66
4 Synthesis of Two-Dimensional (2D) Nanomaterials
multiplier performance and good cyclic stability. In addition, the LaNiO 3 //graphene
asymmetric supercapacitor device has a high energy density of 65.8 Wh kg
−1 at a
power density of 1.8 kW kg
−1 and a specific capacitance retention rate of 92.4%
after 10,000 cycles, which is a key step toward practical application.
4.3 2D Transition Metal Hydroxides
TMHs of Co (i.e., Co(OH) 2 ) and Ni (i.e., nickel hydroxide (Ni(OH) 2 ) were mainly
found in their 2D nanostructures. For instance, a solid-state asymmetric supercapacitor exhibited a high operation voltage of 1.8 V, which is made up of N-doped
graphene as the anode and single layers of β-Co(OH) 2 as the cathode (Gao et al.
2015). In addition, the device showed an exceptionally high energy/power density
of 98.9 Wh kg
−1 /17,981 W kg
−1 and still maintain 93.2% capacitance after 10,000
cycles. The performance similar to LIBS is attributed to the 100% exposure of five
atom thick Co(OH) 2 monolayers to the surface hydrogen atoms, as the electroactive
center of important Faraday redox reactions, as shown in Fig. 4.4b. 2D Ni(OH) 2 is
another kind of high energy density supercapacitor material with excellent electrochemical properties. Ultrathin, independent α-Ni(OH) 2 nanosheets with a thickness
of less than 2 nm were fabricated by microwave-assisted liquid-phase growth method
(Chen et al. 2014). The growth process of the as-prepared α-Ni(OH) 2 nanosheets was
shown in Fig. 4.3h. During the process of electrochemical reaction, most of the atoms
were exposed to these ultrathin 2D nanostructures. At a current density of 1 A g
−1 ,
it shows a maximum specific capacitance of 4172.5 F g
−1 , and even maintained at
2680 F g
−1 , at a higher rate of 16 A g
−1 with 98.5% retention after 2000 cycles.
Layered double hydroxides (LDHs) belong to another two-dimensional (2D)
materials which have unique structure consisting of metal hydroxide layers and
inorganic/organic gallery anions/molecules (Wang and O’Hare 2012; Fan et al.
2015; Gu et al. 2015). Using the general formula of LDHs to express is
[M
II
1−x M
III
x (OH) 2 ]
x+ (A
n− ) x/n ·mH 2 O, where divalent metal cations M
II (M = Mn,
Fe, Co, Ni, Cu, Zn, etc.) and trivalent metal M
III (M = Al, Ga, Ti, Cr, Fe, Co, etc.),
respectively, and A
n− is a charge-balancing anion intercalated between the brucitelike metal hydroxide layers. The easily tailored properties, composition versatility,
and low cost of LDHs have led to surging interest in these materials and many applications such as adsorption (Shao et al. 2012), photochemistry (Cho et al. 2014; Zhao
et al. 2014a), and electrocatalysis (Liang et al. 2015a).
For instance, the NiFe-LDH system has been investigated as an efficient oxygen
evolution reaction (OER) catalyst due to its high activity and stability in basic media;
various strategies have been developed to further improve its performance either by
hybridizing it with carbon materials or by constructing micro/nanostructures (Gong
et al. 2013; Song and Hu 2014). For example, Hu et al. demonstrated that exfoliated
LDH nanosheets showed significantly enhanced performance as compared to their
bulk phase due to their improved intrinsic catalytic activity and conductivity (Tu
et al. 2016). However, the synthesis of exfoliated LDH nanosheets not only required
4 Synthesis of Two-Dimensional (2D) Nanomaterials
multiplier performance and good cyclic stability. In addition, the LaNiO 3 //graphene
asymmetric supercapacitor device has a high energy density of 65.8 Wh kg
−1 at a
power density of 1.8 kW kg
−1 and a specific capacitance retention rate of 92.4%
after 10,000 cycles, which is a key step toward practical application.
4.3 2D Transition Metal Hydroxides
TMHs of Co (i.e., Co(OH) 2 ) and Ni (i.e., nickel hydroxide (Ni(OH) 2 ) were mainly
found in their 2D nanostructures. For instance, a solid-state asymmetric supercapacitor exhibited a high operation voltage of 1.8 V, which is made up of N-doped
graphene as the anode and single layers of β-Co(OH) 2 as the cathode (Gao et al.
2015). In addition, the device showed an exceptionally high energy/power density
of 98.9 Wh kg
−1 /17,981 W kg
−1 and still maintain 93.2% capacitance after 10,000
cycles. The performance similar to LIBS is attributed to the 100% exposure of five
atom thick Co(OH) 2 monolayers to the surface hydrogen atoms, as the electroactive
center of important Faraday redox reactions, as shown in Fig. 4.4b. 2D Ni(OH) 2 is
another kind of high energy density supercapacitor material with excellent electrochemical properties. Ultrathin, independent α-Ni(OH) 2 nanosheets with a thickness
of less than 2 nm were fabricated by microwave-assisted liquid-phase growth method
(Chen et al. 2014). The growth process of the as-prepared α-Ni(OH) 2 nanosheets was
shown in Fig. 4.3h. During the process of electrochemical reaction, most of the atoms
were exposed to these ultrathin 2D nanostructures. At a current density of 1 A g
−1 ,
it shows a maximum specific capacitance of 4172.5 F g
−1 , and even maintained at
2680 F g
−1 , at a higher rate of 16 A g
−1 with 98.5% retention after 2000 cycles.
Layered double hydroxides (LDHs) belong to another two-dimensional (2D)
materials which have unique structure consisting of metal hydroxide layers and
inorganic/organic gallery anions/molecules (Wang and O’Hare 2012; Fan et al.
2015; Gu et al. 2015). Using the general formula of LDHs to express is
[M
II
1−x M
III
x (OH) 2 ]
x+ (A
n− ) x/n ·mH 2 O, where divalent metal cations M
II (M = Mn,
Fe, Co, Ni, Cu, Zn, etc.) and trivalent metal M
III (M = Al, Ga, Ti, Cr, Fe, Co, etc.),
respectively, and A
n− is a charge-balancing anion intercalated between the brucitelike metal hydroxide layers. The easily tailored properties, composition versatility,
and low cost of LDHs have led to surging interest in these materials and many applications such as adsorption (Shao et al. 2012), photochemistry (Cho et al. 2014; Zhao
et al. 2014a), and electrocatalysis (Liang et al. 2015a).
For instance, the NiFe-LDH system has been investigated as an efficient oxygen
evolution reaction (OER) catalyst due to its high activity and stability in basic media;
various strategies have been developed to further improve its performance either by
hybridizing it with carbon materials or by constructing micro/nanostructures (Gong
et al. 2013; Song and Hu 2014). For example, Hu et al. demonstrated that exfoliated
LDH nanosheets showed significantly enhanced performance as compared to their
bulk phase due to their improved intrinsic catalytic activity and conductivity (Tu
et al. 2016). However, the synthesis of exfoliated LDH nanosheets not only required