Oxide ion conductors, mixed conductors and their solid oxide fuel cell applications

facciones adsorbente-adsorbato tienen lugar en cualquier parte de BIBLIOGRAFÍA la superficie y con una frecuencia parecida (según la zona de ener­ gías). Mientras que los materiales ortorrómbicos y fundamental­ 1. SAINT FLOUR, C. y PAPIER, E.: Gas-solid chromatography. A method mente aquél que es muy rico en oxígeno presenta una superficie of measuring surface free energy characteristics of short glass fibers. muy heterogénea ya que todos los centros de adsorción se encuen­ 1. Through Adsorption Isoterms. Ind. Eng. Chem. Prod. Res. Dev., tran en un intervalo muy estrecho de energías a la vez que la fre­ 21 (1982), 337-341. cuencia de dichas interacciones es bastante elevada. 2. LIGNER, G., SDQI, M., JAGIELLO, J., BALARD, H. y PAPIER, E.: Cha­ racterization of specific interactions capacity of solid surfaces by ad­ sorption of alkanes and alkanes. Part II. Adsorption on crystalline sili­ ca laser surfaces. Chromatographia, 29 (1990), 35-38. 4. CONCLUSIONES 3. TARTAJ, J., MOURE, C, DURAN, P., GARCÍA-FIERRO, J. L. y COLINO, J.: Processing and properties of superconducting YBa2Cu307_j^ pow­ Estos resultados de la CIGS permite caracterizar las superficies ders by single-step calcining in air. J. Mater. Sei., 26 (1991), 6135-6143. de polvos YBaCuO. 4. HoBSON, J. P.: Analysis of physical adsorption isotherms on hetero­ Los calores de adsorción indican una débil interacción entre los geneous surfaces at very low pressures. Can. J. Phys., 43 (1965), 1941-1949. aléanos y las superficies de los polvos YBaCuO. 5. RuDZiNSKi, W., NAKSMUNDZKI, A., LEBODA, E. y JARONIEC, M.: New Los valores .hallados para 72 indican que la estructura cristalo­ possibilities of investigating adsorption phenomena by gas chromato­ gráfica influye en la energía superficial; y que para una misma es­ graphy. Chromatography, 1 (1974), 663-667. tructura la desgasificación a temperaturas superiores a 100°C cambia 6. TsuTSUMi, K. y OHSUGA, T. : Surface characterization of modified glass dicha energía debido a la eliminación del agua fisisorbida. fibers by inverse gas chromatography. Colloid & Polymer Sei., 268 Los centros de adsorción de los tres polvos YBaCuO estudiados (190), 38-44. se encuentran en el intervalo de 35 a 43 KJ * mol~^ 7. TsuTSUMi, K. y ABE, Y.: Determination of dispersive and non disper­ La distribución energética o, lo que es lo mismo, la heterogenei­ sive components of the surface free energy of glass fibers. Colloid & dad energética superficial crece en el sentido tetragonal < orto- Polymer Sei., 267 (1989), 637-642. rrómbico original < ortorrómbico tratado. 8. MARTÍN, L., BAUTISTA, C, RUBUI, J. y OTEO, J. L.: Caracterización Esta heterogeneidad superficial está relacionada con el conteni­ de vidrio poroso por cromatografía gaseosa a recubrimiento cero. XXXI do en oxígeno del material YBaCuO; a medida que aumenta dicho Congreso Nacional de Cerámica y Vidrio. Palma de Mallorca. Junio contenido aumenta la heterogeneidad energética. 1991.

BOL.SOC.ESP.CERAM.VIDR. 30 (1991) 6, 461-464 Oxide ion conductors, mixed conductors and their solid oxide fuel cell applications

A. R. WEST University of Aberdeen, Department of Chemistry, Meston Walk, Aberdeen AB9 2UE, Scotland

ABSTRACT.—Oxide ion onductors, mixed conductors and their RESUMEN.—Conductores iónicos óxidos, conductores mixtos solid fuel cell applications. y sus aplicaciones como pilas de combustible de óxidos sólidos.

