Recent trends in supercapacitor technology; basics, history, fabrications, classifications and their application in energy storage materials

3.1 Electrochemical double-layer capacitors

Electrochemical Double-Layer Capacitors (EDLCs), commonly known as supercapacitors or ultracapacitors, represent a promising class of energy storage devices due to their high power density, rapid charge-discharge rates, excellent cycle life, and environmental sustainability. Unlike batteries, EDLCs store energy through electrostatic charge separation at the electrode-electrolyte interface without involving Faradaic reactions ¹⁰ (Figure 3).

Figure 3:

Components of ELDC.

An EDLC typically consists of two carbon-based electrodes immersed in an electrolyte and separated by a porous separator. Upon applying voltage, ions from the electrolyte form an electrical double layer at the electrode surface, facilitating reversible energy storage. The high surface area of carbon materials such as activated carbon, carbon nanotubes, and carbon aerogels enhances ion adsorption, leading to superior capacitance and performance. ³⁴

The functioning of electric double-layer capacitors (EDLCs) heavily depends on electrolyte performance. KOH and H₂SO₄ aqueous electrolytes move electricity efficiently and resist electrical flow well because of their ideal properties for fast charging. They have a restricted capacity to maintain voltage levels. Organic electrolytes and ionic liquids deliver stronger voltage performance but have higher resistance and weak ion movement through the system. ³⁵

The type of raw material used in EDLC electrodes has a direct impact on power output. People prefer using carbon-based electrodes because they provide great electrical performance at lower costs along with long-lasting resistance to chemical effects during manufacturing steps. Carbon nanotubes and aerogels further offer mechanical strength and improved ion transport pathways, enhancing device durability and energy storage capacity. ^{24 , 26}

Overall, EDLCs have proven to be reliable and environmentally friendly energy storage devices suitable for various applications, including electric vehicles, renewable energy systems, portable electronics, and grid stabilization. However, optimizing electrode structure, electrolyte composition, and system design remains essential for further improving their performance and expanding their practical applicability (Figure 3).

3.1.1 Activated carbons

Activated carbon (AC) is the most commonly used electrode material in electrochemical double-layer capacitors (EDLCs) because of its low cost, high surface area, excellent conductivity, and structural stability. The porous structure of activated carbon consists of micropores (less than 2 nm), mesopores (2–50 nm), and macropores (greater than 50 nm), which collectively provide a large surface area for electrolyte ion adsorption and charge storage. ³⁶ However, not all pores contribute equally to the capacitance of EDLCs. Micropores offer high surface area but often restrict ion diffusion due to their small size, while mesopores and macropores facilitate faster ion transport and improve power density. Therefore, the optimal performance of EDLCs largely depends on balancing pore size distribution to allow better ion accessibility and efficient charge storage. To overcome diffusion limitations in micropores, researchers have applied various modification techniques such as chemical activation (using KOH or H₃PO₄), physical activation (using CO₂ or steam), and surface functionalization to improve ion transport within the electrode. Additionally, the use of advanced electrolytes like ionic liquids and solvents with high dielectric constants further enhances the mobility of ions within the pores (Figure 4a). Thus, the strategic design of activated carbon with a hierarchical porous structure and suitable electrolyte composition plays a vital role in improving the energy storage capacity, power density, and overall performance of EDLCs in energy storage applications. ³⁷

Figure 4:

Schematic representation of carbon-based electrode materials used in EDLCs: (a) activated carbons with micro-, meso-, and macropores for ion storage; (b) carbon aerogels with interconnected porous networks for improved conductivity and surface area; (c) carbon nanotubes exhibiting high electrical conductivity and unique tubular morphology for efficient charge transport.

3.2 Pseudocapacitors

Pseudocapacitors are advanced energy storage devices that combine the fast charge-discharge capability of Electric Double Layer Capacitors (EDLCs) with the high energy density of batteries. ⁴⁵ They store charge through Faradaic processes, involving reversible redox reactions at the electrode-electrolyte interface, rather than purely electrostatic mechanisms like in EDLCs. This unique charge storage mechanism significantly enhances their capacitance and energy density, making pseudocapacitors suitable for a wide range of applications, including electric vehicles, portable electronics, and renewable energy systems. ⁴⁶ Performance values for pseudocapacitors depend heavily on electrode materials made up of conducting polymers, metal oxides, and composite mixtures. ⁴⁷

3.2.3 Carbon-based composites

Carbon-based materials linked with graphene and CNTs together with conducting polymers and metal oxides show great promise as a key way to boost pseudocapacitor functionality. ⁵⁴ The combination of different materials in hybrids enhances electric current movement while creating a larger reaction space for improved stability. The use of carbon additives creates a better distribution of pseudocapacitive materials while boosting their reaction rate in batteries. This technique supports better performance by using multiple materials to overcome individual strengths and weaknesses. ⁵⁵ As a result, laboratory-made pseudocapacitors achieve enhanced power delivery with extended lifespan.

Shown in Figure 5 pseudocapacitor charge storage happens by quick surface-based Faradaic reactions. The data points out the key electrode materials used in pseudocapacitors through three groups: Transition metal oxides with best theoretical charge storage capability, conducting polymers that enable flexible power sources and have high electrical conductivity, and combined materials made from both components that show better outcomes than single options. ⁵⁶

Figure 5:

Schematic illustration of pseudocapacitors showing charge storage mechanism through Faradaic reactions and different types of electrode materials: (a) transition metal oxides, (b) conducting polymers, and (c) composite materials.

3.3 Hybrid capacitors

The energy storage devices known as hybrid capacitors unite Electric double-layer capacitor (EDLC) features with pseudocapacitor capabilities by merging non-Faradaic and Faradaic charge storage techniques. ⁵⁷ The devices present a harmonious output of high energy density alongside high power density and extended cycling lifetime which makes them useful for portable electronics and renewable power systems as well as electric vehicles. ⁵⁸ The three main hybrid capacitor categories known as composite hybrid capacitors, asymmetric hybrid capacitors, and battery-type hybrid capacitors obtain their specific design advantages from different storage mechanisms that utilize distinct electrode materials (Figure 6).

Figure 6:

Schematic representation of charge storage mechanisms in different types of hybrid capacitors: (a) composite hybrid capacitors utilizing carbon materials and pseudocapacitive materials for combined non-Faradaic and Faradaic processes; (b) asymmetric hybrid capacitors using different materials for positive (Faradaic) and negative (non-Faradaic) electrodes; and (c) battery-type hybrid capacitors integrating battery-like Faradaic electrodes with EDLC-based non-Faradaic electrodes for enhanced energy and power storage.

4.1 Electrode

The electrode is a key component responsible for energy storage through either electrostatic charge accumulation or Faradaic reactions. Carbon-based materials, particularly activated carbon and carbon nanotubes (CNTs), are widely used due to their high conductivity, large surface area (1–3,000 m²/g), and chemical stability. ⁶⁸ CNTs, with their mesoporous structure, improve ion diffusion and enhance the electrochemical performance of electrodes. Functionalization techniques like oxidation or defect engineering further increase the specific capacitance of CNTs. ⁶⁹ The performance of electrodes becomes limited by resistance which arises from high pore density requiring specified pore sizes between 2 and 5 nm to support efficient ion transport. ⁷⁰ When CNTs are combined with metal oxides (for instance RuO₂ or Co₃O₄) their energy density together with power density significantly increases. ⁷¹

4.2 Electrolyte

Electrolytes enable ion flow between electrodes that directly affects capacitor capacitance power output density and sustained cycling performance. Aqueous electrolytes consisting of H₂SO₄, KOH, or Na₂SO₄ show high ionic conductivity yet their voltage operating range remains restricted whereas organic electrolytes based on acetonitrile achieve wider operational voltages at the expense of lower conductivity. ⁷² Na₂SO₄ operates as a neutral electrolyte due to its ability to match safety features with high conductivity and stability performance hence rendering it suitable for advanced pseudocapacitor applications. ⁷³ The research community investigates ionic liquids as well as hybrid electrolytes to push beyond energy storage thresholds despite the existing limitations. ⁷⁴

