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Research Papers

# Performance Investigation of Dual Layer Yttria-Stabilized Zirconia–Samaria-Doped Ceria Electrolyte for Intermediate Temperature Solid Oxide Fuel CellsOPEN ACCESS

[+] Author and Article Information
Ryan J. Milcarek

Department of Mechanical and
Aerospace Engineering,
Syracuse University,
Syracuse, NY 13244-1240
e-mail: rjmilcar@syr.edu

Kang Wang

Department of Mechanical and
Aerospace Engineering,
Syracuse University,
Syracuse, NY 13244-1240
e-mail: kwang08@syr.edu

Michael J. Garrett

Department of Mechanical and
Aerospace Engineering,
Syracuse University,
Syracuse, NY 13244-1240
e-mail: mjgarret@syr.edu

Jeongmin Ahn

Department of Mechanical and
Aerospace Engineering,
Syracuse University,
Syracuse, NY 13244-1240
e-mail: jeongahn@syr.edu

1Corresponding author.

Manuscript received November 21, 2015; final manuscript received January 30, 2016; published online March 2, 2016. Assoc. Editor: Kevin Huang.

J. Electrochem. En. Conv. Stor. 13(1), 011002 (Mar 02, 2016) (7 pages) Paper No: JEECS-15-1011; doi: 10.1115/1.4032708 History: Received November 21, 2015; Revised January 30, 2016

## Abstract

The performance of yttria-stabilized zirconia (YSZ)–samaria-doped ceria (SDC) dual layer electrolyte anode-supported solid oxide fuel cell (AS-SOFC) was investigated. Tape-casting, lamination, and co-sintering of the NiO–YSZ anode followed by wet powder spraying of the SDC buffer layer and BSCF cathode was proposed for fabrication of these cells as an effective means of reducing the number of sintering stages required. The AS-SOFC showed a significant fuel cell performance of ∼1.9 W cm−2 at 800 °C. The fuel cell performance varies significantly with the sintering temperature of the SDC buffer layer. An optimal buffer layer sintering temperature of 1350 °C occurs due to a balance between the YSZ–SDC contact and densification at low sintering temperature and reactions between YSZ and SDC at high sintering temperatures. At high sintering temperatures, the reactions between YSZ and SDC have a detrimental effect on the fuel cell performance resulting in no power at a sintering temperature of 1500 °C.

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## Introduction

Solid oxide fuel cells (SOFCs) are all solid-state, electrochemical, energy conversion devices that have many advantages including high efficiency and high fuel flexibility [13]. The thick YSZ electrolyte used for SOFCs has good mechanical strength and performance, but requires high operating temperatures (∼1000 °C). While the high temperatures allow for non-noble electrodes, they also introduce new challenges including reduced fuel cell lifetime, increased system maintenance, sealing failure, and material degradation. Recently, efforts have focused on reducing the operating temperature to intermediates temperatures (600–800 °C). One effective way to achieve this goal is to reduce the thickness of the YSZ electrolyte layer, which reduces the ohmic resistance of the fuel cells [4]. In order to maintain mechanical strength, an AS-SOFC, with thin YSZ electrolyte, has been investigated extensively [5]. Further improvements in performance at intermediate temperatures are possible with perovskite cathode materials such as LSCF (La0.6Sr0.4Co0.2Fe0.8O3) [6] or BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3-δ) [7,8] which have superior electrocatalytic activity and oxygen permeability compared with the traditional Sr-doped LaMnO3 (LSM) cathode. Unfortunately, it is not possible to use YSZ electrolyte and LSCF or BSCF cathode directly due to interfacial reactions which significantly degrade the SOFCs performance [9,10]. The interfacial reactions create SrZrO3, as one example, which is a nonconductive material that forms at high sintering temperatures [11]. The undesirable interfacial reactions can be prevented by introducing an SDC or gadolinium-doped ceria (GDC) buffer layer [1218], which eliminates direct contact between YSZ and the perovskite cathode. Thus, a YSZ–SDC dual layer electrolyte is created with four layers in total including the anode and cathode, which has been explored extensively.

