Oxide ion conducting solid electrolytes based on Bi2O3

SOLID STATE Solid State Ionics 89 (1996)

IONICS

179-196

Review

Oxide ion conducting

solid electrolytes

P. Shuk”, H.-D. Wiemh6ferb,

based on B&O,

U. Guth”, W. Gijpeld, M. Greenblatt”‘”

*Department of Chemistry, Rutgers lJniver.siry, Piscutuway, NJ 088550939, USA hInstifute of Inorganic Chemistry, University of Muenster, Wilhelm-Klemm-Str. 8, D-48149, Miinster, Germany ‘Institute of Physical Chemistry, University of Greifswald, Soldtmannsrr. 23, D- 17489, Greifswald, Germany dInstitute ofPhysical and Theoretical Chemistry, University of Tuebingen, Auf der Morgenstelle 8, D-72076, Tuebingen, Received

14 February

1996; accepted

21 February

Germanq

1996

IN MEMORY OF PROFESSOR TAKEHIKO TAKAHASHI

Abstract The high oxide ion conductivity

of solid solutions of bismuth oxide was initially discovered by Takahashi and coworkers. are much better solid electrolytes than the well-known stabilized zirconia. The only difficulty which has prevented their use in high temperature fuel cells and gas sensors up to now is their instability against reduction at low oxygen partial pressures. In this article, we review the structural properties, thermal expansion, electrical conductivity, thermodynamic stability, and surface properties of bismuth oxide and solid solutions of bismuth oxide with face centred cubic, rhombohedral, tetragonal or layer structures.

The bismuth oxide based compounds

Keywords:

Solid electrolyte;

Oxide ion conductivity;

Bismuth oxide; Crystal structure

1. Introduction

ZrO, XT xM;L + 0:

Oxide ion conducting solid electrolytes play a decisive role in electrochemical cells for measuring oxygen activities and thermodynamic data of solid, liquid and gaseous phases. They have found wide applications for gas monitoring and combustion control, in metallurgy, petrology, chemical kinetics and in solid oxide electrolyte fuel cells [l-S]. Presently available solid electrolytes with high oxide ion conductivity are mainly derived from solid solutions of zirconia. Relatively large concentrations of oxide ion vacancies, Vi can be generated in the crystal lattice of ZrO,, if lower valent metal ions are substituted for Zr4+ in ZrO,, e.g. Mgzf, Ca2+ and Y 3+. .

where M.$: is a M2+-ion in a Zr4+ lattice site and 0; is a 02--ion on a regular lattice site. Yttria stabilized zirconia (Zr, _XYX02pX,2), the most often applied solid electrolyte which exhibits high oxide ion conduction at elevated temperatures, has a low dissociation pressure and is non-corrosive. However, compared to B&O,, solid electrolytes based on ZrO, have a relatively low oxide ion conductivity at temperatures below 800 K and require very high sintering temperatures (often higher than 2000 K). Although there have been some reports on B&O, systems earlier [9,10], Takahashi and coworkers [l l] were the first to show that the face centered cubic (fee) phase of Bi,O,, &B&O, stabilized by various aliovalent metal ion substitutions, has the highest

*Corresponding

author.

0167-2738/96/$15.00 01996 PII SO167-2738(96)00348-7

Elsevier Science B.V. All rights reserved

+ XV,,

(1)

180

P. Shuk et ul. I Solid State Ionics 89 (1996) 179-196

oxide ion conductivity of all oxide ion conductors known so far. The conductivity of &Bi,O, is one to two orders of magnitudes higher than that of stabilized zirconia at corresponding temperatures. The highest values of conductivities in the B&O, family of oxides are observed in the high temperature &phase of B&O, with defect fluorite structure. This high temperature phase exhibits two vacant oxide ion sites per unit cell which are statistically disordered. But this phase is only stable in the temperature range 1003- 1097 K. Fortunately, substitution of yttrium or rare earth oxides stabilizes the &phase of B&O, down to room temperature [ 12,131. Recently, several investigations have been performed with the aim of combining the advantages of stabilized B&O, (high conductivity and low sintering temperature) with those of stabilized ZrO, (low dissociation, non-corrosive) [ 1422]. The aim of this article is to review the properties of oxide ion conductive solid electrolytes based on B&O, with particular emphasis on structure, composition and conductivity.

2. Pure bismuth

oxide

cooling, in the temperature range 773-923 K. The structure of the various Bi,O, phases was investigated systematically by Harwig [28-301. Table 1 and Fig. 1 show the characteristic data and transformation temperatures for the different phases of B&O,. The crystal structure of the monoclinic a-B&O, was first determined by Sillen [23]. In the a-Bi,O, structure, layers of bismuth ions, parallel to the (100) plane of the monoclinic unit cell, are separated by layers of oxide ions; the structure is an ordered defect fluorite structure [29,30] with one quarter of the oxygen sites vacant. The (Y+S transition at 1002 K is accompanied by an anomalously large transition enthalpy of 29.6 kJ/mole, which is 2.7 times the heat of fusion. The relative entropy gain due to the o--G transition with respect to the total entropy gain from the solid to the liquid state is 75%. This result indicates a degree of high disorder in the &phase and corresponds to the melting of the sublattice [31]. Sillen [23] also reported on a cubic phase obtained by quenching Bi,O,, that has been fused for a long time in a porcelain crucible. This cubic structure is related to the fluorite structure but has ordered defects in the oxygen sublattice in the (111) direction (Fig. 2). Each Bi3+ ion has six oxygen neighbours arranged at

2.1. Structure

Liquid 1097 K

The polymorphism of B&O, has been the subject of a number of investigations [23-301. Levin and Roth [26] reviewed the work prior to 1964. Four polymorphs of Bi,O, have been reported in the literature. Some confusion about the stability or metastability of the phases resulted from the high tendency of bismuth oxide to incorporate other oxides as impurities. The high temperature fluorite-related S-phase of B&O, was found to be an excellent oxide ion conductor. This phase exists between 1002 K and the melting point of bismuth oxide at 1097 K. Below 1002 K the S-phase transforms to the low-temperature monoclinic a-phase. Large thermal hysteresis is usually observed on cooling the a-phase and one of two intermediate metastable phases may be formed: the tetragonal P-phase at 923 K or the body centred cubic (bee) y-phase at 912 K. Usually these phases transform to the monoclinic a-phase, on further

6

S-pharcl

phase fee

1002 K

923 K

_

912K

-

fee

p - phase tetragonal

fee

‘{

phase bee

a - phase 773 K -

monoclinic (a - phase) y- phase may persist to room temperature

603 K a

phase

monoclinic

Fig. 1. Survey of the temperature regions of stable and metastable phases encountered in Bi,O, [28].

