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

Ultra-High Temperature Calcination of Crystalline α-Fe2O3 and Its Nonlinear Optical Properties for Ultrafast Photonics

As a typical transition metal oxide, α-Fe2O3 has garnered significant attention due to its advantages in nonlinear optical applications, such as strong third-order nonlinearity and fast carrier recovery time. To delve into the nonlinear optical properties of α-Fe2O3, crystalline α-Fe2O3 materials with different microstructures are prepared. The nonlinear optical features of α-Fe2O3 calcined at the previously unexplored ultra-high temperature of >1100°C are emphasized. It is found that α-Fe2O3 exposed to ultra-high temperatures undergoes the phase transition, leading to the formation of Fe3O4. Subsequently, the nonlinear absorption coefficient is measured as -0.6280 cm GW-1 at 1.5 µm. The modulation depth and saturation intensity for the Fe2O3-based saturable absorber at 1.5 µm are 4.20% and 13.94 MW cm-2, respectively. Ultimately, the incorporation of the Fe2O3-based saturable absorber into an Er-doped fiber laser cavity resulted in the achievement of both conventional soliton mode-locking operation with a central wavelength of 1560.3 nm and a pulse duration of 1.13 ps, as well as the dissipative soliton resonance mode-locking operation with a central wavelength near 1564.0 nm. Overall, the phase transition and the nonlinear optical features in iron oxides under ultra-high temperatures are revealed, indicating the great potential in advanced ultrafast photonic applications.

Oxygen vacancies enhancing hierarchical NiCo2S4@MnO2 electrode for flexible asymmetric supercapacitors

The limited energy density of supercapacitors hampers their widespread application in electronic devices. Metal oxides, employed as electrode materials, suffer from low conductivity and stability, prompting extensive research in recent years to enhance their electrochemical properties. Among these efforts, the construction of core-shell heterostructures and the utilization of oxygen vacancy (VO) engineering have emerged as pivotal strategies for improving material stability and ion diffusion rates. Herein, core-shell composites comprising NiCo2S4 nanospheres and MnO2 nanosheets are grown in situ on carbon cloth (CC), forming nanoflower clusters while introducing VO defects through a chemical reduction method. Density functional theory (DFT) results proves that the existence of VO effectively enhances electronic and structural properties of MnO2, thereby enhancing capacitive properties. The electrochemical test results show that NiCo2S4@MnO2-V3 exhibits excellent 1376 F g-1 mass capacitance and 2.06 F cm-2 area capacitance at 1 A g-1. Moreover, NiCo2S4@MnO2-V3//activated carbon (AC) asymmetric supercapacitor (ASC) can achieve an energy density of 39.7 Wh kg-1 at a power density of 775 W kg-1, and maintains 15.5 Wh kg-1 even at 7749.77 W kg-1. Capacitance retention is 73.1 % after 10,000 cycles at 5 A g-1, and coulombic efficiency reaches 100 %, demonstrating satisfactory cycle stability. In addition, the device's excellent flexibility offers broad application prospects in wearable electronic applications.

Binder-Free MOF-Based and MOF-Derived Nanoarrays for Flexible Electrochemical Energy Storage: Progress and Perspectives

The fast development of Internet of Things and the rapid advent of next-generation versatile wearable electronics require cost-effective and highly-efficient electroactive materials for flexible electrochemical energy storage devices. Among various electroactive materials, binder-free nanostructured arrays have attracted widespread attention. Featured with growing on a conductive and flexible substrate without using inactive and insulating binders, binder-free 3D nanoarray electrodes facilitate fast electron/ion transportation and rapid reaction kinetics with more exposed active sites, maintain structure integrity of electrodes even under bending or twisted conditions, readily release generated joule heat during charge/discharge cycles and achieve enhanced gravimetric capacity of the whole device. Binder-free metal-organic framework (MOF) nanoarrays and/or MOF-derived nanoarrays with high surface area and unique porous structure have emerged with great potential in energy storage field and been extensively exploited in recent years. In this review, common substrates used for binder-free nanoarrays are compared and discussed. Various MOF-based and MOF-derived nanoarrays, including metal oxides, sulfides, selenides, nitrides, phosphides and nitrogen-doped carbons, are surveyed and their electrochemical performance along with their applications in flexible energy storage are analyzed and overviewed. In addition, key technical issues and outlooks on future development of MOF-based and MOF-derived nanoarrays toward flexible energy storage are also offered.

