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Manufacturing method for tubular molten carbonate fuel cells and basic cell performance
Journal of Power Sources 285 (2015) 260e265
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Manufacturing method for tubular molten carbonate fuel cells and basic cell performance Makoto Kawase Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa 240-0196, Japan
h i g h l i g h t s A method of manufacturing a tubular MCFC was developed. The key to fastening the cell is the self-shrinking effect of the anode during sintering. The fastening force exerted by the anode is maintained. The generation performance of a tubular MCFC is close to that of a planar MCFC. The tubular MCFC can start from a cold-stopped condition in a few hours.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2014 Received in revised form 7 March 2015 Accepted 18 March 2015 Available online 19 March 2015
The combination of syngas from gasification and high-temperature fuel cells is a candidate for highefficiency power generation systems. Reducing the production cost of fuel cells and gas-cleaning devices is an important issue for commercial application. This study focuses on molten carbonate fuel cells (MCFCs), which are relatively durable against poisoning by impurities in syngas. However, the development of MCFC systems has come to a halt in Japan because the production cost of MCFCs made them commercially infeasible. To reduce the production cost significantly, a tubular MCFC has been developed instead of the conventional planar type. The tubular MCFC requires neither a complex separator nor cell components with high dimensional accuracy. However, there have been no reports about tubular MCFCs because the electrolytes used for these MCFCs are liquid, which makes it difficult to fasten the fuel cell stack without a fastener. In this study, a fastening method is developed by using the self-shrinking effect of anodes during sintering. Using this technique, the tubular MCFC was successfully manufactured. The results of a power generation test for 1000 h show that the cell voltage was kept stable. Moreover, the cell performance was close to that of a conventional planar MCFC. © 2015 Elsevier B.V. All rights reserved.
Keywords: Molten carbonate fuel cell Tubular type Sintered materials Gasification Fastening method
1. Introduction Power generation systems that use gasification and fuel cells are expected to find use in practical applications because highefficiency power generation is provided by the integration of a coal- or biomass-gasification power generation system with fuel cells. Molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs), in which carbon monoxide can also be used as a fuel, are potential technologies for this purpose. Fuel cell manufacturing costs and gas purification costs are issues that must be addressed for practical implementation of these systems. Abundant hydrogen sulfide, hydrogen chloride, and other gaseous impurities are
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jpowsour.2015.03.117 0378-7753/© 2015 Elsevier B.V. All rights reserved.
contained in the product of coal gasification, biomass gasification, and the like. This necessitates gas purification to an acceptable concentration for use in the fuel cell. The purification cost varies according to the desired level of gas purity. The present study focuses on MCFCs, which are relatively robust in the presence of gaseous impurities. Although there are few successful applications of gasification gas in MCFCs [1,2], several successful applications of digestion gas as a fuel have been reported [3,4]. The digestion gas is primarily methane and is used in one type of MCFC: the internal reforming MCFC. The gasification gas is primarily H2 and CO (syngas) and is used in another type of MCFC: the external reforming MCFC. Thus, the external reforming MCFC is selected in order to use gasification gases as fuel. Presently, internal reforming MCFCs are being developed and manufactured in the United States, Korea, and other countries [3,5],
M. Kawase / Journal of Power Sources 285 (2015) 260e265
and the equivalent of 300 MW have been shipped, but almost no development of external reforming MCFCs is being carried out. The suspension of the development of external reforming MCFC is probably because the manufacturing cost was unacceptable to the market [6]. The breakdown of the manufacturing cost for an external reforming MCFC stack developed in Japan in 2005 was reported as that shown in Fig. 1 [7]. In the breakdown, cell components (the anode, cathode, electrolyte, electrolyte matrix, and separator) account for over half the cost, with the separator alone accounting for a quarter of the cost. One reason for the high cost is the requirement for micrometer-scale precision in dimensional accuracy when manufacturing the cell components, and the correspondingly low yield rate. This requirement for high dimensional accuracy results from the low strength of the electrolyte matrix, which in turn is affected by the anode, cathode, and separator connected to it (via thermal expansion, roughness, and so on), with variations from specification resulting in cracking and other problems. It is also necessary to take the time to raise the temperature during start up due to the effect of stress on the electrolyte matrix, which means that it takes time for the cell to reach its operating temperature [8]. Because of these factors, a high precision is currently required for the separator and other components in an MCFC, which increases the manufacturing cost. Turning our attention to the structure of the SOFC currently under development, tubular types exist in addition to the planar types, and research leveraging the benefits of the tubular type is advancing. If a tubular MCFC could be made, then a complex separator would be unnecessary, which could be expected to greatly ease the dimensional accuracy requirements for the cell components, drastically increase the yield rate, and thereby significantly decrease manufacturing costs. Additionally, the influence of one cell on other cells is small because each cell is separated from the others in a tubular MCFC. Therefore, a shorter start up time can be expected in comparison with a planar MCFC. Furthermore, it is possible to replace individual cells that are damaged in a tubular MCFC. However, tubular MCFCs have not yet been reported. This is because the electrolyte for an MCFC cannot be baked onto the electrode as it can in an SOFC, since the electrolyte is a liquid (molten carbonate) and also there is high contact resistance when the electrode and the electrolyte matrix are simply layered. This makes it necessary to fasten them with some kind of force. In the current planar types, these are secured by a fastener at the top of the stack. In this report, we consider the feasibility of a tubular MCFC with a low manufacturing cost that can overcome these issues and reduce start up time.
