Application of Kenaf-derived Carbon as Conductive Electrode Additive in Electric Double-layer Capacitors

As a result of industrialization, technological progress, and the growing global population, energy consumption worldwide is skyrocketing. Researchers are thriving to seek high power density storage device to meet these escalating energy demands. Electrical Double Layer Capacitors (EDLCs) also known as a supercapacitor, are energy storage devices that store energy by Coulombic attraction between ions, polarized solvent on the surface of the activated carbon electrodes to form an electric double layer without involving chemical reactions. EDLCs display some remarkable characteristics such as high-power density, ultrafast charge/discharge rates, and long cycle life, surpassing conventional lithium-ion batteries. These features make EDLCs ideal for a wide range of applications, including portable devices, energy backup systems, and electric vehicles.However, the self-discharge effect in EDLCs is a deadly drawback that decrease the shelf life of EDLCs and impeding its practical applications. This phenomenon refers to the spontaneous voltage decay between the electrodes when the capacitor is under open-circuit potential after being charged, leading to a reduction in the stored charge without any external connection. This voltage declination is a complex process therefore, various mechanisms have been proposed to elucidate this phenomenon. There are primarily four mechanisms leading to the self-discharge. First, the internal short-circuit caused by imperfect seal of EDLCs leading to ohmic leakage current. Second, the uneven distribution of pore sizes in activated carbon electrodes leads to a higher resistance for ions to penetrate deeper into the material, resulting in ion accumulation on the surface during short charging times. This accumulation causes an uneven distribution of electron energy in the electrode material. After disconnecting, electrons rearrange to achieve overall energy consistency, while ions gradually redistribute and move into the smaller micropores within the activated carbon electrode. This phenomenon is referred to as charge redistribution effect. Third, Faradic current in EDLCs occurs due to redox reactions, stemming from the decomposition of the electrolyte or chemical reactions involving impurities within the electrolyte. Lastly, during the charging process, ions accumulate at the electrode surface, creating a concentration gradient between the electrodes and the electrolyte. As a result, ions gradually diffuse back into the bulk electrolyte until reaching equilibrium. Addressing self-discharge is paramount in ongoing research efforts.Electrolyte solutions are critical components in EDLCs, providing a medium for ion transport between the electrodes. The electrolyte solution is typically composed of a supporting electrolyte dissolved in a solvent, and the choice of solvent can significantly impact the performance of the device. This study delves into the impact of electrolyte solvent additives in the electrical double layer of EDLCs, leading to varied self-discharge performances. In this report, five commonly-used organic carbonate solvents in EDLCs, were selected to investigate their effects on the self-discharge phenomenon. Cyclic carbonate solvents, such as EC and PC, are known to possess high dielectric constants, high viscosities and high flash points. In contrast, linear carbonates e.g., DEC, DMC, EMC have low viscosities, low dielectric constants and low flash point. By incorporating carbonate solvents into the 1M TEABF4/PC (Tetraethylammonium tetrafluoroborate/propylene carbonate) commercial electrolyte, the performance of five supercapacitors , minimal deviation in electrochemical analyses such as CV and GCD was observed. However, significant differences emerged in EIS, elucidating the varying degrees of self-discharge phenomena. These differences in self-discharge mechanisms across the five EDLCs stemmed from the distinct dielectric constants and molecule sizes of the five solvent additives (i) Low dielectric constant and large molecule size, as seen in DEC (239.4h) and EMC (165.1h), contributed to increased interfacial impedance which effectively prolonging self-discharge compared to commercial one (145.7h). Conversely, the DMC with smaller interfacial resistance initially exhibited lower voltage decline due to reduced charge redistribution effects. However, its rapid ion diffusion process from carbon pores to electrolyte resulted in the fastest self-discharge of DMC (124.7h). (ii) High dielectric EC competes for solvation coordination, reducing interfacial resistance and enhancing intermolecular interaction. This, in turn, leads to the formation of a denser electric double layer (EDL) with EC, therefore decelerating the self-discharge process (205.9h) in EDLCs. In summary, the study emphasizes the significance of solvent additive selection and demonstrates a simple, safe, and cost-effective method to prolonging the self-discharge duration of DEC to more than 1.5 times that of the commercial one, significantly inhibiting the critical self-discharge issue in supercapacitors. Figure 1

