Hydrophobisation of active carbon surface and effect on the adsorption of water

A technique of surface hydrophobisation has been applied to two microporous carbonaceous adsorbents. A granular active carbon and an activated carbon fibre, both formerly chemically treated in order they preferentially present hydroxyl surface functions, were modified by action of vinyltrimethoxysilane (vtmos) in liquid phase. The resulting samples were characterised using sorption of nitrogen, FTIR, XPS and ²⁹Si MAS-NMR spectroscopy, and elemental analysis. Their stability and heat treatment have also been investigated through thermal analysis.The efficiency of the hydrophobisation treatment was evaluated by static adsorption of water vapour and vapours of chlorinated volatile organic compounds (VOCs): dichloromethane and trichloroethylene. Grafting of the vtmos and development of a “coating” of polysiloxane onto the adsorbent induced a modification of the carbon surface but also a partial filling of the porosity. These modifications accounted for a decrease of both the amounts of water and VOC adsorbed by the hydrophobised materials. However, water uptakes were found to be much lower than those of the VOCs, evidencing an enhanced selectivity of the hydrophobised adsorbents toward VOCs.     

High capacity supercapacitors based on modified activated carbon aerogel

Carbon aerogel derived from a modified resorcinol–formaldehyde (RF) method without supercritical drying step was activated under CO₂ flow and further modified with a surfactant sodium oleate solution. Carbon aerogel, activated carbon aerogel (ACA) and modified activated carbon aerogel (MACA) were characterized by Brunaver, Emmett and Teller(BET) surface measurement, constant-current charge–discharge and cyclic voltammetry(CV). It was found that the specific surface area of the ACA was twice that without activation. At relatively low discharge rates the specific capacitance and energy delivered from the capacitor were improved greatly by the activation of carbon aerogel. To apply the ACA at high discharge rate, a surface modification was introduced. After the surface modification the wettability of the organic electrolyte based on non-polar organic solvent (i.e. propylene carbonate) to the ACA was improved greatly and, as a result, the internal resistance of the capacitor decreased and the specific capacitance and energy delivered increased at all the test discharge rates. The effects from the modification become more marked at higher discharge rates, i.e. at 48mAcm^–2, the energy delivered increased by ca. 70%, which indicates the MACA more suitable as electrode material in electric double-layer capacitors (EDLCs) for high current applications.     

Surface modification of carbonaceous materials for EDLCs application

In this study, carbonaceous materials such as activated carbon and activated carbon aerogel were chemically modified with a surfactant sodium oleate in order to improve their specific capacitance and energy storage in electrochemical double-layer capacitors (EDLCs). Optimal conditions for surface modification of activated carbon have been examined as a surfactant solution concentration of 0.25 wt.% together with a time of 24 h for treatment at 25 °C. Specific capacitance and energy density can be improved significantly by surface modification of carbon materials. The enhancement in specific capacitance and energy density is mainly attributable to improvement in wettability of carbon materials, which results in a higher usable surface area and a smaller internal resistance. The effects from surface modification become more marked at higher discharge rates, at which the internal resistance has a more important impact on the energy delivery. A two times energy density of the original carbon could be achieved for the modified carbon materials at a high discharge rate, which indicates that the modified carbons are more suitable in EDLCs for high current applications. In addition, the modified carbon materials possess excellent cycle stability (the capacity decay was only 4% after 20,000 cycles).     

纳米SiO2填料对离子凝胶电解质及高压超级电容器性能的影响

以SiO2纳米颗粒为填料,通过溶液浇筑法合成了纳米复合离子凝胶电解质,研究了SiO2填料对离子输运的影响规律。基于离子凝胶电解质构筑了准固态电容器,探讨了无机填料对电容器性能的影响,以活性炭为电极、凝胶电解质为隔膜,构筑了准固态双电层电容器。结果表明,SiO2的加入没有改变隔膜电解质的微观形貌,但有效改善了浸润性,提高了离子电导率。高SiO2添加量的隔膜电解质电化学性能更优,当添加8wt% SiO2时凝胶电解质电化学性能最优。SiO2的加入可有效提高活性炭准固态电容器的性能,电容器的比容提升约15%,经4000次循环后容量保持可达100%。电解质高温稳定性良好,器件最高使用温度可达60℃。基于该复合电解质构筑的电容器具有良好的高温性能,电容器比容随温度升高而逐渐提升,60℃时能量密度可达81.36 Wh/kg。     

