α-Fe2O3 nanotubes-reduced graphene oxide composites as synergistic electrochemical capacitor materials

Effect of a solvothermal method using DMF on the dispersibility of rGO, application of rGO as a CDI electrode material, and recovery of sp2-hybridized carbon

Graphene is prized for its large surface area and superior electrical properties. Efforts to maximize the electrical conductivity of graphene commonly result in the recovery of sp2-hybridized carbon in the form of reduced graphene oxide (rGO). However, rGO shows poor dispersibility and aggregation when mixed with other materials without hydrophilic functional groups, This could lead to electrode delamination, agglomeration, and reduced efficiency. This study focuses on the impact of solvothermal reduction on the dispersibility and capacitance of rGO compared with chemical reduction. The results show that the dispersibility of rGO-D obtained through solvothermal reduction using N,N-dimethylformamide improved compared to that obtained through chemical reduction (rGO-H). Furthermore, when utilized as a material for CDI, an improvement in deionization efficiency was observed in the AC@rGO-D-based CDI system compared to AC@rGO-H and AC. However, the specific surface area, a key factor affecting CDI efficiency, was higher in rGO-H (249.572 m2 g-1) than in rGO-D (150.661 m2 g-1). While AC@rGO-H is expected to exhibit higher deionization efficiency due to its greater specific surface area, the opposite was observed. This highlights the effect of the improved dispersibility of rGO-D and underscores its potential as a valuable material for CDI applications.

Stainless Steel Activation for Efficient Alkaline Oxygen Evolution in Advanced Electrolyzers

Designing robust and cost-effective electrocatalysts for efficient alkaline oxygen evolution reaction (OER) is of great significance in the field of water electrolysis. In this study, an electrochemical strategy to activate stainless steel (SS) electrodes for efficient OER is introduced. By cycling the SS electrode within a potential window that encompasses the Fe(II)↔Fe(III) process, its OER activity can be enhanced to a great extent compared to using a potential window that excludes this redox reaction, decreasing the overpotential at current density of 100 mA cm-2 by 40 mV. Electrochemical characterization, Inductively Coupled Plasma - Optical Emission Spectroscopy, and operando Raman measurements demonstrate that the Fe leaching at the SS surface can be accelerated through a Fe → γ-Fe2O3 → Fe3O4 or FeO → Fe2+ (aq.) conversion process, leading to the sustained exposure of Cr and Ni species. While Cr leaching occurs during its oxidation process, Ni species display higher resistance to leaching and gradually accumulate on the SS surface in the form of OER-active Fe-incorporated NiOOH species. Furthermore, a potential-pulse strategy is also introduced to regenerate the OER-activity of 316-type SS for stable OER, both in the three-electrode configuration (without performance decay after 300 h at 350 mA cm-2) and in an alkaline water electrolyzer (≈30 mV cell voltage increase after accelerated stress test-AST). The AST-stabilized cell can still reach 1000 and 4000 mA cm-2 at cell voltages of 1.69 and 2.1 V, which makes it competitive with state-of-the-art electrolyzers based on ion-exchange membrane using Ir-based anodes.

Preparation of Electrodes with β-Nickel Hydroxide/CVD-Graphene/3D-Nickel Foam Composite Structures to Enhance the Capacitance Characteristics of Supercapacitors

