Chromium Poisoning Effects on Surface Exchange Kinetics of La0

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Chromium Poisoning Effects on Surface Exchange Kinetics of La0.6Sr0.4Co0.2Fe0.8O3−δ Yi-Lin Huang,† A. Mohammed Hussain,† Christopher Pellegrinelli,† Chunyan Xiong,†,‡,§ and Eric D. Wachsman*,† †

University of Maryland Energy Research Center, University of Maryland, College Park, Maryland 20742, United States Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China § Wuhan Institute of Technology, Wuhan, Hubei 430205, China ‡

ABSTRACT: The presence of Cr has already been reported in literature to cause severe degradation to La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF). However, fundamental understanding of Cr effects on the surface exchange kinetics is still lacking. For the first time, in situ gas phase isotopic oxygen exchange was utilized to quantitatively determine Cr effect on oxygen exchange kinetics of LSCF powder as a function of temperature and water vapor. Our investigations revealed that the formation of secondary phases such as SrCrO4, Cr2O3, Cr−Co−Fe−O, and La−Co−Fe− O can affect both the oxygen dissociation step and overall surface exchange. Specifically, Cr-containing secondary phases on the surface not only decrease the active sites for surface reactions but also alter the nearby stoichiometry of the LSCF matrix, thereby limiting surface oxygen transport. In addition, water molecules actively participate in the surface reactions and can further block the active sites. KEYWORDS: chromium induced degradation, isotope exchange, surface exchange kinetics, oxygen dissociation, water

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INTRODUCTION

Numerous approaches and characterization techniques were used to study Cr-induced degradation.6 However, the time dependence and surface sensitivity of the degradation process limits the investigation. One major characterization technique to observe the degradation process is electrochemical impedance spectroscopy (EIS). Normally, samples were aged in the presence of Cr-containing alloys, and the changes in EIS impedance spectra under different operating conditions were monitored to determine the degradation process.19−21 Crcontaining secondary phases such as SrCrO4, Cr2O3, and CrO2.5 were identified on the LSCF surface. Other possible surface precipitates from the LSCF perovskite oxide include Co3O4 and Co−Fe−O spinel.12,22−24 Finsterbusch et al.25 reported the direct impact of a thin layer of Cr2O3 on LSCF using isotope exchange depth profiling with a secondary ion mass spectrometer (IEDP-SIMS). Their results suggest that surface exchange coefficient (k) varies with the thickness of Cr2O3 thin film on the LSCF surface. Wang et al. identified active Cr-containing surface species and determined the effect of temperature and Cr−S cross-contamination on k using electrical conductivity relaxation.22,26 However, the direct impact of Cr on the surface exchange kinetics, including

Solid oxide fuel cells (SOFC) are one of the best energy conversion devices for power generation because of their high efficiency and fuel flexibility.1 However, the durability of SOFC cathodes under real operating conditions is still an important issue for commercial implementation.2,3 For SOFCs, high temperature operation is generally required to thermally activate the electrochemical processes in electrodes and oxygen transport in solid electrolytes. Moreover, thermally stable, low cost metal alloy-based interconnects are important to offer electronic conduction to the electrodes. The use of chromium in such high temperature interconnects is difficult to avoid. Flowing of air through the interconnects can drag volatile Cr out, forming various gaseous Cr-containing species that can induce cathode degradation.4−6 In addition, the presence of water vapor can further exacerbate the process.7−10 Mechanisms for Cr-induced degradation vary depending on cathode materials.11,12 In the case of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF), gaseous Cr-containing species react with LSCF and generate Sr−Cr−O nuclei, resulting in the wide distribution of secondary phases on the surface, which eventually decreases the electrochemical performance.6,13,14 Thus, fundamental understanding of chromium poisoning effects on the oxygen reduction reaction (ORR) and the role of water vapor in crosscontamination15−18 are important to prevent Cr-induced degradation. © 2017 American Chemical Society

Received: February 24, 2017 Accepted: April 26, 2017 Published: April 26, 2017 16660

