Showing papers on "Capacitance published in 2012"

Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density

Abstract: Hierarchical flowerlike nickel hydroxide decorated on graphene sheets has been prepared by a facile and cost-effective microwave-assisted method. In order to achieve high energy and power densities, a high-voltage asymmetric supercapacitor is successfully fabricated using Ni(OH)2/graphene and porous graphene as the positive and negative electrodes, respectively. Because of their unique structure, both of these materials exhibit excellent electrochemical performances. The optimized asymmetric supercapacitor could be cycled reversibly in the high-voltage region of 0–1.6 V and displays intriguing performances with a maximum specific capacitance of 218.4 F g−1 and high energy density of 77.8 Wh kg−1. Furthermore, the Ni(OH)2/graphene//porous graphene supercapacitor device exhibits an excellent long cycle life along with 94.3% specific capacitance retained after 3000 cycles. These fascinating performances can be attributed to the high capacitance and the positive synergistic effects of the two electrodes. The impressive results presented here may pave the way for promising applications in high energy density storage systems.

Hydrogenated TiO2 Nanotube Arrays for Supercapacitors

Abstract: We report a new and general strategy for improving the capacitive properties of TiO2 materials for supercapacitors, involving the synthesis of hydrogenated TiO2 nanotube arrays (NTAs). The hydrogenated TiO2 (denoted as H–TiO2) were obtained by calcination of anodized TiO2 NTAs in hydrogen atmosphere in a range of temperatures between 300 to 600 °C. The H–TiO2 NTAs prepared at 400 °C yields the largest specific capacitance of 3.24 mF cm–2 at a scan rate of 100 mV s–1, which is 40 times higher than the capacitance obtained from air-annealed TiO2 NTAs at the same conditions. Importantly, H–TiO2 NTAs also show remarkable rate capability with 68% areal capacitance retained when the scan rate increase from 10 to 1000 mV s–1, as well as outstanding long-term cycling stability with only 3.1% reduction of initial specific capacitance after 10 000 cycles. The prominent electrochemical capacitive properties of H–TiO2 are attributed to the enhanced carrier density and increased density of hydroxyl group on TiO2 surfa...

3D Macroporous Graphene Frameworks for Supercapacitors with High Energy and Power Densities

Abstract: In order to develop energy storage devices with high power and energy densities, electrodes should hold well-defined pathways for efficient ionic and electronic transport. Herein, we demonstrate high-performance supercapacitors by building a three-dimensional (3D) macroporous structure that consists of chemically modified graphene (CMG). These 3D macroporous electrodes, namely, embossed-CMG (e-CMG) films, were fabricated by using polystyrene colloidal particles as a sacrificial template. Furthermore, for further capacitance boost, a thin layer of MnO2 was additionally deposited onto e-CMG. The porous graphene structure with a large surface area facilitates fast ionic transport within the electrode while preserving decent electronic conductivity and thus endows MnO2/e-CMG composite electrodes with excellent electrochemical properties such as a specific capacitance of 389 F/g at 1 A/g and 97.7% capacitance retention upon a current increase to 35 A/g. Moreover, when the MnO2/e-CMG composite electrode was asy...

Micro-Supercapacitors Based on Interdigital Electrodes of Reduced Graphene Oxide and Carbon Nanotube Composites with Ultrahigh Power Handling Performance

Abstract: A novel method for fabricating micro-patterned interdigitated electrodes based on reduced graphene oxide (rGO) and carbon nanotube (CNT) composites for ultra-high power handling micro-supercapacitor application is reported. The binder-free microelectrodes were developed by combining electrostatic spray deposition (ESD) and photolithography lift-off methods. Without typically used thermal or chemical reduction, GO sheets are readily reduced to rGO during the ESD deposition. Electrochemical measurements show that the in-plane interdigital design of the microelectrodes is effective in increasing accessibility of electrolyte ions in-between stacked rGO sheets through an electro-activation process. Addition of CNTs results in reduced restacking of rGO sheets and improved energy and power density. Cyclic voltammetry (CV) measurements show that the specific capacitance of the micro-supercapacitor based on rGO–CNT composites is 6.1 mF cm−2 at 0.01 V s−1. At a very high scan rate of 50 V s−1, a specific capacitance of 2.8 mF cm−2 (stack capacitance of 3.1 F cm−3) is recorded, which is an unprecedented performance for supercapacitors. The addition of CNT, electrolyte-accessible and binder-free microelectrodes, as well as an interdigitated in-plane design result in a high-frequency response of the micro-supercapacitors with resistive-capacitive time constants as low as 4.8 ms. These characteristics suggest that interdigitated rGO–CNT composite electrodes are promising for on-chip energy storage application with high power demands.

