Exploring active, stable, earth-abundant, low-cost, and high-efficiency electrocatalysts is highly desired for large-scale industrial applications toward the low-carbon economy. In this study, we apply a versatile selenizing technology to synthesize Se-enriched Co1–xFexSe2 catalysts on nickel foams for oxygen evolution reactions (OERs) and disclose the relationship between the electronic structures of Co1–xFexSe2 (via regulating the atom ratio of Co/Fe) and their OER performance. Owing to the fact that the electron configuration of the Co1–xFexSe2 compounds can be tuned by the incorporated Fe species (electron transfer and lattice distortion), the catalytic activity can be adjusted according to the Co/Fe ratios in the catalyst. Moreover, the morphology of Co1–xFexSe2 is also verified to strongly depend on the Co/Fe ratios, and the thinner Co0.4Fe0.6Se2 nanosheets are obtained upon selenization treatment, in which it allows more active sites to be exposed to the electrolyte, in turn promoting the OER performance. The Co0.4Fe0.6Se2 nanosheets not only exhibit superior OER performance with a low overpotential of 217 mV at 10 mA cm–2 and a small Tafel slope of 41 mV dec–1 but also possess ultrahigh durability with a dinky degeneration of 4.4% even after 72 h fierce water oxidation test in alkaline solution, which outperforms the commercial RuO2 catalyst. As expected, the Co0.4Fe0.6Se2 nanosheets have shown great prospects for practical applications toward water oxidation.Keywords: cobalt−iron selenides; electrocatalyst; lattice distortion; oxygen evolution; Se-rich effect;

Different crystal facets with different surface atomic configurations and physical/chemical properties will have distinct electrochemical performances during their surface/near-surface redox reactions, and it is important to realize the controllable synthesis of high active surfaces for electrode materials. Herein, using first-principles calculations, the electrochemical performances of different surfaces of β-MnO2 were investigated. Higher surface adsorption pseudocapacitance and lower ion diffusion barrier from the surface to the near surface make the {001} surface of β-MnO2 superior to other surfaces when acting as an electrode material. Moreover, β-MnO2 with a large percentage of the {001} surface was predicted to be obtained through surface F-termination. F-termination decreases the surface energy of the {001} surface while suppressing the growth of {110} surface, which demonstrated as the surface with a much lower electrochemical performance. This work might provide a feasible strategy to synthesize anticipated surfaces with a high electrochemical performance for transition metal oxides.Keywords: adsorption; DFT; diffusion; F-termination; surface energy; β-MnO2;

In situ polymerization of 3,4-ethylenedioxythiophene (EDOT) is achieved on the surface of 2D Ti3C2Tx MXene without using any oxidant, resulting in improved lithium ion storage capability of Ti3C2Tx/poly-EDOT hybrids. A combined experimental and theoretical study revealed the mechanism of charge-transfer-induced polymerization of EDOT, which can be extended to other similar polymers.

•Design a two action sites strategy to break interdependence restriction.•Synergistic effect of two actions sites can beyond the top of the volcano.•Plasma-engraved CoOOH exhibit outstanding OER performance.•Decreased Tafel slope confirm the change of calculative rate-limiting step.Developing effective ways to promote the sluggish oxygen evolution reaction (OER) still remains a great challenge due to the interdependence multiple steps procedure. Herein, we design a two action sites strategy to break interdependence restriction to reduce the calculative overpotential. Density functional theory demonstrated that the introduced oxygen vacancies could accelerate the oxidation of H2O by induced an extra reaction step, corresponding deprotonation of H2O* decomposed into two separated reaction steps: H2O* ↔ (HO + H)* and (HO + H)*↔ HO* + H+ + e-. Meanwhile, experimental observations confirm that two action sites promote the Vo-CoOOH OER performance including a lower onset potential, a lower Tafel slope and an incremental Turnover frequency (TOF) from 0.01 to 0.04 s−1. Through this work, a viable design principle for, but not limited to, CoOOH OER catalyst is proposed: injected hole can activate the synergistic catalytic effect of two actions sites to accelerate the water oxidation.This study can be described as following figure.Download high-res image (271KB)Download full-size image

