NiAl-layered Double Hydroxide/Reduced Graphene Oxide Composite: Microwave-assisted Synthesis and Supercapacitive Properties

Hybridization of layered double hydroxides with functional particles

Layered double hydroxides (LDHs) are a class of materials with useful properties associated with their anion exchange abilities as well as redox and adsorptive properties for a wide range of applications including adsorbents, catalysts and their supports, electrodes, pigments, ceramic precursors, and drug carriers. In order to satisfy the requirements for each application as well as to find alternative applications, the preparation of LDHs with the desired composition and particle morphology and post-synthetic modification by the host-guest interactions have been examined. In addition, the hybridization of LDHs with various functional particles has been reported to design materials of modified, improved, and multiple functions. In the present article, the preparation, the heterostructure and the application of hybrids containing LDHs as the main component are overviewed.

Microwave-Assisted Synthesis as a Promising Tool for the Preparation of Materials Containing Defective Carbon Nanostructures: Implications on Properties and Applications

In this review, we focus on a small section of the literature that deals with the materials containing pristine defective carbon nanostructures (CNs) and those incorporated into the larger systems containing carbon atoms, heteroatoms, and inorganic components.. Briefly, we discuss only those topics that focus on structural defects related to introducing perturbation into the surface topology of the ideal lattice structure. The disorder in the crystal structure may vary in character, size, and location, which significantly modifies the physical and chemical properties of CNs or their hybrid combination. We focus mainly on the method using microwave (MW) irradiation, which is a powerful tool for synthesizing and modifying carbon-based solid materials due to its simplicity, the possibility of conducting the reaction in solvents and solid phases, and the presence of components of different chemical natures. Herein, we will emphasize the advantages of synthesis using MW-assisted heating and indicate the influence of the structure of the obtained materials on their physical and chemical properties. It is the first review paper that comprehensively summarizes research in the context of using MW-assisted heating to modify the structure of CNs, paying attention to its remarkable universality and simplicity. In the final part, we emphasize the role of MW-assisted heating in creating defects in CNs and the implications in designing their properties and applications. The presented review is a valuable source summarizing the achievements of scientists in this area of research.

Mesoporous multi-valence manganese oxides composite nanotubes boosting long-life lithium-ion batteries

Multi-component nano-oxide composite materials may present special synergistic effects as anode materials for lithium-ion batteries. Mesoporous β-MnO₂/Mn₃O₄ composite nanotubes are built here via controlling the deoxidation process of carbon-coating to induce a partial phase transition of high valence manganese dioxides. Compared to single β-MnO₂ nanotubes or Mn₃O₄@C nanotubes, the mesoporous β-MnO₂/Mn₃O₄@C composite nanotubes exhibit superior electrochemical properties. 679 mA h g^-1 of reversible specific capacity and 86% of capacity retention after 1000 cycles at 1 A g^-1 current density are obtained. The excellent performance is attributed to the unique multiple phase transitions regulation phenomena of manganese oxide occurring in the β-MnO₂/Mn₃O₄ composite material during the electrochemical processes, which significantly extends the cycle life of the β-MnO₂/Mn₃O₄ composite material.

Design of ZIF-67 nanoflake derived NiCo-LDH/rGO hybrid nanostructures for aqueous symmetric supercapattery application under alkaline condition

