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Fabrication of supported Ca-doped lanthanum niobate electrolyte layer and NiO containing anode functional layer by electrophoretic deposition
Solid State Ionics 213 (2012) 98–102
Contents lists available at ScienceDirect
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
Fabrication of supported Ca-doped lanthanum niobate electrolyte layer and NiO containing anode functional layer by electrophoretic deposition Francesco Bozza ⁎, Nikolaos Bonanos Risø National Laboratory for Sustainable Energy, Fuel Cells and Solid State Chemistry Division, Technical University of Denmark, P.O. Box 49, 4000 Roskilde, Denmark
a r t i c l e
i n f o
Article history: Received 28 July 2010 Received in revised form 14 April 2011 Accepted 23 May 2011 Available online 6 July 2011 Keywords: Solid oxide fuel cells Ca-doped LaNbO4 Electrophoretic deposition Electrolyte thick film Anode functional layer
a b s t r a c t The technique of electrophoretic deposition (EPD) has been applied for the preparation of a dense calciumdoped lanthanum niobate electrolyte film. La0.995Ca0.005NbO4 (LCN) powder was suspended in a solution of acetylacetone, iodine and water. The effects of suspension composition and deposition conditions were analyzed in order to identify a suitable set of EPD process parameters. The powders were deposited on a composite substrate of LCN, NiO, binder and graphite. A dense 8 μm film of lanthanum niobate supported on a porous substrate was obtained after sintering at 1200 °C. The technique was found to be effective also for the deposition of a mixture of NiO and LCN powders which, after sintering, would form LCN/NiO anode functional layer. Electrochemical characterization of the supported LCN film was performed by applying a LCN/NiO counter electrode. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells based on proton conducting electrolyte have attracted increasing attention as potential candidates for solid oxide fuel cells. Compared to conventional oxygen ion conducting SOFCs they have the advantage of high efficiency, since water is produced at the cathode and dilution of fuel at the anode is avoided. Further the low activation energy for the mobility of the protons means they can operate in a wide range of temperature. At the state of the art, the highest proton conductivity is reported for perovskite-type oxides, such BaCeO3 and SrCeO3 [1,2]. However this class of materials shows a critical reactivity towards CO2 and water, and a mechanical instability [3,4]. Recently, acceptor substituted rare-earth niobates were suggested as an interesting alternative for high temperature proton ceramic fuel cells (PCFC) [5]. This class of materials shows a lower proton conductivity compared to the perovskite-type proton conducting materials, for instance the highest conductivity achieved was of the order of 0.001 S/cm at 800 °C for 1% Ca-doped LaNbO4, more than one order of magnitude lower than the conductivity achieved for the oxide proton conductors having perovskite structure, but have the advantage of high tolerance toward CO2, good mechanical properties and an almost pure proton conductivity. These characteristics make rare-earth niobate electrolytes effective candidates for PCFC working in a range of temperatures between 600 and 800 °C. The techniques employed for cell processing are
⁎ Corresponding author. Tel.: + 45 4677 5621; fax: + 45 4677 5688. E-mail address: [email protected] (F. Bozza). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.05.017
therefore of crucial importance, since thin electrolyte layers and carefully tailored electrodes are required to achieve suitable performances. Processing of supported LaNbO4 electrolyte films has already been reported. The electrolyte thicknesses achieved were 1–2 μm by using pulsed laser deposition technique [6], and 9–20 μm using technique based on powder processing [7–9]. To achieve high cell performance an improvement of the structure of the electrodes–electrolyte interface is required, since the electrode polarization becomes dominant when the electrolyte thickness is reduced and the working temperature decreases. It was shown that an anode functional layer able to increase the TPB lengths can drastically enhance the performance of the cell [10]. However no processing of anode functional layers has been reported in literature for PCFC based on LaNbO4 electrolyte film. In this paper we report the fabrication of both a LCN electrolyte layer- and a composite of LCN/NiO anode functional layer by electrophoretic deposition technique. In EPD process the particles to be deposited are charged by a suitable suspending medium. When an electric field is applied to the suspension, the particles move to one of the electrodes and are deposited on the substrate [11,12]. EPD is a low cost wet process, which requires simple equipment and short deposition times for the manufacturing of both porous and gas tight thick films. The main practical challenge in the application of the EPD technique consists in the choice of a suitable suspending medium which, depending on the properties of the surface of the particles to deposit, can charge the particles and promote their coagulation on the substrate surface so that, at the end of the process, an uniformly distributed and crack free layer of particles can be obtained.
