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Enhanced performance of symmetrical solid oxide fuel cells using a doped ceria buffer layer
Electrochimica Acta 208 (2016) 318–324
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced performance of symmetrical solid oxide fuel cells using a doped ceria buffer layer Dong Tiana,b , Bin Lina,b,c,* , Yang Yangb , Yonghong Chenb,** , Xiaoyong Lub , Zhigao Wangd , Wei Liua , Enrico Traversac,** a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Anhui Key Laboratory of Low temperature Co-fired Material, Huainan Normal University, Huainan 232001, PR China c International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China d State Grid Sichuan Electric Power Research Institute, Chengdu 610072, PR China
A R T I C L E I N F O
Article history: Received 3 December 2015 Received in revised form 27 April 2016 Accepted 27 April 2016 Available online 6 May 2016 Keywords: Symmetrical solid oxide fuel cell La0.8Sr0.2FeO3-d Buffer layer Electrochemical performance
A B S T R A C T
Enhanced performance of symmetrical solid oxide fuel cell (SSOFC) is reported by using a doped ceria buffer layer, which can solve the issues during the operation of traditional solid oxide fuel cells, such as carbon deposition and sulfur poisoning. In this work, cobalt-free perovskite oxide La0.8Sr0.2FeO3-d (LSF) is applied as a novel stable electrode material for symmetrical solid oxide fuel cell. The electrical conductivities of LSF are 141.1 Scm1 and 0.138 Scm1 in air and humidified H2 (3% H2O) at 800 C, respectively. Gadolinium doped ceria (GDC) buffer layer is fabricated by screen printing onto the YSZ electrolyte, which dramatically enhances the electrochemical performance by more than 90 percent at 700 C. The improvement of SSOFC performance is attributed to the elimination of reactivity and the optimization of interface between YSZ electrolyte and LSF electrode. These results demonstrate that the doped ceria buffer layer provides a highly repeatable route for further improving the performance of YSZbased SSOFC, with potentially important implications for developing cost-effective SSOFCs with huge application opportunities. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction As substitutes for traditional power generation devices, solid oxide fuel cells (SOFCs) have demonstrated several advantages, including high efficiency, fuel flexibility, low pollution and longterm stability [1]. However, several issues need to be solved for SOFC commercial development. For instance, Ni-YSZ, the state-ofthe art SOFC anode material promotes carbon or sulfur species deposits on its surface during operation with hydrocarbon fuels, resulting in deactivation of the catalysts. To solve this problem, symmetrical solid oxide fuel cells (SSOFCs) using a redox-stable catalyst as both anode and cathode have been proposed and have attracted significant interest [2–8]. The use of the same material as
* Corresponding author at: Anhui Key Laboratory of Low Temperature Co-fired Materials, Huainan Normal University, Huainan, Anhui, 232001, PR China. ** Corresponding authors. E-mail addresses: [email protected], [email protected] (B. Lin). http://dx.doi.org/10.1016/j.electacta.2016.04.189 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
both anode and cathode simultaneously can simplify the fabrication process, minimize inter-diffusion between components, enhance coking and sulfur tolerance by operating the anode as the cathode in turn [1,9,10], allowing the absorbed carbon and sulfur species burning out by oxidant [11]. Thus, the reliability of SOFC systems will be enhanced by adopting symmetrical electrodes [12]. To enable SSOFCs working at different atmospheres, the electrode materials require suitable characterizatics for both anode and cathode environments, such as stable structure, good catalytic activities and high conductivities. Several types of materials have been used as SSOFC electrodes since the first La0.75Sr0.25Cr0.5Mn0.5O3-d (LSCM) was proposed by Bastidas et al. [2]. Among these materials, perovskite oxides have been evaluated as the most suitable electrodes due to their special structure [13]. Although Co-based perovskite electrodes have shown high performance for SSOFCs, their structure stability is still a big problem. Extensive work was focused on Fe-based
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perovskites [4–7,14], which are promising SSOFC electrode materials with reasonable stability. Zhou et al. [15] showed that a LaSrFe2CrO9d-based symmetrical solid oxide fuel cell achieved a maximum power density of 224 mW/cm2 at 800 C using humidified hydrogen as a fuel, which could be further improved by making composite electrodes with 50 wt.