Recent work in this area has focused on fabrication and sintering of the four layers. To fabricate a multilayered SOFC, a number of techniques including dry pressing, screen printing, dip coating, tape-casting, wet powder spraying, supercritical fluid deposition, chemical solution deposition, flame-assisted vapor deposition, thermal inkjet printing, and pulsed laser deposition have been developed in order to achieve thin YSZ and SDC layers [7,1826]. Wet powder spraying, as a noncontact technique, is suitable for fabricating ultra-thin layers (less than 10 μm) on planes, tubes, and a variety of other substrates and is easy to scale-up from laboratory to industrial fabrication [2729]. However, wet powder spraying and other advanced techniques have been used for the YSZ electrolyte layer to achieve ultra-thin electrolytes (150 nm to 6 μm) and have often required each layer to be sintered separately [7,20,25,26,28,30]. Sintering of each layer separately increases the time and cost of fabrication making the dual layer electrolyte impractical [10,22]. In order to reduce the number of sintering stages, co-sintering, of two or more layers is needed. However, it has been shown that co-sintering of the YSZ–SDC dual layer results in delamination, interfacial reactions, and the resulting reduction in performance [7,20]. Tape-casting technology has been widely used to prepare planar AS-SOFCs and is well established as an economical and scalable option for industrial use [4,3132]. It has been extensively used in the fabrication of electrolytes with thicknesses of 5–10 μm in order to ensure a thin ceramic layer without gas permeability [22,32]. Furthermore, tape-casting of the anode and YSZ electrolyte followed by lamination can allow the anode and electrolyte to be co-sintering thus making the dual layer electrolyte more scalable to industry by reducing the number of sintering stages and the manufacturing cost [32]. Despite the increased electrolyte thickness and resulting ohmic resistance of the cell, it is possible to obtain significant performance with a buffer layer with one less sintering stage [22]. More work is needed to investigate the use of tape-casting, lamination, co-sintering and wet powder spraying of the SDC buffer layer.

Two separate studies have shown that interfacial reactions are possible when sintering the SDC buffer layer and the YSZ electrolyte layer between temperatures of 1100–1350 °C [19,33] and 1400 °C [20]. Despite reactions, Duana et al. demonstrated the effect of buffer layer sintering temperature on the SOFCs performance using BSCF as the cathode and GDC as the buffer layer [34]. The study confirmed an optimal buffer layer sintering temperature could be achieved for GDC–YSZ, which is the result of a trade-off between maximum densification of the buffer layer, reduced area-specific resistance, and maximum performance, all while minimizing interfacial reactions. Since that time, BSCF has been demonstrated as a cathode material capable of significant performance at intermediate and low temperatures with minimal interactions with SDC when the sintering temperature is optimized [35]. No study has investigated the effect of buffer layer sintering temperature on the fuel cells performance using an SDC buffer layer with a BSCF cathode. Despite the two studies performed to understand the SDC–YSZ interactions at different sintering temperatures [20,35] from 1200 to 1500 °C (which are typical temperatures for sintering SDC electrolyte) additional work with the same fabrication technique is needed to uncover the optimal sintering temperature for dual layer performance.

In the present work, the wet powder spraying technique combined with tape-casting, lamination, and co-sintering was applied to fabricate SDC–YSZ dual layer electrolyte, AS-SOFC. The effect of the buffer layer sintering temperature on fuel cell performance is investigated from 1200 to 1500 °C.

