High temperature electrolysis cell

Karsten Agersted, Zeming He, Kjeld Bøhm Andersen Institute for energy conversion and storage, Technical university of Denmark

Risø campus, 4000 Roskilde

Abstract

Tape casting was used as the main tool to fabricate solid oxide cells for high temperature electrolysis. The porous support layer, situated in the oxygen compartment, was made from almost equal volumes of a ceramic backbone from yttria-doped zirconia (TZ3YE) and an electron-conducting perovskite (LSM25). A porous electrode support layer from Samaria doped Ceria (SDC20) and LSM25 made a structural transition to a dense dual-layer electrolyte from SDC20 and Scandium-Yttrium co-doped zirconia (ScYSZ). After sintering the half-cell, support for the counter electrode was applied through screen printing a porous Nb-doped strontium titanate (STN) layer, which was subsequently sintered. The work entailed strength- and conductivity optimisation of the cell support as well as match of the sintering behaviour of the four different layers in the half-cell. After fabrication of the cell structure, electrodes were applied by impregnation with nitrate-salts and subsequent calcination to yield LSC oxygen electrode and Ni-based gas electrode.

Introduction

Recent experiences with using state-of-the-art anode supported solid oxide fuel cells (SOFC) in reverse mode for high temperature electrolysis (HT-SOE) showed marked degradation of both the YSZ-electrolyte, delamination of the oxygen electrode and compositional variations in the micrometer range and different degrees of recrystallisation across the electrode thickness at current densities higher than approximately 0.8 A/cm2 at around 800°C [1-5]. Degradation of the YSZ-electrolyte near the oxygen electrode may be explained by build-up of a high chemical potential of oxygen during electrolysis conditions [1, 6] and the apparently lesser electrode delamination seen at higher temperatures [7] has lead to the hypothesis that some electronic conductivity in the electrolyte may alleviate or reduce the oxygen activity build-up. In addition to the ongoing research on reaching a thorough understanding of the degradation mechanisms and their dependency on operating conditions, search for alternative cell designs has been initiated considering the known degradation mechanisms and aiming at working conditions around 750-800°C, which is regarded as the upper limit for attaining sufficiently long service of the stacking materials, particularly the metallic interconnect [8], and current densities around -2 A/cm2, equivalent to what can be attained today through e.g. PEM-electrolysis [9]. Previously, an all-ceramic cell type based on strontium titanate has been developed [10] and the present cell, cf. Figure 1, encompasses a dual-layer electrode, in which the dense component towards the oxygen electrode has been made from Sm-doped ceria (SDC) exhibiting both higher ionic, hole and electronic conductivity than the component towards the fuel electrode from Sc,Y-doped zirconia (ScYSZ) [11], which in turn is made as thin as possible in order to reduce ohmic resistance and build-up of oxygen chemical potential. The cell support has been moved to the oxygen compartment where the porosity can be minimized, hence

