Temperature controlled diffusion of hydroxide ions in 1D channels of Ni-MOF-74 for its complete conformal hydrolysis to hierarchical Ni(OH)2 supercapacitor electrodes

Sacrificial MOF-derived MnNi hydroxide for high energy storage supercapacitor electrodes via DFT-based quantum capacitance study

Electrochemical energy storage plays a critical role in the transition to clean energy. With the growing demand for efficient and sustainable energy solutions, supercapacitors have gained significant attention due to their high specific capacitance, rapid charge/discharge capabilities, long lifespan, safe operation across various temperatures, and minimal maintenance needs. This study introduces a novel approach for the synthesis of high-performance supercapacitor electrodes by using MnNi-MOF-74 as a precursor. Bimetallic Mn(OH)₂/Ni(OH)₂ hydroxides (MnNi-x, where x = 2, 6, 12) with tailored morphologies were successfully fabricated by treating MnNi-MOF-74 anchored on nickel foam with different concentrations of KOH. Among the various synthesized samples, MnNi-6 exhibited the best performance, with a remarkable specific capacitance of 4031.51 mF cm⁻2 at 2 mA cm⁻2, attributed to its high surface area of 186 m2/g, optimized particle size, and abundant micropores. Furthermore, MnNi-6 demonstrated exceptional thermal stability, positioning it as a promising candidate for high-temperature supercapacitors. It also exhibited excellent cycling stability, retaining 86.34 % of its capacity after 10,000 cycles at 10 mA cm⁻2, highlighting its remarkable durability. Density functional theory (DFT) calculations were conducted to explore the quantum capacitance of the bimetallic hydroxide. The DFT results revealed electron density near the Fermi level, which directly contributes to the high quantum capacitance of Mn(OH)₂/Ni(OH)₂ with a Mn:Ni molar ratio of 3:1. This work underscores the potential of MOF-derived materials as a promising route for the development of high-performance supercapacitor electrodes, paving the way for future advances in electrochemical energy storage technologies.

Functional metal-organic frameworks derived electrode materials for electrochemical energy storage: a review

Pristine metal-organic frameworks (MOFs) are built through self-assembly of electron rich organic linkers and electron deficient metal nodes via coordinate bond. Due to the unique properties of MOFs like highly tunable frameworks, huge specific surface areas, flexible chemical composition, flexible structures and a large volume of pores, they are being used to design the electrode materials for electrochemical energy storage devices. As per the literature, MOFs (including manganese, nickel, copper, and cobalt-based zeolitic imidazolate frameworks (ZIFs), University of Oslo (UiO) MOFs, Hong Kong University of Science and Technology (HKUST) MOFs and isoreticular MOFs (IRMOFs)) have attracted much attention in the field of supercapacitors (SCs)/batteries. According to their dimensionality such as 1D, 2D and 3D, pristine MOFs are mainly used as SC materials. Highly porous materials and their composites are capable for intercalation of metal ions (Na+/Li+). Moreover, the supramolecular features (π⋯π, C-H⋯π, hydrogen bond interactions) of redox stable MOFs provide better insight for electrochemical stability. So, this review provides an in-depth analysis of pure MOFs and MOF derived composites (MOF composites and MOF derived porous carbon) as electrode materials and also discusses their metal ion charge storage mechanism. Finally, we provide our perspectives on the current issues and future opportunities for supercapacitor materials.

Construction of hollow sphere MnOX with abundant oxygen vacancy for accelerating VOCs degradation: Investigation through operando spectroscopycombined with on-line mass spectrometry

To clarify the key role of oxygen vacancy defects on enhancing the oxidative activity of the catalysts, metal-organic frameworks (MOFs) derived MnOX catalysts with different morphologies and oxygen vacancy defects were successfully prepared using a facile in-situ self-assembly strategy with different alkali moderators. The obtained morphologies included three-dimensional (3D) triangular cone stacked MnOX hollow sphere (MnOX-H) and 3D nanoparticle stacked MnOX nanosphere (MnOX-N). Compared to MnOX-N, MnOX-H exhibited higher activity for the oxidation of toluene (T90 = 226 °C). This was mainly due to the large number of oxygen vacancy defects and Mn4+ species in the MnOX-H catalyst. In addition, the hollow structure of MnOX-H not only facilitated toluene adsorption and activation of toluene and also provided more active sites for toluene oxidation. Reaction mechanism studies showed that the conversion of toluene to benzoate could be realized over MnOX-H catalyst during toluene adsorption at room temperature. In addition, abundant oxygen vacancy defects can accelerate the activated oxidation of toluene and the formation of oxidation products during toluene oxidation.

