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Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
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Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell Chunyan Xiong a, Wenlu Li b, Dong Ding c, Jian Pu a,*, Bo Chi a, Jian Li a,d a
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 b International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, Hubei, China c Idaho National Laboratory, P.O. Box 1625, MS 3732, Idaho Falls, ID 83415, USA d Research Institute of Huazhong University of Science and Technology in Shenzhen, Shenzhen, Guangdong 518000, China
article info
abstract
Article history:
Strontium-doped samarium manganite is a potential cathode for solid oxide fuel cells
Received 25 May 2016
(SOFCs) with remarkable high oxygen reduction reaction activity. Here we investigated
Received in revised form
chromium poisoning effect on Sm0.5Sr0.5MnO3 cathode of SOFCs for the first time. The Cr
7 July 2016
caused cathode degradation is studied under current density of 200 mA cm2 and open
Accepted 9 July 2016
circuit potential (OCP) at 750 C. After polarized in the presence of the Crofer22 APU at
Available online xxx
750 C for 1200 min, the polarization resistance decreases from 3.25 U cm2 to 2.25 U cm2, then increases to a stable value of 2.75 U cm2. The degradation rate of SSM is lower than
Keywords:
that of LSM cathode in the same experimental environment. At OCP, the polarization
Solid oxide fuel cell
resistance increases to 7.00 U cm2 and reaches a stable level. SEM and EDX shows the
Cathode
depositions on the Sm0.5Sr0.5MnO3 boundary after applying current for 1200 min, and on
Chromium poisoning
the SSM surface after aging at OCP for 200 h. The Cr depositions are mainly comprised of
Durability
SrCrO4 formed by the nucleation reaction. The results show that SSM is a poisoning
Strontium-doped samarium
tolerant cathode and good replacement for the LSM in IT-SOFCs due to the better elec-
manganite
trochemical performance and the relatively stable characteristics after Cr poisoning. © 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction The demand for clean, secure and sustainable energy sources has stimulated great interests in the electrochemical energy storage and conversion technologies such as batteries, fuel cells, supercapacitors and electrolysers. Solid Oxide Fuel Cells (SOFCs) are considered as one of the most promising energy exchange devices which show advantages of high efficiency,
fuel flexible and low emission. Chromium-forming alloy are widely used as cost-effective metallic interconnect material due to the significant progress on reducing the operation temperature of SOFCs from 1000 C to intermediate temperature between 600 C and 800 C [1,2]. However, the application of the alloy as interconnect still encounters many challenges, even at the reduced temperatures [3,4]. The oxide layer formed on the surface of interconnect alloy after exposure in the operation environment will lead to high electrical
* Corresponding author. Fax: þ86 27 87558142. E-mail address: [email protected] (J. Pu). http://dx.doi.org/10.1016/j.ijhydene.2016.07.061 0360-3199/© 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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resistance and degradations in SOFCs. Furthermore, the volatile Cr species such as CrO3 in the dry air and CrO2(OH)2 in the humid atmosphere can be formed from the oxide scale on surface of interconnect alloy and then deposit on the electrochemical active region of cathode material. The poisoning effect on the performance of cathode has been investigated on the LSM [5,6], LSCF [7,8]. Recently, A site substitution cathode Sm0.5Sr0.5MnO3 (SSM) has received widespread attention for potential application of IT-SOFCs cathode because of the remarkable high oxygen reduction reaction activity [9,10], thermal stability [11], higher electrical conductivity [12] and appropriate thermal expansion coefficient with the conventional electrolyte [13]. However, much investigation was focused on the structure, conductivity and electrochemical performance of SSM, the Cr poisoning effect on the electrochemical performance of SSM cathode is unknown, and Cr tolerant ability is essential for long-term operation of SOFCs. In this study, we report the research results of the Cr poisoning effect on the SSM cathode under the SOFC operation conditions based on microstructure and electrochemical impedance spectroscopy (EIS) analysis.
Experimental GDC (Ce0.9Gd0.1O2d, Fuel Cells Material Ltd) electrolyte powder was pressed into pellets and subsequently sintered at 1450 C for 6 h. The pellets were then polished on both side to obtain parallel surfaces with the dimensions of 8 mm in diameter and thickness of 5 mm Sm0.5Sr0.5MnO3(SSM) powder was prepared by a solegel method using Sm(NO3)3$6H2O, Sr(NO3)2, and Mn(NO3)3 (Sinopharm Chemical Reagent Co. Ltd). The synthesis procedure can be found in the literature [11]. The SSM powder was thoroughly mixed with the 5 wt % ethyl cellulose to obtain cathode paste, which was then screen printed on both side on the centre of GDC pellet surface with an active area of 0.316 cm2. A narrow groove was cut in the
middle of the pellet and Pt wire was arrayed along the groove to act as a reference electrode. The configuration of symmetric cell and test set-up is shown in Fig. 1. A piece of uncoated Crofer22 APU (Fuel Cells Material Ltd), a commercial FeeCr alloy was used as interconnect. The metallic interconnect was directly contacted with the cathode material that can served as the current collector and gas flow passage, similar to that in SOFC stacks. For comparison, a LSM was chosen as cathode and tested in the same conditions owing to the excellent performance in the SOFC. Electrochemical performance of SSM and LSM cathode was evaluated by a Solatron 1260, using a three-electrode configuration. Polarization behaviors of SSM and LSM electrodes were examined under a constant cathodic current density of 200 mA cm2 at 750 C for 1200 min. Current passages were interrupted in test process to collect the electrochemical impedance spectra at OCP. Impedance spectra of SSM were collected at OCP with a frequency range of 102 Hz to 105 Hz and an amplitude of 10 mV. The IeV measurements of SSM cathode with and without the presence of Crofe22 APU after 1200 min with the application of current were carried out between 0 and 1 V DC bias. Measurements were operated at 750 C with dry air every 30 min for 200 h. After 200 h of aging, temperature dependencies were measured for each sample. Polarization resistance (RP) was determined by the difference between the high and low frequency intercepts and ohmic resistance (RU) was measured by the high frequency intercept on the real axis. Impedance curve fittings were carried out by the ZSimpWin (EChem Software). The phase structure of as-prepared SSM powder and after aging with Cr2O3 were characterized by X-ray diffraction measurements (XRD, X'Pert Pro) with Cu Ka radiation, the scan covered 2q angle from 20 to 80 . The microstructure and element distributions of cathode surface contacted with the interconnect were observed using a field emission scanning electron microscope (FE-SEM, FEI Sirion200) with an attached energy dispersive spectrometer (EDS) to illuminate the degradation mechanism of the samples.
Results and discussion Phase analysis
Fig. 1 e Cell configuration and schematic diagram of the experimental setup for the measurement of electrochemical performance of cathode in presence of metallic interconnect.
Fig. 2 shows the XRD patterns of as prepared SSM powder and the SSM mixed with Cr2O3 which calcined at 750 C for 1200 min. The single perovskite phase was obtained for SSM without the presence of any secondary phases. The crystal structure of SSM was orthorhombic (JCPDS:01-089-0791). Based on the XRD analysis, it was concluded that the new phase SrCrO4(JCPDS:01-073-1082) has formed among SSM and Cr2O3(JCPDS:01-084-0312) composite powders. The detection of SrCrO4 indicates that the chemical reaction tend to carry through between segregated Sr from SSM and Cr species. The formation of SrCrO4 may be the most possible reason for cathode degradation caused by volatile Cr6þ from Crcontaining interconnect alloy.
Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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Fig. 2 e XRD patterns of as prepared SSM powder and the SSM mixed with Cr2O3 and calcined at 750 C for 1200 min.
Electrochemical performance In order to evaluate the electrochemical performance of SSM cathode in the stack environment, EIS test was carried out under a constant current of 200 mA cm2 in the presence of
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the Crofer22 APU at 750 C for 1200 min. In Fig. 3, the electrochemical performance of SSM in the absence of Crofer22 APU (Fig. 3a) and LSM in the presence of Crofer22 APU (Fig. 3e) were showed to compare with that of SSM in the presence of the alloy (Fig. 3c). In the absence of Crofer22 APU, the impedance responses of SSM were characterized by a depressed arc. The cathodic current treatment dramatically reduced the initial polarization losses, and improved the SSM electrode performance as indicated by the rapid reduction in RP, which was the same as the LSM cathode. The enhancement of reaction rate under cathodic polarization potential could be explained by the improved surface diffusion and dissociation at the TPB region due to a change in surface composition or formation of oxygen vacancies at the electrode surface [14e16]. As shown in Fig. 3b, the RP decreased from around 3.2 to 1.4 U cm2 in the absence of Crofer22 APU for SSM cathode whereas the RU was stable. However, in the presence of Crofer22 APU, the impedance arcs were shrank in the first 90 min and increased to the platform under a constant current density of 200 mA cm2 (Fig. 3c), which was quite different from the impedance results in the absence of Crofer22 APU. The detailed information of RP and RU are shown in Fig. 3d, the RU maintained stable as the SSM without Crofer22 APU, however, the RP decreased from around 3.3 to 2.3 U cm2 in the first 60 min and increased to 2.8 U cm2 in
Fig. 3 e The impedance curves of SSM in the absence of Crofer22 APU (Fig. 3a) and in the presence of Crofer22 APU (Fig. 3c), the impedance curves of LSM in the presence of Crofer22 APU (Fig. 3e) under the current density of 200 mA cm¡2 at 750 C for 1200 minutes. Fig. 3b, Fig. 3d, Fig. 3f is the change of RU and RP corresponding to Fig. 3a, Fig. 3c, Fig. 3e. Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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the next 180 min and kept stable at 2.8 U cm2 for the rest of 1200 min. The increase of RP from 2.3 U cm2 to 2.8 U cm2 indicates the degradation caused by Cr poisoning cathode effect. The increase of RP originated from the Cr poisoning effect on the ORR of SSM cathode. In fact there are two factors that affect the value of RP: the one is improved electrode activation under the application of the cathodic current, which results in a decrease in RP; the other is cathode Cr-poisoning of cathode caused by Cr volatilization from the oxide scale of interconnect alloy, which increases the value of RP. The final value of RP is determined by the competition of these two mechanisms. The LSM was chosen as cathode to compare with SSM, considering that it is a popular cathode material in SOFC. Fig. 3e shows the impedance spectrum of LSM cathode, the initial RP was much larger than the one for SSM because the electrical conductivity of SSM is about one magnitude higher than LSM presented in the literature [12]. The RP of LSM decreased from the initial 12.8 to 9.8 U cm2 at first 180 min and increased to 12.9 U cm2. The increase of RP from 9.8 U cm2 to 12.9 U cm2 indicates the degradation caused by Cr poisoning LSM effect. However, the increase value of RP for SSM, 0.5 U cm2, was much smaller than that of the LSM cathode which was 3.1 U cm2. In the same testing time, the degradation rate of LSM caused by Cr poisoning was higher than that of SSM under constant current, which may due to that the oxygen exchange kinetics for SSM are much faster than that of LSM [17]. The introduction of Cr vapor in the environment atmosphere will change the oxygen partial pressure, the Cr deposition on the surface of cathode can block the activate sites and oxygen vacancies for ORR. The SSM cathode is more sensitive to the change of oxygen partial pressure, therefore, the response to the Cr poisoning would be quicker than that of LSM. The RP for SSM reached the balance with a smaller value than the initial RP while the RP for LSM increased much larger value than the initial one with the Cr poisoning effect for 1200 min. The SSM cathode showed less Cr poisoning than LSM at the same operation environment with the current applied. From the electrochemical results shown above, SSM exhibited higher performance than LSM under the cathodic current in the presence of Crofer22 APU. Fig. 4 shows the current density as a function of overpotential for SSM and SSM after the Cr poisoning cathode for 1200 min aging at 750 C under a constant current density of 200 mA cm2. The exchange current density i0, the crucial property of cathode, representing the intrinsic ORR rate, can be obtained using ButlereVolmer equation which is expressed as [18,19]:
i ¼ i0[eaaFh/RT eacFh/RT]
(1)
where aa þ ac ¼ n/n
(2)
Fig. 4 e The IR-modified Tafel plots of pure SSM and SSM contacted with Crofer22 APU at 750 C with current passage for 1200 min. electrons employed per O2 molecule (n), the number of occurrences of the rate-limiting step (n); h is the activation overpotential; g is the number of electrons transferred before the rate-determining step (rds) in the reaction sequence, r is the number of electrons transferred during the rds. F is the Faraday constant, R is the gas constant, T is the absolute temperature. In the low overpotential region, the BeV equation is simplified to:
i ¼ i0h(nF/RTn)
(4)
In the high-field cathodic overpotential, i0 can be readily obtained by the following equation:
i ¼ i0eacFh/RT
(5)
The i0 for SSM cathode in the absence and presence with the Crofer22 APU were determined for both the low- and highoverpotential regions by using Equations (4) and (5) according to the Tafel curves shown in Fig. 4. The values were summarized in Table 1. The results shows the i0 for SSM decreased from 175.7 mA cm2 to 155.3 mA cm2 under low potential and from 166.9 mA cm2 to 51.8 mA cm2 under high potential after polarized at 200 mA cm2 for 1200 min in the presence of Cr alloy. As we know, i0 can be deprived from the charge transfer resistance from the impedance diagram according to the equation [18,19]:
i0 ¼ RTn/R1Fn
For the reduction of the oxygen, ac ¼ g/n þ rb
(3)
Where i is the electrode current density; aa and ac are the anodic and cathodic charge transfer coefficients, respectively, containing the symmetry factor (b), the total number of
(6)
Table 1 e Exchange current densities of SSM and SSM with the Cr poisoning cathodes at 750 C. T ( C)
750
i0 Low potential i0 (mA cm2) (mA cm2)
High potential
SSM
SSM with Cr
SSM
SSM with Cr
175.7
155.3
166.9
51.8
Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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However, the overlapping was observed between the high and low frequency arcs shown in Fig. 3b. It was difficult to distinguish the high and low frequency arcs and i0 was not calculated from Eq. (6). Because i0 was related to the charge transfer process, the Cr poisoning effect on cathode can be described by the charge transfer process of SSM with the current passage. Fig. 5 showed the impedance diagrams of SSM with Cr poisoning under open circuit potential (OCP) at 750 C in the air aging for 200 h. The real and fitted results are shown in spot lines and solid lines. The electrochemical circuit model RU(R1Q1)(R2Q2) is used to analyze the data. The circuit elements are in series and inserted in the Fig. 5a. In the models, RP ¼ R1 þ R2. R1 and R2 have the constant phase element (CPE, Q) in parallel to simulate the distribution of relaxation in the real system. Fig. 5a shows that the impedance curves fitted well with the equivalent circuit and R1 represents the polarization resistance associated with the high-frequency charge transfer process and R2 associated with low-frequency oxygen adsorption and dissociation processes [20e22]. According to the fitting results, the tendency of different resistance variation with time is presented in Fig. 5b. After long term aging at OCP, the stability of RU imply the excellent interface contact characteristic and little Cr poisoning effect on electrolyte for long term operation. RP increased from around 3.9 to 7.0 U cm2 in the first 180 h, and kept stable at 7.0 U cm2 to 200 h, which indicates that the Cr poisoning effect on SSM cathode has reached to an equilibration. The value of R1 was stable in 200 h while R2 increased with the same tendency as RP. As it is reported, smaller size for A site element made SSM exhibited
Fig. 5 e The impedance diagrams of SSM with Cr poisoning under open circuit potential (OCP) at 750 C in the air aging for 200 h (a) and the change of R1, R2 and RP (b).
