Effect of the KMnO<Subscript>4</Subscript> concentration on the structure and electrochemical behavior of MnO<Subscript>2</Subscript>

Various MnO₂ structures have been synthesized in this study by a facile method through the manipulation of KMnO₄ concentration. The effect of KMnO₄ concentration on the structure and morphology has been thoroughly investigated by X-ray powder diffraction, field-emission scanning electron microscopy, and nitrogen adsorption–desorption measurements. Birnessite-type MnO₂ flower-like microsphere can be obtained at a lower concentration, while α-MnO₂ nanorods can be obtained at a higher concentration. By increasing the KMnO₄ concentration further, α-MnO₂ nanorods can assemble into urchin-like microspheres and spherical aggregates. The possible formation mechanism based on the experimental results has been proposed to understand their growth procedures. The electrochemical properties of the synthesized materials evaluated by cyclic voltammetry and constant-current charge–discharge cycling techniques are dependent on their pore size distribution and nanostructures. Birnessite-type MnO₂ microsphere showed excellent capacitive behavior with a maximum specific capacitance of 185 F g⁻¹.     

Supercapacitive performance of hydrous ruthenium oxide (RuO<Subscript>2</Subscript>·<Emphasis Type="Italic">n</Emphasis>H<Subscript>2</Subscript>O) thin films deposited by SILAR method

The amorphous hydrous ruthenium oxide (RuO₂·nH₂O) thin films were deposited by a simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method. These films were characterized for their structural, surface morphological, and compositional study by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDAX) techniques. The wettability test was carried out by measuring the water contact angle. The scanning electron microscopy study showed small RuO₂ particles are grouped together to form porous agglomerates. The FT-IR study confirmed the formation of hydrous ruthenium oxide films. The hydrophilic nature of ruthenium oxide (RuO₂·nH₂O) thin films was observed from water contact angle measurement. The presence of Ru and O in the film was confirmed by EDAX analysis. The supercapacitor behavior of these films studied in 0.5 M H₂SO₄ electrolyte showed maximum specific capacitance of 162 F g⁻¹ at 10 mV s⁻¹ scan rate. These films exhibit 80% cycling performance after 2,000 cycles. The charge–discharge studies carried at 1 mA cm⁻² current density revealed the specific power of 3.5 KW kg⁻¹ and specific energy of 29.7 W Kg⁻¹ with 93% coulombic efficiency.     

Preparation and pseudo-capacitance of birnessite-type MnO2 nanostructures via microwave-assisted emulsion method

A microwave-assisted emulsion process has been developed to synthesize birnessite-type MnO₂ one-dimensional (1D) nanostructures. The prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). TEM images confirmed that the particles were composed of nanowires and nanobelts. As a consequence of the small size, such MnO₂ nanostructures exhibit a high specific capacitance of 277 F g⁻¹ at the current density of 0.2 mA cm⁻². Furthermore, the simple synthetic approach may provide a convenient route for the preparation of birnessite-type MnO₂ nanowires and other 1D nanostructured materials on a large scale.     

Synthesis of polypyrrole film by pulse galvanostatic method and its application as supercapacitor electrode materials

Polypyrrole (PPy) film-coated stainless steel electrodes were prepared from aqueous solution containing 0.5 M p-toluene sulphonic acid and 0.1 M pyrrole using pulse galvanostatic method (PGM) and galvanostatic method (GM). The morphology was characterized by scanning electron microscopy. The electrochemical properties of PPy films were investigated with cyclic voltammetry, charge–discharge tests, and ac impedance spectroscopy. The results showed that the PGM-PPy films exhibited higher specific capacitance, better high-rate discharge ability and lower resistance, and were more promising for applications in supercapacitor than GM-PPy films. The specific capacitance (SC) of PGM-PPy films was 403 F g⁻¹ in 1 M H₂SO₄ electrolyte and 281 F g⁻¹ in 1 M NaNO₃ electrolyte.     

