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Deposition of Ni–CGO composite anodes by electrostatic assisted ultrasonic spray pyrolysis method
Materials Research Bulletin 42 (2007) 1674–1682 www.elsevier.com/locate/matresbu
Deposition of Ni–CGO composite anodes by electrostatic assisted ultrasonic spray pyrolysis method Jing-Chiang Chen, Ching-Liang Chang, Ching-Shiung Hsu, Bing-Hwai Hwang * Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan Received 22 August 2006; received in revised form 7 November 2006; accepted 21 November 2006 Available online 3 January 2007
Abstract Deposition of composite films of Ni and Gd-doped ceria was carried out using the electrostatic assisted ultrasonic spray pyrolysis method for the first time. The composite films were highly homogeneous, as revealed by element mapping via energy-dispersive spectrometry. Scanning electron microscope examinations revealed that deposition temperature and electric field strength had profound influence on resultant microstructure, while composition of the precursor solution had little effect. A highly porous cauliflower structure ideal for solid oxide fuel cell anode performance was obtained with a deposition temperature of 450 8C under an electric field introduced by an applied voltage of 12 kV. Films obtained with a lower deposition temperature of 250 8C or a higher applied voltage of 15 kV resulted in denser films with low porosity, while lower applied voltages of 7 or 5 kV resulted in thinner or discontinuous films due to the insufficient electrostatic attraction on the aerosol droplets. As revealed by AC impedance measurement, the area specific resistances of the Ni–CGO anode with porous cauliflower structure were rather low and a value of 0.09 V cm2 at 550 8C was obtained. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; C. Impedance spectroscopy; D. Microstructure
1. Introduction Low temperature (below 650 8C) solid oxide fuel cells (SOFCs) have recently attracted much attention because the cost of materials and fabrication will be dramatically reduced. A lower operating temperature also implies fewer degradation problems, less thermal mismatch between cell components and longer operational life. With high ionic conductivity between 500 and 700 8C, doped cerias have been extensively studied as electrolytes in reduced temperature SOFCs [1–4]. Among them, Ce0.9Gd0.1O1.95 (CGO) is considered to be one of the most promising electrolytes for SOFCs to be operated below 650 8C [2]. While Ni–YSZ (yttria stabilized zirconia) composites are often used as anodes in high temperature SOFCs, Ni–CGO composites have recently been used as anodes in many low or medium temperature SOFCs [5–13]. The conventional solid-state route of preparing mixed oxide electrodes involves ball milling and repeated grinding and sintering. These processes are energy-intensive and time-consuming and the composition homogeneity in the resultant powder is often not satisfactory. Recently, an electrostatic assisted ultrasonic spray pyrolysis (EAUSP)
* Corresponding author. Tel.: +886 7 5252000x4059; fax: +886 7 5254099. E-mail address: [email protected] (B.-H. Hwang). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.11.031
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method was developed in our laboratory to deposit La0.6Sr0.4Co0.2Fe0.8O3 cathode films on CGO substrates successfully [14]. In EAUSP method, an ultrasonic atomizer is used to generate aerosol from a solution containing metal precursors. To constrain the aerosol stream and reduce the loss to open air, an electrostatic field is employed to direct the aerosol droplets towards the substrate as the aerosol-carrying gas exits the guiding tube. A heater beneath the substrate is then used to promote pyrolysis of the aerosol droplets and formation of the desired oxide films. Because EAUSP is a single-step method that can be operated in open air, it is simple and cost-effective. Furthermore, the atomic scale mixing of metal ions in the starting solution renders well dispersion of constitutive phases and elements in the resultant composite films. In this work, EAUSP method is used for the first time to deposit Ni–CGO composite anode films on CGO electrolyte substrates. Because the electrochemical performance of SOFC electrodes depends strongly on their microstructures and an ideal microstructure for Ni–CGO anode should posses high porosity for fuel to flow, homogeneous distribution and dispersion of pores, Ni and CGO phases to increase triple phase boundary (TPB) and good adhesion to the CGO substrates, deposition parameters including deposition temperature, electric field strength and solution composition were varied systematically to investigate their effects on film microstructure, and the best deposition parameter values for an optimum microstructure were identified. The interrelations between microstructure features and deposition conditions were also explored and discussed. Area specific resistances (ASRs) of Ni–CGO anode of optimum microstructure were also obtained by AC impedance measurement and the results were compared to the reported impedances derived from other processing route. 2. Experimental procedures The experimental setup of the EASUP system is shown schematically in Fig. 1. The ultrasonic aerosol generator was operated at 1.65 MHz. The aerosol generated was carried through the glass guiding tube by the carrier gas of nitrogen at a flow rate of 1 l/min. An electrostatic field between the copper heating plate and the tip of a metal needle positioned along the center of the guiding tube was established by applying a DC voltage of 5–15 kV (Acopian, PH030HA1) between them. The distance between the needle tip and the heating plate was 3 cm. The aerosol droplets were directed to the substrate by the electrostatic field after exiting the guiding tube. The heater and thermocouple beneath the heating plate along with the temperature controller were used to control the substrate temperature during deposition.
