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Influence of anode's microstructure on electrochemical performance of solid oxide direct carbon fuel cells
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 1 7 8 4 e1 1 7 9 0
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Influence of anode's microstructure on electrochemical performance of solid oxide direct carbon fuel cells Chao Liu a, Jiangge Pu a, Xing Chen a, Zheng Ma a, Xiao Ding a, Juan Zhou a,*, Shaorong Wang b,** a
School of Energy and Power Engineering, Nanjing University of Science and Technology, 200 Xiaolingwei Street, Nanjing, Jiangsu Province, 210094, China b School of Chemistry and Chemical Engineering, China University of Mining and Technology, 1 Daxue Street, Xuzhou, Jiangsu Province, 221116, China
highlights
graphical abstract
The tubular SO-DCFCs based on cathode supported solid oxide fuel cells. The SO-DCFCs were fabricated by dip-coating
and
co-sintering
techniques. The anode porosity came from the pore former (graphite) in the dipcoating process. When the graphite was 10.1%wt., SO-DCFC showed the best performance and stability.
article info
abstract
Article history:
The microstructure of anode has a significant influence on the whole electrochemical
Received 23 December 2019
performance of solid oxide direct carbon fuel cells (SO-DCFCs). The tubular SO-DCFCs
Received in revised form
based on cathode supported solid oxide fuel cells was fabricated by dip-coating and co-
9 February 2020
sintering techniques. As the anode porosity mainly came from the pore former (graphite)
Accepted 18 February 2020
in the dip-coating process, different contents of graphite were added into the anode
Available online 12 March 2020
slurries. When the graphite was 10.1% wt., the SO-DCFCs showed the best performance and stability. The peak power density reached 242 mW cm2 at 850 C, with carbon black
Keywords: Solid oxide direct carbon fuel cells
(located 5% Fe) as the fuel and air as the oxidant. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
(SO-DCFCs) Porosity of anode Ni-YSZ Carbon fuel * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhou), [email protected] (S. Wang). https://doi.org/10.1016/j.ijhydene.2020.02.119 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 1 7 8 4 e1 1 7 9 0
Introduction Coal as a natural energy source is abundant, cheap, and high energy density [1]. China has the largest coal-fired installed capacity in the world (46.9% of the world's total coal-fired electrical generation). The combined efficiency of thermal is only 48% with advanced ultra-supercritical thermal generation technology, meanwhile, high concentration of pollutants (CO2, SOx and VOCs) will be discharged during the power generation process [2,3]. Direct carbon fuel cells (DCFCs) is a promising power generation technology using coal, which has high energy conversion efficiency with lower pollutant emission. The theoretical conversion efficiency is attain to 100% since the value of △G/△H is slightly more than 1 [4]. According to the different electrolytes, DCFCs mainly have three types, which are direct carbon-molten carbonates fuel cells (MC-DCFCs), direct carbon molten hydroxides fuel cells (MH-DCFCs) and direct carbon solid oxide fuel cells (SODCFCs) [5e7]. Compared with MC-DCFC and MH-DCFC, SODCFC has significant advantages, including no risk of electrolyte degradation and leakage [8,9], fixed carbon reaction area and better stability [7,10,11]. SO-DCFCs usually show a low performance which restricts its application. The key to solve the problem is improving the anode. The porous solid anode of SO-DCFC is usually based on the anode of SOFC. For the fuel carbon is solid, different gasification agents [12] and the contact form between anode and solid carbon fuel also has an important influence on the reaction mechanism. The contact forms are divided into three categories: no contact, direct physical contact and chemical contact (carbon deposited by alkane cracking) [13]. Physical contact is a common method in the use of carbon fuel. The most of researchers [14]think that the electrode reaction process of physical contact carbon firstly occurs the reversed Boudouard reaction of carbon, then the electrochemical oxidation reaction of gas CO at the anode, and a few researchers [15] think that there is also a very small proportion of the electrochemical oxidation reaction of carbon fuel directly at the three-phase boundary (TPB). Therefore, the porosity and microstructure of the anode will have an important impact on the electrochemical performance [16,17]. In order to obtain the required microstructure and porosity of anode, researchers have been using different amount and kind of pore formers such as graphite, corn starch, wheat and organic materials [18,19]. Graphite are formed by aggregates of nanometric particles or sub-micronic dense lamellae. These aggregates can be easily broken down during the ball milling process leading to powders with a broad particle size distribution [20]. On the other hand, graphite in the removal stage results in very uniform distribution of pores, which produces very uniform microstructure without any anisotropy or directionality [21]. Graphite is a good choice of pore former in the process of anode fabrication. In order to study the effect of anode microstructure on electrochemical performance, we prepared different anodes on the cathode supported tubular half cells. Those configuration cells have advantages: i) the outside anode makes it possible to supply the fuel continuously [22,23], ii) The large
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effective area of the anode of the cathode supported cell with tubular structure (10 cm2) provides a high-quality and stable research platform for the research of anode microstructure [24], iii) the anode of the cathode supported cell is usually a thin layer, which makes the fuel electrode produce smaller polarization resistance in the operating process [25e27]. In the research of anode materials [28,29] of SO-DCFCs, the cathode supported tubular SO-DCFCs has obvious advantages. In this paper, four kinds of SO-DCFCs based on cathode supported tubular half cells were fabricated with different porosity anode by using amount of graphite as pore former in the dip-coating fabrication of anode slurry. The aim of this work is to study the influence of the graphite pore former on the microstructure of anode and its associated electrochemical performance in the SO-DCFCs.
