Fibrous MnO2 electrode electrodeposited on carbon fiber for a fuel cell/battery system

A novel fibrous MnO₂ electrode for a fuel cell/battery system is fabricated on carbon fiber by the electrodeposition method. The characteristics of the fibrous MnO₂ electrode are examined by electrochemical impedance spectra, galvanostatic performance and cyclic voltammetry. The experimental results indicate that the fibrous MnO₂ electrodes are superior to pasted electrodes because of the following: (i) better contact between MnO₂ and the electrical conducting material; (ii) high charge-transfer rate because of a smaller diameter than conventional electrodeposited MnO₂ particles (thus it is expected that the specific surface area would be higher); and (iii) a low overpotential. The morphology and the crystal structure of the fibrous MnO₂ electrode are investigated by scanning electron microscopy and X-ray diffraction, respectively. The entire surface of the carbon fiber is found to be coated with γ-MnO₂ after 2 h of electrodeposition at 0.01 A dm⁻² current density.     

Manganese oxide hollow structures with different phases: Synthesis, characterization and catalytic application

Manganese oxide hollow structures with different phases were synthesized by calcination using a γ-MnO₂ hollow structure as a precursor. Our results revealed that the as-synthesized β-MnO₂ and Mn₂O₃ still maintained the morphology of the precursor. Compared with the MnO₂ reported and conventional MnO₂, both the γ-MnO₂ and β-MnO₂ hollow structures showed a higher catalytic activity and an excellent selectivity in the aerobic oxidation of cinnamyl alcohol to cinnamyl aldehyde. The TPR experiments demonstrated that the reduced size of crystallite, hollow nature and higher BET specific surface area were responsible for their higher catalytic activity.     

Preparation and electrochemical profile of Li0.33MnO2 nanorods as cathode material for secondary lithium batteries

We report the preparation of Li_0.33MnO₂ nanorods from γ-MnO₂ nanorods reacted with LiNO₃ by a low temperature solid-state reaction method. The Li_0.33MnO₂ nanorods tend to be oriented along the b-axis, and show an improved rate capability and cycling performance as positive electrode for lithium battery. It delivers a discharge capacity of 199 and 129 mAh/g at the current rate of 0.1 C (20 mA/g) and 2 C, respectively, and keeps 92% of initial capacity over 100 cycles. Li_0.33MnO₂ nanorods reduce both the electrode bulk resistance and charge-transfer resistance for lithium-ion intercalation. The advantage of nanorods results from good electrical conduction, appropriate length of nanorod and small volume expansion from appropriate orientations of tunnels structure.     

Preparation and electrochemical property of three-phase gas-diffusion oxygen electrodes for metal air battery

Three-phase gas-diffusion oxygen electrodes for metal air battery were prepared and characterized. Nano-structured γ-MnO₂ catalysts were synthesized by solid state redox reaction of two compounds, Mn(CH₃COO)₂·4H₂O and C₂H₂O₄·2H₂O. Their crystal phase, morphologies and particle size were characterized by XRD, TEM, respectively. The electrochemical property of three-phase gas-diffusion oxygen electrodes composed of nano-structured γ-MnO₂ catalysts for oxygen reduction was examined by using the linear polarization method in a neutral solution. Besides, the surface morphologies of the catalytic layer of three-phase gas-diffusion oxygen electrodes were also investigated by SEM. Experimental results revealed that these kinds of three-phase gas-diffusion oxygen electrodes have excellent electrochemical performance. The optimal proportion of nano-structured γ-MnO₂ catalysts in the catalytic layer was 60 wt.%. Three-phase gas-diffusion oxygen electrodes composed with nano-structured γ-MnO₂ catalysts appear to be a highly possible candidate for applications in neutral solution metal air battery.     

Reduced graphene oxide anchored with δ-MnO2 nanoscrolls as anode materials for enhanced Li-ion storage

We developed a one-pot in situ synthesis procedure to form nanocomposite of reduced graphene oxide (RGO) sheets anchored with 1D δ-MnO₂ nanoscrolls for Li-ion batteries. The as-prepared products were characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The electrochemical performance of the δ-MnO₂ nanoscrolls/RGO composite was measured by galvanostatic charge/discharge cycling and electrochemical impedance spectroscopy. The results show that the δ-MnO₂ nanoscrolls/RGO composite displays superior Li-ion battery performance with large reversible capacity and high rate capability. The first discharge and charge capacities are 1520 and 810 mAh g⁻¹, respectively. After 50 cycles, the reversible discharge capacity is still maintained at 528 mAh g⁻¹ at the current density of 100 mAh g⁻¹. The excellent electrochemical performance is attributed to the unique nanostructure of the δ-MnO₂ nanoscrolls/RGO composite, the high capacity of MnO₂ and superior electrical conductivity of RGO.     

