Proton Conduction in Sulfonated Organic ... - ACS Publications

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Proton Conduction in Sulfonated Organic−Inorganic Hybrid Monoliths with Hierarchical Pore Structure Martin von der Lehr,† Christopher F. Seidler,‡ Dereje H. Taffa,‡ Michael Wark,‡ Bernd M. Smarsly,† and Roland Marschall*,† †

Institute of Physical Chemistry, Justus-Liebig-University Giessen, 35392 Giessen, Germany Institute of Chemistry, Carl von Ossietzky University Oldenburg, 26129 Oldenburg, Germany

‡

S Supporting Information *

ABSTRACT: Porous organic−inorganic hybrid monoliths with hierarchical porosity exhibiting macro- and mesopores are prepared via sol−gel process under variation of the mesopore size. Organic moieties in the pore walls are incorporated by substituting up to 10% of the silicon precursor tetramethylorthosilicate with bisilylated benzene molecules. After functionalization with sulfonic acid groups, the resulting sulfonated hybrid monoliths featuring a bimodal pore structure are investigated regarding proton conduction depending on temperature and relative humidity. The hierarchical pore system and controlled mesopore design turn out to be crucial for sulfonation and proton conduction. These sulfonated hybrid hierarchical monoliths containing only 10% organic precursor exhibit higher proton conduction at different relative humidities than sulfonated periodic mesoporous organosilica made of 100% bisilylated precursors exhibiting solely mesopores, even with a lower concentration of sulfonic acid groups. KEYWORDS: mesoporosity, proton conduction, surface modification, functionalization, silica

1. INTRODUCTION Ordered mesoporous (2−50 nm)1 silica materials have gained tremendous attention in the last two decades after the reports by Kresge et al., using surfactants as structure-directing agents to gain very narrow pore size distributions with hexagonal pore arrangements in such materials.2,3 This concept was later expanded by utilizing block copolymers as porogens to gain even larger ordered mesopores.4,5 Moreover, the use of certain block copolymers also introduced hierarchical pore geometry in such materials, micropores connecting the mesopores in SBA15 and KIT-6. Being intended for catalytic purposes due to their high surface area, and to improve the mass transport inside the pores of porous silica, macroporous-mesoporous silica materials have been developed,6−8 and have already found industrial application in separation like HPLC, or in electrochemical applications. Bimodal porosity and optimum connectivity in silica materials can improve mass transport, for example, of functionalization agents, leading to increased degrees of functionalization and to improved homogeneity of functional groups inside the pore system.9 © XXXX American Chemical Society

Functionalization of mesoporous silica materials with organic moieties leads to hybrid organic−inorganic materials for a wide range of applications.10 Functional groups can be grafted onto the hydroxyl group-bearing surface of mesoporous silica, the most prominent way being the functionalization with alkoxysilanes.11 As an alternative route, such alkoxysilanes can be hydrolyzed together with the silica precursors for the sol− gel synthesis of the porous hybrid materials. This procedure is called co-condensation, and leads only to the formation of ordered mesopores up to a given percentage of substitution due to phase separation. As a third alternative, organic functionalities can be directly incorporated into the pore walls of the mesoporous materials starting from, for example, bisilylated precursors, resulting in periodic mesoporous organosilica (PMOs).12 The advantage of the latter route as compared to grafting or co-condensation is given by the fact that no organic Received: July 11, 2016 Accepted: September 6, 2016

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DOI: 10.1021/acsami.6b08477 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Hybrid monolith preparation procedure, together with sulfonation of the incorporated benzene rings of BTMB. The sulfonation is performed using standing monoliths without being touched by the magnetic stirring bar.

conductors such as polymers of the Nafion family, is primarily based on the peculiar proton hopping (Grotthuss) mechanism, which benefits from the pores being on the order of a few nanometers in diameter at maximum. Thus, for the silica-based solid proton conductors, narrow pore sizes of MCM-41 had turned out to be advantageous over larger mesopores in SBA15 or SBA-16.17 The advantage of narrow pores in sulfonated silica thin films was recently confirmed, and the density of SO3H-groups improved by Inagaki et al.27 Furthermore, SO3Hmodified PMO materials were prepared recently by different groups,28,29 and tested for proton conduction28 with the aim to improve the loading degree of solid proton conductors. Regularly arranged small mesopores seem beneficial for the proton transport, while it has not yet been investigated whether the degree of sulfonation might be impeded by the relatively low mesopore diameter due to hindered access and diffusion of the sulfonation agent in such a purely mesoporous material. Consequently, the question arises whether highly ordered mesoporous materials exhibiting narrow pore size distributions and defined mesopore shapes outperform porous SiO 2 featuring irregular mesoporosity in proton conduction. Taking into account the aforementioned considerations, improved

moieties range into the pores, reducing the effective pore diameter. Moreover, grafting can lead to an inhomogeneous distribution of moieties, while co-condensation and PMO formation usually result in homogeneous distribution of functional groups, which can be proved by neutron scattering combined with in situ gas sorption experiments.13,14 Functionalization of mesoporous materials with sulfonic (−SO3H) or phosphonic (−PO3H2) acid groups leads to interesting materials for acidic catalysis and proton conduction applications. For example, PO3H2-modified mesoporous silica nanomaterials have been intensively studied by the group of Lu et al. for the preparation of solid proton conductors.15,16 Simultaneously, the group of Wark et al. investigated the sulfonation of ordered mesoporous MCM-41 via either grafting or co-condensation, and the resulting proton conduction properties.17−20 Moreover, composite membranes with proton conducting polymers were prepared and investigated as composite membranes for high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC), and as membranes for desalination or CO2 separation.21−26 In general, enhanced conduction of protons in nanoscaled materials, either modified SiO2 materials or polymeric proton B


