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High performance La3Ni2O7 cathode prepared by a facile sol–gel method for intermediate temperature solid oxide fuel cells
Electrochemistry Communications 22 (2012) 97–100
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
High performance La3Ni2O7 cathode prepared by a facile sol–gel method for intermediate temperature solid oxide fuel cells Zhongliang Lou, Jun Peng, Ningning Dai, Jinshuo Qiao, Yiming Yan, Zhenhua Wang ⁎, Jiawei Wang, Kening Sun ⁎ School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing, 100081, PR China
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 4 June 2012 Accepted 5 June 2012 Available online 13 June 2012 Keywords: Solid oxide fuel cells Cathode La3Ni2O7 F127 Sol–gel method
a b s t r a c t La3Ni2O7 oxide has been synthesized by a facile sol–gel method using a nonionic surfactant (EO)106(PO) 70(EO)106 tri-block copolymer (Pluronic F127) as the chelating agent. The pure phase of La3Ni2O7 has been obtained by sintering at temperatures of 1100 °C for 12 h. La3Ni2O7 oxide as a cathode presents no visible reaction with an Y2O3-stabilized ZrO2 (YSZ) electrolyte. The electrochemical performance of the prepared oxide as a cathode has been investigated. Area specific resistance of La3Ni2O7 material on YSZ electrolyte is as low as 0.39 Ω cm 2 at 750 °C. Furthermore, an anode-supported single-cell configuration of NiO–YSZ/YSZ/La3Ni2O7 has been achieved a maximum power density of 848 mW cm − 2 at 750 °C. The cell performance was stable under a constant current of 0.6 A cm − 2 for over 30 h at 750 °C. These results therefore suggest that La3Ni2O7 synthesized by this sol–gel method is a highly promising cathode material for applications in IT-SOFCs. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are electrochemical devices that convert chemical energy contained within a fuel into electrical energy with high efficiency and low pollutant emissions [1,2]. A critical challenge for such fuel cells is identifying suitable cathode materials for catalyzing the oxygen reduction reaction at a working temperature below 800 °C [3]. Recently, Lan + 1NinO3n + 1 (n = 1, 2 and 3), which can be classified as Ruddlesden–Popper (RP) oxides have attracted great interest for IT-SOFC cathode applications due to important advantages such as high oxygen ionic conductivity, attractive electronic conductivity, moderate temperature expansion coefficient (TEC), and high electrocatalytic activity under oxidizing conditions [4]. According to the reported results, when n=2 or 3, Lan + 1NinO3n + 1 demonstrates faster ionic and electronic transport properties, which may be caused by the increasing number of perovskite layers in the structure [5]. Furthermore, Lan + 1NinO3n + 1 (n > 1) oxides also exhibit good longterm stability [6], which makes them suitable candidates for cathode materials in IT-SOFC applications. Multilayered RP nickelate phases were first reported by Wold in 1959 [7]. However, it was not until 1994 that pure phase La3Ni2O7 [8] was successfully prepared by Goodenough et al. This work opened a way to synthesize the higher-order pure phase La3Ni2O7 and invoked a substantial research increase in the development of applications for La3Ni2O7. To date, several methods have been reported for the synthesis of La3Ni2O7, such as solid-state synthesis [9,10], ⁎ Corresponding authors. Tel./fax: + 86 10 6891 8696. E-mail address: [email protected] (K. Sun). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.06.004
nitrate and citrate routes [8,10], Pechini method [6] and continuous hydrothermal flow synthesis (CHFS) method [11]. However, these methods are usually time consuming and energy intensive with multiple steps required. Amow and Skinner prepared La3Ni2O7 by the Pechini method at 1100 °C over 2 days with one intermittent regrinding step [6]. Weng et al. prepared La3Ni2O7 by heat-treating the coprecipitates of lanthanum and nickel hydroxides at 1150 °C for 12 h, which were prepared using a CHFS system that uses a superheated water flow at 400 °C and 24.1 MPa [11]. Meanwhile, to the best of our knowledge, the electrochemical performance of La3Ni2O7 as IT-SOFC cathode is seldom reported. Hence, a facile synthesis strategy for preparing pure phase La3Ni2O7 and subsequent evaluation for high performance IT-SOFC applications are highly desired. F127 is a poly (ethylene oxide)–poly (propylene oxide)–poly (ethylene oxide) (PEO–PPO–PEO) tri-block copolymer. Such agents are considered to be structure-directing in the synthesis of ordered mesoporous materials based on the PEO–metal complexation [12]. In the present study, we report a facile approach for the synthesis of pure-phase highly-crystallized La3Ni2O7 oxide obtained via a sol–gel route by employing F127 as the chelating agent. A remarkable performance of the as-prepared oxide was observed when it was subsequently investigated for its application as a cathode material on an YSZ electrolyte in IT-SOFCs.
