Investigations of electrochemical double layer capacitor (EDLC) materials – a comparison of test methods

A comparison between various methods for determining the capacitance and resistance of electrochemical double layer capacitors (EDLC) with cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge‐discharge methods and cell voltage jump experiments are presented. Therefore, a commercially available electrochemical double layer capacitor was used. For the validation of the screw cell measurements the electrode foils were isolated and extracted from the commercial capacitor. The results support the adequacy of the screw cell tests for electrode materials on a laboratory scale. The comparison of methods shows in part strong differences. The reasons will therefore be discussed. Consequently, the determination of specific values requires methods which respect the influence of the actual tests or measurement frequency and exhibit the importance of an internationally accepted specification for electrochemical double layer capacitors material investigation.     

Sb掺杂SnO2修饰双电层电容活性炭电极的研究

应用sol-gel浸渍与热处理工艺相结合,在活性炭表面包覆Sb掺杂的SnO2薄膜对电极进行修饰,构成AC-SnO2/KOH/AC-SnO2双电层电容器,测试结果表明,400 mA/g电流密度条件下,修饰后的双电层电容器在0.001～1.5 V相对较高电压区间的放电容量,比AC/KOH/AC双电层电容器在0.001～1.0 V电压区间高36%,但AC-SnO2的单电极比电容仅为AC单电极比电容的91.9%;当电流密度大于400 mA/g,两种电极的大电流性能相当。     

双电层电容器活性炭电极的优化

通过物理方法对双电层电容器用活性炭电极进行改性实验,探讨了活性炭电极的结构（比表面积、孔径分布、孔容）和性能（比电容、充放电特性）的优化问题.改性后活性炭电极BET比表面积从1739.77 m²•g^-1增至2215.40 m²•g^-1,其中微孔比表面积增幅22％,中孔比表面积增幅35％,孔容积也有20%～30％的增幅量,孔径分布更为合理.优化的活性炭电极结构改善了电极材料的电化学特性,比电容量可达424 F•g^-1,增幅10％.     

双电层电容器用不同电解液的电化学性能

研究了双电层电容器(EDLC)用离子液体1-乙基-3-甲基咪唑三氟乙酸盐[EMIm]CF3COO和30%KOH在高比表面积(3 250 m2/g)活性炭电极上的电化学性能.30%KOH的电容特性、大电流充放电性能均优于[EMIm]CF3COO,而[EMIm]CF3COO的比电容随应用电压的提高而增加,使电容器在提高功率密度的同时提高了能量密度.     

Uniaxially oriented carbon monoliths as supercapacitor electrodes

Cylindrical carbon monoliths of 7 mm in diameter and certain heights (1, 2, 3, 4 and 5 mm) are studied as model electrodes for supercapacitors. The monoliths show a narrow microporous structure with average micropore size of 0.73 nm and specific surface area of 1086 m² g⁻¹. The monoliths show straight walls and channels, both arranged along the cylinder axis. The former account for a remarkable electrical conductivity (6.5 S cm⁻¹ at room temperature). The latter allow a rapid ionic transport between the electrolyte bulk and the carbon walls and account for a high specific capacitance at high current density. The cell capacitance and resistance increase linearly with the monolith height according to C = (1.78 ± 0.06)h and ESR = (0.08 ± 0.01)h + (1.67 ± 0.04), respectively. The contribution of the electrolyte resistance, monolith resistance and monolith/collector resistance to ESR is discussed. The cell response time or constant time increases with the monolith height but according to a power dependence, τ = (4.5 ± 0.2)h^{(1.61 ± 0.03)}. The carbon of the monoliths show in KOH electrolyte a specific capacitance of 150 F g⁻¹ and a capacitance per surface area of 14 μF cm⁻².     