F. Bozza, N. Bonanos / Solid State Ionics 213 (2012) 98–102
2. Experimental Commercial La0.995Ca0.005NbO4 (LCN) powder (Cerpotech) was employed for the fabrication of the substrate, anode layer and electrolyte layer and counter electrode. LCN-NiO composite powder (LCN:Ni 60:40 vol. ratio) used for the substrate was prepared by an impregnation method. LCN powders were dispersed in a solution of ethanol and Ni(NO3)2·6H2O (Fluka N97%). The solution was heated at 70 °C until dry. The resulting powder was ground and fired at 600 °C for 2 h. LCN/NiO based membrane was prepared by slurry casting. The slurry was composed of LCN-NiO composite powder 70 wt.%, 9 wt.% graphite (b20 μm, Sigma) and 2 wt.% nanostructured carbon (Super P, TIMCAL) and polyvinylidene difluoride 6020 (PVDF, Solvay Solaxis) 19 wt.% dissolved in N-methylpirrolydone (99%, Aldrich). The slurry was cast on a glass plate, dried at 100 °C, and cut in 15 × 15 mm 2 substrates. 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 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. EPD was performed by suspending commercial LCN powder (Cerpotech) pre-calcined at 1000 °C in a solution of acetylacetone (N99% Aldrich), iodine (99.99%, Aldrich) and deionized water. For the parametric analysis of EPD process graphite foils were employed as substrates. The suspension was sonicated for 60 s before EPD. For the deposition on the LCN/NiO based green membrane, the suspension was sonicated for 60 s, and settled down for 30 min to let
iodine=0.0 mg/l
iodine=0.2 mg/l
iodine=0.1 mg/l
iodine=0.3 mg/l
powder deposited (mg/cm²)
5.0
4.0
3.0
2.0
1.0
0.0 0
10
20 30 water concentration (ml/l)
40
Fig. 1. Amount of LCN powder deposited as a function of the iodine and water content in acetylacetone. Deposition performed at a constant voltage of 30 V and with a LCN concentration of 6 g/l.
undesired agglomerates deposit in the bottom. The supernatant was then separated and used in the EPD process. A composite of LCN powder impregnated with NiO (LCN:Ni 40:60 vol. ratio) was employed for the fabrication of the anode layer. The LCN powder was pre-calcined at 1000 °C before impregnation. The procedure for the impregnation was already described in this paragraph. The half cell processing procedure was the following: the composite powder was first deposited on the green substrate. After deposition the composite suspension was replaced by the electrolyte suspension. The electrolyte powder was then deposited and finally the deposited layers were co-sintered at 1200 °C. The microstructures of the powder deposits and of the sintered samples were characterized by a Scanning Electron Microscope (Zeiss Supra 35). A slurry composed of composite of LCN impregnated with NiO (LCN:Ni 40:60 vol. ratio) and PEG 400 (for synthesis, Merck) was employed for the fabrication of the counter electrode. The LCN powder was pre-calcined at 1000 °C before impregnation. The procedure for the impregnation was already described in this paragraph. The slurry was painted on the electrolyte layer on a 0.2 cm 2 area and sintered at 1200 °C.
8.0
powder deposited (mg/cm²)
In practice solutions of organic solvents, mainly ketones and alcohols, with small amount of electrolytes, generally iodine, have been found to be effective suspending medium for the deposition of ceramic particles by EPD [13,14]. In fuel cell processing, EPD technique has already successfully been applied for the production of dense supported electrolyte films [15–17]. By preliminary studies it was observed that a solution of acetylacetone, iodine and water is an effective suspending medium for the deposition of LCN powders. The present paper reports a systematic analysis of the optimization of the suspension composition and deposition conditions performed by depositing the powders on graphite substrate. The optimized electrophoretic deposition on a green substrate based on a LCN/NiO composite, followed by a cosintering, has made it possible to obtain a dense and crack free LCN electrolyte film supported on a porous LCN/NiO layer. The paper shows that the solution of acetylacetone, iodine and water is effective also for the deposition of LCN and NiO composite powders for anode layer applications. The deposition of the composite powders on the green substrate, followed by the deposition of the electrolyte powder on the as deposited composite layer, and cosintering, results in a LCN electrolyte layer and of LCN/NiO anode functional layer supported on a porous LCN/NiO substrate. The fabrication of an anode functional layer by EPD was already performed by Wang et al. [18], who deposited a mixture of NiO and YSZ suspended in a solution of acetylacetone and iodine. The electrolyte layer was then applied on the sintered anode layer by dip coating. In the present paper we show that is possible to deposit in sequence anode powder and electrolyte powder by EPD and sinter the deposited layers in a single step. The effectiveness of the procedure reported is evidenced by electrochemical characterizations on a quasi-symmetrical cell, obtained by depositing a counter electrode on the electrolyte layer.