•A novel three-dimensional MoS2 framework coupled with Ni(OH)2 nanoparticlesis firstly synthesized.•This facile Ni(OH)2/MoS2 heterostructure delivers an excellent alkaline HER activity.•The interfacial synergistic effect plays a key role in reducing the energy barrier of the initial water dissociation step.•This work demonstrates a potentially interface engineering strategy for accelerating sluggish alkaline HER kinetics.Earth-abundant, noble-metal-free catalysts with outstanding electrochemical hydrogen evolution reaction catalytic activity in alkaline media play a key role in sustainable production of H2 fuel. Herein, the novel three-dimensional Ni(OH)2/MoS2 hybrid catalyst with synergistic effect has been synthesized by a facile approach for efficient alkaline hydrogen evolution reaction. Benefiting from abundant active interfaces, this hybrid catalyst shows high hydrogen evolution catalytic activity in 1 M KOH aqueous solution with an onset overpotential of 20 mV, an overpotential of 80 mV at 10 mA cm−2 and a Tafel slope of 60 mV dec−1. Further theoretical calculations offers a deeper insight of the synergistic effect of Ni(OH)2/MoS2 interface: Ni(OH)2 provides the active sites for hydroxyl adsorption, and MoS2 facilitates adsorption of hydrogen intermediates and H2 generation. This interfacial cooperation leads to a favorable hydrogen and hydroxyl species energetics and reduce the energy barrier of the initial water dissociation step, which is the rate-limiting step of MoS2 catalyst in alkaline media. The combination of experimental and theoretical investigations demonstrates that the sluggish alkaline hydrogen evolution process can be circumvented by rational catalysts interface engineering.Download high-res image (178KB)Download full-size image

•Fabrication of Honeycomb-like composites with dual-templates.•Honeycomb-like composites exhibit excellent lithium and sodium storage capability.•SnO2@C configuration allows volume expansion upon Li+/Na+ uptake and release.•Carbon matrix increase the conductivity and alleviate the volume change of SnO2.Tin oxide (SnO2) has been considered as one of the most promising anodes for advanced rechargeable batteries due to its advantages such as high energy density, earth abundance and environmental friendly. However, its large volume change during the Li-Sn/Na-Sn alloying and de-alloying processes will result in a fast capacity degradation over a long term cycling. To solve this issue, in this work we design and synthesize a novel honeycomb-like composite composing of carbon encapsulated SnO2 nanospheres embedded in carbon film by using dual templates of SiO2 and NaCl. Using these composites as anodes both in lithium ion batteries and sodium-ion batteries, no discernable capacity degradation is observed over hundreds of long term cycles at both low current density (100 mA g−1) and high current density (500 mA g−1). Such a good cyclic stability and high delivered capacity have been attributed to the high conductivity of the supported carbon film and hollow encapsulated carbon shells, which not only provide enough space to accommodate the volume expansion but also prevent further aggregation of SnO2 nanoparticles upon cycling. By engineering electrodes of accommodating high volume expansion, we demonstrate a prototype to achieve high performance batteries, especially high-power batteries.Download high-res image (479KB)Download full-size image

•NiSe nanorods arrays on Ni foam were prepared by one-pot hydrothermal method.•The morphology of NiSe was adjusted by adding different amount of Ni2+.•The areal capacitance of NiSe is as high as 6.81 F g−1 at 5 mA cm−2.•The NiSe//RGO device delivers an energy density of 38.8 Wh kg−1 at 629 W kg−1.To satisfy the ever-increasing demand of smart and miniaturized portable devices, supercapacitors have attracted tremendous attention owing to their short charging time, long cycling lifespan, and reliable safety. Nevertheless, the energy density of pseudocapacitors is the major obstacle because of its kinetically sluggish reactions, low mass loading of active materials, and/or high internal resistance. Herein we develop a one-pot hydrothermal method to synthesize NiSe nanorod arrays on nickel foam (NiSe NRA/NF), which exhibit ultrahigh areal capacitance of 6.81 F cm−2 at current density of 5 mA cm−2 as well as excellent cycling stability. The superior performance of NiSe NRA/NF can be attributed to metallic conductivity of nickel selenides, short diffusion length of ions from electrolyte to electrode materials, and fast electrons transport pathway between active materials and current collectors. For practical applications, an asymmetric supercapacitor was assembled by applying NiSe as positive electrode and reduced graphene oxide as negative electrode, delivering an energy density of 38.8 Wh kg−1 at a power density of 629 W kg−1 and 6.38 Wh kg−1 at a power density of 13.5 kW kg−1. Moreover, the devices retained 90.09% after 3000 cycles at high current density of 3.6 A g−1, showing a promising application prospect.Download high-res image (428KB)Download full-size image

