Small Equivalent Series Resistance Research Articles

CoMnS@rGO nanocomposite grown on NF for high-performance supercapacitor-battery hybrid device and advancing electrochemical sensitivity

The co-electrodeposition approach has been widely used to synthesize the transition metal sulfides/oxides because of remarkable performance of the energy storage devices. In this research, cobalt manganese sulfide (CoMnS) binary compounds are synthesized using the co-electrodeposition approach. The main objective is to investigate the optimal number of depositing periods in cyclic voltammetry while maintaining a constant scan rate. The electrodes without binders are produced using a rapid synthesis approach, which involves assessing binary metal sulfide to achieve a significantly enhanced energy storage capacity. Further, the rGO is incorporated into the CoMnS to enhance the electrochemical properties. The CoMnS@rGO electrode showed the specific capacitance (Cs) of 2357 F/g because of the small equivalent series resistance values as determined through electrochemical impedance spectroscopy (EIS) measurement, indicating a higher level of conductivity. The hybrid electrode showed high values of energy and power densities of 50.8 Wh/kg and 3440 W/kg, respectively. Besides, the nanocomposite electrode is used as a chemical sensor and precisely detects the multiple elements. The estimated value of R2 is 0.994 because of the high sensitivity. The hybrid electrode showed a signal-to-noise ratio (S/N) of 5. Furthermore, based on four observations, the comparative standard deviation (RSD) is calculated to be 1.9 %. The device showed high electrochemical glucose detection sensitivity. The nanocomposite-based electrodes provide a platform for designing novel multifunctional devices.

Effects of binder content on potato starch-derived porous carbon microspheres for high-performance supercapacitors

Potato starch-based carbon microspheres (PCMs) have successfully synthesized from potato starch by a simple strategy based on (NH4)2HPO4 and KOH activation. The PCMs presents a large specific surface area (SSA) of 2048 m2 g−1. The electrochemical behaviors of the PCMs with different binder content in 6 mol L−1 KOH were studied by impedance spectroscopy, cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD). The results show that PCMs-85 (the binder content is 5 wt%) has a small equivalent series resistance (ESR), high specific capacitance, compacted density, and areal density. PCMs-85 exhibited good cycling durability of 100 % after 10,000 cycles at 1 A g−1 and delivered an energy density of 6.35 Wh L−1 and a volume power density of 167.38 W L−1. The corresponding capacitive performances of PCM-85-based symmetrical two-electrode supercapacitors are investigated in 6 mol L−1 KOH, 1 mol L−1 Na2SO4 and 1 mol L−1 TEA.BF4/AN electrolyte. The PCMs supercapacitor with 6 mol L−1 KOH electrolyte exhibits a mass-specific capacitance of 172 F g−1 with volumetric capacitances of 145 F cm−3 at 1 A g−1.

Synergistic effect of Fe (III) doping and MWCNTs integration on the electrochemical performance of nickel sulfide nanoparticles for hybrid supercapacitor applications

In this report, Iron (Fe) doped nickel sulfide (NiS) nanoparticles integrated with multi-walled carbon nanotubes (MWCNTs) have been prepared by hydrothermal and ultrasonication approach. X-rays studies of synthesized samples showed the existence of single-phase hexagonal Fe–NiS along MWCNTs with crystallite size of 13.19 nm. SEM images indicated spherical Fe–NiS particles encapsulated by long and thin thread like MWCNTs. Fe–NiS@MWCNTs was drop casted on nickel foam (NF) for the supercapacitor application. Fe–NiS@MWCNTs/NF offered a specific capacity of 662C g−1 at 5 mV s−1 with excellent cyclic stability. Fe–NiS@MWCNTs/NF maintained 70.2% of initial specific capacity at a high scan rate of 100 mV s−1. For impedance studies, a small equivalent series resistance (ESR = 2.30 Ω) and charge transfer resistance (RCT = 3.12 Ω) were observed for Fe–NiS@MWCNTs/NF. The superior electrochemical performance of Fe–NiS@MWCNTs/NF can be accredited to the large surface area and intervened network of Fe–NiS@MWCNTs. Pseudocapacitive Fe–NiS and highly conductive MWCNTs generate synergistic effects that ultimately boost up the specific capacity of Fe–NiS@MWCNTs/NF electrode. The excellent electrochemical performance of proposes Fe–NiS@MWCNTs/NF as effective electroactive material for supercapacitor applications.