99
7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
0
20
40
60
80
voltage applied (V) Fig. 2. Amount of LCN powder deposited as a function of the voltage applied. The amount of iodine and water in solution was 0.2 g/l and 10 ml/l respectively. The concentration of the powder suspended was 6 g/l.
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powder deposited (mg/cm²)
8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 0.0
2.0
4.0
6.0
8.0
10.0
12.0
powder suspended (g/l) Fig. 3. Amount of LCN powder deposited as a function of the concentration of powder suspended. The amount of iodine and water in solution was 0.2 g/l and 10 ml/l respectively. The voltage applied was 30 V.
Impedance spectroscopy was performed at 800 °C in 3% moisturized hydrogen, and at 300 °C in 3% moisturized hydrogen/nitrogen (6/94 vol. ratio), 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 rig before being analyzed.
3. Results and discussion To obtain a dense, crack free and uniformly distributed green film of powder by EPD technique a careful analysis of the composition of the suspending medium is required. For the deposition of La0.995Ca0.005NbO4 powders a solution of acetylacetone, iodine and water was investigated. Solutions of acetylacetone and iodine were already largely employed for depositions of ceramic powders like dopedZirconia or barium Cerate [16,17]. It is believed that protons in solution are able to charge the suspended particles positively, although details of the charging mechanism need to be further investigated, as does the deposition mechanism and the role of iodine. As shown in Fig. 1 a solution of acetylacetone and iodine can ensure a deposition rate even for La0.995Ca0.005NbO4 powders. Addition of water to the solution can however significantly influence the deposition process. In particular, water concentration in the range between 10 ml/l and 20 ml/l in a solution with 0.2 mg/l of iodine is shown to be the best compromise between the amount of powder deposited and the morphology of the deposited layer. Higher concentrations of water or iodine in solutions result in a non uniform distribution of the deposited layer, and in a reduction of the amount of powder deposited. Similar results were found by Bozza et al. [15] for the EPD of doped lanthanum Gallate powders by employing a solution of acetone, iodine and water. The present paper gives the first report on the effects of addition of water in a solution of acetylacetone and iodine for EPD applications. The optimization of the deposition process requires an accurate analysis of the deposition conditions. Fig. 2 shows the deposition yield as a function of the applied voltage. It can be observed that the deposition yield increases almost linearly with the voltage applied. No
Fig. 4. SEM micrograph of a top view of the La0.995Ca0.005NbO4 powder deposited by EPD on a green LCN/NiO based substrate.
effects of water electrolysis or H2 formation at the electrode were detected on the deposited film, even at high applied voltages. The role of the amount of suspended powder on EPD process was also studied. Fig. 3 shows the deposition yield as a function of the powder concentration. The deposition yield increased with the amount of powder suspended. No relevant influence of this parameter on the morphology of the deposited powders was observed.
Fig. 5. SEM micrograph of a top view of the LCN film by EPD.
Table 1 EPD conditions employed for the deposition of LCN powder. I2 concentration H2O concentration Powder suspended Voltage applied Time of deposition
0.2 mg/l 10 ml/l 6 g/l 25 V 2 min
Fig. 6. SEM micrograph of a cross section of the LCN film by EPD supported on a porous LCN/NiO substrate sintered at 1200 °C for 5 h.
F. Bozza, N. Bonanos / Solid State Ionics 213 (2012) 98–102
Fig. 7. SEM micrograph of a cross section of the LCN/NiO (substrate)–LCN/NiO (anode)– LCN (electrolyte) half cell.
