Showing papers on "Power density published in 2013"

Toward large-scale energy harvesting by a nanoparticle-enhanced triboelectric nanogenerator.

Abstract: This article describes a simple, cost-effective, and scalable approach to fabricate a triboelectric nanogenerator (NG) with ultrahigh electric output. Triggered by commonly available ambient mechanical energy such as human footfalls, a NG with size smaller than a human palm can generate maximum short-circuit current of 2 mA, delivering instantaneous power output of 1.2 W to external load. The power output corresponds to an area power density of 313 W/m2 and a volume power density of 54 268 W/m3 at an open-circuit voltage of ∼1200 V. An energy conversion efficiency of 14.9% has been achieved. The power was capable of instantaneously lighting up as many as 600 multicolor commercial LED bulbs. The record high power output for the NG is attributed to optimized structure, proper materials selection and nanoscale surface modification. This work demonstrated the practicability of using NG to harvest large-scale mechanical energy, such as footsteps, rolling wheels, wind power, and ocean waves.

A high-performance supercapacitor-battery hybrid energy storage device based on graphene-enhanced electrode materials with ultrahigh energy density

Abstract: In pursuing higher energy density with no sacrifice of power density, a supercapacitor-battery hybrid energy storage device—combining an electrochemical double layer capacitance (EDLC) type positive electrode with a Li-ion battery type negative electrode—has been designed and fabricated. Graphene is introduced to both electrodes: an Fe3O4/graphene (Fe3O4/G) nanocomposite with high specific capacity as negative electrode material, and a graphene-based three-dimensional porous carbon material (3DGraphene) with high surface area (∼3355 m2 g−1) as positive electrode material. The Fe3O4/G nanocomposite shows a high reversible specific capacity exceeding 1000 mA h g−1 at 90 mA g−1 and remaining at 704 mA h g−1 at 2700 mA g−1, as well as excellent rate capability and improved cycle stability. Meanwhile the 3DGraphene positive electrode also displays great electrochemical performance. With these two graphene-enhanced electrode materials and using the best recommended industry evaluation method, the hybrid supercapacitor Fe3O4/G//3DGraphene demonstrates an ultrahigh energy density of 147 W h kg−1 (power density of 150 W kg−1), which also remains of 86 W h kg−1 even at high power density of 2587 W kg−1, so far the highest value of the reported hybrid supercapacitors. Furthermore, the energy density of the hybrid supercapacitor is comparable to lithium ion batteries, and the power density also reaches that of symmetric supercapacitors, indicating that the hybrid supercapacitor could be a very promising novel energy storage system for fast and efficient energy storage in the future.

High energy density asymmetric quasi-solid-state supercapacitor based on porous vanadium nitride nanowire anode.

Abstract: To push the energy density limit of asymmetric supercapacitors (ASCs), a new class of anode materials is needed. Vanadium nitride (VN) holds great promise as anode material for ASCs due to its large specific capacitance, high electrical conductivity, and wide operation windows in negative potential. However, its poor electrochemical stability severely limits its application in SCs. In this work, we demonstrated high energy density, stable, quasi-solid-state ASC device based on porous VN nanowire anode and VOx nanowire cathode for the first time. The VOx//VN-ASC device exhibited a stable electrochemical window of 1.8 V and excellent cycling stability with only 12.5% decrease of capacitance after 10 000 cycles. More importantly, the VOx//VN-ASC device achieved a high energy density of 0.61 mWh cm–3 at current density of 0.5 mA cm–2 and a high power density of 0.85 W cm–3 at current density of 5 mA cm–2. These values are substantially enhanced compared to most of the reported quasi/all-solid-state SC devices...

Coaxial fiber supercapacitor using all-carbon material electrodes.

