Carbon Cloth-Based Hybrid Supercapacitors: Recent Progress and Performance Enhancement Strategies

1. Introduction

Interest in developing efficient, safe, and ecologically sustainable electrochemical energy storage systems has become unprecedentedly high, as it has been driven by the surge in the use of electronic devices and renewable energy technologies, such as solar, wind, and geothermal energy. Supercapacitors (SCs), or electrochemical capacitors, as high-power energy storage devices, have been extensively utilized in hybrid automobiles, energy recovery systems, and emergency power supplies, among other applications. However, low energy density, rigidity, and the bulky nature of current SCs have restricted their design and utilization in flexible, portable, and wearable electronics. The ability to bend, twist, and fold has been desired in smart textiles, bendable phones, curved screens, electronic skin, and health monitoring bracelets [1].

Supercapacitors are basically classified depending on their charge storage mechanisms into three classes: electric double-layer capacitors (EDLCs), pseudocapacitors or Faradic capacitors, and hybrid supercapacitors (HSCs) or asymmetric supercapacitors (ASCs). In EDLCs, charges are electrostatically accumulated in the interface between porous electrodes and appropriately sized electrolyte ions. Fast charging and discharging processes occur in the electric double layer that is called the Helmholtz layer. Differently, charges in pseudocapacitors are chemically bonded by Faradic redox reactions between reduced/oxidized electrodes and compatible electrolytes. This mechanism resembles batteries more than capacitors. Such repeated reactions during charging-discharging processes result in a degradation of electrode materials and an increase in the internal resistance. Consequently, pseudocapacitors in most cases exhibit lower cyclic stability compared to EDCLs, but higher energy density. The features of both mechanisms are combined in HSCs or ASCs. The operating voltage of ASCs is generally wider than that of symmetric SCs, which leads to an increase in energy density. As a rule, the electrochemical performance of supercapacitors is basically evaluated through three main experiments, which are cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS).

A variety of nanomaterials have been hybridized in one or both electrodes of HSCs, utilizing their superior specific surface area compared to bulk materials. Metal-organic frameworks (MOFs) [2], layered double hydroxides (LDHs) [3], multivalent metal elements [4], and bimetallic oxide-conducting polymer composites [5] have been widely reported as promising electrodes of HSCs. Various nanomaterials have been simply and cost-effectively prepared via chemical solution–based deposition techniques on different substrates, achieving results for HSCs on par with complex and costly techniques like atomic layer deposition (ALD) [6]. Unlike flat substrates, carbon cloth (CC) is an affordable commercial textile of a three-dimensional (3D) network of carbon microfibers (5-10 μm). In addition, CC is lightweight, safe, breathable, moisture-permeable, and woven, making it suitable for wearable SCs. It has excellent electrical conductivity, mechanical strength, flexibility, chemical stability, and environmental friendliness. Because of these outstanding properties, CC-based hybrid electrodes [7] and treated CC self-standing electrodes [8] for supercapacitors have been of prime interest. Carbon cloth has been employed not only in supercapacitors, but also in other energy storage and conversion devices, including lithium-ion batteries and solar cells [9,10]. Recently, an asymmetric photo-assisted supercapacitor was built based on CeO₂-MnO₂/CC at one end and photosensitive CeO₂-MnO₂/FTO at the other end [11]. In light, the areal capacitance was 4.8 mF/cm², higher than in the dark (2.8 mF/cm²).

On the carbon cloth substrate, the hydrothermal synthesis and HCS performance of conductive polymers, carbon materials, metal compounds, and composite materials were reviewed by Wang and co-authors in 2021 [12]. Interestingly, researchers have still been reporting innovative synthesis techniques, advanced materials, and improved performance of CC-based supercapacitors. In the following content, such recent advances are thoroughly summarized and critically compared, giving comprehensive coverage of challenges and opportunities. Most of the selected original articles herein were published between 2021 and 2026 and indexed in Web of Science. Each section of this review sheds light on effective strategies that are categorized in a roadmap, pointing toward improving carbon cloth-based supercapacitors, as outlined in Figure 1.

Figure 1. A staircase roadmap of effective strategies for improving carbon cloth-based hybrid supercapacitors.

Figure 1. A staircase roadmap of effective strategies for improving carbon cloth-based hybrid supercapacitors.

2. Treatment of Carbon Cloth

Whether used as a self-standing electrode or a current collector substrate in supercapacitors, the surface of carbon cloth requires pre-processing. Chemical and physical treatments are applied to enhance the surface properties as well as supercapacitor performance. At the electrode-electrolyte interface, increasing the overpotentials of the oxygen evolution reaction (OER) or hydrogen evolution reaction (HER) is one strategy to broaden the operating voltage window of supercapacitors, hence elevating the energy. In one study by Xu and others [13], CC was activated in Li₂SO₄ solution, then modified using the amino acid of L-Lysine. In an aqueous electrolyte of Na₂SO₄, the modified CC showcased a potential window versus Ag/AgCl from −0.8 to 0.7 V, wider than that of the unmodified CC from −0.8 to 0.4 V. The OER was impeded by the amphoteric dissociation of L-Lysine, which regulated the pH of the microenvironment between the electrode and the electrolyte. Based on that, a symmetric SC of L-Lysine-modified CC operated in 1.5 V range, which is wider than 1.2 V of the unmodified SC. The increase in voltage yielded a 59.42% increase in energy density of the CC-L-Lysine SC.

There is a proportional relationship between electrode capacitance and surface area that is accessible by electrolyte ions. Based on that, creating pores on CC is an effective strategy especially for self-standing CC electrodes. As proof of this, the CC surface was etched by immersing CC in a mixture of acetic acid and potassium chlorate at 180 ◦C for 12 h, creating nanopores and oxygen-containing functional groups [14]. Therefore, Brunauer–Emmett–Teller (BET) specific surface area of 1.5 m²/g for the untreated CC was increased by two orders of magnitude to 137.9 m²/g for the treated CC. The amount of oxygen attached to the CC was also elevated from 4.6 at. % to 11.8 at. %. The large surface area and the oxygen content improved the areal capacitance from 68 mF/cm² to 7758 mF/cm² at 0.5 mA/cm². Furthermore, Su et al. activated carbon cloth, called ACC, using a solution of MoC_l5, ethylene diamine tetraacetic acid, and ammonium acetate in the electrodeposition process at −1.0 V for 5 min, followed by drying at 60 °C for 12 h, and annealing at 400 °C for 6 h [15]. The resultant mesoporous ACC displayed a surface area of 185.7 m²/g and a pore volume of 0.118 cm³/g, compared to 46.6 m²/g and 0.059 cm³/g of untreated CC. In addition, an increase in oxygen content occurred from 4.9 at. % on CC to 11.7 at. % on ACC. An areal capacitance of 1000 mF/cm² at 2 mA/cm² resulted due to the large surface area and functional groups. Moreover, a perfect cycling stability of 100% capacitance retention during 10000 cycles at 16 mA/cm² was achieved. When the electrodepositing time was prolonged to 10 min, the areal capacitance dropped to 677 mF/cm². This was because of the decrease in the shell layer thickness as measured by TEM images, caused by the rougher edge structure and denser carbon exfoliation. A similar electrochemical activation process of CC in HNO₃ and H₂SO₄ at 3 V for 12 min was reported [16]. Then, the ACC was coated with a biomass-derived carbon (BMC). As expected, the BMC/ACC electrode showed higher areal capacitance (1157 mF/cm² at 4 mA/cm²) than the BMC/CC electrode and ACC electrode.

Coating carbon cloth microfibers with metals is a successful approach to augment the conductivity of CC-based electrodes. For example, CC was coated with dense Ni grains using an electroplating bath containing NiSO₄⋅6H₂O, Na₃C₆H₅O₇, NH₄Cl, NaOH, and NaH₂PO₂ in DI water at 45 °C for 20 min [17]. The Ni-CC prepared by Sudhakaran and co-authors was used as a substrate for a directly electrodeposited β-Co(OH)₂@CoNi₂S₄ core-shell heterostructure. As a result of that, it successfully showed low solution resistance (R_s) of 1.1 ohm and charge transfer resistance (R_ct) of 1.8 ohm. Similar results were obtained but using another way by our group for porous carbon nanostructure derived from date palm fronds. This carbon powder was activated either chemically in NaOH or physically in CO₂, then drop-casted on carbon cloth [18]. An excellent specific surface area of 1011 m²/g, specific capacitance of 126 F/g, and R_ct of 1.8 ohm were obtained for NaOH-activated carbon, superior to 603.5 m²/g, 57 F/g, and 1.5 ohm for the CO₂-activated carbon.

Compared with the abovementioned CC activated by hydrothermal and electrodeposition, a higher BET specific surface area of 329.8699 m²/g was recorded by Akhbar and others using a simple heat treatment at 400 °C for 6 h in the air [19]. The values of pore volume and pore size were 0.183707 cm³/g and 2.2276 nm. On top of ACC, NiCo nanoneedles were coated with NiOOH nanoflakes and CoMoO₄ nanosheets using hydrothermal and dip-dry coating methods. The core-shell heterostructure of the hybrid electrode of ACC@NiCo@NiOOH@CoMoO₄ provided an extraordinary areal capacitance of 2920 mF/cm² at 1 mA/cm² and 2541 mF/cm² at 10 mA/cm². A fabricated ASC of ACC@NiCo@NiOOH@CoMoO₄//ACC offered an excellent areal energy density of 101 mWh /cm² coupled with a remarkable areal power density of 125 mW /cm². It is worth noting that this low-cost supercapacitor was stable by 92% after 10000 cycles of charging and discharging at 8 mA/cm² in the range of 0-1.7 V. Figure 2 shows SEM images and XPS spectra of bare CC and pretreated carbon cloth (PCC) [20]. The pretreatment involved heating at 600 °C in argon then dipping in 60% nitric acid for 6 h. The cracks and structural defects in PCC were ascribed to chemical oxidation, thereby improving the surface wettability. The XPS confirmed that the treatment of CC produced hydroxyl ( ̶ OH), carbonyl (C––O), and carboxylic acid ( ̶ COOH) groups. Likewise, an outstanding specific surface area of 1031.5 m²/g was reached through activating CC in aqueous 5 M KOH at 100 °C for 1 h by Xu et al. [21]. Comparing this result with the untreated CC in the same study, the treated CC had a 33 times higher specific surface area. A rough and porous structure was displayed on the CC surface after the treatment with bases. Besides, this ACC electrode showed better capacitance than ACC treated in mixed acids, where both samples were tested electrochemically in KOH electrolyte. A partial dissolution of CC in H₂SO₄ and HNO₃ was noted, affecting the mechanical strength. Furthermore, they found that the activation in KOH for 30 min or longer led to more stable capacitance than that of the activated electrodes in 0, 10, and 20 min. Such wet chemical methods of activation are simple and widely applied to introduce pores and oxygen groups on the CC surface, leading to improved surface area, wettability, and capacitance of CC-based electrodes.

Figure 2. SEM images of a) Bare CC; and b) pretreated CC. XPS analysis of (c) bare CC; and (b) pretreated CC [20].

Figure 2. SEM images of a) Bare CC; and b) pretreated CC. XPS analysis of (c) bare CC; and (b) pretreated CC [20].