Oxygen Vacancy Engineering of FeOx toward Oxygen-Tolerant Hydrogen Peroxide Reduction for Reliable Bioassays

The accurate determination of hydrogen peroxide (H2O2), an important clinical disease relevant biomarker, is of great importance for the diagnosis and management of illnesses. By using the cathodic monitoring approach, H2O2 can be accurately detected because interfering signals from easily oxidizable endogenous and exogenous species in biofluids can be avoided. However, the simultaneous occurrence of the oxygen reduction reaction (ORR) restricts the practical use of this cathodic method. In this study, via oxygen vacancy modulation, we synthesized FeOx catalysts that can selectively reduce H2O2 over O2. The H2O2 detection system based on this catalyst exhibits an outstanding ORR inhibition ability. Furthermore, by integrating this catalyst with glucose oxidase, a model enzyme, a reliable bioassay system was developed that can selectively detect glucose over a wide variety of interferents in artificially simulated tissue fluids. The bioassay system employing this catalyst in conjunction with oxidases is generally applicable to accurate detect a wide range of biomarkers.

Constructing multiple active sites in iron oxide catalysts for improving carbonylation reactions

Surface engineering is a promising strategy to improve the catalytic activities of heterogeneous catalysts. Nevertheless, few studies have been devoted to investigate the catalytic behavior differences of the multiple metal active sites triggered by the surface imperfections on catalysis. Herein, oxygen vacancies induced Fe2O3 catalyst are demonstrated with different Fe sites around one oxygen vacancy and exhibited significant catalytic performance for the carbonylation of various aryl halides and amines/alcohols with CO. The developed catalytic system displays excellent activity, selectivity, and reusability for the synthesis of carbonylated chemicals, including drugs and chiral molecules, via aminocarbonylation and alkoxycarbonylation. Combined characterizations disclose the formation of oxygen vacancies. Control experiments and density functional theory calculations demonstrate the selective combination of the three Fe sites is vital to improve the catalytic performance by catalyzing the elemental steps of PhI activation, CO insertion and C-N/C-O coupling respectively, endowing combinatorial sites catalyst for multistep reactions.

Vacancy designed 2D materials for electrodes in energy storage devices

Vacancies are ubiquitous in nature, usually playing an important role in determining how a material behaves, both physically and chemically. As a consequence, researchers have introduced oxygen, sulphur and other vacancies into bi-dimensional (2D) materials, with the aim of achieving high performance electrodes for electrochemical energy storage. In this article, we focused on the recent advances in vacancy engineering of 2D materials for energy storage applications (supercapacitors and secondary batteries). Vacancy defects can effectively modify the electronic characteristics of 2D materials, enhancing the charge-transfer processes/reactions. These atomic-scale defects can also serve as extra host sites for inserted protons or small cations, allowing easier ion diffusion during their operation as electrodes in supercapacitors and secondary batteries. From the viewpoint of materials science, this article summarises recent developments in the exploitation of vacancies (which are surface defects, for these materials), including various defect creation approaches and cutting-edge techniques for detection of vacancies. The crucial role of defects for improvement in the energy storage performance of 2D electrode materials in electrochemical devices has also been highlighted.

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.

Mo-doped Co3O4 ultrathin nanosheet arrays anchored on nickel foam as a bi-functional electrode for supercapacitor and overall water splitting

Simple preparation, favorable price and environmental protection have been a long-term challenge in the field of electrochemistry. Herein, we studied and prepared a bifunctional Mo-doped Co₃O₄ ultrathin nanosheets, which has been validated as an effective binder-free electrode material for electrocatalytic water splitting and supercapacitors. The material has a large specific surface area, high electrical conductivity and exposure to more active sites, breaking down the limited performance and range of use of transition metal oxides. Benefiting from intriguing ultrathin property and conductivity, OER and HER process of 0.4Mo-Co₃O₄ have a small Tafel slope of 83.7 and 98 mV dec^-1, respectively. The current density at 10 mA cm^-2 show a low overpotential of 315 and 79 mV and significant stability. The water electrolytic device requires a potential of 1.64 V to reach 10 mA cm^-2, and the potential change is negligible after 12 h of continuous electrolysis. In addition, the manifest improved electrochemical performance of 0.3Mo-Co₃O₄ as supercapacitor electrode material shows high areal capacitance 2815 mF cm^-2 at 1 mA cm^-2, excellent rate performance (85% at 10 mA cm^-2) and retains 90% of the initial capacitance by cycling 5000 at a current density of 10 mA cm^-2. Moreover, 0.3Mo-Co₃O₄||0.3Mo-Co₃O₄ symmetrical supercapacitor has a maximum volumetric energy density of 1.25 mW h cm^-3 at a power density of 7.1 mW cm^-3 and superior cycle life. The influence of doping on electrochemical performance was studied by changing the content of doped metal ions, which is of great significance for the exploration of supercapacitor and electrocatalytic hydrolysis of bifunctional electrode materials.

