Metal-Organic Framework Template Derived Porous CoSe2 Nanosheet Arrays for Energy Conversion and Storage

The Utilization of Metal-Organic Frameworks and Their Derivatives Composite in Supercapacitor Electrodes

Up to now, the mainstream adoption of renewable energy has brought about substantial transformations in the electricity and energy sector. This shift has garnered considerable attention within the scientific community. Supercapacitors, known for their exceptional performance metrics like good charge/discharge capability, strong power density, as well as extended cycle longevity, have gained widespread traction across various sectors, including transportation and aviation. Metal-organic frameworks (MOFs) with unique traits including adaptable structure, highly customizable synthetic methods, and high specific surface area, have emerged as strong candidates for electrode materials. For enhancing the performance, MOFs are commonly compounded with other conducting materials to increase capacitance. This paper provides a detailed analysis of various common preparation strategies and characteristics of MOFs. It summarizes the recent application of MOFs and their derivatives as supercapacitor electrodes alongside other carbon materials, metal compounds, and conductive polymers. Additionally, the challenges encountered by MOFs in the realm of supercapacitor applications are thoroughly discussed. Compared to previous reviews, the content of this paper is more comprehensive, offering readers a deeper understanding of the diverse applications of MOFs. Furthermore, it provides valuable suggestions and guidance for future progress and development in the field of MOFs.

Orbital Occupancy Modulation to Optimize Intermediate Absorption for Efficient Electrocatalysts in Water Electrolysis and Zinc-Ethanol-Air Battery

Spin engineering is a promising way to modulate the interaction between the metal d-orbital and the intermediates and thus enhance the catalytic kinetics. Herein, an innovative strategy is reported to modulate the spin state of Co by regulating its coordinating environment. o-c-CoSe2 -Ni is prepared as pre-catalyst, then in situ electrochemical impedance spectroscopy (EIS) and in situ Raman spectroscopy are employed to prove phase transition, and CoOOH/Co3 O4 is formed on the surface as active sites. In hybrid water electrolysis, the voltage has a negative shift, and in zinc-ethanol-air battery, the charging voltage is lowered and the cycling stability is greatly increased. Coordinated atom substitution and crystalline symmetry change are combined to regulate the absorption ability of reaction intermediates with balanced optimal adsorption. Coordinated atom substitution weakens the adsorption while the crystalline symmetry change strengthens the adsorption. Importantly, the tetrahedral sites are introduced by Ni doping which enables the co-existence of four-coordinated sites and six-coordination sites in o-c-CoSe2 -Ni. The dz2 + dx2 -y2 orbital occupancy decreases after the atomic substitution, while increases after replacing the CoSe6 -Oh field with CoSe6 -Oh /CoSe4 -Td . This work explores a new direction for the preparation of efficient catalysts for water electrolysis and innovative zinc-ethanol-air battery.

In situ selective selenization of ZIF-derived CoSe2 nanoparticles on NiMn-layered double hydroxide@CuBr2 heterostructures for high performance supercapacitors

As an emerging energy storage device, the practical application of supercapacitors (SCs) is currently constrained by their low energy density. Enhancing the capacitance of supercapacitors by leveraging the synergistic effect of multiple components in composite electrodes with well-designed structures can effectively increase their energy density. Here, a wire-sheet-particle hierarchical heterostructured CoSe2@NiMn-layered double hydroxide (NiMn-LDH) @Cu1.8Se/Copper foam (CF) electrode is synthesized via phase pseudomorphic transformation process achieved by selective selenization for Cu and Co elements. Benefiting from the stable support structure of CuBr2, the large specific surface area of NiMn-LDH, and the excellent conductivity of CoSe2, the prepared binder-free electrode shows excellent electrochemical properties. The CoSe2@NiMn-LDH@Cu1.8Se hybrid electrode exhibits a superior specific areal capacitance of 7064 mF cm-2 at 2 mA cm-2 and a stable cyclic performance with 80.11 % capacitance retention after 10,000 cycles. Furthermore, the assembled CoSe2@NiMn-LDH@Cu1.8Se/CF//AC (activated carbon) asymmetric supercapacitor (ASC) achieves an energy density of 36.6 Wh kg-1 when the power density is 760.6 W Kg-1 and retains 87.35 % of the initial capacitance after 5000 cycles. Overall, this pioneering research provided new insight for preparing supercapacitor electrode materials by selective selenization and ration design of the structures.