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higher Mn4þ/Mn3þ reduction ability than that of LSM, the reduction of Mn4þ to Mn3þ can produce more oxygen vacancies which can speed up the transport of surface oxygen to the triple-phase-boundary. Therefore, the value of R1 can be maintained at a very low and stable level, even in the presence of Cr [17]. Therefore, the degradation of RP mainly originates from the increase of R2 which represents the oxygen absorption and dissociation processes. It can be inferred that there would be traces of Cr depositions on the surface of SSM electrode, block the activation sites and hindered the surface absorption and dissociation reactions, which the Cr deposition would be the SrCrO4 phase according to the XRD results. After 200 h of aging, temperature dependencies were measured for the SSM cathode. In Fig. 6, the activation energy of RP for SSM contacted with Crofer22 APU is 147.9 kJ mol1, which is higher than that of RP for SSM without Cr contamination which is 136.2 kJ mol1 reported by Dong [13]. It is obvious that the Cr poisoning increased the activation energy of RP and decreased the ORR activity of SSM cathode.
Composition and microstructure analysis Fig. 7 presents the SEM microstructure of the surface of the SSM cathode and the boundary of SSM/GDC before and after polarization at 200 mA cm2 and 750 C for 1200 min, in comparison, the microstructure of surface for LSM is also shown in Fig. 7(g) and (h). Fig. 7 a, c, e is the surface of as prepared SSM cathode, the surface of SSM after polarization for 1200 min, the surface near the boundary of SSM and GDC in the presence of Crofer22 APU after polarization for 1200 min respectively. Fig. 7 b, d, f is the boundary area between SSM and GDC corresponding to the Fig. 7 a, c, e. The microstructure of as prepared SSM cathode surface was characterized by wellinterconnected SSM grains with the size range of 200e400 nm. The edge of the SSM cathode was clean and uniform in crystal structure as shown in Fig. 7a. Fig. 7c, d presents the SSM cathode surface and the edge under current density of 200 mA cm2 for 1200 min. The grain size and the connection between each particles exhibited similar morphology compared with that of SSM without current
Fig. 6 e Arrhenius of RP for different SSM after aging with Crofer22 APU at the OCP and 750 C for 200 h.
Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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Fig. 7 e The surface of as prepared SSM cathode (a), the surface of SSM after polarization for 1200 min (c), the surface near the boundary of SSM and GDC in the presence of Crofer22 APU after polarization for 1200 min (e), the surface of LSM cathode after polarization for 1200 min (g) and the b, d, f, h is the boundary area between SSM and GDC corresponding to the a, c, e, g.
passage as shown in Fig. 7a and b. It indicates that the current has no obvious effect on the structure of SSM cathode in the absence of Cr. However, significant changes in morphology of SSM cathode were observed after the polarization in the presence of the Crofer22 APU. Deposits were found in the area
near the edge of cathode surface as shown in Fig. 7e. The surface area far from the boundary of cathode and electrolyte was as clean as the SSM in the absence of Crofer22 APU presented in Fig. 7b. However, a deposition zone with a width of approximately 15 mm was observed on the edge of SSM
Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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cathode in the presence of Crofer22 APU. In Fig. 7f, the deposition zone consisted of two kinds of particles with distinct size and shape. Fig. 7g shows the surface of LSM in the presence of Crofer22 APU for 20 h, the surface was clean and the LSM particles was well connected with each other which indicates that no Cr particles was deposited on the surface of LSM cathode. However, as shown in Fig. 7h, the boundary of LSM cathode was covered by the depositional spinel-type particles, which with the size of 4e5 mm. According to the literature, the deposits at the edge of LSM cathode was Cr2O3 and (Cr,Mn)3O4 [5,23e25]. The Cr deposits of LSM cathode were quite different from the Cr deposits of SSM cathode. Fig. 8a shows the detailed information of deposition zone in Fig. 7f and the corresponding EDS analysis. Fig. 8b presents the dense deposits particles with grain size of 4e5 mm close to the SSM cathode. According to the corresponding EDS results,
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the deposits were so dense that substrate SSM cathode can not be fully detected, only Sr, Cr and little Mn was found. The atomic ratio of Sr and Cr was almost 1:1, it can be inferred that the Cr depositional particles were SrCrO4. However, near the GDC electrolyte, bar-like lamellar crystals with size of 0.5e1 mm were formed, the corresponding EDS analysis suggested the lighter layer of sediments were Cr oxide containing parts of Mn. It can be concluded that for SSM cathode material, under the current passage, the Cr deposits prefer to form on the edge of cathode, and presents two distinct Cr oxide particles, one is dense deposits of the SrCrO4 near the SSM electrode, another may be Cr oxide contained Mn element. According to Jiang [25], the formation of Cr deposits can be explained by theory of the nucleation reaction which is also the Cr poisoning mechanism for LSM. However, the deposits were mainly comprised of SrCrO4 but not (Mn,Cr)3O4 for LSM
Fig. 8 e (a) The detailed information of deposition zone is shown in Fig. 7f. The Cr deposition area near the SSM cathode (b), and near the GDC electrolyte (c) and the corresponding EDS (d) and (e) according to the deposition particles in (b) and (c). Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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cathode. The less conductive Cr particles would block ORR active sites and lead to the electrochemical degradation of SSM cathode. The Cr caused electrochemical degradation of cathode was also investigated in the OCP. Fig. 9a shows the surface morphology of SSM after 200 h of aging with Crofer22 APU in the OCP. It can be found that the deposits with crystal size of 2e5 mm were distributed on the SSM electrode layer which was determined to be SrCrO4 by EDS analysis as shown in Fig. 9c. However, the boundary of cathode and electrolyte was clean without any deposits. This indicates that Cr deposition has significant influence on SSM cathode with the current passage. After comparing the electrochemical performance of SSM cathode under the constant current passage with the one in the open circuit potential in the presence of Crofer22 APU, different Cr poisoning phenomenon could be revealed. Under the current passage, Cr deposits were mainly formed on the boundary area of electrode and electrolyte, and presented in two distinct morphologies, mainly comprised of SrCrO4 and Cr oxide. However, the Cr particles were mainly deposited on the surface of electrode not the edge area in the OCP, and the depositional particles were single SrCrO4 phase. According to the literature [7], the nucleation and grain growth of Cr deposition processes can be described as follows:
CrO3 þ SrO ¼ CreSreO(nuclei)
(7)
CreSreO (nuclei) þ CrO3 ¼ Cr2O3
(8)
Cr2O3 þ CrO3 þ SrO ¼ SrCrO4
(9)
The formation of less conductive SrCrO4 particles can block the active sites for the O2 reduction reaction, resulting in the significant increase in polarization resistance, as shown in Figs. 3 and 5. Furthermore, continuous growth of SrCrO4 phase may lead to excess Sr isolation from SSM and the loss of the electrochemical activity. However, according to the electrochemical results shown in Figs. 3d and 5b, the cathode degradation caused by Cr poisoning could be suspended which indicates that there exists a limitation for the effect of Cr poisoning for SSM cathode material. The main deposits for SSM is SrCrO4 phase which has been confirmed by the XRD results because the SrCrO4 is easily formed by chemical reaction between segregated Sr from SSM and Cr2O3. Fig. 10 illustrate the mechanism of the formation of SrCrO4 but not (Mn,Cr)3O4 on the surface of SSM cathode in the
Fig. 9 e The surface (a) and the boundary morphology (b) of SSM after 200 h of aging with Crofer22 APU in the OCP, EDS analysis of the depositional particles on the surface of SSM (c). Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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Fig. 10 e Illustration of the effect of the smaller atomic size of Sm than La on the performance of SSM cathode in the presence of Cr.
