- Email: [email protected]
TiP2O7 composite membrane for high temperature (>150 °C) proton exchange membrane 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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
CsHSO4/TiP2O7 composite membrane for high temperature (>150 C) proton exchange membrane fuel cells Sang-Hyung Lee, Sung-Tae Lee, Dae-Han Lee, Sang-Min Lee, Sang-Soo Han, Sung-Ki Lim* Department of Materials Chemistry and Engineering, Konkuk University, 120, Neungdong-ro, Gwangjin-gu, Seoul 143-701, South Korea
article info
abstract
Article history:
Proton exchange membrane fuel cells (PEMFCs) are one of the most promising clean energy
Received 3 June 2015
technologies converting hydrogen energy to electric power. In this study, a proton-
Received in revised form
conducting electrolyte based on a CsHSO4/TiP2O7 composite membrane and operating at
8 July 2015
> 150 C was fabricated using the vacuum infiltration method and characterized. The
Accepted 25 July 2015
electrical properties were investigated in the high temperature range of ~ 110e190 C, in a
Available online 15 August 2015
dry atmosphere, using an impedance analyzer. The analysis of the resultant phase relationship showed that the composite membrane exhibited the same major peaks as each
Keywords:
individual material (TiP2O7 and CsHSO4) without any secondary phase. The relative density
PEMFCs
of the infiltrated CsHSO4/TiP2O7 composite membrane increased up to 96.76%, indicating
CsHSO4
that most of the pores present in the initial TiP2O7 supporting matrix were infiltrated with
TiP2O7
CsHSO4. It was consistent with the result of scanning electron microscopy. Moreover, the
Composite membrane
composite membranes exhibited low ionic conductivities in the low-temperature region, but at ~ 140 C, the ionic conductivities significantly increased because of the superprotonic phase transition of CsHSO4. The maximum conductivity (~2.38 103 S/cm) was achieved at 190 C under a dry Ar atmosphere. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells have been proposed as a solution to problems such as environmental pollution and depletion of fossil fuels. In the field of proton exchange membrane fuel cells (PEMFCs), DuPont's Nafion® is a widely used conventional protonconducting polymer electrolyte membrane. However, the working temperature of commercial PEMFCs is limited to ~
80 C because of the change in properties suffered by the polymer electrolyte at high temperatures, such as a decrease in thermal resistance [1,4e6]. However, PEMFC systems have some other problems associated with operating at lowworking temperatures. At <100 C, the electrolyte can be destroyed by swelling or shrinkage caused by liquid or vapor water [2,3]. Besides, the Pt catalyst in the anode is poisoned by even very small amounts of CO at low working temperatures.
* Corresponding author. Tel.: þ82 2 450 3500; fax: þ82 2 444 3490. E-mail address: [email protected] (S.-K. Lim). http://dx.doi.org/10.1016/j.ijhydene.2015.07.128 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
12771
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
In general, the permissible concentration of CO is 20 ppm at 80 C [1]. Moreover, it is also very difficult to control the input temperature of the fuel gas supply. Fuel cells with electrolytes operating at high temperatures (150e300 C) present a number of potential advantages over the polymer electrolyte membrane fuel cells described above [4e6]. The higher operation temperature (>150 C) improves electrode kinetics, the electrode reaction kinetics are enhanced, facilitating the replacement of Pt with inexpensive non-noble materials [7,8] and the tolerance of the fuel cell for CO was evaluated [9] besides, CO poisoning of the electrode is considerably suppressed [10]. New electrolyte materials used at high operation temperatures represent a possible solution to the above mentioned problems; therefore, significant effort has been directed towards the development of new proton-conducting materials [11e13]. Inorganic materials such as NH4PO3 [14], CsHSO4 (CHS) [15], CsH2PO4 [16], a-Zr(HPO4)2∙H2O (ZrP) [17], and MP2O7 (M ¼ Sn, Ti, Si, and Ge) [18,19] have been investigated. Among them, CHS has attracted considerable attention owing to its high proton conductivity and phase transition behavior. At 141 C, CHS undergoes a phase transition from a low conductivity phase to a high conductivity phase (superprotonic phase) because of the rearrangement of the hydrogen bond network [15]. This structural transition results in a steep increase in the conductivity (approximately four orders of magnitude, reaching a value of 102 S/cm). Nevertheless, it has not been considered previously for fuel cell applications because of its significant water solubility and inferior mechanical properties. In other words, CHS is unsuitable to be used alone for the electrolyte of PEMFCs. In contrast, MP2O7 (where M ¼ Si, Ti, Sn, Zr, and Hf) is another proton-conducting material possessing high strength and durability. In 1935, Levi et al. [20] reported that MP2O7 compounds can be indexed in a cubic or pseudo-cubic structure over diverse temperature range, characterized by TiO6 octahedral at the corners and P2O7 units at the edges. The crystal structure of TiP2O7 (TPP) is shown in Fig. 1 [21]. TPP has been previously reported to show high proton conductivities between 150 and 300 C under a dry Ar atmosphere [22]. In this study, a CHS/TPP composite membrane was fabricated by impregnating CHS into porous TPP as the supporting matrix. Finally, the properties of the resulting membrane were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), impedance analysis, and densitimetry.
