•Porous carbon nanosheets (PCNSs) are prepared by a facile one-step method.•No post-activation-process is needed for PCNS synthesis.•The PCNSs presents large surface area and abundant nitrogen content.•The PCNSs exhibit excellent electrochemical performance in aqueous electrolytes.A facile one-step pyrolysis route for the synthesis of hierarchically porous carbon nanosheets (PCNSs) derived from Moringa oleifera stems (MOSs) is reported, in which no post-activation-process in needed. The as-prepared PCNSs possesses unique porous nanosheet morphology with high specific surface area of ca. 2250 m2 g−1, large pore volume of ca. 2.3 cm3 g−1, appropriate porosity as well as heteroatom doping (N and O), endowing outstanding electrochemical properties as electrode material for high-performance supercapacitors. The PCNS-based electrodes are investigated in various aqueous electrolytes including 1.0 M Na2SO4, 1.0 M H2SO4, and 6.0 M KOH. The PCNSs exhibit a maximum specific capacitance of ca. 283 F g−1 (0.5 A g−1), excellent rate capability (ca. 72% of capacitance retention even at an ultrahigh current density of 50 A g−1), and a tremendous long-term cycling stability in the three-electrode system. Moreover, the as-assembled PCNS-based symmetric supercapacitor shows a high energy density of ca. 25.8 Wh kg−1 (in 1.0 M Na2SO4 electrolyte) and remarkable long-term cycling stability (almost no capacitance fade in aqueous electrolytes), indicating the promising of the as-prepared PCNSs for electrochemical energy storage and conversion.A facile one-step pyrolysis strategy is designed to synthesize hierarchically porous carbon nanosheets (PCNSs) with unique two-dimensional nanosheet structure, high specific surface area, and outstanding electrochemical properties for high-performance electric double-layer capacitors.Download high-res image (428KB)Download full-size image

•Bagasse-derived hierarchical structured carbon (BDHSC) was synthesized.•Sewage sludge was employed to regulate the morphology and porosity of BDHSC.•The BDHSC-based electrode exhibits excellent supercapacitive performance.Bagasse-derived hierarchical structured carbon (BDHSC) with tunable porosity and improved electrochemical performance is prepared via simple and efficient hydrothermal carbonization combined with KOH activation. Experimental results show that sewage sludge acts as a cheap and efficient structure-directing agent to regulate the morphology, adjust the porosity, and thus improve the supercapacitive performance of BDHSC. The as-resulted BDHSC exhibits an interconnected framework with high specific surface area (2296 m2 g−1), high pore volume (1.34 cm3 g−1), and hierarchical porosity, which offer a more favorable pathway for electrolyte penetration and transportation. Compared to the product obtained from bagasse without sewage sludge, the unique interconnected BDHSC exhibits enhanced supercapacitive performances such as higher specific capacitance (320 F g−1), and better rate capability (capacitance retention over 70.8% at a high current density of 50 A g−1). Moreover, the BDHSC-based symmetric supercapacitor delivers a maximum energy density of over 20 Wh kg−1 at 182 W kg−1 and presents an excellent long-term cycling stability. The developed approach in the present work can be useful not only in production of a variety of novel hierarchical structured carbon with promising applications in high-performance energy storage devices, but also in high-value utilization of biomass wastes and high-ash-content sewage sludge.

