Electrolytic conductivity and resistivity

ELECTROLYTIC CONDUCTIVITY AND RESISTIVITY MEASUREMENTS. Industrial interest in the measurement of electrolytic conductivity (of which electrolytic resistivity is the reciprocal) arises chiefly from its usefulness as a measure of ion concentrations in water solutions. Also, by comparison with other analytical methods, this is relatively simple and inexpensive. 

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The means by which high current densities are obtained can be understood from an examination of the electrolyte conductivity and the interelectrode gap width. These parameters are related to the current through Ohm s law, which states that the current I fiowing in a conductor of resistance Ris directly proportional to the appHed voltage IN... 

Ikonopisov284 has conducted a systematic study of breakdown mechanisms in growing anodic oxides. He has enumerated factors significantly affecting the breakdown (nature of the anodized metal, electrolyte composition and resistivity) as well as those of less importance (current density, surface topography, temperature, etc.). By assuming a mechanism of avalanche multiplication of electrons injected into the oxide by the Schottky mechanism, Ikonopisov has correctly predicted the dependence of Ub on electrolyte resistivity and other breakdown features. 

Finite electrolyte conductivities and ionic current flow lead to ohmic voltage components in electrochemical cells. It is constructive at this point to review the effects of ohmic voltage contributions to driven and driving cells in the case of uniform current distributions. It will be shown that for each type of cell, the ohmic resistance lowers the true overpotential at the electrode interface for a fixed cell voltage even in the case of a uniform current distribution at all points on the electrode. 

Tlie series resistance in these models may be regarded as an access resistance to the electrode interface, dependent on electrolyte conductivity and geometry, but not linked to the polarization processes. As such it belongs to the chapter on contact media, and should be subtracted from measured electrode impedance when analyzing polarization immittance. 

A general problem in the synthesis of those composites is the appearance of conductivity gradients in their thickness direction, which are caused by inhomogeneities in the polypyrrole structure, as a result of difficulties in diffusion of the electrolyte across the host polymer. A method for overcoming these problems has been developed by Wang et al. [375]. PPy/PVC composites can significantly improve uniformity in electrical conductivity and resistance to mechanical delamination when a certain amount of electrolyte is mixed with PVC prior to the electrochemical reaction. After electropolymerization of pyrrole in the presence of electrolyte blended PVC for 20 minutes, the conductivity across the film thickness showed more than ten orders of magnitude difference with respect to film obtained from unblended PVC. 

In addition to the overvoltage losses due to electrode kinetics, one has to take into account the voltage loss in the ion-conducting electrolyte, due to the finite electrolyte conductivity, and (minimized) losses due to the electric resistance of electrode and cell materials, including contact resistances. 

Such structures are known as porous electrodes and they behave quite differently from the effectively planar electrodes used in most other areas of applied electrochemistry. The porous electrode is a mass of particulate reactants (sometimes with additives) with many random and tortuous electrolyte channels between. Real porous electrodes cannot be modelled but their behaviour can be understood qualitatively using a simplified model shown in Fig. M.5 in fact, there are two distinct situations which arise. In the first (Fig. 11.5(a)) the electroactive species is a good electronic conductor (e.g. a metal or lead dioxide here, the electrode reaction will occur initially on the face of the porous electrode in contact with the electrolyte but at the same time, and probably contributing more to the total current, the reaction will occur inside the pore not, however, along the whole depth of the pore because of the fR drop in solution. The potential and current distribution will depend on both the kinetics of electron transfer and the resistance of the electrolyte phase. A quantitative treatment of the straight, circular pore approximation allows a calculation of the penetration depth (the distance down the pore where reaction occurs to a significant extent) and it is found to increase linearly with electrolyte conductivity and the radius of... 

In addition, the cell resistance may be calculated from the electrolyte conductivity and the cell dimensions. Hence, all the terms in the cell voltage equation are readily accessible to measurement. This topic is treated in more detail in Chapter 2, Section 2.3.7. 

Conductivity. The standard unit of conductance is electrolytic conductivity (formerly called specific conductance) k, which is defined as the reciprocal of the resistance of a 1-m cube of liquid at a specified temperature m— ]. See Table 8.33 and the definition of the cell constant. 

Further improvements in anode performance have been achieved through the inclusion of certain metal salts in the electrolyte, and more recently by dkect incorporation into the anode (92,96,97). Good anode performance has been shown to depend on the formation of carbon—fluorine intercalation compounds at the electrode surface (98). These intercalation compounds resist further oxidation by fluorine to form (CF ), have good electrical conductivity, and are wet by the electrolyte. The presence of certain metals enhance the formation of the intercalation compounds. Lithium, aluminum, or nickel fluoride appear to be the best salts for this purpose (92,98). 

Soluble corrosion products may increase corrosion rates in two ways. Firstly, they may increase the conductivity of the electrolyte solution and thereby decrease internal resistance of the corrosion cells. Secondly, they may act hygroscopically to form solutions at humidities at and above that in equilibrium with the saturated solution (Table 2.7). The fogging of nickel in SO2-containing atmospheres, due to the formation of hygroscopic nickel sulphate, exemplifies this type of behaviour. However, whether the corrosion products are soluble or insoluble, protective or non-protective, the... 

An investigation has been made of the factors which control / and D conduction and it has been found that the difference is only one of degree and not of kind . Thus, if the varnish films are exposed to solutions of decreasing water activity, then the resistance falls with increasing concentration of electrolyte, but a point is eventually reached when the type of conduction changes and the films exhibit /-type behaviour. It appears that D films can be converted into / films, the controlling factor being the uptake of water. 

In solid-state batteries, it is extremely favorable to use the solid electrolyte for mechanical support. Despite the larger thickness, which lowers the relative amount for active material in the battery, the advantages are the absence of pinholes of the solid electrolyte, high electronic resistance, and simple multistack fabrication, since the individual cells may be contacted by their electronically conducting current collectors. 

Influence on Electrolyte Conductivity In porous separators the ionic current passes through the liquid electrolyte present in the separator pores. Therefore, the electrolyte s resistance in the pores has to be calculated for known values of porosity of the separator and of conductivity, o, of the free liquid electrolyte. Such a calculation is highly complex in the general case. Consider the very simple model where a separator of thickness d has cylindrical pores of radius r which are parallel and completely electrolyte-filled (Fig. 18.2). Let / be the pore length and N the number of pores (all calculations refer to the unit surface area of the separator). The ratio p = Ud (where P = cos a > 1) characterizes the tilt of the pores and is called the tortuosity factor of the pores. The total pore volume is given by NnrH, the porosity by... 

Ionic (electrolytic) conduction of electric current is exhibited by electrolyte solutions, melts, solid electrolytes, colloidal systems and ionized gases. Their conductivity is small compared to that of metal conductors and increases with increasing temperature, as the resistance of a viscous medium acts against ion movement and decreases with increasing temperature. 

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