•Aluminum nitrate induces growth of flowerlike Ni-Al subcarbonate.•Al-doped mesoporous β-NiS are prepared by sulfidation of Ni-Al precursor.•The β-NiS electrode delivers a high specific capacity and well cycling stability.•The β-NiS//AC hybrid supercapacitor exhibits high energy and power densities.Transition metal sulfides such as NiS and Co9S8 are gaining popularity in electrochemical capacitors owning to low-cost, earth-abundant, and environmentally friendly nature, but their specific capacity/capacitance should be further improved. Here, by doping Al atoms into β-NiS mesoporous nanoflowers, we achieve reversible specific capacity of 697.3C g−1 at a current density of 1 A g−1, while the pure β-NiS electrode only delivers a specific capacity of 311.5C g−1. The improved performance of Al-doped β-NiS could be assigned to its large surface area and mesoporous characteristics, providing abundant electroactive sites and shortening ions transport pathway. Besides, new observed multiple electrochemical redox reactions are also deem as another possible reason to enrich the electrochemical redox species for the improved electrochemical performance caused by Al doping. Theoretical calculation manifests that charges are transferred from Al atoms to both Ni and S atoms that are close to Al atoms, resulting in the appearance of Ni ions-based multiple redox reactions. Moreover, a hybrid supercapacitor composed of Al-doped NiS positive electrode and carbon-based negative electrode is configured, delivering a high energy density of 39.6 W h kg−1 at a power density of 918.8 W kg−1. Even at a high power density of 12296 W kg−1, an energy density of 11.4 W h kg−1 can be still achieved, indicating its great potential for practical applications. Our developed doping approach may reverse the lagging status of nickel sulfides towards high theoretical-capacity capacitors.Download high-res image (175KB)Download full-size image

Because of the advanced nature of their high power density, fast charge/discharge time, excellent cycling stability, and safety, supercapacitors have attracted intensive attention for large-scale applications. Nevertheless, one of the obstacles for their further development is their low energy density caused by sluggish redox reaction kinetics, low electroactive electrode materials, and/or high internal resistance. Here, we develop a facile and simple nitridation process to successfully synthesize hierarchical Ni nanoparticle decorated Ni0.2Mo0.8N nanorod arrays on a nickel foam (Ni–Mo–N NRA/NF) from its NiMoO4 precursor, which delivers a high areal capacity of 2446 mC cm−2 at a current density of 2 mA cm−2 and shows outstanding cycling stability. The superior performance of the Ni–Mo–N NRA/NF can be ascribed to the metallic conductive nature of the Ni–Mo nitride, the fast surface redox reactions for the electrolyte ions and electrode materials, and the low contacted resistance between the active materials and the current collectors. Furthermore, a hybrid supercapacitor (HSC) is assembled using the Ni–Mo–N NRA/NF as the positive electrode and reduced graphene oxide (RGO) as the negative electrode. The optimized HSC exhibits excellent electrochemical performance with a high energy density of 40.9 W h kg−1 at a power density of 773 W kg−1 and a retention of 80.1% specific capacitance after 6000 cycles. These results indicate that the Ni–Mo–N NRA/NF have a promising potential for use in high-performance supercapacitors.

Two-dimensional transition metal dichalcogenides (TMDs) have been widely considered as potential hydrogen evolution reaction (HER) catalysts because of their low cost and good electrochemical stability in acid conditions. The mechanism of hydrogen adsorption on TMDs plays an important role in optimizing HER activity. In this research, a series of TMDs (MX2, M = Co, Cr, Fe, Mn, Mo, Nb, Ni, Re, Sc, Tc, Ti, V, W, Zr, and X = S, Se, Te) in 2H- and 1T-phases were investigated using density functional theory to determine the relationship between hydrogen adsorption free energy (ΔGH) and electronic structure using a simple descriptor. The results showed a positive linear relation between ΔGH and the work required of the H electron to fill the unoccupied electronic states of the TMDs. Based on such linear relationships, the various defects (B-, C-, N-, O-, F-, P-, Se-doping and S-vacancy) were used to activate the inert basal planes of the 2H-phase molybdenum disulfide, which can introduce impurity states in the lower energy level to effectively accommodate the H electron. Furthermore, HER activity can be further optimized with the increasing concentration of the defects. These findings provide a practicable map of the HER performances, as well as indicating an appropriate direction for optimizing HER activity.

