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Novel ceramic composite membranes for low-temperature fuel cells
Journal of Non-Crystalline Solids 356 (2010) 2799–2802
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Novel ceramic composite membranes for low-temperature fuel cells Uma Thanganathan a,⁎, Masayuki Nogami b a b
RCIS (Research Core Interdisciplinary Science), Okayama University, Tsushima-Naka, Okayama 700-8530, Japan Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa, Nagoya 466-8555, Japan
a r t i c l e
i n f o
Article history: Received 18 October 2009 Received in revised form 20 August 2010 Available online 11 October 2010 Keywords: Composite membranes; Low-temperature fuel cell; Inorganic materials; Heteropolyacids;
a b s t r a c t A new class of proton-conducting glass ceramic composite membranes was developed for low-temperature fuel cells. According to characterizations, these membranes demonstrated good mechanical, structural, thermal and textural properties. Moreover, they yielded a high proton conductivity of 10− 2 S cm− 1 at room temperature. An electrode consisting of a heteropolyacid dispersed in a catalyst layer was also prepared and characterized, and the cell performances when using the composite membranes of PWA/PMA doped with either ZrO2 or TiO2 glass together with the heteropoly acid electrode were extremely high as compared to counterparts with a phosphosilicate glass electrolyte, at room temperature. The cell evaluation was performed on two ceramic composite membranes, i.e., PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %), with the same catalyst and with an operation equivalent to that in hydrogen and oxygen fuel cells, and the former membrane was found to present the best properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction At the present time, environmental issues due to increasing emissions of air pollutants and greenhouse gases are spurring the development of clean energy delivery technologies, such as fuel cells. Low-temperature proton exchange membrane fuel cells (PEMFCs) use hydrogen as fuel and their only emission is water. Over the last decade, the interest for utilizing ceramic materials as possible electrolyte membranes for fuel cell applications has grown [1–4]. Ceramics are good candidates because of their thermal, chemical, and mechanical stability, as well as their lower material costs. The proton conduction is highly dependent on the surface coverage of adsorbed water on the inorganic material. While significant advances have been made in recent years, a major restriction of the current technology is the cost and material limitations of the proton conduction membrane. For transport applications, fuel cell companies require more durable, cost effective membrane technologies capable of delivering an enhancement at low temperatures (from room temperature up to 80 °C). As a result, research is being driven towards a wide range of novel organic and inorganic materials with the potential of being good proton conductors and forming coherent membranes. Nafion® and similar membranes suffer serious disadvantages such as high cost, poor hydrophilicity, fuel crossover, and low proton conductivity at low humidity and high temperatures [5]. Consequently, attempts have been made to produce cheaper proton-conducting
⁎ Corresponding author. Tel.: + 81 86 251 8706; fax: + 81 86 251 8705. E-mail address: [email protected] (U. Thanganathan). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.043
glasses that can equal the electrochemical performance of perflourinated ionomers as well as their mechanical, thermal and electrochemical stability. Membranes fabricated from the heteropolyacid (HPA) family are currently the best suited for proton conduction at both low and high temperatures. This paper describes the synthesis and structural properties of novel proton-conducting glass ceramic composites based on heteropolyacids. These materials are both cheaper and more thermally stable than Nafion® and recent studies have already reported on the preparation of highly proton-conducting glasses containing HPAs having provided high power densities at room temperature [6,7]. Since increasing or changing the operating properties of a fuel cell is not an energetically efficient solution, it would be preferable to modify the properties of the proton-conducting membrane. For that purpose, within the framework of the present study, heteropolyacids have been incorporated into the phosphosilicate glass ceramic membrane and either ZrO2 or TiO2 has been added in order to enhance the properties even further. Heteropolyacids (HPAs) are one of the most attractive inorganic modifiers due to their crystalline form. It is in fact the crystallinity that leads to them being highly conductive and thermally stable. Furthermore, HPAs have several hydrated structures depending on their surrounding environment [8]. In the dehydrated phase in polar solvents, the primary structure is called a Keggin unit. The structure, conductivity, thermal and chemical stability of numerous protonated materials have been investigated, and studies on many more will follow. We have already proposed glass ceramic composite membranes with decent performances for H2/O2 fuel cell at low temperatures [6,7]. The synthesis and characterization have been performed of glass composite-based structures that have been made highly proton-conductive through doping with a mixture of PWA and
