1. Introduction

2. Types of Supercapacitors

2.1. Electrochemical Double-Layer Capacitors (EDLCs)

Electrochemical Double-Layer Capacitors (EDLCs) are a prominent type of supercapacitor that store energy primarily through the electrostatic separation of charges at the electrode-electrolyte interface[10]. This process occurs within the electrical double layer, a nanoscale structure formed at the surface of the electrodes when a voltage is applied. EDLCs are highly regarded for their exceptional power density, long cycle life, and rapid charge-discharge capabilities.

2.1.1. Working Principle

The fundamental operating mechanism of EDLCs revolves around the formation of the electrical double layer at the interface between the porous electrode materials and the electrolyte[10]. When a voltage is applied across the electrodes, ions in the electrolyte migrate to the electrode surfaces, creating a layer of opposite charges. This results in two parallel layers of charge, one on the electrode surface and the other in the electrolyte, separated by a very thin solvent layer. The energy stored in an EDLC is purely electrostatic and does not involve any chemical or phase changes, which contributes to its long cycle life and fast response times.

The capacitance (C) of an EDLC is determined by the surface area (A) of the electrode material, the distance between the charges (d), and the dielectric constant (ε) of the electrolyte according to the formula:

C = εA/d

2.1.2. Electrode Materials

The performance of EDLCs is heavily influenced by the properties of the electrode materials used. Key attributes such as high surface area, good electrical conductivity, and chemical stability are crucial for achieving high capacitance and efficient energy storage. Common materials used for EDLC electrodes include:

2.1.2.1. Activated Carbon

Activated carbon is the most widely used material for EDLC electrodes due to its high surface area (typically 1000–3000 m²/g), good electrical conductivity, and cost-effectiveness. It is produced by the activation of carbonaceous materials, such as coconut shells, wood, or coal, creating a highly porous structure.

2.1.2.2. Carbon Nanotubes (CNTs)

CNTs offer excellent electrical conductivity, mechanical strength, and a high surface area. Their unique one-dimensional structure facilitates efficient charge transport and ion diffusion, enhancing the performance of EDLCs[11].

2.1.2.3. Graphene

Graphene, a single layer of carbon atoms arranged in a two-dimensional lattice, has emerged as a promising material for EDLCs due to its exceptional electrical conductivity, high surface area (~2630 m²/g), and mechanical flexibility[12]. Graphene-based electrodes can achieve high capacitance and energy density.

2.1.2.4. Carbon Aerogels

Carbon aerogels are highly porous, low-density materials with a continuous network structure that provides a large surface area and good electrical conductivity. They are synthesized through the sol-gel process followed by pyrolysis.

2.1.3. Electrolytes

The electrolyte in an EDLC plays a critical role in determining its performance, particularly the operating voltage window and ionic conductivity. Common types of electrolytes used in EDLCs include:

2.1.3.1. Aqueous Electrolytes

These electrolytes, such as sulfuric acid (H₂SO₄), potassium hydroxide (KOH), and sodium sulfate (Na₂SO₄), offer high ionic conductivity and are inexpensive[13]. However, their voltage window is limited to about 1.23V due to the decomposition of water.

2.1.3.2. Organic Electrolytes

Organic electrolytes, such as acetonitrile or propylene carbonate solutions of tetraethylammonium tetrafluoroborate (TEABF₄), provide a wider voltage window (up to ~2.7V), which allows for higher energy storage. They have lower ionic conductivity compared to aqueous electrolytes and are more expensive.

2.1.3.3. Ionic Liquids

Ionic liquids are salts in a liquid state at room temperature and offer both high voltage stability (up to ~4V) and good ionic conductivity[14]. Their non-volatility and wide electrochemical stability window make them suitable for high-energy applications, although they can be costly.

2.1.4. Advantages and Applications of EDLCs

Table 1. Classification order Advantages and Applications of EDLCs.

Table 1. Classification order Advantages and Applications of EDLCs.

