RGO form stable cryogels suitable for unidirectional ice crystallization [173, 221,
223]. As-obtained 3D porous monoliths are good supports for the nanocrystalline
gold and silver catalysts [173].
Cryogenic processing is often applied to the preparation of carbon nanotubes
(CNT) in their purification stage. These nanoparticles are usually obtained by
means of catalytic decomposition of hydrocarbons. The application of CNTs
requires separation of metal catalyst by acid dissolution, followed by careful
washing of the reaction products. Separation of the different kinds of nanotubes
that often grow simultaneously during CVD is also performed in the liquid phase.
Air drying of as-obtained CNT suspensions results in significant aggregation of
nanotubes, whereas freeze-drying yields re-dispersible, free flowing powders [226–
229]. Due to the high mechanical strength of CNTs, the synthesis of solid CNT
foams using the same approach results in the formation of stable porous frameworks with variable porosity, especially when a small amount of polymer binder is
added [230–233]. Good electrical conductivity of these materials makes them
suitable electrode material for proton-exchange membrane fuel cells [230, 233].
Both CNTs and graphene-based particulates are well-known as efficient conductive dopants. Cryogel synthesis of composite electrode materials with these
nanocrystalline components causes considerable improvement in the electrochemical performance of Li-ion battery anode materials, which often suffer from insufficient electronic conductivity and the loss of electrical contact between particles
[234–237]. Corresponding enhancement of the electrochemical capacitance and
rate capability is also observed for supercapacitor electrodes made of “carbon–
carbon” composite formed by CNTs and a small amount of reduced graphene oxide
[238]. Along with a better electrical conductivity, the introduction of 1 vol%
multiwalled CNTs could significantly enhance the fracture toughness of
hot-pressed dense alumina ceramics [239]. It is essential in all these cases that the
surface of CNTs should be first oxidized in order to ensure good contact with the
oxide ceramic matrix. The Vickers microhardness and mechanical wear resistance
of copper-based metal matrix composites obtained by spark plasma sintering can
also be improved significantly by adding 4 % of double-wall CNTs [240], while the
corresponding thermal conductivity values decrease compared with pure copper
[241]. Along with doping of other components, the CNTs can also be doped by the
cryogel method. In this case, CNTs have been filled with CdCl 2 solution under
vacuum and carefully washed. Further processing of the freeze-drying product in
H 2 S environment results in the formation of nanocrystalline H 2 S inside the
nanotubes [242].
4 Conclusions
The different materials reviewed in this chapter demonstrate several synthetic
strategies in which the cryogel method can be successfully applied. Two basic
features of cryogel synthesis make this method useful in different situations: the
Inorganic Cryogels
237
223]. As-obtained 3D porous monoliths are good supports for the nanocrystalline
gold and silver catalysts [173].
Cryogenic processing is often applied to the preparation of carbon nanotubes
(CNT) in their purification stage. These nanoparticles are usually obtained by
means of catalytic decomposition of hydrocarbons. The application of CNTs
requires separation of metal catalyst by acid dissolution, followed by careful
washing of the reaction products. Separation of the different kinds of nanotubes
that often grow simultaneously during CVD is also performed in the liquid phase.
Air drying of as-obtained CNT suspensions results in significant aggregation of
nanotubes, whereas freeze-drying yields re-dispersible, free flowing powders [226–
229]. Due to the high mechanical strength of CNTs, the synthesis of solid CNT
foams using the same approach results in the formation of stable porous frameworks with variable porosity, especially when a small amount of polymer binder is
added [230–233]. Good electrical conductivity of these materials makes them
suitable electrode material for proton-exchange membrane fuel cells [230, 233].
Both CNTs and graphene-based particulates are well-known as efficient conductive dopants. Cryogel synthesis of composite electrode materials with these
nanocrystalline components causes considerable improvement in the electrochemical performance of Li-ion battery anode materials, which often suffer from insufficient electronic conductivity and the loss of electrical contact between particles
[234–237]. Corresponding enhancement of the electrochemical capacitance and
rate capability is also observed for supercapacitor electrodes made of “carbon–
carbon” composite formed by CNTs and a small amount of reduced graphene oxide
[238]. Along with a better electrical conductivity, the introduction of 1 vol%
multiwalled CNTs could significantly enhance the fracture toughness of
hot-pressed dense alumina ceramics [239]. It is essential in all these cases that the
surface of CNTs should be first oxidized in order to ensure good contact with the
oxide ceramic matrix. The Vickers microhardness and mechanical wear resistance
of copper-based metal matrix composites obtained by spark plasma sintering can
also be improved significantly by adding 4 % of double-wall CNTs [240], while the
corresponding thermal conductivity values decrease compared with pure copper
[241]. Along with doping of other components, the CNTs can also be doped by the
cryogel method. In this case, CNTs have been filled with CdCl 2 solution under
vacuum and carefully washed. Further processing of the freeze-drying product in
H 2 S environment results in the formation of nanocrystalline H 2 S inside the
nanotubes [242].
4 Conclusions
The different materials reviewed in this chapter demonstrate several synthetic
strategies in which the cryogel method can be successfully applied. Two basic
features of cryogel synthesis make this method useful in different situations: the
Inorganic Cryogels
237