Supercapacitor is an ideal new energy storage device. In order to develop
supercapacitors with excellent performance, from a material perspective, it is
crucial to research and develop carbon electrode materials that are suitable for
supercapacitor applications and have high specific capacities in different
electrolytes. At present, the main focus is on carbon-based materials, such as
activated carbon, glassy carbon, fibers, gels, high-density graphite, foam
obtained by pyrolyzing polymer matrix, carbon nanotubes, highly active mesophase
carbon microspheres and honeycombs with nanopores Shape diamond, etc., as well
as research on materials such as rare metal oxides and conductive polymers.
Among these materials, carbon materials have attracted attention due to their
low price, easy availability, and excellent performance, and have been
successfully commercialized. Therefore, this article intends to elaborate on the
development and research of new carbon materials for supercapacitors.
1 Principle of supercapacitor
According to the energy storage mechanism, supercapacitors are generally
divided into electric double layer capacitors and Faraday quasi-capacitors. The
electric double layer capacitor is based on the electric double layer theory,
and its electrode material is activated carbon with a large specific surface
area. Faraday quasi-capacitors can be divided into two types based on different
electrode materials: metal oxides and conductive polymers. This type of
capacitor mainly uses highly reversible rapid redox reactions that occur on the
surface of active materials and bulk interfaces to store energy. Electric double
layer capacitors are based on the electric double layer theory and use the
interfacial double layer capacitance formed between electrodes and electrolytes
to store energy. Faraday quasicapacitors are based on the Faraday process, which
is produced during the electrochemical change of Faraday charge transfer. It not
only occurs on the surface of the electrode, but also can penetrate deep into
the interior of the electrode, so higher capacitance and energy density can be
obtained than electric double layer capacitors. Recently, hybrid supercapacitors
that combine the advantages of both have been vigorously developed. At present,
new asymmetric supercapacitors have been developed. The two electrode materials
of this supercapacitor are different, which can better improve the performance
of the supercapacitor.
No matter what principle it is based on, supercapacitors can be divided
into four parts: electrodes, electrolytes, current collectors and separators, as
shown in Figure 1. At present, the research hot spots are mainly focused on four
aspects: carbon electrode materials, metal oxide and its hydrate electrode
materials, conductive polymer electrode materials, and hybrid supercapacitors.
The electrolyte needs to be highly conductive and sufficiently electrochemically
stable so that the supercapacitor can operate at the highest possible voltages.
Existing electrolyte materials mainly include solid electrolytes, organic
electrolytes and aqueous solution electrolytes.
Among all electrochemical supercapacitor electrode materials, carbon
materials are the earliest and most mature technology, and their research began
with the related patent published by Beck in 1957. Carbon electrode materials
have a large specific surface and low raw materials, which are conducive to
industrial large-scale production. However, the specific capacity is relatively
low. It is necessary to increase the specific capacity by increasing the
specific surface area of the material. At present, the main research is on
porous carbon materials with high specific surface area and small internal
resistance, (activated) carbon nanotubes, and carbon-containing composite
materials that modify carbon-based materials (such as activated carbon black and
other composite materials) .
2 Development of carbon electrode materials
2.1 Activated carbon powder
Activated carbon has a long history of industrial production and
application. It is also the earliest carbon electrode material used in
supercapacitors. There are rich sources of raw materials for preparing activated
carbon. Petroleum, coal, wood, nut shells, resins, etc. can all be used to
prepare activated carbon powder. The raw materials are different and the
production process is also slightly different. The raw materials are carbonized
and activated after being prepared. The activation methods are divided into
physical activation (using CO2 and H20 steam as activators) and chemical
activation (ZnC12, KOH, etc. are used as activators). The raw materials and
preparation process determine the physical and chemical properties of activated
carbon. Ultra-high specific surface area activated carbon has begun to be used
to increase the capacity of double-layer capacitors. Japan has reported that
ultra-high specific surface area (2500~3000m2/g) activated carbon was developed
using petroleum asphalt as raw material for use as double-layer capacitors, but
this material is not ideal. Therefore, many activated carbons have been
developed taking into account the pore size distribution, apparent density and
other properties. At the same time, the mass specific capacity and volume
specific capacity have been taken into consideration to improve the overall
performance of the capacitor.
