高晶界比CeO2/BaCeO3基复合电解质制备及晶界传导机制研究

CeO2/BaCeO3 based composites are regarded as one of the most promising electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFC) due to their advantages including excellent chemical stability of CeO2 based electrolytes and high open circuit voltage of BaCeO3 based electrolytes. Increasing grain-boundary ratio is an effective approach to improve the ionic conductivity due to the grain-boundary effect. However, it is difficult to fabricate CeO2/BaCeO3 based composites with high grain-boundaries ratio by traditional sintering. Besides, the grain-boundary conduction mechanism of CeO2/BaCeO3 based composites is not clear. In this study, high reactive CeO2/BaCeO3 based nanopowders will be synthesized by a microwave assisted combustion method, and CeO2/BaCeO3 based composites with high grain-boundary ratio will be fabricated by a microwave combined two-step sintering process. Sintering characteristics of CeO2/BaCeO3 based nanopowders and sintering mechanisms of CeO2/BaCeO3 based composites will be clarified, and the electrical properties and grain-boundary conduction mechanism will be studied by EIS fitting and concentration cells designing. Aiming to find the correlation among microstructures, grain-boundary conduction mechanism and electrical performance, and resolve the key scientific issues inhibiting the development of CeO2/BaCeO3 based composite electrolytes, this study will provide theoretical basis and technical support for improving the electrical performance, and lay a scientific foundation for promoting the development of SOFC.

CeO2/BaCeO3基复合电解质兼具CeO2基电解质化学稳定性好和BaCeO3基电解质开路电压高的性能优势，是中温固体氧化物燃料电池的重要电解质候选材料。晶界特性对电解质的电学性能有重要影响，增加晶界比是提高离子电导率的有效途径之一。然而，用传统方法难以制备高晶界比的CeO2/BaCeO3基复合电解质；另外，晶界传导机制目前尚不清楚。本项目拟采用微波燃烧法合成高活性CeO2/BaCeO3基纳米粉体，采用微波两步烧结工艺制备高晶界比的CeO2/BaCeO3基复合电解质；研究粉体烧结特性和电解质烧结机理，并借助于交流阻抗谱图拟合和浓差电池电化学测试，研究电解质的电学性能和晶界传导机制；探讨电解质的微观结构、晶界传导机制与电学性能的相关性规律，解决制约CeO2/BaCeO3基复合电解质发展的关键科学问题，为提高电解质的离子电导率提供理论依据和技术支持，为固体氧化物燃料电池产业化奠定科学基础。

晶界特性是影响固体电解质的重要因素之一。本项目围绕高晶界比CeO2/BaCeO3基复合电解质的制备、电学性能、晶界传导机制开展了系统研究。首先，采用微波燃烧法合成了Sm掺杂CeO2/BaCeO3（SDC-BCS）纳米粉体，研究了微波合成机理与烧结特性。SDC-BCS纳米粉体具有晶粒尺寸小（17.83 nm）、比表面积大（34.04 m2/g）、分散性好（φ=1.32）等优点，通过向SDC-BCS中进一步掺杂Bi元素，其烧成温度降低约150℃，解决了因烧结温度过高而导致的晶界电阻过大、晶界分布均匀性差等问题。然后，研究了Bi掺杂SDC-BCS电解质的烧结动力学和超细晶粒化过程，明确了电解质在不同烧成温度区间的晶粒生长机制（在1100℃以下以晶界扩散为主，在1300℃以上以晶界迁移为主），通过调控晶粒生长过程，采用两步烧结工艺制备了平均晶粒尺寸为100-150nm的高晶界比Bi掺杂SDC-BCS电解质。最后，通过交流阻抗图谱、阻塞电极法、浓差电池（氢、氧）设计，研究了电学性能，明确了载流子传导类型和晶界传导机制。高晶界比Bi掺杂SDC-BCS电解质具有显著的“晶界效应”，其表观单位晶界电导率比微米级同类电解质高1-2个数量级。“晶界效应”并非源于额外的电子电导，而是得益于O-H和氧空位浓度的增大，促进了氧离子和质子传导，其中，氧离子传导的提高效应占主导地位。通过研究，本项目揭示了CeO2/BaCeO3基复合电解质制备工艺、微观结构、晶界传导机制以及电学性能的相关性规律，阐述了高晶界比Bi掺杂SDC-BCS电解质的“晶界效应”及机理，明确了晶界载流子传导类型，可为新型中温SOFCs电解质的研发提供新思路。