Electrochemical Synthesis of CeB6 Nanotubes

doi:10.4236/msce.2014.21010

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Journal of Materials Science and Chemical Engineering, 2014, 2, 57-62

Published Online January 2014 (http://www.scirp.org/journal/msce)

http://dx.doi.org/10.4236/msce.2014.210010

OPEN ACCESS MSCE

Electrochemical Synthesis of CeB6 Nanotubes

H. B. Kushkhov, M. K. Vindizheva, R. A. Mukozheva, A. H. Abazova, M. R. Tlenkopachev

Kabardino-Balkar State University, Nalchik, Russia

Email: [email protected], [email protected]

Received November 2013

ABSTRACT

This work presents the results of joint electroreduction of tetrafluorborate and cerium-ions, and determines the

conditions of electrochemical synthesis of cerium borides in KCl-NaCl melts at the 973 K on tungsten electrode

by the linear and cyclic voltammetry. Based on the current-voltage studies the optimal modes of cerium boride

electrodeposition were found.

KEYWORDS

Molten Chloride; Linear and Cyclic Voltammetry; Cerium Borides; High Temperature Electrosynthesis;

Nanotube s

1. Introduction

Borides of rare earth metals (REM) are widely used in

various fields of modern technology. The electrochemi-

cal synthesis of rare-earth borides at moderate tempera-

tures (973 - 1023 K) is a cost-effective alternative to the

direct solution-phase synthesis. The increased interest in

the development of new efficient methods of producing

rare earth borides are due to remarkable properties of

these materials, such as chemical inertness, heat resis-

tance, a wide range of electrochemical and magnetic pro-

perties, etc. There is an indication of the using possibility

cerium hexaboride for refractory production for use in

neutral or reducing atmosphere and in vacuum at tem-

peratures of 2000˚C or higher [1].

Electroreduction from the molten salts is a specific

method for the preparation of compounds of elements

such as refractory metals, actinides and rare earth metals

[2]. Manifold variations of electrolytic production of me-

tals and compounds based on them—this is a great selec-

tion of solvent, a variety of chemical and electrochemical

characteristics of process and a temperature range, which

is suitable for the process.

Of the various methods for the synthesis of cerium

borides is the closest way to get them through the elec-

trolysis of molten media [3]. Electrolysis was carried out

in graphite crucibles, serving both the anode and a cath-

ode made of graphite or molybdenum. The composition

of the bath electrolysis includes oxides of rare earth met-

als and boric anhydride with additives of fluorides of

alkali and alkaline earth metals to reduce the temperature

and viscosity of the bath. Temperature electrolysis mix-

ture was 1223 K - 1273 K, the voltage on the bath was

3.0 - 15.0 V, current density was 0.3 - 3.0 A/cm2. The

composition of the bath for cerium hexaboride obtaining

was: CeO2 + 2B2O3 + CeF3.

As noted in work [3], the obtaining of the individual

boride phase is practically impossible or very difficult.

The disadvantages are also high temperature of synthesis

and complexity of the product separation from the mol-

ten electrolyte due to the low solubility of borates and

fluoride, contamination by-products, such as borates.

Thus, in view of the increasing use of rare earth metals

and various materials on their basis and with the addition

of rare earth metals in the various fields of science and

techno l ogy, it is becoming an urgent task of obtaining

these materials. A promising way to obtain rare earth,

their alloys with other metals is the electrolysis of molten

salts REM, as well as mixtures thereof.

For effective use of the electrolytic method of produc-

ing of metallic cerium and their alloys and compounds

are necessary to have reliable information about electro-

chemical behavior of complexes formed by cerium ions

in molten salts, and joint electroreduction with compo-

nents connections.

Products quality of rare earth borides is determined by

the purity and dispersion, namely original powder grain

size, from which it is made. The product quality is higher

when the grain size of compounds powder is smaller.

