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Molecules 2020, 25, 1712 4 of 44 increase the efficiency and reduce the energy consumption and costs, many researchers have dedicated their work to the development of alternative low cost materials, and efficiency increase and energy reduction [24]. Although the main challenging issue of the PEMWE technology is the enhancement of oxygen evolution rate, the improvement of membranes with reduced cost, appropriate conductivity, Mloowlecuglaes2c0r2o0s, 2s5-o, xverandenhancedmechanicalpropertiesathighoperatingpressureismandatory4 o[2f544]. 1.1.4. A Short Review on PFSAs (Nafion Polymers) 1.1.4. A Short Review on PFSAs (Nafion Polymers) Nafion is a fluoropolymer made by sulfonated polytetrafluorethylene introduced by DuPont in Nafion is a fluoropolymer made by sulfonated polytetrafluorethylene introduced by DuPont in the mid-1960s. It is commercially available with a thickness between 25 and 250 μm. As for Teflon, its the mid-1960s. It is commercially available with a thickness between 25 and 250 μm. As for Teflon, structure (Figure 1) consists of CF (difluorocarbene) radicals, alternated to CFOCF and with end its structure (Figure 1) consists of C2F2 (difluorocarbene) radicals, alternated to CFOCF22 and with end chains of sulfonic acid SO OH. chains of sulfonic acid SO2OH. Figure 1. Nafion polymer structure; reproduced with permission from [26]. Figure 1. Nafion polymer structure; reproduced with permission from [26]. Main Naffiion characteristics are the following: • High proton conductivity: when the pendant sulfonyl fluoride groups (SO2F) are • High proton conductivity: when the pendant sulfonyl fluoride groups (SO2F) are chemically chemically converted to sulfonic acid (SO3H), the ionic functionality is introduced [27]; converted to sulfonic acid (SO3H), the ionic functionality is introduced [27]; • Water permeability; • Water permeability; • High chemical resistance: only alkaline metals as sodium can damage the Nafion at STP • High chemical resistance: only alkaline metals as sodium can damage the Nafion at STP (standard (standard temperature and pressure); and temperature and pressure); and • High electronic resistance: forcing electrons to flow through the external circuit to generate • High electronic resistance: forcing electrons to flow through the external circuit to generate electrical power. electrical power. The most important parameter to control, in order to keep the ion conductivity high is the relative humidity. Nafion membrane requires water to facilitate the transport of protons through The most important parameter to control, in order to keep the ion conductivity high is the relative either the Grotthus (hopping) mechanism or the vehicle (diffusion) mechanism [28]. This humidity. Nafion membrane requires water to facilitate the transport of protons through either the requirement has led to limiting PEMFC operating temperature to 80 °C as operation above that will Grotthus (hopping) mechanism or the vehicle (diffusion) mechanism [28]. This requirement has led lead to dehydration of the membrane and a su◦bsequent loss in proton conductivity and fuel cell to limiting PEMFC operating temperature to 80 C as operation above that will lead to dehydration performance. However, PEMFC operating above 80 °C can benefit from increased reaction kinetics, of the membrane and a subsequent loss in proton conductivity and fuel cell performance. However, reduced CO poisoning and s◦implified water and thermal management [29]. One method utilised to PEMFC operating above 80 C can benefit from increased reaction kinetics, reduced CO poisoning allow that transition into intermediate temperatures is the use of filler materials within the polymer and simplified water and thermal management [29]. One method utilised to allow that transition into [30]. Alternatively, there are polybenzimidazole membranes that are doped with phosphoric acids intermediate temperatures is the use of filler materials within the polymer [30]. Alternatively, there that operate at even higher temperatures [31]. are polybenzimidazole membranes that are doped with phosphoric acids that operate at even higher Regarding the use in DMFC, Nafion has high ionic conductivity and chemical stability but high temperatures [31]. methanol permeability due to: Regarding the use in DMFC, Nafion has high ionic conductivity and chemical stability but high 1. active transport of protons and water; methanol permeability due to: 2. diffusion through the water-filled pores within the Nafion-structure; 1. active transport of protons and water; 3. diffusion through the Nafion itself. 2. Mdieffthuasinoonl thcrosusgohvethr eiswatner-isfisluleed tphoarteslewaditshintothteheNamfiuocnh-stwruocrtsuereD; MFC performance when c3o. mpdariffeduswioinththHro2 PuEghMtFhCesN. Vafiaorinouitsealpf.proaches to minimize or eliminate methanol crossover have been carried out: increasing membrane thickness [32], increasing the cathode reactant pressure [33], Methanol crossover is an issue that leads to the much worse DMFC performance when compared decreasing both cell temperature [34] and methanol concentration [35]. Another innovative way is to with H2 PEMFCs. Various approaches to minimize or eliminate methanol crossover have been carried modify the membrane using materials that allow obtaining the same chemical and thermal out: increasing membrane thickness [32], increasing the cathode reactant pressure [33], decreasing characteristics of Nafion but with lower crossover and cost. both cell temperature [34] and methanol concentration [35]. Another innovative way is to modify Also, in PEMWE, the commercial Nafion membrane is commonly used as a solid electrolyte the membrane using materials that allow obtaining the same chemical and thermal characteristics of due to its chemical and thermal stability, good proton conductivity, and mechanical strength [36]. A Nafion but with lower crossover and cost. drawback of membranes made of Nafion is that they are known to lose water, and thus ionic conductivity, at temperatures above 100°C [37], which prohibits them from being used for higher temperature water electrolysis. However, high operating temperature offers several advantages from enhanced electrode kinetics and reduced overpotentials [38]. Moreover, higher operation pressure would also be favourable for PEM electrolysis since it would reduce the gas pressurization constraints for storage purposes [39]. Hence, it would be important to develop membranes that canPDF Image | Composite Polymers for Electrolyte Membrane Technologies
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