Novel insight into neutral medium as electrolyte for high-voltage supercapacitors

Abstract: This paper is focused on neutral aqueous medium, i.e.lithium, sodium and potassium sulfate solutions in a wide range of concentrations (0.1–2.5 mol L−1) as promising electrolytes for electrochemical capacitors because they are cheap, non-corrosive and allow applying diverse current collectors. These properties make the capacitor assembling process much easier and cheaper. Additionally, such electrolytes are electrochemically stable and environmentally friendly. Electrochemical investigations carried out especially for 1 mol L−1Li2SO4 aqueous solution confirmed the possibility of efficient capacitor work in a wider voltage range, i.e. even at 2.2 V without any significant capacitance fade during 15 000 cycles. The physicochemical properties of ions (i.e. solvation, diffusion or mobility) and their influence on the capacitor electrochemical behaviour are considered.

WO3–x@Au@MnO2 Core–Shell Nanowires on Carbon Fabric for High‐Performance Flexible Supercapacitors

Abstract: WO3–x@Au@MnO2 core–shell nanowires (NWs) are synthesized on a flexible carbon fabric and show outstanding electrochemical performance in supercapacitors such as high specific capacitance, good cyclic stability, high energy density, and high power density. These results suggest that the WO3–x@Au@MnO2 NWs have promising potential for use in high-performance flexible supercapacitors.

Fiber-based all-solid-state flexible supercapacitors for self-powered systems.

Abstract: All-solid-state flexible supercapacitors based on a carbon/MnO2 (C/M) core–shell fiber structure were fabricated with high electrochemical performance such as high rate capability with a scan rate up to 20 V s–1, high volume capacitance of 2.5 F cm–3, and an energy density of 2.2 × 10–4 Wh cm–3. By integrating with a triboelectric generator, supercapacitors could be charged and power commercial electronic devices, such as a liquid crystal display or a light-emitting-diode, demonstrating feasibility as an efficient storage component and self-powered micro/nanosystems.

Ultrahigh-rate supercapacitors based on eletrochemically reduced graphene oxide for ac line-filtering

Abstract: The recent boom in multifunction portable electronic equipments requires the development of compact and miniaturized electronic circuits with high efficiencies, low costs and long lasting time. For the operation of most line-powered electronics, alternating current (ac) line-filters are used to attenuate the leftover ac ripples on direct current (dc) voltage busses. Today, aluminum electrolytic capacitors (AECs) are widely applied for this purpose. However, they are usually the largest components in electronic circuits. Replacing AECs by more compact capacitors will have an immense impact on future electronic devices. Here, we report a double-layer capacitor based on three-dimensional (3D) interpenetrating graphene electrodes fabricated by electrochemical reduction of graphene oxide (ErGO-DLC). At 120-hertz, the ErGO-DLC exhibited a phase angle of −84 degrees, a specific capacitance of 283 microfaradays per centimeter square and a resistor-capacitor (RC) time constant of 1.35 milliseconds, making it capable of replacing AECs for the application of 120-hertz filtering.

Hierarchical Network Architectures of Carbon Fiber Paper Supported Cobalt Oxide Nanonet for High-Capacity Pseudocapacitors

Abstract: We present a high-capacity pseudocapacitor based on a hierarchical network architecture consisting of Co3O4 nanowire network (nanonet) coated on a carbon fiber paper. With this tailored architecture, the electrode shows ideal capacitive behavior (rectangular shape of cyclic voltammograms) and large specific capacitance (1124 F/g) at high charge/discharge rate (25.34 A/g), still retaining ∼94% of the capacitance at a much lower rate of 0.25 A/g. The much-improved capacity, rate capability, and cycling stability may be attributed to the unique hierarchical network structures, which improves electron/ion transport, enhances the kinetics of redox reactions, and facilitates facile stress relaxation during cycling.

