General solution growth of mesoporous NiCo2O4 nanosheets on various conductive substrates as high-performance electrodes for supercapacitors

Graphene foam supported multilevel network-like NiCo2S4 nanoarchitectures for robust lithium storage and efficient ORR catalysis

Interconnected mesoporous ultrathin nanosheets are assembled to form NiCo₂S₄ nanowall arrays, which are grown on 3D graphene foams to fabricate multilevel network-like composites.

Unique nanopetals of nickel vanadate: crystal structure elucidation and supercapacitive performance

Unique nanopetal array of nickel vanadate is explored for its crystal structure and promising pseudocapacitive performance.

Controllable synthesis of Ni–Co–Mn multi-component metal oxides with various morphologies for high-performance flexible supercapacitors

Ni–Co–Mn multi-component metal oxides with various morphologies were controllably synthesized on carbon cloth for high-performance supercapacitor electrodes.

Interconnected hierarchical NiCo2O4 microspheres as high-performance electrode materials for supercapacitors

Hierarchical NiCo₂O₄ microspheres with large tunnels and abundant mesopores have been prepared, and they exhibit excellent performance in supercapacitor applications.

Enhanced energy density of a supercapacitor using 2D CoMoO4 ultrathin nanosheets and asymmetric configuration

Developing a high energy density micro-supercapacitor still remains a big challenge. In this paper, a two-dimensional (2D) CoMoO₄ ultrathin nanosheet (NS)-based asymmetric supercapacitor (ASC) is fabricated. It is found that the CoMoO₄ NS electrode processes a high specific capacitance (153.2 F g^-1) at a current density of 1 mA cm^-2 and this ASC can deliver an energy density of 0.313 mWh cm^-3 at a power density of 80 mW cm^-3, which is higher than that reported in the literature. Moreover, the ASC can drive a light emitting diode (3 mm diameter, red) to work for 6 min after being charged for 10 s. After 5000 cycles, 77.37% of capacitance still remains. We maintain that the ultrathin thickness can significantly shorten the diffusion paths for both electrons and ions, thus leading to fast electron transport and ion diffusion rates. Our results demonstrate that 2D ultrathin NSs could be a new, promising candidate for energy conversion/storage devices, which could offer more accommodating sites for ion intercalation.

Interrogation of bimetallic particle oxidation in three dimensions at the nanoscale

An understanding of bimetallic alloy oxidation is key to the design of hollow-structured binary oxides and the optimization of their catalytic performance. However, one roadblock encountered in studying these binary oxide systems is the difficulty in describing the heterogeneities that occur in both structure and chemistry as a function of reaction coordinate. This is due to the complexity of the three-dimensional mosaic patterns that occur in these heterogeneous binary systems. By combining real-time imaging and chemical-sensitive electron tomography, we show that it is possible to characterize these systems with simultaneous nanoscale and chemical detail. We find that there is oxidation-induced chemical segregation occurring on both external and internal surfaces. Additionally, there is another layer of complexity that occurs during the oxidation, namely that the morphology of the initial oxide surface can change the oxidation modality. This work characterizes the pathways that can control the morphology in binary oxide materials.

Understanding bimetallic alloy oxidation is key to design of hollow-structured binary oxides and their optimization for applications, e.g., as catalysts. Here the authors combine real-time imaging and chemically-sensitive electron tomography to uncover unexpected complexity in possible morphological outcomes of bimetallic oxidation.

Rational Integration of Inbuilt Aperture with Mesoporous Framework in Unusual Asymmetrical Yolk-Shell Structures for Energy Storage and Conversion

Despite the attractive benefits of hollow structures as electrodes for advanced energy storage-conversion capabilities, one prevailing shortcoming is their compromised structural integrity and volumetric energy density due to the introduction of an ultrathin shell with an excessively underutilized large hollow cavity. Herein, we report a facile and template-free synthetic route to realize unusual asymmetrical yolk-shell (AYs) structures composed of mixed-valence NiCo₂O₄ material. Explicitly, this work highlights the unusual off-central core, an AYs structure that encompasses a hemispherical hollow interior, and a mesoporous solid counterpart. As such, it retains desirable hollow structural characteristics while favorably precludes the excessive unexploited hollow interior space for increased active material packing. Unlike the conventional symmetrical yolk-shell (SYs) which is composed of a porous shell framework radially throughout the structure, the mesoporous solid constitution of the AYs structure offers an inbuilt reinforced framework to support the partial porous shell and concurrently leaves sufficient void for volumetric buffering. Another unique structural feature of the AYs structure is the formation of a submicron aperture or opening on the shell that enhances accessibility of electrolyte diffusion. All of these synergistic structural features of NiCo₂O₄ AYs structures enhance the pseudocapacitive and electrocatalytic properties.

