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Crystalline Bi–amorphous MoOx interfaces anchored on RGO exhibit superior electrocatalytic NRR performance by enhancing the N2 adsorption/activation while suppressing the competitive HER process.

A novel functional flexible transparent film with excellent printability, swellability, degradability, and hydroxyl groups is directly developed from natural cotton.

MoS2 nanomaterials show excellent high theoretical performance in energy storage and hydrogen evolution reaction (HER), whereas they typically suffer from agglomeration and poor electrical conducti...

Developing high-performance electrode materials for energy-storage devices with high energy-power densities, such as sodium ion hybrid capacitors (SIHCs), is of vital importance for applications in electric vehicles and portable electronics. Porous hard carbon is one of the most fascinating anode materials for SIHCs due to the rapid Na+ diffusion. Doping with heteroatoms, such as pyridinic N and thiophene S, may boost both the rate performance and specific capacity. However, it is still very challenging to modulate the content and configuration of N and S dopants efficiently. Furthermore, the trade-off for several storage mechanisms, which is vital for carbon anodes with rapid storage and release of sodium ions, is still scarce. Herein, a simple and efficient method is proposed to regulate the configuration of nitrogen dopants, increase the content of thiophene S, create micropores and adjust the structure of curved graphitic domains of hard carbon. Besides, the evolution mechanism of the structure and component is explored through ex situ XRD and XPS analysis. As-prepared NS-pHC-1.348 (N and S co-doped porous hard carbon with 1.348 g MgCl2 precursor) delivers high reversible capacity (383.9 mA h g−1 at 0.05 A g−1), excellent rate ability (183.2 mA h g−1 at 20 A g−1) and fine cycle stability (287.2 mA h g−1 at 1 A g−1 after 1000 cycles). The ratio of pseudocapacitive behaviors in NS-pHC-1.348 reaches up to 70.19% at 0.2 mV s−1, which could balance the electrochemical kinetics of the cathode and anode in SIHCs. Therefore, SIHCs, assembled from the presodiated NS-pHC-1.348 anode and NPC (N-doped porous carbon) cathode, showed superb power characteristics (92.03 kW kg−1 at 5.98 W h kg−1). This work will contribute to the design of high-performance anodes with tunable multi-active sites and the construction of high-power SIHCs.

Although electrocatalytic nitrogen reduction reaction (NRR) has been considered as an emerging pathway to produce ammonia (NH3) under ambient conditions owing to its low energy consumption, it still lacks efficient the electrocatalysts to dissociate inert N N bonds. Here, we develop an efficient approach to produce vanadium carbide with abundant periodic carbon vacancies (12.5 at. %) and mesoporous structure as electrocatalysts for NRR via a carbothermic reaction. The typical synthesis protocol involves the use of zinc vanadate decorated vanadium pentoxide nanosheets to homogeneously guide the nucleation and growth of metal organic frameworks (MOFs) on their surface, thus facilitating the in-situ formation of unique vanadium carbide during the subsequent carbothermic reaction. Owing to the optimized substrate-adsorbate binding strength, the intrinsic periodic carbon vacancies of the resultant vanadium carbide could act as coordinatively unsaturated sites to adsorb and activate nitrogen through π-back-donation process, thus promoting the reduction of N2 to NH3. As a consequence, a high yield rate and high Faradaic efficiency with good stabilities are achieved for producing NH3 under ambient conditions.

We report a simple process to connect graphene sheets forming graphene hollow microtubes (GHMs), where the tube diameter can be adjusted in the 100–500 nm range by changing reaction conditions. We discovered that graphene sheets can be seamlessly linked to each other if C atoms are substituted for N atoms at the edges of the sheets during annealing of graphene oxide(G-O)-coated electrospun PAN carbon fibers in ammonia atmosphere. The G-O/carbon hybrid nanofibers framework served as a confining template around which graphene sheets curved to form tubular structures. The GHMs formed by this process are similar to (very) large diameter carbon nanotubes (CNTs) with relatively low curvature whose electron field-emission properties include a low turn-on voltage of 0.18 V/μm (at J = 10 μA/cm2), low threshold field of 0.35 V/μm (at J = 10 mA/cm2), and high field-emission stability. This process to produce GHMs can be scaled up to fabricate GHMs of variable diameter in bulk quantities.

Herein, a salt-assisted hydrothermal carbonization (HTC) strategy is applied to fabricate low-surface-area carbon microspheres (as low as 5.5 m2 g−1) for sodium ion batteries (SIBs) by using water containing eutectic salt melt (e.g. NaCl) and sugar (e.g. glucose) as reaction media. The small amount of salt increases the carbon conversion efficiency from 15.0 to 58.3%, and microsphere size from the nanoscale to the microscale. Meanwhile, the specific surface area of carbon microsphere is minimized and the microstructure is optimized. Ex-situ X-ray diffraction (XRD) and kinetic analysis revealed that the narrower lateral width of pseudographitic domains and lower micropore volume are the key factors to promote sodium storage ability and Na ion diffusion. The carbon microsphere anode delivers a capacity of 350 mAh g−1 with 73.0% from the low potential (0–0.2 V) at 100 mA g−1, a high initial Coulombic efficiency (ICE) of 86.1% (excluding conductive carbon black), and an excellent rate capability with capacity of 261 mAh g−1 even at 500 mA g−1. This research highlights a salt-assisted HTC method to synthesize low-surface-area carbon microspheres with superior ICE and energy/power density.

