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A porous yttria-stabilized zirconia layer to eliminate the delamination of air electrode in solid oxide electrolysis cells
Journal of Power Sources 359 (2017) 104e110
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A porous yttria-stabilized zirconia layer to eliminate the delamination of air electrode in solid oxide electrolysis cells Muhammad Shirjeel Khan a, Xiaoyong Xu a, **, Jie Zhao a, Ruth Knibbe b, Zhonghua Zhu a, * a b
School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia
h i g h l i g h t s A porous YSZ layer was applied between the YSZ electrolyte and LSM electrode. High oxygen partial pressure build-up within the defects of YSZ has been prevented. Oxygen evolution reaction is shifted to a porous YSZ-LSM interface. This helps to eliminate the delamination of LSM.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2017 Received in revised form 10 May 2017 Accepted 16 May 2017 Available online 24 May 2017
Delamination of La0.8Sr0.2MnO3-d (LSM) in solid oxide electrolysis cells (SOECs) is usually associated with the high oxygen partial pressure build-up at the LSM-YSZ (yttria-stabilized zirconia) interface. Here we sandwich a porous YSZ layer between the LSM electrode and YSZ electrolyte to release this oxygen pressure. Symmetric cells with and without the porous YSZ layers are prepared and tested in air at 800 C under the current densities of 0.5 and 1 A cm 2 for 100 h. Voltage change is continuously monitored, and impedance spectrum studies have been carried out before and after testing. No delamination has been observed for the samples with the porous YSZ layer even after 100 h. The improved performance for these samples is due to the shift of oxygen evolution reaction from the dense YSZ-LSM interface to a porous YSZ-LSM interface. This shift also helps the oxygen to be easily released instead of going into the pores or grain boundaries of the electrolyte. On the other hand, for the sample without the porous YSZ layer, the LSM is totally delaminated from the electrolyte just after 70 h. © 2017 Elsevier B.V. All rights reserved.
Keywords: Delamination Interface Oxygen Porous layer Solid oxide electrolysis cell
1. Introduction During the recent years, there has been an increasing interest in the development of hydrogen economy to decrease our dependence on fossil fuels and reduce the greenhouse emissions. Hightemperature water electrolysis is a highly efficient method to produce hydrogen as it can effectively utilize the high-temperature waste heat (e.g. from coal or nuclear power plants) and electricity from renewable energy resources (solar, tidal or wind) [1e4]. Solid oxide electrolysis cells (SOECs) are electrochemical devices for high-temperature electrolysis of water. Similar to solid oxide fuel cells (SOFCs), La0.8Sr0.2MnO3-d is the most commonly used air
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Xu), [email protected] (Z. Zhu). http://dx.doi.org/10.1016/j.jpowsour.2017.05.049 0378-7753/© 2017 Elsevier B.V. All rights reserved.
electrode for SOECs. LSM shows a very good performance and stability as an SOFC air electrode. In SOEC mode, however, it undergoes severe degradation, and in the most serious cases, it delaminates from the electrolyte [5e11]. The delamination is attributed to the development of high oxygen partial pressure at the air electrode-electrolyte interface, which is due to the large air electrode overpotential when operating in SOEC mode [12e16]. Some studies suggest that the delamination is caused by the formation of secondary phases such as lanthanum zirconate (La2Zr2O7) and manganese dioxide (MnO2) [17,18]. However, this formation is also a consequence of build-up of high oxygen partial pressure at the air electrode-electrolyte interface. Another study by Chen et al. suggests that the delamination is caused by the disintegration of LSM particles. According to their study, during the SOEC operation, the movement of oxygen ions from YSZ electrolyte into the LSM grains produces tensile strains, which result in the
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between the electrolyte and air electrode. It is hypothesized that this porous layer can shift the sites for oxygen evolution reaction away from the dense YSZ electrolyte-air electrode interface to a porous YSZ electrolyte-air electrode interface. The oxygen will be produced away from the dense YSZ-LSM interface. Therefore, no or much smaller accumulation of O2 into the closed pores/cavities of electrolyte will take place. This will help to reduce the oxygen partial pressure at the dense YSZ-LSM interface, thus eliminating the delamination. The YSZ particles used in the porous layer for our study are much smaller as compared to the previous study [17] and therefore, are more effective to release the oxygen partial pressure. Improved performance for 100 h without any sign of delamination has been observed. 2. Experimental 2.1. Cell fabrication
Fig. 1. Polarization behavior of different samples tested in air at 800 C at different current densities.