Materials and methods Preparation of CHS CHS powder was prepared from an aqueous solution of Cs2CO3 and H2SO4 using the following procedure. Cs2CO3 (99.9%, SigmaeAldrich, USA) was dissolved in deionized water, and methanol was added to obtain a solution with a molar ratio of 1:12:25 for Cs2CO3: H2O: MeOH. After complete dissolution of Cs2CO3, 3 equiv of H2SO4 was added into the solution to induce the precipitation of CHS. Then, the precipitate was vacuum filtered and dried at 100 C for 24 h and
Fig. 1 e Crystal structure of TPP.
stored in a vacuum characterization.
desiccator
prior
to
further
Preparation of TPP as supporting matrix TiO2 (99.9%, High Purity Chemicals, Japan) and 85 wt.% aqueous H3PO4 (99.9%, SigmaeAldrich, USA) were mixed in a molar ratio of 1:2 to form a homogeneous slurry, followed by heating at 200 C for 3 h. The clayish paste obtained was dried at 170 C for 24 h to remove excess phosphoric acid. Then, the preheated powder was calcined at 700 C for 3 h. The resulting calcined powders were uniaxially pressed into pellets of 15 mm diameter and ~1.5 mm thickness. To investigate the potential of TPP as a supporting matrix for CHS, the TPP pellets were annealed from 400, 600 and 800 C for 2 h, respectively.
Preparation of the composite membrane The prepared TPP pellet was filled with CHS molten salt, at 240 C for 48 h, under vacuum. The membrane was subsequently dipped in methanol for 10 s, followed by drying in air for 30 s. The residual CHS on the external surface was removed using sandpaper.
Characterizations The phase analysis of each specimen was performed using an X-ray diffractometer (Rigaku Rint 2000, Cu Ka-radiation, scanning range (2q) 5e50 , step width 0.08 , and scanning speed 10 /min). TEM images were collected on a transmission electron microscope (JEOL-JEM1010, JAPAN) under an
12772
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
accelerating voltage of 80 keV. Flexural strength of each prepared specimen was measured by three-point bending test on a bar of dimensions 4.5 mm 4.5 mm 40 mm at a crosshead speed of 0.5 mm/min (Instron 4024, USA). The microstructure and crystalline structure were determined using a scanning electron microscope (Model JSM-6380, JEOL, JAPAN), and density was calculated by the Archimedes method (ASTM 373-88). The ionic conductivities of the specimens were measured by the silver blocking electrodes using a complex electrochemical impedance analyzer (Zahner, IM6) in the frequency range of 1 Hze3 MHz at an amplitude of 50 mV in the temperature range of 110e190 C under a dry Ar atmosphere. In humidified atmosphere, ionic conductivity had temporarily risen, but more experiment was worthless since it was difficult to obtain reliable data due to the solubility of CHS. Ionic conductivities were calculated using the following equation: L s¼ Rs A
(1)
Fig. 3 e XRD pattern of TPP after annealing at (a) 400 C, (b) 600 C, and (c) 800 C.
where s, L, Rs, and A are the ionic conductivity, specimen thickness, impedance of the specimen, and electrode area, respectively.
Results and discussion Fig. 2 shows the XRD pattern of the synthesized TiP2O7 power having a composition with the molar ratio of 1: 2 for TiO2: H3PO4 and calcined at 700 C. The pattern corresponds exactly to that of the pure TPP phase shown in JCPDS [38-1468]. Fig. 3 shows the XRD pattern of the fabricated pellet using previously synthesized TPP powder at each annealing temperature. The results from the XRD measurement confirmed that all the specimens exhibited similar shaped peaks, without the phase transition or formation of a secondary phase.
Fig. 4 e TEM images of macroscale powder of TPP by annealing at 400 C.
Fig. 4 shows the TEM image of nanoparticle of TPP. To identify the coreeshell structure, the nanoparticle was confirmed by technical TEM tilting, allowing the observation of lattice alignment together with amorphous characteristics. Micrometer-scale crystalline phosphates were fully covered with a continuous amorphous shell film of phosphate. The interface between the core and the shell was clearly noticed. Faint lattice fringes were observed in the shell compared to the well-defined lattice fringes from the core, indicating its crystalline nature.
Table 1 e Comparison of the density of TPP at various annealing temperatures. Sample
Fig. 2 e XRD pattern of the synthesized TPP.
Theoretical TPP 400 C 600 C 800 C
Bulk density (g/cm3)
Relative density (%)
Pore (%)
3.02 1.96 2.19 2.35
e 64.90 72.51 77.81
e 35.10 27.49 22.19
12773
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
The bulk and relative densities of the samples at each annealing temperature are listed in Table 1. All the TPP pellets had an appropriate mechanical strength suitable for the supporting matrix. The relative densities of the obtained pellets varied from 64.90 to ~77.81%. Fig. 5 shows the ionic conductivities of the TPP pellets according to the annealing temperature. A significant decrease in the conductivity following the rise in annealing temperature was observed. It has been previously reported [23,24] that because of a reduction in the non-stoichiometric phosphate phase of TPP, the phosphate “shells” surrounding the crystalline structures decreased with increasing annealing temperature. As listed in Table 1, the TPP pellet annealed at 400 C exhibits the highest porosity of 35.1%, allowing for sufficient CHS impregnation providing higher ionic conductivity. Thus, this was chosen as the optimum composite sample, and subsequent experiments were performed using the TPP annealed at 400 C as the supporting matrix. Fig. 6 shows the XRD patterns of the TPP supporting matrix, pure CsHSO4 (CHS), and CHS/TPP composite membrane by comparing each other. The data for the infiltrated CHS/TPP composite membrane shows that the TPP and CHS crystalline structures were intact following the impregnation. These results indicate that TPP was impregnated with a CHS molten salt at a high temperature (230 C), without any interfacial interaction between the materials, leading to a completely formed composite. The relative density of the TPP supporting matrix pellets was 64.90%. However, in the case of the infiltrated CHS/TPP composite membrane, the relative density increased from 64.90 to 96.76%, indicating that most of the pores in TPP was filled with CHS. Detailed data are listed in Table 2. Table 3 indicated flexural strength which measured by 3 point bending test. According to the table, CHS showed a low flexural strength of 26.83 MPa, TPP showed flexural strength value of 61.11 MPa higher than the CHS. In case of the fabricated CHS/TPP membrane presented enhanced flexural strength of 86.22 MPa than CHS and TPP.
