Bulk and Surface Properties of New Materials for Solid Oxide Fuel Cell Electrodes

为了进一步提高固体氧化物燃料电池(SOFC)的效率和热稳定性，需要能够将燃料电池的工作温度降低到600-800℃的替代电极材料。两类具有良好的电子/离子导电性和低温催化活性的新型阴阳极材料分别是YBaCo4O7结构的层状钴氧化物和Sr2MgMoO6基双钙钛矿结构的钴氧化物。这两种化合物的电子/离子和催化性质都可以通过在不同亚晶格上的各种元素的取代来调节，这也改变了它们的热相稳定性。改善固体氧化物燃料电池性能的另一个非常有前景的策略是使用液态锡阳极高温SOFC。该项目的总体目标是通过基于密度泛函理论的第一性原理计算，在原子水平上更好地了解这些新型SOFC阳极和阴极材料的体相和表面性质。通过研究SOFC与氧和燃料小分子(H2、CH4、CO)的相互作用，我们将分别模拟SOFC阴极侧和阳极侧SOFC工作条件下的过程。我们将重点研究体相和表面结构的热力学稳定性，并在给定的温度和分压下寻找在不同环境中最稳定的表面构型和组成。研究了取代基对电子性质、氧空位形成和相稳定性的影响。氧在块体内部、表面和电极/电解液界面上的迁移动力学应通过计算活化势垒来获得。这些过程控制阴极的摄氧量、阳极的燃料氧化和电极/电解液界面的相稳定性的机理基本上仍不清楚。更好的理解将有助于以知识为基础改进这些新材料类别的化学和热性能，以用于未来的SOFC应用。

Further improvement of the efficiency and thermal stability of solid oxide fuel cells (SOFCs) requires alternative electrode materials that allow to lower the operating temperature of fuel cells to 600-800 C. Two innovative new classes of cathode and anode materials with promising electronic/ionic conductivity and catalytic activity at lower temperatures are layered cobalt oxide compounds with YBaCo4O7 structure and Sr2MgMoO6-based double-perovskites, respectively. The electronic/ionic and catalytic properties of both compounds can be tuned by substitution with a wide variety of elements on the different sublattices, which also modifies their thermal phase stability. Another very promising strategy for improving solid oxide fuel cell performance is to use liquid Sn anodes high-temperature SOFCs. They are distinguished by high robustness and low sensitivity to fuel contaminations.The overall aim of this project is to obtain a better understanding of the bulk and surface properties of these new classes of SOFC anode and cathode materials on an atomistic level by means of first-principles calculations based on density functional theory. By studying the interaction with oxygen and small fuel molecules (H2, CH4, CO) we will simulate the processes that occur during SOFCs working conditions on the cathode and anode sides, respectively. We will focus on the thermodynamic stability of bulk phases and surface structures and search for the most stable surface configurations and compositions in different environments at given temperature and partial pressures. The influence of substituents on the electronic properties, on oxygen vacancy formation and on phase stability will be investigated. Insights into the kinetics of oxygen migration inside the bulk, on the surfaces and at the electrode/electrolyte interfaces shall be obtained by calculation of activation barriers. The mechanisms of these processes that govern the oxygen uptake at the cathode, the fuel oxidation at the anode and the phase stability at the electrode/electrolyte interfaces are still basically unknown. A better understanding shall contribute to a knowledge-based improvement of the chemical and thermal properties of these new classes of materials for future SOFC applications.