Simultaneous precipitation and discharge plasma processing for one-step synthesis of α-Fe2O3-Fe3O4/graphene visible light magnetically separable photocatalysts

A novel facile combination of precipitation and plasma discharge reaction is successfully employed for one-step synthesis of an α-Fe₂O₃-Fe₃O₄ graphene nanocomposite (GFs). The co-existence and anchoring of hematite (α-Fe₂O₃) and magnetite (Fe₃O₄) nanoparticles onto a graphene sheet in the as synthesized GFs were verified by results of XRD, Raman, SEM, TEM, and XPS. HRTEM characterization was used for confirming the bonding between α-Fe₂O₃/Fe₃O₄ nanoparticles and the graphene sheet. Consequently, GFs shows superior photodegrading performance towards methylene blue (MB), compared to individual α-Fe₂O₃/Fe₃O₄ nanoparticles, as a result of band gap narrowing and the electron-hole pair recombination rate reducing. Moreover, GFs allows a good possibility of separating and recycling under an external-magnetic field, suggesting potential in visible-light-promoted photocatalytic applications.

Capacitance enhancement of nitrogen-doped graphene oxide/magnetite with polyaniline or carbon dots under external magnetic field: Supported by theoretical estimation

The effect of conductive materials (polyaniline (PA) or carbon dots (Cdots)) added to supercapacitor consisting of nitrogen-doped graphene oxide (NG) and magnetic nanoparticles (magnetite, Fe₃O₄) was assessed. Small amounts (4 wt%) of Cdots in composites of NG and Fe₃O₄ nanoparticles have shown better supercapacitor performance than the addition of PA. When the external stimulating force (magnetic field, 8.98 mT) was coupled with the electrochemical system, the specific capacitance was highest (2213 F/g at a scan rate of 5 mV/s) and the cyclic retention was 91% after 5000 cycles for the NG/Cdots/Fe₃O₄ composite electrode. These reports show that the adequate ternary composite materials effectively enhance the specific capacitance, increase the specific energy density and maintain the durability of supercapacitors under the magnet. The increase in the specific capacitance under the uniform magnetic field was proportional to the 3/5 power of bulk electrolyte concentration, although the power value was different from the theoretical estimation. The complex capacitance was almost double under the magnetic field due to the convection induced by the Lorentz force. It was also confirmed in comparison with the theoretical estimation that the Lorentz effect was responsible for the reduction of the charge transfer resistance, the increase of the relaxation time constant, the facilitation of the ion diffusion, and hence the increase of the double-layer capacitance. The present results will open a new window for the enhancement mechanisms on the capacitance efficiency under the magnetic field.

Synergistic Effects of Fe2O3 Nanotube/Polyaniline Composites for an Electrochemical Supercapacitor with Enhanced Capacitance

α-Fe₂O₃, which is an attractive material for supercapacitor electrodes, has been studied to address the issue of low capacitance through structural development and complexation to maximize the use of surface pseudocapacitance. In this study, the limited performance of α-Fe₂O₃ was greatly improved by optimizing the nanotube structure of α-Fe₂O₃ and its combination with polyaniline (PANI). α-Fe₂O₃ nanotubes (α-NT) were fabricated in a form in which the thickness and inner diameter of the tube were controlled by Fe(CO)₅ vapor deposition using anodized aluminum oxide as a template. PANI was combined with the prepared α-NT in two forms: PANI@α-NT-a enclosed inside and outside with PANI and PANI@α-NT-b containing PANI only on the inside. In contrast to α-NT, which showed a very low specific capacitance, these two composites showed significantly improved capacitances of 185 Fg⁻¹ for PANI@α-NT-a and 62 Fg⁻¹ for PANI@α-NT-b. In the electrochemical impedance spectroscopy analysis, it was observed that the resistance of charge transfer was minimized in PANI@α-NT-a, and the pseudocapacitance on the entire surface of the α-Fe₂O₃ nanotubes was utilized with high efficiency through binding and conductivity improvements by PANI. PANI@α-NT-a exhibited a capacitance retention of 36% even when the current density was increased 10-fold, and showed excellent stability of 90.1% over 3000 charge–discharge cycles. This approach of incorporating conducting polymers through well-controlled nanostructures suggests a solution to overcome the limitations of α-Fe₂O₃ electrode materials and improve performance.

