Gadolinia-doped ceria mixed with alkali carbonates for SOFC applications: II – An electrochemical insight

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 9 3 7 1 e1 9 3 7 9

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Gadolinia-doped ceria mixed with alkali carbonates for SOFC applications: II e An electrochemical insight M. Benamira a, A. Ringuede´ a, L. Hildebrandt b, C. Lagergren b, R-N. Vannier c, M. Cassir a,* a

Laboratoire d’Electrochimie, Chimie des Interfaces et Mode´lisation pour l’Energie, LECIME, UMR 7575 CNRS, Chimie ParisTech, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France b KTH Royal Institute of Technology, Department of Chemical Engineering and Technology, SE-100 44 Stockholm, Sweden c Unite´ de Catalyse et de Chimie du Solide, UMR 8181 CNRS, Villeneuve d’Ascq, France

article info

abstract

Article history:

Composite materials based on gadolinia-doped ceria (GDC) and alkali carbonates (Li2CO3-

Received 27 June 2011

K2CO3 or Li2CO3-Na2CO3) are potential electrolytes for low temperature solid oxide fuel cell

Received in revised form

applications (LTSOFC). This paper completes a first one dedicated to the thermal, structural

8 October 2011

and morphological study of such compounds; it is fully focussed on their electrical/elec-

Accepted 10 October 2011

trochemical properties in different conditions, temperature, composition and gaseous

Available online 25 November 2011

atmosphere (oxidative or reductive). The influence of the gaseous composition on the Arrhenius conductivity plots is evidenced, in particular under hydrogen atmosphere.

Keywords:

Finally, electrical conductivity determined by impedance spectroscopy is presented as

SOFC

a function of time to highlight the stability of such composites over 6000 h. First results on

Composite

single cells showed performance at 600
C of 60 mW cm2.

Alkali molten carbonates

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Gadolinia-doped ceria

reserved.

Conductivity Electrochemical analysis

1.

Introduction

High temperature fuel cells like the solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) are promising devices for converting chemical energy directly and efficiently into electricity. SOFCs, in particular are suitable to run with a large variety of fuel gas compositions while being relatively insensitive against contaminants in the feed gas stream. However, the commonly used electrolyte, yttria-stabilized zirconia (YSZ), only reaches sufficient oxide ion conductivity at temperatures around 900e1000
C. The high operating temperature causes degradation of the cell components and sealing problems. Thus there is an interest to develop low or intermediate temperature SOFCs (T < 700
C), but such

conditions provoke an increase in cathode polarization and a decrease in the efficiency [1]. Another well-investigated oxide ion conductor at intermediate temperatures (600e700
C) is doped cerium oxide [2e6]. Both samaria-doped CeO2 (SDC) [7] and gadolinia-doped CeO2 (GDC) [8e10] are promising candidates for intermediate temperature applications. Nevertheless, the utilization of ceria-based compounds as electrolyte is impeded by the instability of ceria in reducing environment where Ce4þ can easily be partially reduced to Ce3þ and thus enhance the electronic conductivity. The reduction results not only in a significant decrease of cell voltage and power efficiency, but also in a poor mechanical stability. The effect of conductivity enhancement by the addition of a second phase in solid electrolytes is known since 1929 [11].

* Corresponding author. E-mail address: [email protected] (M. Cassir). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.062