Modulation of Phase Transition in Cobalt Selenide with Simultaneous Construction of Heterojunctions for Highly-Efficient Oxygen Electrocatalysis in Zinc-Air Battery

Phase transformation of cobalt selenide (CoSe2 ) can effectively modulate its intrinsic electrocatalytic activity. However, enhancing electroconductivity and catalytic activity/stability of CoSe2 still remains challenging. Heterostructure engineering may be feasible to optimize interfacial properties to promote the kinetics of oxygen electrocatalysis on a CoSe2 -based catalyst. Herein, a heterostructure consisting of CoSe2 and cobalt nitride (CoN) embedded in a hollow carbon cage is designed via a simultaneous phase/interface engineering strategy. Notably, the phase transition of orthorhombic-CoSe2 to cubic-CoSe2 (c-CoSe2 ) accompanied by in situ CoN formation is realized to build the c-CoSe2 /CoN heterointerface, which exhibits excellent/highly stable activities for oxygen reduction/evolution reactions (ORR/OER). Notably, heterostructure can modulate the local coordination environment and increase Co-Se/N bond lengths. Theoretical calculations show that Co-site (c-CoSe2 ) with an electronic state near Fermi energy level is the main active site for ORR/OER.Energetical tailoring of the d-orbital electronic structure of the Co atom of c-CoSe2 in heterostructure by in situ CoN incorporation lowers thermodynamic barriers for ORR/OER. Attractively, a zinc-air battery with a c-CoSe2 -CoN cathode displays excellent cycling stability (250 h) and charge/discharge voltage loss (0.953/0.96 V). It highlights that heterointerface engineering provides an option for modulating the bifunctional activity of metal selenides with controlled phase transformation.

Phosphorization Engineering on a MOF-Derived Metal Phosphide Heterostructure (Cu/Cu3P@NC) as an Electrode for Enhanced Supercapacitor Performance

A highly conductive and rationally constructed metal-organic framework (MOF)-derived metal phosphide with a carbonaceous nanostructure is a meticulous architecture toward the development of electrode materials for energy storage devices. Herein, we report a facile strategy to design and construct a new three-dimensional (3D) Cu-MOF via a solvent diffusion method at ambient temperature, which was authenticated by a single-crystal X-ray diffraction study, revealing a novel topology of (2,4,7)-connected three-nodal net named smm4. Nevertheless, the poor conductivity of pristine MOFs is a major bottleneck hindering their capacitance. To overcome this, we demonstrated an MOF-derived Cu3P/Cu@NC heterostructure via low-temperature phosphorization of Cu-MOF. The electronic and ionic diffusion kinetics in Cu3P/Cu@NC were improved due to the synergistic effects of the heterostructure. The as-prepared Cu3P/Cu@NC heterostructure electrode delivers a specific capacity of 540 C g-1 at 1 A g-1 with outstanding rate performance (190 C g-1 at 20 A g-1) and cycle stability (91% capacity retention after 10,000 cycles). Moreover, the assembled asymmetric solid-state supercapacitor (ASC) achieved a high energy density/power density of 45.5 Wh kg-1/7.98 kW kg-1 with a wide operating voltage (1.6 V). Long-term stable capacity retention (87.2%) was accomplished after 5000 cycles. These robust electrochemical performances suggest that the Cu3P/Cu@NC heterostructure is a suitable electrode material for supercapacitor applications.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Facile Synthesis of CoSe/Co3O4-CNTs/NF Composite Electrode for High-Performance Asymmetric Supercapacitor

Electrode materials are key factors for supercapacitors to endow them with excellent electrochemical properties. Here, a novel hybrid structure of a CoSe/Co₃O₄-CNTs binder free composite electrode on nickel foam was prepared via a facile flame method, followed by an electrodeposition process. Benefitting from the synergetic effects of the multicomponent (with low resistances of 1.542 Ω cm² and a moderate mesoporous size of 3.12 nm) and the enlarged specific surface area of the composite material (77.4 m² g^-1), the CoSe/Co₃O₄-CNTs composite electrode delivers a high specific capacitance of 2906 F g^-1 at 5 mV s^-1 with an excellent rate stability. The fabricated CoSe/Co3O4-CNTs/NF//AC ASC exhibits a high energy density of 43.4 Wh kg^-1 at 0.8 kW kg^-1 and a long cycle life (92.7% capacitance retention after 10,000 cycles).

