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Molecules 2020, 25, 1712 8 of 44 Some studies have been carried out to investigate experimentally the effect of Nafion membranes with metal oxides, such as; SiO2, TiO2, WO3, as fillers on the performance of DMFCs [80]. As a result of these experiments, the Nafion-modified composite membranes provided higher power density in comparison to the commercial Nafion 115 membrane. Regarding the application in a DMFC, Nafion/TiO2 improved water uptake and reduced methanol absorbance [81] while Nafion/SiO2 showed lower methanol permeability than commercial membrane [82]. Moreover, Nafion membrane modified with both SiO2 and TiO2 were prepared by solvent casting method and studied by Ercelik et al. [83] that investigated the effect of these particles on proton conductivity, water uptake and performance varying temperature. The authors claimed that: • proton conductivity of Nafion-TiO2/SiO2 increased with temperature. The maximum value obtained was 0.255 S cm−1 which is 25% higher than Nafion 117 at 75 ◦C; • water uptake values are similar with those of bare Nafion; and • power densities of MEAs with composite membranes are higher than the MEAs using Nafion 115: at 80 ◦C and 1M of methanol concentration. The maximum power densities obtained by Nafion/TiO2, Nafion/SiO2 and by the MEA with bare Nafion are 641.16 W/m2, 628.68 W/m2 and 612.96 W/m2, respectively. The abovementioned studies demonstrated that the incorporation of inorganic particles like SiO2 and TiO2 provided better performance if compared with the Nafion membrane. Although, the use of metal oxides as filler has enabled many advantages, they too have some problems associated with them. The metal particles are often very difficult to disperse homogenously in the polymer membrane, it would mean that the performance of the composite will not be uniform throughout the bulk of the membrane. Moreover, metal oxides accelerate the degradation of membranes, and so durability studies are required to understand the actual benefits and drawbacks of these fillers. Montmorillite (MMT) is a filler that has attracted much attention recently as Nafion/MMT membranes have been reported to have improved mechanical and thermal properties compared to pristine Nafion membranes [84]. But, the incorporation of this filler into the Nafion matrix does not improve the proton conductivity. Wu et al. [85] prepared the composite membrane via casting solution and reported a slight decrease (about 9%) in proton conductivity compared with pristine Nafion but the methanol crossover decreased more than 90% by loading MMT of only 1% wt [86]. As described above, the utilization of this filler does not contribute to improve the proton conductivity, thus, to minimize the loss in performance Rhee et al. [87] and Lin et al. [88] modified the montmorillonite with an organic sulfonic acid group (MMT-SO3H) with varying the content of the filler. Their studies showed that the proton conductivity of the composite membranes generally declined from that of pristine Nafion membrane with the increase in the inorganic filler content, but the methanol permeability was reduced by up to 90%. The combination of these effects led to an improvement in the performance of a DMFC. In fact, the polarization curve of the MEA with Nafion 115 and composite membrane, realized at 40 ◦C and 2M of methanol concentration, showed that the performance of the DMFC improves initially with increasing the inorganic content, with a maximum power density at 5 wt. % loading. Curves revealed that all composite membranes achieved better performance than Nafion membrane at high current density region. However, the thermal stability of the membrane is not yet adequate and performance at higher temperature and methanol concentration deserves to be investigated. To hinder permeation of methanol, another approach is to develop composite membranes using zeolites. Zeolites are micro porous crystalline materials containing silicon, aluminium and oxygen in their framework. They are based on an infinitely extending three-dimensional network of AlO4 and SiO4 tetrahedra linked by sharing oxygen atoms [89,90]. The chemical structural formula of a zeolite may be expressed by the following [91]: Mx/n[(AlO2)x(SiO2)y]m H2O (10)PDF Image | Composite Polymers for Electrolyte Membrane Technologies
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