Sustainable Cellulose Nanofibers-Mediated Synthesis of Uniform Spinel Zn-Ferrites Nanocorals for High Performances in Supercapacitors

Spinel ferrites are versatile, low-cost, and abundant metal oxides with remarkable electronic and magnetic properties, which find several applications. Among them, they have been considered part of the next generation of electrochemical energy storage materials due to their variable oxidation states, low environmental toxicity, and possible synthesis through simple green chemical processing. However, most traditional procedures lead to the formation of poorly controlled materials (in terms of size, shape, composition, and/or crystalline structure). Thus, we report herein a cellulose nanofibers-mediated green procedure to prepare controlled highly porous nanocorals comprised of spinel Zn-ferrites. Then, they presented remarkable applications as electrodes in supercapacitors, which were thoroughly and critically discussed. The spinel Zn-ferrites nanocorals supercapacitor showed a much higher maximum specific capacitance (2031.81 F g^-1 at a current density of 1 A g^-1) than Fe₂O₃ and ZnO counterparts prepared by a similar approach (189.74 and 24.39 F g^-1 at a current density of 1 A g^-1). Its cyclic stability was also scrutinized via galvanostatic charging/discharging and electrochemical impedance spectroscopy, indicating excellent long-term stability. In addition, we manufactured an asymmetric supercapacitor device, which offered a high energy density value of 18.1 Wh kg^-1 at a power density of 2609.2 W kg^-1 (at 1 A g^-1 in 2.0 mol L^-1 KOH electrolyte). Based on our findings, we believe that higher performances observed for spinel Zn-ferrites nanocorals could be explained by their unique crystal structure and electronic configuration based on crystal field stabilization energy, which provides an electrostatic repulsion between the d electrons and the p orbitals of the surrounding oxygen anions, creating a level of energy that determines their final supercapacitance then evidenced, which is a very interesting property that could be explored for the production of clean energy storage devices.

Embellishing hierarchical 3D core-shell nanosheet arrays of ZnFe2O4@NiMoO4 onto rGO-Ni foam as a binder-free electrode for asymmetric supercapacitors with excellent electrochemical performance

Tailoring hierarchical hybrid core-shell electrodes with impartial microstructural features and excellent electroactive constituents is crucial for the design of high-performance supercapacitors (SCs). Herein, for the first time, we fabricate uniformly aligned porous ZnFe₂O₄ (ZFO) nanosheet arrays onto reduced graphene oxide-garnished conductive Ni foam (rGO-NF) substrates and subsequently embellish the first layer of ZFO nanosheets with morphology-controlled secondary NiMoO₄ nanosheets to achieve a hierarchical 3D core-shell structure of ZnFe₂O₄@NiMoO₄ nanosheet arrays (NSAs) onto rGO-NF for SC applications. Improving the synergistic effect of the core-shell nanoarchitecture with a conductive rGO-NF substrate, the hierarchical 3D ZFO@NMO NSAs tend to have superb electronic conductivity, tailoribility, effective nanoporous channels, and appropriate roadways for rapid ion/electron transfer, which are required for rapid reversible redox reactions, thus reflecting the excellent electrochemical features, including the excellent specific capacitance, good rate performance, and prolonged cyclic performance of the three electrode assemblies for SCs. An asymmetric supercapacitor (ASC) device composed of ZFO@NMO NSAs@rGO-NF as the cathode and MOF-derived hollow porous carbon (MDHPC) as the anode exhibits a high energy density of 58.6 Wh kg^-1 at a power density of 799 W kg^-1 with prolonged cyclic durability (89.6 % after 7000 cycles), thus indicating its potential applicability towards advanced hybrid SCs.

