(695f) Invited Talk: Screening and Characterization of Reduction/Oxidation-Active Reactions of Mixed Ionic-Electronic Conducting Materials for Air Separation

An investigation was performed on the impact of A- and B-site substitution on SrFeO_3-δ reduction/oxidation (redox) extents. SrFeO_3-δ is a promising oxygen carrier for thermochemical air separation due to considerable oxygen capacity and chemical stability across a range of temperature and oxygen partial pressures. A- and B-substitution may increase reducibility while preserving the phase stability of SrFeO_3-δ [1,2]. Singly- and doubly-substituted Sr_1-xA’_x Fe_1-y B’_yO_3-δ (A’ = La, Ba, Ca; B’ = Co, Cr, Cu Mn), where x = 0 to 0.2 and y = 0 to 0.2, were synthesized using the sol-gel method [5] and screened based on their redox activity.

Redox activity screenings were performed via thermogravimetric analysis (TGA, Netzsch STA 449 F3 Jupiter) for a temperature range of 300 to 1200 °C, in 0 and 20% O_2­/Ar. Changes in oxygen non-stoichiometry were quantified from measured mass losses to determine air separation capacity. Larger non-stoichiometry changes were seen in the A-sited doped samples. Of B-site substituents, only Co enhanced SrFeO_3-δ reducibility. Most samples reduced continuously as temperature increased. The largest reduction extents were observed for A’= La and Ba. Samples with A’= Ca generally showed faster reduction kinetics under pure Ar; however, there was indication of a potentially reversible phase change around 850 °C.

Powder X-ray diffraction was performed on samples after synthesis and after TGA to verify phase stability. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed on La_0.1Sr_0.9­FeO_3-δ and Ba_0.1Sr_0.9­FeO_3-δ to investigate particle morphology and elemental distribution before and after thermal reduction and oxidation cycling experiments. SEM showed some smoothing and rounding of particle surfaces after cycling, without significant changes in bulk characteristics and particle sizes. SEM revealed no particle sintering, while EDS showed no evidence of phase segregation in the samples, suggesting promising chemical stability after several thermal cycles.

References:

[1] H. J. M. Bouwmeester, A. J. Burggraaf, Dense ceramic membranes for oxygen separation, in Membr. Sci.Technol., ed. A. J. Burggraaf and L. Cot., Elsevier, 1996, vol.4, pp. 435–528.

[2] C. C. Chang and H. S. Weng, Ind. Eng. Chem. Res., 1993, 32(11), 2930–2933.

[3] H.E. Bush, KP. Schlichting, R.J. Gill, S.M. Jeter, P.G. Loutzenhiser, Design and Characterization of a Novel Upward Flow Reactor for the Study of High-Temperature Thermal Reduction for Solar-Driven Processes, J. Sol. Energy Eng, 2017.

[4] R. Gill, H.E. Bush, P. Haueter, P.G. Loutzenhiser, "Characterization of a 6 kW high-flux solar simulator with an array of xenon arc lamps capable of concentrations of nearly 5000 suns." 2015, 86 (12), 8.

[5] J. Vieten, B. Bulfin, M. Roed, C. Sattler, Citric acid auto-combustion synthesis of Ti-containing perovskites via aqueous precursors. Solid State Ionics, 2018. 315: p. 92-97.