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U. Thanganathan, M. Nogami / Journal of Non-Crystalline Solids 356 (2010) 2799–2802
PMA. Such membranes are expected to have proton conductivities exceeding 10− 1 S cm− 1 at room temperature and superior thermal, mechanical, chemical and electrochemical stabilities [9,10]. This work describes the design and development of a new class of proton-conducting glass composite membranes (PGCMs). The main objective was to increase the power efficiency of these materials at a low temperature. A detailed report is given on the development of two ceramic composite systems, i.e., PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/ 90 mol %) and PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and the characterization of their properties for fuel cell applications. 2. Experimental section
Quantochrome. The pore size distribution was established using the BJH method [13] and specific surface areas were determined by the BET method [14]. Nuclear magnetic resonance (NMR) spectroscopy was carried out on a Varian Unity 400 NMR spectrometer, operating at a spinning speed of 5 kHz. The electrochemical characterization of H2/O2 fuel cells was performed by measuring a polarization curve and carrying out impedance measurements on a Solartron 1287 electrochemical interface with a Solartron 1260 frequency response analyzer. The cell was operated at room temperature (25 °C) with a constant humidity of 30%. The cell conditions were controlled by an NF- fuel cell evaluation system (Japan) and the gas flow rates were 20 and 100 mL min− 1 to the anode and cathode sides, respectively.
2.1. Materials 3. Results and discussion
2.2. Preparation of composite membranes and catalyst layer Ceramic glass composite membranes can be fabricated by a sol–gel process, according to a technique largely utilized in our lab [6,7,11,12]. Metal alkoxides and heteropolyacid were used to synthesize the two porous glass composite membranes: PWA/PMA–TiO2–P2O5–SiO2 (1/1/ 3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %). TEOS was hydrolyzed with water and ethanol (as 0.15N-HCl aq) under stirring for 1 h and then a mixture of PWA and PMA together with either TiO2 or ZrO2 as charged to the above solution. Subsequently, diisoprophyl phosphite was added dropwise under magnetic stirring at room atmosphere for 5 h. This mixture was hydrolyzed a second time with water and ethanol in the ratio of 1:1 (as 0.15N–HCl aq), after which dimethylformamide was added to the bulk solution. The resulting solution was allowed to form a gel through aging at room temperature and was then treated by water-vapor at 150 °C for 15 h, followed by a heat treatment at 600 °C for 12 h to obtain the porous glass. Both composites were prepared under a similar condition at room temperature. The catalyst layers consisted of a porous mixture of the ionomer and platinum-loaded carbon particles, thus allowing contact between the solid, ionomer and gas phases. The catalyst ink was prepared with a Pt/C powder mixed with Nafion® and a PTFE solution, and this was followed by blending the heteropolyacid (PWA and PMA) and solvents (water and ethanol) under ultrasonic stirring at 50 °C for 1 day. Subsequently, the ink was uniformly sprayed on the surface of a carbon sheet, polished with a brush and dried at 80 °C. The catalyst layers were then attached to both sides of a membrane using the catalyst ink, giving rise to a manually fabricated membrane-electrodeassembly (MEA) at room atmosphere. The active area of the electrode was 0.7 cm2, which corresponded to the membrane surface during fuel cell tests. The amount of Pt/C was 0.05 mg cm− 2. 2.3. Equipment and reagents TG/DTA measurements were carried out on a Seiko SSC 5200-TG/ DTA 220 instrument under a nitrogen atmosphere at a heating rate of 10 °C min− 1. Fourier-transform infrared (FTIR) spectra of the glass composite samples were collected with an FTIR spectrometer (JASCOFTIR-460) in the range 400–4000 cm− 1. The average pore diameter was measured from gas adsorption analysis on a NOVA-1000, from