Oxide ion conductors and mixed oxide ion/electronic conduc­ Óxidos conductores mediante iones oxígeno y/O conductores tors fînd application as solid electrolyte and electrode materials, mixtos (iones oxígeno + electrones) son utilizados como elec­ respectively, in solid oxide fuel cells (SOFCs) and oxygen sen­ trolitos sólidos y/o electrodos respectivamente en pilas de com­ sors. The materials requirements for these applications are bustible de óxidos sólidos (SOFCs) y como sensores de oxígeno. reviewed, with particular reference to transport number con­ Los requerimientos exigidos a estos materiales para ser utiliza- siderations. Some factors that influence the magnitude of the bles son revisados haciendo especial énfasis en lo que se refiere oxide ion conductivity of a material are discussed; the al número de transporte. También se discuten algunos factores characteristics of two materials, Cai2Ali4033 and Ba6Ta20ii, que influencian la magnitud de la conductividad iónica de los are summarised. Recent developments in SOFCs are outlined. óxidos, y se hace un resumen de las características de dos ma­ teriales Cai2Ali4033 y Ba5Ta20n» Finalmente se hace mención de los avances más recientes en pilas de combustible del tipo SOFCs.

1. INTRODUCTION. MATERIALS REQUIREMENTS solid electrolytes, in which the transport number of oxide ions is unity or mixed conductors, which exhibit conduction by both ox­ Oxide ion conductors find potential applications in a variety of ide ions and electrons, i. e. solid state electrochemical devices, including fuel cells of the solid oxide type (SOFC), oxygen sensors and oxygen pumps. The solid electrolytes: ti = l materials that are used in these applications fall into two main categories, depending on their electrical properties. They are either mixed conductors: tj+te=l,