High-performance supercapacitor electrode based on naphthoquinone-appended dopamine neurotransmitter as an efficient energy storage material

NQ-DP based organic material was successfuly synthesized and employed as an efficient pseudocapacitor material.

Designing neurotransmitter dopamine-functionalized naphthalene diimide molecular architectures for high-performance organic supercapacitor electrode materials

Naphthalenediimide-dopamine conjugates were successfully synthesized, and the influence of dopamine, a neurotransmitter, on the supercapacitor properties of a NDI scaffold was explored.

Polydiacetylene-Perylenediimide Supercapacitors

Organic supercapacitors have attracted interest as promising "green" and efficient components in energy storage applications. A polydiacetylene derivative coupled with reduced graphene oxide was employed, for the first time, to generate an organic pseudocapacitance-based supercapacitor that exhibited excellent electrochemical properties. Specifically, diacetylene monomers were functionalized with perylenediimide (PDI), spontaneously forming elongated microfibers. Following polymerization through UV irradiation, the PDI-polydiacetylene microfibers were interspersed with reduced graphene oxide (rGO), generating a porous electrode material exhibiting a high surface area and facilitating efficient ion diffusion, both essential preconditions for supercapacitor applications. We show that PDI-polydiacetylene has an important role in enhancing the electrochemical properties as a supercapacitor electrode. Besides stabilizing the microporous electrode organization, the delocalized π electrons in both the PDI residues and conjugated network of the polydiacetylene contributed to a significantly higher capacitance (specific capacitance >600 F g^-1 at 1 A g^-1 current density), longer discharge time, and high power density. The PDI-polydiacetylene-rGO electrodes were employed in a functional supercapacitor device.

Phase Transition Triggers Explosion-like Puffing Process to Make Popcorn-Inspired All-Conductive Anodes for Superb Aqueous Rechargeable Batteries

The major accomplishment of electrochemical energy-storage devices is closely linked to the advent of state-of-the-art techniques to make optimal electrode systems. Herein, we demonstrate a unique popcorn-inspired strategy to develop all-conductive and highly puffed Fe⊂carbon nanopopcorns as superb anodes for rechargeable Ni/Fe batteries. Temperature-dependent systematic studies show that the nanopopcorn evolution mechanism is governed by typical phase variation from Fe₂O₃ nanospheres to dispersed Fe⁰ nanodebris, whose formation induces catalytic reconstruction/conversion from hydrocarbons to graphitic nanolayers while triggering the explosion-like instant puffing process beyond 700 °C. The as-built Fe⊂carbon hybrids with favorable loosened structures, open-up/enlarged surface areas, and intrinsically conducting nature enable great electrochemical reactivity and cyclic stability (reversible capacity higher than ∼420 mA h g^-1 in all cycles without obvious capacity decay), as well as outstanding rate behaviors (∼300 mA h g^-1 is still retained at ∼20 A g^-1). Full-cell devices of NiO@carbon (+)//Fe⊂carbon (-) can exhibit Max. energy/power densities of up to ∼140.8 W h kg^-1 and ∼15.6 kW kg^-1, respectively. This work sheds a fundamental light on arts to configure puffed electrodes for advanced electrodes in various important applications while holding great promise for high-rate/capacity aqueous rechargeable batteries.

