Electrode Materials for Supercapacitors: A Review of Recent Advances

Abstract

The advanced electrochemical properties, such as high energy density, fast charge–discharge rates, excellent cyclic stability, and specific capacitance, make supercapacitor a fascinating electronic device. During recent decades, a significant amount of research has been dedicated to enhancing the electrochemical performance of the supercapacitors through the development of novel electrode materials. In addition to highlighting the charge storage mechanism of the three main categories of supercapacitors, including the electric double-layer capacitors (EDLCs), pseudocapacitors, and the hybrid supercapacitors, this review describes the insights of the recent electrode materials (including, carbon-based materials, metal oxide/hydroxide-based materials, and conducting polymer-based materials, 2D materials). The nanocomposites offer larger SSA, shorter ion/electron diffusion paths, thus improving the specific capacitance of supercapacitors (SCs). Besides, the incorporation of the redox-active small molecules and bio-derived functional groups displayed a significant effect on the electrochemical properties of electrode materials. These advanced properties provide a vast range of potential for the electrode materials to be utilized in different applications such as in wearable/portable/electronic devices such as all-solid-state supercapacitors, transparent/flexible supercapacitors, and asymmetric hybrid supercapacitors.

1. Introduction

Recently, due to the need for preserving natural resources and regulating global energy consumption, there has been a crucial demand for the development of clean energy sources. Among various unconventional electric-power devices, such as supercapacitors (SCs), batteries, and fuel cells, (SCs), which have been used in various fields, (e.g., hybrid cars and electric mass transit vehicles [1], and wearable/portable electronic devices [2], etc.,), SCs have inspired a vast range of research attention owing to their excellent electrochemical performances, including high specific power, superior cycling life and a rapid charging–discharging rate [3]. SCs are a type of capacitor that are also known as ultracapacitors, electrochemical capacitors (ECs) [4], gold capacitors, electrical double-layer capacitors (EDLCs), pseudocapacitors, or power coaches [5,6]. The main combination of SC configuration consists of an electrolyte, current collector, and electrodes with large specific surface area (SSA), which enhances the capacitance 10,000 times more than conventional capacitors [7,8] and thin dielectric separators, which provide higher capacitance for SC compared to conventional batteries [9].

The energy storage in SCs is based on the charge–discharge mechanism at the electrode–electrolyte interface [10] in which the principle is similar to conventional capacitors; however, the charge–discharge process is much faster in SCs [11]. The charge storage range for conventional capacitors is between micro to millifarads while for SCs is between 100–1000 F for each device retaining low equivalent series resistance (ESR) and specific power [12]. SCs could cover both the specific energy and power density (P_d) by several orders of magnitude compared to batteries exploiting proper design and efficient materials, which make them as a flexible energy storage option [12]. The specific energy (energy density) determines how long the energy storage device can be utilized, and the specific power (power density) signifies how rapidly a device can deliver the energy.

The segregating feature of the SC to the conventional capacitor is the absence of dielectric material in the SCs [13]. In comparison to SCs, conventional capacitors store and deliver very high energy density (E_d), but the charge and discharge time of batteries is much slower. SCs also show more safety, a low level of heating, device stability, light-weight, and flexible packaging [14,15,16]. Figure 1, [17], displays the Ragone plot of comparison between conventional capacitor, fuel cells, batteries, and hybrid supercapacitors (HSCs), as various energy storage devices, in terms of power density (P_d) and E_d in which the HSCs show a significant feature of P_d in comparison with batteries and fuel cells but considerably lower P_d than conventional capacitors.

According to the charge storage principle, SCs can be categorized into three groups, including (a) EDLCs that exploit double layer electrodes to store the charge in through non-faradic interactions (electrostatically) [18], (b) pseudocapacitors in which the charge storage mechanism is faradaically through oxidation-reduction reactions associated with their various potential [19], and (c) asymmetric supercapacitors (ASSC), in which, the charge storage mechanism of the two electrodes is not similar, and the storage principle is based on both Faradaic and non-Faradic behaviour of used material [13]. Generally, the “asymmetric” word refers to various charge storage mechanisms corresponding to the positive and negative electrodes in the SC device. According to this, the HSC has been categorized as one of the specific class of the ASSC. The energy storage mechanism of a device with supercapacitive materials as the cathode and a battery material as the anode is called HSC. ASSCs provide a high energy density and power density due to the extended operating voltage window (up to about 2.0 V in the case of aqueous electrolytes, and about 2.7 V in organic electrolytes, and even more than 4.0 V in ionic liquids) by taking the advantages of the two different electrode materials, which could operate at different potential windows. Figure 2 represents the schematic illustration of the EDLCs, pseudocapacitors, and the HSC.

The main characteristics for these energy storage systems, which provide the ability to distinguish one system from others are including energy density, power density, capacitance/capacity, cell voltage, kinetic storage mechanism, i–v relationship, operating temperature, cycle life, self-discharge, and used materials as an electrode, electrolyte, etc. Compared to EDLCs and pseudocapacitors, the HSCs displayed significant advantages such as higher energy density [20], and long cycle life (>100,000) [21], that has been obtained in a higher cell voltage (maximum 0.5) [22]. Lower self-discharge (energy dissipation) is another advantage of HSCs, in comparison with the EDLCs and pseudocapacitor devices [23]. Generally, self-discharge mechanisms, which are associated with various chemical and physical mechanisms, are mainly categorized into three classes: (a) parasitic faradaic reactions, (b) charge redistribution, and (c) ohmic leakage of current between the electrodes [24,25,26]. Accordingly, self-discharge occurs, if one or the combination of these processes is available.

Selecting a standard lifetime as one of the main parameters of the rechargeable energy storage devices is entirely subjected to the nature of the device, and it is highly linked to the operating conditions such as temperature, voltage, charge–discharge current, the depth of the discharge, and at some stages, the humidity [17,27]. It is essential to select the appropriate materials with high purity for both electrolyte and electrodes as it influences the cycling life and the current leakage. Cyclic stability can be estimated through GCD measurements by repeating the cycles continuously at a constant current density [28].

The cycling lifetime of the hybrid system is approximately 5000 cycles, which could vary with respect to the batteries (for non-Li-ion batteries around 500 cycles, and for Na- and Li-ion batteries around 2000 cycles) [29], and the other types of supercapacitor devices (around 10,000 cycles) [30]. The description of the life cycle is highly relient on the application of the corresponding device, where it can be defined as the number of full charge–discharges before the device capacitance/capacity reduces below 80% of the start-of-life value [31,32,33].

In the case of the death state of a battery intended for a hybrid or an ordinary electric vehicle, the life cycle is described as 80% of the corresponding initial capacity [34]. In the following sections, the cycle lifetime results for various energy storage devices are reported as the percentage of the initial capacity/capacitance of the device that remained after a specified number of cycles. In this review, a detailed description of the features of various electrode materials has been given. The electrochemical properties, preparation methods, and the applications of the materials have been discussed in detail. The following features have reviewed comprehensively: (i) the difference between the “pseudocapacitive” and “battery-type” electrode material, (ii) the importance on the selection of the electrode material, and their properties influencing the SCs performance, and (iii) the major categories of materials (carbonaceous materials, TMOs/TM(OH)s, and CPs), and their composites. The advantages and disadvantages of the electrode materials are also briefly compared. The charge storage behaviour of different components has been introduced through recent advances. Then, the importance of the utilization of 2D material (either individually or in combination with other materials) has been comprehensively explained. In the end, an outlook on the significance of the bioderived functional groups into the electrode material, their advantages, and application in various SCs such as transparent SCs, FSCs, and HSCs is given.

2. The Distinction between Terminologies Used: Pseudocapacitive and Battery-Like Electrode Materials

There are various materials, which can be utilized to provide a favorable charge storage mechanism. Generally, the carbonaceous materials are used as capacitive-type electrodes, while transition metal (TM) and conducting polymer-based material can be used as the battery type-electrodes. The battery-type materials can be classified into two groups, including, intercalation-type (intercalation-deintercalation), and conversion-type (two-phase conversion). This classification doesn’t mean that charge storage occurs only in one type of mechanism for the electrode materials. The charge storage process of the battery-type electrode materials is associated with both conversion and intercalation-deintercalation behaviour. There are also different electrode materials that exhibit either capacitive or battery-type behavior, depending upon various potential windows, the electrolyte selection, etc. Thus, to simplify the concept, the electrochemical behaviour of the redox-active materials (whether two-phase conversion or intercalation-type) can be described as “pseudocapacitive behaviour”.

In the field of supercapacitors, the term “pseudocapacitance” is exploited to define the behavior of electrode materials (e.g., RuO₂ [35,36], MnO₂ [37,38]) that exhibit the electrochemical characteristics of a capacitive electrode such as a linear dependency between the stored charge and the potential variation within the certain potential window, however, in this case, the charge storage should be originated from the electron-transfer mechanisms, instead of only relying on the ions accumulation in the electrochemical double layer (EDL), similar to that with activated carbons (AC) [39].

Several types of transition metal oxides (TMOs) have been considered as pseudocapacitive materials in aqueous electrolytes. The charge storage behavior of these materials is associated with their structural and hydration properties as well as the nature of the electrolyte. The structural water provides a large inner surface, which is accessible for ion absorption and leads to improved ion diffusion in the inner surface (e.g., RuO₂·nH₂O). Thus, the influence of surface-bound water is one of the significant issues, which makes a difference between the aqueous and non-aqueous pseudocapacitive materials. For instance, MnO₂ is present in several polymorphs and mainly studied for the pseudocapacitive behavior in a neutral aqueous electrolyte, while it is considered as a battery-like material in alkaline electrolytes. The battery-type behavior in an alkaline electrolyte (e.g., KOH) is attributed to the formation of oxyhydroxides during the charging process [40]. Besides, the spinel compounds (e.g., Mn₃O₄ [41], Fe₃O₄ [42], and Co₃O₄ [43]), display both capacitive and battery-like behavior owing to the phase variations, which occurs during the cycling mechanism.

In various studies, the term “pseudocapacitive” has been used for battery-like material such as Ni(OH)₂ [44,45] or for other materials that display faradic behavior (even for the materials, that are electrochemically irreversible) [46,47]. This may confuse the reader, since the term “capacitance” with the unit of the farad (F), cannot be applied for the material that their charge storage mechanism is only based on the faradic behavior. For these classes of material, the concept of “capacity” (with the unit of C (coulomb), or mAh) is a more significant and suitable term [39]. Figure 3 illustrates the general classification of the pseudocapacitive charge storage mechanism [17].

Among these various processes, the intercalation pseudocapacitance is hardly observed, since the charge storage for the corresponding materials is mostly hybrid, which can be defined as a combination of both “battery-type”, and “capacitive” behaviors [48]. In order to distinguish between the contributions from battery-like energy storage process and intercalative pseudocapacitance mechanism, the corresponding signatures from the electrochemical measurements (e.g., CVs and GCD), as well as the quantitative kinetics analysis, should be considered, [17,40,49], which are illustrated in Figure 4, [49]. According to the Equation (1) [50], the capacitive contribution and the diffusion-controlled contribution can be examined by investigating the CV data at different sweep rates, in which the calculated current “i” follows a power-law relationship with the sweep rate V. The “a” and “b” parameters are changeable, and the b values can be estimated from the slope of the plot of log i vs. log V [49,50].

$$\mathrm{i}\left(\mathrm{V}\right)={\mathrm{av}}^{\mathrm{b}}$$

(1)

As is shown in Figure 4, the “b” value corresponding to EDLC materials is always equal to 1, the CVs is rectangular, and in the case of their GCDs, the potential has a linear dependency on time. Pseudocapacitive materials exhibit the CVs in an almost rectangular shape, and the potential of GCDs is practically linear with respect to time. The corresponding “b” value is close to 1, and as it was mentioned before, the charge storage capability is represented using the F unit. In comparison with EDLCs and pseudocapacitive materials, the battery-type materials show the typical CVs with noticeable redox peaks, and the obvious plateaus are obtained for their corresponding GCDs. The “b” value for the battery-type material, depends on the nature of material, and it can be 0.5 and higher than 0.5 for conventional bulk battery-type materials or those with specific structural design electrode engineering, and for nanomaterial electrodes, respectively [49].

There are several studies, in which the presence of structural water in crystalline oxides resulted in intercalation pseudocapacitance-non-diffusion-controlled proton insertion into the interior of bulk-like oxides. For instance, structural water in layered-hydrated tungsten oxide (WO₃·2H₂O) induced the transition from battery-like electrochemical behavior to pseudocapacitive behavior for proton intercalation in comparison with the anhydrous WO₃ [51].

The substantial difference between battery-like and pseudocapacitive electrode materials is that the charging and discharging time for the pseudocapacitive materials is in the range of seconds—minutes. Therefore, it is worth focusing on emerging pseudocapacitance in order to obtain high energy and high power densities from the same material [40].

3. Electrode Materials

The selection of electrode materials and their fabrication play a crucial role in enhancing the capacitive performance of SCs [52]. Electrodes of SCs must provide thermal stability, high SSA, corrosion resistance, high electrical conductivity, appropriate chemical stability, and suitable surface wettability. They should also be low-cost and environmentally benign. Besides, their capability of transferring the faradic charge is important to enhance the capacitance performance [53,54].

In fact, the specific capacitance factor is not only affected by surface area, but also there are other essential parameters including the simple morphology manipulation, including pore size distributions, as well as the pore shape, pore sizes, and their availability for the electrolyte [11,55,56].

Thus, two of the most important requirements for designing an SC device are (i) improving the electrochemically active sites by selecting the electrode materials with a high SSA, and (ii) tuning the pore size and pore shape (e.g., circle, vertical rectangle, horizontal rectangle, and square in the case of graphene nanopore [57], and cylindrical, spherical, and slit in the case of the biomass-derived carbon [58]) of the electrode material to make them suitable candidates which ease the transportation of electrolyte ions [11,59].

The electrode material having smaller pores offer higher capacitance and, thus, higher E_d. However, the smaller the pore size leads to having a higher the equivalent series resistance (ESR), therefore, decreasing the power density. Based on this, the application of the electrode material affects the selection of the materials. For example, for the applications in which having more peak currents is important, the electrode materials should satisfy the criteria of having greater pores, while for the applications that higher E_d is required, the electrode materials should have smaller pores [60,61].

Furthermore, the suitable size distribution can improve the retention capability, which is the indication of high P_d in an SC device. The effective distribution of micro/mesopores of the electrode material can provide a fast mass and ion transport through a continuous pathway, thus enhance the accessibility of the electrolyte, and make the material an appropriate choice for SC application [62].

The electrode materials are categorized based on three main classes, including carbonaceous materials, TMOs, and conducting polymers (CPs) [63].

Figure 5 shows the classification of the electrode materials for SC application from zero-dimensional (0D) to three-dimensional (3D) structure that is explained in detail through the following sections.

3.1. Nanostructured Carbon-Based Materials

The EDLC devices, capacitance mainly derives from the collected charges at the interface between electrodes and the electrolyte. Consequently, the capacitive performance of an EDLC device is extremely affected by the pore sizes, pore distribution and SSAs of the electrode materials [64,65]. Becker developed the first EDLC utilizing carbon-based materials in 1975 [66].

Mainly, the carbonaceous materials are utilized as electrode material for EDLCs owing to their improved properties such as high surface area, low cost, eco-friendliness, and their facile synthesis methods [67]. The physicochemical characteristics of the carbon-based material depend on the pore size that is approximately less than 1 nm [68].