•Co(OH)2 nanotube arrays were synthesized by electrochemically induced ion-exchange.•Nanowire converted to nanotube caused by kirkendall effect during ion-exchange process.•Activation mechanism was study to understand the reason of electrochemical performance improvement.In this work, a Co(OH)2 nanotube array is grown on Ni foam by an electrochemically induced ion-exchange approach. These arrays can be applied in supercapacitors as electrode materials. In the cyclic voltammetry (CV) charging process, the CO32- in Co(CO3)0.5(OH)·0.11H2O is replaced by OH− in the electrolyte, leading to structural transformation from nanowire to nanotube due to Kirkendall effect. Electrochemical tests reveal that the as-prepared Co(OH)2 nanotubes array on Ni foam show low specific capacitance in the initial cycles. After activation of electrode materials, the delivered specific capacitance is significantly increased. X-ray diffraction and Fourier transform infrared techniques (FTIR) are used to study the activation mechanism of the Co(OH)2 nanotube array upon CV cycling. With the continuous cycling, phase transformation occurred, in which synthesized Co(OH)2 bonds are completely converted to CoOOH after 150 cycles.

•The porous rod-like NiS2 precursor is synthesized by a facile solution method.•The NiS2 delivers a high specific capacitance and excellent cycling stability.•The NiS2//rGO supercapacitor exhibits a high energy and power densities.The square rod-like NiS2 with open ends is synthesized by a general solution method without substrate followed by a post annealing treatment. This method involves a rapid self-assembly and a spontaneous aging process controlled by the time of the solution reaction. And the one-step solution-controlled reaction benefits for the convenient fabrication of metal sulfide precursors. The NiS2 with the length varying from 3 to 8 μm and the width of 2 μm has both open ends, and the BET surface area and average pore diameter of the NiS2 are 59.2 m2 g−1 and 24.4 nm, respectively. The porous NiS2 square rods show a high specific capacitance (1020.2 F g−1 at 1 A g−1 and 534.9 F g−1 at 10 A g−1) as well as excellent cycle life (93.4% of the initial specific capacitance remains after 1000 cycles). Furthermore, an asymmetric supercapacitor device fabricated by using the NiS2 as the cathode and reduced graphene oxide as the anode delivers an energy density of 32.76 Wh kg−1 at a power density of 954 W kg−1. Therefore, the porous square rod-like NiS2 synthesized by an effective route exhibits outstanding electrochemical performance as a promising cathode material for supercapacitors.

•Hollow NiSe–CoSe nanoparticles are first synthesized via a solvothermal approach.•The method is robust to hollow NiSe–CoSe with different NiSe to CoSe ratios.•Synergistic effect of NiSe and CoSe contributes to superior capacity.•Ni–Co–Se-4-2 shows both high power and energy performances.Hollow NiSe−CoSe samples have been synthesized for the first time via a one-pot solvothermal approach. The strategy is robust enough to synthesize NiSe–CoSe nanoparticles with different NiSe to CoSe ratios but with a similar hollow structure. Co ions in the NiSe–CoSe nanoparticles play decisive role for formation of the hollow structure; otherwise, the nanoparticles become solid for the NiSe sample. When used as the positive electroactive materials for energy storage, the NiSe–CoSe samples show excellent electrochemical activity in alkaline electrolyte. Using the synergistic effect between NiSe and CoSe, the electrochemical performance of NiSe–CoSe can be tuned by varying the NiSe to CoSe ratios. The NiSe–CoSe sample with a NiSe to CoSe ratio of 4:2 shows the best electrochemical performance in terms of superior specific capacity, improved rate capability and excellent cycling stability. In addition, the electrochemical performance of NiSe–CoSe sample with a NiSe to CoSe ratio of 4:2 is also evaluated via assembling hybrid supercapacitors with RGO, and the hybrid supercapacitor delivers both high power and energy densities (41.8 Wh kg−1 at 750 W kg−1 and 20.3 Wh kg−1 at 30 kW kg−1).

Glucose intercalated NiMn layered double hydroxide (LDH) is successfully fabricated with a facile one-pot hydrothermal method, which expands interlayer distances to enhance cycling stability and break the bottleneck of Ni-based hydroxide in applications. Electrochemical measurements show that the annealing-treated glucose intercalated NiMn LDH (LDH-GA) delivers a high specific capacity of 1464 F g−1 at a current density of 0.5 A g−1 (852 F g−1 for pristine NiMn LDH). The enhanced performance is contributed to the small sized architectures, lower charge transfer resistance and faster reversible redox reactions. Through enlarging interlayer distance and robustly stabilizing LDH, the cycling stability is dramatically enhanced from 45% to 90% for over 1000 cycles. To further disclose the reason of the enhanced electrochemical performance of NiMn LDH, a molecular dynamics (MD) simulation is implemented to calculate the diffusion of the electrolyte ions, the ionic diffusion coefficient and the ionic conductivity inside the NiMn LDH nanopores for different interlayer distances. Based on the experimental and theoretical results, it suggests that the intercalation of glucose in NiMn LDH could be an effective approach to enhance electrochemical performance, of which it also could be generalized to intercalation of other molecules to stabilize the α-phase of LDH.

