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Study of proton conduction in thulium-doped barium zirconates at high temperatures

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2000 J. Phys. D: Appl. Phys. 33 3112

(http://iopscience.iop.org/0022-3727/33/23/316)

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J. Phys. D: Appl. Phys. 33 (2000) 3112–3120. Printed in the UK PII: S0022-3727(00)13724-6

Study of proton conduction inthulium-doped barium zirconates athigh temperatures

Mouloud Laidoudi†, Ibrahim Abu Talib and Ramli Omar

School of Applied Physics, Faculty of Science and Technology,Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia

E-mail: laidoudi@hotmail.com

Received 3 May 2000

Abstract. The specimens of BaZr1−xTmxO3−α (x = 0.02, 0.05, 0.10 and 0.15, α = x/2)have been prepared and characterized. The formation of the single perovskite phase in thesamples was checked by x-ray diffraction. For the verification of the possible charge carriersin the sintered BaZr1−xTmxO3−α samples, three different electrochemical cell measurementswere carried out. The measurements of electromotive force (emf) of hydrogen and steamconcentration cells showed that the BaZr1−xTmxO3−α ceramic is a protonic conductor and themeasurements of emf of the oxygen concentration cell showed that the BaZr0.90Tm0.10O3−α

sample exhibited poor oxide ion conduction. Proton transport number tH was calculated andwas found to be dependent on the content, x. The BaZr0.90Tm0.10O3−α sample showed thehighest value of proton transport number in the temperature range 500 � T � 900 ◦C.

1. Introduction

Sintered perovskite ceramics such as doped BaCeO3,SrCeO3, BaZrO3 and SrZrO3 by some aliovalent cationslike Y, Yb are high-temperature proton conductors underwater vapour or hydrogen containing atmospheres. Cerateceramics are not chemically stable at high temperatures in aCO2 containing atmosphere, whereas zirconate ceramics arechemically more stable and mechanically stronger [1–3]. Theapplications of these ceramics, include electrolytes in fuelcells [4, 5], hydrogen pumps, steam electrolysers, hydrogenanalysers and hydrogen sensors [6, 7].

The substitution of the trivalent cations for cerium orzirconium in these types of ceramics creates oxygen ionvacancies to maintain electrical neutrality, as in equation (1)[8, 9]:

M+3 → M′Zr/Ce + 1

2 VO (1)

where M+3 denotes a trivalent cation, M′Zr/Ce denotes a

trivalent cation occupying the Zr+4 or Ce+4 site in the latticeand VO denotes oxygen vacancies in the Kroger and Vinknotation. The oxygen ion vacancies play an important role inthe conduction of protons. Protons are generated inside theseoxides at high temperatures either by equilibrium between theoxide ceramic and the water vapour containing atmosphereaccording to equation (2) [10–12]:

H2O(g) + VO → 2Hi + OxO (2)

† Author to whom correspondence should be addressed.

where OxO denotes an oxide ion at the normal lattice site and

Hi denotes the proton, or by equilibrium between the oxideceramic and the hydrogen containing atmosphere.

In this paper, we present the protonic conductivity ofBaZr1−xTmxO3−α (where α denotes oxide ion vacanciesper unit cell), with various Tm contents in the temperaturerange of 400–900 ◦C, and its chemical stability under a CO2

containing atmosphere at elevated temperatures.

2. Experimental

2.1. Sample preparation and phase determination

Samples of BaZr1−xTmxO3−α (x = 0.02, 0.05, 0.10 and0.15) were prepared from BaCO3 (99.997%, Aldrich), ZrO2

(99%, Fluka) and Tm2O3 (99.99%, Aldrich) by solid-statereaction. The starting materials were weighed, mixed andthen calcinated in air at 1450 ◦C for 10 h. This process wascarried out twice with intermediate grindings. The calcinatedpowders were ground manually for 30 min.

The formation of single perovskite phase samples wasdetermined by x-ray powder diffraction (Siemens D5000,Cu Kα radiation, λ = 1.541 78 Å). The ground calcinatedpowders were pressed hydrostatically (10 tons for 5 min) intopellets (12.9 mm in diameter, 1.22–1.38 mm thick). Thesepellets were sintered in air at 1500 ◦C for 10 h.

