Nanostructured ferrite/graphene/polyaniline using for supercapacitor to enhance the capacitive behavior

Graphene nanosheets, polyaniline (PANI), and nanocrystallites of transition metal ferrite {Fe₃O₄ (Mag), NiFe₂O₄ (NiF), and CoFe₂O₄ (CoF)} have been prepared and characterized via XRD, FTIR, SEM, TEM, UV–vis spectroscopy, cyclic voltammetry, galvanostatic charge discharges, and impedance spectroscopy. Electrochemical measurements showed that supercapacitances of hybrid electrodes made of the ternary materials are higher than that of hybrid electrode made of binary or single material. The ternary hybrid CoF/graphene (G)/PANI electrode exhibits a highest specific capacitance reaching 1123 Fg^?1, an energy density of 240 Wh kg^?1 at 1 A g^?1, and a power density of 2680 Wkg^?1 at 1 A g^?1 and outstanding cycling performance, with 98.2% capacitance retained over 2000 cycles. The extraordinary electrochemical performance of the ternary CoF/G/PANI hybrid can be attributed to the synergistic effects of the individual components. The PANI conducting polymer enhances an electron transport. The Ferrite nanoparticles prevent the restocking of the carbon sheets and provide Faradaic processes to increase the total capacitance.     

Nanostructured Fe2O3–graphene composite as a novel electrode material for supercapacitors

Nanostructured Fe₂O₃–graphene composite was successfully fabricated through a facile solution-based route under mild hydrothermal conditions. Well-crystalline Fe₂O₃ nanoparticles with 30–60?nm in size are highly encapsulated in graphene nanosheet matrix, as demonstrated by various characterization techniques. As electrode materials for supercapacitors, the as-obtained Fe₂O₃–graphene nanocomposite exhibits large specific capacitance (151.8?F?g^?1 at 1?A?g^?1), good rate capability (120?F?g^?1 at 6?A?g^?1), and excellent cyclability. The significantly enhanced electrochemical performance compared with pure graphene and Fe₂O₃ nanoparticles may be attributed to the positive synergetic effect between Fe₂O₃ and graphene. In virtue of their superior electrochemical performance, they will be promising electrode materials for high-performance supercapacitors applications.     

Reduced graphene oxide-CoFe2O4 composites for supercapacitor electrode

Reduced graphene oxide-CoFe₂O₄ (RGO-CoFe₂O₄) composite for supercapacitor electrodes has been investigated. On the basis of the morphological and structural characterization of the sample by scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectrometer, the electrochemical performances of the sample were investigated by cyclic voltammetry and chronopotentiometry in 0.50 mol L^?1 Na₂SO₄ aqueous electrolyte. It was shown that the specific capacitance (123.2 F g^?1) of RGO-CoFe₂O₄ composite was highly improved compared with that of RGO (89.9 F g^?1), GO-CoFe₂O₄ (21.1 F g^?1) and CoFe₂O₄ (18.7 F g^?1) at the current density of 5 mA cm^?2. The capacitance retention of about 78.1% for RGO-CoFe₂O₄ after 1000 cycles indicated that it has high cycle stability.     

A facile nitrogen-doped carbon encapsulation of CoFe2O4 nanocrystalline for enhanced performance of lithium ion battery anodes

Nitrogen-doped (N-doped) carbon encapsulation of CoFe₂O₄ nanocrystalline is achieved by a simple pressure-assisted pyrrole pyrolysis method. The CoFe₂O₄/N-doped carbon nanocomposite (CFO/NC) delivers a capacity of 646.2 mAh g^–1 after 80 cycles at 0.1 C, exhibits stable cycling performance at various rates from 0.2 to 1.6 C and retains a capacity of 662.8 mAh g^–1 as the rate returns back to 0.1 C, showing significantly improved lithium storage reversibility compared to the bare CFO. A different lithiation mechanism of CFO/NC above and below the plateau relative to CFO in the first discharge is analyzed in detail based on the potential profiles and cyclic voltammogram curves. Morphology characterization of the cycled electrodes confirms much better integrity of CFO/NC electrode due to the buffer effect of N-doped carbon coating. Electronic conductivity and electrochemical impedance spectroscopy measurements indicate enhanced electrode reaction kinetics of CFO/NC. All the results contribute to its improved electrochemical performance.     

