Lorenz Singheiser

Metallic interconnectors for solid oxide fuel cells – a review

Abstract For planar solid oxide fuel cell (SOFC) designs, ceramic as well as metallic materials are being considered as construction materials for the interconnectors. Compared to the ceramics, mostly compounds on the basis of La-chromite, metallic materials have the advantage of easier fabricability, lower costs as well as higher heat and electrical conductivity. Based on the requirements in respect to oxidation resistance, low thermal expansion coefficient and electrical conductivity of surface oxide scales, Cr-based alloys and high-Cr ferritic steels seem to be the most promising metallic interconnector materials. Whereas Cr-based alloys have recently especially been developed for SOFC application, a large number of ferritic steels are commercially available in a wide range of compositions. However, it seems that the specific combination of properties required for a SOFC interconnector will necessitate the development of a new, specifically designed steel or the modification of an existing commercial steel composition.

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.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

Abstract In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.

The formation of protective alumina-based scales during high-temperature air oxidation of γ-TiAl alloys

The effect of Ag additions on the oxidation behavior of γ-TiAl hasbeen studied. The materials investigated containing 47–50 at.% Aland 0–5 at.% Ag were tested with respect to oxidation resistanceduring exposure in air at 800°C. The exposures up to around 1600 hrshowed that suitable Ag additions can promote formation of long-term,protective, alumina scales on γ-TiAl alloys. Extensive analysesof the oxidation products using optical metallography SEM, XRD, EPMA,and SIMS revealed that Ag stabilizes the Z-phase (Ti5Al3O2) in thesubscale-depletion layer thereby preventing formation of α2-Ti3Alas well as Ti-rich nitrides, which are responsible for the destructionof alumina scales in common γ -TiAl alloys. The best results wereobtained for the alloy Ti–50Al–2Ag; even during exposures aslong as around 1600 hr, this alloy still appeared to form a stable aluminalayer. It was found that high Ag additions of 5% were detrimental afterlonger exposure times due to extensive Ag precipitationat the interface between the alloy and depletion layer, resulting inlocalized formation of rapidly growing, mixed-oxide scales.

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.

Determination of the interfacial fracture energies of cathodes and glass ceramic sealants in a planar solid-oxide fuel cell design

A notched bimaterial bar bend test was applied to identify weak interfaces that influence the thermomechanical performance of solid-oxide fuel cell (SOFC) stacks with planar design. The experiments were focused on the weakest interface of the multilayered cells and on the rigid glass ceramic sealants between metallic interconnects of SOFC stacks. The fracture energies of these interfaces were determined. To test interfaces within the cells, they were glued to steel strips, and the notched cell was used as a stiffener in the test. The weakest part of the cells with composite cathodes was the interface between the functional part of the cathode and the remaining current collector. Values for the interfacial fracture energies of composite cathodes both freshly prepared and after aging were determined. Taking advantage of the crack extension within the anode from the notch-tip to the interface, the fracture energy of the oxidized and reduced anodes was calculated. Sandwich specimens with glass ceramic between the interconnect steel were used to determine the fracture energies for different glass ceramic-steel interfaces. Different combinations of ferritic steel and glass ceramic were tested. The fracture path developed partly along the interface and partly in the glass ceramic, which did not influence the fracture energy. However, a significant improvement of the fracture energy with annealing time was found.

PERFORMANCE OF PLASMA-FACING MATERIALS UNDER INTENSE THERMAL LOADS IN TOKAMAKS AND STELLARATORS

Abstract Beside quasi-stationary plasma operation, short transient thermal pulses with deposited energy densities on the order of several tens of MJ/m2 are a serious concern for next-step devices, in particular, for tokamak devices such as ITER. The most serious of these transient events are plasma disruptions. Here, a considerable fraction of the plasma energy is deposited on a localized surface area in the divertor strike zone region. The timescale of these events is typically on the order of 1 ms. In spite of the fact that a dense cloud of ablation vapor will form above the strike zone, only partial shielding of the divertor armor from incident plasma particles will occur. As a consequence, thermal shock–induced crack formation, vaporization, surface melting, melt layer ejection, and particle emission induced by brittle destruction processes will limit the lifetime of the components. In addition, dust particles (neutron-activated metals or tritium-enriched carbon) are a serious concern from a safety point of view. Other transient heat loads that occasionally occur in magnetic confinement experiments such as instabilities in the plasma positioning (vertical displacement events) also may cause irreversible damage to plasma-facing components (PFCs), particularly to metals such as beryllium and tungsten. Other serious damage to PFCs is due to intense fluxes of 14-MeV neutrons in D-T burning plasma devices. Integrated neutron fluence of several tens of displacements per atom in future thermonuclear fusion reactors will degrade essential physical properties of the components (e.g., thermal conductivity). Another serious concern is the embrittlement of the heat sink and the plasma-facing materials (PFMs). To investigate the performance of carbon-based and metallic PFMs under the aforementioned thermal loads, simulation experiments have been performed in highly specialized high-heat-flux test facilities. The neutron-induced degradation of materials and components was investigated on selected test samples that were irradiated in high-flux material test reactors.