Chemical and electrochemical behavior of carbonate melts containing silicon oxide

Article abstract of DOI:10.1023/A:1019504827943

The optimal conditions of electrochemical synthesis of silicon carbide in carbonate melts containing silicon oxide were determined on the basis of thermodynamic calculations and data of cyclic voltammetry.

Full text of DOI:10.1023/A:1019504827943

Russian Journal of Applied Chemistry, Vol. 75, No. 4, 2002, pp. 562 564. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 4, 2002,
pp. 576 578.
Original Russian Text Copyright
2002 by Devyatkin, Pisanenko, Shapoval.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Chemical and Electrochemical Behavior
of Carbonate Melts Containing Silicon Oxide
S. V. Devyatkin, A. D. Pisanenko, and V. I. Shapoval
Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine
Received January 30, 2001; in final form, July 2001
Abstract The optimal conditions of electrochemical synthesis of silicon carbide in carbonate melts containing
silicon oxide were determined on the basis of thermodynamic calculations and data of cyclic voltammetry.
Carbonate melts have found wide use as electrolytes
for fuel cells [1] and for treatment of castings. The
behavior of molten carbonates has been studied exten-
sively. At the same time, use of carbonates as support-
ing molten electrolytes in electrolysis aimed to obtain
solid deposits is poorly studied. In [2, 3], the possi-
bility of carbon deposition at the cathode in these
media was examined.
high hardness, refractoriness, oxidation resistance, and
semiconducting properties.
The aim of this study was to analyze the ?interac-
tion of molten alkali metal carbonates with silicon
oxide and the electrochemical behavior of this system,
and to choose the optimal conditions of electrochem-
ical synthesis of silicon carbide.
To prognosticate processes occurring in the sys-
0
Alkali metal carbonates react rather well with a tems, the Gibbs energy, G , were calculated for
number of metals [4] and dissolve oxides of refractory
metals and p elements (B, Al, Si, Ge, etc.) [5]. The
range of potentials in which carbonate melts do not
decompose is 1.6-V-wide, but cathodic depolariza-
a number of chemical reactions possible in molten
carbonates at 750 C. The results obtained are pre-
sented in the table. It can be seen that silicon oxide is
infinitely soluble in molten carbonates because of ex-
tion of electrochemical synthesis is possible because change reactions (4) (6). The solubility of carbon di-
oxide in molten carbonates at 750 C is 10 4 mol cm
( 0.22 wt %) [10]. Carbon dioxide dissolves in mol-
ten carbonates without chemical reaction, and, con-
sequently, its solubility is directly proportional to
pressure, i.e., is described by the Raoult law. Having
positive Gibbs energies, molten carbonates of potas-
sium, sodium, and lithium are not susceptible to de-
composition [reactions (2) and (3)]. Introduction of
silicon oxide into a melt leads in all cases to forma-
tion of free carbon dioxide. Consequently, to stabilize
carbonate melts containing silicon oxide, it is neces-
sary to create an excess pressure of carbon dioxide.
Redox reactions (7) (22) have positive Gibbs energies
and do not occur under the given conditions. Thus, it
may be suggested on the basis of thermodynamic cal-
culations that electrochemical synthesis of silicon car-
bide from carbonate melts containing silicon oxide is
possible.
3
of the low energy of formation of silicon and titani-
0
1
um carbides [6] ( G = 65 kJmol for SiC and
f
0
f
1
G = 173 kJmol for TiC at 1000 K). Synthesis
of carbides in molten carbonates may occur in the
given range of potentials. Therefore, it may be con-
sidered promising to use carbonate melts to synthe-
size carbides. Commonly, a binary K CO Li CO
2
3
2
3
(
mp 492 C) or ternary eutectic K CO Na CO
2 3 2 3
Li CO (mp 397 C) are used [7]. However, lithium
2
3
carbonate is poorly soluble in water [8], and, there-
fore, a binary eutectic K CO Na CO (mp 720 C)
2
3
2
3
was chosen as object of study [7].
Potassium and sodium carbonates are well soluble
in water, which makes unnecessary use of acid for
washing of the cathode product. In [9], silicon carbide
was obtained from an SiO (20%) Li CO melt at
2
2
3
1
050 C. However, the synthesis mechanism was not
studied. Electrolysis was performed at elevated tem-
perature because of the high concentration of the
higher-melting component, silicon oxide. Silicon car-
bide is widely used in the industry, since its exhibits
EXPERIMENTAL
Voltammetric measurements were carried out in
two hermetically sealed quartz cells under elevated
1
070-4272/02/7504-0562 $27.00 2002 MAIK Nauka/Interperiodica

