Electrochemical capacitive properties of CNT fibers spun from vertically aligned CNT arrays

Due to their lightweight, large surface area; excellent electrical conductivity; and mechanical strength, carbon nanotube (CNT) fibers show great potentials in serving as both electrode materials and current collectors in supercapacitors. In this paper, the capacitive properties of both as-spun CNT fibers and electrochemically activated CNT fibers have been investigated using cyclic voltammetry and electrochemical impedance spectroscopy. It is found that the as-spun CNT fibers exhibit a very low specific capacitance of 2.6 F g⁻¹, but electrochemically activated CNT fibers show considerably improved specific capacitance. The electrochemical activation has been realized by cyclic scanning in a wide potential window. Different electrolytes have also been examined to validate the applicability of our carbon materials and the activation mechanism. It is believed that such an activation process can significantly improve the surface wetting of the CNT fibers by electrolyte (aqueous Na₂SO₄ solution). The cycling stability and rate-dependence of the capacitance have been studied, and the results suggest practical applications of CNT fibers in electrochemical supercapacitors.     

Water-in-salt electrolytes for high voltage aqueous electrochemical energy storage devices

If were not by their low electrochemical stability, aqueous electrolytes would be the preferred alternative to be used in electrochemical energy storage devices. Their abundance and nontoxicity are key factors for such application, especially in large scale. The development of highly concentrated aqueous electrolytes, so-called water-in-salt electrolytes, has expanded the electrochemical window of aqueous electrolyte up to 3.0 V (whereas salt-in-water electrolytes normally shows up to 1.6 V), showing that water can be an alternative after all. Many devices, ranging from metal-ion batteries to electrochemical capacitors, have been reported recently, making use of such wider electrochemical stability and enhancing devices energy density. Different salts have also been proposed not only to gain in costs but also to improve physicochemical properties.     

Reconstructing Hydrogen Bond Network Enables High Voltage Aqueous Zinc-Ion Supercapacitors

High-voltage aqueous rechargeable energy storage devices with safety and high specific energy are hopeful candidates for the future energy storage system. However, the electrochemical stability window of aqueous electrolytes is a great challenge. Herein, inspired by density functional theory (DFT), polyethylene glycol (PEG) can interact strongly with water molecules, effectively reconstructing the hydrogen bond network. In addition, N, N-dimethylformamide (DMF) can coordinate with Zn²⁺, assisting in the rapid desolvation of Zn²⁺ and stable plating/stripping process. Remarkably, by introducing PEG400 and DMF as co-solvents into the electrolyte, a wide electrochemical window of 4.27 V can be achieved. The shift in spectra indicate the transformation in the number and strength of hydrogen bonds, verifying the reconstruction of hydrogen bond network, which can largely inhibit the activity of water molecule, according well with the molecular dynamics simulations (MD) and online electrochemical mass spectroscopy (OEMS). Based on this electrolyte, symmetric Zn cells survived up to 5000 h at 1 mA cm⁻², and high voltage aqueous zinc ion supercapacitors assembled with Zn anode and activated carbon cathode achieved 800 cycles at 0.1 A g⁻¹. This work provides a feasible approach for constructing high-voltage alkali metal ion supercapacitors through reconstruction strategy of hydrogen bond network.     

A wide potential window aqueous supercapacitor based on LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>–rGO nanocomposite

Aqueous supercapacitors based on neutral solutions have the advantages of high-ionic conductivity, being environmentally friendly, safe, and low cost. However, the operating potential window for most aqueous electrolytes is far lower than that of organic electrolytes that are commonly used in commercial supercapacitors. In this work, we report on the fabrication of a wide potential window, high-energy aqueous asymmetric supercapacitor, without sacrificing power, by using a nanostructured LiMn₂O₄/reduced graphene oxide (LMO–rGO) nanocomposite. We synthesized the uniformly distributed LMO in the LMO–rGO nanocomposite using a co-precipitation route followed by a low-temperature hydrothermal treatment. In a three-electrode cell setup, the specific capacitance of the LMO–rGO nanocomposite electrode at 1 A/g (1.2 mA/cm²) is 268.75 F/g (258 mF/cm²), which shows a dramatic improvement over the sum of the specific capacitances of pristine LMO (162.5 F/g) and pure rGO (29.94 F/g) electrodes in their relative ratios, when used alone. This finding suggests a synergistic coupling of LMO and rGO in the nanocomposite. We also assembled the LMO–rGO nanocomposite, as the positive electrode, with activated carbon, as the negative electrode, into an asymmetric cell configuration. The device shows an ultra-wide potential window of 2.0 V in a neutral aqueous Li₂SO₄ electrolyte, with a maximum energy density of 29.6 Wh/kg (which approaches the commercial lead-acid batteries), power density of up to 7408 W/kg, and an excellent cycle life (5% loss after 6000 cycles). These findings confirm that an LMO–rGO nanocomposite is a promising material to meet the demands of real world energy storage.     

