Electrochemical Polarization Dependence of the Elastic and Electrostatic Driving Forces to Aliovalent Dopant Segregation on LaMnO₃

Abstract

Segregation of aliovalent dopant cations is a common degradation pathway on perovskite oxide surfaces in energy conversion and catalysis applications. Here we focus on resolving quantitatively how dopant segregation is affected by oxygen chemical potential, which varies over a wide range in electrochemical and thermochemical energy conversion reactions. We employ electrochemical polarization to tune the oxygen chemical potential over many orders of magnitude. Altering the effective oxygen chemical potential causes the oxygen nonstoichiometry to change in the electrode. This then influences the mechanisms underlying the segregation of aliovalent dopants. These mechanisms are (i) the formation of oxygen vacancies that couples to the electrostatic energy of the dopant in the perovskite lattice and (ii) the elastic energy of the dopant due to cation size mismatch, which also promotes the reaction of the dopant with O2 from the gas phase. The present study resolves these two contributions over a wide range of effective oxygen pressures. Ca-, Sr-, and Ba-doped LaMnO3 are selected as model systems, where the dopants have the same charge but different ionic sizes. We found that there is a transition between the electrostatically and elastically dominated segregation regimes, and the transition shifted to a lower oxygen pressure with increasing cation size. This behavior is consistent with the results of our ab initio thermodynamics calculations. The present study provides quantitative insights into how the elastic energy and the electrostatic energy determine the extent of segregation for a given overpotential and atmosphere relevant to the operating conditions of perovskite oxides in energy conversion applications.

Ca-doped rare earth perovskite materials for tailored exsolution of metal nanoparticles

“…This segregation of CaO may thus be triggered by the A-site excess stoichiometry in the perovskite resulting from proceeding exsolution of B-site cations (predominantly Fe). A similar segregation of an alkaline earth metal in a perovskite material has already been reported for Sr in LSF (Koo et al, 2018) and for alkaline-earth-metal-doped lanthanum manganite (Kim et al, 2020). However, there is a temperature window between 550 and 575 C where exsolution occurs without CaO segregation.…”

A heavily subtituted manganite in an ordered nanocomposite for long-term energy applications

“…This opposite valence mismatch is the likely driving force for the exchange of overbonded Sr ions with underbonded La ions on the surface (in combination with the electrostatic effects). 50 This is conceptually similar to the Sr ion having a larger ionic radius than the La lattice ion it replaces. 48 Such an instability is strongly alleviated by cationic intermixing (Figs.…”

“…These are ascribed to Sr-based precipitates as a consequence of massive Sr-migration and are responsible for the severe degradation of the surface reactivity owing to a reduced availability of active sites in LSM. 49,50 The top-view SEM images for the materials after ageing (LSM and LSM-SDC in panels b and c, respectively), provide information at lower-magnification and highlight the profound difference between the two surface: Namely, deleterious Sr segregation is fully hindered in LSM-SDC. Please note that the bulk microstructural stability of VANs had been already been proven by TEM (cf.…”

Polar or not polar? The interplay between reconstruction, Sr enrichment, and reduction at the La0.75Sr0.25MnO3 (001) surface

“…First, the cation size mismatch between host and dopant cations has been successfully linked to the observed and computed segregation [24,41,45,46], with larger dopant cations segregating more and segregation onset at lower temperatures. This aspect of segregation is quite well-understood at this point.…”