Studies on Me/Al-layered double hydroxides (Me = Ni and Co) as electrode materials for electrochemical capacitors

Me/Al-layered double hydroxides (Me=Ni and Co) prepared by the chemical co-precipitation method have been shown to be outstanding novel materials for electrochemical capacitors. The crystalline structure and the electrochemical properties of the electrodes have been studied by considering the effect of the mole ratio of nickel/cobalt. X-ray diffraction analysis shows that the materials belong to hexagonal system with layered structure. Cyclic voltammetric measurements indicate that Me/Al-layered double hydroxides with the Ni/Co mole ratio of 4:6 exhibit excellent capacitive properties within the potential range of 0.0-0.6 V versus Hg/HgO in 6 mol/L KOH electrolyte. Charge/discharge behaviors have been observed with the highest specific capacitance values of 960 F/g at the current density of 400 mA/g. Impedance studies show that the enhanced electrical properties and high frequency response are attributed to the presence of Co oxides.     

Electric double‐layer capacitor module with series‐parallel reconfigurable cell voltage equalizers

When electric double‐layer capacitors (EDLCs) are connected in series, a cell voltage imbalance occurs due to nonuniform cell properties. Cell voltage imbalance should be minimized to prolong cycle lives and maximize the available energy of cells. In this study, we propose a series‐parallel reconfigurable cell voltage equalizer that is considered suitable for energy storage systems using EDLCs instead of traditional secondary batteries as the main energy storage sources. The proposed equalizer requires only EDLCs and switches as its main circuit elements, and it utilizes EDLCs not only for energy storage but also for equalization. An equivalent circuit model using equivalent resistors that can be regarded as an index of equalization speed is developed. Current distribution and cell voltage imbalancing during operation are quantitatively generalized. Experimental charge–discharge tests were performed on the EDLC modules to demonstrate the performance of the cell voltage equalizer. All the cells in the modules could be charged/discharged uniformly even when a degradation‐mimicking cell was intentionally included in the module. The resultant cell voltage imbalances and current distributions were in good agreement with those predicted by mathematical analyses. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 181(4): 38–50, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21287     

Active device‐less voltage equalization charger using capacitors,diodes, and an AC power source

Conventional cell/module voltage equalizers or equalization chargers based on traditional DC‐DC converters require numerous switches or transformers as the number of series connections increases; therefore, their cost and complexity tend to increase and their reliability decreases as the number of connections increases. This paper proposes a novel voltage equalization charger that consists only of passive components such as capacitors, diodes, and a transformer. The fundamental operating principle, major features, and derivation of equivalent DC circuits are presented. A symmetrical configuration is also proposed to mitigate the RMS current flowing through energy storage cells in the charging process. Simulations and experimental charging and cycle tests were performed on series‐connected electric double‐layer capacitor modules to demonstrate the equalization performance. The experimental and simulation results were in good agreement, and the voltage imbalances were gradually eliminated as time elapsed even during charge‐discharge cycling. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 181(3): 39‐48, 2012; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21288     

电极浸渍电解液方式对双电层电容器的影响

以活性碳纤维(ACF)电极为研究对象,用质量分数为0.2的氢氧化钾(KOH)作为电解液制作KOH-ACF浸渍电极,研究了浸泡时间、搅拌、真空度、温度等的影响因素,目的使电解液能更有效的进入ACF电极,研究发现,浸泡时间长、真空度高和温度低都有利于电解液进入电极,搅拌可以加快浸泡吸附的速度,制成的双电层电容器比电容可达70F／g。     

Deep eutectic solvent-based supramolecular gel polymer electrolytes for high-performance electrochemical double layer capacitors

Gel polymer electrolytes (GPEs) can avoid the electrolyte leakage risk of electrochemical double layer capacitors (EDLCs). But aqueous GPEs often suffer from narrow electrochemical windows. Herein, a series of deep eutectic solvent (DES)-based supramolecular GPEs are firstly developed for carbon-based EDLCs with wide voltage windows. The as-fabricated DES-based GPE shows an ionic conductivity of ~58 mS cm^?1, which makes the stable voltage window of a carbon-based EDLC reach 2.4 V. The carbon-based EDLC exhibits a specific capacitance of 32.1 F g^?1, an energy density of 24.6 Wh kg^?1 and a capacitance retention of ~90% after 15,000 charge-discharge cycles. Moreover, when quinhydrone is added into the DES-based GPE, the specific capacitance and energy density of the corresponding EDLC can be further expanded to 60 F g^?1 and 43.6 Wh kg^?1, respectively. Therefore, our work may present a universal strategy to prepare novel supramolecular GPEs for high-performance EDLCs with wide voltage windows.