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Molecules 2020, 25, 1712 6 of 44 a wider current density range achieving a maximum power output of 87.5 mW cm−2, 1.3 times higher than Nafion 117. This positive effect of the Nafion/PTFE membrane was also obtained by Nouel et al. [48] and Yu et al. [49] who tested a fuel cell MEA made of Nafion/PTFE comparing results with Nafion 117, 115 and 112. The performance was higher than Nafion 117 and 115 but similar to 112. In an attempt to further enhance the performance of Nafion/PTFE membrane, Chen et al. [50] included zirconium phosphate (ZrP) into the membrane structure and so the Nafion matrix was modified with both PTFE and ZrP/PTFE for comparison. The composite membranes were prepared via two processes: 1. By impregnating PTFE directly in a Nafion/ZROCL2 solution and then annealing it at high temperature; 2. By impregnating the PTFE membrane in a Nafion solution, annealing at high temperature to prepare Nafion/PTFE membrane, then impregnating again in a ZrOCL2 solution. Experimental results indicated that the introduction of ZrP led to reduced methanol crossover and proton conductivity. The impact of proton conductivity is stronger than methanol crossover on DMFC performance, thus, as confirmed by tests conducted on the cell, the performance of ZrP/PTFE was lower than Nafion/PTFE. Most research is focused on the preparation and modification of various proton conductive membranes that are inexpensive and provide better performance and properties than Nafion membranes. To this end, innovative organic materials, which have good thermal and chemical stability and can be easily modified to be used as ionic conductive membranes such as polybenzimidazole (PBI) and polyvinyl alcohols (PVA), were studied [51,52]. Shao et al. [53] and Mollà et al. [54] fabricated Nafion/PVA membranes using casting [55] and impregnation method [56,57] respectively. PVA has higher affinity for water than to methanol (i.e., 55 wt. % and 10 wt. %, respectively), so it can be potentially used for DMFC applications. Both works demonstrated that comparable DMFC performance can be obtained using these membranes. Specifically, Mollà et al. focused on the characterization of Nafion/PVA membranes with varying operating temperature (45, 70 ◦C), thickness of the membrane (19–47 μm) and concentration of methanol (1–2M). The performance of pristine Nafion membrane and Nafion/PVA were roughly equivalent at very low thickness while Nafion/PVA exceeded the pristine Nafion performance only at higher thickness and higher temperature. At any fixed condition; thickness, temperature and methanol concentration, the OCV of Nafion/PVA was higher than pristine Nafion indicating reduced methanol crossover. Hobson et al. [58] presented Nafion-PBI dipped and screen-printed films to investigate the effect on membrane performance. They concluded that the modification of Nafion with PBI by both spin coating and dipping reduced the methanol permeability; however, the benefit of low methanol crossover was counterbalanced by the negative effect of the too high impedance. Since neither of the techniques produced a suitable membrane for DMFC, screen printing was investigated and here methanol permeability was reduced without an increase in impedance. The membranes were then tested in a single cell at 60 ◦C. Using methanol solution of 3.2 M, the cell performance was greatly improved with the current density increased by 42% combined with an increase in maximum power output by 46% as compared with the pristine Nafion membrane. Ainla et al. [59] work on Nafion-PBI membrane was in agreement with the above results. In fact, they demonstrated that the Nafion-PBI membrane has lower methanol permeability and higher conductivity than a commercial membrane. It is important to note that the utilization of these composite membranes led to lower methanol permeability and enhanced the performance only at high methanol concentration. Conductive Polymers such as polyaniline (PANI) and polypyrrole (PPy) have recently been incorporated into Nafion membranes to reduce its methanol permeability [60,61]. Composite Nafion polypyrrole membranes were prepared by two methods: electrodeposition of polypyrrole on Nafion-coated electrodes [62] or by in situ polymerization with a chemical oxidant [63]. Zhu et al. [64] made a membrane by in situ polymerization using Fe(III) and H2 O2 as oxidising agents. The electrostaticPDF Image | Composite Polymers for Electrolyte Membrane Technologies
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