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Structural properties of SOFC anodes and reactivity
PII:
Electrochimica Acta, Vol. 43, Nos 19±20, pp. 3059±3068, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00046-2 0013±4686/98 $19.00 + 0.00
Structural properties of SOFC anodes and reactivity I. Drescher*, W. Lehnert and J. Meusinger Institute of Energy Process Engineering, Research Centre JuÈlich, D-52425 JuÈlich, Germany (Received in Newcastle 21 January 1998) AbstractÐFuel cells convert electrical energy directly into electricity and are thus an alternative to conventional energy conversion techniques. A detailed knowledge of the dierent processes in the fuel cell is necessary to develop fuel cells with high eciency. One aspect is the internal reforming of methane in the anode of SOFC fuel cells. The anode structure is investigated by gas adsorption, diusion and permeation measurements. The reforming reactivity is determined as a function of the anode thickness to characterize the in¯uence of mass transfer limitations. The preparation of the SOFC anode material ends up in a sintered mix of NiO and YSZ. Temperature programmed reduction experiments are carried out in order to de®ne the condition for complete NiO reduction at a low temperature level. # 1998 Published by Elsevier Science Ltd. All rights reserved Key words: BET, diusion, permeation, reforming, structure.
INTRODUCTION Solid oxide fuel cells (SOFC) operate at temperatures between 8008C and 10008C using hydrogen as fuel and atmospheric oxygen as oxidant. The most interesting fuel for SOFC systems is natural gas, which consists mainly of methane. The methane steam reforming reaction according to the overall reaction scheme CH4 2H2 O , 4H2 CO2
1a
is used as the source of hydrogen. This hydrogen can be oxidized electrochemically according to the reaction H2 O2ÿ , H2 O 2eÿ
1b
to produce the electrical power. In order to achieve a high energetic eciency of the fuel cell, internal reforming of the methane is necessary, which takes place directly within the anode. Due to the great dierence between the reaction rates of the endothermic methane reforming reaction (1a) and the exothermic electrochemical hydrogen oxidation (1b), cooling eects arise resulting in intolerable temperature gradients inside the anode structure. In *Author to whom correspondence should be addressed.
order to control the course of methane reforming, detailed knowledge of the reaction rates and the anode properties is necessary. At the Research Center JuÈlich, Ni supported on yttrium stabilized zirconia (YSZ) is used for substrate anodes. They are very thick (about 2 mm), compared to other SOFC anodes, in order to serve as self-supported anodes. This concept allows the application of a very thin electrolyte and therefore lower working temperatures (about 7508C) with high electrical output are possible [1]. In order to develop an adopted anode, which enables internal reforming of natural gas detailed knowledge about the relationship of structural properties and the reforming reaction of SOFC substrate anodes is necessary. Therefore, dierent experiments for structure characterization have been performed. Temperature programmed reduction experiments were carried out in order to de®ne the condition for complete NiO reduction at a low temperature level. The reactivity of the reforming reaction was measured as a function of the thickness of the anode. The results lead to the conclusion that for these anodes the depth of the methane reforming reaction zone is larger than 0.15 mm but smaller than 0.3 mm due to mass transfer limitations [2]. Additionally the anode structure was investigated
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by gas adsorption, measurements.
diusion
and
permeation
EXPERIMENTAL Sample preparation The methane steam reforming reaction was investigated using the SOFC anode material of the Research Centre JuÈlich. This material consists of Ni±8YSZ (8YSZ: 8 mol% Y2O3-stabilized ZrO2) substrate with a standard composition of 50 wt% Ni. This cermet is manufactured by the Coat Mix1 process starting with a powder mixture of NiO (56 wt%) and 8YSZ together with a resin binder. The anode is sintered at 14008C. A more detailed description of the manufacturing process is given in [3]. The sintered anode materials have to be reduced before application. The structural and catalytic properties will depend on the reducing procedure. Therefore the procedure has to be investigated and the material characterization has to be done with the anode reduced under de®ned conditions. Experimental setup Three dierent experimental setups were used. In the ®rst catalyst test equipment, a mixture of pure CH4 with H2O (H2O/CH4=3 mol/mol) was ¯owing at 1.5 bar over both sides of a ¯at piece of the substrate anode. This experimental setup serves for the investigations of the dependence of methane reforming rate on temperature (7008C±9508C) and thickness (0.3±1.2 mm) of the anode under conditions similar to the ones for operating SOFCs. The dry product gas stream was analyzed by a BINOS IR-adsorption-detector for CO, CO2 and CH4. Hydrogen was detected by a thermal conductivity detector (TCD). A detailed description of the experimental setup is published elsewere [2]. The second setup was used for temperature programmed reduction (TPR) experiments, for BET (Brunauer-Emmett-Teller isotherm) and CO, H2 adsorption measurements. The adsorption measurements determine the speci®c surface area of the whole anode material and of the Ni sites. The TPR experiments give information about the reduction properties of the catalyst. For these experiments, the anode was crushed into pieces of 0.5±2 mm diameter. In the TPR experiments the sample (mesh size: 0.5±2 mm) was heated, usually with a temperature gradient of 18C/min, in a reducing environment (10% H2 in Ar or N2). The disappearance of hydrogen was followed via analysis of the euent gas stream by mass spectroscopy. The rate of change in H2 concentration is proportional to the rate of catalyst reduction. This allows the development of a de®ned reduction strategy for Ni-cermet materials. Additional information is available by comparing theoretical H2 consumption (assuming complete re-
duction of all NiO in the catalyst) and the consumption, observed during the TPR experiment. For the BET measurements the reduced and crushed catalyst was contained in a special cell, which can be cooled down to the temperature of liquid nitrogen. After degassing the catalyst at 1258C with pure He for 8 h, a nitrogen in helium gas mixture ¯owed through the catalyst at room temperature. The BET cell was then cooled down in liquid nitrogen. The thermal conductivity detector (TCD) signal became stabilized when no more nitrogen was adsorbed at the catalyst. Now the catalyst was heated up to room temperature again. The TCD signal of the desorbed nitrogen was used for the calculation of the amount of adsorbed nitrogen. This procedure was repeated for dierent nitrogen partial pressures (P(N2) = 13±33 kPa). A pulse of a known volume of nitrogen in He at room temperature was detected by the same TCD and served to normalize the desorption signals of the BET measurements. The adsorption measurements with H2 and CO were done at room temperature with the pulse method. Pulses of known volume of H2 or CO in He were sent to the reduced and degassed catalyst, and were adsorbed only on the free nickel sites. The pulses, detected by the TCD, stopped growing, when no more CO or hydrogen was adsorbed. The detector signal of a pulse of known volume of CO or hydrogen served again for normalization. In order to determine the textural properties of the cermet material, permeation and binary diusion experiments have been performed by the third experimental setup. A modi®cation of the Wicke± Kallenbach cell was used for the diusion experiments [4]. The cell consists of two chambers separated by the porous solid of interest. A gas A (N2 or Ar) ¯ows through the upper part of the cell and gas B (H2 or He) through the lower part (see Fig. 1). When the ¯ow of gas B in the lower compartment is stopped a soap ®lm in a glass burette connected to the lower part of the cell starts to move and the rate of the volume change can be observed. Extrapolation of the observed volume change to zero time equals the net diusion ¯ux density. Both chambers have atmospheric pressure. Thus, isobaric conditions are ful®lled, no viscous ¯ow takes place and the Graham law is valid. r NA MB ÿ 2 NB MA where Ni is the ¯ux density of gas i and Mi the molecular weight of the molecules i. The driving force for permeation of a gas through a porous structure is a pressure gradient over this structure. The permeation measurements were performed in a cell which consists of two parts with the same volume separated by the porous solid. At the beginning both compartments are ®lled
Structural properties of SOFC
3061
Fig. 1. Modi®ed Wicke±Kallenbach diusion cell
with the same gas of the same pressure. In one of the compartments the pressure will then step-wise, be increased and the exponential decay of the pressure dierence in both cells can be observed [5].
