Solid State lonics 18 & 19 (1986) 80%812
North-Holland, Amsterdam
807
ELECTRIC AND ELECTROCHEMICALPROPERTIES OF CATALYTICALLYACTIVE OXYGEN ELECTRODEMATERIALS
A.J. BURGGRAAF, M.P. VAN DIJK
and
K.J. DE VRIES
Twente U n i v e r s i t y of Technology, Department of Chemical Engineering, Laboratory f o r Inorganic
Chemistry, Materials Science and Catalysis, P.O. Box 217, 7500 AE Enschede, The Netherlands
The electrical conductivity has been investigated of some oxygen ion and mixed conducting materials. Electrodes are prepared from thin sputtered layers of these oxides combined with a small Au
or Pt strip. The kinetics of the oxygen reaction has been studied for temperatures of 820-1020 K
and PO- values of 10-~ - 1 atm. respectively.
Highes~ oxygen ion conductivities are found in solid solutions of 0.7 BipO3- 0.3 TbpO~ ~ followed
by 0,7 CeO2 - O.3Tb20~ 5(CT-30) and (TbxGd1_x)2Zr207 with pyrochlore str~cture and R=O'ITGZ-O).
Highest, p-type, electronic conductivitTes are found in CT-30 and TGZ with x=l. TGZ-O was used in
a l l experiments as the solid electrolyte.
Combined current-overvoltage {q) and impedancemeasurements show that with Au strips the nature of
the oxidic materials has a pronounced effect on especially the cathodic polarization, the effect
being larger with larger ~. Introduction of p-type conductivity does not decrease the electrode
resistance. The Butler-Volmer equation is obeyedwith effective cathodic and anodic transfer coefficients close to 0.5 and 1.5 respectively while the effective exchange current varies as the 0.5
power of P02. These results can be interpreted by a mechanism where charge transfer in the metaloxide region is rate controlled by surface diffusion on the oxide.
1. INTRODUCTION
Slow electrochemical reactions reduce the
improve the behaviour of metal electrodes or
efficiency of fuel cells, steam electrolysers
even replace them and they are especially inter-
and oxygen pumpsI-5/I? and complicate the re-
esting for highly selective electrodes in gas
sponse of gas sensors at low temperature6/7/Io.
sensors and electrochemical or - catalytical
For redox reactions in the gasphase both metals
redox reactions (e.g. methanol oxidation, NO
and oxides have been tested as electrode materi-
reduction13-2°/22).
For these reasons they are
als applied on top of an oxygen ionconducting
investigated in our laboratory. Gd2Zr207(zTGZ-G)
electrolyte which is in most cases a solid solu-
was selected in this study as an alternative
tion of InO2-Y2032/5/B/12. I t has been shown
electrolyte material with good ionic conductiv-
that both the nature of the surface of the oxid-
i t y 2° and interesting properties. The structure
ic electrolyte/electrode and of the electrode
consist of pyrochlore domains in a f l u o r i t e
metal play an important role especially at high
matrix. The degree of order can be varied and
temperatures2/5/8-12. Reaction mechanisms and
the boundaries between domains and matrix have a
rate controlling steps are dependenton tempera-
defect chemistry giving rise to rapid oxygen
ture, partial pressure of oxygen and other reac-
diffusion path ways21. Preliminary experiments
tants and on electrode morphology in addition to
with the TGZ system indicate that the CO oxida-
the expected dependenceon material properties.
tion rate at 450°C (r4~0) is dependenton the
As has been discussed oxygen vacanciesB/13-16
the occurence of electronic conductivity9/I0/18
degree of order and that high values can be
obtained23/24. This points to active sites which
and the presence of (catalytically) active
nature depend on ordering degree. Oxidic elec-
sites 3/12-19 most probably play an important
trode materials are investigated in the form of
role. Oxide layers with mixed conductivity and
thin layers applied on top of the electrolyte.
c a t a l y t i c a l l y active centers may therefore
The system (TbxGd1_x)2Zr207(~TGZ-x) was investi-
0 167o2738/86/$ 03.50 © Elsevier Science Publishers B.V.
(North-Holland Physics Pubfishinz Division)
A.J. Burggraa/et al. / Catalytically acti~,e ~).vrgen electrode materials
808
gated because Tb i n t r o d u c e s e l e c t r o n i c conduc-
were not p r e s e n t . The o v e r v o l t a g e measurements
tivity
were c o r r e c t e d by AC impedance a n a l y s i s 24 f o r the
in the pure i o n i c a l l y
conducting system
f o r x=O and i s r e p o r t e d to be a c t i v e in high
f a c t t h a t working and reference e l e c t r o d e do not
temperature gassensors 24/25,
feel the same e q u i p o t e n t i a l plane.
