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Study of phase stability of SrTi0.3Fe0.7O3−δ perovskite in reducing atmosphere: Effect of microstructure
Solid State Ionics 342 (2019) 115064
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Study of phase stability of SrTi0.3Fe0.7O3−δ perovskite in reducing atmosphere: Effect of microstructure Mariano Santaya, Lucia Toscani, Laura Baqué, Horacio E. Troiani, Liliana Mogni
T
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CNEA-CONICET, Department of Materials Characterization, Bariloche Atomic Centre, Av. Bustillo 9500, CP8400, Argentina
A R T I C LE I N FO
A B S T R A C T
Keywords: STF SOFC electrode MIEC Perovskite Microstructure
Increasing SOFC electrode's surface area by modification of its microstructure is a well-known technique to reduce electrode polarization resistance. This is because reduced grain size and increased porosity promote diffusion and surface reactions, thus improving the electrode performance. However, a modified microstructure also causes differences in phase stability and in chemical compatibility with other SOFC materials. In this work, we study the effect of particle size in both the electrode performance and the phase stability under different fuel conditions and temperatures. SrTi0.3Fe0.7O3−δ (STF) is both prepared via solid state reaction (STF-SSR) and also by an alternative sol-gel route (STF-SG). The sintering temperature is reduced dramatically with the sol-gel method, hence inducing a higher porosity and a much smaller grain size. As particle size is reduced the stability under fuel conditions is also diminished, so decomposition induced by segregation of metallic Fe and SrO occurs at lower temperatures for the STF-SG sample. The stability under reducing conditions is studied by combined techniques such as TGA, TPR, XRD, SEM and TEM. Performance as anode and cathode is evaluated by Electrochemical Impedance Spectroscopy (EIS) by using electrolyte supported symmetrical cells. Prior to electrochemical experiments, the reactivity between La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) electrolyte and STF was studied, and also between STF and a Lanthanum Doped Ceria (LDC) buffer layer. It is seen that microstructure also plays a key role in the chemical stability of the STF. The impact of particle size reduction is higher for the anodic polarization resistance, which is reduced twice from STF-SSR to STF-SG.
1. Introduction Solid oxide fuel cell (SOFC) anodes composed of mixed ionic and electronic conducting (MIEC) perovskites are emerging as alternatives to Ni-YSZ anodes, because they can provide improved tolerance to redox cycling, fuel impurities, and hydrocarbon fuels [1–5]. However, their polarization resistance is typically higher than that of Ni-based electrodes, leading to lower cell power densities [6]. Recently, new developments in materials science are aiming to reduce the anode polarization resistance of MIEC materials through surface and microstructure modification [7–11], and improved doping strategies [12–14]. Over the past years, SrTiO3 based perovskites have attracted particular attention due to their good stability under reducing conditions, tolerance to hydrocarbon fuels and their relatively good n-type conductivity [15]. Among these materials, SrTi0.3Fe0.7O3−δ (STF) has been recently found to yield a power density of 1.1 W cm2 at 850 °C in humidified hydrogen, approaching the values achieved in state-of-theart Ni-YSZ anodes [16]. The microstructural modification of traditional electrode materials
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is a common strategy to achieve a better performance at lower temperatures [7,17–19]. In the case of MIEC materials, this is because chemical reactions can occur in the whole surface area of the electrode (they are not restricted only to Triple Phase Boundaries) [20], and so an increase in the electrode surface area has a direct impact in the reaction rate. In addition, when the microstructure is tuned to achieve a more porous electrode, gas diffusion is also favored. All these factors contribute to a reduction in the electrode polarization resistance, which results into higher cell power densities. However, it is also important to consider that smaller grain sizes and more grain boundaries increase the Gibbs energy of the system and result in a thermodynamically less stable material. This also affects chemical reactivity between the material and other SOFC components, which occurs at lower temperatures. In this study, STF perovskites were prepared both by a traditional Solid State Reaction method (STF-SSR) and by an alternative Sol-Gel chemical route (STF-SG). The phase stability at typical operation conditions was studied for both samples and compared. Also, the chemical compatibility with La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM electrolyte) and with La0.4Ce0.6O2−δ (LDC, usually used as protective layer [12,13,16]) was
Corresponding author. E-mail address: [email protected] (L. Mogni).
https://doi.org/10.1016/j.ssi.2019.115064 Received 19 July 2019; Received in revised form 30 August 2019; Accepted 5 September 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
Solid State Ionics 342 (2019) 115064
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Fig. 1. (a) XRD pattern confirming both samples present the perovskite structure. SEM images of (b) STF-SSR and (c) STF-SG powders, showing the detail of the grain size at the same magnification scale.
studied at specific firing temperatures. Finally, the electrochemical performance working as anode and as cathode was evaluated by Impedance Spectroscopy measurements.
