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Evaluating the effect of Pr-doping on the performance of strontium-doped lanthanum ferrite cathodes for protonic SOFCs
Journal Pre-proof Evaluating the effect of Pr-doping on the performance of strontium-doped lanthanum ferrite cathodes for protonic SOFCs Jinming Ma, Zetian Tao, Hongning Kou, Marco Fronzi, Lei Bi PII:
S0272-8842(19)32854-8
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.017
Reference:
CERI 23085
To appear in:
Ceramics International
Received Date: 30 August 2019 Revised Date:
23 September 2019
Accepted Date: 2 October 2019
Please cite this article as: J. Ma, Z. Tao, H. Kou, M. Fronzi, L. Bi, Evaluating the effect of Pr-doping on the performance of strontium-doped lanthanum ferrite cathodes for protonic SOFCs, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.017. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Evaluating the effect of Pr-doping on the performance of strontium-doped lanthanum ferrite cathodes for protonic SOFCs Jinming Ma a,1, Zetian Tao b,1, Hongning Kou c,1, Marco Fronzi d, Lei Bi a∗ a) Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Ningxia Road No.308, Qingdao 266071, China b) Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, China c) Peng Cheng Laboratory, No.2 Xingke 1st street, Nanshan District, Shenzhen, Guangdong, China d) International Research Centre for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi’an, China
∗
Corresponding author. Tel./Fax: +86-532-85951496 Email address: [email protected] and [email protected] (L. Bi)
1. These authors contributed equally to this paper 1
Abstract A Pr-doping strategy was used to improve traditional strontium-doped lanthanum ferrite oxides for proton-conducting solid oxide fuel cells (SOFCs). Three different samples,
La0.5Sr0.5FeO3-δ,
La0.25Pr0.25Sr0.5FeO3-δ,
and
Pr0.5Sr0.5FeO3-δ
,were
successfully prepared. The Pr content was shown to have an obvious influence on the hydration ability of the materials. Hydration was improved at higher Pr-contents, suggesting a promising cathode performance. However, the improved hydration ability did not always lead to an increased fuel cell performance, and it was found that the fuel cell performed best when an appropriate Pr-doping amount was used that resulted in a good compromise between protonic and oxygen-ion conduction. As a result, the optimized composition La0.25Pr025Sr0.5FeO3-δ generated a high peak power density of 616 mW cm-2 and a low polarization resistance of 0.09 at Ω cm2 at 700 °C, which is an encouraging performance for a traditional cathode material.
Keywords: LaFeO3; Cathode; Protonic SOFC; Oxides;
2
1. Introduction Proton-conducting SOFCs (H-SOFCs) are receiving increased attention due to their advantageous low temperature operation, which solves the operating temperature problems of traditional high temperature SOFCs[1-5]Error! Bookmark not defined.. As the cathode is a central component of H-SOFCs, development of cathodes for H-SOFCs has become a popular research subject. In recent decades, several oxide materials were evaluated as cathodes for H-SOFCs and many other types of compounds have been proposed for use in H-SOFC cathodes[6-8]. Discovering new, high performance cathode materials is one direction for improving H-SOFC cathodes[9, 10]. Alternatively, some recent studies have shown that modification of classical cathode materials can also improve the electrochemical performance of H-SOFC cathodes[11, 12]. Among classical cathode materials, Sr-doped LaFeO3 has been reported to be suitable for H-SOFCs due to its low overpotential, which is even lower than the overpotential of Sr-doped LaCoO3[13]. Additionally, Sr-doped LaFeO3 has adequate chemical stability and improved thermal stability when compared with recently proposed cobalt-containing cathodes[14]. Previous studies have demonstrated that Sr-doped LaFeO3 cathodes enabled H-SOFCs to have reasonable fuel cell performance[15]. However, any further improvements in fuel cell performance might be difficult for Sr-doped LaFeO3 cathode because of its moderate oxygen ion conduction and low proton conduction. Therefore, enhancing proton mobility without 3
dramatically impairing the oxygen-ion conduction is a rational way to design Sr-doped LaFeO3-based cathodes for H-SOFCs[16]. In this study, Pr was used to dope Sr-doped LaFeO3 to form new cathode compositions for use in H-SOFCs. Pr was selected as the dopant for Sr-doped LaFeO3 because Pr-containing compounds have demonstrated to show considerable hydration ability and thus potential proton migration ability both in electrolyte material[17] and cathode materials[18]. Pr was used as the dopant for Y-doped BaZrO3, allowing the material to show good protonic conduction[17]. Also, the doping strategy was found to be effective to improve the hydration ability for cathode materials for H-SOFCs. For instance, Zn was found to be beneficial for the hydration of Ba0.5Sr0.5FeO3-based materials[19], while K-doping strategy was demonstrated to improve the hydration ability
of
Ba0.5Sr0.5Co0.8Fe0.2O3
material[20].