4.3 Separator

The membrane separator comprises pores to stop electrical short circuits but enables ion movement through electrodes. A perfect separator needs both high porosity and chemical resistance as well as minimal electrical resistance. ⁷⁵ Performance of the separator depends on separator thickness because thin separators provide low resistance but can weaken the mechanical structure while thick separators enhance durability at the expense of capacitance reduction. IEMs serve as a membrane alternative but their high price reflects the cost factor. ⁷⁶

4.4 Current collector

The current collectors enable the effective transmission of electrons through the electrodes while maintaining proper electric circuit contact. The field research now investigates carbon materials such as graphene and carbon fabric together with CNT films due to their high chemical stability and flexibility and superior conductivity when compared to traditional metal foils. ⁷⁷ Flexible supercapacitor development utilizes conductive polymers including polypyrrole and polyaniline. ⁷⁸ The device performance over time is enhanced by developing 3D architectures together with hierarchical structures and advanced surface coatings. ⁷⁹

A supercapacitor contains fundamental parts with Figure 7 showing their basic layout used for energy storage. Each electrode in the system uses carbon to create an area that builds electric double layers to store energy. The electrodes receive their electrical charges by soaking the electrolyte solid until energy flows in and out of the battery compartment. The separator layer stops electrical connections yet enables movement of ions across the unit. ⁶⁷ Metal foils and carbon films attached to the device serve as conductors that connect energy between the capacitor system and external electrical circuits. This design lets supercapacitors produce powerful energy quickly with durable performance through many cycles. ⁸⁰

Figure 7:

Essential elements of a supercapacitors appear as the carbon-based electrode alongside the conductive electrolyte and ion-permeable separator and the metal foil or carbon film current collector.

5.1 Sol–Gel method

The Sol–Gel method serves materials science with a flexible technique to create homogeneous pure substances that possess specific features. When microparticles in solution unite into a gel network through aggregation the result is complex material structures. The solvent and precursor selection in the Sol–Gel process depends on material properties and applications through three main methods which include colloids, polymers, and alkoxides. The precursor solution needs activation through acid or base addition to begin chemical reactions that construct the gel network. ⁸³ The speed of gelation depends on both temperature levels and chemical precursor concentration along with reaction duration because elevated temperatures may create structural defects. Gel network development affects the container dimensions while post-gelation processes enhance material characteristics. The Sol–Gel method shows versatility in device creation for optoelectronics as well as catalysis and medical applications in device development. ⁵²

The Sol–Gel procedure which frequently produces transition metal oxides reproduces features mentioned in your outline. The method provides an effective way to produce nanoparticles along with nanorods and nanofibers as well as complex materials. The adjustment of temperature along with solvents and surfactants and reaction time allows researchers to perfect material properties. The fabrication of supercapacitor electrodes happens through a Sol–Gel synthesis process. The production of activated carbon fiber (ACF)-nickel hydroxide materials was achieved through synthesis by Yusin and team. The specific capacitance (Cs) measurement on this composite material showed a value of 380 F g⁻¹ indicating energy storage potential. Scientists analyzing material synthesis understand its fundamental operation through investigations of solution concentration structural morphology and volumetric characteristics of the Sol–Gel process. Researchers enhance technology development in energy storage systems and catalysis together with materials detection through the optimization of synthesis variables. ⁵³

Nanoscale control exists through the Sol–Gel method for the composition and structural elements of thin film networks. NiCo₂O₄ films underwent great improvement through this approach resulting in a specific capacitance of 2,157 F g⁻¹ at a low current density of 0.133 mA cm⁻². These recorded findings hold substantial value because the combination of system factors produces enhanced electrochemical features. The high porosity level of the films extends the electrolytic surface contact area. Charge transport is more efficient. The organized interconnected network framework of the film material makes possible continuous electron and ion transport pathways across its entire structure. The material’s structural feature minimizes the operational limitations from diffusion which enhances the kinetics of redox reactions in electrochemical systems. Sol–Gel deposition also produces nanoparticles with customized morphologies and sizes, improving electrochemical performance. The electrolyte and nanoparticle active sites have a lower mass transfer barrier, which makes it easier for ions to move and electrons to hop around. This makes it easier to store and release charge. NiCo₂O₄ films have great electrochemical properties because they have more holes, clear network topologies, and nanoparticles with the best properties. The retention capacitance of 96.5 % shows that these materials are resilient and stable over lengthy cycles, making them promising energy storage materials. ⁸⁴

5.2 Electro-polymerization (electro-deposition)

Electrochemical co-deposition is a technique that can synthesize nanostructured films from conducting polymers (CPs) like polypyrrole, poly (3, 4-ethylenedioxythiophene) (PEDOT), and polyaniline. This technology allows for fine control over the speed of polymerization and the manipulation of film thickness, resulting in films with qualities that are tailored to the user’s specifications. In addition to not requiring the use of hazardous chemicals, it is dependent on the proper processing conditions. The study by Arai ⁸⁵ used electrochemical co-deposition to manufacture nanostructured films that combined CPs with MWCNTs. The researchers derived electrochemically superior films through their experimental process. The researchers explored the development of ultrathin films through the combination of PEDOT with MWCNTs. Research showed that electrochemical co-deposition allowed the group to achieve a specific capacitance value of 296 F g⁻¹ at a concentration level of 1.6 A g⁻¹. The network structure of PEDOT: MWCNT attaches to various branches which exhibits promising energy storage capabilities for these materials systems. According to research Electrochemical co-deposition stands as a process with strong potential to create nanostructured films that show excellent electrochemical properties. ⁵⁵

5.3 Direct coating

Superconductor electrodes are created by spreading active suspension ingredients directly over the material. In direct coating applications, this technique adds powerful substances onto surfaces to enhance their adhesive and electrical conductive properties. ⁸⁶ Scientists maintain electrical conductivity by using PTFE and four types of carbon black materials. Scientific teams developed better SC electrode materials by applying graphitic carbon coatings over hollow CeO₂ nanospheres. Scientists use this technique to tailor how their electrodes function. The best performance of supercapacitors (SCs) comes when actively charged nickel foam is dipped in a slurry blending active material with PTFE and acetylene black. Scientists expect this development may produce better supercapacitors for electric car batteries and portable devices. ⁵⁶

5.4 Chemical vapor deposition (CVD)

CVD technology enables us to produce graphene and its variants through its main applications. This technology also lets us create two-dimensional transition metal dichalcogenides materials. CVD processes let users create materials with precise shapes and designs that work best for porous materials. The CVD process starts with a gaseous material that goes through high-temperature heating from 800 to 1,000 degrees Celsius to develop solid growth on the chosen surface. The extremely high heat in this environment creates perfectly structured and imperfection-free materials that deliver excellent results. ⁸⁷ The CVD method creates graphene as a two-dimensional form of carbon that offers outstanding physical electrical and thermal capabilities. The three ways to produce graphene from graphite include stripping it through cleavage, chemically extracting it from organic solvents, and reducing graphene oxide derived from graphite oxidation.

Through CVD production graphene becomes larger and yields better performance by showing fewer crystal faults in high-quality graphene sheets. Graphene produced through CVD shows great potential in microelectronic development plus battery and measurement tool applications. CVD-prepared transition metal dichalcogenides (TMDs) along with graphene because these materials show distinct electrical and optical properties. ⁸⁸ CVD are used to fabricate vanadium disulfide (VS2) TMDs at room temperature. They manufactured ideal alloys for electrical storage components and catalysts by applying VS2 directly onto graphene and multi-walled carbon nanotube mixtures. Scientists use CVD with metal catalysts especially MgO to develop nanocarbon mixtures. Using CVD and MgO to activate catalysts Lobiak synthesized mixed materials of MWCNTs and graphitic layers. The framework of these mixed materials passes electronic charges faster through electrical cells. The better electron movement within these devices improves both supercapacitor and battery performance. CVD procedures excel at making both graphene and transition metal dichalcogenides alongside hybrid nanocarbon products. CVD technology lets us change how materials appear and work to help create better electronic and energy storage products for different uses. ⁷⁸

Figure 8, demonstrates three proven ways to prepare supercapacitor electrodes with different production outcomes. The Sol–Gel process turns solution ingredients into solid gel material while controlling their composition and opening space to build more active electric surfaces. In Direct Coating the developer applies active materials directly to the current collector creating an affordable and easy production solution suitable for wide-scale manufacturing. The Chemical Vapor Deposition process allows the production of thin films and nanostructures that attain superior purity and strong connection. It provides dependable results and suitable electrical conductivity required for supercapacitor electrodes. ⁷⁹

Figure 8:

The illustration (Figure 8) shows how supercapacitor electrodes can be made through three synthesis methods: Sol–Gel process, direct coating method, and chemical vapor deposition procedure.

