Oxygen Exchange in La0.6Sr0.4Co0.2Fe0.8O3−δ Thin-Film

Aug 5, 2015 - An in operando study of chemical expansion and oxygen surface exchange rates in epitaxial GdBaCo 2 O 5.5 electrodes in a solid-state ...
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Oxygen Exchange in La0.6Sr0.4Co0.2Fe0.8O3−δ Thin-Film Heterostructures under Applied Electric Potential E. Mitchell Hopper,† Edith Perret,†,§ Brian J. Ingram,*,‡ Hoydoo You,† Kee-Chul Chang,† Peter M. Baldo,† Paul H. Fuoss,† and Jeffrey A. Eastman† †

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

‡

ABSTRACT: In situ synchrotron X-ray diffraction was used to investigate oxygen surface exchange behavior in La0.6Sr0.4Co0.2Fe0.8O3−δ/Gd2O3-doped CeO2/Y2O3-stabilized ZrO2 (LSCF/ GDC/YSZ) epitaxial thin-film heterostructures. Applying electrical potentials across the heterostructure under high temperature and controlled oxygen partial pressure conditions resulted in significant changes in oxygen vacancy concentrations due to differing rates of oxygen transport across the LSCF/air surface and LSCF/GDC buried interface. These changes in stoichiometry were correlated with time-dependent out-of-plane LSCF lattice parameter changes. An electrochemical reaction rate analysis was used to determine that the rate constant associated with oxygen exchange at the LSCF/air surface dominates the behavior of the sample as a whole and that the rate of oxygen transport across the LSCF/air surface is smaller than or equal to the rate of oxygen transport across the buried LSCF/GDC interface.

1. INTRODUCTION The kinetics of oxygen exchange at surfaces and interfaces is a major contributor to electrochemical device performance. Understanding the charge transfer and oxygen redox mechanisms presents a significant scientific challenge. Techniques probing charge-transfer reactions typically are used to characterize bulk samples, in which the response depends on both surface exchange and bulk diffusion processes, which are difficult to decouple. Furthermore, isolating local material properties under in operando conditions is challenging using bulk approaches. This is of particular interest in understanding and predicting the behavior of electrochemical energy storage or conversion devices. As an example, solid oxide fuel cells (SOFCs) have great potential for clean power generation with high efficiency, fuel flexibility, and low emissions. Oxygen exchange with the environment and transport through the cathode is particularly important and limits device performance, as these processes are typically associated with the largest overpotential losses.1 Determining how these mechanisms depend on material characteristics, device structures, and operating conditions is crucial for further development of SOFCs. Several techniques have been used to study the oxygen reduction process at SOFC cathodes, including potentiostatic current transient (PCT) or potential step chronoamperometry (PSCA),2 calorimetry,3,4 electrochemical impedance spectroscopy of well-defined model thin films,5−11 secondary ion mass spectrometry (SIMS),12,13 and electrical conductivity relaxation (ECR).14−20 Combining in situ X-ray and electrochemical measurements provides a powerful approach to probe oxygen transport and incorporation in operando at a local level by investigating a wide range of materials properties such as chemical composition, oxidation states, and structural changes. For © 2015 American Chemical Society

example, X-ray diffraction can be used for precise measurement of a material’s lattice parameter, which varies as the oxygen 21−23 vacancy concentration ([V•• at O ]) of the material changes, the specific electric potential witnessed at the localized region being probed. La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) is a material attractive for use as a cathode because of its mixed ionic-electronic conductivity and oxygen reduction catalytic activity.24 We have previously shown that the oxygen vacancy concentration in LSCF thin-film heterostructures responds to applied electric potentials across an electrolyte/LSCF half cell consisting of a thin film of LSCF on a 8% Y2O3-stabilized ZrO2 (YSZ) substrate with a thin Gd2O3-doped CeO2 (GDC) interlayer (Figure 1).22 Because the lattice parameter is linearly related to 21 changes in [V•• O ], observed changes in the LSCF out-of-plane lattice parameter can be attributed to initially slower oxygen vacancy transport rates across the heterostructure LSCF/air surface than across the LSCF/GDC interface. A similar investigation of the related La−Sr−Co−O system used X-ray measurements to show that applying a potential is analogous to changing the oxygen partial pressure (pO2) by a corresponding amount.23 Both of these previous studies indicate that oxygen transport across the LSCF/air surface is rate-limiting compared with transport across the interface between the LSCF and GDC films. The current work takes advantage of the time dependence of lattice strains induced by changes in [V•• O ] under system perturbation, such as applied electric potential, to investigate the behavior of the exchange coefficient and activation energy Received: June 9, 2015 Revised: July 31, 2015 Published: August 5, 2015 19915

