Iron oxide-decorated carbon for supercapacitor anodes with ultrahigh energy density and outstanding cycling stability

Pulsed laser deposited CoFe2O4 thin films as supercapacitor electrodes

The influence of the substrate temperature on pulsed laser deposited (PLD) CoFe2O4 thin films for supercapacitor electrodes was thoroughly investigated. X-ray diffractometry and Raman spectroscopic analyses confirmed the formation of CoFe2O4 phase for films deposited at a substrate temperature of 450 °C. Topography and surface smoothness was measured using atomic force microscopy. We observed that the films deposited at room temperature showed improved electrochemical performance and supercapacitive properties compared to those of films deposited at 450 °C. Specific capacitances of about 777.4 F g-1 and 258.5 F g-1 were obtained for electrodes deposited at RT and 450 °C, respectively, at 0.5 mA cm-2 current density. The CoFe2O4 films deposited at room temperature exhibited an excellent power density (3277 W kg-1) and energy density (17 W h kg-1). Using electrochemical impedance spectroscopy, the series resistance and charge transfer resistance were found to be 1.1 Ω and 1.5 Ω, respectively. The cyclic stability was increased up to 125% after 1500 cycles due to the increasing electroactive surface of CoFe2O4 along with the fast electron and ion transport at the surface.

Reversible control of magnetization in Fe3O4nanoparticles by a supercapacitor

The manipulation of magnetism by electrical means is one of the most intensely pursued research topics of recent times aiming at the development of efficient and low-energy consumption devices in spintronics, microelectronics and bioelectronics. Herein, we successfully tuned the saturated magnetization of Fe3O4by a supercapacitor. Through increasing the surface area of magnetic particles and activation of carbon cloth, fully reversible and robust saturation magnetization variation with low power consumption and remarkable switching speed can be realized on Fe3O4/ionic liquid interfaces at room temperature. The associated magnetism modulation can be attributed to ionic transition between Fe2+and Fe3+resulting from both electrostatic and electrochemical doping. This work paves the way for the development of high-performance spintronic devices.

Sucrose-templated interconnected meso/macro-porous 2D symmetric graphitic carbon networks as supports for α-Fe2O3 towards improved supercapacitive behavior

In this study, ultrahigh electrochemical performance for interconnected meso/macro-porous 2D C@α-Fe₂O₃ synthesized via sucrose-assisted microwave combustion is demonstrated. Hematite (α-Fe₂O₃) synthesized via the same approach gave an encouraging electrochemical performance close to its theoretical value, justifying its consideration as a potential supercapacitor electrode material; nonetheless, its specific capacitance was still low. The pore size distribution as well as the specific surface of bare α-Fe₂O₃ improved from 145 m² g⁻¹ to 297.3 m² g⁻¹ after it was coated with sucrose, which was endowed with ordered symmetric single-layer graphene (2D graphene). Accordingly, the optimized hematite material (2D C@α-Fe₂O₃) showed a specific capacitance of 1876.7 F g⁻¹ at a current density of 1 A g⁻¹ and capacity retention of 95.9% after 4000 cycles. Moreover, the material exhibited an ultrahigh energy density of 93.8 W h kg⁻¹ at a power density of 150 W kg⁻¹. The synergistic effect created by carbon-coating α-Fe₂O₃ resulted in modest electrochemical performance owing to extremely low charge transfer resistance at the electrode–electrolyte interface with many active sites for ionic reactions and efficient diffusion process. This 2D C@α-Fe₂O₃ electrode material has the capacity to develop into a cost-effective and stable electrode for future high-energy-capacity supercapacitors.

In this study, ultrahigh electrochemical performance for interconnected meso/macro-porous 2D C@α-Fe₂O₃ synthesized via sucrose-assisted microwave combustion is demonstrated.

Homogeneous Fe2O3 coatings on carbon nanotube structures for supercapacitors

The combination of carbon nanotubes with transition metal oxides can exhibit complementary charge storage properties for use as electrode materials for next generation energy storage devices. One of the biggest challenges so far is to synthesize homogeneous oxide coatings on carbon nanotube structures preserving their integrity. Here we present the formation of conformal coatings of Fe₂O₃ on vertically aligned carbon nanotubes obtained by atomic layer deposition. We investigate the effect of pristine, nitrogen plasma and water plasma treated carbon nanotube surfaces on the ALD-growth of Fe₂O₃ using ferrocene and ozone precursors. The surface morphology, coating thickness, microstructure and surface chemistry of iron oxide-carbon nanotube composites and their ultimate influence on the electrochemical behavior of the composites are evaluated. The most effective surface functionalization is that achieved by H₂O plasma treatment, whereas untreated carbon nanotubes, despite the lack of active sites in the starting pristine surface, can be coated with an inhomogeneous Fe₂O₃ film.

