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Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs
international journal of hydrogen energy xxx (xxxx) xxx
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Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs € un c, Ali Murat Soydan a, Omer Yıldız b,*, Abdullatif Durg ız Akduman c, Ali Ata c Osman Yag a
Gebze Technical University, Institute of Energy Technologies, TR-41400, Gebze, Kocaeli, Turkey Kocaeli University, Engineering Faculty, Department of Metallurgical and Materials Engineering, TR-41380, Kocaeli, Turkey c Gebze Technical University, Faculty of Engineering, Department of Material Science and Engineering, TR-41400, Gebze, Kocaeli, Turkey b
highlights Production of MT-SOFCs by thermoplastic-extrusion, dip coating, pre-sintering and co-sintering. Microstructural analysis of anode-supported MT-SOFCs in terms of the production processes. The cells produced by co-sintering process have unit cost lower 9% as than the pre-sintered cells. The cells produced by co-sintering have power density lower 24.30% as than the pre-sintered cells.
article info
abstract
Article history:
In the present study, the anode-supported micro-tubular solid oxide fuel cells (MT-SOFCs)
Received 6 June 2019
with an electrolyte thin interlayer were manufactured. The anode support tubes consisting
Received in revised form
of 56 wt% nickel oxide and 44 wt% YSZ (8 mol% yttria (Y2O3) stabilized zirconia (ZrO2)) were
30 July 2019
produced by using the thermo-extrusion method, whereas the electrolyte and cathode
Accepted 19 September 2019
layers were manufactured using the dip-coating method. The half-cells consisting of anode
Available online xxx
and electrolyte were manufactured by using two different methods. In the first method, the
Keywords:
by using the dip-coating method and then exposed to second sintering at 1400 C. In the
Anode-supported micro-tubular
second method, the anode and electrolyte layers were sintered together at 1400 C (co-
SOFC
sintering) in order to produce the half-cells. The half-cells that were produced and then
Performance analysis
coated with cathode solutions by using the dip-coating method and the final cells were
anode-support tubes were pre-sintered at 1200 C, then covered with the electrolyte layer
Thermoplastic extrusion
successfully produced at the end of the sintering at 1150 C. The porosity and shrinkage
Dip-coating
percentage values of these MT-SOFCs differed from each other. The power densities of
Manufacturing cost
these cells were tested at 700 C, 750 C, and 800 C by using H2 gas as fuel and the results of the microstructural and cost analyses were compared. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. € Yıldız). E-mail address: [email protected] (O. https://doi.org/10.1016/j.ijhydene.2019.09.156 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Introduction The anode-supported “micro-tubular solid oxide fuel cells” (MT-SOFCs) were produced by using cold isostatic, co-pressing [1,2], extrusion or co-extrusion [3e12], dip-coating [12e15], and slip casting [14e17], and the performance characteristics were compared in terms of the production method. Although there are many cell designs with different cell geometries [8,15,18e25], the most popular and the most examined geometric structures are the tubular and planar SOFC configurations [3,10,26]. Since the stack production in SOFCs with planar geometry is difficult and complicated, the tubular and micro-tubular SOFCs (MT-SOFCs) are preferred [11,13,26]. Moreover, it is a very important requirement to prevent gas leakages in planar SOFCs. Especially for the high-temperature applications, it is very difficult to establish a stack in a planar configuration for the planar SOFCs [1,2,10,19,26]. The tubular design of the cells might prevent or minimize (at least) the leakage problems and material costs, thus it might become easier to design the simple stacks of tubular cells [1,2,10,19,23,26]. For this reason, the anode-supported tubular SOFCs were produced and analyzed in the present study. The most important limitations against the commercialization of SOFC technology are the high production costs, low performance and productivity, and consequently the high unit cost of energy [11,26,27]. The operational costs and the use of special materials for the operability at high temperatures are the most important factors determining the unit cost of energy [20,27]. Therefore, it is very important to reduce the temperature, to increase the performance of the system and the materials, and thereby to decrease the total unit cost [1,11,18,23]. The reason for preferring the electrolytes working at low temperature is the opportunity of using alternative materials, which have lower costs than the expensive noble metal electrode and interconnect [16,19e21,25]. The ceria- and bismuth-based electrolyte materials were observed to have superior ionic conductance properties when compared to the zirconia derivatives [11]. However, the ceriaand bismuth-oxides are not suitable for long-term use since they transform into metallic form under a low partial pressure of oxygen [19,28,29]. The search for an alternative to 8YSZ from the aspects of stability and cost still continues. Since the ohmic resistance of the electrolyte layer is high, the thickness of the electrolyte should be reduced and it should be produced as a thin film as possible [10,15,24,30]. For this reason, in the cells that contain a thick electrolyte layer with high electrolyte resistance such as 8YSZ, the electrode-supported configurations and mainly the anode-supported tubular cell structures are preferred and examined [2e4,8e10,14e17,23e26]. In the production of SOFCs, generally, three different sintering procedures are employed for the anode, electrolyte, and cathode layers. Regardless of the tubular or planar structure of cells, the support layer is produced and sintered by using the preferred method. The electrolyte and cathode layers are coated by using methods such as spraying or dip-coating. By sintering each layer after the coating procedure, the final cell is obtained. Since reducing the number of sintering operations decreases the costs, the studies were carried out on sintering the different layers together [4,5,7,9e17,23e26]. In this
approach named co-sintering, the anode support layer and electrolyte are sintered together, and then the cathode layer is coated and then re-sintered. Thus, the cell is produced with totally two sintering procedures [4,5,7,9e17,23e26]. In the literature, the studies carried out by using this method focused on the optimization of processes and the performances of cells, but there is no detailed study on the effect on unit cell costs. In the present study, thermoplastic extrusion method has been preferred in the production of anode supported tubes since the thermoplastic extrusion method has the advantages of being a fast and continuous process for mass production. The anode microtubes of the anode-supported MT-SOFCs containing 56 wt% NiO and 34 wt% YSZ were produced using the thermoplastic extrusion method, and then the electrolyte and cathode layers were coated using the dip-coating method. The performances of the pre-sintered and co-sintered cells, which had been sintered in three steps and two steps, were measured using hydrogen (H2) gas as fuel, whereas the SEM method was used to examine their microstructures. The effects of implemented production methods on the cell performances and unit costs were examined.
Experimental procedures All of the analyses, examinations, and tests performed within the scope of this study were carried out in a pilot-scale SOFC production facility founded within the body of NanoTechnology Center of Gebze Technical University’s (GTU) Department of Material Science and Engineering, Faculty of Engineering. In the production of cells, the extrusion, dipcoating, pre-sintering, and co-sintering methods were used. The same devices were used in two cell fabrication methods, and the cells were produced by using the same amount of the materials. Taking the energy and material costs borne during the production phase and ignoring the labor cost, initial investment cost, and the other direct and indirect costs, total production costs were calculated for different production volumes. In the sintering furnace, maximum 1000 cells having 8 cm length and 6.2 mm outer diameter can be sintered at once. This number was set to be the upper limit of cell production. Five different sets of cell numbers as 10, 50, 100, 500, and 1000 cells were determined. By separately measuring the power consumption of machines used in the production, the energy costs were calculated. The material costs were determined over the total amount of the used material by taking the amount of wastes into account.