Abstract: We report a coaxial fiber supercapacitor, which consists of carbon microfiber bundles coated with multiwalled carbon nanotubes as a core electrode and carbon nanofiber paper as an outer electrode. The ratio of electrode volumes was determined by a half-cell test of each electrode. The capacitance reached 6.3 mF cm–1 (86.8 mF cm–2) at a core electrode diameter of 230 μm and the measured energy density was 0.7 μWh cm–1 (9.8 μWh cm–2) at a power density of 13.7 μW cm–1 (189.4 μW cm–2), which were much higher than the previous reports. The change in the cyclic voltammetry characteristics was negligible at 180° bending, with excellent cycling performance. The high capacitance, high energy density, and power density of the coaxial fiber supercapacitor are attributed to not only high effective surface area due to its coaxial structure and bundle of the core electrode, but also all-carbon materials electrodes which have high conductivity. Our coaxial fiber supercapacitor can promote the development of textile ele...

A Dual-Band Rectenna Using Broadband Yagi Antenna Array for Ambient RF Power Harvesting

Abstract: This letter presents a dual-band rectenna that can harvest ambient RF power of GSM-1800 and UMTS-2100 bands efficiently. The novel rectenna is based on a broadband 1 × 4 quasi-Yagi antenna array with bandwidth from 1.8 to 2.2 GHz and high gains of 10.9 and 13.3 dBi at 1.85 and 2.15 GHz, respectively. Also, a dual-band rectifier that can sufficiently enhance the RF-to-dc power conversion efficiency (PCE) at ambient RF power level is designed for the rectenna. Measurement results show that a PCE of 40% and an output dc voltage of 224 mV have been achieved over a 5-k Ω resistor when the dual-tone input power density is 455 μW/m 2. Additionally, output dc voltage varying between 300-400 mV can be obtained by collecting the relatively low ambient RF power.

Active Power Decoupling for High-Power Single-Phase PWM Rectifiers

Abstract: Single-phase pulsewidth modulation rectifiers suffer from ripple power pulsating at twice the line frequency. The ripple power is usually filtered by a bulky capacitor bank or an LC branch, resulting in lower power density. The alternative way is active power decoupling, which uses an active circuit to direct the pulsating power into another energy-storage component. The main dc-link filter capacitor can, therefore, be reduced substantially. This paper proposed a new scheme of active power decoupling. The circuit consists of a third leg, an energy-storage capacitor and a smoothing inductor. The topology combined the advantages of high energy-storage efficiency and low requirement on control bandwidth. Both the pulsating power from the ac source and the reactive power of the smoothing inductors are taken into consideration when deriving the power decoupling scheme. The active power filter's (APF) capacitor voltage control system consists of inner loop pole-placement control and outer loop proportional-resonant control. To enhance the steady-state performance, the capacitor voltage reference is modified in a closed-loop manner. Simulation and experimental results show that the proposed APF scheme has good power decoupling performance and is more suited for high-power applications where switching frequency is limited.

High power density microbial fuel cell with flexible 3D graphene–nickel foam as anode

Abstract: The structure and electrical conductivity of anode play a significant role in the power generation of microbial fuel cells (MFCs). In this study, we developed a three-dimensional (3D) reduced graphene oxide–nickel (denoted as rGO–Ni) foam as an anode for MFC through controlled deposition of rGO sheets onto the nickel foam substrate. The loading amount of rGO sheets and electrode surface area can be controlled by the number of rGO loading cycles. 3D rGO–Ni foam anode provides not only a large accessible surface area for microbial colonization and electron mediators, but also a uniform macro-porous scaffold for effective mass diffusion of the culture medium. Significantly, at a steady state of the power generation, the MFC device with flexible rGO–Ni electrodes produced an optimal volumetric power density of 661 W m−3 calculated based on the volume of anode material, or 27 W m−3 based on the volume of the anode chamber. These values are substantially higher than that of plain nickel foam, and other conventional carbon based electrodes (e.g., carbon cloth, carbon felt, and carbon paper) measured in the same conditions. To our knowledge, this is the highest volumetric power density reported for mL-scale MFC device with a pure strain of Shewanella oneidensis MR-1. We also demonstrated that the MFC device can be operated effectively in a batch-mode at least for a week. These new 3D rGO–Ni electrodes show great promise for improving the power generation of MFC devices.