The
influence
of
Pr-doping
concentration on the material’s structure, hydration ability and final fuel cell performance were studied.
2. Experimental La0.5-xPrxSr0.5FeO3-δ (x=0, 0.25 and 0.5) ceramic oxides were synthesized via a wet chemical route[21]. Briefly, La(NO3)3, Pr(NO3)3, Sr(NO3)2 and Fe(NO3)3 were used as the starting materials and dissolved in water, followed by the addition of citric acid as the complexing agent. The solution was heated under stirring and the ignited, forming black ashes. The ashes were calcined at 950 oC for 3 h to form the target 4
ceramic oxides. La0.5-xPrxSr0.5FeO3-δ (x=0, 0.25 and 0.5) were named LSF, LPSF and PSF, respectively. The La0.5-xPrxSr0.5FeO3-δ powders were mixed with BZY powder and then fired at 950 °C. XRD was used to check the chemical compatibility of these materials. The hydration ability of the La0.5-xPrxSr0.5FeO3-δ (x=0, 0.25 and 0.5) materials was studied using theoretical calculations[22, 23]. We performed all calculations using density functional theory (DFT)[24], as was implemented in the Vienna ab initio simulation package (VASP)[25-27]. The simulation details can be found in our previous studies[20, 28]. To test the performance of the La0.5-xPrxSr0.5FeO3-δ ceramic oxides, these oxides were used as cathodes in H-SOFCs. The BaCe0.7Zr0.1Y0.2O3-δ (BCZY) half-cells were co-pressed and co-fired. The anode powder was prepared by mixing BCZY powder with commercial NiO at a weight ratio of 2 to 3. Then, 20 wt.% of starch was added for creating pores after sintering. BCZY electrolyte powder was deposited onto the BCZY-NiO anode substrate by a co-pressing method and then co-sintered at 1400 oC for 6 h. LSF, LPSF, and PSF was mixed with BZY and then deposited on the BCZY electrolyte and fired at 950 °C. Following firing, complete cells were formed and tested. Scanning electron microscope (SEM) was used to observe the morphologies of the tested cells.
3. Results and Discussion 5
Figure 1 presents the XRD patterns of the LSF, LPSF and PSF materials as well as patterns for the composite LSF-BZY, LPSF-BZY and PSF-BZY materials after firing at 1000 °C. The LSF, LPSF and PSF powders exhibit pure phases without detectable impurities. Further analysis of the lattice parameter of LSF, LPSF and PSF powders indicates a linear shrinkage of the lattice parameter as shown in Figure 1(d), suggesting the replacement of La by Pr reduces the lattice parameter. The reduced lattice parameter suggests the incorporation of Pr into the lattice due to the smaller ionic radius of the Pr cation compared with that of the La cation. The elemental distribution detected by SEM-EDS as shown in Figure 2 also indicates the appearance of Pr for LPSF and PSF powders and the disappearance of Pr for LSF powder. The LSF, LPSF and PSF powders also show good chemical compatibility with the BZY proton-conducting oxide, which is demonstrated by the chemical compatibility tests. No new phases except for the original La0.5-xPrxSr0.5FeO3-δ and BZY phases were found after co-firing the La0.5-xPrxSr0.5FeO3-δ (x=0, 0.25 and 0.5) powders together with BZY at 950 °C for 2 h. It should be noted that the purpose of using the Pr-dopant in Sr-doped LaFeO3 is to enhance the hydration ability of the material, making it more suitable for use in H-SOFCs. Therefore, it is important that the hydration ability of the new doped oxide is tested. According to the formation mechanism of proton defects[29] that is shown in the equation below, in which the lattice oxygen and
∙
∙∙
represents the oxygen vacancy,
×
represents
is the proton defect. The hydration ability of the doped 6
oxide could be anticipated by comparing the proton formation energies of the material. +
∙∙
+
×
⇔ 2
∙
(1)
Therefore, a theoretical calculation was employed to calculate the proton formation energy according to equation 2. ∆Ehydration = E2OH − Edefect − EH 2O
(2)
During the proton formation process, H2O in the form of H-OH occupies one oxygen vacancy with OH, and another H connects with the lattice oxygen to form O-H, as shown in Figure 3. According to the theoretical calculations, the hydration energies for LSF, LPSF, and PSF are -0.68, -1.37 and -1.64 eV, respectively, as shown in Table 1. All the hydration energies for these three materials are negative, indicating that hydration is thermodynamically favourable. PSF has the lowest hydration energy and the hydration energy decreases with increasing amounts of Pr-doping, implying that the formation of protons becomes easier with increased Pr-doping. Therefore, it is expected the fuel cell performance of Pr-doped samples will be better when compared with the Pr-free samples. However, it should also be noted that the hydration energy decreases significantly with the doping of Pr for the transition from LSF to LPSF (from -0.68 eV to -1.37 eV), while the difference is not as apparent for the transition from LPSF to PSF ( from -1.37 eV to -1.64 eV), suggesting that further increases of Pr concentration in the cathode sample do not greatly improve the hydration ability. Figure 4 shows the performance of the BCZY-based cells with LSF, LPSF and 7
