Electrolyte evaluation and engineering for the performance enhancement of electrochemical capacitors

As a consequence of a fast-paced technological evolution along with the acknowledgment of utilizing clean and renewable energy resources over fossil fuels, the importance of energy storage devices is widely recognized. The electrochemical capacitor (EC), commonly known as a supercapacitor or ultracapacitor, is an energy storage device that is already being used in portable consumer electronics, electrification of transportation, and grid-level applications. High power density and long cycle life are the two most prominent properties of ECs, thanks to the electrostatic nature of their charge storage mechanism. These properties are well utilized in a system where ECs are used as a backup power-boosting device to rechargeable batteries. By providing the peak power required, they eventually prolong the battery lifetime.\ua0 However, the relatively low energy density of ECs compared to rechargeable batteries limits their application as a standalone device. In addition, low operating voltage, adverse self-discharge rate, severe leakage current, elevated temperature incompatibility are some of the crucial issues that are preventing the widespread application of ECs. Besides a general discussion about ECs, the main objective of this thesis is to identify and address the above-mentioned critical challenges, and to propose and demonstrate corresponding solutions. Firstly, it is revealed that utilizing a redox-active KBr electrolyte can enhance both operating voltage and capacitance, and hence increases energy density without sacrificing power density or cycle life. Secondly, an evaluation of elevated temperature influence on the capacitive performance of ECs containing ionic liquid (IL) electrolyte demonstrates a high working temperature beyond 120 \ub0C. Thirdly, a systematic investigation of ECs containing IL at elevated temperatures shows a significant increase of the self-discharge rate with temperature and pinpoints the underlying mechanisms; at lower initial voltages the self-discharge rate is dominated by diffusion of electrolyte ions rather than charge redistribution. Fourthly, the addition of a small amount of liquid crystals (LC) in neutral electrolyte shows a reduction of self-discharge and leakage current due to slower diffusion of ions in the device, which is proposed to originate from the anisotropic properties of LC. Finally, by utilizing the thermocapacitive effect, a thermal charging of ECs containing IL is demonstrated, where a high voltage of more than 900 mV could be recovered when two devices in series are exposed to a 60 \ub0C temperature environment

Comparison of Thermally Grown Carbon Nanofiber-Based and Reduced Graphene Oxide-Based CMOS-Compatible Microsupercapacitors

Microsupercapacitors as miniature energy storage devices require complementary metal-oxide-semiconductor (CMOS) compatible techniques for electrode deposition to be integrated in wireless sensor network sensor systems. Among several processing techniques, chemical vapor deposition (CVD) and spin coating, present in CMOS manufacturing facilities, are the two most viable processes for electrode growth and deposition, respectively. To make an argument for choosing either of these techniques to fabricate MSCs utilizable for an on-chip power supply, we need a comparative assessment of their electrochemical performance. Herein, the evaluation of MSCs with CVD-grown carbon nanofiber (CNF)-based and spin-coated reduced graphene oxide (rGO)-based electrodes is reported. The devices are compared for their capacitance, energy and power density, charge retention, characteristic frequencies, and ease of fabrication over a large sweep of scan rates, current densities, and frequencies. The rGO-based MSCs demonstrate 112 mu F cm(-2) at 100 mV s(-1) and a power density of 12.8 mW cm(-2). The CNF-based MSCs show 269.7 mu F cm(-2) and 30.8 mW cm(-2). CVD-grown CNF outperforms spin-coated rGO in capacitive storage at low frequencies, whereas the latter is better in terms of charge retention and high-frequency capacitance response

Aplicação de BI no processo de recuperação de credito: um estudo de caso.

