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Evaluation of SrCo0.8Nb0.2O3-δ, SrCo0.8Ta0.2O3-δ and SrCo0.8Nb0.1Ta0.1O3-δ as air electrode materials for solid oxide electrolysis and reversible solid oxide cells
Electrochimica Acta 321 (2019) 134654
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Evaluation of SrCo0.8Nb0.2O3-d, SrCo0.8Ta0.2O3-d and SrCo0.8Nb0.1Ta0.1O3d as air electrode materials for solid oxide electrolysis and reversible solid oxide cells Muhammad Shirjeel Khan a, Xiaoyong Xu a, **, Mengran Li a, Ateeq-ur Rehman a, Ruth Knibbe b, Anya Josefa Yago c, Zhonghua Zhu a, * a b c
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 Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, Queensland, 4072, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2019 Received in revised form 12 June 2019 Accepted 6 August 2019 Available online 8 August 2019
Strontium cobalt-based perovskites display high electrochemical activity but poor stability for solid oxide electrolysis cells (SOECs). Here, we evaluate the stability of three strontium cobalt-based materials, i.e. SrCo0.8Nb0.2O3-d (SCN), SrCo0.8Ta0.2O3-d (SCT) and SrCo0.8Nb0.1Ta0.1O3-d (SCNT) as air electrodes for SOECs. The electrochemical results show that the area-specific resistance (ASR) for SCN and SCNT cells decreases and then becomes almost constant whereas the ASR of the SCT cell continuously increases, during the 240 h SOEC test. The improved performance of SCN and SCNT cells is ascribed to the lower energy barrier required for the oxide ion movement in the presence of niobium and the less concentration of undesired phases formed during SOEC operation. SCN is further tested under reversible solid oxide cell (RSOC) conditions with 16 h solid oxide fuel cell (SOFC) and 7 h SOEC cycles. The cell presents excellent stability as its impedance increases only slightly from 0.188 to 0.193 U cm2 after 114 h RSOC test. A single cell with NiOSSZ/SSZ/SDC/SCN configuration shows high electrolysis current densities of 0.51, 0.42 and 0.34 A cm2 at 800, 750 and 700 C, respectively, in 10% H2O/90% H2 and 0.79 A cm2 at 800 C in 50% H2O/50% H2 at 1.3 V. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Electrolysis current density Energy barrier Impedance Perovskites Solid oxide electrolysis cells
1. Introduction Solid Oxide Electrolysis Cells (SOECs) have received great attention as a promising energy storage device as they can work in combination with renewable energy resources [1e3]. The waste heat and electrical energy from other energy resources can be used in SOECs to produce H2 and CO by splitting H2O and CO2. SOECs are better energy storage devices than secondary batteries and supercapacitors due to their higher energy storage capacity and lower cost [4]. The major issues currently faced by the SOECs include a high degradation rate of its components and low electrocatalytic activity for oxygen evolution reaction (OER). Lanthanum strontium manganite (LSM) and lanthanum strontium cobalt ferrite (LSCF) are state-of-the-art air electrode materials used for SOECs. However, various researchers have observed LSM delamination from the electrolyte [5e8]. This delamination is * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Xu), [email protected] (Z. Zhu). https://doi.org/10.1016/j.electacta.2019.134654 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
caused by the formation of high oxygen partial pressure at the electrode/electrolyte interface, which is due to the large electrochemical potentials [9,10] and the inter-diffusion of different species [11]. Several studies have proposed different techniques to overcome such delamination and increase the cell lifetime [12e16]. LSCF, on the other hand, has shown quite promising results, as it is a mixed ionic-electronic conductor and offers a lower ASR compared to LSM or LSM/YSZ composite [17,18]. At the same current density, an SOEC with LSCF will have a lower electrode overpotential in comparison to an SOEC with LSM. This, in turn reduces the oxygen partial pressure produced at the interface during electrolysis, potentially avoiding electrode delamination. However, the LSCF suffers from various other degradation phenomena such as secondary phase formation through cation diffusion and pore formation along the electrolyte grain boundaries [19,20]. Various new materials have been proposed over the years to achieve higher OER activity and better long-term stability during the SOEC operation [12,21e28]. Lanthanum strontium cobaltite (LSC) is one such material with a perovskite structure and known
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for its high ionic conductivity. However, the high thermal expansion co-efficient (TEC), sometimes, leads to its delamination from the electrolyte [29]. Lanthanum strontium ferrite (LSF), on the other hand, did not show any sign of delamination as it was tested at 700 C under 0.285 A cm2 for 300 h [30]. A more recent development in this area is the use of highly active and stable perovskite, strontium iron molybdate (SFM). SFM has shown promising results as a symmetric electrode for SOECs. One such cell with SFM/La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM)/SFM configuration produced a current density as high as 0.88 A cm2 at 900 C under an electrolysis voltage of 1.3 V and 60 vol % absolute humidity (AH) [26]. Similarly, an SFM-SDC symmetric electrode with LSGM electrolyte has shown a current density of 0.734 A cm2 at 1.3 V and 850 C when used for co-electrolysis (16%H2O/16%CO2/20%H2/48% N2) [28]. Moreover, it also showed a constant electrolysis voltage under 0.12 A cm2 at 800 C for 103 h without any sign of delamination. The double perovskite, PrBa0.5Sr0.5Co1.5Fe0.5O5þd (PBSCF50), has been demonstrated as the most active material developed so far for SOECs [2]. The maximum electrolysis current density of 1.31 A cm2 at 1.3 V was obtained at 800 C when PBSCF50 was used as an air electrode along with PrBaMn2O5þd as a fuel electrode and LSGM as an electrolyte with 10% H2O/90% H2 on the fuelelectrode side. The cell also displayed excellent stability without any sign of delamination when tested under 0.25 A cm2 for more than 600 h. Nickelates are also strong candidates as air electrodes for SOECs and reversible solid oxide cells (RSOCs). Nano-structured Nd2NiO4þd (NNO) impregnated scandia-stabilized zirconia (SSZ) has shown an electrolysis current density of 1.08 A cm2 at 800 C and 1.3 V [31]. The cell also showed an increase in the electrolysis performance during a short-term test. Another nickelate (Pr2NiO4þd) has also shown a maximum electrolysis current density of 0.9 at 800 C and 1.28 V when using 90% H2O/10% H2 on the fuel electrode side [32]. Most of these studies are focused only on achieving high electrolysis performance. Very few studies have been carried out to determine the long-term stability of air electrode materials under SOEC conditions, and even fewer have been carried out to establish the long-term RSOC stability. The aim of the present research is to study the newly developed strontium cobalt-based perovskites under SOEC and RSOC conditions, both for their activity and longterm stability. We selected strontium-cobalt based materials because of their excellent performance during the solid oxide fuel cell (SOFC) testing, as reported in the recent studies [33,34]. Symmetric cells with three different electrode materials including SrCo0.8Nb0.2O3-d (SCN), SrCo0.8Ta0.2O3-d (SCT) and SrCo0.8Nb0.1Ta0.1O3-d (SCNT), were tested under SOEC conditions for 240 h in air under a current density of 0.5 A cm2 at 800 C. The area-specific resistance (ASR) of the SCNT cell increases slightly, whereas the increase is much larger and continuous for the SCT cell. However, no delamination was observed for any of these cells. SCN, on the other hand, shows a decrease in the ASR during the 240 h SOEC operation. Therefore, it was further tested for its behaviour under RSOC conditions. A single cell with SCN as an air electrode was also tested at different temperatures and fuel conditions to determine the electrolysis current density. The electrochemical behaviour has been explained with the help of other characterization techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). 2. Material and methods 2.1. Cell preparation The electrode powders (SCN, SCT and SCNT) were prepared by
