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The rapid capacity loss caused by the shuttling effect of polysulfides is one of the great challenges of Li–S batteries. In this work, we adopted a simple solid-phase sintering method to synthesize titanium disulfide (TiS2) and further demonstrated it as a superior modifier of separators for Li–S batteries. Two commonly adopted modification processes of separators, including vacuum filtration (VF) and slurry casting (SC) have been used to prepare TiS2/Celgard separators. TiS2-VF/Celgard can better restrain the polysulfide shuttling effect compared with TiS2-SC/Celgard. A TiS2-VF/Celgard-based Li–S battery has a reversible capacity of 771.6 mA h g−1, with a capacity retention of 645.6 mA h g−1 after 500 cycles at 2.0 C, corresponding to a capacity fading rate of ∼0.033% per cycle. This study has shown the potential of TiS2 as a multifunctional modifier of separators for high performance and long cycle life Li–S batteries.

N-doped hollow carbon nanospheres (NHCS) with mesoporous carbon shells is selected as nanoreactors and encapsulated with few-layered MoS2 nanosheets to monitor its electrochemical properties and structural conversion in long-term cycling. Mesoporous walls of NHCS can facilitate electrolyte penetration and provide conductive shells for superior mass diffusion and charge transfer. When serving as SIB anodes, MoS2@NHCS has a capacity value of 371 mAh g-1 at 1 A g-1 with a capacity retention of 94.9% after 100 cycles which is higher than powder form MoS2 and is among the highest values of earlier reported MoS2 electrodes. The origin of gradual capacity decay of MoS2@NHCS anodes in-between 50 to 150 cycles has been investigated. Transmission electron microscopy and ex-situ X-ray diffraction have shown significant morphology change of space-confined MoS2 and the production of MoO3 together with oxygen-deficient MoOx after continuous charge/discharge test. After long-term cycle test of MoS2@NHCS, the ratios of Mo6+ and Mo5+ increased significantly with the decreasing of Mo4+ ratio, while the sulfur from MoS2 transformed from S2− to SO42−. This work highlight the phase transformation and composition change of MoS2 for MoS2-based anode in long-duration charge/discharge tests.

Na3V2(PO4)3 (NVP) is one of the most promising cathode materials for sodium-ion batteries because of its stability, safety, and high reversible capacity. However, the sluggish Na-ion diffusion and poor electronic conductivity of NVP often hinder electrochemical performance, thus requiring compositing with carbon materials, such as graphene to improve the material. In this work, the effect of doping species of graphene on the electrochemical performance of NVP/graphene composites was systematically investigated and vigorously compared. 3D porous NVP fabricated by sol-gel method with 3 nm of carbon coating layers was deposited on graphene sheets with different surface functionalities (GO, N-rGO and P-rGO). NVP/N-rGO composites have low charge transfer resistance and high Na+ diffusion coefficient than that of NVP powder, NVP/P-rGO, and NVP/GO, which delivered a specific capacity of 113.9 mAh g−1 at 0.5C with a capacity retention up to 88.42% after 5000 cycles at 20 C. The superior sodium storage performance derives from the pyridinic and pyrrolic N doping in graphene, which triggers defective and active site numbers but maintains moderate graphitization to accelerate the Na+ and electron transportation. This work was supported by the National Natural Science Foundation of China (Grant No. 51502135), and Primary Research & Developement Plan of Jiangsu Province (BE2016183). The authors are grateful to Prof. Xia Hui, Nanjing University of Science & Technology for access to equipment. The authors are grateful to Prof. Jianmin Ma and Dr Zengxi Wei for discussion and theoretical calculation of NVP capacity.

Here, a free-standing electrode composed of cobalt phosphides (Co2P) supported by cobalt nitride moieties (CoNx) and an N,P-codoped porous carbon nanofiber (CNF) in one-step electrospinning of environmentally friendly benign phosphorous precursors is reported. Physiochemical characterization revealed the symbiotic relationship between a Co2P crystal and surrounding nanometer-sized CoNx moieties embedded in an N,P-codoped porous carbon matrix. Co2P@CNF shows high oxygen reduction reaction and oxygen evolution reaction performance owing to the synergistic effect of Co2P nanocrystals and the neighboring CoNx moieties, which have the optimum binding strength of reactants and facilitate the mass transfer. The free-standing Co2P@CNF air-cathode-based Zn-air batteries deliver a power density of 121 mW cm-2 at a voltage of 0.76 V. The overall overpotential of Co2P@CNF-based Zn-air batteries can be significantly reduced, with low discharge-charge voltage gap (0.81 V at 10 mA cm-2) and high cycling stability, which outperform the benchmark Pt/C-based Zn-air batteries. The one-step electrospinning method can serve as a universal platform to develop other high-performance transition-metal phosphide catalysts benefitting from the synergy effect of transition nitride satellite shells. The free-standing and flexible properties of Co2P@CNF make it a potential candidate for wearable electronic devices.

