The development of earth-abundant, highly active, and corrosion-resistant electrocatalysts to promote the oxygen reduction reaction (ORR) and oxygen and hydrogen evolution reactions (OER/HER) for rechargeable metal-air batteries and water-splitting devices is urgently needed. In this work, Ni Se (0.5 ≤ x ≤ 1) nanocrystals with different crystal structures and compositions have been controllably synthesized and investigated as potential electrocatalysts for multifunctional ORR, OER, and HER in alkaline conditions. A novel hot-injection process at ambient pressure was developed to control the phase and composition of a series of NiSe by simply adjusting the added molar ratio of the nickel resource to triethylenetetramine. Electrochemical analysis reveals that NiSe nanocrystalline exhibits superior OER activity compared to its counterparts and is comparable to RuO in terms of the low overpotential required to reach a current density of 10 mA cm (330 mV), which may benefit from the pyrite-type crystal structure and Se enrichment in NiSe. For the ORR and HER, NiSe nanoparticles achieve the best performance including lower overpotentials and larger apparent current densities. Further investigations demonstrate that NiSe could not only provide an enhanced electrochemical active area but also facilitate electron transfer during the electrocatalytic process, thus contributing to the remarkable catalytic activity. As a practical application, the NiSe electrode enables rechargeable Zn-air battery with a considerable performance including a long cycling lifetime (200 cycles), high specific capacity (609 mA h g based on the consumed Zn), and low overpotential (0.75 V) at 10 mA cm. Meanwhile, the water-splitting cell setup with an anode of NiSe for the HER and a cathode of NiSe for the OER exhibits a considerable performance with low decay in activity of 12.9% under continuous polarization for 10 h. These results suggest the promising potential of nickel selenide nanocrystals as earth-abundant and high-performance electrocatalysts for metal-air batteries and alkaline water splitting.

for fossil fuels. Electrocatalytic water splitting is a promising and environmentally friendly approach for the generation of hydrogen gas with high quality and efficiency. [1][2][3][4] However, the electrochemical hydrogen production is seriously limited by the large overpotential and sluggish kinetics of both cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). [5][6][7] At present, noble metals (e.g., Pt, Ir, and Ru) based catalysts are regarded as the best electrocatalysts, but their scarcity, high cost, poor stability, and single functionalization for only HER (e.g., Pt) or OER (e.g., Ir, Ru) prevent their widespread commercialization. [8][9][10] Hence, it is highly desirable to develop low-cost and efficient bifunctional electrocatalysts for overall water splitting. [11][12][13] Transition metal chalcogenides (TMCs, M = Fe, Co, Ni, Mn, etc., and C = S, Se) have been extensively explored as electroactive materials to promote HER and OER processes owing to the advantages of cost-effective, high abundance, and intrinsically metallic properties. [14][15][16][17][18][19][20] Recently, the design strategies of highly active TMCs electrocatalysts are mainly focused on following principles: (i) anchoring catalysts on conductive substrates to enable a highly dispersed active species and favor the electron transfer; (ii) engineering micro-nanostructures to Exploiting economical and high-performance bifunctional electrocatalysts toward hydrogen and oxygen evolution reactions (HER/OER) is at the heart of overall water splitting in large-scale application. Herein, an in situ and stepwise strategy for synthesizing core-shell Ni 3 (S 1−x Se x ) 2 @NiOOH (0 ≤ x ≤ 1) nanoarray heterostructures on nickel foam with tailored compositions for enhancing water-splitting performance is reported. A series of Ni 3 (S 1−x Se x ) 2 nanostructures is firstly grown on nickel foam via an in situ reaction in a heated polyol solution system. Ni 3 (S 1−x Se x ) 2 @NiOOH nanocomposites are subsequently prepared via electrochemical oxidation and the oxidation degree is systematically investigated by varying the oxidation time. Benefitting from the vertical standing architecture, abundant exposed active sites, and synergetically interfacial enhancement, Ni 3 (S 0.25 Se 0.75 ) 2 @NiOOH heterojunctions with electrochemical polarization for 8 h exhibit superior HER and OER behaviors, achieving a water-splitting current density of 10 mA cm −2 at a small overpotential of 320 mV as well as boosted reaction kinetics and long-term stability. This work should shed light on the controllable synthesis of metal-based hybrid materials and provide a promising direction for developing the highest-performing electrocatalysts based on interfacial and heterostructural regulation for advanced electrochemical energy conversion technologies. www.small-journal.com electrode to receive stable signals. The HER polarization profiles were performed from 0.1 to −0.8 V, while the OER from 1.0 to 2.0 V at a scan speed of 5 mV s −1 . Elect...

In this work, LiFePO4/C composite were synthesized via a green route by using Iron (III) oxide (Fe2O3) nanoparticles, Lithium carbonate (Li2CO3), glucose powder and phosphoric acid (H3PO4) solution as raw materials. The reaction principles for the synthesis of LiFePO4/C composite were analyzed, suggesting that almost no wastewater and air polluted gases are discharged into the environment. The morphological, structural and compositional properties of the LiFePO4/C composite were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman and X-ray photoelectron spectroscopy (XPS) spectra coupled with thermogravimetry/Differential scanning calorimetry (TG/DSC) thermal analysis in detail. Lithium-ion batteries using such LiFePO4/C composite as cathode materials, where the loading level is 2.2 mg/cm2, exhibited excellent electrochemical performances, with a discharge capability of 161 mA h/g at 0.1 C, 119 mA h/g at 10 C and 93 mA h/g at 20 C, and a cycling stability with 98.0% capacity retention at 1 C after 100 cycles and 95.1% at 5 C after 200 cycles. These results provide a valuable approach to reduce the manufacturing costs of LiFePO4/C cathode materials due to the reduced process for the polluted exhaust purification and wastewater treatment.

The solid state reaction method was applied to prepare a series of LiFePO4/C materials by adding various surfactants. The as-prepared LiFePO4/C particles using various surfactants show different electrochemical performances.