Metal-organic framework (MOF)-derived plate-shaped CoS1.097 nanoparticles for an improved hydrogen evolution reaction

Metal-organic framework (MOF)-derived transition metal sulfides are viewed as reliable, cost-effective, and alternative hydrogen evolution reaction (HER)-efficient electrocatalysts. They have been used to replace platinum (and their alloys) for production of renewable energy carriers such as hydrogen. Progress towards development of non-precious transition-metal sulfides through different synthetic routes to obtain unique morphological nanostructures with enhanced HER activity is challenging. We introduced a transition-metal sulfide, cobalt sulfide (CoS_1.097), derived from a cobalt MOF [Co-BPY-DDE] by following facile, one-step solvothermal sulfurization. By varying the sulfurization temperature (from 140 °C to 180 °C) during the solvothermal method, three cobalt-sulfide products were obtained: CoS_1.097-140, CoS_1.097-160, and CoS_1.097-180, respectively. Temperature variation had a vital role in optimizing the HER activity of the electrocatalyst. Besides, notable plate-shaped cobalt sulfide nanoparticles (CoS_1.097-160) required overpotential of 163 mV to deliver a current density of 10 mA cm^-2 with a low Tafel slope of 53 mV dec^-1, thereby demonstrating faster reaction kinetics during the evolution of molecular hydrogen. Furthermore, 25 h of long-term stability of the electrocatalyst reflected its practical applicability in acidic media. CoS_1.097-160 had uniform plate-shaped morphology and large electrochemical active surface area, which contributed to enhanced electrochemical performance through water electrolysis.

Enhanced Performance of Carbon-Selenide Composite with La0.9Ce0.1NiO3 Perovskite Oxide for Outstanding Counter Electrodes in Platinum-Free Dye-Sensitized Solar Cells

For large-scale applications, dye-sensitized solar cells (DSSCs) require the replacement of the scarce platinum (Pt)-based counter electrode (CE) with efficient and cheap alternatives. In this respect, low-cost perovskite oxides (ABO₃) have been introduced as promising additives to composite-based CEs in Pt-free DSSCs. Herein, we synthesized composites from La_0.9Ce_0.1NiO₃ (L) perovskite oxide and functionalized-multiwall-carbon-nanotubes wrapped in selenides derived from metal-organic-frameworks (f-MWCNT-ZnSe-CoSe₂, “F”). L and F were then mixed with carbon black (CB) in different mass ratios to prepare L@CB, F@CB, and L@F@CB composites. The electrochemical analysis revealed that the L@F@CB composite with a mass ratio of 1.5:3:1.5 exhibits better electrocatalytic activity than Pt. In addition, the related DSSC reached a better PCE of 7.49% compared to its Pt-based counterpart (7.09%). This improved performance is the result of the increase in the oxygen vacancy by L due to the replacement of La with Ce in its structure, leading to more active sites in the L@F@CB composites. Moreover, the F@CB composite favors the contribution to the high electrical conductivity of the hybrid carbon nanotube–carbon black, which also offers good stability to the L@F@CB CE by not showing any obvious change in morphology and peak-to-peak separation even after 100 cyclic voltammetry cycles. Consequently, the corresponding L@F@CB-based device achieved enhanced stability. Our work demonstrates that L@F@CB composites with a low cost are excellent alternatives to Pt CE in DSSCs.

NiSe2 Nanoparticles Encapsulated in N-Doped Carbon Matrix Derived from a One-Dimensional Ni-MOF: An Efficient and Sustained Electrocatalyst for Hydrogen Evolution Reaction

The spherical-type NiSe₂ nanoparticles encapsulated in a N-doped carbon (NC) matrix (NiSe₂-T@NC, temperature (T) = 400-800 °C) are derived from a 1D Ni-MOF precursor of the formula [Ni(BPY)(DDE)] [(BPY = 2,2'-bipyridyl), (DDE = 4,4'-dicarboxy diphenyl ether)] via a facile solvothermal technique followed by annealing at different temperatures and selenylation strategies. The combined effect of a NC matrix and the Ni nanoparticles has been optimized during varied annealing processes with subsequent selenylation, leading to the formation of the series NiSe₂-400@NC, NiSe₂-500@NC, NiSe₂-600@NC, NiSe₂-700@NC, and NiSe₂-800@NC, respectively. The variation of annealing temperature plays a vital role in optimizing the catalytic behavior of the NiSe₂-T@NCs. Among different high-temperature annealed products, NiSe₂-600@NC shows superior electrocatalytic performance because of the unique spherical-type morphology and higher specific surface area (57.95 m² g^-1) that provides a large number of electrochemical active sites. The synthesized material exhibits a lower overpotential of 196 mV to deliver 10 mA cm^-2 current density, a small Tafel slope of 45 mV dec^-1 for better surface kinetics, and outstanding durability in an acidic solution, respectively. Consequently, the post stability study of the used electrocatalyst gives insight into surface phase analysis. Therefore, we presume that the synthesized 1D MOF precursor derived NiSe₂ nanoparticles encapsulated in a NC matrix has excellent potential to replace the noble-metal-based electrocatalyst for enhanced hydrogen evolution through simple water electrolysis.