Facile Synthesis of Novel Nanostructured MnO2 Thin Films and Their Application in Supercapacitors

Nanostructured α-MnO₂ thin films with different morphologies are grown on the platinum substrates by a facile solution method without any assistance of template or surfactant. Microstructural characterization reveals that morphology evolution from dandelion-like spheres to nanoflakes of the as-grown MnO₂ is controlled by synthesis temperature. The capacitive behavior of the MnO₂ thin films with different morphologies are studied by cyclic voltammetry. The α-MnO₂ thin films composed of dandelion-like spheres exhibit high specific capacitance, good rate capability, and excellent long-term cycling stability.     

A CTAB-assisted hydrothermal synthesis of VO2(B) nanostructures for lithium-ion battery application

Nanostructured VO₂(B) was synthesized via a combined hydrothermal method using V₂O₅ as a source material and oxalic acid powder as a reductant. Especially, cetyltrimethylammonium bromide (CTAB) was used as template and then three different morphologies of the VO₂(B): nanobelts, nanoflowers and nanoflakes were obtained through the change of the experimental conditions. The morphology and crystalline structure of the prepared products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Furthermore, the electrochemical charge–discharge cycling properties of the VO₂(B) nanostructures in lithium-ion battery were investigated. The results indicated that the belt-like, flower-like and flake-like VO₂(B) nanostructures have the initial specific discharge capacity of 205.2, 254.0 and 56.0 mA h g⁻¹, and that the morphology of VO₂(B) nanostructures can deeply affect the service performance of batteries. According to the experiments, this CTAB-assisted hydrothermal method provides an insight into the preparation and application of nanostructured VO₂(B) as cathode material in lithium-ion battery.     

Charge cycling and impedance characterization of a polyphosphazene solid polymer electrolyte-manganese(IV) oxide intercalation cathode

The compatibility of polyphosphazene (PPZ) polymer electrolytes with MnO₂/C/SPE intercalation cathodes (IC) was investigated. Three-layered laminates of a phosphazene-based solid polymer electrolyte (SPE) film sandwiched between two MnO₂-based ICs (one preloaded with lithium) were constructed. The cathodes were fabricated by either solvent casting or compression techniques. Two different crystal forms of manganese(IV) oxide—λ-MnO₂ and γ-MnO₂—were employed, together with methoxy ethoxy ethoxy PPZ (MEEP) SPE binder material. Carbon black was employed as the electronically conductive phase. One cathode in each laminate was prepared in the ‘chemically intercalated’ form by using LiMn₂O₄ in place of MnO₂. The podand polymer, SMEP, which has better thin film mechanical properties than does MEEP, was complexed with lithium trifluoromethane sulfonate (Li triflate) and used as the SPE. Li⁺ ions were cycled galvanostatically between the two-ICs, through the phosphazene-based SPE layer. The performance of the cell was continuously monitored by electrochemical impedance spectroscopy (EIS) and by measuring the laminate thickness and voltage drop. The method of cathode fabrication (casting vs. pressing) was found to be the primary factor influencing the cycle life.     

Synthesis and electrochemical properties of two types of highly ordered mesoporous MnO2

Mesoporous MnO₂ with uniform nanorod morphology and mesoporous β-MnO₂ were prepared using SBA-15 and KIT-6 as the templates, respectively. XRD, nitrogen adsorption analysis, SEM, TEM and EDX techniques were used for the structural characterization. The electrochemical properties of the MnO₂ samples were studied using alkaline Zn/MnO₂ batteries in a 9 M KOH electrolyte solution. Compared to the commercial electrolytic manganese dioxide (EMD), the discharge capacity of the mesoporous MnO₂ nanorods increased by 74.98%, 119.74% and 146.19% at constant currents of 50, 250 and 500 mA g⁻¹, respectively, while the discharge capacity of the mesoporous β-MnO₂ increased by 63.58%, 95.14% and 100.23%.     

