Sensitive and selective electrochemical sensor for magnolol based on the enhancement effect of multiwalled carbon nanotubes

Herein, insoluble multiwalled carbon nanotubes (MWNTs) were dispersed into N,N-dimethylformamide (DMF) via ultrasonication, resulting in a stable and homogeneous MWNTs suspension. After evaporation of DMF, the surface of glassy carbon electrode was successfully coated with MWNTs film, as confirmed from scanning electron microscopy measurements. In pH 7 phosphate buffer, an irreversible oxidation peak was observed for magnolol, and the peak currents greatly increased on MWNTs film surface. The influences of pH value, amount of MWNTs and accumulation conditions were studied. Based on the remarkable enhancement effect of MWNTs, a new electrochemical sensor with high sensitivity was developed for magnolol. The linear range was from 5 μg?L^?1 to 1 mg?L^?1, and the detection limit was 2 μg?L^?1 (7.51?×?10^?9?M) after a 3-min accumulation. This novel sensor was successfully used to detect the content of magnolol in Chinese traditional medicines, and the recovery was over the range from 98.1 to 99.1 %.     

Binder-free composite electrodes using carbon nanotube networks as a host matrix for activated carbon microparticles

In this research, networks of single-walled carbon nanotubes (SWNTs) were used to host activated carbon (aC) microparticles to fabricate freestanding composite electrodes without the use of polymer binders. The aC-SWNT composite electrodes with up to 50 wt. % aC showed specific surface areas approaching 1000 m²/g and electrical conductivities >36 S/cm. The composite electrodes possessed the properties of both pure SWNT electrodes (e.g. low ohmic drop and rapid ion diffusion) and activated carbon particles (e.g. high specific capacitance). With an interconnected mesoporous microstructure and high electrical conductivity, the CNT networks provide an attractive alternative to polymer binders for forming freestanding electrodes for electrical energy storage devices. Here we show that micron-sized particles can be supported in this framework to utilize the performance enhancement and robustness provided by CNTs. Symmetric electrochemical capacitors fabricated with the electrodes in 6 M potassium hydroxide (KOH) aqueous electrolyte maintained specific capacitances of more than 45 F/g after 30,000 constant-current charge–discharge cycles with a current of 3.6 mA/cm².     

KOH activated carbon fabrics as supercapacitor material

Carbon fabrics from viscose fibers activated with KOH have been investigated as possible electrode materials for electrochemical capacitors. The fibers were first pyrolysed at 400 or 600 °C, then saturated with KOH at C/KOH ratios from 1:3.5 to 1:5 and treated in the temperature range from 700 to 800 °C. The carbon fibers saturated with KOH were analysed by thermogravimetric and differential thermal analysis in order to get information on the temperature dependence of the KOH reaction with carbon. The electrochemical properties of the activated carbons were determined using three-electrode Swagelok^® type capacitors both in 4 M H₂SO₄ and 7 M KOH aqueous electrolytes. Specific capacities of ca. 340 and 270 F/g were determined in acidic and alkaline medium, respectively. We demonstrate that the electrical capacity for both negative and positive electrodes depends on the treatment method. The capacitance values are discussed taking into account the porous texture, the elemental composition and the surface functionality of the activated carbon fibers.     

Activated carbon aerogel as electrode material for coin-type EDLC cell in organic electrolyte

Carbon aerogel (CA) was prepared by a carbonization of resorcinol–formaldehyde (RF) polymer gels, and it was chemically activated with KOH to obtain activated carbon aerogel (ACA) for electrode material for EDLC in organic electrolyte. Coin-type EDLC cells with two symmetrical carbon electrode were assembled using the prepared carbon materials. Electrochemical performance of the carbon electrodes was measured by galvanostatic charge/discharge and cyclic voltammetry methods. Activated carbon aerogel (20.9 F/g) showed much higher specific capacitance than carbon aerogel (7.9 F/g) and commercial activated carbon (8.5 F/g) at a scan rate of 100 mV/s. This indicates that chemical activation with KOH served as an efficient method to improve electrochemical performance of carbon aerogel for EDLC electrode in organic electrolyte. The enhanced electrochemical performance of activated carbon aerogel was attributed to the high effective surface area and the well-developed pore structure with appropriate pore size obtained from activation with KOH.     

