- Email: [email protected]
SOFC system and technology
Solid State Ionics 152 – 153 (2002) 383 – 392 www.elsevier.com/locate/ssi
SOFC system and technology Masayuki Dokiya * Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Accepted 15 March 2002
Abstract Compact size solid oxide fuel cells (SOFCs), which will be operated at reduced temperature, are becoming a frontier of R and D. These compact size SOFCs will fit well with intermittent loads, of which share in energy system is increasing today, whereas the ‘‘conventional SOFCs’’ will be effectively operated with stationary mode. For such compact size SOFCs, throttle down operation following intermittent loads will be profitable because low current density gives higher efficiency. SOFCs are not suitable for quick start up. It was estimated that the hot standby mode would be more acceptable than cold start mode from the viewpoint of heat loss. The merit of internal reforming will also be lost for the reduced operation temperature. In order to overcome this problem, an electrochemical oxidation of deposited carbon was tested and a new direct internal reforming concept using this carbon deposition was proposed. In order to develop cheap compact size, reduced temperature SOFCs, the feasibility of anode supported SOFCs was investigated on Zr(Sc)O2, Zr(Y)O2, Ce(rare earth)O2, (La,Sr)(Ga,Mg)O2 electrolytes, examining applicability of wet co-fire process and electrode activity. The feasibility was confirmed with zirconia, not yet with ceria due to its fragility, and pessimistic with (La,Sr)(Ga,Mg)O2 due to Ni diffusion during co-firing. D 2002 Elsevier Science B.V. All rights reserved. PACS: Electrochemical energy conversion (84.60.D); Fuel cell (84.60.D) Keywords: Solid oxide fuel cells (SOFCs); Combined cycle SOFCs; Reduced temperature SOFCs; Compact SOFCs; Vehicle SOFCs
1. Introduction Solid oxide fuel cells (SOFCs) are at a crossroads. After long years of effort, Siemens – Westinghouse’s tubular cell is now being tested for commercialization at several field sites [1]. There are undoubtedly only three keywords: ‘‘cost, cost, cost.’’ Several cells, such as Mitsubishi Heavy Industry’s [2,3] and Toto’s cells [4], will follow after Siemens – Westinghouse’s cells. The latter cells are promoting cheap mass production by wet processes. * Tel.: +81-45-339-4365; fax: +81-45-339-4374. E-mail address: [email protected] (M. Dokiya).
Recently, there appeared another direction, which orients small size SOFCs for automobile or home/ office/store co-generation. The keywords of such ‘‘compact’’ size SOFCs are ‘‘reduced temperature’’ and ‘‘cheap mass production.’’ The core module concept of the Solid State Energy Conversion Alliance (SECA, USA) is a typical example of this trend [5]. This article will discuss the feasibility of reduced temperature SOFCs for small-scale co-generations and automobiles. Firstly, the background problems in modern energy systems will be briefly discussed. Then, how to make efficient use of SOFCs in the energy system will be discussed, putting attention upon the characteristics of SOFCs, such as internal reforming,
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 3 4 5 - 4
384
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
high efficiency at throttle down, and the difficulty of quick start up. In order to clarify the technological aspects of the problems, some of our experimental results will be introduced about the reduced temperature SOFCs. Finally, a new concept of internal reforming will be proposed in view of how to manage quick start up and/or intermittent operation mode.
2. Background problems in energy system Supply of electricity and transport of goods/person are the basis of modern life. These two sectors are deeply related to mass consumption of energy. The transport sector is well-known for its low energy efficiency. An urgent and continuing problem in the 21st century will be how to conserve energy resources and how to decrease the emission of CO2. This problem can be translated as how to eliminate the mass consumption of energy in developed countries and how to build the most advanced/efficient energy system in the developing countries. This requirement can be summarized as ‘‘highly efficient technology in electricity generation and transportation sector.’’ 2.1. Electricity system When we inspect the present status of power generation technology, the advanced combined cycle (ACC), which is a cascading system of gas and steam turbines, can produce electricity by more than 50% efficiency. As a future technology, SOFCs will offer the highest electricity generation efficiency from fossil fuels, as high as 65– 70%, when they are combined to gas turbines and are operated stationary. The high efficiency of ACC or combined SOFCs is, however, inevitably accompanied by rigidity of system. In other words, these combined systems cannot follow variable intermittent load flexibly, because it is based on the combination of two independent plants. In order to follow peak loads, the pumping up hydraulic power plant is adopted to store electricity, and the introduction of a secondary battery is conceived as a future technology. The storage of electricity is not wise way, because they solely consume electricity in vain. Of course, their raison d’eˆtre as a buffer in electricity distribution system is not denied. Instead of storage of electricity, the intermittent oper-
ation of power plants is also adopted. The daily stop and start (DSS) operation mode is currently adopted even for large-scale steam power stations. It is also cost-consuming to stop large-scale power turbine. How to meet the requirement of intermittent load is an inevitable problem of modern systems. At the end use of energy, requirements are not stationary, especially in home, office, store, etc., and transport usage. If there are small-scale but highly efficient generators, it will be well fit to variable load requirements. The co-generation of electricity and heat is widely recommended to improve energy efficiency in this meaning. SOFCs are expected to offer higher efficiency both for power stations and co-generation than current system. If the downsizing can be as small as a few kilowatts, such a generator can be applied for an enormous number of users such as home, store, office, small factory, etc. Thus, the combination of ACC and/or SOFC-gas turbines for base load and intermittently operable dispersed small SOFCs for peak load will become preferable and one of the key energy systems for the 21st century. 2.2. Transportation system Similarly, we should improve the extremely low energy efficiency in the transportation sector. This problem is also deeply connected to the elimination of hazardous polluting substances, such as SOx, NOx, and suspended particle matter (SPM). SOFCs will be advantageous to improve efficiency and to eliminate pollutants, but not convenient for quick start required for automobiles. The secondary battery is one strong candidate, since it can store offpeak electricity and does not emit pollutants. As is well reported, the polymer electrolyte membrane fuel cells (PEMFC) attracts much attention because it will be able to reduce the demerits of the secondary battery car, especially for recharging time loss, too heavy weight, and expensive cost. However, PEMFC also has serous demerits. The problem is how to reform fuels on automobiles. PEMFC can only be driven by H2. If we use methanol as fuel because of easy reforming, we will lose much of the potential energy of natural gas and/or petroleum through the methanol synthesis step. In addition, we should build a methanol distribution infrastructure. These demerits will be more serious in the case of so-called ‘‘hydrogen fueled car.’’ Note that in
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
the case of coal primary energy system, methanol or dimethyl-ether (DME) could be accepted as liquid fuel from the viewpoint of pollution. The coal gasification is indispensable to eliminate pollutants from coal, and methanol and DME are adequate liquid fuel to be converted from coal gas. If we focus our attention upon efficiency, diesel engines will offer higher efficiency than PEMFC. The demerit of diesel engines is considered to be the emission of pollutants, NOx and SPM. It is expected that highly oxidative burning will be able to decrease the formation of SPM, and NOx produced at oxidative atmosphere can be killed by the addition of urea. If diesel engines can clear the pollutant regulation, only SOFCs will have a chance to be competitive with diesel engines in the viewpoint of efficiency.
