Enhancement of Proton Transport by High Densification of Sulfonic

Article pubs.acs.org/cm

Enhancement of Proton Transport by High Densification of Sulfonic Acid Groups in Highly Ordered Mesoporous Silica Satoru Fujita,* Akihiko Koiwai, Masaya Kawasumi, and Shinji Inagaki Toyota Central Reseach and Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ABSTRACT: Highly ordered sulfonic acid functionalized mesoporous silica films with various pore sizes (2.2−3.9 nm) and acid densities (0.78− 2.3 mmol g−1) were successfully prepared by the Evaporation Induced Self-Assembly (EISA) process using tetramethoxysilane and 3-mercaptopropyltrimethoxysilane as starting materials. The relationship between the proton diffusivity and the water sorption process in the nanopores was clarified in detail by these mesoporous films. The proton conductivity of mesoporous films increased steeply according to the capillary condensation of water in the uniform mesopores. This steep increase of proton conductivity was controlled by the pore size and the acid density described by the Kelvin equation. Notably, the mesoporous electrolyte with the smallest pore size (2.2 nm) and the highest acid density (2.3 mmol g−1) showed the highest proton conductivity at low relative humidity (RH) [e.g., 5.4 × 10−3 S cm−1 (25 °C) at 20% RH]. Furthermore, the proton diffusivity in the mesopores was enhanced by the high densification of sulfonic acid groups even at very small amounts of adsorbed water [λ(H2O/SO3H) = 2]. KEYWORDS: sulfonic acid-functionalized mesoporous silica, proton conductivity

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INTRODUCTION Fuel cells have the potential to become an important energy conversion technology and emerged in the past decade as a keystone for future energy supplies.1−3 Specifically, proton exchange membranes (PEMs) for fuel cells have attracted much attention as clean energy sources because of their high power density and efficiency with low emission levels for numerous applications such as electric vehicles, portable electronics, and residential power generators.4,5 One of the main challenges in current fuel cell research is to develop a membrane that can operate at high temperatures as an alternative to perfluorosulfonic acid polymers such as Nafion.6,7 The morphology of Nafion has been proposed by numerous researchers,8 as a core−shell model,9,10 a lamellar model,11 a sandwich-like model,12 or a rodlike model,13 and there is still ongoing debate in the current literature. For instance, with progressive hydration of Nafion, percolation into the membrane is achieved by the formation of connecting cylinders of water on the mesoscale, resulting in high proton conductivity.14 At low water contents, the continuities of water pathways into the membrane are lost, resulting in a concomitant decrease in proton conductivity. In practice, membranes with high proton conductivity at lower relative humidity (RH) are required in order to operate at elevated temperatures (∼120 °C). There have been a number of studies on new polymeric membranes structurally designed to exhibit high proton conductivity at lower RH.15−20 Most of the synthetic methods utilized for these PEMs resulted in hydrophilic−hydrophobic multiblock copolymers. These materials consisted of ordered network © 2013 American Chemical Society

morphologies with the most versatile nanostructures found in polymeric materials, forming continuous proton pathways. Unfortunately, these membranes swelled in cases of higher ion exchange capacity (IEC). In contrast, mesoporous materials as electrolytes have two advantages: a high density of ionic groups with organic moieties as a proton source in the mesopores, which are formed by the self-assembly of these ionic groups in directional pathways, and inorganic pore walls with mechanical and structural durability. This inorganic wall structure prohibits swelling when hydrated. Moreover, desirable water molecules in the pores are retained by tailoring the sturdy mesostructured channels. Thus, mesoporous materials are expected to realize high proton conductivity at lower RH and allow for significant optimization of electrochemical performance. Consequently, many researchers have devoted their attention to mesoporous materials as electrolytes.21−48 For example, the estimation of proton conductivity for porous silica glasses and xerogels was performed in order to demonstrate their potential as electrolytes.21−26 Additionally, numerous studies using mesoporous electrolytes with phosphate groups,27−35 carboxylic acid groups,34,36 imidazole,37 and mesoporous inorganic oxides38−40 have been reported. A number of researchers have focused on sulfonic acid functionalized mesoporous silica using 3mercaptopropyltrimethoxysilane (MPTMS),34,41−46,48 phenethyltrimethoxy silane (PETMS),34 and 1,3-propanesultone Received: December 10, 2012 Revised: March 29, 2013 Published: April 3, 2013 1584

dx.doi.org/10.1021/cm303950u | Chem. Mater. 2013, 25, 1584−1591

Chemistry of Materials

Article

Figure 1. (a) Synthetic process of sulfonated mesoporous silica films and (b) measurement system of their in-plane proton conductivity.

