- Email: [email protected]
Improved performance of symmetrical solid oxide fuel cells with redox-reversible cermet electrodes
Author’s Accepted Manuscript Improved performance of symmetrical solid oxide fuel cells with redox-reversible cermet electrodes Yonghong Chen, Zhuanxia Cheng, Yang Yang, Weili Yu, Dong Tian, Xiaoyong Lu, Yanzhi Ding, Bin Lin www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)31828-6 http://dx.doi.org/10.1016/j.matlet.2016.11.074 MLBLUE21768
To appear in: Materials Letters Received date: 5 October 2016 Revised date: 17 November 2016 Accepted date: 21 November 2016 Cite this article as: Yonghong Chen, Zhuanxia Cheng, Yang Yang, Weili Yu, Dong Tian, Xiaoyong Lu, Yanzhi Ding and Bin Lin, Improved performance of symmetrical solid oxide fuel cells with redox-reversible cermet electrodes, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.11.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improved performance of symmetrical solid oxide fuel cells with redox-reversible cermet electrodes Yonghong Chena, Zhuanxia Chenga, Yang Yanga, Weili Yub, Dong Tiana, Xiaoyong Lua, Yanzhi Dinga, Bin Lina,b,* a Anhui Key Laboratory of Low temperature Co-fired Material, Huainan Normal University, Huainan 232001, PR China b State Key Lab of Multiphase Flow in Power Engineering, School of Energy & Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China *
Corresponding author. [email protected]
Abstract Keeping the advantages of symmetrical solid oxide fuel cells (SSOFCs) with redox-stable electrodes, novel SSOFCs with redox-reversible cermet electrodes are proposed to solve the major problems of low electronic conductivity ( < 10 S.cm-1) under anode atmosphere and poor electrochemical activity for fuel oxidation. The conventional Ni0.7Co0.3O-Ce0.8Sm0.2O1.9 (NC-SDC) cermet anode is applied simultaneously as a potential redox-reversible cathode for SSOFCs with practical yttria-stabilized zirconia (YSZ) electrolytes. The electrochemical performance of SSOFCs with NC-SDC cermet electrode can be improved via SDC buffer layers and the corresponding maximum power density increases from 140.1 mW.cm-2 to 200.2 mW.cm-2 at 800ºC. The new redox-reversible cermet electrodes of SSOFCs open doors for development of next-generation SSOFC electrode materials. Keywords: Ceramics; Electrical properties; Energy storage and conversion; Symmetrical solid oxide fuel cells; Cermet electrode; Electrochemical properties. 1.
Introduction Symmetrical solid oxide fuel cells (SSOFCs) using a redox-stable catalyst as both anode and
cathode simultaneously have attracted significant interests, which can simplify the cells as well as enhancing coking and sulfur tolerance[1]. The SSOFC electrodes should meet the rather restrictive requirements in both reducing and oxidizing environment, including sufficient conductivity and thermo-mechanical stability as well as competent dual electrocatalytic activity for both hydrogen oxidation
reaction
(HOR)
and
oxygen
reduction
reaction
(ORR)[2].
Since
the
first
La0.75Sr0.25Cr0.5Mn0.5O3-δ was proposed by Irvine[1], several types of materials have been used as SSOFC electrodes, such as LaCrO3-based oxides[3], LaMnO3-based oxides[4], SrFeO3-based oxides[5], SrTiO3-based oxides[4] and K2NiF4-type oxides[6]. However, the relatively low electronic conductivity (<10 S.cm-1) under anode atmosphere and poor electrochemical activity for fuel oxidation at low ______________________________________
temperature have become the major challenges to SSOFCs[4]. The state-of-the-art SOFC anode materials are Ni-yttria stabilized zirconia (Ni-YSZ) cermets, which exhibit good thermomechanical compatibility with practical YSZ electrolyte, excellent electrochemical activity for fuel oxidation at low temperature and very high electronic conductivity (>1000 S.cm-1), but do display serious shortcomings of carbon deposition and sulphur poisoning[7, 8]. In order to solve these issues, the fabrication of alloy anodes (e.g. Ni0.7Co0.3 alloy) and the substitution of ceramic phases (e.g. doped ceria) into the Ni-YSZ anode are successfully developed to avoid or suppress coke formation[9]. Co can interact with the 3d electrons of Ni to form NiCo alloy, consequently reducing nickel carbide formation kinetically[9]. It is reported that the NiCo alloy anodes exhibit higher electrochemical activity for fuel oxidation than pure Ni anode[10]. Additionally, doped ceria as good promoters can participate in the coke removal reaction and provide thermal stability[9]. Keeping the advantages of SSOFCs with redox-stable electrodes, for the first time, the conventional Ni0.7Co0.3O-Ce0.8Sm0.2O1.9 (NC-SDC) cermet anode was applied simultaneously as a potential redox-reversible cathode for SSOFCs with practical YSZ electrolytes. In addition, SDC buffer layers were employed between electrodes and electrolyte to further improve the output performance. 2.