Fig. 6 e XRD pattern of (a) TPP, (b) CHS, (c) CHS-TPP composite membrane.
Fig. 7 shows the microstructure of the TPP supporting matrix and the CHS/TPP composite membrane. As shown in Fig. 7(a), the mean crystalline size of TPP was 0.5e1.5 mm, and the sample contained any amount of pores. Fig. 7(b) shows the TPP covered with CHS. Many pores are visible in the case of TPP supporting matrix, whereas in the infiltrated CHS/TPP composite membrane, the pores in TPP are almost completely filled with CHS. The ionic conductivities of the pure TPP supporting matrix, CHS, and CHS/TPP composite membrane are shown by comparing each other in Fig. 8. The TPP supporting matrix has an ionic conductivity of about 2.67 104 S/cm at 190 C. At the same temperature, CHS has an ionic conductivity of about 1.12 102 S/cm. The CHS/TPP composite membrane has a maximum ionic conductivity of 2.38 103 S/cm. According to these results, the CHS/TPP composite membrane has an ionic conductivity of one order of magnitude higher than the pure TPP. In the case of the composite membrane, the expected value of ~102 S/cm was not achieved, probably because of low ionic conductivity of CHS in the low-temperature region at < 141 C, and at >141 C, because of the interfacial resistance between the two materials. This
Table 2 e Comparison of the density for each sample. Sample Theoretical TPP Prepared TPP CsHSO4-TPP Hybrid membrane
Density (g/cm3)
Relative density (%)
Pore (%)
3.02 1.96 2.92
e 64.90 96.76
e 35.10 3.24
Table 3 e Comparison of the flexural strength for each sample. Sample
Fig. 5 e The ionic conductivities of TPP obtained by annealing at (a) 400 C, (b) 600 C, and (c) 800 C.
CsHSO4 TiP2O7 CsHSO4-TPP Hybrid membrane
Flexural strength (MPa) 26.83 ± 4.74 61.11 ± 4.84 86.22 ± 3.74
12774
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
Fig. 9 e Image of the fabricated composite membrane. phenomenon is not synergy effect but the complementary effect using the tradeoffs of each other. In fact, many of composite materials show similar tendencies [25e29]. Thus the fabrication of this composite membrane significantly improved the downsides such as the poor mechanical properties of the membrane and water solubility of CHS. An image of the fabricated composite membrane is shown in Fig. 9.
Conclusions
Fig. 7 e SEM Images of (a) pure TiP2O7 and (b) CHS-TPP composite membrane ( £ 10,000).
In this study, a CHS/TPP composite electrolyte membrane for use in high-temperature PEMFCs was fabricated. The pure TPP supporting matrix, obtained by annealing at 400 C, had a density of 1.96 g/cm3 (relative density: 64.90%). The composite membrane was prepared by the vacuum infiltration using a CHS molten salt. Even after vacuum infiltration at 240 C for 48 h, all the samples exhibited their original peaks, without phase transition or formation of a secondary phase; however, the relative density increased from 64.90 to 96.76%. The fabricated CHS/TPP composite membrane showed a maximum conductivity of ~ 2.38 103 S/cm at 190 C. The conductivity of the composite membrane decreased in the low-temperature range. However, a rapid increase in the conductivity was observed at >141 C, and the final conductivity was lower than that of the pure CHS electrolyte in the high-temperature range, because of the low conductivity of TPP supporting matrix and the interfacial resistance between the materials. However, the bulk density and strength of the CHS/TPP composite membrane were higher than that of the pure CHS membrane. These results show a high potential of the composite membrane containing CHS and TPP as an electrolyte for fuel cells operating at high temperatures.
Acknowledgments This study was supported by Konkuk University in 2014.
references
Fig. 8 e The ionic conductivity of (a) TPP, (b) CHS, and (c) CHS-TPP composite membrane.
[1] Li O, He R, Gao J, Jensen JQ, Bjerrum NJ. The CO poisoning effect in PEMFCs operational at temperatures up to 200 C. J Electrochem Soc 2003;150:A1599e605.