Intensive attempts have been devoted to solving the inferior cycling stability of Si-based electrode induced by the large volume change of Si. However, the complex synthesis procedures make many strategies much low practical significances. Together with the inferior cycling stability, an easy and scalable fabrication strategy is still a great challenge for implementing Si anode in commercial batteries. This work uses a simple water steam selective etching method to simultaneously engineer the pores and the confinement of commercial Si/SiOx in carbon paper electrodes, leading to a significant improvement in electrode flexibility and cycle life. The as-prepared freestanding lotus-seed-pod-like steam-etched Si/SiOx@CNF electrode shows a high capacity retention of 137% after 1000 cycles at 3 A g−1. It also possesses outstanding electrochemical performance in a flexible lithium-ion full battery with LiCoO2/steam-etched CNF as the cathode, even under bended condition. This simple approach may offer a pathway for the application of Si-based anode in commercialization and/or flexible energy storage devices.

Na-ion capacitors (NICs) are promising energy storage devices in virtue of their merits in combining the high energy densities of secondary batteries and the high power densities of supercapacitors. However, it is still very challenging to achieve a balanced energy-power performance in NIC device due to the kinetic imbalance between the battery-type anode and the capacitive-type cathode. In this work, an NIC device based on carbon materials for both anode and cathode has been reported. As-prepared (polyimide/graphene oxide)-derived carbon (PIGC) anode material shows excellent rate capability, which can deliver a specific capacity of 110 mAh g−1 at high current densities of 5 A g−1. In addition, the N, B co-doped expanded reduced graphite oxide (NBEG) cathode demonstrates a high specific capacitance of 328 F g−1. Due to the improved rate capability of PIGC anode and specific capacitance of NBEG cathode, the imbalance on the energy and power densities between anode and cathode is well addressed. As-assembled PIGC//NBEG device can deliver an energy density of 55 W h kg−1 even at a high power density of 9500 W kg−1. The energy-power properties of PIGC//NBEG are superior to many state-of-the-art NIC devices that using carbon or non-carbon based electrodes. This work offers not only a promising device configuration with superior energy-power properties, but also a guidance for the design strategies on electrode materials for high-throughput energy storage systems. Keywords: Carbon materials, Doping, Porous structure, Sodium ion capacitors, Energy storage

Hybrid ion capacitors that combined high-power density of supercapacitor and high-energy density of battery are drawing attention to insufficient power densities of currently-used lithium-ion batteries (LIBs). Two kinds of layered carbon-based pseudocapacitive materials were used as both anode and cathode for lithium/sodium ion capacitors (LICs/NICs) with balanced energy/power properties. As-assembled NIC and LIC could deliver the energy densities of 125.7 Wh kg−1 and 119.6 Wh kg−1 at the power density of 7941.2 W kg−1 and 8823.5 W kg−1, respectively. The electrochemical properties of NICs were better than that of LICs when the current density was below 4 A g−1, although the working voltage of LIC is higher than that of NICs, and the size of Na+ is larger than that of Li+. Using an instantaneous potential technique, it is found that the increase of the capacity for anode material in low potential region will effectively enhance the electrochemical performance for the full device. Keywords: Sodium/lithium-ion capacitors, High energy-power density, Pseudocapacitive electrodes, Layered materials

An ultrasensitive enzyme-free glucose sensor was facilely prepared by electrodepositing three-dimensional dendritic Cu on a room temperature exfoliated graphite-derived film (RTEG-F). An excellent electrocatalytic performance was demonstrated for glucose by using Cu/RTEG-F as an electrode. In terms of the high conductivity of RTEG-F and the good catalytic activity of the dendritic Cu structures, the sensor demonstrates high sensitivities of 23.237 mA/mM/cm2, R2 = 0.990, and 10.098 mA/mM/cm2, R2 = 0.999, corresponding to the concentration of glucose ranging from 0.025 mM to 1.0 mM and 1.0 mM to 2.7 mM, respectively, and the detection limit is 0.68 μM. In addition, the Cu/RTEG-F electrode demonstrates excellent anti-interference to interfering species and a high stability. Our work provides a new idea for the preparation of high-performance electrochemical enzyme-free glucose sensor.

Ti3C2Tx MXene has drawn considerable attention as anode materials to store sodium ions because of the capability of accommodating the large sodium ions, enabling their intercalation without substantial structural change. However, the limited sodium-ion storage capacity of Ti3C2Tx hinders its real application in sodium-ion batteries (SIBs). To enhance its performance as anode materials in SIBs, here, we introduce nanopores into Ti3C2Tx sheets by sonication, and after which we assemble them with rGO and cellulose nanofibers into an elastic freestanding composite structure by mimicking the wasp nest. The wasp nest-like structure endows the resulting composite with more accessible surfaces of electrode materials to the electrolyte. Further, the nanopores on the Ti3C2Tx sheets and the TiO2 formed from the sonication provide more active sites for sodium storage. As a result, the resulting composite shows a high capacity of 280 mAh g−1 at 100 mA g−1 and remarkable cyclic life with a capacity retention of 84.8% after 1000 cycles at 1 A g−1.

Sodium-ion batteries (SIBs) have attracted tremendous attention as next-generation high-performance and low-cost energy storage devices. However, achieving both high capacity and good cycling performance is still challenging. Tin sulfides are promising SIB anode materials but usually demonstrate a capacity much lower than theoretical values and suffer from large volume expansion during charge-discharge process. Herein, SnS/SnS2 heterostructures are vertically grown on carbon cloth substrates and used as a binder-free self-supporting anode, exhibiting a high Na+ storage capacity (>800 mAh g−1 at 200 mA g−1) and good cycling performance (>400 mAh g−1 after 1000 cycles at 1 A g−1). The excellent sodium-ion storage performance can be attributed to the enhanced charge-transfer kinetics by the SnS/SnS2 heterostructure, the hierarchical structure composed of uniform nanohoneycomb-like SnS/SnS2 structure and the carbon cloth as the self-supporting framework. The superior electrochemical performance by forming heterostructures may inspire the rational design of layered materials for high-performance SIB applications.