Materials MT-SOFCs consist of “nickel oxide (NiO) and 8 mol% yttriastabilized zirconia” (NiO-8YSZ) for the anode support, 8YSZ as the electrolyte, lanthanum strontium manganite (LSM) and YSZ (LSM-YSZ) as the cathode functional layer and LSM for the cathode current collection layer. The anode tubes were made of nickel (II) oxide (NiO) powders with a particle size of 1e2 mm and a specific surface area (SSA) of 3e4 m2 g 1 obtained from Hart Materials, and 13 wt% yttria-stabilized zirconia (which is equal to 8 mol% YSZ, 8YSZ) powders with a particle size of
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Table 1 e Compositions of the cathode and electrolyte slurries. Compositions, wt%
LSM
8YSZ
PVB
DBP
PEG 400
Binary Solvent
Cathode functional Slurry Cathode Current Collector Slurry Electrolyte Slurry
16.4 32.8 e
16.4 e 30
0.1 0.1 0.1
0.05 0.05 0.05
0.05 0.05 0.05
M.E.K-Ethanol M.E.K-Ethanol M.E.K-Ethanol
3e5 mm obtained from IMERYS Fused Material Laufenberg GmbH. Moreover, polymethylmethacrylate (PMMA) powders with an average particle size of <80 mm and polyethylene (PE) with a low density from Alfa Aesar were used as a pore former and binder, respectively. 8YSZ with an SSA of 13.4 m2 g 1 and LSM powders with an SSA of 10.8 m2 g 1 from Fuel Cell Materials were used for electrolyte and cathode slurries, respectively. Polyvinyl Butyral (PVB) and Dibutyl phthalate from Sigma-Aldrich and polyethylene glycol (PEG 400) from Alfa Aesar were used as the binder, plasticizer, and lubricant, respectively. Ethyl methyl ketone (MEK, 99%) and ethanol (99%) from Sigma-Aldrich were mixed and used as the solvent for the coating slurries.
Production of anode-supported micro-tubular cells The anode support layer of the MT-SOFCs was made of starting powders of NiO and YSZ. NiO and YSZ powders’ homogeneity was ensured by grinding with a ball mill for 48 h in a solvent consisting of ethanol and toluene. After 48 h, 5.45 wt % PE and 4.55 wt% PMMA were added to the anode composition to perform the production of the microtube with the extruder device and to obtain a more porous microstructure in the anode support layer. In order to ensure the homogeneity of anode composition consisting of polymer and powders, the grinding operation was continued for 24 h more. After 72 h of mixing procedure, the mixture was poured into a glass container and then air-dried at 80 C. The dried powders containing 56 wt% NiO and 34 wt% YSZ were ground by using the mortar. When the mixture was prepared, it was poured into the mouth of the thermoplastic extruder. The anode support tubes were extruded by using a single-screw extruder made by Rondol Technology. The extruded anode tubes were cut into 65 mm length. The dimensions of anode support tubes were 6.2 mm (external diameter), 4.35 mm (internal diameter), and 0.85 mm (wall thickness).
Preparing the cathode slurries The cathode layer consists of two layers named cathode functional layer and cathode current collector layer. The cathode functional solution was prepared using 16.4% (wt.) 8YSZ and 16.4% LSM, as presented in Table 1. The cathode collection layer was prepared using 32.8% (wt.) LSM. While preparing the cathode solutions, the procedures followed in electrolyte solutions were repeated directly.
Production of anode-supported micro-tubular SOFCs In this study, the half-cells consisting of the anode support and the electrolyte layer were manufactured by using two different methods. In the first method, the anode support tubes cut into 65 mm length after the extrusion process were exposed to 2 h of pre-sintering at 1100 C under air, and then the coating process of the electrolyte layer was initiated. The pre-sintered tubes were coated by dipping them into electrolyte solution at 60 mm/s of pulling rate by using the dipcoating method. In order to remove the solvent in the electrolyte film, they were air-dried for 10 min at 80 C. In order to achieve the desired thickness of electrolyte, this procedure was repeated twice. After the electrolyte-coating procedure, the cells were sintered for 3 h under air at 1400 C and the halfcells were obtained. In the second method, the anode-support tubes cut after the extrusion were exposed to the electrolyte coating process of the first method but without any pre-sintering process. Then, the half-cells consisting of anode support and the
Preparing the electrolyte slurry In order to prepare a homogenous electrolyte solution, the initial materials consisting of YSZ powders, polyvinyl butyral (PVB), PEG-400, and dibutyl phthalate (DBP) were ground by using the planetary mill for 15 h at 350 rpm within an ethyl methyl ketone (MEK) and ethanol solvent. In order to prevent the over-heating during the grinding process, the planetary mill was stopped for 20 min after every 25 min of the grinding process. Before the coating process, the electrolyte solution was exposed to an ultrasonic bath for 10 min and an ultrasonic horn for 1 min. The compositions of electrolyte and cathode slurries are presented in Table 1.
Fig. 1 e Sintering regimes of the anode, electrolyte, and cathode layers.
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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electrolyte layer were sintered together (co-sintering) and the half-cells were obtained. The tips of half-cells produced by using two different methods were coated with paraffin before the cathodecoating process in order to prevent the contact between anode and cathode layers. The half-cells were firstly coated with cathode functional solution consisting of LSM and YSZ by using the dip-coating method at 60 mm/s pulling rate and then kept at 80 C for 10 min for the coating to be dried. Finally, the half-cells coated with LSM-YSZ were submerged into cathode current collection solution consisting of LSM by using the dip-coating method at the pulling rate of 60 mm/s and the coating procedure was completed. The cathode functional layer and the cathode current collection layer were co-sintered at 1100 C. Sintering regimes of the presintered anode, electrolyte, and cathode were showed in detail in Fig. 1. The production process of the anodesupported MT-SOFCs and the images of products are shown in Fig. 2.
Preparing the cells for testing and electrochemical measurements The cathode surface and the anode current collection points were coated with a silver paste (DAD-87) by using a brush. In order to collect the current forming in the anode and cathode surfaces, the silver wire with 0.5 mm diameter (Alfa Aesar, 0.5 mm) was used. The cell to be tested was wrapped using silver wire and then placed into a furnace. Then the alumina tubes ensuring the gas inlet and outlet were connected as shown in Fig. 3. The connection between cell and alumina tube was established by using Ceramabond-569 and
Ceramabond-552-VFG sealing elements in order to prevent fuel leakage at the connection point. The anode and cathode current collecting silver wires were connected to RBL-4881006-400 test device in order to obtain the voltage-current characteristic curves (RBL488 100-60-400, TDI Power Corp., New Jersey, USA). The temperature of the reduction process and flow rate of the reducing gas have a significant effect on the transformation of NiO-YSZ to Ni-YSZ. The reduction of nickel oxide to metallic nickel results in a volume change of 40%. If the volume change during the reduction process is rapid, it causes micro-cracks in the anode structure. These cracks both reduce the mechanical strength of the cell and cause cracking of the electrolyte layer and may cause a short circuit of the cells. In the literature studies, it was observed that deformations in the cell occur more in reducing condition performed in high temperature and humid atmosphere [31,32]. To eliminate deformation possibility during the reduction process low temperature reducing condition was preferred in this study. The details about reducing condition performed at low temperature for NiO, NiO-YSZ are given in the literatures [33,34]. n this study, the reduction process of the anode side was initiated by introducing 100% dry H2 gas at 500 C. The flowing rate of the dry H2 gas was set as a 20 ml/min during the reduction process. The cell was kept 1 h at 500 C, and then the furnace temperature was increased to 700 C with a heating rate of 3C/min and 20 ml/min dry H2 gas flowing rate. The flowing rate of the H2 gas was increased to 40 ml/min after the cell kept 15 min at 700 C. The cell was kept at 700 C for 1 h before the performance tests were performed for 700 C, 750 C, and 800 C. After the performance tests, the temperature of the test furnace was reduced to the room temperature
Fig. 2 e Production process of MT-SOFCs and the photo of cells: a) green anode, b) green anode and electrolyte deposition, c) pre-sintered anode, d) pre-sintered anode and electrolyte deposition, e) sintered electrolyte, f) co-sintered cathodes layers. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Fig. 3 e Preparation of the MT-SOFC for the electrochemical measurements.
at the cooling rate of 5 C/min. During the cooling process, the flow rate of H2 was kept constant at 40 ml/min in order to prevent the oxidation in pre-sintered and co-sintered cells.