Mesoporous Cubic Nanocages Assembled by Coupled Monolayers With 100% Theoretical Capacity and Robust Cycling

High capacity and long cycling often conflict with each other in electrode materials. Despite extensive efforts in structural design, it remains challenging to simultaneously achieve dual high electrochemical properties. In this study, we prepared brand-new completely uniform mesoporous cubic-cages assembled by large d-spacing Ni(OH)2 coupled monolayers intercalated with VO4 3- (NiCMCs) using a biomimetic approach. Such unique mesoporous structural configuration results in an almost full atomic exposure with an amazing specific surface area of 505 m2/g and atomic utilization efficiency close to the theoretical limit, which is the highest value and far surpasses all of the reported Ni(OH)2. Thus, a breakthrough in simultaneously attaining high capacity approaching the 100% theoretical value and robust cycling of 10,000 cycles is achieved, setting a new precedent in achieving double-high attributes. When combined with high-performance Bi2O3 hexagonal nanotubes, the resulting aqueous battery exhibits an ultrahigh energy density of 115 Wh/kg and an outstanding power density of 9.5 kW/kg among the same kind. Characterizations and simulations reveal the important role of large interlayer spacing intercalation units and mesoporous cages for excellent electrochemical thermodynamics and kinetics. This work represents a milestone in developing "double-high" electrode materials, pointing in the direction for related research and paving the way for their practical application.

Selective Reduction of Multivariate Metal-Organic Frameworks for Advanced Electrocatalytic Cathodes in High Areal Capacity and Long-Life Lithium-Sulfur Batteries

Lithium-sulfur batteries hold great promise as next-generation high-energy-density batteries. However, their performance has been limited by the low cycling stability and sulfur utilization. Herein, we demonstrate that a selective reduction of the multivariate metal-organic framework, MTV-MOF-74 (Co, Ni, Fe), transforms the framework into a porous carbon decorated with bimetallic CoNi alloy and Fe3O4 nanoparticles capable of entrapping soluble lithium polysulfides while synergistically facilitating their rapid conversion into Li2S. Electrochemical studies on coin cells containing 89 wt % sulfur loading revealed a reversible capacity of 1439.8 mA h g-1 at 0.05 C and prolonged cycling stability for 1000 cycles at 1 C/1060.2 mA h g-1 with a decay rate of 0.018% per cycle. At a high areal sulfur loading of 6.9 mg cm-2 and lean electrolyte/sulfur ratio (4.5 μL:1.0 mg), the battery based on the 89S@CoNiFe3O4/PC cathode provides a high areal capacity of 6.7 mA h cm-2. The battery exhibits an outstanding power density of 849 W kg-1 at 5 C and delivers a specific energy of 216 W h kg-1 at 2 C, corresponding to a specific power of 433 W kg-1. Density functional theory shows that the observed results are due to the strong interaction between the CoNi alloy and Fe3O4, facilitated by charge transfer between the polysulfides and the substrate.

Recent progress in 1D MOFs and their applications in energy and environmental fields

Metal organic frameworks (MOFs) are porous coordination polymers with adjustable nanostructure, high porosity and large surface areas. These features make MOFs, their derivates and composites all delivered remarkable potential in energy and environmental fields, such as rechargeable batteries, supercapacitors, catalysts, water purification and desalination, gas treatment, toxic matter degradation, etc. In particular, one-dimensional (1D) MOFs have generated extensive attention due to their unique 1D nanostructures. To prepare 1D MOF nanostructures, it is necessary to explore and enhance synthesis routes. In this review, the preparation of 1D MOF materials and their recent process applied in energy and environmental fields will be discussed. The relationship between MOFs' 1D morphologies and the properties in their applications will also be analyzed. Finally, we will also summary and make perspectives about the future development of 1D MOFs in fabrication and applications in energy and environmental fields.