(PS).47 The proton conductivity of sulfonic acid functionalized silica was up to 0.2 S cm−1 at 140 °C and 100% RH. As a comparison, the conductivity for Nafion has been reported as ∼10−2 S cm−1 at 140 °C and 25% RH.49 Moreover, molecular dynamic (MD) simulations revealed a stronger dependence of the conductivity on the water transport for sulfonic acid mesoporous electrolytes.44 A large number of studies using these materials in various states have been performed: dense disks obtained by compressed powders,34,41−44,46 free-standing films,45,47,48 or components in polymer electrolyte membranes such as Nafion.50−52 However, there are no studies of proton conductive properties using highly ordered mesoporous films with various sulfonic acid densities and different pore sizes. A few reports have focused on the proton conductivity dependence on the pore size and the water capillary condensation using porous silica glass and mesoporous inorganic oxide thin films.24,26,30 However, detailed studies explaining the behavior of proton mobility corresponding to the water sorption process in mesopores have not yet emerged, although it is thought that water plays an important role as a proton carrier. In this work, we present the preparation of highly ordered sulfonic acidfunctionalized mesoporous silica films with various pore sizes and acid densities using MPTMS as a precursor of the sulfonic acid group. In particular, we clarify the relationship between the proton diffusivity and the water sorption process in the uniform mesopores.

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lammonium chloride surfactant [CnH2n+1N(CH3)3Cl, n = 12, 14, and 18] (Figure 1a). An ethanol (EtOH) solution of MPTMS and TMOS was added to small amounts of HCl and H2O, and then the mixture was stirred for 1 h at room temperature. The amounts of 2 N HCl (20 μL), H2O (3.0 mL), and EtOH (5.0 mL) were constant. Furthermore, the resultant clear solution was mixed with the surfactant, H2O, EtOH, and 30 wt % H2O2 solution. The surfactant and H2O2 were used as a template and an oxidant, respectively. The EtOH and H2O were used as solvents for the surfactant. The solution was stirred for an additional 2 h at room temperature and dip-coated onto glass or Si substrates with patterning Pt electrodes. The thickness of the film was adjusted to ∼300 nm by controlling the withdrawal speed (10 mm min−1) during the dip coating. The coated films were prepared at different ratios of MPTMS and TMOS, using surfactants with different alkyl-chain lengths. The amounts of MPTMS, TMOS, and surfactants are listed in Table 1, and the quantities of EtOH (10 mL)/H2O (0.1 mL) solvents

Table 1. Amounts of MPTMS, TMOS, and Surfactant for the Synthesis of Sulfonated Mesoporous Silica Films sample

MPTMS, g

TMOS, g

MP10-C18 MP30-C18 MP40-C18 MP40-C14 MP40-C12 MP50-C12

0.22 0.63 0.79 0.79 0.79 0.98

1.46 1.13 0.91 0.91 0.91 0.75

CnH2n+1N(CH3)3C1, g 0.91 1.01 1.21 1.31 1.41 1.51

(n (n (n (n (n (n

= = = = = =

18) 18) 18) 14) 12) 12)

for the surfactant were constant. The amount of H2O2 was adjusted to H2O2/MPTMS = 5. The films were designated as MPm-Cn (m is mol % of the MPTMS contents and n is alkyl-chain length of surfactant). The obtained films were dried at room temperature for 15 h and treated with TMOS and NH3 vapors to stabilize the framework structures. The coated films were placed vertically in a Teflon vessel (100 mL) with 200 μL TMOS in the bottom apart from the films, and heated at 120 °C for 2−4 h using an autoclave. Next, the films were

EXPERIMENTAL SECTION

Preparation of Sulfonic Acid Functionalized Mesoporous Films. The sulfonated mesoporous silica films were prepared by the evaporation induced self-assembly (EISA) approach53,54 from precursor mixtures of 3-mercaptopropyltrimethoxysilane (MPTMS) and tetramethoxysilane (TMOS) in the presence of alkyltrimethy1585