solid-state reaction route, as described elsewhere [33e35]. Appropriate amounts of SrCO3 (99.9%, Aldrich), Co3O4 (99.9%, Aldrich), Nb2O5 (99.9%, Aldrich) and Ta2O5 (99.9%, Aldrich) were ball milled in ethanol for 24 h, followed by drying and pressing in the form of circular pellets. These pellets were then sintered at 1200 C for 10 h, followed by crushing and re-sintering at 1200 C for another 10 h to obtain the perovskite structure. SrCoO3 (SC) powder was also prepared in a similar manner for comparison. The XRD patterns for the prepared powders have been shown in our previous publications [33e35]. The electrode slurry was prepared by mixing the obtained perovskites powders with appropriate amounts of solvent and binder. Symmetric cells were prepared by coating a samaria-doped ceria (SDC) layer on dense scandiastabilized zirconia (SSZ) electrolyte (~200 mm) pellets. Sintering of the SDC layer was carried out at 1350 C for 5 h. Air electrodes were then sprayed on top of the SDC layer and sintered at 950 C for 2 h. The thickness of the air electrode was 11e14 mm, and the active area was 0.283 cm2. These cells were used to determine the electrochemical behaviour of different electrode materials (SC, SCN, SCT and SCNT) under SOEC and RSOC conditions. Symmetric cells have also been studied under SOEC polarization conditions previously by various researchers [15,36e38]. A full cell to determine the electrolysis current density of the cell with SCN as air electrode was also prepared and tested at 800, 750 and 700 C in different fuel conditions. For this purpose, commercial NiO (J. T. Baker) and SSZ (fuel cell materials (Sc2O3)0$10(ZrO2)0.90) powders in a weight ratio of 60:40 were mixed along with 10 wt % dextrin as pore former. The mixture was ball milled with ethanol for 24 h followed by drying. The dried powder was pressed into circular pellets and pre-sintered at 1000 C for 2 h. Functional layer (NiO-SSZ), the electrolyte layer (SSZ) and the barrier layer (SDC) were then spray coated on the presintered pellets, respectively, and co-sintered at 1350 C for 5 h. The air electrode (SCN) was finally spray coated on the electrolyte and sintered at 950 C for 2 h. Silver wires and paste was used for current collection. The final cell was mounted on an alumina tube and sealed with the silver paste. 2.2. Characterization A four-wire resistance measurement method, as described in our previous study [38] and also shown in Fig. S1, was used for the electrochemical characterization of half-cells. The reference electrode (RE) in this case, was placed on the SSZ electrolyte and on the same side as working electrode (WE) to ensure the stable potential of WE under SOEC conditions. The RE, which is circular with a 2 mm diameter, was made of a silver wire attached to the SSZ electrolyte by silver paste. The distance between the WE and RE was 5 mm, which is more than 5 times the thickness of the electrolyte used (~200 mm). This electrode set-up is in accordance with the studies previously carried out [39,40]. A current density of 0.5 A cm2 (SOEC mode) was applied between the WE and counter electrode (CE), and the voltage change was monitored between the sensing (S) and RE for 240 h at 800 C in air. The current was interrupted from time to time to determine the impedance behaviour (EIS) of the WE using Metrohm Autolab instrument in the frequency range of 10 mHze1 MHz with a signal amplitude of 10 mV. The impedance was measured at open circuit potential (0 V) for all cells during the long term SOEC and RSOC tests. However, for the SCN cell tested only for 24 h under SOEC conditions, the impedance was determined both at 0 V and 0.072 V (the voltage obtained under 0.5 A cm2 after 24 h). For RSOC testing, a current density of ±0.5 A cm2 was applied between the WE and CE and the voltage change was monitored between the S and RE. The SOFC operation (þ0.5 A cm2) and the
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SOEC operation (0.5 A cm2) were performed for 16 h and 7 h, respectively. For the full cell testing, H2 was passed through a water bath set at certain temperatures to obtain the required humidity values. The humidity content in the flowing H2 was determined using a humidity sensor before starting the SOEC test. The values obtained at different water bath temperatures were then used as reference, as the sensor could not be used during the experiment. The electrolysis current density was determined at three different temperatures, i.e. 800, 750 and 700 C, with 10% H2O/90% H2 supplied to the fuel electrode and air supplied to the air electrode side. The electrolysis current density was also determined by changing the H2O content to 30% and 50% in H2 at 800 C. After electrochemical testing, samples were prepared for interfacial and cross-sectional analysis. To study the interface, the electrodes were etched away using hydrochloric acid (HCl), whereas, for cross-sectional examination, the samples were mounted in the epoxy resin, followed by vacuum impregnation and drying. The mounted samples were ground and polished to prepare a smooth and scratch-free surface. The samples for the interfacial studies were coated with carbon, while the samples for crosssectional analysis were coated with iridium [41]. The purpose of the coating was to reduce the charging effects from the electron beam during SEM analysis. SEM and EDS analysis were carried out using JEOL-6610, which is equipped with an Oxford 50 mm2 X-Max SDD X-ray detector for simultaneous imaging and elemental analysis. The operating voltage used for the analysis was 15 kV. XRD analysis was carried out at the interface before and after test using a
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Rigaku SmartLab micro-diffraction instrument. 3. Results and discussion 3.1. The SOEC behaviour of half-cells To demonstrate a good performance, the half-cells tested under the SOEC conditions must show a stable voltage during the longterm testing. Fig. 1(aed) shows the electrochemical behaviour of the three electrodes tested under 0.5 A cm2 at 800 C in air for 240 h. Fig. 1a shows that the voltage of the WE for SCN cell decreases during the initial 25 h and then becomes almost constant. The EIS results for this cell (Fig. 1b) show that the ohmic resistance (Rs) of the WE decreases continuously (from 0.078 to 0.042 U cm2 after 240 h), while there is a polarization resistance (Rp) increase (from 0.007 to 0.069 U cm2 after 240 h). Fig. 1a further shows that the voltage of the WE for the SCT cell increases continuously, indicating an increase in the WE ASR. This behaviour is also evident in Fig. 1c, which show an increase in both Rs and Rp of the SCT WE. The WE for the SCNT cell has a voltage between the SCN and the SCT cells. The WE voltage for the SCNT cell decreases initially, followed by an increase and an almost stable behaviour. The EIS patterns for this cell (Fig. 1d) are comparable to the SCN cell, showing a decrease in Rs while an increase in the cell Rp. However, the decrease in the Rs for this cell (0.013 U cm2) is smaller as compared to that for the SCN cell (0.037 U cm2), which indicates the positive effects of niobium (Nb) compared to tantalum (Ta) on the SOEC performance of the cells. The corresponding log f
Fig. 1. (a) Voltage-time graph for three different electrodes (SCN, SCT and SCNT) under the anodic current density of 0.5 A cm2 at 800 C in air, and (b), (c) and (d) impedance plots for the same SCN, SCT and SCNT cells, respectively, obtained at open circuit conditions using four-electrode measurement method.
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Fig. 2. Impedance plots at different voltages using a four-electrode measurement method for SCN WE after it was tested for 24 h in air at 800 C.
vs jZj plots for the three electrodes are shown in Figure S2 (a-c). SC was also tested as a reference and the voltage obtained was much higher compared to all samples, even at the start of the test (Fig. S3a). This voltage increases continuously during the SOEC test. The impedance plot for SC is shown in Fig. S3b, which also confirms the continuous increase in both Rs and Rp of the cell. It is important to reiterate that the impedance plots in Fig. 1(bed) are obtained at open circuit potentials (0 V). The voltage trends in Fig. 1a track closely with the Rs of the samples. The Rp, on the other hand, increases for all samples, even though SCN and SCNT show an almost stable voltage behaviour. This has been illustrated in Figure S4 (a-c). To further explain the conflicting trend
between Rp (Fig. S4) and Fig. 1a, an SCN cell was tested under 0.5 A cm2. The impedance for this cell was performed at 0 V and 0.072 V (the voltage obtained at 0.5 A cm2) after 24 h SOEC operation, and the results are shown in Fig. 2. When a current is drawn from the cell, both the Rs and the Rp is suppressed, but the impact on the Rp is more pronounced. This suggests that the overall contribution of the Rp to the voltage during testing is diminished during the cell operation. This also explains why the voltage trends in Fig. 1 follow the Rs trends rather than the Rp trends. The voltage behaviour with time and log f vs jZj, for the cell shown in Fig. 2, have been illustrated in Figs. S5a and S5b, respectively. In previous studies, it has been observed that the addition of Ta to SrCoO3 enhances the SOFC performance of the cells compared to Nb. However, there are conflicting conclusions as to the origin of this improved performance. A study by Li et al. suggests that the key factor is the lower electronegativity of Ta5þ (1.8) in comparison to Nb5þ (1.87). The electron density, in this case, is more attracted towards the Nb5þ compared to the Ta5þ. This results in a lower oxidation state of cobalt ions for Ta-doped SrCoO3, which the authors argue, increases its oxygen vacancy concentration and improves the oxygen surface exchange kinetics and diffusivity [34]. In another study by Wang et al. [42], the improved performance is attributed to the larger difference in electronegativity between Ta and O in comparison to the Nb and O. The authors argue that this facilitates a stronger TaeO bond, which provides a higher oxygen stoichiometry and oxidation state of cobalt ions, which improves the electrochemical activity and electronic conductivity of Tadoped SrCoO3. However, it would be anticipated that the strong TaeO bond, would be detrimental for SOEC performance, as higher energy would be required to move the oxide ions in the crystal lattice. This has been demonstrated in the present study as improved performance and stability was observed for Nb and Nb, Ta co-doped samples compared to only Ta-doped sample when testing the cells under SOEC conditions. As shown in the previous studies [9,16,43] a high oxygen partial pressure is produced under SOEC conditions at the air electrode/
Fig. 3. SEM images for the cross-section of a) SCN sample after SOEC test, b) SCN sample after RSOC test, c), SCT sample after SOEC test and d) SCNT sample after SOEC test.