3. Merits of fuel cell and SOFC systems 3.1. Internal reforming 3.1.1. Simplicity of SOFCs as a system Fig. 1 compares schematically SOFCs and the other fuel cells. The key point of the fuel cell system is that it needs a fuel-reforming system. This necessity makes the fuel cell plant complicated, and brings about a demerit both for lowering cost and downsizing. SOFCs have an advantage in that they can contain the fuel reformer within themselves as an internal reformer and that they can be of a simple constitution. Other fuel cells require an external fuel reformer and shift converter because the fuel cells can generate electricity only when using H2 as fuel, whereas SOFCs can use CO as fuel also. 3.1.2. High efficiency of SOFCs in a combined cycle In Fig. 1, SOFCs produce electricity, residual fuel, and high temperature waste heat, whereas the other fuel cells produce electricity and low temperature waste heat. The residual fuel and high temperature waste heat can be used in bottoming cycles such as gas turbine. This difference originates from the internal reforming and high temperature operation of SOFCs. The internal reforming recovers irreversible heat loss in SOFCs as heat source for endothermic reforming reaction. SOFCs save fuel, which is required for the heat source of external reforming of
385
the other fuel cells: it amounts up to 25% of fuel. Thus, as a total system, SOFCs can produce higher efficiency than the other fuel cells. 3.1.3. Do SOFCs have high efficiency even as single devices? From the viewpoint of efficiency, it is considered that the external fuel reforming process decreases efficiency remarkably. However, theoretically the situation is fairly complicated. Because the reforming reaction is an endothermic process, the 25% fuel spent at reforming can be recovered in produced CO and H2 as shown in Fig. 1. This relation raises one question: ‘‘Does the external reforming reduce efficiency?’’ The efficiency of fuel cells is mainly governed by fuel utilization rate (Uf) and output voltage (Vout). Textbooks say that the Vout decrease according to the decrease of partial pressure of oxygen (PO2) of fuel, which decreases according to Uf and the Vout drops drastically when Uf exceeds 95%. In actual cases, the internal resistance of cell influences Vout remarkably, especially by those of electrodes. The activity of electrodes is usually governed by the PO2 of fuel, which is Uf again. Thus, we cannot simply conclude that the reforming will reduce the efficiency of fuel cells, since 1. High Uf, usually 85– 90%, of SOFCs is advantageous, but disadvantageous for anode activity. 2. Low Uf of the other fuel cells works contrariwise, and the spent 25% fuel can be recovered in CO and H2. In summary, SOFCs can offer high efficiency because of high Uf, high electrode activity at high temperature, heat recovery by internal reforming, and bottoming cycles using remaining fuel and high temperature wastes heat. How to keep these merits of SOFCs, even when they are not combined with bottoming cycle, will be one key technology for the compact size and reduced temperature SOFCs. 3.1.4. Thermodynamics of fuel cells at reduced temperature The above discussion shows that the high efficiency of SOFCs is mainly originated from the application of bottoming cycle. For the case of compact reduced temperature SOFCs, it will be better to re-examine
386
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
Fig. 1. Internal reforming in SOFCs and the other fuel cells.
the thermochemical relation of fuel cells. Fig. 2 shows three possible cases for electrochemical oxidation (fuel cell oxidation) of reformed hydrocarbon. Case A
CH4 þ 2O2 ! CO2 þ 2H2 O
ð1Þ
Case B
CH4 ! C þ 2H2
ð2Þ
C þ 2H2 þ 2O2 ! CO2 þ 2H2 O
ð3Þ
Case C
CH4 þ H2 O ! CO þ 3H2
ð4Þ
CO þ 3H2 þ 3=2O2 ! CO2 þ 2H2 O
ð5Þ
Case A is advantageous at high temperature but not yet confirmed [6]. Case B was not studied well because of the obstruction by deposited carbon [7,8]: we will discuss later how to utilize case B progressively in order to develop SOFCs, which can start
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
387
at lower power output point than their maximum power output. In the case of conventional SOFCs, the stationary load operation is assumed in order to keep high efficiency and effective cost performance. However, as discussed before, generators applicable for peak load will become more and more important in the future. In this aspect, the high efficiency at low load is a big merit of not only SOFCs but also of fuel cells in general for future electricity system. Imagine a case when we furnish a 3 kW SOFC at home, the device can be operated at 300 W base load for appliances such as a refrigerator with very high efficiency, and can output 3 kW at intermittent peak load with enough high efficiency. In such a case, the ‘‘hot start’’ operation will be possible and the difficult quick ‘‘cold start’’ problem can be eliminated. 3.3. How to start quickly—hot standby or cold standby
Fig. 2. Thermochemical system of reforming, thermal decomposition, and fuel cell generation.
up quickly. Case C becomes advantageous below 650 jC, whereas the steam reforming becomes disadvantageous below this temperature as shown in the upper part of Fig. 2. When fuel becomes higher hydrocarbon, the decomposition reaction (case B) becomes dominant, whereas the reforming still requires high temperature. The combination of high temperature reforming and low temperature fuel cells is a good choice for the external reforming system. The ideal case will be that the internal reforming recovers the heat, and Uf is high, preferably near 95% without decreasing electrode activity, and fuel cell generation (case C) at reduced temperature. The key technology is highly active electrodes and internal reforming at reduced temperature for such a case. 3.2. High efficiency at throttle down—suitability for variable load operation Fuel cells give higher conversion efficiency at lower current density. So they are usually operated
If we assume that there is a cell stack, which has 1 kW/l power output, 5 kg of weight, and 1 J/g K of specific heat. Then this cell requires (1 J g 1 K 1) (5000 g) (700 K) = 3.5 106J 1 kWh to warm up to 700 jC. The heat loss by conduction can be estimated as follows. The cell is cube of 1 l = 10 10 10 cm, with a 1-cm-thick heat insulator having thermal conductivity of 5 10 2 Wm 1 K 1. It will exhaust heat at a rate of (5 10 2 Wm 1 K 1) (6 10 2 m2) (1 10 2 m 1) (700 K) = 210 W at 700 jC. Vacuum container can largely reduce this heat loss as the case of Na –S battery or SOFC of Sulzer. This rough estimation suggests that the heat loss by heating up a cell will be larger than holding the cell at high temperature. The hot standby will become more advantageous when the scale of SOFCs becomes larger, since specific surface per volume will decrease. Now we can come to one tentative conclusion. In order to manage quick start up and response, it will be advantageous to adopt hot standby system for SOFCs. Based on this estimation, we can conceive the following cases. (1) In the case of house use, there is about 200 – 300 W stationary demands for a refrigerator. Thus, we can operate a compact size SOFC at throttled mode with high efficiency. The situation is similar for stores, offices, etc. (2) In the case of automobile, the situation will be not advantageous as the case of home use. The
388
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
electricity produced during hot standby can be stored in the auxiliary battery or connected to the grid network, if possible. However, this may be only adaptable for buses, tracks, and taxis and not suitable for private cars, which are operated only on weekends.