RESULTS AND DISCUSSION Reactivity measurements Measurements to investigate the in¯uences of the structural properties for the methane reforming reaction were done with the ®rst experimental setup. The anode material was a Ni/YSZ cermet as described above. The NiO was gradually reduced in situ to metallic Ni in a gas stream consisting of 5% H2 and 95% N2 at 6008C. The resulting anode
samples with the thickness between 0.36 and 1.9 mm were investigated. The reactivity of the reforming reaction was measured as a function of the thickness of the anode. The aim of these experiments was to determine the depths of the reaction zone in the anode. In Fig. 2 the reactivity related to the surface of the sample is plotted vs the anode thickness. At a reaction temperature of 9158C the reactivity increases linearly with the anode thickness up to a thickness of 0.6 mm. In the range of 1.2 mm down to 0.6 mm no signi®cant dierence in methane conversion could be observed. This result leads to the conclusion that for these anodes with the described structural properties the depth of the methane reforming reaction zone is larger than 0.15 mm but smaller than 0.3 mm [2].
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Fig. 2. Methane conversion rate as a function of the thickness of the anode for the standard material of SOFC anodes
TPR experiments In Fig. 3 the H2 consumption as a function of the sample temperature is shown. 3.155 g of crushed (0.5±2 mm) anode pieces were ®lled in a ceramic reactor tube. The sample was heated from 18C/min to 9008C in a reducing atmosphere of 10% H2 in nitrogen (total ¯ow rate 200 ml/min). Three peaks
Fig. 3. TPR experiment of 3.241 g catalyst. Catalyst shape: crushed to 0.5 mm±2 mm. Atmosphere: 50% H2 in Ar. Flow: 200 ml/min. Temperature gradient: 18C/min. Hydrogen consumption: 611 ml theoretical, 600 ml measured
of H2 consumption exist with maxima at 2608C, 4008C and 6508C. At 8608C the reduction process is completed. In contrast to these measurements, other TPR experiments [6, 7] detected only one signi®cant maximum of reduction at around 4008C± 5008C of Ni/YSZ. Otherwise, Perego et al. [8] measured a similar hydrogen consumption with a reduction maximum at 5008C and a wide maximum at 7008C. It seems to be possible that the wide peak shape of the third maximum at 6508C in Fig. 3 is caused by mass transfer limitations. To check this hypothesis, the experiment was repeated with an increased hydrogen supply. To oer the catalyst more hydrogen during the experiment one can increase the hydrogen concentration in the reducing atmosphere, or increase the time for catalyst reduction. In the second TPR experiment (see Fig. 4) both possibilities were used. A temperature holding point for 8 h at 5008C was chosen to oer the catalyst enough time for reduction before it was heated up to the critical temperature of 6508C. Furthermore, to increase the H2 supply during the whole experiment, a higher hydrogen concentration of 50% in the atmosphere was used. In Fig. 4 one can see the result of the repeated TPR experiment. This huge supply of hydrogen leads to a fully reduced catalyst at a temperature lower than 4508C. Only one signi®cant sharp peak of hydrogen consumption is observable with a maximum at 4108C. The small peak with a maximum at 2608C is still observable. With a third TPR experiment, it was examined whether the mechanical stress (crushing of the anode material) aects the reduction properties.
Structural properties of SOFC
3063 BET experiments
For the determination of the speci®c surface area of a sample, one can use the BET method. The BET equation gives a relation between the adsorbed volume of an adsorbate, the volume of the adsorbed monolayer of adsorbate and the partial pressure of the adsorbate in the gas phase: z 1 c ÿ 1 z I Sz, 1 ÿ zV cVm cVm
Fig. 4. TPR experiment of 3.155 g catalyst. Catalyst shape: crushed to 0.5 mm±2 mm. Atmosphere: 10% H2 in N2. Flow: 200 ml/min. Temperature gradient: 18C/min. Hydrogen consumption: 591 ml theoretical, 542 ml measured
Two cubic pieces (4 4 2 mm, 0.282 g) were cut out of the anode. These samples were reduced in an atmosphere of 10% H2/Ar (total ¯ow rate: 100 ml/ min) at a heating rate of 18C/min. Only one reduction process with a maximum at 4208C was detected (Fig. 5). The small peak with the maximum at 2608C does not exist any longer. This small peak at 2608C is possibly caused by the reduction of NiO, not further bonded to the ceramic support. TPR experiments of bulk NiO have been investigated by Robertson et al. [9]. They reported a single reduction process centered at 3278C (6% H2/N2, heating rate: 4.58C/min, ¯ow rate: 10 ml/min). Shirakawa et al. [7] also reported a single peak at 2948C for reduction of NiO itself (H2/Ar = 3/7, heating rate: 108C/min). Therefore it is fair to assume that some NiO was separated from the sintered structure when the sample was crushed. The cutting out of the catalyst prevents free NiO. At sucient H2 supply, i.e. high H2 concentration in the reduction atmosphere and a low heating rate, the anode material is completely reduced at a temperature of about 4208C (see Fig. 4). One observes a signi®cant sharp peak of H2 consumption. If the H2 supply is not sucient, a second wide peak at 6508C, due to H2 transport limitation, is observed (see Fig. 3). The reduction is completed at temperatures around 4208C. There exists an interaction between NiO and YSZ. Free NiO is reduced at 2608C, whereas NiO, which is held in the sintered YSZ structure, is reduced at 4208C.
