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Carbon-resistant Ni-Zr0.92Y0.08O2-δ supported solid oxide fuel cells using Ni-Cu-Fe alloy cermet as on-cell reforming catalyst and mixed methane-steam as fuel
Journal of Power Sources 303 (2016) 340e346
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Carbon-resistant Ni-Zr0.92Y0.08O2-d supported solid oxide fuel cells using Ni-Cu-Fe alloy cermet as on-cell reforming catalyst and mixed methane-steam as fuel Bin Hua a, b, 1, Meng Li a, 1, Jing-li Luo b, Jian Pu a, Bo Chi a, Jian Li a, c, * a Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China b Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada c Research Institute of Huazhong University of Science and Technology in Shenzhen, Shenzhen 518057, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The NCF-BZCYYb possesses higher activity for CH4 steam reforming compared to Ni-YSZ. The use of NCF-BZCYYb catalyst in the Ni-YSZ supported SOFC is studied. NCF-BZCYYb coated single cell exhibits higher electrochemical performance in CH4. NCF-BZCYYb coated single cell exhibits better durability and carbon resistance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 August 2015 Received in revised form 3 November 2015 Accepted 6 November 2015 Available online xxx
Two types of anode-supported cell are fabricated by tape casting, screen printing and sintering processes. The first one is a conventional anode supported cell (ASC); and the other, namely CASC, contains an extra layer of Ni-Cu/Ni-Fe alloys-BaZr0.1Ce0.7Y0.1Yb0.1O3-d (NCF-BZCYYb) cermet catalyst on the surface of the anode-support. Using CH4-3 mol. % H2O as the fuel, the initial performance of the CASC is moderately improved, compared with that of the ASC; the power density of the CASC and ASC at 500 mA cm2 and 800 C remain stable on the level of 470 mW cm2 for approximately 11 and 0.8 h, respectively, before cell disintegration caused by carbon formation. The performances of the CASC in the fuel of CH433.3 mol. % H2O are significantly increased above the level of the ASC, demonstrating an initial peak power density ranging from 280 to 1638 mW cm2 at temperatures between 600 and 800 C and a stable power density of 485 mW cm2 at 500 mA cm2 and 800 C for 48 h. Carbon deposition in the anode region of the tested CASC cell is not detected, as the NCF-BZCYYb is a more active catalyst than the NiZr0.92Y0.08O2-d (YSZ) anode-support for CH4 steam reforming. © 2015 Elsevier B.V. All rights reserved.
Keywords: Anode-supported cell Methane steam reforming On-cell catalyst Cell performance Carbon deposition
* Corresponding author. Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China. E-mail address: [email protected] (J. Li). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2015.11.029 0378-7753/© 2015 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are recognized as one of the most promising energy conversion technologies considering their ability to efficiently generate electrical energy from chemical energy. A
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major advantage of SOFCs is the fuel flexibility, which means hydrocarbons can be the potential fuel. Directly using hydrocarbons attracts much attention because of the advantages of less expensive and more easily accessible and safer for storage compared to hydrogen. For intermediate temperature SOFCs (IT-SOFCs), Nibased cermets are the state-of-the-art anode materials and also often used as the cell support on account of their high catalytic activity for fuel oxidation, thermal and electrical conductivities and mechanical properties as well [1e6]. When a hydrocarbon fuel, such as methane (CH4), is directly used for IT-SOFCs without prereforming by adding oxygen, steam or carbon dioxide [7e10], it is electrochemically oxidized on the electrolyteeelectrodeefuel gas triple phase boundaries (TPB), producing steam and carbon dioxide with a strong tendency of carbon deposition in the anode [11e14] as Ni is an excellent catalyst for breaking CeH bonds. To alleviate carbon deposition in Ni-based cermet anodes, functionally graded anodes were proposed, modifying the anode or cell support to promote fuel-reforming process and suppress the cracking reaction that is considered responsible for carbon deposition. It has been evidenced that cell performance can be improved greatly by using such functionally graded anode structures [15e19]. In fact the reforming process can be carried out in an extra layer of on-cell reforming catalyst imposed on the conventional Ni-based anode to avoid direct exposure of Ni to methane [12e14,20e22]. In this way, conventional simple cell fabrication method still can be used, without the need to modify the anode or cell support. In our previous work [14], it has been shown recently that CH4 can be on-cell reformed by imposing an extra layer of Ni-Cu/Ni-Fe alloys (NCF)-BaZr0.1Ce0.7Y0.1Yb0.1O3-d (BZCYYb) cermet on the conventional Ni-YSZ anode, in which carbon deposition is completely suppressed during the testing period. The NCF alloy, in situ reduced from Ni0.5Cu0.5Fe2O4 (NCFO) spinel, functions as the catalyst for methane oxidation; and the BZCYYb adsorbs H2O and CO2 [23e25] to promote the reactions of methane oxidation and carbon removal. In the present study, this oncell reforming catalyst was used on a conventional Ni-Zr0.92Y0.08O2d (YSZ) anode-supported cell to demonstrate its applicability in practical SOFCs directly fueled with methane. 2. Experimental 2.1. Catalyst synthesis and cell fabrication NCFO and BZCYYb powders were synthesized by aqueous solution methods with metal nitrates (Sino-Pharm) in the stoichiometric compositions as the precursors, glycine (Sino-Pharm) as the dispersant and EDTA/citric acid (Sino-Pharm) as the chelant agent. The detailed procedures were described in Refs. [13] and [14], respectively. The catalyst powder was prepared by ball-milling the powder mixture consisting of NCFO and BZCYYb at a weight ratio of 80:20 in ethanol for 24 h. To prepare cathode material La0.6Sr0.4Co0.2Fe0.8O3d (LSCF), La(NO3)36H2O, Sr(NO3)2, Co(NO3)26H2O and Fe(NO3)39H2O (Sino-Pharm) were dissolved in distilled water with EDTA/citric acid added as the chelant agent. This solution was vaporized overnight in an oven at 180 C, followed by calcination in air at 800 C for 2 h to form single-phase perovskite LSCF. All the prepared powder materials and reduced NCFO in H2-3 mol. % H2O atmosphere at 800 C for 2 h were subjected to X-ray diffraction (XRD, X'Pert Pro, PAN Analytical B.V.) for phase identification by using Cu Ka radiation generated at 30 kV and 15 mA. The scan was conducted within a 2q range between 20 and 110 at a rate of 10 min1. The anode-supported cells were fabricated by using tape casting-screen printing-sintering processes. To prepare the anodesupport (AS), powder mixture, containing NiO (Inco) and 8 mol. % Y2O3 stabilized ZrO2 (8YSZ, Tosoh) at a weight ratio of 57:43, was