## Experimental

###### Fuel Cell Fabrication.

The AS-SOFCs were prepared using both tape casting and wet powder spraying techniques. The NiO + YSZ (60:40 w/w, NexTech Materials, Lewis Center, OH), YSZ (NexTech Materials), SDC (Fuel Cell Materials, Lewis Center, OH), and BSCF + SDC (70:30 w/w) materials were used for the anode, electrolyte, buffer layer, and cathode, respectively. BSCF powders were prepared by a combined ethylenediaminetetraacetic acid (EDTA)-citrate complexing method [36]. Metal nitrates in analytic grade were made into aqueous solutions. Their precise concentrations were determined by the EDTA titration method. These metal nitrates were then used as the metal ion sources. The required amounts of metal nitrates were set according to the stoichiometry of the desired product. They were made into a mixed aqueous solution, followed by the addition of EDTA and citric acid (CA), where both served as complexing agents to ensure the molecule-level homogeneous mixing of the metal ions in the solution. The molar ratios of metal ions, EDTA, and CA were 1:1:2. NH4OH was applied during the evaporation process to ensure a solution pH value of ∼7. Through heating and stirring, a clear gel was finally obtained. The gel was then heated at 250 °C for 5 hr to form a solid precursor and finally calcined at 900 °C for 5 hrs to result in oxides with the desired final composition and lattice structure. The NiO–YSZ anode and YSZ electrolyte were prepared by tape-casting followed by lamination [37]. To prepare the powders for tape casting, the powders were dried and stored in an oven at 120 °C for 2 hrs to remove the moisture. The dried powders were milled with toluene, ethanol, and blown menhaden fish oil for 24 hrs to achieve a homogeneous solution. Then polyvinyl butyral binder, butyl benzyl phthalate, and polyethylene glycol plasticizer were added into the premixed solution and milled for 12 hrs to achieve uniform slurry. The slurry was then de-aired using a stirring vacuum tank for 5 min. The slurry was then tape casted. After drying, a lamination machine was used to press the anode and electrolyte tape under a pressure of 80 MPa for 5 min at 60 °C. The green tape, with the laminated anode and electrolyte, was co-sintered at 1200 °C for 2 hrs. After sintering the substrates (15 mm in diameter) had an electrolyte thickness of ∼10 μm. The SDC buffer layer was deposited using the wet powder spraying method. The slurry used for wet powder spraying consisting of 46 mL ethanol, 10 mL ethylene glycol, 3 mL glycerol, and 5 g of powder was deposited with a spray gun. The slurry was milled for 2 hrs prior to application. Substrates were then sintered at 1200, 1250, 1300, 1350, 1400, 1450, and 1500 °C for 4 hrs. The cathode layer (0.7 cm2) was then spray deposited and the cells were sintered at 1000 °C for 5 hrs.

###### Characterization.

The current voltage (IV) method was used to measure the fuel cells performance. A digital SourceMeter (Keithley 2420) interfaced with a computer recorded the fuel cells open circuit voltage (OCV), power density, and overpotential. A Solartron 1260 A frequency response analyzer with a Solartron 1287 Potentiostat was used to conduct electrochemical impedance spectroscopy (EIS) of the fuel cells under OCV conditions. EIS was recorded over a frequency range of 106–0.1 Hz with a signal amplitude of 10 mV. The fuel cell cross-sectional morphology was assessed with a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX).

###### Testing Setup.

The IV cell polarization testing was conducted with hydrogen at a flow rate of 100 mL min−1. labview software was used to control the mass flow controllers. Testing was conducted in a tubular furnace, which was heated to the desired temperature at a rate of 5 °C per minute. The fuel cell was sealed on the end of a quartz tube using silver paste. Silver wire and silver paste were used for current collection.

## Results and Discussion

The effect of the SDC buffer layer was assessed by investigating one of the fuel cells prepared with the same fabrication technique, but without the SDC buffer layer. Figure 1 shows the fuel cells performance between 600 and 750 °C. The fuel cell achieved a peak power density of 260 mW cm−2 at 750 °C operating with hydrogen fuel at a flow rate of 100 mL min−1 and air as oxidant. The peak power density achieved is low even for a fuel cell with LSM cathode [25], but BSCF has much higher catalytic activity, which should have resulted in better performance. This result indicates that the undesirable reaction between YSZ and BSCF has occurred, as expected [9,10], with the formation of the SrZrO3 insulating material [19,26].

As shown in our previous study [38], the fuel cell with the SDC buffer layer has slightly higher ohmic resistance than the fuel cell prepared without the buffer layer. This result is attributed to the SDC buffer layer, which can increase the ohmic resistance. However, the addition of the SDC buffer layer reduces the polarization resistance of the electrodes from 1.8 Ω cm2 to 0.58 Ω cm2 at 700 °C. The net effect of adding the SDC buffer layer is a reduction in the total resistance of the fuel cell.