sufficient mechanical strength potentially be attained at lower material employment, due to the slightly lower diffusion resistance of solely oxygen in the pores. Likewise, diffusion resistance may be minimized on the fuel face of the cell by removing the support structure between the electrode and the average gas flow. Diffusion resistances are of little concern if cells are run at relatively low current densities, -0.5-1 A/cm2, but may become important to reduce, particularly if both high fuel conversion and high conversion rates are sought for. Two designs, as shown in Figure 1 have been run in parallel. The 3-layer half-cell design (right side) was employed to investigate if a simpler design would work as well and in that case reduce the ohmic resistance of the cell. The longest SOFC lifetime (tubular Siemens Westinghouse design) has been demonstrated for cell supports from LSM, which is both electronic conducting and stable in oxidizing environments. In order to improve strength over pure LSM, approximately 50 vol% of yttria-stabilised zirconia (TZ3Y) was added to the support composite. Co-sintering LSM and YSZ may lead to reactions between the two and formation of insulating zirconates already from above 1100-1150°C [12], which is lower than temperatures usually needed to densify either YSZ or SDC. However, provided sufficient necking can be obtained between LSM-grains in the composite, only insignificant contribution to the overall resistance may be expected from applying higher sintering temperatures. Several attempts have been made to co-sinter ceria and zirconia [13-14] and keeping the sintering temperature below 1200-1250°C seems to prevent excessive interfacial reaction and formation of resistive reaction products. The strategy for developing a processing route to flat half cells with the desired porosity and conductivity properties entails optimization of the individual layers, support layer, impregnation layer and electrolyte, in order to reach almost identical numbers for linear shrinkage during binder removal and sintering, equivalent temperatures for maximum sintering and finally densification of the electrolyte at temperatures below 1250°C as well as testing mechanical strength and electrical conductivity of the final components and half cells. The relatively low sintering temperatures, being 60-90°C lower than usually employed to reach fully dense zirconia membranes, are sought compensated for by tailoring the sintering properties of the support layer to be as high as possible. The coefficient of thermal expansion of doped ceria depends on both dopant and dopant level, but is usually recorded 10-20% higher than the values of doped zirconia, leading to residual stresses in the cells proportionally to the difference in temperature from the sintering temperature, i.e. be highest at room temperature. Pure ceria does not reach high sintered density in air until above 1600°C but the use of transition metal oxides promotes sintering at lower temperatures (Li, Cu, Co) or controls the region of maximum sintering activity at specific higher temperatures (Fe, Mn), which in this case is of interest for a successful co-sintering of dual-layer electrolyte structures from zirconia- and ceria-based materials [15-16]. Analysis of the sintering kinetics through thermokinetic methods may have both technical purposes, eg. for data reduction and to predict sintering behaviour, and be used for more in-depth analysis of the individual mechanisms of the overall process. The conversion function, U(t, T, a(r), a(p)) is modeled through approximating and separating the conversion in to separate functions: U(t, T, a(r), a(p)) = f(T(t))*g(x) (a) which are dependent on temperature profile, f(T(t)), and degree of conversion, respectively. The conversion dependency may be modeled through a range of well established reaction mechanisms, of which the simplest is a first-order reaction, and the temperature dependent term is described by the well known Arrhenius equation, f(T)= A*exp(-E/RT). Additionally, a model-free approximation of the activation energy may be derived, e.g. using the FRIEDMAN analysis [17] including evaluation of the pre-exponetial factor, A, assuming first-order reactions. Mechanical strength was assessed by measuring the modulus of rupture (MOR) in a ball-on-ring fixture (Ø 25 mm) at low crosshead speed. Resultant numbers have been collected in table 1 and it is seen that the mechanical strength is below 100MPa for all of the three samples, hence insufficiently low, regardless of the sintering temperature. On the other hand, RT-conductivity as measured by the Van-der-Pauw technique seems

to be 2-4 times higher than the previous type of tapes, eg. LZ-26. Mechanical strength results were attributed with surprisingly low Weibull modulus, which may indicate that more drying defects than were readily observed may be present in the tapes. Experimental

