From Thermosetting Resins to Energy Devices: A Review on Polybenzoxazine-Derived Materials for Supercapacitors

Abstract

Polybenzoxazines (PBZs) have garnered significant attention as a versatile class of precursors for the development of advanced carbon-based materials, particularly in the field of electrochemical energy storage. This review comprehensively examines recent progress in the synthesis, structural design, and application of polybenzoxazine-derived materials for supercapacitor electrodes. Owing to their intrinsic nitrogen content, tunable functionality, and excellent thermal and mechanical stability, polybenzoxazines serve as ideal precursors for producing nitrogen-doped porous carbons with high surface areas and desirable electrochemical properties. This review discusses the influence of molecular design, polymerization conditions, and carbonization parameters on the resulting microstructure and performance of the materials. Furthermore, the electrochemical behavior of these materials in both electric double-layer capacitors (EDLCs) and pseudocapacitors is analyzed in detail. Challenges such as optimizing pore architecture, improving conductivity, and achieving scalable synthesis are also addressed. This article highlights emerging trends and offers perspectives on the future development of polybenzoxazine-derived materials for next-generation high-performance supercapacitors.

1. Introduction

The heavy dependence on traditional fossil fuels has caused significant environmental issues. Consequently, there is an urgent demand for clean energy conversion and storage solutions to enhance energy security and build sustainable, carbon-free power systems, which are essential steps toward achieving global carbon neutrality. Despite advancements, fossil fuels remain a dominant energy source, contributing to both energy shortages and environmental concerns. Researchers are increasingly investing in advanced materials to address the growing need for sustainable and renewable energy solutions. The scope covers advancements in flexible energy storage devices like capacitors, supercapacitors, and batteries, as well as energy conversion systems such as solar panels, fuel cells, thermoelectric modules, and wind energy technologies [1,2,3,4,5,6,7]. Notably, fuel cells and supercapacitors are gaining attention as prominent candidates for future energy storage and conversion solutions. Their high efficiency, near-zero greenhouse gas emissions, and ability to support clean, smokeless transportation make them particularly attractive. As dependence on fossil fuels persists, the demand for eco-friendly energy solutions—such as electrochemical supercapacitors and fuel cells—continues to rise. Additionally, the increasing global reliance on electronic and electrical devices has intensified scientific and technological interest in energy storage and renewable energy innovations [8,9,10,11].

Supercapacitors (SCs), or electrochemical capacitors, have emerged as promising energy storage devices due to their rapid charging/discharging, high power density, long cycle life, and excellent safety profile. Commonly used in electric vehicles, renewable energy systems, and aerospace, SCs complement or replace traditional batteries in various applications by offering lightweight design, fast response times, and extended operational life (>10⁵ cycles) [12,13,14,15]. SCs are broadly classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors (PCs). EDLCs store charge electrostatically using carbon-based materials like carbon nanotubes, graphene, and activated carbon. PCs, in contrast, utilize Faradaic redox reactions with materials such as conductive polymers or transition metal compounds to achieve higher capacitance. While SCs offer much higher power density (>10 kW·kg⁻¹) than batteries, their energy density (~10 W·h·kg⁻¹) remains lower. To improve this, research focuses on optimizing electrode materials, electrolytes, and operating voltages [16,17,18], since specific energy (E) is given by Equation (1):

E = ½·C·V²

(1)

A typical SC consists of two carbon-based electrodes, a separator, and an aqueous or organic electrolyte. Due to the cost of conventional carbon materials, heteroatom-doped porous carbons—especially those rich in nitrogen and oxygen—have gained attention as low-cost alternatives. These materials enhance electrostatic attraction and charge storage through surface doping, increasing capacitance and enabling high-performance systems [19,20,21]. To bridge the gap between energy and power densities, supercapacitors combine EDLC and pseudocapacitive mechanisms. This integration maximizes energy storage while retaining fast response and stability, making SCs a promising next step in supercapacitor technology [22,23,24].

Carbon materials, particularly activated carbon, are widely used as electrode components in electrical double-layer capacitors due to their high surface area and conductivity. Carbon particles, ranging in size from a few nanometers to micrometers, have gained widespread attention across various fields, including energy storage and conversion, biomedicine, catalysis, and adsorption [25,26,27]. Their controllable size, high surface-area-to-volume ratio, engineered morphology, and tunable composition make them highly versatile. These physicochemical properties can be precisely tailored through the careful selection of precursors and synthetic strategies. As a result, in the past decade, significant research efforts have been devoted to designing novel molecular precursors to develop functional carbon materials with enhanced properties [28,29,30]. Polymer precursors have emerged as promising candidates for deriving carbon frameworks due to their well-defined molecular architecture, versatile chemical composition, and ability to retain morphological integrity during carbonization. Their adaptability allows for the fabrication of carbon materials with tailored properties, making them attractive for various applications. Over the years, several polymer precursors—such as polyimide, poly(ether imide), polystyrene, polyaniline, and polyacrylonitrile—have been widely explored for their potential in carbon material synthesis [31,32]. Phenolic resins, such as resorcinol formaldehyde (RF) and phenol formaldehyde (PF) are among the most widely used precursors for synthesizing porous carbon spheres, particularly due to their high carbon yield, controllable cross-linking chemistry, excellent thermal stability and easy compatibility with templating or surfactant-assisted synthesis routes [33,34]. More recently, nitrogen-rich or heteroatom-doped carbon materials have been successfully obtained using an in situ process with polymers such as polyamines, polyamides, and polyacrylonitrile [35,36,37]. These nitrogen-doped carbon materials exhibit enhanced physicochemical properties, further expanding their application potential. However, many of these polymers fail to achieve high carbon yields due to molecular fragmentation during carbonization [38,39].