2. Experimental La3Ni2O7 powder was synthesized via a sol–gel route using F127. In a typical procedure, 0.8 g of F127 (Sigma-Aldrich) was dissolved
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Z. Lou et al. / Electrochemistry Communications 22 (2012) 97–100
in 30 mL of water at room temperature. Then, 1.3 g La(NO3)3·6H2O and 0.59 g Ni(NO3)2·6H2O were added to the above solution with vigorous stirring. After being stirred for at least 2 h at room temperature, the homogeneous sol was dried to form a gel in the oven at 100 °C for one day. The gel was calcined at different temperatures (either 1075 °C, 1100 °C or 1150 °C) for 12 h. XRD (X' Pert PRO MPD) (Cu Ka radiation, operated at 40 kV, 40 mA) was used to confirm the crystal structure of La3Ni2O7 powder. A symmetrical half-cell was fabricated by screen-printing the cathode slurry onto both surfaces of YSZ electrolyte and calcining the painted cell at 1000 °C for 4 h in air. An anode-supported fuel cell was prepared by tape-casting Ni/YSZ anodes and YSZ electrolytes, followed by sintering at 1400 °C [13]. The surface and cross-sectional microstructures of the cathode coated onto the YSZ electrolyte were characterized by a scanning electron microscope (FEI, QUANTA-250) and elemental line profiles. The electrical conductivity measurements were performed from 350 to 850 °C in air with a four probing DC technique on sintered La3Ni2O7 using a Keithley 2400 source meter. The thermal expansion coefficient (TEC) was determined using a dilatometer (Netsch DIL 402C/4) over the 40 °C to 1000 °C temperature range. Electrochemical impedance spectra (EIS) of the symmetrical cell of La3Ni2O7/YSZ/La3Ni2O7 over the temperature range 650 to 850 °C were performed on a PARSTAT 2273 in air. I–V curves were carried out with a Fuel Cell Test System (FCTS) produced by Arbin Instruments, with the cathode exposed to ambient air and the anode to humidified (∼3 vol.% H2O) hydrogen at a flow rate of 50 mL min− 1. 3. Results and discussion Fig. 1a shows the XRD patterns of La3Ni2O7 calcined at different temperatures. For the sample calcined at 1075 °C, the main orthorhombic structure of La3Ni2O7 (JCPDS Card No. 50‐0244) was observed together with a hexagonal La2O3 structure suggesting a mixed material. However for the samples calcined at 1100 °C and 1150 °C, the pattern can be clearly indexed as the orthorhombic RP phase structure without any impurity. By using the Scherrer's equation, the mean crystalline size is estimated to be 33, 46, and 56 nm for the samples calcined at 1075, 1100 and 1150 °C, respectively, implying the gradual growth of the particle size along with increasing temperature. The La3Ni2O7 sample was indexed as the Fmmm space group. It is known that alkylene oxide segments of F127 could form crown-ether-type complexes with Ni2 + and La3 + through coordination bonds [12,14], bringing the reaction partners sufficiently close together. Therefore, a pure-phase La3Ni2O7 oxide could be obtained after calcination at relative short time and a low temperature. In order to investigate the compatibility of the prepared sample with an YSZ electrolyte, the La3Ni2O7–YSZ composite was fabricated by sintering at 1000 °C for 4 h. Fig. 1b shows the XRD pattern of La3Ni2O7–YSZ composite. The corresponding peaks of La3Ni2O7 and YSZ are clearly displayed and no impure phases are further observed, indicating that the prepared La3Ni2O7 oxide has good chemical compatibility with the YSZ electrolyte. Fig. 1c shows the electrical conductivity behaviors of the La3Ni2O7 sample over the temperature range from 350 °C to 850 °C in air. The value at 750 °C was 60 S cm− 1, which is in accordance with the result reported by Takahashi et al. [5]. The SEM and elemental line profiles in Fig. 2 show the surface micrograph and the cross-sectional view of an interface of the tri-layer cell (NiO–YSZ/YSZ/La3Ni2O7), which were sintered at 1000 °C for 4 h. As shown in Fig. 2a, the surface of the cathode exhibits a porous morphology which is beneficial to gas diffusion. Fig. 2b shows the cross-section micrograph of the single cell. From this it appears that the cathode and the YSZ electrolyte are tightly adhered to each other without any sign of cracks and delamination. The TEC of La3Ni2O7 was further measured as 12.9×10− 6 K− 1, which is close to that of YSZ (10.8×10− 6 K− 1) [17] over the temperature range of 40–800 °C. It reveals that the La3Ni2O7 cathode and the conventional electrolyte YSZ show good thermal
Fig. 1. XRD patterns of (a) La3Ni2O7 powders calcined at different temperatures and standard JCPDS data and (b) La3Ni2O7–YSZ composite calcined at 1000 °C for 4 h; (c) temperature dependence of electrical conductivity for La3Ni2O7 oxide from 350 °C to 850 °C in air, and lnσT vs. 1000/T plot (inset).