Comprehensive Insight into the Mechanism, Material Selection and Performance Evaluation of Supercapatteries

Electrochemical energy storage devices (EESs) play a crucial role for the construction of sustainable energy storage system from the point of generation to the end user due to the intermittent nature of renewable sources. Additionally, to meet the demand for next-generation electronic applications, optimizing the energy and power densities of EESs with long cycle life is the crucial factor. Great efforts have been devoted towards the search for new materials, to augment the overall performance of the EESs. Although there are a lot of ongoing researches in this field, the performance does not meet up to the level of commercialization. A further understanding of the charge storage mechanism and development of new electrode materials are highly required. The present review explains the overview of recent progress in supercapattery devices with reference to their various aspects. The different charge storage mechanisms and the multiple factors involved in the performance of the supercapattery are described in detail. Moreover, recent advancements in this supercapattery research and its electrochemical performances are reviewed. Finally, the challenges and possible future developments in this field are summarized.

Moderate oxygen-deficient Fe(III) oxide nanoplates for high performance symmetric supercapacitors

As a promising anode material for supercapacitors, Fe₂O₃ has been widely studied but still face the problem of low conductivity. Inducing oxygen vacancy (Vo) into Fe₂O₃ is a widely used approach to tune the conductivity to enhance its capacitive performance, but there is little research on the influence of Vo content. Herein, we report the effect of Vo in Fe₂O₃ nanoplates with various content. We tuned the Vo content by annealing at 200-500 °C. XPS and EPR were taken to characterize the Vo content, ranging from 11% to 26%. Electrochemical results show that FO-300 with 17% Vo has the highest capacity of 301 mAh g^-1, and the capacity of the highest Vo content's (26% Vo) is only 107 mAh g^-1. The symmetric supercapacitor based on FO-300 shows a considerably high energy density of 58.5 Wh kg^-1 at a power density of 9.32 kW kg^-1 and remains 84.6% after 12,000 cycles.

Stabilizing Metallic Iron Nanoparticles by Conformal Graphitic Carbon Coating for High-Rate Anode in Ni-Fe Batteries

Nickel-iron (Ni-Fe) batteries are promising candidates for large-scale energy storage due to their high safety and low cost. However, their power density and cycling efficiency remain limited by the poor kinetics of the Fe anode. Herein, we report high-performance Fe anodes based on active Fe nanoparticles conformally coated with carbon shells, which were synthesized from low-cost precursors using a scalable process. Such core-shell structured C-Fe anodes offer high electrochemical activity and stability. Specifically, a high specific capacity of 208 mAh g^-1 at a current density of 1 A g^-1 (based on the total weight of Fe and C) and a capacity retention of 93% after 2000 cycles at 4 A g^-1 can be achieved. When coupled with a Ni cathode, such a full cell battery can deliver a high energy density of 101.0 Wh kg^-1 at power density of 0.81 kW kg^-1 and 51.6 Wh kg^-1 at 8.2 kW kg^-1 (based on the mass of the electrode materials), among the best energy and power performance among Ni-Fe batteries reported results. Thus, this work may provide an effective and scalable route toward high-performance anodes for high-power and long-life Ni-Fe batteries.

Low-Crystalline FeOOH Nanoflower Assembled Mesoporous Film Anchored on MWCNTs for High-Performance Supercapacitor Electrodes

Crystalline iron oxides/hydroxides are generally preferred as supercapacitor electrode materials instead of the low-crystalline structure, despite the fact that an amorphous phase could have a comprehensive electrochemical performance owing to its structural disorder. Herein, we present a facile and scalable method for preparing amorphous FeOOH nanoflowers@multi-walled carbon nanotubes (FeOOH NFs@MWCNTs) composites. The resulting hybrid nanoflowers hold a distinctive heterostructure composed of a self-assembled amorphous FeOOH nanofilm on the MWCNTs surface. The low-crystalline 1FeOOH NFs@1MWCNTs composites at pH 8 exhibit a high comprehensive capacitive performance, which may be attributed to the advantageous structural features. In a -0.85 to 0 V vs Ag/AgCl potential window, the prepared hybrid electrode delivers a high specific capacitance of 345 F g^-1 at a current density of 1 A g^-1, good cycling stability (76.4% capacity retention over 5000 consecutive cycles), and outstanding rate performance (167 F g^-1 at 11.4 A g^-1). This work may trigger the possibilities of these nanomaterials for further application in supercapacitor electrodes, specifically low-crystalline oxide/hydroxide-based electrode materials.