Figure 2 shows the performance of the fuel cells sintered at 1200, 1250, 1300, 1350, 1400, and 1450 °C at intermediate operating temperatures ranging from 550 to 800 °C. The fuel cell with an SDC buffer layer sintered at 1500 °C did not generate any power. However, the performance of the fuel cell showed a drastic improvement as the sintering temperature increased from 1200 °C to 1350 °C. The peak performances at an operating temperature of 800 °C were 0.17 W cm−2, 0.58 W cm−2, 0.82 W cm−2, and 1.9 W cm−2 operating with hydrogen fuel at a flow rate of 100 mL min−1 and air as oxidant with the SDC buffer layer sintering temperatures of 1200 °C, 1250 °C, 1300 °C, and 1350 °C, respectively. The performance of the fuel cell at 800 °C decreased slightly at a SDC sintering temperature of 1400 °C (1.7 W cm−2) and then decreased drastically when the buffer layer was sintered at 1450 °C to 0.65 W cm−2. Figure 3 shows a comparison of the fuel cell performance at an operating temperature of 750 °C for different sintering temperatures ranging from 1250–1450 °C. As shown, the fuel cell performance is comparable when the sintering temperature is 1250, 1300, and 1450 °C. However, the performance increases significantly at sintering temperatures of 1350 and 1400 °C. The trend is similar to that shown with a GDC buffer layer [34], but with a higher optimal sintering temperature (1350 °C compared to 1250 °C) observed with SDC. Besides the use of a different material, the different fabrication technique used by Duana et al. [34] likely accounts for some of the differences in optimal sintering temperature. Besides demonstrating the optimal sintering temperature, the results also indicate that significant power densities can be achieved at intermediate temperatures down to 600 °C with YSZ electrolyte. For example, the fuel cell sintered at 1350 °C achieved almost 400 mW cm−2 at 600 °C. Using an SDC buffer layer prepared by tape casting, wet powder spraying, and with one less sintering stage than in other studies can be an effective means of generating significant power at intermediate temperatures.

The same trend depicted by the power curves is shown by the electrochemical impedance spectra first as a decrease in total fuel cell resistance with sintering temperature followed by an increase in resistance for temperatures greater than 1350 °C. Low sintering temperatures could not achieve a dense YSZ electrolyte layer or good contact between the SDC and YSZ layers, resulting in high ohmic resistance. The ohmic resistance was 2.1 Ω cm2 for the fuel cell sintered at 1250 °C. At the same time, the sintering temperature is higher than 1100 °C, which can cause the undesirable reaction between YSZ and SDC [19]. At these temperatures, solid-state reactions and Ce–Zr interdiffusion at the YSZ–SDC interface result in the formation of solid solution phases with lower ionic conductivity and reduced fuel cell performance [33]. However, it is known that the reactions are mild at a buffer layer sintering temperature of 1250 °C and did not significantly increase the resistance of the fuel cell in this study. Thus, the poor performance at the low sintering temperature is mainly due to the undense electrolyte and poor YSZ–SDC contact.

By increasing the sintering temperature from 1250 °C to 1350 °C, the fuel cell performances significantly improved from 0.38 W cm−2 to 1.57 W cm−2 at 750 °C. Figure 4 shows that this result is due to the reduction of the fuel cell ohmic resistance despite the reaction between YSZ and SDC, which cannot be neglected at this temperature. Martínez-Amesti et al. confirmed that increasing the sintering temperature improves the YSZ–SDC contact and densification, despite the reactions [33]. However, at buffer layer sintering temperatures from 1350 °C to 1450 °C, the fuel cell performance starts to drop significantly from 1.57 W cm−2 to 0.55 W cm−2 and the ohmic resistance increased from 0.34 Ω cm2 to 1.21 Ω cm2. Figure 3 does not indicate a significant increase in concentration losses with sintering temperature, which means that the reduced performance is not a result of concentration losses. The solid-state reactions, Ce–Zr interdiffusion, and resulting solid solution formation at sintering temperatures greater than 1350 °C become significant and detrimental to the fuel cell performance until no power was generated at a sintering temperature of 1500 °C as the cell was shorted and no OCV could be obtained. To confirm the Ce–Zr interdiffusion, EDX analysis was performed on the buffer layer sintered at 1400 °C. The EDX analysis was performed at the YSZ–SDC interface, in the middle of the SDC buffer layer, and at the SDC–BSCF interface. Figure 5 shows the results of the EDX analysis at each point in the buffer layer. As shown, Ce and Sm were detected at all three points of the buffer layer. However, both Y and Zr were also detected at all three points with the intensity of both decreasing as the SDC–BSCF interface is approached. The interdiffusion of Zr into the SDC buffer layer leads to the formation of solid solution phases that have a detrimental effect on the fuel cell performance as described previously.