Ammonia, as an important carbon-free energy carrier and also an important chemical for producing fertilisers, is mainly synthesized by a traditional Haber–Bosch process with high energy consumption and large amounts of greenhouse gas emissions. Recently, electrocatalytic nitrogen reduction reaction (NRR) has attracted worldwide research attentions as a promising route for achieving green and sustainable ammonia synthesis at ambient conditions. Although exciting advances have been made in the NRR field, the development of electrochemical nitrogen-to-ammonia conversion is still challenging because of the low ammonia yield and unsatisfactory Faradaic efficiency mainly deriving from the poor catalytic activity of catalysts. Herein, various catalyst design strategies for increasing the exposed active sites or altering the electronic structure aiming at improving the apparent activity or intrinsic activity are summarized in this review article. On the basis of effective design strategies, a range of recently reported NRR electrocatalysts, including noble metal-based materials, non-noble metal-based materials, single-metal-atom catalysts, and metal-free materials, are summarized, and the mechanisms of tuning the catalytic activity by applying the design strategies are emphasized based on the combination of theoretical calculations and experimental investigations. It is anticipated that the established correlation between physicochemical properties of catalysts and NRR performance can provide guidance for designing heterogeneous NRR electrocatalysts with high activity, good selectivity, and high stability.

Sulfur cathodes in lithium-sulfur batteries (LSBs) have received a boost in electrochemical performance through developing various sulfur hosts. However, it remains great challenges in achieving fast electron and ion conduction while accommodating the dramatic volume change and suppressing severe intermediate polysulfide dissolution under practically necessary ‘3H’ conditions (high areal sulfur loading, high electrode compactness and high sulfur content). Here a compact 3D interconnected sulfur cathode is reported to satisfy the above requirements. It is constructed by self-assembly of Zn,Co-bimetallic ZIF nanoparticles, following pyrolysis and subsequent melt-diffusion of high-content sulfur. Sulfur filled into an open porous 3D carbon network (3DCN) with abundant N, Co doping and graphitic carbon species and produced a thin sulfur-coating layer on the macroporous surface of 3DCN. Such smart architecture provides multidimensional electron and ion transport pathways and shortened mass and ion diffusion length. The close contact of sulfur species with carbon-based polar host provides facilitated physiochemical adsorption and conversion reaction of polysulfides. At high areal sulfur loading of 10.9 mg cm-2, high sulfur content of 74 wt% in the whole cathode and low electrolyte/sulfur ratio of 6 µL/mg, it delivers high gravimetric/volumetric/areal capacities of 945 mA h g-1/867 mA h cm-3/10.3 mA h cm-2 at 0.1 C (1.83 mA cm-2). At a high rate of 0.5 C (9.13 mA cm-2), it still presents a high capacity of 8.73 mA h cm-2 and maintains 6.35 mA h cm-2 after 200 cycles. Therefore this work provides an instructive paradigm of rational architecture design to fabricate sulfur cathodes for practically viable Li-S batteries.

With the discovery of graphene in 2004, other two-dimensional (2D) layered materials, such as, boron nitride, black phosphorus, transition metal dichalcogenides (TMDCs), etc., have attracted worldwide research interests. In particular, TMDCs have attracted much attention in virtue of their tunable bandgaps and excellent properties. The construction of heterostructures based on graphene and TMDCs is an important strategy to tailor their electronic structures, which has opened up a new era for the next-generation electronic and optoelectronic devices. According to the combination of different 2D heterostructures, they can be divided into two categories: lateral heterostructures and vertical heterostructures. Considering 2D materials are usually grown on specific substrates (e.g. Cu foils), developing cost-effective and eco-friendly transfer methods without degrading their performance is very crucial. In this review article, we summarize the synthesis strategies of 2D heterostructures and discuss the key experimental parameters for their growth, including growth temperature, growth time, and the addition of halide and water. Then, we summarize the transfer methods with respect to different growth substrates. Also, the applications of 2D heterostructures in the field of electronic and optoelectronic devices are briefly introduced. Finally, the challenges ahead for research on 2D heterostructures are proposed.

Developing high-performance anode materials is very crucial for room-temperature sodium-ion batteries (SIBs). Sn-based compounds are promising SIB anode materials for their high theoretical capacities and low costs. However, the intrinsic low conductivity, poor infiltration and irreversible reactions of Sn-based compounds lead to poor rate and cycling performance. In order to address above issues, SnS/SnS2 heterostructures on sulfur-doped reduced graphene oxide (SG) are assembled via a facile alkali ion-assisted growth. As-synthesized SnS/SnS2 heterostructures on SG with assistance of K+, denoted as SnS/SnS2@SG-K, exhibits typical layer-stacked structure and high reversible capacities over 800 mAh g−1 at 50 mA g−1 and 241 mAh g−1 even at a record high current density of 48 A g−1, which are superior to most of the Sn-based SIB anode materials. The excellent rate and cycling performance of SnS/SnS2@SG-K could be ascribed to the layer-stacked structures of SG and heterojunctions between SnS and SnS2 which can accelerate semi-infinite diffusions of Na+ ions and electrons, alleviate polysulfide shuttling problems and reduce volume fluctuation effect. Meanwhile, the pseudocapacitance of oxygen and sulfur-containing groups in SG also contribute to the unprecedented sodium storage performance of as-synthesized SnS/SnS2@SG-K.

