204
7 Nanomaterials for Supercapacitors
at 1 A g
−1 and better cycling performance with 91.6% of initial specific capacitance at 20 A g
−1 , while pristine Co 3 O 4 nanorods exhibited a specific capacitance
of 318 F g
−1 at 1 A g
−1 and 67.1% of initial specific capacitance at 20 A g
−1 .
7.3.2 MnO 2
Manganese dioxide (MnO 2 ) is one of the most potential supercapacitor electrode
materials because of its rich polymorphisms, high theoretical specific capacitance,
high natural abundance, and non-toxicity. Wei et al. (2013) designed a novel 2D
β-MnO 2 networks, in which β-MnO 2 nanowires were highly ordered. The obtained
2D networks exhibited a specific capacitance of 453.0 F g
−1 at 0.5 A g
−1 and a
good cycling performance with 97% retention at 2.5 A g
−1 after 1800 cycles. Porous
MnO 2 nanotubes were prepared via a simple one-step hydrothermal method using
polycarbonate membrane as the template by Huang et al. (2015). The as-prepared
porous MnO 2 nanotubes were about 200 nm in average diameter, which were in
accordance with the average pore diameter of the template. The specific capacitance
of the porous MnO 2 nanotubes electrode was 365 F g
−1 at 0.25 A g
−1 and retained
90.4% after 3000 cycles. Moreover, an asymmetric supercapacitor based on activated graphene as the negative electrode and porous MnO 2 nanotubes as the positive
electrode achieved a maximum power density of 146.2 kW kg
−1 and a large energy
density of 22.5 W h kg
−1 .
MnO 2 -carbon composites were also reported as the electrode. Jia et al. (2018)
synthesized graphene quantum dots (GQDs)/MnO 2 composites as electrodes for
supercapacitors. In the two-step synthesis (Fig. 7.4a), MnO 2 nanosheet arrays were
vertically grown on clean Ni foam substrate through a facile hydrothermal process,
and then GQDs anchored in situ on the surface of MnO 2 nanosheets after the PECVD
process. As shown in Fig. 7.4b, GQDs/MnO 2 heterostructure was vertical and highly
porous. Among the samples with different PECVD deposition time, GQD/MnO 2 -3
exhibited a much higher specific capacitance (1170 F g
−1 at 5 mV s
−1 in 0–1.3 V)
than others. GQD/MnO 2 -3 also delivered better cycle performance with 92.7% of
initial capacitance even after 10,000 cycles (Fig. 7.4c, d). Furthermore, the 2.3 V
aqueous asymmetric supercapacitor based on GQDs/MnO 2 -3 as positive electrode
and nitrogen-doped (NG) graphene as negative electrode achieved outstanding electrochemical performance with a high energy density of 118 W h kg
−1 and a large
power density of 12,351 W kg
−1 . CNTs@MnO 2 core-shell structures grown on Ni
foam were fabricated by Huang et al. (2014). The obtained binder-free capacitor electrodes of ultrathin layered MnO 2 nanosheets coated on the CNTs were prepared via a
floating catalyst CVD process and followed by a facile hydrothermal approach. This
unique CNTs@MnO 2 electrode showed a high specific capacitance of 325.5 F g
−1 at
0.3 A g
−1 and good cycling stability of 90.5% retention after 5000 cycles. Similarly,
Huang et al. (2018) fabricated primary ε-MnO 2 nanosheets and secondary α-MnO 2
nanorod arrays on the surface of carbon fiber by a phase-controlled electrodeposition. The optimized hierarchical architecture (MnO 2 -60, deposited at 60 °C) with
7 Nanomaterials for Supercapacitors
at 1 A g
−1 and better cycling performance with 91.6% of initial specific capacitance at 20 A g
−1 , while pristine Co 3 O 4 nanorods exhibited a specific capacitance
of 318 F g
−1 at 1 A g
−1 and 67.1% of initial specific capacitance at 20 A g
−1 .
7.3.2 MnO 2
Manganese dioxide (MnO 2 ) is one of the most potential supercapacitor electrode
materials because of its rich polymorphisms, high theoretical specific capacitance,
high natural abundance, and non-toxicity. Wei et al. (2013) designed a novel 2D
β-MnO 2 networks, in which β-MnO 2 nanowires were highly ordered. The obtained
2D networks exhibited a specific capacitance of 453.0 F g
−1 at 0.5 A g
−1 and a
good cycling performance with 97% retention at 2.5 A g
−1 after 1800 cycles. Porous
MnO 2 nanotubes were prepared via a simple one-step hydrothermal method using
polycarbonate membrane as the template by Huang et al. (2015). The as-prepared
porous MnO 2 nanotubes were about 200 nm in average diameter, which were in
accordance with the average pore diameter of the template. The specific capacitance
of the porous MnO 2 nanotubes electrode was 365 F g
−1 at 0.25 A g
−1 and retained
90.4% after 3000 cycles. Moreover, an asymmetric supercapacitor based on activated graphene as the negative electrode and porous MnO 2 nanotubes as the positive
electrode achieved a maximum power density of 146.2 kW kg
−1 and a large energy
density of 22.5 W h kg
−1 .
MnO 2 -carbon composites were also reported as the electrode. Jia et al. (2018)
synthesized graphene quantum dots (GQDs)/MnO 2 composites as electrodes for
supercapacitors. In the two-step synthesis (Fig. 7.4a), MnO 2 nanosheet arrays were
vertically grown on clean Ni foam substrate through a facile hydrothermal process,
and then GQDs anchored in situ on the surface of MnO 2 nanosheets after the PECVD
process. As shown in Fig. 7.4b, GQDs/MnO 2 heterostructure was vertical and highly
porous. Among the samples with different PECVD deposition time, GQD/MnO 2 -3
exhibited a much higher specific capacitance (1170 F g
−1 at 5 mV s
−1 in 0–1.3 V)
than others. GQD/MnO 2 -3 also delivered better cycle performance with 92.7% of
initial capacitance even after 10,000 cycles (Fig. 7.4c, d). Furthermore, the 2.3 V
aqueous asymmetric supercapacitor based on GQDs/MnO 2 -3 as positive electrode
and nitrogen-doped (NG) graphene as negative electrode achieved outstanding electrochemical performance with a high energy density of 118 W h kg
−1 and a large
power density of 12,351 W kg
−1 . CNTs@MnO 2 core-shell structures grown on Ni
foam were fabricated by Huang et al. (2014). The obtained binder-free capacitor electrodes of ultrathin layered MnO 2 nanosheets coated on the CNTs were prepared via a
floating catalyst CVD process and followed by a facile hydrothermal approach. This
unique CNTs@MnO 2 electrode showed a high specific capacitance of 325.5 F g
−1 at
0.3 A g
−1 and good cycling stability of 90.5% retention after 5000 cycles. Similarly,
Huang et al. (2018) fabricated primary ε-MnO 2 nanosheets and secondary α-MnO 2
nanorod arrays on the surface of carbon fiber by a phase-controlled electrodeposition. The optimized hierarchical architecture (MnO 2 -60, deposited at 60 °C) with