Electrolytes, Hall effect

Magnetic effects in electrolytic processes have always held a special if somewhat distant interest for electrochemists. In Chapter 5, by Fahidy, an excellent account is given of the fundamentals of this topic and its applications, through magnetohydrodynamics, to electrodeposition and corrosion. Also treated is the basis of the electrolytic Hall effect, which is essential for understanding how electrohydrodynamic forces act on moving ions in a magnetic field. 

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. 

The microscopic approach has been particularly successful in the treatment of the Hall effect in electrolytes, summarized in an earlier overview [5]. As in the case of Hall conductivity, the magnitude of the magnetic field effect on diffusion is very small [6,7] but not negligible in a rigorous sense. The Llelmezs-Musbally formula [6] based on the theory of irreversible thermodynamics for bi-lonic systems ... 

Microwave power and its effect on the electrode/electrolyte interface, 439 Microwave region, Hall experiments, 453 Microwave spectroscopy, intensity modulated photo currents, 508 Microwave transients for nano crystalline desensitized cells, 514 Microwave transmission, as a function of magnetic field, 515 Minority carriers... 

The currents J1 to J4 and thereby the dissolution rate in fluoride media shows a dependence on the nature and concentration of the cation present in the electrolyte. Ji to J4 increase by more than one order of magnitude if a small cation, like Li+, is replaced by a large one, like Rb+ or Cs+. A catalytic effect of the cation on breaking of Si-O bonds is assumed [Hall, Ha 12]. 

Many effects of gas bubbles released at electrodes (on electrolyte flow, mass and heat transport, conduction, etc.) have been well studied in the past. A text with an extensive treatment of this topic is that of Hine [38]. However, in Hall-Heroult cells these effects are worthy of special mention because the relatively high current density, of the order of 1 A cm-2, and temperature make the volumetric gas evolution rate from the anode large. Furthermore, difficulties of measurement on actual cells mean less knowledge of these effects than in many other electrochemical cells. Finally, one effect of the bubble is to make the task difficult in reducing the enormous... 

The multiphysics and multiscale character of the important features of Hall-Heroult cell operation makes difficult laboratory scale experimentation that is relevant to industrial pot operations. For example, cell C E is influenced by the cell-scale flow of the metal and electrolyte, which is determined in turn by the magnetic field which depends on the entire cell current. CE also depends on the finer scale flow due to release of the carbon dioxide bubbles from the anodes. It is generally not possible to examine these two effects simultaneously in the laboratory. Also, the generally hostile environment inside Hall-Heroult cells makes experimentation difficult, and the high cost of modification of full-scale pots further complicates industrial trials. In this environment, numerical or mathematical modeling of pots would be expected to be a useful tool. 

This article has described the Hall-Heroult cell that is the mainstay of the aluminum industry throughout the world. Emphasis has been on the electrochemistry and electrochemical engineering that govern cell performance. The cell operation, electrolyte chemistry, thermodynamics, and electrode kinetics have been reviewed. Some complexities, notably the anode effect and the environmentally important fluoride emissions and anode gas bubbles and their effect on cell voltage, flow, and CE, have been examined. The incorporation of these phenomena, along with current distribution, magnetic fields, electromagnetically driven flow, heat and mass transport, and cell instability into mathematical models was summarized. 

Sillars, R B., S. I. Fletcher, M. Mirzaeian, and R J. Hall. 2011. Effect of activated carbon xerogel pore size on the capacitance performance of ionic liqnid electrolytes. Energy Environmental Science 4 695-706. 

The so-called anode effect on carbon electrodes in fluoride-containing melts is probably the most vivid manifestation of a dielectric FS. This phenomenon was first related to the industrial electrolysis of aluminium in Hall-Heroult cells and, thus, is widely known for more than a 100 years. Nevertheless, a generally acknowledged theory is not achieved yet. Several possible causes have been considered nonwettability of the electrode surface, electrostatic repulsion of the bubbles of the gas evolved at the anode, hydrodynamic crisis of the gas evolution and, finally, the formation of a fluorocarbon dielectric film [18-20]. This latter explanatimi had been developed since then, mainly by Japanese researchers, relatively to the electrochemical production of fluorine [21]. A fluorocarbon film is now widely recognized as a cause of the anode effect both in fluoride and in mixed fluoride-containing electrolytes. 

Reference electrodes of mercury have been used by several investigators in an attempt to measure single electrode potentials. Stastny and Strafelda (5 ) concluded that the zero charge potential of such an electrode in contact with an infinitely dilute aqueous solution is -0.1901V referred to the standard hydrogen electrode. Hall ( ) states that the potential drop across the double layer under these conditions is independent of solution composition when specific adsorption is absent. Daghetti and Trasatti (7, ) have used mercury reference electrodes to study the absolute potential of the fluoride ion-selective electrode and have compared their estimates of ion activities in NaF solutions with those provided by other methods. Their method is based on the assumption that the potential drop across the mercury I solution interface is independent of the electrolyte concentration once the diffuse layer effects are accounted for by the Gouy-Chapman theory. 

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