Charge Storage Behavior Research Articles

Doped Transition Metal Nitride Supercapacitors (DTMSCs): Mechanisms of Charge Storage

Charge storage behavior in nitride nanomaterials depends on a number of particle properties and also on the nature of the electrode (1-9). Vanadium nitride is a promising supercapacitor material on account of pseudocapacitance arising from the surface oxide/oxynitride phase interactions. We have previously reported excellent charge storage behavior of nanoparticulate vanadium nitride on account of the surface reactions occurring on the surface of the oxide exo-shell. Vanadium oxide is a superior pseudocapacitor material but suffers from poor electronic conductivity(10). Forming a thin surface oxide layer on nanoparticle nitrides yields the benefits of the charge storage along with charge storage occurring in the nitride core itself. However, there is evidence showing that the surface oxide suffers from instability in highly alkaline solutions on account of side-reactions. In addition, the core nitride itself becomes a poor conductor when prepared as fine nanoparticles (<10 nm)(3). In this study, we tailor nitride nanoparticles to engineer architectures similar to that shown in Figure 1by the use of suitable dopants. By using wet chemical procedures, we generate a number of doped nitrides with doped surface oxide structure. We use ab-initio first principle studies to guide our dopant selection with improved electronic conductivity of surface oxide/core nitride and stability of surface oxide being parameters driving our dopant selection. We demonstrate in this study doped VN with superior electronic conductivity, high capacity and long cycle life. In-depth surface characterization using XPS and electrochemical impedance spectroscopy (EIS) to probe change in charge-storage mechanisms are conducted herein to obtain fundamental understanding into the nature of the various doped nitride materials. Change in nanoparticle morphology and surface composition are also examined and related to charge storage behavior. Results of these studies will be presented and discussed.

Flexible and transparent nanocrystal floating gate memory devices using silk protein

The charge storage behavior of a floating gate memory device using carbon nanotube-CdS nanostructures embedded in Bombyx mori silk protein matrix has been demonstrated. The capacitance – voltage characteristics in ITO/CNT–CdS-silk composite/Al device exhibits a clockwise hysteresis behavior due to the injection and storage of holes in the quantized valence band energy levels of CdS nanocrystals. The enhanced charge injection resulting in increase in memory window is observed at higher sweeping voltages. Nearly frequency independent hysteresis width over a wide range of 100kHz–2.0MHz, indicates its origin due to the charge storage in nanocrystals. The memory behavior of carbon nanotube–CdS nanostructures/silk nanocomposite devices has also been demonstrated on polyethylene terephthalate substrates, which may provide the way for flexible, transparent and printable electronic devices.

Nitride Supercapacitors: Contribution of Surface Oxide to Charge Storage

With the growing interest in alternative energy sources and the cognitive recognition of the benefits of green energy, there is clearly a pressing need for the development of sustainable and clean energy storage systems (1-3). The scope of electrochemical charge storage devices extends beyond mobile devices into large scale grid level charge storage(4). Load leveling and power quality management applications would be perfectly served by storage of excess charge generated from renewable energy sources in supercapacitors. Supercapacitors are unique electrochemical energy storage devices with high power density and long cyclability. State of the art materials such as high surface area carbons, hydrated ruthenium oxide (5-7) and MnO2 (3, 8-10) suffer from either poor capacity or rate capability. We have previously demonstrated excellent charge storage behavior of nanoparticulate vanadium nitride on account of the surface reactions occurring on the surface of the oxide exo-shell. However, there is limited understanding into the exact nature of the charge storage behavior and its dependence on the synthesis and processing route. In the present work, we explore the effect of materials processing and electrode properties on the capacitive charge storage in VN based supercapacitors. Dependence of capacitance on particle and electrode properties are evaluated and reported here-in. Tailoring the particle size, crystallinity and porosity are of paramount importance to achieve high capacitances at high scan rates. To gain a fundamental understanding into the charge storage mechanism of nitride materials shown in Figure 1, slurries of the nitride were cast on nickel current collectors and characterized used various materials and electrochemical characterization techniques including X-ray photo-electron spectroscopy (XPS), cyclic voltammetry and electrochemical impedance spectroscopy. Results of these studies will be presented and discussed.