Fig. 1. Schematic diagram of the EAUSP system.
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Ce0.9Gd0.1O1.95 substrate disks of 15 mm diameter and 0.5 mm thickness were prepared by solid-state sintering at 1400 8C for 4 h using CGO powder (Rhodia, GDC 91-SY). The starting solution for spray pyrolysis was prepared by dissolving nitrate precursors of Ce(NO3)36H2O (Showa, 98%), Ni(NO3)26H2O (Showa, 98%) and Gd(NO3)36H2O (Aldrich, 99.9%) into a mixed solvent of de-ionized water and ethanol. The volume ratio of de-ionized water:ethanol was 1:1. The total concentration of metal ions in the starting solution was 0.4 M. NiO–CGO films were deposited on the CGO substrates by the EAUSP method under ambient atmosphere. Deposition temperature, electric field strength and solution composition were varied systematically to prepare series of composite oxide films. The deposition time for each film was fixed at 2 min throughout this work. A post-deposition calcination of 800 8C/2 h was carried out for each film to promote crystallization. Further treatment under a humidified H2 atmosphere (1000 8C/0.5 h) was also performed to facilitate the reduction of NiO. Thermogravimetry measurement (Perkin-Elemer, Pyris 1 TGA) was carried out on the starting precursor solution under a nitrogen flow of 10 l/min. The scan rate and span were 10 8C/min and 50–1000 8C, respectively. Phase identification was carried out for the films before and after reduction using an X-ray powder diffractometer (Siemens D5000) with a Cu tube and a quartz monochromator on the diffracted beam side. A scanning electron microscope (SEM, Joel, JSM-6330) was used for observation of the surface morphology and cross-sections. The attached energydispersive spectrometer (EDS) was also used to show the spatial distribution of constitutive elements. Impedances of symmetrical cells with Ni–CGO films of optimum microstructure on both sides of the CGO disks were measured using a potentiostat/galvanostat (Autolab PGSTAT30) with an excitation voltage amplitude of 50 mV. The AC impedance spectra were recorded over the frequency range of 0.003–100 kHz in the temperature of 450–700 8C under a humidified H2 atmosphere. 3. Results and discussion 3.1. TG analysis The TG trace obtained indicates that weight loss of the liquid precursors occurred before 350 8C, therefore, evaporation of solvent and decomposition of intermediate complexes of precursors occurred and finished before this temperature. 3.2. Effect of deposition temperature To investigate the effect of deposition temperature on film morphology, a high deposition temperature of 450 8C and a lower one of 250 8C were employed to deposit composite films under an electrostatic field established by an
Fig. 2. XRD patterns of the films obtained at deposition temperatures of 250 8C and 450 8C.
