Classification of Supertransitive 2-Webs on Surfaces

We obtain a complete classification of supertransitive 2-webs on a closed oriented surface of genus p 1 in terms of asymptotic directions of leaves on the universal covering surface and represent all topological classes of 2-webs by means of compatible pairs of transversal perfect geodesic laminations.     

On Topological Classification of A-Diffeomorphisms of Surfaces

The paper contains a survey of works of the author devoted to topological classification of A-diffeomorphisms on surfaces. In the survey, the following problems are considered.(1) Topological classification of one-dimensional attractors and repellers by means of automorphisms of fundamental group of supports and its representation by hyperbolic homeomorphisms is given.(2) Asymptotic behavior of preimages of stable and unstable manifolds of points belonging to exteriorly situated one-dimensional basic sets on universal covering is investigated (it is proved that if A-diffeomorphism is structurally stable, then these preimages boundedly deviate from geodesic with the same asymptotic direction).(3) Topological classification of an important class of structurally stable diffeomorphisms is obtained.     

Cu2S/Ni3S2 ultrathin nanosheets on Ni foam as a highly efficient electrocatalyst for oxygen evolution reaction

It is an inevitable choice to find efficient and economically-friendly electrocatalysts to reduce the high overpotential of oxygen evolution reaction (OER), which is the key to improve the energy conversion efficiency of water splitting. Herein, we synthesized Cu₂S/Ni₃S₂ catalysts on nickel foam (NF) with different molar ratios of Ni/Cu by a simple two-step hydrothermal method. Cu₂S/Ni₃S₂-0.5@NF (CS/NS-0.5@NF) effectively reduces the overpotential of OER, displaying small overpotentials (237 mV@100 mA cm^?2 and 280 mV@500 mA cm^?2) in an alkaline solution, along with a low Tafel slope of 44 mV dec^?1. CS/NS-0.5@NF also presents an excellent durability at a relatively high current density of 100 mA cm^?2 for 100 h. The excellent performance is benefited by the prominent structural advantages and desirable compositions. The nanosheet has a high electrochemical active surface area and the porous structure is conducive to electrolyte penetration and product release. This work provides an economically-friendly Cu-based sulfide catalyst for effective electrosynthesis of OER.     

Ni3S2-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

One-step synthesis of Ni3S2 nanowires at low temperature as efficient electrocatalyst for hydrogen evolution reaction

Ni₃S₂ is an emerging cost-effective catalyst for hydrogen generation. However, a large amount of reported Ni₃S₂ was synthesized via multi-step approaches and few were fabricated based on the one-step strategies. Herein, we report a facile one-step low-temperature synthesis of Ni₃S₂ nanowires (NWs). In this strategy, a resin containing sulfur element is recommended as a sulfur resource to form Ni₃S₂ NWs. It presents a plausible explanation on the vapor–solid–solid (VSS) growth mechanism according to the results of this experiment and reported in literature that has been published. The Ni₃S₂ NW exhibits a potential ∼199 mV at 10 mA cm⁻² and the long-term durability over 30 h at 20 mA cm⁻² HER operation, better than other reported Ni₃S₂. More importantly, according to replace transition metal foam as the initial metal, other transition metal sulfide can be readily synthesized via this original approach.     

Zeolitic-imidazolate frameworks-derived Co3S4/NiS@Ni foam heterostructure as highly efficient electrocatalyst for oxygen evolution reaction