Figure 6 depicts the SEM cross-sectional morphologies of the fuel cell with the SDC buffer layer at sintering temperatures from 1250 to 1500 °C. At a sintering temperature of 1250 °C closed porosity was observed in the YSZ electrolyte, which results in the high ohmic resistance noted in the EIS data. As the sintering temperature is increased the closed porosity in the YSZ layer reduces and disappears, thus decreasing the ohmic losses. At the same time, the SDC buffer layer is easily observed at sintering temperatures from 1250 to 1350 °C, but becomes less distinguishable at temperatures above 1350 °C. To emphasize this transformation, half-cells were prepared with a thinner YSZ layer and without a BSCF cathode in order to better see the detailed changes between the YSZ and SDC layers alone without the influence of BSCF infiltrated into the SDC layer. Figure 7 shows SEM morphologies of the half-cells sintered at 1200 °C, 1300 °C, 1400 °C, and 1500 °C obtained at higher magnification. At 1200 °C and 1300 °C a clearly defined porous SDC layer is observed. However, as the sintering temperature increases to 1500 °C the SDC layer becomes indistinguishable from the YSZ layer. The disappearance of a clearly defined SDC layer indicates that significant reactions have occurred between SDC and YSZ. Thus, the morphologies confirm a solid solution forms at sintering temperatures above 1350 °C and results in decreased fuel cell performance and increased ohmic resistance.

While the peak performance does occur at 1350 °C, it is clear that the SDC layer is not sintered fully as pores are evident in the cross-sectional morphology at sintering temperatures from 1250 to 1350 °C. The densification of the SDC layer above 1350 °C was assessed by sintering half cells without a BSCF cathode layer and analyzing the SDC surface with SEM. Figure 8 indicates that the SDC layer prepared by wet powder spraying still had significant porosity at a sintering temperature of 1400 °C and 1450 °C. At a sintering temperature of 1500 °C much better densification of the SDC layer occurred and most of the pores were not present. While the SDC layer is not fully dense at a sintering temperature of 1350 °C, it is preventing significant reactions between BSCF and YSZ layers. It is necessary for the SDC buffer layer to be free of pinholes that would allow direct contact between BSCF and YSZ, but it is not necessary for it to be fully dense. In fact, Li et al. noted that having a buffer layer that is not fully densified can be an effective way of increasing the triple phase boundary between the buffer layer and cathode as well as the fuel cell performance [25]. The increase in contact area between SDC and BSCF at an SDC presintered temperature of 1350 °C is also an important reason for the improved performance in this study.

## Conclusion

A dual layer electrolyte AS-SOFC exhibited an extraordinary fuel cell performance of ∼1.9 W cm−2 at 800 °C by wet powder spraying an SDC layer onto the YSZ electrolyte layer. The use of tape-casting and lamination allowed for a reduction in the number of sintering stages typically needed by co-sintering the anode and electrolyte. The fuel cell performance was found to be highly dependent on the sintering temperature of the SDC buffer layer. The peak performance varied by an order of magnitude by increasing the buffer layer sintering temperature from 1200 °C to 1350 °C. After reaching a performance peak, the power density declined significantly at higher sintering temperatures. At the low sintering temperatures, the fuel cell resistance is mainly from the undense electrolyte and poor YSZ–SDC contact. At the high sintering temperatures, the reactions between YSZ and SDC have a significant effect on the fuel cell performance.

