Construction of MOF-derived hollow Ni-Zn-Co-S nanosword arrays as binder-free electrodes for asymmetric supercapacitors with high energy density

Hybridization of metal-organic frameworks and MXenes: Expanding horizons in supercapacitor applications

Metal-organic frameworks (MOFs) and MXenes have gained prominence in the queue of advanced material research. Both materials' outstanding physical and chemical characteristics prominently promote their utilization in diverse fields, especially the electrochemical energy storage (EES) domain. The collective contribution of extremely high specific surface area (SSA), customizable pores, and abundant active sites propose MOFs as integral materials for EES devices. However, conventional MOFs endure low conductivity, constraining their utility in practical applications. The development of hybrid materials via integrating MOFs with various conductive materials stands out as an effective approach to improvising MOF's conductivity. MXenes, formulated as two-dimensional (2D) carbides and nitrides of transition metals, fall in the category of the latest 2D materials. MXenes possess extensive structural diversity, impressive conductivity, and rich surface chemical characteristics. The electrochemical characteristics of MOF@MXene hybrids outperform MOFs and MXenes individually, credited to the synergistic effect of both components. Additionally, the MOF derivatives coupled with MXene, exhibiting unique morphologies, demonstrate outstanding electrochemical performance. The important attributes of MOF@MXene hybrids, including the various synthesis protocols, have been summarized in this review. This review delves into the architectural analysis of both MOFs and MXenes, along with their advanced hybrids. Furthermore, the comprehensive survey of the latest advancements in MOF@MXene hybrids as electroactive material for supercapacitors (SCs) is the prime objective of this review. The review concludes with an elaborate discussion of the current challenges faced and the future outlooks for optimizing MOF@MXene composites.

Al-Based MOF-Derived Amorphous/Crystalline Heterophase Cobalt Sulfides as High-Performance Supercapacitor Materials

Transition metal sulfides (TMSs) are promising electrode materials due to their high theoretical specific capacitance, but sluggish charge transfer kinetics and an insufficient number of active sites hamper their applications in supercapacitors. In this work, a self-sacrificial template strategy was employed to construct Al-based MOF-derived metal sulfides with an amorphous/crystalline (a/c) heterophase, in which aluminum, nitrogen, and carbon species were evenly coordinated in the amorphous phase. The metal sulfides a/c-Co(Al)S-1 and a/c-Co(Al)S-2, originating from the CAU-1 and CoAl-MOF on NF as self-sacrificial templates, were investigated as electrode materials, respectively, in which the a/c-Co(Al)S-1 showed a more excellent electrochemical performance. Through acid etching CAU-1 using Co(NO3)2 followed by sulfuration, the a/c-Co(Al)S-1 with a unique 3D network structure was constructed, whose unique architecture expanded the interfacial contact with the electrolyte and provided vast active sites, accelerating the charge transportation and ion diffusion. Notably, the a/c-Co(Al)S-1 displayed a high specific charge of 1791.8 C g-1 at 1 A g-1, satisfactory cycle stability, and good rate capability. The corresponding assembled a/c-Co(Al)S-1//AC device delivered a high energy density of 77.1 Wh kg-1 at 800 W kg-1 and good durability (87.4% capacitance retention over 10 000 cycles).

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.

Binder-Free MOF-Based and MOF-Derived Nanoarrays for Flexible Electrochemical Energy Storage: Progress and Perspectives

The fast development of Internet of Things and the rapid advent of next-generation versatile wearable electronics require cost-effective and highly-efficient electroactive materials for flexible electrochemical energy storage devices. Among various electroactive materials, binder-free nanostructured arrays have attracted widespread attention. Featured with growing on a conductive and flexible substrate without using inactive and insulating binders, binder-free 3D nanoarray electrodes facilitate fast electron/ion transportation and rapid reaction kinetics with more exposed active sites, maintain structure integrity of electrodes even under bending or twisted conditions, readily release generated joule heat during charge/discharge cycles and achieve enhanced gravimetric capacity of the whole device. Binder-free metal-organic framework (MOF) nanoarrays and/or MOF-derived nanoarrays with high surface area and unique porous structure have emerged with great potential in energy storage field and been extensively exploited in recent years. In this review, common substrates used for binder-free nanoarrays are compared and discussed. Various MOF-based and MOF-derived nanoarrays, including metal oxides, sulfides, selenides, nitrides, phosphides and nitrogen-doped carbons, are surveyed and their electrochemical performance along with their applications in flexible energy storage are analyzed and overviewed. In addition, key technical issues and outlooks on future development of MOF-based and MOF-derived nanoarrays toward flexible energy storage are also offered.

