Danjela Kuščer, Janez Hole, Marko Hrovat, Slavko Bernik, Drago Kolar Jožef Stefan Institute, Ljubljana, Slovenia Keywords- SOFC Solid Oxide Fuel Cells, high temperature SOFC, high temperature fuel cells with solid oxide electrolyte, lanthanum manganites, doping microstructures, electrical resistivity, cathode materials, anode materials, solid electrolytes, YSZ, Yttrium Stabilized cubic Zirconium, environmental friendly, power generation, semiconducting perovskites, TEC, Thermal Expansion Coefficients, XRD analysis, X-Ray Diffraction analysis, EDS microanalysis. Energy Disperse x-ray System microanalysis Abstract' The perovskites with nominal compositions La{IVlni.xAlx)03, strontium doped (Lao,8Sro,2){Mni.xAlx)03 (for x between 0 and 0.94), and substoichiometric (Lao 8Sro,2)o-95(Mno.7Alo.3)03 were evaluated as possible solid oxide fuel cell (SOFC) cathodes. Cell parameters of solid solutions were calculated The electrical and microstructural characteristics and high temperature interactions with yttria stabilised zirconia (YSZ) were studied As compared with "pure" perovskites, doping with strontium and aluminum decreases and increases their specific resistivity, respectively. The incorporation of alumina substantially reduces the sinterability resulting in a rather porous, fine grained microstructure. The partial exchange of lanthanum with strontium and manganese with aluminum oxide significantly depress the reaction rate between perovskites and YSZ. 3 Kiiučne besede: SOFC celice gorivne z elektroliti oksidnimi trdnimi, SOFC celice gorivne z elektroliti oksidnimi trdnimi vlsootemperaturne, manganiti lantanovi, dopiranje, mikrostrukture, upornost električna, materiali katodni, materiali anodni, elektroliti YSZ cirkoniJ " stabifoiran z itrijem, okolno prijazno, proizvodnja energije, perovskiti polprevodni, TEC koeficienti razteznosti termičnih, XRD analiza z uklonom Rentgen žarkov, EDS mikroanaliza z energijsko razpršenimi Rentgen žarki Povzetek- Kot možne materiale za visokotemperaturne gorivne celice s trdnim elektrolitom (SOFC) smo sintetizirali in karakterizirali perovskite z SnaNmi sestavami La(Mni.xAlx)03, s stroncijem dopirane (Lao.8Sro.2)(Mn,,xAlx)03 (x med O m 0,94), m podstehiometri^ega (Lao sS 0 2)0 95(Mn0 7Al0 3)O3. zračuna i smo dimenzije osnovnih celic trdnih raztopin. Študirali smo električne m mikrostrukturne karakteristike ter Skote^^^^ s trdnim elektrolitom. Zamenjava dela mangana z aluminijem poveča specifične upornosti medtenj, ko zamenjava "ntana s stroncijem upornosti zmanjša. Obe vrsti dopiranja zelo zmanjšata reaktivnost med test,ranimi perovskiti m trdnim elektrolitom. INTRODUCTION A fuel celi is a device for direct conversion of chemical energy into electrical energy. Basicaly it consists of cathode, anode and electrolyte. Oxidant is fed to the cathode and reducent (fuel) to the anode. The electrolyte, through which the ion current flows, prevents the mixing of oxidant and fuel. The concept is nearly 160 years old. The principle of fuel cell operation was reported in 1839 by Sir William Grove /1 /. His fuel cell used dilute acid as an electrolyte and oxygen and hydrogen as oxidant and reducent, respectively. High temperature fuel cells with a solid oxide electrolyte (SOFC) work at temperatures around 1000''G. Due to the high operating temperatures the choice of materials is limited mainly to ceramics. The solid electrolyte in SOFC cells is usually yttria stabilised cubic zirconia (YSZ). Cathodes are semiconducting perovskites and anodes are based on the mixture of metallic Ni and YSZ. An extensive and comprehensive review of materials for SOFC is presented in references /2,3/. The oxygen accepts electrons at the cathode and moves as an ion through the dense Zr02 ceramic. At the anode ions combine with fuel and release electrons. The "force" driving oxygen ions through the electrolyte is the concentration gradient of oxygen between the cathode and the anode side. The fuel is hydrogen, a H2/CO mixture, or hydrocarbons because the high operating temperature enables the internal (in situ) reforming of hydrocarbons with water vapour /4-6/. For the typical "working" conditions of an SOFC the open circuit voltage is around 1 V. The advantage of high temperature solid oxide fuel cells for production of electrical energy is their high efficiency of 50-60%, while some estimates are even up to a yield of 70-80%. Also, nitrous oxides are not produced and the amount of CO2 released per kWh, due to the high efficiency, is around 50 percent less than for power sources based on combustion, making SOFC "environmental friendly" power generation /7-11/. The schematic diagram of the solid oxide fuel ceil is shown in Fig. 1. Oxidant (air) is fed to the cathode and reductant (fuel) to the anode. The electrode reactions (for hydrogen as fuel) are: Cathode; Anode: 1/2 O2 + 2e^- H2 + 0^"->H20 + 2e" The crossections of two basic constructions of SOFC, tubular and planar, are shown schematically in Figs. 2 and 3, respectively. Fuel cell elements - anode/solid FUEL a AIR Anode Cathode Fig. 1. FUEL Solid electrolyte Schematic diagram of the solid oxide fuei cell (SOFC). Air is fed to the cathode and fuel to the anode. The electrolyte, through which the ion current is flowing, also prevents the mixing of oxidant and fuel. Ni felt Interconnect Anode Solid electrolyte Cathode Porous zirconia Fig. 2: Crossection of the tubular design of a SOFC (schematically). The porous cathode and its coating of dense solid electrolyte are deposited on the porous Zr02 based carrier tube. Electrical contact with the anode of the next cell is obtained with nickel felt. Air flows through carrier tube and the fuel flows between the tubes. INTERCONNECT CATHODE ELECTROLYTE ANODE FUEL Fig. 3: Crossection of the planar design of a SOFC (schematically). The air and the fuel flow through channels in the interconnect. electrolyte/cathode "sandwiches" are serially connected with an interconnect. Electrical charge flows as electrons through the interconnect and as oxygen ions through the solid electrolyte. In the case of tubular design air flows inside tubes over the cathode and fuel on the outside over anode while in the case of planar design air and fuel are flowing through the channels in the interconnect. Cathode (air electrode) materials, which must withstand high operating temperatures and an oxidizing atmosphere, are at present based mostly on LaMnOs perovskites, doped with alkaline earth oxides, mainly SrO, to decrease specific resistivity. The thermal expansion coefficients (TEC) of LaMnOs is close enough to that of YSZ (11.2x10-6/K and 10.5 x10-6/K, respectively), to prevent cracking or delamination of SOFC components either during high temperature operation or heating / cooling cycles /2,12,13/. Also, SOFC cathode materials must be porous to allow the diffusion of oxygen from the air to the solid electrolyte. It is known that lanthanum perovskites, with the possible exception of LaCrOs /14/, react during high temperature ageing with zirconia from YSZ solid electrolyte, forming La2Zr207 pyrochlore /15-19/. The specific electrical resistivity of La2Zr207 is three to four orders of magnitude higher than that of LaMnOs (reported to be 1.5x10^ ohm.cm /17/, or even 25x103 ohm.cm /20/ at 1000°C), which increases the ceil losses and therefore decrease its yield due to increased internal resistivity. In this paper an evaluation of the electrical and micros-tructural characteristics of La(Mni-xAlx)Os and strontium doped (Lao.8Sro.2)(Mni-xAlx)03 (for x between 0 and 0.94) based materials as possible SOFC cathode materials is described. A range of solid solutions exists between LaMnOs and LaAIOs which enables some "tailoring" of material characteristics /21/. Although the alumina-rich perovskites, i.e. x=0.94, are not interesting for SOFC cathodes due to their high specific resistivities, they are included to cover the entire AI2O3 concentration range. The literature data show that substoichiometry on "A" sites suppresses the reactivity with YSZ (see, for example, /18/) therefore some results on substoichiometric (Lao.8Sro.2)o.95(Mno.7Alo.3)03 will be also presented. EXPERIMENTAL For experimental work, La(0H)3 (Ventron, 99.9%), SrC03 (Ventron, 99.9%), Mn02 (Ventron, 99.9%), and AI2O3 (Alcoa, A-16, +99%) were used. The compositions of the samples were La(Mni-xAlx)03 and (Lao.8Sro.2)o.95(Mni-xAlx)03 (for "x" between 0 and 0.94) and (Lao.8Sro.2)o.95(Mno.7Alo.3)03. The samples were mixed in isopropyl alcohol, pressed into pellets, calcined at 1000°C and fired for 50 hours with intermediate grinding at 1200°C for La(Mni-xAlx)03 and at 1300°C for strontium doped perovskites. During firing pellets were placed on platinum foils. To study the possible interactions at elevated temperatures, perovskites and YSZ powders were mixed in 1:1 molar ratio, pressed into pellets and fired at 1400°C for 300 hours. Samples were placed on platinum foils in alumina crucibles. The results were evaluated by XRD (X-ray powder diffraction analysis), SEM (scanning electron microscopy) and EDS (Energy Dispersive X-ray Microanalysis). A JEOL JXA-840 scanning electron microscope equipped