Transition metals sulfide-based nanomaterials have recently received significant attention as a promising cathode electrode for the oxygen evolution reaction (OER) due to their easily tunable electronic, chemical, and physical properties. However, the poor electrical conductivity of metal-sulfide materials impedes their practical application in energy devices. Herein, firstly nano-sized crystals of cobalt-based zeolitic-imidazolate framework (Co-ZIF) arrays were fabricated on nickel-form (NF) as the sacrificial template by a facile solution method to enhance the electrical conductivity of the electrocatalyst. Then, the Co₃S₄/NiS@NF heterostructured arrays were synthesized by a simple hydrothermal route. The Co-ZIFs derived Co₃S₄ nanosheets are grown successfully on NiS nanorods during the hydrothermal sulfurization process. The bimetallic sulfide-based Co₃S₄/NiS@NF-12 electrocatalyst demonstrated a very low overpotential of 119 mV at 10 mA cm^?2 for OER, which is much lower than that of mono-metal sulfide NiS@NF (201 mV) and ruthenium-oxide (RuO₂) on NF (440 mV) electrocatalysts. Furthermore, the Co₃S₄/NiS@NF-12 electrocatalyst showed high stability during cyclic voltammetry and chronoamperometry measurements. This research work offers an effective strategy for fabricating high-performance non-precious OER electrocatalysts.     

Construction of NiCo2S4/Ni3S2 nanoarrays on Ni foam substrate as an enhanced electrode for hydrogen evolution reaction and supercapacitors

Developing a multifunctional and sustainable electrode material for hydrogen evolution reaction and supercapacitors is a highly feasible avenue for producing the high energy density and renewable energies. In our study, nanostructured NiCo₂S₄/Ni₃S₂/NF nanoarrays are rational developed in experiments via a simple hydrothermal reaction. Ascribed to the 3D nanostructured NiCo₂S₄/Ni₃S₂ with numerous exposure active sites and large contact areas for the electrolyte, the binder-free feature of NiCo₂S₄/Ni₃S₂/NF facilitates a low charge transfer resistance, as well as the synergetic effect of NiCo₂S₄ and Ni₃S₂. The obtained electrocatalyst showed ultrahigh electrocatalytic activity with an overpotential of 111 mV at 10 mA cm⁻² and a Tafel slope of 57 mV dec⁻¹. In addition, the electrode showed an area specific capacity of 6.13 F cm⁻² at 10 mA cm⁻² and superior rate capability (2.72 F cm⁻² at 80 mA cm⁻²), accompanied by excellent cycling stability. This results presented in our work can provide an effective strategy for rational design of other hybrid materials with excellent electrochemical performance in the application of electrocatalysis and supercapacitors.     

Crossed FeCo2S4 nanosheet arrays grown on 3D nickel foam as high-efficient electrocatalyst for overall water splitting

Great efforts in developing low-cost, highly efficient and stable electrocatalysts are to tune the chemical compositions and morphological characteristics for enhancing efficiency of water splitting. In this communication, FeCo₂S₄ nanosheet was grown in situ on nickel foam (FeCo₂S₄/NF) via a facile hydrothermal sulfidization method and served as a high-efficient bifunctional electrocatalyst for overall water splitting. As-synthesized FeCo₂S₄/NF self-supported electrode delivers 20 mA cm^?2 at an overpotential of 259 mV toward OER and 10 mA cm^?2 at an overpotential of 131 mV toward HER in alkaline media. Moreover, when used as both anode and cathode in a two-electrode electrolyzer, only a small cell voltage of 1.541 V is needed to afford a current density of 10 mA cm^?2 for overall water splitting. Bifunctional electrode FeCo₂S₄/NF also revealed a distinguished electrochemical durability during a 12 h stability test at 1.63 V, which would provide a promising water splitting installation for commercial hydrogen production.     

Ultrathin-layered MoS2 hollow nanospheres decorating Ni3S2 nanowires as high effective self-supporting electrode for hydrogen evolution reaction