Synthesis of Ni3S2 and MOF-Derived Ni(OH)2 Composite Electrode Materials on Ni Foam for High-Performance Supercapacitors

Honeycomb-like Ni(OH)₂/Ni₃S₂/Ni foam (NF) was fabricated via a two-step hydrothermal process and subsequent alkalization. Ni₃S₂ with a honeycombed structure was in-situ synthesized on the NF surface by a hydrothermal process. MOF-derived Ni(OH)₂ nanosheets were then successfully grown on the Ni₃S₂/NF surface by a second hydrothermal process and alkaline treatment, and a large number of nanosheets were interconnected to form a typical honeycomb-like structure with a large specific surface area and porosity. As a binder-free electrode, the prepared honeycomb-like Ni(OH)₂/Ni₃S₂/NF exhibited a high specific capacitance (2207 F·g^-1 at 1 A·g^-1, 1929.7 F·g^-1 at 5 mV·s^-1) and a remarkable rate capability and cycling stability, with 62.3% of the initial value (1 A·g^-1) retained at 10 A·g^-1 and 90.4% of the initial value (first circle at 50 mV·s^-1) retained after 5000 cycles. A hybrid supercapacitor (HSC) was assembled with Ni(OH)₂/Ni₃S₂/NF as the positive electrode and activated carbon (AC) as the negative electrode and exhibited an outstanding energy density of 24.5 Wh·kg^-1 at the power density of 375 W·kg^-1. These encouraging results render the electrode a potential candidate for energy storage.

Architecting Nanostructured Co-BTC@GO Composites for Supercapacitor Electrode Application

Herein, we present an innovative graphene oxide (GO)-induced strategy for synthesizing GO-based metal-organic-framework composites (Co-BTC@GO) for high-performance supercapacitors. 1,3,5-Benzene tricarboxylic acid (BTC) is used as an inexpensive organic ligand for the synthesis of composites. An optimal GO dosage was ascertained by the combined analysis of morphology characterization and electrochemical measurement. The 3D Co-BTC@GO composites display a microsphere morphology similar to that of Co-BTC, indicating the framework effect of Co-BTC on GO dispersion. The Co-BTC@GO composites own a stable interface between the electrolyte and electrodes, as well as a better charge transfer path than pristine GO and Co-BTC. A study was conducted to determine the synergistic effects and electrochemical behavior of GO content on Co-BTC. The highest energy storage performance was achieved for Co-BTC@GO 2 (GO dosage is 0.02 g). The maximum specific capacitance was 1144 F/g at 1 A/g, with an excellent rate capability. After 2000 cycles, Co-BTC@GO 2 maintains outstanding life stability of 88.1%. It is expected that this material will throw light on the development of supercapacitor electrodes that hold good electrochemical properties.

Sensing performance and mechanism of carbon dots encapsulated into metal-organic frameworks

Metal-organic frameworks (MOFs) can be combined with nanomaterials and the combined composites have excellent optical properties. Carbon dots (CDs) with tiny particle size, non-toxic and rich surface functional groups are novel fluorescent materials. Carbon dots@metal-organic frameworks (CDs@MOFs) are synthesized by encapsulating CDs into MOFs. CDs@MOFs are promising composites for the preparation of a new generation of fluorescence sensors, which combine the hybrid properties of MOFs and the special optical properties of CDs. Urged as such, we are encouraged to categorize according to the sensing mechanisms. These include fluorescence resonance energy transfer (FRET), aggregation-caused quenching (ACQ), static quenching, dynamic quenching, photo-induced electron transfer (PET), inner filter effect (IFE) and so on. Based on the above mechanisms, CDs@MOFs can specifically interact with target analytes to generate fluorescence quenching. This review covers the research progress of CDs@MOFs in recent five years (with 103 refs), synthetic design of CDs@MOFs and introduces the sensing mechanism. The current challenges and future research directions are discussed briefly. The sensing mechanism and applications of CDs@MOFs.

