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

Leveraging Direct Pyrolysis for the Synthesis of 10 nm Monodispersed Fe3O4/Fe3C NPS@Carbon to Improve SupercapacitANCE in Acidic Electrolyte

The prevailing practice advocates pre-oxidation of electrospun Fe-salt/polymer nanofibers (Fe-salt/polymer Nf) before pyrolysis as advantageous in the production of high-performance FeOx@carbon nanofibers supercapacitors (FeOx@C). However, our study systematically challenges this notion by demonstrating that pre-oxidation facilitates the formation of polydispersed and large FeOx nanoparticles (FeOx@CI-DA) through "external" Fe3+ Kirkendall diffusion from carbon, resulting in subpar electrochemical properties. To address this, direct pyrolysis of Fe-salt/polymer Nf is proposed, promoting "internal" Fe3+ Kirkendall diffusion within carbon and providing substantial physical confinement, leading to the formation of monodispersed and small FeOx nanoparticles (FeOx@CDA). In 1 M H2SO4, FeOx@CDA demonstrates ~2.60× and 1.26× faster SO4 2- diffusivity, and electron transfer kinetics, respectively, compared to FeOx@CI-DA, with a correspondingly ~1.50× greater effective surface area. Consequently, FeOx@CDA exhibits a specific capacity of 161.92 mAhg-1, ~2× higher than FeOx@CI-DA, with a rate capability ~19 % greater. Moreover, FeOx@CDA retains 94 % of its capacitance after 5000 GCD cycles, delivering an energy density of 26.68 Whkg-1 in a FeOx@CDA//FeOx@CDA device, rivaling state-of-the-art FeOx/carbon electrodes in less Fe-corrosive electrolytes. However, it is worth noting that the effectiveness of direct pyrolysis is contingent upon hydrated Fe-salt. These findings reveal a straightforward approach to enhancing the supercapacitance of FeOx@C materials.

Engineering FeOOH/Fe2O3@Carbon Interfaces With Biomass-Derived Carbon Nanodot/Iron Colloids for Efficient Redox-Modulated Dopamine Voltammetric Detection

The Fe2+/Fe3+ redox couple is effective for voltammetric detection of trace dopamine (DA). However, achieving adequate concentrations with high electroactive surface area (ECSA), DA affinity, and fast interfacial charge transfer is challenging. Consequently, most reported Fe-based sensors have a high nanomolar range detection limit (LOD). Herein, we address these limitations by manipulating the phase and morphology of FeOOH/Fe2O3 heterojunctions anchored on sp2-carbon. FeOOH/Fe2O3 is synthesized by variable temperature aging of unique Fe5H9O15/Fe2O3@sp2-carbon colloidal nanoparticles, which form via chelation between biomass-derived carbon nanodots (CNDs) and Fe2+ ions. At 27 °C and 120 °C, Fe5H9O15/Fe2O3@sp2-carbon transforms into β-FeOOH/Fe2O3 nanoparticles and α-FeOOH/Fe2O3 nanosheet, respectively. The β-FeOOH/Fe2O3 interface exhibits higher eg orbital electron occupancy than α-FeOOH/Fe2O3, thereby facilitating oxygen adsorption and the generation of Fe2+/Fe3+ sites near the polarization potential of DA. This facilitates interfacial electron transfer between Fe3+ and DA. Moreover, its nanoparticle morphology enhances ECSA and DA adsorption compared to α-FeOOH/Fe2O3 nanosheets. With a LOD of ~3.11 nM, β-FeOOH/Fe2O3 surpasses the lower threshold in humans (~10 nM) and matches noble-metal sensors. Furthermore, it exhibits selective detection of DA over 10 biochemicals in urine. Therefore, the β-FeOOH/Fe2O3@sp2-C platform holds promise as a low-cost, easy-to-synthesize, and practical voltammetric DA monitor.

Supercapacitors and triboelectric nanogenerators based on electrodes of greener iron nanoparticles/carbon nanotubes composites

The development of supporting materials based on carbon nanotubes (CNTs) impregnated with iron nanoparticles via a sustainable and green synthesis employing plant extract of Punica granatum L. leaves was carried out for the iron nanoparticle modification and the following impregnation into the carbon nanotubes composites (CNT-Fe) that were also coated with polypyrrole (CNT-Fe + PPy) for use as electrode for supercapacitor and triboelectric nanogenerators. The electrochemical characterization of the materials by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) assays revealed that the CNT-Fe + PPy gave rise to better performance due to the association of double-layer capacitance behavior of carbon derivative in association with the pseudocapacitance contribution of PPy resulting in an areal capacitance value 202 mF/ cm2 for the overall composite. In terms of the application of electrodes in triboelectric nanogenerators, the best performance for the composite of CNT-Fe + PPy was 60 V for output voltage and power density of 6 μW/cm2. The integrated system showed that the supercapacitors can be charged directly by the nanogenerator from 0 to 42 mV in 300 s. The successful green synthesis of iron nanoparticles on CNT and further PPy coating provides a feasible method for the design and synthesis of high-performance SCs and TENGs electrode materials. This work provides a systematic approach that moves the research front forward by generating data that underpins further research in self-powered electronic devices.