Reported herein is a facile solution-processed substrate-independent approach for preparation of oriented coordination polymer (Co-BTA) thin-film electrodes for on-chip micro-supercapacitors (MSCs). The Co-BTA-MSCs exhibited excellent AC line-filtering performance with an extremely short resistance-capacitance constant, making it capable of replacing aluminum electrolytic capacitors for AC line-filtering applications.

The applications of potassium ion batteries (KIBs) require the development of advanced electrode materials. The rate performance and cycle stability of anode materials are critical parameters and are closely related to their K + storage mechanisms and structural changes during cycling. This review presents an overview of the electrochemical performance and energy storage mechanisms of currently widely studied anodes for KIBs, including carbon-based, alloy-based, and organic-based anodes. This mini-review covers the emerging operando protocols that are applied to the in-depth mechanistic study of the KIBs, and we hope it will provide guidelines for the design of advanced anodes for KIB technology and hence accelerate the commercialization of KIBs as competitive energy storage devices.

Two-dimensional (2D) porous SiO 2 nanomesh obtained from mixed acid etching of vermiculite (VMT) was successfully used as a catalyst support for CO methanation. Compared with three-dimensional (3D) MCM-41, 2D VMT-SiO 2 provided a superior position for implantation of NiO species. Although NiO/VMT-SiO 2 has a total loading of 10 wt.% NiO as well as the NiO/MCM-41, the NiO particles on VMT-SiO 2 were observed to show a better dispersion as the of 3D MCM-41 channels were easily blocked by large NiO particles. Moreover, on the basis of the H 2 temperature-programming reduction results, NiO particles on VMT-SiO 2 were more easily reduced than those on MCM-41. These characteristics indicated that NiO/VMT-SiO 2 was significantly superior to NiO/MCM-41. The as-obtained NiO/VMT-SiO 2 exhibited a CO conversion of 85.9%, CH 4 selectivity of 78% at 450 °C, 0.1151 s −1 turn over frequency at 320 °C and a gas hourly space velocity of 20745 ml/g/h. All of which were much better than those measured for NiO/MCM-41. We believed that 2D VMT-SiO 2 as a new catalyst support open a prospect application of catalyst.

In this study, a free-standing electrochemical sensor based on 3D graphene foam (GF) with dense carbon nanotubes (CNTs) and gold nanoparticles (GNPs) was developed for the first time. The structure of this hybrid electrode (GF/CNTs/GNPs) was observed by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and transmission electron microscopy (TEM), and its electrochemical properties was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The obtained sensor displayed outstanding electrocatalytic activity toward dopamine (DA) and uric acid (UA) compared with the commercial unmodified glass carbon electrode. Detection of DA and UA with the presented sensor yielded remarkable sensitivity of 12.72 μA μM − 1 cm − 2 and 3.36 μA μM − 1 cm − 2 , the low detection limits of 1.36 nM and 33.03 nM (S/N = 3), with wide linear range of 0.10–48 μM and 0.50–60 μM, respectively. Furthermore, quantification of DA in brain tissue and UA in human urine was realized and the results were found in good agreement with those obtained by using high performance liquid chromatography.

Two-dimensional (2D) metal–organic framework (MOF) nanosheets have been recently regarded as the model electrocatalysts due to their porous structure, fast mass and ion transfer through the thickness, and large portion of exposed active metal centers. Combining them with electrically conductive 2D nanosheets is anticipated to achieve further improved performance in electrocatalysis. In this work, we in situ hybridized 2D cobalt 1,4-benzenedicarboxylate (CoBDC) with Ti3C2Tx (the MXene phase) nanosheets via an interdiffusion reaction-assisted process. The resulting hybrid material was applied in the oxygen evolution reaction and achieved a current density of 10 mA cm–2 at a potential of 1.64 V vs reversible hydrogen electrode and a Tafel slope of 48.2 mV dec–1 in 0.1 M KOH. These results outperform those obtained by the standard IrO2-based catalyst and are comparable with or even better than those achieved by the previously reported state-of-the-art transition-metal-based catalysts. While the CoBDC layer pr...

Si/C@NGs composite containing flake-shaped sub-micron sized silicon (Si) enwrapped by pyrolyzed carbon and natural graphite (NG) was successfully prepared by spray-drying-assisted self-assembly method and was systematically studied as an anode material for lithium-ion batteries. The as-prepared Si/C@NGs composite material has a loading amount of sub-micron sized silicon as low as 6.7 wt%. The Si/C@NGs composite has a hierarchical structure with Si/C embedded into natural graphite which further assembles into larger secondary particles of ∼20–50 μm. Compared with pure silicon, the as-synthesized Si/C@NGs composite has multi-layer carbon coating as well as voids to alleviate the structural changes of Si during charging/discharging, exhibits an initial efficiency of 82.8% and a capacity retention of 428.1 mA h g −1 (1524.0 mA h g −1 vs. Si) after 100 cycles at 0.1 A g −1 . The remarkable cycling performances, high initial cycle efficiency together with low-cost manufacturing process make Si/C@NGs composite appealing for commercial applications.

