酚醛树脂基活性炭材料的制备及其电化学性能

以酚醛树脂（PF）为原料,聚乙二醇（PEG）为造孔剂,采用聚合物共混炭化及水蒸气活化法制备超级电容器电极用活性炭。通过热重（TG）分析探讨了PF、PEG及其共混物（PF-PEG）在升温过程中的热解行为,用N₂-BET法测试比表面积及其孔结构参数。通过测试恒流充放电、循环伏安和交流阻抗曲线分析其电化学性能,研究了活化温度、水蒸气流速及活化时间对活性炭孔结构及电化学性能的影响。结果表明,当活化温度为900℃、水蒸气流速为1 ml·min^-1、活化时间为2 h时制备的活性炭结构和性能相对较好,孔径主要分布在2 nm以下,比电容达到105.4 F·g^-1,具有良好的电容特性。     

Electrical double layer capacitors with activated sucrose-derived carbon electrodes

Electrical double layer capacitors (EDLCs) with activated sucrose-derived carbons (ASCs) as electrodes are reported. The carbons were prepared by the pyrolysis of sucrose followed by the activation with CO₂ gas for 1–5 h at 900 °C to tune the pore size distribution and the specific surface area (SSA). The porosity of the ASCs has been characterized using N₂ and CO₂ adsorption measurements. The activation increased the SSA from ∼200 to 3000 m² g⁻¹ and produced pores mostly in the 0.4–2 nm range. The pyrolysis of sucrose without CO₂ activation produces a carbon with specific capacitance as low as 4 F g⁻¹, whereas selected ASCs exhibit specific capacitance in excess of 160 F g⁻¹ and excellent frequency response in a two-electrode EDLC cell with 1 M H₂SO₄ electrolyte. The activation time of 4 h resulted in the most promising electrochemical performance. Excellent ASC stability was confirmed by extensive electrochemical characterization after 10,000 charge–discharge cycles.     

Nanostructure,porosity and electrochemical performance of chromium carbide derived carbons

This paper presents the results from the investigation of the influence of the chlorination temperature, the carbide crystal structure, the Cr/C ratio and physicochemical properties of CrCl₃ on the morphology, nanostructure, textural properties and electrochemical performance of CDCs. Electron microscopy and its analytical associated techniques reveal that these carbons, mainly composed by disordered graphene layers, evolve into graphitic nanostructures as a result of increasing the Cr/C content, the reaction temperature and the template effect of the etched CrCl₃ halide. Their textural analysis indicates the formation of micro/mesoporous carbons with a pore width below 1.5 nm, surface area as high as 835 m²/g and exhibit capacitive behavior in aqueous electrolyte.     

Model for the impedance analysis in a three electrode solid state electrochemical cell

A T-network model for the determination of the electrode impedances in a three electrode solid state electrochemical cell is postulated and discussed. The model is applied to a Li solid battery where the electrolyte is the polymer (PEO)₈LiCF₃SO₃ and the positive electrode is the insertion electrode V₆O_{13 + x}. The Li metal reference electrode is found to have an impedance considerably greater than that of the working (V₆O₁₃ + x) or the counter electrode (Li). The significance of these impedances is discussed in relation to the geometry and construction of solid electrolyte cells, and to their suitability for potentiostatic measurements.     

Electrochemical behaviour of some transition metal oxides in molten dimethylsulphone at 150°C

In view of the possible application in non-aqueous líthium cells operating at relatively high temperatures, molten dimethylsulphone (DMSO₂) has been used as the electrolyte solvent in lithium cells at 150°C. The stability of lithium in molten DMSO₂ has been found to be good as compared with that observed in organic solvents such as propylene carbonate, thus indicating that the Li⁺/Li system can be used as a suitable reference electrode in this medium.The electrochemical behaviour of some transition metal oxides has been investigated in LIClO₄ solutions in molten DMSO₂. The results obtained from voltammetric and chronopotentiometric measurements have shown a satisfactory behaviour for all the cathodic materials tested. Moreover, electrochemical insertion of Li⁺ ions into the crystal lattice of these oxides is a very fast process. Thus molten DMSO₂ appears to be a very interesting organic solvent usable in high energy density non-aqueous lithium cells.     