3. Utilization of Hybrid Nanomaterials

Incorporating nanostructured materials on the same current collector substrate as a hybrid electrode has been extensively recognized as a powerful strategy for betterment in the electrochemical performance of SCs. Nanomaterials of transition metal compounds, conductive polymers, and carbon forms have been the core active materials of SCs electrodes. The areal capacitances of various CC-based electrodes have been compared at a common areal current. For instance, the areal capacitances in mF/cm² at 1 mA/cm² were 4008 for Ni-Mn metal-organic framework [22], 3834 for MoS₂ [23], 2312 for polyaniline [24], 2120 for tetraaniline [25], 1018 for boron and nitrogen-doped carbon [26], and 494 for SnO₂@PPy [27], all on CC. At 2 mA/cm² in other reports, the areal capacitances in mF/cm² were 4303 for nitrogen, boron, and fluorine co-doped carbon nanotubes (NBF-CNT) [28], 4082 for nitrogen-doped carbon nanotubes (N-CNT) [29], and 1796 for polypyrrole on wolfram carbide [30], all on CC. In the following subsections, detailed cases are reviewed under each class of electrode materials.

3.1. Transition Metal Compounds

Transition metal chalcogenides, hydroxides, nitrides, phosphides, and related compounds have been categorized as pseudocapacitive materials mainly due to their redox activity. As SC electrodes, examples of transition metal compounds on carbon cloth include, but are not limited to, NiS₂, NiO, Ni(OH)₂, Ni₂P [31], Ni₂P/Ni(OH)₂ [32], MnO₂/TiN, Fe₂O₃/TiN [33], WS₂, Ni-WS₂ [34], WS₂/graphene [35], FeNb₂O₆ [36], CuO [37], Ni(OH)₂/CuO [38], gallium oxynitride [39], Co₃O₄/Ni-based MOFs [40], and Co_xMn_3-x(PO₄)₂/MXene [41]. Hybrid CC-based electrodes have been carefully designed and fabricated in complex hierarchical architectures such as hollow spheres of a carbon layer between α-Fe₂O₃ outer and inner shells [42], a core-shell framework of Mn(OH)₂ and CuO on Cu-coated carbon cloth [43], and Ni_xCo_2x(OH)_6x nanoflake layer on MoO₃ arrays on CC [44]. In particular, MnCo₂O₄ hollow nanospheres with multi-hierarchical pores were grown on activated carbon cloth by Zhu and his colleagues, resulting in a specific surface area up to 110.2 m²/g. Their ASC of MnCo₂O₄/ACC//ACC delivered 0.8 mWh/cm³ of energy density at 9.8 mW/cm³ of power density with 87% capacitance retention after 5000 GCD cycles at 45 ° of bending [45]. Another core-shell hierarchical nanostructure was designed by Li and co-authors [46]. Initially, carbon cloth were hydrothermally coated with nickel-doped Ga₂O₄ nanowalls, then with MnO₂ nanoflakes on NiGa₂O₄ nanowalls. The large specific surface area of the 3D core-shell hierarchical nanostructures in the hybrid electrode of NiGa₂O₄@MnO₂/CC offered additional pseudocapacitive sites, hence contributing to improving the CV, GCD, and EIS performance compared to the NiGa₂O₄/CC electrode. Based on that, an ASC of NiGa₂O₄@MnO₂/CC//N-CMK-3/CC was built, showing an energy density of 0.6 Wh/cm³ at a power density of 48 W/cm³. After 5000 cycles at 6 mA/cm², 80% of the initial capacitance was retained. Furthermore, a hybrid electrode of nickel cobalt sulfide/double hydroxide on carbon cloth (abbreviated as NCS@NCOH/CC) was investigated using microscopy and spectroscopy techniques as supplied in Figure 3 [47]. The findings clarified the formation of a core-shell heterostructure and the corresponding interplanar atomic planes. Analyzing in-situ Raman spectra during charging-discharging between −0.1 and 0.4 V revealed that material structure transformations were partially irreversible, leading to a 78% capacitance retention after 7000 cycles.

Metal-organic frameworks (MOFs) with multivalent organic ligands and metal centers tolerate functionalization and possess design changeability along with large surface-to-volume ratios. The groundbreaking work in the design and synthesis of MOFs by Richard Robson, Susumu Kitagawa, and Omar M. Yaghi has been recognized with the Nobel Prize in Chemistry 2025 [48]. Recently, there have been review articles on applying MOFs in supercapacitors highlighting their potential [49,50]. For instance, a hybrid electrode of Co₃O₄@Ni-MOF on carbon cloth exhibited a specific capacitance of 1416 F/g at the current density of 1 A/g, outstanding compared to 410 F/g of Co₃O₄ and 690 F/g of Ni-MOF non-hybrid electrodes [51]. A full cell of Co₃O₄@Ni-MOF/CC electrode and AC electrode delivered an energy density of 69 Wh/kg at the power density of 510 W/kg, and 88% capacitance retention after 5000 cycles. Similarly, a stability of 88% even after 10000 cycles, accompanied by 2.5 mWh/cm³ at 22.8 mW/cm³, was achieved by Song and his team using NiCo₂O₄/Co₂P/Ni₂P/CC as a positive electrode and AC/CC as a negative electrode in polyvinyl alcohol (PVA) with KOH gel electrolyte [52]. These porous hierarchical nanosheets of transition metal oxides and phosphides were derived from MOF of zeolite-imidazolate framework-67 (ZIF-67) as a model during the process of in-situ growth-etching, in addition to the method of ion exchange-carbonization-phosphorization. In a similar approach, Chen and others synthesized NiCo-LDH@CC based on a ZIF-67 crystal, then phosphorated it into NiCoP@CC nanosheets [53]. Outstanding 99.7% of initial capacitance is maintained using NiCoP@CC electrode over 10000 cycles at 6 A/g, which outperforms 93% using NiCo-LDH@CC electrode. A constructed ASC of NiCoP@CC//AC stored a high energy density of 79 Wh/kg at a specific power of 800 W/kg, and kept 90.3% of capacitance after 10000 cycles at 8 A/g.

Figure 3. (a-b) TEM; (c) HRTEM; and (f) SAED images of NCS@NCOH nanoflake. (d-e) Inverse FFT images of area I and II in Figure 3c labelled by the red and yellow dashed rectangular frames. (g-k) EDS mapping of NCS@NCOH heterostructure. (l) XRD patterns of the NCS/CC, NCOH/CC, and NCS@NCOH/CC electrodes, respectively. (m) In-situ Raman spectra of the NCS@NCOH/CC electrode during charging-discharging at 20 A /g [47].

Figure 3. (a-b) TEM; (c) HRTEM; and (f) SAED images of NCS@NCOH nanoflake. (d-e) Inverse FFT images of area I and II in Figure 3c labelled by the red and yellow dashed rectangular frames. (g-k) EDS mapping of NCS@NCOH heterostructure. (l) XRD patterns of the NCS/CC, NCOH/CC, and NCS@NCOH/CC electrodes, respectively. (m) In-situ Raman spectra of the NCS@NCOH/CC electrode during charging-discharging at 20 A /g [47].

Two-dimensional (2D) materials of transition metal carbides recognized as MXenes have recently been employed in SCs. For instance, a flexible MXene (Ti₃C₂T_x) lamellar structure was prepared by vacuum filtration on cellulose nanofibers and considered a negative electrode [54]. As a positive electrode, polyaniline (PANI) was polymerized on CC. This ASC demonstrated an energy density of 31 Wh/kg at 1211 W/kg, with a capacitance stability of 86% during 5000 cycles. Additionally, hybridizing MXene with metal oxide nanoparticles is a promising strategy to improve electrode performance. For example, Ti₃C₂T_x MXene was derived from multilayer Ti₃C₂T_x powder, and Co₃O₄ nanoparticles were prepared using a microplasma discharge reactor, which is a cost-effective, unique synthesis method [55]. This Ti₃C₂T_x/Co₃O₄/CC hybrid electrode possessed a maximum specific capacitance of 577 F/g at 1 A/g, higher by 95% than the Ti₃C₂T_x/CC electrode. A better performance was achieved by Lin et al., who prepared MXene nanosheets from Ti₃AlC₂, then placed MXene onto a CC substrate drop-by-drop to maintain a good quality of the atomic layers. After that, the MXene/CC was coated with PANI and CoNi-LDH by electrodeposition [56]. The incorporation of the MXene decreased the internal resistance of the electrode. Consequently, the hybrid electrode of MXene/PANI/CC presented a specific capacitance of 362 F/g, higher than 210 F/g of non-hybrid electrodes of PANI/CC and 279 F/g of MXene/CC. Among them, another hybrid electrode of MXene/PANI/CoNi-LDH/CC reached the greatest value of 1200 F/g. Its internal resistance was low due to the incorporation of MXene, as calculated from the EIS fitting curve. Combining this positive electrode with negative electrode of an activated carbon powder, called AC, on carbon cloth in a PVA-KOH gel electrolyte, an energy density of 39 Wh/kg was stored at a power density of 400 W/kg. Up to 91% of capacitance was held after 10000 cycles at 5 A/g. As another example of MXene, Fei and co-authors prepared V₂CT_x MXene-coated CC with Ni₃S₂ nanoblock arrays [57]. The hybrid electrode of Ni₃S₂/V₂CT_x/CC had a higher specific capacitance than the individual components. An energy density of 56 Wh/kg at 414 W/kg was delivered by an ASC of Ni₃S₂/V₂CT_x/CC//AC/CC, with 91% cycling stability after 10000 cycles. Such a binder-free 2D layered MXene facilitated ion transport, while the Ni₃S₂ nanoarrays hindered the dilapidation of the V₂CT_x interlayers.

For iron-related compunds, Bi and co-researchers prepared a heterogeneous composite structure of CoS nanosheets and FeOOH nanorods using one-step electrodeposition on carbon cloth [58]. The hybrid FeOOH/CoS/CC electrode exhibited a lower charge transfer resistance (0.68 ohm), thus higher specific capacitance (419 F/g), than that of the pure FeOOH/CC electrode (0.90 ohm and 277 F/g). In a similar example, the crystal structure and conductivity of iron oxide (Fe₂O₃) nanospheres were altered by doping with cobalt using a hydrothermal method, resulting in a 137% enhancement of the areal capacitance of the hybrid Co-Fe₂O₃@CC electrode (316 mF/cm² versus 133 mF/cm² of pure Fe₂O₃@CC electrode) [59]. Furthermore, the maximum volumetric energy density of the ASC of Co-Fe₂O₃@CC//Ni-MnO₂@CC within 2 V (0.96 mWh/cm³) can be almost doubled (2.03 mWh/cm³) in another ASC of MoS₂/Fe₂O₃@CC//Ni-MnO₂@CC [60]. This occurs only by utilizing molybdenum disulfide nanosheets loaded with iron oxide instead of cobalt doping. The MoS₂ in 1 T and 2 H crystal structures had reinforced conductivity, and the long interplanar distance between the two-dimensional nanosheets facilitates the accommodation of electrolyte ions. However, the maximum volumetric power density dropped by nearly half from 29 mW/cm³ of the Co-doped supercapacitor to 16 mW/cm³ of the MoS₂-combined supercapacitor. Although iron oxide used in negative electrodes has considerable theoretical capacitance and environmental friendliness, its energy and power merits are still in a trade-off.