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Fig. 4. EDS maps for SCN, SCT and SCNT samples showing the distribution and precipitation of different species before and after the SOEC test.
electrolyte interface, and delamination/degradation can be avoided if the oxide ions can travel away from the interface. A similar phenomenon is believed to occur in the present system. Nb and Nb, Ta co-doped samples are known for their lower energy barrier (weaker bond with oxygen) for the oxide ion movement in the perovskite structure compared to Ta-doped sample [33]. Because of this low energy barrier, the oxide ions can move easily away from the interface, and hence, low oxygen partial pressure is produced at
the interface. This phenomenon is critical in determining the life of the cell as the high oxygen partial pressure can result in the formation of secondary phases as well as the delamination of air electrode [15,16,44]. As the energy barrier for the oxide ion movement for Ta-doped SrCoO3 is high, it produces high oxygen partial pressure at the electrode-electrolyte interface. Although no delamination has been observed for this sample. However, the formation of large concentrations of different secondary phases has
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Fig. 5. XRD patterns for the SDC surface for SCN, SCT and SCNT air electrodes before and after the SOEC and RSOC test at 800 C in air.
been identified by EDS and XRD, which increases the cell Rs. SC (Figure S3 (a-b)), on the other hand, does not have any substitution element for the generation and movement of oxide ions and therefore shows worst performance among all the materials tested here. SEM images for the cross-sections of the cells on WE side after the SOEC operation have been presented in Fig. 3 (a, c and d). These images clearly show that the air electrode and the SDC barrier layer are intact even after 240 h of electrolysis under 0.5 A cm2. Also, no delamination has been observed for any of the samples. These results suggest that SCN, SCT and SCNT are potential candidates for future SOEC applications. However, during the continuous SOEC operation, the Rs increase of the SCT sample suggests the formation of some undesired phases at the interface between SDC and SCT. As such, it might not be the best candidate as the SOEC air electrode material. EDS analysis was also carried out to analyze the bare SDC surface after the WE was removed using the diluted HCl acid. These results have been presented in Fig. 4 and Fig. S6. For the SCN sample, the SDC surface condition is almost similar before and after the test. Some precipitation of Sr and Co-rich phase has been observed after 240 h SOEC test. For the SCT sample, there is some precipitation of Sr, Co and Ta-rich phases before the test on SDC surface. After the 240 h SOEC test, the concentration of these precipitates increases showing further dissociation of the perovskite structure. The increased concentration of the precipitates is considered a primary reason for the increased Rs of the SCT sample because of the
insulating nature of the precipitates. Similarly, for SCNT samples, some precipitation of Sr, Co and Ta-rich phases on the SDC barrier layer has been observed, however, the concentration of these phases is much less compared to the SCT samples. The lower concentration of these new phases along with improved oxygen diffusion in the presence of Nb might be the reasons for a better performance of SCN and SCNT samples compared to the SCT sample. The XRD analysis of the SDC surface for the different samples after the removal of WE was also carried out and has been presented in Fig. 5(aec). These patterns indicate a shift in the SDC peak position for the SCT and SCNT samples after the SOEC test, indicating a change in the lattice constant of SDC. The lattice constant change can be ascribed to the inter-diffusion of different elements between the air electrodes and SDC during the test, hence forming different phases. For the SCN sample, no such shift has been observed due to the formation of fewer additional phases and therefore no degradation was observed for this material. Fig. 5a shows few small changes in the XRD patterns obtained from the SDC surface for the SCN samples before and after the SOEC test. The characteristic peaks are from the base materials, i.e. SDC. Some other peaks corresponding to SrCoO2.5 and SrCeO3 have also been observed. For the SCT samples (Fig. 5b), the decomposition of the perovskite into Ta2O5 has been identified before and after the test. A small increase in the peak intensity and the number of peaks indicates the additional formation of Ta2O5 during the SOEC test. Strontium species were also found precipitating on the SDC surface
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Fig. 6. a) Stability test for SCN air electrode under ± 0.5 A cm2 at 800 C in air and b) Impedance plots for the same cell after different cycles obtained at open circuit potential using four-electrode system.
in the EDS analysis. The XRD analysis shows that these precipitates might be in the form of SrCoO2.5 and SrCeO3. SCNT sample (Fig. 5c) also shows the formation of Ta2O5 during the SOEC test, suggesting SCN the most promising candidate among these materials, as no significant phase changes were observed as well as the ASR of the cell remains lowest during the 240 h SOEC test. A schematic to demonstrate the location of different phases produced during the test has been presented in Fig. S7.
3.2. The RSOC behaviour of SCN half-cell SCN showed very promising results as a future air electrode material for SOEC applications. Therefore, it was further tested
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under RSOC conditions. The cell and testing setup was similar to our previous study [38]. SCN symmetric cell was cycled between ±0.5 A cm2 and the results are shown in Fig. 6 (a, b). Fig. 6a shows that the absolute value of the WE voltage slightly decreases during the first cycle as the cathodic current (SOFC mode) is applied to it. This decrease in voltage shows that the cell performance is increasing as the ASR (determined from the voltagetime graph) is decreasing continuously for 16 h. The WE was then switched to the anodic current (SOEC mode) and tested for 7 h. Again, a decrease in the absolute value of the voltage can be seen, showing a performance enhancement. This performance increase can be accredited to the interfacial rearrangement of different species (activation) in SCN. This phenomenon has been observed for other electrodes as well [45]. This activation can also be seen in Fig. 6b (obtained at zero dc potential), which shows a decrease in the cell Rp after the 1st SOFC-SOEC cycle. For the next four cycles, the absolute value of the WE voltage increases slowly both during the SOFC and SOEC modes. As a result, an increase in the WE Rs can also be observed in Fig. 6b after 3rd and 5th SOFC-SOEC cycles. The WE ASR increases only from 0.188 to 0.193 U cm2 after 5 cycles. The log f vs jZj for this cell after different cycles has been shown in Fig. S8. It has been demonstrated in Fig. 1 (a, b) that SCN displays very good stability during pure SOEC mode. The slight increase in the ASR of the cell during the RSOC test suggests extra phase changes occurring most probably during the SOFC mode. Moreover, it is also interesting to note that the anodic voltage is lower compared to the cathodic voltage. This can be ascribed to the differences in overpotential associated with the OER (oxygen evolution reaction during anodic mode) and ORR (oxygen reduction reaction during cathodic mode). We propose that the overpotentials are lower in anodic mode due to the formation of oxygen, as pure O2 can significantly decrease the ASR compared to the air. SEM image for the SCN after RSOC operation has been shown in Fig. 3b, illustrating a strong contact between the SDC barrier layer and SCN electrode, and the absence of delamination. The EDS maps at the SDC surface after the removal of SCN (Fig. 7), however, show some evidence of Sr and Co-rich phases. The EDS maps for other elements (oxygen, scandium and cerium) for this cell are shown in Fig. S9. XRD analysis was further carried out at the SDC surface to determine the composition of these phases. These results have been shown in Fig. 5a. The Co-rich phase, as determined from the XRD patterns, is SrCoO2.5. In addition, the formation of Sr2Nb10O27 and SrCeO3 has been indicated by the XRD patterns. Sr2Nb10O27 was not observed in the cell operated in pure SOEC mode. This suggests that this phase has formed during the SOFC component of the RSOC test, and is the source of the slight Rs increase during the RSOC testing. These results confirm that SCN is a potential candidate for
Fig. 7. EDS maps for the interface between the SDC barrier layer and SCN air electrode after RSOC operation at 800 C in air.
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tested at 800, 750 and 700 C to determine the electrolysis current density with 10% H2O/90% H2 on the fuel electrode and air on the air electrode side. This cell was also tested at 800 C in 30% H2O/70% H2 and 50% H2O/50% H2. The results have been presented in Fig. 8 (a, b). The electrolysis current density values obtained at 800, 750 and 700 C are 0.51, 0.42 and 0.34 A cm2 at 1.3 V, respectively (Fig. 8a). The electrolysis current density increases when the H2O content in the flowing gas is increased. Fig. 8b shows that the electrolysis current density at 800 C in 30% H2O/70% H2 is 0.66 A cm2 which further increased to 0.79 A cm2 at 1.3 V in 50% H2O/50% H2 on fuel electrode side. The electrolysis current densities obtained in the present study have been compared with few others reported in the literature (Table 1). The fuel electrode used in our study had a thickness around 750 mm, which makes it quite difficult for the steam to flow efficiently. This is one of the major reasons for the low electrolysis current density and asymmetry observed in the IeV graphs (Fig. 8). Various studies have reported much higher values of electrolysis current densities, however, these studies have either used high performance anode material [2] or techniques such as infiltration [31] or high humidity content [26]. Using a combination of these techniques and optimizing the fuel electrode, the electrolysis current density for SCN can also be greatly improved. Moreover, SCN also shows the excellent long-term stability under pure SOEC conditions (Fig. 1a). These results illustrate the high electrochemical performance of SCN for future SOEC and RSOC applications.
4. Conclusions
Fig. 8. IeV curves for NiO-SSZ/SSZ/SDC/SCN cell (a) at different temperatures in 10% H2O/90% H2 and (b) at 800 C in different fuel conditions.
future RSOC applications. 3.3. SCN single cell A single cell with NiO-SSZ/SSZ/SDC/SCN configuration was
Three different electrode materials, i.e. SCN, SCT and SCNT are evaluated for their use in future SOEC and RSOC applications. All these materials are first tested under 0.5 A cm2 to determine their stability under the SOEC conditions. No delamination from the electrolyte is observed for any of these materials. SCT and SCNT, however, show the formation of some undesired phases as found in EDS maps and XRD patterns. SCN, on the other hand, does not show any significant phase changes and therefore, we further test it under RSOC conditions. During the RSOC test, it does not show any delamination. The ASR of the cell, however, increased slightly from 0.188 to 0.193 U cm2, due to the presence of Sr-rich phase, most probably formed during the SOFC cycle. When used in a single cell configuration with NiO-SSZ/SSZ/SDC, the SCN air electrode produces a high electrolysis current density. The obtained values at 1.3 V are 0.51, 0.42 and 0.34 A cm2 at 800, 750 and 700 C, respectively, in 10% H2O/90% H2 and 0.79 A cm2 at 800 C in 50% H2O/50% H2.