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into a water bath, and the mixture was allowed to react at 313 K for 16 h. The resulting gels were subject to further solvothermal treatment in sealed vials at different temperatures (353 or 368 K) by raising the temperature slowly over 12 h to the desired temperature, followed by different holding times (15 or 27 h in total). The resulting alcogels were washed with methanol for 3 days, replacing the solvent mixture daily. Calcination was carried out at 603 K for 10 h in air (heating and cooling rate: 0.33 K min−1) to remove the porogen PEG. This temperature is low enough to avoid decomposition of the benzenecontaining framework, but high enough to remove all porogen.8 Samples are denoted as HTT XXX/YY, where XXX gives the temperature (in Kelvin) of the solvothermal treatment, and YY represents the respective total time (in hours) of the treatment. Sulfonation was performed adapting a protocol by Sharifi et al.28 The hybrid macro-mesoporous monoliths were put into a special flask, keeping the monoliths standing upward (Figure 1) without being touched by the magnetic stirring bar. The flask was filled with 20% SO3/H2SO4 solution under Ar until the monolith was fully covered. The solution was heated to 353 K for 20 h. After the reaction, the monolith was removed and washed with water for 5 days (stored in water) until the pH of the washing water was neutral, while the solvent was replaced daily. The whole synthesis procedure is outlined in Figure 1. 2.3. Characterization. The porosity of untreated and functionalized materials was investigated in detail using N2 physisorption and mercury intrusion porosimetry. Physisorption measurements were performed in an automated gas adsorption station (Autosorb-6, Quantachrome Corp.), utilized for standard characterization measurements of nanostructured matter by N2 sorption isotherms at T = 77 K. The instrument software supports the standard data reduction algorithms such as the Brunauer−Emmett−Teller (BET) approach, as well as NLDFT kernels for typical pore geometries. The monolithic hybrid rods were placed in standard glass tubes and stabilized at the measurement temperature with T = 77 K kept by liquid nitrogen in standard cryostats. Isotherms were measured up to a relative pressure p/p0 = 0.995. The pore volume was estimated from the amount of adsorbed gas at a relative pressure of p/p0 = 0.95. Pore size distributions and median mesopores sizes were calculated from the adsorption branches of the physisorption measurements by NLDFT calculations, using the kernel for cylindrical pore geometry. Samples were evacuated at T = 393 K for 12 h before measurements. Mercury intrusion measurements were carried out with a Pascal 140/440 porosimeter by Thermo Fisher Scientific, Rodano, Italy, in a pressure range from 0−400 MPa. The ion exchange capacity of the sulfonated hybrid monoliths was determined by titration. A small amount of crushed monolith powder was suspended in a 0.01 M sodium hydroxide solution for 48 h, and the remaining sodium hydroxide was titrated with hydrochloric acid. Elemental analyses of the sulfonated samples were carried out using a Vario micro cube elemental analyzer (Elementar). Scanning electron micrographs were recorded on a Zeiss MERLIN instrument at 3 kV using working distances between 2 and 3 mm. Water sorption measurements were performed on a Quantachrome Aquadyne DVS-2HT, at 353 K. Prior to each measurement, samples were dried under vacuum for several hours at 423 K, transferred to the instrument, and treated in dry nitrogen gas until weight consistency. Additionally, full water physisorption isotherms were measured using Hydrosorb 1000 (Quantachrome Instruments). Prior to the measurements, samples were degassed at 393 K for overnight under a pressure of 1.5 × 10−1 Torr. The water adsorption isotherms were collected at 298 K between 0.05 and 0.95 relative pressures. Proton conduction of compacted embankments of the crushed monoliths was measured with a procedure reported earlier.17 The proton conduction was measured by impedance spectroscopy (IS) using a Zahner electrochemical workstation ZahnerZennium in a frequency range from 1 to 106 Hz with an oscillating voltage of 100 mV. The embankments for the measurements have the dimensions of the sample holder, a diameter of 8 mm and 0.8−1.3 mm thickness. Samples were inserted between two thin carbon gas diffusion layers (8 mm in diameter). The polytetrafluoroethylene (PTFE) specimen

transport in the postfunctionalization step can improve statistical distribution and surface concentration of moieties such as SO3H within the porous material. To investigate the influence of hierarchical pore systems in combination with irregular pore morphologies on proton conduction in inorganic−organic hybrids, we prepared organic−inorganic monoliths via a solvothermal sol−gel process with different mesopore sizes by substituting 5 and 10 wt % of a silica precursor with bis(trimethoxysilyl)benzene (BTMB) to gain benzene moieties in the pore walls of a hierarchical macroporous-mesoporous monolith.8 Such monoliths can be synthesized with high reproducibility, under variation of the meso- and macroporosity. The benzene groups in the pore walls are subsequently modified with SO3H-groups. These materials thus possess a 3D-connected macropore structure, allowing for optimized mass transport and enabling a homogeneous spatial distribution of functional groups, as compared to purely mesoporous materials. The mesopores are formed via a dissolution-reprecipitation-type Ostwald ripening process via in situ formed ammonia. A monolithic material bears the advantage that any species entering the monolith is located inside the macro-mesoporous structure and not on the outer surface of the grains as in case of powder-like materials. The resulting sulfonated silica monoliths were investigated with respect to proton conduction, and the median mesopore diameter was varied to investigate the optimum pore size distribution in such hierarchical pore systems for proton conduction.

