The urgent demand for clean energies and rapid development of modern electronic technologies have led to enthusiastic research on novel energy storage technologies, especially for supercapacitors. The most important part is designing electrode materials with excellent capacitive performance. Layered double hydroxides (LDHs) have sparked intense interest among researchers in the past decade due to the facile tunability of their composition, structure and morphology. Various and fruitful accomplishments have been achieved toward developing LDH-based materials for supercapacitor electrodes. This review outlines the recent advances in the designing of LDH-based electrode materials for supercapacitors. Feasible and practical strategies for improving the capacitive performance of LDH-based materials have been discussed and highlighted in terms of tuning the composition of LDHs, designing the electrode structure and assembling applicable supercapacitor devices. Through the ceaseless efforts of scientists, the capacitive performance and practicability of LDH-based materials have been greatly ameliorated, making them more competitive for modern energy storage applications.

Recent years, there have been massive reports of layered double hydroxides (LDHs) used as electrode materials. However, the synergistic effect between different host metal ions on the electrochemical behavior of LDHs is rarely studied. We prepared a series of LDHs with different host metal combinations and investigated their physiochemical properties and redox behaviors to study the synergistic effects between different active metal ions. According to the experimental and theoretical calculation results, LDHs with dual transition metals possessed lower band-gap energies and higher conductivities than mono-transition metal-based samples. The reduced band-gap energy and enhanced conductivity should be ascribed to the hybridization between 3d-orbitals of different transition atoms. As a result, LDHs with dual transition metals exhibited lower charge transfer resistances and redox potentials as well as longer electron lifetime, implying that they could be oxidized or reduced more easily during the redox process. The lowest charge transfer resistance and longest electron lifetime of NiMn-LDHs signify that the synergistic effect between Ni and Mn is the best. From in situ X-ray photoelectron spectroscopy results, more than 50% Ni atoms in NiMn-LDHs can be oxidized under 0.45 V, which also demonstrates the excellent redox efficiency of NiMn-LDHs.Download high-res image (148KB)Download full-size image

In this work, we develop a bifunctional composite comprised of hierarchical flower-like NiMn-layered double hydroxides (LDH) nanoarrays grown on melamine sponge-derived carbon (SC). This composite material is used as a flexible asymmetric supercapacitor positive electrode and oxygen evaluation reaction (OER) electrocatalyst. Integrated carbon foams with controllable three-dimensional pores are easily obtained and served as conductive backbones for the growth of the flower-like NiMn-LDH nanoarrays. The as-prepared flexible electrode, combining a highly open scaffold-like structure with well-designed electron transfer channels, shows significantly enhanced cyclic stability (99.6% of the initial activity maintained after 5000 cycles) and high energy density (41.0 Wh·Kg−1) in aqueous electrolyte. The as-prepared SC@NiMn-LDH@G//NPC asymmetric supercapacitor can lighten a LED indicator for more than 10 minutes. Additionally, this composite presents extremely low overpotentials (220 mV at 10 mA·cm−2) and low Tafel slopes (30 mV·dec−1) in alkaline medium. This OER performance is one of the best reported so far among the state of art electrocatalysts.

A 3D hybrid nanostructure, in which petal-like ultrathin nickel–aluminum layered double hydroxides (LDHs) were vertically grown on a conductive graphene/polypyrrole (GP) substrate, was designed and fabricated by a facile hydrothermal method. SEM and TEM observations confirmed the successful synthesis of this specially designed nanostructure, in which the conductive substrate ensures very fast electron transfer during the charge–discharge process, whereas the 3D hierarchical structure facilitates rapid ion transfer. The ultrathin LDH nanoflakes (3–5 nm) expose their abundant active sites to the electrolyte, thus generating huge pseudocapacitance. Combining the abovementioned features, this specially designed 3D nanostructured hybrid possesses an exceptional specific capacitance (2395 F g−1 at 1 A g−1), excellent rate performance (retaining 71.8% of capacitance at the current density of 20 A g−1), and remarkable cycling stability (99.6% retention after 10 000 cycles). Moreover, the assembled asymmetric supercapacitor obtained using GP@LDH as a positive electrode and GP-derived carbon as a negative electrode exhibits an ultrahigh energy density of 94.4 W h kg−1 at the power density of 463.1 W kg−1, making GP@LDH very attractive as an electrode material for high performance and low-cost supercapacitors.