Hierarchical Mn3O4 Anchored on 3D Graphene Aerogels via C−O−Mn Linkage with Superior Electrochemical Performance for Flexible Asymmetric Supercapacitor

Flexible asymmetric supercapacitors are more appealing in flexible electronics because of high power density, wide cell voltage, and higher energy density than symmetric supercapacitors in aqueous electrolyte. In virtues of excellent conductivity, rich porous structure and interconnected honeycomb structure, three dimensional graphene aerogels show great potential as electrode in asymmetric supercapacitors. However, graphene aerogels are rarely used in flexible asymmetric supercapacitors because of easily re-stacking of graphene sheets, resulting in low electrochemical activity. Herein, flower-like hierarchical Mn₃O₄ and carbon nanohorns are incorporated into three dimensional graphene aerogels to restrain the stack of graphene sheets, and are applied as the positive and negative electrode for asymmetric supercapacitors devices, respectively. Besides, a strong chemical coupling between Mn₃O₄ and graphene via the C-O-Mn linkage is constructed and can provide a good electron-transport pathway during cycles. Consequently, the asymmetric supercapacitor device shows high rate cycle stability (87.8 % after 5000 cycles) and achieves a high energy density of 17.4 μWh cm⁻² with power density of 14.1 mW cm⁻² (156.7 mW cm⁻³) at 1.4 V.     

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.     

Self‐Assembling Synthesis of Free‐standing Nanoporous Graphene–Transition‐Metal Oxide Flexible Electrodes for High‐Performance Lithium‐Ion Batteries and Supercapacitors

The synthesis of nanoporous graphene by a convenient carbon nanofiber assisted self‐assembly approach is reported. Porous structures with large pore volumes, high surface areas, and well‐controlled pore sizes were achieved by employing spherical silica as hard templates with different diameters. Through a general wet‐immersion method, transition‐metal oxide (Fe₃O₄, Co₃O₄, NiO) nanocrystals can be easily loaded into nanoporous graphene papers to form three‐dimensional flexible nanoarchitectures. When directly applied as electrodes in lithium‐ion batteries and supercapacitors, the materials exhibited superior electrochemical performances, including an ultra‐high specific capacity, an extended long cycle life, and a high rate capability. In particular, nanoporous Fe₃O₄–graphene composites can deliver a reversible specific capacity of 1427.5 mAh g^?1 at a high current density of 1000 mA g^?1 as anode materials in lithium‐ion batteries. Furthermore, nanoporous Co₃O₄–graphene composites achieved a high supercapacitance of 424.2 F g^?1. This work demonstrated that the as‐developed freestanding nanoporous graphene papers could have significant potential for energy storage and conversion applications.     

Li2SO4-polyacrylamide polymer electrolytes for 2.0 V solid symmetric supercapacitors

A neutral polymer electrolyte comprised of lithium sulfate (Li₂SO₄) and polyacrylamide (PAM) was developed. The Li₂SO₄-PAM electrolyte film shows an ionic conductivity up to 10 mS cm^− 1 in 45%RH conditions. Solid double layer capacitors were demonstrated using CNT-graphite electrodes and Li₂SO₄-PAM solid electrolytes. The voltage window of the solid cell was about 2.0 V, identical to that of a Li₂SO₄ liquid cell used as baseline. The demonstrated voltage window is significantly larger than that reported for proton- or hydroxyl-conducting electrolytes, suggesting that the Li₂SO₄-PAM electrolyte is a promising system for high energy density supercapacitors. The solid device also demonstrated excellent rate capability (up to 5 V s^− 1) and good cycle life (beyond 10,000 charge/discharge cycles).     