Experimental Fabrication of SOFC The one-end closed cathode-supported tubular half-cells were composed of (La0.8Sr0.2)0.95MnO3þd (LSM) cathode substrate layer, LSM-Sc0.1Ce0.01Zr0.89O2þd (SSZ) cathode active layer, SSZ electrolyte and NieY0.08Zr0.92O2 (YSZ) anode by dip-coating and co-sintering techniques. The detailed preparation process was reported in our previous work [22,30]. Based on this previous work, four SO-DCFCs with different porous anode were prepared. In the anode fabricating slurry, the weight ratio of graphite and NiO-YSZ is 7.1%, 10.7%, 14.3%, 17.8%, which were called the Cell-1, Cell-2, Cell-3 and Cell-4, respectively. Take Cell-1 for example, Fig. 1(a) is a photograph of the full cell before testing, the length of the tubular cell was 5 cm (anode length ¼ 2.25 cm), the outside diameter of the tube was 1.40 cm, the thickness of the tube was 0.5e0.6 mm, and the active area of the cell was 9.89 cm2. Fig. 1(b) is a pattern photo of the tubular cell, and the crosssectional SEM image is shown in Fig. 1(c) which presents good microstructure and interface bonding, from left to right, followed by Ni-YSZ anode layer, SSZ electrolyte layer, LSMSSZ active layer and LSM supported layer. And the thickness of the anode layer, electrolyte layer and active cathode layer are about 10 mm, 12 mm and 15 mm, respectively.
Measurements In order to accelerate the Boudouard reaction of carbon, the carbon fuels mixed with 5 wt% Fe3O4 catalyst by adding Fe(NO3)3. The Ag mesh was attached to the inner surface of the cathode as current collector, and the Ni mesh was used as current collector on the surface of the anode. The schematic design of the SO-DCFCs testing setup is represented in Fig. 2. The anode chamber is formed by sealing with a ceramic sealant into the alumina tube, and the carbon fuels is placed in the anode chamber to closely contact with the anode surface. In this experiment, the initial mixed carbon fuel is about 1 g. The cathode was opened in the air using compressed air as the oxidant. The testing temperature range is from 850 C to 750 C, with an interval of 50 C. Before testing, H2 (3 vol% steam) was guided to the anode chamber to reduce NiO, which
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H2 flow rate is 40 ml min1 for 2 h, the preliminary electrochemical performance was characterized in H2 atmosphere. Then, the performance of the cell was tested used carbon black as fuel by slowly switching the H2 to carbon dioxide, and the carbon dioxide flow rate is 20 ml min1. A four-probe configuration was adopted in the electrochemical testing. The current density-voltage (IeV) curve and electrochemical impedance spectroscopy (EIS) were obtained by using a CHI604E Electrochemical Workstation in frequency range of 0.1 Hze10 kHz with excitation potential of 20 mV. The microstructure of the ceramic samples was analyzed by a scanning electron microscope (FESEM, Quant 250 FEG).
Results and discussion Before the operation of the cells with carbon as fuel, the performance of the cells was preliminary characterized under 3 vol% humidified H2. Fig. 3(a) and Fig. 3(b) are summaries of IeVeP and EIS curves for Cell-1, Cell-2, Cell-3 and Cell-4 with H2 as fuel. As can be seen from Fig. 3(a), the MPD of the cell is sequentially from Cell-2 > Cell-3 > Cell-4 > Cell-1. Then, the electrochemical performance was tested under carbon fuel. Fig. 3(c) and (d) are the IeVeP and EIS curves of the four SODCFCs. Fig. 3(c) shows the IeV curves of the SO-DCFCs at 850 C. The pore former content increased from 7.1% (Cell-1) to 10.7% (Cell-2), and the cell increased from 42 mW cm2 (Cell-1) to 242 mW cm2 (Cell-2). However, as the pore former content continued to increase to 14.3% (Cell-3), the performance was reduced to 60 mW cm2. When the pore former content continued to increase to 17.8% (Cell-4), the performance continued to drop to 54 mW cm2. The overall trend is that as the pore-forming agent content increases, the MPD of the cell first increases and then decreases. Fig. 3(d) shows the EIS curves of the SO-DCFCs at 850 C. The ohmic resistances of Cell-1, Cell-2, Cell-3 and Cell-4 are 0.88 U cm2, 0.38 U cm2, 0.48 U cm2 and 1.085 U cm2, and the polarization resistances are 13.95 U cm2, 0.8 U cm2, 6.16 U cm2 and 5.49 U cm2, respectively. Compared with the polarization impedance, ohmic impedance changes little with different porosity. The change of polarization resistance indicates the change of gas diffusion and electrode reaction rate. It is speculated that the difference of ohmic and polarization resistance is due to the different porosity and microstructure of anode. Therefore, the microstructure of the four cells’ cross-section is shown in Fig. 4 and Fig. 5.
