Comparison of performance and degradation of large-scale solid oxide electrolysis cells in stack with different composite air electrodes

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Comparison of performance and degradation of large-scale solid oxide electrolysis cells in stack with different composite air electrodes Yifeng Zheng*, Qingshan Li, Tao Chen, Wei Wu, Cheng Xu**, Wei Guo Wang Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo 315201, PR China

article info

abstract

Article history:

Three hydrogen electrode-supported, large-scale solid oxide electrolysis cells (SOECs) with

Received 28 July 2014

different composite air electrodes, namely LSMeYSZ, LSCeGDC, and LSCFeGDC (LSM, LSC,

Received in revised form

and LSCF cells, respectively), were compared for performance and degradation in the same

21 December 2014

three cell stack with an H2O/H2 ratio of 90/10. The initial SOEC performance increased in

Accepted 23 December 2014

the following order: LSM, LSCF, and LSC. The three cells were operated at a constant cur-

Available online 19 January 2015

rent density of 0.50 A cm
2 for 640 h during hydrogen production at 750  C. After the operation, the 10 cm  10 cm large-scale single cell was divided equally into four subparts

Keywords:

from the two diagonal parts with the steam or air inlet area and steam or air outlet area for

Solid oxide electrolysis cell

post-mortem analysis. Delamination mainly occurred in the steam and air inlet areas of

Stack

the LSM and LSC cells, with more severe delamination in the steam inlet area than that in

Large-scale cell

the air inlet area. Agglomeration of Ni in the NieYSZ hydrogen electrode was observed in

Composite air electrode

the LSM and LSCF cells, whereas honeycomb Ni was observed in the steam and air inlet

Degradation

areas of the LSC cell. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The prospect of hydrogen economy requires the development of devices for clean and efficient hydrogen generation and hydrogen-to-electricity energy conversion. High temperature solid oxide cells (SOCs) are promising for such devices because they can be reversibly operated under two modes, namely, power generation and electrolysis. In the power generation mode, the SOC acts as a solid oxide fuel cell (SOFC) and

generates electricity by electrochemically combining fuel with oxidant. In the electrolysis mode, the SOC acts as a solid oxide electrolysis cell (SOEC) and produces chemicals, such as hydrogen, by steam electrolysis [1,2]. Consequently, SOEC technology can be based in SOFC technology, which has made significant progress in recent years. SOEC studies, including those on electrolyte and electrode materials, cell fabrication methods, and stack system designs, are mainly based on SOFC research [3,4].

