- Email: [email protected]
Ni anode functional layer by electrophoretic deposition in a La0.995Ca0.005NbO4 electrolyte based PCFC
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 ) 8 0 2 7 e8 0 3 2
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Characterization of La0.995Ca0.005NbO4/Ni anode functional layer by electrophoretic deposition in a La0.995Ca0.005NbO4 electrolyte based PCFC F. Bozza a,*, W. Schafbauer b, W.A. Meulenberg b, N. Bonanos a a
Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, P.O. Box 49, 4000 Roskilde, Denmark b Forschungszentrum Ju¨lich GmbH, Institute of Energy and Climate Research, IEK-1, 52425 Ju¨lich, Germany
article info
abstract
Article history:
The Electrophoretic Deposition (EPD) technique has been applied to the preparation of
Received 4 August 2011
a porous La0.995Ca0.005NbO4/Ni composite anode layer, deposited on a porous pre-sintered
Received in revised form
La0.995Ca0.005NbO4/Ni support. Powders of La0.995Ca0.005NbO4 and NiO were suspended in
28 October 2011
a solution of acetylacetone, iodine and water. Selectivity in the composition of the
Accepted 1 November 2011
deposited layer was analyzed as a function of the suspension compositions and deposition
Available online 14 December 2011
conditions. A quasi-symmetrical cell was produced by depositing La0.995Ca0.005NbO4 elec-
Keywords:
counter electrode on the dense electrolyte layer by brushing. The performance of the
Solid oxide fuel cells
electrodes was evaluated by electrochemical impedance spectroscopy in 3% wet H2.
Ca-doped LaNbO4
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
trolyte layer on the anode layer by EPD, and by applying a porous La0.995Ca0.005NbO4/Ni
Electrophoretic co-deposition
reserved.
Anode functional layer Selectivity analysis Electrochemical characterization
1.
Introduction
Proton conducting solid oxide fuel cells show the advantage over conventional oxygen ion conducting SOFCs of high efficiency, since water is produced at the cathode and dilution of fuel at the anode is avoided. Such cells potentially operate over a wide range of temperature due to the low activation energy for the mobility of the protons. The highest proton conductivity is reported for perovskite-type oxides, such BaCeO3, SrCeO3 [1,2], which, however show a critical reactivity toward CO2 and water, and a rather poor mechanical strength [3e5]. High grain interior proton conductivity has been reported also for doped-BaZrO3 perovskite-type materials [6];
the low grain boundary conductivity and low sinterability however limit their applications [5]. An effective alternative to this class of materials are the acceptor substituted rare-earth niobates [7,8]. Despite the low proton conductivity compared to perovskite-type proton conducting materials like SrCeO3 and BaZrO3 (0.001 S/cm at 800 C for 1% Ca-doped LaNbO4), they have the advantage of higher stability in CO2 and H2O atmospheres, good mechanical properties and an almost pure proton conductivity. However, due to the low proton conductivity of these materials, thin electrolytes layers and appropriately tailored electrodes are required to achieve suitable performances [9e12].
* Corresponding author. Tel.: þ45 4677 5621; fax: þ45 4677 5688. E-mail address: [email protected] (F. Bozza). 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.2011.11.002
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Recently the authors prepared a 7 mm thick La0.995Ca0.005NbO4 (LCN) electrolyte film and an LCN/Ni anode functional layer supported on a LCN/NiO substrate employing the Electrophoretic Deposition technique (EPD) [12]. This consists in the charging of the particles to be deposited by a suitable suspending medium. When an electric field is applied to the suspension, the particles move to one of the electrodes and deposit in the form of a film [13,14]. For the preparation of the LCN electrolyte layer, a suspending medium based on a solution of acetylacetone, iodine and water was optimized. Such a solution was proved effective also for co-depositing a mixture of LCN and NiO powders for the preparation of the LCN/Ni anode functional layer. Electrochemical characterizations proved the effectiveness of the procedure reported. In the description of the processing of the LCN/Ni anode layer, no analysis has been reported of possible selectivity in the composition of the deposited layer induced by the deposition conditions. As stated in literature, the deposition rate of a ceramic material is strongly affected by the composition of the suspending medium [12,15,16]. Such a strict correlation between suspension composition and deposition rate implies that when a mixture of different types of ceramic particles are co-deposited by the same suspending medium, a mismatch between the composition of the deposited powder and the composition of the deposited layer may be observed. In order to control the co-deposition process, the present paper includes the analysis of the effects of the suspending medium composition and time of deposition on the composition of the deposited powders. The optimized EPD conditions were employed to produce a quasi-symmetrical cell composed of LCN/Ni substrate, LCN/ Ni anode functional layer and LCN electrolyte layer by EPD, and a painted LCN/Ni counter electrode. An evaluation of the electrochemical activity of the LCN/Ni anode functional layer was obtained by measuring the polarization resistance in wet hydrogen of the LCN/Ni anode functional layer and LCN/Ni counter electrode in series.
2.
