Hierarchical NiCo-LDH core/shell homostructural electrodes with MOF-derived shell for electrochemical energy storage.

Constructing hierarchical structure is an effective strategy to boost the electrochemical performance of layered double hydroxide (LDH) materials, but the rational design of such delicate architectures is still challenging. Herein, a unique hierarchical core/shell homostructure with NiCo-LDH nanorods (NCNRs) as core and NiCo-LDH nanosheets (NCNSs) as shell is constructed via in-situ ZIF shell growth and subsequent ion exchange-coprecipitation process. Such novel hierarchical structure provides a large accessible surface area and more exposed electrochemical active sites. The in-situ growth and conversion process contribute to the formation of robust adhesion between the core and the shell, which could facilitate the effective charge and ion diffusion, as well as improve the mechanical stability. Benefiting from the unique structure, the NCNRs@NCNSs electrode exhibits a high capacitance of 2640.2 F g-1, along with the good rate performance and cyclic stability. Furthermore, the as-assembled asymmetric supercapacitor of NCNRs@NCNSs//AC device displays a high energy density of 22.81 Wh kg-1 at the power density of 374.95 W kg-1. This work demonstrates a new strategy for designing hierarchical LDH with core/shell structure as electrode materials for superior electrochemical energy storage.

In Situ Fabrication of a Uniform Co-MOF Shell Coordinated with CoNiO2 to Enhance the Energy Storage Capability of NiCo-LDH via Vapor-Phase Growth.

NiCo-layered double hydroxide (LDH) has attracted increasing attention in recent years for application in supercapacitors (SCs) owing to its high redox activity and intercalating capability. However, the pristine NiCo-LDH is unable to reach theoretical specific capacitance and satisfying rate capability due to the limited electroactive species and a low ion diffusion rate. Here, we demonstrate novel vertically aligned nanosheet arrays of cobalt metal-organic framework (Co-MOF)@CoNiO2 core-shell composites constructed by the in situ grown Co-MOF shell with a uniform and controlled thickness on the CoNiO2 core via a vapor-phase approach. Owing to the intimate contact and synergistic effect between the Co-MOF shell and the CoNiO2 core, the as-synthesized Co-MOF@CoNiO2 displays a high specific capacitance of about 571 F g-1, which is significantly higher than the pristine NiCo-LDH electrode (380 F g-1). Moreover, the capacitive properties of Co-MOF@CoNiO2 can be further boosted to 757.2 F g-1 after cyclic voltammetry oxidation. The easy preparation and high electrochemical performance of the Co-MOF@CoNiO2 composite make it a potential material for SC application. These findings may inspire the exploration and construction of other MOF shell coating metal oxide from various nanostructured LDHs for varied applications. In addition, the as-assembled EO-Co-MOF@CoNiO2/carbon cloth (CC)//activated carbon (AC) device can achieve a high capacitance of 87.67 F g-1. Meanwhile, the asymmetric supercapacitor (ASC) device exhibits a high energy density of 27.4 Wh kg-1 at a power density of 750 W kg-1.

Bimetal-organic framework derived Cu(NiCo)2S4/Ni3S4 electrode material with hierarchical hollow heterostructure for high performance energy storage.

Rational design of electrical active materials with high performance for energy storage and conversion is of great significance. Herein, Cu(NiCo)2S4/Ni3S4, a three-dimensional (3D) hierarchical hollow heterostructured electrode material, is designed by etching the well-defined bimetal organic framework (MOF) via sequential in-situ ion-exchange processes. This trimetallic sulfides with unique structure provide large surface area, hierarchical pore distribution and enhanced electrical conductivity, can enrich the active sites for redox reactions, facilitate electrolyte penetration and rapid charge transfer kinetics. As a result, the Cu(NiCo)2S4/Ni3S4 electrode exhibits a high specific capacitance of 1320 F/g at 1 A/g and excellent rate performance (only 15% of capacitance is attenuated when the current density is increased by 20 times). Furthermore, a fabricated hybrid supercapacitor of Cu(NiCo)2S4/Ni3S4/AC can deliver a maximum energy density of 40.8 Wh/kg, remarkable power density of 7859.2 W/kg and superior cycling stability (85% retention of capacitance after 5000 cycles), demonstrating great potential for practical applications in energy storage and conversion devices.

