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Solid State Ionics 146 (2002) 219 – 224
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Assessment of Ni/YSZ anodes prepared by combustion synthesis
A. Ringuede´ a, D. Bronine b, J.R. Frade a,*
b
a
Ceramics and Glass Engineering Department (UIMC), University of Aveiro, 3810-193 Aveiro, Portugal
Institute of High Temperature Electrochemistry, Ural Division of Russian Academy of Sciences, S. Kovalevskoj 20,
620219 Ekaterinburg, Russia
Received 23 August 2001; received in revised form 3 October 2001; accepted 4 October 2001
Abstract
Homogeneous mixtures of nanocrystalline powders of (NiO + Ni)/YSZ were obtained by combustion synthesis, and used to
prepare Ni/YSZ cermets for symmetrical cermet/YSZ/cermet cells. These cells were prepared by co-pressing, co-firing at 1450
C, and reduction in 10% H2 – 90% N2 at 800 C. The resulting Ni/YSZ cermets are porous and adherent to the electrolyte, and
its metallic and ceramic components are uniformly distributed. Impedance spectroscopy was used to characterise these
symmetrical cells in atmospheres containing H2 and H2O. The impedance spectra show that the electrode reactions comprise at
least two processes with different relaxation frequencies. The low frequency contribution of the polarisation resistance is very
dependent on the partial pressures of H2 and H2O. The contribution at higher frequency is mainly dependent on temperature.
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Ni; YSZ; Combustion synthesis
1. Introduction
The electrode performance of Ni/YSZ cermets is
very dependent on microstructural features, and thus
processing methods [1,2]. The main factors include the
grain size distributions of both components (Ni and
YSZ) and the porosity. The microstructures must thus
be improved to maximise the triple phase boundary,
and the volume fraction of Ni must be sufficient to
attain percolation. Koide et al. [9] estimated a typical
percolation limit of about 30 vol.% of Ni. The lowest
polarisation resistance was found for volume fractions
of about 40 –50% Ni, which also suggests the importance of triple contacts. Results reported for alternative
Ni/TZP cermets [3– 5] compare to those obtained for
Ni/YSZ cermets.
The role of triple contacts, and their effects on the
mechanism of H2 oxidation have been demonstrated by
pattern Ni electrodes deposited onto YSZ [6 –8]. The
effective reaction sites appeared to be located in the
nickel surface near triple contacts, and from the effects
of the partial pressures of hydrogen (pH2) and water
vapour ( pH2O) on the current density Misuzaki and coauthors [7] derived the following solution to describe the
effects of pH2 and pH2O on the electrode conductivity:
rE ¼ di=dE ¼ ½2F=ðRT Þfkkeq pH2 O þ ðk 0 =2Þ
ðKeq pH2 Þ1=2 g
ð1Þ
*
Corresponding author. Tel.: +351-234-370254; fax: +351234-425300.
E-mail address: [email protected] (J.R. Frade).
where keq is the equilibrium constant of reaction
H2 + 1/2O2 () H2O.
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 2 7 3 8 ( 0 1 ) 0 0 9 9 6 - 1
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A. Ringuede´ et al. / Solid State Ionics 146 (2002) 219–224
Impedance spectroscopy also reveals that the
electrochemical oxidation of H2 in Ni/YSZ cermets
or Ni patterns deposited onto YSZ substrates may be
complex, usually comprising two or three processes
[2,9,10]. For example, Primdahl and Mogensen [2]
used an equivalent circuit Rohm(RQ)1(RQ)2(RQ)3 to
fit their results at temperatures in the range 850–
1000 C. Typical relaxation frequencies for the electrode contributions were in the ranges of 2 –5 kHz,
30 –100 Hz, and about 1 Hz, respectively. The contribution at highest frequencies is most sensitive to
the microstructures and shows the highest temperature dependence. At high temperatures (e.g. 1000
C), the low frequency term may become the highest
contribution of the polarisation resistance, at least for
optimised cermet anodes [2]. This contribution shows
strong dependence on anodic overpotential and on
the partial pressures of H2 and/or H2O. The intermediate frequency term remains a minority contribution even for optimised cermets, and was not
observed by other authors [10].
