- Email: [email protected]
Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface
G Model
ARTICLE IN PRESS
CATTOD-9657; No. of Pages 7
Catalysis Today xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface Hansaem Jang a , Jiyoung Eom a , HyungKuk Ju b , Jaeyoung Lee a,b,∗ a Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea b Ertl Center for Electrochemistry and Catalysis, Research Institute for Solar and Sustainable Energies, GIST, Gwangju 500-712, South Korea
a r t i c l e
i n f o
Article history: Received 24 February 2015 Received in revised form 8 June 2015 Accepted 22 June 2015 Available online xxx Keywords: Sn Tin Direct carbon fuel cell Solid oxide fuel cell Carbon fuel cell
a b s t r a c t The catalytic effect of tin (Sn) as a mediator in a direct carbon fuel cell (DCFC) is investigated. Sn is applied onto the Ni–YSZ anode surface with varying in its amount for the Sn quantity optimization. The existence of Sn placed on the anode gives considerably enhanced power density. In addition, the electrochemical behaviors are propense to show the best power density within the region between 15 and 30 mg of Sn per geometric surface area (3.14 cm2 ). After the tests, the distribution of Sn and carbon is confirmed by anode morphology images and analyses through SEM and EPMA/WDS. To better understand the observed phenomena, model experiments are also carried out. The model study suggests that oversupplied Sn mediator becomes agglomerated resulting in less reaction sites, and hence a diminution in power generation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The big motivation has risen to attain a more efficient and less harmful energy conversion system as a global energy consumption keeps increasing every year and environmental concerns enlarge fast simultaneously [1]. One of the efforts for a clean and powerful energy source is a direct carbon fuel cell (DCFC) which only emits pure CO2 [2] able to be captured and then recycled. This system gives an excellent efficiency because it directly converts the fuel into electric energy [2] and it is not subject to Carnot limitations [3]. In addition, any kinds of carbonaceous material including coal [4,5], coke [6,7] and biomass [8] could be employed as a fuel. These solid fuels are readily available in the system as providing us simplicity in treatment and convenience of fuel storage and transport. Solid oxide electrolyte has been widely studied for DCFC due to the benefits including simplicity and fuel flexibility [9–11]. However, using the solid electrolyte results in unfavorable contact between an anode and a fuel due to its large size dimension [12]. This hinders a fuel, an electrolyte and a current collector from forming the triple-phase boundary sites (TPB) in which the
∗ Corresponding author at: Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea. E-mail address: [email protected] (J. Lee).
electrochemical oxidation happens [13], causing more anode polarization and high contact resistance. In addition, formation of TPB with solid fuels is considerably limited than that with gaseous fuels. Besides, the electrochemical reaction on the TPB sites can only occur within no more than 10 m [14–21]. Thus, several approaches have been investigated to overcome anode polarization and one of these is using a liquid metal anode, for example, Ag [22], Bi [23], Fe [24], In [25], Sb [25,26], Sn [23,27–29] and Pb [25,30]. CellTech [27–29] and several groups have studied to foster liquid-state anode using Sn. Liquid Sn anode has considerable oxygen conductivity, high open circuit voltage and ability of tolerance from fuels. However, using Sn as an anode substitute renders a bottleneck in operation. Sn is oxidized into SnO2 working as an insulator, and therefore interfacial conductivity between electrolyte and anode becomes hindered electronically as well as ionically [13]. In order to avoid the oxide layer, operation over 1000 ◦ C is suggested since formation of SnO2 is thermodynamically unfavorable at this temperature [13]. Moreover, this high temperature is also required for the cell operation due to relatively high impedances exhibited in liquid Sn anode cells [25,27]. In metallurgy, carbothermal reduction of cassiterite (Sn ore consisting of SnO2 ) into metallic Sn has been demonstrated [31]. Thermodynamic studies on the reduction of SnO2 corroborate that the state of Sn speciation with carbonaceous material in high temperature regions would remain reduced as Sn [32]. The role of CO as
http://dx.doi.org/10.1016/j.cattod.2015.06.013 0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
2
a reducing agent is also investigated. By Mössbauer spectroscopy, Sn(II) resonance was detected in the sample exposed to CO, indicating a consumption of lattice oxygen, hence the reduction by CO [33]. The following equations are the most widely accepted mechanisms concerning the reduction behavior: SnO2 + 2C = Sn(l) + 2CO(g)
(1)
SnO2 + C = Sn(l) + CO2 (g)
(2)
SnO2 + 2CO = Sn(l) + 2CO2 (g)
(3)
CO2 + C = 2CO(s)
(4)
All the Gibbs free energy values of Eqs. (1)–(3) are negative at the temperature region higher than 641 ◦ C [32]. The reverse Boudouard reaction (Eq. (4)) spontaneously occurs at the temperature region higher than 702 ◦ C [34], over which solid–gas reactions (Eqs. (3) and (4)) are also speculated, while the Gibbs energy values of solid–solid reactions (Eqs. (1) and (2)) are more thermodynamically negative at this range [31,32]. Solid–gas reactions could be favored due to better diffusivity than that of solid–solid. In addition, it is reported that the minimum amount of C to fully reduce 1 mol of SnO2 into Sn is 1.25 mol [32], implying that possibly formed SnO2 on the interface is expected to be reduced into Sn in DCFC systems. In this study, we investigate the catalytic effect of Sn as an interfacial mediator between an anode and a fuel in order to improve TPBs and derive benefits from the use of Sn. Sn is not employed as an anode alternative due to the consideration on SnO2 but applied onto the Ni–YSZ anode surface with varying in its amount for the Sn quantity optimization. Ni is generally used for electrocatalysis that could accelerate carbon reactivity [35,36] and YSZ is a wellknown oxygen ionic conducting material. As our previous study suggested [37], we expect the synergistic effect of Sn combined with Ni–YSZ plays a critical role as a mediator providing ameliorated anode polarization, and hence the enhanced electrochemical reaction of carbon fuels. 