Furthermore, their thermal stability, chemical and electrochemical stability (in various solutions from a strong acidic to basic media), high electrical conductivity, as well as the symmetrical galvanostatic charge–discharge profile and the good rectangular shape of CV curves propose that the carbon-based materials are the useful capacitive materials [54,69,70,71]. The capacitance of the electrodes extremely relies upon the SSA. Consequently, owing to the large SSA and the high porosity of the carbonaceous materials, they have displayed enhanced capacitance [72].

The charge storage mechanism of the carbon-based electrode mainly depends on the formation of a thin layer known as the Helmholtz layer on the surface of electrode material, which is in contact with the electrolyte ions (both non-dissolved and very weakly-dissolved ions). Secondly, the charge storage derives from the formation of a thicker outer Helmholtz layer, which is made through the existence of the solvated ions conducted by strong electrostatic forces. The indicated layer can be prolonged to a thicker layer, the so-called Gouy–Chapman diffuse layer, through solvated ions and the thermal motion. The degree of charge storage in an EDLC device can be described as the potential difference between the thin inner layer and outer thicker layer (the Helmholtz layers and the Gouy–Chapman diffuse layer), which is known as zeta potential (ξ-potential) [73]. Besides, there is a huge necessity to enhance the capacitance of the carbon-based material to obtain higher E_d. Introducing the redox-active moieties has provided the pseudocapacitive carbon-based material through different strategies such as utilizing redox-active functional groups [74], chemical doping of the carbon-based material using redox-active small molecules [75] and combining the aromatic isomers adsorbed onto the structure of carbon material [76].

Carbon-based materials could be used in various forms such as powder, fiber, monoliths, and foils in an SC device. In addition to the pore shape and SSA, there are other influential items such as the structure of the material, and pore size distribution, and electrical conductivity that have an important consequence on the electrochemical performance of an SC device [77,78,79]. There are several common examples of carbon-based material such as, carbon aerogels [80], graphene [81], AC [82], activated carbon fibres (ACF) [82], carbon nanotube (CNT) [83], and carbon cloth(CC) [84] as well as various carbon-based composites, which are suitable to be used as the electrode materials in SC devices, and are explained in detail through the following sections.

3.1.1. Activated Carbon Materials (ACs)

Activated carbons are the most commonly used material for EDLC electrodes with moderate cost [85] that show a complex porous structure comprising micropores (smaller than 2 nm), mesopores (in the range of 2–50 nm), and macropores larger than 50 nm that lead to having a high SSA [11,54,69,86]. Furthermore, although their good electrical conductivitie is lower (1250–2500 Sm⁻¹), still, it is suitable for SCs. According to those properties, activated carbons are considered as useful electrode materials for SCs [82,87,88,89,90,91,92]. Activated carbon can be produced in two steps and involving either chemical or physical activation from various types of carbon-rich organic precursors such as fossil fuels, coke or synthetic polymers, wood, coconut shells [11].

The carbonization step involves the treatment of precursors in the exposure of oxidizing gases, such as CO₂, steam, and air, at high temperatures (700–1200 °C) in an inert atmosphere to obtain the amorphous carbon. In the activation step, the applied temperature is lower (400 to 700 °C). The chemical activation process is carried out in the presence of the activating agents like carbonates, alkalis, chlorides, or acids such as NaOH, KOH, ZnCl₂, and H₃PO₄ that lead to having a porous network in the carbon particles bulk [87,93].

Activated carbon shows plenty of physicochemical properties based on the carbon precursors used and activation methods that have a strong influence on its surface area, porous structure, and the pore size distribution [93].

As mentioned, activated carbon can be obtained from various carbon-rich organic precursors. The obtained coconut shell-based activated carbon exhibits supplementary micropores than the one which is made from charcoal. The research shows that a slighter pore size distribution can be conveniently measured in the case of activated carbon fibre made from activated carbon with the surface area around 2500 m²·g⁻¹ [15]. Since the electrolyte ions with large size cannot inter to the smaller micropores, for activated carbon, the whole SSA does not contribute to the charge storage mechanism [70,71].

Here, a maximum obtainable surface area was estimated by 3000 m²·g⁻¹; however, the effective surface area is obtained between 1000 to 2000 m²·g⁻¹ [93]. Despite those enhancements in the case of specific area and porosity of the activated carbon, there are some reports indicating electrolyte decomposition and dangling bond positions derived by the increase in SSA [94].

The specific capacitance values for the activated carbon electrodes in aqueous electrolytes (between 100 to 300 F·g⁻¹) is higher than in organic solution (<150 F·g⁻¹), since the size of the ions for the organic electrode is larger [11,95] Wang et al., shown that by increasing the SSA of AC from 621 m²·g⁻¹ to 2685 m²·g⁻¹, the specific capacitance of the electrodes was improved from 17.68 F·g⁻¹ to 171.2 F·g⁻¹ [72].

As a result, estimating the pore size suitable for a specified ion size is one of the items that should be considered. Choosing an appropriate method is essential to improve the pore size distribution in the assembly process. According to previous reports, the pore size of 0.8 nm can be suitable in the case of organic electrolytes, while the pore size of 0.4 or 0.7 nm can be useful for the aqueous electrolytes [70,71]. Furthermore, the surface functionalization of the carbon material results in an increase faradaic redox reactions and, consequently, enhancement of about 5 to 10% in capacitance [96]. Achieving a maximum specific capacitance highly depends upon the coordination between the pore size and the ion size [97,98].

One of the significant disadvantages of activated carbon is its low density. Moreno-Fernandez et al. [82] showed that a high specific capacitance value (200 F·g⁻¹) could be attained through the synthesis of the activated carbon fibre monolith that leads to a higher density by three times more. In another study, Li et al. [99] have fabricated activated carbon made by lignite at activation temperatures of 650 °C, which displayed cyclic stability over 3000 cycles, a specific capacitance of 207.5 F·g⁻¹ as an electrode material for SC at a current density of 0.5 A·g⁻¹.

Recently, hybrid lithium-ion SCs have been fabricated using the agar-derived AC as the cathode material, which stores the charge through the electric double-layer capacitance behaviour. The HSC provides both properties of high P_d (like capacitors), and high E_d (like lithium batteries). AC showed an appropriate pore volume and a narrow pore-size distribution that leads to having a large SSA more than 1672 m²·g⁻¹, the high specific capacitance of 210.4 F·g·_AC⁻¹. The HSC provided a high E_d of 308.3 Wh·kg·_AC⁻¹ at a specific power of 0.7 kW·kg·_AC⁻¹ [100].

The electrochemical performance of the ACs could be improved through different strategies such as the Fermi level position modification of the ACs using ultrasonic radiation [101], modification of the AC surface through oxidation process [102], doping sulfur and oxygen functionalities on the porous structure of ACs [103], fabricating a composite using ACs and other carbon nanomaterials (e.g., carbon nanotubes (CNTs) and carbon nanofibers (CNFs), etc.) [104], and inserting polymers into the carbon substrate to form composite electrodes [105,106].

For instance, Cheng et al. [104] developed a new class nanocomposite electrode through the combination of the conventional ACs and carbon black (CB) with CNTs and CNFs. The resultant electrodes showed excellent electrochemical performance in comparison with that electrode of pure AC due to the synergistic effects of combined species. The quaternary nanocomposite electrode (Figure 6a–d) exhibited high capacitance retention in which 91.4% of capacitance remained after 30,000 charging–discharging cycles at 10 A·g⁻¹ (Figure 6e). Compared to the AC electrode, the nanocomposite showed enhanced E_d of 29.6 Wh·L⁻¹ at a P_d of 101.7 kW·L⁻¹ (Figure 6f) as well as a high capacitance of 66.1 F·cm⁻³.

3.1.2. Carbon Nanotubes (CNTs)

Electrodes of CNTs have been developed due to their unique properties, such as their accessible external structure with high SSA and the specific internal network of mesopores, chemical and thermal stability, superior electrical conductivity, and low mass densities.

Furthermore, the mechanical flexibility of the CNTs and their open network provides them with a link to active material, higher interconnection, and to form a continuous distribution that leads to the more utilized SSA [11,107].

The CNT electrodes show lower ESR, and consequently higher power than ACs since the mesoporous structure of CNT networks allows the ions of the electrolyte to diffuse easier and faster [108] (Figure 7a,b). Du et al. decreased the ESR of electrodes outstandingly, through the adopting thin films fabricated by means of a highly concentrated colloidal suspension of CNTs [109]. The CNTs could be fabricated through different methods. Shi et al. [110] stated the importance of CNTs synthesize method on the performance of the electrode. CNTs were grown on 3D porous nickel (Ni) foam by means of the single-step CVD method, in which the resultant electrode exhibited a superior capacitive performance at higher current densities, less ESR, and a capacitance about 127% greater than the CNT electrode fabricated through the conventional transfer technique. However, the low SSA is the limiting factor for CNT application in high-performance SCs.

CNTs could be used as electrode material for SCs in the form of composite along with other materials such as graphene, metal oxides (MOs), polymer. Q. Cheng [108] studied the electrochemical properties of the graphene, CNTs, and their composite electrode material for SC in various electrolytes. Graphene/CNTs electrodes showed superior electrochemical properties (Figure 7c,d) E_d of 155.6 Wh·kg⁻¹, and a maximum P_d of 263.2 kW·kg⁻¹ in ionic liquid as well as a specific capacitance of 290.4 F·g⁻¹. As is shown in Figure 7c, the graphene/CNTs composite exhibited the most rectangular geometry, which is an indication for an outstanding charge dispersion in the electrodes.

Furthermore, they have reported an interesting phenomenon known as “electro-activation”, in which the specific capacitance of graphene and graphene/CNTs not only did not reduce after a long time cycling in the 1 M KCl electrolyte but also have shown a high increase. It has been reported that the ions result in the larger spacing amid the graphene sheets, thus providing a larger surface area, which presents higher available double-layer capacitance (Figure 7e,f).

CNTs can be classified into two main categories such as SWCNTs and multi-walled carbon nanotubes (MWCNTs) [111]. Generally, CNTs can be synthesized via catalytic decomposition of certain hydrocarbons. Several parameters, such as crystalline orders, can be manipulated to obtain a different conformation like SWCNTs and MWCNTs [87]. In the case of the SWCNTs, they usually form bundles, which limits their surface area and only the outermost surface can be available for electrolyte ions [108].

The performance of the single-walled CNTs was profoundly affected by the accessibility of electrode materials and the size of the electrolyte ions [112]. The theoretical SSA for SWCNTs was reported at about 1315 m²·g⁻¹, while the SSA for MWCNTs was smaller than ACs. This smaller SSA leads to a lower E_d for CNT in comparison with AC [87]. However, the MWCNTs mesopores structure provides easier ions movement across the electrode-electrolyte interface, their mechanical stability decreases because of the volume change derived from the frequent intercalation and depletion [113].

The CNT materials can go through the chemical activation using potassium hydroxide to improve the specific capacitance, which leads to enhancing the surface area of CNT to be two or three times higher while the morphology of the CNT is still defined [87].

The morphology and the purity and of the final CNT material are two crucial factors that affect the specific capacitance of the electrodes [87]. For instance, the MWCNT sheet electrodes were synthesized via catalytically-grown CNTs, which had a diameter of 80 Å and a higher SSA of 430 m²·g⁻¹ than another CNT with a maximum specific capacitance of 104 F·g⁻¹ derived from different methods [114]. In another report, the effect of the acid treatment on the specific capacitance of MWCNTs was studied. In comparison with the specific capacitance of the untreated CNTs (~10 F·g⁻¹), the treated MWCNTs provided a specific capacitance of about 50 F·g⁻¹. According to the FTIR spectrum, the obvious peak corresponds to the C–H bond in pristine CNTs became negligible in the acid-treated MWCNTs, which indicated that the acid treatment could chemically modify the C–H defects to introduce more oxygen-containing groups such as carboxyl, carbonyl, and hydroxyl groups to the surface of MWCNTs [115].

Xie et al. [53] synthesized hierarchical porous carbon microtubes (HPNCTs) through the simple carbonization and following KOH activation method utilizing willow catkins. The final product showed a tubular structure, derived from the willow catkins as well as a highly porous morphology as a result of the activation process. The resulting material showed a gravimetric capacitance of 292 F·g⁻¹, the SSA of 1775.7 m²·g⁻¹.

CNT materials could be deposited directly onto the conductive substrates. In this case, the CNTs act as mechanical support, which increases electron transport, thus decreasing the contact resistance between the current collectors and active materials [116].

Furthermore, various classes of materials and composites were used to coat the CNTs to enhance the electrochemical properties of the electrode. The CNT current collectors provide high cyclic stability, high rate capability, as well as enhanced E_d [117].

For instance, Liu et al. [117] fabricated flexible supercapacitors (FSCs) containing high-performance fibrous electrodes using the composite of MnO₂ nanorods, which have been loaded on MXene nanosheets (NSs), and coated on carbon nanotube fibres (CNTFs). In this study, the resultant MnO₂@MXene/CNTF displayed high capacitance retention after 5000 cycles, excellent specific capacitance, and superior flexibility. The CNTFs were applied as a current collector with excellent mechanical and electrical properties.

CNTs could be used as an electrode material along with novel MXene materials [118], TMOs [119], and their composites [117], which are the most common electrode materials for the SC.

3.1.3. Graphene

Graphene is a one-atom-thick layered 2D structure with sp²-bonded carbon atoms, which are arranged in a honeycomb-designed crystal structure [120,121,122,123,124]. Graphene, as a unique carbon material, shows excellent features including superior mechanical, morphological characteristics, and electrical conductivity, as well as high carrier mobility, large surface area, and high chemical stability [125,126,127]. Furthermore, the 2D structure of graphene decreases the thickness of the electrode that leads to obtaining a larger potential window, greater flexibility, chemical, and thermal stability [124]. Due to these properties, graphene is a powerful candidate for energy storage applications [123].

Accordingly, graphene could be utilized as electrode material in the application of high-performance SC, since, in comparison with other carbon-based materials such as ACs and CNTs, graphene-based electrodes do not depend on the distribution of pores in the solid-state [128,129]. Another advantage of utilizing the graphene as an electrode material is the accessibility of both exterior surfaces of the graphene sheet by the electrolyte [68,130,131].

A specific capacitance of 205 F·g⁻¹ has been obtained for the SC fabricated using graphene-based electrodes. The resultant SC showed an E_d of 28.5 Wh·kg⁻¹ and a P_d of 10 W·g⁻¹ [132]. In another study, the flexible thin films of carbon showed that the final graphene material exhibited an electrical conductivity of 5880 S m⁻¹ and a high SSA of 2400 m²·g⁻¹. The obtained specific capacitance and the E_d of the graphene material were reported about 120 F·g⁻¹ and 26 Wh·kg⁻¹, respectively [133].

There are plenty of methods to produce various types of graphene, such as micromechanical exfoliation, epitaxial growth, chemical vapour deposition, unzipping of CNTs, electrochemical and chemical approaches, arch discharge method and intercalation techniques in graphite [68,130,131], Figure 8 illustrates the synthesis of N-doped porous graphene (N-pGr), in which the pGr was obtained through the oxidation of epoxy groups on the surface of reduced graphene oxide(rGO) using H₂O₂, in addition to the hydrolysis of hydroxyl functionalities through the parallel breaking of graphene C–C bonds [134,135].

Nanostructured graphene oxide (GO) could be synthesized with different morphology such as single layers, few layers, and 3D porous structures through the hydrothermal process. The morphology could be controlled through the process, adding various quantities of Ni²⁺. Among the various morphologies, the 3D porous structure displayed an optimum specific capacitance of 352 F·g⁻¹ at a scan rate of 5 mV·s⁻¹ [137].