Experimental
Commercial La0.995Ca0.005NbO4 powder (LCN, Cerpotech) was employed for the fabrication of the substrate, anode layer, electrolyte layer and counter electrode. The powder showed a surface area of 5.93 m2/g. No secondary faces were detected in the LCN powder by X-ray Diffractometry (XRD). A composite of LCN powder impregnated with NiO (LCN:Ni 1:1 vol. ratio) was employed for the fabrication of the anode layer and counter electrode. LCN powder were dispersed in a solution of ethanol and Ni(NO3)2$6H2O (Fluka >97%). The solution was heated at 70 C until dry. The resulting powder was ground and fired at 600 C for 2 h. LCN powder was precalcined at 1100 C before impregnation. Pre-calcined (1100 C) LCN powder was employed for the fabrication of the electrolyte layer. EPD was performed in a 13 ml Teflon box. The substrates used for deposition were fixed between the electrode connected to the negative output of the power sources, and the outer side of Teflon box. A counter-electrode was fixed in
Table 1 e Composition of the LCN/Ni deposited layer and amount of powder deposited by EPD as a function of the iodine and water dissolved in acetylacetone. Powders deposited for 3 min at 25 V. I2 (mg/l) e H2O (ml/l) in acetylacetone 0.1e0 0.1e10 0.1e20 0.2e0 0.2e10 0.2e20
Deposited layer composition (LCN:Ni vol. ratio)
Amount of powder deposited (mg/cm2)
1.35 1.17 1.20 1.51 1.18 1.15
5.0 7.2 7.0 3.3 7.2 7.3
parallel to the substrate at a distance of 20 mm. A 1 cm diameter window in the Teflon box allowed the deposition of the suspended powders on the substrate. Solutions of acetylacetone (>99% Aldrich), iodine (99.99%, Aldrich) and deionized water were employed as suspending mediums. The suspensions were sonicated for 60 s, and allowed to settle for 10 min to let undesired agglomerates deposit in the bottom. The supernatant was then separated and used in the EPD process. Analysis of selectivity of the EPD conditions on the composition of the deposited anode layer was performed by depositing the powders on graphite foils for 3 min at 25 V. The composition of the deposited layer was evaluated by Energy Dispersive Spectroscopy (EDS). The weight ratios between the elements in the deposited layer detected by EDS were converted to LCN:Ni vol. ratios through a calibration curve, obtained by analyzing powders with known LCN:Ni compositions in the range between 60:40 and 40:60 vol. ratios. Pre-sintered LCNeNi substrates were employed for the fabrication of a quasi-symmetrical cell. The substrates were prepared by warm pressing technique and fired at 1100 C for 3 h. Starting powders were processed via Coat-Mix [17] before pressing. The quasi-symmetrical cell processing procedure was the following: the composite powder was first deposited on the reduced substrate. The powders were suspended in a solution
Fig. 1 e Composition of the LCN/Ni deposited layer and amount of powder deposited by EPD as a function of time of deposition. Composition of the suspending medium: 10 ml/l of water and 0.1 mg/l of iodine dissolved in acetylacetone.
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 ) 8 0 2 7 e8 0 3 2
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Fig. 2 e SEM micrograph of a top view of a porous LCN/Ni substrate prepared by warm pressing pre-sintered at 1100 C.
Fig. 4 e SEM micrograph of a cross section of a LCN/Ni support e LCN/Ni anode functional layer e LCN electrolyte e LCN/Ni counter electrode quasi-symmetrical cell.
of acetylacetone, water (10 ml/l) and iodine (10 mg/l), and deposited at 25 V for 3 min. The deposited layer was then sintered at 1100 C for 2 h, and reduced in 6% H2/N2 at 600 C for 2 h. LCN electrolyte layer was then deposited by EPD (20 ml/l of water and 10 mg/l of iodine dissolved in acetylacetone; 2 min of deposition at 25 V) on the LCN/Ni anode layer and sintered at 1275 C for 4 h. A slurry composed of LCN/NiO and PEG 400 (for synthesis, Merck) was employed for the fabrication of the counter electrode. The slurry was painted on the electrolyte layer on a 0.2 cm2 area and sintered at 1275 C for 2 h. A nearly 50 mm thick counter electrode layer was obtained after sintering. The microstructures of the powder deposits and of the sintered samples were characterized by a Scanning Electron Microscope (Zeiss Supra 35). Impedance spectroscopy was performed in a range of temperatures between 800 C and 600 C in 3% moisturized hydrogen, in the frequency range between 0.01 Hz and 1 MHz,
using a Solarton 1260 analyzer. The spectra were corrected from the inductance of the cell before being analyzed.
Fig. 3 e SEM micrograph of a top view of a green LCN/NiO anode layer deposited by EPD. Composition of the suspending medium: 10 ml/l of water and 0.1 mg/l of iodine dissolved in acetylacetone. Powders deposited for 3 min at 25 V.
3.
Results and discussion
Selectivity in the co-deposition process of LCN and NiO powders was checked by analyzing the composition of the deposited layer as a function of iodine and water dissolved in acetylacetone, and as a function of time of deposition. Table 1 summarizes the composition of the LCN/NiO deposited layers as a function of water and iodine dissolved in acetylacetone. The composition of the suspended powder was LCN:Ni 1:1 vol. ratio. It can be observed that EPD process tends to enhance the deposition of LCN powder among NiO powder if only iodine was dissolved in acetylacetone since, in absence of water, compositions of the deposited layer of LCN:Ni 1.35 and 1.50 vol. ratio were detected for respectively 0.1 and 0.2 mg/l of iodine dissolved. The combined effect of iodine and water tends both to balance the deposition rate of the two components in the suspension, with compositions of the deposited layers found to be in the range of LCN:Ni 1.15 and 1.20 vol. ratio, and, in agreement with what observed for the deposition of LCN
Fig. 5 e EIS measurement performed on a LCN electrolyte based quasi-symmetrical cell in 3% wet hydrogen at 800 C.
