Facile Synthesis of A 3D Flower‐Like Mesoporous Ni@C Composite Material for High‐Energy Aqueous Asymmetric Supercapacitors

A 3D flower‐like mesoporous Ni@C composite material has been synthesized by using a facile and economical one‐pot hydrothermal method. This unique 3D flower‐like Ni@C composite, which exhibited a high surface area (522.4 m² g^?1), consisted of highly dispersed Ni nanoparticles on mesoporous carbon flakes. The effect of calcination temperature on the electrochemical performance of the Ni@C composite was systematically investigated. The optimized material (Ni@C 700) displayed high specific capacity (1306 F g^?1 at 2 A g^?1) and excellent cycling performance (96.7 % retention after 5000 cycles). Furthermore, an asymmetric supercapacitor (ASC) that contained Ni@C 700 as cathode and mesoporous carbon (MC) as anode demonstrated high energy density (60.4 W h kg^?1 at a power density of 750 W kg^?1).     

Ultrahigh Energy Density Realized by a Single‐Layer β‐Co(OH)2 All‐Solid‐State Asymmetric Supercapacitor

A conceptually new all‐solid‐state asymmetric supercapacitor based on atomically thin sheets is presented which offers the opportunity to optimize supercapacitor properties on an atomic level. As a prototype, β‐Co(OH)₂ single layers with five‐atoms layer thickness were synthesized through an oriented‐attachment strategy. The increased density‐of‐states and 100 % exposed hydrogen atoms endow the β‐Co(OH)₂ single‐layers‐based electrode with a large capacitance of 2028 F g^?1. The corresponding all‐solid‐state asymmetric supercapacitor achieves a high cell voltage of 1.8 V and an exceptional energy density of 98.9 Wh kg^?1 at an ultrahigh power density of 17 981 W kg^?1. Also, this integrated nanodevice exhibits excellent cyclability with 93.2 % capacitance retention after 10 000 cycles, holding great promise for constructing high‐energy storage nanodevices.     

MnO2 Nanosheets Grown on Nitrogen‐Doped Hollow Carbon Shells as a High‐Performance Electrode for Asymmetric Supercapacitors

A hierarchical hollow hybrid composite, namely, MnO₂ nanosheets grown on nitrogen‐doped hollow carbon shells (NHCSs@MnO₂), was synthesized by a facile in situ growth process followed by calcination. The composite has a high surface area (251 m²g^?1) and mesopores (4.5 nm in diameter), which can efficiently facilitate transport during electrochemical cycling. Owing to the synergistic effect of NHCSs and MnO₂, the composite shows a high specific capacitance of 306 F g^?1, good rate capability, and an excellent cycling stability of 95.2 % after 5000 cycles at a high current density of 8 A g^?1. More importantly, an asymmetric supercapacitor (ASC) assembled by using NHCSs@MnO₂ and activated carbon as the positive and negative electrodes exhibits high specific capacitance (105.5 F g^?1 at 0.5 A g^?1 and 78.5 F g^?1 at 10 A g^?1) with excellent rate capability, achieves a maximum energy density of 43.9 Wh kg^?1 at a power density of 408 W kg^?1, and has high stability, whereby the ASC retains 81.4 % of its initial capacitance at a current density of 5 A g^?1 after 4000 cycles. Therefore, the NHCSs@MnO₂ electrode material is a promising candidate for future energy‐storage systems.     

Nitrogen‐Doped Porous Graphitic Carbon as an Excellent Electrode Material for Advanced Supercapacitors