3
where z = p/p0, partial pressure of N2 normalized by the saturated pressure of N2, c is constant, which is a function of the heat of adsorbate condensation and the heat of adsorbate, Vm the volume of the adsorbed monolayer, V the volume of the adsorbed adsorbate and I, S the intercept and the slope, if the equation is plotted vs z. The ``multi point'' method was used to determine the surface of the sample. For this method the adsorbed volume V of nitrogen was measured by the TCD at dierent partial pressures of nitrogen in He (z = 13%±33%). It is assumed that plotting z/ [(1 ÿ z)V] vs z yields a straight line (see Fig. 6). The slope S = (c ÿ 1)/(cVm) and the intercept I = 1/ (cVm) of the BET isotherm equation are used to determine the adsorbed volume of a monolayer coverage Vm: Vm
1 I S
4
Fig. 5. TPR experiment of 0.282 g catalyst. Catalyst shape: 2 cubic pieces of 4 4 2 mm. Atmosphere: 10% H2 in Ar. Flow: 100 ml/min. Temperature gradient: 18C/ min. Hydrogen consumption: 52.7 ml theoretical, 53 ml measured
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Fig. 6. Typical ``multi point'' BET measurement. The measured points agree with the assumption of linear dependence of equation (3). The error bars correspond to statistical errors
With the knowledge of the monolayer volume, one can calculate the surface area of the catalyst. Normalized by the mass of the catalyst, one obtains the speci®c surface area: 2 m NA Pa Sm 1 Vm , 5 S gcat RT m where NA is the AvogadroÂs number (1/mol), Pa the ambient pressure (atm), Sm the adsorbate cross section area, Sm(N2) = 1.62 10ÿ19 m2, m the mass of used catalyst (g), R the gas constant, 82.1 ml atm/ (K mol) and T the temperature (K). The evaluation of the BET experiments, adsorption of N2 on the total catalyst surface, leads to a speci®c catalyst surface area of 0.43 2 0.03 m2/gcat for the reduced and 0.26 2 0.03 m2/gcat for the unreduced catalyst. The surface area grows with the reduction of NiO to Ni, because the Ni particles are smaller than the NiO particles and therefore more channels in the YSZ structure become available for the gas. One sample of catalyst was reduced in 5% hydrogen in nitrogen at 9008C for 38 days. After this thermal treatment in reducing atmosphere the speci®c surface area was determined to be 0.41 2 0.03 m2/gcat. No signi®cant change in the surface area could be detected. Therefore, it might be concluded that no thermal degradation occurred. Measurement of the Ni sites surface area For the methane steam reforming reaction only Ni in the anode serves as catalyst. The nickel surface area is one important value for the heterogeneously catalyzed methane reforming reaction.
Adsorption experiments were done to estimate the speci®c nickel surface area of the catalyst. Hydrogen and CO adsorb at room temperature only on the nickel sites. The determination of the nickel surface area assumes that the adsorbate is adsorbed only on the Ni sites at a monolayer. The calculation of the surface uses equation (5) with Sm=6.7 10ÿ20 m2 (the Ni cross sectional area). Adsorption measurements with both of these gases lead to a speci®c nickel surface of (8.62 0.2) 10ÿ2 m2/gcat, the dispersion of Ni is 2.46 10ÿ4. The active Ni surface is about 20% of the BET surface. The adsorbed volume of CO was twice the volume of hydrogen. This must be due to the fact that CO on-top adsorption is favored on Ni. With the knowledge of the nickel surface area of the catalyst one can make a rough estimate of the nickel particle size. Assuming that the nickel particles have a spherical shape and that they are in contact only at one point with the substrate, the nickel particle would have a radius of 2 mm. If one assumes that the particles have cubic shape and that they are in contact with only one of the six surfaces of the cube with the substrate, the cubic Ni particles would have the size of 3.43 mm3. Gas diusion and permeation measurements The mass transport of the gases in the porous structure will be described in the frame of the mean transport pore model (MTPM) [10]. This model is based on the assumption that the gas transport takes place in cylindrical capillaries with radii dis-
Structural properties of SOFC tributed around the mean value hri (®rst model parameter). The second model parameter is the squared transport pore radius hr2i and the third parameter is C, the ratio of the porosity and tortuosity. Within the MTPM the steady state diusion of a multicomponent gas mixture can be written in the following form [10]: X YB NA ÿ YA NB dYA NA ÿcT dx DAk DAB B1 B 6 A A 1, . . . ,n
6
DAB C@ AB DkA Chri
r 2 8RT , 3 pMA
7 8
where cT is the total molar concentration of the gas mixture, YA is the mole fraction of the component A and x is the coordinate in the gas transport direction. DkA is the eective diusion coecient of component A in the porous structure, DAB is the eective bulk diusion coecient of the gases A, B and @AB is the gas±gas diusion coecient. The ®rst term on the right side of equation (6) represents the contribution according to Knudsen diusion and the second term the contribution according to the bulk diusion. For binary counter current diusion of gases A and B equation (6) simpli®es to ÿ1 1 ÿ aA YA 1 dYA ÿ cT 9 NA hriCKA CDAB dx
3065 KA
r 2 8RT 3 pMA
aA 1 ÿ MA =MB 1=2
10 11
Integrating the dierential equation with the boundary conditions which state that in the upper part of the cell is pure gas B and in the lower part pure gas A, the net diusion ¯ux, N, is expressed as cT 1 DAB =hriKA N C DAB ln 12 L 1 ÿ a DAB =hriKA To determine the optimum transport parameters hri and C for diusion in the porous structure from the set of experimental net diusion ¯uxes, the experimental values Nexp were ®tted to the theoretical equation by evaluating best values of C for a range of mean transport pore radii hri. The minimum of the sum of squared deviations between the experimental and calculated ¯uxes determines the optimum parameter hri. According to the Beale criterion [4] a 95% con®dence region for the parameter hri*C and C can be calculated (see Fig. 7). From the permeation experiments the permeation B can be calculated according to Fott et al. [5]. ln Dp0 =Dp
2SB t, LV
13
where Dp0 is the pressure dierence (p0: at time zero), S the cross section of the structure, V the volume of the compartments and t the time. With the same assumption as before, that the gases ¯ow through circular capillaries, the permeation can be expressed as [5]
Fig. 7. Con®dence region obtained from binary diusion experiments for one of the oxidized cermets and the optimum parameters obtained from diusion and permeation experiments
