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A direct carbon solid oxide fuel cell stack on a single electrolyte plate fabricated by tape casting technique
Journal of Alloys and Compounds 794 (2019) 294e302
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A direct carbon solid oxide fuel cell stack on a single electrolyte plate fabricated by tape casting technique Wei Wang a, Zhijun Liu a, Yapeng Zhang a, b, Peipei Liu a, Qianyuan Qiu a, Mingyang Zhou a, Meilin Liu a, c, Jiang Liu a, * a
Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou, 510006, China National Engineering Laboratory for Modern Materials Surface Engineering Technology & The Key Lab of Guangdong for Modern Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou, 510650, China c School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA, 30332-0245, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2018 Received in revised form 23 April 2019 Accepted 24 April 2019 Available online 27 April 2019
A four-cell-in-series stack of solid oxide fuel cell is fabricated on a single yttrium-stabilized-zirconia electrolyte plate which is fabricated by tape casting technique. The slurry for the tape casting process is characterized and optimized through Zeta potential and viscosity measurements. The optimal slurry is used to prepare the electrolyte plate, with electrical connecting holes, for the stack. The stack is operated directly with 5 wt% Fe-loaded activated carbon as the fuel. With a total effective area of 8.8 cm2 and carbon fuel of 5 g, this direct carbon solid oxide fuel cell stack shows an open circuit voltage of 3.82 V and an output power of 2.2 W at 820 C. It gives a discharging platform of over 3.0 V for 10.5 h with a constant discharging current of 200 mA, at 800 C. Then voltage drops to zero after discharging for 14 h, releasing a discharging energy of 8.6 W h. This work shows a possibility of developing direct carbon solid oxide fuel cells for applications of portable or distributed power sources. © 2019 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Carbon fuel Electrolyte plate Fuel cell stack Tape casting
1. Introduction Carbon is richly reserved on the earth in forms of coal, plant, and a variety of biomasses. A direct carbon fuel cell (DCFC) can directly convert the chemical energy of carbon into electricity through electrochemical reactions [1e5]. Its electrical conversion efficiency, which is defined as the Gibbs energy change, DG, divided by the enthalpy change, DH, of the complete oxidation of carbon, is slightly over 100%. Besides, the energy density of carbon is as high as ~9 A h g1. Therefore, DCFCs have attracting increasing interest in recent years in developing electricity generators of high efficiency and low emissions. Among different kinds of DCFCs, a direct carbon solid oxide fuel cell (DC-SOFC) has the advantage of all-solid-state configuration. Actually, it is a solid oxide fuel cell (SOFC) directly operated with solid carbon as the fuel [6e15]. According to thermodynamics, CO dominates the equilibrium gas composition of a carbon-oxygen system with exceeding carbon. Thus, at high operating
* Corresponding author. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.jallcom.2019.04.263 0925-8388/© 2019 Elsevier B.V. All rights reserved.
temperature (e.g., ~800 C), CO is produced in the anode chamber through the reaction between the carbon fuel and residual air. When the electrical circuit is closed, CO is electrochemically oxidized by oxygen ion coming from the cathode through the electrolyte to produce CO2 and devote electrons (1). CO þ O2 ¼ CO2 þ 2e
(1)
The CO2 molecules diffuse to the carbon fuel to perform the reverse Boudouard reaction (2) and produce more CO. CO2 þ C ¼ 2CO
(2)
Some of the CO molecules may diffuse back to the anode for the electrochemical oxidation reaction (1) and the others emit from the cell. The coupling of reactions (1) and (2) maintains the continuous operation of a DC-SOFC [10]. Obviously, a DC-SOFC does not need any carrier gas. Note that in the above reaction scheme, the initial CO may also be provided by the direct electrochemical oxidation of carbon
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C þ O2 ¼ CO þ 2e
(3)
At lower operating temperature, the direct complete electrooxidation of carbon is possible C þ 2O2 ¼ CO2 þ 4e
(4)
Except for the advantages of high efficiency and energy density of DCFCs, a DC-SOFC has its own superiority that it does not need any gas feeding or liquid medium, leading to a very simple configuration and a good durability. All these features of a DC-SOFC suggest its promise to be developed for portable or distributed power applications [16,17]. There have been efforts focusing on developing DC-SOFC stacks for portable application [18e22]. A tubular anode-supported segmented-in-series 3-cell-stack has given an open circuit voltage of 3.0 V and a peak power of 2.4 W at an operating temperature of 850 C [23]. Recently, our group has prepared and tested a tubular DC-SOFC of 3-cell-stack with carbon fuel located on the outside of the stack tube, a way allows more carbon stored in the stack (i.e., 17 g carbon loaded outside compared to 3 g filled inside the stack). It gave a peak power of 4.0 W at 800 C. It discharged at a constant current of 1 A for 19 h, giving a discharging energy of 31.6 W h [24]. While the above tubular stack design shows promising feasibility, planar configuration is more suitable for large scale manufacturing which may reduce fabricating cost [25,26]. We have proposed a DC-SOFC stack based on a single electrolyte plate, as shown in Fig. 1 [27]. An electrolyte plate is punched to make some holes (Fig. 1(a)) which are used to electrically connect the cathodes on one side of the plate (Fig. 1(b)) to the anodes on the opposite side (Fig. 1(c)) to form a stack (Fig. 1(d)). Such SOFC stack can give high voltage with small scale and is especially suitable for small or portable application. The electrolyte plate with punched holes is the key component of the stack. Tape casting, which is a standard shaping technique used to produce flat ceramic sheets [28e30], is a proper technique for preparing the electrolyte plate. Because the green plate prepared by tape casting has good plasticity, it is easy to punching holes on it. However, to obtain high quality electrolytes, the key to the technique, i.e., the slurry, needs to be optimized. Here, we report our work on preparing and testing a four-cellin-series stack of DC-SOFC prepared by tape casting technique. First, the slurry used for the technique is optimized by adjusting the
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contents of dispersant, binder, and plasticizers as well as solid loading. Then the optimized formula of slurry is applied to prepare the electrolyte of a four-cell-in-series stack of DC-SOFC. Finally, the performance of the stack is tested and the feasibility of developing this kind of DC-SOFCs for portable or distributed applications is discussed. 2. Experimental 2.1. Preparation of slurry for tape casting Yttria-stabilized-zirconia ((ZrO2)0.92(Y2O3)0.08, YSZ) was used as the electrolyte material. 30 g YSZ (TZ-8Y, Tosoh Corporation, Tokyo, Japan, 99.99%), 1.2 g Al2O3 (Xinfumeng, China, 99.99%) as sintering aid, and a certain amount of triethanolamine (TEA, Shanghai Lingfeng Industry Co., Ltd, China. The following polyvinyl butyral (PVB) and polyethylene glycol 600 (PEG-600) were also purchased from this company.) as dispersant [31] were mixed with ethanol (Sinopharm Group China, A.R.) through ball milling in a planetary ball mill (DM-4L, Nanjing Daran, China) for 1 h. Then, some PVB, dioctyl phthalate (DOP, Jiangsu Linghua, China, A.R.), and PEG-600 were added and the ball milling was continued for 1.5 h to obtain an electrolyte suspension. To optimize the formulae of tape casting slurry, a variety of slurries were made through adjusting the additives and solid loading. TEA of 1e4 wt%, PVB of 5e12 wt%, DOP of 1e8 wt%, PEG of 1e8 wt% and solid loading of 34e48 wt% (solid loading refer to the overall slurry, others refer to YSZ) were respectively investigated. 2.2. Preparation of YSZ electrolyte plates by tape casting technique A prepared slurry was poured onto the glass plate equipped in the tape casting machine (Shenzhen Kejing Technology Company). Then, a leveling blade, with a controlled distance to the surface of the glass plate, moved slowly over the slurry and a film was casted on the glass plate. The film was dried and peeled off. The thickness of the film was controlled to be ~30 mm. Subsequently, several layers of the film were laminated by a thermos-compressor (PCH600C, Tianjin Zhonghuan Furnace Corp) with an applied pressure of 20 MPa and temperature of 60 C for 15 min to form a green electrolyte tape. To optimize the formulae of slurry, pellets with diameter of 13 mm were made by punching a green electrolyte tape with 14 layers of film laminated and sintered at 1450 C for 4 h. They are
Fig. 1. A schematic illustration of a four-cell-in-series SOFC stack based on a single electrolyte plate with punched holes.
