Synthesis of amorphous silica and sulfonic acid functionalized silica used as reinforced phase for polymer electrolyte membrane

Figure 1 shows the preparation process of nano amorphous silica (A-SiO₂). First, 0.5 ml tetraethoxysilane (TEOS, Merck) was added in ethanol solution (20 ml) containing tetra-n-butylammonium bromide (TBAB, Merck) and polyvinylpyrrolidone (PVP, 40 000 M_w, Prolabo) dissolved as cationic and non-ionic surfactants, respectively. For the hydrolysis, 2.5 ml of 1 M solution of nitric acid (HNO₃) was added drop wise and then stirred vigorously at room temperature. The solution had been kept stirring for 1 h under acidic condition. After that, 10 ml of 1 M ammonium hydroxide (NH₄OH) was added into the solution to promote the assembly of the silica–surfactant complex. The resultant sol or gel was dried at 80 °C in air for 24 h and finally calcinated in air at 600 °C for 3 h to remove the organic compounds [2].

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The procedure of Sul-SiO₂ synthesis [3] is presented in figure 2. Firstly, 3-mercatopropyltrimethoxysilane (MPTMS 98%, Merck) was added to 2-propanol solution (99% Merck). Then, hydrogen peroxide (H₂O₂) was added to oxidize the thiol ($-\!\!\!-{\rm SH}$) groups of MPTMS to sulfonic acid groups, and the solution was stirred vigorously at 60 °C. After TEOS (98% Merck) was added, the mixture was stirred about 15 min to obtain homogeneous solution. The hydrolysis occurred when NH₄OH was added. The mixture was stirred for overnight at 60 °C to become a viscous and transparent solution which then was filtered, washed by hot water and ethanol, and dried at 60 °C in Buchi vacuum chamber for 5 h. The volume ratio of MPTMS:TEOS is expressed by (5 − x):x (x = 1, 2, 3, 4), and the products are coded in reference to n% of Sul-SiO₂ (n represents the volume percentage of MPTMS in the precursor solution.

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PVDF powder (average molecular weight of 534 000 by Gel Permeation Chromatography), purchased from Sigma Aldrich, was dissolved in dimethyl formamide (DMF, Merck) at room temperature using magnetic stirrer for 3 h until a transparent solution was obtained. In the other beaker amorphous SiO₂ was dissolved in DMF at room temperature (stirred for 3 h). Then the solution of amorphous SiO₂ in DMF was poured directly into the PVDF solution. After being treated by ultrasonic vibration for 15 min, the mixture was stirred at room temperature for 6 h. The membrane, achieved by casting the resultant solution on a glass substrate, was dried at 80 °C for 2 h. The flow chart of PVDF/SiO₂ composite film preparation was shown in the figure 3.

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The sample structure was characterized by Fourier transform infrared spectroscopy (FTIR, Bruker Equinox 55, 0.5 cm⁻¹ resolution) and powder x-ray diffraction (XRD, Bruker, D8-Advance x-ray diffractometer, Cu-Kα, λ = 1.5406 Å). The morphology and particle size were analyzed by field-emission scanning electron microscope (FESEM, JEOL, JSM-7401F), transmission electron microscope (TEM, JEOL, JEM-1400) and laser diffraction particle size distribution (DLS, Malvern Zetasizer Nano ZS). The thermal properties of hydride composite membranes were analyzed by differential scanning calorimetry (DSC) using Mettler-Toledo 822e DSC instrument. The specific surface area (SSA) was measured by nitrogen adsorption/desorption on Autosorb1C (Quantachrome Instruments).

Ion-exchange capacity (IEC) was defined as the numbers of sulfonic groups (in moles) in 1 g of dry sul-SiO₂. In this method, 100 mg of sul-SiO₂ sample were soaked in 25 ml of 1 M KCl aqueous solution and stirred vigorously at room temperature for 24 h. The released H⁺ was titrated with 5 × 10⁻³ M NaOH solution. The IEC was calculated by equation

$$\begin{equation} {\rm{IEC}} = \frac{{V_{{\rm{NaOH}}} N_{{\rm{NaOH}}} }}{{W_{{\rm{dry}}} }}\;, \end{equation} \tag{ 1 }$$

where V_NaOH, N_NaOH and W_dry are volume of NaOH, concentration of NaOH and weight of the solid powder, respectively.

The thickness of PVDF/SiO₂ membranes was measured by thickness gauge MITUTOYO, Japan (range 0–10 mm, resolution: 0.01 mm).

The FTIR spectrum of A-SiO₂ and Sul-SiO₂ is shown in figure 4. As we can see, the broad band near 3440 cm⁻¹ corresponds to the ${\rm O}-\!\!\!-{\rm H}$ stretching vibration of incomplete silanol group (${\rm Si}-\!\!\!-{\rm OH}$) condensation as well as remaining absorbed water. The vibration's peaks belonged to the SiO₂ groups are assigned to the asymmetric and symmetric stretching modes (observed at 1100 and 808 cm⁻¹) and the bending mode (at 466 cm⁻¹) as a weak band. These results provide an evidence of a successful condensation reaction between Si–OR groups [3].