Previously, we have investigated the processes of joint

electroreduction of rare-earth metals with boron ions in

KCl-NaCl and KCl-NaCl -CsCl melts at different elec-

H. B. KUSHKHOV ET AL.

OPEN ACCESS MSCE

trodes. It is shown that the electroreduction of fluorobo-

rate ion occurs at more positive potentials than deposi-

tion potential of metallic cerium [4-6].

In the works [7-10] 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.

The aim of this work is to study the process of joint

electroreduction of cerium ions with fluoroborate ions in

molten equimolar KCl-NaCl on the tungsten electrode

and the electrochemical synthesis of cerium borides 973

2. Experimental

Chemicals and Apparatus

Experiments were carried out in a sealed quartz cell

(Figure 1) in the argon atmosphere, purified from traces

of moisture and oxygen, which is necessary in order to

obtain reliable results.

In three-electrode cell, the working electrode was the

tungsten (d = 1.0 mm, purity > 99.95%) needle electrode.

Tungsten is chosen as the material for the working elec-

trode, since the boron and cerium insoluble therein [11].

As the reference electrode we used quasi-reversible

glassy-carbon (SU-2000, d = 2.0 mm) rod electrode. The

using of glassy-carbon quasi electrode help us to avoid

the using of oxygen-diaphragms. Oxide ceramics are not

compatible with the halide melts containing rare earth

ions. Glassy-carbon quasi-stationary reference electrode,

apparently, is a compromise electrode, and is determined

by the redox potentials which are established with the

participation of the various components of the molten

medium. Therefore, its value depends on the melt com-

position and temperature. Glassy-carbon quasi-stationary

reference electrode was used in our studies [4], and pre-

viously by the authors [11] in chloride and chloride-

fluoride melts [12-14]. The auxiliary electrode was the

glassy carbon crucible, which was the container for melt

at the same time.

Electroreduction of cerium and tetrafluorborate ions

was investigated by cyclic voltammetry. The current-

voltage dependence was obtained by the electrochemical

complex Autolab PGST 30 (Ecochemic, Holland), which

was paired with computer. It has been estimated value of

ohmic IR drop in the electrolyte at a time-dependent po-

larization mode. The specific conductivity of molten po-

tassium, sodium and cesium chlorides is 0.5 ohm−1cm−1.

At the maximum distance of 0.5 cm between the refer-

ence electrode and the working electrode and a current of

10 mA, scanning rate of 10 V/s the ohmic drop is about

10 - 15 mV. In addition, the electrochemical complex

Figure 1. Scheme of high temperature electrochemical quartz

cell.

Autolab PGST 30 allows a survey of curre nt -voltage

curves with the IR-compensati o n.

Potentiostatic electrolysis was carried out using a

power supply with a current load of up to 5A.

The salts preparation method was following. Sodium

and potassium chlorides qualification «analytical grade»

were recrystallized, calcined in a muffle furnace, mixed

in the desired ratio (an equimolar mixture), and placed in

an alundum crucible into glass. A glass cell was evacu-

ated to a residual pressure of 0.7 Pa, first at room tem-

perature and then heated at progressively stepped up to

473˚C, 673˚C, 873˚C. Then it was filled with inert gas

(argon) and melted.

Cerium ion added to the melt in the form of anhydrous

cerium trichloride (99.9%, ultra- dry, Ltd. “Lanchi ”). To

avoid the formation of oxychlorides, experiments were

performed under purified argon and dried in a sealed cell.

Potassium tetrafluoroborate KBF4 qualification “reagent

grad e ” was washed in HF, than in ethanol, after than it

was dried. All operations with anhydrous salts were car-

ried out in glovebox mBraun Labstar 50 in the argon

atmosphere.

Products of electrolysis were identified by DRON-6

and observed by scanning electron microscope (SEM)

Vega 3 TESCAN. Particle size was measured by laser

diffractive analyzer Fritsch Analysette-22 Nanotech

(Germany).

