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10 Electrochemical Manufacturing Methods Based on Surface …
common that the overall process starts with a mild anodization step before achieving
the final cell voltage, which serves the intercalation and swelling but no formation
of by-products, gas evolution or graphene oxidation [63, 71]. Another approach is
that carbon anodes are used as an electrode pair with an alternating current power
supply so that both electrodes undergo intercalation and the subsequent exfoliation
process [72, 73]. Harmful impact of the cathodic polarization for half of the working
time on either the quality or the quantity of the exfoliation product was not reported;
instead, it was assumed that the cathodic polarization reduce the oxygen content of
the product [63].
Concerning the quality of the carbon source, high-purity graphite and HOPG
[65] are customary; nevertheless, low-quality carbon sources are also suitable for
the exfoliation. An example is the application of used carbon from non-rechargeable
batteries [64]. This can be regarded as a recycling means of used material, and
the low quality of the starting material seems to be tolerable since the exfoliation
product has to be purified anyway (suspending in dimethyl formamide is the most
common way of separation from the aqueous solution for further storage and use).
Compressed carbon black also proved to be suitable for aqueous electrochemical
exfoliation [74]. Since the particle size of carbon powders is not at all smaller than
the typical lateral dimension of the graphene sheets produced (which is at most a
few tens of micrometers), the application of powders enhances the density of the
graphene plane edges in the stating material, which increase the probability of the
intercalation and the subsequent delamination.
Although the application of parallel plates and rods is the most common and the
simplest cell geometry reported, it was suggested that the increase of the process
efficiency requires the completion of the exfoliation of partly separated sheets. This
lead to the cell geometry where the carbon anode is placed to the bottom of the cell
[64] or to a basket [74] from which the partly exfoliated grains cannot escape because
of the sedimentation, and they remain in electrical contact with the rest of the anode.
Hence, the exfoliation can take place also in the case when particles are disintegrated
from the bulk of the anode. This method was named as multiple exfoliation.
The degree of the exfoliation varies from one process to another. The comparison
is difficult due to the diverse nature of the exfoliation products (e.g., the number
of the graphene sheets in the nanoflakes regarded as the end product). The best
results are around 50–60% of the initial mass of the anode [66, 75]. The process
time decreased significantly with the improvement of the exfoliation process, and
high-degree exfoliation electrolysis can now be performed within minutes.
The number of graphene layers delaminating together and making the final product
depends on the overall set of experimental conditions, and it is difficult to establish
a trend. Since the application of the exfoliation products is by far not only the same
as that of pure graphene (including also, among others, paint ingredient [76], supercapacitor materials [67, 68], printed electronic contacts [69] and electrocatalysts
mostly for oxygen reduction [71, 72]), the achievement of the reproducibility of the
process is often enough. Few-layer graphene sheets with a particular thickness range
[64–66, 74] or flakes with a well-defined number of layers [62] all can be useful.
10 Electrochemical Manufacturing Methods Based on Surface …
common that the overall process starts with a mild anodization step before achieving
the final cell voltage, which serves the intercalation and swelling but no formation
of by-products, gas evolution or graphene oxidation [63, 71]. Another approach is
that carbon anodes are used as an electrode pair with an alternating current power
supply so that both electrodes undergo intercalation and the subsequent exfoliation
process [72, 73]. Harmful impact of the cathodic polarization for half of the working
time on either the quality or the quantity of the exfoliation product was not reported;
instead, it was assumed that the cathodic polarization reduce the oxygen content of
the product [63].
Concerning the quality of the carbon source, high-purity graphite and HOPG
[65] are customary; nevertheless, low-quality carbon sources are also suitable for
the exfoliation. An example is the application of used carbon from non-rechargeable
batteries [64]. This can be regarded as a recycling means of used material, and
the low quality of the starting material seems to be tolerable since the exfoliation
product has to be purified anyway (suspending in dimethyl formamide is the most
common way of separation from the aqueous solution for further storage and use).
Compressed carbon black also proved to be suitable for aqueous electrochemical
exfoliation [74]. Since the particle size of carbon powders is not at all smaller than
the typical lateral dimension of the graphene sheets produced (which is at most a
few tens of micrometers), the application of powders enhances the density of the
graphene plane edges in the stating material, which increase the probability of the
intercalation and the subsequent delamination.
Although the application of parallel plates and rods is the most common and the
simplest cell geometry reported, it was suggested that the increase of the process
efficiency requires the completion of the exfoliation of partly separated sheets. This
lead to the cell geometry where the carbon anode is placed to the bottom of the cell
[64] or to a basket [74] from which the partly exfoliated grains cannot escape because
of the sedimentation, and they remain in electrical contact with the rest of the anode.
Hence, the exfoliation can take place also in the case when particles are disintegrated
from the bulk of the anode. This method was named as multiple exfoliation.
The degree of the exfoliation varies from one process to another. The comparison
is difficult due to the diverse nature of the exfoliation products (e.g., the number
of the graphene sheets in the nanoflakes regarded as the end product). The best
results are around 50–60% of the initial mass of the anode [66, 75]. The process
time decreased significantly with the improvement of the exfoliation process, and
high-degree exfoliation electrolysis can now be performed within minutes.
The number of graphene layers delaminating together and making the final product
depends on the overall set of experimental conditions, and it is difficult to establish
a trend. Since the application of the exfoliation products is by far not only the same
as that of pure graphene (including also, among others, paint ingredient [76], supercapacitor materials [67, 68], printed electronic contacts [69] and electrocatalysts
mostly for oxygen reduction [71, 72]), the achievement of the reproducibility of the
process is often enough. Few-layer graphene sheets with a particular thickness range
[64–66, 74] or flakes with a well-defined number of layers [62] all can be useful.