(Tbl_xBix)203 and
(Tbl_xCex)O2+ycalled f u r t h e r BT and CT respectively
were studied because they have a d i f f e r -
ent d e f e c t c h e m i s t r y , and are expected to have
higher c o n d u c t i v i t i e s , w h i l e CeO2 i s an a c t i v e
oxidation catalyst.
3. RESULTS
C o n d u c t i v i t i e s as a f u n c t i o n o f temperature
are given in f i g u r e i .
The h i g h e s t i o n i c conduc-
The present paper concen-
t r a t e s on TGZ w i t h x=O and i (~TGZ-IO0) and on
eQo 70o 6Qo sQo
CT-30 (x=O.3).
2. EXPERIMENTAL
A full
3£o
~qo
3
$
2
,2
E
d e s c r i p t i o n of experimental methods i s
"T
given elsewhere 24. The most i m p o r t a n t f e a t u r e s
are summarized below. E l e c t r o d e l a y e r s of TGZ-O,
I
TGZ-IO0 or CT-30 w i t h a thickness of 150 or 600
nm are a p p l i e d on TGZ-O ceramic substrates by
r a d i o f r e q u e n t s p u t t e r i n g a t 1-1.5 kV in an
o
,0
0
......
1.0
atmosphere of 82% Ar and 18% 02 a t 0 . 6 - 0 . 8 Pa.
1,2
i.:a
1.4
103/T [ K -1]
, i
1.6
1.8
The metal composition was found to d e v i a t e less
than 1% from the t a r g e t composition (X-ray f l u o rescence). The m a c r o s t r u c t u r e of the l a y e r s was
examined by X-ray d i f f r a c t i o n
and S.E.M. Cur-
r e n t - o v e r v o l t a g e measurements were performed
potentiostatically
in a three e l e c t r o d e c e l l .
The p o t e n t i a l was r a i s e d l i n e a r l y
with a rate of
FIGURE i
E l e c t r o n i c and i o n i c c o n d u c t i v i t i e s of i n v e s t i gated oxides at Poe=O"21 atm.
I.CT-30 ( t o t a l ) 2.~T-30 ( e l e c t r . ) 3.BT-30(eCG-30,
i o n i c ) 4.TGZ-IO0 ( t o t a l , e l e c t r . ) 5.CT-30 ( i o n i c )
6.TGZ-O ( p y r . ) 7.TGZ-O ( f l u . ) 8.TGZ-IO0 ( i o n i c )
9. BT-30 ( e l e c t r . ) .
4 mV sec - 1 . The counter e l e c t r o d e c o n s i s t e d of a
225 nm t h i c k s p u t t e r e d porous Pt l a y e r . The
tivities
reference e l e c t r o d e on the o t h e r side of the
f o l l o w e d by CT-30 and Gd2Zr207 (TGZ) w i t h pyro-
electrolyte
chlorephase, TGZ-O w i t h f l u o r i t e
c o n s i s t e d of a spring loaded Pt
~. are found in BT-30 (x=O.7) and CG 30
1
phase and TGZ-
p o i n t c o n t a c t . The working e l e c t r o d e c o n s i s t e d
i00 (~Tb2Zr207+y) have c o n s i d e r a b l y lower i o n i c
of a r i n g of the s p u t t e r e d oxide e l e c t r o d e w i t h
conductivities.
an o u t e r diameter equal to t h a t of the e l e c t r o -
in a l l
l y t e disk ( i 0 mm) and an i n n e r diameter of 5 mm.
energies Ei f o r i o n i c c o n d u c t i v i t y are in o r d e r
Electrical
of decreasing Eivalues (given in parenthesis in
c o n t a c t was made by a spring loaded
The e l e c t r o n i c c o n d u c t i v i t y i s
cases predominantly p type. A c t i v a t i o n
annular s t r i p of Au (or in some cases Pt) w i t h a
kJmole-1): TGZ-IO0 (126), CT-30 and TGZ-O w i t h
geometric c o n t a c t surface of 15 + 3 mm2 and a
fluorite
s t r u c t u r e (=113), TGZ-O w i t h pyrochlore
width of 0.6 + 0 . i mm. From an combination of
s t r u c t u r e and BT-30 (=87). A c t i v a t i o n energies
c o n d u c t i v i t y and c a p a c i t y measurements i t was
f o r e l e c t r o n i c c o n d u c t i v i t y are about equal f o r
concluded t h a t the real c o n t a c t surface i s at
CT-30 and TGZ-IO0 (52-60) f o l l o w e d by BT-30(=40).