* Corresponding author. Tel.: þ86 574 8668 5097. ** Corresponding author. Tel.: þ86 574 8668 5139. E-mail addresses: [email protected] (Y. Zheng), [email protected] (C. Xu). http://dx.doi.org/10.1016/j.ijhydene.2014.12.101 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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Similar to SOFCs, the performance of SOECs critically depends on the electrochemical activity and stability of the electrodes for steam reduction on the hydrogen electrode side and oxygen oxidation reaction on the air electrode side [5]. In SOEC, Ni particle agglomeration (coarsening) in Ni-YSZ hydrogen electrode reduces the concentration of active triple phase boundary (TPB) sites for electron transfer, negatively affecting performance [6]. However, delamination of the air electrode from the electrolyte is considered one of the most severe modes of degradation [7,8]. Efforts have been made to improve the durability of cells and optimize the performance of SOEC electrodes [9,10]. The energy losses caused by electrode polarization, especially that of air electrode, and degradation of air electrode are limiting factors in the performance and practicability of SOECs; thus, considerable efforts have been made to improve the performance of air electrodes [10,11]. Among various air electrodes, (La, Sr)MnO3 (LSM) and LSM-based composites are the most common materials for SOECs because LSM has excellent chemical and thermal compatibility with yttria-stabilized zirconia (YSZ) electrolyte, and reasonable catalytic activity for oxygen oxidation [12,13]. LaCoO3-based perovskite materials, such as La0.6Sr0.4Co0.2Fe0.8O3ed (LSCF) and La0.6Sr0.4CoO3ed (LSC), have been recently proposed as alternatives to the conventional LSM air electrode in both SOFCs and SOECs [1,14,15]. These materials show high electronic conductivity, high oxygen ion conductivity, and high oxygen surface exchange coefficient, and these properties result in rapid kinetics at the gas/electrode interface. These materials typically display mixed electronic and ionic conduction, which indicate their excellent potential as SOC electrodes [14]. However, among LaCoO3based materials, Co-based perovskite materials are negatively affected by the drawback of the reaction with YSZ electrolyte during operation at high temperature [16]. This drawback can be avoided by applying a thin layer of another electrolyte that does not react with YSZ at high temperature, thereby acting as a barrier layer to prevent the interfacial reaction between the YSZ electrolyte and Co-based perovskite air electrode [17]. Ce0.9Gd0.1O1.95 (GDC) layer acts as a barrier to prevent the reaction between YSZ and LSC or LSCF at the electrolyte/air electrode interface [11]. Few studies have compared LSM- and LaCoO3-based air electrodes in SOEC. Performance degradation in terms of area specific resistance (ASR) of LSM, LSCF, and lanthanum strontium ferrite air electrodes under both SOFC and SOEC operating modes was presented by Nguyen Q. Minh [18], who reported that the LSCF electrodes show optimal performance and stability because of their low ASR values. Laguna-Bercero et al. [19] used air electrode materials LSCF and LSMeYSZ with scandia/ceria-stabilized zirconia and NieYSZ as an electrolyte and hydrogen electrode, respectively. They found that both LSCF and LSMeYSZ have good potential as reversible air electrodes in stabilized zirconia-based cells. Further studies are required to characterize the long-term stability of these cells. Hjalmarsson et al. studied the effect of the air electrode and inter-diffusion barrier on the degradation of SOECs [20]. These previous studies mainly focused on the results from symmetrical cells or button cells with a small active area. On the cell or material level, SOEC degradation mainly results from the loss of electrochemical performance in electrodes,

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and this loss is caused by microstructural and stoichiometric changes (reaction between components) in the electrodes because of prolonged operation at high temperature. The performance of small button cells does not represent the performance of larger cells because the scalability of electrodes, particularly air electrodes, is a major concern in SOEC development. To improve the commercial competitiveness and practical application of SOEC technology, large-scale single cells should be assembled into SOEC stacks [21]. When the operation involves large-scale single cells, the performance and degradation of single cells are not the same as those of button cells because factors such as current density, gas flow, and steam molar fraction at the inlet are considered [22]. Stack design also has an important function in performance and degradation because it affects the distribution of current density and steam concentration over large-scale cells. However, systematic research on the performance and durability comparison of large-scale SOECs with LSMeYSZ, LSCeGDC, and LSCFeGDC composite air electrodes is limited. To avoid experimental differences, such as temperature, gas flow, and operation time, in the accurate comparison of different cells, the different single cells assembled in the same SOEC stack was conducted. We evaluated the performance and degradation of large-scale (10 cm  10 cm) SOEC single cells with LSMeYSZ, LSCeGDC, and LSCFeGDC composite air electrodes in the same three cell SOEC stack. The actual performance and degradation of a large-scale single cell during SOEC stack operation were observed. These results may be used as a reference for the improvement of SOEC performance and service life.

Experimental The hydrogen electrode-supported NiOeYSZ/YSZ/LSMeYSZ (designated as LSM cell; LSM: (La0.75Sr0.25)0.95MnO3ed), NiOeYSZ/YSZ/GDC/LSCeGDC (designated as LSC cell; LSC: La0.6Sr0.4CoO3ed), and NiOeYSZ/YSZ/GDC/LSCFeGDC (designated as LSCF cell; LSCF: La0.6Sr0.4Co0.2Fe0.8O3ed) SOEC single cells were used. The single cell had a 400 mm thick NiOeYSZ hydrogen electrode substrate, 10 mm thick NiOeYSZ electrode functional layer, 10 mm thick 8YSZ electrolyte layer, and

Fig. 1 e Schematic diagram of three single cells assembled in a three cell stack for comparison.

