Origin of the Anodic Overvoltage Observed during Fluorine Evolution in KF-2HF

Abstract

International audienceThe kinetics of the fluorine evolution reaction was studied in molten KF-2HF with a horizontal disk electrode in a large potential window. A new model was proposed recently for representing the electrode/electrolyte interface; it includes the presence of a "fluidized" layer between the surface carbon-fluorine film (C-F) generated on the carbon anode during electrolysis and the fluorine gas film. This model was confirmed here to be consistent. The fluidized layer is composed of liquid KF-2HF melt and dissolved fluorine gas. With our electrode geometry, it was shown that the main electroactive surface area is located at the lateral side of the disk electrode. Finally, the contributions of the C-F film, η_C-F, and of the fluidized layer, η_fluid., to the total anodic overvoltage, η_T, were studied using a numerical calculation method. It was shown that both contributions must be taken into account for a global understanding of the fluorine evolution process

Effect of hydrodynamics on Faradaic current efficiency in a fluorine electrolyser

“…This is not surprising when looking at Equations (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) since the four forces acting on bubble speed depend on the diameter in a non-linear way. Figure 6 shows the variation in current efficiency with the hydrogen bubble diameter for a cell current density Io.…”

“…Fluorine electrolysis [1][2][3] is particularly interesting because overpotentials are very large, thermal phenomena are significant [4,5], fluorine emission is not conventional and free convection makes the temperature uniform in the cell [1]. The characteristics of fluorine bubble release are not yet completely understood and a specific study is in progress.…”

A hydraulic model to simulate the hydrodynamics of a fluorine electrolyser

“…characteristics of fluorine bubble release are not yet completely understood [1,4]. The two-phase free flow is mainly due to hydrogen bubbles evolving at the cathode, but large fluorine bubbles also take part in the two-phase movement, as seen in fig.…”

“…H. Roustan [1] has shown that to some extent the large gliding fluorine bubbles drag the electrolyte up along the anode. A numerical model was gradually developed in France to account for all the strongly linked phenomena involved in fluorine electrolysis [2][3][4][5][6]. The final numerical model combined two commercial codes, Flux-Expert (FE) and Estet-Astrid (EA), both distributed by Astek [7,8].…”

“…Fig. 1b) shows a schematic representation of the plume of hydrogen and the bubbles of fluorine that slip along the electrode, partially coated with CFx [4]. The hydrodynamics of a fluorine electrolyser are completely atypical.…”