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Highly CO2‑Tolerant Cathode for Intermediate-Temperature Solid Oxide Fuel Cells: Samarium-Doped Ceria-Protected SrCo0.85Ta0.15O3−δ Hybrid Mengran Li,† Wei Zhou,*,‡ and Zhonghua Zhu*,† †

School of Chemical Engineering, The University of Queensland, St. Lucia, Queensland 4072, Australia Jiangsu National Synergetic Innovation Center for Advanced Materials, State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, P. R. China

‡

S Supporting Information *

ABSTRACT: Susceptibility to CO2 is one of the major challenges for the long-term stability of the alkaline-earth-containing cathodes for intermediate-temperature solid oxide fuel cells. To alleviate the adverse effects from CO2, we incorporated samarium-stabilized ceria (SDC) into a SrCo0.85Ta0.15O3−δ (SCT15) cathode by either mechanical mixing or a wet impregnation method and evaluated their cathode performance stability in the presence of a gas mixture of 10% CO2, 21% O2, and 69% N2. We observed that the CO2 tolerance of the hybrid cathode outperforms the pure SCT15 cathode by over 5 times at 550 °C. This significant enhancement is likely attributable to the low CO2 adsorption and reactivity of the SDC protective layer, which are demonstrated through thermogravimetric analysis, energy-dispersive spectroscopy, and electrical conductivity study. KEYWORDS: IT-SOFC, cathode, CO2 poisoning, ORR, stability

1. INTRODUCTION A solid oxide fuel cell (SOFC) is a promising future energy solution because of its ability to convert chemical energy stored in fuels, such as hydrogen and hydrocarbons, to electricity in a clean and efficient way. Lowering its operating temperature to intermediate temperature (IT; 500−750 °C) is vital for SOFC commercialization because low temperature can lead to significant cost reduction, easy sealing, prolonged system lifetime, and short time for start-up/shut-down procedures.1,2 At lower temperature, however, the SOFC cathode that catalyzes oxygen reduction reaction (ORR) becomes one of the most demanding challenges for IT-SOFC because of its low electroactivity.3,4 Consequently, enormous efforts have been contributed to cathode development to improve the ORR activity at reduced temperature.5−8 Perovskite oxides based on SrCoO3−δ (SC), including some milestone cathodes such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and SrSc0.175Nb0.025Co0.8O3−δ,3,8 are regarded as promising candidates for the IT-SOFC cathode because of their high mixed ionic and electronic conductivities and have undergone © XXXX American Chemical Society

extensive research recently. For example, we reported a tantalum-doped SC perovskite cathode, SrCo0.85Ta0.15O3−δ, that shows a remarkably low polarization resistance of ∼0.1 Ω·cm2 at 550 °C,9 which even outperforms the benchmark BSCF cathode,10 and argued that a tantalum dopant not only can stabilize the perovskite structure of SC but also has beneficial effects on its neighboring cobalt ions for ORR.9,11 However, the instability in the presence of CO2 is a major concern for these SC-based perovskite cathodes.12,13 The interaction between CO2 and these cathodes can seriously deteriorate the cathode performance especially at the IT range by slowing down the kinetics of the cathodes upon oxygen surface exchange.14,15 For example, the SrCo0.8Sc0.2O3‑δ cathode degrades by ∼12 times after only 5 min of exposure to a 10% CO2 atmosphere at 600 °C.16 The CO2 degradation effect is likely related to the basicity of the cathode surface because of Received: October 3, 2016 Accepted: January 3, 2017

