Study of electrochemical properties and the charge/discharge mechanism for Li<Subscript>4</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript>/MnO<Subscript>2</Subscript>-AC hybrid supercapacitor

Spinel Li₄Mn₅O₁₂ was prepared by a sol–gel method. The manganese oxide and activated carbon composite (MnO₂-AC) were prepared by a method in which KMnO₄ was reduced by activated carbon (AC). The products were characterized by XRD and FTIR. The hybrid supercapacitor was fabricated with Li₄Mn₅O₁₂ and MnO₂-AC, which were used as materials of the two electrodes. The pseudocapacitance performance of the Li₄Mn₅O₁₂/MnO₂-AC hybrid supercapacitor was studied in various aqueous electrolytes. Electrochemical properties of the Li₄Mn₅O₁₂/MnO₂-AC hybrid supercapacitor were studied by using cyclic voltammetry, electrochemical impedance measurement, and galvanostatic charge/discharge tests. The results show that the hybrid supercapacitor has electrochemical capacitance performance. The charge/discharge test showed that the specific capacitance of 51.3 F g⁻¹ was obtained within potential range of 0–1.3 V at a charge/discharge current density of 100 mA g⁻¹ in 1 mol L⁻¹ Li₂SO₄ solution. The charge/discharge mechanism of Li₄Mn₅O₁₂ and MnO₂-AC was discussed.     

Self-standing manganese dioxide/graphene carbon nanotubes film electrode for symmetric supercapacitor with high energy density and superior long cycling stability

The low capacitance utilization and capacitance fading of manganese dioxide (MnO₂) is mainly due to poor electro-conductivity and irreversible phase transform. This work proposes a new method of designing hierarchical and binder-free electrode based on MnO₂ material for stable supercapacitor with high specific capacitance. Herein, we fabricated the self-standing electrode of MnO₂ on nitrogen-doped graphene and single wall carbon nanotubes (SWCNTs) self-standing film (NGCF) by electrochemical deposition. As a result, as-prepared MnO₂/NGCF cathode showed excellent electrochemical performance of 489.7 F g^-1 at 1 A g^-1. Assembled symmetric aqueous supercapacitor (SC) manifests high voltage of 2.4 V and presents excellent high energy density of 106.7 Wh kg^-1 at 1200 W kg^-1 and outstanding long-life stability without no decay after 10 000 charge-discharge circuits. This work proposes a new view of designing hierarchical and binder-free electrode with high energy density and long cycling stability based on MnO₂ material for stable symmetric supercapacitor.     

Cooperative effect of hierarchical carbon nanotube arrays as facilitated transport channels for high-performance wire-based supercapacitors

It is demonstrated that hierarchical nanostructures can greatly enhance the performances of a wire-based supercapacitor (WS), meanwhile can also increase the WS's volume and further hamper the improvement of the WS's capacitance per unit volume. Here a type of three dimensional hierarchical MnO₂@carbon nanotube array (CNTA) composites has been designed on stainless steel wires (SSWs) and the cooperative effects of various parameters (such as MnO₂ masses, CNTA lengths and wire lengths) on the electrochemical performances of the wire electrodes are systematically investigated. Results show that the specific capacitance of the electrodes can be optimized by MnO₂ masses and array lengths, while independent to wire lengths. Moreover, the optimized MnO₂@CNTA/SSW electrodes can exhibit high capacitance (9.1 mF cm⁻¹ at 5 mV s⁻¹), good rate-capability (64.46% at 3 A cm⁻³), and high cycle retention (84.8% after 1000 cycles). Furthermore, the assembled all-solid state flat symmetrical WSs show desirable capacitive behaviors (∼0.78 mW h cm⁻³) and good flexibility.     

Electrochemical preparation and properties of nanostructured Co3O4 as supercapacitor material

Nanostructured Co₃O₄ was prepared via a simple two-step process: cathodic electrodeposition of cobalt hydroxide from additive free nitrate bath and then heat treatment at 400 °C for 3 h. The prepared oxide product was characterized by powder X-ray diffraction, infrared spectroscopy, surface area measurement, scanning electron microscopy, and transmission electron microscopy. Morphological characterization showed that the oxide product was composed of porous nanoplates, and BET measurement displayed that the oxide plates have the average pore diameter and the surface area of 4.75 nm and 208.5 m² g⁻¹, respectively. The supercapacitive performance of the nanoplates was evaluated using cyclic voltammetry and charge–discharge tests. A specific capacitance as high as 393.6 F g⁻¹ at the constant current density of 1 A g⁻¹ and an excellent capacity retention (96.5% after 500 charge–discharge cycles) was obtained. These results indicate that Co₃O₄ nanoplates can be recognized as high-performance electrode materials.     

