Influence of nanoporous carbon electrode thickness on the electrochemical characteristics of a nanoporous carbon|tetraethylammonium tetrafluoroborate in acetonitrile solution interface

Electrochemical characteristics for the nanoporous carbon|Et₄NBF₄+acetonitrile interface have been studied by cyclic voltammetry and impedance spectroscopy methods. The influence of the electrolyte concentration and thickness of the nanoporous electrode material on the shape of the cyclic voltammetry and impedance curves has been established and the reasons for these phenomena are discussed. A value of zero charge potential, depending slightly on the structure and concentration of the electrolyte, the region of ideal polarizability and other characteristics have been established. The nanoporous nature of the carbon electrodes introduces a distribution of resistive and capacitive elements, giving rise to complicated electrochemical behaviour. Analysis of the complex plane plots shows that the nanoporous carbon|Et₄NBF₄+acetonitrile electrolyte interface can be simulated by an equivalent circuit, in which two parallel conduction paths in the solid and liquid phases are interconnected by the double-layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte concentration and electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. The value of the solid state phase resistance established is independent of the thickness of the electrode material.     

Influence of electrolyte characteristics on the electrochemical parameters of electrical double layer capacitors

Electrical double layer capacitors based on ideally polarizable nanoporous carbon electrodes in propylene carbonate with the addition of different 1 M Me₃EtNBF₄, Me₂Et₂NBF₄, MeEt₃NBF₄, Et₄NBF₄, Et₃PrNBF₄ and Et₃BuNBF₄ electrolytes have been tested by cyclic voltammetry, chronoamperometry and electrochemical impedance methods. The limits of ideal polarizability, low-frequency limiting capacitance and series resistance, time constant, Ragone plots (energy density vs. power density dependencies) and other characteristics have been discussed. The influence of the electrolyte molar mass on the electrochemical characteristics of the nanoporous carbon electrode cells has been established. The applicability limits of the Srinivasan and Weidner model have been tested.     

活性炭基软包装超级电容器用有机电解液

使用了一种新型的有机电解液(三乙基甲基四氟硼酸铵/(丙烯碳酸酯+乙腈): MeEt₃NBF₄/(AN+PC))和两种传统有机电解液(四乙基四氟硼酸铵/丙烯碳酸酯(Et₄NBF₄/AN)和四乙基四氟硼酸/乙腈(Et₄NBF₄/PC)), 制作成活性炭(AC)基软包装超级电容器. 在不同电压窗口下对新型有机电解液的循环伏安和电化学阻抗谱进行了表征, 并在0-3 V的电压窗口下, 通过循环伏安、电化学阻抗谱、恒流充放电、漏电流、自放电、循环寿命和库仑效率, 对以上三种电解液进行了综合的比较. 结果表明, 新型有机电解液综合了AN和PC各自的优点, 性能优异.     

Application of a novel binder for activated carbon-based electrical double layer capacitors with nonaqueous electrolytes

In this work, we have fabricated activated carbon electrodes using the binder LA135 and assembled electrical double layer capacitors with nonaqueous electrolytes of 1 M tetraethyl ammonium tetrafluoroborate (Et₄NBF₄) in propylene carbonate (PC), 1 M Et₄NBF₄ in acetonitrile (AN), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) ionic liquid, respectively. The main chemical compositions of the binder are polyacrylonitrile and styrene–butadiene rubber. Scanning electron microscope images show that the conductive agents have been uniformly dispersed on the activated carbons in the electrode. The thermal stabilities of electrodes using different binders are studied by thermogravimetric analysis. The electrochemical properties of cells in different nonaqueous electrolytes are characterized by cyclic voltagramms, electrochemical impedance spectra, galvanostatic charge–discharge, leakage current, and cycle life measurements. The capacitor in Et₄NBF₄/AN has the lowest internal resistance and superior high-rate capability, and the one in Et₄NBF₄/PC has the smallest leakage current. The capacitor in EMIMBF₄ has the energy density as high as 35.4 Wh?kg^?1 at a current density of 0.2 A g^?1 (based on the total mass of active materials), which is 1.6 times higher than that of capacitor in PC electrolyte. Besides, the electrochemical properties of capacitors with different binders are comparatively studied. The capacitor using LA135 has the highest specific capacitance and moderate internal resistance comparing with the ones using poly(tetrafluoroethylene), sodium carboxymethyl cellulose + styrene–butadiene rubber or poly (vinylidene fluoride).     

