Ishihara T. (Ed.) Perovskite Oxide for Solid Oxide Fuel Cells (Fuel Cells and Hydrogen Energy)

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12.5 Dopant

As men tioned, acceptor doping is essential for the protonic conduction to occur

in the perovskite oxides, because hydration (Eq. (12.1)) should occur for the

oxide to become proton conducting. For example, 10 mol% Y-doped BaCeO

can be written as BaCe

0.9

0.1

2.95

, which will be the case in a dry atmosphere

and/or at too high temperature, and will become BaCe

0.9

0.1

when it is

fully hydrated by moisture. Comparing the two formulae, it is clear that the

maximum level of proton concentration is equal to the amount of introduced

dopant, i.e., proton concentration is maximally 10 mol% because the Y dopant

concentration is 10 mol%; when the valence of the dopant cation is doubly

smaller, e.g., substitution of Ca (II) for Ce (IV), the maximum concentration of

proton is also doubled. In this regard, the role of the dopant is simply the

creation of the oxide ion vacancy, symbolized by a in the above written for-

mulae, to receive hydration (entrance of water molecules into oxide ion vacan-

cies) to generate protonic charge carriers. However, actually, the proton

conductivity depends strongly on the type of dopant.

Figure 12.6 shows the temperatur e dependence of the conductivity of

BaCe

0.9

0.1

3–a

(M ¼Y, Tm, Yb, Lu, In, or Sc) measured in moist H

(closed

symbols) and in moist O

(open symbols), respectively, in the temperature range

of 4008–9008C [5]. All specimens show the typical conductivity behav ior of

proton conductor, as already explained in Fig. 12.1. The most significant effect

of the dopant is on the magnitude of the conductivity. In cases whose the

dopant cation is Y, Tm, Yb, or Lu, the conductivities are only slightly different.

However, In or Sc doping causes a significant drop in conductivity. The

0.8

0.9

1.0

1.1 1.2 1.3 1.4

1.5

–1

Centigrade temperature/°C

700 600 500 400800900

–2

–1

ln(σT/Scm

–1

K/T

in H

in O

in H

in O

p(H

O)=1.9 kPa

–3

Fig. 12.6 Arrhenius plot

of conductivity of

BaCe

0.9

0.1

3–a

(M ¼Y,

Tm, Yb, Lu, In, or Sc) in

moist H

(closed symbols)

and O

(open symbols)in

the temperature range of

4008–9008C;

p(H

O) ¼1.9 10

Pa.

Dashed lines indicate

isoconductivity lines

in S cm

–1

.(Reprintedfrom

[5] with permission from

Electrochemical Society)

12 Proton Conduction in Cerium- and Zirconium-Based Perovskite Oxides 251

conductivity in moist H

is plotted as a function of the ionic radius of the dopant

cations in Fig. 12.7( a). showing a significant effect of the dopant cation size on

the ionic conductivity. Y

3þ

(ionic radius, r

¼90 pm), which is the largest

dopant, was found to provide the highest conductivity. The decrease in the

ionic radius up to Lu

3þ

¼86.1 pm) results in a gradual decrease in ionic

conductivity. In case of Sc

3þ

¼74.5 pm) and In

3þ

¼80 pm), which are

much smaller than the former four dopants, the ionic conductivity is lower by

nearly one order of magnitude.

Figure 12.7 shows the electrical conductivity of some proton-conducting

perovskite oxides with different dopants in the same concentration collected

from the literature [25–27]. We can see that there is a relationship between

conductivity and the ionic size of the dopants, the same as the foregoing

discussion, and that there is a suitable size of the dopant cation with respect

to the host perovskite oxides. We should remember that the dopant is a minor

component in the total chemical formulae of the proton-conducting perovskite

oxide; as seen in the figure, even dopant at the level of 5 mol% can strongly

affect the conductivity. Why is the conductivity so strongly depen dent on the

kind of dopants?