Pulsed Nanogenerator with Huge Instantaneous Output Power Density

Abstract: A nanogenerator (NG) usually gives a high output voltage but low output current, so that the output power is low. In this paper, we developed a general approach that gives a hugely improved instantaneous output power of the NG, while the entire output energy stays the same. Our design is based on an off-on-off contact based switching during mechanical triggering that largely reduces the duration of the charging/discharge process, so that the instantaneous output current pulse is hugely improved without sacrificing the output voltage. For a vertical contact-separation mode triboelectric NG (TENG), the instantaneous output current and power peak can reach as high as 0.53 A and 142 W at a load of 500 Ω, respectively. The corresponding instantaneous output current and power density peak even approach 1325 A/m(2) and 3.6 × 10(5) W/m(2), which are more than 2500 and 1100 times higher than the previous records of TENG, respectively. For the rotation disk based TENG in the lateral sliding mode, the instantaneous output current and power density of 104 A/m(2) and 1.4 × 10(4) W/m(2) have been demonstrated at a frequency of 106.7 Hz. The approach presented here applies to both a piezoelectric NG and a triboelectric NG, and it is a major advance toward practical applications of a NG as a high pulsed power source.

Membrane-less hydrogen bromine flow battery.

Abstract: In order for the widely discussed benefits of flow batteries for electrochemical energy storage to be applied at large scale, the cost of the electrochemical stack must come down substantially. One promising avenue for reducing stack cost is to increase the system power density while maintaining efficiency, enabling smaller stacks. Here we report on a membrane-less hydrogen bromine laminar flow battery as a potential high-power density solution. The membrane-less design enables power densities of 0.795 W cm(-2) at room temperature and atmospheric pressure, with a round-trip voltage efficiency of 92% at 25% of peak power. Theoretical solutions are also presented to guide the design of future laminar flow batteries. The high-power density achieved by the hydrogen bromine laminar flow battery, along with the potential for rechargeable operation, will translate into smaller, inexpensive systems that could revolutionize the fields of large-scale energy storage and portable power systems.

Three-dimensional nanostructured bilayer solid oxide fuel cell with 1.3 W/cm(2) at 450 °C.

Abstract: Obtaining high power density at low operating temperatures has been an ongoing challenge in solid oxide fuel cells (SOFC), which are efficient engines to generate electrical energy from fuels. Here we report successful demonstration of a thin-film three-dimensional (3-D) SOFC architecture achieving a peak power density of 1.3 W/cm2 obtained at 450 °C. This is made possible by nanostructuring of the ultrathin (60 nm) electrolyte interposed with a nanogranular catalytic interlayer at the cathode/electrolyte interface. We attribute the superior cell performance to significant reduction in both the ohmic and the polarization losses due to the combined effects of employing an ultrathin film electrolyte, enhancement of effective area by 3-D architecture, and superior catalytic activity by the ceria-based interlayer at the cathode. These insights will help design high-efficiency SOFCs that operate at low temperatures with power densities that are of practical significance.

High-Frequency High Power Density 3-D Integrated Gallium-Nitride-Based Point of Load Module Design

Abstract: The demand for the future power supplies that can achieve higher output currents, smaller sizes, and higher efficiencies cannot be satisfied with the conventional technologies. There are limitations in the switch performance, packaging parasitics, layout parasitics, and thermal management that must be addressed to push for higher frequencies and improved power density. To address these limitations, the utilization of Gallium-Nitride (GaN) transistors, 3-D integrated technique, low-profile magnetic substrates, and ceramic substrates with high thermal conductivity are considered. This paper discusses the characteristics of GaN transistors, including the fundamental differences between the enhancement mode and the depletion mode GaN transistors, gate driving, and the deadtime loss, the effect of parasitics on the performance of high-frequency GaN point-of-load (POLs), the 3-D copackage technique to integrate the active layer with low profile low temperature cofired ceramic magnetic substrate, and the thermal design of a high -density module using advanced substrates. The final demonstrators are three 12-1.2-V conversion POL modules: a single-phase 20 A 900 W/in3 2-MHz converter using enhancement mode GaN transistors, a single-phase10-A 800 W/in3 5-MHz converter, and a two-phase 20-A 1100 W/in3 5-MHz converter using the depletion mode GaN transistors. These converters offer unmatched power density compared to state-of-the-art industry products and research.

