YSZ

Solid State Ionics 197 (2011) 13–17

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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i

Cyclic voltammetry characterization of a La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode interfaced to CGO/YSZ☆ Vassilios Ch. Kournoutis a, b, Frank Tietz c, Symeon Bebelis a,⁎ a b c

Department of Chemical Engineering, University of Patras, GR 26504 Patras, Greece Institute of Chemical Engineering and High Temperature Chemical Processes (FORTH/ICE-HT), GR 26504 Patras, Greece Institute of Energy Research (IEF-1), Forschungszentrum Jülich, D-52425 Jülich, Germany

a r t i c l e

i n f o

Article history: Received 1 September 2009 Received in revised form 15 May 2011 Accepted 14 June 2011 Available online 19 July 2011 Keywords: Cyclic voltammetry LSCF Cathodes Mixed conductors Solid oxide fuel cells SOFC

a b s t r a c t The electrochemical characteristics of a La0.8Sr0.2Co0.2Fe0.8O3 − δ cathode electrode interfaced to the CGO layer of a double layer CGO/YSZ electrolyte were studied using cyclic voltammetry, at temperatures of 600 to 850 °C and under oxygen partial pressures ranging from 0.07 to 21 kPa. The aim was to identify the electrochemical processes taking place under cathodic polarization on the basis of differences in the features of the cyclic voltammograms with changing conditions. Depending on temperature, sweep rate and oxygen partial pressure, current peaks appeared both in the forward and backward scans. Furthermore, reversed hysteresis was observed, i.e. higher currents in the backward scan than in the forward scan, with increasing oxygen partial pressure and decreasing temperature. The observed behavior was related to the electrochemical redox of B-sites and concomitant stoichiometry change as well as to the competing reaction of electrochemical oxygen redox, taking also into account the competitive action of chemical reactions occurring in the presence of gaseous oxygen. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The sluggish kinetics for the oxygen reduction reaction (ORR) over conventional cathodes based on La1 − xSrxMnO3 − δ limits severely the commercialization of intermediate temperature (600–800 °C) solid oxide fuel cells (IT-SOFCs). Iron- and cobalt-containing perovskites La1 − x − zSrxCoyFe1 − yO3 − δ (LSCF) [1,2] have recently attracted significant attention as promising alternative cathode materials for IT-SOFCs, mainly due to their high mixed (electronic and ionic) conductivity [3], which results in enlargement of the available electrochemically active area [4,5], and their high oxygen surface exchange coefficients [6,7]. However, these materials are neither thermally (different thermal expansion coefficients) [8] nor chemically (formation of poorly conducting interlayers at high temperature) [9,10] compatible with yttria-stabilized zirconia (YSZ), the standard electrolyte material currently used in SOFCs. As LSCF is chemically and thermally compatible with doped CeO2 [1,11,12], the use of a Sm or Gd doped CeO2 (CSO and CGO, respectively) interlayer between LSCF cathodes and YSZ has been proposed [11,12] to overcome these problems. The evaluation of the electrochemical performance of LSCF perovskites as cathodes in SOFCs is either based on the current density–voltage characteristics of single cells where the same anode is ☆ Presented at the SSI-17 conference in Toronto. ⁎ Corresponding author. E-mail address: [email protected] (S. Bebelis). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.06.007

used [2], or on the individual polarization and impedance characteristics of the cathodes, which directly correlate with their electrocatalytic activity [1,9,13–18]. Recently, cyclic voltammetry was applied for investigation of the formation of oxygen vacancies and of the electrochemical redox of iron and/or cobalt ions in the case of La0.78Sr0.2FeO3 − δ and La0.78Sr0.2Co0.2Fe0.8O3 − δ electrodes interfaced to the Ce0.8Gd0.2O2 − δ (CGO) layer of a double layer CGO/YSZ electrolyte [19]. In the present work cyclic voltammetry is used for identification and study of the electrochemical processes which take place at a porous La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode deposited on the GGO layer of a CGO/YSZ electrolyte, over a wide range of oxygen partial pressures, temperatures and potential sweep rates. 2. Experimental The experiments were carried out in a single chamber cell of volume approximately 30 cm 3 which consisted of a closed at one end quartz tube with its open end mounted in a stainless steel cap. An YSZ disk was suspended inside the tube using gold wires pressed on it between two non-conductive ceramic slabs and connecting the electrodes with the external electric circuit. A platinum gauze (52 mesh) pressed on the working electrode was used as current collector. A three electrode set-up was used, with porous Pt films as auxiliary (counter and reference) electrodes. The perovskite electrode was interfaced to a thin dense CGO layer deposited via screen printing on an YSZ disk (1 mm thickness, 20 mm diameter) and had a geometric area of approximately 1.5 cm 2. The auxiliary electrodes

