Catalytic performance of acid-treated multi-walled carbon nanotube-supported platinum catalyst for PEM fuel cells

Effects of the surface functional groups of multi-walled carbon nanotubes (MWCNTs) as catalyst support for the durability of polymer electrolyte membrane fuel cells (PEMFCs) were examined at high Pt loading conditions. The amount of oxygen functional groups on the MWCNTs surface was increased as the acid treatment time and temperature increased. We found that more functional groups in MWCNTs improved initial Pt dispersion but deteriorated durability due to the reduced carbon corrosion resistance. The experimental results also showed that despite the surface oxidation, Pt/MWCNT catalysts showed highly improved durability than Pt/C catalysts due to the graphitic nature of MWCNTs. Membrane electrode assembly (MEA) fabricated by 4 h acid treated MWCNTs at 25 °C showed 4 times better durability than commercial Pt/C based MEA at the reverse potential operation generated by fuel starvation conditions. We believe that MWCNTs can be effectively used for PEMFCs even at high loading due to their excellent anti-corrosion properties.     

Effect of oxyfluorinated multi-walled carbon nanotube additives on positive temperature coefficient/negative temperature coefficient behavior in high-density polyethylene polymeric switches

Multi-walled carbon nanotubes (MWCNTs) were embedded into high-density polyethylene (HDPE) to improve the electrical properties of HDPE polymeric switches. The MWCNT surfaces were modified by oxyfluorination to improve their positive temperature coefficient (PTC) and negative temperature coefficient (NTC) behaviors in HDPE polymeric switches. HDPE polymeric switches exhibit poor electron mobility between MWCNT particles when the number of oxygen functional groups is increased by oxyfluorination. Thus, the PTC intensity of HDPE polymeric switches was increased by the destruction of the electrical conductivity network. The oxyfluorination of MWCNTs also leads to weak NTC behavior in the MWCNT-filled HDPE polymeric switches. This result is attributed to the reduction of the mutual attraction between the MWCNT particles at the melting temperature of HDPE, which results from a decrease in the surface free energy of the C-F bond in MWCNT particles.     

碳纳米管的表面改性对环氧树脂低温(77 K)冲击性能及热膨胀系数的影响

采用混酸氧化及表面接枝改性的方法制备了表面含不同官能团的多壁碳纳米管(MWCNTs), 并研究了不同MWCNTs对环氧树脂的低温(77 K)抗冲击性能及热膨胀系数(CTE)的影响。结果表明: 通过接枝反应将—NCO基团封端的PEO齐聚物引入MWCNTs表面, 可提高MWCNTs在环氧树脂基体中的分散性, 加强MWCNTs与环氧树脂的界面作用; 相对于纯环氧树脂, 添加质量分数为0.5%的纯MWCNTs、 氧化MWCNTs和表面接枝MWCNTs改性后的环氧树脂的低温冲击强度分别升高了10.27%、 26.13%和32.95%, 而CTE则分别降低了14.79%、 29.59%和40.29%。这表明表面接枝改性MWCNTs可明显提高环氧树脂基体的低温抗冲击性能并降低环氧树脂在玻璃化转变温度下的CTE。     

Rheological and mechanical properties of surface modified multi-walled carbon nanotube-filled PET composite

Poly(ethylene terephthalate) (PET)/multi-walled carbon nanotube (MWCNT) composites were prepared by in situ polymerization. To improve the dispersion of MWCNTs in PET matrix, the surface modified MWCNTs having acid groups (acid-MWCNT) and diamine groups (diamine-MWCNT) were used. The functional groups on the surface of modified MWCNTs were confirmed by infrared (IR) spectrometry. SEM analysis showed better dispersion of diamine-MWCNTs as compared to pristine-MWCNTs and acid-MWCNTs in the PET. The reaction between PET and diamine-MWCNTs was evidenced by the shifting of the G band to a higher frequency in Raman spectroscopy and an increase of the complex viscosity in rheological properties. The composites containing functionalized MWCNTs showed a large increase in the tensile strength and modulus. The PET/diamine-MWCNT composites showed maximum tensile strength and modulus increases by 350% and 290% at 0.5 and 2.0 wt%, respectively, as compared to pure PET.     