2. EXPERIMENTAL SECTION Poly(ethylene glycol) (PEG, 10 000 g mol−1), tetramethyl orthosilicate (TMOS), and urea were obtained from Merck (Darmstadt, Germany). All other chemicals were purchased from Sigma-Aldrich. All reagents were used as received without further purification. 2.1. Precursor Preparation. The route to prepare the precursors for the synthesis of hybrid macro-mesoporous monoliths was discussed in detail before.8 Following a route by Kuroda et al.,30 first anhydrous methanol (88 mL) was added to 80 mL of tetrachlorosilane under argon flow, and the mixture was stirred for 30 min. After distillation, 44 g of the crude product chlorotrimethoxysilane with a boiling point of 385 K was obtained. The whole amount was dissolved in anhydrous tetrahydrofuran (THF, 300 mL), and magnesium turnings (16 g) were added. To this mixture was added a solution of dibromobenzene (32 g) in THF (400 mL) dropwise with sufficient cooling if the exothermic reaction became too intense. After the mixture was stirred for 15 h at room temperature, the products were filtered and the solvent removed. The residue was extracted with dry pentane under inert conditions using Schlenk technique. After removal of the solvent and fractionated distillation in vacuum, the products were obtained. Bis(trimethoxysilyl)benzene (BTMB) was collected at 394 K (0.1 mbar) as a clear liquid, which solidified below 325 K. As a byproduct, bis((trimethoxysilyl)-phenyl)dimethoxysilane (BTPMS) was distilled at 471 K (0.4 mbar) and collected as a yellow liquid. Yields were 30% for the main compound BTMB and 18% for the byproduct BTPMS. 2.2. Preparation of Sulfonated Macro-mesoporous Hybrid Monoliths. The synthesis protocol to prepare macro-mesoporous hybrid monoliths was reported earlier.8 Briefly, BTMB was liquefied at 343 K, and after the addition of TMOS in different weight ratios of BTMB/TMOS of 5/95 (4.04 × 10−4 mol of BTMB and 1.60 × 10−2 mol of TMOS) and 10/90 (8.08 × 10−4 mol BTMB and 1.52 × 10−4 mol TMOS), stirred for 1 h. 2.5 mL of this mixture was added to a solution of acetic acid (0.01 M, 5 mL), urea (0.45 g), and PEG (0.355 g). After being stirred for 25 min at room temperature, the mixture was filtered and filled into 10 mL plastic vials. The vials were immersed C


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ACS Applied Materials & Interfaces Table 1. Structural Parameters of Hybrid Monolith under Varying Solvothermal Treatment sample name TMOS/BTMB 353/15 TMOS/BTMB 353/27 TMOS/BTMB 368/27 TMOS/BTMB 353/15 TMOS/BTMB 353/27 TMOS/BTMB 368/27

spec. surface area SBET [m2/g]

median pore size (NLDFTads) [nm] ∼2

pore volume (NLDFTads) [cc/g]

median macropore size [nm]

macropore volume [cc/g]

0.3

1410

1020

(95/5), HTT

550

(95/5), HTT

800

5.9

0.92

1810

1820

(95/5), HTT

490

11.7

1.04

1670

2110

(90/10), HTT

560

∼2

0.3

2600

980

(90/10), HTT

910

4.9

0.83

3020

1850

(90/10), HTT

540

7.6

0.82

2990

2040

Figure 2. N2 physisorption isotherms with corresponding pore size distributions for hybrid monoliths containing (a,c) 5% or (b,d) 10% BTMB after solvothermal treatment at (■) HTT 353/15, (red ●) HTT 353/27, and (blue ▲) HTT 368/27. holder was located in a gastight stainless steel body (changes in the water content during the measurement can be excluded) with thermocouple access to the holder and two electrical contacts. For electrical contact, an angular moment of 30 cNm was used. This body was connected via a stainless steel tube to a stainless steel water reservoir. Relative humidity (RH) in the cell was controlled by adjusting the temperature of the water tank. Equilibration time at each temperature was at least 30 min. The specific conductivity was calculated according to the formula σ = (1/R)(L/A), where R is the resistance corresponding to the phase angle closest to zero in the Bode diagram, L is the thickness of the sample between the electrodes, and A is the cross-sectional contact area of the electrodes. This analysis procedure is often used to interpret proton conductivities in powders or membranes.31

prepared with various porosities by variation of the solvothermal treatment temperature and time, and the resulting porosity was investigated carefully by mercury intrusion porosimetry and N2 physisorption. Table 1 summarizes the structural properties of the monoliths after calcination, and Figure 2 shows the physisorption isotherms comparing the different monoliths. After solvothermal treatment for 15 h at 353 K, no hysteresis loop resulting from capillary condensation in mesopores can be observed. This finding is in accordance with the pore size distribution calculated via NLDFT adsorption branch analysis from physisorption measurements, showing a pore diameter of ∼2 nm on average. Extending the solvothermal treatment to 27 h, a dissolution-reprecipitationtype Ostwald ripening process via in situ formed ammonia expands these pores to mesopores,32 and a type IV hysteresis loop typical for mesoporous materials can be observed. The shape of the hysteresis loop changed with increasing the solvothermal treatment time further, due to an increase in

3. RESULTS 3.1. Structural Analysis. To investigate the influence of porosity on the proton conductivity of the sulfonated hybrid macro-mesoporous monoliths, non-sulfonated rods were D


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Figure 3. Mercury intrusion porosimetry for non-sulfonated meso-macroporous hybrid monoliths containing (a) 5% BTMB and (b) 10% BTMB after sovothermal treatment at (■) HTT 353/15, (red ●) HTT 353/27, and (blue ▲) HTT 368/27.

macropore formation. The successful incorporation of BTMB was proven by UV absorption (Figure S1). Mercury intrusion data given in Figures 3 hardly show the bimodal porosity of HTT 353/15 and HTT 353/27 samples; only for the HTT 368/27 samples distinct maxima for the mesopores are visible. From N2 physisorption, the mesoporous nature of the monoliths was unambiguously proven. However, mercury intrusion puts high pressure onto the hybrid porous framework, and it seems that only the HTT 368/27 is stable enough to withstand the pressure of the measurement, due to a higher degree of condensation of the framework caused by solvothermal treatment. We have performed consecutive cycles of mercury intrusion runs of HTT 368/27 samples (Figure S2). Interestingly, the monoliths containing 5% BTBM show nearly no difference in macropore volume; the small difference is probably due to residual mercury from the first run. However, the monolith containing 10% BTMB exhibits a distinct framework breakdown. The reason for this behavior due to the pressure of the mercury intrusion measurement is unclear, but it might be a first indication that the skeleton of the monoliths becomes less stable with more organic moieties. Figure 4 shows SEM images of the differently prepared hybrid macro-mesoporous monoliths. The mesopores are buried in the skeleton. From the observation via SEM, the differences in the macropore size between samples with 5% or 10% BTBM can be distinguished. These differences in the sizes of macropores are mainly attributed to the spinodal decomposition, which is affected by the higher amount of organic species in the gelation mixture. Also, the skeleton of the 10% BTMB HTT 368/27 sample seems to be slightly thicker as compared to the other samples. After sulfonation, the structure of the hybrid monoliths is only slightly changed. Table 2 gives the porosity parameters after sulfonation, and corresponding physisorption data and SEM images to show the structural preservation are given in Figures 5 and 6 (see also Figure S3). In case of HTT 353/27 samples, the mesopore diameter increases by 1 nm after sulfonation, and the pore volume increases strongly. It seems that only a longer solvothermal treatment time in combination with higher temperatures strengthens the mesopore network sufficiently to withstand the harsh treatment with oleum used for sulfonation. As a result, the pore diameter for HTT 368/27 does not change after sulfonation. Moreover, the aryl