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a

15

PO2 = 0.07 kPa

i , mA cm-2

o

0

T = 700 C th 10 cycle

-15 -1

v, mV s 50 30 10

-30 -0.9

-0.6

-0.3

0

0.3

UWR , V

b

40 PO2 = 0.07 kPa o

20

i , mA cm-2

were deposited on the other side of the disk, the counter electrode exactly opposite to the working electrode. The geometric areas of the counter and reference electrodes were approximately 1.5 cm 2 and 0.5 cm 2, respectively. Details concerning the test cell and the preparation of the auxiliary electrodes and the electrolyte components can be found elsewhere [16–19]. The perovskite powder was synthesized using the spray-drying technique starting from nitrate precursors [20,21] and calcined at 900 °C in order to develop the perovskite phase [2,20,21]. The stoichiometry of the powder was controlled by optical emission spectroscopy (ICP-OES) and the phase composition was evaluated by X-ray diffraction [2,20]. After calcination, the powder was ground by ball milling for several hours until a mean particle size (d50) of approximately 0.8 μm was obtained. Its specific surface area was 2.2 m2 g− 1 (BET method). The perovskite electrode was screen-printed on the CGO layer and then sintered at 1150 °C for 3 h. Additional details concerning the preparation of the perovskite electrode can be found in earlier studies [16–19]. Characterization of the microstructure of the La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode was performed using scanning electron microscopy which showed that the thickness of the CGO layer was approximately 7 μm while that of the porous perovskite electrode was approximately 90 μm [18]. The electrochemical characterization was carried out in the temperature range of 600 to 850 °C and under flow (30 cm3 STP min− 1) of mixtures of O2 in He (oxygen partial pressure PO2 ranging from 0.07 to 21 kPa), using an Autolab PGSTAT 302 potentiostat– galvanostat controlled by the Autolab GPES software package. Cyclic voltammetry experiments were performed by varying linearly the perovskite electrode potential between 0.3 V and −1 V vs. the Pt reference electrode, at scan rates in the range from 10 to 200 mV s − 1. The presented voltammograms are not corrected for the ohmic component and correspond to those recorded at the 10th cycle of the potential sweep when a steady state had practically been attained, i.e. no further change in the voltammograms was observed between subsequent cycles. The open circuit potential UWR,o was measured before each experiment and, as expected, was practically zero. In order to stabilize the electrocatalytic activity of the electrode, it was pretreated by applying −200 mA for 2 h and then −500 mA for 30 min at 800 °C, under 21% O2 in He mixture.

T = 750 C th 10 cycle

0 -1

v, mV s 50 30 10

-20

-40 -0.9

-0.6

-0.3

0

0.3

UWR , V

c

10 0

i , mA cm-2

14

-10 -20 -1

v, mV s 50 30 10

PO2 = 0.07 kPa

-30

o

T = 600 C th 10 cycle

-40 -0.9

-0.6

-0.3

0

0.3

UWR , V 3. Results and discussion Fig. 1a to c show cyclic voltammograms obtained at PO2 = 0.07 kPa and T = 700, 750 and 600 °C, respectively, at three different potential sweep rates (10, 30 and 50 mV s − 1). At T = 700 °C (Fig. 1a), the main features of the voltammograms are two current peaks, a cathodic peak corresponding to the forward scan (0.3 to −1 V) and an anodic peak corresponding to the backward scan (− 1 V to 0.3 V), similar with those reported in ref. [19] for the La0.78Sr0.2Co0.2Fe0.8O3 − δ/CGO/YSZ system at PO2 = 0.7 kPa. As shown in Fig. 1a and also observed in the aforementioned study, the potential Up,cath corresponding to the cathodic peak shifts to lower values with increasing sweep rate while the opposite behavior is observed for the anodic peak potential Up,anod. This shift of peak potential with changing sweep rate denotes an irreversible electrochemical system [22]. At T = 750 °C (Fig. 1b), two cathodic current peaks appear, the more cathodic peak being wider and described better as a shoulder, while in the reverse scan not distinct anodic peaks appear. Two cathodic current peaks have also been reported by Siebert et al. [23] in the case of the La0.7Sr0.3Co0.8Fe0.2O3 − δ/ CGO system (T= 306 °C, PO2 = 10 kPa, v = 1 mV s − 1), being attributed to successive redox of cobalt sites with different oxidation states. At T = 600 °C (Fig. 1c) the described features of the cyclic voltammograms also appear for a sweep rate equal to 10 mV s − 1. For higher sweep rates, no cathodic peak appears. Normal hysteresis (return current absolutely lower than the forward current) [22] of the current is observed during the reverse (backward) scan, as also observed at all other temperatures at PO2 = 0.07 kPa.