2. Experimental 2.1. Preparation of fuel cells Carbon black powder (ENSACO 350G, Timcal) is employed as a pure carbon source due to its high specific surface area. A commercial anode-supported button-type cell (Ceramic fuel cell power) is selected and composed of Ni–YSZ as an anode, 8YSZ as an electrolyte (8YSZ) and LSM as a cathode. The porosity of an anode is 28.4%, measured by mercury intrusion porosimetry. Cells are assembled with Pt (99.9%, 52 mesh, Alfa Aesar) current collector using Ag paste (Fujikura Kasei) and a sealant (Thermiculite 866) for an air-seal. Commercially available Sn powder (<150 m, Aldrich) is chosen as an interfacial mediator and shows regular particle size distribution – average diameter is ca. 4.5 m measured optically through SEM analysis. The experiments were carried out with varying in applied quantity of Sn: 15, 30 and 60 mg. Sn powder was gently brushed and evenly distributed on the anode surface (geometric surface area: 3.14 cm2 ), and then 300 mg of carbon black was mounted onto the Sn-covered anode surface. In addition, as reference experiments, 300 mg of pure Sn and 300 mg of pure carbon black were tested respectively. The cell–fuel assembly was well connected to a workstation through Pt current collector and then thoroughly air-sealed using a sealant (Aremco ceramabond 668). 2.2. Power performance test procedures The power performance test was carried out with an optimally designed apparatus consisting of an alumina ceramic reactor, a furnace and an electrochemical workstation (NARA Cell-Tech). The
electrochemical tests were performed at different temperatures: 700 ◦ C, 750 ◦ C, 800 ◦ C and 850 ◦ C. The reactor was heated up at a ramping rate of 5 ◦ C min−1 up to the desired temperature. At the temperature region from room temperature to 600 ◦ C, the anodic chamber was continuously purged out using pure Ar gas (99.999%) at a rate of 30 mL min−1 to remove internal O2 , and hence the prevention of fuel oxidation and loss. Meanwhile and until the end of experiments, the cathodic chamber was continuously fed with pure O2 (99.99%) gas at a rate of 50 mL min−1 to provide oxide ion precursors. All gas lines toward the reactor were preheated and thus the gases were kept at 100 ◦ C to prevent a sudden temperature drop since it could influence the cell resistance [38,39]. 2.3. Characterization and model experiments Characterization was carried out with several analyses after performance tests. The change in the surface morphology was magnified with a field emission scanning electron microscope (FE-SEM; S-4700, Hitachi, Japan). The SEM results led to model experiments in order to clearly understand the behaviors and phenomena of Sn powders. An electron probe microanalysis (EPMA-1600; Shimadzu, Japan) equipped with wavelength dispersed X-ray (WDX) was used to determine the elemental quantitative depth-profile of the anode cross-section. Model experiments were carried out to understand agglomeration phenomena of Sn powders. Instead of using a solid oxide cell, Si wafer was employed due to its durability at high temperature region. The experiments were performed under the same temperatures and amounts of Sn as the power performance tests were. In addition, some model experiments were undertaken without covering carbon black over Sn surface in order to figure out the role of carbon. 3. Results and discussion The reactions at the anode in DCFC can be divided into direct and indirect pathway. Direct carbon oxidation reactions (C + nO2− → COn + 2ne− ) happen only at the interface between carbon and anode, which is called triple phase boundary (TPB) where a fuel, an electron conducting material and an ion conducting material meet. However, since both a fuel and an anode are solid-state leading to contact resistance, the reaction is thereby sluggish and has large anode polarization [29]. As our previous study [37] and the other group [40] suggested, Sn could be utilized to overcome this slow kinetic in terms of enhancing interface condition and electrochemical activity. In this sense, we tried to make Sn as an electrochemical mediator on the anode; therefore, carbon would preferentially and favorably oxidize into carbon dioxide. In order to demonstrate the effect of Sn mediator in terms of the quantity, the cells were prepared with different amount of Sn applied onto the anode surface. In addition, as reference experiments, pure Sn and pure carbon black were also tested respectively. Expectedly, the existance of Sn mediator notably enhances the cell performance as shown in Fig. 1. Throughout all the experimental temperature region, 15 mg of Sn shows the best performance, for example, the maximum power density is 136 mW cm−2 while that of carbon black only is less than half at 850 ◦ C. Meanwhile, the power performance test of 30 mg of Sn keeps the second best and almost reached to that of 15 mg of Sn at higher temperatures. It implies that the behaviors including distribution, permeation and mediation of liquid state Sn on the interface of Ni–YSZ pores become most optimized within 15–30 mg of Sn at higher temperatures. Unlike the power density trend of 15 and 30 mg of Sn, that of 60 mg matches that of the Sn reference experiment up to 750 ◦ C
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
3
Fig. 1. Current–potential (I–V) and –power (I–P) curves of three different amount of Sn (䊉 Sn 15 mg, Sn 30 mg and Sn 60 mg) and two references (䊏 carbon black and Sn only) experimented at different temperatures: (a) 700 ◦ C, (b) 750 ◦ C, (c) 800 ◦ C and (d) 850 ◦ C.
and then starts to increase at higher temperatures, implying that Sn partaking the electrochemical reaction up to 750 ◦ C would possibly become oxidized into SnO2 and form oxide layer. This phenomenon, moreover, suggests that 60 mg of Sn cover anode surface and pores too thick and therefore give less chance to carbon particles to meet TPBs and then to be electrochemically oxidized up to 750 ◦ C. At higher temperatures, on the other hand, Sn permeates well into Ni–YSZ anode pores with carbon particles together and hence the enhanced mixed potential than that of the reference experiments. Nonetheless, the curves of 60 mg of Sn are not still in proximity to those of 15 and 30 mg of Sn at higher temperatures due to its excessive distribution thickness. In order to clearly understand the behavior of Sn on the anode surface, the cells after the experiment were magnified with scanning electron microscope (SEM) and the results are shown in Fig. 2. As available in Fig. 2(a), the anode surface after the experiment with 15 mg of Sn is covered with gently distributed Sn and the Ni–YSZ anode pores are exquisitely permeated by Sn. In addition, carbon black is attached not only on Sn but on Ni–YSZ surface as well, meaning that 15 mg is an adequate quantity to mediate the interface between a fuel and an anode. On the other hand, as exhibited in Fig. 2(b), Sn powder agglomerates themselves together to form larger clusters that clog the anode pores, meaning that 60 mg is a superfluous amount for mediating the surface and leads to a hindrance to carbon electrochemical reaction. Moreover, since the cell, experiencing the power performance test, once reached to higher temperatures at which Sn permeation is optimized, the agglomerated Sn remaining on the surface is an evidence of oversupply.