mesopore size (Table 1). As can be seen by data comparison, increasing the solvothermal treatment-time from 15 to 27 h at T = 353 K clearly increases the specific surface area SBET, the mesopore volume, and the mesopore diameter of the hybrid monoliths, resulting in mesopores featuring median pore diameters of 5−6 nm. The effect is more pronounced for monoliths containing less BTMB, indicating that the nonorganic silica part is etched by the in situ formed ammonia. Monoliths containing 10% BTMB exhibit smaller mesopores after being treated for 27 h at 353 K (maximum at 5 nm) as compared to monoliths containing 5% BTMB (maximum at 6 nm). Moreover, the samples HTT 353/27 still contain a large amount of small mesopores around ∼2 nm (Figure 2). The increasing specific surface area observed for elongated treatments at 353 K is unexpected as solvothermal treatment generally reduces the surface area by the elimination of micropores. However, micropores are not removed sufficiently in the present case, as dissolution-reprecipitation-type Ostwald ripening of the silica phase and ammonia etching only occurs at a limited time scale (15 h, heating ramp included), thus resulting in incomplete pore fusion. In this time regime, new mesopores are still generated. Moreover, samples HTT 353/15 show distinct shrinkage behavior (>25%), which in turn leads to a reduced specific surface area as compared to the samples that underwent longer treatments or higher temperatures. Speeding up this process by increasing the solvothermal treatment temperature from 353 to 368 K, the pore diameters and pore volumes increase further; however, SBET is reduced from samples HTT 353/27 to 368/27 expectedly due to increasing size of mesopores. The difference in the mesopore diameter between 5% and 10% BTMB-containing monoliths 11.7 and 7.6 nm, respectively, is even more pronounced, and no more small mesopores 100 cm3/g in the relative pressure range up to 0.1, the uptake of sample HTT 353/15 with 5% BTMB shows less microporosity, while with 10% of BTMB nearly no microporosity is observed. The SEM images in Figure 6 show comparable micrographs in comparison to Figure 4, confirming that the sulfonation process exerts only a minor effect on the macropore structure. Only in some areas can blocked macropores be seen, which have arisen from sample preparation for electron microscopy. Comparing the mercury intrusion results in Tables 1 and 2, one can observe that the macropore structure is only slightly influenced by the oleum treatment. The biggest change in macropore size can be seen for the HTT 353/15 samples, which might result from further condensation of the framework during sulfonation. With increasing solvothermal treatment time and temperature of the pristine hybrid monolith, the stability against oleum increases, and the macropore changes become smaller. Pore size distributions for the sulfonated monoliths from mercury intrusion are shown in Figure S10. G


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Figure 6. SEM images of solvothermally treated samples after sulfonation: (a) 5% BTMB HTT 353/15, (b) 5% BTMB HTT 353/27, (c) 5% BTMB HTT 368/27, (d) 10% BTMB HTT 353/15, (e) 10% BTMB HTT 353/27, and (f) 10% BTMB HTT 368/27.

RH are of the same order of magnitude as for 50% RH, increasing slightly to 10−3 S cm−1, showing that the proton conduction in this case is less influenced by temperature or the water content, but more by the loading and SO3H group density. It needs to be mentioned that the loading with SO3H groups being accessible for ion exchange in all three samples, estimated to lie between 0.7 and 1 mmol g−1, is comparable to other materials in literature,27,29,33 which, however, is surprising because our monoliths contain only 5 wt % of benzene rings, and only those can be sulfonated by our preparation method. Complementary data from elemental analyses are given in Table S1. As can be seen, the amounts of sulfur from elemental analysis show the same trends, but even slightly lower values. In the following, we will discuss values obtained from the titration

proton conduction increases strongly as compared to 50% RH. Sample HTT 353/27, SO3H, shows proton conduction of more than 10−3 S cm−1 at 100% RH, as compared to 10−4 S cm−1 at 50% RH. For the sample HTT 368/27, SO3H, the change is even more drastic, indicating a strong temperature dependence of proton conduction, rising to 10−2 S cm−1. The results indicate that the proton conduction mechanism strongly depends on the water content in the mesopores, especially in the larger mesopores where the SO3H groups are further away from each other. In the material containing small mesopores, no drastic change in proton conduction can be seen in Figure 7b. Even at higher temperatures no conduction drop is visible; the proton conduction increases slightly with temperature. The values for proton conduction of sulfonated HTT 353/15 samples at 100% H


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Figure 7. Proton conduction results for sulfonated hybrid monoliths containing 5% BTMB under (a) 50% RH and (b) ∼100% RH. (■) HTT 353/ 15, (red ●) HTT 353/27, and (blue ▲) HTT 368/27. Lines between data points serve as guides for the eye and have no physical meaning.

Figure 8. Proton conduction results for sulfonated hybrid monoliths containing 10% BTMB under (a) 50% relative humidity (RH) and (b) ∼100% RH for (■) HTT 353/15, (red ●) HTT 353/27, and (blue ▲) HTT 368/27. Lines between data points serve as guides for the eye and have no physical meaning.