MOF-Derived Bifunctional Co0.85Se Nanoparticles Embedded in N-Doped Carbon Nanosheet Arrays as Efficient Sulfur Hosts for Lithium-Sulfur Batteries

Lithium-sulfur batteries possess the merits of low cost and high theoretical energy density but suffer from the shuttle effect of lithium polysulfides and slow redox kinetics of sulfur. Herein, novel Co_0.85Se nanoparticles embedded in nitrogen-doped carbon nanosheet arrays (Co_0.85Se/NC) were constructed on carbon cloth as the self-supported host for a sulfur cathode using a facile fabrication strategy. The interconnected porous carbon-based structure of the Co_0.85Se/NC could facilitate the rapid electron and ion transfer kinetics. The embedded Co_0.85Se nanoparticles can effectively capture and catalyze lithium polysulfides, thus accelerating the redox kinetics and stabilizing sulfur cathodes. Therefore, the Co_0.85Se/NC-S cathode could maintain a stable cycle performance for 400 cycles at 1C and deliver a high discharge specific capacity of 1361, 1001, and 810 mAh g^-1 at current densities of 0.1, 1, and 3C, respectively. This work provides an efficient design strategy for high-performance lithium-sulfur batteries with high energy densities.

The electrochemical kinetics of cerium selenide nano-pebbles: the design of a device-grade symmetric configured wide-potential flexible solid-state supercapacitor

Next-generation portable flexible electronic appliances require liquid-free energy storage supercapacitor devices to eliminate leakage and to support mechanical bending that is compatible with roll-to-roll technologies. Hence, a state-of-the-art process is presented to design a solid-state, wide-potential and flexible supercapacitor through the use of nano-pebbles of cerium selenide via a simple successive ionic layer adsorption and reaction (SILAR) method that could allow an industry scalable route. We strongly believe that this is the first approach amongst physical and chemical routes not only for synthesizing cerium selenide in thin-film form but also using it for device-grade supercapacitor applications. The designed solid-state symmetric supercapacitor assembled from cerium selenide electrodes sandwiched by PVA-LiClO₄ gel electrolyte attains a wide potential window of 1.8 V with capacitance of 48.8 F g^-1 at 2 mV s^-1 and reveals excellent power density of 4.89 kW kg^-1 at an energy density of 11.63 W h kg^-1. The formed device is capable of 87% capacitive retention even at a mechanical bending angle of 175°. Lighting up a strip of 21 parallel connected red LEDs clearly demonstrates the practical use of the designed symmetric solid-state supercapacitor, aiming towards the commercialization of the product in the future.

Bioinspired interfacial engineering of a CoSe2 decorated carbon framework cathode towards temperature-tolerant and flexible Zn-air batteries

A high-performance air electrode is essential for the successful application of flexible Zn-air batteries in wearable devices. However, endowing the electrode-electrolyte interface with high stability and fast electron/ion transportation is still a great challenge. Herein, we report a bioinspired interfacial engineering strategy to construct a cactus-like hybrid electrode comprising CoSe2 nanoparticles embedded in an N-doped carbon nanosheet arrays penetrated with carbon nanotubes (CoSe2-NCNT NSA). Associated with the synergistic effect of highly active CoSe2 nanoparticles and N-doped carbon moieties and a stable 3D interconnected CNT network, the obtained self-standing electrode exhibits satisfactory catalytic activities towards oxygen evolution/reduction and hydrogen evolution, as well as an enhanced electrode-electrolyte interaction/interface area, and thus delivers superior performance for flexible Zn-air batteries. Remarkably, the fabricated flexible Zn-air battery with this CoSe2-NCNT NSA cathode achieves a high peak power density (51.1 mW cm-2), considerable mechanical flexibility, and excellent durability in a wide temperature range of 0 to 40 °C. Furthermore, the assembled Zn-air batteries can efficiently power a water-splitting device that adopts the CoSe2-NCNT NSA as both the anode and cathode, demonstrating promising potential in energy conversion and portable electronic applications.

