The van’t Hoff method is a standard approach for determining reaction enthalpies and entropies, e.g., in the thermochemical reduction of oxides, which is an important process for solar thermochemical fuels and numerous other applications. However, by analyzing the oxygen partial pressure pO₂, e.g., as measured by thermogravimetric analysis (TGA), this method convolutes the properties of the probe gas with the solid-state properties of the examined oxides, which define their suitability for specific applications. The “chemical potential method” is here proposed as an alternative. Using the oxygen chemical potential Δμ_O instead of pO₂ for the analysis, this method does not only decouple gas-phase and solid-state contributions but also affords a simple and transparent approach to extracting the temperature dependence of the reduction enthalpy and entropy, which carries important information about the defect mechanism. For demonstration of the approach, this work considers three model systems; (1) a generic oxide with noninteracting, charge-neutral oxygen vacancy defects, (2) Sr_0.86Ce_0.14MnO_3(1−δ) alloys with interacting vacancies, and (3) a model for charged vacancy formation in CeO₂, which reproduces the extensive experimental TGA data available in the literature. The reduction behavior of these model systems obtained from the chemical potential method is correlated with simulated results for the thermochemical water splitting cycle, highlighting the exceptional behavior of CeO₂, which originates from defect ionization. The theoretical performance limits for solar thermochemical hydrogen within the charged defect mechanism are assessed by considering hypothetical materials described by a variation of the CeO₂ model parameters within a plausible range.

中文翻译：

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化学势分析作为范特霍夫方法的替代方法：太阳能热化学氢的假设极限


范特霍夫方法是确定反应焓和熵的标准方法，例如在氧化物的热化学还原中，这是太阳能热化学燃料和许多其他应用的重要过程。然而，通过分析氧分压 pO ₂ ，例如通过热重分析 (TGA) 测量，该方法将探测气体的特性与所检查氧化物的固态特性进行卷积，这定义了它们对特定应用的适用性。这里提出“化学势法”作为替代方案。使用氧化学势 Δμ _O 代替 pO ₂ 进行分析，该方法不仅可以解耦气相和固态贡献，而且还提供了一种简单透明的方法提取还原焓和熵的温度依赖性，其中包含有关缺陷机制的重要信息。为了演示该方法，本工作考虑了三个模型系统； (1) 具有非相互作用、电中性氧空位缺陷的普通氧化物，(2) 具有相互作用空位的 Sr _0.86 Ce _0.14 MnO _3(1−δ) 合金，以及（ 3）CeO ₂ 中带电空位形成的模型，它再现了文献中可用的大量实验 TGA 数据。通过化学势方法获得的这些模型系统的还原行为与热化学水分解循环的模拟结果相关，突出了CeO ₂ 源自缺陷电离的特殊行为。 通过考虑由 CeO ₂ 模型参数在合理范围内的变化描述的假设材料，评估带电缺陷机制内太阳能热化学氢的理论性能极限。