**5. Carbon-metal hydroxide composites electrode materials**

Among the active materials, metal hydroxides have also been considered promising electrode materials for electrochemical SCs because of extremely high specific capacitance. Metal hydroxide in several forms such as Ni(OH)2, Co(OH)2, NiCo(OH)2, Cu(OH)2, FeOOH have been investigated as electrodes for SC [50–52]. These materials have large internal spaces for fast insertion and desertion of electrolyte ions. Moreover, these metal hydroxides can be synthesized using simple synthetic approaches. Metal hydroxide consists of stacked layers intercalated having interlayer space to occupy more ions hence larger capacitance.

### **5.1 Carbon-nickel hydroxide (Ni(OH)2) composite electrode materials**

Ni(OH)2 is being considered as an attractive candidate as electrode in SCs because of its high theoretical capacitance (2358 F g−1). It can be prepared by a simple and low cost process. It has demonstrated good stability in alkaline electrolytes. Its low electrical conductivity is a barrier to achieve higher capacitance. Therefore, a thin region near the surface of nickel hydroxide contributes to the charge storage process due to diffusion-limited redox reactions. To obtain larger capacitance, it has to be utilized completely in the charge storage process. In this regard, researchers have generally adopted conductive additives to effectively improve utilization of active materials and result in larger capacitance. Kang *et al.* have used the same concept

*Novel Nanomaterials*

simple and effective approach to grow well-aligned 3D cobalt oxide nanowire arrays (Co3O4 NWAs) directly on carbon nanotube fibers (CNTFs) through CVD process [38]. The Co3O4 NWAs/CNFs showed a specific capacitance of 734.25 F cm−3 (2210 mF cm−2) at 1.0 A cm−3 and a high energy density of 13.2 mW h cm−3 at a current density of 1.0 A cm−3. Graphene along with cobalt oxides can be used as a composite material for SCs because of its high conductivity, high surface area, high carrier mobility, and excellent mechanical strength. For example, an in situ synthesised Co3O4/graphene@NF hybrid composite electrode with a thickness of 13 nm exhibited a high specific capacitance of 1.75 F cm−2 at 1 mA cm−2 and a capacitance increase of 12.2% after 5000 cycles at 10 mA cm−2 (**Figure 4(d-f ))** [39]. Tseng *et al.* demonstrate a binder-free and flexible SC based on CoO/graphene hollow nanoballs (GHBs) composite electrode [40]. The as fabricated CoO/GHBs composite electrode exhibits high specific capacitance of 2238 F g−1 at a current density of 1 A g−1 and good rate capability of 1170 F g−1 at a current density of 15 A g−1. The excellent capacitive performance and high rate capability were accomplished by the synergistic combination of conductive GHBs with large surface areas and highly pseudocapacitive CoO. In addition, as fabricated SSC demonstrated a very high power density (6000 W kg−1 at 8.2 W h kg−1), high energy density (16 W h kg−1 at 800 W kg−1), good cycling stability (∼100% capacitance retention after 5000 cycles), and excellent mechanical flexibility at various bending positions. Recently, 3D-carbon aerogels (3D-CA) with appropriate electrical conductivity, high specific surface area and rich dielectric electrochemical stability when combined with the porous cobalt oxides can enabled the fabrication of an composite electrode with outstanding electrochemical performance.Co3O4/CA composite electrode which was synthesized through in situ growth method showed a specific capacitance of 350 F g−1 at 1 A g−1 and Energy density of 23.82 kW kg−1 at a power density of 95.96 W kg−1 (**Figure 4(g-i))** [41]. The as-prepared ASC device could be cycled reversibly in a potential range of 0.0 to 1 V at 1 A g−1 and showed a capacity retention of 210% over 6000 cycles. Zhu *et al.* adopted a facile hydrothermal method to synthesize self-assembled cobalt oxide (CoO) nanorod cluster on 3D-graphene foam (CoO-3DGF) which exhibits a very high performance compared with CoO nanorod clusters grown on Ni foam (680 F g−1) in terms of specific capacitance 980 F g−1 at 1

A g−1 and cycling stability of 103% over 10,000 cycles [42].

