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J. Phys. Chem. B 1999, 103, 9468-9472
High Proton Conductivity in Porous P2O5-SiO2 Glasses Masayuki Nogami,* Ritsuko Nagao, Cong Wong, Toshihiro Kasuga, and Tomokatsu Hayakawa Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa Nagoya, 466-8555, Japan ReceiVed: April 20, 1999; In Final Form: August 9, 1999
High proton conducting P2O5-SiO2 glasses were prepared using the sol-gel method, the electrical conductivities of which were studied in relation to the pore structure and the adsorbed water. The pore properties of SiO2 and P2O5-SiO2 glasses were controlled by addition of formamide during the gel synthesis, the specific surface areas of which were changed from 200 to 900 m2/g-glass. These glasses absorb water molecules on exposure to a humid atmosphere. The conductivity increased with increasing logarithm of water concentration and reached a saturated value above the water vapor pressure of 0.8. The highest conductivity, 2 × 10-2 S/cm at room temperature, was achieved by 5P2O5-95SiO2 glass heated at 700 °C and absorbing the water molecules.
Introduction (H+)
transport in solids has attracted much attention Proton because of its potential use in clean energy fields such as fuel cells, batteries, and chemical sensors. The number of fast protonic conductors, organic or inorganic, crystalline and amorphous, have been prepared during the past two decades,1-3 which are categorized into two types based on their operating temperatures. One is hydrated crystalline compounds,4-7 and the other is perfluorinated ionomers8-11 showing high conductivities of ∼10-2 S/cm below 100 °C, but their chemical degradation limits the use in practical applications. On the other hand, fast proton conducting glasses, if developed, extend beyond the limitation of the above compounds, because of their high chemical and mechanical durability and easy formation of films and plates. Recently, using a sol-gel technique, we prepared gels and glasses exhibiting high conductivities of 10-4-10-2 S/cm at room temperature.12-16 The sol-gel-derived glasses are porous, the pore surfaces of which are terminated with hydroxyl bonds and absorb water in a humid atmosphere. It was found that in these glasses containing both hydroxyl and molecular water the electrical conductivity linearly increases with increasing logarithm of the product of proton and adsorbed-water concentration. The proposed mechanism for proton conduction in these glasses is the dissociation of protons from hydroxyl bonds and the proton hopping between hydroxyl and water molecules. It was found that the energy necessary for the dissociation of the proton decreases with increasing the water concentration. The maximum conductivities at room temperature were ∼10-2 and ∼10-3 S/cm for gel and glass, respectively, although they increased with increasing water concentration. Adsorbed water acts sometimes in harmful ways to decrease the chemical durability of glasses. Water adsorption in the porous glasses is sensitive to the pore structure and the glass composition. The search for finding both the glass composition and the water content which give higher conductivity is necessary for practical applications of the fast proton conducting glasses. The question of how the * Corresponding author. Telephone and Fax: +81 52 735 5285. E-mail:
[email protected].
molecular water contributes to the proton conduction still remains unknown. According to our previous paper,16 we prepared porous P2O5-SiO2 glasses with various surface areas by the sol-gel method and measured the conductivity under a humid atmosphere. The conductivity of 2 × 10-2 S/cm was measured at room temperature, that is the highest value to the best of our knowledge. In this paper, we first discuss the effect of pore structure on both the water adsorption and the proton conductivity. The effect of P2O5 on the proton conduction is also discussed using infrared spectra. Experimental Section SiO2, 2P2O5-98SiO2, and 5P2O5-95SiO2 (in mol %) glasses were prepared through the hydrolysis of tetraethyl orthosilicate (Si(OC2H5)4, 99.9%, Colcote, Japan) and trimethyl phosphate (PO(OCH3)3, >98%, Nacalai tesque, Japan). The starting materials were used as received. Si(OC2H5)4 was hydrolyzed at room temperature with a solution of H2O, C2H5OH, and HCl in molar ratios of 4:4:0.03 per mol of Si(OC2H5)4. For the P2O5SiO2 glasses, PO(OCH3)3 was reacted at room temperature with the Si(OC2H5)4 hydrolyzed with the mixed solution of 1H2O, 1C2H5OH, and 0.0027HCl in mol per mol of Si(OC2H5)4. The resulting homogeneous solution was further hydrolyzed with the solution of 4H2O, 1C2H5OH, and 0.011HCl per mol of the obtained alkoxide complex