Controllable synthesis of MnO2/polyaniline nanocomposite and its electrochemical capacitive property

Polyaniline (PANI) and MnO₂/PANI composites are simply fabricated by one-step interfacial polymerization. The morphologies and components of MnO₂/PANI composites are modulated by changing the pH of the solution. Formation procedure and capacitive property of the products are investigated by XRD, FTIR, TEM, and electrochemical techniques. We demonstrate that MnO₂ as an intermedia material plays a key role in the formation of sample structures. The MnO₂/PANI composites exhibit good cycling stability as well as a high capacitance close to 207 F g⁻¹. Samples fabricated with the facile one-step method are also expected to be adopted in other field such as catalysis, lithium ion battery, and biosensor.     

Flexible electrodes based on polypyrrole/manganese dioxide/polypropylene fibrous membrane composite for supercapacitor

The composites of polypyrrole/manganese dioxide/polypropylene fibrous films (PPy/MnO₂/PPF) have been prepared in situ through chemical oxidation polymerization by using the mixture of FeCl₃·6H₂O and MnO₂ adsorbed on PPF as oxidant in the atmosphere of pyrrole vapor at room temperature. The morphologies and structures of the composites are investigated by using scanning electron microscope and X-ray diffraction spectroscopy. The properties of the capacitor cells assembled by the composites of PPy/MnO₂/PPF are evaluated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy methods. The results reveal that the morphologies, conductivities and capacitance performance of the composites are influenced strongly by the content of MnO₂ in the solution of oxidant. The capacitors assembled by PPy/MnO₂/PPF exhibit the property of quick charge/discharge, and the highest specific capacitance of about 110 F g⁻¹ is obtained when the PPy/MnO₂ content in the composite is about 17.4%.     

The NaxMnO2 materials prepared by a glycine-nitrate method as advanced cathode materials for aqueous sodium-ion rechargeable batteries

Cathodic material for sodium-ion rechargeable batteries based on Na_xMnO₂ were synthesized by glycine nitrate method and subsequent annealing at high temperatures. Different crystal structures with different morphologies were obtained depending on the annealing temperature: hexagonal layeredα-Na_0.7MnO_2.05 nanoplates were obtained at 850 °C, while 3-D tunnel structured Na_0·4MnO₂ and Na_0·44MnO₂, both with rod-like morphology, were obtained at 800 °C and 900 °C, respectively. The investigations of the electrochemical behavior of obtained cathodic materials in aqueous NaNO₃ solution have shown that Na_0·44MnO₂ obtained at 900 °C has shown the best battery performance. Its initial discharge capacities are 123.5 mA h/g, 113.2 mA h/g, and 102.0 mA h/g at the high current densities of 1000, 2000 and 5000 mA/g, respectively.     

Potential of MnO2-based composite and numerous morphological for enhancing supercapacitors performance

The development of materials and electrochemical energy storage (EES) technologies are currently taking the lead and showing excellent performance in the global effort to tackle the issues of sustainable energy supply. Supercapacitors have been widely studied among the EES technologies as they exhibit quick charging rates under high-power conditions. Manganese dioxide (MnO₂) has attracted renewed interest as a promising material due to its high theoretical capacitance and high energy density. However, the widespread application is immediately impacted by low conductivity. Hence, combining nanomaterials and various morphologies of MnO₂ can improve the electrochemical performance of supercapacitors. This paper presents a review based on the composites of nanomaterials/MnO₂ with various morphologies. Their mechanism and practical applications in supercapacitors are introduced in detail. Finally, the challenges and next steps in developing MnO₂ electrode materials are proposed.     

Tailoring carboxyl tubular carbon nanofibers/MnO2 composites for high-performance lithium-ion battery anodes

Three kinds of novel carboxyl modification tubular carbon nanofibers (CMTCFs) and MnO₂ composites materials (CMTCFs/MnO₂) are prepared by combining hyper-crosslinking, liquid phase oxidation and hydrothermal technology. The complex morphology and crystal phase of MnO₂ in CMTCFs/MnO₂ are effectively regulated by adjusting the hydrothermal reaction time. The δ-MnO₂ nanosheet-wrapped CMTCFs (CMTCFs@MNS) are used as anode and compared with the other two CMTCFs/MnO₂. Electrochemical analysis shows that CMTCFs@MNS electrode exhibits a large reversible capacity of 1497.1 mAh g⁻¹ after 300 cycles at 1000 mA g⁻¹ and a long cycling reversible capacity of 400.8 mAh g⁻¹ can be maintained after 1000 cycles at 10 000 mA g⁻¹. CMTCFs@MNS manifests an average reversible capacity of 256.32 mAh g⁻¹ at 10 000 mA g⁻¹ after twelve changes in current density. In addition, the structural superiority of CMTCFs@MNS electrode is clarified by characterizing the microscopic morphology and crystal phase of the electrode after electrochemical performance test.     

