Anodic Passivation of a Graphite Electrode in LiF–KF Melt at 773 K

Abstract

The passivation of a graphite electrode in LiF-KF molten salt at 773 K was studied using cyclic voltammetry and electrochemical impedance spectroscopy. The anodic cyclic voltammogram showed that there are two oxidation current peaks at approximately 1.2 and 4.8 V vs. Pt quasi-reference electrode. We confirmed that carbon fluoride compounds are formed at the first current peak, while the last current peak was predominantly attributed to F 2 evolution. The surfaces of the anodes used for electrolysis were analyzed at the potential of 1.0 V by X-ray photoelectron spectroscopy and Raman spectroscopy, and the anode atmosphere during the experiments was analyzed by gas chromatography. The results show that solid carbon fluoride compounds were formed in the graphite surface and no trace of gaseous carbon fluoride was detected. In addition, the active-passive transition of graphite electrode was observed in the EIS spectra recorded at 1.5 V. We confirmed the reason for the passivation of a graphite anode in LiF-KF molten salt: the formed carbon fluoride compounds layer blocks the transmission of F − anion from the electrolyte to the graphite electrode.

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