Minimum Energy and Capacitance Requirements for Single-Phase Inverters and Rectifiers Using a Ripple Port

Abstract: Converters with a dc port and a single-phase ac port must store energy to buffer the inherent double-frequency power flow at the ac port. The minimum energy storage required to isolate the power ripple from the dc port is presented, and leads to the minimum capacitance required for converters that use capacitive energy storage. This paper presents a ripple power port to manage energy storage and decouple capacitor ripple from power ripple. A ripple power port allows the designer to make a choice of capacitor voltage independent of other system voltages. A combination of an ac link converter and a ripple power port leads to a dramatic increase in reliability: it is shown that converters with nominal ratings up to 200 W can be designed with expected mean-time-between-failure ratings on the order of 1.4 × 106 h-sufficient for hundred-year operation in long-life applications such as photovoltaic converters and LED lamps. This large increase in life is achieved with minimal extra cost.

The Effect of Crystallinity on the Rapid Pseudocapacitive Response of Nb2O5

Abstract: Capacitive energy storage offers several attractive properties compared to batteries, including higher power, faster charging, and a longer cycle life. A key limitation to this electrochemical energy-storage approach is its low energy density and, for this reason, there is considerable interest in identifying pseudocapacitor materials where faradaic reactions are used to achieve greater charge storage. This paper reports on the electrochemical properties of Nb2O5 and establishes that crystalline phases of the material undergo fast faradaic reactions that lead to high specific capacitance in short charging times. In particular, the specific capacitance for the orthorhombic phase at infinite sweep rate reaches ≈400 F g−1, which exceeds that of birnessite MnO2 in nonaqueous electrolyte and is comparable to RuO2 at the same extrapolated rate. The specific capacitances of the orthorhombic and pseudohexagonal phases are much greater than that of the amorphous phase, suggesting that the faradaic reactions which lead to additional capacitive energy storage are associated with Li+ insertion along preferred crystallographic pathways. The ability for Nb2O5 to store charge at high rates despite its wide bandgap and low electronic conductivity is very different from what is observed with other transition metal oxides.

Enhancing electrochemical reaction sites in nickel-cobalt layered double hydroxides on zinc tin oxide nanowires: a hybrid material for an asymmetric supercapacitor device.

Abstract: Conducting nanowires are of particular interest in energy-related research on devices such as supercapacitors, batteries, water splitting electrodes and solar cells. Their direct electrode/current collector contact and highly conductive 1D structure enable conducting nanowires to provide ultrafast charge transportation. In this paper, we report the facile synthesis of nickel cobalt layered double hydroxides (LDHs) on conducting Zn2SnO4 (ZTO) and the application of this material to a supercapacitor. This study also presents the first report of an enhancement of the active faradic reaction sites (electroactive sites) resulting from the heterostructure. This novel material demonstrates outstanding electrochemical performance with a high specific capacitance of 1805 F g−1 at 0.5 A g−1, and an excellent rate performance of 1275 F g−1 can be achieved at 100 A g−1. Furthermore, an asymmetric supercapacitor was successfully fabricated using active carbon as a negative electrode. This asymmetric device exhibits a high energy density of 23.7 W h kg−1 at a power density of 284.2 W kg−1. Meanwhile, a high power density of 5817.2 W kg−1 can be achieved at an energy density of 9.7 W h kg−1. More importantly, this device exhibits long-term cycling stability, with 92.7% capacity retention after 5000 cycles.

"Cut and stick" rubbery ion gels as high capacitance gate dielectrics.

Abstract: A free-standing polymer electrolyte called an ion gel is employed in both organic and inorganic thin-film transistors as a high capacitance gate dielectric. To prepare a transistor, the free-standing ion gel is simply laid over a semiconductor channel and a side-gate electrode, which is possible because of the gel's high mechanical strength.