Optical absorption studies suggest that there will be at least two different

kinds of sites for protons to reside in the oxide crystal lattice [28, 29]. As already

stated, a proton can exist in the oxide by forming hydrogen bonds to the lattice

oxygen. The O–H vibration modes can be observed in infrared absorption

–3.5

–3.0

–2.5

–2.0

–1.5

800

600

400

Ionic radii of dopant, r

/pm

YTmYb

InSc

log (σ/S cm

–1

)

(a):

75 80 85 90

60 70 80

90 100

–4.0

–3.5

–3.0

–2.5

–2.0

–1.5

–1.0

(d): 600°C

(e) 600°C

(e): 800°C

(d): 800°C

(d): 1000°C

(c): 1000°C

(b): 1000°C

InGa

Log (σ/S cm

–1

)

Ionic radii of dopant/pm

Fig. 12.7 Dependence of electrical conductivities of AB

1–x

3–a

in moist atmospheres

plotted on the ionic radii of the dopant M:(a) BaCe

0.9

0.1

3–a

[5], (b) BaCe

0.8

0.2

3–a

[25], (c) BaCe

0.9

0.1

3–a

[25], ( d) SrCe

0.95

0.05

3–a

[26], (e) BaZr

0.95

0.05

3–a

[27]. Con-

ductivities were measured either in wet hydrogen or in wet argon. (The figure on the left

reprinted from [5] with permission from Electrochemical Society)

252 H. Matsumoto

spectroscopy and are different according to the type of dopant. Yugami et al.

and Omata et al. reported that the spectra in the 

region can be decomposed

into four bands [28, 29]. It is notable that the frequencies of the bands are not so

different by the types of dopants, but the dopants affect the fraction of the

bands [28], resulting in different shapes of the infrared absorption spectra with

the type of the dopants. For example, doping of Sc, i.e., SrZr

0.95

0.05

3–a

that

has comparatively low conductivity, results in a high vibrational component of

a high-frequency O–H band, whereas Y-doping that allows high conductivity

for strontium zirconate leads to the growth of low-frequency bands. From these

observations, Omata et al. concluded that the high-frequency O–H bands will

be formed by less mobile protons and the low-frequency modes will be attrib-

uted to mobile protons [29].

It is the lattice oxygen that associates with a proton. Therefore, the difference

in the mobility of protons will probably originate from the different character of

the oxygen atoms with which protons are associated. In the perovskite struc-

ture, the B-site cation coordinates six oxygen atoms to form octahedra that

connect with one another via oxygen atoms to form a crystal lattice . Accord-

ingly, one oxygen atom has twofold coordinat ion with B-site cations. There-

fore, there are at least two kinds of oxygen atoms, i.e., an oxygen connecting to

one dopant cation and one original B-site cation (O in M–O–B) and an oxygen

connecting to two original B-site cations (O in B–O–B). Yugami et al. and

Omata et al. suggested oxygen that has a bond to Sc will probably be trapping;

a proton trapped to the oxygen atom at Sc–O–Zr has strong hydrogen bonds to

be immobilized, resulting in the growth of high-frequency O–H vibration. The

reverse will be true for the Y-doped case [28, 29].

Kreuer et al. proposed an idea of chemical matching; i.e., the dopan t that

provides the least effect on the basicity of adjacent oxygen best chemically

matches, and both the ionic radius and electronegativity of the dopant will

determine the chemical matching [30]. This idea is consistent with the foregoing

discussion; i.e., the dopant that is indistinguishable from the original cations

will add the least effect on the bonding oxygen, resulting in an environment for

the protons similar to that around the original B-site cations.

With respect to the effect of dopants, there is an interesting observation

about the relationship between the conductivity and chemical stability [5]. The

dopant governs not only the condu ctivity but also the stability, and thus the two

properties are likely to have a trade-off relationship. For evaluation of the

chemical stability of the proton-conducting perovskite oxides, the reactivity

with CO

, i.e., the reaction shown in Eq. (12.5), can be used. As shown in

Fig. 12.4, free energy change of the reaction is positive at high temperature, i.e.,

the perovskite is stable. The free energy change decreases with decreasing

temperature and becomes negative at a temperature; that is, the carbonation

reaction will actually occur below the temperature. The critical temperature

decreases with increasing stability of the oxide so that the reaction temperature