As shown in Fig. 3, the open circuit voltages of the experiments at the elevated temperature region with Sn mediator did not follow the theoretical potential of Sn oxidation [34,41]. This implies that Sn was not further oxidized into SnO2 , which can hinder both ionic and electronic conducting behavior [13], but remained in liquid Sn as a mediator. Moreover, it is speculated that the possibly formed SnO2 at lower temperature was reduced and existed as liquid Sn, since Eqs. (1)–(4) are all spontaneous at the experimental temperature. In addition, Fig. 4 is provided in order to demonstrate the electricity was generated by the electrochemical oxidation of C, not of Sn. The discharge test result exhibits that the time for the C over Sn experiments (solid black and red lines) is longer than that for the multiple Sn turnovers (solid blue line), meaning that Sn was not utilized as a fuel until C was almost fully used. Battery mode was also observed, in which metallic liquid Sn started to become electrochemically oxidized into SnO2 like a fuel. In other words, the battery mode could be an evidence that metal was not yet oxidized [42], in this sense, the long term performance result represents that Sn started to turn into SnO2 after ca. 100 min for 60 mg of Sn with carbon. Electron probe micro-analyzer with wave length dispersive spectrometry (EPMA/WDS) renders a quantitative depth-profile data of C and Sn elements on the anode surface cross section as shown in Fig. 5. This shows the permeation effect of Sn (black lines) and carbon black powder (red lines). As available in Fig. 5(a), Sn is well permeated into the Ni–YSZ pores up to 100 m when 15 mg of Sn is applied, meaning that interfacial mediation by Sn could happen even in deep area from the cell surface. Moreover
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7 4
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
Fig. 2. Scanning electron microscope (SEM) images of the anode surface after the experiment with (a) 15 mg of Sn and (b) 60 mg of Sn.
Fig. 3. Comparison between (a) the experimental open circuit voltage of a DCFC test with the different amount of Sn and carbon black (䊏 carbon black, 䊉 Sn 15 mg, Sn 30 mg, Sn 60 mg and Sn only) and (b) the theoretical potential of a partial oxidation of carbon (C + 1/2O2 = CO), a complete oxidation of carbon (C + O2 = CO2 ), a partial oxidation of carbon monoxide (CO + 1/2O2 = CO2 ), and a complete oxidation of tin (Sn + O2 = SnO2 ).
the graph of C is relatively similar to that of Sn, suggesting that Sn permeation affects carbon black particle in some degree. However, in Fig. 5(b), when 60 mg of Sn is applied, the permeated quantity of Sn is considerably limited and only reaches up to approximately
Fig. 4. Discharge test for different amount of tin at the constant voltage, 0.4 V. The total charge is calculated as 1301.6 mAh, 765.2 mAh, respectively corresponding to 15 mg of Sn (solid black line) and 60 mg of Sn (solid red line). As a reference, the experiment with Sn only (solid blue line) is provided. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
60 m, implying that the agglomerated Sn clusters form larger clusters together and then clog the anode pores, leading to a hindrance not only to carbon ingress but to Sn distribution as well. Expectedly, the permeation of C is quite restricted than that shown in Fig. 5(a), showing relatively low power performances as exhibited in Fig. 1. T. Tao et al. [29] reported that the permeation of liquid Sn into porous ceramic material is not facilitated due to its high surface tension (>400 dyn cm-1 ) and large contact angle with ceramics (>90◦ ). This phenomenon was also observed in the experiments as shown in Figs. 2 and 5 especially in the case when Sn was oversupplied. It implies that the oversupplied Sn remained on the anode surface became liquid-state at high temperature region, forming larger cluster with dragging shallowly permeated Sn from porous anode surface. Therefore, the enlarged cluster steadily accumulated and more agglomerated on the anode surface due to the high surface tension of Sn. However, when the adequate amount of Sn was applied onto the anode surface, it was able to better permeate into the pores than when oversupplied as exhibited in Fig. 5, due to the less possibility to be dragged by agglomerated particle. The permeation of Sn also influenced that of C as discussed earlier, and moreover the improvement in reaction sites. For the purpose of understanding agglomeration of Sn powders, model experiments are carried out and shown in Fig. 6. Fig. 6(a)–(c) represents the experiments without covering carbon black over Sn particles and Fig. 6(d)–(l) are with carbon powder; the left-side figures (a, d, g and j) are with 15 mg of Sn, the middle ones (b, e, h and k) are 30 mg, and the right-sides (c, f, i and l) are 60 mg. In Fig. 6(a)–(c), the spherical morphologies are agglomerated Sn
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
5
Fig. 5. Quantitative depth-profile analysis of the anode surface cross-section via electron probe micro-analyzer with wavelength dispersive spectrometry (EPMA/WDS) method in order to identify the residual amount of Sn (solid black lines) and C (solid red lines) after the experiment (a) with the 15 mg of Sn and (b) with the 60 mg of Sn. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. The SEM images of the model experiment to figure out the agglomeration effect shown in the different amount of Sn as a mediator (a, d, g, j: 15 mg; b, e, h, k: 30 mg; c, f, i, l: 60 mg) on the anode surface (a–c) without carbon black powder over Sn particles and (d–l) with carbon black powder over Sn particles. Specifically, (g–i) shows the distribution and coverage of Sn mediator and carbon black fuel and (j–l) shows the capillary effect of liquid Sn.
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7 6
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
Fig. 7. (Left) The schematic diagrams of the expected mechanism for Sn mediator of (a) 15 mg and (b) 60 mg. (Right) The corresponding SEM images of the proposed diagram in juxtaposition and the magnified images of the interface in the yellow box. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
particles and the sizes of which grow prodigiously as the more amount of Sn is applied, while Si wafer remains clear and is explained as dark-gray background. In Fig. 6 (d)–(f), however, diameters of the spheres are noticeably shrunk comparing to the corresponding Sn amount in Fig. 6(a)–(c). It suggests that gravitational pull of carbon black toward Sn powder leads to the distribution of Sn. Therefore, Si wafer is totally covered by well distributed and dispersed Sn particles so the backgrounds are silvery, especially in Fig. 6(d). In addition, irregular shapes existing on the surface other than Sn spheres are carbon particles in Fig. 6(d)–(i). In Fig. 6(d) and (g), most of carbon black particles are placed onto the sub-micron Sn mediator surface, meaning that carbon has strong possibility to meet TPBs and hence excellent power performance. However, in Fig. 6(f) and (i), the coverage of agglomerated Sn is too broad on which carbon particles become considerably located, leading to restriction of access to TPBs. Carbon-anode interfacial interaction is also investigated with the model experiments and exhibited in Fig. 6(j)–(l), which are the magnified images of Fig. 6(g)–(i), respectively. The images show the capillary effect of liquid Sn, implying that the interface between carbon fuel and anode can be well mediated by liquid-state Sn and the poor interaction would be ameliorated. In addition, the capillary phenomenon could effectuate permeation of Sn into Ni–YSZ anode pores with carbon ingress onto TPBs as mentioned in Fig. 5(a). However, liquid Sn is mobile one in the capillaries and pores, and carbon black is mounted onto Sn surfaces for the experiments, suggesting that carbon interaction on TPBs are quite restricted once Sn covers Ni–YSZ anode pores too thick. Considering Sn distribution on anode surface and permeation into anode pores, 15–30 mg of Sn per 3.14 cm2 (geometric surface area)
seems to provide optimized mediation at experimental condition as expected in Fig. 1.
