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Chem. Mater. 2002, 14, 4624-4627
Proton Conduction and Pore Structure in Sol-Gel Glasses Yusuke Daiko, Toshihiro Kasuga, and Masayuki Nogami* Nagoya Institute of Technology, Showa Nagoya 466-8555, Japan Received April 5, 2002. Revised Manuscript Received June 28, 2002
Proton conduction in sol-gel-derived porous glasses was investigated and is related to the pore structure. The porous P2O5-SiO2 glasses absorb water under ambient conditions and consequently increase the electrical conductivity. In glasses having pores ∼5 nm or smaller, the water molecules diffuse through the pores and totally fill the pores. The motion of the water molecules is restricted in small pores, and the absorbed water molecules are well retained even at low humidity. The conductivity of this glass was high in the temperature range down to -20 °C. On the other hand, in glasses with large pores, no confinement effect occurred for the water molecules and the conductivity suddenly decreased below 0 °C.
Introduction Conductors exhibiting high proton conductivities at low temperature have attracted considerable attention because of their potential use in fuel cell electrolytes. A series of perfluorosulfonate ionomers such as Nafion have received significant consideration as possible electrolytes due to their high proton conductivity around room temperature. Despite this, they still have some problems for practical use. The conductivity decreases at temperatures higher than ∼80 °C, because of their inability to contain water.1,2 Nafion films are susceptible to deformation on the basis of their repetition of absorption and desorption of water. In addition, their thermal and chemical degradation at about 100 °C or higher would limit their practical use. Stable inorganic materials with high conductivity over a wide temperature range, if developed, would extend beyond the limitation of the above compounds. Many materials have been proposed as new electrolytes in fuel cells.3-7 Our interest focuses on the preparation of highproton-conducting materials with high thermal durability for the application of the practical fuel cell. Recently, using a sol-gel method, we succeeded in the preparation of P2O5-containing glasses exhibiting conductivities of 10-4 to 10-2 S/cm at room temperature.8-10 Our inorganic glasses, prepared by heating the gels at temper* To whom correspondence should be addressed. Phone/Fax: +81 52 735 5285. E-mail:
[email protected]. (1) Zawodzinski, A. T.; Springer, E. T.; Davey, J.; Jestel, R.; Lopez, C.; Valerio, J.; Gottesfeld, S. J. Electrochem. Soc. 1993, 140, 1981. (2) Hinatsu, T. J.; Mizuhata, M.; Takenaka, H. J. Electrochem. Soc. 1994, 141, 1493. (3) Colomer, T. M.; Anderson, A. M. J. Non-Cryst. Solids 2001, 290, 93. (4) Matsuda, A.; Kanzaki, T.; Kotani, Y.; Tatumisago, M.; Minami, T. Solid State Ionics 2001, 139, 113. (5) Daiko, Y.; Akai, T.; Kasuga, T.; Nogami, M. J. Ceram. Soc. Jpn. 2001, 109, 815. (6) Alberti, G.; Casciola, M.; Palombari, R. J. Membr. Sci. 2000, 172, 233. (7) Antonucci, P. L.; Arico, A. S.; Creti, P.; Ramunni, E.; Antonucci, V. Solid State Ionics 1999, 125, 431. (8) Nogami, M.; Miyamura, K.; Abe, Y. J. Electrochem. Soc. 1997, 144, 2175. (9) Nogami, M.; Abe, Y. Appl. Phys. Lett. 1997, 71, 1323.