Synthesis of copper oxide vegetable sponges and their antibacterial,electrochemical and photocatalytic performance

Copper oxide (CuO) vegetable sponges with mesoporous structure were successfully prepared from pine tree needles cellulose as templates. The sample was characterized by X-ray diffraction, scanning electron microscopy and nitrogen absorption–desorption. In the performance test, the obtained CuO vegetable sponges showed great antibacterial activities, as good as penicillin and kanamycin, especially towards Streptococcus faecalis. They also displayed active photocatalytic degradation of rhodamine B molecules in aqueous solution. Electrochemical data demonstrated that the CuO vegetable sponges were capable of delivering a specific capacitance of 440 F g⁻¹ at a current density of 1 A g⁻¹ and offered good specific capacitance retention of 95.1% after 1,000 continuous charge–discharge cycles even at 5 A g⁻¹.     

Rapid sonochemical synthesis of mesoporous MnO2 for supercapacitor applications

Mesoporous MnO₂ samples with average pore-size in the range of 2–20 nm are synthesized in sonochemical method from KMnO₄ by using a tri-block copolymer, namely, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) as a soft template as well as a reducing agent. The MnO₂ samples are found to be poorly crystalline. On increasing the amplitude of sonication, a change in the morphology of MnO₂ from nanoparticles to nanorods and also change in porosity are observed. A high BET surface area of 245 m² g⁻¹ is achieved for MnO₂ sample. The MnO₂ samples are subjected to electrochemical capacitance studies by cyclic voltammetry (CV) and galvanostatic charge–discharge cycling in 0.1 M aqueous Ca(NO₃)₂ electrolyte. A maximum specific capacitance (SC) of 265 F g⁻¹ is obtained for the MnO₂ sample synthesized in sonochemical method using an amplitude of 30 μm. The MnO₂ samples also possess good electrochemical stability due to their favourable porous structure and high surface area.     

Preparation and characteristics of nanostructured MnO2/MWCNTs using microwave irradiation method

Ball-nanostructured MnO₂/MWCNTs composite was successfully prepared by microwave irradiation. The surface morphology and structures of the composite were examined by scanning electron microscope and X-ray diffraction. Multi-walled carbon nanotubes play a role as sustainment to inhibit MnO₂ nanoplates from collapsing into nanorods. The electrochemical studies indicated that the composite had ideal capacitive performance and high specific capacitances of 298 F g^− 1, 213 F g^− 1 and 198 F g^− 1 at the current density of 2 mA·cm^− 2, 10 mA·cm^− 2 and 20 mA·cm^− 2, respectively. The formation mechanism of nanostructured MnO₂/MWCNTs and the electrochemical behaviour of composites were discussed in detail.     

Self-template route to MnO2 hollow structures for supercapacitors

Birnessite-type MnO₂ hierarchical hollow structures were prepared through a self-template route, by the direct reaction between the aqueous solution of KMnO₄ and solid MnCO₃ precursor crystals, and followed by the removal of MnCO₃ core with HCl. Field emission scanning microscopy (FESEM) images indicate that the shells of hierarchical hollow structures consist of the interconnected sheets with a thickness of about 30 nm, and transmission electron microscopy (TEM) images show that the thickness of the shells can be adjusted over a range from 50 to 80 nm by changing the molar ratio of MnCO₃/KMnO₄. The electrochemical properties of the as-prepared MnO₂ were characterized by cyclic voltammetry (CV) and galvanostatic charge-discharge tests in 1 M Na₂SO₄ solution. The sample obtained at a higher MnCO₃/KMnO₄ molar ratio (i.e., 50:1) shows a relatively higher specific capacitance of 169 F g^− 1 than 111 F g^− 1 of the sample obtained under a lower molar ratio of 25:1 at the current density of 250 mA g^− 1.     

Microwave adsorption of core–shell structured Sr(MnTi)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>12−2<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>19</Subscript>/PANI composites

Polyaniline polymer-coated MnTi-substituted strontium hexaferrite (Sr(MnTi) _x Fe_12−2x O₁₉/PANI, x = 1.0, 1.5, 2.0) composites were synthesized by the oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate. The structure and morphologies of the products were characterized by X-ray diffraction, FT-IR, TGA, SEM, and TEM. In the magnetization for the Sr(MnTi) _x Fe_12−2x O₁₉/PANI composites, it was found that the saturation magnetization (M _s) and coercivity (H _c) decreased after polyaniline coating. The composite under an applied magnetic field exhibited hysteretic loops of ferromagnetic behavior, such as high saturation magnetization (M _s = 12.1–1.9 emu/g) and coercivity (H _c = 0.919–0.084 kG). The composite specimens of core–shell Sr(MnTi) _x Fe_12−2x O₁₉/PANI and thermal plastic resin had a band-width microwave absorption due to the reflection losses from −15 to −35 dB at frequencies between 18 and 40 GHz as observed by a high-frequency network analyzer.     