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Fig. 6 e Arrhenius plot for the EIS contributions measured on a LCN electrolyte based quasi-symmetrical cell.
powders [12], tends to increase the amount of powders deposited. The effect of time of deposition on selectivity was also investigated, by employing a solution of 0.1 mg/l iodine and 10 ml/l water in acetylacetone as suspending medium. In Fig. 1 are reported the compositions of the deposited layer measured in the range of deposition times between 1 and 8 min, and the relative amounts of powder deposited. It can be observed that while the amount of powder deposited increases with the deposition time, no selectivity in the composition of the deposited layer is induced, since the composition of the deposited powders results to be constant (LCN:Ni between 1.13 and 1.18 vol. ratio) in the range of deposition times detected. The small mismatch between the composition of the suspended powder (LCN:Ni 1.1 vol. ratio) and the compositions of the deposited layer may be therefore induced not by the deposition process, but by the process of sedimentation of the powders operated before deposition. The non-selective behavior as a function of deposition time is not in agreement with what observed by Wang et al. [18]. They deposited a mixture of YSZ and NiO by EPD and found that the relative amount of YSZ in the deposited layer was increasing with the time of deposition, obtaining in this way a continuously graded anode functional layer which improved
the performance of the CGO electrolyte based SOFC. The different behaviors observed by Wang and ourselves may be due to the different nature of the suspending medium employed and to the different nature of the suspended powders. The EPD conditions which can minimize selectivity in the co-deposition of LCN and NiO powder were employed for the fabrication of a half cell composed of LCN/Ni support e LCN/Ni anode functional layer by EPD e LCN electrolyte by EPD, after sintering at 1275 C. No reactivity was detected by XRD analysis between LCN powder and NiO powder after heating treatment at 1300 C, which is in agreement with what already observed by Magraso et al. and Tolchard et al. [19,20]. The fabrication of a similar cell was already reported by the authors [10]. The two procedures differ in the nature of the substrates employed: previously the anode layer was deposited on a dense green substrate made of LCN/NiO powder, graphite and binder; in the present paper the anode layer was deposited on a porous pre-sintered LCN/Ni substrate characterized by holes having dimension up to 15 mm (Fig. 2). The different nature of the substrates required different calcination temperatures of both the anode powders and electrolyte powders to be deposited by EPD, and different sintering temperatures, in order to balance the shrinkage mismatch between substrates and deposited layers during sintering. As shown in Fig. 3, the EPD conditions employed for the deposition of the LCN/NiO powder on porous substrate allowed the formation of a crack-free, uniformly distributed green layer of powder, which levels the irregularity of the surface of the substrate and closes its larger pores. In Fig. 4 is reported the SEM image of the cross section of the sintered half cell. The anode layer and substrate reveal a similar microstructure after sintering. The evidence of the anode layer was given by EDS analysis, which revealed a 15 mm thick LCN/Ni interlayer between the substrate and the electrolyte layers characterized by a Ni:La ratio higher than what detected in the substrate. The results described and what already showed in a previously published paper [10] indicate that EPD technique can be successfully applied for the fabrication of a LCN/Ni composite anode functional independently of the characteristics of the substrate employed; the composition of the deposited layer (LCN:NiO ratio) can be easily controlled by adjusting the composition of the suspended powder. In order to obtain an evaluation of the electrochemical behavior of the EPD anode functional layer, EIS measurements
Fig. 7 e EIS measurement performed on a LCN electrolyte based quasi-symmetrical cell in different H2 partial pressures at 600 C.
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Table 2 e Specific resistance and specific capacitances measured for the LCN electrolyte based quasi-symmetrical cell. Temperature ( C) 800 750 700 650 600
Resistance arc I (U cm2)
Resistance arc II (U cm2)
3.35 3.69 4.50 4.98 5.55
0.73 0.97 1.48 2.82 5.49
Capacitance arc II (F/cm2) 1.5 1.6 1.5 1.4 1.4
E-7 E-7 E-7 E-7 E-7
Resistance arc III (U cm2) 0.16 0.18 0.20 0.28 0.40
Capacitance arc III (F/cm2) 1.8 2.6 2.7 2.7 2.0
E-4 E-4 E-4 E-4 E-4
Fig. 8 e EIS measurements for a cell having gold counter electrode and a cell having LCN/Ni counter electrode at 650 C.