Incorporation of manganese dioxide within ultraporous activated graphene for high-performance electrochemical capacitors.

Abstract: Manganese dioxide (MnO2) particles 2–3 nm in size were deposited onto a porous “activated microwave expanded graphite oxide” (aMEGO) carbon scaffold via a self-controlled redox process. Symmetric electrochemical capacitors were fabricated that yielded a specific capacitance of 256 F/g (volumetric: 640 F/cm3) and a capacitance retention of 87.7% after 1000 cycles in 1 M H2SO4; when normalized to MnO2, the specific capacitance was 850 F/g. Asymmetric electrochemical capacitors were also fabricated with aMEGO/MnO2 as the positive electrode and aMEGO as the negative electrode and had a power density of 32.3 kW/kg (for an energy density of 20.8 Wh/kg), an energy density of 24.3 Wh/kg (for a power density of 24.5 kW/kg), and a capacitance retention of 80.5% over 5000 cycles.

Nitrogen doping of graphene and its effect on quantum capacitance, and a new insight on the enhanced capacitance of N-doped carbon

Abstract: Many researchers have used nitrogen (N) as a dopant and/or N-containing functional groups to enhance the capacitance of carbon electrodes of electrical double layer (EDL) capacitors. However, the physical mechanism(s) giving rise to the interfacial capacitance of the N-containing carbon electrodes is not well understood. Here, we show that the area-normalized capacitance of lightly N-doped activated graphene with similar porous structure increased from 6 μF cm−2 to 22 μF cm−2 with 0 at%, and 2.3 at% N-doping, respectively. The quantum capacitance of pristine single layer graphene and various N-doped graphene was measured and a trend of upwards shifts of the Dirac Point with increasing N concentration was observed. The increase in bulk capacitance with increasing N concentration, and the increase of the quantum capacitance in the N-doped monolayer graphene versus pristine monolayer graphene suggests that the increase in the EDL type of capacitance of many, if not all, N-doped carbon electrodes studied to date, is primarily due to the modification of the electronic structure of the graphene by the N dopant. It was further found that the quantum capacitance is closely related to the N dopant concentration and N-doping provides an effective way to increase the density of the states of monolayer graphene.

A Nanostructured Electrochromic Supercapacitor

Abstract: We report the first successful application of an ordered bicontinuous double-gyroid vanadium pentoxide network in an electrochromic supercapacitor. The freestanding vanadia network was fabricated by electrodeposition into a voided block copolymer template that had self-assembled into the double-gyroid morphology. The highly ordered structure with 11.0 nm wide struts and a high specific surface to bulk volume ratio of 161.4 μm–1 is ideal for fast and efficient lithium ion intercalation/extraction and faradaic surface reactions, which are essential for high energy and high power density electrochemical energy storage devices. Supercapacitors made from such gyroid-structured vanadia electrodes exhibit a high specific capacitance of 155 F g–1 and show a strong electrochromic color change from green/gray to yellow, indicating the capacitor’s charge condition. The nanostructuring approach and utilizing an electrode material that has intrinsic electrochemical color-change properties are concepts that can be read...

Preparation of supercapacitor electrodes through selection of graphene surface functionalities.

Abstract: In order to investigate the effect of graphene surface chemistry on the electrochemical performance of graphene/polyaniline composites as supercapacitor electrodes, graphene oxide (G-O), chemically reduced G-O (RG-O), nitrogen-doped RG-O (N-RG-O), and amine-modified RG-O (NH(2)-RG-O) were selected as carriers and loaded with about 9 wt % of polyaniline (PANi). The surface chemistry of these materials was analyzed by FTIR, NEXAFS, and XPS, and the type of surface chemistry was found to be important for growth of PANi that influences the magnitude of increase of specific capacitance. The NH(2)-RG-O/PANi composite exhibited the largest increase in capacitance with a value as high as 500 F g(-1) and good cyclability with no loss of capacitance over 680 cycles, much better than that of RG-O/PANi, N-RG-O/PANi, and G-O/PANi when measured in a three-electrode system. A NH(2)-RG-O/PANi//N-RG-O supercapacitor cell has a capacitance of 79 F g(-1), and the corresponding specific capacitance for NH(2)-RG-O/PANi is 395 F g(-1). This research highlights the importance of introducing -NH(2) to RG-O to achieve highly stable cycling performance and high capacitance values.

