Electrolytes stratification

Electrolyte stratification has been responsible for the failure of many flooded battery banks. In simple terms, this phenomenon can be defined as a build-up of higher strength acid at the bottom of the battery. Stratification occurs because sulfuric acid has a higher density than water and, when formed during the charging process, will sink to the bottom of the battery container. Such behaviour results in a decrease in battery capacity due to uneven utilization of the active material [12]. Moreover, if the resulting concentration gradient is allowed to remain for extended periods, premature failure of the battery can occur. 

How would electrolyte stratification affect the structure and composition of the active masses The data in Figs 3.16 and 3.17 indicate that the electrolyte in the bottom part of the cells reaches a concentration of 1.35 g cm . As evident from Fig. 3.11, the positive plates get progressively passivated at acid concentrations higher than 1.28 g cm. Furthermore,... 

Reduced electrolyte stratification. In flooded batteries, gravitation causes the formation of vertical layers of different electrolyte concentration (i.e. electrolyte stratification). This creates potential differences between the top and bottom parts of vertical cells [19] and leads eventually to slow self-discharge processes. In VRLA batteries, the electrolyte is immobilized, which allows the cells to be positioned (mounted) horizontally as well. Thus, the degree of electrolyte stratification is reduced substantially and hence the service life of the cells is extended. 

A diffusion potential also develops between the H2SO4 solution in the top and bottom parts of cell (vertical electrolyte stratification) [11]. This potential should also be taken into consideration when measuring the EMF of the cell or battery, i.e. the measurement should be performed after levelling of the aeid concentration by height of the cell (battery). 

Figure 2.10 shows a modern lead-acid traction cell designed by Varta with electrolyte circulation compared to a cell with electrolyte stratification. There are many electrochemical systems that will yield favorable accumulators (see Chapters 1 and 10), some of which have reached a very promising state of development. They will have to prove their versatility in practical application, especially with the aspect of economy in the future. 

Electrolyte stratification Figure 5.5 Cell with electrolyte circulation. 

The mutual mechanical mixing of anolyte and catholyte in the bell-jar type electrolyzer is prevented by stratification of the electrolyte due to difference in specific gravity of both liquids, so that no diaphragm is necessary. Simultaneously the motion of the electrolyte from the anode toward the cathode prevents the penetration of hydroxyl ions towards the anode which enables to attain... 

Therefore the correct setting and maintaining of the rate of flow of the brine is of great importance. To prevent a disturbance of natural stratification of the electrolyte by thermal convection the operating temperature of the electrolyzer should not exceed roughly 35 °C. In order to reduce the solubility of chlorine in the electrolyte it is necessary to keep a high chloride content in the brine. 

As already mentioned, stepwise transitions in foam films are observed, as a rule, at thicknesses less then 60 - 70 nm. The number of transitions increases with the increase in surfactant concentration. Manev et al. [351] have observed up to 10 transitions when the NaDoS concentration in the initial aqueous solution was raised to 0.5 mol dm 3 (in the absence of additional electrolyte). Upon increasing the ionic strength (addition of electrolyte or ionic surfactants) the differences in the transition thicknesses decrease. In some cases [351-353] electrolyte inhibits stratification. 

According to Godneva and Klocho (13), when an electrolyte is added to water-dioxane or water-THF, two phases result. In order to bring about this separation, the water-cyclic ether association must be broken down. These authors believe that the hydration of the electrolyte cations is accomplished by the dehydration of the cyclic ether molecules, causing stratification. 

In the beginning of the 20th century, Johnnott and Perrin observed that soap hhns decrease their thickness by several stepwise transitions. The phenomenon was called stratification. Bruil and Lyklema and Friberg et al. studied systematically the effect of ionic surfactants and electrolytes on the occurrence of the stepwise transitions. Keuskamp and Lyklema anticipated that some oscillatory interaction between the Him surfaces must be responsible for the observed phenomenon. Kruglyakov and Kruglyakov and Rovin reported the existence of stradficaHon with emulsion films. 

A. Eklund, D. Simonsson, R. Karlsson, and F. N. Bark, "Theoretical and Experimental Studies of Free Convection and Stratification of Electrolyte in a Lead—Acid Cell," LABAT-89 Conference, Vama, Bulgaria, 1989. 

Fig. 1.3. Schematic representation of processes and electrolyte flow that contribute to acid stratification during (a) discharge and (b) charge [7],...

FM3. Any acid stratification which occurs cannot be overcome by overcharging because there is little free electrolyte and, hence, ineffective mixing. Moreover, water lost cannot be replaced because the battery is expected to remain sealed for life. 

Absorptive glass mat (AGM) Very high porosity of separator High purity and stability of separator Low internal electrical resistance Relatively large separator pores acid stratification -electrolyte drainage risk of internal shorts -risk of thermal runaway -risk of PCL-3... 

Buoyant force (depth of dip) C Local, at the top electrolyte gassing acceleration forces inclination acid level acid stratification No High... 

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