Insights into Nano- and Micro-Structured Scaffolds for Advanced Electrochemical Energy Storage

Adopting a nano- and micro-structuring approach to fully unleashing the genuine potential of electrode active material benefits in-depth understandings and research progress toward higher energy density electrochemical energy storage devices at all technology readiness levels. Due to various challenging issues, especially limited stability, nano- and micro-structured (NMS) electrodes undergo fast electrochemical performance degradation. The emerging NMS scaffold design is a pivotal aspect of many electrodes as it endows them with both robustness and electrochemical performance enhancement, even though it only occupies complementary and facilitating components for the main mechanism. However, extensive efforts are urgently needed toward optimizing the stereoscopic geometrical design of NMS scaffolds to minimize the volume ratio and maximize their functionality to fulfill the ever-increasing dependency and desire for energy power source supplies. This review will aim at highlighting these NMS scaffold design strategies, summarizing their corresponding strengths and challenges, and thereby outlining the potential solutions to resolve these challenges, design principles, and key perspectives for future research in this field. Therefore, this review will be one of the earliest reviews from this viewpoint.

Development of quinoline-based heteroatom polybenzoxazines reinforced graphitic carbon nitride (GCN) carbonisation composites for emerging supercapacitor applications

The current research described in this paper, focuses on the development of a new quinoline-based Mannich-type benzoxazine and its use to obtain advanced carbonisation materials with a high energy storage capacity. Based on this, a quinoline-based benzoxazine monomer (Q-xda) was synthesised by a reaction between 8-hydroxyquinoline, xylylenediamine and paraformaldehyde, and it is characterised by FT-IR and 1H-NMR spectroscopy. Composites were prepared from the benzoxazine and variable weight percentages of graphitic carbon nitride (GCN) (i.e., 5, 10, and 15 wt%). The oxazine ring-opening curing process of the polybenzoxazine composites, and its subsequent pyrolysis reaction was performed; and their chemical structures were confirmed using FT-IR spectroscopy. Also, the thermal and morphological characteristics of the composites were evaluated by XRD, thermogravimetric analysis (TGA), and SEM analyses. According to the results of the thermal experiments, adding GCN reinforcement significantly increased the thermal stability and char yield of the resultant composites. Electrochemical, and hydrophobic investigations were also carried out, and the results of these suggesting that the composites reinforced with 15 wt% GCN exhibit the highest dielectric constant (high κ = 10.2) and contact angle (145°). However, all the crosslinked composites demonstrated a remarkable electrochemical performance as pseudocapacitors. The resulting poly(Q-xda) + 15 wt% GCN electrodes showed a higher capacitance and a lower transferred charge resistance (i.e., 370 F g-1 at 6 A g-1 and 20.8 Ω) than the poly(Q-xda) electrode (i.e., 216 F g-1 at 6 A g-1 and 26.0 Ω). In addition, the poly(Q-xda) + 15% GCN exhibited a cycling efficiency of 96.2% even after 2000 cycles. From these results, it can be concluded that the constructed electrodes perform well in electrochemical operations.

Oxygen defect enriched hematite nanorods @ reduced graphene oxide core-sheath fiber for superior flexible asymmetric supercapacitor

The development of flexible asymmetric supercapacitors with high operating potential, superior energy density, and exceptional rate performance holds significant implications for the advancement of flexible electronics. Herein, oxygen-deficient hematite nanorods @ reduced graphene oxide (Fe2O3-x@RGO) core-sheath fiber was rationally designed and fabricated. The introduction of oxygen defects can simultaneously enhance the conductivity, create a mesoporous crystalline structure, increase active surface area and sites. This leads to a significantly improved electrochemical performance, exhibiting a high specific capacitance of 525.2F cm-3 at 5 mV s-1 and remarkable rate capability (53.7 % retention from 5 to 100 mV s-1). Additionally, a flexible asymmetric supercapacitor was assembled employing Fe2O3-x@RGO fibers as anode and MnO2/RGO fibers as cathode. This design achieved a maximum operating voltage of 2.35 V, high energy density of 71.4 mWh cm-3, and outstanding cycling stability with 97.1 % retention after 5000 cycles. This study proposes a straightforward and efficient strategy to substantially enhance the electrochemical performances of transition metal oxide anodes, thereby promoting their practical application in asymmetric supercapacitors.

