Is YSZ stable in the presence of Cu

PAPER www.rsc.org/materials | Journal of Materials Chemistry

Is YSZ stable in the presence of Cu?†

Juan Carlos Ruiz-Morales,*a Jesus Canales-Vazquez,*b David Marrero-Lopez,a Juan Pena-Martınez,a

Albert Tarancon,*c John T. S. Irvine*d and Pedro Nuneza

Received 30th April 2008, Accepted 5th August 2008

First published as an Advance Article on the web 25th September 2008

DOI: 10.1039/b807378c

XRD, XPS and electrochemical studies show clear evidence of chemical interaction under oxidising and

reducing conditions between YSZ and CuO, both components of a widely studied anode material for

high performance solid oxide fuel cells (SOFCs). The aim of this work is to identify and verify the

nature of this interaction and find means to prevent its formation during the fabrication process of

a fuel cell.

Introduction

The efficiency of a SOFC depends on the adequate properties of

all the cell elements: electrolyte, anode and cathode. The fuel

electrode has important requirements regarding conductivity and

catalytic activity, stability in the presence of several gas species,

chemical and physical compatibility with the other cell compo-

nents and, additionally, a low-cost fabrication process.

As the traditional anode material, Ni–YSZ cermet, exhibits

some limitations, several alternative anode materials have been

proposed in the last few years to develop more efficient SOFCs.

Among them, the most promising results correspond to

Cu-based cermets1–11 and perovskite-based materials such as

chromite–manganites (LSCM),12–14 substituted SrTiO3,15–18 or

double perovskites Sr2MgMoO6�d.19,20 However, mixed oxides

usually demand the use of current collectors due to the fairly low

electronic conductivity and therefore Cu-based cermets offer

significant advantages.

Copper is usually considered as an excellent current collector

with a poor catalytic activity towards hydrocarbon oxidation,

although there are a number of investigations reporting the

catalytic activity of Cu–ZrO2-based systems21,22 for several

reactions involving hydrocarbon-derived species such as CO

oxidation,23,24 methanol steam reforming,25–29 methanol

synthesis,30,31 methanol decomposition,32 methane oxidation,33

propene and toluene oxidation34 and carbon-black oxidation.35

On the other hand, there are few reports regarding the

compatibility between Cu and stabilised zirconias at high

temperatures. This is particularly relevant when considering

that during SOFC fabrication and operation, an interaction

aDpto. Quımica Inorganica, Universidad de La Laguna, Avda, AstrofısicoFrancisco Sanchez s/n, CP 38200 Tenerife, Spain. E-mail: [email protected];Fax: +34 922 318461; Tel: +34 922 318464bRenewable Energy Research Institute, Albacete Science and TechnologyPark, 02006 Albacete, Spain. E-mail: [email protected] Center of Microelectronics CNM-IMB (CSIC), CampusUniversity Autonomous of Bellaterra, 08193 Cerdanyola del Valles,Spain. E-mail: [email protected] of Chemistry, University of St Andrews, North Haugh, StAndrews, Scotland, UK KY16 9ST. E-mail: [email protected]; Fax: +44(0)1334 463808; Tel: +44 (0)1334 463817

† Electronic supplementary information (ESI) available: X-Raydiffraction patterns and XPS spectra. See DOI: 10.1039/b807378c

5072 | J. Mater. Chem., 2008, 18, 5072–5077

between the electrodes and the electrolyte may occur and

consequently, the nature of the reaction product could be

a limiting factor in the fuel cell efficiency. Indeed, unexpected

secondary phases were found after firing Cu-based cermets with

novel ceramic oxide anodes.11

In this work we have explored the origin of these interactions,

their possible effect on the electrochemical performances of

Cu-based systems and defined means to prevent them.