for 1 h. After hydrolyzing, HCONH2 was added in the solution, followed by stirring for further 1 h. The amount of HCONH2 was changed to 1, 2, and 3 mL for 5 g of oxide glass. The obtained solution was clear and left for ∼1 month to form a stiff gel about 0.1 mm thick. The gel was heated in air at 50 °C/h to 600-800 °C and held at that temperature for 2 h. The gel contains physically and chemically absorbed water on the gel particle surfaces. Upon heating, the gel liberates the water molecules and shrinks due to dehydration-condensation of the hydroxyl groups. The porous structure remains unchanged in glass heated below ∼900 °C. The surface area, pore volume, and pore size distribution of samples heated at 600-800 °C were measured by a Quantachrome Corporation NOVA-1000 nitrogen gas sorption analyzer. All the samples were preheated at 250 °C in a vacuum in order
10.1021/jp991277s CCC: $18.00 © 1999 American Chemical Society Published on Web 10/21/1999
High Proton Conductivity in Porous P2O5-SiO2 Glasses
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TABLE 1: Properties of Porous 100SiO2, 2P2O5-98SiO2, and 5P2O5-95SiO2 Glasses sample
composition (mol %)
formamide (mL)
heating temperature (°C)
surface area (m2/g)
pore volume (mL/g)
average pore radius (nm)
100S-1-6 100S-1-7 100S-1-8 100S-2-7 100S-3-7 2P98S-1-6 2P98S-1-7 2P98S-1-8 5P95S-1-6 5P95S-1-7 5P95S-1-8
100SiO2 100SiO2 100SiO2 100SiO2 100SiO2 2P2O5-98SiO2 2P2O5-98SiO2 2P2O5-98SiO2 5P2O5-95SiO2 5P2O5-95SiO2 5P2O5-95SiO2
1 1 1 2 3 1 1 1 1 1
600 700 800 700 700 600 700 800 600 700 800
775 752 594 855 868 779 642 317 504 251 21
0.57 0.48 0.40 0.75 0.89 0.65 0.40 0.22 0.38 0.27 0.03
1.5 1.3 1.3 1.8 2.0 1.7 1.4 1.4 1.5 2.2 2.5
to remove the adsorbed water remaining in pores. The adsorption-desorption isotherm of N2 gas shows a hysteresis that is normally attributed to the existence of pore cavities larger than the opening. Infrared spectra were obtained between 4000 and 2000 cm-1 for the ∼0.1 mm thick samples. The mass of adsorbed water was determined by weighing the sample in the chamber kept at constant temperature and constant water vapor pressure. The conductivity was determined from Cole-Cole plots by an ac method using a Solartron SI 1260 impedance analyzer, where evaporated-gold electrodes were used. The samples for the measurement of conductivity were kept in a chamber to absorb the water equal to the atmosphere. The Cole-Cole plot consisted of a single semicircle and the electrical conductivity was obtained from the intersecting point of the semicircle with the real axis. Results and Discussion Porous Structure of the Sol-Gel-Derived Glasses. In the sol-gel processing using metal alkoxides, the various solvents are used to prevent the liquid-liquid separation during the initial stage of the hydrolysis reaction.17 Further, the effect of solvents has been studied to control the drying rate of gel, especially to obtain the crack-free monolithic gels in a short drying period. Among them, formamide, HCONH2, is considered to be effective to prepare the crack-free gels with larger pores.18 The gels obtained in this work are porous, containing water and solvents incorporated during gel synthesis. Upon heating, the water and solvents in pores evaporated at around 100 °C, followed by gradual removal of water produced by the dehydration-condensation reaction between the hydroxyl groups on the pore surfaces. According to this reaction, the pores were collapsed, which resulted in the reduction of surface and volume of pores. The porous properties strongly reflect the glass composition and the amount of formamide used in the gel synthesis. Pore surface area, pore volume, and the average pore size for glasses heated at 600-800 °C are summarized in Table 1. The specific surface area and pore volume decrease with increasing the heat-treatment temperature, which is due to the polycondensation reaction between the hydroxyl groups of pore surfaces. The size distributions of pores in SiO2 glasses which are prepared using various amount of formamide and heated at 700 °C are shown in Figure 1. It is apparent that the glass prepared by adding large amount of formamide exhibits larger pores in addition to larger surface area and pore volume. Adsorbed Water Molecules and the Proton Conductivity of SiO2 Glasses with Large Surface Area. Pore surfaces of the porous glasses are terminated with hydroxyl bonds and sensitive to air humidity. On exposing the glasses to an ambient air atmosphere, they absorb water. This reaction is monitored by determining the sample weight, which increases with time
Figure 1. Pore size distributions of SiO2 glasses prepared by using various amounts of formamide and heating at 700 °C.