Microstructural characterization of the microcells After the performance analysis, the electrolyte coating thickness and microstructure analyses were performed by using a Philips XL30 SFEG scanning electron microscope (SEM). The porosity rate of the anode support layer was determined using the Archimedes method performed in an aqueous environment.
Results and discussion Performance analysis results Fig. 4 illustrates the power generation density of a presintered cell with 3.3 cm2 cathode active surface area (CASA)
at 700 C, 750 C, and 800 C. The maximum power density of 0.251 W/cm2 at 800 C was obtained from the current density of 0.59 A/cm2. The minimum power density of 0.190 W/cm2 at 700 C was obtained from the current density of 0.41 A/cm2. The SEM image of the pre-sintered cell is presented in Fig. 5. Fig. 6 shows the power density of the co-sintered cell, which was produced using the second method and has an active cathode surface area of 3.1 cm2, at 700 C, 750 C, and 800 C. The maximum power density of 0.187 W/cm2 at 800 C was obtained from the current density of 0.380 A/cm2. The minimum power density of 0.126 W/cm2 at 700 C was achieved from the current density of 0.261 A/cm2. The SEM images of the co-sintered cell are presented in Fig. 7. After the electrochemical tests, the porosity and shrinkage percentage values of the cells produced with the pre-sintering and co-sintering methods were calculated. The anode porosity of co-sintered cell was determined as a 27% by using the Archimedean test, whereas the same parameter was measured as a 31% for the pre-sintered cell. The percentage of
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Fig. 4 e Power generation characteristic of the pre-sintered micro-tubular cell with 5.36 mm diameter, 2 cm cathode length, and 3.3 cm2 CASA.
The power density of the MT-SOFC varies according to the diameter of the cell [35], the electrode design and porosity [36], the electrolyte type, the electrolyte thickness and if the electrolyte is at single or bilayer structure [30]. The Performance of the co-sintered and pre-sintered cells, which were produced in this study, can be improved if the electrodes tailored for more gas diffusion. Monzon et al. [37] have reported that 0.5 W/cm2 power density can be obtained at 800 C using NiYSZ supported MT-SOFC which has 6 mm length and 3.2 mm outer diameter. However, we have obtained 0.25 W/ cm2 power density from the pre-sintered cells having 65 mm length and 6.2 mm outer diameter. As compared with the mentioned literature, the dimensions of our cells are larger. Large cell dimensions can, in principle, cause a large increase in the ohmic resistance of the anode electrode which is attributed to the longer path lengths of collected electrons. If the Ni-YSZ support is optimized and the current collection methods for anode is appropriately improved in these dimensions, it will be possible to approach the value of the literature reported.
Fig. 5 e SEM images of the pre-sintered cell after testing; (a) cross section of the cell tube, and (b) wall cross section of the tube.
shrinkage occurring in the external diameter of cells was 19% for co-sintered cell and 16% for the pre-sintered cell. This result arises from the sintering conditions during the tube production process and the microstructure difference. The higher density of the anode layer of the co-sintered cell, when compared to that of the pre-sintered cell, caused the decrease in tree phase boundary (TPB) density and gas diffusion of the anode structure. For this reason, the co-sintered cells showed lower performance than the pre-sintered cells. As it can be seen in SEM microstructure analyses (Figs. 5 and 7), the electrolyte thickness of co-sintered cells was approx. 14 mm, whereas that of pre-sintered cells was approx. 16 mm. When the electrolyte layer was carefully examined, it was observed that the size pinholes on the electrolyte layers of the co-sintered cells (Fig. 7b) were larger than that of presintered cells (Fig. 5b). This caused the oxygen ion to move a long distance from the cathode-electrolyte surface to the anode-electrolyte surface and thus the co-sintered cells showed lower performance.
Fig. 6 e Power generation characteristic of co-sintered YSZ cell with a diameter of 5.05 mm, cathode length of 2 cm, and CASA of 3.1 cm2.
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Fig. 7 e SEM images of the co-sintered cell; (a) cross section of the cell tube, and (b) wall cross section of the tube.
Cost analysis results The unit cell costs of production for five different production volumes by using pre-sintering and co-sintering methods are presented in Fig. 8. Table 2 and Table 3 were used as a reference to build the cost model. It was observed that the increase in production volume reduced the unit cost of cells dramatically, and the production methods at the co-sintered could yield the cost levels below $1. As the number of cells produced at once (the increase in production volume), the difference between the sintering costs of two methods decreases. The difference between the production costs of 10 pre-sintered cells and 10 co-sintered cells was 20.13%, whereas the difference decreases down to 12.2% when the production volume increases to 1000 cells. Decreasing the number of sintering
operations has a positive effect on the cost minimization. However, considering the cell performances, the peak power density of pre-sintered cells is 24.30% higher than that of cosintered cells. The co-sintering production method yielded the desired improvement in production costs. But, because of the resultant porosity structure and the amount of pores, a loss occurred in the cell performance. For instance, in order to achieve the level of power that is generated by 19 presintered cells, 25 co-sintered cells would be needed. This indicates how a cost-advantage is achieved by preferring the pre-sintered cells over the co-sintered cells in establishing a fuel cell system. Thus, even though the production cost of the co-sintering process is more affordable, the pre-sintering method should be preferred in terms of the unit cost of energy because the performance obtained from the co-sintered cells is at a lower level.
Fig. 8 e Unit costs of production at six different production volumes.
Table 2 e The material and energy cost references.
Unit Price ($)
NiO (kg)
8YSZ Coarse (kg)
LDPE (kg)
PMMA (kg)
DBP (L)
MEK ETHANOL (L) (L)
PEG (L)
PVB (kg)
8YSZ Fine (kg)
LSM (kg)
Energy Cost (kWh)
155
110.3
4.48
61.6
45.92
34.72
61.37
32.92
345
1015
0.144
2.5
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Table 3 e Reference energy cost table of individual production steps. Process
Sintering Ball Milling Mixing Extrusion Dip-coating
Mean Power Consumption per Hour kW
4 0.55 0.25 1.8 0.5
The material costs were defined as the sintering and extrusion other than sintering, the mixing and dip-coating, and the other processes. The effects of these three groups on the total cost are presented in Figs. 9 and 10. The sintering cost of pre-sintered cells constitutes 40% of the total cost, whereas the value of the same parameter is 31% for cosintered cells. The material cost ratio was observed to increase for co-sintered cells. The reason for this is the decrease in total cost and energy costs of co-sintered cells and the
Fig. 9 e The effect of the manufacturing parameters on the Pre-sintering method cost.
Process Time (h)
Energy Cost ($)
Pre
Co
Pre
Co
58 16.66 72 1 3
39.5 16.66 72 1 3
334.08 13.19472 25.92 2.592 2.16
227.52 13.19472 25.92 2.592 2.16
relatively increasing proportion of material costs, which did not change because of the constant level of the amount of material used, to the production cost relative.
Conclusions The processes, in which two different methods named presintering and co-sintering ones, were manufactured and the cost analyzing results were compared. The effects of decreasing the number of sintering on the costs and cell performance were examined. In determining the costs, only the energy and material costs were taken into account. No direct or indirect costs other than these costs were taken into consideration. Decreasing the number of sintering reduced the unit cost of the cell by 9% at high production volumes, but the cell performance decreased by 24.30%. By considering only these data, it can be seen that the co-sintering method does not seem suitable. But some points that should be discussed about the cost in the co-sintering method were not examined here. The effect on the total cost of the production may be much more remarkable because of the decrease in labor and management costs due to the increase in the number of cells produced and the increasing production rates. In order to prevent the decrease in cell performance in further studies, it is necessary to perform detailed process optimization and to consider all the factors affecting the production costs, and then to compare two methods again.