261
2. Consideration of a tubular MCFC fastening method A structure with the cathode on the outside of the cylinder and one with the anode on the outside are both feasible types of tubular MCFC structure. The following fastening methods may reduce the contact pressure between the cathode, matrix, and anode. Fastening by external force Fastening by powder sintering during electrode manufacture
2.1. Fastening by external force Fastening by a tubular metal band, such as a hose band or by a clamp, is a feasible method of fastening by external force. Since it is not possible to cover the entire electrode with a clamp, it is necessary to carry out the fastening with a punching plate as a supporting plate. The issue with this method is that highly accurate material manufacturing is necessary because the fastening force may be concentrated in the vicinity of the clamp. Moreover, because a fastener is necessary for each cell, this method may not be practical from a cost perspective for a stack consisting of several hundred cells. 2.2. Fastening by powder sintering effect during electrode manufacture Both the anode and the cathode are made from Ni-based powders, and electric resistance makes it necessary to sinter them for use as suitable electrodes. It is possible to fasten the inner layers (the electrolyte matrix) through the joining and shrinkage of Ni powder particles that occurs during sintering. A normal cathode is formed from Li-doped NiO through reacting with lithium carbonate, potassium carbonate, sodium carbonate, and other electrolytes in an oxidizing atmosphere after sintering the Ni-based powder. For this reason, when the cathode is on the outside, most of the fastening strength is lost because the powder sintered under oxidizing conditions is dispersed. In contrast, with the anode on the outside, it is highly likely that the fastening force will be maintained, because the coupling of the individual powder grains is maintained so long as the anode is kept in a reducing atmosphere. On the basis of the above considerations, we performed research on a tubular MCFC with the anode on the outside, using the fastening method given by the sintering effect of the Ni powder of the anode material. 3. Manufacture of a tubular MCFC
Fig. 1. Stack cost of MCFC developed in Japan [7].
Fig. 2 shows the manufacturing process for a tubular MCFC. First, the SUS316L punching pipe shown in Fig. 2(a) was used as a tubular separator to shield the gas. This punching part has an outer diameter of 7.9 mm, an inner diameter of 6.0 mm, and a length of 33 mm. The outer diameter of each end is 9.6 mm. The aperture ratio of the punching part is 70%. The dimensional accuracy of the deviation from circularity of the punching pipe is ±0.03 mm, which does not satisfy the accuracy required by the separator in a planar MCFC. The cathode, anode, and electrolyte matrix members are the same as those used in a planar MCFC [9,10], and water, methylcellulose, and glycerin were mixed to form a slurry in the same way as for a planar MCFC. The viscosities of these slurries are adjusted by water to apply without dripping. Table 1 gives the specifications of each member. The thicknesses of the cathode, anode, and electrolyte matrix refer to those in a planar MCFC. Second, the Ni powder slurry for the cathode was applied to the
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M. Kawase / Journal of Power Sources 285 (2015) 260e265
Fig. 2. Manufacturing process of a tubular MCFC.
Table 1 Materials of cell components.