Fig. 1. Effect of sweep rate at PO2 = 0.07 kPa and (a) 700 °C (b) 750 °C and (c) 600 °C on the cyclic voltammograms obtained with the La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode.

Fig. 2a shows cyclic voltammograms obtained at PO2 = 0.2 kPa and T = 800 °C, at different potential sweep rates ranging from 30 to 200 mV s − 1. Under these conditions the main feature of the cyclic voltammograms is a cathodic current peak in the forward scan, while the formation of a shoulder can be discerned in the backward scan. The same behavior was observed over the entire temperature range 700–850 °C, as well as at 650 °C for sweep rates lower than 20 mV s − 1, the relative magnitude of the anodic shoulder (or flat peak) depending on sweep rate and temperature. The potential Up,cath corresponding to the cathodic peak decreased (became more negative) with increasing sweep rate (Fig. 2a and b) as well as with decreasing temperature (Fig. 2b). As shown in Fig. 2c, at PO2 = 0.2 kPa and T = 600 °C the behavior of the system changes, as no distinct current peak is observed in the forward (cathodic) scan, while a current peak is clearly formed during the backward (anodic) scan. Moreover, reversed hysteresis of the current is exhibited over the employed range of sweep rates (50–200 mV s − 1), i.e. the backward current is absolutely higher than the forward one up to a certain potential (Fig. 2c). Similar reversed current hysteresis behavior was also observed at 650 °C, as shown in Fig. 2d where are presented cyclic voltammograms obtained at PO2 = 0.2 kPa and sweep rate v = 50 mV s − 1, over the temperature range of 600 to 850 °C. Reversed current hysteresis in cyclic voltammograms has been reported for the

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Fig. 2. Effect of sweep rate on the cyclic voltammograms at T = 800 °C (a) and on the cathodic peak potential, Up,cath, at different temperatures (b). Effect on the cyclic voltammograms of sweep rate at T = 600 °C (c) and of temperature at sweep rate v = 50 mV s− 1. PO2 = 0.2 kPa. La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode.

both to the relative contribution of the aforementioned two electrochemical processes under different conditions and to the competitive action of chemical reactions that may take place in parallel. These reactions can be the direct adsorption of oxygen from the gas phase as well as the recombination of reduced B-sites with surface oxygen

a

50

i , mA cm-2

0 -50 -100 o

T, C 850 700 650 600

-150 -200

PO = 21 kPa 2

v= 50 mV s th 10 cycle

-1

-250 -0.9

-0.6

-0.3

0

0.3

UWR , V

b

50 PO = 21 kPa 2

0

i , mA cm-2

LSM/YSZ system [24–26], attributed to the increase in the number of oxygen vacancies upon cathodic polarization of the perovskite electrode which results in increase of the electrocatalytic activity for oxygen reduction. It is noted that behavior similar to that observed for PO2 = 0.2 kPa was also observed for PO2 = 0.7 kPa. In this case the threshold temperature for appearance of reversed current hysteresis shifted to 700 °C (at sweep rates above 30 mV s − 1) whilst the appearance of anodic current peak or shoulder was not evident. As shown in Fig. 3a, by increasing oxygen partial pressure to PO2 = 21 kPa no distinct peaks appear in the cyclic voltammograms, while in the entire temperature range of 600 to 850 °C reversed hysteresis of the current is observed following the forward (cathodic) scan. The reversed hysteresis effect becomes more pronounced with decreasing sweep rate (Fig. 3b), indicating that oxygen vacancies are formed in a slow process. Fig. 4 shows the effect of PO2 (Fig. 4a) and of temperature (Fig. 4b) on cyclic voltammograms obtained under conditions where the main feature is the formation of a current peak in the forward (cathodic) scan. The peak potential Up,cath decreases (becomes more negative) with increasing PO2 (Fig. 4a) and decreasing temperature (Fig. 4b). This effect of temperature can also be seen in Fig. 2b and d. Similarly to the results of earlier voltammetric studies concerning LSM electrodes interfaced to YSZ [24–29], La0.7Sr0.3Co0.8Fe0.2O3 − δ electrodes interfaced to CGO [23] as well as La0.78Sr0.2FeO3 − δ and La0.78Sr0.2Co0.2Fe0.8O3 − δ electrodes interfaced to CGO/YSZ [19], the observed peaks are most probably related to electrochemical redox of the iron or/and cobalt ions and concomitant stoichiometry change, x x •• •• according to the reaction OO, perov + 2B + VO, CGO + 2e ⇔ VO, perov + x x 2B′ + OO, CGO where B and B′ denote different oxidation states of the B-sites (e.g. Co 4+ and Co 3+), but also possibly to the competing •• x reaction of oxygen redox: VO, perov + Oad + 2e ⇔ OO, perov. The electrochemical redox of cobalt ions is more probable compared to that of the iron ions, as the relative stability of the iron ions is higher than that of the cobalt ions of the same oxidation number [23,30,31]. On a barely qualitative basis, the observed differences in the features of the voltammograms under different conditions could be attributed