0.3
1 MHz
-Z´´( ·cm²)
800°C 0.2 10 KHz
0.1
101
conditions reported in Table 1, and co-sintered at 1200 °C for 5 h. Fig. 7 shows a cross section of the half cell. A 7 μm dense and almost defects free LCN electrolyte and a porous 10 μm LCN/NiO anode functional layer were obtained. The porosity in the anode layer can be due to the hindering action of NiO on the complete sintering of LCN particles. Such pores in the anode layer, compared to those in the substrate, are smaller and better distributed, thus improving the contact area between the electrolyte and the electrode. The substrate on the other side presents larger holes (up to 10 μm large) which enhance the diffusion of gas. The contrast between the particles in the micrograph picture reveals the presence of both NiO and LCN in the anode layer, confirming that the solution employed as suspending medium was effective for the simultaneous deposition of the two different ceramic materials. A more detailed analysis of the deposition process of the composite powder is in process. The electrochemical behavior of the half cell was analyzed by applying a LCN/NiO counter electrode on the LCN film. In Figs. 8 and 9 are reported the impedance spectra of the quasi symmetrical cell at 800 °C and 300 °C. The impedance at 800 °C (Fig. 8) reveals an series resistance, Rs, of the LCN film of 1.3 Ω cm 2, which is in agreement with the electrochemical measurements already reported in literature by Magraso et al. [6]. The appearance in the spectrum at 300 °C (Fig. 9) of the Rb, Rgb and Rp contributions to the total resistance of the cell, evidence the lack of shortcut paths which could affect the performance of the cell, thus proving the effectiveness of the reported processing procedure.
1 Hz
0 1.1
4. Conclusion
1.2
1.3
1.4
1.5
1.6
1.7
Z´( ·cm²) Fig. 8. Impedance spectrum of a LCN/NiO (substrate)–LCN/NiO (anode)–LCN (electrolyte)–LCN/NiO (counter electrode) cell measured at 800 °C in 3% wet hydrogen.
As a consequence of the reported parametric analysis on graphite substrates, the EPD conditions selected for the deposition on a LCN/NiO based membrane are the ones summarized in Table 1. As shown in Fig. 4, the conditions employed allow to obtain a dense, crack free and hole free film of LCN. The deposited film was fired at 1200 °C for 5 h. In Figs. 5 and 6 are reported the top view and the cross section of the sintered supported film. A dense, crack and hole free 8 μm LCN film supported on a porous LCN/NiO substrate was obtained. The solution of acetylacetone, iodine and water was found to be effective also for the deposition of a mixture of LCN and NiO. The EPD conditions employed for the deposition of LCN-NiO powder are the same employed for the deposition of LCN powder (Table 1), but with a deposition time of 3 min. To fabricate a half cell, the LCN powders were deposited on the green LCN/NiO anode functional layer at the
A dense 10 μm thick LCN electrolyte layer and a porous 3 μm thick LCN/NiO anode layer were fabricated by electrophoretic deposition technique. A solution of acetylacetone, iodine and water was employed as suspending medium. The deposition process of LCN powder was carefully analyzed in order to define the optimal composition of the suspending medium and the optimal deposition condition. The same suspending medium composition was employed for the deposition of the composite LCN/NiO powder. The subsequent deposition of the composite powder and electrolyte powder on a LCN/NiO based substrate and co-sintering allowed to obtain an half cell composed of dense LCN electrolyte, porous LCN/NiO anode layer and porous LCN/NiO substrate. Electrochemical measurements reveal an electrolyte specific resistance of 1.3 Ω cm 2 at 800 °C in reducing atmosphere. 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.
-Z´´(Ω·cm²)
4000
300°C
3000
1 KHz
2000 1000 0
1 MHz 0.1 Hz
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000
Z´(Ω·cm²) Fig. 9. Impedance spectrum of a LCN/NiO (substrate)–LCN/NiO (anode)–LCN (electrolyte)–LCN/NiO (counter electrode) cell measured at 300 °C in 3% wet hydrogen/nitrogen.
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K.C. Liang, A.S. Nowick, Solid State Ionics 61 (1993) 77. N. Bonanos, K.S. Knight, B. Ellis, Solid State Ionics 79 (1995) 161. K. Katahira, Y. Kohchi, T. Shimura, H. Iwahara, Solid State Ionics 138 (2000) 91. M.J. Scholten, J. Schoonman, J.C. van Miltenburg, H.A.J. Oonk, Solid State Ionics 61 (1993) 83. [5] R. Haugsrud, T. Norby, Nat. Mater. 5 (2006) 193. [6] A. Magraso, H. Xuriguera, M. Varela, M.F. Sunding, R. Strandbakke, R. Haugsrud, T. Norby, J. Am. Ceram. Soc. 93 (2010) 1874. [7] M.L. Fontaine, Y. Larring, R. Haugsrud, T. Norby, K. Wiik, R. Bredesen, J. Power Sources 188 (2009) 355.