exhibited a high specific capacitance of 1811 F g**<sup>−</sup>**<sup>1</sup>

**4.4 Carbon-binary metal oxide based composite electrode materials**

Recently, binary metal oxides such as NiCo2O4, NiFe2O4, CoFe2O4, ZnMnO4, and ZnCo2O4 have attracted much attention due to higher electrical conductivity than individual metal oxide and provide higher capacitance due to more affluent redox reaction than individual components [43]. Even though binary meal oxides possess better electrochemical performance than individual metal oxide extremely, they still suffer from inferior rate performance, low utilization rate and poor cycle stability. However, by incorporating carbon based materials improve their conductivity as well as power density due to high surface area, high conductivity and stable chemical properties of carbon based materials [44]. Kumar *et al.* fabricated Carbon black (CB) decorated Ni/Co oxide composite electrode through by using the successive ionic layer adsorption and reaction (SILAR) method [45]. Carbon black (CB) decorated Ni/Co oxide composite electrode with 7% weight percentage of CB

cyclic retention of 92% over 8000 cycles and delivered an impressive high energy

higher than pure Ni/Co oxide composite electrode as well as other carbon embedded composites. Veerasubramani *et al.* have adopted a novel approach to fabricate

at a power density of 151 W Kg**<sup>−</sup>**<sup>1</sup>

at 0.5 mA cm**<sup>−</sup>**<sup>2</sup>

with excellent

, which is significantly

**124**

density of 91 W h Kg**<sup>−</sup>**<sup>1</sup>

#### **Figure 5.**

*(a) A schematic of the growth process of 3D-Ni(OH)2/C/Cu, (b) Morphology of the as-synthesized 3D-Ni(OH)2/C/Cu electrode (inset: large-area uniform porous morphology of the 3D-Ni(OH)2/C/Cu), (c) Specific capacitance of 3DNi(OH)2/C and 3D-Ni(OH)2/C/Cu as a function of the current density based on the galvanostatic charge/discharge measurement [53], (d) Photograph of CF paper coated with cobalt hydroxide nanoflakes and schematic diagram illustrating the loading procedure of cobalt hydroxide on CF, (e) SEM image of bare CF, (f) SEM image of cobalt hydroxide nanoflakes coated on CF (Inset: magnified SEM image of the nanoflakes), (g) variation of specific capacitance with mass loading of each electrode, (h) specific capacitances of CF electrode at scan rates of 5, 10, 20, 50 and 100 mV/s, (i) schematic of flexible SC fabrication, (j) CV curves at bending conditions of 0° and 180° at scan rate of 20 mV/s [56].*

and deposited an ultrathin nickel hydroxide film on carbon-coated 3D porous copper structure in order to prepare binder-free conductive electrode (**Figure 5(a-b)**) [53]. This electrode has short electron path distances and large electrochemical active sites, which improved structural stability for high performance SCs. A carbon coating was used to improve the electron transport behavior and to prevent the oxidation of Cu. Nickel hydroxide supported on mesoporous hollow dendritic threedimensional-nickel exhibited a specific capacitance of 1860 F g−1 at a current density of 1 A g−1 (**Figure 5(c)**). It could retain 86.5% capacitance over 10,000 cycles. Tang *et al.* have prepared an additive-free, nano-architectured nickel hydroxide/carbon nanotube (Ni(OH)2/CNT) electrode for high performance SCs [54]. This Ni(OH)2/ CNT electrode was fabricated by depositing Ni(OH)2 nano-flakes on CNT bundles which were directly grown on Ni foams. The above electrode exhibited the specific capacitance of 3300 F g−1 and an aerial capacitance of 16 Fcm−2. Ma *et al.* have synthesized electrode of Ni(OH)2 nanosheet/3D GF framework using two methods, CVD and hydrothermal [55]. They have compared the capacitive properties of Ni(OH)2 electrode/graphene fiber with Ni(OH)2/Ni foam and Ni(OH)2 nanosheet/ carbon fiber cloth electrodes. Ni(OH)2 electrode with graphene fiber exhibited better performance in terms of specific capacitance and rate capability. The Ni(OH)2 nanosheet/graphene fiber electrode exhibited electrochemical capacitance as high as 2860 F g−1 at a current density of 2 A g−1, and maintains 1791 F g−1 at 30 A g−1.

## **5.2 Carbon-cobalt hydroxide (Co(OH)2) composite electrode materials**

Co(OH)2 has recently received increasing attention as electrode for SC application because of its low cost and high capacitance. Jagadale *et al.* have used cobalt hydroxide nanoflakes which were uniformly loaded on flexible carbon fiber (CF)