2.2. Preparation and characterization of electrochemical cells with STF electrodes Electrochemical performance of electrodes was evaluated by Electrochemical Impedance Spectroscopy (EIS) in a symmetrical cell configuration. Electrolytes were prepared with La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM commercial powder, “Fuel Cell Materials”) dry-pressed and sintered at 1450 °C during 6 h. A Lanthanum Doped Ceria (LDC) buffer layer was added on the LSGM electrolyte surface. The LDC powder was previously prepared via solid-state reaction, milling stoichiometric amounts of La2O3 and CeO2 and synthesized at 1450 °C for 6 h. A LDC ink was prepared using PVB and PVP as binders, with the resulting powders dispersed in α-Terpineol and isopropyl alcohol. The LDC ink was spin coated on the LSGM electrolyte surface and fired at 1450 °C for 6 h. An electrode ink was also prepared with PVB and PVP as binders, dispersed in α-Terpineol, and spin coated on top of the LDC protective layer. The resulting cell was then sintered for 1.5 h at 900 °C in the case of STF-SG, and at 1000 °C with STF-SSR. A silver paste was used to paint a grid on top of the electrodes, and gold grids were used on top as current collectors. Cells were tested at 750 °C by changing the atmosphere from dry synthetic air (Ox) to 10% H2/3% H2O/87% Ar (Red), then back to dry air (re-Ox) and again to 10% H2/3% H2O/87% Ar (reRed), and finally to 10% CH4/3% H2O/87% Ar (Met). In all cases, gas rates of 100 ml·min−1 were used. Impedance spectra were measured using an Autolab PGSTAT32 with frequency analyzer (FRA2) module. Measurements were performed in the frequency range from 1 MHz to 10 mHz with 10 mV of AC perturbation and zero DC bias. Spectra were normalized by the electrode geometric area and divided by two to account for the symmetrical configuration. A chemical compatibility test between STF and La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) was performed before the electrochemical test, consisting in mixing both powders in equal weight amounts and firing them at different temperatures. The resulting powders were analyzed by XRD. The same test was also performed to analyze compatibility between STF and La0.4Ce0.6O2 (LDC), usually used as buffer layer between STF and LSGM [12,13,16]. These tests were also used to determine the optimum thermal treatments in different parts of the fabrication process.
2. Experimental 2.1. Preparation and characterization of STF powders Sr(Ti,Fe)O3−δ perovskites were synthesized by both solid state reaction (STF-SSR) and by a glycine combustion sol-gel method (STF-SG). Preparation of STF-SSR was performed by mixing stoichiometric amounts of SrCO3, Fe2O3 and TiO2 in a ball mill for 1.5 h, and synthetized at 1200 °C for 10 h. The STF-SG sample was prepared by mixing stoichiometric amounts of titanium butoxide (C16H36O4Ti), SrCO3 and Fe(NO3)3·9H2O. Titanium butoxide was first mixed with ethylene glycol and glycine dissolved in distilled water. SrCO3 and iron nitrate also dissolved in distilled water and HNO3 were added to the solution and mixed at ~80 °C until polymerization occurred. Once the solution was sufficiently dry it was thermally treated at 850 °C. The presence of a single phase was confirmed by XRD using a Panalytical Empyrean X-ray diffractometer (XRD) with Cu Kα radiation. The sample microstructure was investigated using Scanning Electron Microscopy (SEM) (Zeiss Crossbeam 340, FEI NanoNova and FEI InspectS50) and Transmission Electron Microscopy (TEM) (FEI TECNAI F20). In order to analyze the effect of microstructure on the stability of the phases in reducing atmosphere, both samples were analyzed by Thermogravimetric analysis (TGA) and Temperature Programmed Reduction (TPR). TGA was performed using a symmetrical thermobalance based on a Cahn 1000 electrobalance, which allows determination of sample mass changes within ± 10 μg in a controlled atmosphere between room temperature and 1100 °C. Powders were heated from room temperature to 900 °C in a dry 5% H2/95% Ar atmosphere, with a heating rate of 10 °C·min−1. TPR experiments were performed in a Micromeritics Chemisorb 2750 equipment to study the reducibility of the samples. This equipment determines H2 uptake from the sample with a Thermal Conductivity Detector (TCD) previously calibrated. Experiments were carried out from room temperature to 900 °C following a heating ramp of 10 °C·min−1 in a flow consisting of 5 mol% H2 in Ar (50 ml·min−1). Samples were pre-treated in pure He flow at 300 °C during 1 h to remove any species that might have been adsorbed in the sample surface. 2
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3. Results
also segregates metallic Fe. In this process, SrO is formed to compensate for the Fe segregation, as can be seen in TEM-EDS analysis shown in Fig. 3(b) and (c). Fig. 4 shows SEM images of the samples before and after reduction-oxidation-reduction (re-Red) cycle at 750 °C in a 10% H2/3% H2O/87% Ar atmosphere. Fig. 4(a) and (b) shows the STF-SSR electrode before and after redox cycle respectively. Fig. 4(b) shows that small particles appear preferentially in the grain boundaries. Fig. 4(c) and (d) shows a STF-SG pressed pellet sintered at 1000 °C, before and after the redox cycle respectively. In this case, there is significant segregation of metallic Fe and SrO particles. This evidence suggests that the smaller grain sizes, and thus the larger amount of grain boundaries, promote cation segregation to the surface causing the decomposition of the sample.