Figure 2. Change in number of adsorbed water molecules per 1 nm2 of surface area for SiO2 glasses, shown in Figure 1, with an exposure to water vapor at 30 °C.
and reaches a saturated level within a few hours. Figure 2 shows the relation between the content of adsorbed water and the partial pressure of water vapor at 30 °C for samples heated at 700 °C, where the adsorbed water is calculated as the number of H2O molecules per 1 nm2 of surface area. The water content increases sigmoidally with the increase of the water vapor pressure and reaches a constant value. The amount of water adsorbed under a vapor pressure of about 0.9 is compared to the volume of pores, indicating that the pores in glass are almost completely filled with water. It is also interesting to notice that the glasses
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Figure 4. Plotting of conductivity shown in Figure 3 against logarithm of the number of water molecules per 1 nm2 of surface area.
Figure 3. Conductivity, measured at 30 °C, of SiO2 with an exposure to water vapor at 30 °C.
prepared using a small amount of formamide absorb a large number of water molecules in the low vapor pressure region, but this is not clear at present and further study is necessary. The H2O molecule is polar and bound with the surface hydroxyl groups with hydrogen-bonding, indicating the condensation of H2O molecules in the smaller pores. Electrical conductivities of glasses without exposure to the ambient air were too small, less than 10-8 S/cm, to measure by the ac method. Upon exposing to ambient air, the conductivity increases as the exposure time increases and approaches a constant level. Figure 3 shows the relation between the conductivity, measured at 30 °C, and water vapor pressure for the glasses heated at 700 °C. It is evident that the conductivity increases with increasing water vapor pressure and reaches a constant value of 10-3-10-4 S/cm at the water vapor pressure above 0.7, irrespective of the porosity of samples. In the previous papers, we reported the electrical conductivity of porous silica glasses containing both hydroxyl bonds and water.15,16 The electrical charge carrier is only proton dissociated from the hydroxyl groups on the pore surfaces, and its activation energy for conduction is related to the energy necessary for the dissociation of the proton from hydroxyl and water molecules. The activation energy linearly decreased with increasing the logarithm of the product of proton and water content. We concluded that the proton conduction is associated with proton hopping between hydroxyl groups and water molecules, and the conductivity increased with increasing the water content. The present glasses are prepared using formamide to form large surface area and large pore volume, indicating that they absorb a large amount of water in pores. Among the adsorbed water molecules, the H2O molecules in first layer of the pore surfaces are strongly hydrogen bonded with the hydroxyl groups and the residual H2O molecules form a liquid state in pores. The experimental results of Figures 2 and 3 suggest that the conductivity is preliminarily determined by the amount of adsorbed water. The conductivities at 30 °C are replotted in Figure 4 as a function of the logarithm of water concentration. Note that conductivities are well represented to be proportional to the logarithm of water concentration, though two separate lines in response to the water content. This result suggests that
Figure 5. Infrared spectra of SiO2 and 2P2O5-98SiO2 glasses heated at 700 °C without exposure to an ambient atmosphere.
the conductivity of porous silica glasses is limited to ∼10-3 S/cm at room temperature for any glass with large surface area. Effect of P2O5 on the Proton Conductivity. The P2O5SiO2 glasses are prepared by reacting PO(OCH3)3 with the previously hydrolyzed Si(OC2H5)4. The phosphorus ions are bound with the silica network consisting of the core structure. Therefore, pore surfaces of the obtained glasses are rich in POH groups. The proton in the POH group is strongly hydrogen bonded with the water molecules compared with that of the SiOH groups. Figure 5 shows the IR spectra of SiO2 and 2P2O598SiO2 glasses heated at 700 °C without exposure to an ambient atmosphere. Two absorption bands peaking at 3680 and 3300 cm-1 with a shoulder around 2900 cm-1 are observed, which are both assigned to the OH stretching. It is known that the position of the OH absorption bands depends on the degree of
High Proton Conductivity in Porous P2O5-SiO2 Glasses
J. Phys. Chem. B, Vol. 103, No. 44, 1999 9471
Figure 6. Plotting of conductivity for SiO2, 2P2O5-98SiO2, and 5P2O5-95SiO2 glasses, measured at 30 °C and water vapor pressure of 0.4, against the product of surface area and the reverse of the pore radius.