Fig. 2 e The schematic design of the SO-DCFCs testing setup.
Fig. 4 shows the cross-section SEM images of the four cells after being tested. From these figures, the electrolytes are in good density, the particles of anode are firmly bonded, the combination of interfaces between electrolyte and anode are very well. In Fig. 4(b), the anode microstructure of Cell-2 are most uniform, and the particles and pores have good penetration, which makes it have enough TPBs for gas fuel reaction. The anode microstructure of Cell-3 is similar to Cell-2, but the particles are slightly looser than Cell-2. Hence, the electrochemical performance of Cell-3 is lower than Cell-2. The porosity of the Cell-4's anode is larger than that in Cell3, which makes part of the pore agglomerate to form a large pore. Therefore, the TPBs of the Cell-4's anode sharply reduced and the performance is worse. Comparison with other three cells, the anode of Cell-1 obviously has less pore, which makes minority of nickel in the anode cannot be fully reduced in the atmosphere of carbon monoxide and the TPBs is not enough. Fig. 5 is the BSE image of the anodes, which intuitively reflect the change of anode porosity. In Fig. 3, Cell-2 has the best performance, and Cell-3 and Cell-4's performance are worse, meanwhile Cell-1 presents a worst performance. The anode microstructure of those four cells is also trend of the change of electrochemical performance. Fig. 6 shows the IeVeP curves and EIS curves of Cell-2 using carbon black as fuel and compressed air as oxidant at different temperatures. In Fig. 6(a), the open circuit voltages at 850 C, 800 C and 750 C are 1.05 V, 1.03 V and 0.98 V, respectively, which are in good agreement with the theoretical values calculated from the Nernst equation. The maximum power density of Cell-2 at 850 C, 800 C and 750 C are 242 mW cm2, 147 mW cm2 and 74 mW cm2, respectively. With the decrease of temperature, the performance decreased obviously. Fig. 6(b) shows the EIS curve for Cell-2 at different temperatures under open circuit conditions. The ohmic resistance of Cell-2 at 850 C, 800 C and 750 C are
Fig. 1 e The SEM images (a) of Cell-1 after operation, the pattern photo (b) of tubular cell, and the appearance picture (c) of cell.
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Fig. 3 e The IeVeP cures (a) and EIS cures (b) for cell using humidified H2 as fuel and IeVeP cures (c) and EIS cures (d) for cell using carbon black as fuel an at 850 C.
Fig. 4 e The SEM images of cross-section of Cell-1(a), Cell-2(b), Cell-3(c) and Cell-4(d).
0.38 U cm2, 0.48 U cm2, and 0.63 U cm2, respectively, and the polarization resistances are 0.80 U cm2, 1.46 U cm2, and 3.34 U cm2, respectively. Compared with the change of ohmic impedance, the change of polarization impedance is relatively large. For the temperature has a double influence on the
boudouard reaction of carbon and the adsorption and dissociation of CO on the anode, the polarization resistance increases rapidly and the electrochemical performance decreases rapidly. Therefore, the SO-DCFC needs to operate in an environment of more than 800 C.
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Fig. 5 e The BSE images of the anodes of Cell-1(a), Cell-2(b), Cell-3(c) and Cell-4(d).
Fig. 6 e The IeVeP (a) and EIS curves (b) of Cell-2 with carbon fuel.
The stability of Cell-2 under constant current discharge was investigated. In order to save carbon fuel, the stability test is carried out at 750 C. Fig. 7 is a constant current discharge curve of Cell-2 under 0.6 A current. Before the carbon fuel is exhausted, the voltage is relatively stable and remains at about 0.8 V until the fuel is completely exhausted 4 h later. Hence, it shows that SO-DCFCs based on cathode supported tubular fuel cells have a good stability. In this experiment, the initial mixed carbon fuel is about 1 g. It is assumed that carbon monoxide after Boudouard reaction of carbon fuel will totally generate carbon dioxide, hence, 4 mol of electrons will be transferred when 1 mol of carbon is reacted. Discharge at the current of 0.6 A for 4 h, the consuming carbon is about 0.45 g. Before constant current discharge, the Boudouard reaction of carbon fuel was kept for about 3 h during the test process from 850 C to 750 C and the changing temperature process.
