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Preparation and evaluation of Ni-based anodes with straight open pores for solid oxide fuel cells
Journal Pre-proof Preparation and evaluation of Ni-based anodes with straight open pores for solid oxide fuel cells Yanting Tian, Xiang Guo, Pingping Wu, Xu Zhang, Zhongquan Nie PII:
S0925-8388(19)34490-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.153244
Reference:
JALCOM 153244
To appear in:
Journal of Alloys and Compounds
Received Date: 12 August 2019 Revised Date:
19 October 2019
Accepted Date: 1 December 2019
Please cite this article as: Y. Tian, X. Guo, P. Wu, X. Zhang, Z. Nie, Preparation and evaluation of Nibased anodes with straight open pores for solid oxide fuel cells, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153244. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Preparation and evaluation of Ni-based anodes with straight open pores for Solid Oxide Fuel Cells Yanting Tian*, Xiang Guo, Pingping Wu, Xu Zhang, Zhongquan Nie College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China Abstract Ni-based anode supports with straight open pores were well prepared using self-organization process. The scanning electron microscopy (SEM) results of YSZ ceramic skeletons showed that honeycomb-like structures with highly ordered straight pores were obtained. The regulation of solid loading and CaCl2 concentration on the morphology of YSZ skeletons were discussed. The YSZ skeleton with solid loading of 10 wt% and CaCl2 concentration of 1.5 mol L-1 delivered a pore size of 99 µm and a porosity of 46.9 %. The solid oxide fuel cell based on Ni-YSZ anode possessed a maximum power density of 253 mW cm-2 and a ohmic resistance of 0.16 Ω cm2 at 800 °C. The pore diameter and porosity of the ceramic supports could be adjusted to expected values by further optimization of the parameters. The self-organization method will become a feasible and valid operation to produce anode supports with appropriate morphologies.
Keywords: Solid oxide fuel cells; Self-organization method; Straight open pores; Ni-based anodes
1. Introduction
Solid oxide fuel cells (SOFCs) possess lots of advantages such as low environmental pollution, high power transformation efficiency and fuel flexibility [1]. As a mixed conductor of ionic and electronic,
*Corresponding author. E-mail address: [email protected] (Yanting Tian).
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porous anodes are one of the key components responsible for the electrochemical performance of SOFCs. Anodes should have superior catalytic activity to promote the oxidation reactions of the fuels and excellent electron mobility for electron transport. Appropriate pore morphology and sufficient porosity are also necessary to facilitate the gas diffusion during the cell operation process. Ni/yttria-stabilized zirconia (YSZ) cermet is the most common material for SOFC anodes based on its high electrocatalytic activity towards the oxidation of fuels [2]. Different kinds of pore-formers such as flour [3-4], starch [5-6], carbon microsphere [7-8] and graphite [9] are normally used to prepare porous Ni/YSZ substrates. Pore structures with various morphologies will be produced after volatilization of pore-formers by heating process [10]. However, the pore size is generally smaller than 10 µm [11]. Larger pores are difficult to form by adding pore-formers into the electrode substrates. Besides, the gas diffusion channels are tortuous and randomly distributed, which prevents the mass transfer process in the anodes. According to Knudsen [12], a cylindrical structure could significantly reduce the gas diffusion resistance. Paper-fibers were used as pore-formers for Ni/YSZ anodes by Pan et al. [13], cylindrical pores with the diameter and length of 1~5 µm and 20~100 µm were obtained. The orientation of the cylindrical pores was perpendicular to the pressure exerted on the anodes as well as the gas transport direction. After further adjusting the orientation of the paper-fibers, Ni/YSZ anode substrates with cylindrical pores parallel to the gas diffusion direction were developed [14]. Cylindrical structure is favorable for gas transport but paper-fibers are still insufficient on the realization of connected gas diffusion channels in thick anode supports. Recently, phase-inversion method has been exploited for preparation of Ni/YSZ anode supports for SOFCs [15-17]. Finger-like macro-porous channels are formed in the planar anodes to decrease the gas
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diffusion resistance. Freeze-casting method has also been employed to prepare porous electrodes with graded directional channels [18-20]. Tree-like structures with pore size varies from 5 µm to 100 µm could be obtained [21]. The low tortuosity of the directional graded channels improves the gas diffusion rate and lowers the electrode polarization of the SOFCs. Both finger-like and tree-like porous structures facilitate the gas diffusion and mass transport in the electrodes. Self-organization process was originally adopted for preparation of honeycomb-like ceramic membranes with straight open pores in the micrometer range [22-24]. The diameter of the pores and porosity of the ceramics could be effectively controlled by the content of solid loadings. In the present work, Ni-based anode supports with well-oriented honeycomb pores were successfully prepared by self-organization method for the first time. The electrochemical performances and impedance spectroscopy analysis of the Ni-based anode supported single cells were investigated systematically.
2. Experimental
2.1 Preparation and characterization of YSZ substrates YSZ ceramics with straight open pores were prepared by alginate self-organization process [23-24]. Sodium alginate (NaAlg, Sinopharm Chemical Reagent Co., Ltd, China) was dissolved in deionized water at a concentration of 1.5 % to form sodium alginate sol. YSZ powders (8YSZ, Ningbo SOFCMAN Energy Technology Co., Ltd, China) were added into the sodium alginate sol with various solid loadings (5, 10 and 15 wt%) and the mixtures were ball-milled for 6 h, followed by degassing in a vacuum-assisted apparatus. The prepared slurries were then poured into a beaker, calcium chloride (Sinopharm Chemical Reagent Co., Ltd, China) solutions were sprayed onto the surface of the YSZ-alginate slurries with varying concentrations of 1, 1.5 and 2 mol L-1. Diffusion-controlled gelation of the mixed slurries was
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allowed for 36 h to form wet gels with unidirectional oriented capillaries. After that, the top and bottom layers of the wet gels were cut away and the rest part of the disks were washed in the deionized water to remove the impurity elements. The gel disks were then immersed into gluconolactone (Aladdin Reagent Co., Ltd, China) solutions and tertiary butanol (TBA, Sinopharm Chemical Reagent Co., Ltd, China) solutions successively to densify the gels and to avoid crack formation during drying process. After that, the samples were dried at room temperature and sintered at 1400 °C for 4 h to obtain the final porous YSZ ceramic. The phase composition of the YSZ ceramic was examined using X-ray diffraction (XRD, DX-2700, Haoyuan). The microstructure and morphology of various samples were examined by a scanning electron microscopy (SEM, JSM-7100F, JEOL). The open porosity of the porous YSZ ceramics after sintering at 1400 °C was measured by Archimedes principle. 2.2 Fabrication and characterization of Ni-based anode supported SOFCs Slurry spin coating method was used to prepare the YSZ electrolyte membranes [25-26]. Two layers of YSZ slurry with a YSZ to organic vehicle weight ratio of 50:50 were spin coated on the porous YSZ supports, and then there layers of slurry with a weight ratio of 30:70, followed by sintering at 1400 °C for 4 h to obtain a dense electrolyte membrane. The La0.7Sr0.3MnO3 (LSM)-YSZ composite cathode was then coated onto the YSZ membrane surface and sintered at 1100 °C for 2 h. The active cathode area was 0.12 cm2. A Ni(NO3)2 solution of 2 mol L-1 was infiltrated into the straight YSZ pores by capillary action and then heated at 400 °C. The infiltration and heating process was repeated for 10 times and the impregnated YSZ supports were finally fired at 700 °C for 1 h.