Effect of doping cobalt on the micro-morphology and electrochemical properties of birnessite MnO2

Birnessite-type MnO₂ nanoparticles are synthesized by mixing KMnO₄ solution directly with ethylene glycol under ambient conditions. When cobalt exists in the solution, the micro-morphology of the products transforms from conglomeration to dispersive state. The result of transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) shows that the product is constructed with nanosphere in sizes of ca. 40 nm. These nanospheres are twisted by nanorods clusters. X-ray diffraction (XRD) pattern shows that the products are birnessite-type. The electrochemical properties of the prepared materials are studied using cyclic voltammetry (CV) and galvanostatic charge–discharge test in aqueous electrolyte. The product shows a very high specific capacity of 326.4 F g⁻¹. These results indicate that cobalt has great effects on the micro-morphology and electrochemical properties of manganese dioxide.     

Graphene/MnO2 hybrid nanosheets as high performance electrode materials for supercapacitors

Graphene/MnO₂ hybrid nanosheets were prepared by incorporating graphene and MnO₂ nanosheets in ethylene glycol. Scanning electron microscopy and transmission electron microscopy analyses confirmed nanosheet morphology of the hybrid materials. Graphene/MnO₂ hybrid nanosheets with different ratios were investigated as electrode materials for supercapacitors by cyclic voltammetry (CV) and galvanostatic charge–discharge in 1 M Na₂SO₄ electrolyte. We found that the graphene/MnO₂ hybrid nanosheets with a weight ratio of 1:4 (graphene:MnO₂) delivered the highest specific capacitance of 320 F g⁻¹. Graphene/MnO₂ hybrid nanosheets also exhibited good capacitance retention on 2000 cycles.     

Sponge-like β-Ni(OH)<Subscript>2</Subscript> nanoparticles: synthesis,characterization and electrochemical properties

The present investigation describes the synthesis of uniform sponge-like Ni(OH)₂ nanoparticles on stainless steel substrates by a two-step successive ionic layer adsorption and reaction method; the study also explores electrochemical properties. The formation of the β-phase Ni(OH)₂ and it’s nanocrystallinity are confirmed by X-ray diffraction and X-ray photoelectron spectroscopy studies. Scanning electron microscopy revealed the formation and random distributions of porous and sponge-like nanoparticles with high Brunauer–Emmett–Teller surface area of 56.4 m² g⁻¹. The maximum specific capacitance of 428 F g⁻¹ was obtained at 5 mV s⁻¹ in a 2 M KOH electrolyte, indicating promising supercapacitor applications with remarkable rate capability. These results suggest the importance of rational design and synthesis of thin nanomaterials for high-performance energy applications.     

Development of redox deposition of birnessite-type MnO2 on activated carbon as high-performance electrode for hybrid supercapacitors

Birnessite-type MnO₂/activated carbon nanocomposites have been synthesized by directly reducing KMnO₄ with activated carbon in an aqueous solution. It is found that the morphologies of MnO₂ grown on activated carbon can be tailored by varying the reaction ratio of activated carbon and KMnO₄. An asymmetric supercapacitor with high energy density was fabricated by using MnO₂/activated carbon (MnO₂/AC) nanocomposite as positive electrode and activated carbon as negative electrode in 1 M Na₂SO₄ aqueous electrolyte. The asymmetric supercapacitor can be cycled reversibly in the cell voltage of 0–2 V, and delivers a specific capacitance of 50.6 F g⁻¹ and a maximum energy density of 28.1 Wh kg⁻¹ (based on the total mass of active electrode materials of 9.4 mg), which is much higher than that of MnO₂/AC symmetric supercapacitor (9.7 Wh kg⁻¹).     