3.2. Conductive Polymers

Conductive polymers, including polypyrrole (PPy), polythiophene (Pth), polyaniline (PANI), and poly(5-nitroindole), have been utilized in CC-based electrodes. A moderate performance of 1.4 mWh/cm³ at 54 mW/cm³ was reached by Li et al., adding a PPy nano-crosslinked network to Co-Fe₂O₃ in an ASC of PPy/Co-Fe₂O₃@CC//Ni-MnO₂@CC [61]. This three-dimensional nanostructured network of PPy/Co-Fe₂O₃ on carbon cloth provided a large specific surface area of 45.229 m²/g, hence an areal capacitance of 704 mF/cm² at 2 mA/cm², which was higher than that of Co-Fe₂O₃ nanospheres on carbon cloth. Regarding PANI, eigenstate polyaniline (EB) on carbon cloth was doped with Mn²⁺ and used as a positive electrode [62]. Higher than the last example, the EB/CC//AC stored 2.6 mWh/cm³ at 18.3 mW/cm³ in PVA-H₂SO₄ (0.3 M Mn²⁺) gel electrolyte. The loose structure of the EB irregular granular particle suffered from weak capacitance stability during charge-discharge cycles of 79% after only 2000 cycles.

In one study, polypyrrole was electrodeposited in SrFeO₃−δ perovskite on carbon cloth (PPy@SFO@CC) by Qiao and others [63]. The external PPy layer greatly improves the charge storage performance by increasing the number of oxygen vacancies in the perovskite, in addition to speeding up electron transport and ion diffusion. Moreover, an energy density of 17 Wh/kg at a power density of 984 W/kg was exhibited by an ASC of NiCo₂O₄@CC//PPy700@SFO@CC, where PPy700@SFO@CC was the negative electrode. The mechanical flexibility of this device was tested at 0, 45, 90, 135, and 180 ° folds, maintaining specific capacitances of 62, 48, 39, 39, and 39 F/g, respectively. After 1000 folds, a retention of 69% of the initial capacitance was measured. In another study, PPy can be used as positive and negative electrodes in symmetric SC. As illustrated in Figure 4, PPy-coated CC electrodes were separated by PVA/H₃PO₄ gel electrolyte and a filter paper, then three SCs connected in series successfully powered an LED [20]. Furthermore, an ASC of SnO₂@PPy/CC//SnO₂@PPy/CC was fabricated by solvothermal and chemical oxidative polymerization [27]. The synergistic benefit of coupling SnO₂ nanosheets with a PPy layer led to higher areal capacitance than single-material electrodes. As well as that, the SnO₂@PPy/CC electrode was mechanically strong, where a 956 g stainless steel autoclave was lifted by this CC-based electrode. At different angles of bending and twisting, the CV curves were still similar even after 200 bending cycles.

Figure 4. (a) Scheme for the preparation of flexible PPy@PCC and fabrication of symmetric device. (b) SEM images of PPy–coated PCC. (c) GCD of single device (SD)-1 and SD-2 in series and parallel connections; (d) LED light-emitting photographic images [20].

Figure 4. (a) Scheme for the preparation of flexible PPy@PCC and fabrication of symmetric device. (b) SEM images of PPy–coated PCC. (c) GCD of single device (SD)-1 and SD-2 in series and parallel connections; (d) LED light-emitting photographic images [20].

In other examples of utilizing conductive polymers in supercapacitors, polyaniline (PANI) was deposited by polymerization of aniline on CC that was doubly activated by the Hummers method and electrochemical treatment [64]. Then, reduced graphene oxide (rGO) was adsorbed on the CC. Such an ACC-PANI@rGO electrode achieved 670 F/g at 0.5 A/g, surpassing 485 for PPy [65], and 309 F/g for oxygen vacancy-Fe₂O₃@PANI [66] at the same current density. A symmetric SC of ACC-PANI@rGO electrodes delivered 112 µWh/cm² at a power density of 0.5 mW/cm² [64]. Additionally, after 500 repetitions of bending at 180 °, 91% of capacitance was retained. Poly(5-nitroindole) is hydrophilic and redox-rich due to the electron-withdrawing nitro ( ̶ NO₂) group. Poly(5-nitroindole) loaded on LaNiO₃/CC prepared by electrodeposition showcased 594 mF/cm² and 808 F/g at 0.5 mA/cm² and 76% retention of after 10000 cycles [67]. Polythiophene (Pth) had high conductivity around 6.21 S/cm; thus, it was utilized to increase the conductivity of NiCo₂S₄ (NCS) coated CC [68]. Moreover, the dip-coating process of the Pth protection layer on NCS nanoneedles lessened agglomeration during charge-discharge cycling, achieving 90% capacitance retention after 5000 cycles. The specific capacitance of the NCS@Pth@CC electrode (1633 F/g at 1 A/g) exceeded both NCS@CC and Pth@CC individual electrodes.

As seen, conductive polymers have been combined with metal compounds or graphene, reinforcing pseudocapacitance, energy, mechanical strength due to flexibility, hydrophilicity, and conductivity of electrodes. However, capacitance stability is still weak because of the pseudocapacitive storage mechanism. As a design strategy, loading metal compounds of graphene in nano-crosslinked network of polymers expands the active surface area. There has been a recent review article on binder-free electrodes for supercapacitors based on conducting polymers on various substrates, not only carbon cloth [69].

3.3. Carbon Nanomaterials

On the whole, hybridizing carbon nanomaterials with pseudocapacitive materials has stabilized the electrode charge-discharge cycles, as no phase change occurs in the EDLC mechanism of carbon-based electrodes. In one approach, carbon nanotubes (CNTs) were attached via the chemical vapor deposition (CVD) technique on carbon cloth, followed by the hydrothermal in situ growth of CuS nanosheets [70]. On top, FeOOH was electrodeposited. As expected, Li and others confirmed that the cycling stability of their cost-effective CC/CNT/CuS electrode was much better than that of the CC/CuS electrode. The electrochemical deposition was also applied to prepare nickel cobalt layered double hydroxide NiCo-LDH on CNTs on CC to act as a positive electrode. Impressively, the fabricated ASC of CC/CNT/NiCo-LDH//CC/CNT/CuS/FeOOH recorded an outstanding volumetric energy density of 3.3 Wh/cm³ at 12.2 W/cm³ and maintained 97.6% of its initial capacitance after 2000 cycles at 20 mA/cm². Along with this, there was excellent electrochemical performance at different scan rates and current densities, in addition to the minor charge transfer resistance of 2.1 ohm. In another approach, CNTs were inserted into the MnO₂/ACC surface via a facile impregnation method [71]. This incorporation of CNTs minimizes volume changes in the MnO₂ structure during charging-discharging cycling, thereby improving capacitance retention.

In another example of carbon nanomaterials, reduced graphene oxide (rGO) was integrated into NiCoFe₂O₄ and PPy on carbon cloth during a scalable sol-gel auto-combustion method by Nabeel and co-authors [72]. The ternary electrode of PPy/rGO/NiCoFe₂O₄/CC demonstrated 223 F/g at 1 A/g, which was higher than 157 F/g of the binary electrode of rGO/NiCoFe₂O₄/CC. In a brief comparison, the gravimetric capacitances in F/g at 0.5 A/g were 670 for PANI@rGO/CC electrode [64], and 170 for rGO/CC electrode [73]. Interestingly, the same rGO from the last report was also deposited on a nickel strip substrate, exhibiting 143 F/g, which is lower than that on carbon cloth. This indeed points out one of the potential advantages of selecting CC as an electrode substrate, as CC contains abundant of woven microfibers.

New graphitic crystallite nanosheets, called GCNs, were coated on NiCo₂S₄ nanorod arrays hydrothermally grown on carbon cloth by Liu et al. [74]. Assisted with polyethyleneimine solution that contains amine groups, the NiCo₂S₄ nanorods became positively charged; hence, GCNs with negative hydroxyls and carboxyls were electrostatically interacted. Such a simple coating process was repeated 3 times, resulting in improved capacitance, rate capability, and cyclic stability of electrodes compared to that repeated for 1, 2, 4, and 5 times, and better than on NiCo₂S₄ with either carbon quantum dots CQDs@NiCo₂S₄/CC or graphene oxide coated by the same method. A value of 51 Wh/kg energy density was obtained at 808 W/kg power density using an ASC of GCNs@NiCo₂S₄/CC//AC/CC. In a different study, graphene nanowalls were vertically aligned on CC, doped with nitrogen and functionalized by oxygen [75]. This hierarchical porous and redox-rich interface had an improved defect density (I_D/I_G=1.70) by Raman spectroscopy, enhancing the wettability and pseudocapacitance of 13 F/cm² at 2 mA/cm². Based on this electrode, the symmetric SC exhibited 1.03 mWh/cm² at 1.34 mW/cm² and 102% capacitance retention after 10000 cycles.

Altogether, the integration of carbon nanomaterials, such as CNTs and graphene, into conductive polymers, MOFs, MXenes, metal oxides, hydroxides, sulfides, nitrides, or phosphides has been clearly proven as a powerful strategy for enhancing SC electrodes. Due to the synergistic effects of each component, the previously discussed cases confirm that the electrochemical performance of hybrid electrodes mostly exceeds that of single-material electrodes. Tables 1–4 compare areal, volumetric, and gravimetric electrochemical performance of recent CC-based symmetric and asymmetric supercapacitors, including their energy and power densities and capacitance retention. It is clearly seen that the energy densities of asymmetric SCs surpass those of symmetric SCs, mainly due to the broadened cell voltage window of the different materials in positive and negative electrodes. Notably, it has been recommended to normalize capacitance, current, energy, and power to the geometric electrode area in cm² and volume in cm³. The gravimetric values may imply overestimated calculations when the mass loading is less than 1 mg/cm².

Table 1. Specific performance of recent carbon cloth-based symmetric supercapacitors.

Electrode on Carbon Cloth	Energy Density at Power Density	Longevity (Cycles)	Ref.
B and N-doped C	1.6 mWh/cm3 @ 12.5 mW/cm3	90% (10000)	[26]
SnO2@PPy	0.7 mWh/cm3 @ 4.7 mW/cm3	91% (10000)	[27]
Tetraaniline (TA)	74.2 mWh/cm2 @ 405.3 mW/cm2	94% (10000)	[76]
Mg-Zn codoped MnO2	0.2 mWh/cm2 @ 2.4 mW/cm2	83% (4000)	[77]
TA	21.1 μWh/cm2 @ 0.5 mW/cm2	100% (20000)	[25]
GaN	27.5 μWh/cm2 @ 0.1 mW/cm2	77% (20000)	[78]
Cd-doped MnO2	7.7 μWh/cm2 @ 1133 μW/cm2	94% (5000)	[79]
MoS2	70.8 Wh/kg @ 1000 W/kg	75% (8000)	[80]
Ni3S2/V2CTx	55.7 Wh/kg @ 413.8 W/kg	91% (10000)	[57]
MXene@CoMnPO4	48 Wh/kg @ 1500 W/kg	93% (5000)	[41]
Oxygen Vacancy-Fe2O3@PANI	12.1 Wh/kg @ 250 W/kg	100% (10000)	[66]
rGO	5.5 Wh/kg @ 1000 W/kg	88% (5000)	[73]
Commercial carbon fiber (T300)	4.6 Wh/kg @ 10 W/kg	92% (1000)	[81]
UIO66 *	0.04 Wh/kg @ 89.9 W/kg	130% (10000)	[82]

* UIO66: Zirconium-based metal-organic framework, comprising 12-connected [Zr₆(μ₃-O)₄(μ₃-OH)₄(O₂C)₁₂] cluster, terephthalic acid and isoreticular series.

Table 1. Specific performance of recent carbon cloth-based symmetric supercapacitors.