A dilatometric and small-angle X-ray scattering study of the electrochemical activation of mesophase pitch-derived carbon in non-aqueous electrolyte solution

The electrochemical activation of certain pitch-derived carbons has been proposed as a promising route towards obtaining high-capacitance electrodes for electrochemical double-layer capacitors. In the present work, the mechanism of electrochemical activation of a graphitizable carbon after calcination and KOH-activation was studied by nitrogen adsorption, electrochemical dilatometry and in situ small-angle X-ray scattering (SAXS). During electrochemical activation, a large capacitance gain from 1 to 121 F/g (at 0 V in a 1 mol/L solution of Et₄NBF₄ in propylene carbonate) was accompanied by a significant irreversible swelling of the electrode by 24% (6%) for activation in the negative (positive) potential range, respectively. In situ SAXS provided clear evidence for the insertion of ions into the latent microporosity of the carbon during electrochemical activation. Thus, the mechanism of electrochemical activation of weakly activated graphitizable carbon is not strictly due to ion intercalation between parallel graphene sheets.     

High-voltage electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials via Al concentration gradient modification

Al₂O₃-modified LiNi_0.5Co_0.2Mn_0.3O₂ cathode material is successfully synthesized via a facile carboxymethyl cellulose (CMC)-assisted wet method followed by a high-temperature calcination process. Al concentration gradient doping and accompanying formation of Al-coating are simultaneously accomplished in the modified samples. XRD and EDS analysis demonstrate that Al element is successfully doped into the crystal lattice with concentration gradient distribution inside the particles, reducing the Li/Ni cation mixing and stabilizing the layered structure. The compact distribution of Al on the surface forms a protective layer between the electrodes and the electrolyte, prohibiting the harmful side reactions and phase transition on the interphase. Compared with the pristine, the modified material with 2000?ppm Al₂O₃ (Al-2000) shows the best high-voltage performance with the capacity retention increased by ~13.3% from 138.3 to 163.0 mAh g^?1 at 1?C in 3.0–4.6?V after 100 cycles. Even under the high current rate of 8?C (1240 mAh g^?1) after 200 cycles, the Al-2000 still exhibits a capacity retention of 88.6%, greater than 80.3% for the pristine. Furthermore, results from the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) measurements confirm the roles of the Al₂O₃ modification in decreasing polarization and electrochemical resistances, contributing to the kinetic process of electrodes.     

Graphene-supported SnO2 nanoparticles prepared by a solvothermal approach for an enhanced electrochemical performance in lithium-ion batteries

SnO₂ nanoparticles were dispersed on graphene nanosheets through a solvothermal approach using ethylene glycol as the solvent. The uniform distribution of SnO₂ nanoparticles on graphene nanosheets has been confirmed by scanning electron microscopy and transmission electron microscopy. The particle size of SnO₂ was determined to be around 5 nm. The as-synthesized SnO₂/graphene nanocomposite exhibited an enhanced electrochemical performance in lithium-ion batteries, compared with bare graphene nanosheets and bare SnO₂ nanoparticles. The SnO₂/graphene nanocomposite electrode delivered a reversible lithium storage capacity of 830 mAh g⁻¹ and a stable cyclability up to 100 cycles. The excellent electrochemical properties of this graphene-supported nanocomposite could be attributed to the insertion of nanoparticles between graphene nanolayers and the optimized nanoparticles distribution on graphene nanosheets.     

Preparation of controlled porosity carbon aerogels for energy storage in rechargeable lithium oxygen batteries

Porous carbon aerogels are prepared by polycondensation of resorcinol and formaldehyde catalyzed by sodium carbonate followed by carbonization of the resultant aerogels in an inert atmosphere. Pore structure of carbon aerogels is adjusted by changing the molar ratio of resorcinol to catalyst during gel preparation and also pyrolysis under Ar and activation under CO₂ atmosphere at different temperatures. The prepared carbons are used as active materials in fabrication of composite carbon electrodes. The electrochemical performance of the electrodes has been tested in a Li/O₂ cell. Through the galvanostatic charge/discharge measurements, it is found that the cell performance (i.e. discharge capacity and discharge voltage) depends on the morphology of carbon and a combined effect of pore volume, pore size and surface area of carbon affects the storage capacity. A Li/O₂ cell using the carbon with the largest pore volume (2.195 cm³/g) and a wide pore size (14.23 nm) showed a specific capacity of 1290 mA h g⁻¹.     

