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Molecules 2020, 25, 1712 12 of 44 Sulfonated Poly (arylene ether sulfone) membranes (SPAES can be useful for methanol fuel cell through the modification of the polymer matrix by introducing inorganic/organic particles such as laponite. Laponite is made of silica tetrahedral and alumina octahedral sheets which have advantageous hygroscopic properties [117]. This inorganic compound was used by Kim et al. [118] to prepare and evaluate the behaviour of the SPAES membrane. Properties of the hybrid membranes for DMFC such as methanol permeability, and proton conductivity were investigated. Authors claimed that methanol permeability was lower than that of a SPAES pure membrane and Nafion using membranes with a small content of Laponite. This is because the presence of Laponite improved the barrier property of the membrane to methanol molecules. This was likely due to the tortuosity of layered silicate and the lower aspect ratio of the particles resulting from their exfoliation increasing methanol diffusion paths through the composite membrane. However, the proton conductivity was very low and further research should be conducted to optimise Laponite loading. Another approach to reduce the undesired swelling property and methanol crossover of sulfonated membrane is to crosslink membranes. This method has been widely investigated by many researchers for crosslinking SPEEK membranes [119,120]. These membranes showed decreased swelling ratio and methanol crossover but decreasing proton conductivity. Following this method, Feng et al. [121] used sufonated poly (arylene ether)s as PEM materials due to their good thermal stability, high glass transistion and excellent mechanical strength. They synthesized sulfonated poly (arylene ether sulfone) copolymers with propenyl groups then crosslinked using benzoyl peroxide varying the crosslinking. Proton conductivity and methanol permeability were evaluated and compared with Nafion 117, and showed that: • the proton conductivity of the SPAES membranes increased from 0.1 to 0.16 S/cm with the increase of temperature from 30 to 70 ◦C which was quite similar to that of Nafion 117: 0.11 to 0.17 S/cm from 30 to 70 ◦C [122]; • the methanol permeability evaluated was lower than that of Nafion117 (2.07 × 10−6 cm2/s): the less pronounced hydrophobic/hydrophilic separation of sulfonated polyaryls compared to Nafion corresponded to narrower, less connected hydrophilic channels and larger separations of the less acidic sulfonic acid functional groups, which affected the permeability of methanol [123]. Poly vinyl alcohol (PVA) is usually synthesized from poly vinyl acetate and commonly used for adhesive, food wrapping, and desalination and pervaporation membranes [124]. Regarding the possible use of a membrane made of PVA in DMFC, it is known that PVA does not have any negative charged ions, like carboxylic and sulfonic acid groups, so the conductivity is very low as compared with Nafion membrane. Therefore, PVA membranes can be used in a fuel cell if negative ions are incorporated within their structure to increase their conductivity [125]. Moreover, several studies have shown how PVA polymer membrane leads to a reduced methanol crossover [126]. Regarding the reduction of methanol crossover, it was demonstrated that the addition of filler into PVA matrix contributes to mitigate this issue: fillers such as SiO2 [127], polyrataxane [128] were reported. Yang et al. [129] used montmorillonite (MMT) as a filler and tested ion conductivity, methanol permeability, current density-potential and power density curves of the PVA/MMT composite polymer showing the following properties: • high ionic conductivity: 0.0368 S cm− 1 , performed by PVA/10wt. % MMT at 30 ◦ C; • methanol permeability: 3–4 × 10− 6 cm2 s− 1 , which was lower than that of Nafion 117 membrane of 5.8 × 10−6 cm2 s−1; • maximum peak power density: 6.77 mW cm−2 at ambient pressure and temperature with the PtRu anode based on Ti-mesh in a 2M H2SO4 + 2M CH3OH solution. 2.3. Other Composite Non-Fluorinated Membranes The modified polyvinylidene fluoride (PVDF) membranes using inorganic additives were prepared with a view of combining the properties of inorganic ion exchanger (high thermal stability, andPDF Image | Composite Polymers for Electrolyte Membrane Technologies
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