is a good measure for their chemical stability. The reaction temperature can be

determined by thermogravimetry (TG) in the CO

stream. The result obtained

12 Proton Conduction in Cerium- and Zirconium-Based Perovskite Oxides 253

for the specimens during cooling in CO

are shown in Fig. 12.8(b) together with

the temperature program in Fig. 12.8(a). In the TG curves, an increase in the

sample weight indicates the occurrence of carbonation in the reaction

(Eq. (12.5)). The mass changes, Dm/m

, were eventually around 15%, and this

is a reasonable quantity to assume the reaction to have taken place. The TG

curves are not so steep and thus the reaction temperature, evaluated by the

inflection points, will include a significant deviation from equilibrium values to

lower temperatures due to the contribution from the kinetics of the carbonation

reaction. However, the TG curves apparent ly shift and reflect a change in the

stability of the materials with variations in the dopants. Y-doping results in

carbonation occurring at the highest temperature. The reaction temperature

decreases with decreasing ionic size of the dopant cations. Figure 12.9 shows the

reaction temperature (open squares) as a function of the dopant cation

radius; the temperature was taken upside down to show the upper plots having

higher chemical stability. A monotoni cal relation can be seen between the

chemical stability and the ionic size of the dopant.

0 100 200 300 400 500

Temperature

Time/min

1100 1050 1000 950 900

m/m

Temperature/°C

M=Y

(a)

(b)

1100°C

900°C

Fig. 12.8 CO

-reactivity test of BaCe

0.9

0.1

3–a

(M ¼Y, Tm, Yb, Lu, In, or Sc).

(a) Temperature program and introduced gases during thermogravimetry (TG) and (b)

mass change, Dm/m

, during cooling the specimens at –0.5 K/min in CO

flow as a function

of temperature; bars show inflection points that were assigned as reaction temperatures.

A crushed sample of 20–30 mg in a Pt pan was heated in a N

stream up to 11008C, and the

TG analysis was then performed; an exposure to CO

during heating was avoided because

the procedure will result in decomposition of the perovskite-type structure that cannot be

recovered at 11008C. (Reprinted from [5] with permission from Electrochemical Society)

254 H. Matsumoto

In Fig. 12.9, the conductivity isotherm at 4008C is also shown. There is a

clear trade-off relationship between the ionic conductivity and chemical stabi-

lity of the trivalent cation-doped barium cerate: Phenomenonologically, the

conductivity and stability are connected and one will decrease in response to the

other increasing. This kind of relationship has often been supposed for different

base perovskites; e.g., cerates generally have high protonic conductivity and

zirconates are chemically stable [31]. It has been demonstrated here that the

relationship will also be true for the variation in the kind of dopant. Figure 12.9

suggests that not only the conductivity but also the chemical stability are

governed by the ionic size of the dopant cation. From a practical perspective,

the relationship between the electrical property and the chemical stability can be

useful in optim izing the dopant cation in a proton-conducting oxide system.

12.6 Proton Hole Mixed Conduction

The foregoing discussion was made on the basis that the proton-conducting

perovskite oxides generally have high ionic transport numbers in moist hydro-

gen. Hydrogen concentration cells generate electromotive forces in agreement

with those predicted by the Nernst equation. For this reason, the oxide can be

used for hydrogen sensors, hydrogen pumps, and fuel cells. Provided we can

additionally introduce electronic conductivity in the proton-conducting solids,

hydrogen permeation will occur throug h the materials. Mixed ionic-electronic

conductors (MIECs) allow gas permeation by the codiffusion of ionic and

electronic charge carriers, referred to in terms of ambipolar diffusion. There

are many examples of mixed conduction in oxides for oxygen permeation

membranes [32–34]. The MIEC having proton as the ionic charge carrier will

serve as a hydrogen-separati ng membrane. No external electric power supply is

75 80 85 90

1050

1025

1000

975

950

Ionic radius of dopant,r

/pm

Temperature/°C

–3.5

–3.0

–2.5

YTmYbLuInSc

log(σ/S cm

–1

)

Fig. 12.9 CO

reaction

temperature [open squares,

assigned by the inflection

points in Fig. 12.8(b)] and

the conductivity isotherm

at 4008C (the same as in

Fig. 12.6) as functions of the

dopant cation radius; the

reaction temperature

was plotted upside down to

ensure the upper plots

having higher chemical

stability. (Reprinted from [5]

with permission from

Electrochemical Society)

12 Proton Conduction in Cerium- and Zirconium-Based Perovskite Oxides 255

necessary, so that the electrodes and the external electrical circuit are excluded

to construct a simple hydrogen separator; the membrane uses the hydrogen

potential gradient as the driving force for the hydrogen transport. In ad dition,

the protonic mixed conductor works as electrodes in combination with the

proton-conducting perovskite oxides to be used as the electrolyte.