DOI: 10.1021/acs.jpcc.5b05505 J. Phys. Chem. C 2015, 119, 19915−19921

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The Journal of Physical Chemistry C

sample with respect to the platinum working electrode was controlled by translating the sample and monitoring the Pt fluorescence signal. A Pilatus 100 K area detector acquired images of the diffracted beam intensity every 100 ms.

3. RESULTS AND DISCUSSION 3.1. General System Description. As described in our previous work,22 the change in [V•• O ] in the LSCF film when a potential is applied is determined by the difference between the vacancy flux in and out of the LSCF film at the LSCF/GDC interface and the LSCF/air surface. The reactions governing oxygen vacancy concentration within the LSCF layer can be written using Kröger−Vink notation22,25 2B•B(LSCF) + 2e′ ↔ 2B×B(LSCF) Figure 1. Schematic of the electrode heterostructure and specular scattering geometry. A focused and monochromatic (16 keV) X-ray beam is incident on the heterostructure with an angle θ and is scattered into an area detector, and X-ray fluorescence can be monitored to probe atomic composition. The reactions listed are related to eqs 1−3

(1)

1 × • × O2(g) + V •• O(LSCF) + 2BB(LSCF) ↔ 2BB(LSCF) + OO(LSCF) 2 (2) × OO(LSCF)

+

B×B(LSCF)

V •• O(GDC)

↔

V •• O(LSCF)

+

× OO(GDC)

(3)

B•B(LSCF)

where and reflect the charge disproportionation reaction on the B site, which is likely to mostly involve the cobalt ions.26 When a cathodic potential is applied to the heterostructure, LSCF B-site ions are reduced at the Pt-LSCF current collector eq 1). Localized holes are transported laterally to the exposed LSCF surface to enable reduction and incorporation of oxygen into the LSCF lattice (eq 2). In the forward direction, these two reactions constitute what is known as the oxygen reduction reaction (ORR), which, for example, occurs on the cathode side of a fuel cell during normal operation. Oxygen ion transport across the buried LSCF/GDC interface is described by eq 3 (and an analogous equation would describe transport across the GDC/YSZ interface). A closed circuit is completed by subsequent oxidation of O×O to O2 at the YSZ/air interface. Figure 2 shows typical transient behavior of the current and out-of-plane lattice strain upon the application of a cathodic potential (i.e., forward reactions in eqs 1− 3) and an anodic potential (reverse reactions in eqs 1−3) at t = 0. As can be seen, the lattice strain increases under cathodic potential (Figure 2a) from 0% at t = 0 to ξ∞, while the current decreases in magnitude to i∞ as t → ∞. The data can be fit with a decaying exponential function with a single characteristic time constant. In the anodic case (Figure 2b) the out-of-plane lattice parameter decreases when the field is applied, and the magnitude of the electrical conductance again decreases. The steady-state strain values at various pO2, temperatures, and applied potentials are shown in Figure 3. These relationships are consistent with our previous report that [V•• O ] increases (decreases) with an applied cathodic (anodic) potential; therefore, the rate of oxygen transport across the LSCF/air surface is initially less than that across the LSCF/ GDC buried interface.22 These two rates must equilibrate as a steady-state lattice strain (i.e., Δ[V•• O ]) is obtained as t → ∞, as shown in Figure 2. Equilibrium can be achieved by varying the oxygen transport rates across the LSCF/air surface or LSCF/ GDC interface. The relaxation behavior of the electric current indicates that the relative change of oxygen transport rates results in a net decrease in conductance. As will be discussed further in the following, this indicates that under both cathodic and anodic conditions, oxygen transport across the buried LSCF/GDC interface must decrease from the initial rate at t =

for oxygen transport across an LSCF thin film heterostructure. An X-ray diffraction approach was used to monitor the time dependence of this lattice strain as a function of applied cathodic and anodic potential at controlled oxygen partial pressures (1.5, 15, and 150 Torr) and temperatures (350−600 °C) to determine the effects of a variety of environmental conditions on interfacial oxygen transport.