Electrochemical characteristics of propylene carbonate solutions of tetraethylammonium tetrafluoroborate

The specific and molar electric conductivities of propylene carbonate solutions of Et₄NBF₄ with concentrations of 0.15–1.4 m were determined at 283.15, 298.15, and 313.15 K. The temperature dependence of the electric conductivity of the system was analyzed. The concentration dependence of electric conductivity was described in terms of the Castell-Amis equation. A comparative analysis of the charge transfer in Et₄NBF₄ and LiAsF₆ systems in propylene carbonate was performed using the concept of quasicrystallinity of concentrated electrolyte solutions. The range of potentials in which the electrolyte solutions are stable on a platinum electrode was determined.     

Synthesis and electrochemical properties of activated carbons and Li4Ti5O12 as electrode materials for supercapacitors

New activated nanoporous carbons, produced by carbonization of mixtures of coal tar pitch and furfural with subsequent steam activation, as well as electrochemically active oxide Li₄Ti₅O₁₂, prepared by thermal co-decomposition of oxalates, were tested and characterized as electrode materials for electrochemical supercapacitors. The phase composition, microstructure, surface morphology and porous structure of the materials were studied. Pure carbon electrodes as well as composite electrodes based on these materials obtained were fabricated. Two types of supercapacitor (SC) cells were assembled and subjected to charge–discharge cycling study at different current rates: (1) symmetric sandwich-type SC cells with identical activated carbon electrodes and different organic electrolytes, and (2) asymmetric hybrid SC cell composed by activated graphitized carbon as a negative electrode and activated carbon–Li₄Ti₅O₁₂ oxide composite as a positive electrode, and an organic electrolyte (LiPF₆–dimethyl carbonate/ethylene carbonate (DMC/EC). Four types of carbons with different specific surface area (1,000–1,600 m² g^?1) and texture parameters, as well as three types of organic electrolytes: Et₄NBF₄–propylene carbonate (PC), LiBF₄–PC and LiPF₆–DMC/EC in the symmetric SC cell, were tested and compared with each other. Capacitance value up to 70 F g^?1 for the symmetric SC, depending on the electrolyte microstructure and conductivity of the carbon material used, and capacitance of about 150 F g^?1 for the asymmetric SC cell, with good cycleability for both supercapacitor systems, were obtained.     

Electrochemical carboxylation of fluorocontaining imines with preparation of fluorinated N-phenylphenylglycines

A possibility of obtaining fluorine-containing N-phenylphenylglycine derivatives at yields of up to 85% via the electrochemical carboxylation of corresponding benzalanilines was shown. The influence of imine's electron structure, the nature of supporting electrolyte and cathodic material on such processes is examined. It was found, that increasing electron accepting ability of the substituents in benzylidene and aniline fragments of the imine molecule lead to decrease of amino acid yields. The dependence of the N-phenyl-p-fluorophenylglycine yield on the cathode material (Zn, GC, Cu, Ag, Pt) and on the nature of the supporting electrolytes (Bu₄NBr, Et₄NBr, Et₄NClO₄, PhCH₂Me₃NClO₄, LiBF₄, LiClO₄, NaBF₄ and KBF₄) was investigated. The highest amino acid yields were obtained at cathodes (GC and Zn) that do not exhibit specific adsorption of fluorine-containing imines, as well as in the presence of background salts (Alk₄NBr) whose cations do not show tendency to strong ion pairing with anion radicals formed by the electrochemical activation of the imines.     

Lithium transport within the solid electrolyte interphase

A LiClO₄ SEI film grown on copper was examined with time-of-flight secondary ion mass spectrometry. The SEI porosity profile and Li⁺ transport processes within the SEI were studied with isotopically labeled ⁶LiBF₄ electrolyte. An ~ 5 nm porous region, into which electrolytes can easily diffuse, was observed at the electrolyte/SEI interface. Below the porous region, a densely packed layer of Li₂O and/or Li₂CO₃ prevents electrolyte diffusion, but Li⁺ transports through this region via ion exchange.     