Cathode Electrolyte matrix Anode
Material
Mean particle diameter
Viscosity of slurry
Porosity after sintering
Ni-3%MgCO3 a-LiAlO2 Ni-2%AlCr
2.2e2.8 mm e 2.6e3.3 mm
15e20 Pa s 40e55 Pa s 15e20 Pa s
68e70 % 53e55 % 63e65 %
trough section while rotating the tubular separator; the outer diameter of the cathode was made approximately 9.7 mm. Thereafter, the slurry was sintered for 1 h at 900 C in an atmosphere of N2/H2 ¼ 93%/7%. The outer diameter contracted to 9.5 mm during sintering. Contraction also occurred in the axial direction, and a gap of approximately 1 mm was produced in the trough section, as shown by Fig. 2(c). Third, a LiAlO2 powder slurry for the electrolyte matrix was applied over the cathode to an outer diameter of 12.0 mm. At this point, the LiAlO2 powder slurry filled the trough gap produced during cathode sintering. After drying the LiAlO2 powder slurry, a Ni powder slurry for the anode was applied to an outer diameter of 14.6 mm. After drying, it was sintered for 3 h at 900 C in an atmosphere of N2/H2/H2O ¼ 37%/3%/60%. After sintering, the anode outer diameter was approximately 14.4 mm. The diameter contracted by approximately 0.2 mm, and a pressure was exerted in the circumferential direction, that is, on the electrolyte matrix, due to this contraction. This contraction of the anode serves the function of a fastener. There were no control values for the thickness of the cathode, electrolyte matrix, or the anode, and strict accuracy management was not necessary, but the thicknesses must be such that a fastening effect is provided by the anode. The permissible range of this thickness is currently under investigation. Finally, for the electrolyte, a slurry of carbonate powder in ethanol was applied to the surface of cathode and anode. The electrolyte was loaded into the electrolyte matrix and the electrodes at 480e650 C in a reducing atmosphere. The electrolyte
was a carbonate mixture of Li2CO3/K2CO3 ¼ 62%/38%. The melting point of this carbonate mixture is 488 C. The quantity of electrolyte was set such that the electrolyte packing ratios (volume of pores filled with electrolyte relative to total pore volume) of the anode, cathode, and electrolyte matrix were 30e50%, 30e50%, and 100%, respectively. 4. Power generation test A power generation test was performed with the tubular MCFC manufactured by the above method in order to confirm its ability to generate power. 4.1. Power generation test apparatus Fig. 3 shows a schematic illustration of the power generation test apparatus and a photograph of a cell. The internal dimensions of the electric furnace were internal diameter 140 mm and height 500 mm. The temperature of the electric furnace was controlled such that the temperature near the cell was 650 C. The flow rate of a fuel gas of H2, CO2, and N2 was regulated by a mass controller and a bubbler was used to add water vapor. The flow rate of an oxidant gas of air and CO2 was regulated by a mass controller and supplied to the cell. The gas composition inside the electric furnace changes as fuel is consumed during power generation. However, the interior volume of the electric furnace is extremely large compared with the size of cell and the flow rate of the supplied fuel gas is small, so the gas inside the electric furnace reaches an almost uniform condition
M. Kawase / Journal of Power Sources 285 (2015) 260e265
263
Fig. 3. Schematic illustration of the power generation test apparatus and photograph of a cell.
in a steady state. When calculating the current density and other parameters, the electrode surface was assumed to be the contact surface between the cathode and the electrolyte matrix.
4.2. Power generation performance Fig. 4 shows the change of currentevoltage characteristics (IeV characteristics) during power generation, with the IeV characteristics of a planar MCFC shown for reference. To obtain these data, a current density of 100 mA cm2 was used as the standard to alter the current density quickly. Because a change in current density corresponds to an extremely small change in the amount of gas relative to the volume of the electric furnace, the fuel gas composition at all current densities is assumed to be equivalent to that at 100 mA cm2. In the figure, Ver. 1 indicates the IeV characteristics for the initial stage of the development, Ver. 2 those for the interim stage, and Ver. 3 those for the characteristics of the present MCFC. The IeV characteristics of a conventional planar MCFC, as calculated from the analysis results of planar MCFC single cell data according to the operating conditions, are included for reference. The initial development Ver. 1 had gas leakage, resulting in an opencircuit voltage lower than the value calculated from the gas conditions at 70 mV, a large IeV characteristic gradient, and poor
Fig. 4. Plot of cell voltage as a function of current density at 650 C.
electrode performance. Modifying the technique in light of the results of Ver. 1, the sintering pattern for the anode was changed in order to promote contraction with anode sintering. Specifically, the anode and electrolyte matrix were degreased at 450 C for 2 h before sintering of the anode. Although the gas leakage improved as a result, the gradient of the IeV characteristics remained unchanged, and no improvement in electrode performance was seen. Furthermore, in addition to changing the amount of binder in the electrolyte matrix slurry in order to prevent cracking of the electrolyte matrix and remedy the gas leakage, the electrolyte packing ratios of the anode and cathode were changed from 50% to 35% in order to increase the electrode reaction area. As a result, Ver. 3 approaches the performance of a conventional planar MCFC. There is still a gap in performance, but the electrode member used in this test was optimized for a planar MCFC, and optimizing the electrode member to a tubular MCFC may improve performance further. Specifically, the binder quantity of the electrode slurry should be adjusted to suit the burning pattern of the electrode and the particle diameter of the electrode material. Fig. 5 shows the effects of temperature on the voltages of tubular and planar MCFCs. Since the cell inlet temperature is set at 580 C and the cell outlet temperature at 660 C in conventional planar MCFC stacks, the temperature characteristics were acquired in the
Fig. 5. Plot of cell voltage as a function of temperature.