were performed on the quasi-symmetrical cell obtained by applying a LCN/Ni counter electrode on the half cell previously described (Figs. 5e7). The spectra measured were corrected for inductance and fitted with a L(RQ)I(RQ)II(RQ)III circuit, where L is the cell inductance, (RQ)I corresponds to the electrolyte arc, (RQ)II to the tailed high frequency arc, and (RQ)III the low frequency arc. Table 2 summarizes the specific resistances and the capacitances calculated at different temperatures. For (RQ)I, only RI can be obtained with confidence, therefore QI is not included in this table. The low frequency arc (RQ)III shows a specific capacitance of the order of 104 F/cm2, which clearly corresponds to an electrode process. The high frequency arc (RQ)II shows a specific capacitance in the order of 107 F/cm2, intermediate between that expected for a grain boundary and for an electrode process. Considering the thickness of the electrolyte (15 mm) and the grain size (w3 mm) it follows that, on average, there electrolyte is 5 grains thick. Assuming the above capacitance is the result of 5 grain boundaries in series, the capacitance of each would be 5 107 F/cm2. Based on a dielectric constant of 20 for LaNbO3 [21], this corresponds to a thickness of 7 nm, characteristic of a grain boundary. The activation energies calculated from the Arrhenius plots (Fig. 6) are 0.10 eV for arc I, 0.40 eV for arc II and 0.19 eV for arc III. These data are not in agreement with what already reported both for a similar LCN film co sintered on an LCN/NiO support [22], and for LCN pellets [7]. The activation energy for arc I results to be unrealistically low and we suggest it may be partly due to parasite resistance (e.g. current collection). In order to understand better the nature of the arcs, p(H2) dependencies analysis of the spectra were performed at 600 C. Fig. 7 shows the EIS spectra measured in atmospheres having compositions in the range between 100% H2 and 5% H2/ N2. All the atmospheres considered are 3% wet. The p(H2) variation strongly affects the behavior of the arc at low frequencies, which indicates that it is related to the electrode polarization. A slight p(H2) dependence was observed in the high frequencies arc. The observed behavior does not,
however, clarify the nature of the arc, since a p(H2) variation could in principle affect the electronic n-type component grain boundary contribution, as well as the charge transfer resistance at the electrodes [23]. A further indication can be obtained by replacing the LCN/ Ni counter electrode with a gold counter electrode. The replacement in principle should not affect grain boundary contribution, while an effective variation of the electrodes contribution is expected, since Ni is replaced with a less active catalyst, and the TPB is strongly reduced. In Fig. 8 is reported the Nyquist plot for a cell having gold counter electrode and a cell having LCN/Ni counter electrode, measured at 650 C. The plot shows that the replacement of the counter electrode does affect the behavior of the high frequency region. The arcs however are characterized by the same capacitance values (107 F/cm2), which suggests that the arcs describe the same mechanism. The difference of the respective resistances moreover is not such huge to justify the replacement of a LCN/Ni electrode with a gold electrode. Similar considerations may be deduced by the respective Bode plot (Fig. 9). Although the high frequency plots do not
Fig. 9 e Bode plot for a cell having gold counter electrode and a cell having LCN/Ni counter electrode at 650 C.
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overlap fully, they show a similar trend, which indicates that they describe the same mechanism. The behavior observed suggests to us that the high frequency arc is due to grain boundary polarization. The evidence fort this is, however, not conclusive, and further investigations are in progress.
4.
Conclusions
An LCN/Ni anode functional layer was prepared by electrophoretic deposition technique and characterized. The influence of the deposition conditions on the composition of the deposited layer was analyzed. EIS measurements on a quasisymmetrical cell were performed to have an evaluation of the electrodes polarization resistance. Even though a complete elucidation of the nature of the observed arcs could not be obtained, we are inclined to attribute a polarization resistance of 0.16 U cm2 at 800 C for the LCN/NiO anode functional layer and the LCN/NiO counter electrode in series.
Ethical statement The work described in the reported article was carried out in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki) for experiments involving humans and EU Directive 2010/63/EU for animal experiments.
Acknowledgments This work has been funded by the EU within the FP7 project Efficient and robust fuel cell with novel ceramic proton conducting electrolyte (EFFIPRO), grant agreement 227560
references
[1] Liang KC, Nowick AS. High temperature protonic conduction in mixed perovskite ceramics. Solid State Ionics 1993;61: 77e81. [2] Bonanos N, Knight KS, Ellis B. Perovskite solid electrolytes e structure, transport-properties and fuel cells applications. Solid State Ionics 1995;79:161e70. [3] Katahira K, Kohchi Y, Shimura T, Iwahara H. Protonic conduction in Zr-substituted BaCeO3. Solid State Ionics 2000; 138:91e8. [4] Scholten MJ, Schoonman J, Van Miltenburg JC, Oonk HAJ. Synthesis of strontium and barium cerate and their reaction with carbon-dioxide. Solid State Ionics 1993;61:83e91. [5] Tao SW, Irvine JTS. A stable, easily sintered protonconducting oxide electrolyte for moderate-temperature fuel cells and electrolyzers. Adv Mater 2006;18:1581e4.
