Facile Synthesis of CoSe/Co3O4-CNTs/NF Composite Electrode for High-Performance Asymmetric Supercapacitor.

Electrode materials are key factors for supercapacitors to endow them with excellent electrochemical properties. Here, a novel hybrid structure of a CoSe/Co3O4-CNTs binder free composite electrode on nickel foam was prepared via a facile flame method, followed by an electrodeposition process. Benefitting from the synergetic effects of the multicomponent (with low resistances of 1.542 Ω cm2 and a moderate mesoporous size of 3.12 nm) and the enlarged specific surface area of the composite material (77.4 m2 g-1), the CoSe/Co3O4-CNTs composite electrode delivers a high specific capacitance of 2906 F g-1 at 5 mV s-1 with an excellent rate stability. The fabricated CoSe/Co3O4-CNTs/NF//AC ASC exhibits a high energy density of 43.4 Wh kg-1 at 0.8 kW kg-1 and a long cycle life (92.7% capacitance retention after 10,000 cycles).

Amine-Assisted Delaminated 2D Ti3C2Tx MXenes for High Specific Capacitance in Neutral Aqueous Electrolytes.

Electrochemical capacitors using neutral aqueous electrolytes are safer and cheaper and allow diverse current collectors compared with the counterparts using organic or acidic/alkaline electrolytes. Two-dimensional (2D) MXenes have been demonstrated as the high-capacitive materials with high rate performance. However, MXene electrodes often exhibit a limited capacitance in neutral electrolytes, where the reversible electrochemical reactions rely greatly on the structural and surface properties of MXenes depending on their synthesis methods. Herein, a simple and highly efficient strategy, which combines HF etching of Ti3AlC2 powder and subsequent amine-assisted delamination at a low temperature, is developed to synthesize 2D Ti3C2Tx MXenes. The comprehensive results demonstrate that the enlarged interlayer spacing and the presence of more -O-containing functional groups synergistically contribute to the improvement of capacitive performance in neutral electrolytes. The 2D Ti3C2Tx MXenes show excellent electrochemical performance in various neutral electrolytes, and a high specific gravimetric capacitance of 149.8 F/g is achieved in 1.0 M Li2SO4. Furthermore, the flexible solid-state supercapacitors (SCs) with a neutral PVA/LiCl gel electrolyte possess a superior areal capacitance (163.1 mF/cm2) and high energy density (17.6 µWh/cm2 at 0.07 mW/cm2), together with high user safety. This work provides a promising guideline of synthesis strategy for high-capacitive MXenes used in neutral electrolytes, which may promote the development of safe and flexible power sources with a high energy density.

Assembling Co3O4 Nanoparticles into MXene with Enhanced electrochemical performance for advanced asymmetric supercapacitors.

MXenes with unique 2D open structure, large surface-area-to-volume ratios, high pseudo-capacitance, and conductivity are attractive for advanced supercapacitor electrodes. However, the restacking issue of MXenes hinders ion accessibility, resulting in the reduction of volume performance, mass load, and speed capability. To address these issues, a facile hydrothermal synthesis strategy is proposed to fabricate Co3O4 nanoparticles-MXene (Co-MXene) composite by the self-assembly process. Co3O4 nanoparticles, introduced in the MXene matrix, effectively prevent self-restacking and shorten ion/electron transport paths. Consequently, the obtained Co-MXene electrode delivers the high-performance of 1081F g-1 at a current density of 0.5 A g-1, surpassing the pristine MXene electrode (89F g-1 at 0.5 A g-1). Being assembled into asymmetric supercapacitors (ASC), a high energy density of 26.06 Wh kg-1 at 700 W kg-1 was realized. After 8000 cycles, the ASC device maintains 83% of initial specific capacitance at 2 A g-1. This work highlights a simple and efficient method for developing high-performance MXene-based electrodes for supercapacitors.

Hierarchically activated porous carbon derived from zinc-based fluorine containing metal-organic framework as extremely high specific capacitance and rate performance electrode material for advanced supercapacitors.

