Decoding and tuning the surface stability of perovskite oxides at the atomic level for faster oxygen exchange kinetics in energy conversion devices

在原子水平上解码和调整钙钛矿氧化物的表面稳定性，以实现能量转换装置中更快的氧交换动力学

• 批准号: 324830457
• 负责人: Professorin Dr. Franziska Heß
• 金额: --
• 依托单位: Nuclear Science and Engineering Department
• 依托单位国家: 德国
• 项目类别: Research Fellowships
• 财政年份: 2017
• 资助国家: 德国
• 起止时间: 2016-12-31 至 2018-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/324830457?language=en
• 关键词: Decoding; tuning; surface; stability; perovskite

Doped perovskite oxides serve as functional electrocatalyst layers in solid oxide fuel cells (SOFCs) because they can attain high electronic and oxide ion conductivity as well as good compatibility with common electrolyte materials. It has recently been identified that aliovalently doped perovskites, such as La1-xSrxMnO3 (LSM), form segregation layers capping the electrode surface under operating conditions, and this process is detrimental for electrode surface activity, thus degrading the material and SOFC performance over time. Reduction of strain energy due to cation size mismatch and electrostatic interaction with a space charge zone in the near-surface region have been proposed as the key mechanisms leading to cation segregation. The segregation is affected by external parameters, such as p(O2), T and applied potential. Because the interplay between the external conditions and material properties (i.e. cation size mismatch, and oxygen vacancy concentration) are not quantitatively understood on the atomistic level, it has not been possible to stabilize these surfaces using knowledge-based approaches. This project aims to quantitatively understand the relations of surface structure with environmental conditions, and material properties, and to predict conditions where stability against surface segregation and electrochemical activity are improved. The proposed computational framework unites the material properties and thermodynamic factors into a single Monte-Carlo model for predicting the evolution of the near-surface region. The expected outcome of the model is the ability to predict the distribution of dopant and host cations at the near-surface region of perovskite oxides, by accounting explicitly for the distribution of oxygen vacancies (giving rise to the proposed space charge zone). As a technologically important model perovskite electrocatalyst system, this research takes La0.8Sr0.2MnO3 as a starting point. The formation energies of oxygen vacancies and SrLa' defects close to the surface will be computed by density functional theory calculations and analyzed in terms of a cluster expansion in order to obtain a lattice Hamiltonian. Monte Carlo simulations of the near-surface region will yield the distribution of oxygen vacancies, as well as the cation segregation profile as a function of T and p(O2). Secondary phase formation will be studied by ab-initio thermodynamics considering formation and interface energies of likely candidates (e.g., SrO, Ruddlesden-Popper phases). Stabilization of the surface by modification with transition metal cations will be studied by considering the effect of surface substitution of Mn by Hf on the Sr segregation profile. This model will make it possible to understand accurately the factors leading to cation segregation and thus provide a first-principles basis for the optimization of surface properties on the atomic scale.

掺杂的钙钛矿型氧化物可以获得较高的电子和氧化物离子导电性以及与普通电解液材料的良好兼容性，可作为固体氧化物燃料电池的功能电催化层。最近发现，低价掺杂的钙钛矿材料，如La1-xSrxMnO3(LSM)，在工作条件下会形成偏析层覆盖在电极表面，这一过程不利于电极表面的活性，从而随着时间的推移而降低材料和SOFC的性能。阳离子尺寸失配引起的应变能降低以及与近表面区空间电荷区的静电相互作用是导致阳离子偏析的主要机制。偏析受p(O2)、T和外加电位等外部参数的影响。由于外界条件和材料性质(即阳离子大小失配和氧空位浓度)之间的相互作用不能在原子水平上定量地理解，因此不可能使用基于知识的方法来稳定这些表面。该项目旨在定量了解表面结构与环境条件和材料性能的关系，并预测提高抗表面偏析稳定性和电化学活性的条件。所提出的计算框架将材料性质和热力学因素统一到一个单一的蒙特卡罗模型中，用于预测近表面区域的演化。该模型的预期结果是能够通过明确考虑氧空位的分布(产生所提议的空间电荷区)来预测钙钛矿氧化物近表面区域的掺杂和主体阳离子的分布。作为一种具有重要技术意义的钙钛矿型电催化体系模型，本研究以La0.8Sr0.2MnO_3为出发点。用密度泛函理论计算了表面附近氧空位和SrLa‘缺陷的形成能，并用团簇展开进行了分析，得到了晶格哈密顿量。近表面区的蒙特卡罗模拟将得到氧空位的分布，以及作为T和p(O2)的函数的阳离子偏析分布。第二相的形成将通过从头算热力学进行研究，考虑可能的候选相(例如，SRO、Ruddlesden-Popper相)的形成和界面能。研究了过渡金属阳离子修饰对表面稳定性的影响，并考虑了Hf取代表面对锶偏析分布的影响。该模型可以准确地理解导致阳离子偏析的因素，从而为原子尺度上的表面性质优化提供第一性原理依据。