The rational design of Cu2-xSe@(Co,Cu)Se2 core-shell structures as bifunctional electrocatalysts for neutral-pH overall water splitting

Highly active and stable bifunctional electrocatalysts for H2 generation from neutral-pH water are desired, but difficult to achieve. The modification of the electronic and crystal structure of a material by element doping, morphology design and constructing a complex is a valid strategy for obtaining high-performance catalysts toward overall water splitting. In this study, a novel Cu2-xSe@(Co,Cu)Se2 core-shell structure with ultrathin (Co,Cu)Se2 nanosheets anchored as a shell on an internal Cu2-xSe core was fabricated, for the first time, by integrating the three above-mentioned modification methods. Benefiting from the synergistic effect between components and the unique structure, the Cu2-xSe@(Co,Cu)Se2 core-shell structure can serve as an efficient bifunctional electrocatalyst for both HERs and OERs in neutral-pH electrolytes with a current density of 10 mA cm-2 at the overpotentials of 106 mV and 396 mV, respectively. Additionally, the material just requires a cell voltage of 1.73 V to afford a current density of 10 mA cm-2 in a neutral two-electrode electrolyzer. Such performances significantly outperform control catalysts and analogues. Even more importantly, the original concept of coordinated regulation presented in this work can broaden our horizons in the design of new and highly efficient catalysts for neutral water splitting.

(Ni,Co)Se@Ni(OH)2 heterojunction nanosheets as an efficient electrocatalyst for the hydrogen evolution reaction

A heterogeneous structure formed by coupling two or more phases can reinforce the activity of active sites and expedite electron transfer, which is conducive to boosting its electrocatalytic activity. Herein, we designed nickel foam supported (NiCo₂)Se@Ni(OH)₂ (NCS@NH) heterojunction nanosheets by a two-step method. First of all, the NiCo₂S₄@Ni(OH)₂ (NiCo₂S₄@NH) nanosheets coated on nickel foam were acquired via a hydrothermal method. In the selenization treatment that followed, NiCo₂S₄@NH was converted into NCS@NH heterogeneous nanosheets in which the selenide nanoparticles decorated on the surface of the Ni(OH)₂ nanosheets formed heterojunction interfaces, and the heterogeneous structure could accelerate electron transfer, thus improving the catalytic activity. The Ni(OH)₂ nanosheets can adequately contact the electrolyte and promote the decomposition of water. Meanwhile, the thickness of the Ni(OH)₂ nanosheets gradually decreases with the increase of Co doping (1.5-2.5 mmol), consequently affecting the HER properties. Notably, when the amount of Co salt added is 2 mmol, NCS@NH exhibited superior HER properties (with a voltage of 253 mV at 100 mA cm^-2) and excellent stability for 24 h.

Iron and nitrogen Co-doped CoSe2 nanosheet arrays for robust electrocatalytic water oxidation

Fe–N–CoSe₂ electrocatalysts with good OER performance and long-term durability were synthesized using an anion and cation co-doping strategy.

Engineering of CoSe2 Nanosheets via Vacancy Manipulation for Efficient Cancer Therapy

CoSe₂ nanosheets with different vacancy associates (i.e., V_SeSe, V_CoCoSe, V_CoSeSe, and V_CoCoCoSe) were synthesized by selecting different templates and calcination temperatures. The nanosheets having higher theoretical adsorption toward cellular membrane phospholipids exerted more damage to both artificial liposomes and cell membranes (V_SeSe > V_CoCoSe > V_CoCoCoSe > V_CoSeSe) and exhibited higher efficiency to kill tumor cells (e.g., A549 lung cancer cells and HeLa cervical cancer cells). Moreover, V_CoCoCoSe CoSe₂ nanosheets can be easily coated with the tumor-targeting protein transferrin (Tf), and these Tf-coated nanosheets (Tf-V_CoCoCoSe) drastically decreased the viability of various types of cancer cells (81%) without significantly damaging normal cells. Importantly, this treatment was more efficient in eliminating tumor tissues than the common chemotherapeutic drug oxaliplatin. Thus, this study offers proof of concept that manipulating the surface vacancies of CoSe₂ nanosheets could provide an efficient route for cancer therapy in a manner that could circumvent or minimize drug resistance development.

Solid-solution hexagonal Ni0.5Co0.5Se nanoflakes toward boosted oxygen evolution reaction

The oxygen evolution reaction (OER) with sluggish kinetics is a bottleneck for the large-scale application of water electrolysis. Herein, solid-solution hexagonal Ni_0.5Co_0.5Se nanoflakes are designed and successfully synthesized via a facile hydrothermal method with a much lower overpotential of 216 mV at 10 mA cm^-2 and a Tafel slope of 37.08 mV dec^-1.