Experimental

X-Ray powder diffraction (XRD) patterns were obtained using

a Philips X’Pert Pro diffractometer, equipped with a primary

monochromator (CuKa1) and a X’Celerator detector. The

patterns were performed using 0.016� steps (30 s per step) in the

15–90� 2q range. Rietveld refinement of the XRD patterns

were performed using FULLPROF software.36 The fits were

performed using a pseudo-Voigt peak-shape function. In the

final cycles, the usual profile parameters (scale factors, back-

ground coefficients, zero-points, half-width, pseudo-Voigt and

asymmetry parameters for the peak-shape) were refined. During

the refinement all the atomic parameters (positions and site

occupation) were fixed and not refined. The experimental errors

presented and determined by Rietveld refinement were smaller

than the dimension of the symbols. All graphics related with

XRD patterns were performed using WinPLOTR software,37

and X’Pert HighScore Plus38 was used for phase identification.

Several composites of YSZ–CuO were prepared to test the

compatibility in oxidising and reducing conditions. The powders,

in an appropriate ratio, were mixed in an agate mortar with

acetone, left to dry and fired at different temperatures ranging

from 800 �C up to 1000 �C for 3 hours. Heating and cooling ramp

rates were fixed at 5 �C min�1 in all cases. The powders were fired

on a platinum foil to prevent reaction with the ceramic

substrates. The composite powders tested in oxidising conditions

were also treated under humidified 5% H2–Ar, in the same

temperature range for 20 hours, to test the compatibility under

reducing conditions.

8 mol% YSZ (Pikem, produced by Daiichi Kigenso

KK-Japan) was used as electrolyte and as a composite element.

Dense YSZ pellets were obtained after uniaxially pressing YSZ

powders at 31 MPa for 1.5 min and further sintering at 1500 �C

This journal is ª The Royal Society of Chemistry 2008

Fig. 1 Experimental XRD pattern for a composite YSZ–CuO (1 : 1

weight), after firing in air, at 950 �C for 3 h, together with the pattern for

the monoclinic ZrO2 obtained from the ICSD database.

for 10 h. CuO used in the composites was from Aldrich (>99%)

and the grain size was <5 mm. Powders of YSZ and CuO were

mixed in an appropriate ratio in addition to a Decoflux (WB41,

Zschwimmer and Schwartz) binder to obtain a slurry, which was

used to attach the electrodes onto the YSZ tape in symmetrical

configuration. The samples were fired at several temperatures for

two hours.

Experimental details for electrochemical measurements can be

found elsewhere.39 The measurements were performed in

symmetrical configuration, under symmetrical atmosphere and

always on cooling.

Two samples were prepared to observe the effect of the

reduction–diffusion zone (under reducing conditions) on the

electrochemical properties of the YSZ electrolyte. Two samples

of dense electrolyte were covered with a YSZ–CuO slurry in

a 4 : 1 ratio, and then fired at 1000 �C for 3 h. After that, one was

fired at 950 �C for 40 hours under humidified 5% H2–Ar and the

second at 700 �C under the same experimental conditions. In

both cases, the cermet anode covering the dense pellet was

removed by polishing the pellet surface, and a dark region where

the anode was in contact with the electrolyte could be observed.

In the case of the sample fired at 950 �C the region seems to go

deep inside the microstructure, whereas for the sample fired at

700 �C sample the dark region was superficial. Both samples and

another YSZ used as a blank were covered with a Pt-ink layer as

current collector. Bulk, grain boundary and total conductivities

were measured by ac impedance spectroscopy.

Optical images of the cross- and top-section of samples were

acquired using a stereomicroscope Leica Zoom 2000.

XPS spectra were obtained on a Physical Electronics

550 spectrometer, using monochromatic AlKa radiation. To

minimise discrepancies due to sample compactness, charging

effects and contact resistance, all the measurement parameters

were verified and adjusted to get the 1s core level of adventitious

carbon at 284.8 eV.

Fig. 2 XRD patterns of a YSZ–CuO (1 : 1 weight) composite, fired in air

in a range of temperatures, for 20 hours each. Arrows indicate m-ZrO2

peaks.

