Three-Dimensional (3D) Ordered Macroporous Bimetallic (Mn,Fe) Selenide/Carbon Composite with Heterojunction Interface for High-Performance Sodium Ion Batteries

Transition-metal selenides have captured significant research attention as anode materials for sodium ion batteries (SIBs) due to their high theoretical specific capacities and excellent electronic conductivity. However, volumetric expansion and inferior cycle life still hinder their practical application. Herein, a three-dimensional (3D) ordered macroporous bimetallic (Mn,Fe) selenide modified by a carbon layer (denoted as 3DOM-MnFeSex@C) composite containing a heterojunction interface is fabricated through selenizing a 3D ordered macroporous Mn-based Prussian Blue analogue single crystal. The 3DOM-MnFeSex@C exhibits hierarchically porous architecture with enhanced mass-transfer efficiency; MnSe and FeSe2 particles are encapsulated into macroporous carbon framework, which can significantly promote the electronic conductivity and maintain the structural integrity. The density functional theory calculation indicates that the heterojunction interface between MnSe and FeSe2 has been successfully engineered so that Na+ can be readily adsorbed and rapidly converted, thus promoting the reaction kinetics and extending the cyclic life. As expected, the 3DOM-MnFeSex@C composite delivers excellent rate performance (277.6 mA h g-1 at 10 A g-1), and prolonged cycling life (191.6 mA h g-1 even after 1000 cycles at 2 A g-1) as a sodium storage anode. The sodium storage mechanism of the composite was further investigated by in situ X-ray diffraction and ex situ high-resolution transmission electron microscopy characterization techniques.

Heterojunction-Promoted Sodium Ion Storage of Bimetallic Selenides Encapsulated in a Carbon Sheath with Boosted Ion Diffusion and Stable Structure

Although metallic chalcogenides are deemed as attractive sodium anode materials recently, the electrochemical performance is severely confined by the liability of structural collapse and sluggish ion diffusion kinetics. Herein a composite of carbon-encapsulated bimetallic selenides MoSe₂-Sb₂Se₃ was prepared by a hydrothermal method on the basis of abundant reaction sites, high activity, an extra built-in electric field generated from heterointerfaces, and synergistic effects between the different components. Equally important, the carbon coating is effective to support the structural stability by restraining the vast volumetric variation to achieve the purpose of improving the cycling performance. The density functional theory calculation results indicate that the band gap is narrowed and that the work function is decreased on the interface of the MoSe₂-Sb₂Se₃ heterojunction, leading to an additional driving force stemming from the introduction of the built-in electric field and the formation of the Sb-Se (Se from MoSe₂) bond. Therefore, the resultant composite presents increased reaction kinetics and good electrochemical properties by acquiring a capacity of 376.0 mA h g^-1 over 580 cycles at 2.0 A g^-1 for the half-cell and 276 mA h g^-1 over 750 cycles at 2 A g^-1 for the full-cell. This work highlights bimetallic selenides with facilitated ion transferability with high performance.

Defective Bimetallic Selenides for Selective CO2 Electroreduction to CO

CO₂ electroreduction (CO₂ RR) to CO is promising for the carbon cycle but still remains challenging. Au is regarded as the most selective catalyst for CO₂ RR, but its high cost significantly hinders its industrial application. Herein, the bimetallic CuInSe₂ is found to exhibit an Au-like catalytic feature: i) the interaction of Cu and In orbitals induces a moderate adsorption strength of CO₂ RR intermediates and favors the reaction pathway; and ii) the hydrogen evolution is energetically unfavorable on CuInSe₂ , as a surface reconstruction along with high energy change will occur after hydrogen adsorption. Furthermore, the Se vacancy is found to induce an electron redistribution, slightly tune the band structure, and optimize the CO₂ RR route of bimetallic selenide. Consequently, the Se-defective CuInSe₂ (V-CuInSe₂ ) achieves a highly selective CO production ability that is comparable to noble metals in aqueous electrolyte, and the V-CuInSe₂ cathode shows a satisfactory performance in an aqueous Zn-CO₂ cell. This work demonstrates that designing cost-effective catalysts with noble-metal-like properties is an ideal strategy for developing efficient electrocatalysts. Moreover, the class of transition bimetallic selenides has shown promising prospects as active and cost-effective electrocatalysts owing to their unique structural, electronic, and catalytic properties.