Uniform Fe2 O3 Nanospheres Anchored on Multilayer Graphene as Anode Materials for High-Rate Ni-Fe Batteries

Ni-Fe battery is one of prospective aqueous alkaline batteries due to its high safety, eco-friendliness and cost-effectiveness. However, the electrochemical performance of Fe-based anodes is limited due to the particle aggregation and low electric conductivity. In this work, iron powder is used as a precursor in a chemical bath deposition method. By optimizing the concentration of HNO3 , a balanced dissolution-crystallization process is achieved to obtain uniform Fe2 O3 nanospheres in size between 60 and 90 nm, which are separately anchored on ultrasonically prepared multilayer graphene (MLG). This composite delivers specific discharge capacities of 191.1 and 160.8 mAh g-1 at the current densities of 2 and 10 A g-1 , respectively. A Ni-Fe battery with the as-prepared Fe2 O3 /MLG as anode and Ni(OH)2 /MLG as cathode exhibits an energy density of 69.5 Wh kg-1 at a high power density of 3931.6 W kg-1 .

Facile syntheses of Fe2O3-rGO and NiCo-LDH-rGO nanocomposites for high-performance electrochemical capacitors

Faraday-type electrode materials and devices for electrochemical capacitors have been widely investigated. However, their applications are severely limited by the preparation method and cost of electrode materials. In this work, high-performance electrochemical capacitors were successfully assembled using Fe₂O₃-decorated reduced graphene oxide (rGO) nanocomposites and NiCo-Layered Double Hydroxides (LDH) as the anode and cathode, respectively. An easy and efficient approach (the modified precipitation method) for the large-scale fabrication was used to prepare Fe₂O₃ and NiCo-LDH, supported by rGO sheets, respectively. The anode material, Fe₂O₃-rGO, exhibited an excellent specific capacitance (C_sp) of 1073 F g^-1 at a current density of 1 A g^-1 and a retention rate of 92 % at 10 A g^-1, while the NiCo-LDH-rGO cathode material provided a C_sp of 1850 F g^-1 at 1 A g^-1 and maintained 84 % at 10 A g^-1. The effective combination of these electrodes for the NiCo-LDH-rGO//Fe₂O₃-rGO electrochemical capacitors resulted in an excellent energy density of 108 Wh/kg at a power density of 884 W/kg, with remarkable cycling stability (80 % after 1000 cycles at 10 A g^-1). We believe that this work, including the proposed method and electrode materials, will advance the further development and commercialization of electrochemical capacitors.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Fe2O3/Porous Carbon Composite Derived from Oily Sludge Waste as an Advanced Anode Material for Supercapacitor Application

It is urgent to improve the electrochemical performance of anode for supercapacitors. Herein, we successfully prepare Fe₂O₃/porous carbon composite materials (FPC) through hydrothermal strategies by using oily sludge waste. The hierarchical porous carbon (HPC) substrate and fine loading of Fe₂O₃ nanorods are all important for the electrochemical performance. The HPC substrate could not only promote the surface capacitance effect but also improve the utilization efficiency of Fe₂O₃ to enhance the pseudo-capacitance. The smaller and uniform Fe₂O₃ loading is also beneficial to optimize the pore structure of the electrode and enlarge the interface for faradaic reactions. The as-prepared FPC shows a high specific capacitance of 465 F g^-1 at 0.5 A g^-1, good rate capability of 66.5% retention at 20 A g^-1, and long cycling stability of 88.4% retention at 5 A g^-1 after 4000 cycles. In addition, an asymmetric supercapacitor device (ASC) constructed with FPC as the anode and MnO₂/porous carbon composite (MPC) as the cathode shows an excellent power density of 72.3 W h kg^-1 at the corresponding power density of 500 W kg^-1 with long-term cycling stability. Owing to the outstanding electrochemical characteristics and cycling performance, the associated materials' design concept from oily sludge waste has large potential in energy storage applications and environmental protection.

Carbon-α-Fe2O3 Composite Active Material for High-Capacity Electrodes with High Mass Loading and Flat Current Collector for Quasi-Symmetric Supercapacitors

In this work, we report the synthesis of an active material for supercapacitors (SCs), namely α-Fe2O3/carbon composite (C-Fe2O3) made of elongated nanoparticles linearly connected into a worm-like morphology, by means of electrospinning followed by a calcination/carbonization process. The resulting active material powder can be directly processed in the form of slurry to produce SC electrodes with mass loadings higher than 1 mg cm−2 on practical flat current collectors, avoiding the need for bulky porous substrate, as often reported in the literature. In aqueous electrolyte (6 M KOH), the so-produced C-Fe2O3 electrodes display capacity as high as ~140 mAh g−1 at a scan rate of 2 mV s−1, while showing an optimal rate capability (capacity of 32.4 mAh g−1 at a scan rate of 400 mV s−1). Thanks to their poor catalytic activity towards water splitting reactions, the electrode can operate in a wide potential range (−1.6 V–0.3 V vs. Hg/HgO), enabling the realization of performant quasi-symmetric SCs based on electrodes with the same chemical composition (but different active material mass loadings), achieving energy density approaching 10 Wh kg−1 in aqueous electrolytes.

