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Mar 28, 2013 - Highly dense sulfonic acid-functionalized mesoporous electrolytes with high proton conductivity under dry conditions were prepared usin...
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Proton Conductivity under Dry Conditions for Mesoporous Silica with Highly Dense Sulfonic Acid Groups Satoru Fujita,*,† Kazuya Kamazawa,*,†,‡ Satoru Yamamoto,† Madhusudan Tyagi,§,∥ Toru Araki,† Jun Sugiyama,† Naoki Hasegawa,† and Masaya Kawasumi† †

Toyota Central Research & Development Laboratories Inc., Nagakute, Aichi 480-1192, Japan Comprehensive Research Organization for Science and Society (CROSS), Tokai, Ibaraki 319-1106, Japan § NIST Center for Neutron Research, Gaithersburg, Maryland 20899, United States ∥ Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States ‡

S Supporting Information *

ABSTRACT: Highly dense sulfonic acid-functionalized mesoporous electrolytes with high proton conductivity under dry conditions were prepared using tetramethoxysilane and 3-mercaptopropyltrimethoxysilane in the presence of surfactants. Impedance spectroscopy and quasielastic neutron scattering measurements showed that the proton conductive properties of the mesoporous electrolytes depended significantly on the sulfonic acid densities on the surface of the mesoporous walls. This finding was also supported by molecular dynamics simulations. The proton conductivity of the mesoporous electrolyte with the highest acid density of 3.1 SO3H molecules/nm2 showed a value of 0.3 mS/cm at 433 K even under dry conditions. This value was higher than that for Nafion by about 2 orders of magnitude. Such high proton conductivity is thought to be induced by proton hopping in the hydrogen-bonded networks that were predominantly formed by neighboring sulfonic acid groups.

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higher relative humidity (RH).7−12 Furthermore, the major approach among hydrocarbon-based PEMs for operation under dry conditions has focused on protonic acid- and base-doped polymers, such as poly(benzimidazole) doped or complexed with phosphoric acid.13−15 Some of the acid-doped polymers exhibited proton conductivity as high as 10−2 S/cm under low RH, but the doped acid was water-soluble and eluted out during fuel cell operation. Additionally, under dry conditions, the proton conductivity of the membranes remained on the order of 10−5 S/cm. In contrast, anhydrous proton conductors based on pyrophosphates and metaphosphates, such as LaP3O9, CeP2O7, and SnP2O7, have attracted much attention as potential electrolytes for next-generation fuel cells that operate in the temperature range between 423 and 673 K.16−21 It is worth noting that there are studies that focused on porous coordination polymers (PCPs) and metal−organic frameworks (MOFs) as anhydrous proton conductors.22−25 This porous structure enhanced the mobility of adsorbed guests such as imidazole, histamine, and triazole and provided high proton conductivities without water support. A PCP-based membrane using histamine achieved a conductivity of over 10−3 S/cm under anhydrous conditions.24 It is also worth mentioning that anhydrous proton conducting systems, comb

INTRODUCTION Proton exchange membranes (PEMs) for fuel cells have attracted much attention as clean energy sources for numerous applications such as electric vehicles, portable electronic devices, and residential power generators because of their high power density and efficiency with low emission levels.1−5 The most commonly used PEMs are based on perfluorosulfonic acid polymers such as Nafion. The key to fast proton transport in perfluorosulfonic acid polymers is thought to be the nanomorphology of sulfonic acid groups, which are designed such that protons can efficiently pass under hydrated conditions. However, at low water content, the continuity of hydrate proton pathways in the membrane is lost, resulting in a concomitant decrease in conductivity. For practical applications, membranes with high proton conductivities at lower humidity are required in order to operate at elevated temperatures. PEMs with high proton conductivity at temperatures between 393 and 473 K are desirable, because operation at higher temperatures can increase catalytic activity, simplify water and heat management, and provide better tolerance of the Pt catalysts against CO poisoning.6 However, Nafion suffers from poor conductivity above the boiling point of water. Developing viable PEMs with high thermal and high proton conductivities above 393 K is a major challenge. Many different approaches have been investigated in response to this challenge. Polymeric sulfonated acid membranes and composite membranes with additives that help to retain water can provide sufficient proton conductivity at temperatures up to 393 K and © 2013 American Chemical Society

Received: July 16, 2012 Revised: February 22, 2013 Published: March 28, 2013 8727

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Figure 1. Procedure of sulfonic acid-functionalized mesoporous silica film obtained by the EISA process.

