Facile synthesis and characterization of MnO<Subscript>2</Subscript> nanomaterials as supercapacitor electrode materials

MnO₂ nanomaterials are synthesized via calcinations in air at various temperatures. Amorphous MnO₂ masses appear between 100 and 300 °C and nanorods form above 400 °C. Transmission and scanning electron microscopy are used to observe the geometries of each material, with further structural analyses conducted using X-ray photoelectron spectroscopy, X-ray diffraction, and BET method. The electrochemical properties are investigated through galvanostatic charge/discharge cycling, electrochemical impedance spectra, and cyclic voltammetry within a three-electrode test cell filled with 1 mol L^?1 Na₂SO₄ solution. The slightly asymmetric galvanostatic cycling curves suggest that the reversibility of the Faradaic reactions are imperfect, requiring a larger time to charge than discharge. The specific capacitances of each sample are calculated and trends are identified, proving that the samples synthesized at higher temperatures exhibit poorer electrochemical behaviors. The highest calculated specific capacitance is 175 F g^?1 by the sample calcinated at 400 °C. However, the lower temperature samples exhibit more favorable geometric properties and higher overall average specific capacitances. For future research, it is suggested that surface modifications such as a carbon coating could be used in conjunction with the MnO₂ nanorods to reach the electrochemical properties required by contemporary industrial applications.     

Enhanced electrochemical performance of Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>F<Subscript>3</Subscript> for Na-ion batteries with nanostructure and carbon coating

Carbon-coating Na₃V₂(PO₄)₂F₃ nanoparticles (NVPF@C NP) were prepared by a hydrothermal assisted sol–gel method and applied as cathode materials for Na-ion batteries. The as-prepared nanocomposites were composed of Na₃V₂(PO₄)₂F₃ nanoparticles with a typical size of ~?100 nm and an amorphous carbon layer with the thickness of ~?5 nm. Cyclic voltammetry, rate and cycling, and electrochemical impedance spectroscopy tests were used to discuss the effect of carbon coating and nanostructure. Results display that the as-prepared NVPF@C NP demonstrates a higher rate capability and better long cycling performance compared with bare Na₃V₂(PO₄)₂F₃ bulk (72 mA h g^?1 at 10 C vs 39 mA h g^?1 at 10 and 1 C capacity retention of 95% vs 88% after 50 cycles). The remarking electrode performance was attributed to the combination of nanostructure and carbon coating, which can provide short Na-ion diffusion distance and rapid electron migration.     

Cathodic electrosynthesis of CuFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/CuO composite nanostructures for high performance supercapacitor applications

In this work, CuFe₂O₄/CuO nanocomposites have been synthesized by galvanostatic cathodic electrodeposition. The obtained nanocomposites were characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, Fourier Transform Infrared, and Brunauer–Emmett–Teller surface area analysis. The electrochemical properties of CuFe₂O₄/CuO nanocomposites were evaluated by cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical impedance spectroscopy in 1.0 M KOH. The CuFe₂O₄/CuO nanocomposites have shown the high specific capacitance of 322.49 F g^?1 at the scan rate of 1 mV s^?1. After 5000 cycles, 92% of this specific capacitance was retained. Although the prepared nanocomposite has shown a mediocre specific capacitance compared to other metal oxide-based materials, the low cost of the starting materials and the ease of preparation make this nanocomposite a good candidate for supercapacitor applications.     

Electrochemical performance of Sn-doped δ-MnO<Subscript>2</Subscript> hollow nanoparticles for supercapacitors

Sn-doped δ-MnO₂ (Sn-MnO₂) hollow nanoparticles have been synthesized via chemical process at room temperature. Many characterizations have been carried out to fully identify the intrinsic information of the as-prepared samples and investigate their electrochemical properties. The results indicate that the morphologies of the samples can be adjusted by changing the concentration of Sn while the capacitance of Sn-MnO₂ nanoparticles increased corresponded with that of the undoped δ-MnO₂ nanoparticles. The specific capacitance of Sn(1 at.%)-MnO₂ is up to 258.2 F g^??1 at a current density of 0.1 A g^??1. What’s more, over 90% of the initial specific capacitance still remains after 1000 cycles at a current density of 2.0 A g^??1, displaying excellent cycling stability.     