5.5 Vacuum filtration

Scientists use vacuum filtration as an easy approach to make nanocomposites at many scales which suits materials research for supercapacitors and related energy storage technologies. A filter membrane under reduced pressure pushes an active material blend through to form a thin solid layer on its surface. Vacuum filtration enables scientists to make precise changes in nanocomposite creation by monitoring precursor mixing rates and filter operation duration under defined atmospheric pressure settings. ⁸⁹

Adekoya, Adekoya ⁹⁰ created thin PPy-encapsulated graphene films with bacterial cellulose paper through vacuum filtration to make supercapacitor electrodes that show strong electrochemical properties. Through their work, they applied graphene films made by vacuum filtration on nickel foam to boost the electrochemical system’s qualities. Ratsoma, Poho ⁹¹ showed that adjusting vacuum filtration settings would improve graphene placement within materials which increases power storage space and enhances the movement of ions. ^{81 , 82}

5.6 Hydrothermal/solvothermal technique

It is possible to ascribe the hydrothermal process to the potency of superheated aqueous solution distribution. It also provides controlled diffusion inside a confined space. Because it is ideal for creating design materials with high purity, nutrition, quality, chemical control, and physical attributes, the process outperforms alternative approaches. It is a low-temperature sintering phenomena that is simple to apply and efficient. ⁹² Nevertheless, there is not much control over the buildup of nanoparticles using this approach. In the supercritical phase, the solvent’s properties – such as its solubility and dielectric constant – change dramatically. As a result of its high efficiency and response rate, the particle size creates optimal conditions for particle production. Solventothermal synthesis is the term for this process if a solvent other than water is utilized. Zinc sulfide decorated graphene (ZnS/G) nanocomposites for SCs were produced by Ramchandran et al., ⁹³ This approach is used to create a variety of SC electrodes, such as hexagonal NiCo₂O₄ nanoparticles (CoWO₄/Co1-xS), reduced graphene oxide (CoS₂-rGO), and hollow rod-like nanoparticles. ⁹⁴ Hydrothermal production of nickel-cobalt hydroxide nanoflowers was employed by Tang et al. ⁹⁵ for SCs. The artificial flower-like nickel-cobalt double hydroxides demonstrated exceptional specific capacities and a higher rate performance, which is expected given that the material as manufactured is intended for use in batteries. The largest capacity is shown by Ni_0.28Co_0.72 (OH)₂, with values of 206.7  mA h g⁻¹ at 1 mV s⁻¹ and 174.3  mA h g⁻¹ at 1 A g⁻¹, accordingly.

Figure 9 shows basic setups for making electrode materials in supercapacitors through both Vacuum Filtration and Hydrothermal approaches. A vacuum filter system layers active material suspensions directly onto a surface under vacuum pressure for making uniformly coated and adherent electrode films. Researchers use this straightforward method to make electrodes that possess thickness control and flexible or binder-free properties. When precursors face heat and pressure in sealed autoclaves during the Hydrothermal Method they produce nanostructured materials with more surface space and pores. The production process generates complex shapes and strong crystals that enhance how supercapacitor electrodes work during electrochemical tests.

Figure 9:

Schematic representation of (a) vacuum filtration method and (b) hydrothermal method for the synthesis of electrode materials in supercapacitors.

5.7 Co-precipitation method

Many powdered drug makers find the process simple. For precipitation determination, the solvent must exceed the solubility limit, and the temperature must allow quick precipitate separation. Because of the rapid precipitation, sample morphology is difficult to control. To solve this problem, Deepa et al., ⁹⁶ used co-precipitation (CPM) to create pure, high-yield WO₃ nanoparticles doped with copper (3D transition metal). This was done at room temperature. The co-precipitation method for nanoparticle manufacturing has many benefits. We precipitate multiple components from a solution to generate homogeneous, well-defined nanostructures. By carefully regulating reaction parameters, including pH, temperature, and precursor concentrations, nanoparticle characteristics can be tailored. Deepa et al. synthesized copper-doped WO₃ nanoparticles using co-precipitation.

Doping alters the electrical and optical characteristics of the WO₃ lattice by adding impurities. Copper dopants make WO₃ nanoparticles better at conducting electricity and working with electricity. These materials help make battery systems and display technology work better for electrochromics. Research groups make different nanostructure types using the co-precipitation method. Through this approach, we made Ni₃(PO₄)₂@GO composite building blocks. The material shows promising potential in supercapacitor applications because of its high capacitance rating. Ni₃(PO₄)₂@GO composite produces 1,329.59 F g⁻¹ Cs under 0.5 A g⁻¹ current without losing its initial capacity over 1,000 charge-discharge cycles.

Nickel phosphate and graphene oxide deliver effective charge movement and ion movement which produces excellent electrochemical results. We created CoFe₂O₄ magnetic nanoparticles through the co-precipitation method with starting material reactions. Magnetic nanoparticles serve several uses regarding magnetic recording since their magnetic properties enable MRI scans and heat-focused treatment. Co-precipitation offers multiple possibilities to make nanostructures with specific features. Changing reaction settings and precursor selection creates nanostructures that function better for electric storage tools plus catalysts and medical imaging. Research progress in this field may create advanced materials for use.

The Co-precipitation making technique shows its production steps for electrodes in Figure 10 which remains a common synthesis method for supercapacitors. Under specific control measures like pH, temperature and stirring speed we create metal ions from solution while a suitable precipitation agent neutralizes them. The combined hydroxide or oxide compounds develop evenly throughout the synthesis space. By passing through filtration steps, then drying and calcination processes, the precipitate material transforms into a nanostructured form that works well for electrochemistry. This technique provides superior cost savings and scalability while producing exact particle arrangements that are perfect for making superior electrode components.

Figure 10:

Schematically representation of co-precipitation method.

5.8 Dealloying method

Dealloying, also known as the selective solvent method, is a sophisticated, adaptable, and cost-effective way to make core–shell, hollow core–shell, porous, and nanoporous-sized metallic materials. This technique creates a highly linked network by removing the more active element from a binary metallic solid solution. These structures have large surface areas, high mechanical strength, and compressive qualities. ⁹⁷ Ates, Kuzgun ⁹⁸ made flexible all solid-state asymmetric supercapacitors (SCs) with nanostructured electrodes from oxidation-assisted dealloying. As positive and negative electrodes, the study used Co₃O₄ flakes and γ-Fe₂O₃ nanoparticles. This shows how flexible this method of making electrodes is. Dealloying has become a key NPM synthesis process over the past decade. He, Shangguan ⁹⁹ studied the fixed voltage dealloying method to make AuAg alloys with sizes ranging from 2 to 6 nm and 20–55 nm. They found that the process created core–shell structures with diameters of 2–4 nm that were in balance with the Ag⁺/Ag. ⁶⁸ Using this dealloying method, researchers made a CuS nanowire-linked nanoplate network that worked very well for electrochemistry at a range of reaction temperatures. Using dealloying, the author created SC Cu₂O electrodes. A free dealloying method in HF and HCl solutions made an electrode made of CuS and metallic glass. ¹⁰⁰ Vapor-phase dealloying produces porous materials in a universal and sustainable manner. This process selectively removes an alloy’s high vapor pressure component, resulting in 3D bicontinuous open nanoporosity. With vapor-phase dealloying, ⁸⁰ it is possible to get back the whole vaporized part and make large, controllable pore-size parts in two dimensions. First-time use of the oxidation dealloying technique to create Co₃O₄ flakes and γFe₂O₃ nanoparticles for flexible SC electrodes, building on previous work. This groundbreaking study demonstrates the potential of dealloying to produce electrode materials with tailored properties for energy storage. Providing unparalleled control over composition, structure, and characteristics, the dealloying approach holds promise for the synthesis of many nanostructured materials. Progress in this discipline will transform energy storage, catalysis, and sensing technology. ¹⁰¹

