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Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells
international journal of hydrogen energy xxx (xxxx) xxx
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Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells Sezer Onbilgin a,b,*, Bora Timurkutluk a,b, Cigdem Timurkutluk a,b,c, Selahattin Celik a,b a
Nigde Omer Halisdemir University, Mechanical Engineering Department, 51245, Nigde, Turkey Nigde Omer Halisdemir University Prof. Dr. T. Nejat Veziroglu Clean Energy Research Center, 51245, Nigde, Turkey c Vestel Defense Industry, Universiteler Mah. Ihsan Dogramaci Bul, Titanyum Blok 17/B Teknokent ODTU, 06800, Ankara, Turkey b
highlights Dip coating, screen printing and tape casting are compared for SOFC electrolyte. Effects of sintering temperature is also considered. Dip coating and screen printing yield porous electrolyte and low cell performance. The cells with tape cast electrolyte provide the highest cell performances.
article info
abstract
Article history:
A comparison of three solid oxide electrolyte fabrication processes, namely dip coating,
Received 7 October 2019
screen printing and tape casting, for planar anode supported solid oxide fuel cells (SOFCs)
Received in revised form
is presented in this study. The effect of sintering temperature (1325e1400 C) is also
23 December 2019
examined. The anode and cathode layers of the anode-supported cells, on the other hand,
Accepted 14 January 2020
are fabricated by tape casting and screen printing, respectively. The quality of the elec-
Available online xxx
trolytes is evaluated via performance measurements, impedance analyses and microstructural investigations of the cells. It is found that the density of the electrolyte increases
Keywords:
with the sintering temperatures for all fabrication methods studied. The results also show
Solid oxide fuel cell
that with the process and fabrication parameters considered in this study, both dip coating
Electrolyte
and screen printing do not yield a desired dense electrolyte structure as proven by open
Yttria stabilized zirconia
circuit potentials measured and SEM photos. The cells with tape cast electrolytes, on the
Tape casting
other hand, provide the highest performances regardless of the electrolyte sintering and
Dip coating
cell operating temperatures. The best peak performance of 0.924 W/cm2 is obtained from
Screen printing
the cell with tape cast electrolyte sintered at 1400 C. SEM investigations and measured open circuit potentials reveal that almost fully dense electrolyte layer can be obtained with a tape cast electrolyte particularly sintered at temperatures higher than 1350 C. Impedance analyses indicate that the main reason behind the significantly higher performances is due to not only increased electrolyte density but a decrease in the interface resistance of the anode functional and electrolyte layer is also responsible. This can be explained by the
* Corresponding author. Nigde Omer Halisdemir University, Mechanical Engineering Department, 51245, Nigde, Turkey. E-mail address: [email protected] (S. Onbilgin). https://doi.org/10.1016/j.ijhydene.2020.01.097 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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load applied during the lamination step in the fabrication of the tape cast electrolyte, providing better powder compaction and adhesion. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The ceramic electrolyte is one of the most important components in solid oxide fuel cells (SOFCs). The main role of the SOFC electrolyte is to transfer oxygen ions generated at the cathode electrode to the anode electrode to complete the electrochemical cycle. Thus, a pure ionic conductivity is desired from SOFC electrolytes to avoid internal shortcircuiting. The ion conduction mechanism in the electrolyte, on the other hand, is based on a vacancy mechanism in the lattice structure and activated at high temperatures. Therefore, the electrolyte material mainly decides the operating temperature of SOFCs. In addition, the solid oxide electrolyte must be free of open porosity so that the fuel and the oxidant can react only by the electrochemical reaction. Otherwise, beside the low cell performance, the cell may be significantly damaged due to local hot spots as a result of the combustion reactions. Therefore, it is essential to manufacture fully dense SOFC electrolytes without any cracks or pinholes, which points out that the fabrication technique of the electrolyte is very substantial. There can be found numerous studies in the literature on various SOFC electrolyte fabrication methods depending on the cell design such as tape casting [1e5], screen printing [6e10], dip coating [11e13], electrophoretic deposition [14e16], spray coating [17e19], electrochemical vapor deposition [20,21], physical vapor deposition [22,23], atomic layer deposition [24e26], electron beam physical vapor deposition [27e29], pulsed laser deposition [30,31], chemical vapor deposition [32], spray pyrolysis [33e35], magnetron sputtering [36e38] and slurry spin coating [39e41]. On the other hand, tape casting, screen printing and dip coating are most favorable methods since they are well-known ceramic processing techniques and offer easy and cheap manufacturing, enabling mass production. Yttria stabilized zirconia (YSZ) is the most common SOFC electrolyte due to showing pure ionic conductivity, acceptable chemical stability and good mechanical properties [42e47]. The highest ionic conductivity is obtained at a doping level of 8 mol % of yttria (8 mol% Y2O3 fully stabilized ZrO2) [48e51]. On the other hand, a typical YSZ electrolyte supported SOFC requires high operating temperatures around 1000 C for YSZ electrolyte to achieve adequate ionic conductivity and therefore acceptable cell performance [52,53]. Such high temperatures threaten long-term stability of not only the cell and but also the balance of plant components and lead to high production costs due to difficulties in production [54e58]. Therefore, the reduction in the operation temperature of SOFCs is crucial for the cost reduction, the long service life with an acceptable degradation and the commercialization of the SOFC systems. One way to lower SOFC operation temperature is to reduce the thickness of the electrolyte by shifting the cell design from electrolyte
supported to anode supported, although the electrolyte supported SOFCs provide relatively strong structure and are less susceptible to mechanical failure. In this respect, compared to electrolyte supported SOFCs, anode supported ones may exhibit higher performances at the same operating temperatures or similar performances at relatively lower operating temperatures. In this study, therefore, planar anode supported cells with various electrolyte layers by using dip coating, screen printing and tape casting methods are fabricated and tested to investigate the effects of the fabrication method on the quality of the electrolyte. The effects of the sintering temperature on the electrolyte microstructure and cell performance are also evaluated.