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The research activities in this field were however initiated in 1973 by Liang who showed that the ionic conductivity of the solid electrolyte LiI may be increased by two orders of magnitude by the addition of fine particles of an inert second phase (Al2O3) [12]. According to the numerous papers published on composite electrolytes, combining the properties of ceramics and molten carbonates, it is nowadays obvious that they present a growing interest for high-temperature fuel cells applications. In particular, ceria-based electrolytes, such as samariadoped ceria (SDC) and gadolinia-doped ceria (GDC) which have significantly higher conductivities (about 2 102 S cm1 at 600
C) than the classical yttria-stabilised zirconia (YSZ), but still 5 times lower than the requested value at 600
C [13]. Doped ceria oxides have been tested with different molten salts (fluorides, sulphates, chlorides), but the most interesting are surely the carbonates [14e40]. A thorough analysis of the literature on such composites has been developed by us in a recent paper [40]. No rigorous explanation on the conductivity mechanisms has ever been published; nevertheless, different assumptions have been formulated. Most of the authors agree with the idea that the enhanced conductivity of the ceria-based oxides/ molten carbonates would be due to the ionic transport of both oxide ions and protons. According to Bin Zhu et al., ceria-based composites electrolytes are also forming highly disordered interfacial regions (“superionic highways”, “percolating conducting paths”) between the doped-ceria phase and the carbonate phase [41]. It was also suggested that there exists not only oxide ion conduction but also proton mobility. This is indicated by the observation that water is formed under fuel cell conditions both at the anode and at the cathode side. It was also published that a CO2/O2 mixture as oxidant leads to higher power densities than N2/O2 [27]. Furthermore, CO2 was detected in the anode outlet gas. This was explained by an additional þ 2 and CO2 CO2 3 conduction and a ternary H , O 3 conduction was assumed. The fact that oxide ions ensure the conductivity in the oxide phase seems perfectly understandable. But the claim that, in the molten carbonate phase and at the interface with the oxide phase, protons are responsible of the ionic transportation is still to be proven. First, because the existence of protons should be explained in the experimental conditions used (residual humidity, HCO 3 .). Second, it is well-known that the ionic conductivity in molten carbonates is mainly ensured by carbonate ions. The situation is surely complex and the real mechanism paths are still controversial. Different approaches can be found, among which a complex ionic transport with intrinsic and extrinsic species and a specific ionic transport at the interface between oxides and carbonates [42]. The possible influence of protons conduction at the interface is also a key aspect to solve. A deeper insight is required including a clear understanding on the melt chemistry relative to carbonates together with possible dissolved species as water and hydroxides, as well on what happens with oxide ions conductivity in presence of carbonates, before and after their melting temperature. Despite quite many publications in this research area, no systematic investigation on this subject is described in literature yet. The existence of a dual Hþ/O2 conduction caused by so called “super ionic highways” in the interface region between the oxide and carbonate phases is questionable.

In a previously published paper, we have focused on the synthesis of GDC/(Li2CO3-K2CO3: 72.7e27.3 mol%) and on GDC/ (Li2CO3-Na2CO3: 52e48 mol%) composite materials and on their structural and morphological characterisations, as well as on their physical and thermal transformations during thermal cycling [41]. It has been shown, in particular, that the composite is stable with very low weight losses of both water and CO2 during thermal cycling and after 168 h ageing. The precise structure of the composite and its regular and reversible evolution with the temperature has been investigated by X-ray diffraction at room-temperature and high-temperature. Two-well separated phases: nanocrystals of GDC (white and finer phase) and well-distributed grey and coarser carbonate phase have been observed by scanning electron microscopy (SEM). After sintering, a better densification has been observed, with a microstructure revealing sub-micrometric particles of GDC and a more uniform distribution of the molten carbonate phase. Finally, the electrical conductivity of such composite has been found stable over 1500 h. This second paper completing the first one on structural and thermal properties [28] of GDC-carbonates composites, is dedicated to a thorough electrochemical study by impedance spectroscopy of such new electrolytes in different conditions (oxidant and reductive atmospheres, annealing temperature, cycling, ageing.), in view of a deeper understanding of the conduction mechanism and the long-term stability of the material. Furthermore, a single cell test was carried out with GDC mixed with carbonates.

2.

Experimental

2.1.

Preparation of GDC-carbonate composite electrolyte

The composite electrolytes were prepared by the solidesolid method. A mixture of Li2CO3 and K2CO3 (purity > 99%, SigmaeAldrich) with a molar ratio of 72.7/27.3 was prepared by grinding and heating (60 min at 650
C in air/CO2). The asprepared carbonate salt and a powder of gadolinium doped ceria (Gd0.1Ce0.9O1.95, GDC) (purity >99.9%, Rhodia) were mixed together in different weight ratios: GDC/LiK20: 80/20 wt%, GDC/LiK30: 70/30 wt%, ground in a mortar and heated at 650
C for 40 min (heating rate 2
C/min). Before being used as electrolytes, the compositions were quenched to room-temperature and then ground again thoroughly. The powder was pressed into pellets with a diameter of 19 mm and a thickness of 0.5e1.5 mm. A second carbonate mixture was Li2CO3Na2CO3 (52e48 mol%) with a molar fraction corresponding to the eutectic. The preparation technique of this sample GDC/ LiNa was the same as for GDC/LiK. A single cell was prepared by a dry press technique. The anode was prepared by mixing 50 vol% NiO (Alfa Aesar, 99%), 40 vol% electrolyte and 10% starch. As cathode, lithiated NiO was prepared by mixing NiO (Alfa Aesar, 99%) and LiOH (Merck, 98%). The mixture was sintered for 12 h at 700
C. The purity was checked by XRD. All the annealed powders were pressed into pellets at a uniaxial pressure of 30 MPa. The pellets were then sintered at 600
C for 1 h in air, with a heating and a cooling rate of 5
C min1.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 9 3 7 1 e1 9 3 7 9

2.2.