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ball-milled for 24 h in toluene/ethanol mixed solvent with fish oil as the dispersant and corn starch as the pore former. It was further milled for another 24 h after adding polyvinyl butyral (Richard E. Mistler Inc.) as the binder and polyethylene glycol (Richard E. Mistler Inc.) as the plasticizer. Such prepared slurry was cast into sheet by using a tape casting machine (LY-250-3, Beijing Oriental Tai Yang Systems). The dried green sheet (1.4 mm thick) was used as the anode support, on one surface of which the pastes of NiO-YSZ (60:40) functional anode (FA) and YSZ electrolyte were screenprinted in sequence, prior to sintering at 1390 C in air for 3 h. To prepare the buffer and cathode, Gd-doped CeO2 (GDC, NIMTE, CAS) and LSCF-GDC (70:30) pastes were then screen-printed successively on the surface of the sintered YSZ electrolyte, followed by sintering separately in air at 1300 C and 950 C for 2 h to complete the fabrication of the anode-supported cells. The NCFO-BZCYYb catalyst was paste-coated on the surface of the anode-support of the cell and sintered at 900 C in air for 2 h. The cells with and without the layer of NCFO-BZCYYb catalyst were designated as ASC (anode-supported cell) and CASC (NCFO-BZCYYb coated anodesupported cell). The final size of the cells was about f13 1 mm and the active area of the cathode was 0.5 cm2. The as-prepared and tested cells were examined by using a scanning electron microscope (SEM, Nova NanoSEM 450, FEI) equipped with an energy dispersive X-ray spectrometer (EDS). 2.2. Reforming activity evaluation The Ni-YSZ anode-support, without the coating of NCFOBZCYYb, is an active catalyst for methane reforming. To compare their activities, catalyst powders of NCFO-BZCYYb and NiO-YSZ (57:43) sintered at 900 and 1390 C for 2 and 3 h, respectively, were prepared, sieved into the particle range of 30e60 mesh. 0.2 g of each catalyst was packed into a quartz tube for activity measurements at temperatures from 600 to 800 C for up to 10 h after reduction in H2 at 800 C for 30 min. CH4, H2O and He at flow rates of 10, 10 (controlled by a high pressure constant flow pump) and 80 ml min1 were fed into the tubular reactor; and the composition of the effluent gas was analyzed by a Varian 3800 gas chromatograph equipped with Hayesep Q, Poraplot Q and 5 Å sieve molecular capillary columns and a thermal conductivity detector (TCD) for the separation and detection of H2, CO, CO2 and CH4. CH4 conversion rate and CO selectivity were calculated, respectively, according to
CH4 conversion ¼
CO selectivity ¼
½CO þ ½CO2 100% ½CO þ ½CO2 þ ½CH4
½CO 100% ½CO þ ½CO2
(1)
(2)
2.3. Cell test For the evaluation of electrochemical performances, Pt paste was painted on the both sides of the prepared cells ASC and CASC and baked in air at 800 C for 2 h; Pt mesh was compressed onto the paste as the current collector and Pt wire was spark-welded to the mesh as the measuring lead. The cell was then sealed on the anode side to an alumina tube by using a Ceramabond® glass sealant (Aremco Product, Inc.) and in-situ reduced in H2-3 mol. % H2O atmosphere at 800 C for 2 h prior to the electrochemical measurements. The open-circuit electrochemical impedance spectra (EIS) and the current densityevoltage (IeV) and current densityepower density (IeP) curves were obtained at temperatures ranging from
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600 to 800 C in H2-3 mol. % H2O and CH4-x mol. % H2O (x ¼ 3 and 33.3) by using an impedance/gain phase analyzer (Solartron 1260) and an electrochemical interface (Solartron 1287). The performance durability was also evaluated at 500 mA cm2 and 800 C. For all the tests, the flow rate for the fuel and air was 150 and 200 ml min1, respectively. 3. Results and discussions 3.1. Phase identification and microstructure examination Fig. 1 shows the cross-sectional microstructure of the asprepared and reduced cells. The LSCF-GDC cathode (~25 mm) was well sintered to the GDC buffer (~2 mm) that was well sintered to the YSZ electrolyte (Fig. 1a), which (~12 mm) was fairly dense with few isolated holes (Fig. 1b). Reduced at 800 C in H2-3 mol. % H2O for 2 h, the Ni-YSZ functional anode (~6 mm) was well adhered to the electrolyte and the Ni-YSZ anode support (Fig. 1a). In the CASC, the on-cell reforming catalyst layer (~35 mm) in both the sintered (Fig. 1c) and reduced (Fig. 1d) forms were seamlessly coated on the anode support. These intimate interfaces between different layers would ensure an uninterrupted transport of oxide ions and electrons upon cell operation. The results of X-ray diffraction, as shown in Fig. S1 in electronic supplementary information (ESI), indicates that the as-synthesized LSCF was single-phase perovskite (JCPDS files 01-082-1961 for La0.6Sr0.4FeO3-d). The crystal structure of other cell component materials, such as the as-synthesized NCFO, BZCYYb and reduced NCFO, were the same as identified in our previous work [14]. 3.2. Cell performance in H2-3 mol. % H2O Fig. 2 shows the initially measured IeV and IeP curves of the
ASC and CASC cells fueled with H2-3 mol. % H2O at temperatures from 600 to 800 C. Both the cells demonstrated a similar open circuit voltage (OCV) around 1.1 V, suggesting the cells were gastightly sealed. The peak power density of the CASC was in the range from 470 to 1600 mW cm2, which was slightly higher than that of the ASC cell, from 400 to 1560 mW cm2. These results indicate that adding a layer of NCF-BZCYYb on-cell reforming catalyst somewhat increased the cell performance. Fig. 3 shows the EIS of the two cells in the same temperature range and atmospheres as above at open circuit voltage. Since the two cells had the same electrodes and electrolyte, and the NCF-BZCYYb on-cell reforming catalyst contained a majority of NCF metallic alloys, the ohmic resistances (RU) of the cells, determined as the value of the high-frequency intercept at the real axis, were almost identical, in the range between 0.71 and 0.12 U cm2; and their electrode polarization resistances (RP), determined as the difference between the high- and low-frequency intercepts at the real axis [26e28], were similar to each other, with that of the CASC was merely around 0.08 U cm2 lower than that of the ASC at each temperature. Assuming that the mass transport polarization is negligible before the peak power density is achieved, which is reasonably true according to the results shown in Fig. 2, and then the cell voltage (V) and power density (P) can be approximately expressed as a function of I as
V ¼ OCV IðRU þ RP Þ
(3)
P ¼ IV
(4)
which may qualitatively explain that the power density of the CASC was marginally higher than that of the ASC, because of the variation in RP. However, the amount of performance increase was fairly small; therefore it might be rather a divergence between two initial
Fig. 1. Cross-sectional microstructures of the as-prepared cell and cell components: (a) sintered ASC; (b) LSCF-GDC cathode/GDC baffle/YSZ electrolyte; (c) NCFO-BZCYYb/NiO-YSZ anode-support; (d) NCF-BZCYYb/Ni-YSZ anode-support and high magnification NCF-BZCYYb (insert).
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Fig. 2. IeV and IeP curves of the ASC and CASC fueled with H2-3 mol. % H2O at temperatures between 650 and 800 C.
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Fig. 4. IeV and IeP curves of the ASC and CASC fueled with CH4-3 mol. % H2O at temperatures between 650 and 800 C.
Fig. 3. Open circuit EIS of the ASC and CASC using H2-3 mol. % H2O as the fuel at temperatures between 650 and 800 C.
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anode. This discrepancy may be caused by the difference in geometric configuration of the cells used in the previous and present studies with dissimilar dimensions of NCF-BCZYYB catalyst and anode. In addition, the present fuel flow rate was 50 ml min1 higher than that utilized before, which may also increase the potential of carbon deposition in the NCF-BCZYYB catalyst and Ni-YSZ anode. It is known that increasing the amount of H2O will promote the reaction of CH4 steam reforming
CH4 þ H2 O ¼ CO þ 3H2
Fig. 5. Time dependence of power density for the ASC and CASC fueled by CH4-3 mol. % H2O at 800 C and 500 mA cm2.
cell measurements than related to the addition of the on-cell reforming catalyst layer. At 800 C and 500 mA cm2, these two cells performed consistently for up to 36 h, as shown in Fig. S2, which also suggests that adding a layer of NCF-BZCYYb on the surface of the anode-support did not affect the cell behavior with H2-3 mol. % H2O as the fuel.