Fig. 7 e The discharge curves of Cell-2 operated at electrical current of 0.6 A at 750 C.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 1 7 8 4 e1 1 7 9 0
Therefore, the carbon fuel utilization is about 78.75% in the discharging process.
Conclusions Based on cathode-supported tubular solid oxide fuel cell, SODCFCs with the composition of Ni þ YSZ/SSZ/SSZ þ LSM/ LSM was prepared by dip-coating and co-sintering techniques. By changing the amount of pore former (graphite) in the anode slurry, different anodes of SO-DCFCs were achieved with different microstructures and porosities. When the amount of graphite was 10.1 wt%, the SO-DCFC exhibited the best performance. The maximum power density was 242 mW cm2 at 850 C, displaying reasonably good performance stability. For carbon contact form is no contact or direct physical contact, these studies provide technical support for the practical application of SO-DCFCs for the future work.
Acknowledgements The authors would like to thank the Jiangsu Natural Science Foundation (BK20170845), the Fundamental Research Funds for the Central Universities (No.30919011236), National Key R&D Program of China (2018YFB1502600) and National Natural Science Foundation of China (51836004).
references
[1] Zhou W, Jiao Y, Li SD, Shao ZP. Anodes for carbon-fueled solid oxide fuel cells. ChemElectroChem 2016;3:193e203. [2] Fan H, Zhang Z, Dong J, Xu W. China's R&D of advanced ultra-supercritical coal-fired power generation for addressing climate change. Therm Sci Eng Prog 2018;5:364e71. [3] Cao T, Huang K, Shi Y, Cai N. Recent advances in hightemperature carbon-air fuel cells. Energy Environ Sci 2017;10:460e90. [4] Wang C, Lu¨ Z, Su C, Li J, Cao Z, Zhu X, et al. Effects of discharge mode and fuel treating temperature on the fuel utilization of direct carbon solid oxide fuel cell. Int J Hydrogen Energy 2019;44:1174e81. [5] Choi SH, Park D-N, Yoon CW, Yoon S-P, Nam SW, Hong S-A, et al. A study on the electrochemical performance of 100-cm2 class direct carbon-molten carbonate fuel cell (DC-MCFC). Int J Hydrogen Energy 2015;40:5144e9. [6] Lim S-H, Yong S-T, Ooi C-W, Chai S-P, Doshi V, Daud WRW. Pyrolysis of palm waste for the application of direct carbon fuel cell. Energy Procedia 2014;61:878e81. [7] Dudek M, Tomov RI, Wang C, Glowacki BA, Tomczyk P, Socha RP, et al. Feasibility of direct carbon solid oxide fuels cell (DC-SOFC) fabrication by inkjet printing technology. Electrochim Acta 2013;105:412e8. [8] Konsolakis M, Kaklidis N, Kyriakou V, Garagounis I, Kraia T, Arenillas A, et al. The combined impact of carbon type and catalyst-aided gasification process on the performance of a direct carbon solid oxide fuel cell. Solid State Ionics 2018;317:268e75.