Supercapacitors have the characteristics of high power density, long cycle life, and fast charge and discharge rates, making them promising alternatives to traditional capacitors and batteries. The use of transition-metal compounds as electrode materials for supercapacitors has been a compelling research topic in recent years because their use can effectively enhance the electrical performance of supercapacitors. The current research on capacitor electrode materials can mainly be divided into the following three categories: carbon-based materials, metal oxides, and conductive polymers. Nickel hydroxide (Ni(OH)2) is a potential electrode material for use in supercapacitors. Depending on the preparation conditions, two crystal phases of nickel hydroxide, α and β, can be produced. When compared to α-NiOH, the structure of β-Ni(OH)2 does not experience ion intercalation. As a result, the carrier transmission rate of α-Ni(OH)2 is slower, and its specific capacitance value is smaller. Its carrier transport rate can be improved by adding conductive materials, such as graphene. β-Ni(OH)2 was chosen as an electrode material for a supercapacitor in this study. Homemade low-pressure chemical vapor deposition graphene (LPCVD-Graphene) conductive material was introduced to modify β-Ni(OH)2 in order to increase its carrier transport rate. The LPCVD method was used to grow high-quality graphene films on three-dimensional (3D) nickel foam substrates. Then, a hydrothermal synthesis method was used to grow β-Ni(OH)2 nanostructures on the 3D graphene/nickel foam substrate. In order to improve the electrical properties of the composite structure, a high-quality graphene layer was incorporated between the nickel hydroxide and the 3D nickel foam substrate. The effect of the conductive graphene layer on the growth of β-Ni(OH)2, as well as its electrical properties and electrochemical performance, was studied. When this β-Ni(OH)2/CVD-Graphene/3D-NF (nickel foam) material was used as the working electrodes of the supercapacitor under a current density of 1 A/g and 3 A/g, they exhibited a specific capacitance of 2015 F/g and 1218.9 F/g, respectively. This capacitance value is 2.62 times higher than that of the structure without modification with a graphene layer. The capacitance value remains at 99.2% even after 1000 consecutive charge and discharge cycles at a current density of 20 A/g. This value also improved compared to the structure without graphene layer modification (94.7%).

Fabrication of Porous Reduced Graphene Oxide Encapsulated Cu(OH)2 Core-shell Structured Carbon Fiber-Based Electrodes for High-Performance Flexible Supercapacitors

To explore next-generation flexible supercapacitors, lightweight, superior conductivity, low cost, and excellent capacitance are the preconditions for practical use. However, subjected to unsatisfactory conductivity, limited surface areas, and poor porosity leading to long ion transport channels, carbon fiber (CF)-based flexible supercapacitors need to further boost the electrochemical properties. Hence, a porous reduced graphene oxide encapsulated Cu(OH)2 core-shell structured CF-based electrode was fabricated through a scalable approach. The inexpensive Cu(OH)2 nanoarrays were controllably grown in situ on a CF substrate, with residual Cu promoting conductivity. Porous graphene oxide (PrGO), which served as the shell, was realized by Ni nanoparticle etching, which not only provided more active sites for capacitance as well as shortened accessible pathways for the ion transport but also effectively alleviated the exfoliation of the internal active materials. Moreover, thanks to this distinctive core-shell architecture, the extra space between the outer PrGO layer and the internal ordered Cu(OH)2 nanoarrays provided increased space for capacitance storage. The assembled PrGO/Cu(OH)2/Cu@CF electrode exhibited an excellent areal capacitance, reaching up to 722 mF cm-2 at a current density of 0.5 mA cm-2, attributed to its superior structure and materials advantages. The resulting PrGO/Cu(OH)2/Cu@CF//AC//CF asymmetric flexible all-solid-state supercapacitor achieved a high energy density of 0.052 mWh cm-2 and exhibited long-term durability. This work proposes a low-cost and effective way to fabricate hierarchically structured electrodes for wearable CF-based supercapacitors.

Fishnet-like double active layer-loaded carbon fiber for electrical double-layer capacitors

The rational design and controllable synthesis of high-performance energy storage materials are important measures to address the growing demand for energy storage devices. This work involves the growth of a fishnet-like Fe₂O₃ nanorod@oxygen-rich carbon layer structure directly onto carbon fiber cloth as a binder-free electrode for symmetric capacitors. The growth of Fe₂O₃ nanorods provided a large specific surface area, and the coating of an oxygen-rich carbon layer protected the Fe₂O₃ nanorods as an active substance. Furthermore, oxidation treatment created rich electrochemically active sites by loading oxygen-containing functional groups onto the composite surface. As a result, the optimal OC@Fe₂O₃-ACC sample exhibited a high areal specific capacitance of 1687 mF cm^-2 at a current density of 1 mA cm^-2. Meanwhile, an excellent capacity retention rate of 58.7% was achieved at 15 mA cm^-2. Finally, the long-term cycling stability was verified with an 80% retention rate of the initial capacitance after 12 000 cycles.

