**6. Carbon-transition metal dichalcogenides (TMDs) composite electrode materials**

TMDs are layered inorganic materials with a chemical configuration of MX2, in which M is a transition metal element (M: Ti, Mo, V, W, Re, Ta), and X can be any chalcogenide element (X: S, Se, Te) (**Figure 7(a)**). Each MX2 unit cell is stacked

#### **Figure 7.**

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**Figure 6.**

to the insertion and extraction of [EMIM]+

offered by the crystalline network of γ-FeOOH during charging-discharging process. γ-FeOOH NSs/CC exhibited a good areal capacitance of 210 mF cm−2 at a current density of 1 mA cm−2 and the ASC device made of γ-FeOOH||APDC (activated polyaniline-derived carbon nanorods) solid-state flexible SCs acquired a high energy density of 1.44 mW h cm−3 at a current density of 3 A g−1 with a cycling stability of 80.5% retention over 2000 cycles (**Figure 6(a-c)**) [62]. An amorphous FeOOH nanoflowers@multi-walled CNT (FeOOH NFs@ MWCNTs) composite was prepared by Sun *et al.* [63]*.* The as-prepared composite electrode displays a high specific capacitance of 345 F g−1 at 1 A g−1 current density and outstanding rate performance (167 F g−1 at 11.4 A g−1) with good cycling stability of 76.4% over 5000 cycles. The outstanding electrochemical performance of the composite electrode is due to the mesoporous structure and high surface area of the electrode materials as well as fast ion/electronic transport and easy accessibility of the active materials to electrolytes. Liu *et al.* demonstrated FeOOH quantum dots (QDs)/graphene hybrid nanosheets, which exhibited a high specific capacitance of 365 F g−1 at a current density of 1 A g−1 with excellent capacitance retention of 89.7% of initial capacitance over 20000 cycles as well as a great rate capability (189 F g−1 at a high current density of 128 A g−1) (**Figure 6(d-f )**) [64]. In addition, specific capacitance

*performance of MnO2//FeOOH-ASC collected at a scan rate of 100 mV s−1 for 5000 cycles [66].*

*(a) Schematic illustrations of the fabrication procedure for the FeOOH//APDC f-SSC electrodes and flexibility and operating status as supercapacitor device, (b) The areal capacitance as a function of the discharge current density (Inset: SEM images of as-prepared* γ*-FeOOH nanosheets on a carbon cloth substrate), (c) CV curves of the FeOOH//APDC f-SSC at bent and flat statuses [62]. (d) Schematic illustration of the synthesis of amorphous FeOOH QDs and amorphous FeOOH/FGS hybrid nanosheets, (e) HRTEM images of the FeOOH QDs (Inset: enlarged HRTEM for FeOOH QDs), (f) The specific capacitances of the FeOOH, functionalized graphene sheet (FGS), and FeOOH/FGS composite electrodes as a function of the scan rate [64]. (g) Highmagnification SEM images of as-prepared 3D FeOOH/rGO/NF, (h) Areal capacitance of FeOOH/NF, rGO/ NF and FeOOH/rGO/NF electrodes calculated from CV curves as a function of scan rate, and (i) Cycling* 

cations through the transport pathways

*(a) Different metal coordination and stacking sequence in TMD unit cells [67]. (b-d) SEM images of aligned MWCNT sheets, MWCNT/MoS2 hybrids, and tightly knotted MoS2/MWCNT and rGO/MWCNT fibers, respectively. (e) CV curves of rGO/MWCNT (cathode) and MoS2-rGO/MWCNT (anode) at different potential windows. (f) Cycle stability test of the fiber-based asymmetric device at 0.55 A cm−3 current density [74]. (g) Optical photographs and (h) SEM images of the MoS2/C composite aerogel. (i) specific capacitances at different current densities and (j) long-term cycle stability at a current density of 6 A g−1 of the MoS2/C composite aerogel electrode material [75].*

together through Vander Waals force in such a way that transition metal layer is present in between the two chalcogen sheets [67]. On the basis of crystal structure, there are two types of phases of TMDs, which are metallic 1T phase with an octahedral structure and semiconducting 2H phase with a trigonal structure. Recently, TMDs have been attracted great attention as SC electrode materials due to their large surface area, low cost, variable oxidation states, high mechanical properties, high chemical stability and easy synthesis [68]. The variable oxidation states, large surface area, and active edges of TMDs allow electrical double layer and fast/reversible redox charge storage mechanisms and offer high energy storage capability in SCs. However, due to the inherently low conductivity, poor cycle life, large volume change during cycling and restacking limits their electrochemical performance as SC electrodes [69]. For example, Soon *et al.* has synthesized sheet-like morphology of MoS2 by chemical vapor deposition method, which has a very large surface area favorable for double layer storage. But due to its poor electrical conductivity, it showed low specific capacitance of ∼100 F g−1 at a scan rate of 1 mV s−1 [70]. Therefore, in order to improve the electrochemical performance of TMDs, they have been compositing with highly conducting/electroactive carbon based supporting materials by various top-down/bottom-up and both synthetic approaches. The synergic effect of carbon-TMDs based composite materials such as carbon offers conductive channels and increasing the interfacial contact, whereas TMDs provide a short ion diffusion path and followed by short electron transport path enhances the overall electrochemical performance of the SC.