Functionalization of textiles with multi-walled carbon nanotubes by a novel dyeing-like process

Multi-walled carbon nanotubes (MWCNTs) were functionalized with oxygen-containing surface groups and subsequently incorporated in cotton and polyester fabrics by a process that mimics the traditional industrial dyeing process. The washing fastness, hydrophobicity and flame retardancy of the functional textiles were evaluated. The MWCNTs surface chemistry was modified by three different routes: (i) liquid phase oxidation with nitric acid, in order to introduce acidic oxygen-containing groups, (ii) thermal treatment of the sample oxidized in (i), in order to remove the carboxylic acid functionalities and (iii) gas phase oxidation with 5% oxygen in nitrogen to incorporate basic and neutral groups. All samples were characterized by temperature programmed desorption, pH at the point of zero charge and N₂ adsorption–desorption isotherms at −196 °C. The effect of the MWCNTs acidity/basicity and of the type of substrate in the nanomaterials incorporation efficiencies and in the performance of the final textile materials was assessed. The scanning electron microscopy images and the whiteness degree values of the functional textiles before and after washing indicated that the incorporation efficiency was higher for the textiles containing the most acidic MWCNTs, especially for the polyester textiles. The immobilization of the less acidic MWCNTs in polyester imparted hydrophobic properties to the fabrics surface; in particular, the polyester samples functionalized with unmodified and O₂-oxidized MWCNTs presented an almost superhydrophobic behaviour. In the case of the cotton-based samples, a hydrophobic behaviour was not achieved. Finally, the flame-retardant properties of both substrates improved upon the MWCNTs immobilization.     

聚乙烯/马来酸酐接枝聚合物修饰多壁碳纳米管

多壁碳纳米管(MWCNTs)与混酸(浓硫酸∶浓硝酸体积比为3∶1)和无水乙二胺进行酸化、胺化反应使MWCNTs表面产生羧基和氨基基团,进而与聚乙烯/马来酸酐接枝聚合物(PE-g-MA)发生开环反应制备PE-g-MWCNTs,以提高MWCNTs在聚乙烯基体中的分散性。采用红外光谱、X-射线光电子能谱(XPS)和拉曼光谱对MWCNTs的化学修饰进行定性表征。结果表明:当MWCNTs的体积分数为0.67%时,MWCNT/PE复合材料的体积电阻率发生渗流突变。MWCNTs的体积分数在0.1%～1.2%时,MWCNT/PE复合材料体积电阻率由1016Ω.m下降至105Ω.m。     

Preparation,characterization and properties of polycaprolactone diol-functionalized multi-walled carbon nanotube/thermoplastic polyurethane composite

Multi-walled carbon nanotubes (MWCNTs) were chemically functionalized to prepare thermoplastic polyurethane (PU) composites with enhanced properties. In order to achieve a high compatibility of functionalized MWCNTs with the PU matrix, polycaprolactone diol (PCL), as one of PU’s monomers, was selectively grafted on the surface of MWCNTs (MWCNT–PCL), while carboxylic acid groups functionalized MWCNTs (MWCNT–COOH) and raw MWCNTs served as control. Both MWCNT–COOH and MWCNT–PCL improved the dispersion of MWCNTs in the PU matrix and interfacial bonding between them at 1 wt% loading fraction. The MWCNT–PCL/PU composite showed the greatest extent of improvement, where the tensile strength and modulus were 51.2% and 33.5% higher than those of pure PU respectively, without sacrificing the elongation at break. The considerable improvement in both mechanical properties and thermal stability of MWCNT–PCL/PU composite should result from the homogeneous dispersion of MWCNT–PCL in the PU matrix and strong interfacial bonding between them.     

Characteristics of dye-sensitized solar cells with surface-modified multi-walled carbon nanotubes as counter electrodes

A polystyrene-based functional block copolymer is employed as a surface modifier for multi-walled carbon nanotube (MWCNT) paste utilized in the fabrication of a MWCNT counter electrode (CE) in dye-sensitized solar cells (DSSCs). The surface modification of MWCNTs paste improves the dispersibility of MWCNTs, resulting in a facilitated fabrication of electrodes through the screen printing procedure, as evidenced by a lower viscosity and more homogeneous paste, as well as a more uniform MWCNT coating. Upon removing organic compounds from the paste through a thermal treatment procedure, the DSSC with the modified CE exhibits enhanced solar energy conversion efficiency (η) compared with that of the neat MWCNT CE. The behavior stems from an improvement in the overall redox reaction kinetics and the short-circuit current (J _sc) of the DSSC. The DSSC also exhibits an improved η value over an extended storage period, which demonstrates a successful combination of processability, performance, and stability of the DSSC achieved by using an optimum amount of surface modifier for MWCNTs.     