Table 1 Electrolysis current density values reported at different fuel conditions and temperatures at 1.3 V Cell configuration
Temperature ( C)
Fuel conditions
Electrolysis current density (A cm2)
NiO-SSZ/SSZ/SDC/SCN (This study)
800
NiO-YSZ/YSZ/LSM-YSZ [46] SFM/LSGM/SFM [26] NiO-YSZ/YSZ/PNO-CGO/PNO [47] PBM(Co, Fe)/LDC/LSGM/PBSCF50-GDC [2] NiO-GDC/YSZ/NNO [22] NiO-YSZ/YSZ/YDC/PNO [32] NiO-YSZ/SSZ/NNO-SSZ [31]
800 900 800 800 800 800 800
10%H2O/90%H2 30%H2O/70%H2 50%H2O/50%H2 30%H2O/70%H2 60%H2O/40%H2 50%H2O/50%H2 10%H2O/90%H2 90%H2O/10%H2 90%H2O/10%H2 50%H2O/50%H2
0.51 0.66 0.79 0.43 0.88 0.78 1.31 0.64 0.9 1.08
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Supplementary material Schematic for the cell tested under SOEC and RSOC conditions; Log f vs jZj graphs for different cells tested under SOEC and RSOC conditions; Voltage-time graph and EIS patterns for SC and SCN cells; Rs and Rp values of three cells under SOEC; EDS maps for different samples before and after SOEC test and after RSOC test; Schematic to determine the location of additional phases produced before and after SOEC operation. Acknowledgements All authors acknowledge the support of an Australian Research Council (ARC) Discovery grants (DP170104660) and Linkage Project grant (LP150100036). Authors Muhammad Shirjeel Khan and Ateeq-ur-Rehman additionally acknowledge the financial support from Australian Government Research Training Program Scholarship (RTP) and UQ Graduate School Scholarship. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134654. Declarations of interest None. References [1] S.D. Ebbesen, S.H. Jensen, A. Hauch, M.B. Mogensen, High temperature electrolysis in alkaline cells, solid proton conducting cells, and solid oxide cells, Chem. Rev. 114 (2014) 10697e10734. [2] A. Jun, J. Kim, J. Shin, G. Kim, Achieving high efficiency and eliminating degradation in solid oxide electrochemical cells using high oxygen-capacity perovskite, Angew. Chem. Int. Ed. 55 (2016) 12512e12515. [3] Y. Zheng, J. Wang, B. Yu, W. Zhang, J. Chen, J. Qiao, J. Zhang, A review of high temperature co-electrolysis of H2O and CO2 to produce sustainable fuels using solid oxide electrolysis cells (SOECs): advanced materials and technology, Chem. Soc. Rev. 46 (2017) 1427e1463. [4] A. Jun, Y.-W. Ju, G. Kim, Solid oxide electrolysis: concluding remarks, Faraday Discuss 182 (2015) 519e528. [5] K. Chen, S.P. Jiang, Failure mechanism of (La,Sr)MnO3 oxygen electrodes of solid oxide electrolysis cells, Int. J. Hydrogen Energy 36 (2011) 10541e10549. [6] M.S. Sohal, J.E. O'Brien, C.M. Stoots, V.I. Sharma, B. Yildiz, A. Virkar, Degradation issues in solid oxide cells during high temperature electrolysis, J. Fuel Cell Sci. Technol. 9 (2012), 011017. [7] A.V. Virkar, Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells, Int. J. Hydrogen Energy 35 (2010) 9527e9543. [8] C. Xu, Y. Wang, L. Jin, J. Ding, X. Ma, W.G. Wang, Degradation of Solid Oxide Electrolyser Cells with Different Anodes, 2012, pp. 97e102. [9] T. Jacobsen, M. Mogensen, The course of oxygen partial pressure and electric potentials across an oxide electrolyte cell, ECS Transactions 13 (2008) 259e273. [10] R. Knibbe, M.L. Traulsen, A. Hauch, S.D. Ebbesen, M. Mogensen, Solid oxide electrolysis cells: degradation at high current densities, J. Electrochem. Soc. 157 (2010) B1209eB1217. [11] S.N. Rashkeev, M.V. Glazoff, Atomic-scale mechanisms of oxygen electrode delamination in solid oxide electrolyzer cells, Int. J. Hydrogen Energy 37 (2012) 1280e1291. [12] N. Ai, N. Li, S. He, Y. Cheng, M. Saunders, K. Chen, T. Zhang, S.P. Jiang, Highly active and stable Er0.4Bi1.6O3 decorated La0.76Sr0.19MnO3þd nanostructured oxygen electrodes for reversible solid oxide cells, J. Mater. Chem. 5 (2017) 12149e12157. [13] K. Chen, N. Ai, S.P. Jiang, Reasons for the high stability of nano-structured (La,Sr)MnO3 infiltrated Y2O3eZrO2 composite oxygen electrodes of solid oxide electrolysis cells, Electrochem. Commun. 19 (2012) 119e122. [14] C. Graves, S.D. Ebbesen, S.H. Jensen, S.B. Simonsen, M.B. Mogensen, Eliminating degradation in solid oxide electrochemical cells by reversible operation, Nat. Mater. 14 (2015) 239e244. [15] N. Li, M. Keane, M.K. Mahapatra, P. Singh, Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ, Int. J. Hydrogen Energy 38 (2013) 6298e6303. [16] P. Moçoteguy, A. Brisse, A review and comprehensive analysis of degradation mechanisms of solid oxide electrolysis cells, Int. J. Hydrogen Energy 38 (2013)
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Birss, CO/CO2 study of high performance La0.3Sr0.7Fe0.7Cr0.3O3ed reversible SOFC electrodes, Fuel Cells 15 (2015) 689e696. [22] F. Chauveau, J. Mougin, J.M. Bassat, F. Mauvy, J.C. Grenier, A new anode material for solid oxide electrolyser: the neodymium nickelate Nd2NiO4þd, J. Power Sources 195 (2010) 744e749. , Z. Shao, Structural and oxygen[23] F. Dong, M. Ni, Y. Chen, D. Chen, M.O. Tade transport studies of double perovskites PrBa1xCo2O5þd(x ¼ 0.00, 0.05, and 0.10) toward their application as superior oxygen reduction electrodes, J. Mater. Chem. 2 (2014) 20520e20529. [24] P. Kim-Lohsoontorn, D.J.L. Brett, N. Laosiripojana, Y.M. Kim, J.M. Bae, Performance of solid oxide electrolysis cells based on composite La0.8Sr0.2MnO3deyttria stabilized zirconia and Ba0.5Sr0.5Co0.8Fe0.2O3d oxygen electrodes, Int. J. Hydrogen Energy 35 (2010) 3958e3966. [25] M.A. Laguna-Bercero, N. Kinadjan, R. Sayers, H. El Shinawi, C. Greaves, S.J. Skinner, Performance of La2-xSrxCo0.5Ni0.5O4±d as an oxygen electrode for solid oxide reversible cells, Fuel Cells 11 (2011) 102e107. [26] Q. Liu, C. Yang, X. Dong, F. Chen, Perovskite Sr2Fe1.5Mo0.5O6d as electrode materials for symmetrical solid oxide electrolysis cells, Int. J. Hydrogen Energy 35 (2010) 10039e10044. nchez, J. Prado-Gonjal, D. Avila-Brande, n, [27] B. Molero-Sa M. Chen, E. Mora V. Birss, High performance La0.3Ca0.7Cr0.3Fe0.7O3d air electrode for reversible solid oxide fuel cell applications, Int. J. Hydrogen Energy 40 (2015) 1902e1910. [28] Y. Wang, T. Liu, S. Fang, F. Chen, Syngas production on a symmetrical solid oxide H2O/CO2 co-electrolysis cell with Sr2Fe1.5Mo0.5O6eSm0.2Ce0.8O1.9 electrodes, J. Power Sources 305 (2016) 240e248. [29] Y. Zheng, Q. Li, T. Chen, W. Wu, C. Xu, W.G. Wang, Comparison of performance and degradation of large-scale solid oxide electrolysis cells in stack with different composite air electrodes, Int. J. Hydrogen Energy 40 (2015) 2460e2472. [30] W. Wang, Y. Huang, S. Jung, J.M. Vohs, R.J. Gorte, A comparison of LSM, LSF, and LSCo for solid oxide electrolyzer anodes, J. Electrochem. Soc. 153 (2006) A2066eA2070. [31] T. Chen, M. Liu, C. Yuan, Y. Zhou, X. Ye, Z. Zhan, C. Xia, S. Wang, High performance of intermediate temperature solid oxide electrolysis cells using Nd2NiO4þd impregnated scandia stabilized zirconia oxygen electrode, J. Power Sources 276 (2015) 1e6. [32] T. Ogier, J.M. Bassat, F. Mauvy, S. Fourcade, J.C. Grenier, K. Couturier, M. Petitjean, J. Mougin, Enhanced performances of structured oxygen electrodes for high temperature steam electrolysis, Fuel Cells 13 (2013) 536e541. [33] M. Li, M. Zhao, F. Li, W. Zhou, V.K. Peterson, X. Xu, Z. Shao, I. Gentle, Z. Zhu, A niobium and tantalum co-doped perovskite cathode for solid oxide fuel cells operating below 500 degrees C, Nat. Commun. 8 (2017) 13990. [34] M. Li, W. Zhou, V.K. Peterson, M. Zhao, Z. Zhu, A comparative study of SrCo0.8Nb0.2O3d and SrCo0.8Ta0.2O3d as low-temperature solid oxide fuel cell cathodes: effect of non-geometry factors on the oxygen reduction reaction, J. Mater. Chem. 3 (2015) 24064e24070. [35] M. Li, W. Zhou, Z. Zhu, Comparative studies of SrCo1xTaxO3d (x¼0.05-0.4) oxides as cathodes for low-temperature solid-oxide fuel cells, ChemElectroChem 2 (2015) 1331e1338. [36] G.A. Hughes, K. Yakal-Kremski, S.A. Barnett, Life testing of LSM-YSZ composite electrodes under reversing-current operation, Phys. Chem. Chem. Phys. 15 (2013) 17257e17262. [37] M. Keane, M.K. Mahapatra, A. Verma, P. Singh, LSMeYSZ interactions and anode delamination in solid oxide electrolysis cells, Int. J. Hydrogen Energy 37 (2012) 16776e16785. [38] M.S. Khan, X. Xu, R. Knibbe, Z. Zhu, Porous scandia-stabilized zirconia layer for enhanced performance of reversible solid oxide cells, ACS Appl. Mater. Interfaces 10 (2) (2018) 25295e25302. [39] F. Mauvy, C. Lalanne, J.-M. Bassat, J.-C. Grenier, H. Zhao, L. Huo, P. Stevens, Electrode properties of Ln2NiO4þd ( Ln ¼ La , Nd , Pr ) : AC impedance and DC polarization studies, J. Electrochem. Soc. 153 (2006) A1547eA1553. [40] T. Ogier, F. Mauvy, J.-M. Bassat, J. Laurencin, J. Mougin, J.-C. Grenier, Overstoichiometric oxides Ln2NiO4þd (Ln ¼ La, Pr or Nd) as oxygen anodic electrodes for solid oxide electrolysis application, Int. J. Hydrogen Energy 40 (2015) 15885e15892. [41] K.O. Hoyt, P.E. Gannon, P. White, R. Tortop, B.J. Ellingwood, H. Khoshuei, Oxidation behavior of (Co,Mn)3O4 coatings on preoxidized stainless steel for solid oxide fuel cell interconnects, Int. J. Hydrogen Energy 37 (2012) 518e529. [42] J. Wang, T. Yang, L. Lei, K. Huang, Ta-Doped SrCoO3d as a promising