Metal-Organic Framework Materials for Electrochemical Supercapacitors

Exploring new materials with high stability and capacity is full of challenges in sustainable energy conversion and storage systems. Metal-organic frameworks (MOFs), as a new type of porous material, show the advantages of large specific surface area, high porosity, low density, and adjustable pore size, exhibiting a broad application prospect in the field of electrocatalytic reactions, batteries, particularly in the field of supercapacitors. This comprehensive review outlines the recent progress in synthetic methods and electrochemical performances of MOF materials, as well as their applications in supercapacitors. Additionally, the superiorities of MOFs-related materials are highlighted, while major challenges or opportunities for future research on them for electrochemical supercapacitors have been discussed and displayed, along with extensive experimental experiences.

Self-Assembly of Metal-Organic Frameworks on Graphene Oxide Nanosheets and In Situ Conversion into a Nickel Hydroxide/Graphene Oxide Battery-Type Electrode

Graphene oxide (GO) has been widely reported as a supercapacitor electrode. Especially, GO is usually utilized to composite with electrochemical active materials, such as transition-metal oxide/hydroxide/sulfide, due to its considerable conductivity and mechanical strength. However, the ideal design and treatment for compositing GO with active materials are still challenging. Herein, an Ni-metal-organic framework (MOF) was self-assembled on GO nanosheets via the solvothermal method and was subsequently etched into the Ni(OH)₂-GO composite electrode material through a gentle hydrolysis strategy. The GO support enables fast electron transport within the composite material, and the nickel hydroxide growth on GO nanosheets can prevent their aggregation, guaranteeing rapid ion migration. The improved Ni(OH)₂-GO battery-type electrode features outstanding stability (capacity retention of 108% at 8000 cycles) and a considerable specific capacity (SC) of 1007.5 C g^-1 at a current density of 0.5 A g^-1. Compared with MOF-derived Ni(OH)₂ obtained through hydrolysis, Ni(OH)₂-GO only contains 7.41% wt GO, while its SC is almost 50% higher. An asymmetric supercapacitor has an energy density of 65.22 W h kg^-1 and a power density of 395.27 W kg^-1 utilizing p-phenylenediamine-functional reduced GO as the negative electrode, and it can maintain 73.08% capacity during 8000 cycles at a current density of 5 A g^-1.

Flower-like Ni3Sn2@Ni3S2 with core-shell nanostructure as electrode material for supercapacitors with high rate and capacitance

To enhance the specific capacitance as well as maintain satisfactory rate performance of nickel hydroxide and nickel sulfide, in this work, the ultra-fine nickel-tin nanoparticles with high conductivity are selected to synthesize Ni₃Sn₂@Ni(OH)₂ and Ni₃Sn₂@Ni₃S₂ nanoflowers. Alloy as the core material improves the electrical conductivity of the composite, and the nanosheets prepared by electrochemical corrosion effectively avoid aggregation as well as increase the active sites of the electrode material. By adjusting the corrosion time, the Ni₃Sn₂@Ni(OH)₂ with better morphology displays a high specific capacitance (1277.37C g^-1 at 1 A g^-1) and good rate performance (1028C g^-1 at 20 A g^-1). After sulfurization, the optimal Ni₃Sn₂@Ni₃S₂ perfectly retains the morphological characterizations of the precursor and exhibits ultra-high specific capacitance (1619.02C g^-1 at 1 A g^-1) as well as outstanding rate performance (1312C g^-1 at 20 A g^-1). The samples before and after vulcanization both have the excellent electrochemical properties, which is attributed to the rational design and construction of the alloy-based core-shell nanostructures. Besides, the all-solid-state hybrid supercapacitor (HSC) is assembled by Ni₃Sn₂@Ni₃S₂ as the positive electrode and activated carbon as the negative electrode, displaying outstanding energy density of 70.54 Wh kg^-1 at 808.67 W kg^-1 and excellent cycling stability (93.21 % after 10,000 cycles). This work provides a novel ingenuity for synthesizing high-performance supercapacitor electrodes.

