Free-standing Two-dimensional Mesoporous ZnCo2O4 Thin Sheets Consisting of 3D Ultrathin Nanoflake Array Frameworks for High Performance Asymmetric Supercapacitor

Electrode Materials, Structural Design, and Storage Mechanisms in Hybrid Supercapacitors

Currently, energy storage systems are of great importance in daily life due to our dependence on portable electronic devices and hybrid electric vehicles. Among these energy storage systems, hybrid supercapacitor devices, constructed from a battery-type positive electrode and a capacitor-type negative electrode, have attracted widespread interest due to their potential applications. In general, they have a high energy density, a long cycling life, high safety, and environmental friendliness. This review first addresses the recent developments in state-of-the-art electrode materials, the structural design of electrodes, and the optimization of electrode performance. Then we summarize the possible classification of hybrid supercapacitor devices, and their potential applications. Finally, the fundamental theoretical aspects, charge-storage mechanism, and future developing trends are discussed. This review is intended to provide future research directions for the next generation of high-performance energy storage devices.

Exploring the redox characteristics of porous ZnCoS@rGO grown on nickel foam as a high-performance electrode for energy storage applications

A supercapattery is a device that combines the properties of batteries and supercapacitors, such as power density and energy density. A binary composite (zinc cobalt sulfide) and rGO are synthesized using a simple hydrothermal method and modified Hummers' method. A notable specific capacity (C_s) of 1254 C g^-1 is obtained in the ZnCoS@rGO case, which is higher than individual C_s of ZnS (975 C g^-1) and CoS (400 C g^-1). For the asymmetric (ASC) device (ZnCoS@rGO//PANI@AC), the PANI-doped activated carbon and ZnCoS@rGO are used as the cathode and anode respectively. A high C_m of 141 C g^-1 is achieved at 1.4 A g^-1. The ASC is exhibited an extraordinary energy density of 45 W h kg^-1 with a power density 5000 W kg^-1 at 1.4 A g^-1. To check the stability of the device, the ASC device is measured for 2000 charging/discharging cycles. The device showed improved coulombic efficiency of 94%. These findings confirmed that the two-dimensional materials provide the opportunities to design battery and supercapacitor hybrid devices.

Nanostructured mixed transition metal oxide spinels for supercapacitor applications

There have been numerous applications of supercapacitors in day-to-day life. Along with batteries and fuel cells, supercapacitors play an essential role in supplementary electrochemical energy storage technologies. They are used as power sources in portable electronics, automobiles, power backup, medical equipment, etc. Among various working electrode materials explored for supercapacitors, nanostructured transition metal oxides containing mixed metals are highly specific and special, because of their stability, variable oxidation states of the constituted metal ions, possibility to tune the mixed metal combinations, and existence of new battery types and extrinsic pseudocapacitance. This review presents the key features and recent developments in the direction of synthesis and electrochemical energy storage behavior of some of the recent morphology-oriented transition metal oxide and mixed transition metal oxide nanoparticles. We also targeted the studies on a few of the recently developed flexible and bendable supercapacitor devices based on these mixed transition metal oxides.

Co3O4@NiMoO4 composite electrode materials for flexible hybrid capacitors

Co₃O₄ nanomaterials as electrodes have been studied widely in the past decade due to their unique structural characteristics. However, their performance does not yet reach the level required for practical applications. It is, nevertheless, an effective strategy to synthesize hybrid electrode materials with high energy density. Herein we prepare Co₃O₄@NiMoO₄ nanowires by a two-step hydrothermal method. The as-obtained sample can be directly used as cathode material of supercapacitors; with specific capacitance of 600 C/g at 1 A/g. An assembled capacitor delivers an energy density of 36.1 Wh/kg at 2700 W/kg, and retains 98.2% of the initial capacity after 8000 cycles.

