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Efficient conversion of methane into power via microchanneled solid oxide fuel cells
Journal of Power Sources 453 (2020) 227848
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Efficient conversion of methane into power via microchanneled solid oxide fuel cells Jingjing Wang a, Dongjie Fan a, Libo Yu a, Tao Wei a, Xun Hu a, Zhengmao Ye a, **, Zhi Wang a, Yi Wang b, Cuncheng Li c, Jianfeng Yao d, Dehua Dong a, * a
School of Material Science and Engineering, University of Jinan, Jinan, 250022, PR China School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China School of Chemistry and Chemical Engineering, University of Jinan, Jinan, 250022, PR China d College of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization of Agricultural and Forest Biomass, Nanjing Forestry University, Nanjing, 210037, Jiangsu, PR China b c
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
� Microchanneled SOFCs were employed to efficiently convert methane into power. � Conversion efficiency is defined to compromise power density and fuel utilization. � Channels structure provides gas diffu sion highway to/from anode functional layer. � Microchanneled anodes achieved the record high methane conversion efficiency.
A R T I C L E I N F O
A B S T R A C T
Keywords: Methane-fueled SOFCs Methane conversion efficiency Gas diffusion Anode supports Microchannel structure
Methane-fueled solid oxide fuel cells (SOFCs) are promising to achieve high energy conversion efficiency while no study focuses on the conversion efficiency of methane into power, which is greatly restrained by gas diffusion within anode supports. This study employs microchanneled anode supports to provide fast gas diffusion pathway. To confirm the advantage, the anodes with half-channels and without channels are also used for comparison. The microchannel structure increases the maximum power density up to 2.5 times because of diminishing or elim inating concentration polarization within anode supports and improving catalyst coating over anode internal surface. As a compromise of fuel utilization and power output, methane conversion efficiency is defined as power output per mol methane input in feeding gas to compare with the reported results, and the microchanneled SOFCs achieve the record high methane conversion efficiency.
1. Introduction Fuel cells can directly convert fuels into electricity and achieve the
higher energy conversion efficiency compared to conventional coal or gas-fired power generation [1]. The direct conversion is performed on proton exchange membrane (PEM) fuel cells at low temperatures
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Ye), [email protected] (D. Dong). https://doi.org/10.1016/j.jpowsour.2020.227848 Received 19 November 2019; Received in revised form 3 February 2020; Accepted 4 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
J. Wang et al.
Journal of Power Sources 453 (2020) 227848
Fig. 1. Facilitated gas diffusion within microchanneled anode supports of methane-fueled SOFCs.
Fig. 2. Flowchart of SOFC preparation process.
(80–250 C) or solid oxide fuel cells (SOFCs) at high temperatures (600–1000 � C). PEM fuel cells only can be operated over hydrogen fuel, which is currently produced mainly via methane steam reforming. The hydrogen production process is energy-intensive, and hydrogen storage and transport are still challenging. In contrast, methane can be directly fed into SOFCs for power generation. In addition, high reaction kinetics can be achieved at high operation temperatures so that noble metals catalysts required in PEM fuel cells can be avoided in SOFCs. Methane is a predominant component of abundant gas resources, such as natural gas [2]. Therefore, SOFCs are promising for converting methane into power, and the conversion efficiency is practically important. Comparing with a lot of studies focusing on the development of anode materials and catalysts to achieve high and stable cell power output [3–5], there are a few studies investigating the fuel utilization of methane-fueled SOFCs [6]. Lin et al., reported a high power density of 1.27 W cm 2 with the feeding of nearly pure methane while fuel utili zation was only 12% [7]. The fuel utilization can be improved via reducing methane concentration in feeding gas. For example, fuel uti lization was increased from 8% to 93% when methane concentration was decreased from 47% to 3%, but cell power density was decreased by one-third [8]. Accordingly, a compromise of sufficient power density and acceptable fuel utilization is required to achieve the high conversion efficiencies of methane into power, and the comprise is due to gas diffusion within anodes [9]. To increase fuel utilization, the unreacted fuel from anodes was reused via recirculation [10,11]. However, the decrease of fuel concentration in the feeding gas will cause the con centration polarization. Therefore, gas diffusion within anode supports greatly affects the conversion efficiency. Ni-based anode supports have been widely used in methane-fueled SOFCs owing to low cost and high catalytic activity, and methane need diffuse through anode supports to reach reaction zone near the anode/electrolyte interface. Conventional anode supports have tortuous pore channels, which limits the gas diffusion and causes the concen tration polarization [12,13]. Our group has developed microchanneled ceramic membranes for oxygen separation and solid oxide cells [14–16]. The channel structure facilitated gas diffusion within supporting elec trodes. Chen et al., established a numerical model for gas transport through the channels and found it is effective in improving the gas transport and the SOFC performance [17]. In this study, as shown in Fig. 1, microchanneled anode supports were employed to facilitate gas diffusion and diminish or eliminate concentration polarization. To the best of our knowledge, there is no study investigating the conversion efficiency of methane-fueled SOFCs. Methane conversion (converted methane/input methane) is generally investigated, but it cannot effectively index the efficiency of converting �
methane into power due to the variation of oxidation products. To compromise the power density and methane utilization, the methane conversion efficiency is defined as power output per mol methane input in the feeding gas. The conversion efficiency over the microchanneled anodes has been investigated, and the improvement of conversion effi ciency by the channel structure was confirmed by comparing with other structure, including conventional tortuous pore structure. The highest conversion efficiency over the microchanneled SOFCs has been achieved after comparing with the results reported in literature. 2. Material and methods 2.1. SOFC preparation The preparation process of SOFCs is presented in Fig. 2. Ceramic powders were provided by Fuel Cell Materials, Ohio, USA. Anode sup ports were prepared by a phase-inversion process. 17.7 g of poly ethersulfone (PESF, Radel-A300) was dissolved in 100 g of N-Methyl-2pyrrolidone (NMP, 99%, Shanghai Macklin Biochemical Co., Ltd, China) by magnetic stirring. 28.32 g of the above solution, ceramic powders (36.99 g of NiO (NiO–F) and 26.42 g of Gd0.1Ce0.9O2 (GDC-TC)) and 0.43 g of polyvinylpyrrolidone (PVP, MW ¼ 130000, Shanghai Dibo Biotechnology, China) were mixed for 48 h by a planetary ball-milling machine (Hefei Ke Jing Materials Technology, China). The prepared slurry was converted to a disc membrane in an aluminum mould by mesh-templating phase-inversion using water as a coagulant, and the mesh has an aperture size of 70 μm. The obtained anode green body was dried in an oven at 60 � C overnight, followed by pre-sintering at 1100 � C for 2 h to obtain NiO-GDC anode supports. To form an electrolyte slurry with a solid loading of 10%, GDC (GDCTC) powder was dispersed in ethanol with 3 wt% of PVP as a dispersant by ball-milling for 24 h. The GDC slurry was coated on the non-channel side of the pre-sintered anode supports by dip-coating. NiO-GDC/GDC half cells were formed after co-sintering at 1380 � C for 5 h. Finally, to prepare full cells, 30 wt% of GDC and 70 wt% of Ba0.5Sr0.5Co0.8Fe0.2O3 (Praxair Surface Technologies) were dispersed in ethanol by ballmilling, and the slurry with the solid loading of 10% was deposited on the GDC electrolyte by spray-coating to form a BSCF/GDC cathode layer after sintering at 950 � C for 2 h. 2.2. Catalyst preparation The catalyst over the NiO-GDC anode scaffold was prepared by a conventional infiltration process through microchannels. A commercial colloid (CeO2 (AC), Naycol Nano Technology, Inc.) with CeO2 2
J. Wang et al.
Journal of Power Sources 453 (2020) 227848
Fig. 3. SEM images of anode supports with different microstructure: (a), microchannels; (b), half-channels; (c), no channels. Scale bars are 300 μm.
2.3. Characterization and test of SOFCs The porosity and pore size distribution of the Ni-GDC anodes were measured by a mercury intrusion porosimetry (AutoPore IV 9500, Micrometrics). The cell feed with methane was tested in a home-made test device. Initially, hydrogen was introduced to reduce NiO and then the feeding gas was switched to CH4/Ar mixture. The cell performance was tested by an electrochemical workstation (Reference 3000, Gamry). Electrochemical impedance spectra were recorded under the OCV within the frequencies between 0.1 Hz and 0.1 MHz. Cell microstructure was examined using a scanning electron microscopy (SEM, Zeiss Neon 40EsB FIBSEM). Exhaust gas was analyzed by gas chromatograph (Shi madzu GC-2014) to further study the effect of pore structure on methane utilization. 3. Results and discussion 3.1. Anode support microstructure As shown in Fig. 3a, numerous microchannels parallel cross anode support with one end open on anode surface and the other end termi nated by a porous layer on the other side of anode [19]. The porous layer acts as an anode functional layer for performing methane oxidation. The microchannel structure can provide fast pathway for methane diffusion from anode surface to the functional layer and also oxidation products diffusion to anode surface [16]. Additionally, these channels facilitate catalyst precursor delivery to the reaction zone [15]. To confirm the structure effect, anode supports with less and half-channels and without channels were also prepared for comparison. The microstructure of the two anode supports is shown in Figs. 3b and 3c.
Fig. 4. Pore size distribution of Ni/GDC anode supports with different microstructure.
nanoparticles suspended in acetic acid was used as a catalyst precursor. The catalyst precursor solutions were coated on the anode internal surface, followed by vacuum to remove trapped bubbles. Then, it was converted into catalyst coating after calcination at 750 � C for 1 h. The CeO2 catalyst loading of 1.75 wt% was used to form stable nanocatalyst network [18].
Fig. 5. Performance of the cells with different structure and a feeding of 30% CH4/Ar: a, power-current-voltage curves; b, electrochemical impedance spectra. 3
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Journal of Power Sources 453 (2020) 227848
Fig. 6. Effect of methane concentration on power output of the cells with different anodes: (a) microchannels; (b) half-channels; (c) no channels, and methane conversion efficiency and maximum power density (d).