Boron‐Doped,Carbon‐Coated SnO2/Graphene Nanosheets for Enhanced Lithium Storage

Heteroatom doping is an effective method to adjust the electrochemical behavior of carbonaceous materials. In this work, boron‐doped, carbon‐coated SnO₂/graphene hybrids (BCTGs) were fabricated by hydrothermal carbonization of sucrose in the presence of SnO₂/graphene nanosheets and phenylboronic acid or boric acid as dopant source and subsequent thermal treatment. Owing to their unique 2D core–shell architecture and B‐doped carbon shells, BCTGs have enhanced conductivity and extra active sites for lithium storage. With phenylboronic acid as B source, the resulting hybrid shows outstanding electrochemical performance as the anode in lithium‐ion batteries with a highly stable capacity of 1165 mA h g^?1 at 0.1 A g^?1 after 360 cycles and an excellent rate capability of 600 mA h g^?1 at 3.2 A g^?1, and thus outperforms most of the previously reported SnO₂‐based anode materials.     

The network gel polymer electrolyte based on poly(acrylate-co-imide) and its transport properties in lithium ion batteries

Poly (acrylate-co-imide)-based gel polymer electrolytes are synthesized by in situ free radical polymerization. Infrared spectroscopy confirms the complete polymerization of gel polymer electrolytes. The ionic conductivity of gel polymer electrolytes are measured as a function of different repeating EO units of polyacrylates. An optimal ionic conductivity of the poly (PEGMEMA₁₁₀₀-BMI) gel polymer electrolyte is determined to be 4.8 × 10^–3 S/cm at 25 °C. The lithium transference number is found to be 0.29. The cyclic voltammogram shows that the wide electrochemical stability window of the gel polymer electrolyte varies from −0.5 to 4.20 V (vs. Li/Li⁺). Furthermore, we found the transport properties of novel gel polymer electrolytes are dependent on the EO design and are also related to the rate capability and the cycling ability of lithium polymer batteries. The relationship between polymer electrolyte design, lithium transport properties and battery performance are investigated in this research.     

Advanced High‐Voltage Aqueous Lithium‐Ion Battery Enabled by “Water‐in‐Bisalt” Electrolyte

A new super‐concentrated aqueous electrolyte is proposed by introducing a second lithium salt. The resultant ultra‐high concentration of 28 m led to more effective formation of a protective interphase on the anode along with further suppression of water activities at both anode and cathode surfaces. The improved electrochemical stability allows the use of TiO₂ as the anode material, and a 2.5 V aqueous Li‐ion cell based on LiMn₂O₄ and carbon‐coated TiO₂ delivered the unprecedented energy density of 100 Wh kg^?1 for rechargeable aqueous Li‐ion cells, along with excellent cycling stability and high coulombic efficiency. It has been demonstrated that the introduction of a second salts into the “water‐in‐salt” electrolyte further pushed the energy densities of aqueous Li‐ion cells closer to those of the state‐of‐the‐art Li‐ion batteries.     

Interfacial Speciation Determines Interfacial Chemistry: X-ray-Induced Lithium Fluoride Formation from Water-in-salt Electrolytes on Solid Surfaces

Super-concentrated “water-in-salt” electrolytes recently spurred resurgent interest for high energy density aqueous lithium-ion batteries. Thermodynamic stabilization at high concentrations and kinetic barriers towards interfacial water electrolysis significantly expand the electrochemical stability window, facilitating high voltage aqueous cells. Herein we investigated LiTFSI/H₂O electrolyte interfacial decomposition pathways in the “water-in-salt” and “salt-in-water” regimes using synchrotron X-rays, which produce electrons at the solid/electrolyte interface to mimic reductive environments, and simultaneously probe the structure of surface films using X-ray diffraction. We observed the surface-reduction of TFSI⁻ at super-concentration, leading to lithium fluoride interphase formation, while precipitation of the lithium hydroxide was not observed. The mechanism behind this photoelectron-induced reduction was revealed to be concentration-dependent interfacial chemistry that only occurs among closely contact ion-pairs, which constitutes the rationale behind the “water-in-salt” concept.     