Electrospun Benzimidazole-Based Polyimide Membrane for Supercapacitor Applications.

A benzimidazole-containing diamine monomer was prepared via a simple one-step synthesis process. A two-step procedure involving polycondensation in the presence of aromatic dianhydrides (4,4′-oxydiphthalic anhydride, ODPA) followed by thermal imidization was then performed to prepare a benzimidazole-based polyimide (BI-PI). BI-PI membranes were fabricated using an electrospinning technique and were hot pressed for 30 min at 200 °C under a pressure of 50 kgf /cm2. Finally, the hot-pressed membranes were assembled into supercapacitors, utilizing high-porosity-activated water chestnut shell biochar as the active material. The TGA results showed that the BI-PI polymer produced in the two-step synthesis process had a high thermal stability (Td5% = 527 °C). Moreover, the hot-press process reduced the pore size in the BI-PI membrane and improved the pore-size uniformity. The hot-press procedure additionally improved the mechanical properties of the BI-PI membrane, resulting in a high tensile modulus of 783 MPa and a tensile strength of 34.8 MPa. The cyclic voltammetry test results showed that the membrane had a specific capacitance of 121 F/g and a capacitance retention of 77%. By contrast, a commercial cellulose separator showed a specific capacitance value of 107 F/g and a capacitance retention of 49% under the same scanning conditions. Finally, the membrane showed both a small equivalent series resistance (Rs) and a small interfacial resistance (Rct). Overall, the results showed that the BI-PI membrane has significant potential as a separator for high-performance supercapacitor applications.

Single and Double Layer of Monoclinic VO2 Ink‐Based Printed and Interdigitated Supercapacitors

Screen printing has received significant attention for manufacturing energy storage devices. However, preparing high‐quality inks is still one of the main challenges. Herein, a homogenous and stable ink based on the monoclinic phase of VO2(M) microparticles has been synthesized by a simple hydrothermal process in only 6 h and through nontoxic solutions. The VO2 ink is printed on Kapton substrate in a single‐layer as well as in a double‐layer arrangement (1‐LP and 2‐LP). The printed 1‐LP VO2 electrode delivers a maximum areal capacitance of 20 mF cm−2 with a small equivalent series resistance of 4 Ω without conducting additives. The interdigitated full‐cell VO2 supercapacitor adequately employs the electric double‐layer and Faradic redox mechanisms in 1.4 V, which is the highest operating voltage reported for symmetric supercapacitors based on pure vanadium oxide electrodes in aqueous inorganic electrolytes. Moreover, the maximum areal energy of the 2‐LP supercapacitor is 0.8 μWh cm−2 at an areal power of 21.0 μW cm−2, which is larger than the 1‐LP (0.2 μWh cm−2 at 17.5 μW cm−2). This improvement is attributed to the homogenously printed double layer of the porous VO2 microparticles. Integrating such supercapacitors into thin‐film electronics could develop portable devices.

A Ten-Minute Synthesis of α-Ni(OH)2 Nanoflakes Assisted by Microwave on Flexible Stainless-Steel for Energy Storage Devices.

Although numerous methods have been widely used to prepare nickel hydroxide materials, there is still a demand for lowering the required heating time, temperature, and cost with maintaining a high-quality nanomaterial for electrochemical energy storage. In this research, we study the relationship between microwave-assisted heating parameters and material properties of nickel hydroxide nanoflakes and evaluate their effect on electrochemical performance. X-ray diffraction spectra show that the samples prepared at the highest temperature of 220 °C have crystallized in the beta phase of nickel hydroxide crystal. While the sample synthesized at 150 °C in 30 min contains both beta and alpha phases. Interestingly, we obtained the pure alpha phase at 150 °C in just 10 min. A scanning electron microscope shows that increasing the temperature and heating time leads to enlarging the diameter of the macro-porous flower-like clusters of interconnected nanoflakes. Electrochemical measurements in potassium hydroxide electrolytes demonstrate that the alpha phase’s electrodes have much higher capacities than samples containing only the beta phase. The maximum areal capacity of 17.7 µAh/cm2 and gravimetric capacity of 35.4 mAh/g are achieved, respectively, at 0.2 mA/cm2 and 0.4 A/g, with a small equivalent series resistance value of 0.887 ohms on flexible stainless-steel mesh as a current collector. These improved nickel hydroxide electrodes can be ascribed to utilizing the diffusion-controlled redox reactions that are detected up to the high scan of 100 mV/s. Such fast charge-discharge processes expand the range of potential applications. Our nickel hydroxide electrode, with its rapid preparation at medium temperature, can be a cost-effective candidate for flexible supercapacitors and batteries.

