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Characterization of solid oxide fuel cell using doped lanthanum gallate
Solid State Ionics 132 (2000) 199–208 www.elsevier.com / locate / ssi
Characterization of solid oxide fuel cell using doped lanthanum gallate a, a a a Kiyoshi Kuroda *, Ikiko Hashimoto , Kazunori Adachi , Jun Akikusa , Yoshitaka Tamou a , Norikazu Komada a , Tatsumi Ishihara b , Yusaku Takita b a
Central Research Institute, Mitsubishi Materials Corporation, 1 -297 Kitabukuro-cho, Omiya, Saitama, 330 -8508, Japan b Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita, 870 -1192, Japan Received 24 December 1999; received in revised form 26 January 2000; accepted 7 February 2000
Abstract The power-generation characteristics and the electrode overpotential of the solid oxide fuel cell (SOFC) using doped lanthanum gallate perovskite-type oxide as an electrolyte were measured at temperatures below that of the typical SOFC using yttria-stabilized zirconia (YSZ) as an electrolyte. The oxide ion conductivity of the electrolyte, La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d (LSGMC), was much higher than that of YSZ. A single cell using LSGMC of 205 mm in thickness showed a power density of 380 mW/ cm 2 at a current density of 0.5 A / cm 2 and a temperature of 6508C by using air and dry hydrogen as oxidant and fuel, respectively. The overpotential of anode was larger than that of the cathode and dominated the overall overpotential. The IR-drop measured by current-interrupting method was in good agreement with the value estimated from the electrical conductivity of the electrolyte. The experimental results indicate that LSGMC is a promising material as an electrolyte for a low-temperature SOFC. The characteristics of electrodes are further discussed in terms of the composition and particle size of the starting powders. 2000 Elsevier Science B.V. All rights reserved. Keywords: Solid oxide fuel cell; Perovskite; Lanthanum gallate; Power generation; Overpotential
1. Introduction Fuel cells are promising energy production systems for the 21st century because of their high efficiency, environmental friendliness, and utilization of a variety of the fuel resources. Great efforts have been made towards the development of the fuel cells. Recently the solid oxide fuel cell (SOFC) with a low-temperature operation ( | 6508C) was focused on *Corresponding author. Tel.: 181-48-641-9901; fax: 181-48642-0545. E-mail address: [email protected] (K. Kuroda).
by several groups [1–5]. Most typical SOFCs using the yttria-stabilized zirconia (YSZ) as an electrolyte were operated at around 10008C. For the distributed heat–power co-generation application of the SOFC, an operation temperature lower than 7008C is highly attractive in view of cost effectiveness, because inexpensive stainless steel may be widely adopted for the structural materials of the SOFC systems. In additon. the wide variety of the fabrication methods of stainless steel enables a complicated structural design. Ishihara et al. reported on a low-temperature SOFC using the perovskite-type oxide of doped
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00659-7
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LaGaO 3 as an electrolyte. In particular, LaGaO 3 where Sr was substituted for the La-site and Mg and Co for the Ga-site (La 0.8 Sr 0.2 Ga 0.8 Mg 0.22x Co x O 32d : LSGMC) [1–3] was highly interesting from the viewpoint of decreasing the operating temperature. The oxide ion conductivity in LSGMC at 6508C is comparable to that of YSZ at 10008C, and the transference number of oxide ion is high (over 0.8). Furthermore, the performance of LSGMC as an electrolyte of the SOFC at 6508C is also comparable to that of YSZ at 10008C. This low-temperature characteristic is a very important feature for the industrial application of the SOFC. The preparation technique, crystal structure, electrical conductivity, and mechanical properties of doped LaGaO 3 (not only Co-doped but also Ni-, Cu-, Fe- and other transition element doped) can be found in the literature [6–21], and the excellent properties of LSGMC as an electrolyte for the SOFC are shown there. The selection of the electrode material is another important issue in obtaining higher power density. Power generation depends on the overpotential originated from the electrode properties together with the ohmic loss due to the electrical resistivity of the electrolyte. In the case of LSGMC, the ohmic loss is expected to be very low due to the high electrical conductivity. In this paper, the performance of the SOFC using LSGMC as the electrolyte at a low temperature, and the measurements of the electrical conductivity and the transference number of oxide ion in LSGMC are reported. With a view to industrial application, a tape-casting method (TC method) that is suitable for mass production was adopted for the preparation of the LSGMC cell in this study.