Road Map for In Situ Grown Binder-Free MOFs and Their Derivatives as Freestanding Electrodes for Supercapacitors

Among several electrocatalysts for energy storage purposes including supercapacitors, metal-organic frameworks (MOFs), and their derivatives have spurred wide spread interest owing to their structural merits, multifariousness with tailor-made functionalities and tunable pore sizes. The electrochemical performance of supercapacitors can be further enhanced using in situ grown MOFs and their derivatives, eliminating the role of insulating binders whose "dead mass" contribution hampers the device capability otherwise. The expulsion of binders not only ensures better adhesion of catalyst material with the current collector but also facilitates the transport of electron and electrolyte ions and remedy cycle performance deterioration with better chemical stability. This review systematically summarizes different kinds of metal-ligand combinations for in situ grown MOFs and derivatives, preparation techniques, modification strategies, properties, and charge transport mechanisms as freestanding electrode materials in determining the performance of supercapacitors. In the end, the review also highlights potential promises, challenges, and state-of-the-art advancement in the rational design of electrodes to overcome the bottlenecks and to improve the capability of MOFs in energy storage applications.

Cycling stability of Fe2O3 nanosheets as supercapacitor sheet electrodes enhanced by MgFe2O4 nanoparticles

The Fe2O3 material is a common active material for supercapacitor electrodes and has received much attention due to its cheap and easy availability and high initial specific capacitance. In the present study, we prepared adhesive-free Fe2O3 sheet electrodes for supercapacitors by growing Fe2O3 material on nickel foam by hydrothermal method. The sheet electrode exhibited a high initial specific capacitance of 863 F g-1, but we found that the sheet lost its specific capacitance too quickly through cyclic stability tests. To solve this problem, Fe2O3/MgFe2O4 composites were grown on nickel foam (NF). It was found through testing that the cycling stability of the sheet electrode gradually increased as the content of MgFe2O4 material increased. When the molar ratio of Fe2O3 to MgFe2O4 material was 1 : 1, the initial specific capacitance of the sheet electrode was 815 F g-1 and the capacitance remained at 81.25% of the initial specific capacitance after 1000 cycles. The better cycling stability results from the more stable structure of the composite, the synergistic effect leading to better reversibility of the reaction.

Highly stable, stretchable, and versatile electrodes by coupling of NiCoS nanosheets with metallic networks for flexible electronics

The rapid development of portable electronics has contributed to an urgent demand for versatile and flexible electrodes of wearable energy storage devices and pressure sensors. We fabricate a stretchable electrode by coupling the nickel-cobalt sulfide (NiCoS) nanosheet layer with Ag@NiCo nanowire (NW) networks. NiCoS wrinkled nanostructure, highly conductive networks, and intense interactions between substrate/networks and active materials/networks endow the electrodes with excellent energy storage capacity, superior electrochemical/mechanical stability, and good conductivity. A high-performance asymmetric supercapacitor is developed using the composite electrode. It operates in a wide potential window of 1.4 V and achieves a maximum energy density of 40.0 W h kg^-1 at a power density of 1.1 kW kg^-1; it also exhibits excellent mechanical flexibility and good waterproof performance. Moreover, a sandwiched capacitive pressure sensor constructed using the same electrodes has a wide sensing range (up to 260 kPa), low detection limit (∼47 mN), fast response (∼66 ms), and excellent mechanical stability (10 000 cycles). This study demonstrates that the appropriate design of the functional electrode facilitates the construction of various high-performance devices, denoting the versatility of our electrodes in the development of wearable electronics.