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Evaluation of SrCo0.8Nb0.2O3-d, SrCo0.8Ta0.2O3-d and SrCo0.8Nb0.1Ta0.1O3d as air electrode materials for solid oxide electrolysis and reversible solid oxide cells Muhammad Shirjeel Khan a, Xiaoyong Xu a, **, Mengran Li a, Ateeq-ur Rehman a, Ruth Knibbe b, Anya Josefa Yago c, Zhonghua Zhu a, * a b c
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 Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, Queensland, 4072, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2019 Received in revised form 12 June 2019 Accepted 6 August 2019 Available online 8 August 2019
Strontium cobalt-based perovskites display high electrochemical activity but poor stability for solid oxide electrolysis cells (SOECs). Here, we evaluate the stability of three strontium cobalt-based materials, i.e. SrCo0.8Nb0.2O3-d (SCN), SrCo0.8Ta0.2O3-d (SCT) and SrCo0.8Nb0.1Ta0.1O3-d (SCNT) as air electrodes for SOECs. The electrochemical results show that the area-specific resistance (ASR) for SCN and SCNT cells decreases and then becomes almost constant whereas the ASR of the SCT cell continuously increases, during the 240 h SOEC test. The improved performance of SCN and SCNT cells is ascribed to the lower energy barrier required for the oxide ion movement in the presence of niobium and the less concentration of undesired phases formed during SOEC operation. SCN is further tested under reversible solid oxide cell (RSOC) conditions with 16 h solid oxide fuel cell (SOFC) and 7 h SOEC cycles. The cell presents excellent stability as its impedance increases only slightly from 0.188 to 0.193 U cm2 after 114 h RSOC test. A single cell with NiOSSZ/SSZ/SDC/SCN configuration shows high electrolysis current densities of 0.51, 0.42 and 0.34 A cm2 at 800, 750 and 700 C, respectively, in 10% H2O/90% H2 and 0.79 A cm2 at 800 C in 50% H2O/50% H2 at 1.3 V. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Electrolysis current density Energy barrier Impedance Perovskites Solid oxide electrolysis cells
1. Introduction Solid Oxide Electrolysis Cells (SOECs) have received great attention as a promising energy storage device as they can work in combination with renewable energy resources [1e3]. The waste heat and electrical energy from other energy resources can be used in SOECs to produce H2 and CO by splitting H2O and CO2. SOECs are better energy storage devices than secondary batteries and supercapacitors due to their higher energy storage capacity and lower cost [4]. The major issues currently faced by the SOECs include a high degradation rate of its components and low electrocatalytic activity for oxygen evolution reaction (OER). Lanthanum strontium manganite (LSM) and lanthanum strontium cobalt ferrite (LSCF) are state-of-the-art air electrode materials used for SOECs. However, various researchers have observed LSM delamination from the electrolyte [5e8]. This delamination is * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Xu), [email protected] (Z. Zhu). https://doi.org/10.1016/j.electacta.2019.134654 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
caused by the formation of high oxygen partial pressure at the electrode/electrolyte interface, which is due to the large electrochemical potentials [9,10] and the inter-diffusion of different species [11]. Several studies have proposed different techniques to overcome such delamination and increase the cell lifetime [12e16]. LSCF, on the other hand, has shown quite promising results, as it is a mixed ionic-electronic conductor and offers a lower ASR compared to LSM or LSM/YSZ composite [17,18]. At the same current density, an SOEC with LSCF will have a lower electrode overpotential in comparison to an SOEC with LSM. This, in turn reduces the oxygen partial pressure produced at the interface during electrolysis, potentially avoiding electrode delamination. However, the LSCF suffers from various other degradation phenomena such as secondary phase formation through cation diffusion and pore formation along the electrolyte grain boundaries [19,20]. Various new materials have been proposed over the years to achieve higher OER activity and better long-term stability during the SOEC operation [12,21e28]. Lanthanum strontium cobaltite (LSC) is one such material with a perovskite structure and known
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for its high ionic conductivity. However, the high thermal expansion co-efficient (TEC), sometimes, leads to its delamination from the electrolyte [29]. Lanthanum strontium ferrite (LSF), on the other hand, did not show any sign of delamination as it was tested at 700 C under 0.285 A cm2 for 300 h [30]. A more recent development in this area is the use of highly active and stable perovskite, strontium iron molybdate (SFM). SFM has shown promising results as a symmetric electrode for SOECs. One such cell with SFM/La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM)/SFM configuration produced a current density as high as 0.88 A cm2 at 900 C under an electrolysis voltage of 1.3 V and 60 vol % absolute humidity (AH) [26]. Similarly, an SFM-SDC symmetric electrode with LSGM electrolyte has shown a current density of 0.734 A cm2 at 1.3 V and 850 C when used for co-electrolysis (16%H2O/16%CO2/20%H2/48% N2) [28]. Moreover, it also showed a constant electrolysis voltage under 0.12 A cm2 at 800 C for 103 h without any sign of delamination. The double perovskite, PrBa0.5Sr0.5Co1.5Fe0.5O5þd (PBSCF50), has been demonstrated as the most active material developed so far for SOECs [2]. The maximum electrolysis current density of 1.31 A cm2 at 1.3 V was obtained at 800 C when PBSCF50 was used as an air electrode along with PrBaMn2O5þd as a fuel electrode and LSGM as an electrolyte with 10% H2O/90% H2 on the fuelelectrode side. The cell also displayed excellent stability without any sign of delamination when tested under 0.25 A cm2 for more than 600 h. Nickelates are also strong candidates as air electrodes for SOECs and reversible solid oxide cells (RSOCs). Nano-structured Nd2NiO4þd (NNO) impregnated scandia-stabilized zirconia (SSZ) has shown an electrolysis current density of 1.08 A cm2 at 800 C and 1.3 V [31]. The cell also showed an increase in the electrolysis performance during a short-term test. Another nickelate (Pr2NiO4þd) has also shown a maximum electrolysis current density of 0.9 at 800 C and 1.28 V when using 90% H2O/10% H2 on the fuel electrode side [32]. Most of these studies are focused only on achieving high electrolysis performance. Very few studies have been carried out to determine the long-term stability of air electrode materials under SOEC conditions, and even fewer have been carried out to establish the long-term RSOC stability. The aim of the present research is to study the newly developed strontium cobalt-based perovskites under SOEC and RSOC conditions, both for their activity and longterm stability. We selected strontium-cobalt based materials because of their excellent performance during the solid oxide fuel cell (SOFC) testing, as reported in the recent studies [33,34]. Symmetric cells with three different electrode materials including SrCo0.8Nb0.2O3-d (SCN), SrCo0.8Ta0.2O3-d (SCT) and SrCo0.8Nb0.1Ta0.1O3-d (SCNT), were tested under SOEC conditions for 240 h in air under a current density of 0.5 A cm2 at 800 C. The area-specific resistance (ASR) of the SCNT cell increases slightly, whereas the increase is much larger and continuous for the SCT cell. However, no delamination was observed for any of these cells. SCN, on the other hand, shows a decrease in the ASR during the 240 h SOEC operation. Therefore, it was further tested for its behaviour under RSOC conditions. A single cell with SCN as an air electrode was also tested at different temperatures and fuel conditions to determine the electrolysis current density. The electrochemical behaviour has been explained with the help of other characterization techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). 2. Material and methods 2.1. Cell preparation The electrode powders (SCN, SCT and SCNT) were prepared by