It is likely that there have been no accepted materials so far as the protonic-

electronic mixed conductors that allow high hydrogen transfer. Therefore, in

contrast to the conventional high-temperature proton conductors (HTPCs), the

protonic-electronic mixed condu ctors are not yet established. We recen tly

found that the protonic-electronic mixed conduction occurs in ruthenium-

doped perovskites, BaCe

0.9–x

0.1

3–a

(x ¼0–0.1) [35]; below is the sum-

mary of our findings.

Hydrogen permeation can be evaluated for a specimen using the following

cell.

at pðH

; suppÞjspecimenj Ar at pðH

; permÞ (12:7)

If one assumes proton (H

), oxide ion (O

2

), conduction electron (e), and

electron hole (h) as ionic and electronic charge carriers, hydrogen evolution at

the permeating side will obey the ambipolar diffusion mechanism, expressed by

Eq. (12.17) [36]:



þ s

2



þ s

ðÞ

pðH

; suppÞ

pðH

; permÞ

(12:8)

in which s

and s

are the partial conductivity of j species and the total

conductivity, respectively, and L is the thickness of the mixed conductor; the

equation assumes equal water vapor pressure in the two compartments and

constant partial conductivities. The ambipolar hydrogen flux is proportional to

ln(p(H

,supp)/p(H

,perm)).

Figure 12.10 shows the hydrogen flux through BaCe

0.9–x

0.1

3–a

(x ¼0.075, 0.1) at 8008C measured with the cell in Eq. (12.7); hydrogen

permeation was not experimentally observed for the Ru-undoped sample

(x ¼0). It is notable that the hydrogen flux is proportional to ln(p(H

,supp)/

p(H

, perm)), suggesting that the hydrogen transport occurred in accordance

with the ambipolar diff usion mechanism in Eq. (12.8). It is evident that

the hydrogen permeation occurs via the ionic-electronic mixed conduction

in the Ru-doped perovskites. The maximum hydrogen flux obtaine d is

6.5 0

8

mol/s cm

(x ¼0.1), equivalent to the internally short-circuited cur-

rent density of 12.5 mA/cm

As explained in previous sections, protonic and oxide-ionic change carriers

are assumable for the proton-conducting perovskite oxides; the SrCe

0.95

0.05

3–a

case has been shown in Fig. 12.2. By observing the change in

water vapor pressure in the permeating compartment, 0.5 of the ionic

256 H. Matsumoto

conduction was estimated to be due to the oxide ions and the other half could be

attributed to protons in the case of BaCe

0.8

0.1

3–a

at 8008C. This fraction

is quite similar to that of the base HTPC of the acceptor-doped barium cerate.

Therefore, the doping of Ru will provide an electronic conduction without

changing the protonic/oxide-ionic fraction of conductivity in the acceptor-

doped BaCeO

To investigate if electron holes or conduction electrons are responsible for the

electronic conduction, soft X-ray absorption spectroscopy (XAS) was employed,

expecting that the electron hole can be observed by this technique [37, 38]. This

technique elucidates unoccupied density of states, and thus electron holes, unoc-

cupied states in the valence band, can be detected. Figure 12.11 shows the XAS

spectra of BaCe

0.9–x

0.1

3–a

(x ¼0 and 0.075) as sintered in air and after

annealed in moist 1% H

at 8008C for 10 h. In the case that the specimens were

untreated after sintering in ambient air containing humidity, the spectra of both

undoped and Ru-doped specimen have peaks just below the top of the valence

band. The peaks at this position are attributable to electron holes present by the

defect equilibrium shown in Eq. (12.3). The undoped specimen annealed in moist

1% H

has no peak at the position, due to the equilibrium leaning to the left as

p(O

) decreases. However, in the case of the Ru-doped sample, the hole peak

remains even after the treatment in the reducing atmosphere. This fact suggests

that the electronic species for the mixed conduction will be attributed to the hole

formed in the Ru-doped materials.