PSF cathodes. It can be seen that the fuel cell performance of the LSF, LPSF and PSF cell reaches 356, 616 and 570 mW cm-2 at 700 °C, respectively. The values of the LPSF cell is clearly higher than the values of the LSF cell, but the LPSF cell is only slightly better than the PSF cell. The fuel cell performance of these cathodes is promising if one considers that no cobalt is used in these cathodes. Even comparing recently reported H-SOFCs using LaNi0.6Fe0.4O3-δ nanofibers which reach 551 mW cm-2 at 700 °C[30], the current LPSF cell show an evident advantage in power output. The performance is also higher than that of the cell using SrSc0.175Nb0.025Co0.8O3-δ cathode that reaches 498 mW cm-2 at 700 °C[31]. This evidence suggests the improvement of hydration for the material is a rational way of designing the cathode for H-SOFCs[32]. Furthermore, the cells used in these tests are normal cells without optimized microstructures, implying that the performance could be enhanced further with proper microstructure optimization[33-37]. As the motivation of using Pr-doping is to improve the material’s hydration ability to enhance the fuel cell performance, the fuel cell testing results in Figure 4 seem to contradict the theoretical predictions of the hydration abilities of the synthesized materials. From the theoretical calculation results, it is expected that the PSF cell would show the highest fuel cell performance because the PSF cell has the lowest hydration energy among the LSF, LPSF and PSF samples. However, the experimental results indicated that LPSF instead of PSF shows the best performance in fuel cell applications. Therefore, other factors beside the hydration ability govern 8
fuel cell performance. Figure 5 shows SEM images of the LSF, LPSF and PSF cells. All three of these cells have a similar microstructure. The electrolyte is approximately 10 µm thick, and the adherence of the electrolyte to the electrodes is good for all three cells. No cracks in the electrolyte or delamination at the interface was observed. The similarity of cell morphology for the three cells suggests that the difference in fuel cell performance is mainly a result of the materials, not from the microstructure. Figure 6 shows impedance plots of three cells. The ohmic resistance of all three cells is similar, which is likely due to the same batch of cells being used for this study. It is generally accepted that the ohmic resistance of the cell mainly comes from the electrolyte and interfacial contacts[38], and the similarity in cell microstructures means the differences in ohmic resistance could be ignored, which leads to the similar ohmic resistances for all three cells. Conversely, the polarization shows some differences between the three cells. The polarization resistance is 0.23, 0.09 and 0.1Ω cm2 for LSF, LPSF, and PSF-based cells, respectively, for the same testing temperature. Like the cell performance above, the polarization of the LPSF cell is much smaller than that of the LSF cell and it is only slightly lower than that of the PSF cell. The ohmic resistance and the polarization resistance for the LPSF cell are 0.229 and 0.09 Ω cm2, respectively. In contrast, the corresponding value for PSF cell is 0.246 and 0.1 Ω cm2, respectively. Although we tried to keep the tested half-cells identical and prepared them in the same way, there may be still some tiny differences from one half-cell to another. The ohmic resistance is mainly related to the electrolyte 9
resistance and the interfacial contact, while the polarization resistance reflects the performance of the electrode. Due to the similarity of the cell structure, the difference in polarization resistance should come from differences in the cathode material used. The hydration ability of the cathodes was shown to follow the order of LSF
parameter, leading to similar oxygen vacancy formation energies for LSF and PSF. Also, oxygen vacancy formation energy does not completely equal to the oxygen migration ability and it has been reported that Pr-doping decreases the oxygen-ion conductivity of the material[39], which might be the cause the slightly lower performance of the PSF-cell when compared to the LPSF-cell. The polarization resistance of LPSF cell and PSF cell is quite similar (only 10% difference), which is probably due to the compromise between the proton and oxygen-ion diffusion ability. Therefore, it can be concluded that the best cathode performance could be achieved by properly balancing the proton and oxygen-ion diffusion ability of the cathode material.