4. Conclusion We investigated the catalytic effect of Sn as a mediator on Ni–YSZ anode surface. As our previous study [37] suggested, Sn rendered enhanced interfacial electrochemical activity. The experiment with 15 mg of Sn shows the best performance with the maximum power density of 136 mW cm−2 at 850 ◦ C. Characterization and discharge test results demonstrate that electricity is generated by C which is employed as an electrochemically oxidisable fuel, not by liquid Sn. The power performance tests and the model experiments suggest that 15–30 mg of Sn is the optimized quantity for interfacial mediating effect due to its excellent properties on distribution, permeation and mediation as a liquid state Sn at higher temperatures. On the other hand, 60 mg of Sn covers Ni–YSZ anode surface and pores too thick and hence a hindrance to carbon access to TPBs, leading to relatively low power density. For the clear understanding, schematic diagrams for each case are visualized with simplified porous Ni–YSZ anode surface in Fig. 7.
Acknowledgments This work was supported by the New & Renewable Energy Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy (No. 20113020030010).
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
References [1] BP plc, BP Staistical Review of World Energy 2014, 63rd ed., BP plc, 2014, pp. 1–45. [2] D. Cao, Y. Sun, G. Wang, J. Power Sources 167 (2007) 250–257. [3] H.A. Liebhafsky, J. Electrochem. Soc. 106 (1959) 1068–1071. [4] T.M. Gür, M. Homel, A.V. Virkar, J. Power Sources 195 (2010) 1085–1090. [5] H. Ju, J. Eom, J.K. Lee, H. Choi, T.-H. Lim, R.-H. Song, J. Lee, Electrochim. Acta 115 (2014) 511–517. [6] C. Li, Y. Shi, N. Cai, J. Power Sources 195 (2010) 4660–4666. [7] F. Guzman, R. Singh, S.S.C. Chuang, Energy Fuels 25 (2011) 2179–2186. [8] A. Elleuch, A. Boussetta, J. Yu, K. Halouani, Y. Li, Int. J. Hydrogen Energy 38 (2013) 16590–16604. [9] N. Nakagawa, M. Ishida, Ind. Eng. Chem. Res. 27 (1988) 1181–1185. [10] T.M. Gür, R.A. Huggins, J. Electrochem. Soc. 139 (1992) L95–L97. [11] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 142 (1995) 2621–2624. [12] J.S. Cronin, J.R. Wilson, S.A. Barnett, J. Power Sources 196 (2011) 2640–2643. [13] T.M. Gür, Chem. Rev. 113 (2013) 6179–6206. [14] J. Mizusaki, H. Tagawa, J. Electrochem. Soc. 138 (1991) 1867–1873. [15] L.G.J. de Haart, R.A. Kuipers, K.J. de Vries, A.J. Burggraaf, J. Electrochem. Soc. 138 (1991) 1970–1975. [16] M.J.L. Østergård, M. Mogensen, Electrochim. Acta 38 (1993) 2015–2020. [17] H. Fukunaga, M. Ihara, K. Sakaki, K. Yamada, Solid State Ion. 86–88 (1996) 1179–1185. [18] C.W. Tanner, K.-Z. Fung, A.V. Virkar, J. Electrochem. Soc. 144 (1997) 21–30. [19] M. Brown, S. Primdahl, M. Mogensen, J. Electrochem. Soc. 147 (2000) 475–485. [20] X. Wang, N. Nakagawa, K. Kato, J. Electrochem. Soc. 148 (2001) A565–A569. [21] S. McIntosh, R.J. Gorte, Chem. Rev. 104 (2004) 4845–4865. [22] T.M. Gur, U.S. Patent (Ed.), Clean Coal Energy, United States (2006).
7
[23] A. Jayakumar, S. Lee, A. Hornés, J.M. Vohs, R.J. Gorte, J. Electrochem. Soc. 157 (2010) B365–B369. [24] I.V. Yentekakis, P.G. Debenedetti, B. Costa, Ind. Eng. Chem. Res. 28 (1989) 1414–1424. [25] A. Jayakumar, J.M. Vohs, R.J. Gorte, Ind. Eng. Chem. Res. 49 (2010) 10237–10241. [26] A. Jayakumar, R. Küngas, S. Roy, A. Javadekar, D.J. Buttrey, J.M. Vohs, R.J. Gorte, Energy Environ. Sci. 4 (2011) 4133. [27] W.A.G. McPhee, M. Boucher, J. Stuart, R.S. Parnas, M. Koslowske, T. Tao, B.A. Wilhite, Energy Fuels 23 (2009) 5036–5041. [28] T. Tao, L. Bateman, J. Bentley, M. Slaney, ECS Trans. 5 (2007) 463–472. [29] T. Tao, M. Slaney, L. Bateman, J. Bentley, ECS Trans. 7 (2007) 1389–1397. [30] M. Anbar, D.F. McMillen, R.D. Weaver, P.J. Jorgensen, U.S. Patent (Ed.), Stanford Research Institute, United States (1976). [31] R. Padilla, H.Y. Sohn, Metall. Trans. B 10B (1979) 109–115. [32] G. Levêque, S. Abanades, Energy Fuels 28 (2014) 1396–1405. [33] M. Batzill, U. Diebold, Prog. Surf. Sci. 79 (2005) 47–154. [34] D.R. Lide, CRC Handbook of Chemistry and Physics, 90th ed., CRC Press, 2009. [35] Y. Nabae, K.D. Pointon, J.T.S. Irvine, J. Electrochem. Soc. 156 (2009) B716. [36] Y. Tang, J. Liu, Int. J. Hydrogen Energy 35 (2010) 11188–11193. [37] H. Ju, S. Uhm, J.W. Kim, R.-H. Song, H. Choi, S.-H. Lee, J. Lee, J. Power Sources 198 (2012) 36–41. [38] K. Wincewicz, J. Cooper, J. Power Sources 140 (2005) 280–296. [39] D.J. Brett, A. Atkinson, N.P. Brandon, S.J. Skinner, Chem. Soc. Rev. 37 (2008) 1568–1578. [40] H. Kan, H. Lee, Appl. Catal. B: Environ. 97 (2010) 108–114. [41] M. Iwase, M.-O. Yasuda, S.-I. Miki, T. Mori, Trans. Jpn. Inst. Met. 19 (1978) 654–660. [42] A. Jayakumar, A. Javadekar, R. Küngas, D.J. Buttrey, J.M. Vohs, R.J. Gorte, ECS Trans. 41 (2012) 149–158.
