Novel Synthesis of Birnessite-Type MnO2 Nanostructure for Water Treatment and Electrochemical Capacitor

Fabrication and characterization of manganese dioxide (MnO2) nanoparticles and its degradation potential of benzene and pyrene

MnO2 nanoparticles have a wide range of applications, including catalytic abilities due to their oxygen reduction potential. Industrial processes and the burning of organic materials released PAHs into the biosphere which have adverse effects on living organisms when continually exposed. In this study, MnO2 nanoparticles were synthesized chemically using sodium thiosulphate as reducing agent. MnO2 nanoparticles were characterized using UV-visible adsorption spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR). A X-Ray Diffraction Spectrophotometer (XRD), a Scanning Electron Microscopy - Energy Dispersive X-Ray Analyzer (SEM-EDAX), and Dynamic Light Scattering (DLS) were used to identify the crystalline nature and particle size of the fabricated MnO2 nanoparticles. Batch adsorption studies were conducted to identify the optimal conditions for better benzene and pyrene adsorption from aqueous solution using MnO2 nanoparticles. They are also effective in degrading benzene and pyrene by batch adsorption as determined by their adsorption isotherms and kinetics.

Ultralight Hierarchically Structured RGO Composite Aerogels Embedded with MnO2/Ti3C2Tx for Efficient Microwave Absorption

Although intensive efforts have been devoted to fabricating Ti₃C₂T_x MXene composites for microwave absorption, it remains a great challenge to achieve excellent MA performance at low loading and thin thickness. Herein, a three-dimensional (3D) lightweight hierarchically structured MnO₂/Ti₃C₂T_x/RGO composite aerogel with abundant heterointerfaces was fabricated via a hydrothermal and chemical reduction self-assembly method. The RGO aerogel embedded with laminated MnO₂/Ti₃C₂T_x provides a lot of heterogeneous interfaces, 3D porous interconnected conductive networks, and reasonable combination of various loss materials for rich interfacial polarization, conductivity loss, multiple reflections and scattering, and good impedance matching. Benefiting from the synergy of different loss mechanisms, the maximum reflection loss (RL) is up to -66.5 dB (>99.9999% energy absorption) at only 10 wt % loading and 2.0 mm thickness, and even at only 1.5 mm thickness, the maximum RL value remains at -36 dB (>99.9% energy absorption). The work provides a promising route to construct 3D hierarchically heterogeneous composite aerogels for efficient MA at thin thickness and low loading.

A Comprehensive Understanding of Interlayer Engineering in Layered Manganese and Vanadium Cathodes for Aqueous Zn-ion Batteries

Rechargeable aqueous zinc-ion batteries (AZIBs) hold a budding technology for large-scale stationary energy storage devices due to their inherent safety, cost-effectiveness, eco-friendly, and acceptable electrochemical performance. However, developing a cathode material with fast kinetics and durable structural stability for Zn 2+ intercalation is still an arduous challenge. Compared with other cathode materials, layered manganese/vanadium (Mn/V) oxides that feature merits of adjustable interlayer spacing and considerable specific capacity have attracted much interest in AZIBs. However, the intrinsic sluggish reaction kinetics, inferior electrical conductivity, and notorious dissolution of active materials still obstruct the realization of their full potentials. Interlayer engineering of pre-intercalation is regarded as an effective solution to overcome these problems. In this review, we start from the crystal structure and reaction mechanism of layered Mn/V oxide cathodes to critical issues and recent progress in interlayer engineering. Finally, some future perspectives are outlined for the development of high-performance AZIBs.

Reversible Transformation between Bipolar Memory Switching and Bidirectional Threshold Switching in 2D Layered K-Birnessite Nanosheets

Birnessite-related manganese dioxides (MnO₂) have recently been studied owing to their diverse low-dimensional layered structures and potential applications in energy devices. The birnessite MnO₂ possesses a layered structure with edge-shared MnO₆ octahedra layer stacked with interlayer of cations. The unique layered structure may provide some distinct electrical properties for the 2D layered nanosheets. In this work, layered K-birnessite MnO₂ samples are synthesized by a hydrothermal method. The resistive switching (RS) devices based on single K-birnessite MnO₂ nanosheets are fabricated by transferring the nanosheets onto SiO₂/Si substrates through a facile and feasible method of mechanical exfoliation. The device exhibits nonvolatile memory switching (MS) behaviors with high current ON/OFF ratio of ∼2 × 10⁵. And more importantly, reversible transformation between the nonvolatile MS and volatile threshold switching (TS) can be achieved in the single layered nanosheet through tuning the magnitude of compliance current (I_cc). To be more specific, a relatively high I_cc (1 mA) can trigger the nonvolatile MS behaviors, while a relatively low I_cc (≤100 μA) can generate volatile TS characteristics. This work not only demonstrates the memristor based on single birnessite-related MnO₂ nanosheet, but also offers an insight into understanding the complex resistive switching types and relevant physical mechanisms of the 2D layered oxide nanosheets.

