Zhang Haiwu, Castelli Ivano E, Santucci Simone, Sanna Simone, Pryds Nini, Esposito Vincenzo
Department of Energy Conversion and Storage, Technical University of Denmark, Anker Engelunds Vej 411, DK-2800 Kgs. Lyngby, Denmark.
Phys Chem Chem Phys. 2020 Oct 7;22(38):21900-21908. doi: 10.1039/d0cp03298k.
Cerium oxide (ceria, CeO2) is one of the most promising mixed ionic and electronic conducting materials. Previous atomistic analysis has widely covered the effects of substitution on oxygen vacancy migration. However, an in-depth analysis of the role of cation substitution beyond trivalent cations has rarely been explored. Here, we investigate soluble monovalent (Li+, Na+, K+, Rb+), divalent (Fe2+, Co2+, Mn2+, Mg2+, Ni2+, Zn2+, Cd2+, Ca2+, Sr2+, Ba2+), trivalent (Al3+, Fe3+, Sc3+, In3+, Lu3+, Yb3+, Y3+, Er3+, Gd3+, Eu3+, Nd3+, Pr3+, La3+) and tetravalent (Si4+, Ge4+, Ti4+, Sn4+, Hf4+, Zr4+) cation substituents. By combining classical simulations and quantum mechanical calculations, we provide an insight into defect association energies between substituent cations and oxygen vacancies as well as their effects on the diffusion mechanisms. Our simulations indicate that oxygen ionic diffusivity of subvalent cation-substituted systems follows the order Gd3+ > Ca2+ > Na+. With the same charge, a larger size mismatch with the Ce4+ cation yields a lower oxygen ionic diffusivity, i.e., Na+ > K+, Ca2+ > Ni2+, Gd3+ > Al3+. Based on these trends, we identify species that could tune the oxygen ionic diffusivity: we estimate that the optimum oxygen vacancy concentration for achieving fast oxygen ionic transport is ≈2.5% for GdxCe1-xO2-x/2, CaxCe1-xO2-x and NaxCe1-xO2-3x/2 at 800 K. Remarkably, such a concentration is not constant and shifts gradually to higher values as the temperature is increased. We find that co-substitutions can enhance the impact of the single substitutions beyond that expected by their simple addition. Furthermore, we identify preferential oxygen ion migration pathways, which illustrate the electro-steric effects of substituent cations in determining the energy barrier of oxygen ion migration. Such fundamental insights into the factors that govern the oxygen diffusion coefficient and migration energy would enable design criteria to be defined for tuning the ionic properties of the material, e.g., by co-substitutions.
氧化铈（二氧化铈，CeO₂）是最具前景的混合离子与电子导电材料之一。先前的原子分析广泛涵盖了取代对氧空位迁移的影响。然而，对于三价阳离子以外的阳离子取代作用的深入分析却鲜有探索。在此，我们研究了可溶性单价（Li⁺、Na⁺、K⁺、Rb⁺）、二价（Fe²⁺、Co²⁺、Mn²⁺、Mg²⁺、Ni²⁺、Zn²⁺、Cd²⁺、Ca²⁺、Sr²⁺、Ba²⁺）、三价（Al³⁺、Fe³⁺、Sc³⁺、In³⁺、Lu³⁺、Yb³⁺、Y³⁺、Er³⁺、Gd³⁺、Eu³⁺丶Nd³⁺、Pr³⁺、La³⁺）和四价（Si⁴⁺、Ge⁴⁺、Ti⁴⁺、Sn⁴⁺、Hf⁴⁺、Zr⁴⁺）阳离子取代基。通过结合经典模拟和量子力学计算，我们深入了解了取代阳离子与氧空位之间的缺陷缔合能及其对扩散机制的影响。我们的模拟表明，低价阳离子取代体系的氧离子扩散率遵循Gd³⁺ > Ca²⁺ > Na⁺的顺序。在电荷相同的情况下，与Ce⁴⁺阳离子的尺寸失配越大，氧离子扩散率越低，即Na⁺ > K⁺，Ca²⁺ > Ni²⁺，Gd³⁺ > Al³⁺。基于这些趋势，我们确定了可以调节氧离子扩散率的物种：我们估计，对于GdxCe1-xO2-x/2、CaxCe1-xO2-x和NaxCe1-xO2-3x/2，在800 K时实现快速氧离子传输的最佳氧空位浓度约为2.5%。值得注意的是，这样的浓度并非恒定不变，而是随着温度升高逐渐向更高值偏移。我们发现，共取代可以增强单取代的影响，其效果超过简单相加的预期。此外，我们确定了优先的氧离子迁移途径，这说明了取代阳离子在确定氧离子迁移能垒方面的电空间效应。对控制氧扩散系数和迁移能的因素的这种基本见解将能够确定设计标准，例如通过共取代来调节材料的离子性质。