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
ARTICLE IN PRESS
CATTOD-9657; No. of Pages 7
Catalysis Today xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface Hansaem Jang a , Jiyoung Eom a , HyungKuk Ju b , Jaeyoung Lee a,b,∗ a Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea b Ertl Center for Electrochemistry and Catalysis, Research Institute for Solar and Sustainable Energies, GIST, Gwangju 500-712, South Korea
a r t i c l e
i n f o
Article history: Received 24 February 2015 Received in revised form 8 June 2015 Accepted 22 June 2015 Available online xxx Keywords: Sn Tin Direct carbon fuel cell Solid oxide fuel cell Carbon fuel cell
a b s t r a c t The catalytic effect of tin (Sn) as a mediator in a direct carbon fuel cell (DCFC) is investigated. Sn is applied onto the Ni–YSZ anode surface with varying in its amount for the Sn quantity optimization. The existence of Sn placed on the anode gives considerably enhanced power density. In addition, the electrochemical behaviors are propense to show the best power density within the region between 15 and 30 mg of Sn per geometric surface area (3.14 cm2 ). After the tests, the distribution of Sn and carbon is confirmed by anode morphology images and analyses through SEM and EPMA/WDS. To better understand the observed phenomena, model experiments are also carried out. The model study suggests that oversupplied Sn mediator becomes agglomerated resulting in less reaction sites, and hence a diminution in power generation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The big motivation has risen to attain a more efficient and less harmful energy conversion system as a global energy consumption keeps increasing every year and environmental concerns enlarge fast simultaneously [1]. One of the efforts for a clean and powerful energy source is a direct carbon fuel cell (DCFC) which only emits pure CO2 [2] able to be captured and then recycled. This system gives an excellent efficiency because it directly converts the fuel into electric energy [2] and it is not subject to Carnot limitations [3]. In addition, any kinds of carbonaceous material including coal [4,5], coke [6,7] and biomass [8] could be employed as a fuel. These solid fuels are readily available in the system as providing us simplicity in treatment and convenience of fuel storage and transport. Solid oxide electrolyte has been widely studied for DCFC due to the benefits including simplicity and fuel flexibility [9–11]. However, using the solid electrolyte results in unfavorable contact between an anode and a fuel due to its large size dimension [12]. This hinders a fuel, an electrolyte and a current collector from forming the triple-phase boundary sites (TPB) in which the
∗ Corresponding author at: Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea. E-mail address: [email protected] (J. Lee).
electrochemical oxidation happens [13], causing more anode polarization and high contact resistance. In addition, formation of TPB with solid fuels is considerably limited than that with gaseous fuels. Besides, the electrochemical reaction on the TPB sites can only occur within no more than 10 m [14–21]. Thus, several approaches have been investigated to overcome anode polarization and one of these is using a liquid metal anode, for example, Ag [22], Bi [23], Fe [24], In [25], Sb [25,26], Sn [23,27–29] and Pb [25,30]. CellTech [27–29] and several groups have studied to foster liquid-state anode using Sn. Liquid Sn anode has considerable oxygen conductivity, high open circuit voltage and ability of tolerance from fuels. However, using Sn as an anode substitute renders a bottleneck in operation. Sn is oxidized into SnO2 working as an insulator, and therefore interfacial conductivity between electrolyte and anode becomes hindered electronically as well as ionically [13]. In order to avoid the oxide layer, operation over 1000 ◦ C is suggested since formation of SnO2 is thermodynamically unfavorable at this temperature [13]. Moreover, this high temperature is also required for the cell operation due to relatively high impedances exhibited in liquid Sn anode cells [25,27]. In metallurgy, carbothermal reduction of cassiterite (Sn ore consisting of SnO2 ) into metallic Sn has been demonstrated [31]. Thermodynamic studies on the reduction of SnO2 corroborate that the state of Sn speciation with carbonaceous material in high temperature regions would remain reduced as Sn [32]. The role of CO as
http://dx.doi.org/10.1016/j.cattod.2015.06.013 0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
2
a reducing agent is also investigated. By Mössbauer spectroscopy, Sn(II) resonance was detected in the sample exposed to CO, indicating a consumption of lattice oxygen, hence the reduction by CO [33]. The following equations are the most widely accepted mechanisms concerning the reduction behavior: SnO2 + 2C = Sn(l) + 2CO(g)
(1)
SnO2 + C = Sn(l) + CO2 (g)
(2)
SnO2 + 2CO = Sn(l) + 2CO2 (g)
(3)
CO2 + C = 2CO(s)
(4)
All the Gibbs free energy values of Eqs. (1)–(3) are negative at the temperature region higher than 641 ◦ C [32]. The reverse Boudouard reaction (Eq. (4)) spontaneously occurs at the temperature region higher than 702 ◦ C [34], over which solid–gas reactions (Eqs. (3) and (4)) are also speculated, while the Gibbs energy values of solid–solid reactions (Eqs. (1) and (2)) are more thermodynamically negative at this range [31,32]. Solid–gas reactions could be favored due to better diffusivity than that of solid–solid. In addition, it is reported that the minimum amount of C to fully reduce 1 mol of SnO2 into Sn is 1.25 mol [32], implying that possibly formed SnO2 on the interface is expected to be reduced into Sn in DCFC systems. In this study, we investigate the catalytic effect of Sn as an interfacial mediator between an anode and a fuel in order to improve TPBs and derive benefits from the use of Sn. Sn is not employed as an anode alternative due to the consideration on SnO2 but applied onto the Ni–YSZ anode surface with varying in its amount for the Sn quantity optimization. Ni is generally used for electrocatalysis that could accelerate carbon reactivity [35,36] and YSZ is a wellknown oxygen ionic conducting material. As our previous study suggested [37], we expect the synergistic effect of Sn combined with Ni–YSZ plays a critical role as a mediator providing ameliorated anode polarization, and hence the enhanced electrochemical reaction of carbon fuels. 2. Experimental 2.1. Preparation of fuel cells Carbon black powder (ENSACO 350G, Timcal) is employed as a pure carbon source due to its high specific surface area. A commercial anode-supported button-type cell (Ceramic fuel cell power) is selected and composed of Ni–YSZ as an anode, 8YSZ as an electrolyte (8YSZ) and LSM as a cathode. The porosity of an anode is 28.4%, measured by mercury intrusion porosimetry. Cells are assembled with Pt (99.9%, 52 mesh, Alfa Aesar) current collector using Ag paste (Fujikura Kasei) and a sealant (Thermiculite 866) for an air-seal. Commercially available Sn powder (<150 m, Aldrich) is chosen as an interfacial mediator and shows regular particle size distribution – average diameter is ca. 4.5 m measured optically through SEM analysis. The experiments were carried out with varying in applied quantity of Sn: 15, 30 and 60 mg. Sn powder was gently brushed and evenly distributed on the anode surface (geometric surface area: 3.14 cm2 ), and then 300 mg of carbon black was mounted onto the Sn-covered anode surface. In addition, as reference experiments, 300 mg of pure Sn and 300 mg of pure carbon black were tested respectively. The cell–fuel assembly was well connected to a workstation through Pt current collector and then thoroughly air-sealed using a sealant (Aremco ceramabond 668). 2.2. Power performance test procedures The power performance test was carried out with an optimally designed apparatus consisting of an alumina ceramic reactor, a furnace and an electrochemical workstation (NARA Cell-Tech). The