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used as samples for relatively density measurement, microstructure characterization, and hydrogen fueled SOFC preparation. The slurry with optimized formula was used to tape cast the electrolyte for DC-SOFC single cell and stack. For this, 10 layers of the casted film were laminated together, and the resulted layer was tailored to make a rectangular plate with a length of 7.6 cm and width of 3.8 cm. A small hole with a diameter of ~0.5 mm on the top and several other holes with a diameter of ~1 mm were made by punching the plate. The plates were sintered in air at 1450 C for 4 h. The thickness of the sintered electrolyte plate was 220 mm. 2.3. Assembling of SOFCs and DC-SOFCs For all the SOFCs operated with hydrogen and DC-SOFCs, cermet of silver and GDC (Ce0.8Gd0.2O1.95, Fuel Cell Materials) with a weight ratio of Ag: GDC ¼ 7 : 3 was used as the material of both cathode and anode. A homogeneous Ag-GDC paste was prepared by mixing silver paste (DAD-87, Shanghai Institute of Synthesized Resins) and GDC powder with binder solution of PVB in terpineol (concentration of 10 wt%) through grinding the mixture in an agate mortar. 2.3.1. Button cells of SOFC operated on hydrogen The Ag-GDC paste was brush-painted on both sides of each electrolyte pellet, and dried at 140 C for 30 min. The process was repeated for 4 times to achieve a required thickness. Subsequently, the painted pellets were sintered at 880 C for 2 h [8]. The effective area of the button cell, i.e., the area of the cathode was 0.2 cm2. The thickness of both the anode and cathode was about 30 mm. Each button cell was sealed on the top of a quartz tube with silver paste as sealing and jointing material. Ag wires were attached to the silver paste as current leading wires. 2.3.2. Planar DC-SOFC Ag-GDC electrodes were coated on the sintered rectangular electrolyte plates, in the same way as for the button cells. A single planar electrolyte cell with completely symmetric cathode and anode was prepared, as shown in Fig. 2(a) and (b). The area of electrodes were both designed as 20 mm 40 mm with an active area of 8 cm2. A four-cell-stack, as shown in Fig. 2(c) and (d), was assembled. Each single cell of the stack was with a size of 22 mm 10 mm which corresponded to an active area of 2.2 cm2, which meant the total effective area of the four-cell-stack was
8.8 cm2. The interval between two adjacent cells was 4 mm. The cells were electrically connected in series by using silver paste to connect the anode of one cell to the cathode of the next cell through the electrical connecting holes. Silver grids were painted on the cathode and anode as current collectors, and silver wires were attached on proper anode and cathode as current leading wires. Activated carbon (Aladdin, Shanghai, China; A.R.) loaded with 5 wt% Fe catalyst [8] by wet agglomeration process [32] was used as the fuel. 5 g of the fuel was filled into an alumina container with a size of 6 cm 3 cm 1.5 cm. Then the planar electrolyte-supported single cell or the four-cell-stack was sealed on the open top of a carbon fuel container to make a DC-SOFC single cell or a stack, as shown in Fig. 2. It is worth noting that no carrier gas is fed in the anode chamber of the DC-SOFCs. 2.4. Characterization The YSZ slurry viscosities were measured using a rotary viscometer (Shenzhen Kejing Technology Company, MSK-SFMVT8S). The stability of these YSZ slurries was investigated through Zeta potential analysis (Brookhaven, USA). The microstructure of the electrolytes were examined using cold field scanning electron microscope (SEM, Hitachi SU8010). The relative density of sintered electrolyte pellet was tested by the Archimedes methods. The button SOFCs were operated with humidified hydrogen (3 vol% H2O, 50 mL min1) as fuel and ambient air as oxidant. The planar DC-SOFCs also use the ambient air as oxidant. Their electrochemical performances were tested using an Iviumstat electrochemical analyser (Ivium Technologies B.V., Netherlands). The impedance was measured in the frequency range of 100 KHz0.01 Hz with a signal amplitude of 10 mV under open circuit condition. 3. Results and discussion 3.1. Optimizing the formulae of slurries for tape casting YSZ electrolyte Zeta potential of a slurry can represent its stability in a certain extent, while viscosity of a slurry affects the ductility of slurry for tape casting process. Both of them are very important for obtaining
Fig. 2. Schematic diagram of illustrations and pictures of DC-SOFC single cell [(a) and (b)] and four-cell-in-series stack [(c) and (d)].
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green tapes with good uniformity and consistency. A good slurry should have a high Zeta potential and a relatively low viscosity. TEA is an important dispersant in YSZ slurry for tape casting. Zeta potential and viscosity change with TEA content of YSZ slurry with solid loading as 40 wt% (no binder and plasticizers) was tested and the results are shown in Fig. 3(a). It can be seen that the viscosity decreases with the increase of TEA content from 0.5 wt% to 2 wt%. It is possible that steric stabilization plays an important role in preventing the particles from agglomeration using organics as solvents [33]. When TEA content increases further, from 2 wt% to 2.5 wt%, the viscosity increases due to mutual cross-linking effect of the excess TEA after its adsorption saturation on the surfaces of YSZ particles. A similar trend in the increase of slurry viscosity beyond the optimum concentration of dispersant has also been reported for Al2O3 [28], AlN [34] and BaTiO3 [35] based slurries. The Zeta potential versus dispersant concentration is opposite to viscosity. The Zeta potential reaches its maximum when the concentration of TEA is about 2 wt%. The decrease of Zeta potential for high concentrations of TEA may be related to the increase of ion concentration
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which leads to a decrease in the double layer thickness [35]. Therefore, combined with the Zeta potential and viscosity measurement results, 2 wt% is selected as the optimal TEA content. The Zeta potential and viscosity changes with different PVB content in slurries with the optimum mass ratio of TEA as 2 wt% and initial mass ratios of DOP, PEG, and solid loading as 4 wt%,4 wt %, and 40 wt%, respectively, were tested and the results are shown in Fig. 3(b). Compared with Fig. 3(a) and (b) shows that the Zeta potential of slurries with addition of binder and plasticizers are lower than those only with dispersant because of synergistic effect. It can be seen from Fig. 3(b) that the viscosity of slurries slowly increases with the addition of PVB increasing from 5 wt% to 8 wt%, then the upward trend becomes more obvious. Meanwhile, the Zeta potential reaches maximum at PVB of 8 wt%. Therefore, 8 wt% PVB content is chosen for the tape casting of YSZ slurry. DOP, functioning as A-type plasticizer, is used to soften the binder polymer chains, allowing them to flow under an applied force. It can also be described as a glass-transition temperature (Tg) modifier or binder solvent [33]. The Zeta potential and viscosity
Fig. 3. Viscosity and Zeta potential with the change of different concentration of TEA (a), PVB (b), DOP (c), PEG (d) and solid loading(marked as SL) (e).
