An A- and B-Site Substitutional Study of SrFeO3−δ Perovskites for Solar Thermochemical Air Separation

Abstract

An A‑ and B‑site substitutional study of SrFeO3−δ perovskites (A’xA1−xB’yB1−yO3−δ, where A = Sr and B = Fe) was performed for a two‑step solar thermochemical air separation cycle. The cycle steps encompass (1) the thermal reduction of A’xSr1−xB’yFe1−yO3−δ driven by concentrated solar irradiation and (2) the oxidation of A’xSr1−xB’yFe1−yO3−δ in air to remove O2, leaving N2. The oxidized A’xSr1−xB’yFe1−yO3−δ is recycled back to the first step to complete the cycle, resulting in the separation of N2 from air and concentrated solar irradiation. A-site substitution fractions between 0 ≤ x ≤ 0.2 were examined for A’ = Ba, Ca, and La. B-site substitution fractions between 0 ≤ y ≤ 0.2 were examined for B’ = Cr, Cu, Co, and Mn. Samples were prepared with a modified Pechini method and characterized with X-ray diffractometry. The mass changes and deviations from stoichiometry were evaluated with thermogravimetry in three screenings with temperature- and O2 pressure-swings between 573 and 1473 K and 20% O2/Ar and 100% Ar at 1 bar, respectively. A’ = Ba or La and B’ = Co resulted in the most improved redox capacities amongst temperature- and O2 pressure-swing experiments.

Factors Governing Oxygen Vacancy Formation in Oxide Perovskites

“…Metal-oxide perovskites have many applications in fields ranging from energy science to electronics: e.g., two-step thermochemical cycles for water and carbon dioxide splitting (WS and CDS, respectively), − thermochemical energy storage, − thermochemical O 2 storage/pumping, − thermochemical air separation, − clean electricity production using solid oxide fuel cells (SOFCs), − solid oxide electrolyzers that produce hydrogen and carbon monoxide from high-temperature electrochemical WS and CDS, − and ferroelectric random-access memory (FRAM). − These applications depend sensitively on the concentration of oxygen vacancies (V O s), which are capable of dictating thermodynamic, electronic, and emergent properties. As examples, V O s reduce H 2 O to H 2 in a two-step thermochemical WS (often referred to as solar thermochemical hydrogen or STCH), ,, absorb O 2 to maintain low O 2 partial pressures in thermochemical O 2 storage/pumping and air separation, − facilitate O 2– transport in SOFCs, ,,, and pin ferroelectric domain walls that impede ferroelectric switching in FRAM. − The concentration of V O s in any given oxide chiefly depends on their formation energy ( E v ), which is a well-vetted microscopic descriptor of performance, especially in applications such as STCH − and SOFCs. ,,, …”

Controlling thermal expansion and phase transitions in Ca1−xSrxMnO3−δ by Sr-content

“…in a perovskite crystal structure, commonly known as ''perovskites'' are widely considered not only as thermoelectric generators, 1,2 but recently have emerged as promising candidates for thermochemical redox cycles in the solar sector. [3][4][5][6] Due to their variable nature with an extraordinary large number of possible A-and B-site cation combinations, including multi-cation solid solutions, and the associated tunability of their thermodynamic properties, they can be utilized in a variety of applications ranging from H 2 O-and CO 2 -splitting (WS/CDS), 3,[7][8][9][10][11] thermochemical storage (TCS), [12][13][14][15] chemical looping combustion (CLC), 16,17 chemical looping partial oxidation of methane (CLPOM) 18 and air separation [19][20][21][22][23][24] to thermochemical oxygen pumping. [25][26][27] Perovskites can be partially reduced and oxized, according to eqn (1) and (2), and are therefore also called non-stoichiometric oxides.…”

Oxygen separation via chemical looping of the perovskite oxide Sr0.8Ca0.2FeO3 in packed bed reactors for the production of nitrogen from air

“…There have also been a number of material screening studies for this application, which consider perovskites in general [20][21][22], and many recent studies which are focused on modifying SrFeO 3 by ion substitution giving perovskites of the form A x Sr 1−x B y Fe 1−y O 3−𝛿 [15,[23][24][25]. Asides from the addition of cobalt, the enthalpy of reduction has also been decreased by the substitution of strontium with both calcium [18,24,26], and barium [15].…”