High-activity and cost-effective transition metal sulfides (TMSs) have attracted tremendous attention as promising catalysts for hydrogen evolution reaction (HER). However, a significant challenge is the simultaneous construction of abundant electrochemical active sites and the fast electronic transmission path to boost a high-efficient HER. Herein, we demonstrate a facile one-step hydrothermal preparation of MoS₂ hollow nanospheres decorating Ni₃S₂ nanowires supported on Ni foam (NF), without any other additional template, surfactant or annealing. In this three-dimensional (3D) heterostructure, the ultrathin-layered MoS₂ hollow nanospheres contribute to the promotion of the total surface area and the electrochemical active sites. Meanwhile, the Ni₃S₂ nanowires are beneficial to the high electrical conductivity. Consequently, the optimized MoS₂/Ni₃S₂/NF-200-24 electrocatalyst presents an extremely superior HER activity to that of individual MoS₂/NF and Ni₃S₂/NF. The HER overpotentials are 85 mV at 10 mA cm⁻² and 189 mV at 100 mA cm⁻², which are also comparable with the state-of-the-art 20% Pt/C/NF electrode at both low and high current.     

Reduced graphene oxide composite Ni3S2 microspheres grown directly on nickel foam as an efficient electrocatalyst for OER

The development of cheap, high-efficiency, and stable oxygen evolution reaction (OER) electrocatalysts is a current research hotspot. In this work, reduced graphene oxide (rGO) composite Ni₃S₂ microspheres grown directly on nickel foam (Ni₃S₂-rGO/NF) were prepared by tube furnace calcination and hydrothermal method. The Ni₃S₂-rGO/NF had excellent OER catalytic activity and stability with an overpotential of 303 mV at the current density of 100 mA cm⁻², which was 100 mV lower than that of Ni₃S₂/NF, and its Tafel slope was as low as 23 mV·dec⁻¹. The main reason for enhancing OER activity of the Ni₃S₂-rGO/NF is due to synergistic effect of Ni₃S₂ microspheres and rGO, which inhibited the production of NiS and refined the micron size of Ni₃S₂. This work offers a new method for developing stable and efficient OER catalysts.     

Co3S4 nanosheets on Ni foam via electrodeposition with sulfurization as highly active electrocatalysts for anion exchange membrane electrolyzer

Co₃S₄ nanosheets on Ni foam (NS/NF) were prepared by sulfurization for various time after calcination of electrodeposited Co(OH)₂. In our FE-SEM images, we observed that Co₃S₄ NS was vertically, or obliquely, deposited on the Ni foam. As a result, the structure contained more active sites, and active sites were highly accessible to the electrolyte for the hydrogen evolution reaction (HER). Furthermore, results of XPS and XRD analysis confirmed S-conversion from Co₃O₄ to Co₃S₄ during sulfurization. 3-Co₃S₄ NS/NF with sulfurization for 3 h exhibited the highest sulfur content, while Co₃S₄ began to desulfurize to Co₉S₈ after sulfurization for 4 h. The 3-Co₃S₄ NS/NF electrocatalyst showed a lowest overpotential of 93 mV at −10 mA/cm², with a Tafel slope of −55.1 mV/dec in N₂-purged 1 M KOH. Also, the single cell anion exchange membrane water electrolyzer (AEMWE) showed a high current density of 431 mA/cm² with cell voltage 2.0 V_cell at 40–45 °C.     

Ni3S2@Co(OH)2 heterostructures grown on Ni foam as an efficient electrocatalyst for water oxidation

It is of high significance to design robust, low-cost and stable electrocatalysts for the oxygen evolution reaction (OER) under alkaline medium. In this communication, we present the exploitation of Ni₃S₂@Co(OH)₂ which directly grown on nickel foam (Ni₃S₂@Co(OH)₂/NF) as a robust and stable electrocatalyst for OER. Such Ni₃S₂@Co(OH)₂/NF-5h demanding overpotential of only 290 mV is less than that of Ni₃S₂@Co(OH)₂/NF-10h (310 mV), Ni₃S₂@Co(OH)₂/NF-2h (320 mV) and Ni₃S₂/NF(350 mV), respectively, to drive a geometrical catalytic current density of 35 mA cm⁻², which is also better than that of noble metal catalyst IrO₂/NF (320 mV). In addition, the Ni₃S₂@Co(OH)₂/NF-5h presents a superior long-term electrocatalytic stability, keeping its activity at 26 mA cm⁻² for 40 h.     