Well-defined polyhedral ZIF-67 metal-organic frameworks (MOFs) are usually synthesized using methanol as solvent. In this work, methanol is replaced with deionized water as a solvent to synthesize ZIF-67 MOFs with unique nanoflake morphology. The ZIF-67 nanoflakes are synthesized directly byin situmethod on reduced graphene oxide (rGO) to obtain ZIF-67/rGO-xprecursors which are further transformed into NiCo-layered double hydroxide nanocomposites (NiCo-LDH/rGO-x,x = 10, 30, 50 and 90 mg of rGO). The NiCo-LDH/rGO-xnanostructured composites are found to be excellent materials for battery type supercapacitor (supercapattery) applications. Among these samples, the NiCo-LDH/rGO-30 composite gives maximum specific capacity of 829 C g^-1(1658 F g^-1) at a current density of 1 A g^-1and high rate capability. The as fabricated 2-electrode symmetric Swagelok deviceNiCo-LDH/rGO-30NiCo-LDH/rGO-30delivered a high energy density of 49.2 Wh kg^-1and a power density of 4511 W kg^-1, and enabled us to glow red, blue and white LED bulbs using three coin cells. The device can show good capacity retention even after 3000 continuous charge-discharge cycles. The NiCo-LDH/rGO-30 composite,in situderived from ZIF-67 MOF in combination with optimal amount of rGO, is an excellent material to deliver both high energy density and high power density in supercapattery devices.

Microwave as a Tool for Synthesis of Carbon-Based Electrodes for Energy Storage

This Spotlight on Applications highlights the significant impact of microwave-assisted methods for synthesis and modification of carbon materials with enhanced properties for electrodes in energy storage applications (supercapacitors and batteries). For the past few years, microwave irradiation has been increasingly used for the synthesis of carbon materials with different morphologies using various precursors. Microwave processing exhibits numerous advantages, such as short processing times, high yield, expanded reaction conditions, high reproducibility, and high purity of products. On this frontier research area, we have discussed microwave-assisted synthesis, defect creation, simultaneous reduction and exfoliation, and heteroatom doping in carbon materials. By careful manipulation of microwave irradiation parameters, the method becomes a powerful and efficient tool to generate different morphologies in carbon-based materials. Other important outcomes are the flexible control over the degree of reduction and exfoliation of graphene derivatives, the generation of defects in graphene-based materials by metals, the intercalation of metal oxides into graphene derivatives, and heteroatom doping of graphene materials. The Spotlight on Applications aims to provide a condensed overview of the current progress in carbon-based electrodes synthesized by microwave, pointing out outstanding challenges and offering a few suggestions to trigger more research endeavors in this important field.

Synthesis of two-Dimensional layered double hydroxide: A systematic overview

Two-dimensional (2D) layered double hydroxides (LDH) are classic materials in fundamental research and practical application. 2D LDH have unique structural features, such as high aspect ratio, high specific surface area,...

A review of the microwave-assisted synthesis of carbon nanomaterials, metal oxides/hydroxides and their composites for energy storage applications

Currently, nanomaterials are considered to be the backbone of modern civilization. Especially in the energy sector, nanomaterials (mainly, carbon- and metal oxide/hydroxide-based nanomaterials) have contributed significantly. Among the various green approaches for the synthesis of these nanomaterials, the microwave-assisted approach has attracted significant research interest worldwide. In this context, it is noteworthy to mention that because of their enhanced surface area, high conducting nature, and excellent electrical and electrochemical properties, carbon nanomaterials are being extensively utilized as efficient electrode materials for both supercapacitors and secondary batteries. In this review article, we briefly demonstrate the characteristics of microwave-synthesized nanomaterials for next-generation energy storage devices. Starting with the basics of microwave heating, herein, we illustrate the past and present status of microwave chemistry for energy-related applications, and finally present a brief outlook and concluding remarks. We hope that this review article will positively convey new insights for the microwave synthesis of nanomaterials for energy storage applications.

Simultaneous voltammetric determination of epinephrine and acetaminophen using a highly sensitive CoAl-OOH/reduced graphene oxide sensor in pharmaceutical samples and biological fluids