Acknowledgments This work was financially supported by the KALE Group under the contract of NewGenSOFC Project. The authors are gratefully acknowledged Ahmet Nazım at the Department of Material Science and Engineering of Gebze Technical University (GTU) for their friendly help.
references
Fig. 10 e The effect of the manufacturing parameters on the Co-sintering method cost.
[1] Guangyao M, Wanyu L, Dingkun P. New solid state fuel cells green power source for 21st century. Ionics 1998;4:451e62. [2] Mahata T, Nair SR, Lenka RK, Sinha PK. Fabrication of Ni-YSZ anode supported tubular SOFC through iso-pressing and cofiring route. Int J Hydrogen Energy 2012;37:3874e82. [3] Pusz J, Mohammadi A, Sammes NM. Fabrication and performance of anode-supported micro-tubular solid oxide fuel cells. J Fuel Cell Sci Technol 2006;3:483e6.
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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[4] Othman MHD, Droushiotis N, Wu Z, Kelsall G, Li K. Novel fabrication technique of hollow fibre support for microtubular solid oxide fuel cells. J Power Sources 2011;196:5035e44. [5] Ahmad SH, Jamil SM, Othman MHD, Rahman MA, Jaafar J, Ismail AF. Co-extruded dual-layer hollow fiber with different electrolyte structure for a high temperature micro-tubular solid oxide fuel cell. Int J Hydrogen Energy 2017;42:9116e24. [6] Dikwal CM, Bujalski W, Kendall K. The effect of temperature gradients on thermal cycling and isothermal ageing of microtubular solid oxide fuel cells. J Power Sources 2009;193:241e8. [7] Othman MHD, Wu Z, Droushiotis N, et al. Single-step fabrication and characterisations of electrolyte/anode duallayer hollow fibres for micro-tubular solid oxide fuel cells. J Membr Sci 2010;351:196e204. [8] Lee S-B, Lim T-H, Song R-H, Shin D-R, Dong S-K. Development of a 700Wanode-supported micro-tubular SOFC stack for APU applications. Int J Hydrogen Energy 2008;33:2330e6. n H, Laguna-Bercero MA, Larrea A, Arias BI, Va rez A, [9] Monzo Levenfeld B. Design of industrially scalable microtubular solid oxide fuel cells based on an extruded support. Int J Hydrogen Energy 2014;39:5470e6. [10] Othman MHD, Droushiotis N, Wu Z, et al. Electrolyte thickness control and its effect on electrolyte/anode duallayer hollow fibres for micro-tubular solid oxide fuel cells. J Membr Sci 2010;365:382e8. [11] Sammes NM, Du Y. Fabrication and characterization of tubular solid oxide fuel cells. Int J Appl Ceram Technol 2007;4(2):89e102. [12] Hsieh W-S, Lin P, Wang S-F. Fabrication of electrolyte supported micro-tubular SOFCs using extrusion and dipcoating. Int J Hydrogen Energy 2013;38:2859e67. [13] Panthi D, Tsutsumi A. Micro-tubular solid oxide fuel cell based on a porous yttria-stabilized zirconia support. Scientific Reports 2014;4(5754):1e6. [14] Zhang L, He HQ, Kwek WR, et al. Fabrication and characterization of anode-supported tubular solid-oxide fuel cells by slip casting and dip coating techniques. J Am Ceram Soc 2009;92:302e10. [15] Sandhu NK, Hanifi AR, Woldnik A, Amiri T, Etsell TH, Luo J, Sarkar Partha. Electrochemical performance of a short tubular solid oxide fuel cell stack at intermediate temperatures. Appl Energy 2016;183:358e68. [16] Navarro ME, Capdevila XG, Morales M, Roa JJ, Segarra M. Manufacturing of anode-supported tubular solid oxide fuel cells by a new shaping technique using aqueous gel-casting. J Power Sources 2012;200:45e52. [17] Dong D, Gao J, Liu X, Meng G. Fabrication of tubular NiO/YSZ anode-support of solid oxide fuel cell by gelcasting. J Power Sources 2007;165:217e23. [18] Howe KS, Thompson GJ, Kendall K. Micro-tubular solid oxide fuel cells and stacks. J Power Sources 2011;196:1677e86. [19] Minh NQ, Takahashi T. Science and technology of ceramic fuel cells. Amsterdam, Netherlands: Elsevier Science B.V.; 1995, ISBN 0-444-89568-X. [20] Singhal SC, Kendal K. High temperature solid oxide fuel cells: fundamentals, design and applications. Oxford, UK: Elsevier Ltd.; 2003, ISBN 1-85617-387-9.
9
ndez-Gonza lez R, Herna ndez E, Savvin S, et al. A novel [21] Ferna microstructured metal-supported solid oxide fuel cell. J Power Sources 2014;272:233e8. [22] Yamaguchi T, Shimizu S, Suzuki T, et al. Design and fabrication of a novel electrode-supported honeycomb SOFC. J Am Ceram Soc 2009;92:S107e11. [23] Suzuki T, Yamaguchi T, Sumi H, Hamamoto K, Fujishiro Y. Microtubular solid-oxide fuel cells for low-temperature operation. Materials Research Society Bulletin 2014;39:805e9. [24] Lee B-T, Rahman AHME, Kim J-H. Novel design of microchanneled tubular solid oxide fuel cells and synthesis using a multipass extrusion process. J Am Ceram Soc 2007;90:1921e5. [25] Kim J-H, Song R-H, Song K-S, et al. Fabrication and characteristics of anode-supported flat-tube solid oxide fuel cell. J Power Sources 2003;122:138e43. [26] Jamil SM, Othman MHD, Rahman MA, et al. Recent fabrication techniques for micro-tubular solid oxide fuel cell support: a review. J Eur Ceram Soc 2015;35:1e22. [27] Thijssen JHJS, Thijssen J. The impact of scale-up and production volume on SOFC manufacturing cost. April 2, 2007. DOE/NETL-XXXX. [28] Takahashi T, Esaka T, Iwahara H. Conduction in Bi2O3-based oxide ion conductors under low oxygen pressure. I. Current blackening of the Bi2O3-Y2O3 electrolyte. J Appl Electrochem 1977;7:299e302. [29] Molenda J, Swierczek K, Zaja˛c W. Functional materials for the IT-SOFC. J Power Sources 2007;173:657e70. [30] Shri Prakash B, Pavitra R, Senthil Kumar S, Aruna ST. Electrolyte bi-layering strategy to improve the performance of an intermediate temperature solid oxide fuel cell: a review. J Power Sources 2018;381:136e55. n H, Laguna-Bercero MA. The influence of the [31] Monzo reducing conditions on the final microstructure and performance of nickel-yttria stabilized zirconia cermets. Electrochim Acta 2016;221:41e7. [32] Lee Y-H, Muroyama H, Matsui T, Eguchi K. Degradation of nickeleyttria-stabilized zirconia anode in solid oxide fuel cells under changing temperature and humidity conditions. J Power Sources 2014;262:451e6. [33] Utigard TA, Wu M, Plascencia G, Marin T. Reduction kinetics of Goro nickel oxide using hydrogen. Chem Eng Sci 2005;60:2061e8. [34] Waldbillig D, Wood A, Ivey DG. Thermal analysis of the cyclic reduction and oxidation behavior of SOFC anodes. Solid State Ion 2005;176:847e59. [35] Cui D, Tu B, Cheng M. Effects of cell geometries on performance of tubular solid oxide fuel cell. J Power Sources 2015;297:419e26. [36] Rabuni MF, Li T, Punmeechao P, Li K. Electrode design for direct-methane micro-tubular solid oxide fuel cell (MTSOFC). J Power Sources 2018;384:287e94. n H, Laguna-Bercero MA. Highly stable microtubular [37] Monzo cells for portable solid oxide fuel cell applications. Electrochim Acta 2016;222:1622e7.