Fig. 1a and b shows the results from the thermal gravimetric analysis (TGA) and the differential thermal analysis (DTA) of respectively the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. Two main weight loss processes can be observed for both samples: one around 200 °C and a second around 400 °C. The first transition peak was the sum of two components indicating the evaporation of weakly and more strongly bound water from room temperature up to 200 °C, there occurred a desorption of physically adsorbed water due to the presence of hydroxyl groups from Si–OH moieties capable of attracting water molecules. The second weight loss around 400 °C was attributed to the final decomposition of the glass matrix network and at temperatures much higher than 400 °C, the glass composite membrane started to degrade. The temperature locations of these weight loss stages were very similar to previous results reported by our group [6,12]. The assignment of the above-mentioned weight loss variations was supported by the DTA results. Fig. 2a and b, presents DTA plots where broad and small exothermic peaks could be observed at approximately 250 °C. There occurred no phase changes up to 600 °C indicating that both the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) ceramic composite membranes were thermally stable at high temperatures because of the temperature-tolerant inorganic SiO2 framework in the composite matrix. Fig. 3a and b shows the FTIR absorption spectra of the two composite membranes. It has been speculated whether the mixture of heteropolyacids (PWA and PMA) can be sufficiently incorporated and
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ZrO2 [Zr(OC4H9)4, Aldrich], TiO2 [Ti(OC4H9)4, Kishida], diisopropyl phosphite (Wako, Japan) and N, N-dimethylformamide (HCON(CH3)2 Kishida, 99 %) were used without further purification. Solvents (TEOS, SiO2 and ethanol) and reagents of analytical grade were commercial products and were used as received. Water purified with a Milli-Q system form Millipore (AQUARIUS/GS-20R, Japan) was employed in all the experiments. Commercial phophomolybdic acid hydrate (H3PMo12O40·nH2O, PMA, Kishida Chemicals), phosphotungstic acid (H3PW12O40, PWA, Aldrich), and tetraethoxysilane (TEOS, Colcoat. Co. Ltd) were also used as supplied.
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Temperature (οC) Fig. 1. TGA plots of (a) the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (b) the PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. All TGA curves were recorded on the glass powders in scans from room temperature to 800 °C, under a nitrogen atmosphere and at a heating rate of 10 °C/min.
U. Thanganathan, M. Nogami / Journal of Non-Crystalline Solids 356 (2010) 2799–2802
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Fig. 2. DTA plots of (a) the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (b) the PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. The DTA curves were recorded on powder samples in scans from room temperature to 800 °C under a nitrogen atmosphere and at a heating rate of 10 °C/min.
dispersed in a glass ceramic matrix (either TiO2–P2O5–SiO2 or ZrO2– P2O5-SiO2) by a sol–gel route [15]. Antisymmetric stretching vibrations of the O–P and O–Si bonds in the P–O–Si linkage were seen at 1050 cm− 1and the O–P bands in the P–O–P linkage were present at 1280 cm− 1. These bands were all assigned to the OH stretching modes of the Si–OH bonds. The broad bands induced around 3700 cm− 1 were attributed to the free and hydrogen-bonded molecular water. In the stretching mode (P), the O–H was strongly hydrogen-bonded with the non-bridging oxygen of a glass matrix. This band was visible at around 2350 cm− 1 [16]. The average pore size and pore volume of the two composite membranes were measured by nitrogen adsorption, and the obtained nitrogen adsorption/desorption isotherms for both tested materials showed a type-I hysteresis loop [Fig. 4a and b]. A total pore volume of 0.28 cm3 g− 1 and 0.16 cm3 g− 1 in addition to average pore sizes of ~2.4 and 2.1 nm were respectively determined for PWA/PMA–TiO2–P2O5– SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %). These result were similar to those presented in previous reports [12]. Materials with a macroporous structure are less able to retain the physisorbed water on the metal oxide surface that is essential for proton transfer. On the other hand, materials with a microporous structure will also hinder proton transfer [17]. The proton conductivity of the
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P/P0 Fig. 3. N2 adsorption–desorption isotherms of the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/ 90 mol %) and the PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. The samples were degassed at 250 °C for 5 h prior to the measurements.
