LaMnO3 air cathodes containing ZrO2 electrolyte for high temperature solid oxide fuel cells

SOLID STATE IOB(S

Solid State Ionics 57 (1992) 295-302 North-Holland

LaMn03 air cathodes containing Zr02 electrolyte for high temperature solid oxide fuel cells T. K e n j o a n d M. N i s h i y a Department of Applied Chemistry, Muroran Institute of Technology, Muroran 050, Japan Received 13 January 1992; accepted for publication 27 March 1992

This study is an attempt to improve LaMnO3 high temperature air cathodes by using an electrode structure which resembles that of porous electrodes of liquid electrolyte fuel cells. Y203-stabilized zireonia (YSZ) was loaded with LaMnO3 and molded into porous electrodeswhich had YSZ-LaMnO3interfaces inside the electrodes. A strong relation was seen between the conductivity of YSZ added and polarization losses, suggestingthat the internal LaMnO3-YSZcontacts are active for 02 reduction. The addition of YSZ prevents the formation of highly resistiveLa2Zr207which has otherwisebeen formed at the electrode-electrolyte interface. This effect also helps to enhance the electrodes.

1. Introduction In low temperature fuel cells such as phosphoric acid fuel cells and alkaline fuel cells, the electrolyte can penetrate into the electrode pores and cover the pore walls with thin electrolyte films [1-3]. The electrochemical reactions can therefore take place not only at the interface between the electrode face and bulk electrolyte, but also at the i'tim-pore wall interfaces. Since the pores penetrated by the electrolyte are distributed throughout the electrode body in a three-dimensional network, the electrode has a much greater reaction area than the nominal electrode area. In solid oxide fuel cells, however, the electrolyte cannot penetrate into the electrode pores, so that their reaction area is usually restricted to a t,~,o-dimensional electrode-electrolyte interface, therefore being apt to be relatively small. Although the disadvantage of the limited reaction area is alleviated by the higher operating temperature, for further improvement of the electrode performance, it is still desirable to keep the reaction area as large as possible. In the previous study, erbia-doped bismuth oxide electrolyte was mixed with a platinum paste electrode to create active electrolyte-electrode interfaces inside the electrode [ 4 ]. The performance was found to increase with increasing electrode thickness, demonstrating that the interfaces introduced into the

electrode are electrochemically active. The present study is a similar attempt to increase the effective reaction area, but using LaMnO3 electrodes. This material has been widely used as the most promising candidate for large scale fuel cell plants. Y203-stabilized zireonia (YSZ) was chosen as an ion conductor to produce internal active sites. It has been found that LaMnO3 undergoes solid phase reaction with YSZ to form La2Zr207 at the electrode-electrolyte interface [5]. The addition of YSZ into the LaMnO3 electrode may have an additional influence on the electrode performance in relation to the formation of interfacial compounds.

2. Experimental 2.1. Preparation o f the electrodes The solid electrolyte used was Y203-stabilized zirconia which was prepared by pressing fine powder (8 tool% Y203, TOSOCo.,Ltd.,TZ-8Y) and then sintering at 1500°C for three hours. LaMnO3 fine powder was prepared by the coprecipitation of the citrate complexes of La and Mn [ 6 ]. When a solution conraining the complexes was heated, a glassy precipitate was obtained. The precursor thus obtained was fired in air at 800°C to give LaMnO3 fine powder.

0167-2738/92/$ 05.00 © 1992 ElsevierSciencePublishers B.V. All fights reserved.

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T. Kenjo, M. Nishiya / LaMnOj air cathodesfor HT SOFCs

The oxide powder was mixed with YSZ powder (8 mol% Y2Oa, Hokko Chemical Co.,Ltd.,) using turpentine oil as a dispersant. The slurry mixture was painted over a 2 cm 2 area on the electrolyte disk and sintered in air at a constant temperature ranging 1000-1300°C for one hour. To improve the electrode contact to the electrolyte disk, an isostatic pressure of 0.7 t o n / c m 2 was applied to the green electrode before heating. The anodes used are platinum paste electrodes (Tokuriki Chemical Co., Ltd., No 8103), which were painted on the other side of disks over the same area. The same paste was painted on the peripheral area and used as reference electrodes.