Ogata et al. [138] prepared the rGO/GO/rGO device, which showed both SC and battery behaviour. The device behaved as a proton-type SC until the operating voltage about 1.2 V, and for voltages higher than 1.5 V, the device provided the redox reaction through the interaction between the oxygenated functional moieties on the rGO and behaving as a battery.

Moreover, Liu et al. [125] solved the problems of water-based electrolytes to function at broader voltage windows, synthesizing a surface compatible and pore accessible graphene for ionic liquid electrolytes. They prepared a mesoporous graphene network which was accessible to the ionic electrolyte and was able to operate a voltage up to 4 V. The synthesized material showed the E_d of 136 Wh·kg⁻¹, and 90 Wh·kg⁻¹ at 80 °C, and at room temperature, respectively.

In another study, a symmetrical SC was assembled utilizing various electrode materials made of GO, polyvinylidene fluoride (PVDF), and KOH with a different ratio. Among all the samples in this study, G1p4K6-2, and G1p9K6-4 showed better electrochemical performance, in which, for the sample G1p9K6-4, the maximum E_d of 78 Wh·kg⁻¹ at P_d of 875 Wk·g⁻¹ and high specific capacitance of 185 F·g⁻¹ at 0.5 A·g⁻¹ was obtained [139]. The samples were named as Gx1px2Kx3-x4, in which G stands for GO, p represents PVDF, K is for KOH, x1 and x2 signify the relative content ratio of PVDF to GO, x3 is the relative content ratio of KOH: (GO + PVDF), x4 is the reaction time, and “t” is the activation time. It was reported that extending the activation time led to having an increase in the effective crystallite size due to the loss of some amorphous carbon in the ignition during the time after completing the activation process. SEM image (Figure 9a) shows the highly porous structures for the sample G1p9K6-4 in comparison with other samples such as G1p4K6-2 (Figure 9b), which is the indication of higher SSA mesopore volume, thus having superior capacitive behaviour of that sample. Figure 9c represents the CV curves of the symmetric SC. The capacitive performance of the HSG device is presented in different galvanostatic charge–discharge current densities and various electrolytes through Figure 9d. The sample G1p9K6-4 having more SSA, revealed a higher P_d and E_d (Figure 9e), and a larger pore volume (Figure 9f) of 2.05 cm³ gr⁻¹ in nano carbons, which ease the ion transport through the mesopores [139].

The graphene nanosheets (GNSs) could be used as electrode material for SC in 2D (also known as graphene paper), or 3D structure. In the case of 2D graphene, the layers, stacking could occur owing to the interplanar $\pi -\pi$ interactions and van der Waals forces between the graphene sheets, which leads to a decrease in SSA, limiting the ion diffusion from the electrolyte, thus decrease in the electrochemical performance. There are various strategies to address this problem such as template-assisted growth [140], adding spacers [141,142], and crumpling the graphene layers [143].

For instance, Wang et al. [141] increased the distance between the graphene layers by incorporating a small quantity of the pillared carbon black (CB) nanoparticles, which are used as a spacer. Therefore, the electrochemical properties of the flexible graphene papers improved owing to the resultant open structure, which eases the charge storage and ion transfer.

Besides, graphene-based hydrogels are another class of electrode material that exhibited excellent electrochemical performance for SC owing to their highly porous structure and abundant active sites. Recently, an FSC device was fabricated using graphene-based composite hydrogel. The composite was obtained through the modification of the functionalized GQDs. The resultant electrode provided an excellent capacitance value of 451.7 F·g⁻¹ and good mechanical flexibility.After 10,000 cycles, 89.0% of capacitance remained [144].

Besides, a redox SC was fabricated using the graphene-based aerogel material as the negative electrode (Nile Blue-conjugated graphene aerogel (NB-GA)), which exhibited excellent pseudocapacitive performance in acidic, neutral and alkaline electrolyte, as well as a high specific capacitance, more than 483 F·g⁻¹ [142]. Incorporating the pseudocapacitive material between the GNSs prevented the agglomeration of the graphene layers, thus increasing the SSA and the supercapacitive performance. Figure 10 [143] shows that the Nile Blue material fulfils the requirements to be suitable as redox-active material including (a) well physical and chemical conjugation with the graphene surface, (b) having electrochemical behaviour, (c) staying stable over the whole range of PH, etc.

3.2. Transition Metal Oxides/Hydroxides-Based Materials

TM-based compounds are the most common redox-active materials, which are used as electrode materials for SCs, mostly HSCs, specifically in oxides and hydroxides form. Generally, there are various TMs, which are mainly used in the structure of TMOs and transition metal hydroxides (TM(OHs)) such as Ni, Fe, Co, Ti, Mo, V and Nb [145].

TMOs are one of the alternative materials for SCs electrodes with high energy and power owing to their higher specific capacitance and lower resistance in comparison with carbon material, as well as their high conductivity [146]. TMOs and CPs are the most common redox pseudocapacitive materials for owing to one of their properties of showing fast-reversible redox reactions, which lead to high specific capacitances and their long operation cycles [9]. There are different TMOs, as B.E. Conway defined, such as RuO₂, Fe₃O₄, MnO₂, NiO, Co₃O₄, IrO₂ [147] as well as V₂O₅ and CuO, [146] in which their charge storage mechanism is based on the faradic behaviour.

Furthermore, there are several examples of various mixed-MOs such as CuO@NiO [148], CuO/Cu₂O [149], Cu/Cu₂O@TiO₂ [150], Fe₂O₃@TiO₂ [151], ZnO@Co₃O₄ [152], Fe₂O₃/Co₃O₄ [153], Fe₂O₃/NiCo₂O₄ [152], Co₃O₄/ZnFe₂O₄ [154], Co₃O₄/NiCo₂O₄ [155], ZnO/ZnFe₂O₄ [156,157,158], Cr₂O₃@TiO₂ [159], NiFe₂O₄/Fe₂O₃ [160], NiO@ZnO [161], and ZnO@ZnCo₂O₄ [162], in which by developing a high surface area and the involving the multiple functionalities in the metal organic framework (MOF), they could be fabricated as electrodes for SCs to provide a high performance. Figure 11 shows various precursors that could be utilized to derive the porous MOs along with the corresponding surface area [163].

Besides, the layered transition metal hydroxide (TM(OH)s) such as Ni(OH)_2, FeOOH, and Co(OH)₂, with a large space between their layers provide suitable the redox reactions, enhancing the capacitance value of the electrodes in SCs. However, the TM(OH)s could offer more charge storage in comparison with their oxide form, the electronic conductivity of their corresponding oxides is higher. TM(OH)s have exhibited lower structure stability, and consequently, lower rate capability or less cycling performance compared to their corresponding oxides [197]. However, there are several strategies to address this limitation and improving the cycling stability of the material, such as layered engineering of the TM(OH)s through the anion and cation doping process. For instance, the cycle life of the α-Ni(OH)₂ can be improved through the partial substitution of other TM cations such as Co²⁺, Ni³⁺, etc., to obtain bimetallic complexes [197].

These species attract the anions to insert into the layer spacing of α-Ni(OH)₂ [198]. The anion insertion improves the cycle stability of the material by altering the structure of the primary α-Ni(OH)₂ that makes the material to act as a pillar to keep structural stability. The intrinsic pillar effect was stated for the first time by Y. Chen et al. [197]. They fabricated an ASSC using MNC and graphene as electrode material (Figure 12a). It was reported that the structural stability of α-(Ni/Co)(OH)₂ was improved owing to the pillar effect of the metaborate groups, which, unlike the external pillar groups in the previous works (such as SO₄²⁻, NO³⁻, and Cl⁻), provides a strong connection between (Ni_0.5Co_0.5)(OH)₂ layers through the strong B-O bonds. (Figure 12b) Besides the enhanced stability, the highly pGr with excellent conductivity led to increasing the E_d and cycling performance (Figure 12c) of the resultant electrode [197].

As it was mentioned in the introduction section, the battery-type TM can be classified into two main groups depending on their electrochemical reaction in HSC device, including intercalation-type and conversion-type (Figure 13) [199]. However, it is worth mentioning that, although TM-based materials are mainly categorized as battery-type materials, they also exhibit capacitive behaviour. For instance, Feng et al. fabricated the MnO_x electrode materials by a simple electrodeposition method, which exhibited excellent capacitive performance through tuning the concentration of complexing agents in the plating electrolyte. They reported that the liquid-phase and solid-phase diffusion were facilitated due to the formation of the highly porous nanostructure and enhancing the contents of hydrous states such as H−O−H, and Mn−O−H for the thick oxide layer [200].

3.2.1. Ruthenium Oxide (RuO₂)

Ruthenium oxide (RuO₂) is one of the most studied TMOs for SC electrodes due to its superior chemical and thermal stability, high metallic type conductivity, broad potential window (1.2 V), highly reversible redox reaction, long cycle life, high theoretical capacitance (1200–2200 F·g⁻¹), and good rate capability [201].

Furthermore, the superior properties of RuO₂ such as its metallic conductivity, field emission behaviour, catalytic activities electrochemical reduction-oxidation properties in both crystalline and amorphous structures, make the RuO₂ to be useful for various practical and theoretical applications, including a thick or thin resistor, ferroelectric films and integrated circuit development, and electronic applications, and the most recent in SCs [202].

In addition, such parameters as annealing temperature, particle size, and the crystallinity affect the pseudocapacitive performance of RuO₂, [115,203], whereas the higher crystallinity of RuO₂ leads to lower specific capacitances because of the compact structure of RuO₂ that limits the insertion/depletion of ions and electrons, resulting to an improved electrochemical impedance [36]. Accordingly, most researches were focused on the amorphous RuO₂ since the faradic redox reactions on both the exterior surface and the bulk of the material deliver a higher specific capacitance [36].

RuO₂ materials are expensive, which could limit their large-scale applications. In order to develop cost-effective materials, the RuO₂ hybrids were coupled with other pseudocapacitive or carbonaceous materials to reduce the amount of RuO₂ without influencing the activity [204]. For instance, a nanosized RuO₂/polyaniline (PANI)/carbon double-shelled hollow spheres nanocomposite was designed through an in situ electro-polymerization process [205].

RuO₂ has three oxidation states as Ru⁴⁺, Ru³⁺, and Ru²⁺ [125,203,206]. The rapid-reversible redox reaction of RuO₂ is shown by Eqquation (2) [207]:

RuO_x(OH)_y + δH⁺ + eH⁺ → RuO_x-δ (OH)_y+δ⋯(0˂δ˃2)

(2)

There are different methods to synthesis the RuO₂ for SC electrode materials. For instance, Zheng et al. [208] synthesized a hydrous RuO₂ through a sol-gel technique, which provided a specific capacitance of 720 F·g⁻¹ at the annealing temperature of 150 °C.

In another research, RuO₂ was prepared through electrodeposition technique for SC application resulting in a stable electrode with a large number of cycles and a specific capacitance of 498 F·g⁻¹ at a scan rate of 5 mV·s⁻¹ [209]. Kim et al. [210] also synthesized RuO₂ loading on carbon via the colloidal method resulting in high specific capacitance of 407 F·g⁻¹ obtained at a 40% RuO₂ loading. The charge storage mechanism of RuO₂ is based on four steps, as the electron hopping (a) within RuO_x·nH₂O, (b) between RuOx·nH2O particles, (c) between electrodes and the current collectors, and d) proton diffusion without RuO_x·nH₂O particles [36,211].

CV curve for RuO₂ was almost rectangular, indicating a suitable capacitive behavior, and compared to another electrode material, RuO₂ displayed lesser ESR, and consequently, higher E_d and P_d than conductive polymer-based SCs and EDLCs. However, their high cost and poor performance at high current densities [104] limit their application, whereas the researchers focused on reducing the loading of RuO₂ via doping with other materials.

The RuO₂-GC electrode was fabricated using the GNSs and CNTs, which were deposited onto CNFs. The combination of the RuO₂ and carbon-based material provided a specific capacitance of 480.3 F·g⁻¹ at 0.6 A·g⁻¹. Furthermore, the composite-based SC showed a high energy density of 30.9 Wh·kg⁻¹ at a power density of 14,000 W·kg⁻¹ [212].

3.2.2. Manganese Dioxide (MnO₂)

MnO₂ is one of the most used electrode materials for SCs due to its superior properties such as outstanding capacitive performance in the aqueous electrolytes, low cost, and low environmental impact, which also make it useful for a broad range of applications such as in biosensor, molecular adsorption, energy storage, ion exchange, and catalysis [213,214].

The lesser cost and higher availability of MnO₂ in comparison with RuO₂-based materials [215,216,217], as well as another superior of properties such as a broad electrochemical potential window, low toxicity and a high theoretical specific capacitance (1370 F·g⁻¹), makes it a promising alternative to replace as electrode materials [218,219,220].

The reversible redox reaction and the existence of various oxidation states for MnO₂ lead to a high pseudocapacitance charge storage mechanism which takes place through two reversible mechanisms involving insertion/reinsertion of protons into the bulk of the electrode and secondly, adsorption/desorption of electrolyte cations onto the electrode surface that occur as the transition between III and IV oxidation states of MnO₂ [219]. Equation (3) represents the charge storage mechanism as follows:

MnO₂ + C⁺ + e⁻ → MnOOC

(3)

The chemical and physical factors, such as the morphology of the MnO₂-based material, play an essential role in the electrochemical performance of MnO_2, such as cyclic stability, which is highly dependent on the crystallinity of the porous MnO₂. The nanocrystalline and the amorphous MnO₂ provide easy accessibility to ion and cation diffusion owing to the porous nature of these compounds [221]. It has been reported that among all crystalline phases, α and δ show the highest specific capacitances [222].

For instance, Godbane et al. [223] considered the effect of the various structure on the charge storage performance of MnO₂, which was prepared in a different allotropic phase such as 1D channels, 2D layers, and 3D interconnected tunnels and reported an optimal specific capacitance 241 F·g⁻¹ for 3D interconnected tunnels and the lowest specific capacitance of 28 F·g⁻¹ for 1D channels. It was also reported that the electrochemical performance was not affected by SSA.

In another study, Sun et al. [224] showed that the energy storage mechanism relied on the number of manganese (Mn) centres as active sites and reported that the enhancement in the percentage of active Mn centres by the decrease in crystal size leads to a higher specific capacitance and in contrast with the previous assumptions, the results in this research showed that the morphological surface to volume ratios, (or SSA) affects the electrochemical performance of MnO₂-based SCs. Furthermore, the physical and chemical factors affect the electrochemical behaviour of the MnO₂, as the hydration state of MnO₂ has a significant influence on the specific capacitance, and the cyclic stability is being dependent on the morphology of the resultant material [225].

Among all various types of MnO₂ nanostructure, the highest E_d and Pd were reported for tubular MnO₂ nanostructure of about 21.1 Wh·kg⁻¹ and 13.33 W·g⁻¹, respectively. Despite these superior properties, SC, including MnO₂-based electrodes, suffers from low specific capacitance values at high charge–discharge rates and insufficient cycling stability owing to MnO₂ dissolution, low conductivity, broad band gaps, and low ionic diffusion constants [226,227].

According to these issues, the flower-like MnO₂/nitrogen-doped graphene nanocomposite was synthesized via a hydrothermal technique, which offered an average specific capacitance of 220 F·g⁻¹ at of 0.5 A·g⁻¹ as well as capacitance retention of 98.3% after 3000 cycles [228].