The proposal of pseudocapacitive materials breaks the barriers between batteries and capacitors, allowing for the possible achievement of a balanced energy-power performance. However, the limited reserve of lithium restricts the practical applications of pseudocapacitive materials in lithium-based systems, whereas sodium resources are abundant with the similar properties to lithium, but there is a lack of suitable pseudocapacitive materials for sodium-based systems. The exploitation of pseudocapacitive materials for Na+ storage is urgent. This study, for the first time, reports a sonochemical approach, involving intercalation and ultrasonic exfoliation processes, to prepare composite K2Mo4O13/α-MoO3 (i.e., KMO) nanorods, and the possible applications of the KMO nanorods in Li+/Na+ pseudocapacitance were evaluated. The as-synthesized KMO possessed a uniform rod-like morphology formed by assembling nanoneedles (or nanobelts) with a large apparent aspect ratio of more than 10. Both in lithium ion batteries (LIBs) and sodium ion batteries (SIBs), the KMO nanorods exhibited efficient pseudocapacitance, which was not observed in the pristine MoO3–Na system. In SIBs, the as-synthesized KMO delivered a capacity of 895 mA h g−1 at 0.02 A g−1, which was much higher than that of the pristine MoO3–Na system. Moreover, it was found that the b values of KMO (i = avb, current i and scan rate v in the CV curves) were over 0.9 at potentials ranging from 1 V to 2 V in SIBs, indicating an obvious pseudocapacitive process. Benefiting from the layered K2Mo4O13 nanorods built with edge-shared distorted MoO6 octahedra, the Na+ ions with a larger size could be intercalated into the spaces between the double MoO6 plates. This sonochemical approach based on the intercalation and exfoliation chemistry opened a new path to prepare molybdenum-based nanostructures for superior Li+/Na+ pseudocapacitance applications on a large scale in a low-carbon and environment-friendly manner.

Lithium-ion hybrid capacitors (LICs) are regarded as one of the most promising next generation energy storage devices. Commercial activated carbon materials with low cost and excellent cycling stability are widely used as cathode materials for LICs, however, their low energy density remains a significant challenge for the practical applications of LICs. Herein, Na0.76V6O15 nanobelts (NaVO) were prepared and combined with commercial activated carbon YP50D to form hybrid cathode materials. Credit to the synergism of its capacitive effect and diffusion-controlled faradaic effect, NaVO/C hybrid cathode displays both superior cyclability and enhanced capacity. LICs were assembled with the as-prepared NaVO/C hybrid cathode and artificial graphite anode which was pre-lithiated. Furthermore, 10-NaVO/C//AG LIC delivers a high energy density of 118.9 Wh kg&minus, 1 at a power density of 220.6 W kg&minus, 1 and retains 43.7 Wh kg&minus, 1 even at a high power density of 21,793.0 W kg&minus, 1. The LIC can also maintain long-term cycling stability with capacitance retention of approximately 70% after 5000 cycles at 1 A g&minus, 1. Accordingly, hybrid cathodes composed of commercial activated carbon and a small amount of high energy battery-type materials are expected to be a candidate for low-cost advanced LICs with both high energy density and power density.

The successful isolation of graphene from graphite in 2004 opened up new avenues to study two-dimensional (2D) systems from layered materials. Since then, research on 2D materials, including graphene, hexagonal-BN (h-BN), transition metal dichalcogenides (TMDs) and black phosphorous, has been extensive, thus leading to various possible applications in the fields of optoelectronics, biomedicine, spintronics, electrochemistry, energy storage and catalysis. However, certain barriers still need to be overcome when dealing with real applications, such as graphene's lack of a bandgap, restricting its use in semiconductor electronics. In this context, a possible solution is to tailor the electronic and optical properties of 2D materials by introducing defects or elemental doping. Although defects play a major role in modifying materials properties, the fact that we call them “defects” might have a negative impact. There has been a long debate on whether structurally perfect materials are equally relevant for modifying the properties and for applications. In this focus article, we clarify that although extra large amounts of defects could be detrimental to the materials properties, well-designed defects might lead to unprecedented properties and interesting applications that pristine materials do not have. Given the relatively short history of research on doped 2D layered materials, our objective is to answer and clarify the following fundamental questions: why does nanomaterial doping offer improved physico-chemical properties? What new applications arise from doping? And what are the current challenges along this line?

Polymeric carbon nitride (PCN) is a stable semiconducting material with an intermediate band gap (2–3 eV), which is efficient for catalysis and optoelectronics. However, it is still a big challenge to synthesize large-area and transferrable PCN films for applications in solid state optoelectronics. In this work, by using nitrogen-doped graphene (NG) as a van der Waals epitaxial substrate, centimeter-size PCN films are synthesized via polymerization of melamine molecules. As-grown PCN/NG films can be then transferred onto other substrates (e.g. SiO2/Si wafers, quartz slides, polymer substrates). Structural characterization reveals a polymerized structure of PCN films with nitrogen-containing heterocycles. By stacking PCN/NG films with graphene films, it is possible to construct a photodetector responsive to near-UV and UV illumination under ambient conditions. The responsivities of the photodetector are 0.59 mA/W and ∼30 μA/W towards 365 nm lamp and 488 nm laser, respectively. Our PCN photodetectors also show fast response times (e.g. ∼0.29 s to 488 nm laser illumination). Furthermore, our PCN photodetector can be fabricated on polymer substrates. As-obtained flexible photodetectors can maintain its photo-response after 100 times bending. Our results clearly demonstrate the possibility of employing large-area carbon-based semiconductors to meet the increasing demands of wearable and portable electronics.

Lithium (Li) metal as anode for Li-S batteries has encountered some issues, eg., dendrite formation and ambient instability, both of which imposed safety problems on the operation and manufacturing of Li metal sulfur batteries. Exploring safer Li metal replacement is thus of fundamental and technical importance for enabling Li-metal-free sulfur batteries. Aluminium (Al) is an appealing Li-alloy anode material for the sake of its high capacity, natural abundance, and safety. Pairing Al-Li alloy with sulfur (S) could be a promising strategy to achieve high-energy rechargeable batteries with improved safety. Herein we show the suppressed dendrite growth and the enhanced ambient stability of Al-Li alloy anode. A Li-metal-free Li-ion sulfur battery was assembled with an Al-Li alloy anode, a sulfurized polyacrylonitrile cathode and a carbonate electrolyte. This Li-ion sulfur full cell exhibited good reversibility and stability, with a slow decaying rate at 0.09% per cycle. The specific energy of the full cell based on the total weight of active materials is estimated to be in a range of 589–762 Wh/kg.