Plasma production of nanomaterials for energy storage: continuous gas-phase synthesis of metal oxide CNT materials via a microwave plasma

In this work we show for the first time that a continuous plasma process can synthesize materials from bulk industrial powders to produce hierarchical structures for energy storage applications. The plasma production process's unique advantages are that it is fast, inexpensive, and scalable due to its high energy density that enables low-cost precursors. The synthesized hierarchical material is comprised of iron oxide and aluminum oxide aggregate particles and carbon nanotubes grown in situ from the iron particles. New aerosol-based methods were used for the first time on a battery material to characterize aggregate and primary particle morphologies, while showing good agreement with observations from TEM measurements. As an anode for lithium ion batteries, a reversible capacity of 870 mA h g-1 based on metal oxide mass was observed and the material showed good recovery from high rate cycling. The high rate of material synthesis (∼10 s residence time) enables this plasma hierarchical material synthesis platform to be optimized as a means for energetic material production for the global energy storage material supply chain.

High performance supercapacitor electrodes based on spinel NiCo2O4@MWCNT composite with insights from density functional theory simulations

In this work, we demonstrated the supercapacitor performance of pristine and composites of spinel NiCo₂O₄ with a multi-walled carbon nanotube (MWCNT) assembled in a two-electrode cell configuration. Spinel NiCo₂O₄ and NiCo₂O₄@MWCNT composites were synthesized via a facile hydrothermal method. The supercapacitive performance of as-synthesized NiCo₂O₄ and NiCo₂O₄@MWCNT fabricated on Ni-foam was studied in a 0.5M K₂SO₄ electrolyte using electrochemical measurement techniques. The symmetric cell configuration of NiCo₂O₄@MWCNT delivers high specific capacitance (374 F/g at 2 A/g) with high energy density and power density (95 Wh/kg and 3 964 W/kg, respectively) compared to that of pristine NiCo₂O₄ electrodes (137 F/g at 0.6 A/g). Furthermore, the energy storage performance of the asymmetric cells of NiCo₂O₄//MWCNT and NiCo₂O₄@MWCNT//MWCNT was studied to enhance cycling stability (retention of 74.85% over 3000 cycles). We have also theoretically studied the supercapacitance performance of pristine NiCo₂O₄ and NiCo₂O₄@SWCNT hybrid structures through its structural and electronic properties using density functional theory predictions. The higher specific capacitance of the NiCo₂O₄@SWCNT hybrid system with high power density and energy density is supported by the enhanced density of states near the Fermi level and increased quantum capacitance of the hybrid structure. We have theoretically computed the diffusion energy barrier of K⁺ ions of the K₂SO₄ electrolyte in the NiCo₂O₄ layer and compared it with the diffusion barrier for Na⁺ ions. The lesser diffusion energy barrier for K⁺ ions in the NiCo₂O₄ layer contributes toward higher energy storage capacity. Thus, owing to superior electrochemical performance of NiCo₂O₄ composites with MWCNTs, it can serve as a high-performance electrode material for supercapacitor applications.

3D nanoflower-like MoSe2 encapsulated with hierarchically anisotropic carbon architecture: a new and free-standing anode with ultra-high areal capacitance for asymmetric supercapacitors

An anisotropic carbon-supported MoSe₂ nanoflowers is designed and acts as an ultra-high areal capacitance of free-standing anode. The energy density of assembled asymmetric supercapacitor is higher than or comparable to that of some Li-ion batteries.

The effects of Ni ions' charge disproportionation on the high electrochemical performance of Ni1−xCoxO nanoparticles

The outstanding electrochemical properties of Ni_1−xCo_xO electrode materials can be attributed to the Ni ion charge disproportionation, which is caused by Co atom doping.

Pine cone mold: a toolbox for fabricating unique metal/carbon nanohybrid electrocatalysts