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changes with the varying content of DOP in slurry with TEA, PVB, PEG, and solid loading as 2 wt%, 8 wt%, 4 wt%, and 40 wt%, respectively, were measured and the results are shown in Fig. 3(c). It can be seen that the viscosity rises slowly with the addition of DOP and remains almost unchanged when the addition is over 3 wt%. Meanwhile the Zeta potential reaches the maximum at the DOP content of 3 wt%. In a similar way, the optimal content of PEG in slurry with TEA, PVB, DOP, and solid loading as 2 wt%, 8 wt%, 3 wt%, and 40 wt%, respectively, is 4 wt%, according to Fig. 3(d). PEG is functioning as B-type plasticizer which works as a lubricant in the tape matrix. It works between the polymer chains, not only allowing a better mobility within the dry tape, but also preventing some of the “cross-linking” between the chains. However, excessive plasticizer will reduce the yield stress and weaken the strength of the tape to an unusable level by preventing polymer ties from forming [36]. In tape casting, it is also very important to obtain high solid loading slurries with good fluidity in order to obtain green tapes with homogenous microstructure and high green density [37]. High solid loading slurry also enables high processing efficiency. With the optimum contents of the organic additives, the influence of solid loading on viscosity and Zeta potential of slurry is measured and the results are shown in Fig. 3(e). The Zeta potential changes little with increasing solid loading of the slurries. While the viscosity increases gradually with the solid loading increasing from 34 to 42 wt%, it rises sharply when the solid loading is over 42 wt%. As mentioned, high viscosity may affect the stretching of slurry and result in voids of casted products. Trading off between high solid loading and low viscosity, a solid loading of 42 wt% may be proper for an efficient tape casting processing. Consequently, the optimized formulae of slurry is that: TEA of 2 wt%, PVB of 8 wt%, DOP of 3 wt%, and PEG of 4 wt%. A solid loading of 42 wt% may be good for tape casting. However, whether it is the best needs to be verified through more experiments, as shown in the following.
loading. It shows that with the increase of solid loading, the relative density of the prepared electrolyte increases. This is because of the higher packing density of a green pellet prepared with a slurry of higher solid loading. However, the relative density of an electrolyte decreases when the solid loading is higher than 42 wt%, which may be caused by higher slurry viscosity and agglomeration of ceramic particles, resulting in the formation of inhomogeneous structure with voids formed on the casted film and tape. The relative density reaches maximum at the solid loading of 42 wt%, which agrees well
3.2. Performances of electrolytes prepared with slurries of different solid loading 3.2.1. Microstructure Relative density is the most representative parameter to characterize the density of electrolytes. Fig. 4 shows the variation of the relative density and thickness (obtained from the SEM images as will be shown later) of sintered electrolyte pellets (laminated with 14 layers of casted film) prepared with slurries of different solid
Fig. 4. The variation of relative density and thickness of electrolytes with different solid loading.
Fig. 5. Surface microstructures (left) and cross-sectional microstructures (right) of electrolytes with different solid loading.
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to the expectation based on the data shown in Fig. 3(e). Fig. 5 shows the SEM images of the surface and section of the sintered pellets prepared with slurries of different solid loading. We can see that when solid loading is lower than 42 wt %, the surface and cross section of the corresponding electrolytes have holes or cracks. When solid loading is above 42 wt%, although the surface has no obvious cracks, big flaws appear in the section. It shows that the solid loading of 42 wt% is the optimum. Meanwhile, the SEM images of the sections of the pellets show that the thickness of the pellets increases with solid loading of the slurry used, as shown in Fig. 4. This is because that higher solid loading of slurry leads to higher packing density of ceramic particles in green pellets, resulting in smaller shrinkage after sintering.
area specific ohmic resistance of a SOFC increases with the solid loading of slurry used to prepare the electrolyte while the polarization resistance of all the cells does not change much. Generally, in an electrolyte-supported SOFC, the resistance of the electrolyte dominates the ohmic resistance of the cell. The thickness of the electrolytes varies with the slurry with different solid loading (as shown later). The conductivity of the electrolytes can be calculated based on the data obtained from the impedance spectra and the results are shown in Fig. 6(c). The conductivity of electrolyte increases with the increasing solid content, and decreases after reaching the maximum at 42 wt%. This agrees well with the relative density of the electrolyte. Consequently, the optimal solid loading of slurry for tape casting YSZ electrolyte is 42 wt%.
3.2.2. Performances of SOFCs operated on hydrogen The electrolyte pellets prepared with slurries of different solid loading are used to make button SOFCs which are tested by using humidified hydrogen as fuel and ambient air as oxidant. Fig. 6 shows their output performances (a), impedance spectra at open circuit voltage (b), and conductivities of the corresponding electrolytes (c), at 800 C. We can see from the Fig. 6(a) that the cell with electrolyte prepared with slurry of 42 wt % solid loading shows the highest open circuit voltage (~1.08 V) and the maximum power density (~225 mW cm2). This result is consistent with the evidence that the electrolyte prepared with slurry of 42 wt% solid loading has the best microstructure and the highest density. From the Nyquist plots of impedance shown in Fig. 6(b), the ohmic resistance Ro (the value of the left intercept of the horizontal axis) and the polarization resistance Rp (the difference between the right and left intercepts) can be identified. It is observed that the
3.3. Performance of planar DC-SOFCs 3.3.1. A single planar DC-SOFC Fig. 7 shows a typical electrochemical performance of a single planar DC-SOFC operated with 5 wt% Fe-loaded activated carbon of 5.0 g as the fuel. Fig. 7(a) shows that the open circuit voltages of the cell increases with temperature. This is consistent with experimental expectation [8]. The cell gives an open circuit voltage of 1.02 V and a maximum power of 2.1 W, which corresponds to a power density of 263 mW cm2, at 820 C. This performance is comparative to a SOFC fueled with hydrogen because Fe loaded on carbon is functioning as an effective catalyst for promoting the reverse Boudouard reaction which produces sufficient CO for the electrochemical reaction [8,38]. Fig. 7(b) shows the impedance spectra of the cell at open circuit voltage. It can be seen that polarization resistance dominates the
Fig. 6. The output performances (a), impedance spectra at open circuit voltage (b) of SOFCs operated on hydrogen and conductivity of electrolyte prepared with slurries of different solid loading (c), all measured at 800 C.
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Fig. 7. The output performance (a), impedance spectra under open circuit voltage (b), a comparison of the impedance spectra to that of a cell operated on humidified hydrogen under 800 C (c), and discharge characteristics at 800 C (d) of a single planar DC-SOFC.
overall resistance of the cell, especially at lower temperature. Fig. 7(c) shows a comparison of the impedance spectra of the cells operated on humidified hydrogen and activated carbon, respectively, at 800 C. The polarization resistance of the DC-SOFC is almost 50% higher than that of hydrogen. There is research showing the large impedance change at low frequency is related to gas diffusion process [39]. It is noted that there is sign of gas diffusion limit for the DC-SOFC at the frequency of around 0.01 Hz. This is because CO concentration in the equilibrium gas product of carbonoxygen system, with excess of carbon, is low at reduced temperature, leading to higher concentration polarization. Fig. 7(d) shows the discharging performance of the single planar DC-SOFC operated at a constant current of 1 A and 800 C. The cell discharges at a platform of over 0.7 V for about 15 h. Then the voltage drops to zero after a total discharging time of about 19.5 h, which means a discharging capacity of 19.5 A h. Theoretically, the charge capacity of carbon (4 electron process) is 8.93 A h g1. Therefore, the practical electrical conversion efficiency of the single planar DC-SOFC can be calculated as ~46%. According to Fig. 7(d), the energy capacity of the cell is estimated to be ~13 W h, corresponding to a mass specific energy of carbon as 2.7 W h g1.
