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10 Electrochemical Manufacturing Methods Based on Surface …
10.5.3 Graphene by Electrochemical Exfoliation
from Solutions with Non-aqueous Solvents
There is much commonality between electrochemical exfoliation form aqueous and
non-aqueous systems (i.e., cell configuration, cell voltage, electrode materials, etc.).
The non-aqueous solvent is often a component in a water-containing solution and
serves either a co-intercalation molecular agent [79], an electrolyte when it is an
ionic liquid [80, 81] or both. Solvent mixtures containing water are often used for
economic reason. The application of either polar non-aqueous solvents [82, 83],
pure ionic liquids [84, 85] or their mixture [86] was rationalized by the quality of
the exfoliated product, although the nature of the specific ionic liquid used has a
fundamental impact on the exfoliation process. Functionalization of the nanosheets
often takes place during exfoliation in organic solvents; hence, the optimization of
the one-step exfoliation and functionalization process is in the focus of research
nowadays.
The systems containing non-aqueous solvent components(s) were partly optimized for single-electrode exfoliation, either cathodic [79] or anodic [80, 81] one.
When the counter electrode is not a carbon material, the opportunity for double
exfoliation cannot be revealed [80]. Double exfoliation is possible in a singleor double-compartment cell, the latter [87] yielding an opportunity to collect two
kinds of exfoliation product since they may significantly differ due to the dissimilar
functionalization effect of cations and anions.
Concerning the appropriateness of various ions for intercalation, only weak trends
can be outlined. The clarification of the roles of individual ions is difficult when a
solvent mixture is used, the components of which (in particular, water) may also take
place in the intercalation and exfoliation process. In anodic exfoliation, the role of
the anions seems to be very similar to the aqueous systems; hence, it is not discussed
here separately. In the case of the cathodic intercalation, the complex of sodium
ion with dimethyl sulfoxide (DMSO) was assumed to be responsible for the good
exfoliation efficiency in DMSO-containing solutions [79]. N,N-dialkylpirrolidinium
cations proved to be suitable for cathodic intercalation and exfoliation (similarly to
teraalkylammonium cations [82]). For the cations containing an organic ring (like
the N,N-dialkylimidasolium cation [85]), the π-π interaction between the graphene
sheets and the intercalating ions decreases the energetic barrier of the intercalation,
similarly to aromatic compounds applied in purely aqueous solutions [88]. Comparative studies concerning the nature of the cations in the exfoliation process are quite
scarce. From a study comparing three different cations with quaternary nitrogen
atoms and with the same anion, one can conclude that the increase in the cation size
is preferred from the viewpoint of the exfoliation efficiency [86].
The thickness of the exfoliated sheets varies from one synthesis method to another.
If the interaction between the solvent/ions used for the exfoliation and the graphene
sheets is strong, the thickness measured may include an additional molecular layer on
each side of the exfoliated sheet. Hence, a thickness of 1.1 nm (like the one obtained
with exfoliation in 1-octyl-3-methyl-imidazolium hexafluorophosphate [81]) means
10 Electrochemical Manufacturing Methods Based on Surface …
10.5.3 Graphene by Electrochemical Exfoliation
from Solutions with Non-aqueous Solvents
There is much commonality between electrochemical exfoliation form aqueous and
non-aqueous systems (i.e., cell configuration, cell voltage, electrode materials, etc.).
The non-aqueous solvent is often a component in a water-containing solution and
serves either a co-intercalation molecular agent [79], an electrolyte when it is an
ionic liquid [80, 81] or both. Solvent mixtures containing water are often used for
economic reason. The application of either polar non-aqueous solvents [82, 83],
pure ionic liquids [84, 85] or their mixture [86] was rationalized by the quality of
the exfoliated product, although the nature of the specific ionic liquid used has a
fundamental impact on the exfoliation process. Functionalization of the nanosheets
often takes place during exfoliation in organic solvents; hence, the optimization of
the one-step exfoliation and functionalization process is in the focus of research
nowadays.
The systems containing non-aqueous solvent components(s) were partly optimized for single-electrode exfoliation, either cathodic [79] or anodic [80, 81] one.
When the counter electrode is not a carbon material, the opportunity for double
exfoliation cannot be revealed [80]. Double exfoliation is possible in a singleor double-compartment cell, the latter [87] yielding an opportunity to collect two
kinds of exfoliation product since they may significantly differ due to the dissimilar
functionalization effect of cations and anions.
Concerning the appropriateness of various ions for intercalation, only weak trends
can be outlined. The clarification of the roles of individual ions is difficult when a
solvent mixture is used, the components of which (in particular, water) may also take
place in the intercalation and exfoliation process. In anodic exfoliation, the role of
the anions seems to be very similar to the aqueous systems; hence, it is not discussed
here separately. In the case of the cathodic intercalation, the complex of sodium
ion with dimethyl sulfoxide (DMSO) was assumed to be responsible for the good
exfoliation efficiency in DMSO-containing solutions [79]. N,N-dialkylpirrolidinium
cations proved to be suitable for cathodic intercalation and exfoliation (similarly to
teraalkylammonium cations [82]). For the cations containing an organic ring (like
the N,N-dialkylimidasolium cation [85]), the π-π interaction between the graphene
sheets and the intercalating ions decreases the energetic barrier of the intercalation,
similarly to aromatic compounds applied in purely aqueous solutions [88]. Comparative studies concerning the nature of the cations in the exfoliation process are quite
scarce. From a study comparing three different cations with quaternary nitrogen
atoms and with the same anion, one can conclude that the increase in the cation size
is preferred from the viewpoint of the exfoliation efficiency [86].
The thickness of the exfoliated sheets varies from one synthesis method to another.
If the interaction between the solvent/ions used for the exfoliation and the graphene
sheets is strong, the thickness measured may include an additional molecular layer on
each side of the exfoliated sheet. Hence, a thickness of 1.1 nm (like the one obtained
with exfoliation in 1-octyl-3-methyl-imidazolium hexafluorophosphate [81]) means