atures above 400 °C, showed no degradation in their conductivity after heating above 100 °C and have potential as the electrolyte of a fuel cell. The sol-gel-derived P2O5-containing glasses are porous. The P5+ ions form POH bonds, which are weakly bonded with the silica network structure and concentrated on the pore surfaces. The proton conduction is promoted by the dissociation of protons from the hydroxyl bonds on the pore surfaces and the proton hopping between hydroxyl groups and water molecules. The POH bonds, strongly hydrogen-bonded with water molecules, are appropriate for increasing the proton conduction, while the SiO2 is useful to increase the mechanical strength of the glass. The conductivity also increases with increasing water content in the pores.11,12 In this sense, a glass having a large pore volume should exhibit a high conductivity, because of its possibility of containing a large amount of water molecules. However, we found out that the glasses meeting these qualifications do not necessarily exhibit high conductivity. Further investigations are needed to understand how the proton conduction is promoted in the sol-gel-derived glasses. We investigated in detail the effect of pore structure and glass composition on the proton conductivity. Here, we show how the pore properties, particularly pore size and pore volume, affect the proton conduction. P2O5-SiO2 glasses with various porous properties were prepared by the sol-gel method using different materials and synthesis conditions. Proton conductivity is discussed in relation to the water molecules confined in the small-sized pores. These studies are very important for the development of fast-proton-conducting glasses. Experiments Porous 5% P2O5-95% SiO2 (mol %) glasses were prepared by the sol-gel method using Si(OC2H5)4, PO(OCH3)3, and (10) Nogami, M.; Matsushita, H.; Kasuga, T.; Hayakawa, T. Electrochem. Solid-State Lett. 1999, 2, 415. (11) Nogami, M.; Nagao, R.; Wong, C. J. Phys. Chem. B 1998, 102, 5772. (12) Nogami, M.; Nagao, R.; Wong, C.; Kasuga, T.; Hayakawa, T. J. Phys. Chem. B 1999, 103, 9468.
10.1021/cm020323p CCC: $22.00 © 2002 American Chemical Society Published on Web 10/04/2002
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Table 1. Specific Surface Area, Pore Volume, Average Pore Radius, and Molar Ratio of P and Si Atoms in Glasses Synthesized from POCl3 or PO(OCH3)3 specific av precursor heating pore surface pore sample of temp vol area radius no. phosphorus catalyst (°C) P/Si (cm3/g) (m2/g) (nm) 1 2 3 4 5 6
PO(OCH3)3 PO(OCH3)3 POCl3 POCl3 POCl3 POCl3
HCl, NH3 HCl HCl HCl HCl HCl
600 600 400 600 650 700
0.058 0.068 0.155 0.137
0.44 0.39 0.52 0.47 0.48 0.40
584 686 130 182 135 131
1.5 1.1 8.1 5.2 7.1 6.1
POCl3. Si(OC2H5)4 was hydrolyzed with a mixed solution of C2H5OH and H2O (as 0.15 N HCl(aq)) in molar ratios of 1:1 per mole of Si(OC2H5)4. After the solution was stirred for 1 h, PO(OCH3)3 was added, followed by stirring for 1 h. The resultant solution was hydrolyzed by adding 4 mol of H2O (as 0.15 N NH3(aq))/mol of Si(OC2H5)4. After hydrolyzing, 1 mL of HCONH2 for 5 g of oxide glass was added to the solution. The solution was also prepared without using NH3 solution. When using POCl3, Si(OC2H5)4 was hydrolyzed with a solution of C2H5OH and H2O (as 0.15 N HCl(aq)) in molar ratios of 2:2, and no HCONH2 was used because of the high reactivity of POCl3. The synthesis of these solutions was done at room temperature. The obtained solutions were clear and were left for several weeks to form stiff gels in Petri dishes under laboratory conditions. The dried gels were heated at 50 °C/h to 400-700 °C and held at that temperature for 5 h, forming glass plates that were about 0.3 mm thick. The P5+ and Si4+ ion contents in glass were determined by inductivity coupled plasma spectroscopy (Shimadzu, ICPS5000). Powdered samples were dissolved in a KOH solution. The specific surface area, pore volume, and pore size distribution were measured using a Quantachrome NOVA 1000 nitrogen gas sorption analyzer. All the samples were heated in advance at 250 °C in a vacuum to remove the absorbed water remaining in the pores. The pore size distribution was determined by the Barrett-Joyner-Halenda (BJH) method.13 The electrical conductivity of the glasses was determined from Cole-Cole plots by an ac method using a Solartron SI-1260 impedance analyzer with frequencies ranging from 1 to 107 Hz. The sample of ∼10 × 10 × 0.3 mm evaporated gold electrodes was dried at 120 °C for 2 h and subsequently kept in a vacuum overnight. Nuclear magnetic resonance (NMR) relaxation measurements were performed using a Varian Unity 400 plus instrument operating at 400 MHz. The spin-lattice relaxation measurement was done using the inversion recovery method. The variable and relaxation delays were 0.01-15 and 15 s, respectively. For the NMR measurements, the sample was immersed in ion exchange water for 2 days to absorb the water into the pores, the surfaces of which were wiped to remove the water. Perfluorooctane was placed above the sample in a tube so that evaporation of the water during the measurement was negligible.
