Corresponding Author

Rotraut Merkle

Max Planck Institute for Solid State Research

References

1.
Wang, L.; Merkle, R.; Mastrikov, Y.A.; Kotomin, E.A.; Maier, J.
Oxygen exchange kinetics on solid oxide fuel cell cathode materials - general trends and their mechanistic interpretation.
2.
Merkle, R.; Mastrikov, Y.A.; Kotomin, E.A.; Kuklja, M.M; Maier, J.
First principles calculations of oxygen vacancy formation and migration in Ba1-xSrxCo1-yFeyO3-δ perovskites
3.
Mastrikov, Y.A.; Merkle, R.; Kotomin, E.A.; Kuklja, M.M; Maier, J.
Formation and migration of oxygen vacancies in La1-xSrxCo1-yFeyO3-& delta; perovskites: insight from ab initio calculations and comparison with Ba1-xSrxCo1-yFeyO3-δ

In collaboration with:

Yu.A. Mastrikov (University of Riga, Latvia)

M.M. Kuklja (University of Maryland, USA)

Department "Physical Chemistry of Solids"

Mobility of oxygen vacancies in perovskites: Insights from experiments and ab-initio calculations

Authors

R. Merkle, E.A. Kotomin, and J. Maier

Departments

Physical Chemistry of Solids (Joachim Maier)

Experimental and ab initio results of fuel cell cathode materials show that both oxygen vacancy concentration and mobility are key factors for the surface reaction kinetics, and that transient electron transfer can lower the oxygen migration barrier.

The efficient conversion of chemical energy (e.g. from fuels such as hydrogen or natural gas) into electrical energy strongly relies on functional materials fulfilling specific demands for the different parts of a fuel cell. For the electrolyte membrane which separates fuel and air compartment, high conductivity for ions and gastightness are the main requirements. On the other hand, the electrode materials have to combine good electronic conductivity with high catalytic activity for oxidation/reduction reactions.

ABO3-δ-type perovskites are a fascinating class of oxides in which the properties (transport, optical, magnetic, catalytic etc) can be varied within an extremely large range by doping or forming solid solutions. It is particularly important that perovskites cannot only be rendered highly electronically conductive, also a high ionic conductivity can be achieved through massive formation of mobile oxygen vacancies. This combination (mixed electronic-ionic conductors) makes them key functional materials for oxygen permeation membranes and catalytically active cathodes in solid oxide fuel cells. For the latter application, the A-sites in the perovskite lattice are typically occupied by large alkaline earth or rare earth cations (La3+, Sr2+, Ba2+) while the B-sites are filled with first-row transition metals (Mn, Fe, Co) exhibiting variable oxidation states. To achieve the desired oxygen ion conductivity (via oxide ion hopping into neighboring oxygen vacancies), the cation composition is chosen such that some oxygen sites remain empty (oxygen deficiency d in ABO3-d). For  good cathode performance, a high exchange rate of the oxygen surface reaction – transforming oxygen molecules from the gas phase into oxide ions in the solid – is decisive.

<p><strong>Fig. 1:</strong> (a) (Ba,Sr)(Co,Fe)O<sub>3-</sub><sub>d</sub> microelectrodes on a YSZ electrolyte, and measured impedance spectrum. (b) Correlation of oxygen surface exchange rate constant <em>k</em> (<em>k</em> &micro; 1/<em>R</em><sub>s</sub> in impedance spectra) in (Ba,Sr)(Co,Fe)O<sub>3-</sub><sub>d</sub> perovskites with ionic conductivity (which is proportional to the product of vacancy concentration and mobility). Data from [1].</p> Zoom Image

Fig. 1: (a) (Ba,Sr)(Co,Fe)O3-d microelectrodes on a YSZ electrolyte, and measured impedance spectrum. (b) Correlation of oxygen surface exchange rate constant k (k µ 1/Rs in impedance spectra) in (Ba,Sr)(Co,Fe)O3-d perovskites with ionic conductivity (which is proportional to the product of vacancy concentration and mobility). Data from [1].