[6] Bohn HG, Schober T. Electrical conductivity of the hightemperature proton conductor BaZr0.9Y0.1O2.95. J Am Cer Soc 2000;83:768e72. [7] Haugsrud R, Norby T. Proton conduction in rare-earth orthoniobates and ortho-tantalates. Nat Mater 2006;5:193e6. [8] Haugsrud R, Norby T. High-temperature proton conductivity in acceptor-doped LaNbO4. Solid State Ionics 2006;177: 1129e35. [9] Fontaine ML, Larring Y, Haugsrud R, Norby T, Wiik K, Bredesen R. Novel high temperature proton conducting fuel cells: production of La0.995Sr0.005NbO4-d electrolyte thin films and compatible cathode architectures. J Power Sources 2009; 188:106e13. [10] Lein HL, Tezuka T, Grande T, Einarsrud MA. Asymmetric proton conducting oxide membranes and fuel cells prepared by aqueous tape casting. Solid State Ionics 2008;179:1146e50. [11] Lin B, Wang S, Liu X, Meng G. Stable proton-conducting Cadoped LaNbO4 thin electrolyte-based protonic ceramic membrane fuel cells by in situ screen printing. J Alloys Compd 2009;478:355e7. [12] Bozza F, Bonanos N. Fabrication of supported Ca-doped lanthanum niobate electrolyte layer and NiO containing anode functional layer by electrophoretic deposition. Solid State Ionics, doi:10.1016/j.ssi.2011.05.017. [13] Sarkar P, Nicholson PS. Electrophoretic deposition (EPD): mechanisms, kinetics, and application to ceramics. J Am Cer Soc 1996;79:1987e2002. [14] Zhitomirsky I. Cathodic electrodeposition of ceramic and organoceramic materials. Fundamental aspects. Adv Colloid Interface Sci 2002;97:279e317. [15] Bozza F, Polini R, Traversa E. Electrophoretic deposition of dense Sr- and mg-doped LaGaO3 electrolyte films on porous La-doped Ceria for intermediate temperature solid oxide fuel cells. Fuel Cells 2008;5:344e50. [16] Dausolier L, Cloots R, Vertrujen V, Moreno R, BurgosMontes O, Ferrari B. YBa2Cu3O7x dispersion in iodine acetone for electrophoretic deposition: surface charging mechanism in a halogenated organic media. J Eur Cer Soc 2011;31:1075e86. [17] Dias FJ, Kampel M. Coat-Mix-Pulver zur Herstellung von Formko¨rpern mit hoher offener Porosita¨t und homogener Mikrostruktur. MACROBUTTON MTEditEquationSection2 DE: 196 18 815.6e45, 10.05.1996. [18] Wang Z, Zhang N, Qiao J, Sun K, Xu P. Improved SOFC performance with continuously graded anode functional layer. Electrochem Commun 2009;11:1120e3. [19] Magraso A, Haugsrud R, Norby T. Preparation and characterization of Ni-LaNbO4 cermet anode supports for proton-conducting fuel cell applications. J Am Cer Soc 2010; 93:2650e5. [20] Tolchard JR, Lein HL, Grande T. Chemical compatibility of proton conducting LaNbO4 electrolyte with potential oxide cathodes. J Eur Cer Soc 2009;29:2823e30. [21] Bonanos N, Bozza F, Ricote S. Unpublished results. [22] Magraso A, Xuriguera H, Varela M, Sunding MF, Strandbakke R, Haugsrud R, et al. Novel fabrication of Cadoped LaNbO4 thin-film proton-conducting fuel cells by pulsed laser deposition. J Am Cer Soc 2010;93:1874e8. [23] Kek D, Bonanos N, Pejovnik S, Mogensen M. The effect of electrode material on the oxidation of H2 at the metal/ Sr0.995Ce0.95Y0.05O2.970 interface. Solid State Ionics 2000;131: 249e60.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Characterization of La0.995Ca0.005NbO4/Ni anode functional layer by electrophoretic deposition in a La0.995Ca0.005NbO4 electrolyte based PCFC F. Bozza a,*, W. Schafbauer b, W.A. Meulenberg b, N. Bonanos a a
Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, P.O. Box 49, 4000 Roskilde, Denmark b Forschungszentrum Ju¨lich GmbH, Institute of Energy and Climate Research, IEK-1, 52425 Ju¨lich, Germany
article info
abstract
Article history:
The Electrophoretic Deposition (EPD) technique has been applied to the preparation of
Received 4 August 2011
a porous La0.995Ca0.005NbO4/Ni composite anode layer, deposited on a porous pre-sintered
Received in revised form
La0.995Ca0.005NbO4/Ni support. Powders of La0.995Ca0.005NbO4 and NiO were suspended in
28 October 2011
a solution of acetylacetone, iodine and water. Selectivity in the composition of the
Accepted 1 November 2011
deposited layer was analyzed as a function of the suspension compositions and deposition
Available online 14 December 2011
conditions. A quasi-symmetrical cell was produced by depositing La0.995Ca0.005NbO4 elec-
Keywords:
counter electrode on the dense electrolyte layer by brushing. The performance of the
Solid oxide fuel cells
electrodes was evaluated by electrochemical impedance spectroscopy in 3% wet H2.
Ca-doped LaNbO4
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
trolyte layer on the anode layer by EPD, and by applying a porous La0.995Ca0.005NbO4/Ni
Electrophoretic co-deposition
reserved.
Anode functional layer Selectivity analysis Electrochemical characterization
1.
Introduction
Proton conducting solid oxide fuel cells show the advantage over conventional oxygen ion conducting SOFCs of high efficiency, since water is produced at the cathode and dilution of fuel at the anode is avoided. Such cells potentially operate over a wide range of temperature due to the low activation energy for the mobility of the protons. The highest proton conductivity is reported for perovskite-type oxides, such BaCeO3, SrCeO3 [1,2], which, however show a critical reactivity toward CO2 and water, and a rather poor mechanical strength [3e5]. High grain interior proton conductivity has been reported also for doped-BaZrO3 perovskite-type materials [6];
the low grain boundary conductivity and low sinterability however limit their applications [5]. An effective alternative to this class of materials are the acceptor substituted rare-earth niobates [7,8]. Despite the low proton conductivity compared to perovskite-type proton conducting materials like SrCeO3 and BaZrO3 (0.001 S/cm at 800 C for 1% Ca-doped LaNbO4), they have the advantage of higher stability in CO2 and H2O atmospheres, good mechanical properties and an almost pure proton conductivity. However, due to the low proton conductivity of these materials, thin electrolytes layers and appropriately tailored electrodes are required to achieve suitable performances [9e12].