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Wave number (cm-1) Fig. 4. FTIR spectra of (a) the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (b) the PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. The spectra were recorded in absorbance mode on powders of the glasses in the wave number range 4000–400 cm− 1.
membranes was measured at room temperature, and was found to be in the order of 10− 2 S cm− 1. The 31P NMR spectra of PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) are displayed in Fig. 5a, b and c, respectively. A chemical shift was observed at 6.9 ppm for PWA/PMA, and its intensity decreased with the further addition of either TiO2–ZrO2. Moreover, significant line broadening of the peaks at 5.5 ppm and 4.7 ppm took place for the composite membranes [Fig. 5b and c]. These changes in intensity and chemical shifts of the proton signals in the complexes supported the idea of the formation of composite membranes between the heteropolyacids and the phosphosilicate. Fuel cell measurements performed on the PWA/PMA–TiO2–P2O5– SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composites demonstrated that the H2/O2 fuel cell performances at room temperature, when using humidified H2 and O2 at a pressure of 1 atm were enhanced as, compared to the original PWA/PMA glass membrane [18]. Similar results were found for PWA doped with TiO2–P2O5–SiO2 or ZrO2–P2O5–SiO2 and PMA doped with TiO2–P2O5–SiO2 or ZrO2–P2O5–SiO2 [19,20]. At 24 °C, the maximum current density was 200 and 135 mA cm− 2 and the maximum power density was 50.5 and 33.5 mW cm− 2 for respectively PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) [Fig. 6a and b] under a hydrogen and oxygen gas feed. The fuel cell test confirmed that the PWA/PMA–TiO2–P2O5–SiO2 glass composite membrane had a superior performance as opposed to its counterpart when utilized in hydrogen and oxygen fuel cells. We have previously reported that the chemical stability of such glass membranes increased significantly in the addition of TiO2 or ZrO2 [21,22]. The Zr4+ ions tend to bond to the P5+ ions, via P–O–Zr bonds in the glass, resulting in the increased durability. However the introduction of the Zr4+ ions caused a decrease of the conductivity. The present study confirmed that the chemical stability of the glass membranes increased also with the chemical bonding of the Ti4+ ions in the glass structure. However, in order for the cell performances of the glass ceramic composite membrane to be satisfactory at a low temperature, it needs to be operated above room temperature as this will enable it to be effective with Pt/C-based electrodes. Further investigations are thus required to develop such a glass membrane exhibiting superior chemical and electrical properties for fuel cell applications.
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Fig. 5. 31P NMR spectra of (i) PWA/PMA, (ii) PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (iii) PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %).
Fig. 6. Polarization curves of (a) the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (b) the PWA/PMA–ZrO2–P2O5–SiO (1/1/3/5/90 mol %) glass composite membranes when employed as electrolytes in H2/O2 fuel cells.
References 4. Summary Low-temperature proton-conducting glass ceramic membranes consisting of PWA/PMA–P2O5–SiO2 mixed with either TiO2 or ZrO2 were synthesized through a sol–gel process. Novel ceramic composite membranes for use in high-performance fuel cells could thus be developed. As determined by TG/DTA measurements under nitrogen, these new nano-engineered glass ceramic proton-conducting membranes presented an excellent thermal stability up to 250 °C, and were also mechanically stable. 31P NMR analysis revealed the existence of protonic transport in the two glass composite membrane. Moreover, the glasses had large pore sizes and surface areas, as determined by characterizations of their textural properties. A maximum current density of 200 mA cm− 2 was obtained for the PWA/PMA–TiO2–P2O5– SiO2 glass composite membrane at 25 °C under a relative humidity of 30%, and further improvements should be able to be achieved by fabrication of novel catalysts for these glass ceramic composite materials, rendering them suitable for low-temperature fuel cells. Acknowledgements This study has been financially supported by the Japan Society for Promotion of Science (JSPS) program.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