Electrical performance

The conductivity was measured in two different laboratories using electrochemical impedance spectroscopy (EIS). At KTH the samples were placed between two current collectors (SS316). In oxidizing atmospheres the current collectors were made of stainless steel covered with gold paste (G3535, Agar Scientific Ltd., England). In reducing atmosphere polished nickel was used. During the measurements, the electrolyte was placed inside a ceramic tube. The atmospheres at the electrodes were controlled independent from each other through gas pipes close to the electrodes. The temperature was measured with a thermocouple which was placed close to the upper electrode. To reduce the contact resistance, a mechanical pressure of 6.7  104 Pa was applied. The whole setup was placed in a furnace. Fig. 1 shows the experimental setup schematically. The sample was heated up to 350
C. Once the temperature was reached, impedance spectroscopy measurements were performed using a Solartron 1250 FRA and a Solartron 1287 potentiostat. Spectra were recorded in a frequency range from 100 kHz to 50 mHz with an amplitude of 10 mV. After the measurement the temperature was increased manually in steps of 5e15
C. The software ZPlot 2.80 (Scribner Assosiates, Inc., 2002) was used to control the EIS measurement. The spectra were analyzed with the software ZView 2.4a (Scribner Assosiates, Inc., 2001). IeV curves were recorded using the same equipment. At the LECIME, the samples were painted with gold paste (Engelhart-Clal) and heat treated at 600
C for 1 h in air. The pellet was placed in a symmetric two-electrode configuration and heated for electrochemical measurements (Fig. 2). Impedance spectroscopy was performed using a PGSTAT 20 potentiostat (Autolab Ecochemie BV, The Netherlands) with amplitude of 20 mV in the 1 MHze10 mHz frequency range. Measurements were carried out only in oxidant atmosphere.

2.3.

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XRD characterisation

The XRD patterns of the composite electrolytes were recorded at room-temperature with a Phillips PW 1390 Diffractometer

Fig. 1 e Experimental setup used at KTH for EIS measurements.

Fig. 2 e Experimental setup used at LECIME for EIS measurements.

using the Cu Ka radiation (lCuKa ¼ 1,54056 Ǻ) with 2q varying from 25 to 65
and a step of 0.02
and 3 s counting time per step.

2.4.

Thermal analysis

Thermo-gravimetric analysis e Differential thermal analysis (TGA-DTA) was carried out on a TGA-DTA SETARAM 92 Thermal Analyser at a rate of 5
C min1 up to 700
C on heating and cooling. A platinum crucible was used.

3.

Results and discussion

3.1.

Ionic conductivity of the composites

The conductivity of the ceria-based composite electrolytes was measured by EIS. Fig. 3 shows the Nyquist plots of GDC/ LiK20 with different amplitudes (20 mV, 50 mV, 100 mV and 200 mV) at 390
C. Two processes are visible: the high frequency semicircle remains independent of the different amplitudes and can therefore be attributed to the ionic conductivity in the bulk electrolyte. The second semicircle at lower frequencies becomes smaller with increasing amplitudes which indicates a process at the electrode; in effect, the kinetics is greatly influenced by the tension amplitudes. The impedance data can be interpreted as two parallel RQ elements (Q: constant phase element) in series in the equivalent circuit. At higher temperatures the first semicircle disappears and the intersection of the curve with the x-axis was taken as the resistance of the electrolyte (Fig. 4). Fig. 5 shows the results of the temperature dependent conductivity measurements of GDC/LiK30 and pure GDC between 300
C and 680
C. The pure GDC sample was thermally treated in the same way as the composite material. The Arrhenius conductivity diagram of the composite can be separated in a low temperature and a high-temperature region, separated by a discontinuity. At temperatures lower than the melting point of the mixed carbonate (Tm < 488
C), also determined by differential thermal analysis (DTA) in a previous paper [40], the total conductivity of the GDCcarbonate composite is relatively close to the bulk