Advantage	Description	Application	Description
High Power Density	Can deliver and absorb energy very quickly, ideal for applications requiring rapid power bursts.	Transportation	Used in electric and hybrid vehicles for regenerative braking and providing quick power bursts for acceleration.
Long Cycle Life	Typically exceeds 1 million cycles, significantly outlasting most batteries.	Consumer Electronics	Powering devices that require rapid charge-discharge cycles, such as cameras, smartphones, and portable power tools.
Fast Charge and Discharge	Can be charged and discharged in seconds to minutes, much faster than batteries.	Renewable Energy Systems	Stabilizing output from solar panels and wind turbines, and providing energy storage for grid balancing.
Wide Operating Temperature Range	Can operate efficiently in a broad range of temperatures.	Industrial Applications	Used in uninterruptible power supplies (UPS), power backup systems, and peak power shaving in industrial equipment.
Maintenance-Free	Requires minimal maintenance compared to batteries.	Grid Energy Storage	Assisting in frequency regulation and load balancing in electrical grids.
Environmentally Friendly	Often made from non-toxic materials and have a lower environmental impact.	Medical Devices	Providing reliable power for emergency medical equipment and portable diagnostic devices.

2.2. Pseudocapacitors

Pseudocapacitors are a type of supercapacitor that store energy through fast and reversible faradaic (redox) reactions at the surface of the electrode materials. Unlike Electrochemical Double-Layer Capacitors (EDLCs) which rely on electrostatic charge separation, pseudocapacitors achieve higher capacitance and energy density by exploiting the rapid redox reactions[15].

2.1.5. Working Principle

The working principle of pseudocapacitors involves faradaic processes, where electron charge transfer occurs between the electrode and electrolyte, leading to oxidation-reduction reactions. These reactions take place at or near the surface of the electrode material and contribute to the overall capacitance. The energy storage mechanism can be described by the following reaction[16]:

Mⁿ⁺ + e^- ↔ M^(n-1)+

where M represents the electroactive species on the electrode surface. The faradaic nature of these reactions allows pseudocapacitors to store more energy compared to EDLCs.

2.2.2. Electrode Materials

Pseudocapacitors utilize a variety of materials that can undergo fast and reversible redox reactions. These materials typically include:

2.2.2.1. Transition Metal Oxides

Metal oxides like ruthenium oxide (RuO₂), manganese oxide (MnO₂), and nickel oxide (NiO) are widely used in pseudocapacitors. They offer high specific capacitance and good conductivity. RuO₂, in particular, exhibits high capacitance but is expensive, leading to the exploration of more cost-effective alternatives like MnO₂ and NiO[16].

2.2.2.2. Conducting Polymers

Polymers such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) are also popular electrode materials for pseudocapacitors. These polymers are conductive and can undergo redox reactions, providing high capacitance and flexibility. However, they may suffer from stability issues over long-term cycling[17].

2.2.2.3. Composite Materials

Combining transition metal oxides with conducting polymers or carbon materials can enhance the overall performance of pseudocapacitors. These composites leverage the high capacitance of metal oxides and the conductivity and stability of polymers or carbon materials[18].

2.2.3. Electrolytes

The choice of electrolyte in pseudocapacitors is crucial for optimizing performance. Electrolytes must be compatible with the electrode materials and support fast ion transport. Common electrolytes include:

2.2.3.1. Aqueous Electrolytes

These provide high ionic conductivity and are generally less expensive. However, their voltage window is limited (up to ~1.23V) due to the electrolysis of water[19].

2.2.3.2. Organic Electrolytes

Organic electrolytes like acetonitrile or propylene carbonate solutions offer a wider voltage window (up to ~2.7V), enabling higher energy density. They have lower ionic conductivity compared to aqueous electrolytes.

2.2.3.3. Ionic Liquids

These are salts that are liquid at room temperature and offer high electrochemical stability and wide voltage windows (up to ~4V)[20]. They are suitable for high-energy applications but are often more expensive.