F.M. Vichi, M.T. Colomer, M.A. Anderson, Electrochem. Solid State 2 (1999) 313. F.M. Vichi, M.I. Tejedor-Tejedor, M.A. Anderson, Chem. Mater. 12 (2000) 1762. S.-P. Tung, B.-J. Hwang, J. Membr. Sci. 241 (2004) 315. M. Yamada, D.L. Li, I. Honma, H.S. Zhou, J. Am. Chem. Soc. 127 (2005) 13092. S.R. Samms, S. Wasmus, R.F. Savinell, J. Electrochem. Soc. 143 (1996) 1498. T. Uma, M. Nogami, J. Non-Cryst. Solids 351 (2005) 3325. T. Uma, M. Nogami, J. Ceramic Soc. Jpn. 114 (2006) 748. Y.S. Kim, F. Wang, M. Hickner, T.A. Zawodzinski, J.E. McGrath, J. Membr. Sci. 212 (2003) 263. O. Nakamura, T. Kodama, I. Ogino, Y. Miyake, Chem. Lett. 1 (1979) 17. O. Nakamura, I. Ogino, Mat. Res. Bull. 17 (1982) 231. T. Uma, M. Nogami, J. Membr. Sci. 280 (2006) 744. T. Uma, M. Nogami, ChemPhyChem. 8 (2007) 2227. E.P. Barret, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 62 (1940) 2839. S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 62 (1938) 309. T. Uma, M. Nogami, Chem. Mater. 19 (2007) 3604. H. Scholze, Glass: Nature, Structure and Properties, Springer, NY, 1991. M.T. Colomer, M.A. Anderson, J. Noncryst. Solids 290 (2001) 93. T. Uma, M. Nogami, Anal. Chem. 80 (2008) 506. T. Uma, M. Nogami, J. Membr. Sci. 323 (2008) 11. T. Uma, M. Nogami, J. Membr. Sci. 334 (2009) 123. M. Nogami, Y. Goto, T. Kasuga, J. Am. Ceram. Soc. 86 (2003) 1504. M. Nogami, M. Suwa, T. Kasuga, Solid State Ionics 166 (2004) 39.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Novel ceramic composite membranes for low-temperature fuel cells Uma Thanganathan a,⁎, Masayuki Nogami b a b
RCIS (Research Core Interdisciplinary Science), Okayama University, Tsushima-Naka, Okayama 700-8530, Japan Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa, Nagoya 466-8555, Japan
a r t i c l e
i n f o
Article history: Received 18 October 2009 Received in revised form 20 August 2010 Available online 11 October 2010 Keywords: Composite membranes; Low-temperature fuel cell; Inorganic materials; Heteropolyacids;
a b s t r a c t A new class of proton-conducting glass ceramic composite membranes was developed for low-temperature fuel cells. According to characterizations, these membranes demonstrated good mechanical, structural, thermal and textural properties. Moreover, they yielded a high proton conductivity of 10− 2 S cm− 1 at room temperature. An electrode consisting of a heteropolyacid dispersed in a catalyst layer was also prepared and characterized, and the cell performances when using the composite membranes of PWA/PMA doped with either ZrO2 or TiO2 glass together with the heteropoly acid electrode were extremely high as compared to counterparts with a phosphosilicate glass electrolyte, at room temperature. The cell evaluation was performed on two ceramic composite membranes, i.e., PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %), with the same catalyst and with an operation equivalent to that in hydrogen and oxygen fuel cells, and the former membrane was found to present the best properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction At the present time, environmental issues due to increasing emissions of air pollutants and greenhouse gases are spurring the development of clean energy delivery technologies, such as fuel cells. Low-temperature proton exchange membrane fuel cells (PEMFCs) use hydrogen as fuel and their only emission is water. Over the last decade, the interest for utilizing ceramic materials as possible electrolyte membranes for fuel cell applications has grown [1–4]. Ceramics are good candidates because of their thermal, chemical, and mechanical stability, as well as their lower material costs. The proton conduction is highly dependent on the surface coverage of adsorbed water on the inorganic material. While significant advances have been made in recent years, a major restriction of the current technology is the cost and material limitations of the proton conduction membrane. For transport applications, fuel cell companies require more durable, cost effective membrane technologies capable of delivering an enhancement at low temperatures (from room temperature up to 80 °C). As a result, research is being driven towards a wide range of novel organic and inorganic materials with the potential of being good proton conductors and forming coherent membranes. Nafion® and similar membranes suffer serious disadvantages such as high cost, poor hydrophilicity, fuel crossover, and low proton conductivity at low humidity and high temperatures [5]. Consequently, attempts have been made to produce cheaper proton-conducting
⁎ Corresponding author. Tel.: + 81 86 251 8706; fax: + 81 86 251 8705. E-mail address: [email protected] (U. Thanganathan). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.043
glasses that can equal the electrochemical performance of perflourinated ionomers as well as their mechanical, thermal and electrochemical stability. Membranes fabricated from the heteropolyacid (HPA) family are currently the best suited for proton conduction at both low and high temperatures. This paper describes the synthesis and structural properties of novel proton-conducting glass ceramic composites based on heteropolyacids. These materials are both cheaper and more thermally stable than Nafion® and recent studies have already reported on the preparation of highly proton-conducting glasses containing HPAs having provided high power densities at room temperature [6,7]. Since increasing or changing the operating properties of a fuel cell is not an energetically efficient solution, it