Ion transport number

Commercially available membranes are usually reinforced with woven, synthetic fabrics to improve the mechanical properties. Several hundred thousand square meters of IX membranes are now produced aimuaHy, and the mechanical and electrochemical properties are varied by the manufacturers to suit the proposed appHcations. The electrochemical properties of most importance for ED are (/) the electrical resistance per unit area of membrane (2) the ion transport number, related to current efficiency (2) the electrical water transport, related to process efficiency and (4) the back-diffusion, also related to process efficiency. 

The ion transport number is defined as the fraction of current carried through the membrane by counterions. If the concentration of fixed charges in the membrane is high compared to the concentration of the ambient solution, then the mobile ions in the IX membrane are mosdy counterions, co-ions are effectively excluded, and the ion transport number then approaches 1. Commercial membranes have ion transport numbers in dilute solutions of ca 0.85—0.95. The relationship between ion transport number and current efficiency is shown in Figure 3 where is the fraction of current carried by the counterions (anions) through the AX membrane and is the fraction of current carried by the counterions (cations) through the CX membrane. The remainder of the current (1 — in the case of the AX membranes and (1 — in the case of the CX membranes is carried by co-ions and... 

Fig. 3. Relationship between current efficiency and ion transport number.

In sodium chloride solutions the ion transport number for Na+ is about 0.4 compared to about 0.6 for CU. Thus a CX membrane would be expected to polarize at lower current densities than an AX membrane. Careful measurements show that CX membranes do polarize at lower current densities however, the effects on pH are not as significant as those found when AX membranes polarize. Such differences ia behavior have beea satisfactorily explaiaed as resultiag from catalysis of water dissociatioa by weaMy basic groups ia the AX membrane surfaces and/or by weaMy acidic organic compounds absorbed on such surfaces (5). 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

The perovskite structure is capable of high anion conductivity when oxide vacancies are introduced, as in, for example, Lai (Sr Co03 (/2 or in the perovskite-related superconductor phases, La2Cu04 and YBa2Cu307. The oxide ion transport number is not unity since such materials are often electronic conductors as well, due to the presence of... 

A battery is a transducer that converts chemical energy into electrical energy and vice versa. It contains an anode, a cathode, and an electrolyte. The anode, in the case of a lithium battery, is the source of lithium ions. The cathode is the sink for the lithium ions and is chosen to optimize a number of parameters, discussed below. The electrolyte provides for the separation of ionic transport and electronic transport, and in a perfect battery the lithium ion transport number will be unity in the electrolyte. The cell potential is determined by the difference between the chemical potential of the lithium in the anode and cathode, AG = —EF. 

For the sake of simplicity, we shall assume again an ideal permselectivity of the ion-exchange membrane. As mentioned before, this assumption is practically justified, since the co-ion transport numbers in modern monopolar ion-exchange membranes, employed, e.g., in electrodialysis, are limited to a few percent. 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

Thus, any ED unit design or optimization exercise relies on quite a great number of engineering parameters, such as ion transport numbers in solution (t+ and t ) and electromembranes (t, ) effective solute (ts) and water (tw)... 

Current-voltage tests to determine the limiting current intensity (/lim), ion transport numbers (ta, tc+), and surface resistances (ra, rc) in anionic and cationic membranes, as well as solute mass transfer coefficient (Am). 

The fraction of the current that is carried by a certain ion is expressed by the ion transport number, which is given by ... 

Electrolytic domain — describes the range of external conditions (- activity of components, temperature or pressure), where the ion transport number of a material is equal to or higher than 0.99 and the material is considered as -> solid electrolyte. Usually this term is related to thermodynamic - equilibrium. 

Solid electrolyte — is a class of solid materials, where the predominant charge carriers are -> ions. This term is commonly used for -> conducting solids with ion -> transport number equal to or higher than 0.99 (see also -> electrolytic domain). Such a requirement can only be satisfied if the -> concentration and -> mobility of ionic -> charge carriers (usually -> vacancies or interstitials) both are relatively high, whilst the content of -> electronic defects is low. See also -> superionics, -> defects in solids, - diffusion, and -> Nernst-Einstein equation. 

Oxide ion transport number, tO2 O, Nd2Eu203F6 , Nd2Ce203F6 A, Nd2Gd203F6 ... 

Fig. 23. The transport number of various rare-earth fluoride stabilized zirconias. Oxide ion transport number, tO2 --------- Electron transport number, re-------- ...

Some of these features are illustrated in Figures 14-18. A rather typical literature plot of current efficiency vs, sodium hydroxide concentration for perfluorosulfonate membranes is shown in Fig. 14. Nation 427 is a 1200-EW sulfonate membrane with fabric reinforcement. Poor hydroxide rejection occurs at catholyte concentrations above 10 wt % but a minimum is seen at higher concentrations, wtih increasing current efficiency from 28 to 40% caustic (9-14 M). The current efficiency of a 1200-EW homogeneous perfluorosulfonate film is shown in more detail over this concentration region in Fig. 15. Sodium ion transport number niol F ), which is equivalent to caustic current efficiency, is plotted vs. both brine anolyte and caustic catholyte concentration. These values were determined using radiotracer techniques, which have proven to be rapid and accurate methods for the determination of membrane performance. " " " A rather sharp maximum is seen at 14 M NaOH, and the influence of brine con-... 

Figure 15. Sodium ion transport number for Nafion 120 us. brine anolyte and caustic catholyte concentrations (Ref. 170).

In Figure 17, sodium ion transport number is plotted vs, catholyte concentration for a homogeneous perfluorocarboxylate film. The current efficiency is now higher than 90% over the entire caustic concentration region studied, although a minimum and maximum in performance is again observed. These features are shifted to lower concentration compared to perfluorosulfonate behavior though. Finally, the performance of a sulfonate-carbox-ylate bilayer membrane, Nafion 901, is plotted in Fig. 18. For such... 

Figure 17. Sodium ion transport number vs. caustic catholyte solution for a perfluorinated carboxylate membrane ( ) anolyte is 5 M NaCl and (O) anolyte and catholyte are identical concentrations of NaOH. (Ref. 149 reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

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