一种新型有机硅离子液体电解液在超级电容中的应用

设计合成了一种新型有机硅室温离子液体(SiN1IL), 并对其化学结构和电化学窗口进行表征, 通过与具有高介电常数的丙烯碳酸酯(PC)/低粘度的乙腈(AN)匹配组成电解液, 其离子电导率达到商业实际应用的要求(19.6 mS·cm^-1). 对以活性炭(AC)为对称电极的超级电容器的电化学性能测试表明, SiN1IL 基电解液与活性炭有很好的界面相容性, 其高倍率充放电、阻抗性能优于商用四乙基四氟硼酸铵(Et₄NBF₄)/PC 电解液, 在电流密度为1000 mA·g^-1的条件下, 工作电压为2.7 V, 其比电容为108 F·g^-1.     

Effect of the nature of the solvent and the supporting electrolyte on the electrochemically activated insertion of CO<Subscript>2</Subscript> into fluorine-containing aromatic imines with the production of amino acids

The effect of the nature of the solvent (DMF, DMSO, CH₃CN, N-methylpyrrolidinone, THF) and supporting electrolyte (Bu₄NBr, Et₄NBr, Et₄NClO₄,Me₄NBr, LiBF₄, NaBF₄, and KBF₄) on the electrochemical activation and carboxylation of fluorine-containing aromatic imines by the action of CO₂ was studied for the case of p-and m-fluorobenzylideneaniline. It was shown that factors promoting the formation of intimate ion pairs between the radical-anions of the imines – the initial products of the processes in the electrochemical activation of the imines – and the cations of the supporting electrolyte (decrease of the polarity of the medium and decrease of the radius of the cation in the supporting electrolyte) significantly reduce the effectiveness of electrochemical carboxylation right down to its complete cessation.     

Analysis of degradation of electrolytic Fe, Co, Ni sulfides and their graphitized analogs in lithium battery using impedance spectroscopy

Changes in parameters of complex-plane plots are analyzed for electrolytic Me _x S _y electrodes (Me = Fe, Co, Ni) and their graphitized analogs contacting electrolyte (ethylene carbonate, dimethyl carbonate, 1 M LiClO₄) in the case of degradation in a layout lithium battery at cycle 15. Impedance measurements in the high- and medium-frequency region of complex-plane plots revealed the key role of the surface solid-phase film and also of resistance of charge transfer through the sulfide material/electrolyte interface in degradation of Me _x S _y electrodes under cycling in the potential range of 2.80?0.02 V vs. Li/Li⁺. The activation energy is determined for processes of charge transfer through the electrode/electrolyte interface.     

A novel Et<Subscript>4</Subscript>NBF<Subscript>4</Subscript> and LiPF<Subscript>6</Subscript> blend salts electrolyte for supercapacitor battery

Electrolytes of 1 M blend salts (LiPF₆ and tetraethylammonium tetrafluoroborate, Et₄NBF₄) have been investigated in supercapacitor battery system with composite LiMn₂O₄ and activated carbon (AC) cathode, and Li₄Ti₅O₁₂ anode. The results obtained with the blend salts electrolytes are compared with those obtained with cells build using standard 1 M LiPF₆ dissolved in ethylene carbonate + dimethyl carbonate + ethyl (methyl) carbonate (EC + DMC + EMC, 1:1:1 wt.%) as electrolyte. It is found that the blend salts electrolyte performs better on both electrochemical and galvanostatic cycling stability, especially cycled at 4 C rate. When the concentration of LiPF₆ is 0.2 M and Et₄NBF₄ is 0.8 M, the capacity retention of the battery is 96.23% at 4 C rate after 5,000 cycles, much higher than that of the battery with standard 1 M LiPF₆ electrolyte, which is only 62.35%. These results demonstrate that the blend salts electrolyte can improve the galvanostatic cycling stability of the supercapacity battery. Electrolyte of 0.2 M LiPF₆ + 0.8 M Et₄NBF₄ in EC + DMC + EMC (1:1:1 wt.%) is a promising electrolyte for (LiMn₂O₄ + AC)/Li₄Ti₅O₁₂.     