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20 mme30 mm thick LSMeYSZ, LSCeGDC, or LSCFeGDC composite air electrode. For LSC and LSCF cells, a 2 mm thick GDC layer was used as a barrier. The overall single cell area for analysis was 10 cm  10 cm, and the active area was 63 cm2. The manufacturing process and parameters of the cells are described in detail in the literature [23,24]. For measurement purposes, the three single cells were assembled in the same three cell stack for comparison according to the schematic diagram [25,26] in Fig. 1, and two stacks were assembled to conduct the test in this study. SUS430 ferritic stainless steel was used to create metal interconnects, which were also adopted as co-flow gas channels. The gas flow channels of interconnects were etched to an area (80 mm  1.5 mm) with a channel tip height of approximately 0.6 mm. To protect from high temperature oxidation and Cr vaporization, the air electrode side of the interconnect was densely coated with NieCr/LSM composite coating by plasma spraying. The sealing material used was Al2O3eSiO2eCaO-based glass, the performance of which has been described in the literature [27]. For full contact between the cell hydrogen electrode anode and metal interconnect, an NiO layer of 70 mme80 mm thickness was printed on the hydrogen electrode side of the cell by screen printing to function as a current-collecting layer. After drying, an approximately 80 mm thick layer with LSM composition was printed on the air electrode side of the cell by screen printing. As shown in Fig. 1, the single cells in the first 3-cell stack (called stack 1) from the bottom to the top corresponded to LSM, LSC, and LSCF cells, respectively. And the single cells in the second 3-cell stack (named stack 2) from the bottom to the top corresponded to LSCF, LSM, and LSC cells, respectively. In the cell assembly process, the voltage leads exited from both sides of the cell according to the schematic diagram in Fig. 2. The cell voltage was detected in real time to obtain the performance variation and degradation of the SOECs. After assembly, the stack was placed on a furnace and heated to 850  C at a rate of 1  C min
1. The temperature remained constant for approximately 2 h. For stack sealing, an external pressure of approximately 10 N cm
2 was loaded on the cell during heat preservation. The stack was subsequently cooled to the working temperature before testing. The hydrogen electrode side of the cell was subjected to nitrogen purging for 10 min during the testing process, and hydrogen and air were fed into the hydrogen and air electrodes of the stack, respectively. The cell hydrogen electrode underwent

more than 3 h of reduction in a hydrogen atmosphere, after which the test began. For the SOEC test, air at 10 sccm cm
2 (standard cubic centimeters per minute per square centimeter) was supplied to the air electrodes of the stack, and an H2O/H2 ratio of 90/10 steam mixture with 1 sccm cm
2 hydrogen gas was supplied to the hydrogen electrodes. The performance and durability of the LSM, LSC, and LSCF cells were evaluated and compared during SOEC operation at 750  C and 800  C. The real time curves of current density versus voltage (IeV) and voltage versus time (Vet) were recorded by a DC stabilized power supply (NB M&C Technology Co., Ltd., China) and the corresponding software. Impedance spectroscopy was measured by a ZAHNER-elektrik IM6ex electrochemical workstation in galvanostatic mode between 20 mHz ~ and 10 kHz. A long-term durability test in electrolysis mode was performed after the initial performance test. Finally, the stack was cooled to room temperature at a rate of 1  C min
1 with hydrogen to protect the hydrogen electrode from oxidization.

Fig. 2 e Schematic diagram of voltage leads arranged inside the stack.

Fig. 3 e IeV curves of three single cells in stacks 1 and 2 with H2O/H2 ¼ 90/10 at (a) 750  C and (b) 800  C.

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The microstructures of the SOECs were observed using a HITACHI S4800 scanning electron microscopy (SEM) system coupled to an energy-dispersive X-ray spectroscopy (EDS) system. Square cells (10 cm  10 cm) were equally divided into four subparts from the two diagonal parts with the steam (steam þ H2) or air inlet area and steam (steam þ H2) or air outlet area for characterization (Fig. 1).

