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Molecules 2020, 25, 1712 11 of 44 polymers are considered to be one of the most promising routes to obtain high performance polymer electrolytes because of their availability, variety of chemical composition and stability in the cell environments. Specifically, poly ether ether ketone, polyvinyl alcohol, and poly arylene ether and their derivatives are currently under investigation. Poly(ether ether ketone)s (PEEKs) [101] are semicrystalline polymers that present high thermal and chemical stability. The sulfonation of PEEK produces copolymers with sulfonic acids into the aromatic backbone; membranes made of these sulfonated polymers show useful properties for DMFCs, such as; low methanol cross-over, good ion conductivity, thermal stability and high mechanical strength [102]. The proton conductivity of SPEEK depends on the sulfonation degree [103], it generally increases with the sulfonation degree but high sulfonation results in high methanol permeability so that its application is limited [104]. The development of SPEEK composite membranes is currently being investigated by using SPEEK for the polymer matrix and modifying it in order to reduce methanol permeability at high sulfonation degree. Many researches have focused on SPEEK-based membrane with phenoxy resin [105], polyphenylsulfone (PPSU) [106], solid heteropolyacids [107], polyaniline [108], SiO2 and zirconium phosphate (ZrP) [109], zeolite [110], polypyrrole [111]. As discussed above, the utilization of montmorillonite and polybenzimidazole into Nafion matrix improved the DMFC performance; in the same way, they can be used to modify the matrix of SPEEK polymer. In fact, Gosalawit et al. [112] used a SPEEK/MMT membrane in their work and compared its performance in a DMFC single cell with pristine SPEEK membrane and Nafion bare membrane. The study confirmed that the performance was higher: current density generated from the MEAs of Nafion 117, SPEEK, SPEEK/MMT 1wt.%, SPEEK/MMT 3wt.% and SPEEK/MMT 5 wt.% membranes at the constant voltage of 0.2 V were 51, 76, 103, 96 and 94 mAcm−2, respectively with the maximum power density of 10, 15, 21, 19 and 18 mWcm−2. However, the thermal stability was significantly reduced. Pasupathi et al. [113] synthetized a non-perfluorinated membrane by casting SPEEK and PBI solution into a glass plate. A SPEEK/PBI membrane enhanced DMFC performance: the maximum power density obtained (45 mW cm2) was two times higher than Nafion 117 at 60 ◦C. Moreover, SPEEK/PBI membranes were found to be extremely stable under DMFC operating conditions up to 60 ◦C. However, their stability dropped considerably at higher temperatures. Experiments are underway to address the stability issue of these membranes at higher temperatures. Generally, sulfonated aromatic polymer membranes require a high sulfonation level to achieve sufficient proton conductivity resulting from the low acidity of the sulfonic groups in the aromatic rings [114]. However, such a high sulfonation level usually makes them excessively swell and even soluble in methanol/water solution which may lead to a loss in mechanical properties and become unavailable in applications [115]. Therefore, they should be modified including organic or inorganic fillers. Jiang et al. [116] investigated the performance of a SPEEK/GO membrane in which GO is sulfonated (SGO) to improve the proton conductivity of the membrane. In fact, the SPEEK/GO membranes exhibit lower ion exchange capacity (IEC) and water uptake than Nafion membrane, and the overall proton conductivity of the membranes remains low. This is due to the lack of proton conductive groups on the pristine GO. Therefore, these membranes are still not quite suitable for use as PEMs in DMFCs. However, by using higher contents of sulfonated GO, these SPEEK/SGO membranes showed even higher IEC and proton conductivity compared to Nafion 112, which makes them particularly attractive as PEMs for the DMFC applications. It is worth noting however that the swelling ratio of membranes increased with the increase of the content of the SGO. DMFCs with SPEEK/SGO showed better performance than those with the plain SPEEK or the pristine SPEEK/GO. With the optimized contents of SGO in SPEEK (3% and 5% wt.), the DMFCs exhibited 38 and 17%, respectively, higher performance than those with Nafion 112 and Nafion 115. Despite of having higher IECs and proton conductivity, the membranes with higher content of the SGO exhibited higher methanol permeability leading to a decrease of the fuel utility and the lifetime of the cathode catalysts, thus low performance. In this regard, for practical applications of the SPEEK/SGO membranes, the contents of SGO in the SPEEK matrices should be well controlled.PDF Image | Composite Polymers for Electrolyte Membrane Technologies
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