Figure 11 shows how dealloying makes nanoporous electrodes for supercapacitors through its chemical process. An alloy with two or more metals is put in a chemical solution where a reactive metal protects the active metal structures while preventing corrosion during the selective etching process. Only the more reactive metal dissolves while the remaining outcome is a hollow metal framework of high surface area. Through slow metal destruction the method produces interconnected metal shapes with an enlarged surface area that functions perfectly for charge storage. The special nanoporous design makes ions transport faster while enabling easy electron flow which produces better electrochemical performance in supercapacitor materials.

Figure 11:

Represented the mechanism of dealloying method.

5.9 Pyrolysis Fangyan

Liu et al., ¹⁰² found direct pyrolysis of kraft lignin to create co-doped nitrogen, oxygen, and sulfur to be cost-effective, low-maintenance, and environmentally friendly. This method synthesizes desired carbon-based chemicals from paper industry waste kraft lignin. Electrochemically, co-doped carbon supercapacitor electrodes work well. Its 91.6 % capacitance retention over 10,000 cycles makes this co-doped carbon-symmetric supercapacitor stable. Its 244.5 F/g specific capacitance at 0.2 A/g indicates good electrode material utilization. At 40.0 A/g, the supercapacitor retains 81.8 % of its specific capacitance, displaying outstanding rate capability. These excellent electrochemical properties demonstrate the energy storage potential of kraft lignin-co-doped carbon composites. The carbon in kraft lignin has a large specific surface area (338–1,307 m²/g). Larger surface areas allow charge storage and ion diffusion in the electrode material, enhancing electrochemical performance. Adding nitrogen, oxygen, and sulfur heteroatoms to the carbon matrix makes it more conductive and increases electrochemical activity, which improves the performance of the supercapacitor. Research teams have created supercapacitors from various raw materials which include rice husk, carbon nanofibers, YCS, bean pulp and nitrogen-doped biocarbons. These resources are inexpensive and easily available as well as earth-friendly copies. Scientists link biomass resources to special production processes that build quality energy storage devices. The direct pyrolysis of kraft lignin creates carbon materials that demonstrate strong electrochemical power for supercapacitors. Research into sustainable energy storage requires improvement of making methods along with alternative raw material sources from biomass materials. ¹⁰³

5.10 In situ polymerization

Researchers ⁷² created AuNP/PANI nanocomposites through in situ polymerization to improve supercapacitor components. HAuCl₄ and aniline assist in generating PANI and AuNP within a matrix by reacting with ammonium peroxydisulfate as the main oxidizer. Research showed that increasing the AuNP amount added to the composite material improved its capacitance. Adjusting the ratio between Au and polymer will enhance the composite capacitance performance. By placing AuNP/PANI activated carbon against regular activated carbon the asymmetrical supercapacitor system performed better results. The best results for supercapacitor performance depend on selecting and placing suitable electrode materials. The unbalanced design raises both energy and power capacity which makes it workable as a practical solution.

The research shows that the AuNP/PANI nanocomposite possesses useful electrochemical qualities for supercapacitor design. The connection of AuNP particles with a PANI matrix lets the composite material store electricity better and transmit it more efficiently. Scientists have examined different materials as well as nanocomposites beyond AuNP/PANI for supercapacitor development. PANI/graphene, NG-PAA/PANI, PANI/TiO₂/graphene nanocomposites and MF/PPy materials represent some of the tested systems. The composite materials stand out as promising options because they bring improved storage area and electrical conductivity together with materials that can be designed to perform better. Researchers continue to develop superior supercapacitor electrode substances out of AuNP/PANI nanocomposites and related materials. Researchers create top-performance battery storage units through better material routines and unique synthesis approaches. ¹⁰⁴

5.11 Electrospinning

The use of MnO₂-coated TiO₂ nanofibers from electrospinning increases the capability of supercapacitor electrodes. The electrospinning process both sets nanofiber shape and controls their energy-storing materials. MnO₂-coated TiO₂ nanofibers show good supercapacitor electrode characteristics. First, the composites’ broad 2.2 V working voltage window maximizes energy storage and device performance. ¹⁰⁵ Nanofibers’ gravimetric capacitance of 111.5 F/g makes them potential supercapacitors. Electrochemical stability makes composites durable and reliable. MnO₂-coated TiO₂ nanofibers are not the only supercapacitor electrode material. TiO₂-carbon nanofibers boost charge storage and rate capacity by combining TiO₂’s huge surface area and carbon’s conductivity. The pseudocapacitive properties of MnO₂ and the conductivity of carbon combine to make MnO₂/carbon nanofibers that have great specific capacitance and cycling stability. Other composites, like PTA, PVA, and GO improve electrochemical performance due to component synergy. MnMoO₄ nanotubes and Ti₃C₂Tx MXene/PAN nanofibers are promising supercapacitor electrodes. These materials are promising energy storage materials because of their high conductivity, changeable form, and large specific surface. Researchers are studying materials and composites to make high-performance supercapacitors for energy storage applications. ¹⁰⁶

5.12 Carbonization method

Carbonizing hierarchical porous carbons (HPCs) by Ajmal, Ali ¹⁰⁷ improves supercapacitor electrode materials. Carbonization heats organic precursors to high temperatures without oxygen to create energy-storing carbonaceous molecules. The high specific capacitance, porosity, and conductivity of HPC composite make it suitable for supercapacitor electrodes. Carbonization has developed numerous supercapacitor-friendly carbon-based electrode materials; besides HPCs. High surface area and pore volume make porous carbon materials effective for ion transport and charge storage. Carbonizing bamboo fibers creates hierarchical pore structures and excellent mechanical properties for high-performance supercapacitors. Jatropha oil cake activated carbon is a renewable supercapacitor electrode precursor. Energy storage is increased by carbonizing organic materials into porous carbon compounds with electrochemical activity and a vast surface area. Birnessite-type MnO₂/carbon composites improve specific capacitance and cycling stability. Palm oil-based supercapacitors save money and the environment by using carbonized palm oil for electrodes. The electrochemical performance of supercapacitors is improved by Co/MnO/CoMn₂O₄@rice husks (RHs) composites, which have a lot of silica and a lot of pores. Carbonized polythiophene (PTh) nanofibers and HPCs have supercapacitor-optimized designs and properties. Porous carbons from laminaria japonica seaweed can make electrodes for eco-friendly energy storage devices. In conclusion, carbonization creates supercapacitor-specific carbon-based electrode materials. Researchers improve energy storage and produce high-performance supercapacitors for various applications. ¹⁰⁸

Figure 12 shows different fabrication methods to make electrodes for supercapacitors together with their specialized structural and electrical benefits. Through the Pyrolysis Method organic precursors transform into porous carbon materials with high electrical conductivity when heated without oxygen present. Producing monomer solutions within composite structures enables excellent electrochemical performance during polymerization on target surfaces. Electrospinning turns liquid or melted polymers into ultrafine nanofiber mats that let ions move fast and build intense capacitance within their 3D network. During Carbonization Method you change organic or polymer sources into carbon-rich conductive parts by heating them to make strong energy storage structures for supercapacitors. ¹⁰⁷

Figure 12:

Schematic representation of synthesis mechanisms for supercapacitor electrode materials: (a) pyrolysis method, (b) in-situ polymerization, (c) electrospinning, and (d) carbonization method.