2.2. Electrochemical and conductivity investigation

For ac conductivity and electromotive force (emf) studies,the two opposite surfaces of the specimen were painted with

0022-3727/00/233112+09$30.00 © 2000 IOP Publishing Ltd

Proton conduction in thulium-doped barium zirconates

2θ (degree)

10 20 30 40 50 60

Inte

nsity

(a.

u.)

(211)(200)

(111)

(110)

Figure 1. XRD patterns of the BaZr1−xTmxO3−α samples. Curves correspond to measurements for (bottom to top) x = 0.02, 0.05, 0.10 and0.15.

a platinum paste (Engelhard A4338) to create electrodes.A coherent contact between the electrolyte and porousPt electrodes was obtained by firing the coated pellets at1000 ◦C for 10 h in air. For conductivity and emf studies,under different atmospheres in the temperature range of400–900 ◦C, a special electrochemical cell was used [13].The cell had two compartments, which enabled gas flow toeach side of the specimen ceramic disc. A high-temperaturecement gasket was used to provide the seal between thecompartments.

Wet nitrogen was prepared by bubbling nitrogen throughdistilled water. The partial pressure of the water vapour,PH2O , in the nitrogen gas was controlled by saturating thewater vapour at a given temperature within an accuracy of±1 ◦C (measured by an Hg thermometer). Dry nitrogenwas produced by passing nitrogen gas through H2SO4 liquidthen through a phosphorous pentaoxide, P2O5, column. Thepartial pressure of hydrogen, PH2 , and that of carbon dioxide,PCO2 , were controlled by mixing the appropriate percentageof each gas (measured by a Kobord mass flow meter) witha N2 atmosphere. Ac conductivity measurements employedthe impedance technique (1 Hz–10 MHz) with a Solartron1255 high-frequency response analyser, programmed via anIBM computer for data collection and analyses (employingsoftware developed in our laboratory [14]). The emf wasmeasured by a Keithley 175A multimeter.

3. Results and discussion

3.1. X-ray diffraction and porosity of the samples

Figure 1 shows the x-ray diffraction patterns of the groundcalcinated powders of the BaZr1−xTmxO3−α (x = 0.02, 0.05,

0.10 and 0.15) ceramics. Formation of the perovskite singlephase of the cubic unit cell can be clearly seen. The 2θ valuesin this work were approximately in accordance with thoseof ASTM standard data for the undoped barium zirconateceramics and with those of our previous work [13]. Figure 2shows the dependence of lattice constant parameter a forBaZr1−xTmxO3−α on the Tm content x. It is evident that thelattice constant of the unit cell increased with increasing x.The porosity η values of prepared specimens were 13, 14,15 and 14% for x = 0.02, 0.05, 0.10 and 0.15, respectively.These values are slightly better than those obtained by Flintet al [15].

3.2. Identification of charge carriers inBaZr1−xTmxO3−α

In order to identify the possible charge carriers in the sinteredBaZr1−xTmxO3−α samples, the following electrochemicalcell measurements were carried out.

3.2.1. Emfs of hydrogen concentration cell. To verifythat BaZr1−xTmxO3−α ceramics are protonic conductors, theemfs of the H2(0.5 atm)+N2(0.5 atm), Pt|BaZr1−xTmxO3−α|Pt, N2(0.95 atm)+ H2(0.05 atm) hydrogen concentration cellwere measured at different temperatures and for x = 0.02,0.05, 0.10 and 0.15. Stable emfs resulting from the differencein hydrogen partial pressure between the two compartmentswere detected. We also observed that the negative terminalwas in the compartment with the higher hydrogen partialpressure. These observations suggest that the samples areprotonic conductors.

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M Laidoudi et al

Tm concentration, x

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

Lat

tice

cons

tant

, a(A

)

4.19

4.20

Figure 2. Dependence of lattice constant a of the BaZr1−xTmxO3−α samples on Tm concentration x.