The half cells were generally manufactured by preparing and casting individual layers, which were then combined through calendaring at temperatures above Tg of the organic binder, although wet-on-wet casting of the layers were also tried in a few cases. Slurries for the individual layers were generally prepared by mixing the powder with part of the solvent and the dispersing agent for 23h in a ball mill with powder-to-ball ratios at 4-5 wt/wt. The particle size distribution (PSD) of the slurry was monitored. For composite components, like the cell support, the other component, dispersant and sintering additives were then added to the slurry, which were milled for another 23h. The particle size distribution (PSD) of the slurry was monitored. If graphite was used for pore former, it was added and the slurry milled for 23h. 50% more milling balls plus 1/3 of the binder were added and milled for 4 h. Subsequently 1/3 of the binder was added and milled for 3h and after addition of the rest of the binder, the slurry was milled overnight, 14-16h. The binder has been prepared in advance from solvent, binders, plasticisers etc. Finally, the slurry was strained through 100micron mesh and the PSD and the viscosity were recorded. PSD and viscosity varies among the slurries and were tailored to both microstructures and casting thicknesses. The slurry was de-aired and cast with the desired combination of height and speed after which it was dried at room temperature. Calendared half cells were further processed through binder removal in air and firing at sintering temperatures for 4 hours, with fewer exceptions fired between 2 and 8 hours each. Final sintering temperature was between 1200°C and 1260°C. After firing half cells, gas electrode support from Nb-doped strontium titanate was applied by screen printing. Nb-doped SrTiO3 (Sr0.94Ti0.9Nb0.1O3, STN94) was used and was prepared using the stoichiometric amounts of SrCO3, TiO2 and Nb2O5 (all materials 99.9% pure, Sigma-Aldrich) by solid state reaction. The printing ink was prepared by mixing ceramic component with dispersant, solvent and binder and was printed with a thickness of about 30 µm, dried and sintered in air at 1225°C for 4 h. The fuel electrode (cathode) of nano-crystalline nickel and subsequently the oxygen electrode (anode) from LSC or LSFC-CGO composite on the reverse face were applied to their respective porous backbones by multiple impregnation and drying steps of nitrate-based precursor solutions. Impregnation was generally carried out at 50°C and supported by de-airing samples and drying slowly until reach of open porosity after which nitrates were decomposed at 700°C. Powders for sintering studies and later for cell components were pretreated or calcined to different temperatures in order to reduce the sintering activity of the pristine powders and to be able to, in combination with graphite pore formers, create porous, yet strong and conductive microstructures. LSM25 powder was calcined for 2 hours at 900°C, 1000°C, 1200°C, 1300°C and TZ3Y to 1100°C. Powders were re-ground after calcination to break up agglomerates and tailor the average grain size to the desired final pore size. Calcined fractions were typically mixed from 2/3 of the powder calcined at the highest temperature and 1/3 of non calcined or powder calcined at the lower temperature. Sintering the LSM-fraction was also promoted by addition of smaller amounts, typically 1-3 wt%, of either Fe3O4, NiO or Co3O4. Sintering of SDC-TC and SDC-HP powders was promoted by admixing sintering aids (1, 2, 5 mol%) of MnCO3 or CuO. Pellets for sintering studies were generally made by mixing the powder(s) with solvent and a binder overnight in a ball mill, drying and pressing uniaxially (50 MPa) and subsequently isostatically (165 MPa) into samples with TD~0,6 and lengths of approximately 7.5 mm, which fitted into the double pushrod dilatometer, NETZSCH model 402 CD. Composites for the cell support were heated at 1 and 5 °C/min in flowing air to between 1000°C and 1300 °C. Zirconia and SDC-powders for the electrolyte were heated in flowing air at heating rates relevant for later processing (1, 5,

10 °C/min) and in cases also at 0.33 °C/min and 20 °C/min. Powders without sintering aids were heated from room temperature to ≈1500°C and powders with sintering aids to 1300 °C or 1400 °C. Binder systems were investigated and optimized to control the shear-thinning effect through mixing binders with different molecular weights and concentration of polar groups. Mechanical strength was assessed by measuring the modulus of rupture (MOR) in a ball-on-ring fixture (Ø: 25mm) at low crosshead speed (~10 μm/min). RT-conductivity of the cell support was assessed by measuring in-plane resistance by the Van-der-Pauw technique [18]. Results

