Estimation of the protonic concentration and mobility in
Ba(Zr
0.81
Yb
0.15
Zn
0.04
)O
3−δ
ceramic
Jong-Sung Park, Jong-Ho Lee, Hae-Weon Lee, Byung-Kook Kim ⁎
Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
abstract article info
Article history:
Received 2 September 2009
Received in revised form 31 August 2010
Accepted 28 September 2010
Available online 26 November 2010
Keywords:
Proton conductor
Barium zirconate
Defect model
ZnO addition
Rare earth doped BaZrO
3
is one of most promising proton conducting oxides as it has high proton
conductivity and sound chemical stability. Sintering aids such as ZnO, however, should be incorporated in
order to improve poor sinterability. In this study, the effects of adding ZnO on proton conductivity of Yb-
doped BaZrO
3
(BZYb) were investigated. From the electrical conductivities measured under various water
vapor pressures, concentration and mobility of the proton were obtained. Proton mobility of BZYb with ZnO
(BZYb–Zn) was smaller than that of BZYb while hydration enthalpy of BZYb–Zn was more negative than
that of BZYb.
© 2010 Published by Elsevier B.V.
1. Introduction
High-temperature proton conductors (HTPCs) are potential
candidates for dense electrolytic membrane materials for hydro-
gen separation (high-temperature steam electrolysis) or power
generation (solid oxide fuel cells) [1–3]. Among the many HTPCs,
BaZrO
3
doped by rare earth elements (RE-BaZrO
3
) has attracted
special attention because of its high proton conductivity and sound
chemical stability against both CO
2
and H
2
O [4–6]. However, it is
well known that RE-BaZrO
3
is very difficult to sinter. Density of
90% can be achieved even when it was sintered at 1700 °C for 10 h
[7,8].
In order to improve poor sinterability, a small amount of ZnO was
added to the RE-BaZrO
3
and almost full density of RE-BaZrO
3
ceramics could be obtained at a temperature of 1300 °C [9–11].
However, electrical conductivity of RE-BaZrO
3
decreased by the
addition of ZnO which was qualitatively ascribed to trapping a mobile
proton by the negatively charged Zn
″
Zr
[11]. The effects of a ZnO
addition on fundamental parameters such as proton mobility, proton
concentration and transference number were not systematically
investigated yet.
Such fundamental parameters could be obtained by analyzing
electrical conductivities measured under various water vapor and
oxygen partial pressures based on the chemical defect model related
with the equilibriums of various charge carriers such as the proton,
oxygen vacancy, electron and hole [12–17]. Total conductivity, σ
total
, is
the sum of the hole, electron, and ion conductivity including the
proton and oxygen vacancy. That is,
σ
total
= σ
ion
+ σ
h
+ σ
e
ð1Þ
where σ
ion
, σ
h
and σ
e
are the partial conductivity of ion, hole and
electron respectively [12–17]. σ
h
and σ
e
are linearly dependent on
(P
O
2
)
1/4
and (P
O
2
)
− 1/4
respectively while σ
ion
is independent of P
O
2
[12–17]. The σ
total
can be expressed by
σ
total
= σ
ion
+ σ
∘
h
P
O
2
1 = 4
+ σ
∘
e
P
O
2
−1 = 4
ð2Þ
where σ
h
∘
and σ
e
∘
are σ
h
and σ
e
at P
O
2
= 1 atm respectively [13,16]. σ
h
becomes dominant under high P
O
2
while σ
e
becomes dominant under
low P
O
2
. In the cases of RE-BaCeO
3
and RE-BaZrO
3
as electrolytes, σ
h
and σ
e
become negligible compared to σ
ion
in the intermediate region
of P
O
2
from 10
−24
to 10
−10
atm below 800 °C. In this intermediate
region of P
O
2
, σ
total
is nearly constant and mainly contributed by σ
ion
[14,18].
In the case of RE-BaCeO
3
, since σ
h
showed a linear dependency on
(P
O
2
)
1/4
, σ
ion
could be obtained by subtracting σ
h
from σ
total
measured
under an oxidizing atmosphere [12–14]. However, σ
h
was much
larger than σ
ion
in RE-BaZrO
3
and thus it is hard to determine the exact
σ
ion
from σ
total
using the linear dependency of σ
h
on (P
O
2
)
1/4
[17–20].
Therefore, in this work, for measuring σ
ion
of BZYb and BZYb–Zn,
electrical conductivity was measured under a reducing atmosphere
with a specific region of oxygen partial pressure in which hole
and electron conductivity became negligible. Although the ionic
conduction range of P
O
2
for RE-BaZrO
3
was known to be from about
10
− 24
to 10
− 10
atm below 800 °C, there could be some
Solid State Ionics 192 (2011) 88–92
⁎ Corresponding author. Tel.: +82 2 958 6703; fax: +82 2 958 5529.
E-mail address: bkkim@kist.re.kr (B.-K. Kim).
0167-2738/$ – see front matter © 2010 Published by Elsevier B.V.
doi:10.1016/j.ssi.2010.09.048
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