Hierarchical coating of MnO2 nanosheets on ZnCo2O4 nanoflakes for enhanced electrochemical performance of asymmetric supercapacitors

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.

Rational design of 2D/1D ZnCo-LDH hierarchical structure with high rate performance as advanced symmetric supercapacitors

Hierarchical microstructures of electrode materials for supercapacitors have attracted strong attention owing to the enlarged surface area, fast charge transportation and enhanced volume expansion. All these advantages of hierarchical structures are available for facilitating the electrochemical performances and stability of electrode materials for advanced supercapacitors. However, few reports focused on the development of transition metal-based homogeneous 2D/1D hierarchical structures use the same material. Herein, the ZnCo-LDH with 3D hierarchical microstructure was self-assembled with 1D nanoneedles and 2D nanosheets that in situ synthesized on Ni foam through a simple and effective strategy. The optimal sample (ZnCo-LDH-2) exhibited the ultra-high specific capacitance of 3871.2 F  g ^-1 at 1 A g ^-1 and excellent cycle life span (capacitance retention of 87.5% after 6000 cycles at 5 A g^-1). Particularly, the current density was as high as 100 A g^-1, the specific capacitance of ZnCo-LDH-2 still remained 1526 F g^-1. Moreover, the symmetric supercapacitor (SSC) fabricated with ZnCo-LDH-2 showed the maximum energy density of 40.3 Wh kg^-1 and power density of 15.08 kW kg^-1.

Zn induced NiCo composites modified by carbon materials as a battery-type electrode material for high-performance supercapacitors

The development of simple preparation and excellent capacity performance electrode materials is the key to energy conversion and storage for supercapacitors. Based on the growth mechanism of crystal, Zn induced NiCo nanosheets and nanoneedles composite structure deposed on Ni foam (ZNC) are successfully attained by a facile one-step method, the growth mechanism of the composite structure is further discussed. Because of its unique composites structure and additional modification of carbon, the carbon modified ZNC (ZNC@C) delivers better energy storage ability (2280 mC cm^-2at 2 mA cm^-2) compare to ZNC. An asymmetric supercapacitor (ASC) is assembled by ZNC@C as the positive electrode and carbonized popcorn as the negative electrode. The ASC exhibits good energy storage performance. Zn also positively affects the adsorption energy to enhance the capacitance property based on Density Functional theory calculation. The simple method for the composite structure by tuning the kinetics behaver of the crystal can provide a new strategy in synthesizing the materials, and the material with a unique structure and high performance will have potential applications in the field of energy storage.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

Facile hydrothermal synthesis of porous MgCo2O4 nanoflakes as an electrode material for high-performance asymmetric supercapacitors

In this work, porous MgCo₂O₄ nanoflakes (MgCo₂O₄ NFs) and MgCo₂O₄ nanocubes (MgCo₂O₄ NCs) have been successfully synthesized through a simple hydrothermal method combined with a post calcination process of the precursor in air. The morphology of the MgCo₂O₄ samples can be easily tuned by changing the hydrothermal temperature and reaction time, respectively. The porous MgCo₂O₄ NFs with an average pore size of 12.5 nm had a BET specific surface area up to 64.9 m² g^-1, which was larger than that of MgCo₂O₄ NCs (19.8 m² g^-1). The MgCo₂O₄ NFs delivered a specific capacitance of 734.1 F g^-1 at 1 A g^-1 and exhibited a considerable rate performance with 74.0% capacitance retention at 12 A g^-1. About 94.2% of its original capacitance could be retained after 5000 charge-discharge cycles at a constant current density of 5 A g^-1. An asymmetric supercapacitor (ASC) was assembled by using MgCo₂O₄ NFs as the positive electrode and AC as the negative electrode, and the ASC had a wide operation voltage of 1.7 V and a high energy density of 33.0 W h kg^-1 at a power density of 859.6 W kg^-1. Such outstanding electrochemical performances make the MgCo₂O₄ NFs a promising candidate for supercapacitor applications. In addition, the simple and scalable synthesis method can be extended to the preparation of other metal oxide-based electrode materials.

Two-Dimensional Transition Metal Oxide and Hydroxide-Based Hierarchical Architectures for Advanced Supercapacitor Materials

The supercapacitor has been widely seen as one of the most promising emerging energy storage devices, by which electricity is converted from chemical energy and stored. Two-dimensional (2D) metal oxides/hydroxides (TMOs/TMHs) are revolutionizing the design of high-performance supercapacitors because of their high theoretical specific capacitance, abundance of electrochemically active sites, and feasibility for assembly in hierarchical structures by integrating with graphitic carbon, conductive polymers, and so on. The hierarchical structures achieved can not only overcome the limitations of using a single material but also bring new breakthroughs in performance. In this article, the research progress on 2D TMOs/TMHs and their use in hierarchical structures as supercapacitor materials are reviewed, including the evolution of supercapacitor materials, the configurations of hierarchical structures, the electrical properties regulated, and the existence of advantages and drawbacks. Finally, a perspective covering directions and challenges related to the development of supercapacitor materials is provided.