212 mW cm 2 for the cells supported on anodes with microchannels, with half-channels and without channels, respectively. Accordingly, the channel structure makes the maximum power density increase up to 2.5 times. From current-voltage curves of the cells, channel structure diminished concentration polarization during the methane conversion through providing fast pathway for gas diffusion, and therefore increased power density [17]. In addition, according to cell resistances shown in Fig. 5b, the channel structure also decreases cell polarization resistances, which is related to catalyst preparation. Through these channels, catalyst can be effective delivered to the reaction zone, which has been demonstrated in previous study [15]. Therefore, the channel structure not only accelerates gas diffusion, but also improves catalyst preparation, resulting in the high power output.
3.2. Porosity and pore size distribution The pore structure of the anode supports was measured by mercury porosimetry. The three anode supports have a similar porosity: 47.5%, 45.5% and 44.8% for the anode supports with microchannels, with halfchannels and without channels, respectively. Thereby, it is feasible to compare the effect of pore structure on anode properties. The pore size distributions are shown in Fig. 4. For the anode without channels, there is a single pore size range between 0.12 and 2.1 μm, with an average pore size of about 0.4 μm. In contrast, the other two anodes show dual pore size ranges, and the small pore size range is similar to that of the anode without channels, which refers to the pores within channel wall and the porous layer. The anode with microchannels shows the smaller channel diameters compared with that with half-channels, which is consistent with SEM images in Fig. 3.
3.4. Methane conversion efficiency 3.4.1. Effect of methane concentration As concentration polarization readily occurs within supporting an odes, methane concentration in feeding gas affects power output and hence the methane conversion efficiency. To investigate the effect, cell performances were tested at different methane concentrations. As shown in Fig. 6, the concentration polarization was diminished as methane concentration was increased. The minimum methane concen trations to eliminate the concentration polarization are about 35%, 60% and 80% for the anodes with microchannels, with half-channels, and without channels respectively, and the corresponding maximum power densities are 590, 453, 330 mW cm 2. Accordingly, without
3.3. Fuel cell performance To investigate the effect of the channel structure on gas diffusion, a mixture of 30% CH4/Ar was fed into the anodes of SOFCs for power generation. Operating temperature has a complicate effect on the per formance of the cells with GDC electrolyte due to its electronic con ductivity. Increasing operation temperature decreases cell resistances and also open circuit voltage, resulting in limited power density increase with temperature when the temperature is above 600 � C (See Fig. S1). Accordingly, the operating temperature in this study was set at 600 � C. As shown in Fig. 5a, the maximum power densities are 545, 393 and 4
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Journal of Power Sources 453 (2020) 227848
Table 1 Comparison of conversion efficiency of methane into power by SOFCs. Anode/electrolyte
Anode catalyst
Temperature(� C)
Maximum power density(mW⋅cm 2)
CH4 conversion efficiency(Wh⋅mol
NiO-YSZ/YSZ NiO-YSZ/YSZ NiO-YSZ/YSZ, NiO-YSZ/YSZ NiO-YSZ/YSZ NiO-YSZ/YSZ NiO-SDC/SDC NiO–GDC/GDC NiO–GDC/GDC NiO–GDC/GDC
No catalyst Doped CeO2 Doped CeO2 CeO2 CeO2 Ag No catalyst No catalyst No catalyst CeO2
825 800 800 800 750 750 700 600 600 600
680 650 231 610 498 542 697 414 520 590
12.18 2.06 0.44 34.16 27.88 0.64 1.82 3.50 7.28 9.44
a b
1
)
References [2] [20] [21] This studya This studya [22] [23] [24] This studya This studyb
Data from Fig. S2. Data from Fig. 6.
concentration polarization, the cell with the microchanneled anode generated the highest maximum power density owing to the improved catalyst preparation. Methane conversion efficiency in this study is defined as power output per mol methane input. Fig. 6d shows the effect of methane concentration on the conversion efficiency and maximum power den sity, which should be considered practically in term of cell power output. The channel structure greatly increases methane conversion efficiency, and the cell with the microchanneled anode has a much higher methane conversion efficiency than the other two cells. For the anodes with half-channels and without channels, maximum power density was increased with methane concentration while methane conversion efficiency was decreased. In contrast, the anode with microchannels demonstrated a slight increase of methane conversion efficiency from the methane concentration of 10% to 30%, followed by a decrease as methane concentration was increased to 35%. Therefore, high methane conversion efficiency and power output can be achieved over the cell with the microchanneled anode. 3.4.2. Comparison of methane conversion efficiency To confirm the high methane conversion efficiency, it was compared with the reported results in Table 1. All cells selected for comparison are button or cone-shaped cells to ensure the same fuel flow pathway. For example, hollow fibre and tubular fuel cells have long flow path along anode surface while button cells have gas flow turned around. According to the definition of methane conversion efficiency (power output per mol methane input in the feeding gas), powder density, cell active area and methane flow rate are required for the calculation. The references are chosen according to the data availability. NiO-YSZ anode-supported cells were also prepared and tested for comparison (Fig. S2). The cells with the microchanneled anodes demonstrate superior methane con version efficiency compared with that reported in literature although maximum power densities are not outstanding. Previous studies used high concentrations of methane or pure methane to diminish concen tration polarization and achieve high power densities due to tortuous pore channel structure. The microchanneled anodes used in this study provides fast gas diffusion pathway and enables the operation without concentration polarization at low methane concentrations so that high methane utilization can be achieved. In addition, the channel structure improves the efficiency of catalyst preparation by infiltration [15], which reduces cell polarization resistance and hence increases power density. Therefore, the high methane conversion efficiencies have been achieved over the microchanneled SOFCs.