Zn–air batteries have attracted significant attention because of their high energy density, environmental friendliness, safety, and low cost. The air cathode of is one of the most expensive cell components and a key factor in determining the performance of Zn–air batteries. As a fuel, O2 availability to the air cathode is determined by the level of both its dissolution and diffusion in an electrolyte, whereby electrocatalysis happens in the three-phase interface where the catalyst, electrolyte, and O2 meet. Maximizing the performance of air cathodes by rational design of the catalyst structure is of significant importance. To date, various electrocatalysts, including heteroatom-doped carbon, transition metal nitrides/oxides/sulfides, and perovskite oxides, have been developed with outstanding oxygen reduction reaction and oxygen evolution activity. More and more researchers are trying to apply electrocatalysts into Zn–air battery prototypes. The aim of this review is to afford a better understanding of air cathodes and provide guidelines to the researchers for the design and construction of high-performance, easy-to-use cathodes for metal–air batteries.

Electrocatalysts for the oxygen reduction reaction (ORR) play a crucial role in renewable-energy technologies, including metal-air batteries and fuel cells. However, development of novel catalysts with high activity and low cost remains a great challenge. Here, we present hierarchical MnO 2 /reduced graphene oxide (MnO 2 /rGO) hybrid nanosheets by using a facile method and study its electrocatalytic performance. Cyclic voltammograms, and rotating disk electrode and rotating ring/disk electrode measurements demonstrate that the hierarchical MnO 2 /rGO hybrid nanosheets exhibit excellent electrocatalytic activity for the ORR in an alkaline medium, as evidenced by their higher cathodic current density, more positive onset potential, lower H 2 O 2 yield, and higher electron transfer number compared to pure rGO. The excellent catalytic activity of the MnO 2 /rGO hybrid nanosheets highlights the importance of the synergetic chemical coupling effect between the ultrathin MnO 2 nanosheets and the graphene layer.

Structure modulation of transition metal phosphides at the atomic scale can significantly modify their catalytic properties. Herein, stacking faults abundant Ni2P/Co2P electrocatalysts with outstanding bifunctional oxygen evolution and hydrogen evolution property have been prepared. The stacking faults are formed by slow dissolution of Ni substrate by Mo6+ and further redeposition of Ni2+ during the growth of Co(OH)F crystals. Unprecedentedly, the distinct mechanistic etching-redeposition pathway dictated by the stacking faults, heterostructures and crystal lattice deformation leads to lattice expansion of Co2P and enables preferred reactants adsorption. Electrolysis cell employing Ni2P/Co2P electrocatalysts as a bifunctional catalyst for both the cathode and the anode delivers a current density of 10 mA cm−2 at a cell voltage of 1.57 V, which is comparable to the integrated Pt/C and IrO2 counterparts. The exceptional electrocatalytic performance of Co2P/Ni2P-x%Mo indicates the redeposition mechanism a new methodology to modulate the structure and surface reactivity of electrocatalysts.

International audience; Energy storage and conversion have attained significant interest owing to its important applications that reduce CO2 emission through employing green energy. Some promising technologies are included metal-air batteries, metal-sulfur batteries, metal-ion batteries, electrochemical capacitors, etc. Here, metal elements are involved with lithium, sodium, and magnesium. For these devices, electrode materials are of importance to obtain high performance. Two-dimensional (2D) materials are a large kind of layered structured materials with promising future as energy storage materials, which include graphene, black phosporus, MXenes, covalent organic frameworks (COFs), 2D oxides, 2D chalcogenides, and others. Great progress has been achieved to go ahead for 2D materials in energy storage and conversion. More researchers will join in this research field. Under the background, it has motivated us to contribute with a roadmap on ‘two-dimensional materials for energy storage and conversion.

The development of freestanding bifunctional air cathodes for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desirable for the next generation of flexible rechargeable metal-air batteries. It remains challenging to achieve efficient OER and ORR bifunctionality on a single lightweight and inexpensive electrode. In this article, a metal-free, and freestanding air cathode based on vertically aligned carbon nanotubes (VACNTs) functionalized with N, P heteroatoms doped carbon is first reported. In addition to the high catalytic activity caused by N, P heteroatoms doping, the importance of efficient gas diffusion and electron transfer provided by the VACNT-GF hierarchical structure is highlighted. The carbonization temperature has been identified to have pronounced effect on catalytic activity, and the samples with P-N bonds have smaller ORR and OER overpotentials, while the quantitative atomic ratio of either P or N has little effect on catalytic activity. The resulting air electrode achieved a high peak power density of 56 mW cm-2 at a current density of 120 mA cm-2, outperforming Pt/C- and IrO2-based rechargeable Zn-air batteries. The zinc-air battery assembled with the air electrode also showed good cyclability, which exceeded that of cells with the Pt/C//IrO2 catalyst. The increase of voltage difference between the charge and discharge platform was 0.2 V for the cell assembled with N,P-doped VACNT-based freestanding air cathode after 75 h of operation at 10 mA cm-2, which was less than half of that of cells with Pt/C//IrO2 catalyst. Impedance analysis further reveals the good performance results from the favorable mass transfer of the electrode. Ministry of Education (MOE) We gratefully acknowledge the National Natural Science Foundation of China for supporting this research through Grant No. 51502135 and the Singapore Ministry of Education for supporting this research through Grant AcRF Tier 1 (Reference No. RG103/16).

