Polymer electrolyte diffusion measurements

Table 18.2 gives an indication of the considerable variation and disagreement concerning the measured values of transference numbers in polymer electrolytes. Diffusion techniques such as PPG NMR measure the flux of both charged and electrically neutral species. If a high concentration of mobile ion pairs or higher neutral... 

Skotheim et al. [286, 357, 362] have performed in situ electrochemistry and XPS measurements using a solid polymer electrolyte (based on poly (ethylene oxide) (PEO) [363]), which provides a large window of electrochemical stability and overcomes many of the problems associated with UHV electrochemistrty. The use of PEO as an electrolyte has also been investigated by Prosperi et al. [364] who found slow diffusion of the dopant at room temperature as would be expected, and Watanabe et al. have also produced polypyrrole/solid polymer electrolyte composites [365], The electrochemistry of chemically prepared polypyrrole powders has also been investigated using carbon paste electrodes [356, 366] with similar results to those found for electrochemically-prepared material. 

H. Yamada, T. Hatanaka, H. Murata, and Y. Morimoto. Measurement of flooding in gas diffusion layers of polymer electrolyte fuel cells with conventional flow field. Journal of the Electrochemical Society 153 (2006) A1748-A1754. 

Electrochemical gas detection instruments have been developed which use a hydrated solid polymer electrolyte sensor cell to measure the concentration of specific gases, such as CO, in ambient air. These instruments are a spin-off of GE aerospace fuel cell technology. Since no liquid electrolyte is used, time-related problems associated with liquid electrolytes such as corrosion or containment are avoided. This paper describes the technical characteristics of the hydrated SPE cell as well as recent developments made to further improve the performance and extend the scope of applications. These recent advances include development of NO and NO2 sensor cells, and cells in which the air sample is transported by diffusion rather than a pump mechanism. 

Hayamizu and Akiba measured self-diffusion coefficients of lithium ion, anion and solvent in the electrolytes for lithium batteries. The self-association of some anions is discussed. Sekhon et u/. investigated the diffusive motion of cations and anions in polyethylene oxide based polymer electrolytes. Translational motion was found above Tg. 

An ionically conductive polymer is applied to the lithium battery, the fuel cell, etc. These polymer electrolytes have a fairly high ionic conductivity. However, general electrochemical measurement could not be performed in such polymer electrolytes. Electrochemical measmement in such polymer electrolytes has been possible only by special electrode systems described earlier. The reason is that the diffusion of ions or redox molecules and the rate of electron transfer are slow in these polymer electrolytes. 

The measurement of diffusion coefficients is particularly important in polymer electrolyte research. One of the desired properties of a potential electrolyte material is the selective mobility of the LE cation. Numerous strategies have been implemented to attain this property [1, 44, 45]. One method of quantifying the selective mobility is the calculation of transference numbers, which is the fraction of the total electric current that the anions and cations carry while passing through an electrolyte. Using the notation of Kalita et al. [46], the transference number for lithium, f, could be expressed as. 

Hayamizu K, AUiara Y, Price W (2000) Correlating the NMR self-diffusion and relaxatirai measurements with ionic conductivity in polymer electrolytes composed of cross-linked poly (ethylene oxide-propylene oxide) doped with LiN(S02CF3)(2). J Chem Phys 113(11) 4785 793... 

The establishment of such interfacial potentials is readily envisaged for cases where the net transport of an electrolyte is prevented because one of its constituents cannot partition. What is perhaps less obvious is that such potentials arise continually within solution phases, even where there is no physical separation into distinct phases. These so-called liquid junction potentials or diffusion potentials play an important role in electrochemical experiments, but because there is no well-defined phase boundary, they are intrinsically more difficult to measure. This chapter discusses how these potentials arise, how they may be calculated, what quantities are associated with them, and how they may be minimised. Finally, interfaces between electrolytes (i.e. those interfaces between immiscible electrolyte solutions (ITIES)) and the application of some of the concepts developed earlier in the chapter to non-standard electrolyte systems, such as polymer electrolytes and room-temperature ionic liquids, will be discussed. 

Various permeable membranes were apphed to decrease the rate of electrolyte diffusion under steady state [67], in particular a collodion membrane first proposed in ref. [68] became rather popular. In contrast to ion-selective membranes, no Donnan potential is established when collodion membrane is used. These membranes are introduced to stabilize the system for long-term operation, not to affect UP as is. Many inert viscous substances and porous inert ceramics can serve for the same purpose, and this approach is widely used in modem technologies of miniaturized reference electrodes for applications. Various types of diaphragms (including polymer and ceramics) are used in industrial pH measurements as well [69]. 

Solid polymer electrolytes for lithium batteries applications are commonly prepared by dissolving a lithium salt in poly(ethylene oxide) (PEO)-based materials. Chiappone et al. investigated these systems by a Li and NMR study yielding local dynamics and mass transport by temperature-dependent Ti and PFG-NMR diffusion measurements. 

Gel nanocomposite polymer electrolytes based on PVDF-HFP were studied by NMR. In this type of electrolyte, BaTiOs and clay were used as nanoparticles and PC + LiCFsSOs as the liquid electrolyte. The filled gel electrolytes presented better mechanical properties than the gels without the filler. The fillers lowered the conductivity by a small amount. The diffusion coefficient measured by NMR indicated that both anion and cation mobilities were reduced by the presence of the filler, but the effect was generally greater for the anions. 

Another use of NMR technique in polymer electrolyte research is the measurement of self-diffusion coefficients. Here the possibility of measurements of different nuclei in one sample is fully used, and the diffusion coefficients of cationic and anionic species and the polymer chain can be obtained, together with the diffusion constant of the sometimes present additives, such as fillers or plasticizers. 

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