Here we report the data presented in the article titled: "Activation in the rate of oxygen release of Sr0.8Ca0.2FeO3−d through removal of secondary surface species with thermal treatment in a CO2-free atmosphere" Luongo, G., Bork., A., Abdala, P. A., Wu, Y., Kountoupi, E., Donat, F., and Müller, C. R The data is organized in two Excel files, one for the data of the figures in the main text and one for the data of the figures in the supplementary information. The data of each figure is organized in different tabs, including sub-figures. ================================================================================================================= FIGURE 1 "(a) Normalized weight upon redox cycling (40 min reduction in N2 and 20 min oxidation in compressed air) collected in the TGA at 500 °C for Sr0.8Ca0.2FeO3-δ; (b) relaxation times of the reduction and oxidation steps, obtained by fitting the temporal change of the normalized sample weight with an exponential decay function, as defined in Equation 5 and 6." a) Raw TGA data not post-processed b) Obtained values of relaxation times by using the equations 5 and 6 in the main text ------------------------------------------------------ FIGURE 2 "Redox performance upon cycling with exposure to CO2 or H2O between two sets of 5 redox cycles at (a), (b) 500 °C or (c), (d) 25 °C. Normalized weight upon 5 redox cycles, followed by exposure (20 min) to (a) 1 %, 5 %, 10 % or 50 % CO2/air, (b) 2 % H2O/air at 500 °C or (c) 50 % CO2/air, (d) 2 % H2O/air at 25 °C, and again 5 redox cycles at 500 °C." a-d) TGA data with blank subtraction ------------------------------------------------------ FIGURE 3 "Relaxation time of the reduction reaction as a function of cycle number. (a) Effect of laboratory storage under ambient conditions with the storage time being specified; (b) effect of exposure to a CO2- or H2O-rich atmosphere, for 20 min, between two sets of 5 redox cycles at 500 °C; (c) effect of temperature on the observed activation phenomenon." a-c) Obtained values of relaxation times by using the equations 5 and 6 in the main text ------------------------------------------------------ FIGURE 4 "XRD and XANES of Sr0.8Ca0.2FeO3-δ. (a) Full pattern refinement of the XRD pattern of Sr0.8Ca0.2FeO3-δ (synchrotron XRD data, λ = 0.1770 Å); (b) XRD patterns of Sr0.8Ca0.2FeO3 δ as-synthesized and after redox cycling (laboratory XRD data, Cu Kα1,2, λ= 1.5405 Å, λ= 1.5444 Å) zoomed in on the y-axis scale to highlight low-intensity peaks. XANES of Sr0.8Ca0.2FeO3-δ as-synthesized and after redox cycling at the (c) Fe K-edge and (d) Sr K-edge." a) Results of the Rietveld refinement (full-pattern analysis) with FullProf software b) Raw XRD data, not post-processed c-d) Normalized XANES spectra ------------------------------------------------------ FIGURE 5 "In-situ XRD measurements during redox cycling of Sr0.8Ca0.2FeO3-δ. The patterns were collected at 500 °C after switching from air to N2; the acquisition time was ~ 65 seconds per pattern, and reported here at (a),(d) first cycle, (b), (e) fifth cycle and (c), (f) tenth cycle. The (110) peak was assigned to a cubic perovskite phase, while the (020), (411) and (020) peaks belong to a brownmillerite phase. (g) Evolution of the integrated intensity normalized with respect to the maximum observed intensity and (h) peak position of the perovskite (110) and brownmillerite (411/002) peaks. The highlighted areas in light green, yellow and blue define the identified three regions (I), (II) and (III) of the phase transition from the perovskite to brownmillerite phase." a-f) Raw XRD data, not post-processed g) Normalized intensity values of the perovskite and brownmillerite peaks obtained by using FullProf software h) Position of the perovskite and brownmillerite peaks obtained by using FullProf software ------------------------------------------------------ FIGURE 6 "XPS collected of as-synthesized Sr0.8Ca0.2FeO3-δ and after 1, 3, 5 and 10 redox cycles in the TGA. Evolution of the (a) Sr 3d, (b) O 1s and (c) C 1s spectra of Sr0.8Ca0.2FeO3 δ upon redox cycling. Fittings of the (d) Sr 3d and (e) O 1s spectra of Sr0.8Ca0.2FeO3-δ." a-c) Raw XPS data, not post-processed d-e) XPS fittings data obtained by using CasaXPS software. Constraints used for the fittings are specified in the text ------------------------------------------------------ FIGURE 7 "Gas profiles collected with an MS at the outlet of the AutoChem during heating up to 700 °C in (a) He and (b) 5 % O2/He (m/z 18 was ascribed to H2O, m/z 32 was ascribed to O2, m/z 44 was ascribed to CO2). (c) In-situ Raman spectra of the as-synthesized Sr0.8Ca0.2FeO3-δ heated up to 700 °C in N2 and held for 1 h; (d) Ex-situ Raman spectra of reference materials, CaCO3 (Sigma-Aldrich, >99.995%), SrCO3 (Sigma-Aldrich, 99.9%), Fe2O3 (Sigma-Aldrich, >99%), Fe3O4 (Sigma-Aldrich, 95%) with 10x or 50x optical magnification, and of Sr0.8Ca0.2FeO3 with 100x optical magnification." a,b) Raw data acquired with a Mass Spectrometer c,d) Raw in-situ Raman data ------------------------------------------------------