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-1

3000

10000/T (K )

20 mV 0

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-1

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-Z''/ohm

2000

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GDC/Li2CO3-K2CO3 GDC/LiK20 GDC GDC

5000

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400

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300

T°C


Fig. 3 e Nyquist impedance diagrams measured at 390 C under ambient air of 80 wt% GDC/LiK20 with different signal amplitudes from 20 to 200 mV.

conductivity of pure GDC. However, the difference between both conductivities tends to increase when lowering the temperature. The activation energy obtained in the case of the GDC/LiK30 sample, around 1.20 eV, is higher than that of pure GDC, 1 eV. Thus, it appears that before 485
C the ionic conductivity is principally controlled by oxide ions, partially inhibited by the presence of solid carbonates. At temperatures around the melting point of the mixed carbonate phase the conductivity of GDC/LiK30: 80/20 wt% increases drastically by 2e3 orders of magnitude whereas the conductivity of pure GDC increases linearly. Carbonates are partially molten in this domain and another conductivity pathway is surely favoured by the presence of the molten phase. Considering the studies of J. Maier on the ionic conduction in boundary regions [44], one may attribute this rise in conductivity to the effect of a superionic pathway at the interface between carbonates and GDC, with the formation of a space charge zone with an excess of microstructural defects allowing a faster movement of ions than in the single phases. It is difficult to judge the importance of this phenomenon; nevertheless, the partial transformation of solid to molten carbonates has surely an important influence. At temperatures higher than 575
C, values of conductivity close to 0.1 S cm1, are obtained. Transport is mainly due to the  þ þ conduction of ions in the molten phase: CO2 3 , Li , K , HCO3 . It is partially inhibited with respect to the pure molten

7

570°C 600°C

6

630°C

3

-Z''/ohm

5 4

3

3

4 0

2 1

5

0 0 -1

1

2

3

4

5

6

7

8

Z'/ohm

Fig. 4 e Nyquist plots of GDC/LiK20 at elevated temperatures (570
C, 600
C and 630
C).

9

10

11

Fig. 5 e Arrhenius plot of the conductivity of GDC/LiK20 under air, annealed at 650
C for 40 min compared to pure GDC.

carbonate phase because of the solid oxide (GDC) environment. The activation energy in this high-temperature domain, of about 0.25 eV, is close to that of pure Li2CO3K2CO3 (62-38 mol%) [43]. In a previous paper, we have shown the influence of the proportion of carbonates in the composite [40]. Conductivities of the GDC/LiK composite with three compositions: 80/20, 70/30 and 60/40 wt%, have been compared. The sample with the highest amount of carbonates (60/40 wt%) has the lowest conductivity at temperatures lower than the melting point (predominantly controlled by the oxide ions mobility) and the highest conductivity after the melting point when the ionic conductivity becomes predominantly controlled by molten carbonates. In the case of the samples with lower carbonate amounts, when the carbonate ratio is high (70/30 wt%), it favours the hightemperature conductivity and when it is low (80/20 wt%), it favours the low-temperature conductivity of oxide ions. It is worth noting that the previous treatment of the composite powder has a significant influence on the Arrhenius plots. In Fig. 6a, three samples of GDC/LiK20 prepared at different temperatures are compared. The powders are annealed at 600, 650, or 680
C for 40 min under air and all of them sintered at 600
C during 1 h. It can be observed that the highest conductivities are reached when annealing is realised at 650
C, which is the temperature selected in this paper. In order to compare the results obtained in both laboratories, we have superimposed Arrhenius diagrams at an annealing temperature of 650
C, as shown in Fig. 6b. The results are similar showing that the experimental conditions are perfectly controlled with both experimental set-ups. Furthermore, no significant differences were found between annealing at 650
C during 40 or 60 min; therefore, both treatments were adopted. In order to better understand the conduction mechanisms in the conditions of a fuel cell, the same kind of analyses were carried out in different atmospheres. Fig. 7a depicts Arrhenius conductivity diagrams of the GDC/LiK20 composite obtained under dry air, cathode (15% O2, 10% CO2 and 75% N2) or anode atmosphere (80% H2, 20% CO2). Under dry air or cathode atmosphere, similar log s vs. 1/T plots were obtained, with a slight increase in conductivity in the second case, in

