Decoding and tuning the surface stability of perovskite oxides at the atomic level for faster oxygen exchange kinetics in energy conversion devices
Physical Chemistry of Solids and Surfaces, Material Characterisation
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
We studied dopant segregation in (Ca, Sr, Ba)-doped LaMnO3 by Density Functional Theory calculations, surface and defect thermodynamics. This material was chosen as a model system for doped perovskite (ABO3) surfaces exposed to harsh reaction conditions at elevated temperatures. These materials undergo surface compositional changes by segregation of aliovalent dopants. In Solid Oxide Fuel Cell cathodes, this surface segregation inhibits the cathode activity, thereby contributing to electrode aging and reducing long-term stability of Solid Oxide Fuel Cells. While it is well-established in the literature that such segregation occurs and can be detrimental for many applications, it is not well-understood why it happens. Establishing a quantitative model based on the abundant experimental data in the literature has so far been impossible, in part due to conflicting experimental results and in part due to the complexity of nonstoichiometric perovskite oxide surfaces. We approach this problem with a Density Functional Theory-based ab-initio thermodynamics model, where we take into account different surface terminations and defects near the surface in the minimization of the surface free energy. We were able to confirm that Sr enrichment at the surface is a result of electrostatic interaction between positively charged oxygen vacancies near the surface and the negatively charged Sr dopant as previously proposed in the literature. In addition, we obtained two more surprising results. The first is that, while it is well-known that polar surfaces can stabilize charged defects, the opposite is also true, that is, charged defects near the surface can stabilize polar surfaces. This means that the favorable surface termination can expose different cations under oxidizing and reducing conditions. The second surprising result is that the stable surface termination exposes stripes of SrO, coexisting with the underlying MnO2 layer at elevated temperatures under reducing conditions. This termination is so stable because it does not have a dipole moment normal to the surface, in contrast to the perfect terminations, LaO, SrO and MnO2. We studied dopant precipitation from 20 % (Ca, Sr, Ba)-doped LaMnO3 under Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolysis Cell (SOEC) conditions by experiment, employing the lateral polarization technique, and by theory, employing a DFT-based defect chemistry model. We were able to demonstrate that precipitation occurs both under SOFC and SOEC conditions and rationalize our findings using theory. Under SOEC conditions, precipitation is driven by an energy reduction (binding oxygen in the condensed phase in the form of a peroxide), while under SOFC conditions, it is driven by increasing entropy due to the release of oxygen. The latter was found to be possible only if the material has an A-site excess in the ppm-range. Because there is no energetic driving force for precipitation under SOFC conditions, this reaction must be treated as a defect reaction on the thermodynamic level. We expanded our defect model to account for the reaction of the precipitated dopant oxide and peroxide with contaminants in the gas phase (H2O, CO2, SOx, CrOx). We found that SOx and CrOx can promote precipitation under SOEC and SOFC conditions, respectively. A noticeable promotion of precipitation is expected at temperatures below 1250 K, suggesting that cells operated at intermediate temperature are more vulnerable to contaminations than cells operated at high temperature. Based on our results, we propose that favoring Ca as a dopant for SOECs will improve electrode durability. Due to its small ion radius, Ca has a higher binding energy in the perovskite than Sr and Ba, resulting in less precipitation under SOEC conditions. Under SOFC conditions, we observe similar behavior for all three dopants because precipitation is driven by entropy rather than energy. It can be mitigated by increasing the entropy of the A-sublattice. This can be achieved by A-site deficiency or by adding more dopants with the same charge as the host cation (La3+).
Publications
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Solid Oxide Fuel Cell Materials and Interfaces. (2019) In: Andreoni W., Yip S. (eds) Handbook of Materials Modeling. Springer, Cham
F. Hess, A.T. Staykov, B. Yildiz, J. Kilner