Surfactant-free microwave hydrothermal synthesis of SnO2 nanosheets as an anode material for lithium battery applications

SnO₂ nanosheets were synthesized using microwave hydrothermal method without using a surfactant and organic solvents. Formation of pure nanocrystalline rutile phase of SnO₂ sample was confirmed by X-ray diffraction (XRD) results and the average crystallite size of SnO₂ sample calculated using Scherrer's formula and XRD data is found to be 6 nm. HR-TEM, SAED and EDX results showed the formation of agglomerated nanosize sheets like morphology with high porous structured SnO₂ powder. Further, the formation of high porous structured SnO₂ powder was confirmed from BET surface area results (59.28 m² g^?1). The electrochemical performance of the lithium-ion battery made up of SnO₂ nanosheets, as an anode, was tested through the cyclic voltammetry and galvanostatic charge-discharge measurements. The galvanostatic charge-discharge results of the lithium-ion battery showed good discharge capacity of 257.8 mAh g^?1 after 50 cycles at a current density of 100 mA g^?1. The improved electrochemical properties may be due to the formation of a unique nanosize sheets type morphology with high porous structured SnO₂ powder. High porous structured nanosize sheets type morphology of SnO₂ can help to reduce the diffusion length and sustain the volume changes during the charging-discharging process.Hence, high porous structured nanosize sheets morphology of SnO₂ prepared using the microwave hydrothermal method without using a surfactant and organic solvents can be a better anode material for lithium ion battery applications.     

General synthesis and morphology control of LiMnPO4 nanocrystals via microwave-hydrothermal route

Olivine structure LiMnPO₄ has been considered as one of the very promising electrodes for lithium-ion batteries because of their low cost, low toxicity and high voltage plateau compared with LiFePO₄. In order to improve the electrochemical performance a key challenge in the field of lithium-ion battery, is to explore and invent suitable synthetic route to control the size and morphology of LiMnPO₄. Here a detailed study exploring the novel route of microwave-hydrothermal (MH) synthesis for successfully obtaining LiMnPO₄ crystals within few minutes is reported and the reaction process discussed in detail. Variation of the synthetic parameters show that a decrease in reactant concentration could lead to LiMnPO₄ nano-platelets orientated in the ac plane with a very high electrochemical performance. The effect of the starting precursor (like, Mn) concentration as a means to tailor LiMnPO₄ electrochemical performance is discussed. The effect of alteration of size, morphology, lattice parameters and crystal structure induced by addition of additives like citric acid (H₃cit) and sodium dodecyl benzene sulfonate (SDBS) is further described and an example of the first reversible discharge of a product treated with H3cit obtained by MH route as high as 89.0 mAh/g, is shown. The general investigation demonstrates that there is a relationship among microwave irradiation condition, crystal structure, morphology and electrochemical performance that can be exploited for the design of next generation lithium batteries.     

Textural and capacitive characteristics of MnO2 nanocrystals derived from a novel solid-reaction route

Nanostructured manganese dioxide (MnO₂) materials were synthesized via a novel room-temperature solid-reaction route starting with Mn(OAc)₂·4H₂O and (NH₄)₂C₂O₄·H₂O raw materials. In brief, the various MnO₂ materials were obtained by air-calcination (oxidation decomposition) of the MnC₂O₄ precursor at different temperatures followed by acid-treatment in 2 M H₂SO₄ solution. The influence of calcination temperature on the structural characteristics and capacitive properties in 1 M LiOH electrolyte of the MnO₂ materials were investigated by X-ray diffraction (XRD), infrared spectrum (IR), transmission electron microscope (TEM) and Brunauer-Emmett-Teller (BET) surface area analysis, cyclic voltammetry, ac impedance and galvanostatic charge/discharge electrochemical methods. Experimental results showed that calcination temperature has a significant influence on the textural and capacitive characteristics of the products. The MnO₂ material obtained at the calcination temperature of 300 °C followed by acid-treatment belongs to nano-scale column-like (or needle-like) γ,α-type MnO₂ mischcrystals. While, the MnO₂ materials obtained at the calcination temperatures of 400, 500, and 600 °C followed by acid-treatment, respectively, belong to γ-type MnO₂ with the morphology of aggregates of crystallites. The γ,α-MnO₂ derived from calcination temperature of 300 °C exhibited a initial specific capacitance lower than that of the γ-MnO₂ derived from the elevated temperatures, but presented a better high-rate charge/discharge cyclability.     

