Membranes cation conductors

Fig. 4. Conductivity of several oxide ceramic anion and cation conductors suitable for fuel cell and selective membrane applications (after Ref. 8).

It has been known for some time that certain antibiotic polypeptides exert a rather specific effect upon natural membranes . Recent observations by Pressman , however, have led to the discovery by Chappell and Crofts that certain polypeptides of the valinomycin and gramicidin group are specific cation conductors. 

Zeolites are essentially cationic conductors having temperature dependence of resistivity typically displayed by ionic conductors (higher mobility of ions at higher temperatures). By appropriate modifications of the zeolite composition, the conductivities of zeolites can be altered to allow their use as solid electrolytes, membranes in ion-selective electrodes, and as host stmctures for cathode materials in battery systems [84M1, 95A1]. 

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. 

Proton exchange membrane fuel cell (PEMFC) working at around 70 °C with a polymer membrane electrolyte, such as Nafion, which is a solid proton conductor (conducting by the H + cation). 

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

Two types of solid ionic conductors are of special interest—those in which metallic cations such as lithium ions can be transported across the polymer membrane, and others in which protons can move from one side of the membrane to the other. The first... 

In electromembrane processes the anions move towards the anode where they are oxidized by releasing electrons to the electrode in an electrochemical reaction. Likewise, the positively charged cations move towards the cathode where they are reduced by receiving electrons from the electrode in an electrochemical reaction. Thus, the transport of ions in an electrolyte solution and ion-exchange membrane between electrodes results in a transport of electrical charges, that is, an electrical current which can be described by the same mathematical relation as the transport of electrons in a metallic conductor, that is, by Ohm s law that is given by ... 

Potentiometry is the measurement of an electrical potential difference between two electrodes (half-ceUs) in an electrochemical cell (Figure 4-1) when the cell current is zero (galvanic cell). Such a cell consists of two electrodes (electron or metallic conductors) that are connected by an electrolyte solution (ion conductor). An electrode, or half-cell, consists of a single metallic conductor that is in contact with an electrolyte solution. The ion conductors can be composed of one or more phases that are either in direct contact with each other or separated by membranes permeable only to specific cations or anions (see Figure 4-1). One of the electrolyte solutions is the unknown or test solution this solution may be replaced by an appropriate reference solution for calibration purposes. By convention, the cell notation is shown so that the left electrode (Mi,) is the reference electrode the right electrode (Mr) is the indicator (measuring) electrode (see later equation 3). ... 

C rystalline membranes arc also available thal consist of a homogeneous mixture of silver sulfide with sulfides of copper(U), lead, or cadmiunt. Toward these divalent cations, electrodes from these materials have electrical responses similar to electrodes of the third kind (Section 23C-3). Note that these divalent sulfides, by themselves, are not conductors and thus do not exhibit ion-selectivc activity. 

Among the cation permeable perfluorinated membranes previously mentioned, only the perfluorosulphonic membranes are proton conductors. Their synthesis is described in this chapter . ... 

The ISE measures the activity of the ions in solution. This activity is related to concentration and thus, in effect, measures the concentration. However, if an ion such as fluoride, which complexes with some metals - Fe or A1 - is to be measured, it must be decomplexed from these cations by the addition of a reagent such as citric acid or EDTA. ISEs for at least 22 ionic species are commercially available. An example is described here for the measurement of fluoride ions in solution. The fluoride electrode uses a LaFj single crystal membrane and an internal reference, bonded into an epoxy body. The crystal is an ionic conductor in which only fluoride ions are mobile. When the membrane is in contact with a fluoride solution, an electrode potential develops across the membrane. This potential, which depends on the level of free fluoride ions in solution, is measured against an external constant reference potential with a digital pH/mv meter or specific ion meter. 

Many solid electrolytes are known today and it can be expected that their importance will further increase, especially for electrochemical devices. For example, beta-alumina solid electrolyte (BASE) is a fast ion conductor, which is used as a membrane in electrochemical cells. It can contain small ions like sodium, which show a high mobility. More classical examples are electrolytes based on lithium or silver iodide where the small cations are very mobile [13]. Note that solid polymer electrolytes are also a rapidly growing field [14]. 

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