Recent Advances on Nitrogen‐Doped Porous Carbons Towards Electrochemical Supercapacitor Applications

Due to ever‐increasing global energy demands and dwindling resources, there is a growing need to develop materials that can fulfil the World's pressing energy requirements. Electrochemical energy storage devices have gained significant interest due to their exceptional storage properties, where the electrode material is a crucial determinant of device performance. Hence, it is essential to develop 3‐D hierarchical materials at low cost with precisely controlled porosity and composition to achieve high energy storage capabilities. After presenting the brief updates on porous carbons (PCs), then this review will focus on the nitrogen (N) doped porous carbon materials (NPC) for electrochemical supercapacitors as the NPCs play a vital role in supercapacitor applications in the field of energy storage. Therefore, this review highlights recent advances in NPCs, including developments in the synthesis of NPCs that have created new methods for controlling their morphology, composition, and pore structure, which can significantly enhance their electrochemical performance. The investigated N‐doped materials a wide range of specific surface areas, ranging from 181.5 to 3709 m2 g−1, signifies a substantial increase in the available electrochemically active surface area, which is crucial for efficient energy storage. Moreover, these materials display notable specific capacitance values, ranging from 58.7 to 754.4 F g−1, highlighting their remarkable capability to effectively store electrical energy. The outstanding electrochemical performance of these materials is attributed to the synergy between heteroatoms, particularly N, and the carbon framework in N‐doped porous carbons. This synergy brings about several beneficial effects including, enhanced pseudo‐capacitance, improved electrical conductivity, and increased electrochemically active surface area. As a result, these materials emerge as promising candidates for high‐performance supercapacitor electrodes. The challenges and outlook in NPCs for supercapacitor applications are also presented. Overall, this review will provide valuable insights for researchers in electrochemical energy storage and offers a basis for fabricating highly effective and feasible supercapacitor electrodes.


Introduction
The rapid progress of technological advancements has led to a significant increase in the consumption of fossil fuels, depleting limited resources and causing massive environmental impacts. [1,2]As a result, renewable energy sources have been identified as the leading candidate for replacing fossil fuels in the future, given their lower environmental impact and sustainability.While solar, hydro, and wind energy are considered simple measures to replace the immediate fossilbased energy requirement, there is a significant issue with power transmission and storage.Therefore, the urgent need to develop energy storage devices to address these issues has arisen. [3][8][9][10][11][12][13][14] The most widely used electrochemical devices include batteries, supercapacitors, and fuel cells. [6]f these electrochemical devices, the first two are commonly used for typical energy storage applications. [7]owever, these devices operate using different mechanisms.Batteries convert electrical energy into chemical energy during charging, while the opposite process occurs during discharging and is classified into primary and secondary batteries. [8]They are constructed by stacking several cells, with every cell containing two electrodes, an electrolyte, and a separator.Energy storage in batteries occurs through reversible redox reactions in the bulk phase of the electrodes. [9]In contrast, electrochemical capacitors store energy via redox reaction or adsorption of ions which can be charged/discharged safely and quickly store energy via a charge and discharge process at the electrode and electrolyte interface.Electrochemical capacitors [10] have gained the attention of scientists due to their remarkable lifespan, low maintenance costs, high power density, and fast charging and discharging capabilities. [11]herefore, researchers in this field are currently trying to fabricate new materials or modify existing ones to improve the performance of supercapacitor devices. [13,12]1 Fundamentals of Electrochemical Energy Storage Devices (EESDs) Electrochemical Capacitors (ECs) are classified into three main classes, namely, electrical double-layer capacitors (EDLC), pseudo capacitors (PC), and a new class that has emerged by combining these two charge storage mechanisms, known as hybrid capacitors (HC).[13][14][15] In the electric double-layer capacitor (EDLC), ions are electrostatically attracted and accumulate on the surface of a porous electrode to form an electrical double layer.In this process, there is a direct charge transfer on the double layer without any charge transfer on the electrode-electrolyte interface.Energy storage devices based on electrodes of carbon materials with a high surface area are usually associated with the EDLC class.[16] Another one is the pseudocapacitors, which involve faradaic reactions [17] and reversible redox reactions between electrolyte ions and surface functional moieties.[18] This class comprises transition metal oxides, conducting polymers, and carbons doped with hetero atoms.[19] Hybrid capacitors that combine a non-faradaic EDLC-type electrode with a faradaic battery-type electrode.[20] This combination leads to much more advanced characteristics such as enhanced energy density, prolonged life cycle, greater safety, greater working potential, and higher efficiency compared to the individual components.[21] Their energy storage is achieved through expeditious, repeatable reversible redox reactions between the electrolyte solution in a pseudo capacitor and the electroactive components on active electrode material. [22] On-half of the hybrid super-capacitor functions as the EDLC, while the other half serves as a pseudocapacitor.[23] They have higher energy densities and voltage, [21] but their slow charge kinetics, arising from the battery-type electrode, leads to poor performance.[24] Figure 1 shows the three types of electrochemical supercapacitors.

Role of Electrode Materials in Electrochemical Energy Storage Devices:
[29][30][31][32] Electroactive materials fabricating electrodes for ECs fall into two categories: carbon-based active materials, [28] primarily used in electric double-layer capacitors and pseudocapacitive active materials.Another category involves metal oxides [29] or heteroatoms incorporated into carbon structures. [30]These materials are referred to as electroactive materials, and various effective synthesis strategies have shown notable improvements in the specific energy of such appliances and their electrical, mechanical, and optical properties.Future concerns about supercapacitors include removing the barrier to using them in devices requiring a high combination of power and energy.Therefore, enhancing energy density31 by designing electrode materials that can maintain a longer lifespan is crucial.Figure 2 shows the typical electrode materials used in supercapacitors.
][46][47][48] However, due to some limitations associated with their high cost and complicated fabrication methods, research is focused on developing electrode materials using economical and abundant resources.Carbon-based materials exhibit remarkable properties as SC electrodes and have gained significant attention.Carbon materials are the most diverse electrode materials used in electric double-layer capacitors (EDLC). [45,46][44][45] Carbon materials are categorized into zero-dimensional, one-dimensional (1-D), two-dimensional (2-D), and three-dimensional (3-D) materials, each with unique properties.Various carbon materials, namely activated carbon, carbon nanofiber, graphene, [47] graphite, [48] graphene oxide, [49,50] hard and soft carbon, metal-organic frameworks, [51] carbon nanotubes, porous carbon, carbon spheres, carbide-derived carbon are described as electrode materials for constructing ECs. [52]Some of these categories are described briefly in the following section.
Activated Carbon (AC): AC possesses a good surface area and is reported as the most widely used material in ECs.It has been given importance in energy devices because of its enhanced properties.The narrow pore size, surface area and conductivity play crucial roles in their performance. [53]ACs with reproducible properties, better porosity, and more uniform pore structures are considered to address this challenges. [54]Their precursors mostly include cotton, bamboo leaves, fish gills, rice, tobacco, corncobs, and dead leaves.Coal is now a precursor material of ACs, resulting in supercapacitors with larger surface and pore sizes. [55]raphene: This two-dimensional, hexagonal, mono-layer form of graphite has attracted global attention since being discovered as one of the thinnest materials.Its unique electronic structure has been attributed to exceptional physical properties. [56]It holds unique properties, including outstanding chemical stability, electrical, thermal, and mechanical characteristics, and a higher surface area.A Young modulus of 1 TP has been measured for graphene, which is nearly 100-300 times more than steel. [54]The breaking strength of graphene is around 42 N/m, and it also exhibits remarkable thermal conductivity and excellent electronic mobility in contrast to other carbon, copper, and silicon analogues.The elevated specific surface area, high electrical conductivity, and great chemical stability are remarkable features of graphene.
Carbon nanofibers: These are 1D graphene materials having the characteristics of being robust macro-structures with pores, flexibility, greater surface-to-volume ratio, and enhanced electrical conductance.Based on the stacking pattern of graphene layers CNFs possess different forms.These layers can be inclined, perpendicular or coiled along the fibre axis.CNFs not only possess the advantage of being used as electrode materials of high performance but also can be used as substrates as a support for other active materials. [59]arbon aerogel (CA): CA comprises a three-dimensional network of nanometer ranged particles with a small interstitial pore. [57]In 1993, the first report was given on using aerogels as an electrode material for EDLC usage.They were first fabricated by using the sol-gel method.Because of EDLC's good capacitance behaviour and higher porosity, they are actively investigated for next-generation energy storage usages, yet improvements in specific surface area, specific capacitance, and maintaining stability and cycle life at higher temperatures are required.Modifications methods are being investigated to fill the gap in their performance. [58]rbon nanotubes (CNTs) are one-dimensional, helically-shaped microtubules composed of carbon hexagons interlaced to form a honeycomb structure, then rolled into a cylindrical tube.The carbon-carbon bonds in these structures are extremely strong, providing exceptional tensile strength and stiffness.The sp 2 hybridization of these structures gives rise to intriguing electrical properties. [59]CNTs are among the thinnest and strongest known fibres, with excellent mechanical, thermal, and electric properties, nondegradable durability, and flexibility in their structure, making them a very versatile material. [60]They possess properties that are higher in dimension compared to graphene.Their use in preparing supercapacitors as power-dense electrodes have been widely reported due to their reduced surface area. [61]One issue that hindered the widespread adoption of CNTs was their susceptibility to contamination by oxidation.CNTs can be classified as arm hair, zigzag, or chiral, based on the orientation of the rolled graphene sheets.Chemical vapour deposition (CVD) is the most promising large-scale method for producing CNTs.Although carbon-based materials hold diverse future application prospects, several aspects, such as their capacitance, conductivity, and stability, still fall short, and structural modifications are needed to address this issues. [62]New methods are being developed, such as doping with heteroatoms, structural regulation, and fabrication of composite materials, to address the limitations in capacitance, conductivity, and stability of carbon materials and to create an excellent electrode material. [63]