P. Shuk et al. I Solid State Ionics 89 (1996) 179-196

SILLEN

model

Bi3r

0

0”

0

GATTOW

WILLIS

Fig. 2. Structure

model

model

models for fluorite related &Bi,O,

[29].

six of the eight comers of a cube; two oxygens at diagonally opposite corners of the cube are missing. Gattow and Schroeder [32] showed by means of high-temperature X-ray powder diffraction that the S-phase of B&O, is fee. They rejected the model of an ordered defect oxygen lattice and gave preference to an oxygen sublattice with a statistical occupation of the sites (occupancy factor of 75%). Willis [3335] replaced each anion site in the fluorite structure by four equivalent sites displaced in the (111) direction from the ideal position. The oxide ions occupy these sites statistically with an occupancy factor 3/16 (Fig. 2). Neutron diffraction studies of Battle et al. [36] showed that S-Bi,O, has a defective-fluorite structure in which 43% of the regular oxide ion sites are randomly occupied, the remaining 1.28 oxide ions per unit cell being displaced from their ideal positions along the (111) directions. This Table 1 Structural

181

high level of disorder is clearly related to the exceptionally high ion conductivity. The large anion shifts and the high temperature factors observed for oxygen and bismuth in S-Bi,O, are reasonable, since the S-phase is stable only in a temperature range close to the melting point [29]. The transition to the metastable tetragonal P-phase may be observed at 923 K on cooling from the melt or from the high temperature S-phase [31]. The tetragonal fl-Bi,O, has a distorted defect-fluorite structure with ordered vacant sites in the anion sublattice [29]. The other metastable bee y-phase may also be observed at 902 K on cooling from the melt or from the high temperature S-phase [31]. This y-phase often persists to room temperature. It is likely that the formation of the metastable phases is affected by traces of impurities. The unit cell parameters of all of the B&O, phases are summarized in Table 1. 2.2. Electrical properties The conductivities of the CY,p, y and S-phases were systematically measured by Hat-wig [28,38]. Typical results which were obtained in repeated heating and cooling runs are presented in Fig. 3 [28]. The electrical conductivity of Bi,O, increases by three orders of magnitude at the cy+S transition at 1002 K. In the cooling direction, a hysteresis occurs and the transition to the intermediate p- and yphases are observed by 80-90 K lower than 1002 K. The transition from the metastable intermediate phases to the stable a-phase is not reproducible and does not occur at a finite temperature [28]. The total conductivity of the cr-phase of B&O, is dominated by the electronic conduction (transference

data of the B&O, phases [29]

Phase Phase stability temperature Temperature (K)

range (K)

Structure a (nm) 6 (nm) c (nm) P’ (“) * May persist to room temperature.

(Y

6

a

Y


1002-1097 1047 fee 0.56595

603-923 916 tetragonal 0.7738

773-912 298 bee

112.977

0.5731

1.0268

*

182

P. Shuk et al. I Solid State Ionics 89 (1996) I79- 196

one out of every four oxygen sites is vacant in a fluorite-type structure; (2) the electronic structure of Bi3+ is characterized by the presence of 6s2 lone pair electrons, leading to the very high polarizability of the cation network which favours oxide ion mobility; (3) the particular ability of Bi3+ to accommodate highly disordered oxygen surroundings. A further advantage for electrochemical application is the tendency of Bi3+ to promote the dissociation of oxygen molecule. Although the S-phase of Bi,O, exhibits the highest oxide ion conductivities known so far, its use is limited because it is only stable in the narrow temperature range 1002-1097 K.

0

-1

z Y m

g-2

4 -3

700

800

900

IO00

1100

2.3. Thermal expansion

T(K)

Fig. 3. Electrical

conductivity,

log u versus temperature

of Bi,O,

P81.

number ti,,<0.002 [39]); holes are the mobile charge carriers. The electronic defect concentrations are determined by impurities. These properties are found in the entire temperature range up to the a+S phase transition. In the temperature range 923-1002 K a rapidly increasing concentration of oxygen vacancies leads to an increase of the ionic conductivity [28]. In the metastable p- and y-phases, the conduction is predominantly ionic [28,39]. The activation enthalpies as determined from the temperature dependence of the ionic conductivity are 132, 95 and 38.5 kJ/mol for the /3-, y- and S-phase, respectively [28]. According to Harwig [28,38], &B&O, exhibits high ionic conductivity with mobile oxide ions as the majority charge carriers. This is in agreement with the results of electromotive force and transference measurements by Takahashi et al. [ 11,401. The conductivity of the a-phase is independent of the oxygen partial pressure at least down to 10e3 Pa [28]. The high intrinsic disorder and the high mobility of the oxide ions are consistent with the change in entropy and the structure of the S-phase. Hightemperature neutron diffraction experiments confirmed that the oxygen sublattice is disordered and liquid-like [29]. Recently, Mairesse [41] summarized the reasons for the high oxide ion conductivity of &Bi,O,: (1)

The thermal expansion coefficients of B&O, were first determined by Gattow and Schroeder [32]. They found very high thermal expansion coefficients for the a-phase of Bi,O, (Table 2). Levin and Roth [26], however, found thermal expansion coefficients that were a factor of two lower than those from high-temperature X-ray investigations. Typical values of the thermal expansion coefficient of different phases of B&O, are summarized in Table 2. The transition from the 6- to the P-phase is accompanied Table 2 The average phases Temperature range (K)

thermal

expansion

coefficients

(Y’ of the B&O,

Middle thermal expansion coefficients (Y’ (10-“/K) a

373-413 473-673 673-848 848-948 948- IO23

12.2 12.4 14.2 14.8

298- 1003 1003- 1098 913-298 923-773

11.0

400-800 940- 1020

12.9

400-800 949-1025 930-840

10.6

P

Y

Reference

6

~321 43.6

23.0

WI 20.0 24.0

20.0

20.2 19.2

1371 99.0%

[371 99.9%

183

P. Shuk et al. I Solid State Ionics 89 (1996) 179- 196

by a large sudden volume change and a deterioration of the mechanical properties of the material.