Este trabalho apresenta um estudo de caso de aplicação da tecnologia de Business Inteligence - BI em conjunto com o processo de planejamento estratégico de modo a proporcionar melhorias em processos operacionais e por sua vez ganhos financeiros. O trabalho baseou-se na aplicação de ferramentas de BI e algoritmos de processamento analítico em uma empresa que atua no mercado de cobrança. A análise do estudo de caso concentra-se em demonstrar os resultados operacionais e financeiros mais eficientes no que tange à cobrança de carteiras de clientes selecionados para os experimentos possibilitando o alcance de objetivos e metas organizacionai

Effect of plasma treatment on electrochemical performance of lignin-based carbon fibers

The abundant and renewable nature of lignin obtained from wood renders it as a sustainable carbon resource for energy storage applications. However, their environmentally unfavorable processing conditions and limited energy storage performance prohibit the use of lignin-based carbon materials\u27 use as supercapacitor electrodes. The material\u27s properties require advancement to overcome the limitation of low specific capacitances. In this study, we report on the impact on the electrochemical performance of inherently hydrophobic lignin-based carbon fibers (LCF) by subjecting them to a mild plasma treatment. The electrode\u27s capacitance was thus increased by 20%, with better rate capability and energy-power performance (11 Wh/kg and 0.8 kW/kg) in the KOH electrolyte. The quantified improvements were attributed to the capacitive functional groups, and enhanced surface wettability, which increased ion accessibility to active surface area improving charge-transfer ability to the surface with more additional functional groups. Remarkably, the selected plasma conditions introduced mostly desirable functional groups that limited any parasitic faradaic reactions prone to affect the device\u27s long-term cycling stability and self-discharge characteristics. Furthermore, the impact of different inherent and introduced oxygen surface functional groups, including COO−, C[sbnd]OH, C[sbnd]O, and C[dbnd]O, on the capacitive performance of these fibers at different device conditions (such as cycling and electrochemical activation) was investigated in different aqueous electrolytes. To ensure environmental favorability, the electrospinning of lignin fibers was conducted using a high molecular fraction of lignin without the inclusion of any fossil-based co-spinning polymers

Finger Number and Device Performance: A Case Study of Reduced Graphene Oxide Microsupercapacitors

Microsupercapacitors (MSCs) are recognized as suitable energy storage devices for the internet of things (IoTs) applications. Herein is described the work conducted to assess the areal energy and power densities of MSCs with 2, 10, 20, and 40 interdigital finger electrodes on a fixed device footprint area (the finger interspacing is fixed at 40 μm, and the finger width and length are allowed to vary to fit the footprint area). The MSCs are based on reduced graphene oxide (rGO) materials and fabricated with a spin-coating and etch method. The performance evaluation indicates a strong dependency of areal capacitance and energy density on the number of fingers, and the maximum (impedance match) power density is also influenced to a relatively large extent, whereas the average power density is not sensitive to the configuration parameters in the present evaluation settings (scan rate 20–200 mV s−1 and current density of 100 μA cm−2). For the rGO-based devices, the equivalent distributed resistance may play an important role in determining the device resistance and power-related performance

Durable Activated Carbon Electrodes with a Green Binder

Herein, the fabrication and electrochemical performance of thick (180−280 μm) activated carbon (AC) electrodes with carbonized lignin-derived carbon fiber (LCF) inclusions are reported. Efforts are taken in fabricating robust free-standing electrodes from an environmentally friendly binder, microfibrillated cellulose (MFC), considering the biologically hazardous nature of other commonly used binders like polytetrafluoroethylene (PTFE), n-methyl-2-pyrrolidone (NMP), and polyvinylidene fluoride (PVDF). Generally, electrodes composed of MFC binder are prone to cracking upon drying, especially with higher mass loadings, which leads to nonflexibility and poor device stability. The LCF inclusions into the AC electrode with MFC binders not only increase flexibility but also contribute to better conductivity in the electrodes. The LCFs act as an intermediate layer among AC particles and serve as conductive pathways, facilitating exposure of more active surfaces to the electrolyte. A thick electrode with high mass loading of 10 mg cm−2 is achieved. The results show that by incorporating 2 wt% LCF to the AC material, the best device with 5 mg cm−2 delivers a specific capacitance of 97 F g−1, while the specific capacitance of the reference AC device without LCF is 85 F g−1

Identification of self-discharge mechanisms of ionic liquid electrolyte based supercapacitor under high-temperature operation