Fig. 7. Stability test of methane-fueled SOFCs with different anode microstructure.
microchannels, with half-channels and without channels, respectively. Previous studies showed the degradation rate of solid oxide cells increased with applied current density [25,26]. In the conversion, the extent of methane oxidation increased with the applied current, as evidenced by the decreased CO and H2 selectivities (See Fig. S3). Accordingly, discharge at high current densities is preferable to achieve high methane utilization. The discharge current densities of the stability tests were chosen at the peak power output. As shown in Fig. 7, the cell with microchanneled anode demonstrated more stable discharge voltage than the other two cells. The cell without channels degraded rapidly. The cell showed the larger initial polarization resistance compared with the other two cells due to the difference in catalyst coating (See Fig. S4), which might cause coke formation and hence the rapid increased polarization resistance within 24 h-operation. It in dicates the effective catalyst coating can prevent coking formation during the methane conversion, which has been confirmed in our pre vious studies [15,18]. 4. Conclusions SOFCs with microchanneled anode supports have demonstrated high efficiency in the conversion of methane into power. Compared with the cell without channels, the cell with channels show the increase of the maximum power density up to 2.5 times (from 212 to 545 mW cm 2). The microchanneled SOFCs demonstrated superior methane conversion efficiency over the reported results in literature. The microchannel structure of anode supports provides fast gas diffusion pathway to diminish or eliminate concentration polarization. In addition, the
3.5. Stability test The stability of the methane conversion is very important for prac tical application. To achieve high power output, the stability tests were conducted at the methane concentrations without obvious centration polarization, which are 30%, 60% and 80% for the cells with 5
J. Wang et al.
Journal of Power Sources 453 (2020) 227848
microchannel structure facilitates catalyst coating over anode scaffold to accelerate methane oxidation and restrain carbon deposition. Therefore, the microchannel structure of anode supports ensures the efficient and stable conversion of methane into power.
[3] Y. Chen, B. deGlee, Y. Tang, Z.Y. Wang, B.T. Zhao, Y.C. Wei, L. Zhang, S. Yoo, K. Pei, J.H. Kim, Y. Ding, P. Hu, F.F. Tao, M.L. Liu, Nat. Energy 3 (2018) 1042–1050. [4] B. Hua, N. Yan, M. Li, Y.Q. Zhang, Y.F. Sun, J. Li, T. Etsell, P. Sarkar, K. Chuang, J. L. Luo, Energy Environ. Sci. 9 (2016) 207–215. [5] Y. Choi, E.C. Brown, S.M. Haile, W. Jung, Nano Energy 23 (2016) 161–171. [6] D.M. Silva-Mosqueda, F. Elizalde-Blancas, D. Pumiglia, F. Santoni, C. BoiguesMunoz, S.J. McPhail, Appl. Energy 235 (2019) 625–640. [7] Y. Lin, Z. Zhan, J. Liu, S. Barnett, Solid State Ionics 176 (2005) 1827–1835. [8] K. Girona, J. Laurencin, J. Fouletier, F. Lefebvre-Joud, J. Power Sources 210 (2012) 381–391. [9] O. Aydin, T. Ochiai, H. Nakajima, T. Kitahara, K. Ito, Y. Ogura, J. Shimano, Int. J. Hydrogen Energy 43 (2018) 17420–17430. [10] D. Saebea, S. Authayanun, Y. Patcharavorachot, A. Arpornwichanop, Energy 94 (2016) 218–232. [11] K. Lee, S. Kang, K.-Y. Ahn, Appl. Energy 205 (2017) 822–833. [12] T. Wang, J. Wang, L. Yu, Z. Ye, X. Hu, G.E. Marnellos, D. Dong, J. Eur. Ceram. Soc. 38 (2018) 5051–5057. [13] H. Sumi, T. Yamaguchi, T. Suzuki, H. Shimada, K. Hamamoto, Y. Fujishiro, Electrochem. Commun. 49 (2014) 34–37. [14] X. Shao, D. Dong, G. Parkinson, C.-Z. Li, J. Mater. Chem. 2 (2014) 410–417. [15] D. Dong, S. Xu, X. Shao, L. Hucker, J. Marin, P. Thang, K. Xie, Z. Ye, P. Yang, L. Yu, G. Parkinson, C.-Z. Li, J. Mater. Chem. 5 (2017) 24098–24102. [16] D. Dong, X. Shao, X. Hu, K. Chen, K. Xie, L. Yu, Z. Ye, P. Yang, G. Parkinson, C.Z. Li, Int. J. Hydrogen Energy 41 (2016) 19829–19835. [17] B. Chen, H.R. Xu, M. Ni, Sci. Bull. 61 (2016) 1324–1332. [18] J. Wang, T. Wang, L. Yu, T. Wei, X. Hu, Z. Ye, Z. Wang, C.E. Buckley, J. Yao, G. E. Marnellos, D. Dong, J. Power Sources 412 (2019) 344–349. [19] X. Shao, D. Dong, G. Parkinson, C.