- Email: [email protected]
Anode substrate with continuous porosity gradient for tubular solid oxide fuel cells
Electrochemistry Communications 38 (2014) 114–116
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Anode substrate with continuous porosity gradient for tubular solid oxide fuel cells Long Chen a, Mutian Yao b, Changrong Xia a,⁎ a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei, Anhui 230026, China b Department of Chemistry, Leland Stanford Junior University, Stanford, CA 94305, United States
a r t i c l e
i n f o
Article history: Received 12 October 2013 Received in revised form 29 October 2013 Accepted 7 November 2013 Available online 14 November 2013 Keywords: Tubular fuel cell Graded anode Phase inversion Skin layer Concentration polarization
a b s t r a c t Tubular solid oxide fuel cells (SOFCs) are fabricated using a modified phase inversion process to obtain anode structure with graded pore distribution. The novel structure is achieved using an additional graphite layer to control the phase separation reaction in the ceramic layer and to remove the skin layer, which always presents in phase inversion process. The graded anode can effectively eliminate the concentration polarization loss at high current density as observed for the anode with the skin layer. In addition, improved peak power density is obtained with the graded-anode based cell, demonstrating that the modified process is promise in fabricating tubular SOFCs. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are considered as a promising avenue for the conversion of chemical energy into electrical powder in terms of its high efficiency, low emissions, and fuel flexibility [1]. SOFC performance is limited by various polarization losses including ohmic polarization, activation polarization, and concentration polarization. Under given operating conditions, these losses are largely dependent on cell materials, electrode microstructures, and geometric parameters [2]. In so far as the anode supported construction is concerned, a graded anode structure such as porosity gradient with decreasing porosity and pore size from the anode surface to the anode-electrolyte interface is preferred, to allow for less hindered gas transportation and rich electrochemical active three-phase boundaries (TPBs). Several processes are thus developed to fabricate gradient anode substrates, including multilayer printing (electrolyte supported) [3], multi-step dry pressing [4], freeze-tape-casting [5,6] and multilayer tape-casting [7]. These processes are successfully demonstrated with the planar design, which is very popular in the SOFC society. However, they are not applicable to the tubular design that has attracted considerable scientific attention due to its high-volumetric power density, good mechanical property, good thermo-cycling behavior, and simple sealing [8]. The tubular anode substrate is fabricated by either plastic mass extrusion or moldassisted dip coating process [8,9], which results in a uniform pore structure, but not a pore-gradient. A special tubular design of hollow-fiber ⁎ Corresponding author. Tel.: +86 551 63607475; fax: +86 551 63601592. E-mail address: [email protected] (C. Xia). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.11.009
SOFCs with asymmetric pore distribution has been prepared by an immersion induced phase inversion method [10–18]. This method results in asymmetric structure typically consisting of three sections: a sponge layer which is rich in TPBs, a highly porous finger-like layer with regular pores for fast gas delivery, and a relatively dense skin layer [19]. The delivery capability is high in the finger-like layer but could be limited by the skin layer, which is much denser than the finger-like layer [11,13]. Therefore, previous researches have shown that concentration polarization is common at high current density with the skin layers [13–15,18]. In this communication, a graphite-assisted phase inversion process is developed to fabricate tubular anode with continuous porosity gradient. The skin layer, which is made of the graphite, is completely removed in the high-temperature heating step. In addition, the graphite layer slows down the phase-inversion reaction, causing a graded structure in the finger-like layer. The novel process is demonstrated with Ni–8 mol%Y2O3–ZrO2 (YSZ) composites, on which single cells are constructed and characterized. 2. Experimental The green tube consists of a NiO–YSZ layer and a graphite layer was fabricated using a multi-step dip coating process with a glass tube as the model, Fig. 1a. NiO (Jinchuan Group, China) and YSZ (Sichuan, China) powders (weight ratio 1:1) were ball-milled for 24 h and used as the raw anode materials. Polymer solution was made from 1-Methyl-2pyrrolidone solvent (NMP, Chemical Pure, Sinopharm Chemical Reagent Co., Ltd, China), polyethersulfone polymer (PESf, Radel A-100, Solvay
L. Chen et al. / Electrochemistry Communications 38 (2014) 114–116
Fig. 1. Schematic illustration for the graphite-assisted phase inversion process. (a) A tube picture and an illustration indicating the NiO–YSZ and graphite (C) slurry layers; (b) 2nd immersion with ethanol, resulting in a sponge like dense layer, which prevents the next step water exchange in this direction; (c) 3rd immersion with water, resulting in a skin graphite layer and a NiO–YSZ layer with graded pore structure; and (d) removal of the skin layer by heating at 1200 °C.