An advanced supercapacitor material based on nitrogen‐doped porous graphitic carbon (NPGC) with high a surface area was synthesized by means of a simple coordination–pyrolysis combination process, in which tetraethyl orthosilicate (TEOS), nickel nitrate, and glucose were adopted as porogent, graphitic catalyst precursor, and carbon source, respectively. In addition, melamine was selected as a nitrogen source owing to its nitrogen‐enriched structure and the strong interaction between the amine groups and the glucose unit. A low‐temperature treatment resulted in the formation of a NPGC precursor by combination of the catalytic precursor, hydrolyzed TEOS, and the melamine–glucose unit. Following pyrolysis and removal of the catalyst and porogent, the NPGC material showed excellent electrical conductivity owing to its high crystallinity, a large Brunauer–Emmett–Teller surface area (S_BET=1027 m² g^?1), and a high nitrogen level (7.72 wt %). The unusual microstructure of NPGC materials could provide electrochemical energy storage. The NPGC material, without the need for any conductive additives, showed excellent capacitive behavior (293 F g^?1 at 1 A g^?1), long‐term cycling stability, and high coulombic efficiency (>99.9 % over 5000 cycles) in KOH when used as an electrode. Notably, in a two‐electrode symmetric supercapacitor, NPGC energy densities as high as 8.1 and 47.5 Wh kg^?1, at a high power density (10.5 kW kg^?1), were achieved in 6 M KOH and 1 M Et₄NBF₄‐PC electrolytes, respectively. Thus, the synthesized NPGC material could be a highly promising electrode material for advanced supercapacitors and other conversion devices.     

Pumpkin‐Derived Porous Carbon for Supercapacitors with High Performance

Pumpkin has been employed for the first time as a renewable, low‐cost precursor for the preparation of porous carbon materials with excellent performance. Unlike most other precursors, pumpkin is rich in sugars and starch, and it has advantageous properties for large‐scale production. The as‐prepared materials adopted a unique morphology that consisted of numerous fused sphere‐like carbon grains with a high specific surface area (2968 m² g^?1), abundant micro and mesopores, and excellent electrochemical properties. The pumpkin‐derived activated carbon (PAC) material not only exhibited a high specific capacitance of 419 F g^?1, but also showed considerable cycling stability, with 93.6 % retention after 10 000 cycles. Moreover, a symmetrical supercapacitor that was based on PAC showed a high energy density of 22.1 W h kg^?1 in aqueous electrolyte. These superior properties demonstrate that PAC holds great promise for applications in electrochemical energy‐storage devices.     

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.     

Synthesis of Phosphorus‐Doped Graphene and its Wide Potential Window in Aqueous Supercapacitors

Phosphorus‐doped (P‐doped) graphene with the P doping level of 1.30 at % was synthesized by annealing the mixture of graphene and phosphoric acid. The presence of P was confirmed by elemental mapping and X‐ray photoelectron spectroscopy, while the morphology of P‐doped graphene was revealed by using scanning electron microscopy and transmission electron microscopy. To investigate the effect of P doping, the electrochemical properties of P‐doped graphene were tested as a supercapacitor electrode in an aqueous electrolyte of 1 M H₂SO₄. The results showed that doping of P in graphene exhibited significant improvement in terms of specific capacitance and cycling stability, compared with undoped graphene electrode. More interestingly, the P‐doped graphene electrode can survive at a wide voltage window of 1.7 V with only 3 % performance degradation after 5000 cycles at a current density of 5 A g^?1, providing a high energy density of 11.64 Wh kg^?1 and a high power density of 831 W kg^?1.     

β‐Co(OH)2 Nanosheets: A Superior Pseudocapacitive Electrode for High‐Energy Supercapacitors

In this work, β‐Co(OH)₂ nanosheets are explored as efficient pseudocapacitive materials for the fabrication of 1.6 V class high‐energy supercapacitors in asymmetric fashion. The as‐synthesized β‐Co(OH)₂ nanosheets displayed an excellent electrochemical performance owing to their unique structure, morphology, and reversible reaction kinetics (fast faradic reaction) in both the three‐electrode and asymmetric configuration (with activated carbon, AC). For example, in the three‐electrode set‐up, β‐Co(OH)₂ exhibits a high specific capacitance of ∼675 F g⁻¹ at a scan rate of 1 mV s⁻¹. In the asymmetric supercapacitor, the β‐Co(OH)₂∥AC cell delivers a maximum energy density of 37.3 Wh kg⁻¹ at a power density of 800 W kg⁻¹. Even at harsh conditions (8 kW kg⁻¹), an energy density of 15.64 Wh kg⁻¹ is registered for the β‐Co(OH)₂∥AC assembly. Such an impressive performance of β‐Co(OH)₂ nanosheets in the asymmetric configuration reveals the emergence of pseudocapacitive electrodes towards the fabrication of high‐energy electrochemical charge storage systems.     

