MnO2 Nanostructures Research Articles

Performance enhancement of asymmetric supercapacitors with bud-like Cu-doped Mn3O4 hollow and porous structures on nickel foam as positive electrodes.

Cu-doped Mn3O4 hollow nanostructures supported on Ni foams as high-performance electrode materials for supercapacitors were successfully synthesized through a facile hydrothermal method and subsequent calcination. The morphology, structure, and electrochemical performance of the as-prepared Mn3O4 nanostructures can be tuned just by varying the Cu doping content. Benefiting from the unique bud-like hollow structure, the 1.5 at% Cu-doped Mn3O4 sample has a high specific capacitance of 257.6 F g−1 at 1 A g−1 and remarkable stability (about 90.6% retention of its initial capacitance after 6000 electrochemical cycles). Besides, an asymmetric supercapacitor (ASC) cell based on the 1.5 at% Cu-doped Mn3O4 exhibits a high specific capacitance of 305.6 F g−1 at 1 A g−1 and an energy density of 108.6 W h kg−1 at a power density of 799.9 W kg−1. More importantly, the ASC shows good long-term stability with 86.9% capacity retention after charging/discharging for 6000 cycles at a high current density of 5 A g−1.

Influence of electrodeposition modes on the electrochemical performance of MnO2 films prepared using anionic MnO4− (Mn7+) precursor

In this work, we report the comparative electrochemical performance of the nanostructured MnO2 films prepared via three different electrodeposition modes, namely, potentiostatic (PS), potentiodynamic (PD), and galvanostatic (GS), using anionic MnO4- (Mn7+) precursor. All the MnO2 films have been found to exhibit the hexagonal crystallographic phase of epsilon manganese dioxide (ε-MnO2, akhtenkite), consistent with the JCPDS card no. 12-0141. The potentiostatically electrodeposited MnO2 film has surface morphology exhibiting a porous microstructure comprising of sparsely distributed grains, which is conducive for easy diffusion of electrolyte ions during electrochemical process. The analyses of atomic force microscopy images reveal that the surface area is maximum for the film deposited via PS mode. Cyclic voltammetry studies reveal that the film deposited via PS mode has the maximum specific capacitance 259.4F/g, followed by 187.1F/g and 180.3F/g for the films deposited via PD and GS modes, respectively at the scan rate of 5mV/s. The galvanostatic charge/discharge measurements at the current density of 1mA/cm2 show the specific capacitance to be maximum 325.6F/g for the MnO2 film prepared via PS mode, followed by 194.6F/g and 182.5F/g for the films prepared by PD and GS modes, respectively. The variation in specific capacitances of the films is attributed to varying morphological features of the films. In conclusion, the PS mode has been found to be the optimum electrodeposition mode to prepare MnO2 films using anionic MnO4− (Mn7+) precursor for improved electrochemical performance of the films.

Direct Interfacial Growth of MnO2 Nanostructure on Hierarchically Porous Carbon for High-Performance Asymmetric Supercapacitors

Developing a composite electrode composed of a carbon-based material and a transition metal oxide is an effective way to address the problems such as poor conductivity and low porosity created by transitional metal oxide electrodes for supercapacitors. In this work, the activated carbon (AC) prepared from enteromorpha prolifera (ACEP) with typical hierarchically porous structure, was used as a substrate to grow MnO2 nanostructures via wet chemical reaction process. The morphology and crystalline phase of the MnO2 could be controlled by facilely adjusting reaction time. For instance, δ-MnO2 nanosheets were anchored on the ACEP in the initial stage (1–6 h), but α-MnO2 nanowires were obtained with the extension of reaction time (7–12 h). The electrode prepared from ACEP@δ-MnO2 nanosheets displays high specific capacitance (345.1 F g–1 at 0.5 A g–1) and excellent cycle stability (i.e., a capacitance retention of 92.8% after 5000 cycles). Moreover, an asymmetric supercapacitor was assembled by using as-prepared ACEP@δ-MnO2 composite as positive electrode and AC as negative electrode. The assembled AC//δ-ACEP@MnO2 supercapacitor is shown to work in a wide voltage range of 0–2.0 V and delivered a high energy density of 31.0 Wh kg–1 at a power density of 500.0 W kg–1.