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LECIME KTH

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particular, in the low temperature region. It should be noted that the presence of CO2 allows carbonation of the molten salt (CO2 þ O2 / CO2 3 ). Therefore, CO2 favours the formation or the renewal of CO2 3 , avoiding the formation of oxides. With the presence of CO2, one would expect even higher conductivity values, mostly after the melting point of the carbonate eutectic. The lower than expected conductivity in the hightemperature region can be attributed to the low ratios of CO2 used in these experiments (10%). Under anode atmosphere, the discontinuity of the log s vs. 1/T plot occurs at a lower temperature, 455
C, instead of 485
C. It seems that the presence of hydrogen favours the conductivity in the lowtemperature region and lowers the melting point. A first hypothesis would be that under hydrogen the nature of the molten phase has changed, i.e. a mixture of carbonates and hydroxides may be formed. Fig. 7b represents Arrhenius conductivity diagrams of the composite with a higher ratio of carbonates, GDC/LiK30, under oxidising (dry air, wet air (10%), pure oxygen or cathode gas), reducing (anode gas) or both anode and cathode atmospheres. No difference can be noted between the measurements obtained under dry air and wet air, as for the previous composite with 20% of carbonates (not shown here). Under oxidising atmosphere, the presence of water does not seem to have any influence on the behaviour of this composite. Under pure cathode gas or under oxygen, the same conductivities are obtained at low temperatures and in the discontinuity region, but a conductivity value 4 times higher than under pure air is

Oxygen gas wet air cathode gas anode gas

-6

Fig. 6 e Arrhenius plots of the conductivity of GDC/LiK20 under air, annealed during 40 min: (a) at Δ) 600
C, ,) 650
C and 3) 680
C; (b) at 650
C at Δ) LECIME and -) KTH.

dry air

anode and cathode gases

-7

700

600

500

400

300

T°C Fig. 7 e Arrhenius plots of the conductivity: (a) of GDC/ LiK20 annealed at 650
C for 60 min under x) Dry air, 6) Anode gas and ,) Cathode gas; (b) of GDC/LiK30 under different gas.

reached at high temperatures. By the one hand, the presence of CO2 increases the proportion of CO2 3 , but contrarily to the composite with the lower ratio of carbonates, it increases significantly the conductivity. By the other hand, the presence of oxygen might favour the following interfacial reactions, producing peroxide or superoxide species known to exist in molten carbonates [45]: O2 þ 1=2O2 /O2 2

(1)

O2 þ 3=2O2 /2O 2

(2)

Even if the amount of superoxide and peroxide species is low, the presence of such species might explain the significant increase in conductivity under oxygen atmosphere at higher temperatures. Under the anode gas, a dramatic increase in conductivity is observed at low as well as at high temperatures. The transition temperature occurs at 415
C, which is inferior to the melting point of Li2CO3-K2CO3 (485
C) and inferior to the value obtained with the 20%carbonates composite. The hypothesis of the presence of another molten salt with a lower melting point seems likely. Working under anode gas could involve the following reactions:

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H2 þ O2 /H2 O þ 2e

(3)

 H2 þ CO2 3 /H2 O þ CO2 þ 2e

(4)

CO2 þ O2 /CO2 3

(5)

and the formation of hydroxides: H2 O þ O2 /2OH

(6)

Hydroxides have lower melting points than carbonates (KOH, 360
C, LiOH, 471
C) [46,47]. If reaction Eqn. 6 occurs, the melting of hydroxides will allow a higher mobility of the ions involved in the conduction mechanism, which will increase the conductivity and lower the melting point of the salt. B. Zhu et al. [41] attributed these high conductivities to the reduction of ceria under reducing atmosphere (partial reduction of Ce4þ into Ce3þ), which may induce an electronic conduction. In effect, in presence of hydrogen, the following redox reaction might partially occur: H2 þ O2 þ 2Ce4þ /H2 O þ 2Ce3þ