Flexible 3D carbon cloth as a high-performing electrode for energy storage and conversion

High-performance energy storage and conversion devices with high energy density, power density and long-term cycling life are of great importance in current consumer electronics, portable electronics and electric vehicles. Carbon materials have been widely investigated and utilized in various energy storage and conversion devices due to their excellent conductivity, mechanical and chemical stability, and low cost. Abundant excellent reviews have summarized the most recent progress and future outlooks for most of the current prime carbon materials used in energy storage and conversion devices, such as carbon nanotubes, fullerene, graphene, porous carbon and carbon fibers. However, the significance of three-dimensional (3D) commercial carbon cloth (CC), one of the key carbon materials with outstanding mechanical stability, high conductivity and flexibility, in the energy storage and conversion field, especially in wearable electronics and flexible devices, has not been systematically summarized yet. In this review article, we present a careful investigation of flexible CC in the energy storage and conversion field. We first give a general introduction to the common properties of CC and the roles it has played in energy storage and conversion systems. Then, we meticulously investigate the crucial role of CC in typical electrochemical energy storage systems, including lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries and supercapacitors. Following a description of the wide application potential of CC in electrocatalytic hydrogen evolution, oxygen evolution/reduction, full-water splitting, etc., we will give a brief introduction to the application of CC in the areas of photocatalytically and photoelectrochemically induced solar energy conversion and storage. The review will end with a brief summary of the typical superiorities that CC has in current energy conversion and storage systems, as well as providing some perspectives and outlooks on its future applications in the field. Our main interest will be focused on CC-based flexible devices due to the inherent superiority of CC and the increasing demand for flexible and wearable electronics.

Facile Synthesis of Metal-Organic Framework-Derived CoSe2 Nanoparticles Embedded in the N-Doped Carbon Nanosheet Array and Application for Supercapacitors

Metal-organic framework (MOF)-derived composites of transition metal oxides and porous carbon show great potential for energy storage applications. Selenylation is an effective strategy to improve the electrochemical properties of electrode materials. A facile one-step derivation and selenylation of MOFs is proposed here to obtain CoSe₂ nanoparticles embedded into an N-doped carbon skeleton material (CoSe₂/NC). Moreover, the composite is directly grown on nickel foam as nanosheet arrays, rather than on other materials as powders. The CoSe₂/NC electrode with special construction exhibits a high capacity of 120.2 mA h g^-1 at 1 A g^-1 and an excellent cyclic ability of 8% loss after 10,000 cycles. An asymmetric supercapacitor CoSe₂/NC-NF//AC displays a maximum energy density of 40.9 W h kg^-1 at 980 W kg^-1. Moreover, the device has demonstrated that it can successfully charge a mobile phone. The outstanding performance indicates promising potential of CoSe₂/NC-NF electrodes for supercapacitors.

A metal–organic framework template derived hierarchical Mo-doped LDHs@MOF-Se core–shell array electrode for supercapacitors

A core–shell array electrode displays high areal capacity and rate capability with a highly conductive framework and a high loading of active materials.

1D-CoSe2 nanoarray: a designed structure for efficient hydrogen evolution and symmetric supercapacitor characteristics

Direct growth of self-supported one-dimensional (1D) nanorod arrays on conducting substrates is highly attractive for electrocatalysis, due to their unique shape, size, and length.

One-step fabrication of a self-supported Co@CoTe2 electrocatalyst for efficient and durable oxygen evolution reactions

The optimal hydrothermal synthesis of a Co@CoTe₂-240 electrode needs an overpotential of 286 mV to achieve a current density of 10 mA cm⁻² and is able to sustain galvanostatic OER electrolysis for 16 hours with little degradation of less than 20 mV.

Aldehyde reduced Co3O4 to form oxygen vacancy and enhance the electrochemical performance for oxygen evolution reaction and supercapacitors

Oxygen vacancy is a feasible approach to boost the electrochemical properties for metal oxides. In this work, a Co₃O₄ with abundant oxygen vacancy is synthesized via aldehyde reduction. After the procedure, the reduced Co₃O₄ exhibits larger electrochemical active surface areas and better electrical conductivity. These outstanding characteristics can improve its performance of catalytic and energy storage. As for catalyst of oxygen evolution reaction, the reduced Co₃O₄ delivers a smaller potential of 1.55 V versus the reversible hydrogen electrode to realize a current density of 10 mA cm^-2 and a lower Tafel slope of 71 mV dec^-1 in alkaline solution, and these values are smaller than those of pristine Co₃O₄. Especially the reduced Co₃O₄ possesses superior stability: the measurements of the polarization curves before and after 15h of stability tests basically coincide. In a supercapacitor, the positive electrode of reduced Co₃O₄ achieves about 1.7 times areal capacitance of pristine Co₃O₄ at current density of 1 mA cm^-2. Significantly, the superior cycling stability is still retained. Also, an aqueous asymmetric supercapacitor is assembled to evaluate the energy storage performance of the R-Co₃O₄. Moreover, the oxygen vacancy formation strategy for Co₃O₄ may be generally extended to other metal oxides for application in energy storage and conversion.