Free-standing WS<Subscript>2</Subscript><Emphasis Type="Strikethrough">-</Emphasis>MWCNTs hybrid paper integrated with polyaniline for high-performance flexible supercapacitor

As two-dimensional layered nanomaterials, the tungsten disulfide (WS₂) nanosheets can be used as building blocks of paper-like electrodes for high-performance FSs. However, poor conductivity and mechanical property of WS₂ nanosheets (NSs) paper greatly hinders their capacitance and/or rate performance. To solve these problems, we fabricated the WS₂-multiwalled carbon nanotubes (MWCNTs)/polyaniline (PANI) composite papers based on liquid exfoliation and electrochemical deposition for high-performance flexible supercapacitors. The WS₂-MWCNTs/PANI with conductive PANI chains linked WS₂ NSs and MWCNTs takes the advantages of high-electronic double-layer capacitance originated from the internal surface areas of MWCNTs and effective pseudocapacitance generated by exfoliated WS₂ NSs and PANI. Electrochemical studies showed that the gravimetric-specific capacitance of WS₂-MWCNTs/PANI can reach ~760.1 F/g at a current density of 1 A/g. A symmetric flexible solid-state supercapacitor was also assembled and studied. The WS₂-MWCNTs/PANI-assembled FS device also has an excellent area specific capacitance of 1158.7 mF/cm² at a current density of 0.5 mA/cm² together with a high-capacity retention of ~82.5% after 2000 cycles.     

Potato starch-based activated carbon spheres as electrode material for electrochemical capacitor

Potato starch-based activated carbon spheres (PACS) were prepared from potato starch by stabilization, carbonization followed by activation with KOH. The obtained PACS are hollow and retain the original morphology of potato starch with decrease in size, as shown by scanning electron microscopy. Modification of textural properties of the PACS was achieved by varying the carbonization temperature and the ratio of KOH/PCS. The results of N₂ adsorption isotherms indicate that the samples prepared are mainly microporous. The electrochemical behaviors of the hollow PACS were studied by galvanostatic charge-discharge, cyclic voltammetry, and impedance spectroscopy. The results indicate that high specific capacitance of 335 F/g is obtained at current density 50 mA/g for PACS with specific surface area 2342 m²/g. Only a slight decrease in capacitance, to 314 F/g, was observed when the current density increases to 1000 mA/g, indicating a stable electrochemical property.     

Activated carbon from coconut leaves for electrical double-layer capacitor

This study investigates the potential of coconut leaves as a precursor to obtain activated carbon. Coconut leaf-activated carbon (CLAC) has been prepared through gas activation process starting with carbonization at 400 °C in nitrogen flow for 3 h. The carbonized coconut leaves were milled using planetary ball milling followed by activation with carbon dioxide (CO₂) at different temperatures ranging from 700 to 1000 °C. The Brunauer–Emmett–Teller (BET) characterization reveals that the surface area of CLACs increases with the increase in activation temperature. Electrodes prepared from CLACs have been used to fabricate electrochemical double-layer capacitors (EDLCs) in order to study the electrochemical behavior using galvanostatic charge–discharge measurements and cyclic voltammetry. The carbon activated at 900 °C delivered the best specific capacitance of 133.4 F/g at current density of 200 mA/g.     

氮/硫共掺杂多孔碳纳米片的制备及其电化学性能

二维多孔碳材料能够提供较短的电解质扩散通道和较快的电子传输过程,因此在能量转换和储存装置中表现出优异的电化学性能.近年来的理论和实验研究表明,两元素共掺杂可使二维多孔碳材料的电化学性能得到明显提高.因此,共掺杂二维多孔碳材料的制备成为目前的研究热点之一.本文以甲基橙-FeCl₃复合物为模板引发剂制备了甲基橙掺杂的聚吡咯纳米管,通过对聚吡咯纳米管与KOH混合物（重量比为1：2）在700 ℃进行热处理,制备了二维石墨烯状氮/硫共掺杂多孔碳纳米片.所制备的氮/硫共掺杂多孔碳纳米片相互连结,形成了多级孔结构.氮气吸附分析表明多级孔结构包含微孔、介孔和大孔,这使所制备的氮/硫共掺杂多孔碳纳米片具有较高的比表面积（1744.58 m²/g）和孔体积（1.01 cm³/g）.共掺杂多孔碳纳米片中的掺杂氮以吡啶氮、吡咯氮和季胺氮形式存在,掺杂硫以噻吩硫和氧化态硫形式存在,二者之间的协同效应能够明显改善碳纳米片表面的浸润性,增加表面电化学活性点.这些特征使所制备的氮/硫共掺杂多孔碳纳米片表现出优异的电化学性能.用氮/硫共掺杂多孔碳纳米片制备的量子点敏化太阳能电池对电极,对多硫电解质再生反应的电催化活性与传统PbS对电极相近,所组装电池的光电转换效率可达到4.30%（100 mW/cm²）.氮/硫共掺杂多孔碳纳米片作为超级电容器电极材料,以6 M（1 M=1 mol/L）KOH为电解质,电流密度为0.4 A/g,比电容达到312.8 F/g.即使电流密度增加到20 A/g,比电容仍达到200.6 F/g,表明其具有较好的倍率性能.     