% Gd0.2Ce0.8O2-d. Yang et al. [16] reported a good performance for Ga-doped lanthanum strontium ferrite (La0.7Sr0.3Fe0.7Ga0.3O3-d, LSFG) as potential symmetrical electrodes. Using humidified hydrogen as fuel, the cell achieved a maximum power density of 489 mW/cm2 at 800 C and exhibited acceptable sulfur tolerance in H2S. Liu et al. [17] used nano-scale La0.6Sr0.4Fe0.9Sc0.1O3-d impregnated into electrolyte backbones as the electrode material, which showed a maximum power density of 560 mW/cm2 and low polarization resistances of 0.015 V.cm2 in air and 0.29 V.cm2 in H2 at 800 C, respectively. The lanthanum strontium cobalt ferrite perovskite family has been proved to be suitable as SOFC electrode materials. However, these materials chemically react with yttria stabilize zirconia (YSZ) electrolyte [18]. This reactivity starts above 1000 C and forms two poorly conducting phases, SrZrO3 and La2Zr2O7 [19]. The cation diffusion may result in the cell collapse [20]. In order to prevent chemical reactions, a thin and dense gadolinium doped cerium oxide (GDC) buffer layer is applied between electrodes and electrolyte, because of its high conductivity and stability [21,22]. Many works focused on the perovskite electrode materials [3–5,7,8]. This type of materials have demonstrated several advantages, including high tolerance of sulfur and coking, good chemical and redox stability [3]. In this work, cobalt-free perovskite oxide La0.8Sr0.2FeO3-d (LSF) was synthesized by a modified Pechini method and applied as a novel stable electrode material for YSZ-based symmetrical solid oxide fuel cells. Gadolinium doped ceria (GDC) buffer layer was fabricated by screen printing onto the YSZ electrolyte, which dramatically enhanced the electrochemical performance. The improvement demonstrates that the doped ceria buffer layer, fabricated using a highly repeatable method, provides a viable method for further improving the performance of YSZ-based SSOFC. 2. Experimental 2.1. Synthesis
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powder was uniaxially pressed at 200 MPa into pellets of 15 mm in diameter, followed by sintering at 1450 C for 5 h in air. In order to prepare the LSF slurry and GDC slurry, ethyecellulose and terpineol (10 wt.%: 90 wt.%) mixture was used as the binder for LSF or GDC powder. The LSF slurry was applied onto both surfaces of the YSZ substrates and then sintered in air at 1100 C for 3 h to prepare the cell with LSF|YSZ|LSF configuration. For the LSF|GDC|YSZ|GDC|LSF configuration, the GDC slurry was printed onto both surfaces of the YSZ electrolyte and sintered in air at 1300 C for 3 h, followed by applying the LSF slurry. For a single cell performance testing, Ag paste was applied as the current collector for both anode and cathode. The cathode area was about 0.2 cm2. Such tests were evaluated in the 650–800 C temperature range. 2.3. Characterization The phase of the synthesized materials was analyzed using powder X-ray diffractometer (XRD, DX2000) with Cu-Ka radiation at 35 kV and 25 mA. The chemical compatibility between the LSF electrode and YSZ electrolyte or GDC buffer layer was also analyzed by XRD. For this aim, LSF powders were mixed with 50 wt.% of YSZ or GDC powders, and then the samples were heated to 1100 C for 10 h in air, or reduced at 800 C for 10 h in wet H2 to simulate the anodic exposure. The electrical conductivity of LSF was measured in air and wet H2 from 800 C to 400 C using a standard four probe DC method. Electrochemical impedance spectroscopy (EIS) measurements of the LSF symmetrical cells were obtained using an IM6 electrochemical workstation (ZAHNER, Germany) in a frequency range of 100 kHz to 0.01 Hz with AC amplitude of 5 mV. The current-voltage curves of single cells were measured using a DC electronic load (IT8511) exposing one side of the cell to ambient air and the other to humidified hydrogen (3% H2O) at a flow rate of 30 mL/min. The microstructure and morphology of the cross section of symmetrical cells were characterized by scanning electron microscope (SEM, EM-3200). 3. Results and discussion As shown in Fig. 1(a), the XRD pattern of the as-prepared La0.8Sr0.2FeO3-d powder in air at 1000 C showed characteristic peaks of the perovskite crystal structure, without any impurities.
La0.8Sr0.2FeO3-d (LSF) and Gd0.2Ce0.8O2-d (GDC) powders were prepared by the modified Pechini method with citric acid as complexing agent. La(NO3)36H2O, Sr(NO3)2, Fe(NO3)39H2O, Gd (NO3)36H2O, Ce(NO3)36H2O were used as raw materials for the synthesis of LSF and GDC. The stoichiometric amounts of the raw materials were dissolved in distilled water to form an aqueous solution, followed by citric acid addition. The molar ratio of citric acid/metal was set at 1.5/1. Ammonia was used for adjusting the pH value of the solution to about 7. The solution was heated under stirring and finally combusted to form the precursors. The LSF and GDC precursors were calcined at 1000 C and 700 C for 3 h in air, respectively. 2.2. Sample preparation The La0.8Sr0.2FeO3-d (LSF) powder was pressed under 220 MPa into several bars (40 mm 5 mm 2 mm) and then calcined at 1150 C for 5 h in air. The electrical conductivity of LSF was measured in air and wet H2 from 800 C to 400 C. Commercially available powder of 8 mol% yttria-stabilized zirconia (8YSZ) was used as electrolyte. In order to obtain a dense electrolyte, YSZ
Fig 1. X-ray diffraction patterns of LSF and GDC, YSZ powders: (a) LSF calcined in air at 1000 C for 3 h, (b) LSF heated in humidified H2 (3% H2O) at 800 C for 10 h, (c) GDC calcined in air at 700 C for 10 h, (d) YSZ powder from commercial.