Novel Dual-Ion Hybrid Supercapacitor Based on a NiCo2O4 Nanowire Cathode and MoO2-C Nanofilm Anode

Cobalt/nickel-based compounds have been extensively used as cathode (positive electrode) materials in alkaline electrolyte for hybrid supercapacitors (HSCs). In these HSCs, however, the anodes (negative electrodes) are almost carbon-based materials that exhibit limited capacitance, leading to relatively low energy density of the device. Herein, we report a novel dual-ion HSC concept, that is, utilizing anion and cation in the electrolyte, respectively, by the two electrodes for charge storage, to promote the device's performance. Based on this, it is possible to exploit cation-consumed metal oxide as a capacitive anode to couple with a cobalt/nickel oxide cathode. As a demonstration, a 1.8 V MoO₂-C/LiOH electrolyte/NiCo₂O₄ HSC device is established. In such a design, NiCo₂O₄ cathode and MoO₂-C anode react with OH^- and Li⁺, respectively, to store energy. With the benefits from enhanced kinetics in NiCo₂O₄ nanowire array (direct electron transport pathway and sufficient electrolyte/ion penetration) and increased stability and electrical conductivity in carbon-encapsulated MoO₂ nanofilm, our device delivers a high capacitance (94.9 F g^-1), high energy density and power density (41.8 Wh kg^-1 and 19922.2 W kg^-1), long cycling stability >3000 times, and good rate capability (∼3.3 s charging/discharging with 43.6% capacitance retention). The dual-ion charge storage concept will stimulate great interest in the design of high-performing all-oxide hybrid electric energy storage systems.

Highly active nickel-cobalt/nanocarbon thin films as efficient water splitting electrodes

Developing low cost, highly active and stable electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) using the same electrolyte has remained a major challenge. Herein, we report a novel and robust material comprised of nickel-cobalt nanoparticles coated on a porous nitrogen-doped carbon (NC) thin film synthesized via a two-step pulsed laser deposition technique. The optimized sample (Ni_0.5Co_0.5/NC) achieved the lowest overpotentials of 176 mV and 300 mV at a current density of 10 mA cm^-2 for HER and OER, respectively. The optimized OER activity might be attributed to the available metal oxide nanoparticles with an effective electronic structure configuration and enhanced mass/charge transport capability. At the same time, the porous nitrogen doped carbon incorporated with cobalt and nickel species can serve as an excellent HER catalyst. As a result, the newly developed electrocatalysts manifest high current densities and strong electrochemical stability in overall water splitting, outperforming most of the previously reported non-precious metal-based catalysts.

High-Performance Supercapacitor Electrode Based on Cobalt Oxide-Manganese Dioxide-Nickel Oxide Ternary 1D Hybrid Nanotubes

We report a facile method to design Co3O4-MnO2-NiO ternary hybrid 1D nanotube arrays for their application as active material for high-performance supercapacitor electrodes. This as-prepared novel supercapacitor electrode can store charge as high as ∼2020 C/g (equivalent specific capacitance ∼2525 F/g) for a potential window of 0.8 V and has long cycle stability (nearly 80% specific capacitance retains after successive 5700 charge/discharge cycles), significantly high Coulombic efficiency, and fast response time (∼0.17s). The remarkable electrochemical performance of this unique electrode material is the outcome of its enormous reaction platform provided by its special nanostructure morphology and conglomeration of the electrochemical properties of three highly redox active materials in a single unit.

Hierarchical core-shell NiCo2O4@NiMoO4 nanowires grown on carbon cloth as integrated electrode for high-performance supercapacitors

Hierarchical core-shell NiCo₂O₄@NiMoO₄ nanowires were grown on carbon cloth (CC@NiCo₂O₄@NiMoO₄) by a two-step hydrothermal route to fabricate a flexible binder-free electrode. The prepared CC@NiCo₂O₄@NiMoO₄ integrated electrode was directly used as an electrode for faradaic supercapacitor. It shows a high areal capacitance of 2.917 F cm⁻² at 2 mA cm⁻² and excellent cycling stability with 90.6% retention over 2000 cycles at a high current density of 20 mA cm⁻². The superior specific capacitance, rate and cycling performance can be ascribed to the fast transferring path for electrons and ions, synergic effect and the stability of the hierarchical core-shell structure.