Facial synthesis of MnO<Subscript>2</Subscript>/three dimensional graphene composite and its application in supercapacitors

MnO₂ nanoparticle/three dimensional graphene composite (MnO₂/3DG) was synthesized by a hydrothermal template-free method and subsequent ultrasonic treatment in KMnO₄ solution. The MnCO₃/3DG particles can be detected after the hydrothermal process, which may be produced through the reaction between Mn²⁺ and \({\text{C}}{{\text{O}}_{\text{3}}}^{{\text{2}} - }\) due to the decarboxylation of GO under the hydrothermal condition. The final product MnO₂/3DG displayed high specific capacitance (324 F g^??1 at 0.4 A g^?1) and good cycle stability (91.1% capacitance retention after 5000 cycles). Furthermore, the asymmetric supercapacitor assembled with MnO₂/3DG and activated carbon (AC) exhibits an energy density of 33.78 Wh kg^?1 at the powder density of 380 W kg^?1. The excellent supercapacitance of the MnO₂/3DG composite may be due to the high pseudocapacitance of the dispersed MnO₂ nanoparticles and the conductive graphene with three dimensional porous microstructure.     

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.     

Investigation for the synthesis of hierarchical Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@MnO<Subscript>2</Subscript> nanoarrays materials and their application for supercapacitor

We designed and fabricated hierarchical Co₃O₄@MnO₂ nanoarrays directly grown on nickel foam by hydrothermal and calcination methods. After the investigation of growth mechanism, we found that the deposition of MnO₂ was based on the self-decomposition of KMnO₄ and the reducibility of Co₃O₄ during the hydrothermal process. Thanks to the hierarchical structure, the obtained electrode exhibited excellent capacitive performance in supercapacitor. It delivered 21.72 F cm^?2 at a current density of 5 mA cm^?2 and retained ~94 % capacitance of initial value after 5000 cycles.     

Three-dimensional heterostructured MnO2/graphene/carbon nanotube composite on Ni foam for binder-free supercapacitor electrode

In this article, three-dimensional (3D) heterostructured of MnO₂/graphene/carbon nanotube (CNT) composites were synthesized by electrochemical deposition (ELD)-electrophoretic deposition (EPD) and subsequently chemical vapour deposition (CVD) methods. MnO₂/graphene/CNT composites were directly used as binder-free electrodes to investigate the electrochemical performance. To design a novel electrode material with high specific area and excellent electrochemical property, the Ni foam was chosen as the substrate, which could provide a 3D skeleton extremely enhancing the specific surface area and limiting the huge volume change of the active materials. The experimental results indicated that the specific capacitance of MnO₂/graphene/CNT composite was up to 377.1 F g^?1 at the scan speed of 200 mV s^?1 with a measured energy density of 75.4 Wh kg^?1. The 3D hybrid structures also exhibited superior long cycling life with close to 90% specific capacitance retained after 500 cycles.     

High-performance flexible supercapacitors based on C/Na<Subscript>2</Subscript>Ti<Subscript>5</Subscript>O<Subscript>11</Subscript> nanocomposite electrode materials

A new solution method to synthesize Na₂Ti₅O₁₁ with titanium powder is presented, and the C/Na₂Ti₅O₁₁ nanocomposite with high specific surface area and tunnel structure as the electrode material has excellent electrochemical performance. The single electrode composed of the C/Na₂Ti₅O₁₁ nanocomposite based on carbon fiber fabric (CFF) has the highest area capacitance of 1066 mF cm^?2 at a current density of 2 mA cm^?2, which is superior to other titanates and Na-ion materials for supercapacitors (SCs). By scan-rate dependence cyclic voltammetry analysis, the capacity value shows both capacitive and faradaic intercalation processes, and the intercalation process contributed 81.7% of the total charge storage at the scan rate of 5 mV s^?1. The flexible symmetric solid-state SCs (C/Na₂Ti₅O₁₁/CFF//C/Na₂Ti₅O₁₁/CFF) based on different C/Na₂Ti₅O₁₁ mass were fabricated, and 7 mg SCs show the best supercapacitive characteristics with an area capacitance of 309 mF cm^?2 and a specific capacitance of 441 F g^?1, it has a maximum energy density of 22 Wh kg^?1 and power density of 1286 W kg^?1. As for practical application, three SCs in series can power 100 green light-emitting diodes (LEDs) to light up for 18 min, which is much longer than our previous work by Wang et al. lighting 100 LEDs for 8 min. Thus, the C/Na₂Ti₅O₁₁ nanocomposite has promising potential application in energy storage devices.     