Experimental Slurry preparation and tape casting of anode layers The anode electrode of the cells fabricated in this study is composed of anode support layer (ASL) and anode functional layer (AFL). Both layers are manufactured by tape casting route. NiO powders (NiO-A, Novamet, New Jersey, USA) and YSZ powders ((Y2O3)0.08(ZrO2)0.92); Nextech Materials, Ohio, USA were weighted at a weight ratio of 70:30 to prepare ASL tape casting slurry. Then, appropriate amounts of solvent (mixture of ethanol and ethyl methyl ketone; Sigma-Aldrich, Munich, Germany) and dispersant (fish oil, Sigma-Aldrich) were included. The solution is ball milled in high-density polyethylene bottles for 24 h accompanied by zirconia balls. Next, plasticizer (polyethylene glycol, Sigma-Aldrich) and binder (polyvinyl butyral, Sigma-Aldrich) were added to the mixture at suitable ratios. The slurry was tape cast with a doctor blade gap of 190 mm on a Mylar strip after another 24 h ball milling process. The tape casting slurry of AFL was prepared and tape cast, similarly. However, this slurry included fine NiO (NiOeF, Novamet) and YSZ powders at a weight ratio of 50:50 as well as active carbon (Sigma-Aldrich) as a pore former. For the electrolytes to be fabricated by screen printing and dip coating, 14 ASL and 3 AFL tapes were stacked and presintered at a temperature of 1350 C for 5 h and 1100 C for 5 h, respectively. On the other hand, for the electrolyte to be fabricated by tape casting, the same numbers of ASL and AFL tapes given above were stacked together with single electrolyte tape and co-sintered at various temperatures. All laminations were achieved via iso-static pressing at under 50 MPa pressure and 60 C temperature for 10 min after uni-axial pressing under 5 MPa for 4 min at room temperature. All laminates were shaped with a laser cutter at desired geometries for the fabrication of disc shaped anode supported cells having 1 cm2 active area.
Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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Electrolyte fabrication YSZ tape casting slurry was prepared similar to those of ASL and AFL. However, the slurry did not include any pore former and was tape cast with a doctor blade gap of 60 mm to obtain thin electrolyte layer. As aforementioned before, 14 ASL tapes, 3 AFL tapes and single electrolyte tape were laminated together to fabricate the cells with tape cast electrolyte. These anode-supported electrolyte structures were then co-sintered at four different temperatures between 1325 C and 1400 C. The same YSZ solution was prepared for screen printing and dip coating processes. The slurry included an appropriate amount of ethyl cellulose binder and terpineol solvent (all from Sigma-Aldrich). The mixture was ball milled for 12 h and the homogenization of the ink was achieved via three rolls mill until the desired viscosity was reached. The ink was then screen printed with a machine on the anode previously sintered at 1350 C and dip coated on the anode pre-sintered at 1100 C. The electrolyte layers coated by screen printing and dip coating were then sintered at 1325e1400 C to investigate the effect of sintering temperature on the electrolyte properties.