Improvement of carbon materials performance by nitrogen functional groups in electrochemical capacitors in organic electrolyte at severe conditions

N-doped activated carbon fibers have been synthesized by using chemically polymerized aniline as source of nitrogen. Commercial activated carbon fibers (A20) were chemically modified with a thin film of polyaniline (PANI) inside the microporosity of the carbon fibers. The modified activated carbon fibers were carbonized at 600 and 800 °C, respectively. In this way, activated carbon fibers modified with surface nitrogen species were prepared in order to analyze their influence in the performance of electrochemical capacitors in organic electrolyte. Symmetric capacitors were made of activated carbon fibers and N-doped activated carbon fibers and tested in a two-electrode cell configuration, using triethylmethylammonium tetrafluoroborate/propylene carbonate (TEMA-BF₄/PC) as electrolyte. The effect of nitrogen species in the degradation or stabilization of the capacitor has been analyzed through floating durability tests using a high voltage charging (3.2 V). The results show higher stabilizing effect in carbonized samples (N-ACF) than in non-carbonized samples and pristine activated carbon fibers, which is attributed to the presence of aromatic nitrogen group, especially positively charged N-functional groups.     

Characterization of grafted polyethylene by contact-angle hysteresis and ESCA studies

Low-density polyethylene (LDPE) has been functionalized in the bulk through dicumyl peroxide (DCP)-initiated grafting of dibutyl maleate (DBM) and vinyltrimethoxysilane (VTMO) in the temperature range from 140 to 200°C. The functionalized surfaces have been characterized through contact angle and contact-angle hysteresis measurements as well as through ESCA studies. The degree of grafting has been determined by infrared spectrophotometry. From the ESCA patterns of the modified polymers, the atomic ratios of oxygen to carbon and silicon to carbon have been calculated. Determination of an empirical relation between contact-angle hysteresis and the oxygen-carbon ratio of the functionalized polymer surfaces has been attempted. It is found that the higher the oxygen-carbon ratio, the lower the contact-angle value for polyethylene. Contact angle-hysteresis increases with increase of the degree of grafting. The total surface energy increases with grafting. The surface energy of the silane-grafted polyethylene (PEgS) is found to be the lower than that of dibutyl maleate-grafted polyethylene (PE_gDBM). © 1993 John Wiley & Sons, Inc.     

Preparation and performance of cobalt-doped carbon aerogel for supercapacitor

Carbon aerogels were prepared by polycondensation of resorcinol with formaldehyde in ambient conditions. The effect of resorcinol-to-catalyst ratio (R/C ratio) on volume shrinkage, BET surface area, and electrochemical property was investigated by changing R/C ratio from 50 to 2000. Carbon aerogel prepared at R/C ratio of 500 showed less than 2% of volume shrinkage and the highest BET surface area (706 m²/g). Specific capacitance of carbon aerogel prepared at R/C ratio of 500 was found to be 81 F/g in 1M H₂SO₄ electrolyte. Cobalt-doped carbon aerogels were then prepared by an impregnation method with a variation of cobalt content, and their performance was investigated. Among the samples prepared, 7 wt% cobalt-doped carbon aerogel showed the highest capacitance (100 F/g) and the most stable cyclability. The enhanced capacitance of cobalt-doped carbon aerogel was attributed to the faradaic redox reactions of cobalt oxide.     

有机添加剂对超级电容器中水系电解液理化性能的影响

以6 mol/L KOH水溶液为电解液,高比表面积的活性炭为活性物质,研究了有机添加剂对体系润湿性、电导率、工作电压窗口及阻抗的影响,测试了超级电容器的电化学性能。结果表明,适量添加有机添加剂可明显抑制体系的极化现象,提高超级电容器的工作电压窗口。添加10vol%异丙醇时,电极材料和电解液间的润湿性大幅提高,比电容从79.3 F/g提高至113.2 F/g。添加20vol%异丙醇时,超级电容器的能量密度达19.4 Wh/kg,体系的电荷转移电阻明显降低,在10 A/g电流密度下的比电容比0.5 A/g时下降13.9%,而不加添加剂时下降30.3%。添加30vol%异丙醇时,电解液电导率迅速下降,比电容降低,电导率是影响比电容的关键因素。     

Plasma-modified polyethylene membrane as a separator for lithium-ion polymer battery