High performance supercapacitors based on reduced graphene oxide in aqueous and ionic liquid electrolytes

Partially reduced graphene oxide (RGO) has been fabricated using hydrobromic acid. Since hydrobromic acid is a weak reductant, some oxygen functional groups which are relatively stable for electrochemical systems remain in RGO. Therefore, RGO can be re-dispersed in water and 2–3 layers of graphene can be observed by transmission electron microscopy, showing excellent affinity with water. RGO facilitates the penetration of aqueous electrolyte and introduces pseudocapacitive effects. Moreover, its capacitive nature is enhanced after cycling measurements. It is concluded that the increase of capacitance is due to the reduction of the oxygen functional groups by the cyclic voltammetry and electrochemical impedance spectroscopy analysis. The electrochemical properties in the ionic liquid electrolyte, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆), are also investigated. At a current density of 0.2 A g⁻¹, the maximum capacitance values of 348 and 158 F g⁻¹ are obtained in 1 M H₂SO₄ and BMIPF₆, respectively.     

Ambient pressure synthesis of La2LiHO3 as a solid electrolyte for a hydrogen electrochemical cell

The capability of La₂LiHO₃ as a H⁻ conductive solid electrolyte has recently been demonstrated, which has suggested the possibility of developing electrochemical devices based on H⁻ conduction. However, the performance of La₂LiHO₃ as a solid electrolyte has not yet been fully clarified. In this study, we prepared La₂LiHO₃ sintered pellets by a conventional solid-state reaction with LiH flux under ambient pressure and characterized the crystal structure and thermal stability (to moisture, ambient air, and oxygen) by neutron and X-ray diffraction measurements. The produced sintered pellets exhibited an activation energy of 69.2 kJ/mol, which is consistent with the value of the sample synthesized by the high-pressure method. The gas-sealing properties of the sintered La₂LiHO₃ pellets as the H⁻ conductive solid electrolyte were confirmed by measuring the electromotive force using a hydrogen concentration cell.     

Physical and electrochemical behavior of affordable Na3SbS4 solid electrolyte at different heat treatment temperatures

High-capacity and affordable all-solid-state Na-ion batteries have gathered increasing interest in recent years owing to low-cost sodium, which contributes to reducing the price of these Na-ion batteries to approximately 70% of that in lithium batteries. However, in terms of electrolyte performance and battery cost, the complete replacement of lithium batteries has a long way to go. In this work, low-cost and high-safety Na₂S·9H₂O materials are used in synthesizing Na₃SbS₄ solid electrolyte, the price of which is only one-fifth that of high-purity Na₂S. The structure and electrochemical properties are studied through X-ray diffraction analysis, Raman spectroscopy, scanning electron microscopy, and electrochemical tests. Results indicate that a multiphase Na₃SbS₄ structure containing cubic and tetragonal phases formed after heat treatment at 300 °C. In addition, a third phase transition of Na₃SbS₄ is inferred after further heating at 600 °C. This phase structure contributes to the improvement of electrochemical performance by promoting increasing ionic conductivity to 0.54 mS cm^?1 at room temperature (25 °C) and reducing activation energy to 0.076 eV. This work provides an affordable material with good electrochemical properties and not only simplifies the preparation but also greatly reduces the risk of the process.     

