Interface polymer/electrolyte, containing

Subianto, S. Mistry, M. K Choudhuty, N. R. Dutta, N. K. Knott, R. (2009). Composite Polymer Electrolyte Containing Ionic Liquid and Functionalized Polyhedral Oligomeric Silsesquioxanes for Anhydrous PEM Applications ACS Appl. Mater. Interfaces, 1,1173- 1182, ISSN 1944-8244. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

By comparing impedance results for polypyrrole in electrolyte-polymer-electrolyte and electrode-polymer-electrolyte systems, Des-louis et alm have shown that the charge-transfer resistance in the latter case can contain contributions from both interfaces. Charge-transfer resistances at the polymer/electrode interface were about five times higher than those at the polymer/solution interface. Thus the assignments made by Albery and Mount,203 and by Ren and Pickup145 are supported, with the caveat that only the primary source of the high-frequency semicircle was identified. Contributions from the polymer/solution interface, and possibly from the bulk, are probably responsible for the deviations from the theoretical expressions/45... 

Numerous efforfs have been made to improve existing fhin-film catalysts in order to prepare a CL with low Pt loading and high Pt utilization without sacrificing electiode performance. In fhin-film CL fabrication, fhe most common method is to prepare catalyst ink by mixing the Pt/C agglomerates with a solubilized polymer electrolyte such as Nation ionomer and then to apply this ink on a porous support or membrane using various methods. In this case, the CL always contains some inactive catalyst sites not available for fuel cell reactions because the electrochemical reaction is located only at the interface between the polymer electrolyte and the Pt catalyst where there is reactant access. 

An interesting result obtained recently shows that the substrate employed may dictate the surface (polymer/solution) properties of even relatively thick polymers.39 For example, with polymers grown from dodecyl sulfate (DS)-containing electrolytes, the nature of the substrate used dictated the hydrophobicity/hydrophilicity of the conducting polymer-solution interface. Polymers grown on a carbon-foil substrate shown to be hydrophobic produced a more hydrophilic polymer surface. Finally, those grown on platinum, a more hydrophilic substrate, produce a more hydrophobic polymer. 

Figure 1. A photoelectrochemlcal cell with a polymer/electrolyte interface containing a light absorbing sensitizer (S) embedded in the polymer. Light absorption may enable a redox reaction of (R) dissolved in the electrolyte. When a semiconductor is the substrate, it is also often the sensitizer. (WE and CE denote working and counter electrodes).

Instrumentation. The interface within a suitably constructed electrochemical cell to be investigated is placed in the sample position of a standard DRIFT accessory for an infrared spectrometer for a typical design, see [328,329]. Examples reported so far deal with solid polymer electrolyte fuel cells where the surface of the anode layer exposed to a mixed gas atmosphere containing both water and methanol is separated from the environment via a Cap2 window [331, 332]. Various oxidized species and penetrating methanol were observed. 

V For example, consider the interface between two types of polymer electrolytes which are both different in nature, yet contain the same type of ions (e.g. the cation and the anion bis-(trifluoromethanesulfonyl)-imlde written TFSP). As above, the activity coefficients of the anions and cations in the same medium are set as equal. The ion activities will be denoted by a and ap in each of the two phases. 

Fig. 29) Two adjacent microelectrodes were derivatized by stepwise electrochemical polymerization. First a polymer viologen film (BPQ )n (related to 4) on one electrode and then a polyvinylferrocene (PVFc) film (related to 11) on the other electrode were obtaineo from the corresponding monomers by reductive and oxidative depositions, respectively. Small spacing between the two microelectrodes is crucial because for these materials the maximum conductivity is much lower than that of polymers such as 14,17, and 20 The redox levels are as follows E (BPQ +/+) = —0.55 V, E°(PVFc /°) = 0.4 V vs SCE. The redox reaction at the interface immersed in an aqueous electrolyte containing LiCIO occurs at a good rate only in one direction because the reaction in Eq. (20) is thermodynamically downhill. 

This discussion shows that the gel electrolyte must match the use of the battery, requiring optimization of the composition of the gel polymer electrolyte, the supporting salt and its concentration, and the solvent. PAN gel electrolytes made using different solvents, lithium salts, and composition will display different behaviors with respect to the ionic conductivity, lithium-ion transference number, electrochemical window, cyclic voltam-metric behavior, and compatibility with electrodes. Table 11.1 lists the ionic conductivity at room temperature of some gel electrolytes based on PAN. Because the PAN chain contains highly polar -CN groups, which exhibit poor compatibility with lithium metal electrodes, the passivation of the interface between the gel electrolyte and lithium metal electrode is crucial. At the same time, PAN has a high crystallization tendency. At elevated temperatures, the liquid electrolyte and plasticizer will separate therefore, the polymer is modified, mainly by copolymerization and cross-linking. 

Since the polyelectrolytes contain only one type of mobile ion, the interpretation of conductivity data is greatly simplified. Polyelectrolytes have significant advantages for applications in electrochemical devices such as batteries. Unlike polymer-salt complexes, polyelectrolytes are not susceptible to the build up of a potentially resistive layer of high or low salt concentration at electrolyte-electrolyte interfaces during charging and discharging. Unfortunately flexible polyelectrolyte films suitable for use in devices have not yet been prepared. 

Hydrogels are crosslinked polymer networks with entrapped solvent. In the case of hydrogels containing polyectrolytes, in addition to solvent, ions and salt can be found in the gel as determined by the Dorman partition. This arises from the exclusion of ions of the same charge that sets a membrane potential at the gel/external electrolyte interface. 

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