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
[2] Mulyazmi, Daud WRW, Majlan EH, Rosli MI. Water balance for the design of a PEM fuel cell system. Int J Hydrogen Energy 2013;38:9409e20. [3] Jeon DH, Kim KN, Baek SM, Nam JH. The effect of relative humidity of the cathode on the performance and the uniformity of PEM fuel cells. Int J Hydrogen Energy 2011;36:12499e511. [4] Yang C, Bocarsly AB. Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells. J Power Sources 2001;103:1e9. [5] Zawodzinski TA, Kavey J, Valerio J, Gottesfeld S. The water content dependence of electro-osmotic drag in protonconducting polymer electrolytes. Electrochimica Acta 1995;40:297e302. [6] Silva RF, Passerini S, Pozio A. Solution-cast Nafion®/ montmorillonite composite membrane with low methanol permeability. Electrochimica Acta 2005;50:2639e45. [7] Heo P, Nagao M, Sano M, Hibino T. A high-performance Mo2C-ZrO2 anode catalyst for intermediate-temperature fuel cells. J Electrochem Soc 2007;154:B53e6. [8] Hibino T, Kobayashi K, Heo P. Oxygen reduction reaction over nitrogen-doped graphene oxide cathodes in acid and alkaline fuel cells at intermediate temperatures. Electrochimica Acta 2013;112:82e9. [9] Heo P, Shibata H, Nagao M, Hibino T, Sano M. Performance of an intermediate-temperature fuel cell using a protonconducting Sn0.9In0.1P2O7 electrolyte. J Electrochem Soc 2006;153(5):A897e901. [10] Cheddie D, Munroe N. A two phase model of an intermediate temperature PEM fuel cell. Int J Hydrogen Energy 2007;32:832e41. [11] Tian JH, Gao PF, Zhang ZY, Luo WH, Shan ZQ. Preparation and performance evaluation of a Nafione-TiO2 composite membrane for PEMFCs. Int J Hydrogen Energy 2008;33:5686e90. [12] Amjadi M, Rowshanzamir S, Peighambardoust SJ, Hosseini MG, Eikani MH. Investigation of physical properties and cell performance of Nafion/TiO2 nanocomposite membranes for high temperature PEM fuel cells. Int J Hydrogen Energy 2010;35:9252e60. [13] Pan J, Zhang H, Chen W, Pan M. Nafione-zirconia nanocomposite membranes formed via in situ solegel process. Int J Hydrogen Energy 2010;35:2796e801. [14] Chen X, Huang Z, Xia C. Fabrication and characterization of solid state proton conductor (NH4)(2)SiP4O13-NH4PO3 for fuel cells operated at 150-250 degrees C. Solid State Ion 2006;177:2413e6.
12775
[15] Bocchetta P, Chiavarotti G, Masi R, Sunseri C, Quarto FD. Nanoporous alumina membranes filled with solid acid for thin film fuel cells at high temperatures. Electrochem Commun 2004;6:923e8. [16] Boysen DA, Haile SM. High-temperature behavior of CsH2PO4 under both ambient and high pressure conditions. Chem Mater 2003;15:727e36. [17] Clearfield A, Stynesj JA. The preparation of crystalline zirconium phosphate and some observations on it ion exchange behavior. Inorg Nucl Chem 1964;26:117e29. [18] Jin Y, shen Y, Hibino T. Proton conduction in metal pyrophosphates (MP2O7) at intermediate temperatures. J Mater Chem 2010;20:6214e7. [19] Nagao M, Kamiya T, Heo P, Tomita A, Hibino T, Sano M. Proton conduction in In3þ-doped SnP2O7 at intermediate temperatures. J Electrochem Soc 2006;153:A1604e9. [20] Levi GR, Peyronel G, Kristallogr Z. Crystal structure of the isomorphous group MP2O7. Sect A 1935;92:190e209. ın MR. A low temperature [21] Senguttuvan P, Rousse G, Palac TiP2O7 polymorph exhibiting reversible insertion of lithium and sodium ions. J Mater Chem A 2013;1:15284e91. [22] Shirai T, Shunsuke S, Yasukazu S. Proton conducting solid electrolytes based on diphosphates. Phosphorous Res Bull 2007;21:31e7. [23] Shirai T, Kuwano J. Proton conductivity and microstructures of the core-shell type solid electrolytes in the MO2-In2O3-P2O5 (M¼Ti, Sn, Zr) systems. Solid State Ion 2009;180:569e74. [24] Tanimoto S, Shiroishi H. Shell-core type proton conducting TiP2O7-based solid electrolytes. Key Eng Mat 2009;388:57e60. [25] Hibino Takashi, Kobayashi Kazuyo. An intermediatetemperature alkaline fuel cell using an Sn0.92Sb0.08P2O7-based hydroxide-ion-conducting electrolyte and electrodes. J Mater Chem A 2013;1:1134e40. [26] Heo P, Shen YB, Kojima K. Fe0.4Ta0.5P2O7-based composite membrane for high-temperature, low-humidity proton exchange membrane fuel cells. Electrochimica Acta 2014;128:287e91. [27] Kobayashi Kazuyo, Nagao Masahiro, Yamamoto Yuta, Heo Pilwon, Hibino Takashi. Rechargeable PEM fuel-cell batteries using porous carbon modified with carbonyl groups as anode materials. J Electrochem Soc 2015;162(8):F868e77. [28] Wang S, Otomo J, Ogura M, Wen C, Nagamoto H, Takahashi H. Preparation and characterization of protonconducting CsHSO4eSiO2. nanocomposite electrolyte membranes. Solid State Ion 2005;176:755e60. [29] Inguanta R, Piazza S, Sunseri C. Impedance spectroscopy characterization of functionalized alumina membranes. Solid State Ion 2005;176:2887e91.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
CsHSO4/TiP2O7 composite membrane for high temperature (>150 C) proton exchange membrane fuel cells Sang-Hyung Lee, Sung-Tae Lee, Dae-Han Lee, Sang-Min Lee, Sang-Soo Han, Sung-Ki Lim* Department of Materials Chemistry and Engineering, Konkuk University, 120, Neungdong-ro, Gwangjin-gu, Seoul 143-701, South Korea
article info
