R Vasant KUMAR' and Hiroyasu IWAHARA 2
I Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB 2 3 QZ, UK
2 Centre for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464, Japan
Contents
List of symbols and abbreviations 1 Introduction
2 Ionic transport in solid electrolytes 3 The role of rare earths in solid electrolytes 4 Oxide ion conductors
4.1 Extrinsic oxide ionic conductors 4.1 1 Zirconia electrolytes 4.1 2 Ceria-based electrolytes 4.2 Intrinsic oxygen ionic conductors
4.2 1 Bi 203-based electrolytes 4.2 2 Pyrochlore-type oxide 4.3 Mixed oxygen-ion/electron conductors 5 Fluoride-ion conducting solid electrolytes
5.1 Fluoride solid electrolytes with fluorite- type structure
5.2 Rare-earth fluorides 5.2 1 Tysonite-type fluorides 5.3 Oxyfluoride solid electrolyte 6 Cation conductors
6.1 Proton conductors
6.1 1 Low-temperature protonic conductors
6.1 2 Protonic defects in oxides 6.1 3 High-temperature proton
conductors 6.2 Metal-ion conductors
6.2 1 Sodium-ion conductors 6.2 2 Lithium-ion conductors
132 6 2 3 Rare-earth-ion conductors 160 132 7 Technological applications of rare-earth- 133 containing solid electrolytes 161
133 7 1 Fuel cells 161
135 7 1 1 Solid oxide fuel cells 162 135 7 1 2 High-temperature fuel cells based
135 upon protonic conductors 165
138 7 2 Chemical sensors 165
140 7 2 1 Oxygen sensors 166
140 7 2 2 Mechanism of air/fuel control 168 143 7 2 3 Operation of the i) oxygen sensor 168 144 7 2 4 Planar-type lambda oxygen
145 sensors 169
7.2 5 Sensors for lean-burn combustion 170 146 7 2 6 A new type of oxygen sensors 171
147 7 3 Fluorine sensors 174
148 7 4 Sensors based on protonically
149 conducting oxides 176
149 7 4 1 Hydrogen sensors 176
149 7 4 2 Humidity sensors 176
7.5 Potential application of rare-earth- 150 containing solid electrolytes in novel gas
150 sensors
7.5 1 SO, sensors 151 7 5 2 HCI sensors
156 8 Concluding remarks 156 References 157
178 178 180 180 181
131
R.V KUMAR and H IWAHARA
List of symbols and abbreviations
A/F air/fuel ratio R universal gas constant
AFC alkaline fuel cell SOFC solid oxide fuel cell
H Cs hydrocarbons SPFC solid polymer fuel cell
emf electromotive force TWC three-way catalyst
EVD electrochemical vapor deposition YBCO Y Ba 2Cu 307 ,O,
F Faraday's constant YSZ yttria stabilized zirconia
ISE ion selective electrode A operating A/F/stoichiometric A/F Nasicon Na super ionic conductor a conductivity
PAFC phosphoric acid fuel cell
1 Introduction
One of the early applications of solid electrolytes was the Nernst glower proposed by Nernst as a new form of electric light (Nernst 1900) Zirconia doped with a rare-earth oxide (such as yttria) can become a conductor of oxygen ions at elevated temperatures resulting in emission of light when passing electric current through it Interest in doped zirconia solid electrolytes intensified in the 1950 s, following the launching of rockets for space research, for application in fuel cells.
Solid electrolytes have also been variously described as Fast Ionic Conductors or Superionic conductors and may cover ionic conductivities within the range of 10- 5 to 1 S/cm with activation energies of 0 1 to 2 e V/atom The levels of ionic conductivity achieved in many of these solid electrolytes are well below their melting points and the values are more typical of liquids than solids In contrast to liquid electrolytes such as the aqueous electrolytic solutions or molten salts, the mobile ions in a solid are limited to one sublattice such that one ionic component can move through a rigid framework provided by the other components.
Fast ionic transport in solids has become a major area of both scientific investigations and technological applications Until a few decades ago, solid-state ionic conductors were considered as exceptions in materials science and their discovery was seen as a fortuitous event More recently, however, fast ionic conduction has been discovered or rediscovered, observed, studied and introduced synthetically in several hundreds of solid materials ranging from ceramics to glass and polymers (Geller 1977, Hagenmuller and Van Gool 1978, Chandra 1980, Tuller and Moon 1988, Laskar and Chandra 1991 and Iwahara 1995) Considerable research has focussed on the theory of ionic transport and its implication for solid state reactions such as corrosion, oxidation and sintering Scientific interest has mainly been driven by applications in many technological areas such as fuel cells, batteries, sensors, electrolysis, electrochromics and displays, optical materials and environmental protection Development of practical devices has in turn raised many interesting scientific and technological questions relating to transport of ions in solids, resulting in the emergence of the field of solid-state ionics as a major scientific area.