disintegration and formation of LSM nanoparticles [5]. All these studies lead to the conclusion that the oxygen partial pressure at the LSM-YSZ interface plays a major role towards the delamination of LSM. Therefore, it is very important to develop some strategies, which can help reduce this pressure. To reduce the high oxygen partial pressure and overcome delamination, different techniques have been developed over the past few years. These include the utilization of mixed ionicelectronic conductors, an addition of oxygen promoters such as palladium (Pd) and infiltration [1,19e21]. Some studies have suggested the use of composites such as LSM-YSZ. However, it also suffers from the delamination, as observed by various researchers [22,23]. In another study, the concept of a Mn-modified YSZ layer was introduced to reduce the Mn diffusion towards the YSZ electrolyte and retard the formation of La2Zr2O7 [17]. However, a continuous performance degradation was observed. In this study, we propose a more simple method to overcome the delamination, by introducing a uniform porous YSZ layer
10 and 20 wt % graphite was added to the YSZ powder, and ball milled with ethanol and the binder for 2 h. The slurry was then spray-coated on a dense YSZ support and sintered at 1200 C for 2 h to have sufficient porosity. LSM (La0.8Sr0.2MnO3-d) ink was prepared, spray-coated on the porous YSZ layer and subsequently sintered at 1100 C for 2 h. The porous YSZ layer and LSM were coated on both sides of YSZ support (850 mm) to prepare a symmetric cell. These cells were named as LSM-10G and LSM-20G for the amount of graphite added. Symmetric cells containing LSM electrode without porous YSZ layer were also prepared for comparison and named as LSM-0G. The effective air electrode area for all the cells was 0.2826 cm2. Silver (Ag) wires and paste were used for current collection for all samples. 2.2. Characterization A four-wire resistance measurement method was used to characterize the polarization behavior of cells. All of the cells were tested at 800 C in air under a current density of 0.5 A cm 2. The newly developed LSM electrode (LSM-20G) was additionally tested under 1 A cm 2. Voltage was continuously monitored until delamination occurred or for a maximum of 100 h if the sample did not show delamination. Electrochemical Impedance Spectroscopy (EIS) was conducted to measure the change in the ohmic (Rs) and
Fig. 2. EIS patterns for LSM-0G before and after test at 0.5 A cm
2
for 70 h.
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Fig. 3. SE micrographs for LSM-0G sample: cross-section a) before and b) after durability testing at 0.5 A cm testing at 0.5 A cm 2 for 70 h.
2
70 h and surface condition of YSZ c) before and d) after durability
carried out to differentiate the porous YSZ and LSM layers.
3. Results and discussion 3.1. Traditional LSM electrode
Fig. 4. XRD patterns for a) LSM-0G before test, b), c) and d) LSM-0G, LSM-10G and LSM-20G, respectively, after polarization test under a current density of 0.5 A cm 2 and e) LSM-20G sample after testing at 1 A cm 2.
polarization resistance (Rp) of the cells before and after the test in the frequency range of 100 kHz to 0.1 Hz. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were carried out before and after testing to observe the microstructures and different phases present. Energy Dispersive Spectroscopy (EDS) was also
The polarization behavior for traditional LSM electrode (LSM0G) under a current density of 0.5 A cm 2 is shown in Fig. 1. The figure shows that there is a decrease in voltage or area-specific resistance (ASR) of LSM-0G during the initial 6e7 h of operation. This reduction has been attributed to the increased performance of LSM under anodic and cathodic polarization conditions [5,8,24]. The sample then shows a gradual increase in voltage until around 67 h. This constant increase illustrates that the degradation is occurring; however, the LSM is still in contact with the electrolyte. A sharp voltage increase at around 70 h is the point that the LSM delaminates from the electrolyte. The voltage increase of LSM-0G can be explained by the EIS patterns shown in Fig. 2. The figure shows that there is an increase in both Rs and Rp of LSM-0G after 70 h of operation. The Rs increased from 1.22 U cm2 to 1.33 U cm2, while Rp has increased from 0.92 U cm2 to 1.22 U cm2. SEM images for the LSM-0G before and after the operation are shown in Fig. 3(aed). Before the polarization test, LSM is in contact with YSZ (Fig. 3a). Fig. 3b shows the cross-section of the anodically polarized surface of LSM-0G after testing for 70 h. It can be seen that LSM was completely delaminated. Fig. 3c presents the surface of YSZ just before the deposition of LSM. YSZ grain can be seen intact along with some pores on the surface. These pores act as accumulation sites for oxygen, which ultimately causes the
M.S. Khan et al. / Journal of Power Sources 359 (2017) 104e110
Fig. 5. EIS patterns for LSM-10G and LSM-20G before and after test at 0.5 and 1 A cm
107
2
for 100 h.