2. Experimental The starting powders for the electrolyte were prepared by the conventional solid state reaction technique with the commercially available powders of La 2 O 3 (99.99%), SrCO 3 (99.9%), Ga 2 O 3 (99.99%), MgO (99.99%) and CoO (99%). These commercial powders were mixed proportionally to La 0.8 Sr 0.2 Ga 0.8 Mg 0.22x Co x O 32d (LSGMC) with x 5 0.05–0.085 (x 5 0.05 denoted LSGMC5, x 5 0.07 to
LSGMC7 and x 5 0.085 to LSGMC8) by ball-milling and then calcined at 1100–12008C in air. The calcined powders were well ground again and mixed with a binder and an organic solvent for 1 day, and the resulting slurry was sheeted in 250–300 mm thick with a tape-casting apparatus. After drying, the sheets were cut into disks or rectangular shapes, and sintered at 1400–15008C in air after removing organic additives at temperatures lower than 10008C. The thicknesses of these sintered specimens were 200–250 mm, and their densities were larger than 98% of the theoretical densities. For comparison, the powders were pressed into rectangular shapes with a single-axis compressing machine. The pressed specimens had a thickness of about 3 mm, and were followed by sintering under the same condition with TC method specimens. No additives were used in the pressing method (P method). The phases in the resulting products were analyzed with powder X-ray diffraction (XRD). The microstructure of the electrolytes were observed by scanning electron microscopy (SEM) for the polished surface after thermally etched. The variation of the composition of the electrolyte was measured by inductivity-coupled plasma emission spectroscopy (ICP) and electron-probe microanalysis (EPMA). The electrical conductivity of the sintered specimens (rectangular: 4 3 40 mm, and the thickness of 200 mm by TC method and 2.5 mm by P method) were measured as a function of temperature by the conventional ac two-probe impedance method from 1.0 Hz to 100 MHz in N 2 gas flow (the PO 2 was estimated as 10 25 atm). The Pt meshes attached to Pt wires were used as the electrodes. The electrodes were attached to the specimens with Pt paste and fired at 9508C for 1 h in air. The transference numbers of oxide ion were estimated from the electromotive forces of oxygen gas concentration cell of H 2 –O 2 . The disk-shaped specimens with a diameter of 25 mm were used for the measurements. The thickness was about 200 mm by the TC method and 2.5 mm by the P method. The oxygen partial pressure was adjusted by adding 3 vol% H 2 O to H 2 . For the power-generation experiment, a sintered disk-shaped electrolyte (40 mm in diameter 3 |200 mm in thickness by the TC method) was used. The cathode material employed was Sm 0.5 Sr 0.5 CoO 32 a , which was prepared as described in a previous report
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[22], since it was found that Sm 0.5 Sr 0.5 CoO 32 a possesses a high activity for a cathodic reaction at a low temperature [1]. The parametric composition of Ni–CeO 2 based oxide cermet was used for the anode material. The anodes were prepared by the reduction of NiO with the varied proportion of Ce 0.8 Sm 0.2 O 22 b , and the particle size of NiO was also chosen as a parameter. Ce 0.8 Sm 0.2 O 22 b is a mixed conductor of oxide ions and electrons, and it is used for the prevention of Ni particles sintering, which results in decreasing the anode activity. The anodes with an effective electrode area of 2.0 cm 2 were first coated with a slurry of a mixture of the electrode powders and an organic binder on a surface of the electrolyte membrane, followed by calcination at 12008C. Then, the cathodes were coated similarly on the another surface of the electrolyte membrane, followed by calcination at 11008C. The overpotentials at the cathode and the anode were monitored by using a reference electrode made of fired Pt paste which was connected to the Pt wire formed on the edge of the cathode side of the electrolyte membrane. As the current collector of the cell membrane, a Pt mesh was attached to the cathode, and a porous-Ni sheet to the anode. Current collectors were contacted with the top and bottom of a cell case made of stainless steel. These cell cases were electrically insulated. The electric current during the power-generation experi-
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ment was taken out directly from the cell cases. The cell case set with a test cell membrane was placed in a uniform temperature area in an electrical furnace with two sets of five rod-type heaters above and below of the cell case set-up. The schematic view of the used single cell membrane testing rig is shown in Fig. 1. The air was supplied directly to the cathode surface through the flow meter at a rate of 500 ml / min (gauged at room temperature), and dry hydrogen was also supplied to the anode surface through the flow meter at a rate of 500 ml / min (gauged at room temperature). All the power generating tests were carried out at 6508C with typical I–V measurements from the open-circuit voltage (OCV) to 0.5 V, and then consecutively, the overpotentials were measured by the current-interrupting method. The microstructures of cross-section of the cell membranes after the tests were observed by SEM.
3. Results and discussion Powder XRD patterns using CuKa radiation were acquired for the LSGMC5 electrolyte fabricated by both the TC and P methods. The formation of LSGMC and / or LSGM single phase was obtained by the conventional solid state reaction [5,6,10,12,14,16,17,19,20], though the single-phase
Fig. 1. Schematic illustration of the test cell membrane placed in the apparatus.
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region of LSGM (not LSGMC) in the quaternary phase diagram was estimated to be very narrow [10]. Fig. 2 shows the XRD patterns of LSGMC5 prepared by both methods. The XRD patterns consisted of peaks due to the LaGaO 3 phase. The composition of LSGMC was analyzed by ICP. The resulting composition was consistent with the initial composition of the raw material mixture. However, the composition as determined by EPMA in the cross-section of the electrolyte (TC method) suggested that the concentrations of Sr, Mg and Co deviated slightly from the designed composition in the range of 20mm depth from the surface. The reason for this deviation in the composition of the specimen prepared by TC method is not clear at present. However, the difference of composition near the surface might affect the apparent properties of LSGMC. This effect which was found to be small will be discussed later in detail. The microstructure of the electrolyte
(TC method) is shown in Fig. 3. The average grain size of the electrolyte was about 10 mm which was the same size as reported for LSGM [11]. In our preliminary experiments, the grain size of the electrolyte changed in accordance with the sintering condition (temperature and heating duration). However, the change in the grain size did not significantly affect the electrical conduction properties of LSGMC. Thus, the TC method is capable of producing a LSGMC membrane suitable as an electrolyte in the SOFC, in terms of the phase and the composition. Fig. 4a shows the temperature dependence of the electrical conductivity of LSGMC prepared by TC method at various Co contents. The electrical conductivity was improved to some extent as the Co concentration increased. On the other hand, Codoping causes an increase of the electronic conduction, and therefore, the electromotive force for H 2 –
Fig. 2. X-ray powder diffraction patterns of the La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d electrolyte fabricated by the (a) tape-casting and (b) pressing method.
K. Kuroda et al. / Solid State Ionics 132 (2000) 199 – 208
Fig. 3. SEM image of microstructure of the La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d electrolyte fabricated by tapecasting method. The image was taken after thermal etching at 13508C for 1 h.
