Investigation of Degradation Mechanisms of LSCF Based SOFC Cathodes — by CALPHAD Modeling and Experiments

LSCF (La_1−xSr_xCo_1−yFeyO_3−δ) is a promising cathode material for intermediate temperature SOFCs (Solid Oxide Fuel Cells). However, the LSCF cathode degrades over an extended period of time. The processes that play a dominant role for the degradation and their relation to cell durability have not been fully understood at the moment. With the developments of computer software and thermodynamic databases, advances have been made in calculating complex phase equilibria and predicting thermodynamic properties of the materials. In order to identify physicochemical degradation mechanisms of LSCF cathodes, investigation of the La-Sr-Co-Fe-O system using computational thermodynamics and designed key experiments was carried out in this work.
The first part of the research work was devoted to establish a self-consistent thermodynamic database of relevant components (La-Sr-Co-Fe-O) using the CALPHAD (CALculation of PHAse Diagrams) approach. Published thermodynamic databases and experimental data related to the La-Sr-Co-Fe-O system were critically reviewed. The thermodynamic descriptions of the La-Co-O, Sr-Co-O and La-Sr-Co-O systems were further improved in order to construct the present thermodynamic database for LSCF, while new thermodynamic modeling of the Co-Fe-O, Sr-Co-Fe-O and La-Sr-Co-Fe-O systems was performed in this work. Calculated phase equilibria in LSCF as functions of composition, temperature, oxygen partial pressure are discussed by comparing with experimental data. Based on the developed thermodynamic database, the “stability windows” of LSC (La_1−xSr_xCoO_3−δ) and LSCF are predicted and presented in Chapter 5 and Chapter 6, respectively. Calculations show that the perovskite phase is stable at high La and Fe content and high oxygen partial pressure. The stability of the perovskite phase is in the trend of LSC<LSCF<LSF. Outside its “stability window”, decomposition or partial decomposition of the perovskite phase takes place. Different secondary phases form under different conditions (temperature, oxygen partial pressure, composition). Taking LSC as an example, the decomposition of the perovskite phase is accompanied with formation of (La,Sr)₂CoO₄ at low oxygen partial pressure, or Sr₆Co₅O₁₅ at low temperature, or Sr₂Co₂O₅ at high Sr content at around 1000°C. With the thermodynamic database, capability of calculating other properties of the LSCF perovskite, such as oxygen non-stoichiometry and cation distribution was also demonstrated.
Experimental investigations on phase stability of LSC, LSCF and LSCF/CGO composites, and applications of the thermodynamic database on analyzing the phase stability are described in the second part of this thesis. An inter-diffusion between LSCF and CGO was detected. The inter-diffusion of La and Ce/Gd between the two phases was further observed to be accompanied by a formation of a halite secondary phase in N2. In addition, it was found that Sr diffuses out of LSCF (i.e. surface segregation), and further reacts with impurities. This phenomenon was observed even at 700°C.
In the last part of this thesis, characterization techniques including Scanning Electron Microscopy (SEM), Secondary ion mass spectroscopy (SIMS) and Transmission Electron Microscopy (TEM) were applied on a tested as well as an “as prepared” LSCF/CGO (Ce_1−xGd_xO_2−δ) composite cathode, in order to reveal the origins of the cell degradation. Issues including LSCF stability, Sr diffusion, LSCF−CGO interaction and impurity segregation were examined. The results show that partial phase separation of LSCF happens mainly at the interface with the CGO barrier layer. The inter-diffusion across the LSCF/CGO cathode – CGO barrier layer interface and the CGO barrier layer – YSZ electrolyte interface happened mainly during sintering, and only to little degree while long-term SOFC testing, and therefore shall not be counted as a major degradation mechanism. The observed Cr enrichment is a likely contributor to the observed electrical degradation whereas the consequences of the increasing sub-micron inhomogeneity are not yet known. The diffusion of Sr through the CGO barrier layer and formation of Sr-Zr phases at the CGO−YSZ interface further contribute to the long term degradation.