The surface of polyethylene (PE) membranes as a separator for lithium-ion polymer battery was modified with acrylonitrile (AN) using the plasma technology. The plasma-induced acrylonitrile coated PE (PiAN-PE) membrane was characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and contact angle measurement. The electrochemical performance of the lithium-ion polymer cell fabricated with the PE and the PiAN-PE membranes were also analyzed. The surface characterization demonstrates that the enhanced adhesion of the PiAN-PE membrane resulted from the increased polar component of surface energy for the PiAN-PE membrane. The presence of the PiAN induced onto the surface of the membrane via the plasma modification plays a critical role in improving the wettability and electrolyte retention, the interfacial adhesion between the electrodes and the separator, the cycle performance of the resulting lithium-ion polymer cell assembly. The PiAN-PE membrane modified by the plasma treatment holds a great potential to be used as a high-performance and cost-effective separator for lithium-ion polymer battery.     

Carbon Cloth Reinforced Carbon Aerogel Films Derived from Resorcinol Formaldehyde

Carbon cloth reinforced RF (Resorcinol Formaldehyde) aerogel films have been produced with extremely high RC ratio (molar ratio of resorcinol to catalyst) or with no catalyst at all. The gels were subcritically dried. Carbon aerogel films were obtained by pyrolysis of the RF aerogel films. The structure of the composite porous films was investigated using infrared spectrometer, optical and electron scanning microscope. The IR measurements indicated that the bottom surface reflectance of the films is higher than the top surface. The microscopical pictures revealed that the bottom surface of the films has a denser structure than the top surface, and the presence of carbon cloth in the sol-gel process strongly influences the structure of the films. The carbon cloth fibers act as catalytic sites. The particles are more likely to form around the fibers instead of within the sol. This leads to a somewhat reduced specific surface area as was shown via N₂ sorption and BET analysis.Using the 4-probe method the electrical conductivity of the carbon cloth reinforced carbon films was found to be higher than that of pure carbon aerogel films.     

Étude des phénomènes de mouillabilité du carbone par le brai liquide

One of the processes used in the manufacture of carbon/carbon composites makes use of impregnation with pitch of a multidirectional structure of carbon rods made of carbon fibres at high temperature followed by carbonization under high pressure. A densification of the composite is obtained by repeated impregnations.Since the mechanical properties of the composites must likely depend on the quality of the interface formed between the carbon rod and the liquid pitch matrix, the wetting properties of pitches of various origins and properties (Table 1) were studied for several carbon rods (Table 2). A tensiometric method based on the weight of pitch lifted during immersion of the carbon rod at 350°C was used (Fig. 1).Measurements of the surface energy of the pitches in the liquid state at 350°C (Table 3) and in the solid state at room temperature (Table 4) show that the surface energy is practically independent of the origin of the pitch and that the surface polarity of the pitch is negligible. On the contrary, the values of the contact angle at 350°C is higher for the petroleum pitch No. 1 than for the coal tar pitches (Table 5).The wettability criteria, i.e. the spreading coefficient S and the wetting tension τ, derived from the measured surface energy and contact angle, confirm the lower wetting ability of the petroleum pitch (Fig. 2). These criteria may explain why the densification process (number of impregnations and carbonization cycles necessary to reach an apparent density of the composite of 1.90) is much more difficult with petroleum pitch. On the contrary, the adhesion criterion factor, expressed as the reversible energy of adhesion between the liquid and the solid at 350°C, has the same value whatever the origin of the pitch. Since the degree of adhesion might influence the quality of the carbon/carbon interface in the composite, the same value of the adhesion criterion factor may explain why all composites, at equivalent density, show about the same compression and torsion strengths.The difference between the wettability and adhesion for the pitches of different origins is due to the difference in the spreading pressure π; this quantity measures the decrease of the surface energy of the carbon rod resulting from the adsorption of the pitch vapor. The calculations reveal that, whatever the surface energy of the carbon rod, the petroleum pitch leads to a larger reduction of the surface energy of the solid than the coal tar pitches (Fig. 3).The wettability and adhesion criteria adopted in this study seem to (at least to a first approximation) explain the dependence of the mechanical properties of the carbon/carbon composites on the type of pitch and on the processing conditions.     