Figure 3 depicts the emfs of BaZr1−xTmxO3−α ceramicsfor x = 0.02, 0.05, 0.10 and 0.15 in the temperature range500–900 ◦C. Also shown are the emfs of the cell at therespective temperatures calculated from Nernst’s equation (3)by assuming the sample was a pure protonic conductor withthe proton transport number equal to one:

emf = RT

2FtH ln

P ′H2

P ′′H2

(3)

where F is the Faraday constant, tH is the proton transportnumber, P ′

H2, P ′′

H2represent the partial pressure of hydrogen

on each side of the specimen, and R is the gas constant.It was found that the emf increased with increasing

temperature. This could be explained as due to the protonicconduction, which dominates the hole conduction. It wasalso noted from figure 3 that samples with x = 0.02 and 0.10give higher emfs. The best result is given by the sample withx = 0.10 at 500 ◦C where the measured emf conformed tothe emf of the cell calculated from Nernst’s equation (3) atthat temperature. This means that at 500 ◦C, the ceramic withx = 0.10 is a pure protonic conductor.

3.2.2. Emfs of steam concentration cell. Protonicconduction in this ceramic was also confirmed bystudying the emf of the N2(wet, PH2O = 0.035 atm),Pt|BaZr0.9Tm0.1O3−α|Pt, N2(dry) steam concentration cellin the temperature range of 500–900 ◦C. A stable emf wasobserved during the treatment and the negative terminal wasin the wet compartment. The emfs of the cell, as shown intable 1, decreased with increasing temperature. The samephenomenon was also observed by other researchers (e.g.[3, 16, 17]), which was attributed to the increase in holeconductivity in the ceramic at higher temperatures.

Table 1. Emf values of the wet N2(PH2O = 0.035 atm)|BaZr0.90Tm0.10O3−α| N2(dry) steam concentration cell at differenttemperatures.

T ( ◦C) 500 600 700 800 900

emf (mV) 20.37 19.84 16.60 11.90 6.68

Table 2. Emf of the O2(1 atm), Pt|BaZr0.90Tm0.10O3−α|Pt, N2(dry)oxygen gas concentration cell at different temperatures.

T ( ◦C) 500 600 700 800 900

emf (mV) 0.73 0.70 0.30 0.16 0.22

3.2.3. Emfs of oxygen gas concentration cell. In order todetermine whether the oxide sample is an ionic conductoror not at high temperatures, the emfs of the O2(1 atm),Pt|BaZr0.9Tm0.10O3−α|Pt, dry N2 oxygen concentration cellwere measured in the temperature range of 500–900 ◦C.Table 2 shows the cell emfs with the electrode in thedry nitrogen compartment being the negative terminal.The obtained emfs were almost negligible compared withthose of the steam concentration cell, suggesting thatthe BaZr0.9Tm0.10O3−α sample is not a good oxide ionicconductor. The emfs obtained are generally in agreementwith those obtained by Iwahara et al [3].

3.3. Total conductivity under wet and dry nitrogenatmospheres

The total conductivity of BaZr1−xTmxO3−α was deducedfrom the high-frequency arc of the impedance spec-tra. Figure 4 shows the impedance spectrum of theBaZr0.9Tm0.1O3−α sample at 600 ◦C under wet N2 with awater vapour partial pressure of 0.035 atm.

3114

Proton conduction in thulium-doped barium zirconates

T(OK)

750 800 850 900 950 1000 1050 1100 1150 1200

EM

F(m

V)

40

50

60

70

80

90

100

110

120

x=0.02

x=0.05

x=0.10

x=0.15

theoritical EMF (tH=1)

Figure 3. Measured and theoretically calculated emfs of the 50%H2 + 50%N2, Pt|BaZr1−xTmxO3−α|Pt, 5%H2+95%N2 hydrogenconcentration cell as a function of temperature.

Z' x 80 (ohm)

0 20 40 60 80 100

-Z"

x 80

(oh

m)

0

20

40

60

80

100T=600OC

high frequency low frequency

PH2O=0.035atm

1 Hz93.2 Hz

1.9565 104 Hz7.7532 104 Hz

5.673 106 Hz

Figure 4. Impedance plots of BaZr0.9Tm0.1O3−α at 600 ◦C under wet N2 (PH2O = 0.035 atm).

Figures 5 and 6 depict ac conductivities of BaZr1−xTmx

O3−α under wet and dry N2, respectively. Both figuresshow the dependence of the total conductivities ofBaZr1−xTmxO3−α on the content, x, and on the temperature,T . It should be noted that as the content, x, increased,the conductivity increased in wet and dry N2, except for

x = 0.05 where the conductivity decreased with increasingx. The apparent activation energies were calculated using theArrhenius equation (4):

σT = A exp

(− Ea

KbT

)(4)

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M Laidoudi et al

103/T (K-1)

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Log

10 (σ

bT/S

cm-1

K)

-5

-4

-3

-2

-1

0

1

x=0.15

x=0.10

x=0.02

x=0.05

PH2O=0.035atm

Figure 5. Arrhenius plots of BaZr1−xTmxO3−α exposed to wet N2 (PH2O = 0.035 atm).