Figure 7. Plotting of conductivity for SiO2, 2P2O5-98SiO2, and 5P2O5-95SiO2 glasses, measured at 30 °C and water vapor pressure of 0.9, against the product of surface area and the reverse of the pore radius.
strength of hydrogen bonding, and shifts to lower wavenumber with increasing strength of the hydrogen bonding.19 Among the absorption bands, the band at 3680 cm-1 is assigned to the hydrogen-bonding free SiOH bonds. On the other hand, the bands at 3300 and 2900 cm-1, observed in the glasses containing P2O5 and their intensities increase with increasing P2O5 content, are assigned to the POH bonds. Abe and co-workers20-22 measured electrical conductivity of different types of oxide glasses containing protons but not alkali ions, and concluded that protons are able to function as electrical charge carriers when they are strongly hydrogen bonded in oxide glasses. They also showed that the conductivity can be estimated from the peak wavenumber of OH-stretching vibration in infrared absorption spectra and the shift to lower wavenumber by 100 cm-1 compared with SiOH bonds gives the conductivity larger by 1 order of magnitude than that in the SiOH bond. According, the difference between these wavenumbers, 3680 and 3300 cm-1 for SiOH and POH, respectively, gives a conductivity for the POH bond that is larger by ∼4 orders of magnitude than that in SiOH. However, the prepared gels are deliquescent to be easily soluble in water owing to P2O5 and not appropriate for practical use. Generally the phosphate materials are lacking in chemical durability and the absorbed water also lowers the chemical stability of glasses, despite the possibility of high conductivities. It is necessary to elucidate the effect of P2O5 content and adsorbed water on the proton conductivity for the development of the practical high conducting glasses.13,14 In the region of the water vapor pressure below ∼0.6, the conductivities increase with increasing the water vapor pressure, which for the glasses containing P2O5 are higher compared with those of SiO2 glasses. In this low water content region, the dissociated proton from the hydroxyl groups moves between the hydroxyl and the hydrogen-bonded water molecules on the pore surfaces. The proton conduction is strongly affected by the nature of the surface properties. In Figure 6, the conductivities, measured at 30 °C and the water vapor pressure of 0.4, for SiO2, 2P2O5-98SiO2, and 5P2O5-95SiO2 glasses are plotted as a function of the product of surface area and the reverse of the pore radius. The values of surface area and pore size correspond to the concentration of the surface OH groups and the adsorbed H2O molecules, respectively. It is apparent that the conductivity linearly increases with increasing the product
of surface area and the reciprocal number of the pore radius and is higher for the glasses containing P2O5. These results strongly suggest that the conductivity is enhanced by the incorporation of P2O5 in glass, despite a difference of their pore surfaces. From the peak positions of OH-stretching modes in infrared spectra, it can be expected that protons in POH bonds give high conductivities larger by ∼4 orders of magnitude than that in SiOH bonds. Because the number of POH bonds on the pore surfaces is not analyzed, the quantitative estimation of increase in the conductivity due to the POH bonds is not done. Nevertheless, it is easy to understand that the P2O5-SiO2 glasses exhibit much higher conductivities compared with SiO2 glasses. Further interesting to notice in Figure 6 is that the P2O5containing glasses exhibit high conductivities, even though these glasses have small surface area of pores. This finding is very important for the development of the glasses exhibiting high conductivities. Figure 7 shows the relation between the conductivities, measured at 30 °C and the water vapor pressure of 0.9, and the product of surface area and the reverse of the pore radius. Under this high water vapor pressure, the pores are filled with water molecules and the conductivity is independent of the water content in glasses. Dependence of Conductivity on Temperature. Figure 8 shows the plots of conductivity vs temperature, measured under the water vapor pressure of 0.9, for three SiO2, 2P2O5-98SiO2, and 5P2O5-95SiO2 glasses heated at 700 °C. It is evident that the conductivity varies exponentially with reciprocal temperature and follows the Arrhenius equation:
σ ) σ0 exp(-E/RT) where E is an activation energy for conduction, T is the temperature, R is the gas constant, and σ0 is a preexponential term called the frequency factor. The activation energies are small: 8.5, 5.8, and 4.3 kJ/mol for SiO2, 2P2O5-98SiO2, and 5P2O5-95SiO2 glasses, respectively, which are much lower than the energies for dissociating the proton from SiOH and POH. We found in the previous works that the activation energy for conduction decreases with increasing the logarithm of the product of proton and water concentration: E ) E0 - k log{[H+][H2O]}, where [H+] and [H2O] are contents of proton and water and k is a constant.15 The first term of this equation, E0,
9472 J. Phys. Chem. B, Vol. 103, No. 44, 1999
Nogami et al. controlling the preparation conditions. Proton conduction is associated with proton hopping between hydroxyl groups and water molecules. It was found that conductivity linearly increases with increasing the product of surface area and the reciprocal number of the pore radius. Protons in POH bonds give high conductivity larger by ∼4 orders of magnitude than that in SiOH bonds. The highest conductivity, ∼2 × 10-2 S/cm at room temperature, was achieved for 5P2O5-95SiO2 glass. References and Notes
Figure 8. Relation between conductivity, measured under water vapor pressure of 0.9, and reciprocal temperature for SiO2, 2P2O5-98SiO2, and 5P2O5-95SiO2 glasses.
is an energy independent of the content of water and corresponds to the energy required to dissociate a proton from the SiOH or POH bonds. Though the energies for dissociation of protons are large, the total energies decrease exponentially with the water content. This means that the proton dissociated from the SiOH or POH bonds moves by hopping through water molecules in pores. The conductivity of ∼2 × 10-2 S/cm at room temperature is the highest value, to our knowledge, in 5P2O5-95SiO2 glasses. Conclusions Using the sol-gel method, the porous P2O5-SiO2 glasses were prepared, the pore properties of which were changed by
(1) Colomban, P. Proton conductors, Chem. Sol. State Mater. 2; Cambridge, London 1992. (2) Bruinink, J. J. Appl. Electrochem. 1972, 2, 239. (3) Glasser, L. Chem. ReV. 1972, 75, 21. (4) Nakamura, M.; Kodama, T.; Ogino, I.; Miyake, Y. Jpn. Patent 76/ 106683, 1977. (5) Shilton, M. G.; Howe, A. T. Mater. Res. Bull. 1977 12, 701. (6) Howe, A. T.; Shilton, M. G. Mater. Res. Bull. 1977, 12, 701. (7) Kobets, L. V.; Kolevich,. T. A.; Umreiko, D. S. Koord. Khim. 1978, 4, 1856. (8) Wolfe, W. R. U.K. Patent 1184321, 1968. (9) Ezzel, B. R.; Carl, W. P.; Mod, W. A. U.S. Patent 4,330,654, 1982; 4,417,969, 1983. (10) Grubb, W. T. J. Electrochem. Soc. 1959, 106, 295. (11) Verbrugge, M. K.; Hill, R. F. J. Electrochem. Soc. 1990, 137, 3770, and references therein. (12) Abe, Y.; Li, G.; Nogami, M.; Kasuga, T.; Hench, L. L. J. Electrochem. Soc. 1996, 143, 144. (13) Nogami, M.; Miyamura, M.; Abe, Y. J. Electrochem. Soc. 1997, 144, 2175. (14) Nogami, M.; Abe, Y. Appl. Phys. Lett. 1997, 71, 1323. (15) Nogami, M.; Abe, Y. Phys. ReV. B 1997, 55, 12108. (16) Nogami, M.; Nagao. R.; Wong, C. J. Phys. Chem. B 1998, 102, 5772. (17) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press Inc., 1990; pp 515-672. (18) Wallace, S.; Hench, L. L. Better Ceramics through Chemistry; Elsevier: 1984; pp 47-52. (19) Scholze, H. Glastech. Ber. 1959, 32, 81; 1959, 32, 142; 1959, 32, 314. (20) Abe, Y.; Shimakawa, H.; Hench, L. L. J. Non-Cryst. Solids 1982, 51, 357. (21) Abe, Y.; Hosono, H.; Ohta, Y.; Hench, L. L. Phys. ReV. 1988, B 38, 10166. (22) Abe, Y.; Hosono, H.; Lee, W. H.; Kasuga, T. Phys. ReV. 1993, B 48, 15621.