DOI: 10.1021/acsami.7b02762 ACS Appl. Mater. Interfaces 2017, 9, 16660−16668

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ACS Applied Materials & Interfaces

Figure 1. Post-analysis on powder samples to determine bulk and surface properties of Cr-aged LSCF. Bulk properties: (a) powder X-ray diffraction on fresh LSCF and Cr-LSCF and green stars (*) denote to the secondary phase SrCrO4 (JCPDS: 35-743). (b) O2 TPD on fresh and Cr-LSCF. Surface characterizations: (c) XPS Cr 2p of Cr-LSCF; black circles: experimental data, red line: the sum of fits, gray line: background, and blue and green lines are fits for each component. (d) Raman spectra of Cr-LSCF shows the corresponding peaks for (a) Cr2O3 and (b) SrCrO4. saturated with normal 16O was heated up under a fixed ramp rate in 18 O2 environment.32−35 TPX provides a quick survey of the temperature ranges where exchange reaction occurs. In isothermal isotope exchange (IIE) with 1:1 ratio of 16O2 and 18O2 (1:1 IIE), the powder sample was heated to a given temperature, and isotopic oxygen was introduced into the system under an isobaric switch.36,37 The concentrations of oxygen isotopologues after surface exchange were then recorded by the QMS downstream. Kinetic parameters can then be quantitatively determined based on the isotopic transient signals at steady state. To study the water effect on isotope exchange, D2O (m/z = 20) was used instead of H2O (m/z = 18) to avoid overlapping signal with 18O fraction (m/z = 18). Also, the cross-contamination between water and Cr on the cathode is believed to cause severe degradation. Therefore, aged LSCF powders were examined by isotope exchange, and the effects of chromium on LSCF as a function of temperature with or without the presence of water vapor were explored.

dissociation and incorporation and the role of water, are still not clear. In this work, we utilize in situ gas phase isotopic oxygen exchange to explore chromium effects on surface exchange kinetics of LSCF, a common cathode for SOFCs. Gas phase isotopic oxygen exchange is a powerful technique to investigate surface reaction kinetics and catalytic activity from a molecular point of view.27−31 By analyzing the gaseous products after surface exchange in real time, the oxygen dissociation rate and k, involving a sequence of surface reaction steps, can be extracted quantitatively from gas phase isotopic oxygen exchange. This technique can further be applied to study the degradation mechanism and the effect of Cr on oxygen transport. Moreover, the surface and bulk properties of posttested samples were evaluated to link the surface exchange kinetics with the surface features.

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ISOTOPE EXCHANGE THEORY Two major reaction steps, dissociative adsorption and incorporation, are involved in oxygen surface exchange on LSCF.36−38 These steps must take place in sequence. At first, gaseous oxygen molecules need to be adsorbed on the LSCF surface, followed by the breaking of the O−O bond. The dissociated oxygen atoms can incorporate into the lattice via a vacancy exchange mechanism. After incorporation, lattice oxygen can transport in the solid by solid-state diffusion. Details of isotope exchange theory were reported in our previous work.36,37 From the inlet 16O2 and 18O2 mixture, the scrambled product 16O18O can be generated only after surface dissociation. Thus, the production rate of 16O18O at steady state specifies the amount of oxygen that can be dissociated on the LSCF surface at a given temperature and oxygen partial pressure (PO2). This parameter is a function of surface area,

EXPERIMENTAL SECTION

LSCF powder samples were used to enable the high surface area required for the surface catalytic study. To avoid the contamination of Cr vapor into the vacuum chamber of quadrupole mass spectrometer (QMS), LSCF (Praxair) powder was aged ex situ in contact with Crcontaining alloy (Crofer APU 22) under the flow of humidified air (3% H2O) for a week to accelerate the Cr degradation. Crofer APU 22 was selected as a Cr-containing source to match real operating conditions. Furthermore, a controlled experiment was designed to distinguish the effects between aging and Cr-induced degradation. For this experiment, LSCF powder was aged in situ in synthetic air for the same period of time. We refer to aged LSCF powder that was in direct contact with Cr-containing alloy (Crofer APU 22) as Cr-LSCF and the aged LSCF powder used in the controlled experiment as aged-LSCF. The test powder was normalized to a surface area of 0.1 m2, determined by BET, and was placed into a quartz microreactor. In temperature-programmed exchange (TPX), the powder sample 16661