2.2.4. Advantages and Disadvantages

Table 2. Advantages and Disadvantages of Ps.

Table 2. Advantages and Disadvantages of Ps.

Aspect	Description
Advantages
High Energy Density	Can store more energy compared to EDLCs due to faradaic reactions.
Fast Charge and Discharge	Capable of rapid charge and discharge cycles, although slower than EDLCs.
Versatility	Wide range of materials can be used, allowing customization for specific applications.
Disadvantages
Material Degradation	Repeated redox cycling can lead to degradation of electrode materials, especially conducting polymers.
Cost	Some high-performance materials, like RuO₂, are expensive.
Lower Power Density	Generally lower power density compared to EDLCs.

2.2.5. Applications

Pseudocapacitors find applications in various fields where high energy density and moderate power density are required:

Table 3. Applications of Ps.

Table 3. Applications of Ps.

Applications	Description
Portable Electronics	Used in devices needing reliable energy storage with high capacity, such as smartphones and tablets.
Electric Vehicles	Complement batteries by providing quick bursts of energy and enhancing overall energy storage capacity.
Grid Energy Storage	Stabilize power supply by storing and releasing energy during peak and off-peak periods.
Medical Devices	Provide consistent and reliable power for critical medical equipment, such as defibrillators.
Renewable Energy Systems	Assist in the storage and distribution of energy from renewable sources like solar and wind power.

2.3. Hybrid Capacitors

Hybrid capacitors represent a unique class of energy storage devices that combine the characteristics of both electrochemical double-layer capacitors (EDLCs) and batteries[21]. By integrating aspects of both capacitor and battery technologies, hybrid capacitors offer a balance between high power density, fast charge-discharge capabilities, and relatively high energy density.

2.3.1. Working Principle

Hybrid capacitors leverage the strengths of both capacitive and battery-type mechanisms for energy storage. Similar to EDLCs, they store energy through the electrostatic separation of charges at the electrode-electrolyte interface, forming an electrical double layer[22]. Additionally, they incorporate a secondary energy storage mechanism, such as a pseudo-faradaic reaction or ion intercalation, akin to batteries. This hybridization allows for higher energy density than traditional capacitors while maintaining the rapid charge-discharge characteristics of EDLCs.

2.3.2. Types of Hybrid Capacitors

Hybrid capacitors can be categorized based on the combination of capacitor and battery components:

2.3.2.1. Lithium-Ion Capacitors (LICs)

LICs combine a lithium-ion battery-type electrode with an EDLC electrode. The lithium-ion electrode provides high energy density through reversible intercalation reactions, while the EDLC electrode offers high power density and rapid charge-discharge cycles[23]. LICs exhibit a good balance between energy and power densities, making them suitable for various applications.

2.3.2.2. Nickel Capacitors (NiCaps)

NiCaps feature a nickel hydroxide electrode (similar to nickel-metal hydride batteries) combined with an EDLC electrode. They offer higher energy density than LICs but with slightly lower power density. NiCaps are particularly well-suited for applications requiring moderate energy storage and extended cycle life[24].

2.3.2.3. Carbon Hybrid Capacitors (CHCs)

CHCs utilize a combination of activated carbon or carbon nanomaterials (similar to those used in EDLCs) with a battery-type electrode, such as lithium-ion or nickel-based materials[25]. These capacitors aim to capitalize on the high power density of carbon-based electrodes and the energy density of battery-type materials, offering a versatile energy storage solution[25].

2.3.3. Advantages and Disadvantages

Table 4. Advantages and Disadvantages of HSc.

Table 4. Advantages and Disadvantages of HSc.