In this work, non-precious metal catalysts (FeCoN) were loaded onto the surface of N-modified graphene (N-G), graphite and activated carbon during a simple yet effective chemical process. The oxygen reduction reaction (ORR) performance of the as-prepared catalysts with different pyrolysis temperatures were evaluated in 0.5 M H2SO4. The use of N-G at a pyrolysis temperature of 750 °C yielded the highest catalyst activity, while both the metal nitrides (MeN) and pyridinic N were demonstrated as the active center for the ORR. The pyridinic-N, which sits surrounding MeN works synergistically with metal nitrides to enhance the catalytic activity of the MeN/N-G composite. A maximum power density output of 130 mW cm−2 at 0.25 V was achieved in a membrane electrode assembly (MEA) using FeCoN-N-G as a cathode catalyst. The mechanism of the ORR at the catalyst and the nature of the active center are discussed.

Cobalt hydroxide nanowires network grown on nitrogen modified microwave exfoliated graphite oxide (NMEG) with a specific capacitance of 610 F/g is successfully synthesized by a chemical precipitation method. Asymmetric-type pseudocapacitors are fabricated with Co(OH) 2 /NMEG and polypyrrole (PPy)/rG-O applied as positive and negative electrodes respectively. The electrochemical properties of the electrodes in three electrode and two electrode systems are systematically investigated in 1 M KOH electrolyte. Various supercapacitor devices, such as rG-O//Co(OH) 2 /NMEG, NG//NG, rG-O//rG-O, and PPy/rG-O//PPy/rG-O are assembled with electrochemical performance evaluated by cyclic voltammetry and galvanostatic charge/discharge measurements. The Co(OH) 2 /NMEG//PPy/rG-O asymmetric supercapacitor cells can achieve a high cell voltage of 1.6 V and an energy density up to 24.9 Wh/kg with an active materials loading of ∼5 mg/cm 2 , significantly higher than that of rG-O//Co(OH) 2 /NMEG (19.3 Wh/kg), NG//NG (16.4 Wh/kg), rG-O//rG-O (15.3 Wh/kg) and PPy/rG-O//PPy/rG-O (9.4 Wh/kg) supercapacitor devices under the same measurement environment. The PPy/rG-O is a superior negative electrode to match cobalt/nickel oxides/hydroxides based positive electrodes for supercapacitor devices.

Three-dimensional (3D) nanoworm-like MoS2 architectures have been successfully synthesized and directly supported on graphene foam/carbon nanotubes (GF/CNT) hybrid films. The sp2-hybridized GF/CNT films provide robust frameworks with an ideal contact for the nucleation and subsequent massive growth of the MoS2 architectures, while acting as an efficient current collector with a conductive contact for binder-free electrodes. The as-prepared hierarchical MoS2@GF/CNT electrode shows capacities of 1368 mA h g−1 and 823 mA h g−1 at current densities of 200 mA g−1 and 5000 mA g−1, and can retain 81.3% of the initial reversible capacity up to 120 cycles.

Compared with thermally exfoliated graphene oxide (TEGO) prepared at high temperature of 800 °C and a long time of 12 h, microwave exfoliated graphene oxide (MEGO) is successfully synthesized at 800 W for 2 min benefiting from “inert and instant heating” of microwave irradiation and employed to optimize electrochemical performance of LiFePO 4 . The as-obtained LiFePO 4 /MEGO exhibits overwhelming superiorities to LiFePO 4 /TEGO, particularly in high-rate performance. Although LiFePO 4 /MEGO delivers the similar specific capacity of 158.1 mAh · g −1 as well as the LiFePO 4 /TEGO at the rate of 0.1 C, the LiFePO 4 /MEGO performs smaller polarization of 53.3 mV resulting in better specific energy of 518.1 Wh · kg −1 and energy efficiency of 89.8%. Even at the high rate of 10 C and 20 C, the LiFePO 4 /MEGO delivers excellent specific energy of 300.3 Wh · kg −1 and 229.7 Wh · kg −1 (with corresponding specific capacity of 104.3 mAh · g −1 and 87.3 mAh · g −1 respectively), much better than those of the LiFePO 4 /TEGO. After 2000 cycles, the LiFePO 4 /MEGO still performs excellent rate capabilities with 91.2% and 83.8% retention respectively.