3.2.3. Nickel Oxide (NiO)

Micro/nanomaterials such as nickel oxide have fascinated a lot of attention by the researcher as electrode material for electrochemical capacitors because of the shortened diffusion paths, fast redox reactions, and a large SSA in the solid phase [229,230,231,232,233].

NiO is one of the other promising pseudocapacitive materials for SCs electrodes owing to its properties such as, the high theoretical specific capacitance of 2584 F·g⁻¹ [234,235,236], thermal stability, chemical stability, facile synthesis, reasonable cost, abundance, and being environmentally friendly [237]. The pseudocapacitive nature of NiO and Ni(OH)₂ are debatable based on previous literature reports [39]. The NiO material can exhibit pseudocapacitive behavior. For instance, the NiO nanoflakes synthesized through the microwave-assisted method displayed a high pseudocapacitive performance as electrode material for supercapacitor, as well as a high conductivity of 33.87 Scm⁻¹ at room temperature, and surface area as high as 206 m²g⁻¹ [238]. In another study, a Co–Ni/Co₃O₄–NiO nano-heterostructures demonstrated a remarkable pseudocapacitance [44].

NiO, as an electrode material, has a lot of benefits in terms of electrochemical strategy, including accuracy, reliability, excellent electrochemical reaction reactivity, and simplicity [237].

NiO was obtained through the transformation of nickel hydroxide using the electrochemical strategy. NiO electrode resulting in this method showed an excellent specific capacitance of 1541 F·g⁻¹ in 1M KOH (at the scan rate of 5 mV·s⁻¹). After 3000 cycles, 91.8% of the capacity retention remained [239]. NiO could be utilized as electrode materials for SCs in the various forms such as nanowires, nanotubes [240], nanoparticles [241], nanoflowers [242], hierarchically nanoporous films [243], nanorods, and hollow nanofibers.

As other electroactive material, the electrochemical performance of the in the NiO material also highly depends on the morphology of the nanostructure while the porous structure provides a large active surface area that corresponds to the further faradic active sites. This structure has also been shown a very short pathway for ions. Besides, the NiO nanorod arrays offer high cycling stability as well as an ultra-high capacitance of 1536 F·g⁻¹ and 2018 F·g⁻¹ at high and low current density [244], respectively, which may be associated with the well-aligned arrays, narrow nanorod and the stable chemical bonding of NiO nanorods on the Ni substrate [237]. Furthermore, NiO nanofibers/Ni [245] provided almost 100% capacitive retention after 8000 cylces, also in the case of NiO nanoflowers [246], 99.7% of the capacitance remained after 1000 cycles.

In another research, a hierarchical shell-core structure was prepared using NiO nanospines and carbon as an inner core through hydrothermal reaction. NiO nanospines offered unimpeded diffusion paths for ion transport that increase the affinity of the electroactive materials to contribute to the redox reactions, whereas the inner carbon core provides physical support to prevent the volumetric variations of NiO nanospines which is associated with its good mechanical property. The obtained electrode showed a high specific capacitance of 1161 F·g⁻¹, a current density of 2 A·g^−1, and even high capacitance retention of 92.4% was obtained after 3000 cycles at a current density of 2 A·g⁻¹ [247]. A sandwich-like nickel oxide/nanoporous nickel/metallic glass (NiO/np-Ni/MG) composite electrode was provided through the free-dealloying method in O₂-rich HF solution. The free-standing electrode showed a high volume-specific capacitance of 745.3 F·cm⁻³ at 0.5 A·cm⁻³ as well as the capacitance retention about 92% after 6000 cycles, which implies on a relatively excellent cyclic stability [248].

3.2.4. Nickel Hydroxide (Ni (OH)₂)

Among TM(OH)s, Nickel hydroxide has recently attracted attention from researchers owing to its excellent properties such as a high theoretical specific capacitance of 2082 F·g⁻¹, good rate capability, ready availability, lower cost, environmentally benign nature, and its excellent thermal stability and high chemical stability in strong alkaline electrolytes [249]. Accordingly, α-Ni(OH)₂ is the most powerful battery-type material for SC application, which is not only used as positive electrode material in nickel-based batteries, but it is also suitable for electrochemical SCs [250,251,252]. The electrochemical performance of nickel-based pseudocapacitor electrode materials is estimated based on the redox reaction of Ni(OH)₂ or NiO in alkaline electrolytes, and it can be shown in two general equations for NiO and Ni(OH)₂ in Equations (4) and (5), respectively [237].

NiO+ OH⁻ ↔ NiOOH+ e⁻

(4)

Ni(OH)₂+ OH⁻ ↔ NiOOH+ H₂O + e⁻

(5)

The supercapacitive performance of the nickel-based materials highly depends on its loading percentage on the electrode, the electrolyte, the morphology and porosity of the material as well as its conductivity and the surface area, whereas the more enhanced depth of the electrochemical process and larger SSA can provide a higher specific capacitance [253,254].

Furthermore, the performance of the Ni(OH)₂ depends on several parameters such as the type of microstructure, the precipitation condition, structural defects, and crystallinity [208,255,256,257], whereas the crystal structure and crystallinity of Ni(OH)₂ have a strong influence on the H⁺ mobility and conducting the charge–discharge rate [258]. For instance, Lang et al. [259] prepared a roughly packed Ni(OH)₂, in which the poor crystallinity nanoflakes were shaped through the chemical precipitation method, and the synthesized sample exhibited a high specific capacitance of 2055 F·g⁻¹. The enhanced capacitance and diffusion rates are due to the availability of OH⁻ ions in the electrolyte for the Ni(OH)₂ nanoflakes.

Furthermore, nanoflower-like Ni(OH)₂ was deposited onto a Ni foam to be used as an electrode for SCs, and the obtained electrode showed an outstanding rate capability with a specific capacitance of 1065 F·g⁻¹ [260]. In another example, amorphous Ni(OH)₂ nano-spheres deposited on a graphite rod and cycled at current densities of 20–70 A·g⁻¹ in 1 M KOH, exhibited a high specific capacitance in the range of 1868–1330 F·g⁻¹ that arises from the amorphous structure and the uneven surfaces [261].

Such as other MOs, besides all the advantages, Ni(OH)₂, also has several drawbacks, like poor conductivity, low stability, and poor cycle performance that occur because of the massive volume alteration during the charging-discharging mechanism. Heteroatom doping of cobalt (Co) and zinc (Zn) could be utilized to improve the electrochemical performance of Ni(OH)₂ [262,263].

Ni(OH)₂ could restrict the rate capabilities due to its semiconductor nature, and the aggregation of Ni(OH)₂ particles leads to decreasing the electrochemical performances. Different nanocomposites have been produced by highly conductive materials such as graphene and activated carbon to address these issues [264]. For instance, the Ni(OH)₂/graphene composites were prepared using a chemical precipitation method in which the loosely 3D structure leads to more active sites and, consequently, more redox reactions. The coupling of graphene leads to enhanced stability and the conductivity of Ni(OH)₂ [265].

The graphene sheet with good conductivity provides a larger surface area, whereas the increased interfacial contact with the electrolyte solution governs the electron transfer and the diffusion rate to the enhanced electrochemical performance. The obtained GNSs/Ni(OH)₂ composite exhibited a specific capacitance of 2053 F·g⁻¹, and 97% of the capacitance remained after 1000 cycles.

Ni(OH)₂ composites could also be coupled with other oxides to increase the electrochemical properties. For example, Co(OH)₂-Ni(OH)₂ composites were synthesized to achieve a higher specific capacitance in comparison with the Ni(OH)₂ or NiO in which this enhancement is attributed to the mesoporous structure and faster ion diffusion in this regular nanostructure [266,267]. After 1000 continuous charge–discharge cycles, the capacitance retention increased from 41.6% for the pure Ni(OH)₂ electrode to 88.2% for the composite-based electrode with the weight ratio of 3:2. The weight ratio of Co(OH)₂/Ni(OH)₂ profoundly affects the SC behaviour [267].

As it was mentioned in Section 2, the phase characteristics of the electrode material could be modulated through the pillar effect.

Recently, Xiao et al. [268] fabricated the pillar-coordinated α-Ni(OH)₂ (Ni-(BO²⁻)-LDH) by exploiting the pillar-coordinated strategy through the etching process of the Ni-MOF precursor, in which the organic ligand was substituted by OH⁻ and BO²⁻ anions. The maintained MOFs structure provided a highly porous frame and abundant channels, which improved the electrolyte diffusion. They have described that the structure of α-Ni(OH)₂ was supported owing to the pillar coordination, which offered a more stable system with a durable redox reaction in strong alkaline electrolyte. The enhanced Ni(BO²⁻)-LDH-4 exhibited a specific capacitance of 1760 F·g⁻¹ at 1 A·g⁻¹, and capacitance retention of 81% after 10,000 charging–discharging cycles. The HSC fabricated using the resultant Ni(BO²⁻)-LDH-4 and 3D-rGO as electrodes, has exhibited a high E_d of 56.5 Wh·kg⁻¹ at the P_d of 0.111 kW·kg⁻¹ (Figure 14) [268].

Besides, the electrochemical performance of the Ni(OH)₂ (NSs) could be improved through growing the material on the carbon-based substrate to take advantage of the highly porous structure of these materials. For instance, Ni(OH)₂ NSs were deposited on the electrochemically activated carbon cloth (ECC) to fabricate the Ni(OH)₂/ECC (HT) composite (ECN-3), which was fabricated through the one-step ethanol-assisted supercritical deposition process (supercritical fluid deposition). Due to the high permeability, good solvent capability, and low surface tension of the supercritical ethanol fluid, it has the ability to propagate the reactant onto the porous structure of CC; thus the Ni(OH)₂ NSs were distributed perfectly on the CC. This provided significant electrochemical performance for the Ni(OH)₂ NSs/ECC flexible electrode. The accessibility of the redox sites to electrolyte ions increased owing to the resultant porous structure and obtained channels. The fabricated flexible solid-state SC (including ECC as the anode, and ECN-3 as the cathode), showed an E_d of 8.3 mWh·cm⁻³ at the P_d of 20 mW·cm⁻³, as well as the capacity retention of 87% after 10,000 cycles at 50 mA·cm⁻² [84] (Figure 15).

3.2.5. Vanadium Pentoxide (V₂O₅)

Vanadium (V) oxide or vanadia is an inorganic-intercalation compound with the formula V₂O₅, which is generally known as vanadium pentoxide. V₂O₅ has attracted a lot of interest from researchers as a suitable electrode for pseudocapacitors in aqueous and organic electrolytes because of its natural abundance, high E_d, low cost, low toxicity, its mixed oxidation states (+2 to +5), ease of fabrication, and high capacitance [269,270]. The specific capacitance of 350 F·g⁻¹ was reported for amorphous V₂O₅ in KCl aqueous electrolyte [271]. V₂O₅ shows an ultra-high theoretical capacitance of 2120 F·g⁻¹ (under the wide potential window of 1 V), that is associated with the higher oxidation state of vanadium, which leads to transfer a higher number of electrons [40]. Nevertheless, low SSA and the low solubility of V₂O₅ limits the specific capacitance improvement. Furthermore, V₂O₅ is diffusely recommended as electrode material for SCs due to its controllable morphology. There are different methods to obtain the various morphology and structure of V₂O₅, such as electrospinning [272], the hydrothermal method [273], template-assisted growth [274], the sol-gel method [275], and so on.

For instance, the V₂O₅ nanofibers were synthesized using the electrospinning method, that provided the specific capacitance of 190 F·g⁻¹ and 250 F·g⁻¹ in 2 M KCl electrolyte and 1 M LiClO₄ electrolyte, respectively [276]. Besides, the V₂O₅ nano-waxberries, nano-wires, and nanoflowers were prepared through the adjustment of the concentration of surfactant using the hydrothermal method [277]. In another study, the V₂O₅ nanoporous were synthesized by a facile capping-agent-assisted precipitation process, which resulted in an interconnected nanoporous network. The resultant nanochannels facilitated the accessibility of ions and improved ion diffusion. The achieved V₂O₅ showed the maximum specific capacitance of 316 F·g⁻¹ and 76% of capacitance retained after 600 cycles [278].

The annealing temperatures have a significant effect on the nanomaterials structure and the electrochemical property of the prepared electrodes using such materials. For instance, the electrochemical performance of the different V₂O₅ films was investigated at different temperature. The specific capacitance of 346 F·g⁻¹ at a scan rate of 5 mV·s⁻¹ was reported for the V₂O₅-based material annealed for an hour at 300 °C which was much higher than those which were annealed at 400 °C (177 F·g⁻¹), 350 °C (226 F·g⁻¹), and 250 °C (241 F·g⁻¹) [279]. In another study, V₂O₅ nanotubes were synthesized through the hydrothermal method, that showed a high specific capacitance of 417.3 mF·cm⁻² at a scan rate of 5 mV·s⁻¹ in 0.5 M Na₂SO₄ and suitable cycling stability with capacitance retention of 80% after 3000 cycles [280].

As it is shown in the previous examples, V₂O₅-based materials suffer from poor cycling performance and slow rate stability, which are due to the structural instability of V₂O₅ during the charge–discharge mechanism, its low conductivity (10⁻² to 10⁻³ S cm⁻¹), and slow electrochemical kinetics. To solve the problem associated with the low electrical conductivity of the V₂O_5, which is the most critical complication, researchers focused on shortening the diffusion length and electrochemical kinetics improvement of the electrolyte [281,282]. Furthermore, carbonaceous materials composites and metal fibres have been prepared to enhance the electronic conductivity of V₂O₅ [283].

In recent decades, to enhance the physicochemical performance of V₂O₅, researchers have focused on the development of the V₂O₅-based nanocomposite materials that can deliver supreme advantages of pure material [284]. Generally, the V₂O₅-based nanocomposites can be categorized into two main groups, including V₂O₅-carbon nanocomposites as well as V₂O₅-metal element nanocomposites such as V₂O₅/CNT [285], V₂O₅/rGO [286], V₂O₅/polypyrrole [287], and V₂O₅/SnO₂ [288].

The E_d and the perdurability of the material could be improved through the synthesis of the composite materials. Heterogeneous structures play a significant role in composites [289].

Recently, the rGO/VO₂ heterostructure was fabricated through a facile hydrothermal process. The resultant material showed a superior characteristic, (in the voltage range between 0.01–3 V (vs. Li/Li+)), to be used as the anode material, due to its high specific capacity, favourable rate capability, which arises from the mixed valency of V ions (V³⁺ and V⁴⁺) within the as-prepared VO₂ matrix that eases the faradic reactions at a low potential. The battery−supercapacitor hybrid (BSH) device was assembled using the rGO@VO₂ as the anode, and the AC@CC as the cathode (Figure 16a). The resultant BSH device provided a high E_d of 126.7 Wh·kg⁻¹ as well as a supreme gravimetric P_d of about 10,000 W·kg⁻¹ (Figure 16b), in the working voltage range between 1 V to 4 V. Figure 16c designates that the 2D VO₂ nanoribbons are tightly attached to the large 2D rGO nanosheet [80].

The graphene-decorated V₂O₅ nanobelts were synthesized using the hydrothermal method, which revealed a high capacitance of 288 F·g⁻¹ at a scan rate of 10 mV·s⁻¹ and 82% of the capacitance remained even after 5000 cycles [290]. Chen et al. [291]. synthesized the V₂O₅-based composites in the nanowire structure, interpenetrating V₂O₅ into conductive porous CNT frames using an in-situ hydrothermal method. The prepared nanocomposite-based electrode exhibited a specific capacitance up to 440 F·g⁻¹ at 0.25 A·g⁻¹. The ASC device developed based on the V₂O₅/CNT hybrid composites showed a high E_d of 16 Wh·kg⁻¹ at a P_d of 75 W·kg⁻¹.