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H.L. Lein, T. Tezuka, T. Grande, M.A. Einarsrud, Solid State Ionics 179 (2008) 1146. B. Lin, S. Wang, X. Liu, G. Meng, J. Alloys Compd. 478 (2009) 355. J.S. Ahn, H. Yoon, K.T. Lee, M.A. Camaratta, E.D. Wachsman, Fuel Cells 5 (2009) 643. P. Sarkar, P.S. Nicholson, J. Am. Ceram. Soc. 79 (1996) 1987. I. Zhitomirsky, Adv. Colloid Interface Sci. 97 (2002) 279. L. Besra, M. Liu, Prog. Mater. Sci 52 (2007) 1. F. Bozza, R. Polini, E. Traversa, Fuel Cells 5 (2008) 344. F. Bozza, R. Polini, E. Traversa, Electrochem. Commun. 8 (2009) 1680. M. Zunic, L. Chevallier, F. Deganello, A. D'Epifanio, S. Licoccia, E. Di Bartolomeo, E. Traversa, J. Power Sources 190 (2009) 4170. [17] T. Hosomi, M. Matsuda, M. Miyake, J. Eur. Ceram. Soc. 27 (2007) 173. [18] Z. Wang, N. Zhang, J. Qiao, K. Sun, P. Xu, Electrochem. Commun. 11 (2009) 1120.
Contents lists available at ScienceDirect
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
Fabrication of supported Ca-doped lanthanum niobate electrolyte layer and NiO containing anode functional layer by electrophoretic deposition Francesco Bozza ⁎, Nikolaos Bonanos Risø National Laboratory for Sustainable Energy, Fuel Cells and Solid State Chemistry Division, Technical University of Denmark, P.O. Box 49, 4000 Roskilde, Denmark
a r t i c l e
i n f o
Article history: Received 28 July 2010 Received in revised form 14 April 2011 Accepted 23 May 2011 Available online 6 July 2011 Keywords: Solid oxide fuel cells Ca-doped LaNbO4 Electrophoretic deposition Electrolyte thick film Anode functional layer
a b s t r a c t The technique of electrophoretic deposition (EPD) has been applied for the preparation of a dense calciumdoped lanthanum niobate electrolyte film. La0.995Ca0.005NbO4 (LCN) powder was suspended in a solution of acetylacetone, iodine and water. The effects of suspension composition and deposition conditions were analyzed in order to identify a suitable set of EPD process parameters. The powders were deposited on a composite substrate of LCN, NiO, binder and graphite. A dense 8 μm film of lanthanum niobate supported on a porous substrate was obtained after sintering at 1200 °C. The technique was found to be effective also for the deposition of a mixture of NiO and LCN powders which, after sintering, would form LCN/NiO anode functional layer. Electrochemical characterization of the supported LCN film was performed by applying a LCN/NiO counter electrode. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells based on proton conducting electrolyte have attracted increasing attention as potential candidates for solid oxide fuel cells. Compared to conventional oxygen ion conducting SOFCs they have the advantage of high efficiency, since water is produced at the cathode and dilution of fuel at the anode is avoided. Further the low activation energy for the mobility of the protons means they can operate in a wide range of temperature. At the state of the art, the highest proton conductivity is reported for perovskite-type oxides, such BaCeO3 and SrCeO3 [1,2]. However this class of materials shows a critical reactivity towards CO2 and water, and a mechanical instability [3,4]. Recently, acceptor substituted rare-earth niobates were suggested as an interesting alternative for high temperature proton ceramic fuel cells (PCFC) [5]. This class of materials shows a lower proton conductivity compared to the perovskite-type proton conducting materials, for instance the highest conductivity achieved was of the order of 0.001 S/cm at 800 °C for 1% Ca-doped LaNbO4, more than one order of magnitude lower than the conductivity achieved for the oxide proton conductors having perovskite structure, but have the advantage of high tolerance toward CO2, good mechanical properties and an almost pure proton conductivity. These characteristics make rare-earth niobate electrolytes effective candidates for PCFC working in a range of temperatures between 600 and 800 °C. The techniques employed for cell processing are
⁎ Corresponding author. Tel.: + 45 4677 5621; fax: + 45 4677 5688. E-mail address: [email protected] (F. Bozza). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.05.017
therefore of crucial importance, since thin electrolyte layers and carefully tailored electrodes are required to achieve suitable performances. Processing of supported LaNbO4 electrolyte films has already been reported. The electrolyte thicknesses achieved were 1–2 μm by using pulsed laser deposition technique [6], and 9–20 μm using technique based on powder processing [7–9]. To achieve high cell performance an improvement of the structure of the electrodes–electrolyte interface is required, since the electrode polarization becomes dominant when the electrolyte thickness is reduced and the working temperature decreases. It was shown that an anode functional layer able to increase the TPB lengths can drastically enhance the performance of the cell [10]. However no processing of anode functional layers has been reported in literature for PCFC based on LaNbO4 electrolyte film. In this paper we report the fabrication of both a LCN electrolyte layer- and a composite of LCN/NiO anode functional layer by electrophoretic deposition technique. In EPD process the particles to be deposited are charged by a suitable suspending medium. When an electric field is applied to the suspension, the particles move to one of the electrodes and are deposited on the substrate [11,12]. EPD is a low cost wet process, which requires simple equipment and short deposition times for the manufacturing of both porous and gas tight thick films. The main practical challenge in the application of the EPD technique consists in the choice of a suitable suspending medium which, depending on the properties of the surface of the particles to deposit, can charge the particles and promote their coagulation on the substrate surface so that, at the end of the process, an uniformly distributed and crack free layer of particles can be obtained.