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Fig. 1(c) shows the XRD pattern of Gd0.2Ce0.8O2-d powder synthesized in air at 700 C. Notably, a pure fluorite structure was obtained without any minor impurity. In order to estimate the phase stability of LSF in the reducing environment, the LSF powder was subsequently heated in humidified hydrogen (3% H2O) at 800 C for 10 h. Fig. 1(b) shows that also in this case only the LSF peaks were detected. The XRD data of LSF in air and hydrogen were refined by Jade 6.5 software. The lattice parameter of the lattice slightly increased from 3.9158 Å in air to 3.9174 Å in hydrogen, probably because of the loss of some lattice oxygen under reducing atmosphere. Therefore, the LSF electrode material is stable under reducing atmosphere. Fig. 2 shows the XRD patterns of the LSF-YSZ and LSF-GDC mixtures after annealing at 1100 C for 10 h in air and reduction at 800 C for 10 h in wet H2, respectively. The weight ratio of LSF and YSZ (GDC) was 1:1. Fig. 2(a) clearly shows that the main peaks of LSF and YSZ were present, but also additional small peaks of an unknown phase were observed, indicating their poor compatibility. Fig. 2(b) shows that only the LSF and GDC diffraction peaks could be found with no additional peaks, indicating that LSF is chemically compatible with GDC both in air and in humidified hydrogen. Fig. 3 shows the temperature dependence of the total electrical conductivity of LSF in air and hydrogen. Obviously, the LSF
Fig. 3. Electrical conductivities of LSF in air (a) and in humidified H2 (b).
conductivity increased with increasing the temperature in the 400–800 C temperature range, exhibiting a semiconducting behavior in both air and hydrogen, which can be explained by the small polaron mechanism. The conductivity at 800 C reach the highest values of 141.1 S/cm and 0.138 S/cm in air and H2, respectively. That can be explained by the fact that LSF is a p-type conductor: exposing LSF to H2, a reducing gas, decreases its conductivity [15,17,23]. The activation energy of LSF can be calculated from the following equation (1): A Ea s ¼ exp ð1Þ T KT
Fig. 2. XRD patterns of (a) LSF + YSZ and (b) LSF + GDC mixtures after calcined at 1100 C for 10 h in air and reduced at 800 C for 10 h in humidified H2.
where A is the pre-exponential constant, T is the absolute temperature, k is the Boltzmann constant, and Ea is activation energy of the conductivity [24]. Fig. 3 shows the activation energy of LSF in both air and H2, the activation energy in H2 (0.96 eV) is much larger than that in air (0.19 eV), caused by the decrease in hole mobility [25]. The electrochemical performance of the LSF electrode was evaluated by electrochemical impedance spectroscopy (EIS) measuremants on symmetrical LSF|YSZ|LSF (Cell A) and LSF| GDC|YSZ|GDC|LSF (Cell B) cells in both air and H2 from 600 C to 800 C. The impedance spectroscopy data were further analyzed using the ZSimpWin software with an equivalent circuit LR(R1Q1) (R2Q2). R is the ohmic resistance of YSZ, and R1Q1, R2Q2 reflect two different electrode processes, including oxygen reduction reaction
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Fig. 5. I-V-P curves of the symmetrical solid oxide fuel cells of (a) Cell A: LSF|YSZ| LSF and (b) Cell B: LSF|GDC|YSZ|GDC|LSF. Fig. 4. (a) The impedance spectra of symmetrical cells in air and humidified H2 (3% H2O) at 800 C; (b) the polarization resistance of symmetrical cells from 650 C to 800 C in air and humidified H2 (3% H2O).
(ORR) and hydrogen oxidation reaction (HOR). During the ORR process, Rp was mainly caused by the non-charge transfer processes at the interface, and by the process of oxygen dissociation into oxygen ions. The Rp of HOR process can be ascribe to both the catalytic activity of hydrogen reduction process and to gas bulk diffusion [4,26]. The ohmic resistance of the YSZ electrolyte was removed to directly show the polarization resistances; the low frequency arc intercept actually reflects the polarization resistance of the symmetrical cells. As shown in Fig. 4 (a), the polarization resistance of the La0.8Sr0.2FeO3-d electrode decreased with adding the GDC buffer layer. The Rp values for the Cell A were 0.48 V.cm2 and 0.92 V.cm2 in air and H2 at 800 C, respectively, while smaller Rp values were measured for the Cell B, 0.30 V.cm2 and 0.58 V.cm2 in air and H2 at 800 C, respectively. As shown in Fig. 4(b), the Ea for Cell A were 1.22 eV and 1.56 eV in air and H2, while the Ea for Cell B with GDC buffer layers were 1.09 eV
Table 1 The polarization resistances of cells at different temperatures (V.cm2). Cells Cell Cell Cell Cell
A (air) A (hydrogen) B (air) B (hydrogen)
800 C
750 C
700 C
650 C
0.48 0.92 0.31 0.58
0.79 2.48 0.53 1.08
1.60 5.90 0.94 2.06
4.08 14.78 2.22 4.40
and 1.15 eV in air and H2, respectively. Table 1 shows the Rp values for both Cell A and Cell B measured at different temperatures. The decrease in and Ea is ascribed to the enhancement of the kinetics of the contact area via the GDC buffer layer, which can provide a larger contact area for electrode and increase the triple-phase boundary length [22–24]. In order to further investigate the electrochemical performance, the two single cells of LSF|YSZ|LSF (Cell A) and LSF|GDC| YSZ|GDC|LSF (Cell B) were fabricated upon 400 mm-thick YSZ electrolytes and tested using humidified H2 (3%H2O) as a fuel and air as oxidant. Fig. 5 shows the typical I–V and I-P curves of the YSZ-supported cells. The open circuit voltages (OCVs) of Cell A (without GDC buffer layer) and Cell B (with GDC buffer layer) were 1.01 V and 1.00 V at 800 C, respectively. The maximum power densities for Cell A were 316 mW/cm2, 165 mW/cm2, and 90 mW/ cm2 at 800 C, 750 C and 700 C, respectively, while the maximum power densities for Cell B were 387 mW/cm2, 254 mW/cm2, and 174 mW/cm2 at 800 C, 750 C and 700 C, respectively. Adding the GDC buffer layer dramatically enhanced the electrochemical performance by more than 90% at 700 C. The improvement of SSOFC performance was attributed to the elimination of chemical reactivity and to the optimization of the interface between the YSZ electrolyte and the LSF electrode. These values are larger than those obtained previously for same similar materials, such as La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-d [27], La0.5Sr0.5Co0.5Ti0.5O3-d [28] and LaSr2Fe2CrO9-d [15]. Fig. 6 shows impedance spectra for Cell A and Cell B under operating condition. No matter Cell A or Cell B, the Rp all decreased with the temperature increased. The polarization resistance were
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Fig. 7. Long-term stability of the symmetrical solid oxide fuel cells of Cell B: LSF| GDC|YSZ|GDC|LSF at 700 C.