Synthesis and capacitive property of δ-MnO2 with large surface area

δ-MnO₂ with layered structure is synthesized in a mixed system of KMnO₄ and C₃H₆O (epoxypropane) by a facile low-temperature hydrothermal method at 90 °C for 24 h. The obtained product is characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and N₂ adsorption–desorption, and its electrochemical property was investigated by cyclic voltammetry method. Experiment results show that the as-synthesized product has a layered structure and a high specific surface area of 188 m² g⁻¹, and C₃H₆O existing in the reaction system plays a crucial role for the formation of δ-MnO₂ particles. Electrochemical characterization indicates that the prepared material exhibits an ideal capacitive behavior with the initial capacitance value of 296 F g⁻¹ in 1 mol L⁻¹ Na₂SO₄ aqueous solution at a scan rate of 5 mV s⁻¹ and good cycling behavior.     

Structure and electrochemical hydrogen storage properties of Pd/Mg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Pd thin films prepared by pulsed laser deposition

Three-layered Pd/Mg_1−x Al _x /Pd (x = 0, 0.13, 0.21, 0.39) thin films were prepared by means of pulsed laser deposition. In the present Al concentration range, X-ray diffraction analyses showed that the Mg_1−x Al _x layer was constituted of a single phase Mg(Al) solid solution. The Mg(Al) grains are preferentially orientated along the c-axis and their size decreased (from 18.5 to 10.5 nm) as the Al content increased. Scanning electron microscopy and atomic force microscopy observations indicated that all the films exhibited a globular surface structure. However, the surface roughness of the films decreased as the Al concentration increased. Rutherford backscattering spectroscopy revealed that the Mg–Al layer density (porosity) was strongly dependent on the Al content. Successive hydriding charge/discharge cycles were performed on the different Pd/Mg_1−x Al_x/Pd films in alkaline media. The highest discharge capacity was obtained with the Pd/Mg_0.79Al_0.21/Pd film, namely ~85 μAh cm⁻² μm⁻¹ or 320 mAh g⁻¹, which corresponds to a H/M atomic ratio of ~0.48 in the Mg–Al layer.     

Chemically activated graphene/porous Si@SiOx composite as anode for lithium ion batteries

Chemically activated graphene/porous Si@SiO_x (CAG/Si@SiO_x) composite has been synthesized via magnesiothemic reduction of mesoporous SiO₂ (MCM-48) to porous Si@SiO_x and dispersing in the suspension of chemically activated graphene oxide (CAGO) followed by thermal reduction. The porous Si@SiO_x particles are well encapsulated in chemically activated graphene (CAG) matrix. The resulting CAG/Si@SiO_x composite exhibits a high reversible capacity and excellent cycling stability up to 763 mAh g⁻¹ at a current density of 100 mA g⁻¹ after 50 cycles. The porous structure of CAG layer and Si@SiO_x is beneficial to accommodate volume expansion of Si during discharge and charge process and the interconnected CAG improves the electronic conductivity of composite.     

Synthesis and characterization of porous-crystalline C/Fe3O4 microspheres by spray pyrolysis with steam oxidation as anode materials for Li-ion batteries

This work introduces a novel synthesis route for the porous-crystalline C/Fe₃O₄ microspheres by spray pyrolysis with post oxidation under a steam atmosphere. The dense-amorphous C/Fe₃O₄ microspheres prepared by spray pyrolysis are firstly annealed at 700 °C for 4 h under 3 %H₂/N₂ atmosphere to crystallize the amorphous carbon and introduce micro- and meso-pores. The reduced Fe₃C nanoparticles are then oxidized into highly-crystalline Fe₃O₄ at 500 °C for 2 h under H₂O_(g)/N₂ atmosphere. The XRD, Raman, XPS, and N₂ sorption analysis confirm the successful formation of C/Fe₃O₄ microspheres with well-developed porous structure and highly conductive graphitic carbon. The porous-crystalline C/Fe₃O₄ microspheres demonstrate a higher specific capacity of ～560 mA h g^?1 at a current density of 50 mA g^?1 than the dense-amorphous microspheres, which corresponds to 92% of its theoretical value. Moreover, the sample has an improved rate capability (240 mA h g^?1 even at 5000 mA g^?1) and stable long-term cycling performance due to the synergy between porous structure and graphitic carbon.     

