Phase Identification of the Layered Perovskite Ce_xSr_2–xMnO₄ and Application for Solar Thermochemical Water Splitting

Abstract

Ruddlesden–Popper (layered perovskite) phases are attracting significant interest because of their unique potential for many applications requiring mixed ionic and electronic conductivity. Here we report a new, previously undiscovered layered perovskite of composition, Ce x Sr2–x MnO4 (x = 0.1, 0.2, and 0.3). Furthermore, we demonstrate that this new system is suitable for solar thermochemical hydrogen production (STCH). Synchrotron radiation X-ray diffraction and transmission electron microscopy are performed to characterize this new system. Density functional theory calculations of phase stability and oxygen vacancy formation energy (1.76, 2.24, and 2.66 eV/O atom, respectively with increasing Ce content) reinforce the potential of this phase for STCH application. Experimental hydrogen production results show that this materials system produces 2–3 times more hydrogen than the benchmark STCH oxide ceria at a reduction temperature of 1400 °C and an oxidation temperature of 1000 °C.

Reduction Thermodynamics of Sr1−xCexMnO3 and CexSr2−xMnO4 Perovskites for Solar Thermochemical Hydrogen Production

“…a) TPTR results demonstrating extents of reduction for SCM10, SCM20, and SCM30 and CSM10, CSM20, and CSM30 in addition to ceria and SLMA6464 under UHP N 2 and b) reduction extent and onset temperature as a function of cerium content. [16,19] (a) (b)…”

“…The results are shown in Figure 8 and are compared with published results for the CSM family from Barcellos et al 2019. [19] Each composition from the SCM family was tested for water splitting at reduction temperatures (T red ) of 1350 and 1400 C and oxidation temperatures (T ox ) of 850 and 1000 C. Slow kinetics of the CSM family prohibit it from oxidizing sufficiently at 850 C in the laboratory setting. Interestingly, at T red ¼ 1400 C and T ox ¼ 1000 C, opposite trends between the SCM and CSM families are observed: SCM20 shows the lowest per cycle H 2 productivity (220 μmol H 2 g À1 ) and SCM30 shows the highest productivity (305 μmol H 2 g À1 ), whereas in the CSM family, [23] CSM20 shows the highest per cycle H 2 productivity (247 μmol H 2 g À1 ) and CSM30 shows the lowest (166 μmol H 2 g À1 ).…”

“…[21][22][23][24] At elevated temperatures, however, SCM has been shown to favor the higher symmetry structures, with SCM10, SCM20, and SCM30 gradually transitioning to the Pm3m cubic phase by T ≅ 210 C, 385 C, 600 C, respectively, and SCM40 reverting back to the tetragonal I4/mcm phase beyond T ≅ 350 C and approaching the cubic Pm3m phase beyond T ≅ 850 C. [24,25] In contrast, CSM possesses a layered Ruddlesden-Popper (RP) perovskite phase throughout the entirety of its composition range, with an additional strontium oxide layer compared with SCM, a tetragonal (I4/mmm) structure, and an A-site coordination of 9. [19] Similar RP perovskites have been shown to undergo a reversible transformation into the orthorhombic Bbcm phase when sufficiently reduced, with minor distortions in the structure's MnO 6 octahedra. [23,26] The crystal structure of a single unit cell of CSM (I4/mmm) and SCM (Pm3m) in their respective phases within the STCH reduction temperature regime are shown (Figure 1).…”

“…Figure 1. The unit cell for a) the C x Sr 2Àx MnO 4 Ruddlesden-Popper-layered perovskite[19] and b) the Sr 1Àx Ce x MnO 3 simple perovskite crystal structures. Purple octahedra represent MnO 6 and the green/yellow atoms represent Sr/Ce atoms that are randomly shared on the A site.…”

Reversible Phase Transformations in Novel Ce‐Substituted Perovskite Oxide Composites for Solar Thermochemical Redox Splitting of CO2

“…[ 14–16 ] Potential metal oxide (MO x ) redox materials should be able to split CO 2 /H 2 O with a high conversion extent to CO/H 2 (in case of a fed‐batch reactor) or be tested under a low CO 2 :CO (H 2 O:H 2 ) molar ratio flow conditions (in case of a flow‐through reactor) to be classified as technologically attractive. A few material candidates resulted in a very low CO 2 (H 2 O) conversion extent, typically with a CO 2 :CO (H 2 O:H 2 ) molar ratio ≥1500:1 in the product stream [ 17–19 ] or in some other cases were tested at a very high oxidant to fuel ratio conditions with a CO 2 :CO (H 2 O:H 2 ) ratio ≥10 000:1. [ 20 ] This often leads to overestimating the material's performance as the results would be substantially different at lower CO 2 :CO (H 2 O:H 2 ) molar ratios.…”