Wettability Modification of Nanomaterials by Low-Energy Electron Flux

Controllable modification of surface free energy and related properties (wettability, hygroscopicity, agglomeration, etc.) of powders allows both understanding of fine physical mechanism acting on nanoparticle surfaces and improvement of their key characteristics in a number of nanotechnology applications. In this work, we report on the method we developed for electron-induced surface energy and modification of basic, related properties of powders of quite different physical origins such as diamond and ZnO. The applied technique has afforded gradual tuning of the surface free energy, resulting in a wide range of wettability modulation. In ZnO nanomaterial, the wettability has been strongly modified, while for the diamond particles identical electron treatment leads to a weak variation of the same property. Detailed investigation into electron-modified wettability properties has been performed by the use of capillary rise method using a few probing liquids. Basic thermodynamic approaches have been applied to calculations of components of solid–liquid interaction energy. We show that defect-free, low-energy electron treatment technique strongly varies elementary interface interactions and may be used for the development of new technology in the field of nanomaterials.     

Physical and chemical properties of polypyrrole–carbon fiber interphases formed by aqueous electrosynthesis

The properties of carbon fibers modified by aqueous electrochemical synthesis of pyrrole has been determined by using the dynamic contact angle analyzer (DCA), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). Electrochemical process parameters such as the initial pyrrole concentration, electrolyte concentration, applied voltage, electrolyte type, and reaction time were systematically varied, and their impact on the polypyrrole–carbon fiber interphases surface free energy and morphology was ascertained. The surface free energies of the polypyrrole–carbon fiber interphases were obtained by using single fiber filaments. SEM analysis of the interphases revealed several distinct surface structures, including smooth, porous, granular, microspheroidal, and leafoidal morphologies. The noncoated but commercially surface oxidized carbon fibers have smooth surface morphology with occasional longitudinal striations. FTIR analysis of the polypyrrole interphases confirmed that the counterions derived from the electrolytes were incorporated into the film. The surface free energies of the electrochemically formed polypyrrole–carbon fiber interphases equivalent to 60–75 dynes/cm, was determined to be up to 40% higher than that for the surface oxidized but unsized carbon fibers equivalent to 50 dynes/cm. This improvement in the surface free energies of the polypyrrole–carbon fiber interphases suggests easy wettability by polymer matrices such as epoxy resin, γ ˜ 47 dynes/cm and, polyimide matrix, γ ˜ 45 dynes/cm. © 1996 John Wiley & Sons, Inc.     

Influence of the interfacial reaction on interfacial adhesion in polyarylacetylene resin–silica glass composites

The influence of interfacial reaction on interfacial adhesion in silica glass/polyarylacetylene resin composites was studied. In order to achieve chemical reaction at the interface, vinyltrimethoxysilane was grafted onto silica glass surface to react with polyarylacetylene resin. The reaction between polyarylacetylene resin and vinyltrimethoxysilane was proved based on the model reaction between phenylacetylene and vinyltrimethoxysilane. At the same time, the modified silica glass surface characteristics were evaluated by contact-angle measurements and surface energy determination. The interfacial adhesion in silica glass/polyarylacetylene resin composites was evaluated by shear strength testing and fracture morphology analysis. It was concluded that polyarylacetylene resin reacted with vinyltrimethoxysilane. Furthermore, due to the reaction between polyarylacetylene resin and vinyltrimethoxysilane at the interface, the interfacial adhesion in composites was significantly increased. The improvement in interfacial adhesion was solely attributed to the interfacial reaction.     

Electrochemical impedance study of polyaniline electrocoated porous carbon foam

Electropolymerization of aniline on mesophase pitch based carbon foam has been studied in order to evaluate the influence of conductive polymer coating on the properties of carbon foam. The surface morphology of the coating was determined by scanning electron microscopy (SEM). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical properties of resulting modified carbon foam samples. Polyaniline (PANI) electrocoated-mesophase pitch based carbon foam showed good capacitor behavior in 0.5 M H₂SO₄. Better capacitive behavior is obtained for 100 and 150 mV/s compared to other scan rates, under these faster scan rates thinner films of PANI coatings were combined with more porous structure of carbon foam. Conductivity of the carbon foam was increased from 9.23 to 13.73 S/cm by electrocoating of PANI.     

