Ceria−Zirconia Solid Solutions (Ce 1−x Zr x O 2−δ , x ≤ 0.2) for Solar Thermochemical Water Splitting: A Thermodynamic Study Yong Hao, †,‡ Chih-Kai Yang, † and Sossina M. Haile* ,† † Materials Science, California Institute of Technology, Pasadena, California 91125, United States ‡ Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, China ABSTRACT: The redox behavior of ceria−zirconia solid solutions (or Zr-substituted ceria, ZSC) with a Zr content of up to 20 mol % is studied by thermogravimetry (TG) between 600 °C and 1490 °C under controlled atmospheres. Thermodynamic properties, specifically standard oxidation enthalpy, ΔH oxd ⊖ , and entropy, ΔS oxd ⊖ , are derived from TG data. The raw TG results show that the extent of reduction is significantly increased (compared with undoped ceria), even at a low Zr substitution level of 5 mol %. Concomitantly, the magnitude of the thermodynamic functions dramatically decreases as a function of Zr content, particularly at low values of oxygen non-stoichiometry, δ (<3 mol %). Thermochemical fuel production from Zr-substituted ceria generally increases with increasing Zr content under both two-temperature and isothermal cycling conditions. In the case of two-temperature cycling, the benefit is accompanied by a penalty in the (computed) steam-to-hydrogen conversion ratio, whereas it is accompanied by a gain in this ratio for isothermal cycling. Overall, introduction of Zr has the potential to enhance solar-driven thermochemical fuel production, depending on the details of cycling conditions and reactor design. 1. INTRODUCTION Ceria (CeO 2 ) and its derivatives constitute a family of important catalyst materials widely adopted in processes requiring reversible reduction and oxidation (i.e., redox) behavior. One of their most prevalent applications is in automotive emissions control. Here, the oxide serves as an active support to metal catalyst particles and provides critical buffering of oxygen in three-way catalytic converters (TWCs), with oxygen being rapidly released or sorbed, in response to a change of gas composition in the exhaust. 1−3 Ceria-based materials have also gained significant attention in recent years as materials to facilitate the solar-driven thermochemical dissociation of water and/or carbon dioxide, 4−7 a process that similarly relies on the redox activity of the oxide. Relevant to both of these applications, the introduction of zirconium as a substitutional element has been shown to be advantageous in increasing what is termed the oxygen storage capacity, or the extent to which the material can change oxidation state. 8−11 For catalytic converters, this corresponds to more-effective buffering. For thermochemical cycling, in which the implica- tions have only just begun to be explored, there is potential for lowering the temperatures of operation and/or increasing the gravimetric fuel production capacity for a given cycling strategy. While the benefits of zirconium substitution into ceria for, particularly, the catalytic applications have long been recognized, remarkably, the bulk redox properties of compositions in the ceria−zirconia solid solution have not been fully described. Specifically, outside of two conflicting studies, 12−14 the variation of oxygen content as a function of temperature and oxygen chemical potential (oxygen partial pressure in the gas phase) for Ce 1−x Zr x O 2−δ materials, particularly at small x (≤0.2), as is relevant to thermochemical fuel production, has not been reported. However, knowledge of this behavior, and the implied thermodynamic functions, enthalpy and entropy of oxidation, are essential inputs to any assessment of the material for thermally driven solar-fuel generation. Deconvolution of the bulk redox properties from those due to the surface may also shed light on the role of the oxide as a support in conventional heterogeneous catalysis applications. Here, we present a comprehensive investigation of the bulk redox thermodynamics of Ce 1−x Zr x O 2−δ (x = 0.05, 0.10, 0.15 and 0.20, hereafter designated Zr##, where ## is the mole percentage of Zr), using thermogravimetry (TG) under conditions relevant to solar fuel synthesis. Oxygen non- stoichiometry (δ) is measured as a function of temperature (600−1490 °C) over a wide range of atmospheres (from strongly reducing to strongly oxidizing), and from these data we extract the enthalpy and entropy of oxidation. The investigation is limited to low Zr content compositions, x ≤ 0.20, to avoid possible formation of the tetragonal phase, 3,14−16 which can be anticipated to display lower diffusivity and is known to have lower oxygen storage capacity than the cubic fluorite structure. 17 Received: August 26, 2014 Revised: September 22, 2014 Article pubs.acs.org/cm © XXXX American Chemical Society A dx.doi.org/10.1021/cm503131p | Chem. Mater. XXXX, XXX, XXX−XXX