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs € un c, Ali Murat Soydan a, Omer Yıldız b,*, Abdullatif Durg ız Akduman c, Ali Ata c Osman Yag a
Gebze Technical University, Institute of Energy Technologies, TR-41400, Gebze, Kocaeli, Turkey Kocaeli University, Engineering Faculty, Department of Metallurgical and Materials Engineering, TR-41380, Kocaeli, Turkey c Gebze Technical University, Faculty of Engineering, Department of Material Science and Engineering, TR-41400, Gebze, Kocaeli, Turkey b
highlights Production of MT-SOFCs by thermoplastic-extrusion, dip coating, pre-sintering and co-sintering. Microstructural analysis of anode-supported MT-SOFCs in terms of the production processes. The cells produced by co-sintering process have unit cost lower 9% as than the pre-sintered cells. The cells produced by co-sintering have power density lower 24.30% as than the pre-sintered cells.
article info
abstract
Article history:
In the present study, the anode-supported micro-tubular solid oxide fuel cells (MT-SOFCs)
Received 6 June 2019
with an electrolyte thin interlayer were manufactured. The anode support tubes consisting
Received in revised form
of 56 wt% nickel oxide and 44 wt% YSZ (8 mol% yttria (Y2O3) stabilized zirconia (ZrO2)) were
30 July 2019
produced by using the thermo-extrusion method, whereas the electrolyte and cathode
Accepted 19 September 2019
layers were manufactured using the dip-coating method. The half-cells consisting of anode
Available online xxx
and electrolyte were manufactured by using two different methods. In the first method, the
Keywords:
by using the dip-coating method and then exposed to second sintering at 1400 C. In the
Anode-supported micro-tubular
second method, the anode and electrolyte layers were sintered together at 1400 C (co-
SOFC
sintering) in order to produce the half-cells. The half-cells that were produced and then
Performance analysis
coated with cathode solutions by using the dip-coating method and the final cells were
anode-support tubes were pre-sintered at 1200 C, then covered with the electrolyte layer
Thermoplastic extrusion
successfully produced at the end of the sintering at 1150 C. The porosity and shrinkage
Dip-coating
percentage values of these MT-SOFCs differed from each other. The power densities of
Manufacturing cost
these cells were tested at 700 C, 750 C, and 800 C by using H2 gas as fuel and the results of the microstructural and cost analyses were compared. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. € Yıldız). E-mail address: [email protected] (O. https://doi.org/10.1016/j.ijhydene.2019.09.156 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Introduction The anode-supported “micro-tubular solid oxide fuel cells” (MT-SOFCs) were produced by using cold isostatic, co-pressing [1,2], extrusion or co-extrusion [3e12], dip-coating [12e15], and slip casting [14e17], and the performance characteristics were compared in terms of the production method. Although there are many cell designs with different cell geometries [8,15,18e25], the most popular and the most examined geometric structures are the tubular and planar SOFC configurations [3,10,26]. Since the stack production in SOFCs with planar geometry is difficult and complicated, the tubular and micro-tubular SOFCs (MT-SOFCs) are preferred [11,13,26]. Moreover, it is a very important requirement to prevent gas leakages in planar SOFCs. Especially for the high-temperature applications, it is very difficult to establish a stack in a planar configuration for the planar SOFCs [1,2,10,19,26]. The tubular design of the cells might prevent or minimize (at least) the leakage problems and material costs, thus it might become easier to design the simple stacks of tubular cells [1,2,10,19,23,26]. For this reason, the anode-supported tubular SOFCs were produced and analyzed in the present study. The most important limitations against the commercialization of SOFC technology are the high production costs, low performance and productivity, and consequently the high unit cost of energy [11,26,27]. The operational costs and the use of special materials for the operability at high temperatures are the most important factors determining the unit cost of energy [20,27]. Therefore, it is very important to reduce the temperature, to increase the performance of the system and the materials, and thereby to decrease the total unit cost [1,11,18,23]. The reason for preferring the electrolytes working at low temperature is the opportunity of using alternative materials, which have lower costs than the expensive noble metal electrode and interconnect [16,19e21,25]. The ceria- and bismuth-based electrolyte materials were observed to have superior ionic conductance properties when compared to the zirconia derivatives [11]. However, the ceriaand bismuth-oxides are not suitable for long-term use since they transform into metallic form under a low partial pressure of oxygen [19,28,29]. The search for an alternative to 8YSZ from the aspects of stability and cost still continues. Since the ohmic resistance of the electrolyte layer is high, the thickness of the electrolyte should be reduced and it should be produced as a thin film as possible [10,15,24,30]. For this reason, in the cells that contain a thick electrolyte layer with high electrolyte resistance such as 8YSZ, the electrode-supported configurations and mainly the anode-supported tubular cell structures are preferred and examined [2e4,8e10,14e17,23e26]. In the production of SOFCs, generally, three different sintering procedures are employed for the anode, electrolyte, and cathode layers. Regardless of the tubular or planar structure of cells, the support layer is produced and sintered by using the preferred method. The electrolyte and cathode layers are coated by using methods such as spraying or dip-coating. By sintering each layer after the coating procedure, the final cell is obtained. Since reducing the number of sintering operations decreases the costs, the studies were carried out on sintering the different layers together [4,5,7,9e17,23e26]. In this
approach named co-sintering, the anode support layer and electrolyte are sintered together, and then the cathode layer is coated and then re-sintered. Thus, the cell is produced with totally two sintering procedures [4,5,7,9e17,23e26]. In the literature, the studies carried out by using this method focused on the optimization of processes and the performances of cells, but there is no detailed study on the effect on unit cell costs. In the present study, thermoplastic extrusion method has been preferred in the production of anode supported tubes since the thermoplastic extrusion method has the advantages of being a fast and continuous process for mass production. The anode microtubes of the anode-supported MT-SOFCs containing 56 wt% NiO and 34 wt% YSZ were produced using the thermoplastic extrusion method, and then the electrolyte and cathode layers were coated using the dip-coating method. The performances of the pre-sintered and co-sintered cells, which had been sintered in three steps and two steps, were measured using hydrogen (H2) gas as fuel, whereas the SEM method was used to examine their microstructures. The effects of implemented production methods on the cell performances and unit costs were examined.
Experimental procedures All of the analyses, examinations, and tests performed within the scope of this study were carried out in a pilot-scale SOFC production facility founded within the body of NanoTechnology Center of Gebze Technical University’s (GTU) Department of Material Science and Engineering, Faculty of Engineering. In the production of cells, the extrusion, dipcoating, pre-sintering, and co-sintering methods were used. The same devices were used in two cell fabrication methods, and the cells were produced by using the same amount of the materials. Taking the energy and material costs borne during the production phase and ignoring the labor cost, initial investment cost, and the other direct and indirect costs, total production costs were calculated for different production volumes. In the sintering furnace, maximum 1000 cells having 8 cm length and 6.2 mm outer diameter can be sintered at once. This number was set to be the upper limit of cell production. Five different sets of cell numbers as 10, 50, 100, 500, and 1000 cells were determined. By separately measuring the power consumption of machines used in the production, the energy costs were calculated. The material costs were determined over the total amount of the used material by taking the amount of wastes into account.