Construction of trifunctional electrode material based on Pt-Coordinated Ce-Based metal organic framework

The main challenge hindering the use of Pt nanoparticles (Pt NPs) for electrochemical applications is their high cost and agglomeration. Herein, a trifunctional electrode material based on a two-dimensional cerium-based metal organic framework (2D Ce-MOF) decorated with Pt NPs is constructed. The large specific surface area of the 2D Ce-MOF can effectively prevent the phenomenon of Pt NPs reaction. The strong synergy between Pt NPs and the 2D Ce-MOF not only significantly enhances electron transport efficiency, but also increases the number of electrochemically reaction reactive sites. As a result, the Ce-MOF@Pt presents excellent performance in the HER (Hydrogen Evolution Reaction), OER (Oxygen Evolution Reaction) and supercapacitor reactions. The Tafel slopes of OER and HER are 47.9 and 188.1 mV dec^-1, respectively. Meanwhile, Ce-MOF@Pt-0.05 shows a specific capacity of 1894F g^-1 at a current density of 1 A g^-1 and remains at 111.5% of the initial capacitance after 3000 cycles. In general, this study highlights the importance of Pt NPs in promoting the electrochemical performance of MOFs and reveals a new way to reduce electrocatalyst prices.

Chemically Synthesized Iron-Oxide-Based Pure Negative Electrode for Solid-State Asymmetric Supercapacitor Devices

Among energy storage devices, supercapacitors have received considerable attention in recent years owing to their high-power density and extended cycle life. Researchers are currently making efforts to improve energy density using different asymmetric cell configurations, which may provide a wider potential window. Many studies have been conducted on positive electrodes for asymmetric supercapacitor devices; however, studies on negative electrodes have been limited. In this study, iron oxides with different morphologies were synthesized at various deposition temperatures using a simple chemical bath deposition method. A nanosphere-like morphology was obtained for α-Fe₂O₃. The obtained specific capacitance (C_s) of α-Fe₂O₃ was 2021 F/g at a current density of 4 A/g. The negative electrode showed an excellent capacitance retention of 96% over 5000 CV cycles. The fabricated asymmetric solid-state supercapacitor device based on α-Fe₂O₃-NF//Co₃O₄-NF exhibited a C_s of 155 F/g and an energy density of 21 Wh/kg at 4 A/g.

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.

Prospects of Using Pseudobrookite as an Iron-Bearing Mineral for the Alkaline Electrolytic Production of Iron

The alkaline electrolytic production of iron is gaining interest due to the absence of CO2 emissions and significantly lower electrical energy consumption when compared with traditional steelmaking. The possibility of using an iron-bearing pseudobrookite mineral, Fe2TiO5, is explored for the first time as an alternative feedstock for the electrochemical reduction process. To assess relevant impacts of the presence of titanium, similar electroreduction processes were also performed for Fe2TiO5·Fe2O3 and Fe2O3. The electroreduction was attempted using dense and porous ceramic cathodes. Potentiostatic studies at the cathodic potentials of -1.15--1.30 V vs. an Hg|HgO|NaOH reference electrode and a galvanostatic approach at 1 A/cm2 were used together with electroreduction from ceramic suspensions, obtained by grinding the porous ceramics. The complete electroreduction to Fe0 was only possible at high cathodic polarizations (-1.30 V), compromising the current efficiencies of the electrochemical process due to the hydrogen evolution reaction impact. Microstructural evolution and phase composition studies are discussed, providing trends on the role of titanium and corresponding electrochemical mechanisms. Although the obtained results suggest that pseudobrookite is not a feasible material to be used alone as feedstock for the electrolytic iron production, it can be considered with other iron oxide materials and/or ores to promote electroreduction.