5.13 Successive ionic layer adsorption and reaction (SILAR)

Ubale, Kale ¹⁰⁹ use the Successive Ionic Layer Adsorption and Reaction (SILAR) method to make the thin Yb₂S₃ supercapacitor electrode material. Depositing precursor ions on a substrate and reacting to form thin films with controlled thickness and composition using SILAR. The Yb₂S₃ thin films developed, showed promising supercapacitor performance, making them suitable energy storage electrodes.

The supercapacitor energy storage system consists of thin Yb₂S₃ sheets and other electrodes. Thin WO₃ films perform well in supercapacitor technology due to superior charge storage capability and strong resistance to electrochemical changes. PANI nanofibers show both good energy transfer and high electrical flow for supercapacitor electrodes. The key materials used for supercapacitor electrodes are Y-doped Sr(OH)₂ and La₂Se₃. Rare earth elements contribute to better charge storage performance in electrical energy devices.

Effective and reliable energy storage appears in birnessite MnO₂ which functions as an electrode material. The materials store electrical charge through pseudo-capacitive reactions at high speed in powerful supercapacitors. The performance of supercapacitor electrodes gets better through the latest manufacturing processes and design methods. To meet the growing need for energy storage in many applications, researchers intend to construct high-performance supercapacitors employing different materials. New materials, such as SILAR-derived Yb₂S₃ thin films, help achieve this goal. ¹¹⁰

5.14 Microwave auxiliary method

One-step microwave-assisted synthesis of high-performance supercapacitor electrode materials in ionic liquid advances materials research. Microwave-assisted synthesis controls material properties, reacts quickly, and heats uniformly. ¹¹¹ This technology easily creates electrode materials with customizable designs and improved electrochemical performance. NiCo₂O₄ nanosheets are suitable for high-power applications because of their specific capacitance of 879 F/g at 0.5 A/g and 343 F/g at 20 A/g. Supercapacitors could use nanosheets for long-term use because they only drop 4.7 % capacitance after 1500cycles. This simple, efficient, and ecologically benign synthesis approach speeds up electrochemical capacitor material development. Microwave-assisted synthesis has produced several supercapacitors besides NiCo₂O₄ nanosheets.

The graphene/PEDOT hybrid improves charge storage and cycling stability by combining the high conductivity of graphene with the electrochemical activity of PEDOT. MoS₂ and MoO₂ work together to make MoS₂/MoO₂@CNT nanocomposites that have great specific capacitance and rate performance. Carbon-supported MnO₂ nanocomposites are another promising supercapacitor electrode. These materials combine MnO₂’s pseudocapacitive properties with carbon’s huge surface area and conductivity to improve charge storage capacity and stability. Hierarchical and large-surface microwave-assisted flower-shaped NiCo₂O₄ microspheres store charge and transport ions efficiently. Supercapacitors made of microwave-assisted zeolitic imidazolate frameworks (ZIF-8) appear promising. With tunable porosity and surface chemistry, these metal-organic frameworks precisely control electrode properties and electrochemical performance. In conclusion, microwave-assisted synthesis produces various and effective supercapacitor electrode materials. By studying different materials and synthesis processes, researchers can increase energy storage and create next-generation supercapacitors. ¹¹²

The two advanced methods that produce supercapacitor electrodes stand out in Figure 13. They are (a) the Successive Ionic Layer Adsorption and Reaction (SILAR) technique and (b) the Microwave-Assisted Synthesis procedure. The SILAR method allows us to grow nanoscale layers on the electrode by taking the material between cation and anion solutions where surface reactions produce thin film structures. The stepwise deposition method allows precise control over film layers which helps make nanostructured materials with superior electrochemical features. Under the Microwave-Assisted Synthesis approach microwave radiation heats reaction mixtures quickly which creates uniform crystals within a short time period. By using these technique scientists achieve more efficient energy use and prepare samples quicker to create superior electrode materials for next-generation supercapacitors.

Figure 13:

Schematic representation of synthesis mechanisms for supercapacitor electrode materials: (a) successive ionic layer adsorption and reaction (SILAR) method and (b) microwave-assisted synthesis method, illustrating the stepwise process involved in each technique.

5.15 Dipping and drying method

The flexible carbon composite electrodes made from carbon nanofibers (CNFs) on cotton fabric improve the flexibility of the flexibility of energy storage devices. Cotton fabric CNFs make lightweight, flexible electrode substrates for conformable and wearable energy storage systems. MnO₂ and activated carbon coatings on CNF-coated cotton fabric increase positive and negative electrode electrochemical activity, respectively. MnO₂ has pseudocapacitive behavior and high specific capacitance, while activated carbon has a high surface area and conductivity, making it an it an ideal electrode material. hybrid carbon-based textile supercapacitors with porous paper and Nafion membrane separators have low self-discharge rates and high positive and negative electrode capacitances of 138 F/g and 134 F/g, respectively. This good performance reveals that supercapacitors can store energy in wearable electronics, smart fabrics, and portable devices. The study also indicates that cheap materials and simple production can generate high-performance supercapacitors. The researchers created a cost-effective and scalable energy storage device production process employing carbon-based and textile substrates.

Other materials and combinations for supercapacitors show potential, besides hybrid carbon-based textiles. High-performance supercapacitors can use ultrafine Fe₃O₄ nanoparticles or graphene on carbon cloth and MoSe₂ on functionalized multiwalled carbon nanotubes due to their high specific capacitance and cycling stability. Graphene fiber electrodes and MnO₂/carbon nanotube/activated carbon composites are effective supercapacitors. These materials can be used to make various high-performance energy storage devices due to their tunable properties, conductivity, and mechanical flexibility. The carbon-based and textile substrates can build flexible and adjustable supercapacitors. Energy storage devices for numerous applications will benefit from advanced material production, electrode design, and device integration. ¹¹⁰

Figure 14 diagram shows the basic steps of the dipping and drying process which makes high-quality supercapacitor materials. A substrate made of current collectors goes into an active electrode material solution or suspension for this process. When submerged the material must be drawn evenly from solution before heating at fixed temperatures which solidifies the coating onto the surface. With affordable setup costs and effortless handling among other benefits this method becomes a preferred choice for mass-producing supercapacitor electrodes (Table 2).

Figure 14:

Schematics view of dipping and drying process.

6.1 Functionalization with metal oxides

Super capacitor performance and storage capacity can be improved when metal oxides are added to electrode materials. Researchers extensively examine the usage of MnO₂, RuO₂, Co₃O₄, Ni(OH)₂, Co(OH)₂, NiO, and their mixtures as metal oxides for supercapacitor improvement. The high capacitance of RuO₂ and additonal charge storage from graphene speed up electronic transfers in RuO₂/graphene substances and TiO₂–SnO₂-doped RuO₂ structures. ^{79 , 84} Both MnO₂/nitrogen-doped graphene and Co₃O₄@NiO plus Ni/Ni(OH)₂ and NiMoO₄/CC metal oxide hybrids exhibit advanced electrochemical performance when their structure is enhanced.