Nature presents delicate and complex materials systems beyond those fathomable by humans, and therefore, extensive effort has been made to utilize or mimic bio-materials and bio-systems in various fields. Biomass, an inexhaustible natural materials source, can also present good opportunities for the development of unprecedented, advanced materials and processing systems. Herein, we demonstrate the use of pine cones as a biomass mold for creating new and useful metal/carbon nanohybrids (MCNHs). The inherent water-induced folding actuation of the cone scales allows the casting of an aqueous solution of a single metal precursor or a binary metal mixture into the cone mold by simply immersing the cone in the solution. The cone actively absorbs aqueous-phase metal precursors through the bract scales and the precursor ions introduced into the cone are anchored to the functional groups of the interior tissues of the cone. Subsequent heat treatment successfully led to the formation of unique MCNHs. Iron, manganese, and cobalt were employed as model metals, binary mixtures of which were also cast into the cone mold to create further versatile MCNHs. Nanoparticulate metals were formed on the carbon supports, where the size, size distribution, and crystallinity of the nanoparticles were highly dependent on the identity of the single-component precursor and the combination of precursors. Consequently, the electrochemical activity of the MCNHs also depended on which metal precursors were cast into the cone mold. The MCNH prepared from the mixture of iron and manganese precursors (M_FeMnCNH) showed the best electrochemical activity. As model applications, M_FeMnCNH was applied to electrode materials for electrochemical charge storage and the oxygen evolution reaction. An electrochemical capacitor cell based on the M_FeMnCNH electrodes showed excellent performance with energy densities of 38.7-54.2 W h kg^-1 at power densities of 16 000-160 kW kg^-1. In addition, M_FeMnCNH demonstrated a low overpotential of 464 mV and fast kinetics with a Tafel slope of 64.6 mV dec^-1 as an electrocatalyst for the oxygen evolution reaction in 1.0 M KOH. These results substantiate that pine cones as a biomass mold show great promise for creating versatile MCNHs through further combination of various precursors.

Photopatterning and Electrochemical Energy Storage Properties of an On-Chip Organic Radical Microbattery

One potential way to fabricate battery-on-chip is photopatterning electrochemical energy storage materials directly on electronics through lithography, but applicable materials are primarily limited to transparent photocurable resins. The transparency of the photoresist would be sacrificed after extra addition of insoluble inorganic battery materials and conductors. Given the importance of radical polymers for their appropriate solubility, optical transparency, and radical robustness, they may have potential application in on-chip energy storage, transport, and conversion devices. Herein, an anodic photoresist is proposed by modifying the MicroChem SU8 resist with a radical polymer poly(2,2,6,6-tetramethyl-4-piperidinyl-N-oxyl methacrylate) and an ionic conductor lithium perchlorate. It can be photopatterned on silicon wafer with 10 μm scale resolution, and it exhibits charge/discharge potentials at ca. 0.68 V versus silver chloride electrode; the coulomb efficiency is regarded as nearly equaling 100%. Although the specific capacity of the photopatterned film electrode is found to be modest, 1 × 10^-5 mA h·cm^-2, it presents 1/8 of its theoretical electron storage ability. All-solid-state half-cells with circular features 30 μm in diameter are prepared by means of overlay exposure using the as-prepared photoresist and lithium perchlorate-modified SU8 as the anodic electrode and solid electrolyte, respectively. These results suggest a promising way of using radical polymers for the integration of electrochemical energy in microelectronics.

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.

Configuring Optimal FeS2@Carbon Nanoreactor Anodes: Toward Insights into Pyrite Phase Change/Failure Mechanism in Rechargeable Ni-Fe Cells

Pyrite FeS₂ has long been a research focus as the alternative anode of rechargeable Ni-Fe cells owing to its eye-catching merits of great earth-abundance, attractive electrical conductivity, and output capacity. However, its further progress is impeded by unsatisfactory cyclic behaviors due to still "ill-defined" phase changes. To gain insights into the pyrite working principles/failure factors, we herein design a core-shell hybrid of a FeS₂@carbon nanoreactor, an optimal anode configuration approaching the practical usage state. The resultant electrodes exhibit a Max. specific capacity of ∼272.89 mAh g^-1 (at ∼0.81 A g^-1), remarkably improved cyclic longevity/stability (beyond ∼80% capacity retention after 10³ cycles) and superior rate capability (∼146.18 mAh g^-1 is remained at ∼20.01 A g^-1) in contrast to bare FeS₂ counterparts. The as-built Ni-Fe full cells can also output impressive specific energy/power densities of ∼87.38 Wh kg^-1/ ∼ 11.54 kW kg^-1. Moreover, a refreshed redox reaction working mechanism of "FeS₂OH ↔FeS₂↔Fe⁰_{(in pyrite domains)}" is redefined based on real-time electrode characterizations at distinct operation stages. In a total cyclic period, the configured pyrite-based anodes would stepwise undergo three critical stages nominally named "retention", "phase transition/coexistence", and "degradation", each of which is closely related to variations on anodic compositions/structures. Combined with optimal electrode configurations and in-depth clarifications on inherent phase conversions, this focus study may guide us to maximize the utilization efficiency of pyrite for all other aqueous electrochemical devices.