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Porous anode supports composed of NiO and YSZ composites (weight ratio of 50:50) were also prepared. After unidirectional oriented channels formed, the top layer of the wet gel was retained instead of resection. The wet gel was then dried and pre-sintered at 1000 °C. Three layers of YSZ slurry with a YSZ to organic vehicle weight ratio of 30:70 were spin coated on the top layer of the NiO-YSZ supports, followed by co-sintering at 1400 °C for 4 h. The cathode was the same with the former cells. To distinguish the two types of Ni-based anodes, the former one was described as Ni-impregnated YSZ anode and the latter one was Ni-YSZ anode. Single cells were tested using the four-terminal method at the operating temperature from 650 °C to 800 °C. The cell temperature was controlled by a digital temperature controller (LU-962UF J8000A1, Anthone Electronics Co. Ltd, China) with a K-type thermocouple located beside the cell. The surfaces of the electrodes were covered by silver conductive adhesive (DAD-87, Shanghai Synthetic Resin Institute, China) for collection of electrons. Quartz tubes were used to fix the single cells and the cell devices were sealed by silver paste. The electrochemical performance of the cells were measured using an electrochemical system (CHI660E, Shanghai CH Instrument Co. Ltd, China). Dry hydrogen at a flow rate of 50 ml min–1 diluted with 100 ml min–1 Ar was used as fuel and ambient air was used as oxidant during the testing process. The purity of both the hydrogen and argon was 99.999%. The frequency ranged from 91 kHz to 0.1 Hz was adopted for impedance spectra measurement at an ac voltage of 10 mV.
3. Results and Discussion
3.1. Microstructure and characterization of porous YSZ skeletons During the self-organization process, ordered capillaries formed through Ca2+ diffusion in the YSZ-alginate slurry. The optical micrograph of the wet gel in top view is shown in Fig. 1. The sample
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exhibited a honeycomb-like pore structure with irregular round pores uniformly distributed on the surface. Fig. 2 shows the cross-sectional SEM micrograph of YSZ ceramic skeleton after sintering at 1400 °C. Highly ordered straight open pores with an average pore size of 100 µm were unidirectional oriented along the diffusion direction of Ca2+. The thickness of the pore walls was 20 µm. The phase composition of the YSZ skeletons was analyzed by X-ray diffraction spectrum, as shown in Fig. 3. After sintering at 1400 °C, the pattern of YSZ skeletons was in conformity with the cubic phase of YSZ [27]. No other impurity diffraction peaks were observed. According to Xue et al. [24], almost all the impurity elements were removed by washing process. A small amount of Ca residual (0.0867 %, measured by X-ray fluorescence method, XRF) was transformed to CaO at high temperature, which was helpful for the YSZ sintering [23]. Fig. 4 shows the surface images of YSZ ceramic skeletons with solid loadings varied from 5 wt% to 15 wt% while keeping the concentration of CaCl2 solution at 1 mol L-1. Uniformly distributed pores with diameters of several tens of micrometers can be observed from all the samples. As shown in Fig. 4a, the substrate with lower solid loading showed larger pores and thinner pore walls. As the solid loading increased, the pore size decreased obviously. The average diameters (D) of the pores with solid loading of 5, 10 and 15 wt% were approximately 148, 132 and 92 µm, respectively. Increasing the YSZ solid loading, however, resulted in an increased thickness of the pore walls. The pore shape of the ceramics changed from irregular round of 5 wt% solid loading to regular round of 10 wt% solid loading. When the solid loading increased to 15 wt%, more YSZ powders were accumulated within the capillary walls, making the thickness of the pore walls obviously increased. The effect of CaCl2 concentration on the morphology of YSZ skeletons is shown in Fig. 5, while the
6
YSZ loading was kept at 10 wt%. There were distinct differences among the morphologies of the YSZ substrates. The average diameter of the pores decreased from 132 µm to 88.3 µm when CaCl2 concentration increased from 1 mol L-1 to 2 mol L-1. The number of Ca2+ reacted with alginate in per unit area increased with increasing CaCl2 concentration, which resulted in an increased number of channels and a decreased pore size. The thickness of the pore walls, however, was not changed noticeably with the same solid loading. The solid loading is the main regulation parameter for the pore diameter, porosity and thus the mechanical strength of the porous ceramics. With the increasing YSZ solid loading, the particles deposited on the pore walls increased. Fig. 6 shows the variations of pore size and porosity with different solid loadings and CaCl2 concentrations. As can be seen, both the pore size and porosity decreased with increasing solid loading. The porosities of the YSZ substrates at the CaCl2 concentration of 1.5 mol L-1 were 55.1, 46.9 and 37.2 % with the solid loading of 5, 10 and 15 wt%, respectively. The substrate with higher solid loading has lower porosity and thicker pore walls which could ensure the mechanical strength of the ceramic. The concentration of CaCl2 solution is another regulatory factor for the formation of ordered capillaries. Higher CaCl2 concentration resulted in reduced pore size, while the thickness of the pore walls did not change noticeably. Therefore, the porosity decreased with the increasing CaCl2 concentration. It is important to note that an appropriate porosity and good mechanical strength are necessary for the SOFC anodes. Consequently, the YSZ skeleton with solid loading of 10 wt% and CaCl2 concentration of 1.5 mol L-1 was selected as the anode support for electrochemical performance evaluation, which possessed a pore size and wall thickness of 99 µm and 35 µm, and a porosity of 46.9 %. 3.2 Cell performance based on Ni-impregnated YSZ substrates
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The cross-sectional SEM image of the YSZ electrolyte membrane is presented in Fig. 7a. The electrolyte membrane was prepared by spin coating method with two layers of higher solid-phase content slurry to block most of the big pores and then three layers of lower solid-phase content slurry to obtain dense electrolyte films. As can be seen, the electrolyte layer was adhered well to the porous supports with a thickness of approximately 110 µm. No visible cracks and holes were observed. This also provides a effective way to prepare dense membranes on honeycomb ceramics. The YSZ skeleton with straight open pores was infiltrated by Ni(NO3)2 solution to form the electron transport layer. Fig. 7b shows the microstructure of impregnated NiO after sintering at 700 °C. Nanoparticles with the size of 50~100 nm were uniformly distributed on the pore walls. The agglomeration phenomenon of NiO particles was not observed. The electrochemical performances of the Ni-impregnated YSZ supported cell were measured from 650 to 800 °C. The anode was reduced in H2 atmosphere at a flow rate of 50 ml min-1 at 650 °C. Ar of 100 ml min-1 was used as dilute gas. The open-circuit voltage (OCV) above 1 V was achieved in 2 minutes and then remained at a nearly stable value (Fig. S1), which indicated a rapid reduction process from NiO to Ni. The straight open pores with diminished tortuosity enables fast diffusion of reaction gas into the inner part of the anodes, therefore, accelerates the reduction rate of the anodes. As shown in Fig. 8a, the OCV of the cell was about 1.08 V throughout the testing process. The maximum power densities (MPDs) were 146, 102, 62 and 30 mW cm-2 at temperatures of 800, 750, 700 and 650 °C, confirming the feasibility of this new designed cell structure. The cell performance is comparable to that of the electrolyte-supported
SOFCs
[28],
which
has
phase-inversion-assisted laser ablation technique.