4. Technological problems In order to reduce the operation temperature, there are several fundamental problems: (1) Are there any electrolytes that have enough high ionic conductivity? (2) Are there any electrodes that show enough high activity? (3) How to fabricate cells by cheap manufacturing process? (4) Is it possible to execute internal reforming even at these reduced temperatures? (5) How to make quick start up and/or quick response to variation of load? There is one consensus to meet the above requirements: it is that the cell is preferably built upon Ni anode substrate and is fabricated by wet process, especially co-fire process. This is because, (1) The usage of many amounts of metal components is preferable to reduce material cost and to respond quickly to temperature swing because of high thermal conductivity. The Ni anode substrate fits well with this requirement. (2) The co-fire process is suitable for mass production and to fabricate very thin electrolyte films, which can reduce the material cost and ionic resistance. 4.1. Feasibility of co-fire process 4.1.1. Electrolytes Table 1 summarizes the ionic conductivity of candidate electrolyte, yttria stabilized zirconia (YSZ), scandia stabilized zirconia (SSZ), rare earth doped ceria (CRO), and lanthanum strontium magnesium galate perovskite (LSGM) measured by us. How much ionic conductivity will be required for electrolyte? If we set 1
Table 1 Candidate electrolyte (V cm 2)
100 100 100 100
Am Am Am Am
YSZ SSZ CGO LSGM
550 jC
600 jC
700 jC
800 jC
900 jC
– – – 0.71
2.2 0.90 (0.59) 0.37
0.77 0.26 0.26 0.15
0.31 0.099 0.13 0.074
0.15 0.046 0.071 –
YSZ: Zr(Y)O2, SSZ: Zr(Sc)O2, LSGM: La0.8Sr0.2Ga0.85Mg0.15 [10], CGO: Ce0.8Gd0.2O2 [9], (0.59); Ce0.9Gd0.1O2 [9], CRO: Ce(rare earth)O2.
V/cm2 as internal resistance for a target cell, preferably less than 0.3 V/cm2 can be allowed for the electrolyte. According to Table 1, for the operation at 600 jC, 15 Am for YSZ, 30 Am for SSZ, 50 Am for CRO, and 80 Am for LSGM will be the allowable limit of electrolyte thickness. These values are not so unrealistic. In the case of CRO electrolytes, we should pay attention to its electronic conduction at reducing atmosphere. Kato et al. [11] estimated that the CRO electrolyte cells would have higher conversion efficiency at 600 jC than 700 jC due to electronic conduction. 4.1.2. Electrodes Table 2 summarizes results we obtained. The interfacial resistances of electrode/electrolyte at open circuit voltage (OCV) state are shown. As anode, NiCSO showed acceptable activity on CGO electrolyte at 600 jC [12,13]. The effectiveness of Ni-SSZ or YSZ anode is not yet confirmed at 600 jC. As cathode, Pt-SSZ [14] and Pt-Ag-SSZ [14] or LSCAg-CGO [12,15] (here, LSC represents (La, Sr) CoO3) showed fairly high activity on YSZ, SSZ or CGO electrolyte, respectively. The demerit of Pt-AgSSZ cathode is that it requires 20 mg/cm2 to show the optimum activity shown in Table 2. It should be noted that CRO cermet electrodes showed fairly high activity on CRO electrolytes, whereas the activity degrades remarkably on SSZ and YSZ electrolytes. This is apparently because of the reaction between zirconia and LSC or CRO [16,17]. Matsuzaki et al. [18,19] successfully mounted CRO cermet electrodes on YSZ electrolyte forming a protective thin CRO layer on YSZ using organo-metal reagents. We have tried to form similar thin (2 Am) CGO layer on SSZ by EPD and co-fire process instead of using costly organo-metal reagents. The reaction between LSC and zirconia seemed likely to
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392 Table 2 Interfacial resistance of new electrodes by ac impedance at OCV (V/cm2)
Cathode Cathode Cathode Cathode Anode Anode
Materials
Electrolyte 600 jC 700 jC
Pt-SSZ Pt-Ag-SSZ (Sm0.6Sr0.4)CoO3-CYO-Ag (La0.2Sr0.2)CoO3-CGO-Ag Ni-CSO Ni-CSO
YSZ YSZ SSZ CGO SSZ CGO
1.25 0.67 7.1 0.83 8.9 0.25
0.14 0.083 0.90 0.19 1.16 0.10
CYO: Ce0.8Y0.2O2, CGO: Ce0.8Gd0.2O2, CSO: Ce0.8Sm0.2O2.
be retarded. However, the formation of solid solution between zirconia and ceria seemed likely to be not stopped because of high co-firing temperature of 1300 jC. The utilization of CRO electrodes on YSZ or SSZ electrolyte is one of technological problems to be solved in future. 4.1.3. Feasibility of co-firing process and performance of SSZ cells Wet co-fire process is considered to be a more costefficient process than dry processes because of low cost of manufacturing device, high efficiency of material utilization, adaptability of mass production. Thus, we have examined the adaptability of co-firing processes for candidate electrolytes, SSZ or YSZ, CRO, and LSGM [10,20,21]. The target cell constitution is Ni-substrate/Ni-electrolyte anode/electrolyte/cathode Electrolyte = YSZ, SSZ, CRO, or LSGM, substrate = 3 mol% YSZ or MgAl2O4, etc. Ni-3 mol% YSZ is used as the main substrate in order to reduce material cost and to increase the mechanical strength. In this viewpoint, more cheap materials such as MgAl2O4 should be examined in the future. Here, the cathode is pasted and fired after cofiring electrolyte on Ni-substrate. The outline of standard process is as follows. NiO and YSZ, SSZ, CRO are ball milled using binder (poly-vinyl-butyral), plasticizer (di-n-butyl phthalate), surfactant (triton-X), peptizer (fish oil), solvents (toluene and iso-propanol), and pore former (polymethyl-meta-acrylate). (1) The slurry was impregnated in paper or cloth. This ‘‘skeleton’’ of paper or cloth was used as pellet or
389
rolled tube. The skeleton was used as green state or after pre-fired at 900 jC. (2) The similar process of step (1) prepared the electrolyte slurry. However, the recipe was changed case by case. (3) The electrolyte slurry was coated on the skeleton pellet or tube by dipping, brushing, spraying, or doctor blade. (4) The electrophoretic deposition (EPD) of electrolyte film was also examined. The results can be summarized as follows: (1) The co-fire process for CRO was not so easy compared to the case of SSZ or YSZ due to the fragility and ill-sinterability of CRO. (2) In the case of LSGM, the co-fire was not yet succeeded, because Ni in substrate diffused vigorously through electrolyte film to cathode side during cofiring. Lowering of co-fire temperature was not effective to stop this diffusion. Taiheyo-cement (Japan) developed LSGM powders, which can be sintered at 1300 jC. By this powder, however, Ni still diffused through the LSGM electrolyte layer to cathode. The SSZ cells thus prepared were tested with humidified hydrogen fuel and air. The cell performance is shown in Figs. 3 and 4. The EPD cell had 20 Am SSZ electrolyte, whereas slurry coated cell had 70-Am-thick electrolyte film. The performance at 600 jC was far from satisfactory compared to that at 700
Fig. 3. Performance of slurry coat/co-fire cell at 700 jC (o 800 jC (5 n) l: Pt-SSZ/SSZ (70 Am)/Ni-SSZ.
.) and
390
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
jC. As can be seen in Figs. 3 and 4, the electrodes occupied large portion of cell resistance compared to that of bulk = electrolyte + substrate. Since the slurry coat cell had thicker electrolyte film and the anode was common, the cathode was improved in the case of slurry coat cell. We can expect further improvement if we use the improved cathode shown in Table 1, since at the test experiments, the optimum preparation condition of cathode was not yet known. 4.2. Internal reforming and carbon deposition—new concept of carbon fueled SOFCs
Fig. 4. Performance of EPD/co-fire cell: Pt-SSZ/SSZ (20 Am)/NiSSZ (1 mm).
