γ-Fe₂O₃ Nanocrystalline Microspheres with Hybrid Behavior of Battery-Supercapacitor for Superior Lithium Storage

Improved Electrochemical Performance of Aqueous Hybrid Supercapacitors Using CrCo2O4 Mesoporous Nanowires: An Innovative Strategy toward Sustainable Energy Devices

High-rate aqueous hybrid supercapacitors (AHSCs) have attracted relevant scientific significance owing to their expected energy density, supercapacitor-level power density, and battery-level energy density. In this work, a bimetallic nanostructured material with chromium-incorporated cobalt oxide (CCO, i.e., CoCr2O4) was prepared via a hydrothermal method to form a stable cubic obelisk structure. Compared with CCO materials prepared using traditional methods, CCO displayed a nanowire structure (50 nm diameter), suggesting an enhanced specific surface area and a large number of active sites for chemical reactions. The electrode possessed a high specific capacitance (2951 F g-1) at a current density of 1 A g-1, minimum Rct (0.135 Ω), and the highest capacitance retention (98.7%), making it an ideal electrode material for AHSCs. Ex situ analysis based on X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) showed a favorable stability of CCO after 10,000 cycles without any phase changes being detected. GGA and GGA + U methods employed in density functional theory (DFT) also highlighted the enhanced metallic properties of CCO originating from the synergistic effect of semiconducting Cr2O3 and Co3O4 materials.

A Study on the Effect of Graphene in Enhancing the Electrochemical Properties of SnO2-Fe2O3 Anode Materials

To enhance the conductivity and volume expansion during the charging and discharging of transition metal oxide anode materials, rGO-SnO₂-Fe₂O₃ composite materials with different contents of rGO were prepared by the in situ hydrothermal synthesis method. The SEM morphology revealed a sphere-like fluffy structure, particles of the 0.4%rGO-10%SnO₂-Fe₂O₃ composite were smaller and more compact with a specific surface area of 223.19 m²/g, the first discharge capacity of 1423.75 mAh/g, and the specific capacity could be maintained at 687.60 mAh/g even after 100 cycles. It exhibited a good ratio performance and electrochemical reversibility, smaller charge transfer resistance, and contact resistance, which aided in lithium-ion transport. Its superior electrochemical performance was due to the addition of graphene, which made the spherical particle size distribution more uniform, effectively lowering the volume expansion during the process of charging and discharging and improving the electrochemical cycle stability of the anode materials.

Reduced graphene oxide (rGO) supported and p yrolytic carbon (PC) coated γ-Fe2O3/PC-rGO composite anode material with enhanced Li-storage performance

As a high-capacity anode material for lithium ion batteries, γ-Fe 2 O 3 is a promising alternative to conventional graphite among multifarious transition metal oxides owing to its high theoretical specific capacity (1007 mAh g -1 ), abundant reserves, good safety and low cost. However, improving the electrical conductivity and overcoming the morphological damage caused by the severe volume expansion during cycling are still the tricky problems to be solved. Herein, a three-dimensional heterostructure composite (γ-Fe 2 O 3 /PC-rGO 60 ) was prepared by a facile solvothermal reaction followed by heat treatment in inert atmosphere. This composite material exhibits a reversible charge specific capacity of 1035 mAh g -1 at the current density of 0.1 A g -1 . After 100 cycles at 0.2 A g -1 , the capacity is increased from 966.2 to 1091.1 mAhg -1 . Even cycled for 200 cycles at 1 A g -1 , the capacity is only decreased from 751.4 to 670.6 mAh g -1 , giving capacity retention of 89.3%. The rGO network supported flexible composite architecture is beneficial for accommodating the volume expansion of the γ-Fe 2 O 3 active material during the lithiation/delithiation process. Besides, the conductive rGO network and the in-situ formed pyrolytic carbon (PC) can provide a smooth electron transmission path and a favorable lithium ion transport channel.

Doping-free bandgap tunability in Fe2O3 nanostructured films

A tunable bandgap without doping is highly desirable for applications in optoelectronic devices. Herein, we develop a new method which can tune the bandgap without any doping. In the present research, the bandgap of Fe2O3 nanostructured films is simply tuned by changing the synthesis temperature. The Fe2O3 nanostructured films are synthesized on ITO/glass substrates at temperatures of 1100, 1150, 1200, and 1250 °C using the hot filament metal oxide vapor deposition (HFMOVD) and thermal oxidation techniques. The Fe2O3 nanostructured films contain two mixtures of Fe2+ and Fe3+ cations and two trigonal (α) and cubic (γ) phases. The increase of the Fe2+ cations and cubic (γ) phase with the elevated synthesis temperatures lifted the valence band edge, indicating a reduction in the bandgap. The linear bandgap reduction of 0.55 eV without any doping makes the Fe2O3 nanostructured films promising materials for applications in bandgap engineering, optoelectronic devices, and energy storage devices.