Lithium perchlorate (tetrafluoroborate)-diethyl carbonate-propylene carbonate electrolyte systems

The method of sections was used to study the solubility of the components of the LiClO₄ (LiBF₄)-diethyl carbonate (DEC)-propylene carbonate (PC) ternary system at 25°C. The phase diagrams of the systems were plotted and discussed. At this temperature, the solid phase of the LiClO₄-DEC-PC system is pure salt while that of the LiBF₄-DEC-PC system is composed of LiBF₄ and its LiBF₄ · DEC and LiBF₄ · PC crystal solvates. The compositions of the crystallized solid phases obtained graphically by using the method of sections were confirmed by XRD analysis.     

Correlation between capacitances of porous carbons in acidic and aprotic EDLC electrolytes

A study based on a total of 41 nanoporous carbons shows that there exists a good correlation between the limiting gravimetric capacitances C_o at low current densities j (1 mA cm⁻²) measured in aprotic (1 M (C₂H₅)₄NBF₄ in acetonitrile) and in acidic (2 M aqueous H₂SO₄) electrolytes. The comparison of the surface-related capacitances (F m⁻²) of well characterized samples with the amount of thermodesorbed CO suggests a strong contribution of CO generating surface groups to charge storage in the acidic electrolyte, but a negligible contribution in the aprotic medium. It also appears that the decrease of the capacitance with current density is similar in both electrolytes. This confirms that the average micropore width and the CO₂ generating surface groups are the main factors which limit the ionic mobility in both electrolytes.     

活性碳纳米管的制备及其在有机电解液中的电容性能研究

以KOH为活化剂对碳纳米管进行化学活化制备双电层电容器用高比表面积活性碳纳米管. 采用TEM和N₂吸附法表征活性碳纳米管的结构, 采用恒流充放电、循环伏安、交流阻抗等评价其在1 mol•L^－1 Et₄NBF₄/PC中的电容性能. 随活化剂用量增大、活化温度升高和活化时间的延长, 活性碳纳米管的比表面积和比电容都呈增大的趋势. 活化剂用量为3∶1, 800 ℃活化4 h制备的活性碳纳米管的比表面积663 m²•g^－1, 比活化前提高了3倍, 其比电容达57.2 F• g^－1, 比活化前提高了2倍. 将活性碳纳米管的比电容与其比表面积相关联, 发现两者之间具有非常好的线性关系, 并分析了原因.     

Preparation and performance of polyvinyl alcohol-based activated carbon as electrode material in both aqueous and organic electrolytes

Porous carbon materials with high surface area and different pore structure have been successfully prepared by phenolic resin combined with polyvinyl alcohol (PVA) and KOH as activation agents. The surface morphology, structure, and specific surface area of the carbon materials were studied by scanning electron microscopy, X-ray diffraction, and nitrogen sorption measurement, respectively. Furthermore, the effects of specific surface area, pore structure, and electrolyte on electrochemical properties were investigated by galvanostatic charge–discharge measurement. The results show that KOH–PVA-activated carbon materials display specific capacitance as high as 218 F?g^?1 in 30 wt.% KOH aqueous electrolyte, 147 F?g^?1 in 1 M LiPF₆/(ethylene carbonate (EC) + dimethyl carbonate) (1:1?v/v), and 115 F?g^?1 in 1 M Et₃MeNBF₄/propylene carbonate organic electrolyte, respectively. In addition, the carbon materials demonstrate long-term cycle stability, especially the AK3P-0.30 in aqueous electrolyte and the AK2P-0.30 with excellent rate capability in organic electrolyte. These reveal that the existence of a micro-mesoporous structure of activated carbon is beneficial to store energy in an aqueous supercapacitor and broad pore size distribution of activated carbon is favorable to energy storage in an organic supercapacitor. The carbon materials with pore size distribution in different ranges improve the electrochemical performance of supercapacitor in different electrolytes. A new pore-expand agent (PVA combining with KOH) was used to obtain porous carbons with enhanced properties for supercapacitor.     

