70
5 Mechanisms Involved in the Electrolytic Fabrication …
material contained Li 2 CO 3 formed by the occurrence of the reaction between the
anodically formed O 2 (reaction 5.5b), with the anode itself and the Li 2 O dissolved
in the melt (reaction 5.3):
O 2 + C + Li 2 O = Li 2 CO 3
(5.9)
The purification of the graphene product from the Li 2 CO 3 component can be
achieved by heating the material at temperatures greater than the evaporation
temperature of lithium carbonate.
5.4 Electrochemical Erosion of Graphite Under
Hydrogen-Containing Atmospheres
The likelihood of the formation of H
+ in the molten LiCl resulting from the hydrolysis
of melt in moist atmospheres was discussed in Sect. 5.3. This procedure is green since
the only raw materials used to produce graphene nanosheets are graphite, water and
electricity. Nevertheless, the disintegration of water in the melt additionally generates
oxygen species in the melt which leads to the formation of Li 2 CO 3 mixed with
graphene nanosheets. Therefore, an extra thermal treatment is needed to remove the
lithium carbonate by-product from the graphene nanosheets.
What’s more, it was found that graphite is able to be exfoliated into single or
few-layer 3D graphene, by cathodic polarization of the material in LiCl melt in Ar
atmosphere containing H 2 (Figs. 4.1h and i, and 4.2 (III)). The proposed mechanism
for this direct formation of graphene in molten salt is summarized in Fig. 5.5.
Under a hydrogen-containing atmosphere, in the absence of protons in the melt,
the electrolysis process begins with the decomposition of LiCl under the influence
of the potential difference applied. It consequently leads to the evolution of Cl 2 on
the anode. The chlorine released can react with the H 2 present in the atmosphere
above the melt to form hydrogen chloride, which then dissolves in the molten salt to
form H
+ . The latter is the responsible for the exfoliation process, as explained in the
Sect. 5.3 [25, 26]. Ideally, this cycle can repeat itself indefinitely until the complete
exfoliation of the graphite cathode. Figure 5.4 illustrates the mechanism proposed
for the formation of graphene in LiCl melt under H 2 containing Ar atmosphere.
It should be mentioned that the molten salt technology developed may provide an
effective strategy for the green and economic fabrication of high-quality graphene,
with an interesting combination of properties including high electronic conductivity, as high as 5.8 × 10
5 S m
−1 , high surface area of up to 500 m
2 g
−1 and high
oxidation resistance of up to 550 °C. The consumables of the process are mainly
electrical energy and water/hydrogen, and no harmful by-product is produced, hence
the process is environmentally safe. The cost and the specific energy consumption
for the preparation of graphene in molten LiCl can be estimated to be about US
$10–20 and 25 kWh per kilogram of the graphene product, respectively [14]. These
5 Mechanisms Involved in the Electrolytic Fabrication …
material contained Li 2 CO 3 formed by the occurrence of the reaction between the
anodically formed O 2 (reaction 5.5b), with the anode itself and the Li 2 O dissolved
in the melt (reaction 5.3):
O 2 + C + Li 2 O = Li 2 CO 3
(5.9)
The purification of the graphene product from the Li 2 CO 3 component can be
achieved by heating the material at temperatures greater than the evaporation
temperature of lithium carbonate.
5.4 Electrochemical Erosion of Graphite Under
Hydrogen-Containing Atmospheres
The likelihood of the formation of H
+ in the molten LiCl resulting from the hydrolysis
of melt in moist atmospheres was discussed in Sect. 5.3. This procedure is green since
the only raw materials used to produce graphene nanosheets are graphite, water and
electricity. Nevertheless, the disintegration of water in the melt additionally generates
oxygen species in the melt which leads to the formation of Li 2 CO 3 mixed with
graphene nanosheets. Therefore, an extra thermal treatment is needed to remove the
lithium carbonate by-product from the graphene nanosheets.
What’s more, it was found that graphite is able to be exfoliated into single or
few-layer 3D graphene, by cathodic polarization of the material in LiCl melt in Ar
atmosphere containing H 2 (Figs. 4.1h and i, and 4.2 (III)). The proposed mechanism
for this direct formation of graphene in molten salt is summarized in Fig. 5.5.
Under a hydrogen-containing atmosphere, in the absence of protons in the melt,
the electrolysis process begins with the decomposition of LiCl under the influence
of the potential difference applied. It consequently leads to the evolution of Cl 2 on
the anode. The chlorine released can react with the H 2 present in the atmosphere
above the melt to form hydrogen chloride, which then dissolves in the molten salt to
form H
+ . The latter is the responsible for the exfoliation process, as explained in the
Sect. 5.3 [25, 26]. Ideally, this cycle can repeat itself indefinitely until the complete
exfoliation of the graphite cathode. Figure 5.4 illustrates the mechanism proposed
for the formation of graphene in LiCl melt under H 2 containing Ar atmosphere.
It should be mentioned that the molten salt technology developed may provide an
effective strategy for the green and economic fabrication of high-quality graphene,
with an interesting combination of properties including high electronic conductivity, as high as 5.8 × 10
5 S m
−1 , high surface area of up to 500 m
2 g
−1 and high
oxidation resistance of up to 550 °C. The consumables of the process are mainly
electrical energy and water/hydrogen, and no harmful by-product is produced, hence
the process is environmentally safe. The cost and the specific energy consumption
for the preparation of graphene in molten LiCl can be estimated to be about US
$10–20 and 25 kWh per kilogram of the graphene product, respectively [14]. These