* Corresponding author. Tel.: þ45 4677 5621; fax: þ45 4677 5688. E-mail address: [email protected] (F. Bozza). 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.2011.11.002
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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 ) 8 0 2 7 e8 0 3 2
Recently the authors prepared a 7 mm thick La0.995Ca0.005NbO4 (LCN) electrolyte film and an LCN/Ni anode functional layer supported on a LCN/NiO substrate employing the Electrophoretic Deposition technique (EPD) [12]. This consists in the charging of the particles to be deposited by a suitable suspending medium. When an electric field is applied to the suspension, the particles move to one of the electrodes and deposit in the form of a film [13,14]. For the preparation of the LCN electrolyte layer, a suspending medium based on a solution of acetylacetone, iodine and water was optimized. Such a solution was proved effective also for co-depositing a mixture of LCN and NiO powders for the preparation of the LCN/Ni anode functional layer. Electrochemical characterizations proved the effectiveness of the procedure reported. In the description of the processing of the LCN/Ni anode layer, no analysis has been reported of possible selectivity in the composition of the deposited layer induced by the deposition conditions. As stated in literature, the deposition rate of a ceramic material is strongly affected by the composition of the suspending medium [12,15,16]. Such a strict correlation between suspension composition and deposition rate implies that when a mixture of different types of ceramic particles are co-deposited by the same suspending medium, a mismatch between the composition of the deposited powder and the composition of the deposited layer may be observed. In order to control the co-deposition process, the present paper includes the analysis of the effects of the suspending medium composition and time of deposition on the composition of the deposited powders. The optimized EPD conditions were employed to produce a quasi-symmetrical cell composed of LCN/Ni substrate, LCN/ Ni anode functional layer and LCN electrolyte layer by EPD, and a painted LCN/Ni counter electrode. An evaluation of the electrochemical activity of the LCN/Ni anode functional layer was obtained by measuring the polarization resistance in wet hydrogen of the LCN/Ni anode functional layer and LCN/Ni counter electrode in series.
2.
Experimental
Commercial La0.995Ca0.005NbO4 powder (LCN, Cerpotech) was employed for the fabrication of the substrate, anode layer, electrolyte layer and counter electrode. The powder showed a surface area of 5.93 m2/g. No secondary faces were detected in the LCN powder by X-ray Diffractometry (XRD). A composite of LCN powder impregnated with NiO (LCN:Ni 1:1 vol. ratio) was employed for the fabrication of the anode layer and counter electrode. LCN powder were dispersed in a solution of ethanol and Ni(NO3)2$6H2O (Fluka >97%). The solution was heated at 70 C until dry. The resulting powder was ground and fired at 600 C for 2 h. LCN powder was precalcined at 1100 C before impregnation. Pre-calcined (1100 C) LCN powder was employed for the fabrication of the electrolyte layer. EPD was performed in a 13 ml Teflon box. The substrates used for deposition were fixed between the electrode connected to the negative output of the power sources, and the outer side of Teflon box. A counter-electrode was fixed in
Table 1 e Composition of the LCN/Ni deposited layer and amount of powder deposited by EPD as a function of the iodine and water dissolved in acetylacetone. Powders deposited for 3 min at 25 V. I2 (mg/l) e H2O (ml/l) in acetylacetone 0.1e0 0.1e10 0.1e20 0.2e0 0.2e10 0.2e20
Deposited layer composition (LCN:Ni vol. ratio)
Amount of powder deposited (mg/cm2)
1.35 1.17 1.20 1.51 1.18 1.15
5.0 7.2 7.0 3.3 7.2 7.3
parallel to the substrate at a distance of 20 mm. A 1 cm diameter window in the Teflon box allowed the deposition of the suspended powders on the substrate. Solutions of acetylacetone (>99% Aldrich), iodine (99.99%, Aldrich) and deionized water were employed as suspending mediums. The suspensions were sonicated for 60 s, and allowed to settle for 10 min to let undesired agglomerates deposit in the bottom. The supernatant was then separated and used in the EPD process. Analysis of selectivity of the EPD conditions on the composition of the deposited anode layer was performed by depositing the powders on graphite foils for 3 min at 25 V. The composition of the deposited layer was evaluated by Energy Dispersive Spectroscopy (EDS). The weight ratios between the elements in the deposited layer detected by EDS were converted to LCN:Ni vol. ratios through a calibration curve, obtained by analyzing powders with known LCN:Ni compositions in the range between 60:40 and 40:60 vol. ratios. Pre-sintered LCNeNi substrates were employed for the fabrication of a quasi-symmetrical cell. The substrates were prepared by warm pressing technique and fired at 1100 C for 3 h. Starting powders were processed via Coat-Mix [17] before pressing. The quasi-symmetrical cell processing procedure was the following: the composite powder was first deposited on the reduced substrate. The powders were suspended in a solution
Fig. 1 e Composition of the LCN/Ni deposited layer and amount of powder deposited by EPD as a function of time of deposition. Composition of the suspending medium: 10 ml/l of water and 0.1 mg/l of iodine dissolved in acetylacetone.
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Fig. 2 e SEM micrograph of a top view of a porous LCN/Ni substrate prepared by warm pressing pre-sintered at 1100 C.