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For various mixed conducting perovskites, effective rate constants of oxygen exchange were determined by a specially designed technique: Microelectrodes are deposited on a Zr0.84Y0.16O1.92 (YSZ) electrolyte and the surface reaction resistance measured by means of electrochemical impedance spectroscopy (Fig. 1(a)). The pronounced increase of the surface exchange rate constant k with the drastically increasing oxygen vacancy concentration between (La,Sr)(Co,Fe,Mn)O3-d and (La,Sr)(Co,Fe,Mn)O3-d perovskites suggests that oxygen vacancies play an important role in the rate-determining step of the oxygen incorporation reaction [1]. It is observed that the rate constant k even further increased when proceeding to  (Ba,Sr)(Co,Fe)O3-d perovskites, in particular to compositions rich in Ba and Co. Figure 1(b) shows roughly a linear correlation of the surface exchange rate constant with the ionic conductivity within the (Ba,Sr)(Co,Fe)O3-d family [1]. The observation of monotonic correlations between rate constant and oxygen ion conductivity (rather than only oxygen vacancy concentration) indicates that the ionic conductivity (i.e. not only vacancy concentration but also their mobility) is important for the oxygen surface reaction. This strongly suggests that migration of oxygen vacancies to adsorbed or dissociated oxygen species is involved in the rate-determining step of the oxygen incorporation reaction.

<p><strong>Fig. 2:</strong> right: transition state of oxygen migration in Ba<sub>0.5</sub>Sr<sub>0.5</sub>FeO<sub>2.875</sub>, the movement of the migrating oxygen O* (light blue) though the "critical AA&rsquo;B cation triangle" (pink) is indicated by the blue arrow; left: Electron density plot of the transition state in the (011) plane (logarithmic scale, densities &gt;1e/&Aring;<sup>3</sup> appear homogeneously red).</p> Zoom Image

Fig. 2: right: transition state of oxygen migration in Ba0.5Sr0.5FeO2.875, the movement of the migrating oxygen O* (light blue) though the "critical AA’B cation triangle" (pink) is indicated by the blue arrow; left: Electron density plot of the transition state in the (011) plane (logarithmic scale, densities >1e/Å3 appear homogeneously red).

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Within the (Ba,Sr)(Co,Fe)O3-d family, the increase in ionic conductivity originates mainly from an increase in the mobility of the oxygen vacancies by more than an order of magnitude with increasing Ba- and/or Co content (only a smaller factor of about two comes from increased vacancy concentration). These experimental findings motivated us to investigate oxygen migration barriers in such perovskites using ab initio calculations, which give a detailed insight in the key parameters on an atomistic level (so far, the study focused on oxygen migration in the bulk, but similar trends are expected to hold also for migration in the surface layer). The transition state of oxygen migration is shown in Fig. 2. The migrating oxygen O* has to pass through a "critical triangle" consisting of the B* cation (to which O* is bound) and two neighboring A* cations. This configuration is geometrically very strained with close ion-ion contacts as illustrated by the electron density plot (the A cations have to relax considerably outwards in order to allow O* to pass through at all). The heights of the migration barriers were calculated by means of density functional theory (VASP code, GGA functional) for a series of (La,Sr)(Co,Fe)O3-d and (Ba,Sr)(Co,Fe)O3-d cation compositions [2,3]. Using a supercell model, different possible local transition state configurations were investigated (e.g. O* attached to Fe or Co, the neighboring two A* cations being various combinations of La, Sr and Ba which have different ionic radii: La3+: 1.36Å, Sr2+: 1.44Å, Ba2+: 1.61Å). For (La,Sr)(Co,Fe)O3-d perovskites, the migration barrier in the most common local configuration with one La and one Sr close to O* was found to be ≈0.8 eV and almost independent on overall cation composition (La/Sr or Co/Fe ratio), which is in good agreement with experimental results.

However, the situation is more complicated for (Ba,Sr)(Co,Fe)O3-d perovskites. The calculated migration barriers for various overall compositions and local ionic configurations are compiled in Fig. 3(a) as a function of two key parameters. The first one characterizes the geometrical restriction in the transition state; it represents the deviation of the A*O* distances in the "critical triangle" from the respective distances in Ba0.5Sr0.5CoO2.875 (chosen as reference distances). The more constrained the saddle point configuration (e.g. two large Ba close to O*, enforcing a decreased Ba*–O* distance), the higher is the migration barrier. In addition, the barriers also increase with increasing oxygen vacancy formation energies (the second parameter in Fig. 3(a)), e.g. in the series from Ba0.5Sr0.5CoO3-d and Ba0.5Sr0.5Co0.8Fe0.2O3-d (0.40-0.42eV) to Ba0.5Sr0.5FeO3-d (0.72eV), where the geometrical constraints hardly vary. Again, the good agreement of calculated barriers with experimental data should be noted [2].