Developing flexible photodetectors is crucial for both military and civil fields. Large-area MoS2 films from several to dozens of layers are controllably synthesized via a facile atmospheric-pressure sulfurization route of predeposited Mo films and transferred onto other substrates (e.g. SiO2/Si wafers, quartz slides, polymers). The flexible photodetectors were fabricated by transferring as-synthesized MoS2 films onto interdigital electrodes patterned on polyethylene terephthalate (PET) substrates. No additional complex lithography positioning techniques were needed during the device fabrication process due to the large area of as-grown atomic thin MoS2 films. As-obtained flexible photodetectors showed responsibilities of ~ 20 mA/W and response time of several seconds. This demonstrates the possibility of employing large-area two-dimensional semiconductors to meet the increasing demands for wearable and portable electronics. Keywords: Flexible photodetector, Two-dimensional material, MoS2

Pristine graphene is known to be nonmagnetic due to its π-conjugated electron system. However, we find that localized magnetic moments can be generated by chemically attaching boron atoms to the graphene sheets. Such spin-polarized states are evidenced by the spin-split of the density of states (DOS) peaks near the Fermi level in scanning tunneling spectroscopy (STS). In the vicinity of several coadsorbed boron atoms, the Coulomb repulsion between multiple spins leads to antiferromagnetic coupling for the induced spin states in the graphene lattice, manifesting itself as an increment of spin-down state at specific regions. Experimental observations and interpretations are rationalized by extensive density functional theory (DFT) simulations.

Hybrid supercapacitors (HSCs) are novel, promising devices having features of both batteries and supercapacitors. Herein, we report HSCs (Li-HSC and Na-HSC in a uniform system) based on an interlayer-expanded MoS2/rGO composite that show ultrahigh energy density and power density as well as superior cycle stability. The 3D network-structured interlayer-expanded MoS2/rGO nanocomposite (3D-IEMoS2@G) was synthesized and employed as the anode. Because the 3D architecture of the graphene skeleton frame delivered sufficient charges and the highly interlayer-expanded MoS2 achieved fast ion diffusion, the as-prepared composite exhibited excellent performance as the anode material for both LIBs and SIBs (1600 mAh g−1 at 100 mA g−1 for the LIB; 580 mAh g−1 at 100 mAh g−1 and 320 mAh g−1 at a high current density of 10 A g−1). When paired with nitrogen-doped hierarchically porous 3D graphene (N-3DG), the obtained Na-HSC surpassed Li-HSC in a uniform system, showing an excellent performance of 140 Wh kg−1 at 630 W kg−1, 43 Wh kg−1 at an ultrahigh power density of 103 kW kg−1 (charge finished within 1.5 s) and no distinct capacity attenuation after over 10000 cycles. Thus, a quantitative kinetic analysis was performed to understand the synergistic effect of the two electrodes and the resulting effect of ions in the hybrid supercapacitors and to further pave a general path for fabricating high-performance HSCs. An energy storage device that combines the advantages of batteries and capacitors has been developed by researchers in China. Batteries store energy electrochemically as charged ions, while supercapacitors store electrical charge electrostatically on a surface. This gives supercapacitors the advantage that they can be charged very quickly, and charged and discharged many times, but they can’t store as much energy as a battery of the same weight. Zheng-Hong Huang from Tsinghua University in Beijing and co-workers created a hybrid device that exhibited both ultra-high energy and power density along with excellent cycle stability. Their structure had an anode made from a composite of interlayer-expanded molybdenum disulfide and graphene oxide. The three-dimensional graphene skeleton supported the electrical charge, while the interlayer-expanded molybdenum disulfide enabled rapid diffusion of ions and provided sufficient energy storage sites. Sodium ion hybrid capacitors is fabricated by interlayer-expanded MoS2/rGO composite and it shows greater performance than lithium ion capacitor.

Electrochemical reduction of water is a renewable way to produce clean hydrogen energy. In order to overcome the high-cost and shortage of noble metals, transition metal compounds involving earth-abundant elements, such as MoS2 and WS2, have been proposed as novel catalysts for hydrogen evolution reaction (HER). Efforts have been made to increase the intrinsic catalytic activity of transition metal dichalcogenides (TMDCs), while alloying same- group elements has not been vastly investigated. Moreover, besides the catalytic activity, the design of free-standing catalytic electrodes is also critical for HER. In this work, we synthesize pyrolytic carbon film with MoxW1-xS2 nanoflakes embedded as a free-standing flexible electrodes for HER catalysis. The pyrolytic carbon acts as conductive and flexible matrix for TMDC alloys with controlled composition, resulting in remarkably enhanced HER activity. The highest HER activity was observed for Mo0.37W0.63S2/C samples with an overpotential of 0.137 V at geometric current densities of jgeo = 10 mA cm−2 and a Tafel slope of 53 mV dec−1. These results now provide low-cost viable alternatives for the design and construction of catalysts based on alloyed TMDCs.