Chemically Treated Activated Carbon for Supercapacitor Electrode Derived from Starch of Solanum Tuberosum

Because of the increasing demands in energy, growth in alternate energy sources is inevitable. By activation followed by carbonization of starch of solanum tuberosum makes a porous carbon network as electrode material for supercapacitor. To achieve an improvement in specific capacitance, activating the starch of solanum tuberosum using chemicals such as hydrochloric acid, phosphoric acid and sulphuric acid were done. The structural and electrochemical performance of activated carbon derived from the starch of solanum tuberosum is evaluated using FTIR, XRD, CV, GCD and EIS. The improved performance of chemically treated starch of solanum tuberosum in energy storage mechanism is ascribed to fast ionic diffusion of the electrolyte into and out of the pores. From the CV analysis, the higher specific capacitance (94 F.g-1) is obtained for sulphuric acid treated activated carbon derived from the starch of solanum tuberosum with good capacity retention ratio. From GCD analysis, 78.3, 56.7, 82.7 and 74.9 Fg-1 of specific capacitance is obtained for POC, POCH, POCP and POCS, respectively. When compared to all the prepared samples, POC exhibited small equivalent series resistance and POCS exhibited small charge transfer resistance.

One-step construction of δ-MnO2 cathodes with an interconnected nanosheet structure on graphite paper for high-performance aqueous asymmetric supercapacitors

Among various supercapacitor materials, transition metal oxides have attracted wide attention due to their high theoretical capacitance, and however a great challenge associated with the related materials is to fabricate nanostructured electrodes with high comprehensive performance including high capacitance, excellent cycle stability and good rate performance by a simple and green method. Herein, a high-performance self-supporting cathode consisting of interconnected δ-MnO2 nanosheets on graphite paper (GP) is developed by water bath only at 40°C. Benefiting from the synergetic effects from the interconnected and open nanosheet structure of δ-MnO2 and the GP substrate including effective contact with the electrolyte, a large specific surface area, efficient stress relief, improved ion diffusion and charge transfer, and small equivalent series resistance, a high specific capacitance of 446.6 F g–1 at 1 A g–1, 1.3 V potential window, and outstanding cycle stability with the capacitance retention of ~ 86.1% after 10000 cycles at 8 A g–1 and Columbic efficiency around 100% during cycling can be achieved for the cathodes. The aqueous asymmetric supercapacitors assembled using the δ-MnO2 cathode and active carbon anode and 1 M NaNO3 aqueous solution electrolyte can deliver a specific energy of 47.2 and 19.8 W h kg−1 at 1150 and 115000 W kg−1, respectively. Moreover, long cycle stability with 87.2% capacitance retention even after 30000 cycles at 4 A g−1 can be maintained by the ASCs. Thanks to the simple and green preparation and high performance of the electrodes, valuable exploration of developing high-performance ASCs can be provided by this work.

Stability Effect of Control Weight on Multiloop COT-Controlled Buck Converter With PI Compensator and Small Output Capacitor ESR

Multiloop constant on-time (COT) controlled buck converter uses two weight resistors for sensing inductor current and detecting output voltage in its inner loop. When an output capacitor with a small equivalent series resistance (ESR) is employed, the converter will suffer the risk of instability due to different weight settings of inductor current and output voltage using a proportional-integral (PI) compensation gain in the outer voltage loop. However, no attention has been paid for their stability effects, which are important for designing the inner loop and outer voltage loop. To address these issues, a piecewise linear model is established for multiloop COT-controlled buck converter. Circuit parameter-related bifurcation behaviors and control weight-related dynamical distributions are explored to demonstrate these stability effects. Furthermore, by deriving an approximate discrete model, a control weight-related stability criterion is expressed explicitly, from which stability boundaries for dividing the normally stable and abnormally unstable operation parameter regions are thereby yielded. Besides, control weight-related transient performances under different circuit parameter settings are analyzed. Finally, PSIM circuit simulations and hardware circuit experiments verify the theoretical analysis well.