Results and discussion The results of the IeV curves for the three single cells with LSMeYSZ, LSCeGDC, and LSCFeGDC composite air electrodes in stack 1 and stack 2 at 750  C and 800  C are shown in Fig. 3a and b. The mixture of hydrogen and steam with H2O/H2 ¼ 90/ 10 was introduced to the hydrogen electrode side of the electrolysis cells, and air was supplied to the air electrodes. In Fig. 3, similar voltage variation curves with current densities were observed for the LSM, LSC or LSCF cells in the two stacks, this confirming the consistency of the SOEC performance of the LSM, LSC or LSCF cells despite the cells assembled in the different positions of different stacks both at 750  C and 800  C. Therefore, on the premise that the performance of different cells in different stacks was similar, the stack 1 at 750  C was selected as the main research object in the discussion below (stack 2 was conducted to durability test at 800  C to confirm the consistency of the microstructure changes of the cells in the two stacks at different temperatures). The open-circuit voltage (OCV) for each of the cells at 750  C was close to the Nernst potential of 0.885 V based on the gas compositions at the electrodes, which indicates excellent sealing performance in Fig. 3. The relationships between voltages and current densities were reasonably linear for each of the three cells, implying that the steam content was sufficient for the tested current density range. The slope of the IeV curve was lowest for the LSC cell and highest for the LSM cell. Low cell voltage at a defined current density indicates good SOEC performance [11,28]. Thus, LSC cell showed the highest performance, LSCF cell showed moderate performance, and LSM cell showed the lowest performance.

Fig. 4 e EIS of three single cells with H2O/H2 ¼ 90/10 at 750  C.

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The electrochemical impedance spectroscopy (EIS) at OCV for the three single cells at 750  C is shown in Fig. 4. The intercepts of the impedance spectra with the real axis at high frequency represent the ohmic resistance (Rs) of the cell and mainly originate from the electrolyte. The intercepts of the impedance spectra with the real axis at low frequency correspond to the total cell resistance (Rt), including the cell Rs and polarization resistance (Rp). The impedance intercept with the real axis between the high and low frequencies is due to the cell Rp, including the charge transfer reaction at the electrode/ electrolyte interface and adsorption/desorption and diffusion inside the porous electrode [29]. The elements associated with the hydrogen electrode and air electrode processes were difficult to distinguish because of the frequency overlap of the different electrode processes. Therefore, the trends in the resistance of the individual processes could not be adequately and satisfactorily distinguished. Rp was the major contributing factor in Rt, as shown in Fig. 4. The Rs values of the LSC and LSCF cells (0.341 and 0.345 U cm2, respectively) were

Fig. 5 e (a) Electrolysis efficiencies and (b) hydrogen production rates of three single cells with H2O/H2 ¼ 90/10 at 750  C.

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slightly higher than that of the LSM cell (0.294 U cm2). The LSC and LSCF cells contained a 2 mm thick GDC barrier layer with resistance reflected in Rs. The Rp values of the LSM, LSC, and LSCF cells were 1.224, 0.294, and 0.311 U cm2, respectively. This result was due to the fact that the conductivities of the air electrodes in the cells decrease in the order of LSC, LSCF, and LSM, according to the results of our previous experiments [15,30,31]. The Rp values of the LSC and LSCF cells were almost four times lower than that of the LSM cell, indicating that the LSCeGDC and LSCFeGDC composite air electrodes (both of the single cells comprised the same hydrogen electrode) showed higher performance than the LSMeYSZ electrode. The LSM, LSC, and LSCF cells used different air electrodes, and the differences in their SOEC performance were due to the effectiveness of these electrodes in oxygen oxidation at the triple phase boundary [11]. Thus, the initial SOEC performance significantly improved from the LSM to LSC cells. The electrolysis efficiency quantifies the heat value of hydrogen produced by electrolysis per unit of electrical energy consumed in SOEC [32]. The comparison of the electrolysis efficiencies and hydrogen production rates for the three single cells at 750  C is shown in Fig. 5. The electrolysis efficiency was calculated using the following equation [32]: he ¼