•Three dimensional nitrogen graphene (3DNG) was synthesized by hydrothermal method.•The 3DNG binder-free electrode exhibits excellent electrochemical performance.•Nitrogen-doping can improve the conductivity and wettability of graphene.•N-configurations play an important role in enhancing the electrochemical behavior.Three dimensional nitrogen-doped graphene (3DNG) with high nitrogen content and improved electrochemical performance is successfully prepared by a facile, lost-cost hydrothermal method with ammonia as reducing-doping agent. The as-prepared 3DNG exhibits a hierarchical and interconnected porous network, which offers favorable pathways for electrolyte penetration and transportation. Remarkably, as binder-free electrode in aqueous electrolyte, the resultant 3DNG-2 with both high nitrogen content (7.71 at%) and large active material density (1.31 g cm−3) exhibits an ultrahigh volumetric capacitance of 437.5 F cm−3 (334.0 F g−1) at current density of 0.5 A g−1 and superior cycling stability (93% capacitance retention after 20 000 cycles at high current density of 10 A g−1). Further analyses indicate that the N-configurations are of great significance to the improvement of electrochemical behavior as well as the N-content. This work provides an effective way to synthesize 3DNG with excellent electrochemical properties for high performance supercapacitor and promotes the in-depth understanding of the enhancement mechanism of N-doping to supercapacitor performance.

Few-layered boron nitride nanosheets (BNNSs) have attracted increasing research interest in the past few years due to their unique material properties. However, the lack of a reliable scale-up production method is an inhibiting issue for their practical applications. In this work, we report a facile one-step and high-yield method for the synthesis of few-layered and hierarchically porous BNNSs through simultaneous etching and in situ nitridation of calcium hexaboride (CaB6) by ammonium chloride under moderate conditions. The output of the few-layered BNNSs is as high as 1.4 g with respect to 1.06 g of starting CaB6 crystals. Transmission electron microscopy and atomic force microscopy characterizations confirm the successful synthesis of few-layered BNNSs, most of which are layered with a thickness less than 3 nm (layer number < 10). The as-prepared BNNSs exhibit a high specific surface area (492–795 m2 g−1) and a high pore volume (0.34–0.50 cm3 g−1). In addition, the as-resulted BNNSs exhibit high and tuneable H2 uptakes from 1.48 to 2.18 wt% at 77 K and at a relatively low pressure of 1.0 MPa, thus guiding the further search of materials for H2 storage. Our results suggest that the simultaneous etching and in situ nitridation of metallic borides is a facile and effective method for reliable production of few-layered BNNSs with hierarchical porosity for potential applications such as gas storage and functional composites.

In this paper, we demonstrate that Moringa oleifera branches, a renewable biomass waste with abundant protein content, can be employed as novel precursor to synthesize three-dimensional heteroatom-doped and hierarchical egg-box-like carbons (HEBLCs) by a facile room-temperature pretreatment and direct pyrolysis process. The as-prepared HEBLCs possess unique egg-box-like frameworks, high surface area, and interconnected porosity as well as the doping of heteroatoms (oxygen and nitrogen), endowing its excellent electrochemical performances (superior capacity, high rate capability, and outstanding cycling stability). Therefore, the resultant HEBLC manifests a maximum specific capacitance of 355 F g–1 at current density of 0.5 A g–1 and remarkable rate performance. Moreover, 95% of capacitance retention of HEBLCs can be also achieved after 20 000 charge–discharge cycles at an extremely high current density (20 A g–1), indicating a prominent cycling stability. Furthermore, the as-assembled HEBLC//HEBLC symmetric supercapacitor displays a superior energy density of 20 Wh kg–1 in aqueous electrolyte and remarkable capacitance retention (95.6%) after 10 000 charge–discharge cycles. This work provides an environmentally friendly and reliable method to produce higher-valued carbon nanomaterials from renewable biomass wastes for energy storage applications.Keywords: cycling stability; electrode materials; heteroatom doping; hierarchical structured carbons; Moringa oleifera; supercapacitors;

In this work, we demonstrate for the first time that three-dimensional honeycomb-like hierarchically structured carbon (HSC) can be prepared by using high-ash-content sewage sludge as a carbon precursor. The fly-silicon process plays a crucial role in the formation of honeycomb-like hierarchical structures. The as-resulted HSC exhibits novel honeycomb-like frameworks, high specific surface area (2839 m2 g−1), large pore volume (2.65 cm3 g−1), interconnected hierarchical porosity, and excellent electrochemical performance. The high specific capacitance of 379 F g−1 as well as excellent rate capability and outstanding cycling stability (over 90% capacitance retention after 20000 cycles even at a high current density of 20 A g−1), makes it suitable for high-performance supercapacitor electrode materials. The assembled HSC//HSC symmetric supercapacitor presents enhanced supercapacitive behavior with a high energy density of 30.5 W h kg−1 in aqueous solution. This strategy provides an effective method to develop high-performance electrode materials derived from other high-ash-content biomass wastes for supercapacitors.