compatibility with each other. Fig. 2c illustrates the elemental line profiles of the element distribution of Y, Zr, La, Ni and O in the electrolyte and cathode regions. No elemental interpenetration between the layers was observed, which is consistent with the XRD results (Fig. 1). Fig. 3a shows the EIS results, in which two arcs are presented corresponding to different electrode processes. The intercept with the real axis at high frequency represents the Ohmic resistance caused by the electrolyte, electrodes and collecting wires. The difference between the real axis intercepts of the impedance arc is considered to be the interfacial resistance of the cathode, denoted as area specific resistance (ASR). It can be seen that the ASR significantly reduces with increasing temperature. The ASRs of the cathode were 2.6, 0.97, 0.39, 0.20 and 0.13 Ωcm2 at 650, 700, 750, 800 and 850 °C, respectively (Fig. 2-inset). ASR represents the overall cathodic properties related on the charge
Z. Lou et al. / Electrochemistry Communications 22 (2012) 97–100
Fig. 2. (a) Surface and (b) cross-section SEM micrographs and (c) EDX linear scan analysis of the relative elemental distribution of the fuel cell with La3Ni2O7 cathode.
transfer, oxygen adsorption/dissociation, surface/bulk diffusion and gasphase diffusion processes [15]. Remarkably, the obtained ASR of the La3Ni2O7 cathode on YSZ electrolyte developed here is lower than that of La2NiO4 on a SDC electrolyte (0.42 Ωcm2) at 800 °C reported elsewhere [16]. This can be ascribed to the high ionic transport property of the higher ordered RP phases of La3Ni2O7 oxide [4]. The well-defined ASR implies that La3Ni2O7 obtained by this method could be considered as a potential cathode material for IT-SOFCs. The performance of the NiO–YSZ anode-supported single cell combined with La3Ni2O7 as a cathode was evaluated. Fig. 3b presents the I–V curves and the corresponding power densities of the single cell with La3Ni2O7 cathode over the temperature range of 650–800 °C with
99
Fig. 3. (a) Nyquist plots for EIS of La3Ni2O7 cathode on YSZ electrolyte measured from 650 °C to 800 °C in air, and Arrhenius plot of ASRs for La3Ni2O7 cathode (inset); (b) performance of a single cell with La3Ni2O7 cathode from 650 °C to 800 °C; (c) the shortterm stability test for anode-supported single cell with La3Ni2O7 cathode at 750 °C.
