10.6 Molten Salt Electrolysis Methods for the Synthesis of Nanostructures
353
molten salt electrolysis technique. These results are as follows: (i) The weight of
the cathode increases at low current density [122]. Achievement of a critical current
density accompanied with the production of molten metal in excess to the intercalation process is required for the cathode delamination. In other works, as long as the
intercalated metal can diffuse away from the surface to the bulk carbon without a
supersaturation of the surface, the delamination process is negligible. (ii) By applying
potential-controlled experiment, delamination of the cathode was detected at electrode potentials where the liquid metal also appeared at the surface of the cathode
[121], which was preceded by the intercalation of the metal at lower overvoltages.
Nevertheless, the accumulation of the liquid metal at the cathode is an inhibiting
factor for the delamination process [128]. (iii) The alkali metal can be found in the
cathode rod after the electrolysis process, especially in its outer layer that was in
direct contact with the molten salt [128].
Quantum chemical calculations also verified the roll-up formation mechanism of
carbon nanotubes from delaminated graphene sheets [130]. The most likely formation
route was characterized with the surface diffusion of the alkali metal atoms toward
the edge of a graphene sheet, whereas halogen atoms remain at the sites of their
initial attachment at any point of the sheet. When the saturation of the sheet surface
with alkali metal and halogen atoms is achieved and they start interacting with each
other, the graphene sheet twists spontaneously to form a tubular structure.
The typical electrolysis time with fixed-polarity molten salt cells is around 4–
5 min. The bottleneck of the process is the cathode consumption that reduces the
active surface area of the cathode, which is counteracted by the degradation-induced
change in the surface morphology that leads to a diminished current density. With
the periodical change of polarity with identical-sized carbon rods, the electrolysis
time could be increased by a factor of four [131], which was accompanied by a more
homogeneous product distribution.
Concerning the electrical control of the cell, the current regulation was the early
solution. The cell voltage was typically below 10 V, although this value includes
all resistive contributions in the circuit (lead wires, contact resistance at the metal
wire/carbon junction, resistance of the carbon electrodes and the ohmic loss within
the melt). The total current was given in the range of 1–10 A, which was correctly
converted to a current density unit in later works only [122]. Potential-controlled
3-electrode setups [121] and the voltage-controlled two-electrode preparative experiments appeared much later [125, 128]. The latter control mode was reported to
help eliminating the cathode blocking by the molten metal (which also floats on the
surface of the molten salt and can cause a short-circuit).
The floating (suspended) carbon particles were not reported to lead to a shortcircuit during the limited-time experiments. After the electrolysis is finished, the
melt is left to solidify in the anode crucible. The solidified salt–carbon blend is then
mixed with water, and the carbon nanoparticles are extracted with a non- or slightly
polar solvent. The common extraction agent is toluene [120, 126], even though a
detailed study concerning the separation efficiency found [132] that ethyl acetate as
separation agent has the highest yield.
353
molten salt electrolysis technique. These results are as follows: (i) The weight of
the cathode increases at low current density [122]. Achievement of a critical current
density accompanied with the production of molten metal in excess to the intercalation process is required for the cathode delamination. In other works, as long as the
intercalated metal can diffuse away from the surface to the bulk carbon without a
supersaturation of the surface, the delamination process is negligible. (ii) By applying
potential-controlled experiment, delamination of the cathode was detected at electrode potentials where the liquid metal also appeared at the surface of the cathode
[121], which was preceded by the intercalation of the metal at lower overvoltages.
Nevertheless, the accumulation of the liquid metal at the cathode is an inhibiting
factor for the delamination process [128]. (iii) The alkali metal can be found in the
cathode rod after the electrolysis process, especially in its outer layer that was in
direct contact with the molten salt [128].
Quantum chemical calculations also verified the roll-up formation mechanism of
carbon nanotubes from delaminated graphene sheets [130]. The most likely formation
route was characterized with the surface diffusion of the alkali metal atoms toward
the edge of a graphene sheet, whereas halogen atoms remain at the sites of their
initial attachment at any point of the sheet. When the saturation of the sheet surface
with alkali metal and halogen atoms is achieved and they start interacting with each
other, the graphene sheet twists spontaneously to form a tubular structure.
The typical electrolysis time with fixed-polarity molten salt cells is around 4–
5 min. The bottleneck of the process is the cathode consumption that reduces the
active surface area of the cathode, which is counteracted by the degradation-induced
change in the surface morphology that leads to a diminished current density. With
the periodical change of polarity with identical-sized carbon rods, the electrolysis
time could be increased by a factor of four [131], which was accompanied by a more
homogeneous product distribution.
Concerning the electrical control of the cell, the current regulation was the early
solution. The cell voltage was typically below 10 V, although this value includes
all resistive contributions in the circuit (lead wires, contact resistance at the metal
wire/carbon junction, resistance of the carbon electrodes and the ohmic loss within
the melt). The total current was given in the range of 1–10 A, which was correctly
converted to a current density unit in later works only [122]. Potential-controlled
3-electrode setups [121] and the voltage-controlled two-electrode preparative experiments appeared much later [125, 128]. The latter control mode was reported to
help eliminating the cathode blocking by the molten metal (which also floats on the
surface of the molten salt and can cause a short-circuit).
The floating (suspended) carbon particles were not reported to lead to a shortcircuit during the limited-time experiments. After the electrolysis is finished, the
melt is left to solidify in the anode crucible. The solidified salt–carbon blend is then
mixed with water, and the carbon nanoparticles are extracted with a non- or slightly
polar solvent. The common extraction agent is toluene [120, 126], even though a
detailed study concerning the separation efficiency found [132] that ethyl acetate as
separation agent has the highest yield.