would be preferable to modify the properties of the proton-conducting membrane. For that purpose, within the framework of the present study, heteropolyacids have been incorporated into the phosphosilicate glass ceramic membrane and either ZrO2 or TiO2 has been added in order to enhance the properties even further. Heteropolyacids (HPAs) are one of the most attractive inorganic modifiers due to their crystalline form. It is in fact the crystallinity that leads to them being highly conductive and thermally stable. Furthermore, HPAs have several hydrated structures depending on their surrounding environment [8]. In the dehydrated phase in polar solvents, the primary structure is called a Keggin unit. The structure, conductivity, thermal and chemical stability of numerous protonated materials have been investigated, and studies on many more will follow. We have already proposed glass ceramic composite membranes with decent performances for H2/O2 fuel cell at low temperatures [6,7]. The synthesis and characterization have been performed of glass composite-based structures that have been made highly proton-conductive through doping with a mixture of PWA and
2800
U. Thanganathan, M. Nogami / Journal of Non-Crystalline Solids 356 (2010) 2799–2802
PMA. Such membranes are expected to have proton conductivities exceeding 10− 1 S cm− 1 at room temperature and superior thermal, mechanical, chemical and electrochemical stabilities [9,10]. This work describes the design and development of a new class of proton-conducting glass composite membranes (PGCMs). The main objective was to increase the power efficiency of these materials at a low temperature. A detailed report is given on the development of two ceramic composite systems, i.e., PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/ 90 mol %) and PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and the characterization of their properties for fuel cell applications. 2. Experimental section
Quantochrome. The pore size distribution was established using the BJH method [13] and specific surface areas were determined by the BET method [14]. Nuclear magnetic resonance (NMR) spectroscopy was carried out on a Varian Unity 400 NMR spectrometer, operating at a spinning speed of 5 kHz. The electrochemical characterization of H2/O2 fuel cells was performed by measuring a polarization curve and carrying out impedance measurements on a Solartron 1287 electrochemical interface with a Solartron 1260 frequency response analyzer. The cell was operated at room temperature (25 °C) with a constant humidity of 30%. The cell conditions were controlled by an NF- fuel cell evaluation system (Japan) and the gas flow rates were 20 and 100 mL min− 1 to the anode and cathode sides, respectively.
2.1. Materials 3. Results and discussion
2.2. Preparation of composite membranes and catalyst layer Ceramic glass composite membranes can be fabricated by a sol–gel process, according to a technique largely utilized in our lab [6,7,11,12]. Metal alkoxides and heteropolyacid were used to synthesize the two porous glass composite membranes: PWA/PMA–TiO2–P2O5–SiO2 (1/1/ 3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %). TEOS was hydrolyzed with water and ethanol (as 0.15N-HCl aq) under stirring for 1 h and then a mixture of PWA and PMA together with either TiO2 or ZrO2 as charged to the above solution. Subsequently, diisoprophyl phosphite was added dropwise under magnetic stirring at room atmosphere for 5 h. This mixture was hydrolyzed a second time with water and ethanol in the ratio of 1:1 (as 0.15N–HCl aq), after which dimethylformamide was added to the bulk solution. The resulting solution was allowed to form a gel through aging at room temperature and was then treated by water-vapor at 150 °C for 15 h, followed by a heat treatment at 600 °C for 12 h to obtain the porous glass. Both composites were prepared under a similar condition at room temperature. The catalyst layers consisted of a porous mixture of the ionomer and platinum-loaded carbon particles, thus allowing contact between the solid, ionomer and gas phases. The catalyst ink was prepared with a Pt/C powder mixed with Nafion® and a PTFE solution, and this was followed by blending the heteropolyacid (PWA and PMA) and solvents (water and ethanol) under ultrasonic stirring at 50 °C for 1 day. Subsequently, the ink was uniformly sprayed on the surface of a carbon sheet, polished with a brush and dried at 80 °C. The catalyst layers were then attached to both sides of a membrane using the catalyst ink, giving rise to a manually fabricated membrane-electrodeassembly (MEA) at room atmosphere. The active area of the electrode was 0.7 cm2, which corresponded to the membrane surface during fuel cell tests. The amount of Pt/C was 0.05 mg cm− 2. 2.3. Equipment and reagents TG/DTA measurements were carried out on a Seiko SSC 5200-TG/ DTA 220 instrument under a nitrogen atmosphere at a heating rate of 10 °C min− 1. Fourier-transform infrared (FTIR) spectra of the glass composite samples were collected with an FTIR spectrometer (JASCOFTIR-460) in the range 400–4000 cm− 1. The average pore diameter was measured from gas adsorption analysis on a NOVA-1000, from