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M. Kawase / Journal of Power Sources 285 (2015) 260e265
range of 580e680 C in this test. As can be seen from the plot, the voltage decreases as temperature decreases, but a striking voltage drop is not evident. This tendency is the same as for planar MCFC. The voltage difference depends on electrode performance, as detailed above, and it is likely that performance can be improved in the future. Fig. 6 shows the time-dependent changes in voltage and internal resistance in a continuous generation test. The voltage is low at the initial stage of operation but stabilizes after 200 h of operation, and almost no fluctuation in voltage is seen after 1000 h of operation. With regard to internal resistance, a temporary drop was seen at 870e970 h, but from 200 h onward, the internal resistance was 0.65 mU cm, with almost no fluctuation. This behavior of the internal resistance suggests that a drop in fastening force does not occur within 1000 h of operation. Moreover, increasing internal resistance caused by corrosion of the separator and the current collector is a cause of voltage decrease in conventional planar MCFC, but the lack of an internal resistance increase in this test suggests that the impact of corrosion on tubular MCFC is small. This may be because there is little corrosion in a tubular MCFC compared with a planar MCFC due to the small contact area between the metal part of the separator and the molten salt. Transfer of the electrolyte is also a possible cause of the voltage fluctuation at the initial stage of operation. The distribution of electrolyte greatly affects the response of the electrode, and so time may be required from impregnation of the electrolyte until it transfers to a steady state.
4.3. Cold start/stop characteristics Planar MCFC have a long cold start/stop time, and there are reports of stacks taking almost a week to start and stop [8]. A large stack can have over 200 cells, and the temperature must be controlled while monitoring the temperature distribution within the stack and on the surface of each cell. This is done to prevent the electrolyte matrix from cracking. In particular, the strain on the electrolyte matrix is likely to increase when the electrolyte melts or solidifies, and so the temperature difference in the cell plane is controlled to keep it within 10e20 C [11]. In contrast, the cells in a tubular MCFC tend not to affect other cells because they are not in direct contact with each other. The structure of a tubular MCFC differs from the structure of a planar
Fig. 7. Cell voltage characteristic of a tubular MCFC when restarted after being stopped with the following test parameters: fuel inlet gas, H2/CO2/N2/H2O ¼ 29/5/60/6 vol%; oxidant inlet gas, O2/CO2/N2 ¼ 15/30/55 vol%.
MCFC in that it also makes the electrolyte matrix unlikely to crack because the entire electrolyte matrix is supported by the anode. Accordingly, we conducted a cold start/stop test on the tubular MCFC. Fig. 7 shows the temperature variation and cell voltage variation during the cold start/stop test. In this test, the temperature was set to decrease from 650 C to 90 C over a 3-h period, and to increase back to 650 C again over another 3-h period. In practice, the process of decreasing the temperature took around 5 h due to the heat capacity of the electric furnace. The voltage during generation prior to stopping was 857 mV, and although the voltage was 843 mV at 7 h after restarting, it returned to 856 mV at 70 h after. This showed that starting (heating) and stopping (cooling) is possible in a few hours. 5. Conclusion A method of manufacturing a tubular MCFC was developed. The key to fastening the cell was the self-shrinking effect of the anode during sintering. As the result of a power generation test, the fastening force exerted by the anode was maintained and the cell was found to have a generation performance close to that of a planar MCFC, although there is room for improvement. It was furthermore found that the tubular MCFC can start from a coldstopped condition in a few hours. The use of this tubular MCFC manufacturing method can be expected to obviate the need for a complex separator, simplify the electrode manufacturing process, and simplify the requirements of electrode manufacturing facilities. It can also be expected to greatly reduce costs in comparison with existing planar MCFC. Furthermore, a reduction in the start/stop time can be expected with the tubular MCFC, in addition to lower separator corrosion because the area of contact between the carbonate and the metal is half that in planar MCFC. Together, these should result in a long lifespan. While issues such as the complexity of the current collection model for cells and a low power density per unit volume remain in the case of stacking the cells, this study suggests that tubular MCFC have potential for practical use. References
Fig. 6. Stabilities of cell voltage and internal resistance in a 1000-h continuous test. Temperature, 650 C; current density, 117 mA cm2; fuel inlet gas, H2/CO2/N2/ H2O ¼ 20/7/67/6 vol%; oxidant inlet gas, O2/CO2/N2 ¼ 12/42/46 vol%.
[1] CHUBU Electric Power Co,Inc., https://www.chuden.co.jp/corpo/publicity/ press2002/0918_1.html (accessed 2014/12/20). [2] K. Yuasa, K. Itou, N. Fujiwara, T. Matsuyama, Development of molten carbonate fuel cell at IHI, in: The 13th FCDIC Fuel Cell Symposium Proceedings, Tokyo, 2006, pp. 207e210. [3] www.fuelcellenergy.com (accessed 2014/12/20).