Comparison of different types of polypyrimidine/CNTs/Pt hybrids in fuel cell catalysis

In this paper, we mainly studied the preparation of platinum-containing composite materials with carbon nanotubes wrapped by polypyrimidine-conjugated polymers and the performance of the composites. The polymer-based carbon nanotubes/Pt catalysts were prepared successfully and confirmed by infrared spectroscopy, XPS, XRD, and TEM images. The performance of polypyrimidine/multi-walled carbon nanotubes (MWCNTs)/Pt and polypyrimidine/double-walled carbon nanotubes (DWCNTs)/Pt was compared with the polypyrimidine/single-walled carbon nanotubes (SWCNTs)/Pt. The amount of the loaded Pt on the polypyrimidine/MWCNTs and polypyrimidine/DWCNTs was calculated to be 50.5 wt% and 52.7 wt% respectively. The effective specific surface area of the polypyrimidine/MWCNTs/Pt (45.7 m²/g) and polypyrimidine/DWCNTs/Pt (42.47 m²/g) was observed by electrochemical cyclic voltammetry. These studies strongly imply that the MWCNTs were better candidates than DWCNTs and SWCNTs in the application of polypyrimidine/CNT materials as catalyst for fuel cells.     

Effect of nickel catalyst on physicochemical properties of carbon xerogels as electrode materials for supercapacitor

Carbon xerogels and Ni-doped carbon xerogels prepared by the sol-gel polymerization were examined to reveal the effect of metallic nickel incorporated in carbon matrix on the physicochemical properties of carbon xerogels and their electrochemical performance for supercapacitor electrode in aqueous 6 M KOH solution. As shown by XRD and XPS measurements, the decomposition of nickel precursor in carbon matrix led to the creation of well-crystalline particles of metallic nickel which gave rise to the changes in the morphology, chemical and porous structure of carbon xerogels. Due to the modification of porous structure the surface area increased from 595 m²/g via 632 m²/g up to 660 m²/g for carbon xerogel, 7 wt% and 10 wt% Ni-doped composites, respectively. The enhancement of the surface area occurred along with diminishing the BJH average pore diameter. The value for nickel free xerogel amounted to 11.35 nm, whereas the value of 5.71 nm was measured for 10 wt% Ni xerogel. The changes in the porous and chemical structure created during the formation of carbon-nickel composites as well as the pseudo-capacitive effects arising from the redox reaction of nickel particles present in carbon matrix brought about a significant improvement of electrode capacitance. Electrochemical measurements showed that in comparison with capacitances measured for nickel free electrode (82.1 F/g calculated using the cyclic voltammetry and 88.8 F/g calculated using the galvanostatic charge/discharge method), the respective capacitances for 10 wt% Ni-doped carbon xerogel increased up to 103.0 F/g and 103.4 F/g. These values correspond to 25 and 16% improvement, respectively.     

电化学阴极沉积制备氧化镍/碳纳米管复合电极的准电容特性

采用催化裂解的方法制备了碳纳米管,其比容量为12F/g.采用碳纳米管作为电极基体,采用阴极电化学还原Ni(NO3)2的方法在碳纳米管基体表面均匀的沉积了纳米氧化镍颗粒并由此制备了氧化镍碳纳米管复合电极材料.采用循环伏安、恒流充放电、交流阻抗及扫描电镜等方法考察了复合电极材料的容量特性、阻抗特性、自放电特性以及电极表观特征.实验表明复合电极具有良好的电化学特性,碳纳米管基体在明显降低氧化镍材料的阻抗的同时还提高了电极材料的电化学容量并拓宽了电极材料的有效工作电位窗,复合电极在6mol/LKOH电解液中比容量达到25F/g且表现了良好的电化学可逆性.与碳纳米管基电容器相比,采用氧化镍复合电极材料组装的电容器具有较低的自放电率.     