Cell fabrication and characterization All anode-supported electrolytes were coated with cathode functional layer (CFL) and cathode current collection layer (CCL) in an order via screen printing after sintering the electrolyte layer. The cathode inks were prepared similar to that of electrolytes for screen printing/dip coating. LSM (SigmaAldrich)/YSZ (wt. % 50/50) was used as CFL, whereas LSM was used as CCL. The pastes were homogenized by means of three rolls mill and first CFL ink was screen printed on the electrolyte layers with a circular active area of 1 cm2. After drying CFL for 30 min at 100 C, CCL ink was screen printed on CFL layer. Both cathode layers were co-sintered at 1050 C for 2.5 h. The electrochemical performances of the cells with various electrolyte layers were evaluated by a fuel cell test station (Arbin Instruments, FCTS, Texas, USA) at 700e800 C operating temperatures under 0.15 NL/min H2 as a fuel and 0.30 NL/min air as an oxidant. Impedance measurements were also performed under the open circuit potential by an impedance analyzer (Solartron Analytical, 1260 A, Hempshire, UK) in a frequency range of 0.1 Hze100 kHz. Anode supported cells were placed between two interconnectors made of Crofer® 22 APU (ThyssenKrupp VDM GmbH, Essen, Germany) plates having gas channels machined for the gas supply as shown in Fig. 1 for the performance and impedance analyses. To obtain better current collection, porous nickel and Crofer mesh were used at the anode and cathode side, respectively. Furthermore, the anode side of the cell and the corresponding interconnector surface were brush painted by NiO anode current collecting paste whereas the cathode side of the cell and the corresponding interconnector surface were brush painted by pure LSM paste as a cathode current collecting paste. Thermiculite® (Flexitallic, Cleckheaton, UK) was used as a sealant. The short stack was then placed into a furnace and heated up to the desired operating temperature. Before the measurements, the anode side of the cells was reduced by pure hydrogen for half an hour. The
Fig. 1 e A photo of the SOFC short stack components.
microstructures of the cells especially the electrolyte layer, on the other hand, were examined by a scanning electron microscope (SEM; Carl Zeiss, Evo 40, London, England).
Results and discussion The performance results at 800 C obtained from the cells having dip coated electrolyte sintered at different temperatures are compared in Fig. 2. Since similar results are obtained at all operating temperatures considered in this study, only the results at 800 C are given here for all cases. Low opencircuit potentials at all sintering temperatures for dip coated electrolytes are noteworthy. Although the open circuit potential shows an increasing trend with the sintering temperature, they are still low compared to theoretical value, which points out that fully dense electrolyte could not be obtained even after sintering 1400 C. On the other hand, it is seen that as a result of the increased open circuit potential with the sintering temperature, the cell performance increases significantly. The cell having 1325 C electrolyte sintering temperature exhibits only 0.036 W/cm2 peak power density while that of the cell with the electrolyte sintered at 1400 C is measured to be 0.336 W/cm2, indicating an increase in the performance to up to almost tenfold. This can be attributed the increase in the density of the electrolyte with the sintering temperature as confirmed by SEM micrographs illustrated in Fig. 3. With the decreased porosity in the electrolyte layer, the amounts of the fuel and oxidant consumed in the electrochemical
Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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Fig. 2 e Effect of sintering temperature of dip coated electrolyte on the cell performance.
reaction increase unlike the chemical combustion reactions, which results in a significant increase in the cell performance. Fig. 4 demonstrates the performances of the cells (at 800 C) with screen printed electrolytes sintered at temperatures between 1325 C and 1400 C. It is seen that slightly higher open circuit potentials can be achieved compared to those of dip coated ones in spite of the same slurry used in both
techniques. On the other hand, similar to results of the cells with dip coated electrolytes, measured open circuit potentials are still well below than the theoretical value, indicating again inadequate electrolyte density. This can be confirmed by SEM images given in Fig. 5. It is seen that the porosity of the electrolyte tends to decrease with increasing the sintering temperature. However, even sintering at 1400 C, which is the highest sintering temperature considered in this study, does not yield a dense electrolyte layer. Further increase in the sintering temperature may provide an electrolyte layer with desired properties; however, this may results in decreased porosity in the anode layers, leading to gas diffusion problems in the anode layer thus low cell performance. Therefore, sintering temperatures higher than 1400 C were not considered in this study. Nevertheless, the performances of the cells having screen printed electrolyte especially with sintering temperatures lower than 1400 C seem to be relatively much higher compared to those of the cells with dip coated electrolyte. This may be due to improved contact between the anode functional layer and the electrolyte with screen printing technique, since there is no load applied in the dip coating process, whereas the machine are set to apply a suitable load via rubber squeegee during the screen printing. This load may provide better adhesion of the electrolyte to the anode functional layer and significantly reduce the anode-electrolyte
Fig. 3 e SEM images of dip coated electrolytes sintering at 1325e1400 C (aed). Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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Fig. 4 e Effect of sintering temperature of screen printed electrolyte on the cell performance.
interface resistance of the cell. The impedance results depicted in Fig. 6 confirm that at the same electrolyte sintering temperatures, the cells with screen printed electrolyte layer show relatively lower ohmic and total resistances, thus higher performances. The frequencies of the highest point of each arc are also indicated in the figure as well as in other impedance plots.