Fig. 6. SE micrographs for the cross-sections of newly developed electrodes a) LSM-20G before test, b) LSM-20G after test under 0.5 A cm under 0.5 A cm 2 for 100 h and d) LSM-20 after test under 1 A cm 2 for 100 h.
delamination [12,14]. Fig. 3d shows the surface condition of YSZ after test, which also confirms the complete delamination. The XRD analysis of this surface shows the presence of La2Zr2O7, which is known to form usually during the sintering of LSM and the SOEC operation. XRD patterns for the LSM-0G before and after testing at 0.5 A cm 2 have been shown in Fig. 4(aeb). The figure compares the phases present in the samples before and after the polarization test. La2Zr2O7 can be seen in addition to LSM and YSZ before and after the test. No LSM has been detected for LSM-0G (Fig. 4b) after
2
for 100 h, c) LSM-10G after testing
testing, as it completely delaminates from the electrolyte surface. 3.2. LSM electrode with porous YSZ layer The cell voltage for the newly developed electrodes (LSM-10G and LSM-20G) under 0.5 and 1 A cm 2 has also been shown in Fig. 1. The voltage for these samples decreases continuously for the 100 h of operation (Fig. 1). As anticipated, the porous YSZ layer increases the cells Rs (Fig. 5) which consequently gives a higher cell voltage (Fig. 1). However, this layer is beneficial in the long run as it
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Fig. 7. SE micrograph for LSM-20G after polarization test at 800 C for 100 h under a current density of 1 A cm sponding EDS analysis carried out at three different points.
eliminates the delamination by providing sufficient space for oxygen removal. The continuous voltage decrease for the newly developed LSM electrodes observed in Fig. 1 can be explained with the help of EIS graphs shown in Fig. 5. There is a decrease in both Rs and Rp of the cell after testing for 100 h. For LSM-10G, the Rs decreases from 2.14 U cm2 to 1.07 U cm2; for LSM-20G, it decreases from 2.56 U cm2 to 1.41 U cm2 after 100 h of operation. Similarly, there is a decrease in Rp of these samples. It decreases from 2.1 U cm2 to 0.48 U cm2 for LSM-10G and from 1.21 U cm2 to 0.59 U cm2 for LSM-20G after 100 h of operation. LSM-20G shows the similar performance even when the current density was increased to 1 A cm 2 (Fig. 1). Fig. 5 shows the EIS plots for LSM-20G before and after testing at 1 A cm 2. Again, a decrease in both Rs and Rp after 100 h can be observed, illustrating a better performance. The decrease in Rp of cells is due to the activation of LSM under anodic and cathodic polarization conditions, as described previously [5,8,24]. Similarly, the decrease in Rs of the cells can be attributed to the combined effect of LSM activation and the p-type electronic conductivity induced in YSZ under high oxygen partial pressure conditions [25,26]. However, further experiments are required to determine the exact values for decrease in ohmic resistance of LSM and p-type conductivity of YSZ. SEM cross-sectional images for the newly developed electrodes before and after the test are shown in Fig. 6(aed). Fig. 6a shows the cross-section of LSM-20G before the polarization test, while Fig. 6b shows the same cross-section after testing for 100 h under
2
showing the contact between the different layers and corre-
0.5 A cm 2. It can be seen that both the porous YSZ layer and LSM layer are intact even after 100 h. No delamination was observed for this sample. SEM images for the cross-sections of LSM-10G after testing under 0.5 A cm 2 and LSM-20G after testing under 1 A cm 2 are also shown in Fig. 6c and d, respectively. Again, the contact between the two layers is evident, showing the absence of delamination even at high current density (1 A cm 2). This illustrates that the porous YSZ layer eventually eliminates the delamination through alleviating the build-up of the high oxygen pressure. XRD patterns for the newly developed LSM electrodes are shown in Fig. 4(cee). Again, La2Zr2O7 can be seen along with LSM and YSZ. Some researchers have proposed that La2Zr2O7 formation is one of the reasons for LSM delamination since it produces uneven surface and weakens the anode-electrolyte interface [17,18]. In our study, La2Zr2O7 was detected in the samples before and after the polarization test. However, we did not observe any delamination. These results indicate that La2Zr2O7 can be one of reasons for airelectrode delamination in SOEC cells however the primary reason is the high oxygen partial pressure build-up at the dense YSZ-LSM interface. From SEM images, it is difficult to distinguish between the porous YSZ and LSM layers. The EDS analysis, therefore, was carried out at three different points of the sample (LSM-20G). Fig. 7 shows the results for EDS analysis performed at LSM-20G cross-section after testing at 1 A cm 2 for 100 h. The elements belonging to the different layers are clear in this figure. The figure also shows that
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oxide ions (O2 ), travelling through the electrolyte, react at the triple phase boundaries (contact point between the pores, LSM and YSZ) to produce electrons and oxygen. In the case of LSM (which is a pure electronic conductor), the triple phase boundary is restricted only to the interface between dense YSZ and LSM. Therefore, all the oxygen will be produced at this interface. If sufficient space is not available for this oxygen to be released, it can accumulate within the pores or grain boundaries of the electrolyte. The increased oxygen partial pressure can ultimately result in the delamination of LSM electrode. For the newly developed air electrodes (LSM-10G and LSM20G), the cell continuously stays in the activation phase. There is a decrease in both Rs and Rp of the cell. This can be attributed to the novel microstructural design which helps to release the oxygen continuously. Fig. 8b shows the working principle of the newly developed LSM electrodes. The introduction of the porous electrolyte layer at the dense electrolyte-air electrode interface shifts the oxygen evolution reaction to the porous YSZ-LSM interface. It also helps for this high oxygen pressure to be easily released, which in-turn reduces the oxygen partial pressure build-up at the LSMdense YSZ interface, thus eliminating delamination. 4. Conclusions
Fig. 8. Schematic for possible mechanisms: a) Traditional LSM anode with pure electronic conduction behavior results in high oxygen partial pressure at the interface and b) Prevention of delamination using a porous YSZ layer as observed for LSM-10G and LSM-20G.