Studies prove that electrochemical behavior of MnO₂@NiO, NiO@Co₃O₄, and similar composites improves when they use 6-amino-4-hydroxy-2-naphthalenesulfonic acid-modified reduced graphene oxide (ANS-rGO) for surface modification. ⁸⁰ Liu et al. demonstrated in their study that Sol–Gel NiO/MnO₂ composite technology reached a specific capacitance of 375 F/g including stable cyclic performance to 97.5 % through 1,000 cycles. ⁸¹ Hydrothermal fabricated NiO/Co₃O₄ composite materials retained high energy levels across different charging rates according to Ref. 82] while MnO₂/CuO nanocomposites achieved 167.2 F/g of energy output and maintained stable cycles. ⁸³

Advanced composites like ZnO@MnO₂/Ni, fabricated via hydrothermal and electrodeposition techniques, achieved a high specific capacitance of 586.8 F/g at 2 A/g. ⁸⁴ Flexible electrode structures using ZnO@C@NiO composites synthesized through chemical bath and hydrothermal methods were reported to enhance stability in solid-state devices. ⁸⁵ Additionally, Sol–Gel synthesized RuO₂/MWCNT and RuO₂/fullerene nanocomposites showed excellent cycle stability (∼100 %) and high capacitance, making them suitable for next-generation supercapacitors. ⁸⁶

TiO₂–SnO₂-doped RuO₂ composites prepared via wet ball milling and precipitation exhibited enhanced electrochemical performance with a specific capacitance of 571 F/g, ⁸⁴ while NF/NiMoO₄/NiMoO₄ composites achieved a remarkable capacitance of 2,121 F/g, demonstrating their suitability for high-performance energy storage. ⁸⁷ ZnO–NiO composites synthesized through co-precipitation and hydrothermal processes further boosted capacitance performance. ⁸⁸ Lastly, SnO₂/Co₃O₄/rGO composites synthesized via co-precipitation exhibited a specific capacitance of 317.2 F/g, highlighting their potential as efficient electrode materials for supercapacitors. ⁸⁸

These findings collectively emphasize that the integration of metal oxides and functionalized nanocomposites significantly enhances the electrochemical characteristics, stability, and specific capacitance of supercapacitor electrodes, paving the way for advanced and cost-effective energy storage solutions across diverse applications.

6.2 Functionalization with polymers

Polymer-based nanocomposite supercapacitors have shown enhanced electrochemical performance through material functionalization techniques (Figure 15). Various polymer composites like carbon/PDA/PMA, NiMoO₄/PANI/CC, PMA-modified pinecone biochar carbon, and hybrid carbon/PMA have been developed to improve specific capacitance, stability, and energy storage properties in supercapacitor electrodes. ⁷³ Ajami et al. utilized pulse potential electrodeposition for the synthesis of MnO₂/poly(o-aminophenol) composites, achieving excellent capacitance and improved electrochemical stability due to synergistic effects between MnO₂ and the polymer matrix. ⁷³ Similarly, graphene/polythiophene (GR/PT) composites were fabricated via spray deposition, providing uniform coating and superior electrode properties for supercapacitor applications. ⁷¹

Figure 15:

Polymerization & coting techniques.

Further developments include MnO₂-doped PANOA composites with a specific capacitance of 365 F/g, and in situ oxidative polymerization of polyoxomolybdate/polyindole composites exhibiting rapid charge-discharge rates and high cycle stability. ⁷³ Anjana et al. synthesized PANOA/MnO₂/MWCNT composites, achieving a remarkable capacitance of 560 F/g, highlighting the potential of hybrid electrode materials. ⁷³ Zhang et al. applied the Sol–Gel method to prepare carbon-PDA/PMA composites with 101 F/g specific capacitance and excellent cycling stability over 10,000 cycles. ⁸³

Oliveira et al. fabricated MWCNT@MnO₂@PPy composites using in situ polymerization, demonstrating good capacitance (272.7 F/g) and improved stability due to hierarchical structure. ⁸⁹ Su et al. synthesized PANI/TiO₂/GO composites with capacitance values of 1,020 F/g at 2 mV/s and excellent cyclic stability, suitable for advanced supercapacitors. ⁹⁰ Gao et al. prepared NiMoO₄-PANI composites through chemical polymerization, achieving superior conductivity, cyclic stability, and high capacitance. ⁹¹

Ramesh et al. employed hydrothermal synthesis to create MnO₂@NGO/PPy composites, enhancing capacitance from 360 F/g to 480 F/g by introducing PPy to improve charge storage. ⁹² Feng et al. synthesized PPy/npcM composites using the Hummer method, enhancing the specific capacitance of the electrode material due to synergistic interactions between PPy and nitrogen-doped carbon nanosheets. ⁹³ Wang et al. fabricated P–CeO₂/PPy composites via in situ chemical oxidation, improving capacitance and charge storage efficiency through functionalization with PABA. ⁸²

Finally, Yu et al. demonstrated the superior performance of PANi-TiO₂ composites (306.5 F/g at 20 mV/s) synthesized via electrochemical deposition and electrospinning, benefiting from the combined properties of PANi’s conductivity and TiO₂’s stability and surface area, leading to advanced supercapacitor electrode materials. ⁸⁰

The illustration 15, shows different ways manufacturers make supercapacitor electrodes using polymerization and coating methods. After creating conductive polymers by monomer polymerization the resulting material is coated onto a base to make an electrode. Monomer molecules connect chemically to create polymer chains during polymerization which happens through chemical bonds or using procedures like electrochemical bonding and photo polymerization. The resulting polymers are applied to the electrode by different coating techniques including spin coating, dip coating, and spray coating. Once applied these production steps let thin polymer films develop that conduct electricity well and take less space while giving better battery efficiency.

9.1 As a source of energy

Supercapacitors boost UPS system performance by enhancing stability in energy storage applications. UPS systems preserve power supply and give users the ability to charge batteries plus prepare for emergencies. UPS supercapacitors have many benefits. These cells store and release energy at high speed to solve power interruption and instability problems. The fast transition is essential to turn on emergency generators right after power breaks. Super capacitors deliver large bursts of energy in a short time thanks to their concentrated power and qualify for UPS tasks that need fast electricity. UPS units require 12–24 V of power which supercapacitors deliver perfectly. Super capacitors operate best in this voltage range and work perfectly for power backup systems. Supercapacitors also last longer than batteries, decreasing UPS maintenance and lifecycle costs. UPS systems using supercapacitors are more reliable, efficient, and perform better, protecting important loads from power outages. UPS units and other critical infrastructure systems will use supercapacitors more as energy storage technology advances. ¹³⁶

9.2 Electrochromism

Electrochromism, a voltage or electrical charge that causes a physical change in color, has exciting uses in supercapacitors. WO₃ (tungsten trioxide) is a potential material for electrochromic supercapacitors due to its vast transparency differences between white and blue. Ashraf et al. investigated WO₃’s smart functionality in SCs. Cai et al. discovered that the voltage or EES color shift of an electrode can indicate the smart functioning of SCs. Zhang et al. created a smarter electrode using electro-deposited WO₃ (e-WO₃). Electrochromic optical density measurements were used to monitor the WO₃ film-based SC color change. Interestingly, EES and optical density were linearly related, allowing EES indicators to evaluate successful color-shift-based SCs. With this new method, combining WO₃ with other materials allows for hybrid SCs. WO₃’s electrochromic characteristics may improve hybrid SC performance and usefulness for specific applications. These advances show the potential of electrochromic supercapacitors in energy storage, smart devices, and wearable electronics, where color-changing capabilities can reveal device status and energy storage capacity. Further research may lead to next-generation electrochromic supercapacitors with improved performance and functionality. ¹³⁷

9.3 Electric composite automobiles

Electric composite automobiles use supercapacitors for energy efficiency and environmental friendliness. For acceleration, these vehicles store kinetic energy from braking in their supercapacitors, improving energy efficiency and minimizing fuel use. Electric composite automobiles have zero emissions, making them more environmentally beneficial than gasoline and diesel cars. Ningbo CSR New Energy Technology’s electric buses use supercapacitors. Compared to traditional electric buses, CSRCAP supercapacitors store and use energy efficiently, reducing energy consumption and improving performance. Major automakers like PSA also use supercapacitors to improve energy efficiency. In 2010, PSA introduced the first supercapacitor car, focusing on the beginning. When the automobile stops with the gear stick in neutral, splitting the engine conserves energy and reduces fuel consumption. Supercapacitors in electric composite cars are a major step toward cleaner, more sustainable transportation. As technology advances, we expect supercapacitor technology and electric car use to improve energy efficiency and sustainability. ¹³⁸

9.4 Supercapacitor battery-hybrid device

Supercapacitor-Battery Hybrid (BSH) devices are essential for electric vehicles, smart grids, and optoelectronic devices. Researchers are investigating alternate battery chemistries for lead-acid, nickel-cadmium, nickel-metal hydride, and lithium-ion batteries to fulfill the growing demand for energy storage solutions with higher energy and power densities. Lithium-air, lithium-sulfur, aluminum-ion, sodium-ion, and aqueous metal-ion batteries are rising in popularity due to their increased energy storage capacity and performance. Zhang et al. recommend constructing electrodes with increased capacitance and electrochemical performance in order to create a battery-supercapacitor hybrid device with higher energy and power densities. BSH devices can improve energy storage and advance technologies that use energy storage by using sophisticated materials and electrode designs. ¹³⁹ Zhang et al.’s novel approach combines EDLC and Li-ion battery strengths. The EDLC-type positive electrode, which often uses graphene and CNT AC for high surface area and rapid charge-discharge, combines with the Li-ion battery’s negative electrode, made of metal oxides (MOs), intercalating chemicals, and their composites for efficient energy storage.

This mix of electrons improves both performance and safety qualities of the battery. The new electrode solution shows a powerful path to create better energy storage methods that need less resources. ¹¹³ The hybrid energy storage system HESS design created by Peng et al. offers EV power solutions in an innovative way. They work to store energy using the SC/BT system when lithium supply stays limited. They reduce their lithium resource consumption by employing supercapacitors to capture power from vehicle brakes. ¹¹⁴ Lu et al.’s study boosts this effort with SC-improved Na-ion batteries. It is groundbreaking to combine a high-rate power surface adsorption-based SC electrode with a high-capacity sodium battery electrode. Synergistic methods generate an increased Na-ion SC electrode with 2,183.5 W/kg power density and 27.9 W/kg energy density.

These findings provide exciting strategies to increase EV energy storage system performance, longevity, and sustainability. Combining supercapacitors’ quick charging with sodium-ion batteries’ high energy density advances electric car energy storage systems. Researchers promote hybrid energy storage technology, making robust, eco-friendly EVs more popular. ¹⁴⁰ In a 1.8 V aqueous electrolyte, MnHCF as the cathode and Fe₃O₄/rGO (reduced graphene oxide) as the anode maintain 82.2 % capacity after 1,000 cycles. Due to its copious components, this combination operates effectively, is affordable, and is accessible. Potassium ions are common battery materials due to their cheapness and availability. Komaba et al. invented a graphite-polyacrylate negative electrode for a 4 V potassium-ion supercapacitor. This idea shows the effort to store energy using abundant materials. ¹⁴¹ Battery supercapacitor hybrids struggle to integrate multivalent ions like aluminum (Al³⁺) and monovalent metal ions, which have larger energy densities. To solve these problems, Li et al. ¹⁴² created a BSH device with an Al_0.2CuFe-PBA negative electrode and an AC positive electrode. This setup shows how BSH technology can optimize performance with different electrode materials. ¹⁴³

9.5 Electro-chemical-flow capacitor

Electrochemical flow capacitors (EFCs) store energy efficiently due to their two electrical layers. Carbon molecules in the EFC store charge, while a slurry of carbon electrolytes and active component controls charge dynamics. A cell precisely regulates this slurry’s electrolyte content. Carbon-based reservoir deposits strategically placed outside the EFC provide energy storage capability. During operation, the uncharged mixture transports the carbon content of the reservoir tank to the flow cell for excitation and energy storage. Large tanks can store the slurry after storing the charge, ready to meet the next energy demand. EFCs’ flexibility lets them cycle loads through numerous charges and discharges, displaying outstanding capacity. Additionally, the liquid electrode made of HQ (hydroquinone) or carbon spheres has a 64 F/g specific capacitance (Cs). Interestingly, this Cs value is 50 % higher than flow able-form electrodes made of carbon. This improved performance shows that EFCs can compete with conventional energy storage options in efficiency and capacity. ¹⁴⁴

9.6 Line-filtering alternating current supercapacitors

Supercapacitors (SC) can replace bulky aluminum electrical condensers (AEC) to reduce system size and preserve performance. Self-sustaining composite electrodes made from single-walled carbon nanotubes by Rangom et al. revolutionized this field. Mesoporous 3D electrodes improve ion transport and performance in thick sheets. With a 601 F cm² capacitance, an −8° phase angle, and a 199s time constant, these SWCNT-based electrodes work at 120 Hz and are very good at what they do. Using a CV parallel tube design with 1 kV⁻¹ enables electrodes to survive cycling at speeds of 200 V s⁻¹. These electrodes can withstand over a million cycles at current densities above 6,400 A/g, preserving over 98 % of their initial capacitance. Such impressive performance metrics demonstrate SWCNT-based composite electrodes’ transformational potential in supercapacitor technology. Their high capacitance, fast ion transport, and excellent cycling stability provide small, high-performance energy storage options for many applications. Researchers are improving electrode design and fabrication methods to improve supercapacitor-based system performance and scalability. ¹⁴⁵

Wu et al. developed large-scale graphene AC line filters using metal joints to mimic reduced graphene oxide (GO) electrodes. The device performed very well with −75.4° of phase angle at 120 Hz and 0.35 ms time constant along with 316 F cm⁻² specific capacitance. The research demonstrates that adding graphene to electrode materials benefits large-scale AC line filter performance though the device loses 2.8 % Cs capacity after 10,000 charge–discharge cycles. ¹²⁰ The research team of Kurra analyzed PEDOT micro-supercapacitors in a 1M H₂SO₄ electrolyte when charging at a rate of 500 V/s. Through their experiments the micro-SC successfully maintained better scanning until 400 Hz along with a −45° phase angle. Real capacitance measurements precise to 9 mF cm² were reliable at a 100 % efficiency level when reaching 7.7 mWh cm³ of energy storage. The system retained 80 % of its capacitance after 10,000 cycles, demonstrating its potential for long-term energy storage. Graphene AC line filters and PEDOT micro-supercapacitors are just two examples of how energy storage technologies can advance. Graphene electrodes meet high-capacity, high-frequency AC applications, whereas PEDOT micro-SCs meet small, high-efficiency energy storage needs. These technologies have the potential to power consumer devices, grid-scale energy storage systems, and other applications, ushering in a new era of sustainable and efficient energy use. ¹⁴⁶

9.7 Micro supercapacitors

Thin films and small batteries are difficult to use in portable devices because of their short lifespan, low power density (Ps), and complex construction. Qi et al. suggest using planar micro-supercapacitors (micro-SCs) to solve these problems, especially in tiny devices. Shao et al. created SCs that are almost solid-state and micro-integrated with cellular graphene sheets in PVA/H₃PO₄ gel electrolytes. These three-dimensional graphene films double as electrolyte ion reservoirs and highly active SC electrodes, improving energy storage efficienc. ¹⁴⁷ Liu et al. were able to change capacitance by 20 % when they used photo-switchable micro-SC films made of composites of diarylethene and graphene. Investigating how light modulation affects ion transport at the diarylethene-graphene interface opens new paths for dynamic energy storage. Lee et al. created multifunctional micro-SCs using self-generated silver and laser-induced growth sintering. These SCs have a higher volumetric energy density of 16.3 mWh/cm³, but a lower power density of 3.54 W/cm³, balancing energy storage and power output for a variety of microelectronics applications. ¹⁴⁸

9.8 Photo supercapacitors

Supercapacitors (SCs) store and stabilize solar cell power output, making photo-supercapacitors (photo-SCs) one of the most energy-efficient technologies. The integration of SCs with photoactive and high-capacity layers is particularly beneficial for autonomous power supply applications such as single-board computers. Xu and colleagues demonstrated that such systems are feasible, with 140 F/g specific capacitance (Cs) and 2 mA/g photo-producing current. ¹⁴⁹ Researchers have integrated SCs with color-sensitized solar cells (DSSCs) atop an anode titanium oxide (ATO) nanotube display to enhance photo-supercapacitor performance. Researchers enhanced the Cs (area) of hydrogenated ATOs from 1.0 mF/cm² to 1 mA/cm² via plasma treatment. This novel approach to charge storage and usage in photo-SC systems could improve their efficiency and stability. Photo-SCs coupled with DSSCs exhibit excellent photoelectric conversion efficiency of 1.64 % because of their strong cycle power, fast response times, and optimal storage productivity. This advances renewable energy technology by allowing for self-sustaining power systems that use solar energy effectively and reliably. Photovoltaic power generation and supercapacitor energy storage can solve intermittent renewable energy problems. Photo-SC systems can reduce power fluctuations, improve energy harvesting efficiency, and enable autonomous and sustainable power solutions for portable electronics and off-grid energy systems by combining the strengths of both technologies. As research in this field continues, materials, device design, and integration techniques will improve photo-SC technology’s performance and scalability, enabling its widespread adoption in the transition to a more sustainable and renewable energy future. ¹⁵⁰