Advanced Polymers, L.L.C.) and polyvinylpyrrolidone dispersant (PVP, K30, Chemical Pure, Sinopharm Chemical Reagent Co., Ltd, China) with weight ratio of NMP: PESf: PVP = 20: 4: 1. The powders were mixed with the polymer solution by 36 h ball-milling to get a well-dispersed NiO–YSZ slurry containing 60 wt.% NiO–YSZ. Graphite slurry with 30 wt.% graphite powder was also prepared by ball-milling. The glass tube (diameter = 1.20 cm) with a closed end was dip coated with the graphite slurry and then immersed into tap water for 5 min. The first immersion caused phase inversion reaction, resulting in a porous solidified graphite layer. After drying, the tube was dip coated with the graphite slurry again to form a transition graphite layer, and subsequently coated with the NiO–YSZ slurry twice. Whereafter, the tube was immersed into ethanol for 30 s. The second immersion coagulated the out-most layer. The glass tube was then removed and the sample was immediately immersed into tap water for 12 h. Third immersion completed the phase inversion. Samples without the graphite layers were also fabricated with the similar process. The dried raw tubes were heated at 1200 °C for 2 h to obtain adequate mechanical strength for applying YSZ electrolyte (TZ-8Y Tosoh, Japan) with the previously reported method [20]. The tubes were finally sintered at 1400 °C for 5 h to densify the electrolyte. A-site non-stoichiometric (La0.85Sr0.15)0.9MnO3 (LSM)-YSZ composite cathode with 60 wt.% LSM was brush painted on the electrolyte, dried and heated at 1200 °C for 2 h. The cathode area is 0.37 cm2. The microstructures were characterized with a scanning electron microscope (SEM, JSM-6700F). The electrochemical performance was measured using an Electrochemical Workstation (VersaSTAT VMC-4) with 10 mL min−1 of humidified (~ 3%H2O) hydrogen as the fuel and stationary air as the oxidant.
115
the literatures. In the second immersion, the reaction occurs at the outmost NiO–YSZ layer with ethanol as the coagulant agent. The sluggish exchange of ethanol and NMP results in a relative dense sponge-like microstructure (Fig. 1b), which is much denser than the graphite skin layer. Thus, when the third immersion is conducted, tap water diffuses through the porous graphite layer rather than the sponge-like NiO– YSZ layer, Fig. 1c. Additionally, the exchange rate of NMP and tap water is decreased by the graphite layer; hence finger-like pores in NiO–YSZ layer become fuzzy, Fig. 1c. When tap water diffuses from the graphite layer at the inner side of the tube to the sponge-like layer at the outer side, an amplitude composition fluctuation leads to the spinodal decomposition. The phase separation takes place simultaneously in the whole system and thus the demixed polymer rich and dilute phases tend to be interconnected to form three-dimensional interpenetrating networks, respectively. The polymer rich phase could be solidified to form the NiO–YSZ network and the dilute phase forms voids. The concentration profile gradient of the diffused water makes the phase separation processes from instantaneous demixing near the inner side to delayed demixing near the outer side. Consequently, as shown in Fig. 1d, a porosity gradient is formed from the inner side with macro-voids to the outer side with sponge like structure when the graphite layers are removed. The former can promote gas diffusion rate and the latter is rich in TPBs. When the graphite layer is not applied, a relatively dense skin layer presents at the inner side. Fig. 2 presents the cross-sectional morphologies of the tested Ni–YSZ anodes, which is ~ 320 μm thick. Fig. 2a clearly demonstrates a gradient distribution in porosity. The average porosity estimated with SEM analysis is 71.6% for the 80-μm-thick area at the inner surface of the tube (the bottom area of Fig. 2a). It decreases to 58.7% and 47.8% for the next two areas. Finally, the porosity decreases to 32.1% for the 80-μm-thick area next to the electrolyte. Fig. 2b shows the sandwich microstructures for the anode prepared without the transition graphite layer. A much porous area is between two relative dense layers, the sponge-like layer and the skin layer, which is about 50 μm thick. Fig. 2c and d compare the microstructures of the inner surfaces. When graphite layer is applied, the inner surface is