-50

o

T = 850 C th 10 cycle

-100

-1

v, mV s 50 75 100 150 200

-150 -200 -250 -0.9

-0.6

-0.3

0

0.3

UWR , V Fig. 3. Effect of temperature, at a sweep rate v = 50 mV s− 1 (a) and of sweep rate, at T = 850 °C (b) on the cyclic voltammograms obtained at PO2 = 21 kPa with the La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode.

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a 30

i , mA cm-2

0

o

T = 750 C -1 v = 50 mV s th 10 cycle

-30 -60 PO , kPa

-90

2

0.7 0.2 0.07

-120 -0.9

-0.6

-0.3

0

0.3

UWR , V

b 40

PO = 0.2 kPa 2

i , mA cm-2

-1

v = 30 mV s th 10 cycle

0 o

T, C 850 800 750 700

-40

-80 -0.9

-0.6

-0.3

0

0.3

UWR , V Fig. 4. Effect on the cyclic voltammograms obtained with the La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode (a) of PO2 , at a sweep rate v = 50 mV s− 1 and T = 750 °C, and (b) of temperature, at v = 30 mV s− 1 and PO2 = 0.2 kPa.

vacancies via gaseous oxygen participation (chemical re-oxidation of x •• x B-sites): VO, perov + ½O2 + 2B′→OO, perov + 2B [19] and compete with electrochemical oxygen adsorption and electrochemical re-oxidation of the reduced B-sites in the anodic scan, respectively. The fact that the cathodic peak is in general the dominant feature of the voltammograms at temperatures above 650 °C for PO2 ranging from 0.07 to 0.7 kPa can be explained by the competing action of oxygen adsorption from the gas phase and chemical re-oxidation of B-sites [19], which prevail over the corresponding electrochemical oxidations under these conditions. The appearance of two cathodic peaks for PO2 = 0.07 kPa at 750 °C (Fig. 1b), with the more cathodic wide peak being better described as a shoulder, can be explained by the occurrence of two distinct reduction processes with different rates under these conditions, e.g., successive electrochemical reduction of B-sites with different oxidation state [23] or electrochemical oxygen and B-site reduction. As the concentration of oxygen vacancies increases with decreasing PO2 [18] accompanied by a decrease in the concentration of B-site transition metal ions at high oxidation state (for reasons of electroneutrality), electrochemical oxygen and B-site reduction could be more probable, in view of the fact that the aforementioned behavior was observed only at PO2 = 0.07 kPa. On this basis, the difference in the number of cathodic peaks appearing at 700 and 750 °C could be related with the comparatively lower concentration of oxygen vacancies at 700 °C which does not favor the appearance of the peak corresponding to electrochemical reduction of oxygen species. On the other hand, the disappearance of the cathodic peak with increasing scan rate above 10 mV/s for PO2 = 0.07 kPa and T = 600 °C (Fig. 1c) may be explained by a shift of the position of the peak to potentials lower than the reversal potential, due to the induced decrease in the rate of the corresponding cathodic reaction with decreasing temperature. It is noted that the features of a solid electrolyte cyclic voltammogram as well as the nature of the provided information are dictated by a number of characteristic time constants with values depending on the conditions and operational parameters of the experiment [32]. This further explains differences in the