Light-driven oxygen evolution from water oxidation with immobilised TiO2 engineered for high performance

Calcination treatments in the range of 500–900 °C of TiO₂ synthesised by the sol–gel resulted in materials with variable physicochemical (i.e., optical, specific surface area, crystallite size and crystalline phase) and morphological properties. The photocatalytic performance of the prepared materials was evaluated in the oxygen evolution reaction (OER) following UV-LED irradiation of aqueous solutions containing iron ions as sacrificial electron acceptors. The highest activity for water oxidation was obtained with the photocatalyst thermally treated at 700 °C (TiO₂-700). Photocatalysts with larger anatase to rutile ratio of the crystalline phases and higher surface density of oxygen vacancies (defects) displayed the best performance in OER. The oxygen defects at the photocatalyst surface have proven to be responsible for the enhanced photoactivity, acting as important active adsorption sites for water oxidation. Seeking technological application, water oxidation was accomplished by immobilising the photocatalyst with the highest OER rate measured under the established batch conditions (TiO₂-700). Experiments operating under continuous mode revealed a remarkable efficiency for oxygen production, exceeding 12% of the apparent quantum efficiency (AQE) at 384 nm (UV-LED system) compared to the batch operation mode.

Copper and Zirconium Codoped O3-Type Sodium Iron and Manganese Oxide as the Cobalt/Nickel-Free High-Capacity and Air-Stable Cathode for Sodium-Ion Batteries

Considering the abundance of iron and manganese within the Earth's crust, the cathode O3-NaFe_0.5Mn_0.5O₂ has shown great potential for large-scale energy storage. Following the strategy of introducing specific heteroelements to optimize the structural stability for energy storage, the work has obtained an O3-type NaFe_0.4Mn_0.49Cu_0.1Zr_0.01O₂ that exhibits enhanced electrochemical performance and air stability. It displays an initial reversible capacity of 147.5 mAh g^-1 at 0.1C between 2 and 4.1 V, a capacity retention ratio exceeding 69.6% after 100 cycles at 0.2C, and a discharge capacity of 70.8 mAh g^-1 at a high rate of 5C, which is superior to that of O3-NaFe_0.5Mn_0.5O₂. The codoping of Cu/Zr reserves the layered O3 structure and enlarges the interlayer spacing, promoting the diffusion of Na⁺. In addition, the structural stability and air stability observed by Cu-doping is well maintained via the incorporation of extra Zr favoring a highly reversible phase conversion process. Thus, this work has demonstrated an efficient strategy for developing cobalt/nickel-free high-capacity and air-stable cathodes for sodium-ion batteries.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.

In Situ Growth of 3D NiFe LDH-POM Micro-Flowers on Nickel Foam for Overall Water Splitting

Rational design and preparation of efficient and durable bifunctional electrocatalyst is an eternal yet challenging goal for sustainable energy conversion processes, such as water splitting. Herein, 3D NiFe layered double hydroxide-polyoxometalate (LDH-POM; polyoxometalate, i.e., K₈ [SiW₁₁ O₃₉ ]·13H₂ O) with micro-flower morphology is in situ grown on Ni foam via a facile one-step hydrothermal method, which can be used as a high-efficient bifunctional catalyst for overall water splitting. The as-prepared catalyst achieves overall water splitting current density of 10 mA cm^-2 at low overpotentials (oxygen evolution reaction (OER): ≈200 mV; hydrogen evolution reaction (HER): ≈156 mV) in 0.1 m KOH over a period of 20 h operation. Experimental investigation and density functional theory calculation indicate that, compared to pristine NiFe LDH, W⁶⁺ in NiFe LDH-POM can effectively minimize the adsorption energy barriers of *OH and therefore improve the kinetics of OER. This result may provide a promising strategy to synthesize 3D LDH micro-flowers by employing POM as a structure-direction agent for catalysis and energy applications.