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DOI: 10.1021/acsami.6b12606 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

and 1400 °C, respectively, until dense, and then polishing them into similar bar shapes, with dimensions of roughly 0.75 cm × 0.2 cm × 0.1 cm. We performed thermogravimetric analysis (TGA) to probe the interactions between the specimen and gas mixture (10% CO2, 69% N2, and 21% O2) from 500 to 650 °C by monitoring the mass change of TGA samples in response to the immediate gas change from air to a 10% gas mixture and also the mass change when the atmosphere changed back to air. The 10% CO2 gas mixture contains 69% N2, 21% air, and 10% CO2 and was purchased from Coregas. Before the test, the samples were first pelletized at the same pressure and crushed to ensure similar grain sizes. EIS was used to study the ORR electroactivity of the cathodes in a symmetrical cell configuration and also to evaluate the cathode polarization resistance stability in the presence of 10% CO2. The electrical conductivity measurement was conducted in a four-probe direct-current (dc) method. A PGSTAT302 Autolab workstation was used for these electrochemical tests, including EIS, single-cell measurement, and the electrical conductivity test. The crystal structures of the samples were characterized using powder X-ray diffraction (XRD). Scanning electron microscopy (SEM; JEOL JSM-7100F) was used to study the microstructures of the samples. Transmission electron microscopy (TEM; Tecnai 20 FEG) was used to perform energy-dispersive X-ray (EDX) analysis on different spots of SCT15 particles before and after CO2 treatment. We performed Fourier transform infrared (FTIR) spectroscopy analyses on the powder samples using a PerkinElmer Spectrum 100 with an attenuated-total-reflection (ATR) objective, and the data were collected within a wavenumber range between 4000 and 750 cm−1.

the presence of the basic alkaline-earth elements (strontium or barium) and surface defects (e.g., oxygen vacancies).12,16−21 The interaction between the cathode and CO2 is considered to be a reaction between a base (cathode) and an acid (CO2) according to Lewis acid−base theory. Consequently, CO2 can not only compete against O2 for limited active oxygen vacancies on the cathode surface22 but also promote carbonate formation and deformation of crystal structures for long-term operation.23 Despite the interaction with CO2, the alkaline-earth elements and oxygen vacancies are important for efficient ORR catalysis.16,24,25 As a result, a trade-off normally exists between the ORR activity and CO2 resistivity. To enhance the cathode resistance against CO2, an effective strategy is to protect the highly active ORR cathodes from CO2 by introducing a protective layer that is oxygen conductive and inert to CO2. When this strategy is adopted, a BSCF cathode structured with a densified La2NiO4+δ (LN)-coned shell has been developed and shows an improved CO2 tolerance because of the MIEC and high CO2 resistance of LN.16 Nevertheless, unconventional microwave-induced plasma has to be used to prevent the unwanted phase interaction between BSCF and LN. Whereas most SC-based perovskite cathodes are chemically compatible with ceria-based oxygen conductors, such as samarium-doped ceria (SDC, Sm0.2Ce0.8O1.9).26 The SDC is reported to show a stable oxygen permeability in the presence of CO227,28 and can effectively alleviate the thermal expansion mismatch of SC-based cathodes. Herein, we chose SrCo0.85Ta0.15O3−δ (SCT15) as the targeted material because of its proven high ORR activity and long-term stability in the absence of CO2.9 A few SCT15−SDC hybrid cathodes were evaluated with respect to their stability to a 10% CO2containing gas mixture with 21% O2, and a systematic investigation is presented to study the effects of SDC on both the electroactivity and stability of cathodes in the absence and presence of 10% CO2.

3. RESULTS AND DISCUSSION The compatibility of SCT15 with SDC was studied in our previous work (Figure 1a−c). SCT15 proved to be chemically