Nanosized Fe3O4-modified activated carbon for supercapacitor electrodes

Nanosized Fe₃O₄-modified activated carbon composites for supercapacitor electrodes have been investigated. Structural and morphological characterizations of activated materials are carried out using X-ray diffraction and scanning electron microscopy, respectively. The electrochemical performances of the composite electrodes are evaluated by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy. The experimental results show that the specific capacitances of the 10 wt % Fe₃O₄-modified activated carbon composite electrode (154.3 F g^?1) is highly improved compared with that of Fe₃O₄ (78.5 F g^?1) and AC (79.2 F g^?1) at the current density of 5 mA cm^?2, respectively. The charge/discharge tests show that it could retain 79.6% of its initial capacitance over 1000 cycles, suggesting its potential application for the fabrication of high-quality supercapacitors.     

Synergistic Ternary Composite (Carbon/Fe3O4@Graphene) with Hollow Microspherical and Robust Structure for Li‐Ion Storage

The electrode materials with hollow structure and/or graphene coating are expected to exhibit outstanding electrochemical performances in energy‐storage systems. 2D graphene‐wrapped hollow C/Fe₃O₄ microspheres are rationally designed and fabricated by a novel facile and scalable strategy. The core@double‐shell structure SPS@FeOOH@GO (SPS: sulfonated polystyrene, GO: graphene oxide) microspheres are first prepared through a simple one‐pot approach and then transformed into C/Fe₃O₄@G (G: graphene) after calcination at 500 °C in Ar. During calcination, the Kirkendall effect resulting from the diffusion/reaction of SPS‐derived carbon and FeOOH leads to the formation of hollow structure carbon with Fe₃O₄ nanoparticles embedded in it. In the rationally constructed architecture of C/Fe₃O₄@G, the strongly coupled C/Fe₃O₄ hollow microspheres are further anchored onto 2D graphene networks, achieving a strong synergetic effect between carbon, Fe₃O₄, and graphene. As an anode material of Li‐ion batteries (LIBs), C/Fe₃O₄@G manifests a high reversible capacity, excellent rate behavior, and outstanding long‐term cycling performance (1208 mAh g^?1 after 200 cycles at 100 mA g^?1).     

Graphitized carbon and graphene modified Fe2O3/Li4Ti5O12 as anode material for lithium ion batteries

Graphitized carbon (GC) and graphene (GE) modified Fe₂O₃/Li₄Ti₅O₁₂ (LTO) composites have been synthesized via a solid‐state reaction, respectively. The structure, morphology and electrochemical performance of the materials have also been characterized with X‐ray diffraction (XRD), scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS) system, X‐ray photoelectron spectrometer (XPS), Fourier transform infrared spectroscopy (FTIR) and electrochemical measurements. The discharge capacities of Fe₂O₃/LTO, GC/Fe₂O₃/LTO and GE/Fe₂O₃/LTO are 100.2 mAh g^?1, 207.5 mAh g^?1 and 238.9 mAh g^?1 after 100 cycles at the current density of 176 mA g^?1. The cyclic stability and rate capability are in the order of GE/Fe₂O₃/LTO > GC/Fe₂O₃/LTO > Fe₂O₃/LTO because of the synergistic effect between GC (GE) and Fe₂O₃/LTO. Copyright © 2016 John Wiley & Sons, Ltd.     