Fabrication of TiO2-Nanotube-Array-Based Supercapacitors

In this work, a simple and cost-effective electrochemical anodization technique was adopted to rapidly grow TiO₂ nanotube arrays on a Ti current collector and to utilize the synthesized materials as potential electrodes for supercapacitors. To accelerate the growth of the TiO₂ nanotube arrays, lactic acid was used as an electrolyte additive. The as-prepared TiO₂ nanotube arrays with a high aspect ratio were strongly adhered to the Ti substrate. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results confirmed that the TiO₂ nanotube arrays were crystallized in the anatase phase. TEM images confirmed the nanotublar-like morphology of the TiO₂ nanotubes, which had a tube length and a diameter of ~16 and ~80 nm, respectively. The electrochemical performance of the TiO₂ nanotube array electrodes was evaluated using the cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) measurements. Excellent electrochemical response was observed for the electrodes based on the TiO₂ nanotube arrays, as the cells delivered a high specific capacitance of 5.12 mF/cm² at a scan rate of 100 mV/s and a current density of 100 µA/cm². The initial capacity was maintained for more than 250 cycles. Further, a remarkable rate capability response was observed, as the cell retained 88% of the initial areal capacitance when the scan rate was increased from 10 to 500 mV/s. The results suggest the suitability of TiO₂ nanotube arrays as electrode materials for commercial supercapacitor applications.

Redox-Active Gel Electrolyte Combined with Branched Polyaniline Nanofibers Doped with Ferrous Ions for Ultra-High-Performance Flexible Supercapacitors

In this work, the effects of utilizing an Fe²⁺/Fe³⁺ redox-active electrolyte and Fe²⁺-doped polyaniline (PANI) electrode material on the performance of an assembled supercapacitor (SC) were studied. The concentration of the redox couple additive in the electrolyte of the SC was optimized to be 0.5 M. With the optimized concentration of 0.4 M Fe²⁺, the doped PANI branched nanofibers electropolymerized onto titanium mesh were much thinner, cleaner, and more branched than normal PANI. A specific capacitance (C_s) of 8468 F g⁻¹ for the 0.4 M Fe²⁺/PANI electrode in the 1 M H₂SO₄ + 0.5 M Fe²⁺/Fe³⁺ gel electrolyte and an energy density of 218.1 Wh kg⁻¹ at a power density of 1854.4 W kg⁻¹ for the resultant SC were achieved, which were much higher than those of the conventional PANI electrode tested in a normal H₂SO₄ electrolyte (404 F g⁻¹ and 24.9 Wh kg⁻¹). These results are among the highest reported for PANI-based SCs in the literature so far and demonstrate the potential effectiveness of this strategy to improve the electrochemical performance of flexible SCs by modifying both the electrode and electrolyte.

Polymer nanocomposites having a high filler content: synthesis, structures, properties, and applications

The recent development of nanoscale fillers, such as carbon nanotubes, graphene, and nanocellulose, allows the functionality of polymer nanocomposites to be controlled and enhanced. However, conventional synthesis methods of polymer nanocomposites cannot maximise the reinforcement of these nanofillers at high filler content. Approaches for the synthesis of high content filler polymer nanocomposites are suggested to facilitate future applications. The fabrication methods address the design of the polymer nanocomposite architecture, which encompasses one, two, and three dimensional morphologies. Factors that hamper the reinforcement of nanostructures, such as alignment, dispersion of the filler and interfacial bonding between the filler and polymer, are outlined. Using suitable approaches, maximum potential reinforcement of nanoscale fillers can be anticipated without limitations in orientation, dispersion, and the integrity of the filler particle-matrix interface. High filler content polymer composites containing emerging materials such as 2D transition metal carbides, nitrides, and carbonitrides (MXenes) are expected in the future.

Nanoforest: Polyaniline Nanotubes Modified with Carbon Nano-Onions as a Nanocomposite Material for Easy-to-Miniaturize High-Performance Solid-State Supercapacitors

This article describes a facile low-cost synthesis of polyaniline nanotube (PANI_NT)–carbon nano-onion (CNO) composites for solid-state supercapacitors. Scanning electron microscopic (SEM) analyses indicate a uniform and ordered composition for the conducting polymer nanotubes immobilized on a thin gold film. The obtained nanocomposites exhibit a brush-like architecture with a specific capacitance of 946 F g⁻¹ at a scan rate of 1 mV s⁻¹. In addition, the nanocomposites offer high conductivity and a porous and well-developed surface area. The PANI_NT–CNO nanocomposites were tested as electrodes with high potential and long-term stability for use in easy-to-miniaturize high-performance supercapacitor devices.