Polyelectrolyte functionalized carbon nanotubes as a support for noble metal electrocatalysts and their activity for methanol oxidation

A highly effective polyelectrolyte functionalization of multi-walled carbon nanotubes (MWCNTs) by poly(diallyldimethylammonium chloride) (PDDA-MWCNTs) was employed for low temperature fuel cell applications. PDDA-MWCNTs were employed as support materials for the in?situ deposition and formation of platinum nanoparticles, via the self-assembly between the negative Pt precursor and positively charged functional groups of PDDA-functionalized MWCNTs. The effect of the functionalization on the deposition and distribution of Pt nanoparticles was investigated in detail. Compared with MWCNTs functionalized by conventional acid-oxidation treatment (AO-MWCNTs), the PDDA-functionalized MWCNTs cause no structural damage on MWCNTs and provide high density and homogeneous surface functional groups for the anchoring Pt nanoparticles. Pt nanoparticles with an average particle size of 1.8 ± 0.4?nm and loading as high as 60?wt% were realized on PDDA-MWCNTs supports. The Pt/PDDA-MWCNTs electrocatalysts show significantly higher electrochemically active surface area and higher electro-catalytic activity for methanol oxidation than that of Pt/AO-MWCNTs and E-TEK Pt/C electrocatalysts.     

The effect of acidic treatment on the lithium storage capacity of multi-walled carbon nanotubes

In this research, multi-walled carbon nanotubes (MWCNTs) were modified by nitric acidic treatment to improve their electrochemical performance. The electrochemical performance of MWCNTs was evaluated by charge and discharge cycles. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectrum analysis techniques were used to characterize the samples. The closed tip or cap structures of pristine MWCNT were opened and the nanotubes were chopped after acidic treatment, which is beneficial to improve the lithium ion insertion/extraction into/from MWCNTs and lithium storage capacity. The graphite crystallinity of acid treated MWCNTs reduced slightly and disordered carbon structures were introduced at the surface of MWCNTs, which led to the large initial irreversible capacity.     

聚丙烯腈基碳纳米管复合材料的制备及其表征

碳纳米管添加到聚合物中对其结构和性能都有深远的影响,本文通过浓硝酸对碳纳米管改性后,采用水相沉淀聚合的方法制备了聚丙烯腈/碳纳米管复合材料,同时研究了碳纳米管经过浓硝酸处理后其化学结构的变化;探讨了碳纳米管对聚丙烯腈复合材料热学和结晶性的影响。研究表明,浓硝酸常温处理不仅能除去杂质,还可以在碳纳米管表面引入羧基等含氧基团;加入碳纳米管后,聚丙烯腈的预氧化温度有一定程度的提前,放热量明显降低,同时对聚合物的结晶度也有一定程度的影响。     

Role of iron oxide impurities in electrocatalysis by multiwall carbon nanotubes: An investigation using a novel magnetically modified ITO electrodes

The role of iron oxide impurities in the electrocatalytic properties of multiwall carbon nanotubes (MWCNTs) prepared by catalytic chemical vapour decomposition method (CCVD) is studied in detail. A novel magnetically modified electrodes have been developed by which MWCNTs were immobilized on indium-tin oxide (ITO) electrodes, without any chemical binders. The electro-catalytic oxidation of dopamine, and reduction of hydrogen peroxide have been studied by cyclic voltammetry on magnetically modified electrodes with (i) MWCNTs with occluded iron oxide impurities (Fe-MWCNTs), (ii) MWCNTs grown on iron oxide nanoparticle particulate films (Io-MWCNTs) and (iii) pristine iron oxide nanoparticle particulate film (Io-NPs). A shift towards less positive potentials for the oxidation of dopamine was observed which is in the order of Fe-MWCNTs < Io-MWCNTs < Io-NPs. Similarly, trend towards less negative potentials for the reduction of hydrogen peroxide was observed. Thus, the electrocatalytic activities displayed by MWCNTs have been attributed to the iron oxide impurities associated with it. The systematic variation was related to the nature of interaction of iron oxide nanoparticles with MWCNT surface.     