presence of Cr. Generally, the size mismatch between the dopant and host cations contributes significantly to the increase of the elastic energy, the segregation of the dopant cations of A-site toward the surface of the manganite-based perovskite oxides can minimize the elastic energy, and formation of secondary phase at the surface of oxides. The larger size mismatch results in more cation rearrangement, and dopant cations separate out more actively than the host cation to form dopant rich secondary phase that are distributed heterogeneously at the surface [26]. The RX is denoted as the size of X, therefore, (RSr RSm)/RSm is 16.1 for SSM, which is higher than 0.59 of (RSr RLa)/RLa in LSM. The Sr is more easier to segregate from SSM, thus the Cr depositional particles is SrCrO4 for SSM rather than (Mn,Cr)3O4 for the LSM cathode. The segregation of the Sr may induce the A-site deficiency of SSM cathode and greatly improve its electrochemical performance and slow down Cr deposition [27]. It can be manifested by the slowly Cr degradation of SSM. The current may help the transform of the segregated Sr oxide and influence on the depositional area for Cr particles. On the other hand, smaller atomic radius of Sm than La may decrease the MneO length by increasing the overlapping area between the O 2p and Mn 3d, resulting in better connections of MneO and less segregation of Mn toward the surface of SSM cathode [11]. Considering the two factors discussed above, the Cr deposits for SSM cathode could be SrCrO4 but not (Mn,Cr)3O4, and the SSM cathode shows higher electrochemical performance under the SOFC stack environment than LSM. However, the way to prevent the Cr poisoning for SSM cathode will be further investigated in the future.
Conclusions The electrochemical performance of Sm0.5Sr0.5MnO3 in the presence of Crofer22 APU at 750 C was investigated under constant current passage and OCP respectively. Based on the XRD, impedance, polarization, SEM-EDS results, the following conclusion could be drawn: (1) The main depositional product is SrCrO4 for SSM cathode which is quite different from the depositional species (Mn,Cr)3O4 for LSM cathode. (2) The Cr particles formed on the surface of SSM in the OCP, but it deposits on the edge of cathode and electrolyte under constant current passage. (3) The performance degradation induced by Cr poisoning cathode could be suspended which implies that SSM cathode can mitigate the poisoning effect, which is different from conventional LSM cathode.
Acknowledgement This research was financially supported by National Natural Science Foundation of China (51271083) and Guangdong Province (2013B090500051), Shenzhen City (JCYJ20140419131733975) and Shandong Province (2015ZDXX0602A02). The SEM characterizations were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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references
[1] Wachsman ED, Lee KT. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935e9. [2] Choy K, Bai W, Charojrochkul S, Steele BCH. The development of intermediate-temperature solid oxide fuel cells for the next millennium. J Power Sources 1998;71:361e9. [3] Chu C-L, Wang J-Y, Lee S. Effects of La0.67Sr0.33MnO3 protective coating on SOFC interconnect by plasmasputtering. Int J Hydrogen Energy 2008;33:2536e46. [4] Liu Y, Chen DY. Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells. Int J Hydrogen Energy 2009;34:9220e6. [5] Jiang SP, Zhang JP, Apateanu L, Foger K. Deposition of chromium species at Sr doped LaMnO3 electrodes in solid oxide fuel cells. I. Mechanism and kinetics. J Electrochem Soc 2000;147:4013e22. [6] Taniguchi S, Kadowaki M, Kawamura H, Yasuo T, Akiyama Y, Miyake Y, et al. Degradation phenomena in the cathode of a solid oxide fuel cell with an alloy separator. J Power Sources 1995;55:73e9. [7] Jiang SP, Zhang S, Zhen YD. Deposition of Cr species at (La,Sr)(Co,Fe)O3 cathodes of solid oxide fuel cells. J Electrochem Soc 2006;153:A127e34. [8] Simner SP, Anderson MD, Engelhard MH, Stevenson JW. Degradation mechanisms of LaSrCoFeO3 SOFC cathodes. Electrochem Solid-State Lett 2006;9:A478e81. [9] Choi Y, Lin MC, Liu M. Rational design of novel cathode materials in solid oxide fuel cells using first-principles simulations. J Power Sources 2010;195:1441e5. [10] Jia L, Wang X, Li W, Li K, Chi B, Pu J, et al. Theoretical study on SmxSr1xMnO3 as a potential solid oxide fuel cell cathode. J Power Sources 2014;253:138e42. [11] Li W, Pu J, Chi B, Jian L. Sm0.5Sr0.5MnO3 as a potential cathode material for intermediate temperature solid oxide fuel cells. Electrochim Acta 2014;141:189e94. [12] Wen T-L, Tu H, Xu Z, Yamamoto O. A study of (Pr,Nd,Sm)1xSrxMnO3 cathode materials for solid oxide fuel cell. Solid State Ionics 1999;121:25e30. [13] Dong F, Chen D, Ran R, Park H, Kwak C, Shao Z. A comparative study of Sm0.5Sr0.5MO3d (M ¼ Co and Mn) as oxygen reduction electrodes for solid oxide fuel cells. Int J Hydrogen Energy 2012;37:4377e87. [14] Jiang SP, Zhang JP, Foger K. Deposition of chromium species at Sr doped LaMnO3 electrodes in solid oxide fuel cells II. Effect on O2 reduction reaction. J Electrochem Soc 2000;147:3195e205.