Fig. 4 e SEM micrograph of a cross section of a LCN/Ni support e LCN/Ni anode functional layer e LCN electrolyte e LCN/Ni counter electrode quasi-symmetrical cell.
of acetylacetone, water (10 ml/l) and iodine (10 mg/l), and deposited at 25 V for 3 min. The deposited layer was then sintered at 1100 C for 2 h, and reduced in 6% H2/N2 at 600 C for 2 h. LCN electrolyte layer was then deposited by EPD (20 ml/l of water and 10 mg/l of iodine dissolved in acetylacetone; 2 min of deposition at 25 V) on the LCN/Ni anode layer and sintered at 1275 C for 4 h. A slurry composed of LCN/NiO and PEG 400 (for synthesis, Merck) was employed for the fabrication of the counter electrode. The slurry was painted on the electrolyte layer on a 0.2 cm2 area and sintered at 1275 C for 2 h. A nearly 50 mm thick counter electrode layer was obtained after sintering. The microstructures of the powder deposits and of the sintered samples were characterized by a Scanning Electron Microscope (Zeiss Supra 35). Impedance spectroscopy was performed in a range of temperatures between 800 C and 600 C in 3% moisturized hydrogen, in the frequency range between 0.01 Hz and 1 MHz,
using a Solarton 1260 analyzer. The spectra were corrected from the inductance of the cell before being analyzed.
Fig. 3 e SEM micrograph of a top view of a green LCN/NiO anode layer deposited by EPD. Composition of the suspending medium: 10 ml/l of water and 0.1 mg/l of iodine dissolved in acetylacetone. Powders deposited for 3 min at 25 V.
3.
Results and discussion
Selectivity in the co-deposition process of LCN and NiO powders was checked by analyzing the composition of the deposited layer as a function of iodine and water dissolved in acetylacetone, and as a function of time of deposition. Table 1 summarizes the composition of the LCN/NiO deposited layers as a function of water and iodine dissolved in acetylacetone. The composition of the suspended powder was LCN:Ni 1:1 vol. ratio. It can be observed that EPD process tends to enhance the deposition of LCN powder among NiO powder if only iodine was dissolved in acetylacetone since, in absence of water, compositions of the deposited layer of LCN:Ni 1.35 and 1.50 vol. ratio were detected for respectively 0.1 and 0.2 mg/l of iodine dissolved. The combined effect of iodine and water tends both to balance the deposition rate of the two components in the suspension, with compositions of the deposited layers found to be in the range of LCN:Ni 1.15 and 1.20 vol. ratio, and, in agreement with what observed for the deposition of LCN
Fig. 5 e EIS measurement performed on a LCN electrolyte based quasi-symmetrical cell in 3% wet hydrogen at 800 C.
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Fig. 6 e Arrhenius plot for the EIS contributions measured on a LCN electrolyte based quasi-symmetrical cell.
powders [12], tends to increase the amount of powders deposited. The effect of time of deposition on selectivity was also investigated, by employing a solution of 0.1 mg/l iodine and 10 ml/l water in acetylacetone as suspending medium. In Fig. 1 are reported the compositions of the deposited layer measured in the range of deposition times between 1 and 8 min, and the relative amounts of powder deposited. It can be observed that while the amount of powder deposited increases with the deposition time, no selectivity in the composition of the deposited layer is induced, since the composition of the deposited powders results to be constant (LCN:Ni between 1.13 and 1.18 vol. ratio) in the range of deposition times detected. The small mismatch between the composition of the suspended powder (LCN:Ni 1.1 vol. ratio) and the compositions of the deposited layer may be therefore induced not by the deposition process, but by the process of sedimentation of the powders operated before deposition. The non-selective behavior as a function of deposition time is not in agreement with what observed by Wang et al. [18]. They deposited a mixture of YSZ and NiO by EPD and found that the relative amount of YSZ in the deposited layer was increasing with the time of deposition, obtaining in this way a continuously graded anode functional layer which improved
the performance of the CGO electrolyte based SOFC. The different behaviors observed by Wang and ourselves may be due to the different nature of the suspending medium employed and to the different nature of the suspended powders. The EPD conditions which can minimize selectivity in the co-deposition of LCN and NiO powder were employed for the fabrication of a half cell composed of LCN/Ni support e LCN/Ni anode functional layer by EPD e LCN electrolyte by EPD, after sintering at 1275 C. No reactivity was detected by XRD analysis between LCN powder and NiO powder after heating treatment at 1300 C, which is in agreement with what already observed by Magraso et al. and Tolchard et al. [19,20]. The fabrication of a similar cell was already reported by the authors [10]. The two procedures differ in the nature of the substrates employed: previously the anode layer was deposited on a dense green substrate made of LCN/NiO powder, graphite and binder; in the present paper the anode layer was deposited on a porous pre-sintered LCN/Ni substrate characterized by holes having dimension up to 15 mm (Fig. 2). The different nature of the substrates required different calcination temperatures of both the anode powders and electrolyte powders to be deposited by EPD, and different sintering temperatures, in order to balance the shrinkage mismatch between substrates and deposited layers during sintering. As shown in Fig. 3, the EPD conditions employed for the deposition of the LCN/NiO powder on porous substrate allowed the formation of a crack-free, uniformly distributed green layer of powder, which levels the irregularity of the surface of the substrate and closes its larger pores. In Fig. 4 is reported the SEM image of the cross section of the sintered half cell. The anode layer and substrate reveal a similar microstructure after sintering. The evidence of the anode layer was given by EDS analysis, which revealed a 15 mm thick LCN/Ni interlayer between the substrate and the electrolyte layers characterized by a Ni:La ratio higher than what detected in the substrate. The results described and what already showed in a previously published paper [10] indicate that EPD technique can be successfully applied for the fabrication of a LCN/Ni composite anode functional independently of the characteristics of the substrate employed; the composition of the deposited layer (LCN:NiO ratio) can be easily controlled by adjusting the composition of the suspended powder. In order to obtain an evaluation of the electrochemical behavior of the EPD anode functional layer, EIS measurements
Fig. 7 e EIS measurement performed on a LCN electrolyte based quasi-symmetrical cell in different H2 partial pressures at 600 C.