High-Loading Nickel Cobaltate Nanoparticles Anchored on Three-Dimensional N-Doped Graphene as an Efficient Bifunctional Catalyst for Lithium-Oxygen Batteries

The lithium-oxygen batteries have been considered as the progressive energy storage equipment for their expected specific energy. To improve the electrochemical catalytic performance in the lithium-oxygen batteries, the NiCo2O4 nanoparticles (NCONPs) are firmly anchored onto the surface of the N-doped reduced graphene oxide (N-rGO) by the hydrothermal method followed by low-temperature calcination. Compared with the pure metallic oxide, the introduction of the rGO can create the high surface area, which give a good performance for ORR (oxygen reduction reaction), and improve the electrical conductivity between the NCONPs. The high-loading NCONPs also ensure the material to have great catalytic activity for OER (oxygen evolution reaction), and the rGO can be protected by the nanoparticles coating against the side reaction with the Li2O2. The as-synthesized NCO@N-rGO composites deliver a specific surface area (about 242.5 m(2) g(-1)), exhibiting three-dimensional (3D) porous structure, which provides a large passageway for the diffusion of the oxygen and benefits the infiltration of electrolyte and the storage of the discharge products. Owing to these special architectures features and intrinsic materials, the NCO@N-rGO cathode delivers a high specific capacity (6716 mAh g(-1)), great rate performance, and excellent cycling stability with cutoff capacity of 1000 mAh g(-1) (112 cycles) in the lithium-oxygen batteries. The improved electrochemical catalytic activity and the special 3D porous structure make the NCO@N-rGO composites be a promising candidate for Li-O2 batteries.

Preparation of Nickel Cobalt Sulfide Hollow Nanocolloids with Enhanced Electrochemical Property for Supercapacitors Application

Nanostructured functional materials with hollow interiors are considered to be good candidates for a variety of advanced applications. However, synthesis of uniform hollow nanocolloids with porous texture via wet chemistry method is still challenging. In this work, nickel cobalt precursors (NCP) in sub-micron sized spheres have been synthesized by a facile solvothermal method. The subsequent sulfurization process in hydrothermal system has changed the NCP to nickel cobalt sulfide (NCS) with porous texture. Importantly, the hollow interiors can be tuned through the sulfurization process by employing different dosage of sulfur source. The derived NCS products have been fabricated into supercapacitor electrodes and their electrochemical performances are measured and compared, where promising results were found for the next-generation high-performance electrochemical capacitors.

Growth of NiCo2O4@MnMoO4 Nanocolumn Arrays with Superior Pseudocapacitor Properties

Three-dimensional heterostructured NiCo2O4@MnMoO4 nanocolumn arrays (NCAs) on Ni foam were first fabricated through an improved two-step hydrothermal process associated with a successive annealing treatment. The hybrid NiCo2O4@MnMoO4 electrode exhibited remarkable pseudocapacitor property with high initial mass specific capacitance of 1705.3 F g(-1) at 5 mA cm(-2), and retained 92.6% after 5000 cycles, compared to the bare NiCo2O4 electrode with 839.1 F g(-1) and 90.9%. The excellent capacitive property of the NiCo2O4@MnMoO4 hydrid was attributed to its high-electron/ion-transfer rate, large electrolyte infiltrate area, and more electroactive reaction sites.

Hierarchical Porous Nickel Cobaltate Nanoneedle Arrays as Flexible Carbon-Protected Cathodes for High-Performance Lithium-Oxygen Batteries

Rechargeable lithium-oxygen (Li-O2) batteries are consequently considered to be an attractive energy storage technology because of the high theoretical energy densities. Here, an effective binder-free cathode with high capacity for Li-O2 batteries, needle-like mesoporous NiCo2O4 nanowire arrays uniformly coated on the flexible carbon textile have been in situ fabricated via a facile hydrothermal process followed by low temperature calcination. Because of the material and structural features, the needle-like NiCo2O4 nanowire arrays (NCONWAs) served as a binder-free cathode exhibits high specific capacity (4221 mAh g(-1)), excellent rate capability, and outstanding cycling stability (200 cycles). This cathode based on nonprecious mesoporous metal oxides nanowire arrays has large open spaces and high surface area, providing numerous catalytically active sites and effective transmission pathways for lithium ion and oxygen, and promises the abundant Li2O2 storage. The fast electron transport by directly anchoring on the substrate ensures fast electrochemical reaction process involved with the every nanowire. Furthermore, a bendable Li-O2 battery assembled by using the flexible NCONWAs as the cathode, can be able to light an LED and shows good rate capability and cyclic stability.