Plasma-assisted nitrogen doping of VACNTs for efficiently enhancing the supercapacitor performance

Nitrogen doping of vertically aligned carbon nanotubes (VACNTs) using plasma-enhanced chemical vapour deposition has been investigated to improve the supercapacitance performance of CNTs. Incorporating electrochemical measurements on the open-ended nitrogen-doped CNTs, showed the achievement of 6 times improvement in the capacitance value. For nitrogen-doped CNTs on silicon substrate, specific capacitance of 60 F g^?1 was obtained in 0.5 M KCl solution, with capacity retention ratio above 90 % after cycled at 0.1 A g^?1 for 5000 cycles. Using this sample, a symmetric supercapacitance was fabricated which showed the power density of 37.5 kW kg^?1. The facile fabrication approach and its excellent capacitance improvement, propose it as an efficient technique for enhancing the supercapacitance performance of the carbon-based electrodes.     

3D porous NiMoO<Subscript>4</Subscript> nanoflakes arrays for advanced supercapacitor electrodes

In recent years, supercapacitors have been considered as one of the auspicious energy storage devices. In this work, two different kinds of mixed metal oxide NiMoO₄ nanoflakes arrays were directly grown on 3D Ni foam. The electrode exhibited high specific capacitance of 2004 F/g at the current density of 2 A/g in 6 M KOH electrolyte. Additionally, it also exhibited low equivalent series resistance of 0.62 Ω and excellent cycling stability (80% capacitance retention after 1000 cycles). With these extraordinary electrochemical properties, the electrode material can be considered as potential candidate for supercapacitor applications.     

Automatic dispersion,long-term stability of multi-walled carbon nanotubes in high concentration electrolytes

Nanoparticles have been known as the useful materials in working fluids for petroleum industry. But the stabilization of nano-scaled materials in water-based working fluids at high salinities is still a big challenge. In this study, we successfully prepared the anionic polymer/multi-walled carbon nanotubes (MWNTs) composites by covalently wrapping of MWNTs with poly (sodium 4-styrenesulfonate) (PSS) to improve the stability of MWNTs in high concentration electrolytes. The PSS/MWNTs composites can automatically disperse in salinity up to 15 wt% NaCl and API brines (8 wt% NaCl?+?2 wt% CaCl₂). Hydrodynamic diameters of composites were measured as a function of ionic strength and API brines by dynamic light scattering (DLS). By varying the concentration of brines, hydrodynamic diameter of PSS/MWNTs composites in brines fluctuated between 545?±?110 nm for 14 days and 673?±?171 nm for 30 days. Above results showed that PSS/MWNTs could be well stable in high salts solutions for a long period of time. After wrapped with PSS, the diameters of nanotubes changed from 30?~?40 to ~?430 nm, the thickness of wrapped polymer is about ~?400 nm by analysis of morphologies. The zeta potentials of PSS/MWNTs composites in various salinity of brines kept at approximately ??41?~???52 mV. Therefore, the well dispersion of PSS/MWNTs in high salinity is due to large negative charges of poly (sodium 4-styrenesulfonate), which provide enough electrostatic repulsion and steric repulsion to hinder compression of electric double layer caused by high concentration electrolytes.     

An investigation into the influence of zinc precursor on the microstructural,photoluminescence, and gas-sensing properties of ZnO nanoparticles

Carbon/silica-nanostructured microfibers were synthesized via electrospinning method using phenol-formaldehyde resin and tetraethyl orthosilicate as carbon and silica precursor with triblock copolymer Pluronic P123 as soft template. The prepared samples show uniform microfiber structure with ~1 μm in diameter and dozens of microns in length. Additionally, the mesopores in the material is about 2–6 nm. When the silica component was removed by HF, the porous carbon microfibers (PCMFs) were obtained. In addition, after the carbon/silica composites were calcined in air, the porous silica microfibers (PSiMFs) were obtained, revealing the converse porous nanostructure as PCMFs. It is a simple way to prepare PCMFs and PSiMFs with silica and carbon as the template to each other. Additionally, PCMFs possess an ultrahigh specific surface area (2,092 m² g^?1) and large pore volume. The electrochemical performance of the prepared PCMF material was investigated in 6.0 M KOH electrolyte. The PCMF electrode exhibits a high specific capacitance (252 F g^?1 at 0.5 A g^?1). Then, superior cycling stability (97 % retention after 4,000 cycles) mainly is due to its unique nanostructure.     

Bamboo-based activated carbon for supercapacitor applications

Bamboo-based activated carbon is synthesized by a simple heat treatment with or without KOH activation, and characterized for possible energy storage applications. The KOH activation introduces a very large surface area of more than 3000 m² g⁻¹ to the bamboo-based activated carbon, resulting in high specific capacitance, energy density, and power density in an aqueous electrolyte. The specific capacitance retention is more than 91% of the original capacitance after 3000 cycles, proving excellent cyclic stability for supercapacitor applications. Our results indicate that the natural resource of common bamboo could be an essential raw material for the energy storage devices.     