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applied voltage of 15 kV. Calculated amounts of metal nitrates were dissolved in the starting solution to obtain a mole ratio of 5:5 for NiO:CGO phases in the deposited films. As shown in the X-ray diffraction (XRD) patterns in Fig. 2, NiO and CGO phases are present in the deposited and then calcined films and no extra phase is observed. After reduction, the NiO phase transformed to crystalline Ni phase as expected. The surface morphologies of films obtained at deposition temperatures of 250 and 450 8C are shown in Fig. 3(a and b). In view of the TGA result, very little evaporation of aerosol droplets had occurred under the deposition temperatures of 250 8C. The very wet droplets resulted in a complete wetting of the substrate, forming a continuous, dense film after deposition. Solvent evaporation and precursor pyrolysis proceeded and finished in the subsequent calcination of 800 8C. The decomposition products, presumably NOx and COx, generated during calcination escaped from the dense film leaving cracks on the surface, as shown in Fig. 3(a). On the other hand, the aerosol droplets were drier under the higher deposition temperature of 450 8C due to the enhanced evaporation. The drier aerosol droplets resulted in a limited wetting upon landing on the substrate and hence granular particles after pyrolysis and solidification (Fig. 3(b)). The granular feature and hence higher porosity are clearly seen in the cross-section views, as compared to that seen in the dense film (Fig. 3(c and d)). Moreover, because the deposition time for each film was fixed at 2 min, the thickness of the dense film obtained under the deposition temperature of 250 8C was much smaller than that of the porous film obtained under the deposition temperature of 450 8C.
Fig. 3. SEM micrographs showing surface morphology and cross-sections of NiO/CGO films obtained at deposition temperatures of: (a and c) 250 8C, (b and d) 450 8C.
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Fig. 4. SEM micrographs showing surface morphology and cross-sections of NiO/CGO films obtained under an applied voltage of: (a and b) 5 kV, (c and d) 7 kV and (e–g) 12 kV.
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3.3. Effect of the electric field strength To investigate the effect of electric field strength on the film morphology, a series of films were deposited under an applied voltage of 15, 12, 7 and 5 kV and a deposition temperature of 450 8C. The starting solution with correct composition was prepared to obtain a mole ratio of 5:5 for NiO:CGO phases in the resultant films. The XRD patterns for films obtained under different applied voltages are similar to that shown in Fig. 2 and not shown. The intensity of NiO and Ni diffraction peaks decreases with decreasing applied voltage owing to the decreasing electrostatic force. Surface morphologies of films obtained at different applied voltages are shown in Figs. 4(a–f) and 3(b). Under the applied voltage of 5 kV, the electrostatic force was so weak that little aerosol droplets were attracted to land on the substrate (Fig. 4(a and b)). Although particles obtained after pyrolysis and solidification showed some degree of agglomeration, their connection was not able to form a continuous film due to the low population. This is evidenced by the CGO grains beneath the discontinuous film as indicated by arrows in Fig. 4(b). As the applied voltage was increased to 7 kV, more aerosol droplets were attracted to the substrate, resulting in a continuous film with some isolated agglomerates of particles on it (Fig. 4(c and d)). These topmost agglomerates were obtained from the aerosol droplets arriving at the final stage of deposition after solidification and agglomeration. When the applied voltage was further increased to 12 kV, even more aerosol droplets were attracted to the substrate, resulting in an even higher degree of particle agglomeration after pyrolysis and solidification. The heavy agglomeration turned the agglomerated particles to a cauliflower-like structure (Fig. 4(e and f)). Finally, when the applied voltage was further increased to 15 kV, the travel time of aerosol droplets before landing was shortened due to the enhanced attraction of
Fig. 5. XRD patterns of the films with different compositions: (a) before and (b) after reduction.
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electrostatic force, resulting in less evaporation of solvent and hence better wetting and denser film on the substrate. The wetter aerosol droplets arriving at the final stage of deposition also resulted in larger particles with less agglomeration after solidification as shown in Fig. 3(b). The more porous film of cauliflower structure obtained under the applied voltage of 12 kV is much thicker than that obtained under the applied voltage of 15 kV (see Figs. 4(g) and 3(d)). 3.4. Effect of NiO–CGO composition Composite films with NiO:CGO mole ratios of 7:3, 6:4 and 5:5 were prepared by properly adjusting the compositions of the starting solutions. The deposition was carried out at a deposition temperature of 450 8C and an applied voltage of 12 kV. The XRD patterns for films of different compositions obtained before and after reduction are shown in Fig. 5(a and b), respectively. The intensity of NiO and Ni diffraction peaks decreases with decreasing concentration of Ni precursor in the starting solution as expected. It is noted that cauliflower structure is observed in all films, and no discernable difference in microstructure was resulted from the composition variation. Therefore, the best composition for electrochemical performance was determined by AC impedance measurement (Section 3.6). 3.5. Element distribution revealed by EDS Element distributions of Ce, Gd and Ni obtained by EDS mapping are shown in Fig. 6(b–d) for the composite NiO– CGO film shown in Fig. 6(a). The homogeneous distribution of all elements observed is typical for all films obtained in this work, indicating that EAUSP method is capable of producing composite films with well dispersion of constitutive phases and elements.
