Material transport electrolytes

Insulating units are installed in pipelines to limit cathodic protection or to separate different materials in a mixed installation. If the pipelines are transporting electrolytes, anodic interference can occur on the pipe interior if a dc voltage, Af/, exists at the insulator of length, L. The current flowing through the insulating unit by anodic polarization of the internal wall of the pipe comes to ... 

These materials are introduced in Chapter 5 and only brief mention of them is necessary here. It is important to appreciate that polymer electrolytes, which consist of salts, e.g. Nal, dissolved in solid cation coordinating polymers, e.g. (CH2CH20) , conduct by quite a different mechanism from crystalline or glass electrolytes. Ion transport in polymers relies on the dynamics of the framework (i.e. the polymer chains) in contrast to hopping within a rigid framework. Intense efforts are being made to make use of these materials as electrolytes in all solid state lithium batteries for both microelectronic medical and vehicle traction applications. 

The t0ff-time has to be as long as necessary for the material transport in the electrolyte to take place, but not longer in order to minimize Ostwald ripening and reach sufficient deposition efficiency. 

If the corrosive medium is an electrolyte solution, the resulting corrosion reactions are electrochemical. This means that material transport in the form of metal ions and charge exchange in the form of electrons take place at the metal-solution phase boundary because of the conductivity caused in the liquid phase by mobile anions and cations and the electron conductivity of the metals. 

Figure 2.1 The basic bipolar electrolytic experiment, shown with material transport directions.

The ionic transport number is directly related to the utilization of the given material as electrolyte or insertion electrode. The + <7 ) ratio is usually of the order of 10" to 10" if the material is supposed to be used as electrolyte. Its measurement is not straightforward and generally has not been made for protonic conductors. 

The diffusion coefficient of holes via the I /l3 redox couple in the solid containing 2.5 wt % carrageenan, 0.3 M KI, 0.03 MI2, and excess water was investigated by an impedance spectroscopy it was ca. 1.7 x 10 cm s , which is almost the same as that in liquid water, showing that the hole transport in the solid is not a problem in comparison with the liquid medium system. Similar results have been obtained also for a solid containing redox electrolyte and organic liquid [57]. In the present solid material, transport of small ions and molecules takes place in the same way as in a liquid, showing that the liquid contained in this solid behaves as if it were a pure liquid. 

Despite the widespread applicability and high capacitance values of activated carbon materials, other electrolyte and ion transport issues may arise and limit the performances of electrodes fabricated from these materials. Fabricating ordered electrode structures seems promising for overcoming this limitation. For example, CNTs have been extensively investigated due to their one-dimensional structures that result in porous electrode networks. 

Type I electrodes, the prevailing type, are three-phase composite media that consist of a solid phase of Pt and electronic support material, an electrolyte phase of ionomer and water, and the gas phase in the porous medium. Gas diffusion is the most effective mechanism of reactant supply and water removal. Yet, CLs with sufficient gas porosity, usually in the range Xp 30-60%, have to be made with thickness of Icl — 10 pm. In this thickness range, proton transport cannot be provided outside of the electrolyte environment. Porous gas diffusion electrodes are, therefore, impregnated with proton-conducting ionomer. The concept of a triple-phase boundary, often invoked for such electrodes, is however inadequate. The amount of the electrochemically active interface is usually controlled by two-phase boundary effects at the interface between Pt and water. 

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