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Table 2 e Specific resistance and specific capacitances measured for the LCN electrolyte based quasi-symmetrical cell. Temperature ( C) 800 750 700 650 600
Resistance arc I (U cm2)
Resistance arc II (U cm2)
3.35 3.69 4.50 4.98 5.55
0.73 0.97 1.48 2.82 5.49
Capacitance arc II (F/cm2) 1.5 1.6 1.5 1.4 1.4
E-7 E-7 E-7 E-7 E-7
Resistance arc III (U cm2) 0.16 0.18 0.20 0.28 0.40
Capacitance arc III (F/cm2) 1.8 2.6 2.7 2.7 2.0
E-4 E-4 E-4 E-4 E-4
Fig. 8 e EIS measurements for a cell having gold counter electrode and a cell having LCN/Ni counter electrode at 650 C.
were performed on the quasi-symmetrical cell obtained by applying a LCN/Ni counter electrode on the half cell previously described (Figs. 5e7). The spectra measured were corrected for inductance and fitted with a L(RQ)I(RQ)II(RQ)III circuit, where L is the cell inductance, (RQ)I corresponds to the electrolyte arc, (RQ)II to the tailed high frequency arc, and (RQ)III the low frequency arc. Table 2 summarizes the specific resistances and the capacitances calculated at different temperatures. For (RQ)I, only RI can be obtained with confidence, therefore QI is not included in this table. The low frequency arc (RQ)III shows a specific capacitance of the order of 104 F/cm2, which clearly corresponds to an electrode process. The high frequency arc (RQ)II shows a specific capacitance in the order of 107 F/cm2, intermediate between that expected for a grain boundary and for an electrode process. Considering the thickness of the electrolyte (15 mm) and the grain size (w3 mm) it follows that, on average, there electrolyte is 5 grains thick. Assuming the above capacitance is the result of 5 grain boundaries in series, the capacitance of each would be 5 107 F/cm2. Based on a dielectric constant of 20 for LaNbO3 [21], this corresponds to a thickness of 7 nm, characteristic of a grain boundary. The activation energies calculated from the Arrhenius plots (Fig. 6) are 0.10 eV for arc I, 0.40 eV for arc II and 0.19 eV for arc III. These data are not in agreement with what already reported both for a similar LCN film co sintered on an LCN/NiO support [22], and for LCN pellets [7]. The activation energy for arc I results to be unrealistically low and we suggest it may be partly due to parasite resistance (e.g. current collection). In order to understand better the nature of the arcs, p(H2) dependencies analysis of the spectra were performed at 600 C. Fig. 7 shows the EIS spectra measured in atmospheres having compositions in the range between 100% H2 and 5% H2/ N2. All the atmospheres considered are 3% wet. The p(H2) variation strongly affects the behavior of the arc at low frequencies, which indicates that it is related to the electrode polarization. A slight p(H2) dependence was observed in the high frequencies arc. The observed behavior does not,
however, clarify the nature of the arc, since a p(H2) variation could in principle affect the electronic n-type component grain boundary contribution, as well as the charge transfer resistance at the electrodes [23]. A further indication can be obtained by replacing the LCN/ Ni counter electrode with a gold counter electrode. The replacement in principle should not affect grain boundary contribution, while an effective variation of the electrodes contribution is expected, since Ni is replaced with a less active catalyst, and the TPB is strongly reduced. In Fig. 8 is reported the Nyquist plot for a cell having gold counter electrode and a cell having LCN/Ni counter electrode, measured at 650 C. The plot shows that the replacement of the counter electrode does affect the behavior of the high frequency region. The arcs however are characterized by the same capacitance values (107 F/cm2), which suggests that the arcs describe the same mechanism. The difference of the respective resistances moreover is not such huge to justify the replacement of a LCN/Ni electrode with a gold electrode. Similar considerations may be deduced by the respective Bode plot (Fig. 9). Although the high frequency plots do not
Fig. 9 e Bode plot for a cell having gold counter electrode and a cell having LCN/Ni counter electrode at 650 C.
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overlap fully, they show a similar trend, which indicates that they describe the same mechanism. The behavior observed suggests to us that the high frequency arc is due to grain boundary polarization. The evidence fort this is, however, not conclusive, and further investigations are in progress.
4.
Conclusions
An LCN/Ni anode functional layer was prepared by electrophoretic deposition technique and characterized. The influence of the deposition conditions on the composition of the deposited layer was analyzed. EIS measurements on a quasisymmetrical cell were performed to have an evaluation of the electrodes polarization resistance. Even though a complete elucidation of the nature of the observed arcs could not be obtained, we are inclined to attribute a polarization resistance of 0.16 U cm2 at 800 C for the LCN/NiO anode functional layer and the LCN/NiO counter electrode in series.
Ethical statement The work described in the reported article was carried out in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki) for experiments involving humans and EU Directive 2010/63/EU for animal experiments.
Acknowledgments This work has been funded by the EU within the FP7 project Efficient and robust fuel cell with novel ceramic proton conducting electrolyte (EFFIPRO), grant agreement 227560
references
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