N-doped microporous carbons with uniform ultramicropores (∼0.50 nm) are facilely prepared by direct carbonization of K+ exchanged meta-aminophenol–formaldehyde resin. These materials give an unprecedented CO2 uptake of 1.67 mmol g−1 at 25 °C and 0.15 bar and superior CO2-over-N2 selectivity (50:1).

The energy storage mechanism of N-doped carbons with low apparent specific surface areas (Brunauer–Emmett–Teller specific surface area determined by N2 adsorption) has puzzled the researchers in the supercapacitor field in recent years. In order to explore this scientific problem, such carbon materials were prepared through pyrolysis of N-rich polymers such as melamine formaldehyde resin and polyaniline. Although these carbons possess low apparent specific surface areas of no more than 60 m2 g−1, their areal capacitance could reach up to an abnormally high value of 252 μF cm−2. The results of systematical materials characterizations and electrochemical measurements show that these carbons contain numerous ultramicropores which could not be detected by the adsorbate of N2 but are accessible to CO2 and electrolyte ions. These ultramicropores play dominant roles in the charge storage process for these low apparent surface area carbons, leading to an energy storage mechanism of electric double layer capacitance. The contribution of pseudocapacitance to the total capacitance is calculated to be less than 15%. This finding challenges the widely accepted viewpoint that the high capacitance of N-doped carbon is mainly attributed to the pseudocapacitance generated from the faradic reactions between nitrogen functionalities and electrolyte.

The salient practical application feature of an ideal supercapacitor is its ability to deliver high energy density stably even at ultrahigh power density. Therefore, a rational design of electrode materials is essentially required for achieving high current, energy and power densities. In this work, a special “in situ replicating” strategy is employed to fabricate double shell hollow carbon spheres with homogeneously doped heteroatoms. The KOH activation introduces micropores to the thin shells of the hollow carbon spheres. Materials characterizations show that these carbon spheres have such merits as large surface area, easy-accessible micropore surface with faradaic reaction sites, and high conductivity. All these result in ultrafast ion transport from electrolyte to the micropores in the carbon spheres and endow the carbon with outstanding capacitive performance, e.g., an unprecedentedly high specific capacitance of 270 F g−1 at a very high current density of 90 A g−1. Moreover, a high energy density of 11.9 Wh kg−1 at a respectable power density of 30,000 W kg−1 is achieved in 6 M KOH electrolyte.

A series of diaminoalkane-intercalated graphenes (DIGs) are successfully synthesized by intercalating graphite oxide with diaminoalkanes, followed by a reduction process using hydrazine as a reductant at room temperature. The as-prepared intercalated graphite oxides (DIGOs) and their reduced products are characterized using a variety of approaches such as X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy and elemental analysis. Electrochemical tests show that the specific capacitances of DIGOs and DIGs decrease with the increase of the interlayer distance, and that the DIGs possess larger capacitance than DIGOs after hydrazine reduction. The ion diffusion in the DIGOs/DIGs follows pseudo-second-order kinetics and is dominated mainly by their pore sizes.

•An ocean biomass, Entromorphra prolifera, has been processed into supercapacitor electrodes.•KOH activation can prepare hierarchical porous carbon.•The as-prepared carbons have high capacitance with good rate capability.•This work provided an approach to value-added products from an ocean biomass.Enteromorpha prolifera (E.prolifera), an ocean biomass, was used as raw materials to prepare active carbons by a two-step strategy (pre-carbonization followed by chemical activation). The as-prepared active carbons have been characterized by a variety of means such as N2 adsorption, field emission scanning electron microscope, transmission electron microscope, Raman spectroscopy. The results showed that the carbons have large surface area and developed porosity with micro-meso hierarchical pore texture. As evidenced by electrochemical measurements, the specific capacitance of the carbons can reach up to 296 F g−1. More importantly, the carbons can maintain a high capacitance of up to 152 F g−1 at a very high current density of 30 A g−1, highlighting the promise of the carbons for high power applications.

