Hang Chu

Controlled synthesis of Mo-doped Ni3S2 nano-rods: an efficient and stable electro-catalyst for water splitting

Fabrication of stable, efficient, and inexpensive bifunctional electro-catalysts for water splitting has become increasingly attractive. Herein, for the first time, the direct growth of Mo-doped Ni3S2 on Ni foams using sodium molybdate as the Mo source at different temperatures is demonstrated. Effects of temperature on the morphology and water splitting performance of Mo-doped Ni3S2 were discussed in detail. It is found that the atomic stoichiometric ratios of Mo and Ni can be controlled by the adjusting of reaction temperature, while the obtained electro-catalysts demonstrate various morphologies, capacitances and chemisorption free energies of hydrogen, which lead to different current densities and hydrogen evolution efficiencies. The electro-catalyst synthesized at 200 °C (200-SMN/NF) demonstrates the best regular morphology and electrochemical properties. When employed in oxygen evolution reactions, 200-SMN/NF demonstrates a low over-potential of 180 mV at 100 mA cm−2. Adapting it as a bifunctional electro-catalyst, a current density of 10 mA cm−2 at a very low cell voltage of 1.53 V and cycling lifespan of more than 15 h was delivered. Further results indicated elevated hydrogen evolution reaction activity, consisting of a moderate 278 mV over-potential at a 100 mA cm−2 hydrogen production current density, a small 72.9 mV dec−1 Tafel slope, and a superior current density compared to that of precious catalyst Pt/C (40%) after −0.53 V. These results underscore the fact that 200-SMN/NF is a high-performance, precious-metal-free electro-catalyst, and provide the foundation for exciting opportunities in water splitting applications.

Controlled Electrodeposition Synthesis of Co–Ni–P Film as a Flexible and Inexpensive Electrode for Efficient Overall Water Splitting

Synthesis of highly efficient and robust catalysts with earth-abundant resources for overall water splitting is essential for large-scale energy conversion processes. Herein, a series of highly active and inexpensive Co-Ni-P films were fabricated by a one-step constant current density electrodeposition method. These films were demonstrated to be efficient bifunctional catalysts for both H2 and O2 evolution reactions (HER and OER), while deposition time was deemed to be the crucial factor governing electrochemical performance. At the optimal deposition time, the obtained Co-Ni-P-2 catalyst performed remarkably for both HER and OER in alkaline media. In particular, it requires -103 mV overpotential for HER and 340 mV for OER to achieve the current density of 10 mA cm-2, with corresponding Tafel slopes of 33 and 67 mV dec-1. Moreover, it outperforms the Pt/C//RuO2 catalyst and only needs -160 mV (430 mV) overpotential for HER (OER) to achieve 200 mA cm-2 current density. Co-Ni-P electrodes were also conducted for the proof-of-concept exercise, which were proved to be flexible, stable, and efficient, further opening a new avenue for rapid synthesis of efficient, flexible catalysts for renewable energy resources.

Novel NiCo2S4@reduced graphene oxide@carbon nanotube nanocomposites for high performance supercapacitors

We present a simple one-step solvothermal method for in situ growth of nickel cobalt sulfide (NiCo2S4) nanoparticles on the reduced graphene oxide (rGO) sheets and carbon nanotube (CNT) without adding any surfactant. The NiCo2S4@rGO@CNT structure was fabricated successfully with the hydrothermal method. When serving as an electrochemical supercapacitor electrode material, the NiCo2S4@rGO@CNT delivers a highly improved specific capacitance of 1242.51 F g−1 at the current density of 2.0 A g−1, as compared with that of pure NiCo2S4 (519.51 F g−1) counterpart. Its durability is mainly owing to the synergistic effects of NiCo2S4, rGO and CNT, further presenting potential applications in energy storage.

Anion-exchange engineering of cookie-like Bi2S3/Bi2MoO6 heterostructure for enhanced photocatalytic activities and gas-sensing properties

Developing efficient visible-light-driven photocatalysts will advance alternative energy technologies, ultimately curbing the environmental pollution associated with fossil fuels. In this work, Bi2S3/Bi2MoO6 photocatalysts with a heterogeneous cookie-like structure were prepared for the first time by in-situ anion exchange at relatively low temperatures. The catalysts exhibited enhanced photocatalytic activity, which we attributed to the photocurrent response, a diminished recombination rate of photogenerated electron-hole pairs, and the existence of a large heterojunction interface. These governing factors were discerned by photoelectrochemical measurements, calculated energy band positions and photoluminescence spectra. Bi2S3/Bi2MoO6 nanocomposites also exhibit better performance in response to gas than bare Bi2MoO6 according to gas sensing tests. Our work, in relaying a feasible method to synthesize Bi2S3/Bi2MoO6-based heterojunction superstructures, and documents a universal preparation method of synthetic heterogeneous complexes, and provides necessary groundwork for the development of next generation semiconductor photocatalytic technology and gas sensor.