Morphology controlling of manganese-cobalt-sulfide nanoflake arrays using polyvinylpyrrolidone capping agent to enhance the performance of hybrid supercapacitors

Transition metal sulfide-based electrode materials are promising candidates for energy storage applications owing to their richer redox-active sites and higher electrical conductivity than their oxide counterparts. Manganese-cobalt-sulfide (MCS) nanoflakes were synthesized on nickel foam in the presence of polyvinylpyrrolidone (PVP) as a capping agent using a one-step hydrothermal method. The variation in the amount of PVP in the reaction solution had a prominent impact on the MCS electrode morphology. PVP altered the morphology of the MCS nanoflakes. Different shapes of interconnecting-nanoflake arrays were formed with different amounts of PVP. The MCS electrode prepared using 0.2 g of PVP (MCS-P2) showed the best efficiency with a specific capacity of 1312 C g^-1 (3215 F g^-1) at 1 A g^-1 and still retained a remarkable capacity of 1000 C g^-1 (2480 F g^-1) at 20 A g^-1. Moreover, the hybrid supercapacitor (HS) device consisting of MCS-P2//reduced graphene oxide (rGO) revealed a high energy density of 48.7 Wh kg^-1 at a corresponding power density of 386 W kg^-1. Even at a higher power density of 10.8 kW kg^-1, a notable energy density of 25.5 Wh kg^-1 was retained. These remarkable results highlight the potential applications of the MCS-P2 electrode material in energy storage.

Cooperative effect of bimetallic MOF-derived CoNi(OH)2@NiCo2S4nanocomposite electrocatalysts with boosted oxygen evolution activity

Highly efficient and inexpensive electrocatalysts for oxygen evolution reaction (OER) are extensively studied for water splitting. Herein, a unique bimetallic nanocomposite CoNi(OH)₂@NiCo₂S₄nanosheet arrays derived from metal-organic-frameworks (MOFs, CoNi-ZIF) is simply fabricated on Ni foam, endowing large specific surface area and outstanding electrical conductivity. Compared with their single-metallic counterparts, the bimetallic composite displays dramatically low overpotential and small Tafel slope as well as outstanding catalytic stability. The overpoptential at 20 mA cm^-2for CoNi(OH)₂@NiCo₂S₄is only 230 mV in comparison with Ni(OH)₂@Ni₃S₂(266 mV), Co(OH)₂@Co₃S₄(294 mV) and RuO₂(η = 302 mV). First-principle calculations based on density functional theory (DFT) are carried out and reveal that the introduction of Ni in Co(OH)₂helps lowered the energy difference of ΔG_OOH*-ΔG_O*, and thereby boosting the OER reactivity. This study provides an effective approach for the rational construction of low-cost metal hybrids.

In-situ construction of heterostructure (Ni, Co)Se2 nanoarrays derived from cone-like ZIF-L for high-performance hybrid supercapacitors

The construction of heterostructure could enhance the electron transfer efficiency and increase the number of active sites, which can further develop high-performance electrode materials of supercapacitors. Herein, (Ni, Co)Se₂ nanorod arrays were prepared based on the NiCo-LDH derived from a conical ZIF-L. Significantly, the single nanorod is composed of interconnected NiSe₂ and CoSe₂ nanoparticles, the heterostructure can expose higher conductivity, more sufficient redox reaction active sites and larger specific surface area. The as-obtained CF@(Ni, Co)Se₂ achieved a high specific capacity of 188.8 mAh g^-1 at the current density of 1.0 A g^-1 and an outstanding cycling stability with a high capacity retention of 90% after 8000 cycles. Finally, an hybrid supercapacitor device composed of activated carbon (AC) as negative electrode and CF@(Ni, Co)Se₂ as positive electrode was designed, which revealed an ideal voltage window of 0-1.6 V and exhibited a great energy density of 36.02 Wh kg^-1 at the power density of 800 W kg^-1, such surpassing energy storage characteristics evidently testify that (Ni, Co)Se₂ nanorod arrays can be as the potential electrode material to promote the development of high-performance supercapacitors.

