In terms of power and energy density, electrochemical double-layer capacitors (EDLCs), also known as supercapacitors or ultracapacitors, are fast energy delivery and storage devices that fall somewhere between batteries/fuel cells, which have a large energy storage capacity, and conventional dielectric capacitors, which offer a high power output. They can be utilized as supplementary or alternative power sources in several applications, such as electric hybrid cars, portable electronics, standby power systems, and telecommunications equipment [10]. As memory backup devices, EDLCs are used due to their long lifespans and high cycle efficiencies. The charge-discharge mechanism (electrosorption) in an electric double layer on porous electrodes serves as their foundation [11]. Ordered mesoporous carbons (OMCs), which have accelerated ionic mobility inside particles, have demonstrated high capacitance and exceptional rate capability as active materials in EDLC systems. Additionally, OMC electrodes have given researchers a platform to conduct fundamental and methodical investigations of electrochemistry, including the impact of electrode shape on ionic and electronic transports [12]. The electrolyte in an EDLC can be an aqueous or non-aqueous liquid or a solid substance such as a conducting polymer. The charged ions are organized in an array at the electrode surface following Helmholtz [13]. Helmholtz established the theoretical basis for the double-layer phenomena. It is utilized in all electrochemical capacitors to store electrical energy. The word “double layer” was first used by Helmholtz in 1853 to refer to two layers of charge at the interface of two distinct metals. He contrasted the metal-to-metal contact and the metal-to-aqueous solution interface in 1879 [14]. The Helmholtz model is the most straightforward technique for estimating the spatial charge distribution at double-layer interfaces. Ions of opposite signs at a distance of d neutralize the charge of a solid electronic conductor. This idea offsets charges from the solid via stiff layers using a parallel plate capacitor model. According to the Gouy-Chapman or diffuse model, opposing ions concentrate in a liquid around a charged solid rather than being permanently affixed to its surface [15]. Charging and discharging are very reversible due to the non-faradaic electrical system. This allows for reliable cycling up to 10⁶ cycles or more and rapid charging and discharging. Finally, there’s little depletion. Since EDLCs require highly conductive electrodes, their primary drawback is the scarcity of electrode materials [16].

In 1941, David C. Grahame proposed the word “pseudocapacitance”, which was later used by Conway and Gileadi in the 1960s to describe electrochemically reversible capacitance not related to EDLCs [17]. Pseudocapacitors are asymmetric supercapacitors that use carbon cathodes and conducting polymers or metal oxides as anodes [18]. When a voltage is applied to the two electrodes of a supercapacitor, electrolyte ions travel to the oppositely charged electrodes, creating two layers at the point of interface. As a result of pseudocapacitor, the electrolyte’s precisely adsorbed ions penetrate the double layer and transmit charge to the electrodes [19]. It uses redox active materials to store electrical charge and energy, unlike EDLCs. The majority of charge transfer and storage is handled by faradaic processes, whereas the EDLC only contributes a minor portion of the charge [20]. Polymer-based electrodes often have less cycle stability than carbon-based electrodes. When doped, polymers can enhance conductivity on the one hand, but they can alter volume, producing swelling and an increase in electrode thickness, which is a risk for any device [16]. Metal oxides (e.g. RuO₂ and MnO₂) and electrically conducting polymers (e.g. polyanilines, polypyrroles, and polythiophenes) are the most often utilized materials for pseudocapacitors [21].

Hybrid supercapacitors, also known as asymmetric supercapacitors, transcend the limitations of EDLCs and pseudocapacitors to give superior performance. Hybrid supercapacitors store energy using both pseudocapacitances (faradaic technique) and double-layer capacitances (non-faradaic approach) [22]. The study focuses on improving combinations to improve the efficiency of EDLCs while maintaining their extended cycle life. The goal is to combine the power capabilities of traditional capacitors with the benefits of pseudocapacitors, lowering cost barriers and boosting certain performance metrics such as energy density, bringing them closer to battery performance. This strategy aims to bridge the gap between capacitors’ quick charge/discharge capabilities and batteries’ larger energy storage capacity [23]. As portable and wearable technology becomes increasingly prevalent, there is an increased need for multi-functional hybrid gadgets. Combining SCs with energy-collecting devices to form a hybrid device is an efficient way to harvest and store energy. To date, all self-charging hybrid SCs employing renewable energy sources such as mechanical, thermal, hydroelectric, solar, piezoelectric, and triboelectric have been thoroughly investigated [24].