Introduction to Porous Carbons
Carbon materials exhibit unique properties, particularly in terms of their surface and structural characteristics, that enable them to deliver specific performance as electrodes in supercapacitors. [64]Porous carbons are widely recognized as high-performance electrode materials for supercapacitors due to their unique features.They can be easily synthesized from natural or synthetic precursors and are known for their excellent electrical conductivity, mechanical strength, thermal stability, and chemical resistance.Porous carbons also possess remarkable surface properties, including high surface area and a broad range of pore sizes.Moreover, they are cost-effective and abundantly available in nature. [65,66]

Incorporation of pores and their importance
The introduction of pores into carbon materials enhances their capacitance performance.Porous carbons are classified based on pore size as microporous, mesoporous, macroporous, and hierarchically permeable carbon materials.Micro, macro and mesopore channels allow exposure to more active sites, better connection with other substances and fast diffusion.An increase in micropores results in enhanced specific capacitance at low current density, but abundant micropores restrict the bulk transfer of electrolyte ions, thus decreasing the capacitance performance.Mesopores with pore sizes less than 10 nm can improve bulk transfer and enhance the surface area, while macropores can increase specific capacitance and pore volume.The micropores, mesopores, and macropores are connected to form a 3D hierarchical structure in porous carbon, which provides better rate performance, improved surface area, and minimizes the diffusion route for electrolyte ions. [67,68]Figure 3 shows the classification of various porous carbons based on their pore sizes.

Porous Carbons
Porous carbons are typically prepared through a combination of carbonization and activation processes.Carbonization involves subjecting carbon precursors to high temperatures ranging from 400 °C to 1000 °C under an inert atmosphere using carbonizing agents.
The resulting product is a nonporous solid carbon known as biochar or coal char.Pore formation is achieved during activation, which uses porogens to create pores in the carbon structure.Pore-forming agents are termed porogens.Activation can be achieved through chemical or physical means.In physical activation, air, [69] CO 2 , [70,71] H 2 O or O 2 are activated to oxidize biochar or coal char.While in chemical activation, H 3 PO 4 , [72,73] Na 2 CO 3 , [74] KOH [75,76] or ZnCl 2 [77][78][79] are also used to oxidize biochar or coal char.Figure 4 shows the different activation strategies to fabricate porous carbons.A new type of porous carbon derived from laminaria japonica was synthesized by Cheng et al. by using a simple carbonization-activation process, with KOH as the activation agent.The resulting porous carbon was analyzed for its structural, electrochemical, and morphological properties.
The results showed that this porous carbon had a very high specific surface area of 1902.42 m 2 g À 1 and contained a total pore volume of 1.26 cm 3 g À 1 .The specific capacitance of this porous carbon was measured at 192 F g À 1 at 0.1 A g À 1 compared to a typical activated carbon having a specific surface area of 1016.4 m 2 g À 1 and a specific capacitance of 169.2 F g À 1 at 0.5 A g À 1 current density. [83]Various porous carbons have been reported through templating routes.This synthesis results in a very fine pore configuration and small pore size.The hard template strategy usually involves Al 2 O 3 , MgO, TiO 2 , ZnO, [80] and SiO 2 . [81]In the soft template fabrication process, the templates comprise organic compounds with different functional groups. [82]In the self-template synthetic process, the porous carbons are fabricated through the direct carbonization of biomass-based organic salts, MOFs, EDTA-based salts or glycolates, regarded as self-template materials. [83,84]Direct pyrolysis is another method used to fabricate porous carbons.This process involves decomposing carbon sources, such as polyionic liquids, biomass, and copolymers, which release gases like CO, NH 3 , CO 2 , and H 2 O that act as porogens.[87] One of the practical challenges in the research of supercapacitors is the limited surface area, low porosity, and inadequate electrochemical reaction of pure carbon materials, leading to low specific capacitance.To address this issue, researchers are exploring the addition of heteroatoms to porous carbon materials, which can increase surface porosity and the number of active sites,   thereby improving their performance.Figure 5 demonstrates the effect of hetero-atom doping on porous carbon.The subpar electrochemical performance observed in MOFs (Metal-Organic Frameworks) and pyrolyzed carbon can be attributed to their narrow pore size distribution and limited efficiency in ion transport.To overcome this challenge, researchers have shifted their focus to flexible foam-like open porous materials as an ideal substrate.These materials possess several advantageous characteristics, including superior deformation tolerance, high conductivity, and three-dimensional interlinked frameworks.By harnessing the potential of flexible foam substrates, it becomes possible to significantly enhance the electrochemical performance of supercapacitors.This approach effectively addresses the limitations associated with MOFs and pyrolyzed carbon, offering improved ion transport efficiency and enabling better utilization of the available surface area.Consequently, the utilization of flexible foam substrates represents a promising strategy to unlock the full potential of supercapacitors and advance their electrochemical performance.