[15] Jiang SP, Love JG, Zhang JP, Hoang M, Ramprakash Y, Hughes AE, et al. The electrochemical performance of LSM/ zirconiaeyttria interface as a function of a-site nonstoichiometry and cathodic current treatment. Solid State Ionics 1999;121:1e10. [16] Hammouche A, Siebert E, Hammou A, Kleitz M, Caneiro A. Electrocatalytic properties and nonstoichiometry of the high temperature air electrode La1 xSrx MnO3. J Electrochem Soc 1991;138:1212e6. [17] Li W, Xiong CY, Jia LC, Pu J, Chi B, Chen X, et al. Strontiumdoped samarium manganite as cathode materials for oxygen reduction reaction in solid oxide fuel cells. J Power Sources 2015;284:272e8. [18] Co AC, Xia SJ, Birss VI. A kinetic study of the oxygen reduction reaction at LaSrMnO3-YSZ composite electrodes. J Electrochem Soc 2005;152:A570e6. [19] Soderberg JN, Co AC, Sirk AHC, Birss VI. Impact of porous electrode properties on the electrochemical transfer coefficient. J Phys Chem B 2006;110:10401e10. [20] Fukunaga H, Koyama M, Takahashi N, Wen C, Yamada K. Reaction model of dense Sm0.5Sr0.5CoO3 as SOFC cathode. Solid State Ionics 2000;132:279e85. [21] Jiang SP, Chen XJ, Chan SH, Kwok JT, Khor KA. (La0.75Sr0.25)(Cr0.5Mn0.5)O3YSZ composite anodes for methane oxidation reaction in solid oxide fuel cells. Solid State Ionics 2006;177:149e57. [22] Liang F, Chen J, Chi B, Pu J, Jiang SP, Jian L. Redox behavior of supported Pd particles and its effect on oxygen reduction reaction in intermediate temperature solid oxide fuel cells. J Power Sources 2011;196:153e8. [23] Xiong C, Li W, Xiao J, Duan N, Pu J, Chi B, et al. Improved chromium-poisoning on lanthanum strontium manganite cathode in presence of a newly developed iron-chromium based interconnect alloy for solid oxide fuel cells. Int J Hydrogen Energy 2015;40:13957e63. [24] Backhaus-Ricoult M, Adib K, St. Clair T, Luerssen B, Gregoratti L, Barinov A. In-situ study of operating SOFC LSM/ YSZ cathodes under polarization by photoelectron microscopy. Solid State Ionics 2008;179:891e5. [25] Jiang SP, Chen X. Chromium deposition and poisoning of cathodes of solid oxide fuel cells e a review. Int J Hydrogen Energy 2014;39:505e31. [26] Lee W, Han JW, Chen Y, Cai Z, Yildiz B. Cation size mismatch and charge interactions drive dopant segregation at the surfaces of manganite perovskites. J Am Chem Soc 2013;135:7909e25. [27] Fu CJ, Sun KN, Chen XB, Zhang NQ, Zhou DR. Electrochemical properties of A-site deficient SOFC cathodes under Cr poisoning conditions. Electrochim Acta 2009;54:7305e12.
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Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell Chunyan Xiong a, Wenlu Li b, Dong Ding c, Jian Pu a,*, Bo Chi a, Jian Li a,d a
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 b International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, Hubei, China c Idaho National Laboratory, P.O. Box 1625, MS 3732, Idaho Falls, ID 83415, USA d Research Institute of Huazhong University of Science and Technology in Shenzhen, Shenzhen, Guangdong 518000, China
article info
abstract
Article history:
Strontium-doped samarium manganite is a potential cathode for solid oxide fuel cells
Received 25 May 2016
(SOFCs) with remarkable high oxygen reduction reaction activity. Here we investigated
Received in revised form
chromium poisoning effect on Sm0.5Sr0.5MnO3 cathode of SOFCs for the first time. The Cr
7 July 2016
caused cathode degradation is studied under current density of 200 mA cm2 and open
Accepted 9 July 2016
circuit potential (OCP) at 750 C. After polarized in the presence of the Crofer22 APU at
Available online xxx
750 C for 1200 min, the polarization resistance decreases from 3.25 U cm2 to 2.25 U cm2, then increases to a stable value of 2.75 U cm2. The degradation rate of SSM is lower than
Keywords:
that of LSM cathode in the same experimental environment. At OCP, the polarization
Solid oxide fuel cell
resistance increases to 7.00 U cm2 and reaches a stable level. SEM and EDX shows the
Cathode
depositions on the Sm0.5Sr0.5MnO3 boundary after applying current for 1200 min, and on
Chromium poisoning
the SSM surface after aging at OCP for 200 h. The Cr depositions are mainly comprised of
Durability
SrCrO4 formed by the nucleation reaction. The results show that SSM is a poisoning
Strontium-doped samarium
tolerant cathode and good replacement for the LSM in IT-SOFCs due to the better elec-
manganite
trochemical performance and the relatively stable characteristics after Cr poisoning. © 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction The demand for clean, secure and sustainable energy sources has stimulated great interests in the electrochemical energy storage and conversion technologies such as batteries, fuel cells, supercapacitors and electrolysers. Solid Oxide Fuel Cells (SOFCs) are considered as one of the most promising energy exchange devices which show advantages of high efficiency,
fuel flexible and low emission. Chromium-forming alloy are widely used as cost-effective metallic interconnect material due to the significant progress on reducing the operation temperature of SOFCs from 1000 C to intermediate temperature between 600 C and 800 C [1,2]. However, the application of the alloy as interconnect still encounters many challenges, even at the reduced temperatures [3,4]. The oxide layer formed on the surface of interconnect alloy after exposure in the operation environment will lead to high electrical
* Corresponding author. Fax: þ86 27 87558142. E-mail address: [email protected] (J. Pu). http://dx.doi.org/10.1016/j.ijhydene.2016.07.061 0360-3199/© 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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resistance and degradations in SOFCs. Furthermore, the volatile Cr species such as CrO3 in the dry air and CrO2(OH)2 in the humid atmosphere can be formed from the oxide scale on surface of interconnect alloy and then deposit on the electrochemical active region of cathode material. The poisoning effect on the performance of cathode has been investigated on the LSM [5,6], LSCF [7,8]. Recently, A site substitution cathode Sm0.5Sr0.5MnO3 (SSM) has received widespread attention for potential application of IT-SOFCs cathode because of the remarkable high oxygen reduction reaction activity [9,10], thermal stability [11], higher electrical conductivity [12] and appropriate thermal expansion coefficient with the conventional electrolyte [13]. However, much investigation was focused on the structure, conductivity and electrochemical performance of SSM, the Cr poisoning effect on the electrochemical performance of SSM cathode is unknown, and Cr tolerant ability is essential for long-term operation of SOFCs. In this study, we report the research results of the Cr poisoning effect on the SSM cathode under the SOFC operation conditions based on microstructure and electrochemical impedance spectroscopy (EIS) analysis.