Novel Heterostructures for Lithium-Sulfur Batteries

IntroductionLithium-ion batteries with their unique high voltage, high energy density chemistries have come to fruition decades of fundamental research conducted in electrochemical systems and materials. The development of a number of plug-in hybrid and fully electric vehicles (EV) using lithium-ion technology and their favorable market response is the harbinger of a long-awaited transition in automotive technology. Despite these advances, there is still a genuine need for increasing energy densities for the successful transition to EVs (1, 2). Advances in lithium-ion anode technology have made it possible to obtain ~1500 mAh/g anodes with superior cycle life (3-5). However, cathode research has been rather stymied in contrast on account of the challenges faced in designing stable, conductive materials with high capacities. Cathode capacities thus far have been limited to ~ 300 mAh/g in high voltage (1, 6) lithium manganese-nickel-cobalt oxide chemistries. Lithium sulfur and lithium-air batteries though well-known but challenging to execute, hold much more promise of matching anode capacities on account of their theoretical capacities of 2567 and 3505 Wh/kg respectively(2). The dissolution of sulfur through the formation of soluble polysulfides and the poor electronic conductivity of sulfur are however, major problems hindering Li-S batteries.It has previously been demonstrated that the use of highly conductive porous carbon matrix and carbon nanotubes can help circumvent these issues (7-10). In addition, the use of a carbon matte as a barrier layer to polysulfide transport has been demonstrated to be effective in retaining high capacity(11). There is however a need to translate this design into thick electrodes capable of delivering high overall electrode capacity. In this study, we coat pristine sulfur particles with carbon and an ionically conducting matrix using a simple chemical method. The thick electrodes were prepared using this procedure and were tested in 2025 coin cells with lithium counter/reference electrodes. We demonstrate excellent charge storage behavior of these composite electrodes at various charge-discharge rates. Extended cycling of these composite electrodes reveals much improved capacity retention. Figure 1 shows the electrochemical results of these different configurations. Results of these studies will be presented and discussed.

Electrochemical Characterization of Liquid Phase Exfoliated Two-Dimensional Layers of Molybdenum Disulfide

We report on the electrochemical charge storage behavior of few-layered flakes of molybdenum disulfide (MoS2) obtained by liquid phase exfoliation of bulk MoS2 powder in 1-dodecyl-2-pyrrolidinone. The specific capacitances of the exfoliated flakes obtained using a 6 M KOH aqueous solution as an electrolyte were found to be an order of magnitude higher than those of bulk MoS2 (∼0.5 and ∼2 mF cm(-2) for bulk and exfoliated MoS2 electrodes, respectively). The exfoliated MoS2 flakes also showed significant charge storage in different electrolytes, such as organic solvents [1 M tetraethylammonium tetrafluoroborate in propylene carbonate (Et4NBF4 in PC)] and ionic liquids [1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)]. The values of specific capacitances obtained using Et4NBF4 in PC and BMIM-PF6 were ∼2.25 and ∼2.4 mF cm(-2), respectively. An analysis of electrochemical impedance spectroscopy using an equivalent circuit modeling was performed to understand the charge storage mechanism of these exfoliated MoS2 flakes using different electrolytes. Our findings indicate that liquid phase exfoliation methods can be used to produce large quantities of electrochemically active, two-dimensional layers of MoS2 and can act as an ideal material in several applications related to electrochemistry.

Lanthanum Fluoride Charge Trapping Layer with Silicon Nanocrystals for Nonvolatile Memory Device Application

Silicon nanocrystals (Si-NCs) embedded in a Lanthanum Fluoride (LaF3) insulating layer were fabricated as a charge trapping layer by a simple Chemical Bath Deposition (CBD) technique. The X-Ray diffraction of the deposited layer shows a polycrystalline LaF3 deposition on silicon. The charge storage behavior of Si-NCs embedded in the LaF3 layer have been investigated in metal-insulator-semiconductor (MIS) structures by electrical characterization, where various interface traps and defects were introduced by thermal annealing treatment. The flat-band voltage shift of capacitance-voltage (C–V) and conductance-voltage (G–V) curves of Si: NC-MIS devices were found to exhibit charge trapping. The current-voltage (I–V) measurement also demonstrate that traps have strong influence on the charge storage behavior, in which the traps and defects at the internal/surface of silicon nanocrystals and the interface states at the LaF3 /Si substrate play different roles, respectively. The flat-band voltage (VFB) shift was about 700 mV, which is agreed well enough to capture charge inside the nanoparticle for nonvolatile memory (NVM) device applications. Thickness-dependent flat-band voltage (VFB) shifts in the MIS structure which can be used as a low-voltage nonvolatile memory.