NOVIEMBRE-DICIEMBRE, 1991 461 A. R. WEST where tj and t^ refer to ionic and electronic transport numbers, CHAROB COMPENSATION MECHANISMS respectively. Solid electrolytes are typified by lime- or yttria-stabilised zirconia. As their name suggests, solid electrolytes are used as the ionically Addition of higher valence cations conducting membrane that separates the gaseous reactants (in these uses, any electronic conduction through the membrane would lead to a partial short circuit with a loss in fuel cell performance or in­ correct readings in a sensor; clearly, the requirement for an oxide ion transport number of unity is crucial. Mixed conductors are typified by reduced perovskites or oxygen- cation vacancies interstitial anions deficient perovskites, such as LaMn03_x, which conduct oxide ions by means of the oxygen vacancies, x and electrons by means of the variable valence of manganese. They are used as a reversi­ Addition of lower valence cations ble cathode in SOFC designs, in which the combination of elec­ tronic and ionic conduction facilitates reaction at the three phase interface: oxygen (or air)/electrode/electrolyte. Oxide ion conductors generally have an «electrolytic domain», which corresponds to the range of oxygen partial pressures over which the oxide ion transport number is unity. Outside this range, anion vacancies interstitial cations mixed conduction occurs, as shown schematically in Fig. 1. At low Fig. 2.—Creation of vacancieslinterstitials in order to maintain chari>e oxygen partial pressures, oxygen gas is released by the material, balance on aliovalent cation substitution. electrons are liberated by the reaction, 20^~ -^ 02+4e and n-type conduction results. Conversely, at high oxygen partial pressures, oxygen is absorbed by the material which picks up electrons from the sample leading to p-type or hole conduction. Zr4+-M/20= One reason for the popularity of stabilised zirconia as a solid elec­ trolyte in both SOFCs and sensors is its wide electrolytic domain. operate or, using Kroger-Vink notation: At very low oxygen partial pressures, e.g. 10"^^ atm, it becomes an n-type mixed conductor but under the usual conditions of SOFC operation it stays within its electrolytic domain. where, x and ' refer to net site charges of -I-1, 0 and — 1, respec­ tively, Y'zr is an yttrium ion on a Zr site and VQ" is an oxide ion vacancy. These vacancies are relatively mobile, with an activation energy of 0.8—1.2 eV, depending on composition and give rise log (f to high ionic conductivity at high temperature, e.g. 0.1 ohm"' cm-' in the composition 91Zr029Y203 at ~ 800- 1.000°C or 0.01 electrolytic ohm"' cm-' at 600°C. domain There do exist materials that have higher conductivity than YSZ (yttria-stabilised zirconia), especially at lower temperatures, but the I ones discovered to date are excluded from practical applications \ / on the basis of cost (scandia-stabilised zirconia) or limited elec­ \ trolytic domain (doped BÍ2O3, also fluorite structure). Never­ \ / theless,, the search for new and improved materials continues. An additional problem with many doped materials, such as YSZ, is that while extensive doping is required to generate a sufficient number of oxide vacancies to give high conductivity, the dopant log P02 Oxygen absorption : O2 + Ae^^^20^~ + 4h ions and oxide vacancies tend to aggregate into immobilised clusters. Oxygen removal : 202--ii^^ O2 + 4e Thus, the Kroger-Vink representation of YSZ, above, shows that positively charged oxide vacancies and substitutional yttrium ions Fig. I .—Electrolytic domain of an oxide ion conductor. with an effective negative charge are present. Inevitably, such op­ positely charged species attract each other to form, initially, dipoles and then larger clusters. In order for the oxide vacancies to move, 2. OXIDE ION CONDUCTION AND ITS OCCURRENCE they must become dissociated from these clusters: this contributes an additional enthalpy term, the enthalpy of dissociation, to the Although zirconia-based ceramics are commonplace, the property overall activation enthalpy for conduction and hence, leads to a of oxide ion conductivity, shown by the cubic stabilised zirconias, lowering of conductivity. In unfavourable cases, materials may 'age' is quite rare. In the vast majority of oxides, the oxygen array forms in which, at a certain temperature, the defect aggregation takes place an immobile sublattice at most temperatures and ionic conduction over a period of time and the conductivity gradually decreases, as occurs, if at all, by means of cation diffusion. In part this is because, shown schematically in Fig. 3. Clearly, such materials are unac­ ceptable for SOFC applications in which operation over a large in many oxides, the oxide ion is the largest ion in the structure and period of time may be required; for 'short burst' applications, such therefore forms a dense, often close packed, array with the cations as in certain types of sensor, this may not be a problem. occupying the smaller interstitial sites within the anion array. Con­ sequently, when a material is doped with a heterovalent cation, the The problem of ageing could perhaps be avoided if it were possible charge compensating mechanism usually involves the creation of to encounter materials that were both stoichiometric (i.e. undoped) cation vacancies or cation interstitials, mechanisms 1 and 4, Fig. 2, and contained a large concentration of oxide ion vacancies/in- and the anion array stays intact. terstitials; often, however, such materials form ordered structures High oxide ion conduction has been found in a small number of on cooling, with a consequent trapping of the mobile species and structure types, mainly fluorite, pyrochlore and perovskite, which lowering of conductivity. An interesting, relatively new oxide ion have been doped to yield anion vacancies, mechanism 3. Thus, in conductor that is both stoichiometric and contains partially occupied the stabilised zirconias (ñuorite structure), mechanisms such as: oxide ion sites is the calcium aluminate Ca,2Al|4033 (or C12A7 in

462 BOL.SOC.ESP.CERAM.VIDR. VOL. 30 - NUM. 6 Oxide ion conductors, mixed conductors and their solid oxide fuel cell applications

presumably due to reaction of the mobile oxides with H2, form­ ing H2O and liberating electrons which become trapped. Under more strongly reducing conditions, at e.g. 800°C, the whole sam­ ple was reduced and became semiconducting, with an overall in­ log ff crease in conductivity (6). In summary then, a material that appeared to be potentially very interesting and whose solid electrolyte properties were subsequently confirmed suffers from two unexpected drawbacks that limit its. possible applications, namely its sensitivity to water at very high temperatures, ~ 1000°C and its reduction in a H2 atmosphere.