Experimental GDC (Ce0.9Gd0.1O2d, Fuel Cells Material Ltd) electrolyte powder was pressed into pellets and subsequently sintered at 1450 C for 6 h. The pellets were then polished on both side to obtain parallel surfaces with the dimensions of 8 mm in diameter and thickness of 5 mm Sm0.5Sr0.5MnO3(SSM) powder was prepared by a solegel method using Sm(NO3)3$6H2O, Sr(NO3)2, and Mn(NO3)3 (Sinopharm Chemical Reagent Co. Ltd). The synthesis procedure can be found in the literature [11]. The SSM powder was thoroughly mixed with the 5 wt % ethyl cellulose to obtain cathode paste, which was then screen printed on both side on the centre of GDC pellet surface with an active area of 0.316 cm2. A narrow groove was cut in the
middle of the pellet and Pt wire was arrayed along the groove to act as a reference electrode. The configuration of symmetric cell and test set-up is shown in Fig. 1. A piece of uncoated Crofer22 APU (Fuel Cells Material Ltd), a commercial FeeCr alloy was used as interconnect. The metallic interconnect was directly contacted with the cathode material that can served as the current collector and gas flow passage, similar to that in SOFC stacks. For comparison, a LSM was chosen as cathode and tested in the same conditions owing to the excellent performance in the SOFC. Electrochemical performance of SSM and LSM cathode was evaluated by a Solatron 1260, using a three-electrode configuration. Polarization behaviors of SSM and LSM electrodes were examined under a constant cathodic current density of 200 mA cm2 at 750 C for 1200 min. Current passages were interrupted in test process to collect the electrochemical impedance spectra at OCP. Impedance spectra of SSM were collected at OCP with a frequency range of 102 Hz to 105 Hz and an amplitude of 10 mV. The IeV measurements of SSM cathode with and without the presence of Crofe22 APU after 1200 min with the application of current were carried out between 0 and 1 V DC bias. Measurements were operated at 750 C with dry air every 30 min for 200 h. After 200 h of aging, temperature dependencies were measured for each sample. Polarization resistance (RP) was determined by the difference between the high and low frequency intercepts and ohmic resistance (RU) was measured by the high frequency intercept on the real axis. Impedance curve fittings were carried out by the ZSimpWin (EChem Software). The phase structure of as-prepared SSM powder and after aging with Cr2O3 were characterized by X-ray diffraction measurements (XRD, X'Pert Pro) with Cu Ka radiation, the scan covered 2q angle from 20 to 80 . The microstructure and element distributions of cathode surface contacted with the interconnect were observed using a field emission scanning electron microscope (FE-SEM, FEI Sirion200) with an attached energy dispersive spectrometer (EDS) to illuminate the degradation mechanism of the samples.
Results and discussion Phase analysis
Fig. 1 e Cell configuration and schematic diagram of the experimental setup for the measurement of electrochemical performance of cathode in presence of metallic interconnect.
Fig. 2 shows the XRD patterns of as prepared SSM powder and the SSM mixed with Cr2O3 which calcined at 750 C for 1200 min. The single perovskite phase was obtained for SSM without the presence of any secondary phases. The crystal structure of SSM was orthorhombic (JCPDS:01-089-0791). Based on the XRD analysis, it was concluded that the new phase SrCrO4(JCPDS:01-073-1082) has formed among SSM and Cr2O3(JCPDS:01-084-0312) composite powders. The detection of SrCrO4 indicates that the chemical reaction tend to carry through between segregated Sr from SSM and Cr species. The formation of SrCrO4 may be the most possible reason for cathode degradation caused by volatile Cr6þ from Crcontaining interconnect alloy.
Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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Fig. 2 e XRD patterns of as prepared SSM powder and the SSM mixed with Cr2O3 and calcined at 750 C for 1200 min.
Electrochemical performance In order to evaluate the electrochemical performance of SSM cathode in the stack environment, EIS test was carried out under a constant current of 200 mA cm2 in the presence of
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the Crofer22 APU at 750 C for 1200 min. In Fig. 3, the electrochemical performance of SSM in the absence of Crofer22 APU (Fig. 3a) and LSM in the presence of Crofer22 APU (Fig. 3e) were showed to compare with that of SSM in the presence of the alloy (Fig. 3c). In the absence of Crofer22 APU, the impedance responses of SSM were characterized by a depressed arc. The cathodic current treatment dramatically reduced the initial polarization losses, and improved the SSM electrode performance as indicated by the rapid reduction in RP, which was the same as the LSM cathode. The enhancement of reaction rate under cathodic polarization potential could be explained by the improved surface diffusion and dissociation at the TPB region due to a change in surface composition or formation of oxygen vacancies at the electrode surface [14e16]. As shown in Fig. 3b, the RP decreased from around 3.2 to 1.4 U cm2 in the absence of Crofer22 APU for SSM cathode whereas the RU was stable. However, in the presence of Crofer22 APU, the impedance arcs were shrank in the first 90 min and increased to the platform under a constant current density of 200 mA cm2 (Fig. 3c), which was quite different from the impedance results in the absence of Crofer22 APU. The detailed information of RP and RU are shown in Fig. 3d, the RU maintained stable as the SSM without Crofer22 APU, however, the RP decreased from around 3.3 to 2.3 U cm2 in the first 60 min and increased to 2.8 U cm2 in
Fig. 3 e The impedance curves of SSM in the absence of Crofer22 APU (Fig. 3a) and in the presence of Crofer22 APU (Fig. 3c), the impedance curves of LSM in the presence of Crofer22 APU (Fig. 3e) under the current density of 200 mA cm¡2 at 750 C for 1200 minutes. Fig. 3b, Fig. 3d, Fig. 3f is the change of RU and RP corresponding to Fig. 3a, Fig. 3c, Fig. 3e. Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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the next 180 min and kept stable at 2.8 U cm2 for the rest of 1200 min. The increase of RP from 2.3 U cm2 to 2.8 U cm2 indicates the degradation caused by Cr poisoning cathode effect. The increase of RP originated from the Cr poisoning effect on the ORR of SSM cathode. In fact there are two factors that affect the value of RP: the one is improved electrode activation under the application of the cathodic current, which results in a decrease in RP; the other is cathode Cr-poisoning of cathode caused by Cr volatilization from the oxide scale of interconnect alloy, which increases the value of RP. The final value of RP is determined by the competition of these two mechanisms. The LSM was chosen as cathode to compare with SSM, considering that it is a popular cathode material in SOFC. Fig. 3e shows the impedance spectrum of LSM cathode, the initial RP was much larger than the one for SSM because the electrical conductivity of SSM is about one magnitude higher than LSM presented in the literature [12]. The RP of LSM decreased from the initial 12.8 to 9.8 U cm2 at first 180 min and increased to 12.9 U cm2. The increase of RP from 9.8 U cm2 to 12.9 U cm2 indicates the degradation caused by Cr poisoning LSM effect. However, the increase value of RP for SSM, 0.5 U cm2, was much smaller than that of the LSM cathode which was 3.1 U cm2. In the same testing