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Figure 2. (a) TEM image and EDS element mapping of Cr-LSCF powder and (b) the corresponding line scan of element distribution. (c) TEM image of two particles at a different spot. (d) Line scan of Co, Fe, and Sr element distribution, determined by EDS. (e) EELS element mapping on the two particles, highlighted by white dashed lines.

inlet gas flow, and the activation energy of the reaction process and can be used to determine the catalytic activity of materials toward oxygen dissociation, as represented in the following relation: ⎛E ⎞ d[16O18O] = ASC exp⎜ d ⎟ ⎝ RT ⎠ dt

experiment, aged without Cr) and Cr-LSCF are shown in Figure 1a. The diffraction peaks of LSCF are labeled with the corresponding diffraction planes, and there are no detectable secondary phases on aged-LSCF. In contrast, a secondary phase SrCrO4 (JCPDS: 35-743) can be identified on Cr-LSCF powder based on the characteristic peaks of SrCrO4 between 25 and 30°.40 Oxygen temperature-programmed desorption (O2 TPD) on aged-LSCF and Cr-LSCF is shown in Figure 1b. Two desorption peaks representing the amount of oxygen vacancies generated as a function of temperature can be identified on aged-LSCF: one low temperature α peak around 300−400 °C and another high temperature β peak. The amount of oxygen released from the lattice is accompanied by the formation of oxygen vacancies in LSCF. The α desorption is attributed to the partial reduction of transition metals (Co and Fe) in LSCF from 4+ to 3+, while the β desorption corresponds to that from 3+ to 2+.41,42 The Cr-LSCF sample shows a decrease in total amount of desorbed oxygen under β desorption peak, as shown in the red curve in Figure 1b, suggesting that the number of oxygen vacancies generated in Cr-LSCF decreases. This observation suggests that a portion of Cr-LSCF perovskite lattice might be damaged and lose oxygen vacancies. Cr concentration in Cr-LSCF quantified by energy dispersive Xray spectroscopy (EDS) analysis is about 3.5% of total cations in bulk. Because the catalytic activity of cathodes is extremely sensitive to surface features, XPS and Raman spectra were used to determine the possible surface species. XPS Cr 2p of Cr-LSCF is shown in Figure 1c. Black circles represent experimental data, and the red line is the sum of fits. On the basis of the literature results,43−46 two major surface Cr-

(1)

where A is the pre-exponential terms and S is the active surface area. C is inlet 16O2 and 18O2 concentrations. Ed is the activation energy for the combination of dissociation and reassociation processes. k is also used to quantitatively describe the degradation in the self-exchange rate between gas and solid. General solutions for diffusion into a sphere with surface exchange39 can be expressed as ⎛ 3k ⎞ M (t ) = 1 − exp⎜ − t ⎟ ⎝ a ⎠ M∞

(2)

Here, M(t) is the accumulated 18O fraction in the solid phase at time t; M(∞) is the total amount of oxygen sites in lattice, and a is the radius of the powder. The apparent activation energy of the surface exchange coefficient (Ek) can also be extracted from the temperature dependence of k.

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RESULTS AND DISCUSSION To link the surface exchange kinetics observed from isotope exchange with surface and bulk properties of test samples, various post-analysis techniques were performed on aged samples, and the results are summarized in Figure 1. Powder X-ray diffraction (XRD) patterns of aged-LSCF (the controlled 16662