Electrode on Carbon Cloth	Energy Density at Power Density	Longevity (Cycles)	Ref.
B and N-doped C	1.6 mWh/cm3 @ 12.5 mW/cm3	90% (10000)	[26]
SnO2@PPy	0.7 mWh/cm3 @ 4.7 mW/cm3	91% (10000)	[27]
Tetraaniline (TA)	74.2 mWh/cm2 @ 405.3 mW/cm2	94% (10000)	[76]
Mg-Zn codoped MnO2	0.2 mWh/cm2 @ 2.4 mW/cm2	83% (4000)	[77]
TA	21.1 μWh/cm2 @ 0.5 mW/cm2	100% (20000)	[25]
GaN	27.5 μWh/cm2 @ 0.1 mW/cm2	77% (20000)	[78]
Cd-doped MnO2	7.7 μWh/cm2 @ 1133 μW/cm2	94% (5000)	[79]
MoS2	70.8 Wh/kg @ 1000 W/kg	75% (8000)	[80]
Ni3S2/V2CTx	55.7 Wh/kg @ 413.8 W/kg	91% (10000)	[57]
MXene@CoMnPO4	48 Wh/kg @ 1500 W/kg	93% (5000)	[41]
Oxygen Vacancy-Fe2O3@PANI	12.1 Wh/kg @ 250 W/kg	100% (10000)	[66]
rGO	5.5 Wh/kg @ 1000 W/kg	88% (5000)	[73]
Commercial carbon fiber (T300)	4.6 Wh/kg @ 10 W/kg	92% (1000)	[81]
UIO66 *	0.04 Wh/kg @ 89.9 W/kg	130% (10000)	[82]

* UIO66: Zirconium-based metal-organic framework, comprising 12-connected [Zr₆(μ₃-O)₄(μ₃-OH)₄(O₂C)₁₂] cluster, terephthalic acid and isoreticular series.

Table 2. Areal performance of recent carbon cloth-based asymmetric supercapacitors.

Positive Electrode on Carbon Cloth	Negative Electrode on Carbon Cloth	Energy Density (mWh/cm2) at Power Density (mW/cm2)	Longevity (Cycles) at Current Density (mA/cm2)	Ref.
MnO2	ACC	140 @ 1	100% (10000) @ 16	[15]
PPy	AC	15 @ 80	105% (1000) @ 2.5	[83]
PPy	PPy	4.1 @ 80	130% (1000) @ 2.5	[83]
ACC	NiVO3@CoNi-MOF/AC	1.27 @ 4	96% (10000) @ 50	[84]
CoNi-MOF	N-doped CC	1.25 @ 4	99% (10000) @ 110	[85]
Ni-Mn-S	AC	0.79 @ 0.85	83% (18,000) @ 25	[86]
CoNiOOH	N-doped CC	0.73 @ 1.60	97% (1000) @ 10	[87]
NiCoMoS	AC	0.60 @ 0.80	83% (15,000) @ 30	[88]
(NiO)0.75(MnO)0.25	AC	0.49 @ 0.80	82% (15000) @ 20	[89]
Ni1.5Co1.5S4	ACC	0.49 @ 4.50	90% (10000) @ 40	[90]
NiCo2O4@NiCoMnS4	AC	0.40 @ 0.85	81% (20000) @ 20	[91]
NiCo2O4/NiFeMn-LDH	ZnMn2O4	0.29 @ 4.38	92% (20000) @10	[92]
MnO2	AC	0.27 @ 1.02	92% (4500) @ 1	[93]
VS2	AC	0.22 @ 4.24	81% (5000) @ 5	[94]
Co(OH)2@CoNi2S4/Ni	AC/Ni	0.21 @ 1.5	96% (5000) @ 20	[17]
Co3O4	CeO2	0.11 @ 3.25	74% (5000) @ 5	[95]
Co3O4-P	Fe2O3-P	0.07 @ 0.38	100% (5000) @ 20	[96]

Table 2. Areal performance of recent carbon cloth-based asymmetric supercapacitors.

Positive Electrode on Carbon Cloth	Negative Electrode on Carbon Cloth	Energy Density (mWh/cm2) at Power Density (mW/cm2)	Longevity (Cycles) at Current Density (mA/cm2)	Ref.
MnO2	ACC	140 @ 1	100% (10000) @ 16	[15]
PPy	AC	15 @ 80	105% (1000) @ 2.5	[83]
PPy	PPy	4.1 @ 80	130% (1000) @ 2.5	[83]
ACC	NiVO3@CoNi-MOF/AC	1.27 @ 4	96% (10000) @ 50	[84]
CoNi-MOF	N-doped CC	1.25 @ 4	99% (10000) @ 110	[85]
Ni-Mn-S	AC	0.79 @ 0.85	83% (18,000) @ 25	[86]
CoNiOOH	N-doped CC	0.73 @ 1.60	97% (1000) @ 10	[87]
NiCoMoS	AC	0.60 @ 0.80	83% (15,000) @ 30	[88]
(NiO)0.75(MnO)0.25	AC	0.49 @ 0.80	82% (15000) @ 20	[89]
Ni1.5Co1.5S4	ACC	0.49 @ 4.50	90% (10000) @ 40	[90]
NiCo2O4@NiCoMnS4	AC	0.40 @ 0.85	81% (20000) @ 20	[91]
NiCo2O4/NiFeMn-LDH	ZnMn2O4	0.29 @ 4.38	92% (20000) @10	[92]
MnO2	AC	0.27 @ 1.02	92% (4500) @ 1	[93]
VS2	AC	0.22 @ 4.24	81% (5000) @ 5	[94]
Co(OH)2@CoNi2S4/Ni	AC/Ni	0.21 @ 1.5	96% (5000) @ 20	[17]
Co3O4	CeO2	0.11 @ 3.25	74% (5000) @ 5	[95]
Co3O4-P	Fe2O3-P	0.07 @ 0.38	100% (5000) @ 20	[96]

Table 3. Volumetric performance of recent carbon cloth-based asymmetric supercapacitors.

Positive Electrode on Carbon Cloth	Negative Electrode on Carbon Cloth	Energy Density (mWh/cm3) at Power Density (mW/cm3)	Longevity (Cycles) at Current Density (mA/cm2)	Ref.
CNT/NiCo-LDH	CNT/CuS/FeOOH	3300 @ 12200	98% (2000) @ 20	[70]
NiGa2O4/MnO2	N-CMK-3 *	590 @ 48000	80% (5000) @ 6	[46]
AC	CuO/Mn(OH)2Cu	6.29 @ 34.31	89% (10000) @ 20	[43]
PPy	AC	5.00 @ 26.70	105% (1000) @ 2.5	[83]
Ni-MnO2	Co-Fe2O3	0.96 @ 28.60	85% (4000) @ 10	[59]
Ni-MnO2	MoS2/Fe2O3	2.03 @ 15.60	78% (3000) @ 10	[60]
NiO	Fe2O3/C	2.87 @ 40.20	82% (10000) @ 20	[42]
Eigenstate polyaniline	AC	2.57 @ 18.30	79% (2000) @ 1	[62]
NiCo2O4/Co2P/Ni2P	AC	2.53 @ 22.77	88% (10000) @ 2	[52]
PPy	PPy	1.00 @ 20.00	130% (1000) @ 2.5	[83]
MnCo2O4	ACC	0.78 @ 9.780	87% (5000) @ 5	[45]

* N-CMK-3: nitrogen-doped ordered mesoporous carbon.

Table 3. Volumetric performance of recent carbon cloth-based asymmetric supercapacitors.

Positive Electrode on Carbon Cloth	Negative Electrode on Carbon Cloth	Energy Density (mWh/cm3) at Power Density (mW/cm3)	Longevity (Cycles) at Current Density (mA/cm2)	Ref.
CNT/NiCo-LDH	CNT/CuS/FeOOH	3300 @ 12200	98% (2000) @ 20	[70]
NiGa2O4/MnO2	N-CMK-3 *	590 @ 48000	80% (5000) @ 6	[46]
AC	CuO/Mn(OH)2Cu	6.29 @ 34.31	89% (10000) @ 20	[43]
PPy	AC	5.00 @ 26.70	105% (1000) @ 2.5	[83]
Ni-MnO2	Co-Fe2O3	0.96 @ 28.60	85% (4000) @ 10	[59]
Ni-MnO2	MoS2/Fe2O3	2.03 @ 15.60	78% (3000) @ 10	[60]
NiO	Fe2O3/C	2.87 @ 40.20	82% (10000) @ 20	[42]
Eigenstate polyaniline	AC	2.57 @ 18.30	79% (2000) @ 1	[62]
NiCo2O4/Co2P/Ni2P	AC	2.53 @ 22.77	88% (10000) @ 2	[52]
PPy	PPy	1.00 @ 20.00	130% (1000) @ 2.5	[83]
MnCo2O4	ACC	0.78 @ 9.780	87% (5000) @ 5	[45]

* N-CMK-3: nitrogen-doped ordered mesoporous carbon.

Table 4. Gravimetric performance of recent carbon cloth-based asymmetric supercapacitors.

Positive Electrode on Carbon Cloth	Negative Electrode on Carbon Cloth	Energy Density (Wh/kg) at Power Density (W/kg)	Longevity (Cycles) at Curren Density (A/g)	Ref.
NiCoS	ACC	107.7 @ 1000	89% (2000) @ 20	[97]
NiS/MnS	AC	111.2 @ 800.0	93% (10000) @ 5	[98]
AC	NiCo2S4/LaNiO3	86.6 @ 400.0	85% (10000) @ 4	[99]
Ni3Se2/NiSe2	CuxFe3-xSe4	84.8 @ 664.0	86% (10000) @ 5	[100]
NiCoP	AC	78.5 @ 799.5	90% (10000) @ 8	[53]
Na+-doped NH4V4O10	Ti3C2Tx MXene	77.7 @ 424.3	106% (8000) @ 6	[101]
CuS/VAGN *	VAGN *	76.7 @ 800.2	84% (10000) @ 8	[102]
Ni0.85Se@ZnSe	AC	74.7 @ 800.0	91% (5000) @ 5	[103]
NiCoMnS4	AC	68.2 @ 850.1	93% (10000) @ 4	[104]
NH4+-intercalated MnO2	AC	63.5 @ 949.8	81% (10000) @ 10	[105]
MnCo2O4@CuCo2O4	AC	62.0 @747.2	91% (10000) @ 8	[106]
MnO2@NiCo2O4@Ti3SiC2	AC	58.0 @ 800.0	62% (5000) @ 10	[107]
NiCo2Mn-LDH@Fe2O3	Fe2O3	53.7 @ 800.0	80% (10000) @ 10	[108]
GCNs@NiCo2S/AC	AC	50.5 @ 807.7	90% (20000) @ 5	[74]
SiC@NiCo2O4	SiC@C	49.9 @ 800.0	89% (10000) @ 10	[109]
MXene/PANI/CoNi-LDH	AC	39.3 @ 400.0	91% (10000) @ 5	[56]
NiCoLDH	NiCoLDH	38.3 @ 1957	90% (10000) @ 5	[110]
NiS2	AC	34.1 @ 561.3	96% (8000) @ 10	[31]
NiCo2Se4	AC	30.7 @ 800.0	70% (2000) @ 5	[111]

* VAGN: vertically aligned graphene nanosheets, ** GCN: graphitic crystallite nanomaterials.

Table 4. Gravimetric performance of recent carbon cloth-based asymmetric supercapacitors.