103

/T (K-1)

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Log

10 (σ

bT/S

cm-1

K)

-5

-4

-3

-2

-1

0

1

x=0.15

x=0.10

x=0.02

x=0.05

Figure 6. Arrhenius plots of BaZr1−xTmxO3−α exposed to dry nitrogen.

where T , A, Ea and Kb are absolute temperature, the pre-exponential factor, activation energy and the Boltzmannconstant, respectively. Figure 7 represents the dependence ofactivation energies on the content x and on the surroundingatmosphere in the temperature range 400–900 ◦C. It is clearthat as x increased, the activation energy decreased, exceptfor the case x = 0.05 where Ea increased with increasing

x. There is not much difference between the values of theconductivity of the sample in wet N2 and dry N2, but there isa clear difference in the activation energies. All the values ofEa in wet N2 are smaller than those in dry N2, except for thecase with x = 0.05 in which the activation energy in wet N2

is higher than that in dry N2. The lowest activation energywas given by the sample with x = 0.15.

3116

Proton conduction in thulium-doped barium zirconates

Tm concentration, x

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

Act

ivat

ion

ener

gy E

a(kJ

mol

-1)

70

80

90

100

110

120

in dry N2

in wet N2

PH2O=0.035atm

Figure 7. The dependence of activation energies on Tm concentration x and on the surrounding atmosphere.

Log (PO2/Pa)

4.3 4.4 4.5 4.6 4.7 4.8

Log

(σ b/

Sm-1

)

-5

-4

-3

-2

-1

0

T=500OC

T=600OC

T=700OC

T=800OC

T=900OC

PH2O=0.035atm

Figure 8. Total conductivity of BaZr0.90Tm0.10O3−α as a function of oxygen partial pressure at various temperatures under a constant watervapour pressure, PH2O = 0.035 atm.

3.4. Ac conductivity under different atmospheres

According to Uchida et al [9] and Kurita et al [7], underatmospheres containing water vapour, H2O, and oxygen, O2,the conductivity of the electron holes, σh, is proportional tothe quadratic root of the oxygen partial pressure, PO2 (i.e.

σh ∝ P1/4O2

). They also pointed out that in atmospheres

containing hydrogen, H2, saturated by water vapour, H2O,

the electronic conductivity, σe, is proportional to the squareroot of the hydrogen partial pressure (i.e. σe ∝ P

+1/2H2

)

and the electron hole conductivity, σh, is proportional to

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M Laidoudi et al

Log (PH2/Pa)

4 5

Log

(σ b/

Sm-1

)

-3

-2

PH2O=0.035atm

T=900oC

T=800oC

T=700oC

T=600oC

T=500oC

Figure 9. Total conductivity of BaZr0.90Tm0.10O3−α as a function of hydrogen partial pressure at various temperatures under a constantwater vapour pressure, PH2O = 0.035 atm.

the negative square root of the hydrogen partial pressure(i.e. σh ∝ P

−1/2H2

). To determine the possible chargecarriers in the BaZr0.9Tm0.10O3−α , ceramics, ac conductivitymeasurements were carried out.

Figure 8 plots the logarithm of the conductivities ofBaZr0.9Tm0.10O3−α against the logarithm of the oxygenpartial pressure at a constant water vapour pressure,PH2O = 0.035 atm. The slopes of the plots are 0.26, 0.71,0.67, 0.54, and 0.73 at 500, 600, 700, 800, and 900 ◦C,respectively. The results show that the conductivity of thissample is strongly dependent on the oxygen partial pressure,PO2 . We attribute this dependence to the hole conduction,as elucidated by Uchida et al [9] who found the predictedslope to be 0.25 (owing to σh ∝ P

1/4O2

). Only the slopecorresponding to the conductivity at 500 ◦C seems to matchthe value predicted by Uchida et al [9].