8
similar
YSZ
porous
structure
prepared
by
Typical ac impedance spectra of the cell measured under open circuit condition are shown in Fig. 8b. Fig. 8c shows the equivalent circuit. The inductance L is attributed to the measurement apparatus and silver wires. The intercept of high-frequency arc with the real axis represents the ohmic resistance (Ro, at 800 °C for example), which is mainly determined by the electrolyte; whereas the (R1, CPE1), (R2, CPE2), and (R3, CPE3) correspond to the intercept distance between the high-frequency arc and low-frequency arc with the real axis, which represents the total interfacial polarization resistances (Rp), including the charge transfer polarization and the concentration polarization of both the electrodes. As shown in Fig. 8b, the Ro significantly decreased with the increasing temperature, typically from 1.68 Ω cm2 at 650 °C to 0.45 Ω cm2 at 800 °C. Table 1 lists the ohmic resistances of electrolyte-supported SOFCs with various electrolyte thicknesses and operating conditions. The ohmic resistances in this study were closed to that of the electrolyte-supported SOFCs, mainly due to its thick electrolyte layer and the long ion migration paths in the pore walls which actually played as a part of electrolyte. The larger Rp indicated that the cell performance was mainly dominated by the electrode polarization. The SEM image of Ni nanoparticles after reduction is displayed in Fig. S2. Serious agglomeration of Ni particles were observed, which resulted in a decreased active triple-phase boundary (TPB) region and an increased cell polarization resistance. Based on the experimental results of Jiao et al. [33], the morphology of Ni particles was apparently influenced by the reduction temperature, which led to different anode performances. A higher reduction temperature could enhance the anode/electrolyte interface by preventing the nickel sintering [34]. The reduction process needs to be evaluated to decrease the electrode polarization resistance in this study. Besides, the impregnated Ni particles were only existed on the surface of the straight pores, the pore walls were almost dense with small amount of pinholes that were
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disconnected for Ni particles to access in (Fig. S3). As a result, a loss of TPB region is exist. Although the cell based on the as-prepared anode showed unsatisfying electrochemical performance, it is expected to be improved by many ways. The first effective way is to add a transition layer between the porous anode and dense electrolyte. A slurry composed of NiO and YSZ could be coated on the anode surface to block the big pores, instead of the first two YSZ layers, which is desired for further deposition of a thin YSZ electrolyte layer on it. Therefore, the thickness of the YSZ membrane could be notably reduced and the ohmic resistance of the cell is expected to be decreased. Moreover, it is reported that a multi-layer anode structure with porosity gradient increased the power density of the single cell from 76 to 101 mW cm−2 at 600 °C in humidified hydrogen [35]. Moon et al. [36] also reported that the maximum power density increased from 300 to 550 mW cm-2 at 700 °C by inserting a functional layer between the electrolyte and anode support. Furthermore, to improve the reaction region at the anode and electrolyte interface, one may also consider increasing the porosity of the pore walls by adding some pore formers into the YSZ solid loading. The Ni contents and TPB region in the substrates could be improved as discussed above. 3.3 Cell performance based on Ni-YSZ substrates The formation process of capillary gel is shown in Fig. 9a. When the calcium chloride solution was sprayed onto the surface of the sodium alginate sol, a reaction layer formed immediately at the interface of the two sols. This so-called primary membrane permitted unidirectional diffusion of Ca2+ into the sodium alginate solution, which led to the formation of ordered capillaries. To simplify the preparation process of electrolyte films on the straight porous skeletons, the primary membrane was retained as a transition layer between the Ni-YSZ support and the YSZ electrolyte. The capillaries located
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near the top membrane were sloped, which was verified in Fig. 9b. The thickness of the primary membrane after sintering at 1400 °C was about 25 µm. Fig. 9c displays the top surface of the primary membrane. Depressions were observed, which were the “egg-box” structures formed by the reaction of sodium alginate and metal ions. This relatively compact structure of the skin layer provides great convenience for subsequent deposition of a dense and thin YSZ electrolyte film on it. From the cross-sectional SEM image of the YSZ electrolyte film (Fig. 10a), it can be seen that a good adherence between the YSZ film and the top membrane was obtained. The thickness of the YSZ film was estimated to be 25 µm, which was significantly reduced compared with that of the Ni-impregnated YSZ supported cell. The surface image also showed a dense electrolyte without pores or cracks (Fig. 10b). The microstructure of Ni-YSZ anode after cell testing is shown in Fig. 10c. The reduction process from NiO to Ni introduced some tiny pores by the volatilization of oxygen. The electrochemical performance of the Ni-YSZ anode supported cell is presented in Fig. 11. As expected, the maximum current density remarkably increased from 530 mA cm-2 of the Ni-impregnated cell to 967 mA cm-2 of the Ni-YSZ supported cell at 800 °C. A maximum power density of 253 mW cm-2 was obtained. However, a serious drop was observed at high current density, which severely restricted the cell performance, whereas such a drop was not observed in the Ni-impregnated cell. The curvature is probably caused by concentration polarization of the Ni-YSZ anodes. Ni-impregnated YSZ supports contain straight open pores and pure YSZ walls, the cell structure is more like a electrolyte supported cell with Ni nanoparticles attached on the YSZ layer, making the reaction sites easily accessible to the reactants and thereby reducing the concentration polarization. In contrast, most of the reaction sites inside the walls of the Ni-YSZ supports are not easily access to the reactants, resulting in a serious concentration
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polarization. The small pores produced by NiO reduction are not sufficient to enable rapid gas transfer within the pore walls. A porous Ni-YSZ wall would be helpful for enhancing the cell performance. The impedance spectra of the cell based on Ni-YSZ substrates were also measured under open circuit condition, as shown in Fig. 11b. Apparently, the area specific resistances of the present cell were much lower than those of the Ni-impregnated cell. At 800 °C, the Ro and Rp values were 0.16 and 0.91 Ω cm2, the lower ohmic resistance presents direct evidence of its thinner electrolyte layer. The primary membrane of the Ni-YSZ substrates is beneficial to the cell preparation as well as the cell performance. In the present study, we have demonstrated that Ni-based anodes with straight open pores can be prepared using alginate self-organization process. A distinctive feature of the anode substrates is that the diameter of the straight open pores is depend on the content of solid loading and the concentration of metal ions. Varying diameters could also be obtained by changing the cross-linking cation with different ionic radius. According to Ahmed et al. [22], capillary ceramics showed a decreased capillary diameter and a increased porosity by changing copper for calcium. We have also prepared a YSZ ceramic using copper chloride of 1 mol/L, the average pore diameter of 35 µm was obtained (Fig. S4). The pore size and porosity of the ceramic skeletons could be adjusted to desired values after further optimizing the parameters of the self-organization reaction. The self-organization method will become a feasible and valid operation to produce porous anodes with appropriate structures. 4. Conclusion Ni-based anodes with straight open pores were fabricated by a self-organization method. The diameter of the pores decreased with increasing solid loading and CaCl2 concentration. Ceramic supports with solid loading of 10 wt% and CaCl2 concentration of 1.5 mol L-1 possessed a pore size of 99 µm and a
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porosity of 46.9 %. The cell based on Ni-impregnated YSZ supportes produced a maximum power density of 146 mW cm-2 and a ohmic resistance of 0.45 Ω cm2 at 800 °C. The Ni-YSZ supported cell with reserved primary membrane obtained a higher power density of 253 mW cm-2 and a lower ohmic resistance of 0.16 Ω cm2. The electrochemical performance of the cells are expected to be improved by further optimization of the cell structure.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Nos. 51602213, 11604236 and 61575139).