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B KA Chrif o KA Chri 1 ÿ f
fÿ1 1
l 2r
ÿ1
hriCp 8Z
14
15
where Z is the viscosity, p the pressure, o = p/4 and l is the mean free path of the molecules. The ®rst term represents the contribution to Knudsen ¯ow, the second the contribution to the slip ¯ow at the wall and the third the viscous ¯ow. The permeation measurements were performed with H2, N2, and He in a pressure range of 1±6 105 Pa. The obtained results for one of the cermets examined are plotted in Fig. 8 in the form B/K vs p/ZK. The obtained linear relationship shows that the Weber equation (14) simpli®es to B o hriCK
hr2 iC p 8Z
16
hriC can now be calculated from the intercept of the plot. A comparison of the hriC values determined from the permeation measurements with the hriC vs C values inside the con®dence region (see Fig. 7) calculated from the diusion experiments results in an optimum parameter set for hri and C for each cermet where gas transport according to permeation and diusion will be described best. The SOFC anodes were carefully oxidized and reduced in three cycles. After each cycle permeation and gas diusion measurements were done. Three samples of one batch have been examined. For all samples it was found that the mean pore radius hri
and C, the ratio of porosity and tortuosity, is smaller for the oxidized than for the reduced anode. This can be explained by the fact that the NiO particles are larger than the Ni particles. A comparison of the results for hri and C of the three samples which have been treated in the same way (e.g. reduced, oxidized, reduced) shows only dierences within the errors. Repeated oxidation and reducing of the samples does not change the structural properties signi®cantly as can be seen in Table 1. Under the assumption that the tortuosity is the same for oxidized and reduced cermets and that no pores will be produced or destroyed during reducing and oxidation, the ratio of the surfaces for reduced and oxidized cermets is 1.5 (pores treated as capillaries). This is in agreement with the ratio of 1.6 obtained from the BET measurements. As can be seen from the BET and the diusion and permeation experiments neither degradation due to thermal treatment nor degradation due to repeated oxidation and reducing could be observed. The structural properties of the SOFC anode are summarized in Table 2. CONCLUSIONS It could be shown that at sucient hydrogen supply the SOFC anode, consisting of Ni/YSZ, is completely reduced at a temperature of about 5008C. As far as the reduction process is regarded, the anode material is usable for SOFC systems operating at low temperatures (T>4508C). Investigations of the anode structure by BET, gas
Fig. 8. Result of a permeation experiment with three dierent gases in a pressure range of 1±6 105 Pa
Structural properties of SOFC
3067
Table 1. The mean pore radii and the C values (C = porosity/tortuosity) of the permeation and gas diusion measurements at three dierent parts of the SOFC anode. The anodes were reduced and oxidized for several times. After each reduction and oxidation hri and C were determined Anode 9.1 9.1 9.1 9.2 9.2 9.2 9.4 9.4 9.4 9.1 9.1 9.1 9.2 9.2 9.2 9.4 9.4 9.4
Treatment r: reduction; o: oxidation cycled reduction ror roror rororor ror roror rororor ror roror rororor cycled oxidation roro rororo rorororo roro rororo rorororo roro rororo rorororo
C/(l)
DC/(l)
hri/(nm)
Dhri/(nm)
0.145 0.152 0.152 0.144 0.148 0.165 0.150 0.149 0.160
0.010 0.009 0.009 0.009 0.011 0.010 0.009 0.009 0.010
610 626 578 641 675 471 562 659 507
75 67 65 77 76 56 70 75 60
0.069 0.066 0.062 0.068 0.078 0.065 0.068 0.069 0.065
0.002 0.002 0.003 0.002 0.003 0.003 0.003 0.003 0.002
407 406 492 440 245 329 485 376 333
32 46 54 46 52 63 35 47 63
Table 2. Structural properties of the SOFC anode Structural property Total surface Ni surface: Dispersion: Ni particle size Open porosity [13]:
Value 2
for the reduced anode 0.4320.03 m /gcat for the unreduced anode 0.2620.03 m2/gcat (no thermal degradation in 5% H2 for 38 days at 9008C) (320% of total anode surface) (8.620.2) 10ÿ2 m2/gcat 2.46 10ÿ4 2.0 mm (radius, spherical model) (cubic model) 3.43 mm3 4023%
diusion and permeation measurements lead to the result, that no degradation due to thermal and atmospheric cycling as well as long thermal treatment was observed. In reactivity measurements the active methane reaction zone due to mass transfer limitations was determined to be between 0.15 and 0.3 mm. Based on these experiments it was shown previously [11, 12] by computer simulations how mass transfer in the anode can in¯uence the rate of steam reforming. REFERENCES 1. H. P. Buchkremer, U. Diekmann and D. StoÈver, Proc. of 5th Int. Symp. of Solid Oxide Fuel Cells, Vol. 97-40, ed. S. C. Singhal, U. Stimming, H. Tagawa and W. Lehnert. The Electrochemical Society, Pennington, NJ, 1997, p. 160.
2. W. Lehnert, J. Meusinger, E. Riensche and U. Stimming, Proc. 2nd Eur. SOFC Forum, ed. B. Thorstensen. ISBN 3-922 148-19-O, Oslo, 1996, p. 143. 3. H. P. Buchkremer, U. Diekmann and D. StoÈver, Proc. 2nd Eur. SOFC Forum, ed. B. Thorstensen. ISBN 3922 148-19-O, Oslo, 1996, p. 221. 4. J. Valus and P. Schneider, Appl. Catal. 1, 355 (1981). 5. P. Fott and G. Petrini, Appl. Catal. 2, 367 (1982). 6. Zanibelli, L., Flego, C., Perego, C. and Rizzo, C., Proc. 1st Eur. Solid Oxide Fuel Cell Forum, Vol. 1, ed. U. Bossel. ISBN 3-922-14-X, Lucern, p. 207. 7. T. Shirakawa, S. Matsuda and A. Fukushima, Proc. 3rd Int. Symp. on Solid Oxide Fuel Cells, ed. S. C. Singhal and H. Iwahara. The Electrochemical Society, Pennigton, NJ, 1993, p. 464. 8. C. Perego, L. Zanibelli, M. Cartrullo and G. Piro, Proc. 3rd Int. Symp. on Solid Oxide Fuel Cells, ed. S. C. Singhal and H. Iwahara. The Electrochemical Society, Pennigton, NJ, 1993, p. 454.
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9. S. D. Robertson, B. D. McNicol, J. H. De Baas, S. C. Kloet and J. W. Jenkins, J. Catal. 37, 424 (1975). 10. P. Schneider and D. Gelbin, Chem. Eng. Sci. 40, 1093 (1985). 11. J. Divisek, W. Lehnert, J. Meusinger and U. Stimming, Proc. of the 5th Int. Symp. of Solid Oxide Fuel Cells, Vol. 97-40, ed. S. C. Singhal, U. Stimming,
H. Tagawa and W. Lehnert. The Electrochemical Society, Pennington, NJ, 1997, p. 993. 12. J. Divisek, D. Froning, W. Lehnert, J. Meusinger and U. Stimming, European workshop on current and potential distributions in complex electrochemical systems, Nancy, 1997. 13. D. Simwonis, A. Naoumidis, F. J. Dias, J. Linke and A. Moropoulou, J. Mater. Res. 12, 1508 (1997).