O 2 cell decreased. Thus, there is a trade-off by the Co substitution in the Ga site. In this paper, the amount of Co in the LSGMC electrolyte was fixed at 5 mol% for the power-generation experiments. Fig. 4b shows the comparison of electrical conductivity of LSGMC5 electrolyte prepared by the TC and P methods. Electrical conductivity of YSZ is also plotted in the figure for comparison. The electrical conductivity of the LSGMC5 electrolyte prepared by the TC method was apparently lower than that of the specimen prepared by the P method. As mentioned before, the concentrations of Sr, Mg and Co near the surface of the TC-method LSGMC electrolyte were slightly deviated from the designed concentration. Therefore it was anticipated that the difference in electrical conductivity will be caused by the deviation in composition at the surface layer. It is wellknown that Co and Sr easily react with Al 2 O 3 . Therefore, the deviation in the chemical composition near the surface may result from the reaction of LSGMC with the Al 2 O 3 holder when the electrolyte
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(TC method) was sintered after tape-casting. Even though the apparent electrical conductivity of the TC method electrolyte is slightly lower than that of the P method one at high temperatures, the electrical conductivity of the TC method electrolyte exhibits the similar electrical conductivity with the P method specimen, in particular, at low temperatures. It is also noted that the electrical conductivity of LSGMC is much higher than that of the conventional electrolytes; YSZ. Thus, in comparison with YSZ of the same thickness, it is expected that LSGMC5 (TC method) could reduce the operating temperature of the SOFC from 900 to 6508C. The temperature dependencies of the transference number of oxide ion in the LSGMC5 electrolytes (TC and P methods) are shown in Fig. 5. The transference number of oxide ion of the TC method specimen was higher than 0.85 in the range of 600–10008C; in particular, it was 0.88 at 6508C. The transference number is almost the same as the reported values [5]. In Fig. 5, the transference number of the P method LSGMC5 at lower than 8008C is slightly higher than that of the TC method one. One cause of the difference is the gas leak of oxygen gas concentration cell of H 2 –O 2 . The transference number was estimated by the ratio of the theoretical electromotive force and the measured terminal voltage of cell of H 2 –O 2 using the LSGMC electrolyte. The terminal voltages were measured from high temperature to low temperature. The borosilicate glass was used as the gas-seal material of the oxygen gas concentration cell, and its softening temperature is 8208C. Therefore, the borosilicate glass was hardened and shrunk below 8008C. Then, it might well be said that microcracks due to the hardening and shrinking of the borosilicate glass were induced in the LSGMC electrolyte (TC method) because the thickness of the electrolyte was very small (0.20 mm). As to the other cause of the difference in the data of the apparent transference number, the change of the composition in the electrolyte is considered. The concentrations of Sr, Mg and Co in the LSGMC electrolyte (TC method) deviated slightly from the designed composition in the range of 20 mm depth from the surface. Therefore, the intrinsic transference number of the LSGMC electrolyte (TC method) reduces. The difference of the transference number of the specimen
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Fig. 4. Arrhenius plots of the electrical conductivity of (a) La 0.8 Sr 0.2 Ga 0.8 Mg 0.22x Co x O 32d with various Co contents and (b) La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d prepared by the tape-casting and pressing methods, respectively. The electrical conductivity of YSZ is also plotted in (b) for comparison.
prepared by the TC and P methods needs further investigation. The fuel cell configuration in this investigation allows separate measurements of the anode and the cathode overpotential; ha and hc , and the voltagedrop across the electrolyte; IR, and the voltage-drop between the reference and the working electrodes; DV from the cell voltage; V V 5VOCV 2 hc 2 ha 2 IR 2 DV
(1)
In most cases, DV is negligible. hc is evaluated from the transient voltage change between the cathode and the reference electrode during the current-interrupting method, and ha between the anode and the reference electrode. If the overall overpotential between the cathode and the anode; hca , would be measured, the following relationship should be theoretically considered
hca 5 hc 1 ha Fig. 5. Temperature dependency of the transference number of oxide ion for the La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d electrolyte fabricated by tape-casting and pressing methods, respectively.
(2)
hc value was too small to be detected in our experiment. Therefore, hca and ha were measured by the current-interrupting method, and then hc was
K. Kuroda et al. / Solid State Ionics 132 (2000) 199 – 208
estimated by the Eq. (2). In our preliminary measurements of each component, hca , hc and ha , it has been confirmed the validity of the Eq. (2). Fig. 6a shows the power generating property of a single cell using LSGMC5 electrolyte prepared by TC method. It is obvious that the output power density was 380 mW/ cm 2 at 0.5 A / cm 2 and 6508C under air and dry hydrogen as oxidant and fuel. The maximum power density of the same cell was estimated to be 410 mW/ cm 2 at 0.73 A / cm 2 . This power density is significantly higher than that of the SOFC using the YSZ electrolyte under development to date. It can be said that the operating temperature of the SOFC may be decreased to 6508C by using LSGMC5 for the electrolyte. Fig. 6b shows the details of the internal resistance of the developed cell. Except for anodic overpotential, it is clear that another large internal resistance is the IR drop due to the electrical resistivity of the electrolyte. Although the overall electrode overpotential consisted of the cathode and the anode components, the overpotential at the anode occupied the most part of the overall overpotential. The small overpotential at the cathode indicates the appropriate choice of the electrode material and the suitable sample preparation method.
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From the measurements of the electrical conductivity as shown in Fig. 4b, it is estimated that the resistivity of the LSGMC5 electrolyte fabricated by TC method is 23 Vcm at 6508C. Using this value, it is also estimated that the IR drop over the electrolyte with the thickness of 205 mm is 0.25 V at 0.5 A / cm 2 . In the experiment as shown in Fig. 6b, it was able to be observed that the IR drop was 0.27 V at 0.5 A / cm 2 . The measured value is in good agreement with the estimated value and is about 50% of the total internal resistance. Table 1 summarizes the anode properties for four different compositions. It is evident that the anodes of Ni mixed with 40 vol% Ce 0.8 Sm 0.2 O 22 b showed better properties in the power density, IR drop, and overpotential than those of pure Ni. The NiO powders of |14 mm showed better properties than that of |1.0 mm. Fig. 7a–c show the SEM images of the anodes of the different compositions. It is observed in Fig. 7a that the pure Ni anode tended to peel off from the electrolyte and agglomerate themselves regardless of the starting NiO particle size. The anode microstructure using NiO powders of |14 mm without Ce 0.8 Sm 0.2 O 22 b is not shown, because the anode completely peeled off from the electrolyte just
Fig. 6. Power generation characteristics of the SOFC using La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d as an electrolyte. Air and dry hydrogen were used as oxidant and fuel, respectively. The fuel cell was operated at 6508C. (a) the I–V curve and I–P curve, and (b) the IR drop and the overpotentials of the anode, ha , and the cathode, hc .