2. EXPERIMENTAL METHODS SrCo0.85Ta0.15O3−δ (SCT15) was synthesized through a solid-state route: stoichiometric mixtures of SrCoO3‑δ (SC; ≥99.9%, Aldrich), Co3O4 (≥99.9%, Aldrich), and Ta2O5 (≥99.9%, Aldrich) were wetball-milled for 24 h, followed by pelletizing and sintering at 1200 °C for 20 h in stagnant air. Samarium-doped ceria (SDC, Sm0.2Ce0.8O1.9; ∼30 m2/g) is a commercial product of Fuel Cell Materials. Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) was prepared through an ethylenediaminetetraacetic acid−citric acid route.3 Symmetrical cells for electrochemical impedance spectroscopy (EIS) studies were fabricated by nitrogen-borne spraying of the cathode ink, which was prepared by ball milling the cathode powders in isopropyl alcohol and terpineol, onto both sides of the SDC (from Fuel Cell Materials) electrolyte dense disk and were subsequently calcined at 1000 °C in stagnant air for 2 h. The SCT15+SDC composite cathodes were prepared through two main routes. One is mixing the SCT15 and SDC powders (60:40 wt %) mechanically by using a ball mill for 2 h to form the cathode ink, and the subsequent steps are the same as those for single-phase cathode fabrication. The other is to infiltrate 10 μL (4.2 wt % SDC) or 20 μL (8.4 wt % SDC) of an ethanol solution, containing SDC nitrate precursors (0.001 mol/ mL), and 10 wt % citric- acid into the SCT15+SDC composite backbone, followed by another calcination at 900 °C for 5 h. These two hybrid cathodes through infiltration are named SCT15+SDC +4.2% and SCT15+SDC+8.4% loadings, respectively. Silver paste was painted onto the cathode, serving as the current collector. The specimens for the electrical conductivity test were fabricated by pressing the SCT15 and SDC powders into pellets, sintering at 1200

Figure 1. Room-temperature powder XRD patterns of (a) a SCT15+SDC mixture treated at 1000 °C for 2 h,9 (b) SCT15, (c) SDC, and (d) SCT15 infiltrated with the SDC precursor followed by 5 h of 900 °C treatment.

compatible with SDC below 1000 °C because no apparent additional phases between SCT15 and SDC were detected from the XRD profiles of the SCT15+SDC mixtures (50:50 wt %) after 1000 °C treatment for 2 h.9 Besides, we also analyzed the crystal structures of the SCT15 powder infiltrated with 10 wt % SDC. As presented in Figure 1d, the major phases are similar to those of the SCT15+SDC mixture, implying that the SDC phase is formed, but some small additional peaks, as B


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Figure 3. High-angle annular dark field (HAADF) images of SCT15 before and after 10 h 10% CO2 treatment at 510 °C and the patterns of the corresponding EDX analysis. Figure 2. Weight percentage changes of (a) SCT15 and (b) SDC as a function of time when the atmosphere changed from air to 10% CO2 gas at different temperatures.

indicated with asterisks, suggest that a small quantity of unknown phases was formed during calcination. The effects of CO2 on SCT15 and SDC were studied using TGA separately. Figure 2 shows that the masses of both SCT15 and SDC increase in response to the addition of a feed of 10% CO2, which indicates that these materials can adsorb CO2 at the IT range. It is interesting to notice that these two samples adsorb more CO2 at higher temperature. Taking SCT15 as an example, during 60 min of exposure to a 10% CO2 atmosphere, the mass increases by ∼0.15% and ∼0.09% at 650 and 510 °C, respectively. A similar phenomenon was also reported for the BSCF material, whose interaction with CO2 is stronger at higher temperature.29 As more oxygen vacancies are formed in SCT15 and SDC at higher temperature,9,30 it is likely that higher levels of oxygen vacancies, which act as CO2 effective adsorption sites, are responsible for the more CO2 captured in the temperature range of 510−650 °C. However, SCT15 and SDC show different mass-change profiles upon interaction with CO2: SCT15 shows an almost linear behavior in adsorbing CO2 as a function of time especially at 510 °C, but the CO2 adsorption process is faster on SDC during the first 5 min but gradually slows until near equilibrium. The continuous CO2 adsorption process on SCT15 is likely a result of the base strontium segregation from the bulk to the surface to react with the acid CO2, which is confirmed by TEM−EDX. We conducted elemental analysis

Figure 4. Weight percentage changes of SCT15 and SDC as a function of time when the atmosphere switched from 10% CO2 to air at 510 °C.