Nitrogen-Doped Porous Carbons
N-doped porous carbons have emerged as promising materials for electrochemical energy storage, owing to their exceptional properties and performance advantages.The incorporation of nitrogen atoms into the carbon framework brings forth distinct benefits, including the introduction of additional pseudocapacitance and the enhancement of the material's overall capacitance.Introducing heteroatoms into porous carbons leads to the occurrence of the pseudo capacitance phenomenon through a Faradaic redox reaction, resulting in a higher specific capacitance value. [88]The chemical stability, electron donor characteristics and electric conductance of carbons can also be enhanced by introducing heteroatoms, resulting in better electronic and crystalline characteristics. [89,90]Figure 6 shows the effects of heteroatom doping on the porous carbon material.Nitrogen atoms are most often used as dopants among several heteroatoms.Nitrogen is more electronegative than carbon, allowing modification of the carbon matrix electronic structure. [90]Nitrogen doping provides extra electrons and amplifies the hydrophilicity and porosity of the material and content of N-doping can also be controlled.Nitrogen doping also facilitates the adsorption of ions by

R e v i e w T H E C H E M I C A L R E C O R D
providing more active sites.N-doped porous carbons have versatile pore sizes and a high specific surface area. [91,92]The synergy between heteroatoms, particularly nitrogen, and the carbon framework in nitrogen-doped porous carbons yields remarkable improvements in the material's electrochemical performance.Nitrogen doping plays a crucial role by introducing additional pseudocapacitance, thereby significantly enhancing the energy storage capacity of the material.Moreover, nitrogen doping enhances the electrical conductivity of the carbon framework, facilitating efficient charge transfer during electrochemical processes.Furthermore, nitrogen incorporation promotes the formation of defects and functional groups within the carbon structure.These defects and functional groups contribute to an increased electrochemically active surface area, providing more sites for charge storage and exchange.This synergistic effect between nitrogen, defects, and functional groups results in superior electrochemical performance, positioning nitrogen-doped porous carbons as highly promising candidates for high-performance supercapacitor electrodes.Usually, the fabrication of N-doped porous carbon moieties involves two processes; one is through the posttreatment method, while the other is through an in-situ strategy.In the post-treatment approach, the porous carbon is thermally treated with melamine, [93] ammonia, cyanamide, [94] urea or polyaniline.While in the in-situ strategy, N-containing moieties, i. e., organic polymers, metal-organic frameworks, ionic liquids, and biomass, undergo direct pyrolysis.Wei et al. fabricated N-doped porous carbon with an enormous surface area by the volatilization of Zn-EDTA complexes in which EDTA had high nitrogen levels and zinc species showed an activation ability.The resultant porous carbon's surface area and morphological characteristics relied on the morphological features of the precursor Zn-EDTA complex. [88]Wang et al. developed a method to prepare N-doped porous carbon through volatilization at high temperatures using the NaCl and KCl salt sealing technique. [99]They added increasing amounts of saturated NaCl solution to egg white, causing the salt components to penetrate the carbon structure at elevated temperatures.Through a complex interaction of water molecules and salt components, it was possible to create Ndoped porous carbon with a mesoporous framework. [95]Das et al. reported the fabrication of N-doped porous carbon by a carbonization activation process that utilized KOH as an activating agent.EDTA being used as precursor of both carbon and nitrogen, promoted the in situ doping of nitrogen with maximum nitrogen content. [96]Wang et al. developed poly Schiff base microspheres via a dispersion polymerization reaction utilizing readily available terephthalaldehyde and aromatic tetraamine.The incorporation of heteroatoms occurred in situ, leading to the binding of nitrogen atoms and their uniform distribution throughout the carbon structure. [97]mall pore size distribution and limited ion transport efficiency in MOFs and pyrolyzed carbon led to poor electrochemical performance in supercapacitors.To address this, flexible foam materials with open porous structures are considered ideal substrates due to their superior deformation tolerance, high conductivity, and interconnected 3D frameworks. [98,99]Zhao et al. (2020) successfully fabricated a composite called N-doped carbon particle-carbon foam (N-CP-CF) from ZIF-8.This composite exhibited a large surface area and exceptional flexibility, enabling it to tolerate significant deformation.As electrodes in a supercapacitor, the N-CP-CF demonstrated outstanding capacitance, reaching 300 F g À 1 at a current density of 0.5 Ag À 1 . [99]olymers are becoming increasingly popular as precursors due to their ease of fabrication and the controllable number of heteroatoms they contain.Among the many available polymers, environmentally stable polypropylene, which has high conductivity, has shown promising results for use in supercapacitors.Zong et al. synthesized nitrogen-doped porous carbons using polypropylene as a carbon precursor, with NaCl/ZnCl 2 mixed salts used as activation agents and templates during the carbonization process. [100]Zhang et al. produced N-doped porous carbon materials by co-pyrolyzing poly(acrylic acid-co-maleic acid), sodium salt (PAMS), and urea, which was used as a nitrogen dopant.The resulting nitrogen-doped porous carbon materials exhibited a high surface area of 181.5 m 2 g À 1 with nitrogen doping content of 10.61 atom %, a tunable porous structure, and improved conductivity. [101]N-rich biomass is now an eminent green source of self-N-doped activated porous carbons.Szabó et al. developed self-N-doped porous carbon materials in a single thermal step using chitosan and poly (ethylene oxide) as a porogen to obtain an ultrafine porous nanostructure as a carbon source.The resulting electrode exhibited promising electrochemical performance due to the interconnected Ndoped carbon and its ultrafine nanostructure. [102]Li et al. reported the synthesis of N-doped hierarchical porous carbon by simultaneously adding KOH and melamine during the synthesis process.The combination of these two additives synergistically influenced pore formation and N-doping.The resulting product had a hierarchical porous structure with a large specific surface area of 2642 m 2 g À . [1 103]

Applications: Architecture Design and Performance Evaluation
The performance of supercapacitors, particularly their power density, energy density, and cycling stability, has been steadily improving thanks to the development of new material synthesis methods, electrolytes, and improved device designs.As research and development continue, supercapacitors are expected to be increasingly important in advancing environmentally friendly and high-performance energy storage technologies. [104,107]To ensure the proper functioning of supercapacitors (SCs), it is important to consider the volume and active masses of the prepared material.Other essential aspects that must be considered include the bulk loading, area, and thickness of both electrodes and separators.Increasing the thickness or mass loading of the respective electrodes while maintaining the same area can improve the capacitance of SCs.However, this strategy may reduce power density and increase the ion diffusion length and electrical resistance.Therefore, the design and performance assessment of the electrode is essential for utilizing SCs effectively. [105]hih et al. studied hybrid electrodes of electrochemically exfoliated graphene with carbon nanotubes (ECG/CNTs).They found that the hierarchical structure of the electrode can improve conductivity and increase the accessible surface area, resulting in enhanced performance.The study also optimized the geometric patterns, active materials, current collector of supercapacitors, and electrolyte concentration, resulting in a high volumetric capacitance of 77.3 F cm À 3 at 5 mVs À 1 and an aerial capacitance of 7.7 mFcm À 2 at 5 mVs À 1 .The ECG/CNTs hybrid electrode also exhibited excellent mechanical flexibility, with a capacitance retention of over 90 % even at a bending radius of less than 0.5 mm, as well as long-term operation stability, with a capacitance retention of over 99 % after 15,000 charge/discharge cycles. [106]i et al. conducted a study that showed increased current density with increasing finger width (from 300 to 600 μm) while keeping other parameters constant.However, when the electrode area was considered, it was observed that the areal capacitance decreased when widths exceeded 500 μm.This suggests that diffusion length becomes more critical at larger widths while conductivity is important.By optimizing all of these parameters, the study achieved a high areal capacitance of 0.7 mFcm À 2 . [107]A comprehensive analysis of various parameters is necessary to evaluate the performance of supercapacitors, and one such parameter is self-discharge.Self-discharge refers to the gradual loss of stored energy or charge over time, even when the supercapacitor is not in use or disconnected from an external circuit.It is an inherent property of supercapacitors and can occur due to charge redistribution, ohmic leakage, and parasitic faradaic reactions.These mechanisms can result in the dissipation of stored charge and reduce supercapacitors' overall performance and energy storage capacity over extended periods.Managing selfdischarge is a fundamental element in the operation and design of supercapacitors since it can affect their overall performance and efficiency in various applications. [108]Rapid discharge of the stored electric energy due to self-discharge and spontaneous voltage decay significantly limits their usefulness.Because of this, studying the causes and consequences of self-discharge and developing effective methods for preventing it is significant. [109]Assessing supercapacitor performance in wearable electronics necessitates a thorough evaluation of its flexibility.However, there exist limited parameters that are currently utilized for this purpose.Li et al. have suggested a systematic testing procedure for analyzing the performance of flexible energy storage systems.Examining device length (L), bending radius of curvature (R), and turning angle (θ) are the primary three parameters that have been investigated in this research.In addition, a leather and fabric softness tester, which is available commercially, can be used to verify the supercapacitor's softness. [110,111]