Energy harvesting using a PZT ceramic multilayer stack

Abstract: In this paper, the interdisciplinary energy harvesting issues on piezoelectric energy harvesting were investigated using a ‘33’ mode (mechanical stress and/or electric field are in parallel to the polarization direction) lead zirconate titanate multilayer piezoelectric stack (PZT-Stack). Key energy harvesting characteristics including the generated electrical energy/power in the PZT-Stack, the mechanical to electrical energy conversion efficiency, the power delivered from the PZT-Stack to a resistive load, the electrical charge/energy transferred from the PZT-Stack to a super-capacitor were systematically addressed. Theoretical models for power generation and delivery to a resistive load were proposed and experimentally affirmed. In a quasi-static regime, 70% generated electrical powers were delivered to matched resistive loads. A 35% mechanical to electrical energy conversion efficiency, which is more than 4 times higher than other reports, for the PZT-Stack had been obtained. The generated electrical power and power density were significantly higher than those from a similar weight and size cantilever-type piezoelectric harvester in both resonance and off-resonance modes. In addition, our study indicated that the capacitance and piezoelectric coefficient of the PZT-Stack were strongly dependent on the dynamic stress. (Some figures may appear in colour only in the online journal)

Graphene-oxide-coated LiNi0.5Mn1.5O4 as high voltage cathode for lithium ion batteries with high energy density and long cycle life

Abstract: Lithium ion batteries are receiving enormous attention as power sources and energy storage devices in the renewable energy field. With the ever increasing demand for higher energy and power density, high voltage cathodes have emerged as an important option for new generation batteries. Here, we report graphene-oxide-coated LiNi0.5Mn1.5O4 as a high voltage cathode and demonstrate that the batteries showed superior cycling performance for up to 1000 cycles. Mildly oxidized graphene oxide coating was found to improve the battery performance by enhancing the conductivity and protecting the cathode surface from undesired reactions with the electrolyte. As a result, the graphene-oxide-coated high voltage cathode LiNi0.5Mn1.5O4 showed 61% capacity retention after 1000 cycles in the cycling test, which converts to only 0.039% capacity decay per cycle. At large current rates of 5 C, 7 C and 10 C, the batteries were able to deliver 77%, 66% and 56% of the 1 C capacity, respectively (1 C = 140 mA g−1). In contrast, the LiNi0.5Mn1.5O4 cathode without graphene oxide coating showed 88.7% capacity retention after only 100 cycles. The promising results demonstrated the potential of developing high energy density batteries with the high voltage cathode LiNi0.5Mn1.5O4 and improving the battery performance by surface modification with mildly oxidized graphene oxide.

Medium frequency transformers for solid-state-transformer applications — Design and experimental verification

Abstract: Solid-state-transformer technology pushes the specifications of electric transformers in the high-power medium-frequency range. This combination results in larger-sized transformers operating at higher frequencies whereby parasitic phenomenon should be carefully accounted for. This paper presents an analytical description of the limiting factors and the connection between frequency/power rating and the available core and copper conductor technology on the power density and efficiency of medium-frequency transformers. Furthermore, two designed transformers for 166kW/20kHz based on two different core materials and cooling systems are presented. Extensive copper, core and cooling system loss measurements on one of these transformers are discussed in order to analyze the transformers' behavior from a practical point of view.