Focused microwaves in electrochemical processes

The effects of high intensity microwave radiation in electrochemistry are summarized and discussed. In situ microwave activation of electrochemical processes has been introduced recently and is possible by placing a carefully designed electrochemical cell directly into a microwave cavity. Self-focusing of intense microwave radiation occurs into a region close to the electrode | solution (electrolyte) interface of a microelectrode placed into the electrochemical cell. The electrode diameter and the electrode material strongly affect the observed mass transport enhancement and temperature effects. Experiments have been conducted to determine the temperature at the electrode surface electrochemically and to quantify the rate of chemical processes which occur in the vicinity of the electrode under high intensity microwave conditions. The effects of microwaves in a wide range of solvent systems from aqueous solutions to organic solvents (DMSO, acetonitrile, DMF, formamide) and in an ionic liquid (BMIM⁺PF₆⁻) have been investigated. Considerable current and temperature enhancements are observed in all solvents and are explained based on the interaction of microwaves with the liquid (electrolyte) and the physical properties of the liquids or solutions.     

Improved activity of a graphene–TiO2 hybrid electrode in an electrochemical supercapacitor

We present a simple and fast approach for the synthesis of a graphene–TiO₂ hybrid nanostructure using a microwave-assisted technique. The microstructure, composition, and morphology were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. The electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. Structural analysis revealed a homogeneous distribution of nanosized TiO₂ particles on graphene nanosheets. The material exhibited a high specific capacitance of 165 F g⁻¹ at a scan rate of 5 mV s⁻¹ in 1 M Na₂SO₄ electrolyte solution. Theenhanced supercapacitance property of these materials could be ascribed to the increased conductivity of TiO₂ and better utilization of graphene. Moreover, the material exhibited long-term cycle stability, retaining ∼90% specific capacitance after 5000 cycles, which suggests that it has potential as an electrode material for high-performance electrochemical supercapacitors.     

Enhancing electrochemical performance of Fe3O4/graphene hybrid aerogel with hydrophilic polymer

Graphene hybrid aerogels have attracted attention as electrode materials because of their unique porous architectures. However, their electrochemical performance is limited by the intrinsic hydrophobicity and the ease of aggregation of graphene nanosheets. We demonstrate a unique methodology to produce graphene hybrid aerogels through assembly of graphene nanosheets, nanometer‐scale ferroferric oxide (Fe₃O₄), and hydrophilic poly(vinyl alcohol) (PVA) into three‐dimensional hierarchical macrostructures. Electrochemical performance measurements exhibit a significant improvement in the specific capacitance of this ternary hybrid aerogel with remarkable cycling stability. Specifically, the specific capacitance is nearly 6.6 times higher than that of the neat graphene aerogel, and a cycling capacitance retention rate of 99% was achieved after 2000 cycles at a high current density of 0.5 A g^?1. Electrochemical impedance spectroscopy measurements demonstrate a lower resistance in the Fe₃O₄/graphene/PVA aerogel electrode compared with that of both neat graphene and Fe₃O₄/graphene aerogel electrodes. The obtained graphene hybrid aerogels with outstanding cycling performance and high energy density are very promising as electrode materials for supercapacitors. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45566.     

KOH activation of ordered mesoporous carbons prepared by a soft-templating method and their enhanced electrochemical properties

Ordered mesoporous carbon (COU-2) was synthesized by a soft-templating method. The COU-2 mesoporous carbon was activated by using KOH to improve its porosity. The mesopore size of COU-2 was 5.5 nm and did not change by the KOH activation. But, the BET surface area of COU-2 largely increased from 694 to 1685 m²/g and total pore volume was increased from 0.54 to 0.94 cm³/g after the KOH activation. The large increase of micropore volume is due to the increase of the surface area. Electrochemical cyclic voltammetry measurements were conducted in aqueous (1 M sulfuric acid) and organic (1 M tetraethyl ammonium tetrafluoroborate/polypropylene carbonate) electrolyte solutions. The KOH-activated COU-2 carbon shows superior capacitances over the COU-2 carbon and a commercial microporous carbon both in aqueous and organic electrolyte solutions. These results suggest that the carbons having regularly-interconnected uniform mesopores and micropores in thin pore walls are desirable for the electrodes in electrochemical double-layer capacitors.     

