4.2 2D Transition Metal Oxides
61
as-prepared holey CAN have the hole size of 10 nm and its reversible capacities are
1324 mAh g
−1 and 566 mAh g
−1 at the current density of 0.4 A g
−1 and 0.1 A g
−1
for lithium and sodium-ion storage, respectively.
Eom et al. (2016) synthesized 2D oxide nanomaterials via imitating a single GO
sheet. Unlike the common mixing method for sandwich type or core/shell materials,
GO is used for collecting metal ions on the carbon sheet surface and sacrificing
to produce a 2D oxide nanofoil consisting of nanocrystals. The negatively charged
GO surface adsorbs positively charged Co ions. Then, the mixture of GO and these
Co ions forms the hybrid of rGO/Co(OH) 2 /Co 3 O 4 under hydrothermal conditions.
Finally, thermal annealing under air condition will leave porous nanofoils of Co 3 O 4 .
Since the reactants are easy to diffuse, the porous shape is extremely advantageous
for the electrode material. Moreover, vacancies along the nanocrystal network can
offer a short path for mass transmission (Wang et al. 2016). In addition, the Co 3 O 4
nanoparticles on the reduced GO (rGO) sheets act as catalysts for the degradation of
the carbon backbone during thermal oxidation. Furthermore, due to the existence of
oxidation defect regions, rGO can be degraded at a comparatively low temperature
compared to the other carbon nanomaterials. The TEM image (Fig. 4.2f) obviously
displays porous Co 3 O 4 nanofoils, which mimics the original 2D morphology of
GO. As shown in Fig. 4.2g, the Co 3 O 4 nanofoils show a good reversible capacity
of 1279.2 mAh g
−1 after 50 cycles. According to the conversion mechanism from
Co 3 O 4 to Li 2 O and Co, this capacity far exceeds the theoretical capacity of Co 3 O 4 .
The preparation process of Co 3 O 4 nanofoils is clearly shown in Fig. 4.2h.
4.2.2 MnO x
Among all TMOs, the manganese oxides (MnO x ) like MnO 2 , Mn 2 O 3 have shown
excellent applications in many fields including catalysis, magnetic materials, electronics, supercapacitors, etc. (Umek et al. 2009). In addition, MnO x attract great
interest as promising electrode materials because of their non-toxic nature, abundance, and low cost. Importantly, MnO x have shown excellent structural flexibility
with good chemical and physical properties which express various applications in
heterogeneous catalysis, electrocatalysis, supercapacitors, or rechargeable batteries
(Han et al. 2008; Cheng et al. 2013; Yan et al. 2010).
MnO 2 as a typical TMO has attracted much attention because of its excellent
theoretical specific capacitance and rich redox activity (Shimamoto et al. 2013; Xiao
and Cao 2015; Fang et al. 2013; Cai et al. 2016). In order to overcome its poor
electrical conductivity and cycling crystal expansion/contraction induced flaking off
during repeated cycling processes, conducting carbon materials such as graphene
and CNTs were used as supporting materials (Zhao et al. 2015a; Mei and Zhang
2015; Li et al. 2014). Recently, several interesting MnO 2 -carbon-based composites
have been reported.
Li et al. (2016) present a novel flexible thin MnO 2 /graphene film (MGF) electrode
based on MnO 2 -graphene combination composites as supercapacitor electrodes. In
61
as-prepared holey CAN have the hole size of 10 nm and its reversible capacities are
1324 mAh g
−1 and 566 mAh g
−1 at the current density of 0.4 A g
−1 and 0.1 A g
−1
for lithium and sodium-ion storage, respectively.
Eom et al. (2016) synthesized 2D oxide nanomaterials via imitating a single GO
sheet. Unlike the common mixing method for sandwich type or core/shell materials,
GO is used for collecting metal ions on the carbon sheet surface and sacrificing
to produce a 2D oxide nanofoil consisting of nanocrystals. The negatively charged
GO surface adsorbs positively charged Co ions. Then, the mixture of GO and these
Co ions forms the hybrid of rGO/Co(OH) 2 /Co 3 O 4 under hydrothermal conditions.
Finally, thermal annealing under air condition will leave porous nanofoils of Co 3 O 4 .
Since the reactants are easy to diffuse, the porous shape is extremely advantageous
for the electrode material. Moreover, vacancies along the nanocrystal network can
offer a short path for mass transmission (Wang et al. 2016). In addition, the Co 3 O 4
nanoparticles on the reduced GO (rGO) sheets act as catalysts for the degradation of
the carbon backbone during thermal oxidation. Furthermore, due to the existence of
oxidation defect regions, rGO can be degraded at a comparatively low temperature
compared to the other carbon nanomaterials. The TEM image (Fig. 4.2f) obviously
displays porous Co 3 O 4 nanofoils, which mimics the original 2D morphology of
GO. As shown in Fig. 4.2g, the Co 3 O 4 nanofoils show a good reversible capacity
of 1279.2 mAh g
−1 after 50 cycles. According to the conversion mechanism from
Co 3 O 4 to Li 2 O and Co, this capacity far exceeds the theoretical capacity of Co 3 O 4 .
The preparation process of Co 3 O 4 nanofoils is clearly shown in Fig. 4.2h.
4.2.2 MnO x
Among all TMOs, the manganese oxides (MnO x ) like MnO 2 , Mn 2 O 3 have shown
excellent applications in many fields including catalysis, magnetic materials, electronics, supercapacitors, etc. (Umek et al. 2009). In addition, MnO x attract great
interest as promising electrode materials because of their non-toxic nature, abundance, and low cost. Importantly, MnO x have shown excellent structural flexibility
with good chemical and physical properties which express various applications in
heterogeneous catalysis, electrocatalysis, supercapacitors, or rechargeable batteries
(Han et al. 2008; Cheng et al. 2013; Yan et al. 2010).
MnO 2 as a typical TMO has attracted much attention because of its excellent
theoretical specific capacitance and rich redox activity (Shimamoto et al. 2013; Xiao
and Cao 2015; Fang et al. 2013; Cai et al. 2016). In order to overcome its poor
electrical conductivity and cycling crystal expansion/contraction induced flaking off
during repeated cycling processes, conducting carbon materials such as graphene
and CNTs were used as supporting materials (Zhao et al. 2015a; Mei and Zhang
2015; Li et al. 2014). Recently, several interesting MnO 2 -carbon-based composites
have been reported.
Li et al. (2016) present a novel flexible thin MnO 2 /graphene film (MGF) electrode
based on MnO 2 -graphene combination composites as supercapacitor electrodes. In