Present endeavour on development of the support layer is based on previous results, which have been shown in table 1 together with the present results of mechanical strength and electrical conductivity. 2/3 of the LSM fraction in the series LZ05-LZ07 was precalcined at 1250°C and 1/3 not pre-calcined. Specimens were made without pore formers and results show that both strength and conductivity depends on the LSM:YSZ-ratio and that conductivity depends strongly on the percolation of the LSM-network. In the following series (T9, LZ22, TZ-2) with smaller variations of the LSM:YSZ-ratio at high levels of porosity, results show that decent strength (MOR= 200-300 MPa) can be achieved through the YSZ-network, but electrical conductivity was poor in all cases. These samples, as well as the following series (LZ27-LZ29) employs pre-calcined LSM-fractions, composed from 2/3 calcined at 1300°C and 1/3 calcined at 950°C, and particularly LZ-27-LZ29 clearly illustrates the other point learned that strength and conductivity is relatively insensitive to the starting amount of pore former, hence more dependent on the sintering ability of the LSM- and YSZ-fractions. The LZ39-type of support as the start of the present endeavour gave hope for achieving sufficiently high electrical conductivity, but mechanical strength was recorded relatively low for all. Electrical conductivity is shown in Figure 2 and it is observed it scales almost linearly with the sintering shrinkage. Figure 3 features average pore size and open porosity as function of sintering temperature and an optimum seems to exist for sintering around 1250°C. Microstructure may be evaluated from the polished cross section shown in Figure 4. Bearing in mind that the linear shrinkage both during binder removal and during sintering of the cell support must follow the behaviour of the electrolyte, ScYSZ, a test matrix was set up, which would test the influence from varying the calcination temperatures of the two LSM-fractions, three different sintering aids and pre-calcination of a fraction of the ScYSZ-electrolyte. Pressed pellets were used out of convenience, and although results of eg. the linear shrinkage cannot be transferred directly to the linear shrinkage of a tape-cast body of similar composition, relative measures of shrinkage as well as significant temperatures of onset and maximum sintering activity will be valid. Calcined fractions of ScYSZ at 1100°C reduces linear shrinkage and increases slightly onset temperature but does not alter significantly the temperatures for maximum shrinkage, so they will only be useful tools when more porous electrolytes are asked for. Sintering of TZ3YE takes place over a broad temperature interval 1080-1200°C and the onset around 800°C makes it well suited for co-sintering with both ScYSZ and calcined fractions of LSM. Adding more alumina promotes sintering only marginally. Sintering onset for ScYSZ is at 840°C and temperature for maximum sintering activity is at about 1080°C, but gas tight components are only developed after reaching to above 1150°C. In the range 1 > LSM:YSZ > 0,67 only small changes are seen for both temperatures of maximum shrinkage and onset as well as the linear shrinkage, which means that a robust sintering behaviour can be attained using a small surplus of YSZ, which also would provide the desired strength

to the composite, cf. table 1. In the range 1.3 > LSM:YSZ > 1, shrinkage increases by ~4%. Increasing the calcination temperature of the low-temperature fraction (900°C to 1000°C) increases the temperature of maximum sintering by 20°C and reduces slightly the linear shrinkage, which makes it possible to shift sintering temperatures a bit up, while maintaining the shrinkage level. Employing three calcined LSM-fractions (900°C, 1000°C, 1200°C) increases shrinkage by 9% over just two fractions, meaning that early-stage necking of LSM promotes sintering. The addition of sintering aids indicated that Cobalt increases shrinkage if LSM:YSZ = 0.67 but not if LSM:YSZ = 1. It also shifts the temperature of sintering onset to ~100°C lower, but not the temperature of maximum sintering. Nickel promotes sintering by shifting temperatures for onset and maximum shrinkage to 30°C and 50°C lower values, respectively. Measuring the conductivity of sintered pellets confirmed the earlier observed trend that it is roughly proportional to the linear shrinkage and the content of LSM, provided 0.8 < LSM:TSZ < 1.2. Specimens, where sintering was promoted by nickel, further showed superior conductivity over comparative samples. These results led to the new formulation MZ-03 shown in Table 1, where the LSM fraction is composed from 1:2 fractions of powder calcined at 1000°C and 1200°C, and sintering promoted by small additions of nickel oxide. Both numbers of strength and conductivity are higher than previous porous samples, but this has been achieved partly on the expense of the amount of open porosity, as may be seen from comparing Figure 4 and Figure 5.