with a Tracor-Norlhern energy dispersive system (EDS) was used for overall microstructural and compositional analysis. Samples prepared for SEM were mounted in epoxy in cross-sectional orientation and then polished using standard metallographic techniques. Prior to analysis in the SEM, the samples were coated with carbon to provide electrical conductivity and avoid charging effects. In the case of standardless analysis the Tracor SQ standardless quantitative analysis program using multiple least-squares analysis and ZAF matrix correction procedure was used. Samples were analyzed under the following conditions; acceleration voltage 25 kV; probe current 250 pA; spectra acquisition time 100 s. The phases in the sintered samples were determined using a Philips X-ray powder diffractometer using CuK« radiation at a step size of 0.02° in the range 2 9 = 20° to 70°. Electrical d.c. resistance was measured with four point method on sintered pellets with diameter of 6 mm and thickness of 2 mm with a Keithley 196 multimeter and a Keithley 580 Micro-ohmmeter instrument in the temperature range 20 - 925°C in air. Unfritted Pt electrodes were deposited as contacts on both sides of the samples and fired for 30 minutes at 1100 °C. Fig. 4.a: The microstructure of LaMnOs MMM Ä* * W' > s , ^ ^ ' r m Fig. 4.b: The microstructure of La(Mno.8Aio.2)03 RESULTS AND DISCUSSION The microstructures of La(Mni-xAlx)03, (Lao.aSro.a) (Mni-xAlx)03, and (Lao.8Sro.2)o.95(Mno.7Alo,3)03 are in Figs. 4, 5, and 6, respectively. The microstructures are porous and average grain diameters decrease with increasing AI2O3 content. Fig. 4.c: The microstructure of La(Mno.5Aio.5)03 Fig. 4.d: The microstructure of La(Mno.06Alo.94)03 tSmSM Fig. 5.b: The microstructure of (Lao.sSro. 2) (Mno. 7AI0.3) O3 The nnicrostructure of Lao.8Sro.2Mn03 is well sintered (Fig. 5.a), but more porous than that of LaMnOs (Fig. 4.a). Data in the literature indicate that the partial replacement of the lanthanum with the strontium increased the density of sintered materials up to 10% substitution. For higher concentrations the densities of sintered samples decrease /22/. Substoichiometry on "A" sites increases the sintered density (Fig. 5.b -stoichiometric (Lao.8Sro.2)(IVIno,7Alo.3)03. and Fig. 6 -substoichiometric (Lao.sSro.2)0.95(lVlno.7Alo.3)03. Dark gray inclusions in Fig. 5.d (alumina rich (Lao.8Sro.2)(i\/lno.o6Alo.94)03 composition) are, according to the results of X-ray analysis and EDS microanalysis, LaSrAI04 compound. In Table I and Table II the calculated cell parameters of La(Mni-xAlx)03 and (Lao.8Sro.2)(Mni-xAlx)03 solid solutions, respectively, are given. Cell parameters were calculated from uncalibrated X-ray powder diffractome-ter data using the PARAM program. Lal\/ln03, LaAlOa and solid solutions are indexed by a hexagonal cell, only Lao.sSro,2Mn03 is indexed by monoclinic cell. The Fig. 5.c: The microstructure of (Lao.8Sro.2)(Mno.5Alo.5)03 mS'M mMm Ml Pi^tltf^ill äÄlliS® H Fig. 5.a: The microstructure of (Lao.8Sro.2)Mn03 % - Fig. 5.d: The microstructure of (Lao.8Sro.2)(Mno.o6Aio.94)03 number in the bracket indicates tine accuracy of the last significant digit. Subunit ceil volumes are also shown in Fig. 7. Vertical bars indicate the accuracy of the calculated volume of perovskite subunit cells (the volume of unit cells in Table I and in Table II, divided by the number of subunit cells or pseudo-cells in the unit cell, i.e. 4 in the case of a monoclinic cell and 6 in the case of a hexagonal cell). The values obtained for LaMnOs, LaAIOs, and Lao sSro aMnOs are in fair agreement with data in JCPDS-ICDD 32-484, JCPDS-ICDD 31 -22, and JCPDS-ICDD 40-1100 cards, respectively. The cell volume of La(Mni-xAlx)03 linearly decreases with increasing alumina content from x=0 to x=0.4 which is the limit of solid solubility of LaAIOs in LaMnOs at 1200°C. It is due to the smaller ionic radius of A|3+ (A|3+ 0,535 Ä and Mn3+ 0.645 A). Strontium substituted perovskites (Lao.8Sro.2)(Mni-xAlx)03 have smaller cell volumes as their counterparts with the same aluminum / manganese ratio. The ionic radius of Sr2+ and La3+ are similar (La3+ 1.36AandSr2+ 1.44 Ä), therefore the contraction of volume occurs either due to formation of ^oxygen vacancies or smaller Mn4+ ions (Mn4+--=0.53 Ä). Oxygen vacancies and/or Mn4+ ions are needed for charge compensation when the La3+ ion on "A" site is exchanged by the lower valence Sr2+ ion. Fig. 6: The microstructure of substoichiometric (Lao.8Sro.2)o.95(Mno.7Alo.3)03 60 ro < < 56 : (La0,8SrO.2)(Miil-xAlx) La(MnI-xAl 0,4 0,6 X 0,8 Fig. 7: Volume of perovsl