(7)

followed by the formation of hydroxides according to reaction Eqn. (6). Once again, apart of a slight electronic conduction in the GDC phase, one may consider a molten hydroxide phase or a mixed hydroxide-carbonate phase lowering the melting point and enhancing the ionic conductivity in the molten phase. Considering the conduction in the whole electrolyte, it is likely that the electronic conduction in GDC is blocked by the molten salt surrounding, which is a pure ionic conductor. In the case of a single cell, Fig. 7b, the conductivity in the low and high-temperature regions is as high as under the anode gas, showing the probable influence of the same interfacial reactions. However, the transition is closer to that of dry air, which means that the carbonate phase is predominant and other molten phases cannot be hypothesized. In order to compare two types of carbonate melts, the Arrhenius conductivity diagrams of the GDC/LiNa30 (70/30 wt%) composite material composite was analyzed under dry air and cathode atmosphere and the GDC/LiNa20 (80/20 wt%) only under air, Fig. 8. As expected, the conductivity increases with the amount of carbonates in the mixture at temperatures higher than the melting point (500
C). It can also be observed that the values of conductivity obtained under anodic gas are higher than those obtained under dry air, and the jump in conductivity occurs at a lower temperature (490
C). Whatever the carbonate melt, Li/K or Li/Na, the same effect is observed under anodic atmosphere, but more accentuated in the case of Li-K.

3.2.

Fig. 8 e Arrhenius plots of the conductivity of GDC/LiNa30 under dry air and anode gas. Conductivity of GDC/LiNa20 under dry air is reported for comparison.

(TGA) of the two cycles is weak; it corresponds to a variation of mass lower than 3%, showing the good stability of the mixture of carbonates. After the release of the water adsorbed on the surface towards 140
C (cycle 1), a very weak variation of mass is observed in the second cycle. The DTA curves relative to the two cycles are perfectly superimposed, revealing an intense peak at 484
C, in agreement with the melting point of the Li2CO3-K2CO3 (62-38 mol%) eutectic (488
C). When cooling, two exothermic peaks are observed, at 537
C and 444
C, they correspond to the beginning of Li2CO3 crystallization and to the end of the eutectic formation (further details can be found in [40]). It was also previously demonstrated that after ageing during 168 h at 500
C under air, the shape of the TGA and DTA curves is almost the same than before ageing [40]. The effect of cycling was also followed by impedance spectroscopy. Measurements were carried out during several rises and descents of the temperature, on various samples, under various conditions. The average duration of each test is two weeks. As an example, Fig. 10 shows the Arrhenius conductivity diagrams of the GDC/LiK20 composite under humidified air. No significant difference can be observed when

Effect of cycling and ageing

In order to check the stability of these composite materials, cycling and ageing tests were realized. Thorough TGA and DTA analyses coupled to mass spectrometry have been developed by us in a previous paper [40]. Our purpose here is to analyze specifically the effect of cycling on the GDC/LiK20 composite by differential and thermogravimetric thermal analyses. A test of two cycles was carried out on this material after annealing at 650
C during 40 min, as shown in Fig. 9. Each cycle comprises a rise and a descent of the temperature. Indeed, the signal related to the loss of mass

Fig. 9 e Two successive TGA and DTA plots of the GDC/ LiK20 composite, annealed at 650
C for 40 min, are reported. The experiment was carried out under air with a rate of 5
C minL1.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 9 3 7 1 e1 9 3 7 9

comparing the two cycles. The same results were obtained under dry air and cathode atmosphere. It can be deduced that the composite material has a good stability during cycling. A study of ageing was carried out during 168 h at a temperature of 500
C on GDC/LiK20 pellets (previously annealed at 650
C during 40 min and sintered at 600
C for 1 h). These pellets were withdrawn from the furnace progressively before being analyzed by XRD. Before 96 h, only the intense peaks of GDC are visible. Very low intensity peaks corresponding to carbonates, Li2CO3 and Li2CO3-K2CO3, are only detected after 96 h (not shown here). Fig. 11 shows the XRD patterns obtained at ambient temperature (25
C) after 168 h of ageing. The peaks corresponding to GDC are clearly detected, but few peaks relative to lithium carbonates or to LiK eutectic can also be observed, showing that ageing has a slight effect on the crystallinity of carbonates after cooling to ambient temperature. It can be concluded that, apart from the appearance of carbonate peaks, the composite structure seems to be stable all over the 168 h with no drastic changes. Finally, the conductivity of the GDC/LiK30 sample, was measured during 6000 h under air at 600
C (temperature after the melting point), as described in Fig. 12. An initial value around 9  102 S cm1 was obtained, relatively stable during 3500 h (8e9  102 S cm1). Then, it slowly decreased up to 4000 h before stabilizing around 7  102 S cm1. It can be deduced that the conductivity is almost stable over a relatively long period which is of great interest for the fuel cell application. This value is significantly higher than that of pure GDC at 600
C, around 1.5  102 S cm1, knowing that the preparation of the composite and the experimental conditions could be optimised in the future.