In this study, space charge distribution in Electric Double Layer Capacitor (EDLC) for energy storage has been examined by Pulsed Electro-Acoustic (PEA) Method. Also, ketjen black was used for polarized electrodes instead of conventionally used acetylene black in order to improve the capacitance of EDLC. Usually, ketjen black exhibits higher electronic conductivity compared to acetylene black, so that ketjen black could be considered as conducting filler. Moreover, N2 adsorption-desorption isotherm as well as TEM observation revealed that ketjen black used in this study has different type of pore structures compared to acetylene black. Therefore, the space charge and capacitance of prepared EDLC using ionic liquid was evaluated. As the results, it was found that the ketjen black containing EDLC showed fairly high charge density and high capacitance compared to acetylene black containing one. In addition, temperature characteristic of EDLC containing ketjen black and acetylene black with best quantity were measured on temperature from 0degC to 50degC. It is understood that ketjen black as conducting material and ionic liquid based EDLC was superior to organic electrolyte based EDLC in all temperature on capacitance. Also, internal resistance of EDLC containing ketjen black and ionic liquid based EDLC can be decrease.

In this study, we report the development result of a supercapacitor as a power supply system for wireless sensor network (WSN) systems. Our supercapacitor is an electric double layer capacitor (EDLC) using an environmentally friendly electrolyte, which is made of a potassium hydroxide (KOH) solution and a charcoal and a steel plate. The EDLC using a charcoal, a steel and KOH as element materials (charcoal EDLC) does not contain heavy metals and a toxic substance. The charcoal EDLC, even if it is discarded by failure, does not almost influence the environment. Moreover, since a KOH solution is used for the electrolyte, there is no fear of ignition and so it is safe. We fabricated the charcoal EDLC and performed demonstration experiment. In the demonstration experiment with a WSN system, as power supplies, we used a 10 W solar cell and a charcoal EDLC with the power storage of 25 Wh. The WSN system continuously operating for 24 hours with the power consumption of about 0.7 W outdoors was used in the experiment. As a result, we could confirm that the WSN system was operated successfully even in the nighttime through the charcoal EDLC. In the nighttime, the WSN system consumed 35 % (8.4 Wh) of power stored in the charcoal EDLC. We conclude that our environmentally friendly charcoal EDLC is one of solutions for a reliable and safe power supply for the WSN system.

The space charge distribution in electric double layer capacitor (EDLC) for energy storage has been examined by pulsed electro-acoustic (PEA) method since 2002. It is found that the measuremental result can be influenced by the reflection and penetration of the sound wave when the space charge distribution of EDLC is measured by PEA method, because the EDLC has a five layer structure consisting of three materials (aluminum, cellulose, and activated carbon). Then, we calculated reflection wave components that influence on charge density from the acoustic impedance and relative permittivity of the materials. As a result, it has been understood that the changes in the space charge distribution of EDLC and the charge characteristic of the EDLC are almost corresponding. The use of the PEA method can be expected as a method of evaluating the accumulation charge of EDLC by measuring the space charge distribution of it. In this paper, a polarized electrode is made to develop EDLC and a ratio of surface area and average pore diameter of polarized electrode is measured by nitrogen adsorption method at 77K. Also, relation between a ratio of surface area and average pore diameter and space charge distributions of EDLC are discussed.

Commercial activated carbon (AC) was improved and its performance in electric double layer (dl) capacitors was evaluated as compared to the original AC. Improvement of AC and its optimization were carried out through CO 2 reactivation using a NiCl 2 catalyst at different temperatures. After improvements, both specific capacitance and high rate capability of capacitors were increased. For AC improved at 850°C for 3 h, its discharge specific capacitance increases up to 51.84 F/g, an increase of about 20% as compared to the original AC. The electric dl capacitor with improved AC as active material can be cycled at very high current densities and its resistance capacitance time constant is much lower than that with original AC, proving that its high rate capability was improved significantly. Moreover, the significant increase in electrical conductivity for improved AC suggested that it is not necessary to use a conductor within the polarizable electrodes. Improved AC seems to be a suitable material for the fabrication of electric dl capacitors with high power density.