(5)
and decreases the tendency of carbon deposition. Thus steam reforming of CH4 was conducted at a steam to carbon ratio of 1:1 (reaction 5) and temperatures from 600 to 800 C using the Ni-YSZ sintered at 1390 C for 3 h and NCF-BZCYYb sintered at 900 C for 2 h to simulate the Ni-YSZ anode-support and NCF-BZCYYb catalyst in the prepared cells, as shown in Fig. 6. The CH4 conversion rate of the NCF-BZCYYb catalyst was in the range from 60.61 to 96.67% (Fig. 6a) and remained stable on the level of 96% at 800 C during the 10 h test (Fig. 6b). In comparison, that of the Ni-YSZ anodesupport was significantly lower in the range from 6.53 to 62.98% (Fig. 6a) and decreased continuously with testing time to below 30% (Fig. 6b) due to the carbon formation on the active Ni surfaces [12e14,16,17,30e33]. On the other hand, the CO selectivity of the NCF-BZCYYb catalyst was in the range from 90 to 100%, which was also remarkably higher than that of the Ni-YSZ anode-support from
3.3. Cell performance in CH4-x mol. % H2O Fig. 4 demonstrates the initial performance of the ASC and CASC cells fueled with CH4-3 mol. % H2O at temperatures from 650 to 800 C. The OCVs of these two cells were similar to each other, in the range between 1.09 and 1.15, and increased with the increase in testing temperature. The peak power density of the ASC was 347, 620, 1035 and 1432 mW cm2 at temperatures of 650, 700, 750 and 800 C, respectively, which is close to that of 320, 603, 1029 and 1530 mW cm2 for the CASC. These results are in agreement with those reported previously [29] and indicate that, fueled with CH43 mol. % H2O, the initial performance of the ASC and CASC was equivalent to each other, similar to the situation using H2-3 mol. % H2O as the fuel (Fig. 2). It is further noted that there is no significant difference in the initial performance of the same type cells fueled with either H2-3 mol. % H2O or CH4-3 mol. % H2O. On account of merely 3 mol. % H2O in the fuel, this phenomenon may suggest that CH4 was cracked in to C and H2 by the thick anode-support before it reached the functional anode; and only H2 was electrochemically oxidized with a lower thermodynamic barrier than that for direct CH4 oxidation. However, during a prolonged operation, the performance of the ASC and CASC in CH4-3 mol. % H2O was pronouncedly different, as shown in Fig. 5. The performance of the ASC remained stable on the level of 470 mW cm2 for merely less than 1 h at 500 mA cm2 and 800 C; whereas that of the CASC lasted for 11 h. It was observed that the cells were completely disintegrated at the end of the tests possibly due to the dusting of Ni component in the anode-support caused by carbon formation [30]; and the addition of the NCF-BZCYYb catalyst layer alleviated the degree of carbon deposition. It is acknowledged that such result is not exactly the same as that obtained in our previous study [14], where carbon deposition was completely suppressed during a test of an YSZsupported half-cell for up to 24 h at 800 C by imposing an extra layer of NCF-BZCYYb catalyst on the YSZ-supported thin Ni-YSZ
Fig. 6. CH4 steam reforming activity of the on-cell NCF-BZCYYb catalyst and Ni-YSZ anode-support in CH4-50 mol. % H2O (S/C ¼ 1): (a) temperature dependence of CH4 conversion rate and CO selectivity; (b) time dependence of CH4 conversion rate at 800 C.
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Fig. 8. Time dependence of power density for the ASC and CASC fueled by CH433.3 mol. % H2O atmosphere at 800 C and 500 mA cm2.
YSZ anode-support, and prevented carbon deposition in the NCFBZCYYb catalyst, the Ni-YSZ anode-support and even the Ni-YSZ functional anode, as shown in Fig. S3 for SEM examination and EDS compositional analysis of the CASC tested for 48 h. The absence of carbon in the anode region of the CASC also suggests that the CH4 in the fuel was almost completely reformed into CO and H2 in the NCF-BZCYYb layer, and the produced CO and H2 were electrochemically oxidized in the Ni-YSZ functional anode, forming CO2 and H2O, which then migrated back to the NCF-BZCYYb layer for the use of CH4 reforming via reactions (5) and (6)
CH4 þ CO2 ¼ 2CO þ 2H2 Fig. 7. IeV and IeP curves of the ASC and CASC fueled by CH4-33.3 mol. % H2O at temperatures between 600 and 800 C.
20 to 70% (Fig. 6a). These results indicate that the NCF-BZCYYb is a highly active catalyst for CH4 steam reforming and resistant to carbon deposition, as demonstrated in our previous investigations [13,14]. Guided by the above reforming results, the ASC and CASC were further evaluated with CH4-33.3 mol. % H2O as the fuel, in which the amount of H2O decreased from that used in the reforming tests to increase the fuel concentration for a higher cell performance. Fig. 7 is the initial IeV and IeP curves at various temperatures between 600 and 800 C. Alike those shown above in CH4-3 mol. % H2O, the values of OCV of ASC and CASC were similar to each other; however, they were lower than those obtained in CH4-3 mol. % H2O, ranging from 1.05 to 1.09 V at temperatures between 600 and 800 C. This reason for the lowered OCVs here may be the higher H2O content in the fuel that increases the oxygen partial pressure at the anode/ electrolyte interface. The peak power density of the CASC was 280, 543, 936, 1245 and 1638 mW cm2, which is noticeably higher than 209, 429, 755, 1048 and 1283 mW cm2 of the ASC, at 600, 650, 700, 750 and 800 C, respectively. Fig. 8 is the performance durability at 500 mA cm2 and 800 C, showing that the life of the ASC was around 11 h, while that of the CASC was longer than 48 h under the same condition. This performance improvement is attributed to the addition of the NCF-BZCYYb catalyst, which was proved to possess a substantially higher activity for CH4 steam reforming than the Ni-
(6)
4. Conclusions Ni-YSZ anode-supported cells, with (CASC) and without (ASC) the NCF-BZCYYb cermet catalyst imposed on the anode-support, were fabricated and investigated comparatively to demonstrate the feasibility of the NCF-BZCYYb as an on-cell reforming catalyst for direct utilization of CH4 as a fuel. Based on the results obtained, the following conclusions are made. (1) The use of NCF-BZCYYb on-cell reforming catalyst does not affect the performance of the cell in H2-3 mol. % H2O. The ASC and CASC cells demonstrate comparable initial and prolonged performances. (2) Fueled by CH4-3 mol. % H2O, the initial performance of the ASC and CASC cells remains similar, and the use of the NCFBZCYYb catalyst moderately improves the performance stability of the cell. Carbon deposition cannot be avoided in both the cells, which causes cell disintegration. (3) The NCF-BZCYYb possesses a higher activity of CH4 steam reforming than that of the Ni-YSZ anode-support; therefore the initial performance and durability of the CASC are substantially increased in CH4-33.3 mol. % H2O without detectable carbon deposition. Acknowledgment This research was financially supported by the National Natural Science Foundation of China (U1134001), the City of Shenzhen
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(20140416165218) and Huazhong University of Science and Technology (2015650011). XRD and SEM characterizations were performed with the assistance of the Analytical and Testing Center of Huazhong University of Science and Technology.
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Appendix A. Supplementary data [17]
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.11.029.