SnSe2 /FeSe2 Nanocubes Capsulated in Nitrogen-Doped Carbon Realizing Stable Sodium-Ion Storage at Ultrahigh Rate

Metal selenides have attracted increasing attention recently as anodes for sodium-ion batteries (SIBs) because of their large capacities, high electric conductivity, as well as environmental benignity. However, the application of metal selenides is hindered by the huge volume variation, which causes electrode structure devastation and the consequent degrading cycling stability and rate capability. To overcome the aforementioned obstacles, herein, SnSe₂ /FeSe₂ nanocubes capsulated in nitrogen-doped carbon (SFS@NC) are fabricated via a facile co-precipitation method, followed by poly-dopamine wrapping and one-step selenization/carbonization procedure. The most remarkable feature of SFS@NC is the ultra-stability under high current density while delivering a large capacity. The synergistic effect of dual selenide components and core-shell architecture mitigates the volume effect, alleviates the agglomeration of nanoparticles, and further improves the electric conductivity. The as-prepared SFS@NC nanocubes present a high capacity of 408.1 mAh g^-1 after 1200 cycles at 6 A g^-1 , corresponding to an 85.3% retention, and can achieve a capacity of 345.0 mAh g^-1 at an extremely high current density of 20 A g^-1 . The outstanding performance of SFS@NC may provide a hint to future material structure design strategy, and promote further developments and applications of SIBs.

Conversion Reaction Mechanism of Ultrafine Bimetallic Co-Fe Selenides Embedded in Hollow Mesoporous Carbon Nanospheres and Their Excellent K-Ion Storage Performance

Potassium-ion batteries (KIBs) are considered as promising alternatives to lithium-ion batteries owing to the abundance and affordability of potassium. However, the development of suitable electrode materials that can stably store large-sized K ions remains a challenge. This study proposes a facile impregnation method for synthesizing ultrafine cobalt-iron bimetallic selenides embedded in hollow mesoporous carbon nanospheres (HMCSs) as superior anodes for KIBs. This involves loading metal precursors into HMCS templates using a repeated "drop and drying" process followed by selenization at various temperatures, facilitating not only the preparation of bimetallic selenide/carbon composites but also controlling their structures. HMCSs serve as structural skeletons, conductive templates, and vehicles to restrain the overgrowth of bimetallic selenide particles during thermal treatment. Various analysis strategies are employed to investigate the charge-discharge mechanism of the new bimetallic selenide anodes. This unique-structured composite exhibits a high discharge capacity (485 mA h g^-1 at 0.1 A g^-1 after 200 cycles) and enhanced rate capability (272 mA h g^-1 at 2.0 A g^-1 ) as a promising anode material for KIBs. Furthermore, the electrochemical properties of various nanostructures, from hollow to frog egg-like structures, obtained by adjusting the selenization temperature, are compared.

Construction of Bimetallic Selenides Encapsulated in Nitrogen/Sulfur Co-Doped Hollow Carbon Nanospheres for High-Performance Sodium/Potassium-Ion Half/Full Batteries

Metallic selenides have been widely investigated as promising electrode materials for metal-ion batteries based on their relatively high theoretical capacity. However, rapid capacity decay and structural collapse resulting from the larger-sized Na⁺ /K⁺ greatly hamper their application. Herein, a bimetallic selenide (MoSe₂ /CoSe₂ ) encapsulated in nitrogen, sulfur-codoped hollow carbon nanospheres interconnected reduced graphene oxide nanosheets (rGO@MCSe) are successfully designed as advanced anode materials for Na/K-ion batteries. As expected, the significant pseudocapacitive charge storage behavior substantially contributes to superior rate capability. Specifically, it achieves a high reversible specific capacity of 311 mAh g^-1 at 10 A g^-1 in NIBs and 310 mAh g^-1 at 5 A g^-1 in KIBs. A combination of ex situ X-ray diffraction, Raman spectroscopy, and transmission electron microscopy tests reveals the phase transition of rGO@MCSe in NIBs/KIBs. Unexpectedly, they show quite different Na⁺ /K⁺ insertion/extraction reaction mechanisms for both cells, maybe due to more sluggish K⁺ diffusion kinetics than that of Na⁺ . More significantly, it shows excellent energy storage properties in Na/K-ion full cells when coupled with Na₃ V₂ (PO₄ )₂ O₂ F and PTCDA@450 °C cathodes. This work offers an advanced electrode construction guidance for the development of high-performance energy storage devices.

Metal Organic Framework-Templated Synthesis of Bimetallic Selenides with Rich Phase Boundaries for Sodium-Ion Storage and Oxygen Evolution Reaction

Two-phase or multiphase compounds have been evidenced to exhibit good electrochemical performance for energy applications; however, the mechanism insights into these materials, especially the performance improvement by engineering the high-active phase boundaries in bimetallic compounds, remain to be seen. Here, we report a bimetallic selenide heterostructure (CoSe₂/ZnSe) and the fundamental mechanism behind their superior electrochemical performance. The charge redistribution at the phase boundaries of CoSe₂/ZnSe was experimentally and theoretically proven. Benefiting from the abundant phase boundaries, CoSe₂/ZnSe exerts low Na⁺ adsorption energy and fast diffusion kinetics for sodium-ion batteries and high activity for oxygen evolution reaction. As expected, excellent sodium storage capability, specifically a superb cyclic stability of up to 800 cycles for the Na₃V₂(PO₄)₃∥CoZn-Se full cell, and efficient water oxidation with a small overpotential of 320 mV to reach 10 mA cm^-2 were obtained. This work demonstrates the importance of phase boundaries in bimetallic compounds to boost the performance in various fields.