A lightweight, flexible, and highly efficient energy management strategy is highly desirable for flexible electronic devices to meet a rapidly growing demand. Herein, Ni–Co–S nanosheet array is successfully deposited on graphene foam (Ni–Co–S/GF) by a one‐step electrochemical method. The Ni–Co–S/GF composed of Ni–Co–S nanosheet array which is vertically aligned to GF and provides a large interfacial area for redox reactions with optimum interstitials facilitates the ions diffusion. The Ni–Co–S/GF electrodes have high specific capacitance values of 2918 and 2364 F g−1 at current densities of 1 and 20 A g−1, respectively. Using such hierarchical Ni–Co–S/GF as the cathode, a flexible asymmetric supercapacitor (ASC) is further fabricated with polypyrrple(PPy)/GF as the anode. The flexible asymmetric supercapacitors have maximum operation potential window of 1.65 V, and energy densities of 79.3 and 37.7 Wh kg−1 when the power densities are 825.0 and 16100 W kg−1, respectively. It's worth nothing that the ASC cells have robust flexibility with performance well maintained when the devices were bent to different angles from 180° to 15° at a duration of 5 min. The efficient electrochemical deposition method of Ni–Co–S with a preferred orientation of nanosheet arrays is applicable for the flexible energy storage devices. Published version

Cost-effective synthesis of carbon nanospheres with a desirable mesoporous network for diversified energy storage applications remains a challenge. Herein, a direct templating strategy is developed to fabricate monodispersed N-doped mesoporous carbon nanospheres (NMCSs) with an average particle size of 100 nm, a pore diameter of 4 nm, and a specific area of 1093 m2 g-1 . Hexadecyl trimethyl ammonium bromide and tetraethyl orthosilicate not only play key roles in the evolution of mesopores but also guide the assembly of phenolic resins to generate carbon nanospheres. Benefiting from the high surface area and optimum mesopore structure, NMCSs deliver a large specific capacitance up to 433 F g-1 in 1 m H2 SO4 . The NMCS electrodes-based symmetric sandwich supercapacitor has an output voltage of 1.4 V in polyvinyl alcohol/H2 SO4 gel electrolyte and delivers an energy density of 10.9 Wh kg-1 at a power density of 14014.5 W kg-1 . Notably, NMCSs can be directly applied through the mask-assisted casting technique by a doctor blade to fabricate micro-supercapacitors. The micro-supercapacitors exhibit excellent mechanical flexibility, long-term stability, and reliable power output.

Co3O4 nanoparticles are grown on nitrogen modified microwave exfoliated graphite oxide (NMEG) with weight ratio controlled from 10% to 70%. Electrochemical performance reveals that the obtained Co3O4/NMEG composite as Li-ion battery anode exhibits improved cycle stability, excellent reversible capacity, high current rate performance, and reduced irreversible capacity loss in the initial cycle compared to pure Co3O4 without graphene or Co3O4 on thermally reduced graphene oxide (tRG-O). The 70%Co3O4/NMEG composite has initial irreversible capacity of 230 mAh g−1 (first cycle efficiency of 77%), and 910 mAh g−1 of capacity is retained after 100 cycles. The 70%Co3O4/tRG-O delivers a reversible capacity of 750±20 mAh g−1, and the irreversible capacity loss during the first cycle is 700±20 mAh g−1. Nitrogen functional groups in NMEG, especially pyridinic and pyrrolic N are advantageous for the Co3O4 growth. Furthermore, the N modification is effective in reducing the oxygen content of chemically prepared graphene and hence is good for Co3O4 dispersion and the amelioration of first cycle efficiency, demonstrating the potential of NMEG based composite as high performance Li-ion battery anode materials.

Three dimensional, self-supported α-Fe2O3/Ppy composite electrode with enhanced specific areal capacity and rate performance was successfully fabricated by a simple, low-cost, two-steps process consisting of direct heating of iron foil in air and subsequent coating of conducting polymer Ppy on the α-Fe2O3 nanoflakes. By using α-Fe2O3/Ppy as the anode materials with iron foil as the current collector, the unique structure affords a highly conductive pathway for electron, a short diffusion length for ions, a fast mass transport channel for electrolyte, and sufficient void space for accommodating large volume variations during Li intercalation/diintercalation for Li-ion battery. A relatively high specific capacity of 0.42 mA h/cm2 can be achieved at 0.1 mA/cm2 even after 100 charge/discharge cycles, with a plateau potential of 1 V and nearly 100% Coulombic efficiency, suggesting the feasibility to use this unique 3D nanostructured hybrid composite for microbattery in both small and large scale applications.

The development of high-efficiency Zn−air batteries represents one of the emerging energy storage and conversion technologies. In this work, hierarchically structured N, P co-doped graphene (NPHG) has been synthesized through the chemical polymerization of aniline and phytic acid in the presence of SiO2 nanoparticles and applied as air cathodes for rechargeable Zn−air batteries, owing to its high bifunctional oxygen electrocatalytic activities. The P−N bond, rather than P−C or P=N bonds, in NPHG is proposed to be responsible for the high catalytic activity. The rechargeable Zn−air batteries assembled from this NPHG exhibited much higher reversibility and better stability as compared with a benchmark Pt/C catalyst.