Materials MT-SOFCs consist of “nickel oxide (NiO) and 8 mol% yttriastabilized zirconia” (NiO-8YSZ) for the anode support, 8YSZ as the electrolyte, lanthanum strontium manganite (LSM) and YSZ (LSM-YSZ) as the cathode functional layer and LSM for the cathode current collection layer. The anode tubes were made of nickel (II) oxide (NiO) powders with a particle size of 1e2 mm and a specific surface area (SSA) of 3e4 m2 g 1 obtained from Hart Materials, and 13 wt% yttria-stabilized zirconia (which is equal to 8 mol% YSZ, 8YSZ) powders with a particle size of
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Table 1 e Compositions of the cathode and electrolyte slurries. Compositions, wt%
LSM
8YSZ
PVB
DBP
PEG 400
Binary Solvent
Cathode functional Slurry Cathode Current Collector Slurry Electrolyte Slurry
16.4 32.8 e
16.4 e 30
0.1 0.1 0.1
0.05 0.05 0.05
0.05 0.05 0.05
M.E.K-Ethanol M.E.K-Ethanol M.E.K-Ethanol
3e5 mm obtained from IMERYS Fused Material Laufenberg GmbH. Moreover, polymethylmethacrylate (PMMA) powders with an average particle size of <80 mm and polyethylene (PE) with a low density from Alfa Aesar were used as a pore former and binder, respectively. 8YSZ with an SSA of 13.4 m2 g 1 and LSM powders with an SSA of 10.8 m2 g 1 from Fuel Cell Materials were used for electrolyte and cathode slurries, respectively. Polyvinyl Butyral (PVB) and Dibutyl phthalate from Sigma-Aldrich and polyethylene glycol (PEG 400) from Alfa Aesar were used as the binder, plasticizer, and lubricant, respectively. Ethyl methyl ketone (MEK, 99%) and ethanol (99%) from Sigma-Aldrich were mixed and used as the solvent for the coating slurries.
Production of anode-supported micro-tubular cells The anode support layer of the MT-SOFCs was made of starting powders of NiO and YSZ. NiO and YSZ powders’ homogeneity was ensured by grinding with a ball mill for 48 h in a solvent consisting of ethanol and toluene. After 48 h, 5.45 wt % PE and 4.55 wt% PMMA were added to the anode composition to perform the production of the microtube with the extruder device and to obtain a more porous microstructure in the anode support layer. In order to ensure the homogeneity of anode composition consisting of polymer and powders, the grinding operation was continued for 24 h more. After 72 h of mixing procedure, the mixture was poured into a glass container and then air-dried at 80 C. The dried powders containing 56 wt% NiO and 34 wt% YSZ were ground by using the mortar. When the mixture was prepared, it was poured into the mouth of the thermoplastic extruder. The anode support tubes were extruded by using a single-screw extruder made by Rondol Technology. The extruded anode tubes were cut into 65 mm length. The dimensions of anode support tubes were 6.2 mm (external diameter), 4.35 mm (internal diameter), and 0.85 mm (wall thickness).
Preparing the cathode slurries The cathode layer consists of two layers named cathode functional layer and cathode current collector layer. The cathode functional solution was prepared using 16.4% (wt.) 8YSZ and 16.4% LSM, as presented in Table 1. The cathode collection layer was prepared using 32.8% (wt.) LSM. While preparing the cathode solutions, the procedures followed in electrolyte solutions were repeated directly.
Production of anode-supported micro-tubular SOFCs In this study, the half-cells consisting of the anode support and the electrolyte layer were manufactured by using two different methods. In the first method, the anode support tubes cut into 65 mm length after the extrusion process were exposed to 2 h of pre-sintering at 1100 C under air, and then the coating process of the electrolyte layer was initiated. The pre-sintered tubes were coated by dipping them into electrolyte solution at 60 mm/s of pulling rate by using the dipcoating method. In order to remove the solvent in the electrolyte film, they were air-dried for 10 min at 80 C. In order to achieve the desired thickness of electrolyte, this procedure was repeated twice. After the electrolyte-coating procedure, the cells were sintered for 3 h under air at 1400 C and the halfcells were obtained. In the second method, the anode-support tubes cut after the extrusion were exposed to the electrolyte coating process of the first method but without any pre-sintering process. Then, the half-cells consisting of anode support and the
Preparing the electrolyte slurry In order to prepare a homogenous electrolyte solution, the initial materials consisting of YSZ powders, polyvinyl butyral (PVB), PEG-400, and dibutyl phthalate (DBP) were ground by using the planetary mill for 15 h at 350 rpm within an ethyl methyl ketone (MEK) and ethanol solvent. In order to prevent the over-heating during the grinding process, the planetary mill was stopped for 20 min after every 25 min of the grinding process. Before the coating process, the electrolyte solution was exposed to an ultrasonic bath for 10 min and an ultrasonic horn for 1 min. The compositions of electrolyte and cathode slurries are presented in Table 1.
Fig. 1 e Sintering regimes of the anode, electrolyte, and cathode layers.
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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electrolyte layer were sintered together (co-sintering) and the half-cells were obtained. The tips of half-cells produced by using two different methods were coated with paraffin before the cathodecoating process in order to prevent the contact between anode and cathode layers. The half-cells were firstly coated with cathode functional solution consisting of LSM and YSZ by using the dip-coating method at 60 mm/s pulling rate and then kept at 80 C for 10 min for the coating to be dried. Finally, the half-cells coated with LSM-YSZ were submerged into cathode current collection solution consisting of LSM by using the dip-coating method at the pulling rate of 60 mm/s and the coating procedure was completed. The cathode functional layer and the cathode current collection layer were co-sintered at 1100 C. Sintering regimes of the presintered anode, electrolyte, and cathode were showed in detail in Fig. 1. The production process of the anodesupported MT-SOFCs and the images of products are shown in Fig. 2.
Preparing the cells for testing and electrochemical measurements The cathode surface and the anode current collection points were coated with a silver paste (DAD-87) by using a brush. In order to collect the current forming in the anode and cathode surfaces, the silver wire with 0.5 mm diameter (Alfa Aesar, 0.5 mm) was used. The cell to be tested was wrapped using silver wire and then placed into a furnace. Then the alumina tubes ensuring the gas inlet and outlet were connected as shown in Fig. 3. The connection between cell and alumina tube was established by using Ceramabond-569 and
Ceramabond-552-VFG sealing elements in order to prevent fuel leakage at the connection point. The anode and cathode current collecting silver wires were connected to RBL-4881006-400 test device in order to obtain the voltage-current characteristic curves (RBL488 100-60-400, TDI Power Corp., New Jersey, USA). The temperature of the reduction process and flow rate of the reducing gas have a significant effect on the transformation of NiO-YSZ to Ni-YSZ. The reduction of nickel oxide to metallic nickel results in a volume change of 40%. If the volume change during the reduction process is rapid, it causes micro-cracks in the anode structure. These cracks both reduce the mechanical strength of the cell and cause cracking of the electrolyte layer and may cause a short circuit of the cells. In the literature studies, it was observed that deformations in the cell occur more in reducing condition performed in high temperature and humid atmosphere [31,32]. To eliminate deformation possibility during the reduction process low temperature reducing condition was preferred in this study. The details about reducing condition performed at low temperature for NiO, NiO-YSZ are given in the literatures [33,34]. n this study, the reduction process of the anode side was initiated by introducing 100% dry H2 gas at 500 C. The flowing rate of the dry H2 gas was set as a 20 ml/min during the reduction process. The cell was kept 1 h at 500 C, and then the furnace temperature was increased to 700 C with a heating rate of 3C/min and 20 ml/min dry H2 gas flowing rate. The flowing rate of the H2 gas was increased to 40 ml/min after the cell kept 15 min at 700 C. The cell was kept at 700 C for 1 h before the performance tests were performed for 700 C, 750 C, and 800 C. After the performance tests, the temperature of the test furnace was reduced to the room temperature
Fig. 2 e Production process of MT-SOFCs and the photo of cells: a) green anode, b) green anode and electrolyte deposition, c) pre-sintered anode, d) pre-sintered anode and electrolyte deposition, e) sintered electrolyte, f) co-sintered cathodes layers. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Fig. 3 e Preparation of the MT-SOFC for the electrochemical measurements.
at the cooling rate of 5 C/min. During the cooling process, the flow rate of H2 was kept constant at 40 ml/min in order to prevent the oxidation in pre-sintered and co-sintered cells.
