Klaus Hilpert

Chromium Vapor Species over Solid Oxide Fuel Cell Interconnect Materials and Their Potential for Degradation Processes

Alloys protected from corrosion by a chromia scale and LaCrO{sub 3}-based perovskites are used as materials for the interconnect of solid oxide fuel cells (SOFCs). The chromium vaporization of these materials was studied by thermochemical modeling. Partial pressures of the vaporizing species were determined for different O{sub 2} and H{sub 2}O concentrations in the oxidizing gas. CrO{sub 2}(OH){sub 2}(g) and CrO{sub 3}(g) are the most abundant species in air with and without humidity, respectively. The potential of the Cr-containing vapor species for the degradation of the electrical properties of an SOFC was analyzed by thermodynamic computations. The electrochemical reduction of the Cr-containing vapor species at the cathode/electrolyte/gas phase boundary can lead to polarization losses.

Chromium Poisoning of Perovskite Cathodes by the ODS Alloy Cr5Fe1Y2O3 and the High Chromium Ferritic Steel Crofer22APU

The alloys Cr5Fe1Y 2 O 3 and the ferritic steel Crofer22APU are typical alloys used as solid oxide fuel cell (SOFC) interconnect materials. Alloy Cr5Fe1Y 2 O 3 is an oxide dispersion strengthened (ODS) alloy developed by Plansee, Reutte, Austria, for use at high temperature. A typical material for medium-temperature SOFC, is the high chromium ferritic steel Crofer22APU supplied by Thyssen Krupp VDM, Germany. The two alloys form different oxide scales which affect chromium poisoning. Chromium vaporization as source term and electrochemical degradation of La 1-x Sr x MnO 3 (LSM) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) describing the poisoning were studied for the two alloys. The dynamics of the chromium deposition in porous perovskite cathodes was studied by a dc method and impedance spectroscopy. Electrical degradation of the LSM cathode by alloy Cr5Fe1Y 2 O 3 was significantly higher than for Crofer22APU. The microstructure of the cells was studied after measurements by scanning and energy filtering transmission electron microscopy. Significant amounts of chromium were observed at the TPB in the functional layer of cells, with the LSM cathode giving insight into the degradation mechanism. Cells tested with the LSCF cathode clearly show Cr poisoning. Formation of large SrCrO 4 crystals was observed on the surface of the LSCF cathode.

Chromium Vaporization from High-Temperature Alloys I. Chromia-Forming Steels and the Influence of Outer Oxide Layers

The vaporization of chromium species from chromia scales limits the applicability of chromia-forming steels at high temperatures and is one of the major reasons for degradation in the development of planar solid oxide fuel cells (SOFCs). Cr(VI) vaporized from the interconnector is reduced at the cathode and deposits in the form of solid Cr(III)-oxide, thereby inhibiting the electrochemical processes. This work presents the first systematic study on the Cr vaporization of Cr-, Fe-, Ni-, and Co-based alloys in air and in H 2 atmospheres at high temperatures. The influence of outer oxide layers of (Cr,Mn) 3 O 4 , (Fe, Cr) 3 O 4 , Co 3 O 4 , TiO 2 , and Al 2 O 3 on the Cr vaporization is investigated. It is shown that the Cr vaporization of chromia-forming steels can be reduced by more than 90% by alloying. An estimate of the expected degradation effects on planar SOFC designs for the use of uncoated interconnector materials is used to show that in order to achieve the desired lifetimes for SOFC systems, additional Cr-retention coatings are necessary. Additionally, equilibrium vaporization measurements are carried out for pure Cr 2 O 3 (s) in humid air in order to elucidate controversies in the literature concerning the thermodynamic data of CrO 2 (OH) 2 (g).

Crystallisation kinetics in AO-Al2O3-SiO2-B2O3 glasses (A = Ba, Ca, Mg)

The crystallisation kinetics of AO-Al2O3-SiO2-B2O3glasses (A = Ba, Ca, Mg) was investigated using DTA, XRD, and microstructural studies. Moreover, the influence of nucleating agentssuch as TiO2, ZrO2, Cr2O3, and Ni on MgO base glasses waselucidated. The glasses are of interest for the development ofsealants in Solid Oxide Fuel Cells (SOFC). The activation energy ofcrystal growth, Ea, was evaluated for the different glassesusing the modified Kissinger equation. The preparation method of theglasses seems to determine whether surface or bulk nucleation is thedominant mechanism. The Ea values vary between 330 and622 kJ/mol. The nucleating agents tend to enhance Ea exceptZrO2. An increase of the Al2O3 concentration induces phaseseparation and decreases Ea. The results are discussed onthe basis of the structural role and chemical properties of the Alions as well as with respect to the possible use of the glasses inSOFC.

Chemical Interactions Between Aluminosilicate Base Sealants and the Components on the Anode Side of Solid Oxide Fuel Cells

The chemical interactions between several aluminosilicate glasses and components at the anode side of SOFC have been investigated. Severe reactions occur if the modifier ions are Ba and Ca. MgO base sealants have been investigated in detail. The formation of some detrimental phases are seen. Cordierite forms with many reaction mixtures of sealants with ZrO 2 stabilized with 8 mol % Y 2 O 3 (as electrolyte) or nickel (as amide). On the other hand, with oxide dispersion strengthened Cr 5 FelY 2 O 3 and steel. another detrimental phase, cristobalite, is favored. There appears to be a competitive formation of these two detrimental phases in some cases. In some of the interactions none of the detrimental phases appears. The diffusion behavior of the cations across the interfaces has been investigated. Among all the cations, chromium diffuses the maximum. The diffusion coefficient of chromium for the diffusion couples of different sealants with ODS alloy have been determined.