W x C and Pt-W x C ( x =1 or 2) nano-catalysts supported on carbon black were synthesized using a simple co-impregnation and thermal reduction method. An enhanced performance in the electro-reduction of oxygen in an alkaline media is achieved when Pt is incorporated into the W x C catalyst. In the presence of W x C, Pt loading in carbon-supported Pt-W x C catalysts can be reduced to as low as 5% while the oxygen reduction reaction (ORR) performance of the Pt-W x C catalyst remains comparable to that of a commercially available 20% Pt/C catalyst. This indicates that the Pt-W x C composite material is more cost-efficient in ORR than the conventional Pt catalysts since WC is three orders of magnitude less expensive than Pt. W x C alone is not an excellent ORR catalyst, but its addition can greatly enhance the ORR activity of Pt where it serves as an electronic promoter by preventing Pt from oxidation and modifying Pt d-band structure via charge flow from W 2 C to Pt which is confirmed by XPS and UPS. It is found by XRD and TEM characterizations that W x C may play a role as a structural promoter by preventing agglomeration of Pt particles. On the other hand, Pt is found to promote the formation of W x C during the synthesis and also to stabilize the W x C during ORR. The findings on the synergistic effects of this hybrid material are important in assisting the future design of more efficient and durable ORR catalysts.

A series of functionalized graphene derived from graphite oxide (GO) and doped with polypyrrole (PPy) are prepared to systematically investigate the combined effect of surface functionalization and polymer hybridization on supercapacitor performance. Surface chemistry of graphene is found to be of significant importance to PPy growth and PPy/graphene electrochemical performance improvement. High specific capacitance has been achieved by N doped graphene/polypyrrole (NG–PPy) electrode with a value of 393.67 Fg −1 , which is larger than those of –NH 2 modified graphene/PPy (225.33 Fg −1 ), GO/PPy (165.25 Fg −1 ), reduced GO/PPy electrodes (150.00 Fg −1 ) under the same experimental conditions. Incorporation of N into the graphene network enhances electronic transfer efficiency and improved graphene surface wettability, therefore, largest synergistic effect is achieved on NG–PPy. Introduction of N doping to carbon material before the growth of conducting polymer is essential to prepare carbon/conducting polymer composites with high cycling stability and specific capacitance for supercapacitor application.

Multi-walled carbon nanotubes (MWCNTs) are functionalized with 2-pyrridone functional groups by wrapping poly(vinylpyrrolidone) polymers around it under high-powered sonication. The presence of 2-pyrridone functional groups facilitates the complexation of metal ions (Fe 3+ , Co 2+ ). The best oxygen reduction catalyst, MWCNT–FeCo, is synthesized by annealing poly(vinylpyrrolidone)/MWCNTs/Fe–Co complex in ammonia at 900 °C and exhibits nearly four-electron oxygen pathway ( n ∼3.8). X-ray diffraction pattern shows that low loading of metallic Fe nanoparticles (0.79 wt.%) and bimetallic FeCo nanoparticles (0.34 wt.%) is formed on the MWCNT–FeCo catalysts. X-ray photoelectron spectroscopy reveals that ∼1 at.% of nitrogen has been doped into MWCNT–FeCo catalysts.

In order to investigate the effect of graphene surface chemistry on the electrochemical performance of graphene/polyaniline composites as supercapacitor electrodes, graphene oxide (G-O), chemically reduced G-O (RG-O), nitrogen-doped RG-O (N-RG-O), and amine-modified RG-O (NH(2)-RG-O) were selected as carriers and loaded with about 9 wt % of polyaniline (PANi). The surface chemistry of these materials was analyzed by FTIR, NEXAFS, and XPS, and the type of surface chemistry was found to be important for growth of PANi that influences the magnitude of increase of specific capacitance. The NH(2)-RG-O/PANi composite exhibited the largest increase in capacitance with a value as high as 500 F g(-1) and good cyclability with no loss of capacitance over 680 cycles, much better than that of RG-O/PANi, N-RG-O/PANi, and G-O/PANi when measured in a three-electrode system. A NH(2)-RG-O/PANi//N-RG-O supercapacitor cell has a capacitance of 79 F g(-1), and the corresponding specific capacitance for NH(2)-RG-O/PANi is 395 F g(-1). This research highlights the importance of introducing -NH(2) to RG-O to achieve highly stable cycling performance and high capacitance values.