Facile Solvothermal Synthesis Ni(OH)<Subscript>2</Subscript> Nanostructure for Electrochemical Capacitors

Nickel hydroxide nanosheets were successfully synthesized by facile solvothermal method without any template. The structure and morphology of the as-prepared sample were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy. The observations revealed the formation of hexagonal phase β-Ni(OH)₂ nanosheets with an average diameter of about 100–120 nm. Electrochemical studies were carried out using cyclic voltammetry and galvanostatic charge–discharge tests, respectively. A maximum specific capacitance of 2,342 F g⁻¹, which is the highest reported for a β-Ni(OH)₂ electrode, could be achieved in 6 mol L⁻¹ KOH electrolyte within the potential range of 0–0.50 V (vs. SCE) for the obtained β-Ni(OH)₂ electrode at 0.4 A g⁻¹, suggesting its potential application in the electrode material for electrochemical capacitors.     

Electrochemical method integrating exfoliation and in-situ growth to synthesize MoS2 nanosheets/MnO2 heterojunction for performance-enhanced supercapacitor

Two-dimensional (2D) molybdenum disulfide (MoS₂) nanomaterials have become one of the promising options for constructing excellent supercapacitors. However, the application of MoS₂ materials is limited by low energy density, and the difficulty of large-scale and low-cost preparation seriously hinders its practical application in the field of energy storage. Here, the exfoliation of the MoS₂ nanosheets and the loading of MnO₂ nanoparticles on the MoS₂ nanosheets are realized in one step by electrochemical method. A series of characterization methods have fully confirmed that the electrochemical method has successfully prepared the MoS₂ nanosheet/MnO₂ (MoS₂ NS/MnO₂) heterojunction. The experimental results show that the MoS₂ NS/MnO₂ heterojunction has better electrochemical performance than a single MoS₂ nanosheet. It has a good capacitance even in a neutral solution, and its specific capacitance is 275 F g^?1 at a current density of 2 A g^?1. In addition, a supercapacitor device based on MoS₂ NS/MnO₂ heterojunction was constructed, which not only exhibited excellent capacitive performance, but also exhibited 10,000 charge-discharge cycle stability under 10 A g^?1 conditions. This work provides an experimental basis for the preparation of 2D nanosheets and the large-scale preparation of functionalized 2D material heterojunctions by electrochemical methods.     

Supercapacitive properties of polyaniline/hydrous RuO2 composite electrode

A composite electrode based on polyaniline (PANI) and hydrous RuO₂ is prepared by electrochemical deposition of PANI onto hydrous RuO₂ (PANI/RuO₂) and its supercapacitive properties are investigated using cyclic voltammetry. The specific capacitances of PANI/RuO₂ and hydrous RuO₂ electrodes are determined to be 708 and 517 F g⁻¹ at 5 mV s⁻¹, respectively. Simple electrodeposition of PANI on the hydrous RuO₂ can achieve comparatively greater capacitance values.     

Design of an “all solid-state” supercapacitor based on phosphoric acid doped polybenzimidazole (PBI) electrolyte

The effectiveness of phosphoric acid doped polybenzimidazole as a polymer electrolyte membrane to fabricate an all solid-state super capacitor has been explored using hydrous RuO₂/carbon composite electrodes (20 wt.%) of surface area 250 m² g⁻¹ with many intrinsic advantages. The electrochemical evaluation of these super capacitors through cyclic voltammetry, charge/discharge and impedance measurements demonstrate the utility of this type of thin, compact and flexible supercapacitor capable of functioning at 150 °C to yield a maximum capacitance of about 290 F g⁻¹ along with a life of more than 1,000 cycles. A power density of 300 W kg⁻¹ and energy density of 10 Wh kg⁻¹ have been accomplished although the equivalent series resistance (ESR) of about 3.7 Ω needs to be reduced further for high rated applications.     