Self-supported metal-organic framework-based nanostructures as binder-free electrodes for supercapacitors

Metal-organic frameworks (MOFs), an interesting class of functional inorganic materials, have recently emerged as suitable electrode materials or templates/precursors of electrode materials for supercapacitors (SCs). The key in utilizing MOF-based electrode materials is to address the low electronic conductivity and poor stability issues. Therefore, the rational design and fabrication of self-supported binder-free electrodes is considered the most promising strategy to address these challenges. In this review, we summarize the recent advances in the design and manufacture of self-supported MOF-based nanostructures and their use as binderless electrodes for SCs, especially over the last five years. The synthesis strategies for constructing pristine MOFs, MOF composites and MOF derivative arrays are overviewed. By highlighting the advantages and challenges of each class of electrode materials, we hope that this review will provide some insights into the rational design of MOF-based electrode materials to promote the future development of this highly exciting field.

Dual-ligand strategies to assemble S, N-containing metal organic framework nanoflowers for hybrid supercapacitors

Ni-MOF [Ni(Tdc)(Bpy)]n was successfully prepared, and the Ni-MOF//AC hybrid supercapacitor exhibited superior energy density and cycling stability.

Tuning the Nanoparticle Interfacial Properties and Stability of the Core-Shell Structure in Zn-Doped NiMoO4@AWO4

The ability to tune the interfacial region in core-shell nanocomposites with a surface reconstruction as a source for surface energy (de)stabilization is presented. We consider Zn-doped nickel molybdate (NiMoO₄) (ZNM) as a core crystal structure and AWO₄ (A = Co or Mg) as a shell surface. Based on the density-functional theory method, the interfacial models of Zn-doped NiMoO₄@AWO₄ (ZNM@AW) core@shell structures are simulated and revealed to undergo surface reconstruction on the (-110) and (-202) surfaces of the AW shells, where the surface degradation of ZNM@MW(-110) is observed. The theoretical simulation is validated against the electrochemical performance of supercapacitor studies. To verify, we synthesize the hierarchical ZNM@AW core@shell semiconductor structured nanocomposites grown on a nickel foam conductive substrate using a facile and green two-step hydrothermal method. The morphology and chemical and electrochemical properties of the hierarchically structured nanocomposites are characterized in detail. The performance of the core@shell is significantly affected by the chosen intrinsic properties of metal oxides and exhibited high performance compared to a single-component system in supercapacitors. The proposed asymmetric device, Zn-doped NiMoO₄@CoWO₄ (ZNM@CW)||activated carbon, exhibits a superior pseudo-capacitance, delivering a high areal capacitance of 0.892 F cm^-2 at a current density of 2 mA cm^-2 and an excellent cycling stability of 96% retention of its initial capacitance after 1000 charge-discharge cycles. These fundamental theoretical and experimental insights with the extent of the surface reconstruction sufficiently explain the storage properties of the studied materials.

Oxygen-vacancy abundant alpha bismuth oxide with enhanced cycle stability for high-energy hybrid supercapacitor electrodes

Bi₂O₃ is an outstanding electrode material due to its high theoretical specific capacity. Hence, the synthesis of δ-Bi₂O₃ materials with high oxygen-vacancy contents could improve their electrochemical performances but causes easy conversion to α-Bi₂O₃ with low oxygen-vacancy contents, leading to poor cycling stability and limited practical applications. To overcome these problems, an effective strategy for constructing high oxygen vacancies α-Bi₂O₃ on activated carbon fiber paper (ACFP) is developed in this study. To this end, ACFP/Bi(OH)₃ is first synthesized by the solvothermal method and then converted to ACFP/α-Bi₂O₃ by in situ electrochemical activation. The proposed innovative electrochemical method quickly and easily introduces oxygen vacancies while preserving the three-dimensional structure, thereby promoting the charge transfer and ions diffusion in ACFP/α-Bi₂O₃. Consequently, the specific capacity of ACFP/α-Bi₂O₃ reaches 906C g^-1 at 1 A g^-1, and the capacity retention remains above 70% after 3000 cycles, a value higher than that of δ-Bi₂O₃ (45%). Furthermore, the hybrid supercapacitor device assembled by ACFP/α-Bi₂O₃ delivers a maximum energy density of 114.9 Wh kg^-1 at 900 W kg^-1 and outstanding cycle stability with 73.56 % retention after 5500 cycles. In sum, the proposed ACFP/α-Bi₂O₃ with high performance and good stability looks promising for use as bismuth-based anode materials in supercapacitors and aqueous batteries.