Three-dimensional carbon foam-metal oxide-based asymmetric electrodes for high-performance solid-state micro-supercapacitors

A three-dimensional carbon foam (CF)-based asymmetric planar micro-supercapacitor is fabricated by the direct spray patterning of active materials on an array of interdigital electrodes. The solid-state asymmetric micro-supercapacitor comprises the CF network with pseudocapacitive metal oxides (manganese oxide (MnO), iron oxide (Fe₂O₃)), where CF-MnO composite as a positive electrode, and CF-Fe₂O₃ as negative electrode for superior electrochemical performance. The micro-supercapacitor, CF-MnO//CF-Fe₂O₃, attains an ultrahigh supercapacitance of 18.4 mF cm^-2 (2326.8 mF cm^-3) at a scan rate of 5 mV s^-1. A wider potential window of 1.4 V is achieved with a high energy density of 5 μW h cm^-2. The excellent cyclic stability is confirmed by 86.1% capacitance retention after 10 000 electrochemical cycles.

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

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

Ultralight Flexible Electrodes of Nitrogen-Doped Carbon Macrotube Sponges for High-Performance Supercapacitors

Light-weight and flexible supercapacitors with outstanding electrochemical performances are strongly desired in portable and wearable electronics. Here, ultralight nitrogen-doped carbon macrotube (N-CMT) sponges with 3D interconnected macroporous structures are fabricated and used as substrate to grow nickel ferrite (NiFe₂ O₄ ) nanoparticles by vapor diffusion-precipitation and in situ growth. This process effectively suppresses the agglomeration of NiFe₂ O₄ , enabling good interfacial contact between N-CMT sponges and NiFe₂ O₄ . More remarkably, the as-synthesized NiFe₂ O₄ /N-CMT composite sponges can be directly used as electrodes without additional processing that could cause agglomeration and reduction of active sites. Benefiting from the tubular structure and the synergetic effect of NiFe₂ O₄ and N-CMT, the NiFe₂ O₄ /N-CMT-2 exhibits a high specific capacitance of 715.4 F g^-1 at a current density of 1 A g^-1 , and 508.3 F g^-1 at 10 A g^-1 , with 90.9% of capacitance retention after 50 000 cycles at 1 A g^-1 in an alkaline electrolyte. Furthermore, flexible supercapacitors are fabricated, yielding areal specific capacitances of 1397.4 and 1041.2 mF cm^-2 at 0.5 and 8 mA cm^-2 , respectively. They also exhibit exceptional cycling performance with capacitance retention of 92.9% at 1 mA cm^-2 after 10 000 cycles under bending. This work paves a new way to develop flexible, light-weight, and high-performance energy storage devices.

An efficient and stable coral-like CoFeS2 for wearable flexible all-solid-state asymmetric supercapacitor applications

CoFeS2 was synthesized by an organic solvothermal method as a positive electrode for flexible asymmetric supercapacitors, which showed high specific capacitance and good stability.

The α-Fe2O3/graphite anode composites with enhanced electrochemical performance for lithium-ion batteries

The α-Fe₂O₃/graphite composites were prepared by a thermal decomposition method using the expanded graphite as the matrix. The α-Fe₂O₃ nanoparticles with the size of 15-30 nm were embedded into interlayers of graphite, forming a laminated porous nanostructure with a main pore distribution from 2 to 20 nm and the Brunauer-Emmett-Teller surface area of 33.54 m² g^-1. The porous structure constructed by the graphite sheets can alleviate the adverse effects caused by the huge volume change of the α-Fe₂O₃ grains during the charge/discharge process. The composite electrode exhibits a high reversible capacity of 1588 mAh g^-1 after 100 cycles at 100 mA g^-1, 702 mAh g^-1 at 5 A g^-1, 460 mAh g^-1 at 10 A g^-1 after 160 cycles, respectively, showing good cycle stability and outstanding rate capability at high current densities.