Three-dimensional Bi2O3/Ti microspheres as an advanced negative electrode for hybrid supercapacitors

Herein, we report a novel, low-temperature solvothermal method to grow 3D-Bi2O3 flower-like microspheres on Ti substrates as a binder-free negative electrode for supercapacitor applications. The Bi2O3/Ti electrode showed an areal capacitance of 1.65 F cm-2 at 4 mA cm-2. Moreover, the 3D-NiCo2O4||3D-Bi2O3 hybrid device delivered high energy and power densities of 31.17 μW h cm-2 and 7500 μW cm-2, respectively. The more optimal energy storage performance based on the strong adhesion of the current collector and self-assembled three-dimensional nanostructures permits efficient electron and ion transportation.

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.

Thermodynamic and Kinetic Influence of Oxygen Vacancies on the Solar Water Oxidation Reaction of α-Fe2O3 Photoanodes

To reveal the role of oxygen vacancies in the solar water oxidation of α-Fe₂O₃ photoanodes, the kinetic and thermodynamic properties that are closely related to the water oxidation reaction of the α-Fe₂O₃ photoanode containing oxygen vacancies were investigated. Compared with the pristine α-Fe₂O₃ photoanode, the presence of surface oxygen vacancies can improve the water oxidation activity and stability of the α-Fe₂O₃ photoanode simultaneously, but the bulk oxygen vacancies have a negative effect on the water oxidation performance of the α-Fe₂O₃ photoanode. In thermodynamics, our investigations revealed that the presence of surface oxygen vacancies narrows the space charge region width of the α-Fe₂O₃ photoanode, which could boost the charge separation and transfer efficiency of the α-Fe₂O₃ photoanode during water oxidation. Because the surface property and hydrophilicity of α-Fe₂O₃ are modified owing to the presence of surface oxygen vacancies, the water oxidation kinetics of the α-Fe₂O₃ photoanode with surface oxygen vacancies is obviously boosted. Our findings in the present work provide comprehensive understanding of the thermodynamic and kinetic differences for α-Fe₂O₃ photoanodes with and without oxygen vacancies for solar water oxidation.

Hierarchical bimetallic hydroxide/chalcogenide core-sheath microarrays for freestanding ultrahigh rate supercapacitors

Transition metal compounds (TMCs) either crystalline or amorphous exhibit specific advantages in electrochemical energy storage. To integrate their merits into one electrode, we have herein developed hierarchical bimetallic hydroxide/chalcogenide core-sheath microarrays on nickel foam (NF) for freestanding high-efficiency supercapacitors, wherein interior crystalline metal chalcogenides serve as highly conductive pivots and exterior amorphous bimetallic hydroxides provide rich ion diffusion channels. With the synergic effect of the unique structure and bimetallic composition, the as-prepared Ni(OH)₂-Co(OH)₂/NiSe-Ni₃S₂/NF electrode displays an ultrahigh areal specific capacitance of 19.01 F cm^-2 at 15 mA cm^-2, which can be retained as 6.01 F cm^-2 even at 125 mA cm^-2. To the best of our knowledge, such excellent tolerance of ultrafast ion insertion/extraction at high current density is rare among NF-based free-standing electrodes. The asymmetric supercapacitor by assembling with activated carbon as the negative electrode delivers a volumetric capacitance of 3.93 F cm^-3 at 30 mA cm^-2, corresponding to an energy density of 13.9 mW h cm^-3 at a power density of 200 mW cm^-3. A capacitance retention of 82.5% was observed after 4000 cycles, together with an average 97% coulombic efficiency. This work may provide a facile strategy to construct hierarchical microarrays for efficient energy storage devices.

Sulfur vacancy-tailored NiCo2S4 nanosheet arrays for the hydrogen evolution reaction at all pH values

Sulfur vacancy-tailored NiCo₂S₄ nanosheet arrays as efficient electrocatalysts for the hydrogen evolution reaction at all pH values.