Carbon-based nanomaterials such as activated carbon, carbon nanotube (CNT), and graphene are extensively applied in energy conversion and storage while h-BN is rarely employed in electrochemical energy systems because of the wide bandgap and insulating nature of boron nitride  (BN). However, hexagonal boron nitride (h-BN) can actually be used to alleviate the thermal deformation of conventional organic separators, weak solid electrolyte interface layers of metal anodes, and electrocatalyst poisoning in electrochemical systems due to its chemical and thermal stability, and high mechanical strength ^[1][2][9,79]. In recent years, BCN has come to be regarded as a promising electrode material for next generation SCs since it combines the excellent physicochemical properties of h-BN and graphene. 

Theoretical calculations and experimental studies have demonstrated that the composite of h-BN and graphene can be applied in SCs ^{[3][4][5][6][7][8][9]}[15,16,80,81,82,83,84]. In recent years, h-BN has come to be regarded as an ideal candidate for fabricating van der Waals heterostructures when combining it with graphene due to its remarkable mechanical, thermal, and electronic properties ^{[3][4][7][8][9]}[15,16,82,83,84]. This class of h-BN/graphene heterostructures can be tuned for exceptional properties such as regulating the intrinsic electronic structure of h-BN and enhancing the carrier mobility of graphene, or used for SCs ^[10][85]. Byun and co-workers utilized the assembly of h-BN and reduced graphite oxide (rGO) to construct h-BN/graphene-based van der Waals heterostructure nanocomposites through electrostatic interaction ^[11][86].

Zheng et al. reported a well-constructed h-BN/graphene heterostructure material which showed a maximum capacitance of 134 F g⁻¹ and excellent cycling stability (~96% retention @ 10,000 cycles at 10 A g⁻¹) based on the liquid-phase exfoliation method ^[12][88]. Pati et al. fabricated a 2D/3D heterostructure material of h-BN/rGO with the maximum capacitance of 304 F g⁻¹ (at 1 A g⁻¹) in alkaline conditions and a good rate capability (98% of the initial capacitance after 10,000 cycles) via a hydrothermal assembly strategy ^[13][89]. Saha et al. elaborately investigated the effect of different concentrations of rGO within the hBN/graphene composite on the electrical and electrochemical properties of an h-BN/rGO heterostructure via a simple pyrolysis method ^[14][90]. It was found that increasing the rGO amount caused the transition from pseudocapacitance to EDLC and the sheet-like h-BN/rGO superlattice exhibited its highest specific capacitance of ~960 F g⁻¹ at a scan rate of 10 mV s⁻¹. Saha and co-workers subsequently found that a fluorine (F)-doped h-BN/rGO superlattice exhibited a decreased bandgap (~1.79 eV) compared to the h-BN/rGO superlattice (∼2.1 eV) ^[15][87]. The electrochemical activity of electrode materials changes from n-type semiconductor to p-type when element doping happens. The h-BN/rGO superlattice exhibits an enhanced specific capacitance (942 F g⁻¹, 10 mV s⁻¹) compared to graphene/rGO because charge transfer occurs not only on the surface of the electrode material, but also in the interior of h-BN/rGO superlattice at specific redox potentials. Conversely, F doping triggers the shift of Fermi level towards the valance band of the electrode, that is, to a lower energy level compared with the redox potential of the electrolyte. The charge transfer of the F-doped h-BN/rGO superlattice occurs from the electrolyte to the electrode at the interface, leading to an increased capacitance (1250 F g⁻¹, 10 mV s⁻¹) in comparison to the EDLC capacitance.

Owing to the similar lattice constant with graphene, high mechanical strength, as well as the pseudocapacitive nature of h-BN, h-BN can serve as a 2D substrate and electrolyte channel in h-BN/graphene heterostructure nanocomposites, thus effectively enhancing the electrochemical performance and flexibility of supercapacitors ^[4][16]. However, the synthesis of these h-BN/graphene heterostructures always suffers from problems such as a high cost and low efficiency. The traditional micromechanical cleavage of bulk h-BN and graphite crystals can maintain the complete lattice of the parent materials while the low yield limits its practical application. The CVD method can realize the direct growth of h-BN on graphene, but the complex transfer procedure and expensive catalysts also hinder the large-scale preparation. Liquid exfoliation combined with vacuum filtering is an alternative low-cost method to fabricate such h-BN/graphene heterostructures, whereas the limitations, such as poor dispersity and the small lateral sizes of the products, still need to be solved ^[11][86]. To conquer these shortcomings, a potential effective solution is to functionalize h-BN or graphene in advance. For example, ball milling allows large-scale production of aminated or hydroxylated h-BN/graphene. Then, h-BN/graphene heterostructures can be scaled up through filtering or printing.

Ternary BCN materials also show improved supercapacitor behavior compared to carbon-based materials ^[16][91]. Heteroatom doping not only modifies the surface polarity, but also affects the pseudocapacitive effect of carbon materials. The specific capacitance of BCN material is 247 F g⁻¹ at 2 mV s⁻¹, which is more than twice that of dopant-free carbon materials (111 F g⁻¹ at 2 mV s⁻¹) ^[16][91]. As compared to h-BN/graphene heterostructures, the synthesis of BCN nanomaterials is more productive and easier to modify. In the past decade, various BCN nanomaterials have been successfully used in supercapacitor applications. For example, Iyyamperumal et al. synthesized vertically aligned BCN nanotubes (VA-BCNs) with high specific capacitance (321.0 F g⁻¹) from a single melamine diborate precursor using the CVD method ^[17][92]. Owing to the synergetic effects arising from the heteroatom doping and the well-aligned nanotube structure, VA-BCNs display a higher specific capacitance than nonaligned BCNs (167.3 F g⁻¹) and undoped multiwalled carbon nanotubes (117.3 F g⁻¹). Subsequently, Zhou et al. fabricated a high-performance electrochemical capacitor based on vertically aligned BC₂N nanotube arrays (VA-BC₂NNTAs) via a one-step solvothermal method using NaBH₄, NaN₃, and hexadecyl trimethyl ammonium bromide (CTAB) as starting materials at low temperature ^[18][93]. The well-aligned nonbuckled tubular structure of VA-BC₂NNTAs means it possesses an extremely high specific capacitance (547 F g⁻¹) and maintains an excellent rate capability and durability. Dou et al. reported a BCN graphene electrode material which is prepared through the thermal annealing of graphene oxide and melamine diborate mixtures at 600–1000 °C ^[19][94]. The specific capacitance of BCN (130.7 F g⁻¹ at 0.2 A g⁻¹) is nearly 1.7 times that of an undoped graphene electrode (77.4 F g⁻¹). Wu et al. prepared 3D B and N co-doped graphene aerogels (BN-GAs) in the presence of GO and ammonia boron trifluoride using a hydrothermal method, showing larger specific capacitance (62 F g⁻¹) and higher energy density (≈8.65 W h kg ⁻¹) and power density (≈1600 W kg ⁻¹) with respect to undoped GAs in all-solid-state supercapacitors (ASSSs) ^[20][95]. Element doping on the honeycomb structure will introduce a large number of defects in BCN materials and increase the number of active sites for electron transfer ^[21][22][96,97]. Wang and co-workers successfully prepared bandgap-tunable porous BCNNs using boric acid, urea, and glucose precursors by annealing and exfoliating ^[23][98]. 

In spite of the remarkable achievements of BCN materials applied in SCs, challenges still exist to develop facile and effective approaches for the preparation of nanostructured BCN materials. Similar to the synthesis of h-BN/graphene heterostructure nanocomposites, the disadvantages such as being expensive and requiring toxic starting materials, sophisticated instruments, and laborious operations hinders the practical application of BCN materials. From the point of view of large-scale production and green synthesis, new techniques should select nontoxic, low-cost, and easily accessible raw materials. Furthermore, pre/post-processing steps should be avoided to simplify the synthesis procedure. In addition, the compositions of BCNs should be easily adjustable to determine the optimum B, C, and N source ratio. The defect sites in BCN skeletons can act as active sites for charge transfer reactions, which is beneficial for electrode materials to improve the electrochemical performance. It is important for the new synthesis techniques to prepare BCN nanomaterials with large SSA and abundant active and conductive sites.

In recent years, many strategies such as designing and fabricating porous and defective BCN nanomaterials with diverse structures and constructing heterostructures or nanocomposites have been applied to boost the electrochemical performance of BCN-based supercapacitors ^{[24][25][26][27][28]}[100,101,102,103,104]. For example, Shi et al. fabricated a BCN-polyaniline (PANI)-based electrochemical capacitor which possesses a high voltage window of 3.0 V (1 M Et₄N BF₄ as the electrolyte) and ultrahigh energy density of 67.1 W h kg^{−1 [29]}[105]. PANI modification changed the EDLC behavior and stacked-layer structure of BCNNs, providing a promising strategy to configure BCN-based composite electrodes for other energy storage devices.

Owing to the structural merits, 1D BCN nanotubes/nanofibers exhibit tremendous performance for energy storage materials because the unique hollow tubular structure can effectively improve the ion diffusion channel properties of BCNNT-based SCs ^[30][31][106,107]. For instance, Li⁺ ions can diffuse into sites either on the outer or the inner surface of hollow BCNNTs and can be inserted within the BCN layers of BCNNTs ^[32][108]. Xu et al. successfully synthesized a Na_0.76V₆O₁₅@BCNNT cathode with excellent capacity, good cyclic stability, and an energy density of 238.7 Wh kg⁻¹ (at 200 W kg⁻¹) for lithium-ion capacitors (LICs) ^[33][109]. Liang et al. reported a strategy for the in situ growth of BCNNTs on carbon fibers and assembled a symmetric supercapacitor with a BCNNT electrode in a 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM·BF₄) electrolyte ^[34][110]. The 1D physical structure of BCNNTs is advantageous in maintaining the structural integrity and resisting the damage caused by the swift adsorption/desorption of the electrolyte during long-period charge/discharge cycles at high temperatures, leading to an enhancement in the high-temperature cycle stability. Benefitting from the synergy between the high conductivity of BCNNTs and the lamellar structure of MoS₂, Tu and co-workers manufactured a highly dense BCN nanofiber core with a MoS₂ shell for a high-performance supercapacitor (446.3 A g⁻¹ at current density of 0.25 A g⁻¹) ^[35][111]. In short, 1D BCN materials possess large SSA, high structural stability, and excellent mechanical strength, leading to an enhanced capacitance and electrical conductivity. New advancements such as combining metal oxides, carbon fibers, and other 2D materials are regarded as efficient strategies to fabricate 1D BCN-based hybrid electrode materials for high-performance SCs. Hybridization with pseudocapacitive materials is conducive for BCN-based SC devices to enlarge their energy density values. However, it should be noted that the weak interaction between 1D BCN materials and other materials tends to cause the uneven distribution or untight contact on the BCN surface. Therefore, finding a new strategy to combine 1D BCNs with functional materials still remains a challenge.

Focusing on SCs, 2D BCNNs and their nanocomposites as advanced materials have been widely used due to their large energy storage capabilities, astonishing electronic conductivity, and concomitant mechanical properties ^[36][112]. However, the inherent restacking and stability of 2D honeycomb structures hamper the further increase in capacitance. Pore/defect engineering and introducing a complementary high-performance 2D counterpart material can improve the surface area and alleviate the stability issue of both 2D materials, respectively, providing enhanced synergistic interplayed energy storage opportunities ^[37][113]. Panda et al. obtained hierarchal porous BCNNs (p-BCN) with extremely high SSA (3310.4 m² g⁻¹) by KOH activation at high temperatures ^[38][114]. It was found that the assembled symmetric p-BCN device possessed high energy and power densities (17 W h Kg⁻¹ and 4000 W kg⁻¹) with high cycling stability. Zhang and co-workers developed a high-performance MSC using a 2D BCN nanomesh as the electrode material with pseudocapacitive charge storage capacity ^[39][115]. Shi et al. designed a BCN-assisted built-in electric field in a heterostructure and successfully broadened the voltage window of aqueous supercapacitors (1.2 V to 2 V in MnO/MnS@BCN-based symmetrical supercapacitors) by utilizing the synergistic effect ^[40][116]. Nasrin and co-workers constructed an MXene/BCN heterostructure electrode which shows a high specific capacitance of 1173 F g⁻¹ (at 2 A g⁻¹) and an energy density of 45 Wh kg^{−1 [41]}[117]. Notably, the as-assembled solid-state device exhibits an ultra-high cyclability without any degradation after 100,000 cycles (100% capacitive retention).

BCNNs are well suited for SC applications owing to their extraordinary thermal and electrical conductivities, high SSA, excellent tensile strength and good flexibility. Unfortunately, single BCN layers tend to restack because of the interaction between adjacent BCN layers. Serious agglomeration issues will reduce the SSA and may prevent the diffusion of electrolyte ions between BCN layers. However, defects will accelerate the charge transfer reaction and an appropriate porosity may improve the electrical conductivity. Hence, it is still a challenge and desirable to design new routes to prepare porous and defective BCN nanomaterials. In addition, coupling with other pseudocapacitive materials can also alleviate the restacking issue of BCNNs and enhance the electrochemical performance of 2D BCN-based SCs.

Another alternative type of electrode material platform for the assembly of BCN-based SCs is 3D nanostructured materials. Compared with 1D and 2D nanomaterials, the infinite growth from 3D features in their spatial structure make 3D BCN-based nanomaterials possess abundant active sites and porous-and-loose characteristics, which is beneficial for supercapacitors with high capacitance performance ^[42][43][118,119]. Tabassum et.al reported a BCNNT architecture entangled on a 3D melamine-foam-derived carbon skeleton with high surface area and hierarchical porosity which displayed a large capacitance of 344 F g⁻¹ at a current density of 1 A g^{−1 [44]}[120]. In addition, the as-prepared 3D BCN could be used as electrodes in a symmetric supercapacitor (presenting a high energy density of 19.8 W h kg⁻¹ and elevated power density of 5074 W kg⁻¹) and the negative electrode in an asymmetric hybrid supercapacitor (energy density of 72 W h kg⁻¹ and elevated power density of 22,732 W kg⁻¹). Zou and co-workers fabricated B, N co-doped holey graphene aerogels (BN-HGA) with an SSA of 249 m² g⁻¹ and rich B-N motifs for flexible SCs ^[45][121]. The rich B-N motifs in the BN-HGA electrode cause the high surface polarity and abundant redox sites for the enhanced pseudocapacitance (a capacitance of 456 F g⁻¹ at 1 A g⁻¹ in three-electrode systems using sulfuric acid as electrolyte). Meanwhile, the integrated carbon matrix and the hierarchical 3D network facilitate the fast ion diffusion in the electrode and adsorption in the high-viscosity gel electrolyte, resulting in a high specific capacitance (345 mF cm⁻² at 1 mA cm⁻²) and outstanding rate performance (80% retention at 20 mA cm⁻²) for all-solid-state flexible supercapacitors based on the symmetric BN-HGA electrodes. Liu et al. subsequently constructed a self-supported fluorine-doped BCN (F-BCN) aerogel material for a symmetric supercapacitor with a maximum energy density of 11.75 Wh kg⁻¹ and 83% retention after 5000 charge and discharge cycles ^[46][122]. Fluorine doping leads to an increase in the defect density, expanding of the interlayer spacing, massive electrochemical active sites, and faster diffusion of ions in the electrode, thereby promoting the specific capacitance of 524.9 F g⁻¹ at a specific current of 1 A g⁻¹ for F-BCN. In addition, people have tried to change conventional 2D BCNNs into 3D architectures using other 2D materials. Tu and co-workers have successively achieved the assembly of 3D MXene/BCN microflowers and BCN/rGO broccoli for all-solid-state flexible MSCs with remarkable mechanical flexibility ^[47][48][123,124].

3D BCN materials possess a highly porous structure, eminent SSA, as well as excellent mechanical and electrical properties. The unique 3D structure of BCNs not only provides extra electron moving channels, but also offers high electrochemical performance such as large capacitance and excellent cycling stability. The template-assisted method, hydrothermal synthesis, CVD, and pyrolysis have been adopted to prepare active 3D BCN electrode materials. However, the electrical conductivity will decrease with the existence of macro-pores in 3D BCN, resulting in a reduction in the energy and power density values. In order to solve this problem, constructing 3D BCN-based composites with other 2D materials is an alternative to increase the accessible area for electrolyte diffusion and improve the charge transfer reaction, thus enhancing the capacitance and energy density.

The recent development of BCN-based electrodes, fabrication approaches, and their electrochemical performance as SCs are summed up in Table 1. Table 1 reveals that BCN-based electrode materials exhibit an excellent electrochemical performance through large specific capacitance, outstanding energy density, and power density in a high potential window compared to the electrolyte. Moreover, BCN-based electrodes also present good capacity retention over a long cycle, demonstrating that BCN is a promising high-performance electrode material for flexible device storage systems.