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Table 1 Comparison as for the anode conditions of the SOFC using La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d (LSGMC5) as an electrolyte. The average particle size of Ce 0.8 Sm 0.2 O 22 b is around about 1.0 mm. Cathode materials are fixed at Sm 0.5 Sr 0.5 CoO 32 a in all the tests. Power generating experiments were operated at 6508C for air and dry hydrogen as oxidant and fuel, respectively Specimen ID
a
b
c
d
Average particle size of NiO (mm) Volume ratio of Ni / Ce 0.8 Sm 0.2 O 22 b OCV (V) At cell voltage of 0.7 V Current density (A / cm 2 ) Power density (mW/ cm 2 )
|1.0 100 / 0 1.12
|1.0 60 / 40 1.13
|14 100 / 0 1.16
|14 60 / 40 1.20
0.26 180
0.39 270
0.34 240
0.54 380
(,0.50) – 0.26
0.61 310 0.24
0.51 260 0.24
0.72 360 0.25
0.28 0.38 0.35 0.03
0.25 0.31 0.24 0.07
0.41 0.39 –a –a
0.27 0.24 0.23 0.01
240
330
260
410
0.48 0.50
0.50 0.65
0.54 0.48
0.56 0.73
At current density of 0.5 A / cm 2 Cell voltage (V) Power density (mW/ cm 2 ) Estimated IR-drop by the resistivity and the Thickness of the LSGMC5 electrolyte (V) IR drop (V) Overall overpotential (V) Overpotential at anode (V) Overpotential at cathode (V) Estimated maximum power density (mW/ cm 2 ) At estimated maximum power density Cell voltage (V) Current density (A / cm 2 ) a
Overpotential was not measured because of the failure of adequate contact between the reference electrode and the electrolyte.
after the power generation. On the other hand, the anodes of Ni with 40 vol% Ce 0.8 Sm 0.2 O 22 b seem to show good contact with the electrolyte. This implies that good electrical conduction through the interface between the anode and the electrolyte results in the reduction of the internal resistance of the cell. Since the cathode overpotential was very small, its composition was fixed. Fig. 7d shows the image of the cathode. Sm 0.5 Sr 0.5 CoO 32 a particles seem to be slightly fused. The particle size affects the morphology of the anode microstructure. Fig. 7c shows that the large particles form frameworks and are surrounded by small particles. On the other hand, in the anodes in Fig. 7b all particles are almost same size and seem to mix at random. The large particles are not observed in Fig. 7b, and the NiO particles as a starting material in Fig. 7c are larger than that of Fig. 7b, then, it is presumed that the large particles in Fig. 7c are Ni, and are effective for forming the framework of the anodes and increasing the triple-phaseboundaries among Ni, Ce 0.8 Sm 0.2 O 22 b , and fuel atmosphere. Therefore, it is considered that the
anodes consisting of large NiO particles as a starting material are suitable for the highly active anode resulting in a higher power density (see Table 1). Matsuzaki et al. [23] and Kawashima et al. [24] reported that the ratio of the particle size of YSZ and NiO as the starting powders for the anode affects the overpotential of the anode in the SOFC using the YSZ electrolyte. The range of the ratio of the particle size, YSZ / NiO50.01–0.10, showed the lowest overpotential. The reports [23,24] conclude that the ratio of the particle size of the anode components, in their case using YSZ and Ni, affected the distribution of these components by the filling of the YSZ particles in the opening between the Ni particles. In our case, the ratio of the particle size of the anode components as the starting powders, Ce 0.8 Sm 0.2 O 22 b (|1 mm) / NiO (|14 mm), was 0.071, consistent with the previous reports [23,24]. Apart from anodic overpotential, another large internal resistance of the cell is the IR drop due mainly to the electrical resistivity of the electrolyte. However, this resistance can be reduced by decreasing the thickness of the electrolyte. Therefore, the
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Fig. 7. SEM images of microstructure for the interface between, (a–c) the anode and the electrolyte and (d) the cathode and the electrolyte, respectively. (a) Pure NiO with particle size of |1.0 mm (b) mixture of NiO with particle size of |1.0 mm and additive Ce 0.8 Sm 0.2 O 22 b , (c) mixture of NiO with particle size of |14 mm and additive Ce 0.8 Sm 0.2 O 22 b . The volume ratio of Ni and the additive Ce 0.8 Sm 0.2 O 22 b was 60 / 40. The average particle size of Ce 0.8 Sm 0.2 O 22 b , and Sm 0.5 Sr 0.5 CoO 32 a is around 1.0 mm. The image of the anode with pure NiO of particle size of |14 mm is not shown, because the anode peeled off from the electrolyte just after the power generation.
decrease of the anodic overpotential is an important matter for further improvement in terms of power density of the cell. The approach can be optimization of the composition, particle size of each component and microstructure. This investigation of the electrodes coupled with LSGMC as the electrolyte showed a promising starting point for the further study.
4. Summary We have characterized the power generation and the electrode overpotential of a single cell using LSGMC5 as an electrolyte in the temperature range lower than that of the typical SOFC using YSZ. The oxide ion conductivity of LSGMC5 was higher than that of YSZ. A single cell using LSGMC5 with 205
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mm in thickness showed the power density of 380 mW/ cm 2 at the current density of 0.5 A / cm 2 and 6508C under air and dry hydrogen as oxidant and fuel, respectively. The IR drop was 0.27 V which was in good agreement with the predicted value of 0.25 V. The overpotential for the anode was 0.23 V and larger than 0.01 V for the cathode at the same current density of 0.5 A / cm 2 , and dominated the overall overpotential. It is confirmed that LSGMC is a promising material as an electrolyte for a SOFC operating at 6508C with high power density. In addition, this study reveals the possibility for the SOFC of using a LSGMC electrolyte prepared by a tape-casting method and the Ni–Ce 0.8 Sm 0.2 O 22 b cermet as for the anode.
Acknowledgements The authors would like to thank M. Uzuki and Y. Tamura for preparation of the electrolyte cell membranes and T. Yamada for measurement of the electrical conductivity. The financial supports of Ministry of International Trade and Industry (MITI) of Japan for promoting energy-related technologies, and Grant-in-Aid from Ministry of Education, Culture, and Sports of Japan are gratefully acknowledged.