Template method has been used as an important method to prepare porous materials. However, there are few reports about template method employing potassium chloride as template. Here, potassium chloride is employed as a template to prepare the porous carbon, and transition metal nitrates (Fe(NO3)2,d Co(NO3)2, Ni(NO3)2) are introduced to catalyze graphitization and to result different carbon structure during carbonization (denoted as Fe@C, Co@C and Ni@C). The Fe@C shows a formicary-like structure with an about 20 nm pore diameter and the Co@C displays a completely compact structure. Whereas, the Ni@C exhibits a foam-like structure with hierarchical porous structure consisting of macroporous frameworks, mesopores and ultrathin porous walls (∼5 nm). Its macropore and mesopore diameter is around 100 nm and 4 nm, respectively, its specific surface area is 464.5 m2 g−1. When adopted as anode material, the Ni@C presents much outstanding rate and cycling capability for lithium and sodium storage than Fe@C and Co@C, the capacity for sodium storage is 260 mAh g−1 after 100 cycles at 100 mA g−1 and 92 mAh g−1 after 1000 cycles at 1A g−1, and the capacity for lithium storage is 683 mAh g−1 after 1000 cycles at a current density of 1 A g−1.

Exploring anode materials with excellent rate performance and high initial coulombic efficiency (ICE) is crucial for lithium/sodium-ion batteries (LIBs/SIBs). However, it is still very challenging to achieve this goal in a cost-effective way, particularly for SIBs. Herein, graphite oxide, was treated in ammonia atmosphere for a balance between the oxygen- and nitrogen-contained functional groups and yielded nitrogen-doped expanded graphite oxide (NEGO). Electrochemical characterizations were systematically carried out in ether and ester-based electrolytes to shed light on the storage mechanism of NEGO in SIBs. The ICE of NEGO employed in ether-based electrolyte improves to 72.08% from that in ester-based electrolyte (24.73%). Moreover, the as-synthesized NEGO exhibits ∼125 mA h g−1 and ∼110 mA h g−1 capacities in ether and ester-based electrolytes, respectively, even under a record high current density (30 A g−1). Expanded surface area and nitrogen doping significantly increase the active sites and decrease the electrical resistivity from 140 Ω (EGO) to 40 Ω (NEGO) by removing excess oxygen. Moreover, small amounts of residual oxygen, particularly quinone and carboxyl, along with nitrogen occupied sites offer additional pseudocapacitance. Considering the advantages in scale-up and cost-effective production, NEGO is a promising low-cost anode material for SIBs. This study also provides strategies for the design of electrolyte for SIBs to realize practical applications in power-grid energy storage.

Noble-metal-free, durable and high-efficiency electrocatalysts for oxygen reduction and evolution reaction (ORR/OER) are vital for rechargeable Zn-air batteries (ZABs). Herein, a flexible and free-standing carbon fiber membrane immobilized with atomically dispersed Fe-N4 /C catalysts (Fe/SNCFs-NH3 ) is synthesized and used as air cathode for ZABs. The intertwined carbon fibers with hierarchical nanopores facilitate the gas transportation, electrolyte infiltration and electron transfer. The larger specific surface area after NH3 activation exposes a high concentration of Fe-N4 /C sites that embedded in the carbon matrix. Modulation of local atomic configurations by sulfur doping in Fe/SNCFs-NH3 catalyst leads to excellent ORR and enhanced OER activities. As-synthesized Fe/SNCFs-NH3 catalyst demonstrates a positive half-wave potential of 0.89 V and a small Tafel slope of 70.82 mV dec-1 , outperforming the commercial Pt/C (0.86 V/94.74 mV dec-1 ) and most reported M-Nx /C (M = Fe, Co, Ni) catalysts. Experimental characterizations and theoretical calculations uncover the crucial role of S doping in regulating ORR and OER activity. The liquid-state ZABs with Fe/SNCFs-NH3 catalyst as air cathode deliver a large peak power density of 255.84 mW cm-2 and a long-term cycle durability over 1000 h. Solid-state ZAB shows stable cycling at various flat/bent/flat states, demonstrating great prospects in portable and wearable device applications. This article is protected by copyright. All rights reserved.

The sodium-ion battery (SIB) has been considered as one of the most important and promising candidates for large-scale storage of electrical energy. Developing cost-effective but high-performance anode materials is crucial for SIBs. Herein a hard carbon-based anode material is synthesized from polyurethane foam. In order to boost its performance, NaCl, one of the most abundant and low-cost salts in the ocean, is used to intercalate hard carbon. To the best of our knowledge, reports on NaCl intercalation in sodium ion battery anode materials are very scarce so far. After Na+ intercalation, the as-obtained sample delivers a capacity of over 210 mAh g−1 at 20 mA g−1. Moreover, a 90 mAh g−1 specific capacity with over 78% retention can be achieved after 1000 cycles at 1 A g−1 current density, which is 100% higher than that of untreated samples. The much enhanced performance can be attributed to a synergetic effect of both heteroatom doping and Na+ intercalation which can improve the electronic/ionic conductivity and enlarge the lattice spacing of the hard carbon as well. This work demonstrates that NaCl-intercalated nitrogen-rich hard carbons are very promising in their ability to serve as a kind of low-cost but efficient anode material for SIBs.

Large-area (∼cm2) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide (TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature. In particular, we synthesized stacked graphene and WxMo1–xS2 alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec–1 and 96 mV onset potential (at current density of 10 mA cm–2) when the heterostructure alloy was annealed at 300 °C. These results indicate that heterostructures formed by graphene and W0.4Mo0.6S2 alloys are far more efficient than WS2 and MoS2 by at least a factor of 2, and they are superior compared to other reported TMD systems. This strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER). Furthermore, the catalytic activity of the alloy is stable over time, i.e., the catalytic activity does not experience a significant change even after 1000 cycles. Using...

