Synthesis of Sub-2 nm Iron-Doped NiSe2 Nanowires and Their Surface-Confined Oxidation for Oxygen Evolution Catalysis

Cyclic voltammetry electrodynamic deposition of Co9-xMnxS8 nanosheet arrays for electrocatalytic hydrogen evolution

Room-temperature cyclic voltammetry (CV) electrodynamic deposition is proposed herein for the first time to deposit Co_9-xMn_xS₈ nanosheet arrays. The incorporation of Mn spin states induces atomic distortion, which contributes to more active edge sites and the fine-tuning of the electronic structure. The Co_9-xMn_xS₈ (x = 4.5) catalyst exhibits enhanced catalytic activity toward the hydrogen evolution reaction (HER) when compared with pristine Co₉S₈. This work offers a promising strategy for the design of highly efficient HER electrocatalysts.

Highly Efficient Oxygen Evolution Reaction Enabled by Phosphorus Doping of the Fe Electronic Structure in Iron-Nickel Selenide Nanosheets

The electronic structure of active sites is critically important for electrochemical reactions. Here, the authors report a facile approach to independently regulate the electronic structure of Fe in Ni0.75 Fe0.25 Se2 by P doping. The resulting electrode exhibits superior catalytic performance for the oxygen evolution reaction (OER) showing a low overpotential (238 mV at 100 mA cm-2 , 185 mV at 10 mA cm-2 ) and an impressive durability in an alkaline medium. Additionally, the mass activity of 328.19 A g-1 and turnover frequency (TOF) of 0.18 s-1 at an overpotential of 500 mV are obtained for P─Ni0.75 Fe0.25 Se2 which is much higher than that of Ni0.75 Fe0.25 Se2 and RuO2 . This work presents a new strategy for the rational design of efficient electrocatalysts for OER.

Lattice-Strain Engineering of Homogeneous NiS0.5 Se0.5 Core-Shell Nanostructure as a Highly Efficient and Robust Electrocatalyst for Overall Water Splitting

Developing highly-efficient non-noble-metal electrocatalysts for water splitting is crucial for the development of clean and reversible hydrogen energy. Introducing lattice strain is an effective strategy to develop efficient electrocatalysts. However, lattice strain is typically co-created with heterostructure, vacancy, or substrate effects, which complicate the identification of the strain-activity correlation. Herein, a series of lattice-strained homogeneous NiS_x Se_{1- x} nanosheets@nanorods hybrids are designed and synthesized by a facile strategy. The NiS_0.5 Se_0.5 with ≈2.7% lattice strain exhibits outstanding activity for hydrogen and oxygen evolution reaction (HER/OER), affording low overpotentials of 70 and 257 mV at 10 mA cm^-2 , respectively, as well as excellent long-term durability even at a large current density of 100 mA cm^-2 (300 h), significantly superior to other benchmarks and the precious metal catalysts. Experimental and theoretical calculation results reveal that the generated lattice strain decreases the metal d-orbital overlap, leading to a narrower bandwidth and a closer d-band center toward the Fermi level. Thus, NiS_0.5 Se_0.5 possesses favorable H* adsorption kinetics for HER and lower energy barriers for OER. This work provides a new insight to regulate the lattice strain of advanced catalyst materials and further improve the performance of energy conversion technologies.

Carbon Dioxide Conversion with High-Performance Photocatalysis into Methanol on NiSe2/WSe2

Climate change has been recognized as a threatening environmental problem around the world. CO2 is considered to be the main component of greenhouse gas. By using solar energy (light energy) as the energy source, photocatalytic conversion is one of the most effective technologies to reveal the clean utilization of CO2. Herein, using sodium tungstate, nickel nitrate, and selenium powder as the main raw materials, the high absorption and utilization of WSe2 for light energy and the high intrinsic conductivity of NiSe2 were combined by a hydrothermal method to prepare NiSe2/WSe2 and hydrazine hydrate as the reductant. Then, high-performance NiSe2/WSe2 photocatalytic material was prepared. The characterization results of XRD, XPS, SEM, specific surface area, and UV-visible spectroscopy show that the main diffraction peak of synthesized NiSe2/WSe2 is sharp, which basically coincides with the standard card. After doping NiSe2, the morphology of WSe2 was changed from a flake shape to smaller and more trivial crystal flakes, which demonstrates richer exposed edges and more active sites; the specific surface area increased from 3.01 m2 g−1 to 8.52 m2 g−1, and the band gap becomes wider, increasing from 1.66 eV to 1.68 eV. The results of a photocatalytic experiment show that when the prepared NiSe2/WSe2 catalyst is used to conduct photocatalytic reduction of CO2, the yield of CH3OH is significantly increased. After reaction for 10 h, the maximum yield could reach 3.80 mmol g−1, which presents great photocatalytic activity.