the bottom layer with relatively small particles is YSZ (Pt2), while the top layer with relatively larger particles is LSM (Pt3). Pt1 corresponds to the dense electrolyte (YSZ). The strong contact between all layers after test is also evident, confirming the absence of delamination.
A new microstructural design has been developed to overcome the delamination of the LSM air electrode during the solid oxide electrolysis operation. The results show that this design helps to reduce the oxygen partial pressure at the air electrode-electrolyte interface by shifting the oxygen evolution reaction sites from the dense YSZ-LSM interface to a porous YSZ-LSM interface. This shift helps to alleviate the oxygen partial pressure accumulation within the pores and grain boundaries of the electrolyte and eliminate the delamination. Acknowledgements We acknowledge the support of an Australian Research Council (ARC) Linkage Project Grant (LP150100036). Author Muhammad Shirjeel Khan acknowledges the financial support from Australian Government Research Training Program Scholarship (RTP) and UQ Graduate School Scholarship. References
3.3. Possible mechanism It is know that when an SOEC cell is biased, it produces oxygen on the air electrode side. Associated with this oxygen production is an increase in oxygen partial pressure at the air electrodeelectrolyte interface in accordance with the Nernst equation. The air electrode overpotential is a product of the air electrode polarization resistance and the current density. As such, as the current density increases, the air electrode overpotential increases and the oxygen partial pressure at the air electrode-electrolyte interface increases. It has been reported previously that under SOEC conditions, the performance of the cell initially increases due to the activation of LSM [5,8,24]. Our study also shows that for traditional LSM electrode (LSM-0G), there is an initial drop in the voltage or ASR of the cell (Fig. 1). However, due to the continuous production of oxygen at the interface, degradation becomes a dominant phenomenon as compared to activation. With the passage of time, more and more oxygen accumulates within the grain boundaries/ pores of the electrolyte, until the LSM completely delaminates from the electrolyte. The oxygen production at the dense YSZ-LSM interface has been illustrated in Fig. 8a. It can be seen that the
[1] K. Chen, N. Ai, S.P. Jiang, Electrochem. Commun. 19 (2012) 119e122. [2] S.D. Ebbesen, S.H. Jensen, A. Hauch, M.B. Mogensen, Chem. Rev. 114 (2014) 10697e10734. [3] Q. Fu, C. Mabilat, M. Zahid, A. Brisse, L. Gautier, Energy & Environ. Sci. 3 (2010) 1382. [4] M.A. Laguna-Bercero, J. Power Sources 203 (2012) 4e16. [5] K. Chen, S.P. Jiang, Int. J. Hydrog. Energy 36 (2011) 10541e10549. mez, D. Hotza, Renew. Sustain. Energy Rev. 61 (2016) 155e174. [6] S.Y. Go [7] G.A. Hughes, K. Yakal-Kremski, S.A. Barnett, Phys. Chem. Chem. Phys. 15 (2013) 17257e17262. [8] M. Keane, M.K. Mahapatra, A. Verma, P. Singh, Int. J. Hydrog. Energy 37 (2012) 16776e16785. [9] M.A. Laguna-Bercero, R. Campana, A. Larrea, J.A. Kilner, V.M. Orera, J. Power Sources 196 (2011) 8942e8947. [10] M.A. Laguna-Bercero, J.A. Kilner, S.J. Skinner, Chem. Mater. 22 (2010) 1134e1141. [11] N.Q. Minh, ECS Trans. 35 (2011) 2897e2904. [12] T. Jacobsen, M. Mogensen, ECS Trans. 13 (2008) 259e273. [13] R. Knibbe, M.L. Traulsen, A. Hauch, S.D. Ebbesen, M. Mogensen, J. Electrochem. Soc. 157 (2010) B1209eB1217. [14] A.V. Virkar, Int. J. Hydrog. Energy 35 (2010) 9527e9543. [15] J. Wang, Y. Zhang, T. Liang, C. Deng, J. Xu, J. Power Sources 208 (2012) 415e420. [16] Y. Zhang, K. Chen, C. Xia, S.P. Jiang, M. Ni, Int. J. Hydrog. Energy 37 (2012) 13914e13920. [17] N. Li, M. Keane, M.K. Mahapatra, P. Singh, Int. J. Hydrog. Energy 38 (2013) 6298e6303.
