7.4 Hybrid Capacitors
213
exert the advantages of both pseudocapacitances and EDLCs at the same time, thus
increasing the operating voltage window (about 1.6–2.0 V) and improving the energy
density of supercapacitors. In addition, it needs to be emphasized that the supercapacitor devices based on two different metal oxides, which serve as the cathode and
the anode, respectively, also belong to the hybrid supercapacitor.
7.4.1 Metal Oxide Cathode and Carbon Material
Anode/Metal Oxide Anode
Owing to the outstanding electrical properties and excellent electrochemical stability,
graphene-based electrodes have attracted great interest. Pseudocapacitive materials
are favorable materials to form nanocomposites with graphene. It was reported that
a composite of graphene oxide and MnO 2 on nickel foam current collector achieved
a high capacitance of 211.2 F g
−1 (Chen et al. 2010). However, the actual capacitance is still far from the expected value. Sumboja et al. (2013) reported a flexible
free-standing paper electrode of reduced graphene oxide (RGO)/MnO 2 . The paper
electrode combines the high conductivity of RGO and the large specific capacitance of MnO 2 . A high areal capacitance of 897 mF cm
−2 was achieved for the
RGO/MnO 2 paper with a loading mass of 3.7 mg cm
−2 . Additionally, the hybrid
supercapacitor device based on RGO as anode and RGO/MnO 2 as cathode was also
manufactured, which yield a high areal power density of 3.8 mW cm
−2 at 1 A g
−1
with an energy density of 11.5 μW h cm
−2 . The device possessed a maximum
areal energy of 35.1 μW h cm
−2 at a power density of 37.5 μW cm
−2 . Zhang
et al. (2014a, b) reported a flexible, all-solid-state hybrid supercapacitor based on
MnO 2 /graphene as the cathode and CNT/graphene as the anode. The device could
be operated under a high voltage region of 0–1.8 V and exhibited a maximum energy
density of 31.8 W h kg
−1 .
To realize high energy density, Xia et al. (2015) manufactured the hybrid supercapacitor device using a nanocomposite of functionalized graphene sheets (FGS)
decorated with Fe 2 O 3 quantum dots (≈2 nm) (Fe 2 O 3 /FGS) as anode and MnO 2 /FGS
as cathode (Fig. 7.10a). The Fe 2 O 3 /FGS composite not only possessed high surface
area but also had excellent electronic conductivity, thus achieving good performance.
Furthermore, MnO 2 /FGS nanocomposite also exhibited high specific capacitance,
which could match the anode materials, enhancing the specific energy of supercapacitors. As a result, a high energy density of 50.7 Wh kg
−1 was obtained at a power
density of 100 W kg
−1 (Fig. 7.10b). In addition, the asymmetric device also exhibited
a good cycling stability with 95% of initial capacitance retained after 5000 cycles
(Fig. 7.10c).
213
exert the advantages of both pseudocapacitances and EDLCs at the same time, thus
increasing the operating voltage window (about 1.6–2.0 V) and improving the energy
density of supercapacitors. In addition, it needs to be emphasized that the supercapacitor devices based on two different metal oxides, which serve as the cathode and
the anode, respectively, also belong to the hybrid supercapacitor.
7.4.1 Metal Oxide Cathode and Carbon Material
Anode/Metal Oxide Anode
Owing to the outstanding electrical properties and excellent electrochemical stability,
graphene-based electrodes have attracted great interest. Pseudocapacitive materials
are favorable materials to form nanocomposites with graphene. It was reported that
a composite of graphene oxide and MnO 2 on nickel foam current collector achieved
a high capacitance of 211.2 F g
−1 (Chen et al. 2010). However, the actual capacitance is still far from the expected value. Sumboja et al. (2013) reported a flexible
free-standing paper electrode of reduced graphene oxide (RGO)/MnO 2 . The paper
electrode combines the high conductivity of RGO and the large specific capacitance of MnO 2 . A high areal capacitance of 897 mF cm
−2 was achieved for the
RGO/MnO 2 paper with a loading mass of 3.7 mg cm
−2 . Additionally, the hybrid
supercapacitor device based on RGO as anode and RGO/MnO 2 as cathode was also
manufactured, which yield a high areal power density of 3.8 mW cm
−2 at 1 A g
−1
with an energy density of 11.5 μW h cm
−2 . The device possessed a maximum
areal energy of 35.1 μW h cm
−2 at a power density of 37.5 μW cm
−2 . Zhang
et al. (2014a, b) reported a flexible, all-solid-state hybrid supercapacitor based on
MnO 2 /graphene as the cathode and CNT/graphene as the anode. The device could
be operated under a high voltage region of 0–1.8 V and exhibited a maximum energy
density of 31.8 W h kg
−1 .
To realize high energy density, Xia et al. (2015) manufactured the hybrid supercapacitor device using a nanocomposite of functionalized graphene sheets (FGS)
decorated with Fe 2 O 3 quantum dots (≈2 nm) (Fe 2 O 3 /FGS) as anode and MnO 2 /FGS
as cathode (Fig. 7.10a). The Fe 2 O 3 /FGS composite not only possessed high surface
area but also had excellent electronic conductivity, thus achieving good performance.
Furthermore, MnO 2 /FGS nanocomposite also exhibited high specific capacitance,
which could match the anode materials, enhancing the specific energy of supercapacitors. As a result, a high energy density of 50.7 Wh kg
−1 was obtained at a power
density of 100 W kg
−1 (Fig. 7.10b). In addition, the asymmetric device also exhibited
a good cycling stability with 95% of initial capacitance retained after 5000 cycles
(Fig. 7.10c).