Furthermore, the graphene/V₂O₅-xerogel nanocomposites were fabricated utilizing a hydrothermal approach. The prepared electrode provided a good specific capacitance of 195.4 F·g⁻¹ at a current density of 1 A·g⁻¹. The electrical conductivity improvement was due to the presence of the graphene, which also improved the SSA of the V₂O₅-xerogels approximately twice in comparison with the V₂O₅-only xerogels [292]. Besides, Zhang et al. [293] synthesized the 3D porous V₂O₅ hierarchical spheres consisting of stacked platelets through the precipitation of precursor microspheres in aqueous solution at room temperature and the subsequent calcination. The as-obtained V₂O₅ hierarchical spheres showed an excellent high-rate capability. The porous V₂O₅ hierarchical spheres exhibited a high capacity of up to 473 C·g⁻¹ at 0.2 A·g⁻¹ as a battery-typed electrode.

Furthermore, crystalline α-V₂O₅ powders were successfully exfoliated into wrinkled V₂O₅ nanoribbons through a novel processing method based on specific microwave heating and were used to form a strongly bound V₂O₅/rGO hybrid material using a simple cation-mediated self-assembly mechanism and further thermal annealing treatment. The yield nanoribbons showed a specific capacitance of 423.6 F·g⁻¹ at 1 A·g⁻¹. The SSC developed using the hybrid material provides a high E_d of 58.8 Wh·kg⁻¹ at and an average P_d of 500 W·kg⁻¹ as well as an excellent high-rate capability. The specific capacitance remained nearly constant after up to 1800 charge–discharge cycles [294].

In another study, the reduced GO/vanadium pentoxide (V₂O₅)/PANI (GVP) nanocomposite was fabricated through in situ single-step chemical process, and the as-prepared material showed a high E_d of 54.62 Wh·kg⁻¹, the specific capacitance of 273 F·g⁻¹ (specific capacity of 328.5 C·g⁻¹), and P_d of 1636.5 W·kg⁻¹ at a current density of 1 A·g⁻¹. The symmetric and asymmetric SCs using V₂O₅-based composites as electrode displayed high E_d and a wide voltage window that could compete with batteries [295].

Table 1. A comparison of the recent metal oxide-based electrode materials performance utilized in various types of SC devices. (Cs: specific capacitance).

Nanocomposite	Cs of the Electrode Material at Current Density (A·g−1) or at Scan Rate (mV·s−1)	Cs of the SC at Current Density (A·g−1) or at Scan Rate (mV·s−1)	Ed of the SC	Pd of the SC	Capacitance Retention (%)/Number of Cycles/Current Density (A·g−1) or at Scan Rate (mV·s−1)	Electrolyte	Type of the SC Device	Ref.
MnO2/GPCNs	438 F·g−1 at 0.5 A·g−1	72.4 F·g−1	50.2 Wh·kg−1	516 W·kg−1	67.8/-/50 A·g−1 for the electrode, and 77.8/-/0.5 to 50 A·g−1 for the device	Aqueous 1.0 M Na2SO4	ASSC (MnO2/GPCN-SS//GPCN-SS)	[296]
Mn3O4@NiCo2O4@NiO	1905 F·g−1 at 1 A·g−1	216 F·g−1 at 1 A·g−1	76.8 Wh·kg−1	800 W·kg−1	92/10,000	Aqueous 2 M KOH	ASSC (MNN//AC)	[297]
MnO2-coated graphene paper	385.2 F·g−1 at 1 mV·s−1	76.8 mF·cm−2 at 0.05 mA·cm−2	6.14 mWh·cm−2	36 mW·cm−2	82.2/5000/-	Aqueous 0.1 M Na2SO4	Flexible solid-state ASSC (MnO2-coated graphene paper//graphene paper)	[298]
MnO2/CNTs	1980 Fm−2 at 10 A.m−2	-	177 Wh.m−2	250 W.m−2	87/3000/10 A.m−2	Aqueous 1.0 M Na2SO4	SSC	[299]
CNTs/MnO2	1229 F·g−1 at 1 A·g−1	152 F·g−1 at 0.3 1 A·g−1	84.6 Wh·kg−1	190 W·kg−1	94.4/10,000/- for the electrode, and 91/10,000/- for the device	Aqueous 1 M Na2SO4	ASSC (CNTs/MnO2//AC)	[300]
Mno2/CNTs-60	3.54 Fcm−2 at 1 mA·cm−2	-	93.8 μWh.cm−2	193 μW.cm−2	>98/1000/- for the electrode	Polymer gel PVA-LiCl (6 M)	SSC	[301]
MnO2/CNTs	201 F·g−1 at 20 A·g−1	-	13.3 Wh·kg−1	600 W·kg−1	70/10,000/1 A·g−1 for the electrode	Aqueous 1 M Na2SO4	ASSC (AC//MnO2/CNTs)	[302]
MnO2-AgCNT-CC	up to 325 F·g−1 at 1 A·g−1	-	-	-	-	Aqueous 0.5 M Na2SO4	FSC	[303]
CoMCNFs@MnO2	265 F·g−1 at 0.5 A·g−1	66 F·g−1 at 0.3 A·g−1	19.27 Wh·kg−1	217.51 W·kg−1	98.7/10,000/for the electrode, and 94.9/10,000/for the device	Aqueous 3 M KOH	ASSC (CoMCNFs@MnO2//CNFs)	[304]
Mn3O4/CNFs	300.7 F·g−1 at 5 mV·s−1	-	79.4 mWh·cm−2	-	100/7500/-	Aqueous 1 M Na2SO4	SSC	[305]
Mn3O4/Ni(OH)2	742 F·g−1 at 1 A·g−1	43 F·g−1 at 0.2 A·g−1	15.3 Wh·kg−1	168.8 W·kg−1	-	Aqueous 1 M KOH	ASSC (Mn3O4/Ni(OH)2//AC)	[306]
Porous graphene@Mn3O4	208.3 F·g−1	60.1 F·g−1 at 0.5 A·g−1	30.1 Wh·kg−1	475 W·kg−1	86/2000/-	Aqueous 1 M Na2SO4	ASSC (porous graphene@Mn3O4//AG)	[307]
Zn2SnO4/SnO2/CNT	702 F·g−1 at 1 A·g−1	-	98 Wh·kg−1	1000 W·kg−1	93/15,000	Aqueous 6 M KOH	SSC	[308]
CuO@NiO/PANI/MWCNT	1372 F·g−1 at 5 mV·s−1	-	-	-	83/1500/-	Aqueous 3 M NaOH	-	[309]
NiO/PANI/CNT	356 F·g−1 at 5 mV·s−1	-	-	-	93.4/-/-	Aqueous 0.1 M, 10 mL KCl in acetonitrile	-	[310]
ZnO/MnOx	556 F·g−1 at 1 A·g−1	-	16 Wh·kg−1	225 W·kg−1	96/10,000/-	Aqueous 1 M Na2SO4	SSC	[311]
RuO2-CNFs	188 F·g−1 at 1 mA·cm−2	-	-	-	93/3000/-for the electrode	Aqueous 6 M KOH	-	[312]
RuO2−ITO nanopillars	1235 F·g−1 at 50 mV·s−1	-	-	-	75/4000/-for the electrode	Aqueous 0.5 M H2SO4	-	[313]
Co3O4@RuO2 nanosheet	-	3.42 F·cm−3	1.21 mWh·cm−3	13.29	-	Aqueous 3 M KOH	ASSC (Co3O4@RuO2 nanosheet/Co9S8)	[314]
Co3O4@RuO2 nanosheet	-	4.28 F·cm−3	1.44 mWh·cm−3	0.89 W.cm−3	-	Solid-state PVA/KOH gel	ASSC (Co3O4@RuO2 nanosheet/Co9S8)	[314]
carbon/Fe3O4	536 F·g−1 at 3 A·g−1	368 F·g−1 at 1 A·g−1	42 Wh·kg−1	14.5 k W·kg−1	77/20,000/20 A·g−1	Aqueous 3.5 M KOH	SSC	[315]
Fe2O3/GNSs/CNTs	675.7 F·g−1 at 1 A·g−1	169.5 F·g−1 at 0.5 A·g−1	60.3 Wh·kg−1	-	82.2/5000/5 A·g−1	Aqueous 6 M KOH	All-solid-state ASSC	[316]
PANI/rGO/Fe2O3 @ CC	1124 F·g−1 at 0.25 A·g−1	-	-	-	82/10,000/-	Aqueous 1 M H2SO4	-	[317]
CNT−Fe3O4	373 F·g−1 at 10 mV·s−1	135.2 F·g−1 at 10 mV·s−1	37 Wh·kg−1	10 k W·kg−1	100/15,000/-	Aqueous 1 M Na2SO4	HASSC (CNT−Fe3O4//CNT−Mn3O4)	[318]
CNT−Mn3O4	453 F·g−1 at 10 mV·s−1	135.2 F·g−1 at 10 mV·s−1	37 Wh·kg−1	10 k W·kg−1	100/15,000/-	Aqueous 1 M Na2SO4	HASSC (CNT−Fe3O4//CNT−Mn3O4)	[276]
(Fe,Cr)2O3 oxide layer	45.92 mF·cm−2 at 5 mV·s−1	16.88 mF·cm−2 at 5 mV·s−1	0.57 mWh·cm−2	200 mW·cm−2	97/10 000/-for the electrode, and 90/10 000/-for the device	Polymer gel LiClO4−PC	Flexible solid-state SSC	[306]
WS2@NiCo2O4/CC	2449.9 mF·cm−2 at 1 mA·cm−2	-	45.67 Wh·kg−1	992.83 W·kg−1	85.59/5000/2 mA·cm−2	Aqueous 3 M KOH	Flexible solid-state SSC	[319]
CoO/NiO−Cu@CuO	2035 mF·cm−2 at 2 mA·cm−2	-	69.3 Wh·kg−1	1080 W·kg−1	90.3/5000/	Solid-state PVA/KOH hydrogel	Solid-state aASSC	[320]
NiO@Nanofiber-based composite	-	161 at 1.0 A·g−1	22 Wh·kg−1	491.9 W·kg−1	∼96.0/5000/10 A·g−1	Li gel	Flexible transparent SSC	[321]
Ni(OH)2@CCF	502.00 at 1 A·g−1	131.43 F·g−1 at 0.25 A·g−1	35.78 Wh·kg−1	0.35 k W·kg−1	102.61 even at a bending angle of 180°	Aqueous 6 M KOH	Flexible and wearable SSC	[322]
Ni-Mn-oxides	1215.5 F·g−1 at 2 mV·s−1	196.5 F·g−1 at 1.5 A·g−1	132.3 Wh·kg−1	1651 W·kg−1	97/3000/-	Aqueous 1 M Na2SO4	ASSC (Ni-Mn-oxide//AC)	[323]
V2O5/Pt/conductive printing paper	343 F·g−1 at 10 mV·s−1	160 F·g−1	355 Wh·kg−1	0.2 kW·kg−1	91/5000/-	Urea−LiClO4−PVA (ULP)	Wearable SSC pseudocapacitor	[324]
VO2/graphene@NiS2 hybrid aerogel	1280.0 F·g−1 at 1 A·g−1	222.2 F·g−1 at 2 mV·s−1	60.2 Wh·kg−1	350.0 W·kg−1	86.2/10,000/-	PVA/KOH gel	All-solid state ASSC (VO2/graphene@NiS2 hybrid aerogel//graphene aerogel)	[325]
Gradient oxygen-vacant V2O5/PEDOT	-	614 F·g−1 at 0.5 A·g−1	85 Wh·kg−1	250 W·kg−1	122/50 000	Aqueous 1 M Na2SO4	SSC	[326]

summarizes the electrochemical measurement results of different metal oxide-based electrode materials and their corresponding fabricated SCs using various types of electrolytes (e.g., liquid, solid/quasi solid-state, and redox-active electrolytes). As it is shown in Table 1, various types of supercapacitors, including SSCs, ASSCs, FSCs, and HSCs can be fabricated utilizing different TMOs and TM(OH)s composited with highly porous and conductive materials such as carbonaceous (e.g., CNTs, CCF, CB, etc.) and CPs, which provide rapid ionic diffusion as well as ion transport owing to their inter- and intraplanar pores. In several examples, the electrical conductivity of the TM(OH)s and TM(OH)s composite have been increased by utilizing the 2D nanomaterials (graphene, GPCNs, etc.), which leads to faster charge transfer owing to the 2D structure. The capacitive performance of the composite-based electrode materials and their corresponding supercapacitor was improved by introducing TMOs and TM(OH)s to carbonaceous materials which increased the hydrophilicity and the conductivity of carbon-based materials offering both EDLC and faradic behaviour to the resultant composite. Besides, in comparison with different mixed TMOs, the ternary metal-based nanomaterials (composed of three various MOs), exhibited excellent electrochemical performance owing to the combination of advantages between three components that can display extraordinary structural stability and synergies.

3.3. Conducting Polymer-Based Material (CPs)

Different CPs are another pseudocapacitive electrode material that has been widely researched because of their relatively high capacitance, high E_d, adjustable redox activity through chemical modification, good conductivity in doped states, high voltage windows as well as facile production, low environmental impact, and low cost [327,328,329].

CPs can store the charge in its bulk, since no structural alterations such as phase changes occur during the charge/discharge mechanism. Due to this, CPs can offer a higher capacitance because of the larger surface areas and redox storage capabilities [330,331,332,333].

In comparison with other carbon-based electrode materials, CPs have higher conductivity, capacitance, and low equivalent series resistance [334,335]. The specific capacitances in CPs derive from the fast-reversible redox reactions resulting from the π-conjugated polymer chains [90], in which the ions move to the polymer backbone through the oxidation process, also known as doping, and during the reduction process or de-doping, they are released back into the electrolyte solution. However, despite all the advantages, the reduction-oxidation in CPs causes mechanical stress and consequently limiting the stability and degradation through several charge–discharge cycles [13,336]. One of the other disadvantages CPs which hinders their performance is the low power densities, arise from the slow ion diffusion rates in bulk [337].

Sharma et al. [338] perceived a 50% decline in initial capacitance of the PPy-based SC electrode, just after 1000 cycles. Accordingly, the loss of Cs of PANI nanorods was about 29.5% after 1000 cycles. Therefore, the main concern related to CP-based electrodes is low cycling stability.

There are various methods for the synthesis of CPs, and among all, the oxidation of monomers via chemical or electrochemical approaches is the most commonly used approach [201].

The synthesized polymers arise in either two or three general states. As-formed polymers tend to be in the oxidized or “p-doped” state rather than the reduced or “n-doped” state. In the p-doped state, the polymer backbone is positively charged and exhibits a high electronic conductivity (between l to 100 Scm⁻¹) [339]. Currently, p-dopable polymers are attracting more attention from researchers in comparison with non-dopable polymers due to their stable performances [330].

The CP-based electrodes for SC exist in three various configurations including (a) the p–p configuration in which both electrode materials are the same p-doped polymer (b) the SCs n-p type in which both electrode materials made from the same polymers, but one is positively charged (p-doped) and the other one is negatively charged (n-doped) electrodes and (c) p-doped and p–p′ SCs containing two various p-doped CPs with various oxidation and reduction electroactivities [334,339,340,341,342,343,344,345,346,347]. Notably, the n-p type polymer conformation has massive potential for high E_d and P_d [335].