3.1. Sol-gel and SSR synthesis methods Fig. 1(a) shows the X-ray diffraction patterns of both STF-SSR and STF-SG samples. The formation of a single perovskite phase can be observed in both cases despite the synthesis temperature was reduced from 1200 °C (SSR method) to 850 °C with the sol-gel synthesis method presented here. Fig. 1(b) and (c) shows that, because of the lower synthesis temperature, the STF-SG sample shows a smaller grain size (~100 nm) and a higher porosity than the STF-SSR sample (~1 μm).
3.2. Phase stability 3.3. Chemical compatibility Grain size reduction and increased porosity are known to improve electrode performance because gaseous diffusion and surface processes are favored [11]. However, it is important to consider that the kinetics of reduction is faster for the sample with sub-micrometric grain size (prepared by sol-gel) compared to the sample with micrometric grain size (prepared by SSR), even when the chemical composition is the same. This leads to decomposition of the STF-SG sample in conditions in which STF-SSR is stable (at equal temperature, atmosphere and time of reduction). Fig. 2 shows the comparison between TGA (blue) and TPR (green) independent analysis of both STF-SSR (dotted) and STF-SG (straight lines) samples. The TPR curves show a first peak at ~390 °C and at ~480 °C for the STF-Sol-gel and STF-SSR samples respectively, which is associated with Fe+4/Fe+3 partial reduction to Fe+2. This process is accompanied by a faster mass loss due to oxygen releases up to ~500 °C and ~600 °C respectively, observed in the TGA curves. Two small contributions appear at around 600 °C and 700 °C, which are more evident in the STF-SG sample. These contributions may be due to incipient reduction to Fe0 although Ti+4 partial reduction to Ti+3 [21] cannot be excluded at these temperatures. A second large peak in the TPR curve starts at around 740 °C for the STF-SG and at 800 °C for the STF-SSR samples, which corresponds to Fe reduction to Fe0, and is associated with decomposition of the sample. From the TPR it is clear that all processes in the STF-SG occur at lower temperatures than in the STFSSR sample. Also, from the TGA data it is clear that the STF-SG sample losses more mass over the whole process, which means a greater Fe reduction at lower temperatures. Fig. 3(a) shows an XRD pattern of the STF-SG and STF-SSR reduced at 750 °C in a dry 10% H2/90% Ar atmosphere. It can be seen that whereas the STF-SSR is stable under such conditions, the STF-SG decomposes in different perovskite phases, and
3.3.1. Reactivity between STF and LSGM The chemical compatibility of STF-SSR with LSGM was studied by mixing the powders in equal weight amounts, and firing for 6 h at 1000 °C, 1200 °C and 1400 °C. The diffraction patterns are shown in Fig. 5(a). It can be seen that as sintering temperature increases the diffraction peaks of STF start to shift to lower angles, whereas LSGM peaks shift to higher angles. These shifts are evidence of changes in the lattice parameters (Fig. 5(b)), suggesting interdiffusion of cations between the two perovskite phases. For temperatures as high as 1400 °C, the two perovskite phases eventually form a single phase, a clear evidence of chemical reaction between STF-SSR and LSGM. This reactivity is the reason of a poor electrochemical response of an STF electrode directly deposited on LSGM, in comparison with the results obtained when a LDC buffer layer is deposited between both (results not shown here). 3.3.2. Reactivity between STF and LDC The chemical compatibility of STF-SG and STF-SSR with LDC was also studied. Mixtures of the powders in equal weight amounts were fired at 1000 °C, 1075 °C and 1150 °C for 1.5 h, which are typical thermal treatments found in the literature for firing electrode ink to the electrolyte [12,13,16]. The resulting diffraction patterns are shown in Fig. 6. No reaction was seen for either sample at 1000 °C (not shown). For STF-SG a third phase appears at 1075 °C, which could be indexed with LaSrFeO4. This third phase becomes even more significant at 1150 °C, where the LDC and STF peaks also become closer, presumably forming a new perovskite phase due to interdiffusion of cations, and a new LDC oxide with less La. It can be seen that reactivity between STF and LDC strongly depends on STF microstructure. That is, for STF-SSR no reaction is seen at 1075 °C, and the mentioned LaSrFeO4 third phase peak appears only slightly at 1150 °C. For this reasons, in order to avoid chemical reaction with LDC, the STF-SG electrode ink was fired at 900 °C and the STF-SSR at 1000 °C. 3.4. Electrochemical performance Electrochemical Impedance Spectroscopy (EIS) tests were performed both in cathode (Ox and re-Ox) and anode (Red, re-Red and Met) conditions. Fig. 7 shows Nyquist and Bode plots of the cells with STF-SG and STF-SSR working as cathode in dry air at 750 °C, before (Ox) and after (re-Ox) reduction. Fig. 8 shows Nyquist and Bode plots of the cells with STF-SG and STF-SSR working as anodes at 750 °C in a 10% H2/87% Ar/3% H2O atmosphere (Red), and also in a 10% CH4/ 87% Ar/3% H2O atmosphere (Met). It can be noted that, as cathode, there is not a significant difference in the total polarization resistance of both samples, despite the differences in microstructure. However, in the Bode plot the maximum appears at lower frequencies for the STF-SSR than for STF-SG. In both cases, the EIS spectra show at least two contributions, with the low