**127**

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors*

paper as electrode for SC (**Figure 5(d)**) [56]. The carbon fiber was basically used to provide unique porous nanostructure offering low ion diffusion and charge transfer resistance to the electrode (**Figure 5(e, f )**). The electrode exhibited maximum specific capacitance of 386.5 F g−1 at a current density of 1 mA cm−2 with a mass loading of 2.5 mg cm−2 (**Figure 5(g, h)**). An energy density of 133.5 W h kg−1 has been obtained with power density of and 1769 W kg−1. The carbon fiber has improved the cyclic stability of 92% over 2000 cycles. To check applicability of electrodes, these electrodes further employed to fabricate flexible solid state supercapacitor. CV curves of SC at bending conditions of 0° and 180° at scan rate of 20 mV/s. It is clearly seen that the area under curve doesn't change significantly after bending which proves that SC is highly flexible and does not lose its structural integrity

Two possible reactions are suggested for the electrochemical reactions of

Co OH CoOOH H O e (*OH*)<sup>2</sup> <sup>2</sup>

CoOOH OH Co H O e *O*2 2

Co(OH)2 nano-sheet-decorated graphene-CNT composite structure has been designed for SC application [58]. Suspensions method was used to prepare graphene-CNT composite by sonication and vacuum filtration. The graphene-CNT composite may offer high porosity with high conductivity, chemical stability and a three-dimensional structure. The vertically aligned Co(OH)2 nano-sheets were then deposited on 3D graphene-CNT composite by solution based process. The ASC of Co(OH)2 with graphene-CNT has shown a specific capacitance of 310 F g−1. The electrode exhibited an energy density of 172 W h kg−1 and maximum power density

of 198 kW kg−1 in ionic liquid electrolyte 1-ethyl-3-methylimidazoliumbis

with active materials, exhibiting good cycling stability and lifetime.

**5.3 Carbon-iron oxy hydroxide (FeOOH) composite electrode materials**

FeOOH has been recognized is an attractive electrode material for SC due to low cost, high theoretical specific capacitance, and broad potential window. In addition, the unique tunnel structure of FeOOH with open permeable channels are beneficial for ion transportation and shorten the diffusion path for electrolyte ion diffusion [60]. However, the poor electrical conductivity and low specific surface area limited the use of FeOOH as a potential electrode for SC, which limited specific capacitance and rate capability [61]. Alternatively, composite system by assembling FeOOH on the carbon based supporting materials (AC, carbon black, graphene, etc.) can be enhance the capacitive performance. Shen *et al.* synthesized radiating γ-FeOOH Nanosheets on CC substrate (γ-FeOOH NSs/CC) by a simple one-step electrodeposition method and investigated its pseudocapacitive behaviour in a typical ionic liquid [1-ethyl-3-methylimidazolium bis imide (EMIM-NTF2)] through electrochemical quartz crystal microbalance (EQCM). The charge storage is mainly due

(trifluoromethanesulfone)imide (EMI-TFSI). Zhang *et al.* have deposited Co(OH)2 on multi-walled CNT which were grown on the carbon paper substrate [59]. The composite electrode showed the specific capacitance of 1083 F g−1 determined at a current density of 0.83 A g−1 in aqueous electrolyte. CNTs were added to Co(OH)2 in order to improve the electrical conductivity of the electrode. The interconnected nanosheets of the Co(OH)2 would help to facilitate the contact of the electrolyte

− − + = ++ (6)

− − + = ++ (7)

*DOI: http://dx.doi.org/10.5772/intechopen.95460*

under bending conditions (**Figure 5(i, j)**).

Co(OH)2 in KOH electrolyte [57]:

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors DOI: http://dx.doi.org/10.5772/intechopen.95460*

paper as electrode for SC (**Figure 5(d)**) [56]. The carbon fiber was basically used to provide unique porous nanostructure offering low ion diffusion and charge transfer resistance to the electrode (**Figure 5(e, f )**). The electrode exhibited maximum specific capacitance of 386.5 F g−1 at a current density of 1 mA cm−2 with a mass loading of 2.5 mg cm−2 (**Figure 5(g, h)**). An energy density of 133.5 W h kg−1 has been obtained with power density of and 1769 W kg−1. The carbon fiber has improved the cyclic stability of 92% over 2000 cycles. To check applicability of electrodes, these electrodes further employed to fabricate flexible solid state supercapacitor. CV curves of SC at bending conditions of 0° and 180° at scan rate of 20 mV/s. It is clearly seen that the area under curve doesn't change significantly after bending which proves that SC is highly flexible and does not lose its structural integrity under bending conditions (**Figure 5(i, j)**).