Adjacent to the cell support, the first versions of the porous oxygen-electrode support were based on an existing CGO10 tape in order to evaluate co-sintering behaviour and possible interface reactions. The microstructure of this was much coarser than required for the electrode support, cf. Figure 6, but it revealed a poor match and adherence between the cell support from LSM-YSZ-composite and the porous electrode support from ceria, cf. Figure 6, if sintered below ~1250°C and generally a good match and adherence to the dense ceria-part of the electrolyte. Following versions of the electrode support layer included a moderate concentration of LSM (10-15 vol%) as well as pre-calcined fractions of the ceria, in order to develop the desired finer pore size distribution. One example of this generation has been shown in Figure 4, which features ~30 vol% porosity and average pore size ~3 μm. Not shown in this micrograph however, adherence between the cell support and the electrode support was still problematic, particularly in the designs employing both the porous and the dense ceria-layers. In order to improve adherence the most recent version of this electrode support layer was produced with similar LSM-concentration and fractions as in the cell support and using 2/3 of the SDC-powder in a pre-calcined (1100°C) condition, cf. Figure 5. However, the latter sintered to higher density than wished for and a better optimum will lie somewhere in between. Adherence between cell and electrode supports was generally better if only three layers were used, namely cell support, electrode support and ScYSZ-electrolyte, cf. Figure 1. It is probably due to the lack of stress created by the dense ceria-layer, which otherwise was present, as the ceria only shrinks about 80% of the average half-cell shrinkage on sintering, cf. Figure 7 and further has a relatively high coefficient of thermal expansion. The fact is supported by conductivity measurements, which showed that the “intrinsic” conductivity on LZ27 and to some extent also on LZ28 was not retrieved if co-sintered in 4-layer laminates, as opposed to 3-layer laminates. Similarly, edge bend are higher on 4-layer laminates and omitting the dense CGO-layer seemingly reduces internal stresses after sintering.

In order to further study co-sintering of SDC and ScYSZ powders and explore routes to better matching the processing characteristics, eg. sintering shrinkage of the bilayer component, two SDC-powders, SDC-TC (Sw= 7m2/g, d50=0,4μm) and SDC-HP (Sw= 12 m2/g, d50=0,38μm) were mixed with MnCO3 as sintering aid in different proportions, 0.5 mol%, 1 mol%, 2 mol% Mn and the results compared to similar experiments with ScYSZ-powder. The results in Figure 7 outlines the difference between the pristine powders as well as the impact of adding 0.5 mol% manganese to the SDC-HP powder. More manganese reduces the total shrinkage but doesn’t change significantly the characteristic sintering temperatures or the activation energy. Shrinkage and sintered density of SDC was proportional to the powder surface area and the SDC powders with manganese tend to reach higher densities when heating rates are lowered. The activation energies of the two SDC powders with 0.5 mol% manganese do not follow a similar trend, cf. Figure 8, and the SDC-HP matches better ScYSZ. Due to a slightly higher activation energy of the ScYSZ powder, cf. Figure 8, low heating rates tend to reduce sintering activity at the later stages of sintering, hence also the shrinkage and density. However, this may to some extent be useful to make co-sintering of SDC and ScYSZ more compatible. The extrapolated onset temperature for sintering ScYSZ powder is ~40°C lower than for SDC-HP powder at low heating rates (1°C/min.), but as this difference almost disappear at higher heating rates (10 °C/min.) sintering may be optimized further by heating relatively quickly to the onset of sintering at about 1050°C and then conduct the rest of the sintering at low heating rates and long holding times.