Internal reforming is one of the merits of SOFCs over the other fuel cells. However, this advantage will be uncertain when the operation temperature is lowered. As discussed before, the equilibrium is not favorable for steam reforming (Fig. 2). However, the steam reforming can be ‘‘conjugated’’ with the electrochemical oxidation (fuel cell reactions) of CO and H2. If the produced CO and H2 are consumed quickly, the
Fig. 5. Dry CH4 and deposited carbon fueled SOFC (FeO – CGO/YSZ (1.5 mm)/LSMO at 1000 jC.
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
equilibrium can be shifted. A serious problem is that this temperature is the region of carbon deposition as can be seen in Fig. 2. Fig. 2 shows that when fuel becomes higher hydrocarbon, the thermal decomposition temperature goes down, whereas the steam reforming still requires high temperatures. Much addition of steam is effective to retard the deposition of carbon. Undoubtedly, however, much steam recycle is not a favorable choice to obtain high efficiency. The key issue is the deposition of carbon. If we can accept the carbon deposition, there can be a new internal reforming system. Let us assume that a SOFC cell is at hot standby, hydrocarbon fuels are introduced directly, and then they will decompose to carbon and H2. Hydrogen will be consumed immediately in a following SOFC or even in PEMFC. Questions are: (1) can steam or carbon dioxide reform the deposited carbon? and (2) can the carbon be electrochemically oxidized? In order to answer this question, we tried to examine the feasibility of direct carbon fueled SOFCs [22,23]. Using commercially available YSZ tubes (10 mm outer diameter with 1.5 mm thickness), we consisted SOFCs with Fe –O anode. In Fig. 5, dry CH4 was passed at shunt current state, then at open circuit state. After that, dry Ar was passed at shunt current state. The current at shunt current state under dry CH4 and Ar flow indicated that the deposited carbon could be electrochemically oxidized by a SOFC cell. Ni anode lost activity quickly. Reforming by steam or CO2 proceeded but not so smoothly on Fe– O anode. The experiments were done only at 1000 jC, so too many questions remain for future works. Such SOFCs will be fairly convenient for quick start up from hot standby state, since H2 can be produced quickly. The deposited carbon will serve as the fuel stock in SOFC itself and consumed during hot standby, in other words, as carbon battery.
5. Summary The applicability of compact size SOFCs, which will be operated at reduced temperatures, was discussed for small co-generation at homes, offices, stores and automobiles. High efficiency of SOFCs at throttle down operation will be profitable for intermittent loads. For difficulty of quick start up, hot standby mode was recommended from a viewpoint of
391
heat loss. In order to develop cheap compact size, reduced temperature SOFCs, the feasibility of anode supported SOFCs was investigated on SSZ, YSZ, CRO, and LSGM electrolytes examining the applicability of wet co-fire processes and electrode activity. The feasibility is confirmed with zirconia, not yet with ceria due to its fragility, and pessimistic with LSGM due to Ni diffusion during co-firing. In order to manage the quick start up and usage of various fuels, an electrochemical oxidation of deposited carbon was tested. A new direct internal reforming concept was proposed using this carbon fueled SOFC. Experimental results are not yet enough to show the feasibility of this carbon fueled SOFC and internal reforming at reduced temperature.
Acknowledgements This research was supported by the proposal-based new industry creative project of New Energy and Industrial Development Organization (NEDO) of Japan. The author expresses deep thanks to NEDO and also to Dr. T. Kato and Dr. S. Wang, Electrotechnical Laboratory, and Dr. N.T. Lan, Yokohama National University, for their collaboration both in the experiments and discussion.
References [1] W.L. Lundberg, S.E. Veyo, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 78 – 87. [2] Y. Sakaki, A. Nakanishi, M. Hattori, H. Miyamoto, H. Aiki, K. Takenobu, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 72 – 77. [3] J. Iritani, K. Kougami, N. Komiyama, K. Nagata, K. Ikeda, K. Tomida, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 63 – 71. [4] M. Kuroishi, S. Furuya, K. Hiwatashi, K. Omoshiki, A. Ueno, M. Aizawa, K. Tsujimoto, U. Uchida, H. Yoshikawa, K. Shukuri, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 88 – 93. [5] S.C. Singhal, in: O. Yamamoto, et al. (Eds.), Proc. of Intl. Symp. Fuel Cells for Vehicles, Nagoya, The Electrochem. Soc, Japan, Nov. 2000, pp. 26 – 36.
392
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
[6] M. Ihara, C. Yokoyama, A. Abudula, R. Kato, H. Komiyama, K. Yamada, J. Electrochem. Soc. 146 (1999) 2481 – 2487. [7] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 143 (1996) 1161 – 1168. [8] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 142 (1996) 2621 – 2624. [9] S. Wang, T. Kobayashi, M. Dokiya, T. Hashimoto, J. Electrochem. Soc. 147 (2000) 3606 – 3609. [10] T.N. Lan, K. Kobayashi, Z. Cai, I. Takahashi, K. Yasumoto, M. Dokiya, S. Wang, T. Kato, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 1042 – 1050. [11] T. Kato, K. Nozaki, S. Wang, S. Negishi, S. Nagata, in: M. Dokiya, S.C. Singhal (Eds.), Proc. 6th Intl. Symp. SOFC, Honolulu. The Electrochem. Soc., Oct. 1999, pp. 116 – 124. [12] S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwashita, M. Dokiya, in: H. Yokokawa, S.C. Singhal (Ed.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 607 – 616. [13] S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwashita, M. Dokiya, J. Electrochem. Soc. (submitted for publication). [14] K. Sasaki, J. Tamura, M. Dokiya, S. Badwal (Eds.), Solid State Ionics, in press. [15] S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwa-
[16] [17] [18]
[19]
[20]
[21]
[22]
[23]
shita, M. Dokiya, Solid State Ionics (submitted for publication). T. Horita, N. Sakai, H. Yokokawa, M. Dokiya, K. Kawada, Jan Van herle, K. Sasaki, J. Electroceram. 1 (1997) 155 – 164. A. Tsoga, A. Gupta, A. Naumididi, P. Nikokopoulos, Acta Mater. 48 (2000) 4709 – 4714. I. Yasuda, Y. Baba, T. Ogiwara, H. Yakabe, Y. Matsuzaki, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ USA, June 3 – 8, 2001, pp. 131 – 139. Y. Matsuzaki, I. Yasuda, in: S. Badwal (Eds.), Ext. Abstr. of SSI-2001, Cairns, The Intl. Soc. Solid State Ionics, CSIRO, Australia, July 8 – 13, 2001, p. 131. Z. Cai, T.N. Lan, S. Wang, M. Dokiya, S. Badwal (Eds.), Solid State Ionics, Proc. of SSI-2001, Cairns, July 8 – 13, in press. K. Kobayashi, I. Takahashi, M. Shiono, M. Dokiya, S. Badwal (Eds.), Solid State Ionics, Proc. of SSI-2001, Cairns, July 8 – 13, in press. M. Dokiya, K. Yasumoto, J. Tadani, in: O. Yamamoto, et al. (Eds.), Proc. of Intl. Symp. Fuel Cells for Vehicles, Nagoya. The Electrochem. Soc, Japan, Nov. 2000, pp. 26 – 36. K. Yasumoto, K. Furusho, J. Tadani, T.N. Lan, S. Cai, M. Dokiya, in: S. Badwal (Ed.), Ext. Abstr. of SSI-2001, Cairns, The Intl. Soc. Solid State Ionics, CSIRO, Australia, July 8 – 13, 2001, p. 123.