Redox capacitance of poly-3,4-ethylenedioxythiophene studied by cyclic voltammetry and faradaic impedance spectroscopy

The paper presents results of studying the electrochemical properties of poly-3,4-ethylenedioxythiophene films using the methods of cyclic voltammetry and faradaic impedance spectroscopy in acetonitrile and propylene carbonate solutions of different electrolytes: LiClO₄, Bu₄NBF₄, Bu₄NPF₆. The effect of the film synthesis conditions, the nature of anion and solvent, and the supporting electrolyte concentration on the film redox capacitance is discussed. Main attention is paid to the comparison of values characterizing the redox capacitance of a poly-3,4-ethylenedioxythiophene film C _lf (determined using the faradaic impedance method) and C _cv (determined using the cyclic voltammetry method) and the studies of their dependence on the varied experimental factors. The experimental C _lf vs. E curves are analyzed using the relationships of a model of a uniform film.     

Spectroelectrochemical study of poly-3,4-ethylenedioxythiophene films in the presence of different supporting electrolytes

Steady-state absorption spectra of poly-3,4-ethylenedioxythiophene (PEDOT) films at different oxidation degrees and their differential cyclic voltabsorptograms are studied in 0.1 M LiClO₄, Bu₄NBF₄, and Bu₄NPF₆ solutions in acetonitrile. Three major absorption bands are obtained in the film electronic spectra: a complex band with a pronounced maximum at λ = 600 nm that corresponds to π → π* electronic transitions in reduced fragments of a PEDOT film and two absorption bands (at λ_max = 850 nm and λ_max > 1100 nm) corresponding to two oxidized film fragments. It is shown that the position and shape of absorption bands are practically independent of the nature of dopant anions, which points to the absence of pronounced interactions with positively charged polymer fragments. An attempt is made to analyze the obtained spectroelectrochemical data qualitatively to estimate the extinction coefficients and concentrations of absorbing particles and their variation in the course of redox processes.     

Adsorption phenomena on a mercury electrode in aqueous cryptand[2.2.2] solutions by the background of tetraalkylammonium salts

The electrochemical behavior of cryptand[2.2.2] (Cry) is studied on a mercury electrode in aqueous solutions of tetraalkylammonium tetrafluoroborates (Me₄N⁺, Et₄N⁺, and Bu₄N⁺). Cryptand [2.2.2] is shown to exhibit high surface activity in Me₄ NBF₄ nd Et₄NBF₄ solutions. Based on the model of two parallel capacitors supplemented by the Frumkin adsorption isotherm, the adsorption parameters of Cry by the background of Me₄BNF₄ were calculated using the regression analysis methods. The calculated dependences of the differential capacitance on the potential adequately agree with experimental curves. The adsorption characteristics of Cry in the studied solutions are compared with those in MgSO₄ solutions. By the background of Bu₄NBF₄, Cry molecules and Bu₄N⁺ cations exhibit very close surface activity and form a mixed adsorption layer.     

Effects of Supporting Electrolytes on Spectroelectrochemical and Electrochromic Properties of Polyaniline‐poly(styrene sulfonic acid) and Poly(ethylenedioxythiophene)‐poly(styrene sulfonic acid)‐based Electrochromic Device

Electrochromic devices are fabricated by using polyaniline (PANI) doped with poly(styrene sulfonic acid) (PSS) as coloring electrodes, poly(ethylenedioxythiophene)‐poly(styrene sulfonic acid) (PEDOT‐PSS) as complementary electrodes, and hybrid polymer electrolytes as gel electrolytes. The device based on LiClO₄‐based electrolyte (weight ratio of PMMA:PC:LiClO₄ = 0.7:1.1:0.3) shows the highest optical contrast and coloration efficiency (333 cm²/C) after 1200 cycles in these devices, and the color changes from pale yellow (?0.5 V) to dark blue (+2.5 V). The spectroelectrochemical and electrochromic switching properties of electrochromic devices are investigated, the maximum optical contrast (ΔT%) of electrochromic device for ITO|PANI‐PSS‖PMMA‐PC‐LiClO₄‐SiO₂‖PEDOT‐PSS|ITO are 31.5% at 640 nm, and electrochromic device based on LiClO₄‐based electrolyte with SiO₂ shows faster response time than that based on LiClO₄‐based electrolyte without SiO₂.