electrochemical tests were performed at different temperatures: 700 ◦ C, 750 ◦ C, 800 ◦ C and 850 ◦ C. The reactor was heated up at a ramping rate of 5 ◦ C min−1 up to the desired temperature. At the temperature region from room temperature to 600 ◦ C, the anodic chamber was continuously purged out using pure Ar gas (99.999%) at a rate of 30 mL min−1 to remove internal O2 , and hence the prevention of fuel oxidation and loss. Meanwhile and until the end of experiments, the cathodic chamber was continuously fed with pure O2 (99.99%) gas at a rate of 50 mL min−1 to provide oxide ion precursors. All gas lines toward the reactor were preheated and thus the gases were kept at 100 ◦ C to prevent a sudden temperature drop since it could influence the cell resistance [38,39]. 2.3. Characterization and model experiments Characterization was carried out with several analyses after performance tests. The change in the surface morphology was magnified with a field emission scanning electron microscope (FE-SEM; S-4700, Hitachi, Japan). The SEM results led to model experiments in order to clearly understand the behaviors and phenomena of Sn powders. An electron probe microanalysis (EPMA-1600; Shimadzu, Japan) equipped with wavelength dispersed X-ray (WDX) was used to determine the elemental quantitative depth-profile of the anode cross-section. Model experiments were carried out to understand agglomeration phenomena of Sn powders. Instead of using a solid oxide cell, Si wafer was employed due to its durability at high temperature region. The experiments were performed under the same temperatures and amounts of Sn as the power performance tests were. In addition, some model experiments were undertaken without covering carbon black over Sn surface in order to figure out the role of carbon. 3. Results and discussion The reactions at the anode in DCFC can be divided into direct and indirect pathway. Direct carbon oxidation reactions (C + nO2− → COn + 2ne− ) happen only at the interface between carbon and anode, which is called triple phase boundary (TPB) where a fuel, an electron conducting material and an ion conducting material meet. However, since both a fuel and an anode are solid-state leading to contact resistance, the reaction is thereby sluggish and has large anode polarization [29]. As our previous study [37] and the other group [40] suggested, Sn could be utilized to overcome this slow kinetic in terms of enhancing interface condition and electrochemical activity. In this sense, we tried to make Sn as an electrochemical mediator on the anode; therefore, carbon would preferentially and favorably oxidize into carbon dioxide. In order to demonstrate the effect of Sn mediator in terms of the quantity, the cells were prepared with different amount of Sn applied onto the anode surface. In addition, as reference experiments, pure Sn and pure carbon black were also tested respectively. Expectedly, the existance of Sn mediator notably enhances the cell performance as shown in Fig. 1. Throughout all the experimental temperature region, 15 mg of Sn shows the best performance, for example, the maximum power density is 136 mW cm−2 while that of carbon black only is less than half at 850 ◦ C. Meanwhile, the power performance test of 30 mg of Sn keeps the second best and almost reached to that of 15 mg of Sn at higher temperatures. It implies that the behaviors including distribution, permeation and mediation of liquid state Sn on the interface of Ni–YSZ pores become most optimized within 15–30 mg of Sn at higher temperatures. Unlike the power density trend of 15 and 30 mg of Sn, that of 60 mg matches that of the Sn reference experiment up to 750 ◦ C
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
3
Fig. 1. Current–potential (I–V) and –power (I–P) curves of three different amount of Sn (䊉 Sn 15 mg, Sn 30 mg and Sn 60 mg) and two references (䊏 carbon black and Sn only) experimented at different temperatures: (a) 700 ◦ C, (b) 750 ◦ C, (c) 800 ◦ C and (d) 850 ◦ C.
and then starts to increase at higher temperatures, implying that Sn partaking the electrochemical reaction up to 750 ◦ C would possibly become oxidized into SnO2 and form oxide layer. This phenomenon, moreover, suggests that 60 mg of Sn cover anode surface and pores too thick and therefore give less chance to carbon particles to meet TPBs and then to be electrochemically oxidized up to 750 ◦ C. At higher temperatures, on the other hand, Sn permeates well into Ni–YSZ anode pores with carbon particles together and hence the enhanced mixed potential than that of the reference experiments. Nonetheless, the curves of 60 mg of Sn are not still in proximity to those of 15 and 30 mg of Sn at higher temperatures due to its excessive distribution thickness. In order to clearly understand the behavior of Sn on the anode surface, the cells after the experiment were magnified with scanning electron microscope (SEM) and the results are shown in Fig. 2. As available in Fig. 2(a), the anode surface after the experiment with 15 mg of Sn is covered with gently distributed Sn and the Ni–YSZ anode pores are exquisitely permeated by Sn. In addition, carbon black is attached not only on Sn but on Ni–YSZ surface as well, meaning that 15 mg is an adequate quantity to mediate the interface between a fuel and an anode. On the other hand, as exhibited in Fig. 2(b), Sn powder agglomerates themselves together to form larger clusters that clog the anode pores, meaning that 60 mg is a superfluous amount for mediating the surface and leads to a hindrance to carbon electrochemical reaction. Moreover, since the cell, experiencing the power performance test, once reached to higher temperatures at which Sn permeation is optimized, the agglomerated Sn remaining on the surface is an evidence of oversupply.
As shown in Fig. 3, the open circuit voltages of the experiments at the elevated temperature region with Sn mediator did not follow the theoretical potential of Sn oxidation [34,41]. This implies that Sn was not further oxidized into SnO2 , which can hinder both ionic and electronic conducting behavior [13], but remained in liquid Sn as a mediator. Moreover, it is speculated that the possibly formed SnO2 at lower temperature was reduced and existed as liquid Sn, since Eqs. (1)–(4) are all spontaneous at the experimental temperature. In addition, Fig. 4 is provided in order to demonstrate the electricity was generated by the electrochemical oxidation of C, not of Sn. The discharge test result exhibits that the time for the C over Sn experiments (solid black and red lines) is longer than that for the multiple Sn turnovers (solid blue line), meaning that Sn was not utilized as a fuel until C was almost fully used. Battery mode was also observed, in which metallic liquid Sn started to become electrochemically oxidized into SnO2 like a fuel. In other words, the battery mode could be an evidence that metal was not yet oxidized [42], in this sense, the long term performance result represents that Sn started to turn into SnO2 after ca. 100 min for 60 mg of Sn with carbon. Electron probe micro-analyzer with wave length dispersive spectrometry (EPMA/WDS) renders a quantitative depth-profile data of C and Sn elements on the anode surface cross section as shown in Fig. 5. This shows the permeation effect of Sn (black lines) and carbon black powder (red lines). As available in Fig. 5(a), Sn is well permeated into the Ni–YSZ pores up to 100 m when 15 mg of Sn is applied, meaning that interfacial mediation by Sn could happen even in deep area from the cell surface. Moreover
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7 4
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
Fig. 2. Scanning electron microscope (SEM) images of the anode surface after the experiment with (a) 15 mg of Sn and (b) 60 mg of Sn.