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[9] Liu R, Zhao C, Li J, Zeng F, Wang S, Wen T, et al. A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells. J Power Sources 2010;195:480e2. [10] Li X, Zhu Z, De Marco R, Bradley J, Dicks A. Evaluation of raw coals as fuels for direct carbon fuel cells. J Power Sources 2010;195:4051e8. [11] Cooper JF, Selman JR. Analysis of the carbon anode in direct carbon conversion fuel cells. Int J Hydrogen Energy 2012;37:19319e28. [12] Xu H, Chen B, Zhang H, Sun Q, Yang G, Ni M. Modeling of direct carbon solid oxide fuel cells with H2O and CO2 as gasification agents. Int J Hydrogen Energy 2017;42:15641e51. [13] Li C, Yi H, Lee D. On-demand supply of slurry fuels to a porous anode of a direct carbon fuel cell: attempts to increase fuel-anode contact and realize long-term operation. J Power Sources 2016;309:99e107. [14] Tang Y, Liu J. Effect of anode and Boudouard reaction catalysts on the performance of direct carbon solid oxide fuel cells. Int J Hydrogen Energy 2010;35:11188e93. [15] Li C, Shi Y, Cai N. Mechanism for carbon direct electrochemical reactions in a solid oxide electrolyte direct carbon fuel cell. J Power Sources 2011;196:754e63. [16] Shao X, Rickard WDA, Dong D, Dang H, Saunders M, Dodd A, et al. High performance anode with dendritic porous structure for low temperature solid oxide fuel cells. Int J Hydrogen Energy 2018;43:17849e56. [17] Zhou J, Liu Q, Zhang L, Pan Z, Chan SH. Influence of pore former on electrochemical performance of fuel-electrode supported SOFCs manufactured by aqueous-based tapecasting. Energy 2016;115:149e54. [18] Amaya DM, Estrada D, Hotza D, Rodrigues Neto JB, Escobar JA. Porous Cu/YSZ anodes processed by aqueous tape casting for IT-SOFC. J Eur Ceram Soc 2017;37:5233e7. [19] Clemmer RMC, Corbin SF. Influence of porous composite microstructure on the processing and properties of solid oxide fuel cell anodes. Solid State Ionics 2004;166:251e9. [20] Sanson A, Pinasco P, Roncari E. Influence of pore formers on slurry composition and microstructure of tape cast supporting anodes for SOFCs. J Eur Ceram Soc 2008;28:1221e6. [21] Lee JH, Heo JW, Lee DS, Kim J, Kim GH, Lee HW, et al. The impact of anode microstructure on the power generating characteristics of SOFC. Solid State Ionics 2003;158:225e32. [22] Zhou J, Ye XF, Shao L, Zhang XP, Qian JQ, Wang SR. A promising direct carbon fuel cell based on the cathodesupported tubular solid oxide fuel cell technology. Electrochim Acta 2012;74:267e70. [23] Rehman Su, Song R-H, Lee J-W, Lim T-H, Park S-J, Lee S-B. Fabrication and characterization of La0.65Sr0.3MnO3d/(Y2O3) 0.08(ZrO2)0.92/Gd0.1Ce0.9O2d tri-composite cathodesupported tubular direct carbon solid oxide fuel cell. Ceram Int 2017;43:1086e91. e E. Electrochemical [24] Ma Q, Tietz F, Leonide A, Ivers-Tiffe performances of solid oxide fuel cells based on Y-substituted SrTiO3 ceramic anode materials. J Power Sources 2011;196:7308e12. [25] Zhou J, Zhao CH, Ye XF, Wang SR, Wen TL. Improvement of cathode support tubular solid oxide fuel cells. Electrochem Solid State Lett 2012;15:B57e60. [26] Fergus JW. Electrolytes for solid oxide fuel cells. J Power Sources 2006;162:30e40. [27] George RA, Bessette NF. Reducing the manufacturing cost of tubular SOFC technology. J Power Sources 1998;71:131e7. [28] Li J, Wei B, Wang C, Zhou Z, Lu Z. High-performance and stable La0.8Sr0.2Fe0.9Nb0.1O3-delta anode for direct carbon
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solid oxide fuel cells fueled by activated carbon and corn straw derived carbon. Int J Hydrogen Energy 2018;43:12358e67. [29] Kulkarni A, Ciacchi FT, Giddey S, Munnings C, Badwal SPS, Kimpton JA, et al. Mixed ionic electronic conducting
perovskite anode for direct carbon fuel cells. Int J Hydrogen Energy 2012;37:19092e102. [30] Zhao C, Liu R, Wang S, Wang Z, Qian J, Wen T. Fabrication and characterization of a cathode-supported tubular solid oxide fuel cell. J Power Sources 2009;192:552e5.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Influence of anode's microstructure on electrochemical performance of solid oxide direct carbon fuel cells Chao Liu a, Jiangge Pu a, Xing Chen a, Zheng Ma a, Xiao Ding a, Juan Zhou a,*, Shaorong Wang b,** a
School of Energy and Power Engineering, Nanjing University of Science and Technology, 200 Xiaolingwei Street, Nanjing, Jiangsu Province, 210094, China b School of Chemistry and Chemical Engineering, China University of Mining and Technology, 1 Daxue Street, Xuzhou, Jiangsu Province, 221116, China
highlights
graphical abstract
The tubular SO-DCFCs based on cathode supported solid oxide fuel cells. The SO-DCFCs were fabricated by dip-coating
and
co-sintering
techniques. The anode porosity came from the pore former (graphite) in the dipcoating process. When the graphite was 10.1%wt., SO-DCFC showed the best performance and stability.