The CPs electrode materials have limited pseudocapacitors from reaching their potential due to a lack of proficient n-doped conducting material and low cycling stability [334]. Various types of CPs commonly used for SC electrodes, including PANI, polythiophene (PTh), PPy, polyindole (PIND), polyfurane, poly(p-phenylene), poly(p-phenylenevinylene), poly 3,4-ethylenedioxythiophene (PEDOT) besides different derivatives, in which PTh is either p-doped or n-doped, while PANI and PPy can be only p-dopable, and their n-doping potential is much lesser than the reduction potential of the common electrolyte [346,347,348].

Conducting polymer hydrogels are successful candidates for FSC electrodes [349]. There are different strategies to synthesing the polymer hydrogel including (a) in situ polymerization of monomers into the insulating polymer matrix [349,350], (b) introducing crosslinkers through the polymerization process in order to develop the gelation [351]. However, the resultant CP hydrogels fabricated by the first method often provide a favourable mechanical performance; their electrochemical performance is not satisfying. The CP hydrogels synthesized through the second method have shown good electrochemical behaviour. Nevertheless, the processability and fabrication of the self-standing hydrogel with a favorable mechanical performance is difficult. Besides, synthesizing a polymer hydrogel electrolyte, which has both high electronic conductivity, and good electrochemical performance is challenging.

For instance, Yang et al. [352] exploited a simple macromolecular interaction assisted route to synthesize a self-standing CP hydrogel composed by PANI and PEDOT. The resultant hydrogel showed a high mechanical performance, good electronic conductivity, and good electrochemical properties simultaneously, owing to the synergic effect between PANI and PEDOT. The assembled all-hydrogel-state SC device exhibited good areal capacitance of 808.2 mF·cm⁻², as well as a high E_d of 0.63 mWh·cm⁻³ at a high P_d of 28.42 mW·cm⁻³ (Figure 17).

Conducting polymer hydrogels could also be used as the electrolyte in FSCs due to their resistance against the deformation and physical damage. For instance, an FSC was fabricated by inserting the PEI−PVA−Bn-LiCl hydrogel electrolyte, (polyethyleneimine/polyvinyl alcohol/4-formylphe-nylboronic acid/Lithium chloride), within the two MWCNT electrodes, which showed a specific capacitance of 16.7 mF·cm⁻², an enlarged operating potential window of 1.4 V, as well as high cycling stability, and superior mechanical stability [353] (Figure 18).

3.3.1. Polyaniline (PANI)

PANI is one of the most common CPs electrode material owing to its high conductivity, light-weight, superior capacitive performance for energy storage, facile synthesis, and low cost [354]. Furthermore, PANI is mechanically flexible, environment-friendly [355].

Besides these advantages, PANI is susceptible to rapid degradation in performance because of the repetitive charge–discharge cycles, (swelling and shrinkage). To address this problem the combination of PANI with carbon materials was utilized as the PANI layer, and it was coated on MOs/carbon composite to form a ternary hybrid composite (PANI/MOs/carbon) [355], which provided the reinforced cyclic stability for PANI and improved the capacitance as well [356].

Furthermore, PANI shows a wide range of electrochromic properties, such as exhibiting various colours, which is due to its many oxidation and protonation forms. These characteristics make it a useful candidate for the fabrication of electrochromic SCs [335].

Here, Wang et al. [342] synthesized PANI as the active material for an SC through an electrochemical polymerization method. It provided a high specific capacitance of 950 F·g⁻¹ at 1 A·g⁻¹. Here, it was reported that the array in unique nanowire structure leads to the reduction of diffusion paths, also the charge transfer resistance, which allows the material to have a high specific capacitance at high current densities.

The pathway of the polymerization (solution (Sol), emulsion (Eml), interfacial (Int)) affects the capacitance value of the CPs, whereas PANI-Int nanosphere, PANI-Eml nano-petal, and PANI-Sol nanocrystal, [357] were exhibited the Cs of, 300, 424 and 460 F·g⁻¹, the E_d of 15, 21.2, and 23 Wh·kg⁻¹, respectively at P_d of 200 W·kg⁻¹.

3.3.2. Polypyrrole (PPy)

The requirement of developing energy storage devices for high-performance electrode materials can be derived from the structure-controlled conductive polymer hydrogels with mechanical flexibility and adjustable electrochemical properties. PPy is one of these conductive polymers that attract too much attention from the researchers. It has a higher density and greater flexibility compared to other CPs. It can endure a fast redox reaction for the charge storage, as well as showing a high electrical conductivity value between 10 to 500 S·cm⁻¹ [358].

For example, a conductive polypyrrole hydrogels nanostructure was synthesized through an interfacial polymerization technique and obtained the 3D porous PPy network that showed excellent rate capabilities and high specific capacitance of 380 F·g⁻¹ [345].

In another example, by coating the lignin with PPy, the Lignin-PPy composite was prepared through the polymerization of PPy in a process with and without the existence of methyl orange that is resulting in the development of PPy films of nanotubular and globular morphology. The experiment was conducted with the composite of carbon-based materials in nitrogen atoms by pyrolysis in an N₂ atmosphere. It was reported that the SSA of the synthesized materials improved by 10 times more than that of pure carbon materials [359].

3.4. Nanocomposite-Based Materials

Nanocomposite-based electrodes integrated carbon-based material with either CPs or MOs and put them together to achieve a faradaic (chemical) and a non-faradaic (physical) charge storage mechanism in a single electrode.

Pseudocapacitive materials increase the capacitance of the nanocomposite while the SSA increases owing to the presence of the carbonaceous materials. There are various types of nanocomposites for the SC electrodes such as carbon–carbon composites, carbon-MOs composites, carbon-CPs composites and MOs-CPs composites that in the last case CNTs can be used as a supporting backbone for homogeneous distribution of CPs or MOs which leads to producing a high electric double-layer capacitance and pseudocapacitance. The specific capacitance of such categories of electrodes is higher than individual carbon, CPs, or MOs electrodes [63,360].

3.4.1. Carbon–Carbon Composites

The high capacitance in the carbon material arises from the effective SSA, which is reachable to the electrolyte ions. Therefore, improving the SSA of the carbon-based materials leads to having higher energy and P_d in SC. There are various types of carbon materials which can be used to build the carbon-based composites. In the case of graphene, the dispersion is problematic and must be improved. In order to find a solution, non-covalent functionalized graphene was synthesized [81]. Other drawbacks are associated with GO in which the material tends to restack and aggregate due to the presence of the van der Waals forces through the chemical reduction process, which leads to having a less accessible surface area to the electrolyte ions. The restacking issue can be solved by providing the intrapore for the solution using the SWCNTs [67].

The electrode fabricated using the SWCNTs inserted in graphene aerogels provided a high specific capacitance of 245.5 F·g⁻¹ in 5M KOH. The graphene aerogel SC showed excellent cycle stability and a high rate capability [361].

3.4.2. Carbon-MOs Composites

As it was mentioned before, TMOs have low specific capacitance, poor electronic conductivity, and low electrochemical stability. The nanocomposite made of the combination of TMOs and the carbon material can improve the performance of the SCs. Therefore, in addition to the MOs and mixed MOs, there are different composites of MOs and carbon-based material, which has exhibited a higher surface area than those of pure metal oxide and carbon material [362].

One of the most important benefits of CNTs is that they can go through the surface functionalization process by various materials such as CPs and MOs, or they can be oxidized to solve the problem with the presence of some impurities. Accordingly, there is a lot of research carried out to compare the performance of the nanocomposite with pure materials. For instance, the composite of MnO₂/CNT was prepared through a facile hydrothermal method, and the MnO₂/CNT nanocomposite electrode showed a higher rate capability and specific capacitance in comparison with the pure MnO₂ and CNT electrodes. It was reported that the high specific capacitance for the prepared nanocomposite could be due to the high SSA of the MnO₂ and the highly porous structure [363].

In another study, MnO₂/CNS exhibited ultra-high stability, with 96.1% of the capacitance retention at a current density of 5 A·g⁻¹ even after 5000 cycles. It was reported that the superior capacity and cycling stability of MnO₂/CNS were mainly because of the strong coupling between the carbon NSs and the in situ formed MnO₂ nanosheet arrays (NSAs) with large SSA [364]. Furthermore, various hybrid materials such as Co₃O₄/CNTs [365], Co₃O₄/grapheme [366,367] and Co₃O₄/CNFs [368] were synthesized with a huge surface area and consequently an improved electrical conductivity.

Besides, an ASC device was fabricated using MnO₂ and graphene, whereas the pure graphene was used as the anode and the composite of MnO₂-coated/graphene was used as the cathode. The pure graphene electrodes showed capacitance of 245 F·g⁻¹ at a current density of 1 mA while after depositing the MnO₂, the capacitance increased up to 328 F·g⁻¹ at the same current density [337].

A hybrid NiO/GF(graphene foam) nanocomposite was synthesized for the fabrication of a hybrid electrode for ASSCs that displayed a high specific capacitance of 1225 F·g⁻¹ at 2 A·g⁻¹. An ASSC was fabricated using HPNCNTs (hierarchical porous nitrogen-doped CNTs) as the negative electrode and NiO/GF as the positive electrode as well as in the aqueous KOH as the electrolyte, which showed a superior electrochemical performance due to the excellent capacitance of individual NiO/GF and HPNCNTs, and the synergetic effect between the two electrodes. For the resultant device, a high E_d of 32 Wh·kg⁻¹ at a P_d of 700 W·kg⁻¹ has been reported. Even after 2000 cycles, 94% of the capacitance remained [369].

Furthermore, the synthesized CoNi₂S₄/graphene (CNS/GR) nanocomposite exhibited a specific capacitance of 2009.1 F·g⁻¹ at 1 A·g^−1, and the capacitance could be reserved at 755.4 F·g⁻¹ at 4 A·g⁻¹ after 2000 cycles [370]. In another study, Perera et al. [371] fabricated a coin cell type ASC exploiting carbon fiber as a cathode and V₂O₅-CNT as an anode that showed P_d of 5.26 kW·kg⁻¹ and an E_d of around 46.3Wh·kg⁻¹.

Besides, the synthesized CNTs/NCSMnO₂ composites showed a specific capacitance of 312.5 F·g⁻¹ at 1 A·g⁻¹ with capacitance retention of 92.7% after 4000 cycles. The ASC was fabricated using ACs (-ve electrode) and the CNTs/NCS/MnO₂ composite (+ve electrode), which displayed a high specific capacitance and a stable potential window of 1.8 V with E_d of 27.3 Wh·kg⁻¹ at P_d of 4500 W·kg⁻¹ [165].

Another alternative is the RuO₂-based composites in which the composition of RuO₂ and the carbonaceous materials such as AC, CNTs, carbon aerosols, and graphene provides higher capacitive properties and lesser cost in comparison with the pure RuO₂ based electrodes [372,373,374]. The MWCNT/RuO₂ composite was assembled by inserting the RuO₂ into MWCNTs in an acidic solution showing a maximum specific capacitance as high as 628 F·g⁻¹ for different loading of the RuO₂ films [375].

As it was mentioned before, CNTs could be utilized to support the hollow-structured nanoparticles. For instance, one of the main problems of the RuO₂-based electrodes are cracking the RuO₂ structure leading to poor long-term stability, which is due to the developed strain during the charge–discharge cycles [376,377]. In order to address this issue, Wang et al. [378] utilized a novel core–shell-templated method to assemble a CNT-supported hollow-structured RuO₂ nanoparticle (hRuO₂/CNT), in which there was no crack formation reported for the nanocomposite electrode. The fabricated electrode also has provided a high specific capacitance of 655.0 F·g⁻¹ at a current density of 5 A·g⁻¹. Guan et al. [379] fabricated the composite using needle-like Co₃O₄ deposited on graphene as electrode materials for SC. The resultant electrodes show a specific capacitance of 157 F·g⁻¹ at a current density of 0.1 A·g⁻¹.

Furthermore, there many examples of nanocomposites, or the single-phase materials of Ni_xCo_3-xS₄ were synthesized with excellent energy storage properties. For instance, nanowires [380], nanoneedles [381], and (NSs) [382], of NiCo₂S₄/(GO) [383], Ni_0.3Co_2.7S₄, NiCo₂S₄ hierarchical structures [384] and NiCo₂S₄/(CNTs) [385], etc. For instance, NiCo₂S₄ nanotubes were synthesized through the sacrificial templates, which showed the capacitance of 933 F·g⁻¹ at 1 A·g⁻¹ [386].

Ternary NiCo₂S₄ provides more redox reactions due to having a higher electronic conductivity in comparison with Ni_xCo_3-xS₄ that decrease the charge transfer resistance, and consequently, a slighter interior resistance (IR) loss at higher current density. Therefore, a grater P_d and larger rate capability and can be obtained [376]. Besides, Ni–Co–Mn hydroxide nanoneedles were synthesized through a facile hydrothermal technique, which provided a specific capacitance of about 1400 F·g⁻¹, higher E_d and P_d (30 Wh·kg⁻¹ and 39 kW·kg⁻¹, respectively), in comparison with the Ni–Co–S electrodes. There was no report of the capacitance loss even after 3000 cycles [387].

Carbon-based materials and its composites that are used for the SC electrodes can be compared in terms of various parameters, including surface area, pore size, specific capacitance, stability, rate capability, and cost. The composites consist of the MOs, and the carbon materials are one of the powerful candidates for the electrode materials, which can provide an excellent specific capacitance, good performance, and good rate capability as well as stability and less cost [376].

For instance, a ternary nanocomposite has been fabricated using the MnO₂ nanorods and rGO NSs, which were reinforced on the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) (Figure 19a–e). The resultant composite (MGP) was used as electrode material for SC application and showed an improved specific capacitance of 633 F·g⁻¹ at a current density of 0.5 A·g⁻¹. The device remained stable 100% even after and 5000 cycles. The capacitance of the material is associated with both double-layer and pseudocapacitive mechanisms. Figure 19f,g shows asymmetrical CV curves for all different electrode materials (MnO₂, MG(MnO₂/rGO), and MGP), in which a larger area under the CV, dedicated to MGP, is an indication for a higher supercapacitance behaviour MGP compared to MnO₂ and MG [388].

Besides, W. Peng fabricated the NiCo₂S₄/rGO nanospheres composite through a one-step hydrothermal method. The resultant composites showed an excellent specific capacitance of 1406 F·g⁻¹ and outstanding cycle stability of 82.36% after 2000 charge–discharge cycles at the current density of 1 A·g⁻¹, that were higher than the pure NiCo₂S₄ material (792 F·g⁻¹ and 64.77% after the same number of cycles) [389].

In another study, iron cyclotetraphosphate (Fe₂P₄O₁₂)–carbon composite was utilized as an electrode material for SCs. They reported that the phosphorus doping in the porous carbon structure and the presence of the functional groups in the phytic acid-derived carbon had increased the redox couple, thus improving the pseudocapacitive performance of the composite, which attributed to the highly reversible redox reaction. However, capacitive behaviour has been reported as the predominant charge storage mechanism, which mainly occurred in the acidic medium. They have displayed that the charge storage behaviour and the stability of the composite do not only depend on the electrode material but also are highly dependent on the electrolyte selection. Fe₂P₄O₁₂ composite exhibited high stability in the 0.5 M H₂SO₄, a capacitance of 251 F·g⁻¹ at a current density of 1 A·g⁻¹, as well as a constant capacitance without any loss even after 9000 cycles [390].