MOF-Derived ZnSe/N-Doped Carbon Composites for Lithium-Ion Batteries with Enhanced Capacity and Cycling Life

In this work, three different morphologies of ZnSe/N-doped carbon (NC) composites are synthesized using ZIF-8 by a facile calcination process. By adjusting the particle size of precursor ZIF-8, the morphology and size of the product ZnSe/NC can be controlled. The as-prepared ZnSe/NC composites show excellent cyclic stability and rate capability as anode materials in lithium-ion batteries (LIBs). Especially, the as-obtained ZnSe/NC-300 exhibits reversible discharge capacity of 724.4 mAh g^-1 after 500 cycles at 1 A g^-1. The introduction of N-doped carbon can significantly improve the conductivity of ZnSe and promotes the transfer of electrons. And mesoporous structure is conducive to the penetration of electrolyte in active materials, increases the contact area, and alleviates the volume expansion during the charge-discharge process. Thus, ZnSe/NC composites provide a new insight into the development of anode materials for next-generation high-performance LIBs.

A 3D self-supported coralline-like CuCo2S4@NiCo2S4 core-shell nanostructure composite for high-performance solid-state asymmetrical supercapacitors

Rational construction of three dimensional (3D) composite structure is an important method to flexible supercapacitor electrodes and has been extensively developed. In this work, a 3D self-supported CuCo₂S₄@NiCo₂S₄ core-shell nanostructure grown on Nickel (Ni) foam, constructed by a hydrothermal method, was used as a novel supercapacitor electrode material. The unique structure possesses a large, specific surface area, rapid diffusion of electrolyte ions by numerous channels and avoids the use of additives and adhesives. The high electrical conductivity of the CuCo₂S₄ nanoneedle arrays can speed up electronic transmission. At a current density of 1 A g^-1, the electrode material exhibits a high specific capacity of 539.2 C g^-1 and cycling stability with 100% capacity retention after 5000 cycles in 3 M KOH. Furthermore, when the obtained CuCo₂S₄@NiCo₂S₄ was used as the positive electrode and an activated carbon was used as the negative electrode, a solid-state asymmetric supercapacitor was assembled. More importantly, the obtained solid-state asymmetric supercapacitor demonstrated excellent electrochemical performance. When the power density was 400 W kg^-1, it delivered a high density of 23.4 W h kg^-1 with a high voltage window of 1.6 V, thus demonstrating that the material has the potential for use as an efficient electrode for electrochemical capacitors. Due to its comprehensive electrochemical performance, the CuCo₂S₄@NiCo₂S₄ solid-state asymmetric supercapacitor effectively operated a red LED.

Structural Engineering of Low-Dimensional Metal-Organic Frameworks: Synthesis, Properties, and Applications

Low-dimensional metal-organic frameworks (LD MOFs) have attracted increasing attention in recent years, which successfully combine the unique properties of MOFs, e.g., large surface area, tailorable structure, and uniform cavity, with the distinctive physical and chemical properties of LD nanomaterials, e.g., high aspect ratio, abundant accessible active sites, and flexibility. Significant progress has been made in the morphological and structural regulation of LD MOFs in recent years. It is still of great significance to further explore the synthetic principles and dimensional-dependent properties of LD MOFs. In this review, recent progress in the synthesis of LD MOF-based materials and their applications are summarized, with an emphasis on the distinctive advantages of LD MOFs over their bulk counterparties. First, the unique physical and chemical properties of LD MOF-based materials are briefly introduced. Synthetic strategies of various LD MOFs, including 1D MOFs, 2D MOFs, and LD MOF-based composites, as well as their derivatives, are then summarized. Furthermore, the potential applications of LD MOF-based materials in catalysis, energy storage, gas adsorption and separation, and sensing are introduced. Finally, challenges and opportunities of this fascinating research field are proposed.

