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7 Nanomaterials for Supercapacitors
Supercapacitors can bridge the power and energy gap between batteries and
traditional physical capacitors and combine the advantages of them, such as quick
charge/discharge, long cycling life, high reliability, and wide range of working
adaptability. On the basis of the charge storage mechanism, supercapacitors can
be categorized into two types: electrical double-layer capacitors (EDLCs) and
pseudocapacitors (Wang et al. 2016; Zheng et al. 2017).
The capacitive process of EDLCs is based on the charge separation caused by
the directional arrangement of electrons and ions at the interface of electrode and
electrolyte. During the charging process, electrons are transferred from the positive
electrode to the negative electrode. Under the electric field force, positive ions and
negative ions in the solution will move directionally to the two poles and form a tight
electrical double layer on the electrode surface to generate a potential difference.
During the discharge process, the charges on the electrode will release automatically
through the external circuit to supply the power. Meanwhile, the ions on the electrode
surface will migrate back to the electrolyte, and the tight electrical double layer will
disappear.
For pseudocapacitors, the charge storage is due to the highly reversible redox
reactions of the active material and its chemical adsorption/desorption. Compared
to EDLCs, which occur only on the surface of the electrode, the charge storage of
Faraday pseudocapacitor materials can occur not only on the surface of the electrode
but also in the bulk phase of the materials. Therefore, Faraday pseudocapacitors
can achieve a much higher specific capacitance than EDLCs, which can generally
reach 10–100 times of the capacitance of EDLCs. However, the power density of
the pseudocapacitors is lower than that of the EDLCs, because the Faraday process
is generally slower than the non-Faraday process. In addition, the pseudocapacitor
materials usually have a poor cycling stability, which is like the battery.
The mechanisms of EDLCs and pseudocapacitors indicate that the specific capacitance of supercapacitors is closely related to the interface of electrode materials and
electrode. Therefore, the microstructure of electrode material is an important factor
determining the performance of a supercapacitor. Nanostructured materials have been
extensively studied as electrode materials of supercapacitor because they can provide
more active surface area, facilitate the transport of electrolytes, and accommodate
volume changes during charging/discharging.
In this chapter, we mainly discussed the application of nanostructured electrode
materials in supercapacitors. The synthetic methods and electrochemical performance of nanostructured carbon materials, transition metal oxides, conductive polymers, and MXenes were systematically reviewed. The hybrid capacitor, which is
composed of two different electrode materials, was also summarized. In addition,
we put forward our own prospect for the further development of nanostructured
materials for supercapacitors.
7 Nanomaterials for Supercapacitors
Supercapacitors can bridge the power and energy gap between batteries and
traditional physical capacitors and combine the advantages of them, such as quick
charge/discharge, long cycling life, high reliability, and wide range of working
adaptability. On the basis of the charge storage mechanism, supercapacitors can
be categorized into two types: electrical double-layer capacitors (EDLCs) and
pseudocapacitors (Wang et al. 2016; Zheng et al. 2017).
The capacitive process of EDLCs is based on the charge separation caused by
the directional arrangement of electrons and ions at the interface of electrode and
electrolyte. During the charging process, electrons are transferred from the positive
electrode to the negative electrode. Under the electric field force, positive ions and
negative ions in the solution will move directionally to the two poles and form a tight
electrical double layer on the electrode surface to generate a potential difference.
During the discharge process, the charges on the electrode will release automatically
through the external circuit to supply the power. Meanwhile, the ions on the electrode
surface will migrate back to the electrolyte, and the tight electrical double layer will
disappear.
For pseudocapacitors, the charge storage is due to the highly reversible redox
reactions of the active material and its chemical adsorption/desorption. Compared
to EDLCs, which occur only on the surface of the electrode, the charge storage of
Faraday pseudocapacitor materials can occur not only on the surface of the electrode
but also in the bulk phase of the materials. Therefore, Faraday pseudocapacitors
can achieve a much higher specific capacitance than EDLCs, which can generally
reach 10–100 times of the capacitance of EDLCs. However, the power density of
the pseudocapacitors is lower than that of the EDLCs, because the Faraday process
is generally slower than the non-Faraday process. In addition, the pseudocapacitor
materials usually have a poor cycling stability, which is like the battery.
The mechanisms of EDLCs and pseudocapacitors indicate that the specific capacitance of supercapacitors is closely related to the interface of electrode materials and
electrode. Therefore, the microstructure of electrode material is an important factor
determining the performance of a supercapacitor. Nanostructured materials have been
extensively studied as electrode materials of supercapacitor because they can provide
more active surface area, facilitate the transport of electrolytes, and accommodate
volume changes during charging/discharging.
In this chapter, we mainly discussed the application of nanostructured electrode
materials in supercapacitors. The synthetic methods and electrochemical performance of nanostructured carbon materials, transition metal oxides, conductive polymers, and MXenes were systematically reviewed. The hybrid capacitor, which is
composed of two different electrode materials, was also summarized. In addition,
we put forward our own prospect for the further development of nanostructured
materials for supercapacitors.