<p><strong>Fig. 3:</strong> (a) Oxygen migration barriers for various local cation configurations in (Ba,Sr)(Co,Fe)O<sub>3-</sub><sub>d</sub>, calculated by means of density functional theory (lines are guide for the eye only). Triangle up: local BaBa configuration in critical triangle, diamond: Ba,Sr, triangle down: Sr,Sr. The blue arrow emphasizes the dependence of the barriers on geometrical criteria, the red arrow on calculated oxygen vacancy formation energies. Modified from [2] with permission, copyright 2012, The Electrochemical Society. (b) Migration barrier and partial electron density transfer from migrating O* to neighboring B cation in the transition state vs vacancy formation energy. Abbreviations "LSCF" "BSCF" etc. indicate various overall cation compositions of (La,Sr)(Co,Fe)O<sub>3-</sub><sub>d</sub> and (Ba,Sr)(Co,Fe)O<sub>3-</sub><sub>d</sub>. Modified from [3] with permission, copyright 2013, Royal Society.</p> Zoom Image

Fig. 3: (a) Oxygen migration barriers for various local cation configurations in (Ba,Sr)(Co,Fe)O3-d, calculated by means of density functional theory (lines are guide for the eye only). Triangle up: local BaBa configuration in critical triangle, diamond: Ba,Sr, triangle down: Sr,Sr. The blue arrow emphasizes the dependence of the barriers on geometrical criteria, the red arrow on calculated oxygen vacancy formation energies. Modified from [2] with permission, copyright 2012, The Electrochemical Society. (b) Migration barrier and partial electron density transfer from migrating O* to neighboring B cation in the transition state vs vacancy formation energy. Abbreviations "LSCF" "BSCF" etc. indicate various overall cation compositions of (La,Sr)(Co,Fe)O3-d and (Ba,Sr)(Co,Fe)O3-d. Modified from [3] with permission, copyright 2013, Royal Society.

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At first glance, such a correlation of the ion migration barrier with the reaction energy of a redox reaction (formation of an oxygen vacancy is accompanied by the reduction of two neighboring transition metal cations) may appear surprising. Here the ab initio calculations offer additional insight. Indeed, for all materials in the transition state some electron density is transferred from the migrating O* to the neighboring B* cation. This corresponds to a "partial reduction" of the B cation. Thus, it is not surprising that the largest electron transfer occurs in those perovskites which reveal the lowest vacancy formation energy ((Ba,Sr)(Co,Fe)O3-d with high Ba and Co content) as shown in Fig. 3(b). A lower electron density of the migrating O* decreases its effective radius, facilitating the passing through the critical triangle, and thus reducing the migration barrier. On the other hand, the larger vacancy formation energies in (La,Sr)(Co,Fe)O3-d lead to negligibly small electron transfer so that the further variation in the formation energy hardly changes the migration barrier. Larger vacancy formation energies also lead to lower equilibrium vacancy concentrations.

The lower oxygen vacancy formation energies in (Ba,Sr)(Co,Fe)O3-d perovskites can be finally traced back to two reasons: compared to (La,Sr)(Co,Fe)O3-d, the transition metal cations in (Ba,Sr)(Co,Fe)O3-d have a higher formal oxidation state which makes them more easily reducible. Within the (Ba,Sr)(Co,Fe)O3-d family where Fe and Co oxidation states depend only on the oxygen deficiency d but not on the Ba/Sr ratio, the decreased vacancy formation energy for materials with high Ba and Co content is caused by a kind of "internal tensile strain": Incorporation of the large Ba2+ on the perovskite's A-site expands the unit cell, with the consequence that the cobalt cation turns out to be too small for its octahedron position. This facilitates the reduction of cobalt to lower oxidation states (thus increasing the Co ionic radius), resulting in the decreased oxygen vacancy formation energy. Since cobalt changes its oxidation states more easily than Fe, this effect is most pronounced for cobalt-rich perovskite compositions [1,2].

To summarize, our combined experimental and theoretical ab initio study revealed that a high concentration and mobility of oxygen vacancies is important for the oxygen incorporation surface reaction, i.e. the process which largely limits the performance of solid oxide fuel cells. The unusually low oxygen migration barriers in (Ba,Sr)(Co,Fe)O3-d perovskites are closely related to an electron density redistribution in the transition state. Here it is interesting to note that the ionic conductivity of Ba0.5Sr0.5Co0.8Fe0.2O3-d even exceeds that of YSZ, the typical electrolyte in solid oxide fuel cells. Unfortunately, the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-d with the lowest barrier and highest reaction rate is found to suffer from severe problems under real fuel cell operation conditions (carbonate formation from CO2 in air, decomposition into hexagonal-phase perovskite, detrimental reactivity with electrolyte materials). However, the detailed understanding of its unique properties can serve as important guideline in the search of alternative, catalytically highly active materials for fuel cell electrodes and permeation membranes.


 
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