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[18] P. Moçoteguy, A. Brisse, Int. J. Hydrog. Energy 38 (2013) 15887e15902. [19] K. Chen, N. Ai, S.P. Jiang, Int. J. Hydrog. Energy 39 (2014) 10349e10358. [20] P. Hjalmarsson, X. Sun, Y.-L. Liu, M. Chen, J. Power Sources 223 (2013) 349e357. [21] S.J. Kim, G.M. Choi, Solid State Ion. 262 (2014) 303e306. [22] K. Chen, N. Ai, S.P. Jiang, Int. J. Hydrog. Energy 37 (2012) 10517e10525. [23] J. Kim, H.-I. Ji, H.P. Dasari, D. Shin, H. Song, J.-H. Lee, B.-K. Kim, H.-J. Je, H.-
W. Lee, K.J. Yoon, Int. J. Hydrog. Energy 38 (2013) 1225e1235. [24] M. Liang, B. Yu, M. Wen, J. Chen, J. Xu, Y. Zhai, J. Power Sources 190 (2009) 341e345. , A.R. West, Chem. Mater. 27 (2015) 1552e1558. [25] N. Maso [26] N.Q. Minh, T. Takahashi, Chapter 3-electrical conduction in ceramics, in: Science and Technology of Ceramic Fuel Cells, Elsevier Science Ltd, Oxford, 1995, pp. 41e68.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A porous yttria-stabilized zirconia layer to eliminate the delamination of air electrode in solid oxide electrolysis cells Muhammad Shirjeel Khan a, Xiaoyong Xu a, **, Jie Zhao a, Ruth Knibbe b, Zhonghua Zhu a, * a b
School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia
h i g h l i g h t s A porous YSZ layer was applied between the YSZ electrolyte and LSM electrode. High oxygen partial pressure build-up within the defects of YSZ has been prevented. Oxygen evolution reaction is shifted to a porous YSZ-LSM interface. This helps to eliminate the delamination of LSM.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2017 Received in revised form 10 May 2017 Accepted 16 May 2017 Available online 24 May 2017
Delamination of La0.8Sr0.2MnO3-d (LSM) in solid oxide electrolysis cells (SOECs) is usually associated with the high oxygen partial pressure build-up at the LSM-YSZ (yttria-stabilized zirconia) interface. Here we sandwich a porous YSZ layer between the LSM electrode and YSZ electrolyte to release this oxygen pressure. Symmetric cells with and without the porous YSZ layers are prepared and tested in air at 800 C under the current densities of 0.5 and 1 A cm 2 for 100 h. Voltage change is continuously monitored, and impedance spectrum studies have been carried out before and after testing. No delamination has been observed for the samples with the porous YSZ layer even after 100 h. The improved performance for these samples is due to the shift of oxygen evolution reaction from the dense YSZ-LSM interface to a porous YSZ-LSM interface. This shift also helps the oxygen to be easily released instead of going into the pores or grain boundaries of the electrolyte. On the other hand, for the sample without the porous YSZ layer, the LSM is totally delaminated from the electrolyte just after 70 h. © 2017 Elsevier B.V. All rights reserved.
Keywords: Delamination Interface Oxygen Porous layer Solid oxide electrolysis cell
1. Introduction During the recent years, there has been an increasing interest in the development of hydrogen economy to decrease our dependence on fossil fuels and reduce the greenhouse emissions. Hightemperature water electrolysis is a highly efficient method to produce hydrogen as it can effectively utilize the high-temperature waste heat (e.g. from coal or nuclear power plants) and electricity from renewable energy resources (solar, tidal or wind) [1e4]. Solid oxide electrolysis cells (SOECs) are electrochemical devices for high-temperature electrolysis of water. Similar to solid oxide fuel cells (SOFCs), La0.8Sr0.2MnO3-d is the most commonly used air
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Xu), [email protected] (Z. Zhu). http://dx.doi.org/10.1016/j.jpowsour.2017.05.049 0378-7753/© 2017 Elsevier B.V. All rights reserved.