Design of trimetallic sulfide hollow nanocages from metal-organic frameworks as electrode materials for supercapacitors

Transition metal sulfides (TMSs) are the most used electrode materials for supercapacitors (SCs). However, they still suffer from unsatisfactory electrochemical properties. Designing a hollow mixed TMS nanostructure with a well-defined chemical composition and shape is an effective strategy to tackle this issue, yet remains challenging. Herein, using a bimetallic zeolitic imidazolate framework (Zn-Co-ZIF) with various Zn/Co ratios as the template, a series of trimetallic sulfide (Ni-Zn-Co-S) hollow nanocages were successfully prepared by sequential nickel nitrate etching, co-precipitation and vulcanization. As an electrode material for a three-electrode SC in an aqueous alkaline electrolyte, the Ni-Zn-Co-S-0.25 electrode achieves an ultra-high specific capacitance of 1930.9 at 1 A g^-1 with a good rate performance (64.5% at 10 A g^-1). In order to further prove the advantage of the as-prepared Ni-Zn-Co-S-0.25 material, it was assembled into an asymmetric energy storage device using an activated carbon (AC) anode. The Ni-Zn-Co-S-0.25//AC cell exhibits an outstanding energy storage capability (32.8 W h kg^-1 at 864.8 W kg^-1) with a splendid cyclic life (retaining ∼92.2% of the initial capacitance after 10 000 cycles). The excellent electrochemical performance of Ni-Zn-Co-S-0.25 is ascribed to the merits of the trimetallic sulfide hollow nanocage i.e., good electronic conductivity, a large active surface area, fast charge transfer, rich redox reactions and the synergic effect of different metal ions.

Flexible nickel disulfide nanoparticles-anchored carbon nanofiber hybrid mat as a flexible binder-free cathode for solid-state asymmetric supercapacitors

Nickel disulfide nanoparticles (NiS₂NPs)-anchored carbon nanofibers (NiS₂NPs@CNF) hybrid mats were fabricated via the sequential process of stabilization and carbonization of electrospun polyacrylonitrile-based fibers followed by hydrothermal growth of NiS₂NPs on the porous surface of CNFs. The vertical growth of NiS₂NPs on entire surfaces of porous CNFs appeared in the SEM images of hybrid mat. The hierarchical NiS₂NPs@CNF core-shell hybrid nanofibers with 3D interconnected network architecture can endow continuous channels for easy and rapid ionic diffusion to access the electroactive NiS₂NPs. The conductive and interconnected CNF core could facilitate electron transfer to the NiS₂shell. Moreover, the porous CNF as a buffering matrix can resist volumetric deformation during the long-term charge-discharge process. The NiS₂NPs@CNF electrode can yield high specific capacitance (916.3 F g^-1at 0.5 A g^-1) and reveal excellent cycling performances. The solid-state asymmetric supercapacitor (ASC) was fabricated with NiS₂NPs@CNF mat as a binder-free positive electrode and activated carbon cloth as a negative electrode. As-assembled ASC not only produce high specific capacitance (364.8 F g^-1at 0.5 A g^-1) but also exhibit excellent cycling stability (∼92.8% after 5000 cycles). The ASC delivered a remarkably high energy density of 129.7 Wh kg^-1at a power density of 610 W kg^-1. These encouraging results could make this NiS₂NPs@CNF hybrid mat a good choice of cathode material for the fabrication of flexible solid-state ASC for various flexible/wearable electronics.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.

One-pot construction of highly oriented Co-MOF nanoneedle arrays on Co foam for high-performance supercapacitor

Highly oriented Co-MOF nanoneedle arrays arein situconstructed on Co foam (Co-MOF@Co) by using a one-pot solvothermal strategy. As-prepared Co-MOF@Co can be directly served as a binder-free electrode for supercapacitor, which exhibits wonderful electrochemical performances, i.e. high specific capacitance (12783.0 mF cm^-2or 1164.2 F g^-1), exceptional cycling stability (90.5% retention over 10 000 cycles at 250 mA cm^-2) with a loading of 10.98 mg cm^-2. Meanwhile, an asymmetric supercapacitor of AC//Co-MOF@Co delivers a high ratability (87% retention upon ten-fold current density) and high energy density of 43.4 W h kg^-1at the power density of 145.1 W kg^-1.