Capacitance properties of poly(3,4-ethylenedioxythiophene)/carbon nanotubes composites

The electrochemical properties of composites prepared from an electrically conducting polymer poly(3,4-ethylenedioxythiophene), i.e. PEDOT and multiwalled carbon nanotubes (CNTs) have been investigated for supercapacitor application. The novel composite material was prepared by chemical or electrochemical polymerization of EDOT directly on the nanotubes or from a homogenous mixture of PEDOT and CNTs. Acetylene black (AB) has been also used as a composite component in order to evaluate whether nanotubes are giving improved properties or not. Electrodes prepared from such composites were used in supercapacitors operating in acidic (1 M H₂SO₄), alkaline (6M KOH) and organic (1 M TEABF₄ in AN) electrolytic solutions. The capacitance values were estimated by galvanostatic, voltammetry and impedance spectroscopy techniques with two- or three-electrode cell configuration. Due to the open mesoporous network of nanotubes, the easily accessible electrode/electrolyte interface allows quick charge propagation in the composite material and an efficient reversible storage of energy in PEDOT during subsequent charging/discharging cycles. The composites with AB supply quite good capacitance results, however, nanotubes as electrode component gave definitively a more homogenous dispersion of PEDOT that should give a better charge propagation. The values of capacitance for PEDOT/carbon composites ranged from 60 to 160 F/g and such material has a good cycling performance with a high stability in all the electrolytes. Organic medium is especially interesting because of higher energy stored. Another quite important advantage of this composite is its significant volumetric energy because of the high density of PEDOT.     

Variation in chemical,colloidal and electrochemical properties of carbon nanotubes with the degree of carboxylation

Multiwalled carbon nanotubes (CNTs) were carboxylated via microwave irradiation where the treatment time was varied to alter the degree of functionalization, and as many as one in 15 carbons in the CNT could be oxidized. Chemical, physical, electrochemical, and colloidal behavior of the carboxylated CNTs was studied. All properties changed with the degree of functionalization to a point beyond which they appeared to remain constant. The surface area increased from 173.9 to 270.9 m²/g while the critical coagulation concentration (CCC) values increased from 142.14 to 168.69 mM in the presence of NaCl, and the corresponding increase was from 0.97 to 5.32 mM in the presence of MgCl₂. As seen from cyclic voltammetry curves, the functionalized CNTs showed mainly non-Faradic interactions with Na₂SO_4, but showed Faradic behaviors in alkaline KOH.     

Adhered Supported Carbon Nanotubes

Carbon nanotubes (NTs) in excess of 200 m long are grown by catalytic pyrolysis of hydrocarbon vapors. The nanotubes grow continuously without the typical extinction due to catalyst encapsulation. A woven metal mesh supports the nanotubes creating a metal supported nanotube (MSNT) structure. The 140 m wide mesh openings are completely filled by 70 nm diameter multiwalled nanotubes (MWNTs). The MWNTs are straight, uniform and highly crystalline. Their wall thickness is about 10 nm (30 graphite layers). The adherent NTs are not removed from the support in a Scotch tape pull test. A 12.5 cm² capacitor made from two MSNT structures immersed in 1 M KCl has a capacitance of 0.35 F and an equivalent series resistance of 0.18 . Water flows through the MSNT at a flow velocity of 1 cm/min with a pressure drop of 15 inches of water. With the support removed, the MWNTs naturally form a carbon nanocomposite (CNC) paper with a specific area of 80 m²/gm, a bulk density of 0.21 g/cm³, an open pore fraction of 0.81, and a resistivity of 0.16 -cm.     

Thermal transport of oil and polymer composites filled with carbon nanotubes

We studied the thermal transport properties of multi-walled carbon nanotubes (MWNTs) in polymer and oil matrices. The thermal conductivity of the oils and polymers increased linearly when adding tubes. We observe a particularly high increase in the thermal diffusivity of carbon-nanotube-loaded liquid crystal polymers (6×10⁻⁵ cm²/s wt%), which is due to a spontaneous alignment of the MWNTs. Carbon nanotubes increased the thermal conductivity of oil by a factor of three for 20 wt% loading. We found little or no dependence of the thermal enhancement on the specific flavor of multiwall nanotubes used in the composites. Carbon nanotubes are excellent nanoscale fillers for composites in thermal management application.