Wintersweet‐Flower‐Like CoFe2O4/MWCNTs Hybrid Material for High‐Capacity Reversible Lithium Storage

CoFe₂O₄/multiwalled carbon nanotubes (MWCNTs) hybrid materials were synthesized by a hydrothermal method. Field emission scanning electron microscopy and transmission electron microscopy analysis confirmed the morphology of the as‐prepared hybrid material resembling wintersweet flower “buds on branches”, in which CoFe₂O₄ nanoclusters, consisting of nanocrystals with a size of 5–10 nm, are anchored along carbon nanotubes. When applied as an anode material in lithium ion batteries, the CoFe₂O₄/MWCNTs hybrid material exhibited a high performance for reversible lithium storage. In particular, the hybrid anode material delivered reversible lithium storage capacities of 809, 765, 539, and 359 mA h g^?1 at current densities of 180, 450, 900, and 1800 mA g^?1, respectively. The superior performance of CoFe₂O₄/MWCNTs hybrid materials could be ascribed to the synergistic pinning effect of the wintersweet‐flower‐like nanoarchitecture. This strategy could also be applied to synthesize other metal oxide/CNTs hybrid materials as high‐capacity anode materials for lithium ion batteries.     

Ammonia-treatment assisted fully encapsulation of Fe2O3 nanoparticles in mesoporous carbons as stable anodes for lithium ion batteries

To improve the initial coulombic efficiency and bulk density of ordered mesoporous carbons, active Fe₂O₃ nanoparticles were introduced into tubular mesopore channels of CMK-5 carbon, which possesses high specific surface area (>1700 m²·g^?1) and large pore volume (>1.8 cm³·g^?1). Fine Fe₂O₃ nanoparticles with sizes in the range of 5–7 nm were highly and homogenously encapsulated into CMK-5 matrix through ammonia-treatment and subsequent pyrolysis method. The Fe₂O₃ loading was carefully tailored and designed to warrant a high Fe₂O₃ content and adequate buffer space for improving the electrochemical performance. In particular, such Fe₂O₃ and mesoporous carbon composite with 47 wt% loading exhibits a considerably stable cycle performance (683 mAh·g^?1 after 100 cycles, 99% capacity retention against that of the second cycle) as well as good rate capability. The fabrication strategy can effectively solve the drawback of single material, and achieve a high-performance lithium electrode material.     

Coal tar residues-based nanostructured activated carbon/Fe3O4 composite electrode materials for supercapacitors

Oxygen-rich activated carbon with a three-dimensional network structure was prepared by chemical activation of coal tar residues with potassium hydroxide and subsequent carbonization treatment. Nanostructured Fe₃O₄/AC composites were then prepared by simple chemical coprecipitation method and were used as active electrode materials for supercapacitors. The electrochemical behaviors of Fe₃O₄/AC nanocomposites were characterized by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy in 1.0 M Na₂SO₃ electrolyte. It was shown that the specific capacitance of Fe₃O₄/AC nanocomposites reached 150 F g^?1 at a current density of 3.0 A g^?1 and was a great improvement over Fe₃O₄ or AC alone. Furthermore, as-prepared Fe₃O₄/AC nanocomposites exhibited long cycle life without obvious capacitance fading even after 1,000 charge/discharge cycles. Compared with pure Fe₃O₄ and AC, the significant enhanced electrochemical performance of Fe₃O₄/AC nanocomposites could be reasonably attributed to the positive synergetic effect between Fe₃O₄ and AC.     

Superior Electrochemical Properties of Nanofibers Composed of Hollow CoFe2O4 Nanospheres Covered with Onion‐Like Graphitic Carbon

Nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion‐like carbon are prepared by applying nanoscale Kirkendall diffusion to the electrospinning process. Amorphous carbon nanofibers embedded with CoFe₂@onion‐like carbon nanospheres are prepared by reduction of the electrospun nanofibers. Oxidation of the CoFe₂‐C nanofibers at 300 °C under a normal atmosphere produces porous nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion‐like carbon. CoFe₂ nanocrystals are transformed into the hollow CoFe₂O₄ nanospheres during oxidation through a well‐known nanoscale Kirkendall diffusion process. The discharge capacities of the carbon‐free CoFe₂O₄ nanofibers composed of hollow nanospheres and the nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion‐like carbon are 340 and 930 mA h g^?1, respectively, for the 1000th cycle at a current density of 1 A g^?1. The nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion‐like carbon exhibit an excellent rate performance even in the absence of conductive materials.     

