Showing papers on "Capacitance published in 2015"

Nitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy storage

Abstract: Carbon-based supercapacitors can provide high electrical power, but they do not have sufficient energy density to directly compete with batteries. We found that a nitrogen-doped ordered mesoporous few-layer carbon has a capacitance of 855 farads per gram in aqueous electrolytes and can be bipolarly charged or discharged at a fast, carbon-like speed. The improvement mostly stems from robust redox reactions at nitrogen-associated defects that transform inert graphene-like layered carbon into an electrochemically active substance without affecting its electric conductivity. These bipolar aqueous-electrolyte electrochemical cells offer power densities and lifetimes similar to those of carbon-based supercapacitors and can store a specific energy of 41 watt-hours per kilogram (19.5 watt-hours per liter).

All-Solid-State Symmetric Supercapacitor Based on Co3O4 Nanoparticles on Vertically Aligned Graphene

Abstract: We have synthesized the hybrid supercapacitor electrode of Co3O4 nanoparticles on vertically aligned graphene nanosheets (VAGNs) supported by carbon fabric. The VAGN served as an excellent backbone together with the carbon fabric, enhancing composites to a high specific capacitance of 3480 F/g, approaching the theoretical value (3560 F/g). A highly flexible all-solid-state symmetric supercapacitor device was fabricated by two pieces of our Co3O4/VAGN/carbon fabric hybrid electrode. The device is suitable for different bending angles and delivers a high capacitance (580 F/g), good cycling ability (86.2% capacitance retention after 20 000 cycles), high energy density (80 Wh/kg), and high power density (20 kW/kg at 27 Wh/kg). These excellent electrochemical performances, as a result of the particular structure of VAGN and the flexibility of the carbon fabric, suggest that these composites have an enormous potential in energy application.

Negative capacitance in a ferroelectric capacitor.

Abstract: The Boltzmann distribution of electrons poses a fundamental barrier to lowering energy dissipation in conventional electronics, often termed as Boltzmann Tyranny. Negative capacitance in ferroelectric materials, which stems from the stored energy of a phase transition, could provide a solution, but a direct measurement of negative capacitance has so far been elusive. Here, we report the observation of negative capacitance in a thin, epitaxial ferroelectric film. When a voltage pulse is applied, the voltage across the ferroelectric capacitor is found to be decreasing with time--in exactly the opposite direction to which voltage for a regular capacitor should change. Analysis of this 'inductance'-like behaviour from a capacitor presents an unprecedented insight into the intrinsic energy profile of the ferroelectric material and could pave the way for completely new applications.

Controlled Growth of NiMoO4 Nanosheet and Nanorod Arrays on Various Conductive Substrates as Advanced Electrodes for Asymmetric Supercapacitors

Abstract: Hierarchical NiMoO4 architectures assembled from well-aligned uniform nanosheets or nanorods are successfully grown on various conductive substrates using a facile and effective general approach. Importantly, the nanostructures of NiMoO4 can be easily controlled to be nanosheets or nanorods by using different solvents. By virtue of their intriguing structure features, NiMoO4 nanosheets as integrated additive-free electrodes for supercapacitors manifest higher Faradaic capacitance than NiMoO4 nanorods. Moreover, an asymmetric supercapacitor (ASC) is constructed using the as-prepared NiMoO4 nanosheets as the positive electrode and activated carbon (AC) as the negative electrode. The optimized ASC with an extended operating voltage range of 0–1.7 V displays excellent electrochemical performance with a high energy density of 60.9 Wh kg−1 at a power density of 850 W kg−1 in addition to superior rate capability. Furthermore, the NiMoO4//AC ASC device exhibits remarkable cycling stability with 85.7% specific capacitance retention after 10 000 cycles. The results show that these NiMoO4-based nanostructures are promising for high-energy supercapacitors.

Ultrahigh volumetric capacitance and cyclic stability of fluorine and nitrogen co-doped carbon microspheres

Abstract: Highly porous nanostructures with large surface areas are typically employed for electrical double-layer capacitors to improve gravimetric energy storage capacity; however, high surface area carbon-based electrodes result in poor volumetric capacitance because of the low packing density of porous materials. Here, we demonstrate ultrahigh volumetric capacitance of 521 F cm−3 in aqueous electrolytes for non-porous carbon microsphere electrodes co-doped with fluorine and nitrogen synthesized by low-temperature solvothermal route, rivaling expensive RuO2 or MnO2 pseudo-capacitors. The new electrodes also exhibit excellent cyclic stability without capacitance loss after 10,000 cycles in both acidic and basic electrolytes at a high charge current of 5 A g−1. This work provides a new approach for designing high-performance electrodes with exceptional volumetric capacitance with high mass loadings and charge rates for long-lived electrochemical energy storage systems. Carbon-based supercapacitors often suffer from poor volumetric capacitance due to the low packing density which arises from attempts to increase the electrode surface area. Here, in contrast, the authors fabricate N and F co-doped non-porous solid carbon spheres and achieve exceptional performances.