Polyaniline, often known as black aniline, is produced by 1- and 4-coupling of aniline monomers in various oxidation states. Its structure is defined by changing benzenoid and quinonoid ratios, which are investigated using FT-IR spectroscopy, revealing its distinct electrochemical capabilities and usefulness for a wide range of applications [29]. Polyaniline is a highly conductive polymer commonly utilized as a cost-effective agent in electrochemical capacitors due to its changeable oxidation state, stoichiometric composition, ability to become strongly doped and de-doped, and assignment of the largest specific pseudo capacitance [30]. Polyaniline is a key polymer in advanced materials research due to its electron-rich structure, high electrical conductivity, and processing flexibility. Over the last decade, it has received attention for its varied qualities, which make it perfect for use in electronics, sensors, SCs, and other cutting-edge technologies [31]. Polyaniline is widely used in industrial and biomedical applications due to its easy synthesis, affordability, excellent electrical conductivity, biocompatibility, low toxicity, and environmental stability. However, it faces challenges such as low solubility in standard solvents, infusibility, poor processability, and a gradual decline in conductivity over time. To address these issues, various strategies have been developed, including redoping with functionalized organic acids, copolymerizing with PANI derivatives or other polymers, and creating blends or nanocomposites with other materials [32]. The properties of polyaniline are shown in Fig. 2 [33].

Figure 2.

Properties of polyaniline

Polyaniline has been one of the most actively researched conductive polymers in recent years, owing to its ease of manufacture and great thermal stability. PANI can be produced using chemical polymerization by oxidation or electrochemical polymerization. Its characteristics vary depending on its synthesis methods and protonation state. Polyaniline comes in two forms: reduced (leucoemeraldine) and oxidized (pernigraniline). Emeraldine base is the neutral form, which is half reduced and half oxidized. Emeraldine salt, a highly conductive polymer, is produced when proton-doped polyaniline is made with acids. Emeraldine salt is frequently produced via oxidative polymerization in acidic conditions (pH 1–3) using oxidants like potassium dichromate and ammonium persulfate (APS) [32]. Emeraldine is the most conductive kind of PANI. Because EB is semiconducting, it has a lesser conductivity than ES, which is metallic. When the emeraldine base is protonated to generate the emeraldine salt, a structural alteration occurs as a result of proton-induced spin unpairing. As a result, the band becomes half-filled, possibly leading to a metallic state in which each repeating unit has a positive charge owing to protonation, as well as an accompanying counter ion. The conductivity is also modified by the synthesis procedures and settings used. The conductivity of EB has been shown to increase with acid doping [34]. The chemical characteristics of PANI do not change with the dopant since it does not make any connection with the main polymer chain; nevertheless, the dopant just remains near the polymer chain to enable carrier activity [35]. The chemical specifications for polyaniline are mentioned in Table 1 [36].

Heterocyclic organic conjugated polymers include PPy. Its monomer structure is a ring with five members. One atom is swapped out for a nitrogen atom in the five-membered conjugated carbon ring. The 1960s saw the first reports of PPy. Pyrrole’s nitrogen atom also had a role in this material’s conjugation. It is an amorphous polymer with weak strength and solubility characteristics [37]. Due to its low cost and favorable electrical characteristics, PPy is the most often used naturally conducting polymer in energy producers (batteries and solar cells) and in other fields including sensors, actuators, and the biomedical industry [38]. A pyrrole monomer was first chemically oxidized with hydrogen peroxide to produce PPy, a black powdered material. Polypyrrole has insulating qualities when it is undoped and pure. Even though it is doped with halogenic electron acceptors such as iodine or bromine, its conductivity remains constant at 10⁻⁵ s/m. Bulk PPy possesses 15 % crystallinity, with the crystalline component in the monoclinic phase, despite being non-crystalline and amorphous. Electrochemically generated PPy is 1 μm thick and yellow-black. The polymer changes and becomes more stable in air and thermally stable up to 300 °C as the protonation concentration rises. However, the loss of impurity anions may result in heat deterioration [39]. The properties of PPy are shown in Fig. 3. [40]

Figure 3.

Properties of polypyrrole

Conductive polymers are an interesting family of materials distinguished by their ability to transmit electricity, which is uncommon among typical organic polymers. This conductivity is due to their unique molecular structure, the polymer backbone is made up of a conjugated structure of alternating single and double bonds. This conjugation delocalizes π-electrons, allowing charge carriers like electrons or holes to flow freely throughout the material.

Because of its potential for broad and creative applications, this distinct trait has caught the interest of researchers and scientists from a variety of fields, including academia and industry. Polypyrrole is a particularly remarkable conducting polymer. Its broad usage in a variety of applications is because of its high electrical conductivity, environmental stability, and simplicity of synthesis. PPy has found applications in the production of electrical devices like as transistors and diodes, as well as the development of sophisticated sensors capable of detecting chemical or biological molecules with great sensitivity. It is also used in batteries as a conductive material for energy storage, as well as microactuators, which use electrical energy to generate mechanical motion on a small size. PPy is also utilized in antielectrostatic coatings, which minimize static electricity accumulation, and in biomedical applications, including as drug delivery systems and bioactive scaffolds for tissue engineering. PPy may be manufactured utilizing a variety of processes, the most frequent being the polymerization of its monomer, pyrrole. This process can take place by electrochemical oxidation, in which an electric current drives the polymerization reaction, or through chemical oxidation, in which chemical oxidants are used to start and promote the reaction. These methods can be carried out in a range of solvents, including organic solvents and aqueous media, offering versatility in tailoring the properties of the resulting polymer for specific applications. This versatility and PPy’s intrinsic features confirm its position as a critical material in advancing basic research and commercial applications [41].