electrode for SOECs. LSM shows a very good performance and stability as an SOFC air electrode. In SOEC mode, however, it undergoes severe degradation, and in the most serious cases, it delaminates from the electrolyte [5e11]. The delamination is attributed to the development of high oxygen partial pressure at the air electrode-electrolyte interface, which is due to the large air electrode overpotential when operating in SOEC mode [12e16]. Some studies suggest that the delamination is caused by the formation of secondary phases such as lanthanum zirconate (La2Zr2O7) and manganese dioxide (MnO2) [17,18]. However, this formation is also a consequence of build-up of high oxygen partial pressure at the air electrode-electrolyte interface. Another study by Chen et al. suggests that the delamination is caused by the disintegration of LSM particles. According to their study, during the SOEC operation, the movement of oxygen ions from YSZ electrolyte into the LSM grains produces tensile strains, which result in the
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between the electrolyte and air electrode. It is hypothesized that this porous layer can shift the sites for oxygen evolution reaction away from the dense YSZ electrolyte-air electrode interface to a porous YSZ electrolyte-air electrode interface. The oxygen will be produced away from the dense YSZ-LSM interface. Therefore, no or much smaller accumulation of O2 into the closed pores/cavities of electrolyte will take place. This will help to reduce the oxygen partial pressure at the dense YSZ-LSM interface, thus eliminating the delamination. The YSZ particles used in the porous layer for our study are much smaller as compared to the previous study [17] and therefore, are more effective to release the oxygen partial pressure. Improved performance for 100 h without any sign of delamination has been observed. 2. Experimental 2.1. Cell fabrication
Fig. 1. Polarization behavior of different samples tested in air at 800 C at different current densities.
disintegration and formation of LSM nanoparticles [5]. All these studies lead to the conclusion that the oxygen partial pressure at the LSM-YSZ interface plays a major role towards the delamination of LSM. Therefore, it is very important to develop some strategies, which can help reduce this pressure. To reduce the high oxygen partial pressure and overcome delamination, different techniques have been developed over the past few years. These include the utilization of mixed ionicelectronic conductors, an addition of oxygen promoters such as palladium (Pd) and infiltration [1,19e21]. Some studies have suggested the use of composites such as LSM-YSZ. However, it also suffers from the delamination, as observed by various researchers [22,23]. In another study, the concept of a Mn-modified YSZ layer was introduced to reduce the Mn diffusion towards the YSZ electrolyte and retard the formation of La2Zr2O7 [17]. However, a continuous performance degradation was observed. In this study, we propose a more simple method to overcome the delamination, by introducing a uniform porous YSZ layer
10 and 20 wt % graphite was added to the YSZ powder, and ball milled with ethanol and the binder for 2 h. The slurry was then spray-coated on a dense YSZ support and sintered at 1200 C for 2 h to have sufficient porosity. LSM (La0.8Sr0.2MnO3-d) ink was prepared, spray-coated on the porous YSZ layer and subsequently sintered at 1100 C for 2 h. The porous YSZ layer and LSM were coated on both sides of YSZ support (850 mm) to prepare a symmetric cell. These cells were named as LSM-10G and LSM-20G for the amount of graphite added. Symmetric cells containing LSM electrode without porous YSZ layer were also prepared for comparison and named as LSM-0G. The effective air electrode area for all the cells was 0.2826 cm2. Silver (Ag) wires and paste were used for current collection for all samples. 2.2. Characterization A four-wire resistance measurement method was used to characterize the polarization behavior of cells. All of the cells were tested at 800 C in air under a current density of 0.5 A cm 2. The newly developed LSM electrode (LSM-20G) was additionally tested under 1 A cm 2. Voltage was continuously monitored until delamination occurred or for a maximum of 100 h if the sample did not show delamination. Electrochemical Impedance Spectroscopy (EIS) was conducted to measure the change in the ohmic (Rs) and
Fig. 2. EIS patterns for LSM-0G before and after test at 0.5 A cm
2
for 70 h.
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Fig. 3. SE micrographs for LSM-0G sample: cross-section a) before and b) after durability testing at 0.5 A cm testing at 0.5 A cm 2 for 70 h.
2
70 h and surface condition of YSZ c) before and d) after durability
carried out to differentiate the porous YSZ and LSM layers.