2. EXPERIMENTAL DETAILS Targets with compositions of La0.6Sr0.4Co0.2Fe0.8O3−δ and Ce0.8Gd0.2O1.9 were used to deposit LSCF and GDC films by pulsed laser deposition onto single crystal (001)-oriented YSZ substrates. Phase purity of the films was confirmed with X-ray diffraction, and both GDC and LSCF films were determined to be epitaxially oriented with the YSZ substrate, such that (001) YSZ ∥ (001) GDC ∥ (001) LSCF and [100] YSZ ∥ [100] GDC ∥ [110] LSCF, where (001) is the surface plane and the LSCF orientation is described using pseudocubic indices. A surface roughness ji. In fact, from the sign of the LSCF lattice parameter change, we know that js < ji at t = 0 and that the two fluxes become equal as t → ∞. It should also be noted that under conditions in which this limitation on the maximum oxygen vacancy concentration is approached, an additional limiting term related to the barrier height, ΔGi, associated with incremental V•• O formation energy is expected to dominate; that is, Kfi will be inversely related to C. A single exponential time constant is observed; therefore, the condition in eq 11 is confirmed under the reported experimental conditions. 3.3. Analysis. The analysis of the relaxation time constants represents a powerful tool for investigating in operando local properties of electrochemically active films and allows comparison between location-dependent properties and average device performance (e.g., cell resistance). Furthermore, the observed relationships provide important insight into the relative magnitudes of the effective reaction constants and the relative change of transport rates at the LSCF surface (Kfs, Krs) and the LSCF/GDC interface (Kfi, Kri)22 On the basis of the above observations, at t = 0 in Figure 2, ji > js, and as t → ∞, the two oxygen transport currents modulate such that at steady state, ji = js. While there are multiple

where d is the LSCF film thickness. This differential equation can be solved by utilizing the following boundary conditions: At t = 0 the structure has an initial vacancy concentration, C0, and at long times (t → ∞) the vacancy concentration reaches a steady-state value, C∞, once the incoming and outgoing fluxes are equal in magnitude (i.e., a steady state has been achieved). This gives rise to the following ⎡ ⎛ K′ ⎞ ⎤ C(t ) − C0 = 1 − exp⎢ −⎜ ⎟t ⎥ ⎣ ⎝d ⎠⎦ C∞ − C0

(13)

(11)

where K′ = (Kfi + Kri + Kfs + Krs) and C∞ = (Kfi + Krs)/K′. In the current work, changes in [V•• O ] were measured indirectly by monitoring the LSCF film’s out-of-plane lattice parameter, which increases as [V•• O ] increases and vice versa. Because the film is constrained to the thick YSZ substrate, the lattice parameter expands only in the out-of-plane direction.22 The LSCF lattice strain ξ arising from changes in the lattice parameter c is extracted from the refracted peak position using the relationship q Δc(t ) ξ(t ) = = 0 −1 c0 q(t ) (12) where q0 and q(t) correspond to the centroid location of an LSCF Bragg peak at open-circuit voltage and at time t after the 19918

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the free energy and the oxygen chemical potential by the following relationship