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exposed to NH3 vapor at 100 °C for 2 h in the same manner. The stabilized films were immersed in EtOH solution (100 mL) with 100 mg of conc. HCl aq. at 60 °C for 3 h for removal of surfactant from the films. Characterizations. X-ray diffraction (XRD) patterns of the coated films were measured using a Rigaku RINT-TTR diffractometer with Cu Kα radiation (50 kV and 300 mA) in the region of 2θ = 1.5−7° with 0.01° steps and a scan speed of 1° min−1. Transmission electron microscope (TEM) images were recorded with a JEOL JEM-2000EX instrument at an accelerated voltage of 200 kV. The samples for TEM were scratched off, and the mesoporous thin film was microtome-cut on the substrate. Krypton adsorption−desorption isotherms at 87 K were obtained using a Quantachrome AUTOSORB-1 with a low surface area measurement system. The mesoporous films on the substrates (0.3 × 1.5 cm, 3−5 pieces, 2−3 mg for films only) were placed in a special cell for the measurements of isotherms. The sample weights were measured by a precision weighing balance (Sartorius SC 2). Prior to each adsorption measurement, samples were outgassed at 120 °C for 24 h. The Barrett−Joyner−Halanda (BJH) pore-size distributions and pore volumes and the Brunauer−Emmett−Teller (BET) surface areas were calculated from the krypton adsorption isotherms using parameters of krypton. 29Si MAS NMR and 13C CP MAS NMR spectra were recorded on a Bruker MSL-300WB spectrometer at 59.62 and 75.47 MHz for 29Si and 13C, respectively. Chemical shifts were referenced to tetramethylsilane and glycine, respectively. Acid amounts of the films were measured by acid−base titrations. In a typical procedure, the scratched film from the substrate (0.1 g) was suspended in 20 g of 2 M aqueous NaCl solution. The resulting suspension was stirred at room temperature for 15 h until equilibrium was reached and then titrated by dropwise addition of 0.1 M NaOH. Water Vapor Sorption and Proton Conductivity. The water vapor adsorption−desorption isotherms (water uptakes) of the mesoporous films were measured using a quartz crystal microbalance (QCM) chip. The QCM consists of a quartz disk with both sides coated with Pt electrodes. As comparison, Nafion thin film (360 nm) on a quartz disk was obtained from a Nafion solution (20 wt %). The sorption measurements were performed using a conventional flow type reactor in the range of 0 to 80% RH at 25 °C by recording the change in the frequency of the chip at every sorption step. RH was controlled by bubbling N2 gas through temperature-controlled water. For the Nafion 112 membrane, the adsorption isotherms of water vapor were measured by means of a volumetric method using a Bell Japan Belsorp 18 apparatus. In-plane proton conductivity of the mesoporous electrolyte on the substrate was measured by electrochemical impedance spectroscopy using a Hioki 3532−50 LCR Hitester instrument (Figure 1b). The frequency range was 50 Hz−50 MHz. Before measuring the proton conductivity, the mesoporous films were washed by 0.1 M HCl at room temperature for 2 h and pure water at 80 °C for 2 h and then dried at 100 °C for 2 h. The films were allowed to equilibrate at the desired RH in the range of 10−90% at 25 °C. The RH was controlled by bubbling N2 gas through a temperaturecontrolled impinger. The total gas flow was 200 mL min−1. The conductivity was calculated using the following equation:

Figure 2. XRD patterns of propyl-sulfonated mesoporous silica films: (a) MP10-C18, (b) MP30-C18, (c) MP40-C18, (d) MP40-C14, (e) MP40-C12, and (f) MP50-C12.

Figure 3. TEM image of microtome-cut sample for MP40-C18 film on Si substrate.