Cobalt/Nickel Ions-Assisted Synthesis of Laminated CuO Nanospheres Based on Cu(OH)2 Nanorod Arrays for High-Performance Supercapacitors.

The development for environmentally friendly energy conversion and storage equipment has given rise to tremendous research efforts as a result of the growing requirements for environmental friendly resources and the rapid consumption of traditional fossil fuel. Herein, a novel hierarchical CoO/NiO-Cu@CuO heterostructure is successfully devised and synthesized. Cobalt/nickel ions are used to generate novel CoO/NiO-doped laminated CuO nanospheres through the facile in situ wet oxidation combined with cation exchange and calcination strategies. As a result, the electrochemical supercapacitance of the as-prepared CoO/NiO-Cu@CuO electrode can reach 875 C cm-2 (2035 mF cm-2), which exhibits much better electrochemical performance compared to other precursor electrodes at a same current density of 2 mA cm-2. Moreover, an excellent rate capacity of 1395 mF cm-2 (50 mA cm-2) can be achieved when measured at a relative high current density; 90.3% of the initial supercapacitance remains even after 5000 cycles. Furthermore, the as-prepared hierarchical hybrid of laminated CoO/NiO-CuO nanospheres in situ generated on three-dimensional (3D) porous Cu foam is applied to prepare a solid-state asymmetric supercapacitor equipment unit. The fabricated equipment unit shows an energy density of 69.3 W h kg-1 at a power density of 1080 W kg-1. Additionally, the commercially applied 2.5 V light-emitting-diode indicator with blue light can be energized for 4 min when two as-fabricated supercapacitor devices are in series connection. The unique hierarchical heterostructure of the novel laminated nanospheres combined with the 3D grid structure brings about the outstanding electrochemical capacitor performances. This strategy for the fabrication of hierarchical heterostructure electrodes could have an enormous potential for high-performance electrochemical equipment.

Improving the rate capability of ultrathin NiCo-LDH nanoflakes and FeOOH nanosheets on surface electrochemically modified graphite fibers for flexible asymmetric supercapacitors.

A fiber asymmetric supercapacitor system is designed with NiCo-LDH nanoflakes and FeOOH nanosheets anchored on electrochemically activated graphite fibers as positive electrode and negative electrode, respectively. Due to the formation of COMetal bonding, the oxygen-functionalized carbon on electrochemically activated graphite fibers can bind strongly with NiCo-LDH and FeOOH, which assists in establishing the fast electron transfer routes and fluent ion transport avenues. Both NiCo-LDH and FeOOH anchored on electrochemically activated graphite fibers display a high rate performance, 80% and 87.3% of the electric capacity can be reserved with the current density increasing from 2 to 20 A g-1 and 2 to 10 A g-1, respectively, while the NiCo-LDH and FeOOH deposited on untreated graphite fibers can only retain 45% and 40%. The fabricated novel solid-state fiber asymmetric supercapacitor device exhibits an expanded operation potential window of 1.8 V with a maximum energy density (130 W h kg-1) when the power density is 1.8 kW kg-1. Furthermore, a high energy density (81 W h kg-1) is still achieved at a superhigh power density (10.8 kW kg-1). Additionally, a good cycling stability of the solid-state fiber asymmetric supercapacitor can be obtained (90% capacity retention after 10,000 cycles).

Synthesis of polypyrrole coated melamine foam by in-situ interfacial polymerization method for highly compressible and flexible supercapacitor.