Osteoblast cell response to surface-modified carbon nanotubes

In order to investigate the interaction of cells with modified multi-walled carbon nanotubes (MWCNTs) for their potential biomedical applications, the MWCNTs were chemically modified with carboxylic acid groups (–COOH), polyvinyl alcohol (PVA) polymer and biomimetic apatite on their surfaces. Additionally, human osteoblast MG-63 cells were cultured in the presence of the surface-modified MWCNTs. The metabolic activities of osteoblastic cells, cell proliferation properties, as well as cell morphology were studied. The surface modification of MWCNTs with biomimetic apatite exhibited a significant increase in the cell viability of osteoblasts, up to 67.23%. In the proliferation phases, there were many more cells in the biomimetic apatite-modified MWCNT samples than in the MWCNTs–COOH. There were no obvious changes in cell morphology in osteoblastic MG-63 cells cultured in the presence of these chemically-modified MWCNTs. The surface modification of MWCNTs with apatite achieves an effective enhancement of their biocompatibility.     

Growth of spin-capable multi-walled carbon nanotubes and flexible transparent sheet films

Iron-catalyzed spin-capable multi-walled carbon nanotubes (MWCNTs) were grown on a SiO2 wafer by chemical vapor deposition, which was carried out at 780 degrees C using C2H2 and H2 gases. We fabricated a flexible transparent film using the spun MWCNTs. The MWCNT sheets were produced by being continuously pulled out from well-aligned MWCNTs grown on a substrate. The MWCNT sheet films were manufactured by simply carrying out direct coating on a flexible film or glass. The thickness of the sheet film decreased remarkably when alcohol was sprayed on the surface of the sheet. The alcohol spraying increased the transmittance and decreased the electrical resistance of the MWCNT sheet films. The sheets obtained after alcohol spraying had a resistance of -699 omega and a transmittance of 81%-85%. The MWCNT sheet films were heated by applying direct current. The transparent heaters showed a rapid thermal response and uniform distribution of temperature. In addition, we tested the field emission of the sheet films. The sheet films showed a turn-on voltage of -1.45 V/microm during field emission.     

Corrugation of the external surface of multiwall carbon nanotubes by catalytic oxidative etching and its effect on their decoration with metal nanoparticles

It was established that partial combustion of carbon constituting the walls of multiwall carbon nanotubes (MWCNTs) catalyzed by previously deposited CaCO₃ nanoparticles converts parallel graphene layers in a multiwall structure to aggregates formed by nano-onions with a diameter of 5–12 nm. The areas with positive curvature of graphene layers on the external surface of air-etched MWCNTs played the role of docking stations for nickel nanoparticles inserted by sonochemical deposition after removal of the CaCO₃. The nickel nanoparticles were located exclusively at the tops of the onions. Formation of nanoscale curvature at the MWCNT support surfaces decreased the average size of Ni nanocrystals at similar loading of 50–60 wt% from 8 to 2 nm. Partial catalytic combustion did not change the concentration of surface carbonyl groups measured by titration, which attributes the observed phenomena directly to the corrugation of the MWCNT surfaces. The catalytic tests revealed a significant increase of catalytic activity of supported Ni catalyst due to corrugating of the external surface of the MWCNT support. After oxidative etching of the MWCNTs, the rate of chloroacetophenone hydrogenation measured with a Ni–MWCNT catalyst increased by a factor of 2 without change in selectivity yielding chlorophenylethanol as the main product.     

Effect of surface functionalization on mechanical properties and decomposition kinetics of high performance polyetherimide/MWCNT nano composites

In this investigation, Polyetherimide (PEI) reinforced with multi-walled carbon nanotube (MWCNT) using novel melt blending technique. Surface of MWCNTs are modified by acid treatment as well as by plasma treatment. PEI nano composites with 2 wt% treated MWCNT shows about 15% improvement in mechanical properties when compared to unfilled PEI. The thermal decomposition kinetics of PEI/MWCNT nano composites has been critically analyzed by using Coats – Redfern model. The increase in activation energy for thermal degradation by 699 kJ/mol for 2 wt% MWCNT implies improvement in the thermal properties of PEI. Studies under Fourier Transform Infrared Spectroscopy (FTIR) and Transmission Electron Microscopy (TEM) depict significant interfacial adhesion with uniform dispersion of MWCNT in polymer matrix due to surface functionalization. 0.5 wt% chemically modified MWCNT shows typical alignment of MWCNT. There is a significant improvement in mechanical properties and thermal properties for surface functionalized MWCNT reinforced.     

