Silver–chromium oxide interactions in SOFC environments

doi:10.1016/j.jpowsour.2009.02.036

Journal of Power Sources

Volume 191, Issue 2, 15 June 2009, Pages 465-472

https://doi.org/10.1016/j.jpowsour.2009.02.036 Get rights and content

Abstract

Interactions between silver and chromium oxide were investigated during SOFC-relevant extended exposures pertaining to secondary phase formation and associated electrical contact degradation. In one case, Ag or Ag₂O and Cr₂O₃ powders were mixed, pressed into pellets and thermally treated in air at 700 °C, 800 °C and 900 °C for up to 1000 h. X-ray diffraction revealed AgCrO₂ and trace Cr₂O₃ in the specimens treated at 700 °C and 800 °C; however, Cr₂O₃, Ag and only trace amounts of AgCrO₂ were detected at 900 °C. In another case, silver contact paste was used in area specific resistance (ASR) measurements of ferritic stainless steel (FSS), with and without (Co,Mn)₃O₄ coatings, in 800 °C air for greater than 1500 h. A distinct reaction layer, having AgCrO₂ stoichiometry, was observed to form between the Cr-containing corrosion-products on the uncoated FSS and the silver contact paste yielding a 500% increase in ASR over 1500 h. No significant chemical interactions and ASR losses were observed with the (Co,Mn)₃O₄ coated FSS at this interface. Analyses and implications of the observed interactions of Cr₂O₃ and Ag within SOFC cathode environments are presented and discussed.

Introduction

Silver metal exhibits many physical and chemical properties that are driving the development of silver based SOFC applications such as sealing approaches, interconnect alloys, and composite cathodes [1], [2], [3], [4], [5], [6], [7], [8], [9]. These properties including: high electrical conductivity, hydrogen dissociation and oxidation catalysis, noble metal oxidation resistance/chemical stability, and relatively low cost are also advantageous in the form of contact pastes for fabrication and developmental testing of SOFC components [10], [11]. A key downfall to the use of silver is the low melting point (∼962 °C), which is exacerbated by its substantial volatility under SOFC operating temperatures (e.g., 650–850 °C) [6], [7], [8], [9], [12]. Silver metal loss for braze applications has been shown to reach 2 × 10⁻⁹ g cm⁻² s at 850 °C [12]. Given the nature of silver metal at these elevated temperatures, the effectiveness of seals, contacts, and interfaces may be substantially deteriorated after only short exposures to SOFC operation at these temperatures, thus mitigating the potential benefits of developing SOFC systems based on silver for long duration operation. The implications of silver reactions and transport therefore have the potential to convolute the performance and degradation characteristics of SOFC testing at component, cell and stack level.

In addition to silver, ferritic stainless steels (FSSs) have seen heavy integration into SOFC designs by virtue of their compatible thermal expansion coefficient, ease in fabrication and low cost. Planar SOFC designs often utilize a FSS interconnect (IC), which separates fuel and oxidant gasses and connect individual cells into a stack. During operation, a Cr₂O₃-based thermally grown oxide (TGO) layer forms and grows predominantly on the FSS IC/cathode interface, which leads to increased ASR and eventuates in TGO layer spallation. In addition, Cr-transport (from TGO layers in either or both solid and gas phases) into cathodes and cathode/electrolyte interfaces has been established as a mechanism for substantial SOFC stack performance degradation [13], [14], [15], [16], [17], [18]. A variety of protective coatings and deposition techniques have been investigated for the FSS SOFC(IC) application. More promising coating compositions include various doped lanthanum chromite and manganite perovskites, e.g., La_1−xCa_xCrO₃ and La_1−xSr_xMnO₃, and various Co, Fe, Ni, Cu and/or Mn-containing spinels, e.g., (Co,Mn)₃O₄ and (Cu,Mn)₃O₄ deposited by a range of solution and vapor-based approaches including thermal or plasma spray, slurry coating and physical or chemical vapor deposition [13], [15], [17], [19].

Inexpensive silver leads and contact pastes, foils, and meshes are often used in SOFC materials development, e.g., ASR testing of FSS SOFC(ICs), which may influence chromium transport, TGO layer growth and ASR depending upon the extent of reaction between chromia and silver. The negative effects of silver may diminish its benefits due to interfacial degradation in virtually all aspects of SOFC application in temperature ranges supporting a prevalence of SOFC development from 700 °C to 900 °C. Therefore, the objective of this study was to investigate the interaction of chromia-based TGO layers based upon degradation of current collection as opposed to cathode/electrolyte activity. Further, a common IC coating, (Co,Mn)₃O₄, was also evaluated with respect to conductivity degradation to evaluate potential impacts for the primary application of Ag-containing contact pastes and seals. The study was designed to not only evaluate the use of silver in moderate temperature ranges, but also to explore the upper temperature limits of silver containing contacts, where maximum stack performance may be required. The chemical stability was determined by X-ray diffraction analysis of silver/silver oxide and chromium oxide powders, in addition to thermodynamic equilibrium simulation while conductivity loss was determined by ASR measurement of FSS joined with silver.

Section snippets

Thermodynamic modeling

Thermodynamic equilibrium calculations of experimental systems were performed using the TERRA program with pressure, temperature and composition as key constraints [20]. Calculations were made using silver and chromium in moist air (400–1000 °C) to evaluate equilibrium solid, vapor and gas species within the TERRA database, which includes most of the refractory compounds properties within SGTE and JANAF-NIST databases. In addition, an ideal solid solution model was employed to approximate mixing

Results

Thermodynamic equilibrium calculations considering an initial 1:1 mass ratio of Ag:Cr in air (3 vol% H₂O) were plotted in the temperature range from 400 °C to 1000 °C to cover the complete useful range of SOFC operation. Fig. 2 shows the volatilization behavior of Ag and Cr-containing species, which indicate substantial vapor pressures of CrO₂(OH)₂ and Ag at elevated temperatures [21]. Thermodynamic equilibrium mass fractions of condensed phase species vs. temperature are presented in Fig. 3. Fig.

Thermodynamic modeling

The potential for Ag and Cr volatility under SOFC operating temperature is illustrated in Fig. 2, which shows appreciable and increasing equilibrium vapor pressure of CrO₂(OH)₂ and Ag in air at elevated temperatures. Compared with this simulation, Cr and Ag activities may effectively decrease in SOFC application when compounded with other metals and/or oxides. Regardless, the availability of silver and/or Cr-containing vapor may drive reactions with other components and create deleterious

Conclusions

The formation of silver chromite (AgCrO₂) is shown to be thermodynamically stable over the 700–800 °C temperature range, while the AgCrO₂ decomposes to Ag and Cr₂O₃ at temperatures above 800 °C, suggesting a temperature limit for SOFC operation using silver contacts/seals/leads with FSS interconnects. Temperatures above 800 °C indicate substantial metallic silver volatilization as observed by condensation on adjoining support substrates. Long duration testing, 500–1500 h, indicates a strong

Acknowledgements

This work was supported by the MSU-HiTEC program and is funded by the United States Department of Energy under Award No. DE-AC06-76RL01830. Any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the DOE.

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Silver–chromium oxide interactions in SOFC environments

Abstract

Introduction

Section snippets

Thermodynamic modeling

Results

Thermodynamic modeling

Conclusions

Acknowledgements

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

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Solid State Ionics

Int. J. Hydrogen Energy

J. Hydrogen Energy

J. Power Sources

Electrochem. Solid-State Lett.

J. Mater. Sci.