Potato Chip-Like 0D Interconnected ZnCo2O4 Nanoparticles for High-Performance Supercapacitors

Zinc cobaltite (ZnCo2O4) is an emerging electrode material for supercapacitors due to its rich redox reactions involving multiple oxidation states and different ions. In the present work, potato chip-like 0D interconnected ZnCo2O4 nanoparticles (PIZCON) were prepared using a solvothermal approach. The prepared material was characterized using various analytical methods, including X-ray powder diffraction and scanning electron microscopy. The possible formation mechanism of PIZCON was proposed. The PIZCON electrode material was systematically characterized for supercapacitor application. The areal capacitance of PIZCON was 14.52 mF cm−2 at 10 µA cm−2 of current density, and retention of initial capacitance was 95% at 250 µA cm−2 following 3000 continuous charge/discharge cycles. The attained measures of electrochemical performance indicate that PIZCON is an excellent supercapacitor electrode material.

Novel Two-Dimensional Porous Materials for Electrochemical Energy Storage: A Minireview

Two dimensional (2D) porous materials have great potential in electrochemical energy conversion and storage. Over the past five years, our research group has focused on Simple, Mass, Homogeneous and Repeatable Synthesis of various 2D porous materials and their applications for electrochemical energy storage especially for supercapacitors (SCs). During the experimental process, through precisely controlling the experimental parameters, such as reaction species, molar ratio of different ions, concentration, pH value of reaction solution, heating temperature, and reaction time, we have successfully achieved the control of crystal structure, composition, crystallinity, morphology, and size of these 2D porous materials including transition metal oxides (TMOs), transition metal hydroxides (TMHOs), transition metal oxalates (TMOXs), transition metal coordination complexes (TMCCs) and carbon materials, as well as their derivatives and composites. We have also named some of them with CQU-Chen (CQU is the initialism of Chongqing University, Chen is the last name of Lingyun Chen), such as CQU-Chen-Co-O-1, CQU-Chen-Ni-O-H-1, CQU-Chen-Zn-Co-O-1, CQU-Chen-Zn-Co-O-2, CQU-Chen-OA-Co-2-1, CQU-Chen-Co-OA-1, CQU-Chen-Ni-OA-1, CQU-Chen-Gly-Co-3-1, CQU-Chen-Gly-Ni-2-1, CQU-Chen-Gly-Co-Ni-1, etc. The introduction of 2D porous materials as electrode materials for SCs improves the energy storage performances. These materials provide a large number of active sites for ion adsorption, supply plentiful channels for fast ion transport and boost electrical conductivity and facilitate electron transportation and ion penetration. The unique 2D porous structures review is mainly devoted to the introduction of our contribution in the 2D porous nanostructured materials for SC. Finally, the further directions about the preparation of 2D porous materials and electrochemical energy conversion and storage applications are also included.

Electrochemical Performance of 2D-Hierarchical Sheet-Like ZnCo2O4 Microstructures for Supercapacitor Applications

With the rapid improvement of the global economy, the role of energy has become even more vital in the 21st century. In this regard, energy storage/conversion devices have become a major, worldwide research focus. In response to this, we have prepared two-dimensional (2D)-hierarchical sheet-like ZnCo2O4 microstructures for supercapacitor applications using a simple hydrothermal method. The 2D-hierarchical sheet-like morphologies with large surface area and smaller thickness enhanced the contact area of active material with the electrolyte, which increased the utilization rate. We investigated the electrochemical performance of sheet-like ZnCo2O4 microstructures while using Cyclic voltammetry (CV), Galvanostatic charge-discharge (GCD), and Electrochemical impedance spectroscopy (EIS) analysis. The electrochemical results demonstrated that the ZnCo2O4 electrode possesses 16.13 mF cm−2 of areal capacitance at 10 µA cm−2 of current density and outstanding cycling performance (170% of capacitance is retained after 1000 cycles at 500 µA cm−2). The high areal capacitance and outstanding cycling performance due to the unique sheet-like morphology of the ZnCo2O4 electrode makes it an excellent candidate for supercapacitor applications.