Accurate Capacitance Modeling and Characterization of Organic Thin-Film Transistors

This paper presents analysis of the charge storage behavior in organic thin-film transistors (OTFTs) by means of admittance characterization, compact modeling, and 2-D device simulation. The measurements are performed for frequencies ranging from 100 Hz to 1 MHz and bias potentials from zero to -3 V on top-contact OTFTs that employ air-stable and high-mobility dinaphtho-thieno-thiophene as the organic semiconductor. It is demonstrated that the dependence of the intrinsic OTFT gate-source and gate-drain capacitances on the applied voltages agrees very well with Meyer's capacitance model. Furthermore, the impact of parasitic elements, including fringe current and contact impedance, is investigated. The parameters used for the simulation and modeling of all the dynamic characteristics correspond closely to those extracted from static measurements. Finally, the implications of the admittance measurements are also discussed relating to the OTFTs dynamic performance, particularly the cutoff frequency and the charge response time.

Charge storage behavior of nanostructures based on SiGe nanocrystals embedded in Al2O3 matrix

The charge storage behavior of nanostructures based on Si1−x Ge x (0 ≤ x ≤ 1) nanocrystals (NCs) in an Al2O3 matrix was investigated. The structures have been grown by RF magnetron sputtering and subsequently annealed at temperatures ranging from 700 °C to 1000 °C for 30 min in nitrogen ambient. The stoichiometry of the SiGe NCs and the alumina crystalline structure were found to be significantly dependent on the RF power and the annealing temperature. The sizes of the SiGe NCs and their distribution were investigated by grazing incidence small angle X-ray scattering (GISAXS). The capacitance-voltage (C-V) and conductance-voltage (G-V) measurements were performed to investigate the charge trapping characteristics of the memory structures. The C-V hysteresis width depends on variations in the crystalline structure resulting from different annealing temperatures. It is also shown that charge injection is governed by the Fowler-Nordheim tunnel mechanism for higher electric fields.

Pseudo-Capacitors: SPPS Deposition and Electrochemical Analysis of α-MoO3 and Mo2N Coatings

Solution precursor plasma spraying (SPPS) is a novel thermal spray process in which a solution precursor is injected into the high-temperature zone of a DC-arc plasma jet to allow solvent evaporation from the precursor droplets, solute precipitation, and precipitate pyrolysis prior to substrate impact. This investigation explored the potential of SPPS to fabricate α-MoO3 coatings with fine grain sizes, high porosity levels, and high surface area: characteristics needed for application as pseudo-capacitor electrodes. Since molybdenum nitride has shown a larger electrochemical stability window and higher specific area capacitance, the α-MoO3 deposits were subsequently converted into molybdenum nitride. A multistep heat-treatment procedure resulted in a topotactic phase-transformation mechanism, which retained the high surface area lath-shaped features of the original α-MoO3. The electrochemical behaviors of molybdenum oxide and molybdenum nitride deposits formed under different deposition conditions were studied using cyclic voltammetry to assess the influence of the resulting microstructure on the charge storage behavior and potential for use in pseudo-capacitors.

High Performance Hybrid Supercapacitor Based on Graphene-Supported Ni(OH)2-Nanowires and Ordered Mesoporous Carbon CMK-5

In this work, graphene-supported Ni(OH)2-nanowires were prepared through a facile hydrothermal method, and their charge storage behavior was studied in comparison with ordered mesoporous carbon CMK-5. The as-prepared Ni(OH)2-nanowires displays ultrafast charge-discharge rate, owing to their unique nanostructure. The comparison investigation demonstrates that the capacitance of graphene-supported Ni(OH)2-nanowires mainly arises from battery behavior, rather than pseudocapacitive behavior. Finally, the graphene-supported Ni(OH)2-nanowires and CMK-5 were used as the positive and negative electrode, respectively, to form a high performance hybrid supercapacitor which can deliver a maximum specific power density of 40840 W/kg with a high energy density of about 17.3 Wh/kg. Thereby, this hybrid supercapacitor should be a promising system for fast energy storage.