For this study, three novel types of sensors comprised of CoAl-layered double oxyhydroxide (CoAl-LDH), CoAl-LDH/reduced graphene oxide (rGO), and CoAl-OOH/rGO nanosheets were successfully fabricated on glassy carbon electrodes (GCEs) and employed for the electrochemical detection of epinephrine (EP) and acetaminophen (AC). Interestingly, we found that the CoAl-OOH/rGO/GCE was more suitable for the determination of EP and AC in contrast to the CoAl-LDH and CoAl-OOH/rGO sensors. Differential pulse voltammetry results revealed that the CoAl-OOH/rGO/GCE delivered excellent electrocatalytic activity. The sensitivities and detection limits for the simultaneous measurement of EP and AC were 12.2 μA μM^-1 cm^-2, 0.023 μM L^-1, and 4.87 μA μM^-1 cm^-2, 0.058 μM L^-1, respectively. Especially, the as-obtained CoAl-OOH/rGO/GCE was successfully utilized for the detection in pharmaceutical samples and biological fluids with satisfactory results. Owing to its outstanding electrocatalytic activity and superior sensitivity, the CoAl-OOH/rGO/GCE could be beneficial to construct a promising electrochemical sensor for the detection of EP and AC.

Tuning the structural properties of cadmium-aluminum layered double hydroxide for enhanced photocatalytic dye degradation

The distinctive layered structure, chemical stability and tunability of layered double hydroxides (LDHs) have led to extensive investigations in various areas of photocatalysis, including photocatalytic water splitting, carbon dioxide photoreduction, and degradation of organic pollutants. Here, a series of visible light active cadmium-aluminum layered double hydroxides (CdAl LDHs) with various Cd2+ : Al3+ ratios is synthesized via the reaction-diffusion framework (RDF) leading thereby to a hierarchal spherical structure of the LDH. The aim of this study is to develop an optimal CdAl LDH photocatalyst that is activated by solar light irradiation and tested for methylene blue (MB) degradation. The structural and physicochemical properties of the synthesized materials are determined by several imaging and spectroscopic techniques. The photocatalytic study reveals a strong dependence of the photocatalytic activity of the CdAl LDH on the cationic ratio with an optimal performance at a ratio Cd2+ : Al3+ equal to 3 : 1. A mechanism is proposed whereby the activity is ascribed to the formation of intermediate reactive oxidative species (ROS) during the photodegradation reactions and scrutinised by invoking different ROS quenchers and corroborated by density functional theory (DFT) calculations.

Preparation of Layered Double Hydroxides toward Precisely Designed Hierarchical Organization

Layered double hydroxides (LDHs) are a class of materials with useful properties associated with their anion exchange abilities for a wide range of materials’ applications including adsorbent, catalyst and its support, ceramic precursor, and drug carrier. In order to satisfy the requirements for the detailed characterization and the practical application, the preparation of LDHs with varied composition and particle morphology has been examined extensively. The versatility of the preparation methods led LDHs with varied composition and micro/macroscopic morphology, which makes the application of LDHs more realistic. In the present review article, synthetic methods of LDHs are overviewed in order to highlight the present status of the LDHs for practical application.

All-Solid-State Flexible Asymmetric Supercapacitor with Good Cycling Performance and Ultra-Power Density by Electrode Materials of Core-Shell CoNiO2@NiAl-Layered Double Hydroxide and Hollow Spherical α-Fe2O3

High electrochemical performance of asymmetric supercapacitor (ASC) depends on exquisite nanostructure design and synthesis of electrodes, including structural controllable design and synthesis of high theoretical performance materials and nanocomposite materials. Herein, a template-free hierarchical core-shell nanostructure of CoNiO₂@NiAl-layered double hydroxide (NiAl-LDH) and α-Fe₂O₃ with a hollow spherical structure composed of nanoparticles are successfully prepared. The CoNiO₂@NiAl-LDH as the cathode electrode and the hollow spherical α-Fe₂O₃ as the anode electrode of the ASC devices exhibit superior electrochemical performance. The gel of polyvinyl alcohol (PVA) and KOH acts as the solid electrolyte and the separator to assemble into the all-solid-state flexible ASC devices. Two of the CoNiO₂@NiAl-LDH//α-Fe₂O₃ ASC devices in series are fabricated to meet the voltage requirement of mobile energy equipment, which exhibit a maximum energy density of 65.68 Wh kg^-1 at the power density of 369.45 W kg^-1. Interestingly, in addition to many advantages that the solid electrolyte in ASC devices already have, we find that it can be an alternative way of solving the problem of iron oxide cycling performance, and of course it can also be used as a reference for other materials with poor cycling performance.