Synergistic Interaction of Clusters of Iron Oxide Nanoparticles and Reduced Graphene Oxide for High Supercapacitor Performance

Supercapacitors have been recognized as one of the more promising energy storage devices, with great potential use in portable electronics and hybrid vehicles. In this study, a composite made of clusters of iron oxide (Fe3O4-γFe2O3) nanoparticles and reduced graphene oxide (rGO) has been developed through a simple one-step solvothermal synthesis method for a high-performance supercapacitor electrode. Electrochemical assessment via cyclic voltammetry, galvanostatic charge-discharge experiments, and electrochemical impedance spectroscopy (EIS) revealed that the Fe3O4-γFe2O3/rGO nanocomposite showed much higher specific capacitance than either rGO or bare clusters of Fe3O4-γFe2O3 nanoparticles. In particular, specific capacitance values of 100 F g-1, 250 F g-1, and 528 F g-1 were obtained for the clusters of iron oxide nanoparticles, rGO, and the hybrid nanostructure, respectively. The enhancement of the electrochemical performance of the composite material may be attributed to the synergistic interaction between the layers of graphene oxide and the clusters of iron oxide nanoparticles. The intimate contact between the two phases eliminates the interface, thus enabling facile electron transport, which is key to attaining high specific capacitance and, consequently, enhanced charge-discharge time. Performance evaluation in consecutive cycles has demonstrated that the composite material retains 110% of its initial capacitance after 3000 cycles, making it a promising candidate for supercapacitors.

Rational La-doped hematite as an anode and hydrous cobalt phosphate as a battery-type electrode for a hybrid supercapacitor

In recent years, modern appliances require high energy density with a burst power supply. Hybrid supercapacitors show high performance based on high energy density without compromising power density and stability over thousands of charge-discharge cycles. In this work, the optimized hybrid electrodes using lanthanum-doped hematite (lanthanum-doped iron oxide) noted as 7.5%La-HMT as a negative electrode and hydrous cobalt phosphate (CoPO) as a battery-type positive electrode have been successfully fabricated via a simple hydrothermal method and a facile co-precipitation method, respectively. The 7.5%La-HMT showed excellent electrochemical performance due to doping of rare-earth La³⁺ metal ions, resulting in improvised active sites and reduction in the equivalent resistance. The 7.5%La-HMT operated at a high potential window (0 to -1.2 V) with an ultra-high specific capacitance (S_p) of 1226.7 F g^-1 at 1 A g^-1 with capacitance retention of 89.3% over 1000 cycles. CoPO could be operated at a high working window (0 to 0.45 V) with a specific capacity of 121.7 mA h g^-1 at a current density of 2 A g^-1 with capacitance retention of 85.4% over 1000 cycles. The configured CoPO//KOH//10%La-HMT aqueous hybrid capacitor device (Aq-HSC) could be operated at a potential window of 1.6 V and delivered a maximum energy density (E.D) of 83.6 W h kg^-1 at a power density (P.D) of 3.2 kW kg^-1 with S_p of 235.0 F g^-1 at 2 A g^-1 and 89.0% S_p retention over 5000 cycles. The simplicity of the synthesis methods for CoPO and 7.5%La-HMT along with their superior super-capacitive properties make them suitable for advanced electrical devices and hybrid vehicles.

SrFeO3–δ: A Novel Fe4+↔Fe2+ Redox Mediated Pseudocapacitive Electrode in Aqueous Electrolyte

Pseudocapacitor offers both high energy and high power making them suitable for grid scale electrochemical energy storage to harness the renewable energy produced through solar, wind and tides. To overcome...

Single-Step Preparation of Ultrasmall Iron Oxide-Embedded Carbon Nanotubes on Carbon Cloth with Excellent Superhydrophilicity and Enhanced Supercapacitor Performance

Nanocomposites consisting of carbon materials and metal oxides are generally preferred as anodes in electrochemical energy storage. However, their low capacitance limits the achieved energy density of supercapacitors (SCs) in aqueous electrolytes. Herein, we propose a rapid combustion strategy to construct a novel electrode architecture-ultrasmall Fe₂O₃ anchoring on carbon nanotubes (FeO-CNT)-as a superhydrophilic and flexible anode for SCs. In 1 M Na₂SO₄ aqueous electrolyte, such an FeO-CNT-20 anode presents a high capacitance of 483.4 mF cm^-2 (326 F g^-1) at 1 mA cm^-2. The aqueous asymmetric supercapacitor devices (ASCs) assembled by FeO-CNT-20 and MnO₂ present a maximum operating potential of 2.0 V with a high areal energy density of 0.11 mWh cm^-2 at a power density of 0.5 mW cm^-2. The flexible solid-state ASCs display an energy density of 0.99 mWh cm^-3 at 14.3 mW cm^-3. The rapidly prepared FeO-CNT not only offers an attractive electrode for SCs but also would open up exciting new avenues to the rational design and large-scale preparation of Fe₂O₃-based nanocomposites for electrochemical energy storage.