Chodankar N, Ghoreishian SM, Kwak CH, Huh YS, Han YK. Electroactive Ultra-Thin rGO-Enriched FeMoO4 Nanotubes and MnO2 Nanorods as Electrodes for High-Performance All-Solid-State Asymmetric Supercapacitors

A flexible asymmetric supercapacitor (ASC) with high electrochemical performance was constructed using reduced graphene oxide (rGO)-wrapped redox-active metal oxide-based negative and positive electrodes. Thin layered rGO functionality on the positive and the negative electrode surfaces has promoted the feasible surface-active sites and enhances the electrochemical response with a wide operating voltage window. Herein we report the controlled growth of rGO-wrapped tubular FeMoO4 nanofibers (NFs) via electrospinning followed by surface functionalization as a negative electrode. The tubular structure offers the ultrathin-layer decoration of rGO inside and outside of the tubular walls with uniform wrapping. The rGO-wrapped tubular FeMoO4 NF electrode exhibited a high specific capacitance of 135.2 F g-1 in Na2SO4 neutral electrolyte with an excellent rate capability and cycling stability (96.45% in 5000 cycles) at high current density. Meanwhile, the hydrothermally synthesized binder-free rGO/MnO2 nanorods on carbon cloth (rGO-MnO2@CC) were selected as cathode materials due to their high capacitance and high conductivity. Moreover, the ASC device was fabricated using rGO-wrapped FeMoO4 on carbon cloth (rGO-FeMoO4@CC) as the negative electrode and rGO-MnO2@CC as the positive electrode (rGO-FeMoO4@CC/rGO-MnO2@CC). The rationally designed ASC device delivered an excellent energy density of 38.8 W h kg-1 with a wide operating voltage window of 0.0-1.8 V. The hybrid ASC showed excellent cycling stability of 93.37% capacitance retention for 5000 cycles. Thus, the developed rGO-wrapped FeMoO4 nanotubes and MnO2 nanorods are promising hybrid electrode materials for the development of wide-potential ASCs with high energy and power density.

δ-MnO2 nanoflower/graphite cathode for rechargeable aqueous zinc ion batteries

Manganese oxide (MnO2) is one of the most promising intercalation cathode materials for zinc ion batteries (ZIBs). Specifically, a layered type delta manganese dioxide (δ-MnO2) allows reversible insertion/extraction of Zn2+ ions and exhibits high storage capacity of Zn2+ ions. However, a poor conductivity of δ-MnO2, as well as other crystallographic forms, limits its potential applications. This study focuses on δ-MnO2 with nanoflower structure supported on graphite flake, namely MNG, for use as an intercalation host material of rechargeable aqueous ZIBs. Pristine δ-MnO2 nanoflowers and MNG were synthesized and examined using X-ray diffraction, electron spectroscopy, and electrochemical techniques. Also, performances of the batteries with the pristine δ-MnO2 nanoflowers and MNG cathodes were studied in CR2032 coin cells. MNG exhibits a fast insertion/extraction of Zn2+ ions with diffusion scheme and pseudocapacitive behavior. The battery using MNG cathode exhibited a high initial discharge capacity of 235 mAh/g at 200 mA/g specific current density compared to 130 mAh/g which is displayed by the pristine δ-MnO2 cathode at the same specific current density. MNG demonstrated superior electrical conductivity compared to the pristine δ-MnO2. The results obtained pave the way for improving the electrical conductivity of MnO2 by using graphite flake support. The graphite flake support significantly improved performances of ZIBs and made them attractive for use in a wide variety of energy applications.

Mixed solvent exfoliated transition metal oxides nanosheets based flexible solid state supercapacitor devices endowed with high energy density

Mixed solvent exfoliated transition metal oxide nanosheet/SWCNT composite based flexible supercapacitors demonstrate large specific capacitance and high energy density.

One-pot hydrothermal synthesis of novel 3D starfish-like δ-MnO2 nanosheets on carbon fiber paper for high-performance supercapacitors

Novel 3D starfish-like δ-MnO₂ nanosheets with a hierarchical nanostructure supported on carbon fiber paper were synthesized through a facile hydrothermal method.