In this work, a hierarchically activated porous carbon (APC) was synthesized using fluorine-containing metal-organic framework via facile combined carbonization and KOH activation treatments. The influences of activation conditions on the surface structures and electrochemical performance of APC were systematically studied. Afterwards, the electrochemical responses of APC electrode were further assessed from the cyclic voltammetry and galvanostatic charge-discharge examinations by 6 M KOH electrolyte. The as-obtained APC electrode delivered the high specific capacitances of 540.8 and 280 F g-1 at 1 and 500 A g-1, correspondingly with superior capacitance retention of 94% after 250,000 cycles even at 100 A g-1, which is showing that its outstanding capacitance, remarkable rate capacity, and very-long cyclic life. Furthermore, the as-assembled APC-based symmetrical supercapacitor offers a superb energy density of 19 Wh kg-1 at 182 W kg-1, indicating its large-scale application. Thus, this work proposes a potential route to synthesize highly efficient porous carbon material for the future development of energy storage systems.

High Specific Capacitance of the Electrodeposited MnO2 on Porous Foam Nickel Soaked in Alcohol and its Dependence on Precursor Concentration.

In this work, we used the mixed solution of manganese acetate and sodium sulfate to deposit manganese dioxide on the three-dimensional porous nickel foam that was previously soaked in alcohol, and then the effects of solution concentrations on their capacitance properties were investigated. The surface morphology, microstructure, elemental valence and other information of the material were observed by scanning electron microscope (SEM), Transmission Electron Microscope (TEM), X-ray photoelectron spectroscopy (XPS), etc. The electrochemical properties of the material were tested by Galvanostatic charge-discharge (GCD), Cyclic Voltammetry (CV), Chronoamperometry (CA), Electrochemical impedance spectroscopy (EIS), etc. The MnO2 electrode prepared at lower concentrations can respectively reach a specific capacitance of 529.5 F g-1 and 237.3 F g-1 at the current density of 1 A g-1 and 10 A g-1, and after 2000 cycles, the capacity retention rate was still 79.8% of the initial capacitance, and the energy density can even reach 59.4 Wh Kg-1, while at the same time, it also has a lower electrochemical impedance (Rs = 1.18 Ω, Rct = 0.84 Ω).

Surface Roughness: A Crucial Factor To Robust Electric Double Layer Capacitors.

Electric double layer capacitors (EDLCs) usually show high rate performance and long cycling spans but inferior specific capacitance, which are mainly created by restriction of the charge storage mechanism. To improve the capacitive performance, traditional methods include enlarging surface area, optimizing porous structures, and readjusting functional groups through heteroatom doping to electrode materials. Besides that, another promising approach is suggested, which is to enhance surface roughness of the electrode materials for ion storage and transport. To prove this view, two porous carbon materials were fabricated by activation-calcination methods, which allowed the materials to have identical surface area, porous structures, and surface composition but the surface roughness. Further electrochemical measurements exhibited that the optimal sample with higher roughness has remarkable specific capacitance (up to 562 F g-1), and the increment rate is more than 50% when compared with contrast sample (367 F g-1). Therefore, optimization of the surface roughness of electrode materials is another efficient route to make robust EDLCs.

High-Performance Flexible Solid-State Asymmetric Supercapacitors Based on Bimetallic Transition Metal Phosphide Nanocrystals.