Microwave and RF p-i-n Diode Model for Time-Domain Simulation

A dynamic time-domain model suitable for simulating the behavior of high-speed high-power p-i-n diodes is presented. This time-domain model accurately describes not only the charge storage behavior in the p-i-n diode in forward bias, but also the reverse-bias capacitance and resistance as a function of reverse voltage. A SPICE implementation of the time-domain model is fully described and a spreadsheet is being made available to the microwave community. The time-domain model is verified with experimental data and good agreement was obtained in both diode bias states. Three applications describing the linear, nonlinear, and transient behavior of the p-i-n diode simulated using the time-domain model are also presented. This improved time-domain model and associated SPICE implementation allows full modeling of high-speed high-frequency p-i-n diodes.

Memory Characteristics of Metal-Oxide-Semiconductor Structures Based on Ge Nanoclusters-Embedded GeOx Films Grown at Low Temperature

The memory devices constructed from the Ge-nanoclusters embedded GeO(x) layer deposited by the laser-assisted chemical vapor deposition (LACVD) system were fabricated. The Ge nanoclusters were observed by a high-resolution transmission electron microscopy. Using the capacitance versus voltage (C-V) and the conductance versus voltage (G-V) characteristics measured under various frequencies, the memory effect observed in the C-V curves was dominantly attributed to the charge storage in the Ge nanoclusters. Furthermore, the defects existed in the deposited film and the interface states were insignificant to the memory performances. Capacitance versus time (C-t) measurement was also executed to evaluate the charge retention characteristics. The charge storage and retention behaviors of the devices demonstrated that the Ge nanoclusters grown by the LACVD system at low temperature are promising for memory device applications.

Functionalization of Polypropylene with High Dielectric Properties: Applications in Electric Energy Storage

Biaxial-oriented polypropylene (BOPP) thin films are currently used as dielectrics in state-of-the-art capacitors that show many advantages, such as low energy loss and high breakdown strength, but a limited energy density ( 600 MV/m. The PP-OH dielectric demonstrates a linear reversible charge storage behavior with high releasing energy density > 7 J/cm3 (2 - 3 times of BOPP) after an applied electric field at E = 600 MV/m, without showing any significant increase of energy loss and remnant polarization at zero electric field. On the other hand, a cross-linked polypropylene (x-PP) exhibits an e ~ 3, which is independent of a wide range of temperatures and frequencies, slim polarization loops, high breakdown strength (E = 650 MV/m), narrow breakdown distribution, and reliable energy storage capacity > 5 J/cm3 (double that of state-of-the-art BOPP capacitors), without showing any increase in energy loss.

Investigation of Pseudocapacitive Charge-Storage Behavior in Highly Conductive Ordered Mesoporous Tungsten Oxide Electrodes

Herein, a pseudocapacitive charging behavior of highly conductive ordered mesoporous tungsten oxide (m-WO3-X) is investigated. For this purpose, various electrochemical analysis methods such as cyclic voltammetry (CV), galvanostatic charge–discharge experiment and electrochemical impedance spectroscopy (EIS) were employed. From CV experiment, a relationship analysis between voltammetric charge and scan rate resulted in total (67 C g–1), outer (61 C g–1) and inner charge (6 C g–1), which was related with the well-developed crystalline structure of m-WO3-X. In galvanostatic charge–discharge profiles with change of applied current, a more severe decrease of cathodic capacity than the anodic one was observed, which was attributed to larger cathodic resistance. This resistance dependency on potential was clarified with EIS fitting analysis. Here, the charge transfer resistance and Warburg diffusion resistance became larger with increasing potential, which was relevant to the change of oxidation state during re...

Reversible Electrical Reduction and Oxidation of Graphene Oxide

We demonstrate that graphene oxide can be reversibly reduced and oxidized using electrical stimulus. Controlled reduction and oxidation in two-terminal devices containing multilayer graphene oxide films are shown to result in switching between partially reduced graphene oxide and graphene, a process which modifies the electronic and optical properties. High-resolution tunneling current and electrostatic force imaging reveal that graphene oxide islands are formed on multilayer graphene, turning graphene into a self-assembled heterostructure random nanomesh. Charge storage and resistive switching behavior is observed in two-terminal devices made of multilayer graphene oxide films, correlated with electrochromic effects. Tip-induced reduction and oxidation are also demonstrated. Results are discussed in terms of thermodynamics of oxidation and reduction reactions.