3.2. Ba6Ta20H

This material has an oxygen-deficient, perovskite related struc­ T"1 ture and therefore potential oxide ion conductivity. Key points about its properties are as follows (7): Fig. 3.—Decrease in conductivity with time due to formation of defect clusters and immobilisation of oxide ion vacancies. I) ac impedance measurements showed a moderately high bulk conductivity, e.g. 3.7x10"^ ohm~^ cm"^ at 800°C, with a oxide ratio notation). It contains an aluminate framework of temperature-dependent activation energy in the range 0.93 to 1.22 stoichiometry [Al 14032]^^" which contains cavities for the calcium eV. ions and sites for an additional oxide ion, with an occupancy fac­ tor of 1/6. The activation energy for conduction, 0.74 eV, is quite II) The bulk conductivity was independent of atmosphere, but low and is less than that of YSZ, probably because there is no the grain boundary conductivity increased with increasing oxygen dissociation barrier to overcome. However, the relatively small partial pressure in the conductivity cell, suggesting the grain boun­ number of oxide ions that are potentially mobile (one in 33 at best) dary to be a p-type semiconductor. On heating at 800°C in O2, the means that at high temperatures, > 600°C, the conductivity is about grain boundary conductivity increased significantly and effective­ an order of magnitude lower than that of YSZ. ly short-circuited in the sample. III) Oxygen concentration cell measurements indicated an ox­ 3. MATERIALS CASE HISTORIES ide ion transport number in the range 0.5 to 0.65 over the range 450 to 850°C. A water concentration cell, at constant pOj, in­ 3.1. Cai2Ali4033 dicated the absence of any hydrogen-conducting species.

This material, referred to above, is an interesting new oxide ion IV) Thermoelectric measurements indicated the conductivity to conductor. Some key points about it, which illustrate the metho­ be p-type; this was obtained from the polarity of the emf generated dology of research into potential new ceramic electrolytes, are as when the sample placed in a temperature gradient. follows: In conclusion, this material is an interesting mixed conductor with I) Prior to electrical conductivity measurement, crystallographic p-type electronic conductivity and oxide ion conductivity. studies had shown the existence of partially occupied ion sites. This, together with a cubic structure (and hence isotropic properties) and a relatively low melting temperature (- 1360°C), indicated pro­ 4. SOLID OXIDE FUEL CELLS mising possibilities for oxide ion conduction. There is currently great interest, worldwide, in the development II) ac impedance measurements showed that it was possibly, of SOFC system for power generadon (8). They have the potential by sintering, to obtain materials with small grain boundary advantages of high energy conversion efficiency, simplicity of design resistances and a bulk conductivity that was only 8-10 times lower and high-grade waste heat, leading to possible combined heat and than that of YSZ (1, 2).

III) Transport number measurements in a gas concentration cell showed the electronic transport number to be effectively zero (O2 concentration cell) and the hydrogen transport number also to be zero (H2O concentration cell with constant PO2). These results together showed the oxide ion transport number to be unity (3).

IV) Doping experiments to partially replace Ca or Al failed to yield improved conductivities (4).

V) Ca,2Al,4033 was found to pick up water vapour at — 1000°C resulting in a decrease in conductivity. This was attributed to the reaction of the mobile oxide ions with H2O to form im­ mobile hydroxides. Water is eliminated and the conductivity Cathode reaction : O2 + 4e 20^" recovered on heating at - 1350°C (5). Anode reaction : CO + 0"" CÜ2 + 2e H2 + 0= H2Ü + 2e VI) The electrical properties were found to be unstable in a Overall reaction : Ü2 + 2CÜ/2H2 • 2CÜ2/2H2O reducing atmosphere (5% H2, 95% Nj). At low temperatures, e. g. 400°C, a resistive outer layer forms around a ceramic sample, Fig. 4.—Schematic operation of a solid oxide fuel cell.

NOVIEMBRE-DICIEMBRE, 1991 463 R. M. C. MARQUES, J. R. FRADE, F. M. B. MARQUES

CURRENT FLOW configuration, separated by an electronically conducting bipolar plate made from LaCr03 ceramic. Opposite sides of the bipolar plate are grooved, with the grooves running perpendicular to each other in the two faces, and these act as channels for the flow of air and Ni,Zr02 fuel gases. YSZ During operation of the SOFC, current is drawn from the cell in the direction perpendicular to the plates. LaMn03 La Cr03 BIPOLAR PLATE CELL REPEAT ACKNOWLEDGEMENTS UNIT Research support from the Science and Engineering Research Council is gratefiilly acknowledged.