Aspect	Description
Advantages
High Energy Density	Offers higher energy density compared to traditional capacitors, approaching levels of some batteries.
High Power Density	Retains the rapid charge-discharge capabilities of EDLCs, enabling quick energy transfer.
Long Cycle Life	Generally exhibits longer cycle life than batteries due to the capacitor-like mechanism of charge storage.
Wide Operating Temperature Range	Suitable for use in diverse environments due to their robust electrochemical properties.
Disadvantages
Complex Design	Incorporating multiple electrode materials and energy storage mechanisms can increase manufacturing complexity.
Limited Energy Density	While higher than traditional capacitors, the energy density of hybrid capacitors may still be lower than that of some batteries.
Cost	The hybrid nature and specialized components may lead to higher production costs compared to conventional capacitors.

2.3.4. Applications

Hybrid capacitors find applications in various industries where a balance between energy and power density is required:

Table 5. Application of HSc.

Table 5. Application of HSc.

Applications	Description
Automotive	Used in hybrid and electric vehicles for applications such as regenerative braking and power delivery during acceleration.
Renewable Energy	Employed in grid-level energy storage systems to stabilize output from intermittent sources like solar and wind.
Consumer Electronics	Integrated into portable electronic devices, providing a combination of high energy density and rapid charging capabilities.
Industrial Applications	Utilized in power backup systems, uninterruptible power supplies (UPS), and peak load shaving in industrial settings.
Aerospace	Deployed in satellites and spacecraft for energy storage during periods of high power demand or eclipses.

2.4. Materials for Supercapacitors

The performance of supercapacitors heavily depends on the materials used for the electrodes and electrolytes.

2.4.1. Electrode Materials

2.4.1.1. Carbon-Based Materials

Carbon-based materials are widely employed in supercapacitors due to their high surface area, excellent electrical conductivity, chemical stability, and abundance[26]. These materials play a crucial role in enhancing the capacitance, energy density, and cycling stability of supercapacitors[27]. Here, we delve into various carbon-based materials utilized in supercapacitor electrodes:

2.4.1.1.1. Activated Carbon

Activated carbon is a porous carbon material with a highly developed surface area, typically ranging from 500 to 3000 m²/g[28]. It is produced by the activation of carbonaceous precursors such as coconut shells, wood, or coal. The activation process creates a network of micropores and mesopores, providing ample surface area for ion adsorption[8].

2.4.1.1.2. Carbon Nanotubes (CNTs)

Carbon nanotubes are cylindrical nanostructures composed of carbon atoms arranged in a hexagonal lattice. They possess exceptional electrical conductivity, mechanical strength, and high aspect ratios. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) are the two primary types used in supercapacitor electrodes[29]

2.4.1.1.3. Graphene

Graphene is a two-dimensional material consisting of a single layer of carbon atoms arranged in a honeycomb lattice. It exhibits exceptional electrical conductivity, high mechanical strength, and large specific surface area (~2630 m²/g). Graphene-based materials are synthesized through various methods, including mechanical exfoliation, chemical vapor deposition (CVD), and chemical reduction of graphene oxide[8].

2.4.1.1.4. Carbon Aerogels

Carbon aerogels are lightweight, porous materials with a three-dimensional network structure composed of interconnected carbon nanoparticles[30]. They are synthesized through the sol-gel polymerization of resorcinol and formaldehyde followed by carbonization and supercritical drying.

Table 6. Advantage, Disadvantage of applications of carbon based materials.

Table 6. Advantage, Disadvantage of applications of carbon based materials.

Carbon-Based Material	Advantages	Disadvantages	Applications
Activated Carbon	- High surface area for ion adsorption, leading to high capacitance.	- Relatively low electrical conductivity compared to other carbon-based materials.	- Portable electronics - Automotive (regenerative braking) - Grid energy storage - Industrial applications
Carbon Nanotubes (CNTs)	- Exceptional electrical conductivity and mechanical strength.	- Cost-intensive synthesis methods.	- Aerospace - Energy harvesting - Flexible and wearable electronics
Graphene	- Highest electrical conductivity and large specific surface area among carbon-based materials.	- Challenges in large-scale production and uniformity.	- Energy storage systems - Flexible and transparent electrodes for optoelectronic devices - Water purification membranes
Carbon Aerogels	- Ultralow density and high porosity, suitable for lightweight and portable applications.	- Complex fabrication process.	- Aerospace - Energy-efficient buildings - Environmental sensors and monitoring systems
Carbon-Based Composites	- Tailored properties and synergistic effects with other components (e.g., metal oxides, conducting polymers).	- Complexity in material synthesis and optimization.	- Hybrid electric vehicles - Renewable energy storage systems - Medical device

2.4.1.2. Carbon-Based Composites

Carbon-based composites integrate carbon materials with other functional components, such as metal oxides, conducting polymers, or heteroatom-doped carbons[31]. These composites are designed to synergistically enhance the electrochemical performance of supercapacitors by leveraging the unique properties of each constituent material.

Examples of Carbon-Based Composites

1. 

Carbon/Metal Oxide Composites:

• Graphene/MnO₂: Combines the high conductivity of graphene with the pseudocapacitive properties of manganese oxide[32].
• CNTs/RuO₂: Integrates carbon nanotubes with ruthenium oxide to enhance capacitance and conductivity.
• Activated Carbon/NiO: Merges activated carbon with nickel oxide for improved energy storage capacity.

2. 

Carbon/Conducting Polymer Composites:

• Graphene/Polyaniline (PANI): Blends graphene with polyaniline to combine high conductivity and pseudocapacitance[33].
• CNTs/Polypyrrole (PPy): Combines carbon nanotubes with polypyrrole for improved charge storage and mechanical flexibility.
• Activated Carbon/Polythiophene (PTh): Integrates activated carbon with polythiophene to enhance capacitance and stability[34].

3. 

Heteroatom-Doped Carbon Composites:

• N-Doped Graphene: Graphene doped with nitrogen atoms to enhance conductivity and electrochemical performance[35].
• S-Doped CNTs: Carbon nanotubes doped with sulfur to improve capacitance and charge transfer properties[36].
• Boron-Doped Activated Carbon: Activated carbon doped with boron for better charge storage and cycling stability[37].

Advantages and Disadvantages of Carbon-Based Composites

Table 7. Advantage and Disadvantage of carbon based composite materials.

Applications of Carbon-Based Composites

Table 8. Applications of carbon based Composite Materials.

3. Significant Advancements in Supercapacitor Technology Include

4. Applications

Table 9. Applications of Supercapacitors.