Facile synthesis of MnCo2S4 nanosheets as a binder-free electrode material for high performance supercapacitor applications

This paper reported an easy synthesis of MnCo2S4 (MCS) nanosheets by a one-pot solvothermal method for high performance supercapacitor electrode material applications. The obtained MCS nanosheets with an ultrathin thickness...

Conversion of Amorphous MOF Microspheres into a Nickel Phosphate Battery-Type Electrode Using the "Anticollapse" Two-Step Strategy

Metal-organic frameworks (MOFs) have attracted great attention as templates for preparation of functional porous materials owing to their adjustable structures, rich porosity, and controllable components. However, collapsed templates during the conversion process hinder their application and synthesis of derivatives. In this study, we demonstrate a novel two-step etching strategy during which amorphous MOF microspheres are initially transformed into nickel hydroxide and then subsequently transformed into microspherical nickel phosphates. Through this strategy, the prepared nickel phosphates maintain the microspherical morphology of MOFs but with no MOF residuals, exhibiting ultrahigh specific surface area, uniform pore size, and good structural robustness. Examined as a supercapacitor electrode, they show an outstanding specific capacity of 820 C g^-1 at 0.5 A g^-1 and remarkable cycling stability of 88% capacity retention after 10 000 cycles. Moreover, an asymmetric supercapacitor constructed utilizing reduced graphene cross-linked with p-phenylenediamine oxide (PPD-rGO) as the cathode displays a preeminent energy density of 64.56 Wh kg^-1 at a power density of 507 W kg^-1. This strategy has important significance in guiding the preparation of high-performance MOF-derived electrodes.

Recent progress on pristine two-dimensional metal-organic frameworks as active components in supercapacitors

Two-dimensional (2D) metal-organic frameworks (MOFs) are a new generation of 2D materials that can provide uniform active sites and unique open channels as well as excellent catalytic abilities, interesting magnetic properties, and reasonable electrical conductivities. Thus, these MOFs are uniquely qualified for use in applications in energy-related fields or portable devices because they possess fast charge and discharge ability, high power density, and ultralong cycle life factors. There has been worldwide research interest in 2D conducting MOFs, and numerous techniques and strategies have been developed to synthesize these MOFs and their derivatives. Thus, this is the opportune time to review recent research progress on the development of 2D MOFs as electrodes in supercapacitors. This review covers synthetic design strategies, electrochemical performances, and working mechanisms. We will divide these 2D MOFs into two types on the basis of their conductive aspects: 2D conductive MOFs and 2D layered MOFs (including pillar-layered MOFs and 2D nanosheets). The challenges and perspectives of 2D MOFs are also provided.

Alkali-induced Transformation of Ni-MOF into Ni(OH)2 Nanostructured Flowers for Efficient Electrocatalytic Methanol Oxidation Reaction

Post treatment of metal-organic frameworks (MOFs) is widely employed to develop efficient electrocatalysts with better catalytic properties. But the complex processes of post treatment generally led to the collapse of the original structures of MOFs, making the preservation of their pristine hierarchical porous structure a great challenge. Herein, we propose the strategy of alkali treatment of Ni-MOF to transform it into Ni(OH)₂ with similar morphology and enhanced electrocatalytic properties for methanol oxidation reaction (MOR). The structure and electrocatalytic properties of as-obtained Ni(OH)₂ nanostructured flowers were seriously depended on the alkali concentrations. As the result, Ni(OH)₂ obtained from Ni-MOF treated by 0.25 M NaOH (noted as Ni(OH)₂ -0.25) performs 1.5 and 2.5 times larger current density than those of Ni(OH)₂ -0.025 and Ni(OH)₂ -0.5 for MOR. Moreover, the electrocatalytic process and mechanism of MOR on the catalyst of Ni(OH)₂ -0.25 are also revealed. Hence, this ex situ conversion strategy of alkali treatment for Ni-MOF uncovered the transformation of MOFs in alkaline solution and develops robust electrocatalyst for practical application of methanol fuel cells.