High-Accuracy Capacitance Monitoring of DC-Link Capacitor in VSI Systems by LC Resonance

Monitoring the condition of a dc-link capacitor is important for the reliability-critical voltage source inverter (VSI) systems. With the ever increasing application of a metalized poly propylene film (MPPF) capacitor, a new challenge for dc-link capacitor condition monitoring (CM) is presented due to the very small equivalent series resistance and capacitance end-of-life criterion (typically 2%–5%) of an MPPF capacitor. To improve the accuracy of capacitance estimation, a novel quasi-online CM method is proposed in this paper, which introduces an LC resonance intentionally when system is not in operation. Experimental work is carried out to validate the method and the results show that 1% capacitance estimation accuracy can be achieved in a 3-kW induction machine variable frequency drive system. Because of the signal measurement at large amplitude (e.g., nominal current) and the time-based multi-parameters identification algorithm, the method is accurate and inherently insensitive to the system parameters variation, measurement noise, and effect of temperature drift. In practice, the method can be implemented using the system available sensors with no need of hardware modification. Moreover, since the resonance current is limited and decays shortly, the condition of a dc-link capacitor can be monitored safely and frequently on-site in a VSI system.

High performance hierarchical porous carbon derived from distinctive plant tissue for supercapacitor

It is generally acknowledged that the activation method and component of the precursor are of great importance for making porous carbon. In this study, four plant materials belong to one genus were selected as optimized plant material to produce hierarchical porous carbon for supercapacitors, the influence of initial structure was discussed. All the produced porous carbons have large specific surface area (higher than 2342 m2 g−1), high microporosity (more than 57%), and high pore volume (larger than 1.32 cm3 g−1). All the samples show characteristic of electrical double layer capacitance, and the onion-based porous carbon obtain highest specific capacitance of 568 F g−1 at the current density of 0.1 A g−1. With the current density rising from 1 A g−1 to 50 A g−1, the specific capacitance only decreases for 20%. After 5000 cycles, all the samples show relatively high capacitance retention (up to 97%). Two-step acid pickling has washed most impurities and directly lead to small equivalent series resistance (lower than 0.2 Ω). The samples show high power density and energy density (71 W h kg−1@180 W kg−1, 210 kW kg−1@33 W h kg−1). This study open an avenue to create high-performance hierarchical porous carbon based on plant architecture.

High performance asymmetric supercapacitor having novel 3D networked polypyrrole nanotube/N-doped graphene negative electrode and core-shelled MoO3/PPy supported MoS2 positive electrode

3D networked polypyrrole (PPy) nanotube/N-doped graphene (NDG) has been fabricated through a facile two-step method, in-situ polymerization assisted by MoO3 template and a microwave-assisted hydrothermal method at a short time. The MoO3 template-assisted polymerization is demonstrated as an effective way to obtain PPy nanotubes with controlled thickness. It is found that the PPy nanotube formation and N doping of reduced graphene oxide occur simultaneously during the microwave process at short time. Due to the unique structure and excellent electrical conductivity, the PPy nanotube/NDG shows extremely small equivalent series resistance of 1.7 Ω, charge transfer resistance (Rc) of 0.1 Ω, pore resistance (Rd) of 0.1 Ω, and specific capacitance (Csp) of 292 F g−1 at 5 mV s−1. Also, a novel MoS2-decorated core-shelled MoO3/PPy has also been fabricated through a facile method. Owing to the unique structure and rich redox activities, MoS2-decorated core-shelled MoO3/PPy shows excellent performance with Csp of 527 F g−1 at 5 mV s−1 and low charge transfer resistance of 0.5 Ω. Furthermore, an aqueous asymmetric supercapacitor, consisting of MoS2/MoO3/PPy positive electrode and PPy nanotube/NDG negative electrode, has been constructed. The asymmetric supercapacitor shows an excellent energy density of 43.2 Wh kg−1 at power density of 674 W kg−1. The asymmetric supercapacitor also shows an excellent retention of 126% after 5000 cycles, possess high potential for application in the energy storage.