Fabrication of In2O3/In2S3 heterostructures for enhanced photoelectrochemical performance

Constructing core/shell heterojunction has always been an effective strategy for photoelectrochemical (PEC) water splitting owing to special morphology characterization and band structure. Herein, we synthesized a series of In₂O₃/In₂S₃ core/shell structure photoanodes via a simple two-step hydrothermal method to improve the PEC performance of In₂O₃. Various methods were employed to investigate the influence of sulfurization time on the morphologies, microstructures, photoelectrochemical properties and band structures of the as-prepared photoanodes. The results indicated that the In₂O₃/In₂S₃-5 possessed stronger visible light absorption, faster charge transfer rate and higher electron carrier density, which resulted in an excellent PEC performance. Under visible light irradiation, the photocurrent density of the In₂O₃/In₂S₃-5 photoanode reached 0.53 mA cm⁻² at 1.23 V vs RHE in 1 M NaOH solution, which was about twice as high as that of the pristine In₂O₃. Furthermore, the onset potential of the In₂O₃/In₂S₃-5 photoanode had an obvious negative shift (~200 mV) when compared to the pure In₂O₃ nanorod photoanode.     

Coal as an option for power generation in U.S. territories of the Pacific

General considerations relating to the use of coal in U.S. territories and trust territories of the Pacific suggest that coal is a viable option for power generation. Future coal supplies, principally from Australia and the west coast of America, promise to be more than adequate, but the large bulk carriers (100,000–200,000 dwt) expected to serve Far Eastern markets will probably not be able to land coal directly in the territories because of inadequate port facilities. Hence, smaller than Panamax-class vessels (60,000 dwt) or some arrangement utilizing self-loading barges or lighters would have to be used. Except for Guam, with peak power requirements on the order of 175 MW_e, most territories have current, albeit inadequate, installations of 1–25 MW_e. Turnkey, conventional-coal-fired, electrical-power generating systems are available in that size range. Fluidized bed combustion is another option currently being commercialized. Its use has clear environmental advantages and a variety of fuels (e.g. coal, heavy oils, biomass, etc.) may be employed without interruption of power generation. U.S. environmental laws, such as the Clean Air Act, are now applicable to Guam and American Samoa; the trust territories are exempt. When United Nations trusteeship terminates, the current unclear position of the Commonwealth of the Northern Marianas will cease and the laws will probably apply. Nonetheless, the small power requirements of many small islands will qualify for exemption from the New Source Performance Standards called for in the Clean Air Act.The principal problems with coal use in the territories, apart from the shallow draft of most harbors, are the limited amount of land available and the high capital costs associated with conversion. Ocean dumping of ash and sludge can be permitted under existing Environmental Protection Agency regulations, and barge-mounted power installations are not out of the question. The feasibility of converting from oil-fired to coal-fired electrical-power generating systems must be determined with site-specific information.     

Bimetallic-metal organic framework-derived Ni3S2/MoS2 hollow spheres as bifunctional electrocatalyst for highly efficient and stable overall water splitting

Herein, strongly coupled Ni₃S₂/MoS₂ hollow spheres derived from NiMo-based bimetal-organic frameworks are successfully synthesized for overall water splitting via a one-pot solvothermal method followed by sulfurization. A well-defined hollow spherical structure with a heterointerface between Ni₃S₂ and MoS₂ is constructed using solvothermal and sulfurization processes. Owing to their bimetallic heterostructure, porous hollow carbon structure with large surface area, and numerous exposed active sites, the Ni₃S₂/MoS₂ hollow spheres are found to be efficient electrocatalysts for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The heterostructured Ni₃S₂/MoS₂ hollow spheres show small overpotentials of 303 and 166 mV to reach a current density of 10 mA cm^?2 for the OER and HER in 1.0 M KOH, respectively. Furthermore, an overall water-splitting electrolyzer consisting of the Ni₃S₂/MoS₂ hollow spheres as both the anode and cathode requires a very low cell voltage of 1.62 V to drive a current density of 10 mA cm^?2 with outstanding long-term stability for 100 h. Our findings offer a new pathway for the design and synthesis of electrochemically advanced bifunctional catalysts for various energy storage and conversion applications.     

