4.4 MXenes
69
conductivity, intercalation of spontaneous ions in solution and capacitance. To further
enhance electrochemical performance of Ti 3 C 2 electrode materials, one straightforward strategy is the introduction of Ti 3 C 2 . Conducting polymers, such as polyaniline
(PANI), is famous for its large specific surface area (SSA) and relatively high specific
capacitance (Liu et al. 2015). In particular, through some methods such as providing
faradaic reactions for additional capacity increasing the electric conductivity for
faster ion transport, and improving surface wettability for more active sites, nitrogencontaining functional groups in PANI can improve electrochemical performance of
Ti 3 C 2 (Yan et al. 2010). Multilayered PANI-Ti 3 C 2 was synthesized by Ren et al.
(2018) via in situ polymerization by low temperature, which could reduce the oxidation of Ti 3 C 2 during the polymerization. PANI-Ti 3 C 2 images were shown in Fig. 4.4a,
homogeneous PANI nanoparticles cover the surfaces of lamellar Ti 3 C 2 , which results
in the increase in SSA of PANI-Ti 3 C 2 comparing with Ti 3 C 2 . When used as supercapacitor electrode material, PANI-Ti 3 C 2 showed enhanced performance comparing
with Ti 3 C 2 .
Yu et al. (2018) mixed a delaminated Ti 3 C 2 T x MXene with CNTs to prepare a
Ti 3 C 2 T x /CNT self-supporting composite film by a facile vacuum filtration method.
Figure 4.4b shows the cross-sectional SEM images of the Ti 3 C 2 T x /CNT composite
film. It is clearly seen that the film still maintains a certain lamellar structure, and
carbon nanotubes are interposed between the Ti 3 C 2 T x nanoflakes. The thickness of
the Ti 3 C 2 T x /CNT composite film is about 2 μm. Carbon nanotubes look like multilayer fishing nets and wrap the Ti 3 C 2 T x nanoflakes, which can effectively suppress
the Ti 3 C 2 T x nanoflakes from stacking and enhance the electrochemical performance.
When used as a binder-free anode, the Ti 3 C 2 T x /CNT film reveals a high reversible
capacity up to 489 mA h g
−1 together with good cycling performance (Fig. 4.4c).
Subsequently, an activated carbon electrode and the Ti 3 C 2 T x /CNT film as the cathode
and the anode, respectively, which delivered a high energy density of 67 Wh kg
−1
and a good capacity retention of 81.3% after 5000 cycles, to assemble lithium-ion
capacitor (LIC). This is the first time that a Ti 3 C 2 T x /CNT film is utilized as an anode
material for LICs.
Zheng et al. (2018) selected Ti 3 C 2 , Ti 2 C, and V 2 C as representative MXenes and
successfully synthesized CNTs on them by microwave irradiation using carbon fibers
(C f ) as the ignitor. The whole preparation process is illustrated in Fig. 4.4e. In such
synthesized CNTs@Ti 3 C 2 , the CNTs serve as spacers preventing the restacking of
MXene, and bridge the gap between Ti 3 C 2 interlayers, thereby forming a conductive
network. On the other hand, Ti 3 C 2 presents as a substrate with fine thermal conductivity, high surface area, and catalytically active sites, which facilitate the growth of
CNTs. Benefiting from these merits, CNTs@Ti 3 C 2 hybrids as anodes of LIBs exhibit
excellent electrochemical performance compared with pristine Ti 3 C 2 and recently
reported MXene composites.
69
conductivity, intercalation of spontaneous ions in solution and capacitance. To further
enhance electrochemical performance of Ti 3 C 2 electrode materials, one straightforward strategy is the introduction of Ti 3 C 2 . Conducting polymers, such as polyaniline
(PANI), is famous for its large specific surface area (SSA) and relatively high specific
capacitance (Liu et al. 2015). In particular, through some methods such as providing
faradaic reactions for additional capacity increasing the electric conductivity for
faster ion transport, and improving surface wettability for more active sites, nitrogencontaining functional groups in PANI can improve electrochemical performance of
Ti 3 C 2 (Yan et al. 2010). Multilayered PANI-Ti 3 C 2 was synthesized by Ren et al.
(2018) via in situ polymerization by low temperature, which could reduce the oxidation of Ti 3 C 2 during the polymerization. PANI-Ti 3 C 2 images were shown in Fig. 4.4a,
homogeneous PANI nanoparticles cover the surfaces of lamellar Ti 3 C 2 , which results
in the increase in SSA of PANI-Ti 3 C 2 comparing with Ti 3 C 2 . When used as supercapacitor electrode material, PANI-Ti 3 C 2 showed enhanced performance comparing
with Ti 3 C 2 .
Yu et al. (2018) mixed a delaminated Ti 3 C 2 T x MXene with CNTs to prepare a
Ti 3 C 2 T x /CNT self-supporting composite film by a facile vacuum filtration method.
Figure 4.4b shows the cross-sectional SEM images of the Ti 3 C 2 T x /CNT composite
film. It is clearly seen that the film still maintains a certain lamellar structure, and
carbon nanotubes are interposed between the Ti 3 C 2 T x nanoflakes. The thickness of
the Ti 3 C 2 T x /CNT composite film is about 2 μm. Carbon nanotubes look like multilayer fishing nets and wrap the Ti 3 C 2 T x nanoflakes, which can effectively suppress
the Ti 3 C 2 T x nanoflakes from stacking and enhance the electrochemical performance.
When used as a binder-free anode, the Ti 3 C 2 T x /CNT film reveals a high reversible
capacity up to 489 mA h g
−1 together with good cycling performance (Fig. 4.4c).
Subsequently, an activated carbon electrode and the Ti 3 C 2 T x /CNT film as the cathode
and the anode, respectively, which delivered a high energy density of 67 Wh kg
−1
and a good capacity retention of 81.3% after 5000 cycles, to assemble lithium-ion
capacitor (LIC). This is the first time that a Ti 3 C 2 T x /CNT film is utilized as an anode
material for LICs.
Zheng et al. (2018) selected Ti 3 C 2 , Ti 2 C, and V 2 C as representative MXenes and
successfully synthesized CNTs on them by microwave irradiation using carbon fibers
(C f ) as the ignitor. The whole preparation process is illustrated in Fig. 4.4e. In such
synthesized CNTs@Ti 3 C 2 , the CNTs serve as spacers preventing the restacking of
MXene, and bridge the gap between Ti 3 C 2 interlayers, thereby forming a conductive
network. On the other hand, Ti 3 C 2 presents as a substrate with fine thermal conductivity, high surface area, and catalytically active sites, which facilitate the growth of
CNTs. Benefiting from these merits, CNTs@Ti 3 C 2 hybrids as anodes of LIBs exhibit
excellent electrochemical performance compared with pristine Ti 3 C 2 and recently
reported MXene composites.