First report on synthesis of ZnFe2O4 thin film using successive ionic layer adsorption and reaction: Approach towards solid-state symmetric supercapacitor device

Recent advances on surface mounted metal-organic frameworks for energy storage and conversion applications: Trends, challenges, and opportunities

Establishing green and reliable energy resources is very important to counteract the carbon footprints and negative impact of non-renewable energy resources. Metal-organic frameworks (MOFs) are a class of porous material finding numerous applications due to their exceptional qualities, such as high surface area, low density, superior structural flexibility, and stability. Recently, increased attention has been paid to surface mounted MOFs (SURMOFs), which is nothing but thin film of MOF, as a new category in nanotechnology having unique properties compared to bulk MOFs. With the advancement of material growth and synthesis technologies, the fine tunability of film thickness, consistency, size, and geometry with a wide range of MOF complexes is possible. In this review, we recapitulate various synthesis approaches of SURMOFs including epitaxial synthesis approach, direct solvothermal method, Langmuir-Blodgett LBL deposition, Inkjet printing technique and others and then correlated the synthesis-structure-property relationship in terms of energy storage and conversion applications. Further the critical assessment and current problems of SURMOFs have been briefly discussed to explore the future opportunities in SURMOFs for energy storage and conversion applications.

Binder-Free Zinc-Iron Oxide as a High-Performance Negative Electrode Material for Pseudocapacitors

The interaction between cathode and anode materials is critical for developing a high-performance asymmetric supercapacitor (SC). Significant advances have been made for cathode materials, while the anode is comparatively less explored for SC applications. Herein, we proposed a high-performance binder-free anode material composed of two-dimensional ZnFe₂O₄ nanoflakes supported on carbon cloth (ZFO-NF@CC). The electrochemical performance of ZFO-NF@CC as an anode material for supercapacitor application was examined in a KOH solution via a three-electrode configuration. The ZFO-NF@CC electrode demonstrated a specific capacitance of 509 F g^-1 at 1.5 A g^-1 and was retained 94.2% after 10,000 GCD cycles. The ZFO-NF@CC electrode showed exceptional charge storage properties by attaining high pseudocapacitive-type storage. Furthermore, an asymmetric SC device was fabricated using ZFO-NF@CC as an anode and activated carbon on CC (AC@CC) as a cathode with a KOH-based aqueous electrolyte (ZFO-NF@CC||AC@CC). The ZFO-NF@CC||AC@CC yielded a high specific capacitance of 122.2 F g^-1 at a current density of 2 A g^-1, a high energy density of 55.044 Wh kg^-1 at a power density of 1801.44 W kg^-1, with a remarkable retention rate of 96.5% even after 4000 cycles was attained. Thus, our results showed that the enhanced electrochemical performance of ZFO-NF@CC used as an anode in high-performance SC applications can open new research directions for replacing carbon-based anode materials.

Microwave-assisted thermochemical conversion of Zr-FeOOH nanorods to Zr-ZnFe2O4 nanorods for bacterial disinfection and photo-Fenton catalytic degradation of organic pollutants

Herein, we report a CoO_x-loaded Zr-doped ZnFe₂O₄ (CoO_x/Zr-ZFO) NR photocatalyst synthesized by successive microwave and wet impregnation methods for bacterial inactivation and degradation of organic pollutants. For the first time, microwave treatment was used for Zn attachment on hydrothermally synthesized self-assembled Zr-FeOOH NRs to produce Zr-doped ZnFe₂O₄ (Zr-ZFO) NRs. The lowest bandgap energy (1.96 eV) enables for significant absorption in the visible light region, which helps to improve bacteria degradation inactivation efficiency. Further, various metal oxides (Cu, Ag and Co) were loaded onto the surface of photocatalysts (Zr-ZFO NRs) by a wet impregnation method. As-synthesized CoO_x/Zr-ZFO-3 NRs were systematically characterized and used as photocatalysts for inactivation of E. coli and S. aureus and degradation of organic pollutants. The CoO_x/Zr-ZFO-3 NR photocatalyst exhibited better inactivation efficiency (99.4 %) than other metal oxide-loaded Zr-ZFO NRs (Ag₂O_x-loaded Zr-ZFO NRs (33.6 %), CuO_x-loaded Zr-ZFO NRs (77.6 %)). Additionally, the optimum CoO_x/Zr-ZFO-3 NR photocatalyst showed 98.3 % and 98.1 % degradation efficiencies for BPA and orange II dye, respectively, under visible light irradiation (λ ≥ 420 nm). Therefore, this work affords a novel, simple and rapid approach for the development of photocatalysts which active in visible light for bacterial disinfection and organic degradation.