CHEMICAL AND ELECTROCHEMICAL BEHAVIOR OF CARBONATE MELTS
563
0
Gibbs energies, G , of reactions (1) (22) at 750 C
0
0
Reaction
G , kJ
Reaction
K CO = K O+ C+ O
G , kJ
Li CO = Li O+ CO
(1) 88.892
(2) 187.731
(3) 233.970
(12) 629.839 ( 1.632)
(13) 193.326 ( 1.002)
(14) 202.542 ( 1.050)
(15) 395.868 ( 1.026)
(16) 723.953 ( 1.878)
(17) 1055.266 ( 1.367)
2
3
2
2
2
3
2
2
Na CO = Na O+ CO
CO = CO+ 1/2O2
2
3
2
2
2
K CO = K O+ CO
CO= C+ 1/2O2
2
3
2
2
Li CO + SiO = Li SiO + CO (4) 88.478
CO = C+ O2
2
3
2
2
3
2
2
Na CO + SiO = Na SiO + CO (5) 46.478
SiO = Si+ O2
2
2
3
2
2
3
2
K CO + SiO = K SiO + CO
(6) 39.428
SiO + CO = SiC+ 2O
2
3
2
2
3
2
2 2 2
Li CO = 2Li+ C+ 3/2O
(7) 919.656 ( 1.588)* Li CO + SiO = Li O+ SiC+ 2O (18) 1144.158 ( 1.482)
2
3
2
2 3 2 2 2
Na CO = 2Na+ C+ 3/2O
(8) 844.991 ( 1.459) Na CO + SiO = Na O+ SiC+ 2O (19) 1242.996 ( 1.482)
2 3 2 2 2
2
3
2
K CO = 2K+ C+ 3/2O
(9) 850.496 ( 1.469) K CO + SiO = K O+ SiC+ 2O
(20) 1289.236 ( 1.670)
2
3
2
2
3
2
2
2
Li CO = Li O+ C+ O
(10) 484.761 ( 1.256) CO + Na SiO = Na O+ SiC+ 2O (21) 896.616 ( 1.162)
2
3
2
2
2
2
3
2
2
Na CO = Na O+ C+ O
(11) 583.599 ( 1.512) CO + K SiO = K O+ SiC+ 2O
(22) 942.856 ( 1.222)
2
3
2
2
2
2
3
2
2
*
The electromotive force V in a circuit in which a given current-forming reaction proceeds is given in parentheses.
pressure of carbon dioxide at 750 C. In one case, num carbide cannot be found in the literature, and,
a platinum crucible served as a container for the melt therefore, it is impossible to assess the possibility
and an auxiliary electrode, with platinum wire (S = of formation of a compound of platinum and carbon
2
0
.2 cm ) as working electrode and oxygen platinum at 1023 K. With increasing silicon oxide concentration
reference electrode in air. In the other, glassy carbon in the melt, a new peak of anodic current appears in
2
crucible with working electrode (S = 0.8 cm ) and voltammetric curves (Fig. 1c) on both platinum and
reference electrode made of glassy carbon rods were glassy carbon electrodes. The potential at which this
used. Prior to experiments, reagents of chemically current peak is observed is independent of silicon ox-
pure grade were recrystallized and silicon oxide was
dried at 300 C. As cathode materials in electrolysis
were used platinum and stainless steel. After elec-
trolysis, cathode deposits were analyzed by X-ray
diffraction analysis, Auger spectroscopy, and electron
microscopy.
The rise in current observed in the anodic range
of potentials in voltammetric curves obtained on a
glassy carbon electrode in carbonate melts containing
no SiO is due to oxygen evolution, and the rise in
2
the cathodic region, to deposition of carbon and al-
kali metal (Fig. 1a). The range of potentials in which
there are no redox reactions is 0.5 V narrower in the
case of glassy carbon electrodes, compared with that
for platinum electrodes. This is due to the formation
of carbon oxides on glassy carbon subjected to anodic
polarization. In titration of the melt with silicon oxide
4
3
to a concentration of 5 10 mol cm , which corre-
sponds to the limiting solubility of carbon dioxide, the
rise in current observed in the cathodic range of po-
tentials in the voltammetric curves obtained on a plat-
inum electrode is due to discharge of carbon dioxide
dissolved in the melt to elemental carbon (Fig. 1b).
A peak of anodic current, associated with oxidation of
carbon, is observed in the reverse run. No anodic cur-
rent peak is observed on the glassy carbon electrode.
Fig. 1. Cyclic voltammetric curves obtained in (a) carbo-
nate melt and (b, c) in carbonate melt containing SiO . Tem-
2
1
perature 750 C, potential sweep rate 0.1 V s . (I) Cur-
rent and (E) potential. Electrode: (a, c) glassy carbon and
3
5
Stable platinum monocarbide exists only in the gas
phase [11]. Thermodynamic characteristics of plati-
(b) platinum. SiO concentration (mol cm ): (b) 1 10
2
5
and (c) 9 10
.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 4 2002

564
DEVYATKIN et al.
hardness, easily scratched glass, and possessed semi-
conducting properties.
CONCLUSIONS
1) Carbonate melts containing silicon oxide are
(
unstable because of the occurrence of an exchange
reaction to give carbon dioxide. This process can be
stabilized by creating a carbon dioxide atmosphere
with elevated pressure over the melt.
(2) The optimal conditions of electrochemical syn-
thesis of silicon carbide in carbonate melts containing
silicon oxide are found.
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2
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4
3
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4
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9
. Diagramma plavkosti solevykh sistem (Fusion Dia-
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2
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2
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8
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1
0 20 m. The carbide coating was dense, had high
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 4 2002