Eco-Friendly, Direct Deposition of Metal Nanoparticles on Graphite for Electrochemical Energy Conversion and Storage

Simple, green, and energy-efficient methods for preparing electroactive materials used to generate and store renewable energy are important for a sustainable future. In this study, we showed that noble and certain non-noble metal nanoparticles can be deposited on graphite without the aid of any reducing agent. This method of reducing metal ions to metal nanoparticles by graphite involves only one step (i.e., immersion into a solution) and one chemical (i.e., a metal salt). Hence, the method is exceedingly simple, green, and does not require any energy input. Large amounts of metal nanoparticles are generated both on the surface and deep into the bulk of graphite (∼100 μm). Despite the simplicity of this method, the metal deposited on graphite showed good electrocatalytic performance for ethanol oxidation and oxygen evolution reactions and also functioned as electrodes for supercapacitors. This method is thus ideal for preparing electrocatalytic materials and electrochemical energy storage devices due to its simplicity and environmental sustainability. The simplicity of the method is due to the inherent reducing potential of graphite (i.e., a material that is generally perceived as inert). Results from analyses showed that functionalization of the reactive edges in the regions of defects allowed the graphite to serve as a reducing agent. Increasing the amount of defects (e.g., via chemical or simple mechanical treatments) is shown to be the fundamental principle for increasing the reactivity of graphite.

Facile synthesis of carbon nanotube-supported NiO//Fe2O3 for all-solid-state supercapacitors

We have successfully prepared iron oxide and nickel oxide on carbon nanotubes on carbon cloth for the use in supercapacitors via a simple aqueous reduction method. The obtained carbon cloth-carbon nanotube@metal oxide (CC-CNT@MO) three-dimensional structures combine the high specific capacitance and rich redox sites of metal oxides with the large specific area and high electrical conductivity of carbon nanotubes. The prepared CC-CNT@Fe₂O₃ anode reaches a high capacity of 226 mAh·g^-1 at 2 A·g^-1 with a capacitance retention of 40% at 40 A·g^-1. The obtained CC-CNT@NiO cathode exhibits a high capacity of 527 mAh·g^-1 at 2 A·g^-1 and an excellent rate capability with a capacitance retention of 78% even at 40 A·g^-1. The all-solid-state asymmetric supercapacitor fabricated with these two electrodes delivers a high energy density of 63.3 Wh·kg^-1 at 1.6 kW·kg^-1 and retains 83% of its initial capacitance after 5000 cycles. These results demonstrate that our simple aqueous reduction method to combine CNT and metal oxides reveals an exciting future in constructing high-performance supercapacitors.

Three-step-in-one synthesis of supercapacitor MWCNT superparamagnetic magnetite composite material under flow

Composites of multi-walled carbon nanotubes (MWCNTs) and superparamagnetic magnetite nanoparticles, Fe3O4@MWCNT, were synthesized in DMF in a vortex fluidic device (VFD). This involved in situ generation of the iron oxide nanoparticles by laser ablation of bulk iron metal at 1064 nm using a pulsed laser, over the dynamic thin film in the microfluidic platform. The overall processing is a three-step in one operation: (i) slicing MWCNTs, (ii) generating the superparamagnetic nanoparticles and (iii) decorating them on the surface of the MWCNTs. The Fe3O4@MWCNT composites were characterized by transmission electron microscopy, scanning transmission electron microscope, TG analysis, X-ray diffraction and X-ray photoelectron spectroscopy. They were used as an active electrode for supercapacitor measurements, establishing high gravimetric and areal capacitances of 834 F g-1 and 1317.7 mF cm-2 at a scan rate of 10 mV s-1, respectively, which are higher values than those reported using similar materials. In addition, the designer material has a significantly higher specific energy of 115.84 W h kg-1 at a specific power of 2085 W kg-1, thereby showing promise for the material in next-generation energy storage devices.

Hydrangea-like α-Ni1/3Co2/3(OH)2 Reinforced by Ethyl Carbamate "Rivet" for All-Solid-State Supercapacitors with Outstanding Comprehensive Performance