Fig. 6. (a) SEI micrograph and corresponding distribution of elements: (b) Ce, (c) Gd and (d) Ni obtained by EDS mapping.
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3.6. AC impedance measurement AC impedance measurements were carried out for composite films with NiO:CGO mole ratios of 7:3, 6:4 and 5:5 prepared under a deposition temperature of 450 8C and an applied voltage of 12 kV. The arc shape of impedance spectra shown in Fig. 7 is typical for all spectra obtained in different measurement temperatures. The ASRs of Ni–CGO anodes, as determined from the intercepts of the depressed arcs on the Z0 axis, are shown in Fig. 8. It is clearly seen in Fig. 8 that the anode with Ni:CGO mole ratio of 6:4 has the lowest ASRs. In AC impedance measurement, an initial stabilization period of 5 h in the furnace (700 8C) was allowed for the composite anodes. Stable microstructure of the composite anodes was reached because reproducible ASR values were obtained after the stabilization period. A separate XRD examination revealed that the corresponding grain sizes of Ni and CGO were 28 and 55 nm, respectively. Therefore, Ni and CGO grains remained very fine even after the prolonged AC impedance measurement. Ni–CGO anodes were prepared by Liu and his co-workers by slurry coating method [8], they found that the ASR decreased significantly when the commercial NiO and CGO raw powders were replaced by that derived from a glycine–nitrate process. For example, the ASR value at 550 8C was decreased from 2.15 V cm2 for the anodes prepared using commercially available powders to 0.14 V cm2 for those prepared using powders derived from a glycine–nitrate process. Because the corresponding ASR values of current anodes lie between 0.09 and 0.12 V cm2 (Fig. 8), it is concluded that the EAUSP is a promising method to prepare Ni–CGO anodes with a large amount of TPBs.
Fig. 7. AC impedance spectra of the Ni–CGO//CGO//Ni–CGO cells measured at 550 8C under a humidified H2 atmosphere.
Fig. 8. ASR values as functions of measurement temperature.
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4. Summary and conclusion Ni–CGO composite films were successfully deposited on CGO substrates using EAUSP method. The composite films were highly homogeneous with well dispersion of constitutive phases and elements. While deposition temperature and electric field strength have profound influence on resultant microstructure, composition of the precursor solution has little effect. A highly porous cauliflower structure ideal for SOFC anode performance was obtained with a deposition temperature of 450 8C under an electric field introduced by an applied voltage of 12 kV. Films obtained with a lower deposition temperature of 250 8C or a higher applied voltage of 15 kV resulted in denser films with low porosity, while lower applied voltages of 7 or 5 kV resulted in thinner or discontinuous films due to the insufficient electrostatic attraction on the aerosol droplets. Low ASR values of current Ni–CGO anodes, as revealed by AC impedance measurement, indicate that EAUSP is a promising method to prepare Ni–CGO anodes. Acknowledgements Thanks are due to National Science Council of Taiwan for financial support under the project number NSC-932216-E-110-013. Donation of CGO powder by Rhodia is also gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
S.C. Singhal, Solid State Ionics 135 (2000) 305. B.C.H. Steele, Solid State Ionics 129 (2000) 95. M. Mogensen, N.M. Sammes, G.A. Tompett, Solid State Ionics 129 (2000) 63. M. Sahibzada, B.C.H. Steele, K. Hellgardt, D. Barth, A. Effendi, D. Mantzavinos, I.S. Metcalfe, Chem. Eng. Sci. 55 (2000) 3077. S. Suda, M. Itagaki, E. Node, S. Takahashi, M. Kawano, H. Yoshida, T. Inagaki, J. Eur. Ceram. Soc. 26 (2006) 593. X. Fang, G. Zhu, C. Xia, X. Liu, G. Meng, Solid State Ionics 168 (2004) 31. K. Higashinakagawa, S. Sameshima, Y. Hirata, J. Ceram. Process. Res. 5 (1) (2004) 84. S. Zha, W. Rauch, M. Liu, Solid State Ionics 166 (2004) 241. B. Rosch, H. Tu, A.O. Stfrmer, A.C. Mqller, U. Stimming, Solid State Ionics 175 (2004) 113. J.B. Wang, J.-C. Jang, T.-J. Huang, J. Power Sources 122 (2003) 122. S. Primdahl, Y.L. Liuz, J. Electrochem. Soc. 149 (11) (2002) A1466. C. Xia, M. Liu, Solid State Ionics 152–153 (2002) 423. X. Zhang, S. Ohara, R. Maric, K. Mukai, T. Fukui, H. Yoshida, M. Nishimura, T. Inagaki, K. Miura, J. Power Sources 83 (1999) 170. C.-S. Hsu, B.-H. Hwang, J. Electrochem. Soc. 153 (8) (2006) A1478.
