Steven P. Simner

Thermal Growth and Performance of Manganese Cobaltite Spinel Protection Layers on Ferritic Stainless Steel SOFC Interconnects

To protect solid oxide fuel cells (SOFCs) from chromium poisoning and improve metallic interconnect stability, manganese cobaltite spinel protection layers with a nominal composition of Mn 1 . 5 Co 1 . 5 O 4 were thermally grown on Crofer22 APU, a ferritic stainless steel. Thermal, electrical, and electrochemical investigations indicated that the spinel protection layers not only significantly decreased the contact area specific resistance (ASR) between a LSF cathode and the stainless steel interconnect, but also inhibited the subscale growth on the stainless steel by acting as a barrier to the inward diffusion of oxygen. A long-term thermal cycling test demonstrated excellent structural and thermomechanical stability of these spinel protection layers, which also acted as a barrier to outward chromium cation diffusion to the interconnect surface. The reduction in the contact ASR and prevention of Cr migration achieved by application of the spinel protection layers on the cathode side of ferritic stainless steel interconnects resulted in improved stability and electrochemical performance of SOFCs.

Structure and Conductivity of Thermally Grown Scales on Ferritic Fe-Cr-Mn Steel for SOFC Interconnect Applications

With the development of solid oxide fuel cells (SOFCs) that operate in the intermediate temperature range of 650-800°C, ferritic stainless steels have become promising candidate materials for interconnects in SOFC stacks. The SOFC interconnect requires that the alloy possess not only excellent surface stability, but also high electrical conductivity through the oxide scale that forms at elevated temperatures and contributes to the alloys surface stability. It appears that ferritic Fe-Cr-Mn alloys may be potential candidates due to the formation of an electrically conductive scale containing (Mn, Cr) 3 O 4 spinel. To improve the understanding of scale growth on manganese-containing ferritic stainless steels and evaluate their suitability for use in SOFC interconnects, the oxidation behavior (i.e., growth kinetics, composition, and structure of the oxide scale) and the scale electrical conductivity of a commercially available Fe-Cr-Mn steel developed specifically for SOFC applications were investigated. The results are reported and compared with those of conventional ferritic stainless steel compositions.

Interaction between La(Sr)FeO3 SOFC cathode and YSZ electrolyte

Anode-supported yttria-stabilized zirconia (YSZ) solid oxide fuel cells utilizing a Sr-doped LaFeO3 (LSF) cathode show improved performance with the incorporation of a Sm-doped CeO2 layer between the cathode and YSZ electrolyte. Detailed X-ray diffraction (XRD) analysis of LSF–YSZ reaction mixtures indicates no strontium or lanthanum zirconate formation between these materials even at 1400 °C. However, a significant shift in the LSF diffraction peaks is readily apparent and corresponds to a unit cell volume expansion. The most likely scenario for the change in volume is considered to be the incorporation of Zr4+ cations in the perovskite structure. The presence of the Zr cations subsequently results in reduced electrical conductivity of the cathode, potentially explaining the need for the aforementioned ceria interlayer.

SOFC Performance with Fe-Cr-Mn Alloy Interconnect

The performance of anode-supported SOFC’s utilizing a Mn-containing ferritic stainless steel (Crofer22 APU) as the cathode current collector were assessed. Three cathodes were considered: (La0.8Sr0.2)0.99MnO3, (La0.8Sr0.2)0.99FeO3and (La0.6Sr0.4)0.98Fe0.8C00.2O3 (all samples incorporated a Sm-doped CeO2 interlayer between the cathode and the thin film YSZ electrolyte). Inclusion of the Fe-Cr alloy caused rapid degradation in all samples, which was attributed to solid-state reactivity at the cathode-Crofer interface, in addition to Cr volatilization from the alloy and subsequent condensation/reaction within the cathode and at the cathode-electrolyte interface. In situ- HTXRD was used to access the cathode-Crofer reaction products. Pre-oxidation of the Crofer at 800 degrees C for 500 hours to form a protective (Mn,Cr)3O4 spinel coating resulted in a marginal reduction of the cell degradation rate.

Performance Variability of La(Sr)FeO3 SOFC Cathode with Pt, Ag and Au Current Collectors

This investigation details relatively short-term (500 hours) performance characteristics (in particular cathode conditioning and degradation) of anode-supported SOFCs utilizing La(Sr)FeO3 cathodes with platinum, silver or gold current collectors. Conditioning or degradation characteristics were found to be highly dependant on the current collector used. An additional area of study considered transient cell behavior following rest periods at open circuit voltage (zero current). Three types of behavior were observed (1) decreased performance after OCV hold, (2) increased performance after OCV hold, and (3) no discernible change. The type of transient behavior was found to be dependent on the noble metal current collector utilized, and the time at which the transient behavior was assessed. Longer testing times indicated more pronounced transient characteristics.