Improving the self-conductivity and structural stability of electrode materials is a key strategy to improve the energy density, rate performance, and cycle life of supercapacitors. Controlled intercalation of ethyl carbamate (CH₃CH₂OCONH₂) as the rivet between Ni-Co hydroxide layers can be used to obtain sufficient ion transport channels and robust structural stability of hydrangea-like α-Ni_1/3Co_2/3(OH)₂ (NC). Combining the improved electronic conductivity offered by the coexistence of Ni²⁺ and Co²⁺ optimizing itself electronic conductivity and the addition of carbon nanotubes (CNTs) as the electron transport bridge between the active material and the current collector and the large specific surface area (296 m² g^-1) reducing the concentration polarization, the capacitance retention ratio of NC-CNT from 0.2 to 20 A g^-1 is up to 93.4% and its specific capacitance is as high as 1228.7 F g^-1 at 20 A g^-1. The large total hole volume (0.40 cm³ g^-1) and wide crystal plane spacing (0.71 nm) provide an adequate space to withstand structure deformation during charge/discharge processes and enhance the structural stability of the NC material. The capacitance fading ratio of NC-CNT is only 4.5% at 10 A g^-1 for 10 000 cycles. The aqueous supercapacitor (NC-CNT//AC) and all-solid-state supercapacitor (PVA-NC-CNT//PVA-AC) exhibit high energy density (35.2 W h kg^-1 at 100.0 W kg^-1 and 35.4 W h kg^-1 at 100.7 W kg^-1), ultrahigh rate performance (the specific capacitances at 20 A g^-1 are 92.8 and 87.2% compared to that at 0.5 A g^-1), and long cycling life span (the specific capacitances after 100 000 cycles at 10 A g^-1 are 91.5 and 90.8% compared with that of their initial specific capacitances), respectively. Therefore, hydrangea-like NC could be a promising material for advanced next-generation supercapacitors.

Synthesis of FeP nanotube arrays as negative electrode for solid-state asymmetric supercapacitor

Recently, metal phosphides have attracted considerable attention as promising electrode materials for supercapacitors. In this work, FeP nanotube arrays have been successfully synthesized on carbon cloth using ZnO nanorod arrays as the sacrificial templets, via a phosphidation process. The dimensions of the FeP nanotubes are characterized using SEM and TEM showing the diameter to be approximately 200 nm and with a wall thickness of 50-100 nm. The tubular structure of FeP nanotubes provides a facile ion pathway and reduced inner inactive material, thus they are favorable for supercapacitor applications. As a result, the as-synthesized FeP nanotube arrays deliver an improved specific capacitance of 149.11 F g^-1 and a high areal capacitance of 300.1 mF cm^-2 at a current density of 1 mA cm^-2. Furthermore, an MnO₂//FeP solid-state asymmetric supercapacitor was fabricated with a high areal capacitance of 142 mF cm^-2, which indicates the great potential of FeP nanotube arrays to be a high-performing negative electrode material for supercapacitors.

Voltage-Control of Magnetism in All-Solid-State and Solid/Liquid Magnetoelectric Composites

The control of magnetism by means of low-power electric fields, rather than dissipative flowing currents, has the potential to revolutionize conventional methods of data storage and processing, sensing, and actuation. A promising strategy relies on the utilization of magnetoelectric composites to finely tune the interplay between electric and magnetic degrees of freedom at the interface of two functional materials. Albeit early works predominantly focused on the magnetoelectric coupling at solid/solid interfaces; however, recently there has been an increased interest related to the opportunities offered by liquid-gating techniques. Here, a comparative overview on voltage control of magnetism in all-solid-state and solid/liquid composites is presented within the context of the principal coupling mediators, i.e., strain, charge carrier doping, and ionic intercalation. Further, an exhaustive and critical discussion is carried out, concerning the suitability of using the common definition of coupling coefficient α C = Δ M Δ E   to compare the strength of the interaction between electricity and magnetism among different magnetoelectric systems.

Iron oxide-based nanomaterials for supercapacitors

As highly efficient and clean electrochemical energy storage devices, supercapacitors (SCs) have drawn widespread attention as promising alternatives to batteries in recent years. Among various electrode materials, iron oxide materials have been widely studied as negative SC electrode materials due to their broad working window in negative potential, ideal theoretical specific capacitance, good redox activity, abundant availability, and eco-friendliness. However, iron oxides still suffer from the problems of low stability and poor conductivity. In this review, recent progress in iron oxide-based nanomaterials, including Fe₂O₃, Fe₃O₄, Fe_xO_y, and FeOOH, as electrode materials of SCs, is discussed. The nanostructure design and various synergistic effects of nanocomposites for improving the electrochemical performance of iron oxides are emphasized. Research on iron oxide-based symmetric/asymmetric SCs is also discussed. Future outlooks regarding iron oxides for SCs are likewise proposed.