Ionic liquids (ILs) are promising electrolytes for supercapacitors (SCs) aimed for high-temperature applications, where increased ionic conductivity results in superior capacitive performance compared to room temperature (RT) performance. However, an increased temperature also accelerates the self-discharge rate that adversely affects energy retention and restricts the usage of SCs in standalone applications. In this study, a detailed electrochemical investigation on the self-discharge behaviour of carbon-based SCs containing an IL, 1-Ethyl-3-methylimidazolium acetate (EMIM Ac), has been carried out in the temperature range RT - 60\ua0\ub0C, and the underlying self-discharge mechanisms are identified. The results reveal that at a high voltage of 1.5\ua0V, the self-discharge is characterized by a combination of charge redistribution and diffusion at both RT and 60\ua0\ub0C. At 60\ua0\ub0C, the diffusion-controlled mechanism dominates at lower voltages over the charge redistribution effect, while at RT both mechanisms contribute to a similar extent. The observed difference in the self-discharge mechanism between RT and 60\ua0\ub0C is explained in terms of a decreased RC time constant (τRC) at elevated temperature, and the same conclusions are potentially applicable to other IL-containing SCs as well

Factors Affecting Apparel Pattern Grading Accuracy: Existing Software Solutions Comparison and Development of New Solution

Every so often, grading is not 100% accurate due to the conventional system for calculating the grading incre­ment. The aim of this study was to develop a new calculation system of grading increment provided by different software, e.g. Lectra, Gerber, Optitex, Boke CAD etc., and to develop a new mathematical solution that enhances grading precision. For this experiment, three different spec sheets of different buyers were collected, and then combined and drawn to a solitary sketch for both front and back including all points of measures (POM) for a more easy comparison. The solutions for the presence of diagonal and curve measurements were provided with examples using various tools and techniques of different professional garment CAD software. The benefit of the new approach is not only reduced errors of grading but also guaranteed garment fit without distorting style features. However, the drawbacks of the measurement method are complicated and time-consuming. They revolve around the fact that iterative fitting and adjustments are mandatory to improve the fit before bulk production. The study revealed that this new system slightly increases calculation time, whereas the sample approval time for order execution reduces considerably

Investigation of palladium current collectors for vertical graphene-based microsupercapacitors

As microsystems are reduced in size and become integrated in the Internet of Things (IoT), they require an adequate power supply which can be integrated at the same size scale. Microsupercapacitors (MSCs), if coupled with on-chip harvesters, can offer solutions for a self-sustaining, on-chip power supply. However, the implementation of reliable MSC wafer-scale production compatible with CMOS technology remains a challenge. Palladium (Pd) is known as a CMOS compatible metal and, in this paper, we investigate the use of Pd as a contact material for vertical graphene (VG) electrodes in wafer-scale MSC fabrication. We show that a Ti diffusion barrier is required to prevent short-circuiting for the successful employment of Pd contacts. The fabricated MSCs demonstrate a capacitance of 1.3 μF/cm2 with an energy density of 0.42 μJ/cm2. Thus, utilization of a Ti diffusion barrier with a CMOS compatible Pd metal electrode is a step towards integrating MSCs in semiconductor microsystems

Short Communication: Evaluation of a New Rapid Diagnostic Test for Quality Assurance by Kala Azar Elimination Programme in Bangladesh

In Bangladesh, serological tests have been widely used for the primary screening of visceral leishmaniasis (VL). Several serologic tests are available for the diagnosis of VL. Selection of the best test is important to permit diagnostic differentiation between symptomatic and asymptomatic patients and to reduce cross-reactivity. We evaluated the effectiveness of a new serological test “Onsite Leishmania Ab Rapid Test” as a part of “quality assurance” activities for the kala azar elimination programme of the Government of Bangladesh. Plasma samples of 100 parasitologically confirmed cases of VL along with 101 healthy controls were tested, and “Onsite Leishmania Ab Rapid Test” strip tests were positive in 94 out of 100 confirmed VL cases, whereas four out of 51 healthy subjects from the VL endemic areas also tested positive. All the 50 healthy volunteers tested negative. Thus, the sensitivity and specificity of “Onsite Leishmania Ab Rapid Test” strip test were found to be 94% (95% CI: 87–98) and 96% (95% CI: 90–99), respectively. This study showed that the performance of the “Onsite Leishmania Ab Rapid Test” strip tests was up to the recommended level