Vth Vop

of the galvanostatic test (0.50 A cm
2) for 640 h at 750  C are plotted in Fig. 6a. The corresponding ASRs and electrolysis efficiencies are presented in Fig. 6b. In Fig. 6a, the initial voltages of the LSM, LSC, and LSCF cells were 1.382, 1.113, and 1.170 V, respectively. The degradation of the three cells increased with time. Rapid degradation of the LSM cell at approximately 228 h was observed. The degradation rates of the LSM, LSC, and LSCF cells were 4.89%, 1.27%, and 4.33% kh
1, respectively. The degradation rate of the LSC cell was lower than that of the Materials and Systems Research Inc. cell (4.26% kh
1) [33]. In Fig. 6b, ASR increased from 1.04 U cm2 to 1.21 U cm2, 0.41 U cm2 to 0.47 U cm2, and 0.57 U cm2 to 0.765 U cm2 for the LSM, LSC, and LSCF cells, respectively. The electrolysis efficiency decreased from 93.3% to 86.7%, 116.8%e115%, and 109.7%e102.6% for the LSM, LSC, and LSCF cells, respectively. The heat value of hydrogen produced by the electrolysis of consumed electrical energy decreased with increasing time of SOEC operation. Only a part of the electrical energy was converted to hydrogen internal energy after long-term SOEC operation. Hence, the electrolysis efficiency decreased. However, the electrolysis efficiencies of the LSC and LSCF cells were still higher than 100% after the

(1)

where Vth is the thermal-neutral voltage of the cell, and Vop is the cell operating voltage. The thermal-neutral voltage for a single cell is 1.285 V at 750  C. The initial electrolysis efficiencies for the three cells were approximately 145%, and the electrolysis efficiencies decreased with increasing current density, as shown in Fig. 5a. The SOEC operated endothermically under low current densities, which was in accordance with theory. However, the electrolysis efficiencies decreased at higher current densities because of increasing operating voltage. Such increases were due to the larger amount of heat generated from SOEC internal resistance than that required for water decomposition at high current densities. Thus, SOEC operated exothermically. Both the LSC and LSCF cells showed electrolysis efficiencies higher than 100% at current densities below 0.65 A cm
2. By contrast, the electrolysis efficiency of the LSM cell was less than 100% when the current density exceeded 0.40 A cm
2. The LSC cell showed the highest electrolysis efficiency, which suggests that this cell had the lowest heat value of hydrogen produced by electrolysis per unit of electrical energy consumed. By contrast, the LSM cell had the highest heat value of hydrogen produced by electrolysis of electrical energy consumed. As shown in Fig. 5b, the LSC cell had higher efficiency than the LSCF and LSM cells, and the efficiency increased with increasing hydrogen production rate. The difference in the hydrogen production rate among the three cells increased with increasing applied voltage. When a voltage of 1.1 V was applied, the LSC cell showed a hydrogen production rate of 218.9 NmL cm
2 h
1, whereas the LSCF and LSM cells showed hydrogen production rates of 159.2 and 99.5 NmL cm
2 h
1, respectively. After the performance comparison of the three single cells, the durability of these cells was compared under constant galvanostatic electrolysis conduction, and the voltages across the cells were continuously monitored. The cell voltage curves

Fig. 6 e (a) Cell voltage curves and (b) ASRs and electrolysis efficiencies of three single cells under constant galvanostatic electrolysis (0.50 A cm¡2) with H2O/H2 ¼ 90/ 10 at 750  C.

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operation, thereby indicating the excellent performance of these cells. C. Graves et al. [34] reported that degradation at the NieYSZ electrode is dominant at a low current density (0.25 A cm
2), whereas at a high current density (0.5 A cm
2), the NieYSZ electrode continues to degrade, but the serial resistance and degradation in the air electrode start contributing to the total loss in performance. To obtain data on the performance degradation for the three large-scale cells, a series of EIS data was obtained at a time interval of 40 he48 h during the operation. Rs and Rp were plotted as a function of time based on the EIS results in Fig. 7. In Fig. 7, the results from the EIS measurements show that Rs and Rp increased faster for the LSM cell than those in the LSC and LSCF cells. Rs of the LSM cell increased rapidly at approximately 228 h, which was in accordance with the rapid LSM cell degradation shown in Fig. 6a. This finding indicates the degradation and deteriorated contact at the electrode/electrolyte interface of the LSM cell. After 640 h, the air electrode delaminated from the electrolyte. Furthermore, Rp of the LSC cell was almost stable, whereas Rp of the LSCF cell increased during the entire SOEC

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Fig. 8 e Change in bode plot of three single cells before and after galvanostatic electrolysis.