Sintering of lanthanum chromite using strontium vanadate

Abstract Small proportions (5 and 10 wt.%) of strontium vanadate (Sr 3 (VO 4 ) 2 ) were added to strontium-doped lanthanum chromite (La 0.85 Sr 0.15 CrO 3 ) to produce high density fuel cell interconnect materials in air at 1550°C without adversely affecting the desirable properties of the material. Compositions investigated were shown to have good electrical conductivity at SOFC operating temperatures in air and reducing environments, phase stability from room temperature to 1000°C, negligible thermal expansion mismatch with yttria-stabilized zirconia electrolytes and relatively low dilation at p O 2 10 −16 atm.

Sintering mechanisms in strontium doped lanthanum chromite

The sintering behavior of (La0.7Sr0.3)xCrO3 (0.95 ≤ x ≤ 1.05) is investigated to compare liquid phase sintering phenomena occuring in stoichiometric and non-stoichiometric compositions. Shrinkage analysis revealed marked contrast between the densification characteristics of the A-site enriched (x > 1.00) and A-site depleted (x < 1.00) materials. A-site depleted samples typically exhibited a single liquid phase sintering event at 1250 °C attributed to the melting of an exsoluted SrCrO4 phase. A-site enriched samples indicated two rapid shrinkage events due to the melting of SrCrO4, and a Sr2.67(CrO4)2 phase with a melting temperature of 1450 °C. Sr2.67(CrO4)2 was shown to evolve from a decomposition reaction between SrCrO4 and La2CrO6, detected together in A-site enriched samples from 800–1000 °C. Maximum densities (93% theoretical density) were achieved for (La0.7Sr0.3)xCrO3 x = 1.00 after sintering at 1700 °C for two hours.

Sintering and Properties of Mixed Lanthanide Chromites

A variety of strontium-doped lanthanide chromite compositions were synthesized from a mixed lanthanide (Ln) precursor predominantly consisting of La, Nd, Ce, and Pr. Samples were initially analyzed to assess their sintering characteristics. The most promising sintering behavior was observed for samples of the general formula Ln 0.85 Sr 0 15Cr1 z M z O 3 , where M=transition metal elements Co, Cu, Ni, and V, and 0.02 ≤ z ≤ 0.1. For most compositions >90% theoretical density was attained at 1450°C, though a 5 mol % Cu-doped compound could be sintered to high density as low as 1250°C. High density samples were subsequently analyzed with respect to phase stability, thermal expansion, electrical conductivity in air and reducing atmospheres, and dilation at low oxygen partial pressures. Virtually all samples indicated an orthorhombic- to rhombohedral-phase transformation between 750 and 850°C, and nonlinear Arrhenius electrical conductivity behavior with a positive inflection around 650°C indicative of increased carrier concentrations. Both phenomena are related to the influence of the additional A site cations (in particular Ce in the case of increased conductivity at elevated temperatures). Ln 0.85 Sr 0.15 Cr 0.95 Cu 0.05 O 3 was the only sample that exhibited linear conductivity behavior and no discernible structure transformation, thought to be related to the precipitation of a Ce 0.5 Nd 0.5 O 1.75 second phase.

Electrochemical Performance and Stability of the Cathode for Solid Oxide Fuel Cells II. Role of Ni Diffusion on LSM Performance

The sintering of a standard (La0.8Sr0.2)0.98MnO3 (LSM-20) solid oxide fuel cell cathode composition (in the temperature range of 1050-1200oC) on anode-supported cells utilizing a Ni-YSZ anode and thin YSZ electrolyte (<10 μm thickness) has revealed the need for a protective ceria interlayer to prevent a detrimental interaction between the YSZ and the LSM. The interaction, however, is not the typically assumed formation of insulating La- and/or Sr-zirconate, but rather the result of Ni diffusion from the anode through the YSZ electrolyte and into the LSM resulting in coarsening and increased densification of the LSM microstructure. As an alternative to the use of a protective ceria interlayer, the presence of YSZ in the cathode material was able to suppress coarsening of LSM, thereby significantly improving the electrochemical performance.

Development of (Mn,Co)3O4 Protection Layers for Ferritic Stainless Steel Interconnects

A spinel-based surface protection layer has been developed for alloy SOFC current collectors and bi-polar gas separators. The (Mn,Co)3O4 spinel with a nominal composition of Mn1.5Co1.5O4 demonstrates an excellent electrical conductivity and thermal expansion match to ferritic stainless steel interconnects. A slurry-coating technique provides a viable approach for fabricating protective layers of the spinel onto the steel interconnects. Thermally grown protection layers of Mn1.5Co1.5O4 have been found not only to significantly decrease the contact resistance between a LSF cathode and stainless steel interconnect, but also inhibits the sub-scale growth on the stainless steel. The combination of the inhibited sub-scale growth, good thermal expansion matching between the spinel and the stainless steel, and the closed-pore structure contribute to the excellent structural and thermomechanical stability of these spinel protection layers, which was verified by a long-term thermal-cycling test. The spinel protection layers can also act effectively to prevent outward diffusion of chromium from the interconnect alloy, preventing subsequent chromium migration into the cathode and contact materials. PNNL is currently engaged in studies intended to optimize the composition, microstructure, and fabrication procedure for the spinel protection layers.