Evaluating the Role of Nanostructured Current Collectors in Energy Storage Capability of Supercapacitor Electrodes with Thick Electroactive Materials Layers

AbstractElectroactive materials (especially pseudocapacitive materials) are generally in the form of ultrathin conformal coating in supercapacitor electrodes based on nanostructured current collectors; thus, the resultant low mass loading of electroactive materials largely limits the applications of nanostructured current collectors. Here, supercapacitor electrodes with nickel nanorod arrays as nanostructured current collectors and MnO2 as electroactive materials are fabricated to study the role of nanostructured current collectors in determining the energy storage capability when electroactive materials are in thick layer rather than ultrathin conformal coating. Electrochemical analysis reveals that Ni nanorods could create numerous electrical conductive tunnels in the thick‐layer electrodes to dramatically alleviate the contact resistance at the electroactive‐material/current‐collector interface. With 1 µm thick MnO2 layer, the Ni nanorods based electrodes have much higher areal capacitance than those with Ni foils as current collectors, which is more than six times of that with the same MnO2 mass loading or more than 18 times of that with the same 1 µm thick MnO2 layer. Moreover, better rate capability and higher structural stability is maintained in Ni nanorods based electrodes even with 3 µm thick MnO2 layer. These results open up new opportunities for nanostructured current collectors to construct supercapacitors with superior energy storage capability.

A sensitive turn on fluorescent probe for detection of biothiols using MnO2@carbon dots nanocomposites

Presently, the combination of carbon quantum dots (CQDs) and metal oxide nanostructures in one frame are being considered for the sensing of purine compounds. In this work, a combined system of CQDs and MnO2 nanostructures was used for the detection of anticancer drugs, 6-Thioguanine (6-TG) and 6-Mercaptopurine (6-MP). The CQDs were synthesized through microwave synthesizer and the MnO2 nanostructures (nanoflowers and nanosheets) were synthesized using facile hydrothermal technique. The CQDs exhibited excellent fluorescence emission at 420nm when excited at 320nm wavelength. By combining CQDs and MnO2 nanostructures, quenching of fluorescence was observed which was attributed to fluorescence resonance energy transfer (FRET) mechanism, where CQDs act as electron donor and MnO2 act as acceptor. This fluorescence quenching behaviour disappeared on the addition of 6-TG and 6-MP due to the formation of Mn-S bond. The detection limit for 6-TG (0.015μM) and 6-MP (0.014μM) was achieved with the linear range of concentration (0–50μM) using both MnO2 nanoflowers and nanosheets. Moreover, the as-prepared fluorescence-sensing technique was successfully employed for the detection of bio-thiol group in enapril drug. Thus a facile, cost-effective and benign chemistry approach for biomolecule detection was designed.

Carbon Quantum Dot-Induced MnO2 Nanowire Formation and Construction of a Binder-Free Flexible Membrane with Excellent Superhydrophilicity and Enhanced Supercapacitor Performance.

Manganese oxides (MnO2) are regarded as typical and promising electrode materials for supercapacitors. However, the practical electrochemical performance of MnO2 is far from its theoretical value. Nowadays, numerous efforts are being devoted to the design and preparation of nanostructured MnO2 with the aim of improving its electrochemical properties. In this work, ultralong MnO2 nanowires were fabricated in a process induced by carbon quantum dots (CQDs); subsequently, a binder-free flexible electrode membrane was easily obtained by vacuum filtration of the MnO2 nanowires. The effects of the CQDs not only induced the formation of one-dimensional nanostructured MnO2, but also significantly improved the wettability between electrode and electrolyte. In other words, the MnO2 membrane demonstrated a superhydrophilic character in aqueous solution, indicating the sufficient and abundant contact probability between electrode and electrolyte. The binder-free flexible MnO2 electrode exhibited a preeminent specific capacitance of 340 F g-1 at 1 A g-1; even when the current density reached 20 A g-1, it still maintained 260 F g-1 (76% retention rate compared to that at 1 A g-1). Moreover, it also showed good cycling stability with 80.1% capacity retention over 10 000 cycles at 1 A g-1. Furthermore, an asymmetric supercapacitor was constructed using the MnO2 membrane and active carbon as the positive and negative electrodes, respectively, which exhibited a high energy density of 33.6 Wh kg-1 at 1.0 kW kg-1, and a high power density of 10 kW kg-1 at 12.5 Wh kg-1.