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Polarization curves were measured at 900°C in air. The ohmic loss was eliminated by the current interruption method [ 7 ]. The load current was supplied from a dc source and interrupted periodically to generate rectangular current pulses. The pulsed current was passed between the cathode and anode, the cathode potential being measured against the reference electrode on an oscilloscope screen. Fig. 1 shows a rectangular current pulse and a decay curve of the cathode potential resulting from the pulse, which typically appeared on the oscilloscope screen. A gradually decreasing polarization tail follows an instantaneous drop due to an ohmic drop. In a sufficiently long time after the current interruption, the tail reaches a horizontal line which is the cathode potential for zero current. It was used as the base for the potential standard. The frequency of the pulse was adjusted to be low enough for the decay curve to reach the base line within the zero current period. In this study, a frequency range of 10-100 Hz was employed, depending on the electrode materials used. The potential values free of the ohmic loss, i.e., polarizations, were obtained by subtracting the instantaneous drop from the total potential drop.

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LaMnO3 electrodes were prepared by painting the oxide slurries on YSZ disks and sintering at various

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2.2. Measurement o f the polarization

3. Results and discussion

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Fig. 1. A decay curve of Lao.ssMnO3cathode (bottom) appeared when a current pulse shown in the top was passed through the cell. temperatures. One of their potential responses to a current pulse is shown in the top of fig. 2. Both the rising and decay curves are seen to become horizontal within 1 ms, indicating that the reaction reaches the steady state in this time period. The polarizations of these electrodes were accordingly measured in a frequency of 100 Hz which is low enough for the attainment of the steady state. Their polarization curves are shown in fig. 3. It is seen that the electrodes sintered at higher temperatures give greater polarization losses. After finishing the polarization measurement, the electrodes were taken off the YSZ disks and traces left on the disks were examined by the X-ray diffractometry. The Xray diffraction patterns of the traces, that is, those of the electrode-electrolyte interfaces are shown in fig. 4. Besides strong peaks due to YSZ, those of La2Zr207 are observed, indicating that LaMnO3 reacts with ZrO2 to form La2Zr2OT, i.e., 2LaMnO3+ 2ZrO2-*La2Zr207 + Mn203. The Mn203 peaks to be observed cannot be seen, probably because the

297

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Fig. 2. The potential responses of LaMnO3 (top) and of Lao.,MnO~ (bottom) electrodesto rectangularcurrent pulses. Mn203, if any, strongly absorbs the X-ray emitted from a Cu target. The L a 2 Z r 2 0 7 layer formed at the interface is increasingly resistive for oxide-ion conduction with increasing amount of the interracial compound. Since the more interfacial compound results from the higher sintering temperature, as indicated by the Xray diffraction patterns in fig. 4, the polarization losses of the LaMnO3 electrodes become greater as the sintering temperature rises, as seen in the polarization data in fig. 3. This finding agrees with that of previous studies [ 5,8 ]. It has been found that nonstoichiometric compounds, Lat_xMnO3, are much less reactive with ZrO2 than LaMnO3, so that the former electrodes give much lower polarization losses than the latter [ 9 ]. To confirm this, an Lao.ssMnO3 electrode was prepared and its polarization losses were measured. The bottom of fig. 2 shows the potential response of the electrode. It is seen that the time period required for the reaction to reach the steady state is as long as 40

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Fig. 3. Polarizationcurvesof LaMnO3electrodessinteredat various temperatures.Top and bottom indicatehigh current density and low current densityre~ons, respectively. ms, being much longer than that of the LaMnO3 electrode. The current pulse of 10 Hz was therefore used for this electrode. The polarization curves obtained are shown in fig. 5. Much lower polarization losses are seen than those of the LaMnO3 electrodes (see fig. 3). The X-ray diffraction pattern of the interface for the nonstoichiometric electrode is also shown at the bottom of fig. 4. No peaks of La2Zr207 can be seen, indicating that the solid state reaction does not take place at a sintering temperature as high as 1300°C. The smaller polarization losses of the Lao.ssMnO3 electrode is thus explained as resulting from a lower interfacial resistance due to no intervention by highly resistive compounds. The Lao.ssMnO3 electrode is also in contrast with the LaMnO3 electrode in the effect of sintering temperature. In the former, the electrode sintered at a higher temperature yields a larger polarization loss, but the latter electrode exhibits the inverse tendency (see figs. 3 and 5). It is expected that a higher sin-