Hierarchical flower-like Ni–Co layered double hydroxide nanostructures: synthesis and super performance

Three-dimensional hierarchical flower-like Ni–Co LDHs have been prepared with a large specific surface area and expanded interlayer spacing as an adsorbent for removing anionic dyes and as an electrode for supercapacitors.

Theoretical and experimental investigations of the electronic/ionic conductivity and deprotonation of Ni3−xCoxAl-LDHs in an electrochemical energy storage system

A schematic illustration of the mechanism of enhanced electrochemical performance by doping Co species.

Flexible Asymmetric Supercapacitors Based on Nitrogen-Doped Graphene Hydrogels with Embedded Nickel Hydroxide Nanoplates

To push the energy density limit of supercapacitors (SCs), new electrode materials with hierarchical nano-micron pore architectures are strongly desired. Graphene hydrogels that consist of 3 D porous frameworks have received particular attention but their capacitance is limited by electrical double layer capacitance. In this work, we report the rational design and fabrication of a composite hydrogel of N-doped graphene (NG) that contains embedded Ni(OH)₂ nanoplates that is cut conveniently into films to serve as positive electrodes for flexible asymmetric solid-state SCs with NG hydrogel films as negative electrodes. The use of high-power ultrasound leads to hierarchically porous micron-scale sheets that consist of a highly interconnected 3 D NG network in which Ni(OH)₂ nanoplates are well dispersed, which avoids the stacking of NG, Ni(OH)₂ , and their composites. The optimal SC device benefits from the compositional features and 3 D electrode architecture and has a high specific areal capacitance of 255 mF cm^-2 at 1.0 mA cm^-2 and a very stable, high output cell voltage of 1.45 V, which leads to an energy density of 80 μW h cm^-2 even at a high power of 944 μW cm^-2 , considerably higher than that reported for similar devices. The devices exhibit a high rate capability and only 8 % capacitance loss over 10 000 charging cycles as well as excellent flexibility with no clear performance degradation under strong bending.

Electrostatic-Induced Assembly of Graphene-Encapsulated Carbon@Nickel-Aluminum Layered Double Hydroxide Core-Shell Spheres Hybrid Structure for High-Energy and High-Power-Density Asymmetric Supercapacitor

Achieving high energy density while retaining high power density is difficult in electrical double-layer capacitors and in pseudocapacitors considering the origin of different charge storage mechanisms. Rational structural design became an appealing strategy in circumventing these trade-offs between energy and power densities. A hybrid structure consists of chemically converted graphene-encapsulated carbon@nickel-aluminum layered double hydroxide core-shell spheres as spacers among graphene layers (G-CLS) used as an advanced electrode to achieve high energy density while retaining high power density for high-performance supercapacitors. The merits of the proposed architecture are as follows: (1) CLS act as spacers to avoid the close restacking of graphene; (2) highly conductive carbon sphere and graphene preserve the mechanical integrity and improve the electrical conductivity of LDHs hybrid. Thus, the proposed hybrid structure can simultaneously achieve high electrical double-layer capacitance and pseudocapacitance resulting in the overall highly active electrode. The G-CLS electrode exhibited high specific capacitance (1710.5 F g^-1 at 1 A g^-1) under three-electrode tests. An ASC fabricated using the G-CLS as positive electrode and reduced graphite oxide as negative electrode demonstrated remarkable electrochemical performance. The ASC device operated at 1.4 V and delivered a high energy density of 35.5 Wh kg^-1 at a 670.7 W kg^-1 power density at 1 A g^-1 with an excellent rate capability as well as a robust long-term cycling stability of up to 10 000 cycles.