humidified hydrogen (∼3 vol.% H2O) as the fuel and static air as the oxidant. As shown in Fig. 3b, the peak power densities of the fuel cells with the La3Ni2O7 cathode were 473, 652, 848 and 1138 mW cm− 2 at 650, 700, 750 and 800 °C, respectively. The cell showed stable performance with no obvious degradation in 30 h under a constant current density of 0.6 Acm− 2 at 750 °C, as shown in Fig. 3c. These results indicate that La3Ni2O7 prepared by this method is an attractive cathode material for IT-SOFCs. 4. Conclusions In this work we have demonstrated that a sol–gel method using F127 as the chelating agent is a facile and effective strategy of
100
Z. Lou et al. / Electrochemistry Communications 22 (2012) 97–100
synthesizing La3Ni2O7 for IT-SOFC applications. Pure orthorhombic RP phased La3Ni2O7 was prepared after calcination at relative short time and a minimum temperature and used as a cathode when combined with an YSZ electrolyte and a NiO–YSZ anode to build a IT-SOFC. Herein the prepared La3Ni2O7 oxide based IT-SOFC shows low ASR of 0.39 Ω cm2 and remarkable peak power density of 848 mW cm− 2 at 750 °C. The cell shows stable performance under a constant current density of 0.6 A cm− 2 for 30 h at 750 °C. Overall these preliminary results suggest that La3Ni2O7 derived from this preparation is a promising cathode material for IT-SOFCs. Acknowledgment This work was supported by the “National Natural Science Foundation of China” under contract No. 21076023 and No. 21006006. References [1] J.R. Wilson, A.T. Duong, M. Gameiro, H.Y. Chen, K. Thornton, D.R. Mumm, S.A. Barnett, Electrochemistry Communications 11 (2009) 1052. [2] S.W. Tao, J.T.S. Irvine, Nature Materials 2 (2003) 320.
[3] D.J.L. Brett, A. Atkinson, N.P. Brandon, S.J. Skinner, Chemical Society Reviews 37 (2008) 1568. [4] S. Choi, S. Yoo, J.-Y. Shin, G. Kim, Journal of the Electrochemical Society 158 (2011) B995. [5] S. Takahashi, S. Nishimoto, M. Matsuda, M. Miyake, Journal of the American Ceramic Society 93 (2010) 2329. [6] G. Amow, I.J. Davidson, S.J. Skinner, Solid State Ionics 177 (2006) 1205. [7] A. Wold, R.J. Arnott, Journal of Physics and Chemistry of Solids 9 (1959) 176. [8] Z. Zhang, M. Greenblatt, J.B. Goodenough, Journal of Solid State Chemistry 108 (1994) 402. [9] C.D. Ling, D.N. Argyriou, G.Q. Wu, J.J. Neumeier, Journal of Solid State Chemistry 52 (2000) 517. [10] M.D. Carvalho, F.M.A. Costa, I.D.S. Pereira, A. Wattiaux, J.M. Bassat, J.C. Grenier, M. Pouchard, Journal of Materials Chemistry 7 (1997) 2107. [11] X.L. Weng, P. Boldrin, I. Abrahams, S.J. Skinner, J.A. Darr, Chemistry of Materials 19 (2007) 4382. [12] P.D. Yang, D.Y. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Nature 396 (1998) 152. [13] X. Zhou, K. Sun, J. Gao, S. Le, N. Zhang, P. Wang, Journal of Power Sources 191 (2009) 528–533. [14] J. Kim, J.K. Kim, S. Heo, H.S. Lee, Thin Solid Films 503 (2006) 60. [15] S.-W. Baek, J.H. Kim, J. Bae, Solid State Ionics 179 (2008) 1570. [16] M. Chen, B.H. Moon, S.H. Kim, B.H. Kim, Q. Xu, B.G. Ahn, Fuel Cells 12 (2012) 86. [17] H. Liu, X. Zhu, M. Cheng, Y. Cong, W. Yang, Chemical Communications 47 (2011) 2378.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
High performance La3Ni2O7 cathode prepared by a facile sol–gel method for intermediate temperature solid oxide fuel cells Zhongliang Lou, Jun Peng, Ningning Dai, Jinshuo Qiao, Yiming Yan, Zhenhua Wang ⁎, Jiawei Wang, Kening Sun ⁎ School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing, 100081, PR China
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 4 June 2012 Accepted 5 June 2012 Available online 13 June 2012 Keywords: Solid oxide fuel cells Cathode La3Ni2O7 F127 Sol–gel method