Compressible and flexible supercapacitors have aroused enormous interest of many scientific researchers for potential applications in wearable electronic products. However, the design and construction of the electrode with superior mechanical as well as electrical properties still face a lot of challenges. In present work, melamine foam/polypyrrole (MF/PPy) electrode with high deformation-tolerance and excellent electrochemical performance is prepared by in-situ interfacial polymerization of polypyrrole on commercial melamine foam, where PPy nanoparticles with size of 700 nm are uniformly anchored on the MF skeletons. The electrochemical characterizations show that the electrode exhibits excellent specific area capacitance of 2.685 F cm-2 at 2 mA cm-2 and good cyclic stability with more than 80% of capacitance remained after 3000 cycles. Furthermore, a symmetrical aqueous supercapacitor is assembled and exhibits an excellent energy density up to 75.95 µWh cm-2 at the power density of 5.82 mW cm-2 and excellent cycling stability as the current density increases by 10 times. Even under a high strain of 70%, about 95.76% of the initial capacitance is retained after 500 consecutive compressions. These outstanding performances enable the MF/PPy composite a promising candidate for potential applications in compressible and flexible electrochemical energy storage devices.

Electrochemical synthesis of NiCo layered double hydroxide nanosheets decorated on moderately oxidized graphene films for energy storage.

The introduction of oxygenous functional groups onto graphene can provide additional pseudocapacitance for supercapacitors. However, how to balance the amount of introduced oxygenous functional groups and the reduced electrical conductivity arising from the disruption of the conjugated system remains a big challenge. Here, a controllable strategy is reported to prepare moderately oxidized reduced graphene oxide (MORGO) via an electrochemical oxidation process. The MORGO not only has oxygenous groups with appropriate quantities, but also preserves the highly crystalline structure of the π-π conjugated carbon framework. As a result, the MORGO films showed superior electrochemical properties to the pristine RGO films and other previously reported RGO films. Furthermore, the oxygenous groups and the conductivity of MORGO films can be easily adjusted by controlling the oxidation time. A hierarchical composite of NiCo-layered double hydroxide nanosheet arrays on MORGO films (MORGO/NiCo-LDH) was also constructed via electrochemical deposition to combine the advantages of electric double-layer electrode materials and faradaic electrode materials. The flexible solid-state supercapacitor fabricated with MORGO/NiCo-LDH film electrodes exhibits a high energy density (0.51 mW h cm-3), as well as a long cycle life (88.2% capacitance retention after 10 000 cycles).

Boosting Electrochemistry of Manganese Oxide Nanosheets by Ostwald Ripening during Reduction for Fiber Electrochemical Energy Storage Device.

The poor electronic conductivity of MnO x severely limits the practical application as high-performance electrode materials for faradaic pseudocapacitors. Herein, a facile vapor reduction method is demonstrated for the treatment of MnO x with hydrazine hydrate (HH) to improve the electronic conductivity. The HH vapor treatment without annealing process not only introduces oxygen vacancies to form oxygen-deficient MnO x, but also leads to obvious structural transformation from highly aggregated and poorly crystallized MnO x nanorobs and nanoparticles into uniformly orientated and highly crystallized MnO x nanosheets via the Ostwald ripening process. Compared with pristine MnO x on carbon fiber (CF-MnO x), the reduced CF-MnO x exhibits a highly improved specific capacitance of 1130 mF cm-1 (434 F g-1) with excellent rate capability and cycling stability. Our results have shown that the moderate concentration of oxygen vacancies and highly uniform orientation of reduced MnO x endow the electrode with a fast electron and ion transport, respectively. Moreover, a flexible fiber asymmetric supercapacitor (ASC) device with high-energy and power density based on the as-prepared reduced CF-MnO x as a cathode and electrochemically activated graphene oxide on carbon fiber (CF-ArGO) as an anode is fabricated. The MnO x//ArGO ASC device delivers a high volumetric capacitance of 1.9 F cm-3, a maximum energy density of 1.06 mWh cm-3, and a volumetric power density of 371.3 mW cm-3. The present work opens a new way for oxygen vacancy introduction and structural modification of metal oxide as high-performance materials for energy storage applications.