Trends in Stability of Perovskite Oxides

Abstract

Perovskite oxides with general formula AMO 3 have a large variety of applications as dielectrics and piezoelectrics, [1] ferroelectrics [2] and/or ferromagnetic materials, [3] among others. Rare earth and alkaline earth metal perovskites are useful as catalysts for hydrogen generation, [4] as oxidation catalysts for hydrocarbons, [5] and as effective and inexpensive electrocatalysts for state-of-the-art fuel cells, [6] mainly due to the possibility of tuning their mixed ionic-electronic conductivity through substitution of A and M and subsequent formation of oxygen vacancies. Despite the general interest in perovskites, so far there have been no ab initio studies devoted to their formation energies, and the trends in stability are unknown.Among the available theoretical techniques to investigate perovskites, DFT is an appealing candidate, since it has proved useful for understanding metals and alloys at the atomic scale. [7] Nevertheless, the well-known shortcoming of DFT in describing strongly correlated systems has prevented its use for the estimation of properties such as band gaps and electron localization-delocalization of oxides, and there are numerous corrections. [8] Despite these limitations, Figure 1 a shows the experimental formation energies from elements and O 2 of 20 perovskites at 298 K and the corresponding standard DFT energies using the RPBE-GGA [9] exchange-correlation functional. The simulations are able to reproduce trends in the formation energies, and the calculated energies are shifted by about 0.75 eV compared to experiments. The A component is Y, La, Ca, Sr, or Ba, while M is a 3d metal from Ti to Cu. However, it is possible to combine the formation energies of these compounds with those of their sesquioxides (A 2 O 3 and M 2 O 3 ), rutile dioxides (MO 2 ), monoxides (AO and MO), and O 2 to reproduce the energetics of several reactions (Figure 1 b-d). The reactions are shown in the Supporting Information. The excellent correspondence between experiments and theory shows that DFT very accurately captures the mixing energies between oxides. The chemical reaction depicted in Figure 1 a and the way of representing its Gibbs energy, are given by Equations (1) and (2).

In Situ XANES/XPS Investigation of Doped Manganese Perovskite Catalysts

“…According to studies on La 0.6 Ca 0.4 CoO 3 [27,28], the changed structure of the Ca 2p spectra is consistent with the formation of surficial Ca(OH) 2 The spectra presented in Figure 5a,b have been recorded at photon energies of 568 and 750 eV respectively, resulting in kinetic energies of the photoelectrons of about 220 eV. This corresponds to an information depth of roughly 1 nm [34].…”

“…Finally, samples at x Ca = 0.3 and 0.5 show a higher reversibility in UHV post reaction analysis. These findings thus indicate a higher (electro) chemical stability at intermediate x Ca , which is in contrast to the monotonic decrease of the formation energy of manganite perovskites from the metal elements and O 2 with increasing Ca-doping [2]. The hereby observed maximum stability at intermediate x Ca is in good agreement with environmental transmission electron microscopy results comparing low/intermediate x Ca to high Ca-doping [5], as well as stability analysis in the whole doping range by ex situ cyclic voltammetry (see supplementary information in [5]).…”

“…Upon in situ activation in 0.1 mbar water vapor at a sample bias of +2.5 V against the grounded chamber our data show the emergence of a second Ca species represented by the peaks labeled A 1 and A 2 . According to studies on La 0.6 Ca 0.4 CoO 3 [27,28] this could correspond to formation of surficial Ca(OH) 2 and CaO. Reoxidation in 0.1 mbar O 2 fully restores the virgin state of the surface (blue curves in Figure A3a).…”

“…At present, theoretical works on the catalytic activity of perovskites (ABO 3 ) are based on strong approximations, such as a frozen, defect free surface [2,3]. Those consider the bonding strength of the B-site cation to oxygen as a universal descriptor for oxygen evolution activity of perovskites.…”

Perovskite cathodes for NaBH 4 /H 2 O 2 direct fuel cells

“…Platinum-based catalysts, similarly to gold, palladium and silver, possess good catalytic activity for H 2 O 2 reduction reaction (HPRR) but have the disadvantage of high cost [9]. Perovskite-type materials, with general chemical formula ABO 3 where A is a lanthanide or an alkaline earth metal and B is a transition metal, are known to be good electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells and other energy conversion devices, both at low and high temperatures [12][13][14][15][16][17].…”