distinctly porous, Fig. 2c. On the contrary, Fig. 2d demonstrates a much dense surface for the anode with the skin layer. Fig. 3a presents the cell voltage and power density for the anode with the skin layer. The peak power densities are 0.27, 0.40 and 0.51 W cm− 2 at 750, 800, and 850 °C, respectively. The cell performance is close to the tubular SOFC prepared with the phase-inversion process as reported by Wang and Liu [15], who have achieved a peak power density of 0.35 W cm−2 at 800 °C using Ni–YSZ as the anode, 22-μm-thick YSZ as the electrolyte, and LSM-YSZ as the cathode. It is noted that, at 800 and 850 °C, the slopes of the I–V curves change
3. Results and discussion Precious reports have shown that different non-solvents can obtain various pore morphologies [21,22]. In this work, the first phase inversion takes place at the outmost graphite layer with water as the coagulant agent. This reaction results in a porous graphite layer, which should have the similar microstructure as the skin layer reported in
Fig. 2. SEM micrographs for cross-sectional microstructure of the anode (a) with pore gradient, (b) with the skin layer, (c) the inner surface of (a), and (d) the inner surface of (b). Insets of (c) and (d) depict the cross-section view of the electrode-electrolyte interface of the corresponding cells.
116
L. Chen et al. / Electrochemistry Communications 38 (2014) 114–116
polarization is observed even at the current density of 2.2 A cm− 2 (850 °C) or 1.8 A cm−2 (800 °C). Since the two cells herein have very similar electrolytes and cathodes (insets in Fig. 2c,d), the difference in performance must be caused by the difference in anode microstructures. That is, the concentration polarization must be attributed to the skin layer. So, the graded structure can effectively eliminate concentration polarization loss. It is noted that its durability should be further investigated. 4. Conclusions A graphite-assisted phase inversion process is developed to fabricate tubular SOFCs. The process not only removes the relative dense skin layer but also results in continuous porosity gradient in the Ni–YSZ anode substrates. Concentration polarization loss is thus successfully eliminated. In addition, the graded anode exhibits improved electrochemical performance compared with that of normal asymmetric structure. Therefore, the modified process is novel in fabricating tubular SOFCs with graded anode substrates. Acknowledgment We gratefully acknowledge the financial supports from the National Nature Science Foundation of China (51372239) and the Ministry of Science and Technology of China (2012CB215403). References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 3. Current–voltage (I–V) curves and the corresponding power densities for the cells with (a) and without (b) the skin layer.
[9] [10] [11]
with the current densities. At 850 °C, the absolute value of the segment slope for I N 1.58 A cm− 2 is about 1.34, much higher than the slope (0.42) for I b 1.42 A cm−2, suggesting high concentration polarization when I exceeds 1.58 A cm−2 [23–25]. At 800 °C, concentration polarization starts to limit the cell performance at about 1.47 A cm−2. Analogous concentration polarization is also observed by Wang and Liu using a tubular cell with the skin layer at the current density about 1.1 A cm−2 (800 °C) [15]. Fig. 3b presents the cell performance for the anode with graded pore structure. The peak power densities are 0.35, 0.50 and 0.66 W cm−2 at 750, 800, and 850 °C, respectively, higher than those for the anode with the skin layer. The fuel utilization estimated with the equation in ref. [16] increases about 20% at 800 °C. These results are consistent with the previous reports that the graded structure has advantage of high electrochemical activity [3–7]. No concentration
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