Improvement of hydrothermally synthesized MnO2 electrodes on Ni foams via facile annealing for supercapacitor applications

Nanostructured manganese dioxide (MnO₂) is deposited on nickel foams by a hydrothermal synthesis route. As-deposited MnO₂ thin films are largely amorphous. Facile post-deposition annealing significantly improves the electrochemical performance of the MnO₂ thin films via changing their morphology, phase, and crystallinity. The specific capacitance of the MnO₂ electrode increases with the annealing temperature and reaches an optimal value of 244 F g^?1 (at the current density of 1 A g^?1) in a neutral 1 M Na₂SO₄ electrolyte for a specimen annealed at 500 °C. Furthermore, when an alkaline 5 M KOH electrolyte is used, an exceptionally high capacitance of 950 F g^?1 is achieved at the current density of 2 A g^?1. The cost-effective facile synthesis, high specific capacitance, and good cycle stability of these MnO₂-based electrodes enable their applications in high-performance supercapacitors.     

Enhanced supercapacitive performances of hierarchical porous nanostructure assembled from ultrathin MnO2 nanoflakes

Ultrathin mesoporous nanoflakes of α-MnO₂ films were easily prepared by mixing potassium permanganate (KMnO₄) with hydrochloric acid in an aqueous solution. The obtained α-MnO₂ has a tunnel structure composed of an edge-shared network of [MnO₆] octahedra. Scanning electron micrograph observations revealed that the obtained MnO₂ materials have 2D frameworks which consisted of highly porous interconnected nanoflakes. TEM image shows that the ultrathin nanoflakes almost have the thickness about 3–5 nm. Nitrogen desorption analyses showed that these ultrathin MnO₂ nanoflakes exhibited large surface area up to 102 m² g^?1. The electrochemical performances of the synthesized α-MnO₂ ultrathin nanoflakes as supercapacitor electrode materials were studied by cyclic voltammetry cycling in 1 M Na₂SO₄ solution. The result showed that mesoporous MnO₂ with 2D frameworks exhibit a high capacitance of 328 Fg^?1 with 84 % stability for 1000 cycles. Thus, the obtained ultrathin α-MnO₂ nanoflakes are suitable for its use as supercapacitor electrode materials.     

High Volumetric Energy Density Asymmetric Supercapacitors Based on Well‐Balanced Graphene and Graphene‐MnO2 Electrodes with Densely Stacked Architectures

The well‐matched electrochemical parameters of positive and negative electrodes, such as specific capacitance, rate performance, and cycling stability, are important for obtaining high‐performance asymmetric supercapacitors. Herein, a facile and cost‐effective strategy is demonstrated for the fabrication of 3D densely stacked graphene (DSG) and graphene‐MnO₂ (G‐MnO₂) architectures as the electrode materials for asymmetric supercapacitors (ASCs) by using MnO₂‐intercalated graphite oxide (GO‐MnO₂) as the precursor. DSG has a stacked graphene structure with continuous ion transport network in‐between the sheets, resulting in a high volumetric capacitance of 366 F cm^–3, almost 2.5 times than that of reduced graphene oxide, as well as long cycle life (93% capacitance retention after 10 000 cycles). More importantly, almost similar electrochemical properties, such as specific capacitance, rate performance, and cycling stability, are obtained for DSG as the negative electrode and G‐MnO₂ as the positive electrode. As a result, the assembled ASC delivers both ultrahigh gravimetric and volumetric energy densities of 62.4 Wh kg^–1 and 54.4 Wh L^–1 (based on total volume of two electrodes) in 1 m Na₂SO₄ aqueous electrolyte, respectively, much higher than most of previously reported ASCs in aqueous electrolytes.     