To evaluate the reactivity between the two layers of the electrolyte, elemental profiling by EDS was done on polished cross sections of two samples, sintered at 1250°C or 1313°C, cf. Figures 9-10. When the electron beam accelerated by 10-15 kV travels across a sharp interface, the response from the excitation volume looks like a gradual change in concentration over a distance of 0.8-1.1 μm, so if profiles change over longer distances actual diffusion may have taken place. By visual inspection of the contrast between ScYSZ and SDC in figure 9, inter-diffusion seems obvious, and analyzing the various elemental profiles individually reveals that zirconium diffuses at all temperatures and that both cerium and scandium diffuses at the higher temperature. Even at 1250 °C cerium may diffuse.

The gas electrode support layer from STN94, which was applied after sintering the half-cell, attains good bonding the electrolyte, cf. Figure 5, and the final porosity at about 20 vol% and average pore size of ~1.5 μm seems perfect for electrode impregnation.

Conclusions and outlook

Given the requirement that the electrolyte must sinter to full density, optimisation of the composition and processing of the other layers led to manufacture of a flat and relatively strong half-cell, which lend itself to application of a suitable electrode support layer on the fuel face. At the time of reporting, electrodes are to be impregnated into the support layers on both faces, hence no performance data on conversion efficiency and capacity can be given yet. However, the electrolyte thicknesses were reduced, cp. Figures 4 and 5, the electrical conductivity of the support layer was increased and it is therefore believed that cells may perform decently; not excellent because strength and bonding between the various layers were achieved partly on the expense of the porosity on the oxygen face, why the cells may suffer from mass-transport limitations at higher current densities.

However, open porosity of the cell support and oxygen electrode support may still be optimised. The “window” for processing, in particular for sintering the half cells, has turned out to be very narrow. The upper temperature limit at around 1250°C set by inter-diffusion between zirconia- and ceria-phases is slightly lower than the ~1300°C usually employed to sinter the zirconia-part of the electrolyte and cell support to full density. Undertaking comparative sintering studies on pressed pellets turned out useful and in combination with more general knowledge on how to control the green density and sintering performance of the various substances, when employed in tape casting and screen printing procedures revealed the necessary trends and facts for the development.

Acknowledgement

Development of the STN94 impregnation layer through funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 256730 is gratefully acknowledged together with inspiring and fruitful collaboration with colleagues at the institute for energy conversion and storage.

Literature

[1] Virkar, A.V., Int. J. Hydrogen Energy 35 (16), 9527-43, (2010) [2] Hauch, A. et al., Solid State ionics, 192 (1) 547-51, (2011) [3] Knibbe, R. et al., J. Electrochem. Soc. 157 (8) B1209-17 (2010) [4] J. Schefold, et al., J. Electrochem. Soc. 159 (2) A137 (2012) [5] Tietz et al, Journal of Power Sources 223, 129 (2013) [6] Mogensen, M. & Jacobsen, T. ECS Transactions, 25 (2) 1315-1320 (2009) [7] Hauch, A. et al, J. of the Electrochemical Soc., 155, B1184-93 (2008) [8] Yang, Z.G. & Stevenson, J.W., in “Handbook of Fuel Cells” vol. 5, p. 531-542, ed. W. Vielstich et al., Viley & sons (2009) [9] Anderson, E. “Recent advances in cost and efficiency for PEM electrolysis”, Hanover Fair, (25-04-2012) [10] Mao, X. et al. “Solid oxide cells for steam electrolysis” Poster #1082, ECerS XII, Stockholm (2011) [11] Carter et al. in “Handbook of Fuel Cells” vol. 5, 477-488, ed. W. Vielstich et al., Viley & sons (2009) [12] Hessler-Wyser, A. et al., J. Mater. Sci. 46, 4532–4539 (2011) [13] Eguchi, K. et al. Solid State Ionics 135, 589–594 (2000) [14] Wang, D. et al, J. of Alloys and Compounds 505, 118–124 (2010) [15] C. Kleinlogel & L. J. Gauckler, Solid State Ionics, 135 [1–4] 567–73 (2000) [16] J.S. Hardy et al. PNNL-20908 (2011) [17] H. L. Friedman, J. Polym. Sci., C6 175 (1965) [18] Bonanos, N. BC-note 694, DTU Energy conversion (2004)

Figure 1: Two versions of the oxygen-electrode supported electrolysis cell. Right features a simplified version,

with only single-layer electrolyte.