Table 3 The crystal phases observed in annealed and quenched of (Bi,O,),-<(Ln,O,), (? unidentified phase) [49]

ho, 3. Solid electrolytes oxide

based on modified

bismuth

In their original work, Takahashi and coworkers demonstrated that the high temperature phase with high ionic conductivity can be stabilised to lower temperatures in a similar way as known for cubic zirconia, i.e. by substitution of aliovalent cations for Bi. Bismuth oxide easily forms solid solutions with many other metal oxides. Different structures are found including fee, rhombohedral and tetragonal structures on the B&O,-rich side of the binary systems [42,43]. The stability region of the high ionic conductivity phases can be extended to room temperature by incorporation of 22-27 mol% WO, [44], 25-43 mol% Y,O, [45], 35-50 mol% Gd,O, [46], 17.5-45 mol% Er,O, [47], 28.5-50 mol% Dy,O, [48], 30-40 mol% Sm,O, [49], 15-26 mol% Nb,O, and 20-25 mol% Ta,O, [50], lo-35 mol% Pr,O, 66 [51,52], and 30-50 mol% Tb203_5 [52,53]. 3.1. Solid electrolytes Bi,O,

Crystal phase observed Quenched

specimen

Ln

x

Annealed specimen

La

0.15-0.28 0.30-0.40

Rhomb Rhomb + LaOF-type LaOF-type Rhomb + fee Rhomb Rhomb + LaOF-type LaOF-type Rhomb + ? fee fee Rhomb fee Rhomb + fee fee LaOF-type Rhomb + ? fee fee fee+? fee fee fee+? Tetragonal Tetragonal + ? fee fee fee+?

Nd

Sm

Rhomb

0.50 0.10-0.30 0.40 0.50 0. IO-O.20 0.30 0.35

DY

0.40 0.50 0.20

Er

0.30-0.40 0.20

Yb

-

0.25-0.30 0.10 0.20 0.30

La

based on solid solutions of

3.1.1. Structure Most of the ion conducting solid solutions of B&O, form with the fee structure of the high temperature &B&O, phase. In addition, a rhombohedral structure corresponding to the rhombohedral Bi,O,-SrO structure (analysed by Sillen et al. [54]) or a rhombohedral LaOF-structure were found [55]. The type of structure that forms is largely dependent on the ion radius of the substituted metal cations and on their concentration. Some examples of the relationship between the structure and the composition in solid solutions of Bi,O, with rare earth oxides are summarized in Table 3. Fig. 4 shows the regions of fee and rhombohedral phases which are stable at room temperature as a function of the ionic radius of Ln3+ and its content [49]. The thin lines show the limits of accuracy due to the concentration measurement at relatively low and high Ln,O, contents. Generally the rhombohedral phase is formed in the systems doped with Ln,O, (Ln = La,

specimens

-

Nd

-

Sm

-

Gd

-Y -

I 0

0.2

I

I

0.4

0.6

Er

I 0.8

1.0

x

Fig. 4. Stability ranges of the rhombohedral and fee phases of (Bi,O,),_,(Ln,O,)” in the ionic radius vs. composition diagram [491.

Nd, Sm, Gd), with relatively large rare earth ion radii and relatively low x in (B&O,), _,(Ln,O,)x. The fee phase of B&O, is stabilized by cations with smaller cationic radii than Bi3+ and relatively high concentrations. Verkerk et al. [48,58] assumed that

184

P. Shuk et al. I Solid State lonics 89 (1996)

stabilization of the relatively loose high temperature &phase structure occurs by a contraction of this structure due to the substituent. If the difference between the ionic radius of Bi3+ (0.111 nm [59]) and the substituted Ln3+ is large, the substitution will result in a large distortion of the host lattice and only a small amount of substituent is necessary for supplying the energy required to stabilize the fee phase of Bi,O,. Conversely, a small difference between the ionic radii of Bi3+ and Ln3+ requires a large amount of substituent to stabilize the fee phase, as for example is the case for Gd3+ (Fig. 5). Too large a difference between the ionic radii of the Ln3+ and Bi3+ ions makes the fee phase unstable [48]. A large number of ternary and quaternary Bi,O,based oxides have been synthesized and characterized [60-631. The substitution of two different metal oxides instead of only one favored the stabilization of the S-phase down to room temperature at distinctly lower concentration of the oxides [62]. This cooperative effect was attributed to the entropy increase in the quaternary systems, which is the main factor determining the defect properties of the fee phase. It was shown by Battle et al. [64] that in

0.35

0.30

0.25 -2 * 0.20

0.15

L

0.098

0.100

Fig. 5. x,,,, the minimum phase in Bi,O,),_,(Ln,O,), [481.