Self‐Template Synthesis of Hybrid Porous Co3O4–CeO2 Hollow Polyhedrons for High‐Performance Supercapacitors

In this work, hybrid porous Co₃O₄–CeO₂ hollow polyhedrons have been successfully obtained via a simple cation‐exchange route followed by heat treatment. In the synthesis process, ZIF‐67 polyhedron frameworks are firstly prepared, which not only serve as a host for the exchanged Ce3⁺ ions but also act as the template for the synthesis of hybrid porous Co₃O₄–CeO₂ hollow polyhedrons. When utilized as electrode materials for supercapacitors, the hybrid porous Co₃O₄–CeO₂ hollow polyhedrons delivered a large specific capacitance of 1288.3 F g^?1 at 2.5 A g^?1 and a remarkable long lifespan cycling stability (<3.3 % loss after 6000 cycles). Furthermore, an asymmetric supercapacitor (ASC) device based on hybrid porous Co₃O₄–CeO₂ hollow polyhedrons was assembled. The ASC device possesses an energy density of 54.9 W h kg^?1, which can be retained to 44.2 W h kg^?1 even at a power density of 5100 W kg^?1, indicating its promising application in electrochemical energy storage. More importantly, we believe that the present route is a simple and versatile strategy for the preparation of other hybrid metal oxides with desired structures, chemical compositions and applications.     

An Approach to Preparing Ni–P with Different Phases for Use as Supercapacitor Electrode Materials

Herein, we describe a simple two‐step approach to prepare nickel phosphide with different phases, such as Ni₂P and Ni₅P₄, to explain the influence of material microstructure and electrical conductivity on electrochemical performance. In this approach, we first prepared a Ni–P precursor through a ball milling process, then controlled the synthesis of either Ni₂P or Ni₅P₄ by the annealing method. The as‐prepared Ni₂P and Ni₅P₄ are investigated as supercapacitor electrode materials for potential energy storage applications. The Ni₂P exhibits a high specific capacitance of 843.25 F g^?1, whereas the specific capacitance of Ni₅P₄ is 801.5 F g^?1. Ni₂P possesses better cycle stability and rate capability than Ni₅P₄. In addition, the Fe₂O₃//Ni₂P supercapacitor displays a high energy density of 35.5 Wh kg^?1 at a power density of 400 W kg^?1 and long cycle stability with a specific capacitance retention rate of 96 % after 1000 cycles, whereas the Fe₂O₃//Ni₅P₄ supercapacitor exhibits a high energy density of 29.8 Wh kg^?1 at a power density of 400 W kg^?1 and a specific capacitance retention rate of 86 % after 1000 cycles.     

Design of Advanced MnO/N‐Gr 3D Walls through Polymer Cross‐Linking for High‐Performance Supercapacitor

Three‐dimensional, vertically aligned MnO/nitrogen‐doped graphene (3D MnO/N‐Gr) walls were prepared through facile solution‐phase synthesis followed by thermal treatment. Polyvinylpyrrolidone (PVP) was strategically added to generate cross‐links to simultaneously form 3D wall structures and to incorporate nitrogen atoms into the graphene network. The unique wall features of the as‐prepared 3D MnO/N‐Gr hybirdes provide a large surface area (91.516 m² g^?1) and allow for rapid diffusion of the ion electrolyte, resulting in a high specific capacitance of 378 F g^?1 at 0.25 A g^?1 and an excellent charge/discharge stability (93.7 % capacity retention after 8000 cycles) in aqueous 1 m Na₂SO₄ solution as electrolyte. Moreover, the symmetric supercapacitors that were rationally designed by using 3D MnO/N‐Gr hybrids exhibit outstanding electrochemical performance in an organic electrolyte with an energy density of 90.6 Wh kg^?1 and a power density of 437.5 W kg^?1.     

