Metal–Organic Framework Templated 3D Hierarchical ZnCo 2 O 4 @Ni(OH) 2 Core–Shell Nanosheet Arrays for High‐Performance Supercapacitors

Uniform P-Doped MnMoO4 Nanosheets for Enhanced Asymmetric Supercapacitors Performance

Manganese molybdate has garnered considerable interest in supercapacitor research owing to its outstanding electrochemical properties and nanostructural stability but still suffers from the common problems of transition metal oxides not being able to reach the theoretical specific capacitance and lower electrical conductivity. Doping phosphorus elements is an effective approach to further enhance the electrochemical characteristics of transition metal oxides. In this study, MnMoO4·H2O nanosheets were synthesized on nickel foam via a hydrothermal route, and the MnMoO4·H2O nanosheet structure was successfully doped with a phosphorus element using a gas-solid reaction method. Phosphorus element doping forms phosphorus-metal bonds and oxygen vacancies, thereby increasing the charge storage and conductivity of the electrode material. The specific capacitance value is as high as 2.112 F cm-2 (1760 F g-1) at 1 mA cm-2, which is 3.2 times higher than that of the MnMoO4·H2O electrode (0.657 F cm-2). The P-MnMoO4//AC ASC device provides a high energy density of 41.9 Wh kg-1 at 666.8 W kg-1, with an 84.5% capacity retention after 10,000 charge/discharge cycles. The outstanding performance suggests that P-MnMoO4 holds promise as an electrode material for supercapacitors.

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.

Zeolitic Imidazolate Framework-Derived Zn/Co-S@Ni(OH)2 Nanoarrays with Excellent Energy Storage and Electrocatalytic Performance

The design and development of high-performance electrochemical electrode materials are crucial for energy storage and conversion systems. This work reports a facile preparation of a self-supported Zn/Co-S@Ni(OH)2 array electrode in which a Zn/Co-S nanosheet is derived from a leaf-like zeolitic imidazolate framework (Zn/Co-ZIF-L). The core-shell structure provides multiple benefits such as enhanced electrical conductivity, an abundance of exposed active sites, and strong electronic interactions between Zn/Co-S and ultra-thin Ni(OH)2 nanosheets, facilitating faster charge transfer. Consequently, Zn/Co-S@Ni(OH)2 demonstrates remarkable electrochemical characteristics as an electrode material for supercapacitors with an area capacitance of 12.9 F cm-2 at a current density of 2 mA cm-2 in 2 M KOH. The assembled asymmetric supercapacitor device achieves a high energy density of 0.95 mW h cm-2, while showing excellent longevity with a retention of 90.9% over 5000 cycles. Additionally, the Zn/Co-S@Ni(OH)2 arrays demonstrate significant oxygen evolution reaction activity with an overpotential of 242 mV at 10 mA cm-2 in 1 M KOH and significant stability for more than 100 h. This work provides a valuable approach for synthesizing bifunctional electrode materials for both energy storage and electrocatalysis applications.

Design of Ni(OH)2 Nanosheets@NiMoO4 Nanofibers' Hierarchical Structure for Asymmetric Supercapacitors

Transition-metal-based materials show great promise for energy conversion and storage due to their excellent chemical properties, low cost, and excellent natural properties. In this paper, through simple strategies such as classical electrospinning, air calcination, and the one-step hydrothermal method, a large area of Ni(OH)₂ nanosheets were grown on NiMoO₄ nanofibers, forming NiMoO₄@Ni(OH)₂ nanofibers. The one-dimensional nanostructure was distributed with loose nanosheets, and this beneficial morphology made charge-transfer and diffusion more rapid, so the newly developed material showed good capacitance and conductivity. Under the most suitable experimental conditions, the optimal electrode exhibited the highest specific capacitance (1293 F/g at 1 A/g) and considerable rate capability (56.8% at 10 A/g) under typical test conditions. Most interestingly, the corresponding asymmetrical capacitors exhibited excellent electrochemical cycle stability, maintaining 77% of the original capacitance. NiMoO₄@Ni(OH)₂ nanofibers were verified to be simple to prepare and to have good performances as energy-storage devices within this experiment.