0.102 0.104 r ion @ml

0.106

value of x required to stabilize the fee vs. the ionic radius (r,,,,) of Ln’+

179-196

(B&O,), _JY203)+ oxide ion ordering occurs along the (111) and (110) directions. Subsequent neutron scattering and X-ray diffraction experiments on fluorite-type solid solutions (B&O,), _.,(Ln,O,), (Ln=Y, Er, Yb) have revealed substantial shortrange ordering, the extent of which increases with increasing content of the substituent [65-671. A new phase, found by Watanabe et al. in the bismuth-rich region of the systems Bi,O,-Y,O, and B&O,-Ho,O, has a [W [691 Bi 0,765Sr0,2350,,383-tyPe layered structure with hexagonal symmetry and shows high oxide ion conductivity [70-721. 3.1.2. Ionic conduction Verkerk et al. [48,58] studied the relationship of the ionic radius of the Ln3+ substituent ion and the minimum amount of Ln,O, (xmin) required to stabilize the fee phase of Bi,O, and the affect of these factors on the conductivity. The correlation between the ionic radius and x,,,~” is given in Fig. 5. There are two opposing tendencies that must be optimized for maximal conductivity [48]: first, the ionic conductivity increases with increasing ionic radius. Second, x,,,~” increases with increasing ionic radius (Fig. 5), however, a high x,,,~” value results in low oxide ion conductivity. Nevertheless, the influence of the ionic radius on the ionic conductivity is smaller than the affect of the Ln,O, content. Therefore optimum ionic conductivity is achieved by lowering x,,,~“. The highest oxide ion conductivity occurs at the lowest x,,,~” required to stabilize the fee phase for B&O, stabilized by Er,O,, as shown in Fig. 6 [48]. In a highly defective structure a completely random arrangement of oxygen vacancies is unfavourable and is only possible in small domains. A model of the short-range ordered units, or ordered microdomains proposed for (Bi,O,),,,,(Ln,O,),,,, by Verkerk et al. [73] is shown in Fig. 7. For this composition every tetrahedron consists of three Bi3+-ions and one Ln3+-ion, denoted as a (Bi,Ln)tetrahedron. Fig. 7 shows the oxide ions of a (001) plane at z = 3 14. The cations above and below this plane are indicated. The oxide ions are displaced in the direction of the lanthanide ions. It is clear from Fig. 7 that there are two different O-O distances. Indeed, from diffuse neutron scattering studies of

P. Shuk et al. I Solid State Ionics 89 (1996)

/“--

r-

713 K

Yb3’

0.098

Er

3+

0.100

Y3+Dy

3+

3+ Gd

0.102 0.104 r IO” (nm)

0.106

Fig. 6. Electrical conductivity, log (T of (Bi,O,),_,(Ln,O,), x,,, vs. the ionic radius of the substituted Ln’+ [48].

l 02Fig.

7.

(Bi,O,),

A

0 model

,,(Ln,W,

-oxygen vacancy

of

the

25 [731.

n

ordered

-Ln’+

for

0 - Bi3+

microdomain

unit

for

179-196

185

B&O, solid solutions at lower temperatures (<870 K) two types of O-O distances have been identified: 0.268 nm and 0.290 nm respectively [73]. At about the same temperature (870 K) a knee in the Arrhenius plot of B&O, stabilized with Y,O, [45], Gd,O, [46], Er,O, [47] or Dy,O, [48] was reported. The anomaly was ascribed to a change in the defect structure. The activation energy of the conductivity is determined by the strength of the Ln-0 bond and by the energy necessary for 02- ions to migrate through the tetrahedral planes. In the low temperature region the effect of the Ln-0 bond strength is predominant. Only oxide ions in the (Bi,Ln)-tetrahedron are mobile. Migration of an oxide ion from one tetrahedron to an empty site in the next one involves breaking of the Ln-0 bond and passage through (Ln,Bi)-, (Bi,Ln)or (B&)-tetrahedral planes. At about 870 K the lattice disorders, resulting in a increase of the Ln-0 distance. In the high temperature region all the oxide ions take part in the conductivity process and there are no preferential diffusion paths. Detailed calculations are necessary to determine whether the strength of the Ln-0 bond or the energy to migrate through a tetrahedral plane is predominant at high temperatures [73]. Typical solid electrolytes based on Bi,O, are summarized in Table 4. Most of them show fee or rhombohedral crystal structure. Fig. 8 shows, that solid electrolytes based on Bi,O, have one or two orders of magnitude higher oxide ion conductivity than zirconia. stabilized The y-type B&V, _XMx05,5_y (M = Cu, Co, Ni) also present very attractive conductivities, especially at low temperatures and will be discussed separately. 3.1.3. Hole and electron conduction The partial electronic conductivity of Bi,O,-based oxide ion conductors was studied by cell voltage (e.m.f.) and polarization measurements by Takahashi et al. [75]. The hole (a,), electron (a,) and oxide ion (ui) conductivities versus oxygen partial pressure ~(0,) in Bi,.,,Y,,,,O,,, are given in Fig. 9. The partial pressure dependence of the electron and hole conductivities correspond to aP -p( 0,)’ ‘4 and c,, which indicate that the following defect P(O*)-“4, equilibria are established in the presence of the large and constant concentration of oxygen vacancies:

186

P. Shuk et al. I Solid State lonics 89 (1996)

Table 4 Typical solid electrolytes

179-196

based on B&O, [12,13,41,47,48,52,56,57] Oxide ion conductivity

Composition

q (S/cm)

Structure

173 K

923 K

1153 K

% xssY,, &, 92 BiO 15 Bi,, &%, , ,O, 44T Bi,, ,,Ba,, , ,O

4.6. IO-’ _

3.8. 1om2 _

6.0. IO-’

1.1.10-2

5.0. IO_’ 0.7

0.02 2.3 (1073 K) 0.27

Bi,,.,,f’b,, &: :P’ Bi,,,,Y,, &, s Bi,, &dO ,,O, 5 Bi,, ,,Tb,, ,,O ,4+fi

lo-’ 1.3. lo-2 3.5.1o-J 3.4. lo-’

1.1 0.11 0.056 0.012

Bi,, x,,Tb,, &, 5+fi Bi,, ,,DY,~.,,O, 5 Bi,, 71Ho,1.250 5 Bi,, 7&2501 J Bi,,.J% #, 5 Bi,, ,5Tml, &, 5 Bi,, 7 Yb,, 20, 5 Bi 7&I ,sO, 0 5 Bi,, xoSmO 2,P1 5

4.2.10-’ 1.35.10-* l.35~lo~2 1.27.10-’ 2.1. 1o-2 7.10-’ 8.0. IO-’ 3.3.10-’ 2.3.10-1

0.28 0.12 0.17 0.135 0.23 0.08 0.074 0.037 0.02

Bi,, x,$‘ro & Is+6 Bi,,,,Nb,,,,O, hS Bi,, xc,Ta,,2,PI ,(, Bi,, &o,, r201 hX Bi,,.,,W, ,,O I 6X B&V,, &r,, ,A 35

3.0.10+ 1.1.10-* 5.0. lo-1 6.0. IO-’ l.o~lo-* 5.6. 1O-2

0.02 0.11 0.0 1 0.017 0.04 1 0.13

_ 0.35 0.26 0.28 0.75 0.46 0.40 0.43 0.52 0.40 0.40 0.28 0.15 0.15 0.50 0.073 0.11 0.15 _

o-

T(K) 1200

1100

1000

900

0

973

10”

800

K 873

-2

-

K

773

I

-10 -5

-3

I

-I

I -log

8

fee fee rhomb rhomb fee fee fee fee fee fee fee fee fee fee fee fee fee rhomb fee fee tetrag fee tetrag

I 3

I 5

I

I 7

PCJ 2 ) (Pa)

Fig. 9. Oxygen partial pressure dependence of the hole and electron conductivities of (Bi,O,),, ,,(Y,O,),, 27 solid electrolyte 9

10

II F4

Fig. 8. Oxide ion conductivity solid electrolytes [28,47,74].