Positive Electrode on Carbon Cloth	Negative Electrode on Carbon Cloth	Energy Density (Wh/kg) at Power Density (W/kg)	Longevity (Cycles) at Curren Density (A/g)	Ref.
NiCoS	ACC	107.7 @ 1000	89% (2000) @ 20	[97]
NiS/MnS	AC	111.2 @ 800.0	93% (10000) @ 5	[98]
AC	NiCo2S4/LaNiO3	86.6 @ 400.0	85% (10000) @ 4	[99]
Ni3Se2/NiSe2	CuxFe3-xSe4	84.8 @ 664.0	86% (10000) @ 5	[100]
NiCoP	AC	78.5 @ 799.5	90% (10000) @ 8	[53]
Na+-doped NH4V4O10	Ti3C2Tx MXene	77.7 @ 424.3	106% (8000) @ 6	[101]
CuS/VAGN *	VAGN *	76.7 @ 800.2	84% (10000) @ 8	[102]
Ni0.85Se@ZnSe	AC	74.7 @ 800.0	91% (5000) @ 5	[103]
NiCoMnS4	AC	68.2 @ 850.1	93% (10000) @ 4	[104]
NH4+-intercalated MnO2	AC	63.5 @ 949.8	81% (10000) @ 10	[105]
MnCo2O4@CuCo2O4	AC	62.0 @747.2	91% (10000) @ 8	[106]
MnO2@NiCo2O4@Ti3SiC2	AC	58.0 @ 800.0	62% (5000) @ 10	[107]
NiCo2Mn-LDH@Fe2O3	Fe2O3	53.7 @ 800.0	80% (10000) @ 10	[108]
GCNs@NiCo2S/AC	AC	50.5 @ 807.7	90% (20000) @ 5	[74]
SiC@NiCo2O4	SiC@C	49.9 @ 800.0	89% (10000) @ 10	[109]
MXene/PANI/CoNi-LDH	AC	39.3 @ 400.0	91% (10000) @ 5	[56]
NiCoLDH	NiCoLDH	38.3 @ 1957	90% (10000) @ 5	[110]
NiS2	AC	34.1 @ 561.3	96% (8000) @ 10	[31]
NiCo2Se4	AC	30.7 @ 800.0	70% (2000) @ 5	[111]

* VAGN: vertically aligned graphene nanosheets, ** GCN: graphitic crystallite nanomaterials.

4. Optimization of Structural Properties

It is imperative to tune the structural properties of electrode materials to improve the performance of supercapacitors. The crystallinity, morphology, and conductivity of nanomaterials can be tuned by optimizing the synthesis conditions, such as chemical reaction time, solvent, temperature, and precursors. As an example, our group synthesized Co₃O₄ nanoflakes hydrothermally and Co₃O₄ octahedra solvothermally, both on carbon cloth [112]. Then, both samples were converted to Co₉S₈ by another hydrothermal reaction at 160 °C for 20 h. Although the two products possessed the same crystal phase and chemical composition, their morphologies were still distinct. The specific capacitance of CC/Co₉S₈ nanoflake electrodes was 12 times higher than that of CC/Co₉S₈ octahedra. As another example, the electrode properties were affected by the number of electrodeposition cycles. Core−shell nanoarrays of NiCoP@NiCoS on carbon cloth were electrodeposited with 10 cycles on CC, and produced the best morphology and performance compared to the same composite deposited in 5 and 15 cycles [113]. During a few cycles, a thin NiCoS layer was deposited on the NiCoP nanowires, while the nanomaterial’s structure was dilapidated after extended deposition cycles, weakening electrolyte ions transport and capacitance. More examples are systematically discussed in the following subsections.

4.1. Effects of Synthesis Time and Temperature

To clarify the effect of reaction time, Zou and coworkers developed four samples of nanoflakes on MnO₂ nanothorns on porous carbon cloth in 1, 2, 3, and 6 h of hydrothermal reaction [114]. The growth of nanoflakes increased over time. In the 3 hour-reaction, the (Ni)MOF nanoflakes uniformly coated the MnO₂ nanothorns, resulting in the highest areal specific capacitance of 1780 mF/cm² at 4 mA/cm² among the other coated samples. In the 6-hour reaction, closely packed (Ni)MOF nanoflakes hindered the rate of electron transport to the underlying MnO₂ nanothorns. Similarly, the reaction time affected the morphology and aggregation of ZnMn₂O₄ nanoparticles on CC [115]. As shown in Figure 5a–h, the results of the samples that were hydrothermally deposited for 7, 9, and 12 h, respectively, called ZMO-7, ZMO-9, and ZMO-12, demonstrated that the reaction time of 9 h led to the optimum morphological and electrochemical properties. The specific capacitance of ZMO-9 was 499 F/g, corresponding to a specific capacity of 300 C/g at 1 A/g. The deposition time also affects the conductive polymer doping in electrodes. To elaborate on this, thermally activated CC was coated with PPy using oxidative chemical vapor deposition (oCVD) at 30, 60, and 90 min [83]. The longest time produced the highest doping degree as indicated by Raman spectroscopy, yielding the best electrode performance of 300 mF/cm² (1498 mF/cm³) in that article. Regarding the effect of heating time, our team prepared the alpha phase of nickel hydroxide at 150 °C in just 10 minutes by microwave-assisted synthesis using nickel acetate as a precursor [116]. Besides, mixed alpha and beta phases at 150 °C in 30 min and the beta phase at 220 °C in 30 min were also prepared and identified according to XRD spectra in Figure 5i. Although all samples on CC exhibited similar nanoflake morphology, the alpha-phase-based electrode showcased a higher capacity as compared in Figure 5j,k. Such an increase in charge storage was explained by comparing the nickel oxidation states; the difference between +2 and +3.6 in α-Ni(OH)₂ and γ-NiOOH, respectively, was wider than the difference between +2 and +3 in β-Ni(OH)₂ and β-NiOOH, respectively [117].

In demonstrating the temperature effects, Ni microparticles were initially electrodeposited on CC to enhance the conductivity, then four samples of MoO₃ thin films were electrodeposited at 25, 50, 70, and 90 °C [118]. The MoO₃/Ni/CC synthesized at 90 °C displayed optimum electrochemical properties in both K₂SO₄ and KOH electrolytes. Increasing the deposition temperature increased MoO₃ thin film loading mass on Ni-enhanced CC, structural integrity, and active material-flexible substrate adhesion. In the hydrothermal method, Co(OH)₂ nanoneedles were vertically grown on CC at an optimum reaction temperature of 120 °C [119]. This condition produced the best morphological and electrochemical performance (591 F/g at 0.5 A/g) compared to Co(OH)₂ prepared at 80, 100, 140, and 160 °C. By a selenization method, NiSe₂/CoSe₂/CC electrodes were prepared from Ni-Co precursors at different temperatures of 300, 350, 400, and 450 °C [120]. The temperature of 400 °C was found to be the optimal condition for a perfect transformation to NiSe₂/CoSe₂/CC via an Ostwald ripening mechanism. This electrode exhibited 1558 F/g at 1 A/g, higher than the electrodes prepared at the other temperatures.

Recently, a facile, safe, and environmentally friendly method was innovated for fast preliminary activation of CC and for nanoparticle synthesis. Three pulses of high-temperature thermal shock (HTS) at ~800 °C for 100 ms were applied to CC pretreated by mixed acids [121]. After dropping a precursor of transition metal oxides on the activated CC, another three pulses at ~1000 °C were applied to obtain Co₃O₄ nanoparticles. The ultrafast-prepared Co₃O₄/ACC electrode showed 408 mF/cm² at 0.2 mA/cm², and a capacity retention of 97% following 20000 cycles at 2 mA/cm². Using the same HTS, MnO_x nanoparticles were prepared on ACC, which achieved 456 mF/cm² at 0.2 mA/cm². It is worth noting that the mass loading of each oxide was 3.5 mg/cm², and the total time of the HTS was 20 s. As presented in the previous examples, the temperature and time of heating, chemical reaction, electrodeposition, and other methods affect one or more material properties, such as morphology, crystal phase, and doping, which in turn affect electrode surface area, redox activity, and conductivity.

4.2. Effects of Synthesis at Room Temperature

For nanomaterials of metal compounds, the chemical co-precipitation technique can be cost-effectively used at room temperature. Using this simple method, Samyn and others synthesized copper(II) hexacyanoferrate(II) (CuHCF) nanoparticle powder at room temperature, which was mixed with activated carbon and polyvinylidene difluoride (PVDF) binder, then plated on fibers of carbon cloth [122]. This CuHCF@CC positive electrode exhibited a specific capacity of 43 C/g in K₂SO₄ electrolyte and 24 C/g in Na₂SO₄ electrolyte, at 0.2 A/g. Using the same method, potassium nickel(II) hexacyanoferrate(III) nanoparticles were prepared at room temperature and adhered to carbon cloth by PVA binder. At 0.4 A/g, 199 F/g in Na₂SO₄ and 169 F/g in K₂SO₄ were reported by the last-mentioned authors [123]. Without a binder, Alhebshi successfully deposited a conformal coating of cobalt hydroxide chloride, Co₂(OH)₃Cl, nanoflakes on carbon cloth at room temperature in 1 h [124]. The nanoflakes evolved from nanoparticles that appeared during the first 10 min of chemical bath deposition, as confirmed in Figure 6a–h. The specific capacitance of the nanoflakes was 313 F/g at 1 A/g, while it was 293 F/g for the nanoparticles due to morphological features, crystallinity, and conductivity. A higher specific capacitance of 521 F/g at 0.5 A/g was reached by Ag-doped MnO₂ nanowires directly grown on CC by a chemical reaction at 25 °C for 24 h [125]. Silver doping concentrations of 1, 3, and 5% affect the MnO₂ growth, as captured in Figure 6i–n. Besides, the growth of Ag-doped MnO₂ nanowires on CC produced a larger BET specific area and a higher specific capacitance compared to the doped compound prepared via the same experiment but without CC. In addition, the capacitance retention of 91% during 2000 cycles at 1 A/g confirmed that MnO₂ had firm bonds with the CC surface, minimizing exfoliation during charge-discharge cycles. Carbon nanomaterials can also be fabricated under ambient conditions. In a sustainable approach, porous graphene was derived from lignin on carbon cloth through direct laser writing (DLW) [[126]. The used lignin is commercially available at a low cost, and the DLW method is safer and faster than other methods to prepare graphene with toxic chemicals. Excellent flexibility and 157 mF/cm² at 0.1 mA/cm² were achieved by this green-fabricated electrode. Notably, synthesis at room temperature is safe, low-cost, and facile. However, it may yield lower-quality materials in terms of purity and crystallinity compared to energy-consuming synthesis methods that require high temperatures.

4.3. Effects of Precursor Compounds and Ratios

One of our studies focused on the effect of nickel precursors used in the electrospinning technique on the nanostructure of nickel oxide on carbon cloth [127]. It was found that the NiO electrode derived from nickel acetate, abbreviated NiO-A, had a well-crystalline phase and a robust nanofiber structure. Consequently, it possessed superior electrochemical performance over the other NiO electrode derived from nickel nitrite, abbreviated NiO-N, as compared in Figure 7a–h. Using 0.1 mol/L of nickel nitrite resulted initially in nanofibers but turned to NiO nanoparticles after calcination. When the concentration increased to 0.2 mol/L, the electrospinning was not stable due to the high ionic conductivity of nitrate. As the ionic conductivity of the acetate is lower, NiO nanofibers were stably electrospun using 0.2 mol/L of nickel acetate and maintained their morphology after calcination. Figure 7i,j shows that hybridizing with CNTs on both positive and negative electrodes as an ASC of NiO/CNT/CC//CNT/CC significantly improved the CV and GCD curves over another ASC of NiO/CC//CNT/CC.