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ACS Applied Materials & Interfaces containing species are identified: SrCrO4 (green lines) with bonding energies of 580.2 and 589.2 eV, and Cr2O3 (blue lines) with bonding energies of 577.2 and 586.4 eV. Raman spectra of Cr-LSCF shows the corresponding peaks for SrCrO4 and Cr2O3, in agreement with XPS results, as shown in Figure 1c.47 TEM was used to further determine possible secondary phases on Cr-LSCF. Figure 2a shows a TEM image on CrLSCF powder and the corresponding element mapping using EDS. On the basis of the element mapping, a Sr−Cr−O secondary phase located in the center of powder clustering can be observed: only Sr and Cr signals exist without any La, Co, or Fe signals. A line scan of the distribution of elements across the particle is shown in Figure 2b. In the line scan, each element signal near the Sr−Cr−O phase (shown by circles in Figure 2a) changes, suggesting that the formation of the Sr−Cr−O phase may affect the nonstoichiometry of LSCF matrix near this region. In addition, we also observed other secondary phases in other regions of the Cr-LSCF powder. TEM image of two adjacent particles is shown in Figure 2c. Due to the high energy of Sr L edge (∼2000 eV), it is difficult to determine Sr concentration using electron energy loss spectroscopy (EELS). Therefore, a line scan of element mapping from EDS was conducted to determine the existence of Sr, as shown in Figure 2d. Co and Fe signals are observed across the two particles, while no detectable Sr signal is seen. The corresponding EELS element mappings on particles A and B are shown in Figure 2e. La and Cr concentrations in Figure 2e show the mutual exclusion of La and Cr signals from each other in these two particles. In particle A, a high concentration of Cr is detected, while there is no observable Cr signal in particle B. In contrast, La signal can only be detected in particle B, but not in particle A, while both Co (lower concentration) and Fe signals can be detected in both particles.48 On the basis of these TEM results, secondary phases Cr−Co−Fe−O and La−Co−Fe−O are identified in Cr-LSCF.24 For isotope exchange experiments, temperature-programmed isotope exchange (TPX) was performed to provide a quick survey of surface exchange kinetics across a wide range of temperatures, as shown in Figure 3. Figure 3a shows TPX of fresh LSCF (closed symbols) and Cr-LSCF (open symbols). Comparing fresh LSCF with Cr-LSCF, a delay of onset temperature for exchange reaction on Cr-LSCF from 300 to 400 °C suggests that the surface exchange process deteriorates and that a higher temperature is required to thermally activate the oxygen exchange process. Figures 3b and c show O2 and D2O signals of TPX of CrLSCF with the presence of both O2 and water. The onset temperature for oxygen exchange does not change with the presence of water. Isotopically labeled D218O is observed with an onset temperature of 400 °C, and water exchange on CrLSCF has an exchange peak around 500 °C. On the basis of TPX results, 1:1 IIE was performed on agedLSCF and Cr-LSCF at different temperatures to extract kinetic parameters. Figures 4a−d show isotope exchange results on aged-LSCF at different temperatures. Dots are experimental data, and the best fit results are shown in lines. Compared with that of fresh LSCF,36 the surface kinetics of aged-LSCF does not show any significant degradation. Oxygen dissociation and surface exchange on aged-LSCF have performances similar to those of fresh LSCF. Through comparison of 1:1 IIE of agedLSCF with Cr-LSCF, the Cr-induced degradation can be determined. Figures 4e−h show 1:1 IIE of Cr-LSCF at different

Figure 3. (a) TPX of fresh LSCF (closed symbols) and Cr-LSCF (open symbols) in 25000 ppm of O2 only; TPX of Cr-LSCF in 25000 ppm of O2 with the presence of 6000 ppm D2O: (b) O2 signal and (c) D2O signal.

temperatures. As shown in Figure 4e, the dissociation ability of Cr-LSCF is deteriorated, and there is no observable exchange at 350 °C. Even at 500 °C in Figure 4h, there is only a limited amount of 16O18O that is produced, suggesting that surface catalytic activity of LSCF degrades, and even higher temperature is necessary to thermally activate the oxygen surface exchange process. To identify the role of water in isotope exchange on CrLSCF, in situ isotope exchange was performed with the presence of water. Figure 5 shows 1:1 IIE of LSCF with the presence of water at different temperatures ranging between 350 to 500 °C with best fit results. O2 signals at different temperatures in Figures 5a−d show that the presence of water changes the O2 exchange curves. The formation of isotopically labeled D218O can be observed in Figures 5e−h based on the water signals at different temperatures, indicating that water molecules actively participate in the exchange reaction. The presence of water further suppresses the formation of 16O18O, suggesting that competitive adsorption may play an important role. The temperature dependence of extracted kinetic parameters is shown in Figures 6a and b, and the percentage change of oxygen dissociation rates on Cr-LSCF at different temperatures 16663


