Precipitation of dopants on acceptor-doped LaMnO3±δ revealed by defect chemistry from first principles

Abstract

Perovskite oxides degrade at elevated temperatures while precipitating dopant-rich particles on the surface. A knowledge-based improvement of surface stability requires a fundamental and quantitative understanding of the dopant precipitation mechanism on these materials. We propose that dopant precipitation is a consequence of the variation of dopant solubility between calcination and operating conditions in solid oxide fuel cells (SOFCs) and electrolyzer cells (SOECs). To study dopant precipitation, we use 20% (D = Ca, Sr, Ba)-doped LaMnO 3+δ (LDM20) as a model system. We employ a defect model taking input from density functional theory calculations. The defect model considers the equilibration of LDM20 with a reservoir consisting of dopant oxide (DO), peroxide (DO 2 ), and O 2 in the gas phase. The equilibrated non-stoichiometry of the A-site and B-site as a function of temperature, T, and oxygen partial pressure, p(O 2 ), reveals three regimes for LDM20: A-site deficient (oxidizing conditions), A-site rich (atmospheric conditions), and near-stoichiometric (reducing conditions). Assuming an initial A/B non-stoichiometry, we compute the dopant precipitation boundaries in a p-T phase diagram. Our model predicts precipitation both under reducing (DO) and under highly oxidizing conditions (DO 2 ). We found precipitation under anodic, SOEC conditions to be promoted by large dopant size, while under cathodic, SOFC conditions precipitation is promoted by initial A-site excess. The main driving forces for precipitation are oxygen uptake by the condensed phase under oxidizing conditions and oxygen release assisted by B-site vacancies under reducing conditions. Possible strategies for mitigating dopant precipitation under in electrolytic and fuel cell conditions are discussed.

Cation deficiency enables reversal of dopant segregation at perovskite oxide surfaces under anodic potential

“…The formation of the B-site vacancies ðV 000 Mn Þ under anodic conditions is in line with a recent theoretical and computational work from our group which predicted the formation of a large amount of B-site vacancies in La 0.8 Sr 0.2 -MnO 3 under anodic conditions. 38 In other papers, it was reported that undoped LaMnO 3 contained similar amounts of Aand B-site vacancies under oxidizing conditions, 44,62 and also, some other perovskites are known to exhibit B-site deciency such as Pr 0.5 Sr 0.5 Co 1−y O 3 (10%) 63 and Sr 0.9 La 0.1 Ti 1−y O 3 (6%). 64 Also, we point out the redox-active species on the LCM surface in eqn (3).…”

“…Mn is thought to be the redox active species in doped LaMnO 3 , 44,65 and we have also assumed that in our previous theoretical work connecting defect reactions to surface segregation. 38 This knowledge comes from bulk defect equilibria studies on the manganite system. 44,65 From the XAS measurements, we found that Mn is indeed redox-active in the bulk, but at the surface it is the lattice oxygen that was redox-active.…”

“…In addition to the elastic and electrostatic energy contributions, we believe that we must also consider the defect equilibria in the perovskite oxide in the segregation in the form of dopant oxide precipitates, and reversal of such segregation on perovskite oxides, as we have recently studied theoretically. 38 Eqn (1) demonstrates the known defect reaction that can explain the dopant segregation (forward) under anodic conditions and dopant re-incorporation (backward) under cathodic conditions. 36,39 ( = Sr on the La lattice site, = Mn on the Mn lattice site, = La vacancy, = Mn with single positive charge on the Mn lattice site)…”

Sr Surface Enrichment in Solid Oxide Cells – Approaching the Limits of EDX Analysis by Multivariate Statistical Analysis and Simulations

“…In comparison to Zr and O, this effect is the strongest for Sr, where the elemental profile is even shifted slightly towards the surface, whereas for La and Mn the profiles' points of inflection are shifted towards the bulk region of the complexion. While the general phenomenon of Mn [71][72][73] and Sr [7,[24][25][26][73][74][75][76][77] surface segregation in LSM has been reported for various conditions and doping concentrations, Sr segregation has even been linked to cell performance. [22,23,27,38] Note that this simulation, performed without any polarization and neglecting the oxygen partial pressure, yields the same results as experiments under various experimental conditions.…”

Theoretical Study of the Phonon and Electrical Conductivity Properties of Pure and Sr-Doped LaMnO3 Thin Films

“…The properties of LMO have been studied intensively in the last few years since it was found that the partial substitution of La by Ca, Sr or Ba results in structural changes and the occurrence of colossal magnetoresistance near the temperatures of the spin ordering of Mn ions [2]. Hess et al [3] have studied 20% (Ca, Sr, Ba)-doped LMO using a DFT-based defect chemistry model. Sr substitution at the A-site of LMO nanoparticles could be used for energy applications because it shows a higher current value and higher conductivity, as described by Gupta et al [4].…”