Synthesis of 3D flower-like hierarchical NiCo-LDH microspheres with boosted electrochemical performance for hybrid supercapacitors

3D flower-like NiCo-LDH microspheres synthesized via conformal alkaline hydrolysis strategy possess superior specific capacitance and stability, and the assembled HSC exhibits prominent power/energy density and durability.

Improved ionic diffusion and interfacial charge/mass transfer of ZIF-67-derived Ni–Co-LDH electrodes with bare ZIF-residual for enhanced supercapacitor performance

The crystallization of Ni–Co-LDH and the morphology of the hierarchical structure can be simply optimized to achieve improved ionic diffusion and reduce reaction resistance of charge/mass transfer between the electrode/electrolyte interfaces.

Cobalt-based metal–organic frameworks as functional materials for battery applications

Research progress on cobalt-based metal–organic frameworks as functional materials for battery applications has been presented.

The Evolution in Electrochemical Performance of Honeycomb-Like Ni(OH)2 Derived from MOF Template with Morphology as a High-Performance Electrode Material for Supercapacitors

Ni(OH)2 derived from an MOF template was synthesized as an electrode material for supercapacitors. The electrochemical performance of the electrode was adjusted by effectively regulating the morphology of Ni(OH)2. The evolution of electrochemical performance of the electrode with morphology of Ni(OH)2 was highlighted in detail, based on which honeycomb-like Ni(OH)2 was successfully synthesized, and endowed the electrode with outstanding electrochemical performance. For the three-electrode testing system, honeycomb-like Ni(OH)2 exhibited a very high specific capacitance (1865 F·g-1 at 1 A·g-1, 1550 F·g-1 at 5 mV·s-1). Moreover, it also presented an excellent rate capability and cycling stability, due to 59.46 % of the initial value (1 A·g-1) being retained at 10 A·g-1, and 172% of initial value (first circle at 50 mV·s-1) being retained after 20,000 cycles. With respect to the assembled hybrid supercapacitor, honeycomb-like Ni(OH)2 also displayed superior electrochemical performance, with a high energy density (83.9 Wh·kg-1 at a power density of 374.8 W·kg-1). The outstanding electrochemical performance of Ni(OH)2 should be attributed to its unique honeycomb-like structure, with a very high specific surface area, which greatly accelerates the transformation and diffusion of active ions.

In situ semi-transformation from heterometallic MOFs to Fe-Ni LDH/MOF hierarchical architectures for boosted oxygen evolution reaction

Metal-organic frameworks (MOFs) with large surface area, abundant coordination metal centers and tunable structures are regarded as promising electrocatalysts for the water splitting reaction. However, the less accessible active sites and poor stability of MOFs hinder their potential practical applications. Hierarchical double-layer hydroxide (LDH)/MOF electrocatalysts that combine the advantages of two materials are expected to overcome these drawbacks. Herein, we develop a simple and universal strategy, in situ pseudomorphic transformation, to construct hierarchical LDH/MOF electrocatalysts. Accordingly, ultra-thin Fe-Ni LDH nanosheets are in situ produced in the heterometallic MOF during the transformation process. Profiting from the abundant metal sites and the extended electron transport channel from the inserted ultra-thin LDH arrays, the hierarchical Fe-Ni LDH/MOFs exhibit striking electrochemical activities for the oxygen evolution reaction (OER). In particular, the as-synthesized Fe-Ni LDH/MOF-b2 delivers the best OER performance, exhibiting an ultralow overpotential (255 mV at 10 mA cm^-2), minimum Tafel slope (24 mV dec^-1) and outstanding cycling durability. Meanwhile, the evolution process of the hierarchical Fe-Ni LDH/MOF has been monitored with the controllable in situ semi-transformation strategy. This also provides an opportunity to decipher the original active species for the OER process. Mechanism analysis indicates that the bimetallic MOF and bimetallic LDH are both active species, and the excellent OER performance of hierarchical Fe-Ni LDH/MOF could be attributed to the effect of "a whole greater than the sum of the parts".