Microstructural characterization of the microcells After the performance analysis, the electrolyte coating thickness and microstructure analyses were performed by using a Philips XL30 SFEG scanning electron microscope (SEM). The porosity rate of the anode support layer was determined using the Archimedes method performed in an aqueous environment.
Results and discussion Performance analysis results Fig. 4 illustrates the power generation density of a presintered cell with 3.3 cm2 cathode active surface area (CASA)
at 700 C, 750 C, and 800 C. The maximum power density of 0.251 W/cm2 at 800 C was obtained from the current density of 0.59 A/cm2. The minimum power density of 0.190 W/cm2 at 700 C was obtained from the current density of 0.41 A/cm2. The SEM image of the pre-sintered cell is presented in Fig. 5. Fig. 6 shows the power density of the co-sintered cell, which was produced using the second method and has an active cathode surface area of 3.1 cm2, at 700 C, 750 C, and 800 C. The maximum power density of 0.187 W/cm2 at 800 C was obtained from the current density of 0.380 A/cm2. The minimum power density of 0.126 W/cm2 at 700 C was achieved from the current density of 0.261 A/cm2. The SEM images of the co-sintered cell are presented in Fig. 7. After the electrochemical tests, the porosity and shrinkage percentage values of the cells produced with the pre-sintering and co-sintering methods were calculated. The anode porosity of co-sintered cell was determined as a 27% by using the Archimedean test, whereas the same parameter was measured as a 31% for the pre-sintered cell. The percentage of
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Fig. 4 e Power generation characteristic of the pre-sintered micro-tubular cell with 5.36 mm diameter, 2 cm cathode length, and 3.3 cm2 CASA.
The power density of the MT-SOFC varies according to the diameter of the cell [35], the electrode design and porosity [36], the electrolyte type, the electrolyte thickness and if the electrolyte is at single or bilayer structure [30]. The Performance of the co-sintered and pre-sintered cells, which were produced in this study, can be improved if the electrodes tailored for more gas diffusion. Monzon et al. [37] have reported that 0.5 W/cm2 power density can be obtained at 800 C using NiYSZ supported MT-SOFC which has 6 mm length and 3.2 mm outer diameter. However, we have obtained 0.25 W/ cm2 power density from the pre-sintered cells having 65 mm length and 6.2 mm outer diameter. As compared with the mentioned literature, the dimensions of our cells are larger. Large cell dimensions can, in principle, cause a large increase in the ohmic resistance of the anode electrode which is attributed to the longer path lengths of collected electrons. If the Ni-YSZ support is optimized and the current collection methods for anode is appropriately improved in these dimensions, it will be possible to approach the value of the literature reported.
Fig. 5 e SEM images of the pre-sintered cell after testing; (a) cross section of the cell tube, and (b) wall cross section of the tube.
shrinkage occurring in the external diameter of cells was 19% for co-sintered cell and 16% for the pre-sintered cell. This result arises from the sintering conditions during the tube production process and the microstructure difference. The higher density of the anode layer of the co-sintered cell, when compared to that of the pre-sintered cell, caused the decrease in tree phase boundary (TPB) density and gas diffusion of the anode structure. For this reason, the co-sintered cells showed lower performance than the pre-sintered cells. As it can be seen in SEM microstructure analyses (Figs. 5 and 7), the electrolyte thickness of co-sintered cells was approx. 14 mm, whereas that of pre-sintered cells was approx. 16 mm. When the electrolyte layer was carefully examined, it was observed that the size pinholes on the electrolyte layers of the co-sintered cells (Fig. 7b) were larger than that of presintered cells (Fig. 5b). This caused the oxygen ion to move a long distance from the cathode-electrolyte surface to the anode-electrolyte surface and thus the co-sintered cells showed lower performance.
Fig. 6 e Power generation characteristic of co-sintered YSZ cell with a diameter of 5.05 mm, cathode length of 2 cm, and CASA of 3.1 cm2.
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
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Fig. 7 e SEM images of the co-sintered cell; (a) cross section of the cell tube, and (b) wall cross section of the tube.
Cost analysis results The unit cell costs of production for five different production volumes by using pre-sintering and co-sintering methods are presented in Fig. 8. Table 2 and Table 3 were used as a reference to build the cost model. It was observed that the increase in production volume reduced the unit cost of cells dramatically, and the production methods at the co-sintered could yield the cost levels below $1. As the number of cells produced at once (the increase in production volume), the difference between the sintering costs of two methods decreases. The difference between the production costs of 10 pre-sintered cells and 10 co-sintered cells was 20.13%, whereas the difference decreases down to 12.2% when the production volume increases to 1000 cells. Decreasing the number of sintering
operations has a positive effect on the cost minimization. However, considering the cell performances, the peak power density of pre-sintered cells is 24.30% higher than that of cosintered cells. The co-sintering production method yielded the desired improvement in production costs. But, because of the resultant porosity structure and the amount of pores, a loss occurred in the cell performance. For instance, in order to achieve the level of power that is generated by 19 presintered cells, 25 co-sintered cells would be needed. This indicates how a cost-advantage is achieved by preferring the pre-sintered cells over the co-sintered cells in establishing a fuel cell system. Thus, even though the production cost of the co-sintering process is more affordable, the pre-sintering method should be preferred in terms of the unit cost of energy because the performance obtained from the co-sintered cells is at a lower level.
Fig. 8 e Unit costs of production at six different production volumes.
Table 2 e The material and energy cost references.
Unit Price ($)
NiO (kg)
8YSZ Coarse (kg)
LDPE (kg)
PMMA (kg)
DBP (L)
MEK ETHANOL (L) (L)
PEG (L)
PVB (kg)
8YSZ Fine (kg)
LSM (kg)
Energy Cost (kWh)
155
110.3
4.48
61.6
45.92
34.72
61.37
32.92
345
1015
0.144
2.5
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
8
international journal of hydrogen energy xxx (xxxx) xxx
Table 3 e Reference energy cost table of individual production steps. Process
Sintering Ball Milling Mixing Extrusion Dip-coating
Mean Power Consumption per Hour kW
4 0.55 0.25 1.8 0.5
The material costs were defined as the sintering and extrusion other than sintering, the mixing and dip-coating, and the other processes. The effects of these three groups on the total cost are presented in Figs. 9 and 10. The sintering cost of pre-sintered cells constitutes 40% of the total cost, whereas the value of the same parameter is 31% for cosintered cells. The material cost ratio was observed to increase for co-sintered cells. The reason for this is the decrease in total cost and energy costs of co-sintered cells and the
Fig. 9 e The effect of the manufacturing parameters on the Pre-sintering method cost.
Process Time (h)
Energy Cost ($)
Pre
Co
Pre
Co
58 16.66 72 1 3
39.5 16.66 72 1 3
334.08 13.19472 25.92 2.592 2.16
227.52 13.19472 25.92 2.592 2.16
relatively increasing proportion of material costs, which did not change because of the constant level of the amount of material used, to the production cost relative.
Conclusions The processes, in which two different methods named presintering and co-sintering ones, were manufactured and the cost analyzing results were compared. The effects of decreasing the number of sintering on the costs and cell performance were examined. In determining the costs, only the energy and material costs were taken into account. No direct or indirect costs other than these costs were taken into consideration. Decreasing the number of sintering reduced the unit cost of the cell by 9% at high production volumes, but the cell performance decreased by 24.30%. By considering only these data, it can be seen that the co-sintering method does not seem suitable. But some points that should be discussed about the cost in the co-sintering method were not examined here. The effect on the total cost of the production may be much more remarkable because of the decrease in labor and management costs due to the increase in the number of cells produced and the increasing production rates. In order to prevent the decrease in cell performance in further studies, it is necessary to perform detailed process optimization and to consider all the factors affecting the production costs, and then to compare two methods again.