Integrated Energy Aerogel of N,S-rGO/WSe2/NiFe-LDH for Both Energy Conversion and Storage

High-performance active materials for energy-storage and energy-conversion applications require a novel class of electrodes: ones with a structure conducive to conductivity, large specific surface area, high porosity, and mechanical robustness. Herein, we report the design and fabrication of a new ternary hybrid aerogel. The process entails an in situ assembly of 2D WSe2 nanosheets and NiFe-LDH nanosheets on a 3D N,S-codoped graphene framework, accomplished by a facile hydrothermal method and electrostatic self-assembly technology. The obtained nanocomposite architecture maximizes synergistic effects among its three 2D-layer components. To assess the performance of this hybrid material, we deployed it as an advanced electrode in overall water splitting and in a supercapacitor. Results in both scenarios attest to its excellent electrochemical properties. Specifically, serving as a catalyst in an oxygen evolution reaction, our nanocomposite requires overpotentials of 1.48 and 1.59 V to obtain current densities of 10 and 100 mA cm-2, respectively. The hybrid material also efficiently electrocatalyzes hydrogen evolution reactions in base solution, necessitating overpotentials of -50 and -237 mV for current densities of 1.0 and 100 mA cm-2, respectively. The 3D hybrid, when applied to a symmetric supercapacitor device, achieves 125.6 F g-1 capacitance at 1 A g-1 current density. In summary, our study elucidates a new strategy to maximize efficiency via synergetic effects that is likely applicable to other 2D materials.

Large-scale controlled synthesis of porous two-dimensional nanosheets for the hydrogen evolution reaction through a chemical pathway

Molybdenum disulfide (MoS2) is an extensively studied promising non-noble catalyst because of its remarkable performance for the hydrogen evolution reaction (HER). However, the primary factors that affect its catalytic activity have not been analysed comprehensively and quantitatively; this impedes the further design and development of MoS2-based electrocatalysts. Herein, using novel porous MoS2 nanosheets prepared via a controlled and scalable KOH-assisted exfoliation pathway, we methodically studied the contributions of bore edge active sites to the catalytic activity towards the HER. To make the preparation safer, 2H-MoS2 instead of 1T-MoS2 that needs to be prepared with butyllithium has been chosen to synthesize porous MoS2 nanosheets. A comparative study revealed that the overpotential of porous MoS2 nanosheets exhibited an extreme point, which was predominantly due to the different densities of bore edge active sites derived from different quantities of KOH. Amazingly, the HER performance of MoS2 nanosheets experienced the most obvious improvement after these nanosheets were treated with 37.5 wt% KOH. A series of tests and density functional theory calculations were conducted to explain the experimental results, which were consistent with each other. Furthermore, this method has been proven to be universal since porous WS2 and SnS2 nanosheets have been prepared by the same route. This study presents novel insights and reveals a new, controlled, and scalable chemical avenue for programming electrocatalysts based on MoS2 or other layered materials.

Template-free solvothermal preparation of ternary FeNi2S4 hollow balloons as RuO2-like efficient electrocatalysts for the oxygen evolution reaction with superior stability

Ternary transition metal sulfide hollow structures are promising materials for energy storage and conversion applications. However, their synthetic methods are rather complicated and time-consuming. In this work, for the first time, with FeNi2S4 hollow balloons (FNSH) as the quintessential example, a template-free solvothermal method has been developed to synthesize ternary transition metal sulfide hollow balloons. The corresponding formation mechanism is deeply investigated, and the main factors affecting the formation of the hollow balloons are confirmed to be reaction time and temperature. FNSH/graphene aerogel (FNSH/GA) is further fabricated, which has been proven to be a highly efficient electrocatalyst for the oxygen evolution reaction (OER). The FNSH/GA exhibits excellent performance for the OER with a small overpotential of 273 mV at 10 mA cm−2 in 1.0 M KOH, which is even superior to the typical RuO2 catalyst. Furthermore, the FNSH/GA maintains the excellent performance for the OER even after a 10 h chronoamperometric test. Moreover, the synthetic method is universal and can be widely extended to other ternary transition metal sulfide hollow balloons.

Dual-Functional Starfish-like P-Doped Co–Ni–S Nanosheets Supported on Nickel Foams with Enhanced Electrochemical Performance and Excellent Stability for Overall Water Splitting

Dual-functional electrocatalysts have recently been reported to improve the conversion and storage of energy generated from overall water splitting in alkaline electrolytes. Herein, for the first time, a shape-controlled synthesis of starfish-like Co-Ni-S nanosheets on three-dimensional (3D) hierarchically porous nickel foams (Co-Ni-S/NF) via a one-step hydrothermal method was developed. The influence of reaction time on the nanosheet structure and properties was intensively studied. After 11 h reaction, the Co-Ni-S/NF-11 sample displays the most regular structure of nanosheets and the most outstanding electrochemical properties. As to water splitting, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) required overpotentials of 284.3 and 296 mV, respectively, to provide a current density of 100 mA cm-2. The marvelous electrochemical performance can be attributed to the conductive networks of 3D layered porous nickel skeletons that are highly interconnected, which provided a large specific area and highly active sites. To further enhance the electrochemical performances of the electrocatalyst, the influence of the doping of the P element was also studied. The results proved that the P-doped Co-Ni-S/NF maintains the starfish structure and demonstrates outstanding properties, providing a current density of 100 mA cm-2 with only 187.4 and 292.2 mV overpotentials for HER and OER, respectively. It exhibited far more excellent properties than reported dual-functional electrocatalysts. Additionally, when used as an overall water-splitting catalyst, P-Co-Ni-S/NF can provide a 10 mA cm-2 current density at a given cell voltage of 1.60 V in 1 M KOH, which is competitive to the best-known electrocatalysts, with high long-term stability.