Anode materials for lithium ion batteries obtained by mild and uniformly controlled oxidation of natural graphite

Modification of natural graphite for anode materials has been a recent focus of research and development. Here we report that a common natural graphite, whose electrochemical performance is very poor, can be modified by solutions of (NH₄)₂S₂O₈, concentrated nitric acid solution, or green chemical solutions such as aqueous solutions of hydrogen peroxide and ceric sulfate. All treatments result in marked improvement of the electrochemical performance, including reversible capacity, coulombic efficiency in the first cycle, and cycling behavior. The main reason is the effective removal of active defects in natural graphite, formation of a new dense surface film consisting of oxides, improvement of the graphite stability, and introduction of more nanochannels/micropores. As a result, these changes inhibit the decomposition of electrolytes, prevent the movement of graphene planes along the a-axis direction, and provide more passages and storage sites for lithium. They are mild and the uniformity of the product can be well controlled. Pilot experiments show economic promise for their application in industry to manufacture anode materials for lithium ion batteries.Presented at the 3rd International Meeting on Advanced Batteries and Accumulators, 16–20 June 2002, Brno, Czech Republic     

Does thiol-functionalized viologen monolayer memorize anion present when forming the monolayer?

We investigated the effect of the presence of an additive anion while forming a self-assembled monolayer of a thiol-functionalized redox active species upon the behavior in aqueous solutions. A bromide salt was added in an acetonitrile solution of a thiol-functionalized viologen (viologen-thiol: N-pentyl-N′-(11-mercapto)undecyl-4,4′-bipyridinium bishexafluorophosphate) in which the self-assembled monolayer was formed on a polycrystalline Au electrode. We examined the structure and electrochemical behavior of the resulting monolayer-modified electrode in three different aqueous electrolyte solutions. The viologen-thiol monolayer prepared in the presence of Br⁻ exhibited obviously different behavior in both KBr and KF electrolyte solutions from that prepared in the absence of Br⁻. On the other hand, the difference was minor in KPF₆ solution. The difference of the electrochemical behavior was represented by the coverage of the viologen-thiol and the formal potential of viologen dication/radical cation redox couple. The memory that the viologen-thiol monolayer was formed in the presence of Br⁻ was retained in KF and KBr solutions, though it was erased in the electrolyte solution containing PF₆⁻, which is a softer anion than Br⁻. However, no definitive difference of the film structure was deduced from the electroreflectance study in regard to the monomer content and the average orientation of the viologen moiety.     

Electrochemical reduction of CO2 with high current density in a CO2 + methanol medium II. CO formation promoted by tetrabutylammonium cation

The cation of the supporting electrolyte was found to play an important role in the electrochemical reduction of highly concentrated CO₂ in a CO₂ + methanol medium. Electroreduction of CO₂ with tetrabutylammonium (TBA) salts yielded CO as the main product, while methyl formate was predominantly formed when lithium salts were used as supporting electrolytes. The latter supporting electrolytes showed a higher overvoltage than the former. When TBA salt was used, the reduction of CO₂ was catalysed by TBA ion to yield CO^−.₂. This intermediate may be stabilized by forming an ion pair, {TBA⁺---CO^−.₂}, or by being adsorbed on the electrode surface as CO^−._2ad. Then CO^−.₂ reacts with CO₂ to produce CO. The hydrophobic atmosphere at the electrode provided by TBA ion may be adequate for CO production. Lithium ion, on the other hand, suppressed the reduction of CO₂.     

Enhanced electrochemical properties of polyethylene oxide-based composite solid polymer electrolytes with porous inorganic–organic hybrid polyphosphazene nanotubes as fillers

Solid composite polymer electrolytes consisting of polyethylene oxide (PEO), LiClO₄, and porous inorganic–organic hybrid poly (cyclotriphosphazene-co-4, 4′-sulfonyldiphenol) (PZS) nanotubes were prepared using the solvent casting method. Differential scanning calorimetry and scanning electron microscopy were used to determine the characteristics of the composite polymer electrolytes. The ionic conductivity, lithium ion transference number, and electrochemical stability window can be enhanced after the addition of PZS nanotubes. The electrochemical impedance showed that the conductivity was improved significantly. Maximum ionic conductivity values of 1.5 × 10⁻⁵ S cm⁻¹ at ambient temperature and 7.8 × 10⁻⁴ S cm⁻¹ at 80 °C were obtained with 10 wt.% content of PZS nanotubes, and the lithium ion transference number was 0.35. The good electrochemical properties of the solid-state composite polymer electrolytes suggested that the porous inorganic–organic hybrid polyphosphazene nanotubes had a promising use as fillers in SPEs and the PEO₁₀–LiClO₄–PZS nanotube solid composite polymer electrolyte might be used as a candidate material for lithium polymer batteries.     

