88
Y. Li et al.
products), protons and electrons. For traditional carbon carriers (such as Vulcan XC72), they are rich in micropores and have deep chapping, which makes it difficult
to form a three-phase interface. When Pt is deposited on its surface, the utilization
rate of Pt decreases, which is due to the fact that reactants and proton-conducting
polyelectrolytes (such as Nafion) are not easy to enter micropores. However, carbon
carriers with pore sizes larger than 50 nm have low specific surface area and large
internal resistance [23]. Therefore, the performance of carbon-supported catalyst is
affected by the pore structure, pore size, and conductivity of MPC. Wang et al. [25]
reported that N-doped NiFe/3D MPC was used for high-performance ORR/OER
catalysts. Under the condition of 0.1 M KOH, the η10 of OER was 340 mV and the
half-wave potential of ORR was 0.862 V, which was due to the chemical interaction
between the nanodots and the substrate, and the increase of catalytic sites exposed
to electrolyte at nNiFe LDH nanodots.
4.1.1.4 Carbon Nanofiber
Carbon nanofibers (CNFs) are fibrous nanocarbon materials formed by crimping
multi-layer graphite sheets, which have high strength, light weight, good thermal
conductivity, and high electrical conductivity [26, 27]. According to its microstructural characteristics, it can be divided into fishbone, flat plate, ribbon, and tubular
[28]. The diameter is generally 10 to 500 nm, and the length is distributed in the
range of 0.5 to 100 m. It is a one-dimensional carbon material between carbon
nanotubes and common carbon fibers. It has high crystallinity, good electrical and
thermal conductivity. Carbon nanofibers have the characteristics of low density, high
specific modulus, high specific strength, high electrical conductivity, and thermal
stability of ordinary carbon fibers grown by chemical vapor deposition; it also has
the advantages of small number of defects, large aspect ratio, large specific surface
area, compact structure, etc. [29]. It is a high-performance fiber, which not only
has the inherent characteristics of carbon materials, but also has the flexibility and
processability of textile fibers, so it has a good application prospect in catalyst carrier
materials [30, 31].
As a new type of carbon material, CNFs have good electrochemical catalytic
activity. Compared with other catalyst carriers, CNFs have many advantages [32],
mainly as follows:
(1) Carbon nanofibers have a large specific surface area, usually 50 ~ 300m
2 g
−1 ,
which enables metal catalyst particles to be better positioned on the surface of
carbon nanofibers, thus facilitating the loading and dispersion of metal catalyst. At the same time, carbon nanofibers contain almost no micropores, and
their pore volume is mainly composed of mesopores formed by hollow parts of
carbon nanofibers and macropores and mesopores formed by intertwining and
stacking of carbon fibers. The mesoporous structure is favorable for reducing
the resistance of reactants or products to migrate out of the surface of the catalyst, thereby alleviating the diffusion problem caused by catalytic reaction, and is
Y. Li et al.
products), protons and electrons. For traditional carbon carriers (such as Vulcan XC72), they are rich in micropores and have deep chapping, which makes it difficult
to form a three-phase interface. When Pt is deposited on its surface, the utilization
rate of Pt decreases, which is due to the fact that reactants and proton-conducting
polyelectrolytes (such as Nafion) are not easy to enter micropores. However, carbon
carriers with pore sizes larger than 50 nm have low specific surface area and large
internal resistance [23]. Therefore, the performance of carbon-supported catalyst is
affected by the pore structure, pore size, and conductivity of MPC. Wang et al. [25]
reported that N-doped NiFe/3D MPC was used for high-performance ORR/OER
catalysts. Under the condition of 0.1 M KOH, the η10 of OER was 340 mV and the
half-wave potential of ORR was 0.862 V, which was due to the chemical interaction
between the nanodots and the substrate, and the increase of catalytic sites exposed
to electrolyte at nNiFe LDH nanodots.
4.1.1.4 Carbon Nanofiber
Carbon nanofibers (CNFs) are fibrous nanocarbon materials formed by crimping
multi-layer graphite sheets, which have high strength, light weight, good thermal
conductivity, and high electrical conductivity [26, 27]. According to its microstructural characteristics, it can be divided into fishbone, flat plate, ribbon, and tubular
[28]. The diameter is generally 10 to 500 nm, and the length is distributed in the
range of 0.5 to 100 m. It is a one-dimensional carbon material between carbon
nanotubes and common carbon fibers. It has high crystallinity, good electrical and
thermal conductivity. Carbon nanofibers have the characteristics of low density, high
specific modulus, high specific strength, high electrical conductivity, and thermal
stability of ordinary carbon fibers grown by chemical vapor deposition; it also has
the advantages of small number of defects, large aspect ratio, large specific surface
area, compact structure, etc. [29]. It is a high-performance fiber, which not only
has the inherent characteristics of carbon materials, but also has the flexibility and
processability of textile fibers, so it has a good application prospect in catalyst carrier
materials [30, 31].
As a new type of carbon material, CNFs have good electrochemical catalytic
activity. Compared with other catalyst carriers, CNFs have many advantages [32],
mainly as follows:
(1) Carbon nanofibers have a large specific surface area, usually 50 ~ 300m
2 g
−1 ,
which enables metal catalyst particles to be better positioned on the surface of
carbon nanofibers, thus facilitating the loading and dispersion of metal catalyst. At the same time, carbon nanofibers contain almost no micropores, and
their pore volume is mainly composed of mesopores formed by hollow parts of
carbon nanofibers and macropores and mesopores formed by intertwining and
stacking of carbon fibers. The mesoporous structure is favorable for reducing
the resistance of reactants or products to migrate out of the surface of the catalyst, thereby alleviating the diffusion problem caused by catalytic reaction, and is