comparably small (Figure 8b), showing the advantage of the high loading for proton transport lowering the dependence on water content. Increasing the RH to ∼100%, the proton conduction of the HTT 353/27, SO3H sample reaches an outstanding value of 0.1 S cm−1 (Figure 8b). Also, the samples HTT 353/15, SO3H and HTT 368/27, SO3H reach proton conduction values of more than 10−2 S cm−1 at 413 K. Again, in the materials containing small mesopores, proton conduction seems to be independent of the water content, and less dependent on temperature. In the midsized mesopores of HTT 353/27, enough water is retained inside the pores. However, the slope in the proton conduction increasing as a function of temperature at higher RH indicates a stronger dependence on the water content. As compared to sulfonated benzene-PMOs with hexagonally arranged mesopores containing 100% BTMB and thus higher loading, the here presented sulfonated samples featuring bimodal porosity containing only 10% BTMB show 2 orders of magnitude higher proton conduction measured under identical conditions.27 To correlate the proton conductivities not only with IEC and surface area but also with water uptake, we have performed water sorption measurements at proton conductivity measurement conditions, at a temperature of T = 353 K. As can be seen in Figure 9, at low RH the water uptake of the samples containing small mesopores (HTT 353/15, SO3H, 5% or 10% BTMB) is actually, as expected, higher than in samples possessing larger mesopores, due to capillary forces. It is reasonable to assume that such capillary forces are also present

experiments, also when comparing literature data. Thus, it is even more astonishing that our monolithic samples possessing bimodal porosity and only 5% sulfonated BTMB already reach proton conduction values comparable to benzene-based PMOs with hexagonally arranged mesopores containing 100% BTMB and being sulfonated, which bear a higher loading of SO3H groups.28 This effect is even more pronounced when the loading with SO3H groups is increased by incorporating 10% BTMB into the hybrid macro-mesoporous monoliths followed by sulfonation (Figure 8). Already at 50% RH, the sulfonated samples HTT 353/15 and HTT 353/27 exhibit proton conduction values close to 10−2 S cm−1, at loadings with SO3H groups of 1.8 and 1.1 mmol g−1, respectively. The HTT 368/27, SO3H sample shows values of 10−4 S cm−1 (loading: 1 mmol g−1). For all of these samples (10 wt % BTMB), the proton conduction increases due to the increased loading as compared to 5% BTMB samples, and the better accessibility due to the increased macropores. Especially the HTT 353/27, SO3H sample shows an outstanding increase by 2 orders of magnitude. In this case, the midsized mesopores seem to lead to optimum proton conduction conditions. Moreover, the HTT 353/15, SO3H sample containing small mesopores again exhibits the highest loading with SO3H groups, which might be due to capillary forces inside the mesopores being relevant during the sulfonation process. As compared to Figure 7a and b, the influence of water and temperature on the proton conduction of this sample is I


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(compare Table 2) achieved for the HTT 353/15 samples as compared to their respective counterparts with larger mesopores. The numbers even suggest the substitution of more than one SO3H group per aryl ring, which is very unusual, but can be explained by the high concentration of oleum in the pores due to capillary forces. At high RH the total water uptake in the HTT 353/15 samples is of course lower than in the samples with larger mesopores due to the narrow pores and small pore volume. Moreover, the water uptake at high RH (measurements above 80% RH were not possible at 353 K) correlates with the proton conduction values and the specific surface area. The samples with the highest water uptake give also the highest proton conduction at T = 353 K, in the exact same order. The samples possessing medium-sized mesopores (HTT 353/27, SO3H) exhibit the highest water uptake and the highest proton conduction. Even the corresponding non-sulfonated samples showed reasonable high water uptake at high RH. Although the water uptake curves look similar to those of proton conducting polymer membranes,34 a direct comparison is aggravated by the fact that our samples exhibit a rigid framework in which the proton conduction does not depend on

Figure 9. Water adsorption performed at 353 K for sulfonated and non-sulfonated hybrid monoliths.

during sulfonation, resulting in a high amount of oleum present in the mesopores, explaining the much higher loadings

Figure 10. Comparison between (a) sulfonated hexagonal PMO as reported by ref 26, and (b) our monolithic samples with bimodal porosity and less ordered mesopore network. Atomic structures depict the situation on the inner pore walls. J


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ACS Applied Materials & Interfaces the flexibility of the polymer backbone. While the nanostructure of polymer-based membranes forms in the presence of water, in case of our siliceous materials the nanoscaled porosity is independent of the uptake of water. Additional water sorption data measured as full isotherms at 298 K are given in Figure S11. The highest water uptake is observed for both samples of the series HTT 353/27, representing the midrange pore size. As compared to the non-sulfonated sample (HTT 353/27 with 5% BTMB), the amount of incorporated water is slightly higher and can be explained by an enhanced surface polarity resulting from the SO3H groups attached to the aromatic moieties. Moreover, shifting of the capillary condensation to lower relative pressure with higher loading can be observed, indicating more hydrophilic samples. At small values for p/p0 = 0.1−0.3, one can observe that the sample possessing the smallest mesopore size shows the highest water uptake, which is the aforementioned interpretation based on strong capillary forces occurring in systems with such small mesopores. Comparing measurements at 298 and 323 K (Figure S12) for the highest proton conducting samples, a type V isotherm can be observed at both temperatures. For HTT 353/27 (10%) SO3H sample, water uptake slightly decreased with temperature, due to larger macropores holding less water back inside the material at elevated temperatures. These results from water sorption are complementary to the water adsorption data measured at 353 K (Figure 9), and in agreement with the proton conduction measurements.