a b s t r a c t La3Ni2O7 oxide has been synthesized by a facile sol–gel method using a nonionic surfactant (EO)106(PO) 70(EO)106 tri-block copolymer (Pluronic F127) as the chelating agent. The pure phase of La3Ni2O7 has been obtained by sintering at temperatures of 1100 °C for 12 h. La3Ni2O7 oxide as a cathode presents no visible reaction with an Y2O3-stabilized ZrO2 (YSZ) electrolyte. The electrochemical performance of the prepared oxide as a cathode has been investigated. Area specific resistance of La3Ni2O7 material on YSZ electrolyte is as low as 0.39 Ω cm 2 at 750 °C. Furthermore, an anode-supported single-cell configuration of NiO–YSZ/YSZ/La3Ni2O7 has been achieved a maximum power density of 848 mW cm − 2 at 750 °C. The cell performance was stable under a constant current of 0.6 A cm − 2 for over 30 h at 750 °C. These results therefore suggest that La3Ni2O7 synthesized by this sol–gel method is a highly promising cathode material for applications in IT-SOFCs. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are electrochemical devices that convert chemical energy contained within a fuel into electrical energy with high efficiency and low pollutant emissions [1,2]. A critical challenge for such fuel cells is identifying suitable cathode materials for catalyzing the oxygen reduction reaction at a working temperature below 800 °C [3]. Recently, Lan + 1NinO3n + 1 (n = 1, 2 and 3), which can be classified as Ruddlesden–Popper (RP) oxides have attracted great interest for IT-SOFC cathode applications due to important advantages such as high oxygen ionic conductivity, attractive electronic conductivity, moderate temperature expansion coefficient (TEC), and high electrocatalytic activity under oxidizing conditions [4]. According to the reported results, when n=2 or 3, Lan + 1NinO3n + 1 demonstrates faster ionic and electronic transport properties, which may be caused by the increasing number of perovskite layers in the structure [5]. Furthermore, Lan + 1NinO3n + 1 (n > 1) oxides also exhibit good longterm stability [6], which makes them suitable candidates for cathode materials in IT-SOFC applications. Multilayered RP nickelate phases were first reported by Wold in 1959 [7]. However, it was not until 1994 that pure phase La3Ni2O7 [8] was successfully prepared by Goodenough et al. This work opened a way to synthesize the higher-order pure phase La3Ni2O7 and invoked a substantial research increase in the development of applications for La3Ni2O7. To date, several methods have been reported for the synthesis of La3Ni2O7, such as solid-state synthesis [9,10], ⁎ Corresponding authors. Tel./fax: + 86 10 6891 8696. E-mail address: [email protected] (K. Sun). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.06.004
nitrate and citrate routes [8,10], Pechini method [6] and continuous hydrothermal flow synthesis (CHFS) method [11]. However, these methods are usually time consuming and energy intensive with multiple steps required. Amow and Skinner prepared La3Ni2O7 by the Pechini method at 1100 °C over 2 days with one intermittent regrinding step [6]. Weng et al. prepared La3Ni2O7 by heat-treating the coprecipitates of lanthanum and nickel hydroxides at 1150 °C for 12 h, which were prepared using a CHFS system that uses a superheated water flow at 400 °C and 24.1 MPa [11]. Meanwhile, to the best of our knowledge, the electrochemical performance of La3Ni2O7 as IT-SOFC cathode is seldom reported. Hence, a facile synthesis strategy for preparing pure phase La3Ni2O7 and subsequent evaluation for high performance IT-SOFC applications are highly desired. F127 is a poly (ethylene oxide)–poly (propylene oxide)–poly (ethylene oxide) (PEO–PPO–PEO) tri-block copolymer. Such agents are considered to be structure-directing in the synthesis of ordered mesoporous materials based on the PEO–metal complexation [12]. In the present study, we report a facile approach for the synthesis of pure-phase highly-crystallized La3Ni2O7 oxide obtained via a sol–gel route by employing F127 as the chelating agent. A remarkable performance of the as-prepared oxide was observed when it was subsequently investigated for its application as a cathode material on an YSZ electrolyte in IT-SOFCs.