4. Conclusions Pr doped LSF materials were synthesised as potential cathode materials for H-SOFCs. Pr was found to be beneficial for the hydration ability of La0.5-xPrxSr0.5FeO3-δ oxides and the hydration energy of the oxides decreased with increased amounts of Pr, suggesting more facile proton mobility in the high Pr-containing oxides. However, the fuel cell results showed that the PSF cell did not yield the highest fuel cell performance, while the LPSF cell had the best cell performance. Electrochemical studies suggested that a proper compromise between the oxygen and proton conduction is critical for cathode performance, which provides guidance for future cathode designs. 11
Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province (Grant No.: ZR2018JL017). The Work was Supported by Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province
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18
Captions Figure 1. XRD patterns for the (a) LSF, (b) LPSF and (c) PSF powders before and after firing with BZY powder; (d) the calculated lattice parameters for the powders. Figure 2. Elemental analysis of the LSF, LPSF and PSF powders by using SEM-EDS. Figure 3. Scheme for the proton formation process in the PSF oxide. Figure 4. Fuel cell performance for different cells tested at 700 oC. Figure 5. SEM images of the (a) LSF, (c) LPSF, (e) PSF single cells and the (b) LSF, (d) LPSF, (f) PSF cathodes. Figure 6. Impedance plots of the cells tested at 700 °C.
Table 1. Hydration energies for LSF, LPSF and PSF.
19
Table 1 Material Hydration energy (eV)
LSF -0.68
LPSF -1.37
PSF -1.64
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(19)32854-8
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.017
Reference:
CERI 23085
To appear in:
Ceramics International
Received Date: 30 August 2019 Revised Date:
23 September 2019
Accepted Date: 2 October 2019
Please cite this article as: J. Ma, Z. Tao, H. Kou, M. Fronzi, L. Bi, Evaluating the effect of Pr-doping on the performance of strontium-doped lanthanum ferrite cathodes for protonic SOFCs, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.017. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Evaluating the effect of Pr-doping on the performance of strontium-doped lanthanum ferrite cathodes for protonic SOFCs Jinming Ma a,1, Zetian Tao b,1, Hongning Kou c,1, Marco Fronzi d, Lei Bi a∗ a) Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Ningxia Road No.308, Qingdao 266071, China b) Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, China c) Peng Cheng Laboratory, No.2 Xingke 1st street, Nanshan District, Shenzhen, Guangdong, China d) International Research Centre for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi’an, China
∗
Corresponding author. Tel./Fax: +86-532-85951496 Email address: [email protected] and [email protected] (L. Bi)
1. These authors contributed equally to this paper 1
Abstract A Pr-doping strategy was used to improve traditional strontium-doped lanthanum ferrite oxides for proton-conducting solid oxide fuel cells (SOFCs). Three different samples,
La0.5Sr0.5FeO3-δ,
La0.25Pr0.25Sr0.5FeO3-δ,
and
Pr0.5Sr0.5FeO3-δ
,were
successfully prepared. The Pr content was shown to have an obvious influence on the hydration ability of the materials. Hydration was improved at higher Pr-contents, suggesting a promising cathode performance. However, the improved hydration ability did not always lead to an increased fuel cell performance, and it was found that the fuel cell performed best when an appropriate Pr-doping amount was used that resulted in a good compromise between protonic and oxygen-ion conduction. As a result, the optimized composition La0.25Pr025Sr0.5FeO3-δ generated a high peak power density of 616 mW cm-2 and a low polarization resistance of 0.09 at Ω cm2 at 700 °C, which is an encouraging performance for a traditional cathode material.