epoxy resin. The cross-sectional image showed a clear hexagonal arrangement of uniform mesopores with a large single crystal-like domain size (at least 500 nm in width), which indicates a highly aligned structure of mesochannels parallel to the substrate surface. The other mesoporous films (MPn-Cm) also showed clear TEM images of hexagonal structure and onedimensional channels in the scratched samples (Figure 4a−f). These results indicate that all of the mesoporous films (MPnCm) have well-defined and aligned 2D hexagonal mesostructures. The lattice parameters [d-spacing of (10) reflections and unit cells in 2D-hexagonal] determined from XRD are listed in Table 2. Figure 5 shows krypton adsorption−desorption isotherms at 87 K for the mesoporous films. They showed type IV isotherms for MP10-C18, MP30-C18, MP40-C18, and MP40-C14 or type I isotherms for MP40-C12 and MP50-C12. The films had narrow pore-size distributions and mean pore diameters ranging from 2.2 to 3.9 nm. The pore diameters, pore volumes, and BET surface areas estimated from the krypton adsorption isotherms are also listed in Table 2. Figure 6 shows 13C CP and 29Si MAS NMR spectra of a representative MP40-C18 film, which were measured as a scratched sample from several films on a substrate. The 13C CP MAS NMR spectrum displayed three peaks at 53, 18, and 12 ppm corresponding to three carbons of the propyl linker between Si and SO3H (Figure 6a). The signal at 53 ppm is assigned to the carbon directly attached to the SO3H group. The absence of a signal at around 22 ppm due to carbons

σ = d(SR ) where σ is the conductivity, d is the film thickness, S is the area of cross section, and R is the intersection of the semicircle.

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RESULTS AND DISCUSSION Characterization of Mesoporous Electrolyte. The XRD patterns of the sulfonated mesoporous films (MPn-Cm) are shown in Figure 2. They showed sharp peaks at low angles (2θ = 2−3.5°) and an additional one or two weak peaks at higher angle (2θ = 4−7°). These peaks are assignable as (h0) reflections of a two-dimensional hexagonal with the mesochannel aligned parallel to the substrate surface. Figure 3 shows a TEM image of microtome-cut MP40-C18 film embedded in 1586



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Figure 5. Krypton sorption isotherms (left side) and pore distributions (right side) on propyl-sulfonated mesoporous silica films: (a) MP10C18, (b) MP30-C18, (c) MP40-C18, (d) MP40-C14, (e) MP40-C12, (f) MP50-C12.

g−1) is significantly higher compared to that of Nafion. The usual equivalent weights (EW) of Nafion are 800−1200, which correspond to 0.71−1.25 mmol g−1 of acid density. Nafion with its higher acid density usually has much swelling and unfavorable dissolution properties and, thus, is not an effective electrolyte. The volume acid densities were calculated using the densities of the mesoporous films (Table 3). The maximum volume acid density (3.29 mmol cm−3) for MP50-C12 is much larger than that for hydrated Nafion (1.3 mmol cm−3 for Nafion 112 with EW = 1094 at 100% RH). The mesoporous film offers a great advantage in that a large amount of sulfonic acid groups can be incorporated in the framework with no swelling and dissolution behavior and preservation of rigid pore structure. The sulfonic acid groups of the mesoporous silica films are located on the pore surface as shown in Figure 7. Thus, the local acid density on the pore surface is even higher than that in the case of homogeneous distribution (Table 3). The sulfonated mesoporous silica films prepared here can be classified into three types: (a) low acid density and large pore, (b) medium acid density and medium pore, and (c) high acid density and small pore (Figure 7). Water Vapor Adsorption and Proton Conductivity. Figure 8a,b shows the water uptake λ = [H2O]/[−SO3H] (25 °C) for different RHs of the sulfonated mesoporous silica films. As a comparison, Nafion 112 membrane and Nafion thin film for the adsorption process are also shown. The isotherms showed a gentle increase at low RH, a steep increase at medium RH, and almost a plateau at high RH. The gentle increase at low RH is due to the adsorption of water on the pore surface and the clustering around the sulfonic acid groups. The steep increase and the plateau correspond to the capillary condensation into the mesopores and adsorption on the outer surface of the mesoporous films, respectively. At medium RH, the water uptake per sulfonic acid group of the sulfonated mesoporous silica films was significantly higher, by a factor of 2, by capillary condensation than that of Nafion membrane and ionomer. The RH where capillary condensation occurs was shifted to a lower value upon decreasing pore size and increasing acid density of the sulfonated mesoporous film. The RH shift could be explained reasonably by the following Kelvin equation:

Figure 4. TEM images of the sulfonated mesoporous silica films: (a) MP10-C18, (b) MP30-C18, (c) MP40-C18, (d) MP40-C14, (e) MP40-C12, and (f) MP50-C12.