l e a s t a f a c t o r i0 s m a l l e r . Thermovoltaic e f f e c t s
For the TGZ system i t
has been proven t h a t
A.J. Burggraaf et al. / Catalytically active oxygen electrode materials
809
the electron hole conductivity is due to a small
Butler Volmer equation [ i ] in which diffusion
polaron mechanism in the Tb sublattice with
mobilities of 10-10-10-8 m2(Vs)- I at 970 K
phenomena are incorporated and which can be
reduced to equation [ 2 ] .
depending on the Tb concentration26. The Tb4+/
Tb3+ ratio hardly depends on composition and
reaches a maximum value of 0.25 at Po2=latm. The
exp(~aq*) - exp(-Cc~*)
i =
"- + i -1£ a e x p ( ~ *) + i -£~
lexp(-~*)
]01
[1]
sputtered layers have a small porosity (SEM
observation) but are rather strongly textured
with {100} crystal planes oriented parallel to
the surface. After heat treatment sharpened
with q*=~F/R and index £ indicating a l i m i t i n g
current.
The behaviour at not too large q values is
X-ray diffraction lines indicate a good crystall i n i t y ( f l u o r i t e structure). Typical current(i)
1000
t
overvoltage (y) curves for TGZ-O/IO0 and CT-30
are given in f i g . 2 . In all configurations anodic
%
6~-,C
100
.
.
-~,oo
.
.
.
o
.
~oo
10
-
SOC
-
730 c
_~
electrolyte : T G Z ~
600 nm CT30/Au
500
3
I
cathode
1 -~6o
l~[mV]
.
.
-~0o
.
.
.
0
,
,
~o
iO0
FIGURE 2
Polarization curves for oxides with Au strip
contacts at 727°C and PO =0.21 atm on TGZ-O.
2
1.uncovered electrolyte, 2.600nm sputtered TGZ-O
3.600nm sputtered CT-3O, 4.150nm sputt. TGZ-IO0
currents are much larger then cathodic ones and
so cathodic polarization is more severe. With Au
-2~o
P02 = 0.21 (~tm
o
2~o
[mv ]
~o
FIGURE 3.
Typical plot of log i vs q for a CT-30 layer
(600nm) with Au strip at P02=0.21 atm.
represented by Tafel plots (Fig.3) from which i 0
can be deduced and by the electrode resistance
Rel as determined from AC impedance measurement24. Rel and i 0 are interrelated by Rel =
(~/i)Tr, o=RT/[(~a+~c)Fio]. Results are presented
in figure 4 and table 1. The behaviour of the
uncovered electrolyte (TGZ-O). A decrease in
sputtered layers as characterized by Rel follows
the same sequence as given above for larger
polarization is obtained with a sputtered layer
values but with smaller differences. For the
contact strips polarization is largest for the
of the same material (TGZ-O), a smaller decrease
higher temperatures charge transfer coefficients
with CT-30 layers. At ~=500 mV, i-values are
amount ~c~0.5 and ~a=l.5. Temperature decrease
results in a decrease of c a and an increase of
increased by factors of about 11 and 6 respeclayers. Obviously introduction of Tb in TGZ-O
c" Activation enthalpies of Rel are about i00
kJmole-lfor all oxide-Au configurations except
layers increases the (cathodic) polarization.
The results can be analyzed by means of a
for CT-30 at the higher temperatures. The i-~
curves can be f i t t e d with equation [ 2 ] .
t i v e l y . No improvement is achieved with TGZ-IO0
810
A.J. Burggraaf et al. / Catalytically active oxygen electrode materials
TABLE 1.
Transfer c o e f f i c i e n t s ~ and a c t i v a t i o n e n t h a l p i e s
AH o f the e l e c t r o d e r e s i s t a n c e R^I . Numbers in
the ~ colums i n d l c a t e low and hlgh temperature
l i m i t s r e s p e c t i v e l y . PO=0.21~ atm except f o r
moment. I n t r o d u c t i o n of p type c o n d u c t i v i t y
marks i and 2 w i t h P02=~.OI and 0.43 atm. Thickness of s p u t t e r e d l a y e r s i s 600 nm except mark 3
( w i t h 150 nm)
e l e c t r o d e m a t r i x has an i m p o r t a n t e f f e c t when Au
Important conclusions can be drawn at t h i s
increases the e l e c t r o d e r e s i s t a n c e and p o l a r i z a t i o n . Furthermore the c h a r a c t e r of the o x i d i c
s t r i p s are used. With Pt s t r i p s and in a i r Rel
values are independent of the o x i d i c m a t e r i a l
Electrode
~a
~c
configuration
_+0.2
bareTGZO/Au
1.6
AH
_+0.2
kJmol - I
(Table i and Figure 4) so in t h i s case the e l e c t r o d e behaviour is determined mainly by the
m e t a l . Comparing Au and Pt on CT-30 ( f i g u r e 5)
TGZOtAu
i
0.8-0.6
i01
1.1-1.6
0.7-0.6
106
2
1.5
0.6
ii0
i.i
0.7
240-140
3
1.3-1.5
0.6-0.5
100-150
1.3-1.5
0.7-0.4
100-150
i.I
0.7
240-140
1.2-1.5
0.4-0.5
TGZO rPt
CT30 tAu
CT30 rAu
CT30 zPt
TGZO tAu
120
1.4-1.7
TGZO rAu
TGZO~Au
0.35
3
we observe t h a t at l a r g e r ~ values (=400 mv) the
600 nm CT30
eledrdyte : TGZ 0
P02 = 0.21 ATM
512]
=L
q. imvl
99
-5~
4
7
!