Metal Oxide and Hydroxide-Based Aqueous Supercapacitors: From Charge Storage Mechanisms and Functional Electrode Engineering to Need-Tailored Devices

Energy storage devices that efficiently use energy, in particular renewable energy, are being actively pursued. Aqueous redox supercapacitors, which operate in high ionic conductivity and environmentally friendly aqueous electrolytes, storing and releasing high amounts of charge with rapid response rate and long cycling life, are emerging as a solution for energy storage applications. At the core of these devices, electrode materials and their assembling into rational configurations are the main factors governing the charge storage properties of supercapacitors. Redox-active metal compounds, particularly oxides and hydroxides that store charge via reversible valence change redox reactions with electrolyte ions, are prospective candidates to optimize the electrochemical performance of supercapacitors. To address this target, collaborative investigations, addressing different streams, from fundamental charge storage mechanisms and electrode materials engineering to need-tailored device assemblies, are the key. Over the last few years, significant achievements in metal oxide and hydroxide-based aqueous supercapacitors have been reported. This work discusses the most recent achievements and trends in this field and brings into the spotlight the authors' viewpoints.

All-Transparent Stretchable Electrochromic Supercapacitor Wearable Patch Device

Flexible and stretchable electrochromic supercapacitor systems are widely considered as promising multifunctional energy storage devices that eliminate the need for an external power source. Nevertheless, the performance of conventional designs deteriorates significantly as a result of electrode/electrolyte exposure to atmosphere as well as mechanical deformations for the case of flexible systems. In this study, we suggest an all-transparent stretchable electrochromic supercapacitor device with ultrastable performance, which consists of Au/Ag core-shell nanowire-embedded polydimethylsiloxane (PDMS), bistacked WO₃ nanotube/PEDOT:PSS, and polyacrylamide (PAAm)-based hydrogel electrolyte. Au/Ag core-shell nanowire-embedded PDMS integrated with PAAm-based hydrogel electrolyte prevents Ag oxidation and dehydration while maintaining ionic and electrical conductivity at high voltage even after 16 days of exposure to ambient conditions and under application of mechanical strains in both tensile and bending conditions. WO₃ nanotube/PEDOT:PSS bistacked active materials maintain high electrochemical-electrochromic performance even under mechanical deformations. Maximum specific capacitance of 471.0 F g^-1 was obtained with a 92.9% capacity retention even after 50 000 charge-discharge cycles. In addition, high coloration efficiency of 83.9 cm² C^-1 was shown to be due to the dual coloration and pseudocapacitor characteristics of the WO₃ nanotube and PEDOT:PSS thin layer.

Electrodeposited Nanostructured CoFe2O4 for Overall Water Splitting and Supercapacitor Applications

To contribute to solving global energy problems, a multifunctional CoFe2O4 spinel was synthesized and used as a catalyst for overall water splitting and as an electrode material for supercapacitors. The ultra-fast one-step electrodeposition of CoFe2O4 over conducting substrates provides an economic pathway to high-performance energy devices. Electrodeposited CoFe2O4 on Ni-foam showed a low overpotential of 270 mV and a Tafel slope of 31 mV/dec. The results indicated a higher conductivity for electrodeposited compared with dip-coated CoFe2O4 with enhanced device performance. Moreover, bending and chronoamperometry studies suggest excellent durability of the catalytic electrode for long-term use. The energy storage behavior of CoFe2O4 showed high specific capacitance of 768 F/g at a current density of 0.5 A/g and maintained about 80% retention after 10,000 cycles. These results demonstrate the competitiveness and multifunctional applicability of the CoFe2O4 spinel to be used for energy generation and storage devices.

Iron phosphide anchored nanoporous carbon as an efficient electrode for supercapacitors and the oxygen reduction reaction

Inspired by their distinctive properties, transition metal phosphides have gained immense attention as promising electrode materials for energy storage and conversion applications. The introduction of a safe and large-scale method of synthesizing a composite of these materials with carbon is of great significance in the fields of electrochemical and materials sciences. In the current effort, we successfully synthesize an iron phosphide/carbon (FeP/C) with a high specific surface area by the pyrolysis of the gel resulting from the hydrothermal treatment of an iron nitrate–phytic acid mixed solution. In comparison with the blank (P/C), the as-synthesized FeP/C appears to be an efficient electrode material for supercapacitor as well as oxygen reduction reaction (ORR) applications in an alkaline medium in a three-electrode system. In the study of supercapacitors, FeP/C shows areal capacitance of 313 mF cm⁻² at 1.2 mA cm⁻² while retaining 95% of its initial capacitance value after 10 000 cycles, while in the ORR, the synthesized material exhibits high electrocatalytic activity with an onset potential of ca. 0.86 V vs. RHE through the preferred four-electron pathway and less than 6% H₂O₂ production calculated in the potential range of 0.0–0.7 V vs. RHE. The stability is found to be better than those of the benchmark Pt/C (20 wt%) catalyst.