132
2 Ionic transport in solid electrolytes
Solid electrolytes are characterized by a high ionic conductivity with a low activation en- ergy relative to "normal" defective solids This requires the presence of a large concentra- tion of charge carriers with high mobilities, such that the coulombically charged particles move rapidly among near-equivalent atomic sites or within a disordered host medium.
High ionic conductivity in solids is achieved in a variety of ways as described below.
Some solid electrolytes such as a-Ag I are characterized by a high degree of intrinsic disorder in the Ag+ ion sublattice, resulting in a random distribution of Ag+ ions over an excess number of equivalent sites, leading to ion hopping between crystallographic sites.
Another striking feature of many such soft ionic crystals is the high entropy increase of a solid-solid phase transition which brings the material to a good conductivity phase The values of entropy changes and the diffusion coefficient of the mobile ions are closer to those found in molten salts A number of solid electrolytes such as -alumina and Ca F 2, on the other hand, reach high conductivity values without passing a first-order solid-solid phase transition.
In some other solid electrolytes, the intrinsic disorder is created as a consequence of forming a metastable phase, e g by rapid solidification of a melt to form a glass.
Polymeric and glassy solid electrolytes comprise some of the most recent additions to solid-state ionics Although there is no long-range order of any type, disorder in glasses and polymers far exceeds that in crystals The entropy of the glassy state (supercooled liquid) is high and such electrolytes are deemed to have liquid-like properties.
In another group of materials, fast ionic conductivity is introduced extrinsically by doping For example, addition of aliovalent Y 203 to Zr O 2, not only stabilizes the cubic fluorite structure, but also leads to the formation of oxygen vacancies to compensate for the charge difference between Zr 4+ and y 3 + At high concentrations of the dopant, typically at 8-15 mol%, a considerable disorder is created in the oxygen sublattice, leading to ionic conductivity A special case of extrinsic doping occurs in composite materials where high levels of charge carriers are created only at interfaces owing to the limited solubility of the insulating phase in the matrix of the ionically conducting phase, for example in the Ca F 2-A 1203 composite electrolyte.
3 The role of rare earths in solid electrolytes
In order for an ion to move easily in a solid, a special array of atoms (or ions) in the material is necessary This array requires either several large clearances for the mobile ions or a number of lattice defects through which a certain ion can move The former structure can be constructed by large ions as host constituents and the latter can be induced by the introduction of some dopants Rare-earth ions act as host ions of the crystal lattice in the former case and as dopants in the latter.
In general, ionic radii of multivalent ions are small The ionic radii of rare-earth elements, however, are relatively large, even if they are tri or tetravalent and, furthermore,
Table 1
Typical solid electrolytes containing rare earth elements Mobile ion Solid electrolytes Notes: crystal structure etc.
02 Zr O 2-Ln 203, Hf O 2-Ln 203, Fluorite-type tetravalent oxide Ce O 2-Ln 2O 3, Th O 2-Ln 203,
Ce O 2-MO (M = Ca, Sr, etc )
Bi 203-Ln 203, Fluorite-type, rhombohedral-type bismuth sesquioxide Zr 2Gd 207, Zr 2Sm 207, Pyrochlore-type
Ti 2Y 207, Ti 25 m 207
F Ca F 2-Ln F 3, Sr F 2-Ln F 3, Ba F 2-Ln F 3 Fluorite-type Ln F 3-MF 2(M =Ca, Sr, etc ) Tysonite-type La 2 ,03-Ca F 2, La OF, Zr O 2-Ln F 3 Oxyfluorides
H+ (Na H) Gd Si 4,1 2 Nasicon-type
Sr Ce, x Ln O 3-_,, Ba Celx Ln O 3_ Perovskite-type, in H 2 or H 20 at high temperature
Li+ YPO 4-Li 3PO 4 Zircon-type
Na+ Na 2SO 4-Ln 2(SO 4) High-temperature phase Na 3Gd Si 4O,12, Na 3Sc 2P 3012 Nasicon-type
Eu 2+ Eu-3 "-alumina Layer structure
Gd 3+ Gd-"-alumina Na+-conductor? layer structure
a Ln, rare-earth element.
their valency in general is very stable Rare-earth ions as the constituent of solid electrolytes have the following features: ( 1) they behave as stable trivalent cations with large ionic radii; ( 2) they provide different sizes of trivalent ions; ( 3) their electronegativity is rather small and they function as a constituent with strong ionicity; ( 4) they provide a variety of ions, for example, tetravalent or divalent ions which are stable under given conditions; and ( 5) they exhibit characteristics expected of 4 f electrons As rare-earth elements are all quite similar in their chemical properties, it may be possible to select the most appropriate ion for optimum properties such as conductivity and most favorable practical usage, without significantly affecting the chemical properties.
