Single ionic polymer electrolyte

Abstract The chapter begins by discussing the characters and composition of polymer electrolytes for electrochromic devices. It then describes the four types of the polymer electrolytes dry solid polymer electrolyte, gel polymer electrolyte, porous gel polymer electrolyte and composite solid polymer electrolyte, their preparation procedures and properties especially ion conductivity of the samples. Finally, new types of polymer electrolytes including proton-conducting, alkaline, single ionic polymer electrolytes and electrolytes with ionic liquids are also introduced. 

Single ionic polymer electrolyte based on 2-acrylamide-2-methy 1-1-propane sulphonic acid (AMPS) copolymer... 

Various single ion polymer electrolyte backbone structures are possible. The fixed anion is commonly carboxylate, sulfonate, or borate. Some structures are shown in Figure 10.34. The ionic conductivity of these structures can be as high as 7 x 10 S/cm. 

Due to the reduced number of charge carriers, Lh ions and anions have a high capability to form ion pairs. Therefore, the ionic conductivity of a single ion polymer electrolyte is lower than that of double ion polymer electrolytes. To facilitate dissociation and increase the ionic conductivity, major future improvements are expected to lie in the following directions ... 

Polyphosphazenes bearing crown ethers (12-crown-4,15-crown-5 and 18-crown-6) as single or as mixed substituents with trifluoroethoxy or methoxy-ethoxyethoxy groups were synthesized by Cowie [601,602] and Allcock [484] and their conductivity studied because it was shown that the incorporation of crown ether molecules into a polymer electrolyte could increase their ionic conductivity. In these macromolecules, the crown ether units were linked to the backbone through oxymethylene spacer groups. 

In addition to the modified electrodes described in the previous sections, which usually involve a conductive substrate and a single film of modifying material, more complicated structures have been described. Typical examples (Figure 14.2.4) include multiple films of different polymers (e.g., bilayer structures), metal films formed on the polymer layer (sandwich structures), multiple conductive substrates under the polymer film (electrode arrays), intermixed films of ionic and electronic conductor (biconductive layers), and polymer layers with porous metal or minigrid supports (solid polymer electrolyte or ion-gate structures) (6,7). These often show different electrochemical properties than the simpler modified electrodes and may be useful in applications such as switches, amplifiers, and sensors. 

Single lithium ion conducting polymer electrolytes have been prepared by the copolymerizahon of hthium(4-styrenesulfonyl)(trifluoromethanesulfonyl) imide and methoxy-poly(ethylene glycol) acrylate [117]. The highest ionic conduchvihes for the copolymer electrolytes are 7.6 x 10 S cm at 25 °C and then grow to reach 10 " S cm at 60 °C, at a raho of ethylene oxide to Li+ of 20.5. 

The aforementioned polymeric electrolytes have been effectively used in polymer electrolyte fuel cells operating up to In order to study the single cell performance and apart from the high ionic conductivity of the membrane, several parameters residing the MEA constmction must be taken into account in order to have optimum performance of the cell. Some of these parameters are the amount of the catalyst the ionomer-binder used at the electrodes and its percentage, electrode surface and the preparation method, pressure and the temperature of the MEA assembling and design and constmction parameters of the cell. ... 

Gel and solid polymer electrolytes aim to combine the function of the electrolyte and separator into a single component to reduce the number of parts in an ES and increase the potential window through the higher stability offered by a polymer matrix. A gel electrolyte incorporates a liquid electrolyte into a microporous polymer matrix that holds in the liquid electrolyte through capillary forces, creating a solid polymer film. The chosen separator must be insoluble in the desired electrolyte and provide adequate ionic conductivity. Non-polar rigid polymers such as PTFE, PVA, PVdF, and cellulose acetate offer good ion conductivity when used as gel electrolytes [114]. Based on the data in Table 4.9, the ionic conductivity of EtMeIm+Bp4 is 14 mS.cm". Ionic conductivity of the same imidazolium salt used as a gel electrolyte in a PVdF matrix retains 5 mS.cm [115]. 

Li, W., Xing, Y., Xing, X., Li, Y, Yang, G., and Xu, L. (2013) PVDF-based composite microporous gel polymer electrolytes containing a novel single ionic conductor Si02 (Li" ). Electrochim. Acta, 112, 183-190. 

In this area of solid polymer electrolytes, polybenzimidazole (FBI) doped with KOH has to be mentioned, as it (Fig. 5.5) is known for its high thermal and chemical stability, even if it is not water soluble, which is desired for ion-solvating polymers [61-65]. Ionic conductivities between 5 X 10 and 1X 10 S/cm were obtained for FBI using KOH with a concentration of 6 M at 70-90°C [65]. These membranes were assembled with commercial electrodes from E-Teck and the resulting MEA was then incorporated into a single cell. The performance in a H2/O2 fuel cell test was reported and a current of 620x 10 A/cm at 0.6 V was obtained. However, measurements were performed at 50 °C under pressurized H2 and O2, which results in a significant improvement in performance compared with the use of nonpressurized fuels [65]. 

Blending a polymer electrolyte with a polymer matrix that is beneficial to ionic conduction. For example, a single ion conductor mixture is prepared by blending a high molecular weight PEO with a polymer electrol5de, poly(lithium 2-(4-carboxyl hexafluorobutyl acyloxy) ethyl methacrylate). However, the ionic conductivity is low, 4 x 10 S/cm at 30°C. 

A single-ion conductor containing a PEO unit can also be prepared for gel polymer electrolytes. For example, the lithium salt prepared as shown in Figure 11.3 is plasticized with EC or PC solvent, though the LP-ion transference number decreases with the amount of solvent added. When the number of EO units is greater than three, it behaves as a single-ion conductor and follows the VTF equation. The ionic conductivity remains relatively constant with varying numbers of EO units. 

FIGURE 7.7 Structure of mixed-substituent phenoxy/oligoethyleneoxy polyphosphazenes and single-substituent aryloxy polyphosphazenes with oligoethyleneoxy units attached to the aromatic rings in the para position. (Reprinted from Solid State Ionics, 156, Allcock, H.R. and Kellam, E.C., The synthesis and applications of novel aryloxy/oligoethyleneoxy substituted polyphosphazenes as solid polymer electrolytes, 401-414, Copyright 2003, with permission from Elsevier.)... 

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