Single atom tungsten doped ultrathin α-Ni(OH)2 for enhanced electrocatalytic water oxidation

Electrocatalytic water oxidation is a rate-determining step in the water splitting reaction. Here, we report one single atom W⁶⁺ doped Ni(OH)₂ nanosheet sample (w-Ni(OH)₂) with an outstanding oxygen evolution reaction (OER) performance that is, in a 1 M KOH medium, an overpotential of 237 mV is obtained reaching a current density of 10 mA/cm². Moreover, at high current density of 80 mA/cm², the overpotential value is 267 mV. The corresponding Tafel slope is measured to be 33 mV/dec. The d⁰ W⁶⁺ atom with a low spin-state has more outermost vacant orbitals, resulting in more water and OH⁻ groups being adsorbed on the exposed W sites of the Ni(OH)₂ nanosheet. Density functional theory (DFT) calculations confirm that the O radical and O-O coupling are both generated at the same site of W⁶⁺. This work demonstrates that W⁶⁺ doping can promote the electrocatalytic water oxidation activity of Ni(OH)₂ with the highest performance.

Electrocatalytic water splitting for hydrogen and oxygen generation provides an attractive path to obtain clean energy, but the half reaction of oxygen evolution remains the bottleneck for the progress. Here, the authors show single atom tungsten doped ultrathin α-Ni(OH)₂ exhibits enhanced performance in electrocatalytic water oxidation.

Ligand-Solvent Compatibility: The Unsung Hero in the Digestive Ripening Story

Digestive ripening (DR) is a process where a polydisperse nanocrystal (NC) system is converted into a monodisperse one with the aid of thermal heating of NCs in the presence of an excess surface-active organic ligand called digestive ripening agent (DRA) and a solvent. Here, we demonstrate that the solvent-DRA compatibility influences the final size and size distribution of the NCs in a significant manner. Accordingly, in this study, using the DR of gold NCs as the test case with alkanethiol (decanethiol/C10HT) and fluorinated thiol (1 H,1 H,2 H,2 H-perfluorodecanethiol/C10FT) as DRA's and toluene and α,α,α-trifluoro-toluene (TFT) and their combination as solvents, we clearly establish that alkanethiols result in best-quality NCs after DR in toluene while the fluorinated thiols provide reasonably monodispersed NCs in TFT. Our results also ascertain that even when DR is carried out in a mixture of solvents, as long as the compatible solvent is the major component, the DR process results in reasonably monodisperse NCs. As soon as the amount of uncompatible solvent exceeds a threshold limit, there is perceptible increase in the polydispersity of the NCs. We conclude that the polarity of the solvent, which affects the buildup of ligated atoms/clusters, plays a key role in controlling the size distributions of the NCs.

Bifunctional Electrocatalysts for Overall Water Splitting from an Iron/Nickel-Based Bimetallic Metal-Organic Framework/Dicyandiamide Composite

Pyrolysis of a bimetallic metal-organic framework (MIL-88-Fe/Ni)-dicyandiamide composite yield a Fe and Ni containing carbonaceous material, which is an efficient bifunctional electrocatalyst for overall water splitting. FeNi₃ and NiFe₂ O₄ are found as metallic and metal oxide compounds closely embedded in an N-doped carbon-carbon nanotube matrix. This hybrid catalyst (Fe-Ni@NC-CNTs) significantly promotes the charge transfer efficiency and restrains the corrosion of the metallic catalysts, which is shown in a high OER and HER activity with an overpotential of 274 and 202 mV, respectively at 10 mA cm^-2 in alkaline solution. When this bifunctional catalyst was further used for H₂ and O₂ production in an electrochemical water-splitting unit, it can operate in ambient conditions with a competitive gas production rate of 1.15 and 0.57 μL s^-1 for hydrogen and oxygen, respectively, showing its potential for practical applications.