Ultrathin Ti3C2Tx (MXene) Nanosheet-Wrapped NiSe2 Octahedral Crystal for Enhanced Supercapacitor Performance and Synergetic Electrocatalytic Water Splitting

Metal selenides, such as NiSe₂, have exhibited great potentials as multifunctional materials for energy storage and conversation. However, the utilization of pure NiSe₂ as electrode materials is limited by its poor cycling stability, low electrical conductivity, and insufficient electrochemically active sites. To remedy these defects, herein, a novel NiSe₂/Ti₃C₂T_x hybrid with strong interfacial interaction and electrical properties is fabricated, by wrapping NiSe₂ octahedral crystal with ultrathin Ti₃C₂T_x MXene nanosheet. The NiSe₂/Ti₃C₂T_x hybrid exhibits excellent electrochemical performance, with a high specific capacitance of 531.2 F g^-1 at 1 A g^-1 for supercapacitor, low overpotential of 200 mV at 10 mA g^-1, and small Tafel slope of 37.7 mV dec^-1 for hydrogen evolution reaction (HER). Furthermore, greater cycling stabilities for NiSe₂/Ti₃C₂T_x hybrid in both supercapacitor and HER have also been achieved. These significant improvements compared with unmodified NiSe₂ should be owing to the strong interfacial interaction between NiSe₂ octahedral crystal and Ti₃C₂T_x MXene, which provides enhanced conductivity, fast charge transfer as well as abundant active sites, and highlight the promising potentials in combinations of MXene with metal selenides for multifunctional applications such as energy storage and conversion.

NiSe2 nanooctahedra as anodes for high-performance sodium-ion batteries

Octahedral NiSe₂ was fabricated by a facile hydrothermal method. It showed excellent Na ion storage performance when used as an anode material for sodium-ion batteries.

Synthesis from a layered double hydroxide precursor for a highly efficient oxygen evolution reaction

A hybrid of Co₃Se₄ and FeSe₂ prepared via a facile hydrothermal method achieves an efficient OER activity during water splitting.

Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation

As one of the most remarkable oxygen evolution reaction (OER) electrocatalysts, metal chalcogenides have been intensively reported during the past few decades because of their high OER activities. It has been reported that electron-chemical conversion of metal chalcogenides into oxides/hydroxides would take place after the OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during the OER for these electrocatalysts, has not been understood yet; therefore a direct observation for the electrocatalytic water oxidation process, especially at nano or even angstrom scale, is urgently needed. In this research, by employing advanced Cs-corrected transmission electron microscopy (TEM), a step by step oxidational evolution of amorphous electrocatalyst CoS _x into crystallized CoOOH in the OER has been in situ captured: irreversible conversion of CoS _x to crystallized CoOOH is initiated on the surface of the electrocatalysts with a morphology change via Co(OH)₂ intermediate during the OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process has also been confirmed by multiple applications of X-ray photoelectron spectroscopy (XPS), in situ Fourier-transform infrared spectroscopy (FTIR), and other ex situ technologies. Moreover, on the basis of this discovery, a high-efficiency electrocatalyst of a nitrogen-doped graphene foam (NGF) coated by CoS _x has been explored through a thorough structure transformation of CoOOH. We believe this in situ and in-depth observation of structural evolution in the OER measurement can provide insights into the fundamental understanding of the mechanism for the OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for water splitting.

A Zinc Cobalt Sulfide Nanosheet Array Derived from a 2D Bimetallic Metal-Organic Frameworks for High-Performance Supercapacitors

Porous ternary metal sulfide integrated electrode materials with abundant electroactive sites and redox reactions are very promising for supercapacitors. Herein, a porous zinc cobalt sulfide nanosheet array on Ni foam (Zn-Co-S/NF) was constructed by facile growth of 2D bimetallic zinc/cobalt-based metal-organic framework (Zn/Co-MOF) nanosheets with leaf-like morphology on NF, followed by additional sulfurization. The Zn-Co-S/NF nanosheet array acted directly as a supercapacitor electrode showing much better electrochemical performance (2354.3 F g^-1 and 88.6 % retention over 1000 cycles) when compared with zinc cobalt sulfide powder (355.3 F g^-1 and 75.8 % retention over 1000 cycles), which originates from good electrical conductivity and mechanical stability, abundant electroactive sites, and facilitated transportation of electrons and electrolyte ions due to the unique nanosheet array structure. An asymmetric supercapacitor (ASC) device assembled from Zn-Co-S/NF and activated carbon electrodes can deliver a highest energy density of 31.9 Wh kg^-1 and a maximum power density of 8.5 kW kg^-1 . Most importantly, this ASC also shows good cycling stability (71.0 % retention over 10000 cycles). Furthermore, a red LED can be powered by two connected ASCs, and thus as-synthesized Zn-Co-S/NF has great potential for practical applications.