Effect of thickness and reaction media on properties of ZnO thin films by SILAR

Zinc oxide (ZnO) is one of the most promising metal oxide semiconductor materials, particularly for optical and gas sensing applications. The influence of thickness and solvent on various features of ZnO thin films deposited at ambient temperature and barometric pressure by the sequential ionic layer adsorption and reaction method (SILAR) was carefully studied in this work. Ethanol and distilled water (DW) were alternatively used as a solvent for preparation of ZnO precursor solution. Superficial morphology, crystallite structure, optical and electrical characteristics of the thin films of various thickness are examined applying X-ray diffraction (XRD) system, scanning electron microscopy, the atomic force microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy, photoluminescence spectroscopy, Hall effect measurement analysis and UV response study. XRD analysis confirmed that thin films fabricated using ethanol or DW precursor solvents are hexagonal wurtzite ZnO with a preferred growth orientation (002). Furthermore, it was found that thin films made using ethanol are as highly crystalline as thin films made using DW. ZnO thin films prepared using aqueous solutions possess high optical band gaps. However, films prepared with ethanol solvent have low resistivity (10^-2 Ω cm) and high electron mobility (750 cm²/Vs). The ethanol solvent-based SILAR method opens opportunities to synthase high quality ZnO thin films for various potential applications.

SILAR Deposition of Metal Oxide Nanostructured Films

Methods for the fabrication of thin films with well controlled structure and properties are of great importance for the development of functional devices for a large range of applications. SILAR, the acronym for Successive Ionic Layer Adsorption and Reaction, is an evolution and combination of two other deposition methods, the Atomic Layer Deposition and Chemical Bath Deposition. Due to a relative simplicity and low cost, this method has gained increasing interest in the scientific community. There are, however, several aspects related to the influence of the many parameters involved, which deserve further deepening. In this review article, the basis of the method, its application to the fabrication of thin films, the importance of experimental parameters, and some recent advances in the application of oxide films are reviewed. At first the fundamental theoretical bases and experimental concepts of SILAR are discussed. Then, the fabrication of chalcogenides and metal oxides is reviewed, with special emphasis to metal oxides, trying to extract general information on the effect of experimental parameters on structural, morphological and functional properties. Finally, recent advances in the application of oxide films prepared by SILAR are described, focusing on supercapacitors, transparent electrodes, solar cells, and photoelectrochemical devices.

A Sensitive Visible Light Photodetector Using Cobalt-Doped Zinc Ferrite Oxide Thin Films

In this study, a highly sensitive trilayer photodetector using Co-doped ZnFe₂O₄ thin films annealed at 400 °C was synthesized successfully. Trilayer-photodetector devices with a film stack of 5 at % Co-doped-zinc-ferrite-thin-film/indium-tin-oxide on p⁺-Si substrates were fabricated by radio-frequency sputtering. The absorbance spectra, photoluminescence spectra, transmission electron microscopy images, and I-V characteristics under various conditions were comprehensively investigated. The outstanding performance of trilayer-photodector devices was measured, including a high photosensitivity of 181 and a fast photoresponse time with a rise time of 10.6 ms and fall time of 9.9 ms under 630 nm illumination. Therefore, the Co-doped ZnFe₂O₄ thin film is favorable for potential photodetector applications in visible light regions.

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.