2. Experimental La3Ni2O7 powder was synthesized via a sol–gel route using F127. In a typical procedure, 0.8 g of F127 (Sigma-Aldrich) was dissolved
98
Z. Lou et al. / Electrochemistry Communications 22 (2012) 97–100
in 30 mL of water at room temperature. Then, 1.3 g La(NO3)3·6H2O and 0.59 g Ni(NO3)2·6H2O were added to the above solution with vigorous stirring. After being stirred for at least 2 h at room temperature, the homogeneous sol was dried to form a gel in the oven at 100 °C for one day. The gel was calcined at different temperatures (either 1075 °C, 1100 °C or 1150 °C) for 12 h. XRD (X' Pert PRO MPD) (Cu Ka radiation, operated at 40 kV, 40 mA) was used to confirm the crystal structure of La3Ni2O7 powder. A symmetrical half-cell was fabricated by screen-printing the cathode slurry onto both surfaces of YSZ electrolyte and calcining the painted cell at 1000 °C for 4 h in air. An anode-supported fuel cell was prepared by tape-casting Ni/YSZ anodes and YSZ electrolytes, followed by sintering at 1400 °C [13]. The surface and cross-sectional microstructures of the cathode coated onto the YSZ electrolyte were characterized by a scanning electron microscope (FEI, QUANTA-250) and elemental line profiles. The electrical conductivity measurements were performed from 350 to 850 °C in air with a four probing DC technique on sintered La3Ni2O7 using a Keithley 2400 source meter. The thermal expansion coefficient (TEC) was determined using a dilatometer (Netsch DIL 402C/4) over the 40 °C to 1000 °C temperature range. Electrochemical impedance spectra (EIS) of the symmetrical cell of La3Ni2O7/YSZ/La3Ni2O7 over the temperature range 650 to 850 °C were performed on a PARSTAT 2273 in air. I–V curves were carried out with a Fuel Cell Test System (FCTS) produced by Arbin Instruments, with the cathode exposed to ambient air and the anode to humidified (∼3 vol.% H2O) hydrogen at a flow rate of 50 mL min− 1. 3. Results and discussion Fig. 1a shows the XRD patterns of La3Ni2O7 calcined at different temperatures. For the sample calcined at 1075 °C, the main orthorhombic structure of La3Ni2O7 (JCPDS Card No. 50‐0244) was observed together with a hexagonal La2O3 structure suggesting a mixed material. However for the samples calcined at 1100 °C and 1150 °C, the pattern can be clearly indexed as the orthorhombic RP phase structure without any impurity. By using the Scherrer's equation, the mean crystalline size is estimated to be 33, 46, and 56 nm for the samples calcined at 1075, 1100 and 1150 °C, respectively, implying the gradual growth of the particle size along with increasing temperature. The La3Ni2O7 sample was indexed as the Fmmm space group. It is known that alkylene oxide segments of F127 could form crown-ether-type complexes with Ni2 + and La3 + through coordination bonds [12,14], bringing the reaction partners sufficiently close together. Therefore, a pure-phase La3Ni2O7 oxide could be obtained after calcination at relative short time and a low temperature. In order to investigate the compatibility of the prepared sample with an YSZ electrolyte, the La3Ni2O7–YSZ composite was fabricated by sintering at 1000 °C for 4 h. Fig. 1b shows the XRD pattern of La3Ni2O7–YSZ composite. The corresponding peaks of La3Ni2O7 and YSZ are clearly displayed and no impure phases are further observed, indicating that the prepared La3Ni2O7 oxide has good chemical compatibility with the YSZ electrolyte. Fig. 1c shows the electrical conductivity behaviors of the La3Ni2O7 sample over the temperature range from 350 °C to 850 °C in air. The value at 750 °C was 60 S cm− 1, which is in accordance with the result reported by Takahashi et al. [5]. The SEM and elemental line profiles in Fig. 2 show the surface micrograph and the cross-sectional view of an interface of the tri-layer cell (NiO–YSZ/YSZ/La3Ni2O7), which were sintered at 1000 °C for 4 h. As shown in Fig. 2a, the surface of the cathode exhibits a porous morphology which is beneficial to gas diffusion. Fig. 2b shows the cross-section micrograph of the single cell. From this it appears that the cathode and the YSZ electrolyte are tightly adhered to each other without any sign of cracks and delamination. The TEC of La3Ni2O7 was further measured as 12.9×10− 6 K− 1, which is close to that of YSZ (10.8×10− 6 K− 1) [17] over the temperature range of 40–800 °C. It reveals that the La3Ni2O7 cathode and the conventional electrolyte YSZ show good thermal
Fig. 1. XRD patterns of (a) La3Ni2O7 powders calcined at different temperatures and standard JCPDS data and (b) La3Ni2O7–YSZ composite calcined at 1000 °C for 4 h; (c) temperature dependence of electrical conductivity for La3Ni2O7 oxide from 350 °C to 850 °C in air, and lnσT vs. 1000/T plot (inset).