A promising mechanical ball-milling method to synthesize carbon-coated Co<Subscript>9</Subscript>S<Subscript>8</Subscript> nanoparticles as high-performance electrode for supercapacitor

A Co₉S₈/C nanocomposite has been prepared using a solid-state reaction followed by a facile mechanical ball-milling treatment with sucrose as the carbon source. The phases, morphology, and detailed structures of Co₉S₈/C nanocomposite are well characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. Our experimental results show that not only a process of particle size reduction, the ball-milling treatment also promotes the carbon and Co₉S₈ combining with each other more effectively to form an ultrafine nanocomposite. When used as an electrode material in supercapacitor, Co₉S₈/C nanocomposite exhibits a high initial specific capacitance of 756.2 F g^?1 at 1 A g^?1 and excellent cycling stability with 73.4% retention after 2000 cycles. Its outstanding electrochemical properties are mainly attributed to the nanosize of particles and amorphous carbon layer coating on its surface.     

Electrochemical co-deposition and characterization of MnO2/SWNT composite for supercapacitor application

Manganese oxide/single-wall carbon nanotubes (MnO₂/SWNT) composite was co-deposited by the potentiostatic method on a graphite slice. Morphological and structural performances for MnO₂/SWNT composite were characterized by means of scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The wall surface of SWNT was wrapped by ramsdellite MnO₂ nanoparticles to fabricate MnO₂/SWNT coaxial nanotubes, which further interconnected other MnO₂ particles to form the porous MnO₂/SWNT composite. The electrochemical properties were examined by cyclic voltammograms, galvanostatic charge and discharge and electrochemical impedance spectrum. A high specific capacitance of 421 F g^?1 was obtained for overall MnO₂/SWNT composite electrode at the constant current density of 1 A g^?1 in 3 mol L^?1 KCl solution.     

Synthesis and characterization of Cd-doped ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> microspheres as supercapacitor electrodes

In this paper, we demonstrate the effects of Cd-doping ZnMn₂O₄ on structural and electrochemical performance. Cd-doped ZnMn₂O₄ spheres with diameters of about 2 μm were successfully synthesized by a facile hydrothermal method at 200 °C for 18 h. The fabricated Cd-doped ZnMn₂O₄ samples were characterized by X-ray diffraction, scanning electron microscopy, Brunauer Emmett Teller surface area analyzer and X-ray photoelectron spectroscopy. The electrochemical performance was investigated by cyclic voltammetry and electrochemical impedance spectrometry. The experimental results show that the synthesized spherical Cd-doped ZnMn₂O₄ exhibit far better rate capability and cyclic stability than that of pure spinel porous ZnMn₂O₄ microspheres. The result of cyclic voltammetry measurement indicates that the obtained Cd-doped ZnMn₂O₄ microspheres exhibited the high specific capacitance of 364 Fg^?1 at 2 mV/s.     

Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> nanorods with secondary plate-like nanostructures; preparation,characterization and application as high performance electrode material in supercapacitors

Mn₃O₄ nanorods with secondary plate-like structures were prepared through precipitation from a 0.005 M manganese chloride bath, under the applying direct current mode (i = 2 mA cm^?2). The structural analysis through XRD and FTIR confirmed that the deposited nanopowder has pure monoclinic phase of Mn₃O₄. Further morphological assessment through SEM proved the product to have the Mn₃O₄ nanorods in large quantity, which constructed the secondary plate-like building blocks. Cyclic voltammetric and charge–discharge experiments on the product indicated the prepared Mn₃O₄ to possess high specific capacitance (SC) values of 298 F g^?1, as well as an outstandingly durable cycling stability (95.1 % of initial capacity after discharging 1000 cycles).     