Fig. 6 e Impedance results (at 800 C) of the cells with dip coated and screen printed electrolytes sintered at 1325 C and 1375 C.
The performances of the cells having tape cast electrolyte layers sintered at different temperatures are shown in Fig. 7. It is seen that the open circuit potentials are higher than those obtained from dip coated and screen printed electrolytes. Furthermore, except the electrolyte sintering temperature (co-
Fig. 5 e SEM images of screen printed electrolytes sintering at 1325e1400 C (aed). Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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Fig. 7 e Performance comparison of the cells having tape cast electrolytes with various sintering temperatures. sintering) of 1325 C, all cells exhibit an open circuit potential close to the theoretical value. This can be elucidated by that tape casting, unlike the dip coating and screen printing, provides a dense electrolyte layer especially at sintering temperatures higher than 1350 C, which can be validated by SEM images given in Fig. 8. Fig. 9 shows the impedance results (at
800 C) of the cells with tape cast electrolytes sintered at different temperatures. As expected, both ohmic and total resistances of the cell increase with decreasing the electrolyte sintering temperature, confirming the performance results. The charge transfer and gas diffusion resistances also vary depending on the electrolyte sintering temperature, since this temperature is also co-sintering temperature of ASL-AFLelectrolyte structure for the cells with a tape cast electrolyte, thus deciding not only the final microstructure of the electrolyte layer but also that of both ASL and AFL. As it can be seen from the figure, all tape cast electrolyte layers seem to have relatively higher density. This may be due to not only the composition of the tape casting slurry, but also two-step lamination process by uni-axial and iso-static pressing applied after tape casting process. The applied loads in these pressing are much more higher than that of in the screen printing and as aforementioned before no load was applied in the dip coating route. The loads applied in the pressing steps provide a better compaction of the powder and a better adhesion between the anode functional and electrolyte layer as well as resulting in increased electrolyte density as shown in Fig. 10, which compares the effect of electrolyte fabrication method on the cell resistances for 1350 C electrolyte sintering temperature. As a result of the decreased resistances, all cells with a tape cast electrolyte layer exhibit
Fig. 8 e SEM images of tape cast electrolytes sintering at 1325e1400 C (aed). Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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Fig. 9 e Impedance results (at 800 C) of the cells with tape cast electrolytes sintered at different temperatures.
Fig. 10 e Impedance results (at 800 C) of the cells with dip coated, screen printed and tape cast electrolytes sintered at 1400 C.
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The performance results show that open circuit potentials of the cells with dip coated and screen printed electrolytes are ranging between 0.6 and 0.8 V at all sintering temperatures studied. However, they show an increasing trend with the sintering temperature as expected due increased electrolyte density. These values as being lower than the theoretical value indicate a porous electrolyte structure as confirmed by SEM micrographs. On the other hand, especially at electrolyte sintering temperatures lower than 1400 C, screen printed electrolytes provide significantly higher peak performances compared to dip coated ones. This may be attributed to a better adhesion of the electrolyte layer to the anode functional layer and improved electrolyte density due to applied load during screen printing, while there is no load applied during the dip coating process. As a result, the cells with screen printed electrolytes show relatively low ohmic and total resistances as confirmed by the impedance results. Therefore, considering the range of sintering temperature and the properties of the slurry used in dip coating and screen printing of the electrolyte in this study, it is found that it is not possible to obtain fully dense electrolyte layer with these methods. Nevertheless, better results may be obtained by changing the fabrication and processing parameters such as the composition of the coating slurry and the number of coating. The best performances, on the other hand, are obtained from the cells with tape cast electrolytes. These cells exhibit higher peak power at all operating and electrolyte sintering temperatures studied. Furthermore, the open circuit potentials close to theoretical value are obtained at all electrolyte sintering temperature higher than 1350 C from all cells with tape cast electrolyte layer, indicating a dense electrolyte layer as confirmed by SEM investigations. This may be as a result of the lamination steps in the fabrication process of the cells with tape cast electrolytes. The loads applied in these steps may result in a dense electrolyte layer due to better compaction of the powders and improved contact between the anode functional and the electrolyte layer, reducing the cell resistance.
references higher performances independent from the sintering temperature, compared to the cells with dip coated or screen printed electrolytes. The maximum performance is obtained from the cell having tape cast electrolyte sintered at 1400 C as 0.924 W/cm2 peak power density, while those for dip coated and screen printed electrolyte are only 0.336 W/cm2 and 0.379 W/cm2, respectively.
Conclusion In this study, dip coating, screen printing and tape casting are evaluated as an SOFC electrolyte fabrication technique. The cells having various electrolytes are manufactured in this respect and the electrolyte sintering temperatures of 1325e1400 C are also considered. The cells are characterized via performance and impedance measurements. Microstructural observations via SEM are also performed to investigate the electrolyte fabrication method on the electrolyte quality.