Results and discussions

Preliminary studies in the system CuO–cubic-YSZ showed the

presence of secondary phases, Fig. 1.

The extra reflections observed by XRD corresponded to

monoclinic zirconia, Fig. 1 and 2, which fully agreed with Simner

et al.,40,41 who previously reported the reaction of YSZ with

compounds containing just 2% CuO to yield monoclinic zirconia.

No traces of other impurities were detected.

Simner et al. also suggested that this reaction is very sensitive

to YSZ calcination temperature, e.g. pre-firing YSZ at high

temperature (1200 �C) seemed to inhibit the reaction between

a copper containing oxide and YSZ at lower temperatures.

However in our case, the reaction is not affected by this treat-

ment, (Fig. S1, ESI†).

m-ZrO2 seems to be produced independently of the Cu source,

e.g. (La0.8Sr0.2)0.98Fe0.98Cu0.02O3�d,40,41 CuNb2O6,42 or simply

CuO11,43,44 mixed with YSZ and fired at relatively low tempera-

tures, 900–1000 �C, for 2–3 h produce the same result. One

should note that an intimate mixture of powders is not necessary

to produce the aforementioned reaction. Indeed YSZ powders on

a slightly pressed thin CuO layer, led to the same formation of

m-ZrO2 after firing at 1000 �C for 3 hours.


Additional studies performed to evaluate the sensitivity of

this reaction with the Cu content and temperature showed that

even the addition of just 1% weight of CuO was enough to

produce monoclinic zirconia, Fig. 3. The amount of monoclinic

zirconia formed did not depend on the CuO concentration

above 5% wt. The firing temperature should be below 900 �C to

avoid YSZ destabilisation, as shown in Fig. 2. This was double

checked by firing, at 800 �C for 120 hours, a composite prepared

by impregnation of YSZ with copper acetate, which decom-

poses to form CuO, and no trace of the monoclinic zirconia was

detected.

On the other hand, the same thermal treatments under mildly

reducing conditions did not result in the formation of m-ZrO2,

(Fig. S2, ESI†). This is clear indication that Cu2+ is responsible of

the formation of m-ZrO2.

J. Mater. Chem., 2008, 18, 5072–5077 | 5073

Fig. 4 Evolution of (a) phase concentration and (b) unit cell volumes

determined by Rietveld refinement as a function of the annealing

temperature. (The multiphase refinements converged to final agreement

factors about: Rwp ¼ 15.5% and Rexp ¼ 7.2%).

Fig. 3 XRD patterns for composites YSZ–CuO, different ratios in

weight, fired in air, at 950, 3 h. (Arrows indicate peaks from the mono-

clinic zirconia).

The origin of the YSZ destabilisation due to the presence of

Cu2+ has not been described in the literature, although there exist

certain possible routes to explain this process. Firstly, CuO may

interact with Y3+ to produce secondary phases liberating m-ZrO2

from the stabilised cubic YSZ. A second route could be via

substitution of Zr4+ in the cubic YSZ for Cu2+.

The reaction between Y and Cu could lead to the formation of

the blue-greenish compound Y2Cu2O5 as observed by Lemaire

et al.45 and Ran et al.46–48 in a similar situation, e.g. when firing

CuO with 3 mol% yttria-stabilised tetragonal zirconia (3Y-TZP)

at temperatures above 850 �C. A long XRD run carried out in

a 1 : 1 YSZ–CuO composite, fired at 975 �C for 3 hours showed

no trace of Y2Cu2O5 (Fig. S3, ESI†). It should be noted

that CuO–YSZ mixture calcined at 1000 �C presents �30 %wt of

m-ZrO2 and if the formation of this phase was due to yttria

depletion to form Y2Cu2O5, this last phase would have been

detected by XRD. Alternatively, the possible formation of

Y2Cu2O5 may be monitored by increasing the segregation of

Y2O3 as the authors suggested, increasing the firing temperature

of YSZ. However, the XRD pattern of a 1 : 1 CuO : YSZ

composite fired at 1000 �C for 3 h with YSZ previously fired 15 h

at 1200 �C, (Fig. S1, ESI†), did not show any trace of Y2Cu2O5.