Engineering three-dimensional hybrid supercapacitors and microsupercapacitors for high-performance integrated energy storage

Abstract: Supercapacitors now play an important role in the progress of hybrid and electric vehicles, consumer electronics, and military and space applications. There is a growing demand in developing hybrid supercapacitor systems to overcome the energy density limitations of the current generation of carbon-based supercapacitors. Here, we demonstrate 3D high-performance hybrid supercapacitors and microsupercapacitors based on graphene and MnO2 by rationally designing the electrode microstructure and combining active materials with electrolytes that operate at high voltages. This results in hybrid electrodes with ultrahigh volumetric capacitance of over 1,100 F/cm3. This corresponds to a specific capacitance of the constituent MnO2 of 1,145 F/g, which is close to the theoretical value of 1,380 F/g. The energy density of the full device varies between 22 and 42 Wh/l depending on the device configuration, which is superior to those of commercially available double-layer supercapacitors, pseudocapacitors, lithium-ion capacitors, and hybrid supercapacitors tested under the same conditions and is comparable to that of lead acid batteries. These hybrid supercapacitors use aqueous electrolytes and are assembled in air without the need for expensive “dry rooms” required for building today’s supercapacitors. Furthermore, we demonstrate a simple technique for the fabrication of supercapacitor arrays for high-voltage applications. These arrays can be integrated with solar cells for efficient energy harvesting and storage systems.

Probing the Mechanism of High Capacitance in 2D Titanium Carbide Using In Situ X-Ray Absorption Spectroscopy

Abstract: The field of supercapacitors (electrochemical capacitors) is constantly evolving. The global motivation is to create devices that possess a significant energy density without compromising the power density. To achieve this goal, new materials must be discovered and complex electrode architectures developed.

Control of I–V Hysteresis in CH3NH3PbI3 Perovskite Solar Cell

Abstract: Mismatch of current (I)-voltage (V) curves with respect to the scan direction, so-called I–V hysteresis, raises critical issue in MAPbI3 (MA = CH3NH3) perovskite solar cell. Although ferroelectric and ion migration have been proposed as a basis for the hysteresis, origin of hysteresis has not been apparently unraveled. We report here on the origin of I–V hysteresis of perovskite solar cell that was systematically evaluated by the interface-dependent electrode polarizations. Frequency (f)-dependent capacitance (C) revealed that the normal planar structure with the TiO2/MAPbI3/spiro-MeOTAD configuration showed most significant I–V hysteresis along with highest capacitance (10–2 F/cm2) among the studied cell configurations. Substantial reduction in capacitance to 10–3 F/cm2 was observed upon replacing TiO2 with PCBM, indicative of the TiO2 layer being mainly responsible for the hysteresis. The capacitance was intensively reduced to 10–5 F/cm2 and C–f feature shifted to higher frequency for the hysteresis-fre...

Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells

Abstract: Despite spectacular advances in conversion efficiency of perovskite solar cell many aspects of its operating modes are still poorly understood. Capacitance constitutes a key parameter to explore which mechanisms control particular functioning and undesired effects as current hysteresis. Analyzing capacitive responses allows addressing not only the nature of charge distribution in the device but also the kinetics of the charging processes and how they alter the solar cell current. Two main polarization processes are identified. Dielectric properties of the microscopic dipolar units through the orthorhombic-to-tetragonal phase transition account for the measured intermediate frequency capacitance. Electrode polarization caused by interfacial effects, presumably related to kinetically slow ions piled up in the vicinity of the outer interfaces, consistently explain the reported excess capacitance values at low frequencies. In addition, current–voltage curves and capacitive responses of perovskite-based solar ...