SOFC system and technology Masayuki Dokiya * Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Accepted 15 March 2002
Abstract Compact size solid oxide fuel cells (SOFCs), which will be operated at reduced temperature, are becoming a frontier of R and D. These compact size SOFCs will fit well with intermittent loads, of which share in energy system is increasing today, whereas the ‘‘conventional SOFCs’’ will be effectively operated with stationary mode. For such compact size SOFCs, throttle down operation following intermittent loads will be profitable because low current density gives higher efficiency. SOFCs are not suitable for quick start up. It was estimated that the hot standby mode would be more acceptable than cold start mode from the viewpoint of heat loss. The merit of internal reforming will also be lost for the reduced operation temperature. In order to overcome this problem, an electrochemical oxidation of deposited carbon was tested and a new direct internal reforming concept using this carbon deposition was proposed. In order to develop cheap compact size, reduced temperature SOFCs, the feasibility of anode supported SOFCs was investigated on Zr(Sc)O2, Zr(Y)O2, Ce(rare earth)O2, (La,Sr)(Ga,Mg)O2 electrolytes, examining applicability of wet co-fire process and electrode activity. The feasibility was confirmed with zirconia, not yet with ceria due to its fragility, and pessimistic with (La,Sr)(Ga,Mg)O2 due to Ni diffusion during co-firing. D 2002 Elsevier Science B.V. All rights reserved. PACS: Electrochemical energy conversion (84.60.D); Fuel cell (84.60.D) Keywords: Solid oxide fuel cells (SOFCs); Combined cycle SOFCs; Reduced temperature SOFCs; Compact SOFCs; Vehicle SOFCs
1. Introduction Solid oxide fuel cells (SOFCs) are at a crossroads. After long years of effort, Siemens – Westinghouse’s tubular cell is now being tested for commercialization at several field sites [1]. There are undoubtedly only three keywords: ‘‘cost, cost, cost.’’ Several cells, such as Mitsubishi Heavy Industry’s [2,3] and Toto’s cells [4], will follow after Siemens – Westinghouse’s cells. The latter cells are promoting cheap mass production by wet processes. * Tel.: +81-45-339-4365; fax: +81-45-339-4374. E-mail address: [email protected] (M. Dokiya).
Recently, there appeared another direction, which orients small size SOFCs for automobile or home/ office/store co-generation. The keywords of such ‘‘compact’’ size SOFCs are ‘‘reduced temperature’’ and ‘‘cheap mass production.’’ The core module concept of the Solid State Energy Conversion Alliance (SECA, USA) is a typical example of this trend [5]. This article will discuss the feasibility of reduced temperature SOFCs for small-scale co-generations and automobiles. Firstly, the background problems in modern energy systems will be briefly discussed. Then, how to make efficient use of SOFCs in the energy system will be discussed, putting attention upon the characteristics of SOFCs, such as internal reforming,
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 3 4 5 - 4
384
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
high efficiency at throttle down, and the difficulty of quick start up. In order to clarify the technological aspects of the problems, some of our experimental results will be introduced about the reduced temperature SOFCs. Finally, a new concept of internal reforming will be proposed in view of how to manage quick start up and/or intermittent operation mode.
2. Background problems in energy system Supply of electricity and transport of goods/person are the basis of modern life. These two sectors are deeply related to mass consumption of energy. The transport sector is well-known for its low energy efficiency. An urgent and continuing problem in the 21st century will be how to conserve energy resources and how to decrease the emission of CO2. This problem can be translated as how to eliminate the mass consumption of energy in developed countries and how to build the most advanced/efficient energy system in the developing countries. This requirement can be summarized as ‘‘highly efficient technology in electricity generation and transportation sector.’’ 2.1. Electricity system When we inspect the present status of power generation technology, the advanced combined cycle (ACC), which is a cascading system of gas and steam turbines, can produce electricity by more than 50% efficiency. As a future technology, SOFCs will offer the highest electricity generation efficiency from fossil fuels, as high as 65– 70%, when they are combined to gas turbines and are operated stationary. The high efficiency of ACC or combined SOFCs is, however, inevitably accompanied by rigidity of system. In other words, these combined systems cannot follow variable intermittent load flexibly, because it is based on the combination of two independent plants. In order to follow peak loads, the pumping up hydraulic power plant is adopted to store electricity, and the introduction of a secondary battery is conceived as a future technology. The storage of electricity is not wise way, because they solely consume electricity in vain. Of course, their raison d’eˆtre as a buffer in electricity distribution system is not denied. Instead of storage of electricity, the intermittent oper-
ation of power plants is also adopted. The daily stop and start (DSS) operation mode is currently adopted even for large-scale steam power stations. It is also cost-consuming to stop large-scale power turbine. How to meet the requirement of intermittent load is an inevitable problem of modern systems. At the end use of energy, requirements are not stationary, especially in home, office, store, etc., and transport usage. If there are small-scale but highly efficient generators, it will be well fit to variable load requirements. The co-generation of electricity and heat is widely recommended to improve energy efficiency in this meaning. SOFCs are expected to offer higher efficiency both for power stations and co-generation than current system. If the downsizing can be as small as a few kilowatts, such a generator can be applied for an enormous number of users such as home, store, office, small factory, etc. Thus, the combination of ACC and/or SOFC-gas turbines for base load and intermittently operable dispersed small SOFCs for peak load will become preferable and one of the key energy systems for the 21st century. 2.2. Transportation system Similarly, we should improve the extremely low energy efficiency in the transportation sector. This problem is also deeply connected to the elimination of hazardous polluting substances, such as SOx, NOx, and suspended particle matter (SPM). SOFCs will be advantageous to improve efficiency and to eliminate pollutants, but not convenient for quick start required for automobiles. The secondary battery is one strong candidate, since it can store offpeak electricity and does not emit pollutants. As is well reported, the polymer electrolyte membrane fuel cells (PEMFC) attracts much attention because it will be able to reduce the demerits of the secondary battery car, especially for recharging time loss, too heavy weight, and expensive cost. However, PEMFC also has serous demerits. The problem is how to reform fuels on automobiles. PEMFC can only be driven by H2. If we use methanol as fuel because of easy reforming, we will lose much of the potential energy of natural gas and/or petroleum through the methanol synthesis step. In addition, we should build a methanol distribution infrastructure. These demerits will be more serious in the case of so-called ‘‘hydrogen fueled car.’’ Note that in
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
the case of coal primary energy system, methanol or dimethyl-ether (DME) could be accepted as liquid fuel from the viewpoint of pollution. The coal gasification is indispensable to eliminate pollutants from coal, and methanol and DME are adequate liquid fuel to be converted from coal gas. If we focus our attention upon efficiency, diesel engines will offer higher efficiency than PEMFC. The demerit of diesel engines is considered to be the emission of pollutants, NOx and SPM. It is expected that highly oxidative burning will be able to decrease the formation of SPM, and NOx produced at oxidative atmosphere can be killed by the addition of urea. If diesel engines can clear the pollutant regulation, only SOFCs will have a chance to be competitive with diesel engines in the viewpoint of efficiency.
