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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.

A hierarchically structured free-standing V2O5/vertically aligned carbon nanotube/graphene foam (V2O5-VA-CNTs/GF) coated by PEDOT was designed. Instead of forming a thick coating layer around, the V2O5 nanobelts disperse uniformly among the CNTs forest without severe aggregations. The PEDOT-V2O5-VA-CNTs/GF delivered a reversible capacity of 296.8 mAh g−1 at 1C, and has capacity retention of 113.3 mAh g−1 at 5C after 1000 cycles. First principles calculations indicate the addition of VA-CNTs to V2O5 electrode could improve the electronic conductivity and facilitate Li-ion adsorption, which lead to the outstanding Li-ion storage and conversion behaviour.

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.

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.

The design of electrocatalysts with lower overpotential is of great significance for water splitting. Herein, cobalt hydroxide carbonate (CCH) has been used as a model to demonstrate the boost of its oxygen evolution reaction (OER) activity by atomic doping of W6+ (W-CCH). The 5 at % W doping reduced the OER overpotential of CCH by 95.3 mV at 15 mA cm−2, and increased the current density by 2.8 times at 1.65 V. 5%W-PCCH || 5%W-CCH-based electrolyzer only required a potential of 1.65 V to afford 10 mA cm−2 for full water splitting. The W6+ in CCH are active sites for O2– adsorption and induced an incesaed electron density near the Fermi level, which facilitates the charge transfer during electrocatalysis. The W6+ doping has been validated as an efficient booster for transition-metal carbonate hydroxides-based electrocatalysts, which has half or more than half-filled d-bands.

Two-dimensional metal-organic framework (MOF) nanosheets have attracted considerable research interest as electrocatalysts, and thermal annealing is important to boost their conductivity. The effect of annealing atmosphere on the electrochemical performance of 2D MOFs and their catalytic center structure have been investigated. The Co-MOF/H2 synthesized by annealing of 2D MOF under a H2 atmosphere has shown a significantly enhanced catalytic activity compared with those annealed under an Ar atmosphere (Co-MOF/Ar). The Co-MOF/H2 has 2-3 graphitic layers of graphitic carbon coating and presents a large amount of high valent Co2+. H2 annealing leads to a fast reduction of Co-MOF to Co/CoOx nanoparticles and catalyzes the growth of CNTs with MOF feed as carbon source. The Co-MOF/H2 shows a high electrocatalytic activity which requires an overpotential of 312 mV to reach a current density of 10 mA cm-2. A Co-MOF/H2-based water electrolyzer requires a potential of 1.619 V to reach a current density of 10 mA cm-2 for overall water splitting in 1.0 M KOH. After 25 h of continuous operation for water electrolysis, the Co-MOF/H2-based cell has shown a negligible increase in the overpotential, indicating its superior durability compared to the 2D Co-MOF.

In the last decade, Li-O2 batteries have been a research focus due to its ultrahigh energy density as chemical power sources and being rechargeable through oxygen reduction and evolution reactions. Besides catalyzing the oxygen reduction and evolution, the rationally designed air-cathode to accommodate the insoluble discharge products and allow efficient transfer of reactants is critical for Li-O2 batteries. To date, the development of a wide variety of electrocatalyst has driven the application-directed research in Li-O2 batteries. This review provides a comprehensive overview of the O2-electrodes for Li-O2 batteries, with an emphasis on the O2-electrodes synthesis, working mechanism, and overall performance evaluation. The aim of this review is to afford a better understanding of Li-O2 cathodes and to provide guidelines for researchers to design and construct high-performance, easy-to-use cathodes for Li-O2 batteries.

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.

Here, a free-standing electrode composed of cobalt phosphides (Co

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.

In recent years, graphene has been considered as a potential “miracle material” that will revolutionize the Li-ion battery (LIB) field and bring a huge improvement in the performance of LIBs. However, despite the large number of publications every year, practical prototypes of graphene-based batteries are still few and no commercial products have entered large-scale production so far. In this review, we will start from a brief introduction of the working mechanism of LIBs, important concepts such as the solid–electrolyte interface (SEI), graphene and the production methods of graphene and graphene based composites followed by the review of graphene and graphene composites as electrode materials, highlighting the role graphene plays in the composite materials and the advantages and drawbacks of these materials. The previous sections laid the foundation of the focus of discussion, which is the application of graphene based materials in full cell prototypes, the difficulties they face and efforts to solve the various problems preventing the implementation of graphene based materials in practical commercial cells.