Fig. 2. TGA (blue) and TPR (green) curves of STF-SSR (dotted) and STF-SG (straight lines) samples, performed at a temperature increase rate of 10 °C·min−1 in a dry 5% H2/95% Ar atmosphere. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 3. (a) XRD data of the STF-SSR (green) and STF-SG (blue) samples reduced at 750 °C in a dry 10% H2/90% Ar atmosphere. (b) and (c) TEM images of the STF-SG electrode after reduction at 750 °C in wet 10% H2/90% Ar. EDS analysis (not shown) reveals segregation of Fe and SrO in this soft reducing condition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Comparison between STF porous electrode and STF dense pellet before and after redox cycles. STF-SSR porous electrode (a) before and (b) after reduction-oxidation-reduction cycle at 750 °C in a 10% H2/87% Ar/3% H2O atmosphere. STF-SG pressed and sintered pellet (c) before, and (d) after the same reduction-oxidation-reduction cycle.
reduction. The impedance response of STF as anode in wet 10% H2 shows the presence of at least two arcs in both electrodes. In contrast to that observed for cathode response, the anode polarization resistance of
frequency arc probably corresponding to O2-gas diffusion. The STF-SG sample reduces slightly the polarization resistance after re-oxidation (re-Ox), whereas both samples show a shift to higher frequencies in the Bode plot. These behaviors suggest an activation of the electrodes after 4
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Fig. 5. Chemical compatibility test between STF-SSR and LSGM electrolyte. (a) XRD of STF-SSR/LSGM powder mixtures sintered at different temperatures and (b) calculated lattice parameters of the XRD data.
Fig. 6. XRD of LDC powder mixed with (a) STF-SG and (b) STF-SSR.
Fig. 7. Impedance spectra (Bode to the left and Nyquist to the right) of symmetrical STF-SSR and STF-SG cells in dry synthetic air, before (Ox) and after reduction (reOx). 5
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Fig. 8. Impedance spectra (Bode to the left and Nyquist to the right) of symmetrical STF-SSR and STF-SG cells in wet 10% H2 atmosphere (Red), and in a wet 10% CH4 atmosphere (Met).
chemical route, achieving much lower sintering temperature (i.e. 350 °C lower) than the sintering temperature needed for the Solid State Reaction method, and thus obtaining smaller grain sizes and higher porosity. However, STF-SG was observed to be more unstable in reducing atmospheres, showing segregation of metallic Fe and SrO at lower temperatures as compared to STF-SSR. Interdiffusion of cations between STF-SSR and LSGM was observed, confirming the need of a buffer protective layer. Formation of a secondary phase and interdiffusion of cations between the STF and the LDC protective layer was also observed, especially for the STF-SG sample with lower particle size. However, this reaction does not seem to affect the cell performance negatively. EIS measurements reveal that both samples have similar polarization resistances working as cathode. However, STF-SG works remarkably better than STF-SSR as anode, both in H2 and in CH4 atmospheres. It was also observed that after reduction (re-Ox), both samples show a shift to higher frequencies in the Bode plot. These changes suggest activation of the surface, which may be induced by surface Fe reduction. Acknowledgments
Fig. 9. Comparison of polarization resistance of STF-SG and STF-SSR, changing from dry air to reducing atmospheres, following a redox cycle.
The authors would like to acknowledge to ANPCyT PICT-2016-2965 and CONICET PIP 0565. Also, authors would like to acknowledge to Alberto Baruj and Paula Troyon for their assistance with SEM. No competing interests to declare.
STF-SG is almost half of the STF-SSR value, indicating that surface modification has a higher impact in the fuel oxidation process. The RedOx cycling reduces the polarization resistance, but shifts in frequency become less evident (results not shown here) due to the major overlapping between arcs. As can be expected, the polarization resistances of both samples in 10% CH4 are higher than in 10% H2, being the performance of STF-SG also superior to that of the STF-SSR. Finally, Fig. 9 summarizes the comparison between the polarization resistance (Rp) for both samples in different gas conditions. It can be observed that cathode polarization resistance is one order of magnitude lower than anode polarization resistance. STF-SG presents the lower Rp for all conditions, showing larger differences as anode in H2 and CH4 fuels.