One‐Pot Synthesis of Pomegranate‐Structured Fe3O4/Carbon Nanospheres‐Doped Graphene Aerogel for High‐Rate Lithium Ion Batteries

A unique hierarchically nanostructured composite of iron oxide/carbon (Fe₃O₄/C) nanospheres‐doped three‐dimensional (3D) graphene aerogel has been fabricated by a one‐pot hydrothermal strategy. In this novel nanostructured composite aerogel, uniform Fe₃O₄ nanocrystals (5–10 nm) are individually embedded in carbon nanospheres (ca. 50 nm) forming a pomegranate‐like structure. The carbon matrix suppresses the aggregation of Fe₃O₄ nanocrystals, avoids direct exposure of the encapsulated Fe₃O₄ to the electrolyte, and buffers the volume expansion. Meanwhile, the interconnected 3D graphene aerogel further serves to reinforce the structure of the Fe₃O₄/C nanospheres and enhances the electrical conductivity of the overall electrode. Therefore, the carbon matrix and the interconnected graphene network entrap the Fe₃O₄ nanocrystals such that their electrochemical function is retained even after fracture. This novel hierarchical aerogel structure delivers a long‐term stability of 634 mA h g^?1 over 1000 cycles at a high current density of 6 A g^?1 (7 C), and an excellent rate capability of 413 mA h g^?1 at 10 A g^?1 (11 C), thus exhibiting great potential as an anode composite structure for durable high‐rate lithium‐ion batteries.     

A Core–Shell Fe/Fe2O3 Nanowire as a High‐Performance Anode Material for Lithium‐Ion Batteries

The preparation of novel one‐dimensional core–shell Fe/Fe₂O₃ nanowires as anodes for high‐performance lithium‐ion batteries (LIBs) is reported. The nanowires are prepared in a facile synthetic process in aqueous solution under ambient conditions with subsequent annealing treatment that could tune the capacity for lithium storage. When this hybrid is used as an anode material for LIBs, the outer Fe₂O₃ shell can act as an electrochemically active material to store and release lithium ions, whereas the highly conductive and inactive Fe core functions as nothing more than an efficient electrical conducting pathway and a remarkable buffer to tolerate volume changes of the electrode materials during the insertion and extraction of lithium ions. The core–shell Fe/Fe₂O₃ nanowire maintains an excellent reversible capacity of over 767 mA h g^?1 at 500 mA g^?1 after 200 cycles with a high average Coulombic efficiency of 98.6 %. Even at 2000 mA g^?1, a stable capacity as high as 538 mA h g^?1 could be obtained. The unique composition and nanostructure of this electrode material contribute to this enhanced electrochemical performance. Due to the ease of large‐scale fabrication and superior electrochemical performance, these hybrid nanowires are promising anode materials for the next generation of high‐performance LIBs.     

Green Template‐Free Synthesis of Hierarchical Shuttle‐Shaped Mesoporous ZnFe2O4 Microrods with Enhanced Lithium Storage for Advanced Li‐Ion Batteries

In the work, a facile and green two‐step synthetic strategy was purposefully developed to efficiently fabricate hierarchical shuttle‐shaped mesoporous ZnFe₂O₄ microrods (MRs) with a high tap density of ～0.85 g cm³, which were assembled by 1D nanofiber (NF) subunits, and further utilized as a long‐life anode for advanced Li‐ion batteries. The significant role of the mixed solvent of glycerin and water in the formation of such hierarchical mesoporous MRs was systematically investigated. After 488 cycles at a large current rate of 1000 mA g^?1, the resulting ZnFe₂O₄ MRs with high loading of ～1.4 mg per electrode still preserved a reversible capacity as large as ～542 mAh g^?1. Furthermore, an initial charge capacity of ～1150 mAh g^?1 is delivered by the ZnFe₂O₄ anode at 100 mA g^?1, resulting in a high Coulombic efficiency of ～76 % for the first cycle. The superior Li‐storage properties of the as‐obtained ZnFe₂O₄ were rationally associated with its mesoprous micro‐/nanostructures and 1D nanoscaled building blocks, which accelerated the electron transportation, facilitated Li⁺ transfer rate, buffered the large volume variations during repeated discharge/charge processes, and provided rich electrode–electrolyte sur‐/interfaces for efficient lithium storage, particularly at high rates.     