Composite Electrodes for Electrochemical Supercapacitors

Manganese dioxide nanofibers with length ranged from 0.1 to 1 μm and a diameter of about 4–6 nm were prepared by a chemical precipitation method. Composite electrodes for electrochemical supercapacitors were fabricated by impregnation of the manganese dioxide nanofibers and multiwalled carbon nanotubes (MWCNT) into porous Ni plaque current collectors. Obtained composite electrodes, containing 85% of manganese dioxide and 15 mass% of MWCNT, as a conductive additive, with total mass loading of 7–15 mg cm⁻², showed a capacitive behavior in 0.5-M Na₂SO₄ solutions. The decrease in stirring time during precipitation of the nanofibers resulted in reduced agglomeration and higher specific capacitance (SC). The highest SC of 185 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹ for mass loading of 7 mg cm⁻². The SC decreased with increasing scan rate and increasing electrode mass.     

Multiwall carbon nanotube supported poly(3,4-ethylenedioxythiophene)/manganese oxide nano-composite electrode for super-capacitors

MWCNT-PSS/PEDOT/MnO₂ nano-composite electrodes were fabricated by generating pseudo-capacitive poly(3,4-ethylenedioxythiophene) (PEDOT)/MnO₂ nano-structures on poly(styrene sulfonate) (PSS) dispersed multiwalled carbon nanotubes (MWCNTs). PSS dispersed MWCNTs (MWCNT-PSS) facilitated the growth of PEDOT and MnO₂ into nano-rods with large active surface area and good electrical conductivity. The ternary MWCNT-PSS/PEDOT/MnO₂ nano-composite electrode was studied for the application in super-capacitors, and exhibited excellent capacitive behavior between −0.2 V and 0.8 V (vs. saturated Ag/AgCl electrode) with high reversibility. Specific capacitance of the nano-composite electrode was found as high as 375 F g⁻¹. In contrast, specific capacitance of MWCNT-PSS/MnO₂ and MWCNT-PSS nano-composite electrodes is 175 F g⁻¹ and 15 F g⁻¹, respectively. Based on cyclic voltammetric studies and cycle-life tests, the MWCNT-PSS/PEDOT/MnO₂ nano-composite electrode gave a highly stable and reversible performance up to 2000 cycles. Our studies demonstrate that the synergistic combination of MWCNT-PSS, PEDOT and MnO₂ has advantages over the sum of the individual components.     

Composite electrodes for electrochemical supercapacitors

Manganese dioxide and Ag-doped manganese dioxide powders were prepared by a chemical precipitation method using KBH₄ as a reducing agent. The powders were studied by X-ray analysis, thermogravimetry, and electron microscopy. Composite electrodes for electrochemical supercapacitors (ES) were fabricated by impregnation of slurries of the precipitated powders and carbon black into porous nickel foam current collectors. In the composite electrodes, carbon black nanoparticles formed a secondary conductivity network within the nickel foam cells. Obtained composite electrodes, containing manganese dioxide and 20 wt% carbon black with total mass loading of 50 mg cm⁻², showed a capacitive behavior in the 0.5 M Na₂SO₄ solutions. The capacitive behavior of the composite electrodes can be improved by mixing of manganese dioxide and carbon black in solutions or using Ag-doped manganese dioxide powders. The highest specific capacitance (SC) of 150 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹. The electrodes showed good cycling behavior with no loss in SC during 1,000 cycles.     

Pseudo-capacitance properties of porous metal oxide nanoplatelets derived from hydrotalcite-like compounds

Nanoplatelets of metal oxides with interesting porous structure were obtained by thermal treatment of Ni/Al hydrotalcite. Structural and surface properties of the porous oxides were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM and HRTEM), and N₂ adsorption–desorption. The electrochemical performance of the electrodes was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and constant current charge–discharge measurements. Ni/Al hydrotalcite calcined at 450 °C (NA-450) displayed a maximum specific capacitance (419.0 F g⁻¹) due to the porous structure with the highest specific surface area (142.3 m² g⁻¹) and small pore size (4.4 nm). The present study shows the potential of NiO nanoplatelets composite material for electrochemical pseudo-capacitors.     