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SOFCs, Ceram. Int. 45 (2019) 9866-9870. [31] Y.J. Xue, H. Miao, C.R. He, J.X. Wang, M. Liu, S.S. Sun, Q. Wang, W.G. Wang, Electrolyte supported solid oxide fuel cells with the super large size and thin ytterbia stabilized zirconia substrate, J. Power Sources 279 (2015) 610-619. [32] L.Y. Wei, J.J. Zhang, F.Y. Yu, W.M. Zhang, X.X. Meng, N.T. Yang, S.M. Liu, A novel fabrication of yttria-stabilized-zirconia dense electrolyte for solid oxide fuel cells by 3D printing technique, Int. J. hydrogen energy 44 (2019) 6182-6191. [33] Z.J. Jiao, A. Ueno, Y. Suzuki, N. Shikazono, Study on the influences of reduction temperature on nickel-yttria-stabilized zirconia solid oxide fuel cell anode using nickel oxide-film electrode, J. Power Sources 328 (2016) 377-384. [34] Z.J. Jiao, N. Shikazono, Quantitative study on the correlation between solid oxide fuel cell Ni-YSZ composite anode performance and reduction temperature based on three-dimensional reconstruction, J. Electrochem. Soc. 162 (2015) F571-F578. [35] C.M. An, J.H. Song, I. Kang, N. Sammes, The effect of porosity gradient in a Nickel/Yttria Stabilized Zirconia anode for an anode-supported planar solid oxide fuel cell, J. Power Sources 195 (2010) 821-824. [36] H. Moon, S.-D. Kim, S.-H. Hyun, H.-S. Kim, Development of IT-SOFC unit cells with anode-supported thin electrolytes via tape casting and co-firing, Int. J. hydrogen energy 33 (2008) 1758-1768.
Figure Captions Fig. 1. Optical micrograph in top view of wet gel.
17
Fig. 2. Cross sectional SEM image of YSZ skeleton with 5 wt% solid loading and 2 mol L-1 CaCl2 solution. Fig. 3. XRD pattern of YSZ skeleton after sintering at 1400 °C for 4h. Fig. 4. Surface images of the YSZ skeletons with different solid loading: (a) 5 wt%, (b) 10 wt% and (c) 15 wt%. (d) Cross section of YSZ skeleton with 15 wt% solid loading. Fig. 5. Surface images of the YSZ skeletons with different CaCl2 concentrations: (a) 1 mol L-1, (b) 1.5 mol L-1 and (c) 2 mol L-1. (d) Cross section of the YSZ skeleton with 1.5 mol L-1 CaCl2. Fig. 6. Variations of pore size (a) and porosity (b) with different solid loadings and CaCl2 concentrations. Fig. 7. (a) Cross-sectional SEM micrograph of the electrolyte membrane. (b) Microstructure of impregnated NiO particles after sintering at 700 °C. Fig. 8. (a) I-V and I-P curves and (b) EIS of Ni-impregnated YSZ anode supported cells at different temperatures. (c) The equivalent circuit. Fig. 9. (a) The scheme of alginate-based capillary gel formation. (b) Cross-sectional SEM micrograph of the Ni-YSZ composite support and the primary membrane. (c) Surface image of the primary membrane. Fig. 10. (a) Cross-sectional SEM micrograph of the YSZ membrane, (b) surface image of the YSZ membrane and (c) microstructure of Ni-YSZ anode after test. Fig. 11. (a) I-V and I-P curves and (b) EIS of Ni-YSZ supported cells at different temperatures.
Table Captions Table 1 Comparison of the Ro for previously-reported YSZ-supported cells and the cell in this work.
18
Table 1 Comparison of the Ro for previously-reported YSZ-supported cells and the cell in this work. SOFC structure
Electrolyte thickness (µm)
Temp. (°C)
Ro (Ω cm2)
Ni-YSZ/YSZ/Pt [29]
500 (Dry pressing)
750
1.2
LSCF-SDC/SDC/YSZ/SDC/BSCF-SDC
78
850
0.19
[30]
(Dry pressing/heating/quenching/calcining) 200
0.33
(Dry pressing/calcining/polishing) NiO-GDC/5YSZ/LSM-5YSZ [31]
70
830
0.34
800
0.42
sintering method)
750
0.49
500 (3D printing)
850
0.5
(tape-casting,
Ag-GDC/YSZ/Ag-GDC [32]
multilayer-lamination
and
500 (Dry pressing) This work
110
0.64 800
0.45
200 µm
Fig. 1. Optical micrograph in top view of wet gel.
(b)
(a)
YSZ films
(c) Ni YSZ
Fig. 10. (a) Cross-sectional SEM micrograph of the YSZ membrane, (b) surface image of the YSZ membrane and (c) microstructure of Ni-YSZ anode after test.
Diffusion direction
Fig. 2. Cross sectional SEM image of YSZ skeleton with 5 wt% solid loading and 2 mol L-1 CaCl2 solution.
(a) 5 wt% YSZ, D = 148 µm
(b) 10 wt% YSZ, D = 132 µm
(c) 15 wt% YSZ, D = 92 µm
(d) 1 mol L-1 CaCl2, 15 wt% YSZ
Fig. 4. Surface images of the YSZ skeletons with different solid loading: (a) 5 wt%, (b) 10 wt% and (c) 15 wt%. (d) Cross section of YSZ skeleton with 15 wt% solid loading.
(a) 1 mol L-1 CaCl2, D = 132 µm
(c) 2 mol L-1 CaCl2, D = 88.3 µm
(b) 1.5 mol L-1 CaCl2, D = 99 µm
(d) 1.5 mol L-1 CaCl2, 10 wt% YSZ
Fig. 5. Surface images of the YSZ skeletons with different CaCl2 concentrations: (a) 1 mol L-1, (b) 1.5 mol L-1 and (c) 2 mol L-1. (d) Cross section of the YSZ skeleton with 1.5 mol L-1 CaCl2.