The values of polarisation resistance obtained by
electrochemical impedance spectroscopy (EIS) may be
significantly lower than results obtained by other
methods such as galvanostatic current interruption
(GCI), as reported by Jiang et al. [4,5,11]. The activation energy is also stronger for the polarisation resistance results obtained with impedance spectroscopy
than for results obtained by GCI. However, the differences between the EIS and GCI results decrease with
increasing water vapour partial pressure [11]. These
findings were thus explained by assuming that extra
water vapour is generated by the anodic reaction (in
GCI conditions) causing a local increase in its partial
pressure, and thus lowering one of the main contributions of the polarisation resistance. Impedance spectroscopy is thus often preferred to study the role of
microstructural features and/or gas composition on
the oxidation of H2 in Ni-based cermets.
Partial oxidation of H2 may thus cause local
increase of water vapour partial pressure, yielding
some unexpected results obtained for the dependence
of electrode conductivity (rE) in the range of low
fractions of H2. For example, some results [7,11]
indicate an increase in rE with decrease in H2, thus
contradicting Eq. (1). Impedance spectroscopy results
reported for Ni/TZP cermets show that the low frequency contribution of the polarisation resistance
decreases with increasing %H2 whereas other contributions at higher frequency tend to increase [3].
Ni/YSZ cermets often degrade in working conditions, and this has been ascribed to decrease of
porosity [1], and/or coarsening of Ni particles [12],
causing decrease in electrode conductivity and increase in polarisation. Itoh et al. [1] demonstrated that
a fraction of large YSZ particles might prevent coarsening and volume contraction of Ni/YSZ cermets,
thus contributing to retain high conductivity and low
polarisation.
Current collecting might also play a major role on
the measured polarisation resistance values, as found
on comparing results obtained with collectors made of
gold, platinum and nickel pastes [10]. These authors
suggested that the degradation of the anode may be
due to interdiffusion of the metal in the Ni/YSZ
cermet and in the current collector, and suggested
that the same metal should be used in the cermet and
in the current collector. However, poor contact between the current collect and the cermet may also affect the results, as suggested by the differences between impedance spectra obtained with Ni mesh and
Ni paste.
The present work reports results obtained for Ni/
YSZ cermet anodes prepared by a combustion synthesis of the cermet powders to ensure a very homogeneous distribution of the metallic and ceramic
components. These powders are suitable for co-firing the electrolyte and the cermet anode in a single
step.
2. Experimental procedure
A combustion synthesis method was used to obtain
NiO/YSZ powders, as described elsewhere [13].
Nitrate precursors (ZrO(NO3)26H2O, Y(NO3)36H2O,
O, and Ni(NO3)26H2O) were mixed in the required
proportions and melted together with urea on a hot
plate, and then introduced in a furnace at 600C, where
the combustion reaction took place in less than 2 min.
The rapid increase of temperature was monitored by
inserting a thermocouple in the reacting melt. This
short reaction time ensured homogeneity and yielded
nanocrystalline mixtures of YSZ, NiO and traces of
metallic Ni, as found by X-ray diffraction. Crystallite
sizes of about 30– 40 nm were estimated from peak
A. Ringuede´ et al. / Solid State Ionics 146 (2002) 219–224
broadening, and also from BET specific surface area
(S) measurements, which were used to estimate the
average diameter U = 6/(Sq), q being the powder density.
The combustion synthesised cermet powders were
used to obtain symmetrical cells cermet/YSZ/cermet.
A relatively thick YSZ electrolyte pellet was pressed
first, and thinner cermet layers were then co-pressed
onto both surfaces of the YSZ pellet. These symmetrical cells were co-fired at 1450 C for 90 min, to
densify the YSZ electrolyte layer, and to attain good
adherence of the cermet layers (Fig. 1). The resulting
cermet layers remained porous even at these relatively
high firing temperatures.
The nickel oxide in the cermet layers was reduced
to metallic Ni in 90% N2 + 10% H2, at 800 C,
yielding a porous cermet with homogeneous distribution of the metallic and ceramic components, as
found by scanning electron microscopy (Fig. 1) and
microprobe analysis. The solubility of nickel or
nickel oxide in yttria stabilised zirconia is very low
both in the as prepared powders and after reduction.
The amount of cermet powder was adjusted to obtain
porous Ni/YSZ layers with thickness of about 100
mm. The average grain sizes were under 1 mm for
both components of the cermet (Ni and YSZ), and the
pore size distribution is nearly bimodal with submicron and larger pores. One did not find significant
ageing effects in N2 + H2 atmospheres, at temperatures in the range of 681– 900 C, and for up to 3
221
weeks. Fast degradation occurred only after exposition to methane with pH2O = 0.045 atm, due to
carbon deposition.