Due to the low density of nanostructured materials, it is still a big challenge to realize high volumetric performance instead of high specific gravimetric capacity with many state-of-the-art electrodes for compact electrochemical energy storage. Moreover, developing high-performance flexible and binder-free electrode materials is also crucial for their future applications in diverse fields, such as portable electronics and wearable devices. In this work, we designed and synthesized a Ni3V2O8/carbon cloth (CC) hierarchical structure as a flexible anode for sodium-ion batteries. Morphology-controllable growth of different Ni3V2O8/CC hierarchical structures is achieved by optimizing the synthesis parameters (e.g. the growth temperatures). The high mass loading (4 mg cm−2), ultra-high areal specific capacity (2.6 mA h cm−2 at a current density of 500 mA g−1), no addition of binders or other additives and good flexibility facilitate their application in sodium-ion batteries.

Sodium-ion capacitors (NICs) are considered an important candidate for large-scale energy storage in virtue of their superior energy&ndash, power properties, as well as availability of rich Na+ reserves. To fabricate high-performance NIC electrode material, a hydrothermal method was proposed to synthesize sulfur-doped reduced graphene oxide (SG), which exhibited unique layered structures and showed excellent electrochemical properties with 116 F/g capacitance at 1 A/g as the cathode of NICs from 1.6 V to 4.2 V. At the power&ndash, energy density over 5000 W/kg, the SG demonstrated over 100 Wh/kg energy density after 3500 cycles, which indicated its efficient durability and superior power&ndash, energy properties. The addition of a sulfur source in the hydrothermal process led to the higher specific surface area and more abundant micropores of SG when compared with those of reduced graphene oxide (rGO), thus SG exhibited much better electrochemical properties than those shown by rGO. Partially substituting surface oxygen-containing groups of rGO with sulfur-containing groups also facilitated the enhanced sodium-ion storage ability of SG by introducing sufficient pseudocapacitance.

Paper-supported electrodes with high flexibility have attracted much attention in flexible Li-ion batteries. However, they are restricted by the heavy inactive paper substrate and large volume change during the lithiation-delithiation process, which will lead to low capacity and poor rate capability and cyclability. Converting the paper substrate to carbon fiber by carbonization can substantially eliminate the "dead mass", but it becomes very brittle. This study reports a water-steam selective etching strategy that successfully addresses these problems. With the help of steam etching, pores are created, and transition-metal oxides are embedded into the fiber. These effectively accommodate the volume change and enhances the kinetics of ion and electron transport. The pores release the mechanical stress from bending, ensuring the sufficient bendability of carbonized paper. Benefiting from these merits, the steam-etched samples show high flexibility and possess outstanding electrochemical performance, including ultra-high capacity and superior cycling stability with capacity retention over 100% after 1500 cycles at 2 A g-1. Furthermore, a flexible Li-ion full battery using the steam-etched Fe2O3@CNF anode and LiFePO4/steam-etched CNF cathode delivers a high capacity of 623 mAh g-1 at 100 mA g-1 and stable electrochemical performances under the bent state, holding great promise for next-generation wearable devices.

Air-cathodes are a critical component for microbial fuel cells (MFCs) and need to have high catalytic performance for the oxygen reduction reaction (ORR). As an important two-dimensional material, graphene has been explored in various applications including ORR catalysts for MFCs. However, the reported graphene for MFC cathodes was usually small flakes/powders, which cannot be directly coated onto metal meshes without binders. Here, we report a binder-free nitrogen-doped graphene (NG) sheet in situ grown on nickel mesh as an efficient catalyst layer for MFC air-cathodes. By optimizing the growth parameters of NG, the maximum power density of MFCs based on NG can be boosted up to 1470 ± 80 mW m−2, which is 32% higher than that of the conventional Pt/C air-cathode. The optimized NG air-cathode has a low internal resistance (21 ± 3 Ω), only 20% of that of the Pt/C air-cathode. These results provide a proof-of-concept for the binder-free NG air-cathode as an alternative to the costly Pt cathode for MFCs.

The traditional fabrication of graphene-based devices requires polymer-assisted transfer of graphene and a removal procedure of polymer coatings. Here, we propose to turn this process on its head and demonstrate a novel strategy of polymer-coated graphene as an optically antireflective and transparent electrode used in a graphene/silicon (G/Si) solar cell. No additional polymer removal and antireflection coatings (e.g. TiO2 colloids) are needed in our strategy. By engineering the thickness of polymer protective coatings, the light absorption and short-circuit current density of graphene solar cells can be greatly enhanced. We also showed that retaining the polymer coatings avoided the degradation of electrical conductivity of graphene films. With HNO3 doping applied on PMMA-coated G/Si solar cells, the PCEs can reach up to 13.34%. The long-term stabilities of HNO3 doped G/Si solar cells are also improved by using fluoropolymer (CYTOP) coatings on graphene. Our approach provides a novel fabrication method of transparent graphene electrodes for graphene-based optoelectronic devices with excellent light absorption.

In recent years, research on two-dimensional (2D) materials including graphene and transition metal dichalcogenides (TMDCs), especially molybdenum and tungsten disulfides (MoS2 and WS2), has rapidly developed. In order to meet the increasing demands of using these 2D materials in fields as diverse as optoelectronics and sensing, heteroatom doping has become an effective method to tune their electronic and physico-chemical properties. This review discusses versatile doping methods applied to graphene and TMDCs, the corresponding changes to their properties, and their potential applications. Future perspectives and new emerging areas are also presented.

Storing more energy in a limited device area is very challenging but crucial for the applications of flexible and wearable electronics. Metal vanadates have been regarded as a fascinating group of materials in many areas, especially in lithium-ion storage. However, there has not been a versatile strategy to synthesize flexible metal vanadate hybrid nanostructures as binder-free anodes for Li-ion batteries so far. A convenient and versatile synthesis of Mx Vy Ox+2.5y @carbon cloth (M = Mn, Co, Ni, Cu) composites is proposed here based on a two-step hydrothermal route. As-synthesized products demonstrate hierarchical proliferous structure, ranging from nanoparticles (0D), and nanobelts (1D) to a 3D interconnected network. The metal vanadate/carbon hybrid nanostructures exhibit excellent lithium storage capability, with a high areal specific capacity up to 5.9 mAh cm-2 (which equals to 1676.8 mAh g-1 ) at a current density of 200 mA g-1 . Moreover, the nature of good flexibility, mixed valence states, and ultrahigh mass loading density (over 3.5 mg cm-2 ) all guarantee their great potential in compact energy storage for future wearable devices and other related applications.