Deposition of Ni–CGO composite anodes by electrostatic assisted ultrasonic spray pyrolysis method Jing-Chiang Chen, Ching-Liang Chang, Ching-Shiung Hsu, Bing-Hwai Hwang * Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan Received 22 August 2006; received in revised form 7 November 2006; accepted 21 November 2006 Available online 3 January 2007
Abstract Deposition of composite films of Ni and Gd-doped ceria was carried out using the electrostatic assisted ultrasonic spray pyrolysis method for the first time. The composite films were highly homogeneous, as revealed by element mapping via energy-dispersive spectrometry. Scanning electron microscope examinations revealed that deposition temperature and electric field strength had profound influence on resultant microstructure, while composition of the precursor solution had little effect. A highly porous cauliflower structure ideal for solid oxide fuel cell anode performance was obtained with a deposition temperature of 450 8C under an electric field introduced by an applied voltage of 12 kV. Films obtained with a lower deposition temperature of 250 8C or a higher applied voltage of 15 kV resulted in denser films with low porosity, while lower applied voltages of 7 or 5 kV resulted in thinner or discontinuous films due to the insufficient electrostatic attraction on the aerosol droplets. As revealed by AC impedance measurement, the area specific resistances of the Ni–CGO anode with porous cauliflower structure were rather low and a value of 0.09 V cm2 at 550 8C was obtained. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; C. Impedance spectroscopy; D. Microstructure
1. Introduction Low temperature (below 650 8C) solid oxide fuel cells (SOFCs) have recently attracted much attention because the cost of materials and fabrication will be dramatically reduced. A lower operating temperature also implies fewer degradation problems, less thermal mismatch between cell components and longer operational life. With high ionic conductivity between 500 and 700 8C, doped cerias have been extensively studied as electrolytes in reduced temperature SOFCs [1–4]. Among them, Ce0.9Gd0.1O1.95 (CGO) is considered to be one of the most promising electrolytes for SOFCs to be operated below 650 8C [2]. While Ni–YSZ (yttria stabilized zirconia) composites are often used as anodes in high temperature SOFCs, Ni–CGO composites have recently been used as anodes in many low or medium temperature SOFCs [5–13]. The conventional solid-state route of preparing mixed oxide electrodes involves ball milling and repeated grinding and sintering. These processes are energy-intensive and time-consuming and the composition homogeneity in the resultant powder is often not satisfactory. Recently, an electrostatic assisted ultrasonic spray pyrolysis (EAUSP)
* Corresponding author. Tel.: +886 7 5252000x4059; fax: +886 7 5254099. E-mail address: [email protected] (B.-H. Hwang). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.11.031
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method was developed in our laboratory to deposit La0.6Sr0.4Co0.2Fe0.8O3 cathode films on CGO substrates successfully [14]. In EAUSP method, an ultrasonic atomizer is used to generate aerosol from a solution containing metal precursors. To constrain the aerosol stream and reduce the loss to open air, an electrostatic field is employed to direct the aerosol droplets towards the substrate as the aerosol-carrying gas exits the guiding tube. A heater beneath the substrate is then used to promote pyrolysis of the aerosol droplets and formation of the desired oxide films. Because EAUSP is a single-step method that can be operated in open air, it is simple and cost-effective. Furthermore, the atomic scale mixing of metal ions in the starting solution renders well dispersion of constitutive phases and elements in the resultant composite films. In this work, EAUSP method is used for the first time to deposit Ni–CGO composite anode films on CGO electrolyte substrates. Because the electrochemical performance of SOFC electrodes depends strongly on their microstructures and an ideal microstructure for Ni–CGO anode should posses high porosity for fuel to flow, homogeneous distribution and dispersion of pores, Ni and CGO phases to increase triple phase boundary (TPB) and good adhesion to the CGO substrates, deposition parameters including deposition temperature, electric field strength and solution composition were varied systematically to investigate their effects on film microstructure, and the best deposition parameter values for an optimum microstructure were identified. The interrelations between microstructure features and deposition conditions were also explored and discussed. Area specific resistances (ASRs) of Ni–CGO anode of optimum microstructure were also obtained by AC impedance measurement and the results were compared to the reported impedances derived from other processing route. 