-Z. Li, J. Mater. Chem. 1 (2013) 9641–9644. [20] Y. Chen, Y. Zhang, Y. Lin, Z. Yang, D. Su, M. Han, F. Chen, Nano Energy 10 (2014) 1–9. [21] J.M. Lee, Y.G. Kim, S.J. Lee, H.S. Kim, S.P. Yoon, S.W. Nam, S.D. Yoon, J.W. Yun, J. Appl. Electrochem. 44 (2014) 581–588. [22] X. Wu, X. Zhou, Y. Tian, X. Kong, J. Zhang, W. Zuo, X. Ye, K. Sun, Int. J. Hydrogen Energy 40 (2015) 16484–16493. [23] A. Ideris, E. Croiset, M. Pritzker, A. Amin, Int. J. Hydrogen Energy 42 (2017) 23118–23129. [24] J. Ding, J. Liu, Y. Feng, G. Yin, Int. J. Hydrogen Energy 36 (2011) 7649–7655. [25] M.Z. Khan, M.T. Mehran, R.-H. Song, S.-B. Lee, T.-H. Lim, Int. J. Hydrogen Energy 43 (2018) 12346–12357. [26] Y. Tao, S.D. Ebbesen, M.B. Mogensen, J. Power Sources 328 (2016) 452–462.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Prof. D. H. Dong acknowledges the startup funding provided by the University of Jinan. This work is supported by the National Natural Science Foundation of China (51872123). The study is a part of the projects of Natural Science Foundation of Shandong Province (ZR2017MEM022) and Shandong Province Key Research and Develop ment Program (2018GGX102037). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227848. References [1] A. Buonomano, F. Calise, M.D. d’Accadia, A. Palombo, M. Vicidomini, Appl. Energy 156 (2015) 32–85. [2] Y. Jiao, L. Zhang, W. An, W. Zhou, Y. Sha, Z. Shao, J. Bai, S.-D. Li, Energy 113 (2016) 432–443.
6
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Efficient conversion of methane into power via microchanneled solid oxide fuel cells Jingjing Wang a, Dongjie Fan a, Libo Yu a, Tao Wei a, Xun Hu a, Zhengmao Ye a, **, Zhi Wang a, Yi Wang b, Cuncheng Li c, Jianfeng Yao d, Dehua Dong a, * a
School of Material Science and Engineering, University of Jinan, Jinan, 250022, PR China School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China School of Chemistry and Chemical Engineering, University of Jinan, Jinan, 250022, PR China d College of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization of Agricultural and Forest Biomass, Nanjing Forestry University, Nanjing, 210037, Jiangsu, PR China b c
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
� Microchanneled SOFCs were employed to efficiently convert methane into power. � Conversion efficiency is defined to compromise power density and fuel utilization. � Channels structure provides gas diffu sion highway to/from anode functional layer. � Microchanneled anodes achieved the record high methane conversion efficiency.
A R T I C L E I N F O
A B S T R A C T
Keywords: Methane-fueled SOFCs Methane conversion efficiency Gas diffusion Anode supports Microchannel structure
Methane-fueled solid oxide fuel cells (SOFCs) are promising to achieve high energy conversion efficiency while no study focuses on the conversion efficiency of methane into power, which is greatly restrained by gas diffusion within anode supports. This study employs microchanneled anode supports to provide fast gas diffusion pathway. To confirm the advantage, the anodes with half-channels and without channels are also used for comparison. The microchannel structure increases the maximum power density up to 2.5 times because of diminishing or elim inating concentration polarization within anode supports and improving catalyst coating over anode internal surface. As a compromise of fuel utilization and power output, methane conversion efficiency is defined as power output per mol methane input in feeding gas to compare with the reported results, and the microchanneled SOFCs achieve the record high methane conversion efficiency.
1. Introduction Fuel cells can directly convert fuels into electricity and achieve the
higher energy conversion efficiency compared to conventional coal or gas-fired power generation [1]. The direct conversion is performed on proton exchange membrane (PEM) fuel cells at low temperatures
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Ye), [email protected] (D. Dong). https://doi.org/10.1016/j.jpowsour.2020.227848 Received 19 November 2019; Received in revised form 3 February 2020; Accepted 4 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
J. Wang et al.
Journal of Power Sources 453 (2020) 227848
Fig. 1. Facilitated gas diffusion within microchanneled anode supports of methane-fueled SOFCs.
Fig. 2. Flowchart of SOFC preparation process.
