Permselectivity and conductance

Ehrenbeck, C., and K. Juttner. 1996. Ion conductivity and permselectivity measurements of polypyrrole membranes at variable states of oxidation. Electrochim Acta 41 1815. 

In 1973, Dupont began to commercialize their first perfluorosulfonic add cation exchange membrane, Nafion. Since then until now, Nafion has been attracting much attention because of its superb chemical and thermal stability, high ionic conductivity, excellent permselectivity and good mechanical strength. Many approaches have been proposed to use this unique material as a modifier of electrochemical electrode surfaces. 

In conclusion, it can be claimed that a combination of kinetic and equilibrium conductance and membrane potential measurements provides a powerful method for investigating the permselective properties of membranes of low fixed charge density. Such methods should be applicable also to other polymers useful in hyperfiltration if they can be prepared in the form of homogeneous membranes. 

High-ionic permselectivity combining simultaneously a high conductivity and a moderate water uptake... 

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. 

Polymeric ion exchange membranes are a particular class of these ion-containing polymers. We have been interested in understanding their microstructure in order to explain membrane characteristics such as permselectivity, ionic conductivity, and water diffusion. 

As discussed elsewhere in this text, there are two types of dense, hydrogen-permeable metal membranes to consider from the perspective of module scale-up and design thin metal foils and permselective metal layers formed on a porous support. Another class of hydrogen-permeable inorganic membranes - dense proton-conducting ceramic membranes - are still under development and are addressed in Chapter 2. 

It is important to choose high permselective membranes with high conductivity and low leakage of water molecules or neutral molecules. The membrane production... 

Similar to metal electrode materials, conducting polymers stimulation requires reversible reactions driven under stimulation currents to ensure low voltage excursion and reduced production of by-products. Reports suggest that conducting polymers, especially PPy, are susceptible to irreversible oxidation [120]. Overoxidation of conducting polymers was reported to cause permselectivity changes, loss of conductivity, and de-doping [58,121-122]. 

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. 

In addition to high permselectivity, the membrane must have low-elec trical resistance. That means it is conductive to counterions and does not unduly restrict their passage. Physical and chemical stabihty are also required. Membranes must be mechanically strong and robust, they must not swell or shrink appreciably as ionic strength changes, and they must not wrinkle or delorm under thermal stress. In the course of normal use, membranes may be expec ted to encounter the gamut of pH, so they should be stable from 0 < pH < 14 and in the presence of oxidants. 

Using dilatometry in parallel with cyclic voltammetry (CV) measurements in lmolL 1 LiC104 EC-l,2-dimethoxy-ethane (DME), Besenhard et al. [87] found that over the voltage range of about 0.8-0.3 V (vs. Li/Li+), the HOPG crystal expands by up to 150 percent. Some of this expansion seems to be reversible, as up to 50 percent contraction due to partial deintercalation of solvated lithium cations was observed on the return step of the CV. It was concluded [87] that film formation occurs via chemical reduction of a solvated graphite intercalation compound (GIC) and that the permselective film (SEI) in fact penetrates into the bulk of the HOPG. It is important to repeat the tests conducted by Besenhard et al. [87] in other EC-based electrolytes in order to determine the severity of this phenomenon. 

A special case of interfaces between electrolytes are those involving membranes. A membrane is a thin, ion-conducting interlayer (most often solid but sometimes also a solution in an immiscible electrolyte) separating two similar liquid phases and exhibiting selectivity (Fig. 5.1). Nonselective interlayers, interlayers uniformly permeable for all components, are called diaphragms. Completely selective membranes (i.e., membranes that are permeable for some and impermeable for other substances) are called permselective membranes. 

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. 

Br > Cl > I. This conductance was not time-dependent. On the other hand, Cl conductance stimulated by the Ca2+ ionophore A23187 (2.5 pM) or elevation in the free Ca2+ levels in the pipet from 100 to 500 nM was time-dependent and exhibited a nonlinear current-voltage relationship that was outwardly rectifying. The permselectivity of this Ca2+-stimulated conductance was 1 > Br > cr, distinguishing it from the cAMP-stimulated Cfr conductance. As both calcium ionophore A23187 and cAMP were shown to elevate the Cl conductance of the cornea, it is quite likely that these two types of Cr channels are present in the corneal epithelium [96,106,114],... 

Cox et al. (1995) portray a new approach to thermochemical gasification of biomass to hydrogen. The process is based on catalytic steam gasification of biomass with concurrent separation of hydrogen in a membrane reactor that employs a permselective membrane to separate the hydrogen as it is produced. The process is particularly well-suited for wet biomass and may be conducted at temperatures as low as 575 K. 

The boundary condition (4.4.52c) states that the normal component of the cationic flux at the membrane vanishes at the insulating portion of the membrane surface and is equal to a given constant i throughout the conducting site. The boundary condition (4.4.52d) asserts the vanishing of the normal component of the anionic flux at the membrane, corresponding to the ideal permselectivity of the latter. Finally, the boundary conditions (4.4.52e,f) state that the surface r == 0, 0 < x < 6 is that of symmetry. We observe that c(r,x), [Pg.150]

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