3.3.2. Four-cell-in-series stack of DC-SOFC Fig. 8 shows the performances of a four-cell-in-series DC-SOFC stack. Similar to the situation of the single planar DC-SOFC, both open circuit voltage and power density of the stack increase with operating temperature, as shown in Fig. 8(a). At 820 C, the open circuit voltage of the stack is 3.82 V and the maximum output power is 2.2 W, which corresponds a power density of 251 mW cm2.
Fig. 8(b) shows that the ohmic resistance and the polarization resistance of the stack at 820 C are 1.79 U cm2 and 2.14 U cm2, respectively. As a comparison, the performance of one unit cell of the four-cell-in-series stack is shown in Fig. 8(c). The open circuit voltage of the unit cell at 820 C is 0.96 V, 0.06 V lower than the planar single DC-SOFC. Its maximum power density is 266 mW cm2, comparable with the single planar DC-SOFC. Note that the open circuit voltage of the stack is exactly 4 times of the unit cell, and the power density of the unit cell is also close to that of the single planar DC-SOFC. Impedance measurements also show that the resistance of the stack is almost 4 times of that of the unit cell. All these evidences suggest that the cells constituting the stack are well consistent with each other in performance. Fig. 8(d) shows the discharging performance of the four-cell-inseries stack of DC-SOFC operated at a constant current of 200 mA and a temperature of 800 C. There is a discharging platform of over 3.0 V which lasts for 10.5 h. Then the voltage drops to zero after discharging for 14 h. The discharged energy estimated from Fig. 8(d) is 8.6 W h. This is better than a previous reported tubular segmented-in-series 3-cell-stack of DC-SOFC with an effective area of 10.2 cm2, which gives a discharging energy of 6.9 W h at 800 C [7]. Comparing the performances of the four-cell-stack and the single planar DC-SOFC, we can see that the stack gives higher voltage and output power. While the slightly higher output power (2.2 W) may be attributed to the larger effective area (8.8 cm2 compared to 8.0 cm2 of the single DC-SOFC), its significantly higher voltage is obtained from the stack configuration. Note that the power density and the discharging energy are still lower than those
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Fig. 8. The output performance (a) and impedance spectra under open circuit voltage (b) of a four-cell-in-series stack of DC-SOFC; (c) The output performance of a unit cell of the stack; (d) Discharging characteristic of the stack at 800 C.
of the single DC-SOFC. More work is needed on engineering to improve the performance of the stack of DC-SOFC and to develop it for practical applications.
2018PY11), and the Chinese Education Ministry Key Laboratory of Resource Chemistry. References
4. Conclusions A DC-SOFC stack, which can supply high voltage with small stack size, can be made on a single YSZ electrolyte plate, with punching holes, prepared by tape casting technique. The optimal formula of slurry for preparing the electrolyte plate is with mass ratios of TEA, PVB, DOP, and PEG-600 to the ceramic power (YSZ with 4 wt% Al2O3) as 2 wt%, 8 wt%, 3 wt%, and 4 wt%, respectively, and the solid loading (mass ratio of the ceramic powder to the slurry) as 42%. A successfully prepared four-cell-in-series stack of DC-SOFC gives an open circuit voltage of 3.82 V and an output power density of 250 mW cm2 at 820 C. It discharges at a constant current of 200 mA under 800 C, showing a discharging platform of over 3.0 V for 10.5 h and releasing a total discharging energy of 8.6 W h. This work demonstrates the feasibility of developing DC-SOFCs for portable and distributed applications. Acknowledgement This work was supported by the National Natural Science Foundation of China (NSFC, No. 91745203, U1601207), the Special Funds of Guangdong Province Public Research and Ability Construction (No. 2014A010106008), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200), the Fundamental Research Funds for the Central Universities (No.
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2207e2216. [14] F.Y. Yu, Y.P. Zhang, L. Yu, W.Z. Cai, L.L. Yuan, J. Liu, L.M. Lin, All-solid-state direct carbon fuel cells with thin yttrium-stabilized-zirconia electrolyte supported on nickel and iron bimetal-based anodes, Int. J. Hydrogen Energy 41 (2016) 9048e9058. [15] L. Zhang, J. Xiao, Y.M. Xie, Y.B. Tang, J. Liu, M.L. Liu, Behavior of strontium- and magnesium-doped gallate electrolyte in direct carbon solid oxide fuel cells, J. Alloys Compd. 608 (2014) 272e277. [16] J. Liu, Y. Liu, Y. B. Tang, Y. H. Bai, A Direct Carbon Solid Oxide Fuel Cell Power Supply System, Chinese Patent: CN102130354 A. [17] T.Y. Cao, K. Huang, Y.X. Shi, N.S. Cai, Recent advances in high-temperature carboneair fuel cells, Energy Environ. Sci. 10 (2017) 460e490. [18] N.M. Sammes, Y. Du, R. Bove, Design and fabrication of a 100 W anode supported micro-tubular SOFC stack, J. Power Sources 145 (2005) 428e434. [19] H.Y. Jung, S.H. Choi, H. Kim, J.W. Son, J. Kim, H.W. Lee, J.H. Lee, Fabrication and performance evaluation of 3-cell SOFC stack based on planar 10 cm10 cm anode-supported cells, J. Power Sources 159 (2006) 478e483. [20] J. Sui, J. Liu, An electrolyte-supported SOFC stack fabricated by slip casting technique, ECS Trans. 7 (2007) 633e637. [21] C. Jin, J. Liu, L.H. Li, Y.H. Bai, Electrochemical properties analysis of tubular NiOeYSZ anode-supported SOFCs fabricated by the phase-inversion method, J. Membr. Sci. 341 (2009) 233e237. [22] J. Ding, J. Liu, A novel design and performance of cone-shaped tubular anodesupported segmented-in-series solid oxide fuel cell stack, J. Power Sources 193 (2009) 769e773. [23] Y.H. Bai, Y. Liu, Y.B. Tang, Y.M. Xie, J. Liu, Direct carbon solid oxide fuel cellda potential high performance battery, Int. J. Hydrogen Energy 36 (2011) 9189e9194. [24] X.Q. Wang, J. Liu, Y.M. Xie, Q. Zhou, F.Y. Yu, W.Z. Cai, Y.P. Zhang, M.L. Liu, A high performance direct carbon solid oxide fuel cell stack for portable applications, Acta Phys. -Chim. Sin. 33 (2017) 1614e1620. [25] S.J. Kim, M.-B. Choi, M. Park, H. Kim, J.-W. Son, J.-H. Lee, B.-K. Kim, H.-W. Lee, S.-G. Kim, K.J. Yoon, Acceleration tests: degradation of anode-supported planar solid oxide fuel cells at elevated operating temperatures, J. Power Sources 360 (2017) 284e293. [26] G. Brus, H. Iwai, M. Mozdzierz, Y. Komatsu, M. Saito, H. Yoshida, J.S. Szmyd, Combining structural, electrochemical, and numerical studies to investigate the relation between microstructure and the stack performance, J. Appl.