operation, resulting in a lower LSC cell degradation rate. The Rs values of the LSC and LSCF cells were almost stable and close to each other in the early stage of SOEC, but started to increase significantly at 270 h. After 270 h, Rs of the LSC cell increased faster than that of the LSCF cell, which indicates that the contact at the electrode/electrolyte interface was more highly deteriorated for the LSC cell than that for the LSCF cell. The results from Fig. 7 also show that Rs increased more substantially than Rp, indicating that cell degradation may be more associated with the increase in Rs than Rp. The Bode plots of the three cells before and after SOEC operation are shown in Fig. 8. The main electrode response of the LSM cell increased from approximately 2 Hze10 Hz before and after galvanostatic electrolysis operation. Moreover, the summit frequencies of the LSC and LSCF cells were approximately 1 Hz before and after galvanostatic electrolysis operation. Previous studies reported that the electrochemical processes at the NieYSZ electrode occur within 1 Hze10 Hz

Fig. 7 e (a) Rs and (b) Rp of three single cells with time obtained from the EIS data.

Fig. 9 e Cell voltage curves of three single cells in stack 2 under constant galvanostatic electrolysis (0.50 A cm¡2) with H2O/H2 ¼ 90/10 at 800  C.

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[34]. Thus, this process may be also mainly ascribed to the NieYSZ hydrogen electrode in this study. The stack 2 was also conducted to durability test (0.50 A cm
2) at 800  C for about 600 h in this study, the cell voltage curves are shown in Fig. 9. It was conducted to confirm whether the results were the same even when the sample order was rearranged in the stack at different temperatures. After testing the two stacks, comparison of degradation in the three single cells was conducted by post-mortem analysis. The 10 cm  10 cm large-scale single cell was equally divided into four subparts from the two diagonal parts with the steam or air inlet area and steam or air outlet area, as shown in Fig. 1. The observed post-mortem analysis of the cells in the two stacks was similar, this confirming the consistency of the microstructure changes of the LSM, LSC or LSCF cells despite the cells assembled in the different positions of different stacks both at 750  C and 800  C. Therefore, on the premise

that the microstructure changes of different cells in different stacks after electrolysis were similar, the results presented below were from the stack 1. Typical SEM microstructures of the cross-section of the three cells before and after electrolysis are shown in Fig. 10. The interfaces of the three cells were combined well before the test (Fig. 10a0, 10b0 and 10c0). As shown in Fig. 10a1 and 10b1, delamination of the LSMeYSZ air electrode layer by the electrolyte may contribute to the degradation of the LSM cell, whereas no delamination was observed in Fig. 10c1 and 10d1 after the test. Delamination mainly occurred in the steam and air inlet areas of the 10 cm  10 cm cell. Delamination in the steam inlet area was more severe than that in the air inlet area. Similarly, delamination of the LSCeGDC air electrode was also observed in the LSC cell (Fig. 10a2 and 10b2). Delamination in the LSC cell mainly occurred in the steam and air inlet areas, but delamination occurred in the GDC barrier instead of in the YSZ

Fig. 10 e SEM images of the cross-section (fracture surface) of the three cells before and after electrolysis, before: (a0) LSM cell, (b0) LSC cell, (c0) LSCF cell; after: LSM cell (a1) steam inlet area, (b1) air inlet area, (c1) steam outlet area, and (d1) air outlet area; LSC cell (a2) steam inlet area, (b2) air inlet area, (c2) steam outlet area, and (d2) air outlet area; LSCF cell (a3) steam inlet area, (b3) air inlet area, (c3) steam outlet area, and (d3) air outlet area.

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Fig. 10 e (continued).

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electrolyte. However, as shown in Fig. 10a3 to 10d3, no delamination was observed at the interface of the LSCF cell compared with the LSM and LSC cells, thereby indicating that the LSCF cell had satisfactory interface stability. Air electrode delamination resulted in a reduced electrochemically active area and increased ohmic loss. Delamination of the air electrode was possibly due to the accumulation of high oxygen pressure at the air electrode/electrode interface [7,8]. The images in Fig. 10 may be explained as follows. First, delamination mainly occurred in the steam and air inlet areas of the LSM and LSC cells, and delamination in the steam inlet area was more severe than that in the air inlet area. The reaction of H2O electrolysis is expressed as: Hydrogen electrode reaction :

Air electrode reaction :