Origins of Boosted Charge Storage on Heteroatom‐Doped Carbons

Although tremendous efforts have been devoted to understanding the origin of boosted charge storage on heteroatom‐doped carbons, none of the present studies has shown a whole landscape. Herein, by both experimental evidence and theoretical simulation, it is demonstrated that heteroatom doping not only results in a broadened operating voltage, but also successfully promotes the specific capacitance in aqueous supercapacitors. In particular, the electrolyte cations adsorbed on heteroatom‐doped carbon can effectively inhibit hydrogen evolution reaction, a key step of water decomposition during the charging process, which broadens the voltage window of aqueous electrolytes even beyond the thermodynamic limit of water (1.23 V). Furthermore, the reduced adsorption energy of heteroatom‐doped carbon consequently leads to more stored cations on the heteroatom‐doped carbon surface, thus yielding a boosted charge storage performance.     

Design of Advanced MnO/N‐Gr 3D Walls through Polymer Cross‐Linking for High‐Performance Supercapacitor

Three‐dimensional, vertically aligned MnO/nitrogen‐doped graphene (3D MnO/N‐Gr) walls were prepared through facile solution‐phase synthesis followed by thermal treatment. Polyvinylpyrrolidone (PVP) was strategically added to generate cross‐links to simultaneously form 3D wall structures and to incorporate nitrogen atoms into the graphene network. The unique wall features of the as‐prepared 3D MnO/N‐Gr hybirdes provide a large surface area (91.516 m² g^?1) and allow for rapid diffusion of the ion electrolyte, resulting in a high specific capacitance of 378 F g^?1 at 0.25 A g^?1 and an excellent charge/discharge stability (93.7 % capacity retention after 8000 cycles) in aqueous 1 m Na₂SO₄ solution as electrolyte. Moreover, the symmetric supercapacitors that were rationally designed by using 3D MnO/N‐Gr hybrids exhibit outstanding electrochemical performance in an organic electrolyte with an energy density of 90.6 Wh kg^?1 and a power density of 437.5 W kg^?1.     

Effect of Dibenzo-18-crown-6 on the Polymer Electrolyte/Lithium Anode Interface

Photocured polymer electrolytes, applied onto a porous polypropylene separator, with conduction by lithium cations (1 × 10⁻⁴ S cm⁻¹ at 20°C) are designed. The polymer is formed from a 1 : 1 mixture of oligourethane dimethacrylate and polypropylene glycol monomethacrylate, which are capable of undergoing polymerization via double bonds in a liquid organic electrolyte (0.5 M LiClO₄) in a 1 : 1 mixture of propylene carbonate and dimethoxyethane. The polymer electrolyte comprises a polymer composition (20 wt %), a liquid electrolyte (78 wt %), and a photoinducer (2 wt %). Effect of insertion of dibenzo-18-crown-6 into the electrolyte on its electrochemical characteristics (on the electrolyte/Li interface) is investigated. Dependences of the bulk conductivity and exchange currents at the interface on the temperature and storage duration are studied at different crown ether contents.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 646–650.Original Russian Text Copyright © 2005 by Yarmolenko, Efimov.     

Improving the performance of all-solid-state supercapacitors by modifying ionic liquid gel electrolytes with graphene nanosheets prepared by arc-discharge

Ionic liquid gel polymers have widely been used as the electrolytes in all-solid-state supercapacitors, but they suffer from low ionic conductivity and poor electrochemical performance. Arc discharge is a fast, low-cost and scalable method to prepare multi-layered graphene nanosheets, and as-made graphene nanosheets （denoted as ad-GNSs） with few defects, high electrical conductivity and high thermal stability should be favorable conductive additive materials. Here, a novel ionic liquid gel polymer electrolyte based on an ionic liquid （EM1MNTF2） and an copolymer （P（VDF-HFP）） was modified by the addition of ad-GNSs as an ionic conducting promoter. This modified gel electrolyte shows excellent thermal stability up to 400 ℃ and a wide electrochemical window of 3 V. An all-solid-state supercapacitor based on commercial activated carbon was fabricated using this modified ionic liquid gel polymer electrolyte, which shows obviously improved electrochemical behaviors compared with those of the corresponding all-solid-state supercapacitor using pure ionic liquid gel polymer electrolyte. Specially, smaller internal resistance, higher specific capacitance, better rate performance and cycling stability are achieved. These results indicate that the ionic liquid gel polymers modified by ad-GNSs would be promising and suitable gel electrolytes for high performance all-solid-state electrochemical devices.