2. Experimental procedures The experimental setup of the EASUP system is shown schematically in Fig. 1. The ultrasonic aerosol generator was operated at 1.65 MHz. The aerosol generated was carried through the glass guiding tube by the carrier gas of nitrogen at a flow rate of 1 l/min. An electrostatic field between the copper heating plate and the tip of a metal needle positioned along the center of the guiding tube was established by applying a DC voltage of 5–15 kV (Acopian, PH030HA1) between them. The distance between the needle tip and the heating plate was 3 cm. The aerosol droplets were directed to the substrate by the electrostatic field after exiting the guiding tube. The heater and thermocouple beneath the heating plate along with the temperature controller were used to control the substrate temperature during deposition.
Fig. 1. Schematic diagram of the EAUSP system.
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Ce0.9Gd0.1O1.95 substrate disks of 15 mm diameter and 0.5 mm thickness were prepared by solid-state sintering at 1400 8C for 4 h using CGO powder (Rhodia, GDC 91-SY). The starting solution for spray pyrolysis was prepared by dissolving nitrate precursors of Ce(NO3)36H2O (Showa, 98%), Ni(NO3)26H2O (Showa, 98%) and Gd(NO3)36H2O (Aldrich, 99.9%) into a mixed solvent of de-ionized water and ethanol. The volume ratio of de-ionized water:ethanol was 1:1. The total concentration of metal ions in the starting solution was 0.4 M. NiO–CGO films were deposited on the CGO substrates by the EAUSP method under ambient atmosphere. Deposition temperature, electric field strength and solution composition were varied systematically to prepare series of composite oxide films. The deposition time for each film was fixed at 2 min throughout this work. A post-deposition calcination of 800 8C/2 h was carried out for each film to promote crystallization. Further treatment under a humidified H2 atmosphere (1000 8C/0.5 h) was also performed to facilitate the reduction of NiO. Thermogravimetry measurement (Perkin-Elemer, Pyris 1 TGA) was carried out on the starting precursor solution under a nitrogen flow of 10 l/min. The scan rate and span were 10 8C/min and 50–1000 8C, respectively. Phase identification was carried out for the films before and after reduction using an X-ray powder diffractometer (Siemens D5000) with a Cu tube and a quartz monochromator on the diffracted beam side. A scanning electron microscope (SEM, Joel, JSM-6330) was used for observation of the surface morphology and cross-sections. The attached energydispersive spectrometer (EDS) was also used to show the spatial distribution of constitutive elements. Impedances of symmetrical cells with Ni–CGO films of optimum microstructure on both sides of the CGO disks were measured using a potentiostat/galvanostat (Autolab PGSTAT30) with an excitation voltage amplitude of 50 mV. The AC impedance spectra were recorded over the frequency range of 0.003–100 kHz in the temperature of 450–700 8C under a humidified H2 atmosphere. 3. Results and discussion 3.1. TG analysis The TG trace obtained indicates that weight loss of the liquid precursors occurred before 350 8C, therefore, evaporation of solvent and decomposition of intermediate complexes of precursors occurred and finished before this temperature. 3.2. Effect of deposition temperature To investigate the effect of deposition temperature on film morphology, a high deposition temperature of 450 8C and a lower one of 250 8C were employed to deposit composite films under an electrostatic field established by an
Fig. 2. XRD patterns of the films obtained at deposition temperatures of 250 8C and 450 8C.