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Characterization of solid oxide fuel cell using doped lanthanum gallate a, a a a Kiyoshi Kuroda *, Ikiko Hashimoto , Kazunori Adachi , Jun Akikusa , Yoshitaka Tamou a , Norikazu Komada a , Tatsumi Ishihara b , Yusaku Takita b a
Central Research Institute, Mitsubishi Materials Corporation, 1 -297 Kitabukuro-cho, Omiya, Saitama, 330 -8508, Japan b Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita, 870 -1192, Japan Received 24 December 1999; received in revised form 26 January 2000; accepted 7 February 2000
Abstract The power-generation characteristics and the electrode overpotential of the solid oxide fuel cell (SOFC) using doped lanthanum gallate perovskite-type oxide as an electrolyte were measured at temperatures below that of the typical SOFC using yttria-stabilized zirconia (YSZ) as an electrolyte. The oxide ion conductivity of the electrolyte, La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d (LSGMC), was much higher than that of YSZ. A single cell using LSGMC of 205 mm in thickness showed a power density of 380 mW/ cm 2 at a current density of 0.5 A / cm 2 and a temperature of 6508C by using air and dry hydrogen as oxidant and fuel, respectively. The overpotential of anode was larger than that of the cathode and dominated the overall overpotential. The IR-drop measured by current-interrupting method was in good agreement with the value estimated from the electrical conductivity of the electrolyte. The experimental results indicate that LSGMC is a promising material as an electrolyte for a low-temperature SOFC. The characteristics of electrodes are further discussed in terms of the composition and particle size of the starting powders. 2000 Elsevier Science B.V. All rights reserved. Keywords: Solid oxide fuel cell; Perovskite; Lanthanum gallate; Power generation; Overpotential
1. Introduction Fuel cells are promising energy production systems for the 21st century because of their high efficiency, environmental friendliness, and utilization of a variety of the fuel resources. Great efforts have been made towards the development of the fuel cells. Recently the solid oxide fuel cell (SOFC) with a low-temperature operation ( | 6508C) was focused on *Corresponding author. Tel.: 181-48-641-9901; fax: 181-48642-0545. E-mail address: [email protected] (K. Kuroda).
by several groups [1–5]. Most typical SOFCs using the yttria-stabilized zirconia (YSZ) as an electrolyte were operated at around 10008C. For the distributed heat–power co-generation application of the SOFC, an operation temperature lower than 7008C is highly attractive in view of cost effectiveness, because inexpensive stainless steel may be widely adopted for the structural materials of the SOFC systems. In additon. the wide variety of the fabrication methods of stainless steel enables a complicated structural design. Ishihara et al. reported on a low-temperature SOFC using the perovskite-type oxide of doped
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00659-7
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LaGaO 3 as an electrolyte. In particular, LaGaO 3 where Sr was substituted for the La-site and Mg and Co for the Ga-site (La 0.8 Sr 0.2 Ga 0.8 Mg 0.22x Co x O 32d : LSGMC) [1–3] was highly interesting from the viewpoint of decreasing the operating temperature. The oxide ion conductivity in LSGMC at 6508C is comparable to that of YSZ at 10008C, and the transference number of oxide ion is high (over 0.8). Furthermore, the performance of LSGMC as an electrolyte of the SOFC at 6508C is also comparable to that of YSZ at 10008C. This low-temperature characteristic is a very important feature for the industrial application of the SOFC. The preparation technique, crystal structure, electrical conductivity, and mechanical properties of doped LaGaO 3 (not only Co-doped but also Ni-, Cu-, Fe- and other transition element doped) can be found in the literature [6–21], and the excellent properties of LSGMC as an electrolyte for the SOFC are shown there. The selection of the electrode material is another important issue in obtaining higher power density. Power generation depends on the overpotential originated from the electrode properties together with the ohmic loss due to the electrical resistivity of the electrolyte. In the case of LSGMC, the ohmic loss is expected to be very low due to the high electrical conductivity. In this paper, the performance of the SOFC using LSGMC as the electrolyte at a low temperature, and the measurements of the electrical conductivity and the transference number of oxide ion in LSGMC are reported. With a view to industrial application, a tape-casting method (TC method) that is suitable for mass production was adopted for the preparation of the LSGMC cell in this study.