S.D. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. F. Zhao, A.V. Virkar, J. Power Sources 141 (2005) 79. A.C. Muller, D. Herbstritt, E. Ivers-Tiffee, Solid State Ionics 152 (2002) 537. P. Holtappels, C. Sorof, M.C. Verbraeken, S. Rambert, U. Vogt, Fuel Cells 6 (2006) 113. S.W. Sofie, J. Am. Ceram. Soc. 90 (2007) 2024. Y. Chen, J. Bunch, T.S. Li, Z.P. Mao, F.L. Chen, J. Power Sources 213 (2012) 93. A. Chung Min, S. Jung-Hoon, K. Inyong, N. Sammes, J. Power Sources 195 (2010) 821. T. Suzuki, Z. Hasan, Y. Funahashi, T. Yamaguchi, Y. Fujishiro, M. Awano, Science 325 (2009) 852. Y.H. Bai, J. Liu, H.B. Gao, C. Jin, J. Alloys Compd. 480 (2009) 554. C. Yang, W. Li, S. Zhang, L. Bi, R. Peng, C. Chen, W. Liu, J. Power Sources 187 (2009) 90. N. Droushiotis, M.H.D. Othman, U. Doraswami, Z.T. Wu, G. Kelsall, K. Li, Electrochem. Commun. 11 (2009) 1799. C.H. Yang, C. Jin, F.L. Chen, Electrochem. Commun. 12 (2010) 657. M.H.D. Othman, N. Droushiotis, Z.T. Wu, G. Kelsall, K. Li, J. Power Sources 196 (2011) 5035. M.H.D. Othman, N. Droushiotis, Z. Wu, G. Kelsall, K. Li, Adv. Mater. 23 (2011) 2480. H. Wang, J. Liu, Int. J. Hydrogen Energy 37 (2012) 4339. W. Sun, N.Q. Zhang, Y.C. Mao, K.N. Sun, Electrochem. Commun. 20 (2012) 117. C. Yang, C. Jin, M. Liu, F. Chen, Electrochem. Commun. 34 (2013) 231. S. Peng, Y. Wei, J. Xue, Y. Chen, H. Wang, Int. J. Hydrogen Energy 38 (2013) 10552. W.N. Yin, B. Meng, X.X. Meng, X.Y. Tan, J. Alloys Compd. 476 (2009) 566. M.F. Liu, J.F. Gao, D.H. Dong, X.Q. Liu, G.Y. Meng, Int. J. Hydrogen Energy 35 (2010) 10489. Z.S. Li, C.Z. Jiang, J. Polym. Sci. B Polym. Phys. 43 (2005) 498. C. Jin, C. Yang, F. Chen, J. Membr. Sci. 363 (2010) 250. S. Kakac, A. Pramuanjaroenkij, X.Y. Zhou, Int. J. Hydrogen Energy 32 (2007) 761. J.W. Kim, A.V. Virkar, K.Z. Fung, K. Mehta, S.C. Singhal, J. Electrochem. Soc. 146 (1999) 69. A.V. Virkar, J. Chen, C.W. Tanner, J.W. Kim, Solid State Ionics 131 (2000) 189.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Anode substrate with continuous porosity gradient for tubular solid oxide fuel cells Long Chen a, Mutian Yao b, Changrong Xia a,⁎ a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei, Anhui 230026, China b Department of Chemistry, Leland Stanford Junior University, Stanford, CA 94305, United States
a r t i c l e
i n f o
Article history: Received 12 October 2013 Received in revised form 29 October 2013 Accepted 7 November 2013 Available online 14 November 2013 Keywords: Tubular fuel cell Graded anode Phase inversion Skin layer Concentration polarization
a b s t r a c t Tubular solid oxide fuel cells (SOFCs) are fabricated using a modified phase inversion process to obtain anode structure with graded pore distribution. The novel structure is achieved using an additional graphite layer to control the phase separation reaction in the ceramic layer and to remove the skin layer, which always presents in phase inversion process. The graded anode can effectively eliminate the concentration polarization loss at high current density as observed for the anode with the skin layer. In addition, improved peak power density is obtained with the graded-anode based cell, demonstrating that the modified process is promise in fabricating tubular SOFCs. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are considered as a promising avenue for the conversion of chemical energy into electrical powder in terms of its high efficiency, low emissions, and fuel flexibility [1]. SOFC performance is limited by various polarization losses including ohmic polarization, activation polarization, and concentration polarization. Under given operating conditions, these losses are largely dependent on cell materials, electrode microstructures, and geometric parameters [2]. In so far as the anode supported construction is concerned, a graded anode structure such as porosity gradient with decreasing porosity and pore size from the anode surface to the anode-electrolyte interface is preferred, to allow for less hindered gas transportation and rich electrochemical active three-phase boundaries (TPBs). Several processes are thus developed to fabricate gradient anode substrates, including multilayer printing (electrolyte supported) [3], multi-step dry pressing [4], freeze-tape-casting [5,6] and multilayer tape-casting [7]. These processes are successfully demonstrated with the planar design, which is very popular in the SOFC society. However, they are not applicable to the tubular design that has attracted considerable scientific attention due to its high-volumetric power density, good mechanical property, good thermo-cycling behavior, and simple sealing [8]. The tubular anode substrate is fabricated by either plastic mass extrusion or moldassisted dip coating process [8,9], which results in a uniform pore structure, but not a pore-gradient. A special tubular design of hollow-fiber ⁎ Corresponding author. Tel.: +86 551 63607475; fax: +86 551 63601592. E-mail address: [email protected] (C. Xia). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.11.009