Table 2. Physicochemical Properties of Sulfonated Mesoporous Silica Films sample

d10,a nm

a,b nm

D,c nm

Vp,d cm3 g−1

SBET,e m2 g−1

MP10-C18 MP30-C18 MP40-C18 MP40-C14 MP40-C12 MP50-C12

4.1 3.7 3.6 3.2 2.9 2.7

4.8 4.3 4.1 3.7 3.3 3.1

3.9 3.4 3.2 3.0 2.7 2.2

0.51 0.40 0.36 0.27 0.26 0.30

620 760 810 910 830 980

a

d-spaceing of (10) reflection. ba = 2/√3d10. cPore diameter. dPore volume. eBET surface area.

attached to SH groups indicated that SH groups were almost quantitatively converted to SO3H groups. The 29Si MAS NMR spectrum showed two Q peaks [Q3: Si(OH)(OSi)3 and Q4: Si(OSi)4] originating from the TMOS precursor and two T peaks [T2: SiC(OH)(OSi)2 and T3: SiC(OSi)3] originating from the MPTMS precursor. The intensity ratio of the T and Q signals correlated with the mixture ratio of the TMOS and MPTMS precursors (40/60) for MP40-C18. Table 3 lists the acid densities measured by the acid−base titration for the sulfonated mesoporous silicas. The acid density ranges from 0.78 to 2.30 mmol g−1. These values are almost comparable to the amounts of sulfonate groups (0.6−2.9 mmol g−1) in the mesoporous films assuming 100% conversion of SH to SO3H groups. The acid density of MP50-C12 (2.3 mmol

ln(P /P0) = − 2γVm cos θ /(rRT )

where P/P0 is the relative vapor pressure at the capillary condensation, γ and Vm are the surface tension and molecular 1587



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Figure 6. 13C CP (a) and 29Si NMR (b) spectra of MP40-C18 film.

molar volume of water, respectively, r is the pore radius, R and T are the gas constant and absolute temperature, respectively, and θ is the contact angle between the pore surface and water molecule. According to the Kelvin equation, RH (P/P0 × 100) becomes small with decreases in r and θ. The increase in acid density in the mesopore surface decreases θ due to the hydrophilic property of the sulfonic acid group and r due to the bulkiness. The large adsorption−desorption hysteresis loop became small with a decrease in pore size. Similar hysteresis behavior was also observed for mesoporous silicas with different pore sizes and explained by thermodynamics.55 Figure 9 shows Cole−Cole plots measured at 25 °C in 90% RH for the mesoporous films. All mesoporous films show a depressed semicircle, which is assigned to the bulk and grain boundary, with a small tail at low frequency assigned to electrode contribution. Conductivity was calculated from the lowfrequency intercept on the real axis. Figure 8c shows proton conductivity of the sulfonated mesoporous films as a function of RH. The proton conductivities were very high (6.0 × 10−3 to 3.4 × 10−2 S cm−1) at high RH where the mesopores were completely filled with water. The proton conductivity dropped steeply and jumped 2−3 orders of magnitude at medium RH in accordance with the steep increase and decrease in adsorption of water, respectively, due to the capillary condensation. To the best of our knowledge, this is the first report showing in detail such steep changes in proton conductivity according to the capillary condensation of water for mesoporous films. RH at the steep change in proton conductivity shifted lower with the decrease in pore size and increase in acid density, which corresponded well with the water vapor adsorption−desorption

Table 3. Acid Densities for Sulfonated Mesoporous Silica Films sample MP10-C18 MP30-C18 MP40-C18 MP40-C14 MP40-C12 MP50-C12 a

weight acid density,a mmol g−1 0.78 1.52 1.80 1.82 1.80 2.30

(0.6) (1.7) (2.3) (2.3) (2.3) (2.9)

density of film, g cm−3

volume acid density, mmol cm−3

surface acid density, molecules nm−2

0.74 0.98 1.11 1.25 1.40 1.43

0.58 1.49 1.90 2.27 2.52 3.29

0.75 1.21 1.34 1.20 1.31 1.41

() Theoretical weight acid density.

Figure 7. Schematic illustrations of the pore structures for the sulfonated mesoporous silicas with the different pore sizes and the acid densities: (a) MP10-C18, (b) MP30-C18, MP40-C18, MP40-C14, MP40-C12, and (c) MP50-C12.