12

13

[751.

(K-I)

versus temperature

0,

+ 2V,

20:

tj

H 200” + 4h’,

(2)

for selected

2V,

+ 4e’ + 0,.

(3)

I? Shuk et al. I Solid State Ionics 89 (1996) 179- 196

The hole and electron conductivity of Bi,,,,Y,,,,O,,, can be expressed by the following equations [75]: ~~,/S*cm-’

= 5.0. 102[p(0,)lbar]“4 .exp[-106kJI(RT~mol)],

u,/S.cm-’

(4)

= 3.4. 105[p(02)lbar-“4 . exp[-213

kJI(RT. mol)].

(5)

Bouwmeester et al. [76] performed isothermal oxygen permeability experiments on sintered and obtained a p-type electronic Bi,.,,Er,.,,O,., conductivity similar to that of the Y substituted phase: a,/S.cm-’

187

these crystals were twinned. Bi,VO,., is in fact the upper limit of solid solution formation in B&O; xVO,s with 0.86-(x< 1. On heating, the room-temperature a-Bi,VO 5.s phase transforms through the P-phase (stable at 720-840 K) to the tetragonal y-Bi,VO,, phase [77]. a-Bi2V05,5 can be indexed in a face-centered orthorhombic cell with a = 0.5533(l), b=0.5611(1) and c= 1.5288 nm. The mean cell parameters of the p form seems to be tetragonal with a = b = 1.1285(S), and c = 1.542( 1) nm at 775 K. The y-phase is tetragonal with a = 0.3988(2), c = 1.542( 1) nm at 885 K and space group 14/mmm [78]. Thus the prototype y-cell is a subcell of the (Y and the p cells according to the relationship [77]:

= 3.0. 102[p(0,)lbar]“4 . exp[-91

kJI(RT.

mol)].

(6)

These values of hole and electron conductivities of Bi,O, solid electrolytes are much lower than the oxide ion conductivity in a broad oxygen partial pressure range. Defect electron conductivity dominates oxide ion conductivity only at very low oxygen partial pressures p(02)<10-‘6 Pa at 1000 K.

ap#2d2a,,

b,#d2a,,

cs#c,.

The idealized structure of the oxygen vacancies, is presented

y-phase, in Fig.

without 10 with

3.2. BIMEVOX solid electrolytes The latest generation of attractive oxide ion conducting solids is known under the short term BIMEVOX (BI-bismuth, ME-dopant metal, V-vanadium, OX-oxygen). It denotes a family of multinary oxides based on Bi2V0,,, [77-971. This phase is a solid solution of Bi,O,*VO,,, whose compositions covers the range -66.7 to 70.4% Bi,O,. The major part of a recent review by Mairesse [41] focuses on BIMEVOX, obtained from the parent compound BiZV05,5 by substituting other cations for vanadium. 3.2.1. Structure The structure of B&VO,., is of Aurivillius-type [93] with [Bi20212+ layers sandwiched between defect [VO,,,(V,),,,]*perovskite-like slabs, with the perovskite slab containing oxygen vacancies Vi responsible for the high oxide ion conductivity [77,78]. Bush et al. [94] first identified Bi,VO,,, as an orthorhombic phase with unit cell parameters a = 1.662, b = 1.684 and c = 1.55 nm. Owing to the large unit cell volume, Mairesse [41] presumed that

Fig. 10. The ideal structure

of y-type

Bi,VO,

5.

188

P. Shuk et al. I Solid State Ionics 89 (1996) I79- 196

typical alternating B&O, layers and VO, perovskite like sheets. Compared with the ideal tetragonal structure, the structure of B&VO,,, exhibits a splitting of all the atomic positions except 0( 1). The vanadium is in a highly distorted octahedral site. It is assumed that the numerous oxygen vacancies in the perovskite-like sheets, and the correlated shifts of the cations from their ideal positions are responsible for the high oxide ion mobility and the low activation energy in this phase. In the past it has been shown that the partial substitution of other metal ions for V suppresses the r-+P or y+a transitions, allowing the highly conducting y-structure to be stabilized to room temperature. Recently Lazure et al. [97] reported about possible substitution for vanadium with both Co and Bi. 3.2.2. Electrical conductivity The compound Bi,VO,,, exhibits a relatively large oxide ion conductivity in the high-temperature y-phase between 840 and 1050 K [77]. Many elements can substitute for vanadium to form y-type solid solutions. However, some of the y-type compounds are metastable at room temperature [41]. In order to achieve better conductivity at lower temperatures, vanadium was partially substituted by other elements including Cu, Ni or Zn [82-841, Pb [86], MO [87], W [SS], Co [89,97], Ti, Zr, Sn and Pb [90]. Depending on the chemical nature of the substituent M in Bi2V,_,M,0s.s_y, the critical concentration values x,,, limiting the y-solid solution domain differ considerably and have a maximum value (x =0.5) with Nb and Sb [95,96]. However, the minimum value of x is often around 0.10 independent of the formal valence of M [41]. Furthermore, the best conductivities are always obtained with an x value close to 0.10, whatever the dopant cation in All the BIMEVOX (M=Cu, B&V, -xMxG5.5-1% Ni, Zn and Co) exhibit the same temperature dependence of the conductivity with a more distinct change of the slope in the case of Zn (Fig. 11). Double substitutions in Bi,VO,,S, either on the V site only, or simultaneously on both the Bi and V sites, do not improve the oxide ion conductivity relative to the singly substituted phases [84]. Recently Yan and Greenblatt [90] reported the highest oxide ion con-

T (K) 1000 900 800

700

600

500

-1

-2 -z x D B -3

-4

8

10

12

14

16

18

:

+4(K-l) Fig. 11. Oxide ion conductivity of selected with M=Cu, Co, Ni and Zn [41].

