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Oxide Ion Conductors, Mixed Conductors and Their Solid Oxide Fuel Cell Applications

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Oxide Ion Conductors, Mixed Conductors and Their Solid Oxide Fuel Cell Applications 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
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