Rare-earth elements are vital constituents of many prominent solid electrolyte systems such as the oxygen-ion conductor in fluorite structures (Dell and Hooper 1978), the fluorine-ion conductor in tysonite-type trifluorides (Reau and Portier 1978), the protonic conductor in doped perovskite phases (Iwahara et al 1981 a), and the trivalent cationic conductor in hexagonal O-alumina-type compounds (Verstegen et al 1973) Typical solid electrolytes containing rare-earth elements are listed in table 1 It is worth noting that rare-earth elements have a high profile in conferences directly concerned with solid electrolytes If we consider the last two International Conferences on Solid State Ionics (held every two years in various locations around the world), of the 190 oral presentations and 260 posters in the last Conference, held in Singapore in December 1995, and the 360 oral presentations in the more recent conference held in Hawaii in November 1997,
more than one-third of the papers were directly concerned with solid-state ionic systems containing rare-earth elements.
4 Oxide ion conductors
4.1 Extrinsic oxide ionic conductors
Inorganic crystalline materials which exhibit oxygen-ion conduction are mainly found among the oxides of tetravalent cations which crystallize with the fluorite structure (e g.
Ce O 2) or distorted fluorite structure (e g Zr O 2) Introduction of divalent or trivalent cations into the lattice stabilizes the cubic fluorite structure and also leads to the formation of oxygen-ion vacancies in order to retain charge neutrality Oxygen-ion conduction occurs as a result of this defect structure.
4.1 1 Zirconia electrolytes
In a fluorite-type oxide, whose chemical formula is expressed as MO 2, the structure type is named after Ca F 2, in which the cation/anion radius ratio is such that the anions achieve a simple cubic packing with the cations occupying half the available sites with 8-fold coordination The packing of the anions is not as compact as those found in other oxides such as spinel or corundum-type structures, which have closest packing of oxide ions.
Pure zirconia has a distorted fluorite (monoclinic) structure at room temperature, which transforms to a tetragonal structure at above 1200 °C and finally to a cubic form at > 2300 °C The cubic fluorite form has a crystal structure as shown in fig la The exact transformation temperature and behavior are probably very sensitive to any impurity present and also influenced by hysteresis If the Zr is partially replaced by a divalent or trivalent cation with relatively large ionic radius, the fluorite structure can be stabilized at lower temperatures This "stabilized zirconia" is often metastable at room temperature and does not decompose to the thermodynamically stable phases.
Rare-earth elements are the most commonly used stabilizers in the zirconia electrolytes.
Rare-earth oxides can dissolve in relatively large amounts in zirconia, and can thus substitute for tetravalent zirconium ions resulting in a large concentration of oxide- ion vacancies in order to compensate for the charge imbalance In this "defect fluorite structure", the oxygen ions are mobile at elevated temperatures by an oxygen vacancy mechanism as shown in fig lb Therefore the rare-earth stabilized zirconia is a good oxygen-ion conductor at elevated temperatures In summary, the rare-earth elements dissolved in zirconia serve both as a stabilizer for the cubic fluorite structure and as a dopant to create oxygen-ion vacancies which are indispensable for ionic conduction.
When a rare-earth oxide, e g Y 203, is dissolved in Zr O 2, the following defect equilibria can be expressed, using Kr 6 ger-Vink notation:
Y 203 2 Y + Vo + 30 ( 4 1)
The addition of rare-earth oxides thus introduces oxygen-ion conductivity by affecting the defect equilibria Increasing the concentration of dopant leads to an increase in the
(a) Crystal structure 02- S Zr
2 O 2 2 02 02 02-
Zr 4 + Zr A' Zr*'
O_ _ 0 : 02 O 02 02-
Y 3' Zr'4 Zr 4 +
02 02 O o O O 02 O o 02-
Zr' Y 3 Zr'
o 2 o 2 O o o-
Zr 4 + Zr+ y 3+
02 o o O 02 O o o o 2- (b) Oxide ion vacancies
Content of rare earth oxide / mol%
Fig 2 Conductivities of rare-earth doped zirconias.
