Nitrogen, sulfur co-doped hierarchically porous carbon from rape pollen as high-performance supercapacitor electrode

In Situ Self-Assembly of Nitrogen-Doped 3D Flower-like Hierarchical Porous Carbon and Its Application for Supercapacitors

The hierarchical porous carbon-based materials derived from biomass are beneficial for the enhancement of electrochemical performances in supercapacitors. Herein, we report the fabrication of nitrogen-doped 3D flower-like hierarchical porous carbon (NPC) assembled by nanosheets using a mixture of urea, ZnCl2, and starch via a low-temperature hydrothermal reaction and high-temperature carbonization process. As a consequence, the optimized mass ratio for the mixture is 2:2:2 and the temperature is 700 °C. The NPC structures are capable of electron transport and ion diffusion owing to their high specific surface area (1498.4 m2 g-1) and rich heteroatoms. Thereby, the resultant NPC electrodes display excellent capacitive performance, with a high specific capacitance of 249.7 F g-1 at 1.0 A g-1 and good cycling stability. Remarkably, this implies a superior energy density of 42.98 Wh kg-1 with a power density of 7500 W kg-1 in organic electrolyte for the symmetrical supercapacitor. This result verifies the good performance of as-synthesized carbon materials in capacitive energy storage applications, which is inseparable from the hierarchical porous features of the materials.

Mesoporous graphitic carbon electrodes derived from boat-fruited shells of Sterculia Foetida for symmetric supercapacitors for energy storage applications

In recent years, intensive research efforts have focused on translating biomass waste into value-added carbon materials broadcasted for their significant role in energy and environmental applications. For the first time, high-performance carbonaceous materials for energy storage applications were developed from the multi-void structure of the boat-fruited shells of Sterculia Foetida (SF). In that view, synthesized mesoporous graphitic activated carbon (g-AC) via the combination of carbonization at various elevating temperatures of 700, 800, and 900 °C, respectively, and alkali activation by KOH, with a high specific surface area of 1040.5 m2 g-1 and a mesopore volume of 0.295 cm3 g-1. In a three-electrode configuration, the improved electrode (SF-K900) exhibited excellent electrochemical behavior, which was observed in an aqueous electrolyte (1 M H2SO4) with a high specific capacitance of 308.6 F/g at a current density of 1 A/g, owing to the interconnected mesopore structures and high surface area of SF-K900. The symmetric supercapacitor (SSC) delivered the specific capacitance of 138 F/g at 1 A/g with a high energy density (ED) of 13.4 Wh/kg at the power density (PD) of 24.12 kW/kg with remarkable cycle stability and supercapacitive retention of 93% over 5000 cycles. Based on the findings, it is possible to develop low-cost active electrode materials for high-rate performance SSC using mesoporous g-AC derived from SF boat-fruited shells.

Freeze-Drying-Assisted ZIF-67 Template-Derived Co@NCS Porous Composite as Sulfur Cathode Host for Improved Li-S Battery Performance: Deconvolution of Diffusive and Capacitive Li+ Storage

A zeolitic imidazole framework (ZIF-67) template-derived cobalt@nitrogen-doped carbon-sulfur composite (ZIF-Co@NCS) was synthesized using a freeze-drying method and explored for lithium-sulfur (Li-S) batteries. Material characterizations confirmed the formation of the ZIF-Co@NCS composite. To fabricate the Li-S battery in a coin-type CR-2032 cell, the as-synthesized ZIF-Co@NCS composite having a sulfur content of ∼65% was used as a cathode material and coupled with a lithium metal anode. This battery demonstrated exceptional cycling stability over 600 charge-discharge cycles with a steady capacity of 550 mAh g-1 at 0.05 C-rate. The good electrochemical performance of the cathode was ascribed to the nanofeatures associated with the freeze-dried porous carbon structure, which offered enough space for the sulfur cathode. The enhanced trapping of polysulfide facilitated by the presence of the Co-N sites in the ZIF-Co@NCS composite led to excellent cycling stability. The derived cathode composite showed a high specific capacity, outstanding rate capability, and excellent cyclic stability rendering it a promising candidate for high-performance Li-S batteries. Detailed Li+ ion charge storage examined by means of Dunn's method revealed a significant capacitive mode of charge storage both at peak currents and nonpeak currents. The post-mortem analysis of the cycled cathode using X-ray diffraction and scanning electron microscopy at different depth-of-discharge (DOD) values revealed the stepwise formation of discharge products.

Mushroom-like cobalt nickle metaphosphate@nickel diselenide core-shell nanorods for asymmetric supercapacitors

Although transition metal metaphosphates (TMPOs) display special physical/chemical features and high theoretical capacities, their applications for supercapacitors (SCs) are still restricted by their low energy densities and inferior cycling stability. Herein, a novel strategy has been proposed to address these issues through in situ construction of cobalt nickle metaphosphate (Co_0.2Ni_0.8(PO₃)₂)@nickel diselenide (NiSe₂) core-shell heterostructure on carbon paper (CP) as a self-supporting flexible electrode for SCs. Particularly, this unique mushroom-like porous nanoarchitecture assembled by one-dimensional (1D) Co_0.2Ni_0.8(PO₃)₂ nanorods and zero-dimensional (0D) NiSe₂ nanospheres can expose abundant active sites and afford multi-dimensional channels, which favors rapid electron ions/electron transfer, accelerates the reaction kinetics, and alleviates volume changes during charging/discharging processes. Profiting from its well-aligned 1D/0D nanostructure and strong synergistic effect between Co_0.2Ni_0.8(PO₃)₂ and NiSe₂, the Co_0.2Ni_0.8(PO₃)₂@NiSe₂/CP electrode delivers a specific capacity of 219.4 mAh/g/0.414 mAh cm^-2 at 1 A/g and good cycling stability with capacity retention of 90.7% after 5000 cycles, outperforming many previously reported TMPO-based electrodes in literature. Impressively, an asymmetric supercapacitor (ASC) device assembled with Co_0.2Ni_0.8(PO₃)₂@NiSe₂ as cathode and porous carbon as anode achieves an energy density of 69.2 Wh kg^-1 at 736.0 W kg^-1 and maintains a capacity retention of 97.6% after 20,000 charge-discharge cycles. This work provides an efficient approach to design multi-dimensional hybrid nanomaterials for high-performance SCs.

Two-step electrodeposition synthesis of iron cobalt selenide and nickel cobalt phosphate heterostructure for hybrid supercapacitors

Exploring novel heterostructure with multiscale nanoarchitectures and modulated electronic structure is crucial to improve the electrochemical properties of electrode materials for supercapacitors (SCs). In this study, a two-step electrodeposition approach which involves suitable efficient procedures, is leading to in-situ preparation of iron cobalt selenide (Fe0.4Co0.6Se2) @ nickel cobalt phosphate (NiCo(HPO4)2·3H2O, denoted as NiCo-P) hybrid nanostructure on carbon cloth (CC) substrate. Particularly, depositing two-dimensional (2D) NiCo-P nanosheets on the surface of Fe0.4Co0.6Se2 nanobelts results in formation of well-organized Fe0.4Co0.6Se2@NiCo-P nanocomposite with large surface area, hierarchical porous nanoarchitecture as well as numerous electroactive sites, leading to enhanced electroactivity and accelerated mass/electron transfer. Benefiting from its unique nanoarchitecture and synergistic effect of two components, the obtained free-standing Fe0.4Co0.6Se2@NiCo-P electrode demonstrates gravimetric capacity (Cm)/volumetric capacity (Cd) of 202.3 mAh/g/319.6 mAh cm-3 at 1 A g-1 and good cyclic stability (83.9% capacity retention over 5000 cycles), which are superior to those of pure Fe0.4Co0.6Se2 and NiCo-P electrodes. Impressively, it was established that an aqueous hybrid supercapacitor (HSC) based on Fe0.4Co0.6Se2@NiCo-P and rape pollen derived hierarchical porous carbon (RPHPC) achieves gravimetric energy density (Em)/volumetric energy density (Ed) of 64.4 Wh kg-1/10.7 mWh cm-3 and a long cycle life with 90.3% capacity retention over 10,000 cycles. This report offers a perspective to design selenide/phosphate heterostructure on conducting substrate for electrochemical energy storage applications.

H3PO4/KOH Activation Agent for High Performance Rice Husk Activated Carbon Electrode in Acidic Media Supercapacitors

H₃PO₄/KOH combined solution is proposed as a new effective activation agent for activated carbon production from rice husk. Several activated carbon samples were produced by using different volumes of the utilized acid and alkali individually, in addition to the combined solution. FTIR results indicated that the mixed agent partially decomposed the chemical compounds on the rice husk char surface, resulting in an increase in the surface area. Moreover, XRD and EDS analyses showed the presence of a considerable amount of amorphous silica. Electrochemical measurements concluded that the volume of the activation agent solution should be optimized for both single and mixed activation agents. Numerically, for 0.3 g treated rice husk char, the maximum specific capacitance was observed at 7, 10 and 14 mL of H₃PO₄, KOH (3 M) and mixed (1:1 by volume) activation agents, respectively; the determined specific capacitance values were 73.5, 124.2 and 241.3 F/g, respectively. A galvanostatic charging/discharging analysis showed an approximate symmetrical triangular shape with linear voltage versus time profile which indicates very good electrochemical performance as an electrode in the supercapacitors application. The stability of the proposed activated carbon was checked by performing a cyclic voltammetry measurement for 1000 cycles at 2 mV/s and for 30,000 cycles at 10 mV/s. The results indicate an excellent specific capacitance retention, as no losses were observed.

A Review on Production and Surface Modifications of Biochar Materials via Biomass Pyrolysis Process for Supercapacitor Applications

Biochar (BC) based materials are solid carbon enriched materials produced via different thermochemical techniques such as pyrolysis. However, the non-modified/non-activated BC-based materials obtained from the low-temperature pyrolysis of biomass cannot perform well in energy storage applications due to the mismatched physicochemical and electrical properties such as low surface area, poor pore features, and low density and conductivity. Therefore, to improve the surface features and structure of the BC and surface functionalities, surface modifications and activations are introduced to improve its properties to achieve enhanced electrochemical performance. The surface modifications use various activation methods to modify the surface properties of BC to achieve enhanced performance for supercapacitors in energy storage applications. This article provides a detailed review of surface modification methods and the application of modified BC to be used for the synthesis of electrodes for supercapacitors. The effect of those activation methods on physicochemical and electrical properties is critically presented. Finally, the research gap and future prospects are also elucidated.

B, O and N Codoped Biomass-Derived Hierarchical Porous Carbon for High-Performance Electrochemical Energy Storage

High specific surface area, reasonable pore structure and heteroatom doping are beneficial to enhance charge storage, which all depend on the selection of precursors, activators and reasonable preparation methods. Here, B, O and N codoped biomass-derived hierarchical porous carbon was synthesized by using KCl/ZnCl2 as a combined activator and porogen and H3BO3 as both boron source and porogen. Moreover, the cheap, environmentally friendly and heteroatom-rich laver was used as a precursor, and impregnation and freeze-drying methods were used to make the biological cells of laver have sufficient contact with the activator so that the layer was deeply activated. The as-prepared carbon materials exhibit high surface area (1514.3 m2 g−1), three-dimensional (3D) interconnected hierarchical porous structure and abundant heteroatom doping. The synergistic effects of these properties promote the obtained carbon materials with excellent specific capacitance (382.5 F g−1 at 1 A g−1). The symmetric supercapacitor exhibits a maximum energy density of 29.2 W h kg−1 at a power density of 250 W kg−1 in 1 M Na2SO4, and the maximum energy density can reach to 51.3 W h kg−1 at a power density of 250 W kg−1 in 1 M BMIMBF4/AN. Moreover, the as-prepared carbon materials as anode for lithium-ion batteries possess high reversible capacity of 1497 mA h g−1 at 1 A g−1 and outstanding cycling stability (no decay after 2000 cycles).

Biomass-derived hierarchical N, P codoped porous 3D-carbon framework@TiO2 hybrids as advanced anode for lithium ion batteries

Advanced anode materials with high theoretical capacity and rate capability are urgently required for next generation lithium ion batteries (LIBs). In this study, hierarchical N, P codoped porous 3D-carbon framework@TiO₂ nanoparticle hybrid (N, PC@TiO₂) is synthesized by using pollen as biomass precursor through a facile template assisted sol-gel methode and exhibits hierarchical porous hollow structure with plenty of redox active sites and enhanced specific surface area. Compared with N, P codoped porous micro-carbon sphere framework and TiO₂ porous hollow microspheres anodes, the N, PC@TiO₂ anode shows superior reversible capacity of 687.3 mAh g^-1 at 0.1 A g^-1 after 200 cycles and 440.5 mAh g^-1 after 1000 cycles at 1 A g^-1. The excellent performance can be attributed to the rational hierarchical porous hollow structure and the synergetic contributions from the N, P codoped-carbon and TiO₂ components, which enhance Li⁺ storage capability, accelerate the reaction kinetics and stabilize the electrode structure and interface during charge/discharge process. This study suggests a practical strategy to prepare novel anode material with abundant natural resource and facile synthetic route, and the optimized hybrid anode with outstanding Li⁺ storage properties provides hopeful application prospect in advanced LIBs and other energy storage devices.

Preparation of porous biochar and its application in supercapacitors

In this study, economical porous biochar was prepared from an apricot shell and used as an electrode material for a supercapacitor, showing excellent capacitance, cycling stability and rate performance.

Biochar for agronomy, animal farming, anaerobic digestion, composting, water treatment, soil remediation, construction, energy storage, and carbon sequestration: a review

In the context of climate change and the circular economy, biochar has recently found many applications in various sectors as a versatile and recycled material. Here, we review application of biochar-based for carbon sink, covering agronomy, animal farming, anaerobic digestion, composting, environmental remediation, construction, and energy storage. The ultimate storage reservoirs for biochar are soils, civil infrastructure, and landfills. Biochar-based fertilisers, which combine traditional fertilisers with biochar as a nutrient carrier, are promising in agronomy. The use of biochar as a feed additive for animals shows benefits in terms of animal growth, gut microbiota, reduced enteric methane production, egg yield, and endo-toxicant mitigation. Biochar enhances anaerobic digestion operations, primarily for biogas generation and upgrading, performance and sustainability, and the mitigation of inhibitory impurities. In composts, biochar controls the release of greenhouse gases and enhances microbial activity. Co-composted biochar improves soil properties and enhances crop productivity. Pristine and engineered biochar can also be employed for water and soil remediation to remove pollutants. In construction, biochar can be added to cement or asphalt, thus conferring structural and functional advantages. Incorporating biochar in biocomposites improves insulation, electromagnetic radiation protection and moisture control. Finally, synthesising biochar-based materials for energy storage applications requires additional functionalisation.

Flute-type porous carbon derived from soybean shells for high-performance all-solid-state symmetric supercapacitors

Flute-type porous carbon was successfully prepared from soybean shells through convenient methods. The influence of mass ratio on the structure and electrochemical performance of porous carbon obtained from soybean shells was investigated in detail. The obtained porous carbon exhibited a micro-tube morphology structure with a specific surface area of 2802 m² g⁻¹, pore volume of 1.36 cm³ g⁻¹, and appropriate pore size distribution. The porous carbon showed good electrochemical properties as an electrode material for supercapacitors. The optimal porous carbon SSAC4 exhibited high specific capacitance of 465 F g⁻¹ (1 A g⁻¹) and 287 F g⁻¹ (20 A g⁻¹) in a three-electrode system with 6 M KOH electrolyte. In addition, the as-assembled SSAC4-based all-solid-state supercapacitors delivered a high specific capacitance of 294 F g⁻¹ at 0.1 A g⁻¹ and excellent cycling stability of 86.2% after 10 000 cycles at 5 A g⁻¹.

Flute type porous carbon is successfully derived from soybean shell through a convenient method. The porous carbon shows good electrochemical properties as an electrode material for supercapacitors.

Inorganic salt-induced synthesis of lignin derived hierarchical porous carbon with self-embedded quantum dots and ultrahigh mesoporosity for supercapacitors

Lignin-based hierarchical porous carbon with self-embedded carbon quantum dots for supercapacitor electrodes.

Fe3O4 Nanoparticles on 3D Porous Carbon Skeleton Derived from Rape Pollen for High-Performance Li-Ion Capacitors

Herein, a three-dimensional (3D) Fe₃O₄@C composite with hollow porous structure is prepared by simple solution method and calcination treatment with biomass waste rape pollen (RP) as a carbon source, which is served as an anode of Li-ion capacitor (LIC). The 3D interconnected porous structure and conductive networks facilitate the transfer of ion/electron and accommodate the volume changes of Fe₃O₄ during the electrochemical reaction process, which leads to the excellent performance of the Fe₃O₄@C composite electrode. The electrochemical analysis demonstrates that the hybrid LIC fabricated with Fe₃O₄@C as the anode and activated carbon (AC) as the cathode can operate at a voltage of 4.0 V and exhibit a high energy density of 140.6 Wh kg⁻¹ at 200 W kg⁻¹ (52.8 Wh kg⁻¹ at 10 kW kg⁻¹), along with excellent cycling stability, with a capacity retention of 83.3% over 6000 cycles. Hence, these encouraging results indicate that Fe₃O₄@C has great potential in developing advanced LICs electrode materials for the next generation of energy storage systems.

N/S Co-doped microporous carbon derived from PSSH-Melamine salt solution as cathode host for Lithium-Selenium batteries

Selenium cathode attracts great attention due to its high theoretical volumetric capacity and better electrical conductivity than sulfur cathode. Herein, N/S co-doped microporous carbon (NS-K-PC) is designed and prepared as Se host by a spray drying process of the poly(styrenesulfonic acid)-melamine salt solution followed by carbonization and activation process. The as-prepared NS-K-PC shows a very high micropore contribution of 94.8% in the total surface area, and a total N/S heteroatom doping level of 2.5 wt% in the carbon matrix. The NS-K-PC/Se cathode delivers a high reversible capacity of 499.2 mA h g^-1 at 0.1C, superior rate capacity of 324 mA h g^-1 at 8C, and great cycling stability with a capacity decay of 0.081% per cycle over 500 cycles at 1C. Additionally, a comparative study demonstrates that NS-K-PC/Se cathode with the carbonate-based electrolytes exhibit better cycling stability than those with ether-based electrolytes primarily resulted from a direct solid-solid conversion of Se to Li₂Se bypassing the formation of soluble polyselenides.

The Effect of Modifications of Activated Carbon Materials on the Capacitive Performance: Surface, Microstructure, and Wettability

In this review, the efforts done by different research groups to enhance the performance of the electric double-layer capacitors (EDLCs), regarding the effect of the modification of activated carbon structures on the electrochemical properties, are summarized. Activated carbon materials with various porous textures, surface chemistry, and microstructure have been synthesized using several different techniques by different researchers. Micro-, meso-, and macroporous textures can be obtained through the activation/carbonization process using various activating agents. The surface chemistry of activated carbon materials can be modified via: (i) the carbonization of heteroatom-enriched compounds, (ii) post-treatment of carbon materials with reactive heteroatom sources, and (iii) activated carbon combined both with metal oxide materials dan conducting polymers to obtain composites. Intending to improve the EDLCs performance, the introduction of heteroatoms into an activated carbon matrix and composited activated carbon with either metal oxide materials or conducting polymers introduced a pseudo-capacitance effect, which is an additional contribution to the dominant double-layer capacitance. Such tricks offer high capacitance due to the presence of both electrical double layer charge storage mechanism and faradic charge transfer. The surface modification by attaching suitable heteroatoms such as phosphorus species increases the cell operating voltage, thereby improving the cell performance. To establish a detailed understanding of how one can modify the activated carbon structure regarding its porous textures, the surface chemistry, the wettability, and microstructure enable to enhance the performance of the EDLCs is discussed here in detail. This review discusses the basic key parameters which are considered to evaluate the performance of EDLCs such as cell capacitance, operating voltage, equivalent series resistance, power density, and energy density, and how these are affected by the modification of the activated carbon framework.

One-step production of N-O-P-S co-doped porous carbon from bean worms for supercapacitors with high performance

In recent years, multi-heteroatom-doped hierarchical porous carbons (HPCs) derived from natural potential precursors and synthesized in a simple, efficient and environmentally friendly manner have received extensive attention in many critical technology applications. Herein, bean worms (BWs), a pest in bean fields, were innovatively employed as a precursor via a one-step method to prepare N-O-P-S co-doped porous carbon materials. The pore structure and surface elemental composition of carbon can be modified by adjusting KOH dosage, exhibiting a high surface area (S _BET) of 1967.1 m² g^-1 together with many surface functional groups. The BW-based electrodes for supercapacitors were shown to have a good capacitance of up to 371.8 F g^-1 in 6 M KOH electrolyte at 0.1 A g^-1, and good rate properties with 190 F g^-1 at a high current density of 10 A g^-1. Furthermore, a symmetric supercapacitor based on the optimal carbon material (BWPC_1/3) was also assembled with a wide voltage window of 2.0 V, demonstrating satisfactory energy density (27.5 W h kg^-1 at 200 W kg^-1) and electrochemical cycling stability (97.1% retention at 10 A g^-1 over 10 000 charge/discharge cycles). The facile strategy proposed in this work provides an attractive way to achieve high-efficiency and scalable production of biomass-derived HPCs for energy storage.

Nitrogen- and Oxygen-Containing Three-Dimensional Hierarchical Porous Graphitic Carbon for Advanced Supercapacitor

Three-dimensional hierarchical porous graphitic carbon (HPGC) were synthesized via one-step carbonization-activation and a catalytic strategy. The method can not only improve the graphitization degree of carbon materials, but also offer plentiful interfaces for charge accumulation and short paths for ion/electron transport. Polypyrrole, potassium hydroxide, and nickel acetate were used as the carbon precursors, activating agent, and catalyst, respectively. The retraction and dissolution of Ni caused the change of pore size in the material and led to the interconnected micro/nano holes. Nickel acetate played a significant role in enhancing the electrical conductivity, introducing pseudocapacitance, and promoting ion diffusion. In the supercapacitor, HPGC electrode exhibited a remarkable specific capacitance of 336.3 F g⁻¹ under 0.5 A g⁻¹ current density and showed high rate capability, even with large current densities applied (up to 50 A g⁻¹). Moreover, HPGC showed optimal cycling stability with 97.4% capacitance retention followed by 3000 charge-discharge cycles. The excellent electrochemical performances coupled with a facile large-scale synthesis procedure make HPGC a promising alternative for supercapacitors.

Cation intercalated one-dimensional manganese hydroxide nanorods and hierarchical mesoporous activated carbon nanosheets with ultrahigh capacitance retention asymmetric supercapacitors

We have reported the electrochemical performance of K⁺ ion doped Mn(OH)₄ and MnO₂ nanorods as a positive electrode and a highly porous activated carbon nanosheet (AC) made from Prosopis Juliflora as negative electrode asymmetric supercapacitor (ASC) with high rate capability and capacity retention. The cation K⁺ doped Mn(OH)₄ and MnO₂ nanorods with large tunnel sizes allow the electrolyte to penetrate through a well-defined pathway and hence benefits from the intercalation pseudocapacitance and surface redox reactions. As a result, they exhibit good electrochemical performance in neutral aqueous electrolytes. More specifically, the K⁺-Mn(OH)₄ nanorods exhibit higher capacitance values than K⁺-MnO₂ nanorods due to the homogenous distribution of 1D nanorods and optimum amount of OH bonds. The fabricated K⁺-Mn(OH)₄ symmetric electrochemical Pseudocapacitor shows very high energy density of 10.11 Wh/kg and high-power density of 51.04 W/kg over the range of 1.0 V in aqueous electrolyte. The energy density of AC||K⁺-Mn(OH)₄ ASC is improved significantly compared to those of symmetric supercapacitors. The fabricated ASC exhibits a wide working voltage window (1.6 V), high power (143.37 W/kg) and energy densities (41.38 Wh/kg) at 0.2 A g^-1, and excellent cycling behavior with 107.3% capacitance retention after 6000 cycles at 2 A g^-1 indicating the promising practical applications in electrochemical supercapacitors.

Dual potassium salt-assisted lyophilization of natural fibres for the high-yield synthesis of one-dimensional carbon microtubes for supercapacitors and the oxygen reduction reaction

Natural fibre-derived carbon microtubes exhibit excellent performances as supercapacitor electrodes and oxygen reduction electrocatalysts via dual-potassium-salt-assisted freeze-drying and post-nitrogen doping.

N, O and P co-doped honeycomb-like hierarchical porous carbon derived from Sophora japonica for high performance supercapacitors

Novel N, O and P co-doped honeycomb-like hierarchically porous carbon (N-O-P-HHPC) materials with a large specific surface area from Sophora japonica were prepared via a one-step activation and carbonization method and used as an electrode for supercapacitors. The results indicate that as-prepared N-P-HHPC with a large specific surface area (2068.9 m² g⁻¹) and N (1.5 atomic%), O (8.4 atomic%) and P (0.4 atomic%) co-doping has a high specific capacitance of 386 F g⁻¹ at 1 A g⁻¹. Moreover, a 1.8 V symmetrical SC was assembled from the N-O-P-HHPC-3 electrode using 1 M Na₂SO₄ gel electrolyte, presenting a high energy density (28.4 W h kg⁻¹ at 449.9 W kg⁻¹) and a long life cycling stability with only 7.3% capacitance loss after 10 000 cycles. Furthermore, the coin-type symmetrical SC using EMIMBF4 as electrolyte presents an ultrahigh energy density (80.8 W h kg⁻¹ at 1500 W kg⁻¹). When the two coin-type symmetrical SCs are connected in series, eight red light-emitting diodes (LEDs) and a small display screen can be powered. These results demonstrate as-prepared N, O and P co-doped HHPC is a considerable candidate as a carbon electrode for energy storage devices.

N, O and P co-doped honeycomb-like hierarchical porous carbon (N-P-HHPC-3) derived from Sophora japonica displays an ultrahigh energy density (80.8 W h kg⁻¹ at 1500 W kg⁻¹) and outstanding long-term stability.