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applied voltage of 15 kV. Calculated amounts of metal nitrates were dissolved in the starting solution to obtain a mole ratio of 5:5 for NiO:CGO phases in the deposited films. As shown in the X-ray diffraction (XRD) patterns in Fig. 2, NiO and CGO phases are present in the deposited and then calcined films and no extra phase is observed. After reduction, the NiO phase transformed to crystalline Ni phase as expected. The surface morphologies of films obtained at deposition temperatures of 250 and 450 8C are shown in Fig. 3(a and b). In view of the TGA result, very little evaporation of aerosol droplets had occurred under the deposition temperatures of 250 8C. The very wet droplets resulted in a complete wetting of the substrate, forming a continuous, dense film after deposition. Solvent evaporation and precursor pyrolysis proceeded and finished in the subsequent calcination of 800 8C. The decomposition products, presumably NOx and COx, generated during calcination escaped from the dense film leaving cracks on the surface, as shown in Fig. 3(a). On the other hand, the aerosol droplets were drier under the higher deposition temperature of 450 8C due to the enhanced evaporation. The drier aerosol droplets resulted in a limited wetting upon landing on the substrate and hence granular particles after pyrolysis and solidification (Fig. 3(b)). The granular feature and hence higher porosity are clearly seen in the cross-section views, as compared to that seen in the dense film (Fig. 3(c and d)). Moreover, because the deposition time for each film was fixed at 2 min, the thickness of the dense film obtained under the deposition temperature of 250 8C was much smaller than that of the porous film obtained under the deposition temperature of 450 8C.
Fig. 3. SEM micrographs showing surface morphology and cross-sections of NiO/CGO films obtained at deposition temperatures of: (a and c) 250 8C, (b and d) 450 8C.
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Fig. 4. SEM micrographs showing surface morphology and cross-sections of NiO/CGO films obtained under an applied voltage of: (a and b) 5 kV, (c and d) 7 kV and (e–g) 12 kV.
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3.3. Effect of the electric field strength To investigate the effect of electric field strength on the film morphology, a series of films were deposited under an applied voltage of 15, 12, 7 and 5 kV and a deposition temperature of 450 8C. The starting solution with correct composition was prepared to obtain a mole ratio of 5:5 for NiO:CGO phases in the resultant films. The XRD patterns for films obtained under different applied voltages are similar to that shown in Fig. 2 and not shown. The intensity of NiO and Ni diffraction peaks decreases with decreasing applied voltage owing to the decreasing electrostatic force. Surface morphologies of films obtained at different applied voltages are shown in Figs. 4(a–f) and 3(b). Under the applied voltage of 5 kV, the electrostatic force was so weak that little aerosol droplets were attracted to land on the substrate (Fig. 4(a and b)). Although particles obtained after pyrolysis and solidification showed some degree of agglomeration, their connection was not able to form a continuous film due to the low population. This is evidenced by the CGO grains beneath the discontinuous film as indicated by arrows in Fig. 4(b). As the applied voltage was increased to 7 kV, more aerosol droplets were attracted to the substrate, resulting in a continuous film with some isolated agglomerates of particles on it (Fig. 4(c and d)). These topmost agglomerates were obtained from the aerosol droplets arriving at the final stage of deposition after solidification and agglomeration. When the applied voltage was further increased to 12 kV, even more aerosol droplets were attracted to the substrate, resulting in an even higher degree of particle agglomeration after pyrolysis and solidification. The heavy agglomeration turned the agglomerated particles to a cauliflower-like structure (Fig. 4(e and f)). Finally, when the applied voltage was further increased to 15 kV, the travel time of aerosol droplets before landing was shortened due to the enhanced attraction of
Fig. 5. XRD patterns of the films with different compositions: (a) before and (b) after reduction.