Preparation of the Nanostructured Radioisotope Metallic Oxide by Neutron Irradiation for Use as Radiotracers

Metallic oxides manganese dioxide (MnO2), samarium oxide (Sm2O3), and dysprosium oxide (Dy2O3) with nanorod-like structures were synthesized by the hydrothermal synthesis method, respectively. Subsequently, the nanostructured radioisotopes MnO2 with Mn-56, Sm2O3 with Sm-153, and Dy2O3 with Dy-165 were prepared by neutron irradiation from the HANARO research reactor, respectively. The three different elements, Mn, Sm, and Dy, were selected as radiotracers because these elements can be easily gamma-activated from neutrons (activation limits: 1 picogram (Dy), 1–10 picogram (Mn), 10–100 picogram (Sm)). Furthermore, the synthesized radioisotopes can be used as radiotracers in Prompt Gamma Neutron Activation Analysis as the rare earth metals Dy and Sm were not present in the Korean environment. The successful synthesis of the radioisotope metallic oxides was confirmed by Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectrometry (EDS), X-ray Diffraction (XRD) analysis, and gamma spectroscopy analysis. The synthesized nanostructured radioisotope metallic oxides may be used as radiotracers in scientific, environmental, engineering, and industrial fields.

Nitrogen-doped hollow porous carbon nanospheres coated with MnO2 nanosheets as excellent sulfur hosts for Li–S batteries

In this work, nitrogen-doped hollow porous carbon nanospheres coated with MnO2 nanosheets (NHPC@MnO2) were prepared as a novel sulfur host for the cathode of lithium–sulfur battery. N-doping of carbon and deposition of the inherently polar MnO2 promote chemical binding of the host with sulfur and its reduction products, known as polysulfides. Meanwhile, proper N-doping can improve the electron conductivity of carbon, and the nanosheet structure may help to guarantee facile electron- and lithium-ion transport through MnO2. Attributed to these advantages, the NHPC@MnO2/S cathode with a high sulfur content (70 wt% and 2.6 mg cm−2) exhibited an excellent cycle stability: its capacity retention was 93% within 100 cycles at 0.5 C. It also displayed a good rate capability: discharge capacities being ∼1130 mAh g−1 at 0.2 C, ∼1000 mAh g−1 at 0.5 C, ∼820 mAh g−1 at 1 C, and ∼630 mAh g−1 at 2 C. Our work demonstrates the synergistic effect of MnO2 nanostructure and N-doped carbon nanospheres for enhanced performance of lithium–sulfur battery cathodes.

Electrochemical Magnetization Switching and Energy Storage in Manganese Oxide filled Carbon Nanotubes

The ferrimagnetic and high-capacity electrode material Mn3O4 is encapsulated inside multi-walled carbon nanotubes (CNT). We show that the rigid hollow cavities of the CNT enforce size-controlled nanoparticles which are electrochemically active inside the CNT. The ferrimagnetic Mn3O4 filling is switched by electrochemical conversion reaction to antiferromagnetic MnO. The conversion reaction is further exploited for electrochemical energy storage. Our studies confirm that the theoretical reversible capacity of the Mn3O4 filling is fully accessible. Upon reversible cycling, the Mn3O4@CNT nanocomposite reaches a maximum discharge capacity of 461 mA h g−1 at 100 mA g−1 with a capacity retention of 90% after 50 cycles. We attribute the good cycling stability to the hybrid nature of the nanocomposite: (1) Carbon encasements ensure electrical contact to the active material by forming a stable conductive network which is unaffected by potential cracks of the encapsulate. (2) The CNT shells resist strong volume changes of the encapsulate in response to electrochemical cycling, which in conventional (i.e., non-nanocomposite) Mn3O4 hinders the application in energy storage devices. Our results demonstrate that Mn3O4 nanostructures can be successfully grown inside CNT and the resulting nanocomposite can be reversibly converted and exploited for lithium-ion batteries.