Graphene is widely used as promising electronic material and devices, owing to its exceptional electronic and optoelectronic properties. Up to now, defect-free graphene has been limited to the method for controllable, reproducible and scalable mass production. A simple, green, and nontoxic approach for large-scale preparation of high quality graphene is produced by exfoliation of graphite sheets collaborated with intercalant (FeCl2) under hydrothermal conditions, the absence of defects or oxides in graphene with a yield up to 10 wt% can be a practical application and industrial process such as optical limiters, transparent conductors, and sensors. This new process could potentially be improved to give a yield of up to 35 wt% of the starting graphite mass with sediment recycling. We show with experiments and theories that exfoliation graphene is the result of a combined action by diminishing the van der Waals interactions between graphite layers and the shear force drove by the Brownian motion of H2O and FeCl2 molecules. Hydrothermal exfoliation has potential applications in the exfoliation of other layered materials (e.g. BN, MoS2) and carbon nantubes, and in the synthesis of intercalation compounds, nanoribbons, and nanoparticles, thus opening new ways of exfoliation engineering.

•SnO2@C porous microspheres were prepared via ethanol-thermal carbonization combined with simple steam activation.•The as-prepared SnO2@C microspheres have hierarchical porosity and novel mosaic structure.•The resulted SnO2@C microspheres exhibit high specific capacitance (420 F g−1) and good electro-cycling stability.•The maximum energy density of SnO2@C microspheres is up to 34.2 Wh kg−1.Mosaic-structured SnO2@C microspheres with hierarchical porosity were designed and synthesized via an ethanol-thermal carbonization and steam activation method. SnO2 nanoparticles were embedded uniformly within the porous carbon microspheres. These SnO2@C porous microspheres have been characterized by XRD, FESEM, TEM, XPS and tested as electrode materials for high-performance supercapacitors. The SnO2 nanoparticles were embedded within the hierarchically porous carbon microspheres, and thus the conductivity and stability was improved. Furthermore, the channel of porous carbon microspheres make the electrolyte fully contacts with SnO2 nanoparticles, hence the as-prepared SnO2@C porous microspheres possess the characteristic of pseudocapacitors and ECDLs. In addition, uniformly spherical SnO2@C particles possess the enhanced volumetric density of the electrode materials make the further charge storage in supercapacitor. Experimental results show that the SnO2@C microspheres exhibit a super specific capacitance up to 420 F g−1 in galvanostatic charge/discharge measurements and high energy density of 34.2 Wh kg−1, it also possess a good conductivity as well as electro-cycling stability.

Carbon microtubes (CMTs) with large inner spacing were prepared by a simple in situ template approach under moderate conditions, in which metallic Zn powder and ethanol were used as raw material. The morphology and microstructure of the CMTs were characterized by SEM, TEM, HRTEM, XRD, and Raman spectrum. The as-obtained CMTs have a homogenous morphology with an internal diameter of about 1 μm, a wall thickness of 30 nm and several microns in length. The electrochemical performance of CMTs was investigated by cyclic voltammetry and galvanostatic charge/discharge measurements. Experimental results show that the CMT material presented a high specific capacitance of 185 F g−1 at current density of 200 mA g−1. The better capacitive behavior of the CMTs demonstrates a promising electrode material for electrochemical supercapacitors.Highlights► Carbon microtubes (CMTs) were synthesized by a simple in situ template method. ► Metallic Zn powder and ethanol were used as raw material for CMT synthesis. ► The as-obtained CMTs present a high specific capacitance of 185 F g−1 at current density of 200 mA g-1. ► This work may provide an alternative to synthesize high-purity CMTs with large inner spacing and better electrochemical performance for supercapacitor.