Facile Green Synthesis of Pseudocapacitance-Contributed Ultrahigh Capacity Fe2(MoO4)3 as an Anode for Lithium-Ion Batteries

The investigation into the use of earth-abundant elements as electrode materials for lithium-ion batteries (LIBs) is becoming more urgent because of the high demand for electric vehicles and portable devices. Herein, a new green synthesis strategy, based on a facile solid-state reaction with the assistance of water droplets' vapor, was conducted to prepare Fe₂(MoO₄)₃ nanosheets as anode materials for LIBs. The obtained sample possesses a two-dimensional stacked nanosheet construction with open gaps providing a much higher surface area compared to the bulk sample conventionally synthesized. The nanosheet sample delivers an ultrahigh reversible capacity (1983.6 mA h g^-1) at a current density of 100 mA g^-1 after 400 cycles, which could be related to the contribution of pseudocapacitance. The enhancement in cyclability and rated performance with an interesting increased capacity could be caused by the effect of electrochemical milling and the in situ formation of metallic particles in its lithium-ion storage mechanism.

Liquid-Phase Synthesis of Iron Oxide Nanostructured Materials and Their Applications

Owing to their high natural abundance, low cost, easy availability, and excellent magnetic properties, considerable interest has been devoted to the synthesis and applications of iron oxide nanostructured materials. Liquid-phase synthesis methods are economical and environmentally friendly with low energy consumption and volatile emissions, and as such have received much attention for the preparation of iron oxide nanostructured materials. Herein, the liquid-phase synthesis methods of iron oxide nanostructured materials including the co-precipitation method, microemulsion method, conventional hydrothermal and solvothermal methods, microwave-assisted heating method, sonolysis method, and other methods are summarized and reviewed. Many iron oxide nanostructured materials, self-assembled nanostructures, and nanocomposites have been successfully prepared, which are of great significance to enhance their structure-dependent properties and applications. The specific roles of liquid-phase chemical reaction parameters in regulating the chemical composition, structure, crystallinity, morphology, particle size, and dispersive behavior of the as-prepared iron oxide nanostructured materials are emphasized. The biomedical, environmental, and electrochemical energy storage applications of iron oxide nanostructured materials are discussed. Finally, challenges and perspectives are proposed for future investigations on the liquid-phase synthesis and applications of iron oxide nanostructured materials.

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.

A review of the α-Fe2O3 (hematite) nanotube structure: recent advances in synthesis, characterization, and applications

α-Fe2O3 nanotubes are exceptional one-dimensional transition metal oxide materials with low density, large surface area, promising electrochemical and photoelectrochemical properties, which are widely investigated in lithium-ion batteries, photoelectrochemical devices, gas sensors, and catalysis. They have drawn significant attention to the fields of energy storage and conversion, and environmental sensing and remediation due to the increase in the global energy crisis and environmental pollution. Many efforts have been made toward controlling the morphology or impurity doping to improve the intrinsic properties of α-Fe2O3 nanotubes. In this review, we introduce the synthesis methods and physicochemical properties of α-Fe2O3 nanotubes. The fabrication conditions, which are important for the physicochemical properties of materials, are also listed to describe the synthesis processes. Furthermore, the development and breakthrough of various applications in batteries, supercapacitors, photoelectrochemical devices, environmental remediation, and sensors are systematically reviewed. Finally, some of the current challenges and future perspectives for α-Fe2O3 nanotubes are discussed. We believe that this timely and critical mini-review will stimulate extensive studies and attract more attention, further improving the development of the α-Fe2O3 (hematite) nanotube structure.

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.

Structural Reorganization-Based Nanomaterials as Anodes for Lithium-Ion Batteries: Design, Preparation, and Performance

In recent years, with the growing demand for higher capacity, longer cycling life, and higher power and energy density of lithium ion batteries (LIBs), the traditional insertion-based anodes are increasingly considered out of their depth. Herein, attention is paid to the structural reorganization electrode, which is the general term for conversion-based and alloying-based materials according to their common characteristics during the lithiation/delithiation process. This Review summarizes the recent achievements in improving and understanding the lithium storage performance of conversion-based anodes (especially the most widely studied transition metal oxides like Mn-, Fe-, Co-, Ni-, and Cu-based oxides) and alloying-based anodes (mainly including Si-, Sn-, Ge-, and Sb-based materials). The synthesis schemes, morphological control and reaction mechanism of these materials are also included. Finally, viewpoints about the challenges and feasible improvement measures for future development in this direction are given. The aim of this Review is to shed some light on future electrode design trends of structural reorganization anode materials for LIBs.