•Unravel contrastive pseudocapacitance and similar EDLC in T and H phases of MoS2.•Possible pseudocapacitance introduced by Mo-edge and vacancy of H-MoS2.•EDLC of MoS2 is comparable with graphene, which agrees with previous experiments.Two-dimensional MoS2 is a promising candidate for high performance electrochemical capacitors. However, the understanding of different capacitive performances of MoS2 with different phases is still limited. Herein, the pseudocapacitive and electrical double layer characters in T and H phases of MoS2 monolayer are systematically studied, by using first-principles calculations and molecular dynamic simulations. The electronic levels referenced to the vacuum level are utilized to discuss the intrinsic pseudocapacitive characters. The large density of states of T-MoS2 located at 0.25 V versus the standard hydrogen electrode indicates possible redox pseudocapacitance. Contrastively, the large band gap of H-MoS2 limits its pseudocapacitive behavior. Furthermore, the reaction with the Li atom is employed to shed more light on the possible redox behavior. The different shifts of Fermi levels in H and T phase confirm their different capacitive properties. Moreover, the electrical double layer capacitances character of MoS2 in aqueous solution is also investigated. The H-MoS2 exhibits similar double layer character with T-MoS2. The calculated double layer capacitances of MoS2 are comparable with graphene, and can reach 9.07 μF/cm2 and 7.42 μF/cm2 for anode and cathode, respectively. The combined calculations shed more light to explore other two-dimensional transition-metal dichalcogenides as supercapacitors electrode materials.

Two-dimensional (2D) transition-metal (TM) compound nanomaterials, due to their high-surface-area and large potential charge capability of TM atoms, have been widely investigated as electrochemical capacitors. However, the understanding of charge-storage mechanisms of 2D transition-metal compounds as electrode materials is still limited. In this study, using density functional theory computations, we systematically investigate the electrochemical properties of monolayer VS2 and V2C. Their electronic structures show a significant electron storage capability of around 0.25 V, referenced to the standard hydrogen electrode, and indicate redox pseudocapacitance characteristics as cathodes. The different charge densities visually confirm that excess electrons tend to localize in the vanadium atoms nearby contact-adsorbed Li ions, corresponding to the redox of vanadium atoms. In contrast, only the electric double layer acts as a charge-storage mechanism in the V2C monolayer. However, the O saturation would induce redox pseudocapacitance in the V2C monolayer. Furthermore, the calculated metallic behavior and low Li ion diffusion barriers substantiate that V2C and VS2 monolayers would manifest low resistance in the charging process. Our findings provide insights for the different charge-storage mechanism of VS2 and V2C monolayers.

Nickel cobalt sulphides with different stoichiometric nickel and cobalt contents have been synthesized and used as pseudocapacitive materials for supercapacitors. The as-employed polyol method is robust enough to use a one-pot synthesis of nickel cobalt sulphides with similar porous morphology and crystal structure at different nickel and cobalt ratios. The electrochemical performance of the nickel cobalt sulphides can be easily tuned by the varying the Ni and Co content. Owing to the combined contributions from both Ni and Co ions, the bimetallic Ni–Co sulphides show a superior pseudocapacitance compared to monometallic Co and Ni sulphides in terms of high specific capacitance, excellent rate capability and long cycle stability. In particular, the Ni1.5Co1.5S4 sample shows the highest specific capacitance of 1093 F g−1 at 1 A g−1, superior rate capability of 69% capacitance retention after a 50-fold increase in current densities, and longer cycle stability with increased specific capacitance of 108% of capacitance retention after 2000 cycles. In addition, the Ni1.5Co1.5S4 was also used to assemble an asymmetric supercapacitor with reduced graphene oxide and attains excellent capacitive performance with high specific capacitance (113 F g−1 at 1 A g−1), high energy density (37.6 W h kg−1 at 775 W kg−1) and high power density (23.25 kW kg−1 at 17.7 W h kg−1).