[18]
References [1] Y. Leng, S. Chan, K. Khor, S. Jiang, Performance evaluation of anode-supported solid oxide fuel cells with thin film YSZ electrolyte, Int. J. Hydrogen Energy 29 (2004) 1025e1033. [2] C. Yang, W. Li, S. Zhang, L. Bi, R. Peng, C. Chen, W. Liu, Fabrication and characterization of an anode-supported hollow fiber SOFC, J. Power Sources 187 (2009) 90e92. [3] D. Simwonis, A. Naoumidis, F. Dias, J. Linke, A. Moropoulou, Material characterization in support of the development of an anode substrate for solid oxide fuel cells, J. Mater. Res. 12 (6) (Jun 1997) 1508e1518. [4] Z. Wang, J. Qian, J. Cao, S. Wang, T. Wen, A study of multilayer tape casting method for anode-supported planar type solid oxide fuel cells (SOFCs), J. Alloys Compd. 437 (2007) 264e268. [5] J. Ahn, S. Omar, H. Yoon, J. Nino, E. Wachsman, Performance of anodesupported solid oxide fuel cell using novel ceria electrolyte, J. Power Sources 195 (2010) 2131e2135. [6] J. Kong, K. Sun, D. Zhou, N. Zhang, J. Mu, J. Qiao, Ni-YSZ gradient anodes for anode-supported SOFCs, J. Power Sources 166 (2007) 337e342. [7] S. McIntosh, R. Gorte, Direct Hydrocarbon Solid Oxide Fuel Cells, Chem. Rev. 104 (2004) 4845e4865. [8] H. Kim, C. Lu, W. Worrell, J. Vohs, R. Gorte, Cu-Ni Cermet Anodes for Direct Oxidation of Methane in Solid-Oxide Fuel Cells, J. Electrochem. Soc. 149 (2002) A247. [9] T. Hibino, A. Hashimoto, M. Yano, M. Suzuki, M. Sano, Ru-catalyzed anode materials for direct hydrocarbon SOFCs, Electrochimica Acta 48 (2003) 2531e2537. [10] K. Walters, A. Dean, H. Zhu, R. Kee, Homogeneous kinetics and equilibrium predictions of coking propensity in the anode channels of direct oxidation solid-oxide fuel cells using dry natural gas, J. Power Sources 123 (2003) 182e189. [11] J. Koh, Y. Yoo, J. Park, H. Lim, Carbon deposition and cell performance of NiYSZ anode support SOFC with methane fuel, Solid State Ionics 149 (2002) 157e166. [12] B. Hua, M. Li, B. Chi, J. Li, Enhanced electrochemical performance and carbon deposition resistance of NieYSZ anode of solid oxide fuel cells by in situ formed Ni-MnO layer for CH4 on-cell reforming, J. Mater. Chem. A 2 (2014) 1150. [13] B. Hua, M. Li, W. Zhang, J. Pu, B. Chi, J. Li, Methane On-Cell Reforming by Alloys Reduced from Ni0.5Cu0.5Fe2O4 for Direct-Hydrocarbon Solid Oxide Fuel Cells, J. Electrochem. Soc. 161 (2014) F569eF575. [14] B. Hua, M. Li, J. Pu, B. Chi, J. Li, BaZr0.1Ce0.7Y0.1Yb0.1O3-d enhanced coking-free
[19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29] [30] [31] [32] [33]
on-cell reforming for direct-methane solid oxide fuel cells, J. Mater. Chem. A 2 (2014) 12576. X. Meng, X. Gong, N. Yang, Y. Yin, X. Tan, Z. Ma, Carbon-resistant Ni-YSZ/CuCeO2-YSZ dual-layer hollow fiber anode for micro tubular solid oxide fuel cell, Int. J. Hydrogen Energy 39 (2014) 3879e3886. H. Kan, S. Hyun, Y. Shul, H. Lee, Improved solid oxide fuel cell anodes for the direct utilization of methane using Sn-doped Ni/YSZ catalysts, Catal. Commun. 11 (2009) 180e183. H. Kan, H. Lee, Sn-doped Ni/YSZ anode catalysts with enhanced carbon deposition resistance for an intermediate temperature SOFC, Appl. Catal. B Environ. 97 (2010) 108e114. , Mitigation of D. La Rosa, A. Sin, M.L. Faro, G. Monforte, V. Antonucci, A. Arico carbon deposits formation in intermediate temperature solid oxide fuel cells fed with dry methane by anode doping with barium, J. Power Sources 193 (2009) 160e164. L. Liu, K. Sun, X. Zhou, X. Li, M. Zhang, N. Zhang, Sulfur tolerance improvement of Ni-YSZ anode by alkaline earth metal oxide BaO for solid oxide fuel cells, Electrochem. Commun. 19 (2012) 63e66. Z. Zhan, S.A. Barnett, An octane-fueled solid oxide fuel cell, Science 308 (2005) 844e847. X. Ye, S. Wang, Z. Wang, L. Xiong, X. Sun, T. Wen, Use of a catalyst layer for anode-supported SOFCs running on ethanol fuel, J. Power Sources 177 (2008) 419e425. B. Huang, X. Zhu, W. Hu, Y. Wang, Q. Yu, Characterization of the Ni-ScSZ anode with a LSCMeCeO2 catalyst layer in thin film solid oxide fuel cell running on ethanol fuel, J. Power Sources 195 (2010) 3053e3059. L. Yang, S. Wang, K. Blinn, M. Liu, Z. Liu, Z. Cheng, Enhanced Sulfur and Coking Tolerance of a Mixed Ion Conductor for SOFCs: BaZr0.1Ce0.7Y0.1Yb0.1O3-d, Science 326 (2009) 126e129. C. Tu, R. Chien, V. Schmidt, S. Lee, C. Huang, C. Tsai, Thermal stability of Ba(Zr0.8xCexY0.2)O2.9 ceramics in carbon dioxide, J. Appl. Phys. 105 (2009) 103504. M. Li, B. Hua, S. Jiang, P. Jian, B. Chi, J. Li, 25 Electrochemical performance and carbon deposition resistance of M-BaZr0.1Ce0.7Y0.1Yb0.1O3-d (M ¼ Pd, Cu, Ni or NiCu) anodes for solid oxide fuel cells, Sci. Rep. 5 (2014) 7667. S. Jiang, S. Badwal, An electrode kinetics study of H oxidation on Ni/Y2O3-ZrO2 cermet electrode of the solid oxide fuel cell, Solid State Ionics 123 (1999) 209e224. B. Hua, W. Zhang, M. Li, X. Wang, B. Chi, J. Pu, J. Li, Improved microstructure and performance of Ni-based anode for intermediate temperature solid oxide fuel cells, J. Power Sources 247 (2014) 170e177. M. Li, B. Hua, S.P. Jiang, J. Pu, B. Chi, J. Li, BaZr0.1Ce0.7Y0.1Yb0.1O3-d as highly active and carbon tolerant anode for direct hydrocarbon solid oxide fuel cells, Int. J. Hydrogen Energy 39 (2014) 15975e15981. Y. Lin, Z. Zhan, J. Liu, S. Barnett, Direct operation of solid oxide fuel cells with methane fuel, Solid State Ionics 176 (2005) 1827e1835. S. McIntosh, R. Gorte, Direct Hydrocarbon Solid Oxide Fuel Cells, Chem. Rev. 104 (2004) 4845e4865. H. Kan, H. Lee, Enhanced stability of Ni-Fe/GDC solid oxide fuel cell anodes for dry methane fuel, Catal. Commun. 12 (2010) 36e39. E. Nikolla, J. Schwank, S. Linic, Hydrocarbon steam reforming on Ni alloys at solid oxide fuel cell operating conditions, Catal. Today 136 (2008) 243e248. L. da Costa, A. da Silva, F. Noronha, L. Mattos, The study of the performance of Ni supported on gadolinium doped ceria SOFC anode on the steam reforming of ethanol, Int. J. Hydrogen Energy 37 (2012) 5930e5939.