Crossed ferric oxide nanosheets supported cobalt oxide on 3-dimensional macroporous Ni foam substrate used for diesel soot elimination under self-capture contact mode

Crossed Fe2O3 nanosheets supported cobalt oxide nanoparticles on three-dimensionally macroporous nickel foam substrate (xCo/Fe-NF) was designed and successfully prepared through a facile hydrothermal and impregnation route. These catalysts showed high catalytic soot combustion activities under self-capture contact mode. The three-dimensional macroporous structures of Ni foam and the crossed Fe2O3 nanosheets constituted macroporous voids can greatly increase the contact efficiency between soot particulates and catalysts. The interaction between Co and Fe facilitated the activation of the Fe-O bond and increased the amounts of active oxygen species, thus improving the redox property of the catalysts. The 0.6Co/Fe-NF catalyst exhibited the highest turnover frequency (TOF) for soot combustion, which is in good accordance with the largest amount of active oxygen species. Based upon the catalytic performance and multiple characterization results, two reaction pathways for soot oxidation are identified, namely, the direct oxidation by the activated oxygen species via oxygen vacancies and the NOx-aided soot oxidation.

Self-supported nanoporous NiCo2O4 nanowires with cobalt-nickel layered oxide nanosheets for overall water splitting

Water splitting via the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in producing H2 and O2 is a very important process in the energy field. Developing an efficient catalyst which can be applied to both HER and OER is crucial. Here, a bifunctional catalyst, CFP/NiCo2O4/Co0.57Ni0.43LMOs, has been successfully fabricated. It exhibits remarkable performance for OER in 0.1 M KOH producing a current density of 10 mA cm(-2) at an overpotential of 0.34 V (1.57 V vs. RHE), better than that of the commercial Ir/C (20%) catalyst. Simultaneously, it also exhibits good catalytic performance for HER in 0.5 M H2SO4 producing a current density of 10 mA cm(-2) at an overpotential of 52 mV and a Tafel slope of 34 mV dec(-1), approaching that of the commercial Pt/C (20%) nanocatalyst. Particularly, CFP/NiCo2O4/Co0.57Ni0.43LMOs present better durability under harsh OER and HER cycling conditions than commercial Ir/C and Pt/C. Furthermore, an H-type electrolyzer was fabricated by applying CFP/NiCo2O4/Co0.57Ni0.43LMOs as the cathode and anode electrocatalyst, which can be driven by a single-cell battery. This bifunctional catalyst will be very promising in overall water splitting.

Porous Iron Cobaltate Nanoneedles Array on Nickel Foam as Anode Materials for Lithium-Ion Batteries with Enhanced Electrochemical Performance

A monocrystalline and porous FeCo2O4 nanoneedles array growing directly on a nickel foam substrate was obtained by a hydrothermal technique accompanying with combustion of the one-dimensional precursor. The average length of the FeCo2O4 nanoneedles is approximately 2 μm, while the diameter of the root segment of the nanoneedle can be estimated to be around 100 nm, which gradually reduces to only several nanometers at the top. When the as-prepared porous FeCo2O4 nanoneedles array with a high surface area of 58.49 m(2) g(-1) was applied as binder-free electrode in lithium-ion batteries, it exhibited satisfactory electrochemical performance, such as outstanding reversibility (Coulombic efficiency of approximately 92-95%), high specific capacity (1962 mAh g(-1) at the current density of 100 mA g(-1)), and excellent rate performance (discharge capacity of 875 mAh g(-1) at the current density of 2000 mA g(-1)), due to the various favorable conditions. Undoubtedly, the simple but effective strategy can be expanded to other high-performance binary metal-oxide materials.

One-dimensional metal oxide-carbon hybrid nanostructures for electrochemical energy storage

Numerous metal oxides (MOs) have been considered as promising electrode materials for electrochemical energy storage devices, including lithium-ion batteries (LIBs) and electrochemical capacitors (ECs), because of their outstanding features such as high capacity/capacitance, low cost, as well as environmental friendliness. However, one major challenge for MO-based electrodes is the poor cycling stability derived from the large volume variation and intense mechanic strain, which are inevitably generated during repeated charge/discharge processes. Nanostructure engineering has proven to be one of the most effective strategies to improve the electrochemical performance of MO-based electrode materials. Among various nanostructures, one-dimensional (1D) metal oxide-carbon hybrid nanostructures might offer some solution for the challenging issues involved in bulk MO-based electrode materials for energy storage devices. Herein, we give an overview of the rational design, synthesis strategies and electrochemical properties of such 1D MO-carbon structures and highlight some of the latest advances in this niche area. It starts with a brief introduction to the development of nanostructured MO-based electrodes. We will then focus on the advanced synthesis and improved electrochemical performance of 1D MO-carbon nanostructures with different configurations, including MO-carbon composite nanowires, core-shell nanowires and hierarchical nanostructures. Lastly, we give some perspective on the current challenges and possible future research directions in this area.