As the core component of supercapacitors, electrode materials play a crucial role in determining performance. The most commonly used electrode materials include porous carbon materials, metal oxides, and conductive polymers. Among these, metal oxides exhibit the highest specific capacitance but suffer from high costs and poor cycle stability. Similarly, conductive polymers offer excellent capacitance and conductivity but are limited by poor thermal stability and mechanical deformability, restricting their practical applications. In contrast, porous carbon materials have gained significant attention due to their exceptional electrical and thermal conductivity, high-temperature resistance, and strong corrosion resistance, making them highly suitable for energy storage applications [40,41,42,43]. Polybenzoxazine (PBZ) has recently emerged as an advanced class of phenolic thermosetting polymers, synthesized via the ring-opening polymerization of the oxazine moiety in benzoxazine monomers (Figure 1). This process occurs through thermal curing without requiring a catalyst or curing agent, allowing PBZs to form inter- and intramolecular hydrogen bonds. Due to their unique structural and chemical properties, PBZs are widely utilized in coatings, low-dielectric materials, and aerospace applications. The use of amine precursors in benzoxazine synthesis enriches the resulting polymer network with nitrogen heteroatoms, making PBZs highly suitable for producing heteroatom-doped carbon materials. PBZs also offer several advantageous characteristics, including molecular design flexibility, high char yield (35–80 wt.%), high cross-linking density, and excellent dimensional stability with minimal volume shrinkage. These materials exhibit remarkable thermal resistance, high glass transition temperatures (T_g), and flame-retardant properties, making them one of the most promising polymeric precursors for carbon material synthesis [44,45,46,47]. To obtain heteroatom-doped carbon from PBZ precursors, various advanced synthetic approaches have been employed, including soft templating via organic–organic self-assembly, hard templating, and modified sol–gel methods. Additionally, carbon materials derived from PBZs—such as foams, nanofibers, nanospheres, aerogels, and xerogels—have been synthesized using carbonization and graphitization techniques [48,49,50,51]. These materials possess controlled pore distribution, pseudocapacitive properties, and high charge stability, making them highly relevant for energy storage applications, particularly in supercapacitors and batteries [52,53].

This review details the use of PBZs in the development of advanced carbon materials for supercapacitors, highlighting the role of structured polymer frameworks in energy storage applications. Additionally, it explores the combination of PBZ-based materials with metal oxides and conductive polymers to create hybrid structures that bridge the gap between capacitors and batteries, offering high power density along with improved energy density. This review discusses the results of the literature reports in several sections. The first section deals about the supercapacitor performance of novel benzoxazines and their copolymers synthesized using several raw materials such as eugenol, apigenin, vanillin, melamine, furfurylamine, tetraethylene pentaamine, and boric acid. This section highlights the importance of several dopants into the carbon framework, like nitrogen, oxygen, boron and phosphorous in enhancing the supercapacitor performance. In the next section, the role of metal oxides along with PBZ carbon is discussed in detail. The formation of PBZ carbon with metal oxides—Ni, Mn, Co and bimetaloxides—NiMn and NiCo and the effect of carbonization temperature and redox active electrolytes in enhancing the electrochemical performance have been discussed in detailed. The third sections deal with PBZ composites with CNTs, graphitic carbon nitride, silica and graphene oxide. Here, the structure stability of the composites and the enhanced conductivity of the composites in enhancing the supercapacitor performance have been discussed. Finally, it concludes with a discussion on challenges, future perspectives, and potential avenues for improving performance through hybridization, scalable synthesis, and structural design innovations.

2. Supercapacitor Performance of Novel Benzoxazines and Their Copolymers

Recent research highlights the promise of novel benzoxazine-based materials and their copolymers in enhancing the performance of supercapacitors. By tailoring monomer structures or blending them into copolymers, enhanced electrochemical properties, such as higher capacitance, better cycle life, and increased energy density could be achieved. Thirukumaran et al. [38] introduced nitrogen-enriched mesoporous carbon ropes (NMCRs), produced from polybenzoxazine—an advanced thermosetting polymer—as active electrode materials for supercapacitors. The benzoxazine monomer was synthesized using eugenol and tetraethylenepentamine. The NMCRs were generated via straightforward carbonization followed by activation using aqueous KOH. Their research highlighted the significant influence of the nitrogen-rich polybenzoxazine precursor’s composition on boosting electrochemical performance. The final NMCRs contained notable nitrogen (5.26 wt.%) and oxygen (7.31 wt.%) levels [38], attributed due to the intrinsic heteroatoms present in the polybenzoxazine [54,55].

The Brunauer–Emmett–Teller (BET) surface analysis revealed that the NMCRs possessed a surface area of approximately 300 m²/g, with an average pore size of 3.0 nm and a pore volume of 0.003 cm³/g. High-resolution imaging revealed a unique rope-like porous morphology, which contributes to the enhanced surface area and offers additional electrochemically active sites [56,57]. Elemental mapping confirmed the distribution of carbon, nitrogen, and oxygen elements within the NMCR structure (Figure 2). Enhanced electrochemical behavior was achieved through modifications to both the surface properties and electrical conductivity of the carbon material [58,59]. Performance testing indicated a specific capacitance of 60 F/g in a 2 M KOH solution under a current density of 1 A/g. This improved result is attributed to the material’s distinct porous framework and excellent conductivity.

In a related study by Thirukumaran et al. [60], a cost-efficient and straightforward method (involving Mannich condensation, thermal polymerization, carbonization and activation) was introduced to synthesize nitrogen-doped mesoporous carbon using a novel polybenzoxazine precursor. Two carbon structures were fabricated from the same precursor, made from apigenin, furfuryl amine, and formaldehyde. The product obtained through thermal treatment was named APFC-N (apigenin-furfurylamine-based carbon obtained through normal carbonization), while the one prepared via an aerogel route was termed APFC-G (apigenin-furfurylamine-based carbon obtained through gelation). Nitrogen adsorption–desorption studies revealed that both materials exhibited characteristics of type I and IV isotherms, indicating a considerable presence of mesopores—particularly pronounced in APFC-G [61]. BET analysis showed surface areas of 248 m²/g for APFC-N and 635 m²/g for APFC-G. APFC-G displayed a narrow pore size distribution between 20–50 nm, as well as notable macropores with voids larger than 0.5 µm, making it well suited for electrochemical use (Figure 3a,b). Electrochemical evaluation of APFC-G, which was activated in KOH at 900 °C, revealed a peak specific capacitance of 120 F/g in a 1 M H₂SO₄ electrolyte at a current density of 0.5 A/g (Figure 4). The high performance is credited to its oxygen- and nitrogen-rich surface and large surface area [62,63]. Impressively, it retained excellent stability, with no meaningful loss in capacitance observed over 25,000 charge–discharge cycles, highlighting its long-term durability. Moreover, the APFC-G electrode displays a low solution resistance of 2.2 Ω, indicating good electronic conductivity.

Thubsuang et al. [64] synthesized carbon microspheres based on polybenzoxazine (PBZ) using a simple preparation method (Mannich polycondensation) that utilized a mixture of formaldehyde and dimethylformamide (DMF) as solvents. Successful formation of PBZ microspheres was achieved at F/DMF weight ratios of 0.4 and 0.6. Upon carbonization, the materials displayed a high nitrogen content, and structural analysis confirmed an amorphous nature with regions of partial graphitization. Notably, the carbon derived from a 0.4 F/DMF ratio showed a superior specific capacitance of 275.1 F g⁻¹, outperforming the reference carbon (PBZ carbon synthesized using pure DMF) (198.9 F g⁻¹) when tested at 0.05 A g⁻¹. This performance boost was linked to a synergistic effect of electric double-layer capacitance and pseudocapacitance, the latter being enhanced by nitrogen and oxygen surface groups [65,66]. After CO₂ activation, the specific surface area of the 0.4 ratio sample increased markedly from 349 m² g⁻¹ to 859 m² g⁻¹, resulting in a further rise in capacitance to 424.7 F g⁻¹—more than double that of the reference. These results indicate that an F/DMF ratio of 0.4 is optimal for producing high-performance carbon microspheres, with improvements mainly driven by surface chemistry and increased porosity [67,68].

Meanwhile, Mohammed et al. [44] developed a new type of porous organic polymer (POP) by coupling Cr-TPA-4BZ-Br₄ with tetraethynylpyrene (Py-T) (Figure 5). The precursor benzoxazine monomers—Cr-TPA-4BZ (Dibenzo-crown ether-triphenylamine-based benzoxazine) and its brominated version Cr-TPA-4BZ-Br₄—were synthesized through a conventional three-step pathway, involving CH=N formation, sodium borohydride reduction, and a Mannich-type condensation. These monomers were then linked via a Sonogashira cross-coupling reaction to create Cr-TPA-4BZ-Py-POP. Upon further carbonization, this yielded poly(Cr-TPA-4BZ-Py-POP)-800, a carbonaceous material with graphite-like domains. This material showed a high CO₂ adsorption capacity of 4.4 mmol g⁻¹ at 273 K. Electrochemical evaluations revealed a significant specific capacitance of 397.2 F g⁻¹ at a current density of 0.5 A g⁻¹, along with excellent stability, retaining 94% of its capacitance over extended cycling. The enhanced performance was largely attributed to an optimized ratio (C = 73.5%, N = 8.9% and O = 17.6%) of heteroatom doping [69,70], which contributed to improved structural features and electrochemical behavior.

Wan et al. [71] developed nitrogen-doped porous carbon materials with high surface areas and well-defined micro-/mesoporous architectures by employing a novel nitrile-functionalized polybenzoxazine. The process combined a soft-templating strategy (Surfactant F127 was used as soft templating agent) with KOH chemical activation. Both the use of the soft template and the activation temperature were found to significantly affect the resulting pore structure and surface characteristics [72]. Compared to the primarily microporous NPC-0, the hierarchical pore systems in NPC-1 and NPC-2 demonstrated superior capacitive behavior. This enhancement was linked to a large surface area (up to 2036 m²/g), balanced pore distribution, robust electrical conductivity, and the inclusion of nitrogen (2.33–5.32 wt.%) and oxygen (10.26–14.22 wt.%) functionalities within the carbon framework [73,74]. Notably, NPC-2, activated at 700 °C, delivered the highest specific capacitance of 362.4 F/g at a current density of 1 A/g in a KOH aqueous solution (Figure 6).

The material also exhibited excellent rate performance and long-term durability, preserving 94.7% of its capacitance after 5000 charge–discharge cycles. Moreover, the solid adsorbent NPC-1, thermally activated at 600 °C, achieved the highest CO₂ adsorption capacity under 1 bar—recording 6.20 mmol g⁻¹ at 0 °C and 3.95 mmol g⁻¹ at 25 °C. It also demonstrated high CO₂/N₂ selectivity and could be reused effectively without performance loss. These exceptional properties, both as supercapacitor electrodes and CO₂ adsorbents, were primarily ascribed to their extensive surface area, tailored pore architecture, superior electrical conductivity, and the incorporation of nitrogen functionalities within the carbon framework [75,76]. In a related study, Liu et al. [77] developed nitrogen and oxygen co-doped porous carbon materials (NOPC-x and NOPC-bis-CN-x) [NOPC and NOPC-bis-CN are carbon materials derived from different benzoxazine monomers, Boz-Va (benzoxazine synthesized from vanillin, aniline and paraformaldehde) and Boz-bis-VaCN (benzoxazine synthesized from diaminodiphenyl ether, 2-(4-hydroxy-3-methoxybenzylidene) malanonitrile and paraformaldehyde)] derived from bio-based polybenzoxazines through a soft-template strategy. They found that increasing the cyano content in the monomers improved surface area, pore structure, and graphitization level. Notably, the NOPC-bis-CN-3 variant achieved a remarkable surface area of 2347 m² g⁻¹ and abundant mesopores (20–40 nm), along with a higher density of electrochemically active nitrogen [N-1 (pyridinic N), N-2 (pyridonic N), and N-3 (quarternary N)] and oxygen (O-2, O-3) species [78,79]. Electrochemical testing confirmed the superior performance of the NOPC-bis-CN-x series over the NOPC-x group. Specifically, NOPC-bis-CN-3 delivered the highest specific capacitance of 167.3 F g⁻¹ at 1 A g⁻¹, retained more than 80% of its capacitance at 10 A g⁻¹, and demonstrated strong pseudocapacitive behavior and excellent cycling durability. The symmetric device assembled with this material showed minimal decline in energy density—from 10.8 to 9.0 Wh kg⁻¹—as power density increased from 45 to 4500 W kg⁻¹ (see Figure 7). The convergence of outstanding electrochemical features, eco-friendly origins, and straightforward synthesis renders NOPC-bis-CN-3 a top contender for next-generation supercapacitor applications.

M.M. Samy and colleagues [46] developed an innovative bio-based benzoxazine monomer, termed VFBZ-CN, synthesized through the condensation reaction of vanillin, formaldehyde, and furfurylamine—compounds derived from renewable natural sources. Characterization via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) indicated that VFBZ-CN underwent thermal curing at a relatively low onset temperature of 196 °C, a property attributed to cyano functionalities enhancing oxazine ring-opening. Upon curing at 250 °C, the resulting polymer displayed impressive thermal resilience, with a 10% weight loss temperature (Td₁₀) of 379 °C, surpassing traditional benzoxazine analogs. Electrochemical assessments revealed that poly(VFBZ-CN)-800 achieved a gravimetric capacitance of 506 F g⁻¹, significantly exceeding that of poly(VFBZ-CN)-700, which reached only 171 F g⁻¹ at 0.5 A g⁻¹ in an alkaline KOH medium. Moreover, the poly(VFBZ-CN)-800 electrode retained 99.43% of its capacitance after 2000 cycles at a high current density of 10 A g⁻¹, underscoring its cycling durability. This enhanced electrochemical efficiency was primarily ascribed to its porous carbon architecture and elevated nitrogen/oxygen content [80,81]. Apart from energy storage, these N- and O-enriched microporous carbons derived from VFBZ-CN also exhibited strong CO₂ adsorption capacity, reinforcing their multifunctionality and alignment with sustainable technologies.

Zhang and colleagues [82] developed nitrogen and phosphorus co-doped carbon materials (C/P-Cs) derived from nonporous polybenzoxazine, using polybenzoxazine as the carbon base and melamine polyphosphate as a dual-source dopant for nitrogen and phosphorus. These carbon structures exhibited notably high atomic concentrations of nitrogen (5.5%) and phosphorus (5.1%), which played a key role in enhancing their electrochemical properties—specifically in terms of capacitance, charge–discharge efficiency, and operational stability as supercapacitor electrodes. Among them, the optimized variant (C/P-20-1) delivered an impressive specific capacitance of 203.0 F g⁻¹ at a current density of 0.5 A g⁻¹, and still maintained 173.2 F g⁻¹ under a much higher load of 20 A g⁻¹. The electrode’s durability was also remarkable, preserving 90.1% of its capacitance over 5000 cycles at 5 A g⁻¹. When configured into a symmetric supercapacitor, this material demonstrated a peak energy density of 11.45 Wh kg⁻¹ at a power density of 50 W kg⁻¹, and a maximum power output of 25 kW kg⁻¹ at an energy density of 5.55 Wh kg⁻¹. Even after undergoing 10,000 cycles of charging and discharging at 5 A g⁻¹, the device retained 79.9% of its original capacitance. The superior electrochemical behavior of the C/P-20-1 electrode and the symmetric device was attributed to a combination of factors: (i) the presence of functional nitrogen and phosphorus species (such as pyridinic and pyrrolic N), which boosted pseudocapacitive effects and overall charge storage [41]; (ii) nitrogen doping, which enhanced electrical conductivity and charge transfer rate [83]; and (iii) phosphorus doping, which notably contributed to the material’s long-term cycling resilience [84].

Wang et al. [40] developed a range of oxygen-rich porous carbon materials by copolymerizing a diacetal-type benzoxazine (ACE-a) with melamine in varying proportions, utilizing a straightforward, template-free synthesis approach. The inclusion of melamine introduced a substantial amount of reactive C–N bonds into the network during polymer formation, which contributed to the creation of abundant micropores during the carbonization and activation phases [85], thanks to melamine’s self-sacrificing behavior (see Figure 8). This decomposition process not only promoted micropore generation but also increased the oxygen content in the resulting carbon, improved its surface wettability, and lowered internal resistance. Among the materials synthesized, CA3MK showed outstanding characteristics, including a high BET surface area of 1383.9 m² g⁻¹, a large pore volume of 0.748 cm³ g⁻¹, and a notable surface oxygen content of 66.2%. Electrochemical evaluations revealed impressive performance, with a specific capacitance of 430 F g⁻¹ in 0.5 M H₂SO₄ and 194 F g⁻¹ in 6 M KOH at 0.5 A g⁻¹ (Figure 9). Moreover, the material demonstrated excellent cycling stability and strong performance at high current densities. These results underscore the promise of oxygen-functionalized porous carbons in supercapacitor technologies, driven by their high surface area, well-developed porosity, and abundant oxygen-containing groups [86,87].

Bai et al. [88] developed boron and nitrogen co-doped porous carbons (BNPC-X) by carbonizing boron-containing polybenzoxazines followed by chemical activation. In this synthesis, the benzoxazine resin functioned as both the carbon and nitrogen precursor, while boric acid provided the boron dopant. Among the series, BNPC-0.15 showed notable elemental incorporation, containing 2.97 wt.% boron and 2.43 wt.% nitrogen, with a homogeneous spatial distribution (Figure 10). This material delivered a high specific capacitance of 286 F g⁻¹ at 0.05 A g⁻¹ and retained a commendable 174 F g⁻¹ at 1 A g⁻¹, maintaining 92% of its initial capacitance after 1000 cycles in 6 M KOH. The synergistic effects of boron and nitrogen functionalities enhanced electrolyte interaction and introduced additional redox activity, leading to improved electrochemical performance [89,90]. These features highlight BNPC-X as a strong candidate for advanced supercapacitor electrode applications.

3. Supercapacitor Performance of Polybenzoxazine/Bimetal Oxides

The integration of polybenzoxazine-based carbons with bimetallic oxides is one of the emerging combinations that has gained notable attention. This synergy merges the high surface area and conductivity of carbon materials with the redox-active sites provided by bimetallic oxides, leading to significant performance enhancements in electrochemical energy storage. Thirukumaran et al. [91] developed a hierarchical framework in which electroactive materials are securely integrated onto a carbon scaffold, promoting optimal exposure of active sites and facilitating efficient ion and electron transport. The carbon base is derived from polybenzoxazine (Pbz), synthesized from a benzoxazine monomer. The carbonized Pbz features a rigid network enriched with nitrogen and oxygen heteroatoms, which contributes to enhanced supercapacitor (SC) performance. Furthermore, incorporation of pseudocapacitive metal hydroxides like Ni(OH)₂ and Mn(OH)₂ supports reversible Faradaic reactions. The final products—nitrogen-rich porous carbon (NRPC), along with its composites NRPC/Mn, NRPC/Ni, and NRPC/NiMn—demonstrate pore volumes between 0.18 and 0.42 cm³ g⁻¹. The NRPC displayed a tangled porous network containing a variety of pore sizes, including prominent macropores. SEM analysis revealed that the NRPC/Mn composite developed flake- or petal-like formations, while NRPC/Ni exhibited a deformed petal structure with sharp, spike-like tips. These morphological elements were embedded within the carbon framework. Interestingly, co-incorporation of Mn and Ni gave rise to a vertically oriented, three-dimensional flower-like architecture formed by interlinked petal structures. These petals were anchored by a hollow central core, contributing to mechanical robustness and offering abundant electroactive sites conducive to efficient ion storage in supercapacitor applications [92,93] (Figure 11).

In contrast to NRPC containing only monometallic components, the NRPC/NiMn composite exhibits a notably higher specific surface area of 365 m² g⁻¹ and a greater pore volume of 0.42 cm³ g⁻¹, attributed to its distinctive flower-like architecture. Electrochemical measurements reveal a significant specific capacitance of 1825 F g⁻¹ for NRPC/NiMn, retaining 78% of its initial capacity after 2500 charge–discharge cycles. This performance is driven by the synergistic effects of bimetallic active sites and heteroatom doping, which collectively promote strong pseudocapacitive behavior. Additionally, the porous carbon framework acts as a highly conductive matrix, enhancing electron mobility and supporting electrochemical double-layer capacitance (EDLC) through efficient ion transport and surface adsorption. Asrafali et al. [15] designed a novel supercapacitor configuration integrating heteroatom-enriched carbon, bimetallic oxide nanostructures, and redox-active electrolytes to significantly boost electrochemical efficiency. The system leverages dual pseudocapacitive mechanisms: one originating from the electrode material—specifically, nitrogen and oxygen-doped carbon derived from polybenzoxazine blended with NiCo bimetallic oxides—and the other from the electrolyte, utilizing iodide-containing redox-active species such as KI and RbI. The polymer precursor, synthesized via Mannich condensation of eugenol and ethylenediamine, yields a heteroatom-doped carbon (HC) upon calcination at 800 °C. The resulting HC/NiCo@800 °C structure features a hierarchical assembly of interlinked 3D and ultrathin 2D morphologies [94], as confirmed by SEM and TEM imaging. The flower-like architecture, formed without observable thermal degradation, provides a high surface area, interconnected pore networks, and abundant electroactive sites, all of which contribute to improved electrolyte accessibility and charge storage kinetics [95,96]. Remarkably, devices tested with redox-active RbI and KI electrolytes achieved elevated specific capacitances of 2334 F g⁻¹ and 2076 F g⁻¹ at 1 A g⁻¹, respectively. The asymmetric device HC/NiCo@800 °C//HC operating in RbI demonstrated a peak specific capacitance of 232 F g⁻¹ with 89.04% retention after 5000 cycles, along with a low solution resistance (Rs) of 0.75 Ω and charge transfer resistance (Rct) of 0.77 Ω (Figure 12). The superior electrochemical response in RbI is credited to the rapid transport dynamics of Rb⁺ and I⁻ ions, which enhance ion–solvent and solvent–solvent interactions. This configuration achieved an impressive energy density of 96.57 Wh kg⁻¹ while maintaining a power density of 850 W kg⁻¹. The integration of functionalized porous carbon, dual-metal oxides, and redox-active electrolytes demonstrates a promising strategy for developing high-performance supercapacitors with excellent durability, high capacitance, and energy output, paving the way for broader practical applications.

4. Supercapacitor Performance of Polybenzoxazine Composites

Polybenzoxazine composites, particularly when combined with other functional materials like carbon nanotubes (CNTs), graphitic carbon nitride (g-C₃N₄), graphene oxide (GO) and silica (SiO₂), enable the development of high-capacitance, stable, and conductive electrode architectures. Ge et al. [97] introduced a straightforward yet efficient approach for producing highly porous carbon nanofiber (CNF) membranes with superior mechanical flexibility and multifunctional performance. Their method involved multicomponent electrospinning followed by in situ polymerization using polybenzoxazine (PBZ) as a novel carbon precursor. The incorporation of SnCl₂ enhanced both the spinnability and thermal stability of the precursor nanofibers (Figure 13 and Figure 14). During the carbonization process, SnO₂ nanoclusters—ranging from 20 to 40 nm—were uniformly embedded throughout the carbon network and anchored onto the nanofiber surfaces via in situ synthesis. This led to the formation of a heterogeneous nanostructure that imparted a plasticizing effect, enabling the resulting SnO₂/CNF membrane to withstand large deformations while preserving its structural integrity. The membrane exhibited a high specific surface area of 1415 m²/g and a pore volume of 0.82 cm³/g. It also achieved an energy density of 16.25 Wh/kg and a power density of 1.03 kW/kg, significantly surpassing traditional porous CNF electrodes. Moreover, the membrane maintained excellent electrochemical performance under bending, highlighting its potential for use as flexible electrodes in wearable energy storage systems. The enhanced performance was primarily due to (i) the strong interfacial interaction between the SnO₂ clusters and the carbon matrix, which minimized resistance, and (ii) the well-developed porosity that facilitated rapid ion transport [98,99].

Wu et al. [34] developed uniform porous yolk-shell carbon nanospheres (PYCNs) via a two-step coating strategy, using resorcinol–formaldehyde (RF) resin spheres as the initial core material. RF resin was selected for its affordability, high carbon yield, and structural stability. The process involved coating the RF spheres with two layers: an inner dense silica layer and an outer composite shell made of polybenzoxazine and silica (PB/SiO₂). Tetraethyl orthosilicate (TEOS) served as the silica source, while a mixture of resorcinol, formaldehyde, and ethylenediamine (EDA) was used for the PB layer. Transmission electron microscopy revealed that the final carbon shell measured about 20 nm in thickness, enclosing a central void of approximately 40 nm, with a carbon core approximately 600 nm in diameter—matching the original size of the solid carbon spheres (SCS). Surface area analysis showed that SCS possessed a higher specific surface area of 606 m² g⁻¹ and a micropore volume of 0.25 cm³ g⁻¹, in contrast to PYCNs, which exhibited values of 486 m² g⁻¹ and 0.15 cm³ g⁻¹, respectively. Despite the slightly lower surface area, the yolk-shell design, mesoporosity, and nitrogen doping in PYCNs significantly enhanced ion transport and diffusion [100,101]. Consequently, PYCNs outperformed SCS in supercapacitor applications, achieving a specific capacitance of 236 F g⁻¹ at a current density of 0.5 A g⁻¹, compared to 176 F g⁻¹ for SCS.

Du et al. [102] introduced a co-assembly strategy for producing nitrogen-doped hollow mesoporous carbon spheres (N-HMCS) with tunable morphologies. This method employed cetyl-3-methyl ammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) in conjunction with polybenzoxazine (PBZ), synthesized from phenol, formaldehyde, and ethylenediamine, via electrostatic interactions. In the process, CTAB served as a structural template, TEOS functioned as a silica precursor, and the PF oligomers with ethylenediamine acted as carbon and nitrogen sources within a modified Stöber synthesis. Ethylenediamine also facilitated both TEOS hydrolysis and PF oligomer polymerization. The resulting core-shell particles displayed morphology-dependent surface features—silica@PB-0.1 had a smooth, spherical structure with a distinct core-shell interface, while silica@PB-0.4 featured a rougher surface with prominent protrusions [103,104]. Among the derived carbon spheres, N-HMCS-0.1 delivered the highest specific capacitance of 307 F g⁻¹, outperforming its counterparts (198, 206, and 192 F g⁻¹ for N-HMCS-0.05, 0.2, and 0.4, respectively) (Figure 15). Moreover, N-HMCS-0.1 maintained 83% capacitance retention at elevated current densities. A symmetric supercapacitor assembled using N-HMCS-0.1 and a 6 M KOH electrolyte attained an energy density of 11.2 Wh kg⁻¹ at 660.8 W kg⁻¹, and 10 Wh kg⁻¹ at 9000.5 W kg⁻¹. The superior electrochemical performance of N-HMCS-0.1 is ascribed to its fine particle size, thin shell, and high surface area, which together enhance charge storage and ion accessibility [105,106]. Additionally, the mesoporous architecture ensures efficient ion transport, while the nitrogen functionalities promote pseudocapacitive behavior through redox activity [107,108].

Wan et al. [109] developed a method for producing graphene oxide (GO) and nitrogen-doped porous carbon (NC) nanocomposites aimed at enhancing supercapacitor electrode performance. The synthesis involved a polybenzoxazine (PBZ)-based ring-opening polymerization followed by KOH activation, leveraging both hydrogen bonding and covalent interactions between GO and benzoxazine. Nanocomposites with different GO loadings were fabricated, revealing that the inclusion of GO significantly altered the surface characteristics, porosity, and electrical conductivity of the materials, thereby improving their electrochemical behavior [110,111,112,113]. Among the samples, the electrode containing 1.29 wt.% GO exhibited the highest specific capacitance of 405.6 F g⁻¹ at 1.0 A g⁻¹ in a 6 M KOH electrolyte. This electrode also showed excellent rate performance (267.8 F g⁻¹ at 40 A g⁻¹) and strong cycling durability, retaining 95.8% of its capacitance after 5000 cycles. Moreover, symmetric supercapacitor devices assembled with the GO/NC material in 1 M Na₂SO₄ operated within a broad 1.8 V voltage range, delivering an energy density of 38.6 Wh kg⁻¹ at 180 W kg⁻¹ and sustaining 19.9 Wh kg⁻¹ at a power density of 32.4 kW kg⁻¹.

Selvaraj et al. [10] reported the synthesis of a novel quinoline-based Mannich-type benzoxazine monomer (Q-xda), derived from the reaction of 8-hydroxyquinoline, xylylenediamine, and paraformaldehyde. This monomer was used to fabricate high-performance carbon-based materials for energy storage. Composites were formulated by incorporating graphitic carbon nitride (GCN) at varying concentrations (5, 10, and 15 wt.%) into the poly(Q-xda) matrix. The presence of GCN notably improved the thermal resistance and char yield of the composites [114,115]. Electrochemical tests confirmed pseudocapacitive behavior, with poly(Q-xda) + 15 wt.% GCN showing the highest specific capacitance of 370 F g⁻¹ at a current density of 6 A g⁻¹. The composites with 5 and 10 wt.% GCN exhibited capacitances of 294 and 310 F g⁻¹, respectively, while the pristine poly(Q-xda) showed a lower value of 216 F g⁻¹. The poly(Q-xda) + 15 wt.% GCN sample also demonstrated superior charge transport characteristics, with a reduced charge transfer resistance (20.8 Ω) compared to that of the unmodified polymer (26.0 Ω), and maintained 96.2% of its capacitance after 2000 cycles. In a related study by the same group [9], a facile and scalable approach was introduced for producing nitrogen-rich porous carbon (NRPC), which was further integrated with graphitic carbon nitride and magnetite (g-C₃N₄/Fe₃O₄) to fabricate a functional nanocomposite. The difunctional benzoxazine monomer (NP-ha), synthesized from nonylphenol, hexane-1,6-diamine, and paraformaldehyde, served as the precursor. The process involved direct carbonization, KOH activation, and subsequent hydrothermal treatment to embed Fe₃O₄ and g-C₃N₄ into the NRPC matrix. Characterization using FE-SEM and HR-TEM confirmed uniform dispersion of the nanophases [116]. (Figure 16) The resulting composite exhibited a high specific surface area of 479.6 m² g⁻¹ and demonstrated effective pseudocapacitive behavior, highlighting its potential for use in advanced supercapacitor applications.

Compared to the Fe₃O₄ and g-C₃N₄/Fe₃O₄ electrodes, the NRPC/g-C₃N₄/Fe₃O₄ electrode exhibited lower charge transfer resistance and higher capacitance. Specifically, the NRPC/g-C₃N₄/Fe₃O₄ electrode achieved the highest specific capacitance of 385 F g⁻¹ at 1 A g⁻¹, outperforming Fe₃O₄ (112 F g⁻¹) and g-C₃N₄/Fe₃O₄ (150 F g⁻¹). Additionally, the cycling efficiency of the NRPC/g-C₃N₄/Fe₃O₄ electrode remained at 94.3% after 2000 cycles. These results demonstrate that the incorporation of NRPC into g-C₃N₄/Fe₃O₄ significantly enhances its potential for use in high-performance supercapacitors [117,118].

5. Supercapacitor Performance of Polybenzoxazine with Other Polymers

Polybenzoxazine-based thermosets, known for their high nitrogen content and substantial char yield, are gaining attention as sustainable precursors for nitrogen-doped carbon materials. A key consideration in upcycling these materials is the reduction in both energy input and processing costs. The combination of PBZ with other functional polymers, including polyaniline (PANI), polypyrrole (PPy), or porous organic polymers (POPs), leads to multifunctional electrode materials that deliver a balanced combination of energy and power densities, making them suitable for both commercial and high-performance supercapacitor applications. In line with this goal, Sharma et al. [119] demonstrated a method for producing carbon materials under relatively mild carbonization conditions, eliminating the need for chemical activation. Electrochemical analysis using a three-electrode system revealed that the carbon derived from guaiacol-based polybenzoxazine (C-GP81), which features 6.4% nitrogen incorporation, delivered a notable specific capacitance of 700 F g⁻¹ at 10 A g⁻¹. This indicates excellent charge storage and rate capability, making it a strong candidate for supercapacitor electrode applications.

The material also achieved an energy density of 48 Wh kg⁻¹ at a power density of 8400 W kg⁻¹ in the same three-electrode setup. Its performance in an acidic medium is credited to the synergistic effects of a well-developed surface area and a favorable composition of nitrogen (pyridinic, pyrrolic, and graphitic) and oxygen (quinone) functional groups [41,120]. When evaluated in a symmetric supercapacitor device, C-GP81 delivered a specific capacitance of 76 F g⁻¹ at 0.5 A g⁻¹, which gradually declined to 40% after 10,000 cycles, reflecting moderate cycling stability. The device also reached a peak energy density of 10 Wh kg⁻¹ at a power density of 2400 W kg⁻¹ (Figure 17). Overall, these findings underscore the promise of eco-friendly carbonization strategies for transforming polybenzoxazine resins into high-performance, heteroatom-enriched carbon materials for energy storage applications [121,122].

Tiwari et al. [25] leveraged the molecular design flexibility of polybenzoxazine to fabricate polybenzoxazine colloidal spheres using phloroglucinol, polyethylenimine, and formaldehyde as multifunctional precursors, employing a simple template-free extended sol–gel method (Figure 18). The kinetics of particle formation were controlled by adjusting synthesis parameters such as the stoichiometric ratio of the reactants, precursor concentration, and solvent ratio, which influenced the morphology, particle size, and heteroatom content in the polymeric particles [123]. The heteroatom-doped carbon spheres, containing 28% nitrogen and 20.5% oxygen (as determined by CHNS analysis), were obtained by subjecting the polybenzoxazine particles to moderate carbonization conditions, resulting in a partial graphitic structure.

The uniform morphology, coupled with a significant BET surface area of 221 m² g⁻¹ and high heteroatom content in the resulting carbon structure, highlights their potential as active materials for supercapacitor electrodes [124,125]. The N, O co-doped carbon structure exhibited an impressive specific capacitance of 728 F g⁻¹ at a current density of 10 A g⁻¹, resulting in a maximum energy density of 56 W-h kg⁻¹ and a maximum power density of 14,246 W kg⁻¹. Further electrochemical performance evaluation was conducted using a flexible all-solid-state symmetric two-electrode system to simulate real-time conditions. The active material displayed a notable specific capacitance of 50.3 F g⁻¹ at 0.2 A g⁻¹. Additionally, the material maintained approximately 86% capacitance retention after 2500 cycles in the asymmetric two-electrode configuration, confirming the reversible nature of the device with negligible changes in resistance before and after cyclic tests. These results underscore the potential of fabricating high-performance supercapacitors by precisely modulating the surface characteristics and functionality of polybenzoxazine precursors through the careful selection of multifunctional precursors in the extended sol–gel method [126,127].

Murugan et al. [1] reported the development of a series of polybenzoxazine-co-copper metal-organic frameworks (PABz-co-Cu MOFs) and their covalently cross-linked membrane composites with poly(imidazole-diphosphoric acid) (PIDPA) in various weight ratios (80/20, 60/20, 50/50, 40/60, and 20/80%). These hybrid membranes were fabricated using a sequential thermal curing method at different temperatures, resulting in a networked polymer structure (Figure 19). Among the formulations, the 50/50 wt.% PABz-co-Cu MOFs-graft-PIDPA membrane displayed the best performance as a high-temperature proton exchange membrane fuel cell (HT-PEMFC) material, outperforming both pristine PA-PIDPA and unmodified PABz-co-Cu MOFs. The enhanced performance was attributed to the presence of large voids between the particles, which effectively retained phosphoric acid (PA), crucial for proton conduction [128,129]. At the optimal 50/50 ratio, the PA-doped membrane achieved a proton conductivity of 7.57 × 10⁻² S cm⁻¹, an open-circuit voltage (OCV) of 0.91 V, and a peak power density of 0.729 W cm⁻²—significantly higher than those of the pure PABz-co-Cu MOFs membrane (3.21 × 10⁻² S cm⁻¹ conductivity, 0.43 V OCV, and 0.357 W cm⁻² power density).

Furthermore, the 50/50 membrane composite showed a specific capacitance of 387 F g⁻¹ at a current density of 1 A g⁻¹, exceeding the 187 F g⁻¹ achieved by the unmodified MOFs. This research offers a cost-efficient strategy to design covalently bonded PABz-co-Cu MOFs-graft-PIDPA networks enriched with imidazole, diphosphoric acid, and 5-sulfo salicylic acid functional groups, showcasing strong potential for both HT-PEMFC and supercapacitor technologies.

Table 1. Comparative data showing the different synthesis process for carbon materials and their properties.

Carbon Materials	Synthesis Method	Properties
Nitrogen-enriched mesoporous carbon ropes, NCMR [38]	Carbonization and KOH activation.	SA = 300 m2 g−1; pore size = 3 nm; pore volume = 0.003 cm3 g−1; Cs = 60 F g−1 @ 1 A g−1 (2 M KOH)
Apigenin and furfurylamine-based Bzo, APFC-N and APFC-G [60]	Gelation, calcination and KOH activation.	SA = 248 m2 g−1 (APFC-N) and 635 m2 g−1 (APFC-G); pore size = 2–5 nm; Cs (APFC-G) = 120 F g−1 @ 0.5 A g−1 (1 M H2SO4)
Carbon microspheres [64]	Gelation, carbonization and CO2 activation.	SA = 859 m2 g−1; pore size = 1 nm; Cs = 424.7 F g−1 @ 0.5 A g−1 (1 M H2SO4)
Porous organic polymer, Cr-TPA-4Bz-PY-POP [44]	Sonogashira–Hagihara cross-coupling and carbonization.	Pore size = 4.17 nm; Cs = 397.2 F g−1 @ 0.5 A g−1 (1 M KOH)
Nitrogen-doped porous carbon, NPC-2 [71]	Soft templating method and KOH activation.	SA = 2036 m2 g−1; Cs = 362.4 F g−1 @ 1 A g−1 (1 M KOH)
Nitrogen and oxygen-doped porous carbon, NOPC-bis-CN-3 [77]	Soft templating method and KOH activation.	SA = 2347 m2 g−1; pore size = 20–40 nm; Cs = 167.3 F g−1 @ 1 A g−1 (6 M KOH)
Vanillin-malonitrile-based PBz, poly(VFBZ-CN) 800 [46]	Carbonization and KOH activation.	SA = 560 m2 g−1; Cs = 506 F g−1 @ 0.5 A g−1 (1 M KOH)
Nitrogen and phosphorous co-doped carbon, C/P-20-1 [82]	Carbonization.	SA = 29 m2 g−1; Cs = 203 F g−1 @ 0.5 A g−1 (1 M H2SO4)
Diacetyl-type Bzo carbon, CA3MK [40]	Gelation and curing.	SA = 1383.9 m2 g−1; pore volume = 0.748 cm3 g−1; Cs = 430 F g−1 (0.5 M H2SO4) and 194 F g−1 (6 M KOH) @ 0.5 A g−1
Boron and nitrogen co-doped porous carbon, BNPC-0.15 [88]	Carbonization and KOH activation.	Cs = 286 F g−1 @ 0.5 A g−1 (6 M KOH)
Nitrogen rich porous carbon, NRPC/NiMn [91]	Carbonization, KOH activation and hydrothermal reaction.	SA = 365 m2 g−1; pore volume = 0.42 cm3 g−1; Cs = 1825 F g−1 @ 1 A g−1 (1 M KOH)
Hetero atom-doped carbon, HC/NiCo@800C [15]	Carbonization, KOH activation and hydrothermal reaction.	Cs = 2334 F g−1 @ 1 A g−1 (1 M RbI) and 2076 F g−1 @ 1 A g−1 (1 M KI)
Carbon nano fibers, SnO2/CNF [97]	Template polymerization using PVB, electrospinning and carbonization.	SA = 1415 m2 g−1; pore volume = 0.82 cm3 g−1; Cs = 118 F g−1 @ 0.5 A g−1 (2 M HCl)
Porous yolk shell nanospheres, CPYCNs [34]	Layer-by-layer coating, ultra-sonication and carbonization.	SA = 486 m2 g−1; pore volume = 0.15 cm3 g−1; Cs = 236 F g−1 @ 0.5 A g−1 (6 M KOH)
Nitrogen-doped hollow mesoporous carbon spheres, N-HMCS (0.1) [102]	Stobber synthesis and polymerization.	SA = 636 m2 g−1; pore volume = 1.60 cm3 g−1; Cs = 307 F g−1 @ 0.5 A g−1 (6 M KOH)
Nitrogen-doped porous carbon/graphene oxide composites, GO/NC-2 [109]	Ring opening polymerization and KOH activation.	SA = 1345.8 m2 g−1; pore volume = 0.53 cm3 g−1; Cs = 405.6 F g−1 @ 1 A g−1 (6 M KOH)
Quinoline-based PBz/graphitic carbon nitride, poly(Q-xda) + 15 wt.% GCN [10]	Pyrolysis and ring-opening polymerization.	Cs = 370 F g−1 @ 6 A g−1 (1 M KOH)
Nitrogen rich porous carbon/graphitic carbon nitride/magnetite, NRPC/g-C3N4/Fe3O4 [9]	Carbonization, KOH activation, sonication and ageing.	SA = 497.6 m2 g−1; Cs = 385 F g−1 @ 1 A g−1 (1 M KOH)
Guaiacol-based PBz carbon, C-GP81 [119]	Carbonization.	Pore size = 200–300 µm; Cs = 700 F g−1 @ 10 A g−1 (0.1 M H2SO4)
Hetero-doped carbon spheres, C-P-PEI [25]	Sol-gel method and carbonization.	SA = 221 m2 g−1; pore size = 5.1 nm; pore volume = 0.28 cm3 g−1; Cs = 728 F g−1 @ 10 A g−1 (0.1 M H2SO4)
PBz and poly(imidazole diphosphoric acid)-based carbon, PABz-co-Cu MOFs-graft-PIPDA (50/50) [1]	Thermal curing.	Cs = 387 F g−1 @ 1 A g−1 (3 M KOH)

SA = surface area; C_s = specific capacitance.

shows the list of different carbon materials synthesized from PBZ and their properties.

PBZ naturally contain nitrogen and oxygen, making them ideal for heteroatom doping during pyrolysis. This enhances Faradaic activity, wettability, and electrical conductivity, boosting pseudocapacitance and electrolyte access. With rising interest in green chemistry, PBZ precursors are increasingly synthesized from biomass sources like natural phenols and amines. Advanced methods, such as foaming and templating, enable the creation of hierarchically porous structures (micro-, meso-, and macropores), which improve ion transport and boost power and energy density. PBZ-derived carbons are often composited with graphene, CNTs, or conductive polymers to enhance conductivity and mechanical strength, supporting the development of flexible, wearable supercapacitors. However, the field presents contradictions: Some studies credit micropores with high EDLC [59], while others highlight mesopores for better ion compatibility [130]. The effect of KOH activation is debated—beneficial in some cases [15], but linked to pore collapse in others [59]. There is also no agreement on the best electrolyte type, or whether co-doping (e.g., N and S) improves or destabilizes performance. Key research gaps include: understanding capacity fading (e.g., heteroatom leaching, pore blockage), limited data on cycling beyond 10,000 cycles, and a lack of studies on all-solid-state or stretchable devices. Additionally, there is minimal insight into the cost, scalability, and environmental impact of PBZ synthesis, calling for life cycle and techno-economic assessments.

6. Conclusions and Future Directions

Polybenzoxazine-derived supercapacitors offer a promising alternative for energy storage applications due to their tunable properties, high stability, and potential for large-scale production. The research landscape on polybenzoxazine-derived supercapacitors is rapidly evolving, focusing on (i) Sustainable materials: The integration of bio-derived PBZs into supercapacitor electrodes is a major step toward sustainable energy storage solutions, aligning with the principles of green chemistry. These materials are gaining attention due to their eco-friendliness, cost-effectiveness, and excellent electrochemical properties, making them ideal candidates for next-generation supercapacitor electrodes. (ii) Enhanced performance: Heteroatom doping involves introducing elements like nitrogen (N), oxygen (O), sulfur (S), phosphorus (P), or boron (B) into the carbon framework derived from polybenzoxazines. This process modifies the electronic structure, improves charge distribution, and enhances redox activity, leading to higher specific capacitance and energy storage efficiency. Nitrogen and oxygen doping introduce functional groups, such as -NH₂, -OH, -C=O, and -COOH, that participate in Faradaic redox reactions, significantly boosting capacitance. Research results show that N-doped carbon materials show up to a 50% increase in capacitance compared to undoped carbons. Particularly, graphitic-N and pyridinic-N doping have been shown to improve conductivity and charge transfer rates. Oxygen and sulfur doping improve surface hydrophilicity, ensuring better electrolyte penetration and reducing charge transfer resistance. (iii) Composite materials combine PBZ-derived carbon with other high-performance materials like graphene and metal oxides to leverage synergistic effects that boost energy storage capabilities. Metal oxides (MnO₂, Fe₂O₃) and conducting polymers (polyaniline, polypyrrole) introduce Faradaic charge storage mechanisms, significantly increasing capacitance. Graphene reinforce the PBZ-derived carbon matrix, preventing electrode degradation over repeated charge–discharge cycles. Future work may explore scalable manufacturing techniques for PBZ-based supercapacitor materials and their integration into next-generation energy storage systems. These innovations will be crucial in meeting the increasing demand for high-performance, environmentally friendly, and cost-effective supercapacitors.

Future research should aim at low-cost, eco-friendly, and scalable polymerization and carbonization techniques, possibly incorporating bio-based benzoxazine monomers and solvent-free processes. It should emphasize controllable synthesis of multi-scale porous architectures (micro-, meso-, and macropores) to optimize ion transport and electrolyte accessibility while preserving high surface areas. Addressing degradation mechanisms and ensuring thermal and electrochemical stability over extended cycles remain crucial for commercial viability. Investigating the integration of polybenzoxazine-based materials into flexible or solid-state supercapacitors could expand their application in wearable and portable electronics. By focusing on these areas, researchers can unlock the full potential of polybenzoxazine-derived materials in next-generation supercapacitor technologies.