Fig. 6. Impedance spectra of Cell A (a), Cell B (b) and the comparison (c) under operating condition.
morphology of YSZ electrolyte and GDC buffer layer are corresponding to (a) and (b), respectively. Fig. 8(c) and (d) indicate the cross-section microstructures of Cell A and Cell B, respectively. From the images, one can see that both LSF anode and LSF cathode are highly porous enough and the interface between LSF electrode and electrolyte is adhered well without any separation [31]. Comparing with Fig. 8 (c) (the cell without GDC buffer layer), Fig. 8 (d) (with buffer layer) exists a small grain size layer (GDC) and the thickness of GDC buffer layer is about 2 mm. In short, the observed enhanced performance of SSOFCs using a GDC buffer layer, which is very important for industrialization of SSOFCs in the future, could be ascribed to many advantages in SSOFCs with a GDC buffer layer, such as elimination of reaction area between LSF electrodes and YSZ electrolyte, improvement of the surface oxygen exchange rate, increase in the surface roughness of electrolyte membranes, and optimization of the thermal compatibility between the LSF electrodes and YSZ electrolyte.
4. Conclusion 0.197 V.cm2 and 0.118 V.cm2 for Cell A and Cell B at 800 C, respectively. The better performance of Cell B is confirmed to be mainly ascribed to the lower interfacial polarization resistance. As well known, the high frequency arc intercept with the x axis represents the ohmic resistance of the cell. As shown in Fig. 6 (c), the ohmic resistance (Ro) of Cell A is 0.58 V.cm2 while Ro of Cell B is 0.64 V.cm2. This can be explained by the electrolyte thickness increase due by the addition of the GDC buffer layers. However, when the temperature decreased to 700 C, the ohmic resistance of Cell B was smaller than that for the Cell A at 700 C, suggesting that the thinner GDC buffer layer can improve the charge-transfer processes and ionic current collection between the electrode and electrolyte at lower temperatures [28,29]. The long-term stability of the Cell B with a GDC buffer layer was further investigated at 700 C with a constant output voltage of 0.7 V, and Fig. 7 shows the dependence of the power density on the elapsed time. After operating at 700 C for 48 h, the power density of the single cell remained almost constant, indicating that LSF electrodes with GDC buffer layers for SSOFC can be stable in practical long-time applications [30]. The morphology of the symmetrical cells after testing are shown in Fig. 8. The surface
A cubic fluorite Gd0.2Ce0.8O2-d (GDC) powder was prepared by a modified Pechini method and further investigated as a buffer layer for symmetrical solid oxide fuel cell with stable La0.8Sr0.2FeO3-d (LSF) electrode materials. The electrical conductivity of LSF was 141.1 S/cm and 0.138 S/cm in air and humidified H2 (3% H2O) at 800 C, respectively. The GDC buffer layers, which were fabricated by screen printing onto the YSZ electrolyte, dramatically enhanced the electrochemical performance. The polarization resistance decreased from 0.48 to 0.30 V.cm2 in air and from 0.92 to 0.58 V.cm2 in humidified hydrogen at 800 C, respectively. The maximum power density of the SSOFC increased from 316 mW/cm2 to 387 mW/cm2 at 800 C and the polarization resistance of SSOFC decreased from 0.197 V.cm2 to 0.118 V.cm2 at 800 C, respectively. The improvement in SSOFC performance is attributed to the elimination of reactivity and the optimization of interface between the YSZ electrolyte and the LSF electrode. These results demonstrate that the doped ceria buffer layer provides a highly repeatable route for further improving the performance of YSZ-based SSOFC, with potentially important implications for developing cost-effective SSOFCs with huge application opportunities.