Bi2V,j,Mo,,0,

ductivity value in BIMEVOX family ((r=4. S/cm at 500 K) in y-Bi,V,,ssTio,ls0,,4~~. 3.2.3. Electrochemical Two compositions

5-1

10e4

properties of the BIMEVOX family, and Bi,V,,,Ni,,,O,,,_, were %V,PO. IO,., -y studied as solid electrolytes in electrochemical cells with various electrode materials such as gold and strontium substituted lanthanum manganite [79]. Significant, about an order of a few percent, electronic contribution to the total conductivity was evident, which plays a decisive role in the electrode polarization. In marked contrast with common conventional oxide ion conductors such as stabilized zirconia, the electrode polarization decreases with decreasing oxygen partial pressure, because of the increasing electronic conductivity. The cathodic voltage limit of the stability range of Bi,V,.,(Cu/Ni),,,O,.,_, solid electrolytes was determined using the relaxation potential of a tip electrode after a reduction pulse and was found to be

P. Shuk et al. I Solid State Ionics 89 (1996) 179- 196

189

below -300 mV with respect to air [79]. At 800 K this value is equivalent to an oxygen partial pressure of 5. 1O-4 Pa.

4. Mixed conductors

based on bismuth

oxide

Mixed conductors based on B&O, are of principal interest as electrode materials and semipermeable membranes for use in electro-catalysis and in gas separation. Solid solutions of B&O, with mixedvalent ions can lead to mixed electronic and ionic conduction. Studies of a number of B&O,-based mixed conductors e.g. Bi,03-Tb,O, [52,53,56,981001, B&O,-Pr,O,, [51,52,98,99], B&O,-BiVO, [101,102] and Bi,03-Y,03-TiO,_, [103] have been reported. Some excellent mixed-conducting materials with high oxide ion conductivity, close to the conductivity of &Bi,O, and comparable p-type electronic conductivity, were found in solid solutions of Bi,O, with Tb,O,. The hole conductivity decreases with decreasing Tb4+ concentration and oxygen pressure [ 1011. Some molybdates, Bi,Mo,O,,, P-Bi,Mo,O,,, Bi,Mo,O, and Bi,MoO, have shown very high ionic-electronic conductivity even at temperatures as low as 573 K [104-1061. A two-phase system consisting an intimate mixture of an electronic conductor and an oxide ion conducting phase is another approach to obtain a mixed electron-ion conducting material. Both phases should exhibit continuous conduction paths in the mixture. Shen et al. [107] recently found promising results for mixed conduction in the ceramic twophase system consisting of oxide ion conducting Bi ,,75Y,,250,.5 and the electronic conducting tetragonal phase Bi,CuO,_,. Mixed-conducting oxides based on Bi,O, used as electrode materials can have a number of advantages in comparison with traditional noble metal electrodes. In the case of metal electrodes (e.g. Pt), charge transfer between the electrode and the solid electrolyte occurs at or near the three-phase gas/ electrode/solid electrolyte interface (Fig. 12). A potentially better electrode material is one for which oxygen exchange is not limited to the three-phase contact region. At a mixed-conducting electrode, the

solid electrolyte v;;,o; solid electrolyte

Fig. 12. Scheme of electrode reaction at the gas/solid electrolyte/ electrode interface for: (a) metal electrodes, (b) mixed-conducting electrodes.

charge transfer reaction occurs over the whole surface, because both oxide ions and electrons are mobile, and so polarization losses are reduced [99,108]. Mixed conductors based on B&O, are presented in Table 5 in comparison with some other mixed conductors.

5. Electronic surface properties electrolytes

of Bi,O,

solid

Recently the electronic surface properties of Bi,O,-based solid electrolytes have been studied by UV- and X-ray-photoelectron-spectroscopies (UPS, XPS) as well as electron-energy-loss-spectroscopy (EELS) [109,110]. UPS yields information on the work function of electrons and the ionisation energy of the surface. Using solid metal-metal oxide mixtures (such as Fe-FeO) as solid reference contacts during the photoemission experiments, the position of the Fermi level was obtained with relation to the valence band edge and for the corresponding oxygen partial pressure (fixed by the metal-metal oxide system) [ill]. In addition EELS was used to determine the energy difference between the highest occupied and lowest unoccupied electronic levels, i.e. the band gap between the valence band and the conduction band. A band gap E, =2.8 eV was derived from the EEL spectrum [ 1 lo]. The data from UPS and EELS suggest the band scheme in Fig. 13.

P. Shuk et al. I Solid State tonics 89 (1996)

190 Table 5 Properties

of mixed-conducting

Bi,, KSPrO,$,

oxides Ionic transport

Composition

s+8

179-196

Ionicconductivity u, (S/cm)

Temperature

number (t,,,, ) 0.93

0.91

973

Bi,175%2501s+6

0.92

973

Bi o w,Tb,, o,s:::+fi Bi,, ,,,Tb,, J’, s+8 (Bit, 7sYI, J), A, v5

0.88 0.9 1 0.85 0.86

0.29 0.2 1

(Pro, H31)“M Ce,,.,,,Tb,,.&-, La,, ,Sr,,.,,,Co,,,Fe,, Bi,, &o,,& 2s

0.18 0.007 0.51

Bi,, d'rO,,O

A

N [cpsl

Fig. 13. Band scheme for cubic stabilized Bi,,,,Y,,,,O, s as derived from UPS- and EELS results (at T=873 K). The energy scale is referred to the energy E, of the valence band edge [I 101.

