Oxide ion conductors, mixed conductors and their solid oxide fuel

Oxide ion conductors, mixed conductors and their solid oxide fuel

Oxide ion conductors, mixed conductors and their solid oxide fuel cell applications
facciones adsorbente-adsorbato tienen lugar en cualquier parte de
la superficie y con una frecuencia parecida (según la zona de energías). Mientras que los materiales ortorrómbicos y fundamentalmente aquél que es muy rico en oxígeno presenta una superficie
muy heterogénea ya que todos los centros de adsorción se encuentran en un intervalo muy estrecho de energías a la vez que la frecuencia de dichas interacciones es bastante elevada.
4.
CONCLUSIONES
Estos resultados de la CIGS permite caracterizar las superficies
de polvos YBaCuO.
Los calores de adsorción indican una débil interacción entre los
aléanos y las superficies de los polvos YBaCuO.
Los valores .hallados para 72 indican que la estructura cristalográfica influye en la energía superficial; y que para una misma estructura la desgasificación a temperaturas superiores a 100°C cambia
dicha energía debido a la eliminación del agua fisisorbida.
Los centros de adsorción de los tres polvos YBaCuO estudiados
se encuentran en el intervalo de 35 a 43 KJ * mol~^
La distribución energética o, lo que es lo mismo, la heterogeneidad energética superficial crece en el sentido tetragonal < ortorrómbico original < ortorrómbico tratado.
Esta heterogeneidad superficial está relacionada con el contenido en oxígeno del material YBaCuO; a medida que aumenta dicho
contenido aumenta la heterogeneidad energética.
BIBLIOGRAFÍA
1.
SAINT FLOUR, C . y PAPIER, E . : Gas-solid chromatography. A method
of measuring surface free energy characteristics of short glass fibers.
1. Through Adsorption Isoterms. Ind. Eng. Chem. Prod. Res. Dev.,
21 (1982), 337-341.
2.
LIGNER, G., SDQI, M., JAGIELLO, J., BALARD, H . y PAPIER, E . : Cha-
racterization of specific interactions capacity of solid surfaces by adsorption of alkanes and alkanes. Part II. Adsorption on crystalline silica laser surfaces. Chromatographia, 29 (1990), 35-38.
3.
TARTAJ, J., MOURE, C , DURAN, P., GARCÍA-FIERRO, J. L. y COLINO,
4.
J.: Processing and properties of superconducting YBa2Cu307_j^ powders by single-step calcining in air. J. Mater. Sei., 26 (1991), 6135-6143.
HoBSON, J. P.: Analysis of physical adsorption isotherms on heterogeneous surfaces at very low pressures. Can. J. Phys., 43 (1965),
1941-1949.
5.
RuDZiNSKi, W., NAKSMUNDZKI, A . , LEBODA, E . y JARONIEC, M . : New
possibilities of investigating adsorption phenomena by gas chromatography. Chromatography, 1 (1974), 663-667.
6. TsuTSUMi, K. y OHSUGA, T. : Surface characterization of modified glass
fibers by inverse gas chromatography. Colloid & Polymer Sei., 268
(190), 38-44.
7. TsuTSUMi, K. y ABE, Y.: Determination of dispersive and non dispersive components of the surface free energy of glass fibers. Colloid &
Polymer Sei., 267 (1989), 637-642.
8.
MARTÍN, L . , BAUTISTA, C , RUBUI, J. y OTEO, J. L . : Caracterización
de vidrio poroso por cromatografía gaseosa a recubrimiento cero. XXXI
Congreso Nacional de Cerámica y Vidrio. Palma de Mallorca. Junio
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
solid fuel cell applications.
RESUMEN.—Conductores iónicos óxidos, conductores mixtos
y sus aplicaciones como pilas de combustible de óxidos sólidos.
Oxide ion conductors and mixed oxide ion/electronic conductors fînd application as solid electrolyte and electrode materials,
respectively, in solid oxide fuel cells (SOFCs) and oxygen sensors. The materials requirements for these applications are
reviewed, with particular reference to transport number considerations. Some factors that influence the magnitude of the
oxide ion conductivity of a material are discussed; the
characteristics of two materials, Cai2Ali4033 and Ba6Ta20ii,
are summarised. Recent developments in SOFCs are outlined.
Óxidos conductores mediante iones oxígeno y/O conductores
mixtos (iones oxígeno + electrones) son utilizados como electrolitos sólidos y/o electrodos respectivamente en pilas de combustible de óxidos sólidos (SOFCs) y como sensores de oxígeno.