2. Experimental The starting powders for the electrolyte were prepared by the conventional solid state reaction technique with the commercially available powders of La 2 O 3 (99.99%), SrCO 3 (99.9%), Ga 2 O 3 (99.99%), MgO (99.99%) and CoO (99%). These commercial powders were mixed proportionally to La 0.8 Sr 0.2 Ga 0.8 Mg 0.22x Co x O 32d (LSGMC) with x 5 0.05–0.085 (x 5 0.05 denoted LSGMC5, x 5 0.07 to
LSGMC7 and x 5 0.085 to LSGMC8) by ball-milling and then calcined at 1100–12008C in air. The calcined powders were well ground again and mixed with a binder and an organic solvent for 1 day, and the resulting slurry was sheeted in 250–300 mm thick with a tape-casting apparatus. After drying, the sheets were cut into disks or rectangular shapes, and sintered at 1400–15008C in air after removing organic additives at temperatures lower than 10008C. The thicknesses of these sintered specimens were 200–250 mm, and their densities were larger than 98% of the theoretical densities. For comparison, the powders were pressed into rectangular shapes with a single-axis compressing machine. The pressed specimens had a thickness of about 3 mm, and were followed by sintering under the same condition with TC method specimens. No additives were used in the pressing method (P method). The phases in the resulting products were analyzed with powder X-ray diffraction (XRD). The microstructure of the electrolytes were observed by scanning electron microscopy (SEM) for the polished surface after thermally etched. The variation of the composition of the electrolyte was measured by inductivity-coupled plasma emission spectroscopy (ICP) and electron-probe microanalysis (EPMA). The electrical conductivity of the sintered specimens (rectangular: 4 3 40 mm, and the thickness of 200 mm by TC method and 2.5 mm by P method) were measured as a function of temperature by the conventional ac two-probe impedance method from 1.0 Hz to 100 MHz in N 2 gas flow (the PO 2 was estimated as 10 25 atm). The Pt meshes attached to Pt wires were used as the electrodes. The electrodes were attached to the specimens with Pt paste and fired at 9508C for 1 h in air. The transference numbers of oxide ion were estimated from the electromotive forces of oxygen gas concentration cell of H 2 –O 2 . The disk-shaped specimens with a diameter of 25 mm were used for the measurements. The thickness was about 200 mm by the TC method and 2.5 mm by the P method. The oxygen partial pressure was adjusted by adding 3 vol% H 2 O to H 2 . For the power-generation experiment, a sintered disk-shaped electrolyte (40 mm in diameter 3 |200 mm in thickness by the TC method) was used. The cathode material employed was Sm 0.5 Sr 0.5 CoO 32 a , which was prepared as described in a previous report
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[22], since it was found that Sm 0.5 Sr 0.5 CoO 32 a possesses a high activity for a cathodic reaction at a low temperature [1]. The parametric composition of Ni–CeO 2 based oxide cermet was used for the anode material. The anodes were prepared by the reduction of NiO with the varied proportion of Ce 0.8 Sm 0.2 O 22 b , and the particle size of NiO was also chosen as a parameter. Ce 0.8 Sm 0.2 O 22 b is a mixed conductor of oxide ions and electrons, and it is used for the prevention of Ni particles sintering, which results in decreasing the anode activity. The anodes with an effective electrode area of 2.0 cm 2 were first coated with a slurry of a mixture of the electrode powders and an organic binder on a surface of the electrolyte membrane, followed by calcination at 12008C. Then, the cathodes were coated similarly on the another surface of the electrolyte membrane, followed by calcination at 11008C. The overpotentials at the cathode and the anode were monitored by using a reference electrode made of fired Pt paste which was connected to the Pt wire formed on the edge of the cathode side of the electrolyte membrane. As the current collector of the cell membrane, a Pt mesh was attached to the cathode, and a porous-Ni sheet to the anode. Current collectors were contacted with the top and bottom of a cell case made of stainless steel. These cell cases were electrically insulated. The electric current during the power-generation experi-
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ment was taken out directly from the cell cases. The cell case set with a test cell membrane was placed in a uniform temperature area in an electrical furnace with two sets of five rod-type heaters above and below of the cell case set-up. The schematic view of the used single cell membrane testing rig is shown in Fig. 1. The air was supplied directly to the cathode surface through the flow meter at a rate of 500 ml / min (gauged at room temperature), and dry hydrogen was also supplied to the anode surface through the flow meter at a rate of 500 ml / min (gauged at room temperature). All the power generating tests were carried out at 6508C with typical I–V measurements from the open-circuit voltage (OCV) to 0.5 V, and then consecutively, the overpotentials were measured by the current-interrupting method. The microstructures of cross-section of the cell membranes after the tests were observed by SEM.
3. Results and discussion Powder XRD patterns using CuKa radiation were acquired for the LSGMC5 electrolyte fabricated by both the TC and P methods. The formation of LSGMC and / or LSGM single phase was obtained by the conventional solid state reaction [5,6,10,12,14,16,17,19,20], though the single-phase
Fig. 1. Schematic illustration of the test cell membrane placed in the apparatus.
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region of LSGM (not LSGMC) in the quaternary phase diagram was estimated to be very narrow [10]. Fig. 2 shows the XRD patterns of LSGMC5 prepared by both methods. The XRD patterns consisted of peaks due to the LaGaO 3 phase. The composition of LSGMC was analyzed by ICP. The resulting composition was consistent with the initial composition of the raw material mixture. However, the composition as determined by EPMA in the cross-section of the electrolyte (TC method) suggested that the concentrations of Sr, Mg and Co deviated slightly from the designed composition in the range of 20mm depth from the surface. The reason for this deviation in the composition of the specimen prepared by TC method is not clear at present. However, the difference of composition near the surface might affect the apparent properties of LSGMC. This effect which was found to be small will be discussed later in detail. The microstructure of the electrolyte
(TC method) is shown in Fig. 3. The average grain size of the electrolyte was about 10 mm which was the same size as reported for LSGM [11]. In our preliminary experiments, the grain size of the electrolyte changed in accordance with the sintering condition (temperature and heating duration). However, the change in the grain size did not significantly affect the electrical conduction properties of LSGMC. Thus, the TC method is capable of producing a LSGMC membrane suitable as an electrolyte in the SOFC, in terms of the phase and the composition. Fig. 4a shows the temperature dependence of the electrical conductivity of LSGMC prepared by TC method at various Co contents. The electrical conductivity was improved to some extent as the Co concentration increased. On the other hand, Codoping causes an increase of the electronic conduction, and therefore, the electromotive force for H 2 –
Fig. 2. X-ray powder diffraction patterns of the La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d electrolyte fabricated by the (a) tape-casting and (b) pressing method.
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Fig. 3. SEM image of microstructure of the La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d electrolyte fabricated by tapecasting method. The image was taken after thermal etching at 13508C for 1 h.
