5. Combining a high porosity and a high electrical conductivity

Unlike other carbonaceous materials, another remarkable feature of carbon xerogels is that despite being materials with a high porosity they also show low electrical resistivity, i.e., they are good electrical conductors, which are usually opposed characteristics. This is because good electrical conductors are usually materials with ordered carbonaceous structures, with condensed aromatic rings to a greater or lesser extent that have some delocalized π electrons that are free to migrate in the plane, making these kinds of material electrically conductive. This is the case of graphite or graphene [46]. On the other hand, porous materials have disordered structures due to their numerous defects, voids, etc., which are responsible for their high porosity. Such structural defects are not favorable for the movement of the electrons, and therefore porous material is usually synonymous with insulation. In fact, a lot of current research is focused on obtaining the optimal combination of both properties (porosity and electric conductivity) in a single material. The approaches for achieving this involve doping techniques or creating composites of porous materials with highly conductive ones such as carbon nanofibers or graphene. In this way, the resultant composite or doped material retains the porosity of the matrix, but its inherent electrical resistivity is reduced [41, 47–52].

when carbon xerogels with specific surface areas of 2400 m2 g<sup>1</sup> are produced, i.e., more

obtain a high porosity, the carbonization/activation conditions must be more intense, i.e., more residence time is required. As a result, although extra microporosity is created, the carbon

Figure 8 presents HRTEM images of the nanostructure of a carbon xerogel and YP-50F. As can be seen, both materials are composed of interconnected carbon sheets. Nevertheless, the structure of this carbon xerogel is dense and compact unlike that of YP-50F where there are some voids. According to Canal-Rodríguez et al. [41], these structural differences possibly

This special feature of carbon xerogels, i.e., the combination of a tailored porosity with a high electrical conductivity, makes them perfect candidates for a wide variety of applications in many different areas such as (i) clean energy, i.e., for use as active materials in energy storage devices (this application will be discussed in detail in the following sections) or even in energy generation devices such as fuel cells [53–55]; (ii) environmental issues and better use of natural resources, in water-treatment and in the desalination of brackish water devices that function on the basis of capacitive deionization [56–62]; (iii) biotechnology, where they are used as

Electrical energy storage is required in numerous applications, not only as a result of the increasing number of electronic devices that we use in our daily life but also as a means to ensure a more rational and sustainable use of energy resources. Thus, there is an increasing demand for more efficient energy storage devices in telecommunications, stand-by power systems, uninterruptible power supply systems, electric/hybrid vehicles, energy recovery systems, burst and regenerative power in industry and transportation, electric grid modulation or

. This is because to

Carbon Xerogels: The Bespoke Nanoporous Carbons http://dx.doi.org/10.5772/intechopen.71255 77

porous xerogels, their electrical conductivity also increases to ca. 240 S m<sup>1</sup>

structure has more time to reorganize and order itself.

Figure 8. HRTEM images of a carbon xerogel (a) and YP-50F (b).

explain the differences in their electrical conductivity.

supports of biomolecules as active materials in sensors [63, 64], etc.

6. Nanoporous carbon xerogels in energy storage

as complements to renewable energies.

However, even without doping, carbon xerogels have high electrical conductivities of 110 S m<sup>1</sup> when their SBET is around 1600 m<sup>2</sup> g<sup>1</sup> , which is double the electrical conductivity of a commercially activated carbon (50 S m<sup>1</sup> ) with the same specific area (YP-50F of Kuraray) [41]. This is due to their peculiar carbonaceous structure. On the one hand, as mentioned above, the polymeric structure of carbon xerogels consists of interconnected carbonaceous nodules with the most characteristic porosity between them. These nodules make up the carbonaceous network described in Figure 2. Such polycondensed structures are susceptible to ordering due to the effect of temperature resulting in a polymeric structure formed by ordered nodules interconnected with each other. The end product is a very special macroscopically porous material, but ordered at the level of nodules. As a consequence, carbon xerogels differ from active carbons in that they are able to combine a perfect porosity and electrical conductivity. On the other hand, during the carbonization/activation step not only is microporosity generated, but also the carbonaceous structure of the polymeric network undergoes a certain ordering and reorganization due to the high temperatures used (i.e., 1000C). In fact,

Figure 8. HRTEM images of a carbon xerogel (a) and YP-50F (b).