Los requerimientos exigidos a estos materiales para ser utilizables son revisados haciendo especial énfasis en lo que se refiere
al número de transporte. También se discuten algunos factores
que influencian la magnitud de la conductividad iónica de los
óxidos, y se hace un resumen de las características de dos materiales Cai2Ali4033 y Ba5Ta20n» Finalmente se hace mención
de los avances más recientes en pilas de combustible del tipo
SOFCs.
1.
solid electrolytes, in which the transport number of oxide ions is
unity or mixed conductors, which exhibit conduction by both oxide ions and electrons, i. e.
INTRODUCTION. MATERIALS REQUIREMENTS
Oxide ion conductors find potential applications in a variety of
solid state electrochemical devices, including fuel cells of the solid
oxide type (SOFC), oxygen sensors and oxygen pumps. The
materials that are used in these applications fall into two main
categories, depending on their electrical properties. They are either
NOVIEMBRE-DICIEMBRE, 1991
solid electrolytes: ti = l
mixed conductors: tj+te=l,
461
A. R. WEST
where tj and t^ refer to ionic and electronic transport numbers,
respectively.
Solid electrolytes are typified by lime- or yttria-stabilised zirconia.
As their name suggests, solid electrolytes are used as the ionically
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 incorrect 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 oxygendeficient 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 reversible cathode in SOFC designs, in which the combination of electronic 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,
mixed conduction occurs, as shown schematically in Fig. 1. At low
oxygen partial pressures, oxygen gas is released by the material,
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.
One reason for the popularity of stabilised zirconia as a solid electrolyte in both SOFCs and sensors is its wide electrolytic domain.
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.
log (f
electrolytic
domain
I
\
/
\
\
Oxygen absorption : O2 + Ae^^^20^~
Oxygen removal
/
+ 4h
log P02
: 202--ii^^ O2 + 4e
Fig. I .—Electrolytic domain of an oxide ion conductor.
2. OXIDE ION CONDUCTION AND ITS OCCURRENCE
Although zirconia-based ceramics are commonplace, the property
of oxide ion conductivity, shown by the cubic stabilised zirconias,
is quite rare. In the vast majority of oxides, the oxygen array forms
an immobile sublattice at most temperatures and ionic conduction
occurs, if at all, by means of cation diffusion. In part this is because,
in many oxides, the oxide ion is the largest ion in the structure and
therefore forms a dense, often close packed, array with the cations
occupying the smaller interstitial sites within the anion array. Consequently, when a material is doped with a heterovalent cation, the
charge compensating mechanism usually involves the creation of
cation vacancies or cation interstitials, mechanisms 1 and 4, Fig. 2,
and the anion array stays intact.
High oxide ion conduction has been found in a small number of
structure types, mainly fluorite, pyrochlore and perovskite, which
have been doped to yield anion vacancies, mechanism 3. Thus, in
the stabilised zirconias (ñuorite structure), mechanisms such as:
462
CHAROB COMPENSATION MECHANISMS
Addition of higher valence cations
cation vacancies
interstitial anions
Addition of lower valence cations
anion vacancies
interstitial cations
Fig. 2.—Creation of vacancieslinterstitials in order to maintain chari>e
balance on aliovalent cation substitution.
Zr4+-M/20=
operate or, using Kroger-Vink notation:
where, x and ' refer to net site charges of -I-1, 0 and — 1, respectively, 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
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
ohm"' cm-' at 600°C.
There do exist materials that have higher conductivity than YSZ
(yttria-stabilised zirconia), especially at lower temperatures, but the
ones discovered to date are excluded from practical applications
on the basis of cost (scandia-stabilised zirconia) or limited electrolytic domain (doped BÍ2O3, also fluorite structure). Nevertheless,, 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
ions and oxide vacancies tend to aggregate into immobilised clusters.
Thus, the Kroger-Vink representation of YSZ, above, shows that
positively charged oxide vacancies and substitutional yttrium ions
with an effective negative charge are present. Inevitably, such oppositely charged species attract each other to form, initially, dipoles
and then larger clusters. In order for the oxide vacancies to move,
they must become dissociated from these clusters: this contributes
an additional enthalpy term, the enthalpy of dissociation, to the
overall activation enthalpy for conduction and hence, leads to a
lowering of conductivity. In unfavourable cases, materials may 'age'
in which, at a certain temperature, the defect aggregation takes place
over a period of time and the conductivity gradually decreases, as
shown schematically in Fig. 3. Clearly, such materials are unacceptable for SOFC applications in which operation over a large
period of time may be required; for 'short burst' applications, such
as in certain types of sensor, this may not be a problem.