(b)
(a) Electrolyte
Fig. 7. (a) Cross-sectional SEM micrograph of the electrolyte membrane. (b) Microstructure of impregnated NiO particles after sintering at 700 °C.
(c)
Fig. 8. (c) The equivalent circuit.
(a) (b)
(c)
Primary membrane
Fig. 9. (a) The scheme of alginate-based capillary gel formation. (b) Cross-sectional SEM micrograph of the Ni-YSZ composite support and the primary membrane. (c) Surface image of the primary membrane.
Highlights (1) Ni-based anode supports with straight open pores were prepared using self-organization process. (2) The pore size and the porosity of the ceramic supports could be regulated by the content of solid loadings and the concentration of metal ions. (3) The YSZ skeleton with solid loading of 10 wt% and CaCl2 concentration of 1.5 mol L-1 delivered a pore size of 99 µm and a porosity of 46.9 %. (4) The cell based on Ni-YSZ anode possessed a maximum power density of 253 mW cm-2 and a ohmic resistance of 0.16 Ω cm2 at 800 °C.
We declare that we have no conflict of interest.
S0925-8388(19)34490-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.153244
Reference:
JALCOM 153244
To appear in:
Journal of Alloys and Compounds
Received Date: 12 August 2019 Revised Date:
19 October 2019
Accepted Date: 1 December 2019
Please cite this article as: Y. Tian, X. Guo, P. Wu, X. Zhang, Z. Nie, Preparation and evaluation of Nibased anodes with straight open pores for solid oxide fuel cells, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153244. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Preparation and evaluation of Ni-based anodes with straight open pores for Solid Oxide Fuel Cells Yanting Tian*, Xiang Guo, Pingping Wu, Xu Zhang, Zhongquan Nie College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China Abstract Ni-based anode supports with straight open pores were well prepared using self-organization process. The scanning electron microscopy (SEM) results of YSZ ceramic skeletons showed that honeycomb-like structures with highly ordered straight pores were obtained. The regulation of solid loading and CaCl2 concentration on the morphology of YSZ skeletons were discussed. The YSZ skeleton with solid loading of 10 wt% and CaCl2 concentration of 1.5 mol L-1 delivered a pore size of 99 µm and a porosity of 46.9 %. The solid oxide fuel cell based on Ni-YSZ anode possessed a maximum power density of 253 mW cm-2 and a ohmic resistance of 0.16 Ω cm2 at 800 °C. The pore diameter and porosity of the ceramic supports could be adjusted to expected values by further optimization of the parameters. The self-organization method will become a feasible and valid operation to produce anode supports with appropriate morphologies.
Keywords: Solid oxide fuel cells; Self-organization method; Straight open pores; Ni-based anodes
1. Introduction
Solid oxide fuel cells (SOFCs) possess lots of advantages such as low environmental pollution, high power transformation efficiency and fuel flexibility [1]. As a mixed conductor of ionic and electronic,
*Corresponding author. E-mail address: [email protected] (Yanting Tian).
1
porous anodes are one of the key components responsible for the electrochemical performance of SOFCs. Anodes should have superior catalytic activity to promote the oxidation reactions of the fuels and excellent electron mobility for electron transport. Appropriate pore morphology and sufficient porosity are also necessary to facilitate the gas diffusion during the cell operation process. Ni/yttria-stabilized zirconia (YSZ) cermet is the most common material for SOFC anodes based on its high electrocatalytic activity towards the oxidation of fuels [2]. Different kinds of pore-formers such as flour [3-4], starch [5-6], carbon microsphere [7-8] and graphite [9] are normally used to prepare porous Ni/YSZ substrates. Pore structures with various morphologies will be produced after volatilization of pore-formers by heating process [10]. However, the pore size is generally smaller than 10 µm [11]. Larger pores are difficult to form by adding pore-formers into the electrode substrates. Besides, the gas diffusion channels are tortuous and randomly distributed, which prevents the mass transfer process in the anodes. According to Knudsen [12], a cylindrical structure could significantly reduce the gas diffusion resistance. Paper-fibers were used as pore-formers for Ni/YSZ anodes by Pan et al. [13], cylindrical pores with the diameter and length of 1~5 µm and 20~100 µm were obtained. The orientation of the cylindrical pores was perpendicular to the pressure exerted on the anodes as well as the gas transport direction. After further adjusting the orientation of the paper-fibers, Ni/YSZ anode substrates with cylindrical pores parallel to the gas diffusion direction were developed [14]. Cylindrical structure is favorable for gas transport but paper-fibers are still insufficient on the realization of connected gas diffusion channels in thick anode supports. Recently, phase-inversion method has been exploited for preparation of Ni/YSZ anode supports for SOFCs [15-17]. Finger-like macro-porous channels are formed in the planar anodes to decrease the gas
2
diffusion resistance. Freeze-casting method has also been employed to prepare porous electrodes with graded directional channels [18-20]. Tree-like structures with pore size varies from 5 µm to 100 µm could be obtained [21]. The low tortuosity of the directional graded channels improves the gas diffusion rate and lowers the electrode polarization of the SOFCs. Both finger-like and tree-like porous structures facilitate the gas diffusion and mass transport in the electrodes. Self-organization process was originally adopted for preparation of honeycomb-like ceramic membranes with straight open pores in the micrometer range [22-24]. The diameter of the pores and porosity of the ceramics could be effectively controlled by the content of solid loadings. In the present work, Ni-based anode supports with well-oriented honeycomb pores were successfully prepared by self-organization method for the first time. The electrochemical performances and impedance spectroscopy analysis of the Ni-based anode supported single cells were investigated systematically.
2. Experimental
2.1 Preparation and characterization of YSZ substrates YSZ ceramics with straight open pores were prepared by alginate self-organization process [23-24]. Sodium alginate (NaAlg, Sinopharm Chemical Reagent Co., Ltd, China) was dissolved in deionized water at a concentration of 1.5 % to form sodium alginate sol. YSZ powders (8YSZ, Ningbo SOFCMAN Energy Technology Co., Ltd, China) were added into the sodium alginate sol with various solid loadings (5, 10 and 15 wt%) and the mixtures were ball-milled for 6 h, followed by degassing in a vacuum-assisted apparatus. The prepared slurries were then poured into a beaker, calcium chloride (Sinopharm Chemical Reagent Co., Ltd, China) solutions were sprayed onto the surface of the YSZ-alginate slurries with varying concentrations of 1, 1.5 and 2 mol L-1. Diffusion-controlled gelation of the mixed slurries was
3
allowed for 36 h to form wet gels with unidirectional oriented capillaries. After that, the top and bottom layers of the wet gels were cut away and the rest part of the disks were washed in the deionized water to remove the impurity elements. The gel disks were then immersed into gluconolactone (Aladdin Reagent Co., Ltd, China) solutions and tertiary butanol (TBA, Sinopharm Chemical Reagent Co., Ltd, China) solutions successively to densify the gels and to avoid crack formation during drying process. After that, the samples were dried at room temperature and sintered at 1400 °C for 4 h to obtain the final porous YSZ ceramic. The phase composition of the YSZ ceramic was examined using X-ray diffraction (XRD, DX-2700, Haoyuan). The microstructure and morphology of various samples were examined by a scanning electron microscopy (SEM, JSM-7100F, JEOL). The open porosity of the porous YSZ ceramics after sintering at 1400 °C was measured by Archimedes principle. 2.2 Fabrication and characterization of Ni-based anode supported SOFCs Slurry spin coating method was used to prepare the YSZ electrolyte membranes [25-26]. Two layers of YSZ slurry with a YSZ to organic vehicle weight ratio of 50:50 were spin coated on the porous YSZ supports, and then there layers of slurry with a weight ratio of 30:70, followed by sintering at 1400 °C for 4 h to obtain a dense electrolyte membrane. The La0.7Sr0.3MnO3 (LSM)-YSZ composite cathode was then coated onto the YSZ membrane surface and sintered at 1100 °C for 2 h. The active cathode area was 0.12 cm2. A Ni(NO3)2 solution of 2 mol L-1 was infiltrated into the straight YSZ pores by capillary action and then heated at 400 °C. The infiltration and heating process was repeated for 10 times and the impregnated YSZ supports were finally fired at 700 °C for 1 h.