Nanostructured ZnFe2O4: An Exotic Energy Material

More people, more cities; the energy demand increases in consequence and much of that will rely on next-generation smart materials. Zn-ferrites (ZnFe₂O₄) are nonconventional ceramic materials on account of their unique properties, such as chemical and thermal stability and the reduced toxicity of Zn over other metals. Furthermore, the remarkable cation inversion behavior in nanostructured ZnFe₂O₄ extensively cast-off in the high-density magnetic data storage, 5G mobile communication, energy storage devices like Li-ion batteries, supercapacitors, and water splitting for hydrogen production, among others. Here, we review how aforesaid properties can be easily tuned in various ZnFe₂O₄ nanostructures depending on the choice, amount, and oxidation state of metal ions, the specific features of cation arrangement in the crystal lattice and the processing route used for the fabrication.

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.

Low-temperature-synthesized Mn-doped Bi2Fe4O9 as an efficient electrode material for supercapacitor applications

Manganese-doped Bi2Fe4O9, a new material synthesized at a low temperature with a micro-rectangular-shaped particles, is used for supercapacitor applications.

Utilizing Waste Thermocol Sheets and Rusted Iron Wires to Fabricate Carbon-Fe3 O4 Nanocomposite-Based Supercapacitors: Turning Wastes into Value-Added Materials

The synthesis of porous activated carbon (specific surface area=1883 m²  g^-1 ), Fe₃ O₄ nanoparticles, and carbon-Fe₃ O₄ (C-Fe₃ O₄ ) nanocomposites from local waste thermocol sheets and rusted iron wires is demonstrated herein. The resulting carbon, Fe₃ O₄ nanoparticles, and C-Fe₃ O₄ composites are used as electrode materials for supercapacitor applications. In particular, C-Fe₃ O₄ composite electrodes exhibit a high specific capacitance of 1375 F g^-1 at 1 A g^-1 and longer cyclic stability with 98 % capacitance retention over 10 000 cycles. Subsequently, an asymmetric supercapacitor, namely, C-Fe₃ O₄ ∥Ni(OH)₂ /carbon nanotube device, exhibits a high energy density of 91.1 Wh kg^-1 and a remarkable cyclic stability, with 98 % capacitance retention over 10 000 cycles. Thus, this work has important implications not only for the fabrication of low-cost electrodes for high-performance supercapacitors, but also for the recycling of waste thermocol sheets and rusted iron wires for value-added reuse.

NiCo2O4 nanosheets grown on interconnected honeycomb-like porous biomass carbon for high performance asymmetric supercapacitors

An interconnected honeycomb-like structure of the HLPBC/NiCo₂O₄ composite was successfully prepared by carbonization combined with a facile hydrothermal process.

Facile Synthesis of Self-Assembled Flower-Like Mesoporous Zinc Oxide Nanoflakes for Energy Applications

Flower-shaped self-assembled zinc oxide (ZnO) nanoflakes were successfully synthesized via a temperature-controlled hydrothermal method. The crystallinity and phase formation of the compound were determined from powder X-ray diffraction (PXRD) result. Surface morphology investigations reveal the self-assembled ZnO nanoflakes to form a spherical flower-like structure. In addition, the particle size was determined from high-resolution transmission electron microscope measurement as 18[Formula: see text]nm which is in accord with XRD and UV results. X-ray photo electron spectroscopy studies reveal the chemical composition and oxidation state of the ZnO nanoparticle. The specific surface area was calculated, and mesoporous nature was confirmed using Brunauer–Emmett–Teller analysis. Results support the superior interaction between the electrode and electrolyte ions through surface pores. Capacitive performance of the ZnO electrode material was determined using cyclic voltammetry and galvanostatic charge/discharge studies, and a maximum specific capacitance of 322[Formula: see text]F/g was obtained at 5[Formula: see text]mV/sec. Electrochemical impedance spectrum reveals the materials fast charge transfer kinetics.

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.

Contact angle measurements: a preliminary diagnostic tool for evaluating the performance of ZnFe2O4 nano-flake based supercapacitors

The contact angle of an electrolyte in contact with an electrode is used as an empirical diagnostic tool to pre-evaluate the performance of a supercapacitor prior to the actual fabrication of the device.