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.

Three-Dimensional Core-Branch α-Fe2O3@NiO/Carbon Cloth Heterostructured Electrodes for Flexible Supercapacitors

A convenient and scalable hydrothermal method was developed for the fabrication of the core-branch Fe₂O₃@NiO nanorods arrays directly grown on flexible carbon cloth (denoted as Fe₂O₃@NiO/CC). Such a unique architecture was applied as an electrode of the supercapacitors. As a result, the Fe₂O₃@NiO/CC exhibited a high areal capacitance ~800 mF cm^-2 at 10 mA cm^-2, which was about 10 times increase with respect to Fe₂O₃ nanorods array grown on carbon cloth (Fe₂O₃/CC). The Fe₂O₃@NiO/CC also had the long life cycle (96.8 % capacitance retention after 16,000 cycles) and remarkable rate capability (44.0 % capacitance loss at a very large current density of 100 mA cm^-2). The superior performance of the Fe₂O₃@NiO/CC should be ascribed to the reduction of the contact resistance and the free-standing structure of the flexible electrode. This study provides a novel strategy to construct high-performance flexible electrode materials with unique core-branch structure by incorporating two different pseudocapacitive materials.

Hollow nanostructures of metal oxides as emerging electrode materials for high performance supercapacitors

Comparative study of TMO based hollow and solid nanostructures for supercapacitor applications.

Interface metallization enabled an ultra-stable Fe2O3 hierarchical anode for pseudocapacitors

Despite significant advances in cathode materials, developing high-performance anodes remains a key challenge for future pseudocapacitors. Fe₂O₃ has been considered as a promising anode candidate due to its high theoretical capacitance, environmental benignity, and earth-abundant characteristics. However, the low electronic conductivity and poor cyclability of Fe₂O₃ significantly limit its practical application. In this work, a 3D nickel-metalized carbon nanofiber network was developed to deposit an Fe₂O₃ nanosheet anode. The nickel layer not only improved the electronic conductivity and the wettability of the 3D carbon substrate but also benefit the stability of the Fe₂O₃/carbon interfaces and the stress-release upon cycling. As a result, the newly designed Fe₂O₃ anode composite exhibited a high areal capacitance of 1.80 F cm⁻² at a high mass loading of 4.2 mg cm⁻¹ and ultra-high capacitance retention of 85.1% after successive 100 000 cycles, outperformed most of the reported Fe₂O₃-based anode materials. Extended the interface metallization method to a MnO₂ cathode, excellent capacitance retention of 108.2% was reached after 26 000 cycles, suggesting a potentially broad application of such an interface-management method in elevating the stability of metal oxide materials in various pseudocapacitive energy storage devices.

An ultra-stable Fe₂O₃ anode composite was developed by nickel metallizing the interfaces between the Fe₂O₃ nanosheets and the carbon nanofiber substrate. High capacitance retention of 85.1% after 100 000 cycles was reached under a high mass loading of 4.2 mg cm⁻¹.

Porous Fe2O3 Modified by Nitrogen-Doped Carbon Quantum Dots/Reduced Graphene Oxide Composite Aerogel as a High-Capacity and High-Rate Anode Material for Alkaline Aqueous Batteries

Carbon quantum dots (CQDs) as novel types of emerging materials have aroused tremendous attention in recent years. Herein, we report for the first time a new application of 3D CQD-based composite aerogels as excellent electrode materials for alkaline aqueous batteries. The scalable graphitic CQDs are prepared with high yields (>40%) and further utilized to fabricate the novel nitrogen-doped CQDs/reduced graphene oxide/porous Fe₂O₃ (N-CQDs/rGO/Fe₂O₃) composite aerogels with different contents of Fe₂O₃. Benefiting from the unique 3D network composite aerogel structure with a high surface area and hierarchical porous structure as well as the synergistic effect of high-capacity Fe₂O₃ and highly conductive and stable N-CQDs/rGO, the composite aerogels achieve enhanced electrochemical properties with ultrahigh specific capacity, admirable rate property, and superior cycling performance. Furthermore, the N-CQDs/rGO/Fe₂O₃-1 electrode (Fe₂O₃, 34.9 wt %) exhibits the best rate capability (72.1, 58.9, and 46.2% capacity retention at 5, 50, and 100 A g^-1, respectively) and cycle performance (80.4% capacity retention at 3 A g^-1 over 5000 cycles), while the N-CQDs/rGO/Fe₂O₃-3 electrode (Fe₂O₃, 62.3 wt %) displays the highest specific capacity (274.1 mA h g^-1 at 1 A g^-1). The current research provides a valuable guidance for developing high-performance 3D CQD-based composite aerogels for application in energy storage systems.