Direct growth of nickel-cobalt oxide nanosheet arrays on carbon nanotubes integrated with binder-free hydrothermal carbons for fabrication of high performance asymmetric supercapacitors

A high performance asymmetric supercapacitor (ASC) has been fabricated by using nickel oxide-cobalt oxide nanosheets (NiO–CoO NSs), which were directly grown on carbon nanotubes (CNTs) and hydrothermal carbon spheres (HTCs) as positive and negative electrodes, respectively. Both electrode materials are binder-free prepared by using a catalytic chemical vapour deposition (CVD) approach followed by a facile hydrothermal method for cathode and a one-step environmental-friendly route called hydrothermal carbonization for anode. Using NiO–CoO NSs@CNTs and HTCs, which were directly grown on Ni foam, not only leads to a very small equivalent series resistance, but also provides an impressive capacitive performance. The assembled ASC exhibits remarkable capacitive performance over a broad operational potential window ensuring outstanding energy densities (84.625 Wh. kg−1 at 3 A g−1). Other noteworthy features of the prepared supercapacitor include its superior power density (7810 W kg−1 at 9.5 A g−1) and cycling stability with capacitance retention almost over 80% after 6000 cycles. These electrochemical results point to NiO–CoO NSs@CNTs-Ni//HTCs-Ni based asymmetric supercapacitors as a promising remedy for energy crisis and environmental deterioration problems.

The supercapacitive performance of woollen-like structure of CuO thin films prepared by the chemical method

The woollen-like structure of Copper Oxide (CuO) thin films has been synthesized via chemical method using aqueous ammonia as a complexing agent which is directly used as an electrode material for supercapacitor application. CuO electrode achieved maximum specific capacitance of 576 F/g, exhibited for 5 mV/s scan rate with the small Equivalent Series Resistance (ESR) value of 0.72 Ω. The highest energy density, power density and Coulomb efficiency of CuO electrode was observed having values of 19.65 Wh/kg, 916.66 W/kg and 82.01% respectively at current density of 1 mA/cm2. CuO thin films are found to have monoclinic structure as confirmed from X-ray Diffraction (XRD). It is observed that as the deposition time enhances the crystallite size also increases, whereas micro-strain, dislocation density and distortion parameters decrease. The FESEM results confirmed woollen-like structure of CuO thin films. Surface wettability shows hydrophilic nature with the liquid interface. The EDS spectra confirmed the Cu and O elements present in the prepared samples. The BET analysis shows that the CuO thin film has high surface area of 39.58 m2/g with mesoporous characteristics. These results show that electrode will be a potential material in the supercapacitor applications besides it is cost-effective.

Electrochemical characterization of NaClO4–mixed rice starch as a cost-effective and environment-friendly electrolyte

Being eco-friendly, cost-effective, easy to synthesize and handle, starch-based electrolytes have tremendous advantages for electrochemical device fabrications. Starch electrolytes synthesized by crosslinking process result in stable, flexible, free-standing films which can be molded in different shapes and sizes. Rice starch–based material, prepared using glutaraldehyde as crosslinker and NaClO4 as ion-providing salt, resulted in high conductivity (~ 0.01 S/cm) and fast-relaxing (ion relaxation time in micro seconds) electrolytes having wide electrochemical stability window (~ 3 V). Resistance of highest conducting sample, having 0.8 mm thickness and 1 × 1 cm2 area is ~ 6 Ω. The synthesized electrolyte is easily moldable in desired shape and size and hence devices may be fabricated with smaller equivalent series resistance. The estimated relaxation time is 62 μs indicating that fabricated devices will be able to deliver energy in shorter time. The approximate preparation cost of the material (1 × 1 × 0.08 cm3) is INR 20 (~ 0.3USD). The low cost, good technical parameters, and flexibility make it a promising material for electrochemical device fabrication.