3. Results and discussion 3.1. Traditional LSM electrode
Fig. 4. XRD patterns for a) LSM-0G before test, b), c) and d) LSM-0G, LSM-10G and LSM-20G, respectively, after polarization test under a current density of 0.5 A cm 2 and e) LSM-20G sample after testing at 1 A cm 2.
polarization resistance (Rp) of the cells before and after the test in the frequency range of 100 kHz to 0.1 Hz. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were carried out before and after testing to observe the microstructures and different phases present. Energy Dispersive Spectroscopy (EDS) was also
The polarization behavior for traditional LSM electrode (LSM0G) under a current density of 0.5 A cm 2 is shown in Fig. 1. The figure shows that there is a decrease in voltage or area-specific resistance (ASR) of LSM-0G during the initial 6e7 h of operation. This reduction has been attributed to the increased performance of LSM under anodic and cathodic polarization conditions [5,8,24]. The sample then shows a gradual increase in voltage until around 67 h. This constant increase illustrates that the degradation is occurring; however, the LSM is still in contact with the electrolyte. A sharp voltage increase at around 70 h is the point that the LSM delaminates from the electrolyte. The voltage increase of LSM-0G can be explained by the EIS patterns shown in Fig. 2. The figure shows that there is an increase in both Rs and Rp of LSM-0G after 70 h of operation. The Rs increased from 1.22 U cm2 to 1.33 U cm2, while Rp has increased from 0.92 U cm2 to 1.22 U cm2. SEM images for the LSM-0G before and after the operation are shown in Fig. 3(aed). Before the polarization test, LSM is in contact with YSZ (Fig. 3a). Fig. 3b shows the cross-section of the anodically polarized surface of LSM-0G after testing for 70 h. It can be seen that LSM was completely delaminated. Fig. 3c presents the surface of YSZ just before the deposition of LSM. YSZ grain can be seen intact along with some pores on the surface. These pores act as accumulation sites for oxygen, which ultimately causes the
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Fig. 5. EIS patterns for LSM-10G and LSM-20G before and after test at 0.5 and 1 A cm
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2
for 100 h.
Fig. 6. SE micrographs for the cross-sections of newly developed electrodes a) LSM-20G before test, b) LSM-20G after test under 0.5 A cm under 0.5 A cm 2 for 100 h and d) LSM-20 after test under 1 A cm 2 for 100 h.
delamination [12,14]. Fig. 3d shows the surface condition of YSZ after test, which also confirms the complete delamination. The XRD analysis of this surface shows the presence of La2Zr2O7, which is known to form usually during the sintering of LSM and the SOEC operation. XRD patterns for the LSM-0G before and after testing at 0.5 A cm 2 have been shown in Fig. 4(aeb). The figure compares the phases present in the samples before and after the polarization test. La2Zr2O7 can be seen in addition to LSM and YSZ before and after the test. No LSM has been detected for LSM-0G (Fig. 4b) after
2
for 100 h, c) LSM-10G after testing
testing, as it completely delaminates from the electrolyte surface. 3.2. LSM electrode with porous YSZ layer The cell voltage for the newly developed electrodes (LSM-10G and LSM-20G) under 0.5 and 1 A cm 2 has also been shown in Fig. 1. The voltage for these samples decreases continuously for the 100 h of operation (Fig. 1). As anticipated, the porous YSZ layer increases the cells Rs (Fig. 5) which consequently gives a higher cell voltage (Fig. 1). However, this layer is beneficial in the long run as it
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Fig. 7. SE micrograph for LSM-20G after polarization test at 800 C for 100 h under a current density of 1 A cm sponding EDS analysis carried out at three different points.
eliminates the delamination by providing sufficient space for oxygen removal. The continuous voltage decrease for the newly developed LSM electrodes observed in Fig. 1 can be explained with the help of EIS graphs shown in Fig. 5. There is a decrease in both Rs and Rp of the cell after testing for 100 h. For LSM-10G, the Rs decreases from 2.14 U cm2 to 1.07 U cm2; for LSM-20G, it decreases from 2.56 U cm2 to 1.41 U cm2 after 100 h of operation. Similarly, there is a decrease in Rp of these samples. It decreases from 2.1 U cm2 to 0.48 U cm2 for LSM-10G and from 1.21 U cm2 to 0.59 U cm2 for LSM-20G after 100 h of operation. LSM-20G shows the similar performance even when the current density was increased to 1 A cm 2 (Fig. 1). Fig. 5 shows the EIS plots for LSM-20G before and after testing at 1 A cm 2. Again, a decrease in both Rs and Rp after 100 h can be observed, illustrating a better performance. The decrease in Rp of cells is due to the activation of LSM under anodic and cathodic polarization conditions, as described previously [5,8,24]. Similarly, the decrease in Rs of the cells can be attributed to the combined effect of LSM activation and the p-type electronic conductivity induced in YSZ under high oxygen partial pressure conditions [25,26]. However, further experiments are required to determine the exact values for decrease in ohmic resistance of LSM and p-type conductivity of YSZ. SEM cross-sectional images for the newly developed electrodes before and after the test are shown in Fig. 6(aed). Fig. 6a shows the cross-section of LSM-20G before the polarization test, while Fig. 6b shows the same cross-section after testing for 100 h under
2
showing the contact between the different layers and corre-
