Protic ionic liquids: Fuel cell applications
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Introduction 1
Polymer electrolyte fuel cells (PEFCs) are an important class of fuel cells because their characteristics, such as high power density, low operation temperature, short startup time, small size, and light weight, make them promising candidates for use in domestic and vehicle power sources. However, their high production cost is the most serious bottleneck for wide application and commercialization. Therefore, the main goal of recent studies on PEFCs has been the reduction of cost.2–4 One solution is operating PEFCs under high temperatures (>100°C) without humidification. The expected benefits of such a system are that a water management system is not required, and high utilization of waste heat, easy thermal control by a radiator, reduction of CO poisoning, and improvement of activity of a Pt catalyst are realized.5 In a conventional humidified PEFC system, a polymer electrolyte membrane, such as Nafion, has sulfonic acid groups in its chemical structure. By absorbing water, the sulfonic acid groups can dissociate, and the resultant hydronium cations conduct protons.6 Therefore, conventional PEFCs should usually be operated at temperatures lower than 80°C with humidification to keep water inside the membranes. In order to develop non-humidified intermediate-temperature fuel cells, new anhydrous proton conductors should be explored as an alternative to aqueous systems. Ionic liquids (ILs) are molten salts that exhibit melting temperatures (Tm) at around room temperature (also see the
Introductory article in this issue).7–9 Typical ILs (e.g., 1-methyl-3ethylimidazolium bis(trifluoromethanesulfone)imide ([C2mim] [NTf2] or [C2mim][TFSI])) exhibit high thermal stability (>300°C), negligible vapor pressure, and nonflammability.10 Additionally, ILs can self-dissociate,11 thus exhibiting high ionic conductivity even in the absence of a molecular solvent, which may interfere with the advantages of ILs (e.g., thermal stability).12 These unique characteristics are desirable for electrolytes in electrochemical devices such as lithium batteries, electric double layer capacitors, and dye sensitized solar cells, since such devices should be safer and more stable (see the Navarra article in this issue).13–16 ILs can be categorized into two groups: Typical ILs, such as [C2mim][NTf2], are generally synthesized by the quaternization (alkylation) of the corresponding amine, followed by an anion exchange reaction (Scheme 1a). Such ILs have no exchangeable protons (i.e., no active protons) in their chemical structure and are thus called aprotic ionic liquids (AILs). On the other hand, ILs that are typically synthesized by the neutralization reactions of a Brønsted acid (proton donor) and a Brønsted base (proton acceptor) (Scheme 1b) have exchangeable (active) protons and are called protic ionic liquids (PILs).17 PILs generally show similar characteristics to AILs. However, the active protons induce hydrogen bonding in PILs, which allows for unique applications. The Angell group and other research groups have reported the preserva
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