materials, surfaces, and interfaces comprising the LSCF/GDC/ YSZ heterostructure, significant changes occurring in the electrical resistance and oxygen exchange current rates are likely to occur only at the LSCF/air surface and the LSCF/ GDC buried interface as a sample evolves to reach steady state when a potential is applied. Changes in charge-transport resistance across the surface and buried interface are expected to be significant relative to changes associated with the [V•• O] dependence of resistance in the LSCF film. Likewise, extremely small relative changes in oxygen vacancy concentrations in the GDC and YSZ layers will have negligible effects on measured changes in electrical resistance (ionic transport rates) and are known to result in unmeasurable changes in the out-of-plane lattice constants of those materials.22 Thus, to better understand the changes associated with reaching steady state, one can focus on considering the outcomes of the following scenarios involving changes in the oxygen exchange currents across the LSCF surface and/or buried interface: (1) js increases, while ji remains constant. (2) ji decreases, while js remains constant. (3) js increases and ji decreases. (4) js decreases and ji decreases by an even larger amount. (5) ji and js both increase, but the rate of increase of js is larger than that of js. The observed decrease in electrical conductance as steady state is approached under both anodic and cathodic conditions indicates that neither scenario 1 nor 5 can occur. Likewise, scenarios 2 and 4 are not feasible, as from inspection of the forward direction terms of eqs 6 and 9, it can be seen that buried interface and surface rates change in opposite directions as C increases. Therefore, #3 is the only plausible scenario leading to steady state. Furthermore, because the net electrical conductance decreases as steady state is approached, we can conclude that the decrease in ji must be larger than the increase in js. Interestingly, by considering that C∞ < 0.233 (from eq 15) and Kfs < (1 − Co)Kfi/Co (i.e., ΔC < 0 as experimentally observed) then Ks > 3.34Ki, which indicates that the effective rate constant K′ is dominated by Ks, even though ji ≥ js. The values of K′ obtained from the fits to the strain relaxation (Figure 2a) measured at various bias voltages, temperatures, and pO2, shown in Figure 3, can be further analyzed to gain insight into the thermal and oxygen partial pressure dependence of the transport properties of the interfaces. Consider bias, temperature, and pO2 dependences by expressing K′ explicitly as (cathodic polarizations)

E = ΔGs − μO , where μO = 2

(17)

Reaction constants measured under cathodic potentials using a pO2 of 1.5 Torr are shown as a function of 1/T on an Arrhenius plot in Figure 4. The slope of the line for each set of

Figure 4. Arrhenius plot of exchange coefficients under cathodic potential as a function of inverse temperature at a pO2 of 1.5 Torr. The slope is proportional to the activation energy for oxygen reduction.

data is equal to (−E − 2αeη)/kB. While the surface exchange coefficients all increase monotonically with increasing voltage magnitude, the activation energy is similar for all applied potentials, ∼1.1 eV. This is in agreement with previous reports of 1.1 eV determined for 1 μm thick LSCF films on GDC using the ECR19,20 and other techniques.11 The effective overpotential can also be determined from this plot; at a given T and pO2, the change in K′ is directly related to changes in η. This change in overpotential across the cathode is ∼70 mV for every 1 V applied across the entire cell. The contribution of the overpotential term in eq 17 to the Arrhenius slope in Figure 4 is therefore ∼0.1 eV for the largest potentials used in this experiment. The bias dependence of K′ is shown in Figure 5 for several oxygen partial pressures. It shows that K′ is smaller for smaller bias voltages, as expected from eq 16. The activation energy of the interface decreases with increasing overpotential for a cathodic bias and vice versa for an anodic bias. Because the forward and reverse reaction constants across the LSCF/GDC interface are exponential functions of inverse temperature, the magnitude of the term with reduced activation energy increases rapidly and dominates the other term, thereby explaining why K′ increases with increasing positive or negative bias. On close examination of Figure 5a, it can be seen that the change in reaction constant with bias is not symmetric between the cathodic and anodic cases. The steady-state vacancy concentration shows similar asymmetry (Figure 3). This could indicate that it is energetically easier to add an oxygen vacancy to the LSCF layer (cathodic conditions) than to remove one (anodic conditions), that is, that α is not equal to 0.5. It may also be due to factors not explicitly included in our model, such as vacancy formation energy changes at low vacancy concentrations. In short, the bias dependence may be affected by characteristics of the electrolyte and details of the surface and interface barriers in addition to temperature and

K ′ = K fi + K fs ⎡ ΔGi + 2αeη ⎤ •• i ] exp⎢ − = k 0,i[V O(GDC) ⎥ kB T ⎦ ⎣ ⎤ ⎡ G e 2 α η Δ + s s + k 0,s(pO1/2 ⎥ 2 ) exp⎢ − kB T ⎦ ⎣