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Fig. 8. SEM microstructures for the symmetrical solid oxide fuel cells of (a, c) Cell A: LSF|YSZ|LSF and (b, d) Cell B: LSF|GDC|YSZ|GDC|LSF.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced performance of symmetrical solid oxide fuel cells using a doped ceria buffer layer Dong Tiana,b , Bin Lina,b,c,* , Yang Yangb , Yonghong Chenb,** , Xiaoyong Lub , Zhigao Wangd , Wei Liua , Enrico Traversac,** a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Anhui Key Laboratory of Low temperature Co-fired Material, Huainan Normal University, Huainan 232001, PR China c International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China d State Grid Sichuan Electric Power Research Institute, Chengdu 610072, PR China
A R T I C L E I N F O
Article history: Received 3 December 2015 Received in revised form 27 April 2016 Accepted 27 April 2016 Available online 6 May 2016 Keywords: Symmetrical solid oxide fuel cell La0.8Sr0.2FeO3-d Buffer layer Electrochemical performance
A B S T R A C T
Enhanced performance of symmetrical solid oxide fuel cell (SSOFC) is reported by using a doped ceria buffer layer, which can solve the issues during the operation of traditional solid oxide fuel cells, such as carbon deposition and sulfur poisoning. In this work, cobalt-free perovskite oxide La0.8Sr0.2FeO3-d (LSF) is applied as a novel stable electrode material for symmetrical solid oxide fuel cell. The electrical conductivities of LSF are 141.1 Scm1 and 0.138 Scm1 in air and humidified H2 (3% H2O) at 800 C, respectively. Gadolinium doped ceria (GDC) buffer layer is fabricated by screen printing onto the YSZ electrolyte, which dramatically enhances the electrochemical performance by more than 90 percent at 700 C. The improvement of SSOFC performance is attributed to the elimination of reactivity and the optimization of interface between YSZ electrolyte and LSF electrode. These results demonstrate that the doped ceria buffer layer provides a highly repeatable route for further improving the performance of YSZbased SSOFC, with potentially important implications for developing cost-effective SSOFCs with huge application opportunities. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction As substitutes for traditional power generation devices, solid oxide fuel cells (SOFCs) have demonstrated several advantages, including high efficiency, fuel flexibility, low pollution and longterm stability [1]. However, several issues need to be solved for SOFC commercial development. For instance, Ni-YSZ, the state-ofthe art SOFC anode material promotes carbon or sulfur species deposits on its surface during operation with hydrocarbon fuels, resulting in deactivation of the catalysts. To solve this problem, symmetrical solid oxide fuel cells (SSOFCs) using a redox-stable catalyst as both anode and cathode have been proposed and have attracted significant interest [2–8]. The use of the same material as
* Corresponding author at: Anhui Key Laboratory of Low Temperature Co-fired Materials, Huainan Normal University, Huainan, Anhui, 232001, PR China. ** Corresponding authors. E-mail addresses: [email protected], [email protected] (B. Lin). http://dx.doi.org/10.1016/j.electacta.2016.04.189 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
both anode and cathode simultaneously can simplify the fabrication process, minimize inter-diffusion between components, enhance coking and sulfur tolerance by operating the anode as the cathode in turn [1,9,10], allowing the absorbed carbon and sulfur species burning out by oxidant [11]. Thus, the reliability of SOFC systems will be enhanced by adopting symmetrical electrodes [12]. To enable SSOFCs working at different atmospheres, the electrode materials require suitable characterizatics for both anode and cathode environments, such as stable structure, good catalytic activities and high conductivities. Several types of materials have been used as SSOFC electrodes since the first La0.75Sr0.25Cr0.5Mn0.5O3-d (LSCM) was proposed by Bastidas et al. [2]. Among these materials, perovskite oxides have been evaluated as the most suitable electrodes due to their special structure [13]. Although Co-based perovskite electrodes have shown high performance for SSOFCs, their structure stability is still a big problem. Extensive work was focused on Fe-based
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perovskites [4–7,14], which are promising SSOFC electrode materials with reasonable stability. Zhou et al. [15] showed that a LaSrFe2CrO9d-based symmetrical solid oxide fuel cell achieved a maximum power density of 224 mW/cm2 at 800 C using humidified hydrogen as a fuel, which could be further improved by making composite electrodes with 50 wt.