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7 Nanomaterials for Supercapacitors
perfect rectangle. Currently, the specific capacitance of RuO 2 has been reported to
be up to 1300 F g
−1 , far exceeding the specific capacitance of double-layer materials, which greatly improves the energy density of supercapacitors. Although RuO 2
shows good electrochemical behavior, its high cost restricts practical application in
the field of supercapacitors to some extent. Therefore, the researchers investigated
other non-noble metal oxides as supercapacitor electrode materials, such as cobalt
oxide, manganese dioxide, and nickel oxide, all of which showed their potential as
pseudocapacitor materials.
7.3.1 Co 3 O 4
Among various metal oxides, Co 3 O 4 is considered a highly promising material for supercapacitors due to its good redox property, high theoretical specific
capacitance (3560 F g
−1 ), and great reversibility. Co 3 O 4 -based materials with
various structures/morphologies showed a clear difference in the electrochemical
performance.
Two-dimensional (2D) nanostructure not only possessed high specific surface
area but also facilitated the ion transfer during the charge-discharge process. Feng
et al. (2015) fabricated sub-3 nm atomic layers Co 3 O 4 nanofilms via a hydrothermal
reaction without surfactants and substrates. Because of the ultrathin thickness and
large size, the as-prepared atomic layers Co 3 O 4 nanofilms achieved high specific
capacitances of 1400 F g
−1 at 1 A g
−1 and 1276 F g
−1 at 8 A g
−1 . Furthermore,
the Co 3 O 4 nanofilms also showed a good cycling performance with a small decrease
(less than 3%) after 1500 cycles. Meher and Rao (2011) synthesized 2D layered
Co 3 O 4 with high porosity under hydrothermal conditions. The pores of the obtained
2D flakes are quite narrow (~1.85 nm) and uniform. The specific capacitance of
the sample was 548 F g
−1 at 8 A g
−1 and retained 66% at 32 A g
−1 . Furthermore,
the layered Co 3 O 4 delivered outstanding cycling performance with 97.5% of initial
capacitance after 2000 charge-discharge cycles at 16 A g
−1 .
Inspired by oxygen vacancies, Wang et al. (2014) prepared reduced mesoporous
Co 3 O 4 nanowires via a facile solution reduction method. Due to the numerous oxygen
vacancies on the nanowires’ surface caused by the treatment of NaBH 4 , the reduced
Co 3 O 4 nanowires achieved better electrochemical performance than the pristine
Co 3 O 4 nanowires. Supercapacitors assembled with the reduced Co 3 O 4 nanowires
showed a high capacitance of 978 F g
−1 at 2 A g
−1 . Furthermore, the capacitance of
the reduced Co 3 O 4 nanowires is about 883 F g
−1 after 2000 cycles, which indicates
that the oxygen vacancies are stable during the cycling test. Zhang et al. (2014a, b)
fabricated hollow Co 3 O 4 polyhedral particles with a porous shell by the heat treatment of ZIF-67 microcrystals. As the electrode, the material delivered a large specific
capacitance of 1100 F g
−1 at 1.25 A g
−1 and excellent cycling performance with little
capacitance decay even after 6000 cycles at 6.25 A g
−1 .
Preparing composite materials can make use of synergistic effects of both
components. Yang et al. (2018) prepared oxygen-vacancy abundant ultrafine
7 Nanomaterials for Supercapacitors
perfect rectangle. Currently, the specific capacitance of RuO 2 has been reported to
be up to 1300 F g
−1 , far exceeding the specific capacitance of double-layer materials, which greatly improves the energy density of supercapacitors. Although RuO 2
shows good electrochemical behavior, its high cost restricts practical application in
the field of supercapacitors to some extent. Therefore, the researchers investigated
other non-noble metal oxides as supercapacitor electrode materials, such as cobalt
oxide, manganese dioxide, and nickel oxide, all of which showed their potential as
pseudocapacitor materials.
7.3.1 Co 3 O 4
Among various metal oxides, Co 3 O 4 is considered a highly promising material for supercapacitors due to its good redox property, high theoretical specific
capacitance (3560 F g
−1 ), and great reversibility. Co 3 O 4 -based materials with
various structures/morphologies showed a clear difference in the electrochemical
performance.
Two-dimensional (2D) nanostructure not only possessed high specific surface
area but also facilitated the ion transfer during the charge-discharge process. Feng
et al. (2015) fabricated sub-3 nm atomic layers Co 3 O 4 nanofilms via a hydrothermal
reaction without surfactants and substrates. Because of the ultrathin thickness and
large size, the as-prepared atomic layers Co 3 O 4 nanofilms achieved high specific
capacitances of 1400 F g
−1 at 1 A g
−1 and 1276 F g
−1 at 8 A g
−1 . Furthermore,
the Co 3 O 4 nanofilms also showed a good cycling performance with a small decrease
(less than 3%) after 1500 cycles. Meher and Rao (2011) synthesized 2D layered
Co 3 O 4 with high porosity under hydrothermal conditions. The pores of the obtained
2D flakes are quite narrow (~1.85 nm) and uniform. The specific capacitance of
the sample was 548 F g
−1 at 8 A g
−1 and retained 66% at 32 A g
−1 . Furthermore,
the layered Co 3 O 4 delivered outstanding cycling performance with 97.5% of initial
capacitance after 2000 charge-discharge cycles at 16 A g
−1 .
Inspired by oxygen vacancies, Wang et al. (2014) prepared reduced mesoporous
Co 3 O 4 nanowires via a facile solution reduction method. Due to the numerous oxygen
vacancies on the nanowires’ surface caused by the treatment of NaBH 4 , the reduced
Co 3 O 4 nanowires achieved better electrochemical performance than the pristine
Co 3 O 4 nanowires. Supercapacitors assembled with the reduced Co 3 O 4 nanowires
showed a high capacitance of 978 F g
−1 at 2 A g
−1 . Furthermore, the capacitance of
the reduced Co 3 O 4 nanowires is about 883 F g
−1 after 2000 cycles, which indicates
that the oxygen vacancies are stable during the cycling test. Zhang et al. (2014a, b)
fabricated hollow Co 3 O 4 polyhedral particles with a porous shell by the heat treatment of ZIF-67 microcrystals. As the electrode, the material delivered a large specific
capacitance of 1100 F g
−1 at 1.25 A g
−1 and excellent cycling performance with little
capacitance decay even after 6000 cycles at 6.25 A g
−1 .
Preparing composite materials can make use of synergistic effects of both
components. Yang et al. (2018) prepared oxygen-vacancy abundant ultrafine