Three dimensional (3D) hierarchical network configurations are composed of NiCo2S4 nanotube @Ni–Mn layered double hydroxide (LDH) arrays in situ grown on graphene sponge. The 3D graphene sponge with robust hierarchical porosity suitable for as a basal growth has been obtained from a colloidal dispersion of graphene oxide using a simple directional freeze-drying technique. The high conductive NiCo2S4 nanotube arrays grown on 3D graphene shows excellent pseudocapacity and good conductive support for high-performance Ni–Mn LDH. The 3D NiCo2S4@Ni–Mn LDH/GS shows a high specific capacitance (Csp) 1740 mF cm–2 at 1 mA cm–2, even at 10 mA cm–2, 1267.9 mF cm–2 maintained. This high-performance composite electrode proposes a new and feasible general pathway as 3D electrode configuration for energy storage devices.Keywords: graphene; nanotube array; NiCo2S4; supercapaitor; three-dimensional;

•Hollow spiny shell Ni–Mn precursors were synthesized via a simple hydrothermal method.•The morphology of porous hollow spiny shell Ni–Mn oxides inherits the microstructure of precursors after thermal treatment.•The Ni–Mn oxide obtained at 300 °C delivers a maximum capacitance of 1140 F g−1 at 1 A g−1 after 1000th cycles.Hollow spiny shell Ni–Mn precursors composed of one-dimensional nanoneedles were synthesized via a simple hydrothermal method without any template. The hollow Spiny shell Ni–Mn oxides are obtained under thermal treatment at different temperatures. The BET surface areas of Ni–Mn oxides reach up to 112 and 133 m2 g−1 when calcination temperatures occur at 300 and 400 °C, respectively. The electrochemical performances of as-synthesized hollow spiny shell Ni–Mn oxides gradually die down with annealing temperatures increasing. The porous hollow spiny shell Ni–Mn oxide obtained at 300 °C delivers a maximum capacitance of 1140 F g−1 at a high current density of 1 A g−1 after 1000th cycles and the specific capacitance of Ni–Mn oxide will increase with cycling times increasing. So, porous hollow spiny shell Ni–Mn oxide obtained at low annealing temperature can form a competitive electrode material for supercapacitors.

•α phase Ni–Co hydroxides have been synthesized via a precursor conversion method.•The method is robust to synthesis α phase hydroxides with different Ni and Co ratios.•Electrochemical properties can be improved by tuning the Ni and Co content.•Bimetallic Ni–Co hydroxides exhibit excellent supercapacitive performance.•The mechanism that the interlayer species are beneficial to the cycling stability is proposed.A facile and novel precursor conversion method is successfully developed to grow Ni–Co hydroxides with α phase structure. The method is robust and versatile enough to be employed to prepare α phase Ni–Co hydroxides with similar interconnected flower-like nanostructures even at different Ni and Co contents. When used as the electroactive materials for supercapacitors, the bimetallic Ni–Co hydroxides deliver much higher specific capacitance with improved rate capability and cycling stability than monometallic Ni hydroxide. Especially, the Ni0.5Co0.5 hydroxide exhibits the highest specific capacitance of 1600 F g−1 at a current density of 1 A g−1. Meanwhile, the mechanism for better cycling stability of the bimetallic Ni–Co hydroxides is studied by FT-IR measurement. It is found that the introduction of Co ions contributes to the stable existence of ethylene glycol molecules in the interlayer space of the α phase Ni–Co hydroxides, resulting in superior cycling stability of the bimetallic Ni–Co hydroxides.

•NiCo2S4 nanotube arrays have been synthesized based on the anion-exchange reaction.•The specific structure achieves high utilization efficiency of NiCo2S4 at high mass loading.•Ultrahigh specific capacitance with superior rate performance has been achieved.•The asymmetric supercapacitor cell has been assembled.•The cell delivers superior capacitive performance at high mass loading.Self-standing NiCo2S4 nanotube arrays have been in situ grown on Ni foam by the anion-exchange reaction and directly used as the electrode for supercapacitors. The NiCo2S4 nanotube in the arrays effectively reduces the inactive material and increases the electroactive surface area because of the ultrathin wall, which is quite competent to achieve high utilization efficiency at high electroactive materials mass loading. The NiCo2S4 nanotube arrays hybrid electrode exhibits an ultrahigh specific capacitance of 14.39 F cm−2 at 5 mA cm−2 with excellent rate performance (67.7% retention for current increases 30 times) and cycling stability (92% retention after 5000 cycles) at a high mass loading of 6 mg cm−2. High areal capacitance (4.68 F cm−2 at 10 mA cm−2), high energy density (31.5 Wh kg−1 at 156.6 W kg−1) and high power density (2348.5 W kg−1 at 16.6 Wh kg−1) can be achieved by assembling asymmetric supercapacitor with reduced graphene oxide at a total active material mass loading as high as 49.5 mg. This work demonstrates that NiCo2S4 nanotube arrays structure is a superior electroactive material for high-performance supercapacitors even at a mass loading of potential application-specific scale.

A 3D highly conductive urchin-like NiCo2S4 nanostructure has been successfully prepared using a facile precursor transformation method. Remarkably, the NiCo2S4 electroactive material demonstrates superior electrochemical performance with ultrahigh high-rate capacitance, very high specific capacitance, and excellent cycling stability.