exhibit lower loadings, and samples containing 10% BTMB exhibit higher or comparable loadings (sulfonated PMO with hexagonally arranged cylindrical mesopores: 1.42 mmol g−1). As described before, at 50% RH conditions, small mesopores subjected to short solvothermal treatment (HTT 353/15) and sulfonation favor proton conduction. This is a finding being comparable to reports comparing MCM-41, SBA-15, and SBA16 for proton conduction, in which MCM-41 turned out advantageous due to its smaller pore diameter.17 Moreover, proton conduction increases for the HTT 353/15, SO3H sample by increasing RH from 50% to 100%, but the slope upon temperature enhancement remains comparable small, while for the HTT 368/27, SO3H sample and sulfonated MCM-41-like benzene-PMO, this slope is quite high, indicating a proton conduction that is more water- and temperaturedependent. The larger is the slope, the more water-dependent is the proton conduction. The low temperature dependence of proton conduction in small mesopores can be an indication for a different proton transport mechanism as compared to larger pores, more depending on proton hopping via Grotthuss mechanism35 than diffusion of water or hydronium (vehicle mechanism).36 In pores being only 2 nm in diameter, diffusion pathways for protons would be very short, and no bulk water can be assumed. Thus, only moderate thermal excitation of water or hydronium diffusion can be expected, but the conductivity depends on the mobility of the protons via hopping. Activation energies calculated from Arrhenius plots given in Figures S13 and S14 underline this hypothesis. Activation energies for small pore materials HTT 353/15 vary only from 0.08 to 0.4 eV, indicating Grotthuss-type mechanism.37 While activation energies can vary with IEC in even such a range, we can assume the same type of proton conduction mechanism. In samples with larger mesopores, for example, HTT 368/27, SO3H, bulk water condensation can be expected inside the pores for extended vehicle mechanism. The more pronounced temperature dependence is an indication for a more diffusiondepending and thermally activated proton conduction mechanism.36 Again, activation energies confirm this conclusion, as they increase to 0.4−0.6 eV for HTT 368/27 at 100% RH indicating vehicle mechanism.37 Interestingly, at 50% RH the activation energies even for the large-mesopore materials are much lower as compared to those at 100% RH. As indicated in an earlier work on ordered mesoporous silica materials functionalized with SO3H, in such pore systems water is inhomogeneously distributed at low hydration levels, mainly covering the pore walls building a network with the attached sulfonic acid groups.20 In the present case, this explains the low activation energy inside HTT 368/27 at low RH, and a Grotthuss-ruled transport will most probably occur along the pore walls including water and SO3− groups. In comparison, in model systems like CH3−SO3H/water mixtures, the proton conduction mechanism was found to be depending on the water content. At low degrees of hydration, strong evidence for mainly vehicle mechanism was found by NMR.38 On the contrary, in our present material systems, the opposite trend can be found, as low degrees of hydration activation energies suggest Grotthuss-like proton transport. The comparability is also questionable because our materials exhibit a rigid silica framework providing less degrees of freedom for the proton conduction. As compared to sulfonated MCM-41 materials formed by oxidation of grafted mercaptosilane

4. DISCUSSION In this study, proton conducting sulfonated hybrid monoliths were synthesized and compared with literature data28 of a sulfonated benzene-PMO possessing a 2D hexagonal pore structure. For a detailed discussion, the main differences between these types of materials have to be clarified. Sulfonated benzene-PMOs possessing a 2D hexagonal mesopore structure exhibit pore walls purely made of BTMB (or BTEB), which are condensed around a structure-directing template. For example, octadecyltrimethylammonium chloride can be used as surfactant, forming micelles, the shape of which depends on the concentration. The hexagonal arrangement of rod-like micelles and the subsequent condensation of BTMB around these liquid crystals lead to a defined 2D hexagonal pore arrangement after template removal, with highly ordered mesopores and very narrow pore size distribution of the resulting PMO.28 In contrast, the mesopores in our hybrid monoliths derive from a dissolution-reprecipitation-type Ostwald ripening process via in situ formed ammonia during the solvothermal process; thus those mesopores are both irregular in their shape and in their mutual arrangement, leading to the broader pore size distribution shown in Figure 2. Moreover, they are connected via macropores in the monoliths presented here. By controlling the solvothermal treatment conditions, we are further able to control the amount and the size of mesopores. These structural differences are depicted in Figure 10. Our monoliths exhibit either comparable median mesopore sizes or slightly larger median mesopore sizes after sulfonation as compared to the reported hexagonal sulfonated benzenePMO, by the samples HTT 353/15 and HTT 353/27 (compare Table 2). Nevertheless, the pore size distribution is much broader, due to formation of mesopores via dissolutionreprecipitation, and not via a templating process. In terms of SO3H loading, sulfonated samples containing 5% BTMB K


Research Article


PMO materials functionalized identically, the monolithic materials gave 2 orders of magnitude higher proton conduction, although only containing maximum 10% BTMB, reaching up to 10−1 S cm−1. These results show that bimodal porosity and nonordered mesopore networks are worth being investigated further to improve solid proton conductors. Infiltration with proton conducting polymers would enable the preparation of self-standing membranes with high inorganic content for HTPEMFC applications. Moreover, these findings could also be interesting for the optimization of continuous flow acidic catalysis.

species, the calculated average activation energies are significantly lower.39 In general, our 5% hybrid monolithic samples with bimodal porosity and nonordered mesopores already reach proton conduction values comparable to those of benzene-PMOs with hexagonally arranged mesopores containing 100% BTMB, however being sulfonated with lower loading. The reason is that the macropores of the here presented hybrid monoliths improve the mass transport of either oleum for sulfonation (resulting in a more effective sulfonation and even a more homogeneous distribution and accessibility of SO3H groups; unfortunately this could not be verified yet) or water during the proton conduction measurement, reducing the activation energy for proton transport. Another advantage is the nonordered mesopore structure with slightly broader pore size distribution for proton conduction. As was already known from other materials and applications, especially when transport phenomena have to be taken into account (e.g., in catalysis), pore disorder can be crucial to reduce pore blocking effects and transport constraints.40 Highly ordered narrow pores, like in hexagonal benzene-PMO, can be easily blocked, while a nonordered porosity exhibits many different types of pore channels, for proton or water transport. Finally, when preparing the hybrid monoliths with 10% BTMB, the HTT 353/15, SO3H sample (see Table 2) possesses higher loadings than previously reported benzenePMO samples with SO3H-functionalized benzene rings.28 This observation is another indication for the advantage of our macroporous-mesoporous hybrids, leading to improved sulfonation even for smaller mesopores due to less pore blocking and better mass transport. Moreover, the proton conduction values are much higher, reaching the maximum for HTT 353/ 27, SO3H. Already at 333 K, the proton conduction under similar conditions (∼100% RH) is 1 order of magnitude higher than that of sulfonated benzene-PMO, rising up to 0.1 S cm−1. Thus, this sample reaches proton conduction values of cocondensed SO3H-MCM-41 samples (reported loading of 2.3 mmol g−1)18 although only being sulfonated with 1.1 mmol g−1. However, our explanations need to be supported by studying more porous materials with hierarchical pore structure, as we plan to do in the near future. It will be interesting to see how pure silica monoliths, hybrid bimodal monoliths with ordered mesopores, or powders with three-dimensional pores (ordered and nonordered) perform after sulfonation in proton conduction. To investigate detailed structure property relationships, we plan to perform magic angle spinning pulsed field gradient (MAS-PFG)-NMR as further future studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08477. Absorption spectra of our hybrid monoliths, thermogravimetric analyses, additional water sorption isotherms, additional SEM images, additional mercury intrusion data, and Arrhenius plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Rüdiger Ellinghaus, Felix Badaczewski, and Rafael Meinusch (Justus-Liebig-University Giessen) for performing the physisorption measurements and providing a pure silica monolith as reference. The Department of Chemistry from Philipps-University Marburg is acknowledged for performing elemental analyses. R.M. gratefully acknowledges funding in the Emmy-Noether program (MA 5392/3-1) of the German Research Foundation DFG, and financial support by the Max-Buchner-Research Foundation through a research scholarship (3313). C.F.S. and M.W. were supported by the DFG in project WA 1116/24. This project was supported by the Laboratory of Materials Research (LaMa) of the Justus-LiebigUniversity Giessen, Germany.