Two-dimensional porous nickel oxalate thin sheets constructed by ultrathin nanosheets as electrode materials for high-performance aqueous supercapacitors

2D porous nickel oxalate thin sheets constructed by ultrathin nanosheets were first synthesized by using a simple hydrothermal method. The resulting porous thin sheets exhibited superior supercapacitor performance.

Interfacial synthesis of micro-cuboid Ni0.55Co0.45C2O4 solid solution with enhanced electrochemical performance for hybrid supercapacitors

Efficient charge and energy storage relies essentially on the development of innovative electrode materials with enhanced electrochemical kinetics. Herein, Ni_0.55Co_0.45C₂O₄ solid solution was successfully synthesized by a liquid-liquid interfacial reaction. The observation of the morphologies of Ni_0.55Co_0.45C₂O₄ depicts a peculiar micro-cuboid structure composed of nanoparticles in the size range of 13 to 23 nm, benefiting the increase in the contribution of surface-controlled reactions to charge storage processes. The results from X-ray diffraction and thermogravimetric analysis demonstrate the similarity of the crystal structure and thermal decomposition behavior between Ni_0.55Co_0.45C₂O₄ and CoC₂O₄, and indicate that the CoC₂O₄ lattice plays a role as the fundamental framework in the solid solution instead of NiC₂O₄. The electrochemical measurements show that Ni_0.55Co_0.45C₂O₄ achieves a higher specific capacity of 562 C g^-1 at a current density of 1 A g^-1 than its counterpart NiC₂O₄/CoC₂O₄ hybrids, due to this the alternative arrangement of nickel and cobalt species in the solid solution expedites the diffusion process of active ions during the electrochemical reaction. Depending on the enhancement of the electrochemical stability in the solid solution, Ni_0.55Co_0.45C₂O₄ electrodes retain 87.4% of the initial capacity after 4000 cycles. The assembled Ni_0.55Co_0.45C₂O₄//AC hybrid supercapacitor attains an energy density of 38.5 W h kg^-1 at a power density of 799 W kg^-1 with a long cycling life (80% of the initial capacitance after 10 000 cycles). The excellent electrochemical performance suggests that Ni_0.55Co_0.45C₂O₄ is a promising candidate electrode material for supercapacitors.

Preparation of mesoporous CoNiO2 hexagonal nanoparticles for asymmetric supercapacitors via a hydrothermal microwave carbon bath process

Mesoporous CoNiO₂ hexagonal nanoparticles prepared via a hydrothermal microwave carbon bath process instead of the conventional calcination method have excellent pseudocapacitance properties.

Solvothermal synthesis of NiWO4 nanostructure and its application as a cathode material for asymmetric supercapacitors

This study proposes a facile solvothermal synthesis of nickel tungstate (NiWO4) nanowires for application as a novel cathode material for supercapacitors. The structure, morphology, surface area and pore distribution were characterized and their capacitive performances were investigated. The results showed that the NiWO4 nanowires synthesized in ethylene glycol solvent could offer a high specific capacitance of 1190 F g-1 at a current density of 0.5 A g-1 and a capacitance retaining ratio of 61.5% within 0.5-10 A g-1. When used as a cathodic electrode of an asymmetric supercapacitor (ASC), the NiWO4 nanowire based device can be cycled reversibly in a high-voltage region of 0-1.7 V with a high specific capacitance of 160 F g-1 at 0.5 A g-1, which therefore contributed to an energy density of 64.2 W h kg-1 at a power density of 425 W kg-1. Moreover, 92.8% of its initial specific capacitance can be maintained after 5000 consecutive cycles (5 A g-1). These excellent capacitive properties make NiWO4 a credible electrode material for high-performance supercapacitors.

Size and crystallinity control of two-dimensional porous cobalt oxalate thin sheets: tuning surface structure with enhanced performance for aqueous asymmetric supercapacitors

Porous cobalt oxalate thin sheets with enhanced performance were synthesized under hydrothermal condition.