T. Kenjo, M, Nishiya / LaMn03 air cathodesfor HT SOFCs

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L a M n O 3 powder was mixed with Y S Z in a weight ratio of 1 :I and sintcred on the Y S Z disks. The polarization curves of the electrodes are shown in fig. 6 as a function of the Y203 content in the Y S Z added. Comparing this with fig. 3, one can see that the addition of Y S Z markedly improves the performance. Similarly to the previous section,the electrodes were removed from the electrolyte disks and the X-ray diffractionpatterns of the interfaceswere taken. The patterns obtained arc shown in the top of fig. 7. N o formation of interfacialcompounds is seen, resembling the case of the Lao.asMnO3 electrodes.The bottom of fig.7 isthat of the mixed electrodeitself.There are La2Zr207 peaks in addition to those of L a M n O 3 and YSZ, indicating that the solid state reaction occurs between the latter two oxides. The remaining L a M n O 3 in the electrode thus becomes La-deficient by the amount consumed by the reaction. This effect leads to the same situation as seen in the electrode starting with Lao.ssMnO3: Just as the Lao.gsMnO3 electrode is unreactivc with YSZ, so also is the mixed electrode which is made La-
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reaction with YSZ. The lower polarization losses of the electrodes containing YSZ can therefore be explained as being partly due to the unreactive nature of the electrodes. Although this is not an expected result, but just fortunate one, this method is very helpful to enhance the electrodes. The key factor to make LaMnO3 unreactive is probably to use a very fine powder of YSZ, which thereby permits to start the solid state reaction with LaMnO3 before the disk electrolyte does the same reaction. It is also interesting to note that the polarization losses of the mixed electrodes depend on the Y203 content of YSZ added. Calculating the polarization resistance (Rp) from the slopes of each polarization curve in fig. 6, Rp is plotted against the Y203 content (see fig. 8 ). The plots pass through the minimum at 8 mol% Y 2 0 3 and increase with increasing Y203 content. This behavior is very similar to the relation between the resistivity of YSZ versus Y203 content, as shown with triangles in the same figure. The

parallelism between the polarization resistance and electrolyte resistivity strongly suggests that the YSZ added acts as an oxide-ion conductor. Fig. 9 shows the optical micrographs of the LaMnO3 electrode containing YSZ in a weight ratio of 1 : I. They were taken under polarized light so as to make the image of YSZ distinct from that of LaMnO3: White part indicates YSZ and black ones LaMnO3. It is obvious from these pictures that the mixed electrode is not a uniform mixture of fine particles where two oxides are separated completely from each other, but is an agglomerate mixture of the two components, as suggested in a previous paper [4]. If the mixing ratio of the two oxides is appropriate, there will be an appreciable amount of agglomerates which can penetrate through the electrode thickness, contacting with the electrolyte disk at one end and the current collector at the other end. Since each agglomerate can pass ions or electrons respectively, the agglomerate mixture may be pictured as an interpenetrating subnetwork of ionic and electronic paths which contact with each other inside the electrode. If O2 gas is supplied sufficiently throughout the electrode, the electrochemical reduction of O2 gas is also possible at these internal contacts as well as the diskelectrode interface; the electrons needed to reduce O2 molecules are supplied through the electron conducting paths while the 0 2- ions resulting from the O2 reduction are transported through the ion conducting paths. The above situation is very analogous to that of a porous electrode penetrated by liquid

300

T. Kenjo, M. Nishiya / LaMnOs air cathodes for H T SOFCs

Fig. 9. Optical micrographs of the LaMnO3-YSZ mixed electrode. LaMnO3: YSZ= 1 : 1 (weight). They were taken under polarized light. White parts indicate YSZ and black ones LaMnO3.

electrolyte. The latter porous electrode has been analyzed on the basis of a single pore covered with an electrolyte film [ 1-3 ]. The same pore model may be applied to the mixed electrode. Fig. 10 shows the pore model used. It involves three main elements: ( 1 ) thin film electrolyte, (2) pore wall and (3) gas diffusing pore, representing the YSZ and LaMnO3 agglomerates, and interagglomerate space, respectively. The thin film is assumed to be gas permeable, since the YSZ agglomerate can pass gas between the sintered particles. The electrochemical reaction takes place in the film-pore wall interface with the local polarization resistance k, and then the resulting 02- ions flow through the film having

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the resistivity p. The diffusional resistance of 02 gas and the electronic resistance of pore wall are assumed to be negligible in comparison with p and k. Simple mathematical analysis based on the model leads to the following equation for the polarization resistance Rp [ 3 ]: Rp =

