4
400
X. Ge et al. / Electrochimica Acta 54 (2009) 4397–4402
If the CaS formed had a low solubility in the salt melt, it could
In the present work, an attempt was also made to explain the for-
mation of the needle-like structure close to the surface as shown
in Fig. 3c and d. For the needle structure, further investigations
show that these are actually tubes in micrometer size, the formation
of which can also be attributed to the electrochemical deposition.
It is known from earlier work [26–28] that Cu nanotube can be
synthesized by the direct electrochemical deposition on a porous
membrane with very careful treatment to obtain the ideal orien-
tation. Nanotubes without very good orientation even can grow
from a copper coil without any template and additives [29]. Under
the present experimental conditions, the number of pores on the
outer layer of the pellet is significant as can be seen in Fig. 3a. With
the removal of sulphur during the electrolysis, the porosity would
increase. The likelihood of copper getting deposited along the pores
to form a tube would be high, in analogy with a porous mem-
brane. Consequently, micrometer size pores close to the surface of
the pellet would lead to the formation of the tube structure in the
micrometer size (Fig. 3d). However, preliminary experiments sug-
gest that these two structures, which are confined to the outer layer
can, to a smaller extent, be controlled by a stringent control of the
chemistry of the salt mixture and the purity of the gas atmosphere
as further sulphur depletion from the surface can occur leading to
higher porosity.
be present in the form of Ca2+ and S . As electrolysis proceeds
2−
further, CaS will dissociate into Ca2 and S . The S ions would
be transported to the anode during the electrolysis and would be
oxidized at the anode to form elemental sulphur according to the
anodic reaction (3).
+
2−
2−
During the present series of experiments, the graphite anode
almost remained intact after every experimental run, confirming
the negligible formation of the anodic product, CS2.
EDS analysis of the different locations in a partially reduced pel-
let as shown in Fig. 2, indicates that the reduction pathway proceeds
from outside of the pellet to the inside. Content of copper decreases
towards the interior of the pellet, while Ca and S contents increase. It
should be pointed out that the sample is very clean and the amount
of salt, such as Na, Cl, was insignificant, even though the porous
structure is probably to be filled with molten salts during elec-
trolysis. It is admitted that the high solubility of CaCl2 and NaCl
in distilled water may contribute to some extent. However, it also
indicates that wetting could be poor between the reduced porous
copper and salt melts. This phenomenon means that the separation
of copper and molten salts is quite easy.
Fig. 3 shows the SEM images of Cu S pellets before and after
2
electrolysis. Fig. 3a shows the surface image of sintered pellet before
electrolysis. It is seen the surface of the sintered Cu S pellet consists
The current efficiency and energy consumption can be estimated
from the current–time plots during electrolysis in combination
with corresponding sulphur contents in the electrolysis products
2
of particles with sizes varying in a wide range. After 20 h electroly-
sis, the pellet presents a clear appearance of copper (inset of Fig. 3b).
The particles are of ∼10 m, and well sintered, showing a more
complicated nodular structure. This can probably be attributed to
by chemical analysis. In the case of pure Cu S, shown in Fig. 4a,
2
◦
the relatively low melting point of copper (1083 C) and long elec-
trolysis time of 20 h. It should be noted that Fig. 3b is taken from the
inner section of the reduced pellet. This shows mainly the existence
of nodular structure. Fig. 3c shows the structures corresponding
to the outer part of the pellet. Here, it can be seen that, besides
the nodular structure, two other interesting structures, dendritic
as well as needle crystals, can be found. Fig. 3d presents a clear
view of the needle structure. The typical dendritic structure shown
in Fig. 3c is more likely to be formed by electrodeposition from the
salt, not from the reduction of Cu S.
2
In the present experiments, no special steps were taken to purify
the salt mixtures such as purification by chlorine gas. Further, high
purity argon gas was used in the electrolysis experiments as sup-
plied without taking extra precautions for the removal of traces of
oxygen, moisture and carbon dioxide. This would mean that the
following possible side reactions might occur:
◦
Cu S + 1.5O = Cu O + SO ,
ꢀG (1073 K) = −272.09 kJ
◦
(9)
(10)
(11)
2
2
2
2
Cu S + O = 2Cu + SO ,
ꢁG (1073 K) = −181.55 kJ
2
2
2
◦
2
Cu + 0.5O = Cu O,
ꢁG (1073 K) = −90.55 kJ
2
2
Even assuming that the partial pressure of SO2 as 1 atm, the
equilibrium oxygen pressures of the above reactions are around
−9
1
.5 × 10 atm, which means the above equations are highly pos-
sible.
Since CaCl2 is very sensitive to the moisture, HCl gas may be
generated by the reaction:
CaCl (salt) + 2H O (gas) = Ca(OH) (solid) + 2HCl (salt)
(12)
2
2
2
The generated HCl can react with Cu O according to the reaction:
2
◦
Cu O + 2HCl = 2CuCl + H O,
ꢀG (1073 K) = −76.89 kJ (13)
2
2
CuCl has
a
low decomposition voltage of Ed(1073 K,
CuCl) = 0.939 V, Cu can be deposited to form the dendritic crystals
on the surface. Another possibility is that Cu S may dissolve in the
2
salt melt to form Cu and S , and then Cu+ gets deposited on the
+
2−
Fig. 4. Typical current–time plots for pellet reduction in molten CaCl2–NaCl at
800 C. Pellets are sintered at 400 C in argon: (a) Cu2S pellet, and (b) Cu2S/FeS pellet.
◦
◦
surface. This would explain the observation in Fig. 3c.