Rational Design of Ni Nanoparticles on N‐Rich Ultrathin Carbon Nanosheets for High‐Performance Supercapacitor Materials: Embedded‐ Versus Anchored‐Type Dispersion

Highly dispersed Ni nanoparticles (NPs) and abundant functional N‐species were integrated into ultrathin carbon nanosheets by using a facile and economical sol–gel route. Embedded‐ and anchored‐type configurations were achieved for the dispersion of Ni NPs in/on N‐rich carbon nanosheets. The anchored‐type composite exhibited outstanding pseudocapacitance of 2200 F g^?1 at 5 A g^?1 with unusual rate capability and extraordinary cyclic stability over 20 000 cycles with little capacitance decay. Aqueous asymmetric supercapacitors fabricated with this composite cathode demonstrated a high energy density of 51.3 Wh kg^?1 at a relatively large power density of 421.6 W kg^?1, along with outstanding cyclic stability. This approach opens an attractive direction for enhancing the electrochemical performances of metal‐based supercapacitors and can be generalized to design high‐performance energy‐storage devices.     

Strong and Robust Polyaniline‐Based Supramolecular Hydrogels for Flexible Supercapacitors

We report a supramolecular strategy to prepare conductive hydrogels with outstanding mechanical and electrochemical properties, which are utilized for flexible solid‐state supercapacitors (SCs) with high performance. The supramolecular assembly of polyaniline and polyvinyl alcohol through dynamic boronate bond yields the polyaniline–polyvinyl alcohol hydrogel (PPH), which shows remarkable tensile strength (5.3 MPa) and electrochemical capacitance (928 F g^?1). The flexible solid‐state supercapacitor based on PPH provides a large capacitance (306 mF cm^?2 and 153 F g^?1) and a high energy density of 13.6 Wh kg^?1, superior to other flexible supercapacitors. The robustness of the PPH‐based supercapacitor is demonstrated by the 100 % capacitance retention after 1000 mechanical folding cycles, and the 90 % capacitance retention after 1000 galvanostatic charge–discharge cycles. The high activity and robustness enable the PPH‐based supercapacitor as a promising power device for flexible electronics.     

Hierarchically Porous Carbons Derived from Metal–Organic Framework/Chitosan Composites for High‐Performance Supercapacitors

Hierarchically porous carbon materials with high surface areas are promising candidates for energy storage and conversion. Herein, the facile synthesis of hierarchically porous carbons through the calcination of metal–organic framework (MOF)/chitosan composites is reported. The effects of the chitosan (CS) additive on the pore structure of the resultant carbons are discussed. The corresponding MOF/chitosan precursors could be readily converted into hierarchically porous carbons (NPC‐V, V=1, 2, 4, and 6) with much higher ratios of meso‐/macropore volume to micropore volume (V_meso‐macro/V_micro). The derived carbon NPC‐2 with the high ratio of V_meso‐macro/V_micro=1.47 demonstrates a high specific surface area of 2375 m² g^?1, and a high pore volume of 2.49 cm³ g^?1, as well as a high graphitization degree, in comparison to its counterpart (NPC) without chitosan addition. These excellent features are favorable for rapid ion diffusion/transport, endowing NPC‐2 with enhanced electrochemical behavior as supercapacitor electrodes in a symmetric electrode system, corresponding to a high specific capacitance of 199.9 F g^?1 in the aqueous electrolyte and good rate capability. Good cycling stability is also observed after 10 000 cycles.     

Construction of CoO/Co‐Cu‐S Hierarchical Tubular Heterostructures for Hybrid Supercapacitors

Hierarchical hollow structures for electrode materials of supercapacitors could enlarge the surface area, accelerate the transport of ions and electrons, and accommodate volume expansion during cycling. Besides, construction of heterostructures would enhance the internal electric fields to regulate the electronic structures. All these features of hierarchical hollow heterostructures are beneficial for promoting the electrochemical properties and stability of electrode materials for high‐performance supercapacitors. Herein, CoO/Co‐Cu‐S hierarchical tubular heterostructures (HTHSs) composed of nanoneedles are prepared by an efficient multi‐step approach. The optimized sample exhibits a high specific capacity of 320 mAh g^?1 (2300 F g^?1) at 2.0 A g^?1 and outstanding cycling stability with 96.5 % of the initial capacity retained after 5000 cycles at 10 A g^?1. Moreover, an all‐solid‐state hybrid supercapacitor (HSC) constructed with the CoO/Co‐Cu‐S and actived carbon shows a stable and high energy density of 90.7 Wh kg^?1 at a power density of 800 W kg^?1.     