Fabrication and characterization of ZnO/MnO2 and ZnO/TiO2 flexible nanocomposites for energy storage applications

ZnO based nanostructure composites are attractive for high-tech applications of energy conversion and energy storage due to wide range in operating potential window. This study presents about fabrication of ZnO/MnO2 and ZnO/TiO2 composites via hydrothermal method in two consecutive steps. Furthermore lignocelluloses fiber directly collected form self-growing plant Monochoria Vagina, were incorporated in fabricated ZnO/MnO2 and ZnO/TiO2 composites for development of flexible and bulk paper composite electrode. The structural and morphological analysis were carried out using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), respectively. XRD analysis confirm the crystalline structures for ZnO/MnO2 and ZnO/TiO2 samples with crystallite size ∼62 nm and ∼63 nm, respectively. SEM images shows the sheet-like and nanorods morphology for ZnO and MnO2 nanostructures, respectively. Fourier Transform Infrared (FTIR) Spectroscopy absorbance spectrum reveal the composite formation of ZnO/MnO2/LC and ZnO/TiO2/LC as bulk paper electrodes whereas cyclic Voltammetry measurement showed the capacitive behavior of composite paper electrodes for the suitable applications in supercapacitors and batteries.

Transparent and Flexible Supercapacitors with Networked Electrodes.

Transparent and flexible energy storage devices have received immense attention due to their suitability for innovative electronics and displays. However, it remains a great challenge to fabricate devices with high storage capacity and high degree of transmittance. This study describes a simple process for fabrication of supercapacitors with ≈75% of visible transparency and areal capacitance of ≈3 mF cm-2 with high stability tested over 5000 cycles of charging and discharging. The electrodes consist of Au wire networks obtained by a simple crackle template method which are coated with MnO2 nanostructures by electrodeposition process. Importantly, the membrane separator itself is employed as substrate to bring in the desired transparency and light weight while additionally exploiting its porous nature in enhancing the interaction of electrolyte with the active material from both sides of the substrate, thereby enhancing the storage capacity. The method opens up new ways for fabricating transparent devices.

MnO2-nanowire@NiO-nanosheet core-shell hybrid nanostructure derived interfacial Effect for promoting catalytic oxidation activity

Nanostructure-derived interfacial effect plays a great role in enhancing catalytic activities. Herein, a MnO2 nanowires@NiO nanosheets core-shell hybrid nanostructure was successfully prepared by uniformly decorating NiO nanosheets on the one-dimensional MnO2 nanowires. The physi-chemical properties of the MnO2 and MnO2@NiO were characterized by using XRD, BET, SEM, H2-TPR and XPS techniques. Compared to the single MnO2 nanowires, the MnO2@NiO nanocomposite with better low-temperature reducibility and more active surface oxygen species exhibited much better performance in complete oxidation of benzene, giving the temperatures for 100% benzene conversion of 320°C under the conditions of 1000ppm benzene in air and space velocity of 120,000mLg−1h−1 while the value over pure MnO2 was 380°C. The novel hetero-interface constructed between MnO2 and NiO core-shell nanostructures might make a great contribution on this significantly promoting effect.

