Electrochemical Behavior of Boron in LiF‐NaF‐KF‐ Melts

The electrochemical reduction of B(III) to B(0) in KBF 4 -LiF-NaF-KF melts has been studied by voltammetric and chronopotentiometric methods. Glassy carbon, Pt, and Ag were used as working electrode materials. Only in the case of Ag was the reduction not complicated by interaction between boron and the electrode material. On a silver electrode B(III) was reduced to B(0) in a single irreversible step in the KBF 4 concentration range up to 5.7 x 10 -2 mole percent (m/o). The cathodic half-wave potential was -1.34 V vs. an Ag/AgCl reference electrode at 700°C. The diffusion coefficient of BF 4 - at 700°C was determined to be 2.06 x 10 -5 cm 2 s -1 . Further increase of the KBF 4 concentration above 5.7 x 10 -2 m/o leads to a change in the reduction process. An ohmic resistance control becomes the limiting factor of the boron electroreduction process. The apparent surface resistance changes from 3.0 to 21.6 Ω cm 2 as the temperature decreases from 700 to 550°C, respectively. Furthermore at KBF 4 concentrations higher than 5.7 x 10 -2 m/o a second reduction peak and a corresponding anodic peak appeared on the voltammograms. These peaks were attributed to formation of alkali metal borides.