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4. Conclusions SrTi0.3Fe0.7O3−δ single perovskite phase was obtained by a Sol-Gel 6
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Study of phase stability of SrTi0.3Fe0.7O3−δ perovskite in reducing atmosphere: Effect of microstructure Mariano Santaya, Lucia Toscani, Laura Baqué, Horacio E. Troiani, Liliana Mogni
T
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CNEA-CONICET, Department of Materials Characterization, Bariloche Atomic Centre, Av. Bustillo 9500, CP8400, Argentina
A R T I C LE I N FO
A B S T R A C T
Keywords: STF SOFC electrode MIEC Perovskite Microstructure
Increasing SOFC electrode's surface area by modification of its microstructure is a well-known technique to reduce electrode polarization resistance. This is because reduced grain size and increased porosity promote diffusion and surface reactions, thus improving the electrode performance. However, a modified microstructure also causes differences in phase stability and in chemical compatibility with other SOFC materials. In this work, we study the effect of particle size in both the electrode performance and the phase stability under different fuel conditions and temperatures. SrTi0.3Fe0.7O3−δ (STF) is both prepared via solid state reaction (STF-SSR) and also by an alternative sol-gel route (STF-SG). The sintering temperature is reduced dramatically with the sol-gel method, hence inducing a higher porosity and a much smaller grain size. As particle size is reduced the stability under fuel conditions is also diminished, so decomposition induced by segregation of metallic Fe and SrO occurs at lower temperatures for the STF-SG sample. The stability under reducing conditions is studied by combined techniques such as TGA, TPR, XRD, SEM and TEM. Performance as anode and cathode is evaluated by Electrochemical Impedance Spectroscopy (EIS) by using electrolyte supported symmetrical cells. Prior to electrochemical experiments, the reactivity between La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) electrolyte and STF was studied, and also between STF and a Lanthanum Doped Ceria (LDC) buffer layer. It is seen that microstructure also plays a key role in the chemical stability of the STF. The impact of particle size reduction is higher for the anodic polarization resistance, which is reduced twice from STF-SSR to STF-SG.
1. Introduction Solid oxide fuel cell (SOFC) anodes composed of mixed ionic and electronic conducting (MIEC) perovskites are emerging as alternatives to Ni-YSZ anodes, because they can provide improved tolerance to redox cycling, fuel impurities, and hydrocarbon fuels [1–5]. However, their polarization resistance is typically higher than that of Ni-based electrodes, leading to lower cell power densities [6]. Recently, new developments in materials science are aiming to reduce the anode polarization resistance of MIEC materials through surface and microstructure modification [7–11], and improved doping strategies [12–14]. Over the past years, SrTiO3 based perovskites have attracted particular attention due to their good stability under reducing conditions, tolerance to hydrocarbon fuels and their relatively good n-type conductivity [15]. Among these materials, SrTi0.3Fe0.7O3−δ (STF) has been recently found to yield a power density of 1.1 W cm2 at 850 °C in humidified hydrogen, approaching the values achieved in state-of-theart Ni-YSZ anodes [16]. The microstructural modification of traditional electrode materials
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is a common strategy to achieve a better performance at lower temperatures [7,17–19]. In the case of MIEC materials, this is because chemical reactions can occur in the whole surface area of the electrode (they are not restricted only to Triple Phase Boundaries) [20], and so an increase in the electrode surface area has a direct impact in the reaction rate. In addition, when the microstructure is tuned to achieve a more porous electrode, gas diffusion is also favored. All these factors contribute to a reduction in the electrode polarization resistance, which results into higher cell power densities. However, it is also important to consider that smaller grain sizes and more grain boundaries increase the Gibbs energy of the system and result in a thermodynamically less stable material. This also affects chemical reactivity between the material and other SOFC components, which occurs at lower temperatures. In this study, STF perovskites were prepared both by a traditional Solid State Reaction method (STF-SSR) and by an alternative Sol-Gel chemical route (STF-SG). The phase stability at typical operation conditions was studied for both samples and compared. Also, the chemical compatibility with La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM electrolyte) and with La0.4Ce0.6O2−δ (LDC, usually used as protective layer [12,13,16]) was
Corresponding author. E-mail address: [email protected] (L. Mogni).
https://doi.org/10.1016/j.ssi.2019.115064 Received 19 July 2019; Received in revised form 30 August 2019; Accepted 5 September 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) XRD pattern confirming both samples present the perovskite structure. SEM images of (b) STF-SSR and (c) STF-SG powders, showing the detail of the grain size at the same magnification scale.
studied at specific firing temperatures. Finally, the electrochemical performance working as anode and as cathode was evaluated by Impedance Spectroscopy measurements.