Benzoate anions-intercalated cobalt-nickel layered hydroxide nanobelts as high-performance electrode materials for aqueous hybrid supercapacitors

Layered metal hydroxide salts (LHSs) have recently gained extensive interests as an efficient electrode material for supercapacitors (SCs). Herein, we report, for the first time ever, the synthesis of a cobalt-nickel layered hybrid organic-inorganic LHS that was intercalated with benzoate anions (B-CoNi-LHSs) and observe a high performance as electrode materials for hybrid supercapacitors (HSCs). B-CoNi-LHSs were synthesized by using a co-precipitation method, where sodium benzoate was added dropwise to cobalt and nickel salt solution, without the addition of any organic solvent or surfactant. Due to the intercalation of anions and synergistic interactions of the multi-metallic components, the B-CoNi-LHSs electrode showed a high specific capacity of 570 C g^-1 (specific capacitance of 1267 F·g^-1) at 1 A g^-1, excellent rate performance (65% from 1 to 10 A g^-1) and outstanding cycling performance (81.09% over 8000 cycles), in comparison to the mono-metallic counterparts. An HSC device, assembled by using B-CoNi-LHSs as the positive electrode and activated carbon (AC) as the negative one, exhibited a power density of 780 W kg^-1 at the energy density of 31.7 Wh kg^-1, and 8543 W kg^-1 at 18.1 Wh kg^-1. Results from this study show that the organic-inorganic hybrids of layered dual-metal hydroxides intercalated with benzoate anions may be a viable candidate as electrode materials for high-performance SCs.

Facile and scalable synthesis of α-Fe2O3/γ-Fe2O3/Fe/C nanocomposite as advanced anode materials for lithium/sodium ion batteries

To develop low-cost advanced anode materials for lithium/sodium ion batteries, the chemical reaction equilibrium of Fe(NO₃)₃ and glucose in hot aqueous solution is creatively used to fabricate a new α-Fe₂O₃/γ-Fe₂O₃/Fe/C nanocomposite with the primary particle sizes concentrated at 25-80 nm. As anodes for lithium ion batteries, it exhibits a discharge capacity of ∼878 mAh g^-1 after 200 cycles at a current density of 200 mA g^-1. Moreover, even after 1000 cycles at a current density of 3200 mA g^-1, the discharge capacity is as high as ∼532 mAh g^-1, with a capacity retention of over than 100% against that of the second cycle. As anodes for sodium ion batteries, the nanocomposite displays a stable discharge capacity of ∼400 mAh g^-1 at a current density of 100 mA g^-1 and no obvious capacity degradation happens after 200 cycles. During cycling, the α-Fe₂O₃/γ-Fe₂O₃/Fe/C nanocomposite electrodes shows high structural stability and relatively faster reaction kinetics, which should be responsible for its excellent electrochemical performance. This work provides a facile and scalable route to synthesize high-performance and low-cost Fe₂O₃-based nanocomposite for the secondary batteries.

High-performance mesoporous γ-Fe2O3 sphere/graphene aerogel composites towards enhanced lithium storage

Transition metal oxides have recently been demonstrated as highly attractive anodes for high-capacity lithium ion batteries, whose electrochemical properties could be further improved through rational architecture design and incorporating reliable conductive network. Herein, mesoporous γ-Fe₂O₃ spheres/graphene aerogel composites were synthesized via a solvothermal pathway followed by suitable annealing. Experimental results reveal the uniform mesoporous structure and well-dispersed γ-Fe₂O₃ spheres with the size of 300-400 nm embedded in the mesopores of the graphene aerogel network. Compared with α-Fe₂O₃/graphene aerogel and pure γ-Fe₂O₃, the as-synthesized composite delivers, at the first cycle, a high discharging capacity of 1080 mAh g^-1 at current density of 200 mA g^-1. Even at much higher current density of 8000 mA g^-1, satisfactory discharging capacities of 421.5 mAh g^-1 can still be achieved. Upon 100 charging-discharging cycles, the specific capacity of as high as 890.5 mAh g^-1 at 200 mA g^-1 is maintained. The enhanced electrochemical properties could be attributed to their favorable three-dimensional graphene aerogel network, which accounts for the improved structural stability and electronic conductivity of γ-Fe₂O₃ during the lithiation/delithiation process.

Controlled Synthesis of Magnetic Iron Oxide Nanoparticles: Magnetite or Maghemite?