(80–250 C) or solid oxide fuel cells (SOFCs) at high temperatures (600–1000 � C). PEM fuel cells only can be operated over hydrogen fuel, which is currently produced mainly via methane steam reforming. The hydrogen production process is energy-intensive, and hydrogen storage and transport are still challenging. In contrast, methane can be directly fed into SOFCs for power generation. In addition, high reaction kinetics can be achieved at high operation temperatures so that noble metals catalysts required in PEM fuel cells can be avoided in SOFCs. Methane is a predominant component of abundant gas resources, such as natural gas [2]. Therefore, SOFCs are promising for converting methane into power, and the conversion efficiency is practically important. Comparing with a lot of studies focusing on the development of anode materials and catalysts to achieve high and stable cell power output [3–5], there are a few studies investigating the fuel utilization of methane-fueled SOFCs [6]. Lin et al., reported a high power density of 1.27 W cm 2 with the feeding of nearly pure methane while fuel utili zation was only 12% [7]. The fuel utilization can be improved via reducing methane concentration in feeding gas. For example, fuel uti lization was increased from 8% to 93% when methane concentration was decreased from 47% to 3%, but cell power density was decreased by one-third [8]. Accordingly, a compromise of sufficient power density and acceptable fuel utilization is required to achieve the high conversion efficiencies of methane into power, and the comprise is due to gas diffusion within anodes [9]. To increase fuel utilization, the unreacted fuel from anodes was reused via recirculation [10,11]. However, the decrease of fuel concentration in the feeding gas will cause the con centration polarization. Therefore, gas diffusion within anode supports greatly affects the conversion efficiency. Ni-based anode supports have been widely used in methane-fueled SOFCs owing to low cost and high catalytic activity, and methane need diffuse through anode supports to reach reaction zone near the anode/electrolyte interface. Conventional anode supports have tortuous pore channels, which limits the gas diffusion and causes the concen tration polarization [12,13]. Our group has developed microchanneled ceramic membranes for oxygen separation and solid oxide cells [14–16]. The channel structure facilitated gas diffusion within supporting elec trodes. Chen et al., established a numerical model for gas transport through the channels and found it is effective in improving the gas transport and the SOFC performance [17]. In this study, as shown in Fig. 1, microchanneled anode supports were employed to facilitate gas diffusion and diminish or eliminate concentration polarization. To the best of our knowledge, there is no study investigating the conversion efficiency of methane-fueled SOFCs. Methane conversion (converted methane/input methane) is generally investigated, but it cannot effectively index the efficiency of converting �
methane into power due to the variation of oxidation products. To compromise the power density and methane utilization, the methane conversion efficiency is defined as power output per mol methane input in the feeding gas. The conversion efficiency over the microchanneled anodes has been investigated, and the improvement of conversion effi ciency by the channel structure was confirmed by comparing with other structure, including conventional tortuous pore structure. The highest conversion efficiency over the microchanneled SOFCs has been achieved after comparing with the results reported in literature. 2. Material and methods 2.1. SOFC preparation The preparation process of SOFCs is presented in Fig. 2. Ceramic powders were provided by Fuel Cell Materials, Ohio, USA. Anode sup ports were prepared by a phase-inversion process. 17.7 g of poly ethersulfone (PESF, Radel-A300) was dissolved in 100 g of N-Methyl-2pyrrolidone (NMP, 99%, Shanghai Macklin Biochemical Co., Ltd, China) by magnetic stirring. 28.32 g of the above solution, ceramic powders (36.99 g of NiO (NiO–F) and 26.42 g of Gd0.1Ce0.9O2 (GDC-TC)) and 0.43 g of polyvinylpyrrolidone (PVP, MW ¼ 130000, Shanghai Dibo Biotechnology, China) were mixed for 48 h by a planetary ball-milling machine (Hefei Ke Jing Materials Technology, China). The prepared slurry was converted to a disc membrane in an aluminum mould by mesh-templating phase-inversion using water as a coagulant, and the mesh has an aperture size of 70 μm. The obtained anode green body was dried in an oven at 60 � C overnight, followed by pre-sintering at 1100 � C for 2 h to obtain NiO-GDC anode supports. To form an electrolyte slurry with a solid loading of 10%, GDC (GDCTC) powder was dispersed in ethanol with 3 wt% of PVP as a dispersant by ball-milling for 24 h. The GDC slurry was coated on the non-channel side of the pre-sintered anode supports by dip-coating. NiO-GDC/GDC half cells were formed after co-sintering at 1380 � C for 5 h. Finally, to prepare full cells, 30 wt% of GDC and 70 wt% of Ba0.5Sr0.5Co0.8Fe0.2O3 (Praxair Surface Technologies) were dispersed in ethanol by ballmilling, and the slurry with the solid loading of 10% was deposited on the GDC electrolyte by spray-coating to form a BSCF/GDC cathode layer after sintering at 950 � C for 2 h. 2.2. Catalyst preparation The catalyst over the NiO-GDC anode scaffold was prepared by a conventional infiltration process through microchannels. A commercial colloid (CeO2 (AC), Naycol Nano Technology, Inc.) with CeO2 2
J. Wang et al.
Journal of Power Sources 453 (2020) 227848
Fig. 3. SEM images of anode supports with different microstructure: (a), microchannels; (b), half-channels; (c), no channels. Scale bars are 300 μm.