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Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells Sezer Onbilgin a,b,*, Bora Timurkutluk a,b, Cigdem Timurkutluk a,b,c, Selahattin Celik a,b a
Nigde Omer Halisdemir University, Mechanical Engineering Department, 51245, Nigde, Turkey Nigde Omer Halisdemir University Prof. Dr. T. Nejat Veziroglu Clean Energy Research Center, 51245, Nigde, Turkey c Vestel Defense Industry, Universiteler Mah. Ihsan Dogramaci Bul, Titanyum Blok 17/B Teknokent ODTU, 06800, Ankara, Turkey b
highlights Dip coating, screen printing and tape casting are compared for SOFC electrolyte. Effects of sintering temperature is also considered. Dip coating and screen printing yield porous electrolyte and low cell performance. The cells with tape cast electrolyte provide the highest cell performances.
article info
abstract
Article history:
A comparison of three solid oxide electrolyte fabrication processes, namely dip coating,
Received 7 October 2019
screen printing and tape casting, for planar anode supported solid oxide fuel cells (SOFCs)
Received in revised form
is presented in this study. The effect of sintering temperature (1325e1400 C) is also
23 December 2019
examined. The anode and cathode layers of the anode-supported cells, on the other hand,
Accepted 14 January 2020
are fabricated by tape casting and screen printing, respectively. The quality of the elec-
Available online xxx
trolytes is evaluated via performance measurements, impedance analyses and microstructural investigations of the cells. It is found that the density of the electrolyte increases
Keywords:
with the sintering temperatures for all fabrication methods studied. The results also show
Solid oxide fuel cell
that with the process and fabrication parameters considered in this study, both dip coating
Electrolyte
and screen printing do not yield a desired dense electrolyte structure as proven by open
Yttria stabilized zirconia
circuit potentials measured and SEM photos. The cells with tape cast electrolytes, on the
Tape casting
other hand, provide the highest performances regardless of the electrolyte sintering and
Dip coating
cell operating temperatures. The best peak performance of 0.924 W/cm2 is obtained from
Screen printing
the cell with tape cast electrolyte sintered at 1400 C. SEM investigations and measured open circuit potentials reveal that almost fully dense electrolyte layer can be obtained with a tape cast electrolyte particularly sintered at temperatures higher than 1350 C. Impedance analyses indicate that the main reason behind the significantly higher performances is due to not only increased electrolyte density but a decrease in the interface resistance of the anode functional and electrolyte layer is also responsible. This can be explained by the
* Corresponding author. Nigde Omer Halisdemir University, Mechanical Engineering Department, 51245, Nigde, Turkey. E-mail address: [email protected] (S. Onbilgin). https://doi.org/10.1016/j.ijhydene.2020.01.097 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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load applied during the lamination step in the fabrication of the tape cast electrolyte, providing better powder compaction and adhesion. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The ceramic electrolyte is one of the most important components in solid oxide fuel cells (SOFCs). The main role of the SOFC electrolyte is to transfer oxygen ions generated at the cathode electrode to the anode electrode to complete the electrochemical cycle. Thus, a pure ionic conductivity is desired from SOFC electrolytes to avoid internal shortcircuiting. The ion conduction mechanism in the electrolyte, on the other hand, is based on a vacancy mechanism in the lattice structure and activated at high temperatures. Therefore, the electrolyte material mainly decides the operating temperature of SOFCs. In addition, the solid oxide electrolyte must be free of open porosity so that the fuel and the oxidant can react only by the electrochemical reaction. Otherwise, beside the low cell performance, the cell may be significantly damaged due to local hot spots as a result of the combustion reactions. Therefore, it is essential to manufacture fully dense SOFC electrolytes without any cracks or pinholes, which points out that the fabrication technique of the electrolyte is very substantial. There can be found numerous studies in the literature on various SOFC electrolyte fabrication methods depending on the cell design such as tape casting [1e5], screen printing [6e10], dip coating [11e13], electrophoretic deposition [14e16], spray coating [17e19], electrochemical vapor deposition [20,21], physical vapor deposition [22,23], atomic layer deposition [24e26], electron beam physical vapor deposition [27e29], pulsed laser deposition [30,31], chemical vapor deposition [32], spray pyrolysis [33e35], magnetron sputtering [36e38] and slurry spin coating [39e41]. On the other hand, tape casting, screen printing and dip coating are most favorable methods since they are well-known ceramic processing techniques and offer easy and cheap manufacturing, enabling mass production. Yttria stabilized zirconia (YSZ) is the most common SOFC electrolyte due to showing pure ionic conductivity, acceptable chemical stability and good mechanical properties [42e47]. The highest ionic conductivity is obtained at a doping level of 8 mol % of yttria (8 mol% Y2O3 fully stabilized ZrO2) [48e51]. On the other hand, a typical YSZ electrolyte supported SOFC requires high operating temperatures around 1000 C for YSZ electrolyte to achieve adequate ionic conductivity and therefore acceptable cell performance [52,53]. Such high temperatures threaten long-term stability of not only the cell and but also the balance of plant components and lead to high production costs due to difficulties in production [54e58]. Therefore, the reduction in the operation temperature of SOFCs is crucial for the cost reduction, the long service life with an acceptable degradation and the commercialization of the SOFC systems. One way to lower SOFC operation temperature is to reduce the thickness of the electrolyte by shifting the cell design from electrolyte
supported to anode supported, although the electrolyte supported SOFCs provide relatively strong structure and are less susceptible to mechanical failure. In this respect, compared to electrolyte supported SOFCs, anode supported ones may exhibit higher performances at the same operating temperatures or similar performances at relatively lower operating temperatures. In this study, therefore, planar anode supported cells with various electrolyte layers by using dip coating, screen printing and tape casting methods are fabricated and tested to investigate the effects of the fabrication method on the quality of the electrolyte. The effects of the sintering temperature on the electrolyte microstructure and cell performance are also evaluated.
