Effect of the Cathode Material on the Kinetics of Silicon Ion Electroreduction in the KCl-CsCl-K2SiF6 Melt

Electrolysis of molten salts is one of the promising methods for the preparation of silicon and its materials due to a possibility of controlling the composition and morphology. However, data on the influence of various factors on the kinetics of silicon electrodeposition are needed. The influence of the material of the cathode support on the kinetics of silicon ion electroreduction in the low-fluoride 57KCl-43CsCl (wt %) melt with an additive of 2.8 wt % K2SiF6 at 730 degrees C is studied by cyclic voltammetry and chronoamperometry. Both interacting materials and those indifferent toward silicon are chosen as supports: glassy carbon, silver, and nickel. The electroreduction of silicon ions at the glassy carbon electrode occurs in the potential range more negative than -0.5 V, that at the silver electrode occurs in the range more negative than 0.05 V, and silicon ions are reduced at the nickel electrode in a range more negative than 0.40 V vs. potential of the silicon quasi-reference electrode. The cathode process is not electrochemically reversible for all studied supports. According to the chronoamperometry measurements, the stage of new phase nucleation at the cathode exerts no effect on the kinetics of the process. Presumably, the irreversibility in the case of glassy carbon and silver can be induced by a delayed discharge, whereas the electrodeposition of silicon at the nickel electrode is accompanied by the formation of nickel silicides. The diffusion coefficients of silicon ions to the glassy carbon electrode are estimated from the voltammetric and chronoamperometric dependences as equal to 1.5 x 10(-5) and 1.2 x 10(-5) cm(2)/s, respectively.