F. Bozza, N. Bonanos / Solid State Ionics 213 (2012) 98–102
2. Experimental Commercial La0.995Ca0.005NbO4 (LCN) powder (Cerpotech) was employed for the fabrication of the substrate, anode layer and electrolyte layer and counter electrode. LCN-NiO composite powder (LCN:Ni 60:40 vol. ratio) used for the substrate was prepared by an impregnation method. LCN powders were dispersed in a solution of ethanol and Ni(NO3)2·6H2O (Fluka N97%). The solution was heated at 70 °C until dry. The resulting powder was ground and fired at 600 °C for 2 h. LCN/NiO based membrane was prepared by slurry casting. The slurry was composed of LCN-NiO composite powder 70 wt.%, 9 wt.% graphite (b20 μm, Sigma) and 2 wt.% nanostructured carbon (Super P, TIMCAL) and polyvinylidene difluoride 6020 (PVDF, Solvay Solaxis) 19 wt.% dissolved in N-methylpirrolydone (99%, Aldrich). The slurry was cast on a glass plate, dried at 100 °C, and cut in 15 × 15 mm 2 substrates. 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 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. EPD was performed by suspending commercial LCN powder (Cerpotech) pre-calcined at 1000 °C in a solution of acetylacetone (N99% Aldrich), iodine (99.99%, Aldrich) and deionized water. For the parametric analysis of EPD process graphite foils were employed as substrates. The suspension was sonicated for 60 s before EPD. For the deposition on the LCN/NiO based green membrane, the suspension was sonicated for 60 s, and settled down for 30 min to let
iodine=0.0 mg/l
iodine=0.2 mg/l
iodine=0.1 mg/l
iodine=0.3 mg/l
powder deposited (mg/cm²)
5.0
4.0
3.0
2.0
1.0
0.0 0
10
20 30 water concentration (ml/l)
40
Fig. 1. Amount of LCN powder deposited as a function of the iodine and water content in acetylacetone. Deposition performed at a constant voltage of 30 V and with a LCN concentration of 6 g/l.
undesired agglomerates deposit in the bottom. The supernatant was then separated and used in the EPD process. A composite of LCN powder impregnated with NiO (LCN:Ni 40:60 vol. ratio) was employed for the fabrication of the anode layer. The LCN powder was pre-calcined at 1000 °C before impregnation. The procedure for the impregnation was already described in this paragraph. The half cell processing procedure was the following: the composite powder was first deposited on the green substrate. After deposition the composite suspension was replaced by the electrolyte suspension. The electrolyte powder was then deposited and finally the deposited layers were co-sintered at 1200 °C. The microstructures of the powder deposits and of the sintered samples were characterized by a Scanning Electron Microscope (Zeiss Supra 35). A slurry composed of composite of LCN impregnated with NiO (LCN:Ni 40:60 vol. ratio) and PEG 400 (for synthesis, Merck) was employed for the fabrication of the counter electrode. The LCN powder was pre-calcined at 1000 °C before impregnation. The procedure for the impregnation was already described in this paragraph. The slurry was painted on the electrolyte layer on a 0.2 cm 2 area and sintered at 1200 °C.