•Highly ordered mesoporous carbon (OMC) was synthesized.•The as-synthesized OMC was of high surface area and uniform mesoporous structure.•The OMC was prepared as an SPME coating using Nafion as a binder.•The OMC/Nafion coating exhibited excellent extraction selectivity towards aromatic compounds.In this study, ordered mesoporous carbon (OMC) with large surface area (1019 m2 g−1), uniform mesoporous structure (pore size distribution centering at 4.2 nm) and large pore volume (1.46 cm3 g−1) was synthesized using 2D hexagonally mesoporous silica MSU-H as the hard template and sucrose as the carbon precursor. The as-synthesized OMC was immobilized onto a stainless steel wire using Nafion as a binder to prepare an OMC/Nafion solid-phase microextraction (SPME) coating. The extraction characteristics of the OMC/Nafion coating were extensively investigated using a wide range of analytes including non-polar (light petroleum and benzene homologues) and polar compounds (amines and phenols). The OMC/Nafion coating exhibited much better extraction efficiency towards all selected analytes than that of a multi-walled carbon nanotubes/Nafion coating with similar length and thickness, which is ascribed to its high surface area, well-ordered mesoporous structure and large pore volume. When the OMC/Nafion coating was used to extract a mixture containing various kinds of analytes, it possessed excellent extraction selectivity towards aromatic non-polar compounds. In addition, the feasibility of the OMC/Nafion coating for application in electrochemically enhanced SPME was demonstrated using protonated amines as model analytes.

N-doped microporous carbons with uniform ultramicropores (∼0.50 nm) are facilely prepared by direct carbonization of K+ exchanged meta-aminophenol–formaldehyde resin. These materials give an unprecedented CO2 uptake of 1.67 mmol g−1 at 25 °C and 0.15 bar and superior CO2-over-N2 selectivity (50:1).

The urgent demand for clean energies and rapid development of modern electronic technologies have led to enthusiastic research on novel energy storage technologies, especially for supercapacitors. The most important part is designing electrode materials with excellent capacitive performance. Layered double hydroxides (LDHs) have sparked intense interest among researchers in the past decade due to the facile tunability of their composition, structure and morphology. Various and fruitful accomplishments have been achieved toward developing LDH-based materials for supercapacitor electrodes. This review outlines the recent advances in the designing of LDH-based electrode materials for supercapacitors. Feasible and practical strategies for improving the capacitive performance of LDH-based materials have been discussed and highlighted in terms of tuning the composition of LDHs, designing the electrode structure and assembling applicable supercapacitor devices. Through the ceaseless efforts of scientists, the capacitive performance and practicability of LDH-based materials have been greatly ameliorated, making them more competitive for modern energy storage applications.

A 3D hybrid nanostructure, in which petal-like ultrathin nickel–aluminum layered double hydroxides (LDHs) were vertically grown on a conductive graphene/polypyrrole (GP) substrate, was designed and fabricated by a facile hydrothermal method. SEM and TEM observations confirmed the successful synthesis of this specially designed nanostructure, in which the conductive substrate ensures very fast electron transfer during the charge–discharge process, whereas the 3D hierarchical structure facilitates rapid ion transfer. The ultrathin LDH nanoflakes (3–5 nm) expose their abundant active sites to the electrolyte, thus generating huge pseudocapacitance. Combining the abovementioned features, this specially designed 3D nanostructured hybrid possesses an exceptional specific capacitance (2395 F g−1 at 1 A g−1), excellent rate performance (retaining 71.8% of capacitance at the current density of 20 A g−1), and remarkable cycling stability (99.6% retention after 10000 cycles). Moreover, the assembled asymmetric supercapacitor obtained using GP@LDH as a positive electrode and GP-derived carbon as a negative electrode exhibits an ultrahigh energy density of 94.4 W h kg−1 at the power density of 463.1 W kg−1, making GP@LDH very attractive as an electrode material for high performance and low-cost supercapacitors.