Cathodic processes at electrodes in potassium fluoride-hydrofluoric acid (1/2) and potassium hydroxide-water (1/2) melts and the effect of adsorbed catalyst poisons.

 Title: Cathodic processes at electrodes in potassium fluoride-hydrofluoric acid (1/2) and potassium hydroxide-water (1/2) melts and the effect of adsorbed catalyst poisons. Author: Gao, Lijun. Abstract: The industrial production of F$\sb2$ is through the electrolysis process from HF melt. The reaction of $\rm HF\to F\sb2+H\sb2$ includes the anodic production of F$\sb2$ and cathodic H$\sb2$ evolution. It is the latter process on which this thesis study has been concentrated. The objectives of the research were to characterize the polarization behaviour of the electrocatalyst material, a Ni-Mo-Cd composite, for the H$\sb2$ evolution reaction (HER) from the KF$\cdot$2HF melt, its comparison with that from the analogous KOH$\cdot$2H$\sb2$O hydrate melt, and the mechanism and kinetics of the HER. It was found in the work described in this thesis that the high surface area of the electrodeposited material, Ni-Mo-Cd composite alloy, played an important role in its superior performance over a smooth surface electrode, e.g. mild-steel. The real, effective area of the electrode during the polarization process also varies in the different electrolyte solutions, as was observed through comparison between the KF$\cdot$2HF and KOH$\cdot$2H$\sb2$O melts. Contact angle measurements at the solid/melt/H$\sb2$ gas interphase and experimental in situ determinations of double-layer capacitance, C$\rm\sb{dl},$ were carried out, and the effect of wettability and accessibility of electrolytes to the inner pores of the porous surface in association with the surface tension was discussed. The "unfolding" of the electrochemically accessible area of a porous Pt electrode in KOH electrolyte of increasing conductivity was demonstrated experimentally for the first time. The transmission-line model and constant phase element (CPE) concept were used in the interpretation of the a.c. impedance results. A new method for in situ determination of C$\rm\sb{dl}$ was developed by means of analysis of initial (i.e. over short times of 2 to 100 $\mu$s) potential-relaxation transients in the HER. Its effectiveness was examined in comparison with the results of other methods, e.g. cyclic voltammetry and a.c. impedance measurement, based on a well-known prototype system of Pt electrodes in KOH solutions. A theoretical treatment was developed to describe the full potential-relaxation transient behaviour in terms of the roles of double-layer and H pseudocapacitance (C$\rm\sb{\phi,H})$ relaxation processes. The representation of the experimental data through computer simulation provided rate constants and other parameters of the reaction. It is shown how the o.p.d. H coverage, $\rm\theta\sb H,$ and the associated pseudocapacitance of the electrode can be quantitatively evaluated from the calculation involving rate constants. The presence of catalyst poisons in the electrolytes has significant influence in the industrial electrolysis process. In the present work, the effects of As$\rm\sb2O\sb3$ and (NH$\sb2)\sb2$C=S as poisons for the HER on the kinetics and adsorption behaviour were quantitatively characterized. The effects on the HER were theoretically treated, taking account of poison coverage $\rm\theta\sb P$ and a lateral interaction factor, g, at the electrode surface. Related $\rm\theta\sb H,\ \theta\sb P,$ g and C$\rm\sb{\phi,H}$ quantities were evaluated from the experimental data obtained, for the HER, at the Pt electrode in KF$\cdot$2HF and KOH solutions. Date: 1994 URI: http://hdl.handle.net/10393/6785

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