% Gd0.2Ce0.8O2-d. Yang et al. [16] reported a good performance for Ga-doped lanthanum strontium ferrite (La0.7Sr0.3Fe0.7Ga0.3O3-d, LSFG) as potential symmetrical electrodes. Using humidified hydrogen as fuel, the cell achieved a maximum power density of 489 mW/cm2 at 800 C and exhibited acceptable sulfur tolerance in H2S. Liu et al. [17] used nano-scale La0.6Sr0.4Fe0.9Sc0.1O3-d impregnated into electrolyte backbones as the electrode material, which showed a maximum power density of 560 mW/cm2 and low polarization resistances of 0.015 V.cm2 in air and 0.29 V.cm2 in H2 at 800 C, respectively. The lanthanum strontium cobalt ferrite perovskite family has been proved to be suitable as SOFC electrode materials. However, these materials chemically react with yttria stabilize zirconia (YSZ) electrolyte [18]. This reactivity starts above 1000 C and forms two poorly conducting phases, SrZrO3 and La2Zr2O7 [19]. The cation diffusion may result in the cell collapse [20]. In order to prevent chemical reactions, a thin and dense gadolinium doped cerium oxide (GDC) buffer layer is applied between electrodes and electrolyte, because of its high conductivity and stability [21,22]. Many works focused on the perovskite electrode materials [3–5,7,8]. This type of materials have demonstrated several advantages, including high tolerance of sulfur and coking, good chemical and redox stability [3]. In this work, cobalt-free perovskite oxide La0.8Sr0.2FeO3-d (LSF) was synthesized by a modified Pechini method and applied as a novel stable electrode material for YSZ-based symmetrical solid oxide fuel cells. Gadolinium doped ceria (GDC) buffer layer was fabricated by screen printing onto the YSZ electrolyte, which dramatically enhanced the electrochemical performance. The improvement demonstrates that the doped ceria buffer layer, fabricated using a highly repeatable method, provides a viable method for further improving the performance of YSZ-based SSOFC. 2. Experimental 2.1. Synthesis
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powder was uniaxially pressed at 200 MPa into pellets of 15 mm in diameter, followed by sintering at 1450 C for 5 h in air. In order to prepare the LSF slurry and GDC slurry, ethyecellulose and terpineol (10 wt.%: 90 wt.%) mixture was used as the binder for LSF or GDC powder. The LSF slurry was applied onto both surfaces of the YSZ substrates and then sintered in air at 1100 C for 3 h to prepare the cell with LSF|YSZ|LSF configuration. For the LSF|GDC|YSZ|GDC|LSF configuration, the GDC slurry was printed onto both surfaces of the YSZ electrolyte and sintered in air at 1300 C for 3 h, followed by applying the LSF slurry. For a single cell performance testing, Ag paste was applied as the current collector for both anode and cathode. The cathode area was about 0.2 cm2. Such tests were evaluated in the 650–800 C temperature range. 2.3. Characterization The phase of the synthesized materials was analyzed using powder X-ray diffractometer (XRD, DX2000) with Cu-Ka radiation at 35 kV and 25 mA. The chemical compatibility between the LSF electrode and YSZ electrolyte or GDC buffer layer was also analyzed by XRD. For this aim, LSF powders were mixed with 50 wt.% of YSZ or GDC powders, and then the samples were heated to 1100 C for 10 h in air, or reduced at 800 C for 10 h in wet H2 to simulate the anodic exposure. The electrical conductivity of LSF was measured in air and wet H2 from 800 C to 400 C using a standard four probe DC method. Electrochemical impedance spectroscopy (EIS) measurements of the LSF symmetrical cells were obtained using an IM6 electrochemical workstation (ZAHNER, Germany) in a frequency range of 100 kHz to 0.01 Hz with AC amplitude of 5 mV. The current-voltage curves of single cells were measured using a DC electronic load (IT8511) exposing one side of the cell to ambient air and the other to humidified hydrogen (3% H2O) at a flow rate of 30 mL/min. The microstructure and morphology of the cross section of symmetrical cells were characterized by scanning electron microscope (SEM, EM-3200). 3. Results and discussion As shown in Fig. 1(a), the XRD pattern of the as-prepared La0.8Sr0.2FeO3-d powder in air at 1000 C showed characteristic peaks of the perovskite crystal structure, without any impurities.
La0.8Sr0.2FeO3-d (LSF) and Gd0.2Ce0.8O2-d (GDC) powders were prepared by the modified Pechini method with citric acid as complexing agent. La(NO3)36H2O, Sr(NO3)2, Fe(NO3)39H2O, Gd (NO3)36H2O, Ce(NO3)36H2O were used as raw materials for the synthesis of LSF and GDC. The stoichiometric amounts of the raw materials were dissolved in distilled water to form an aqueous solution, followed by citric acid addition. The molar ratio of citric acid/metal was set at 1.5/1. Ammonia was used for adjusting the pH value of the solution to about 7. The solution was heated under stirring and finally combusted to form the precursors. The LSF and GDC precursors were calcined at 1000 C and 700 C for 3 h in air, respectively. 2.2. Sample preparation The La0.8Sr0.2FeO3-d (LSF) powder was pressed under 220 MPa into several bars (40 mm 5 mm 2 mm) and then calcined at 1150 C for 5 h in air. The electrical conductivity of LSF was measured in air and wet H2 from 800 C to 400 C. Commercially available powder of 8 mol% yttria-stabilized zirconia (8YSZ) was used as electrolyte. In order to obtain a dense electrolyte, YSZ
Fig 1. X-ray diffraction patterns of LSF and GDC, YSZ powders: (a) LSF calcined in air at 1000 C for 3 h, (b) LSF heated in humidified H2 (3% H2O) at 800 C for 10 h, (c) GDC calcined in air at 700 C for 10 h, (d) YSZ powder from commercial.