Electronic conductivity is predicted for such a solid, if EF as a function of ~(0,) approaches the conduction band edge to within of about 0.5 eV. However, disordered solid compounds and in particular solid electrolytes often show additional electronic states in the band gap near the band edges (band tailing). Such localized electronic states, due to the high ionic disorder in the oxide lattice, were found in bismuth oxide as well as in stabilized zirconia [ 110,112,113], Therefore, electronic conductivity can arise in these systems by electron hopping through the localised electronic states near the band edges. The Fermi level of bismuth oxide at 900 K is 0.7-0.9 eV below the conduction band

Reference

T(K)

1.2 0.38 0.22

973 973 973 973

0.0147 1.8 0.00324

873 1073 873

WI WI [=I 1521 [521 [2Il [IO61 [IO61 [I061

edge. This limits the electrolytic domain of the solid electrolyte towards low oxygen activities, i.e. under reducing conditions. Potentials of -1 volt should already lead to electronic conduction in the bismuth oxide electrolyte at the cathode. The electrolytic domain at low oxygen partial pressure depends on the composition. The band gap, being practically temperature independent in the temperature range 700-900 K, increases with Y,O, concentration and the position of the Fermi level E, measured from the valence band edge E, shifts slightly to higher values: for Bi,.,,Y,,,,O, ,5 E,= 2.8 eV and for Bi o,60Yo,400,,5 E,=3.2 eV. The position of the Fermi level with Fe/Fe0 reference contacts was E,-E,=2.1 eV for Bi,,75Y0,250,,5 and E,E,=2.3 eV for Bi0,60Y0,400,,5. These results are comparable with energy gap data (E,=3.1 eV) for &B&O, thin films deduced from optical adsorption spectroscopy [ 1141. XPS-spectra of the 01s level of Bi, _,YIO,,, (x= 0.25-0.40) samples at higher temperatures (7001000 K) can be deconvoluted into two contributions, one with a maximum at Eb= 529.0eV and a second at 531.4 eV. These two oxygen peaks are attributed to lattice oxygen (lower binding energy) and adsorbed oxygen or hydroxyl groups (higher binding energy), respectively. Similar results have been obtained for thin oxide layers on Bi-metal [115] or Bi containing superconducting oxide phase [ 116,117]. If the temperature increases above 673 K, the lattice oxygen peak becomes more intense due to desorption of adsorbed species.

P. Shuk et al. I Solid Stare Ionics 89 (1996) 179-196

6. Electrochemical

properties

The electrochemical behavior of the electrode/ solid electrolyte systems depends on many factors such as oxygen partial pressure, temperature, microstructure of the electrode and composition of solid electrolyte and electrode materials. Sputtered Pt and Pt-gauze electrodes were investigated on ZrO,Y,O,, CeO,-Gd,O, and Bi,O,-Er,O, solid electrolytes by Verkerk et al. [ 1181 by means of d-c measurements as a function of temperature and oxygen partial pressure. The electrode process was found to be strongly influenced by the nature of the solid electrolyte. The electrode resistance for Pt electrodes on B&O,-Er,O, solid electrolytes was found to be much lower than on ZrO,-Y,O, and CeO*-Gd,O, solid electrolytes. With zirconia- and ceria-based solid electrolyte materials, diffusion of atomic oxygen on the Pt electrode was assumed to be the rate-determining step in the electrode process, whereas for bismuth oxide based materials diffusion of adsorbed gas on the oxide surfaces was more likely the rate-determining step [ 1181. A surface control in the oxygen exchange kinetics has been found by Boukamp et al. [ 119,120] from the direct measurement of the gas-phase isotope ratios in “0 exchange experiments on solid solutions of B&O, with Er,O, or Tb,O,.,. For Pt electrodes on Bi,O,Er,O, solid electrolytes frequency dispersion measurements showed that two circuit elements are sufficient to describe the impedance, a resistance R and a Warburg impedance, W connected in parallel. From a comparison with other solid electrolyte/ electrode combinations, it was concluded that R was controlled by the diffusion of atomic oxygen along the electrolyte surface to the electrode. A Warburg impedance was typical for electrodes on B&O,based solid electrolytes and it was suggested that bulk transport of electronic charge carriers in the solid electrolytes was the rate determining process of the Warburg coefficient. At high oxygen partial pressure ~(0,) the d-c resistance of the electrode process was determined by the resistance R (diffusion on the surface of the solid electrolyte) and at low ~(0,) by the Warburg impedance (diffusion through the solid electrolyte) [ 1181. The behavior of Ag, Pt and La, _,Sr,CoO, (x=00.7) electrodes on Bi,_YEr,yO,,, (y=O.15, 0.20)

191

solid electrolytes was studied by Nagamoto and lnoue [ 1211 by means of a current interruption method. The electrode resistance of noble metals was found to be approximately proportional to ~(0,)~“* suggesting that the rate-determining step is the diffusion of dissociatively adsorbed oxygen. The dependence of the resistance of cobaltite electrodes on the oxygen partial pressure varied from - 1/4th order at lower temperature to - 1/2th at higher temperature. It was concluded that charge transfer as well as diffusion are rate controlling, and that the degree of contribution of each process changes with temperature. The contribution of this diffusion to the electrode impedance decreased with increasing Sr concentration in lanthanum cobaltite [121]. It was demonstrated by Tanabe and Fukushima 11221 that the preparation method of the electrode material had a great influence on its electrochemical properties. It was shown that a thermal decomposition method for cobaltite electrode preparation gave favorable polarization properties of the electrode/electrolyte system. The anodic and cathodic polarization behavior of sputtered porous gold electrodes on Bi,,,,Er,,,,O,,, solid electrolytes were studied by Vinke et al. [ 123,124] as a function of temperature and oxygen partial pressure using a three-electrode cell. Comparison with the polarization behavior of sputtered Pt electrodes on the same solid electrolyte composition showed little effect of the electrode material on the exchange current densities. This result indicates that the solid electrolyte surface was actively participating in the oxygen transfer process. The noble metal electrode serves merely as an electron current collector. Analysis of the electrode impedance showed strong influence of surface diffusion on the electrode reaction [124]. The polarization resistance of doped cobaltite electrodes on Bi,,,,Y,,,,O, .5 solid electrolytes was determined from the steady-state current density overpotential curves, which were measured in a three electrode arrangement [ 125,126]. Cobaltites are active mixed conducting electrodes, particularly at low temperatures. At temperatures below 800 K the polarization resistance of all investigated cobaltite electrodes was below the value of active silver electrodes (Fig. 14). This finding is connected with relatively weakly bound oxygen species formed at the surface of the perovskite-type oxide. These