Deeply, tuning the molar ratios of precursors alters structural properties and hence the electrochemical performance. In one study, Liang and co-researchers discovered a creative way to grow metal-organic frameworks on carbon cloth, then converted them into porous hollow CoNiOOH nanorods [87]. A hydrothermally grown Ni(OH)₂ nanosheet on carbon cloth was used as a precursor in a solvothermal reaction at 120 °C for 12 h. Then, the resultant solid nanorods of CoNi-MOF were electrochemically induced to hollow nanorods of CoNiOOH. The molar ratios of Co(NO₃)₂⋅6H₂O and 2,5-dihydroxyterephthalic acid (H₄DOBDC) were tuned as 1:1, 2:1, and 3:1, and symbolized as CoNi-MOF-1, CoNi-MOF-2 and CoNi-MOF-3 electrodes, respectively. By comparing the electrochemical performance, the most favorable amount of organic ligand was found in CoNi-MOF-2 using a 2:1 ratio. Furthermore, combining this positive electrode with a nitrogen-doped activated carbon cloth (NAC) electrode produced an areal energy of 0.7 mWh/cm² at 1.6 mW/cm² and displayed 99% capacitance retention after 10000 cycles. In another study, Das and others optimized a heterostructure of copper iron selenide (Cu_xFe_3-xSe) by changing the molar ratio of Cu/Fe in the precursor solution of an electrodeposition process [100]. The results revealed that the stoichiometric ratio of x = 1.5, denoted as CFS-1.5/CC, yields the optimal electrode compared to other ratios of x = 0, 1, 2, and 3. The obtained sample has nanoneedles of iron selenide and flower-like clusters of copper selenide, which were dense flake-like clusters in the CFS-1/CC. By increasing the Cu/Fe ratio to 2:1, the nanoneedles in the CFS-2/CC became mainly covered by extra clusters. Based on that, a selenide-based supercapacitor of CFS-1.5/CC negative electrode and Ni₃Se₂/NiSe₂/CC positive electrode operated at 1.6 V showed an excellent energy density of 85 Wh/kg at 664 W/kg. Another bimetallic chalcogenide was prepared by Jin et al. by optimizing the feeding ratio in a hydrothermal reaction [128]. Nanopetal (Ni,Co)Se₂@CC and nanoagaric (Ni,Co)Se₂@CC were produced when the molar ratio of Ni/Co was 2:1 and 1:2, respectively. The nanopetal-structured electrode showed better capacitance, internal resistance, and stability than the nanoagaric one. The DFT calculations revealed that the nanopetal (Ni,Co)Se₂ had an adsorption energy of OH− of −2.6386 eV, greatly higher than −1.8820 eV of nanoagaric (Ni,Co)Se₂. As a result, an ASC of nanopetal (Ni,Co)Se₂@CC//AC@CC stored a maximum energy density of 75 Wh/kg at a power density of 250 W/kg.

In a balanced comparison of different methods, two samples of iron oxide/hydrophilic carbon cloth (Fe₂O₃@hCC) were synthesized, one by hydrothermal (HT) and the second by electrodeposition (E), then converted to iron nitride [129]. Figure 8 compares their morphology, crystallinity, specific capacity, and EIS. The electrodeposition produced amorphous Fe₂N but with a higher specific capacity by 66%. All the above-mentioned cases obviously emphasize the synergetic effects of synthesis conditions on the tunable properties of materials, and consequently on the electrochemical performance of electrodes. Table 5 and Table 6 present lists of the areal and gravimetric capacitances, respectively, of recent carbon cloth-based electrodes, along with their morphologies and synthesis methods. It can be concluded from the tables that the capacitances of Ni-based electrodes of nanosheets and 3D nanostructures are at the top. They have been considered favorable morphologies for supercapacitor electrodes due to their large specific surface area and redox-active sites. Although graphene has a nanosheet morphology as well, its capacitance is lower than metal compound nanosheets, based on their different charge storage mechanisms, as explained earlier.

5. Integration of Supercapacitor Components

Enhancement of supercapacitor performance requires careful integration of electrodes and electrolytes on current collector substrates. The nature of electrolytes used in supercapacitors affects the charge storage performance and electrode lifetime. To elaborate on such effects, six aqueous electrolytes of Na₂SO₄, Li₂SO₄, K₂SO₄, H₂SO₄, LiCl, and KCl were tested with MoS₂@CC binder-free electrodes [139]. Among them, using LiCl and H₂SO₄ electrolytes activated the highest capacitances but lowest cycling stability, ascribed to corrosion as indicated by the morphological and elemental imaging after cycling. On the other hand, better stability (94% after 5000 cycles) but lower specific capacitance (223 F/g) were obtained in Na₂SO₄ electrolyte with the same electrode. Such a compromise between capacitance and stability is still a challenge. Nevertheless, in the same Na₂SO₄ electrolyte, MnO₂@CC cathode induced by cetyltrimethylammonium bromide (CTAB) nanomicellar and AC anode provided 1011 mF/cm² at 1 mA/cm² and maintained an extraordinary 127% capacitance after 20000 long cycles at 30 mV/s [93]. Such increase in capacitance during cycling is discussed in detail in section 6 of this review. In an additional case for Na₂SO₄ electrolyte, an Aquivion membrane was used as a polymer electrolyte when dipped in 1 M Na₂SO₄ solution for 3 h [140]. This electrolyte membrane was then inserted between a carbon xerogel (CX) electrode and MnO₂ electrode, both screen-printed on CC (Figure 9). The GCD curves specify the positive potential of the MnO₂/CC electrode (0.95 V) and the negative potential of the CX/CC electrode (− 0.85 V) versus the reversible hydrogen electrode (RHE), in addition to the cell voltage of 1.8 V of the asymmetric supercapacitor. Using this prototype, an LED module consisting of 30 LEDs in a ring was powered for 50 s in normal and bent conditions.

5.1. Conformal Coating on Carbon Cloth

The direct deposition of nanomaterials on current collector substrates, instead of a physical pasting of nanomaterial powder on substrates using insulating binders, plays a substantial role in augmenting the electrochemical performance of electrodes. As a typical demonstration, two electrodes of Ni(OH)₂ were simply prepared at room temperature using chemical bath deposition by Alhebshi et al. (Figure 10) [141]. The first one was directly deposited on carbon cloth, while the second was drop-cast on carbon cloth. Both resultant Ni(OH)₂ samples consisted of multi-phase crystalline nanoflakes. The conformal coating of nanoflake-based film around each carbon microfiber provided greater contact with electrolyte ions than nanoflakes aggregated in a flower-like morphology that filled gaps in CC. Therefore, the conformal Ni(OH)₂/CC resulted in not only five times higher specific capacitance, but also better cyclic stability than the drop-cast electrode. Interestingly, Kordek-Khalil and co-researchers [129] found that an amorphous Fe₂N film electrodeposited on hydrophilic carbon cloth showed a 66% higher specific capacity than a crystalline Fe₂N prepared by the hydrothermal method, first as a powder, then drop-cast onto the hydrophilic carbon cloth. Similarly, they prepared Fe₂O₃ by the two methods and confirmed the advantage of the electrodeposition technique over the hydrothermal method with drop-casting. In a more accurate comparison, two electrodes of tungsten trioxide (WO₃) nanotube bundles were made by Li et al. via a safe hydrothermal method, replacing strong corrosive acids with NaHSO₃. The first electrode was in-situ anchored on carbon cloth, while the second electrode was loaded with the prepared WO₃ powder with a binder [137]. Both had similar XRD, Raman, SEM, and TEM results, but the binder-free electrode had better CV, GCD, EIS, and capacitance results than the pasted WO₃ electrode and the bare CC electrode. Inserting substrates inside the hydrothermal autoclaves is a simple yet powerful strategy to produce direct nucleation and growth of nanostructured thin films for binder-free electrodes.

In novel approaches, electrodeposition and cyclic voltammetry passivation (CVP) and oxidation were applied to coat CC with Ni, then oxidized to a porous nickel oxide layer, resulting in strong interfacial adhesion to the substrate [142]. The electrode showcased 1736 mF/cm² at 1 mA/cm², and 99% mass retention after high-power ultrasonication. Moreover, the supercapacitor maintained nearly its initial capacitance under 180 ^° bending. A molten salt method is a one-step reaction that was used for direct deposition of ZnMn₂O₄ nanoparticles on CC [92]. Within a potential window from −1.2 to 0 V, this electrode exhibited an excellent areal specific capacitance of 2863 mF/cm² at 5 mA/cm². Additionally, the capacitance retention was 93% after 12000 cycles at 20 mA/cm². Printing on CC is a promising technique for large-scale supercapacitor fabrication. In specific, carbon xerogel (CX) and manganese dioxide (MnO₂) were prepared as electrically conductive inks, then screen-printed on CC [143]. An ASC based on printed CX and MnO₂ on CC achieved a low self-discharge rate with a voltage retention of 72% after 22 hours, a high specific capacitance of 213 F/g, and an energy density of 24 Wh/kg at 180 W/kg.

In another creative way, conductive porous carbon nanoflakes (CPCN) were derived from a Co-based MOF on carbon cloth, then hybridized with MnO₂ and Au nanoparticles [144]. The CPCN as a skeleton offered abundant mass loading of 2.86 mg/cm², while the presence of gold increased the charge transfer between electrode and electrolyte, leading to a higher specific capacitance. In three-electrode measurements, an aqueous electrolyte of Na₂SO₄ was used. The Au-MnO₂/CPCN electrode displayed a leakage current of 0.048 mA at 1 V. As a consequence, an output voltage of 1 V dropped to 0.743 V after 24 h in self-discharge conditions. A sodium ion adsorption energy of 3.744 eV was calculated by Density-functional theory (DFT) of the electrode, and a large density of states (DOS) was found near the Fermi level. In two-electrode measurements, an organic electrolyte of sodium hexafluorophosphate (NaPF₆) with ethylene carbonate (EC), propylene carbonate (PC), and fluoroethylene carbonate (FEC) was used. An ASC of Au-MnO₂/CPCN/CC//AC/CC exhibited 72 Wh/kg at 80 W/kg. A higher performance was reached in a similar example. CPCN was derived from ZnCo-MOFs conformally on carbon cloth by annealing and etching, then hydrothermally decorated with CdSe nanoparticles [136]. The CdSe/CPCN on CC was used as a positive electrode and activated carbon on nickel foam as a negative electrode. Regarding the electrolyte, they used NaPF₆ dissolved in EC, PC, and FEC as an additive. The fabricated an ASC of CdSe/CPCN/CC//AC/CC stored 139 Wh/kg at 590 W/kg, and retained up to 89% of its capacitance following 10000 cycles. The 3D CPCN structure promoted electron transport and Na⁺ diffusion while inhibiting nanoparticle disintegration and aggregation. Furthermore, DFT simulations indicated that the CdSe/CPCN hybrid material improved the sodium ion adsorption energy to −1.0692 eV and promoted electrode conductivity. Besides, when the nickel foam was replaced by carbon cloth, the authors noticed better mechanical flexibility. In bending states at angles of 100, 60, and 0 °, two supercapacitors connected in parallel powered an LED array. Clearly, this is an evidence of the powerful flexibility of CC-based supercapacitors.