Experimental The Ni0.7Co0.3O powders were synthesized by a modified Pechini method with Co(NO3)2·6H2O
and Ni(NO3)2·6H2O as raw materials, and followed by calcination at 850℃ for 3 h. Similarly, the pure fluorite phase Ce0.8Sm0.2O1.9 powders were achieved under 700℃ calcination. The crystal structure of NC-SDC (70:30 wt.%) was identified by X-ray diffractometer (XRD). The redox reversibility of NC-SDC cermet electrode was investigated by XRD during three redox cycles of reduction (humidified H2) and re-oxidation (air) at 850℃ for 3 h. The NC-SDC powders were pressed into bars (200 MPa) and sintered at 1300℃ for 5 h. The electrical conductivity was conducted at 450-800℃ in air by DC four-probe method. Thermal expansion behavior was measured with NETZSCH DIL 402PC dilatometer at 100-1000℃. The cermet cathode polarization resistance was performed in air by two-electrode AC impedance method. NC or NC-SDC slurry were printed onto the both side of SDC and fired at 1000℃ for 3 h. NC-SDC slurry was applied onto both surfaces of YSZ and calcined at 1100ºC for 3 h to prepare NC-SDC|YSZ|NC-SDC. For NC-SDC|SDC|YSZ|SDC|NC-SDC, SDC slurry was printed onto both surfaces of YSZ and sintered at 1300ºC for 3 h, followed by applying NC-SDC slurry. The current 2
collector is silver paste. The electrode area was 0.2 cm2 and the operation temperature was 650-800ºC. IM6 Electrochemical Workstation (ZAHNER, Germany) was used to get electrochemical impedance spectra (frequency range: 100 kHz-0.01 Hz, AC amplitude: 5 mV). The ZsimpWin software was used to obtain the equivalent circuit model. The single cells were tested by the DC Electronic Load (IT8511). Scanning electron microscope (SEM, EM-3200) was used to characterize the microstructure of symmetrical cells. 3.