Kinetic Insights into the Mechanism of Oxygen Reduction Reaction on Fe2O3/C Composites

Since the inception of cobalt phthalocyanine for oxygen reduction reaction (ORR), non-platinum group metals have been the central focus in the area of fuel-cell electrocatalysts. Besides Fe-N_x active sites, a large variety of species are formed during the pyrolysis, but studies related to their ORR activity have been given less importance in the literature. Fe₂O₃ is one among them, and this study describes the role of Fe₂O₃ in the ORR. The Fe₂O₃ is carefully synthesized on various carbon supports and characterized using X-ray photoelectron spectroscopy (XPS) spectra, high-resolution transmission electron microscopy (HRTEM) images, and surface area analysis. The characterization techniques reveal that the Fe₂O₃ nanoparticles are present in the pores of the carbon supports, having a particle size ranging from 4 to 15 nm. The current density of the ORR on Fe₂O₃/C catalysts is increased compared with bare carbon supports, as discerned from the rotating ring-disk electrode (RRDE) voltammetry experiments, demonstrating the role of size-confined Fe₂O₃ nanoparticles. The overall number of electrons in the ORR is increased by the introduction of Fe₂O₃ on the carbon support. Based on the kinetic analysis, the ORR on Fe₂O₃/C follows a pseudo-4-electron or 2+2-electron ORR, where the first 2-electron ORR to H₂O₂ and second 2-electron H₂O₂ reduction reaction (HPRR) to H₂O are assigned to the graphitic carbon (carbon defects) and Fe₂O₃ active sites, respectively. Theoretical studies indicate that the role of Fe₂O₃ is to decrease the free energy of O₂ adsorption and reduce the energy barrier for the reduction of *OOH to OH^-. The onset potential estimated from the free energy diagram is 0.42 V, matching with the HPRR activity demonstrated using the potential-dependent rate constants plot. Fe₂O₃/C shows higher stability by retaining 95% of the initial activity even after 20 000 cycles.

Microwave-Enabled Size Control of Iron Oxide Nanoparticles on Reduced Graphene Oxide

Nanoparticle-functionalized 2D material networks are promising for a wide range of applications, but in situ formation of nanoparticles is commonly challenged by rapid growth. Here, we demonstrate controlled synthesis of small and dispersed iron oxide nanoparticles on reduced graphene oxide (rGO) networks through rapid localized heating with microwaves with low-cost iron nitrate as the precursor. The strong coupling of the microwave radiation with the rGO network rapidly heats the network locally to decompose the iron nitrate and generate iron oxide nanoparticles, while cessation of microwaves leads to rapid cooling to minimize crystal growth. Small changes in the microwave reaction time (<1 min) led to very large changes in the iron oxide morphology. The solid-state microwave syntheses produced narrower nanoparticle size distribution than conventional heating. These results illustrate the potential of solid-state microwave syntheses to control the nanoparticle size on 2D materials through rapid localized heating under the microwave process conditions, which should be extendable to a variety of transition metal oxide-rGO systems.

Dramatically comprehensive improved electrochemical performances of symmetric and asymmetric supercapacitors under external magnetic field

Recently, magnetic field (MF) has been described as a potential way to improve the properties of electrode materials, which significantly enhances the ion diffusion behavior and material wettability. We first synthezised for the first time a comparable carbon-based electrode material (GCA) including active carbon, reduced graphene and carbon nanotubes through a facile stirring method for further research under MF. Herein, 0.15 T of MF induced by two NdFeB magnets was applied on the supercapacitor devices to enhance the energy density, which increased by about 62% for the symmetric supercapacitor (SSC: from 11.2 to 18.1 W h kg^-1). An asymmetric supercapacitor composed of the prepared GCA as the anode and NiCoFe/NiCoFe-OH as the cathode was also assembled for research. And it was found that the whole electrochemical performance significantly improved (for example, energy density increased by about 22% for the asymmetric supercapacitor, i.e., from 50.6 to 61.4 W h kg^-1).

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.

High electrochemical performance of Ni-foam supported Ti3C2TxMXene/rGO nanocomposite

Two-dimensional (2D) materials have attracted extensive attention owing to their unique electronic/physiochemical properties, and wide application potential in energy storage and conversion. However, 2D materials are often tendency to aggregate due to the strong van der Waals interactions, leading to gradually decrease of an efficient mass transfer pathway and accessible surface area for the electrolyte. Here, we demonstrate an efficient approach for large-scale production of a hybrid nanostructures (Ti₃C₂T_x/rGO) based on ultrathin MXene nanosheets anchored on layered reduced graphene (rGO) supported by porous Ni-foam via a plain chemical dipping method followed by high temperature annealing process. Ti₃C₂T_x/rGO electrode exhibits a porous structure, excellent ionic and electrical conductivities, and remarkable specific capacitance. Furthermore, it shows ultra-high cycle stability, for example, 88.70% of its specific capacity can be maintained through 3000 cycles. This kind of porous nanostructure and integrated design idea is significant to design other energy storage modules.