article info
abstract
Article history:
The microstructure of anode has a significant influence on the whole electrochemical
Received 23 December 2019
performance of solid oxide direct carbon fuel cells (SO-DCFCs). The tubular SO-DCFCs
Received in revised form
based on cathode supported solid oxide fuel cells was fabricated by dip-coating and co-
9 February 2020
sintering techniques. As the anode porosity mainly came from the pore former (graphite)
Accepted 18 February 2020
in the dip-coating process, different contents of graphite were added into the anode
Available online 12 March 2020
slurries. When the graphite was 10.1% wt., the SO-DCFCs showed the best performance and stability. The peak power density reached 242 mW cm2 at 850 C, with carbon black
Keywords: Solid oxide direct carbon fuel cells
(located 5% Fe) as the fuel and air as the oxidant. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
(SO-DCFCs) Porosity of anode Ni-YSZ Carbon fuel * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhou), [email protected] (S. Wang). https://doi.org/10.1016/j.ijhydene.2020.02.119 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 1 7 8 4 e1 1 7 9 0
Introduction Coal as a natural energy source is abundant, cheap, and high energy density [1]. China has the largest coal-fired installed capacity in the world (46.9% of the world's total coal-fired electrical generation). The combined efficiency of thermal is only 48% with advanced ultra-supercritical thermal generation technology, meanwhile, high concentration of pollutants (CO2, SOx and VOCs) will be discharged during the power generation process [2,3]. Direct carbon fuel cells (DCFCs) is a promising power generation technology using coal, which has high energy conversion efficiency with lower pollutant emission. The theoretical conversion efficiency is attain to 100% since the value of △G/△H is slightly more than 1 [4]. According to the different electrolytes, DCFCs mainly have three types, which are direct carbon-molten carbonates fuel cells (MC-DCFCs), direct carbon molten hydroxides fuel cells (MH-DCFCs) and direct carbon solid oxide fuel cells (SODCFCs) [5e7]. Compared with MC-DCFC and MH-DCFC, SODCFC has significant advantages, including no risk of electrolyte degradation and leakage [8,9], fixed carbon reaction area and better stability [7,10,11]. SO-DCFCs usually show a low performance which restricts its application. The key to solve the problem is improving the anode. The porous solid anode of SO-DCFC is usually based on the anode of SOFC. For the fuel carbon is solid, different gasification agents [12] and the contact form between anode and solid carbon fuel also has an important influence on the reaction mechanism. The contact forms are divided into three categories: no contact, direct physical contact and chemical contact (carbon deposited by alkane cracking) [13]. Physical contact is a common method in the use of carbon fuel. The most of researchers [14]think that the electrode reaction process of physical contact carbon firstly occurs the reversed Boudouard reaction of carbon, then the electrochemical oxidation reaction of gas CO at the anode, and a few researchers [15] think that there is also a very small proportion of the electrochemical oxidation reaction of carbon fuel directly at the three-phase boundary (TPB). Therefore, the porosity and microstructure of the anode will have an important impact on the electrochemical performance [16,17]. In order to obtain the required microstructure and porosity of anode, researchers have been using different amount and kind of pore formers such as graphite, corn starch, wheat and organic materials [18,19]. Graphite are formed by aggregates of nanometric particles or sub-micronic dense lamellae. These aggregates can be easily broken down during the ball milling process leading to powders with a broad particle size distribution [20]. On the other hand, graphite in the removal stage results in very uniform distribution of pores, which produces very uniform microstructure without any anisotropy or directionality [21]. Graphite is a good choice of pore former in the process of anode fabrication. In order to study the effect of anode microstructure on electrochemical performance, we prepared different anodes on the cathode supported tubular half cells. Those configuration cells have advantages: i) the outside anode makes it possible to supply the fuel continuously [22,23], ii) The large
11785
effective area of the anode of the cathode supported cell with tubular structure (10 cm2) provides a high-quality and stable research platform for the research of anode microstructure [24], iii) the anode of the cathode supported cell is usually a thin layer, which makes the fuel electrode produce smaller polarization resistance in the operating process [25e27]. In the research of anode materials [28,29] of SO-DCFCs, the cathode supported tubular SO-DCFCs has obvious advantages. In this paper, four kinds of SO-DCFCs based on cathode supported tubular half cells were fabricated with different porosity anode by using amount of graphite as pore former in the dip-coating fabrication of anode slurry. The aim of this work is to study the influence of the graphite pore former on the microstructure of anode and its associated electrochemical performance in the SO-DCFCs.