Electrochem. 47 (2017) 979e989. [27] J. Liu, L. Zhang, Y. Liu, L. L.Yuan, A Single Electrolyte Solid Oxide Fuel Cell Stack, Chinese Patent: CN103956504. [28] M.X. Yu, J.X. Zhang, X.G. Li, H.Q. Liang, H. Zhong, Y.S. Li, Y.S. Duan, D.L. Jiang, X.J. Liu, Z.R. Huang, Optimization of the tape casting process for development of high performance alumina ceramics, Ceram. Int. 41 (2015) 14845e14853. [29] R.K. Nishihora, P.L. Rachadel, M.G.N. Quadri, D. Hotza, Manufacturing porous ceramic materials by tape castingda review, J. Eur. Ceram. Soc. 38 (2018) 988e1001. [30] R. Bulatova, M. Gudik-Sørensen, M.D. Negra, K.B. Andersen, A. Kaiser, C.R.H. Bahl, The effect of tape casting operational parameters on the quality of adjacently graded ceramic film, Ceram. Int. 42 (2016) 4663e4671. [31] W.-C.J. Wei, S.C. Wang, F.Y. Ho, Electrokinetic properties of colloidal zirconia powders, J. Am. Ceram. Soc. 82 (1999) 3385e3392. [32] W.Z. Cai, Q. Zhou, Y.M. Xie, J. Liu, A facile method of preparing Fe-loaded activated carbon fuel for direct carbon solid oxide fuel cells, Fuel 159 (2015) 887e893. [33] B.-M. Jutta, S. Wolfgang, W. Gerhard, M.H. Wolfgang, H. Friedrich, S. Katharina, R. Andreas, The function of polymers in the tape casting of alumina, Adv. Mater. 4 (1992). [34] M. Descamps, G. Moreau, M. Mascart, B. Thierry, Processing of aluminium nitride powder by the tape-casting process, J. Eur. Ceram. Soc. 13 (1994) 221e228. [35] T. Chartier, E. Jorge, Dispersion properties of batio3 tape-casting slurries, J. Eur. Ceram. Soc. 11 (1993) 387e393. [36] A. Ceylan, E. Suvaci, H. Mandal, Role of organic additives on non-aqueous tape casting of sialon ceramics, J. Eur. Ceram. Soc. 31 (2011) 167e173. [37] Y.H. Wang, X.Q. Liu, G.Y. Meng, Dispersion and stability of 8mol.% yttria stabilized zirconia suspensions for dip-coating filtration membranes, Ceram. Int. 33 (2007) 1025e1031. [38] M. Konsolakis, N. Kaklidis, V. Kyriakou, I. Garagounis, T. Kraia, A. Arenillas, ndez, R. Strandbakke, G.E. Marnellos, The combined impact of carbon J.A. Mene type and catalyst-aided gasification process on the performance of a direct carbon solid oxide fuel cell, Solid State Ionics 317 (2018) 268e275. [39] J. Nielsen, M. Mogensen, SOFC LSM:YSZ cathode degradation induced by moisture: an impedance spectroscopy study, Solid State Ionics 189 (2011) 74e81.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A direct carbon solid oxide fuel cell stack on a single electrolyte plate fabricated by tape casting technique Wei Wang a, Zhijun Liu a, Yapeng Zhang a, b, Peipei Liu a, Qianyuan Qiu a, Mingyang Zhou a, Meilin Liu a, c, Jiang Liu a, * a
Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou, 510006, China National Engineering Laboratory for Modern Materials Surface Engineering Technology & The Key Lab of Guangdong for Modern Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou, 510650, China c School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA, 30332-0245, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2018 Received in revised form 23 April 2019 Accepted 24 April 2019 Available online 27 April 2019
A four-cell-in-series stack of solid oxide fuel cell is fabricated on a single yttrium-stabilized-zirconia electrolyte plate which is fabricated by tape casting technique. The slurry for the tape casting process is characterized and optimized through Zeta potential and viscosity measurements. The optimal slurry is used to prepare the electrolyte plate, with electrical connecting holes, for the stack. The stack is operated directly with 5 wt% Fe-loaded activated carbon as the fuel. With a total effective area of 8.8 cm2 and carbon fuel of 5 g, this direct carbon solid oxide fuel cell stack shows an open circuit voltage of 3.82 V and an output power of 2.2 W at 820 C. It gives a discharging platform of over 3.0 V for 10.5 h with a constant discharging current of 200 mA, at 800 C. Then voltage drops to zero after discharging for 14 h, releasing a discharging energy of 8.6 W h. This work shows a possibility of developing direct carbon solid oxide fuel cells for applications of portable or distributed power sources. © 2019 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Carbon fuel Electrolyte plate Fuel cell stack Tape casting
1. Introduction Carbon is richly reserved on the earth in forms of coal, plant, and a variety of biomasses. A direct carbon fuel cell (DCFC) can directly convert the chemical energy of carbon into electricity through electrochemical reactions [1e5]. Its electrical conversion efficiency, which is defined as the Gibbs energy change, DG, divided by the enthalpy change, DH, of the complete oxidation of carbon, is slightly over 100%. Besides, the energy density of carbon is as high as ~9 A h g1. Therefore, DCFCs have attracting increasing interest in recent years in developing electricity generators of high efficiency and low emissions. Among different kinds of DCFCs, a direct carbon solid oxide fuel cell (DC-SOFC) has the advantage of all-solid-state configuration. Actually, it is a solid oxide fuel cell (SOFC) directly operated with solid carbon as the fuel [6e15]. According to thermodynamics, CO dominates the equilibrium gas composition of a carbon-oxygen system with exceeding carbon. Thus, at high operating
* Corresponding author. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.jallcom.2019.04.263 0925-8388/© 2019 Elsevier B.V. All rights reserved.