abstract
Article history:
Proton exchange membrane fuel cells (PEMFCs) are one of the most promising clean energy
Received 3 June 2015
technologies converting hydrogen energy to electric power. In this study, a proton-
Received in revised form
conducting electrolyte based on a CsHSO4/TiP2O7 composite membrane and operating at
8 July 2015
> 150 C was fabricated using the vacuum infiltration method and characterized. The
Accepted 25 July 2015
electrical properties were investigated in the high temperature range of ~ 110e190 C, in a
Available online 15 August 2015
dry atmosphere, using an impedance analyzer. The analysis of the resultant phase relationship showed that the composite membrane exhibited the same major peaks as each
Keywords:
individual material (TiP2O7 and CsHSO4) without any secondary phase. The relative density
PEMFCs
of the infiltrated CsHSO4/TiP2O7 composite membrane increased up to 96.76%, indicating
CsHSO4
that most of the pores present in the initial TiP2O7 supporting matrix were infiltrated with
TiP2O7
CsHSO4. It was consistent with the result of scanning electron microscopy. Moreover, the
Composite membrane
composite membranes exhibited low ionic conductivities in the low-temperature region, but at ~ 140 C, the ionic conductivities significantly increased because of the superprotonic phase transition of CsHSO4. The maximum conductivity (~2.38 103 S/cm) was achieved at 190 C under a dry Ar atmosphere. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells have been proposed as a solution to problems such as environmental pollution and depletion of fossil fuels. In the field of proton exchange membrane fuel cells (PEMFCs), DuPont's Nafion® is a widely used conventional protonconducting polymer electrolyte membrane. However, the working temperature of commercial PEMFCs is limited to ~
80 C because of the change in properties suffered by the polymer electrolyte at high temperatures, such as a decrease in thermal resistance [1,4e6]. However, PEMFC systems have some other problems associated with operating at lowworking temperatures. At <100 C, the electrolyte can be destroyed by swelling or shrinkage caused by liquid or vapor water [2,3]. Besides, the Pt catalyst in the anode is poisoned by even very small amounts of CO at low working temperatures.
* Corresponding author. Tel.: þ82 2 450 3500; fax: þ82 2 444 3490. E-mail address: [email protected] (S.-K. Lim). http://dx.doi.org/10.1016/j.ijhydene.2015.07.128 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
12771
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
In general, the permissible concentration of CO is 20 ppm at 80 C [1]. Moreover, it is also very difficult to control the input temperature of the fuel gas supply. Fuel cells with electrolytes operating at high temperatures (150e300 C) present a number of potential advantages over the polymer electrolyte membrane fuel cells described above [4e6]. The higher operation temperature (>150 C) improves electrode kinetics, the electrode reaction kinetics are enhanced, facilitating the replacement of Pt with inexpensive non-noble materials [7,8] and the tolerance of the fuel cell for CO was evaluated [9] besides, CO poisoning of the electrode is considerably suppressed [10]. New electrolyte materials used at high operation temperatures represent a possible solution to the above mentioned problems; therefore, significant effort has been directed towards the development of new proton-conducting materials [11e13]. Inorganic materials such as NH4PO3 [14], CsHSO4 (CHS) [15], CsH2PO4 [16], a-Zr(HPO4)2∙H2O (ZrP) [17], and MP2O7 (M ¼ Sn, Ti, Si, and Ge) [18,19] have been investigated. Among them, CHS has attracted considerable attention owing to its high proton conductivity and phase transition behavior. At 141 C, CHS undergoes a phase transition from a low conductivity phase to a high conductivity phase (superprotonic phase) because of the rearrangement of the hydrogen bond network [15]. This structural transition results in a steep increase in the conductivity (approximately four orders of magnitude, reaching a value of 102 S/cm). Nevertheless, it has not been considered previously for fuel cell applications because of its significant water solubility and inferior mechanical properties. In other words, CHS is unsuitable to be used alone for the electrolyte of PEMFCs. In contrast, MP2O7 (where M ¼ Si, Ti, Sn, Zr, and Hf) is another proton-conducting material possessing high strength and durability. In 1935, Levi et al. [20] reported that MP2O7 compounds can be indexed in a cubic or pseudo-cubic structure over diverse temperature range, characterized by TiO6 octahedral at the corners and P2O7 units at the edges. The crystal structure of TiP2O7 (TPP) is shown in Fig. 1 [21]. TPP has been previously reported to show high proton conductivities between 150 and 300 C under a dry Ar atmosphere [22]. In this study, a CHS/TPP composite membrane was fabricated by impregnating CHS into porous TPP as the supporting matrix. Finally, the properties of the resulting membrane were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), impedance analysis, and densitimetry.