Graphene oxide (GO) is reduced and functionalized with primary amine by a one-pot solvothermal process using ethylene glycol as solvent and ammonia water as nitrogen precursor at 180 °C for 10 h. The reaction is featured by nucleophilic substitution of –COOH and C–O–C groups by the ammonia radicals. The presence of primary amine in graphene is identified by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and proton nuclear magnetic resonance spectra. The atomic ratio of N to C can be tuned within 2–10% by changing the ammonia water to GO weight ratio in the reaction system. The resulting amine modified graphene (NH2-G) is an excellent electrochemical material and up to 217.8 F/g high specific capacitance is measured on NH2-G electrode at a current density of 0.4 A/g, superior over commercially available GO, chemically reduced GO, activated carbon, and CNT. The as-prepared NH2-G shows excellent cycle stability, with negligible decrease of specific capacitance value after thousand cycles.

Program for New Century Excellent Talents in Fujian Province [X12103]; Natural Science Foundation of China [20701031]; Natural Science Foundation of Fujian Province of China [C0710045]

We present the electronic structure evolution from graphite oxide to thermally reduced graphite oxide. Most functional groups were removed by thermal reduction as indicated by high resolution X-ray photoelectron spectroscopy, and the electrical conductivity increased 6 orders compare with the precursor graphite oxide. X-ray absorption spectroscopy reveals that the thermally reduced graphite oxide shows several absorption peaks similar to those of pristine graphite, which were not observed in graphite oxide or chemically reduced graphite oxide. This indicates the better restoration of graphitic electronic conjugation by thermal reduction. Furthermore, the significant increased intensity of Raman 2D band of thermally reduced graphite oxide compared with graphite oxide also suggests the restoration of graphitic electronic structure (π orbital). These results provide useful information for fundamental understanding of the electronic structure of graphite oxide and thermally reduced graphite oxide.

Program for New Century Excellent Talents in Fujian Province [X12103]; Natural Science Foundation of China [20701031]; Natural Science Foundation of Fujian Province of China [C0710045]

Amine-modified carbon micro/sub-microtubes that encapsulate magnetite cores have been synthesized by decomposing a ferrocene/hexabromobenzene mixture in the presence of ammonia under solvothermal conditions at 250°C for 24 h in a one-step process. The as-prepared carbon microstructures (NH 2 -ME-CMTs) were 90-2000nm in diameter and from tens to hundreds of micrometers in length that could be tuned in various solvents. The surface of the carbon microtubes can be modified with amino groups by the synthetic process, as confirmed by infra-red (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Ammonia in the reaction system plays a key role in the formation of the microtube morphology and was the source of the surface functionalization groups. Fluorescent fluorescein isothiocyanate (FITC) was selected as a model compound and successively attached to the amino groups of the carbon microtubes. This result confirms the reactivity of the amino groups on the surface of the carbon microtubes. The inner magnetite cores were removed after immersion in 1 M HCl solution at room temperature over two months, and hollow carbon microtubes (NH 2 -H-CMTs) were obtained. The magnetite-encapsulated carbon microtubes and the hollow carbon microtubes were coated on gold electrodes to prepare carbon microtube-modified gold electrodes. The two electrodes have been used to investigate the oxidative properties of dopamine (DA) and ascorbic acid (AA). Different from the magnetite-encapsulated microtube-modified electrode, the hollow microtube-modified electrode can be utilized in the selective detection of DA in the presence of a large excess of AA. The electrochemical behaviour of DA and AA on the hollow carbon microtubes modified with amino groups is similar to that on carbon nanotubes. This result suggests that the one-step synthesis method will not change the electrochemical properties or break the backbone structure of the carbon microtubes.

Increasing energy and power demands have continued to stimulate the development of new electrochemical energy storage devices. Supercapacitors, well-known energy storage systems characterized by a high power density and long cycle life, have experienced a rapid progress benefiting from fast advancements in electrode materials. However, for those conventional supercapacitors assembled through a thin film preparation technique, the conductive agent and polymer binder will inevitably account for a large amount of ‘dead volume’, which should be further diminished through a better design of the supercapacitor architecture. Here, we present a comprehensive review on recent research progress on the design of integrated electrode architectures, especially the binder-free nanoarray electrodes. By means of an integration of highly-ordered active nanomaterials and a current collector, the binder-free nanoarrays can provide a larger active surface area, faster electron-transport route, easier ion diffusion and superior structural stability, thus leading to a substantially improved cycling and rate performance. This work will narrow its focus on two independent aspects of binder-free architectures: the design of electrode materials and the construction of current collectors. In addition, we also discuss and review future research directions and the remaining challenges in materials development for advanced supercapacitors.