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Fig. 1(c) shows the XRD pattern of Gd0.2Ce0.8O2-d powder synthesized in air at 700 C. Notably, a pure fluorite structure was obtained without any minor impurity. In order to estimate the phase stability of LSF in the reducing environment, the LSF powder was subsequently heated in humidified hydrogen (3% H2O) at 800 C for 10 h. Fig. 1(b) shows that also in this case only the LSF peaks were detected. The XRD data of LSF in air and hydrogen were refined by Jade 6.5 software. The lattice parameter of the lattice slightly increased from 3.9158 Å in air to 3.9174 Å in hydrogen, probably because of the loss of some lattice oxygen under reducing atmosphere. Therefore, the LSF electrode material is stable under reducing atmosphere. Fig. 2 shows the XRD patterns of the LSF-YSZ and LSF-GDC mixtures after annealing at 1100 C for 10 h in air and reduction at 800 C for 10 h in wet H2, respectively. The weight ratio of LSF and YSZ (GDC) was 1:1. Fig. 2(a) clearly shows that the main peaks of LSF and YSZ were present, but also additional small peaks of an unknown phase were observed, indicating their poor compatibility. Fig. 2(b) shows that only the LSF and GDC diffraction peaks could be found with no additional peaks, indicating that LSF is chemically compatible with GDC both in air and in humidified hydrogen. Fig. 3 shows the temperature dependence of the total electrical conductivity of LSF in air and hydrogen. Obviously, the LSF
Fig. 3. Electrical conductivities of LSF in air (a) and in humidified H2 (b).
conductivity increased with increasing the temperature in the 400–800 C temperature range, exhibiting a semiconducting behavior in both air and hydrogen, which can be explained by the small polaron mechanism. The conductivity at 800 C reach the highest values of 141.1 S/cm and 0.138 S/cm in air and H2, respectively. That can be explained by the fact that LSF is a p-type conductor: exposing LSF to H2, a reducing gas, decreases its conductivity [15,17,23]. The activation energy of LSF can be calculated from the following equation (1): A Ea s ¼ exp ð1Þ T KT
Fig. 2. XRD patterns of (a) LSF + YSZ and (b) LSF + GDC mixtures after calcined at 1100 C for 10 h in air and reduced at 800 C for 10 h in humidified H2.
where A is the pre-exponential constant, T is the absolute temperature, k is the Boltzmann constant, and Ea is activation energy of the conductivity [24]. Fig. 3 shows the activation energy of LSF in both air and H2, the activation energy in H2 (0.96 eV) is much larger than that in air (0.19 eV), caused by the decrease in hole mobility [25]. The electrochemical performance of the LSF electrode was evaluated by electrochemical impedance spectroscopy (EIS) measuremants on symmetrical LSF|YSZ|LSF (Cell A) and LSF| GDC|YSZ|GDC|LSF (Cell B) cells in both air and H2 from 600 C to 800 C. The impedance spectroscopy data were further analyzed using the ZSimpWin software with an equivalent circuit LR(R1Q1) (R2Q2). R is the ohmic resistance of YSZ, and R1Q1, R2Q2 reflect two different electrode processes, including oxygen reduction reaction
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Fig. 5. I-V-P curves of the symmetrical solid oxide fuel cells of (a) Cell A: LSF|YSZ| LSF and (b) Cell B: LSF|GDC|YSZ|GDC|LSF. Fig. 4. (a) The impedance spectra of symmetrical cells in air and humidified H2 (3% H2O) at 800 C; (b) the polarization resistance of symmetrical cells from 650 C to 800 C in air and humidified H2 (3% H2O).
(ORR) and hydrogen oxidation reaction (HOR). During the ORR process, Rp was mainly caused by the non-charge transfer processes at the interface, and by the process of oxygen dissociation into oxygen ions. The Rp of HOR process can be ascribe to both the catalytic activity of hydrogen reduction process and to gas bulk diffusion [4,26]. The ohmic resistance of the YSZ electrolyte was removed to directly show the polarization resistances; the low frequency arc intercept actually reflects the polarization resistance of the symmetrical cells. As shown in Fig. 4 (a), the polarization resistance of the La0.8Sr0.2FeO3-d electrode decreased with adding the GDC buffer layer. The Rp values for the Cell A were 0.48 V.cm2 and 0.92 V.cm2 in air and H2 at 800 C, respectively, while smaller Rp values were measured for the Cell B, 0.30 V.cm2 and 0.58 V.cm2 in air and H2 at 800 C, respectively. As shown in Fig. 4(b), the Ea for Cell A were 1.22 eV and 1.56 eV in air and H2, while the Ea for Cell B with GDC buffer layers were 1.09 eV
Table 1 The polarization resistances of cells at different temperatures (V.cm2). Cells Cell Cell Cell Cell
A (air) A (hydrogen) B (air) B (hydrogen)
800 C
750 C
700 C
650 C
0.48 0.92 0.31 0.58
0.79 2.48 0.53 1.08
1.60 5.90 0.94 2.06
4.08 14.78 2.22 4.40
and 1.15 eV in air and H2, respectively. Table 1 shows the Rp values for both Cell A and Cell B measured at different temperatures. The decrease in and Ea is ascribed to the enhancement of the kinetics of the contact area via the GDC buffer layer, which can provide a larger contact area for electrode and increase the triple-phase boundary length [22–24]. In order to further investigate the electrochemical performance, the two single cells of LSF|YSZ|LSF (Cell A) and LSF|GDC| YSZ|GDC|LSF (Cell B) were fabricated upon 400 mm-thick YSZ electrolytes and tested using humidified H2 (3%H2O) as a fuel and air as oxidant. Fig. 5 shows the typical I–V and I-P curves of the YSZ-supported cells. The open circuit voltages (OCVs) of Cell A (without GDC buffer layer) and Cell B (with GDC buffer layer) were 1.01 V and 1.00 V at 800 C, respectively. The maximum power densities for Cell A were 316 mW/cm2, 165 mW/cm2, and 90 mW/ cm2 at 800 C, 750 C and 700 C, respectively, while the maximum power densities for Cell B were 387 mW/cm2, 254 mW/cm2, and 174 mW/cm2 at 800 C, 750 C and 700 C, respectively. Adding the GDC buffer layer dramatically enhanced the electrochemical performance by more than 90% at 700 C. The improvement of SSOFC performance was attributed to the elimination of chemical reactivity and to the optimization of the interface between the YSZ electrolyte and the LSF electrode. These values are larger than those obtained previously for same similar materials, such as La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-d [27], La0.5Sr0.5Co0.5Ti0.5O3-d [28] and LaSr2Fe2CrO9-d [15]. Fig. 6 shows impedance spectra for Cell A and Cell B under operating condition. No matter Cell A or Cell B, the Rp all decreased with the temperature increased. The polarization resistance were
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Fig. 7. Long-term stability of the symmetrical solid oxide fuel cells of Cell B: LSF| GDC|YSZ|GDC|LSF at 700 C.
