Sm~(3+),Sr~(2+)共掺杂对CeO_2基电解质性能影响的密度泛函理论+U计算

Sm~(3+),Sr~(2+)共掺杂CeO_2的离子电导率被证实可高达Sm~(3+)掺杂CeO_2离子电导率的近两倍,然而,共掺杂对CeO_2电导率的作用机理尚不明确.本文利用第一性原理计算的密度泛函理论+U方法,对Sm~(3+)和Sr~(2+)共掺杂的CeO_2进行了系统的研究,对比Sm~(3+)或Sr~(2+)单掺杂的CeO_2体系,计算并分析了共掺杂体系的电子态密度、能带结构、氧空位形成能以及氧空位迁移能等微观属性.计算结果表明,Sm~(3+),Sr~(2+)的共掺杂对CeO_2基电解质性能的提高具有协同效应,二者的共掺杂不仅能协同抑制CeO_2体系的电子电导率,还能在单掺杂CeO_2的基础上进一步降低氧空位形成能,Sm~(3+)的存在还有助于降低Sr~(2+)对氧空位的俘获作用,而Sr~(2+)的加入则能够在Sm~(3+)掺杂CeO_2的基础上进一步降低最低氧空位迁移能,爬坡式弹性能带方法计算表明共掺杂体系的氧空位迁移能最低可达0.314/0.295 eV,低于Sm~(3+)掺杂CeO_2的最低氧空位迁移能.研究揭示了Sm~(3+),Sr~(2+)共掺杂对CeO_2电导率的协同作用机理,对进一步研发其他高性能的共掺杂电解质材料具有重要的指导意义.

DFT+U calculation of Sm3+ and Sr2+ co-doping effect on performance of CeO2-based electrolyte

Solid oxide fuel cells (SOFCs) have been attracting people's attention for their high energy conversion efficiency, good fuel compatibility, no precious metal catalysts, and pollution-free emissions. However, the high operating temperature (800-1200℃) of the traditional SOFC can reduce the long-term stability and cause the difficulties in either the selecting of material or the sealing of SOFC. Therefore, great efforts have been devoted to developing the intermediate temperature SOFC (IT-SOFC), which works at 600-800℃. In the IT-SOFC, the ionic conductivity of doped CeO₂-based electrolyte has a significant advantage relative to that of the conventional yttria-stabilized zirconia (YSZ) electrolyte. For example, at 600℃, the ionic conductivity of Sm-doped CeO₂ is 0.02 S/cm much higher than that of the traditional YSZ electrolyte (only 0.0032 S/cm). Therefore, the doped CeO₂-based electrolyte is a very promising electrolyte for IT-SOFC.Recently, the co-doping of two different elements into CeO₂ has become a hot research topic. The ionic conductivity of Sm³⁺ and Sr²⁺ co-doped CeO₂ has proved to be nearly twice as high as that of Sm³⁺ doped CeO₂ (SDC). However, the mechanism for the co-doping effect on the conductivity of CeO₂ is not clear. In this paper, Sm³⁺ and Sr²⁺ co-doped CeO₂ is systematically studied using the DFT+U method. The microscopic properties of the Sm³⁺ and Sr²⁺ co-doped CeO₂ including electronic density of states, band structure, oxygen vacancy formation energy and oxygen vacancy migration energy and so on have been calculated and analyzed by comparing with those of the Sm³⁺ or Sr²⁺ single doped CeO₂. The calculation results indicate that Sm³⁺ and Sr²⁺ co-doping has a synergistic effect on the performance improvement of CeO₂-based electrolyte, which can not only suppress the electronic conductivity of doped CeO₂ system, but also can reduce the oxygen vacancy formation energy on the basis of single doped CeO₂. The existence of Sm³⁺ can help to reduce the trapping effect of Sr²⁺ on oxygen vacancies, meanwhile the addition of Sr²⁺ can further reduce the minimum oxygen vacancy migration energy on the basis of SDC. Calculations by the climbing image nudged elastic band (CINEB) method indicate that the oxygen vacancy migration energy of the co-doped system can reach as low as 0.314/0.295 eV, which is lower than the minimum oxygen vacancy migration energy of SDC. Our research reveals the synergistic mechanism for Sm³⁺ and Sr²⁺ co-doping effect on the conductivity of CeO₂, which is of great instructive significance for the further research and development of other high-performance co-doped electrolyte materials in IT-SOFC.