5. Conclusion

References

1. Akinyele, D.O.; Rayudu, R.K. Review of energy storage technologies for sustainable power networks. Sustain. Energy Technol. Assess. 2014, 8, 74–91, doi:10.1016/j.seta.2014.07.004.
2. Afif, A., et al., Advanced materials and technologies for hybrid supercapacitors for energy storage–A review. Journal of Energy Storage, 2019. 25: p. 100852.
3. Gupta, G.K.; Sagar, P.; Srivastava, M.; Singh, A.K.; Singh, J.; Srivastava, S.K.; Srivastava, A. Excellent supercapacitive performance of graphene quantum dots derived from a bio-waste marigold flower (Tagetes erecta). Int. J. Hydrogen Energy 2021, 46, 38416–38424, . [CrossRef]
4. Gupta, G.K.; Sagar, P.; Pandey, S.K.; Srivastava, M.; Singh, A.K.; Singh, J.; Srivastava, A.; Srivastava, S.K.; Srivastava, A. In Situ Fabrication of Activated Carbon from a Bio-Waste Desmostachya bipinnata for the Improved Supercapacitor Performance. Nanoscale Res. Lett. 2021, 16, 1–12, . [CrossRef]
5. Fu, W.; Turcheniuk, K.; Naumov, O.; Mysyk, R.; Wang, F.; Liu, M.; Kim, D.; Ren, X.; Magasinski, A.; Yu, M.; et al. Materials and technologies for multifunctional, flexible or integrated supercapacitors and batteries. Mater. Today 2021, 48, 176–197, . [CrossRef]
6. Sagar, P.; Gupta, G.K.; Srivastava, M.; Srivastava, A.; Srivastava, S.K. Tagetes erecta as an organic precursor: synthesis of highly fluorescent CQDs for the micromolar tracing of ferric ions in human blood serum. RSC Adv. 2021, 11, 19924–19934, . [CrossRef]
7. Gupta, G.K.; Sagar, P.; Srivastava, M.; Singh, A.K.; Singh, J.; Srivastava, S.K.; Srivastava, A. Hydrothermally synthesized nickel ferrite nanoparticles integrated reduced graphene oxide nanosheets as an electrode material for supercapacitors. J. Mater. Sci. Mater. Electron. 2024, 35, 1–15, . [CrossRef]
8. Gupta, G., A Comprehensive Review on Various Techniques Used for Synthesizing Graphene Quantum Dots. 2024.
9. Yan, J., et al., Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities. Advanced Energy Materials, 2014. 4(4): p. 1300816.
10. Wu, J. Understanding the Electric Double-Layer Structure, Capacitance, and Charging Dynamics. Chem. Rev. 2022, 122, 10821–10859, . [CrossRef]
11. Le Xie, J.; Guo, C.X.; Li, C.M. Construction of one-dimensional nanostructures on graphene for efficient energy conversion and storage. Energy Environ. Sci. 2014, 7, 2559–2579, . [CrossRef]
12. Yin, H., Surface modification of porous carbon materials for electrochemical energy storage applications. 2022, Université Paul Sabatier-Toulouse III.
13. Azam, M.A.; Pembuatan, U.F.K.; Seman, R.N.A.R.; Aziz, M.F.A.; Izamshah, R.; Kasim, M.S. Electrochemical Performance of Molybdenum Disulfide Supercapacitor Electrode in Potassium Hydroxide and Sodium Sulfate Electrolytes. Int. J. Recent Technol. Eng. 2020, 8, 5499–5503, . [CrossRef]
14. Niu, H.; Wang, L.; Guan, P.; Zhang, N.; Yan, C.; Ding, M.; Guo, X.; Huang, T.; Hu, X. Recent Advances in Application of Ionic Liquids in Electrolyte of Lithium Ion Batteries. J. Energy Storage 2021, 40, . [CrossRef]
15. Srivastava, A., et al., Fabrication Of Mnfe2o4/Rgo Nanostructure for Stable and Enhanced Super Capacitive Performance. Fabrication Of Mnfe2o4/Rgo Nanostructure for Stable and Enhanced Super Capacitive Performance, 2023.
16. Srivastava, A., et al., High-Performance Electrode Based on Nife2o4 Nanoparticles Architecture R-Go Nanosheets For Supercapacitors. Gopal K. and Srivastava, Monika and Anwar, Sharmistha and Srivastava, Sanjay K. and Sagar, Pinky, High-Performance Electrode Based on Nife2o4 Nanoparticles Architecture R-Go Nanosheets For Supercapacitors.
17. Gupta, G.K. and K.K. Shandilya, Hierarchical Ni-Mn Double Layered/Graphene Oxide with Excellent Energy Density for Highly Capacitive Supercapacitors. Mater Sci, 2023. 11: p. 004.
18. Gupta, G.K., et al., Hierarchical NiMn Double Layered/Graphene with Excellent Energy Density for Highly Capacitive Supercapacitors. 2021.
19. Gupta, G.K., Development of grapheme Ni Mn layered nanosheets for ultra highSupercapacitance and its performance.
20. Melot, B.C.; Tarascon, J.-M. Design and Preparation of Materials for Advanced Electrochemical Storage. Accounts Chem. Res. 2013, 46, 1226–1238, . [CrossRef]
21. Zhao, J.; Burke, A.F. Electrochemical capacitors: Materials, technologies and performance. Energy Storage Mater. 2020, 36, 31–55, . [CrossRef]
22. Ratajczak, P.; Suss, M.E.; Kaasik, F.; Béguin, F. Carbon electrodes for capacitive technologies. Energy Storage Mater. 2019, 16, 126–145, . [CrossRef]
23. Yao, F., D.T. Pham, and Y.H. Lee, Carbon-based materials for lithium-ion batteries, electrochemical capacitors, and their hybrid devices. ChemSusChem, 2015. 8(14): p. 2284-2311.
24. Trepanier, N., Food as a window into daily life in fourteenth century Central Anatolia. 2008: Harvard University.
25. Tan, H.; Sun, L.; Zhang, Y.; Wang, K.; Zhang, Y. Metal Phosphides as Promising Electrode Materials for Alkali Metal Ion Batteries and Supercapacitors: A Review. Adv. Sustain. Syst. 2022, 6, . [CrossRef]
26. Miao, L.; Song, Z.; Zhu, D.; Li, L.; Gan, L.; Liu, M. Recent advances in carbon-based supercapacitors. Mater. Adv. 2020, 1, 945–966, . [CrossRef]
27. Wang, T.; Chen, H.C.; Yu, F.; Zhao, X.; Wang, H. Boosting the cycling stability of transition metal compounds-based supercapacitors. Energy Storage Mater. 2018, 16, 545–573, . [CrossRef]
28. Rugayah, A., A. Astimar, and N. Norzita, Preparation and characterization of activated carbon from palm kernel shell by physical activation with steam. Journal of Oil Palm Research, 2014. 26(3): p. 251-264.
29. Parveen, N., Synthesis of Biowaste Activated Carbon for Water Purification: A Comprehensive Review. 2024.
30. Tan, D.; Zhao, J.; Gao, C.; Wang, H.; Chen, G.; Shi, D. Carbon Nanoparticle Hybrid Aerogels: 3D Double-Interconnected Network Porous Microstructure, Thermoelectric, and Solvent-Removal Functions. ACS Appl. Mater. Interfaces 2017, 9, 21820–21828, . [CrossRef]
31. Paul, R., et al., Carbon nanotubes, graphene, porous carbon, and hybrid carbon-based materials: Synthesis, properties, and functionalization for efficient energy storage, in Carbon Based Nanomaterials for Advanced Thermal and Electrochemical Energy Storage and Conversion. 2019, Elsevier. p. 1-24.
32. Wang, Y.; Lai, W.; Wang, N.; Jiang, Z.; Wang, X.; Zou, P.; Lin, Z.; Fan, H.J.; Kang, F.; Wong, C.-P.; et al. A reduced graphene oxide/mixed-valence manganese oxide composite electrode for tailorable and surface mountable supercapacitors with high capacitance and super-long life. Energy Environ. Sci. 2017, 10, 941–949, . [CrossRef]
33. Tong, Z.; Yang, Y.; Wang, J.; Zhao, J.; Su, B.-L.; Li, Y. Layered polyaniline/graphene film from sandwich-structured polyaniline/graphene/polyaniline nanosheets for high-performance pseudosupercapacitors. J. Mater. Chem. A 2014, 2, 4642–4651, . [CrossRef]
34. Nejati, S.; Minford, T.E.; Smolin, Y.Y.; Lau, K.K.S. Enhanced Charge Storage of Ultrathin Polythiophene Films within Porous Nanostructures. ACS Nano 2014, 8, 5413–5422, . [CrossRef]
35. Li, W.; Lü, H.-Y.; Wu, X.-L.; Guan, H.; Wang, Y.-Y.; Wan, F.; Wang, G.; Yan, L.-Q.; Xie, H.-M.; Wang, R.-S. Electrochemical performance improvement of N-doped graphene as electrode materials for supercapacitors by optimizing the functional groups. RSC Adv. 2015, 5, 12583–12591, . [CrossRef]
36. Kim, J.H.; Ko, Y.-I.; Kim, Y.A.; Kim, K.S.; Yang, C.-M. Sulfur-doped carbon nanotubes as a conducting agent in supercapacitor electrodes. J. Alloy. Compd. 2020, 855, 157282, . [CrossRef]
37. Lee, Y.-G.; An, G.-H. Synergistic Effects of Phosphorus and Boron Co-Incorporated Activated Carbon for Ultrafast Zinc-Ion Hybrid Supercapacitors. ACS Appl. Mater. Interfaces 2020, 12, 41342–41349, . [CrossRef]
38. Jiang, J.; Zhang, Y.; Nie, P.; Xu, G.; Shi, M.; Wang, J.; Wu, Y.; Fu, R.; Dou, H.; Zhang, X. Progress of Nanostructured Electrode Materials for Supercapacitors. Adv. Sustain. Syst. 2017, 2, . [CrossRef]
39. Kinloch, I.A.; Suhr, J.; Lou, J.; Young, R.J.; Ajayan, P.M. Composites with carbon nanotubes and graphene: An outlook. Science 2018, 362, 547–553, doi:10.1126/science.aat7439.
40. Zhao, Z.; Xia, K.; Hou, Y.; Zhang, Q.; Ye, Z.; Lu, J. Designing flexible, smart and self-sustainable supercapacitors for portable/wearable electronics: from conductive polymers. Chem. Soc. Rev. 2021, 50, 12702–12743, . [CrossRef]
41. Huang, T.; Yang, X.; Xiao, J.; Gao, H.; Wang, Y.; Liu, H.; Wang, G. Advancing low-dimensional flexible energy devices for wearable technology. J. Mater. Chem. A 2024, . [CrossRef]
42. Bocchetta, P.; Frattini, D.; Ghosh, S.; Mohan, A.M.V.; Kumar, Y.; Kwon, Y. Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices. Materials 2020, 13, 2733, . [CrossRef]
43. Benzigar, M.R.; Dasireddy, V.D.B.C.; Guan, X.; Wu, T.; Liu, G. Advances on Emerging Materials for Flexible Supercapacitors: Current Trends and Beyond. Adv. Funct. Mater. 2020, 30, . [CrossRef]
44. Akin, M.; Zhou, X. Recent advances in solid-state supercapacitors: From emerging materials to advanced applications. Int. J. Energy Res. 2022, 46, 10389–10452, . [CrossRef]
45. Khan, H.A.; Tawalbeh, M.; Aljawrneh, B.; Abuwatfa, W.; Al-Othman, A.; Sadeghifar, H.; Olabi, A.G. A comprehensive review on supercapacitors: Their promise to flexibility, high temperature, materials, design, and challenges. Energy 2024, . [CrossRef]
46. Ren, W.; Ding, C.; Fu, X.; Huang, Y. Advanced gel polymer electrolytes for safe and durable lithium metal batteries: Challenges, strategies, and perspectives. Energy Storage Mater. 2020, 34, 515–535, . [CrossRef]
47. Dubal, D.P.; Ayyad, O.; Ruiz, V.; Gómez-Romero, P. Hybrid energy storage: the merging of battery and supercapacitor chemistries. Chem. Soc. Rev. 2015, 44, 1777–1790, . [CrossRef]
48. Muzaffar, A.; Ahamed, M.B.; Deshmukh, K.; Thirumalai, J. A review on recent advances in hybrid supercapacitors: Design, fabrication and applications. Renew. Sustain. Energy Rev. 2018, 101, 123–145, . [CrossRef]
49. Reddy, P.H.; Amalraj, J.; Ranganatha, S.; Patil, S.S.; Chandrasekaran, S. A review on effect of conducting polymers on carbon-based electrode materials for electrochemical supercapacitors. Synth. Met. 2023, 298, . [CrossRef]
50. Gao, D.; Luo, Z.; Liu, C.; Fan, S. A survey of hybrid energy devices based on supercapacitors. Green Energy Environ. 2023, 8, 972–988, . [CrossRef]