temperature (e.g., ~800 C), CO is produced in the anode chamber through the reaction between the carbon fuel and residual air. When the electrical circuit is closed, CO is electrochemically oxidized by oxygen ion coming from the cathode through the electrolyte to produce CO2 and devote electrons (1). CO þ O2 ¼ CO2 þ 2e
(1)
The CO2 molecules diffuse to the carbon fuel to perform the reverse Boudouard reaction (2) and produce more CO. CO2 þ C ¼ 2CO
(2)
Some of the CO molecules may diffuse back to the anode for the electrochemical oxidation reaction (1) and the others emit from the cell. The coupling of reactions (1) and (2) maintains the continuous operation of a DC-SOFC [10]. Obviously, a DC-SOFC does not need any carrier gas. Note that in the above reaction scheme, the initial CO may also be provided by the direct electrochemical oxidation of carbon
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C þ O2 ¼ CO þ 2e
(3)
At lower operating temperature, the direct complete electrooxidation of carbon is possible C þ 2O2 ¼ CO2 þ 4e
(4)
Except for the advantages of high efficiency and energy density of DCFCs, a DC-SOFC has its own superiority that it does not need any gas feeding or liquid medium, leading to a very simple configuration and a good durability. All these features of a DC-SOFC suggest its promise to be developed for portable or distributed power applications [16,17]. There have been efforts focusing on developing DC-SOFC stacks for portable application [18e22]. A tubular anode-supported segmented-in-series 3-cell-stack has given an open circuit voltage of 3.0 V and a peak power of 2.4 W at an operating temperature of 850 C [23]. Recently, our group has prepared and tested a tubular DC-SOFC of 3-cell-stack with carbon fuel located on the outside of the stack tube, a way allows more carbon stored in the stack (i.e., 17 g carbon loaded outside compared to 3 g filled inside the stack). It gave a peak power of 4.0 W at 800 C. It discharged at a constant current of 1 A for 19 h, giving a discharging energy of 31.6 W h [24]. While the above tubular stack design shows promising feasibility, planar configuration is more suitable for large scale manufacturing which may reduce fabricating cost [25,26]. We have proposed a DC-SOFC stack based on a single electrolyte plate, as shown in Fig. 1 [27]. An electrolyte plate is punched to make some holes (Fig. 1(a)) which are used to electrically connect the cathodes on one side of the plate (Fig. 1(b)) to the anodes on the opposite side (Fig. 1(c)) to form a stack (Fig. 1(d)). Such SOFC stack can give high voltage with small scale and is especially suitable for small or portable application. The electrolyte plate with punched holes is the key component of the stack. Tape casting, which is a standard shaping technique used to produce flat ceramic sheets [28e30], is a proper technique for preparing the electrolyte plate. Because the green plate prepared by tape casting has good plasticity, it is easy to punching holes on it. However, to obtain high quality electrolytes, the key to the technique, i.e., the slurry, needs to be optimized. Here, we report our work on preparing and testing a four-cellin-series stack of DC-SOFC prepared by tape casting technique. First, the slurry used for the technique is optimized by adjusting the
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contents of dispersant, binder, and plasticizers as well as solid loading. Then the optimized formula of slurry is applied to prepare the electrolyte of a four-cell-in-series stack of DC-SOFC. Finally, the performance of the stack is tested and the feasibility of developing this kind of DC-SOFCs for portable or distributed applications is discussed. 2. Experimental 2.1. Preparation of slurry for tape casting Yttria-stabilized-zirconia ((ZrO2)0.92(Y2O3)0.08, YSZ) was used as the electrolyte material. 30 g YSZ (TZ-8Y, Tosoh Corporation, Tokyo, Japan, 99.99%), 1.2 g Al2O3 (Xinfumeng, China, 99.99%) as sintering aid, and a certain amount of triethanolamine (TEA, Shanghai Lingfeng Industry Co., Ltd, China. The following polyvinyl butyral (PVB) and polyethylene glycol 600 (PEG-600) were also purchased from this company.) as dispersant [31] were mixed with ethanol (Sinopharm Group China, A.R.) through ball milling in a planetary ball mill (DM-4L, Nanjing Daran, China) for 1 h. Then, some PVB, dioctyl phthalate (DOP, Jiangsu Linghua, China, A.R.), and PEG-600 were added and the ball milling was continued for 1.5 h to obtain an electrolyte suspension. To optimize the formulae of tape casting slurry, a variety of slurries were made through adjusting the additives and solid loading. TEA of 1e4 wt%, PVB of 5e12 wt%, DOP of 1e8 wt%, PEG of 1e8 wt% and solid loading of 34e48 wt% (solid loading refer to the overall slurry, others refer to YSZ) were respectively investigated. 2.2. Preparation of YSZ electrolyte plates by tape casting technique A prepared slurry was poured onto the glass plate equipped in the tape casting machine (Shenzhen Kejing Technology Company). Then, a leveling blade, with a controlled distance to the surface of the glass plate, moved slowly over the slurry and a film was casted on the glass plate. The film was dried and peeled off. The thickness of the film was controlled to be ~30 mm. Subsequently, several layers of the film were laminated by a thermos-compressor (PCH600C, Tianjin Zhonghuan Furnace Corp) with an applied pressure of 20 MPa and temperature of 60 C for 15 min to form a green electrolyte tape. To optimize the formulae of slurry, pellets with diameter of 13 mm were made by punching a green electrolyte tape with 14 layers of film laminated and sintered at 1450 C for 4 h. They are
Fig. 1. A schematic illustration of a four-cell-in-series SOFC stack based on a single electrolyte plate with punched holes.
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used as samples for relatively density measurement, microstructure characterization, and hydrogen fueled SOFC preparation. The slurry with optimized formula was used to tape cast the electrolyte for DC-SOFC single cell and stack. For this, 10 layers of the casted film were laminated together, and the resulted layer was tailored to make a rectangular plate with a length of 7.6 cm and width of 3.8 cm. A small hole with a diameter of ~0.5 mm on the top and several other holes with a diameter of ~1 mm were made by punching the plate. The plates were sintered in air at 1450 C for 4 h. The thickness of the sintered electrolyte plate was 220 mm. 2.3. Assembling of SOFCs and DC-SOFCs For all the SOFCs operated with hydrogen and DC-SOFCs, cermet of silver and GDC (Ce0.8Gd0.2O1.95, Fuel Cell Materials) with a weight ratio of Ag: GDC ¼ 7 : 3 was used as the material of both cathode and anode. A homogeneous Ag-GDC paste was prepared by mixing silver paste (DAD-87, Shanghai Institute of Synthesized Resins) and GDC powder with binder solution of PVB in terpineol (concentration of 10 wt%) through grinding the mixture in an agate mortar. 2.3.1. Button cells of SOFC operated on hydrogen The Ag-GDC paste was brush-painted on both sides of each electrolyte pellet, and dried at 140 C for 30 min. The process was repeated for 4 times to achieve a required thickness. Subsequently, the painted pellets were sintered at 880 C for 2 h [8]. The effective area of the button cell, i.e., the area of the cathode was 0.2 cm2. The thickness of both the anode and cathode was about 30 mm. Each button cell was sealed on the top of a quartz tube with silver paste as sealing and jointing material. Ag wires were attached to the silver paste as current leading wires. 2.3.2. Planar DC-SOFC Ag-GDC electrodes were coated on the sintered rectangular electrolyte plates, in the same way as for the button cells. A single planar electrolyte cell with completely symmetric cathode and anode was prepared, as shown in Fig. 2(a) and (b). The area of electrodes were both designed as 20 mm 40 mm with an active area of 8 cm2. A four-cell-stack, as shown in Fig. 2(c) and (d), was assembled. Each single cell of the stack was with a size of 22 mm 10 mm which corresponded to an active area of 2.2 cm2, which meant the total effective area of the four-cell-stack was
8.8 cm2. The interval between two adjacent cells was 4 mm. The cells were electrically connected in series by using silver paste to connect the anode of one cell to the cathode of the next cell through the electrical connecting holes. Silver grids were painted on the cathode and anode as current collectors, and silver wires were attached on proper anode and cathode as current leading wires. Activated carbon (Aladdin, Shanghai, China; A.R.) loaded with 5 wt% Fe catalyst [8] by wet agglomeration process [32] was used as the fuel. 5 g of the fuel was filled into an alumina container with a size of 6 cm 3 cm 1.5 cm. Then the planar electrolyte-supported single cell or the four-cell-stack was sealed on the open top of a carbon fuel container to make a DC-SOFC single cell or a stack, as shown in Fig. 2. It is worth noting that no carrier gas is fed in the anode chamber of the DC-SOFCs. 2.4. Characterization The YSZ slurry viscosities were measured using a rotary viscometer (Shenzhen Kejing Technology Company, MSK-SFMVT8S). The stability of these YSZ slurries was investigated through Zeta potential analysis (Brookhaven, USA). The microstructure of the electrolytes were examined using cold field scanning electron microscope (SEM, Hitachi SU8010). The relative density of sintered electrolyte pellet was tested by the Archimedes methods. The button SOFCs were operated with humidified hydrogen (3 vol% H2O, 50 mL min1) as fuel and ambient air as oxidant. The planar DC-SOFCs also use the ambient air as oxidant. Their electrochemical performances were tested using an Iviumstat electrochemical analyser (Ivium Technologies B.V., Netherlands). The impedance was measured in the frequency range of 100 KHz0.01 Hz with a signal amplitude of 10 mV under open circuit condition. 3. Results and discussion 3.1. Optimizing the formulae of slurries for tape casting YSZ electrolyte Zeta potential of a slurry can represent its stability in a certain extent, while viscosity of a slurry affects the ductility of slurry for tape casting process. Both of them are very important for obtaining
Fig. 2. Schematic diagram of illustrations and pictures of DC-SOFC single cell [(a) and (b)] and four-cell-in-series stack [(c) and (d)].