Fig. 1a and b shows the results from the thermal gravimetric analysis (TGA) and the differential thermal analysis (DTA) of respectively the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. Two main weight loss processes can be observed for both samples: one around 200 °C and a second around 400 °C. The first transition peak was the sum of two components indicating the evaporation of weakly and more strongly bound water from room temperature up to 200 °C, there occurred a desorption of physically adsorbed water due to the presence of hydroxyl groups from Si–OH moieties capable of attracting water molecules. The second weight loss around 400 °C was attributed to the final decomposition of the glass matrix network and at temperatures much higher than 400 °C, the glass composite membrane started to degrade. The temperature locations of these weight loss stages were very similar to previous results reported by our group [6,12]. The assignment of the above-mentioned weight loss variations was supported by the DTA results. Fig. 2a and b, presents DTA plots where broad and small exothermic peaks could be observed at approximately 250 °C. There occurred no phase changes up to 600 °C indicating that both the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) ceramic composite membranes were thermally stable at high temperatures because of the temperature-tolerant inorganic SiO2 framework in the composite matrix. Fig. 3a and b shows the FTIR absorption spectra of the two composite membranes. It has been speculated whether the mixture of heteropolyacids (PWA and PMA) can be sufficiently incorporated and
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ZrO2 [Zr(OC4H9)4, Aldrich], TiO2 [Ti(OC4H9)4, Kishida], diisopropyl phosphite (Wako, Japan) and N, N-dimethylformamide (HCON(CH3)2 Kishida, 99 %) were used without further purification. Solvents (TEOS, SiO2 and ethanol) and reagents of analytical grade were commercial products and were used as received. Water purified with a Milli-Q system form Millipore (AQUARIUS/GS-20R, Japan) was employed in all the experiments. Commercial phophomolybdic acid hydrate (H3PMo12O40·nH2O, PMA, Kishida Chemicals), phosphotungstic acid (H3PW12O40, PWA, Aldrich), and tetraethoxysilane (TEOS, Colcoat. Co. Ltd) were also used as supplied.
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Temperature (οC) Fig. 1. TGA plots of (a) the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (b) the PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. All TGA curves were recorded on the glass powders in scans from room temperature to 800 °C, under a nitrogen atmosphere and at a heating rate of 10 °C/min.
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Fig. 2. DTA plots of (a) the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (b) the PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. The DTA curves were recorded on powder samples in scans from room temperature to 800 °C under a nitrogen atmosphere and at a heating rate of 10 °C/min.
dispersed in a glass ceramic matrix (either TiO2–P2O5–SiO2 or ZrO2– P2O5-SiO2) by a sol–gel route [15]. Antisymmetric stretching vibrations of the O–P and O–Si bonds in the P–O–Si linkage were seen at 1050 cm− 1and the O–P bands in the P–O–P linkage were present at 1280 cm− 1. These bands were all assigned to the OH stretching modes of the Si–OH bonds. The broad bands induced around 3700 cm− 1 were attributed to the free and hydrogen-bonded molecular water. In the stretching mode (P), the O–H was strongly hydrogen-bonded with the non-bridging oxygen of a glass matrix. This band was visible at around 2350 cm− 1 [16]. The average pore size and pore volume of the two composite membranes were measured by nitrogen adsorption, and the obtained nitrogen adsorption/desorption isotherms for both tested materials showed a type-I hysteresis loop [Fig. 4a and b]. A total pore volume of 0.28 cm3 g− 1 and 0.16 cm3 g− 1 in addition to average pore sizes of ~2.4 and 2.1 nm were respectively determined for PWA/PMA–TiO2–P2O5– SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %). These result were similar to those presented in previous reports [12]. Materials with a macroporous structure are less able to retain the physisorbed water on the metal oxide surface that is essential for proton transfer. On the other hand, materials with a microporous structure will also hinder proton transfer [17]. The proton conductivity of the
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Wave number (cm-1) Fig. 4. FTIR spectra of (a) the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (b) the PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composite membranes. The spectra were recorded in absorbance mode on powders of the glasses in the wave number range 4000–400 cm− 1.