observed characteristics of the voltammograms with changing sweep rate, temperature and PO2 . Under conditions where the anodic peak is the main feature reversed hysteresis of the current was observed for PO2 equal to 0.2 kPa (Fig. 2c and d), which implies that the prevailing mechanism under these conditions is the electrochemical redox of B-sites. Furthermore, it denotes that the competing action of the chemical re-oxidation of the Bsites is not prevailing in this case compared to their electrochemical oxidation, which can be explained by the reduced redox stability of the cobalt sites [30,31]. In view of the fact that oxygen vacancy concentration decreases with increasing PO2 , the threshold temperature for appearance of reverse hysteresis of the current shifts to higher values with increasing PO2 . Indeed, at PO2 = 0.7 kPa this threshold temperature shifts to 700 °C, while at PO2 equal to 21 kPa reversed hysteresis of the current appears at all temperatures (Fig. 3). Furthermore, no current peaks were observed in this case due to the competing action of the chemical reactions which are favored at high oxygen partial pressure. On the contrary, no reversed hysteresis effect was observed at PO2 = 0.07 kPa (Fig. 1), which can be explained by the high number of oxygen vacancies at this reduced oxygen partial pressure. Differences observed between the voltammograms obtained in the present work at PO2 = 0.7 kPa (Fig. 4a) and those reported in Ref. [19] for a La0.78Sr0.2Co0.2Fe0.8O3 − δ electrode at the same oxygen partial pressure can be partly explained by the difference [20] in the concentration of oxygen vacancies (oxygen stoichiometry change δ) and activation energy of oxygen surface adsorption between the La0.8Sr0.2Co0.2Fe0.8O3 − δ and La0.78Sr0.2Co0.2Fe0.8O3 − δ electrodes. The reported [2] improved performance as SOFC cathode of the A-site deficient La0.78Sr0.2Co0.2Fe0.8O3 − δ electrode compared to that of the La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode has been also attributed [2] to the aforementioned difference. In addition to the different composition, the two LSCF electrodes used in the present study and in Ref. [19] had significantly different thickness, although similar microstructure, which can also partly account for the observed differences concerning the features of the corresponding cyclic voltammograms. As already mentioned, the observed shift of peak potentials with changing sweep rate (Figs. 1 and 2) denotes an irreversible electrochemical system [22] and, under certain assumptions, its dependence on sweep rate can be used to extract information about the kinetics of the underlying reactions [19,22]. On the other hand, the observed shift of the cathodic peak potential to more negative potentials with decreasing temperature (Figs. 2b and d, 4b) and increasing PO2 (Fig. 4a) is in agreement with a reaction–diffusion mechanism [22], more clearly figured out in the case of prevailing electrochemical oxygen redox. 4. Conclusions Cyclic voltammetry was used for electrochemical characterization of a porous La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode deposited on the GGO layer of a double layer CGO/YSZ electrolyte. Cyclic voltammograms were recorded over a wide range of oxygen partial pressures (0.07 to 21 kPa), temperatures (600 to 850 °C) and potential sweep rates (10 to 200 mV s− 1), varying the La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode potential between 0.3 V and −1 V vs. a Pt reference electrode exposed to the same gas atmosphere as the perovskite electrode. Depending on the particular conditions, cathodic and anodic current peaks were observed corresponding to potential and currents dependent on scan rate, temperature and PO2 . Furthermore, reversed hysteresis of the current following the cathodic scan was observed for PO2 above 0.07 kPa and below a certain temperature for each particular PO2 . The current peaks were related to the electrochemical redox of the B-site transition metal ions, most probably cobalt ions, and the concomitant stoichiometry change but also to the competitive reaction of electrochemical oxygen redox. Differences in the features