Results and discussion Fig. 1(a) shows the XRD patterns of NC-SDC cermet after three redox cycles. After calcination at
1300℃ for 5 h, only peaks corresponding to NC with cubic phase and SDC with fluorite phase can be detected, thereby NC is chemical stable with SDC electrolyte. After reduction, only peaks corresponding to face-centered cubic (fcc) Ni0.7Co0.3 alloy and fluorite SDC can be detected for all the samples, which indicates the repeated transformation from Ni0.7Co0.3O-SDC to Ni0.7Co0.3-SDC. Fig. 1(b) shows the XRD patterns of typical NiCo alloy structures after reduction. The XRD patterns of NC-SDC are very similar to Co (JCPDS-15-0806) or Ni (JCPDS 04-0850) with an fcc structure. All the peak positions lie between the position of pure Co and pure Ni, which affirms the formation of fcc-NiCo alloy rather than individual metals of Co and Ni. Thus, NC-SDC cermet is confirmed to be completely redox-reversible[10]. As expected (Fig. 1(c)), the SDC (~12×10-6 K-1) additions dramatically reduce the average thermal expansion coefficient (TEC) values of NC (~15.5×10-6 K-1) cathode[11]. Obviously, it is in favor of the SOFC system in maintaining long-term stability and enduring thermal cycles[12]. As expected (Fig. 1(d)), the addition of SDC decreases the conductivity of the composite cathode, compared to pure NC cathode[13]. The activation energy (Ea) curves of NC-SDC are almost linear, which means that NC-SDC are all in favor of the small polaron conduction mechanism[14]. Generally, the additive of oxygen-ion conducting phase SDC produce more obstruction of the electrode matrix, which blocks the flow of electronic current and thus decreases conductivity[15]. The polarization resistance (Rp) is an effective indicator to evaluate the catalytic ability of cathode for ORR. As shown in Fig. 2(a, b), the NC-SDC shows similar impedance spectra and the same electrochemical process with pure NC. As shown in Fig. 2(c), the Rp values dramatically decreased with substitution of SDC with an optimum content of 30 wt.%. The RP values of pure NC are 0.097, 0.179, 0.328 and 0.822 Ω.cm2 at 800, 750, 700 and 650℃, respectively. The RP values of NC-SDC are 3
0.071, 0.106, 0.151 and 0.292 Ω.cm2 at 800, 750, 700 and 650℃, respectively, preceding over pure NC. The lower cathodic polarization resistance is mainly due to the extended triple-phase boundary length[16]. After operating for 24 h, the Rp of the symmetrical cell remains almost constant, indicating that the NC-SDC cermet cathode is stable in practical long-time application. Fig. 2(d) shows the fine structure layer of NC-SDC/SDC, the visual appearance of the electrolyte/cathode interface indicates that the porous NC-SDC well attaches to the electrolyte and no third reactive layer appears. Therefore, the low Rp can be explain legitimately. Two 400 μm-thick YSZ electrolyte-supported cells of NC-SDC|YSZ|NC-SDC (Cell-a) and NC-SDC|SDC|YSZ|SDC|NC-SDC (Cell-b) were prepared to further evaluate the output performance measured with air as oxidant and humidified hydrogen as fuel. The I-V-P curves of SSOFCs are shown in Fig. 3(a, b). The open circuit voltages of 1.00 and 1.01 V were obtained at 800ºC for Cell-a and Cell-b, respectively. The maximum power densities of Cell-a were 140.1, 90.2, 51.5 and 24.7 mW.cm-2 at 800, 750, 700 and 650ºC, respectively. The maximum power densities of Cell-b were 200.2, 126.7, 75.7 and 40.5 mW.cm-2 at 800, 750, 700 and 650ºC, respectively. Obviously, the electrochemical performance is improved via SDC buffer layers by more than 65% at 650ºC, owing to the interface optimization between NC-SDC electrode and YSZ electrolyte. The impedance spectra of Cell-a and Cell-b are also measured under operating conditions (Fig. 3(a, b)). At 800ºC, the Rp of 0.741 and 0.492 Ω.cm2 were obtained for Cell-a and Cell-b, respectively. The performance of Cell-b is much higher than that of Cell-a, owing to the lower interfacial polarization resistance. As thin SDC (2-3 μm) buffer layer can improve the charge-transfer processes and ionic current collection between the electrode and electrolyte[17]. Fig. 3(c) shows the Rp of the cells as a function of temperature, the Cell-b displays much lower Rp at all temperatures. Fig. 3(d) shows the typical cross section of the cells after testing. Porous NC-SDC cermet electrode perfectly adhered on the dense SDC/YSZ electrolyte. Overall, SSOFCs with SDC buffer layers are promising to raise industrialization, which enhance compatibility and surface oxygen exchange rate. 4.