M. Kawase / Journal of Power Sources 285 (2015) 260e265 [4] G. Bush, A. Skok, MW size fuel cell power is here, in: Proceedings of 2005 Fuel Cell Seminar, California, 2005, pp. 264e267. [5] www.poscoenergy.com (accessed 2014/12/20). [6] New Energy and Industrial Technology Development Organization (NEDO), MCFC research development 7th meeting report No. 3-2-3, www.nedo.go.jp/ content/100097165.pdf (accessed 2014/12/20). [7] New Energy and Industrial Technology Development Organization (NEDO), MCFC Research Development for Realization and Cost Reduction (FY2005) FY2005 Annual Report, 2006, pp. 306e314. [8] New Energy and Industrial Technology Development Organization (NEDO),
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MCFC research development First meeting report No.5-2, www.nedo.go.jp/ content/100091521.pdf (accessed 2014/12/20). [9] M. Yoshikawa, H. Morita, Y. Mugikura, Development of 1kW Class MCFC Stack, CRIEPI report, 2009. M09001. [10] M. Yoshikawa, Y. Mugikura, Molten carbonate fuel cell, electrolyte matrix, aqueous solvent, cell performance, J. Soc. Powder Technol. Jpn. 48 (2011) 370e375. [11] T. Ogawa, M. Yamaguchi, M. Tooi, Y. Ohtake, T. Matsuyama, MCFC kousekisou daiyouryou sutaku seizougijutu no kaihatu, J. Fuel Cell. Technol. 5 (1) (2005) pp.12e15.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Manufacturing method for tubular molten carbonate fuel cells and basic cell performance Makoto Kawase Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa 240-0196, Japan
h i g h l i g h t s A method of manufacturing a tubular MCFC was developed. The key to fastening the cell is the self-shrinking effect of the anode during sintering. The fastening force exerted by the anode is maintained. The generation performance of a tubular MCFC is close to that of a planar MCFC. The tubular MCFC can start from a cold-stopped condition in a few hours.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2014 Received in revised form 7 March 2015 Accepted 18 March 2015 Available online 19 March 2015
The combination of syngas from gasification and high-temperature fuel cells is a candidate for highefficiency power generation systems. Reducing the production cost of fuel cells and gas-cleaning devices is an important issue for commercial application. This study focuses on molten carbonate fuel cells (MCFCs), which are relatively durable against poisoning by impurities in syngas. However, the development of MCFC systems has come to a halt in Japan because the production cost of MCFCs made them commercially infeasible. To reduce the production cost significantly, a tubular MCFC has been developed instead of the conventional planar type. The tubular MCFC requires neither a complex separator nor cell components with high dimensional accuracy. However, there have been no reports about tubular MCFCs because the electrolytes used for these MCFCs are liquid, which makes it difficult to fasten the fuel cell stack without a fastener. In this study, a fastening method is developed by using the self-shrinking effect of anodes during sintering. Using this technique, the tubular MCFC was successfully manufactured. The results of a power generation test for 1000 h show that the cell voltage was kept stable. Moreover, the cell performance was close to that of a conventional planar MCFC. © 2015 Elsevier B.V. All rights reserved.
Keywords: Molten carbonate fuel cell Tubular type Sintered materials Gasification Fastening method
1. Introduction Power generation systems that use gasification and fuel cells are expected to find use in practical applications because highefficiency power generation is provided by the integration of a coal- or biomass-gasification power generation system with fuel cells. Molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs), in which carbon monoxide can also be used as a fuel, are potential technologies for this purpose. Fuel cell manufacturing costs and gas purification costs are issues that must be addressed for practical implementation of these systems. Abundant hydrogen sulfide, hydrogen chloride, and other gaseous impurities are
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jpowsour.2015.03.117 0378-7753/© 2015 Elsevier B.V. All rights reserved.
contained in the product of coal gasification, biomass gasification, and the like. This necessitates gas purification to an acceptable concentration for use in the fuel cell. The purification cost varies according to the desired level of gas purity. The present study focuses on MCFCs, which are relatively robust in the presence of gaseous impurities. Although there are few successful applications of gasification gas in MCFCs [1,2], several successful applications of digestion gas as a fuel have been reported [3,4]. The digestion gas is primarily methane and is used in one type of MCFC: the internal reforming MCFC. The gasification gas is primarily H2 and CO (syngas) and is used in another type of MCFC: the external reforming MCFC. Thus, the external reforming MCFC is selected in order to use gasification gases as fuel. Presently, internal reforming MCFCs are being developed and manufactured in the United States, Korea, and other countries [3,5],
M. Kawase / Journal of Power Sources 285 (2015) 260e265
and the equivalent of 300 MW have been shipped, but almost no development of external reforming MCFCs is being carried out. The suspension of the development of external reforming MCFC is probably because the manufacturing cost was unacceptable to the market [6]. The breakdown of the manufacturing cost for an external reforming MCFC stack developed in Japan in 2005 was reported as that shown in Fig. 1 [7]. In the breakdown, cell components (the anode, cathode, electrolyte, electrolyte matrix, and separator) account for over half the cost, with the separator alone accounting for a quarter of the cost. One reason for the high cost is the requirement for micrometer-scale precision in dimensional accuracy when manufacturing the cell components, and the correspondingly low yield rate. This requirement for high dimensional accuracy results from the low strength of the electrolyte matrix, which in turn is affected by the anode, cathode, and separator connected to it (via thermal expansion, roughness, and so on), with variations from specification resulting in cracking and other problems. It is also necessary to take the time to raise the temperature during start up due to the effect of stress on the electrolyte matrix, which means that it takes time for the cell to reach its operating temperature [8]. Because of these factors, a high precision is currently required for the separator and other components in an MCFC, which increases the manufacturing cost. Turning our attention to the structure of the SOFC currently under development, tubular types exist in addition to the planar types, and research leveraging the benefits of the tubular type is advancing. If a tubular MCFC could be made, then a complex separator would be unnecessary, which could be expected to greatly ease the dimensional accuracy requirements for the cell components, drastically increase the yield rate, and thereby significantly decrease manufacturing costs. Additionally, the influence of one cell on other cells is small because each cell is separated from the others in a tubular MCFC. Therefore, a shorter start up time can be expected in comparison with a planar MCFC. Furthermore, it is possible to replace individual cells that are damaged in a tubular MCFC. However, tubular MCFCs have not yet been reported. This is because the electrolyte for an MCFC cannot be baked onto the electrode as it can in an SOFC, since the electrolyte is a liquid (molten carbonate) and also there is high contact resistance when the electrode and the electrolyte matrix are simply layered. This makes it necessary to fasten them with some kind of force. In the current planar types, these are secured by a fastener at the top of the stack. In this report, we consider the feasibility of a tubular MCFC with a low manufacturing cost that can overcome these issues and reduce start up time.