3. Results and Discussion

Cyclic current-voltage curves in the KCl-NaCl chloride

H. B. KUSHKHOV ET AL.

OPEN ACCESS MSCE

melt by adding cerium trichloride and potassium fluoro-

borate are shown in Figure 2. Curve 1 in this figure

represents the voltammogram of background electrolyte

—equimolar molten KCl-NaCl. The absence of any

waves in it, and low leakage current at relatively high

negative potentials allows us to draw conclusions about

the cleanliness of the background electrolyte. When we

add in the background melt cerium trichloride C(CeCl3)

= 4,3 × 10−4 mol/cm3 (Figure 2(a), curve 2) at potentials

−(2.2 ÷ 2.3) V relative glassy-carbon quasi-stationary

reference electrode on voltammogram appears well re-

producible reduction wave of cerium ions. The fluorobo-

rate ions reduction wave observed at potentials −(1.3 ÷

1.5) V (Figure 2(b), curve 2).

To determine the sequence of the process of joint elec-

troreduction fluoroborate ion and cerium complex ions

tungsten electrode polarization at different potentials of

return was held (Figure 3), corresponding to a reduction

potential of boron, potential of joint electroreduction and

potential recovery of pure cerium. This shooting is possi-

ble to correlate the waves observed on the anode and ca-

thode regions of the voltammograms in cyclic polariza-

tion.

This picture can be assumed that the shift of reduction

potential of complex halide cerium ions to the region of

more positive values of the potential was not only due to

the changing nature of the substrate, but also the inte-

raction of cerium with the deposited boron observed.

Pre-wave, which was observed on the voltammograms

before a wave of pure cerium reduction corresponds to

the reducing of cerium on deposited boron.

With increasing concentration of fluoroborate ion with

respect to the initial concentration of cerium chloride com-

plexes in the cyclic voltammogram (Figure 4) are

merged wave electroreduction fluoroborate ion and chlo-

ride complexes of cerium in the stretched along the axis

(a) (b)

Figure 2. Cyclic voltammograms of NaCl-KCl melt on

tungsten electrode (vs SU) adding (a) cerium trichloride,

C(CeCl3) = 4.30 × 10−4 mol/cm3 (curve 2). V = 0.1 V/s. S =

0.21 cm2; (b) potassium fluoroborate, C(KBF4) = 3.1 × 10 −4

mol/cm3 (curve 2). V = 0.2 V/s. S = 1.6 cm2. Curve 1, bac k-

ground electrolyte. T = 973 K.

Figure 3. Cyclic voltammograms of NaCl-K Cl-СеCl3 (3.1 ×

10−4 mol/cm3) KBF4 (3.1 × 10−4 mol/cm3) melt at different

return potentials, V: 1, (−2, 5); 2, (−2.2); 3, (−2. 0); 4, (−1.6),

5, 1.0. Т = 973 К. V = 0.05 В/c. S = 1.6 cm2.

(a) (b) (c)

Figure 4. Cyclic voltammograms at different return poten-

tials, V: 1, 3.0; 2, 2.6; 3, 2.2; 4, 2.0; 5, 1.5. Т = 973 К. V = 0.1

V/s. S = 1.6 cm2: a) NaCl-KCl-СеCl3 (3.1 × 10−4 mol/cm3)

KBF4 (3.1 × 10−4 mol/cm3); (b) NaCl-KCl-СеCl3 (3.1 × 10−4

mol/cm3) KBF4 (6.0 × 10−4 mol/cm3); (c) NaCl-KCl-СеCl3

(3.1 × 10−4 mol/cm3) KBF4 (15 × 10−4 mol/cm3).

of the wave potentials of reduction, which we attribute to

the formation alloys cerium with boron. Further inc-

reasing the concentration of fluoroborate ion n the melt

leads to the formation only of boride phases.

Our investigations can be concluded that the electro-

synthesis of cerium borides is conducted only in the ki-

netic mode. Consequently, the electrochemical synthesis

process can be represented like successive stages:

• reducing of more electropositive component (boron),

• reducing of more electronegative component (cerium)

on pre-selected boron,

• mutual diffusion of cerium and boron to form the

different boride phases up to the higher boride CeB6.