Conclusion Novel SSOFCs with redox-reversible cermet electrodes were proposed to solve the major
problems of low electronic conductivity and poor electrochemical activity. The conventional NC-SDC cermet anode was applied simultaneously as a potential redox-reversible cathode for SSOFCs with practical YSZ electrolytes. The NC-SDC cermet cathode with oxidation stability is reversibly 4
transformed to a reducing-stable NC-SDC alloy anode, which exhibit desirable chemical and thermal compatibility with electrolyte. The RP of NC-SDC cermet cathode is 0.071 Ω.cm2 at 800℃, preceding over pure NC (0.097 Ω.cm2). The electrochemical performance was improved via SDC buffer layers. The maximum power density increases from 140.1 to 200.2 mW.cm-2 at 800ºC and the RP decreases from 0.741 to 0.492 Ω.cm2 at 800ºC, respectively. The new SSOFCs with redox-reversible cermet electrodes open doors for development of next-generation SSOFC electrode materials. Acknowledgments We thank the support from the National Natural Science Foundation of China (No. 51102107, No. 51202080 and No. 21404015) and Anhui Science and Technology Project (1206c0805038).
References: [1] D.M. Bastidas, S.W. Tao, J.T.S. Irvine, J Mater Chem, 16 (2006) 1603-1605. [2] J.C. Ruiz-Morales, D. Marrero-Lopez, J. Canales-Vazquez, J.T.S. Irvine, Rsc Adv, 1 (2011) 1403-1414. [3] B. Lin, S. Wang, X. Liu, G. Meng, J Alloy Compd, 490 (2010) 214-222. [4] C. Su, W. Wang, M.L. Liu, M.O. Tade, Z.P. Shao, Adv Energy Mater, 5 (2015). [5] Q. Liu, X. Dong, G. Xiao, F. Zhao, F. Chen, Advanced Materials, 22 (2010) 5478-5482. [6] G. Yang, C. Su, R. Ran, M.O. Tade, Z. Shao, Energy and Fuels, 28 (2014) 356-362. [7] S. Sengodan, S. Choi, A. Jun, T.H. Shin, Y.W. Ju, H.Y. Jeong, J. Shin, J.T.S. Irvine, G. Kim, Nature Materials, 14 (2015) 205-209. [8] S. Tao, J.T.S. Irvine, Nature Materials, 2 (2003) 320-323. [9] W. Wang, C. Su, Y.Z. Wu, R. Ran, Z.P. Shao, Chemical Reviews, 113 (2013) 8104-8151. [10] J.S. O'Brien, J.B. Giorgi, J Power Sources, 200 (2012) 14-20. [11] M. Mogensen, N.M. Sammes, G.A. Tompsett, Solid State Ionics, 129 (2000) 63-94. [12] L. Xiong, S.R. Wang, Y.S. Wang, T.L. Wen, J Alloy Compd, 453 (2008) 356-360. [13] Z. Shao, S.M. Halle, Nature, 431 (2004) 170-173. [14] D.P. Karim, A.T. Aldred, Phys Rev B, 20 (1979) 2255-2263. [15] J. Kim, W.Y. Seo, J. Shin, M.L. Liu, G. Kim, J Mater Chem A, 1 (2013) 515-519. [16] S.B. Adler, Chemical Reviews, 104 (2004) 4791-4843. [17] G. Constantin, C. Rossignol, P. Briois, A. Billard, L. Dessemond, E. Djurado, Solid State Ionics, 249 (2013) 98-104.
5
Fig. 1 (a, b) XRD patterns of NC-SDC cermet after three redox cycles; (c) Thermal expansion curves of NC and NC-SDC; (d) Arrhenius plots of conductivity for NC and NC-SDC.
Fig. 2 The impedance spectra of NC (a) and NC-SDC (b) cermet cathodes in air; (c) The corresponding Arrhenius plots of Rp; (d) SEM cross-sectional image of the tested cell.
6
Fig. 3 The V-I-P curves and corresponding electrochemical impedance spectra of the single cells: (a) NC-SDC|YSZ|NC-SDC and (b) NC-SDC|SDC|YSZ|SDC|NC-SDC; (c) The Arrhenius plots of Rp with temperature for the single cells; (d) SEM cross-sectional images for NC-SDC electrode after testing. Graphical abstract:
7
Highlights
Novel SSOFCs with redox-reversible cermet electrodes are proposed. Ni0.7Co0.3O-Ce0.8Sm0.2O1.9 (NC-SDC) is used as new SSOFC electrode materials. NC-SDC cermet electrode is confirmed to be completely redox-reversible.