Keywords: LaFeO3; Cathode; Protonic SOFC; Oxides;
2
1. Introduction Proton-conducting SOFCs (H-SOFCs) are receiving increased attention due to their advantageous low temperature operation, which solves the operating temperature problems of traditional high temperature SOFCs[1-5]Error! Bookmark not defined.. As the cathode is a central component of H-SOFCs, development of cathodes for H-SOFCs has become a popular research subject. In recent decades, several oxide materials were evaluated as cathodes for H-SOFCs and many other types of compounds have been proposed for use in H-SOFC cathodes[6-8]. Discovering new, high performance cathode materials is one direction for improving H-SOFC cathodes[9, 10]. Alternatively, some recent studies have shown that modification of classical cathode materials can also improve the electrochemical performance of H-SOFC cathodes[11, 12]. Among classical cathode materials, Sr-doped LaFeO3 has been reported to be suitable for H-SOFCs due to its low overpotential, which is even lower than the overpotential of Sr-doped LaCoO3[13]. Additionally, Sr-doped LaFeO3 has adequate chemical stability and improved thermal stability when compared with recently proposed cobalt-containing cathodes[14]. Previous studies have demonstrated that Sr-doped LaFeO3 cathodes enabled H-SOFCs to have reasonable fuel cell performance[15]. However, any further improvements in fuel cell performance might be difficult for Sr-doped LaFeO3 cathode because of its moderate oxygen ion conduction and low proton conduction. Therefore, enhancing proton mobility without 3
dramatically impairing the oxygen-ion conduction is a rational way to design Sr-doped LaFeO3-based cathodes for H-SOFCs[16]. In this study, Pr was used to dope Sr-doped LaFeO3 to form new cathode compositions for use in H-SOFCs. Pr was selected as the dopant for Sr-doped LaFeO3 because Pr-containing compounds have demonstrated to show considerable hydration ability and thus potential proton migration ability both in electrolyte material[17] and cathode materials[18]. Pr was used as the dopant for Y-doped BaZrO3, allowing the material to show good protonic conduction[17]. Also, the doping strategy was found to be effective to improve the hydration ability for cathode materials for H-SOFCs. For instance, Zn was found to be beneficial for the hydration of Ba0.5Sr0.5FeO3-based materials[19], while K-doping strategy was demonstrated to improve the hydration ability
of
Ba0.5Sr0.5Co0.8Fe0.2O3
material[20].
The
influence
of
Pr-doping
concentration on the material’s structure, hydration ability and final fuel cell performance were studied.
2. Experimental La0.5-xPrxSr0.5FeO3-δ (x=0, 0.25 and 0.5) ceramic oxides were synthesized via a wet chemical route[21]. Briefly, La(NO3)3, Pr(NO3)3, Sr(NO3)2 and Fe(NO3)3 were used as the starting materials and dissolved in water, followed by the addition of citric acid as the complexing agent. The solution was heated under stirring and the ignited, forming black ashes. The ashes were calcined at 950 oC for 3 h to form the target 4
ceramic oxides. La0.5-xPrxSr0.5FeO3-δ (x=0, 0.25 and 0.5) were named LSF, LPSF and PSF, respectively. The La0.5-xPrxSr0.5FeO3-δ powders were mixed with BZY powder and then fired at 950 °C. XRD was used to check the chemical compatibility of these materials. The hydration ability of the La0.5-xPrxSr0.5FeO3-δ (x=0, 0.25 and 0.5) materials was studied using theoretical calculations[22, 23]. We performed all calculations using density functional theory (DFT)[24], as was implemented in the Vienna ab initio simulation package (VASP)[25-27]. The simulation details can be found in our previous studies[20, 28]. To test the performance of the La0.5-xPrxSr0.5FeO3-δ ceramic oxides, these oxides were used as cathodes in H-SOFCs. The BaCe0.7Zr0.1Y0.2O3-δ (BCZY) half-cells were co-pressed and co-fired. The anode powder was prepared by mixing BCZY powder with commercial NiO at a weight ratio of 2 to 3. Then, 20 wt.% of starch was added for creating pores after sintering. BCZY electrolyte powder was deposited onto the BCZY-NiO anode substrate by a co-pressing method and then co-sintered at 1400 oC for 6 h. LSF, LPSF, and PSF was mixed with BZY and then deposited on the BCZY electrolyte and fired at 950 °C. Following firing, complete cells were formed and tested. Scanning electron microscope (SEM) was used to observe the morphologies of the tested cells.