Patterning Graphene Surfaces with Iron-Oxide-Embedded Mesoporous Polypyrrole and Derived N-Doped Carbon of Tunable Pore Size

This study develops a novel strategy, based on block copolymer self-assembly in solution, for preparing two-dimensional (2D) graphene-based mesoporous nanohybrids with well-defined large pores of tunable sizes, by employing polystyrene-block-poly(ethylene oxide) (PS-b-PEO) spherical micelles as the pore-creating template. The resultant 2D nanohybrids possess a sandwich-like structure with Fe₂ O₃ nanoparticle-embedded mesoporous polypyrrole (PPy) monolayers grown on both sides of reduced graphene oxide (rGO) nanosheets (denoted as mPPy-Fe₂ O₃ @rGO). Serving as supercapacitor electrode materials, the 2D ternary nanohybrids exhibit controllable capacitive performance depending on the pore size, with high capacitance (up to 1006 F/g at 1 A/g), good rate performance (750 F/g at 20 A/g) and excellent cycling stability. Furthermore, the pyrolysis of mPPy-Fe₂ O₃ @rGO at 800 °C yields 2D sandwich-like mesoporous nitrogen-doped carbon/Fe₃ O₄ /rGO (mNC-Fe₃ O₄ @rGO). The mNC-Fe₃ O₄ @rGO nanohybrids with a mean pore size of 12 nm show excellent electrocatalytic activity as an oxygen reduction reaction (ORR) catalyst with a four-electron transfer nature, a high half-wave-potential of +0.84 V and a limiting current density of 5.7 mA/cm² , which are well comparable with those of the best commercial Pt/C catalyst. This study takes advantage of block copolymer self-assembly for the synthesis of 2D multifunctional mesoporous nanohybrids, and helps to understand the control of their structures and electrochemical performance.

Synthesis, design and sensing applications of nanostructured ceria-based materials

Cerium-based materials possess redox properties due to the presence of dual valence states of Ce³⁺ and Ce⁴⁺.

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.

Enhanced Pseudocapacitive Performance of α-MnO2 by Cation Preinsertion

Although the theoretical capacitance of MnO₂ is 1370 F g^-1 based on the Mn³⁺/Mn⁴⁺ redox couple, most of the reported capacitances in literature are far below the theoretical value even when the material goes to nanoscale. To understand this discrepancy, in this work, the electrochemical behavior and charge storage mechanism of K⁺-inserted α-MnO₂ (or K_xMnO₂) nanorod arrays in broad potential windows are investigated. It is found that electrochemical behavior of K_xMnO₂ is highly dependent on the potential window. During cyclic voltammetry cycling in a broad potential window, K⁺ ions can be replaced by Na⁺ ions, which determines the pseudocapacitance of the electrode. The K⁺ or Na⁺ ions cannot be fully extracted when the upper cutoff potential is less than 1 V vs Ag/AgCl, which retards the release of full capacitance. As the cyclic voltammetry potential window is extended to 0-1.2 V, enhanced specific capacitance can be obtained with the emerging of new redox peaks. In contrast, the K⁺-free α-MnO₂ nanorod arrays show no redox peaks in the same potential window together with much lower specific capacitance. This work provides new insights on understanding the charge storage mechanism of MnO₂ and new strategy to further improve the specific capacitance of MnO₂-based electrodes.

Recent advances in graphene-based hybrid nanostructures for electrochemical energy storage

In recent years, graphene has emerged as a promising candidate for electrochemical energy storage applications due to its large specific surface area, high electrical conductivity, good chemical stability, and strong mechanical flexibility. Moreover, its unique two-dimensional (2D) nanostructure can be used as an ideal building block for controllable functionalization with other active components and the resulting graphene-based hybrids exhibit desirable properties for improved energy storage capability. This review summarizes the most recent progress on graphene and graphene-based hybrid nanostructures for three frontier electrochemical energy storage device applications, i.e., lithium-ion batteries, lithium-sulfur batteries and supercapacitors. Finally, we outline the future perspectives and trends in this research field including several challenges and opportunities.

Spinous α-Fe2O3 hierarchical structures anchored on Ni foam for supercapacitor electrodes and visible light driven photocatalysts

Spinous α-Fe₂O₃ hierarchical structures grown on a Ni foam substrate have been successfully obtained.