H2 O þ 2e
/H2 þ O2


1 O2
/ O2 þ 2e
2

(2)

(3)

Generally, both the steam content and pressure were relatively high in the steam inlet area of the stack (Fig. 1). Therefore, more O2
was produced in this area than in the air inlet area according to Equation (2), and more oxygen ions were transported through the electrolyte via bulk and grain boundary diffusion from the hydrogen electrode to the air electrode. More oxygen ions at the electrolyte grain boundaries and air electrode side interface were oxidized to oxygen gas (Equation (3)) to provide the required electrons for current flow. This area had higher oxygen pressure than the air inlet area. Therefore, more severe delamination mainly

occurred in the steam inlet area of the LSM and LSC cells. Second, both air content and pressure were relatively high in the air inlet area of the stack (Fig. 1), and oxygen gas was simultaneously produced at the electrolyte grain boundaries and air electrode side interface. The acting force of air (external air) to the air electrode was opposite that of the internal oxygen pressure, which resulted in air electrode delamination in the air inlet area of the LSM and LSC cells. Third, delamination of the LSCeGDC air electrode occurred in the GDC barrier instead of the YSZ electrolyte, suggesting that the air electrode reaction (Equation (3)) occurred at the LSCeGDC/GDC interface in the LSC cell. Fourth, regardless of the similar structures of the LSCF and LSC cells, no delamination appeared in the LSCF cell. The thermal expansion coefficient (TEC) of different materials should be considered. The TECs of the LSC, LSCF, and GDC are 20, 15.4, and 12  10
6 K
1, respectively [35]. Therefore, the difference in TECs between LSCF and GDC was much smaller than that between LSC and GDC, indicating that the LSCFeGDC/GDC interface in the LSCF cell was more stable than the LSCeGDC/ GDC interface in the LSC cell. This result was consistent with the slight change in Rs (Fig. 7a). After delamination, the LSMeYSZ air electrode was peeled from the YSZ electrolyte, whereas the LSCeGDC air electrode was peeled from the GDC barrier. The SEM micrographs of the YSZ surface in contact with the LSMeYSZ air electrode of the LSM cell (with the corresponding EDS spot scan shown in Fig. 11b1) and the GDC surface in contact with the LSCeGDC air electrode of the LSC cell (with the corresponding EDS spot

Fig. 11 e (a1) SEM image of the YSZ surface in contact with the LSMeYSZ air electrode of the LSM cell and (b1) the corresponding EDS spot scan; (a2) SEM image of the GDC surface in contact with the LSCeGDC air electrode of the LSC cell and (b2) the corresponding EDS spot scan.

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Fig. 12 e SEM images of the NieYSZ hydrogen electrode of the three cells (a0) before, and after electrolysis: LSM cell (a1) steam inlet area, (b1) air inlet area, (c1) steam outlet area, and (d1) air outlet area; LSC cell (a2) steam inlet area, (b2) air inlet area, (c2) steam outlet area, and (d2) air outlet area; LSCF cell (a3) steam inlet area, (b3) air inlet area, (c3) steam outlet area, and (d3) air outlet area (the circled regions indicate the agglomeration of Ni or the honeycomb structure of Ni).

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Fig. 12 e (continued). scan shown in Fig. 11b2) are shown in Fig. 11a1 and 11a2. As shown in Fig. 11a1, the surface of the YSZ electrolyte was characterized by the formation of LSM nanoparticles, which was consistent with reference [4]. These nanoparticles were observed by EDS, and are shown in Fig. 11b1 (the peak of Zr was the YSZ matrix). The nanoparticles were uniformly distributed on the YSZ surface. These nanoparticles most likely resulted from the migration or incorporation of oxygen ions from YSZ to LSM grains. This incorporation resulted in the shrinkage of the LSM lattice, which induced local tensile strains within LSM particles and subsequent formation of microcracks and nanoparticles at the interface [4]. Nanoparticle formation may weaken the air electrode/electrolyte interface, resulting in delamination of the LSMeYSZ air electrode, particularly in the steam and air inlet areas in the cells

under high internal partial pressures of oxygen. However, in Fig. 11a2, the surface of the GDC barrier was characterized by the formation of LSC nanoparticle clusters, as observed by EDS (Fig. 11b2). The nanoparticle clusters formed on the GDC surface most likely resulted from the migration or incorporation of oxygen ions from GDC to LSC grains. This incorporation also resulted in the shrinkage of the LSC lattice, which induced local tensile strains within LSC particles and subsequent formation of microcracks and nanoparticle cluster at the interface. The microstructure and morphology on the GDC surface in the LSC cell were different from those on the YSZ surface in the LSM cell. The incorporated GDC phase in LSCeGDC or incorporated YSZ phase in LSMeYSZ may affect the formation of nanoparticle clusters or nanoparticles of LSC or LSM grains at the interface. Details of the different