Experimental Slurry preparation and tape casting of anode layers The anode electrode of the cells fabricated in this study is composed of anode support layer (ASL) and anode functional layer (AFL). Both layers are manufactured by tape casting route. NiO powders (NiO-A, Novamet, New Jersey, USA) and YSZ powders ((Y2O3)0.08(ZrO2)0.92); Nextech Materials, Ohio, USA were weighted at a weight ratio of 70:30 to prepare ASL tape casting slurry. Then, appropriate amounts of solvent (mixture of ethanol and ethyl methyl ketone; Sigma-Aldrich, Munich, Germany) and dispersant (fish oil, Sigma-Aldrich) were included. The solution is ball milled in high-density polyethylene bottles for 24 h accompanied by zirconia balls. Next, plasticizer (polyethylene glycol, Sigma-Aldrich) and binder (polyvinyl butyral, Sigma-Aldrich) were added to the mixture at suitable ratios. The slurry was tape cast with a doctor blade gap of 190 mm on a Mylar strip after another 24 h ball milling process. The tape casting slurry of AFL was prepared and tape cast, similarly. However, this slurry included fine NiO (NiOeF, Novamet) and YSZ powders at a weight ratio of 50:50 as well as active carbon (Sigma-Aldrich) as a pore former. For the electrolytes to be fabricated by screen printing and dip coating, 14 ASL and 3 AFL tapes were stacked and presintered at a temperature of 1350 C for 5 h and 1100 C for 5 h, respectively. On the other hand, for the electrolyte to be fabricated by tape casting, the same numbers of ASL and AFL tapes given above were stacked together with single electrolyte tape and co-sintered at various temperatures. All laminations were achieved via iso-static pressing at under 50 MPa pressure and 60 C temperature for 10 min after uni-axial pressing under 5 MPa for 4 min at room temperature. All laminates were shaped with a laser cutter at desired geometries for the fabrication of disc shaped anode supported cells having 1 cm2 active area.
Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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Electrolyte fabrication YSZ tape casting slurry was prepared similar to those of ASL and AFL. However, the slurry did not include any pore former and was tape cast with a doctor blade gap of 60 mm to obtain thin electrolyte layer. As aforementioned before, 14 ASL tapes, 3 AFL tapes and single electrolyte tape were laminated together to fabricate the cells with tape cast electrolyte. These anode-supported electrolyte structures were then co-sintered at four different temperatures between 1325 C and 1400 C. The same YSZ solution was prepared for screen printing and dip coating processes. The slurry included an appropriate amount of ethyl cellulose binder and terpineol solvent (all from Sigma-Aldrich). The mixture was ball milled for 12 h and the homogenization of the ink was achieved via three rolls mill until the desired viscosity was reached. The ink was then screen printed with a machine on the anode previously sintered at 1350 C and dip coated on the anode pre-sintered at 1100 C. The electrolyte layers coated by screen printing and dip coating were then sintered at 1325e1400 C to investigate the effect of sintering temperature on the electrolyte properties.