Nickel Cobaltite: A Positive Electrode Material for Hybrid Supercapacitors

The increased demand of energy due to the recent technological advances in diverse fields such as portable electronics and electric vehicles is often hindered by the poor capability of energy-storage systems. Although supercapacitors (SCs) exhibit higher power density than state-of-the art batteries, their insufficient energy density remains a major challenge. An emerging concept of hybrid supercapacitors (HSCs) with the combination of one capacitive and one battery electrode in a single cell holds a great promise to deliver high energy density without sacrificing power density and cycling stability. This Minireview elaborates the recent advances of use of nickel cobaltite (NiCo₂ O₄ ) as a potential positive electrode (battery-like) for HSCs. A brief introduction on the structural benefits and charge storage mechanisms of NiCo₂ O₄ was provided. It further shed a light on composites of NiCo₂ O₄ with different materials like carbon, polymers, metal oxides, and others, which altogether helps in increasing the electrochemical performance of HSCs. Finally, the key scientific challenges and perspectives on building high-performance HSCs for future-generation applications were reviewed.

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.

Recent advances in metal-organic framework-based electrode materials for supercapacitors

Exploring porous electrode materials with designed micro/nano-structures is an effective way to realize high-performance supercapacitors (SCs). A metal-organic framework (MOF) is a porous crystalline material with a periodic structure formed by coordination of metal ions/clusters and organic ligands. Due to the excellent properties (e.g., large specific surface area, high porosity and tailorable structure), MOFs have been widely used in diverse applications. This Frontier article highlights the recent progress in the synthesis of MOF-based micro/nano-structured electrode materials including pristine MOFs, MOF composites and MOF derivatives, and their application in SCs. Furthermore, the challenges of MOF-based electrode materials and possible solutions are also discussed.

MOF-derived ZnCo2O4@NiCo2S4@PPy core-shell nanosheets on Ni foam for high-performance supercapacitors

The ZnCo₂O₄@NiCo₂S₄@PPy core-shell nanosheets material is prepared by directly growing leaf-like ZnCo₂O₄ nanosheets derived from the metal-organic framework (MOF) on Ni foam (NiF) via chemical bath deposition and annealing methods and then combining with NiCo₂S₄ and PPy via electrodeposition methods. The special core-shell structure formed by MOF-derived ZnCo₂O₄, NiCo₂S₄ and PPy creates a bi-interface, which could significantly promote the contact between electrode and electrolyte, provide more active sites and accelerate electron/ion transfer. And the combination of these three materials also produces a strong synergistic effect, which could further improve the capacitive performance of the electrode. Therefore, the ZnCo₂O₄@NiCo₂S₄@PPy/NiF electrode exhibits the maximum areal capacitance (3.75 F cm^-2) and specific capacitance (2507.0 F g^-1) at 1 mA cm^-2 and 0.5 A g^-1, respectively. Moreover, its capacitance retention rate is still 83.2% after 5000 cycles. In addition, a coin-type hybrid supercapacitor is assembled and displays a high energy density of 44.15 Wh kg^-1 and good cycling performance.

A visible-light active p-n heterojunction NiFe-LDH/Co3O4 supported on Ni foam as photoanode for photoelectrocatalytic removal of contaminants

Light and electricity are the most prevalent energy sources in natural environment. Herein, a visible-light active Ni foam@NiFe-LDH/Co₃O₄ composite was successfully prepared by loading 1D Co₃O₄ nanowires on the surface of 2D NiFe-LDH nanosheets to be a p-n heterojunction supported on the 3D Ni foam through hydrothermal method, which can be used as photoanode directly for photoelectrocatalytic (PEC) process to simultaneously remove bisphenol (BPA) and Cr(VI) from water. This unique Ni foam-based photoanode modified by NiFe-LDH/Co₃O₄ heterojunction can fully expose the active sites, enhance visible-light absorption and facilitate the migration and separation of photogenerated carriers, thus obtained a boosted efficiency for simultaneous removal of BPA and Cr(VI) under a low applied voltage. Furthermore, the convenient recyclability and excellent stability of the as-prepared Ni foam@NiFe-LDH/Co₃O₄ also show a great potential in environmental purification.

Ni(OH)2 nanodot-decorated Co-Co LDH/C hollow nanocages for a high performance supercapacitor

A novel Co-Co LDH/C/Ni(OH)2 nanostructure was constructed by loading Ni(OH)2 nanodots on hollow Co-Co LDH/C nanocages derived from MOFs. The Co-Co LDH/C/Ni(OH)2 nanostructure revealed a high specific capacitance of up to 1426 F g-1 at 1 A g-1 and an outstanding rate capability with 90.2% retention at 10 A g-1 owing to the cooperative effect of the Ni(OH)2 nanodots and hollow Co-Co LDH/C nanocages. The electrochemical kinetic analysis showed that the Co-Co LDH/C/Ni(OH)2 electrode was dominated by surface capacitance control, demonstrating the origins of performance improvement. This work may provide an effective strategy by combining nanodots with hollow porous structures for low-cost and efficient energy storage materials.