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electrostatic force, resulting in less evaporation of solvent and hence better wetting and denser film on the substrate. The wetter aerosol droplets arriving at the final stage of deposition also resulted in larger particles with less agglomeration after solidification as shown in Fig. 3(b). The more porous film of cauliflower structure obtained under the applied voltage of 12 kV is much thicker than that obtained under the applied voltage of 15 kV (see Figs. 4(g) and 3(d)). 3.4. Effect of NiO–CGO composition Composite films with NiO:CGO mole ratios of 7:3, 6:4 and 5:5 were prepared by properly adjusting the compositions of the starting solutions. The deposition was carried out at a deposition temperature of 450 8C and an applied voltage of 12 kV. The XRD patterns for films of different compositions obtained before and after reduction are shown in Fig. 5(a and b), respectively. The intensity of NiO and Ni diffraction peaks decreases with decreasing concentration of Ni precursor in the starting solution as expected. It is noted that cauliflower structure is observed in all films, and no discernable difference in microstructure was resulted from the composition variation. Therefore, the best composition for electrochemical performance was determined by AC impedance measurement (Section 3.6). 3.5. Element distribution revealed by EDS Element distributions of Ce, Gd and Ni obtained by EDS mapping are shown in Fig. 6(b–d) for the composite NiO– CGO film shown in Fig. 6(a). The homogeneous distribution of all elements observed is typical for all films obtained in this work, indicating that EAUSP method is capable of producing composite films with well dispersion of constitutive phases and elements.
Fig. 6. (a) SEI micrograph and corresponding distribution of elements: (b) Ce, (c) Gd and (d) Ni obtained by EDS mapping.
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3.6. AC impedance measurement AC impedance measurements were carried out for composite films with NiO:CGO mole ratios of 7:3, 6:4 and 5:5 prepared under a deposition temperature of 450 8C and an applied voltage of 12 kV. The arc shape of impedance spectra shown in Fig. 7 is typical for all spectra obtained in different measurement temperatures. The ASRs of Ni–CGO anodes, as determined from the intercepts of the depressed arcs on the Z0 axis, are shown in Fig. 8. It is clearly seen in Fig. 8 that the anode with Ni:CGO mole ratio of 6:4 has the lowest ASRs. In AC impedance measurement, an initial stabilization period of 5 h in the furnace (700 8C) was allowed for the composite anodes. Stable microstructure of the composite anodes was reached because reproducible ASR values were obtained after the stabilization period. A separate XRD examination revealed that the corresponding grain sizes of Ni and CGO were 28 and 55 nm, respectively. Therefore, Ni and CGO grains remained very fine even after the prolonged AC impedance measurement. Ni–CGO anodes were prepared by Liu and his co-workers by slurry coating method [8], they found that the ASR decreased significantly when the commercial NiO and CGO raw powders were replaced by that derived from a glycine–nitrate process. For example, the ASR value at 550 8C was decreased from 2.15 V cm2 for the anodes prepared using commercially available powders to 0.14 V cm2 for those prepared using powders derived from a glycine–nitrate process. Because the corresponding ASR values of current anodes lie between 0.09 and 0.12 V cm2 (Fig. 8), it is concluded that the EAUSP is a promising method to prepare Ni–CGO anodes with a large amount of TPBs.
Fig. 7. AC impedance spectra of the Ni–CGO//CGO//Ni–CGO cells measured at 550 8C under a humidified H2 atmosphere.
Fig. 8. ASR values as functions of measurement temperature.
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4. Summary and conclusion Ni–CGO composite films were successfully deposited on CGO substrates using EAUSP method. The composite films were highly homogeneous with well dispersion of constitutive phases and elements. While deposition temperature and electric field strength have profound influence on resultant microstructure, composition of the precursor solution has little effect. A highly porous cauliflower structure ideal for SOFC anode performance was obtained with a deposition temperature of 450 8C under an electric field introduced by an applied voltage of 12 kV. Films obtained with a lower deposition temperature of 250 8C or a higher applied voltage of 15 kV resulted in denser films with low porosity, while lower applied voltages of 7 or 5 kV resulted in thinner or discontinuous films due to the insufficient electrostatic attraction on the aerosol droplets. Low ASR values of current Ni–CGO anodes, as revealed by AC impedance measurement, indicate that EAUSP is a promising method to prepare Ni–CGO anodes. Acknowledgements Thanks are due to National Science Council of Taiwan for financial support under the project number NSC-932216-E-110-013. Donation of CGO powder by Rhodia is also gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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