Cell fabrication and characterization All anode-supported electrolytes were coated with cathode functional layer (CFL) and cathode current collection layer (CCL) in an order via screen printing after sintering the electrolyte layer. The cathode inks were prepared similar to that of electrolytes for screen printing/dip coating. LSM (SigmaAldrich)/YSZ (wt. % 50/50) was used as CFL, whereas LSM was used as CCL. The pastes were homogenized by means of three rolls mill and first CFL ink was screen printed on the electrolyte layers with a circular active area of 1 cm2. After drying CFL for 30 min at 100 C, CCL ink was screen printed on CFL layer. Both cathode layers were co-sintered at 1050 C for 2.5 h. The electrochemical performances of the cells with various electrolyte layers were evaluated by a fuel cell test station (Arbin Instruments, FCTS, Texas, USA) at 700e800 C operating temperatures under 0.15 NL/min H2 as a fuel and 0.30 NL/min air as an oxidant. Impedance measurements were also performed under the open circuit potential by an impedance analyzer (Solartron Analytical, 1260 A, Hempshire, UK) in a frequency range of 0.1 Hze100 kHz. Anode supported cells were placed between two interconnectors made of Crofer® 22 APU (ThyssenKrupp VDM GmbH, Essen, Germany) plates having gas channels machined for the gas supply as shown in Fig. 1 for the performance and impedance analyses. To obtain better current collection, porous nickel and Crofer mesh were used at the anode and cathode side, respectively. Furthermore, the anode side of the cell and the corresponding interconnector surface were brush painted by NiO anode current collecting paste whereas the cathode side of the cell and the corresponding interconnector surface were brush painted by pure LSM paste as a cathode current collecting paste. Thermiculite® (Flexitallic, Cleckheaton, UK) was used as a sealant. The short stack was then placed into a furnace and heated up to the desired operating temperature. Before the measurements, the anode side of the cells was reduced by pure hydrogen for half an hour. The
Fig. 1 e A photo of the SOFC short stack components.
microstructures of the cells especially the electrolyte layer, on the other hand, were examined by a scanning electron microscope (SEM; Carl Zeiss, Evo 40, London, England).
Results and discussion The performance results at 800 C obtained from the cells having dip coated electrolyte sintered at different temperatures are compared in Fig. 2. Since similar results are obtained at all operating temperatures considered in this study, only the results at 800 C are given here for all cases. Low opencircuit potentials at all sintering temperatures for dip coated electrolytes are noteworthy. Although the open circuit potential shows an increasing trend with the sintering temperature, they are still low compared to theoretical value, which points out that fully dense electrolyte could not be obtained even after sintering 1400 C. On the other hand, it is seen that as a result of the increased open circuit potential with the sintering temperature, the cell performance increases significantly. The cell having 1325 C electrolyte sintering temperature exhibits only 0.036 W/cm2 peak power density while that of the cell with the electrolyte sintered at 1400 C is measured to be 0.336 W/cm2, indicating an increase in the performance to up to almost tenfold. This can be attributed the increase in the density of the electrolyte with the sintering temperature as confirmed by SEM micrographs illustrated in Fig. 3. With the decreased porosity in the electrolyte layer, the amounts of the fuel and oxidant consumed in the electrochemical
Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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reaction increase unlike the chemical combustion reactions, which results in a significant increase in the cell performance. Fig. 4 demonstrates the performances of the cells (at 800 C) with screen printed electrolytes sintered at temperatures between 1325 C and 1400 C. It is seen that slightly higher open circuit potentials can be achieved compared to those of dip coated ones in spite of the same slurry used in both
techniques. On the other hand, similar to results of the cells with dip coated electrolytes, measured open circuit potentials are still well below than the theoretical value, indicating again inadequate electrolyte density. This can be confirmed by SEM images given in Fig. 5. It is seen that the porosity of the electrolyte tends to decrease with increasing the sintering temperature. However, even sintering at 1400 C, which is the highest sintering temperature considered in this study, does not yield a dense electrolyte layer. Further increase in the sintering temperature may provide an electrolyte layer with desired properties; however, this may results in decreased porosity in the anode layers, leading to gas diffusion problems in the anode layer thus low cell performance. Therefore, sintering temperatures higher than 1400 C were not considered in this study. Nevertheless, the performances of the cells having screen printed electrolyte especially with sintering temperatures lower than 1400 C seem to be relatively much higher compared to those of the cells with dip coated electrolyte. This may be due to improved contact between the anode functional layer and the electrolyte with screen printing technique, since there is no load applied in the dip coating process, whereas the machine are set to apply a suitable load via rubber squeegee during the screen printing. This load may provide better adhesion of the electrolyte to the anode functional layer and significantly reduce the anode-electrolyte
Fig. 3 e SEM images of dip coated electrolytes sintering at 1325e1400 C (aed). Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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interface resistance of the cell. The impedance results depicted in Fig. 6 confirm that at the same electrolyte sintering temperatures, the cells with screen printed electrolyte layer show relatively lower ohmic and total resistances, thus higher performances. The frequencies of the highest point of each arc are also indicated in the figure as well as in other impedance plots.
Fig. 6 e Impedance results (at 800 C) of the cells with dip coated and screen printed electrolytes sintered at 1325 C and 1375 C.