O 2 cell decreased. Thus, there is a trade-off by the Co substitution in the Ga site. In this paper, the amount of Co in the LSGMC electrolyte was fixed at 5 mol% for the power-generation experiments. Fig. 4b shows the comparison of electrical conductivity of LSGMC5 electrolyte prepared by the TC and P methods. Electrical conductivity of YSZ is also plotted in the figure for comparison. The electrical conductivity of the LSGMC5 electrolyte prepared by the TC method was apparently lower than that of the specimen prepared by the P method. As mentioned before, the concentrations of Sr, Mg and Co near the surface of the TC-method LSGMC electrolyte were slightly deviated from the designed concentration. Therefore it was anticipated that the difference in electrical conductivity will be caused by the deviation in composition at the surface layer. It is wellknown that Co and Sr easily react with Al 2 O 3 . Therefore, the deviation in the chemical composition near the surface may result from the reaction of LSGMC with the Al 2 O 3 holder when the electrolyte
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(TC method) was sintered after tape-casting. Even though the apparent electrical conductivity of the TC method electrolyte is slightly lower than that of the P method one at high temperatures, the electrical conductivity of the TC method electrolyte exhibits the similar electrical conductivity with the P method specimen, in particular, at low temperatures. It is also noted that the electrical conductivity of LSGMC is much higher than that of the conventional electrolytes; YSZ. Thus, in comparison with YSZ of the same thickness, it is expected that LSGMC5 (TC method) could reduce the operating temperature of the SOFC from 900 to 6508C. The temperature dependencies of the transference number of oxide ion in the LSGMC5 electrolytes (TC and P methods) are shown in Fig. 5. The transference number of oxide ion of the TC method specimen was higher than 0.85 in the range of 600–10008C; in particular, it was 0.88 at 6508C. The transference number is almost the same as the reported values [5]. In Fig. 5, the transference number of the P method LSGMC5 at lower than 8008C is slightly higher than that of the TC method one. One cause of the difference is the gas leak of oxygen gas concentration cell of H 2 –O 2 . The transference number was estimated by the ratio of the theoretical electromotive force and the measured terminal voltage of cell of H 2 –O 2 using the LSGMC electrolyte. The terminal voltages were measured from high temperature to low temperature. The borosilicate glass was used as the gas-seal material of the oxygen gas concentration cell, and its softening temperature is 8208C. Therefore, the borosilicate glass was hardened and shrunk below 8008C. Then, it might well be said that microcracks due to the hardening and shrinking of the borosilicate glass were induced in the LSGMC electrolyte (TC method) because the thickness of the electrolyte was very small (0.20 mm). As to the other cause of the difference in the data of the apparent transference number, the change of the composition in the electrolyte is considered. The concentrations of Sr, Mg and Co in the LSGMC electrolyte (TC method) deviated slightly from the designed composition in the range of 20 mm depth from the surface. Therefore, the intrinsic transference number of the LSGMC electrolyte (TC method) reduces. The difference of the transference number of the specimen
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Fig. 4. Arrhenius plots of the electrical conductivity of (a) La 0.8 Sr 0.2 Ga 0.8 Mg 0.22x Co x O 32d with various Co contents and (b) La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d prepared by the tape-casting and pressing methods, respectively. The electrical conductivity of YSZ is also plotted in (b) for comparison.
prepared by the TC and P methods needs further investigation. The fuel cell configuration in this investigation allows separate measurements of the anode and the cathode overpotential; ha and hc , and the voltagedrop across the electrolyte; IR, and the voltage-drop between the reference and the working electrodes; DV from the cell voltage; V V 5VOCV 2 hc 2 ha 2 IR 2 DV
(1)
In most cases, DV is negligible. hc is evaluated from the transient voltage change between the cathode and the reference electrode during the current-interrupting method, and ha between the anode and the reference electrode. If the overall overpotential between the cathode and the anode; hca , would be measured, the following relationship should be theoretically considered
hca 5 hc 1 ha Fig. 5. Temperature dependency of the transference number of oxide ion for the La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d electrolyte fabricated by tape-casting and pressing methods, respectively.
(2)
hc value was too small to be detected in our experiment. Therefore, hca and ha were measured by the current-interrupting method, and then hc was
K. Kuroda et al. / Solid State Ionics 132 (2000) 199 – 208
estimated by the Eq. (2). In our preliminary measurements of each component, hca , hc and ha , it has been confirmed the validity of the Eq. (2). Fig. 6a shows the power generating property of a single cell using LSGMC5 electrolyte prepared by TC method. It is obvious that the output power density was 380 mW/ cm 2 at 0.5 A / cm 2 and 6508C under air and dry hydrogen as oxidant and fuel. The maximum power density of the same cell was estimated to be 410 mW/ cm 2 at 0.73 A / cm 2 . This power density is significantly higher than that of the SOFC using the YSZ electrolyte under development to date. It can be said that the operating temperature of the SOFC may be decreased to 6508C by using LSGMC5 for the electrolyte. Fig. 6b shows the details of the internal resistance of the developed cell. Except for anodic overpotential, it is clear that another large internal resistance is the IR drop due to the electrical resistivity of the electrolyte. Although the overall electrode overpotential consisted of the cathode and the anode components, the overpotential at the anode occupied the most part of the overall overpotential. The small overpotential at the cathode indicates the appropriate choice of the electrode material and the suitable sample preparation method.
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From the measurements of the electrical conductivity as shown in Fig. 4b, it is estimated that the resistivity of the LSGMC5 electrolyte fabricated by TC method is 23 Vcm at 6508C. Using this value, it is also estimated that the IR drop over the electrolyte with the thickness of 205 mm is 0.25 V at 0.5 A / cm 2 . In the experiment as shown in Fig. 6b, it was able to be observed that the IR drop was 0.27 V at 0.5 A / cm 2 . The measured value is in good agreement with the estimated value and is about 50% of the total internal resistance. Table 1 summarizes the anode properties for four different compositions. It is evident that the anodes of Ni mixed with 40 vol% Ce 0.8 Sm 0.2 O 22 b showed better properties in the power density, IR drop, and overpotential than those of pure Ni. The NiO powders of |14 mm showed better properties than that of |1.0 mm. Fig. 7a–c show the SEM images of the anodes of the different compositions. It is observed in Fig. 7a that the pure Ni anode tended to peel off from the electrolyte and agglomerate themselves regardless of the starting NiO particle size. The anode microstructure using NiO powders of |14 mm without Ce 0.8 Sm 0.2 O 22 b is not shown, because the anode completely peeled off from the electrolyte just
Fig. 6. Power generation characteristics of the SOFC using La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d as an electrolyte. Air and dry hydrogen were used as oxidant and fuel, respectively. The fuel cell was operated at 6508C. (a) the I–V curve and I–P curve, and (b) the IR drop and the overpotentials of the anode, ha , and the cathode, hc .