2.3. Characterization and test of SOFCs The porosity and pore size distribution of the Ni-GDC anodes were measured by a mercury intrusion porosimetry (AutoPore IV 9500, Micrometrics). The cell feed with methane was tested in a home-made test device. Initially, hydrogen was introduced to reduce NiO and then the feeding gas was switched to CH4/Ar mixture. The cell performance was tested by an electrochemical workstation (Reference 3000, Gamry). Electrochemical impedance spectra were recorded under the OCV within the frequencies between 0.1 Hz and 0.1 MHz. Cell microstructure was examined using a scanning electron microscopy (SEM, Zeiss Neon 40EsB FIBSEM). Exhaust gas was analyzed by gas chromatograph (Shi madzu GC-2014) to further study the effect of pore structure on methane utilization. 3. Results and discussion 3.1. Anode support microstructure As shown in Fig. 3a, numerous microchannels parallel cross anode support with one end open on anode surface and the other end termi nated by a porous layer on the other side of anode [19]. The porous layer acts as an anode functional layer for performing methane oxidation. The microchannel structure can provide fast pathway for methane diffusion from anode surface to the functional layer and also oxidation products diffusion to anode surface [16]. Additionally, these channels facilitate catalyst precursor delivery to the reaction zone [15]. To confirm the structure effect, anode supports with less and half-channels and without channels were also prepared for comparison. The microstructure of the two anode supports is shown in Figs. 3b and 3c.
Fig. 4. Pore size distribution of Ni/GDC anode supports with different microstructure.
nanoparticles suspended in acetic acid was used as a catalyst precursor. The catalyst precursor solutions were coated on the anode internal surface, followed by vacuum to remove trapped bubbles. Then, it was converted into catalyst coating after calcination at 750 � C for 1 h. The CeO2 catalyst loading of 1.75 wt% was used to form stable nanocatalyst network [18].
Fig. 5. Performance of the cells with different structure and a feeding of 30% CH4/Ar: a, power-current-voltage curves; b, electrochemical impedance spectra. 3
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Fig. 6. Effect of methane concentration on power output of the cells with different anodes: (a) microchannels; (b) half-channels; (c) no channels, and methane conversion efficiency and maximum power density (d).
212 mW cm 2 for the cells supported on anodes with microchannels, with half-channels and without channels, respectively. Accordingly, the channel structure makes the maximum power density increase up to 2.5 times. From current-voltage curves of the cells, channel structure diminished concentration polarization during the methane conversion through providing fast pathway for gas diffusion, and therefore increased power density [17]. In addition, according to cell resistances shown in Fig. 5b, the channel structure also decreases cell polarization resistances, which is related to catalyst preparation. Through these channels, catalyst can be effective delivered to the reaction zone, which has been demonstrated in previous study [15]. Therefore, the channel structure not only accelerates gas diffusion, but also improves catalyst preparation, resulting in the high power output.
3.2. Porosity and pore size distribution The pore structure of the anode supports was measured by mercury porosimetry. The three anode supports have a similar porosity: 47.5%, 45.5% and 44.8% for the anode supports with microchannels, with halfchannels and without channels, respectively. Thereby, it is feasible to compare the effect of pore structure on anode properties. The pore size distributions are shown in Fig. 4. For the anode without channels, there is a single pore size range between 0.12 and 2.1 μm, with an average pore size of about 0.4 μm. In contrast, the other two anodes show dual pore size ranges, and the small pore size range is similar to that of the anode without channels, which refers to the pores within channel wall and the porous layer. The anode with microchannels shows the smaller channel diameters compared with that with half-channels, which is consistent with SEM images in Fig. 3.
3.4. Methane conversion efficiency 3.4.1. Effect of methane concentration As concentration polarization readily occurs within supporting an odes, methane concentration in feeding gas affects power output and hence the methane conversion efficiency. To investigate the effect, cell performances were tested at different methane concentrations. As shown in Fig. 6, the concentration polarization was diminished as methane concentration was increased. The minimum methane concen trations to eliminate the concentration polarization are about 35%, 60% and 80% for the anodes with microchannels, with half-channels, and without channels respectively, and the corresponding maximum power densities are 590, 453, 330 mW cm 2. Accordingly, without
3.3. Fuel cell performance To investigate the effect of the channel structure on gas diffusion, a mixture of 30% CH4/Ar was fed into the anodes of SOFCs for power generation. Operating temperature has a complicate effect on the per formance of the cells with GDC electrolyte due to its electronic con ductivity. Increasing operation temperature decreases cell resistances and also open circuit voltage, resulting in limited power density increase with temperature when the temperature is above 600 � C (See Fig. S1). Accordingly, the operating temperature in this study was set at 600 � C. As shown in Fig. 5a, the maximum power densities are 545, 393 and 4
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Table 1 Comparison of conversion efficiency of methane into power by SOFCs. Anode/electrolyte
Anode catalyst
Temperature(� C)
Maximum power density(mW⋅cm 2)
CH4 conversion efficiency(Wh⋅mol
NiO-YSZ/YSZ NiO-YSZ/YSZ NiO-YSZ/YSZ, NiO-YSZ/YSZ NiO-YSZ/YSZ NiO-YSZ/YSZ NiO-SDC/SDC NiO–GDC/GDC NiO–GDC/GDC NiO–GDC/GDC
No catalyst Doped CeO2 Doped CeO2 CeO2 CeO2 Ag No catalyst No catalyst No catalyst CeO2
825 800 800 800 750 750 700 600 600 600
680 650 231 610 498 542 697 414 520 590
12.18 2.06 0.44 34.16 27.88 0.64 1.82 3.50 7.28 9.44
a b
1
)
References [2] [20] [21] This studya This studya [22] [23] [24] This studya This studyb
Data from Fig. S2. Data from Fig. 6.