Two-Dimensional Planar Supercapacitor Based on Zinc Oxide/Manganese Oxide Core/Shell Nano-architecture

We report the fabrication and performance of zinc oxide core/manganese oxide shell (ZnO/MnO2) three-dimensional nanostructured planar supercapacitor. In this particular fabrication, ZnO nanorods were grown over FTO substrate followed by the successive ionic layer adsorption and reaction (SILAR) deposition of MnO2 nanostructure on ZnO nanorods. The quality of the ZnO/MnO2 electrode was optimized by performing various cycles of SILAR deposition of MnO2 layer over ZnO core. The electrode with 7 cycles of MnO2 deposition demonstrated maximum electrochemical performance with area capacitance of 14mFcm−2 and the same condition was utilized for the fabrication of a symmetric planar supercapacitor. This ZnO/MnO2 3D architecture planar supercapacitor showed maximum area capacitance of 5mFcm−2 in 1M Na2SO4 aqueous electrolyte with good energy density of 4.07×10−7Whcm−2 and power density of 3.19mWcm−2. Moreover, this planar supercapacitor shows an excellent electrochemical performance with comparable long term cyclic stability.

Surfactant-Mediated Resistance to Surface Oxidation in MnO Nanostructures.

The intrinsic physical properties of nanostructures of metals and their oxides are altered when they are prone to surface oxidation in ambient atmosphere. To overcome this limitation, novel synthesis methodologies are required. In this study, solid octahedral shapes of MnO limit the inward oxygen diffusion compared to that of the MnO-nanoparticle-assembled octahedra. In addition to morphology control, which restricts the thickness of the Mn3O4 surface layer, the binding chemistry of the surfactants plays an essential role. For example, the Mn3O4 surface layer is 0.4 nm thinner with trioctylphosphine oxide than with trioctylamine as the surfactant. The nanostructures were prepared by varying the surfactants, surfactant-to-precursor molar ratio, accelerating agent, and reaction heating rate. The surface oxidation of MnO nano-octahedra was probed by Rietveld analysis of X-ray diffraction patterns and X-ray photoelectron spectroscopy and characterized by magnetic measurements, as the presence of ferrimagnetic Mn3O4 shell on the antiferromagnetic MnO core provides an exchange coupling at the core–shell interface. Thicker the Mn3O4 shell, higher is the exchange-biased hysteresis loop shift.

Controlling the nanoscale morphology and structure of the ZnO/MnO2 system for efficient transparent supercapacitors

Abstract

Tunable morphology and property of a MnO2/carbonized cotton textile hybrid electrode for electrochemical capacitors

There is an increasing demand of low cost, flexible, stable, and environmentally benign power sources for emerging wearable electronic equipment. Herein, we develop electrochemical capacitor electrodes based on MnO2/carbonized cotton textile with high mass-loading and tunable morphology. After a simple carbonization process, the cotton cloth with high surface area (585 m2 g−1) served as 3D binder-free and flexible scaffolds to anchor MnO2 nanostructures. The morphology of MnO2 nanostructures was tuned and optimized into curled sheet-like, which provided large surface area and could also release large local stress. Electrochemical measurements showed that the curled sheet-like MnO2 had a specific capacitance of 465 F g−1 at 0.1 A g−1, and exhibited an excellent cyclic stability with a specific capacitance retention ratio of 95% after 5000 cycles (at 10 A g−1). Due to the flexible nature of cotton textile, the hybrid electrodes could be bent freely, and the capacitance and cyclability almost remained unchanged even at a bending angle of 150°. Such flexible and stable electrodes from low cost and environmentally benign biomass offer new development potentials for energy storage and wearable electronic applications.

High energy density asymmetric supercapacitors based on MOF-derived nanoporous carbon/manganese dioxide hybrids

Nanoporous carbon materials with high specific surface areas have attracted increasing attention for electrochemical energy storage applications. Metal organic frameworks (MOF) as a good precursor for nanoporous carbon could be used as a promising substrate to improve the energy density of the electrode because of its high specific surface area and high porosity. Here we fabricated MOF-derived nanoporous carbon (MOF-NPC) by direct calcination of zine-based MOF with different carbonization temperatures and synthesized MOF-NPC/MnO2 hybrids via a self-controlled redox process with MnO2 nanostructures well confining in the porous structures of MOF-NPC. An asymmetric supercapacitor has been developed using MOF-NPC/MnO2 hybrids as positive electrode and MOF-NPC as negative electrode in a neutral aqueous Na2SO4 electrolyte. Due to the excellent structure of MOF-NPC as a porous scaffold, optimized asymmetric device could be reversibly cycled in the voltage range of 0–2.2V, and exhibits a maximum energy density of 76.02Whkg−1 (for a power density of 2.20kWkg−1) and a maximum power density of 22.00kWkg−1 (for an energy density of 49.56Whkg−1), demonstrating its strong potential for the practical applications of energy storage devices.