Today, magnetic nanoparticles are present in multiple medical and industrial applications. We take a closer look at the synthesis of magnetic iron oxide nanoparticles through the co-precipitation of iron salts in an alkaline environment. The variation of the synthesis parameters (ion concentration, temperature, stirring rate, reaction time and dosing rate) change the structure and diameter of the nanoparticles. Magnetic iron oxide nanoparticles are characterized by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). Magnetic nanoparticles ranging from 5 to 16 nm in diameter were synthesized and their chemical structure was identified. Due to the evaluation of Raman spectra, TEM and XRD, the magnetite and maghemite nanoparticles can be observed and the proportion of phases and the particle size can be related to the synthesis conditions. We want to highlight the use of Raman active modes A1g of spinel structured iron oxides to determine the content of magnetite and maghemite in our samples. Magnetite nanoparticles can be derived from highly alkaline conditions even without establishing an inert atmosphere during the synthesis. The correlation between the particle properties and the various parameters of the synthesis was modelled with linear mixture models. The two models can predict the particle size and the oxidation state of the synthesized nanoparticles, respectively. The modeling of synthesis parameters not only helps to improve synthesis conditions for iron oxide nanoparticles but to understand crystallization of nanomaterials.

Electrode Materials, Electrolytes, and Challenges in Nonaqueous Lithium-Ion Capacitors

Among the various energy-storage systems, lithium-ion capacitors (LICs) are receiving intensive attention due to their high energy density, high power density, long lifetime, and good stability. As a hybrid of lithium-ion batteries and supercapacitors, LICs are composed of a battery-type electrode and a capacitor-type electrode and can potentially combine the advantages of the high energy density of batteries and the large power density of capacitors. Here, the working principle of LICs is discussed, and the recent advances in LIC electrode materials, particularly activated carbon and lithium titanate, as well as in electrolyte development are reviewed. The charge-storage mechanisms for intercalative pseudocapacitive behavior, battery behavior, and conventional pseudocapacitive behavior are classified and compared. Finally, the prospects and challenges associated with LICs are discussed. The overall aim is to provide deep insights into the LIC field for continuing research and development of second-generation energy-storage technologies.

Etching-Assisted Crumpled Graphene Wrapped Spiky Iron Oxide Particles for High-Performance Li-Ion Hybrid Supercapacitor

From graphene oxide wrapped iron oxide particles with etching/reduction process, high-performance anode and cathode materials of lithium-ion hybrid supercapacitors are obtained in the same process with different etching conditions, which consist of partially etched crumpled graphene (CG) wrapped spiky iron oxide particles (CG@SF) for a battery-type anode, and fully etched CG for a capacitive-type cathode. The CG is formed along the shape of spikily etched particles, resulting in high specific surface area and electrical conductivity, thus the CG-based cathode exhibits remarkable capacitive performance of 210 F g^-1 and excellent rate capabilities. The CG@SF can also be ideal anode materials owing to spiky and porous morphology of the particles and tightly attached crumpled graphene onto the spiky particles, which provides structural stability and low contact resistance during repetitive lithiation/delithiation processes. The CG@SF anode shows a particularly high capacitive performance of 1420 mAh g^-1 after 270 cycles, continuously increases capacity beyond the 270th cycle, and also maintains a high capacity of 170 mAh g^-1 at extremely high speeds of 100 C. The full-cell exhibits a higher energy density up to 121 Wh kg^-1 and maintains high energy density of 60.1 Wh kg^-1 at 18.0 kW kg^-1 . This system could thus be a practical energy storage system to fill the gap between batteries and supercapacitors.

Stable Lithium Storage in Nitrogen-Doped Carbon-Coated Ferric Oxide Yolk-Shell Nanospindles with Preserved Hollow Space

Iron oxide (Fe₂ O₃ ) is a promising anode material for next-generation high-energy lithium-ion batteries owing to its high theoretical specific capacity, but it suffers from unstable electrochemistry, as represented by a significant volume variation upon (de)lithiation and unstable solid-electrolyte interface. To target these issues, a double-coating synthetic route has been developed to prepare a yolk-shell-structured γ-Fe₂ O₃ /nitrogen-doped carbon composite, in which spindle-like γ-Fe₂ O₃ cores are encapsulated in the highly conductive carbon shell. Through precisely controlling the void space between the γ-Fe₂ O₃ core and the carbon shell, volume variation in γ-Fe₂ O₃ during (de)lithiation is well accommodated, while the composite maintains an intact and relatively dense structure, which stabilizes the solid-electrolyte interface and is beneficial for improving the practical energy density of the material. With a stabilized (de)lithiation electrochemistry and a synergistic storage effect between the two active components, the composite enables excellent lithium storage performance, in terms of reversible capacity, cycling ability, and rate capability.