3. Merits of fuel cell and SOFC systems 3.1. Internal reforming 3.1.1. Simplicity of SOFCs as a system Fig. 1 compares schematically SOFCs and the other fuel cells. The key point of the fuel cell system is that it needs a fuel-reforming system. This necessity makes the fuel cell plant complicated, and brings about a demerit both for lowering cost and downsizing. SOFCs have an advantage in that they can contain the fuel reformer within themselves as an internal reformer and that they can be of a simple constitution. Other fuel cells require an external fuel reformer and shift converter because the fuel cells can generate electricity only when using H2 as fuel, whereas SOFCs can use CO as fuel also. 3.1.2. High efficiency of SOFCs in a combined cycle In Fig. 1, SOFCs produce electricity, residual fuel, and high temperature waste heat, whereas the other fuel cells produce electricity and low temperature waste heat. The residual fuel and high temperature waste heat can be used in bottoming cycles such as gas turbine. This difference originates from the internal reforming and high temperature operation of SOFCs. The internal reforming recovers irreversible heat loss in SOFCs as heat source for endothermic reforming reaction. SOFCs save fuel, which is required for the heat source of external reforming of
385
the other fuel cells: it amounts up to 25% of fuel. Thus, as a total system, SOFCs can produce higher efficiency than the other fuel cells. 3.1.3. Do SOFCs have high efficiency even as single devices? From the viewpoint of efficiency, it is considered that the external fuel reforming process decreases efficiency remarkably. However, theoretically the situation is fairly complicated. Because the reforming reaction is an endothermic process, the 25% fuel spent at reforming can be recovered in produced CO and H2 as shown in Fig. 1. This relation raises one question: ‘‘Does the external reforming reduce efficiency?’’ The efficiency of fuel cells is mainly governed by fuel utilization rate (Uf) and output voltage (Vout). Textbooks say that the Vout decrease according to the decrease of partial pressure of oxygen (PO2) of fuel, which decreases according to Uf and the Vout drops drastically when Uf exceeds 95%. In actual cases, the internal resistance of cell influences Vout remarkably, especially by those of electrodes. The activity of electrodes is usually governed by the PO2 of fuel, which is Uf again. Thus, we cannot simply conclude that the reforming will reduce the efficiency of fuel cells, since 1. High Uf, usually 85– 90%, of SOFCs is advantageous, but disadvantageous for anode activity. 2. Low Uf of the other fuel cells works contrariwise, and the spent 25% fuel can be recovered in CO and H2. In summary, SOFCs can offer high efficiency because of high Uf, high electrode activity at high temperature, heat recovery by internal reforming, and bottoming cycles using remaining fuel and high temperature wastes heat. How to keep these merits of SOFCs, even when they are not combined with bottoming cycle, will be one key technology for the compact size and reduced temperature SOFCs. 3.1.4. Thermodynamics of fuel cells at reduced temperature The above discussion shows that the high efficiency of SOFCs is mainly originated from the application of bottoming cycle. For the case of compact reduced temperature SOFCs, it will be better to re-examine
386
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
Fig. 1. Internal reforming in SOFCs and the other fuel cells.
the thermochemical relation of fuel cells. Fig. 2 shows three possible cases for electrochemical oxidation (fuel cell oxidation) of reformed hydrocarbon. Case A
CH4 þ 2O2 ! CO2 þ 2H2 O
ð1Þ
Case B
CH4 ! C þ 2H2
ð2Þ
C þ 2H2 þ 2O2 ! CO2 þ 2H2 O
ð3Þ
Case C
CH4 þ H2 O ! CO þ 3H2
ð4Þ
CO þ 3H2 þ 3=2O2 ! CO2 þ 2H2 O
ð5Þ
Case A is advantageous at high temperature but not yet confirmed [6]. Case B was not studied well because of the obstruction by deposited carbon [7,8]: we will discuss later how to utilize case B progressively in order to develop SOFCs, which can start
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
387
at lower power output point than their maximum power output. In the case of conventional SOFCs, the stationary load operation is assumed in order to keep high efficiency and effective cost performance. However, as discussed before, generators applicable for peak load will become more and more important in the future. In this aspect, the high efficiency at low load is a big merit of not only SOFCs but also of fuel cells in general for future electricity system. Imagine a case when we furnish a 3 kW SOFC at home, the device can be operated at 300 W base load for appliances such as a refrigerator with very high efficiency, and can output 3 kW at intermittent peak load with enough high efficiency. In such a case, the ‘‘hot start’’ operation will be possible and the difficult quick ‘‘cold start’’ problem can be eliminated. 3.3. How to start quickly—hot standby or cold standby
Fig. 2. Thermochemical system of reforming, thermal decomposition, and fuel cell generation.
up quickly. Case C becomes advantageous below 650 jC, whereas the steam reforming becomes disadvantageous below this temperature as shown in the upper part of Fig. 2. When fuel becomes higher hydrocarbon, the decomposition reaction (case B) becomes dominant, whereas the reforming still requires high temperature. The combination of high temperature reforming and low temperature fuel cells is a good choice for the external reforming system. The ideal case will be that the internal reforming recovers the heat, and Uf is high, preferably near 95% without decreasing electrode activity, and fuel cell generation (case C) at reduced temperature. The key technology is highly active electrodes and internal reforming at reduced temperature for such a case. 3.2. High efficiency at throttle down—suitability for variable load operation Fuel cells give higher conversion efficiency at lower current density. So they are usually operated
If we assume that there is a cell stack, which has 1 kW/l power output, 5 kg of weight, and 1 J/g K of specific heat. Then this cell requires (1 J g 1 K 1) (5000 g) (700 K) = 3.5 106J 1 kWh to warm up to 700 jC. The heat loss by conduction can be estimated as follows. The cell is cube of 1 l = 10 10 10 cm, with a 1-cm-thick heat insulator having thermal conductivity of 5 10 2 Wm 1 K 1. It will exhaust heat at a rate of (5 10 2 Wm 1 K 1) (6 10 2 m2) (1 10 2 m 1) (700 K) = 210 W at 700 jC. Vacuum container can largely reduce this heat loss as the case of Na –S battery or SOFC of Sulzer. This rough estimation suggests that the heat loss by heating up a cell will be larger than holding the cell at high temperature. The hot standby will become more advantageous when the scale of SOFCs becomes larger, since specific surface per volume will decrease. Now we can come to one tentative conclusion. In order to manage quick start up and response, it will be advantageous to adopt hot standby system for SOFCs. Based on this estimation, we can conceive the following cases. (1) In the case of house use, there is about 200 – 300 W stationary demands for a refrigerator. Thus, we can operate a compact size SOFC at throttled mode with high efficiency. The situation is similar for stores, offices, etc. (2) In the case of automobile, the situation will be not advantageous as the case of home use. The
388
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
electricity produced during hot standby can be stored in the auxiliary battery or connected to the grid network, if possible. However, this may be only adaptable for buses, tracks, and taxis and not suitable for private cars, which are operated only on weekends.