Electrochemical Performance of Few-Layer Graphene Nano-Flake Supercapacitors Prepared by the Vacuum Kinetic Spray Method

A few-layer graphene nano-flake thin film was prepared by an affordable vacuum kinetic spray method at room temperature and modest low vacuum conditions. In this economical approach, graphite microparticles, a few layers thick, are deposited on a stainless-steel substrate to form few-layer graphene nano-flakes using a nanoparticle deposition system (NPDS). The NPDS allows for a large area deposition at a low cost and can deposit various metal oxides at room temperature and low vacuum conditions. The morphology and structure of the deposited thin films are alterable by changing the scan speed of the deposition. These changes were verified by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy. The electrochemical performances of the supercapacitors, fabricated using the deposited films and H3PO4–PVA gel electrolytes with different concentrations, were measured using a 2-electrode cell. The electrochemical performance was evaluated by cyclic voltammetry, galvanostatic Charge–discharge, and electrochemical impedance spectroscopy. The proposed affordable fabricated supercapacitors show a high areal capacitance and a small equivalent series resistance.

Bi-Stability Phenomenon in Constant On-Time Controlled Buck Converter With Small Output Capacitor ESR

This paper reports the finding of bi-stability phenomenon in a constant on-time (COT) controlled buck converter with a small output capacitor equivalent series resistance (ESR). In the previously published literatures, it has been reported that the COT controlled buck converter with a small output capacitor ESR operates in the reduced-frequency periodic oscillation state. However, it is newly validated by numerical simulations that for some specified initial states, such a buck converter with the same circuit parameters can operate in a chaotic oscillation state as well, reflecting the existence of bi-stable oscillation states. Based on the piecewise continuous model of COT controlled buck converter, the bi-stability phenomenon associated with two sets of initial states is revealed by numerical simulations and verified by PSIM circuit simulations. Consequently, when a small output capacitor ESR is used, the coexisting attractors' behavior of two bi-stable oscillation states appears in such a COT controlled buck converter, leading to the emergence of bi-stability phenomenon.

Enhancement of electrochemical performance based on symmetrical poly-(3,4-ethylenedioxythiophene) coated polyvinyl alcohol/graphene oxide/manganese oxide microfiber for supercapacitor

In this study, a symmetrical poly (3, 4-ethylenedioxythiophene) (PEDOT) coated on poly (vinyl alcohol) (PVA)-graphene oxide (GO)-manganese oxide (MnO2) microfibers (PVA-GO-MnO2/PEDOT) supercapacitor was successfully prepared using a combination of two facile techniques; electrospinning and electropolymerisation. The FESEM analysis revealed the uniform distribution of manganese oxide nanoparticles on the surface of cross-linking PVA-GO microfibers and a cauliflower-like morphology was observed upon deposition of PEDOT on the surface of PVA-GO-MnO2 microfibers. The chemical composition of PVA-GO-MnO2/PEDOT and oxidation state of manganese were characterised using Raman and X-Ray photoelectron spectroscopies. The inclusion of MnO2 and PEDOT in the microcomposite proved the enhancement of specific capacitance where PVA-GO-MnO2/PEDOT exhibited a specific capacitance of 144.66 F/g compared to PVA-MnO2/PEDOT (107.22 F/g), PVA-GO/PEDOT (94.73 F/g) and PEDOT (62.86 F/g). A wide potential window (1.8 V) was achieved for PVA-GO-MnO2/PEDOT with an excellent capacitance retention of 91.18% suggesting an ideal capacitive behaviour and good cycling stability. PVA-GO-MnO2/PEDOT microcomposite also showed an improved specific energy and specific power with small equivalent series resistance (34.5 Ω) and charge transfer resistance (0.62 Ω). This demonstrated that symmetric electrode of PVA-GO-MnO2/PEDOT can offer a great promise in producing high-performance supercapacitors.