Hydrothermally, NiCo₂O₄ nanoarrays were directly grown on CC under conditions of 120 °C for 4 h, followed by calcination at 300 °C for 2 h [145]. Interestingly, oxygen vacancies were introduced in NiCo₂O₄ by a solvothermal treatment in a glycol with 1.2 g NaOH at 120 °C for 16 h, avoiding high temperature and strong reducing agents. The calculated specific surface area was 124 m²/g. An ASC of NiCo₂O₄/CC//AC/CC offered 44 Wh/kg at 281 W/kg. Such defect engineering of oxygen vacancies is considered an effective strategy for increasing the electrocatalytic active sites. Hierarchical porous triangle-like NiCo₂O₄ nanosheets were derived from MOF nanoarrays, grown on SiC nanowires on CC, and used as a negative electrode [109]. On positive other electrode, carbon nanosheet arrays were derived from MOF on the SiC nanowires on CC. This ASC exhibited 50 Wh/kg at 800 W/kg, which is higher than the previous example. Furthermore, 89% capacitance was maintained over 10000 cycles at 10 A/g, and excellent flexibility at bending angles of 0, 90, and 180 °. As concluded in the study, there were four possible factors for the enhanced supercapacitor performance. First, the absence of insulating binders prevented the formation of inactive spots on the electrode. Second, the porous nanosheet array morphology provided a larger active area accessible to the electrolyte. Third, the interwoven conductive nanowires facilitated electron transfer. Fourth, the core-shell nanosheets maintained structural stability.

5.2. Adjusting Electrode Mass Loading

In large-scale production of electrodes, the mass loading of active materials on the substrate may vary from the reported laboratory-scale fabrication. Accordingly, the mass loading should be adjusted to meet practical considerations. It has still been challenging to control mass loading, especially in conformal coating synthesis methods. Nevertheless, a high mass loading of 12 mg/cm² of zeolite imidazole framework (ZIF-67) microrods on CC was achieved by Bai and others, considered a commercial-level amount [146]. The preparation process of ZIF included crystallization, electrodeposition, and seed-oriented growth. It was found that a better mass was loaded in a longer dipping time of 72 h than 24 h. The heterostructure of vertically aligned microrods on CC offered approachable pathways and electroactive locations, thus demonstrating an excellent areal energy density of 380 mWh/cm² at 1600 mW/cm² using ASC of ZIF-67/CC//AC/CC in PVA/ LiOH gel electrolyte. The same mass loading, 12 mg/cm², was measured for another electrode of MnO₂ on CC, which is higher than the commonly reported cases (< 1 mg/cm²) [147]. A hydrothermal reaction at 160 °C for 12 h followed by annealing at 400 °C for 2 h and treatment in N₂-plasma created oxygen vacancies on MnO₂. The ASC of MnO₂/CC//ACC delivered 46 Wh/kg at 420 W/kg, equivalent to 0.56 mWh/cm² at 5.14 mW/cm². Similar results of 0.49 mWh/cm² at 4.5 mW/cm² were measured for Ni_1.5Co_1.5S₄/CC//ACC by Chen and co-researchers [90]. Produced by hydrothermal reactions, the mass loading of vertical Ni_1.5Co_1.5S₄ nanorods on CC was 5.54 mg/cm², which is relatively high. Similarly, 5 mg/cm² of was directly loaded on CC by Javed et al. using the hydrothermal conditions of 135 °C for 16 h, then sintering at 350 °C for 2 h [135]. Interestingly, ZnCo₂O₄ nanowires were formed in a nanonest-like structure. The electrode exhibited 467 F/g, which was equivalent to 1320 C/g, at 1.2 A/g. At 15 mV/s, the charge storage kinetics were a combination of surface capacitive charge storage of 42% and diffusion-controlled charge storage of 62%. The contribution of the surface capacitive process increased when the scan rate increased.

Much higher mass loading of 20.4 mg/cm² of Mn₃O₄/NiMoO₄@NiCo-LDH was reported by Liang et al. The CC was ultrasonically cleaned with deionized water and absolute ethyl alcohol for 30 min, followed by drying at 60 °C for 12 h [148]. Then, Mn₃O₄ was electrochemically deposited on CC, resulting in a mass loading of 7.4 mg/cm². Above them, NiMoO₄ was grown by a hydrothermal method; the mass loading of NiMoO₄ was 7.6 mg/cm². Finally, NiCo-LDH with a mass of 5.4 mg/cm² was also hydrothermally synthesized on Mn₃O₄/NiMoO₄. The fabricated ASC of Mn₃O₄/NiMoO₄@NiCo-LDH//AC/CC delivered a maximum energy density of 23 Wh/kg.

Wu and co-authors investigated the interesting relationship between the mass loading of vanadium nitride (VN) on CC and the areal and gravimetric capacitances [149]. The areal capacitance of VN/CC with 28.3 mg/cm² of 3.3 F/cm² was higher than the electrodes with 6.5, 14.0, and 21.2 mg/cm². On the other hand, the gravimetric capacitance decreased by nearly 21% with increasing mass loading from 6.5 to 28.3 mg/cm². The charge transfer resistance slightly increased from 0.162 to 0.447 ohm with increasing mass loading. The mass loading was controlled by optimizing the number of dip-dry repetitions during the coating of CC with VO_x. All electrodes were nitridated in ammonia at 550 °C for 2 h. In the two aforementioned examples, the porous structure of the electrodes played a crucial role in enhancing the contact between electrode charges and electrolyte ions. Overall, seed-oriented growth of microrods on CC, subsequent hydrothermal routes of two or more materials on CC, or dip-dry repetitions of one material on CC, all have been suggested as promising future directions to overcome the limited mass loading challenge.

5.3. Testing Electrolytes at Low and High Temperatures

Most supercapacitor performance reported in the literature has been tested at room temperature. For some commercial applications, energy storage devices require working properly under cold or hot conditions. Although the common usage of aqueous electrolyte solutions in supercapacitors is safer than organic electrolytes, the presence of water limits their durability at freezing or evaporating temperatures. In a recent research by Zhang and others [150], a supercapacitor with a CC@Co(OH)₂:Mn₂₊@T₃ positive electrode and a CC@T₃ negative electrode was assembled in polyvinyl alcohol/sodium alginate hydrogel electrolyte. This anti-freezing hydrogel was abbreviated as PSET30, as it includes polyvinyl alcohol (P), sodium alginate (S), ethylene glycol (E), and 30 mg of Ti₃C₂T_x (T). At 0 °C, areal capacitance of 52 mF/cm² and R_ct of 33.4 ohm were calculated, greatly better than 2.4 mF/cm² and 223.1 ohm at 25 °C. This phenomenon was ascribed to the availability of rich hydrogen bonds in the PSET30 hydrogel, which was stronger at 0 °C, leading to strengthening of the porous network of PSET30 with Ti₃C₂T_x, thus enhancing the electron conductivity. At room temperature, the electron movement was further impeded because some pores were blocked by free ethylene glycol molecules.

At 35, 0, −10, −20, and −30 °C, the areal capacitances of an ASC of NiCo₂O₄@NiMn-LDH/CC//AC/CC were measured as 1608, 1532, 1517, 878, and 253 mF/cm², respectively [151]. PVA/KOH was prepared by adding 1.2 mL glutaraldehyde (1 vol%) as a crosslinking agent and 30 μL H₂SO₄ (1 M) as an initiating agent. This gel film has the ability of withstand temperature changes without losing its elasticity, and it freezes at around −20 °C. From 0 to 60 °C, a CC/PPy symmetric SC with a PVA-H₃PO₄ gel electrolyte was tested by Liu et al. [152]. With increasing temperature, they found that the areal specific capacitance increased, and the internal resistance decreased, attributed to increasing ionic conductivity of the electrolyte. However, the cycling stability decreased, and the self-discharge became worse at high temperatures, where the long chain in PPy became more breakable. To quantify the self-discharge effect of a supercapacitor, its voltage was recorded under open-circuit conditions after being charged. As a result, an ASC of MnCo₂O₄@CuCo₂O₄/CC//AC was charged at 1.6 V, then self-discharged until 0.88 V after 70 h [106]. As summarized, gel electrolytes such as PSET30, PVA-H₃PO₄, and PVA/KOH with glutaraldehyde have been alternatively proposed to withstand a range of temperatures. Despite that, self-discharge and weak cycling stability remain current issues.

6. Evaluation of Electrochemical Stability

For practical applications, testing the cycling stability of supercapacitors in a full-cell configuration is a key requirement. It is worth noting that excellent stability of electrodes tested in a half-cell configuration does not necessarily lead to similar performance in full cells. For example, a nearly perfect cyclic stability of 99.7% over 30000 cycles at 3 mA/cm² was achieved using an electrode of oxygen vacancy-containing cobalt-based metal-organic framework (O-v-CoFe-MOF) nanoparticles on CC [153]. A maximum areal capacitance of 57 mF/cm² was calculated at 0.5 mA/cm². However, when used as a negative electrode with an ACC positive electrode in PVA/ H₂SO₄ gel electrolyte, the supercapacitor preserved 77% of its capacitance after 15000 cycles at 5 mA/cm². Analyzing factors affecting supercapacitor stability and tracking recent strategies for stability improvement are reviewed in this section.

Although extraordinary capacitances and energy have been recorded using the NiCo-LDHs-based asymmetric supercapacitors, as presented in the previous sections, some challenges have impeded cycling stability, hence delaying their utilization in practical applications. The hydroxides have inherently low conductivity, and their volume changes during the charge-discharge cycles. One of the innovative solutions was validated by in-situ implanting carbon quantum dots (CQDs) on NiCo-LDHs [130]. After 5000 cycles, a 54% capacitance retention of the NiCo-LDHs@CC electrode was improved to 83% of the CQDs/NiCo-LDHs@CC electrode, both at 30 mA/cm². Luo and co-authors investigated the primary factors using SEM and TEM before and after cycling. During 5000 cycles, cracks and deformation occurred in the NiCo-LDHs that were easily exfoliated from the carbon fibers, while the CQDs/NiCo-LDHs remained conformally and firmly attached to the carbon fibers. The robust cactus-like nanopillars evolved by an electrostatic interaction between NiCo-LDH positive ions and carboxyl and hydroxyl groups on CQDs. Such a strong attachment by the in-situ implantation of CQDs decreased internal resistance and charge transfer resistance. The CQDs were also coupled with P-Fe₂O₃ on CC through a one-pot hydrothermal process by Wang and others [154]. The hybrid electrode exhibited a specific capacitance of 1787 F/g, which was the highest among Fe₂O₃, P-Fe₂O₃, and CQDs/Fe₂O₃ electrodes, all on CC. Employing CQDs/P-Fe₂O₃ as a negative electrode and MnO₂ as a positive electrode, both on CC substrates in KOH, this ASC kept 91.31% capacitance after 1000 cycles. In addition, at 640 W/kg, an excellent 103 Wh/kg was recorded at an operating voltage of 1.6 V.

One of the highest energy densities of carbon cloth-based ASC was reached by Zhang et al. using NiCoS/ACC//ACC in PVA/KOH [97]. The CC was activated at 500 °C for 2 h, then hydrothermally coated with NiCoAl, which in turn converted to porous NiCoS nanosheets by a second hydrothermal reaction. The flexible device operated at 2 V and offered 108 Wh/kg with a power density of 1000 Wk/g. However, after 2000 cycles at 20 A/g, it holed 89% of its capacitance. In a recent article, NiCo₂S₄ was combined with a perovskite structure of lanthanum nickel oxide (LaNiO₃) on CC by Noormohammadi [99]. After 10000 cycles at 4 A/g, 85% capacitance retention was observed using an ASC of LaNiO₃/NiCo₂S₄/CC//AC/CC. Besides, an energy density of 87 Wh/kg was evaluated at a power density of 400 W/kg. It was found that the layers in this perovskite promoted cyclic stability. Moreover, the device was tested at different bending angles with almost no change in its CV and GCD curves, confirming its mechanical flexibility.