Materials and methods Preparation of CHS CHS powder was prepared from an aqueous solution of Cs2CO3 and H2SO4 using the following procedure. Cs2CO3 (99.9%, SigmaeAldrich, USA) was dissolved in deionized water, and methanol was added to obtain a solution with a molar ratio of 1:12:25 for Cs2CO3: H2O: MeOH. After complete dissolution of Cs2CO3, 3 equiv of H2SO4 was added into the solution to induce the precipitation of CHS. Then, the precipitate was vacuum filtered and dried at 100 C for 24 h and
Fig. 1 e Crystal structure of TPP.
stored in a vacuum characterization.
desiccator
prior
to
further
Preparation of TPP as supporting matrix TiO2 (99.9%, High Purity Chemicals, Japan) and 85 wt.% aqueous H3PO4 (99.9%, SigmaeAldrich, USA) were mixed in a molar ratio of 1:2 to form a homogeneous slurry, followed by heating at 200 C for 3 h. The clayish paste obtained was dried at 170 C for 24 h to remove excess phosphoric acid. Then, the preheated powder was calcined at 700 C for 3 h. The resulting calcined powders were uniaxially pressed into pellets of 15 mm diameter and ~1.5 mm thickness. To investigate the potential of TPP as a supporting matrix for CHS, the TPP pellets were annealed from 400, 600 and 800 C for 2 h, respectively.
Preparation of the composite membrane The prepared TPP pellet was filled with CHS molten salt, at 240 C for 48 h, under vacuum. The membrane was subsequently dipped in methanol for 10 s, followed by drying in air for 30 s. The residual CHS on the external surface was removed using sandpaper.
Characterizations The phase analysis of each specimen was performed using an X-ray diffractometer (Rigaku Rint 2000, Cu Ka-radiation, scanning range (2q) 5e50 , step width 0.08 , and scanning speed 10 /min). TEM images were collected on a transmission electron microscope (JEOL-JEM1010, JAPAN) under an
12772
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
accelerating voltage of 80 keV. Flexural strength of each prepared specimen was measured by three-point bending test on a bar of dimensions 4.5 mm 4.5 mm 40 mm at a crosshead speed of 0.5 mm/min (Instron 4024, USA). The microstructure and crystalline structure were determined using a scanning electron microscope (Model JSM-6380, JEOL, JAPAN), and density was calculated by the Archimedes method (ASTM 373-88). The ionic conductivities of the specimens were measured by the silver blocking electrodes using a complex electrochemical impedance analyzer (Zahner, IM6) in the frequency range of 1 Hze3 MHz at an amplitude of 50 mV in the temperature range of 110e190 C under a dry Ar atmosphere. In humidified atmosphere, ionic conductivity had temporarily risen, but more experiment was worthless since it was difficult to obtain reliable data due to the solubility of CHS. Ionic conductivities were calculated using the following equation: L s¼ Rs A
(1)
Fig. 3 e XRD pattern of TPP after annealing at (a) 400 C, (b) 600 C, and (c) 800 C.
where s, L, Rs, and A are the ionic conductivity, specimen thickness, impedance of the specimen, and electrode area, respectively.
Results and discussion Fig. 2 shows the XRD pattern of the synthesized TiP2O7 power having a composition with the molar ratio of 1: 2 for TiO2: H3PO4 and calcined at 700 C. The pattern corresponds exactly to that of the pure TPP phase shown in JCPDS [38-1468]. Fig. 3 shows the XRD pattern of the fabricated pellet using previously synthesized TPP powder at each annealing temperature. The results from the XRD measurement confirmed that all the specimens exhibited similar shaped peaks, without the phase transition or formation of a secondary phase.
Fig. 4 e TEM images of macroscale powder of TPP by annealing at 400 C.
Fig. 4 shows the TEM image of nanoparticle of TPP. To identify the coreeshell structure, the nanoparticle was confirmed by technical TEM tilting, allowing the observation of lattice alignment together with amorphous characteristics. Micrometer-scale crystalline phosphates were fully covered with a continuous amorphous shell film of phosphate. The interface between the core and the shell was clearly noticed. Faint lattice fringes were observed in the shell compared to the well-defined lattice fringes from the core, indicating its crystalline nature.
Table 1 e Comparison of the density of TPP at various annealing temperatures. Sample
Fig. 2 e XRD pattern of the synthesized TPP.
Theoretical TPP 400 C 600 C 800 C
Bulk density (g/cm3)
Relative density (%)
Pore (%)
3.02 1.96 2.19 2.35
e 64.90 72.51 77.81
e 35.10 27.49 22.19
12773
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
The bulk and relative densities of the samples at each annealing temperature are listed in Table 1. All the TPP pellets had an appropriate mechanical strength suitable for the supporting matrix. The relative densities of the obtained pellets varied from 64.90 to ~77.81%. Fig. 5 shows the ionic conductivities of the TPP pellets according to the annealing temperature. A significant decrease in the conductivity following the rise in annealing temperature was observed. It has been previously reported [23,24] that because of a reduction in the non-stoichiometric phosphate phase of TPP, the phosphate “shells” surrounding the crystalline structures decreased with increasing annealing temperature. As listed in Table 1, the TPP pellet annealed at 400 C exhibits the highest porosity of 35.1%, allowing for sufficient CHS impregnation providing higher ionic conductivity. Thus, this was chosen as the optimum composite sample, and subsequent experiments were performed using the TPP annealed at 400 C as the supporting matrix. Fig. 6 shows the XRD patterns of the TPP supporting matrix, pure CsHSO4 (CHS), and CHS/TPP composite membrane by comparing each other. The data for the infiltrated CHS/TPP composite membrane shows that the TPP and CHS crystalline structures were intact following the impregnation. These results indicate that TPP was impregnated with a CHS molten salt at a high temperature (230 C), without any interfacial interaction between the materials, leading to a completely formed composite. The relative density of the TPP supporting matrix pellets was 64.90%. However, in the case of the infiltrated CHS/TPP composite membrane, the relative density increased from 64.90 to 96.76%, indicating that most of the pores in TPP was filled with CHS. Detailed data are listed in Table 2. Table 3 indicated flexural strength which measured by 3 point bending test. According to the table, CHS showed a low flexural strength of 26.83 MPa, TPP showed flexural strength value of 61.11 MPa higher than the CHS. In case of the fabricated CHS/TPP membrane presented enhanced flexural strength of 86.22 MPa than CHS and TPP.