Synthesis of porous MnCo2O4 microspheres with yolk–shell structure induced by concentration gradient and the effect on their performance in electrochemical energy storage

In this study, novel spherical yolk–shell MnCo₂O₄ powders with concentration gradient have been synthesized.

Self-assembly of ultrathin mesoporous CoMoO4 nanosheet networks on flexible carbon fabric as a binder-free anode for lithium-ion batteries

Novel hierarchical CoMoO₄ networks assembled by ultrathin mesoporous nanosheets are directly grown on flexible carbon fabric as integrated anodes for highly efficient and reversible lithium storage.

Facile preparation of free-standing rGO paper-based Ni–Mn LDH/graphene superlattice composites as a pseudocapacitive electrode

A novel film electrode was assembled via a simple filtration process, with an rGO paper as the substrate and Ni–Mn LDH/graphene superlattice composites as the functional layer.

Hierarchical mesoporous NiCo2O4 hollow nanocubes for supercapacitors

In the present work, mesoporous NiCo₂O₄ hollow nanocubes are synthesized using a “coordinating etching & precipitating” (CEP) route.

Buckypaper templating Ni–Co hydroxide nanosheets as free-standing electrodes for ultrathin and flexible supercapacitors

A novel sheet-on-tube hierarchical structure with a uniform distribution of Ni–Co hydroxides on BP acts as a promising supercapacitor.

Highly effective synthesis of NiO/CNT nanohybrids by atomic layer deposition for high-rate and long-life supercapacitors

Uniform and high-efficiency NiO films coated on CNTs with controllable thickness were synthesized by an ALD method, exhibiting an excellent performance for supercapacitor applications.

One-step synthesis of nickel cobalt sulphides particles: tuning the composition for high performance supercapacitors

NiS₂–CoS₂ composites with different Ni and Co molar ratios for supercapacitors (SCs) were synthesized by one-step hydrothermal co-deposition method. Among them, Ni/Co/S-1 (Ni : Co ratio of 1 : 1) possesses the highest C_m value at the same current densities.

Facile synthesis of NiCoMnO4 nanoparticles as novel electrode materials for high-performance asymmetric energy storage devices

Hydrothermally synthesized NiCoMnO₄ NPs showed a high capacitance of 510 F g⁻¹ with high mass loaded electrodes (∼10 mg cm⁻²). Integrated with RGO NSs, their viability was testified for high-performance asymmetric supercapacitors.

Facile synthesis of two-dimensional (2D) nanoporous NiO nanosheets from metal–organic frameworks with superior capacitive properties

2D nanoporous NiO nanosheets were synthesized from metal–organic frameworks as precursors using a facile method, which exhibited superior capacitive properties.

High performance electrochemical capacitor materials focusing on nickel based materials

Of the two major capacitances contributing to electrochemical storage devices, pseudo-capacitance, which results from the reversible faradaic reactions, can be much higher than the electric double layer capacitance.

Template-free synthesis of 3D hierarchical nanostructured NiCo2O4 mesoporous ultrathin nanosheet hollow microspheres for excellent methanol electrooxidation and supercapacitors

Novel 3D hierarchical NiCo₂O₄ mesoporous ultrathin nanosheets hollow microspheres upon a facile template-free solvothermal method followed air-annealing shows excellent methanol electrooxidation and supercapacitors performance.

Mn0.5Co2.5O4 nanofibers sandwiched in graphene sheets for efficient supercapacitor electrode materials

With unique sandwich-like structures, rich active sites, and boosted electrical conductivity, the Mn_0.5Co_2.5O₄@G composite demonstrates superior electrochemical performances for supercapacitors.

Fabrication of carbon-coated NiO supported on graphene for high performance supercapacitors

We report a feasible strategy for the synthesis of the carbon-coated NiO nanoparticles supported on graphenes and investigate their application as supercapacitor electrodes.