Experimental Fabrication of SOFC The one-end closed cathode-supported tubular half-cells were composed of (La0.8Sr0.2)0.95MnO3þd (LSM) cathode substrate layer, LSM-Sc0.1Ce0.01Zr0.89O2þd (SSZ) cathode active layer, SSZ electrolyte and NieY0.08Zr0.92O2 (YSZ) anode by dip-coating and co-sintering techniques. The detailed preparation process was reported in our previous work [22,30]. Based on this previous work, four SO-DCFCs with different porous anode were prepared. In the anode fabricating slurry, the weight ratio of graphite and NiO-YSZ is 7.1%, 10.7%, 14.3%, 17.8%, which were called the Cell-1, Cell-2, Cell-3 and Cell-4, respectively. Take Cell-1 for example, Fig. 1(a) is a photograph of the full cell before testing, the length of the tubular cell was 5 cm (anode length ¼ 2.25 cm), the outside diameter of the tube was 1.40 cm, the thickness of the tube was 0.5e0.6 mm, and the active area of the cell was 9.89 cm2. Fig. 1(b) is a pattern photo of the tubular cell, and the crosssectional SEM image is shown in Fig. 1(c) which presents good microstructure and interface bonding, from left to right, followed by Ni-YSZ anode layer, SSZ electrolyte layer, LSMSSZ active layer and LSM supported layer. And the thickness of the anode layer, electrolyte layer and active cathode layer are about 10 mm, 12 mm and 15 mm, respectively.
Measurements In order to accelerate the Boudouard reaction of carbon, the carbon fuels mixed with 5 wt% Fe3O4 catalyst by adding Fe(NO3)3. The Ag mesh was attached to the inner surface of the cathode as current collector, and the Ni mesh was used as current collector on the surface of the anode. The schematic design of the SO-DCFCs testing setup is represented in Fig. 2. The anode chamber is formed by sealing with a ceramic sealant into the alumina tube, and the carbon fuels is placed in the anode chamber to closely contact with the anode surface. In this experiment, the initial mixed carbon fuel is about 1 g. The cathode was opened in the air using compressed air as the oxidant. The testing temperature range is from 850 C to 750 C, with an interval of 50 C. Before testing, H2 (3 vol% steam) was guided to the anode chamber to reduce NiO, which
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H2 flow rate is 40 ml min1 for 2 h, the preliminary electrochemical performance was characterized in H2 atmosphere. Then, the performance of the cell was tested used carbon black as fuel by slowly switching the H2 to carbon dioxide, and the carbon dioxide flow rate is 20 ml min1. A four-probe configuration was adopted in the electrochemical testing. The current density-voltage (IeV) curve and electrochemical impedance spectroscopy (EIS) were obtained by using a CHI604E Electrochemical Workstation in frequency range of 0.1 Hze10 kHz with excitation potential of 20 mV. The microstructure of the ceramic samples was analyzed by a scanning electron microscope (FESEM, Quant 250 FEG).
Results and discussion Before the operation of the cells with carbon as fuel, the performance of the cells was preliminary characterized under 3 vol% humidified H2. Fig. 3(a) and Fig. 3(b) are summaries of IeVeP and EIS curves for Cell-1, Cell-2, Cell-3 and Cell-4 with H2 as fuel. As can be seen from Fig. 3(a), the MPD of the cell is sequentially from Cell-2 > Cell-3 > Cell-4 > Cell-1. Then, the electrochemical performance was tested under carbon fuel. Fig. 3(c) and (d) are the IeVeP and EIS curves of the four SODCFCs. Fig. 3(c) shows the IeV curves of the SO-DCFCs at 850 C. The pore former content increased from 7.1% (Cell-1) to 10.7% (Cell-2), and the cell increased from 42 mW cm2 (Cell-1) to 242 mW cm2 (Cell-2). However, as the pore former content continued to increase to 14.3% (Cell-3), the performance was reduced to 60 mW cm2. When the pore former content continued to increase to 17.8% (Cell-4), the performance continued to drop to 54 mW cm2. The overall trend is that as the pore-forming agent content increases, the MPD of the cell first increases and then decreases. Fig. 3(d) shows the EIS curves of the SO-DCFCs at 850 C. The ohmic resistances of Cell-1, Cell-2, Cell-3 and Cell-4 are 0.88 U cm2, 0.38 U cm2, 0.48 U cm2 and 1.085 U cm2, and the polarization resistances are 13.95 U cm2, 0.8 U cm2, 6.16 U cm2 and 5.49 U cm2, respectively. Compared with the polarization impedance, ohmic impedance changes little with different porosity. The change of polarization resistance indicates the change of gas diffusion and electrode reaction rate. It is speculated that the difference of ohmic and polarization resistance is due to the different porosity and microstructure of anode. Therefore, the microstructure of the four cells’ cross-section is shown in Fig. 4 and Fig. 5.
Fig. 2 e The schematic design of the SO-DCFCs testing setup.