Fig 1 Fluorite-type Zr O 2, its crystal structure; and oxide ion vacancies.
conductivity until a maximum is reached after which the conductivity begins to decrease, either limited by the solid solution range or due to the formation of vacancy clusters with lower mobility This onset of substantial ordering in the defect structure not only leads to a decrease in the conductivity but also to an increase in the activation energy values Ionic conductivity data at 1073 K for several rare-earth-zirconia systems are shown in fig 2 (Tannenberger et al 1965) The ionic radius of the dopant rare-earth element directly affects the value of ionic conductivity in zirconia as shown in fig 3 (Adachi 1988) As the ionic radius is increased from that of Sc 3 + at 1 12 A to Nd 3+ at 1 25 A, the ionic conductivity at 1273 K is observed to decrease by almost an order of magnitude from 0.32 S/cm to 0 04 S/cm.
In the stabilized zirconia doped with a rare-earth oxide such as yttria, the vacancy concentration is determined by the dopant concentration, for a wide range of temperature and partial pressure of oxygen, as given by the equation
lVl = (lYl.
I
( 4.2)
E O 3 0.2
i O 1
0 fi
Fig 3 Conductivity of rare-earth doped zirconia
na fincrionn nf inni rill (R Penrinted frn
0.10 0 11 0 12 -.
Adachi 1988 by permission of the publisher, Ionic radius(nm) Elsevier Science Ltd )
1 g 4 A _ fleac reprsenttio ____ :_
vu-~
~ ~~o
'F fig 4 A schematic representation oflog Px 2 ionic transference number vs logpo 2. At higher temperatures and low Po 2 values, n-type electronic conductivity may appear as a result of the following equilibria:
O +-+ 2 02 (g) + VO+ 2 e', ( 4 3)
while at high Po 2 values, p-type electronic conductivity may dominate as a result of the following equilibria:
½O 2 (g) + V O O + 2 h' ( 4 4)
Since electronic mobilities are usually much higher than ionic mobilities, the ionic conductivity can be dominant only when a very large concentration of ionic defects are present in the solid For a given temperature, there is a range of Po values within which eq ( 4 2) is dominant, and the conductivity, predominantly ionic, is independent of oxygen pressures, as shown in fig 4 This Po 2 range is referred to as the electrolytic or ionic domain of the solid electrolyte at that temperature As the temperature is increased the electrolytic domain is diminished A schematic representation of the electrolytic domain as a function of temperature for a typical oxide-ion conductor is represented in fig 5 In order to operate a given oxygen-ion conductor as an useful solid electrolyte, it is important to know the ionic domain of the material Areas close to the domain boundary represent mixed ionic and electronic conduction.
Sc\ L
T Er Ho Dy
Gd
Zr Nd
lI l G
_
I
-12 I > 1 1.00 5
__
10 0 10
-W
N -20 30 _ 40
T C
2000 1500 1000 600
3 5 7 9 11 13 Fig 5 Electrolytic domain of typical
I /T (x 1 O ) (< ) oxygen conducting electrolytes.
Among the zirconia systems, yttria-stabilized zirconia is the most familiar oxygen- ion conductor, with widespread practical applications in sensors (Seiyama 1988-94), oxygen pumps (Fouletier et al 1975), fuel cells (Minn and Takahashi 1995), and steam electrolysers (Donitz and Erdle 1985) At a dopant concentration of around 8 mol% of Y 203, the zirconia solid electrolyte can achieve a conductivity value of 10-1 S/cm at
1273 K with an activation energy of 0 8 e V.
4.1 2 Ceria-based electrolytes
The tetravalent cerium ion is large ( 1 01 A) in ionic size compared to the zirconium ion ( 0 8 A), and its dioxide Ce O 2 crystallizes to a fluorite-type structure even at room temperatures and without the need for any stabilizers Ceria can also dissolve a large concentration of trivalent rare-earth oxides, while retaining the fluorite structure In the solid solution, when the trivalent rare-earth ions partially replace the tetravalent Ce 4+, a corresponding concentration of oxygen vacancies is created in a manner as described for the zirconia solid electrolytes As shown in table 2, the conductivity of doped ceria is, in general, higher than that of stabilized zirconia Figure 6 shows Arrhenius plots of conductivity of various ceria-based solid electrolytes, also comparing them with pure ceria and yttria-stabilized zirconia systems (Eguchi et al 1992) It should be pointed out that the activation energies for the fluorite-type oxide ionic conductors fall within the range 0.6 to 1 4 e V and are relatively high in comparison with the truly fast ion conductors such as Ag I-based electrolytes and some /i-aluminas which exhibit room-temperature conductivities of 0 12 and 0 03 S/cm with activation energies of 0 07 and 0 15 e V, respectively.