Facile Synthesis of Nitrogen‐Containing Mesoporous Carbon for High‐Performance Energy Storage Applications

Porous carbon with high specific surface area (SSA), a reasonable pore size distribution, and modified surface chemistry is highly desirable for application in energy storage devices. Herein, we report the synthesis of nitrogen‐containing mesoporous carbon with high SSA (1390 m² g^?1), a suitable pore size distribution (1.5–8.1 nm), and a nitrogen content of 4.7 wt % through a facile one‐step self‐assembly process. Owing to its unique physical characteristics and nitrogen doping, this material demonstrates great promise for application in both supercapacitors and encapsulating sulfur as a superior cathode material for lithium–sulfur batteries. When deployed as a supercapacitor electrode, it exhibited a high specific capacitance of 238.4 F g^?1 at 1 A g^?1 and an excellent rate capability (180 F g^?1, 10 A g^?1). Furthermore, when an NMC/S electrode was evaluated as the cathode material for lithium–sulfur batteries, it showed a high initial discharge capacity of 1143.6 mA h g^?1 at 837.5 mA g^?1 and an extraordinary cycling stability with 70.3 % capacity retention after 100 cycles.     

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.     

Conductive Microporous Covalent Triazine‐Based Framework for High‐Performance Electrochemical Capacitive Energy Storage

Nitrogen‐enriched porous nanocarbon, graphene, and conductive polymers attract increasing attention for application in supercapacitors. However, electrode materials with a large specific surface area (SSA) and a high nitrogen doping concentration, which is needed for excellent supercapacitors, has not been achieved thus far. Herein, we developed a class of tetracyanoquinodimethane‐derived conductive microporous covalent triazine‐based frameworks (TCNQ‐CTFs) with both high nitrogen content (>8 %) and large SSA (>3600 m² g^?1). These CTFs exhibited excellent specific capacitances with the highest value exceeding 380 F g^?1, considerable energy density of 42.8 Wh kg^?1, and remarkable cycling stability without any capacitance degradation after 10 000 cycles. This class of CTFs should hold a great potential as high‐performance electrode material for electrochemical energy‐storage systems.     

Porous Nitrogen‐Doped Carbon Nanotubes Derived from Tubular Polypyrrole for Energy‐Storage Applications

Porous nitrogen‐doped carbon nanotubes (PNCNTs) with a high specific surface area (1765 m² g^?1) and a large pore volume (1.28 cm³ g^?1) have been synthesized from a tubular polypyrrole (T‐PPY). The inner diameter and wall thickness of the PNCNTs are about 55 nm and 22 nm, respectively. This material shows extremely promising properties for both supercapacitors and for encapsulating sulfur as a superior cathode material for high‐performance lithium–sulfur (Li‐S) batteries. At a current density of 0.5 A g^?1, PNCNT presents a high specific capacitance of 210 F g^?1, as well as excellent cycling stability at a current density of 2 A g^?1. When the S/PNCNT composite was tested as the cathode material for Li‐S batteries, the initial discharge capacity was 1341 mAh g^?1 at a current rate of 1 C and, even after 50 cycles at the same rate, the high reversible capacity was retained at 933 mAh g^?1. The promising electrochemical energy‐storage performance of the PNCNTs can be attributed to their excellent conductivity, large surface area, nitrogen doping, and unique pore‐size distribution.     

High Power- and Energy-Density Supercapacitors through the Chlorine Respiration Mechanism

Supercapacitor represents an important electrical energy storage technology with high-power performance and superior cyclability. However, currently commercialized supercapacitors still suffer limited energy densities. Here we report an unprecedentedly respiring supercapacitor with chlorine gas iteratively re-inspires in porous carbon materials, that improves the energy density by orders of magnitude. Both electrochemical results and theoretical calculations show that porous carbon with pore size around 3 nm delivers the best chlorine evolution and adsorption performance. The respiring supercapacitor with multi-wall carbon nanotube as the cathode and NaTi₂(PO₄)₃ as the anode can store specific energy of 33 Wh kg⁻¹ with negligible capacity loss over 30 000 cycles. The energy density can be further improved to 53 Wh kg⁻¹ by replacing NaTi₂(PO₄)₃ with zinc anode. Furthermore, thanks to the extraordinary reaction kinetics of chlorine gas, this respiring supercapacitor performs an extremely high-power density of 50 000 W kg⁻¹.