Degradation mechanisms in La₀.₈Sr₀.₂CoO₃ as oxygen electrode bond layer in solid oxide electrolytic cells (SOECs)

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009.Cataloged from PDF version of thesis.Includes bibliographical references (p. 100-104).High temperature steam electrolysis is an efficient process and a promising technology to convert electricity and steam or a mixture of steam and CO₂, into H₂ or syn-gas (H₂2 + CO) respectively. It is carried out in Solid Oxide Electrolytic Cells (SOECs). At the high temperature of operation, above 8000[degree] C, loss in the rate of hydrogen (or syn gas) production by SOECs has been observed. This loss of performance has been a scientific and technological challenge. The goal of this thesis is to identify the mechanisms for the loss in the electrochemical performance of SOECs due to the oxygen electrode and bond layer degradation. Our specific research objectives were focused on two main mechanisms: 1) Cr transport into the oxygen electrode and bond layer, and 2) Long-range segregation of cations in the bond layer. For SOECs provided by Ceramatec Inc. for this analysis, La₀.₈Sr₀.₂CoO₃ (LSC) was the bond layer and A₀.₈Sr₀.₂MnO₃ (ASM*) was the oxygen electrode, both comprised of perovskite structure. The approach in thesis integrated complementary spectroscopy and microscopy techniques in a novel manner to carry out the 'post-mortem' analysis of SOECs from a high level to a high resolution. Raman spectroscopy was employed to identify secondary phases on the top surface of LSC near the interconnect interphase. Surface chemistry and microstructure of the air electrode and the bond layer was studied using scanning Auger Electron Spectroscopy (AES) with nano-probe capability.(cont.) High-resolution analysis of the cation distribution in the bulk of the LSC bond layer was achieved by employing Energy Dispersive X-ray Analysis (EDX) coupled with Scanning Transmission Electron Microscopy (STEM). Electrochemical treatment and characterization was performed to isolate the mechanism(s) governing the long-range segregation of cations, leading to the dissociation of the LSC bond layer. Less-conducting, secondary phases of Cr₂O₃, LaCrO₃, La₂CrO₆ and Co₃0₄ were identified on the top surface of LSC bond layer. The bond layer exhibited: 1) presence of Cr, with average Cr-fraction of approximately 0.07 at the surface of its grains, and 2) surface composition variation locally, with La/Co ranging widely from 0.67 to 16.37 compared to the stoichiometric La/Co value of 0.8. Sr and Co cations migrated from the bond layer structure to the LSC/interconnect interface, over a distance of 10-20 microns. Furthermore, STEM/EDX results showed the presence of phase separated regions at the nano-scale rich in Cr and La but lacking Co, and vice-versa. This indicates the dissociation of bond layer bulk structure at nano-scale. Cr fraction in LSC bulk varied from 10 to 33%, which is higher than the average Cr-content at the surface of LSC grains. The maximum Sr fraction observed in LSC bulk was 4.16%, confirming the migration of Sr to LSC/interconnect interface.(cont.) We hypothesize that the long-range transport of Sr, Co, and Cr cations can be caused by two primary mechanisms: 1) Driven by Cr-related thermodynamics, where the Crcontaning species (i.e. at the vicinity of the interconnect) could thermodynamically favor the presence of select cations (i.e. Sr and Co) at the region interfacing the interconnect. 2) Driven by the electronic or oxygen ion current. To test these hypotheses and to isolate the governing mechanism, we simulated controlled electrochemical conditions on reference cells having ASM electrodes coated with LSC, on both sides of SSZ electrolyte, without any Cr-containing layers on the LSC bond layer. The reference cells degraded even in the absence of Cr. AES results showed that the microstructure and surface composition of the reference cells stayed stable and uniform upon the electrochemical treatment, in spite of the degradation. Thus, this thesis concludes that the Cr-related thermodynamics could be the dominant mechanism driving the uneven dissociation and segregation of cations in LSC as observed in the stack cells. As a mechanism for Cr-deposition in the LSC bond layer, we suggest that a thermodynamically-favored reaction between the La-enriched phase (at the surface of the LSC grains) and the volatile Cr-species (Cr0₃ and CrO₂(OH)) is responsible for the formation of poorly-conducting secondary phases. This interaction is likely to be limited by the presence of the segregated La-O-species which can serve as a nucleation agent for this reaction.by Vivek Inder Sharma.S.M