Electrochemical Performance of 2D-Hierarchical Sheet-Like ZnCo2O4 Microstructures for Supercapacitor Applications

With the rapid improvement of the global economy, the role of energy has become even more vital in the 21st century. In this regard, energy storage/conversion devices have become a major, worldwide research focus. In response to this, we have prepared two-dimensional (2D)-hierarchical sheet-like ZnCo2O4 microstructures for supercapacitor applications using a simple hydrothermal method. The 2D-hierarchical sheet-like morphologies with large surface area and smaller thickness enhanced the contact area of active material with the electrolyte, which increased the utilization rate. We investigated the electrochemical performance of sheet-like ZnCo2O4 microstructures while using Cyclic voltammetry (CV), Galvanostatic charge-discharge (GCD), and Electrochemical impedance spectroscopy (EIS) analysis. The electrochemical results demonstrated that the ZnCo2O4 electrode possesses 16.13 mF cm−2 of areal capacitance at 10 µA cm−2 of current density and outstanding cycling performance (170% of capacitance is retained after 1000 cycles at 500 µA cm−2). The high areal capacitance and outstanding cycling performance due to the unique sheet-like morphology of the ZnCo2O4 electrode makes it an excellent candidate for supercapacitor applications.

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.

Defective Fe3+ self-doped spinel ZnFe2O4 with oxygen vacancies for highly efficient photoelectrochemical water splitting

Defective Fe³⁺ self-doped spinel ZnFe₂O₄ with abundant oxygen vacancies exhibits largely enhanced photoelectrochemical performance.

A facile one-step hydrothermal approach to synthesize hierarchical core-shell NiFe2O4@NiFe2O4 nanosheet arrays on Ni foam with large specific capacitance for supercapacitors

In this contribution, NiFe2O4@NiFe2O4 nanosheet arrays (NSAs) with three-dimensional (3D) hierarchical core-shell structures were synthesized by a facile one-step hydrothermal method and they were used as electrode materials for supercapacitors (SCs). The NiFe2O4@NiFe2O4 composite electrode showed a high specific capacitance of 1452.6 F g-1 (5 mA cm-2). It also exhibited a superior cycling stability (93% retention after 3000 cycles). Moreover, an asymmetric supercapacitor (ASC) was constructed utilizing NiFe2O4@NiFe2O4 NSAs and activated carbon (AC) as the positive and negative electrode, respectively. The optimized ASC shows extraordinary performances with a high energy density of 33.6 W h kg-1 at a power density of 367.3 W kg-1 and an excellent cycling stability of 95.3% capacitance retention over 3000 cycles. Therefore, NiFe2O4@NiFe2O4 NSAs have excellent pseudocapacitance properties and are good electrode materials for high energy density.

Facile synthesis of core-shell nanostructured hollow carbon nanospheres@nickel cobalt double hydroxides as high-performance electrode materials for supercapacitors

Core-shell nanostructured hollow carbon nanospheres@nickel cobalt double hydroxides (HCNs@NiCo-LDH) were fabricated using a facile hydrothermal method and investigated as high-performance electrode materials for supercapacitors. HCNs were acquired by a successive polymerization, carbonization and etching process, which was subsequently wrapped by ultrathin NiCo-LDH nanosheets. The HCNs@NiCo-LDH electrode achieved a high specific capacitance (2558 F g^-1 at 1 A g^-1) and outstanding rate capability with 74.9% capacitance retention after a 20-fold increase in current density. Capacitances of 2405, 2310, 2168, 2006 and 1916 F g^-1 can be achieved at rates of 3, 5, 10, 15 and 20 A g^-1, respectively, which are much higher than the specific capacitances of most reported carbon loaded NiCo-LDH. Specifically, the assembled HCNs@NiCo-LDH//graphene asymmetric supercapacitor displayed distinguished capacitive behaviors with a prominent specific capacitance of 172.8 F g^-1 and eminent cycling stability with 93.5% capacitance retention after 3000 cycles. These remarkable electrochemical properties indicate that the unique HCNs@NiCo-LDH core-shell electrode is highly promising for application in energy storage fields.