•Fe3O4/rGO composites were prepared through a facile hydrolysis process.•The composites exhibit specific capacitance of 350.6 F g−1 at 1 mV s−1 and 157.6 F g−1 at 100 mV s−1.•The composites show much higher specific capacitances than either pure rGO or pure Fe3O4.•The good electrochemical performance can be attributed to the positive synergistic effect.Fe3O4/reduced graphene oxide (rGO) composites were prepared through a facile hydrolysis route and subsequent annealing process and used as the electroactive materials for supercapacitors. The as-prepared Fe3O4/rGO composites exhibit much higher specific capacitances than either pure rGO or pure Fe3O4, meanwhile, the electrochemical performance of Fe3O4/rGO composites with different amount of RGO has been compared. The optimized Fe3O4/rGO composites (FG-0.8) exhibit the highest specific capacitance of 350.6 F g−1 at 1 mV s−1, 157.6 F g−1 of the specific capacitance can still be retained when the scan rate increases 100 times to 100 mV s−1, indicating a good rate capability. These Fe3O4/rGO composites also show excellent cycling performance without any decrease of specific capacitance after 10,000 cycles. These fascinating performances can be attributed to the positive synergistic effect between Fe3O4 and rGO.

NiCo2S4 porous nanotubes are synthesised by a sacrificial template method based on the Kirkendall effect. The as-prepared NiCo2S4 nanotube electrode shows a specific capacitance of 1093 F g−1 at a current density of 0.2 A g−1 (933 F g−1 at 1 A g−1).

•NiMoO4·H2O nanoclusters are synthesized via a facile and rapid microwave assisted method.•The formation mechanism of the NiMoO4·H2O is confirmed using FTIR technology.•NiMoO4 nanoclusters show 680 F g−1 at 1 A g−1, and good reversibility.NiMoO4·H2O nanoclusters with one-dimensional nanorods have been synthesized via a facile and rapid microwave assisted method. The formation mechanism of the NiMoO4·H2O is confirmed as speedy self-assembly process via controlling the reaction time using FTIR technology. The as-prepared NiMoO4·H2O nanoclusters have a uniform size of 3 μm and are composed of NiMoO4·H2O nanorods of about length of 1 μm and diameter of 100 nm, which shows a specific capacitance of 680 F g−1 at a current density of 1 A g−1, and good reversibility with a cycling efficiency of 98% after 1000 cycles. The excellent electrochemical performance makes the microwave assisted synthesis of NiMoO4·H2O nanoclusters a promising electrode material for supercapacitors.

Using first-principles calculations, we investigated the effect of the synergistic mechanism on the properties of a SnO2 and graphene hybrid structure, including the stability, electronic, and Li diffusion performance. The stable interface formed by C–O covalent bonds not only improves the structural stability but also makes the hybrid structure more conductive than the semiconducting SnO2. The calculated binding energies and diffusion barriers show that Li tends to insert into the interface, and a new pathway along the SnO2(110) direction for Li rapid diffusion with a low barrier is found.

First-principles plane-wave pseudopotential calculations have been performed to study the formation energy, electronic structure and optical properties of V-, N- and V/N-doped TiO2. The calculated results show that V:N defect pairs tend to bind to each other, which facilitates the enhancement of the concentration of N dopant compared with N-doped case. In addition, the incorporation of V into N-doped TiO2 lattice induces the appearance of passivated shallow acceptor and donor levels near the VBM and CBM, which would act as capture traps for photoexcited holes or electrons. Furthermore, the optical absorption coefficient spectra show that V/N codoping can enhance the optical absorption region, which can be attributed to lower excitation energy from the N 2p orbitals to V t2gt2g orbitals rather than a band gap narrowing. These findings give a reasonable explanation of recent experimental results.

Total energy and electronic structure calculations for Fe3Al1−xSix alloys with DO3-type structure have been performed to understand structural and magnetic properties using plane-wave pseudopotential method. With the increase of Si content, the equilibrium lattice constants reduce linearly and the saturation magnetic moments per unit cell of Fe3Al1−xSix alloys decrease, which are in good agreement with the experimental results. To illustrate the origin of magnetism of Fe3Al1−xSix alloys, spin-polarized density of states and local magnetic moments are investigated. The results show that the difference in local moments of Fe(A,C) and Fe(B) is due to different local environment in DO3-type structure and the relative affinity of Si and Al for Fe. In addition, to analyze the effect of Si on the magnetism of Fe3Al, the Mulliken charge and bond population are used to study the charge transfer and the bonding types, respectively. The calculated results give a reasonable explanation of recent experimental findings.