Therefore, the second option was investigated. Dongare

et al.49,50 and Bhagwat et al.51 reported the stabilisation of cubic

zirconia with Cu in samples containing just 2 mol% Cu, after

firing at 500 �C. Analogous stabilisations of cubic zirconia have

been reported for other transition metals as Mn, Co, Fe to

produce materials with special catalytic properties.52 Similarly,

a possible substitution of Zr4+ for Cu2+ in the YSZ lattice could

allow the formation of Cu-stabilised cubic zirconia, liberating

monoclinic ZrO2 in the process. The variation of the unit cell

volumes and the quantification of the different phases as

a function of the firing temperature were investigated by Rietveld

refinements, Fig. 4. The cell volume for YSZ increases with the

firing temperature above 925 �C, when secondary phases appear.

In addition, the unit cell volume for CuO is independent of

the firing temperature (inset of Fig. 4b) and the m-ZrO2

volume undergoes a slight increase. The concentration of m-ZrO2

increases up to 30% at 1000 �C; on the other hand, the

CuO content in the mixture varies only by 2% between 900

and 1000 �C.

5074 | J. Mater. Chem., 2008, 18, 5072–5077

These results demonstrate that only a small degree of Cu

substitution is required to destabilise cubic YSZ to form m-ZrO2,

which is not entirely surprising if one considers the proximity

of the tetragonal + cubic phase to the cubic phase boundary to

8 mol% yttria in the ZrO2–Y2O3 phase diagram. In this study, the

monoclinic phase has been observed throughout, although it is

quite likely that it is the tetragonal phase that forms at that

temperature and this transforms to monoclinic on processing of

powders. Such a transformation is routinely encountered in

studies of these zirconia systems. This also explains the increase

of YSZ cell volume with increasing the firing temperature as

higher levels of yttria in doped zirconia produce an increase of

the volume cell. The absence of segregated yttria or phases

containing yttria, e.g. Y2Cu2O5, seems to confirm this behaviour.

One should note that the formation of m-ZrO2, which exhibits

a very low conductivity, will affect the ohmic resistance of the

composite electrode material under study. The mechanical

properties of monoclinic zirconia may also cause degradation

through either phase transformation to tetragonal at high

temperatures or through hydrothermal ageing at lower temper-

atures. However the polarisation resistance values do not show

evidence that this has an impact on the performance.


It has been reported that Cu may diffuse from the anode

microstructure towards the dense electrolyte,8 also in several

other materials such as the superconducting phases

YBa2Cu3O7�x. Vasiliev et al.53 reported and identified the

diffusion of metallic Cu through the grain boundaries of YSZ

during their preparation on Si substrates with YSZ buffer layers

at low temperatures 700–750 �C.

A slurry of CuO:YSZ was painted on a dense YSZ pellet, and

the resulting specimen was then fired at 1000 �C for 3 hours.

After that, it was heated at 950 �C, under wet 5% H2–Ar for

10 hours. The surface of the electrolyte, Fig. 5a and the cross-

section of the sample, Fig. 5b indicate a dark reaction zone. As

the only materials involved were Cu and YSZ, one may assume

that Cu is responsible for such reaction. However, it should

be noted that the composite must be strongly attached to the

electrolyte to observe the aforementioned dark zone. EDAX

analysis performed inside this 100 mm layer do not reveal the

presence of Cu, although the same tests in the anode layer show

clear evidence of Cu. Consequently the Cu content should be

below the detection limit of this technique (typically 5%), and/or

it may occur that Cu promotes YSZ reduction near the interface.

On the other hand, the electrode layer was removed by polishing

and the electrolyte interface in contact with the electrode showed

a very dark colour, Fig. 5c. A XRD pattern of this zone, (inset of

Fig. 5d) indicates the presence of Cu at trace levels.

XPS studies were carried out, in a sample reduced at 950 �C

under humidified 5% H2–Ar, to understand the surface compo-

sition of the Cu–YSZ interface. Photoelectron peaks associated

Fig. 5 Optical pictures of (a) top-view of a cell with Cu–YSZ cermet in

contact with a dense YSZ pellet (1.2 mm thickness), after annealing in

reducing conditions. (b) Magnified view of a cross-section, showing the

anode layer, a reaction zone and the rest of the electrolyte dense pellet. (c)

Top-view of the same sample after removing the electrode by polishing,

revealing a dark region that corresponds with the electrode contact zone.

(d) XRD pattern of the dark zone shown in (c). The dark line corresponds

to the pattern obtained after one polishing process. The cubic-YSZ peaks

appear distorted at lower angles. This distortion almost disappears after

a second polishing process (mid line). The light line corresponds to the

back of the YSZ sample used as reference of free Cu. Just traces of

metallic Cu could be verified from X-ray as shows the inset of Fig. 5d.


to Cu 2p and Zr 3d core levels were studied on both bulk YSZ

and Cu–YSZ interface. For Cu core level on bulk YSZ no

significant peaks were observed, whilst at the Cu–YSZ interface

a doublet peak was observed corresponding to Cu2p3/2 and

Cu2p1/2 at 932.4 and 952.3 eV respectively, (Fig. S4, ESI†). The

binding energies for the observed copper core lines are charac-

teristic of copper metal and copper(I) oxide. Albeit copper metal

has binding energies very close to those for Cu2O making it is not

possible to distinguish between the two compounds, the presence

of copper(II) oxide can be discarded (the Cu2p3/2 line for CuO is

shifted further 1.3 eV above).54 Quantitative compositional

analysis of the Cu–YSZ interface yields a copper-to-zirconium

ratio (NCu–NZr) around 3%.

Similar spectra were observed for the Zr core level on both

bulk YSZ and Cu–YSZ interface. Two different doublets for

each Zr 3d5/2/Zr 3d3/2 core levels were observed, 181.2/184.2 eV

and 183.3/185.95 eV, respectively, Fig. 6. These two sets of peaks

can be attributed to changes in local chemical and physical

environment or to the presence of mixed valence states for

zirconium, Zr4+ and Zr3+, on the surface of the sample. As

changes in local environment are usually associated to smaller

energy shifts (less than 0.5 eV), the measured shift is more likely

to be related to mixed valence states (probably limited to a very

narrow band close to the surface). Indeed, Pomfret et al.55 have

reported evidence of zirconium reduction on the surface for

yttria-stabilized zirconia after exposing YSZ to a reducing

environment at elevated temperatures; this has also been

observed after exposing a dense YSZ pellet under humidified

5% H2–Ar at 950 �C for 20 h (Fig. S5, ESI†). This reduction of

zirconium could be promoted by copper, as previously reported

for nickel in Ni–ZrO2 plating.56 Hence, a combination of

processes would be the origin of the reaction region, i.e. an

exolution process involving metallic Cu (from the previously

incorporated CuO) and a very superficial partial reduction of

Zr(IV) to Zr(III) promoted by Cu.

Additional studies were carried out to reveal the effect of this

reduction zone in the electrochemical properties of dense YSZ

electrolytes, Fig. 7. The higher the temperature during the

reduction process, the lower the conductivity of the YSZ will be,

Fig. 7. Both, bulk and grain boundary contributions are deeply

affected by the reaction zone, Fig. 7a and 7b. In general the bulk

and grain boundaries conductivities are between �20% and

�30% lower than the values corresponding to the YSZ blank, for

Fig. 6 XPS core level spectra of Zr 3d in the apparent reaction zone.

J. Mater. Chem., 2008, 18, 5072–5077 | 5075

Fig. 7 (a) Impedance spectra, (b) overall conductivity and (c) bulk and grain boundary conductivities of three dense YSZ samples, one of them used as

a blank. The other two were previously covered with a slurry of YSZ–Cu-based anode (80 : 20 % weight) and fired at 1000 �C in air for 3 h. After that they

were fired under reducing conditions (humidified 5% H2–Ar) at 950 �C and 700 �C. The activation energies of each process match very well the values of

the YSZ used as reference, indicating that the main charge-transport mechanism remains unaffected by the reaction zone.

the sample treated at 700 �C under H2. At 950 �C the degradation

is more noticeable, i.e. an increase of �50% and 100% in the

resistance values was observed. Hence even firing at low

temperatures as 700 �C and with just an apparent superficial

reaction zone, the effect in the conductivity of the YSZ is very

important and should not be neglected. Moreover, one should

remark that these results refer to very thick electrolytes and

therefore the effect of this reaction may be much worse in thin

YSZ electrolytes.

This is entirely consistent with what might be considered

feasible redox chemistry and seems clear evidence that CuO

exolves Cu metal. It seems that as this incorporation of CuO and

exolution of Cu releases further secondary phases and under

kinetically controlled series of redox processes, one might expect

this not to be reversed and so secondary phases would build up,

even at temperatures below 950 �C.

In fact, the irreversibility of this degradation was experimen-

tally verified. The sample fired at 700 �C, was heated again under

oxidising conditions and the resistances were always higher than

the values obtained in the previous cycle. It should be expected

that long time operation would produce a deeper reaction zone

inside the pellet, resulting in poorer conductivity values with the

5076 | J. Mater. Chem., 2008, 18, 5072–5077

time. This effect may explain the recently published results by

Gorte et al.9,10 They found that annealing a Cu-impregnated

YSZ-based composite under reducing conditions at temperatures

above 700 �C produced a marked degradation of the fuel cell

performance under study. These results were explained by the

authors as due to the agglomeration of Cu particles. We think

that our findings regarding the existence of a reaction zone in the

electrode–electrolyte interface, even at very superficial level,

could partly explain such degradation in the electrochemical

response.

Conclusions

In summary, it has been shown that CuO reacts with YSZ under

oxidising conditions and Cu is then exolved under reducing

conditions even at low temperatures.

Firing YSZ with a material containing Cu2+ always results in

the formation of m-ZrO2 even when the CuO content was as low

as 1% in weight, though this may be avoided by firing under

reducing conditions. XRD analysis suggests that Cu2+ destabil-

ises the YSZ, resulting in the formation of Y-rich cubic and


monoclinic zirconia, ruling out the formation of Y2Cu2O5, at

least under our experimental conditions.

The formation of m-ZrO2 does not affect the polarisation

resistance values and its low conductivity is compensated with

the excellent electronic contribution from Cu. In any case the

formation m-ZrO2 could be avoided by firing at temperatures

below 900 �C under oxidising conditions.

Finally, XPS studies show evidence of metallic Cu and/or Cu(I)

inside of the YSZ electrolyte together with a partial reduction of

Zr(IV) to Zr(III). Although the electrolyte seems to be affected

just in the surface layer in contact with the Cu-based anode,

impedance measurements reveals a non-reversible increase of at

least �25% in the overall resistance compared to the as-prepared

YSZ, at 700 �C. In this case an alternative to overcome this

situation could be via fuel cell operation at very low tempera-

tures, perhaps below 600 �C.

The impregnation methods could be an ideal solution to

overcome both problems, though always working and/or

fabricating at low temperatures (<850 �C and <700 �C under

oxidising and reducing conditions, respectively).

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Is YSZ stable in the presence of Cu

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