Figure 2: Room-temperature conductivity of the LZ39-cell support after sintering in two different half-cell designs, cf. Figure 1, to different temperatures (1100-1300°C) yielding a range of densities expressed through the linear shrinkage.

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Room-temperature conductivity of LZ39 composite

LZ39-CM51-YSZ139

LZ39-TC13-CM51-YSZ139

Figure 3: Open porosity and average pore size of the LZ39-cell support after sintering the LZ39-CM51-YSZ139 combination to different temperatures (1100-1300°C).

Figure 4: Polished cross section of the cell with LZ39 support after sintering at 1250°C. None of the dual

electrolyte layers achieved full density. Open porosity in the cell support is ~30 vol% and in the electrode support ~21 vol%.

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Figure 5: Half cell from ScYSZ-177; D-TC-03; P-TC-06; MZ-03 after sintering at 1260°C. Additional STN94

electrode support has subsequently been sintered at 1225°C.

Figure 6: Half cell from LZ39, CGO10 and ScYSZ electrolyte, cf. Figure 1, right. Note the poor adherence between

cell and electrode supports.

Figure 7: Sintering shrinkage from dilatometry on pressed pellets of ScYSZ, SDC-powders without sintering aids

and SDC-HP with 0.5 mol% Mn.

Figure 8: Activation energy from sintering profiles of SDC-TC and SDC-HP with 0.5 mol% Mn and ScYSZ powders

derived through assuming 1st order kinetics (Friedman analysis). Heating rates were 1, 5, 10 °C/min.

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Figure 9: Inter-diffusion of elements after sintering bilayer electrolytes from ScYSZ and SDC-HP with 0.5 mol%

Mn visualized by micrographs from polished cross sections on the two samples also shown in Figure 10. Sintering temperatures were 1313°C (left) and 1250°C (right).

Figure 10: Diffusion length as function of sintering temperature. Sample 1 has been sintered at 1250°C and sample 2 at 1313°C.

Table 1: Mechanical strength and room temperature electrical conductivity of selected samples from LSM-YSZ-composites, which were used for development of the cell support. LZ39 and MZ-03 were developed recently.

Sintering at 1150°C Sintering at 1200°C Sintering at 1250°C * and at 1260°C

Sample LSM:YSZ vol:vol

Pore former Vol%

MOR MPa

Mw σ @ RT S/cm

MOR MPa

Mw σ @ RT S/cm

MOR MPa

Mw σ @ RT S/cm

LZ05 65:35 0 360 2.5 54 608 6.4 97 120 * LZ06 1:1 0 707 6.8 28 862 3.0 33 43 * LZ07 35:65 0 618 2.3 0.42 997 1.9 3.6 8.2 *

T9 52:48 27 36 2.9 1.2 44 3.7 1.9 LZ22 1:1 30 194 14.3 1.3 239 8.8 1.9 TZ-2 48:52 37 285 4.2 0.38 392 3.7 0.48

LZ27 55:45 15 67 23 3.5 63 4.7 4.3 54 3.1 5.2 LZ28 55:45 20 55 2.1 3 62 3.8 3.9 50 8.4 4.5 LZ29 55:45 25 54 3.0 2.6 47 5.4 3.2 66 4.3 4.3

LZ39 55:45 10 - - 26 71 2.9 27 58 2.3 32 MZ-03 48:52 16 186 2.8 6.8 216 3.6 11.3