2

⎛ pO ⎞ 1 kBT ·ln⎜⎜ 2 ⎟⎟ 2 ⎝ patm ⎠

(16)

where k0 is a standard pre-exponential exchange coefficient independent of temperature, pO2, and electrochemical bias for each reaction. Because oxygen transport across both interfaces is thermally activated eq 16, measuring the exchange coefficients at multiple temperatures for a given overpotential and pO2 makes it possible to determine the activation energy for oxygen transport through the system. In this case, the measured activation energy corresponds to transport across the interface with the dominant rate constant, which we have established is Kfs. This surface activation energy, E, is related to 19919

DOI: 10.1021/acs.jpcc.5b05505 J. Phys. Chem. C 2015, 119, 19915−19921

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sample temperature to better understand their behavior. This analysis reveals that in the LSCF/GDC/YSZ system the rate constant associated with exchange at the LSCF/air surface dominates the behavior of the sample as a whole. In future planned studies, oxygen exchange behavior at the LSCF/air surface will be further investigated with perturbations of other environments, such as CO2 and H2O gas-phase concentrations. A key feature of the synchrotron-based approach used is that it measures local material properties, and thus can be used to map changes in surface exchange behavior across specific surfaces or interfaces of a heterostructure, whereas conductance (i.e., electric current) measurements provide averaged behavior of the sample. Furthermore, depth-dependent measurements are also possible with grazing-incidence X-rays. When combined with electrochemical measurements, a comparison of local versus average performance can provide valuable insight into performance-limiting processes and structures. For example, this would be very useful for materials with spatial composition variation or to compare different grains in a polycrystalline material. The ability to measure surface and interface exchange behavior of materials under realistic operating conditions is an important step in better understanding the behavior of electrochemical devices for energystorage and energy-transfer applications.

Figure 5. (a) Surface exchange coefficient (k) determined from lattice strain as a function of applied potential at 500 °C for oxygen partial pressures of 1.5, 15, and 150 Torr. (b) Comparison between surface exchange coefficient determined from strain and current relaxation at 500 °C and an oxygen partial pressure of 15 Torr.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

pO2. Nevertheless, these qualitative explanations are helpful in understanding the interfacial properties. A final observation is that an advantage of the synchrotron measurement technique used in this study over conventional methods of measuring surface exchange is that it gives information about the local behavior of a material. In the current work, this effect can be observed by comparing the surface exchange coefficients determined from the characteristic relaxation times of the lattice strain relaxation with those calculated from changes in the induced current (Figure 5b). The rate constants calculated from the current relaxation are smaller than those based on lattice strain and have a weaker dependence on applied potential. This occurs because the current is an averaged measurement based on oxygen vacancy motion across the entire LSCF film, while the lattice strains are extracted from Bragg peak shifts at a specific distance from the working electrode. The effective potential experienced at specific points along the film surface decreases exponentially with increasing distance from the top Pt current collector due to sheet resistance effects, and thus oxygen reaction rates determined from synchrotron measurements reflect the local behavior where the film is experiencing one specific potential.

§

E.P.: University of Fribourg, Department of Physics and Fribourg Centre for Nanomaterials, Chemin du Musée 3, CH1700 Fribourg, Switzerland. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was supported by the Solid State Energy Conversion Alliance (SECA) program and Fossil Energy Program of the U.S. Department of Energy (E.M.H., K.-C.C., B.J.I.), as well as by Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (P.M.B., E.P., J.A.E., P.H.F., H.Y.). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DEAC02-06CH11357. Notes

The authors declare no competing financial interest.

■

4. CONCLUSIONS In situ X-ray diffraction techniques in combination with electrochemical measurements provide a powerful means for measuring local oxygen exchange kinetics at material surfaces and interfaces. The effective time constant for oxygen exchange across the LSCF/air surface and the LSCF/GDC buried interface were measured independently of bulk diffusivity by observing the change in oxygen vacancy concentration of cathode thin films under applied potentials. These reaction coefficients can be modified by changing the oxygen partial pressure in the gas environment, the applied potential, and the

ACKNOWLEDGMENTS We thank L. Yan and P. A. Salvador of Carnegie Mellon University for providing the samples used in this study.

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