2.2. Preparation and characterization of electrochemical cells with STF electrodes Electrochemical performance of electrodes was evaluated by Electrochemical Impedance Spectroscopy (EIS) in a symmetrical cell configuration. Electrolytes were prepared with La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM commercial powder, “Fuel Cell Materials”) dry-pressed and sintered at 1450 °C during 6 h. A Lanthanum Doped Ceria (LDC) buffer layer was added on the LSGM electrolyte surface. The LDC powder was previously prepared via solid-state reaction, milling stoichiometric amounts of La2O3 and CeO2 and synthesized at 1450 °C for 6 h. A LDC ink was prepared using PVB and PVP as binders, with the resulting powders dispersed in α-Terpineol and isopropyl alcohol. The LDC ink was spin coated on the LSGM electrolyte surface and fired at 1450 °C for 6 h. An electrode ink was also prepared with PVB and PVP as binders, dispersed in α-Terpineol, and spin coated on top of the LDC protective layer. The resulting cell was then sintered for 1.5 h at 900 °C in the case of STF-SG, and at 1000 °C with STF-SSR. A silver paste was used to paint a grid on top of the electrodes, and gold grids were used on top as current collectors. Cells were tested at 750 °C by changing the atmosphere from dry synthetic air (Ox) to 10% H2/3% H2O/87% Ar (Red), then back to dry air (re-Ox) and again to 10% H2/3% H2O/87% Ar (reRed), and finally to 10% CH4/3% H2O/87% Ar (Met). In all cases, gas rates of 100 ml·min−1 were used. Impedance spectra were measured using an Autolab PGSTAT32 with frequency analyzer (FRA2) module. Measurements were performed in the frequency range from 1 MHz to 10 mHz with 10 mV of AC perturbation and zero DC bias. Spectra were normalized by the electrode geometric area and divided by two to account for the symmetrical configuration. A chemical compatibility test between STF and La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) was performed before the electrochemical test, consisting in mixing both powders in equal weight amounts and firing them at different temperatures. The resulting powders were analyzed by XRD. The same test was also performed to analyze compatibility between STF and La0.4Ce0.6O2 (LDC), usually used as buffer layer between STF and LSGM [12,13,16]. These tests were also used to determine the optimum thermal treatments in different parts of the fabrication process.
2. Experimental 2.1. Preparation and characterization of STF powders Sr(Ti,Fe)O3−δ perovskites were synthesized by both solid state reaction (STF-SSR) and by a glycine combustion sol-gel method (STF-SG). Preparation of STF-SSR was performed by mixing stoichiometric amounts of SrCO3, Fe2O3 and TiO2 in a ball mill for 1.5 h, and synthetized at 1200 °C for 10 h. The STF-SG sample was prepared by mixing stoichiometric amounts of titanium butoxide (C16H36O4Ti), SrCO3 and Fe(NO3)3·9H2O. Titanium butoxide was first mixed with ethylene glycol and glycine dissolved in distilled water. SrCO3 and iron nitrate also dissolved in distilled water and HNO3 were added to the solution and mixed at ~80 °C until polymerization occurred. Once the solution was sufficiently dry it was thermally treated at 850 °C. The presence of a single phase was confirmed by XRD using a Panalytical Empyrean X-ray diffractometer (XRD) with Cu Kα radiation. The sample microstructure was investigated using Scanning Electron Microscopy (SEM) (Zeiss Crossbeam 340, FEI NanoNova and FEI InspectS50) and Transmission Electron Microscopy (TEM) (FEI TECNAI F20). In order to analyze the effect of microstructure on the stability of the phases in reducing atmosphere, both samples were analyzed by Thermogravimetric analysis (TGA) and Temperature Programmed Reduction (TPR). TGA was performed using a symmetrical thermobalance based on a Cahn 1000 electrobalance, which allows determination of sample mass changes within ± 10 μg in a controlled atmosphere between room temperature and 1100 °C. Powders were heated from room temperature to 900 °C in a dry 5% H2/95% Ar atmosphere, with a heating rate of 10 °C·min−1. TPR experiments were performed in a Micromeritics Chemisorb 2750 equipment to study the reducibility of the samples. This equipment determines H2 uptake from the sample with a Thermal Conductivity Detector (TCD) previously calibrated. Experiments were carried out from room temperature to 900 °C following a heating ramp of 10 °C·min−1 in a flow consisting of 5 mol% H2 in Ar (50 ml·min−1). Samples were pre-treated in pure He flow at 300 °C during 1 h to remove any species that might have been adsorbed in the sample surface. 2
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3. Results
also segregates metallic Fe. In this process, SrO is formed to compensate for the Fe segregation, as can be seen in TEM-EDS analysis shown in Fig. 3(b) and (c). Fig. 4 shows SEM images of the samples before and after reduction-oxidation-reduction (re-Red) cycle at 750 °C in a 10% H2/3% H2O/87% Ar atmosphere. Fig. 4(a) and (b) shows the STF-SSR electrode before and after redox cycle respectively. Fig. 4(b) shows that small particles appear preferentially in the grain boundaries. Fig. 4(c) and (d) shows a STF-SG pressed pellet sintered at 1000 °C, before and after the redox cycle respectively. In this case, there is significant segregation of metallic Fe and SrO particles. This evidence suggests that the smaller grain sizes, and thus the larger amount of grain boundaries, promote cation segregation to the surface causing the decomposition of the sample.