Super-capacitive performance depending on different crystal structures of MnO2 in graphene/MnO2 composites for supercapacitors

In the present study, we synthesize nanoneedle structures of MnO₂/graphene nanocomposites (N-RGO/MnO₂) and birnessite-type MnO₂/graphene nanocomposites (B-RGO/MnO₂). The morphologies and microstructures of as-prepared composites are characterized by X-ray diffractometry, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Our characterizations indicate that nanoneedle structures of MnO₂ and birnessite-type MnO₂ are successfully formed on graphene surfaces. Capacitive properties of the N-RGO/MnO₂ and B-RGO/MnO₂ electrodes are measured using cyclic voltammetry, galvanostatic charge/discharge tests, and electrochemical impedance spectroscopy in a three-electrode experimental setup using a 1 M Na₂SO₄ aqueous solution as the electrolyte. The N-RGO/MnO₂ electrode displays a specific capacitance as high as 327.5 F g^?1 at 10 mV s^?1, which is higher than that of a B-RGO/MnO₂ electrode (248.5 F g^?1). It is believed that the nanoneedle structure of MnO₂ shows excellent electrochemical properties than birnessite-type MnO₂ for the electrode materials for supercapacitors.     

Samarium-doped Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles with improved magnetic and supercapacitive performance: a novel preparation strategy and characterization

Sm³⁺-doped magnetite (Fe₃O₄) nanoparticles were synthesized through a one-pot facile electrochemical method. In this method, products were electrodeposited on a stainless steel (316L) cathode from an additive-free 0.005 M Fe(NO₃)₃/FeCl₂/SmCl₃ aqueous electrolyte. The structural characterizations through X-ray diffraction, field-emission electron microscopy, and energy-dispersive X-ray indicated that the deposited material has Sm³⁺-doped magnetite particles with average size of 20 nm. Magnetic analysis by VSM revealed the superparamagnetic nature of the prepared nanoparticles (Ms = 41.89 emu g^?1, Mr = 0.12 emu g^?1, and H _Ci = 2.24 G). The supercapacitive capability evaluation of the prepared magnetite nanoparticles through cyclic voltammetry and galvanostat charge–discharge showed that these materials are capable to deliver specific capacitances as high as 207 F g^?1 (at 0.5 A g^?1) and 145 F g^?1 (at 2 A g^?1), and capacity retentions of 94.5 and 84.6% after 2000 cycling at 0.5 and 1 A g^?1, respectively. The results proved the suitability of the electrosynthesized nanoparticles for use in supercapacitors. Furthermore, this work provides a facile electrochemical route for the synthesis of lanthanide-doped magnetite nanoparticles.     

Synthesis of NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> onto carbon current collector films as the flexible electrode material for supercapacitors

NiCo₂O₄ is one of the well-known pseudocapacitive material with higher specific capacitance. In this work, carbon nanotubes (CNT) integrated carbon fibers (CF) are used as the flexible substrate for the growth of NiCo₂O₄ nanoneedle arrays by an in-situ hydrothermal approach to obtain NiCo₂O₄/CC flexible electrode for supercapacitors application. The NiCo₂O₄ nanoneedles with diameters of 40–50 nm were formed on the hydroxyl-functionalized CC. This hybrid electrode NiCo₂O₄/CC not only exhibits a high specific capacitance of 249.69 F g^?1, but also shows a favourable cycling stability of 63.3% retention after 1000 cycles at high mass loading. In addition, the proposed method provides a simple and effective strategy for preparing flexible electrode materials for supercapacitor and enables the perfect combination of pseudocapacitance and double layer capacitance.     

Successful synthesis of interconnected Co<Subscript>0.85</Subscript>Se nanosheets with high pore volume and its electrochemical performance in supercapacitors

Interconnected Co_0.85Se nanosheets have been prepared by a facile hydrothermal method via tuning reaction time to control the chemical constitution and the morphology. The nanosheets morphology of Co_0.85Se offers sufficient electron transfer and short ion diffusion pathway, which can favor the fast transfer of electrolyte ions. The Co_0.85Se electrode exhibits specific capacitance of 980 F g^??1 at 10 A g^??1 with high cycling life stability (8.3% loss after 5000 cycles) and good conductivity. The assembled Co_0.85Se//AC asymmetric supercapacitor (ASC) device exhibits a high energy density of 46.2 Wh kg^??1 at a power density of 807.4 W kg^??1 and still maintained 29.3 Wh kg^??1 at a power density of 15981.8 W kg^??1 with excellent cycling performance (90.01% capacitance retention over 5000 cycles). The impressive results indicate that such unique interconnected Co_0.85Se nanosheets are promising electrode materials for high-performance supercapacitors.