Fast Diffusion of O2 on Nitrogen-Doped Graphene to Enhance Oxygen Reduction and Its Application for High-Rate Zn-Air Batteries

N-doped graphene (NDG) was investigated for oxygen reduction reaction (ORR) and used as air-electrode catalyst for Zn-air batteries. Electrochemical results revealed a slightly lower kinetic activity but a much larger rate capability for the NDG than commercial 20% Pt/C catalyst. The maximum power density for a Zn-air cell with NDG air cathode reached up to 218 mW cm^-2, which is nearly 1.5 times that of its counterpart with the Pt/C (155 mW cm^-2). The equivalent diffusion coefficient (D_E) of oxygen from electrolyte solution to the reactive sites of NDG was evaluated as about 1.5 times the liquid-phase diffusion coefficient (D_L) of oxygen within bulk electrolyte solution. Combined with experiments and ab initio calculations, this seems counterintuitive reverse ORR of NDG versus Pt/C can be rationalized by a spontaneous adsorption and fast solid-state diffusion of O₂ on ultralarge graphene surface of NDG to enhance effective ORR on N-doped-catalytic-centers and to achieve high-rate performance for Zn-air batteries.

Cascading Boost Effect on the Capacity of Nitrogen-Doped Graphene Sheets for Li- and Na-Ion Batteries

Specific capacity and cyclic performance are critically important for the electrode materials of rechargeable batteries. Herein, a capacity boost effect of Li- and Na-ion batteries was presented and clarified by nitrogen-doped graphene sheets. The reversible capacities progressively increased from 637.4 to 1050.4 mAh g^-1 (164.8% increase) in Li-ion cell tests from 20 to 185 cycles, and from 187.3 to 247.5 mAh g^-1 (132.1% increase) in Na-ion cell tests from 50 to 500 cycles. The mechanism of the capacity boost is proposed as an electrochemical induced cascading evolution of graphitic N to pyridinic and/or pyrrolic N, during which only these graphitic N adjacent pyridinic or pyrrolic structures can be taken precedence. The original and new generated pyridinic and pyrrolic N have strengthened binding energies to Li/Na atoms, thus increased the Li/Na coverage and delivered a progressive capacity boost with cycles until the entire favorable graphitic N transform into pyridinic and pyrrolic N.

Enabling a High Performance of Mesoporous α-Fe2O3 Anodes by Building a Conformal Coating of Cyclized-PAN Network

The mesoporous α-Fe2O3/cyclized-polyacrylonitrile (C-PAN) composite was synthesized by a rapid and facile two-step method. The electrode was fabricated without conductive carbon addictive and employed as anode for lithium-ion batteries. Results demonstrate that building a conformal coating of a C-PAN network can provide a strong adhesion with active materials and contribute excellent electronic conductivity to the electrode, which can relieve the huge volume changes during a lithiation/delithiation process and accelerate the charge transfer rate. The material exhibited high reversible capacity of ca. 996 mAh g(-1) after 100 cycles at 0.2C, 773 mAh g(-1) at 1C and 655 mAh g(-1) at 2C, respectively, showing well-enhanced cycling performance and superior rate capacity, and also exhibiting significantly improved power density and energy density compared to the traditional graphite materials. Our results provide a facile and efficient way to enhance the performance of α-Fe2O3 anode material, which also can be applied for other oxide anode materials.

Boric Acid Assisted Reduction of Graphene Oxide: A Promising Material for Sodium-Ion Batteries

Reduced graphene oxide, an intensively investigated material for Li-ion batteries, has shown mostly unsatisfactory performance in Na-ion batteries, since its d-spacing is believed to be too small for effective insertion/deinsertion of Na(+) ions. Herein, a facile method was developed to produce boron-functionalized reduced graphene oxide (BF-rGO), with an enlarged interlayer spacing and defect-rich structure, which effectively accommodates the sodiation/desodiation and provides more active sites. The Na/BF-rGO half cells exhibit unprecedented long cycling stability, with ∼89.4% capacity retained after 5000 cycles (0.002% capacity decay per cycle) at 1000 mA·g(-1) current density. High specific capacity (280 mAh·g(-1)) and great rate capability were also delivered in the Na/BF-rGO half cells.

Etching synthesis of iron oxide nanoparticles for adsorption of arsenic from water

Nano-iron oxide prepared by an etching method is good adsorbent for arsenic removal from water.

Mesoporous and carbon hybrid structures from layered molecular precursors for Li-ion battery application: the case of β-In2S3

Mesoporous and carbon hybrid structures were constructed from layered molecular precursors as high performance anode materials for the Li-ion battery application.