Metal-organic-framework derived hollow manganese nickel selenide spheres confined with nanosheets on nickel foam for hybrid supercapacitors

Metal-organic framework (MOF) derived nanoarchitectures have special features, such as high surface area (SA), abundant active sites, exclusive porous networks, and remarkable supercapacitive performance when compared to traditional nanoarchitectures. Herein, we propose a viable strategy for the synthesis of hollow manganese nickel selenide spheres comprising nanosheets supported on the nickel foam (denoted as MNSe@NF) from the MOF. The MNSe nanostructures can demonstrate enriched active sites, and shorten the ion-electron diffusion pathways. When the MNSe@NF electrode is used as a cathode electrode for a hybrid supercapacitor, the electrode reflected impressive supercapacitive properties with a high capacity of 325.6 mA h g-1 (1172.16 C g-1) at 2 A g-1, an exceptional rate performance of 86.6% at 60 A g-1, and remarkable longevity (3.2% capacity decline after 15 000 cycles). Also, the assembled MNSe@NF∥AC@NF hybrid supercapacitors employing activated carbon on the nickel foam (AC@NF, anode electrode) and MNSe@NF (cathode electrode) revealed an impressive energy density of 66.1 W h kg-1 at 858.45 W kg-1 and an excellent durability of 94.1% after 15 000 cycles.

Overview of transition metal-based composite materials for supercapacitor electrodes

Supercapacitors (SCs) can bridge the gap between batteries and conventional capacitors, playing a critical role as an efficient electrochemical storage device in intermittent renewable energy sources. Transition metal-based electrode materials have been investigated extensively as a class of electrode materials for SC application, but they have some limitations due to the sluggish ion/electron diffusion and inferior electronic conductivity, restricting their electrochemical performances towards energy storage. Developing advanced transition metal-based electrode materials is crucial for high energy density along with high specific power and fast charging/discharging rates towards high performance SCs. In this review, we highlight the state-of-the-art of transition metal-based electrode materials (transition metal oxides and their composites, transition metal sulfides and their composites, and transition metal phosphides and their composites), focusing on specific morphologies, components, and power characteristics. We also provide future prospects for transition metal-based electrode materials for SCs and hope this review will shed light on the achievement of higher performance and hold great promise in vast applications for future energy storage and conversion.

Synergistic effect of microwave heating and hydrothermal methods on synthesized Ni2CoS4/GO for ultrahigh capacity supercapacitors

A simple and efficient strategy that takes advantages of the synergistic effect of microwave heating method and hydrothermal method is used to synthesize Ni₂CoS₄/graphene oxide (MH-Ni₂CoS₄/GO). Firstly, Ni₂CoS₄ nanoparticles are observed to grow uniformly on the surface of GO. Then the obtained MH-Ni₂CoS₄/GO electrode is tested and it demonstrates ultrahigh specific capacitance of 2675.0 F g^-1 at the current densities of 2 A g^-1, fantastic stability of 95.0% even after 2000 cycles at 30 A g^-1 and excellent rate capability of 89.7% with current density increasing from 2 A g^-1 to 30 A g^-1. Moreover, the assembled AC//MH-Ni₂CoS₄/GO asymmetric supercapacitor also delivers a good specific capacitance of 126.5 F g^-1 at 0.5 A g^-1, outstanding stability of 97.0% after 2000 cycles at 5.0 A g^-1, and an ultrahigh energy density of 59.6 Wh kg^-1 at power density of 497.6 W kg^-1. This work provides an approach to synthesize electrode materials with superior excellent performances and it can be easily scaled up for practical applications in supercapacitors.