Optimal Design and Tradeoff Analysis of Planar Transformer in High-Power DC–DC Converters

Abstract: The trend toward high power density, high operating frequency, and low profile in power converters has exposed a number of limitations in the use of conventional wire-wound magnetic component structures. A planar magnetic is a low-profile transformer or inductor utilizing planar windings, instead of the traditional windings made of Cu wires. In this paper, the most important factors for planar transformer (PT) design including winding loss, core loss, leakage inductance, and stray capacitance have individually been investigated. The tradeoffs among these factors have to be analyzed in order to achieve optimal parameters. Combined with an application, four typical winding arrangements have been compared to illustrate their advantages and disadvantages. An improved interleaving structure with optimal behaviors is proposed, which constructs the top layer paralleling with the bottom layer and then in series with the other turns of the primary, so that a lower magnetomotive force ratio m can be obtained, as well as minimized ac resistance, leakage inductance, and even stray capacitance. A 1.2-kW full-bridge dc-dc converter prototype employing the improved PT structure has been constructed, over 96% efficiency is achieved, and a 2.7% improvement, compared with the noninterleaving structure, is obtained.

Green Synthesis of NiO Nanobelts with Exceptional Pseudo-Capacitive Properties

Abstract: As one key electric energy storage device, supercapacitors have been playing an important role in many applications such as electric vehicles. [ 8–10 ] Based on the mechanism of charge storage, supercapacitors can be generally categorized into two types. Electrochemical double-layer capacitors (EDLCs) store electric energy by accumulating charges from the electrolyte at the surface of the solid electrode, [ 8 ] and carbon-based materials with a high surface area are usually used as electrodes in this type of supercapacitors. In pseudocapacitors, on the other hand, the charges are stored via some fast redox reactions taking place at the surface or near-surface regions of the electrode. [ 9 ] Transition metal oxides and conducting polymers are usually used as the electroactive materials in this type of supercapactiors. Even though the pseudocapacitors generally exhibit higher specifi c capacitance than the EDLCs, they usually suffer from poor cycling stability because of the battery-like redox reactions. Nickel oxide (NiO) has been intensively studied for lithium-ion batteries and supercapacitors because of its highly electroactive nature. [ 11–21 ] It has an unusually high theoretical specifi c capacitance of 2584 F g − 1 , [ 14 ] making it very attractive as the electrode material for pseudocapacitors. Additionally, as a transition metal oxide, NiO has relatively higher electrical conductivity compared to other metal oxides/hydroxides or polymers. [ 12 ] Despite these attractive features, the real specifi c capacitance obtained from NiO materials with a wide range of unique nanostructures [ 15 , 22–27 ] is still far below the theoretical value, likely because the pseudocapacitance is mainly from a region near the surface. It is thus of great interest to develop

High performance of a solid-state flexible asymmetric supercapacitor based on graphene films

Abstract: Solid-state flexible energy storage devices hold the key to realizing portable and flexible electronic devices. Achieving fully flexible energy storage devices requires that all of the essential components (i.e., electrodes, separator, and electrolyte) with specific electrochemical and interfacial properties are integrated into a single solid-state and mechanically flexible unit. In this study, we describe the fabrication of solid-state flexible asymmetric supercapacitors based on an ionic liquid functionalized-chemically modified graphene (IL-CMG) film (as the negative electrode) and a hydrous RuO(2)-IL-CMG composite film (as the positive electrode), separated with polyvinyl alcohol-H(2)SO(4) electrolyte. The highly ordered macroscopic layer structures of these films arising through direct flow self-assembly make them simultaneously excellent electrical conductors and mechanical supports, allowing them to serve as flexible electrodes and current collectors in supercapacitor devices. Our asymmetric supercapacitors have been optimized with a maximum cell voltage up to 1.8 V and deliver a high energy density (19.7 W h kg(-1)) and power density (6.8 kW g(-1)), higher than those of symmetric supercapacitors based on IL-CMG films. They can operate even under an extremely high rate of 10 A g(-1) with 79.4% retention of specific capacitance. Their superior flexibility and cycling stability are evident in their good performance stability over 2000 cycles under harsh mechanical conditions including twisted and bent states. These solid-state flexible asymmetric supercapacitors with their simple cell configuration could offer new design and fabrication opportunities for flexible energy storage devices that can combine high energy and power densities, high rate capability, and long-term cycling stability.