Fig. 6 e XRD pattern of (a) TPP, (b) CHS, (c) CHS-TPP composite membrane.
Fig. 7 shows the microstructure of the TPP supporting matrix and the CHS/TPP composite membrane. As shown in Fig. 7(a), the mean crystalline size of TPP was 0.5e1.5 mm, and the sample contained any amount of pores. Fig. 7(b) shows the TPP covered with CHS. Many pores are visible in the case of TPP supporting matrix, whereas in the infiltrated CHS/TPP composite membrane, the pores in TPP are almost completely filled with CHS. The ionic conductivities of the pure TPP supporting matrix, CHS, and CHS/TPP composite membrane are shown by comparing each other in Fig. 8. The TPP supporting matrix has an ionic conductivity of about 2.67 104 S/cm at 190 C. At the same temperature, CHS has an ionic conductivity of about 1.12 102 S/cm. The CHS/TPP composite membrane has a maximum ionic conductivity of 2.38 103 S/cm. According to these results, the CHS/TPP composite membrane has an ionic conductivity of one order of magnitude higher than the pure TPP. In the case of the composite membrane, the expected value of ~102 S/cm was not achieved, probably because of low ionic conductivity of CHS in the low-temperature region at < 141 C, and at >141 C, because of the interfacial resistance between the two materials. This
Table 2 e Comparison of the density for each sample. Sample Theoretical TPP Prepared TPP CsHSO4-TPP Hybrid membrane
Density (g/cm3)
Relative density (%)
Pore (%)
3.02 1.96 2.92
e 64.90 96.76
e 35.10 3.24
Table 3 e Comparison of the flexural strength for each sample. Sample
Fig. 5 e The ionic conductivities of TPP obtained by annealing at (a) 400 C, (b) 600 C, and (c) 800 C.
CsHSO4 TiP2O7 CsHSO4-TPP Hybrid membrane
Flexural strength (MPa) 26.83 ± 4.74 61.11 ± 4.84 86.22 ± 3.74
12774
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
Fig. 9 e Image of the fabricated composite membrane. phenomenon is not synergy effect but the complementary effect using the tradeoffs of each other. In fact, many of composite materials show similar tendencies [25e29]. Thus the fabrication of this composite membrane significantly improved the downsides such as the poor mechanical properties of the membrane and water solubility of CHS. An image of the fabricated composite membrane is shown in Fig. 9.
Conclusions
Fig. 7 e SEM Images of (a) pure TiP2O7 and (b) CHS-TPP composite membrane ( £ 10,000).
In this study, a CHS/TPP composite electrolyte membrane for use in high-temperature PEMFCs was fabricated. The pure TPP supporting matrix, obtained by annealing at 400 C, had a density of 1.96 g/cm3 (relative density: 64.90%). The composite membrane was prepared by the vacuum infiltration using a CHS molten salt. Even after vacuum infiltration at 240 C for 48 h, all the samples exhibited their original peaks, without phase transition or formation of a secondary phase; however, the relative density increased from 64.90 to 96.76%. The fabricated CHS/TPP composite membrane showed a maximum conductivity of ~ 2.38 103 S/cm at 190 C. The conductivity of the composite membrane decreased in the low-temperature range. However, a rapid increase in the conductivity was observed at >141 C, and the final conductivity was lower than that of the pure CHS electrolyte in the high-temperature range, because of the low conductivity of TPP supporting matrix and the interfacial resistance between the materials. However, the bulk density and strength of the CHS/TPP composite membrane were higher than that of the pure CHS membrane. These results show a high potential of the composite membrane containing CHS and TPP as an electrolyte for fuel cells operating at high temperatures.
Acknowledgments This study was supported by Konkuk University in 2014.
references
Fig. 8 e The ionic conductivity of (a) TPP, (b) CHS, and (c) CHS-TPP composite membrane.
[1] Li O, He R, Gao J, Jensen JQ, Bjerrum NJ. The CO poisoning effect in PEMFCs operational at temperatures up to 200 C. J Electrochem Soc 2003;150:A1599e605.