Pt electrodes. The film thickness was adjusted to ∼300 nm by controlling the dipping speed (10 mm/min). The optimum initial precursor ratio was TMOS/MPTMS/C18-TMACl/HCl/H2O/ EtOH = 0.0088−0.0050/0.0012−0.0050/0.0053/0.0005/0.05/0.23. The coated film (as-made film) obtained by EISA was 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 of TMOS in the bottom, apart from the films, and heated at 393 K for 2−4 h using an autoclave. The films were also exposed to 28% NH3 vapor at 373 K for 2 h in the same manner. The stabilized films were immersed in EtOH solution (100 mL) with 0.1 g of concentrated auqeous HCl at 333 K for 3 h to remove the surfactant from the films. The samples prepared using 10, 30, and 50 mol % of MPTMS are designated as MP10-f, MP30-f and MP50-f, respectively. Mesoporous Silica Powder for Quasielastic Neutron Scattering Measurements. We prepared sulfonic acidfunctionalized mesoporous silica powder with similar pore structures and sulfonic acid densities to the sulfonic acid-functionalized mesoporous silica films, because it was impossible to measure proton diffusion in the thin film by QENS measurements. Sulfonic acid-functionalized mesoporous silica powder was synthesized from a mixture of MPTS, tetraethoxysilane (TEOS), and the C18-TMACl surfactant in a basic medium of tetramethylammonium hydroxide (TMAOH) 10 wt % solution. The molar ratio was TEOS/MPTMS/C18-TMACl/TMAOH/ H2O = 0.9, 0.7, 0.5/0.1, 0.3, 0.5/0.4, 0.5, 0.6/0.36/130. The mixture was stirred for 3 h at room temperature and was then refluxed at 369 K for 48 h. After the reaction, the resulting powder was filtered and washed with water. Furthermore, this powder was hydrothermally treated (powder/water = 2 g/50 g) at 373 K for 24 h using a 100 mL Teflon vessel. After hydrothermal treatments, the resulting powder was filtered and washed with water. Next, the powder was immersed in EtOH solution (200 mL) with 2 mL of concentrated aqueous HCl at 333 K for 3 h to remove the C18-TMACl surfactant. The mercaptopropyl group was oxidized with 30 wt % H2O2 solution at room temperature for 3 h (H2O2/MPTMS = 5). The samples prepared using 10, 30, and 50 mol % of MPTMS are designated as MP10-p, MP30-p, and MP50-p, respectively. Mesoporous silica (MS) was synthesized using TEOS, TMAOH, C18-TMACl, and H2O. The molar ratio was TEOS/C18-TMACl/TMAOH/ H2O = 1/0.12/0.36/130. The mixture was stirred for 3 h at room temperature and was then refluxed at 369 K for 48 h. After the reaction, the resulting powder was filtered and washed with