Figure 8. Water uptakes (a, b) and proton conductivities (c) of sulfonic acid functionalized mesoporous silica films at 25 °C versus different RH values. Solid and open circles show water adsorption and desorption processes, respectively. Water uptakes of Nafion 112 and Nafion thin film show the adsorption process (a, b). 1588



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depended on the acid density (the equivalent weight) because the acid density drastically changes the morphologies of proton pathways due to the flexible backbone structure and the different microphase separation behavior.56 Therefore, the almost constant value of Dσ for the mesoporous films with different sulfonic acid group densities is very unique and one of the characteristic features of mesoporous silica electrolytes with rigid pore structures. The Dσ values for all mesoporous electrolytes at high RH are almost comparable to Dσ of fully hydrated Nafion (2−7 × 10−10 m2 s−1) but lower than the selfdiffusivity of bulk water (9.3 × 10−9 m2 s−1).57 Mesoporous silicas can incorporate a large amount of sulfonic acid groups preserving the rigid pore structure, which results in achievement of high proton conductivity (∼10−2 S cm−1) as shown in Figure 8c. Figure 11 plots Dσ against the filling ratio of water in Figure 9. Cole−Cole plots measured at 25 °C in 90% RH for the mesoporous films.

isotherms. Thus, the RH position at the steep change can be controlled by the pore size and the acid density of mesoporous silica according to the Kelvin equation. MP50-C12 film with the smallest pore size and the largest acid density showed the highest proton conductivity, 2.2−3.4 × 10−2 S cm−1 (25 °C) at RH over 40%. Additionally, this film still showed a high proton conductivity of 5.4 × 10−3 S cm−1 even at low RH of 20%, whereas MP10-C18 with the largest pore size and the lowest acid density showed a large hysteresis in proton conductivity, corresponding to the water vapor adsorption−desorption isotherm. The proton diffusion coefficients (Dσ) were calculated using the Nernst−Einstein equation:

Figure 11. Relationship between proton diffusion coefficient and water filling ratio in the mesopores: (a) adsorption and (b) desorption process.

Dσ = kTσ /NHe 2

the mesopores Fp, which were calculated by the following equation:

NH = (ρIEC/1000)NA

Fp = Vw /Vp

where k is Boltzmann’s constant, T is the absolute temperature, σ is the proton conductivity, NH is the concentration of the protonic volume calculated from the IEC value, e is the electric charge, NA is Avogadro’s constant, and ρ is the density of the film calculated from the weight and the thickness of the film on the substrate (Table 3). Dσ is plotted as a function of RH for various films (Figure 10). The Dσ values at high RH where the mesopores were filled with water were almost comparable (1−3 × 10−10 m2 s−1) for the films in spite of the different acid densities. For Nafion, water and proton mobilities strongly

where Vw is the volume of adsorbed water. Dσ steeply increased at around Fp = 0.4 and reached the maxima at Fp = 1 for the mesoporous films except for MP50-C12 and MP10-C18. This indicates that more than 40% filling of mesopores with water is necessary for exhibiting high diffusion of protons for mesoporous films with moderate acid densities and pore sizes. Surprisingly, MP50-C12 with the highest acid density and the smallest pore size showed a steep increase in Dσ at around Fp = 0.2. Only 20% filling of mesopores is enough for exhibiting high proton diffusion. This is mainly attributed to the exceptionally high surface acid density (1.41 groups nm−2, Table 3). The high density of sulfonic acid groups can effectively make proton channels with a smaller amount of water (Figure 12a). MP10-C18 with the lowest volumetric acid density (0.58 mmol cm−3, Table 3) showed an increase in Dσ at around Fp = 0.7 because a large amount water is necessary to make the proton channels (Figure 12b). Figure 13 plots Dσ against the number of water molecules per sulfonic acid site (λ = [H2O]/[−SO3H]). Dσ of the mesoporous films with the moderate acid density increased steeply at λ = 4−12, which is almost the same behavior as that of Nafion.58 However, MP50C12 showed high proton diffusivity at fewer water molecules (λ = 2), while MP10-C18 needed more water molecules (λ = 21) for high proton diffusivity. Figure 14 shows proton conductivities of MP50-C12 at various RHs (90, 50, and

Figure 10. Relationship between proton diffusion coefficient and relative humidity for various sulfonic acid-functionalized mesoporous silica films: (a) adsorption and (b) desorption process. 1589



