4 Catalyst Materials for Oxygen Reduction Reaction
89
especially suitable for liquid phase reaction systems containing macromolecular
substances to participate in the reaction.
(2) Carbon nanofibers have the characteristic of inactive surface chemistry, which
enables them to exist stably under strong acidic and alkaline conditions. At the
same time, carbon nanofibers have certain stability in air at normal temperature.
(3) Carbon nanofibers have unique electronic characteristics. CNFs graphite sheet
has delocalized π electrons, which makes it to have good conductivity similar to
graphite. However, because the graphite layer of CNFs is continuously curled in
space, the electronic characteristics of CNFs are obviously different from that
of complete graphite, and the distribution and density of electron clouds are also
slightly different. The surface structure of CNFs synthesized by vapor deposition
inevitably introduces defects and carbon nanofiber end atoms, which have higher
energy and can further affect their electronic properties. Research shows that
curling of CNTs graphite sheet can cause further hybridization between sp
2
orbit of carbon and d orbit of Ni and other catalysts, and its acting force is far
greater than that between catalyst and graphite [33]. Therefore, when CNTs are
used as catalyst carrier, They are more conducive to the positioning and loading
of metal catalyst on its surface, to make the catalyst more evenly dispersed on
the surface of CNTs, and to improve the activity of the catalyst.
(4) Carbon nanofibers have good mechanical stability. Almost every carbon atom
in CNFs forms a stable covalent bond with the surrounding three carbon atoms.
The structure is tight, both ends are closed, and there is no unbroken lone pair.
This structure makes CNFs have good mechanical stability. At the same time,
large π bonds formed between carbon atoms have higher binding energy, which
greatly improves the stretching ability of CNFs. Some studies have shown that
the surface structure of CNFs hardly changes after ultrasonic dispersion, while
for activated carbon, many fragments can be obviously found.
(5) Carbon nanofibers have good thermal conductivity. The unique structure of
CNFs makes its thermal conductivity have great difference between the direction perpendicular to the axis and the direction parallel to the axis. Its thermal
conductivity along the axis direction is almost comparable to that of diamond.
However, the thermal conductivity perpendicular to the axial direction is very
poor and hardly has thermal conductivity.
(6) Adjustability of carbon nanofiber surface structure. The microstructure of CNFs
obtained by different preparation methods is different. Therefore, by changing
the preparation method and preparing different CNFs for appropriate systems,
it is beneficial to improve their loading capacity or catalytic capacity.
4.1.1.5 Graphene
Simply put, graphene is a single-layer graphite sheet with only one carbon atom thick,
and carbon and carbon atoms are closely arranged in a sp
2 heterozygous manner into
a honeycomb six-membered ring structure (Fig. 4.1a). The distance between carbon
and carbon atoms is about 0.142 nm. There are three σ bonds in each lattice of
89
especially suitable for liquid phase reaction systems containing macromolecular
substances to participate in the reaction.
(2) Carbon nanofibers have the characteristic of inactive surface chemistry, which
enables them to exist stably under strong acidic and alkaline conditions. At the
same time, carbon nanofibers have certain stability in air at normal temperature.
(3) Carbon nanofibers have unique electronic characteristics. CNFs graphite sheet
has delocalized π electrons, which makes it to have good conductivity similar to
graphite. However, because the graphite layer of CNFs is continuously curled in
space, the electronic characteristics of CNFs are obviously different from that
of complete graphite, and the distribution and density of electron clouds are also
slightly different. The surface structure of CNFs synthesized by vapor deposition
inevitably introduces defects and carbon nanofiber end atoms, which have higher
energy and can further affect their electronic properties. Research shows that
curling of CNTs graphite sheet can cause further hybridization between sp
2
orbit of carbon and d orbit of Ni and other catalysts, and its acting force is far
greater than that between catalyst and graphite [33]. Therefore, when CNTs are
used as catalyst carrier, They are more conducive to the positioning and loading
of metal catalyst on its surface, to make the catalyst more evenly dispersed on
the surface of CNTs, and to improve the activity of the catalyst.
(4) Carbon nanofibers have good mechanical stability. Almost every carbon atom
in CNFs forms a stable covalent bond with the surrounding three carbon atoms.
The structure is tight, both ends are closed, and there is no unbroken lone pair.
This structure makes CNFs have good mechanical stability. At the same time,
large π bonds formed between carbon atoms have higher binding energy, which
greatly improves the stretching ability of CNFs. Some studies have shown that
the surface structure of CNFs hardly changes after ultrasonic dispersion, while
for activated carbon, many fragments can be obviously found.
(5) Carbon nanofibers have good thermal conductivity. The unique structure of
CNFs makes its thermal conductivity have great difference between the direction perpendicular to the axis and the direction parallel to the axis. Its thermal
conductivity along the axis direction is almost comparable to that of diamond.
However, the thermal conductivity perpendicular to the axial direction is very
poor and hardly has thermal conductivity.
(6) Adjustability of carbon nanofiber surface structure. The microstructure of CNFs
obtained by different preparation methods is different. Therefore, by changing
the preparation method and preparing different CNFs for appropriate systems,
it is beneficial to improve their loading capacity or catalytic capacity.
4.1.1.5 Graphene
Simply put, graphene is a single-layer graphite sheet with only one carbon atom thick,
and carbon and carbon atoms are closely arranged in a sp
2 heterozygous manner into
a honeycomb six-membered ring structure (Fig. 4.1a). The distance between carbon
and carbon atoms is about 0.142 nm. There are three σ bonds in each lattice of