Chemical compatibility of the LaFeO3 base perovskites (La0.6Sr0.4)zFe0.8M0.2O3 − δ (z = 1, 0.9; M = Cr, Mn, Co, Ni) with yttria stabilized zirconia

Abstract Physicochemical compatibilities of the series of LaFeO 3 base perovskites La 0.6 Sr 0.4 Fe 0.8 M 0.2 O 3 − δ and ( La 0.6 Sr 0.4 ) 0.9 Fe 0.8 M 0.2 O 3 − δ ( M = Cr , Mn , Co , Ni ) with the solid electrolyte yttria stabilized (8 mol% Y 2 O 3 ) cubic zirconia, 8YSZ, were investigated. Powder mixtures of the perovskites with 8YSZ were annealed at 1000 °C for time periods up to 1300 h. The quenched samples were analyzed by XRD and SEM/EDX for identifying the reaction products. The results provide a working hypothesis for the development of improved compositions of cathode materials in solid oxide fuel cells.

Influence of Nucleating Agents on the Chemical Interaction of MgO ­ Al2 O 3 ­ SiO2 ­ B 2 O 3 Glass Sealants with Components of SOFCs

The chemical interactions between different MgO-SiO 2 -Al 2 O 3 -B 2 O 3 base glass sealants and components of solid oxide fuel cells (SOFCs) were investigated. The SOFC materials considered are ZrO 2 stabilized with 8 mol % Y 2 O 3 (as electrolyte and part of the anode), Ni (as part of the anode), and the oxide-dispersion-strengthened (ODS) alloy Cr5Fe1Y 2 O 3 (as interconnect). Glass compositions with no nucleating agent and with ZrO 2 , Cr 2 O 3 , or Ni as the nucleating agents were prepared. Powder mixtures of these sealants with the mentioned SOFC materials, as well as the sealant/material interfaces, were characterized by X-ray diffraction and scanning electron microscopy and energy dispersive spectroscopy in order to determine possible reaction phases and the diffusion behavior of different cations. Formation of cordierite and cristobalite as detrimental phases was detected in many of the mixtures. The formation of these phases can be suppressed if Cr 2 O 3 or Ni is added to the glass as the nucleating agent. The most interesting feature of these results is the absence of the cordierite phase for all reaction powder mixtures it Cr 2 O 3 is used as the nucleating agent. The sealants with Cr 2 O 3 and Ni as nucleating agents formed a reaction zone at the interface with ODS, rich in Cr and Mg. A parabolic reaction rate equation describing the growth of the reaction zone thickness, the diffusion coefficient of chromium, and rate constants was evaluated.

Chromium Poisoning of the Porous Composite Cathode Effect of Cathode Thickness and Current Density

Electrochemical behavior of the (Lao 0.65 Sr 0.3 MnO 3 (LSM)/porous functional layer(FL)/8 mol % Y 2 O 3 -stabilized ZrO 2 (8YSZ) electrolyte} half cell in the presence of a Cr source was studied by dc method and impedance spectroscopy. The porous FL was made of a LSM and 8YSZ powder mixture. The degradation rate of the cell was found to depend strongly on the FL thickness. The increase in the thickness of the FL up to 13 μm leads to a significant reduction of the Cr poisoning of the cathode. The effect of high current density (up to 0.5 A/cm 2 ) on the long-term performance of half cells in the presence of chromium source was investigated. A few processes occur near the {LSM/SYSZ/gas} contact under current load in the presence of a chromium source: chemical transformations of this contact accompanied by the formation of chromium oxide under a relatively high current load, destruction of the electrolyte and penetration of chromium into the electrolyte along the grain boundaries. The chemical nature and rate of the transformations taking place near the {LSM/8YSZ/gas} contact under current load, most probably, determine the electrochemical performance of the cell for the initial operation period whereas the two other processes could contribute to the degradation during long-term performance.

Chemical compatibility of (La{sub 0.6}Ca{sub 0.4}){sub x}Fe{sub 0.8}M{sub 0.2}O{sub 3} with yttria-stabilized zirconia

LaFeO{sub 3}-based perovskites are of interest as cathode material for the development of advanced solid oxide fuel cells operating at temperatures significantly lower than 900 C. The compatibility of the selected materials is necessary to guarantee long-term operation of a solid oxide fuel cell. Systematic investigations on the reactivity between the solid electrolyte yttria-stabilized zirconia (ZrO{sub 2} + 8 mole percent Y{sub 2}O{sub 3}) and the perovskites (La{sub 0.6}Ca{sub 0.4}){sub x}Fe{sub 0.8}M{sub 0.2}O{sub 3} (x = 1, 0.9; M = Cr, Mn, Co, Ni) were carried out. The results obtained are discussed.

Vaporization of Sr- and Mg-Doped Lanthanum Gallate and Implications for Solid Oxide Fuel Cells

Vaporization of the La0.85Sr0.15Ga0.85Mg0.15O2.85, and La0.90Sr0.10Ga0.80Mg0.20O2.85 perovskite phases was investigated by the use of Knudsen effusion mass spectrometry in the temperature range of 1618-1886 K. The partial pressures of the gaseous species O2, Mg, Sr, SrO, Ga, GaO, Ga2O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of thermodynamic activities of the components at 1800 K. The results are compared with thermodynamic data of LaGaO3~s! without additives. Implications of the present data for the potential use of the material in solid oxide fuel cell technology are discussed as well.