polymers with amphoteric proton-transfer functionalities obtained by supramolecular organization, have been reported.26 This electrolyte constructs nanoscale ion-conducting channels and provides insight into designing a new type of anhydrous proton conductor at high temperature under dry conditions. Similarly, mesoporous electrolytes also construct nanoscale ion-conducting channels formed by the self-assembly of ionic groups such as sulfonic acids in directional pathways.27−29 The most striking characteristic of mesoporous materials with inorganic pore walls is their mechanical and thermal durability. Therefore, mesoporous materials are expected to function as anhydrous proton conductive electrolytes at high temperatures. In order to enhance the proton conductivity under dry conditions, the presence of neighboring sulfonic acid groups as proton carriers plays an important role in the proton transfer.27,29,30 However, there are no studies focused on the relationship between proton conductivity under dry condition and sulfonic acid density in mesopores using a film that is applicable as a fuel cell electrolyte. In the present study, in order to obtain mesoporous films with various loadings of sulfonic acid sites, an evaporation-induced selfassembly (EISA) templating process is adopted.31,32 Using these mesoporous films, we report the proton conductivity at high temperatures under dry conditions and demonstrate high proton conductivities of mesoporous electrolytes at a low RH. Furthermore, we propose a proton conductive mechanism of the films based on the results of infrared (IR) spectra, molecular dynamics (MD) simulations, and quasielastic neutron scattering (QENS) measurements.

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EXPERIMENTAL SECTION Mesoporous Silica Film for Proton Conductivity Measurements. The formation of sulfonic acid-functionalized films was accomplished using tetramethoxysilane (TMOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) precursors in an acidic medium containing cetyltrimethylammonium chloride [CnH2n+1N(CH3)3Cl, n = 18; C18-TMACl] surfactant by the EISA templating process (Figure 1). Small amounts of HCl and H2O were added to a mixture of TMOS, MPTMS, and ethanol (EtOH), and this mixture was then stirred for 1 h at room temperature. The C18-TMACl surfactant and a 30 wt % H2O2 solution as the oxidant of −SH to −SO3H were added to the resultant clear solution. The amounts of H2O2 were adjusted to H2O2/MPTMS = 5. The solution was then stirred for 2 h at room temperature. The mixture of acid-functionalized silica precursors/ surfactant was dip-coated onto a glass substrate with patterning 8728

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Figure 2. Schematic illustration of the simulation system (a) and molecular dynamics (MD) simulations for sulfonic acid groups (0.5 molecules/nm2) on the surface of mesoporous silica [top view (b) and cross section (c)].

0.1 M NaOH. The acid density (SO3H molecules/nm2) in sulfonic acid-functionalized mesoporous silica was calculated from IEC and BET specific surface area obtained by nitrogen or krypton adsorption. Proton Conductivity. The film was washed with 0.1 M HCl at room temperature for 2 h and pure water at 353 K for 2 h and then dried at 373 K for 2 h. Proton conductivity was measured by electrochemical impedance spectroscopy using a Hioki 3532-50 LCR Hitester instrument. The frequency range was from 5 Hz to 50 MHz. Before collecting the impedance spectra, each film was heated at 433 K for 5 h with flowing N2 gas and then allowed to equilibrate at the desired temperature and the desired RH in the range of 0, 5, and 10%. The RH was controlled by bubbling N2 gas through a temperature-controlled impinger. The total gas flow was 200 mL/min. Simulation. Molecular dynamics (MD) simulations were carried out to investigate the proton conductive mechanism for the model system of propylsulfonic acid-functionalized mesoporous silica corresponding to the experimental materials. MD simulations were performed using the Forcite module in Materials Studio (Accelrys, Inc.) with the COMPASS force field and the Ewald summation method. Figure 2 shows a schematic illustration of the simulation system. Surface models were constructed by cleaving a silica crystal. Propylsulfonic acid units were added at several acid densities (0.5−3.0 SO3H molecules/nm2) and residual sites were terminated by OH groups (Figure 2). The dynamics of the sulfonic acid groups were examined at 1 ns intervals at 298 K. The temperature was controlled by the Berendsen method for the NVT ensemble. Quasielastic Neutron Scattering. The high-flux backscattering spectrometer (HFBS) at the NIST Center for Neutron Research (NCNR),33 Gaithersburg, MD, was employed to measure proton diffusion in the powder samples by QENS. The samples were transferred from inert gas packaging into aluminum foil wrapped annular configurations inside a He drybox to prevent moisture uptake. The samples were then sealed in aluminum sample cans under He atmosphere. All the data was reduced and fit using the DAVE software developed at NIST.34 All error bars shown in this paper denote ±1σ. A self-diffusion coefficient was calculated by fitting a quasielastic broadening signal at the elastic peak. The line broadening can be modeled in our system with a scattering function S(Q,ω) given by