4. Technological problems In order to reduce the operation temperature, there are several fundamental problems: (1) Are there any electrolytes that have enough high ionic conductivity? (2) Are there any electrodes that show enough high activity? (3) How to fabricate cells by cheap manufacturing process? (4) Is it possible to execute internal reforming even at these reduced temperatures? (5) How to make quick start up and/or quick response to variation of load? There is one consensus to meet the above requirements: it is that the cell is preferably built upon Ni anode substrate and is fabricated by wet process, especially co-fire process. This is because, (1) The usage of many amounts of metal components is preferable to reduce material cost and to respond quickly to temperature swing because of high thermal conductivity. The Ni anode substrate fits well with this requirement. (2) The co-fire process is suitable for mass production and to fabricate very thin electrolyte films, which can reduce the material cost and ionic resistance. 4.1. Feasibility of co-fire process 4.1.1. Electrolytes Table 1 summarizes the ionic conductivity of candidate electrolyte, yttria stabilized zirconia (YSZ), scandia stabilized zirconia (SSZ), rare earth doped ceria (CRO), and lanthanum strontium magnesium galate perovskite (LSGM) measured by us. How much ionic conductivity will be required for electrolyte? If we set 1
Table 1 Candidate electrolyte (V cm 2)
100 100 100 100
Am Am Am Am
YSZ SSZ CGO LSGM
550 jC
600 jC
700 jC
800 jC
900 jC
– – – 0.71
2.2 0.90 (0.59) 0.37
0.77 0.26 0.26 0.15
0.31 0.099 0.13 0.074
0.15 0.046 0.071 –
YSZ: Zr(Y)O2, SSZ: Zr(Sc)O2, LSGM: La0.8Sr0.2Ga0.85Mg0.15 [10], CGO: Ce0.8Gd0.2O2 [9], (0.59); Ce0.9Gd0.1O2 [9], CRO: Ce(rare earth)O2.
V/cm2 as internal resistance for a target cell, preferably less than 0.3 V/cm2 can be allowed for the electrolyte. According to Table 1, for the operation at 600 jC, 15 Am for YSZ, 30 Am for SSZ, 50 Am for CRO, and 80 Am for LSGM will be the allowable limit of electrolyte thickness. These values are not so unrealistic. In the case of CRO electrolytes, we should pay attention to its electronic conduction at reducing atmosphere. Kato et al. [11] estimated that the CRO electrolyte cells would have higher conversion efficiency at 600 jC than 700 jC due to electronic conduction. 4.1.2. Electrodes Table 2 summarizes results we obtained. The interfacial resistances of electrode/electrolyte at open circuit voltage (OCV) state are shown. As anode, NiCSO showed acceptable activity on CGO electrolyte at 600 jC [12,13]. The effectiveness of Ni-SSZ or YSZ anode is not yet confirmed at 600 jC. As cathode, Pt-SSZ [14] and Pt-Ag-SSZ [14] or LSCAg-CGO [12,15] (here, LSC represents (La, Sr) CoO3) showed fairly high activity on YSZ, SSZ or CGO electrolyte, respectively. The demerit of Pt-AgSSZ cathode is that it requires 20 mg/cm2 to show the optimum activity shown in Table 2. It should be noted that CRO cermet electrodes showed fairly high activity on CRO electrolytes, whereas the activity degrades remarkably on SSZ and YSZ electrolytes. This is apparently because of the reaction between zirconia and LSC or CRO [16,17]. Matsuzaki et al. [18,19] successfully mounted CRO cermet electrodes on YSZ electrolyte forming a protective thin CRO layer on YSZ using organo-metal reagents. We have tried to form similar thin (2 Am) CGO layer on SSZ by EPD and co-fire process instead of using costly organo-metal reagents. The reaction between LSC and zirconia seemed likely to
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392 Table 2 Interfacial resistance of new electrodes by ac impedance at OCV (V/cm2)
Cathode Cathode Cathode Cathode Anode Anode
Materials
Electrolyte 600 jC 700 jC
Pt-SSZ Pt-Ag-SSZ (Sm0.6Sr0.4)CoO3-CYO-Ag (La0.2Sr0.2)CoO3-CGO-Ag Ni-CSO Ni-CSO
YSZ YSZ SSZ CGO SSZ CGO
1.25 0.67 7.1 0.83 8.9 0.25
0.14 0.083 0.90 0.19 1.16 0.10
CYO: Ce0.8Y0.2O2, CGO: Ce0.8Gd0.2O2, CSO: Ce0.8Sm0.2O2.
be retarded. However, the formation of solid solution between zirconia and ceria seemed likely to be not stopped because of high co-firing temperature of 1300 jC. The utilization of CRO electrodes on YSZ or SSZ electrolyte is one of technological problems to be solved in future. 4.1.3. Feasibility of co-firing process and performance of SSZ cells Wet co-fire process is considered to be a more costefficient process than dry processes because of low cost of manufacturing device, high efficiency of material utilization, adaptability of mass production. Thus, we have examined the adaptability of co-firing processes for candidate electrolytes, SSZ or YSZ, CRO, and LSGM [10,20,21]. The target cell constitution is Ni-substrate/Ni-electrolyte anode/electrolyte/cathode Electrolyte = YSZ, SSZ, CRO, or LSGM, substrate = 3 mol% YSZ or MgAl2O4, etc. Ni-3 mol% YSZ is used as the main substrate in order to reduce material cost and to increase the mechanical strength. In this viewpoint, more cheap materials such as MgAl2O4 should be examined in the future. Here, the cathode is pasted and fired after cofiring electrolyte on Ni-substrate. The outline of standard process is as follows. NiO and YSZ, SSZ, CRO are ball milled using binder (poly-vinyl-butyral), plasticizer (di-n-butyl phthalate), surfactant (triton-X), peptizer (fish oil), solvents (toluene and iso-propanol), and pore former (polymethyl-meta-acrylate). (1) The slurry was impregnated in paper or cloth. This ‘‘skeleton’’ of paper or cloth was used as pellet or
389
rolled tube. The skeleton was used as green state or after pre-fired at 900 jC. (2) The similar process of step (1) prepared the electrolyte slurry. However, the recipe was changed case by case. (3) The electrolyte slurry was coated on the skeleton pellet or tube by dipping, brushing, spraying, or doctor blade. (4) The electrophoretic deposition (EPD) of electrolyte film was also examined. The results can be summarized as follows: (1) The co-fire process for CRO was not so easy compared to the case of SSZ or YSZ due to the fragility and ill-sinterability of CRO. (2) In the case of LSGM, the co-fire was not yet succeeded, because Ni in substrate diffused vigorously through electrolyte film to cathode side during cofiring. Lowering of co-fire temperature was not effective to stop this diffusion. Taiheyo-cement (Japan) developed LSGM powders, which can be sintered at 1300 jC. By this powder, however, Ni still diffused through the LSGM electrolyte layer to cathode. The SSZ cells thus prepared were tested with humidified hydrogen fuel and air. The cell performance is shown in Figs. 3 and 4. The EPD cell had 20 Am SSZ electrolyte, whereas slurry coated cell had 70-Am-thick electrolyte film. The performance at 600 jC was far from satisfactory compared to that at 700
Fig. 3. Performance of slurry coat/co-fire cell at 700 jC (o 800 jC (5 n) l: Pt-SSZ/SSZ (70 Am)/Ni-SSZ.
.) and
390
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
jC. As can be seen in Figs. 3 and 4, the electrodes occupied large portion of cell resistance compared to that of bulk = electrolyte + substrate. Since the slurry coat cell had thicker electrolyte film and the anode was common, the cathode was improved in the case of slurry coat cell. We can expect further improvement if we use the improved cathode shown in Table 1, since at the test experiments, the optimum preparation condition of cathode was not yet known. 4.2. Internal reforming and carbon deposition—new concept of carbon fueled SOFCs
Fig. 4. Performance of EPD/co-fire cell: Pt-SSZ/SSZ (20 Am)/NiSSZ (1 mm).
Internal reforming is one of the merits of SOFCs over the other fuel cells. However, this advantage will be uncertain when the operation temperature is lowered. As discussed before, the equilibrium is not favorable for steam reforming (Fig. 2). However, the steam reforming can be ‘‘conjugated’’ with the electrochemical oxidation (fuel cell reactions) of CO and H2. If the produced CO and H2 are consumed quickly, the
Fig. 5. Dry CH4 and deposited carbon fueled SOFC (FeO – CGO/YSZ (1.5 mm)/LSMO at 1000 jC.
