Permselectivity with conducting polymer

This material was first synthesized in the middle 1960s by E.I. Du Pont de Nemours and Co., and was soon recognized as an outstanding ion conductor for laboratory as well as for industrial electrochemistry. The perfluorinated polymeric backbone is responsible for the good chemical and thermal stability of the polymer. Nation membrane swollen with an electrolyte solution shows high cation conductivity, whereas the transport of anions is almost entirely suppressed. This so-called permselectivity (cf. Section 6.2.1) is a characteristic advantage of Nation in comparison with classical ion-exchange polymers, in which the selective ion transport is usually not so pronounced. 

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

Flemion is quite different from prior membranes in that it is based on specific perfluorinated copolymers with pendant carboxylic acid as a functional group. The introduction of carboxylic functions in the polymer has realized high permselectivity in cation transport with high conductivity, which is indispensable to electrochemical application of ion exchange membranes. 

Ion-exchange membranes (lEM s) are polymeric materials that contain ionic functionalities chemically bound to the polymer backbone. The bound ionic functionalities can be anionic or cationic. In order to maintain electroneutrality within the membrane, each bound ionic site must be paired with an ion of the opposite charge. When the membrane Is swollen with an appropriate solvent, these ions become mobile and can be exchanged with other ions of like chau ge. The Internal structure of certain lEM s such as Naflon (23) results in a property known as permselectivity. Permselective lEM s reject Ions of the same charge as the bound ionic sites, l.e. in Nafion vltually all the ionic conductivity through the membrane is due to the mobile cations. 

Kruczek and Matsuura in their studies on characterization of gas separation properties of SPPO films have reported similar trends for permeabilities and permselectivities for O2 and N2. They have also reported CO2/CH4 permeability ratio of 43 corresponding to CO2 permeability of 11 Barrer for SPPO. The authors have conducted a detailed study on the effect of mono-, di- and trivalent cation substitutions of SPPO membranes on their gas separation performances. The thus substituted polymers were more permeable to gases than the hydrogen form of SPPO without any loss in the permeability ratios. The improved gas transport properties (separation factor for O2/N2 of 7.65 corresponding to 67.3% of O2 in the permeate when the membrane was used for oxygen enrichment of air) of SPPO with a degree of substitution of 18.5% and in the Mg " form for O2/N2 gas pair placed the polymer above the upper-bound line. 

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