Hybridized Graphene for Supercapacitors: Beyond the Limitation of Pure Graphene

Graphene-based supercapacitors have been attracting growing attention due to the predicted intrinsic high surface area, high electron mobility, and many other excellent properties of pristine graphene. However, experimentally, the state-of-the-art graphene electrodes face limitations such as low surface area, low electrical conductivity, and low capacitance, which greatly limit their electrochemical performances for supercapacitor applications. To tackle these issues, hybridizing graphene with other species (e.g., atom, cluster, nanostructure, etc.) to enlarge the surface area, enhance the electrical conductivity, and improve capacitance behaviors are strongly desired. In this review, different hybridization principles (spacers hybridization, conductors hybridization, heteroatoms doping, and pseudocapacitance hybridization) are discussed to provide fundamental guidance for hybridization approaches to solve these challenges. Recent progress in hybridized graphene for supercapacitors guided by the above principles are thereafter summarized, pushing the performance of hybridized graphene electrodes beyond the limitation of pure graphene materials. In addition, the current challenges of energy storage using hybridized graphene and their future directions are discussed.

Controllable design of defect-rich hybrid iron oxide nanostructures on mesoporous carbon-based scaffold for pseudocapacitive applications

The controllable design of functional nanostructures for energy and environmental applications represents a critical yet challenging technology. The existing fabrication strategies focus mainly on increasing the number of accessible active sites. However, these techniques generally necessitate complex chemical agents and suffer from limited experimental conditions delivering high costs, low yields, and poor reproducibility. The present work reports a new strategy for controllable synthesis of a hybrid system including mixed iron oxide nanostructures enriched with non-stoichiometric Fe21.34O32 and Fe3+[Fe5/33+□1/32+]O4 phases, which possess a high concentration of oxygen and Fe2+ vacancies, and a mesoporous carbon-based scaffold (MCS), which was dervied from coffee residues, with graphitic surface and perforated architecture. The nanoperforates acted as trapping sites to localise the FexOy nanoparticles, thereby boosting the density of accessible active sites. Additionally, at the interfacial regions between the FexOy crystallites, a high density of oxygen vacancies with an oriented pattern was shown to create superlattice structures. The energy storage functionality of the defect-rich MCS/FexOy nanostructure with nanoperforated architecture was investigated, where the results exhibited a high gravimetric capacitance of 540 F g-1 at a current density of 1 A g-1 with outstanding capacitance retention of 73.6% after 14 000 cycles.

Advanced materials and technologies for supercapacitors used in energy conversion and storage: a review

Supercapacitors are increasingly used for energy conversion and storage systems in sustainable nanotechnologies. Graphite is a conventional electrode utilized in Li-ion-based batteries, yet its specific capacitance of 372 mA h g−1 is not adequate for supercapacitor applications. Interest in supercapacitors is due to their high-energy capacity, storage for a shorter period and longer lifetime. This review compares the following materials used to fabricate supercapacitors: spinel ferrites, e.g., MFe2O4, MMoO4 and MCo2O4 where M denotes a transition metal ion; perovskite oxides; transition metals sulfides; carbon materials; and conducting polymers. The application window of perovskite can be controlled by cations in sublattice sites. Cations increase the specific capacitance because cations possess large orbital valence electrons which grow the oxygen vacancies. Electrodes made of transition metal sulfides, e.g., ZnCo2S4, display a high specific capacitance of 1269 F g−1, which is four times higher than those of transition metals oxides, e.g., Zn–Co ferrite, of 296 F g−1. This is explained by the low charge-transfer resistance and the high ion diffusion rate of transition metals sulfides. Composites made of magnetic oxides or transition metal sulfides with conducting polymers or carbon materials have the highest capacitance activity and cyclic stability. This is attributed to oxygen and sulfur active sites which foster electrolyte penetration during cycling, and, in turn, create new active sites.

Synthesis of a Co3V2O8/CNx hybrid nanocomposite as an efficient electrode material for supercapacitors

Cobalt vanadium oxide/carbon nitride composite (Co₃V₂O₈/CN_x) was synthesized by solvothermal method. This Co₃V₂O₈/CN_x composite was applied for asymmetric supercapacitor application.