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
equilibrium can be shifted. A serious problem is that this temperature is the region of carbon deposition as can be seen in Fig. 2. Fig. 2 shows that when fuel becomes higher hydrocarbon, the thermal decomposition temperature goes down, whereas the steam reforming still requires high temperatures. Much addition of steam is effective to retard the deposition of carbon. Undoubtedly, however, much steam recycle is not a favorable choice to obtain high efficiency. The key issue is the deposition of carbon. If we can accept the carbon deposition, there can be a new internal reforming system. Let us assume that a SOFC cell is at hot standby, hydrocarbon fuels are introduced directly, and then they will decompose to carbon and H2. Hydrogen will be consumed immediately in a following SOFC or even in PEMFC. Questions are: (1) can steam or carbon dioxide reform the deposited carbon? and (2) can the carbon be electrochemically oxidized? In order to answer this question, we tried to examine the feasibility of direct carbon fueled SOFCs [22,23]. Using commercially available YSZ tubes (10 mm outer diameter with 1.5 mm thickness), we consisted SOFCs with Fe –O anode. In Fig. 5, dry CH4 was passed at shunt current state, then at open circuit state. After that, dry Ar was passed at shunt current state. The current at shunt current state under dry CH4 and Ar flow indicated that the deposited carbon could be electrochemically oxidized by a SOFC cell. Ni anode lost activity quickly. Reforming by steam or CO2 proceeded but not so smoothly on Fe– O anode. The experiments were done only at 1000 jC, so too many questions remain for future works. Such SOFCs will be fairly convenient for quick start up from hot standby state, since H2 can be produced quickly. The deposited carbon will serve as the fuel stock in SOFC itself and consumed during hot standby, in other words, as carbon battery.
5. Summary The applicability of compact size SOFCs, which will be operated at reduced temperatures, was discussed for small co-generation at homes, offices, stores and automobiles. High efficiency of SOFCs at throttle down operation will be profitable for intermittent loads. For difficulty of quick start up, hot standby mode was recommended from a viewpoint of
391
heat loss. In order to develop cheap compact size, reduced temperature SOFCs, the feasibility of anode supported SOFCs was investigated on SSZ, YSZ, CRO, and LSGM electrolytes examining the applicability of wet co-fire processes and electrode activity. The feasibility is confirmed with zirconia, not yet with ceria due to its fragility, and pessimistic with LSGM due to Ni diffusion during co-firing. In order to manage the quick start up and usage of various fuels, an electrochemical oxidation of deposited carbon was tested. A new direct internal reforming concept was proposed using this carbon fueled SOFC. Experimental results are not yet enough to show the feasibility of this carbon fueled SOFC and internal reforming at reduced temperature.
Acknowledgements This research was supported by the proposal-based new industry creative project of New Energy and Industrial Development Organization (NEDO) of Japan. The author expresses deep thanks to NEDO and also to Dr. T. Kato and Dr. S. Wang, Electrotechnical Laboratory, and Dr. N.T. Lan, Yokohama National University, for their collaboration both in the experiments and discussion.
References [1] W.L. Lundberg, S.E. Veyo, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 78 – 87. [2] Y. Sakaki, A. Nakanishi, M. Hattori, H. Miyamoto, H. Aiki, K. Takenobu, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 72 – 77. [3] J. Iritani, K. Kougami, N. Komiyama, K. Nagata, K. Ikeda, K. Tomida, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 63 – 71. [4] M. Kuroishi, S. Furuya, K. Hiwatashi, K. Omoshiki, A. Ueno, M. Aizawa, K. Tsujimoto, U. Uchida, H. Yoshikawa, K. Shukuri, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 88 – 93. [5] S.C. Singhal, in: O. Yamamoto, et al. (Eds.), Proc. of Intl. Symp. Fuel Cells for Vehicles, Nagoya, The Electrochem. Soc, Japan, Nov. 2000, pp. 26 – 36.
392
M. Dokiya / Solid State Ionics 152 – 153 (2002) 383–392
[6] M. Ihara, C. Yokoyama, A. Abudula, R. Kato, H. Komiyama, K. Yamada, J. Electrochem. Soc. 146 (1999) 2481 – 2487. [7] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 143 (1996) 1161 – 1168. [8] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 142 (1996) 2621 – 2624. [9] S. Wang, T. Kobayashi, M. Dokiya, T. Hashimoto, J. Electrochem. Soc. 147 (2000) 3606 – 3609. [10] T.N. Lan, K. Kobayashi, Z. Cai, I. Takahashi, K. Yasumoto, M. Dokiya, S. Wang, T. Kato, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 1042 – 1050. [11] T. Kato, K. Nozaki, S. Wang, S. Negishi, S. Nagata, in: M. Dokiya, S.C. Singhal (Eds.), Proc. 6th Intl. Symp. SOFC, Honolulu. The Electrochem. Soc., Oct. 1999, pp. 116 – 124. [12] S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwashita, M. Dokiya, in: H. Yokokawa, S.C. Singhal (Ed.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ, USA, June 3 – 8, 2001, pp. 607 – 616. [13] S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwashita, M. Dokiya, J. Electrochem. Soc. (submitted for publication). [14] K. Sasaki, J. Tamura, M. Dokiya, S. Badwal (Eds.), Solid State Ionics, in press. [15] S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwa-
[16] [17] [18]
[19]
[20]
[21]
[22]
[23]
shita, M. Dokiya, Solid State Ionics (submitted for publication). T. Horita, N. Sakai, H. Yokokawa, M. Dokiya, K. Kawada, Jan Van herle, K. Sasaki, J. Electroceram. 1 (1997) 155 – 164. A. Tsoga, A. Gupta, A. Naumididi, P. Nikokopoulos, Acta Mater. 48 (2000) 4709 – 4714. I. Yasuda, Y. Baba, T. Ogiwara, H. Yakabe, Y. Matsuzaki, in: H. Yokokawa, S.C. Singhal (Eds.), Proc. 7th Intl. Symp. SOFC, Tsukuba, The Electrochem. Soc., Pennington, NJ USA, June 3 – 8, 2001, pp. 131 – 139. Y. Matsuzaki, I. Yasuda, in: S. Badwal (Eds.), Ext. Abstr. of SSI-2001, Cairns, The Intl. Soc. Solid State Ionics, CSIRO, Australia, July 8 – 13, 2001, p. 131. Z. Cai, T.N. Lan, S. Wang, M. Dokiya, S. Badwal (Eds.), Solid State Ionics, Proc. of SSI-2001, Cairns, July 8 – 13, in press. K. Kobayashi, I. Takahashi, M. Shiono, M. Dokiya, S. Badwal (Eds.), Solid State Ionics, Proc. of SSI-2001, Cairns, July 8 – 13, in press. M. Dokiya, K. Yasumoto, J. Tadani, in: O. Yamamoto, et al. (Eds.), Proc. of Intl. Symp. Fuel Cells for Vehicles, Nagoya. The Electrochem. Soc, Japan, Nov. 2000, pp. 26 – 36. K. Yasumoto, K. Furusho, J. Tadani, T.N. Lan, S. Cai, M. Dokiya, in: S. Badwal (Ed.), Ext. Abstr. of SSI-2001, Cairns, The Intl. Soc. Solid State Ionics, CSIRO, Australia, July 8 – 13, 2001, p. 123.