Powder-sintered technology for manufacturing thick metal hydride electrodes has been investigated. A slurry consisting of hydrogen storage alloy mixed with fine nickel powder and some additives was pasted onto porous metal substrate. The results with porous metal substrates were compared with those obtained with nickel foam, perforated nickel strips, perforated copper foil and copper mesh. The effect of sintering parameters on the electrochemical properties of the sintered metal hydride electrode has been characterized. The results show that activation and high-rate dischargeability of the sintered hydride electrode was strongly improved compared to that obtained with conventional pasting processes.

Nickel cobalt sulphides with different stoichiometric nickel and cobalt contents have been synthesized and used as pseudocapacitive materials for supercapacitors. The as-employed polyol method is robust enough to use a one-pot synthesis of nickel cobalt sulphides with similar porous morphology and crystal structure at different nickel and cobalt ratios. The electrochemical performance of the nickel cobalt sulphides can be easily tuned by the varying the Ni and Co content. Owing to the combined contributions from both Ni and Co ions, the bimetallic Ni–Co sulphides show a superior pseudocapacitance compared to monometallic Co and Ni sulphides in terms of high specific capacitance, excellent rate capability and long cycle stability. In particular, the Ni1.5Co1.5S4 sample shows the highest specific capacitance of 1093 F g−1 at 1 A g−1, superior rate capability of 69% capacitance retention after a 50-fold increase in current densities, and longer cycle stability with increased specific capacitance of 108% of capacitance retention after 2000 cycles. In addition, the Ni1.5Co1.5S4 was also used to assemble an asymmetric supercapacitor with reduced graphene oxide and attains excellent capacitive performance with high specific capacitance (113 F g−1 at 1 A g−1), high energy density (37.6 W h kg−1 at 775 W kg−1) and high power density (23.25 kW kg−1 at 17.7 W h kg−1).

In situ polymerization of 3,4-ethylenedioxythiophene (EDOT) is achieved on the surface of 2D Ti3C2Tx MXene without using any oxidant, resulting in improved lithium ion storage capability of Ti3C2Tx/poly-EDOT hybrids. A combined experimental and theoretical study revealed the mechanism of charge-transfer-induced polymerization of EDOT, which can be extended to other similar polymers.

Two-dimensional (2D) transition-metal (TM) compound nanomaterials, due to their high-surface-area and large potential charge capability of TM atoms, have been widely investigated as electrochemical capacitors. However, the understanding of charge-storage mechanisms of 2D transition-metal compounds as electrode materials is still limited. In this study, using density functional theory computations, we systematically investigate the electrochemical properties of monolayer VS2 and V2C. Their electronic structures show a significant electron storage capability of around 0.25 V, referenced to the standard hydrogen electrode, and indicate redox pseudocapacitance characteristics as cathodes. The different charge densities visually confirm that excess electrons tend to localize in the vanadium atoms nearby contact-adsorbed Li ions, corresponding to the redox of vanadium atoms. In contrast, only the electric double layer acts as a charge-storage mechanism in the V2C monolayer. However, the O saturation would induce redox pseudocapacitance in the V2C monolayer. Furthermore, the calculated metallic behavior and low Li ion diffusion barriers substantiate that V2C and VS2 monolayers would manifest low resistance in the charging process. Our findings provide insights for the different charge-storage mechanism of VS2 and V2C monolayers.

Two-dimensional transition metal dichalcogenides (TMDs) have been widely considered as potential hydrogen evolution reaction (HER) catalysts because of their low cost and good electrochemical stability in acid conditions. The mechanism of hydrogen adsorption on TMDs plays an important role in optimizing HER activity. In this research, a series of TMDs (MX2, M = Co, Cr, Fe, Mn, Mo, Nb, Ni, Re, Sc, Tc, Ti, V, W, Zr, and X = S, Se, Te) in 2H- and 1T-phases were investigated using density functional theory to determine the relationship between hydrogen adsorption free energy (ΔGH) and electronic structure using a simple descriptor. The results showed a positive linear relation between ΔGH and the work required of the H electron to fill the unoccupied electronic states of the TMDs. Based on such linear relationships, the various defects (B-, C-, N-, O-, F-, P-, Se-doping and S-vacancy) were used to activate the inert basal planes of the 2H-phase molybdenum disulfide, which can introduce impurity states in the lower energy level to effectively accommodate the H electron. Furthermore, HER activity can be further optimized with the increasing concentration of the defects. These findings provide a practicable map of the HER performances, as well as indicating an appropriate direction for optimizing HER activity.