3.4.3. Carbon-CPs Composites

There are different carbonaceous materials that can be coupled with CPs to form a nanocomposite. For instance, the nanocomposite using the activated carbon as the cathode and CPs as the anode can provide a higher E_d and P_d in comparison with EDLCs and an enhanced cycling performance compared to pseudocapacitors [360].

As an example, a composite electrode was fabricated through the colloidal self-assembly method using graphene-MWCNT-PPY nanofibers [83,391,392,393,394,395].

The freestanding PEDOT-PSS/SWCNTs composites exhibited the specific capacitance of 104 F·g⁻¹ at 0.2 A·g⁻¹, P_d of 825 W·kg⁻¹, and E_d of 7 Wh·kg⁻¹. Furthermore, 90% of the capacitance left after 1000 cycles [396].

In another study, PEDOT/PSS and MWNT composites showed a specific capacitance of 100 F·g⁻¹ [397].

There are various types of electrodes fabricated using PANI. However, the low rate capability and relatively poor cycle stability limit their applications; still, single-phase PANI materials have been extensively considered as promising electrodes for SCs. There are various nanocomposites formed through the coupling of PANI nanostructures and carbon-based materials that show a better performance than that of PANI or carbon-based electrodes in which carbon material plays an important role because they can be considered as the efficient stabilizer for PANI materials with high availability in different morphologies [398].

For example, rGO aerogel-PANI composite was synthesized through the electro-deposition method in which the PANI arrays deposited on the rGO aerogel. Generally, the poor cyclability, E_d, and flexibility of rGO aerogels limit its application. The prepared composite has shown a rich open pore network and high capacitance due to the unique capacitance of the PANI as well as a high conductivity which arise from the cross-linked framework of 3D aerogel [399].

The specific capacitance of the resultant composite was reported of 432 F·g⁻¹ at a current density of 1 A·g^−1, and the stability of the materials in various bending conditions was outstanding. It provided an E_d of 25 Wh·kg^−1, and 85% of the capacitance remained after 10,000 cycles [399]. In another study, an FSC, was fabricated using the prepared composite of PANI/etched carbon fiber cloth (eCFC), which exhibited 88% capacity retention and a higher specific capacitance (1035 F·g⁻¹ at 1 A·g⁻¹), compared to that of PANI nanowires at all current densities [400].

The graphene/PANI nanofiber composite was synthesized through the one-step in-situ polymerization method showing the specific capacitance of 526 F·g⁻¹ at 0.2 A·g⁻¹ current density [401]. The fabricated GNSs/PANI composite showed capacitance of 532.3 F·g⁻¹ at 2 mV·s^−1, and 99.6% capacitance has remained at 50 mV·s⁻¹ [402]. Besides, a CNT hydrogel coupled with PANI has shown a specific capacitance of 680 mF·cm⁻² at 1 mA·cm⁻² [403]. In another study, the synthesized PANI nanorods/graphite NS composite showed a specific capacitance of 1665 F·g⁻¹ at a current density of 1 A·g⁻¹ [404].

Han et al. [405] fabricated electrodes using the composite of PEDOT/PSS and GO in 1M H₂SO_4, showing a specific capacitance of 108 F·g⁻¹ and capacity retention of 78% after 1200 cycles. Furthermore, the symmetric (PPy/PPy) and asymmetric (PPy/AC) SCs have been fabricated utilizing Cladophora algae-derived cellulose as a binder which has exhibited the capacitance in the range of 0.45 F to 3.8 F [406].

In another study, three different composites have been synthesized through in-situ polymerization using GNSs, CNTs, and PANI. The GNSs/PANI composite exhibited a higher specific capacitance of 1046 F·g⁻¹. The capacitance for the GNSs/CNT/PANI and CNT/PANI composites were reported of 1035 F·g⁻¹ and 780 F·g⁻¹. The capacitance reduced only 6% for GNSs/CNT/PANI, 52% GNSs/PANI, and 67% for CNT/PANI after 1000 cycles [407].

Polyphenylene diamine and hydrogen exfoliated GNSs have been prepared, which exhibited a specific capacitance of 248 F·g⁻¹ at 2 A·g^−1, and the assembled electrode showed an E_d of 8.6 Wh·kg⁻¹ and a P_d of 0.5 kW·kg⁻¹ [408].

In another study, the prepared graphene-based PVA composites showed 10 times enhancement in Young’s modulus with a tensile strength 150% better as well as the graphene loading of 1.8 vol % [409]. The PEDOT/PSS CNTs composite showed a specific capacitance in the range of 85 F·g⁻¹ to 150 F·g⁻¹. The E_d was reported up to 0.92 Wh·kg⁻¹, and P_d varied from 100 W·kg⁻¹ to 3000 W·kg⁻¹ [335].

A hybrid 2D MoS₂ NSs and 1D PANI nanowires were used as the SC electrode. The MoS₂/PANI hybrid electrode showed capacitance of about 485 F·g⁻¹ at a current density of 1 mAcm². The higher performance was reported about 812 F·g^−1, which was for the 1:2 ratio of MoS₂ and PANI that can be associated with the synergistic interactions between conductive 2D MoS₂ NSs with a large surface area and 1D PANI. The related E_d to that ratio and also the P_d was reported as high as 112 W·kg⁻¹ and 0.6 kW·kg⁻¹, respectively [410].

3.4.4. MOs-CPs Composites

The MOs/CPs-based material is another alternative composite for electrode material which provides an enhanced electrochemical characteristic in SCs resulting from the compatibility between the MOs and CPs [23,411].

The specific capacitance, rate capability, and cyclic stability can be increased, in comparison with pure MOs and pure CPs material for electrodes, by developing the composites of MOs coupled with CPs, which leads to improved conductivity of the electrodes.

For instance, the PPy coated MoO₃ was synthesized through the in-situ polymerization method, which exhibited the specific capacitance of 110 F·g⁻¹ at a current density of 100 m A·g⁻¹ as well as an E_d of 20 Wh·kg⁻¹ at P_d of 75 W·kg⁻¹. ASC was assembled using the AC as the –ve electrode and the PPy/MoO₃ as the +ve electrode and in 0.5 M K₂SO₄ aqueous solution that showed an E_d of 12 Wh·kg⁻¹ at P_d of 3 kW·kg⁻¹ [412].

The PANI/MnO₂ nanocomposite was [413] prepared through the exchange reaction between PANI, and n-octadecyltrimethylammonium-intercalated MnO₂ in N-methyl-2-pyrrolidone solvent in which the Fourier transform infrared (FTIR) spectroscopy results indicated the desirable interactions between MnO₂ layers and intercalated PANI. It also has been found that the PANI still shows a proper conductivity after insertion into the layered MnO₂. The composite exhibited a specific capacitance of 330 F·g⁻¹ at the current density of 1 A·g⁻¹ that was 187 F·g⁻¹ and 208 F·g⁻¹ higher than pristine PANI and MnO₂, respectively, which is the result of the synergistic effect of PANI and MnO₂. After 1000 cycles, 94% of the initial capacitance remained.

The MOs-CPs composite could be used extensively as an electrode material for both flexible and non-flexible planar SCs to improve different parameters such as the capacitance, E_d, and power density.

As an example, Raj et al. [414] synthesized Co₃O₄-PIND, (cobalt oxide-conducting polyindole), as electrode material for a non-flexible SC, which was fabricated in a free-binder system and showed a specific capacitance of 1805 F·g⁻¹ and 1625 F·g⁻¹ at the current density of 2 A·g⁻¹ at 25 A·g⁻¹, respectively.

There is also various electrode material composite, fabricated using carbon-based material, MOs and CPs, in which the ternary nanocomposite has exhibited improved electrochemical performance compared to those of corresponding individual components as electrode materials for SC.

4. Two-Dimensional (2D) Electrode Materials and Their Modification Methods

Since the tremendous discovery of exfoliated monocrystalline graphitic films in 2004 [415], the 2D material with a layered structure has appealed much interest from scientists due to their fantastic properties.

During 16 years after the successful mechanical exfoliation of graphene from three-dimensional (3D) graphite, various 2D nanomaterial has been obtained through various preparation and exfoliation methods, from different layered precursor materials such as hexagonal boron nitride (hBN) [416], metal nitrides (MNs) [416], TMOs [417], black phosphorus (BP) [418], transition metal dichalcogenides (TMDs) [419], metal-organic frameworks (MOFs) [420], covalent organic frameworks (COFs) [421], layered double hydroxides (LDHs) [422], graphitic carbon nitride (g-C₃N₄) [423], and different MAX phase (e.g., Ti₂AlC) [424], as well as other precursors, which are utilized to obtain the 2D transition metal carbides, carbonitrides, and nitrides called “MXene” [425].

The structure and electrochemical properties (including the energy storage mechanism and etc.,) of various 2D materials, as well as their application as the electrode in SC devices, was explained in detail in our previous study [426] (Figure 20).

As it was mentioned in the previous sections, the efficiency of the SC device could be enhanced mainly by improving the electrode materials and modifying the electrolyte material. Here, we have summarized different modification methods, which provide more interesting outlooks on the study of 2D materials, in addition to their exclusive intrinsic properties. There are some common modification techniques to enlarge the surface area, which lead to enhancing the charge storage ability of the electrode materials, thus improving the properties of 2D materials. These techniques can be categorized into several groups including alloying [427], hybridization [428] intercalation [429], nanosizing, the top-down method [430], vertical/lateral heterojunctions [431], external field modification [432], strain engineering [433], and stacking order of the atomic layers [434].

There are different studies with regards to the enhancement of the capacitive performance of the 2D material through a core-shell formation strategy.

For instance, the hierarchical NiCo₂O₄@polypyrrole core-shell nanowires were embedded onto the hemp-derived carbon (HDC) microfiber as fiber-shaped electrode material for flexible planar SC, for which the as-fabricated electrode (HDC@NiCo₂O₄@PPy) exhibited an excellent rate performance as a specific capacitance of 2055 F·g⁻¹ was attained at 1 A·g⁻¹ that was higher than that of the HDC@NiCo₂O₄ composites which exhibited 1506 F·g⁻¹ under the same conditions. The specific capacitance (742 F·g⁻¹) was remained still higher than that of the HDC@NiCo₂O₄ composites (402 F·g⁻¹ at 50 A·g⁻¹), even after increasing the current density up to 50 A·g⁻¹. After 5000 cycles, 90% of the initial capacity was left. The fabricated fiber-shaped all-solid-state SSC showed high physical flexibility, good stability, and an E_d of 17.5 Wh·kg⁻¹ at P_d of 500 W·kg⁻¹ [435]. The improvements in the pseudocapacitive performance arise from the high electrical conductivity of carbon microfiber and the PPY.

Yim et al. developed the nanospheres in core–shell structure through the electrodeposition method utilizing the 2D arrayed of polystyrene (PS) monolayer as the core and the RuO₂ nanoparticles as the shell, in which the resultant nanospheres showed an excellent capacitive performance including a superior areal capacitance, well bonded to the current collector materials compared to that of planar RuO₂-based electrode (Figure 21) [436].

There are several examples of different 2D materials that have been utilized for SC applications in addition to the mentioned examples in this review so far, and in our previous review, [426] in which we have described and highlighted the development of the 2D electrode materials such as MXene, MOFs, TMDs, COFs, MNs, BP, and perovskite (PSK).

Recently, three new β-ketoenamine-linked COFs have been fabricated using Schiff-base [3 + 3] polycondensations of 1,3,5-triformylphloroglucinol (TFP-3OHCHO) along with three tris(aminophenyl)-presenting derivatives-featuring amino, carbazole, and pyridine linkages, respectively under the solvothermal conditions [437].

β-ketoenamine connected naphthalene based porous covalent organic framework (TFP-NDA-COF) has been developed through a simple Schiff base condensation polymerization reaction between triformylphloroglucinol (TFP) and 1,5-napthalenediamine (NDA). Further, this material was utilized for the SC application in which the resultant electrode showed a specific capacitance up to 379 F·g⁻¹ at 2 mV·s⁻¹ scan rate and 348 F·g⁻¹ at the current density of 0.5 A·g⁻¹. It has shown an excellent supercapacitive behaviour and high chemical stability (75%) even after 8000 cycles which could be associated with the high SSA, high-porosity in the polymer network, the presence of naphthalene moiety for facile π conjugation as well as nitrogen as a hetero atom for redox activity [438].

The synthesized TFP-COFs were used in the SC and for gas storage applications and showed enhanced performance such as excellent crystallinity and superior electrochemical specific capacitance of 291.1 F·g⁻¹ as well as a high BET SSA up to 686 m²·g⁻¹. The improved performance can be associated with the conjugated enamine structures with redox-active triphenyl amino, carbazole, and pyridine units. The presence of more highly planar monomer units in the hexagonal structures of the synthesized COFs provided a stronger quadrupolar interaction with the included CO₂ molecules [437].

It has been reported that the PEDOT-modified DAAQ-TFP COF films exhibited excellent high-rate performance and a better current response in the CV compared to that of pristine DAAQ-TFP COF film that can be associated with the wiring effect of PEDOT chains which ease the electron transport between electrodes and the redox-active sites of the DAAQ-TFP COF. Though, the ion transfer is enhanced through the vertical pore channels by shortening the diffusion length [439].Romero et al. [440] developed the COFs of polyimine with many metal ions, including Fe³⁺, Co²⁺, and Ni²⁺.

There are several disadvantages for COFs, such as less stability and electrical conductivity, which limits their application. A strategy has been exploited to eliminate these restrictions in which the CPs were confined within the porous frameworks that enhanced the cycle stability along with maintaining the mechanical stability and electrical conductivity [364]. For instance, Dichtel et al. [441] obtained a very high-rate charging (10−1600 °C) with higher E_d by exploiting this procedure via a simple electro-polymerization of EDOT into the COF pores.

Chandra et al. developed the COFs [TpPa-(OH)₂, TpBD-(OH)₂] as electrode materials for SCs. Figure 22 [442] illustrates the XRD pattern, the N₂ adsorption curves, and the pore size distribution along with the TEM images of COFs. The resultant TpPa-(OH)₂ presented a specific capacitance of 416 F·g⁻¹ at a current density of 0.5 A·g⁻¹ with 66% Cs retention after 10,000 cycles. The high specific capacitance is ascribed to the specific molecular control of redox-active units in the COF.

Recently, a COF complex was prepared as electrode material for flexible and extensible SCs using a template of hydroxyl-ended hyperbranched polymer (OHP) through a simple solid-state mechanical mixing process. Further, the COF@OHP complex was impregnated on a microporous carbon nanotube film (CNTF) to build a composite membrane (CHCM). The electrode showed superior electrochemical performance, including a high gravimetric capacitance of 249 F·g⁻¹ as well as a superior lifespan in which 80% of initial capacitance remained after 10,000 charge–discharge cycles [443].

Furthermore, to obtain a high power/energy density SC device, the advance electrode materials are required to provide desirable properties depending upon the specific application. There are various approaches to improving the electrochemical properties of the “old” pseudocapacitive material such as MnO₂ and RuO_x to obtain the “advanced” pseudocapacitive material such as TMDs (such as MoS₂, etc.,) and MOFs [426,444] For example, the functionalized of hybridization of 2D-TMDs with various carbon-based supporting materials to increase more active sites, and ease the charge transferability has been reported in several studies [418,445,446,447], (Figure 23) [428].

TMDs have fascinated a lot of interests owing to their excellent physicochemical properties including high surface area, semiconducting ability, low cost, high chemical stability, high mechanical performance, facile synthesis by means of different fabrication techniques such as mechanical exfoliation, direct sonication in solvents, chemical vapour deposition (CVD) and, etc. [419,448,449,450].