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 2 4 6 0 e2 4 7 2

formations of nanoparticle clusters and nanoparticles in the two cells will be investigated further. As shown in Fig. 8, the electrochemical processes may mainly occur in the NieYSZ hydrogen electrode during galvanostatic electrolysis operation. Therefore, the microstructures of the NieYSZ hydrogen electrode before and after the electrolysis test of the three cells were further investigated. The SEM microstructures of the NieYSZ hydrogen electrode before and after the electrolysis test of the three cells are shown in Fig. 12. The NieYSZ hydrogen electrode of the 10 cm  10 cm cell was equally divided into four subparts for characterization. Compared with the microstructures of the hydrogen electrode before the test (Fig. 12a0), severe agglomeration of Ni in the NieYSZ structure was observed in the LSM cell (the circled regions of Fig. 12a1 to 12d1). The steam and air inlet areas and steam and air outlet areas of the LSM cell showed Ni agglomeration. The porosity of the NieYSZ hydrogen electrode decreased after SOEC operation. Agglomeration of Ni reduced Ni electrical interconnectivity as well as contact area with the current collector [6]. The steam partial pressure reached 90% in the SOEC, and the diffusion process of steam was much more difficult than that of hydrogen. The decrease in porosity resulted in a more difficult steam diffusion process. This result was consistent with the rapid change in Rp in Fig. 7b. Moreover, agglomeration of Ni in the NieYSZ structure was also observed in the steam and air inlet areas and steam outlet area of the LSCF cell (the circled regions of Fig. 12a3 to 12c3). However, no Ni agglomeration in the NieYSZ structure occurred in the LSC cell (Fig. 12a2 to 12d2). Honeycomb Ni structures were observed in the steam and air inlet areas (the circled regions of Fig. 12a2 and b2). The honeycomb Ni may increase the catalytic activity of Ni, resulting in the stability of Rp for the LSC cell in Fig. 7b. The NieYSZ electrode, YSZ electrolyte, and GDC barrier of the LSC and LSCF cells were identical. Thus, the difference could only be due to the LSC and LSCF materials. Given that the conductivity of LSC is much higher than that of LSCF [15,30], the electron and oxygen ion could be transported easily in the LSC cell, and H2O electrolysis occurred easily. The honeycomb structure of Ni in the NieYSZ electrode of the LSC cell may be beneficial because catalytic activity increased. Thus, this phenomenon could explain the almost stable Rp of the LSC cell, as shown in Fig. 7b.

Conclusions Three large-scale SOECs with different composite air electrodes were compared for performance and degradation in the same three cell stack. The initial SOEC performance increased in the following order: LSM, LSCF, and LSC cells. The three cells operated at a constant current density of 0.50 A cm
2 for 640 h. After the operation, the 10 cm  10 cm large-scale single cell was equally divided into four subparts from the two diagonal parts with the steam or air inlet area and steam or air outlet area for post-mortem analysis. Delamination mainly occurred in the steam and air inlet areas of the LSM and LSC cells, and with delamination in the steam inlet area was more severe than that in the air inlet area. No delamination occurred at the interface of the LSCF cell. Agglomeration of Ni

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in the NieYSZ structure was observed in the LSM and LSCF cells, which reduced the concentration of active TPB sites for electron transfer. Honeycomb Ni was observed in the steam and air inlet areas of the LSC cell, which may increase the catalytic activity of Ni.

Acknowledgments This work was supported by the China Postdoctoral Science Foundation (2012M521208), Zhejiang Provincial Natural Science Foundation of China, and the Chinese Academy of Sciences (Project Position kgcx2-yw-314).

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