through TEM−EDX at the surface and bulk of SCT15 particles with and without treatment in 10% CO2 for 10 h, respectively. As shown in Figure 3, the SCT15 free of CO2 treatment has similar cation contents at spots 1 and 2, but the specimen with CO2 treatment exhibits a much higher level of strontium, which is likely in the form of carbonate, near the surface compared with cobalt and tantalum cations. Therefore, it can be concluded that CO2 is one of the major reasons for strontium segregation, and the continuous mass increase of SCT15 in CO2 is a result of the continuous diffusion of strontium to the C


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Figure 5. Electrical conductivities of (a) SDC and (b) SCT15 upon exposure to 10% CO2 at 500 °C for 167 min after stabilization in air.

Figure 8. ASRs of SCT15, SCT15+SDC, and SDC-infiltrated SCT15+SDC cathodes as a function of the temperature in flowing air.

elements, such as strontium, in SDC, so that no further increase of adsorbed CO2 occurs. When the CO2-containing gas mixture is removed from the atmosphere at 510 °C, most of CO2 desorbs from SDC in the first 30 min, but SCT15 shows a different response to CO2 removal: nearly no CO2, which is captured during the first 60 min of CO2 treatment, is released from SCT15 (Figure 4). This is a sign suggesting that the CO2 adsorption process on SCT15 is nearly irreversible at 510 °C and is not sensitive to the CO2 concentration change. The slight mass gains after a change of the atmosphere in SCT15 can be ascribed to the remaining CO2 in the furnace chamber. Besides, the formation of carbonates on the SCT15 surface during the first 60 min of exposure to CO2 is also another main reason for the thereafter no release of CO2 from SCT15. In contrast, however, SDC shows a reversible low-CO2 adsorption and very low reactivity with CO2 due to its relatively lower basic surface, so that the mass of SDC almost decreases to its original mass only 30 min after the CO2 flux is removed. The electronic and ionic conductivities are important for efficient ORR catalysis of the cathode. Therefore, we evaluated the short-term stability of the electrical conductivity of SCT15 and SDC at 500 °C in the presence of 10% CO2 through a fourprobe dc method, respectively. The electrical conductivity of these two samples consists of the electronic and ionic conductivity: the electronic conductivity dominates in SCT15, but the ionic conductivity dominates in SDC. From Figure 5, negligible electrical conductivity changes for both SCT15 and SDC samples suggest that 10% CO2 in the atmosphere does

Figure 6. SEM micrographs of cross sections of SCT15, SCT15+SDC, and SCT15+SDC with different SDC loading cathodes under investigations.

surface to form carbonate. Similar segregations of alkaline-earth elements are also reported for other materials containing alkaline-earth elements in the presence of CO2 but with different oxygen partial pressures.22,23,31 For the SDC sample, however, the increase of the mass slows significantly after 20 min at 510 °C, indicating that an equilibrium will be reached on the SDC surface, where all of the effective adsorption sites are saturated with CO2. This different CO2 adsorption behavior between SCT15 and SDC may arise from the lack of basic

Figure 7. BSE micrographs of (a) SCT15+SDC, (b) SCT15+SDC+4.2% SDC loading, and (c) SCT15+SDC+8.4% SDC loading cathodes. The brighter particles shown in the images are SDC particles, and examples of SDC and infiltrated SDC particles are indicated by red and black rectangles, respectively. D


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Figure 9. (a) Relative and (b) real ASR value changes with time for the studied cathodes when the flowing air is replaced by flowing 10% CO2containing air at 550 °C. (c) ASR change of the SCT15+SDC+42% loading cathode as a function of time in the presence of 10% CO2 at 600 and 650 °C. The slopes shown in the figure are the estimated slopes of a linearly increasing ASR profile as a function of time, especially after 5 min of exposure to 10% CO2.