Supercapacitive properties of composite electrodes consisting of polyaniline, carbon nanotube, and RuO2

Three types of composite supercapacitor electrodes were prepared; electroactive polyaniline (PANI), PANI/multi-walled carbon nanotube (CNT), and PANI/CNT/RuO₂. Specifically, the PANI and PANI/CNT were prepared by polymerization, and PANI/CNT/RuO₂ was prepared by electrochemical deposition of RuO₂ on the PANI/CNT matrix. Cyclic voltammetry between −0.2 and 0.8 V (vs. Ag/AgCl) at various scan rates was performed to investigate the supercapacitive properties in an electrolyte solution of 1.0 M H₂SO₄. The PANI/CNT/RuO₂ electrode showed the highest specific capacitance at all scan rates (e.g., 441 and 392 F g⁻¹ at 100 and 1,000 mV s⁻¹, respectively). In contrast, the PANI/CNT electrode demonstrated the best capacitance retention (66%) after 10⁴ cycles. Additional analysis including morphology and complex impedance spectroscopy suggested that with small loading of RuO₂, an increase in capacitance was observed, but dissolution and/or detachment of RuO₂ species from the electrode might occur during cycling to reduce the cycle performance.     

Manganese oxide precipitated into activated carbon electrodes for electrochemical capacitors

Manganese oxide was prepared at different pH and temperatures and then precipitated into activated carbon by the chemical impregnation method. Size distributions of manganese oxide sol were also measured by light scattering. The electrodes were annealed in nitrogen gas at different temperatures. In addition, electrochemical characterization was carried out using cyclic voltammetry (CV) at a scan rate of 25 mV s⁻¹ and chronopotentiometry (CP) with constant-current (10 mA cm⁻²). Maximum capacitance of 461.3 F g⁻¹ was obtained in a 0.1 M Na₂SO₄ solution for manganese oxide prepared under optimum conditions (pH = 13.11 and T = 25 oC) and annealed at a temperature of 195 oC. The manganese oxide particle size decreased with annealing. This probably leads to increased specific capacitance. Using X-ray photoelectron spectroscopy (XPS) the results reveal that manganese oxide species are transformed from hydroxide to oxide after annealing.     

Electrooxidation and dischargeability of transition-metal borides as possible anodic materials in neutral aqueous electrolytes

A number of transition-metal borides were studied as anodic materials for neutral aqueous batteries. These borides are shown to have considerably high electrochemical activities in neutral electrolytes. The discharge capacities for TiB₂ reach 1,350 mAh g⁻¹ at a constant current density of 50 mA g⁻¹, exceeding those for all the metal electrodes reported so far. Amorphous CoB_x can deliver a discharge capacity of >650 mAh g⁻¹, and even simply ball-milled FeB_x can also give a discharge capacity of >200 mAh g⁻¹. These results suggest the possible use of boride compounds as a large family of new anodic materials for constructing neutral aqueous batteries with high electrochemical capacity and rate capability.     

Interconnected SnO2/graphene+CNT network as high performance anode materials for lithium-ion batteries

As a metal oxide with a high theoretical capacity, SnO₂ is considered to be one of the promising alternative anode materials in lithium-ion batteries. However, the pulverization of electrodes caused by the large volume expansion of SnO₂ during repeated charge/discharge hinders its practical application. Here, SnO₂ nanoparticles decorated on a 3D carbon network structure formed by the interconnection of graphene and CNT (SnO₂/G + CNT), which is designed and successfully synthesized via in situ chemical synthesis and thermal treatment. In this structure, the SnO₂ with nanosized can increase energy storage points and decrease the ions transport length, the carbon network can build a high conductive network that facilitates electron transport and alleviate the volume expansion to prevent electrode pulverization. In addition, graphene has a high specific surface area effect that facilitates lithium-ion storage, and the CNT also supports the graphene frame to make the carbon skeleton structure more stable, and provides a large number of ion transport channels, increasing the active sites of the reaction. Due to this excellent structure with synergistic effects, the SnO₂/G + CNT electrode exhibits superior reversible capacity (1227.2 mAh g^-1 at 0.1 A g^-1 after 200 cycles), superior rate capacity (549.3 mAh g^-1 at 3.0 A g^-1) and long cycle stability (1630.1 mAh g^-1 at 0.5 A g^-1 after 1000 cycles).     