time, the degradation rate of LSM caused by Cr poisoning was higher than that of SSM under constant current, which may due to that the oxygen exchange kinetics for SSM are much faster than that of LSM [17]. The introduction of Cr vapor in the environment atmosphere will change the oxygen partial pressure, the Cr deposition on the surface of cathode can block the activate sites and oxygen vacancies for ORR. The SSM cathode is more sensitive to the change of oxygen partial pressure, therefore, the response to the Cr poisoning would be quicker than that of LSM. The RP for SSM reached the balance with a smaller value than the initial RP while the RP for LSM increased much larger value than the initial one with the Cr poisoning effect for 1200 min. The SSM cathode showed less Cr poisoning than LSM at the same operation environment with the current applied. From the electrochemical results shown above, SSM exhibited higher performance than LSM under the cathodic current in the presence of Crofer22 APU. Fig. 4 shows the current density as a function of overpotential for SSM and SSM after the Cr poisoning cathode for 1200 min aging at 750 C under a constant current density of 200 mA cm2. The exchange current density i0, the crucial property of cathode, representing the intrinsic ORR rate, can be obtained using ButlereVolmer equation which is expressed as [18,19]:
i ¼ i0[eaaFh/RT eacFh/RT]
(1)
where aa þ ac ¼ n/n
(2)
Fig. 4 e The IR-modified Tafel plots of pure SSM and SSM contacted with Crofer22 APU at 750 C with current passage for 1200 min. electrons employed per O2 molecule (n), the number of occurrences of the rate-limiting step (n); h is the activation overpotential; g is the number of electrons transferred before the rate-determining step (rds) in the reaction sequence, r is the number of electrons transferred during the rds. F is the Faraday constant, R is the gas constant, T is the absolute temperature. In the low overpotential region, the BeV equation is simplified to:
i ¼ i0h(nF/RTn)
(4)
In the high-field cathodic overpotential, i0 can be readily obtained by the following equation:
i ¼ i0eacFh/RT
(5)
The i0 for SSM cathode in the absence and presence with the Crofer22 APU were determined for both the low- and highoverpotential regions by using Equations (4) and (5) according to the Tafel curves shown in Fig. 4. The values were summarized in Table 1. The results shows the i0 for SSM decreased from 175.7 mA cm2 to 155.3 mA cm2 under low potential and from 166.9 mA cm2 to 51.8 mA cm2 under high potential after polarized at 200 mA cm2 for 1200 min in the presence of Cr alloy. As we know, i0 can be deprived from the charge transfer resistance from the impedance diagram according to the equation [18,19]:
i0 ¼ RTn/R1Fn
For the reduction of the oxygen, ac ¼ g/n þ rb
(3)
Where i is the electrode current density; aa and ac are the anodic and cathodic charge transfer coefficients, respectively, containing the symmetry factor (b), the total number of
(6)
Table 1 e Exchange current densities of SSM and SSM with the Cr poisoning cathodes at 750 C. T ( C)
750
i0 Low potential i0 (mA cm2) (mA cm2)
High potential
SSM
SSM with Cr
SSM
SSM with Cr
175.7
155.3
166.9
51.8
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However, the overlapping was observed between the high and low frequency arcs shown in Fig. 3b. It was difficult to distinguish the high and low frequency arcs and i0 was not calculated from Eq. (6). Because i0 was related to the charge transfer process, the Cr poisoning effect on cathode can be described by the charge transfer process of SSM with the current passage. Fig. 5 showed the impedance diagrams of SSM with Cr poisoning under open circuit potential (OCP) at 750 C in the air aging for 200 h. The real and fitted results are shown in spot lines and solid lines. The electrochemical circuit model RU(R1Q1)(R2Q2) is used to analyze the data. The circuit elements are in series and inserted in the Fig. 5a. In the models, RP ¼ R1 þ R2. R1 and R2 have the constant phase element (CPE, Q) in parallel to simulate the distribution of relaxation in the real system. Fig. 5a shows that the impedance curves fitted well with the equivalent circuit and R1 represents the polarization resistance associated with the high-frequency charge transfer process and R2 associated with low-frequency oxygen adsorption and dissociation processes [20e22]. According to the fitting results, the tendency of different resistance variation with time is presented in Fig. 5b. After long term aging at OCP, the stability of RU imply the excellent interface contact characteristic and little Cr poisoning effect on electrolyte for long term operation. RP increased from around 3.9 to 7.0 U cm2 in the first 180 h, and kept stable at 7.0 U cm2 to 200 h, which indicates that the Cr poisoning effect on SSM cathode has reached to an equilibration. The value of R1 was stable in 200 h while R2 increased with the same tendency as RP. As it is reported, smaller size for A site element made SSM exhibited
Fig. 5 e The impedance diagrams of SSM with Cr poisoning under open circuit potential (OCP) at 750 C in the air aging for 200 h (a) and the change of R1, R2 and RP (b).
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higher Mn4þ/Mn3þ reduction ability than that of LSM, the reduction of Mn4þ to Mn3þ can produce more oxygen vacancies which can speed up the transport of surface oxygen to the triple-phase-boundary. Therefore, the value of R1 can be maintained at a very low and stable level, even in the presence of Cr [17]. Therefore, the degradation of RP mainly originates from the increase of R2 which represents the oxygen absorption and dissociation processes. It can be inferred that there would be traces of Cr depositions on the surface of SSM electrode, block the activation sites and hindered the surface absorption and dissociation reactions, which the Cr deposition would be the SrCrO4 phase according to the XRD results. After 200 h of aging, temperature dependencies were measured for the SSM cathode. In Fig. 6, the activation energy of RP for SSM contacted with Crofer22 APU is 147.9 kJ mol1, which is higher than that of RP for SSM without Cr contamination which is 136.2 kJ mol1 reported by Dong [13]. It is obvious that the Cr poisoning increased the activation energy of RP and decreased the ORR activity of SSM cathode.
Composition and microstructure analysis Fig. 7 presents the SEM microstructure of the surface of the SSM cathode and the boundary of SSM/GDC before and after polarization at 200 mA cm2 and 750 C for 1200 min, in comparison, the microstructure of surface for LSM is also shown in Fig. 7(g) and (h). Fig. 7 a, c, e is the surface of as prepared SSM cathode, the surface of SSM after polarization for 1200 min, the surface near the boundary of SSM and GDC in the presence of Crofer22 APU after polarization for 1200 min respectively. Fig. 7 b, d, f is the boundary area between SSM and GDC corresponding to the Fig. 7 a, c, e. The microstructure of as prepared SSM cathode surface was characterized by wellinterconnected SSM grains with the size range of 200e400 nm. The edge of the SSM cathode was clean and uniform in crystal structure as shown in Fig. 7a. Fig. 7c, d presents the SSM cathode surface and the edge under current density of 200 mA cm2 for 1200 min. The grain size and the connection between each particles exhibited similar morphology compared with that of SSM without current
Fig. 6 e Arrhenius of RP for different SSM after aging with Crofer22 APU at the OCP and 750 C for 200 h.
Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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Fig. 7 e The surface of as prepared SSM cathode (a), the surface of SSM after polarization for 1200 min (c), the surface near the boundary of SSM and GDC in the presence of Crofer22 APU after polarization for 1200 min (e), the surface of LSM cathode after polarization for 1200 min (g) and the b, d, f, h is the boundary area between SSM and GDC corresponding to the a, c, e, g.
passage as shown in Fig. 7a and b. It indicates that the current has no obvious effect on the structure of SSM cathode in the absence of Cr. However, significant changes in morphology of SSM cathode were observed after the polarization in the presence of the Crofer22 APU. Deposits were found in the area
near the edge of cathode surface as shown in Fig. 7e. The surface area far from the boundary of cathode and electrolyte was as clean as the SSM in the absence of Crofer22 APU presented in Fig. 7b. However, a deposition zone with a width of approximately 15 mm was observed on the edge of SSM
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cathode in the presence of Crofer22 APU. In Fig. 7f, the deposition zone consisted of two kinds of particles with distinct size and shape. Fig. 7g shows the surface of LSM in the presence of Crofer22 APU for 20 h, the surface was clean and the LSM particles was well connected with each other which indicates that no Cr particles was deposited on the surface of LSM cathode. However, as shown in Fig. 7h, the boundary of LSM cathode was covered by the depositional spinel-type particles, which with the size of 4e5 mm. According to the literature, the deposits at the edge of LSM cathode was Cr2O3 and (Cr,Mn)3O4 [5,23e25]. The Cr deposits of LSM cathode were quite different from the Cr deposits of SSM cathode. Fig. 8a shows the detailed information of deposition zone in Fig. 7f and the corresponding EDS analysis. Fig. 8b presents the dense deposits particles with grain size of 4e5 mm close to the SSM cathode. According to the corresponding EDS results,
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the deposits were so dense that substrate SSM cathode can not be fully detected, only Sr, Cr and little Mn was found. The atomic ratio of Sr and Cr was almost 1:1, it can be inferred that the Cr depositional particles were SrCrO4. However, near the GDC electrolyte, bar-like lamellar crystals with size of 0.5e1 mm were formed, the corresponding EDS analysis suggested the lighter layer of sediments were Cr oxide containing parts of Mn. It can be concluded that for SSM cathode material, under the current passage, the Cr deposits prefer to form on the edge of cathode, and presents two distinct Cr oxide particles, one is dense deposits of the SrCrO4 near the SSM electrode, another may be Cr oxide contained Mn element. According to Jiang [25], the formation of Cr deposits can be explained by theory of the nucleation reaction which is also the Cr poisoning mechanism for LSM. However, the deposits were mainly comprised of SrCrO4 but not (Mn,Cr)3O4 for LSM
Fig. 8 e (a) The detailed information of deposition zone is shown in Fig. 7f. The Cr deposition area near the SSM cathode (b), and near the GDC electrolyte (c) and the corresponding EDS (d) and (e) according to the deposition particles in (b) and (c). Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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cathode. The less conductive Cr particles would block ORR active sites and lead to the electrochemical degradation of SSM cathode. The Cr caused electrochemical degradation of cathode was also investigated in the OCP. Fig. 9a shows the surface morphology of SSM after 200 h of aging with Crofer22 APU in the OCP. It can be found that the deposits with crystal size of 2e5 mm were distributed on the SSM electrode layer which was determined to be SrCrO4 by EDS analysis as shown in Fig. 9c. However, the boundary of cathode and electrolyte was clean without any deposits. This indicates that Cr deposition has significant influence on SSM cathode with the current passage. After comparing the electrochemical performance of SSM cathode under the constant current passage with the one in the open circuit potential in the presence of Crofer22 APU, different Cr poisoning phenomenon could be revealed. Under the current passage, Cr deposits were mainly formed on the boundary area of electrode and electrolyte, and presented in two distinct morphologies, mainly comprised of SrCrO4 and Cr oxide. However, the Cr particles were mainly deposited on the surface of electrode not the edge area in the OCP, and the depositional particles were single SrCrO4 phase. According to the literature [7], the nucleation and grain growth of Cr deposition processes can be described as follows:
CrO3 þ SrO ¼ CreSreO(nuclei)
(7)
CreSreO (nuclei) þ CrO3 ¼ Cr2O3
(8)
Cr2O3 þ CrO3 þ SrO ¼ SrCrO4
(9)
The formation of less conductive SrCrO4 particles can block the active sites for the O2 reduction reaction, resulting in the significant increase in polarization resistance, as shown in Figs. 3 and 5. Furthermore, continuous growth of SrCrO4 phase may lead to excess Sr isolation from SSM and the loss of the electrochemical activity. However, according to the electrochemical results shown in Figs. 3d and 5b, the cathode degradation caused by Cr poisoning could be suspended which indicates that there exists a limitation for the effect of Cr poisoning for SSM cathode material. The main deposits for SSM is SrCrO4 phase which has been confirmed by the XRD results because the SrCrO4 is easily formed by chemical reaction between segregated Sr from SSM and Cr2O3. Fig. 10 illustrate the mechanism of the formation of SrCrO4 but not (Mn,Cr)3O4 on the surface of SSM cathode in the
Fig. 9 e The surface (a) and the boundary morphology (b) of SSM after 200 h of aging with Crofer22 APU in the OCP, EDS analysis of the depositional particles on the surface of SSM (c). Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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Fig. 10 e Illustration of the effect of the smaller atomic size of Sm than La on the performance of SSM cathode in the presence of Cr.
presence of Cr. Generally, the size mismatch between the dopant and host cations contributes significantly to the increase of the elastic energy, the segregation of the dopant cations of A-site toward the surface of the manganite-based perovskite oxides can minimize the elastic energy, and formation of secondary phase at the surface of oxides. The larger size mismatch results in more cation rearrangement, and dopant cations separate out more actively than the host cation to form dopant rich secondary phase that are distributed heterogeneously at the surface [26]. The RX is denoted as the size of X, therefore, (RSr RSm)/RSm is 16.1 for SSM, which is higher than 0.59 of (RSr RLa)/RLa in LSM. The Sr is more easier to segregate from SSM, thus the Cr depositional particles is SrCrO4 for SSM rather than (Mn,Cr)3O4 for the LSM cathode. The segregation of the Sr may induce the A-site deficiency of SSM cathode and greatly improve its electrochemical performance and slow down Cr deposition [27]. It can be manifested by the slowly Cr degradation of SSM. The current may help the transform of the segregated Sr oxide and influence on the depositional area for Cr particles. On the other hand, smaller atomic radius of Sm than La may decrease the MneO length by increasing the overlapping area between the O 2p and Mn 3d, resulting in better connections of MneO and less segregation of Mn toward the surface of SSM cathode [11]. Considering the two factors discussed above, the Cr deposits for SSM cathode could be SrCrO4 but not (Mn,Cr)3O4, and the SSM cathode shows higher electrochemical performance under the SOFC stack environment than LSM. However, the way to prevent the Cr poisoning for SSM cathode will be further investigated in the future.
Conclusions The electrochemical performance of Sm0.5Sr0.5MnO3 in the presence of Crofer22 APU at 750 C was investigated under constant current passage and OCP respectively. Based on the XRD, impedance, polarization, SEM-EDS results, the following conclusion could be drawn: (1) The main depositional product is SrCrO4 for SSM cathode which is quite different from the depositional species (Mn,Cr)3O4 for LSM cathode. (2) The Cr particles formed on the surface of SSM in the OCP, but it deposits on the edge of cathode and electrolyte under constant current passage. (3) The performance degradation induced by Cr poisoning cathode could be suspended which implies that SSM cathode can mitigate the poisoning effect, which is different from conventional LSM cathode.
Acknowledgement This research was financially supported by National Natural Science Foundation of China (51271083) and Guangdong Province (2013B090500051), Shenzhen City (JCYJ20140419131733975) and Shandong Province (2015ZDXX0602A02). The SEM characterizations were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061
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Please cite this article in press as: Xiong C, et al., Chromium poisoning effect on strontium-doped samarium manganite for solid oxide fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.061