Investigation of Perovskite Structures as Oxygen-Exchange Redox Materials for Hydrogen Production from Thermochemical Two-Step Water-Splitting Cycles

This study addresses the synthesis, characterization, and thermochemical redox performance evaluation of perovskites and parent structures (Ruddlesden–Popper phases) as a class of oxygen-exchange materials for hydrogen generation via solar two-step water splitting. The investigated materials are La_<i>x</i>Sr_1–<i>x</i>MO₃ (M = Mn, Co, Fe), Ba_<i>x</i>Sr_1–<i>x</i>(Co,Fe)­O₃, LaSrCoO₄, and LaSrFeO₄, also used as mixed ionic-electronic conductors in fuel cells. Temperature-programmed reduction, powder X-ray diffraction, and thermogravimetric analysis were used to obtain a preliminary assessment of these materials performances. Most of the perovskites studied here stand out by larger thermal reduction capabilities and oxygen vacancies formation at modest temperatures in the range 1000–1400 °C when compared with reference nonstoichiometric compounds such as spinel ferrites or fluorite-structured ceria-based materials. In addition, these materials offer noticeable access to metallic valence transitions during reoxidation in steam atmosphere that are not available in stoichiometric oxides. The promising behaviors characterized here are discussed in regard to the crystal chemistry of the perovskite and parent phases

Tuning of Water and Hydroxide Content of Intercalated Ruddlesden–Popper-type Oxides in the PrSr₃Co_1.5Fe_1.5O_10−δ System

A series of hydration experiments of the Ruddlesden–Popper phase PrSr₃Co_1.5Fe_1.5O_10−δ with varying levels of oxygen nonstoichiometry were performed with the goal to clarify phase formation and underlying mechanisms and driving forces. The hydration reaction is most intense for partly reduced samples with a vacancy concentration corresponding to δ ≈ 1. Fully oxidized samples show little or no tendency toward hydration. Presence of oxygen vacancies acts as a prerequisite for hydration. Probably, the basicity of the materials owing to A-site cations is another contributing factor to the hydration ability. Under CO₂ free conditions pure hydrates and oxide hydroxides are formed. In CO₂-containing atmosphere, additional carbonate anions are easily incorporated into the hydrate, probably at the expense of hydroxyl groups. The <i>I-</i>centered PrSr₃Co_1.5Fe_1.5O₈(OH)₂·1H₂O achieves a highly expanded <i>c</i>-axis upon the topochemical insertion reactions. In situ powder synchrotron X-ray diffraction (SXRD) shows that the hydrate converts to an oxide hydroxide, PrSr₃Co_1.5Fe_1.5O₈(OH)₂, at 70 °C with a primitive orthorhombic unit cell. Upon heating above 170 °C, an <i>I-</i>centered product is formed for which further dehydroxylation occurs at around 400–500 °C. Rietveld refinement of SXRD data shows that the absorbed water molecules fill the tetrahedral voids of the [AO]^RS rock salt layer of the monoclinic hydrate

Tuning of Water and Hydroxide Content of Intercalated Ruddlesden–Popper-type Oxides in the PrSr₃Co_1.5Fe_1.5O_10−δ System

A series of hydration experiments of the Ruddlesden–Popper phase PrSr₃Co_1.5Fe_1.5O_10−δ with varying levels of oxygen nonstoichiometry were performed with the goal to clarify phase formation and underlying mechanisms and driving forces. The hydration reaction is most intense for partly reduced samples with a vacancy concentration corresponding to δ ≈ 1. Fully oxidized samples show little or no tendency toward hydration. Presence of oxygen vacancies acts as a prerequisite for hydration. Probably, the basicity of the materials owing to A-site cations is another contributing factor to the hydration ability. Under CO₂ free conditions pure hydrates and oxide hydroxides are formed. In CO₂-containing atmosphere, additional carbonate anions are easily incorporated into the hydrate, probably at the expense of hydroxyl groups. The <i>I-</i>centered PrSr₃Co_1.5Fe_1.5O₈(OH)₂·1H₂O achieves a highly expanded <i>c</i>-axis upon the topochemical insertion reactions. In situ powder synchrotron X-ray diffraction (SXRD) shows that the hydrate converts to an oxide hydroxide, PrSr₃Co_1.5Fe_1.5O₈(OH)₂, at 70 °C with a primitive orthorhombic unit cell. Upon heating above 170 °C, an <i>I-</i>centered product is formed for which further dehydroxylation occurs at around 400–500 °C. Rietveld refinement of SXRD data shows that the absorbed water molecules fill the tetrahedral voids of the [AO]^RS rock salt layer of the monoclinic hydrate

Tuning of Water and Hydroxide Content of Intercalated Ruddlesden–Popper-type Oxides in the PrSr₃Co_1.5Fe_1.5O_10−δ System

A series of hydration experiments of the Ruddlesden–Popper phase PrSr₃Co_1.5Fe_1.5O_10−δ with varying levels of oxygen nonstoichiometry were performed with the goal to clarify phase formation and underlying mechanisms and driving forces. The hydration reaction is most intense for partly reduced samples with a vacancy concentration corresponding to δ ≈ 1. Fully oxidized samples show little or no tendency toward hydration. Presence of oxygen vacancies acts as a prerequisite for hydration. Probably, the basicity of the materials owing to A-site cations is another contributing factor to the hydration ability. Under CO₂ free conditions pure hydrates and oxide hydroxides are formed. In CO₂-containing atmosphere, additional carbonate anions are easily incorporated into the hydrate, probably at the expense of hydroxyl groups. The <i>I-</i>centered PrSr₃Co_1.5Fe_1.5O₈(OH)₂·1H₂O achieves a highly expanded <i>c</i>-axis upon the topochemical insertion reactions. In situ powder synchrotron X-ray diffraction (SXRD) shows that the hydrate converts to an oxide hydroxide, PrSr₃Co_1.5Fe_1.5O₈(OH)₂, at 70 °C with a primitive orthorhombic unit cell. Upon heating above 170 °C, an <i>I-</i>centered product is formed for which further dehydroxylation occurs at around 400–500 °C. Rietveld refinement of SXRD data shows that the absorbed water molecules fill the tetrahedral voids of the [AO]^RS rock salt layer of the monoclinic hydrate

Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO₃ perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO₃ network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO₃ network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation