A facile synthesis of hierarchical porous carbon derived from renewable lignin for high-performance supercapacitor

In this work, we proposed a facile one-pot pyrolysis method to conveniently manufacture lignin-derived carbon materials with graded porous construction for use in supercapacitors. The renewable lignin was selected as precursor, while the potassium citrate was used as a pore-forming agent. The properties of the prepared lignin-derived carbon (LAC) and the performance for supercapacitor application were thoroughly evaluated. The LAC at optimal preparation conditions shows a layered porous structure with a large specific surface area of 3174 cm² g⁻¹ and pore volume of 2.796 cm³ g⁻¹, where the specific capacitance reach to 241 F g⁻¹ at 1 A g⁻¹ scan rate in 6 M KOH electrolyte solution. At the same time, the specific capacitance remains at 220 F g⁻¹ even at an excessive scan velocity of 20 A g⁻¹, while the capacitance retention is still close to 91.3%. The capacitance retention rate is stable above 95% after 10,000 charge/discharge cycles, which shows the desired long-time stability. All these results demonstrate the outstanding properties of the new prepared LAC material and the considerable application potential in the field of electrical energy storage.     

Perovskite SrCo0.9Nb0.1O3−δ as an Anion‐Intercalated Electrode Material for Supercapacitors with Ultrahigh Volumetric Energy Density

We have synthesized and characterized perovskite‐type SrCo_0.9Nb_0.1O_3−δ (SCN) as a novel anion‐intercalated electrode material for supercapacitors in an aqueous KOH electrolyte, demonstrating a very high volumetric capacitance of about 2034.6 F cm⁻³ (and gravimetric capacitance of ca. 773.6 F g⁻¹) at a current density of 0.5 A g⁻¹ while maintaining excellent cycling stability with a capacity retention of 95.7 % after 3000 cycles. When coupled with an activated carbon (AC) electrode, the SCN/AC asymmetric supercapacitor delivered a specific energy density as high as 37.6 Wh kg⁻¹ with robust long‐term stability.     

Synthesis and Characterization of Aramid Fiber‐Reinforced Polyimide/Carbon Black Composites and Their Use in a Supercapacitor

The aramid fabric (AF )‐reinforced polyimide (PI ) composites filled with carbon black (CB ) were fabricated by means of a thermal imidization and their mechanical, thermal and electrochemical behaviors were comparatively investigated. Experimental results showed that the tensile strength of composite increased with an increase of the CB , meanwhile, the addition of 5 wt% CB and AF increased the tensile modulus to 5682.0 MPa . The superior mechanical properties of the composites were attributed to the good dispersion and effective stress transfer between the polymer and CB , as evidenced by the results from X‐ray diffraction (XRD ) and morphological studies. Besides, the thermal‐nonoxidative stability of PI was significantly improved by the incorporation of CB and AF . Furthermore, the CB /AF /PI composite was employed as the supercapacitor electrode in the 6 mol/L KOH aqueous electrolyte solution, which exhibited a specific capacitance of 510 F•g⁻¹ at 10 mV •s⁻¹. It also exhibited excellent long‐term stability, and the energy density was stable with the increase in the power density. The super performance of the composite electrode is attributed to the synergistic effects of CB particles and organic polymer.     

Hierarchically Porous Nickel Oxide Nanosheets Grown on Nickel Foam Prepared by One‐Step In Situ Anodization for High‐Performance Supercapacitors

Porous NiO nanosheets are successfully grown on nickel foam substrate through an in situ anodization by using molten KOH as the electrolyte. High‐purity NiO is directly obtained by this one‐step method without any subsequent treatment. The obtained NiO supported on nickel foam is used as a binder‐free electrode for a supercapacitor and its pseudocapacitive behavior has been investigated by cyclic voltammetry and galvanostatic charge–discharge tests in a 6 M aqueous solution of KOH. Electrochemical data demonstrates that this binder‐free electrode possesses ultrahigh capacitance (4.74 F cm^?2 at 4 mA cm^?2), excellent rate capability, and cycling stability. After 1000 cycles, the areal capacitance value is 9.4 % lower than the initial value and maintains 85.4 % of the maximum capacitance value.     

Excellent Capacitive Performance of a Three‐Dimensional Hierarchical Porous Graphene/Carbon Composite with a Superhigh Surface Area

Three‐dimensional hierarchical porous graphene/carbon composite was successfully synthesized from a solution of graphene oxide and a phenolic resin by using a facile and efficient method. The morphology, structure, and surface property of the composite were investigated intensively by a variety of means such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption, Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). It is found that graphene serves as a scaffold to form a hierarchical pore texture in the composite, resulting in its superhigh surface area of 2034 m²g^?1, thin macropore wall, and high conductivity (152 S m^?1). As evidenced by electrochemical measurements in both EMImBF₄ ionic liquid and KOH electrolyte, the composite exhibits ideal capacitive behavior, high capacitance, and excellent rate performance due to its unique structure. In EMImBF₄, the composite has a high energy density of up to 50.1 Wh kg^?1 and also possesses quite stable cycling stability at 100 °C, suggesting its promising application in high‐temperature supercapacitors. In KOH electrolyte, the specific capacitance of this composite can reach up to an unprecedented value of 186.5 F g^?1, even at a very high current density of 50 A g^?1, suggesting its prosperous application in high‐power applications.     

A hexaazatriphenylene-based porous organic polymer for high performance supercapacitor

Porous organic polymers (POPs) with high physiochemical stability and pseudocapacitive activity are crucial for supercapacitors with high specific capacitance and long cycle life. We report herein a hexaazatrinaphthylene-based POP (HPOP-1) for high-performance supercapacitor by introducing redox-active hexaazatrinaphthylene (HATN) moiety through Sonogashira–Hagihara coupling reaction. HATN moiety can undergo a proton-induced electron transfer redox reaction, which endows HPOP-1 with high pseudocapacitive activity. As electrode materials for supercapacitor application, HPOP-1 exhibits high specific capacitance (667 F g⁻¹ at 0.5 A g⁻¹) and long-term cyclic stability (90% capacitance retention after 10,000 cycles at 5 A g⁻¹) in a three-electrode system with 1 M H₂SO₄ as the electrolyte. In addition, HPOP-1 also exhibits a specific capacitance of 376 F g⁻¹ at 0.5 A g⁻¹ in 1 M KOH electrolyte. An asymmetric supercapacitor was further fabricated with HPOP-1 as negative electrode and rGO as positive electrode, respectively. The device delivers a specific capacitance of 63 F g⁻¹ at 0.5 A g⁻¹ and a rate performance of 37 F g⁻¹ at 5 A g⁻¹. Our work provides a facile approach for the design and preparation of pseudocapacitive POPs with high specific capacitance and long cycle life.     

Hydrothermal synthesis of reduced graphene sheets/Fe2O3 nanorods composites and their enhanced electrochemical performance for supercapacitors

Reduced graphene nanosheets/Fe₂O₃ nanorods (GNS/Fe₂O₃) composite has been fabricated by a hydrothermal route for supercapacitor electrode materials. The obtained GNS/Fe₂O₃ composite formed a uniform structure with the Fe₂O₃ nanorods grew on the graphene surface and/or filled between the graphene sheets. The electrochemical performances of the GNS/Fe₂O₃ hybrid supercapacitor were tested by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests in 6 M KOH electrolyte. Comparing with the pure Fe₂O₃ electrode, GNS/Fe₂O₃ composite electrode exhibits an enhanced specific capacitance of 320 F g⁻¹ at 10 mA cm⁻² and an excellent cycle-ability with capacity retention of about 97% after 500 cycles. The simple and cost-effective preparation technique of this composite with good capacitive behavior encourages its potential commercial application.     

Potentiostatically deposited nanostructured α-Co(OH)2: A high performance electrode material for redox-capacitors

A high specific capacitance was obtained for α-Co(OH)₂ potentiostatically deposited onto a stainless-steel electrode in 0.1 M Co(NO₃)₂ electrolyte at −1.0 V vs. Ag/AgCl. The structure and surface morphology of the obtained α-Co(OH)₂ were studied by using X-ray diffraction analysis and scanning electron microscopy. A network of nanolayered α-Co(OH)₂ sheets was obtained; the average thickness of individual α-Co(OH)₂ sheets was 10 nm, and the thickness of the deposit was several micrometers. The capacitive characteristics of the α-Co(OH)₂ electrodes were investigated by means of cyclic voltammetry and constant current charge–discharge cycling in 1 M KOH electrolyte. A specific capacitance of 860 F g⁻¹ was obtained for a 0.8 mg cm⁻² α-Co(OH)₂ deposit. The specific capacitance did not decrease significantly for the active mass loading range of 0.1–0.8 mg cm⁻² due its layered structure, which allowed easy penetration of electrolyte and effective utilization of electrode material even at a higher mass. This opens up the possibility of using such materials in supercapacitor applications.     

Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing,flexible electrode for supercapacitors

MnO₂ nanowires were electrodeposited onto carbon nanotube (CNT) paper by a cyclic voltammetric technique. The as-prepared MnO₂ nanowire/CNT composite paper (MNCCP) can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that the MNCCP electrode displayed specific capacitances as high as 167.5 F g⁻¹ at a current density of 77 mA g⁻¹. After 3000 cycles, the composite paper can retain more than 88% of initial capacitance, showing good cyclability. The CNT paper in the composite acted as a good conductive and active substrate for flexible electrodes in supercapacitors, and the nanowire structure of the MnO₂ could facilitate the contact of the electrolyte with the active materials, and thus increase the capacitance.     

Hierarchical Activated Mesoporous Phenolic‐Resin‐Based Carbons for Supercapacitors

A series of hierarchical activated mesoporous carbons (AMCs) were prepared by the activation of highly ordered, body‐centered cubic mesoporous phenolic‐resin‐based carbon with KOH. The effect of the KOH/carbon‐weight ratio on the textural properties and capacitive performance of the AMCs was investigated in detail. An AMC prepared with a KOH/carbon‐weight ratio of 6:1 possessed the largest specific surface area (1118 m² g^?1), with retention of the ordered mesoporous structure, and exhibited the highest specific capacitance of 260 F g^?1 at a current density of 0.1 A g^?1 in 1 M H₂SO₄ aqueous electrolyte. This material also showed excellent rate capability (163 F g^?1 retained at 20 A g^?1) and good long‐term electrochemical stability. This superior capacitive performance could be attributed to a large specific surface area and an optimized micro‐mesopore structure, which not only increased the effective specific surface area for charge storage but also provided a favorable pathway for efficient ion transport.     

High Energy Density Heteroatom (O,N and S) Enriched Activated Carbon for Rational Design of Symmetric Supercapacitors

Carbon-based symmetric supercapacitors (SCs) are known for their high power density and long cyclability, making them an ideal candidate for power sources in new-generation electronic devices. To boost their electrochemical performances, deriving activated carbon doped with heteroatoms such as N, O, and S are highly desirable for increasing the specific capacitance. In this regard, activated carbon (AC) self-doped with heteroatoms is directly derived from bio-waste (lima-bean shell) using different KOH activation processes. The heteroatom-enriched AC synthesized using a pretreated carbon-to-KOH ratio of 1:2 (ONS@AC-2) shows excellent surface morphology with a large surface area of 1508 m² g⁻¹. As an SC electrode material, the presence of heteroatoms (N and S) reduces the interfacial charge-transfer resistance and increases the ion-accessible surface area, which inherently provides additional pseudocapacitance. The ONS@AC-2 electrode attains a maximum specific capacitance (C_sp) of 342 F g⁻¹ at a specific current of 1 Ag⁻¹ in 1 m NaClO₄ electrolyte at the wide potential window of 1.8 V. Moreover, as symmetric SCs the ONS@AC-2 electrode delivers a maximum specific capacitance (C_sc) of 191 F g⁻¹ with a maximum specific energy of 21.48 Wh kg⁻¹ and high specific power of 14 000 W kg⁻¹ and excellent retention of its initial capacitance (98 %) even after 10000 charge/discharge cycles. In addition, a flexible supercapacitor fabricated utilizing ONS@AC-2 electrodes and a LiCl/polyvinyl alcohol (PVA)-based polymer electrolyte shows a maximum C_sc of 119 F g⁻¹ with considerable specific energy and power.     

Electrophoretic Deposition of Binder‐Free MnO2/Graphene Films for Lithium‐Ion Batteries

Binder‐free, nano‐sized needlelike MnO₂ ‐submillimeter‐sized reduced graphene oxide (nMnO₂‐srGO ) hybrid films with abundant porous structures were fabricated through electrophoretic deposition and subsequent thermal annealing at 500 °C for 2 h. The as‐prepared hybrid films exhibit a unique hierarchical morphology, in which nMnO₂ with a diameter of 20—50 nm and a length of 300—500 nm is randomly anchored on both sides of srGO . When evaluated as binder‐free anodes for lithium‐ion half‐cell, the nMnO₂‐srGO composites with a content of 76.9 wt% MnO₂ deliver a high capacity of approximately 1652.2 mA •h•g⁻¹ at a current density of 0.1 A•g⁻¹ after 200 cycles. The high capacity remains at 616.8 mA •h•g⁻¹ (ca. 65.1% capacity retention) at a current density as high as 4 A•g⁻¹. The excellent electrochemical performance indicates that the nMnO₂‐srGO hybrid films could be a promising anode material for lithium ion batteries (LIBs ).     

Facile synthesis of biomass-derived hierarchical porous carbon microbeads for supercapacitors

Using the facile method of solvent evaporation, the leonardite fulvic acids (LFA)-based porous carbon microbeads (PCM) have been successfully prepared at ambient pressure, followed by carbonization and KOH activation (a low mass ratio alkali/LFA = 1.5) in an inert atmosphere. The effects of KOH treatment on pore structures and the formation mechanism of the PCM were discussed. The results showed that the sample exhibited remarkable improvement in textural properties. The activated carbon microbeads had high surface area (2269 m² g^?1), large pore volume (1.97 cm³ g^?1), and displayed excellent capacitive performances, compared with carbon powder. The porous carbon material electrodes with the “porous core structure” behaved superiorly at a specific capacitance of 320 F g^?1 at a current density of 0.05 A g^?1 in 6 M KOH electrolyte, which could still remain 193 F g^?1 when the current density increased to 100 A g^?1. Remarkably, in the 1 M TEABF₄/PC electrolyte, the PCM samples could reach 156 F g^?1 at 0.05 A g^?1, possess an outstanding energy density of 39.50 Wh kg^?1, and maintain at 22.05 Wh kg^?1 even when the power density rose up to 5880 W kg^?1. The balance of structural characteristic and high performance makes the porous carbon microbeads a competitive and promising supercapacitor electrode material.     

Cold-Resistant Nitrogen/Sulfur Dual-Doped Graphene Fiber Supercapacitors with Solar–Thermal Energy Conversion Effect

Although graphene fiber-based supercapacitors are promising for wearable electronic devices, the low energy density of electrodes and poor cold resistance of aqueous electrolytes limit their wide application in cold environments. Herein, porous nitrogen/sulfur dual-doped graphene fibers (NS-GFs) are synthesized by hydrothermal self-assembly followed by thermal annealing, exhibiting an excellent capacitive performance of 401 F cm⁻³ at 400 mA cm⁻³ because of the synergistic effect of heteroatom dual-doping. The assembled symmetric all-solid-state supercapacitor with polyvinyl alcohol/H₂SO₄/graphene oxide gel electrolyte exhibits a high capacitance of 221 F cm⁻³ and a high energy density of 7.7 mWh cm⁻³ at 80 mA cm⁻³. Interestingly, solar–thermal energy conversion of the electrolyte with 0.1 wt % graphene oxide extends the operating temperature range of the supercapacitor to 0 °C. Furthermore, the photocatalysis effect of the dual-doped heteroatoms increases the capacitance of NS-GFs. At an ambient temperature of 0 °C, the capacitance increases from 0 to 182 F cm⁻³ under 1 sun irradiation because of the excellent solar light absorption and efficient solar–thermal energy conversion of graphene oxide, preventing the aqueous electrolyte from freezing. The flexible supercapacitor exhibits a long cycle life, good bending resistance, reliable scalability, and ability to power visual electronics, showing great potential for outdoor electronics in cold environments.     

大碳层间距的沥青基多级孔碳材料的制备及其在超级电容器中的应用

基于KOH活化法,以纳米级片层多孔MgO为模板剂,制备大碳层间距的沥青基超级电容器用多级孔碳材料。考察了模板剂添加量对多孔碳材料孔分布、碳层间距等理化性能及电化学性能的影响。结果表明模板剂添加量为沥青质量的25%时,多孔碳材料比表面积、孔体积分别为2 634 m~2·g~(-1)、1.12 cm~3·g~(-1),碳层间距高达0.374 nm,用于超级电容器电极材料时,1和20A·g~(-1)电流密度下的比电容分别为338和277 F·g~(-1),经过10 000次循环恒电流充放电,1 A·g~(-1)下容量保持率为93.5%,展现了优异的电化学性能。     

One-Pot Hydrothermal Synthesis of Ternary 1T-MoS2/Hexa-WO3/Graphene Composites for High-Performance Supercapacitors

A new ternary composite of 1T-molybdenum disulfide, hexagonal tungsten trioxide, and reduced graphene oxide (M-W-rGO) is synthesized by using a one-pot hydrothermal process. The synergetic effect of 1T-MoS₂ and hexa-WO₃ nanoflowers improves the electrochemical performance for supercapacitors by inducing additional active sites and hexagonal tunnels, respectively, which lead to high storage capacity and easy transfer of electrolyte ions. The ternary M-W-rGO composite has a high specific capacitance of 836 F g⁻¹ at 1 A g⁻¹, which is nearly twice that of binary composites of M-rGO and W-rGO with high capacitance retention of 86.35 % after 3000 cycles at a high current density of 5 A g⁻¹. This study provides a new ternary composite that can be used as an electrode material for high-performance supercapacitors.     

Rational Design of Porous Structured Nickel Manganese Sulfides Hexagonal Sheets-in-Cage Structures as an Advanced Electrode Material for High-Performance Electrochemical Capacitors

The design of hierarchical electrodes comprising multiple components with a high electrical conductivity and a large specific surface area has been recognized as a feasible strategy to remarkably boost pseudocapacitors. Herein, we delineate hexagonal sheets-in-cage shaped nickel–manganese sulfides (Ni-Mn-S) with nanosized open spaces for supercapacitor applications to realize faster redox reactions and a lower charge-transfer resistance with a markedly enhanced specific capacitance. The hybrid was facilely prepared through a two-step hydrothermal method. Benefiting from the synergistic effect between Ni and Mn active sites with the improvement of both ionic and electric conductivity, the resulting Ni-Mn-S hybrid displays a high specific capacitance of 1664 F g⁻¹ at a current density of 1 A g⁻¹ and a capacitance of 785 F g⁻¹ is maintained at a current density of 50 A g⁻¹, revealing an outstanding capacity and rate performance. The asymmetric supercapacitor device assembled with the Ni-Mn-S hexagonal sheets-in-cage as the positive electrode delivers a maximum energy density of 40.4 Wh kg⁻¹ at a power density of 750 W kg⁻¹. Impressively, the cycling retention of the as-fabricated device after 10 000 cycles at a current density of 10 A g⁻¹ reaches 85.5 %. Thus, this hybrid with superior capacitive performance holds great potential as an effective charge-storage material.     

Aqueous Manganese Dioxide Ink for Paper‐Based Capacitive Energy Storage Devices

We report a simple approach based on a chemical reduction method to synthesize aqueous inorganic ink comprised of hexagonal MnO₂ nanosheets. The MnO₂ ink exhibits long‐term stability and continuous thin films can be formed on various substrates without using any binder. To obtain a flexible electrode for capacitive energy storage, the MnO₂ ink was printed onto commercially available A4 paper pretreated with multiwalled carbon nanotubes. The electrode exhibited a maximum specific capacitance of 1035 F g^?1 (91.7 mF cm^?2). Paper‐based symmetric and asymmetric capacitors were assembled, which gave a maximum specific energy density of 25.3 Wh kg^?1 and a power density of 81 kW kg^?1. The device could maintain a 98.9 % capacitance retention over 10 000 cycles at 4 A g^?1. The MnO₂ ink could be a versatile candidate for large‐scale production of flexible and printable electronic devices for energy storage and conversion.     

Ultrathin Mn Doped Ni-MOF Nanosheet Array for Highly Capacitive and Stable Asymmetric Supercapacitor

In this study, we demonstrate that an Mn-doped ultrathin Ni-MOF nanosheet array on nickel foam (Mn_0.1-Ni-MOF/NF) serves as a highly capacitive and stable supercapacitor positive electrode. The Mn_0.1-Ni-MOF/NF shows an areal capacity of 6.48 C cm⁻² (specific capacity C: 1178 C g⁻¹) at 2 mA cm⁻² in 6.0 m KOH, outperforming most reported MOF-based materials. More importantly, it possesses excellent cycle stability to maintain 80.6 % capacity after 5000 cycles. An asymmetric supercapacitor device utilizing Mn_0.1-Ni-MOF/NF as the positive electrode and activated carbon as the negative electrode attains a high energy density of 39.6 Wh kg⁻¹ at 143.8 Wkg⁻¹ power density with a capacitance retention of 83.6 % after 5000 cycles.     

Electrochemical performance of Co–Al layered double hydroxide nanosheets mixed with multiwall carbon nanotubes

Regular hexagonal Co–Al layered double hydroxides (Co–Al LDH) were synthesized by urea-induced homogeneous precipitation. This material proved to be nanosheets by scanning electron microscopy and X-ray diffraction measurements. The electrochemical capacitive behavior of the nanosheets in 1 M KOH solution were evaluated by constant current charge/discharge and cyclic voltammetric measurements, showing a large specific capacitance of 192 F·g⁻¹ even at the high current density of 2 A·g⁻¹. When multiwall carbon nanotubes (MWNTs) were mixed with the Co–Al LDH, it was found that the specific capacitance and long-life performance of all composite electrodes at high current density are superior to pure LDH electrode. When the added MWNTs content is 10 wt%, the specific capacitance increases to 342.4 F·g⁻¹ and remains at a value of 304 F·g⁻¹ until the 400th cycle at 2 A·g⁻¹, showing that this is a promising electrode material for supercapacitors working at heavy load. According to the electrochemical impedance spectra, MWNTs greatly increase the electronic conductivity between MWNTs and the surface of Co–Al LDH, which consequently facilitates the access of ions in the electrolyte and electrons to the electrode/electrolyte interface.