The problem of ageing could perhaps be avoided if it were possible
to encounter materials that were both stoichiometric (i.e. undoped)
and contained a large concentration of oxide ion vacancies/interstitials; often, however, such materials form ordered structures
on cooling, with a consequent trapping of the mobile species and
lowering of conductivity. An interesting, relatively new oxide ion
conductor that is both stoichiometric and contains partially occupied
oxide ion sites is the calcium aluminate Ca,2Al|4033 (or C12A7 in
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, forming H2O and liberating electrons which become trapped. Under
more strongly reducing conditions, at e.g. 800°C, the whole sample was reduced and became semiconducting, with an overall increase in conductivity (6).
log ff
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
T"1
Fig. 3.—Decrease in conductivity with time due to formation of defect clusters
and immobilisation of oxide ion vacancies.
oxide ratio notation). It contains an aluminate framework of
stoichiometry [Al 14032]^^" which contains cavities for the calcium
ions and sites for an additional oxide ion, with an occupancy factor of 1/6. The activation energy for conduction, 0.74 eV, is quite
low and is less than that of YSZ, probably because there is no
dissociation barrier to overcome. However, the relatively small
number of oxide ions that are potentially mobile (one in 33 at best)
means that at high temperatures, > 600°C, the conductivity is about
an order of magnitude lower than that of YSZ.
3. MATERIALS CASE HISTORIES
3.1.
Cai2Ali4033
This material, referred to above, is an interesting new oxide ion
conductor. Some key points about it, which illustrate the methodology of research into potential new ceramic electrolytes, are as
follows:
I) Prior to electrical conductivity measurement, crystallographic
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 promising possibilities for oxide ion conduction.
II) ac impedance measurements showed that it was possibly,
by sintering, to obtain materials with small grain boundary
resistances and a bulk conductivity that was only 8-10 times lower
than that of YSZ (1, 2).
This material has an oxygen-deficient, perovskite related structure and therefore potential oxide ion conductivity. Key points about
its properties are as follows (7):
I) ac impedance measurements showed a moderately high bulk
conductivity, e.g. 3.7x10"^ ohm~^ cm"^ at 800°C, with a
temperature-dependent activation energy in the range 0.93 to 1.22
eV.
II) The bulk conductivity was independent of atmosphere, but
the grain boundary conductivity increased with increasing oxygen
partial pressure in the conductivity cell, suggesting the grain boundary to be a p-type semiconductor. On heating at 800°C in O2, the
grain boundary conductivity increased significantly and effectively short-circuited in the sample.
III) Oxygen concentration cell measurements indicated an oxide 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, indicated the absence of any hydrogen-conducting species.
IV) Thermoelectric measurements indicated the conductivity to
be p-type; this was obtained from the polarity of the emf generated
when the sample placed in a temperature gradient.
In conclusion, this material is an interesting mixed conductor with
p-type electronic conductivity and oxide ion conductivity.
4. SOLID OXIDE FUEL CELLS
There is currently great interest, worldwide, in the development
of SOFC system for power generadon (8). They have the potential
advantages of high energy conversion efficiency, simplicity of design
and high-grade waste heat, leading to possible combined heat and
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 immobile hydroxides. Water is eliminated and the conductivity
recovered on heating at - 1350°C (5).
Cathode reaction : O2 + 4e
20^"
Anode reaction :
CÜ2 + 2e
CO + 0""
H2 + 0=
VI) The electrical properties were found to be unstable in a
reducing atmosphere (5% H2, 95% Nj). At low temperatures, e.
g. 400°C, a resistive outer layer forms around a ceramic sample,
NOVIEMBRE-DICIEMBRE, 1991
Overall reaction :
Ü2 + 2CÜ/2H2
H2Ü + 2e
•
2CÜ2/2H2O
Fig. 4.—Schematic operation of a solid oxide fuel cell.
463
R. M. C. MARQUES, J. R. FRADE, F. M. B. MARQUES
CURRENT FLOW
Ni,Zr02
YSZ
LaMn03
CELL
REPEAT
UNIT
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
fuel gases.
During operation of the SOFC, current is drawn from the cell
in the direction perpendicular to the plates.