The maximum value of conductivity for ceria-based electrolytes is attained at a certain concentration of the dopant oxides, in a manner similar to the zirconia electrolytes As shown in fig 7 (Takahashi and Iwahara 1966), the conductivity of Ce O 2 doped with
Zr O 2 ( 15 mole % C)O)
Th O 2 ( 15 mole % Y 015)
I I_ I I I I
_ _ = _
Table 2
Conductivity of fluorite-type solid oxide electrolytes
Electrolyte o at Activation Electrolyte a at Activation
1273 K energy 1273 K energy
(S/cm) (e V) (S/cm) (e V)
10 mol% Y 203-Zr O 2 0 1 0 8 8 mol% Y 203-Th O 2 0 0048 1 1 10 mol% Sm 203-Zr O 2 0 058 0 95 10 mol% La 203-Ce O 2 0 08 0 9 8 mol% Yb 203-Zr O 2 0 088 0 75 15 mol% Ca O-Ce O 2 O 025 0 75 10 mol% Sc 203-Zr O 2 0 25 0 65 10 mol% Gd 2O 3-Ce O 2 0 5 0 7
Temperature I C 900 800 700 600 500
'0.7 0 8 0 9 1 0 1 1
I
j
A'
U
1.2 1 3 1 4 103-T-1 /K-1
Fig 6 Arrhenius plots for ionic conductivities of ceria-based oxides (Reprinted from Eguchi et al 1992 by permission of the publisher, Elsevier Science Ltd )
Fig 7 Conductivities of (Ce O 2)1_,(La OL 5)x in
air. X In (Ce O 0,t(La O,j).
La Os 5reaches a maximum value at about 20 mol% of the dopant concentration, although the upper limit of solid solution formation range extends far beyond this value The association of oxide-ion vacancies at high concentrations leads to a decrease in the mobility of the vacancies, which in turn diminishes progressively the effect of increased concentration (Anderson et al 1983) The lattice parameters of defect fluorite-type Ce O 2
change monotonously with the dopant content, i e with the concentration of oxygen vacancies It is possible to change the lattice parameter of the defect fluorite Ce O 2, at
3
2
2
E U 3
'
gl
0
I O oI
* O
* 4 D O
0 *
I "" "
_
_ I
0
-2
U -4 b
-6
-8
-50 -40 -30 -20 -10 0 10 20 30 40
Log l po 2 (bar)l
Fig 8 Ionic and electronic conductivities of Ce O 2-Gd O,, ( 10 mol%) at 773 and 1000 K (Reprinted from Steele et al 1994 by permission of the publisher, The Institute of Energy, London )
a fixed concentration of vacancies, by using different rare-earth ions For example, in the ternary system (Ce O 2) -(,,+y)(Gd Ol s)x(YO 1 5)y, the lattice parameter can be controlled by changing the contents of Gd and Y respectively An increase in Gd content (x) results in an increase in the lattice constant, while an increase in Y content (y) leads to a decrease in the lattice constant It was found that at x = 0 09 and y = 0 01, the lattice constant of the doped Ce O 2is identical to that of the non-doped (pure) Ce O 2and that the conductivity exhibits a maximum value in this system (Zhen et al 1987) This opens up the prospect of designing the size of crystal lattice and the concentration of defects in the oxide-ionic conductors by using a combination of trivalent rare-earth ions with similar chemical properties Ceria can also dissolve divalent alkali-earth oxides such as Sr O and Ca O resulting in ionic conductivity at elevated temperatures (Yahiro et al 1986).
The electrolytic domain for a ceria-based electrolyte, at any given temperature, is narrower than that of a zirconia-based electrolyte, such that electronic conductivity is acquired in reducing atmospheres As shown in fig 8 (Steele et al 1994), the electrolytic domain of Ce O 2-Gd O 15 ( 10 mol%) at 1000 K is such that the electrolyte is not suitable for use at oxygen pressures below 10-1°O atm The high ionic conductivity, however, makes the electrolyte of considerable interest at low temperatures and/or in environments of moderate oxygen potential.
4.2 Intrinsic oxygen ionic conductors 4.2 1 Bi 203-based electrolytes
Bi 203is monoclinic at room temperature, and exhibits electronic conductivity On heating to 1050 K, Bi 203 undergoes a phase change to a cubic fluorite structure ( 6-Bi 203), which
Ce 9Gdo 1 t s I
I~~~~~~~~~