Hierarchically Porous Ni-Co Oxide for High Reversibility Asymmetric Full-Cell Supercapacitors

Abstract: Hierarchically porous Ni-Co oxide powder is successfully synthesized by a facile chemical bath deposition method. The structure and composition of Ni-Co oxide are confirmed by transmission electron microscopy and energy dispersive X-ray analysis. Scanning electron microscopy characterization indicates that the Ni-Co oxide has architecture of numerous microflowers with porous flakes. The pseudocapacitive behavior of the Ni-Co oxide powder is investigated by cyclic voltammgrams (CVs) and galvanostatic charge-discharge tests in alkali solution. The Ni-Co oxide shows a good reversibility with a high specific capacitance (834.93 Fg−1 at 1 mVs−1 scan rate). This active material was also used to manufacture a Ni-Co oxide//AC (Active Carbon) asymmetric supercapacitor. The Ni-Co oxide//AC asymmetric supercapacitor shows not only a high specific capacitance (60 Fg−1 with 1 mVs−1 scan rate), but also a high reversibility where its specific capacitance remains at 37 Fg−1 at a high current density of 20 mAcm−2.

Hierarchically aminated graphene honeycombs for electrochemical capacitive energy storage

Abstract: Graphene with mediated surface properties and three-dimensional hierarchical architectures show unexpected performance in energy conversion and storage. To achieve advanced graphene electrode supercapacitors, manipulating the graphene building-blocks into hierarchical nanostructured carbon materials with large electrical double layer capacitance and pseudo-capacitance is a key issue. Here, it is shown that the hierarchically aminated graphitic honeycombs (AGHs) with large surface area for electrical double layer capacitance, tunable surface chemistry for pseudo-capacitance, mediated 3D macropores for ion buffering, and low-resistant pathways for ion diffusion are fabricated for electrochemical capacitive energy storage application through a facile high vacuum promoted thermal expansion and subsequent amination process. In the initial stage of amination (∼200 °C), NH3 reacts with carboxylic acid species to form mainly intermediate amides and amines through nucleophilic substitution. As the temperature increases, the intramolecular dehydration and decarbonylation will take place to generate thermally more stable heterocyclic aromatic moieties such as pyridine, pyrrole, and quaternary type N sites. The AGH exhibits a promising prospect in supercapacitor electrodes with high capacitance (e.g. maximum gravimetric capacitance 207 F g−1 and specific capacitance 0.84 F m−2 at a scan rate of 3 mV s−1) and extraordinary stability (e.g. 97.8% of capacitance retention after 3000 cycles, and 47.8% of capacitance maintaining at a high scan rate of 500 mV s−1 comparing with that at 3 mV s−1). This provides a novel structure platform for catalysis, separation, and drug delivery, which require fast mass transfer through mesopores, reactant reservoirs, and tunable surface chemistry.

High-performance three-dimensional nanoporous NiO film as a supercapacitor electrode

Abstract: A three-dimensional nanoporous NiO film was fabricated using a two-step process through an electrochemical route. The as-prepared NiO film exhibited a highly nanoporous structure and high surface area (264 m2 g−1). The textural characterization of the film and its electrochemical performance as an electrochemical electrode were investigated. The electrode showed a high specific capacitance (1776 F g−1), power density (89 Wh kg−1), and energy density (16.5 kW kg−1). In addition, the electrode exhibited high and stable specific capacitance retention after a long cycle test in KOH solution. More importantly, the power density met the power requirements of the Partnership for a New Generation of Vehicles (PNGV).

