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

Abstract

ABSTRACT.The electrochemical reduction of B(III) to 8(0) in KBF4-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 8(111) was reduced to B(0) in a single irreversible step in the KBF4 concentration range up to 5.7 X 10 mole percent (mb). The cathodic half-wave potential was -1.34 V vs. an Ag/AgC1 reference electrode at 700°C. The diffusion coefficient of BF at 700°C was determined to be 2.06 X i0' cm2 s'. Further increase of the KBF4 concentration above 5.7 x 10-2 mb 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 11 cm2 as the temperature decreases from 700 to 550°C, respectively. Furthermore at KBF4 concentrations higher than 5.7 X 10-2 mb a second reduction peak and a corresponding anodic peak appeared on the voltammograms. These peaks were attributed to formation of alkali metal borides. InfrodudionDue to various unique physical and chemical properties, refractory metal borides have attracted increasing attention by researchers recently."7 Molten salts provide some possibilities for implementing a technology for refractory metal boride deposition either by using chemical reaction with the substrate' or electrolysis. Two approaches of the latter method are available: (1) boron electrochemical deposition on a refractory metal substrate followed by diffusion into the metal phase2 or (ii) a direct electrochemical synthesis. The electrochemical synthesis has the advantage compared to the galvanodiffusion method, that the solid-state diffusion, which is often a rate-determining Electrochemical Society Active Member.step, is avoided. Further, it allows production not only of coatings but also powders. Most investigations3"7 on the electrochemical synthesis of refractory metal borides are concerned with development of coating technologies. Therefore more knowledge about the mechanism of the electrochemical deposition of boron and refractory metals is needed.Unlike the electrochemistry of the refractory metals, the electrochemical behavior of boron is not clear. Investigations concerning the mechanism of the cathodic reduction of boron ions"4'6'8'20 do not give a comprehensive idea of the mechanism and the kinetics of the boron electrodeposition. It was shown"° for platinum electrodes that the reduction process B(III) -B(0) proceeds reversibly in fluoride melts. However this reaction was shown by several authors to be irreversible at glassy carbon elec- Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, inc. 3179 trodes."2° These authors as well as others'8"9 suggested that the process of boron(III) reduction in chloride-fluoride and fluoride melts proceeded in one step without the participation of inte...

Electrochemical Synthesis of CeB6 Nanotubes

“…In the works [7][8][9][10] the electrochemical behavior of boron, the laws of electrode processes at its refining, the solubility of boron compounds in molten alkali metal chlorides are studied. In literature the processes of joint electroreduction cerium and boron ions in halide melts on a tungsten electrode are poorly understood.…”

A review on the methods of preparation of elemental boron

“…And, for LiFKFB 2 O 3 melts, B 2 O 3 firstly reacted with LiFKF to form KBF 4 , which was then reduced to element boron according to Eqn . In addition, Polyakova et al . found that the electrochemical reduction of B(III) to B(0) on a sliver electrode in the KBF 4 LiFNaFKF melts depended on the concentration of KBF 4 .…”

Insights into the Electrochemistry of Deposition of Boron from KCl-KF-NaBF4 Melt

“…/ is cathodic half-peak potential), is larger than the value required for the reversible process of formation of an insoluble substance ( (0.071/n)*V at 1073 K where, n is the number of electrons transferred for the reduction process) [33,34], indicating that the reduction of boron species on platinum electrode from KCl-KF-NaBF 4 melt is not reversible. (Fig.…”

“…Polyakova et al [34] reported that the broadening of the cathodic peak in CV occurred due to the ohmic control of deposition of boron. These authors presented linear plots of peak current and peak potentials against the square root of scan rate and concluded that a surface boron film formed on the electrode rendered the process to be controlled by ohmic resistance.…”

“…These authors presented linear plots of peak current and peak potentials against the square root of scan rate and concluded that a surface boron film formed on the electrode rendered the process to be controlled by ohmic resistance. The "apparent surface resistance" ( app R ) of the electrolyte in the pores of the boron film covering the electrode can be obtained from the reciprocal of the slope of peak current density ( P j ) vs. peak potential ( P E ) plots as reported by Polyakova et al [34]. The plots for cathodic peak current density vs. cathodic peak potential for the melt A show a linear behavior.…”