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Carbon-resistant Ni-Zr0.92Y0.08O2-d supported solid oxide fuel cells using Ni-Cu-Fe alloy cermet as on-cell reforming catalyst and mixed methane-steam as fuel Bin Hua a, b, 1, Meng Li a, 1, Jing-li Luo b, Jian Pu a, Bo Chi a, Jian Li a, c, * a Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China b Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada c Research Institute of Huazhong University of Science and Technology in Shenzhen, Shenzhen 518057, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The NCF-BZCYYb possesses higher activity for CH4 steam reforming compared to Ni-YSZ. The use of NCF-BZCYYb catalyst in the Ni-YSZ supported SOFC is studied. NCF-BZCYYb coated single cell exhibits higher electrochemical performance in CH4. NCF-BZCYYb coated single cell exhibits better durability and carbon resistance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 August 2015 Received in revised form 3 November 2015 Accepted 6 November 2015 Available online xxx
Two types of anode-supported cell are fabricated by tape casting, screen printing and sintering processes. The first one is a conventional anode supported cell (ASC); and the other, namely CASC, contains an extra layer of Ni-Cu/Ni-Fe alloys-BaZr0.1Ce0.7Y0.1Yb0.1O3-d (NCF-BZCYYb) cermet catalyst on the surface of the anode-support. Using CH4-3 mol. % H2O as the fuel, the initial performance of the CASC is moderately improved, compared with that of the ASC; the power density of the CASC and ASC at 500 mA cm2 and 800 C remain stable on the level of 470 mW cm2 for approximately 11 and 0.8 h, respectively, before cell disintegration caused by carbon formation. The performances of the CASC in the fuel of CH433.3 mol. % H2O are significantly increased above the level of the ASC, demonstrating an initial peak power density ranging from 280 to 1638 mW cm2 at temperatures between 600 and 800 C and a stable power density of 485 mW cm2 at 500 mA cm2 and 800 C for 48 h. Carbon deposition in the anode region of the tested CASC cell is not detected, as the NCF-BZCYYb is a more active catalyst than the NiZr0.92Y0.08O2-d (YSZ) anode-support for CH4 steam reforming. © 2015 Elsevier B.V. All rights reserved.
Keywords: Anode-supported cell Methane steam reforming On-cell catalyst Cell performance Carbon deposition
* Corresponding author. Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China. E-mail address: [email protected] (J. Li). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2015.11.029 0378-7753/© 2015 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are recognized as one of the most promising energy conversion technologies considering their ability to efficiently generate electrical energy from chemical energy. A
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major advantage of SOFCs is the fuel flexibility, which means hydrocarbons can be the potential fuel. Directly using hydrocarbons attracts much attention because of the advantages of less expensive and more easily accessible and safer for storage compared to hydrogen. For intermediate temperature SOFCs (IT-SOFCs), Nibased cermets are the state-of-the-art anode materials and also often used as the cell support on account of their high catalytic activity for fuel oxidation, thermal and electrical conductivities and mechanical properties as well [1e6]. When a hydrocarbon fuel, such as methane (CH4), is directly used for IT-SOFCs without prereforming by adding oxygen, steam or carbon dioxide [7e10], it is electrochemically oxidized on the electrolyteeelectrodeefuel gas triple phase boundaries (TPB), producing steam and carbon dioxide with a strong tendency of carbon deposition in the anode [11e14] as Ni is an excellent catalyst for breaking CeH bonds. To alleviate carbon deposition in Ni-based cermet anodes, functionally graded anodes were proposed, modifying the anode or cell support to promote fuel-reforming process and suppress the cracking reaction that is considered responsible for carbon deposition. It has been evidenced that cell performance can be improved greatly by using such functionally graded anode structures [15e19]. In fact the reforming process can be carried out in an extra layer of on-cell reforming catalyst imposed on the conventional Ni-based anode to avoid direct exposure of Ni to methane [12e14,20e22]. In this way, conventional simple cell fabrication method still can be used, without the need to modify the anode or cell support. In our previous work [14], it has been shown recently that CH4 can be on-cell reformed by imposing an extra layer of Ni-Cu/Ni-Fe alloys (NCF)-BaZr0.1Ce0.7Y0.1Yb0.1O3-d (BZCYYb) cermet on the conventional Ni-YSZ anode, in which carbon deposition is completely suppressed during the testing period. The NCF alloy, in situ reduced from Ni0.5Cu0.5Fe2O4 (NCFO) spinel, functions as the catalyst for methane oxidation; and the BZCYYb adsorbs H2O and CO2 [23e25] to promote the reactions of methane oxidation and carbon removal. In the present study, this oncell reforming catalyst was used on a conventional Ni-Zr0.92Y0.08O2d (YSZ) anode-supported cell to demonstrate its applicability in practical SOFCs directly fueled with methane. 2. Experimental 2.1. Catalyst synthesis and cell fabrication NCFO and BZCYYb powders were synthesized by aqueous solution methods with metal nitrates (Sino-Pharm) in the stoichiometric compositions as the precursors, glycine (Sino-Pharm) as the dispersant and EDTA/citric acid (Sino-Pharm) as the chelant agent. The detailed procedures were described in Refs. [13] and [14], respectively. The catalyst powder was prepared by ball-milling the powder mixture consisting of NCFO and BZCYYb at a weight ratio of 80:20 in ethanol for 24 h. To prepare cathode material La0.6Sr0.4Co0.2Fe0.8O3d (LSCF), La(NO3)36H2O, Sr(NO3)2, Co(NO3)26H2O and Fe(NO3)39H2O (Sino-Pharm) were dissolved in distilled water with EDTA/citric acid added as the chelant agent. This solution was vaporized overnight in an oven at 180 C, followed by calcination in air at 800 C for 2 h to form single-phase perovskite LSCF. All the prepared powder materials and reduced NCFO in H2-3 mol. % H2O atmosphere at 800 C for 2 h were subjected to X-ray diffraction (XRD, X'Pert Pro, PAN Analytical B.V.) for phase identification by using Cu Ka radiation generated at 30 kV and 15 mA. The scan was conducted within a 2q range between 20 and 110 at a rate of 10 min1. The anode-supported cells were fabricated by using tape casting-screen printing-sintering processes. To prepare the anodesupport (AS), powder mixture, containing NiO (Inco) and 8 mol. % Y2O3 stabilized ZrO2 (8YSZ, Tosoh) at a weight ratio of 57:43, was