Two possible reactions are suggested for the electrochemical reactions of Co(OH)2 in KOH electrolyte [57]:

$$\text{Co}(\text{OH})\_2 + \text{OH}^- = \text{CoOOH} + \text{H}\_2\text{O} + \text{e}^- \tag{6}$$

$$\text{CoOOH} + \text{OH}^- = \text{CoO}\_2 + \text{H}\_2\text{O} + \text{e}^- \tag{7}$$

Co(OH)2 nano-sheet-decorated graphene-CNT composite structure has been designed for SC application [58]. Suspensions method was used to prepare graphene-CNT composite by sonication and vacuum filtration. The graphene-CNT composite may offer high porosity with high conductivity, chemical stability and a three-dimensional structure. The vertically aligned Co(OH)2 nano-sheets were then deposited on 3D graphene-CNT composite by solution based process. The ASC of Co(OH)2 with graphene-CNT has shown a specific capacitance of 310 F g−1. The electrode exhibited an energy density of 172 W h kg−1 and maximum power density of 198 kW kg−1 in ionic liquid electrolyte 1-ethyl-3-methylimidazoliumbis (trifluoromethanesulfone)imide (EMI-TFSI). Zhang *et al.* have deposited Co(OH)2 on multi-walled CNT which were grown on the carbon paper substrate [59]. The composite electrode showed the specific capacitance of 1083 F g−1 determined at a current density of 0.83 A g−1 in aqueous electrolyte. CNTs were added to Co(OH)2 in order to improve the electrical conductivity of the electrode. The interconnected nanosheets of the Co(OH)2 would help to facilitate the contact of the electrolyte with active materials, exhibiting good cycling stability and lifetime.

#### **5.3 Carbon-iron oxy hydroxide (FeOOH) composite electrode materials**

FeOOH has been recognized is an attractive electrode material for SC due to low cost, high theoretical specific capacitance, and broad potential window. In addition, the unique tunnel structure of FeOOH with open permeable channels are beneficial for ion transportation and shorten the diffusion path for electrolyte ion diffusion [60]. However, the poor electrical conductivity and low specific surface area limited the use of FeOOH as a potential electrode for SC, which limited specific capacitance and rate capability [61]. Alternatively, composite system by assembling FeOOH on the carbon based supporting materials (AC, carbon black, graphene, etc.) can be enhance the capacitive performance. Shen *et al.* synthesized radiating γ-FeOOH Nanosheets on CC substrate (γ-FeOOH NSs/CC) by a simple one-step electrodeposition method and investigated its pseudocapacitive behaviour in a typical ionic liquid [1-ethyl-3-methylimidazolium bis imide (EMIM-NTF2)] through electrochemical quartz crystal microbalance (EQCM). The charge storage is mainly due

*Novel Nanomaterials*

**Figure 5.**

and deposited an ultrathin nickel hydroxide film on carbon-coated 3D porous copper structure in order to prepare binder-free conductive electrode (**Figure 5(a-b)**) [53]. This electrode has short electron path distances and large electrochemical active sites, which improved structural stability for high performance SCs. A carbon coating was used to improve the electron transport behavior and to prevent the oxidation of Cu. Nickel hydroxide supported on mesoporous hollow dendritic threedimensional-nickel exhibited a specific capacitance of 1860 F g−1 at a current density of 1 A g−1 (**Figure 5(c)**). It could retain 86.5% capacitance over 10,000 cycles. Tang *et al.* have prepared an additive-free, nano-architectured nickel hydroxide/carbon nanotube (Ni(OH)2/CNT) electrode for high performance SCs [54]. This Ni(OH)2/ CNT electrode was fabricated by depositing Ni(OH)2 nano-flakes on CNT bundles which were directly grown on Ni foams. The above electrode exhibited the specific capacitance of 3300 F g−1 and an aerial capacitance of 16 Fcm−2. Ma *et al.* have synthesized electrode of Ni(OH)2 nanosheet/3D GF framework using two methods, CVD and hydrothermal [55]. They have compared the capacitive properties of Ni(OH)2 electrode/graphene fiber with Ni(OH)2/Ni foam and Ni(OH)2 nanosheet/ carbon fiber cloth electrodes. Ni(OH)2 electrode with graphene fiber exhibited better performance in terms of specific capacitance and rate capability. The Ni(OH)2 nanosheet/graphene fiber electrode exhibited electrochemical capacitance as high as 2860 F g−1 at a current density of 2 A g−1, and maintains 1791 F g−1 at 30 A g−1.