Designing vertically aligned porous NiCo2O4@MnMoO4 Core@Shell nanostructures for high-performance asymmetric supercapacitors

NiCo₂O₄@MnMoO₄ core@shell nanostructures are synthesized as electrode material using hydrothermal method for the fabrication of asymmetric supercapacitor (ASC) device. The NiCo₂O₄@MnMoO₄ electrode shows better electrochemical performance with specific capacitance (SC) of 1821 F/g at current density of 5 A/g and cycling stability of 94%. The NiCo₂O₄@MnMoO₄ core@shell electrode shows better SC compared to pure NiCo₂O₄ and MnMoO₄ electrodes. An ASC device is fabricated using NiCo₂O₄@MnMoO₄ as a positive and rGO/Fe₂O₃ as negative electrode materials. Remarkably, the fabricated device shows a SC of 294 F/g at current density 4 A/g, with an energy density of 91.87 Wh/kg at a power density of 374.15 W/kg. The device shows good reversibility with cycling stability of 68% after 2,000 cycles. The ASC device is used to illuminate nine green color LEDs for 35 min. Therefore, the present report provides a simple method to fabricate efficient and stable energy storage devices for industrial applications.

Iron oxides/graphene hybrid structures - Preparation, modification, and application as fillers of polymer composites

The current status of knowledge regarding magnetic hybrid structures based on graphene or carbon nanotubes with various forms of iron oxides is reviewed. The paper starts with a summary of the preparation and properties of iron oxide nanoparticles, both untreated and coated with silica or polymer layers. In the next section, organic-inorganic hybrid materials obtained as a result of a combination of graphene or carbon nanotubes and iron chemical compounds are characterized and discussed. These hybrids constitute an increasing percentage of all consumable high performance biomedical, electronic, and energy materials due to their valuable properties and low production costs. The potential of their application as components of materials used in corrosion protection, catalysis, spintronics, biomedicine, photoelectrochemical water splitting and groundwater remediation, as well as magnetic nanoparticles in polymer matrices, are also presented. The last part of this review article is focused on reporting the most recent developments in design and the understanding of the properties of polymer composites reinforced with nanometer-sized iron oxide/graphene and iron oxide/carbon nanotubes hybrid fillers. The discussion presents comparative analysis of the magnetic, electromagnetic shielding, electrical, thermal, and mechanical properties of polymer composites with various iron oxide/graphene structures. It is shown that the introduction of hybrid filler nanoparticles into polymer matrices enhances both the macro- and microproperties of final composites as a result of synergistic effects of individual components and the simultaneous formation of an oriented filler network in the polymer. The reinforcing effect is related to the structure and geometry of hybrid nanoparticles applied as a filler, the interactions between the filler particles, their concentration in a composite, and the method of composite processing.

Fabrication of iron oxide-CNT based flexible asymmetric solid state supercapacitor device with high cyclic stability

Integration of high surface area nanostructures with conducting and deformable electrodes at large scale are of significant importance for flexible supercapacitors with high cyclic stability and low cost. Here, we report water assisted meter scale growth of aligned iron oxide and CNT 1D nanostructures on flexible stainless steel mesh for asymmetric supercapacitor device applications. Electron microscopic investigations revealed the uniform coverage of both iron oxide and CNT forest nanostructures over one meter length of SS mesh. Both iron oxide and CNT nanostructures were tested for supercapacitor electrode material in neutral electrolytes. Further, asymmetric solid state devices were fabricated and connected in serial fashion to demonstrate glowing of LEDs as well as rotation of 5 V micro fan. In addition, at bending angle of 90°, device showed 68% increase whereas, at 180° it showed 13% decrease in capacitance. The calculated specific capacitance for single device is found to be 14.4 mF cm^-2. Corresponding energy density and power density are found to be 3 μW-hr cm^-2 and 0.74 mW cm^-2 respectively. The device showed remarkable capacitance retention of 87% over 25 000 charge discharge cycles. The flexible nature with remarkable cyclic stability of solid state iron oxide/CNT device is suitable for low cost flexible and wearable supercapacitor applications.

Polypyrrole-coated Fe2O3 nanotubes constructed from nanoneedles as high-performance anodes for aqueous asymmetric supercapacitors

Asymmetric supercapacitors (ASCs) show promising potential for electrochemical energy storage applications. However, the energy density of ASCs is limited by the poor electrochemical performance of anodes. To achieve high-performance ASCs, herein, Fe₂O₃ nanotubes constructed from Fe₂O₃ nanoneedles were fabricated by employing MnO₂ nanotubes as a self-sacrificing template, and then a layer of polypyrrole (PPy) was coated through an in situ chemical oxidative polymerization method to enhance their performance. The electrochemical tests indicate that the resultant PPy-coated Fe₂O₃ nanotubes (Fe₂O₃@PPy) exhibit a high areal capacitance of 530 mF cm^-2 at 1 mA cm^-2 and good cycling stability, which are superior to those of the Fe₂O₃ nanotubes. The superior performance of the Fe₂O₃@PPy nanotubes can be attributed to the synergistic effect between the PPy shell and Fe₂O₃ core, in which the conducting PPy shell not only works as a superhighway for charge transport, but also stabilizes the Fe₂O₃ nanotubes during charge-discharge processes. When the Fe₂O₃@PPy nanotubes were assembled with MnO₂ nanotubes, the as-assembled ASCs possess a high cell voltage of 2.0 V and deliver a high energy density of up to 51.2 Wh kg^-1 at a power density of 285.4 W kg^-1 with an excellent cycling stability (83.5% capacitance retention over 5000 cycles).