Electrochimica Acta, Vol. 43, Nos 19±20, pp. 3059±3068, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00046-2 0013±4686/98 $19.00 + 0.00
Structural properties of SOFC anodes and reactivity I. Drescher*, W. Lehnert and J. Meusinger Institute of Energy Process Engineering, Research Centre JuÈlich, D-52425 JuÈlich, Germany (Received in Newcastle 21 January 1998) AbstractÐFuel cells convert electrical energy directly into electricity and are thus an alternative to conventional energy conversion techniques. A detailed knowledge of the dierent processes in the fuel cell is necessary to develop fuel cells with high eciency. One aspect is the internal reforming of methane in the anode of SOFC fuel cells. The anode structure is investigated by gas adsorption, diusion and permeation measurements. The reforming reactivity is determined as a function of the anode thickness to characterize the in¯uence of mass transfer limitations. The preparation of the SOFC anode material ends up in a sintered mix of NiO and YSZ. Temperature programmed reduction experiments are carried out in order to de®ne the condition for complete NiO reduction at a low temperature level. # 1998 Published by Elsevier Science Ltd. All rights reserved Key words: BET, diusion, permeation, reforming, structure.
INTRODUCTION Solid oxide fuel cells (SOFC) operate at temperatures between 8008C and 10008C using hydrogen as fuel and atmospheric oxygen as oxidant. The most interesting fuel for SOFC systems is natural gas, which consists mainly of methane. The methane steam reforming reaction according to the overall reaction scheme CH4 2H2 O , 4H2 CO2
1a
is used as the source of hydrogen. This hydrogen can be oxidized electrochemically according to the reaction H2 O2ÿ , H2 O 2eÿ
1b
to produce the electrical power. In order to achieve a high energetic eciency of the fuel cell, internal reforming of the methane is necessary, which takes place directly within the anode. Due to the great dierence between the reaction rates of the endothermic methane reforming reaction (1a) and the exothermic electrochemical hydrogen oxidation (1b), cooling eects arise resulting in intolerable temperature gradients inside the anode structure. In *Author to whom correspondence should be addressed.
order to control the course of methane reforming, detailed knowledge of the reaction rates and the anode properties is necessary. At the Research Center JuÈlich, Ni supported on yttrium stabilized zirconia (YSZ) is used for substrate anodes. They are very thick (about 2 mm), compared to other SOFC anodes, in order to serve as self-supported anodes. This concept allows the application of a very thin electrolyte and therefore lower working temperatures (about 7508C) with high electrical output are possible [1]. In order to develop an adopted anode, which enables internal reforming of natural gas detailed knowledge about the relationship of structural properties and the reforming reaction of SOFC substrate anodes is necessary. Therefore, dierent experiments for structure characterization have been performed. Temperature programmed reduction experiments were carried out in order to de®ne the condition for complete NiO reduction at a low temperature level. The reactivity of the reforming reaction was measured as a function of the thickness of the anode. The results lead to the conclusion that for these anodes the depth of the methane reforming reaction zone is larger than 0.15 mm but smaller than 0.3 mm due to mass transfer limitations [2]. Additionally the anode structure was investigated
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by gas adsorption, measurements.
diusion
and
permeation
EXPERIMENTAL Sample preparation The methane steam reforming reaction was investigated using the SOFC anode material of the Research Centre JuÈlich. This material consists of Ni±8YSZ (8YSZ: 8 mol% Y2O3-stabilized ZrO2) substrate with a standard composition of 50 wt% Ni. This cermet is manufactured by the Coat Mix1 process starting with a powder mixture of NiO (56 wt%) and 8YSZ together with a resin binder. The anode is sintered at 14008C. A more detailed description of the manufacturing process is given in [3]. The sintered anode materials have to be reduced before application. The structural and catalytic properties will depend on the reducing procedure. Therefore the procedure has to be investigated and the material characterization has to be done with the anode reduced under de®ned conditions. Experimental setup Three dierent experimental setups were used. In the ®rst catalyst test equipment, a mixture of pure CH4 with H2O (H2O/CH4=3 mol/mol) was ¯owing at 1.5 bar over both sides of a ¯at piece of the substrate anode. This experimental setup serves for the investigations of the dependence of methane reforming rate on temperature (7008C±9508C) and thickness (0.3±1.2 mm) of the anode under conditions similar to the ones for operating SOFCs. The dry product gas stream was analyzed by a BINOS IR-adsorption-detector for CO, CO2 and CH4. Hydrogen was detected by a thermal conductivity detector (TCD). A detailed description of the experimental setup is published elsewere [2]. The second setup was used for temperature programmed reduction (TPR) experiments, for BET (Brunauer-Emmett-Teller isotherm) and CO, H2 adsorption measurements. The adsorption measurements determine the speci®c surface area of the whole anode material and of the Ni sites. The TPR experiments give information about the reduction properties of the catalyst. For these experiments, the anode was crushed into pieces of 0.5±2 mm diameter. In the TPR experiments the sample (mesh size: 0.5±2 mm) was heated, usually with a temperature gradient of 18C/min, in a reducing environment (10% H2 in Ar or N2). The disappearance of hydrogen was followed via analysis of the euent gas stream by mass spectroscopy. The rate of change in H2 concentration is proportional to the rate of catalyst reduction. This allows the development of a de®ned reduction strategy for Ni-cermet materials. Additional information is available by comparing theoretical H2 consumption (assuming complete re-
duction of all NiO in the catalyst) and the consumption, observed during the TPR experiment. For the BET measurements the reduced and crushed catalyst was contained in a special cell, which can be cooled down to the temperature of liquid nitrogen. After degassing the catalyst at 1258C with pure He for 8 h, a nitrogen in helium gas mixture ¯owed through the catalyst at room temperature. The BET cell was then cooled down in liquid nitrogen. The thermal conductivity detector (TCD) signal became stabilized when no more nitrogen was adsorbed at the catalyst. Now the catalyst was heated up to room temperature again. The TCD signal of the desorbed nitrogen was used for the calculation of the amount of adsorbed nitrogen. This procedure was repeated for dierent nitrogen partial pressures (P(N2) = 13±33 kPa). A pulse of a known volume of nitrogen in He at room temperature was detected by the same TCD and served to normalize the desorption signals of the BET measurements. The adsorption measurements with H2 and CO were done at room temperature with the pulse method. Pulses of known volume of H2 or CO in He were sent to the reduced and degassed catalyst, and were adsorbed only on the free nickel sites. The pulses, detected by the TCD, stopped growing, when no more CO or hydrogen was adsorbed. The detector signal of a pulse of known volume of CO or hydrogen served again for normalization. In order to determine the textural properties of the cermet material, permeation and binary diusion experiments have been performed by the third experimental setup. A modi®cation of the Wicke± Kallenbach cell was used for the diusion experiments [4]. The cell consists of two chambers separated by the porous solid of interest. A gas A (N2 or Ar) ¯ows through the upper part of the cell and gas B (H2 or He) through the lower part (see Fig. 1). When the ¯ow of gas B in the lower compartment is stopped a soap ®lm in a glass burette connected to the lower part of the cell starts to move and the rate of the volume change can be observed. Extrapolation of the observed volume change to zero time equals the net diusion ¯ux density. Both chambers have atmospheric pressure. Thus, isobaric conditions are ful®lled, no viscous ¯ow takes place and the Graham law is valid. r NA MB ÿ 2 NB MA where Ni is the ¯ux density of gas i and Mi the molecular weight of the molecules i. The driving force for permeation of a gas through a porous structure is a pressure gradient over this structure. The permeation measurements were performed in a cell which consists of two parts with the same volume separated by the porous solid. At the beginning both compartments are ®lled
Structural properties of SOFC
3061
Fig. 1. Modi®ed Wicke±Kallenbach diusion cell
with the same gas of the same pressure. In one of the compartments the pressure will then step-wise, be increased and the exponential decay of the pressure dierence in both cells can be observed [5].