The Autolab spectrometer (ECO Chimie) was used
to characterise the cermet/YSZ/cermet cells in wet
hydrogen or wet N2 + H2 atmospheres, and in the
temperature range 681– 884 C. Impedance spectroscopy with a frequency range 10 3 – 104 Hz was
sufficient to detect the relevant contributions of the
electrode processes. Ni mesh current collectors were
used to avoid using different metals in the anode and
for current collecting [10]. A YSZ oxygen sensor was
used to monitor the oxygen partial pressure, and the
values of water vapour partial pressure were adjusted
by bubbling gases through water at known temperatures. The nominal water vapour partial pressure was
estimated by assuming that equilibrium is attained in
these conditions. However, the oxygen partial pressure
measurements indicate that the true values of water
vapour partial pressure differ from the nominal values.
Corrected values of water vapour partial pressure were
thus estimated as follows:
pH2 O ¼ keq pH2 ðpO2 Þ1=2
ð2Þ
where keq is the equilibrium constant of reaction
H2 + 1/2O2 () H2O.
3. Results and discussion
Fig. 1. SEM microstructures of cermets obtained from combustion
synthesised powders.
Figs. 2 and 3 show impedance spectra obtained for
cermet/YSZ/cermet cells. The ohmic resistance of the
electrolyte RYSZ corresponds to a local minimum in
the high frequency range, or the intercept of the
electrode arc in the upper limit of the frequency range.
At relatively low temperatures (Fig. 2), the spectra
nearly reduce to a single somewhat depressed electrode and asymmetrical arc, with peak frequency in a
typical range of 20 – 50 Hz. However, this peak
frequency tends to increase with temperature, and
attains values of about 1 kHz at temperatures close
to 900 C (Fig. 3), as usually found for the contribution with strongest temperature dependence [2]. In
addition, these high temperature spectra show a low
frequency process at frequencies in the range of 1– 10
Hz. This low frequency contribution is dependent on
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A. Ringuede´ et al. / Solid State Ionics 146 (2002) 219–224
Fig. 2. Impedance spectra obtained with symmetrical cermet/YSZ/
cermet cells in H2 at 681 C and with water vapour partial pressure
of about 0.045 atm, and at 761 C with water vapour partial pressure
of about 0.049 atm.
the partial pressures of H2 and water vapour, and is
much less dependent on temperature, as reported by
other authors [2].
The Nyquist plots of the spectra obtained for the
present Ni/YSZ cermet anodes did not show the intermediate frequency contribution reported by Primdhal
and Mogensen [2]. Alternative representations of the
impedance data [14,15] were thus also used to assess if
the spectra included any additional contribution. The
modulus representation tends to show the contributions
with very small capacitance, as reported in the literature
[14], and thus fails to show any relevant additional
contribution with much higher capacitance, and thus
lower relaxation frequency (Fig. 4). The admittance
representation, log(A00) versus log( f ), confirms that
typical spectra reduce to two main contributions.
Fig. 3. Impedance spectra obtained with symmetrical cermet/YSZ/
cermet cells, at 884 C and for the following conditions: 15% H2,
with water vapour partial pressure pH2O = 0.031 atm (circles);
100% H2 with water vapour partial pressure pH2O = 0.043 atm
(triangles).
Fig. 4. Alternative admittance plots of the results shown in Fig. 3 to
demonstrate that the electrode processes reduce to two contributions.
The contribution of the polarisation resistance at
relatively high frequencies Rmf predominates at relatively low temperatures and conceals the low frequency
resistance contribution Rlf. Actually, the deviations
from a single arc (Fig. 2) occur mainly in the high
frequency side, suggesting an additional contribution,
at still higher frequencies. However, this interpretation
is debatable and other interpretations may be found for
similar deviations in the high frequency (left) side of
impedance spectra. Note also that the intermediate
frequency contribution found by Primdahl and Mogensen [2] should correspond to deviations in the right side
of the Nyquist plots.