not have a significant adverse effect on the electronic conductivity of SCT15 and the ionic conductivity of SDC. The microstructures of the cathodes under investigation are also studied using SEM and are presented in Figures 6 and 7. Figure 6 shows the cross sections of the targeted cathodes and confirms that all of the cathodes attach well with the SDC surface and have similar cathode thicknesses. Because SDC has a larger mean atomic number of ∼25.52 than SCT15 (∼19.18), backscattered electrons were used to characterize the topography of SDC particles on the SCT15 cathode: a high mean atomic number leads to a high backscattering intensity. From the backscattered electron images as shown in Figure 7, we found that the SCT15 cathode is covered by small SDC particles with ∼0.5 μm size for the SCT15+SDC cathode and is additionally covered by nanosized SDC particles for infiltrationloaded SCT15+SDC. Further secondary-electron SEM images (Figure S1) also confirm the topography of SDC particles on the SCT15 cathode. It can be clearly seen that the SDC nanoparticles incorporated by infiltration significantly increase the interfaces between SCT15 and SDC. The ORR activity of the cathodes in air is tested using electrochemical impedance analysis in a configuration of the SDC-based symmetrical cell from 500 to 700 °C. The area specific resistance (ASR), as calculated from the impedance

spectra, characterizes the cathode performance: a lower value reflects a higher activity over ORR. As shown in Figure 8, pure SCT15 exhibits the lowest ASRs at 500−700 °C among all of the cathodes under study. Incorporating the SDC phase degrades the ASR values as well as the activation energy of the pure SCT15 cathode mainly because of the lower electrical conductivity of SDC in comparison to SCT15. Further infiltration of SDC also makes it slightly less active in catalyzing ORR especially above 600 °C compared to the SCT15+SDC cathode. Nevertheless, the activation energy is noticeably lowered to 96−97 kJ/mol, which can even be comparable to that of the pure SCT15 cathode (∼102 kJ/mol). The enhanced activation energy may be attributable to the infiltrated SDC nanoparticles (shown in Figure 9), which improve the three phase boundaries among SCT15, SDC, and air. The cathode tolerance against CO2 was evaluated in the presence of 10% CO2. The relative ASR change is recorded as a function of time after a 10% CO2 gas mixture is fluxed in. As presented in Figure 9a, the electroactivity of SCT15 degrades dramatically by nearly 18 times in the first 5 min and linearly degrades by 22 times after 1 h of CO2 exposure. The increasing rate of polarization resistance of SCT15 is about 0.012 Ω·cm2/ min after ∼5 min of exposure to 10% CO2. The observed significant ORR deterioration as a result of CO2 mainly arises E


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treatment at 517 °C for 3 h. The spectra (Figure S2) show that the peaks at 1436 and 857 cm−1 wavenumbers, which are characteristic of a carbonate group,18,23 become far more intense after 10% CO2 treatment for SCT15 than for SCT15 with infiltrated SDC, implying that more carbonates are formed in SCT15 after the treatment compared to those with SDC incorporated. This can be explained by the demonstrated low CO2 adsorption and reactivity of SDC nanoparticles on the surface of SCT15. Therefore, the spectra data further confirm that incorporating SDC hinders the interaction between SCT15 and CO2 at IT. Moreover, the interface between SCT15 and SDC can serve as a stable active site for ORR in the presence of CO2 due to the proved unaffected ionic conductivity of SDC and the electronic conductivity of SCT15 (Figure 5). Therefore, the size increase of such a SCT15/SDC interface is another reason for the enhanced stability of the cathode performance in the presence of CO2 for the hybrid cathodes. Consequently, the incorporated SDC particles can serve as a discontinued protective layer against CO2 for the SCT15 cathode mainly because of the aforementioned relatively low CO2 adsorption and reactivity of SDC, as well as unaffected mixed conductivities of the SCT15/SDC interface in the CO2containing atmosphere. Furthermore, we also applied this strategy to the BSCF cathode, which is one of the benchmark cathodes for ITSOFCs but susceptible to CO2, by simply mechanical mixing of the BSCF cathode with SDC before fabrication onto the electrolyte. Figure 10 provides the impedance spectra of both the BSCF and BSCF-SDC cathodes in a configuration of the symmetrical cell before and after 30 min of 10% CO2 treatment. In the presence of 10% CO2, the ASR increases by ∼46 times after only 32 min of exposure for the BSCF cathode, but the ASR only goes up by 6 times for the BSCF+SDC cathode. Therefore, it can be concluded that the CO2 tolerance of doped SC can be significantly enhanced by the incorporation of a SDC protective layer.

