Synthesis and structure of high-purity BaCe_0.25Mn_0.75O₃: an improved material for thermochemical water splitting

Abstract

Solar thermochemical hydrogen production (STCH) via redox-active metal oxides is an approach for direct solar-driven hydrogen generation typically using a high-temperature redox cycle involving refractory oxides and steam. Typical cycles involve high-temperature reduction of oxides to form oxygen vacancies, followed by lower temperature reaction between oxygen vacancies and steam where the oxide is re-oxidized and the steam is reduced to hydrogen. Only a few materials have demonstrated reversible cycling under the typically harsh STCH conditions (e.g. 1500°C reduction, 900°C re-oxidation) and critical questions remain on the true reversibility of non-stoichiometric multi-cation oxide systems, significantly hampered by the lack of single-phase samples for these material systems. To date, most STCH processes have relied on CeO2 as a benchmark active material, but more recently, the 12R phase of BaCe0.25Mn0.75O3 (BCM) has demonstrated greater hydrogen-generation potential at lower peak temperatures. However, previous reports of 12R-BCM have included large fractions, > 10 wt%, of secondary phases, which complicate analysis of the stability and performance. A comprehensive understanding of the redox mechanism and reversibility of the process in BCM can only be achieved with nearly single-phase samples which, to date, have been difficult to produce. Here two approaches to BCM synthesis are reported: solid state and sol–gel-based routes. It is demonstrated that both routes can be tuned to produce the 12R structure with > 97 wt% yield when annealed ≥1450°C. Herein synchrotron-based diffraction measurements of rhombohedral 12R-BCM enabled characterization of the anisotropy between thermal expansion along the c-axis and within the ab plane. The impact of high-temperature redox cycling on the stability and phase fraction of the 12R-BCM polytype was also investigated. These results offer two viable routes for synthesis of high-purity 12R-BCM critically needed for evaluating the efficacy of BCM as a STCH material and validate its ability to split water at lower temperatures over extended numbers of redox cycles.

Influence of the Rare Earth Cation on the Magnetic Properties of Layered 12R-Ba4M4+Mn3O12 (M = Ce, Pr) Perovskites

“…A summary of the weight fractions of each phase identified via Rietveld refinement is presented in Table S2, confirming the high phase purity of 12R-BCM and 12R-BPM. This sample of 12R-BCM has a phase purity of 98.39 ± 0.1 wt %, consistent with previous reports using the same synthesis . Very small (<1 wt %) amounts of impurity phases Ba 4 Mn 3 O 10 , BaCeO 3 , and CeO 2 were identified via Rietveld refinement.…”

“…Comparing the low- and room-temperature PXRD data (Figures and S1) indicates that for both materials, no structural differences or symmetry lowering are present between 100 and 300 K, beyond anisotropic thermal expansion similar to that observed at elevated temperatures for 12R-BCM . Lattice parameters and refinement details are listed in Table S1.…”

“…Synthesis of 12R-BCM was performed by an established solid-state method. , BaCO 3 (99.999%, Sigma-Aldrich) and Ce 2 (CO 3 ) 3 ·H 2 O (99.9%, Sigma-Aldrich) precursor powders were first dried while MnO 2 (99.9%, Alfa Aesar) was used as-received. Stoichiometric amounts of each precursor powder were ground with an agate mortar and pestle prior to weighing and mixing.…”

“…In this work, we embark on a higher precision examination of the magnetic and thermodynamic properties of 12R-BCM and study 12R-BPM for the first time. We synthesize high phase purity 12R-BCM and 12R-BPM via established methods , and study their crystal structures using Rietveld refinements of synchrotron powder X-ray diffraction (PXRD) data. Magnetic and thermodynamic properties are investigated via DC magnetization, AC susceptibility, and heat capacity measurements.…”

Synchrotron-based techniques for characterizing STCH water-splitting materials

“…In situ XRD measurements (e.g., high temperature and controlled gaseous environments) provide a valuable route for deriving relationships between atmospheric redox conditions and material structure (i.e., structure-property relationships) (Metcalfe et al, 2019;Mastronardo et al, 2020). The advantage of XRD with a synchrotron source is exemplified by Bell et al (2022), where impurity phases at less than 0.4 weight percent were identified within the high-purity STCH material BCM, as well as by Strange et al (2022), where a novel BCM polytype was identified and found to form under high-temperature reducing conditions. The results described in both studies would have not been observed with conventional laboratory-source XRD.…”