As reviewed, nickel-based sulfides, selenides, and oxides have been considered as promising electrodes. Adil et al. electrodeposited mixed NiS/MnS on ACC, then combined the NiS/MnS/ACC positive electrode with the ACC negative electrode in a KOH aqueous solution [98]. This ASC operated at 1.6 V and exhibited 111 Wh/kg at 800 Wh/kg in addition to maintaining 93% of initial capacitance over 10000 cycles at 5 A/g. These electrochemical merits were superior to ASCs of the last-mentioned NiCoS/ACC//ACC and LaNiO₃/NiCo₂S₄/CC//AC/CC. Nevertheless, cyclic stability needs further improvements to meet commercial preferences. Wan and others combined NiCoS with Ni-Mn-S on CC by electrodeposition [155]. An ASC of Ni-Mn-S@NiCo₂S₄/CC//porous carbon/CC promoted a stability of 96% after 10000 cycles at 8 A/g, which is better than the previous examples in this section. Such improvement was caused by the core-shell heterostructure of nanosheets. On the other hand, its energy and power densities of 59 Wh/kg at 345 W/kg are lower than theirs. At 5 A/g, the capacitance retention of an ASC of NiCo₂Se₄/CC//AC/CC supercapacitor was 88% after 20000 cycles [156], while it was 95% after 10000 cycles for an ASC of NiCo₂O₄/CC//rGO/CC [157], as shown in Figure 11.

Such a stability issue can be overcome using electrodes whose capacitance undergoes a continuous increment during charge-discharge cycles. In an interesting case, 107% of the initial capacitance was increased over 20000 cycles using a Co(OH)F/CoS₂//AC supercapacitor in aqueous KOH by Liu et al. [158]. The transfer of ions and electrons during long cycles was facilitated by a hollow structure, CoS₂ derived from ZIF-67, conjugated with nanowires of Co(OH)F. Therefore, the following redox reactions are possibly accelerated:

CoS₂ + OH⁻ ↔ CoS₂OH + H₂O + e⁻

(1)

CoS₂OH + OH⁻ ↔ CoS2O + H₂O + e⁻

(2)

Co(OH)F + OH⁻ ↔ CoOF + H₂O + e⁻

(3)

CoOF + OH⁻ ↔ Co(OH)OF + e⁻

(4)

In addition, a high energy density of 64 Wh/kg occurred at a power density of 800 W/kg. Besides, a mass of 1.0 mg/cm² of Co(OH)F/CoS₂ was loaded on CC, far from commercial scales of electrode materials. A similar increase of the initial capacitance by 106% after 8000 cycles at 6 A/g was recently reported by Zhou and co-researchers using Na⁺-doped NH₄V₄O₁₀ nano-array on CC, symbolized as NaNVO@CC [101]. It was set as a positive electrode, and Ti₃C₂T_x MXene as a negative electrode with a mass ratio of 0.40 mg: 1.90 mg to balance the asymmetric supercapacitor. An energy density of 78 Wh/kg at 424 W/kg was reached. The cyclic stability was improved due to the gradual activation of the layered crystal structures, in addition to the strong ionic bonds affected by the doped sodium ions.

Zhao et al. found that the addition of iron in the NiCoFe-LDH/CC electrode increases the capacitance by 142% of the initial capacitance following 4000 CV cycles at 100 mV/s [159]. This rare result was better than the retention of 99% using a NiCo-LDH/CC electrode under the same CV cycling conditions. By comparing SEM images before and after cycling, it was found that the thickness of the NiCoFe-LDH nanosheet obviously increased with no structural collapse. Therefore, the doped Fe created additional electroactive sites during the CV test. Even in the full-cell testing, 127% of the capacitance and of an ASC of doped NiCoFe-LDH/CC//AC/CC increased after 5000 CV cycles at 200 mV/s, compared to 99% by another ASC of undoped NiCo-LDH/CC//AC/CC reported in the same study. Moreover, 65 Wh/kg at 83 W/kg was produced by the Fe-doped ASC, much higher than 25 Wh/kg at 94 W/kg produced by the undoped ASC.

Nanoporous aniline and 1,5-diaminoanthraquinone (DAA) were electrochemically copolymerized on carbon cloth after functionalization [132]. The value of R_ct decreased from 1.064 to 0.229 ohm with increasing aniline concentration. Surprisingly, the capacitance maintained 127% after 10000 cycles, as the authors attributed this behavior to the availability of nanopores. The same group conducted more investigation on what is called the “improving with using” phenomenon for poly(1,5-diaminoanthraquinone) (PDAA) electrodeposited CC electrodes [160]. After 50000 charge-discharge cycles, an unusual cyclic stability of 194.8% was recorded. Using XPS, SEM, and additional experiments before and after 50000 cycles, they concluded that the encapsulated 1,5-diaminoanthraquinone (DAA) doped with H₂SO₄ and electropolymerized in the immobilized PDAA nanoparticles during these long cycles, enhancing the capacitance. Regarding the charge transfer resistance, the FCC@PDAA electrode deposited in 3 h had a much smaller value of 0.391 ohm than 345.5 ohm of the CC@PDAA electrode with the same deposition time. It was an increase by three orders of magnitude gained by attaching carboxyl groups to the commercial CC via an oxidizing process. This functional group enhanced the interfacial interaction of the polymers with carbon. The “improving with using” effect has been further reported for other conductive polymers, such as aniline with diphenylamine on functionalized carbon cloth (FCC@PANI-DPA) nanoparticles, and aniline with triphenylamine (FCC@PANI-TPA) nanowire network, showing capacitance retentions of 102% and 106%, respectively, after 10000 CV cycles at 100 mV/s [133]. Based on the presented studies herein, improving with using” phenomenon, that can occur by doping or electropolymerization during cycling, is a promising solution for the cycling stability issue. Future research direction should include in-situ experiments to analyze this phenomenon in the promising hybrid nanomaterial-based electrodes. There is a recent review article that focuses on improving the cycling stability of layered double hydroxides for supercapacitors [161].

7. Current Challenges and Future Directions

In this review, recent articles on carbon cloth-based supercapacitors have been critically compared and categorized according to four main strategies for improving their performance. They are presented as promising future directions as well: (1) applying surface treatment and activation; (2) utilizing hybrid nanomaterials-based electrodes; (3) optimizing the synthesis-structure-property relationship; and (4) integrating supercapacitor components. Extensively reported experiments using carbon cloth with hybrid nanomaterials have achieved advanced supercapacitor performance. It is noteworthy that some high-capacitance supercapacitors still require enhanced energy density, while others with high energy density need stable charge and discharge cycles over extended periods. Still others require improved mechanical flexibility of the nanomaterials deposited on the carbon cloth. Furthermore, increasing the loading of active materials onto the carbon cloth without impeding ion diffusion and charge transport must be balanced. Additionally, while exploiting redox reactions in hybrid materials on carbon cloth increases capacitance, it often leads to slower charge and discharge rates and material degradation after numerous cycles.

Therefore, future approaches include fabricating shell-core nanostructures, MOFs, MXenes, and CQDs to prevent the electrochemical degradation of active materials. To translate such research into commercial manufacturing, methods for depositing materials onto carbon cloth must be scalable to produce a uniform, homogeneous coating around each microfiber of the cloth, at a cost-effective price. For example, coating carbon fabric with nanomaterials using dip-coating, spray-coating, or roll-to-roll techniques continues to garner scientific and practical attention. Given the importance of sustainability, supercapacitors should ideally be manufactured at standard temperatures by green methods, without hazardous solvents, and from recyclable or biodegradable components such as biochar, lignin-derived carbon, and organic redox molecules. This aligns with the global trend toward environmentally friendly energy storage technologies. Ultimately, future directions for supercapacitor applications have been suggested as follows:

• Integration of energy conversion, harvesting, and storage technologies into a carbon cloth-based system such as supercapacitors with solar cells, piezoelectric, thermoelectric energy harvesters, or batteries.
• Combination of different functions in a single hybrid carbon cloth such as energy storage with sensing, catalysis, or heat management.
• Artificial intelligence-assisted material simulation for the prediction of optimal hybrid structures and compatible electrolyte compositions.
• Woven carbon cloth with microelectronics in smart textiles, flexible robots, medical devices, the Internet of Things, and the aerospace industry.

8. Conclusion

Supercapacitors among electrochemical energy storage systems demonstrate higher energy density than conventional capacitors but lower than batteries. Although supercapacitors have been found in multiple current applications such as transportation, renewable energy systems, and electronics, the low energy density and rigidity of supercapacitors have still limited their use. For wearable electronics, smart textiles, bendable devices, and flexible robots, the supercapacitor electrodes need to be flexible enough. Carbon cloth that consists of woven microfibers is a perfect candidate for next-generation supercapacitors. It is flexible, lightweight, conductive, mechanically strong, chemically stable, breathable, and affordable. To unlock the capabilities of CC, its surface requires activation and treatment. As reviewed, chemical activation, electrochemical activation, acid/base treatment, heat treatment, amino-acid modification, and metal coating have proven efficacious in enhancing capacitance, voltage range, conductivity, stability, and electrolyte accessibility in CC-based electrodes.

On activated CC, utilization of hybrid nanomaterials couples both charge storage mechanisms of EDLC and pseudocapacitors. Hierarchical architectures such as core-shell structures, hollow nanospheres, nanosheets, nanowalls, and nanoflakes increase active sites and improve ion-electron transport. On one hand, conductive polymers and transition metal compounds, including oxides, hydroxides, sulfides, phosphides, nitrides, chalcogenides, MOF-derived structures, and MXene-based composites, offer high energy density due to their redox reactions with compatible electrolytes. Unfortunately, they usually suffer swelling and structural degradation over extended charging-discharging cycles. On the other hand, carbon nanomaterials such as CNTs and rGO possess almost perfect stability but low energy density. Hybridizing two or more of those materials as hybrid CC-based electrodes achieves high energy density and excellent cycling stability. To succeed in such hybridization, synthesis-structure-property relationships should be investigated. Optimization of synthesis parameters such as reaction time, temperature, precursor type, precursor ratio, solvent, and deposition method is a prime practice to obtain favorable morphology, crystallinity, and conductivity of CC-based hybrid electrodes.

The integration of supercapacitor components implies that there is a strong attachment of electrode materials to the current collector in addition to full electrolyte accessibility. Such integration affects device performance on the whole. Some acidic electrolytes can cause high capacitance but weak electrode stability due to corrosion, in contrast to natural electrolytes. It has been clearly seen that the direct deposition of active materials conformally on carbon microfibers preserves the 3D nature of the cloth, lower resistance, better adhesion, and mechanical flexibility. This strategy is more effective than drop-casting powders with binders on CC as a 2D flat bulky electrode. One of the remaining challenges has been the balance of increasing electrode mass loading to meet manufacturing requirements without blocking ion diffusion and charge transfer. Other challenges have been the stability of electrolytes at high and low operating temperatures, electrode cycling stability, self-discharge, and large-scale fabrication techniques. Future directions have been suggested as deep understanding of tunable properties of nanomaterials, integrating supercapacitors with other devices into a carbon cloth-based system, and artificial intelligence-assisted material designs.