Iron oxides nanobelt arrays rooted in nanoporous surface of carbon tube textile as stretchable and robust electrodes for flexible supercapacitors with ultrahigh areal energy density and remarkable cycling-stability

We report a significant advance toward the rational design and fabrication of stretchable and robust flexible electrodes with favorable hierarchical architectures constructed by homogeneously distributed α-Fe₂O₃ nanobelt arrays rooted in the surface layer of nanoporous carbon tube textile (NPCTT). New insight into alkali activation assisted surface etching of carbon and in-situ catalytic anisotropic growth is proposed, and is experimentally demonstrated by the synthesis of the Fe₂O₃ nanobelt arrays/NPCTT. The Fe₂O₃/NPCTT electrode shows excellent flexibility and great stretchability, especially has a high specific areal capacitance of 1846 mF cm⁻² at 1 mA cm⁻² and cycling stability with only 4.8% capacitance loss over 10,000 cycles at a high current density of 20 mA cm⁻². A symmetric solid-state supercapacitor with the Fe₂O₃/NPCTT achieves an operating voltage of 1.75 V and a ultrahigh areal energy density of 176 µWh cm⁻² (at power density of 748 µW cm⁻²), remarkable cycling stability, and outstanding reliability with no capacity degradation under repeated large-angle twisting. Such unique architecture improves both mechanical robustness and electrical conductivity, and allows a strong synergistic attribution of Fe₂O₃ and NPCTT. The synthetic method can be extended to other composites such as MnO nanosheet arrays/NPCTT and Co₃O₄ nanowire arrays/NPCTT. This work opens up a new pathway to the design of high-performance devices for wearable electronics.

Carbon Transition-metal Oxide Electrodes: Understanding the Role of Surface Engineering for High Energy Density Supercapacitors

Supercapacitors store electrical energy by ion adsorption at the interface of the electrode-electrolyte (electric double layer capacitance, EDLC) or through faradaic process involving direct transfer of electrons via oxidation/reduction reactions at one electrode to the other (pseudocapacitance). The present minireview describes the recent developments and progress of carbon-transition metal oxides (C-TMO) hybrid materials that show great promise as an efficient electrode towards supercapacitors among various material types. The review describes the synthetic methods and electrode preparation techniques along with the changes in the physical and chemical properties of each component in the hybrid materials. The critical factors in deriving both EDLC and pseudocapacitance storage mechanisms are also identified in the hope of pointing to the successful hybrid design principles. For example, a robust carbon-metal oxide interaction was identified as most important in facilitating the charge transfer process and activating high energy storage mechanism, and thus methodologies to establish a strong carbon-metal oxide contact are discussed. Finally, this article concludes with suggestions for the future development of the fabrication of high-performance C-TMO hybrid supercapacitor electrodes.

Magnetite ultrafine particles/porous reduced graphene oxide in situ grown onto Ni foam as a binder-free electrode for supercapacitors

Here, we report a simple and green electrochemical route to fabricate a porous network of a Fe3O4 nanoparticle-porous reduced graphene oxide (p-rGO) nanocomposite supported on a nickel-foam substrate, which is directly used as a binder-free charge storage electrode. Through this method, pristine Fe3O4 NPs/Ni, p-rGO/Ni and Fe3O4 NPs@p-rGO/Ni electrodes are fabricated and compared. In the fabricated Fe3O4 NPs@p-rGO/Ni electrode, the porous rGO sheets served as a conductive network to facilitate the collection and transportation of electrons during the charge/discharge cycles, improving the conductivity of magnetite NPs and providing a larger specific surface area. As a result, the Fe3O4 NPs@p-rGO/Ni exhibited a specific capacitance of 1323 F g-1 at 0.5 A g-1 and 79% capacitance retention when the current density is increased 20 times, where the Fe3O4 NPs/Ni electrode showed low specific capacitance of 357 F g-1 and 43% capacity retention. Furthermore, the composite electrode kept 95.1% and 86.7% of its initial capacitances at the current densities of 1 and 4 A g-1, respectively, which were higher than those of a Fe3O4/NF electrode at similar loads (i.e. 80.4% and 65.9% capacitance retentions at 1 and 4 A g-1, respectively). These beneficial effects proved the synergistic contribution between p-rGO and Fe3O4. Hence, such ultrafine magnetite particles grown onto a porous reduced GO network directly imprinted onto a Ni substrate could be a promising candidate for high performance energy storage aims.

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.