SOFCs with asymmetric pore distribution has been prepared by an immersion induced phase inversion method [10–18]. This method results in asymmetric structure typically consisting of three sections: a sponge layer which is rich in TPBs, a highly porous finger-like layer with regular pores for fast gas delivery, and a relatively dense skin layer [19]. The delivery capability is high in the finger-like layer but could be limited by the skin layer, which is much denser than the finger-like layer [11,13]. Therefore, previous researches have shown that concentration polarization is common at high current density with the skin layers [13–15,18]. In this communication, a graphite-assisted phase inversion process is developed to fabricate tubular anode with continuous porosity gradient. The skin layer, which is made of the graphite, is completely removed in the high-temperature heating step. In addition, the graphite layer slows down the phase-inversion reaction, causing a graded structure in the finger-like layer. The novel process is demonstrated with Ni–8 mol%Y2O3–ZrO2 (YSZ) composites, on which single cells are constructed and characterized. 2. Experimental The green tube consists of a NiO–YSZ layer and a graphite layer was fabricated using a multi-step dip coating process with a glass tube as the model, Fig. 1a. NiO (Jinchuan Group, China) and YSZ (Sichuan, China) powders (weight ratio 1:1) were ball-milled for 24 h and used as the raw anode materials. Polymer solution was made from 1-Methyl-2pyrrolidone solvent (NMP, Chemical Pure, Sinopharm Chemical Reagent Co., Ltd, China), polyethersulfone polymer (PESf, Radel A-100, Solvay
L. Chen et al. / Electrochemistry Communications 38 (2014) 114–116
Fig. 1. Schematic illustration for the graphite-assisted phase inversion process. (a) A tube picture and an illustration indicating the NiO–YSZ and graphite (C) slurry layers; (b) 2nd immersion with ethanol, resulting in a sponge like dense layer, which prevents the next step water exchange in this direction; (c) 3rd immersion with water, resulting in a skin graphite layer and a NiO–YSZ layer with graded pore structure; and (d) removal of the skin layer by heating at 1200 °C.
Advanced Polymers, L.L.C.) and polyvinylpyrrolidone dispersant (PVP, K30, Chemical Pure, Sinopharm Chemical Reagent Co., Ltd, China) with weight ratio of NMP: PESf: PVP = 20: 4: 1. The powders were mixed with the polymer solution by 36 h ball-milling to get a well-dispersed NiO–YSZ slurry containing 60 wt.% NiO–YSZ. Graphite slurry with 30 wt.% graphite powder was also prepared by ball-milling. The glass tube (diameter = 1.20 cm) with a closed end was dip coated with the graphite slurry and then immersed into tap water for 5 min. The first immersion caused phase inversion reaction, resulting in a porous solidified graphite layer. After drying, the tube was dip coated with the graphite slurry again to form a transition graphite layer, and subsequently coated with the NiO–YSZ slurry twice. Whereafter, the tube was immersed into ethanol for 30 s. The second immersion coagulated the out-most layer. The glass tube was then removed and the sample was immediately immersed into tap water for 12 h. Third immersion completed the phase inversion. Samples without the graphite layers were also fabricated with the similar process. The dried raw tubes were heated at 1200 °C for 2 h to obtain adequate mechanical strength for applying YSZ electrolyte (TZ-8Y Tosoh, Japan) with the previously reported method [20]. The tubes were finally sintered at 1400 °C for 5 h to densify the electrolyte. A-site non-stoichiometric (La0.85Sr0.15)0.9MnO3 (LSM)-YSZ composite cathode with 60 wt.% LSM was brush painted on the electrolyte, dried and heated at 1200 °C for 2 h. The cathode area is 0.37 cm2. The microstructures were characterized with a scanning electron microscope (SEM, JSM-6700F). The electrochemical performance was measured using an Electrochemical Workstation (VersaSTAT VMC-4) with 10 mL min−1 of humidified (~ 3%H2O) hydrogen as the fuel and stationary air as the oxidant.
