Journal of Materials Science and Chemical Engineering, 2014, 2, 57-62
Published Online January 2014 (
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
Received November 2013
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.
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 themthis 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-
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 ohm1cm1.
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
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
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
3. Results and Discussion
Cyclic current-voltage curves in the KCl-NaCl chloride
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 × 104 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-
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 × 104 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 ×
104 mol/cm3) KBF4 (3.1 × 104 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 × 104 mol/cm3)
KBF4 (3.1 × 104 mol/cm3); (b) NaCl-KCl-СеCl3 (3.1 × 104
mol/cm3) KBF4 (6.0 × 104 mol/cm3); (c) NaCl-KCl-СеCl3
(3.1 × 104 mol/cm3) KBF4 (15 × 104 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
−− −−−
+ →+−+
( )
6y 3
CeClF3eCe6y ClyF
−− −−−
+→ +−+
qBpCeCe B+=
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-
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
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) × 103 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 × 104 mol/cm3) KBF4 (6.0 × 104
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 restmixture of
NaCl-KCl; voltage bath (2.6 ÷ 2.8) V, time electrolysis
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-
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 F42.48 2.5 50 - 100 CeB4 90 - 110 HM
2) Molar ratio CeCl3:KBF4 = 1: 2
NaCl—39.88; KC l —50.75; CeCl34.53; KBF44.83 2.6 80 - 100 CeB6 50 - 70 HM
3) Molar ratio CeCl3:KB F4 = 1:5
NaCl—37.39; KC l—47.59; CeCl34.25; KBF 410.76 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.
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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