Graphene-Oxide-Coated LiNi0.5Mn1.5O4 As High Voltage Cathode for Lithium Ion Batteries With High Energy Density and Long Cycle Life

Abstract: Lithium ion batteries are receiving enormous attention as power sources and energy storage devices in the renewable energy field. With the ever increasing demand for higher energy and power density, high voltage cathodes have emerged as an important option for new generation batteries. Here, we report graphene-oxide-coated LiNi0.5Mn1.5O4 as a high voltage cathode and demonstrate that the batteries showed superior cycling performance for up to 1000 cycles. Mildly oxidized graphene oxide coating was found to improve the battery performance by enhancing the conductivity and protecting the cathode surface from undesired reactions with the electrolyte. As a result, the graphene-oxide-coated high voltage cathode LiNi0.5Mn1.5O4 showed 61% capacity retention after 1000 cycles in the cycling test, which converts to only 0.039% capacity decay per cycle. At large current rates of 5 C, 7 C and 10 C, the batteries were able to deliver 77%, 66% and 56% of the 1 C capacity, respectively (1 C = 140 mA g−1). In contrast, the LiNi0.5Mn1.5O4 cathode without graphene oxide coating showed 88.7% capacity retention after only 100 cycles. The promising results demonstrated the potential of developing high energy density batteries with the high voltage cathode LiNi0.5Mn1.5O4 and improving the battery performance by surface modification with mildly oxidized graphene oxide.

High energy density nanocomposite capacitors using non-ferroelectric nanowires

Abstract: A high energy density nanocomposite capacitor is fabricated by incorporating high aspect ratio functionalized TiO2 nanowires (NWs) into a polyvinylidene-fluoride matrix. These nanocomposites exhibited energy density as high as 12.4 J/cc at 450 MV/m, which is nine times larger than commercial biaxially oriented polypropylene polypropylene capacitors (1.2 J/cc at 640 MV/m). Also, the power density can reach 1.77 MW/cc with a discharge speed of 2.89 μs. The results presented here demonstrate that nanowires can be used to develop nanocomposite capacitors with high energy density and fast discharge speed for future pulsed-power applications.

Minimizing the instant and accumulative effects of salt permeability to sustain ultrahigh osmotic power density.

Abstract: We have investigated the instant and accumulative effects of salt permeability on the sustainability of high power density in the pressure-retarded osmosis (PRO) process experimentally and theoretically. Thin-film composite (TFC) hollow-fiber membranes were prepared. A critical wall thickness was observed to ensure sufficient mechanical stability and hence a low salt permeability, B. The experimental results revealed that a lower B was essential to enhance the maximum power density from 15.3 W/m(2) to as high as 24.3 W/m(2) when 1 M NaCl and deionized water were feeds. Modeling work showed that a large B not only causes an instant drop in the initial water flux but also accelerates the flux decline at high hydraulic pressures, leading to reduced optimal operating pressure and maximal power density. However, the optimal operating pressure to harvest energy can be greater than one-half of the osmotic pressure gradient across the membrane if one can carefully design a PRO membrane with a large water permeability, small B value, and reasonably small structural parameter. It was also found that a high B accumulates salts in the feed, leads to the oversalinization of the feed, and largely lowers both the water flux and power density along the membrane module. Therefore, a low salt permeability is highly desirable to sustain high power density not only locally but also throughout the whole module.

Thresholds for igniting exothermic reactions in Al/Ni multilayers using pulses of electrical, mechanical, and thermal energy

Abstract: We use pulses of electrical, mechanical, and thermal energy to determine the ignition thresholds of self-propagating reactions in Al/(Ni-7 V) and Al/Inconel multilayers The energy density and power density required to initiate reactions in a Al/(Ni-7 V) foil with a 50 nm bilayer is compared for all three techniques to demonstrate the importance of heat loss on ignition thresholds and its dependence on the test volume and the surrounding thermal resistance In addition, ignition is shown to occur at temperatures as low as 232 °C when heat losses are very small suggesting that ignition can be controlled by atomic mixing in the solid state The experiments demonstrate that the ignition threshold drops with increasing ignition volume, and it rises with increasing bilayer spacing and with increasing intermixed thickness These trends are also supported by an analytical model we derive to predict the effects of ignition volume, multilayer microstructure, and physical properties on the ignition threshold We calculate an activation energy of 773 ± 13 kJ/mol for solid state mixing based on measured ignition temperatures