water. An acid solvent extraction technique (HCl/EtOH) was used to remove the C18-TMACl surfactant at 333 K. Similarly, bromine-functionalized mesoporous silica (Br-MS) was formed by TEOS, 3-bromopropyltrimethoxysilane (BPTMS), TMAOH, and H2O under basic conditions using the C18-TMACl surfactant. The molar ratio was TEOS/BPTMS/C18-TMACl/ TMAOH/H2O = 0.9/0.1/0.4/0.36/130. In the same manner, the mixture was stirred for 3 h at room temperature and was then refluxed at 369 K for 48 h. After the reaction, the resulting powder was filtered and washed with water. An acid solvent extraction technique was used to remove the C18-TMACl surfactant.

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CHARACTERIZATION The formation of a mesostructure was confirmed by powder X-ray diffraction patterns (XRD) obtained using a Rigaku RINT-TTR diffractometer with Cu Kα radiation (50 kV and 300 mA) in the region of 2θ = 1°−8° with a 0.01° step and a scan speed of 1°/min. Nitrogen adsorption at 77 K for determining the Brunauer−Emmett−Teller (BET) specific surface areas, pore volumes, and pore sizes of the powder samples was carried out with a Quantachrome NOVA apparatus. Krypton adsorption measurements at 87 K for determining BET specific surface areas, pore volumes, and pore sizes of the thin film were carried out on a Quantachrome AUTOSORB-1 apparatus. The pore size distribution was derived from the adsorption branches of the isotherms using the Barrett−Joyner−Halenda (BJH) method. Transmission electron microscope (TEM) images were recorded with a JEOL JEM-2000EX instrument at an accelerated voltage of 200 kV. The samples were prepared by scratching films on the substrate. The 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. Infrared (IR) spectra of the electrolytes were collected using a Thermo Nicolet NEXUS 670 FT-IR with diffuse reflectance cell under flowing nitrogen gas (100 mL/min) at various temperatures. The IR spectra were measured at a resolution of 4 cm−1 using CaF2 as a reference sample. The adsorption isotherms of water vapor were measured by means of a volumetric method using a BEL JAPAN BELSORP 18 apparatus. The samples for FT-IR and water uptake measurements were the mesoporous electrolyte scraped away from the glass substrate. The ion exchange capacity (IEC) in sulfonic acid-functionalized mesoporous silica was confirmed by acid−base titrations. In a typical procedure, 0.1 g of solid 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

2

2

S(Q , ω) = {δ(ω) + e−Q ⟨u ⟩ /3L(Q , ω)} ⊗ R(Q , ω) + BG

(1)

where Q is the momentum transfer, ω is the frequency, and u is the atomic displacement. In the above equation, R(Q,ω) is the instrumental resolution function, δ(ω) is a delta function 2

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mesochannels aligned parallel to the substrate surface. TEM images of MP10-f and MP30-f also showed a hexagonal mesostructure with one-dimensional channel structures. These results indicate that MP10-f and MP30-f films have well-defined and aligned 2D hexagonal mesostructures. In contrast, the structures of MP50-p and MP50-f with the highest loading of −SO3H (Table 1) showed a disordered phase with wormhole structures (Figures 3c,f and 4c,f). This result indicates that the incorporation of a terminal-bonded MPTMS precursor in the mesoporous matrix reduces the structural stability of the mesophase. Figure 5 shows nitrogen and krypton adsorption− desorption isotherms and pore distributions of the sulfonic acidfunctionalized mesoporous silica. The BET specific surface areas and pore sizes of the films are also shown in Table 1.

representing the elastic peak, BG is the background due to atomic vibrations, and L(Q,ω) is a Lorentzian function that measures the quasielastic broadening with the peak width Γ(Q) as given in eq 2. L(Q , ω) =

Γ(Q ) 1 π [ω 2 + Γ(Q )2 ]

(2)

We used a vanadium standard for instrumental resolution.