Acknowledgments This work was financially supported by the KALE Group under the contract of NewGenSOFC Project. The authors are gratefully acknowledged Ahmet Nazım at the Department of Material Science and Engineering of Gebze Technical University (GTU) for their friendly help.
references
Fig. 10 e The effect of the manufacturing parameters on the Co-sintering method cost.
[1] Guangyao M, Wanyu L, Dingkun P. New solid state fuel cells green power source for 21st century. Ionics 1998;4:451e62. [2] Mahata T, Nair SR, Lenka RK, Sinha PK. Fabrication of Ni-YSZ anode supported tubular SOFC through iso-pressing and cofiring route. Int J Hydrogen Energy 2012;37:3874e82. [3] Pusz J, Mohammadi A, Sammes NM. Fabrication and performance of anode-supported micro-tubular solid oxide fuel cells. J Fuel Cell Sci Technol 2006;3:483e6.
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156
international journal of hydrogen energy xxx (xxxx) xxx
[4] Othman MHD, Droushiotis N, Wu Z, Kelsall G, Li K. Novel fabrication technique of hollow fibre support for microtubular solid oxide fuel cells. J Power Sources 2011;196:5035e44. [5] Ahmad SH, Jamil SM, Othman MHD, Rahman MA, Jaafar J, Ismail AF. Co-extruded dual-layer hollow fiber with different electrolyte structure for a high temperature micro-tubular solid oxide fuel cell. Int J Hydrogen Energy 2017;42:9116e24. [6] Dikwal CM, Bujalski W, Kendall K. The effect of temperature gradients on thermal cycling and isothermal ageing of microtubular solid oxide fuel cells. J Power Sources 2009;193:241e8. [7] Othman MHD, Wu Z, Droushiotis N, et al. Single-step fabrication and characterisations of electrolyte/anode duallayer hollow fibres for micro-tubular solid oxide fuel cells. J Membr Sci 2010;351:196e204. [8] Lee S-B, Lim T-H, Song R-H, Shin D-R, Dong S-K. Development of a 700Wanode-supported micro-tubular SOFC stack for APU applications. Int J Hydrogen Energy 2008;33:2330e6. n H, Laguna-Bercero MA, Larrea A, Arias BI, Va rez A, [9] Monzo Levenfeld B. Design of industrially scalable microtubular solid oxide fuel cells based on an extruded support. Int J Hydrogen Energy 2014;39:5470e6. [10] Othman MHD, Droushiotis N, Wu Z, et al. Electrolyte thickness control and its effect on electrolyte/anode duallayer hollow fibres for micro-tubular solid oxide fuel cells. J Membr Sci 2010;365:382e8. [11] Sammes NM, Du Y. Fabrication and characterization of tubular solid oxide fuel cells. Int J Appl Ceram Technol 2007;4(2):89e102. [12] Hsieh W-S, Lin P, Wang S-F. Fabrication of electrolyte supported micro-tubular SOFCs using extrusion and dipcoating. Int J Hydrogen Energy 2013;38:2859e67. [13] Panthi D, Tsutsumi A. Micro-tubular solid oxide fuel cell based on a porous yttria-stabilized zirconia support. Scientific Reports 2014;4(5754):1e6. [14] Zhang L, He HQ, Kwek WR, et al. Fabrication and characterization of anode-supported tubular solid-oxide fuel cells by slip casting and dip coating techniques. J Am Ceram Soc 2009;92:302e10. [15] Sandhu NK, Hanifi AR, Woldnik A, Amiri T, Etsell TH, Luo J, Sarkar Partha. Electrochemical performance of a short tubular solid oxide fuel cell stack at intermediate temperatures. Appl Energy 2016;183:358e68. [16] Navarro ME, Capdevila XG, Morales M, Roa JJ, Segarra M. Manufacturing of anode-supported tubular solid oxide fuel cells by a new shaping technique using aqueous gel-casting. J Power Sources 2012;200:45e52. [17] Dong D, Gao J, Liu X, Meng G. Fabrication of tubular NiO/YSZ anode-support of solid oxide fuel cell by gelcasting. J Power Sources 2007;165:217e23. [18] Howe KS, Thompson GJ, Kendall K. Micro-tubular solid oxide fuel cells and stacks. J Power Sources 2011;196:1677e86. [19] Minh NQ, Takahashi T. Science and technology of ceramic fuel cells. Amsterdam, Netherlands: Elsevier Science B.V.; 1995, ISBN 0-444-89568-X. [20] Singhal SC, Kendal K. High temperature solid oxide fuel cells: fundamentals, design and applications. Oxford, UK: Elsevier Ltd.; 2003, ISBN 1-85617-387-9.
9
ndez-Gonza lez R, Herna ndez E, Savvin S, et al. A novel [21] Ferna microstructured metal-supported solid oxide fuel cell. J Power Sources 2014;272:233e8. [22] Yamaguchi T, Shimizu S, Suzuki T, et al. Design and fabrication of a novel electrode-supported honeycomb SOFC. J Am Ceram Soc 2009;92:S107e11. [23] Suzuki T, Yamaguchi T, Sumi H, Hamamoto K, Fujishiro Y. Microtubular solid-oxide fuel cells for low-temperature operation. Materials Research Society Bulletin 2014;39:805e9. [24] Lee B-T, Rahman AHME, Kim J-H. Novel design of microchanneled tubular solid oxide fuel cells and synthesis using a multipass extrusion process. J Am Ceram Soc 2007;90:1921e5. [25] Kim J-H, Song R-H, Song K-S, et al. Fabrication and characteristics of anode-supported flat-tube solid oxide fuel cell. J Power Sources 2003;122:138e43. [26] Jamil SM, Othman MHD, Rahman MA, et al. Recent fabrication techniques for micro-tubular solid oxide fuel cell support: a review. J Eur Ceram Soc 2015;35:1e22. [27] Thijssen JHJS, Thijssen J. The impact of scale-up and production volume on SOFC manufacturing cost. April 2, 2007. DOE/NETL-XXXX. [28] Takahashi T, Esaka T, Iwahara H. Conduction in Bi2O3-based oxide ion conductors under low oxygen pressure. I. Current blackening of the Bi2O3-Y2O3 electrolyte. J Appl Electrochem 1977;7:299e302. [29] Molenda J, Swierczek K, Zaja˛c W. Functional materials for the IT-SOFC. J Power Sources 2007;173:657e70. [30] Shri Prakash B, Pavitra R, Senthil Kumar S, Aruna ST. Electrolyte bi-layering strategy to improve the performance of an intermediate temperature solid oxide fuel cell: a review. J Power Sources 2018;381:136e55. n H, Laguna-Bercero MA. The influence of the [31] Monzo reducing conditions on the final microstructure and performance of nickel-yttria stabilized zirconia cermets. Electrochim Acta 2016;221:41e7. [32] Lee Y-H, Muroyama H, Matsui T, Eguchi K. Degradation of nickeleyttria-stabilized zirconia anode in solid oxide fuel cells under changing temperature and humidity conditions. J Power Sources 2014;262:451e6. [33] Utigard TA, Wu M, Plascencia G, Marin T. Reduction kinetics of Goro nickel oxide using hydrogen. Chem Eng Sci 2005;60:2061e8. [34] Waldbillig D, Wood A, Ivey DG. Thermal analysis of the cyclic reduction and oxidation behavior of SOFC anodes. Solid State Ion 2005;176:847e59. [35] Cui D, Tu B, Cheng M. Effects of cell geometries on performance of tubular solid oxide fuel cell. J Power Sources 2015;297:419e26. [36] Rabuni MF, Li T, Punmeechao P, Li K. Electrode design for direct-methane micro-tubular solid oxide fuel cell (MTSOFC). J Power Sources 2018;384:287e94. n H, Laguna-Bercero MA. Highly stable microtubular [37] Monzo cells for portable solid oxide fuel cell applications. Electrochim Acta 2016;222:1622e7.
Please cite this article as: Soydan AM et al., Production, performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.156