Characterization of MoS2–Graphene Composites for High-Performance Coin Cell Supercapacitors

Abstract: Two-dimensional materials, such as graphene and molybdenum disulfide (MoS2), can greatly increase the performance of electrochemical energy storage devices because of the combination of high surface area and electrical conductivity. Here, we have investigated the performance of solution exfoliated MoS2 thin flexible membranes as supercapacitor electrodes in a symmetrical coin cell arrangement using an aqueous electrolyte (Na2SO4). By adding highly conductive graphene to form nanocomposite membranes, it was possible to increase the specific capacitance by reducing the resistivity of the electrode and altering the morphology of the membrane. With continued charge/discharge cycles the performance of the membranes was found to increase significantly (up to 800%), because of partial re-exfoliation of the layered material with continued ion intercalation, as well as increasing the specific capacitance through intercalation pseudocapacitance. These results demonstrate a simple and scalable application of layered...

Bio-inspired beehive-like hierarchical nanoporous carbon derived from bamboo-based industrial by-product as a high performance supercapacitor electrode material

Abstract: Bio-inspired beehive-like hierarchical nanoporous carbon (BHNC) with a high specific surface area of 1472 m2 g−1 and a good electronic conductivity of 4.5 S cm−1 is synthesized by carbonizing the industrial waste of bamboo-based by-product. The BHNC sample exhibits remarkable electrochemical performances as a supercapacitor electrode material, such as a high specific capacitance of 301 F g−1 at 0.1 A g−1, still maintaining a value of 192 F g−1 at 100 A g−1, negligible capacitance loss after 20 000 cycles at 1 A g−1, and a high power density of 26 000 W kg−1 at an energy density of 6.1 W h kg−1 based on active electrode materials in an aqueous electrolyte system. Moreover, an enhanced power density of 42 000 W kg−1 at a high energy density of 43.3 W h kg−1 is obtained in an ionic liquid electrolyte system, which places the BHNC-based supercapacitors in the Ragone chart among the best energy–power synergetic outputting properties ever reported for carbon-based supercapacitors.

Cellulose Nanofibril/Reduced Graphene Oxide/Carbon Nanotube Hybrid Aerogels for Highly Flexible and All-Solid-State Supercapacitors

Abstract: A novel type of highly flexible and all-solid-state supercapacitor that uses cellulose nanofibril (CNF)/reduced graphene oxide (RGO)/carbon nanotube (CNT) hybrid aerogels as electrodes and H2SO4/poly(vinyl alcohol) (PVA) gel as the electrolyte was developed and is reported here. These flexible solid-state supercapacitors were fabricated without any binders, current collectors, or electroactive additives. Because of the porous structure of the CNF/RGO/CNT aerogel electrodes and the excellent electrolyte absorption properties of the CNFs present in the aerogel electrodes, the resulting flexible supercapacitors exhibited a high specific capacitance (i.e., 252 F g–1 at a discharge current density of 0.5 A g–1) and a remarkable cycle stability (i.e., more than 99.5% of the capacitance was retained after 1000 charge–discharge cycles at a current density of 1 A g–1). Furthermore, the supercapacitors also showed extremely high areal capacitance, areal power density, and energy density (i.e., 216 mF cm–2, 9.5 mW c...

Novel scalable synthesis of highly conducting and robust PEDOT paper for a high performance flexible solid supercapacitor

Abstract: A novel synthetic strategy has been developed for the preparation of a highly conducting polyethylenedioxythiophene (PEDOT) phase on flexible cellulose paper by inducing surfactant-free interfacial polymerization at the interface of two immiscible liquids. The process is highly scalable in that very large flexible PEDOT papers can be prepared in 2–3 h under laboratory conditions. The PEDOT paper possesses efficiently packed π-conjugated chains and offers the possibility of increased doping levels, confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-Vis measurement. This useful development was achieved by slow polymerization, coupled with the high dielectric constant of the interface, which stabilised the counter-ions by hydrogen bonding. This gave improved intra-chain charge mobility, leading to conductivity as high as 375 S cm−1, much higher than for PEDOT prepared in n-butanol, typically 30 S cm−1. A low sheet resistance of 3 Ω □−1 was achieved by multiple coating, and this was found to be stable even after two months in ambient conditions and under a variety of flexible and bending conditions. A flexible solid-state supercapacitor with overall thickness 0.17 mm made from the PEDOT paper and PVA–H2SO4 as the solid electrolyte exhibited a volumetric energy density of 1 mW h cm−3. The specific capacitance measured per unit mass of PEDOT in the system was 115 F g−1, together with a high volumetric capacitance of 145 F cm−3. These observed values were significantly higher than those for bulk PEDOT tested on solid current collectors, and also than the highest values quoted in the literature. The flexible devices were found to be very stable during charge–discharge cycling under twisted and bending conditions over more than 3800 cycles. A 3.6 V inter-digitized flexible device was also made using a single PEDOT paper, and was found to be sufficiently powerful to cause an LED to glow under flexible conditions.