Transition metal phosphides (TMPs) have recently emerged as an important type of electrode material for use in supercapacitors thanks to their intrinsically outstanding specific capacity and high electrical conductivity. Herein, we report the synthesis of bimetallic CoxNi1-xP ultrafine nanocrystals supported on carbon nanofibers (CoxNi1-xP/CNF) and explore their use as positive electrode materials of asymmetric supercapacitors. We find that the Co:Ni ratio has a significant impact on the specific capacitance/capacity of CoxNi1-xP/CNF, and CoxNi1-xP/CNF with an optimal Co:Ni ratio exhibits an extraordinary specific capacitance/capacity of 3514 F g-1/1405.6 C g-1 at a charge/discharge current density of 5 A g-1, which is the highest value for TMP-based electrode materials reported by far. Our density functional theory calculations demonstrate that the significant capacitance/capacity enhancement in CoxNi1-xP/CNF, compared to the monometallic NiP/CNF and CoP/CNF, originates from the enriched density of states near the Fermi level. We further fabricate a flexible solid-state asymmetric supercapacitor using CoxNi1-xP/CNF as positive electrode material, activated carbon as negative electrode material, and a polymer gel as the electrolyte. The supercapacitor shows a specific capacitance/capacity of 118.7 F g-1/166.2 C g-1 at 20 mV s-1, delivers an energy density of 32.2 Wh kg-1 at 3.5 kW kg-1, and demonstrates good capacity retention after 10000 charge/discharge cycles, holding substantial promise for applications in flexible electronic devices.

Rationally designed CuCo2O4@Ni(OH)2 with 3D hierarchical core-shell structure for flexible energy storage.

Composite electrodes that possess both rational structures and appropriate integration are needed to deliver high electrochemical performance in energy storage devices. In this paper, a flexible and binder-free electrode material based on a heterogeneous core-shell structure of CuCo2O4@Ni(OH)2 nanosheets grown on carbon cloth was fabricated by a simple method. The unique three-dimensional hierarchical structure gives the electrode a large specific surface area, which enables rapid response and increases of specific capacitance. The CuCo2O4@Ni(OH)2/carbon fiber cloth (CFC) composite electrode exhibited a specific capacitance of 2160 F g-1 at 1 A g-1 and a good rate capability energy of 82.7% at 20 A g-1. A flexible all-solid-state asymmetric supercapacitor (FAASC) was assembled with the CuCo2O4@Ni(OH)2/CFC electrode as the positive electrode, and activated carbon (AC)/CFC as the negative electrode. This device showed both a high energy density and power density (58.9 W h kg-1 at a power density of 400 W kg-1), and good long-term cycling stability. Furthermore, the assembled CuCo2O4@Ni(OH)2/CFC//AC/CFC devices were capable of driving a blue light-emitting diode after a short charge. The remarkable performance of this CuCo2O4@Ni(OH)2/CFC electrode indicates that this heterogeneous structure has great potential for applications in flexible high-performance energy storage devices.

Multidimensional Hierarchical Fabric-Based Supercapacitor with Bionic Fiber Microarrays for Smart Wearable Electronic Textiles.

Flexible textile-based supercapacitors (SCs) have attracted a lot of attention, with the artificial intelligence technology and smart wearable electronic textiles developing rapidly. However, energy-storage performance of common textile-based SCs is always restricted with the low-dimensional substrates (i.e., one-dimensional fibers or two-dimensional fabrics), and hence flexible textile-based SCs with multifarious hierarchical substrates are highly desired. Herein, a multidimensional hierarchical fabric electrode model with a bionic fiber microarray structure has been designed, inspired by the "grasp effect" of the sophisticated arrangement structures of hedgehog spines, and the bionic assembled SCs exhibit an enhanced specific areal capacitance (245.5 mF/cm2 at 1 mV/cm2), compared with the planar fabric-based SCs (41.6 mF/cm2), and a high energy density (21.82 µWh/cm2 at 0.4 mW/cm2). Besides, the SCs also show a stable capacitance ratio of 83.9% after 10 000 cycles and a mere capacitance loss under different bending states. As a proof of concept, an all-fabric smart electronic switch is fabricated with self-power and wearable properties, along with some other trial applications. Such a hierarchical fabric with a bionic fiber microarray structure is believed to enhance the performance of the assembled SCs. We foresee that the multidimensional hierarchical fabric would bring more promising prospects for flexible textile-based energy-storage systems and be used in smart wearable textile applications.

A Novel Radiation Method for Preparing MnO2/BC Monolith Hybrids with Outstanding Supercapacitance Performance.