Boosting the energy density of supercapacitors by encapsulating a multi-shelled zinc-cobalt-selenide hollow nanosphere cathode and a yolk-double shell cobalt-iron-selenide hollow nanosphere anode in a graphene network

The practical exploration of electrode materials with complex hollow structures is of considerable significance in energy storage applications. Mixed-metal selenides (MMSs) with favorable architectures emerge as new electrode materials for supercapacitor (SC) applications owing to their excellent conductivity. Herein, a facile and effective metal-organic framework (MOF)-derived strategy is introduced to encapsulate multi-shelled zinc-cobalt-selenide hollow nanosphere positive and yolk-double shell cobalt-iron-selenide hollow nanosphere negative electrode materials with controlled shell numbers in a graphene network (denoted as G/MSZCS-HS and G/YDSCFS-HS, respectively) for SC applications. Due to the considerable electrical conductivity and unique structures of both electrodes, the G/MSZCS-HS positive and G/YDSCFS-HS negative electrodes exhibit remarkable capacities (∼376.75 mA h g^-1 and 293.1 mA h g^-1, respectively, at 2 A g^-1), superior rate performances (83.4% and 74%, respectively), and an excellent cyclability (96.8% and 92.9%, respectively). Furthermore, an asymmetric device (G/MSZCS-HS//G/YDSCFS-HS) has been fabricated with the ability to deliver an exceptional energy density (126.3 W h kg^-1 at 902.15 W kg^-1), high robustness of 91.7%, and a reasonable capacity of 140.3 mA h g^-1.

NiSe2/Ni(OH)2 Heterojunction Composite through Epitaxial-like Strategy as High-Rate Battery-Type Electrode Material

Constructing heterojunction is a promising way to improve the charge transfer efficiency and can thus promote the electrochemical properties. Herein, a facile and effective epitaxial-like growth strategy is applied to NiSe₂ nano-octahedra to fabricate the NiSe₂-(100)/Ni(OH)₂-(110) heterojunction. The heterojunction composite and Ni(OH)₂ (performing high electrochemical activity) is ideal high-rate battery-type supercapacitor electrode. The NiSe₂/Ni(OH)₂ electrode exhibits a high specific capacity of 909 C g^-1 at 1 A g^-1 and 597 C g^-1 at 20 A g^-1. The assembled asymmetric supercapacitor composed of the NiSe₂/Ni(OH)₂ cathode and p-phenylenediamine-functional reduced graphene oxide anode achieves an ultrahigh specific capacity of 303 C g^-1 at 1 A g^-1 and a superior energy density of 76.1 Wh kg^-1 at 906 W kg^-1, as well as an outstanding cycling stability of 82% retention for 8000 cycles at 10 A g^-1. To the best of our knowledge, this is the first example of NiSe₂/Ni(OH)₂ heterojunction exhibiting such remarkable supercapacitor performance. This work not only provides a promising candidate for next-generation energy storage device but also offers a possible universal strategy to fabricate metal selenides/metal hydroxides heterojunctions.

Metal–organic frameworks with different spatial dimensions for supercapacitors

Recent progress in MOF materials for SCs with different spatial dimensions, such as 2D MOFs, including conductive MOFs and nanosheets, and 3D MOFs, categorized as single metallic and multiple metallic MOFs, are reviewed.

Integration of fluorescent probes into metal–organic frameworks for improved performances

Recent years have witnessed a rapid development of fluorescent probes in both analytical sensing and optical imaging. Enormous efforts have been devoted to the regulation of fluorescent probes during their development, such as improving accuracy, sensitivity, selectivity, recyclability and overcoming the aggregation-caused quenching effect. Metal–organic frameworks (MOFs) as a new class of crystalline porous materials possess abundant host–guest chemistry, based on which they display a great application potential in regulating fluorescent probes. This review summarized the research works on the regulation of fluorescent probes using MOFs, with emphasis on the methods of integrating fluorescent probes into MOFs, the regulation effects of MOFs on fluorescent probes, the superiorities of MOFs in regulating fluorescent probes, and the outlook of this subject. It is desirably hoped that this review can provide a useful reference for the researchers interested in this field.