Investigation of the quasi-ternary system LaMnO3–LaCoO3–“LaCuO3”. II: The series LaMn0.25?xCo0.75?xCu2xO3?δ and LaMn0.75?xCo0.25?xCu2xO3?δ

This paper investigates the crystal structure, thermal expansion, and electrical conductivity of two series of perovskites (LaMn_0.25−x Co_0.75−x Cu_2x O_3−δ and LaMn_0.75−x Co_0.25−x Cu_2x O_3−δ with x = 0, 0.025, 0.05, 0.1, 0.15, 0.2, and 0.25) in the quasi-ternary system LaMnO₃–LaCoO₃–“LaCuO₃”. The Mn/Co ratio was found to have a stronger influence on these properties than the Cu content. In comparison to the Co-rich series (LaMn_0.25−x Co_0.75−x Cu_2x O_3−δ), the Mn-rich series (LaMn_0.75−x Co_0.25−x Cu_2x O_3−δ) showed a much higher Cu solubility. All compositions in this series were single-phase materials after calcination at 1100 °C. The Co-rich series showed higher thermal expansion coefficients (α_max = 19.6 × 10⁻⁶ K⁻¹) and electrical conductivity (σ_max = 730 S/cm at 800 °C) than the Mn-rich series (α_max = 10.6 × 10⁻⁶ K⁻¹, σ_max = 94 S/cm at 800 °C). Irregularities in the thermal expansion curves indicated phase transitions at 150–350 °C for the Mn-rich series, while partial melting occurred at 980–1000 °C for the Co-rich series with x > 0.15. I. Arul Raj—on leave from Central Electrochemical Research Institute, Karaikudi, 630006 India.     

Crystal structure and negative thermal expansion of solid solution Y<Subscript>2</Subscript>W<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>12</Subscript>

A new series of solid solutions Y₂W_3−x Mo _x O₁₂ (0.5 ≤ x ≤ 2.5) were successfully synthesized by the solid state method. Their crystal structure and negative thermal expansion properties were studied using high-temperature X-ray powder diffraction and the Rietveld method. All samples of rare earth tungstates and molybdates were found to crystallize in the same orthorhombic structure with space group Pnca, and show the negative thermal expansion phenomena related to transverse vibration of bridging oxygen atoms in the structure. Thermal expansion coefficients (TEC) of Y₂W_3−x Mo _x O₁₂ were determined as −16.2 × 10⁻⁶ K⁻¹ for x = 0.5 and −16.5 × 10⁻⁶ K⁻¹ for x = 2.5 in the identical temperature range of 200–800 °C. High-temperature XRD data and bond length analysis suggest that the difference between W–O and Mo–O bond is responsible for the change of TECs after the element substitution in this series of solid solutions.     

Effect of Bivalent Cation Ca-Doping on Magnetic Properties in Multiferroic YMnO<Subscript>3</Subscript> Manganites

Effect of bivalent cation Ca-doping on magnetic properties in multiferroic YMnO₃ manganites was systemically studied by DC and AC magnetic measurements. Series of samples were prepared by solid-state reaction method with composition Y_1−x Ca _x MnO₃ (x=i/8,i=2,3,…,7). The results show that each of the zero-field-cooling (ZFC) curves has a clear cusp and it shifts to higher temperature with increasing Ca-doping x. From the χ ⁻¹–T curve, the temperature of charge-ordering (CO) state T _CO is given at about 244.0 K and this indicates that the CO state is dominated by the cooperative Jahn–Teller effect in Y_1/2Ca_1/2MnO₃ with small A-site cation, for which there is a different behavior compared to the other manganites of larger 〈r _A 〉. When x=2/8 and 7/8 for Y_1−x Ca _x MnO₃, the magnetization curve shows strong irreversibility between ZFC and FC data. The variation of AC susceptibility presents a maximum cusp at the beginning temperature of the bifurcation between ZFC and ZF curve and shifts to higher temperature with increasing measurement frequency for AC susceptibility. These results can be characterized by spin-glass, indicating that much more itinerant e_g electrons are promoting the magnetic transition.