SDC buffer layer dramatically enhances the electrochemical performance.
Redox-reversible cermet electrodes open doors for SSOFC development.
8
PII: DOI: Reference:
S0167-577X(16)31828-6 http://dx.doi.org/10.1016/j.matlet.2016.11.074 MLBLUE21768
To appear in: Materials Letters Received date: 5 October 2016 Revised date: 17 November 2016 Accepted date: 21 November 2016 Cite this article as: Yonghong Chen, Zhuanxia Cheng, Yang Yang, Weili Yu, Dong Tian, Xiaoyong Lu, Yanzhi Ding and Bin Lin, Improved performance of symmetrical solid oxide fuel cells with redox-reversible cermet electrodes, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.11.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improved performance of symmetrical solid oxide fuel cells with redox-reversible cermet electrodes Yonghong Chena, Zhuanxia Chenga, Yang Yanga, Weili Yub, Dong Tiana, Xiaoyong Lua, Yanzhi Dinga, Bin Lina,b,* a Anhui Key Laboratory of Low temperature Co-fired Material, Huainan Normal University, Huainan 232001, PR China b State Key Lab of Multiphase Flow in Power Engineering, School of Energy & Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China *
Corresponding author. [email protected]
Abstract Keeping the advantages of symmetrical solid oxide fuel cells (SSOFCs) with redox-stable electrodes, novel SSOFCs with redox-reversible cermet electrodes are proposed to solve the major problems of low electronic conductivity ( < 10 S.cm-1) under anode atmosphere and poor electrochemical activity for fuel oxidation. The conventional Ni0.7Co0.3O-Ce0.8Sm0.2O1.9 (NC-SDC) cermet anode is applied simultaneously as a potential redox-reversible cathode for SSOFCs with practical yttria-stabilized zirconia (YSZ) electrolytes. The electrochemical performance of SSOFCs with NC-SDC cermet electrode can be improved via SDC buffer layers and the corresponding maximum power density increases from 140.1 mW.cm-2 to 200.2 mW.cm-2 at 800ºC. The new redox-reversible cermet electrodes of SSOFCs open doors for development of next-generation SSOFC electrode materials. Keywords: Ceramics; Electrical properties; Energy storage and conversion; Symmetrical solid oxide fuel cells; Cermet electrode; Electrochemical properties. 1.
Introduction Symmetrical solid oxide fuel cells (SSOFCs) using a redox-stable catalyst as both anode and
cathode simultaneously have attracted significant interests, which can simplify the cells as well as enhancing coking and sulfur tolerance[1]. The SSOFC electrodes should meet the rather restrictive requirements in both reducing and oxidizing environment, including sufficient conductivity and thermo-mechanical stability as well as competent dual electrocatalytic activity for both hydrogen oxidation
reaction
(HOR)
and
oxygen
reduction
reaction
(ORR)[2].
Since
the
first
La0.75Sr0.25Cr0.5Mn0.5O3-δ was proposed by Irvine[1], several types of materials have been used as SSOFC electrodes, such as LaCrO3-based oxides[3], LaMnO3-based oxides[4], SrFeO3-based oxides[5], SrTiO3-based oxides[4] and K2NiF4-type oxides[6]. However, the relatively low electronic conductivity (<10 S.cm-1) under anode atmosphere and poor electrochemical activity for fuel oxidation at low ______________________________________
temperature have become the major challenges to SSOFCs[4]. The state-of-the-art SOFC anode materials are Ni-yttria stabilized zirconia (Ni-YSZ) cermets, which exhibit good thermomechanical compatibility with practical YSZ electrolyte, excellent electrochemical activity for fuel oxidation at low temperature and very high electronic conductivity (>1000 S.cm-1), but do display serious shortcomings of carbon deposition and sulphur poisoning[7, 8]. In order to solve these issues, the fabrication of alloy anodes (e.g. Ni0.7Co0.3 alloy) and the substitution of ceramic phases (e.g. doped ceria) into the Ni-YSZ anode are successfully developed to avoid or suppress coke formation[9]. Co can interact with the 3d electrons of Ni to form NiCo alloy, consequently reducing nickel carbide formation kinetically[9]. It is reported that the NiCo alloy anodes exhibit higher electrochemical activity for fuel oxidation than pure Ni anode[10]. Additionally, doped ceria as good promoters can participate in the coke removal reaction and provide thermal stability[9]. Keeping the advantages of SSOFCs with redox-stable electrodes, for the first time, the conventional Ni0.7Co0.3O-Ce0.8Sm0.2O1.9 (NC-SDC) cermet anode was applied simultaneously as a potential redox-reversible cathode for SSOFCs with practical YSZ electrolytes. In addition, SDC buffer layers were employed between electrodes and electrolyte to further improve the output performance. 2.