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of the obtained voltammograms under different conditions as well as differences with voltammograms reported in previous studies for electrodes of similar composition were discussed on the basis of the relative contribution of these two electrochemical processes and the competitive action of chemical reactions occurring in parallel, with O2 participation. Acknowledgments The authors gratefully acknowledge financial support by the Integrated Project “Real-SOFC” (SES6-CT-2003-502612). They also thank Dr A. Mai, Forschungszentrum Jülich (IEF-1), Germany and Dr N. Kotsionopoulos, Department of Chemical Engineering, University of Patras, Greece for the preparation of the perovskite electrode as well as Dr V. Drakopoulos, FORTH/ICE-HT, Greece for the SEM characterization of the electrode. References [1] A. Esquirol, N.P. Brandon, J.A. Kilner, M. Mogensen, J. Electrochem. Soc. 151 (2004) A1847–A1855 (and references therein). [2] A. Mai, V.A.C. Haanappel, S. Uhlenbruck, F. Tietz, D. Stöver, Solid State Ionics 176 (2005) 1341–1350. [3] Y. Teraoka, H.M. Zhang, K. Okamoto, N. Yamazoe, Mater. Res. Bull. 23 (1988) 51–58. [4] J. Fleig, J. Power Sources 105 (2002) 228–238. [5] S.B. Adler, J.A. Lane, B.C.H. Steele, J. Electrochem. Soc. 143 (1996) 3554–3564. [6] S.J. Benson, R.J. Chater, J.A. Kilner, in: T.A. Ramanarayanan, W.L. Worell, H.L. Tuller, A.C. Khandkar, M. Mogensen, W. Göpel (Eds.), Proc. 3rd Intern. Symp. on Ionic and Mixed Conductors III, The Electrochemical Society Proceedings Series, PV 97–24, Pennington, NJ, U.S.A., 1998, pp. 596–609. [7] A. Esquirol, J. Kilner, N. Brandon, Solid State Ionics 175 (2004) 63–67.

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[8] A. Petric, P. Huang, F. Tietz, Solid State Ionics 135 (2000) 719–725. [9] H.Y. Tu, Y. Takeda, N. Imanishi, O. Yamamoto, Solid State Ionics 117 (1999) 277–281. [10] F.M. Figueiredo, J.A. Labrincha, J.R. Frade, F.M.B. Marques, Solid State Ionics 101–103 (1997) 343–349. [11] H. Uchida, S. Arisaka, M. Watanabe, Electrochem. Solid State Lett. 2 (1999) 428–430. [12] A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) 4709–4714. [13] J. Liu, A.C. Co, S. Paulson, V.I. Birss, Solid State Ionics 177 (2006) 377–387. [14] D.Z. de Florio, R. Muccillo, V. Esposito, E. Di Bartolomeo, E. Traversa, J. Electrochem. Soc. 152 (2005) A88–A92. [15] N. Grunbaum, L. Dessemond, J. Fouletier, F. Prado, A. Caneiro, Solid State Ionics 177 (2006) 907–913. [16] S. Bebelis, N. Kotsionopoulos, A. Mai, F. Tietz, J. Appl. Electrochem. 37 (2007) 15–20. [17] S. Bebelis, N. Kotsionopoulos, A. Mai, D. Rutenbeck, F. Tietz, Solid State Ionics 177 (2006) 1843–1848. [18] V.Ch. Kournoutis, F. Tietz, S. Bebelis, Fuel Cells 9 (2009) 852–860. [19] S. Bebelis, V. Kournoutis, A. Mai, F. Tietz, Solid State Ionics 179 (2008) 1080–1084. [20] A. Mai, PhD Thesis, Ruhr-Univ. Bochum, Schriften des FZJ, Energietechnik, Vol. 31 (2004). [21] P. Kountouros, R. Förthmann, A. Naoumidis, G. Stochniol, S. Syskakis, Ionics 1 (1995) 40–50. [22] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., Wiley & Sons, Inc., New York, 2001. [23] E. Siebert, C. Roux, A. Boréave, F. Gaillard, P. Vernoux, Solid State Ionics 183 (2011) 40–47. [24] H.Y. Lee, W.S. Cho, S.M. Oh, H.-D. Wiemhöfer, W. Göpel, J. Electrochem. Soc. 142 (1995) 2659–2664. [25] Y. Jiang, S. Wang, Y. Zhang, J. Yan, W. Li, J. Electrochem. Soc. 145 (1998) 373–378. [26] A. Hammouche, E. Siebert, A. Hammou, M. Kleitz, A. Caneiro, J. Electrochem. Soc. 138 (1991) 1212–1216. [27] X.J. Chen, S.H. Chan, K.A. Khor, Solid State Ionics 164 (2003) 17–25. [28] X.J. Chen, K.A. Khor, S.H. Chan, Solid State Ionics 167 (2004) 379–387. [29] X.J. Chen, S.H. Chan, K.A. Khor, Electrochem. Solid State Lett. 7 (2004) A144–A147. [30] J. Yoo, A. Verma, S. Wang, A. Jacobson, J. Electrochem. Soc. 152 (2005) A497–A505. [31] A. Mai, F. Tietz, D. Stöver, Solid State Ionics 173 (2004) 35–40. [32] C.G. Vayenas, A. Ioannides, S. Bebelis, J. Catal. 129 (1991) 67–87.