Polythiophenes are polymers of sulfur heterocycles. Polythiophenes has the formula (C₄H₂S)_n and is solid in color and insoluble. The rings are connected at locations 2 and 5. The alkyl substituents in positions 3 and 4 are composed of poly(alkyl thiophene)(s). These are likewise colorful solids, but they dissolve in organic solvents [42]. Polythiophene shares some structural similarities with PPy. The main distinction is that PTh has a sulfur-substituted group, whereas PPy contains NH. Polythiophene is a low-bandgap semiconductor material. Polythiophene has excellent electrical conductivity, thermal consistency, environmental stability, and processability. Furthermore, the transparency of PTh has resulted in unexpected optical properties, such as photoluminescence. Polythiophene is an intrinsic conducting polymer, featuring conjugated double bonds in the middle [43]. Researchers and industry are very interested in PTh polymer because of its excellent environmental stability, low band gap energy, and better thermal stability. Additionally, the PTh composites attracted a lot of interest due to their fascinating characteristics, which comprised electrical, semiconducting, and optical activity, in addition to their exceptional mechanical properties and processing simplicity. Due to their enhanced interaction with metal electrodes and environmental resilience, they are frequently used in solar cells. Combining the two allowed for the creation of a unique hybrid variety with enhanced electrical characteristics since n-type semiconducting particles are more efficient at transporting holes than PTh matrix. The transparency of PTh also provides notable optical properties [44]. It has been shown that the structure of the thiophene molecules affects the band gap and optical characteristics of completely conjugated cyclo[n]thiophenes, which have been investigated both experimentally and theoretically. Furthermore, it is anticipated that electronic memory devices will make use of PTh and its derivatives. For the past two decades, these materials have been the primary emphasis. In this class of materials, it is simple to inject electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) [45]. The polymerization process is complicated by the inert sulfur present in the thiophene ring, which has a high oxidation potential. PTh has typically been created using three methods: chemical oxidative polymerization, metal-catalyzed coupling reaction, and electropolymerization. Nevertheless, there are benefits to chemical oxidative polymerization over other techniques, such as the ability to choose different monomers and an appropriate catalyst that promotes high yield, simplicity of synthesis, and optimal electrical conductivity. The characteristics of the materials in chemical oxidative synthesis are determined by the type of dopant anion and the reaction circumstances. High electrical conductivity for PTh with iodine vapor postdoping was observed in several experiments [46]. The role of PTh in various fields is shown in Fig. 4.

Figure 4.

Role of polythiophene

For SC electrodes, poly(3,4-ethylenedioxythiophene) (PEDOT) is a very appealing conducting polymer because of its superior electrical conductivity, stability, and electrochemical performance. PEDOT is perfect for advanced energy applications because it works in concert with other conductive materials like graphene or carbon nanotubes to improve energy storage capacity, charge transfer efficiency, and overall electrode durability [47]. Poly(3,4-ethylenedioxythiophene) undergoes oxidative polymerization using an electrochemical current or suitable oxidants (chemically). Of the several possible morphologies of nanostructures, the anisotropic transport and optical characteristics of fibers and tubes have drawn special attention. Using a porous alumina substrate, PEDOT nanowires and nanotubes were created using electrochemical polymerization [48]. Poly (3,4-ethylene dioxythiophene) has higher cycle stability than typical π-conjugated polymers such as polyaniline and polypyrrole due to its lower structural flaws. The ethylenedioxy group replaces the two βH atoms in 3,4-ethylene dioxythiophene (EDOT), allowing polymerization to occur exclusively at the α-site. Unfortunately, PEDOT’s high relative molecular weight yields a low theoretical specific capacitance. Furthermore, the agglomeration of synthesized PEDOT due to the poor solubility of EDOT monomer in the polymerization system utilizing water as a solvent causes a significantly lower observed specific capacitance than the predicted specific capacitance [49]. PEDOT is a polymer that is insoluble in water. By doping polystyrene sulfonate, a water-soluble polyelectrolyte, during the polymerization of PEDOT and creating the conjugated polymer PEDOT/PSS, the solubility issue of PEDOT was resolved [42]. In order to create a hydrophilic polyelectrolyte system that can form films, PEDOT polymers are usually p-doped. Sodium polystyrene sulfonate, a typical doping agent, acts as a charge-balancing counter-ion to PEDOT for polymerization. The p-doped PEDOT/PSS improves conductivity and thermal stability while increasing PEDOT dispersion in aqueous solution. PEDOT’s great cytocompatibility and high charge delivery capability make it a potential material for tissue regeneration [50]. Because of their electrochemical characteristics, conductive polymers are widely used in applications such as drug delivery systems, electrochemical transistors, actuators, electrochromic devices, and smart membranes. Nevertheless, little is known about the basic physical and chemical processes that underlie the doping and dedoping procedures, which modify conductivity by adding or removing charge carriers. To maximize the functionality and dependability of conductive polymer-based devices and increase their potential uses in cutting-edge technologies, it is imperative to clarify these intricate processes [51].