Figure 10. Comparison of the (a) BSCF and (b) BSCF+SDC cathode impedance profiles in response to ∼30 min of 10% CO2 treatment.

4. CONCLUSIONS SDC is incorporated in the SCT15 cathode through both mixing and infiltration methods in order to enhance the resistivity of the SCT15 cathode against CO2 at IT. Our study reveals different CO2 adsorption mechanisms between SCT15 and SDC at SOFC operating temperatures, and the reactivity of SCT15 with CO2 is much higher than that of SDC. Because of the low CO2 adsorption and reactivity of SDC and the unaffected mixed conductivities of the SCT15/SDC interface, the SCT15+SDC composite cathode is found to be far less susceptible in the presence of 10% CO2 compared to the pure SCT15 cathode. This strategy is also found to be effective in improving CO2 tolerance of other promising cathode materials containing alkaline-earth metals, such as BSCF.

from the competition for active adsorption sites between O2 and CO2,29,32 as well as the formed carbonation that inhibits the oxygen-exchange process on the cathode surface.15 For the cathode hybrid with SDC, however, the degradation is significantly alleviated: the ASR value increases by only 5 times after 1 h of exposure to 10% CO2 for the SCT15+SDC cathode and increases by only ∼3 times for the SCT15+SDC cathode with 8.4 wt % SDC loading at 550 °C. It is important to note from Figure 9b that the cathode performance becomes better compared to that of the pure SCT15 cathode as the load of SDC increases, although the ASRs of these composite cathodes are not as low as those of pure SCT15 in the absence of CO2. Moreover, the ASR increasing rate (when the exposure time >5 min) is significantly reduced when the cathode is infiltrated with SDC: e.g., the SCT15+SDC+4.2% loading degrades at a rate of ∼0.0055 Ω·cm2/min, which is less than half the rate of the pure SCT15 cathode. Additionally, the polarization resistance of the cathode infiltrated with 4.2 wt % SDC remains below 0.1 Ω·cm2 at 650 °C when exposed to 10% CO2 for 48 min, showing an acceptable cathode performance in a CO2-containing atmosphere (Figure 9c). To further study the effects of SDC on the CO2 tolerance of the cathode, we compared the spectra of FTIR-ATR among SCT15 and SCT15 infiltrated with 0.86 wt % SDC before and after 10% CO2

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12606. Secondary-electron SEM of cathode microstructures and FTIR-ATR spectra of samples before and after CO2 treatment (PDF) F


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.Z.). *E-mail: [email protected] (Z.Z.). ORCID

Zhonghua Zhu: 0000-0003-2144-8093 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate technical support from the Centre for Microscopy and Microanalysis at the University of Queensland. This work was financially supported by the Australian Research Council (Grant DP130102151) and also received project support from a Queensland Smart Futures Fund, Research Partnerships Program. M.L. acknowledges additional financial support from a scholarship from the China Scholarship Council. W.Z. acknowledges Open Funding from the State Key Laboratory of Material-oriented Chemical Engineering (No. KL15-04).

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ABBREVIATIONS SOFC, solid oxide fuel cells IT, intermediate temperature ORR, oxygen reduction reaction ASR, area specific resistance SCT15, SrCo0.85Ta0.15O3−δ SDC, samarium-doped ceria BSCF, Ba0.5Sr0.5Co0.8Fe0.2O3−δ SE, secondary electron BSE, backscattered electron

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