*(a) A schematic of the growth process of 3D-Ni(OH)2/C/Cu, (b) Morphology of the as-synthesized 3D-Ni(OH)2/C/Cu electrode (inset: large-area uniform porous morphology of the 3D-Ni(OH)2/C/Cu), (c) Specific capacitance of 3DNi(OH)2/C and 3D-Ni(OH)2/C/Cu as a function of the current density based on the galvanostatic charge/discharge measurement [53], (d) Photograph of CF paper coated with cobalt hydroxide nanoflakes and schematic diagram illustrating the loading procedure of cobalt hydroxide on CF, (e) SEM image of bare CF, (f) SEM image of cobalt hydroxide nanoflakes coated on CF (Inset: magnified SEM image of the nanoflakes), (g) variation of specific capacitance with mass loading of each electrode, (h) specific capacitances of CF electrode at scan rates of 5, 10, 20, 50 and 100 mV/s, (i) schematic of flexible SC* 

*fabrication, (j) CV curves at bending conditions of 0° and 180° at scan rate of 20 mV/s [56].*

**5.2 Carbon-cobalt hydroxide (Co(OH)2) composite electrode materials**

Co(OH)2 has recently received increasing attention as electrode for SC application because of its low cost and high capacitance. Jagadale *et al.* have used cobalt hydroxide nanoflakes which were uniformly loaded on flexible carbon fiber (CF)

**126**

#### **Figure 6.**

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

to the insertion and extraction of [EMIM]<sup>+</sup> cations through the transport pathways 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

**129**

**Figure 7.**

*composite aerogel electrode material [75].*

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors*

0.19 W cm−3 with maximum energy density of 0.48 mW h cm−3.

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

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

*(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* 

of the SC increased to 1243 F g−1 at 5 mV s−1 while the voltage window was extended from -0.8 to 0 V to -1.25 to 0 V but the cycling performance declined sharply. Wei *et al.* synthesized ultrathin α-FeOOH nanorods/graphene oxide (GO) composite by hydrothermal method, which exhibited high specific capacitance of 127 F g−1 at a current density of 10 A g−1, good cyclic performance of 85% capacitance retention over 2000 cycles, and excellent rate capability (100 F g−1 at 20 A g−1) as compared to than bare α-FeOOH nanorods [65]. The outstanding electrochemical performance of α-FeOOH nanorods/GO composite is due to its unique structure, which provides fast electron/ions transport and high charging/discharging rate. 3D FeOOH/ reduced graphene oxide/Ni foam (FeOOH/rGO/NF) based hybrid electrodes fabricated by the electrodeposition of FeOOH nanosheets on the rGO/Ni foam surface exhibited an exception high areal capacitance of 406.5 mF cm−2 at a scan rate of 10 mV s−1, which is 10-fold higher than the bare FeOOH/NF electrode (**Figure 6(g-i)**) [66]. This high areal capacitance of FeOOH/rGO/NF is due to the improved conductivity and increased surface area, which not only provide a superior pathway for electron transfer, but also offer more active sites for energy storage. In addition, an ASC device made of 3D FeOOH/rGO/NF electrode as anode and MnO2@TiN electrode as cathode attained a remarkable maximum power density of

*DOI: http://dx.doi.org/10.5772/intechopen.95460*

**electrode materials**

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors DOI: http://dx.doi.org/10.5772/intechopen.95460*

of the SC increased to 1243 F g−1 at 5 mV s−1 while the voltage window was extended from -0.8 to 0 V to -1.25 to 0 V but the cycling performance declined sharply. Wei *et al.* synthesized ultrathin α-FeOOH nanorods/graphene oxide (GO) composite by hydrothermal method, which exhibited high specific capacitance of 127 F g−1 at a current density of 10 A g−1, good cyclic performance of 85% capacitance retention over 2000 cycles, and excellent rate capability (100 F g−1 at 20 A g−1) as compared to than bare α-FeOOH nanorods [65]. The outstanding electrochemical performance of α-FeOOH nanorods/GO composite is due to its unique structure, which provides fast electron/ions transport and high charging/discharging rate. 3D FeOOH/ reduced graphene oxide/Ni foam (FeOOH/rGO/NF) based hybrid electrodes fabricated by the electrodeposition of FeOOH nanosheets on the rGO/Ni foam surface exhibited an exception high areal capacitance of 406.5 mF cm−2 at a scan rate of 10 mV s−1, which is 10-fold higher than the bare FeOOH/NF electrode (**Figure 6(g-i)**) [66]. This high areal capacitance of FeOOH/rGO/NF is due to the improved conductivity and increased surface area, which not only provide a superior pathway for electron transfer, but also offer more active sites for energy storage. In addition, an ASC device made of 3D FeOOH/rGO/NF electrode as anode and MnO2@TiN electrode as cathode attained a remarkable maximum power density of 0.19 W cm−3 with maximum energy density of 0.48 mW h cm−3.