Porous honeycomb structures formed from interconnected MnO2 sheets on CNT-coated substrates for flexible all-solid-state supercapacitors

The use of lightweight and easily-fabricated MnO2/carbon nanotube (CNT)-based flexible networks as binder-free electrodes and a polyvinyl alcohol/H2SO4 electrolyte for the formation of stretchable solid-state supercapacitors was examined. The active electrodes were fabricated from 3D honeycomb porous MnO2 assembled from cross-walled and interconnected sheet-architectural MnO2 on CNT-based plastic substrates (denoted as honeycomb MnO2/CNT textiles).These substrates were fabricated through a simple two-step procedure involving the coating of multi-walled carbon nanotubes (MWCNTs) onto commercial textiles by a dipping-drying process and subsequent electrodeposition of the interconnected MnO2 sheets onto the MWCNT-coated textile. With such unique MnO2 architectures integrated onto CNT flexible films, good performance was achieved with a specific capacitance of 324 F/g at 0.5 A/g. A maximum energy density of 7.2 Wh/kg and a power density as high as 3.3 kW/kg were exhibited by the honeycomb MnO2/CNT network device, which is comparable to the performance of other carbon-based and metal oxide/carbon-based solid-state supercapacitor devices. Specifically, the long-term cycling stability of this material is excellent, with almost no loss of its initial capacitance and good Coulombic efficiency of 82% after 5000 cycles. These impressive results identify these materials as a promising candidate for use in environmentally friendly, low-cost, and high-performance flexible energy-storage devices.

3D hierarchical assembly of ultrathin MnO2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in Li- doped ionic liquid

Building of hierarchical core-shell hetero-structures is currently the subject of intensive research in the electrochemical field owing to its potential for making improved electrodes for high-performance micro-supercapacitors. Here we report a novel architecture design of hierarchical MnO₂@silicon nanowires (MnO₂@SiNWs) hetero-structures directly supported onto silicon wafer coupled with Li-ion doped 1-Methyl-1-propylpyrrolidinium bis(trifluromethylsulfonyl)imide (PMPyrrBTA) ionic liquids as electrolyte for micro-supercapacitors. A unique 3D mesoporous MnO₂@SiNWs in Li-ion doped IL electrolyte can be cycled reversibly across a voltage of 2.2 V and exhibits a high areal capacitance of 13 mFcm⁻². The high conductivity of the SiNWs arrays combined with the large surface area of ultrathin MnO₂ nanoflakes are responsible for the remarkable performance of these MnO₂@SiNWs hetero-structures which exhibit high energy density and excellent cycling stability. This combination of hybrid electrode and hybrid electrolyte opens up a novel avenue to design electrode materials for high-performance micro-supercapacitors.

Merging of Kirkendall growth and Ostwald ripening: CuO@MnO2 core-shell architectures for asymmetric supercapacitors

Fabricating hierarchical core-shell nanostructures is currently the subject of intensive research in the electrochemical field owing to the hopes it raises for making efficient electrodes for high-performance supercapacitors. Here, we develop a simple and cost-effective approach to prepare CuO@MnO₂ core-shell nanostructures without any surfactants and report their applications as electrodes for supercapacitors. An asymmetric supercapacitor with CuO@MnO₂ core-shell nanostructure as the positive electrode and activated microwave exfoliated graphite oxide (MEGO) as the negative electrode yields an energy density of 22.1 Wh kg⁻¹ and a maximum power density of 85.6 kW kg⁻¹; the device shows a long-term cycling stability which retains 101.5% of its initial capacitance even after 10000 cycles. Such a facile strategy to fabricate the hierarchical CuO@MnO₂ core-shell nanostructure with significantly improved functionalities opens up a novel avenue to design electrode materials on demand for high-performance supercapacitor applications.

Self-assembly of mesoporous nanotubes assembled from interwoven ultrathin birnessite-type MnO2 nanosheets for asymmetric supercapacitors

Porous nanotubes comprised of MnO₂ nanosheets were fabricated with a one-pot hydrothermal method using polycarbonate membrane as the template. The diameter and thickness of nanotubes can be controlled by choice of the membrane pore size and the chemistry. The porous MnO₂ nanotubes were used as a supercapacitor electrode. The specific capacitance in a three-electrode system was 365 F g⁻¹ at a current density of 0.25 A g⁻¹ with capacitance retention of 90.4% after 3000 cycles. An asymmetric supercapacitor with porous MnO₂ nanotubes as the positive electrode and activated graphene as the negative electrode yielded an energy density of 22.5 Wh kg⁻¹ and a maximum power density of 146.2 kW kg⁻¹; these values exceeded those reported for other MnO₂ nanostructures. The supercapacitor performance was correlated with the hierarchical structure of the porous MnO₂ nanotubes.