Recent progress on hollow array architectures and their applications in electrochemical energy storage

The structural design of electrode materials is one of the most important factors that determines the electrochemical performance of energy storage devices. In recent years, hollow micro-/nanoarray structures have been widely explored for energy applications due to their unique structural advantages. Their complex hollow interior and shell arrays enable fast ion diffusion/transport, provide abundant active sites and accommodate volume changes. Moreover, the direct contact of hollow arrays with substrates enhances the mechanical stability during long-term cycling. To date, huge progress has been achieved in the rational design of various hollow array architectures. However, a review on this topic has been rarely reported. Herein, the multifunctional merits and typical synthetic strategies for hollow array structures are analyzed in detail. Furthermore, their applications in electrochemical energy storage (such as supercapacitors and batteries) are summarized. The development and challenges of hollow arrays in terms of substrates, technique improvement and material innovation are discussed. Finally, their applications for energy storage and conversion are prospected.

Construction of self-supported hierarchical NiCo-S nanosheet arrays for supercapacitors with ultrahigh specific capacitance

Transition metal bimetallic sulfides derived from metal-organic frameworks (MOFs) hold great promise for energy-related applications. Here, a facile two-step MOF-engaged strategy is developed to grow ultrathin nickel-cobalt sulfide nanosheet arrays (NiCo-S) on Ni foam with robust adhesion, which provides a large specific surface area and excellent electric conductivity. The optimal self-supported NiCo-S electrode exhibits the best electrochemical performance as a binder-free electrode for supercapacitors with an ultrahigh specific capacitance of 3724 F g-1 at a current density of 1 A g-1 and maintains 1680 F g-1 at 20 A g-1, outperforming recently reported best values based on nickel-cobalt sulfides and oxide/hydroxide counterparts. The results demonstrate that the in situ growth of conductive Ni3S2, the presence of Co(OH)2 and the synergy between bimetals help contribute to the superior capacity. Most importantly, electronic and valence states are carefully investigated to reveal the synergetic effect and it is evidenced that the greatly decreased energy barrier differences between two redox pairs (Ni2+/Ni3+ and Co2+/Co3+) result in higher electrochemical performance. This work might shed light on the origin of high capacitance obtained from bimetallic compound based electrochemical energy storage devices.

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.

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.

An amino-functionalized metal-organic framework nanosheet array as a battery-type electrode for an advanced supercapattery

An amino-functionalized metal-organic framework nanosheet array supported on nickel foam (NH2-Co-MOF-NS/NF) was fabricated and served as a binder-free battery-type electrode, which exhibited maximum areal specific capacities of 6.7 C cm-2 (1861 μA h cm-2) and 2.9 C cm-2 (806 μA h cm-2) in a three-electrode device and in a supercapattery device, respectively. Moreover, the advanced supercapattery device delivered a high energy density of 0.351 mW h cm-2 at a power density of 1.70 mW cm-2, with superior capacity retention of 91.8% after 5000 cycles. This work can provide an efficient strategy to construct amino-functionalized 2D MOF nanosheets for energy storage and conversion.

Room-Temperature Fabrication of a Nickel-Functionalized Copper Metal⁻Organic Framework (Ni@Cu-MOF) as a New Pseudocapacitive Material for Asymmetric Supercapacitors

A nickel-functionalized copper metal-organic framework (Ni@Cu-MOF) was prepared by a facile volatilization method and a post-modification synthesis method at room temperature. The obtained Ni@Cu-MOF electrode delivered a high capacitance of 526 F/g at 1 A/g and had a long-term cycling stability (80% retention after 1200 cycles at 1 A/g) in a 6 M KOH aqueous solution. Furthermore, an asymmetric supercapacitor device was assembled from this Ni@Cu-MOF and activated carbon electrodes. The fabricated supercapacitor delivered a high capacitance of 48.7 F/g at 1 A/g and a high energy density of 17.3 Wh/kg at a power density of 798.5 kW/kg. This study indicates that the Ni@Cu-MOF has great potential for supercapacitor applications.

Mesoporous nickel cobalt manganese sulfide yolk–shell hollow spheres for high-performance electrochemical energy storage

Mesoporous Ni–Co–Mn sulfide yolk–shell hollow spheres have been prepared via a self-template route and show excellent electrochemical performance in supercapacitors.