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0.1−0.2 eV60,61 and shows lower values than that of sulfonic acid functionalized mesoporous film. Furthermore, the Ea for proton transfer of sulfonated mesoporous silica films in porous electrolytes is similar to those reported for mesoporous silica xerogels (0.08−0.32 eV)26 and porous electrolytes such as metal−organic framework (MOF) materials (0.16−0.32 eV).62−64 Indeed, it is generally considered that both the Grotthuss and vehicle mechanism are responsible for the proton conductivity in hydrated membranes. The proton transfer mechanism is elucidated from the ratio of Dσ/DH2O (self-diffusion of water in mesopores). Interparticle selfdiffusion (DH2O) of capillary condensed water in mesoporous silica (MCM-41) with 2D hexagonal structure measured by magic-angle spinning pulsed field gradient (MAS PFG) NMR shows anisotropic self-diffusion coefficients that can be assigned to diffusion parallel and perpendicular to the channel axis.65 For instance, DH2O at 25 °C parallel and perpendicular to the channel axis estimated by an Arrhenius plot are ∼6 × 10−10 m2 s−1 and ∼10−11 m2 s−1, respectively, lower than that of bulk water.57 This lower self-diffusion indicates the inhibition of water mobility by the hydrogen-bond network in adsorbed water in the mesopores, resulting in proton transfer in the confined water in the restricted narrow pores.

Figure 12. Water sorption at (a) high and (b) low acid density at low RH and proton channel formation.

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CONCLUSION Highly ordered sulfonated mesoporous silica films with various pore sizes (2.2−3.9 nm) and acid densities (0.78−2.3 mmol g−1) were prepared by the EISA process with the mixture precursors of MPTMS and TMOS. The mesoporous silica films are of great advantage in that a large amount of sulfonic acid groups can be incorporated in the mesopores with no swelling and dissolution properties while preserving the uniform pore structure. The proton conductivity increased steeply at medium RH according to the capillary condensation of water in the uniform mesopores. The RH position at the steep increase can be controlled by the pore size and the acid density according to the Kelvin equation. The proton diffusion coefficients after filling the mesopores with water are almost comparable for the films in spite of the large differences in acid densities. The mesoporous silica films with the highest surface acid density (1.41 groups nm−2) showed exceptionally high proton diffusivity even at a low water filling ratio of mesopores (only 20%) and few water molecules (λ = 2). The relationship between the proton diffusivity and the water sorption process in the uniform and rigid nanopores was first revealed using the highly ordered sulfonic acid-functionalized mesoporous silica films, and the information gleaned from this study will be useful for further understanding of proton diffusivity in nanochannels with water.

Figure 13. Relationship between proton diffusion coefficient and the number of water molecules per ionic site (λ = H2O/−SO3H) for various sulfonic acid-functionalized mesoporous silica films: (a) adsorption and (b) desorption process.

20%) above 25 °C and Arrhenius plots of the conductivities and activation energies (Ea) of MP50-C12.

Figure 14. (a) Proton conductivities of MP50-C12 at various RH (90, 50, and 20%) above 25 °C and (b) Arrhenius plots of the conductivities and activation energies (Ea) of MP50-C12.

The proton conductivities increased with increasing temperature and indicated high proton conductivities of 0.16, 0.11, and 0.0041 S cm−1 for RH 90, 50, and 20%, respectively, at 100 °C. The Ea was obtained by linear regression of an Arrhenius equation:

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σ = σ0 exp( −Ea /k bT)

AUTHOR INFORMATION

Corresponding Author

*Fax: +81-561-63-5743. Tel: +81-561-71-8070. E-mail: [email protected].

where σ0 is a pre-exponential parameter and kb is Boltzmann’s constant. The values of Ea at RH 20, 50, and 90% were estimated to be 0.26, 0.20, and 0.19 eV, respectively. These values indicate a Grotthuss mechanism for the proton conduction. Activation energy values typically attributed to Grotthuss transfer via water molecules are in the range of 0.14− 0.40 eV.59 The magnitude of Ea decreased with increasing RH, because water molecules as mobile species were added. As a comparison, the Ea for hydrated Nafion has been reported as

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Matthias Thommes (Quantachrome Instruments) for analyses of Kr adsorption measurements. 1590



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