Cubic boron nitride nanorods (cBNNRs) were synthesized through a simple reaction between boron tribromide and sodium amide in lithium bromide molten salt medium at 600 °C in a sealed autoclave system. The cBNNRs observed by transmission electron microscopy have various diameters ranging from 8 to 25 nm with aspect ratios of about 10–20. X-ray diffraction and selected area electron diffraction pattern revealed the cubic phase of cBNNRs with a lattice constant a=3.625 Å. The B:N ratio obtained by energy dispersive X-ray spectroscopy was 1:1.12, and Fourier transform infrared spectrum showed a characteristic absorption at 1106 cm–1. The possible formation mechanism of cBNNRs in molten salt medium is also discussed.Graphical abstractOne-dimensional cubic boron nitride (cBN) nanorods with diameters ranging from 8 to 25 nm and aspect ratios of about 10–20 were synthesized under moderate conditions, in which boron tribromide and sodium amide were used as reactants and LiBr molten salt as growth medium. Highlights► One-dimensional cubic boron nitride nanorods were synthesized under moderate conditions. ► LiBr molten salt was used as a medium for the growth of one-dimensional cBN nanorods. ► X-ray diffraction and selected area electron diffraction pattern revealed the cubic phase of BN nanorods. ► This work may provide an alternative for transformation and growth of cubic boron nitride under moderated conditions.

•Sulfur-doped carbon microspheres (SCMSs) were prepared using starch as precursor.•Simple hydro-sulfur-thermal carbonization (HSTC) was developed to prepare SCMSs.•Results show that sulfur atoms were introduced into carbon network in SCMSs.•The SCMSs present high surface area and enhanced hydrogen storage capacity.Sulfur-doped carbon microspheres (SCMSs) with large surface area were prepared on large scale by a simple hydro-sulfur-thermal carbonization (HSTC) method using starch as carbon precursor. The morphology and composition of the resulted SCMSs were characterized by scanning emission microscopy, X-ray diffraction, Raman spectrum, and X-ray photoelectron spectroscopy. Experimental results show that monodispersed SCMSs can be obtained by the HSTC of starch under high concentration conditions (up to 1.0 g mL−1) by this HSTC method. The as-prepared SCMSs exhibit an enhanced hydrogen storage capacity (3.3 wt% at 77 K and 0.7 MPa) in comparison with the sulfur-free carbon microspheres (0.98 wt%).Sulfur-doped carbon microspheres (SCMSs) were synthesized successfully through a simple one-pot hydro-sulfur-thermal carbonization (HSTC) using starch as precursor. The as-prepared SCMSs exhibit high specific surface and enhanced hydrogen storage capacity.

In this work, we demonstrate for the first time that three-dimensional honeycomb-like hierarchically structured carbon (HSC) can be prepared by using high-ash-content sewage sludge as a carbon precursor. The fly-silicon process plays a crucial role in the formation of honeycomb-like hierarchical structures. The as-resulted HSC exhibits novel honeycomb-like frameworks, high specific surface area (2839 m2 g−1), large pore volume (2.65 cm3 g−1), interconnected hierarchical porosity, and excellent electrochemical performance. The high specific capacitance of 379 F g−1 as well as excellent rate capability and outstanding cycling stability (over 90% capacitance retention after 20000 cycles even at a high current density of 20 A g−1), makes it suitable for high-performance supercapacitor electrode materials. The assembled HSC//HSC symmetric supercapacitor presents enhanced supercapacitive behavior with a high energy density of 30.5 W h kg−1 in aqueous solution. This strategy provides an effective method to develop high-performance electrode materials derived from other high-ash-content biomass wastes for supercapacitors.