Results and Discussion Properties of Sol-Gel-Derived Glasses. Two types of glasses were synthesized using PO(OCH3)3 and POCl3 as the P5+ ion components, and the concentrations of the P5+ and Si4+ ions were determined by an ICP analyzer. The concentration of Si4+ ions was consistent with the nominal glass composition within experimental error. On the other hand, the P5+ content was initially dependent on the chemistry and decreased in the glasses prepared using PO(OCH3)3 as the raw material. Shown in Table 1 is the concentration ratio of P to Si in the glasses. The P/Si ratio in the glasses (nos. 1 and 2) (13) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.
Figure 1. Nitrogen gas adsorption-desorption isotherms of sample nos. 1 and 3. Open and closed symbols indicate the data during adsorption and desorption, respectively.
prepared using PO(OCH3)3 was about 0.06, reduced to half its nominal value of 0.105. PO(OCH3)3 is chemically stable and hardly reacts with either H2O or the hydrolyzed Si(OC2H5)4,14-16 which remains unchanged in the gel. Upon heating the gel, the PO(OCH3)3 is decomposed to form the PO43+ monomer, bonding with the SiO4 network structure. Some of the P5+ ions evaporate without reacting with the silica networks during heating, resulting in a decreased content of the P5+ ion. The formation of the PO43+ group was also confirmed from the 31P NMR spectra. On the other hand, the glasses (nos. 3 and 4) prepared using POCl3 exhibited the nominal content of P5+ ions, all of which remained in the glass. Typical isotherms of the nitrogen adsorption and desorption are shown in Figure 1 for sample nos. 1 and 3. Both samples exhibit hysteresis curves but different dependences on the partial gas pressure. The pore volumes, calculated from the N2 gas adsorption isotherms, are 0.44 and 0.52 cm3/g for sample nos. 1 and 3, respectively. It is evident that the glass prepared from POCl3 has a large pore volume compared with the PO(OCH3)3-derived glass. We previously discussed the proton conduction in the sol-gel-derived glasses.11,12,17,18 The obtained glasses are porous, and the water molecules are bonded with the POH and SiOH on the pore surfaces. Compared with the SiOH bonds, the POH bonds are strongly bonded with the water molecules and appropriate for increased proton conductivity.12 The conductivity also increases with the content of the water molecules. From these points of view, glass no. 3 should exhibit a high proton conductivity, because of its high values of both the P5+ ion content and the pore volume. However, the result was not necessarily the same. The conductivities were measured in a relative humidity of 80% at 30 °C and were 2.0 × 10-2 and 8.4 × 10-4 S/cm for sample nos. 1 and 3, respectively. Note that sample no. 1 exhibits a high conductivity, even though it has a low P2O5 content and small pore volume compared with sample no. 3. (14) D’apuzzo, M.; Aronne, A.; Esposito, S.; Pernice, P. J. Sol-Gel Sci. Technol. 2000, 17, 247. (15) Livage, J.; Barboux, P.; Vandenborre, T, M.; Schmutz, C.; Taulelle, F. J. Non-Cryst. Solids 1992, 147, 148, 18. (16) Schrotter, J.; Cardenas, A.; Smaihi, M.; Hovnanian, N. J. SolGel Sci. Technol. 1995, 4, 195. (17) Nogami, M.; Abe, Y. Phys. Rev. B 1997, 55, 12108. (18) Nogami, M.; Daiko, Y.; Akai, T.; Kasuga, T. J. Phys. Chem. B 2001, 105, 4653.