membranes was measured at room temperature, and was found to be in the order of 10− 2 S cm− 1. The 31P NMR spectra of PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) are displayed in Fig. 5a, b and c, respectively. A chemical shift was observed at 6.9 ppm for PWA/PMA, and its intensity decreased with the further addition of either TiO2–ZrO2. Moreover, significant line broadening of the peaks at 5.5 ppm and 4.7 ppm took place for the composite membranes [Fig. 5b and c]. These changes in intensity and chemical shifts of the proton signals in the complexes supported the idea of the formation of composite membranes between the heteropolyacids and the phosphosilicate. Fuel cell measurements performed on the PWA/PMA–TiO2–P2O5– SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) glass composites demonstrated that the H2/O2 fuel cell performances at room temperature, when using humidified H2 and O2 at a pressure of 1 atm were enhanced as, compared to the original PWA/PMA glass membrane [18]. Similar results were found for PWA doped with TiO2–P2O5–SiO2 or ZrO2–P2O5–SiO2 and PMA doped with TiO2–P2O5–SiO2 or ZrO2–P2O5–SiO2 [19,20]. At 24 °C, the maximum current density was 200 and 135 mA cm− 2 and the maximum power density was 50.5 and 33.5 mW cm− 2 for respectively PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %) [Fig. 6a and b] under a hydrogen and oxygen gas feed. The fuel cell test confirmed that the PWA/PMA–TiO2–P2O5–SiO2 glass composite membrane had a superior performance as opposed to its counterpart when utilized in hydrogen and oxygen fuel cells. We have previously reported that the chemical stability of such glass membranes increased significantly in the addition of TiO2 or ZrO2 [21,22]. The Zr4+ ions tend to bond to the P5+ ions, via P–O–Zr bonds in the glass, resulting in the increased durability. However the introduction of the Zr4+ ions caused a decrease of the conductivity. The present study confirmed that the chemical stability of the glass membranes increased also with the chemical bonding of the Ti4+ ions in the glass structure. However, in order for the cell performances of the glass ceramic composite membrane to be satisfactory at a low temperature, it needs to be operated above room temperature as this will enable it to be effective with Pt/C-based electrodes. Further investigations are thus required to develop such a glass membrane exhibiting superior chemical and electrical properties for fuel cell applications.
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Fig. 5. 31P NMR spectra of (i) PWA/PMA, (ii) PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (iii) PWA/PMA–ZrO2–P2O5–SiO2 (1/1/3/5/90 mol %).
Fig. 6. Polarization curves of (a) the PWA/PMA–TiO2–P2O5–SiO2 (1/1/3/5/90 mol %) and (b) the PWA/PMA–ZrO2–P2O5–SiO (1/1/3/5/90 mol %) glass composite membranes when employed as electrolytes in H2/O2 fuel cells.
References 4. Summary Low-temperature proton-conducting glass ceramic membranes consisting of PWA/PMA–P2O5–SiO2 mixed with either TiO2 or ZrO2 were synthesized through a sol–gel process. Novel ceramic composite membranes for use in high-performance fuel cells could thus be developed. As determined by TG/DTA measurements under nitrogen, these new nano-engineered glass ceramic proton-conducting membranes presented an excellent thermal stability up to 250 °C, and were also mechanically stable. 31P NMR analysis revealed the existence of protonic transport in the two glass composite membrane. Moreover, the glasses had large pore sizes and surface areas, as determined by characterizations of their textural properties. A maximum current density of 200 mA cm− 2 was obtained for the PWA/PMA–TiO2–P2O5– SiO2 glass composite membrane at 25 °C under a relative humidity of 30%, and further improvements should be able to be achieved by fabrication of novel catalysts for these glass ceramic composite materials, rendering them suitable for low-temperature fuel cells. Acknowledgements This study has been financially supported by the Japan Society for Promotion of Science (JSPS) program.
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