0.5 A cm 2. It can be seen that both the porous YSZ layer and LSM layer are intact even after 100 h. No delamination was observed for this sample. SEM images for the cross-sections of LSM-10G after testing under 0.5 A cm 2 and LSM-20G after testing under 1 A cm 2 are also shown in Fig. 6c and d, respectively. Again, the contact between the two layers is evident, showing the absence of delamination even at high current density (1 A cm 2). This illustrates that the porous YSZ layer eventually eliminates the delamination through alleviating the build-up of the high oxygen pressure. XRD patterns for the newly developed LSM electrodes are shown in Fig. 4(cee). Again, La2Zr2O7 can be seen along with LSM and YSZ. Some researchers have proposed that La2Zr2O7 formation is one of the reasons for LSM delamination since it produces uneven surface and weakens the anode-electrolyte interface [17,18]. In our study, La2Zr2O7 was detected in the samples before and after the polarization test. However, we did not observe any delamination. These results indicate that La2Zr2O7 can be one of reasons for airelectrode delamination in SOEC cells however the primary reason is the high oxygen partial pressure build-up at the dense YSZ-LSM interface. From SEM images, it is difficult to distinguish between the porous YSZ and LSM layers. The EDS analysis, therefore, was carried out at three different points of the sample (LSM-20G). Fig. 7 shows the results for EDS analysis performed at LSM-20G cross-section after testing at 1 A cm 2 for 100 h. The elements belonging to the different layers are clear in this figure. The figure also shows that
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oxide ions (O2 ), travelling through the electrolyte, react at the triple phase boundaries (contact point between the pores, LSM and YSZ) to produce electrons and oxygen. In the case of LSM (which is a pure electronic conductor), the triple phase boundary is restricted only to the interface between dense YSZ and LSM. Therefore, all the oxygen will be produced at this interface. If sufficient space is not available for this oxygen to be released, it can accumulate within the pores or grain boundaries of the electrolyte. The increased oxygen partial pressure can ultimately result in the delamination of LSM electrode. For the newly developed air electrodes (LSM-10G and LSM20G), the cell continuously stays in the activation phase. There is a decrease in both Rs and Rp of the cell. This can be attributed to the novel microstructural design which helps to release the oxygen continuously. Fig. 8b shows the working principle of the newly developed LSM electrodes. The introduction of the porous electrolyte layer at the dense electrolyte-air electrode interface shifts the oxygen evolution reaction to the porous YSZ-LSM interface. It also helps for this high oxygen pressure to be easily released, which in-turn reduces the oxygen partial pressure build-up at the LSMdense YSZ interface, thus eliminating delamination. 4. Conclusions
Fig. 8. Schematic for possible mechanisms: a) Traditional LSM anode with pure electronic conduction behavior results in high oxygen partial pressure at the interface and b) Prevention of delamination using a porous YSZ layer as observed for LSM-10G and LSM-20G.
the bottom layer with relatively small particles is YSZ (Pt2), while the top layer with relatively larger particles is LSM (Pt3). Pt1 corresponds to the dense electrolyte (YSZ). The strong contact between all layers after test is also evident, confirming the absence of delamination.
A new microstructural design has been developed to overcome the delamination of the LSM air electrode during the solid oxide electrolysis operation. The results show that this design helps to reduce the oxygen partial pressure at the air electrode-electrolyte interface by shifting the oxygen evolution reaction sites from the dense YSZ-LSM interface to a porous YSZ-LSM interface. This shift helps to alleviate the oxygen partial pressure accumulation within the pores and grain boundaries of the electrolyte and eliminate the delamination. Acknowledgements We acknowledge the support of an Australian Research Council (ARC) Linkage Project Grant (LP150100036). Author Muhammad Shirjeel Khan acknowledges the financial support from Australian Government Research Training Program Scholarship (RTP) and UQ Graduate School Scholarship. References
3.3. Possible mechanism It is know that when an SOEC cell is biased, it produces oxygen on the air electrode side. Associated with this oxygen production is an increase in oxygen partial pressure at the air electrodeelectrolyte interface in accordance with the Nernst equation. The air electrode overpotential is a product of the air electrode polarization resistance and the current density. As such, as the current density increases, the air electrode overpotential increases and the oxygen partial pressure at the air electrode-electrolyte interface increases. It has been reported previously that under SOEC conditions, the performance of the cell initially increases due to the activation of LSM [5,8,24]. Our study also shows that for traditional LSM electrode (LSM-0G), there is an initial drop in the voltage or ASR of the cell (Fig. 1). However, due to the continuous production of oxygen at the interface, degradation becomes a dominant phenomenon as compared to activation. With the passage of time, more and more oxygen accumulates within the grain boundaries/ pores of the electrolyte, until the LSM completely delaminates from the electrolyte. The oxygen production at the dense YSZ-LSM interface has been illustrated in Fig. 8a. It can be seen that the
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