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ball-milled for 24 h in toluene/ethanol mixed solvent with fish oil as the dispersant and corn starch as the pore former. It was further milled for another 24 h after adding polyvinyl butyral (Richard E. Mistler Inc.) as the binder and polyethylene glycol (Richard E. Mistler Inc.) as the plasticizer. Such prepared slurry was cast into sheet by using a tape casting machine (LY-250-3, Beijing Oriental Tai Yang Systems). The dried green sheet (1.4 mm thick) was used as the anode support, on one surface of which the pastes of NiO-YSZ (60:40) functional anode (FA) and YSZ electrolyte were screenprinted in sequence, prior to sintering at 1390 C in air for 3 h. To prepare the buffer and cathode, Gd-doped CeO2 (GDC, NIMTE, CAS) and LSCF-GDC (70:30) pastes were then screen-printed successively on the surface of the sintered YSZ electrolyte, followed by sintering separately in air at 1300 C and 950 C for 2 h to complete the fabrication of the anode-supported cells. The NCFO-BZCYYb catalyst was paste-coated on the surface of the anode-support of the cell and sintered at 900 C in air for 2 h. The cells with and without the layer of NCFO-BZCYYb catalyst were designated as ASC (anode-supported cell) and CASC (NCFO-BZCYYb coated anodesupported cell). The final size of the cells was about f13 1 mm and the active area of the cathode was 0.5 cm2. The as-prepared and tested cells were examined by using a scanning electron microscope (SEM, Nova NanoSEM 450, FEI) equipped with an energy dispersive X-ray spectrometer (EDS). 2.2. Reforming activity evaluation The Ni-YSZ anode-support, without the coating of NCFOBZCYYb, is an active catalyst for methane reforming. To compare their activities, catalyst powders of NCFO-BZCYYb and NiO-YSZ (57:43) sintered at 900 and 1390 C for 2 and 3 h, respectively, were prepared, sieved into the particle range of 30e60 mesh. 0.2 g of each catalyst was packed into a quartz tube for activity measurements at temperatures from 600 to 800 C for up to 10 h after reduction in H2 at 800 C for 30 min. CH4, H2O and He at flow rates of 10, 10 (controlled by a high pressure constant flow pump) and 80 ml min1 were fed into the tubular reactor; and the composition of the effluent gas was analyzed by a Varian 3800 gas chromatograph equipped with Hayesep Q, Poraplot Q and 5 Å sieve molecular capillary columns and a thermal conductivity detector (TCD) for the separation and detection of H2, CO, CO2 and CH4. CH4 conversion rate and CO selectivity were calculated, respectively, according to
CH4 conversion ¼
CO selectivity ¼
½CO þ ½CO2 100% ½CO þ ½CO2 þ ½CH4
½CO 100% ½CO þ ½CO2
(1)
(2)
2.3. Cell test For the evaluation of electrochemical performances, Pt paste was painted on the both sides of the prepared cells ASC and CASC and baked in air at 800 C for 2 h; Pt mesh was compressed onto the paste as the current collector and Pt wire was spark-welded to the mesh as the measuring lead. The cell was then sealed on the anode side to an alumina tube by using a Ceramabond® glass sealant (Aremco Product, Inc.) and in-situ reduced in H2-3 mol. % H2O atmosphere at 800 C for 2 h prior to the electrochemical measurements. The open-circuit electrochemical impedance spectra (EIS) and the current densityevoltage (IeV) and current densityepower density (IeP) curves were obtained at temperatures ranging from
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600 to 800 C in H2-3 mol. % H2O and CH4-x mol. % H2O (x ¼ 3 and 33.3) by using an impedance/gain phase analyzer (Solartron 1260) and an electrochemical interface (Solartron 1287). The performance durability was also evaluated at 500 mA cm2 and 800 C. For all the tests, the flow rate for the fuel and air was 150 and 200 ml min1, respectively. 3. Results and discussions 3.1. Phase identification and microstructure examination Fig. 1 shows the cross-sectional microstructure of the asprepared and reduced cells. The LSCF-GDC cathode (~25 mm) was well sintered to the GDC buffer (~2 mm) that was well sintered to the YSZ electrolyte (Fig. 1a), which (~12 mm) was fairly dense with few isolated holes (Fig. 1b). Reduced at 800 C in H2-3 mol. % H2O for 2 h, the Ni-YSZ functional anode (~6 mm) was well adhered to the electrolyte and the Ni-YSZ anode support (Fig. 1a). In the CASC, the on-cell reforming catalyst layer (~35 mm) in both the sintered (Fig. 1c) and reduced (Fig. 1d) forms were seamlessly coated on the anode support. These intimate interfaces between different layers would ensure an uninterrupted transport of oxide ions and electrons upon cell operation. The results of X-ray diffraction, as shown in Fig. S1 in electronic supplementary information (ESI), indicates that the as-synthesized LSCF was single-phase perovskite (JCPDS files 01-082-1961 for La0.6Sr0.4FeO3-d). The crystal structure of other cell component materials, such as the as-synthesized NCFO, BZCYYb and reduced NCFO, were the same as identified in our previous work [14]. 3.2. Cell performance in H2-3 mol. % H2O Fig. 2 shows the initially measured IeV and IeP curves of the
ASC and CASC cells fueled with H2-3 mol. % H2O at temperatures from 600 to 800 C. Both the cells demonstrated a similar open circuit voltage (OCV) around 1.1 V, suggesting the cells were gastightly sealed. The peak power density of the CASC was in the range from 470 to 1600 mW cm2, which was slightly higher than that of the ASC cell, from 400 to 1560 mW cm2. These results indicate that adding a layer of NCF-BZCYYb on-cell reforming catalyst somewhat increased the cell performance. Fig. 3 shows the EIS of the two cells in the same temperature range and atmospheres as above at open circuit voltage. Since the two cells had the same electrodes and electrolyte, and the NCF-BZCYYb on-cell reforming catalyst contained a majority of NCF metallic alloys, the ohmic resistances (RU) of the cells, determined as the value of the high-frequency intercept at the real axis, were almost identical, in the range between 0.71 and 0.12 U cm2; and their electrode polarization resistances (RP), determined as the difference between the high- and low-frequency intercepts at the real axis [26e28], were similar to each other, with that of the CASC was merely around 0.08 U cm2 lower than that of the ASC at each temperature. Assuming that the mass transport polarization is negligible before the peak power density is achieved, which is reasonably true according to the results shown in Fig. 2, and then the cell voltage (V) and power density (P) can be approximately expressed as a function of I as
V ¼ OCV IðRU þ RP Þ
(3)
P ¼ IV
(4)
which may qualitatively explain that the power density of the CASC was marginally higher than that of the ASC, because of the variation in RP. However, the amount of performance increase was fairly small; therefore it might be rather a divergence between two initial
Fig. 1. Cross-sectional microstructures of the as-prepared cell and cell components: (a) sintered ASC; (b) LSCF-GDC cathode/GDC baffle/YSZ electrolyte; (c) NCFO-BZCYYb/NiO-YSZ anode-support; (d) NCF-BZCYYb/Ni-YSZ anode-support and high magnification NCF-BZCYYb (insert).
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Fig. 2. IeV and IeP curves of the ASC and CASC fueled with H2-3 mol. % H2O at temperatures between 650 and 800 C.
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Fig. 4. IeV and IeP curves of the ASC and CASC fueled with CH4-3 mol. % H2O at temperatures between 650 and 800 C.
Fig. 3. Open circuit EIS of the ASC and CASC using H2-3 mol. % H2O as the fuel at temperatures between 650 and 800 C.
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anode. This discrepancy may be caused by the difference in geometric configuration of the cells used in the previous and present studies with dissimilar dimensions of NCF-BCZYYB catalyst and anode. In addition, the present fuel flow rate was 50 ml min1 higher than that utilized before, which may also increase the potential of carbon deposition in the NCF-BCZYYB catalyst and Ni-YSZ anode. It is known that increasing the amount of H2O will promote the reaction of CH4 steam reforming
CH4 þ H2 O ¼ CO þ 3H2
Fig. 5. Time dependence of power density for the ASC and CASC fueled by CH4-3 mol. % H2O at 800 C and 500 mA cm2.
cell measurements than related to the addition of the on-cell reforming catalyst layer. At 800 C and 500 mA cm2, these two cells performed consistently for up to 36 h, as shown in Fig. S2, which also suggests that adding a layer of NCF-BZCYYb on the surface of the anode-support did not affect the cell behavior with H2-3 mol. % H2O as the fuel.