Fig. 3. Comparison between (a) the experimental open circuit voltage of a DCFC test with the different amount of Sn and carbon black (䊏 carbon black, 䊉 Sn 15 mg, Sn 30 mg, Sn 60 mg and Sn only) and (b) the theoretical potential of a partial oxidation of carbon (C + 1/2O2 = CO), a complete oxidation of carbon (C + O2 = CO2 ), a partial oxidation of carbon monoxide (CO + 1/2O2 = CO2 ), and a complete oxidation of tin (Sn + O2 = SnO2 ).
the graph of C is relatively similar to that of Sn, suggesting that Sn permeation affects carbon black particle in some degree. However, in Fig. 5(b), when 60 mg of Sn is applied, the permeated quantity of Sn is considerably limited and only reaches up to approximately
Fig. 4. Discharge test for different amount of tin at the constant voltage, 0.4 V. The total charge is calculated as 1301.6 mAh, 765.2 mAh, respectively corresponding to 15 mg of Sn (solid black line) and 60 mg of Sn (solid red line). As a reference, the experiment with Sn only (solid blue line) is provided. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
60 m, implying that the agglomerated Sn clusters form larger clusters together and then clog the anode pores, leading to a hindrance not only to carbon ingress but to Sn distribution as well. Expectedly, the permeation of C is quite restricted than that shown in Fig. 5(a), showing relatively low power performances as exhibited in Fig. 1. T. Tao et al. [29] reported that the permeation of liquid Sn into porous ceramic material is not facilitated due to its high surface tension (>400 dyn cm-1 ) and large contact angle with ceramics (>90◦ ). This phenomenon was also observed in the experiments as shown in Figs. 2 and 5 especially in the case when Sn was oversupplied. It implies that the oversupplied Sn remained on the anode surface became liquid-state at high temperature region, forming larger cluster with dragging shallowly permeated Sn from porous anode surface. Therefore, the enlarged cluster steadily accumulated and more agglomerated on the anode surface due to the high surface tension of Sn. However, when the adequate amount of Sn was applied onto the anode surface, it was able to better permeate into the pores than when oversupplied as exhibited in Fig. 5, due to the less possibility to be dragged by agglomerated particle. The permeation of Sn also influenced that of C as discussed earlier, and moreover the improvement in reaction sites. For the purpose of understanding agglomeration of Sn powders, model experiments are carried out and shown in Fig. 6. Fig. 6(a)–(c) represents the experiments without covering carbon black over Sn particles and Fig. 6(d)–(l) are with carbon powder; the left-side figures (a, d, g and j) are with 15 mg of Sn, the middle ones (b, e, h and k) are 30 mg, and the right-sides (c, f, i and l) are 60 mg. In Fig. 6(a)–(c), the spherical morphologies are agglomerated Sn
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
5
Fig. 5. Quantitative depth-profile analysis of the anode surface cross-section via electron probe micro-analyzer with wavelength dispersive spectrometry (EPMA/WDS) method in order to identify the residual amount of Sn (solid black lines) and C (solid red lines) after the experiment (a) with the 15 mg of Sn and (b) with the 60 mg of Sn. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. The SEM images of the model experiment to figure out the agglomeration effect shown in the different amount of Sn as a mediator (a, d, g, j: 15 mg; b, e, h, k: 30 mg; c, f, i, l: 60 mg) on the anode surface (a–c) without carbon black powder over Sn particles and (d–l) with carbon black powder over Sn particles. Specifically, (g–i) shows the distribution and coverage of Sn mediator and carbon black fuel and (j–l) shows the capillary effect of liquid Sn.
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7 6
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
Fig. 7. (Left) The schematic diagrams of the expected mechanism for Sn mediator of (a) 15 mg and (b) 60 mg. (Right) The corresponding SEM images of the proposed diagram in juxtaposition and the magnified images of the interface in the yellow box. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
particles and the sizes of which grow prodigiously as the more amount of Sn is applied, while Si wafer remains clear and is explained as dark-gray background. In Fig. 6 (d)–(f), however, diameters of the spheres are noticeably shrunk comparing to the corresponding Sn amount in Fig. 6(a)–(c). It suggests that gravitational pull of carbon black toward Sn powder leads to the distribution of Sn. Therefore, Si wafer is totally covered by well distributed and dispersed Sn particles so the backgrounds are silvery, especially in Fig. 6(d). In addition, irregular shapes existing on the surface other than Sn spheres are carbon particles in Fig. 6(d)–(i). In Fig. 6(d) and (g), most of carbon black particles are placed onto the sub-micron Sn mediator surface, meaning that carbon has strong possibility to meet TPBs and hence excellent power performance. However, in Fig. 6(f) and (i), the coverage of agglomerated Sn is too broad on which carbon particles become considerably located, leading to restriction of access to TPBs. Carbon-anode interfacial interaction is also investigated with the model experiments and exhibited in Fig. 6(j)–(l), which are the magnified images of Fig. 6(g)–(i), respectively. The images show the capillary effect of liquid Sn, implying that the interface between carbon fuel and anode can be well mediated by liquid-state Sn and the poor interaction would be ameliorated. In addition, the capillary phenomenon could effectuate permeation of Sn into Ni–YSZ anode pores with carbon ingress onto TPBs as mentioned in Fig. 5(a). However, liquid Sn is mobile one in the capillaries and pores, and carbon black is mounted onto Sn surfaces for the experiments, suggesting that carbon interaction on TPBs are quite restricted once Sn covers Ni–YSZ anode pores too thick. Considering Sn distribution on anode surface and permeation into anode pores, 15–30 mg of Sn per 3.14 cm2 (geometric surface area)
seems to provide optimized mediation at experimental condition as expected in Fig. 1.