3.3.

Single cell experiments

Single cells with GDC/LiK30 as electrolyte, LixNi1xO/electrolyte as cathode and Ni/electrolyte as anode were tested at different temperatures with pure hydrogen (30 mL min1) as fuel and pure oxygen (30 mL min1) as oxidant (Fig. 13). At 465
C, the performance of the cell is very low with 8.0 mW cm2 at a potential of 0.51 V. The temperature is lower

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Fig. 11 e XRD pattern at 25
C of the GDC/LiK20 composite, annealed at 650
C during 40 min and sintered at 600
C 1 h, after an ageing duration of 168 h at 500
C.

than the melting temperature of the salt (Tm ¼ 488
C) and the mobile charge carriers are most probably oxide ions through the GDC bulk or along the space charge region between the two phases. Above the melting point, the ionic conductivity of the electrolyte and the performance of the cell become higher. At 505
C, a maximum of 30.9 mW cm2 were reached and at 550
C, 58.8 mW cm2 were detected. Further heating to 600
C did not increase the performance significantly but lowered the OCV (open-circuit-voltage) by 43 mV. Although these results are not optimized and would require more attention on the single cell preparation and more precise control of the grain size of gadolinia-doped ceria, to be compared to the literature, they clearly show the feasibility of a fuel cell system with a composite electrolyte.

5.

Conclusions

This study, focussed mainly on the electrical properties of GDC/LiK composites, is the complement of a previous study on the structural and morphological characterisation of such

18

-1

Log (σ /S.cm-1)

-2

-3

-4

-5

-6

First First increasing rise First decreasing descent rise Second increasing descent Second decreasing

-7

700

600

500

T°C

400

300

Fig. 10 e Arrhenius plots of the conductivity of GDC/LiK20 under wet air. Two forwardebackward cycles are shown, beginning from low temperatures.

Fig. 12 e Evolution of the conductivity vs. time for the GDC/ LiK30 composite at 600
C under air.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 9 3 7 1 e1 9 3 7 9

references

Fig. 13 e Single cell test on Ni-GDC/LiK30//GDC/LiK30// LixNi1LxO-GDC/LiK30 system, with pure hydrogen (30 mL minL1) as fuel and pure oxygen (30 mL minL1) as oxidant, at different temperatures.

materials. Arrhenius conductivity plots obtained by impedance spectroscopy clearly show a discontinuity between lower temperatures dominated mainly by the conduction of oxide ions and higher temperatures where the conductivity of carbonates is predominant. In between, around the carbonates melting point lies a zone where different assumptions can be given. These conductivity plots are sensible to the gaseous atmosphere, in particular under hydrogen where the discontinuity occurs at lower temperatures suggesting the presence of a new composition of the molten phase that might be provoked by a localized electronic conductivity (reduction of Ce(IV) into Ce(III)). A specific conduction pathway is probably happening in the interface between the oxide and the carbonate phases, involving oxide, oxygen reduced species and even other species. This is complex and would require sophisticated analyses (XPS, Tof-SIMS, NMR.) to be able to give a rational answer. The results obtained with GDC/LiNa are quite similar. Ageing and cycling studies, including Arrhenius plots, TGA/DTA and XRD analyses show the stability of the composite and its behaviour. A study of the conductivity of GDC/LiK30 over 6000 h showed a relative stability, beginning with 9  102 S cm1 and declining after 3500 h before stabilizing at 7  102 S cm1. A single cell test with GDC/carbonates composite showed the feasibility of such systems, even if a performance of only 60 mW cm1 were obtained at 600
C; better results could be expected by optimizing both the setup and cell components, in particular the composite composition and structure.

Acknowledgements We wish to acknowledge very warmly Prof. Go¨ran Lindbergh for his valuable scientific advices. XRD facilities in Lille (France) are supported by the Conseil Regional du Nord-Pas de Calais, and the European Regional Development Fund (FEDER).

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