Experimental The Ni0.7Co0.3O powders were synthesized by a modified Pechini method with Co(NO3)2·6H2O
and Ni(NO3)2·6H2O as raw materials, and followed by calcination at 850℃ for 3 h. Similarly, the pure fluorite phase Ce0.8Sm0.2O1.9 powders were achieved under 700℃ calcination. The crystal structure of NC-SDC (70:30 wt.%) was identified by X-ray diffractometer (XRD). The redox reversibility of NC-SDC cermet electrode was investigated by XRD during three redox cycles of reduction (humidified H2) and re-oxidation (air) at 850℃ for 3 h. The NC-SDC powders were pressed into bars (200 MPa) and sintered at 1300℃ for 5 h. The electrical conductivity was conducted at 450-800℃ in air by DC four-probe method. Thermal expansion behavior was measured with NETZSCH DIL 402PC dilatometer at 100-1000℃. The cermet cathode polarization resistance was performed in air by two-electrode AC impedance method. NC or NC-SDC slurry were printed onto the both side of SDC and fired at 1000℃ for 3 h. NC-SDC slurry was applied onto both surfaces of YSZ and calcined at 1100ºC for 3 h to prepare NC-SDC|YSZ|NC-SDC. For NC-SDC|SDC|YSZ|SDC|NC-SDC, SDC slurry was printed onto both surfaces of YSZ and sintered at 1300ºC for 3 h, followed by applying NC-SDC slurry. The current 2
collector is silver paste. The electrode area was 0.2 cm2 and the operation temperature was 650-800ºC. IM6 Electrochemical Workstation (ZAHNER, Germany) was used to get electrochemical impedance spectra (frequency range: 100 kHz-0.01 Hz, AC amplitude: 5 mV). The ZsimpWin software was used to obtain the equivalent circuit model. The single cells were tested by the DC Electronic Load (IT8511). Scanning electron microscope (SEM, EM-3200) was used to characterize the microstructure of symmetrical cells. 3.
Results and discussion Fig. 1(a) shows the XRD patterns of NC-SDC cermet after three redox cycles. After calcination at
1300℃ for 5 h, only peaks corresponding to NC with cubic phase and SDC with fluorite phase can be detected, thereby NC is chemical stable with SDC electrolyte. After reduction, only peaks corresponding to face-centered cubic (fcc) Ni0.7Co0.3 alloy and fluorite SDC can be detected for all the samples, which indicates the repeated transformation from Ni0.7Co0.3O-SDC to Ni0.7Co0.3-SDC. Fig. 1(b) shows the XRD patterns of typical NiCo alloy structures after reduction. The XRD patterns of NC-SDC are very similar to Co (JCPDS-15-0806) or Ni (JCPDS 04-0850) with an fcc structure. All the peak positions lie between the position of pure Co and pure Ni, which affirms the formation of fcc-NiCo alloy rather than individual metals of Co and Ni. Thus, NC-SDC cermet is confirmed to be completely redox-reversible[10]. As expected (Fig. 1(c)), the SDC (~12×10-6 K-1) additions dramatically reduce the average thermal expansion coefficient (TEC) values of NC (~15.5×10-6 K-1) cathode[11]. Obviously, it is in favor of the SOFC system in maintaining long-term stability and enduring thermal cycles[12]. As expected (Fig. 1(d)), the addition of SDC decreases the conductivity of the composite cathode, compared to pure NC cathode[13]. The activation energy (Ea) curves of NC-SDC are almost linear, which means that NC-SDC are all in favor of the small polaron conduction mechanism[14]. Generally, the additive of oxygen-ion conducting phase SDC produce more obstruction of the electrode matrix, which blocks the flow of electronic current and thus decreases conductivity[15]. The polarization resistance (Rp) is an effective indicator to evaluate the catalytic ability of cathode for ORR. As shown in Fig. 2(a, b), the NC-SDC shows similar impedance spectra and the same electrochemical process with pure NC. As shown in Fig. 2(c), the Rp values dramatically decreased with substitution of SDC with an optimum content of 30 wt.%. The RP values of pure NC are 0.097, 0.179, 0.328 and 0.822 Ω.cm2 at 800, 750, 700 and 650℃, respectively. The RP values of NC-SDC are 3