One-pot synthesis of porous nickel cobalt sulphides: tuning the composition for superior pseudocapacitance

Abstract: Nickel cobalt sulphides with different stoichiometric nickel and cobalt contents have been synthesized and used as pseudocapacitive materials for supercapacitors. The as-employed polyol method is robust enough to use a one-pot synthesis of nickel cobalt sulphides with similar porous morphology and crystal structure at different nickel and cobalt ratios. The electrochemical performance of the nickel cobalt sulphides can be easily tuned by the varying the Ni and Co content. Owing to the combined contributions from both Ni and Co ions, the bimetallic Ni–Co sulphides show a superior pseudocapacitance compared to monometallic Co and Ni sulphides in terms of high specific capacitance, excellent rate capability and long cycle stability. In particular, the Ni1.5Co1.5S4 sample shows the highest specific capacitance of 1093 F g−1 at 1 A g−1, superior rate capability of 69% capacitance retention after a 50-fold increase in current densities, and longer cycle stability with increased specific capacitance of 108% of capacitance retention after 2000 cycles. In addition, the Ni1.5Co1.5S4 was also used to assemble an asymmetric supercapacitor with reduced graphene oxide and attains excellent capacitive performance with high specific capacitance (113 F g−1 at 1 A g−1), high energy density (37.6 W h kg−1 at 775 W kg−1) and high power density (23.25 kW kg−1 at 17.7 W h kg−1).

Electrodeposition of high-capacitance 3D CoS/graphene nanosheets on nickel foam for high-performance aqueous asymmetric supercapacitors

Abstract: Electrochemical energy storage devices that encompass the capability of offering both excellent capacitance and rate performance have always be in high demand. Herein, we present a simple and green two-step electrodeposition process to fabricate a high-performance 3D CoS/graphene hybrid network with a nanosheet structure on Ni foam. The nanosheet-like CoS is tightly wrapped and anchored by the graphene layer and the two different material species are nicely integrated together, leading to increased conductivity and enlarged electroactive surface area of the electrode materials. The CoS/graphene composites exhibit an impressive specific capacitance of 3785 F g−1 at a current density of 1 A g−1, a favorable rate capability with 82% retention at 20 A g−1. A CoS/graphene‖activated carbon asymmetric supercapacitor fabricated in 2 M KOH solution exhibits a maximum energy density of 29 Wh kg−1 at the power density of 800 W kg−1, and a power density of 40.0 kW kg−1 (at the energy density of 11.0 Wh kg−1). Furthermore, 70% capacitance retention was obtained after 10 000 cycles within the potential window of 0–1.6 V. The excellent performance of the CoS/graphene composites demonstrated in this work has revealed the promising potential of adopting the CoS/graphene hybrid network for high performance supercapacitors.

Hierarchical Fe3O4@Fe2O3 Core–Shell Nanorod Arrays as High-Performance Anodes for Asymmetric Supercapacitors

Abstract: Anode materials with relatively low capacitance remain a great challenge for asymmetric supercapacitors (ASCs) to pursue high energy density. Hematite (α-Fe2O3) has attracted intensive attention as anode material for ASCs, because of its suitable reversible redox reactions in a negative potential window (from 0 V to −1 V vs Ag/AgCl), high theoretical capacitance, rich abundance, and nontoxic features. Nevertheless, the Fe2O3 electrode cannot deliver large volumetric capacitance at a high rate, because of its poor electrical conductivity (∼10–14 S/cm), resulting in low power density and low energy density. In this work, a hierarchical heterostructure comprising Fe3O4@Fe2O3 core–shell nanorod arrays (NRAs) is presented and investigated as the negative electrode for ASCs. Consequently, the Fe3O4@Fe2O3 electrode exhibits superior supercapacitive performance, compared to the bare Fe2O3 and Fe3O4 NRAs electrodes, demonstrating large volumetric capacitance (up to 1206 F/cm3 with a mass loading of 1.25 mg/cm2), a...