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REFERENCES

(1) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez-reinoso, F.; Rouquerol, J.; Sing, K. S. W. Pure Appl. Chem. 2015, 87, 1051−1069. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a LiquidCrystal Template Mechanism. Nature 1992, 359, 710−712. (3) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenkert, J. L. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates. J. Am. Chem. Soc. 1992, 114, 10834−10843. (4) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G.; Chmelka, B.; Stucky, G. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores. Science (Washington, DC, U. S.) 1998, 279, 548−552. (5) Kleitz, F.; Hei Choi, S.; Ryoo, R. Cubic Ia3d Large Mesoporous Silica: Synthesis and Replication to Platinum Nanowires, Carbon Nanorods and Carbon nanotubesElectronic Supplementary Information (ESI) Available: TEM Images of Mesoporous Cubic Silica and Pt Networks, XRD Patterns during Forma. Chem. Commun. 2003, 17, 2136−2137.

5. CONCLUSIONS Hybrid organic−inorganic monoliths with hierarchical pore structure were prepared from TMOS and BTMB with ratios of 90:10 and 95:5, and the resulting monoliths were sulfonated with SO3H groups at the benzene rings, thus enabling proton conduction. It turned out that fine-tuning of the mesoporous structure is possible to get optimum proton conduction at T = 413 K. Samples with medium-sized mesopores of around 6 nm gave the highest conduction values, while samples with small median mesopore diameter show proton conduction depending less on relative humidity and temperature. The broader pore size distribution and the bimodal porosity of the hybrid monoliths were found to be crucial for mass transport and high proton conduction. As compared to mesoporous benzeneL


Research Article


Membranes for Desalination Application. J. Mater. Chem. 2011, 21, 7401−7409. (25) Klaysom, C.; Moon, S. H.; Ladewig, B. P.; Lu, G. Q. M.; Wang, L. The Influence of Inorganic Filler Particle Size on Composite IonExchange Membranes for Desalination. J. Phys. Chem. C 2011, 115, 15124−15132. (26) Khan, A. L.; Klaysom, C.; Gahlaut, A.; Li, X.; Vankelecom, I. F. J.; SPEEK. and Functionalized Mesoporous MCM-41 Mixed-Matrix Membranes for Gas Separation. Procedia Eng. 2012, 44, 1902−1905. (27) Fujita, S.; Koiwai, A.; Kawasumi, M.; Inagaki, S. Enhancement of Proton Transport by High Densi Fi Cation of Sulfonic Acid Groups in Highly Ordered Mesoporous Silica. Chem. Mater. 2013, 25, 1584− 1591. (28) Sharifi, M.; Köhler, C.; Tölle, P.; Frauenheim, T.; Wark, M. Proton Conductivity of SO3H-Functionalized Benzene-Periodic Mesoporous Organosilica. Small 2011, 7, 1086−1097. (29) Gehring, J.; Schleheck, D.; Luka, M.; Polarz, S. AerosolSynthesis of Mesoporous Organosilica Nanoparticles with Highly Reactive, Superacidic Surfaces Comprising Sulfonic Acid Entities. Adv. Funct. Mater. 2014, 24, 1140−1150. (30) Suzuki, J.; Shimojima, A.; Fujimoto, Y.; Kuroda, K. Stable Silanetriols That Contain Tert-Alkoxy Groups: Versatile Precursors of Siloxane-Based Nanomaterials. Chem. - Eur. J. 2008, 14, 973−980. (31) Mikhailenko, S.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Solid Electrolyte Properties of Sulfonic Acid Functionalized Mesostructured Porous Silica. Microporous Mesoporous Mater. 2002, 52, 29−37. (32) Nakanishi, K. Sol-Gel Process of Oxides Accompanied by Phase Separation. Bull. Chem. Soc. Jpn. 2006, 79, 673−691. (33) Tölle, P.; Köhler, C.; Marschall, R.; Sharifi, M.; Wark, M.; Frauenheim, T. Proton Transport in Functionalised Additives for PEM Fuel Cells: Contributions from Atomistic Simulations. Chem. Soc. Rev. 2012, 41, 5143−5159. (34) Zhang, H.; Li, J.; Tang, H.; Lin, Y.; Pan, M. Hydrogen crossover through perfluorosulfonic acid membranes with variable side chains and its influence in fuel cell lifetime. Int. J. Hydrogen Energy 2014, 39, 15989−15995. (35) Grotthuss, C. J. T. Sur La Décomposition de L’eau et Des Corps Qu’elle Tient En Dissolution À L’aide de L’électricité Galvanique. Ann. Chim. 1806, 58, 54−73. (36) Kreuer, K.; Paddison, S. J.; Spohr, E.; Schuster, M. Transport in Proton Conductors for Fuel-Cell Applications: Simulations, Elementary Reactions, and Phenomenology. Chem. Rev. 2004, 104, 4637− 4678. (37) Ogawa, T.; Kamiguchi, K.; Tamaki, T.; Imai, H.; Yamaguchi, T. Differentiating Grotthuss Proton Conduction Mechanisms by Nuclear Magnetic Resonance Spectroscopic Analysis of Frozen Samples. Anal. Chem. 2014, 86, 9362−9366. (38) Telfah, A.; Majer, G.; Kreuer, K. D.; Schuster, M.; Maier, J. Formation and Mobility of Protonic Charge Carriers in Methyl Sulfonic Acid-Water Mixtures: A Model for Sulfonic Acid Based Ionomers at Low Degree of Hydration. Solid State Ionics 2010, 181, 461−465. (39) Marschall, R. New Solid Proton Conductors: Functionalized Mesoporous SiO2 Materials for Application in High Temperature PEM Fuel Cell Membranes Doktor Der Naturwissenschaften; Leibniz University Hannover, 2008. (40) Rolison, D. R. Catalytic Nanoarchitectures - The Importance of Nothing and the Unimportance of Periodicity. Science (Washington, DC, U. S.) 2003, 299, 1698−1701.