8.0
powder deposited (mg/cm²)
In practice solutions of organic solvents, mainly ketones and alcohols, with small amount of electrolytes, generally iodine, have been found to be effective suspending medium for the deposition of ceramic particles by EPD [13,14]. In fuel cell processing, EPD technique has already successfully been applied for the production of dense supported electrolyte films [15–17]. By preliminary studies it was observed that a solution of acetylacetone, iodine and water is an effective suspending medium for the deposition of LCN powders. The present paper reports a systematic analysis of the optimization of the suspension composition and deposition conditions performed by depositing the powders on graphite substrate. The optimized electrophoretic deposition on a green substrate based on a LCN/NiO composite, followed by a cosintering, has made it possible to obtain a dense and crack free LCN electrolyte film supported on a porous LCN/NiO layer. The paper shows that the solution of acetylacetone, iodine and water is effective also for the deposition of LCN and NiO composite powders for anode layer applications. The deposition of the composite powders on the green substrate, followed by the deposition of the electrolyte powder on the as deposited composite layer, and cosintering, results in a LCN electrolyte layer and of LCN/NiO anode functional layer supported on a porous LCN/NiO substrate. The fabrication of an anode functional layer by EPD was already performed by Wang et al. [18], who deposited a mixture of NiO and YSZ suspended in a solution of acetylacetone and iodine. The electrolyte layer was then applied on the sintered anode layer by dip coating. In the present paper we show that is possible to deposit in sequence anode powder and electrolyte powder by EPD and sinter the deposited layers in a single step. The effectiveness of the procedure reported is evidenced by electrochemical characterizations on a quasi-symmetrical cell, obtained by depositing a counter electrode on the electrolyte layer.
99
7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
0
20
40
60
80
voltage applied (V) Fig. 2. Amount of LCN powder deposited as a function of the voltage applied. The amount of iodine and water in solution was 0.2 g/l and 10 ml/l respectively. The concentration of the powder suspended was 6 g/l.
100
F. Bozza, N. Bonanos / Solid State Ionics 213 (2012) 98–102
powder deposited (mg/cm²)
8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 0.0
2.0
4.0
6.0
8.0
10.0
12.0
powder suspended (g/l) Fig. 3. Amount of LCN powder deposited as a function of the concentration of powder suspended. The amount of iodine and water in solution was 0.2 g/l and 10 ml/l respectively. The voltage applied was 30 V.
Impedance spectroscopy was performed at 800 °C in 3% moisturized hydrogen, and at 300 °C in 3% moisturized hydrogen/nitrogen (6/94 vol. ratio), 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 rig before being analyzed.
3. Results and discussion To obtain a dense, crack free and uniformly distributed green film of powder by EPD technique a careful analysis of the composition of the suspending medium is required. For the deposition of La0.995Ca0.005NbO4 powders a solution of acetylacetone, iodine and water was investigated. Solutions of acetylacetone and iodine were already largely employed for depositions of ceramic powders like dopedZirconia or barium Cerate [16,17]. It is believed that protons in solution are able to charge the suspended particles positively, although details of the charging mechanism need to be further investigated, as does the deposition mechanism and the role of iodine. As shown in Fig. 1 a solution of acetylacetone and iodine can ensure a deposition rate even for La0.995Ca0.005NbO4 powders. Addition of water to the solution can however significantly influence the deposition process. In particular, water concentration in the range between 10 ml/l and 20 ml/l in a solution with 0.2 mg/l of iodine is shown to be the best compromise between the amount of powder deposited and the morphology of the deposited layer. Higher concentrations of water or iodine in solutions result in a non uniform distribution of the deposited layer, and in a reduction of the amount of powder deposited. Similar results were found by Bozza et al. [15] for the EPD of doped lanthanum Gallate powders by employing a solution of acetone, iodine and water. The present paper gives the first report on the effects of addition of water in a solution of acetylacetone and iodine for EPD applications. The optimization of the deposition process requires an accurate analysis of the deposition conditions. Fig. 2 shows the deposition yield as a function of the applied voltage. It can be observed that the deposition yield increases almost linearly with the voltage applied. No
Fig. 4. SEM micrograph of a top view of the La0.995Ca0.005NbO4 powder deposited by EPD on a green LCN/NiO based substrate.
effects of water electrolysis or H2 formation at the electrode were detected on the deposited film, even at high applied voltages. The role of the amount of suspended powder on EPD process was also studied. Fig. 3 shows the deposition yield as a function of the powder concentration. The deposition yield increased with the amount of powder suspended. No relevant influence of this parameter on the morphology of the deposited powders was observed.
Fig. 5. SEM micrograph of a top view of the LCN film by EPD.