6
~
Y~
, J3
!0 4_
--
/
6o0 ~oo-2oo
o
260 ~do
FIGURE 5.
Anodic and cathodic p o l a r i z a t i o n curves f o r CT-30
l a y e r s w i t h Pt or Au s t r i p contacts a t
Po~=O'21atm
i ~nd 2: Pt e l e c t r o d e , 728 and 684°C resp.
3 and 4: Au e l e c t r o d e , 728 and 684°c resp.
i
•
--50(3
i:
U
the c u r r e n t s are l a r g e r f o r Pt w i t h a f a c t o r of
/'i//'
about 3-10 depending on the temperature. For
lo34
/~"'
T<600°C and in a i r Au s t r i p e l e c t r o d e s however
have a s m a l l e r e l e c t r o d e r e s i s t a n c e than Pt.
l
21
09
lY~ ,,,/
10
11
103/T [K 1]
F i n a l l y the e f f e c t of P02 can be represented
12
FIGURE 4
Scaled t o t a l e l e c t r o d e r e s i s t a n c e Rel of several
oxide l a y e r metal combinations.
I=CT-30/Pt,
2=TGZ-O/Pt, 3=TGZ-O/Au,
4=TGZ-IOO/Au, 5=CT-30/Au, 6=CT-30/Au,
7=uncovered TGZ-O/Au. All l a y e r s are 600 nm
t h i c k except 4 and 6 (150nm).
by Rel~ pm w i t h m=-0.47 f o r a l l oxides w i t h Au
O2
s t r i p s and f o r P02=I0-5-I atm. I f we remind t h a t
and m are i n t e r r e l a t e d 11/12 and t h a t ~a~l.5
and ~ : 0 . 5 f u r t h e r a n a l y s i s z9/24 leads to the
c
conclusion t h a t there i s only one mechanism being
able to reproduce these combination of c and m
values. This i s a charge t r a n s f e r process w i t h
A.J. Burggraaf et al. / Catalytically active oxygen electrode materials
Ca=cc=l were both anodic and cathodic p o l a r i z a -
steps where r e s p e c t i v e l y l i m i t a t i o n
81 1
by d i f f u s i o n
t i o n are r a t e c o n t r o l l e d by a d i f f u s i o n process
or by charge t r a n s f e r occurs. Reaction 4 y i e l d s
from an adsorption s i t e to an electrochemical
values of ~ = i / 2 , r=O, ya=Yc=2 and v=2. With ~c =
Yc/V + rx~ and Ca=(Ya/V + r-r×~) 12/19/24 t h i s
r e a c t i o n s i t e probably on the oxide surface. A
full
discussion is given elsewhere I I / 1 9 / 2 4 , some
arguments are given below.
r e s u l t s in Ca=cc=l which was required by the
model.
I f we define a r a t i o Q=(ioL2/2DCo ) I / 2 then Q
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E.C. Subbarao and H.S. M a i t i , Solid State
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2.
separated by a distance L and with an
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eq. i (with Ca=cc=l) reduces to
i = i O , a p p [ e x p ( l . 5 ~ ) * -exp(-O.5~*}]
with iO,ap p is a f u n c t i o n of ( i o / Q ) .
[2]
In t h i s
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concentration on the r e a c t i o n s i t e Cr<< Co but
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[3]
with V is a vacant s i t e
11. D.Y. Wang and A.S. Nowick, J. Electrochem.
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02(g) + Vad(C) + e' = O~,ad(C)
O'2,ad + Vr,s (C}
[4a]
= O~(C) + Vad(C) [4b]
O~(C} + Vr,s(C) + e = 20'(C)
o
2[0'(C} + Vo°°(C) + e = 0o + Vr,s(C) ]
[4c]
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[4d]
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A.Z Burggraaf et al. / Catal.vticall3, active oxygen electrode materials
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