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Figure 2. Distribution curves of pore size, determined using the BJH method, for sample nos. 1 and 3.
Figure 3. Weight change (closed symbols) and conductivity (open symbols) of sample nos. 1 (circles) and 3 (squares) against exposure time in a relative humidity of 80% at 30 °C.
The nitrogen gas adsorption-desorption experiments were performed for the other glasses prepared in this study. Sample nos. 2 and 4-6 exhibit isotherm curves that resemble those of nos. 1 and 3, respectively. The pore size distributions, determined using the BJH method, for sample nos. 1 and 3, are shown in Figure 2. Note that sample no. 1 exhibits a narrow pore size distribution with an average radius of 1.5 nm, whereas sample no. 3 has a wide distribution ranging from 5 to 20 nm and an average radius of 8 nm. The pore surface area, pore volume, and average pore size are summarized in Table 1. It is interesting to note that the glasses having a large surface area and small pores are obtained using PO(OCH3)3. Electrical Conductivity and Pore Structure. The electrical conductivities of the glasses, measured without exposure to an ambient atmosphere after their heat treatment, are lower than ∼1 × 10-7 S/cm at room temperature. When the glasses are exposed to ambient air, they absorb water. Correspondingly, the conductivity increases and reaches a constant level within several tens of minutes. As mentioned above, the conductivity is not unconditionally determined by the pore volume. In this section, we will discuss the proton conduction as related to the pore structure. The weight change and conductivity for sample nos. 1 and 3 were measured during exposure to a relative humidity of 80% at 30 °C and are shown in Figure 3 as a function of exposure time. The absorption of water in sample no. 1 takes place in several tens of minutes and approaches a constant value corresponding to the pore volume, indicating that the total pores in the sample
Daiko et al.
Figure 4. Relative humidity dependence of the proton conductivity, measured at 30 °C, for sample nos. 1 and 3. Open and closed symbols indicate the absorption and desorption branches of humidity, respectively.
are filled with the water molecules. It is considered that the water absorption in the porous materials is diffusion controlled through the small pores. The experimental data for sample no. 1 were well fitted to Fick’s equation, suggesting a diffusion coefficient of 2.8 × 10-7 cm2/s. On the other hand, the water absorption in sample no. 3 did not follow a simple Fick’s equation with a single diffusion coefficient. The total amount of the absorbed water was only half the total pore volume. The conductivity increases with time and approaches a constant level within a period shorter than that of the water absorption (see Figure 3). These results indicate that the conductivity does not necessarily increase in proportion to the amount of absorbed water molecules. We previously found that the conductivity linearly increases with increasing logarithm of the number of water molecules with separate lines in response to the water concentration.11,12,17,18 The present results also suggest that the pore structure of the glass preliminarily controls the water absorption, resulting in the determination of the proton conduction. Electrical Conductivity as a Function of Humidity and Temperature. The conductivities, measured after exposure to ambient humidity for a long period at 30 °C, are plotted in Figure 4 as a function of the relative humidity (RH). The conductivities were measured upon increasing the relative humidity from 30% to 90%, followed by decreasing it from 90% to 30%. It is evident that the conductivity increases with increasing relative humidity, but its humidity dependence is substantially different between the two samples. In sample no. 1, the conductivity is ∼1 × 10-6 S/cm at 30% RH, and rapidly increases with increasing humidity, reaching a constant value of ∼1 × 10-2 S/cm at a humidity above ∼70% RH. Of further interest in Figure 4 is that sample no. 1 maintains a high conductivity of ∼10-2 S/cm with decreasing humidity, though the conductivity gradually decreases at a humidity below ∼50% RH. This result indicates that the glass, exposed once to high humidity, holds the water in its pores and exhibits a high conductivity irrespective of the humidity change. This finding is important for the practical application of the sol-gel-derived porous glasses, because they allow a simple water management, consequently resulting in a significant decrease in the operating cost of the fuel cell. On the other hand, sample no. 3 exhibits a gradual increase in the conductivity with
Proton Conduction in Sol-Gel Glasses
Figure 5. Temperature dependence of the proton conductivity of sample nos. 1 and 3.