(5)
and decreases the tendency of carbon deposition. Thus steam reforming of CH4 was conducted at a steam to carbon ratio of 1:1 (reaction 5) and temperatures from 600 to 800 C using the Ni-YSZ sintered at 1390 C for 3 h and NCF-BZCYYb sintered at 900 C for 2 h to simulate the Ni-YSZ anode-support and NCF-BZCYYb catalyst in the prepared cells, as shown in Fig. 6. The CH4 conversion rate of the NCF-BZCYYb catalyst was in the range from 60.61 to 96.67% (Fig. 6a) and remained stable on the level of 96% at 800 C during the 10 h test (Fig. 6b). In comparison, that of the Ni-YSZ anodesupport was significantly lower in the range from 6.53 to 62.98% (Fig. 6a) and decreased continuously with testing time to below 30% (Fig. 6b) due to the carbon formation on the active Ni surfaces [12e14,16,17,30e33]. On the other hand, the CO selectivity of the NCF-BZCYYb catalyst was in the range from 90 to 100%, which was also remarkably higher than that of the Ni-YSZ anode-support from
3.3. Cell performance in CH4-x mol. % H2O Fig. 4 demonstrates the initial performance of the ASC and CASC cells fueled with CH4-3 mol. % H2O at temperatures from 650 to 800 C. The OCVs of these two cells were similar to each other, in the range between 1.09 and 1.15, and increased with the increase in testing temperature. The peak power density of the ASC was 347, 620, 1035 and 1432 mW cm2 at temperatures of 650, 700, 750 and 800 C, respectively, which is close to that of 320, 603, 1029 and 1530 mW cm2 for the CASC. These results are in agreement with those reported previously [29] and indicate that, fueled with CH43 mol. % H2O, the initial performance of the ASC and CASC was equivalent to each other, similar to the situation using H2-3 mol. % H2O as the fuel (Fig. 2). It is further noted that there is no significant difference in the initial performance of the same type cells fueled with either H2-3 mol. % H2O or CH4-3 mol. % H2O. On account of merely 3 mol. % H2O in the fuel, this phenomenon may suggest that CH4 was cracked in to C and H2 by the thick anode-support before it reached the functional anode; and only H2 was electrochemically oxidized with a lower thermodynamic barrier than that for direct CH4 oxidation. However, during a prolonged operation, the performance of the ASC and CASC in CH4-3 mol. % H2O was pronouncedly different, as shown in Fig. 5. The performance of the ASC remained stable on the level of 470 mW cm2 for merely less than 1 h at 500 mA cm2 and 800 C; whereas that of the CASC lasted for 11 h. It was observed that the cells were completely disintegrated at the end of the tests possibly due to the dusting of Ni component in the anode-support caused by carbon formation [30]; and the addition of the NCF-BZCYYb catalyst layer alleviated the degree of carbon deposition. It is acknowledged that such result is not exactly the same as that obtained in our previous study [14], where carbon deposition was completely suppressed during a test of an YSZsupported half-cell for up to 24 h at 800 C by imposing an extra layer of NCF-BZCYYb catalyst on the YSZ-supported thin Ni-YSZ
Fig. 6. CH4 steam reforming activity of the on-cell NCF-BZCYYb catalyst and Ni-YSZ anode-support in CH4-50 mol. % H2O (S/C ¼ 1): (a) temperature dependence of CH4 conversion rate and CO selectivity; (b) time dependence of CH4 conversion rate at 800 C.
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Fig. 8. Time dependence of power density for the ASC and CASC fueled by CH433.3 mol. % H2O atmosphere at 800 C and 500 mA cm2.
YSZ anode-support, and prevented carbon deposition in the NCFBZCYYb catalyst, the Ni-YSZ anode-support and even the Ni-YSZ functional anode, as shown in Fig. S3 for SEM examination and EDS compositional analysis of the CASC tested for 48 h. The absence of carbon in the anode region of the CASC also suggests that the CH4 in the fuel was almost completely reformed into CO and H2 in the NCF-BZCYYb layer, and the produced CO and H2 were electrochemically oxidized in the Ni-YSZ functional anode, forming CO2 and H2O, which then migrated back to the NCF-BZCYYb layer for the use of CH4 reforming via reactions (5) and (6)
CH4 þ CO2 ¼ 2CO þ 2H2 Fig. 7. IeV and IeP curves of the ASC and CASC fueled by CH4-33.3 mol. % H2O at temperatures between 600 and 800 C.
20 to 70% (Fig. 6a). These results indicate that the NCF-BZCYYb is a highly active catalyst for CH4 steam reforming and resistant to carbon deposition, as demonstrated in our previous investigations [13,14]. Guided by the above reforming results, the ASC and CASC were further evaluated with CH4-33.3 mol. % H2O as the fuel, in which the amount of H2O decreased from that used in the reforming tests to increase the fuel concentration for a higher cell performance. Fig. 7 is the initial IeV and IeP curves at various temperatures between 600 and 800 C. Alike those shown above in CH4-3 mol. % H2O, the values of OCV of ASC and CASC were similar to each other; however, they were lower than those obtained in CH4-3 mol. % H2O, ranging from 1.05 to 1.09 V at temperatures between 600 and 800 C. This reason for the lowered OCVs here may be the higher H2O content in the fuel that increases the oxygen partial pressure at the anode/ electrolyte interface. The peak power density of the CASC was 280, 543, 936, 1245 and 1638 mW cm2, which is noticeably higher than 209, 429, 755, 1048 and 1283 mW cm2 of the ASC, at 600, 650, 700, 750 and 800 C, respectively. Fig. 8 is the performance durability at 500 mA cm2 and 800 C, showing that the life of the ASC was around 11 h, while that of the CASC was longer than 48 h under the same condition. This performance improvement is attributed to the addition of the NCF-BZCYYb catalyst, which was proved to possess a substantially higher activity for CH4 steam reforming than the Ni-
(6)
4. Conclusions Ni-YSZ anode-supported cells, with (CASC) and without (ASC) the NCF-BZCYYb cermet catalyst imposed on the anode-support, were fabricated and investigated comparatively to demonstrate the feasibility of the NCF-BZCYYb as an on-cell reforming catalyst for direct utilization of CH4 as a fuel. Based on the results obtained, the following conclusions are made. (1) The use of NCF-BZCYYb on-cell reforming catalyst does not affect the performance of the cell in H2-3 mol. % H2O. The ASC and CASC cells demonstrate comparable initial and prolonged performances. (2) Fueled by CH4-3 mol. % H2O, the initial performance of the ASC and CASC cells remains similar, and the use of the NCFBZCYYb catalyst moderately improves the performance stability of the cell. Carbon deposition cannot be avoided in both the cells, which causes cell disintegration. (3) The NCF-BZCYYb possesses a higher activity of CH4 steam reforming than that of the Ni-YSZ anode-support; therefore the initial performance and durability of the CASC are substantially increased in CH4-33.3 mol. % H2O without detectable carbon deposition. Acknowledgment This research was financially supported by the National Natural Science Foundation of China (U1134001), the City of Shenzhen
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(20140416165218) and Huazhong University of Science and Technology (2015650011). XRD and SEM characterizations were performed with the assistance of the Analytical and Testing Center of Huazhong University of Science and Technology.
[15]
[16]
Appendix A. Supplementary data [17]
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.11.029.