La Cr03 BIPOLAR PLATE
ACKNOWLEDGEMENTS
Research support from the Science and Engineering Research
Council is gratefiilly acknowledged.
REFERENCES
Fig. 5.—Flat plate SOFC configuration.
power, CHP, applications. The principle of SOFC operation is
shown in Fig. 4. The central component is the oxide ion conducting membrane, YSZ, either in a tubular or flat plate configuration. Its two opposite surfaces are in contact with solid electrodes,
a Ni/Zr02 cermet (anode) and a lanthanum manganite (cathode).
Fuel gas (methane, CO, H2, natural gas, etc.) is passed over the
anode and air or oxygen over the cathode. At the cathode, oxygen
molecules dissociate and pick up electrons from the external circuit to form oxide ions which then diffuse through the YSZ membrane. These react at the fuel electrode with the fuel gas(es) and
liberate electrons to the external circuit where they act as a source
of power. The reactions involved in the process are given in Fig. 4.
Most of the early development work on SOFCs has been carried
out by Westinghouse, using a tubular YSZ electrolyte. There is now
increasing interest, for its ease of fabrication, in a flat plate configuration. Fig. 5. This comprises three-layer sandwiches of Ni
cermet, YSZ and LaMnOg, which are stacked into a mulfilayer
1.
LACERDA, M., IRVINE, F. P., CLASSER, F. P. and WEST, A. R.: High
2.
LACERDA, M., IRVINE, J. T. S., LACHOWSKI, E. E., CLASSER, F. P.
oxide ion conductivity in Ca,2Al,4033. Nature, 332, 525-526 (1988).
and WEST, A. R.: Ceramic processing of Cai2Al,4033 for high oxide
ion conductivity. Br. Ceram. Trans. J., 87, 191-194 (1988).
3. IRVINE, J. T. S., LACERDA, M. and WEST, A. R.: Oxide ion conduc-
tivity in Ca,2Al,4033. Mat. Res. Bull., 23, 1033-1038 (1988).
4. IRVINE, J. T. S. and WEST, A. R.: Ca,2Al,4033 solid electrolytes doped
with zinc and phosphorous. Solid State Ionics, 40/41, 896-899 (1990).
5. IRVINE, J. T. S. and WEST, A. R.: Ca,2Al,4033 a possible high
temperature moisture sensor. /. Appl. Electrochem., 19, 410-412 (1989).
6. LACERDA, M., WEST, A. R. and IRVINE, J. T. S.: Electrical proper-
ties of Cai2Ali4033: effect of hydrogen reduction. /. Electrochem.
Soc, submitted.
7. GOTO, T. and WEST, A. R.: ac impedance and transport number
measurements of Ba5Ta20ii, MRS Symposium. Solid State Ionics, in
press.
8. YAMAMOTO, O., DOYIKA, M. and TAGAWA (eds.): Solid Oxide Fuel
Cells, Proc. Int. Symp., Nagoya, Japan, Nov. 1989, Publ. Science
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 solutions with mixed valence dopants.
RESUMEN.—Propiedades de transporte en soluciones sólidas
basadas en circona con dopantes de valencia mixta.
Yttria stabilized zirconia and two derived materials (with partial replacement of ceria for zirconia and ceria for yttria) where
characterized in terms of structure and transport properties in
air and under reducing conditions. From electrical conductivity temperature dependence it was concluded that ceria additions promote a small increase in activation energy for defect
mobility, and a decrease in defect association energies. Ceria
doped materials exhibit a pronounced aging effect under reducing conditions probably related to an ordering process involving the cation sublattice and enhanced interaction with oxygen
vacancies.
Circonia estabilizada con ytria y dos materiales derivados (con
reemplazamiento parcial de ceria por circonia y ceria por ytria)
fueron caracterizados en términos de estructura y propiedades
de transporte en aire y bajo condiciones reductor as. Desde la
dependencia de la conductividad eléctrica con la temperatura
se ha concluido que adiciones de ceria promueven un pequeño
aumento de la energía de activación por movilidad de defectos,
y una disminución en las energías asociadas de los defectos. Los
materiales dopados con ceria muestran un efecto de envejecimiento pronunciado bajo condiciones reductoras, probablemente referido a un proceso de ordenamiento envolvente de subredes catiónicas e interacción intensificada con las vacantes de
oxígeno.
464
BOL.SOC.ESP.CERAM.VIDR. VOL. 30 - NUM. 6