increasing humidity up to ∼80%, at which the conductivity rapidly increases and reaches ∼5 × 10-2 S/cm. The conductivity of sample no. 3 reversibly changes with the changed humidity. Figure 5 shows the conductivity dependence on temperature. The conductivities were measured in a constant humidity of 90% RH, except at temperatures below 0 °C. For conductivity measurement below 0 °C, the sample was kept at 90% RH and 5 °C and then the temperature was decreased to below 0 °C, at which the conductivities were measured. Note that the conductivities for sample no. 1 are well represented by one Arrhenius equation down to ∼ -20 °C, at which the conductivity deviates from the linear relation to follow a different line with a large activation energy. On the other hand, the conductivity of sample no. 3 abruptly decreases at around 0 °C. These results strongly suggest that the water absorbed in the porous glasses have a different effect on the proton conduction in the two glasses having different porous properties. The freezing temperature of water in these glasses was determined by the differential scanning calorimetric experiment and was -20 and 0 °C for sample nos. 1 and 3, respectively, consistent with the discontinuous points observed in the conductivity-temperature curves. The water molecules confined in the small-sized pores are limited in motion, and the freezing temperature decreases as the pore size decreases. The high activation energy for proton conduction in the temperature range below the freezing point indicates that the solid water molecules do not work to accelerate the proton hopping. These results indicate that glasses having small pores exhibit high conductivities even at temperatures below 0 °C, and are quite necessary to manage the water to maintain high conductivity. NMR Study of Proton Conduction. How the water molecules absorbed in the small-sized pores affect the proton transfer was also discussed. The proton motion in the porous glasses was investigated on the basis of the measurement of the relaxation dynamics using 1H NMR spectroscopy. The spin-lattice relaxation was measured by using the standard inversion recovery technique, typical spectra of which are illustrated in the inset of Figure 6. The magnetization intensities, M(τ), at time τ are normalized to the equilibrium value, Mo, and are plotted in Figure 6 versus time, where the data for the free water are also shown for comparison. It is evident that the recovery of the magnetization exhibits a single-exponential behavior, and the spin-lattice relaxation times, T1, are determined from the slope of
Chem. Mater., Vol. 14, No. 11, 2002 4627
Figure 6. Logarithm plots of the spin-lattice relaxation time for sample nos. 1 (circles) and 3 (squares) and free water (triangles) as a function of time. The inset is the relaxation recovery spectrum of a proton in sample no. 1.
Figure 7. Relationship between the spin-lattice relaxation time of a proton and the inverse of the average pore radius. The values in the ordinate are shown as the deviation from the relaxation time of free water.
the straight lines. We found that the relaxation time decreases for water molecules absorbed in the glasses with small pores. The deviations of the relaxation time from that of the free water are plotted in Figure 7 as a function of the inverse of the pore size. They seem to have a linear relationship. Sample no. 1 has pores with an average size of 1.8 nm, and the motion of the water molecules is restricted by the small-sized pores. Thus, the restricted motion of the water molecules produces a lower relaxation time, and the freezing temperature decreases below 0 °C. These liquid states of water molecules confined in the small pores act to accelerate the proton hopping, resulting in high conductivity at low temperatures. Conclusion We demonstrated the effect of pore size on proton conductivity in sol-gel-derived porous glasses. Porous glasses having pores less than 5 nm exhibited high conductivities even under 50% RH and at temperatures down to -20 °C. It was concluded that proton motion is restricted in water molecules confined in the small-sized pores, resulting in a decrease of the freezing temperature in water. We could conclude that glasses having small pores are suitable for high conductivities at low temperatures and humidities. Acknowledgment. This research was partly supported by a Grant-in-Aid for Scientific Research (No. 1255188) from the Ministry of Education, Science, and Culture of Japan. CM020323P