RESULTS AND DISCUSSION Reactivity measurements Measurements to investigate the in¯uences of the structural properties for the methane reforming reaction were done with the ®rst experimental setup. The anode material was a Ni/YSZ cermet as described above. The NiO was gradually reduced in situ to metallic Ni in a gas stream consisting of 5% H2 and 95% N2 at 6008C. The resulting anode
samples with the thickness between 0.36 and 1.9 mm were investigated. The reactivity of the reforming reaction was measured as a function of the thickness of the anode. The aim of these experiments was to determine the depths of the reaction zone in the anode. In Fig. 2 the reactivity related to the surface of the sample is plotted vs the anode thickness. At a reaction temperature of 9158C the reactivity increases linearly with the anode thickness up to a thickness of 0.6 mm. In the range of 1.2 mm down to 0.6 mm no signi®cant dierence in methane conversion could be observed. This result leads to the conclusion that for these anodes with the described structural properties the depth of the methane reforming reaction zone is larger than 0.15 mm but smaller than 0.3 mm [2].
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Fig. 2. Methane conversion rate as a function of the thickness of the anode for the standard material of SOFC anodes
TPR experiments In Fig. 3 the H2 consumption as a function of the sample temperature is shown. 3.155 g of crushed (0.5±2 mm) anode pieces were ®lled in a ceramic reactor tube. The sample was heated from 18C/min to 9008C in a reducing atmosphere of 10% H2 in nitrogen (total ¯ow rate 200 ml/min). Three peaks
Fig. 3. TPR experiment of 3.241 g catalyst. Catalyst shape: crushed to 0.5 mm±2 mm. Atmosphere: 50% H2 in Ar. Flow: 200 ml/min. Temperature gradient: 18C/min. Hydrogen consumption: 611 ml theoretical, 600 ml measured
of H2 consumption exist with maxima at 2608C, 4008C and 6508C. At 8608C the reduction process is completed. In contrast to these measurements, other TPR experiments [6, 7] detected only one signi®cant maximum of reduction at around 4008C± 5008C of Ni/YSZ. Otherwise, Perego et al. [8] measured a similar hydrogen consumption with a reduction maximum at 5008C and a wide maximum at 7008C. It seems to be possible that the wide peak shape of the third maximum at 6508C in Fig. 3 is caused by mass transfer limitations. To check this hypothesis, the experiment was repeated with an increased hydrogen supply. To oer the catalyst more hydrogen during the experiment one can increase the hydrogen concentration in the reducing atmosphere, or increase the time for catalyst reduction. In the second TPR experiment (see Fig. 4) both possibilities were used. A temperature holding point for 8 h at 5008C was chosen to oer the catalyst enough time for reduction before it was heated up to the critical temperature of 6508C. Furthermore, to increase the H2 supply during the whole experiment, a higher hydrogen concentration of 50% in the atmosphere was used. In Fig. 4 one can see the result of the repeated TPR experiment. This huge supply of hydrogen leads to a fully reduced catalyst at a temperature lower than 4508C. Only one signi®cant sharp peak of hydrogen consumption is observable with a maximum at 4108C. The small peak with a maximum at 2608C is still observable. With a third TPR experiment, it was examined whether the mechanical stress (crushing of the anode material) aects the reduction properties.
Structural properties of SOFC
3063 BET experiments
For the determination of the speci®c surface area of a sample, one can use the BET method. The BET equation gives a relation between the adsorbed volume of an adsorbate, the volume of the adsorbed monolayer of adsorbate and the partial pressure of the adsorbate in the gas phase: z 1 c ÿ 1 z I Sz, 1 ÿ zV cVm cVm
Fig. 4. TPR experiment of 3.155 g catalyst. Catalyst shape: crushed to 0.5 mm±2 mm. Atmosphere: 10% H2 in N2. Flow: 200 ml/min. Temperature gradient: 18C/min. Hydrogen consumption: 591 ml theoretical, 542 ml measured
Two cubic pieces (4 4 2 mm, 0.282 g) were cut out of the anode. These samples were reduced in an atmosphere of 10% H2/Ar (total ¯ow rate: 100 ml/ min) at a heating rate of 18C/min. Only one reduction process with a maximum at 4208C was detected (Fig. 5). The small peak with the maximum at 2608C does not exist any longer. This small peak at 2608C is possibly caused by the reduction of NiO, not further bonded to the ceramic support. TPR experiments of bulk NiO have been investigated by Robertson et al. [9]. They reported a single reduction process centered at 3278C (6% H2/N2, heating rate: 4.58C/min, ¯ow rate: 10 ml/min). Shirakawa et al. [7] also reported a single peak at 2948C for reduction of NiO itself (H2/Ar = 3/7, heating rate: 108C/min). Therefore it is fair to assume that some NiO was separated from the sintered structure when the sample was crushed. The cutting out of the catalyst prevents free NiO. At sucient H2 supply, i.e. high H2 concentration in the reduction atmosphere and a low heating rate, the anode material is completely reduced at a temperature of about 4208C (see Fig. 4). One observes a signi®cant sharp peak of H2 consumption. If the H2 supply is not sucient, a second wide peak at 6508C, due to H2 transport limitation, is observed (see Fig. 3). The reduction is completed at temperatures around 4208C. There exists an interaction between NiO and YSZ. Free NiO is reduced at 2608C, whereas NiO, which is held in the sintered YSZ structure, is reduced at 4208C.