Pseudocapacitive materials, such as binary metal sulfides, show great promise as electrode candidates for energy storage devices due to their higher specific capacitance than that of mono-metal sulfides and binary metal oxides, but generally suffer from low energy densities when assembled in supercapacitor devices. To push the energy density limit of pseudocapacitive materials in devices, a new class of electrode materials with favorable architectures is strongly needed. Here, rationally designed and coaxially grown Ni–Co–S and polypyrrole on vertically aligned carbon nanotube (VA-CNT) arrays/3D carbon fiber paper (CFP) is presented as a novel freestanding electrode for energy storage devices. Our study has revealed that the catalyst preparation is the key step and the presence of an Al2O3 buffer layer is essential for the growth of VA-CNTs. The 3D hierarchical VA-CNTs/CFP allows high areal loading and high utilization efficiency of the active materials. The binder-free asymmetric energy storage devices with Ni–Co–S/VA-CNTs/CFP as the positive electrode and polypyrrole/VA-CNTs/CFP as the negative electrode, respectively, demonstrate a high energy density of 82 W h kg−1 at 200 W kg−1. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Carbon nanotubes (CNTs) have attracted extensive research interest due to their unique structure and properties such as high surface area, extraordinary mechanical properties, high electronic conductivity and chemical stability. These remarkable characteristics of CNTs, along with the inherent benefits of a carbon material, make CNTs promising candidates for fuel cell applications. In this review, we summarize and compare the recent research and development on CNT-based fuel cells, particularly focussing on CNTs as a catalytic support for enhanced electro-catalytic activity, metal-free electro-catalysts for the oxygen reduction reaction and fillers in the polymer electrolyte membrane in fuel cells.

Carbon nanotubes (CNTs) have attracted extensive research interest due to their unique structure and properties such as high surface area, extraordinary mechanical properties, high electronic conductivity and chemical stability. These remarkable characteristics of CNTs, along with the inherent benefits of a carbon material, make CNTs promising candidates for fuel cell applications. In this review, we summarize and compare the recent research and development on CNT-based fuel cells, particularly focussing on CNTs as a catalytic support for enhanced electro-catalytic activity, metal-free electro-catalysts for the oxygen reduction reaction and fillers in the polymer electrolyte membrane in fuel cells.

Many researchers have used nitrogen (N) as a dopant and/or N-containing functional groups to enhance the capacitance of carbon electrodes of electrical double layer (EDL) capacitors. However, the physical mechanism(s) giving rise to the interfacial capacitance of the N-containing carbon electrodes is not well understood. Here, we show that the area-normalized capacitance of lightly N-doped activated graphene with similar porous structure increased from 6 μF cm−2 to 22 μF cm−2 with 0 at%, and 2.3 at% N-doping, respectively. The quantum capacitance of pristine single layer graphene and various N-doped graphene was measured and a trend of upwards shifts of the Dirac Point with increasing N concentration was observed. The increase in bulk capacitance with increasing N concentration, and the increase of the quantum capacitance in the N-doped monolayer graphene versus pristine monolayer graphene suggests that the increase in the EDL type of capacitance of many, if not all, N-doped carbon electrodes studied to date, is primarily due to the modification of the electronic structure of the graphene by the N dopant. It was further found that the quantum capacitance is closely related to the N dopant concentration and N-doping provides an effective way to increase the density of the states of monolayer graphene.

Different C–N bonding configurations in nitrogen (N) doped carbon materials have different electronic structures. Carbon materials doped with only one kind of C–N bonding configuration are an excellent platform for studying doping effects on the electronic structure and physical/chemical properties. Here we report synthesis of single layer graphene doped with pure pyridinic N by thermal chemical vapour deposition of hydrogen and ethylene on Cu foils in the presence of ammonia. By adjusting the flow rate of ammonia, the atomic ratio of N and C can be modulated from 0 to 16%. The domain like distribution of N incorporated in graphene was revealed by the imaging of Raman spectroscopy and time-of-flight secondary ion mass spectrometry. The ultraviolet photoemission spectroscopy investigation demonstrated that the pyridinic N efficiently changed the valence band structure of graphene, including the raising of density of π states near the Fermi level and the reduction of work function. Such pyridinic N doping in carbon materials was generally considered to be responsible for their oxygen reduction reaction (ORR) activity. The 2e reduction mechanism of ORR on our CNxgraphene revealed by rotating disk electrode voltammetry indicated that the pyridinic N may not be an effective promoter for ORR activity of carbon materials as previously expected.

MnO2 supported on graphene oxide (GO) made from different graphite materials has been synthesized and further investigated as electrode materials for supercapacitors. The structure and morphology of MnO2-GO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Nitrogen adsorption-desorption. As demonstrated, the GO fabricated from commercial expanded graphite (denoted as GO(1)) possesses more functional groups and larger interplane gap compared to the GO from commercial graphite powder (denoted as GO(2)). The surface area and functionalities of GO have significant effects on the morphology and electrochemical activity of MnO2, which lead to the fact that the loading amount of MnO2 on GO(1) is much higher than that on GO(2). Elemental analysis performed via inductively coupled plasma optical emission spectroscopy confirmed higher amounts of MnO2 loading on GO(1). As the electrode of supercapacitor, MnO2-GO(1) nanocomposites show larger capacitance (307.7 F g-1) and better electrochemical activity than MnO2-GO(2) possibly due to the high loading, good uniformity, and homogeneous distribution of MnO2 on GO(1) support.