Synthesis of a nanoporous FeP/C material through a two-step method involving hydrothermal and carbonization processes for supercapacitors and the oxygen reduction reaction.

A Porous and Conductive Graphite Nanonetwork Forming on the Surface of KCu7S4 for Energy Storage

A flexible all-solid-state supercapacitor is fabricated by building a layer of porous and conductive nanonetwork on the surface of KCu₇S₄ nanowires supported on the carbon fiber fabric, where the porous and conductive nanonetwork is assembled by graphite nanoparticles. This porous graphite layer plays a key role in providing ion diffusion channels to access the KCu₇S₄ through the pores for electrochemical reactions and forming electron transport pathways from the graphite network to the electronic collector of the carbon fiber fabric. This flexible supercapacitor exhibits excellent electrochemical performance with high specific capacitance of 408 F g^-1 at a current density of 0.5 A g^-1 and high energy density of 36 Wh kg^-1 at a power density of 201 W kg^-1. Moreover, it is cost-effective, easy to scale up and environmentally friendly with high flexibility. Our investigation demonstrates that such a porous and conductive nanonetwork could be used to improve the charge storage efficiency for a wide range of electrode materials.

Carbon Nanodots as Feedstock for a Uniform Hematite-Graphene Nanocomposite

High degrees of dispersion are a prerequisite for functional composite materials for applications in electronics such as sensors, charge and data storage, and catalysis. The use of small precursor materials can be a decisive factor in achieving a high degree of dispersion. In this study, carbon nanodots are used to fabricate a homogeneous, finely dispersed Fe₂ O₃ -graphene composite aerogel in a one-step conversion process from a precursor mixture. The laser-assisted conversion of small size carbon nanodots enables a uniform distribution of 6.5 nm Fe₂ O₃ nanoparticles during the formation of a highly conductive carbon matrix. Structural and electrochemical characterization shows that the features of both material entities are maintained and successfully integrated. The presence of Fe₂ O₃ nanoparticles has a positive effect on the active surface area of the carbon aerogel and thus on the capacitance of the material. This is demonstrated by testing the performance of the composite in supercapacitors. Faradaic reactions are exploited in an aqueous electrolyte through the high accessible surface of the incorporated small Fe₂ O₃ nanoparticles boosting the specific capacitance of the 3D turbostratic graphene network significantly.

All-Metal-Organic Framework-Derived Battery Materials on Carbon Nanotube Fibers for Wearable Energy-Storage Device

The ever-increasing demands for portable and wearable electronics continue to drive the development of high-performance fiber-shaped energy-storage devices. Metal-organic frameworks (MOFs) with well-tunable structures and large surface areas hold great potential as precursors and templates to form porous battery materials. However, to date, there are no available reports about fabrication of wearable energy-storage devices on the utilization of all-MOF-derived battery materials directly grown on current collectors. Here, MOF-derived NiZnCoP nanosheet arrays and spindle-like α-Fe2O3 on carbon nanotube fibers are successfully fabricated with impressive electrochemical performance. Furthermore, the resulting all-solid-state fiber-shape aqueous rechargeable batteries take advantage of large specific surface area and abundant reaction sites of well-designed MOF-derived electrode materials to yield a remarkable capacity of 0.092 mAh cm-2 and admirable energy density of 30.61 mWh cm-3, as well as superior mechanical flexibility. Thus, this research may open up exciting opportunities for the development of new-generation wearable aqueous rechargeable batteries.

Electrochemical Performance of Iron Oxide Nanoflakes on Carbon Cloth under an External Magnetic Field

In this work, the iron oxide (Fe2O3) nanoflakes on carbon cloth (Fe2O3@CC) were triumphantly prepared and served as the electrode of supercapacitors. By applying an external magnetic field, we first find that the magnetic field could suppress the polarization phenomenon of electrochemical performance. Then, the influences of the mono-/bi-valent cations on the electrochemical properties of the Fe2O3@CC were investigated under a large external magnetic field (1 T) in this work. The chemical valences of the cations in the aqueous electrolytes (LiNO3 and Ca(NO3)2) have almost no influences on the specific capacitance at different scan rates. As one of important parameters to describe the electrochemical properties, the working potential window of the Fe2O3@CC electrode was also investigated in this work. The broad potential window in room-temperature molten salt (LiTFSI + LiBETI (LiN(SO2CF3)2 + LiN(SO2C2F5)2)) has been obtained and reached 1.2 V, which is higher than that of the traditional aqueous electrolyte (~0.9 V).