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Table 1 Comparison as for the anode conditions of the SOFC using La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 32d (LSGMC5) as an electrolyte. The average particle size of Ce 0.8 Sm 0.2 O 22 b is around about 1.0 mm. Cathode materials are fixed at Sm 0.5 Sr 0.5 CoO 32 a in all the tests. Power generating experiments were operated at 6508C for air and dry hydrogen as oxidant and fuel, respectively Specimen ID
a
b
c
d
Average particle size of NiO (mm) Volume ratio of Ni / Ce 0.8 Sm 0.2 O 22 b OCV (V) At cell voltage of 0.7 V Current density (A / cm 2 ) Power density (mW/ cm 2 )
|1.0 100 / 0 1.12
|1.0 60 / 40 1.13
|14 100 / 0 1.16
|14 60 / 40 1.20
0.26 180
0.39 270
0.34 240
0.54 380
(,0.50) – 0.26
0.61 310 0.24
0.51 260 0.24
0.72 360 0.25
0.28 0.38 0.35 0.03
0.25 0.31 0.24 0.07
0.41 0.39 –a –a
0.27 0.24 0.23 0.01
240
330
260
410
0.48 0.50
0.50 0.65
0.54 0.48
0.56 0.73
At current density of 0.5 A / cm 2 Cell voltage (V) Power density (mW/ cm 2 ) Estimated IR-drop by the resistivity and the Thickness of the LSGMC5 electrolyte (V) IR drop (V) Overall overpotential (V) Overpotential at anode (V) Overpotential at cathode (V) Estimated maximum power density (mW/ cm 2 ) At estimated maximum power density Cell voltage (V) Current density (A / cm 2 ) a
Overpotential was not measured because of the failure of adequate contact between the reference electrode and the electrolyte.
after the power generation. On the other hand, the anodes of Ni with 40 vol% Ce 0.8 Sm 0.2 O 22 b seem to show good contact with the electrolyte. This implies that good electrical conduction through the interface between the anode and the electrolyte results in the reduction of the internal resistance of the cell. Since the cathode overpotential was very small, its composition was fixed. Fig. 7d shows the image of the cathode. Sm 0.5 Sr 0.5 CoO 32 a particles seem to be slightly fused. The particle size affects the morphology of the anode microstructure. Fig. 7c shows that the large particles form frameworks and are surrounded by small particles. On the other hand, in the anodes in Fig. 7b all particles are almost same size and seem to mix at random. The large particles are not observed in Fig. 7b, and the NiO particles as a starting material in Fig. 7c are larger than that of Fig. 7b, then, it is presumed that the large particles in Fig. 7c are Ni, and are effective for forming the framework of the anodes and increasing the triple-phaseboundaries among Ni, Ce 0.8 Sm 0.2 O 22 b , and fuel atmosphere. Therefore, it is considered that the
anodes consisting of large NiO particles as a starting material are suitable for the highly active anode resulting in a higher power density (see Table 1). Matsuzaki et al. [23] and Kawashima et al. [24] reported that the ratio of the particle size of YSZ and NiO as the starting powders for the anode affects the overpotential of the anode in the SOFC using the YSZ electrolyte. The range of the ratio of the particle size, YSZ / NiO50.01–0.10, showed the lowest overpotential. The reports [23,24] conclude that the ratio of the particle size of the anode components, in their case using YSZ and Ni, affected the distribution of these components by the filling of the YSZ particles in the opening between the Ni particles. In our case, the ratio of the particle size of the anode components as the starting powders, Ce 0.8 Sm 0.2 O 22 b (|1 mm) / NiO (|14 mm), was 0.071, consistent with the previous reports [23,24]. Apart from anodic overpotential, another large internal resistance of the cell is the IR drop due mainly to the electrical resistivity of the electrolyte. However, this resistance can be reduced by decreasing the thickness of the electrolyte. Therefore, the
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Fig. 7. SEM images of microstructure for the interface between, (a–c) the anode and the electrolyte and (d) the cathode and the electrolyte, respectively. (a) Pure NiO with particle size of |1.0 mm (b) mixture of NiO with particle size of |1.0 mm and additive Ce 0.8 Sm 0.2 O 22 b , (c) mixture of NiO with particle size of |14 mm and additive Ce 0.8 Sm 0.2 O 22 b . The volume ratio of Ni and the additive Ce 0.8 Sm 0.2 O 22 b was 60 / 40. The average particle size of Ce 0.8 Sm 0.2 O 22 b , and Sm 0.5 Sr 0.5 CoO 32 a is around 1.0 mm. The image of the anode with pure NiO of particle size of |14 mm is not shown, because the anode peeled off from the electrolyte just after the power generation.
decrease of the anodic overpotential is an important matter for further improvement in terms of power density of the cell. The approach can be optimization of the composition, particle size of each component and microstructure. This investigation of the electrodes coupled with LSGMC as the electrolyte showed a promising starting point for the further study.
4. Summary We have characterized the power generation and the electrode overpotential of a single cell using LSGMC5 as an electrolyte in the temperature range lower than that of the typical SOFC using YSZ. The oxide ion conductivity of LSGMC5 was higher than that of YSZ. A single cell using LSGMC5 with 205
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mm in thickness showed the power density of 380 mW/ cm 2 at the current density of 0.5 A / cm 2 and 6508C under air and dry hydrogen as oxidant and fuel, respectively. The IR drop was 0.27 V which was in good agreement with the predicted value of 0.25 V. The overpotential for the anode was 0.23 V and larger than 0.01 V for the cathode at the same current density of 0.5 A / cm 2 , and dominated the overall overpotential. It is confirmed that LSGMC is a promising material as an electrolyte for a SOFC operating at 6508C with high power density. In addition, this study reveals the possibility for the SOFC of using a LSGMC electrolyte prepared by a tape-casting method and the Ni–Ce 0.8 Sm 0.2 O 22 b cermet as for the anode.
Acknowledgements The authors would like to thank M. Uzuki and Y. Tamura for preparation of the electrolyte cell membranes and T. Yamada for measurement of the electrical conductivity. The financial supports of Ministry of International Trade and Industry (MITI) of Japan for promoting energy-related technologies, and Grant-in-Aid from Ministry of Education, Culture, and Sports of Japan are gratefully acknowledged.
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