Table 1 EPD conditions employed for the deposition of LCN powder. I2 concentration H2O concentration Powder suspended Voltage applied Time of deposition
0.2 mg/l 10 ml/l 6 g/l 25 V 2 min
Fig. 6. SEM micrograph of a cross section of the LCN film by EPD supported on a porous LCN/NiO substrate sintered at 1200 °C for 5 h.
F. Bozza, N. Bonanos / Solid State Ionics 213 (2012) 98–102
Fig. 7. SEM micrograph of a cross section of the LCN/NiO (substrate)–LCN/NiO (anode)– LCN (electrolyte) half cell.
0.3
1 MHz
-Z´´( ·cm²)
800°C 0.2 10 KHz
0.1
101
conditions reported in Table 1, and co-sintered at 1200 °C for 5 h. Fig. 7 shows a cross section of the half cell. A 7 μm dense and almost defects free LCN electrolyte and a porous 10 μm LCN/NiO anode functional layer were obtained. The porosity in the anode layer can be due to the hindering action of NiO on the complete sintering of LCN particles. Such pores in the anode layer, compared to those in the substrate, are smaller and better distributed, thus improving the contact area between the electrolyte and the electrode. The substrate on the other side presents larger holes (up to 10 μm large) which enhance the diffusion of gas. The contrast between the particles in the micrograph picture reveals the presence of both NiO and LCN in the anode layer, confirming that the solution employed as suspending medium was effective for the simultaneous deposition of the two different ceramic materials. A more detailed analysis of the deposition process of the composite powder is in process. The electrochemical behavior of the half cell was analyzed by applying a LCN/NiO counter electrode on the LCN film. In Figs. 8 and 9 are reported the impedance spectra of the quasi symmetrical cell at 800 °C and 300 °C. The impedance at 800 °C (Fig. 8) reveals an series resistance, Rs, of the LCN film of 1.3 Ω cm 2, which is in agreement with the electrochemical measurements already reported in literature by Magraso et al. [6]. The appearance in the spectrum at 300 °C (Fig. 9) of the Rb, Rgb and Rp contributions to the total resistance of the cell, evidence the lack of shortcut paths which could affect the performance of the cell, thus proving the effectiveness of the reported processing procedure.
1 Hz
0 1.1
4. Conclusion
1.2
1.3
1.4
1.5
1.6
1.7
Z´( ·cm²) Fig. 8. Impedance spectrum of a LCN/NiO (substrate)–LCN/NiO (anode)–LCN (electrolyte)–LCN/NiO (counter electrode) cell measured at 800 °C in 3% wet hydrogen.
As a consequence of the reported parametric analysis on graphite substrates, the EPD conditions selected for the deposition on a LCN/NiO based membrane are the ones summarized in Table 1. As shown in Fig. 4, the conditions employed allow to obtain a dense, crack free and hole free film of LCN. The deposited film was fired at 1200 °C for 5 h. In Figs. 5 and 6 are reported the top view and the cross section of the sintered supported film. A dense, crack and hole free 8 μm LCN film supported on a porous LCN/NiO substrate was obtained. The solution of acetylacetone, iodine and water was found to be effective also for the deposition of a mixture of LCN and NiO. The EPD conditions employed for the deposition of LCN-NiO powder are the same employed for the deposition of LCN powder (Table 1), but with a deposition time of 3 min. To fabricate a half cell, the LCN powders were deposited on the green LCN/NiO anode functional layer at the
A dense 10 μm thick LCN electrolyte layer and a porous 3 μm thick LCN/NiO anode layer were fabricated by electrophoretic deposition technique. A solution of acetylacetone, iodine and water was employed as suspending medium. The deposition process of LCN powder was carefully analyzed in order to define the optimal composition of the suspending medium and the optimal deposition condition. The same suspending medium composition was employed for the deposition of the composite LCN/NiO powder. The subsequent deposition of the composite powder and electrolyte powder on a LCN/NiO based substrate and co-sintering allowed to obtain an half cell composed of dense LCN electrolyte, porous LCN/NiO anode layer and porous LCN/NiO substrate. Electrochemical measurements reveal an electrolyte specific resistance of 1.3 Ω cm 2 at 800 °C in reducing atmosphere. 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.
-Z´´(Ω·cm²)
4000
300°C
3000
1 KHz
2000 1000 0
1 MHz 0.1 Hz
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000
Z´(Ω·cm²) Fig. 9. Impedance spectrum of a LCN/NiO (substrate)–LCN/NiO (anode)–LCN (electrolyte)–LCN/NiO (counter electrode) cell measured at 300 °C in 3% wet hydrogen/nitrogen.
102
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