Highly crystalline macroporous β-MnO2: Hydrothermal synthesis and application in lithium battery

A highly crystalline macroporous β-MnO₂ was hydrothermally synthesized using stoichiometric reaction between KMnO₄ and MnCl₂. The as-prepared material has a pore size of ca. 400 nm and a shell thickness of 300-500 nm. The formation of the macroporous morphology is related to self-assembling from nanowires of α-MnO₂, and could be obtained at high reactant concentrations (e.g., 0.8 M KMnO₄) but not at low ones (e.g., below 0.04 M KMnO₄). Compared to conventional bulk β-MnO₂ processing very low capacity, our macroporous material exhibits good electrochemical activity, e.g., obtaining an initial discharge capacity of 251 mAh g⁻¹ and sustaining as ca. 165 mAh g⁻¹ at 10 mA g⁻¹. The electrochemical activity of the as-prepared β-MnO₂ is related to its macroporous morphology and small shell thickness; the former leads to that electrolyte can flood pore of the material and its inner surface is available for lithium ion diffusion, while the latter helps to release the stress from phase transformation during the initial discharging. The X-ray diffraction characterizations of the macroporous β-MnO₂ electrodes suggest that, upon initial discharging, such a β-MnO₂ will be irreversibly transformed to an orthorhombic Li_xMnO₂ and then cycled within the new developed phase in the subsequent lithium insertion/extraction processes.     

Cathodic activity of deep-sea manganese nodule in Leclanché-type test dry cell

The present study has examined the performance of the manganese nodule by incorporating it into the Leclanché cathode mix of D-size test cells.It has been found that, with the particular species collected from the Pacific Ocean, off-shore California, the value of O.C.V. was about 1.62 V, which is quite close to the value with naturally occurring manganese dioxide ores of battery grade. An intermittent discharge on a 4 Ω load, down to a cut-off voltage of 0.75 V, showed approximately half the service duration time obtained with E.M.D.However, when taking into consideration a low MnO₂ content of about 38% in the deep-sea ore tested and the various constituent elements present therein together, the exhibited performance as battery cathode active material was quite surprising.Digestion of the deep-sea nodule with hot sulfuric acid revealed that almost all the original content of MnO₂ was retained in the digested residue, while a 10–20 per cent portion of the accompanying elements was brought into the digesting solution. The “up-graded” MnO₂ product of nodule showed a better battery performance in accordance with the increase in MnO₂ content.These results suggest that the MnO₂ phase present in the manganese nodule might be quite similar to the one possessed by the cathode active γ-MnO₂ species, and that the MnO₂ crystalline net work formation mechanisms involved in “autoclave” of the deep sea might have something in common with the anode oxidation product synthesized by electrolysis.     

The synthesis of Li(Ni1/3Co1/3Mn1/3)O2 using eutectic mixed lithium salt LiNO3–LiOH

A lithium-ion battery cathode material, Li(Ni_1/3Co_1/3Mn_1/3)O₂, with excellent electrochemical properties was prepared via two-step isothermal sintering, using eutectic lithium salts (0.38LiOH·H₂O–0.62LiNO₃) mixed with Co, Ni, or Mn hydroxides. Based on analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), a thermogravimetric-differential scanning calorimetric (TG–DSC) analyzer, and Fourier-transform Infrared (FT-IR), this synthetic process consists of procedures including lithium salt melting, permeation, reaction, crystalline transformation, and crystallization. Due to the lower melting point of the eutectic molten salts compared with that of the single lithium salt, a relatively mild synthetic condition (low temperature) is needed, and the product can be highly crystallized with low cation mixing, which facilitates maintenance of the precursor morphology. The electrochemical properties of the product were investigated by constant current discharge–charge and cyclic voltammetry. The results show that the initial discharge capacity is 160 mhA g⁻¹, with excellent cycling stability even after 50 cycles. We conclude that this novel eutectic molten salt method is a promising and practical approach for synthesizing cathode materials for lithium-ion batteries.