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green tapes with good uniformity and consistency. A good slurry should have a high Zeta potential and a relatively low viscosity. TEA is an important dispersant in YSZ slurry for tape casting. Zeta potential and viscosity change with TEA content of YSZ slurry with solid loading as 40 wt% (no binder and plasticizers) was tested and the results are shown in Fig. 3(a). It can be seen that the viscosity decreases with the increase of TEA content from 0.5 wt% to 2 wt%. It is possible that steric stabilization plays an important role in preventing the particles from agglomeration using organics as solvents [33]. When TEA content increases further, from 2 wt% to 2.5 wt%, the viscosity increases due to mutual cross-linking effect of the excess TEA after its adsorption saturation on the surfaces of YSZ particles. A similar trend in the increase of slurry viscosity beyond the optimum concentration of dispersant has also been reported for Al2O3 [28], AlN [34] and BaTiO3 [35] based slurries. The Zeta potential versus dispersant concentration is opposite to viscosity. The Zeta potential reaches its maximum when the concentration of TEA is about 2 wt%. The decrease of Zeta potential for high concentrations of TEA may be related to the increase of ion concentration
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which leads to a decrease in the double layer thickness [35]. Therefore, combined with the Zeta potential and viscosity measurement results, 2 wt% is selected as the optimal TEA content. The Zeta potential and viscosity changes with different PVB content in slurries with the optimum mass ratio of TEA as 2 wt% and initial mass ratios of DOP, PEG, and solid loading as 4 wt%,4 wt %, and 40 wt%, respectively, were tested and the results are shown in Fig. 3(b). Compared with Fig. 3(a) and (b) shows that the Zeta potential of slurries with addition of binder and plasticizers are lower than those only with dispersant because of synergistic effect. It can be seen from Fig. 3(b) that the viscosity of slurries slowly increases with the addition of PVB increasing from 5 wt% to 8 wt%, then the upward trend becomes more obvious. Meanwhile, the Zeta potential reaches maximum at PVB of 8 wt%. Therefore, 8 wt% PVB content is chosen for the tape casting of YSZ slurry. DOP, functioning as A-type plasticizer, is used to soften the binder polymer chains, allowing them to flow under an applied force. It can also be described as a glass-transition temperature (Tg) modifier or binder solvent [33]. The Zeta potential and viscosity
Fig. 3. Viscosity and Zeta potential with the change of different concentration of TEA (a), PVB (b), DOP (c), PEG (d) and solid loading(marked as SL) (e).
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changes with the varying content of DOP in slurry with TEA, PVB, PEG, and solid loading as 2 wt%, 8 wt%, 4 wt%, and 40 wt%, respectively, were measured and the results are shown in Fig. 3(c). It can be seen that the viscosity rises slowly with the addition of DOP and remains almost unchanged when the addition is over 3 wt%. Meanwhile the Zeta potential reaches the maximum at the DOP content of 3 wt%. In a similar way, the optimal content of PEG in slurry with TEA, PVB, DOP, and solid loading as 2 wt%, 8 wt%, 3 wt%, and 40 wt%, respectively, is 4 wt%, according to Fig. 3(d). PEG is functioning as B-type plasticizer which works as a lubricant in the tape matrix. It works between the polymer chains, not only allowing a better mobility within the dry tape, but also preventing some of the “cross-linking” between the chains. However, excessive plasticizer will reduce the yield stress and weaken the strength of the tape to an unusable level by preventing polymer ties from forming [36]. In tape casting, it is also very important to obtain high solid loading slurries with good fluidity in order to obtain green tapes with homogenous microstructure and high green density [37]. High solid loading slurry also enables high processing efficiency. With the optimum contents of the organic additives, the influence of solid loading on viscosity and Zeta potential of slurry is measured and the results are shown in Fig. 3(e). The Zeta potential changes little with increasing solid loading of the slurries. While the viscosity increases gradually with the solid loading increasing from 34 to 42 wt%, it rises sharply when the solid loading is over 42 wt%. As mentioned, high viscosity may affect the stretching of slurry and result in voids of casted products. Trading off between high solid loading and low viscosity, a solid loading of 42 wt% may be proper for an efficient tape casting processing. Consequently, the optimized formulae of slurry is that: TEA of 2 wt%, PVB of 8 wt%, DOP of 3 wt%, and PEG of 4 wt%. A solid loading of 42 wt% may be good for tape casting. However, whether it is the best needs to be verified through more experiments, as shown in the following.
loading. It shows that with the increase of solid loading, the relative density of the prepared electrolyte increases. This is because of the higher packing density of a green pellet prepared with a slurry of higher solid loading. However, the relative density of an electrolyte decreases when the solid loading is higher than 42 wt%, which may be caused by higher slurry viscosity and agglomeration of ceramic particles, resulting in the formation of inhomogeneous structure with voids formed on the casted film and tape. The relative density reaches maximum at the solid loading of 42 wt%, which agrees well
3.2. Performances of electrolytes prepared with slurries of different solid loading 3.2.1. Microstructure Relative density is the most representative parameter to characterize the density of electrolytes. Fig. 4 shows the variation of the relative density and thickness (obtained from the SEM images as will be shown later) of sintered electrolyte pellets (laminated with 14 layers of casted film) prepared with slurries of different solid
Fig. 4. The variation of relative density and thickness of electrolytes with different solid loading.
Fig. 5. Surface microstructures (left) and cross-sectional microstructures (right) of electrolytes with different solid loading.
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to the expectation based on the data shown in Fig. 3(e). Fig. 5 shows the SEM images of the surface and section of the sintered pellets prepared with slurries of different solid loading. We can see that when solid loading is lower than 42 wt %, the surface and cross section of the corresponding electrolytes have holes or cracks. When solid loading is above 42 wt%, although the surface has no obvious cracks, big flaws appear in the section. It shows that the solid loading of 42 wt% is the optimum. Meanwhile, the SEM images of the sections of the pellets show that the thickness of the pellets increases with solid loading of the slurry used, as shown in Fig. 4. This is because that higher solid loading of slurry leads to higher packing density of ceramic particles in green pellets, resulting in smaller shrinkage after sintering.
area specific ohmic resistance of a SOFC increases with the solid loading of slurry used to prepare the electrolyte while the polarization resistance of all the cells does not change much. Generally, in an electrolyte-supported SOFC, the resistance of the electrolyte dominates the ohmic resistance of the cell. The thickness of the electrolytes varies with the slurry with different solid loading (as shown later). The conductivity of the electrolytes can be calculated based on the data obtained from the impedance spectra and the results are shown in Fig. 6(c). The conductivity of electrolyte increases with the increasing solid content, and decreases after reaching the maximum at 42 wt%. This agrees well with the relative density of the electrolyte. Consequently, the optimal solid loading of slurry for tape casting YSZ electrolyte is 42 wt%.