超级电容器准固态聚合物电解质膜的研究

制备了一种介于水凝胶和全固态聚合物电解质之间的聚合物电解质膜,用于活性炭电子双电层电容器。测试表明使用该聚合物电解质膜的双电层电容器的容量为2 15mA·h,其容量、功率特性与KOH水溶液电容器相当。电容器的循环伏安曲线,稳定的充放电循环曲线及交流阻抗谱说明该种聚合物电解质膜在碳基超级电容器的使用电压范围(0～1V)内是稳定的,而且聚合物电解质膜电容器表现出良好的可逆性和循环特性。     

碳气凝胶改性全氟磺酸质子交换膜的性能

利用苯酚-甲醛体系的碳气凝胶对全氟磺酸质子交换膜进行改性,考察了碳气凝胶粒径以及添加量对改性效果的影响,并将其应用于电吸附脱盐过程分析了其脱盐效果。结果表明,采用平均粒径为9.76 nm,比表面积为590 m²/g且质量分数为2%添加量的中孔碳气凝胶改性后的全氟磺酸质子交换膜在含水率、保水率、溶胀度以及导电性方面性能优异,并且应用于电吸附脱盐过程效果明显。     

Preparation and Application of Nano-composite Poly(vinyl alcohol) Gel Electrolyte in Electrochemical Capacitor

A nano-composite polymer gel electrolyte was prepared using titanium oxide nanowire, poly(vinyl alcohol) (PVA), lithium salt and organic solvent N-methyl-2-pyrrolidone (NMP). The obtained electrolyte has the potential for application in electrochemical capacitor, the PVA in it is in an amorphous state. The ionic conductivities of electrolytes increased after addition of the nanowire, and the electrolyte with 3%(w) of nanowire exhibited the highest ionic conductivity of 3.2 mS/cm at 20℃, as measured by electrochemical impedance spectroscopy. The temperature dependence of the conductivity was found to be in agreement with the Arrhenius equation. Functioning as separator and electrolyte, this nano-composite PVA gel electrolyte was used to assemble the electrochemical capacitor with active carbon film as electrodes. The compositing of nanowire may extend the life of electrochemical capacitors as they keep more than 90% of their capacitance after 5000 cycles of charging and discharging.     

The effect of surface energy on the heat transfer enhancement of paraffin wax/carbon foam composites

The influence of carbon foam surface energy on heat transfer through paraffin wax/carbon foam composite was investigated. Carbon foam samples were surface treated and their corresponding surface energy values were measured. A theoretical model was formulated to analyze the mass of paraffin wax absorbed for both pristine and surface activated carbon foam samples based on the concept foam wettability. An experimental study was carried out for heating of the wax/carbon foam composite samples to study the phase change heat transfer due to the melting of wax within the foam matrices. The above studies showed that a greater mass of wax was absorbed within the activated carbon foam samples as compared to the pristine sample which can be due to their greater wettability. This resulted in an improvement in heat transfer rate for the activated samples. The total energy storage rate for the activated composite samples was compared with the pristine sample for the same heating duration and an enhancement of more than 18% was observed for the two activated samples. These studies revealed that the surface energy of carbon foams can play an important role in improving the overall thermal performance of wax/carbon foam composites.     

Electric double layer capacitors with gelled polymer electrolytes based on poly(ethylene oxide) cured with poly(propylene oxide) diamines

Using a gel electrolyte for electric double layer capacitors usually encountered a drawback of poor contact between the electrolyte and the electrode surface. A gel electrolyte consisting of poly(ethylene oxide) crosslinked with poly(propylene oxide) as a host, propylene carbonate (PC) as a plasticizer, and LiClO₄ as a electrolytic salt was synthesized for double layer capacitors. Diglycidyl ether of bisphenol-A was blended with the polymer precursors to enhance the mechanical properties and increase the internal free volume. This gel electrolyte showed an ionic conductivity as high as 2 × 10⁻³ S cm⁻¹ at 25 °C and was electrochemically stable over a wide potential range (ca. 5 V). By sandwiching this gel-electrolyte film with two activated carbon cloth electrodes (1100 m² g⁻¹ in surface area), we obtained a capacitor with a specific capacitance of 86 F g⁻¹ discharged at 0.5 mA cm⁻², while the capacitance was 82 F g⁻¹ for a capacitor equipped with a liquid electrolyte of 1 M LiClO₄/PC. The capacitance decrease with the current density was less significant for the gel-electrolyte capacitor. We found that the less restricted ion diffusion near the electrolyte/electrode interface led to the smaller overall resistance of the gel-electrolyte capacitor. The high performance of the gel-electrolyte capacitor has demonstrated that the developed polymer network not only facilitated ion motion in the electrolyte bulk phase but also gave an intimate contact with the carbon surface. The side chains of the polymer in the amorphous phase could stretch across the boundary layer at the electrolyte/electrode interface to come into contact with the carbon surface, thus improving transport of Li⁺ ions by the segmental mobility in polymer.