Solid-state electric double layer capacitors fabricated with plastic crystal based flexible gel polymer electrolytes: Effective role of electrolyte anions

This paper proposes a direct current (DC) power supply circuit consisted of a battery and an electrical double layer capacitor (EDLC). An inverter-fed induction motor (IM) drive is controlled using the proposed power supply circuit. This power supply circuit can is a power regeneration system consisting of a battery connected to an EDLC through a step-up and down chopper. By switching control of the step-up and down chopper, the chopper can control charge and discharge of the EDLC. Therefore, the EDLC enables us to DC energy with the chopper. When the inverter-fed IM drive generates a power with regenerative braking, the power energy is charged to the EDLC. The EDLC discharges the energy power when the IM is accelerated or load is added to the IM. By controlling the energy of the EDLC suitably, the proposed power supply circuit can reduce the load of battery. The utility of the proposed method is examined through computer simulations and experimental results.

Electric double layer capacitors (EDLCs), which store free charges on the electrode surface via non-Faradaic process, balanced by the electric double layer on the electrolyte side, exhibit excellent cycle stability and high power density. Though EDLCs are considered as promising energy storage devices, the charges stored on the electrode surface in EDLCs are much lower than those in batteries. Ionic liquids (ILs), as a new type of electrolytes in EDLCs, are capable to deliver high energy density, due to their excellent physicochemical properties and wide electrochemical window. In this review, we focus on the widely studied IL electrolytes for EDLCs, including pure ILs, IL/IL binary electrolytes, IL/organic solvent mixtures, as well as functionalized ILs, with attention on the relationship between the structures of different IL-based electrolytes and the energy storage properties in EDLCs. For imidazolium- and ammonium-based IL electrolytes which are most widely studied in EDLCs, the former generally have higher gravimetric specific capacitance, while the latter exhibit wider electrochemical window. The modifications of functional group substituted can be an effective strategy to enhance the gravimetric specific capacitance of the latter and thus improve the energy density of EDLCs.

In the study of an electric vehicle (EV), an energy storage system which combines a battery and an electric double layer capacitor (EDLC) has been proposed. Introducing of EDLCs to an EV drive system, energy management become more efficient in term of using regenerative braking energy and saving battery energy. In this study, we propose a series or parallel changeover system using a battery and an EDLC for a small E V. This system can change over the composition of electric sources which battery and the EDLC bank in series or parallel depending on the voltage level of EDLC bank. The operating voltage range of EDLC bank is expanded from maximum to zero by the proposed system. In order to confirm the merit of the proposed method, two prototype EVs that one has a conventional DC-DC converter system and the other has the proposed changeover system are prepared and measured on automobile racing circuit. The effectiveness of proposed changeover system be verified and presented by experimental results.