02468

0.00

0.02

0.04

0.06

0.08

x = 0.075

x =

0.1

(

,supp

), Pt|BaCe

0.9–x

0.1

3-α

|Pt, Ar (p

,perm

)

,perm

/μmol s

–1

–2

ln( p

,supp

,perm

)

Sample thickness = 0.5 mm

800

Fig. 12.10 Hydrogen permeation rate through BaCe

0.825

0.1

0.075

3–a

and BaCe

0.8

0.1

3–a

at 8008C as a function of the logarithm of the hydrogen pressure ratio

(hydrogen supply side to permeate side) [35]. Sample thickness ¼0.5 mm. H

/Ar mixtures

were fed to the left compartment of cell (12.7), and argon gas was passed to sweep the

hydrogen evolved in the right compartment. The gases contained water vapor at 1.9 10

Pa and were fed at 30 mL/min. The partial pressures of hydrogen in the Ar sweep gas, p(H

perm) was determined by gas chromatography. (Reprinted from [35] with permission from

Electrochemical Society)

12 Proton Conduction in Cerium- and Zirconium-Based Perovskite Oxides 257

It is not clear how the Ru doping results in the formation of an electron

hole in reducing atmosphere. However, it is possible to assume a kind of energy

level produced by the Ru-4d orbitals, which locate just above the valence band

and accept electrons from the valence band to generate electron holes. Addi-

tional experiments, e.g., determination of the valence of Ru and the conductiv-

ity dependences on p(O

) and p(H

O), will provide a definite model for the

protonic-electronic mixed conduction in Ru-doped perovskites.

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12 Proton Conduction in Cerium- and Zirconium-Based Perovskite Oxides 259

Chapter 13

Mechanisms of Proton Conduction

in Perovskite-Type Oxides

K.D. Kreuer

13.1 Introduction

The proton is the only ion of chemical significance that has no electron shell on

its own. It, therefore, strongly polarizes its environment, which leads to high

binding energies. Speaking in terms of transport, this corresponds to a high

degree of self-localization and implies a strong coupling of long-range proton

motion to the dynamics of the proton environment. This is actually one com-

mon feature of all proton conduction mechanisms in quite diverse families of

compounds [1].

In the most trivial case, the proton is just diffusing as part of a protonat ed

species (e.g., H

), which allows for an overall proton transport provided the

unprotonated species (e.g., H

O) are also mobile (vehicle mechanism [2, 3]). In

most proton conductors, however, the proton is actually transferred between

different bin ding sites. As just pointed out, this has to involve a strong ‘‘proto-

n–phonon coupling,’’ which was first recognized for proton conduction in ice by

Fischer et al. in the late 1960s [4]. Their considerations were surely inspired

by the mechanisms of electron transfer in solution, i.e., Marcu s theory [5], and

by the theory of incoherent phonon-assisted hydrogen tunneling in metals that

was developed at the same time [6]. All these mechanisms have in common that

the mobile species (prot on, electron, hydrogen) are following a moving struc-

tural pattern (e.g., proton solvation shell), and it is the activation of this

‘‘structural diffusion’’ that is assumed to control the total activation enthalpy

of, e.g., proton diffusion. In the case of proton mobility within hydrogen-

bonded liquids, this is usually the rearrangement of the hydrogen bond pattern,

which requires thermally activated hydrogen bond-breaking and bond-forming

processes. This effect has particularly been studied for the mobility of excess

protons in wate r. Today’s understanding of proton conduction in water is

K.D. Kreuer (*)

Max-Planck-Institut fu

r Festk

orperforschung, Heisenbergstr. 1, D-70569, Stuttgart,

Germany

e-mail: Kreuer@fkf.mpg.de

T. Ishihara (ed.), Perovskite Oxide for Solid Oxide Fuel Cells,

Fuel Cells and Hydrogen Energy, DOI 10.1007/978-0-387-77708-5_13,

Ó Springer ScienceþBusiness Media, LLC 2009

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