Oxygen vacancy-engineered Fe2O3 nanoarrays as free-standing electrodes for flexible asymmetric supercapacitors

The charge storage performance of Fe₂O₃ nanoarrays (NAs) as negative electrodes are limited by their poor conductivity and rate capability. Herein, we have reported the delicate interfacial engineering on carbon cloth (CC) fibers and oxygen vacancy (V_O) generation on Fe₂O₃ nanorod arrays to boost the capacitive performance. Polydopamine-derived nitrogen-doped carbon layers were fabricated on CC fibers to govern the growth of FeOOH NAs. Rich V_Os were generated in Fe₂O₃ NAs to construct a unique heterostructure with a crystalline core and amorphous shell via successive N₂ thermal treatment and chemical reduction. Optimized by 2 h chemical reduction, the V_O-rich Fe₂O₃ NA electrode, featuring a charged voltage of -1.10 V, exhibited a high areal specific capacitance of 2.63 F cm^-2 at 0.5 mA cm^-2 and 0.12 F cm^-2 even at 60 mA cm^-2. Impressively, 86.7% specific capacitance was retained after 10 000 cycles at 10 mA cm^-2. The flexible asymmetric supercapacitor by assembling free-standing CN-Fe₂O₃-2 h (negative electrode) and MnO₂ (positive electrode) showed an energy density of 1.33 mW h cm^-3 at 15.4 mW cm^-3. To the best of our knowledge, these results are the record performance for Fe₂O₃-based electrodes. The two-step interfacial engineering reported in this study may open a new door in the design of high energy-density electrodes for advanced energy storage.

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.

Amorphous FeOOH nanorods and Co3O4 nanoflakes as binder-free electrodes for high-performance all-solid-state asymmetric supercapacitors

We have built a high-performance supercapacitor anode based on carbon fabric (CF), vertically aligned graphene nanosheets (VAGN) and FeOOH nanorods.

Micrometer-Sized Porous Fe2 N/C Bulk for High-Areal-Capacity and Stable Lithium Storage

High-capacity anodes of lithium-ion batteries generally suffer from poor electrical conductivity, large volume variation, and low tap density caused by prepared nanostructures, which make it an obstacle to achieve both high-areal capacity and stable cycling performance for practical applications. Herein, micrometer-sized porous Fe₂ N/C bulk is prepared to tackle the aforementioned issues, and thus realize both high-areal capacity and stable cycling performance at high mass loading. The porous structure in Fe₂ N/C bulk is beneficial to alleviate the volumetric change. In addition, the N-doped carbon conducting networks with high electrical conductivity provide a fast charge transfer pathway. Meanwhile, the micrometer-sized Fe₂ N/C bulk exhibits a higher tap density than that of commercial graphite powder (1.03 g cm^-3 ), which facilitates the preparation of thinner electrode at high mass loadings. As a result, a high-areal capacity of above 4.2 mA h cm^-2 at 0.45 mA cm^-2 is obtained at a high mass loading of 7.0 mg cm^-2 for LIBs, which still maintains at 2.59 mA h cm^-2 after 200 cycles with a capacity retention of 98.8% at 0.89 mA cm^-2 .

In situ growth of heterostructured Sn/SnO nanospheres embedded in crumpled graphene as an anode material for lithium ion batteries

Heterostructured Sn/SnO core–shell nanospheres embedded in graphene have been prepared by heating tin oleate coated on the surface of sodium carbonate crystals, and they exhibit excellent electrochemical performance for LIBs.