The electrochemical processes that occurring during

the formation of cerium borides can be represented by

the following equations:

( )

4x x

BFCl3e B4xFxCl

−− −−−

+ →+−+

(1)

H. B. KUSHKHOV ET AL.

OPEN ACCESS MSCE

( )

6y 3

CeClF3eCe6y ClyF

−− −−−

+→ +−+

(2)

qBpCeCe B+=

(3)

Results obtained at investigation of the joint electro-

reduction of cerium halide ions and tetrafluoroborate ions

were taken as a basis for the practical implementation of

electrochemical synthesis of cerium hexaborides CeB6.

4. Electrochemical Synthesis of Cerium

Borid e

The electrosynthesis of cerium borides nanotubes was

performed in a molten mixture of NaCl-KCl-CeCl3-KBF 4

at 973 K on tungsten electrode in the range up −2.4 to

−2.8 V to relative a quasi-stationary glassy carbon elec-

trode.

The select of electrolytic bath components was done

on the basis of thermodynamic analysis and kinetic mea-

surements of joint electrowinning of cerium and boron

from halide melts. From the compounds of boron and

cerium, which do not contain oxygen, cerium chloride

and potassium tetrafluoroborate are fairly low melting

point and good solubility in KCl-NaCl melt. This solvent

was chosen because the decomposition voltage of the

molten mixture KCl-NaCl more stress decomposition

melts CeCl3 and KBF4, and because the alkali metal

chlorides are highly soluble in water. These properties

are necessary to the washing of cerium borides (Figure

5).

The individual phase of boron, higher boride CeB6 and

the mixture of phases, including CeB4 (Figure 6) were

obtained in depending on the composition and the syn-

thesis parameters. The purpose of electrosynthesis opti-

mization was to obtain higher boride CeB6 with most

valuable properties.

When we chose the concentration ratios of CeCl3 and

KBF4, we take into account the first stage of electro-

synthesis, during which the reducing of more electro-

positive boron was done. Electroreduction of the cerium

was started when KBF4 concentration was ended. In these

temperature conditions the optimum concentration of

KBF4 is about (1.0 ÷ 1.5) × 10−3 mol/cm3. According to

our study, at higher concentrations of KBF4 the cerium

borides getting were complicated by instability of cath-

ode deposit.

(а) (b) (c)

Figure 5. “Cathode-salt pear” (a), the product of electroly-

sis before washing (b), and the resulting powder after

washing (c).

Figure 6. Radiographs of cerium boride powder obtained in

KCl-NaCl-CeCl3 (3.1 × 10−4 mol/cm3) KBF4 (6.0 × 10−4

mol/cm3) melt on tungsten electrode. U = −2.5 B: a) the line

1, CeB6; 2, CeB4; b) 1, CeB 6; 2, CeB4; 3, B.

The ceruim borides electrosynthesis was held in po-

tentio and galvanostatic modes. It was observed that

these modes are not equal. At galvanostatic electrolysis

the true value of the current density is known only in the

initial period of time, because during electrolysis varies

significantly in cathode area. In most cases we used the

potentiostatic electrolysis because the voltage (potential)

determines the course of the reactions and monitors the

reaction of deposition. If the anode material is glassy

carbon and the voltage in the bath U < −1.8 V, the cath-

ode deposit consists mainly is boron. Provided the volt-

age U = −(1.8 - 2.5) V the mixture of different phases (B

and CeB4) was obtained. If the voltage U = −(2.5 - 2.8) V,

the cathode deposit consists from higher boride CeB6.

The duration of the electrosynthesis was affected to

the composition of the cathode deposits. The data in Ta-

ble 1 show the dependence of the phase composition of

the cathode deposits from the duration of electrolysis in

the electrolyte of optimal composition, as well as tem-

perature and voltage.