Fig. 6. Impedance spectra of Cell A (a), Cell B (b) and the comparison (c) under operating condition.
morphology of YSZ electrolyte and GDC buffer layer are corresponding to (a) and (b), respectively. Fig. 8(c) and (d) indicate the cross-section microstructures of Cell A and Cell B, respectively. From the images, one can see that both LSF anode and LSF cathode are highly porous enough and the interface between LSF electrode and electrolyte is adhered well without any separation [31]. Comparing with Fig. 8 (c) (the cell without GDC buffer layer), Fig. 8 (d) (with buffer layer) exists a small grain size layer (GDC) and the thickness of GDC buffer layer is about 2 mm. In short, the observed enhanced performance of SSOFCs using a GDC buffer layer, which is very important for industrialization of SSOFCs in the future, could be ascribed to many advantages in SSOFCs with a GDC buffer layer, such as elimination of reaction area between LSF electrodes and YSZ electrolyte, improvement of the surface oxygen exchange rate, increase in the surface roughness of electrolyte membranes, and optimization of the thermal compatibility between the LSF electrodes and YSZ electrolyte.
4. Conclusion 0.197 V.cm2 and 0.118 V.cm2 for Cell A and Cell B at 800 C, respectively. The better performance of Cell B is confirmed to be mainly ascribed to the lower interfacial polarization resistance. As well known, the high frequency arc intercept with the x axis represents the ohmic resistance of the cell. As shown in Fig. 6 (c), the ohmic resistance (Ro) of Cell A is 0.58 V.cm2 while Ro of Cell B is 0.64 V.cm2. This can be explained by the electrolyte thickness increase due by the addition of the GDC buffer layers. However, when the temperature decreased to 700 C, the ohmic resistance of Cell B was smaller than that for the Cell A at 700 C, suggesting that the thinner GDC buffer layer can improve the charge-transfer processes and ionic current collection between the electrode and electrolyte at lower temperatures [28,29]. The long-term stability of the Cell B with a GDC buffer layer was further investigated at 700 C with a constant output voltage of 0.7 V, and Fig. 7 shows the dependence of the power density on the elapsed time. After operating at 700 C for 48 h, the power density of the single cell remained almost constant, indicating that LSF electrodes with GDC buffer layers for SSOFC can be stable in practical long-time applications [30]. The morphology of the symmetrical cells after testing are shown in Fig. 8. The surface
A cubic fluorite Gd0.2Ce0.8O2-d (GDC) powder was prepared by a modified Pechini method and further investigated as a buffer layer for symmetrical solid oxide fuel cell with stable La0.8Sr0.2FeO3-d (LSF) electrode materials. The electrical conductivity of LSF was 141.1 S/cm and 0.138 S/cm in air and humidified H2 (3% H2O) at 800 C, respectively. The GDC buffer layers, which were fabricated by screen printing onto the YSZ electrolyte, dramatically enhanced the electrochemical performance. The polarization resistance decreased from 0.48 to 0.30 V.cm2 in air and from 0.92 to 0.58 V.cm2 in humidified hydrogen at 800 C, respectively. The maximum power density of the SSOFC increased from 316 mW/cm2 to 387 mW/cm2 at 800 C and the polarization resistance of SSOFC decreased from 0.197 V.cm2 to 0.118 V.cm2 at 800 C, respectively. The improvement in SSOFC performance is attributed to the elimination of reactivity and the optimization of interface between the YSZ electrolyte and the LSF electrode. These results demonstrate that the doped ceria buffer layer provides a highly repeatable route for further improving the performance of YSZ-based SSOFC, with potentially important implications for developing cost-effective SSOFCs with huge application opportunities.
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Fig. 8. SEM microstructures for the symmetrical solid oxide fuel cells of (a, c) Cell A: LSF|YSZ|LSF and (b, d) Cell B: LSF|GDC|YSZ|GDC|LSF.
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