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RESULTS AND DISCUSSION Synthesis of Sulfonic Acid-Functionalized Mesoporous Silica. Figures 3 and 4 and Table 1 show XRD patterns, TEM

Figure 3. XRD patterns of sulfonic acid-functionalized mesoporous silica: (a) MP10-p, (b) MP30-p, (c) MP50-p, (d) MP10-f, (e) MP30-f, and (f) MP50-f.

images, and physicochemical properties, respectively, of the sulfonic acid-functionalized mesoporous silicas. The XRD patterns of MP10-p and MP30-p showed sharp peaks around 2θ = 2° reflection peaks and two weak peaks at a higher angle (2θ = 3°−5°). These peaks are assigned to (10), (11), and (20) (Figure 3a,b). This pattern is consistent with the two-dimensional (2D) hexagonal mesostructure, corresponding to the TEM result (Figure 4a,b). Similarly, MP10-f and MP30-f showed sharp peaks at low angle (2θ = 2°−3°) and an additional one or two weak peaks at a higher angle (2θ = 4°−7°). These peaks are assignable as (h0) reflections of a 2D hexagonal mesostructure with

Figure 5. Nitrogen (a−c) and krypton (d−f) adsorption−desorption isotherms and pore distributions for sulfonic acid-functionalized mesoporous silica: (a) MP10-p, (b) MP30-p, (c) MP50-p, (d) MP10-f, (e) MP30-f, and (f) MP50-f.

Figure 4. TEM images of sulfonic acid-functionalized mesoporous silica: (a) MP10-p, (b) MP30-p, (c) MP50-p, (d) MP10-f, (e) MP30-f, and (f) MP50-f. 8730

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Table 1. Physicochemical Properties of Sulfonic Acid-Functionalized Mesoporous Silica sample

meso struct

SBET (m2·g−1)

pore volume (cm3·g−1)

pore diameter (nm)

IEC (mmol·g−1)

SO3H molecules·nm−2

MP10-p MP30-p MP50-p MP10-f MP30-f MP50-f MS Br-MS

2d-hex 2d-hex wormhole 2d-hex 2d-hex wormhole 2d-hex 2d-hex

1087 941 531 623 756 390 1185 1155

1.27 0.78 0.61 0.51 0.40 0.15 1.49 1.38

3.1 2.1 2.0 3.9 3.4 1.7 3.6 3.3

0.47 1.52 2.21 0.41 1.52 2.25

0.22 0.96 2.51 0.49 1.18 3.10

Figure 6. 13C CP and 29Si MAS NMR spectra of MP50-p before (a) and after (b) oxidation using H2O2 and of MP30-f after oxidation (c).

The adsorption−desorption isotherms showed type IV for MP10-p, MP30-p, MP10-f, and MP30-f and type I for MP50-p and MP50-f. All mesoporous materials had narrow pore-size distributions and mean pore diameters ranging from 1.7 to 3.9 nm. Figure 6 shows 13C CP and 29Si MAS NMR spectra for mesoporous electrolytes before and after oxidation using H2O2. The 13 C CP MAS NMR spectrum of MP50-p before oxidation displayed two intense signals with NMR shifts of 27.6 ppm (C1 and C2 carbons, adjacent to the −SH functionality) and 11.5 ppm

(C3 carbons), confirming the presence of mercaptopropyl groups anchored to the pore walls (Figure 6a). After oxidation, the NMR spectrum displayed three peaks at 53.4, 17.1, and 10.6 ppm, corresponding to three carbons of the propyl linker between Si and SO3H (Figure 6b,c). The signal at 53.4 ppm is assigned to the carbon directly attached to the SO3H group. The absence of a signal at around 27.6 ppm due to carbons attached to SH groups indicated that the SH groups were almost quantitatively converted to SO3H groups. The 29Si MAS NMR spectrum shows two Q 8731