3.2.2. Performances of SOFCs operated on hydrogen The electrolyte pellets prepared with slurries of different solid loading are used to make button SOFCs which are tested by using humidified hydrogen as fuel and ambient air as oxidant. Fig. 6 shows their output performances (a), impedance spectra at open circuit voltage (b), and conductivities of the corresponding electrolytes (c), at 800 C. We can see from the Fig. 6(a) that the cell with electrolyte prepared with slurry of 42 wt % solid loading shows the highest open circuit voltage (~1.08 V) and the maximum power density (~225 mW cm2). This result is consistent with the evidence that the electrolyte prepared with slurry of 42 wt% solid loading has the best microstructure and the highest density. From the Nyquist plots of impedance shown in Fig. 6(b), the ohmic resistance Ro (the value of the left intercept of the horizontal axis) and the polarization resistance Rp (the difference between the right and left intercepts) can be identified. It is observed that the
3.3. Performance of planar DC-SOFCs 3.3.1. A single planar DC-SOFC Fig. 7 shows a typical electrochemical performance of a single planar DC-SOFC operated with 5 wt% Fe-loaded activated carbon of 5.0 g as the fuel. Fig. 7(a) shows that the open circuit voltages of the cell increases with temperature. This is consistent with experimental expectation [8]. The cell gives an open circuit voltage of 1.02 V and a maximum power of 2.1 W, which corresponds to a power density of 263 mW cm2, at 820 C. This performance is comparative to a SOFC fueled with hydrogen because Fe loaded on carbon is functioning as an effective catalyst for promoting the reverse Boudouard reaction which produces sufficient CO for the electrochemical reaction [8,38]. Fig. 7(b) shows the impedance spectra of the cell at open circuit voltage. It can be seen that polarization resistance dominates the
Fig. 6. The output performances (a), impedance spectra at open circuit voltage (b) of SOFCs operated on hydrogen and conductivity of electrolyte prepared with slurries of different solid loading (c), all measured at 800 C.
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Fig. 7. The output performance (a), impedance spectra under open circuit voltage (b), a comparison of the impedance spectra to that of a cell operated on humidified hydrogen under 800 C (c), and discharge characteristics at 800 C (d) of a single planar DC-SOFC.
overall resistance of the cell, especially at lower temperature. Fig. 7(c) shows a comparison of the impedance spectra of the cells operated on humidified hydrogen and activated carbon, respectively, at 800 C. The polarization resistance of the DC-SOFC is almost 50% higher than that of hydrogen. There is research showing the large impedance change at low frequency is related to gas diffusion process [39]. It is noted that there is sign of gas diffusion limit for the DC-SOFC at the frequency of around 0.01 Hz. This is because CO concentration in the equilibrium gas product of carbonoxygen system, with excess of carbon, is low at reduced temperature, leading to higher concentration polarization. Fig. 7(d) shows the discharging performance of the single planar DC-SOFC operated at a constant current of 1 A and 800 C. The cell discharges at a platform of over 0.7 V for about 15 h. Then the voltage drops to zero after a total discharging time of about 19.5 h, which means a discharging capacity of 19.5 A h. Theoretically, the charge capacity of carbon (4 electron process) is 8.93 A h g1. Therefore, the practical electrical conversion efficiency of the single planar DC-SOFC can be calculated as ~46%. According to Fig. 7(d), the energy capacity of the cell is estimated to be ~13 W h, corresponding to a mass specific energy of carbon as 2.7 W h g1.
3.3.2. Four-cell-in-series stack of DC-SOFC Fig. 8 shows the performances of a four-cell-in-series DC-SOFC stack. Similar to the situation of the single planar DC-SOFC, both open circuit voltage and power density of the stack increase with operating temperature, as shown in Fig. 8(a). At 820 C, the open circuit voltage of the stack is 3.82 V and the maximum output power is 2.2 W, which corresponds a power density of 251 mW cm2.
Fig. 8(b) shows that the ohmic resistance and the polarization resistance of the stack at 820 C are 1.79 U cm2 and 2.14 U cm2, respectively. As a comparison, the performance of one unit cell of the four-cell-in-series stack is shown in Fig. 8(c). The open circuit voltage of the unit cell at 820 C is 0.96 V, 0.06 V lower than the planar single DC-SOFC. Its maximum power density is 266 mW cm2, comparable with the single planar DC-SOFC. Note that the open circuit voltage of the stack is exactly 4 times of the unit cell, and the power density of the unit cell is also close to that of the single planar DC-SOFC. Impedance measurements also show that the resistance of the stack is almost 4 times of that of the unit cell. All these evidences suggest that the cells constituting the stack are well consistent with each other in performance. Fig. 8(d) shows the discharging performance of the four-cell-inseries stack of DC-SOFC operated at a constant current of 200 mA and a temperature of 800 C. There is a discharging platform of over 3.0 V which lasts for 10.5 h. Then the voltage drops to zero after discharging for 14 h. The discharged energy estimated from Fig. 8(d) is 8.6 W h. This is better than a previous reported tubular segmented-in-series 3-cell-stack of DC-SOFC with an effective area of 10.2 cm2, which gives a discharging energy of 6.9 W h at 800 C [7]. Comparing the performances of the four-cell-stack and the single planar DC-SOFC, we can see that the stack gives higher voltage and output power. While the slightly higher output power (2.2 W) may be attributed to the larger effective area (8.8 cm2 compared to 8.0 cm2 of the single DC-SOFC), its significantly higher voltage is obtained from the stack configuration. Note that the power density and the discharging energy are still lower than those
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Fig. 8. The output performance (a) and impedance spectra under open circuit voltage (b) of a four-cell-in-series stack of DC-SOFC; (c) The output performance of a unit cell of the stack; (d) Discharging characteristic of the stack at 800 C.
of the single DC-SOFC. More work is needed on engineering to improve the performance of the stack of DC-SOFC and to develop it for practical applications.
2018PY11), and the Chinese Education Ministry Key Laboratory of Resource Chemistry. References
4. Conclusions A DC-SOFC stack, which can supply high voltage with small stack size, can be made on a single YSZ electrolyte plate, with punching holes, prepared by tape casting technique. The optimal formula of slurry for preparing the electrolyte plate is with mass ratios of TEA, PVB, DOP, and PEG-600 to the ceramic power (YSZ with 4 wt% Al2O3) as 2 wt%, 8 wt%, 3 wt%, and 4 wt%, respectively, and the solid loading (mass ratio of the ceramic powder to the slurry) as 42%. A successfully prepared four-cell-in-series stack of DC-SOFC gives an open circuit voltage of 3.82 V and an output power density of 250 mW cm2 at 820 C. It discharges at a constant current of 200 mA under 800 C, showing a discharging platform of over 3.0 V for 10.5 h and releasing a total discharging energy of 8.6 W h. This work demonstrates the feasibility of developing DC-SOFCs for portable and distributed applications. Acknowledgement This work was supported by the National Natural Science Foundation of China (NSFC, No. 91745203, U1601207), the Special Funds of Guangdong Province Public Research and Ability Construction (No. 2014A010106008), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200), the Fundamental Research Funds for the Central Universities (No.
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