3. Results and Discussion 5
Figure 1 presents the XRD patterns of the LSF, LPSF and PSF materials as well as patterns for the composite LSF-BZY, LPSF-BZY and PSF-BZY materials after firing at 1000 °C. The LSF, LPSF and PSF powders exhibit pure phases without detectable impurities. Further analysis of the lattice parameter of LSF, LPSF and PSF powders indicates a linear shrinkage of the lattice parameter as shown in Figure 1(d), suggesting the replacement of La by Pr reduces the lattice parameter. The reduced lattice parameter suggests the incorporation of Pr into the lattice due to the smaller ionic radius of the Pr cation compared with that of the La cation. The elemental distribution detected by SEM-EDS as shown in Figure 2 also indicates the appearance of Pr for LPSF and PSF powders and the disappearance of Pr for LSF powder. The LSF, LPSF and PSF powders also show good chemical compatibility with the BZY proton-conducting oxide, which is demonstrated by the chemical compatibility tests. No new phases except for the original La0.5-xPrxSr0.5FeO3-δ and BZY phases were found after co-firing the La0.5-xPrxSr0.5FeO3-δ (x=0, 0.25 and 0.5) powders together with BZY at 950 °C for 2 h. It should be noted that the purpose of using the Pr-dopant in Sr-doped LaFeO3 is to enhance the hydration ability of the material, making it more suitable for use in H-SOFCs. Therefore, it is important that the hydration ability of the new doped oxide is tested. According to the formation mechanism of proton defects[29] that is shown in the equation below, in which the lattice oxygen and
∙
∙∙
represents the oxygen vacancy,
×
represents
is the proton defect. The hydration ability of the doped 6
oxide could be anticipated by comparing the proton formation energies of the material. +
∙∙
+
×
⇔ 2
∙
(1)
Therefore, a theoretical calculation was employed to calculate the proton formation energy according to equation 2. ∆Ehydration = E2OH − Edefect − EH 2O
(2)
During the proton formation process, H2O in the form of H-OH occupies one oxygen vacancy with OH, and another H connects with the lattice oxygen to form O-H, as shown in Figure 3. According to the theoretical calculations, the hydration energies for LSF, LPSF, and PSF are -0.68, -1.37 and -1.64 eV, respectively, as shown in Table 1. All the hydration energies for these three materials are negative, indicating that hydration is thermodynamically favourable. PSF has the lowest hydration energy and the hydration energy decreases with increasing amounts of Pr-doping, implying that the formation of protons becomes easier with increased Pr-doping. Therefore, it is expected the fuel cell performance of Pr-doped samples will be better when compared with the Pr-free samples. However, it should also be noted that the hydration energy decreases significantly with the doping of Pr for the transition from LSF to LPSF (from -0.68 eV to -1.37 eV), while the difference is not as apparent for the transition from LPSF to PSF ( from -1.37 eV to -1.64 eV), suggesting that further increases of Pr concentration in the cathode sample do not greatly improve the hydration ability. Figure 4 shows the performance of the BCZY-based cells with LSF, LPSF and 7
PSF cathodes. It can be seen that the fuel cell performance of the LSF, LPSF and PSF cell reaches 356, 616 and 570 mW cm-2 at 700 °C, respectively. The values of the LPSF cell is clearly higher than the values of the LSF cell, but the LPSF cell is only slightly better than the PSF cell. The fuel cell performance of these cathodes is promising if one considers that no cobalt is used in these cathodes. Even comparing recently reported H-SOFCs using LaNi0.6Fe0.4O3-δ nanofibers which reach 551 mW cm-2 at 700 °C[30], the current LPSF cell show an evident advantage in power output. The performance is also higher than that of the cell using SrSc0.175Nb0.025Co0.8O3-δ cathode that reaches 498 mW cm-2 at 700 °C[31]. This evidence suggests the improvement of hydration for the material is a rational way of designing the cathode for H-SOFCs[32]. Furthermore, the cells used in these tests are normal cells without optimized microstructures, implying that the performance could be enhanced further with proper microstructure optimization[33-37]. As the motivation of using Pr-doping is to improve the material’s hydration ability to enhance the fuel cell performance, the fuel cell testing results in Figure 4 seem to contradict the theoretical predictions of the hydration abilities of the synthesized materials. From the theoretical calculation results, it is expected that the PSF cell would show the highest fuel cell performance because the PSF cell has the lowest hydration energy among the LSF, LPSF and PSF samples. However, the experimental results indicated that LPSF instead of PSF shows the best performance in fuel cell applications. Therefore, other factors beside the hydration ability govern 8