Constructing high-performance electrode materials using core–shell ZnCo2O4@PPy nanowires for hybrid batteries and water splitting

Heterogeneity can be used as a promising method to improve the electrochemical performance of electrode materials; thus, ZnCo₂O₄@PPy samples were prepared using a facile hydrothermal route and an electrochemical deposition process. The as-prepared products possess a specific capacitance of 605 C g⁻¹ at a current density of 1 A g⁻¹. The asymmetric supercapacitor (ASC) possesses an energy density of 141.3 W h kg⁻¹ at a power density of 2700.5 W kg⁻¹ and capacity retention of 88.1% after 10 000 cycles, indicating its promising potential for energy devices. ZnCo₂O₄@PPy-50 exhibited an excellent OER performance and outstanding HER performance in alkaline media. As an advanced bifunctional electrocatalyst for overall water splitting, a voltage of 1.61 V at a current density of 50 mA cm⁻² outperforms the majority of noble-metal-free electrocatalysts.

Heterogeneity can be used as a promising method to improve the electrochemical performance of electrode materials; thus, ZnCo₂O₄@PPy samples were prepared using a facile hydrothermal route and an electrochemical deposition process.

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.

The effects of Ni ions' charge disproportionation on the high electrochemical performance of Ni1−xCoxO nanoparticles

The outstanding electrochemical properties of Ni_1−xCo_xO electrode materials can be attributed to the Ni ion charge disproportionation, which is caused by Co atom doping.

Nitrogen-doped carbon integrated nickel–cobalt metal phosphide marigold flowers as a high capacity electrode for hybrid supercapacitors

The fabrication of advanced MOF-derived multicomponent NiCoP/NC marigold flowers electrode for high-performance hybrid supercapacitors.

Metal–organic framework templated fabrication of Cu7S4@Ni(OH)2 core–shell nanoarrays for high-performance supercapacitors

Core–cell Cu₇S₄@Ni(OH)₂ nanorod arrays were fabricated by using metal–organic frameworks as templates, and showed high specific capacitance, superior rate capacity and excellent cycling stability for 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.

Hierarchically Porous W-Doped CoP Nanoflake Arrays as Highly Efficient and Stable Electrocatalyst for pH-Universal Hydrogen Evolution

It is still challenging to develop high-efficiency and low-cost non-noble metal-based electrocatalysts for hydrogen evolution reaction (HER) in pH-universal electrolytes. Herein, hierarchically porous W-doped CoP nanoflake arrays on carbon cloth (W-CoP NAs/CC) are synthesized via facile liquid-phase reactions and a subsequent phosphorization process. The W-CoP NAs/CC hybrid can be directly employed as a binder-free electrocatalyst and delivers superior HER performance in pH-universal electrolytes. Especially, it delivers very low overpotentials of 89, 94, and 102 mV to reach a current density of 10 mA cm^-2 in acidic, alkaline, and neutral electrolytes, respectively. Furthermore, it shows a nearly 100% Faradaic efficiency as well as superior long-term stability with no decreasing up to 36 h in pH-universal electrolytes. The outstanding electrocatalytic performance of W-CoP NAs/CC can be mainly attributed to the porous W-doped nanoflake arrays, which not only afford rich exposed active sites, but also accelerate the access of electrolytes and the diffusion of H₂ bubbles, thus efficiently promoting the HER performance. This work provides a new horizon to rationally design and synthesize highly effective and stable non-noble metal phosphide-based pH-universal electrocatalysts for HER.

Construction of NiCo2O4 nanosheet-decorated leaf-like Co3O4 nanoarrays from metal–organic framework for high-performance hybrid supercapacitors

Leaf-like Co₃O₄@NiCo₂O₄ nanoarrays with an excellent energy storage performance was designed by a MOF-guided strategy.

Wrapping CuCo2S4 arrays on nickel foam with Ni2(CO3)(OH)2 nanosheets as a high-performance faradaic electrode

Ternary metal sulfides represent a new class of faradaic electrode material outperforming their oxide counterparts.

Zeolitic imidazolate framework-derived Co3S4@Co(OH)2 nanoarrays as self-supported electrodes for asymmetric supercapacitors

Core–shell Co₃S₄@Co(OH)₂ nanosheet arrays with enhanced electrochemical capacitive performance were designed using a ZIF-engaged strategy.