## Acknowledgements

This work was supported by the National Science Foundation under Grant No. CBET-1403405 and an award from Empire State Development's Division of Science, Technology and Innovation (NYSTAR) through the Syracuse Center of Excellence, under Award No. #C120183.

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Duana, Z. , Yang, M. , Yan, A. , Hou, Z. , Dong, Y. , Chong, Y. , Cheng, M. , and Yang, W. , 2006, “ Ba0.5Sr0.5Co0.8Fe0.2O3−δ as a Cathode for IT-SOFCs With a GDC Interlayer,” J. Power Sources, 160(1), pp. 57–64.
Wang, K. , Ran, R. , Zhou, W. , Gu, H. , Shao, Z. , and Ahn, J. , 2008, “ Properties and Performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + Sm0.2Ce0.8O1.9 Composite Cathode,” J. Power Sources, 179(1), pp. 60–68.
Zhou, W. , Shao, Z. , and Jin, W. , 2006, “ Synthesis of Nanocrystalline Conducting Composite Oxide Based on a Non-Ion Selective Combined Complexing Process for Functional Applications,” J. Alloys Compd., 426(1–2), pp. 368–374.
Ohrui, H. , Matushima, T. , and Hirai, T. , 1998, “ Performance of a Solid Oxide Fuel Cell Fabricated by Co-Firing,” J. Power Sources, 71(1–2), pp. 185–189.
Wang, K. , Zeng, P. , and Ahn, J. , 2012, “ Performance Investigation of YSZ-SDC Solid Oxide Fuel Cells,” ASME Paper No. FuelCell2012-91429.
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Wang, K. , Ran, R. , Zhou, W. , Gu, H. , Shao, Z. , and Ahn, J. , 2008, “ Properties and Performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + Sm0.2Ce0.8O1.9 Composite Cathode,” J. Power Sources, 179(1), pp. 60–68.
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Wang, K. , Zeng, P. , and Ahn, J. , 2012, “ Performance Investigation of YSZ-SDC Solid Oxide Fuel Cells,” ASME Paper No. FuelCell2012-91429.

## Figures

Fig. 1

Fuel cell performance without the SDC buffer layer operating with hydrogen fuel at a flow rate of 100 mL min−1 and air as oxidant at different operating temperatures

Fig. 2

Fuel cell performance at different sintering temperatures of the SDC buffer layer (a) 1200 °C, (b) 1250 °C, (c) 1300 °C, (d) 1350 °C, (e) 1400 °C, and (f) 1450 °C operating with hydrogen fuel at a flow rate of 100 mL min−1 and air as oxidant at different operating temperatures

Fig. 3

Fuel cell performance comparison at an operating temperature of 750 °C with the SDC buffer layer sintered between 1250 °C and 1450 °C operating with hydrogen fuel at a flow rate of 100 mL min−1 and air as oxidant

Fig. 4

Ohmic resistance, polarization resistance, and total resistance for different sintering temperatures of the SDC buffer layer from 1250 °C to 1450 °C at an operating temperature of 750 °C with hydrogen fuel at a flow rate of 100 mL min−1 and air as oxidant

Fig. 5

EDX analysis of the YSZ–SDC interface, in the middle of the SDC buffer layer and at the SDC–BSCF interface of the fuel cell sintered at 1400 °C

Fig. 6

Cross-sectional morphologies of the fuel cell with SDC buffer layer sintered at (a) 1250 °C, (b) 1300 °C, (c) 1350 °C, (d) 1400 °C, (e) 1450 °C, and (f) 1500 °C

Fig. 7

Cross-sectional morphologies of the fuel cell without a cathode layer showing changes in the YSZ–SDC interface at a SDC sintering temperature of (a) 1200 °C, (b) 1300 °C, (c) 1400 °C, and (d) 1500 °C

Fig. 8

SEM morphologies of the SDC buffer layer surface at different sintering temperatures of (a) 1400 °C (b) 1450 °C, and (c) 1500 °C

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