fuel cell performance. Figure 5 shows SEM images of the LSF, LPSF and PSF cells. All three of these cells have a similar microstructure. The electrolyte is approximately 10 µm thick, and the adherence of the electrolyte to the electrodes is good for all three cells. No cracks in the electrolyte or delamination at the interface was observed. The similarity of cell morphology for the three cells suggests that the difference in fuel cell performance is mainly a result of the materials, not from the microstructure. Figure 6 shows impedance plots of three cells. The ohmic resistance of all three cells is similar, which is likely due to the same batch of cells being used for this study. It is generally accepted that the ohmic resistance of the cell mainly comes from the electrolyte and interfacial contacts[38], and the similarity in cell microstructures means the differences in ohmic resistance could be ignored, which leads to the similar ohmic resistances for all three cells. Conversely, the polarization shows some differences between the three cells. The polarization resistance is 0.23, 0.09 and 0.1Ω cm2 for LSF, LPSF, and PSF-based cells, respectively, for the same testing temperature. Like the cell performance above, the polarization of the LPSF cell is much smaller than that of the LSF cell and it is only slightly lower than that of the PSF cell. The ohmic resistance and the polarization resistance for the LPSF cell are 0.229 and 0.09 Ω cm2, respectively. In contrast, the corresponding value for PSF cell is 0.246 and 0.1 Ω cm2, respectively. Although we tried to keep the tested half-cells identical and prepared them in the same way, there may be still some tiny differences from one half-cell to another. The ohmic resistance is mainly related to the electrolyte 9
resistance and the interfacial contact, while the polarization resistance reflects the performance of the electrode. Due to the similarity of the cell structure, the difference in polarization resistance should come from differences in the cathode material used. The hydration ability of the cathodes was shown to follow the order of LSF
parameter, leading to similar oxygen vacancy formation energies for LSF and PSF. Also, oxygen vacancy formation energy does not completely equal to the oxygen migration ability and it has been reported that Pr-doping decreases the oxygen-ion conductivity of the material[39], which might be the cause the slightly lower performance of the PSF-cell when compared to the LPSF-cell. The polarization resistance of LPSF cell and PSF cell is quite similar (only 10% difference), which is probably due to the compromise between the proton and oxygen-ion diffusion ability. Therefore, it can be concluded that the best cathode performance could be achieved by properly balancing the proton and oxygen-ion diffusion ability of the cathode material.
4. Conclusions Pr doped LSF materials were synthesised as potential cathode materials for H-SOFCs. Pr was found to be beneficial for the hydration ability of La0.5-xPrxSr0.5FeO3-δ oxides and the hydration energy of the oxides decreased with increased amounts of Pr, suggesting more facile proton mobility in the high Pr-containing oxides. However, the fuel cell results showed that the PSF cell did not yield the highest fuel cell performance, while the LPSF cell had the best cell performance. Electrochemical studies suggested that a proper compromise between the oxygen and proton conduction is critical for cathode performance, which provides guidance for future cathode designs. 11
Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province (Grant No.: ZR2018JL017). The Work was Supported by Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province
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Captions Figure 1. XRD patterns for the (a) LSF, (b) LPSF and (c) PSF powders before and after firing with BZY powder; (d) the calculated lattice parameters for the powders. Figure 2. Elemental analysis of the LSF, LPSF and PSF powders by using SEM-EDS. Figure 3. Scheme for the proton formation process in the PSF oxide. Figure 4. Fuel cell performance for different cells tested at 700 oC. Figure 5. SEM images of the (a) LSF, (c) LPSF, (e) PSF single cells and the (b) LSF, (d) LPSF, (f) PSF cathodes. Figure 6. Impedance plots of the cells tested at 700 °C.
Table 1. Hydration energies for LSF, LPSF and PSF.
19
Table 1 Material Hydration energy (eV)
LSF -0.68
LPSF -1.37
PSF -1.64
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: