Lanthanum Germanate-Based Apatites as Electrolyte for SOFCs

Lanthanum Germanate-Based Apatitesas Electrolyte for SOFCsD. Marrero-López1*, P. Díaz-Carrasco1, J. Peña-Martínez2, J. C. Ruiz-Morales3,J. R. Ramos-Barrado1

1 Departamento de Física Aplicada I, Laboratorio de Materiales y Superficies (Unidad Asociada al C.S.I.C.), Universidad de Málaga,29071 Málaga, Spain2 Instituto de Energías Renovables, Parque Tecnológico, Universidad de Castilla La Mancha, 02006 Albacete, Spain3 Departamento de Química Inorgánica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain

Received February 05, 2010; accepted March 18, 2010

1 Introduction

Electrolytes with apatite-type structure have been studiedin last years due to their high ionic conductivity at low tem-perature and potential application in solid oxide fuel cells(SOFCs) [1–8]. The dominant ion conduction mechanism inthese materials is via interstitial oxide ions, in contrast to thecommonly used fluorite and perovskite-based electrolytes,where the conduction occurs via oxygen vacancies [9–13].Several lanthanum silicate apatites exhibit higher ionic con-ductivity than the state-of-the-art Zr0.84Y0.16O1.92 (YSZ) elec-trolyte at intermediate temperature range (600–800 °C). Forthis reason, they have been considered as a promising alter-native to YSZ for SOFC applications [14, 15]. However, sever-al drawbacks of these silicates must be overcome before usingthem as SOFC electrolytes. For instance, they require highsintering temperatures (around 1,600 °C) to obtain fullydense ceramic materials, which are necessary to separate thegases of anode and cathode compartments. Furthermore, theionic conductivity of silicates depends strongly on synthesismethod, sintering temperature, microstructure and porosity

of ceramic materials [16–18]. Despite the fact that several wetchemical methods have been used to prepare silicates withlow particle size, most of them require sintering temperaturesas high as 1,600 °C to obtain the required densification[19–21]. On the other hand, the compatibility and perfor-mance studies of different electrodes with silicates showedthat silica migrates towards the electrode/electrolyte inter-face [22, 23]. As a result, the electrochemical reaction zonesare partially blocked in this region, reducing the performanceof these electrodes. Otherwise, a ceria buffer layer depositedbetween the silicate electrolyte and electrode materialsimproves the efficiency of these electrodes and provides sta-bility to the fuel cell during operation conditions [24].

Germanium apatites have been less considered for fuel cellapplications due to the high cost of germanium, comparedto silica, and possibly lower stability at high temperature

–[*] Corresponding author, [email protected]

AbstractGermanate apatites with composition La10–xGe5.5Al0.5O26.75–

3x/2 have been evaluated for the first time as possible electro-lytes for solid oxide fuel cells (SOFCs). Different electrodematerials have been considered in this study, i.e. manganite,ferrite, nickelates and cobaltite as cathode materials; andNiO–CGO composite and chromium–manganite as anodes.The chemical compatibility and electrochemical perfor-mance of these electrodes with La9.8Ge5.5Al0.5O26.45 havebeen studied by X-ray powder diffraction (XRPD) and impe-dance spectroscopy. The XRPD analysis did not reveal

appreciable bulk reactivity with the formation of reactionproducts between the germanate electrolyte and these elec-trodes up to 1,200 °C. However, a significant cation interdif-fusion was observed by energy dispersive spectroscopy(EDS) at the electrode/electrolyte interface, which leads to asignificant decrease of the performance of these electrodes.

Keywords: Apatite-Type Electrolyte, Area-Specific Resis-tance, Lanthanum Germanate, SOFC

FUEL CELLS 11, 2011, No. 1, 65–74 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 65

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DOI: 10.1002/fuce.201000024

Marrero-López et al.: Lanthanum Germanate-Based Apatites as Electrolyte for SOFCs

[25–27]. However, germanates have the advantage of the lowdensification temperature by using adequate synthesis routes[28–30]. Moreover, the total conductivity of germanates is notseriously affected by the microstructure in contrast to sil-icates, and large amounts of substitutions can be carried outin the Ge-site without affecting considerably the ionic con-ductivity and therefore reducing fabrication costs. For theaforementioned reasons, germanates could be of interest forSOFC applications at intermediate temperatures, althoughfurther investigation on the chemical compatibility and stabil-ity with different electrode materials is required.

The aim of this work is to evaluate the use of lanthanumgermanates, with apatite type structure, as SOFC electrolyteswith different electrode materials. Chemical compatibility isevaluated by X-ray powder diffraction (XRPD) and energydispersive spectroscopy (EDS). Area-specific resistance (ASR)with the different electrode materials are studied by impe-dance spectroscopy using symmetrical cells.

2 Experimental

2.1 Synthesis and Powder Characterisation

Polycrystalline powders of La9.8Ge5.5Al0.5O26.45 were pre-pared via a freeze-drying precursor method as describedelsewhere [30]. It should be commented that this compositionexhibits lower ionic conductivity than those containinghigher lanthanum content, e.g. La10Ge5.5Al0.5O26.75 [30],however, La9.8Ge5.5Al0.5O26.45 phase shows hexagonal sym-metry (space group P63/m) in the whole temperature rangestudied 25–1,000 °C in contrast to samples with higherlanthanum composition, which present a phase transforma-tion from triclinic to hexagonal symmetry around 750 °C [27,30]. It is worth noting that a phase transition could be detri-mental for practical application in SOFCs due to the mechani-cal stress caused during the thermal processes and for thisreason the composition La9.8Ge5.5Al0.5O26.45 was used in thisstudy.

Electrode materials were synthesised by sol–gel or freeze-drying routes. The starting reactants were metal nitratessupplied by Aldrich: La(NO3)3

.6H2O (99.99%), Sr(NO3)2

(99.9%), Ca(NO3)2.4H2O (99%), Fe(NO3)3

.9H2O (98%),Mn(NO3)2

.6H2O (99.99%), Cr(NO3)3.9H2O (99%),

Ni(NO3)2.6H2O (99%) and Cu(CH3COO)2 (99%). These were

previously studied by thermogravimetric analysis to deter-mine the correct cation composition. The synthesis procedurewas similar to that reported previously [24, 31]. Commercialpowders of La0.8Sr0.2MnO3–d (LSM, Praxair specialty ceram-ics) and Ce0.9Gd0.1O2–d (CGO, Rhodia) were also used.NiO–CGO composite powders, 60 wt.% of NiO, were pre-pared from CGO powders and an ethanol solution containingnickel nitrate. They were mixed and dried under stirring at35 °C and then fired at 900 °C for 1 h to obtain the compositematerial.

XRPD patterns were recorded using a PANalytical X’PertPRO automated diffractometer equipped with a Ge(111) pri-mary monochromator and the X’Celerator detector. The scanswere collected in the 2h range (10–100°) with 0.016 step for1–2 h. XRPD studies were also performed to evaluate thechemical compatibility of La9.8Ge5.5Al0.5O26.45 with the differ-ent electrodes listed in Table 1. Mixtures of electrolyte andelectrodes powders were prepared in a 1:1 (wt.%) ratio,ground in an agate mortar and then fired in the temperaturerange between 800 and 1,200 °C for 5–24 h, depending on theelectrode composition (Table 1). The powder mixtures werethen cooled to room temperature (RT) and analysed byXRPD. The highest firing temperature used in the chemicalcompatibility studies was 1,200 °C, because this temperaturewas sufficient to obtain a good adhesion between the elec-trode and electrolyte materials as discussed below. Phaseidentification and quantification were performed with X’PertHighScore Plus v.2.2d software [32]. In the Rietveld methodthe usual parameters (zero-points, scale factors, backgroundcoefficients, pseudo–Voigt and asymmetry parameters for thepeak-shape) were refined. The atomic parameters were fixedand not refined.

Table 1 Firing temperature (Td), ASR and activation energies (Ea) for the different electrode and composite electrode materials investigated withLa9.8Ge5.5Al0.5O26.45.

Composition Abbreviation Td (°C) ASR (X cm2) Ea (eV)

800 °C 600 °C –

Pt (cathode) Pt 900 0.80 266 2.28La0.8Sr0.2MnO3–d LSM 1,100 6.34 229 1.34La0.6Sr0.4Co0.2Fe0.8O3–d LSCF0.8 950 1.23 89 1.40La0.6Sr0.4Co0.8Fe0.2O3–d LSCF0.2 950 0.78 45.3 1.2750 wt.% La0.6Sr0.4Co0.8Fe0.2O3–d + Ce0.9Gd0.1O1.95 LSCF0.2 + CGO 950 0.37 18.8 1.16CGO/La0.6Sr0.4Co0.8Fe0.2O3–d CGO/LSCF0.2 950 0.12 3.66 1.26CGO/La0.75Sr0.25Cr0.5Mn0.5O3–d (cathode) CGO/LSCM 1,050 5.08 205 1.31La2Ni0.8Cu0.2O4 LNC 1,100 – – –Pt (anode) Pt 900 27.7 565 1.0860 wt.% NiO + CGO (composite) Ni + CGO 1,200 2.22 21.5 0.7860 wt.% NiO + La9.8Ge5.5Al0.5O26.45 Ni + germanate 1,200 65.7 647 0.89CGO/La0.75Sr0.25Cr0.5Mn0.5O3–d (anode) LSCM 1,050 2.49 41.5 1.13

ASR values were obtained in air and humidified 5%H2–Ar for the cathode and anode materials, respectively.

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2.2 Conductivity and Area-Specific Resistance Measurements

As-prepared La9.8Ge5.5Al0.5O26.45 powders were uniaxiallypressed into disks of 13 mm of diameter and 1 mm of thick-ness under a pressure of 250 MPa. The green pellets were sin-tered at 1,200 °C for 1 h in air, reaching relative densitiesranging from 90 to 95%. After that, a platinum paste wasdeposited on each side of the pellets and fired at 900 °C for30 min. The overall conductivity of the pellets was deter-mined by impedance spectroscopy using a 1260 SolartronFRA with an AC signal of 50 mV in the 0.01–106 Hz fre-quency range and temperature from 800 to 250 °C with acooling rate of 5 °C min–1 and stabilisation time of 15 minbetween consecutive measurements.

For ASR measurements, symmetrical electrodes were coat-ed on both sides of the electrolyte using a slurry preparedwith the electrode powders and Decoflux™ (WB41, Zschim-mer and Schwarz) as binder material. The symmetrical cellswere fired, depending on the electrode composition, between950 and 1,200 °C for 1 h in air (Table 1). Afterwards, Pt-basedink was applied onto the electrodes to obtain a current collec-tor layer and finally fired at 900 °C for 30 min. A CGO bufferlayer between the electrolyte and electrode materials was alsoinvestigated in order to prevent cation interdiffusion betweenthe materials. The CGO buffer layer was fired onto germanateelectrolyte at 1,200 °C for 1 h.

The ASR values were obtained under symmetrical atmo-spheres (air or humidified 5%H2–Ar) in a two electrode con-figuration. Fuel gases were humidified by bubbling through agas-washer at a temperature of 20 °C. The gas flow fuel ratewas set at 30 mL min–1, using a mass–flow controller. Impe-dance spectra of the electrochemical cells were collectedusing the Solartron 1260 FRA, at open circuit voltage (OCV).The spectra were fitted to equivalent circuits using the ZViewsoftware [33].

Scanning electron microscopy (SEM) images wereobtained on a JEOL SM-6490LV electron microscope com-bined with EDS to analyse the microstructure, connectivitybetween electrodes and electrolyte layers, or any evidence ofdegradation after electrochemical characterisation.

3 Results and Discussion

3.1 Conductivity

The overall conductivity for La9.8Ge5.5Al0.5O26.45 is shownin Figure 1. For comparison the conductivity data of severalsolid electrolytes, commonly used in SOFCs, are also plottedin this figure. As can be observed the conductivity ofLa9.8Ge5.5Al0.5O26.45 is somewhat lower than those of relatedsilicates with similar composition, especially in the low tem-perature range (e.g. 0.011 S cm–1 for La9.8Ge5.5Al0.5O26.45 and0.025 S cm–1 for La10Si5.5Al0.5O26.75 at 700 °C). The values ofconductivity of this germanate are also lower compared tothose of YSZ and doped ceria electrolytes, e.g. 0.02 S cm–1 forYSZ and 0.041 S cm–1 for CGO at 700 °C in air. Although, the

conductivity of this germanate is somewhat lower than thatof common SOFC electrolytes, it has the advantage of the lowdensification temperature and thin film electrolytes could beused to reduce the ohmic losses of the electrolyte and toachieve high performance in a SOFC.

3.2 Chemical Compatibility

The electrochemical performance of an electrode dependson its electrocatalytic properties in addition to the characteris-tic of the interface between electrolyte and electrodes [34]. Inthis sense the chemical compatibility between the fuel cellcomponents plays a critical role in the performance of aSOFC. An excessive interdiffusion of cations between theelectrolyte and the electrode layers could create new phasesat the interface. Depending on the nature of these phases, theelectrochemical reaction sites might be partially blocked, re-ducing the oxygen transference and consequently the cell per-formance. For these reasons, it is fundamental to know thechemical compatibility between the cell components and theoptimum sintering temperature in order to minimise the pos-sible formation of reaction products at the electrode/electro-lyte interfaces during fabrication and operation of the fuelcell.

The chemical compatibility of La9.8Ge5.5Al0.5O26.45 wasevaluated by XRPD with several electrode materials listed inTable 1. Powder mixtures of electrolyte and electrodes1:1 wt.% were calcined at different temperatures and thenanalysed at RT by XRPD (Figure 2).

As can be observed in Figure 2, the XRPD patterns do notshow significant structural changes or additional phases afterfiring the powder mixtures between RT and 1,200 °C for 24 h.

1.0 1.2 1.4 1.6 1.8 2.0-7

-6

-5

-4

-3

-2

-1800 700 600 500 400 300

La9.8Ge5.5Al0.5O26.45

Zr0.84

Y0.16

O1.92

Ce0.9Gd0.1O1.95

La10Si5.5Al0.5O26.75 ref.[24]

log(

σ / S

cm-1)

103/T (K-1)

T / ºC

Fig. 1 Arrhenius plot of the overall conductivity of La9.8Ge5.5Al0.5O26.45and different commercial solid electrolyte materials: Zr0.84Y0.16O1.92(Tosoh), Ce0.8Gd0.1O1.95 (NexTech) and La10Si5.5Al0.5O26.75 [24].

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However, a detailed analysis of the XRPD patterns revealsmall shifts in the diffraction peak positions of certain phasesas the firing temperature increases (Figure 2a and c). Forinstance, the diffraction peaks for CGO and germanate phasesare slightly shifted with respect to the starting position at RTand a similar trend is observed in the phases of the LSCF/apatite mixture.

Small amounts of reaction products are only found in theLNC–germanate mixture after firing at 1,200 °C, which wasidentified as La2GeO5 (Figure 2b). This phase is usually pres-ent in germanates with higher lanthanum content [27]. In thissystem one can also observe a substantial diffraction peakbroadening, in several peaks of the XRPD pattern of the ger-manate structure, with the temperature increase (Figure 2b),which is accompanied with a slight expansion of the volumecell from 619.5 Å3 at RT to 620.3 Å3 at 1,200 °C. As previouslycommented, La10–xGe5.5Al0.5O26.75–3x/2 phases with x < 0.2show triclinic structure at RT with broader diffraction peakscompared to the phases with hexagonal symmetry for x ≥ 0.2[30]. Thus, the diffraction peak broadening observed for the

germanate in the LNC–apatite mixture could be explained byan increase of lanthanum content in the apatite structure dueto lanthanum migration from the nickelate to the germanate,resulting in a slight change of symmetry of the apatite fromhexagonal to triclinic. This seems to be also confirmed by thepresence of La2GeO5 as reaction product, which is usuallyfound in germanates with excess of lanthanum.

In the case of CGO–germanate mixture, the unit cell vol-ume of both fluorite and apatite structure increases slightlywith the temperature (619.11–621.69 Å3 for the germanate,and 159.03–159.74 Å3 for CGO, in the temperature range of25–1,200 °C). Interdifussion of Ce and La into the germanateand ceria phases respectively cannot be ruled out consideringthe similar ionic radius of both cations.

These results indicate that germanate apatites show lowbulk reactivity with the investigated electrodes of Table 1,because reaction products are only found in the LNC–germa-nate system at 1,200 °C. Therefore, one should expect highperformance of these electrodes in contact with the germa-nate-based electrolytes. However, interdiffusion of elements

20 25 3 35

1200ºC

1000ºC

900ºC

800ºC

2θ/º

RT•

La9.8

Ge5.5

Al0.5

O26.45

CGO

30.0 30.5 31.0 31.5

2θ/º

(c)

20 25 30 35

1000 ºC

900 ºC

800 ºC

2θ/º

RT•

La9.8

Ge5.5

Al0.5

O26.45

La0.6

Sr0.4

Co0.8

Fe0.2

O3

29.5 30.0 30.5 31.0

2θ/º

(a)

20 25 30 35 40

1200ºC

1000ºC

900ºC

800ºC

2θ/º

RT•

La9.8Ge5.5Al0.5O26.45

NiO

29.5 30.0 30.5 31.02θ/º

(d)

20 25 30 35

La9.8

Ge5.5

Al0.5

O26.45

La2Ni0.8Cu0.2O4

* ** *1200ºC

1000ºC

900ºC

800ºC

2θ/º

RT

La2GeO5

•

*

30.0 30.6 31.2

2θ/º

*

(b)

Fig. 2 XRPD patterns for (a) LSCF0.2 + germanate, (b) LNC + germanate, (c) CGO + germanate and (d) NiO + germanate powder mixtures (1:1 wt.%)at RT and after firing between 800 and 1,200 °C. The different phases are labelled in the figure.

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or segregation of minor reaction products at the electrode/electrolyte interface may have negative effects on the elec-trode performance as observed previously in related silicates[22–24].

3.3 SEM and EDS Analysis

Combined SEM and EDS studies were performed tofurther investigate the chemical compatibility between thegermanate and the electrode materials.

The microstructure of SEM images after the electrochemi-cal tests reveal an adequate porosity and adherence of theelectrodes with thickness ranging between 25 and 35 lm (Fig-ures 3a and 4a). Despite the fact that non appreciable chemi-cal reaction was observed by standard XRPD, the EDS analy-sis show significant interdiffusion of cations between theelectrolyte and electrode layers. Representative EDS analysisis shown in Figures 3b and 4b. In the case of LSM/germanateinterface, a deficiency of Sr near the LSM interface and simul-taneously incorporation of Sr in the electrolyte are observed.Mn incorporation into the electrolyte is difficult to detect due

to the overlapping of the Mn and La peaks, although this can-not be ruled out because small extra peaks ascribed to thiselement are visible at the electrolyte interface (Figure 3b). Inaddition, Ge is also detectable in the LSM layer. The esti-mated variation of the atomic concentration with the distanceat the interface is shown in Figure 3c.

A significant interdiffusion of cations is also observed inthe CGO/germanate interface after sintering at 1,200 °C, in-volving diffusion of La to CGO, and Ce to the apatite phase(Figure 4b). Germanium was also detected in a region of3–5 lm towards the CGO layer. On the other hand, Ce diffu-sion inside the electrolyte material was not observed at theinterface between the LSCF0.2 + CGO composite and germa-nate layers sintered at lower temperature (950 °C). This indi-cates that incorporation of Ce in the apatite structure occursat high sintering temperatures. It should be considered thatseveral cerium containing apatites have been reported pre-viously [35, 36]. In addition, iron and cobalt were not detectedin the electrolyte material, although small quantities of Srwere observed in the electrolyte as well as LSM/germanateinterface.

MnMnLaLaAl

interface

La

La

La

Ge

Sr

6 7 8

germanate

1 2 3 4 5 6 7 8

Ge

Sr

La Mn

Mn

La

La

La

LSM

Energy / keV

1 3 4

interface

0 1 2 3 4 5 6 7 80

20

40

60

Ato

mic

/ %

d /µm

Mn La Sr Ge Al

(b)

(c)

(a)

interface

Fig. 3 (a) SEM image of the cross-section at the LSM/germanate interface, (b) EDS spectra in different points across the interface and (c) variation of theatomic fraction with the distance to the interface.

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3.4 Electrode Polarisation and Serial Resistance of SymmetricalCells

Representative impedance spectra of symmetrical cells areshown in Figures 5 and 6. The spectra were fitted usingequivalent circuits consisting of a serial association of (RQ)elements ascribed to electrolyte or electrode processes, whereR is a resistance and Q is a constant phase element in parallel.The constant phase element is related to the ideal capacitanceby:

Ci RiQi1n

Ri1

In the high temperature range (T > 650 °C) electrode andinductive processes are observed in the impedance spectraand they were fitted with the following equivalent circuit:LRs(RQ)HF(RQ)LF, where L is an autoinductance attributed tothe equipment, Rs is the serial resistance associated with theoverall ohmic losses of the symmetrical cell and the subscriptHF and LF denote the high and low frequency electrode pro-cesses, respectively (Figure 5a and b). The low and high fre-quency processes show capacitance values varying between

10–5 and 0.1 F cm–2 and relaxation frequencies of 0.1 Hz–30 kHz and therefore they could be ascribed to electrodeprocesses. An additional arc is observed at intermediate fre-quencies in the cells with a CGO buffer layer (Figure 5a). Thisnew contribution has capacitance values of ∼5 × 10–8 F cm–1,being similar to that the grain boundary one ∼10–9 F cm–1, sothat it could be ascribed to interfacial reactivity between CGOand germanate layers in agreement with XRPD and EDS anal-ysis.

In the low temperature range (T < 650 °C) electrolyte con-tributions are observed and the following equivalent circuitwas considered for the fitting: (RQ)B(RQ)GB(RQ)E, where thesubscripts B and GB denote the bulk and grain boundary con-tributions of the electrolyte, respectively, and E is the elec-trode processes (Figure 6). As can be observed in this figurethe electrolyte contribution is not significantly affected inthose cells with iron–cobaltite electrodes. On the contrary, thecells with CGO buffer layers show a different spectrum withtwo separated contributions associated to bulk at high fre-quency and interfacial processes at intermediate frequencies(grain-boundary of the electrolyte or reactivity at the elec-trode/electrolyte interface). It should be also commentedthat the bulk conductivity in La10–xGe5.5Al0.5O26.75–3x/2 series

Ce

Ce

Ce

Ce

Ge

LaLa

La

1 2 3

interface

germanateLa

Al La

1 2 3 4 5 6 7 8

GdCe

Ce

La

Ce

Ce

La

Ce

interface

Energy / keV

5 6 7

Ge Ce

CGO

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

20

40

60

80

100

Ato

mic

/ %

d / µm

Ce La Gd Ge Al

(a) (b)

(c) interface

Fig. 4 (a) SEM image of the cross-section at the CGO/germanate interface, (b) EDS spectra in different points across the interface and (c) variation ofthe atomic fraction with the distance to the interface.

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increases with the lanthanum deficiency in the lowtemperature range [30], so that the lower bulk resis-tance in cells with CGO buffer is possibly explainedby a decrease of lanthanum content in the electro-lyte and this is consistent with the XRPD and EDSresults. It is also clear that the total resistance(RB + RGB) in the cells with CGO layer is somewhatlarger than those without CGO layer.

The temperature dependence of the ASR in staticair for the different cathode materials is shown inFigure 7a. The manganite-based electrodes (LSMand LSCM) exhibit the highest ASR values, i.e.5–6 X cm2 at 800 °C (Table 1) and they increasewith the firing temperature of the electrodes. Thesevalues are comparable to those obtained for silicateswith similar composition [24]. Iron–cobaltites showthe lowest ASR values, about 0.78 X cm2 at 800 °C,and these values are improved when compositeof LSCF and CGO are used (i.e. 0.37 X cm2 wasobtained at 800 °C with 50%CGO–LSCF composite,which is smaller than that of the cell with Pt elec-trodes). It should be noted that the lowest ASR val-ues are correlated with the highest ionic–electronicconduction in cobaltite cathodes compared to ferriteand manganite-based cathodes (Table 1). On theother hand, the use of a CGO layer between theelectrolyte and electrode materials improves signifi-cantly the ASR values (e.g. 0.12 X cm2 for LSCF0.2/CGO/germanate cell at 800 °C), although the serialresistance increases, as discussed later. It should benoted that a similar behaviour was observed withrelated silicate apatites [24].

One would expect comparable serial resistancevalues between the cells with Pt-electrodes and

those with symmetrical electrodes, con-sidering negligible ohmic resistance ofthe electrodes compared to the electro-lyte and that additional ohmic losses donot occur at the interface between thematerial layers. As can be observed inFigure 7b the serial resistance of cellswith cobaltite cathodes is comparable tothe cell with Pt electrodes; however,manganites and cells with CGO bufferlayer exhibit larger values of serial resis-tance, which suggests a significant block-ing conduction through the electrolyte/electrode interface as consequence ofinterdiffusion of elements between thesematerials. These results are somewhatdifferent to those found with silicates,where the serial resistance was almost in-dependent on the electrode used, and itsuggests a higher reactivity of germa-nates in comparison to silicates. It can bealso observed that the serial resistance of

420-0.5

0.0

0.5

1.0

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316 kHz

400 Hz

15 kHz

30 kHz

Pt LSCF

0.2

LSCF0.2/CGO LSCF

0.2+CGO

-Z´´

/ Ω·c

m2

Z´/ Ω·cm2

interfacialreactivity

500 Hz

1 kHz RHF

QHFRs

QLF

RLF

L(a)

(b)

0 2 4 6 8

0

1

2

3

4

500 kHz

0.3 Hz

20 Hz

60 % NiO+CGO LSCM/CGO (5% H

2)

Z´ / Ω·cm2

-Z´´

/ Ω·c

m2 20 Hz

Fig. 5 Impedance spectra of (a) Pt, LSCF0.2/germanate, LSCF0.2 + CGO-composite/germanate and LSCF0.2/CGO/germanate symmetrical cells in air and (b)NiO + CGO-composite/germanate and LSCM/CGO/germanate symmetrical cells inhumidified 5% H2–Ar at 750 °C. The solid line is the fitting result obtained with theequivalent circuits. The serial resistance was subtracted for better comparison of thespectra.

0 5000 10000 150000

5000

10000

0.5 kHz

18 kHz

250 kHz

Pt LSCF0.2

LSCF0.2/CGO

-Z´´

/ Ω·c

m2

Z´/ Ω·cm2

350ºC

290 kHz

RB

QB QGB

RGB

QE

RE

Fig. 6 Impedance spectra of Pt/germanate, LSCF0.2/germanate and LSCF0.2/CGO/germanatesymmetrical cells at 350 °C in air. The grain boundary process of the electrolyte is observed at highfrequency.

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LSCF0.2 + CGO composite fixed on the apatite type electro-lyte at 950 °C is similar to the cell with Pt-electrodes, indicat-ing that low sintering temperatures avoid the reactivitybetween CGO and germanates.

The temperature dependence of ASR values for severalanode materials in humidified 5%H2–Ar are shown in Fig-ure 8b. Pt, as anode material, exhibits rather high ASR values,i.e. 27.7 X cm2 at 800 °C (Table 1), although these valuesare lower than those for NiO + germanate composite, i.e.65.7 X cm2 at 800 °C). It should be commented that low ASR

values for NiO–La9SrSi6O26 + d composite, i.e. 144 X cm2 at600 °C, have been reported by Brisse et al. [37]. NiO–CGOcomposite exhibits the lowest ASR values about 2.22 X cm2 at800 °C, which is also similar to LSCM using a ceria layer2.49 X cm2.

The temperature dependence of the serial resistance forthe anode materials (Figure 8b) shows that the cells withCGO layer have the largest values, as also occurs with thecathode materials. Nevertheless, the serial resistance of thecell with 60 wt.% NiO + CGO does not increase considerably.

0.9 1.0 1.1 1.2 1.3 1.4 1.5

10-1

100

101

102

103

104

800 700 600 500 400

PtLSMLSCF0.8

LSCF0.2

LSCF0.2+CGOLSCF0.2/CGO

ASR

/ Ω

·cm

2

103/T (K-1)

T/ ºC

0.9 1.0 1.1 1.2 1.3 1.4 1.5101

102

103

104

105800 700 600 500 400

PtLSMLSCF0.2

LSCF0.2+CGOLSCF0.2/CGO

Rs(S

/L) [

Ω·c

m]

103/T (K-1)

T / ºC

(a) (b)

Fig. 7 (a) ASR values and (b) serial resistance for the symmetrical cells with the different cathode materials.

0.9 1.0 1.1 1.2 1.3 1.4 1.5

10-1

100

101

102

103

104

800 700 600 500 400

Pt 60 % NiO+germanate 60 % NiO+CGOLSCM/CGO (5 % H2)LSCM/CGO (air)

AS

R /

Ω·c

m2

103/T (K-1)

T / ºC

0.9 1.0 1.1 1.2 1.3 1.4 1.5101

102

103

104

800 700 600 500 400

Pt60 % Ni+CGOLSCM/CGO (5% H

2-Ar)

LSCM/CGO (air)

Rs(S

/L) [

Ω·c

m]

103/T (K-1)

T / ºC

(a) (b)

Fig. 8 (a) ASR values and (b) serial resistance for the symmetrical cells with the different anode materials.

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In summary, the performance of these electrodes with ger-manate electrolytes is inferior compared to silicate investi-gated in a previous work [24]. This is consequence of thestronger reactivity of germanates, which leads to a significantincrease of the ASR and serial resistance of the symmetricalcells. Fuel cell studies were not carried out with germanateapatite electrolytes, however, one would expect low values ofpower densities, taking into account the rather high ASR val-ues deduced from the symmetrical cells, especially for theanode materials (Table 1).

4 Conclusion

The germanate apatite-type electrolyte with compositionLa9.8Ge5.5Al0.5O26.45 was studied as a possible candidate forSOFC electrolyte. The chemical compatibility and electro-chemical performance of this electrolyte with several elec-trode materials, frequently used in SOFC, were investigated.Chemical compatibility carried out by XRPD did not revealappreciable bulk reactivity with the formation of reactionproducts between germanates and the investigated electrodematerials up to 1,200 °C. However, EDS analysis showed asignificant interdiffusion of cations at the electrolyte/elec-trode interface. This results in a blocking of the charge trans-fer processes at the interface and rendering high values ofASR of these electrodes. In addition, the overall serial resis-tance of the symmetrical cells is significantly increased, evi-dencing the negative effects of the reactivity between the ger-manate electrolyte and these electrodes. The use of a CGObuffer layer improves the ASR values, but increases the serialresistance. Alternative methods to fix the CGO buffer layeron germanate electrolyte at lower temperature will likelyimprove these results.

Acknowledgements

This work was supported by the Spanish Research Pro-gramme (TEC2007-60996). The authors wish to thank “Minis-terio de Educación y Ciencia” for a “Juan de La Cierva fellow-ships” (J.P.-M.) and a “Ramón y Cajal fellowship” (J.C.R.-M.).

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