4
Porous anode supports composed of NiO and YSZ composites (weight ratio of 50:50) were also prepared. After unidirectional oriented channels formed, the top layer of the wet gel was retained instead of resection. The wet gel was then dried and pre-sintered at 1000 °C. Three layers of YSZ slurry with a YSZ to organic vehicle weight ratio of 30:70 were spin coated on the top layer of the NiO-YSZ supports, followed by co-sintering at 1400 °C for 4 h. The cathode was the same with the former cells. To distinguish the two types of Ni-based anodes, the former one was described as Ni-impregnated YSZ anode and the latter one was Ni-YSZ anode. Single cells were tested using the four-terminal method at the operating temperature from 650 °C to 800 °C. The cell temperature was controlled by a digital temperature controller (LU-962UF J8000A1, Anthone Electronics Co. Ltd, China) with a K-type thermocouple located beside the cell. The surfaces of the electrodes were covered by silver conductive adhesive (DAD-87, Shanghai Synthetic Resin Institute, China) for collection of electrons. Quartz tubes were used to fix the single cells and the cell devices were sealed by silver paste. The electrochemical performance of the cells were measured using an electrochemical system (CHI660E, Shanghai CH Instrument Co. Ltd, China). Dry hydrogen at a flow rate of 50 ml min–1 diluted with 100 ml min–1 Ar was used as fuel and ambient air was used as oxidant during the testing process. The purity of both the hydrogen and argon was 99.999%. The frequency ranged from 91 kHz to 0.1 Hz was adopted for impedance spectra measurement at an ac voltage of 10 mV.
3. Results and Discussion
3.1. Microstructure and characterization of porous YSZ skeletons During the self-organization process, ordered capillaries formed through Ca2+ diffusion in the YSZ-alginate slurry. The optical micrograph of the wet gel in top view is shown in Fig. 1. The sample
5
exhibited a honeycomb-like pore structure with irregular round pores uniformly distributed on the surface. Fig. 2 shows the cross-sectional SEM micrograph of YSZ ceramic skeleton after sintering at 1400 °C. Highly ordered straight open pores with an average pore size of 100 µm were unidirectional oriented along the diffusion direction of Ca2+. The thickness of the pore walls was 20 µm. The phase composition of the YSZ skeletons was analyzed by X-ray diffraction spectrum, as shown in Fig. 3. After sintering at 1400 °C, the pattern of YSZ skeletons was in conformity with the cubic phase of YSZ [27]. No other impurity diffraction peaks were observed. According to Xue et al. [24], almost all the impurity elements were removed by washing process. A small amount of Ca residual (0.0867 %, measured by X-ray fluorescence method, XRF) was transformed to CaO at high temperature, which was helpful for the YSZ sintering [23]. Fig. 4 shows the surface images of YSZ ceramic skeletons with solid loadings varied from 5 wt% to 15 wt% while keeping the concentration of CaCl2 solution at 1 mol L-1. Uniformly distributed pores with diameters of several tens of micrometers can be observed from all the samples. As shown in Fig. 4a, the substrate with lower solid loading showed larger pores and thinner pore walls. As the solid loading increased, the pore size decreased obviously. The average diameters (D) of the pores with solid loading of 5, 10 and 15 wt% were approximately 148, 132 and 92 µm, respectively. Increasing the YSZ solid loading, however, resulted in an increased thickness of the pore walls. The pore shape of the ceramics changed from irregular round of 5 wt% solid loading to regular round of 10 wt% solid loading. When the solid loading increased to 15 wt%, more YSZ powders were accumulated within the capillary walls, making the thickness of the pore walls obviously increased. The effect of CaCl2 concentration on the morphology of YSZ skeletons is shown in Fig. 5, while the
6
YSZ loading was kept at 10 wt%. There were distinct differences among the morphologies of the YSZ substrates. The average diameter of the pores decreased from 132 µm to 88.3 µm when CaCl2 concentration increased from 1 mol L-1 to 2 mol L-1. The number of Ca2+ reacted with alginate in per unit area increased with increasing CaCl2 concentration, which resulted in an increased number of channels and a decreased pore size. The thickness of the pore walls, however, was not changed noticeably with the same solid loading. The solid loading is the main regulation parameter for the pore diameter, porosity and thus the mechanical strength of the porous ceramics. With the increasing YSZ solid loading, the particles deposited on the pore walls increased. Fig. 6 shows the variations of pore size and porosity with different solid loadings and CaCl2 concentrations. As can be seen, both the pore size and porosity decreased with increasing solid loading. The porosities of the YSZ substrates at the CaCl2 concentration of 1.5 mol L-1 were 55.1, 46.9 and 37.2 % with the solid loading of 5, 10 and 15 wt%, respectively. The substrate with higher solid loading has lower porosity and thicker pore walls which could ensure the mechanical strength of the ceramic. The concentration of CaCl2 solution is another regulatory factor for the formation of ordered capillaries. Higher CaCl2 concentration resulted in reduced pore size, while the thickness of the pore walls did not change noticeably. Therefore, the porosity decreased with the increasing CaCl2 concentration. It is important to note that an appropriate porosity and good mechanical strength are necessary for the SOFC anodes. Consequently, the YSZ skeleton with solid loading of 10 wt% and CaCl2 concentration of 1.5 mol L-1 was selected as the anode support for electrochemical performance evaluation, which possessed a pore size and wall thickness of 99 µm and 35 µm, and a porosity of 46.9 %. 3.2 Cell performance based on Ni-impregnated YSZ substrates
7
The cross-sectional SEM image of the YSZ electrolyte membrane is presented in Fig. 7a. The electrolyte membrane was prepared by spin coating method with two layers of higher solid-phase content slurry to block most of the big pores and then three layers of lower solid-phase content slurry to obtain dense electrolyte films. As can be seen, the electrolyte layer was adhered well to the porous supports with a thickness of approximately 110 µm. No visible cracks and holes were observed. This also provides a effective way to prepare dense membranes on honeycomb ceramics. The YSZ skeleton with straight open pores was infiltrated by Ni(NO3)2 solution to form the electron transport layer. Fig. 7b shows the microstructure of impregnated NiO after sintering at 700 °C. Nanoparticles with the size of 50~100 nm were uniformly distributed on the pore walls. The agglomeration phenomenon of NiO particles was not observed. The electrochemical performances of the Ni-impregnated YSZ supported cell were measured from 650 to 800 °C. The anode was reduced in H2 atmosphere at a flow rate of 50 ml min-1 at 650 °C. Ar of 100 ml min-1 was used as dilute gas. The open-circuit voltage (OCV) above 1 V was achieved in 2 minutes and then remained at a nearly stable value (Fig. S1), which indicated a rapid reduction process from NiO to Ni. The straight open pores with diminished tortuosity enables fast diffusion of reaction gas into the inner part of the anodes, therefore, accelerates the reduction rate of the anodes. As shown in Fig. 8a, the OCV of the cell was about 1.08 V throughout the testing process. The maximum power densities (MPDs) were 146, 102, 62 and 30 mW cm-2 at temperatures of 800, 750, 700 and 650 °C, confirming the feasibility of this new designed cell structure. The cell performance is comparable to that of the electrolyte-supported
SOFCs
[28],
which
has
phase-inversion-assisted laser ablation technique.