Polycrystalline iron oxide nanoparticles prepared by C-dot-mediated aggregation and reduction for supercapacitor application

A facile strategy to directly prepare three-dimensional, multicomponent, multiphase oxide by solvothermal method and C-dots-mediated aggregation and reduction is demonstrated.

ZnO–graphene–polyaniline nanoflowers: solution synthesis, formation mechanism and electrochemical activity

Morphology dependent electrochemical activity of conducting ZnO–graphene–polyaniline nanocomposite.

Preparation of binder-free CuO/Cu2O/Cu composites: a novel electrode material for supercapacitor applications

The results of XRD and XPS demonstrate that CuO/Cu₂O/Cu is prepared successfully via a facile, eco-friendly, one-step template-free growth process. SEM figures show that cubic CuO/Cu₂O/Cu uniformly and densely covers a skeleton of nickel foam.

Synergistic Effect between Ultra-Small Nickel Hydroxide Nanoparticles and Reduced Graphene Oxide sheets for the Application in High-Performance Asymmetric Supercapacitor

Nanoscale electrode materials including metal oxide nanoparticles and two-dimensional graphene have been employed for designing supercapacitors. However, inevitable agglomeration of nanoparticles and layers stacking of graphene largely hamper their practical applications. Here we demonstrate an efficient co-ordination and synergistic effect between ultra-small Ni(OH)₂ nanoparticles and reduced graphene oxide (RGO) sheets for synthesizing ideal electrode materials. On one hand, to make the ultra-small Ni(OH)₂ nanoparticles work at full capacity as an ideal pseudocapacitive material, RGO sheets are employed as an suitable substrate to anchor these nanoparticles against agglomeration. As a consequence, an ultrahigh specific capacitance of 1717 F g⁻¹ at 0.5 A g⁻¹ is achieved. On the other hand, to further facilitate ion transfer within RGO sheets as an ideal electrical double layer capacitor material, the ultra-small Ni(OH)₂ nanoparticles are introduced among RGO sheets as the recyclable sacrificial spacer to prevent the stacking. The resulting RGO sheets exhibit superior rate capability with a high capacitance of 182 F g⁻¹ at 100 A g⁻¹. On this basis, an asymmetric supercapacitor is assembled using the two materials, delivering a superior energy density of 75 Wh kg⁻¹ and an ultrahigh power density of 40 000 W kg⁻¹.

Mesoporous ZnS-NiS Nanocomposites for Nonenzymatic Electrochemical Glucose Sensors

Mesoporous ZnS-NiS composites are prepared via ion- exchange reactions using ZnS as the precursor. The prepared mesoporous ZnS-NiS composite materials have large surface areas (137.9 m(2) g(-1)) compared with the ZnS precursor. More importantly, the application of these mesoporous ZnS-NiS composites as nonenzymatic glucose sensors was successfully explored. Electrochemical sensors based on mesoporous ZnS-NiS composites exhibit a high selectivity and a low detection limit (0.125 μm) toward the oxidation of glucose, which can mainly be attributed to the morphological characteristics of the mesoporous structure with high specific surface area and a rational composition of the two constituents. In addition, the mesoporous ZnS-NiS composites coated on the surface of electrodes can be used to modify the mass transport regime, and this alteration can, in favorable circumstances, facilitate the amperometric discrimination between species. These results suggest that such mesoporous ZnS-NiS composites are promising materials for nonenzymatic glucose sensors.

One-step synthesis of copper compounds on copper foil and their supercapacitive performance

Nanowire-like Cu(OH)₂ arrays, microflower-like CuO standing on Cu(OH)₂ nanowires and hierarchical CuO microflowers are directly synthesized via a simple and cost-effective liquid–solid reaction.

Template-free synthesis of hierarchically porous NaCoPO4–Co3O4 hollow microspheres and their application as electrocatalysts for glucose

Hierarchically porous NaCoPO₄–Co₃O₄ hollow microspheres are synthesized via annealing the hollow microsphere precursor, which was prepared via a template-free method.