115
the literatures. In the second immersion, the reaction occurs at the outmost NiO–YSZ layer with ethanol as the coagulant agent. The sluggish exchange of ethanol and NMP results in a relative dense sponge-like microstructure (Fig. 1b), which is much denser than the graphite skin layer. Thus, when the third immersion is conducted, tap water diffuses through the porous graphite layer rather than the sponge-like NiO– YSZ layer, Fig. 1c. Additionally, the exchange rate of NMP and tap water is decreased by the graphite layer; hence finger-like pores in NiO–YSZ layer become fuzzy, Fig. 1c. When tap water diffuses from the graphite layer at the inner side of the tube to the sponge-like layer at the outer side, an amplitude composition fluctuation leads to the spinodal decomposition. The phase separation takes place simultaneously in the whole system and thus the demixed polymer rich and dilute phases tend to be interconnected to form three-dimensional interpenetrating networks, respectively. The polymer rich phase could be solidified to form the NiO–YSZ network and the dilute phase forms voids. The concentration profile gradient of the diffused water makes the phase separation processes from instantaneous demixing near the inner side to delayed demixing near the outer side. Consequently, as shown in Fig. 1d, a porosity gradient is formed from the inner side with macro-voids to the outer side with sponge like structure when the graphite layers are removed. The former can promote gas diffusion rate and the latter is rich in TPBs. When the graphite layer is not applied, a relatively dense skin layer presents at the inner side. Fig. 2 presents the cross-sectional morphologies of the tested Ni–YSZ anodes, which is ~ 320 μm thick. Fig. 2a clearly demonstrates a gradient distribution in porosity. The average porosity estimated with SEM analysis is 71.6% for the 80-μm-thick area at the inner surface of the tube (the bottom area of Fig. 2a). It decreases to 58.7% and 47.8% for the next two areas. Finally, the porosity decreases to 32.1% for the 80-μm-thick area next to the electrolyte. Fig. 2b shows the sandwich microstructures for the anode prepared without the transition graphite layer. A much porous area is between two relative dense layers, the sponge-like layer and the skin layer, which is about 50 μm thick. Fig. 2c and d compare the microstructures of the inner surfaces. When graphite layer is applied, the inner surface is distinctly porous, Fig. 2c. On the contrary, Fig. 2d demonstrates a much dense surface for the anode with the skin layer. Fig. 3a presents the cell voltage and power density for the anode with the skin layer. The peak power densities are 0.27, 0.40 and 0.51 W cm− 2 at 750, 800, and 850 °C, respectively. The cell performance is close to the tubular SOFC prepared with the phase-inversion process as reported by Wang and Liu [15], who have achieved a peak power density of 0.35 W cm−2 at 800 °C using Ni–YSZ as the anode, 22-μm-thick YSZ as the electrolyte, and LSM-YSZ as the cathode. It is noted that, at 800 and 850 °C, the slopes of the I–V curves change
3. Results and discussion Precious reports have shown that different non-solvents can obtain various pore morphologies [21,22]. In this work, the first phase inversion takes place at the outmost graphite layer with water as the coagulant agent. This reaction results in a porous graphite layer, which should have the similar microstructure as the skin layer reported in
Fig. 2. SEM micrographs for cross-sectional microstructure of the anode (a) with pore gradient, (b) with the skin layer, (c) the inner surface of (a), and (d) the inner surface of (b). Insets of (c) and (d) depict the cross-section view of the electrode-electrolyte interface of the corresponding cells.
116
L. Chen et al. / Electrochemistry Communications 38 (2014) 114–116
polarization is observed even at the current density of 2.2 A cm− 2 (850 °C) or 1.8 A cm−2 (800 °C). Since the two cells herein have very similar electrolytes and cathodes (insets in Fig. 2c,d), the difference in performance must be caused by the difference in anode microstructures. That is, the concentration polarization must be attributed to the skin layer. So, the graded structure can effectively eliminate concentration polarization loss. It is noted that its durability should be further investigated. 4. Conclusions A graphite-assisted phase inversion process is developed to fabricate tubular SOFCs. The process not only removes the relative dense skin layer but also results in continuous porosity gradient in the Ni–YSZ anode substrates. Concentration polarization loss is thus successfully eliminated. In addition, the graded anode exhibits improved electrochemical performance compared with that of normal asymmetric structure. Therefore, the modified process is novel in fabricating tubular SOFCs with graded anode substrates. Acknowledgment We gratefully acknowledge the financial supports from the National Nature Science Foundation of China (51372239) and the Ministry of Science and Technology of China (2012CB215403). References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 3. Current–voltage (I–V) curves and the corresponding power densities for the cells with (a) and without (b) the skin layer.