Tubular graphene nanoribbons with attached manganese oxide nanoparticles for use as electrodes in high-performance supercapacitors

Graphene nanoribbons (GNRs) with tubular shaped thin graphene layers were prepared by partially longitudinal unzipping of vapor-grown carbon nanofibers (VGCFs) using a simple solution-based oxidative process. The GNR sample has a similar layered structure to graphene oxide (GO), which could be readily dispersed in isopropyl alcohol to facilitate electrophoretic deposition (EPD). GO could be converted to graphene after heat treatment at 300 °C. The multilayer GNR electrode pillared with open-ended graphene tubes showed a higher capacitance than graphene flake and pristine VGCF electrodes, primarily due to the significantly increased surface area accessible to electrolyte ions. A GNR electrode with attached MnO₂ nanoparticles was prepared by EPD method in the presence of hydrated manganese nitrate. The specific capacitance of GNR electrode with attached MnO₂ could reach 266 F g⁻¹, much higher than that of GNR electrode (88 F g⁻¹) at a discharge current of 1 A g⁻¹. The hydrophilic MnO₂ nanoparticles attached to GNRs could act as a redox center and nanospacer to allow the storage of extra capacitance.     

Manganese dioxide nanosheets decorated on MXene (Ti3C2Tx) with enhanced performance for asymmetric supercapacitors

The incorporation of nanosized pseudocapacitive materials and structure design are general strategies to enhance the electrochemical performance of MXene-based materials. Herein, the decoration of manganese dioxide (MnO₂) nanosheets on MXene (Ti₃C₂T_x) surfaces was prepared by a facile liquid phase coprecipitation method. Ti₃C₂T_x is initially modified by polydopamine (PDA) coating to ensure the homogeneous distribution of MnO₂ nanosheets and tight and close connections between MnO₂ and the Ti₃C₂T_x backbone. Due to the obtained three-dimensional (3D) nanostructure, facilitating electron transport within the electrode and promoting electrolyte ion accessibility, the δ-MnO₂@Ti₃C₂T_x-0.06 electrode yields superior electrochemical performances, such as a rather large areal capacity of 1233.1 mF cm^?2 and high specific capacitance of 337.6 F g^?1 at 2 mV s^?1, as well as high cyclic stability for 10000 cycles. Furthermore, δ-MnO₂@Ti₃C₂T_x-0.06 composites are employed as positive electrodes, and activated carbon (AC) materials act as negative electrodes with an aqueous electrolyte of 1 M Na₂SO₄ to assemble asymmetric supercapacitors. The prototype device is reversible at cell voltages from 0 to 1.8 V, and manifests a maximum energy density of 31.4 Wh kg^?1 and a maximum power density of 2700 W kg^?1. These encouraging results show enormous possibilities for energy storage applications.     

Encapsulation of manganese oxides nanocrystals in electrospun carbon nanofibers as free-standing electrode for supercapacitors

Flexible composites with manganese oxides (MnO_x) nanocrystals encapsulated in electropun carbon nanofibers were successfully fabricated via a simple and practical combination of electrospinning and carbonization process. The as-formed MnO_x/carbon nanofibers composites have a rough surface with MnO_x nanoparticles well embedded in the carbon nanofibers backbones. When used as electrodes for supercapacitor, the resulting MnO_x/carbon nanofiber composites exhibit good electrochemical performance with a specific capacitance of 174.8 F g⁻¹ at 2 mV s⁻¹ in 0.5 M Na₂SO₄ electrolyte, a good rate capability at high current density and long-term cycling stability. It is expected that such freestanding composites could be promising electrodes for high-performance supercapacitors.     

The additive-free electrode based on the layered MnO2 nanoflowers/reduced,graphene oxide film for high performance supercapacitor

The MnO₂ nanoflowers/reduced graphene oxide composite is coated on a nickel foam substrate (denoted as MnO₂ NF/RGO @ Ni foam) via the layer by layer (LBL) self-assembly technology without any polymer additive, following the soft chemical reduction. The layered MnO₂ NF/RGO composite is uniformly anchored on the Ni foam skeleton to form the 3D porous framework, and the interlayers have access to lots of ions channels to improve the electron transfer and diffusion. This special construction of 3D porous structure is beneficial to the enhancement of electrochemical property. The specific capacitance is up to 246 F g⁻¹ under the current density of 0.5 A g⁻¹. After 1000 cycles, it can retain about 93%, exhibiting excellent cycle stability. The electrochemical impedance spectroscopy measurements confirm that MnO₂ NF/RGO @ Ni foam electrode has lower R_ESR and R_CT values when compared to MnO₂ @ Ni foam and RGO @ Ni foam. This study opens a new door to the preparation of composite electrodes for high performance supercapacitor.