Contact interface between the active materials and the current collector is essential for electron transfer rate and the mechanical properties of an electrode. In this study, various types of contacts between the active material (graphene sheets) and the metal current collector are created through different fabrication methods, and their impacts to the electrochemical performance are investigated. Intimate “sheet contact” is observed between graphene sheets and Ni foam after KOH in situ activation, which is believed to facilitate the electron transfer. The anode that is tailored to have “sheet contact” delivers a reversible capacity of 1457 mAh g–1 at 0.1 C, which is higher than the electrode obtained by a commercial drop-casting method and comparable to most of the high-end graphene-based anodes. In addition, the anode delivers a high capacity of 173 mAh g–1 with a short charging time of 56 s, indicating its promising use as a high-rate LIB anode.

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.

A new type of three-dimensional (3D) electrochemical sensor was prepared by combining carbon nanotubes on graphene foam (GF/CNT) with molecularly imprinted polymer (MIP), which affords simultaneous identification and quantification towards target compound. The hybrid sensor shows ultralow detection limit of 6.67 × 10−16 M (S/N = 3), wide range of 2 × 10−15 M to 1 × 10−12 M, and superb selectivity for dopamine (DA) detection. Keywords: Electrochemical sensor, Dopamine, Graphene network, Carbon nanotube, Molecularly imprinted polymer

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.

Developing non-precious metal catalysts with both high efficiency and long-term stability is the top priority for hydrogen evolution reactions (HER). Herein, we present a facile two-step method to synthesize Zn, N co-doped molybdenum carbide nanosheets (Zn-N-MoC-H NSs) by using bi-metal oxides of ZnMoO4 as a unique precursor. Zn not only serves as a template to form a porous structure on MoC nanosheets during volatilizing at high temperatures, but also acts as a doping source for Zn doping in MoC. The N-containing carbon source realizes N doping of MoC. Benefitting from Zn, N co-doping and the porous nanosheet structure with a large electrochemical surface area, Zn-N-MoC-H NSs lead to enhanced HER activity in an acidic electrolyte (0.5 M H2SO4) with a low onset potential of -66 mV vs. RHE (1 mA cm-2), overpotential of 128 mV (10 mA cm-2), small Tafel slope of 52.1 mV dec-1 and persistent long-term stability. Density functional theory calculations reveal that Zn, N co-doping can synergistically weaken the strong Mo-H bonding, improve absorbed hydrogen atom (Hads) desorption and lead to faster HER kinetics. This study provides new insights into the use of Zn as a template and electronic regulator toward efficient catalysis and applications in energy storage and conversion.

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.

We report the fabrication of a three-dimensional free-standing nitrogen-doped porous graphene/graphite foam by in situ activation of nitrogen-doped graphene on highly conductive graphite foam (GF). After in situ activation, intimate "sheet contact" was observed between the graphene sheets and the GF. The sheet contact produced by in situ activation is found to be superior to the "point contact" obtained by the traditional drop-casting method and facilitates electron transfer. Due to the intimate contact as well as the use of an ultralight GF current collector, the composite electrode delivers a gravimetric capacity of 642 mAh g(-1) and a volumetric capacity of 602 mAh cm(-3) with respect to the whole electrode mass and volume (including the active materials and the GF current collector). When normalized based on the mass of the active material, the composite electrode delivers a high specific capacity of up to 1687 mAh g(-1), which is superior to that of most graphene-based electrodes. Also, after ∼90 s charging, the anode delivers a capacity of about 100 mAh g(-1) (with respect to the total mass of the electrode), indicating its potential use in high-rate lithium-ion batteries.

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.

A one-step electrochemical method is applied to coaxially deposit highly conductive Co0.85Se nanosheets composed of ultrasmall Co0.85Se nanocrystals on three-dimensional (3D) CoNi2S4 nanotube arrays supported on graphene foam (GF) (Co0.85Se@ CoNi2S4/GF) as an electrode for flexible solid-state supercapacitors. The Co0.85Se@CoNi2S4/GF electrode has an areal capacitance of 5.25 F cm−2 at 1 mA cm−2 and good rate capability (2.65 F cm−2 at 20 mA cm−2). A solid-state asymmetric supercapacitor with Co0.85Se@NiCo2S4/GF as the cathode and hollow carbon spheres (HCSs) as the anode in a PVA/KOH electrolyte was assembled which shows an energy density of 46.5 W h kg−1 at a power density of 750 W kg−1, and 89.0% capacity retention after 10 000 cycles over a potential window of up to 1.55 V. These results demonstrate that the electrodeposition method is applicable for engineering of 3D microstructured electrodes and also provides an efficient strategy for fabricating transition metal selenide based nanodevices.

