Carbon cryogels exhibiting low density, good electronic conductivity, and
significant flexibility are also very useful for the development of composite anodes
for lithium-ion batteries, because the electrochemically active components of the
batteries often suffer from significant volume expansion and related integrity loss
during electrochemical cycling [207, 208]. The mechanical strength of carbon
cryogels is often insufficient for a number of applications. This problem can be
solved by the reinforcement of cryogels using ultradisperse particles and fibers of
other substances that can be introduced at the early stages of polymer precursor
synthesis [206, 209, 210]. A similar approach can be used for modification of the
sorption properties of carbon cryogels by creating a composite with zeolites [211],
while the electronic conductivity can be further enhanced by the introduction of
graphene colloid into the starting mixture for RF polycondensation [212].
3.2 Carbon-Based Particulate Materials
Cryogenic processing of carbon-based nanomaterials appeared during the last two
decades is similar to the processing of their oxide counterparts. The two main tasks
solved by freezing and freeze-drying of carbon-containing suspensions are the
isolation of nanoparticles from the dispersion medium and the directed formation
of 2D and 3D structures from these particles. In the preparation of carbon-based
nanomaterials, since supercritical drying is applied more frequently than freezedrying, the products of freeze-drying are also often called “aerogels.”
Taking into account the fact that, in many studies, single-layer graphene
nanosheets have not been isolated and investigated, it is more reasonable to call
them “graphene materials” [213]. Graphene-based cryogels have been prepared by
the reduction of exfoliated graphite oxide [214–217] by templated chemical vapor
deposition (CVD)-based synthesis from CH 4 on MgO catalyst [218], and by
ultrasonic agitation of flake graphite in a specially selected nonpolar solvent
[219]. It seems that the latter kind of cryogel is closest to graphene nanomaterials
because true graphene consisting of carbon atoms should be hydrophobic [220].
Re-dispersibility of the other cryogel products in water could be realized by the
oxidation of their surface. However, some of these cryogel-based materials demonstrate outstanding functional properties, such as a specific surface area of over
2,000 m
2 g
À1 , high methane storage capacity [218], high electrochemical capacitance in aqueous alkaline electrolytes [214], and good electrochemical performance
as anode material of Li-ion batteries [217].
Increasing application of graphene oxide as a precursor for graphene-based
materials has revealed a number of interesting properties of the reduced graphene
oxides and the poorly conducting but hydrophilic graphene oxide. First of all, the
graphene oxide-based materials (GO) have enhanced sorption activities [221–223]
and good catalytic properties [224, 225]. They are usually obtained by the oxidative
exfoliation of graphite, while their subsequent reduction results in the synthesis of a
number of reduced graphene oxides (RGO) of different C:O ratios. Both GO and
236
O.A. Shlyakhtin
significant flexibility are also very useful for the development of composite anodes
for lithium-ion batteries, because the electrochemically active components of the
batteries often suffer from significant volume expansion and related integrity loss
during electrochemical cycling [207, 208]. The mechanical strength of carbon
cryogels is often insufficient for a number of applications. This problem can be
solved by the reinforcement of cryogels using ultradisperse particles and fibers of
other substances that can be introduced at the early stages of polymer precursor
synthesis [206, 209, 210]. A similar approach can be used for modification of the
sorption properties of carbon cryogels by creating a composite with zeolites [211],
while the electronic conductivity can be further enhanced by the introduction of
graphene colloid into the starting mixture for RF polycondensation [212].
3.2 Carbon-Based Particulate Materials
Cryogenic processing of carbon-based nanomaterials appeared during the last two
decades is similar to the processing of their oxide counterparts. The two main tasks
solved by freezing and freeze-drying of carbon-containing suspensions are the
isolation of nanoparticles from the dispersion medium and the directed formation
of 2D and 3D structures from these particles. In the preparation of carbon-based
nanomaterials, since supercritical drying is applied more frequently than freezedrying, the products of freeze-drying are also often called “aerogels.”
Taking into account the fact that, in many studies, single-layer graphene
nanosheets have not been isolated and investigated, it is more reasonable to call
them “graphene materials” [213]. Graphene-based cryogels have been prepared by
the reduction of exfoliated graphite oxide [214–217] by templated chemical vapor
deposition (CVD)-based synthesis from CH 4 on MgO catalyst [218], and by
ultrasonic agitation of flake graphite in a specially selected nonpolar solvent
[219]. It seems that the latter kind of cryogel is closest to graphene nanomaterials
because true graphene consisting of carbon atoms should be hydrophobic [220].
Re-dispersibility of the other cryogel products in water could be realized by the
oxidation of their surface. However, some of these cryogel-based materials demonstrate outstanding functional properties, such as a specific surface area of over
2,000 m
2 g
À1 , high methane storage capacity [218], high electrochemical capacitance in aqueous alkaline electrolytes [214], and good electrochemical performance
as anode material of Li-ion batteries [217].
Increasing application of graphene oxide as a precursor for graphene-based
materials has revealed a number of interesting properties of the reduced graphene
oxides and the poorly conducting but hydrophilic graphene oxide. First of all, the
graphene oxide-based materials (GO) have enhanced sorption activities [221–223]
and good catalytic properties [224, 225]. They are usually obtained by the oxidative
exfoliation of graphite, while their subsequent reduction results in the synthesis of a
number of reduced graphene oxides (RGO) of different C:O ratios. Both GO and
236
O.A. Shlyakhtin