The preparation of LaSr<Subscript>3</Subscript>Fe<Subscript>3</Subscript>O<Subscript>10 − <Emphasis Type="Italic">δ</Emphasis></Subscript> and its electrochemical performance

LaSr₃Fe₃O_10 ? δ powders were synthesized by hydrothermal method and characterized by XRD and SEM. The XRD patterns showed that the sample calcined at 1000 °C was single phase and the sample calcined at 900 °C had tiny amount of LaSrFeO₄ phase. The single-phase LaSr₃Fe₃O_10 ? δ powders were used to prepare test electrode. The capacitive behaviors of LaSr₃Fe₃O_10 ? δ electrode were analyzed by cyclic voltammetry, galvanostatic charge-discharge techniques, and electrochemical impedance spectroscopy. The electrochemical results showed a capacity as high as 470 F g^?1 at a scan rate of 1 mV s^?1 and 380 F g^?1 at a charge-discharge current density of 0.1 A g^?1 in 6 M KOH solution. The electrode showed good cyclic stability since its capacitive retention is 87.1% after 1000 charge-discharge cycles. The electrochemical performances suggest that LaSr₃Fe₃O_10 ? δ could be a potential candidate as a capacitive electrode material.     

A Microwave Synthesis of Mesoporous NiCo2O4 Nanosheets as Electrode Materials for Lithium‐Ion Batteries and Supercapacitors

A facile microwave method was employed to synthesize NiCo₂O₄ nanosheets as electrode materials for lithium‐ion batteries and supercapacitors. The structure and morphology of the materials were characterized by X‐ray diffraction, field‐emission scanning electron microscopy, transmission electron microscopy and Brunauer–Emmett–Teller methods. Owing to the porous nanosheet structure, the NiCo₂O₄ electrodes exhibited a high reversible capacity of 891 mA h g^?1 at a current density of 100 mA g^?1, good rate capability and stable cycling performance. When used as electrode materials for supercapacitors, NiCo₂O₄ nanosheets demonstrated a specific capacitance of 400 F g^?1 at a current density of 20 A g^?1 and superior cycling stability over 5000 cycles. The excellent electrochemical performance could be ascribed to the thin porous structure of the nanosheets, which provides a high specific surface area to increase the electrode–electrolyte contact area and facilitate rapid ion transport.     

Synthesis of Co3O4/Carbon Heteroaerogels with Ultrahigh Capacitance via Polyethyleneimine Intercalation of Co2BIM4 Nanosheets

The development of metal–organic frameworks (MOFs)-based supercapacitors have attracted intense concentration in recent years due to their regularly arranged porous and tunable pore sizes. However, the performance of the MOFs-derived supercapacitors is also low because of their poor electrical conductivity and rarely accessible active sites. In the present work, we developed a Co-MOF (namely Co₂BIM₄, BIM=benzimidazole) nanosheets derived Co₃O₄/nitrogen-doped carbon (Co₂BIM₄-Co₃O₄/NC) heteroaerogel as a novel supercapacitor electrode. The 3D Co₂BIM₄-Co₃O₄/NC heteroaerogels were obtained by directly intercalating polyethyleneimine (PEI) into the interlayers of Co₂BIM₄ nanosheets and following by carbonizing the resulting Co₂BIM₄/PEI composite. The Co₂BIM₄-Co₃O₄/NC electrode possessed 3D conductive framework with an overlapped hetero-interface and expanded interlayers, leading to fast and stable charge transfer/diffusion and an enhanced pseudocapacitance performance. Therefore, the Co₂BIM₄-Co₃O₄/NC electrode showed ultrahigh capacitance of 2568 F g⁻¹ at 1 A g⁻¹, 1747 F g⁻¹ at 10 A g⁻¹, and excellent long cycling time with a capacitance preservation of 92.7 % following 10000 cycles at 10 A g⁻¹, which is very promising for applications in supercapacitors and other energy storage devices.     