Figure 9 plots the logarithm of the conductivity, log σb,against the logarithm of the hydrogen partial pressure,log PH2 , at a constant water vapour partial pressure,PH2O = 0.035 atm. The slopes of the plots are −0.01, 0.02,−0.07, −0.08, and −0.05 at 500, 600, 700, 800, and 900 ◦C,respectively. These values are negligible compared with thatof −0.5, expect for the electron hole conductivity. The resultssuggest that the bulk of the conductivity is independent of thehydrogen partial pressure.

3.5. Proton transport number of BaZr1−xTmxO3−α

The proton transport numbers of BaZr1−xTmxO3−α atdifferent temperatures were calculated from the emfsof the H2(0.5 atm)+N2(0.5 atm), Pt|BaZr1−xTmxO3−α|Pt,

N2(0.95 atm)+H2(0.05 atm) hydrogen concentration cellusing Nernst’s equation (3).

Figure 10 shows the dependence of the proton transportnumber tH on the content x and on the temperature. Thedata obtained show that, in the measurement temperaturerange of 500–900 ◦C, the sample doped with x = 0.10 hasthe highest proton transport numbers varying from 0.85 toalmost 1. tH is approximately equal to 1 at 500 ◦C and isalmost constant in the temperature range 600–700 ◦C. Fromthese results, we suggest that the BaZr0.9Tm0.10O3−α sampleis a good protonic conductor under a hydrogen atmosphereat high temperatures.

3.6. Chemical stability

To investigate whether the BaZr0.9Tm0.10O3−α sample ischemically stable under a CO2 atmosphere, the sample, in theform of powder, was exposed to a atmosphere containing 50%N2 and 50% CO2 at 650 ◦C for 9 h. Figure 11 shows XRDpatterns of the untreated and the treated BaZr0.9Tm0.10O3−α

powder samples in a CO2 containing atmosphere. It wasfound that no new peak appeared in the spectrum of the treatedsample. These results indicate that the BaZr0.9Tm0.10O3−α

sample is stable under CO2 at high temperatures. Yajimaet al also reported the stability of ceramic samples based onSrZrO3 under CO2 at high temperatures [2].

4. Conclusion

In this paper we demonstrated the formation of the singleperovskite phase in BaZr1−xTmxO3−α ceramic samplesprepared by solid-state reaction. Emf measurements of

3118

Proton conduction in thulium-doped barium zirconates

T (oC)

400 500 600 700 800 900 1000

Prot

on tr

ansp

ort n

umbe

r t H

0.5

0.6

0.7

0.8

0.9

1.0

1.1

x=0.10

x=0.02

x=0.05

x=0.15

Figure 10. Proton transport number, tH , of BaZr1−xTmxO3−α as a function of temperature measured by the hydrogen concentration cell,H2(0.5 atm)+N2(0.5 atm), Pt|BaZr1−xTmxO3−α|Pt, N2(0.95 atm)+H2(0.05 atm).

2θ (degree)

10 20 30 40 50 60

Inte

nsit

y (a

. u.)

(a)

(b)

(110)

(111)

(200)(211)

Figure 11. XRD patterns of the BaZr0.90Tm0.10O3−α powder (a) untreated and (b) treated in CO2 (0.5 atm) at 650 ◦C for 9 h.

hydrogen and steam concentration cells confirmed that thesesamples are protonic conductors. Electron hole conductionin the sample was observed by studying its bulk conductivityvariation with oxygen partial pressure. Emf measurementsof the oxygen gas concentration cell exhibited poor oxide

ionic conduction in the sample. We also showed that thesample is a poor electronic conductor through the study ofits bulk conductivity against hydrogen partial pressure atconstant water vapour pressure. It was also demonstratedthat BaZr1−xTmxO3−α ceramics are very stable under CO2

3119

M Laidoudi et al

at high temperatures. Proton transport number measurementsshowed that the sample characterized by x = 0.10 is apure protonic conductor at T = 500 ◦C and its protontransport number decreased with increasing temperature. Weexpect to improve both the ac conductivity and the protontransport number by using a binder (e.g. ethylene glycol)during pressing of the pellets and by using high-purity ZrO2.This will be the subject of our next study.

Acknowledgments

The authors thank the Ministry of Science, Technology andEnvironment, Malaysia for the IRPA grant (09-02-02-0006)to carry out this project.

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