this material, La(5.5) WO11.25-δ was doped in the W position by using Re and Mo; the chosen stoichiometry was La(5.5) W0.8 M0.2 O11.25-δ . This work presents the electrochemical characterization of these two compounds under reducing conditions, the H2 separation properties, as well as the influence of the H2 concentration in the feed stream, degree of humidification, and operating temperature. Doping with both Re and Mo enabled the magnitude of H2 permeation to be enhanced, reaching unrivaled values of up to 0.095 mL min(-1) cm(-2) at 700 °C for a La(5.5) W0.8 Re0.2 O11.25-δ membrane (760 μm thick). The spent membranes were investigated by using XRD, SEM, and TEM on focused-ion beam lamellas. Furthermore, the stability in CO2 -rich and H2 S-containing atmospheres was evaluated, and the compounds were shown to be stable in the atmospheres studied.

Improved contacting by the use of silver in solid oxide fuel cells up to an operating temperature of 800 °C

In the following, a contacting variant for solid oxide fuel cells will be presented in which the conductivity of the interconnect is ensured by contact elements made of fine silver. To this end, the interconnect has holes through which the contact elements of fine silver (99.9 wt% Ag) are introduced and then pressed. This pressing process and the thermal expansion of the silver during heating leads to a gastight joint. The silver penetrations are additionally soldered to render them capable of withstanding temperature cycling. Contact resistance measurements and corrosion studies at 800 °C in air or Ar/4 vol.% H2/3 vol.% H2O demonstrate the functionality of the contacting variant under the described conditions. The experimental results indicate that contacting by means of silver contact elements ensures long-term stability up to operating temperatures of 800 °C. Current transmission via the silver contact elements means that a large number of materials are conceivable as the interconnect material. In the following application, an FeCrAl steel (1.4767, Aluchrom Y Hf—trade name Krupp Thyssen Nirosta) with 5.7 wt.% aluminium was used. At the operating temperature, a dense aluminium oxide layer forms on its surface which prevents the vaporization, for example of chromium oxide species, during fuel cell operation.

Screening of A-Substitution in the System A0.68Sr0.3Fe0.8Co0.2O3 − δ for SOFC Cathodes

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-δ system. The considered elements included La, Pr, Sm, Nd, Er, Eu, Gd, Dy, and Ba. The multicomponent oxides were prepared following a complexation-polymerization-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 (sweep) 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 oxide) 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.

Synthesis and Characterization of Nonsubstituted and Substituted Proton-Conducting La6–xWO12–y

Mixed proton-electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic-electronic conductivity. Lanthanum tungstate La(6-x)WO(12-y) (with y = 1.5x + δ and x = 0.5-0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 °C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La(6-x)WO(12-y) and for all substituted La(6-x)WO(12-y) compounds was measured in the temperature range of 400-900 °C in wet (2.5% H2O) and dry mixtures of 4% H2 in Ar. The greatest improvement in the electrical characteristics was found in the case of 20 mol % substitution with both Re and Mo. After treatment in 100% H2 at 800 °C, the compounds remained unchanged as confirmed with XRD, Raman, and SEM.

Improvement of transport properties and hydrogen permeation of chemically-stable proton-conducting oxides based on the system BaZr1-x-yYxMyO3-δ

The structural and transport properties as well as the chemical stability of a series of proton-conducting oxides based on yttrium-doped barium zirconate were investigated. Specifically, Pr-, Fe- and Mn-doped BaZr1-x-yYxMyO3-δ compounds were prepared by solid state reaction. The compound exhibiting the highest total and protonic conductivity at elevated temperatures under reducing atmospheres was BaZr0.8Y0.15Mn0.05O3-δ. Temperature-programmed reduction experiments revealed a particular redox behavior related to the Mn-species under selected conditions. The hydrogen permeation was thoroughly studied as a function of the temperature, hydrogen concentration and the humidification degree in the sweep gas. Moreover, the transient processes induced by alternate step changes in the humidification degree of the sweep gas were analysed. The highest steady hydrogen evolution flow exceeded 0.03 ml min−1 cm−2 (0.9 mm-thick membrane) at 1000 °C for the humidified sweep gas. The stability of BaZr0.8Y0.15M0.05O3-δ under operation-relevant atmospheres (CO2-rich reducing atmosphere at high temperature) was tested using different techniques (X-ray diffraction (XRD), Raman, SEM, TEM and TG) and the results showed that this material is stable even when exposed to 115 ppm H2S.

Oxidation behaviour of ferrous alloys used as interconnecting material in solid oxide fuel cells

Under operating conditions in the solid oxide fuel cell (SOFC), metallic interconnect plates form electrically insulating or poor-conducting oxide scales (e.g. Cr2O3, Al2O3) at their surface which increase the contact resistance from one fuel cell membrane to the next. In order to minimize electric losses in a fuel cell stack, the formation of oxide scales on the interconnect surface must either be prevented or the oxide scale formed must have sufficient electrical conductivity. In the present work, investigations were carried out on the corrosion behaviour of different FeCrAl and FeCrMn alloys, some of which were coated with nickel (Ni). Information about ageing of these alloys on the anode side of the fuel cell was obtained by means of contact resistance measurements and scanning electron microscopy. The results reveal that FeCrMn(LaTi) alloys and Ni-coated interconnects exhibit low ageing rates and are thus suitable for use on the anode side of SOFCs.

Thin BaCe0.8Gd0.2O3 − δ Protonic Electrolytes on Porous Ce0.8Gd0.2O1.9 – Ni Substrates

Thin proton-conducting electrolytes with 20 μm thickness and a nominal composition BaCe 0.8 Gd 0.2 O 3-δ were prepared over porous substrates composed of Ce 0.8 Gd 0.2 O 1.9 (CGO)-Ni. The fabrication method includes the simultaneous cofiring and solid-state reaction at 1300°C of three screen-printed layers: mixed conducting anode layer, CGO, and an upper-coated BaCO 3 layer. The resulting asymmetric membranes with 5 × 5 cm 2 geometry are gastight. The solid-state reaction resulted in the formation of (i) an electrolyte with large (∼2-5 μm) grains and high packing density and (ii) anode layer comprising finely grained BaCe 0.8 Gd 0.2 O 3-δ and Ni particles with a thickness around 5 μm. The layer morphology, phase distribution, and lattice composition has been investigated by X-ray diffraction, energy-dispersive spectroscopy scanning electron microscopy, and secondary-ion mass spectrometry. The preparation of the substrate by warm-pressing and tailoring of the porosity and sintering activity are also thoroughly described. DC-electrical conductivity and ac-electrochemical characterization showed that the supported Gd-doped barium cerate electrolyte produced by solid-state reaction has interesting proton-conduction properties for application in solid oxide fuel cells, hydrogen separation, and sensors.

Tape Casting as a Multi Purpose Shaping Technology for Different Applications in Energy Issues

Tape casting is widely used in industrial scale for production of multilayer ceramic capacitors or substrates for different applications. In 2009, it was successfully introduced as standard shaping technology for 3 (BSCF) are shown. The entire scope from the preparation of the used powders, the different manufacturing steps and their optimization potential up to the final tape-cast product will be discussed. The influence of the use of pore forming agents, heat treatment or other parameters during processing will be described in detail. Finally, the option of sequential tape casting of different materials for graded structures as a future step in shaping technology will be presented for different applications.