8
similar
YSZ
porous
structure
prepared
by
Typical ac impedance spectra of the cell measured under open circuit condition are shown in Fig. 8b. Fig. 8c shows the equivalent circuit. The inductance L is attributed to the measurement apparatus and silver wires. The intercept of high-frequency arc with the real axis represents the ohmic resistance (Ro, at 800 °C for example), which is mainly determined by the electrolyte; whereas the (R1, CPE1), (R2, CPE2), and (R3, CPE3) correspond to the intercept distance between the high-frequency arc and low-frequency arc with the real axis, which represents the total interfacial polarization resistances (Rp), including the charge transfer polarization and the concentration polarization of both the electrodes. As shown in Fig. 8b, the Ro significantly decreased with the increasing temperature, typically from 1.68 Ω cm2 at 650 °C to 0.45 Ω cm2 at 800 °C. Table 1 lists the ohmic resistances of electrolyte-supported SOFCs with various electrolyte thicknesses and operating conditions. The ohmic resistances in this study were closed to that of the electrolyte-supported SOFCs, mainly due to its thick electrolyte layer and the long ion migration paths in the pore walls which actually played as a part of electrolyte. The larger Rp indicated that the cell performance was mainly dominated by the electrode polarization. The SEM image of Ni nanoparticles after reduction is displayed in Fig. S2. Serious agglomeration of Ni particles were observed, which resulted in a decreased active triple-phase boundary (TPB) region and an increased cell polarization resistance. Based on the experimental results of Jiao et al. [33], the morphology of Ni particles was apparently influenced by the reduction temperature, which led to different anode performances. A higher reduction temperature could enhance the anode/electrolyte interface by preventing the nickel sintering [34]. The reduction process needs to be evaluated to decrease the electrode polarization resistance in this study. Besides, the impregnated Ni particles were only existed on the surface of the straight pores, the pore walls were almost dense with small amount of pinholes that were
9
disconnected for Ni particles to access in (Fig. S3). As a result, a loss of TPB region is exist. Although the cell based on the as-prepared anode showed unsatisfying electrochemical performance, it is expected to be improved by many ways. The first effective way is to add a transition layer between the porous anode and dense electrolyte. A slurry composed of NiO and YSZ could be coated on the anode surface to block the big pores, instead of the first two YSZ layers, which is desired for further deposition of a thin YSZ electrolyte layer on it. Therefore, the thickness of the YSZ membrane could be notably reduced and the ohmic resistance of the cell is expected to be decreased. Moreover, it is reported that a multi-layer anode structure with porosity gradient increased the power density of the single cell from 76 to 101 mW cm−2 at 600 °C in humidified hydrogen [35]. Moon et al. [36] also reported that the maximum power density increased from 300 to 550 mW cm-2 at 700 °C by inserting a functional layer between the electrolyte and anode support. Furthermore, to improve the reaction region at the anode and electrolyte interface, one may also consider increasing the porosity of the pore walls by adding some pore formers into the YSZ solid loading. The Ni contents and TPB region in the substrates could be improved as discussed above. 3.3 Cell performance based on Ni-YSZ substrates The formation process of capillary gel is shown in Fig. 9a. When the calcium chloride solution was sprayed onto the surface of the sodium alginate sol, a reaction layer formed immediately at the interface of the two sols. This so-called primary membrane permitted unidirectional diffusion of Ca2+ into the sodium alginate solution, which led to the formation of ordered capillaries. To simplify the preparation process of electrolyte films on the straight porous skeletons, the primary membrane was retained as a transition layer between the Ni-YSZ support and the YSZ electrolyte. The capillaries located
10
near the top membrane were sloped, which was verified in Fig. 9b. The thickness of the primary membrane after sintering at 1400 °C was about 25 µm. Fig. 9c displays the top surface of the primary membrane. Depressions were observed, which were the “egg-box” structures formed by the reaction of sodium alginate and metal ions. This relatively compact structure of the skin layer provides great convenience for subsequent deposition of a dense and thin YSZ electrolyte film on it. From the cross-sectional SEM image of the YSZ electrolyte film (Fig. 10a), it can be seen that a good adherence between the YSZ film and the top membrane was obtained. The thickness of the YSZ film was estimated to be 25 µm, which was significantly reduced compared with that of the Ni-impregnated YSZ supported cell. The surface image also showed a dense electrolyte without pores or cracks (Fig. 10b). The microstructure of Ni-YSZ anode after cell testing is shown in Fig. 10c. The reduction process from NiO to Ni introduced some tiny pores by the volatilization of oxygen. The electrochemical performance of the Ni-YSZ anode supported cell is presented in Fig. 11. As expected, the maximum current density remarkably increased from 530 mA cm-2 of the Ni-impregnated cell to 967 mA cm-2 of the Ni-YSZ supported cell at 800 °C. A maximum power density of 253 mW cm-2 was obtained. However, a serious drop was observed at high current density, which severely restricted the cell performance, whereas such a drop was not observed in the Ni-impregnated cell. The curvature is probably caused by concentration polarization of the Ni-YSZ anodes. Ni-impregnated YSZ supports contain straight open pores and pure YSZ walls, the cell structure is more like a electrolyte supported cell with Ni nanoparticles attached on the YSZ layer, making the reaction sites easily accessible to the reactants and thereby reducing the concentration polarization. In contrast, most of the reaction sites inside the walls of the Ni-YSZ supports are not easily access to the reactants, resulting in a serious concentration
11
polarization. The small pores produced by NiO reduction are not sufficient to enable rapid gas transfer within the pore walls. A porous Ni-YSZ wall would be helpful for enhancing the cell performance. The impedance spectra of the cell based on Ni-YSZ substrates were also measured under open circuit condition, as shown in Fig. 11b. Apparently, the area specific resistances of the present cell were much lower than those of the Ni-impregnated cell. At 800 °C, the Ro and Rp values were 0.16 and 0.91 Ω cm2, the lower ohmic resistance presents direct evidence of its thinner electrolyte layer. The primary membrane of the Ni-YSZ substrates is beneficial to the cell preparation as well as the cell performance. In the present study, we have demonstrated that Ni-based anodes with straight open pores can be prepared using alginate self-organization process. A distinctive feature of the anode substrates is that the diameter of the straight open pores is depend on the content of solid loading and the concentration of metal ions. Varying diameters could also be obtained by changing the cross-linking cation with different ionic radius. According to Ahmed et al. [22], capillary ceramics showed a decreased capillary diameter and a increased porosity by changing copper for calcium. We have also prepared a YSZ ceramic using copper chloride of 1 mol/L, the average pore diameter of 35 µm was obtained (Fig. S4). The pore size and porosity of the ceramic skeletons could be adjusted to desired values after further optimizing the parameters of the self-organization reaction. The self-organization method will become a feasible and valid operation to produce porous anodes with appropriate structures. 4. Conclusion Ni-based anodes with straight open pores were fabricated by a self-organization method. The diameter of the pores decreased with increasing solid loading and CaCl2 concentration. Ceramic supports with solid loading of 10 wt% and CaCl2 concentration of 1.5 mol L-1 possessed a pore size of 99 µm and a
12
porosity of 46.9 %. The cell based on Ni-impregnated YSZ supportes produced a maximum power density of 146 mW cm-2 and a ohmic resistance of 0.45 Ω cm2 at 800 °C. The Ni-YSZ supported cell with reserved primary membrane obtained a higher power density of 253 mW cm-2 and a lower ohmic resistance of 0.16 Ω cm2. The electrochemical performance of the cells are expected to be improved by further optimization of the cell structure.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Nos. 51602213, 11604236 and 61575139).