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Figure 4. 1:1 IIE of aged-LSCF, the controlled experiment in which LSCF powder had been aged in synthesis dry air in situ for a week, at (a) 375, (b) 400, (c) 425, and (d) 450 °C with best fits. 1:1 IIE of Cr-LSCF in which LSCF powder had been in contact with Cr-containing alloy Crofer APU 22 in humidified air ex situ for a week at (e) 350, (f) 375, (g) 400, and (h) 425 °C with best fits. The temperature region for isotope exchange is selected differently due to the decrease in oxygen transport kinetics after Cr aging.

LSCF with the presence of water at 400 and 450 °C, respectively, as listed in Table 1. The effect of Cr on surface exchange coefficient (k) of LSCF was quantitatively determined using gas phase isotopic oxygen exchange. An in-depth discussion of the variations in activation energy using different characterization techniques is given in our previous work.36,37 Figure 6b shows an Arrhenius plot of k on all tested samples. The mechanistic probe sections for k include both dissociation and incorporation processes. The apparent activation energy of k (Ek) for aged-LSCF is 45.4 kJ/ mol, which is similar to the value for fresh LSCF.36,37 Ek for CrLSCF shows a decrease in activation down to 30.3 kJ/mol. The presence of water further decreases the apparent activation energy of overall surface exchange to 20.8 kJ/mol. Therefore,

is summarized in Table 1. Figure 6a shows Arrhenius plot of 16 18 O O production rate. Aged-LSCF has a similar performance with fresh LSCF with apparent activation energy (Edis) of 77 kJ/mol.36 Compared with fresh LSCF, the oxygen dissociation rates for Cr-LSCF decrease at all tested temperatures. Only 9.0, 13.7, and 24.9% of dissociated oxygen were observed on CrLSCF at 350, 400, and 450 °C with a higher Edis of 102 kJ/mol. With the presence of water (purple line), the oxygen dissociation on Cr-LSCF is further prohibited, and Edis increases to 134.7 kJ/mol, possibly due to the competitive adsorption of water and oxygen. Water molecules may preferably bond to the LSCF surface, resulting in a decrease of active sites for oxygen exchange. Compared with fresh LSCF, only 2.8 and 9.1% of dissociated oxygen were observed on Cr16664


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Figure 5. 1:1 IIE of Cr-LSCF at different temperatures with the presence of water: O2 signals at (a) 350, (b) 400, (c) 450, and (d) 500 °C with best fits. D2O signals at (e) 350, (f) 400, (g) 450, and (h) 500 °C. D2O partial pressure is 6 × 10−3 atm.

the decreased k is the result of a decrease in the pre-exponential term, possibly due to surface blocking, as shown in Figure 7. Also, the changes in the apparent activation energy for both oxygen dissociation and overall surface exchange for Cr-LSCF suggest that the Cr-induced degradation depends on several other factors, and the decrease in active surface sites might not be the only reason. An oxygen transport mechanism is proposed to explain Cr-poising effect on surface exchange kinetics. With the presence of gaseous Cr-containing species, secondary phases such as Sr−Cr−O nuclei can be formed on the LSCF surface, leading to the changes of LSCF stoichiometry in the near region of matrix LSCF, as shown in Figure 7a. These changes in the near surface region can cause different energy barriers for oxygen transport. Therefore, oxygen transport may have different pathways. One is across the Sr−Cr−O phase, which is inactive due to the insulating

layer of Cr-containing species. The second pathway is across the surface nonstoichiometric region, wherein its surface configuration is altered, and oxygen transport is only partially active. Moreover, we showed that the near surface stoichiometry of LSCF changes as Sr precipitates.14 It is also suggested in literature49−51 that a slight Sr deficiency in the A site of LSCF promotes the formation of oxygen vacancies, leading to an enhancement in surface exchange rate. However, once the level of Sr deficiency exceeds the limit, the perovskite structure is damaged, deteriorating the surface exchange properties. Our results show that Cr-LSCF has a concentration of oxygen vacancy and surface exchange rate lower than those of fresh LSCF. This suggests that the perovskite structure of this nonstoichiometric region may be damaged, resulting in a partially active region. The third pathway is across the normal LSCF surface, which is expected to be active. The formation of 16665