Summary Monolayers of transition metal dichalcogenides (TMDs), such as MoS2 and WS2, have recently triggered worldwide research interest due to their remarkable optical and electronic properties. More fascinatingly is the fact that these monolayers could also adopt various morphologies with exposed edges that include triangular, hexagonal or star-shaped clusters, in addition to nanoribbons. Exciting progress has been recently achieved in the synthesis, characterization, device fabrication and functionalization of these monolayer and few-layer TMDs. This article firstly reviews the properties of bulk and monolayer/few-layer TMDs. The “top-down” and “bottom-up” synthesis routes for different TMDs are then summarized. Raman spectroscopy is now becoming a key tool used to characterize atomically thin TMDs, and this review will show the latest advances using this spectroscopic technique. Here we also summarize the most relevant characterization techniques, optical/electronic device fabrication, functionalization and defect engineering of TMDs. There are numerous opportunities for applications and multiple challenges to overcome, and this review will be instructive and useful to researchers working in the area of 2-dimensional materials, as well as scientists and engineers interested in their applications in electronics, optics, catalysis, energy and many others.

The hollow core of a carbon nanotube (CNT) provides a unique opportunity to explore the physics, chemistry, biology, and metallurgy of different materials confined in such nanospace. Here, we investigate the nonequilibrium metallurgical processes taking place inside CNTs by in situ transmission electron microscopy using CNTs as nanoscale resistively heated crucibles having encapsulated metal nanowires/crystals in their channels. Because of nanometer size of the system and intimate contact between the CNTs and confined metals, an efficient heat transfer and high cooling rates (similar to 10(13) K/s) were achieved as a result of a flash bias pulse followed by system natural quenching, leading to the formation of disordered amorphous-like structures in iron, cobalt, and gold. An intermediate state between crystalline and amorphous phases was discovered, revealing a memory effect of local short-to-medium range order during these phase transitions. Furthermore, subsequent directional crystallization of an amorphous iron nano wire formed by this method was realized under controlled Joule heating. High-density crystalline defects were generated during crystallization due to a confinement effect from the CNT and severe plastic deformation involved.

Lithium-ion capacitors (LICs) are novel advanced electrochemical energy storage (EES) systems integrating both battery and capacitor functions. Most efforts for developing high-power LICs are currently dedicated to nanostructure design of battery-type anodes, which in general results in low packing densities and cannot fundamentally improve the slow Faradaic reaction. Up to now, little attention has been focused on the effects of porous carbon cathodes and the reasonable matching of cathode/anode on the power performance of LICs. Herein, a novel nitrogen-enriched mesoporous carbon nanospheres/graphene (N-GMCS) nanocomposite is demonstrated, which shows simultaneously hierarchical porous structure, 3D conductive network, as well as very high mass density. When such N-GMCS cathode is coupled with prelithiated microcrystalline graphite (PLMG) anode, the integrated device shows quite high packing density which is highly desirable in EES systems. In particular, the PLMG anode in N-GMCS//PLMG system breaks the limitation of slow Faradaic reaction and lithium-ion bulk diffusion, providing an ultrafast capacitor-like electrochemical response. Quite attractive maximum energy density (80 W h kg −1 , 68.6 W h L −1 ) and state-of-the-art maximum power density (352 kW kg −1 , 292 kW L −1 ) can be achieved in N-GMCS//PLMG, which are 5 and 2.8 times as large as those of the supercapacitor counterpart, respectively.

Magnesium citrate and potassium citrate are two commonly used food additives in our daily life. Herein, we prepared nitrogen-doped hierarchical porous carbon nanosheets (N-HPCNSs) through direct pyrolysis of their mixtures and subsequent NH3 treatment. The as-prepared N-HPCNS shows hierarchical porosity (specific surface area of 1735 m2 g−1 and pore volume of 1.71 cm3 g−1), and a moderate nitrogen doping of 1.7%. Moreover, it can be effectively applied in various energy storage/conversion systems. When used as supercapacitor electrodes, it shows a high specific capacitance of 128 F g−1 in organic electrolytes and retains 45% of the original capacitance even at an ultrahigh current density of 100 A g−1. It can also serve as an effective sulfur carrier in lithium–sulfur batteries. The N-HPCNS/sulfur cathode shows high discharge capacities of 1209 mA h g−1 at 0.2C and 493 mA h g−1 even at 4C. Over 500 charge/discharge cycles at 1C, it still retains a high discharge capacity of 486 mA h g−1 with an ultralow capacity loss of 0.051% per cycle and a high average coulombic efficiency of 99.4%.

Nitrogen-enriched porous carbon nanofiber networks (NPCNFs) were successfully prepared by using low-cost melamine and polyacrylonitrile as precursors via electrospinning followed by carbonization and NH3 treatments. The NPCNFs exhibited inter-connected nanofibrous morphology with a large specific surface area, well-developed microporous structure, relatively high-level nitrogen doping and great amount of pyridinic nitrogen. As free-standing new anode materials in lithium-ion batteries (LIBs), the NPCNFs showed ultrahigh capacity, good cycle performance and superior rate capability with a reversible capacity of as high as 1323 mA h g−1 at a current density of 50 mA g−1. These attractive characteristics make the NPCNFs materials very promising anode candidates for high-performance LIBs and, as free-standing electrode materials to be used in other energy conversion and storage devices.