Structural and electrochemical studies of a hexaphenylbenzene pyrolysed soft carbon as anode material in lithium batteries

XRD, SEM micrographs, BET analyses and typical electrochemical experiments (cyclic voltammetry, step voltammetry and Li insertion/deinsertion at constant current) have been carried out to characterize a new type of soft carbons obtained by pyrolysis of hexaphenylbenzene (HPB). By means of XRD and cyclic voltammetry at least three different type of sites for lithium storage were found. The first is graphite like type with d₀₀₂ graphene layer distance greater than pure graphite; the second is associated to disordered volumes among crystallities and the third is represented by Li sites at the hydrogen-terminated edges of hexagonal carbon fragments, characterized by higher energy in comparison with simple insertion sites. These last two types of sites are able to store some extra lithium, compared to pure graphite. BET analyses and cyclic voltammetries demonstrate the key role of the milling time on the characteristics and properties of this HPB pyrolysed carbon. Specific capacities shown by this pyrolysed material in Li coin-type cell have been also reported.     

Anodic lithium ion battery material with negative thermal expansion

Negative thermal expansion materials will effectively counteract possible severe expansion and contraction due to the insertion and extraction of Li ions in lithium ion batteries. Herein, negative thermal expansion ZrScMo₂VO₁₂ and its carbon-coating composites are prepared as electrode material in lithium ion batteries by a heating treatment route. The galvanostatic charge/discharge process, cyclic voltammetry measurement and electrochemical impedance spectroscopy are tested to relate their thermal expansion and electrochemical properties. The initial specific capacity reaching 1062 mA h g^-1 at the current density of 0.2 A g^-1 is obtained with ideal negative thermal expansion properties. The reversible specific capacity still remains stable at 310 mA h g^-1 for that material coated with carbon after 100 cycles. The corresponding theoretical simulations and in situ XRD patterns propose a Li ion storage mechanism based on Li ion insertion process in open framework structure. As a proof-of-concept research, this work paves a way to the promising application of negative thermal expansion materials in lithium ion batteries and other energy storage systems.     

In situ transmission FTIR spectroelectrochemistry: A new technique for studying lithium batteries

In situ transmission FTIR spectra are measured during the electrochemical insertion of lithium into phospho-olivine FePO₄. The spectroelectrochemical cell consists of a composite FePO₄ cathode, a lithium metal anode, and an electrolyte of 1 M LiPF₆ in a 1:1 mixture of ethylene carbonate and diethyl carbonate (EC-DEC). Bands belonging to the electrolyte and cathode are identified in the infrared spectra of the in situ cells. The antisymmetric PO₄³⁻ bending vibrations (ν₄) are used to monitor Li⁺ insertion into FePO₄. Discharging produces spectral changes that are consistent with the formation of phospho-olivine LiFePO₄, yet the electrolyte bands are not affected by the discharging process. The in situ infrared experiments confirm the two-phase mechanism for lithium insertion into FePO₄. Moreover, the experiments demonstrate the ability to collect in situ transmission FTIR spectra of functioning electrode materials in lithium batteries. Unfortunately, lithium plating occurs on the optical window when the Li//FePO₄ half-cells are charged. The use of an intercalation anode such as graphite could alleviate this problem; however, this avenue of research is not explored in this study.     

Preparation and electrochemical characterization of MnOOH nanowire–graphene oxide

MnOOH nanowire–graphene oxide composites are prepared by hydrothermal reaction in distilled water or 5% ammonia aqueous solution at 130 °C with MnO₂–graphene oxide composites which are synthesized by a redox reaction between KMnO₄ and graphene oxide. Powder X-ray diffraction (XRD) analyses and energy dispersive X-ray analyses (EDAX) show MnO₂ is deoxidized to MnOOH on graphene oxide through hydrothermal reaction without any extra reductants. The electrochemical capacitance of MnOOH nanowire–graphene oxide composites prepared in 5% ammonia aqueous solution is 76 F g⁻¹ at current density of 0.1 A g⁻¹. Moreover, electrochemical impedance spectroscopy (EIS) suggests the electrochemical resistance of MnOOH nanowire–graphene oxide composites is reduced when hydrothermal reaction is conducted in ammonia aqueous solution. The relationship between the electrochemical capacitance and the structure of MnOOH nanowire–graphene oxide composites is characterized by cyclic voltammetry (CV) and field emission scanning electron microscopy (FESEM). The results indicate the electrochemical performance of MnOOH nanowire–graphene oxide composites strongly depends on their morphology.