Fabrication and electrochemical performances of hierarchical porous Ni(OH)2 nanoflakes anchored on graphene sheets

Abstract: Hierarchical porous Ni(OH)2 nanoflakes anchored on graphene sheets has been fabricated by a facile chemical precipitation approach. The as-prepared Ni(OH)2/graphene composite as a electrode material for supercapacitors displays ultrahigh specific capacitance, superior cycling performance, and excellent rate capability. A maximum specific capacitance of 2194 F g−1 could be obtained at 2 mV s−1 in 6 M KOH aqueous solution. Meanwhile, the electrode exhibits excellent long cycle life along with 95.7% specific capacitance retained after 2000 cycle tests. Such composite is a highly promising candidate as electrode material for broad applications in energy conversion/storage systems.

A novel inner current suppressing method for modular multilevel converters

Abstract: Ideally, the inner (the upper or lower arm) current of a modular multilevel converter (MMC) is assumed to be the sum of a dc component and an ac component of the fundamental frequency. However, this current is usually distorted and the peak/RMS value of it is increased compared with the theoretical result. This is because ac current flows through the submodule (SM) capacitors and the capacitor voltages fluctuate with time. The increased currents will increase power losses and may threaten the safe operation of the power devices and capacitors. This paper proposes a novel close-loop method for suppression of the inner current in MMC. This method is very simple and is implemented in stationary frame, and no harmonic extraction algorithm is needed. Hence, it can be applied to single-phase or three-phase MMC. What is more important, this method does not influence the balancing of the SM capacitor voltages. Simulation and experimental results show that the proposed method can suppress the peak and RMS values of the inner currents dramatically. Meanwhile, the harmonic contents in the output current can also be suppressed satisfactorily even when the SM capacitor voltage ripple factor is as large as about ±19%. Therefore, the proposed method can also be adopted to reduce the SM capacitance requirement.

Porous Co3O4 nanowires derived from long Co(CO3)0.5(OH)·0.11H2O nanowires with improved supercapacitive properties

Abstract: Porous Co3O4 nanowires with large aspect ratio have been obtained by annealing long Co(CO3)0.5(OH)·0.11H2O precursor nanowires synthesized by a facile hydrothermal method. The results show that the amount of the additive (urea) has an important impact on the morphology of the as-synthesized cobalt-carbonate-hydroxide intermediate, where the uniformity and the overall structure can be controlled by changing the urea concentration. After the heat treatment, the as-obtained phase-pure Co3O4 nanowires with a well retained structure are applied as the electrode material for supercapacitors, and the sample exhibits excellent performance with a high specific capacitance of 240 F g−1 after 2000 charge/discharge cycles, corresponding to a retention of 98% of the initial capacitance.

Self-assembly of well-ordered whisker-like manganese oxide arrays on carbon fiber paper and its application as electrode material for supercapacitors

Abstract: Self-assembled well-ordered whisker-like manganese dioxide (MnO2) arrays on carbon fiber paper (MOWAs) were synthesized via a simple in situ redox replacement reaction between potassium permanganate (KMnO4) and carbon fiber paper (CFP) without any other oxidant or reductant addition. The CFP serves as not only a sacrificial reductant and converts aqueous permanganate (MnO4−) to insoluble MnO2 in this reaction, but also a substrate material and guarantees MnO2 deposition on the surface. The electrochemical properties were examined by cyclic voltammograms (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) in a three-electrode cell. According to the CV results, the ordered MOWAs yield high-capacitance performance with specific capacitance up to 274.1 F g−1 and excellent long cycle-life property with 95% of its specific capacitance kept after 5000 cycles at the current density of 0.1 A g−1. The high-performance hybrid composites result from a synergistic effect of large surface area and high degree of ordering of the ultrathin layer of MnO2 nanowhisker arrays, combined with the flexible CFP substrate and can offer great promise in large-scale energy storage device applications.