Pseudocapacitive Lithium Storage of Cauliflower-Like CoFe2O4 for Low-Temperature Battery Operation

Binary transition-metal oxides (BTMOs) with hierarchical micro–nano-structures have attracted great interest as potential anode materials for lithium-ion batteries (LIBs). Herein, we report the fabrication of hierarchical cauliflower-like CoFe₂O₄ (cl-CoFe₂O₄) via a facile room-temperature co-precipitation method followed by post-synthetic annealing. The obtained cauliflower structure is constructed by the assembly of microrods, which themselves are composed of small nanoparticles. Such hierarchical micro–nano-structure can promote fast ion transport and stable electrode–electrolyte interfaces. As a result, the cl-CoFe₂O₄ can deliver a high specific capacity (1019.9 mAh g⁻¹ at 0.1 A g⁻¹), excellent rate capability (626.0 mAh g⁻¹ at 5 A g⁻¹), and good cyclability (675.4 mAh g⁻¹ at 4 A g⁻¹ for over 400 cycles) as an anode material for LIBs. Even at low temperatures of 0 °C and −25 °C, the cl-CoFe₂O₄ anode can deliver high capacities of 907.5 and 664.5 mAh g⁻¹ at 100 mA g⁻¹, respectively, indicating its wide operating temperature. More importantly, the full-cell assembled with a commercial LiFePO₄ cathode exhibits a high rate performance (214.2 mAh g⁻¹ at 5000 mA g⁻¹) and an impressive cycling performance (612.7 mAh g⁻¹ over 140 cycles at 300 mA g⁻¹) in the voltage range of 0.5–3.6 V. Kinetic analysis reveals that the electrochemical performance of cl-CoFe₂O₄ is dominated by pseudocapacitive behavior, leading to fast Li⁺ insertion/extraction and good cycling life.     

Hydrothermal synthesis of reduced graphene sheets/Fe2O3 nanorods composites and their enhanced electrochemical performance for supercapacitors

Reduced graphene nanosheets/Fe₂O₃ nanorods (GNS/Fe₂O₃) composite has been fabricated by a hydrothermal route for supercapacitor electrode materials. The obtained GNS/Fe₂O₃ composite formed a uniform structure with the Fe₂O₃ nanorods grew on the graphene surface and/or filled between the graphene sheets. The electrochemical performances of the GNS/Fe₂O₃ hybrid supercapacitor were tested by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests in 6 M KOH electrolyte. Comparing with the pure Fe₂O₃ electrode, GNS/Fe₂O₃ composite electrode exhibits an enhanced specific capacitance of 320 F g⁻¹ at 10 mA cm⁻² and an excellent cycle-ability with capacity retention of about 97% after 500 cycles. The simple and cost-effective preparation technique of this composite with good capacitive behavior encourages its potential commercial application.     

From nickel oxalate dihydrate microcubes to NiS<Subscript>2</Subscript> nanocubes for high performance supercapacitors

In this study, NiS₂ nanocubes were successfully synthesized by a novel facile solvothermal method using NiC₂O₄·2H₂O microstructures and used as an electrode for high-performance supercapacitors. The electrochemical properties of the prepared NiS₂ electrode were studied using galvanostatic charge–discharge analysis, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) studies. Its maximum specific capacitance was 2077 F g^?1 at a constant current density of about 0.65 A g^?1. Further, the EIS results confirmed the pseudocapacitive nature of the NiS₂ electrode. The experimental results suggested that the NiS₂ electro-active material demonstrates excellent electrochemical performance with high specific capacitance, low resistance, and excellent cycling stability.