261
2. Consideration of a tubular MCFC fastening method A structure with the cathode on the outside of the cylinder and one with the anode on the outside are both feasible types of tubular MCFC structure. The following fastening methods may reduce the contact pressure between the cathode, matrix, and anode. Fastening by external force Fastening by powder sintering during electrode manufacture
2.1. Fastening by external force Fastening by a tubular metal band, such as a hose band or by a clamp, is a feasible method of fastening by external force. Since it is not possible to cover the entire electrode with a clamp, it is necessary to carry out the fastening with a punching plate as a supporting plate. The issue with this method is that highly accurate material manufacturing is necessary because the fastening force may be concentrated in the vicinity of the clamp. Moreover, because a fastener is necessary for each cell, this method may not be practical from a cost perspective for a stack consisting of several hundred cells. 2.2. Fastening by powder sintering effect during electrode manufacture Both the anode and the cathode are made from Ni-based powders, and electric resistance makes it necessary to sinter them for use as suitable electrodes. It is possible to fasten the inner layers (the electrolyte matrix) through the joining and shrinkage of Ni powder particles that occurs during sintering. A normal cathode is formed from Li-doped NiO through reacting with lithium carbonate, potassium carbonate, sodium carbonate, and other electrolytes in an oxidizing atmosphere after sintering the Ni-based powder. For this reason, when the cathode is on the outside, most of the fastening strength is lost because the powder sintered under oxidizing conditions is dispersed. In contrast, with the anode on the outside, it is highly likely that the fastening force will be maintained, because the coupling of the individual powder grains is maintained so long as the anode is kept in a reducing atmosphere. On the basis of the above considerations, we performed research on a tubular MCFC with the anode on the outside, using the fastening method given by the sintering effect of the Ni powder of the anode material. 3. Manufacture of a tubular MCFC
Fig. 1. Stack cost of MCFC developed in Japan [7].
Fig. 2 shows the manufacturing process for a tubular MCFC. First, the SUS316L punching pipe shown in Fig. 2(a) was used as a tubular separator to shield the gas. This punching part has an outer diameter of 7.9 mm, an inner diameter of 6.0 mm, and a length of 33 mm. The outer diameter of each end is 9.6 mm. The aperture ratio of the punching part is 70%. The dimensional accuracy of the deviation from circularity of the punching pipe is ±0.03 mm, which does not satisfy the accuracy required by the separator in a planar MCFC. The cathode, anode, and electrolyte matrix members are the same as those used in a planar MCFC [9,10], and water, methylcellulose, and glycerin were mixed to form a slurry in the same way as for a planar MCFC. The viscosities of these slurries are adjusted by water to apply without dripping. Table 1 gives the specifications of each member. The thicknesses of the cathode, anode, and electrolyte matrix refer to those in a planar MCFC. Second, the Ni powder slurry for the cathode was applied to the
262
M. Kawase / Journal of Power Sources 285 (2015) 260e265
Fig. 2. Manufacturing process of a tubular MCFC.
Table 1 Materials of cell components.