Fig. 4 shows the cross-section SEM images of the four cells after being tested. From these figures, the electrolytes are in good density, the particles of anode are firmly bonded, the combination of interfaces between electrolyte and anode are very well. In Fig. 4(b), the anode microstructure of Cell-2 are most uniform, and the particles and pores have good penetration, which makes it have enough TPBs for gas fuel reaction. The anode microstructure of Cell-3 is similar to Cell-2, but the particles are slightly looser than Cell-2. Hence, the electrochemical performance of Cell-3 is lower than Cell-2. The porosity of the Cell-4's anode is larger than that in Cell3, which makes part of the pore agglomerate to form a large pore. Therefore, the TPBs of the Cell-4's anode sharply reduced and the performance is worse. Comparison with other three cells, the anode of Cell-1 obviously has less pore, which makes minority of nickel in the anode cannot be fully reduced in the atmosphere of carbon monoxide and the TPBs is not enough. Fig. 5 is the BSE image of the anodes, which intuitively reflect the change of anode porosity. In Fig. 3, Cell-2 has the best performance, and Cell-3 and Cell-4's performance are worse, meanwhile Cell-1 presents a worst performance. The anode microstructure of those four cells is also trend of the change of electrochemical performance. Fig. 6 shows the IeVeP curves and EIS curves of Cell-2 using carbon black as fuel and compressed air as oxidant at different temperatures. In Fig. 6(a), the open circuit voltages at 850 C, 800 C and 750 C are 1.05 V, 1.03 V and 0.98 V, respectively, which are in good agreement with the theoretical values calculated from the Nernst equation. The maximum power density of Cell-2 at 850 C, 800 C and 750 C are 242 mW cm2, 147 mW cm2 and 74 mW cm2, respectively. With the decrease of temperature, the performance decreased obviously. Fig. 6(b) shows the EIS curve for Cell-2 at different temperatures under open circuit conditions. The ohmic resistance of Cell-2 at 850 C, 800 C and 750 C are
Fig. 1 e The SEM images (a) of Cell-1 after operation, the pattern photo (b) of tubular cell, and the appearance picture (c) of cell.
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Fig. 3 e The IeVeP cures (a) and EIS cures (b) for cell using humidified H2 as fuel and IeVeP cures (c) and EIS cures (d) for cell using carbon black as fuel an at 850 C.
Fig. 4 e The SEM images of cross-section of Cell-1(a), Cell-2(b), Cell-3(c) and Cell-4(d).
0.38 U cm2, 0.48 U cm2, and 0.63 U cm2, respectively, and the polarization resistances are 0.80 U cm2, 1.46 U cm2, and 3.34 U cm2, respectively. Compared with the change of ohmic impedance, the change of polarization impedance is relatively large. For the temperature has a double influence on the
boudouard reaction of carbon and the adsorption and dissociation of CO on the anode, the polarization resistance increases rapidly and the electrochemical performance decreases rapidly. Therefore, the SO-DCFC needs to operate in an environment of more than 800 C.
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Fig. 5 e The BSE images of the anodes of Cell-1(a), Cell-2(b), Cell-3(c) and Cell-4(d).
Fig. 6 e The IeVeP (a) and EIS curves (b) of Cell-2 with carbon fuel.
The stability of Cell-2 under constant current discharge was investigated. In order to save carbon fuel, the stability test is carried out at 750 C. Fig. 7 is a constant current discharge curve of Cell-2 under 0.6 A current. Before the carbon fuel is exhausted, the voltage is relatively stable and remains at about 0.8 V until the fuel is completely exhausted 4 h later. Hence, it shows that SO-DCFCs based on cathode supported tubular fuel cells have a good stability. In this experiment, the initial mixed carbon fuel is about 1 g. It is assumed that carbon monoxide after Boudouard reaction of carbon fuel will totally generate carbon dioxide, hence, 4 mol of electrons will be transferred when 1 mol of carbon is reacted. Discharge at the current of 0.6 A for 4 h, the consuming carbon is about 0.45 g. Before constant current discharge, the Boudouard reaction of carbon fuel was kept for about 3 h during the test process from 850 C to 750 C and the changing temperature process.
Fig. 7 e The discharge curves of Cell-2 operated at electrical current of 0.6 A at 750 C.
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Therefore, the carbon fuel utilization is about 78.75% in the discharging process.
Conclusions Based on cathode-supported tubular solid oxide fuel cell, SODCFCs with the composition of Ni þ YSZ/SSZ/SSZ þ LSM/ LSM was prepared by dip-coating and co-sintering techniques. By changing the amount of pore former (graphite) in the anode slurry, different anodes of SO-DCFCs were achieved with different microstructures and porosities. When the amount of graphite was 10.1 wt%, the SO-DCFC exhibited the best performance. The maximum power density was 242 mW cm2 at 850 C, displaying reasonably good performance stability. For carbon contact form is no contact or direct physical contact, these studies provide technical support for the practical application of SO-DCFCs for the future work.
Acknowledgements The authors would like to thank the Jiangsu Natural Science Foundation (BK20170845), the Fundamental Research Funds for the Central Universities (No.30919011236), National Key R&D Program of China (2018YFB1502600) and National Natural Science Foundation of China (51836004).
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