For symmetrical cells, the overall electrode behaviour comprises contributions of both electrodes, and the
polarisation resistance for a single electrode thus
reduces to Rp=(R1 + R2)/2, where R1 and R2 represent
the two electrode contributions of impedance spectra
[10]. The fitting parameters extracted from the impedance spectra were thus used to obtain the moderately
high and low frequency terms of a single cermet
electrode, Rmf = R1/2 and Rlf = R2/2. Only at the highest
temperatures could one obtain results for the low
frequency term Rlf with typical values in the range of
0.2 V cm2, except possibly in very dry atmospheres
and/or for low partial pressures of H2. The high
frequency contribution is the most sensitive to changes
in temperature, with an activation energy in the range
1.12– 1.15 eV (Fig. 5), which is significantly higher
than the value of 0.8 eV found by Primdahl and
A. Ringuede´ et al. / Solid State Ionics 146 (2002) 219–224
Fig. 5. Temperature dependence of the polarisation resistance and its
main contribution (in the moderately high frequency range)
obtained in H2. Slight changes in water vapour partial pressure
occurred and are shown dashed.
Mogensen [2]. Jiang and Ramprakash [5] reported
similar values for the polarisation resistance of Ni/
TZP cermet anodes obtained by the current interruption
method. However, higher values of activation energy
were reported for results obtained under low overpotential, and much higher activation energy (1.69
eV) was reported for the polarisation resistance results
obtained by impedance spectroscopy.
On extrapolating the results obtained for Rmf, and
assuming that Rlf remains nearly unchanged, one
obtains a prediction for the polarisation resistance of
about Rp = 0.2 V cm2 at 1000 C, in atmosphere of H2
with values of water vapour partial pressure in the
range of 0.04– 0.05; this is close to the best results
reported for optimised cermet microstructures [2], and
also much better than reported by other authors [10].
Further improvements might still be attained by lowering the relatively high thickness of our cermets
(about 100 mm), and possibly also by optimising other
microstructural features. These changes are mainly
related to the high frequency (microstructural) contribution of the polarisation resistance.
The results obtained for different %H2 at 884 C
are shown in Fig. 6. The low frequency term Rlf drops
significantly with increasing %H2 but the dependence
is stronger than predicted by Eq. (1). These differences may be partly due to the increase in water
vapour partial pressure when the %H2 decreases from
100% to about 20% (also shown in Fig. 6). Fig. 7
223
Fig. 6. Dependence of the low and high frequency contributions of
the polarisation resistance on the %H2 in H2 + N2 + H2O atmospheres.
clearly demonstrates the effects of water vapour content, in agreement with Eq. (1). The main effect is
exerted on the low frequency contribution Rlf, as
reported in the literature [2,7].
The results in Figs. 6 and 7 also suggest that the
moderately high frequency contribution of the polarisation resistance increases slightly with increasing
%H2 and decreases with increasing water vapour
partial pressure. Similar trends have been reported in
the literature, namely references [2] for the effects of
water vapour and for the effects of hydrogen [3].
Fig. 7. Values of the low and high frequency contributions of the
polarisation resistance obtained for H2 with variable water vapour
partial pressure.
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A. Ringuede´ et al. / Solid State Ionics 146 (2002) 219–224
However, Jiang and Badwal [3] argued that changes
in oxygen partial pressure might be the true reason for
the changes in Rmf observed on varying the %H2 at
constant nominal water vapour content.
[2]
[3]
4. Conclusions
Powders obtained by combustion synthesis are
suitable to prepare Ni/YSZ cermet electrodes for symmetrical cells, cermet/YSZ/cermet. These cells can be
prepared by co-pressing and co-firing the different
layers of these cells. High temperature co-firing did
not spoil the microstructure of the cermets, which retain
relatively good electrochemical performance in atmospheres containing H2 and H2O. The electrode reactions
comprise two main processes. At relatively low temperatures, the overall behaviour is determined by the
high frequency contribution of the polarisation resistance. This contribution shows significant temperature
dependence with activation energy slightly above 1.1
eV. The low frequency contribution of the polarisation
resistance is much less dependent on temperature but is
very dependent on the partial pressures of H2 and water
vapour. The values of moderately high frequency
contribution of the polarisation resistance obtained in
the present work are somewhat higher than reported for
optimised cermets. This indicates that one must seek
further improvements by optimising the cermet microstructures (e.g. by lowering their thickness and/or the
firing temperatures).
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
Acknowledgements
[13]
This work was supported by the EC (project
TMRX-CT93-0130).
[14]
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