Dynamical effects of composite output capacitors on current‐mode controlled buck converter with constant current load

The discrete mapping model of current-mode controlled buck converter with constant current load, taking account of composite output capacitors (parallel connection of two different types of capacitor branches, i.e. electrolytic capacitors and ceramic capacitors), is established. Based on the model, dynamical effects of varying output capacitance and equivalent series resistance (ESR) are investigated by bifurcation behaviours. The period of low-frequency oscillation among coexisting fast-slow scale instability is derived by exploring the loci of eigenvalues, while the operating regions are estimated. Time-domain simulation and experimental waveforms are provided for verification of the theoretical analysis, indicating the existences of subharmonic oscillation and coexisting fast-slow scale instability in the converter with variation of output capacitance and ESR. Research results reveal that the low-frequency oscillation can be eventually eliminated due to a relatively large (or small) ESR and the capacitance in the same branch presents to identical tendency of dynamical effects on the converter. Moreover, the interaction effects between two parallel capacitor branches are demonstrated. It illustrates that the low-frequency oscillation can be removed with smaller (or larger) ESR or capacitance in one branch of the composite output capacitors while larger (or smaller) ESR or capacitance in the other branch.

Investigation of Self Supporting Paper-like Structures Fabricated with Few-Layer Exfoliated Graphene Platelets and Composites with Birnessite-MnO2 As Electrode Materials for Electric Double-Layer Capacitor and Redox Capacitor

Few-layer exfoliated graphene platelets are ultrathin particles of graphite that can also be thought of as short stacks of graphene sheets prepared through proprietary manufacturing processes. Several grades and sizes with thicknesses ranging from 1-20 nanometers and width ranging from 1-50µm are manufactured by XG Sciences, Lansing, MI, USA. Unique manufacturing processes for these platelet materials are non-oxidizing; thus, yielding a pristine graphitic surface of sp2 carbon molecules. This makes these platelet particles especially suitable for applications requiring high electrical and thermal conductivities. The surface areas of the resulting platelets predominantly consist of the open surface areas of the graphene basal planes. Open surface areas of the graphene basal planes as well the interlayer spaces between the graphene sheets coupled with high electrical conductivities are an important criteria for electrodes in many electrochemical energy storage technologies. Unique morphology of the platelets offer the potential to structure self-standing electrodes by doing away with inactive components such as current collectors and thus, enhance the energy density. Surface modification of the platelets through the deposition of transition metal oxides on the graphene basal planes has the potential to enhance the energy storage capacity due to the participation of two different components in the energy storage process. Few-layer exfoliated graphene platelets can be dispersed in various aqueous and non-aqueous solvents depending upon the nature of the interaction between their surfaces and the solvent molecules. Sometimes this nature of interaction can be altered by modifying the surfaces of the few-layer exfoliated graphene platelets with polymer surfactants that can control their hydrophobicity or hydrophilicity. Transition metal oxides can be deposited on few-layer exfoliated graphene platelet surface through the interaction with precursor materials in wet-chemical synthesis procedure. Presence of polymer surfactants in the synthesis procedure can bring in even further surface interaction phenomena and alter the structure of the deposited transition metal oxides. Dispersions of few-layer exfoliated graphene platelets and composites with transition metal oxides can be used to manufacture electrode structures either by coating directly onto metallic substrates with the help of a binder material or through the fabrication of self-standing structures without a binder material. Two types of few-layer exfoliated graphene platelets have been investigated in this research: Grade-M particles (15µm average diameter) have an average thickness of approximately 6-8 nanometers and a typical surface area of 120-150 m2/g; and Grade-C, generally consisting of aggregates of particles with diameter less than 2µm and a particle thickness of few nanometers with a surface area of 750 m2/g. First part of this research investigated platelets with different surface areas, particle sizes and structures combined with an aqueous electrolyte as electric double-layer capacitor electrode materials. A binder free self-standing bilayer composite electrode fabricated with Grade-M particles and Grade-C particles was found to retain near-ideal double-layer capacitive characteristics at high scan rate. Reasonably high specific capacitance values normalized with respect to both the weight of the Grade-C particles and the weight of the self-standing electrode were obtained at high current rate. Small equivalent series resistance (ESR) and charge transfer resistance (Rct) of the self-standing electrode was found to be the cause of this desirable behavior. Second part of this research investigated a composite of Birnessite-MnO2 with Grade-M platelet as a redox capacitor electrode material in a self-standing structure. It was found that a self-standing electrode with Birnessite-MnO2 coated platelet exhibits improved impedance characteristics compared to pure Grade-M platelet based electrode. This may be explained by the fact that the platelets remain separated in the electrode structure due to the metallic deposition and thus, improving the impedance characteristics.