Takacs M, Scheffe J R, Steinfeld A
Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland.
Phys Chem Chem Phys. 2015 Mar 28;17(12):7813-22. doi: 10.1039/c4cp04916k.
This work encompasses the thermodynamic characterization and critical evaluation of Zr(4+) doped ceria, a promising redox material for the two-step solar thermochemical splitting of H2O and CO2 to H2 and CO. As a case study, we experimentally examine 5 mol% Zr(4+) doped ceria and present oxygen nonstoichiometry measurements at elevated temperatures ranging from 1573 K to 1773 K and oxygen partial pressures ranging from 4.50 × 10(-3) atm to 2.3 × 10(-4) atm, yielding higher reduction extents compared to those of pure ceria under all conditions investigated, especially at the lower temperature range and at higher pO2. In contrast to pure ceria, a simple ideal solution model accounting for the formation of isolated oxygen vacancies and localized electrons accurately describes the defect chemistry. Thermodynamic properties are determined, namely: partial molar enthalpy, entropy, and Gibbs free energy. In general, partial molar enthalpy and entropy values of Zr(4+) doped ceria are lower. The equilibrium hydrogen yields are subsequently extracted as a function of the redox conditions for dopant concentrations as high as 20%. Although reduction extents increase greatly with dopant concentration, the oxidation of Zr(4+) doped ceria is thermodynamically less favorable compared to pure ceria. This leads to substantially larger temperature swings between reduction and oxidation steps, ultimately resulting in lower theoretical solar energy conversion efficiencies compared to ceria under most conditions. In effect, these results point to the importance of considering oxidation thermodynamics in addition to reduction when screening potential redox materials.
这项工作包括对锆（4+）掺杂二氧化铈进行热力学表征和批判性评估，锆（4+）掺杂二氧化铈是一种很有前景的氧化还原材料，可用于将水和二氧化碳两步太阳能热化学分解为氢气和一氧化碳。作为一个案例研究，我们对5摩尔%锆（4+）掺杂二氧化铈进行了实验研究，并给出了在1573 K至1773 K的高温以及4.50×10⁻³ 大气压至2.3×10⁻⁴ 大气压的氧分压下的氧非化学计量测量结果，在所有研究条件下，与纯二氧化铈相比，其还原程度更高，尤其是在较低温度范围和较高氧分压下。与纯二氧化铈不同，一个考虑孤立氧空位和局域电子形成的简单理想溶液模型准确地描述了缺陷化学。确定了热力学性质，即：偏摩尔焓、熵和吉布斯自由能。一般来说，锆（4+）掺杂二氧化铈的偏摩尔焓和熵值较低。随后提取了高达20%掺杂剂浓度下平衡氢产率与氧化还原条件的函数关系。尽管还原程度随着掺杂剂浓度的增加而大幅提高，但与纯二氧化铈相比，锆（4+）掺杂二氧化铈的氧化在热力学上不太有利。这导致还原和氧化步骤之间的温度波动大幅增大，最终在大多数条件下与二氧化铈相比理论太阳能转换效率更低。实际上，这些结果表明在筛选潜在的氧化还原材料时，除了考虑还原之外，还需要考虑氧化热力学的重要性。