Ion Transport and Storage in Ionic Polymer Bending Actuators

ABSTRACTThe actuation of ionic polymer actuators is mainly caused by the ion transport and excess ions storage in the membrane and electrodes. To quantify the charge transport behavior, a time domain method based on Poisson-Nernst-Planck equations was applied. The time domain transient current in response to a step voltage can provide insights on the charge transport and storage behaviors in the membranes. In this study, we investigate the charge transport behavior of Aquivion ionomer with different uptakes of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-Tf). A critical uptake and voltage independent of charge transport behavior were observed. The results also show that bending actuations of the Aquivion membrane with 40wt% EMI-Tf is much larger than that of Nafion, indicating that the shorter flexible side chain ionomer possesses a better electromechanical coupling between the excess ions and the membrane backbones, while not affect the actuation speed.

Micropatternable elastic electrets based on a PDMS/carbon nanotube composite

This paper reports the fabrication and performance of an elastic electret based on polydimethylsiloxane (PDMS)/carbon-nanotube (CNT) nanocomposites. The composite formulation concentrates CNTs near the surface of the PDMS and thereby combines the excellent electrical properties and room temperature micropatternability of the CNT with the elasticity of PDMS. The fabrication approach preserves the good rheological properties of unfilled PDMS and also efficiently utilizes the CNTs. The material was corona charged and the charge storage behavior was characterized using surface potential measurements. Substantial improvements in charge storage capacity and stability were observed compared to either pure PDMS or CNTs on the surface of PDMS at room temperature over a 280 h measurement period. The power generation of the corona-charged elastic composite was initially demonstrated through a ball drop experiment.

Charge Storage Behaviors of Ge Nanocrystals Embedded in SiO&lt;SUB&gt;2&lt;/SUB&gt; for the Application in Non-Volatile Memory Devices

Ge nanocrystals distributed in the SiO2 of metal-oxide-semiconductor structure are synthesized by low-energy Ge ion implantation with various energies and doses. Their charge storage behaviors are influenced by both the ion implantation dose and energy. The larger flatband voltage shift achieved by increasing either the implantation dose or energy is explained by the locations and concentration of the charge trapping sites. The smaller charge loss achieved by decreasing the implantation dose or increasing the implantation energy is explained by the co-existence of the charge leakage to the gate electrode and the lateral charge loss to the adjacent Ge nanocrystals.

Comparison of Charge Storage Behavior of Electrons and Holes in a Continuous Ge Nanocrystal Layer

The charge storages behaviors of electrons and holes injected into a continuous nc-Ge layer embedded in SiO2 synthesized by the Ge ion implantation technique are investigated. The hole injection is found to exhibit a much higher charging speed as well as a better retention in the nc-Ge layer as compared to the electron injection. The better hole retention is explained in terms of slower charge loss to the Si substrate as well as slower lateral charge diffusion as a result of the larger oxide barrier height. The faster hole injection is also attributed to the less significant lateral diffusion of injected holes during the charging period. The results indicate that the oxide barrier height plays an important role in the charge storage behaviors of electrons and holes injected into the continuous nc-Ge layer.

Low Temperature Growth of Silicon Structures for Application in Flash Memory Devices

AbstractAn in-depth study of the structural and electrical properties of silicon (Si) films deposited by a novel low temperature technique at temperatures less than 400°C in a 13.56 MHz RF PECVD reactor is reported. The method is based on substrates having to undergo some initial preparatory steps (IPS) before the deposition of Si films in the PECVD chamber. The optical band gap of Si films deposited using this novel technique narrowed to 1.25 eV from 1.78 eV using the traditional a-Si:H deposition recipe. No annealing of any form was performed on the films to attain this band gap. Furthermore, photosensitivities for these films under various deposition conditions were of order 100 compared to 104 for a-Si:H films deposited under like conditions. Using metal-insulator-semiconductor devices, the Si films grown by this novel technique exhibit charge storage and memory behaviour unlike their amorphous counterparts. However, device endurance has been found to be inadequate, probably due to the presence of some contaminants - notably interstitial oxygen - which has been found elsewhere to have adverse effects on the electrical characteristics of Si films. If well harnessed, we suggest Si structures grown by this novel growth technique could be well-suited for flash memory applications, particularly 3-D flash which requires process temperatures to be less than 400 °C.