solid-state reaction route, as described elsewhere [33e35]. Appropriate amounts of SrCO3 (99.9%, Aldrich), Co3O4 (99.9%, Aldrich), Nb2O5 (99.9%, Aldrich) and Ta2O5 (99.9%, Aldrich) were ball milled in ethanol for 24 h, followed by drying and pressing in the form of circular pellets. These pellets were then sintered at 1200 C for 10 h, followed by crushing and re-sintering at 1200 C for another 10 h to obtain the perovskite structure. SrCoO3 (SC) powder was also prepared in a similar manner for comparison. The XRD patterns for the prepared powders have been shown in our previous publications [33e35]. The electrode slurry was prepared by mixing the obtained perovskites powders with appropriate amounts of solvent and binder. Symmetric cells were prepared by coating a samaria-doped ceria (SDC) layer on dense scandiastabilized zirconia (SSZ) electrolyte (~200 mm) pellets. Sintering of the SDC layer was carried out at 1350 C for 5 h. Air electrodes were then sprayed on top of the SDC layer and sintered at 950 C for 2 h. The thickness of the air electrode was 11e14 mm, and the active area was 0.283 cm2. These cells were used to determine the electrochemical behaviour of different electrode materials (SC, SCN, SCT and SCNT) under SOEC and RSOC conditions. Symmetric cells have also been studied under SOEC polarization conditions previously by various researchers [15,36e38]. A full cell to determine the electrolysis current density of the cell with SCN as air electrode was also prepared and tested at 800, 750 and 700 C in different fuel conditions. For this purpose, commercial NiO (J. T. Baker) and SSZ (fuel cell materials (Sc2O3)0$10(ZrO2)0.90) powders in a weight ratio of 60:40 were mixed along with 10 wt % dextrin as pore former. The mixture was ball milled with ethanol for 24 h followed by drying. The dried powder was pressed into circular pellets and pre-sintered at 1000 C for 2 h. Functional layer (NiO-SSZ), the electrolyte layer (SSZ) and the barrier layer (SDC) were then spray coated on the presintered pellets, respectively, and co-sintered at 1350 C for 5 h. The air electrode (SCN) was finally spray coated on the electrolyte and sintered at 950 C for 2 h. Silver wires and paste was used for current collection. The final cell was mounted on an alumina tube and sealed with the silver paste. 2.2. Characterization A four-wire resistance measurement method, as described in our previous study [38] and also shown in Fig. S1, was used for the electrochemical characterization of half-cells. The reference electrode (RE) in this case, was placed on the SSZ electrolyte and on the same side as working electrode (WE) to ensure the stable potential of WE under SOEC conditions. The RE, which is circular with a 2 mm diameter, was made of a silver wire attached to the SSZ electrolyte by silver paste. The distance between the WE and RE was 5 mm, which is more than 5 times the thickness of the electrolyte used (~200 mm). This electrode set-up is in accordance with the studies previously carried out [39,40]. A current density of 0.5 A cm2 (SOEC mode) was applied between the WE and counter electrode (CE), and the voltage change was monitored between the sensing (S) and RE for 240 h at 800 C in air. The current was interrupted from time to time to determine the impedance behaviour (EIS) of the WE using Metrohm Autolab instrument in the frequency range of 10 mHze1 MHz with a signal amplitude of 10 mV. The impedance was measured at open circuit potential (0 V) for all cells during the long term SOEC and RSOC tests. However, for the SCN cell tested only for 24 h under SOEC conditions, the impedance was determined both at 0 V and 0.072 V (the voltage obtained under 0.5 A cm2 after 24 h). For RSOC testing, a current density of ±0.5 A cm2 was applied between the WE and CE and the voltage change was monitored between the S and RE. The SOFC operation (þ0.5 A cm2) and the
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SOEC operation (0.5 A cm2) were performed for 16 h and 7 h, respectively. For the full cell testing, H2 was passed through a water bath set at certain temperatures to obtain the required humidity values. The humidity content in the flowing H2 was determined using a humidity sensor before starting the SOEC test. The values obtained at different water bath temperatures were then used as reference, as the sensor could not be used during the experiment. The electrolysis current density was determined at three different temperatures, i.e. 800, 750 and 700 C, with 10% H2O/90% H2 supplied to the fuel electrode and air supplied to the air electrode side. The electrolysis current density was also determined by changing the H2O content to 30% and 50% in H2 at 800 C. After electrochemical testing, samples were prepared for interfacial and cross-sectional analysis. To study the interface, the electrodes were etched away using hydrochloric acid (HCl), whereas, for cross-sectional examination, the samples were mounted in the epoxy resin, followed by vacuum impregnation and drying. The mounted samples were ground and polished to prepare a smooth and scratch-free surface. The samples for the interfacial studies were coated with carbon, while the samples for crosssectional analysis were coated with iridium [41]. The purpose of the coating was to reduce the charging effects from the electron beam during SEM analysis. SEM and EDS analysis were carried out using JEOL-6610, which is equipped with an Oxford 50 mm2 X-Max SDD X-ray detector for simultaneous imaging and elemental analysis. The operating voltage used for the analysis was 15 kV. XRD analysis was carried out at the interface before and after test using a
3
Rigaku SmartLab micro-diffraction instrument. 3. Results and discussion 3.1. The SOEC behaviour of half-cells To demonstrate a good performance, the half-cells tested under the SOEC conditions must show a stable voltage during the longterm testing. Fig. 1(aed) shows the electrochemical behaviour of the three electrodes tested under 0.5 A cm2 at 800 C in air for 240 h. Fig. 1a shows that the voltage of the WE for SCN cell decreases during the initial 25 h and then becomes almost constant. The EIS results for this cell (Fig. 1b) show that the ohmic resistance (Rs) of the WE decreases continuously (from 0.078 to 0.042 U cm2 after 240 h), while there is a polarization resistance (Rp) increase (from 0.007 to 0.069 U cm2 after 240 h). Fig. 1a further shows that the voltage of the WE for the SCT cell increases continuously, indicating an increase in the WE ASR. This behaviour is also evident in Fig. 1c, which show an increase in both Rs and Rp of the SCT WE. The WE for the SCNT cell has a voltage between the SCN and the SCT cells. The WE voltage for the SCNT cell decreases initially, followed by an increase and an almost stable behaviour. The EIS patterns for this cell (Fig. 1d) are comparable to the SCN cell, showing a decrease in Rs while an increase in the cell Rp. However, the decrease in the Rs for this cell (0.013 U cm2) is smaller as compared to that for the SCN cell (0.037 U cm2), which indicates the positive effects of niobium (Nb) compared to tantalum (Ta) on the SOEC performance of the cells. The corresponding log f
Fig. 1. (a) Voltage-time graph for three different electrodes (SCN, SCT and SCNT) under the anodic current density of 0.5 A cm2 at 800 C in air, and (b), (c) and (d) impedance plots for the same SCN, SCT and SCNT cells, respectively, obtained at open circuit conditions using four-electrode measurement method.