In recent years, environmentally friendly energy storage devices have gained increasing attention due to the growing requirement of energy storage for sustainable energy applications. Among the various existing energy storage devices, supercapacitors (electrical double layer capacitors (EDLC) and hybrid capacitors (HC)) are considered as the most promising short-term energy storage devices [1]. EDLCs store energy in the electrical double layer, where the adsorption of ions is based mainly on the electrostatic interactions. The unique characteristics of EDLCs allow them to replace or combine with batteries and fuel cells in applications where the high-power pulses are important, such as the different energy recuperation systems and peak power sources, hybrid electric vehicles, wind turbines, digital communication devices, cameras and mobile phones, laptops, etc. [1]. Porous carbon materials are considered to be the most promising electrode materials for portable supercapacitors due to the high surface area, good electrical conductivity, good chemical stability, low gravimetric density and low cost [2]. The electrical charge accumulated in EDLC depends on the electrochemically active surface area and, thus, on the porosity and hierarchical porous structure of a carbon material. In addition, the presence of mesopores in porous carbon materials determines the power density of EDLC having a strong effect on the rate of mass transfer (diffusion and migration) and adsorption rate of charge carriers inside the hierarchical porous matrix. Therefore, the characteristics of micro- and mesoporous carbon materials (especially the ratio of micropore and mesopore surface areas and pore volumes) have to be optimized to improve further the specific energy and power density of EDLCs [1,2]. The objective of this study was to investigate the applicability limits of carbon material derived from granulated white sugar (GWS carbon) by hydrothermal carbonization (HTC) method combined with subsequent zinc chloride activation step of hydrochar, for supercapacitor electrodes. Synthesized GWS carbon material was used as an electrode material in the two-electrode single cells of EDLC filled with 1 M triethylmethylammonium tetrafluoroborate (TEMABF4) solution in acetonitrile (AN) or 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid (EMImBF4) as the electrolytes. Cyclic voltammetry (CV), constant current charge/discharge (CC), electrochemical impedance spectroscopy (EIS) and constant power discharge (CP) methods were used to study the electrochemical performance of EDLC. The EIS, CV and CC measurement results show that the values of specific capacitance are somewhat higher for EMImBF4 (135 F g-1) electrolyte compared to the TEMABF4 in AN (110 F g-1). The CP test results show that at low constant power values the stored energy is higher for EDLC based on EMImBF4 ionic liquid (48 W h kg-1) compared with EDLC based on TEMABF4 in AN electrolyte (39 W h kg-1). However, the best capacitance retention and shortest relaxation time constant were established for EDLCs in 1 M TEMABF4 solution in AN due to the lower viscosity and higher electrical conductivity compared to the ionic liquid based electrolytes and EDLC based on 1 M TEMABF4 solution in AN electrolyte delivers substantially higher energy at higher constant power values. Thus, the GWS carbon material synthesized using cheap and abundant GWS as the starting material, show good electrochemical performance in TEMABF4 in AN as well in EMImBF4 ionic liquid and is promising carbon material for the high energy and power density supercapacitor application. Acknowledgements This research was supported by the EU through the European Regional Development Fund (Centers of Excellence, 2014-2020.4.01.15-0011 and 3.2.0101–0030, TeRa project SLOKT12026T. Higher education specialization stipends in smart specialization growth areas 2014-2020.4.02.16-0026) and Institutional Research Grant IUT20–13. This work was partially supported by Estonian Research Council grants PUT1033 and PUT55.

Electric double-layer capacitors (EDLCs) are promising devices for sustainable energy storage. However, EDLC components, such as separators and electrodes composed of activated carbon and conductive additives, are derived from fossil resources. To reduce this dependency, an EDLC was assembled using a separator and electrodes derived from hardwood kraft lignin, while still relying on fossil-based carbon black (CB) as the conductive additive. To achieve more sustainable EDLCs, this study developed all the conductive carbon, separator, and electrodes from softwood kraft lignin (SKL). When SKL was carbonized at 900 °C, it showed poor electrical conductivity and was unsuitable as a conductive additive. The carbon structures became more ordered with higher temperatures, and SKL-carbons prepared at 1,300–2,000 °C showed comparable conductivity to CB. The EDLCs with 1 wt % of these SKL-carbons exhibited higher capacitance and energy density than reference EDLCs with 1 and 5 wt % CB. Furthermore, a turbostratic (T) structure formed at 2,500 °C, enhancing conductivity and EDLC performance. SKL-carbon prepared at 2,800 °C exhibited a graphite structure in addition to the T structure, achieving the highest conductivity (0.54 S cm⁻1), but the resulting EDLC showed low power density. Thus, SKL-carbon prepared at 2,500 °C was the best conductive additive for EDLCs.

ABSTRACTActivated carbon fiber cloth (ACFC) electrodes whose surface was modified by “pulsed cold plasma”, i.e., low-temperature plasma generated by a pulsed electric power in argon-oxygen mixed gas at a reduced pressure, were applied to electric double layer (EDL) capacitors with an organic electrolyte composed of propylene carbonate and tetraethylammonium tetrafluoroborate (TEABF4). The treatment of the ACFC electrodes with the pulsed cold plasma increased total capacitance in the EDL capacitors. The observed increase in the total capacitance was ascribed mainly to an ascertained increase in capacitance of a negative ACFC electrode involving TEA+cation adsorption/desorption with the cold plasma treatment. No obvious increase in capacitance of a positive ACFC electrode involving BF4- anion adsorption/desorption was observed with the plasma treatment. The chemical and electrochemical characteristics of a treated ACFC interface were found to be favorable for TEA+cation adsorption/desorption.