The optimal duration of the high-temperature electro-

chemical synthesis for prepare of CeB6 is 90 - 120 min-

utes. Thus, the synthesis of cerium borides was deter-

mined by the following interrelated parameters: the

composition of the electrolytic bath, the voltage and the

temperature. The optimal values of these parameters was

as follows: the composition of the melt, wt. %: CeCl3

(3.5 ÷ 7.0), KBF4 (4.5 ÷ 10.0), the rest—mixture of

NaCl-KCl; voltage bath −(2.6 ÷ 2.8) V, time electrolysis

H. B. KUSHKHOV ET AL.

OPEN ACCESS MSCE

is 90 ÷ 120 min, the temperature is 973 K.

Phase composition of the “cathode-salt pears” identi-

fied by X-ray analysis using a DRON-6 (Fig ure 7).

Particle size was measured by laser diffraction ana-

lyzer Fritsch Analysette-22 (Figure 7), and the order of

50 - 100 nm. The surface of the resulting powders have

also examined using the digital scanning electron micro-

scope Vega 3 TESCAN (Figure 8).

The yield of the single-phase CeB6 was 0.20 - 0.30

g/А × hour. Specific surface area of ultra-dispersive pow-

ders of CeB6 was 5 - 10 m2/g.

Our work was focused on the cathode deposit treat-

ment. The comparative radiographs were made before

and after different options of the cathode deposit wash-

ing.

The experiments showed that the best option of powders

Table 1. Electrochemical synthesis parameters, T = 973 K, cathode—W.

Electrolyte composition, wt.% Voltage E, V Time

, min Phase Particle size

1) Molar ratio CeCl3:KB F4 = 1:1

NaCl—40.86; KC l —52.01; CeCl3—4.66; KB F4—2.48 −2.5 50 - 100 CeB4 90 - 110 HM

2) Molar ratio CeCl3:KBF4 = 1: 2

NaCl—39.88; KC l —50.75; CeCl3—4.53; KBF4—4.83 −2.6 80 - 100 CeB6 50 - 70 HM

3) Molar ratio CeCl3:KB F4 = 1:5

NaCl—37.39; KC l—47.59; CeCl3—4.25; KBF 4—10.76 −2.7 90 - 100 CeB6 70 - 90 HM

Figure 7. Particle size distribution obtained by the electrochemical synthesis of 973K melt composition, wt.%: KCl (39.92) -

NaCl (50.8)-KBF4 (4.57)-CeCl3 (4.68); i = 0.3 A/cm2.

Figure 8. SEM images of the CeB6.

H. B. KUSHKHOV ET AL.

OPEN ACCESS MSCE

washing was the washing in distilled water, post-treat-

ment with ammonium hydroxide solution and washing

by KF than distilled water by decantation and centrifuga-

tion then by washing with double-distilled water.

Thus, to obtain reliable information on the phase

composition of the synthesized compounds by electroly-

sis and the possibility of direct electrochemical synthesis

CeB6 nanotubes in halide melts.

5. Conclusion

The joint electroreduction of tetrafluorborate and ce-

rium-ions was conducted in equmolar NaCl-KCl melt on

tungsten electrode at 973 K by cyclic voltammetry. The

analysis of voltammograms was shown that the electro-

synthesis in studied systems proceeds in the kinetic mode

because reducing potentials of boron and cerium is very

different. The resul ts of this research found that under

certain conditions, the concentrations of cerium and bo-

ron and certain anionic composition of the melt are pos-

sible for their joint electroreduction.

Synthesis of cerium borides nanotubes was carried out

by potentiostatic electrolysis of molten KCl-NaCl, con-

taining CeCl3 and KBF4. Electrolysis performed on tung-

sten electrode in the range of −2.4 to −2.8 V relatively of

the quasi-stationary glassy-carbon electrode. The influ-

ence of the electrolyte composition, temperature, current

density, voltage and the duration of electrolysis on the

synthesis products was studied. An optimal parameter for

getting cerium boride CeB6 nanotubes was found.

Acknowled gements

The work was done using equipment of Access Center

“X-ray diagnosis of materials” with the financial support

of the Ministry of Education and Science of Russian Fed-

eration, the state contract No. 16552.11.7074.

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