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peaks at −101 and −110 ppm [Q3, Si(OH)(OSi)3; Q4, Si(OSi)4] originating from the TEOS and TMOS precursors. Two additional peaks from −56 and −66 ppm are T peaks [T2, SiC(OH)(OSi)2; T3, SiC(OSi)3] originating from the MPTMS precursor. The presence of T peaks confirms the incorporation of the organosilane moieties as part of the silica wall structure. On the basis of the results of XRD, TEM, and sorption measurements, it is clear that the pore structure and acid density of the sulfonic acid-functionalized mesoporous silica powders are almost identical with those of the mesoporous films. Proton Conductivity. Figure 7a shows proton conductivities of sulfonic acid-functionalized mesoporous silica films with

Figure 8. In situ high temperature diffuse reflectance IR spectra of MP50-f under flowing nitrogen gas in the 4000−2000 cm−1 range (a) and of MP30-f and MP50-f in the 2800−2000 cm−1 range (b).

aliphatic CH2 groups. In Figure 7, the data for Nafion112 (IEC = 0.9 mmol/g) and sulfonated polystyrene (SPS) (IEC = 5.37 mmol/g) measured under dry conditions are shown for comparison. Under dry condition, proton conductivities of both MP30-f and MP50-f increased with increasing temperature. The proton conductivity of MP10-f with the lowest acid density was too low to obtain reliable data. Upon comparing the data obtained under dry condition, it is very clear that proton conductivity of MP50-f with the highest acid density, i.e., 3.1 SO3H molecules/nm2, was higher than that of MP30-f (0.3 mS/cm at 423 K for MP50-f). Consequently, proton conductivity is found to be enhanced by the high acid density by about 2 orders of magnitude compared with that of MP30-f. Furthermore, the proton conductivity of MP50-f was higher than that of Nafion by 1 order of magnitude. Figure 9 shows water uptake of MP50-f and Nafion at 298 K. Although the two were

Figure 7. (a) Proton conductivities of MP50-f and MP30-f under dry (RH 0%) and low RH conditions above 373 K. (b) Arrhenius plots of the conductivities and activation energies (Ea) of MP50-f and MP30-f. Ea was estimated from the slope.

different acid densities under dry (RH 0%) conditions and low RH above 373 K. All mesoporous films were maintained at the desired temperature after heating at 433 K. Parts a and b of Figure 8 show in situ high-temperature IR spectra of MP50-f in the 4000−2000 cm−1 range and MP50-f and MP30-f in the 2800−2000 cm−1 range, respectively. The OH stretching vibrations of the silanol groups (Si−OH) and adsorbed water appear between 3700 and 3000 cm−1 in Figure 8a. The signal intensity of the OH stretching mode between 3700 and 3000 cm−1 decreased with increasing temperature. At 433 K, the absence of a signal at ca. 3700 cm−1 attributed to the isolated Si−OH and the broad band around 3600 cm−1 suggests that Si−OH groups on the surface interacted with Si−OH and sulfonic acid groups through hydrogen bonding.35,36 Furthermore, in Figure 8b, a band at 2376 cm−1 was observed at both room temperature and 433 K in MP50-f with the highest sulfonic acid group density. This band is due to the stretching modes of direct interactions among sulfonic acid groups.37 On the other hand, for MP30-f, this band was observed only at high temperature. Additionally, the IR signal in the 3000− 2850 cm−1 region corresponds to the C−H stretching vibrations of

Figure 9. Water uptakes of MP50-f and Nafion at 298 K. 8732

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Figure 10. Atomic surface models of the silica walls with various acid densities (0.5−3.0 SO3H molecules/nm2).