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Fig. 2. Impedance plots at different voltages using a four-electrode measurement method for SCN WE after it was tested for 24 h in air at 800 C.
vs jZj plots for the three electrodes are shown in Figure S2 (a-c). SC was also tested as a reference and the voltage obtained was much higher compared to all samples, even at the start of the test (Fig. S3a). This voltage increases continuously during the SOEC test. The impedance plot for SC is shown in Fig. S3b, which also confirms the continuous increase in both Rs and Rp of the cell. It is important to reiterate that the impedance plots in Fig. 1(bed) are obtained at open circuit potentials (0 V). The voltage trends in Fig. 1a track closely with the Rs of the samples. The Rp, on the other hand, increases for all samples, even though SCN and SCNT show an almost stable voltage behaviour. This has been illustrated in Figure S4 (a-c). To further explain the conflicting trend
between Rp (Fig. S4) and Fig. 1a, an SCN cell was tested under 0.5 A cm2. The impedance for this cell was performed at 0 V and 0.072 V (the voltage obtained at 0.5 A cm2) after 24 h SOEC operation, and the results are shown in Fig. 2. When a current is drawn from the cell, both the Rs and the Rp is suppressed, but the impact on the Rp is more pronounced. This suggests that the overall contribution of the Rp to the voltage during testing is diminished during the cell operation. This also explains why the voltage trends in Fig. 1 follow the Rs trends rather than the Rp trends. The voltage behaviour with time and log f vs jZj, for the cell shown in Fig. 2, have been illustrated in Figs. S5a and S5b, respectively. In previous studies, it has been observed that the addition of Ta to SrCoO3 enhances the SOFC performance of the cells compared to Nb. However, there are conflicting conclusions as to the origin of this improved performance. A study by Li et al. suggests that the key factor is the lower electronegativity of Ta5þ (1.8) in comparison to Nb5þ (1.87). The electron density, in this case, is more attracted towards the Nb5þ compared to the Ta5þ. This results in a lower oxidation state of cobalt ions for Ta-doped SrCoO3, which the authors argue, increases its oxygen vacancy concentration and improves the oxygen surface exchange kinetics and diffusivity [34]. In another study by Wang et al. [42], the improved performance is attributed to the larger difference in electronegativity between Ta and O in comparison to the Nb and O. The authors argue that this facilitates a stronger TaeO bond, which provides a higher oxygen stoichiometry and oxidation state of cobalt ions, which improves the electrochemical activity and electronic conductivity of Tadoped SrCoO3. However, it would be anticipated that the strong TaeO bond, would be detrimental for SOEC performance, as higher energy would be required to move the oxide ions in the crystal lattice. This has been demonstrated in the present study as improved performance and stability was observed for Nb and Nb, Ta co-doped samples compared to only Ta-doped sample when testing the cells under SOEC conditions. As shown in the previous studies [9,16,43] a high oxygen partial pressure is produced under SOEC conditions at the air electrode/
Fig. 3. SEM images for the cross-section of a) SCN sample after SOEC test, b) SCN sample after RSOC test, c), SCT sample after SOEC test and d) SCNT sample after SOEC test.
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Fig. 4. EDS maps for SCN, SCT and SCNT samples showing the distribution and precipitation of different species before and after the SOEC test.
electrolyte interface, and delamination/degradation can be avoided if the oxide ions can travel away from the interface. A similar phenomenon is believed to occur in the present system. Nb and Nb, Ta co-doped samples are known for their lower energy barrier (weaker bond with oxygen) for the oxide ion movement in the perovskite structure compared to Ta-doped sample [33]. Because of this low energy barrier, the oxide ions can move easily away from the interface, and hence, low oxygen partial pressure is produced at
the interface. This phenomenon is critical in determining the life of the cell as the high oxygen partial pressure can result in the formation of secondary phases as well as the delamination of air electrode [15,16,44]. As the energy barrier for the oxide ion movement for Ta-doped SrCoO3 is high, it produces high oxygen partial pressure at the electrode-electrolyte interface. Although no delamination has been observed for this sample. However, the formation of large concentrations of different secondary phases has
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Fig. 5. XRD patterns for the SDC surface for SCN, SCT and SCNT air electrodes before and after the SOEC and RSOC test at 800 C in air.
been identified by EDS and XRD, which increases the cell Rs. SC (Figure S3 (a-b)), on the other hand, does not have any substitution element for the generation and movement of oxide ions and therefore shows worst performance among all the materials tested here. SEM images for the cross-sections of the cells on WE side after the SOEC operation have been presented in Fig. 3 (a, c and d). These images clearly show that the air electrode and the SDC barrier layer are intact even after 240 h of electrolysis under 0.5 A cm2. Also, no delamination has been observed for any of the samples. These results suggest that SCN, SCT and SCNT are potential candidates for future SOEC applications. However, during the continuous SOEC operation, the Rs increase of the SCT sample suggests the formation of some undesired phases at the interface between SDC and SCT. As such, it might not be the best candidate as the SOEC air electrode material. EDS analysis was also carried out to analyze the bare SDC surface after the WE was removed using the diluted HCl acid. These results have been presented in Fig. 4 and Fig. S6. For the SCN sample, the SDC surface condition is almost similar before and after the test. Some precipitation of Sr and Co-rich phase has been observed after 240 h SOEC test. For the SCT sample, there is some precipitation of Sr, Co and Ta-rich phases before the test on SDC surface. After the 240 h SOEC test, the concentration of these precipitates increases showing further dissociation of the perovskite structure. The increased concentration of the precipitates is considered a primary reason for the increased Rs of the SCT sample because of the
insulating nature of the precipitates. Similarly, for SCNT samples, some precipitation of Sr, Co and Ta-rich phases on the SDC barrier layer has been observed, however, the concentration of these phases is much less compared to the SCT samples. The lower concentration of these new phases along with improved oxygen diffusion in the presence of Nb might be the reasons for a better performance of SCN and SCNT samples compared to the SCT sample. The XRD analysis of the SDC surface for the different samples after the removal of WE was also carried out and has been presented in Fig. 5(aec). These patterns indicate a shift in the SDC peak position for the SCT and SCNT samples after the SOEC test, indicating a change in the lattice constant of SDC. The lattice constant change can be ascribed to the inter-diffusion of different elements between the air electrodes and SDC during the test, hence forming different phases. For the SCN sample, no such shift has been observed due to the formation of fewer additional phases and therefore no degradation was observed for this material. Fig. 5a shows few small changes in the XRD patterns obtained from the SDC surface for the SCN samples before and after the SOEC test. The characteristic peaks are from the base materials, i.e. SDC. Some other peaks corresponding to SrCoO2.5 and SrCeO3 have also been observed. For the SCT samples (Fig. 5b), the decomposition of the perovskite into Ta2O5 has been identified before and after the test. A small increase in the peak intensity and the number of peaks indicates the additional formation of Ta2O5 during the SOEC test. Strontium species were also found precipitating on the SDC surface
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Fig. 6. a) Stability test for SCN air electrode under ± 0.5 A cm2 at 800 C in air and b) Impedance plots for the same cell after different cycles obtained at open circuit potential using four-electrode system.
in the EDS analysis. The XRD analysis shows that these precipitates might be in the form of SrCoO2.5 and SrCeO3. SCNT sample (Fig. 5c) also shows the formation of Ta2O5 during the SOEC test, suggesting SCN the most promising candidate among these materials, as no significant phase changes were observed as well as the ASR of the cell remains lowest during the 240 h SOEC test. A schematic to demonstrate the location of different phases produced during the test has been presented in Fig. S7.
3.2. The RSOC behaviour of SCN half-cell SCN showed very promising results as a future air electrode material for SOEC applications. Therefore, it was further tested
7
under RSOC conditions. The cell and testing setup was similar to our previous study [38]. SCN symmetric cell was cycled between ±0.5 A cm2 and the results are shown in Fig. 6 (a, b). Fig. 6a shows that the absolute value of the WE voltage slightly decreases during the first cycle as the cathodic current (SOFC mode) is applied to it. This decrease in voltage shows that the cell performance is increasing as the ASR (determined from the voltagetime graph) is decreasing continuously for 16 h. The WE was then switched to the anodic current (SOEC mode) and tested for 7 h. Again, a decrease in the absolute value of the voltage can be seen, showing a performance enhancement. This performance increase can be accredited to the interfacial rearrangement of different species (activation) in SCN. This phenomenon has been observed for other electrodes as well [45]. This activation can also be seen in Fig. 6b (obtained at zero dc potential), which shows a decrease in the cell Rp after the 1st SOFC-SOEC cycle. For the next four cycles, the absolute value of the WE voltage increases slowly both during the SOFC and SOEC modes. As a result, an increase in the WE Rs can also be observed in Fig. 6b after 3rd and 5th SOFC-SOEC cycles. The WE ASR increases only from 0.188 to 0.193 U cm2 after 5 cycles. The log f vs jZj for this cell after different cycles has been shown in Fig. S8. It has been demonstrated in Fig. 1 (a, b) that SCN displays very good stability during pure SOEC mode. The slight increase in the ASR of the cell during the RSOC test suggests extra phase changes occurring most probably during the SOFC mode. Moreover, it is also interesting to note that the anodic voltage is lower compared to the cathodic voltage. This can be ascribed to the differences in overpotential associated with the OER (oxygen evolution reaction during anodic mode) and ORR (oxygen reduction reaction during cathodic mode). We propose that the overpotentials are lower in anodic mode due to the formation of oxygen, as pure O2 can significantly decrease the ASR compared to the air. SEM image for the SCN after RSOC operation has been shown in Fig. 3b, illustrating a strong contact between the SDC barrier layer and SCN electrode, and the absence of delamination. The EDS maps at the SDC surface after the removal of SCN (Fig. 7), however, show some evidence of Sr and Co-rich phases. The EDS maps for other elements (oxygen, scandium and cerium) for this cell are shown in Fig. S9. XRD analysis was further carried out at the SDC surface to determine the composition of these phases. These results have been shown in Fig. 5a. The Co-rich phase, as determined from the XRD patterns, is SrCoO2.5. In addition, the formation of Sr2Nb10O27 and SrCeO3 has been indicated by the XRD patterns. Sr2Nb10O27 was not observed in the cell operated in pure SOEC mode. This suggests that this phase has formed during the SOFC component of the RSOC test, and is the source of the slight Rs increase during the RSOC testing. These results confirm that SCN is a potential candidate for
Fig. 7. EDS maps for the interface between the SDC barrier layer and SCN air electrode after RSOC operation at 800 C in air.