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 0 ( 2 0 1 5 ) 1 2 7 7 0 e1 2 7 7 5
[2] Mulyazmi, Daud WRW, Majlan EH, Rosli MI. Water balance for the design of a PEM fuel cell system. Int J Hydrogen Energy 2013;38:9409e20. [3] Jeon DH, Kim KN, Baek SM, Nam JH. The effect of relative humidity of the cathode on the performance and the uniformity of PEM fuel cells. Int J Hydrogen Energy 2011;36:12499e511. [4] Yang C, Bocarsly AB. Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells. J Power Sources 2001;103:1e9. [5] Zawodzinski TA, Kavey J, Valerio J, Gottesfeld S. The water content dependence of electro-osmotic drag in protonconducting polymer electrolytes. Electrochimica Acta 1995;40:297e302. [6] Silva RF, Passerini S, Pozio A. Solution-cast Nafion®/ montmorillonite composite membrane with low methanol permeability. Electrochimica Acta 2005;50:2639e45. [7] Heo P, Nagao M, Sano M, Hibino T. A high-performance Mo2C-ZrO2 anode catalyst for intermediate-temperature fuel cells. J Electrochem Soc 2007;154:B53e6. [8] Hibino T, Kobayashi K, Heo P. Oxygen reduction reaction over nitrogen-doped graphene oxide cathodes in acid and alkaline fuel cells at intermediate temperatures. Electrochimica Acta 2013;112:82e9. [9] Heo P, Shibata H, Nagao M, Hibino T, Sano M. Performance of an intermediate-temperature fuel cell using a protonconducting Sn0.9In0.1P2O7 electrolyte. J Electrochem Soc 2006;153(5):A897e901. [10] Cheddie D, Munroe N. A two phase model of an intermediate temperature PEM fuel cell. Int J Hydrogen Energy 2007;32:832e41. [11] Tian JH, Gao PF, Zhang ZY, Luo WH, Shan ZQ. Preparation and performance evaluation of a Nafione-TiO2 composite membrane for PEMFCs. Int J Hydrogen Energy 2008;33:5686e90. [12] Amjadi M, Rowshanzamir S, Peighambardoust SJ, Hosseini MG, Eikani MH. Investigation of physical properties and cell performance of Nafion/TiO2 nanocomposite membranes for high temperature PEM fuel cells. Int J Hydrogen Energy 2010;35:9252e60. [13] Pan J, Zhang H, Chen W, Pan M. Nafione-zirconia nanocomposite membranes formed via in situ solegel process. Int J Hydrogen Energy 2010;35:2796e801. [14] Chen X, Huang Z, Xia C. Fabrication and characterization of solid state proton conductor (NH4)(2)SiP4O13-NH4PO3 for fuel cells operated at 150-250 degrees C. Solid State Ion 2006;177:2413e6.
12775
[15] Bocchetta P, Chiavarotti G, Masi R, Sunseri C, Quarto FD. Nanoporous alumina membranes filled with solid acid for thin film fuel cells at high temperatures. Electrochem Commun 2004;6:923e8. [16] Boysen DA, Haile SM. High-temperature behavior of CsH2PO4 under both ambient and high pressure conditions. Chem Mater 2003;15:727e36. [17] Clearfield A, Stynesj JA. The preparation of crystalline zirconium phosphate and some observations on it ion exchange behavior. Inorg Nucl Chem 1964;26:117e29. [18] Jin Y, shen Y, Hibino T. Proton conduction in metal pyrophosphates (MP2O7) at intermediate temperatures. J Mater Chem 2010;20:6214e7. [19] Nagao M, Kamiya T, Heo P, Tomita A, Hibino T, Sano M. Proton conduction in In3þ-doped SnP2O7 at intermediate temperatures. J Electrochem Soc 2006;153:A1604e9. [20] Levi GR, Peyronel G, Kristallogr Z. Crystal structure of the isomorphous group MP2O7. Sect A 1935;92:190e209. ın MR. A low temperature [21] Senguttuvan P, Rousse G, Palac TiP2O7 polymorph exhibiting reversible insertion of lithium and sodium ions. J Mater Chem A 2013;1:15284e91. [22] Shirai T, Shunsuke S, Yasukazu S. Proton conducting solid electrolytes based on diphosphates. Phosphorous Res Bull 2007;21:31e7. [23] Shirai T, Kuwano J. Proton conductivity and microstructures of the core-shell type solid electrolytes in the MO2-In2O3-P2O5 (M¼Ti, Sn, Zr) systems. Solid State Ion 2009;180:569e74. [24] Tanimoto S, Shiroishi H. Shell-core type proton conducting TiP2O7-based solid electrolytes. Key Eng Mat 2009;388:57e60. [25] Hibino Takashi, Kobayashi Kazuyo. An intermediatetemperature alkaline fuel cell using an Sn0.92Sb0.08P2O7-based hydroxide-ion-conducting electrolyte and electrodes. J Mater Chem A 2013;1:1134e40. [26] Heo P, Shen YB, Kojima K. Fe0.4Ta0.5P2O7-based composite membrane for high-temperature, low-humidity proton exchange membrane fuel cells. Electrochimica Acta 2014;128:287e91. [27] Kobayashi Kazuyo, Nagao Masahiro, Yamamoto Yuta, Heo Pilwon, Hibino Takashi. Rechargeable PEM fuel-cell batteries using porous carbon modified with carbonyl groups as anode materials. J Electrochem Soc 2015;162(8):F868e77. [28] Wang S, Otomo J, Ogura M, Wen C, Nagamoto H, Takahashi H. Preparation and characterization of protonconducting CsHSO4eSiO2. nanocomposite electrolyte membranes. Solid State Ion 2005;176:755e60. [29] Inguanta R, Piazza S, Sunseri C. Impedance spectroscopy characterization of functionalized alumina membranes. Solid State Ion 2005;176:2887e91.