One-Pot Synthesis of Tunable Crystalline Ni3S4@Amorphous MoS2 Core/Shell Nanospheres for High-Performance Supercapacitors

Abstract: Transition metal sulfides gain much attention as electrode materials for supercapacitors due to their rich redox chemistry and high electrical conductivity. Designing hierarchical nanostructures is an efficient approach to fully utilize merits of each component. In this work, amorphous MoS2 is firstly demonstrated to show specific capacitance 1.6 times as that of the crystalline counterpart. Then, crystalline core@amorphous shell (Ni3S4@MoS2) is prepared by a facile one-pot process. The diameter of the core and the thickness of the shell can be independently tuned. Taking advantages of flexible protection of amorphous shell and high capacitance of the conductive core, Ni3S4@amorphous MoS2 nanospheres are tested as supercapacitor electrodes, which exhibit high specific capacitance of 1440.9 F g−1 at 2 A g−1 and a good capacitance retention of 90.7% after 3000 cycles at 10 A g−1. This design of crystalline core@amorphous shell architecture may open up new strategies for synthesizing promising electrode materials for supercapacitors.

Few-layered MoSe2 nanosheets as an advanced electrode material for supercapacitors

Abstract: We report the synthesis of few-layered MoSe2 nanosheets using a facile hydrothermal method and their electrochemical charge storage behavior. A systematic study of the structure and morphology of the as-synthesized MoSe2 nanosheets was performed. The downward peak shift in the Raman spectrum and the high-resolution transmission electron microscopy images confirmed the formation of few-layered nanosheets. The electrochemical energy-storage behavior of MoSe2 nanosheets was also investigated for supercapacitor applications in a symmetric cell configuration. The MoSe2 nanosheet electrode exhibited a maximum specific capacitance of 198.9 F g(-1) and the symmetric device showed 49.7 F g(-1) at a scan rate of 2 mV s(-1). A capacitance retention of approximately 75% was observed even after 10 000 cycles at a high charge-discharge current density of 5 A g(-1). The two-dimensional MoSe2 nanosheets exhibited a high specific capacitance and good cyclic stability, which makes it a promising electrode material for supercapacitor applications.

Highly Flexible Freestanding Porous Carbon Nanofibers for Electrodes Materials of High-Performance All-Carbon Supercapacitors.

Abstract: Highly flexible porous carbon nanofibers (P-CNFs) were fabricated by electrospining technique combining with metal ion-assistant acid corrosion process. The resultant fibers display high conductivity and outstanding mechanical flexibility, whereas little change in their resistance can be observed under repeatedly bending, even to 180°. Further results indicate that the improved flexibility of P-CNFs can be due to the high graphitization degree caused by Co ions. In view of electrode materials for high-performance supercapacitors, this type of porous nanostructure and high graphitization degree could synergistically facilitate the electrolyte ion diffusion and electron transportation. In the three electrodes testing system, the resultant P-CNFs electrodes can exhibit a specific capacitance of 104.5 F g–1 (0.2 A g–1), high rate capability (remain 56.5% at 10 A g–1), and capacitance retention of ∼94% after 2000 cycles. Furthermore, the assembled symmetric supercapacitors showed a high flexibility and can del...

Superelastic supercapacitors with high performances during stretching.

Abstract: A fiber-shaped supercapacitor that can be stretched over 400% is developed by using two aligned carbon nanotube/polyaniline composite sheets as electrodes. A high specific capacitance of approximately 79.4 F g(-1) is well maintained after stretching at a strain of 300% for 5000 cycles or 100.8 F g(-1) after bending for 5000 cycles at a current density of 1 A g(-1). In particular, the high specific capacitance is maintained by 95.8% at a stretching speed as high as 30 mm s(-1).

MnO2 Nanorods Intercalating Graphene Oxide/Polyaniline Ternary Composites for Robust High-Performance Supercapacitors

Abstract: New ternary composites of MnO2 nanorods, polyaniline (PANI) and graphene oxide (GO) have been prepared by a two-step process. The 100 nm-long MnO2 nanorods with a diameter ~20 nm are conformably coated with PANI layers and fastened between GO layers. The MnO2 nanorods incorporated ternary composites electrode exhibits significantly increased specific capacitance than PANI/GO binary composite in supercapacitors. The ternary composite with 70% MnO2 exhibits a highest specific capacitance reaching 512 F/g and outstanding cycling performance, with ~97% capacitance retained over 5000 cycles. The ternary composite approach offers an effective solution to enhance the device performance of metal-oxide based supercapacitors for long cycling applications.