High-performance symmetric electrochemical capacitor based on graphene foam and nanostructured manganese oxide

Abstract: We have fabricated a symmetric electrochemical capacitor with high energy and power densities based on a composite of graphene foam (GF) with ∼80 wt% of manganese oxide (MnO2) deposited by hydrothermal synthesis. Raman spectroscopy and X-ray diffraction measurements showed the presence of nanocrystalline MnO2 on the GF, while scanning and transmission electron microscopies showed needle-like manganese oxide coated and anchored onto the surface of graphene. Electrochemical measurements of the composite electrode gave a specific capacitance of 240 Fg−1 at a current density of 0.1 Ag−1 for symmetric supercapacitors using a two-electrode configuration. A maximum energy density of 8.3 Whkg−1 was obtained, with power density of 20 kWkg−1 and no capacitance loss after 1000 cycles. GF is an excellent support for pseudo-capacitive oxide materials such as MnO2, and the composite electrode provided a high energy density due to a combination of double-layer and redox capacitance mechanisms.

Bendable polymer electrolyte fuel cell using highly flexible Ag nanowire percolation network current collectors

Abstract: This study reports a polymer electrolyte fuel cell based on polydimethylsiloxane coated with a flexible current-collecting layer of Ag nanowire percolation networks The reactive area of the bendable fuel cell was 9 cm2 and showed the maximum absolute power of 639 mW (the power density was 71 mW cm−2) under various bending conditions Impedance spectra of the operating cell revealed that ohmic and Faradaic resistances decreased under the bent condition Overall, the degree of bending improves the cell performances The structural modeling result showed that decrease of the resistance and corresponding performance enhancement were due to the increased compressive force normal to the membrane electrode assembly, which was investigated through finite element simulation of the stress within the bendable fuel cell

Scalable High-Power Redox Capacitors with Aligned Nanoforests of Crystalline MnO2 Nanorods by High Voltage Electrophoretic Deposition

Abstract: It is commonly perceived that reduction–oxidation (redox) capacitors have to sacrifice power density to achieve higher energy density than carbon-based electric double layer capacitors. In this work, we report the synergetic advantages of combining the high crystallinity of hydrothermally synthesized α-MnO2 nanorods with alignment for high performance redox capacitors. Such an approach is enabled by high voltage electrophoretic deposition (HVEPD) technology which can obtain vertically aligned nanoforests with great process versatility. The scalable nanomanufacturing process is demonstrated by roll-printing an aligned forest of α-MnO2 nanorods on a large flexible substrate (1 inch by 1 foot). The electrodes show very high power density (340 kW/kg at an energy density of 4.7 Wh/kg) and excellent cyclability (over 92% capacitance retention over 2000 cycles). Pretreatment of the substrate and use of a conductive holding layer have also been shown to significantly reduce the contact resistance between the alig...

Room temperature all-silicon photonic crystal nanocavity light emitting diode at sub-bandgap wavelengths

Abstract: Silicon is now firmly established as a high performance photonic material. Its only weakness is the lack of a native electrically driven light emitter that operates CW at room temperature, exhibits a narrow linewidth in the technologically important 1300-1600 nm wavelength window, is small and operates with low power consumption. Here, an electrically pumped all-silicon nano light source around 1300-1600 nm range is demonstrated at room temperature. Using hydrogen plasma treatment, nano-scale optically active defects are introduced into silicon, which then feed the photonic crystal nanocavity to enhance the electrically driven emission in a device via Purcell effect. A narrow (Δλ=0.5 nm) emission line at 1515 nm wavelength with a power density of 0.4mW/cm2 is observed, which represents the highest spectral power density ever reported from any silicon emitter. A number of possible improvements are also discussed, that make this scheme a very promising light source for optical interconnects and other important silicon photonics applications. © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.