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The Raman spectrum in figure 5 shows the intensive band at 1048 cm⁻¹ in sample of sul-SiO₂ 80%, this band was assigned to the symmetric mode of the sulfo functional (SO₃) group of the corresponding sulfonic acid [4].

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X-ray diffraction patterns of pure SiO₂ and Sul-SiO₂ 80% are shown in figure 6. Both XRD patterns have a broad peak between 17° and 30°, centered at 22.5°, typical for the amorphous silica [5].

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TEM image of dried amorphous SiO₂ was shown in figure 7(a). The distribution of average particle diameter in the range 8–23 nm is presented in figure 7(b). In addition, the particle size distribution curve (figure 7(c)) shows that there are three groups of particle size. The large particle size (268 nm) indicates that silica particles tend to agglomerate together during storage. It is well known that nanoparticles have low stability due to their high surface energy [6]. The formation of silica nanoparticles was achieved by the cooperative assembly of anionic silicate with a cationic surfactant and a non-ionic polymer. The tetra-n-butylammonium cations (TBA⁺) combined with anionic silicate through electrostatic interaction in the basic solution, which caused a decrease in their polarity. PVP, as a non-ionic surfactant, was used to protect the silica-TBA particles due to a weak interaction between ionic and non-ionic hydrophilic groups [7]. The synthesis procedure of silica nanoparticles cannot be used for silica modification with MPTMS because of the latter's high viscosity which would cause problems in filtration and removal of organic surfactants from the final product. Unlike the process of A-SiO₂, we cannot remove surfactants of Sul-SiO₂ by calcination at high temperature since the organic chains of modified silica, particularly, $-\!\!\!-{\rm CH}_2{\rm CH}_2{\rm CH}_2-\!\!\!-{\rm SH}$ or $-\!\!\!-{\rm CH}_2{\rm CH}_2{\rm CH}_2-\!\!\!-{\rm SO}_3{\rm H}$ chains, may be burnt.

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An SEM image (figure 8(a)) shows the morphology of Sul-SiO₂ powder. From TEM images of Sul-SiO₂ 80% (figures 8(b) and 8(c)), we can see that the average particle diameter of this sample was in the range of 20–30 nm. The Sul-SiO₂ particles are normally larger than A-SiO₂ because the former were synthesized without surfactants.

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With increasing MPTMS concentration from 20 to 80% in the original solution, the specific surface area of Sul-SiO₂ samples remained unchanged, around 300 m² g⁻¹. We observed that the specific surface area (SSA) of A-SiO₂ is 411.6 m² g⁻¹, higher than SSA of all the Sul-SiO₂ samples (figure 9). This may be explained by the presence of organic groups in the materials [8].

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The ion exchange capacity of the materials tends to increase with the increasing MPTMS content from 20 to 80% in the initial solution (figure 10), but the IEC of Sul-SiO₂ was reduced when no TEOS was added to the starting solution, which composed of MPTMS, 2-propanol and H₂O₂.

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As shown in figure 11 and table 1, five points were chosen for the thickness-gauge measurement, and the average membrane thickness was obtained by the mean value. The thickness of PVDF is about 60 μm (the commercial DuPont proton exchange membrane nafion is 50 μm of thickness).

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The melting point and the crystallization degree of the pristine PVDF and PVDF/SiO₂ composites were measured by DSC, performed at 10 °C min^–1 heating rate under nitrogen gas atmosphere. The crystallization (X_c) of PVDF and PVDF/SiO₂ composites was calculated from equation

$$\begin{equation} X_{\rm c} = \frac{{\Delta H_m }}{{W(\Delta H_m^0 )}} \times 100\% , \end{equation} \tag{ 2 }$$

where ΔH_m is the experimental heat of fusion and W is the PVDF content in the PVDF/SiO₂ composites. A value of 90.4 J g⁻¹ was used for Δ H_m⁰ of PVDF crystallized under large super-cooling conditions, which is essential for the formation of α crystals [9].

The crystalline degree of PVDF decreased from 32.3 to 23.7% with the addition of 3 wt% SiO₂, however, increases to 31.4% when amorphous SiO₂ content reaches 5%.

DSC thermograms for PVDF and PVDF/SiO₂ hybrid composites are shown in figure 12. The PVDF homopolymer has a melting temperature (T_m) of 161 °C, which does not change with the presence of dispersed SiO₂. The melting points for the hybrid composites were observed also at around 162 °C. Thus, the addition of SiO₂ particles up to 5 wt% inconsiderably affects the thermal properties of PVDF film.

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