[9] [10] [11]
with the current densities. At 850 °C, the absolute value of the segment slope for I N 1.58 A cm− 2 is about 1.34, much higher than the slope (0.42) for I b 1.42 A cm−2, suggesting high concentration polarization when I exceeds 1.58 A cm−2 [23–25]. At 800 °C, concentration polarization starts to limit the cell performance at about 1.47 A cm−2. Analogous concentration polarization is also observed by Wang and Liu using a tubular cell with the skin layer at the current density about 1.1 A cm−2 (800 °C) [15]. Fig. 3b presents the cell performance for the anode with graded pore structure. The peak power densities are 0.35, 0.50 and 0.66 W cm−2 at 750, 800, and 850 °C, respectively, higher than those for the anode with the skin layer. The fuel utilization estimated with the equation in ref. [16] increases about 20% at 800 °C. These results are consistent with the previous reports that the graded structure has advantage of high electrochemical activity [3–7]. No concentration
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
S.D. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. F. Zhao, A.V. Virkar, J. Power Sources 141 (2005) 79. A.C. Muller, D. Herbstritt, E. Ivers-Tiffee, Solid State Ionics 152 (2002) 537. P. Holtappels, C. Sorof, M.C. Verbraeken, S. Rambert, U. Vogt, Fuel Cells 6 (2006) 113. S.W. Sofie, J. Am. Ceram. Soc. 90 (2007) 2024. Y. Chen, J. Bunch, T.S. Li, Z.P. Mao, F.L. Chen, J. Power Sources 213 (2012) 93. A. Chung Min, S. Jung-Hoon, K. Inyong, N. Sammes, J. Power Sources 195 (2010) 821. T. Suzuki, Z. Hasan, Y. Funahashi, T. Yamaguchi, Y. Fujishiro, M. Awano, Science 325 (2009) 852. Y.H. Bai, J. Liu, H.B. Gao, C. Jin, J. Alloys Compd. 480 (2009) 554. C. Yang, W. Li, S. Zhang, L. Bi, R. Peng, C. Chen, W. Liu, J. Power Sources 187 (2009) 90. N. Droushiotis, M.H.D. Othman, U. Doraswami, Z.T. Wu, G. Kelsall, K. Li, Electrochem. Commun. 11 (2009) 1799. C.H. Yang, C. Jin, F.L. Chen, Electrochem. Commun. 12 (2010) 657. M.H.D. Othman, N. Droushiotis, Z.T. Wu, G. Kelsall, K. Li, J. Power Sources 196 (2011) 5035. M.H.D. Othman, N. Droushiotis, Z. Wu, G. Kelsall, K. Li, Adv. Mater. 23 (2011) 2480. H. Wang, J. Liu, Int. J. Hydrogen Energy 37 (2012) 4339. W. Sun, N.Q. Zhang, Y.C. Mao, K.N. Sun, Electrochem. Commun. 20 (2012) 117. C. Yang, C. Jin, M. Liu, F. Chen, Electrochem. Commun. 34 (2013) 231. S. Peng, Y. Wei, J. Xue, Y. Chen, H. Wang, Int. J. Hydrogen Energy 38 (2013) 10552. W.N. Yin, B. Meng, X.X. Meng, X.Y. Tan, J. Alloys Compd. 476 (2009) 566. M.F. Liu, J.F. Gao, D.H. Dong, X.Q. Liu, G.Y. Meng, Int. J. Hydrogen Energy 35 (2010) 10489. Z.S. Li, C.Z. Jiang, J. Polym. Sci. B Polym. Phys. 43 (2005) 498. C. Jin, C. Yang, F. Chen, J. Membr. Sci. 363 (2010) 250. S. Kakac, A. Pramuanjaroenkij, X.Y. Zhou, Int. J. Hydrogen Energy 32 (2007) 761. J.W. Kim, A.V. Virkar, K.Z. Fung, K. Mehta, S.C. Singhal, J. Electrochem. Soc. 146 (1999) 69. A.V. Virkar, J. Chen, C.W. Tanner, J.W. Kim, Solid State Ionics 131 (2000) 189.