Hierarchical 3D NiFe2O4@MnO2 core–shell nanosheet arrays on Ni foam for high-performance asymmetric supercapacitors

Hierarchical NiFe₂O₄@MnO₂ core–shell nanosheet arrays synthesized via a two-step hydrothermal method with successive annealing exhibited outstanding electrochemical performance.

Anchoring Ultrafine ZnFe2O4/C Nanoparticles on 3D ZnFe2O4 Nanoflakes for Boosting Cycle Stability and Energy Density of Flexible Asymmetric Supercapacitor

Heterostructure-based metal oxide thin films are recognized as the leading material for new generation, high-performance, stable, and flexible supercapacitors. However, morphologies, like nanoflakes, nanotubes, nanorods, and so forth, have been found to suffer from issues related to poor cycle stability and energy density. Thus, to circumvent these problems, herein, we have developed a low-cost, high surface area, and environmentally benign self-assembled ZnFe₂O₄ nanoflake@ZnFe₂O₄/C nanoparticle heterostructure electrode via anchoring ZnFe₂O₄ and carbon nanoparticles using an in situ biomediated green rotational chemical bath deposition approach for the first time. The synthesized ZnFe₂O₄ nanoflake@ZnFe₂O₄/C nanoparticle heterostructure thin films demonstrate an excellent specific capacitance of 1884 F g^-1 at a current density of 5 mA cm^-2. Additionally, all solid-state flexible asymmetric supercapacitor devices were designed on the basis of ZnFe₂O₄ nanoflake@ZnFe₂O₄/C nanoparticle heterostructures as the negative electrode and reduced graphene oxide and energy density of 81 Wh kg^-1 at a power density of 3.9 kW kg^-1. Similarly, the asymmetric device exhibits ultralong cycle stability of 35 000 cycles by losing only 2% capacitance. The excellent performance of the device is attributed to the self-assembled organization of the heterostructures. Moreover, the in situ biomediated green strategy is also applicable for the synthesis of other metal oxide and carbon-based heterostructure electrodes.

Uniformly Dispersed ZnFe2O4 Nanoparticles on Nitrogen-Modified Graphene for High-Performance Supercapacitor as Electrode

A facile strategy has been adopted for the preparation of ZnFe₂O₄/NRG composite by anchoring ultrasmall ZnFe₂O₄ nanoparticles on nitrogen-doped reduced graphene (denoted as NRG) for high-performance supercapacitor electrode. Remarkably, the growth of ZnFe₂O₄ nanocrystals, the reduction of graphitic oxide and the doping of nitrogen to graphene have been simultaneously achieved in one process. It is found that the NRG employed as substrate can not only control the formation of nano-sized ZnFe₂O₄, but also guarantee the high dispersion without any agglomeration. Benefiting from this novel combination and construction, the hybrid material has large surface area which can provide high exposure of active sites for easy access of electrolyte and fast electron transport. When served as supercapacitor electrode, the ZnFe₂O₄/NRG composite exhibits a favorable specific capacitance of 244 F/g at 0.5 A/g within the potential range from −1 to 0 V, desirable rate stability (retain 131.5 F/g at 10 A/g) and an admirable cycling durability of 83.8% at a scan rate of 100 mV/s after 5000 cycles. When employed as symmetric supercapacitor, the device demonstrates favorable performance. These satisfactory properties of the ZnFe₂O₄/NRG composite can make it be of great promise in the supercapacitor application.

Low-crystalline mesoporous CoFe2O4/C composite with oxygen vacancies for high energy density asymmetric supercapacitors

A low-crystalline mesoporous cobalt ferrite and carbon composite (L-CoFe₂O₄/C) material was prepared via a sol–gel approach and calcination process. An L-CoFe₂O₄/C//AC asymmetric supercapacitor exhibited high energy density and power density.

Facile synthesis of a MnFe2O4/rGO nanocomposite for an ultra-stable symmetric supercapacitor

A symmetric supercapacitor based on a MnFe₂O₄/rGO nanocomposite prepared from expanded graphite.