For instance, an ASC device was fabricated by using flowerlike MoS₂/GNSs as the negative and MnO₂/GNSs as the positive electrodes in which the MoS₂/GNSs electrode displayed excellent electrochemical performance, such as high specific capacitance of 320 F·g⁻¹ at 2 A·g⁻¹. In addition, the resultant all-solid-state ASC achieved a remarkable E_d of 78.9 Wh·kg⁻¹ at a P_d of 284.1 W·kg⁻¹ [451].

Besides all the advantages, the interlayer interactions and high surface energy of the 2D-TMDs layers result in an increase in the restacking possibility of the layers, thus decreasing the number of active sites [452]. One of the efficient solutions to overcome the above disadvantages as well as increase the TMD activity for real advanced applications is to fabricate the new functional hybrid materials by coupling the 2D-TMDs with a highly conductive supporting material [419,445,453].

Furthermore, boron nitride (BN) is another excellent member of the 2D material, which has exhibited a superior dielectric property, high thermal conductivity. The BN layered material has shown superior stability and excellent dispersion in the organic electrolyte. With all these properties, BN material cannot be utilized alone as the SC electrode material, since its bandgap is large (between 4 eV to 5 eV). However, this drawback could be addressed through the bandgap engineering of the BN materials [454].

Recently, the PANI-coated CNTs has been grown on the exfoliated BN sheets to improve the electrochemical properties of the BN material. The resultant nanocomposite exhibited a specific capacitance of 515 F·g⁻¹ as well as an E_d of 46 Wh·kg⁻¹ in a three-electrode system.After 10,000 cycles, 98% of the capacitance remained. The nanohybrid ternary composite has also been used as electrode material in an ASSC, which provides an excellent E_d of 62 Wh·kg⁻¹ and capacitance retention about 91% after 5000 cycles [455]. In another study, a ternary nanocomposite was synthesized through the hydrothermal method using rGO, exfoliated BN NSs, and Ni(OH)₂. The resultant nanocomposite (rGO/BN/Ni(OH)₂) exhibited a maximum specific capacitance of 349 F·g⁻¹ as well as a high E_d of 48 Wh·kg⁻¹, and a P_d of 118 W·kg⁻¹ in 0.5 M KOH at a current density of 0.5 mA·cm⁻² [456].

BP is another strong candidate that is comprised of phosphorene layers connected by the Van der Waals interaction [457]. One of the attractive characteristics of the BP material is the interlayer distance (0.55 nm), which is larger than that of graphene. This criterion makes the BP material to be suitable as electrode materials, owing to their capability of enhancing the ion intercalation, thus improving the electrochemical properties of the corresponding SC device [458].

Recently, the R-BP/SPC (red-black phosphorus/sulfonated porous carbon) composite was synthesized via the sonochemical method. The hybrid composite was utilized as the electrode in a SSC, which provided a high specific capacitance of 364.5 F·g⁻¹ at a current density of 0.5 A·g⁻¹. They reported superior capacitance retention of 89% after 10,000 cycles [459].

The oxygen vacancy is a significant factor that affects the ionic diffusion and conductivity of electrode materials, where the enhanced number of oxygen vacancies can decline the diffusion resistance [460] as a result, the porous networks can deliver higher SSA and increased the number of oxygen vacancies, thus improving the specific capacity/capacitance.

Similar PSKs exhibit different electrochemical behaviours in various types of electrolytes (pseudocapacitive behaviour and battery-like behavior). The charge storage performance of the PSKs with battery-like behaviour should be calculated by specific capacity (mAh·g⁻¹ or C·g⁻¹) rather than specific capacitance (F·g⁻¹) [461].

The electrode materials with ultrathin nanosheet structures can achieve high specific capacity/capacitance and long-term stability because such structure decreases the strain resulted from the volume change during the charge–discharge process, thus providing more contact between the active material and electrolyte due to the high SSA [462].

PSK material is one of the novel alternative materials, which have attracted a lot of attention as electrode materials for SCs. PSK could be utilized as the anode material for SCs to provide an anion intercalation process. PSK oxides are promising multi-functional materials due to their enhanced physicochemical properties and the controllable oxygen vacancy number, which could be tuned through various techniques to improve the properties of the PSK oxides material as anodes for anion-intercalation SC. The number of oxygen vacancies and the high SSA of PSK oxides could enhance the electrochemical performance of SCs.

The electrical properties of PSKs could be changed through altering the temperature, in which most PSKs achieve a high conductivity at high temperature due to their structure with high oxygen diffusion rate and stability, which are different from that of those metals in the structure [463,464].

For example, the sphere La_0.85Ca_0.15MnO₃ (LCM) and LaMnO₃ (LMO) have been compared in terms of the morphology, chemical properties, specific capacitance and intrinsic resistance of the electroactive material itself. The specific capacitance was reported of 198 F·g⁻¹ and 187 F·g⁻¹ at the current density of 0.5 A·g⁻¹ for La0.85Sr0.15MnO₃ and LaMnO₃, respectively [465].

Recently, the effect of A-site substitution on the crystalline structure of LMO PSK by using Ca or Sr substituentswas studied. The LCM and La_0.85Sr_0.15MnO₃ (LSM) electrodes showed a specific capacitance of 140.5 mF·cm⁻² and 129.0 mF·cm⁻² at 0.5 mA·cm⁻² in an aqueous electrolyte, respectively, which are four times higher than that of LMO (Figure 24) [466].

Sharma et al. prepared strontium titanate as the electrode material for SSC through the sol-gel method. The resultant electrode displayed a high capacitance of 592 F·g⁻¹ at 5 mV·s⁻¹ because of the mesoporous structure, and large SSA, of the electrode materials as well as a high mass transfer rate of electrolytic ions. The fabricated SSC using strontium titanate electrodes and aqueous electrolyte exhibited a high specific capacitance of 212.5 F·g⁻¹ at 0.63 A·g⁻¹, high E_d of 27.8 Wh·kg⁻¹ with a P_d of 1921 W·kg⁻¹. Furthermore, the device showed high cycling stability of 99% after 5000 cycles, and good capacitance retention of 74.5% at 2.5 A·g⁻¹. The flexible and solid-state symmetric cell also exhibited high energy and P_d of 3.62 Wh·kg⁻¹ and 965W·kg⁻¹, respectively [467].

PSK materials are also a powerful candidate for flexible energy storage/conversion, such as flexible PSK solar cells and FSCs [2,468].

Table 2. A comparison of various 2D nanomaterials utilized as the electrode in SC devices.

2D Nanomaterial	Type of the Corresponding Composite-Based Electrode Material	Charge Storage Behavior of the Resultant Composite-Based Electrode Material	Cs of the Electrode Material at Current Density (A·g−1) or at Scan Rate (mV·s−1)	Capacitance Retention (%)/Number of Cycles	Ref.
MXene	CoF/MXene	The faradaic charge storage mechanism of CoF NPs and the EDLC behaviour of MXene flakes resulted in a battery-like charge−discharge behaviour for the resultant composite.	1268.75 F·g−1 at 1 A·g−1	97/5000	[469]
BN	Ternary nanohybrids (mK-BN/CNT/PANI)	The combination of EDLC and pseudocapacitive behaviour. (Pure BN and modified BN exhibited EDLC behaviour mainly. mK-BN showed the maximum pseudocapacitive involvement (28%) with respect to BN and mH-BN. The maximum pseudocapacitive contribution has been reported for the ternary composites.)	515 F·g−1	98/10,000	[455]
h-BN	(h-BN)/rGO	A prominent pseudo nature combined with EDLC was observed due to the EDLC behaviour of graphene and the intrinsic pseudocapacitive nature of h-BN heteroatoms.	∼824 F·g−1 at 4 A·g−1	100/4000	[454]
BP	BP thin film	EDLC behaviour		94.3/50,000	[470]
TMDs	MoS2@HCNBs	The domination of EDLC with slight faradaic pseudocapacitance, which arises from the faradaic charge transfer Mo in MoS2 structure	560 F·g−1 at 0.2 A·g−1	94.4/5000	[471]
PSK	Mesoporous CoTiO3 PSK	Pseudocapacitive behavior of the CoTiO3 microrods	608.4 F·g−1 at 5 mV·s−1	82.3/1950	[472]
LDHs	NiCo-LDHs	The EDLC behaviour and faradic behaviour contribution	1272 C·g−1 at 2 A·g−1	103.9/5000
COFs	pyridyl-hydroxyl functionalized COFs	The domination of EDLC due to the high surface area of COFs (through the formation of the EDLC between the COF surface and the H+ ions) along with a pseudocapacitance, which increased by increasing the number of OH groups	546 F·g−1 at 0.5 A·g−1	100/10,000	[473]
MOFs	NiCo-MOF@PNTs	The pseudocapacitive behaviour, which was derived from the faradaic reactions of Ni2+/Ni3+ and Co2+/Co3+, as well as an extra pseudocapacitance due to the presence of the PNTs.	1109 F·g−1 at 0.5 A·g−1	79.1/10,000	[474]

summarizes the recent advances on the 2D nanomaterials as the electrode for supercapacitor devices as well as their reported charge storage mechanisms. As is shown in Table 2, the electrochemical performance of the supercapacitor can be improved using nanocomposites made by materials, which display different charge storage mechanism to obtain a battery-type nanocomposite. 2D material such as BN NSs and MXene flakes exhibit EDLC behaviour, while MOFs, COFs, and PSK can provide pseudocapacitance. Moreover, the EDLC behaviour of the 2D-based electrode materials can be improved through the incorporation of the carbonaceous materials, as the extra pseudocapacitance can be obtained by the utilization of various TMOs and CPs.

5. Applications and Future Perspectives

Recent studies on the development of various electrode materials, the investigation of the SC operation and their charge storage mechanisms have enabled novel perspectives on the utilization of advanced electrode materials in the asymmetric flexible hybrid supercapacitor (ASFHSC) increasingly in the future. The configuration of hybrid-asymmetric superconductor provides the advantage of having a wide operating potential window, which results from the unique potential range of cathodes and anodes [475]. However, the HSCs exploit the advantages of both battery and supercapacitor electrodes, to provide high energy density and power density, respectively. Still, there are several challenges with regard to the fabrication of such systems, including the cost-efficient and scalable fabrication methods, developing the effective procedures to provide more control on the surface features (e.g., porosity, morphology, etc.) of the materials.

FSCs have commanded a lot of concentration as one of the most important energy storage research focuses owing to their extraordinary capacitive behaviour, higher power density, higher rate capability, and higher cyclic stability in comparison with batteries and higher E_d compared to the traditional capacitors [476], which provide them with a vast range of potential to be utilized in various applications. Each category of SC has its own specific features and applications, such as in portable/wearable electronic devices as well as in computer and mobile phones [2,477], high power sources for heavy lift trucks or cranes [478,479], the self-powered SCs in medical applications (e.g., flexible artificial skin, artificial intelligence, portable or implantable medical/biomedical devices) [480,481], solar applications [482,483,484]. There are several SC companies such as ELTON (Russia), Nesscap (Korea), CAP-XX (Australia), and Nippon Chemicon (Japan), which are developing various kind of SCs for different applications [485].

Generally, the selection of the electrode material is one of the intense issues in the fabrication of an FSCs to obtain the appropriate mechanical strength and a high capacitance using the free-standing and binder-free electrodes, which are the main constituents of FSCs [476]. The electrode material for FSCs, as other classes of SCs, should fulfil the requirements of having a superior electrical conductivity, high tensile strain, large SSA, favourable chemical and thermal stability, high flexibility, wide potential window, and having enormous surface functional groups. For instance, among the carbon-based materials, graphene is one of the most powerful candidates for electrode material in FSCs owing to its large specific surface and high flexibility, which gives graphene the ability to be bent into large angles and provides a wide variety on the microstructures [486,487]. Recently, aerogels exhibited promising properties for 3D printed energy storage applications. For instance, graphene aerogels are commonly exploited owing to their high specific surface area, mechanical properties as well as tunable pore structures and porosity [488]. Therefore, as it has been reported in different studies [476,487] that carbonaceous materials could be used as a backbone, in the composite material along with redox-active materials such as MOs and CPs, or could be integrated with novel 2D materials such as MXene [489,490] to fabricate the appropriate flexible film as the electrode material for flexible/transparent SCs. Furthermore, incorporating the redox-active small molecules and bioderived functional groups into the pseudocapacitive material, as well as their utilization to increase the capacitance of the carbon-based material, have been provided more efficient, lower cost, non-toxic electrodes for SC application through eco-friendly synthesis [491,492].

6. Conclusions

The charge storage mechanisms, electrochemical behaviour and the principles of these materials have been investigated through several examples of recent advances. In this review, we have also described the distinction between the term “asymmetric” and “hybrid” as well as the difference between various electrochemical behaviour for the electrode materials including “battery-type” and “pseudocapacitive” behaviour. Besides, the main characteristic of the energy storage system including capacitance/capacity, operating temperature, energy density, power density, cell voltage, kinetic storage mechanism, I–V characteristics, cycling lifetime, self-discharge, and the nature of electrode materials have been briefly explained. The electrochemical signatures of EDLC, pseudocapacitive, and battery-like materials, including the CVs, GCDs, and i–v relationship, have been investigated. Three main groups of the electrode material include carbon-based material, metal oxides/hydroxides, and conductive polymers. Carbonaceous materials are promising electrodes to provide high specific surface area, favorable chemical and thermal stability as well as low electrical resistance, however their low energy density, which arises from their surface or quasi-surface energy storage, limits their application for large-scale. MOs such as RuO₂, V₂O₅, and MnO₂, exhibited an outstanding improvement as electrode materials (both the cathode and the anode) due to their variety, excellent specific capacitance, and environmental friendlyness. However, their low electrical conductivity is one of the drawbacks, which weakens their rate capability and consequently the P_d of the corresponding material. Conducting polymers have shown facile processability, improved conductivity, favourable flexibility, and advanced E_d owing to their redox-active heteroatoms; however, there are some drawbacks such as volume shrinking/expansion, which restrict the cyclic performance of the CPs during the charging–discharging process. Apart from the mentioned advantage of different materials, still, there are several drawbacks for each group of materials. For example, in the case of the pseudocapacitive materials, the mass loading and the thickness of the active material must be accurately considered, since when the mass loading/thickness of the active material is very low, or the charge–discharge mechanisms are directed at a low current density, the energy performance may be observed as high mistakenly.

The composite-based or hybrid electrodes designed employing 1D, 2D, and 3D nanomaterials exhibited higher SSA, offer shorter diffusion paths for electrons and ions, thus improving the pseudocapacitance. The recent examples of the developed 2D nanomaterial such as hBN, MNs, BP, TMOs, TMDs, MOFs, COFs, MXene, and PSK has been given. Furthermore, we have extensively reviewed the importance of different nanocomposite made of various carbonaceous materials, TMOs, TM(OH)s, CPs, and 2D material, which provided advanced properties for the electrode material such as larger SSA, higher conductivity (to provide a favourable charge transfer kinetics through the high availability for the electrolyte), facile manipulation of morphology (e.g., pore shape, particle distributions/dimensions), higher mechanical, chemical, and thermal stability as well as enhanced electrochemical properties such as higher specific capacitance, energy and P_d for the corresponding SC compared to that of the pure electrode material. In addition to the appropriate material selection, the electrochemical properties of the energy storage devices can be enhanced through the proper combination of the positive and negative electrodes, especially in the case of hybrid devices, where the battery-type and supercapacitor-type materials with various chemical natures are responsible for providing the high energy and high power densities, respectively.