192

P. Shuk et al. I Solid State tonics 89 (1996)

Table 7 Fqrilibrium oxygen pressure trolytes from electrochemical

T W) IO00

900

700

800

179-196

of (B&O,), _,(Me,O,), measurements [ 1331

Composition

Equilibrium sure fog ~(0~)

Bi@, (Bi,O,),_X(Y,O,)X

(Bi,W-JDy,O,),

(Bi,W-X(Er,O,),

IO

11

12 F4

13

14

(1h L%,% ,CoO, CV, Pr,,,% $00, (3), Nd, ,Sr,, $00, $L

oxygen

pres-

(Pa)

873 K

973 K

8.01+0.02 8.45~0.10 8.8OkO.07 9.23kO.10 860~0.07 8.90t0.10 9.31 kO.10 8.63kO.10 9.01~0.10 9.55kO.10

5.8OkO.04 6.09kO.08 6.43 20.05 6.8OkO.08 6.18+0&l 6.44kO.08 6.71 lrO.08 6.30t0.08 6.56kO.08 6.92kO.08

15

(K-’ )

Fig. 14. Temperature dependence of polarization resistivity rP of various electrodes with Bi 0 7SY0250,.5 solid electrolyte [126]: Ag SrCo,,Pe,,

x=0.25 0.30 0.37 x=0.30 0.35 0.40 x=0.27 0.32 0.37

solid elec-

(4).

(5).

According to Takahashi et al. [ 1281 the overall decomposition reaction of B&O, solid electrolytes is described by: (B&O,),-,(Me,O,),

~(Bi,O,),-x~,(Me,O,),

+

2cuBi + 3a/2 0,. oxygen species are assumed temperatures and to enhance step of the electrode reaction charge transfer step [ 1271.

to be active at low the rate determining corresponding to the

7. Stability range At high temperatures, solid electrolytes based on B&O, are unstable towards a reduction to metallic Bi under low oxygen pressure. For practical applications, it is very important to know the limiting oxygen partial pressure where reduction occurs.

(7)

Thermodynamic data on S-B&O, and their solid solution have been obtained by electrochemical measurements. Data shown in Table 6 indicates good agreement between various investigators. Verkerk and Burggraaf [132] showed that the range of ~(0,) where (B&O,), _X(Ln,O,), solid electrolytes are stable against reduction did not increase by addition of Dy,O, or Er,O, with respect to pure S-Bi,O,. However, later electrochemical investigations of Berezovskaya et al. [133] showed a considerable increase of stability of (Bi,O,), _X(Ln,O,), solid electrolytes with increasing dopant concentration (Table 7).

Table 6 Temperature dependence of the standard Gibbs free energy for the formation of S-B&O, and solid electrolytes electrochemical measurements (referred to 1 mole B&O,) Composition

AG”=a+bT -a

S-Bi,O, S-B&O, &Bi,O, Bi o @Y,.,, 0 15 Bi 0 &rO 4&I 5

(kJ/mol)

561k2 558 56455 561’5 55927

(kJ/mol)

based on Bi,O,

from

Reference b.10” (kJ/(mol-K))

Validity range (K)

26529 272 268+5 271+10 268213

795-1095 991-1095 949- 1076 750-950 750-950

[12gl 112% 11301 [I311 [I321

P. Shuk et al. I Solid State Ionics 89 (1996)

Furthermore, in the rhombohedral Bi,O,-SrO system, the stability in reducing atmospheres increases with increasing Sr concentration. The oxygen partial pressure for decomposition of Bi,O, determined from the known thermodynamic data were compared with calculated values of decomposition oxygen partial pressure of the (Bi,O,),,,,(SrO), 4X rhombohedral phase (Table 8). Further, the cubic phase in B&O,-Ln,O, systems has been found to be unstable below about 970 K. It undergoes a transformation to a mechanically unstable rhombohedral phase [ 1341. The phase transformation can be suppressed by addition of aliovalent dopants (e.g. ZrO,), which is assumed to suppress the cation interdiffusion [ 1351. Wang et al. [ 1361 obtained unexpectedly favorable results on yttria stabilized bismuth oxide which showed an extended stability interval for oxygen partial pressures down to lOpI6 Pa at 973 K. These data were confirmed by Jurado et al. [137]. The potential use of B&OX-based solid electrolytes in solid oxide fuel cells was examined by Virkar et al. [135,138] and very recently reviewed by Azad et al. [139]. It was shown that by carefully optimising the conduction properties in two-layer electrolytes consisting of a thin layer of zirconia on B&O,-based materials the interface partial pressure of oxygen can be maintained high enough to prevent electrolyte reduction. The range of oxygen partial pressures over which B&O, can be used as a solid electrolyte with predominantly ionic conductivity is clearly more limited in comparison to stabilized ZrO, due to its smaller band gap (about 3 eV compared to 5.2 eV for ZrO, solid electrolytes). Finally, solid electrolytes based on Bi,O, are stable at oxygen partial pressure

Table 8 A comparison of the calculated values of the decomposition oxygen partial pressure of pure Bi,O, and a rhombohedral phase in the Bi,O,-St-G system [134] Temperature

Decomposition

T (K)

~(0,)

oxygen

partial pressure

(Pa)

(BizG,),,.5,(Sr%

43

B&G,

853 903

4.96. IO-“’ 8.04. IO-’

1.61. IO-’ 3.37.10-”

943

6.032.10-K

3.03. to-’

p(O,)>

193

I79- 196

10-6-10-7

Pa at 973 K and p(O,)>lO-‘-

10e9 Pa at 873 K.

8. Conclusions Solid solutions with fluorite-type, fee, rhombohedral, tetragonal and layer structures based on Bi,O, are excellent candidates for use as solid electrolytes and electrodes in electrochemical devices. They are applicable at much lower temperatures than devices with stabilized zirconia. Their primary disadvantage at present is the instability against reduction and the limited electrolytic domain. However, development of solid solutions with appropriate composition and improved properties can be expected. Due to the flexibility with which bismuth oxide forms compounds with widely variable compositions and many different substitutions, there is a large potential for further improvements in the properties for medium and low temperature applications. Mixed conductors based on bismuth oxide solid solutions represent an independent branch of compounds which are potentially useful as a general class of electrode materials.

Acknowledgments This work was partially supported by the Alexander von Humboldt Foundation (Germany).

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