Introduction: Electric double layer capacitors (EDLCs), which store electricity by adsorption and desorption of ions, are commercialized for mobile devices and vehicles due to their high-rate capability and long cycle life. However, the lower energy densities of EDLCs than batteries limit their applications. Thus, much attention has been paid to improving their energy densities (E), described by the equation E = 1/2 CV2, where C is the capacitance, and V is the operating cell voltage1). Here, we focused on aqueous electrolytes, which are superior to organic electrolytes used in most commercial EDLCs, in terms of safety, cost, ionic conductivity, and permittivity2). However, the narrow potential window by theoretical water splitting voltage limits the operating voltage of aqueous EDLCs. Recently, highly concentrated aqueous electrolytes, referred to as hydrate-melts3), have been developed, enabling high-voltage operation of aqueous energy-storage devices, including aqueous EDLCs. In this context, it is desired to investigate the influences of electrolyte compositions on double layer capacitances in concentrated aqueous electrolytes. In most previous studies, the performance evaluation, including capacitance in each electrolyte, was performed with activated carbon electrodes used for commercial EDLCs, demonstrating the feasibility of relatively concentrated aqueous electrolytes for EDLCs4). However, the capacitance of activated carbon electrodes may be affected by various non-intrinsic factors including viscosity. Therefore, capacitance evaluation, eliminating various effects of non-EDL related factors, is required to gain a fundamental understanding of the electric double layer in highly concentrated aqueous electrolytes as well as establish an electrolyte design strategy to improve energy density.In this study, we studied the EDL capacitance in concentrated aqueous electrolytes. To exclude the influence of electrolyte viscosity, we used a flat carbon plate as a model electrode and evaluated the EDL capacitance by electrochemical impedance spectroscopy (EIS). The EDL capacitance was evaluated in aqueous electrolytes with various concentrations of alkali-metal salts to gain basic insights into the design of aqueous electrolytes for EDLCs. Experimental: EIS was performed in the frequency range of 100 kHz–1 Hz at 30°C with three-electrode cells (with glassy carbon, Pt mesh, and Ag/AgCl as a working, counter, and reference electrodes, respectively) to evaluate the capacitances in 100 Hz at various potentials in aqueous electrolytes. KN(SO2F)2 (KFSI) with concentrations ranging from 1.1 mol kg-1 (m) to 30.8 m, 61.7 m K(N(SO2F)2)0.55(SO3CF3)0.45 (K(FSI)0.55(OTf)0.45), 21 m NaFSI, 21 m LiN(CF3SO2)2 (LiTFSI) were used as electrolytes. Results & Discussion: The EIS results show that the capacitance of glassy carbon in KFSI/H2O reached its maximum at 22.2 m (Fig.1a). Further increasing the salt concentration up to 61.7 m K(FSI)0.55(OTf)0.45/H2O monotonically decreased the capacitance. This trend is different from those reported for activated carbon electrodes, at which the capacitance monotonically increased with increasing concentration5). This difference may result from the electrolyte viscosity, which can differently affect the flat glassy carbon and porous activated carbon. We also studied various alkaline-metal salt electrolytes at the same 21 m concentrations (Fig.1b). The capacitance at glassy carbon electrodes increased in the order of LiTFSI<NaFSI<KFSI, suggesting that larger cations result in higher capacitances. In the presentation, we will discuss how the electrolyte components and concentrations dominate EDL structures and capacitances on carbon electrodes in aqueous electrolytes. Acknowledgement: This study was partially supported by JSPS KAKENHI Grant Number JP 24H00483 and Mitsubishi Foundation.

Challenges with prediction of battery electrolyte electrochemical stability window and guiding the electrode – electrolyte stabilization

A method for leveling power generation was examined for a power supply system that uses wind power generation for a stand-alone radio base station. One such possible method is to convert the surplus electric power to hydrogen, which enables stable energy storage that can be later used to produce electrical energy via a fuel cell. In order to efficiently produce hydrogen from wind power, it has been suggested that an electric double layer capacitor (EDLC) can be used as the electric power buffer medium for the system. In this research, the effect of an EDLC installation in the system was examined using numerical analysis. In a system that is constantly converting electrical energy generated into hydrogen, there is an appropriate load capacity for the hydrogen generator that corresponds to the amount of the energy generated. By setting the appropriate capacity and expanding the range of load operation of the hydrogen generator, the electrical charging and discharging of the EDLC are balanced, and the buffer ability improves. As a result, even if the EDLC capacity was extremely small, it was shown to contribute to the stabilization of the amount of the hydrogen generation. Moreover, a reduction in the hydrogen generator load capacity was observed with installation of the EDLC.