3
where z = p/p0, partial pressure of N2 normalized by the saturated pressure of N2, c is constant, which is a function of the heat of adsorbate condensation and the heat of adsorbate, Vm the volume of the adsorbed monolayer, V the volume of the adsorbed adsorbate and I, S the intercept and the slope, if the equation is plotted vs z. The ``multi point'' method was used to determine the surface of the sample. For this method the adsorbed volume V of nitrogen was measured by the TCD at dierent partial pressures of nitrogen in He (z = 13%±33%). It is assumed that plotting z/ [(1 ÿ z)V] vs z yields a straight line (see Fig. 6). The slope S = (c ÿ 1)/(cVm) and the intercept I = 1/ (cVm) of the BET isotherm equation are used to determine the adsorbed volume of a monolayer coverage Vm: Vm
1 I S
4
Fig. 5. TPR experiment of 0.282 g catalyst. Catalyst shape: 2 cubic pieces of 4 4 2 mm. Atmosphere: 10% H2 in Ar. Flow: 100 ml/min. Temperature gradient: 18C/ min. Hydrogen consumption: 52.7 ml theoretical, 53 ml measured
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Fig. 6. Typical ``multi point'' BET measurement. The measured points agree with the assumption of linear dependence of equation (3). The error bars correspond to statistical errors
With the knowledge of the monolayer volume, one can calculate the surface area of the catalyst. Normalized by the mass of the catalyst, one obtains the speci®c surface area: 2 m NA Pa Sm 1 Vm , 5 S gcat RT m where NA is the AvogadroÂs number (1/mol), Pa the ambient pressure (atm), Sm the adsorbate cross section area, Sm(N2) = 1.62 10ÿ19 m2, m the mass of used catalyst (g), R the gas constant, 82.1 ml atm/ (K mol) and T the temperature (K). The evaluation of the BET experiments, adsorption of N2 on the total catalyst surface, leads to a speci®c catalyst surface area of 0.43 2 0.03 m2/gcat for the reduced and 0.26 2 0.03 m2/gcat for the unreduced catalyst. The surface area grows with the reduction of NiO to Ni, because the Ni particles are smaller than the NiO particles and therefore more channels in the YSZ structure become available for the gas. One sample of catalyst was reduced in 5% hydrogen in nitrogen at 9008C for 38 days. After this thermal treatment in reducing atmosphere the speci®c surface area was determined to be 0.41 2 0.03 m2/gcat. No signi®cant change in the surface area could be detected. Therefore, it might be concluded that no thermal degradation occurred. Measurement of the Ni sites surface area For the methane steam reforming reaction only Ni in the anode serves as catalyst. The nickel surface area is one important value for the heterogeneously catalyzed methane reforming reaction.
Adsorption experiments were done to estimate the speci®c nickel surface area of the catalyst. Hydrogen and CO adsorb at room temperature only on the nickel sites. The determination of the nickel surface area assumes that the adsorbate is adsorbed only on the Ni sites at a monolayer. The calculation of the surface uses equation (5) with Sm=6.7 10ÿ20 m2 (the Ni cross sectional area). Adsorption measurements with both of these gases lead to a speci®c nickel surface of (8.62 0.2) 10ÿ2 m2/gcat, the dispersion of Ni is 2.46 10ÿ4. The active Ni surface is about 20% of the BET surface. The adsorbed volume of CO was twice the volume of hydrogen. This must be due to the fact that CO on-top adsorption is favored on Ni. With the knowledge of the nickel surface area of the catalyst one can make a rough estimate of the nickel particle size. Assuming that the nickel particles have a spherical shape and that they are in contact only at one point with the substrate, the nickel particle would have a radius of 2 mm. If one assumes that the particles have cubic shape and that they are in contact with only one of the six surfaces of the cube with the substrate, the cubic Ni particles would have the size of 3.43 mm3. Gas diusion and permeation measurements The mass transport of the gases in the porous structure will be described in the frame of the mean transport pore model (MTPM) [10]. This model is based on the assumption that the gas transport takes place in cylindrical capillaries with radii dis-
Structural properties of SOFC tributed around the mean value hri (®rst model parameter). The second model parameter is the squared transport pore radius hr2i and the third parameter is C, the ratio of the porosity and tortuosity. Within the MTPM the steady state diusion of a multicomponent gas mixture can be written in the following form [10]: X YB NA ÿ YA NB dYA NA ÿcT dx DAk DAB B1 B 6 A A 1, . . . ,n
6
DAB C@ AB DkA Chri
r 2 8RT , 3 pMA
7 8
where cT is the total molar concentration of the gas mixture, YA is the mole fraction of the component A and x is the coordinate in the gas transport direction. DkA is the eective diusion coecient of component A in the porous structure, DAB is the eective bulk diusion coecient of the gases A, B and @AB is the gas±gas diusion coecient. The ®rst term on the right side of equation (6) represents the contribution according to Knudsen diusion and the second term the contribution according to the bulk diusion. For binary counter current diusion of gases A and B equation (6) simpli®es to ÿ1 1 ÿ aA YA 1 dYA ÿ cT 9 NA hriCKA CDAB dx
3065 KA
r 2 8RT 3 pMA
aA 1 ÿ MA =MB 1=2
10 11
Integrating the dierential equation with the boundary conditions which state that in the upper part of the cell is pure gas B and in the lower part pure gas A, the net diusion ¯ux, N, is expressed as cT 1 DAB =hriKA N C DAB ln 12 L 1 ÿ a DAB =hriKA To determine the optimum transport parameters hri and C for diusion in the porous structure from the set of experimental net diusion ¯uxes, the experimental values Nexp were ®tted to the theoretical equation by evaluating best values of C for a range of mean transport pore radii hri. The minimum of the sum of squared deviations between the experimental and calculated ¯uxes determines the optimum parameter hri. According to the Beale criterion [4] a 95% con®dence region for the parameter hri*C and C can be calculated (see Fig. 7). From the permeation experiments the permeation B can be calculated according to Fott et al. [5]. ln Dp0 =Dp
2SB t, LV
13
where Dp0 is the pressure dierence (p0: at time zero), S the cross section of the structure, V the volume of the compartments and t the time. With the same assumption as before, that the gases ¯ow through circular capillaries, the permeation can be expressed as [5]
Fig. 7. Con®dence region obtained from binary diusion experiments for one of the oxidized cermets and the optimum parameters obtained from diusion and permeation experiments