not compared at high temperatures, the quantity of adsorbed water was larger in MP50-f than in Nafion at low RH. The wateradsorption isotherm suggests that MP50-f has strong affinity for H2O due to the strong hydrogen-bonding interaction of sulfonic acid groups and H2O. It can therefore be presumed that adsorbed water, as a proton carrier, remains on the surface of the mesopores even at high temperature. It is worth noting that MP50-f has conductivity comparable to that of SPS, although the amount of sulfonic acid groups in SPS (IEC = 5.57 mmol/g) is higher than that of MP50-f (IEC = 2.25 mmol/g). This indicates that the local high density of sulfonic acid groups in the mesopores allows for a higher conductivity. The activation energy (Ea) was obtained by linear regression of an Arrhenius equation

for proton transfer. This also agrees with the results of the energy barrier of proton transfer calculated by the MD simulation.29 Figure 10 shows detailed atomistic surface models of the silica walls with various acid densities. Below 1.5 SO3H molecules/nm2, the sulfonic acid groups formed hydrogen bonds to the OH groups on the silica surface. Above 2.0 SO3H molecules/nm2, the hydrogen bonds constructed a continuous network even at 298 K. This means that the proton conduction pathways are connected throughout the entire surface by a hydrogen bond network, resulting in the suppression of Ea for the proton jumping from one sulfonic acid group to the nearest neighboring sulfonic acid site even under dry condition. In comparison, the values of Ea for Nafion and SPS under dry condition are 47 and 49 kJ/mol, respectively. These values are slightly higher than the values for MP30-f and MP50-f. These data show the importance of proton channel continuity in the electrolytes. The mesoporous electrolyte has a rigid silica mesopore backbone that supports the presence of sulfonic acid groups at high density within the pores. This construction allows proton channel continuity in the pores even under dry conditions and permits proton transfer with lower Ea compared with Nafion and SPS electrolytes. Based on IR spectra

σ = σ0 exp( −Ea /k bT )

where σ0 is a pre-exponential parameter and kb is Boltzmann’s constant. The values of Ea for MP30-f and MP50-f are estimated to be 45 and 43 kJ/mol, respectively, under dry conditions (Figure 7b). This demonstrates that the increase in the loading density of sulfonic acid groups in pores reduces Ea 8733

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Figure 11. (a) QENS spectra at 300 K of MS and Br-MS. (b) QENS spectra at 300 K of MP10-p, MP30-p, and MP50-p. (c) Temperature dependences of QENS spectra of MP50-p. (d) fwhm vs Q for 370 K of MP50-p using a jump diffusion model.

is shown in Figure 11c as compared with that at 300 K. The QENS spectrum also becomes broad with increasing temperature. The quasielastic scattering is fitted well by delta and Lorentz functions (eq 1) convoluted with instrumental resolution. The Q dependence of the obtained full width at half-maximum (fwhm) [Γfwhm(Q)] of the Lorentzian (eq 2) for MP50-p at 370 K is shown in Figure 11d. The curve is fitted well by a jump diffusion model and is shown in Figure 11d (the equation is also shown in the inset of the figure). This indicates that the proton of a sulfonic acid group can selfdiffuse even in the absence of water, as in the case for superprotonic conductors.38 The numerical values of proton self-diffusion coefficient (D), average diffusion length (S ), and residence times (τ) obtained from the fitting curve of Figure 11d are 9.11 × 10−6 cm2/s, 0.43 nm, and 3.5 × 10−9 s, respectively. The diffusion length of proton self-diffusion obtained by QENS measurements is 0.43 nm, which is consistent with distance to the nearest neighboring sulfonic acid site (≈ 0.4 nm for 2.5−3.0 SO3H molecules/nm2) obtained by the present MD simulations. On the basis of the results of macroscopic proton conductivities for MP50-f (3.1 SO3H molecules/nm2), the following model is proposed. Under anhydrous conditions, the continuity of the hydrogen bond network is maintained by high densification of sulfonic acid groups. As for proton transport, proton conduction seems to be related to proton self-diffusion of the sulfonic acid group within the hydrogen bond network. It can therefore be presumed that the distances of proton transport are elongated by increasing the number of neighbors in the vicinity of the sulfonic acid site. Thus, a high concentration of hydrogen bonds, which are formed by highly dense sulfonic acid groups, is necessary to increase anhydrous proton conductivity efficiently.