Porous Carbon
When designing porous carbons for high capacitance applications, pore size and pore distribution are essential considerations.The pore size and distribution of the carbon material determine the accessibility of the electrolyte to the surface area of the carbon, which affects the double-layer capacitance.While porous carbon materials offer many benefits, their poor power density and capacitance have hindered their widespread commercial use.To overcome these limitations and enhance their practical viability in SCs, recent efforts have focused on improving the morphology and pore structure of these materials.Developing porous carbon materials with a suitable structure is essential to address this challenges. [112]rali et al. used interconnected activated carbon that showed a specific capacitance and surface area of 210 F g À 1 at 1 Ag À 1 and 929.9 m 2 g À 1 , respectively, and it remained stable for over 10,000 cycles at 5 Ag À 1 in 1 M H 2 SO 4 electrolyte. [113]ang et al. produced porous carbons with tunable pores using ammonium lignosulfonate and an activating agent, NaCl.The temperature was changed from 800 to 990 0 C, enhancing its conductivity.Ma-NaCl-700 showed a capacitance of 215 F g À 1 at 0.25 Ag À 1 and an exceptional rate capability of 156 F g À 1 in 6M KOH at 50 Ag À 1 due to a suitable heteroatom ratio.If the heteroatom content is too low, the catalytic activity may not be enhanced, and if it is too high, it may decrease the material's surface area and pore volume.Therefore, the optimum ratio of the heteroatom in porous carbons is significant for improving catalytic activity by increasing their surface area and porosity. [114]Song et al. used an economic and ecological alternative to heavy metals and their salts, using KCl to grow carbon nanotubes.Notably, the catalytic properties of potassium chloride are unique and irreplaceable, albeit with a narrow temperature range.KCl itself is not a catalytic material, but rather it acts as a catalyst precursor.During the carbonization process, the KCl undergoes a chemical reaction, forming catalytic active sites that promote the growth of carbon nanotubes.These active sites can be potassium carbonate (K 2 CO 3 ) or potassium oxide (K 2 O), the catalysts for carbon nanotube growth.Furthermore, KCl is recyclable and inexpensive.The resulting product, CC-KCl-700, features distinctive porous carbon nanotubes and is notable for its specific capacitance of 334.4 F g À 1 .It gives a retention rate of 87.2 % over 10,000 cycles.The Faradaic efficiency was determined to be 100 % after 10000 cycles. [115]ihua et al. has proposed another effective process for regulating the pore morphologies of various porous carbons by co-pyrolyzing organic compounds and cotton stalks. [123]he mesopore volume and specific surface area were determined to be 0.24 cm 3 g À 1 and 1510 m 2 g À 1, respectively, resulting in a specific capacitance of 345 F g À 1 at 1 A g À 1 , exceptional rate performance with a 69 % retention rate at 50 A g À 1 using 6 M KOH, 1 M Na 2 SO 4 , and an ionic liquid as an electrolyte.This work expands the possibilities for copyrolysis of various precursors to develop porous carbons and highlights the necessity of balancing SSA and porous structure in supercapacitors' capacitance and rate performance. [116]Hydrothermal treatment of fungus, followed by carbonization of the material, was the method that Conglai et al. used to fabricate highly porous and layered carbon nanosheet structures. [124]This material possesses a high-power density, significantly less pore size organization, a large specific surface area of 1103 m 2 g À 1 , and an interlinked and hierarchical porous framework.These characteristic features contribute to the material's high volumetric and specific capacitance of 360 F cm À 3 and 374 F g À 1 , respectively, with a remarkable capacitance retention of 99 % over 10000 cycles.This study highlights a practical approach that utilizes economic and environment-friendly biomass material (fungus, Auricularia) to fabricate new storage materials, enabling high-performance supercapacitor applications. [117]in et al. have reported a porous carbon-based material characterized by a remarkable SSA of 3710 m 2 g À 1 .This material was synthesized using β-cyclodextrin, which resulted in abundant porosity and a large pore volume that significantly enhanced its electrochemical properties and achieved a remarkable capacitance equivalent to 416 F g À 1 at 0.5 A g À 1 118 .In another work, Ming et al. successfully used inexpensive anthracite to make it an ideal candidate in SCs. [127]This activated porous carbon material has an impressively large SSA and porous volume of 550.7 m 2 g À 1 and 2.168 cm 3 g À 1, respectively.The specific capacitance was determined to be 433 F g À 1 at a current density of 0.5 A g À 1 .The material maintained an excellent specific capacitance of 230 F g À 1 , even when subjected to a current density of 50 A g À 1 , resulting in a retention rate of 61.7 % over 30,000 cycles. [119]If we compare the energy density of supercapacitors with batteries, the lower density related to SCs is one of the issues that has severely hampered their practical implementation.Thus, it has become an essential demand for the next generation SCs to improve both their specific capacitance and energy density.To accomplish this goal, it is necessary to consider several parameters, including pore morphology, heteroatom ratio, surface area, electrode architecture and the presence of structural defects.These parameters heavily affect the electrochemical activity of Ndoped porous carbons, which plays a vital role in developing supercapacitors. [120]Designing porous carbons for highcapacitance applications necessitates careful consideration of pore size and distribution.Pore characteristics influence the accessibility of electrolytes to the carbon surface, thereby impacting double-layer capacitance.Despite their advantages, limited power density and capacitance have hindered the widespread commercial use of porous carbon materials.Recent efforts have aimed to enhance the morphology and pore structure of porous carbons, aiming to overcome these limitations and increase their practical viability in supercapacitors.Optimizing pore morphology, heteroatom ratio, surface area, electrode architecture, and structural defects is crucial for advancing the development of next-generation supercapacitors with improved specific capacitance and energy density.

Nitrogen-doped Porous Carbon
Although carbon-derived electrodes have numerous desirable characteristics, their limited energy density significantly impedes commercialization.To overcome this, current research efforts are focused on enhancing their energy density and minimizing the cost of electric double-layer capacitors (EDLCs) while preserving their power density.Multiple approaches are being investigated to synthesize carbon-based materials, including increasing the surface area, optimizing the pore size of electrode materials and the pore size distribution, integrating heteroatoms such as oxygen, nitrogen, phosphorus, and sulfur, and operating cell voltage.Nitrogen has garnered considerable attention among these heteroatoms due to its higher electronegativity, comparable size to carbon, and ease of synthesis.Nitrogen possesses a higher electronegativity of 3.04 than carbon, which induces polarization and donates its lone pairs, resulting in structural and reactive variations.Incorporating nitrogen atoms into carbon materials can affect a capacitor's capacitance by altering the material's electronic properties.In certain cases, N-doping can enhance capacitance by modifying the charge storage behavior and improving the material's conductivity, thereby increasing its potential to store and release electrical charge.123] Xu et al. used rice straw to produce N-doped porous carbon via a hydrothermal treatment, which was then activated with KHCO 3 (Figure 6a).The addition of melamine in this work enhanced mesoporosity and a specific surface area of 2786.5 m 2 g À 1 , as determined in Figure 6b.The resulting product exhibited a remarkable specific capacitance of 317 Fg À 1 at 1 Ag À 1 current density (Figure 6c). [124]Long et al. have produced a surface-modified three-dimensional hierarchical porous carbon using a slightly modified KOH reactivation technique with PPy micro sheets as the precursor material.This carbon material has been functionalized in three dimensions.The product that was produced, which is known as THPC, possesses a highly favourable 3D hierarchical permeable nanostructure, having appropriate pore volume, a substantial SSA equal to 2870 m 2 g À 1 , a significant dopant quantity in which oxygen is 12.4 wt.% and nitrogen is 7.7 wt.% with excellent electrical conductance equivalent to 5.6 S cm À 1 .THPC exhibits extraordinary capacity when used as a material for SC, in addition to superior cyclic stability and long-term durability. [125]iamiao et al. modified nitrogen-doped permeable carbon sheets with graphene oxide (GO).The use of urea increased nitrogen content from 3.08 at% to 4.12 at% in the carbon structure.The resulting reduced graphene oxide nanosheets (rGONS) displayed variable pore structures and a high ratio of nitrogen doping.Following the electrochemical studies, the capacitance of rGO/NS was checked in 6M KOH electrolyte and determined to be 199.5 F g À 1 at 0.5 A g À 1 with stability of 94.4 %. [126] As demonstrated by Rika et al., the activation method was used to create three-dimensional doped carbon, which incorporated self-doping atoms derived from waste bread. [134]The required pore shape was achieved by significantly raising the temperature of physical activation.The remarkable specific capacitance was 202 F g À 1 at 1 A g À 1 in 1 M H 2 SO 4 .The total volume and SSA were also determined and found to be 0.391 cm 3 g À 1 and 610.50 m 2 g À 1 , respectively.The as-synthesized electrode material was manufactured in a solid coin configuration without any binders.In addition, the carbon created by heating it to 850 °C had a power density of 156.71 W Kg À 1 and a low ohmic resistance of 0.11 ohms.Its highest energy density was 11.61 Wh Kg À 1 . [127]iang et al. reported a feasible CO(NH 2 ) 2 -C 6 H 5 O 7 (NH 4 ) 3 -co-assisted C 6 H 5 K 3 O 7 (UA-co-assisted P) strategy to make nanohybrid NPC with a specific capacitance and specific surface area of 203.9 F g À 1 and 3280.16m 2 g À 1 , respectively. [135]These remarkable features are attributed to the synergistic effects of urea and ammonium citrate.Its cyclic stability is 88.7 % over 50 cycles, indicating its potential practical application. [128]A hierarchical 3D structure of porous and nitrogen-doped carbon electrodes was synthesized by Junfeng et al. using a suitable oxidation approach with magnesium sulfate (MgSO 4 ) employed as an "interlayer" and protective factor under alkaline conditions. [136]This methodology resulted in morphological changes and an increased specific surface area while maintaining a high nitrogen content.
Furthermore, the calcination process induced the generation of additional pores, enhancing the capacitance of the respective electrode material.Furthermore, incorporating MgSO 4 into the nanosheets' interlayer effectively blocked the nanosheets stacking in the final product, which resulted in a specific capacitance of 215 F g À 1 at 1 A g À 1 and stability of 96.1 % after 5000 discharge/charge cycles. [130]Zhang et al. designed a method for producing porous N-doped activated carbon on a large scale via one step using low-cost chemicals and commercial activated carbon by a chemical vapour deposition method (Figure 7a).The fabricated NAC material with 3.1wt % N represents a specific area of 1186 m 2 g À 1 and a specific capacitance of 427 F g À 1 in 1 M H 2 SO 4 .After performing 20,000 cycles, 98.2 % of the capacity is retained at 20 A g À 1 .The energy densities of synthesized NAC were 87.8 and 17.2 Wh kg À 1 in acidic and organic electrolytes, respectively.The results of the electrochemical studies are presented in Figure 7(b-g).
Furthermore, the variations in capacitance before and after treatment are also illustrated in Figure 7h. [129]Zhen et al. have successfully developed a highly efficient method for producing carbon composite electrode materials from biomass, which exhibits exceptional electrochemical performance. [138]The method utilizes zeolitic imidazolate framework ZIF-8 as a nitrogen source, and the carbon derived from wood pulp forms a conductive network that facilitates the movement of electrons.The as-synthesized composite has a specific capacitance of 270.74 F g À 1 and a surface area of 593.52 m 2 g À 1 .This composite material demonstrates a stability of 98.4 % over 10,000 cycles, indicating its excellent electrochemical stability.This research represents a promising method for producing interconnected porous and nitrogen-doped carbon composites that exhibit outstanding performance in energy storage applications. [131,132]Using a HIPE (high internal phase emulsion) template, Xeu et al. synthesized polyaniline (PANI) arrays on porous carbon for effective N-doping. [139]he material's shape, structure, and electrochemical performance can be changed by altering the aniline content and polymerisation time.Initially, the specific surface area (SSA) and specific capacitance (SC) of the porous carbon increase with an increase in aniline concentration or polymerization time.However, beyond a certain limit, polyaniline (PANI) tends to nucleate and detach from the surface, decreasing nitrogen content.At a current density of 1 A g À 1 , the SSA and SC were 1769 m 2 g À 1 and 241 F g À 1 , respectively.The material exhibited good energy storage performance with a retention rate of 97.8 % over 10,000 cycles. [133]Wang et al. developed a rapid and versatile approach to synthesizing twodimensional NPC nanosheets. [140]These nanosheets serve as a model system to investigate the correlation between carbon surface characteristics and electrolyte redox chemistry.The specific capacitance of these nanosheets was calculated to be 251 F g À 1 , and their SSA was determined to be 1243 m 2 g À 1 .
The cyclic stability of these materials was estimated to be 86.5 % over 30,000 cycles, demonstrating their potential for long-term energy storage applications. [134]he biomass-derived porous carbons synthesized in the presence of magnesium nitrate yield a large BET surface area of 2700 m 2 g À 1 , attributed to the activating effect of potassium that enhances the production of micro/mesopores.The resulting porous carbon material exhibits positive electrochemical capacitance performance when utilised as electrodes in supercapacitors.It shows a specific capacitance value of 279 F g À 1 at 1 A g À 1 , exceptional cycling stability of 89 % over 10,000 cycles at 2 A g À 1 , and a rate capability of 235 F g À 1 at 30 A g À 1 . [135]Zheng et al. developed a novel method for synthesizing nitrogen-doped porous carbons using industrial alkali lignin (IAL) through urea-assisted carbonization. [142]The combination of urea and alkali had a synergistic effect, resulting in the formation of nitrogendoped materials.The key factor in this process was the presence of an alkali species, which played a critical role.The resulting porous carbon material exhibited a high specific surface area (SSA) of 1482 m 2 g À 1 and a nitrogen concentration of 7.78 %.The specific capacitance was 283.4 F g À 1 , indicating that this material has the potential to be a valuable electrode material for supercapacitors. [136]nna et al. synthesized nitrogen-doped porous carbons using gelatin and green algae, which is cost-effective, environmentally friendly, and renewable.This material was reported via carbonization at 800 °C under nitrogen. [143]Their analysis showed that the capacitance values of the samples were significantly affected by the morphological characteristics of the carbon electrodes with low nitrogen content.The carbon sample had a nitrogen content of 0.52 % by weight, an SSA of 2270 m 2 g À 1 , and a specific capacitance of 287 F g À 1 at 5 A g À 1 .The retention rate after 10,000 cycles was 86 %, indicating a promising performance rate.The nanostructured active carbon electrode exhibited exceptional capacitive performance and was produced using an economical and straightforward process, making it a promising candidate for use in supercapacitors.The materials' unique composition and properties, including an acceptable distribution of pore sizes and adequate electrochemically active sites, were responsible for these impressive results. [137]According to research conducted by Wei et al., a practical approach utilizing dual-hydroxide activation linked to the self-temple was identified as the most effective method for producing hierarchical NPC using waste shells. [144]The resulting material has a morphology like a honeycomb, an SSA of 246 m 2 g À 1 , a specific capacitance value of 300 F g À 1 , a high graphitization degree, an appropriate doping ratio and stability of 79.8 % over 50000 cycles.These factors collectively contribute to its favorable characteristics of rapid ions/ electron transport, ample charge storage, and notable pseudocapacitance. [138]In a separate study, Xu and colleagues employed hydrothermal carbonization and ZnCl 2 activation to produce NPCs with varying nitrogen ratios. [145]They investigated the impact of nitrogen ratio on the electrochemical performance of NPCs.The sample named NPC7.5 had an SSA of 1465.6 m2 g À 1 and exhibited a decrease in the quantity of less active quaternary N.They also observed that the amount of electrochemically active pyridine N increased as the urea to chitosan ratio increased from 0 to 7.5.Consequently, this adjustment in nitrogen ratio resulted in improved electrochemical performance, with NPC7.5 achieving the highest specific capacitance of 303.2 F g À 1 . [139]heng et al. successfully fabricated a hierarchical structured porous carbon through an activation with KOH. [146]his material has a large surface area, facilitating ion conduction and adsorption.Additionally, it has a specific capacitance value of 312 F g À 1 at 0.5 A g À 1 in 6M KOH. [140]n another work, Zhou and his colleagues successfully synthesized a hierarchically permeable carbon material with a 3D architecture using a sustainable approach involving in situ templating, KOH activation, and the addition of urea from pectin. [116]The resulting material showed a specific capaci-tance of 338 F g À 1 at 1 Ag À 1 , with a stability of 83 % at 50 A g À 1 using 6 M KOH and 1 M Na 2 SO 4 electrolyte.This response is due to its high SSA, rich heteroatom ratios, appropriate micropore/mesopore size, and unique 3D carbon architecture. [141]Bai et al. synthesized nitrogen-doped porous carbon (NDPC) using the TIPS method, as shown in Figure 8a, [147] which involves sequential carbonization and activation with konjac/polyacrylonitrile and an N source.This combination led to an enlarged SSA of 2125 m 2 g À 1 , as shown in Figure 8b-c, high nitrogen doping content of 1.54 atom %, and an enhanced hierarchically porous structure, as shown in Figures 8d-i from SEM images.The fabricated material showed promising potential, with a 390 F g À 1 capacitance resulting from the union of pseudo and EDL capacitance and an enhanced stability of 70 %.These findings could inspire researchers to develop future NDPC materials for efficient energy applications. [142]Gong et al. used a green one-step approach to produce N-doped porous carbon. [147]In this carbonization process, doping of the N atom and graphitization took place in a single step using K 2 FeO 4 (Figure 9a).

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SEM images showed that the N-doped porous carbon had a 3-D interlinked framework with enormous macropores (Figure 9b,c), while TEM images illustrated the microporous framework (Figure 9 d-f).The resulting product showed an ideal EDLC response in Figure 10g.This NPC material had an outstanding specific capacitance of 332 Fg À 1 at a current density of 0.5 A/g and a high surface area of 1770 m 2 g À 1 , with a nitrogen doping level of 5.9 atom% after 10 hours of synthesis. [143]riya et al. has successfully synthesized nitrogen-functionalized microporous carbon through a self-doping solvothermal approach, using ethylene diamine as a precursor. [149]The resulting products showed nitrogen functionalization within the carbon framework, and activation with KOH disrupted the materials' micro-porous morphology.The researchers discovered a correlation between the KOH/C ratio fluctuations resulting from activation and the microporosity, specific area, and nitrogen ratio present in the materials.The NC-2 material exhibited an SSA of 1202.8 m 2 g À 1 , determined through the BET technique, and a pore volume of 0.596 cm 3 g À 1 .The exceptional supercapacitor response (353 F g À 1 ) was due to successfully tuning the material's structure. [144]Hua et al. utilized Zn-bioMOF to synthesize NPC material using the etchants-free method. [150]Etchantfree method refers to a process or technique that does not involve using any etchant or chemical substance that removes material from a surface.In this study, the sample named H-NCNs-8 was considered the best material due to its hierarchical porosity and a high degree of graphitization.These factors contributed to its high capacitive performance and 94.5 % stability even after 40,000 cycles.NPC was also This method allows other researchers to synthesize carbon-based materials derived from polysaccharides. [151]hang et al. synthesized NPC from houttuynia biomass, utilizing a three-dimensional hierarchically porous architecture resulting in unique flower-like structures. [152]The resulting material exhibited an SSA of 2090 m 2 g À 1 and a specific capacitance of 473.5 F g À 1 at 1 A g-1 , with stability over 50 % when subjected to a current density of 20 Ag À 1 .These findings suggest that the innovative 3D porous material could be used as an efficient electrode in supercapacitor applications.Xiaoguang et al. developed a flexible 2D hierarchical porous carbon material through a one-step process of etching potassium oxalate with the blowing effect of melamine. [153]he combined effect of potassium oxalate/melamine tailored the physiochemical properties of the glucose-driven carbons.The resulting 2D-NPC exhibited improved energy storage properties, such as a hierarchical pore structure, 2D carbon nanosheet structure, exceptional flexibility, high nitrogen content (6.1 at.%), and a high SSA of 1730 m 2 g À 1 and specific capacitance of 523 F g À 1 in 6M KOH.The symmetric supercapacitor showed a high energy density of 108 Wh kg À 1 at 900 W kg À 1 .This fabrication method provides a one-step procedure for producing higher-performance electrodes.Figure 10 (a-k) provides schematic diagrams, device fabrication, SEM images, and electrochemical performance results. [145]u et al. effectively synthesized a PC material incorporating nitrogen dopant.They used a synthesis process that involved the synchronized addition of KOH and melamine as additives.The combined effect of these two additives synergistically promotes pore formation and N-doping in the final product.The carbon-based material with a hierarchical morphology produced has a high specific capacitance and surface area of 715 F g À 1 at 1 A g À 1 and 2642 m 2 g À 1, respectively.The as-synthesized material was also tested at 100 A g À 1 , which still gave the capacitance value of 526 F g À 1 .

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Additionally, the material has remarkable durability, retaining almost the same preliminary capacitance even after 5000 cycles at 5 A g À 1 , further proving the material's higher performance.Because of the increased presence of N in the material, it has incredible cycling stability and remarkable specific capacitance.Notably, the structural doping approach employed in this study to enhance energy density without compromising cycling stability offers a reliable strategy for material modification, surpassing traditional surface modification techniques. [146]Designing porous carbons for high capacitance applications requires careful consideration of parameters such as pore size, distribution, and morphology.Recent studies have investigated various strategies to enhance the capacitance of porous carbons, including interconnected activated carbon, tunable pore structures, catalyst precursor utilization, co-pyrolysis techniques, and hydrothermal treatment.These approaches have demonstrated significant improvements in specific capacitance, surface area, and retention rates, highlighting the potential of porous carbons for advanced supercapacitor applications.However, further research is needed to optimize the balance between these parameters and explore novel approaches to enhance the energy density of supercapacitors, bridging the gap between current capabilities and practical implementation.

Role of Pores of Nanostructured Carbons
To optimize the electrochemical performance of supercapacitors, it is crucial to carefully consider the pore structures and distribution within the electrode materials.These factors are pivotal in determining supercapacitors' overall efficiency and effectiveness.It is widely recognized that micro/mesopores in the electrode material greatly aid in ion transport by providing improved inter-pore connections.As a result, carbon-based electrodes with meso-or micropores are preferred for supercapacitor applications over those without such pores.Recently, carbon materials with three-dimensional interconnected hierarchical structures have gained significant attention as promising candidates due to their porosity, which provides a large surface area and continuous 3D pathways for efficient ion conduction and mobility within the device.. [147,148] In another study, pore design was studied to improve the capacitance of electrode materials.The main factors associated with pore design include pore size, distribution, and morphology, which significantly affect charge transfer and ion transport processes, ultimately affecting the electrode material's capacitance and rate capabilities. [149]The relationship between the pore size and energy density in porous carbons with a restricted size distribution is non-linear, meaning that changes in pore size do not result in proportional changes in energy density.This indicates that achieving maximum power density requires the electric voltage to influence the precise pore size, which tends to saturate at high potentials.However, optimizing the distribution of pore sizes can lead to higher energy density.Therefore, developing monodispersed porous carbons with the appropriate pore sizes is highly desirable as optimal electrocatalysts in supercapacitors (SCs).The pore aspect ratio, between pore diameter and length, examines the pore shape of regular mesoporous carbons.The ion transfer process kinetics can be described using Equation (1) by applying the principles of traditional electrochemistry. [158] In the context of ion transport, the variables denoted as τ, L, and D correspond to the time, length, and coefficient associated with the transportation of ions, respectively.
Yu et al. developed a hydrogel carbonization approach to creating highly porous carbons with customizable pore shapes and excellent specific surface area.Polyacrylamide was used as a pore-forming agent, creating closed pores that were expanded and revealed by activation with potassium hydroxide.The resulting highly porous carbons (HPCs), such as HPCs-60, had a maximum specific surface area of 3381 m 2 /g and an optimal pore size distribution.The HPCs showed a specific capacitance of 441 F g À 1 and a good energy density of 10.9 Wh kg À 1 , even after 20,000 cycles with a modest 10 % capacity loss.The synthetic process offers promise for creating tunable PC materials suitable for various electrochemistry applications, and HPC-based supercapacitors show flexibility, cycling stability, and potential for series connection to meet energy requirements. [150]garaj et al. synthesized NPC nanosheets from biomass waste using cost-effective and environmentally safe methods.The nanosheets had well-defined micropores and mesopores in all three dimensions, ideal for supercapacitor electrodes.Surface oxygen groups contributed to their exceptional double-layer chemical and pseudocapacitance properties.The BDPCNS electrode had a maximum specific capacity of 290 F g À 1 at 1 A g À 1 , 93.4 % stability over 1000 cycles, and a high energy density of 40 Wh kg À 1 at 208.5 W kg À 1 power density.These remarkable features were due to the large interfacial area, unique framework, low charge-transfer resistance, and minimal structural resistance. [151]Nanostructured pores are critical in improving the performance of supercapacitors by boosting their surface area, facilitating ion transport, enhancing electrolyte accessibility, controlling pore size and distribution, and enabling surface functionalization.These attributes synergistically boost capacitance, enhance power density, and prolong cycle life, establishing nanostructured pores as a crucial element in developing highperformance supercapacitor materials.Optimizing pore structures in electrode materials is crucial for enhancing the electrochemical performance of supercapacitors.Carbonbased electrodes with well-defined micro/mesopores offer improved ion transport and interconnectivity, making them highly desirable for supercapacitor applications.Threedimensional interconnected hierarchical carbon materials have emerged as promising candidates, thanks to their large surface area and efficient ion conduction pathways.Pore design, including size, distribution, and morphology, significantly influences charge transfer, ion transport, capacitance, and rate capabilities of electrode materials.By carefully considering pore size and distribution and developing monodispersed porous carbons, researchers can create optimal electrocatalysts that exhibit enhanced supercapacitor performance.Nanostructured pores play a crucial role in increasing surface area, promoting ion transport, facilitating electrolyte accessibility, controlling pore characteristics, and enabling surface modifications.This ultimately leads to high-performance supercapacitors with improved capacitance, power density, and stability.

Effects of Surface Areas
In a study to improve the specific capacitance of PC, [167] it has been suggested that there is a linear correlation between SSA and capacitance, as shown in Equation 2.
The hypothesis that increasing the area of the PC efficiently improves specific capacitance has been put for-

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ward.However, experimental findings have not supported this notion, as no such linear correlation has been observed. [117,152]In a theoretical framework, assuming an average EDL (Electrical Double Layer) capacitance of 20 μFcm À 2 and a PC material with a BET (Brunauer-Emmett-Teller) surface area of 920 m 2 g À 1 , it would be theoretically possible to achieve the highest capacitance of 184 F g À 1 .
However, experimental results suggest that the actual specific capacitance obtained was only 80 F g À 1 .The study shows that only 44 % of the added surface area contributes to the observed capacitance, indicating that increasing the surface area of PC materials may not significantly improve capacitance.The estimate of SSA for these materials is done using gas adsorption/desorption isotherms such as N 2 , Ar, or CO 2 .
During ion transport through pores, some surface areas may not be fully accessible due to size differences between ions and pores.Achieving optimal capacitance in porous carbon materials involves balancing specific surface area and pore structure.Larger surface areas, predominantly consisting of micropores, contribute to increased specific surface area and enhance volumetric-specific capacitance.Therefore, careful consideration of both specific surface area and volumetric energy density is crucial when designing capacitive carbons to attain optimal results. [153]To improve the capacitance value and rate capability of electrochemical double-layer capacitors (EDLCs), there is a significant need for a hierarchical porous structure that has evenly distributed micropores.Nitrogen doping can further enhance the specific capacitance of porous carbon materials, as demonstrated by Liu et al., who used cellulose microfibers derived from rice straw as the precursor for synthesizing nitrogen-doped porous carbon via a hydrothermal process.In this study, urea was used as a nitrogen dopant, and K 2 CO 3 was used as an activating agent to increase the porosity of the resulting carbon material (Figure 11 c, e). [168]he fibrous structure was maintained after carbonization, with only some microfiber shrinkage occurring due to the volatilization of biochar at elevated temperatures.However, direct carbonization resulted in fragmentized granules, which can be attributed to the activation by K 2 CO 3 (Figure 11d).The resulting product obtained had a high surface area of 2788.7 m 2 g À 1 with an average 1.65 atom% nitrogen doping and exceptional specific capacitance of 380.1 F g À 1 at 0.5 A g À 1 current density (figure 11 f-g). [155]Hua et al. reported a simple method for maintaining the porosity of PC materials by co-pyrolyzing oxidized coal with cotton stalks.This process produces interconnected structures with enhanced conductivity, a high SSA, and ample mesopores.The resulting porous carbons exhibit intriguing characteristics, including a substantial SSA and volume of 1510 m 2 g À 1 and 0.24 cm 3 g À 1, respectively.These properties contribute to a huge specific capacitance of 345 F g À 1 at 1 A g À 1 and improved stability of 69 % 50 A g À 1 .Moreover, when these porous carbons are constructed into a symmetric supercapacitor using clean EMIM BF 4 electrolyte, they exhibit an excessive energy density of 56 Wh kg À 1 at 741 W kg À 1 and 16 Wh kg À 1 at 17903 W kg À 1 , which is a notable development over the previous result.This work expands the possibilities for fabricating porous carbons through co-pyrolysis. [156]ian et al. has developed a highly effective technique for synthesizing carbon material with enormous surface area (SSA) and a hierarchical porous structure.Their process involves using a freeze-drying method on Agar-Arg gel and carefully activating KHCO 3 , resulting in carbon material with a remarkably large SSA of 3184 m 2 g À 1 .The resulting high SSA porous carbon exhibits favourable properties, such as a hierarchical porous structure and heteroatom doping, contributing to its exceptional specific capacity, remarkable rate performance, and extraordinary cycling stability.This is due to its prolonged charge retention ability.Additionally, when fabricating a symmetric supercapacitor with Agar-Na 2 SO 4 electrolyte, this porous carbon demonstrates impressive cyclic stability and an energy density of 35.5 Wh kg À 1 . [157]ptimizing the specific capacitance of porous carbon materials is a complex and multifaceted task.The linear correlation between specific surface area and capacitance remains elusive, but innovative strategies like nitrogen doping and co-pyrolysis show promise in enhancing electrochemical energy storage devices.Further research is needed to explore the interplay between surface area, pore structure, and dopants to unlock the full potential of porous carbons and advance energy storage technologies.By continually refining our understanding of these intricate materials, we can pave the way for more efficient and sustainable energy storage solutions.The comparison between SSA, retention rate and specific capacitance of Nitrogen-doped porous carbons synthesized from different materials is given in Table 1.

Challenges and Outlook
Porous N-doped carbon-based supercapacitors have undergone a remarkable development in recent years.However, their high energy density requirements still limit their practical application.Further efforts are needed to enhance their performance and explore new storage mechanism models for NPCs.Continuing progress in this field is essential to overcome the current limitations and unlock the full potential of high-performance porous and N-doped carbons in supercapacitor technology.The review paper highlighted the extraordinary potential of supercapacitors (SCs) based on porous and nitrogen-doped carbon materials.SCs possess unique properties, such as high-power density, extended cycle life, environmental sustainability, a wide operating temperature range, and versatility in size and form supercapacitor technology is the maintenance of performance at high temperatures, as some conventional electrolytes and electrode materials can degrade or lose performance under elevated temperature conditions.However, nitrogen-doped carbon materials have shown good thermal stability, making them promising for applications that require high-temperature stability, where conventional supercapacitors may not be suitable.Future research efforts  may focus on developing nitrogen-doped carbon materials with even higher thermal stability to enable their use in demanding high-temperature environments, including automotive, aerospace, and industrial applications.3. Eco-Friendly and Sustainable Energy Storage: Nitrogendoped carbon materials are typically derived from renewable and sustainable carbon sources, such as biomass or waste materials, and can be synthesized using environmentally friendly methods.This makes them attractive for environmentally friendly and economically feasible energy storage applications, as they offer the potential for low-cost and sustainable production of supercapacitor electrodes.Future research could focus on further improving the sustainability and scalability of the synthetic strategies for nitrogen-doped carbon materials and exploring their potential for recycling and upcycling to minimize their environmental impact.4. Integration with Other Advanced Materials: Nitrogendoped carbon materials can be combined with advanced materials such as metal oxides, polymers, and nanomaterials to create hybrid materials with synergistic properties that enhance supercapacitor performance.Future research could explore developing new composite materials and architectures incorporating nitrogen-doped carbon materials to further improve energy storage performance and expand the range of potential applications for supercapacitors.5. Emerging Applications: Nitrogen-doped carbon materials possess unique properties that could create advanced opportunities for supercapacitors in emerging applications.
For instance, supercapacitors based on nitrogen-doped carbon materials have promising potential in wearable, flexible, and stretchable electronics applications.Future research could further develop and optimise nitrogendoped carbon-based supercapacitors for these emerging applications while exploring new areas where their unique properties can be harnessed.

Improve electrode design:
The strategic development of electrode design is crucial in determining supercapacitor electrode performance.Careful design and optimization of electrodes can result in superior electrode performance, thus improving the overall function of the supercapacitor.The potential benefits of cross-disciplinary work in materials science, chemistry, and engineering are vast, offering exciting opportunities for discoveries and technological advancements.However, such work can also pose significant challenges, including the need for effective communication, collaboration, and coordination among researchers from different disciplines.Overcoming these challenges will require a commitment to open-mindedness, flexibility, and a willingness to learn from one another.Successful cross-disciplinary work will ultimately require bridging gaps between different fields and integrating knowledge and expertise in new and innovative ways.

Conclusions
The rapid advancement of nanotechnology has led to the miniaturization of electronic devices, which requires the development of compact and efficient energy storage systems.The design of supercapacitor electrodes, including their structure, pore shape, surface area, and functional groups, strongly affects their performance.Porous carbons are a popular electrode material due to their low cost, non-toxicity, and accessibility.When nitrogen is incorporated into porous carbons, their properties are enhanced, resulting in improved supercapacitor performance.These enhanced properties include a high surface area, specific capacitance, excellent retention rate, a high aspect ratio of defects, and a twodimensional geometry.These properties make nitrogendoped carbon materials potentially suitable as high-performance electrode materials for energy-related applications.
Furthermore, the synthesis methods allow for easy control of various properties, including pore size, structure, and distribution.The two-dimensional geometry of nitrogendoped carbon materials makes them ideal for designing ultrathin electrodes, which can be advantageous for energy storage devices like supercapacitors.However, the energy density of nitrogen-doped carbon materials is limited, so their commercialization requires further development of composites/hybrids with other nanomaterials.

Figure 1 .
Figure 1.Scheme for three types of electrochemical capacitors.

Figure 2 .
Figure 2. Different types of electrode materials used in supercapacitors . 2024, 24, e202300161 (5 of 26) © 2023 The Authors.The Chemical Record published by The Chemical Society of Japan and Wiley-VCH GmbH

Figure 3 .
Figure 3. Classification of porous carbon based on pore dimensions.

Figure 4 .
Figure 4. Conventional activation methods to prepare porous carbons.

Figure 5 .
Figure 5.Effect of heteroatom doping on porous carbon.
. 2024, 24, e202300161 (10 of 26) © 2023 The Authors.The Chemical Record published by The Chemical Society of Japan and Wiley-VCH GmbH

Figure 8 .
Figure 8.(a) synthetic method of NPCs (b-c) BET results showing SSA (d-i) SEM images at different temperatures after nitrogen doping.Reprinted with permission from [142].Copyright [2020] American Chemical Society.

Figure 10 .
Figure 10.(a) Fabrication of the supercapacitor using coin cells (b-c) CV curves (d) GCD curves (e) specific capacitance (f) Ragone plot (g) Synthesis via pyrolysis and activation process with their SEM images (h) fabricated device demonstration (i) CV curves (j) GCD curves (k) synthetic scheme of as-synthesized material Reprinted with permission from [145].Copyright [2020] American Chemical Society.
. 2024, 24, e202300161 (19 of 26) © 2023 The Authors.The Chemical Record published by The Chemical Society of Japan and Wiley-VCH GmbH factor, making them attractive for future applications.The prospects of supercapacitors based on nitrogen-doped carbon materials are promising, as these materials have shown significant potential in enhancing SC performance.As research and development in supercapacitor technology continue to advance, we can expect to see the ever-increasing applications of SCs in various fields, including transportation, renewable energy, aerospace, and more.Below are some potential prospects for supercapacitors based on nitrogendoped carbon materials: 1. Improved Energy Storage Performance: Nitrogen-doped carbon materials can exhibit enhanced energy storage functionality, including increased specific capacitance, improved rate capability, and enhanced cycling stability.Incorporating nitrogen atoms can introduce additional pseudo capacitance, resulting in higher energy storage capacity and improved overall performance of supercapacitors.Future research and development efforts can focus on optimizing the doping level, type, and distribution of nitrogen atoms in carbon materials to further enhance their energy storage performance.2. High-Temperature Stability: One of the challenges facing

Table 1 .
Comparison Table of Nitrogen-doped porous carbons.
Chem.Rec.2024, 24, e202300161 (21 of 26) © 2023 The Authors.The Chemical Record published by The Chemical Society of Japan and Wiley-VCH GmbH