A novel facile process for fabrication of amorphous MnO2/bamboo charcoal monolith hybrids (MnO2/BC) for potential supercapacitor applications using γ-irradiation methods is described. The structural, morphological and electrochemical properties of the MnO2/BC hybrids have been investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. The combination of BC (electrical double layer charge) and MnO2 (pseudocapacitance) created a complementary effect, which enhanced the specific capacitance and good cyclic stability of the MnO2/BC hybrid electrodes. The MnO2/BC hybrids showed a higher specific capacitance (449 F g-1 at the constant current density of 0.5 A g-1 over the potential range from ⁻0.2 V to 0.8 V), compared with BC (101 F g-1) in 1 M of Na2SO4 aqueous electrolyte. Furthermore, the MnO2/BC hybrid electrodes showed superior cycling stability with 78% capacitance retention, even after 10,000 cycles. The experimental results demonstrated that the high performance of MnO2/BC hybrids could be a potential electrode material for supercapacitors.

Facile preparation of 3D hierarchical coaxial-cable-like Ni-CNTs@beta-(Ni, Co) binary hydroxides for supercapacitors with ultrahigh specific capacitance.

A facile chemical method for Co doping Ni-CNTs@α-Ni(OH)2 combining with an in situ phase transformation process is successfully proposed and employed to synthesize three-dimensional (3D) hierarchical Ni-CNTs@ß-(Ni, Co) binary hydroxides. This strategy can effectively maintain the coaxial-cable-like structure of Ni-CNTs@α-Ni(OH)2 and meanwhile increase the content of Co as much as possible. Eventually, the specific capacitances and electrical conductivity of the composites are remarkably enhanced. The optimized composite exhibits high specific capacitances of 2861.8F g-1 at 1A g-1 (39.48F cm-2 at 15mAcm-2), good rate capabilities of 1221.8F g-1 at 20A g-1 and cycling stabilities (87.6% of capacitance retention after 5000cycles at 5A g-1). The asymmetric supercapacitor (ASC) constructed with the as-synthesized composite and activated carbon as positive and negative electrode delivers a high specific capacitance of 287.7F g-1 at 1A g-1. The device demonstrates remarkable energy density (96Whkg-1) and high power density (15829.4Wkg-1). The retention of capacitance remains 83.5% at the current density of 5A g-1 after 5000cycles. The charged and discharged samples are further studied by ex situ electron energy loss spectroscopy (EELS) analysis, XRD and SEM to figure out the reasons of capacitance fading. Overall, it is believable that this facile synthetic strategy can be applied to prepare various nanostructured metal hydroxide/CNT composites for high performance supercapacitor electrode materials.

3D architecture of a graphene/CoMoO(4) composite for asymmetric supercapacitors usable at various temperatures.

Designing and optimizing the electrode materials and studying the electrochemical performance or cycle life of the supercapacitor under different working conditions are crucial to its practical application. Herein, we proposed a rational design of 3D-graphene/CoMoO4 nanoplates by a facile two-step hydrothermal method. Owing to the high electron transfer rate of graphene and the high activity of the CoMoO4 nanoplates, the three-dimensional electrode architectures achieved remarkable electrochemical performances with high areal specific capacitance (1255.24F/g at 1A/g) and superior cycling stability (91.3% of the original specific capacitance after 3000 cycles at 1A/g). The all-solid-state asymmetric supercapacitor composed of 3D-graphene/CoMoO4 and activated carbon (AC) exhibited a specific capacitance of 109F/g at 0.2A/g and an excellent cycling stability with only 12.1% of the initial specific capacitance off after 3000 cycles at 2A/g. The effects of temperature and charge-discharge current densities on the charge storage capacity of the supercapacitor were also investigated in detail for practical applications.

Carbon nanocages as supercapacitor electrode materials.

Supercapacitor electrode materials: Carbon nanocages are conveniently produced by an in situ MgO template method and demonstrate high specific capacitance over a wide range of charging-discharging rates with high stability, superior to the most carbonaceous supercapacitor electrode materials to date. The large specific surface area, good mesoporosity, and regular structure are responsible for the excellent performance.