Hess Franziska, Yildiz Bilge
Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2021 Feb 14;154(6):064702. doi: 10.1063/5.0035691.
Perovskite oxides degrade at elevated temperatures while precipitating dopant-rich particles on the surface. A knowledge-based improvement of surface stability requires a fundamental and quantitative understanding of the dopant precipitation mechanism on these materials. We propose that dopant precipitation is a consequence of the variation of dopant solubility between calcination and operating conditions in solid oxide fuel cells (SOFCs) and electrolyzer cells (SOECs). To study dopant precipitation, we use 20% (D = Ca, Sr, Ba)-doped LaMnO (LDM20) as a model system. We employ a defect model taking input from density functional theory calculations. The defect model considers the equilibration of LDM20 with a reservoir consisting of dopant oxide (DO), peroxide (DO), and O in the gas phase. The equilibrated non-stoichiometry of the A-site and B-site as a function of temperature, T, and oxygen partial pressure, p(O), reveals three regimes for LDM20: A-site deficient (oxidizing conditions), A-site rich (atmospheric conditions), and near-stoichiometric (reducing conditions). Assuming an initial A/B non-stoichiometry, we compute the dopant precipitation boundaries in a p-T phase diagram. Our model predicts precipitation both under reducing (DO) and under highly oxidizing conditions (DO). We found precipitation under anodic, SOEC conditions to be promoted by large dopant size, while under cathodic, SOFC conditions precipitation is promoted by initial A-site excess. The main driving forces for precipitation are oxygen uptake by the condensed phase under oxidizing conditions and oxygen release assisted by B-site vacancies under reducing conditions. Possible strategies for mitigating dopant precipitation under in electrolytic and fuel cell conditions are discussed.
钙钛矿氧化物在高温下会降解，同时在表面析出富含掺杂剂的颗粒。基于知识的表面稳定性改进需要对这些材料上的掺杂剂析出机制有基本的定量理解。我们提出，掺杂剂析出是固体氧化物燃料电池（SOFC）和电解槽（SOEC）中煅烧和运行条件下掺杂剂溶解度变化的结果。为了研究掺杂剂析出，我们使用20%（D = Ca、Sr、Ba）掺杂的LaMnO（LDM20）作为模型体系。我们采用了一个从密度泛函理论计算获取输入的缺陷模型。该缺陷模型考虑了LDM20与由掺杂剂氧化物（DO）、过氧化物（DO）和气相中的O组成的储库之间的平衡。A位和B位的平衡非化学计量比作为温度T和氧分压p(O)的函数，揭示了LDM20的三种状态：A位缺陷（氧化条件）、A位富（大气条件）和近化学计量比（还原条件）。假设初始的A/B非化学计量比，我们在p-T相图中计算掺杂剂析出边界。我们的模型预测了在还原（DO）和高氧化条件（DO）下的析出。我们发现，在阳极SOEC条件下，大的掺杂剂尺寸会促进析出，而在阴极SOFC条件下，初始的A位过量会促进析出。析出的主要驱动力是氧化条件下凝聚相对氧的吸收以及还原条件下B位空位辅助的氧释放。讨论了在电解和燃料电池条件下减轻掺杂剂析出的可能策略。