Free-standing α-Co(OH)2/graphene oxide thin films fabricated through delamination and reassembling of acetate anions intercalated α-Co(OH)2 and graphene oxide in water

A novel hydrothermal process is demonstrated to prepare acetate anions intercalated α-Co(OH)2 that can be delaminated in water without any additional anion exchange processes. Positively charged Co(OH)2 nanosheets with lateral size of hundreds of nanometers and thickness less than 2 nm can be obtained by dispersing the as-obtained α-Co(OH)2 into water followed by sonication. The exfoliated Co(OH)2 nanosheets can be restacked into its original structure with different interlayer d-spacings. A flexible free-standing film with stacking Co(OH)2 nanosheets and graphene oxide (GO) layers can be obtained through flocculation of the Co(OH)2 nanosheets with GO nanosheets suspensions followed by a vacuum filtration, but the content of Co(OH)2 has to be kept under a low value so as to obtain films with flexible nature. Electrochemical tests show that this kind of film is not suitable to be used as electrode material for supercapacitor and lithium ion battery, because the content of active material is not high and the compacted junction between opposite charged nanosheets will prevent the electrolyte from diffusing into the interlayer space.

A comparative study of Ni–Mn layered double hydroxide/carbon composites with different morphologies for supercapacitors

A variety of carbon materials varying from 0D to 2D, i.e. 0D nanoparticles, 1D carbon nanotubes (CNTs) and 2D reduced graphene oxide (rGO) are selected to in situ combine with Ni–Mn layered double hydroxide (LDH) to prepare electrode materials for supercapacitors.

Rational design of sandwich-like exfoliated nickel hydroxide–carbon nanotubes as a novel electrode for supercapacitors

A composite of nickel hydroxide–carbon nanotubes (Ni(OH)₂–CNTs) was successfully fabricated as an electrode material for supercapacitors.

2D materials for renewable energy storage devices: Outlook and challenges

We review cost-effective, clean and durable alternative energy devices based on 2D materials.

Bath temperature controlled phase stability of hierarchical nanoflakes CoS2 thin films for supercapacitor application

In the present study, CoS₂ thin-film electrodes are synthesized at different bath temperatures using a simple chemical bath deposition (CBD) method.

Binary Nickel-Cobalt Oxides Electrode Materials for High-Performance Supercapacitors: Influence of its Composition and Porous Nature

Nickel-cobalt oxides were prepared by coprecipitation of their hydroxides precursors and a following thermal treatment under a moderate temperature. The preformed nickel-cobalt bimetallic hydroxide exhibited a flower-like morphology with single crystalline nature and composed of many interconnected nanosheets. The ratio of Ni to Co in the oxides could easily be controlled by adjusting the composition of the original reactants for the preparation of hydroxide precursors. It was found that both the molecular ratio of Ni to Co and the annealing temperature had significant effects on their porous structure and electrochemical properties. The effect of the Ni/Co ratio on the pseudocapacitive properties of the binary oxide was investigated in this work. The binary metal oxide with the exact molar ratio of Ni:Co = 0.8:1 annealed at 300 °C, showing an optimum specific capacitance of 750 F/g. However, too high an annealing temperature would lead to a large crystal size, a low specific surface area, as well as a much lower pore volume. With the use of the binary metal oxide with Ni:Co = 0.8:1 and activated carbon as the positive and negative electrode, respectively, the assembled hybrid capacitor could exhibit a high-energy density of 34.9 Wh/kg at the power density of 875 W/kg and long cycling life (86.4% retention of the initial value after 10000 cycles).

Anchoring cobalt oxide nanoparticles on to the surface multiwalled carbon nanotubes for improved supercapacitive performances

SEM and TEM images of Co₃O₄ nanodots anchored on to the surface of multiwalled carbon nanotubes.

Synthesis of exfoliated titanium dioxide nanosheets/nickel–aluminum layered double hydroxide as a novel electrode for supercapacitors

A facile composite of titanium dioxide nanosheets/nickel–aluminum layered double hydroxide (TiO₂/Ni–Al LDH) is fabricated successfully as the electrode material for supercapacitors.

Recent development of metal hydroxides as electrode material of electrochemical capacitors

Recent research on electrochemical capacitors using transition metal hydroxides as electrode materials is reviewed.