3.1. Sol-gel and SSR synthesis methods Fig. 1(a) shows the X-ray diffraction patterns of both STF-SSR and STF-SG samples. The formation of a single perovskite phase can be observed in both cases despite the synthesis temperature was reduced from 1200 °C (SSR method) to 850 °C with the sol-gel synthesis method presented here. Fig. 1(b) and (c) shows that, because of the lower synthesis temperature, the STF-SG sample shows a smaller grain size (~100 nm) and a higher porosity than the STF-SSR sample (~1 μm).
3.2. Phase stability 3.3. Chemical compatibility Grain size reduction and increased porosity are known to improve electrode performance because gaseous diffusion and surface processes are favored [11]. However, it is important to consider that the kinetics of reduction is faster for the sample with sub-micrometric grain size (prepared by sol-gel) compared to the sample with micrometric grain size (prepared by SSR), even when the chemical composition is the same. This leads to decomposition of the STF-SG sample in conditions in which STF-SSR is stable (at equal temperature, atmosphere and time of reduction). Fig. 2 shows the comparison between TGA (blue) and TPR (green) independent analysis of both STF-SSR (dotted) and STF-SG (straight lines) samples. The TPR curves show a first peak at ~390 °C and at ~480 °C for the STF-Sol-gel and STF-SSR samples respectively, which is associated with Fe+4/Fe+3 partial reduction to Fe+2. This process is accompanied by a faster mass loss due to oxygen releases up to ~500 °C and ~600 °C respectively, observed in the TGA curves. Two small contributions appear at around 600 °C and 700 °C, which are more evident in the STF-SG sample. These contributions may be due to incipient reduction to Fe0 although Ti+4 partial reduction to Ti+3 [21] cannot be excluded at these temperatures. A second large peak in the TPR curve starts at around 740 °C for the STF-SG and at 800 °C for the STF-SSR samples, which corresponds to Fe reduction to Fe0, and is associated with decomposition of the sample. From the TPR it is clear that all processes in the STF-SG occur at lower temperatures than in the STFSSR sample. Also, from the TGA data it is clear that the STF-SG sample losses more mass over the whole process, which means a greater Fe reduction at lower temperatures. Fig. 3(a) shows an XRD pattern of the STF-SG and STF-SSR reduced at 750 °C in a dry 10% H2/90% Ar atmosphere. It can be seen that whereas the STF-SSR is stable under such conditions, the STF-SG decomposes in different perovskite phases, and
3.3.1. Reactivity between STF and LSGM The chemical compatibility of STF-SSR with LSGM was studied by mixing the powders in equal weight amounts, and firing for 6 h at 1000 °C, 1200 °C and 1400 °C. The diffraction patterns are shown in Fig. 5(a). It can be seen that as sintering temperature increases the diffraction peaks of STF start to shift to lower angles, whereas LSGM peaks shift to higher angles. These shifts are evidence of changes in the lattice parameters (Fig. 5(b)), suggesting interdiffusion of cations between the two perovskite phases. For temperatures as high as 1400 °C, the two perovskite phases eventually form a single phase, a clear evidence of chemical reaction between STF-SSR and LSGM. This reactivity is the reason of a poor electrochemical response of an STF electrode directly deposited on LSGM, in comparison with the results obtained when a LDC buffer layer is deposited between both (results not shown here). 3.3.2. Reactivity between STF and LDC The chemical compatibility of STF-SG and STF-SSR with LDC was also studied. Mixtures of the powders in equal weight amounts were fired at 1000 °C, 1075 °C and 1150 °C for 1.5 h, which are typical thermal treatments found in the literature for firing electrode ink to the electrolyte [12,13,16]. The resulting diffraction patterns are shown in Fig. 6. No reaction was seen for either sample at 1000 °C (not shown). For STF-SG a third phase appears at 1075 °C, which could be indexed with LaSrFeO4. This third phase becomes even more significant at 1150 °C, where the LDC and STF peaks also become closer, presumably forming a new perovskite phase due to interdiffusion of cations, and a new LDC oxide with less La. It can be seen that reactivity between STF and LDC strongly depends on STF microstructure. That is, for STF-SSR no reaction is seen at 1075 °C, and the mentioned LaSrFeO4 third phase peak appears only slightly at 1150 °C. For this reasons, in order to avoid chemical reaction with LDC, the STF-SG electrode ink was fired at 900 °C and the STF-SSR at 1000 °C. 3.4. Electrochemical performance Electrochemical Impedance Spectroscopy (EIS) tests were performed both in cathode (Ox and re-Ox) and anode (Red, re-Red and Met) conditions. Fig. 7 shows Nyquist and Bode plots of the cells with STF-SG and STF-SSR working as cathode in dry air at 750 °C, before (Ox) and after (re-Ox) reduction. Fig. 8 shows Nyquist and Bode plots of the cells with STF-SG and STF-SSR working as anodes at 750 °C in a 10% H2/87% Ar/3% H2O atmosphere (Red), and also in a 10% CH4/ 87% Ar/3% H2O atmosphere (Met). It can be noted that, as cathode, there is not a significant difference in the total polarization resistance of both samples, despite the differences in microstructure. However, in the Bode plot the maximum appears at lower frequencies for the STF-SSR than for STF-SG. In both cases, the EIS spectra show at least two contributions, with the low