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where d is the thickness of electrode. It is obvious that p is directly proportional to the resistivity of YSZ (Rvsz), the proportional constant depending on the morphology of YSZ agglomerates. Although k is not expressed as a function of Rvsz, one can expect that it also increases monotonically with increasing Rvsz. Considering these qualitative dependencies ofp and k on Rvsz in combination with eq. ( 1 ), it is expected that Rp and Rvsz follow essentially the same function of any variables affecting the YSZ resistivity. When the Y203 content is taken as the variable changing Rvsz, the plots of Rp versus Y203 content will give a similar curve to the same plots of Rysz if the mixed electrode has the expected active sites inside the electrode. This is, in fact, observed in fig. 8, suggesting that there are active sites inside the electrode which help to enhance the electrode. The total area of the active sites contained in the above electrodes will depend on the YSZ/LaMnO3 ratio. The more YSZ is contained in the electrode, the more LaMnO3 will be isolated like islands sur-

T. Kenjo, M. Nishiya / LaMnOj air cathodes for H T SOFCs

rounded by a "YSZ sea", and vice versa. The total active area is therefore expected to be maximized when these two components are mixed in an equal amount. Fig. 11 shows the plots of 1/Rp, a measure of performance, against the weight ratio of YSZ/ LaMnO3. As expected, the performance attains the maximum at this ratio = 1. A similar experiment was carried out using Lao.ssMnO3 electrodes instead of LaMnO3. The 1/ Rp values for them are plotted in the same figure. It is seen that the curve obtained is parallel to, but above E

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301

that for the LaMnO3 electrode. Remembering that LaMnO3 is more reactive with YSZ than Lao.ssMnO3, one can explain the larger polarization loss of the former as being due to highly resistive LaeZr207 formed inside the electrode. The Lao.ssMnO3 electrode free of such an intervention can provide internal active sites which are electrochemically more active and more conductive for O 2- ions. Eq. ( 1 ) predicts that 1/Rp increases with increasing electrode thickness d for fixed p and k values. This arises from that the electrochemical reaction occurs not only at the electrode-electrolyte interface, but also in a zonal area extending along the thickness. If the expected trend is observed experimentally, this will provide another evidence for existence of the internal active sites. To test this, mixed electrodes having various thicknesses were prepared under a constant YSZ/LaMnO3 ratio, and their polarization resistance was measured. The results are shown in fig. 12 as a function of the electrode weight, which was used as a measure of the thickness. The solid line in the figure is the best fitting curve to eq. ( 1 ) calculated using p=0.039 f2.mg/cm 2 and k = l . 2 F~.mg3/cm 6. The 1/Rp value is seen to increase with the thickness in such an asymptotical manner as expected from eq. ( 1 ). This behavior also supports the view that the YSZ added creates internal active sites, thus increasing the effective reaction area of the electrodes.

4. Conclusion

The LaMnO3 electrode could be enhanced by mixing with Y203-stabilized zirconia (YSZ). The doping effect of YSZ is ascribable partly to La-deficient LaMn03 which results from the consumption of La by the solid state reaction with ZrO2, and partly to electrochemically active YSZ-LaMnO3 interfaces which are formed inside the electrode. The former makes the electrode less reactive with YSZ disks, thus permitting the direct contact of electrode and electrolyte. The latter increases the effective reaction area by creating additional active sites inside the electrode.

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T. Kenjo, M. Nishiya / LaMn03 air cathodesfor HT SOFCs

References [ 1 ] IL Mund, G. Richter and F. yon Sturm, J. Electrochem. SOc. 124 (1977) 1. [2] J. Giner and C. Hunter, J. Electrochem. Soc. 116 (1969) 1124. [3] T. Kenjo, J. Electrochem. Soc. 132 (1985) 1583. [4] 1". Kenjo, S. Osawa and K. Fujikawa, J. Electrochem. Soc. 138 (1991) 349.

[ 5 ] O. Yamamoto, Y. Takeda, R. Kanno and M. Nod& Solid State Ionics 22 (1987) 241. [ 6 ] M. Hirabayashi, Kagaku to Kogyo 40 (1987) 992. [ 7 ] D.Y. Wang and A.S. Nowick, J. Electrochem. SOc. 126 (1979) 1166. [8 ] H. Yokokawa, N. Sasaki, T. Kawada and M. Dokiya, Denki Kagaku 57 (1989) 821. [9] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Denki Kagaku 57 ((1989) 829.