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.

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.

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

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 Ti

In this paper, using Ni 3 S 2 @MoS 2 as an example, we report the successful design and synthesis of a novel hybrid core/shell metal sulfides with a conductive Ni 3 S 2 core by a green, scalable and one-step solution strategy. When they are tested as supercapacitor electrodes, the Ni 3 S 2 @MoS 2 heterostructure exhibits about 2 times the capacitance (848 F g −1 ) compared to the pristine Ni 3 S 2 sample (425 F g −1 ), excellent rate capability (46.6% capacity retention at 20 A g −1 ) and outstanding cycling stability (91% retention after 2000 cycles). The enhancement is ascribed to the robust hierarchical core/shell structures which provide an increased reaction area and a close contact of electrolyte with the active material. In addition, a highly conductive 1D core material endows the quick transport of electrons along Ni 3 S 2 nanorods to Ni foam. It is prospected that such novel hybrids can offer great potential promise in large-scale energy storage device applications.

Transition metal nitrides (TMN)/N modified graphene (N-rG-O) composites are prepared by annealing of Metal n+ -EN (EN: ethylene diamine) chelate and G-O composite. Li-storage and cycling behavior of TMN/N-rG-O composite have been evaluated by galvanostatic discharge-charge and cyclic voltammetry in coin cells with Li-metal as counter electrode. FeN/N-rG-O, CoN/N-rG-O, FeCoN/N-rG-O, NiN/N-rG-O are prepared using this method, and all have shown pronounced cyclability and considerable high capacity value. Even a low loading of TMN (12.5% ±2.0%) on graphene can significantly improve the Li-ion storage capacity of graphene. For example, FeN/N-rG-O with FeN weight ratio of only 12.5% has an initial reversible capacity of 665 ± 10 mAhg −1 at a current density of 50 mA g −1 , and the capacity value further increase consistently to 698 ±10 mAhg −1 after 50 cycles. The excellent cyclability, low charge/discharge potentials (

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 synthesis of graphene in both high quality and quantity via economic ways is highly desirable and meaningful for practical applications. Here we report a simple, green and cost- effective multiple electrochemical exfoliation approach to high quality and high yield (nearly 50%) graphene flakes by using graphite rod from spent zinc–carbon as graphene source. The graphite rod (anode) and platinum (cathode) were placed vertically at bottom and top of the electrochemical cell, with protonic acid (i.e, H 2 SO 4 , H 3 PO 4 or H 2 C 2 O 4 ) aqueous solution as electrolyte. The vertical cell configuration enables multiple exfoliation process to improve both the quality and yield of graphene sheets from electrochemical exfoliation of graphite. After nitrogen doping, the exfoliated graphene flakes processes excellent electrocatalytic activity, stability and toxicity tolerance for oxygen reduction reaction in alkaline solution.

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.

The demand of highly efficient energy storage system has significantly increased along with the rapid development of electric vehicles and hybrid electric vehicles. However, the energy and power density of standard energy storage devices, such as Li-ion batteries or supercapacitors still cannot meet the requirement of the state-of-the-art electric vehicles. Herein, free-standing Li4Ti5O12/graphene foam (LTO/GF) composite is synthesized via a hydrothermal method, and applied as anode material for Li-ion hybrid supercapacitor. The as-synthesized LTO/GF delivers specific capacities of 186, 179 and 175 mAh g−1 at 0.2, 0.5 and 1 C, respectively. Li-ion hybrid supercapacitors have been assembled with LTO/GF as anodes and activated carbon as cathodes, which have energy densities of 46 and 26 Wh kg−1 at power densities of 625 and 2500 W kg−1, respectively. Furthermore, the hybrid supercapacitor exhibits a superior cycle performance with capacity retention of 83% after 4000 cycles at 1 A g−1. GF sponge substrate can speed up ions and electrons transport with short diffusion lengths and large electrode/electrolyte contact area. Hierarchically structured LTO/GF electrode is lightweight, flexible, and is promising for energy storage applications.

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.