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References [1] Y. Leng, S. Chan, K. Khor, S. Jiang, Performance evaluation of anode-supported solid oxide fuel cells with thin film YSZ electrolyte, Int. J. Hydrogen Energy 29 (2004) 1025e1033. [2] C. Yang, W. Li, S. Zhang, L. Bi, R. Peng, C. Chen, W. Liu, Fabrication and characterization of an anode-supported hollow fiber SOFC, J. Power Sources 187 (2009) 90e92. [3] D. Simwonis, A. Naoumidis, F. Dias, J. Linke, A. Moropoulou, Material characterization in support of the development of an anode substrate for solid oxide fuel cells, J. Mater. Res. 12 (6) (Jun 1997) 1508e1518. [4] Z. Wang, J. Qian, J. Cao, S. Wang, T. Wen, A study of multilayer tape casting method for anode-supported planar type solid oxide fuel cells (SOFCs), J. Alloys Compd. 437 (2007) 264e268. [5] J. Ahn, S. Omar, H. Yoon, J. Nino, E. Wachsman, Performance of anodesupported solid oxide fuel cell using novel ceria electrolyte, J. Power Sources 195 (2010) 2131e2135. [6] J. Kong, K. Sun, D. Zhou, N. Zhang, J. Mu, J. Qiao, Ni-YSZ gradient anodes for anode-supported SOFCs, J. Power Sources 166 (2007) 337e342. [7] S. McIntosh, R. Gorte, Direct Hydrocarbon Solid Oxide Fuel Cells, Chem. Rev. 104 (2004) 4845e4865. [8] H. Kim, C. Lu, W. Worrell, J. Vohs, R. Gorte, Cu-Ni Cermet Anodes for Direct Oxidation of Methane in Solid-Oxide Fuel Cells, J. Electrochem. Soc. 149 (2002) A247. [9] T. Hibino, A. Hashimoto, M. Yano, M. Suzuki, M. Sano, Ru-catalyzed anode materials for direct hydrocarbon SOFCs, Electrochimica Acta 48 (2003) 2531e2537. [10] K. Walters, A. Dean, H. Zhu, R. Kee, Homogeneous kinetics and equilibrium predictions of coking propensity in the anode channels of direct oxidation solid-oxide fuel cells using dry natural gas, J. Power Sources 123 (2003) 182e189. [11] J. Koh, Y. Yoo, J. Park, H. Lim, Carbon deposition and cell performance of NiYSZ anode support SOFC with methane fuel, Solid State Ionics 149 (2002) 157e166. [12] B. Hua, M. Li, B. Chi, J. Li, Enhanced electrochemical performance and carbon deposition resistance of NieYSZ anode of solid oxide fuel cells by in situ formed Ni-MnO layer for CH4 on-cell reforming, J. Mater. Chem. A 2 (2014) 1150. [13] B. Hua, M. Li, W. Zhang, J. Pu, B. Chi, J. Li, Methane On-Cell Reforming by Alloys Reduced from Ni0.5Cu0.5Fe2O4 for Direct-Hydrocarbon Solid Oxide Fuel Cells, J. Electrochem. Soc. 161 (2014) F569eF575. [14] B. Hua, M. Li, J. Pu, B. Chi, J. Li, BaZr0.1Ce0.7Y0.1Yb0.1O3-d enhanced coking-free
[19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29] [30] [31] [32] [33]
on-cell reforming for direct-methane solid oxide fuel cells, J. Mater. Chem. A 2 (2014) 12576. X. Meng, X. Gong, N. Yang, Y. Yin, X. Tan, Z. Ma, Carbon-resistant Ni-YSZ/CuCeO2-YSZ dual-layer hollow fiber anode for micro tubular solid oxide fuel cell, Int. J. Hydrogen Energy 39 (2014) 3879e3886. H. Kan, S. Hyun, Y. Shul, H. Lee, Improved solid oxide fuel cell anodes for the direct utilization of methane using Sn-doped Ni/YSZ catalysts, Catal. Commun. 11 (2009) 180e183. H. Kan, H. Lee, Sn-doped Ni/YSZ anode catalysts with enhanced carbon deposition resistance for an intermediate temperature SOFC, Appl. Catal. B Environ. 97 (2010) 108e114. , Mitigation of D. La Rosa, A. Sin, M.L. Faro, G. Monforte, V. Antonucci, A. Arico carbon deposits formation in intermediate temperature solid oxide fuel cells fed with dry methane by anode doping with barium, J. Power Sources 193 (2009) 160e164. L. Liu, K. Sun, X. Zhou, X. Li, M. Zhang, N. Zhang, Sulfur tolerance improvement of Ni-YSZ anode by alkaline earth metal oxide BaO for solid oxide fuel cells, Electrochem. Commun. 19 (2012) 63e66. Z. Zhan, S.A. Barnett, An octane-fueled solid oxide fuel cell, Science 308 (2005) 844e847. X. Ye, S. Wang, Z. Wang, L. Xiong, X. Sun, T. Wen, Use of a catalyst layer for anode-supported SOFCs running on ethanol fuel, J. Power Sources 177 (2008) 419e425. B. Huang, X. Zhu, W. Hu, Y. Wang, Q. Yu, Characterization of the Ni-ScSZ anode with a LSCMeCeO2 catalyst layer in thin film solid oxide fuel cell running on ethanol fuel, J. Power Sources 195 (2010) 3053e3059. L. Yang, S. Wang, K. Blinn, M. Liu, Z. Liu, Z. Cheng, Enhanced Sulfur and Coking Tolerance of a Mixed Ion Conductor for SOFCs: BaZr0.1Ce0.7Y0.1Yb0.1O3-d, Science 326 (2009) 126e129. C. Tu, R. Chien, V. Schmidt, S. Lee, C. Huang, C. Tsai, Thermal stability of Ba(Zr0.8xCexY0.2)O2.9 ceramics in carbon dioxide, J. Appl. Phys. 105 (2009) 103504. M. Li, B. Hua, S. Jiang, P. Jian, B. Chi, J. Li, 25 Electrochemical performance and carbon deposition resistance of M-BaZr0.1Ce0.7Y0.1Yb0.1O3-d (M ¼ Pd, Cu, Ni or NiCu) anodes for solid oxide fuel cells, Sci. Rep. 5 (2014) 7667. S. Jiang, S. Badwal, An electrode kinetics study of H oxidation on Ni/Y2O3-ZrO2 cermet electrode of the solid oxide fuel cell, Solid State Ionics 123 (1999) 209e224. B. Hua, W. Zhang, M. Li, X. Wang, B. Chi, J. Pu, J. Li, Improved microstructure and performance of Ni-based anode for intermediate temperature solid oxide fuel cells, J. Power Sources 247 (2014) 170e177. M. Li, B. Hua, S.P. Jiang, J. Pu, B. Chi, J. Li, BaZr0.1Ce0.7Y0.1Yb0.1O3-d as highly active and carbon tolerant anode for direct hydrocarbon solid oxide fuel cells, Int. J. Hydrogen Energy 39 (2014) 15975e15981. Y. Lin, Z. Zhan, J. Liu, S. Barnett, Direct operation of solid oxide fuel cells with methane fuel, Solid State Ionics 176 (2005) 1827e1835. S. McIntosh, R. Gorte, Direct Hydrocarbon Solid Oxide Fuel Cells, Chem. Rev. 104 (2004) 4845e4865. H. Kan, H. Lee, Enhanced stability of Ni-Fe/GDC solid oxide fuel cell anodes for dry methane fuel, Catal. Commun. 12 (2010) 36e39. E. Nikolla, J. Schwank, S. Linic, Hydrocarbon steam reforming on Ni alloys at solid oxide fuel cell operating conditions, Catal. Today 136 (2008) 243e248. L. da Costa, A. da Silva, F. Noronha, L. Mattos, The study of the performance of Ni supported on gadolinium doped ceria SOFC anode on the steam reforming of ethanol, Int. J. Hydrogen Energy 37 (2012) 5930e5939.