ELECTROCHEMICAL-BEHAVIOR OF BORON IN LIF-NAF-KF-MELTS

The electrochemical reduction of B(III) to B(0) in KBF4-LiF-NaF-KF mel ts has been studied by voltammetric and chronopotentiometric methods, Glassy carbon, Pt, and Ag were used as working electrode materials. On ly 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 KBF4 concent ration range up to 5.7 x 10(-2) mole percent (m/o). The cathodic half- wave potential was -1.34 V us, an Ag/AgCl reference electrode at 700 d egrees C. The diffusion coefficient of BF4- at 700 degrees C was deter mined to be 2.06 X 10(-5) cm(2) s(-1). Further increase of the KBF4 co ncentration above 5.7 x 10(-2) m/o leads to a change in the reduction process. An ohmic resistance control becomes the limiting factor oi th e boron electroreduction process. The ''apparent surface resistance'' changes from 3.0 to 21.6 Ohm cm(2) as the temperature decreases from 7 00 to 550 degrees C, respectively. Furthermore at KBF4 concentrations higher than 5.7 x 10(-2) m/o a second reduction peak and a correspondi ng anodic peak appeared on the voltammograms. These peaks were attribu ted to formation of alkali metal borides.