concentration polarization, the cell with the microchanneled anode generated the highest maximum power density owing to the improved catalyst preparation. Methane conversion efficiency in this study is defined as power output per mol methane input. Fig. 6d shows the effect of methane concentration on the conversion efficiency and maximum power den sity, which should be considered practically in term of cell power output. The channel structure greatly increases methane conversion efficiency, and the cell with the microchanneled anode has a much higher methane conversion efficiency than the other two cells. For the anodes with half-channels and without channels, maximum power density was increased with methane concentration while methane conversion efficiency was decreased. In contrast, the anode with microchannels demonstrated a slight increase of methane conversion efficiency from the methane concentration of 10% to 30%, followed by a decrease as methane concentration was increased to 35%. Therefore, high methane conversion efficiency and power output can be achieved over the cell with the microchanneled anode. 3.4.2. Comparison of methane conversion efficiency To confirm the high methane conversion efficiency, it was compared with the reported results in Table 1. All cells selected for comparison are button or cone-shaped cells to ensure the same fuel flow pathway. For example, hollow fibre and tubular fuel cells have long flow path along anode surface while button cells have gas flow turned around. According to the definition of methane conversion efficiency (power output per mol methane input in the feeding gas), powder density, cell active area and methane flow rate are required for the calculation. The references are chosen according to the data availability. NiO-YSZ anode-supported cells were also prepared and tested for comparison (Fig. S2). The cells with the microchanneled anodes demonstrate superior methane con version efficiency compared with that reported in literature although maximum power densities are not outstanding. Previous studies used high concentrations of methane or pure methane to diminish concen tration polarization and achieve high power densities due to tortuous pore channel structure. The microchanneled anodes used in this study provides fast gas diffusion pathway and enables the operation without concentration polarization at low methane concentrations so that high methane utilization can be achieved. In addition, the channel structure improves the efficiency of catalyst preparation by infiltration [15], which reduces cell polarization resistance and hence increases power density. Therefore, the high methane conversion efficiencies have been achieved over the microchanneled SOFCs.
Fig. 7. Stability test of methane-fueled SOFCs with different anode microstructure.
microchannels, with half-channels and without channels, respectively. Previous studies showed the degradation rate of solid oxide cells increased with applied current density [25,26]. In the conversion, the extent of methane oxidation increased with the applied current, as evidenced by the decreased CO and H2 selectivities (See Fig. S3). Accordingly, discharge at high current densities is preferable to achieve high methane utilization. The discharge current densities of the stability tests were chosen at the peak power output. As shown in Fig. 7, the cell with microchanneled anode demonstrated more stable discharge voltage than the other two cells. The cell without channels degraded rapidly. The cell showed the larger initial polarization resistance compared with the other two cells due to the difference in catalyst coating (See Fig. S4), which might cause coke formation and hence the rapid increased polarization resistance within 24 h-operation. It in dicates the effective catalyst coating can prevent coking formation during the methane conversion, which has been confirmed in our pre vious studies [15,18]. 4. Conclusions SOFCs with microchanneled anode supports have demonstrated high efficiency in the conversion of methane into power. Compared with the cell without channels, the cell with channels show the increase of the maximum power density up to 2.5 times (from 212 to 545 mW cm 2). The microchanneled SOFCs demonstrated superior methane conversion efficiency over the reported results in literature. The microchannel structure of anode supports provides fast gas diffusion pathway to diminish or eliminate concentration polarization. In addition, the
3.5. Stability test The stability of the methane conversion is very important for prac tical application. To achieve high power output, the stability tests were conducted at the methane concentrations without obvious centration polarization, which are 30%, 60% and 80% for the cells with 5
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microchannel structure facilitates catalyst coating over anode scaffold to accelerate methane oxidation and restrain carbon deposition. Therefore, the microchannel structure of anode supports ensures the efficient and stable conversion of methane into power.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Prof. D. H. Dong acknowledges the startup funding provided by the University of Jinan. This work is supported by the National Natural Science Foundation of China (51872123). The study is a part of the projects of Natural Science Foundation of Shandong Province (ZR2017MEM022) and Shandong Province Key Research and Develop ment Program (2018GGX102037). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227848. References [1] A. Buonomano, F. Calise, M.D. d’Accadia, A. Palombo, M. Vicidomini, Appl. Energy 156 (2015) 32–85. [2] Y. Jiao, L. Zhang, W. An, W. Zhou, Y. Sha, Z. Shao, J. Bai, S.-D. Li, Energy 113 (2016) 432–443.
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