(6) Antonietti, M.; Berton, B.; Göltner, C.; Hentze, H.-P. Synthesis of Mesoporous Silica with Large Pores and Bimodal Pore Size Distribution by Templating of Polymer Latices. Adv. Mater. 1998, 10, 154−159. (7) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Hierarchically Ordered Oxides. Science 1998, 282, 2244−2246. (8) von der Lehr, M.; Weidmann, C.; Mascotto, S.; Smarsly, B. M. Hierarchically Ordered Monolithic Silica with Bimodal Porosity Obtained by Hydrolysis and Condensation of 1,4-Bis(trimethoxysilyl)arenes. Chem. Ing. Tech. 2013, 85, 1700−1706. (9) Stoeckel, D.; Kübel, C.; Hormann, K.; Höltzel, A.; Smarsly, B. M.; Tallarek, U. Morphological Analysis of Disordered MacroporousMesoporous Solids Based on Physical Reconstruction by Nanoscale Tomography. Langmuir 2014, 30, 9022−9027. (10) Stein, A. Advances in Microporous and Mesoporous Solids Highlights of Recent Progress. Adv. Mater. 2003, 15, 763−775. (11) El Kadib, A.; Chimenton, R.; Sachse, A.; Fajula, F.; Galarneau, A.; Coq, B. Functionalized Inorganic Monolithic Microreactors for High Productivity in Fine Chemicals Catalytic Synthesis. Angew. Chem., Int. Ed. 2009, 48, 4969−4972. (12) Hoffmann, F.; Cornelius, M.; Morell, J.; Fröba, M. Silica-Based Mesoporous Organic-Inorganic Hybrid Materials. Angew. Chem., Int. Ed. 2006, 45, 3216−3251. (13) Sharifi, M.; Marschall, R.; Wilhelm, M.; Wallacher, D.; Wark, M. Detection of Homogeneous Distribution of Functional Groups in Mesoporous Silica by Small Angle Neutron Scattering and in Situ Adsorption of Nitrogen or Water. Langmuir 2011, 27, 5516−5522. (14) Sharifi, M.; Wallacher, D.; Wark, M. Distribution of Functional Groups in Periodic Mesoporous Organosilica Materials Studied by Small-Angle Neutron Scattering with in Situ Adsorption of Nitrogen. Beilstein J. Nanotechnol. 2012, 3, 428−437. (15) Jin, Y. G.; Qiao, S. Z.; Xu, Z. P.; Yan, Z.; Huang, Y.; da Costa, J. C.; Lu, G. Q. Phosphonic Acid Functionalized Silicas for Intermediate Temperature Proton Conduction. J. Mater. Chem. 2009, 19, 2363− 2372. (16) Jin, Y. G.; Qiao, S. Z.; Xu, Z. P.; Costa, J. C. D.; Da Lu, G. Q. Porous Silica Nanospheres Functionalized with Phosphonic Acid as Intermediate-Temperature Proton Conductors. J. Phys. Chem. C 2009, 113, 3157−3163. (17) Marschall, R.; Bannat, I.; Caro, J.; Wark, M. Proton Conductivity of Sulfonic Acid Functionalised Mesoporous Materials. Microporous Mesoporous Mater. 2007, 99, 190−196. (18) Marschall, R.; Rathousky, J.; Wark, M. Ordered Functionalized Silica Materials with High Proton Conductivity. Chem. Mater. 2007, 19, 6401−6407. (19) Marschall, R.; Bannat, I.; Feldhoff, A.; Wang, L.; Lu, G. Q.; Wark, M. Nanoparticles of Mesoporous SO3H-Functionalized SiMCM-41 with Superior Proton Conductivity. Small 2009, 5, 854−859. (20) Marschall, R.; Tölle, P.; Cavalcanti, W. L.; Wilhelm, M.; Köhler, C.; Frauenheim, T.; Wark, M. Detailed Simulation and Characterization of Highly Proton Conducting Sulfonic Acid Functionalized Mesoporous Materials under Dry and Humidified Conditions. J. Phys. Chem. C 2009, 113, 19218−19227. (21) Wilhelm, M.; Jeske, M.; Marschall, R.; Cavalcanti, W. L.; Tölle, P.; Köhler, C.; Koch, D.; Frauenheim, T.; Grathwohl, G.; Caro, J.; Wark, M. New Proton Conducting Hybrid Membranes for HTPEMFC Systems Based on Polysiloxanes and SO3H-Functionalized Mesoporous Si-MCM-41 Particles. J. Membr. Sci. 2008, 316, 164−175. (22) Gomes, D.; Marschall, R.; Nunes, S. P.; Wark, M. Development of Polyoxadiazole Nanocomposites for High Temperature Polymer Electrolyte Membrane Fuel Cells. J. Membr. Sci. 2008, 322, 406−415. (23) Klaysom, C.; Marschall, R.; Wang, L.; Ladewig, B. P.; Lu, G. Q. M. Synthesis of Composite Ion-Exchange Membranes and Their Electrochemical Properties for Desalination Applications. J. Mater. Chem. 2010, 20, 4669. (24) Klaysom, C.; Marschall, R.; Moon, S.-H.; Ladewig, B. P.; Lu, G. Q. M.; Wang, L. Preparation of Porous Composite Ion-Exchange M


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