compatibility with each other. Fig. 2c illustrates the elemental line profiles of the element distribution of Y, Zr, La, Ni and O in the electrolyte and cathode regions. No elemental interpenetration between the layers was observed, which is consistent with the XRD results (Fig. 1). Fig. 3a shows the EIS results, in which two arcs are presented corresponding to different electrode processes. The intercept with the real axis at high frequency represents the Ohmic resistance caused by the electrolyte, electrodes and collecting wires. The difference between the real axis intercepts of the impedance arc is considered to be the interfacial resistance of the cathode, denoted as area specific resistance (ASR). It can be seen that the ASR significantly reduces with increasing temperature. The ASRs of the cathode were 2.6, 0.97, 0.39, 0.20 and 0.13 Ωcm2 at 650, 700, 750, 800 and 850 °C, respectively (Fig. 2-inset). ASR represents the overall cathodic properties related on the charge
Z. Lou et al. / Electrochemistry Communications 22 (2012) 97–100
Fig. 2. (a) Surface and (b) cross-section SEM micrographs and (c) EDX linear scan analysis of the relative elemental distribution of the fuel cell with La3Ni2O7 cathode.
transfer, oxygen adsorption/dissociation, surface/bulk diffusion and gasphase diffusion processes [15]. Remarkably, the obtained ASR of the La3Ni2O7 cathode on YSZ electrolyte developed here is lower than that of La2NiO4 on a SDC electrolyte (0.42 Ωcm2) at 800 °C reported elsewhere [16]. This can be ascribed to the high ionic transport property of the higher ordered RP phases of La3Ni2O7 oxide [4]. The well-defined ASR implies that La3Ni2O7 obtained by this method could be considered as a potential cathode material for IT-SOFCs. The performance of the NiO–YSZ anode-supported single cell combined with La3Ni2O7 as a cathode was evaluated. Fig. 3b presents the I–V curves and the corresponding power densities of the single cell with La3Ni2O7 cathode over the temperature range of 650–800 °C with
99
Fig. 3. (a) Nyquist plots for EIS of La3Ni2O7 cathode on YSZ electrolyte measured from 650 °C to 800 °C in air, and Arrhenius plot of ASRs for La3Ni2O7 cathode (inset); (b) performance of a single cell with La3Ni2O7 cathode from 650 °C to 800 °C; (c) the shortterm stability test for anode-supported single cell with La3Ni2O7 cathode at 750 °C.
humidified hydrogen (∼3 vol.% H2O) as the fuel and static air as the oxidant. As shown in Fig. 3b, the peak power densities of the fuel cells with the La3Ni2O7 cathode were 473, 652, 848 and 1138 mW cm− 2 at 650, 700, 750 and 800 °C, respectively. The cell showed stable performance with no obvious degradation in 30 h under a constant current density of 0.6 Acm− 2 at 750 °C, as shown in Fig. 3c. These results indicate that La3Ni2O7 prepared by this method is an attractive cathode material for IT-SOFCs. 4. Conclusions In this work we have demonstrated that a sol–gel method using F127 as the chelating agent is a facile and effective strategy of
100
Z. Lou et al. / Electrochemistry Communications 22 (2012) 97–100
synthesizing La3Ni2O7 for IT-SOFC applications. Pure orthorhombic RP phased La3Ni2O7 was prepared after calcination at relative short time and a minimum temperature and used as a cathode when combined with an YSZ electrolyte and a NiO–YSZ anode to build a IT-SOFC. Herein the prepared La3Ni2O7 oxide based IT-SOFC shows low ASR of 0.39 Ω cm2 and remarkable peak power density of 848 mW cm− 2 at 750 °C. The cell shows stable performance under a constant current density of 0.6 A cm− 2 for 30 h at 750 °C. Overall these preliminary results suggest that La3Ni2O7 derived from this preparation is a promising cathode material for IT-SOFCs. Acknowledgment This work was supported by the “National Natural Science Foundation of China” under contract No. 21076023 and No. 21006006. References [1] J.R. Wilson, A.T. Duong, M. Gameiro, H.Y. Chen, K. Thornton, D.R. Mumm, S.A. Barnett, Electrochemistry Communications 11 (2009) 1052. [2] S.W. Tao, J.T.S. Irvine, Nature Materials 2 (2003) 320.
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