and MD simulations, proton conduction takes place predominantly via the hydrogen bonds of the sulfonic acid groups. Furthermore, this high proton conductivity of mesoporous materials is strongly related to not only a high density of SO3H groups but also physisorbed water and surface Si−OH groups. Under a small amount of water, i.e., 5 and 10% RH, proton conductivity of MP50-f was drastically enhanced compared with the dry condition (Figure 7). In fact, proton conductivity was 2 mS/cm at 433 K in the case of 10% RH. The magnitude of Ea also decreased with increasing humidity, because water molecules as proton carriers were added. The mesoporous electrolytes loaded with sulfonic acid groups at high acid density are, therefore, found to promote efficient proton transport even under low RH conditions. Quasielastic Neutron Scattering. Figure 11a shows the QENS spectra at 300 K for MS and Br-MS samples. In Br-MS, the sulfonic acid group [−SO3H atomic weight (M) = 81] in MP10-p is replaced by Br (M = 79.9) (Supporting Information), which is of an equivalent amount and almost the same molecular mass as the sulfonic acid group. The purpose of these studies was to separately investigate hydrogen motions of the sulfonic acid group and the rest of the hydrogen atoms that appear in the molecule. As can be seen, the spectral widths of MS and Br-MS were the same as that of the instrumental resolution obtained by the vanadium standard. From these results, it is clear that the motions of the hydrogens of the propyl chains (−CH2−CH2−CH2−) of the organic sulfonic acid and the silanol groups (Si−OH) of the pore surface were not detected in the observable time window of the quasielastic scattering by HFBS, indicating that these hydrogens are not dynamic. Figure 11b shows the variation of the QENS spectra with various acid densities, namely, the spectra for MP10-p, MP30-p, MP50-p, and MS at 300 K. It is clear that the QENS spectrum becomes broad with increasing density of sulfonic acid groups. From this, we can conclude that broadened spectra are due to the hydrogen motions in sulfonic acid. The spectrum of MP50-p with the highest acid density of 2.5 SO3H molecules/nm2 at 370 K

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CONCLUSIONS

(1) As the acid density increased, sulfonic acid groups on the surface of mesoporous silica walls were bound to neighboring sulfonic acid groups by hydrogen bonds.

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(2) Self-diffusive behavior of the proton of the sulfonic acid group in dried mesoporous silica was detected, for the first time, by QENS measurements. (3) The dense hydrogen bonds were found to form a network based on MD simulations. Such a network presumably enhances proton transport throughout the pore channels, resulting in high proton conductivity at high temperatures. (4) Under dry conditions, the continuity of the hydrogen bond network is maintained by the high densification of sulfonic acid groups, and such a network is thought to induce proton-hopping conduction mainly between the neighboring sulfonic acid groups. (5) High proton conductivity of mesoporous materials under dry conditions is strongly related to not only a high density of SO3H groups but also adsorbed water and surface Si−OH groups.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns for MS and Br-MS showing 2D hexagonal structure (Figure S1a,b) and nitrogen adsorption desorption isotherms and pore distributions for MS and Br-MS (Figure S2a,b). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*S.F.: Phone: +81-561-71-8070. Fax: +81-561-63-5743. E-mail: [email protected]. K.K.: Phone: +81-29-2195300. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Y. Fukushima and N. Hoshikawa (Toyota Central R&D Laboratories., Inc.) for fruitful discussions. J.S. is partially supported by Grant-in-Aid for Scientific Research on Innovative Areas 23108003, MEXT, Japan.

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REFERENCES

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