5. Combining a high porosity and a high electrical conductivity

Figure 7. Different kinds of pores according to the curvature of their surface.

76 Porosity - Process, Technologies and Applications

the porosity of the matrix, but its inherent electrical resistivity is reduced [41, 47–52].

when their SBET is around 1600 m<sup>2</sup> g<sup>1</sup>

commercially activated carbon (50 S m<sup>1</sup>

However, even without doping, carbon xerogels have high electrical conductivities of 110 S m<sup>1</sup>

[41]. This is due to their peculiar carbonaceous structure. On the one hand, as mentioned above, the polymeric structure of carbon xerogels consists of interconnected carbonaceous nodules with the most characteristic porosity between them. These nodules make up the carbonaceous network described in Figure 2. Such polycondensed structures are susceptible to ordering due to the effect of temperature resulting in a polymeric structure formed by ordered nodules interconnected with each other. The end product is a very special macroscopically porous material, but ordered at the level of nodules. As a consequence, carbon xerogels differ from active carbons in that they are able to combine a perfect porosity and electrical conductivity. On the other hand, during the carbonization/activation step not only is microporosity generated, but also the carbonaceous structure of the polymeric network undergoes a certain ordering and reorganization due to the high temperatures used (i.e., 1000C). In fact,

, which is double the electrical conductivity of a

) with the same specific area (YP-50F of Kuraray)

Unlike other carbonaceous materials, another remarkable feature of carbon xerogels is that despite being materials with a high porosity they also show low electrical resistivity, i.e., they are good electrical conductors, which are usually opposed characteristics. This is because good electrical conductors are usually materials with ordered carbonaceous structures, with condensed aromatic rings to a greater or lesser extent that have some delocalized π electrons that are free to migrate in the plane, making these kinds of material electrically conductive. This is the case of graphite or graphene [46]. On the other hand, porous materials have disordered structures due to their numerous defects, voids, etc., which are responsible for their high porosity. Such structural defects are not favorable for the movement of the electrons, and therefore porous material is usually synonymous with insulation. In fact, a lot of current research is focused on obtaining the optimal combination of both properties (porosity and electric conductivity) in a single material. The approaches for achieving this involve doping techniques or creating composites of porous materials with highly conductive ones such as carbon nanofibers or graphene. In this way, the resultant composite or doped material retains when carbon xerogels with specific surface areas of 2400 m2 g<sup>1</sup> are produced, i.e., more porous xerogels, their electrical conductivity also increases to ca. 240 S m<sup>1</sup> . This is because to obtain a high porosity, the carbonization/activation conditions must be more intense, i.e., more residence time is required. As a result, although extra microporosity is created, the carbon structure has more time to reorganize and order itself.

Figure 8 presents HRTEM images of the nanostructure of a carbon xerogel and YP-50F. As can be seen, both materials are composed of interconnected carbon sheets. Nevertheless, the structure of this carbon xerogel is dense and compact unlike that of YP-50F where there are some voids. According to Canal-Rodríguez et al. [41], these structural differences possibly explain the differences in their electrical conductivity.

This special feature of carbon xerogels, i.e., the combination of a tailored porosity with a high electrical conductivity, makes them perfect candidates for a wide variety of applications in many different areas such as (i) clean energy, i.e., for use as active materials in energy storage devices (this application will be discussed in detail in the following sections) or even in energy generation devices such as fuel cells [53–55]; (ii) environmental issues and better use of natural resources, in water-treatment and in the desalination of brackish water devices that function on the basis of capacitive deionization [56–62]; (iii) biotechnology, where they are used as supports of biomolecules as active materials in sensors [63, 64], etc.