Cathode Electrolyte matrix Anode
Material
Mean particle diameter
Viscosity of slurry
Porosity after sintering
Ni-3%MgCO3 a-LiAlO2 Ni-2%AlCr
2.2e2.8 mm e 2.6e3.3 mm
15e20 Pa s 40e55 Pa s 15e20 Pa s
68e70 % 53e55 % 63e65 %
trough section while rotating the tubular separator; the outer diameter of the cathode was made approximately 9.7 mm. Thereafter, the slurry was sintered for 1 h at 900 C in an atmosphere of N2/H2 ¼ 93%/7%. The outer diameter contracted to 9.5 mm during sintering. Contraction also occurred in the axial direction, and a gap of approximately 1 mm was produced in the trough section, as shown by Fig. 2(c). Third, a LiAlO2 powder slurry for the electrolyte matrix was applied over the cathode to an outer diameter of 12.0 mm. At this point, the LiAlO2 powder slurry filled the trough gap produced during cathode sintering. After drying the LiAlO2 powder slurry, a Ni powder slurry for the anode was applied to an outer diameter of 14.6 mm. After drying, it was sintered for 3 h at 900 C in an atmosphere of N2/H2/H2O ¼ 37%/3%/60%. After sintering, the anode outer diameter was approximately 14.4 mm. The diameter contracted by approximately 0.2 mm, and a pressure was exerted in the circumferential direction, that is, on the electrolyte matrix, due to this contraction. This contraction of the anode serves the function of a fastener. There were no control values for the thickness of the cathode, electrolyte matrix, or the anode, and strict accuracy management was not necessary, but the thicknesses must be such that a fastening effect is provided by the anode. The permissible range of this thickness is currently under investigation. Finally, for the electrolyte, a slurry of carbonate powder in ethanol was applied to the surface of cathode and anode. The electrolyte was loaded into the electrolyte matrix and the electrodes at 480e650 C in a reducing atmosphere. The electrolyte
was a carbonate mixture of Li2CO3/K2CO3 ¼ 62%/38%. The melting point of this carbonate mixture is 488 C. The quantity of electrolyte was set such that the electrolyte packing ratios (volume of pores filled with electrolyte relative to total pore volume) of the anode, cathode, and electrolyte matrix were 30e50%, 30e50%, and 100%, respectively. 4. Power generation test A power generation test was performed with the tubular MCFC manufactured by the above method in order to confirm its ability to generate power. 4.1. Power generation test apparatus Fig. 3 shows a schematic illustration of the power generation test apparatus and a photograph of a cell. The internal dimensions of the electric furnace were internal diameter 140 mm and height 500 mm. The temperature of the electric furnace was controlled such that the temperature near the cell was 650 C. The flow rate of a fuel gas of H2, CO2, and N2 was regulated by a mass controller and a bubbler was used to add water vapor. The flow rate of an oxidant gas of air and CO2 was regulated by a mass controller and supplied to the cell. The gas composition inside the electric furnace changes as fuel is consumed during power generation. However, the interior volume of the electric furnace is extremely large compared with the size of cell and the flow rate of the supplied fuel gas is small, so the gas inside the electric furnace reaches an almost uniform condition
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Fig. 3. Schematic illustration of the power generation test apparatus and photograph of a cell.
in a steady state. When calculating the current density and other parameters, the electrode surface was assumed to be the contact surface between the cathode and the electrolyte matrix.
4.2. Power generation performance Fig. 4 shows the change of currentevoltage characteristics (IeV characteristics) during power generation, with the IeV characteristics of a planar MCFC shown for reference. To obtain these data, a current density of 100 mA cm2 was used as the standard to alter the current density quickly. Because a change in current density corresponds to an extremely small change in the amount of gas relative to the volume of the electric furnace, the fuel gas composition at all current densities is assumed to be equivalent to that at 100 mA cm2. In the figure, Ver. 1 indicates the IeV characteristics for the initial stage of the development, Ver. 2 those for the interim stage, and Ver. 3 those for the characteristics of the present MCFC. The IeV characteristics of a conventional planar MCFC, as calculated from the analysis results of planar MCFC single cell data according to the operating conditions, are included for reference. The initial development Ver. 1 had gas leakage, resulting in an opencircuit voltage lower than the value calculated from the gas conditions at 70 mV, a large IeV characteristic gradient, and poor
Fig. 4. Plot of cell voltage as a function of current density at 650 C.
electrode performance. Modifying the technique in light of the results of Ver. 1, the sintering pattern for the anode was changed in order to promote contraction with anode sintering. Specifically, the anode and electrolyte matrix were degreased at 450 C for 2 h before sintering of the anode. Although the gas leakage improved as a result, the gradient of the IeV characteristics remained unchanged, and no improvement in electrode performance was seen. Furthermore, in addition to changing the amount of binder in the electrolyte matrix slurry in order to prevent cracking of the electrolyte matrix and remedy the gas leakage, the electrolyte packing ratios of the anode and cathode were changed from 50% to 35% in order to increase the electrode reaction area. As a result, Ver. 3 approaches the performance of a conventional planar MCFC. There is still a gap in performance, but the electrode member used in this test was optimized for a planar MCFC, and optimizing the electrode member to a tubular MCFC may improve performance further. Specifically, the binder quantity of the electrode slurry should be adjusted to suit the burning pattern of the electrode and the particle diameter of the electrode material. Fig. 5 shows the effects of temperature on the voltages of tubular and planar MCFCs. Since the cell inlet temperature is set at 580 C and the cell outlet temperature at 660 C in conventional planar MCFC stacks, the temperature characteristics were acquired in the
Fig. 5. Plot of cell voltage as a function of temperature.