0.071, 0.106, 0.151 and 0.292 Ω.cm2 at 800, 750, 700 and 650℃, respectively, preceding over pure NC. The lower cathodic polarization resistance is mainly due to the extended triple-phase boundary length[16]. After operating for 24 h, the Rp of the symmetrical cell remains almost constant, indicating that the NC-SDC cermet cathode is stable in practical long-time application. Fig. 2(d) shows the fine structure layer of NC-SDC/SDC, the visual appearance of the electrolyte/cathode interface indicates that the porous NC-SDC well attaches to the electrolyte and no third reactive layer appears. Therefore, the low Rp can be explain legitimately. Two 400 μm-thick YSZ electrolyte-supported cells of NC-SDC|YSZ|NC-SDC (Cell-a) and NC-SDC|SDC|YSZ|SDC|NC-SDC (Cell-b) were prepared to further evaluate the output performance measured with air as oxidant and humidified hydrogen as fuel. The I-V-P curves of SSOFCs are shown in Fig. 3(a, b). The open circuit voltages of 1.00 and 1.01 V were obtained at 800ºC for Cell-a and Cell-b, respectively. The maximum power densities of Cell-a were 140.1, 90.2, 51.5 and 24.7 mW.cm-2 at 800, 750, 700 and 650ºC, respectively. The maximum power densities of Cell-b were 200.2, 126.7, 75.7 and 40.5 mW.cm-2 at 800, 750, 700 and 650ºC, respectively. Obviously, the electrochemical performance is improved via SDC buffer layers by more than 65% at 650ºC, owing to the interface optimization between NC-SDC electrode and YSZ electrolyte. The impedance spectra of Cell-a and Cell-b are also measured under operating conditions (Fig. 3(a, b)). At 800ºC, the Rp of 0.741 and 0.492 Ω.cm2 were obtained for Cell-a and Cell-b, respectively. The performance of Cell-b is much higher than that of Cell-a, owing to the lower interfacial polarization resistance. As thin SDC (2-3 μm) buffer layer can improve the charge-transfer processes and ionic current collection between the electrode and electrolyte[17]. Fig. 3(c) shows the Rp of the cells as a function of temperature, the Cell-b displays much lower Rp at all temperatures. Fig. 3(d) shows the typical cross section of the cells after testing. Porous NC-SDC cermet electrode perfectly adhered on the dense SDC/YSZ electrolyte. Overall, SSOFCs with SDC buffer layers are promising to raise industrialization, which enhance compatibility and surface oxygen exchange rate. 4.
Conclusion Novel SSOFCs with redox-reversible cermet electrodes were proposed to solve the major
problems of low electronic conductivity and poor electrochemical activity. The conventional NC-SDC cermet anode was applied simultaneously as a potential redox-reversible cathode for SSOFCs with practical YSZ electrolytes. The NC-SDC cermet cathode with oxidation stability is reversibly 4
transformed to a reducing-stable NC-SDC alloy anode, which exhibit desirable chemical and thermal compatibility with electrolyte. The RP of NC-SDC cermet cathode is 0.071 Ω.cm2 at 800℃, preceding over pure NC (0.097 Ω.cm2). The electrochemical performance was improved via SDC buffer layers. The maximum power density increases from 140.1 to 200.2 mW.cm-2 at 800ºC and the RP decreases from 0.741 to 0.492 Ω.cm2 at 800ºC, respectively. The new SSOFCs with redox-reversible cermet electrodes open doors for development of next-generation SSOFC electrode materials. Acknowledgments We thank the support from the National Natural Science Foundation of China (No. 51102107, No. 51202080 and No. 21404015) and Anhui Science and Technology Project (1206c0805038).