Cryopolymerization enables anisotropic polyaniline hybrid hydrogels with superelasticity and highly deformation-tolerant electrochemical energy storage

The development of energy storage devices that can endure large and complex deformations is central to emerging wearable electronics. Hydrogels made from conducting polymers give rise to a promising integration of high conductivity and versatility in processing. However, the emergence of conducting polymer hydrogels with a desirable network structure cannot be readily achieved using conventional polymerization methods. Here we present a cryopolymerization strategy for preparing an intrinsically stretchable, compressible and bendable anisotropic polyvinyl alcohol/polyaniline hydrogel with a complete recovery of 100% stretching strain, 50% compressing strain and fully bending. Due to its high mechanical strength, superelastic properties and bi-continuous phase structure, the as-obtained anisotropic polyvinyl alcohol/polyaniline hydrogel can work as a stretching/compressing/bending electrode, maintaining its stable output under complex deformations for an all-solid-state supercapacitor. In particular, it achieves an extremely high energy density of 27.5 W h kg⁻¹, which is among that of state-of-the-art stretchable supercapacitors.

Energy storage devices that can endure large and complex deformations are central to the development of wearable electronics. Here the authors present a cryopolymerization strategy for preparing an anisotropic polyvinyl alcohol/polyaniline hydrogel for flexible supercapacitor electrodes.

Metal oxide-based supercapacitors: progress and prospectives

Distinguished by particular physical and chemical properties, metal oxide materials have been a focus of research and exploitation for applications in energy storage devices. Used as supercapacitor electrode materials, metal oxides have certified attractive performances for fabricating various supercapacitor devices in a broad voltage window. In comparison with single metal oxides, bimetallic oxide materials are highly desired for overcoming the constraint of the poor electric conductivity of single metal oxide materials, achieving a high capacitance and raising the energy density at this capacitor-level power. Herein, we investigate the principal elements affecting the properties of bimetallic oxide electrodes to reveal the relevant energy storage mechanisms. Thus, the influences of the chemical constitution, structural features, electroconductivity, oxygen vacancies and various electrolytes in the electrochemical behavior are discussed. Moreover, the progress, development and improvement of multifarious devices are emphasized systematically, covering from an asymmetric to hybrid configuration, and from aqueous to non-aqueous systems. Ultimately, some obstinate and unsettled issues are summarized as well as a prospective direction has been given on the future of metal oxide-based supercapacitors.

Self-Assembled Nickel Pyrophosphate-Decorated Amorphous Bimetal Hydroxides 2D-on-2D Nanostructure for High-Energy Solid-State Asymmetric Supercapacitor

To obtain a supercapacitor with a remarkable specific capacitance and rate performance, a cogent design and synthesis of the electrode material containing abundant active sites is necessary. In present work, a scalable strategy is developed for preparing 2D-on-2D nanostructures for high-energy solid-state asymmetric supercapacitors (ASCs). The self-assembled vertically aligned microsheet-structured 2D nickel pyrophosphate (Ni₂ P₂ O₇ ) is decorated with amorphous bimetallic nickel cobalt hydroxide (NiCo-OH) to form a 2D-on-2D nanostructure arrays electrode. The resulting Ni₂ P₂ O₇ /NiCo-OH 2D-on-2D array electrode exhibits peak specific capacity of 281 mA hg^-1 (4.3 F cm^-2 ), excellent rate capacity, and cycling stability over 10 000 charge-discharge cycles in the positive potential range. The excellent electrochemical features can be attributed to the high electrical conductivity and 2D layered structure of Ni₂ P₂ O₇ along with the Faradic capacitance of the amorphous NiCo-OH nanosheets. The constructed Ni₂ P₂ O₇ /NiCo-OH//activated carbon based solid-state ASC cell operates in a high voltage window of 1.8 V with an energy density of 78 Wh kg^-1 (1.065 mWh cm^-3 ) and extraordinary cyclic stability over 10 000 charge-discharge cycles with excellent energy efficiency (75%-80%) over all current densities. The excellent electrochemical performance of the prepared electrode and solid-state ASC device offers a favorable and scalable pathway for developing advanced electrodes.

MnO2/ZnCo2O4 with binder-free arrays on nickel foam loaded with graphene as a high performance electrode for advanced asymmetric supercapacitors

ZnCo₂O₄ nanosheets were successfully arrayed on a Ni foam surface with graphene using a hydrothermal method followed by annealing treatment; then MnO₂ nanoparticles were electrodeposited on the ZnCo₂O₄ nanosheets to obtain a synthesized composite binder-free electrode named MnO₂/ZnCo₂O₄/graphene/Ni foam (denoted as MnO₂/ZnCo₂O₄/G/NF). After testing the binder-free composite electrode of MnO₂/ZnCo₂O₄/G/NF via cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy testing, we found that it exhibited ultrahigh electrochemical properties, with a high specific areal capacitance of 3405.21 F g⁻¹ under a current density of 2 A g⁻¹, and wonderful cycling stability, with 91.2% retention after 5000 cycles. Moreover, an asymmetric supercapacitor (ASC) based on MnO₂/ZnCo₂O₄/G/NF//G/NF was successfully designed. When tested, the as-designed ASC can achieve a maximum energy density of 46.85 W h kg⁻¹ at a power density of 166.67 W kg⁻¹. Finally, the ASC we assembled can power a commercial red LED lamp successfully for more than 5 min, which proves its practicability. All these impressive performances indicate that the MnO₂/ZnCo₂O₄/graphene composite material is an outstanding electrode material for electrochemical capacitors.

Schematic illustration of formation process of MnO₂/ZnCo₂O₄/G/NF composite electrode.

Formation of ZnCo2O4@MnO2 core–shell electrode materials for hybrid supercapacitor

In this work, ZnCo₂O₄@MnO₂ core–shell structures are successfully prepared on nickel foam by a simple hydrothermal approach.