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range of 580e680 C in this test. As can be seen from the plot, the voltage decreases as temperature decreases, but a striking voltage drop is not evident. This tendency is the same as for planar MCFC. The voltage difference depends on electrode performance, as detailed above, and it is likely that performance can be improved in the future. Fig. 6 shows the time-dependent changes in voltage and internal resistance in a continuous generation test. The voltage is low at the initial stage of operation but stabilizes after 200 h of operation, and almost no fluctuation in voltage is seen after 1000 h of operation. With regard to internal resistance, a temporary drop was seen at 870e970 h, but from 200 h onward, the internal resistance was 0.65 mU cm, with almost no fluctuation. This behavior of the internal resistance suggests that a drop in fastening force does not occur within 1000 h of operation. Moreover, increasing internal resistance caused by corrosion of the separator and the current collector is a cause of voltage decrease in conventional planar MCFC, but the lack of an internal resistance increase in this test suggests that the impact of corrosion on tubular MCFC is small. This may be because there is little corrosion in a tubular MCFC compared with a planar MCFC due to the small contact area between the metal part of the separator and the molten salt. Transfer of the electrolyte is also a possible cause of the voltage fluctuation at the initial stage of operation. The distribution of electrolyte greatly affects the response of the electrode, and so time may be required from impregnation of the electrolyte until it transfers to a steady state.
4.3. Cold start/stop characteristics Planar MCFC have a long cold start/stop time, and there are reports of stacks taking almost a week to start and stop [8]. A large stack can have over 200 cells, and the temperature must be controlled while monitoring the temperature distribution within the stack and on the surface of each cell. This is done to prevent the electrolyte matrix from cracking. In particular, the strain on the electrolyte matrix is likely to increase when the electrolyte melts or solidifies, and so the temperature difference in the cell plane is controlled to keep it within 10e20 C [11]. In contrast, the cells in a tubular MCFC tend not to affect other cells because they are not in direct contact with each other. The structure of a tubular MCFC differs from the structure of a planar
Fig. 7. Cell voltage characteristic of a tubular MCFC when restarted after being stopped with the following test parameters: fuel inlet gas, H2/CO2/N2/H2O ¼ 29/5/60/6 vol%; oxidant inlet gas, O2/CO2/N2 ¼ 15/30/55 vol%.
MCFC in that it also makes the electrolyte matrix unlikely to crack because the entire electrolyte matrix is supported by the anode. Accordingly, we conducted a cold start/stop test on the tubular MCFC. Fig. 7 shows the temperature variation and cell voltage variation during the cold start/stop test. In this test, the temperature was set to decrease from 650 C to 90 C over a 3-h period, and to increase back to 650 C again over another 3-h period. In practice, the process of decreasing the temperature took around 5 h due to the heat capacity of the electric furnace. The voltage during generation prior to stopping was 857 mV, and although the voltage was 843 mV at 7 h after restarting, it returned to 856 mV at 70 h after. This showed that starting (heating) and stopping (cooling) is possible in a few hours. 5. Conclusion A method of manufacturing a tubular MCFC was developed. The key to fastening the cell was the self-shrinking effect of the anode during sintering. As the result of a power generation test, the fastening force exerted by the anode was maintained and the cell was found to have a generation performance close to that of a planar MCFC, although there is room for improvement. It was furthermore found that the tubular MCFC can start from a coldstopped condition in a few hours. The use of this tubular MCFC manufacturing method can be expected to obviate the need for a complex separator, simplify the electrode manufacturing process, and simplify the requirements of electrode manufacturing facilities. It can also be expected to greatly reduce costs in comparison with existing planar MCFC. Furthermore, a reduction in the start/stop time can be expected with the tubular MCFC, in addition to lower separator corrosion because the area of contact between the carbonate and the metal is half that in planar MCFC. Together, these should result in a long lifespan. While issues such as the complexity of the current collection model for cells and a low power density per unit volume remain in the case of stacking the cells, this study suggests that tubular MCFC have potential for practical use. References
Fig. 6. Stabilities of cell voltage and internal resistance in a 1000-h continuous test. Temperature, 650 C; current density, 117 mA cm2; fuel inlet gas, H2/CO2/N2/ H2O ¼ 20/7/67/6 vol%; oxidant inlet gas, O2/CO2/N2 ¼ 12/42/46 vol%.
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MCFC research development First meeting report No.5-2, www.nedo.go.jp/ content/100091521.pdf (accessed 2014/12/20). [9] M. Yoshikawa, H. Morita, Y. Mugikura, Development of 1kW Class MCFC Stack, CRIEPI report, 2009. M09001. [10] M. Yoshikawa, Y. Mugikura, Molten carbonate fuel cell, electrolyte matrix, aqueous solvent, cell performance, J. Soc. Powder Technol. Jpn. 48 (2011) 370e375. [11] T. Ogawa, M. Yamaguchi, M. Tooi, Y. Ohtake, T. Matsuyama, MCFC kousekisou daiyouryou sutaku seizougijutu no kaihatu, J. Fuel Cell. Technol. 5 (1) (2005) pp.12e15.