References
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Figure Captions Fig. 1. Optical micrograph in top view of wet gel.
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Fig. 2. Cross sectional SEM image of YSZ skeleton with 5 wt% solid loading and 2 mol L-1 CaCl2 solution. Fig. 3. XRD pattern of YSZ skeleton after sintering at 1400 °C for 4h. Fig. 4. Surface images of the YSZ skeletons with different solid loading: (a) 5 wt%, (b) 10 wt% and (c) 15 wt%. (d) Cross section of YSZ skeleton with 15 wt% solid loading. Fig. 5. Surface images of the YSZ skeletons with different CaCl2 concentrations: (a) 1 mol L-1, (b) 1.5 mol L-1 and (c) 2 mol L-1. (d) Cross section of the YSZ skeleton with 1.5 mol L-1 CaCl2. Fig. 6. Variations of pore size (a) and porosity (b) with different solid loadings and CaCl2 concentrations. Fig. 7. (a) Cross-sectional SEM micrograph of the electrolyte membrane. (b) Microstructure of impregnated NiO particles after sintering at 700 °C. Fig. 8. (a) I-V and I-P curves and (b) EIS of Ni-impregnated YSZ anode supported cells at different temperatures. (c) The equivalent circuit. Fig. 9. (a) The scheme of alginate-based capillary gel formation. (b) Cross-sectional SEM micrograph of the Ni-YSZ composite support and the primary membrane. (c) Surface image of the primary membrane. Fig. 10. (a) Cross-sectional SEM micrograph of the YSZ membrane, (b) surface image of the YSZ membrane and (c) microstructure of Ni-YSZ anode after test. Fig. 11. (a) I-V and I-P curves and (b) EIS of Ni-YSZ supported cells at different temperatures.
Table Captions Table 1 Comparison of the Ro for previously-reported YSZ-supported cells and the cell in this work.
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Table 1 Comparison of the Ro for previously-reported YSZ-supported cells and the cell in this work. SOFC structure
Electrolyte thickness (µm)
Temp. (°C)
Ro (Ω cm2)
Ni-YSZ/YSZ/Pt [29]
500 (Dry pressing)
750
1.2
LSCF-SDC/SDC/YSZ/SDC/BSCF-SDC
78
850
0.19
[30]
(Dry pressing/heating/quenching/calcining) 200
0.33
(Dry pressing/calcining/polishing) NiO-GDC/5YSZ/LSM-5YSZ [31]
70
830
0.34
800
0.42
sintering method)
750
0.49
500 (3D printing)
850
0.5
(tape-casting,
Ag-GDC/YSZ/Ag-GDC [32]
multilayer-lamination
and
500 (Dry pressing) This work
110
0.64 800
0.45
200 µm
Fig. 1. Optical micrograph in top view of wet gel.
(b)
(a)
YSZ films
(c) Ni YSZ
Fig. 10. (a) Cross-sectional SEM micrograph of the YSZ membrane, (b) surface image of the YSZ membrane and (c) microstructure of Ni-YSZ anode after test.
Diffusion direction
Fig. 2. Cross sectional SEM image of YSZ skeleton with 5 wt% solid loading and 2 mol L-1 CaCl2 solution.
(a) 5 wt% YSZ, D = 148 µm
(b) 10 wt% YSZ, D = 132 µm
(c) 15 wt% YSZ, D = 92 µm
(d) 1 mol L-1 CaCl2, 15 wt% YSZ
Fig. 4. Surface images of the YSZ skeletons with different solid loading: (a) 5 wt%, (b) 10 wt% and (c) 15 wt%. (d) Cross section of YSZ skeleton with 15 wt% solid loading.
(a) 1 mol L-1 CaCl2, D = 132 µm
(c) 2 mol L-1 CaCl2, D = 88.3 µm
(b) 1.5 mol L-1 CaCl2, D = 99 µm
(d) 1.5 mol L-1 CaCl2, 10 wt% YSZ
Fig. 5. Surface images of the YSZ skeletons with different CaCl2 concentrations: (a) 1 mol L-1, (b) 1.5 mol L-1 and (c) 2 mol L-1. (d) Cross section of the YSZ skeleton with 1.5 mol L-1 CaCl2.
(b)
(a) Electrolyte
Fig. 7. (a) Cross-sectional SEM micrograph of the electrolyte membrane. (b) Microstructure of impregnated NiO particles after sintering at 700 °C.
(c)
Fig. 8. (c) The equivalent circuit.
(a) (b)
(c)
Primary membrane
Fig. 9. (a) The scheme of alginate-based capillary gel formation. (b) Cross-sectional SEM micrograph of the Ni-YSZ composite support and the primary membrane. (c) Surface image of the primary membrane.
Highlights (1) Ni-based anode supports with straight open pores were prepared using self-organization process. (2) The pore size and the porosity of the ceramic supports could be regulated by the content of solid loadings and the concentration of metal ions. (3) The YSZ skeleton with solid loading of 10 wt% and CaCl2 concentration of 1.5 mol L-1 delivered a pore size of 99 µm and a porosity of 46.9 %. (4) The cell based on Ni-YSZ anode possessed a maximum power density of 253 mW cm-2 and a ohmic resistance of 0.16 Ω cm2 at 800 °C.
We declare that we have no conflict of interest.