Synthesis and loading-dependent characteristics of nitrogen-doped graphene foam/carbon nanotube/manganese oxide ternary composite electrodes for high performance supercapacitors

The ternary composite electrodes, nitrogen-doped graphene foam/carbon nanotube/manganese dioxide (NGF/CNT/MnO2), have been successfully fabricated via chemical vapor deposition (CVD) and facile hydrothermal method. The morphologies of the MnO2 nanoflakes presented the loading-dependent characteristics and the nanoflake thickness could also be tuned by MnO2 mass loading in the fabrication process. The correlation between their morphology and electrochemical performance was systematically investigated by controlling MnO2 mass loading in the ternary composite electrodes. The electrochemical properties of the flexible ternary electrode (MnO2 mass loading of 70%) exhibited a high areal capacitance of 3.03F/cm2 and a high specific capacitance of 284F/g at the scan rate of 2mV/s. Moreover, it was interesting to find that the capacitance of the NGF/CNT/MnO2 composite electrodes showed a 51.6% increase after 15,000 cycles. The gradual increase in specific capacitance was due to the formation of defective regions in the MnO2 nanostructures during the electrochemical cycles of the electrodes, which further resulted in increased porosity, surface area, and consequently increased electrochemical capacity. This work demonstrates a rarely reported conclusion about loading-dependent characteristics for the NGF/CNT/MnO2 ternary composite electrodes. It will bring new perspectives on designing novel ternary or multi-structure for various energy storage applications.

Electrochemical supercapacitive performance of potentiostatically cathodic electrodeposited nanostructured MnO2 films

Nanostructured MnO2 films were prepared via cathodic electrodeposition under potentiostatic condition. X-ray diffraction (XRD) analyses reveal that the deposited films possess the hexagonal phase of epsilon manganese dioxide (e-MnO2). Fourier transform infrared (FTIR) spectroscopy studies also confirm the manganese dioxide phase of the deposited films. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies show that the film deposited at the potential of 0.2 V has a porous network structure which is made of sparsely distributed grains. Cyclic voltammetry studies show the maximum specific capacitance to be 259.4 F/g at the scan rate of 5 mV/s for the film deposited at the potential of 0.2 V, while the chrono charge-discharge measurements on the film exhibit the maximum specific capacitance to be 325.6 F/g at the current density of 1 mA/cm2. The variation in specific capacitance values of the films deposited at different potentials is attributed to different morphologies of the films.

Electrodeposition of MnO2-Based Nanomaterials As Carbon-Free and Binder-Free Cathodes for Li-O2 Battery

Carbon based nanomaterials have been widely used as cathode materials for Li-O2 batteries because of their high conductivity, large surface area, and good ORR catalytic activity. However, instability of carbon under Li-O2 operation poses problems. During recharge, carbon can decompose to form lithium carbonate at high charging potential1, but also react chemically with the discharge product Li2O2 2. Metal oxides, like MnO2 and RuO2, have been investigated as cathode catalysts in Li-O2 batteries3. In this study, MnO2 nanostructure is electrodeposited on stainless steel mesh and applied as a carbon-free and binder-free cathode for non-aqueous Li-O2 battery. To maintain good adhesion and electrical contact between deposited layer and the substrate, the mesh is pre-treated. Benefiting from enhanced electronic contact and large surface area of MnO2, the electrochemical performance is improved. Material characterization and electrochemical impedance spectroscopy (EIS) technique are also utilized to systematically study the electrochemical processes of these carbon-free MnO2-based electrodes.