9.9 Upheaval chargeable supercapacitors

Reusing heat energy to power sensors and portable devices is a promising energy-harvesting method. Waste heat management and usage depend on thermoelectric energy conversion. However, low output voltage and insufficient power for energy storage necessitate the use of voltage amplifiers and capacitors to maximize system performance. ¹⁵¹

The Beck effect, which utilizes temperature-dependent electrochemical redox potential, rapid thermal ion diffusion, and thermal self-charging supercapacitors (SCs), can address these issues. Using the temperature gradient, two electrodes at different temperatures drive electrochemical processes and store energy.

In a unique way, traditional thermoelectricity generates high voltage from a temperature gradient. Graphene and polyaniline (PANI) electrodes covered with tri-polystyrene sulfonic acid (PSSH) sheets and thermally induced electrochemical processes make it possible to charge without using extra energy. Despite a 5 K temperature difference, the SC thermal charger reaches 1200 F/m² capacitance (Cs) and 38 mV voltage.

Al-Zubaidi et al. studied thermal induction at the solid-liquid interface and in ionic electrolytes. Wang et al. explained SC electrode materials’ thermal charge phenomena, allowing energy recovery during charging and discharging cycles. ¹⁵²

To study stress-induced self-charging in SCs, Zhao et al. used heat induction and a polymer-based electrolyte of polyethylene oxide (PEO) treated with sodium hydroxide. Gold (Au) electrodes mixed with multi-walled carbon nanotubes (MWCNTs) created a 10 mV K⁻¹ temperature difference at 4.5 k, along with 1.35 mJ/cm² of energy density and 1.03 mF cm⁻² of capacitance. ¹⁵³ These advances demonstrate the potential of waste heat for energy harvesting, a sustainable and efficient way to power electronics. Optimization of materials and device designs will enhance the efficiency and usefulness of thermally-induced energy harvesting systems.

9.10 Self-healing supercapacitors

Self-healing supercapacitors (SCs) pioneered by Li et al. improve energy storage. Li et al. employed charcoal electrodes and polyampholytic gel-type electrolytes to make self-repairing hydrogel SCs with higher energy density. Self-healing, mechanically stable polyampholyte hydrogels show how material design affects long-lasting energy storage systems.

Low-temperature pyrolyzing biological waste generates Biochar (BC), which enhances electrode mechanical strength and electrical conductivity while reducing graphene oxide. This synergistic combination gives SCs energy densities above 30 Wh/kg at ambient temperature and 90 % power retention after 5,000 cycles. These self-healing SCs are versatile and effective in a variety of working conditions, since their energy density climbs to 10.5 W/kg at 500 W/kg and 300 °C. ¹⁵⁴ This groundbreaking discovery inspires researchers to examine polyampholyte chains’ water phase activity to improve SC self-healing and durability. Wang et al. created flexible SCs using rGO-based electrodes and self-healing polymers to withstand mechanical stress and operate effectively in tough settings. The material retained 82.4 % and 54.2 % of its initial capacity, respectively, after stretching and multiple restorative treatments. Self-healing SCs are durable and adaptable, offering versatile and robust energy storage options for wearable electronics and structurally integrated energy systems. Energy storage systems will innovate due to material design, production, and self-healing processes. High-performance, self-healing, and flexible SCs can increase energy storage in many sectors and applications. ¹⁵⁵

9.11 Form memory CS (SMSC)

Shape memory supercapacitors (SMSCs), developed solve the structural and functional collapse problems of elastic supercapacitors. SMSCs with shape memory properties allow devices to keep their original shape after repeated stress. Heating to a specified temperature initiates the shape memory effect, allowing the SMSC to return to its predetermined shape. The integration of SMSCs into memory textile fabrics enhances their functionality. We can integrate SMSCs into clothes and upholstery to create smart fabrics that remember their setup and automatically cool when overheated. This capacity improves wearer comfort and safety and shows SMSC technology’s ability to create intelligent and adaptive materials for many applications. ¹⁵⁶

9.12 Piezoelectric supercapacitors

People review supercapacitors (SCs) as potential replacements for energy storage systems because these devices charge fast and last long. On the other hand normal Li-ion batteries only last a specific time period. The research team of Xing developed an effective technique through combining piezoelectric separation with lithium-ion batteries and self-powered cells to convert energy efficiently. ¹³² SCs show strong potential to solve the problems present in today’s battery systems. Song et al. showed how a PVDF film could collect energy as it served as both a spacer and a separator for SC systems to generate superior energy and performance results. Maitra and Associates studied whether a fish swimming bladder pore could serve as a bio-piezoelectric separator in self-charging ASCs. The research showed that this technology fits well into many different kinds of equipment systems. Different medical implant devices rely on piezoelectric supercapacitors to power operations at microwatt and milliwatt levels. This type of energy storage supplies reliable power to its users. Insulin pumps and cardiac pacemakers demand these SCs for their medical purposes since they must work effectively in space-limited areas with fast recharging while maintaining reliable performance over time. Enhancing SC research will benefit every industrial sector through efficient power storage solutions. SCs stand out because of their unique traits to create customized solutions for different industrial needs. ¹³⁴

The piezoelectric supercapacitor technology helps power many medical devices that need power between microwatts and milliwatts. Using this form of storage produces dependable and efficient results. SCs deliver unmatched power capabilities to insulin pumps and cardiac pacemakers because they offer the best features for medical device operations. Research progress will continue to make SCs more important as storage solutions for various industries. SCs deliver customized energy storage options because of their special mix of features and functions.

9.13 Aircrafts and protection

Supercapacitors hold key importance for various critical aerospace functions because of their special features. Ordinary batteries can no longer power aircraft systems because supercapacitors deliver immediate power with superb battery life. Supercapacitors deliver quick power to airbag deployment systems and military vehicles as well as backup electronics at any temperature. Supercapacitors also power fire control systems and black boxes in armored vehicles and tanks, preserving data during emergencies. Supercapacitors are used in advanced aerospace systems, such as step-departure approach actuator systems in launch vehicles, satellites, and spaceship onboard organizations. Aerospace applications require compactness and efficiency, and their high-power density meets these criteria. Supercapacitors help aircraft systems succeed and stay safe by powering GPS-guided missiles, projectiles, or emergency portable radios. Supercapacitors are likely to play a larger part in aerospace systems as technology advances and research improves their performance, durability, and integration. Supercapacitors will continue as vital aerospace parts by performing diverse critical operations to guarantee civilian and military missions succeed. ¹⁵⁷

9.14 As energy storage devices

Supercapacitors fulfill dual functions by capturing renewable energy from solar together with wind resources. Solar energy systems achieve efficient energy storage through photovoltaic panel-generated energy that supercapacitors transform into reliable sustainable power. Superchargers under wind energy applications protect power flow by storing energy produced by wind turbines to deliver uninterrupted performance in varying wind patterns. These supercapacitors are specifically put together as either cells or modules to support system voltage requirements. ¹⁵¹ The combination of solar-supercapacitor technologies shows promise for developing electricity systems which power electric vehicle charging stations together with portable devices. Solar-supercapacitor systems exist across parking lots and roadways and gardens to supply users with convenient and eco-friendly charging stations using renewable power sources. Solar-supercapacitor systems could be the flexible energy solution to fulfill future energy requirements because they minimize fossil fuel consumption and protect environmental health while demand for clean energy solutions continues to expand. ^{136 , 152}