Fig. 2. TGA (blue) and TPR (green) curves of STF-SSR (dotted) and STF-SG (straight lines) samples, performed at a temperature increase rate of 10 °C·min−1 in a dry 5% H2/95% Ar atmosphere. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 3. (a) XRD data of the STF-SSR (green) and STF-SG (blue) samples reduced at 750 °C in a dry 10% H2/90% Ar atmosphere. (b) and (c) TEM images of the STF-SG electrode after reduction at 750 °C in wet 10% H2/90% Ar. EDS analysis (not shown) reveals segregation of Fe and SrO in this soft reducing condition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Comparison between STF porous electrode and STF dense pellet before and after redox cycles. STF-SSR porous electrode (a) before and (b) after reduction-oxidation-reduction cycle at 750 °C in a 10% H2/87% Ar/3% H2O atmosphere. STF-SG pressed and sintered pellet (c) before, and (d) after the same reduction-oxidation-reduction cycle.
reduction. The impedance response of STF as anode in wet 10% H2 shows the presence of at least two arcs in both electrodes. In contrast to that observed for cathode response, the anode polarization resistance of
frequency arc probably corresponding to O2-gas diffusion. The STF-SG sample reduces slightly the polarization resistance after re-oxidation (re-Ox), whereas both samples show a shift to higher frequencies in the Bode plot. These behaviors suggest an activation of the electrodes after 4
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Fig. 5. Chemical compatibility test between STF-SSR and LSGM electrolyte. (a) XRD of STF-SSR/LSGM powder mixtures sintered at different temperatures and (b) calculated lattice parameters of the XRD data.
Fig. 6. XRD of LDC powder mixed with (a) STF-SG and (b) STF-SSR.
Fig. 7. Impedance spectra (Bode to the left and Nyquist to the right) of symmetrical STF-SSR and STF-SG cells in dry synthetic air, before (Ox) and after reduction (reOx). 5
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Fig. 8. Impedance spectra (Bode to the left and Nyquist to the right) of symmetrical STF-SSR and STF-SG cells in wet 10% H2 atmosphere (Red), and in a wet 10% CH4 atmosphere (Met).
chemical route, achieving much lower sintering temperature (i.e. 350 °C lower) than the sintering temperature needed for the Solid State Reaction method, and thus obtaining smaller grain sizes and higher porosity. However, STF-SG was observed to be more unstable in reducing atmospheres, showing segregation of metallic Fe and SrO at lower temperatures as compared to STF-SSR. Interdiffusion of cations between STF-SSR and LSGM was observed, confirming the need of a buffer protective layer. Formation of a secondary phase and interdiffusion of cations between the STF and the LDC protective layer was also observed, especially for the STF-SG sample with lower particle size. However, this reaction does not seem to affect the cell performance negatively. EIS measurements reveal that both samples have similar polarization resistances working as cathode. However, STF-SG works remarkably better than STF-SSR as anode, both in H2 and in CH4 atmospheres. It was also observed that after reduction (re-Ox), both samples show a shift to higher frequencies in the Bode plot. These changes suggest activation of the surface, which may be induced by surface Fe reduction. Acknowledgments
Fig. 9. Comparison of polarization resistance of STF-SG and STF-SSR, changing from dry air to reducing atmospheres, following a redox cycle.
The authors would like to acknowledge to ANPCyT PICT-2016-2965 and CONICET PIP 0565. Also, authors would like to acknowledge to Alberto Baruj and Paula Troyon for their assistance with SEM. No competing interests to declare.
STF-SG is almost half of the STF-SSR value, indicating that surface modification has a higher impact in the fuel oxidation process. The RedOx cycling reduces the polarization resistance, but shifts in frequency become less evident (results not shown here) due to the major overlapping between arcs. As can be expected, the polarization resistances of both samples in 10% CH4 are higher than in 10% H2, being the performance of STF-SG also superior to that of the STF-SSR. Finally, Fig. 9 summarizes the comparison between the polarization resistance (Rp) for both samples in different gas conditions. It can be observed that cathode polarization resistance is one order of magnitude lower than anode polarization resistance. STF-SG presents the lower Rp for all conditions, showing larger differences as anode in H2 and CH4 fuels.
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