The performances of the cells having tape cast electrolyte layers sintered at different temperatures are shown in Fig. 7. It is seen that the open circuit potentials are higher than those obtained from dip coated and screen printed electrolytes. Furthermore, except the electrolyte sintering temperature (co-
Fig. 5 e SEM images of screen printed electrolytes sintering at 1325e1400 C (aed). Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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Current density (A/cm2)
Fig. 7 e Performance comparison of the cells having tape cast electrolytes with various sintering temperatures. sintering) of 1325 C, all cells exhibit an open circuit potential close to the theoretical value. This can be elucidated by that tape casting, unlike the dip coating and screen printing, provides a dense electrolyte layer especially at sintering temperatures higher than 1350 C, which can be validated by SEM images given in Fig. 8. Fig. 9 shows the impedance results (at
800 C) of the cells with tape cast electrolytes sintered at different temperatures. As expected, both ohmic and total resistances of the cell increase with decreasing the electrolyte sintering temperature, confirming the performance results. The charge transfer and gas diffusion resistances also vary depending on the electrolyte sintering temperature, since this temperature is also co-sintering temperature of ASL-AFLelectrolyte structure for the cells with a tape cast electrolyte, thus deciding not only the final microstructure of the electrolyte layer but also that of both ASL and AFL. As it can be seen from the figure, all tape cast electrolyte layers seem to have relatively higher density. This may be due to not only the composition of the tape casting slurry, but also two-step lamination process by uni-axial and iso-static pressing applied after tape casting process. The applied loads in these pressing are much more higher than that of in the screen printing and as aforementioned before no load was applied in the dip coating route. The loads applied in the pressing steps provide a better compaction of the powder and a better adhesion between the anode functional and electrolyte layer as well as resulting in increased electrolyte density as shown in Fig. 10, which compares the effect of electrolyte fabrication method on the cell resistances for 1350 C electrolyte sintering temperature. As a result of the decreased resistances, all cells with a tape cast electrolyte layer exhibit
Fig. 8 e SEM images of tape cast electrolytes sintering at 1325e1400 C (aed). Please cite this article as: Onbilgin S et al., Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.097
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Fig. 9 e Impedance results (at 800 C) of the cells with tape cast electrolytes sintered at different temperatures.
Fig. 10 e Impedance results (at 800 C) of the cells with dip coated, screen printed and tape cast electrolytes sintered at 1400 C.
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The performance results show that open circuit potentials of the cells with dip coated and screen printed electrolytes are ranging between 0.6 and 0.8 V at all sintering temperatures studied. However, they show an increasing trend with the sintering temperature as expected due increased electrolyte density. These values as being lower than the theoretical value indicate a porous electrolyte structure as confirmed by SEM micrographs. On the other hand, especially at electrolyte sintering temperatures lower than 1400 C, screen printed electrolytes provide significantly higher peak performances compared to dip coated ones. This may be attributed to a better adhesion of the electrolyte layer to the anode functional layer and improved electrolyte density due to applied load during screen printing, while there is no load applied during the dip coating process. As a result, the cells with screen printed electrolytes show relatively low ohmic and total resistances as confirmed by the impedance results. Therefore, considering the range of sintering temperature and the properties of the slurry used in dip coating and screen printing of the electrolyte in this study, it is found that it is not possible to obtain fully dense electrolyte layer with these methods. Nevertheless, better results may be obtained by changing the fabrication and processing parameters such as the composition of the coating slurry and the number of coating. The best performances, on the other hand, are obtained from the cells with tape cast electrolytes. These cells exhibit higher peak power at all operating and electrolyte sintering temperatures studied. Furthermore, the open circuit potentials close to theoretical value are obtained at all electrolyte sintering temperature higher than 1350 C from all cells with tape cast electrolyte layer, indicating a dense electrolyte layer as confirmed by SEM investigations. This may be as a result of the lamination steps in the fabrication process of the cells with tape cast electrolytes. The loads applied in these steps may result in a dense electrolyte layer due to better compaction of the powders and improved contact between the anode functional and the electrolyte layer, reducing the cell resistance.
references higher performances independent from the sintering temperature, compared to the cells with dip coated or screen printed electrolytes. The maximum performance is obtained from the cell having tape cast electrolyte sintered at 1400 C as 0.924 W/cm2 peak power density, while those for dip coated and screen printed electrolyte are only 0.336 W/cm2 and 0.379 W/cm2, respectively.
Conclusion In this study, dip coating, screen printing and tape casting are evaluated as an SOFC electrolyte fabrication technique. The cells having various electrolytes are manufactured in this respect and the electrolyte sintering temperatures of 1325e1400 C are also considered. The cells are characterized via performance and impedance measurements. Microstructural observations via SEM are also performed to investigate the electrolyte fabrication method on the electrolyte quality.
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