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tested at 800, 750 and 700 C to determine the electrolysis current density with 10% H2O/90% H2 on the fuel electrode and air on the air electrode side. This cell was also tested at 800 C in 30% H2O/70% H2 and 50% H2O/50% H2. The results have been presented in Fig. 8 (a, b). The electrolysis current density values obtained at 800, 750 and 700 C are 0.51, 0.42 and 0.34 A cm2 at 1.3 V, respectively (Fig. 8a). The electrolysis current density increases when the H2O content in the flowing gas is increased. Fig. 8b shows that the electrolysis current density at 800 C in 30% H2O/70% H2 is 0.66 A cm2 which further increased to 0.79 A cm2 at 1.3 V in 50% H2O/50% H2 on fuel electrode side. The electrolysis current densities obtained in the present study have been compared with few others reported in the literature (Table 1). The fuel electrode used in our study had a thickness around 750 mm, which makes it quite difficult for the steam to flow efficiently. This is one of the major reasons for the low electrolysis current density and asymmetry observed in the IeV graphs (Fig. 8). Various studies have reported much higher values of electrolysis current densities, however, these studies have either used high performance anode material [2] or techniques such as infiltration [31] or high humidity content [26]. Using a combination of these techniques and optimizing the fuel electrode, the electrolysis current density for SCN can also be greatly improved. Moreover, SCN also shows the excellent long-term stability under pure SOEC conditions (Fig. 1a). These results illustrate the high electrochemical performance of SCN for future SOEC and RSOC applications.
4. Conclusions
Fig. 8. IeV curves for NiO-SSZ/SSZ/SDC/SCN cell (a) at different temperatures in 10% H2O/90% H2 and (b) at 800 C in different fuel conditions.
future RSOC applications. 3.3. SCN single cell A single cell with NiO-SSZ/SSZ/SDC/SCN configuration was
Three different electrode materials, i.e. SCN, SCT and SCNT are evaluated for their use in future SOEC and RSOC applications. All these materials are first tested under 0.5 A cm2 to determine their stability under the SOEC conditions. No delamination from the electrolyte is observed for any of these materials. SCT and SCNT, however, show the formation of some undesired phases as found in EDS maps and XRD patterns. SCN, on the other hand, does not show any significant phase changes and therefore, we further test it under RSOC conditions. During the RSOC test, it does not show any delamination. The ASR of the cell, however, increased slightly from 0.188 to 0.193 U cm2, due to the presence of Sr-rich phase, most probably formed during the SOFC cycle. When used in a single cell configuration with NiO-SSZ/SSZ/SDC, the SCN air electrode produces a high electrolysis current density. The obtained values at 1.3 V are 0.51, 0.42 and 0.34 A cm2 at 800, 750 and 700 C, respectively, in 10% H2O/90% H2 and 0.79 A cm2 at 800 C in 50% H2O/50% H2.
Table 1 Electrolysis current density values reported at different fuel conditions and temperatures at 1.3 V Cell configuration
Temperature ( C)
Fuel conditions
Electrolysis current density (A cm2)
NiO-SSZ/SSZ/SDC/SCN (This study)
800
NiO-YSZ/YSZ/LSM-YSZ [46] SFM/LSGM/SFM [26] NiO-YSZ/YSZ/PNO-CGO/PNO [47] PBM(Co, Fe)/LDC/LSGM/PBSCF50-GDC [2] NiO-GDC/YSZ/NNO [22] NiO-YSZ/YSZ/YDC/PNO [32] NiO-YSZ/SSZ/NNO-SSZ [31]
800 900 800 800 800 800 800
10%H2O/90%H2 30%H2O/70%H2 50%H2O/50%H2 30%H2O/70%H2 60%H2O/40%H2 50%H2O/50%H2 10%H2O/90%H2 90%H2O/10%H2 90%H2O/10%H2 50%H2O/50%H2
0.51 0.66 0.79 0.43 0.88 0.78 1.31 0.64 0.9 1.08
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Supplementary material Schematic for the cell tested under SOEC and RSOC conditions; Log f vs jZj graphs for different cells tested under SOEC and RSOC conditions; Voltage-time graph and EIS patterns for SC and SCN cells; Rs and Rp values of three cells under SOEC; EDS maps for different samples before and after SOEC test and after RSOC test; Schematic to determine the location of additional phases produced before and after SOEC operation. Acknowledgements All authors acknowledge the support of an Australian Research Council (ARC) Discovery grants (DP170104660) and Linkage Project grant (LP150100036). Authors Muhammad Shirjeel Khan and Ateeq-ur-Rehman additionally acknowledge the financial support from Australian Government Research Training Program Scholarship (RTP) and UQ Graduate School Scholarship. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134654. Declarations of interest None. References [1] S.D. Ebbesen, S.H. Jensen, A. Hauch, M.B. Mogensen, High temperature electrolysis in alkaline cells, solid proton conducting cells, and solid oxide cells, Chem. Rev. 114 (2014) 10697e10734. [2] A. Jun, J. Kim, J. Shin, G. Kim, Achieving high efficiency and eliminating degradation in solid oxide electrochemical cells using high oxygen-capacity perovskite, Angew. Chem. Int. Ed. 55 (2016) 12512e12515. [3] Y. Zheng, J. Wang, B. Yu, W. Zhang, J. Chen, J. Qiao, J. Zhang, A review of high temperature co-electrolysis of H2O and CO2 to produce sustainable fuels using solid oxide electrolysis cells (SOECs): advanced materials and technology, Chem. Soc. Rev. 46 (2017) 1427e1463. [4] A. Jun, Y.-W. Ju, G. Kim, Solid oxide electrolysis: concluding remarks, Faraday Discuss 182 (2015) 519e528. [5] K. Chen, S.P. Jiang, Failure mechanism of (La,Sr)MnO3 oxygen electrodes of solid oxide electrolysis cells, Int. J. Hydrogen Energy 36 (2011) 10541e10549. [6] M.S. Sohal, J.E. O'Brien, C.M. Stoots, V.I. Sharma, B. Yildiz, A. Virkar, Degradation issues in solid oxide cells during high temperature electrolysis, J. Fuel Cell Sci. Technol. 9 (2012), 011017. [7] A.V. Virkar, Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells, Int. J. Hydrogen Energy 35 (2010) 9527e9543. [8] C. Xu, Y. Wang, L. Jin, J. Ding, X. Ma, W.G. Wang, Degradation of Solid Oxide Electrolyser Cells with Different Anodes, 2012, pp. 97e102. [9] T. Jacobsen, M. Mogensen, The course of oxygen partial pressure and electric potentials across an oxide electrolyte cell, ECS Transactions 13 (2008) 259e273. [10] R. Knibbe, M.L. Traulsen, A. Hauch, S.D. Ebbesen, M. Mogensen, Solid oxide electrolysis cells: degradation at high current densities, J. Electrochem. Soc. 157 (2010) B1209eB1217. [11] S.N. Rashkeev, M.V. Glazoff, Atomic-scale mechanisms of oxygen electrode delamination in solid oxide electrolyzer cells, Int. J. Hydrogen Energy 37 (2012) 1280e1291. [12] N. Ai, N. Li, S. He, Y. Cheng, M. Saunders, K. Chen, T. Zhang, S.P. Jiang, Highly active and stable Er0.4Bi1.6O3 decorated La0.76Sr0.19MnO3þd nanostructured oxygen electrodes for reversible solid oxide cells, J. Mater. Chem. 5 (2017) 12149e12157. [13] K. Chen, N. Ai, S.P. Jiang, Reasons for the high stability of nano-structured (La,Sr)MnO3 infiltrated Y2O3eZrO2 composite oxygen electrodes of solid oxide electrolysis cells, Electrochem. Commun. 19 (2012) 119e122. [14] C. Graves, S.D. Ebbesen, S.H. Jensen, S.B. Simonsen, M.B. Mogensen, Eliminating degradation in solid oxide electrochemical cells by reversible operation, Nat. Mater. 14 (2015) 239e244. [15] N. Li, M. Keane, M.K. Mahapatra, P. Singh, Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ, Int. J. Hydrogen Energy 38 (2013) 6298e6303. [16] P. Moçoteguy, A. Brisse, A review and comprehensive analysis of degradation mechanisms of solid oxide electrolysis cells, Int. J. Hydrogen Energy 38 (2013)
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