This review surveyed the research works for the regulation of fluorescent probes with metal–organic frameworks based on host–guest chemistry.

Rational design of flower-like cobalt–manganese-sulfide nanosheets for high performance supercapacitor electrode materials

The design and development of composite materials with novel structures is one of the effective ways to enhance the electrochemical properties of supercapacitors.

Green synthesis of ZnO–Co3O4 nanocomposite using facile foliar fuel and investigation of its electrochemical behaviour for supercapacitors

Currently, the sustainable fabrication of supercapacitors with enhanced properties is one of the significant research hotspots.

Rapid in situ growth of β-Ni(OH)2 nanosheet arrays on nickel foam as an integrated electrode for supercapacitors exhibiting high energy density

Ni(OH)2 has been widely investigated as a prospective electrode material because of its high theoretical capacitance and relatively low cost. However its synthesis usually needs a complex and lengthy process, and a binder is generally used for fabricating Ni(OH)2 based electrodes. In this work, a self-supporting binder-free β-Ni(OH)2@nickel foam (NF) integrated electrode was prepared by the in situ growth of β-Ni(OH)2 on NF using a rapid and facile approach. This approach consists of two processing steps: (1) the pre-treatment of NF with an acid and (2) the quick in situ electrochemical synthesis of β-Ni(OH)2 on the NF in the KOH electrolyte within half a minute under an applied voltage. The β-Ni(OH)2@NF integrated electrode possesses a three-dimensional network structure of nanosheet arrays and exhibits excellent electrochemical performance. Its areal capacity is 3.68 mA h cm-2 at a current density of 2 mA cm-2, and the capacity can retain 115.8% of its initial value even after 2000 cycles at a current density of 15 mA cm-2. Moreover, the as-assembled β-Ni(OH)2@NF//activated carbon (AC) asymmetric supercapacitor (ASC) exhibits a high energy density of 74.2 W h kg-1 with a power density of 776.9 W kg-1 and excellent cycling stability (89.9% retained after 10 000 cycles). This work provides an efficient, facile and economic method for fabricating Ni(OH)2 based integrated electrodes for high-performance supercapacitors.

"HOT" Alkaline Hydrolysis of Amorphous MOF Microspheres to Produce Ultrastable Bimetal Hydroxide Electrode with Boosted Cycling Stability

Nickel/cobalt hydroxide is a promising battery-type electrode material for supercapacitors. However, its low cycle stability hinders further applications. Herein, Ni_0.7 Co_0.3 (OH)₂ core-shell microspheres exhibiting extreme-prolonged cycling life are successfully synthesized, employing Ni-Co-metal-organic framework (MOF) as the precursor/template and a specific hydrolysis strategy. The Ni-Co-MOF and KOH aqueous solution are separated and heated to 120 °C before mixing, rather than mixing before heating. Through this hydrolysis strategy, no MOF residual exists in the product, contributing to close stacking of the hydroxide nanoflakes to generate Ni_0.7 Co_0.3 (OH)₂ microspheres with a robust core-shell structure. The electrode material exhibits high specific capacity (945 C g^-1 at 0.5 A g^-1 ) and unprecedented cycling performance (100% after 10 000 cycles). The fabricated asymmetric supercapacitor delivers an energy density of 40.14 Wh kg^-1 at a power density of 400.56 W kg^-1 and excellent cycling stability (100% after 20 000 cycles). As far as is known, it is the best cycling performance for pure Ni/Co(OH)₂ .

Lithium ferrite (α-LiFe5O8) nanorod based battery-type asymmetric supercapacitor with NiO nanoflakes as the counter electrode

The fabricated battery-type NiO//α-LiFe₅O₈ cell could deliver a specific energy of 30 W h Kg⁻¹ at a specific power of 621 W kg⁻¹ with 90.5% capacity retention at the end of 5000 GCD cycles.