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B KA Chrif o KA Chri 1 ÿ f
fÿ1 1
l 2r
ÿ1
hriCp 8Z
14
15
where Z is the viscosity, p the pressure, o = p/4 and l is the mean free path of the molecules. The ®rst term represents the contribution to Knudsen ¯ow, the second the contribution to the slip ¯ow at the wall and the third the viscous ¯ow. The permeation measurements were performed with H2, N2, and He in a pressure range of 1±6 105 Pa. The obtained results for one of the cermets examined are plotted in Fig. 8 in the form B/K vs p/ZK. The obtained linear relationship shows that the Weber equation (14) simpli®es to B o hriCK
hr2 iC p 8Z
16
hriC can now be calculated from the intercept of the plot. A comparison of the hriC values determined from the permeation measurements with the hriC vs C values inside the con®dence region (see Fig. 7) calculated from the diusion experiments results in an optimum parameter set for hri and C for each cermet where gas transport according to permeation and diusion will be described best. The SOFC anodes were carefully oxidized and reduced in three cycles. After each cycle permeation and gas diusion measurements were done. Three samples of one batch have been examined. For all samples it was found that the mean pore radius hri
and C, the ratio of porosity and tortuosity, is smaller for the oxidized than for the reduced anode. This can be explained by the fact that the NiO particles are larger than the Ni particles. A comparison of the results for hri and C of the three samples which have been treated in the same way (e.g. reduced, oxidized, reduced) shows only dierences within the errors. Repeated oxidation and reducing of the samples does not change the structural properties signi®cantly as can be seen in Table 1. Under the assumption that the tortuosity is the same for oxidized and reduced cermets and that no pores will be produced or destroyed during reducing and oxidation, the ratio of the surfaces for reduced and oxidized cermets is 1.5 (pores treated as capillaries). This is in agreement with the ratio of 1.6 obtained from the BET measurements. As can be seen from the BET and the diusion and permeation experiments neither degradation due to thermal treatment nor degradation due to repeated oxidation and reducing could be observed. The structural properties of the SOFC anode are summarized in Table 2. CONCLUSIONS It could be shown that at sucient hydrogen supply the SOFC anode, consisting of Ni/YSZ, is completely reduced at a temperature of about 5008C. As far as the reduction process is regarded, the anode material is usable for SOFC systems operating at low temperatures (T>4508C). Investigations of the anode structure by BET, gas
Fig. 8. Result of a permeation experiment with three dierent gases in a pressure range of 1±6 105 Pa
Structural properties of SOFC
3067
Table 1. The mean pore radii and the C values (C = porosity/tortuosity) of the permeation and gas diusion measurements at three dierent parts of the SOFC anode. The anodes were reduced and oxidized for several times. After each reduction and oxidation hri and C were determined Anode 9.1 9.1 9.1 9.2 9.2 9.2 9.4 9.4 9.4 9.1 9.1 9.1 9.2 9.2 9.2 9.4 9.4 9.4
Treatment r: reduction; o: oxidation cycled reduction ror roror rororor ror roror rororor ror roror rororor cycled oxidation roro rororo rorororo roro rororo rorororo roro rororo rorororo
C/(l)
DC/(l)
hri/(nm)
Dhri/(nm)
0.145 0.152 0.152 0.144 0.148 0.165 0.150 0.149 0.160
0.010 0.009 0.009 0.009 0.011 0.010 0.009 0.009 0.010
610 626 578 641 675 471 562 659 507
75 67 65 77 76 56 70 75 60
0.069 0.066 0.062 0.068 0.078 0.065 0.068 0.069 0.065
0.002 0.002 0.003 0.002 0.003 0.003 0.003 0.003 0.002
407 406 492 440 245 329 485 376 333
32 46 54 46 52 63 35 47 63
Table 2. Structural properties of the SOFC anode Structural property Total surface Ni surface: Dispersion: Ni particle size Open porosity [13]:
Value 2
for the reduced anode 0.4320.03 m /gcat for the unreduced anode 0.2620.03 m2/gcat (no thermal degradation in 5% H2 for 38 days at 9008C) (320% of total anode surface) (8.620.2) 10ÿ2 m2/gcat 2.46 10ÿ4 2.0 mm (radius, spherical model) (cubic model) 3.43 mm3 4023%
diusion and permeation measurements lead to the result, that no degradation due to thermal and atmospheric cycling as well as long thermal treatment was observed. In reactivity measurements the active methane reaction zone due to mass transfer limitations was determined to be between 0.15 and 0.3 mm. Based on these experiments it was shown previously [11, 12] by computer simulations how mass transfer in the anode can in¯uence the rate of steam reforming. REFERENCES 1. H. P. Buchkremer, U. Diekmann and D. StoÈver, Proc. of 5th Int. Symp. of Solid Oxide Fuel Cells, Vol. 97-40, ed. S. C. Singhal, U. Stimming, H. Tagawa and W. Lehnert. The Electrochemical Society, Pennington, NJ, 1997, p. 160.
2. W. Lehnert, J. Meusinger, E. Riensche and U. Stimming, Proc. 2nd Eur. SOFC Forum, ed. B. Thorstensen. ISBN 3-922 148-19-O, Oslo, 1996, p. 143. 3. H. P. Buchkremer, U. Diekmann and D. StoÈver, Proc. 2nd Eur. SOFC Forum, ed. B. Thorstensen. ISBN 3922 148-19-O, Oslo, 1996, p. 221. 4. J. Valus and P. Schneider, Appl. Catal. 1, 355 (1981). 5. P. Fott and G. Petrini, Appl. Catal. 2, 367 (1982). 6. Zanibelli, L., Flego, C., Perego, C. and Rizzo, C., Proc. 1st Eur. Solid Oxide Fuel Cell Forum, Vol. 1, ed. U. Bossel. ISBN 3-922-14-X, Lucern, p. 207. 7. T. Shirakawa, S. Matsuda and A. Fukushima, Proc. 3rd Int. Symp. on Solid Oxide Fuel Cells, ed. S. C. Singhal and H. Iwahara. The Electrochemical Society, Pennigton, NJ, 1993, p. 464. 8. C. Perego, L. Zanibelli, M. Cartrullo and G. Piro, Proc. 3rd Int. Symp. on Solid Oxide Fuel Cells, ed. S. C. Singhal and H. Iwahara. The Electrochemical Society, Pennigton, NJ, 1993, p. 454.
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9. S. D. Robertson, B. D. McNicol, J. H. De Baas, S. C. Kloet and J. W. Jenkins, J. Catal. 37, 424 (1975). 10. P. Schneider and D. Gelbin, Chem. Eng. Sci. 40, 1093 (1985). 11. J. Divisek, W. Lehnert, J. Meusinger and U. Stimming, Proc. of the 5th Int. Symp. of Solid Oxide Fuel Cells, Vol. 97-40, ed. S. C. Singhal, U. Stimming,
H. Tagawa and W. Lehnert. The Electrochemical Society, Pennington, NJ, 1997, p. 993. 12. J. Divisek, D. Froning, W. Lehnert, J. Meusinger and U. Stimming, European workshop on current and potential distributions in complex electrochemical systems, Nancy, 1997. 13. D. Simwonis, A. Naoumidis, F. J. Dias, J. Linke and A. Moropoulou, J. Mater. Res. 12, 1508 (1997).