Constructing 3D hierarchical Zn0.2Cd0.8S microspheres for the improved visible-light-driven photocatalytic performance

Nanorod assembled three-dimensional (3D) hierarchical Zn_0.2Cd_0.8S microspheres were successfully prepared by a facile one-pot microwave hydrothermal approach using ethylenediamine (EN) as a template agent. The optimal 3D hierarchical Zn_0.2Cd_0.8S microspheres (ZCS-30) is obtained when adding 30 mL into the synthetic system, which can function as a highly active photocatalyst for H₂ evolution under visible-light irradiation with the wavelength of 420 nm, delivering an activity approximately 2.5 and 7.5 times higher than that of Zn_0.2Cd_0.8S nanoparticles and CdS counterparts, respectively, and giving an apparent quantum efficiency (AQE) of 7.4%. Furthermore, the ZCS-30 photocatalyst shows the good stability after the catalytic H₂ evolution for 15 h (5 cycles). In addition, the ZCS-30 photocatalyst exhibits the excellent degradation of Rhodamine B (Rh B) almost up to 80% under the visible light irradiation over a period of 150 min. The results demonstrate that ZCS-30 can serve as a promising visible-light-driven bifunctional photocatalyst for water splitting and dye degradation.     

Hydrothermal synthesis of pyramid-like In2S3 film for efficient photoelectrochemical hydrogen generation

Herein, the pyramid-like In₂S₃ film was synthesized for photoelectrochemical (PEC) hydrogen generation using a facile hydrothermal route for the first time. The hydrothermal time was crucial to determine the morphology and composition of the as-prepared films. When the hydrothermal time lasted for 24 h, the pyramid-like In₂S₃ film could be achieved. With the increased hydrothermal time, the photocurrent first increased and then decreased, and the pyramid-like In₂S₃ film prepared for 24 h showed the highest photocurrent. After an annealing treatment, the photocurrent could be improved to 2.7 mA cm^?2 at 0.78 V vs. RHE, and a substantial hydrogen generation was observed. Compared to previously reported In₂S₃ films with different morphologies, the pyramid-like In₂S₃ film exhibited better PEC property. It was revealed that the remaining In₂O₃ after the synthesis of In₂S₃ had a great influence on the PEC performance. The pyramid-like In₂S₃ film with a moderate amount of In₂O₃ induced an efficient charge separation and transfer, as well as good light-absorption ability, which led to the superior PEC performance. This work has demonstrated great potentials of the pyramid-like In₂S₃ film for PEC hydrogen generation and opened a promising avenue towards the design and fabrication of novel pyramid-like nanostructures.     

Ordered CoO/CMK-3 nanocomposites as the anode materials for lithium-ion batteries

A novel ordered mesoporous carbon hybrid composite, CoO/CMK-3, is prepared by an infusing method using Co(NO₃)₂·6H₂O as the cobalt source. The products are characterized by X-ray diffraction, transmission electron microscopy and N₂ adsorption-desorption analysis techniques. It is observed that the CoO nanoparticles are loaded in the channels of mesoporous carbon. The mesopore structure of CMK-3 is destroyed gradually with increasing of the CoO content. The electrochemical properties of samples as the anode materials for lithium-ion batteries are studied by galvanostatic method. The results show that the CoO/CMK-3 composites have higher reversible capacities (more than 700 mAh g⁻¹) and better cycle performance in comparison with the pure mesoporous carbon (CMK-3). Based on the above results, a mechanism is proposed to explain the reason of such a substantial improvement of electrochemical performance in the CoO/CMK-3 composites.