2.2 Activated carbon fiber
Activated carbon fiber (ACF) is an efficient active adsorption material and
environmentally friendly engineering material that has better performance than
activated carbon. The preparation of ACF generally involves stabilizing the
organic precursor fiber at low temperature (200-400°C), followed by
carbonization activation (700-1000°C). Organic fibers used as ACF precursors
mainly include cellulose-based, polyacrylonitrile-based, asphalt-based,
phenolic-based, polyvinyl alcohol, etc. The first four types are mainly
commercialized. The application of activated carbon fibers in double-layer
capacitors has attracted more and more attention.
In the early days, Panasonic Electric Company of Japan used activated
carbon powder as raw material to prepare electrodes for electric double layer
capacitors. Later developed models used phenolic activated carbon fibers with
excellent conductivity, average pore diameter of 2 to 5nm, and a specific
surface area of 1500 to 3000m2/g. Activated carbon The advantages of fiber are
high mass specific capacity and good electrical conductivity, but low apparent
density. H. Nakagawa developed high-density activated carbon fiber (HD-ACF)
using a hot pressing method, and used this HD-ACF to make supercapacitor
electrodes. For unit capacitors of the same size, the capacitance of capacitors
using HD-ACF as electrodes is significantly improved.
2.3 Carbon aerogel
Carbon aerogel is a new lightweight nanoporous amorphous carbon material
developed by Pekala. PW et al. first discovered [3]. It is characterized by high
specific surface area, wide range of density changes, adjustable structure,
special properties in electrical, thermal, optical and other aspects, and has
broad application prospects. In particular, its large specific surface area and
high conductivity make it a supercapacitor. Ideal electrode material. Carbon
aerogels generally use resorcin and formaldehyde as raw materials. The two
undergo a polycondensation reaction under the catalysis of sodium carbonate to
form resorcinol-formaldehyde gel. The solvent in the pores is removed by
supercritical drying to form RF air condensation. Glue, RF aerogel is carbonized
in an inert atmosphere to obtain a carbon aerogel that maintains its network
structure. By adjusting the ratio of resorcinol to catalyst and the
concentration of the gel, the network structure of the carbon aerogel can be
controlled. The Lorentz Livermore National Laboratory in the United States, with
the support of the U.S. Department of Energy, researched and developed carbon
airgel carbon electrode capacitors. PowerStor has commercialized supercapacitors
with carbon aerogel electrodes.
2.4 Carbon nanotubes
From the perspective of the electricity storage principle of capacitors,
carbon nanotubes are ideal electrode materials. First of all, carbon nanotubes
are hollow tubes with a large specific surface area, especially single-wall
nanotubes, which are conducive to the formation of electric double layer
capacitance. In addition, the carbon in the carbon nanotube is sp hybridized,
and three hybrid bonds are used to form a ring. Generally, a six-membered ring
is formed, and there is one hybrid bond left. This hybrid bond can be connected
to a faradaic reaction. Functional groups (such as hydroxyl, carboxyl, etc.).
Therefore, carbon nanotubes can not only form an electric double layer
capacitor, but are also an ideal material that can fully utilize the principle
of pseudo-capacitance energy storage. 【
3 Outlook
Although carbon-based supercapacitors have been successfully
commercialized, there are still many problems in carbon electrode materials that
need to be further improved in order to further improve the performance of the
capacitor. At present, porous carbon materials with large mass specific capacity
are relatively easy to obtain, but carbon materials with large volume specific
capacity and good long-term application stability are difficult to obtain. In
order to coordinately solve the two contradictory issues of electric capacity
and long-term application stability, In terms of performance indicators, some
new activated carbon raw materials and activation technologies need to be
developed. The pore size distribution of carbon materials is a key factor
affecting the performance of supercapacitors, and good control of the pore
structure of carbon electrode materials requires further exploration. In
addition, the internal resistance of carbon-based capacitors is relatively
large, and the structure of the material itself also needs to be improved.
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