References: [1] D.M. Bastidas, S.W. Tao, J.T.S. Irvine, J Mater Chem, 16 (2006) 1603-1605. [2] J.C. Ruiz-Morales, D. Marrero-Lopez, J. Canales-Vazquez, J.T.S. Irvine, Rsc Adv, 1 (2011) 1403-1414. [3] B. Lin, S. Wang, X. Liu, G. Meng, J Alloy Compd, 490 (2010) 214-222. [4] C. Su, W. Wang, M.L. Liu, M.O. Tade, Z.P. Shao, Adv Energy Mater, 5 (2015). [5] Q. Liu, X. Dong, G. Xiao, F. Zhao, F. Chen, Advanced Materials, 22 (2010) 5478-5482. [6] G. Yang, C. Su, R. Ran, M.O. Tade, Z. Shao, Energy and Fuels, 28 (2014) 356-362. [7] S. Sengodan, S. Choi, A. Jun, T.H. Shin, Y.W. Ju, H.Y. Jeong, J. Shin, J.T.S. Irvine, G. Kim, Nature Materials, 14 (2015) 205-209. [8] S. Tao, J.T.S. Irvine, Nature Materials, 2 (2003) 320-323. [9] W. Wang, C. Su, Y.Z. Wu, R. Ran, Z.P. Shao, Chemical Reviews, 113 (2013) 8104-8151. [10] J.S. O'Brien, J.B. Giorgi, J Power Sources, 200 (2012) 14-20. [11] M. Mogensen, N.M. Sammes, G.A. Tompsett, Solid State Ionics, 129 (2000) 63-94. [12] L. Xiong, S.R. Wang, Y.S. Wang, T.L. Wen, J Alloy Compd, 453 (2008) 356-360. [13] Z. Shao, S.M. Halle, Nature, 431 (2004) 170-173. [14] D.P. Karim, A.T. Aldred, Phys Rev B, 20 (1979) 2255-2263. [15] J. Kim, W.Y. Seo, J. Shin, M.L. Liu, G. Kim, J Mater Chem A, 1 (2013) 515-519. [16] S.B. Adler, Chemical Reviews, 104 (2004) 4791-4843. [17] G. Constantin, C. Rossignol, P. Briois, A. Billard, L. Dessemond, E. Djurado, Solid State Ionics, 249 (2013) 98-104.
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Fig. 1 (a, b) XRD patterns of NC-SDC cermet after three redox cycles; (c) Thermal expansion curves of NC and NC-SDC; (d) Arrhenius plots of conductivity for NC and NC-SDC.
Fig. 2 The impedance spectra of NC (a) and NC-SDC (b) cermet cathodes in air; (c) The corresponding Arrhenius plots of Rp; (d) SEM cross-sectional image of the tested cell.
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Fig. 3 The V-I-P curves and corresponding electrochemical impedance spectra of the single cells: (a) NC-SDC|YSZ|NC-SDC and (b) NC-SDC|SDC|YSZ|SDC|NC-SDC; (c) The Arrhenius plots of Rp with temperature for the single cells; (d) SEM cross-sectional images for NC-SDC electrode after testing. Graphical abstract:
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Highlights
Novel SSOFCs with redox-reversible cermet electrodes are proposed. Ni0.7Co0.3O-Ce0.8Sm0.2O1.9 (NC-SDC) is used as new SSOFC electrode materials. NC-SDC cermet electrode is confirmed to be completely redox-reversible.
SDC buffer layer dramatically enhances the electrochemical performance.
Redox-reversible cermet electrodes open doors for SSOFC development.
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