Oxygen Nonstoichiometry and Defect Equilibria of Yttrium Manganite Perovskites with Strontium A-Site and Aluminum B-Site Doping

Abstract

In this work, we investigate YMnO3 perovskites with Sr2+ and Al3+ doping, Y1–x Sr x Mn1–y Al y O3, for use in thermochemical H2O or CO2 splitting for the first time. Oxygen nonstoichiometries (δ) with x = 0.1 or 0.2 and y = 0.4 or 0.6 were measured over a wide range of temperatures and oxygen partial pressures (pO2). Experiments were performed from 973 to 1173 K and pO2 from 1.24 × 10–20 to 2.26 × 10–13 atm in a high-temperature tubular reactor coupled with residual gas analysis by delivering controlled mixtures of H2 and H2O. Higher temperature and pO2 experiments, from 1173 to 1473 K and 1.61 × 10–4 to 3.23 × 10–2 atm, respectively, were performed via thermogravimetric analysis by delivering controlled mixtures of O2 and Ar. In the low-temperature and pO2 region, each material showed low sensitivity to changes in pO2; thus, the slopes of the equilibrium δ isotherms were small. These experiments, under precisely controlled reaction conditions, help to explain some of the discrepancies in the literature regarding Y0.5Sr0.5MnO3 that, in some cases, highlight its superior behavior compared to ceria and other perovskite compositions. Defect models considering oxygen vacancy formation accompanied by either reduction of Mn4+ to Mn3+ or Mn4+ to Mn2+ along with the formation of extended defects were formulated and tested to describe the chemical equilibria of the synthesized perovskites. Y0.8Sr0.2Mn0.4Al0.6O3 could be well described by Mn4+ to Mn3+, but other compositions showed mixed results because of phase transformations, as confirmed by powder X-ray diffraction.

Experimental protocols for the assessment of redox thermodynamics of nonstoichiometric oxides: A case study of YMnO3‐δ

“…In compounds such as CaMnO 3 , in which the reduction of Mn is from 4+ to a lower oxidation state, a lower enthalpy value has been measured, just 175 kJ (mol‐O) −1 in the slightly oxygen deficient cubic phase 10 . Compositional intermediates between the LnMnO 3 (Ln = lanthanide) and AMnO 3 (A = alkaline earth) generally result in intermediate values of enthalpy, particularly within the perovskite system, 27 and many such materials have been found to be more favorable for thermochemical fuel production than the end‐members, 28 although some debate exists concerning doped YMnO 3‐ δ 29 . In the present study of undoped YMnO 3‐ δ , the apparent contradiction of strong sensitivity to crystal structure in regard to the difference between the ferrielectric and centrosymmetric phases in YMnO 3‐ δ but little sensitivity to structure between the centrosymmetric phase and a hypothetical perovskite form of YMnO 3‐ δ emerges as a surprising puzzle.…”

Characterization of Zr‐Doped Ceria and Sr‐Doped La−Mn Perovskites as Redox Intermediates for Solar Chemical‐Looping Reforming of Methane

“…Material Synthesis: Ce 0.9 Zr 0.1 O 2 (CZO10) and La 0.6 Sr 0.4 MnO 3 (LSM40) were synthesized using a modified Pechini method described in prior work. [19,26,27] Briefly, stoichiometric ratios of nitrate precursors were dissolved in 20 mL of deionized water; then, citric acid was added at a 3:2 molar ratio with metal cations. Nitrate precursors consisted of Ce(NO 3 ) 3 •6H 2 O (Sigma-Aldrich, 238538) and ZrO(NO 3 ) 2 •xH 2 O (Sigma-Aldrich, 251275) for CZO10 and La(NO 3 ) 3 •6H 2 O (Sigma-Aldrich, 331937), Sr(NO 3 ) 2 (Sigma-Aldrich, 243426), and Mn(NO 3 ) 2 (Sigma-Aldrich, 340707) for LSM40.…”

Thermodynamic Guiding Principles for Designing Nonstoichiometric Redox Materials for Solar Thermochemical Fuel Production: Ceria, Perovskites, and Beyond

“…[20] Researchers then sought new materials that can achieve high fuel yield at appreciably lower reduction temperatures; examples include doped ceria [21][22][23][24][25][26] and perovskites. [27][28][29][30][31][32][33][34][35][36] Although these materials could be reduced at more easily achieved conditions, they were more difficult to oxidize, necessitating a lower temperature and/or larger supply of oxidizer. [27,28] Consequently, the energy burden only shifted from the reduction to the oxidation step, resulting in little or no improvement in process efficiency.…”