REFERENCES

Fig. 5.—Flat plate SOFC configuration. 1. LACERDA, M., IRVINE, F. P., CLASSER, F. P. and WEST, A. R.: High oxide ion conductivity in Ca,2Al,4033. Nature, 332, 525-526 (1988). 2. LACERDA, M., IRVINE, J. T. S., LACHOWSKI, E. E., CLASSER, F. P. power, CHP, applications. The principle of SOFC operation is and WEST, A. R.: Ceramic processing of Cai2Al,4033 for high oxide shown in Fig. 4. The central component is the oxide ion conduc­ ion conductivity. Br. Ceram. Trans. J., 87, 191-194 (1988). ting membrane, YSZ, either in a tubular or flat plate configura­ 3. IRVINE, J. T. S., LACERDA, M. and WEST, A. R.: Oxide ion conduc­ tion. Its two opposite surfaces are in contact with solid electrodes, tivity in Ca,2Al,4033. Mat. Res. Bull., 23, 1033-1038 (1988). a Ni/Zr02 cermet (anode) and a lanthanum manganite (cathode). 4. IRVINE, J. T. S. and WEST, A. R.: Ca,2Al,4033 solid electrolytes doped Fuel gas (methane, CO, H2, natural gas, etc.) is passed over the with zinc and phosphorous. Solid State Ionics, 40/41, 896-899 (1990). anode and air or oxygen over the cathode. At the cathode, oxygen 5. IRVINE, J. T. S. and WEST, A. R.: Ca,2Al,4033 a possible high molecules dissociate and pick up electrons from the external cir­ temperature moisture sensor. /. Appl. Electrochem., 19, 410-412 (1989). cuit to form oxide ions which then diffuse through the YSZ mem­ 6. LACERDA, M., WEST, A. R. and IRVINE, J. T. S.: Electrical proper­ brane. These react at the fuel electrode with the fuel gas(es) and ties of Cai2Ali4033: effect of hydrogen reduction. /. Electrochem. liberate electrons to the external circuit where they act as a source Soc, submitted. of power. The reactions involved in the process are given in Fig. 4. 7. GOTO, T. and WEST, A. R.: ac impedance and transport number Most of the early development work on SOFCs has been carried measurements of Ba5Ta20ii, MRS Symposium. Solid State Ionics, in out by Westinghouse, using a tubular YSZ electrolyte. There is now press. increasing interest, for its ease of fabrication, in a flat plate con­ 8. YAMAMOTO, O., DOYIKA, M. and TAGAWA (eds.): Solid Oxide Fuel figuration. Fig. 5. This comprises three-layer sandwiches of Ni Cells, Proc. Int. Symp., Nagoya, Japan, Nov. 1989, Publ. Science cermet, YSZ and LaMnOg, which are stacked into a mulfilayer House Co., Tokyo.

BOL. SOC. ESP. CERAM. VIDR. 30 (1991) 6, 464-468

Transport properties of zircoma based solid solutions with mixed valence dopants

R. M. C. MARQUES, J. R. FRADE, F. M. B. MARQUES Ceramics and Glass Engineering Department, University of Aveiro, 3800 Aveiro, Portugal

ABSTRACT.—Transport properties of zircoma based solid solu­ RESUMEN.—Propiedades de transporte en soluciones sólidas tions with mixed valence dopants. basadas en circona con dopantes de valencia mixta.

Yttria stabilized zirconia and two derived materials (with par­ Circonia estabilizada con ytria y dos materiales derivados (con tial replacement of ceria for zirconia and ceria for yttria) where reemplazamiento parcial de ceria por circonia y ceria por ytria) characterized in terms of structure and transport properties in fueron caracterizados en términos de estructura y propiedades air and under reducing conditions. From electrical conductivi­ de transporte en aire y bajo condiciones reductor as. Desde la ty temperature dependence it was concluded that ceria addi­ dependencia de la conductividad eléctrica con la temperatura tions promote a small increase in activation energy for defect se ha concluido que adiciones de ceria promueven un pequeño mobility, and a decrease in defect association energies. Ceria aumento de la energía de activación por movilidad de defectos, doped materials exhibit a pronounced aging effect under reduc­ y una disminución en las energías asociadas de los defectos. Los ing conditions probably related to an ordering process involv­ materiales dopados con ceria muestran un efecto de envejeci­ ing the cation sublattice and enhanced interaction with oxygen miento pronunciado bajo condiciones reductoras, probablemen­ vacancies. te referido a un proceso de ordenamiento envolvente de subre- des catiónicas e interacción intensificada con las vacantes de oxígeno.

464 BOL.SOC.ESP.CERAM.VIDR. VOL. 30 - NUM. 6