Effective load transfer by a chromium carbide nanostructure in a multi-walled carbon nanotube/copper matrix composite

Multi-walled carbon nanotube (MWCNT) reinforced copper (Cu) matrix composites, which exhibit chromium (Cr) carbide nanostructures at the MWCNT/Cu interface, were prepared through a carbide formation using CuCr alloy powder. The fully densified and oriented MWCNTs dispersed throughout the composites were prepared using spark plasma sintering (SPS) followed by hot extrusion. The tensile strengths of the MWCNT/CuCr composites increased with increasing MWCNTs content, while the tensile strength of MWCNT/Cu composite decreased from that of monolithic Cu. The enhanced tensile strength of the MWCNT/CuCr composites is a result of possible load-transfer mechanisms of the interfacial Cr carbide nanostructures. The multi-wall failure of MWCNTs observed in the fracture surface of the MWCNT/CuCr composites indicates an improvement in the load-bearing capacity of the MWCNTs. This result shows that the Cr carbide nanostructures effectively transferred the tensile load to the MWCNTs during fracture through carbide nanostructure formation in the MWCNT/Cu composite.     

Induced NH<Subscript>2</Subscript> bonding of carbon nanotubes using NH<Subscript>3</Subscript> plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition was used to modify the multiwall carbon nanotubes (MWCNTs) using ammonia (NH₃) plasma. For various durations of NH₃ plasma treatment, a scanning electron microscope, X-ray, Raman spectroscopy and contact angle measurement were used to ascertain several characteristics of the MWCNTs. The experimental results show that: (1) the length of the MWCNTs is reduced, if the duration of the plasma treatment is increased; (2) the NH₃ plasma treatment can incorporate amine (NH₂ ⁻) or amino (NH⁻) functional groups onto the MWCNT surface; (3) the plasma treated carbon nanotubes become more hydrophilic.     

多壁碳纳米管的辐照处理及其对多壁碳纳米管/环氧树脂复合材料热性能的影响

在空气中用高频高压电子加速器辐照多壁碳纳米管(MWCNTs),采用红外光谱、能谱分析、拉曼光谱和透射电镜表征分析辐照处理对碳纳米管结构的影响;通过原位复合法制备MWCNT/环氧树脂(EP)复合材料.采用场发射扫描电镜、热失重分析和动态力学分析研究辐照处理MWCNTs对环氧树脂热稳定性的影响.结果表明:电子束辐照处理使MWCNTs表面接入了少量的含氧基团,同时破坏了MWCNTs的完整结构,当辐照剂量为170 kGy时,接枝含氧基团的量最多(约为4％),且结构破坏程度较小.与原始MWCNT/EP体系相比,经电子束辐照处理后的MWCNTs在EP中分散得更均匀,并能使材料的最大热分解温度和玻璃化转变温度较纯EP有所提高,在EP中加入质量分数0.5％的经170kGy辐照处理后的MWCNTs,能够使材料的最大热分解温度和玻璃化转变温度分别提高约14℃和8℃.     

Carbon nanotube modification using gum arabic and its effect on the dispersion and tensile properties of carbon nanotubes/epoxy nanocomposites

In this study, the effects of a MWCNT treatment on the dispersion of MWCNTs in aqueous solution and the tensile properties of MWCNT/epoxy nanocomposites were investigated. MWCNTs were treated using acid and gum arabic, and MWCNT/epoxy nanocomposites were fabricated with 0.3 wt.% unmodified, oxidized and gum-treated MWCNTs. The dispersion states of the unmodified, oxidized, and Gum-treated MWCNTs were characterized in distilled water. The tensile strengths and elastic modulus of the three nanocomposites were determined and compared. The results indicated that the gum treatment produced better dispersion of the MWCNTs in distilled water and that gum-treated MWCNT/epoxy nanocomposites had a better tensile strength and elastic modulus than did the unmodified and acid-treated MWCNT/epoxy nanocomposites. Scanning electron microscope examination of the fracture surface showed that the improved tensile properties of the gum-treated MWCNT/epoxy nanocomposites were attributed to the improved dispersion of MWCNTs in the epoxy and to interfacial bonding between nanotubes and the epoxy matrix.