4. Conclusion We investigated the catalytic effect of Sn as a mediator on Ni–YSZ anode surface. As our previous study [37] suggested, Sn rendered enhanced interfacial electrochemical activity. The experiment with 15 mg of Sn shows the best performance with the maximum power density of 136 mW cm−2 at 850 ◦ C. Characterization and discharge test results demonstrate that electricity is generated by C which is employed as an electrochemically oxidisable fuel, not by liquid Sn. The power performance tests and the model experiments suggest that 15–30 mg of Sn is the optimized quantity for interfacial mediating effect due to its excellent properties on distribution, permeation and mediation as a liquid state Sn at higher temperatures. On the other hand, 60 mg of Sn covers Ni–YSZ anode surface and pores too thick and hence a hindrance to carbon access to TPBs, leading to relatively low power density. For the clear understanding, schematic diagrams for each case are visualized with simplified porous Ni–YSZ anode surface in Fig. 7.
Acknowledgments This work was supported by the New & Renewable Energy Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy (No. 20113020030010).
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013
G Model CATTOD-9657; No. of Pages 7
ARTICLE IN PRESS H. Jang et al. / Catalysis Today xxx (2015) xxx–xxx
References [1] BP plc, BP Staistical Review of World Energy 2014, 63rd ed., BP plc, 2014, pp. 1–45. [2] D. Cao, Y. Sun, G. Wang, J. Power Sources 167 (2007) 250–257. [3] H.A. Liebhafsky, J. Electrochem. Soc. 106 (1959) 1068–1071. [4] T.M. Gür, M. Homel, A.V. Virkar, J. Power Sources 195 (2010) 1085–1090. [5] H. Ju, J. Eom, J.K. Lee, H. Choi, T.-H. Lim, R.-H. Song, J. Lee, Electrochim. Acta 115 (2014) 511–517. [6] C. Li, Y. Shi, N. Cai, J. Power Sources 195 (2010) 4660–4666. [7] F. Guzman, R. Singh, S.S.C. Chuang, Energy Fuels 25 (2011) 2179–2186. [8] A. Elleuch, A. Boussetta, J. Yu, K. Halouani, Y. Li, Int. J. Hydrogen Energy 38 (2013) 16590–16604. [9] N. Nakagawa, M. Ishida, Ind. Eng. Chem. Res. 27 (1988) 1181–1185. [10] T.M. Gür, R.A. Huggins, J. Electrochem. Soc. 139 (1992) L95–L97. [11] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 142 (1995) 2621–2624. [12] J.S. Cronin, J.R. Wilson, S.A. Barnett, J. Power Sources 196 (2011) 2640–2643. [13] T.M. Gür, Chem. Rev. 113 (2013) 6179–6206. [14] J. Mizusaki, H. Tagawa, J. Electrochem. Soc. 138 (1991) 1867–1873. [15] L.G.J. de Haart, R.A. Kuipers, K.J. de Vries, A.J. Burggraaf, J. Electrochem. Soc. 138 (1991) 1970–1975. [16] M.J.L. Østergård, M. Mogensen, Electrochim. Acta 38 (1993) 2015–2020. [17] H. Fukunaga, M. Ihara, K. Sakaki, K. Yamada, Solid State Ion. 86–88 (1996) 1179–1185. [18] C.W. Tanner, K.-Z. Fung, A.V. Virkar, J. Electrochem. Soc. 144 (1997) 21–30. [19] M. Brown, S. Primdahl, M. Mogensen, J. Electrochem. Soc. 147 (2000) 475–485. [20] X. Wang, N. Nakagawa, K. Kato, J. Electrochem. Soc. 148 (2001) A565–A569. [21] S. McIntosh, R.J. Gorte, Chem. Rev. 104 (2004) 4845–4865. [22] T.M. Gur, U.S. Patent (Ed.), Clean Coal Energy, United States (2006).
7
[23] A. Jayakumar, S. Lee, A. Hornés, J.M. Vohs, R.J. Gorte, J. Electrochem. Soc. 157 (2010) B365–B369. [24] I.V. Yentekakis, P.G. Debenedetti, B. Costa, Ind. Eng. Chem. Res. 28 (1989) 1414–1424. [25] A. Jayakumar, J.M. Vohs, R.J. Gorte, Ind. Eng. Chem. Res. 49 (2010) 10237–10241. [26] A. Jayakumar, R. Küngas, S. Roy, A. Javadekar, D.J. Buttrey, J.M. Vohs, R.J. Gorte, Energy Environ. Sci. 4 (2011) 4133. [27] W.A.G. McPhee, M. Boucher, J. Stuart, R.S. Parnas, M. Koslowske, T. Tao, B.A. Wilhite, Energy Fuels 23 (2009) 5036–5041. [28] T. Tao, L. Bateman, J. Bentley, M. Slaney, ECS Trans. 5 (2007) 463–472. [29] T. Tao, M. Slaney, L. Bateman, J. Bentley, ECS Trans. 7 (2007) 1389–1397. [30] M. Anbar, D.F. McMillen, R.D. Weaver, P.J. Jorgensen, U.S. Patent (Ed.), Stanford Research Institute, United States (1976). [31] R. Padilla, H.Y. Sohn, Metall. Trans. B 10B (1979) 109–115. [32] G. Levêque, S. Abanades, Energy Fuels 28 (2014) 1396–1405. [33] M. Batzill, U. Diebold, Prog. Surf. Sci. 79 (2005) 47–154. [34] D.R. Lide, CRC Handbook of Chemistry and Physics, 90th ed., CRC Press, 2009. [35] Y. Nabae, K.D. Pointon, J.T.S. Irvine, J. Electrochem. Soc. 156 (2009) B716. [36] Y. Tang, J. Liu, Int. J. Hydrogen Energy 35 (2010) 11188–11193. [37] H. Ju, S. Uhm, J.W. Kim, R.-H. Song, H. Choi, S.-H. Lee, J. Lee, J. Power Sources 198 (2012) 36–41. [38] K. Wincewicz, J. Cooper, J. Power Sources 140 (2005) 280–296. [39] D.J. Brett, A. Atkinson, N.P. Brandon, S.J. Skinner, Chem. Soc. Rev. 37 (2008) 1568–1578. [40] H. Kan, H. Lee, Appl. Catal. B: Environ. 97 (2010) 108–114. [41] M. Iwase, M.-O. Yasuda, S.-I. Miki, T. Mori, Trans. Jpn. Inst. Met. 19 (1978) 654–660. [42] A. Jayakumar, A. Javadekar, R. Küngas, D.J. Buttrey, J.M. Vohs, R.J. Gorte, ECS Trans. 41 (2012) 149–158.
Please cite this article in press as: H. Jang, et al., Ameliorated performance in a direct carbon fuel cell using Sn mediator on Ni–YSZ anode surface, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.013