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secondary phase to extract kinetic parameters for further understanding the surface exchange kinetics caused by Crpoisoning effects.

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CONCLUSIONS In this study, Cr-induced degradation of surface reaction kinetics on LSCF is directly observed via isotope exchange. Surface Cr-containing species are identified as SrCrO4 and Cr2O3 on Cr-aged samples. Other secondary phases can also be identified as Sr−Co−Fe−O and La−Co−Fe−O phases. Cr effects on oxygen dissociation and overall surface exchange of LSCF as a function of temperature were determined separately. Isotope exchange results show that aging in the presence of Cr induces the degradation in both oxygen dissociation and overall surface reaction kinetics of LSCF, leading to changes in the apparent activation energies on each step. In addition, water and O2 coexchange on LSCF shows that water actively participates in surface exchange, and the presence of water shows a high impact on oxygen exchange on Cr-aged LSCF, possibly due to competitive adsorption. An oxygen transport model on degraded LSCF is proposed based on isotope exchange results. Formation of the secondary phases not only limits the available surface sites for reaction to occur but also decreases the electrochemical activity in the near region due to the changes in nonstoichiometry of LSCF matrix. The overall surface exchange kinetics of LSCF is the sum of each oxygen transport pathway. Our isotope exchange results were performed on powder samples. However, microstructure could have a significant effect on how our observed intrinsic properties manifest themselves in sintered cathodes. Fundamental understanding of chromium effects as well as crosscontamination effects on the ORR is critical for the development of new cathodes with high tolerance toward Crinduced degradation.

Figure 6. Arrhenius plot of LSCF with respect to reciprocal temperature with different aging/testing conditions for (a) the dissociation ability and (b) surface exchange coefficient (k).

Table 1. Oxygen Dissociation Rate (mol m−2 s−1) on Fresh LSCF at Different Temperatures and the Oxygen Dissociation Rate on Cr-LSCF with and without the Presence of Water temperature (°C) LSCF (mol m−2 s−1) Cr-LSCF Cr-LSCF with water

300 1.4 × 10−9

350 4.5 × 10−7

400

450

1.7 × 10−6

3.4 × 10−6

percentage of original (%) 4.1 × 10−8 2.3 × 10−7 (9.0%) (13.7%) 4.8 × 10−8 (2.8%)

8.5 × 10−7 (24.9%) 3.1 × 10−7 (9.1%)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yi-Lin Huang: 0000-0002-1886-3352 Eric D. Wachsman: 0000-0002-0667-1927 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors wish to acknowledge the support of the U.S. Department of Energy, NETL, Contract DEFE0009084.

Figure 7. (a) Formation of Sr−Cr−O nuclei on the LSCF surface, leading to the changes of nonstoichiometry of LSCF in the near region. (b) Oxygen transport can be divided into three different surface pathways: the electrochemically inactive pathway through the Sr−Cr−O secondary phase, the partially active pathway in the effective region, and the normal pathway on the LSCF surface.

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other secondary phases such as Cr−Co−Fe−O and La−Co− Fe−O may also contribute to oxygen transport with a limited reaction rate. The overall surface exchange kinetics observed from isotope exchange is a macro-view of overall exchange across this region and is a sum of reactions across these different pathways, leading to changes of the apparent activation energies for dissociation and surface exchange. Because the formation of secondary phases is a function of aging time,47 isotope exchange can be used to determine the time dependent Cr degradation process. As a future outlook, isotope exchange experiments can be conducted on each 16666


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