Li-air cells based on Li foil as an anode electrode, freestanding carbon nanotube/nanofiber mixed buckypaper as an air ͑cathode͒ electrode, and organic electrolyte were assembled. The air electrode was made with single-wall carbon nanotube ͑SWNT͒ and carbon nanofiber ͑CNF͒ without any binder. The discharge capacity was strongly dependent on both the discharge current density and the thickness of the air electrode. A discharge capacity as high as 2500 mAh/g was obtained for an air electrode at a thickness of 20 m with a discharge current density of 0.1 mA/cm 2 ; however, it was reduced to 400 mAh/g when the thickness of the air electrode was increased to 220 m. For a 66 m thick air electrode, the discharge capacity decreased from 1600 to 340 mAh/g when the discharge current density increased from 0.1 to 0.5 mA/cm 2. The scanning electron microscope images on surfaces of the air electrode from a fully discharged cell showed that the voids at the air side were almost fully filled by the solid deposition; however, the voids at the membrane side were still wide open.

A model for predication of the gravimetric and volumetric energy densities of Li-air batteries using aqueous electrolytes is developed. The theoretical gravimetric/volumetric capacities and energy densities are calculated based on the minimum weight of the electrolyte and volume of air electrode needed for completion of the electrochemical reaction with Li metal as an anode electrode. It was determined that both theoretical gravimetric/volumetric capacities and energy densities are dependent on the porosity of the air electrode. For instance, at a porosity of 70%, the maximum theoretical cell capacities are 435 mAh/g and 509 mAh/cm 3 in basic electrolyte, and 378 mAh/g and 452 mAh/cm 3 in acidic electrolyte. The maximum theoretical cell energy densities are 1300 Wh/kg and 1520 Wh/L in basic electrolyte, and 1400 Wh/kg and 1680 Wh/L in acidic electrolyte. The significant deduction of cell capacity from specific capacity of Li metal is due to the bulky weight requirement from the electrolyte and air electrode materials. In contrast, the Li-air battery using a nonaqueous electrolyte does not consume electrolyte during the discharge process and has high cell energy density. For Li-air batteries using both aqueous and nonaqueous electrolytes, the weight increases by 8-13% and the volume decreases by 8-20% after the cell is fully discharged.

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Carbon nanotubes (CNTs) demonstrate extraordinary properties and show great promise in enhancing out-of-plane properties of traditional polymer composites and enabling functionality, but current manufacturing challenges hinder the realization of their potential. This paper presents a method to fabricate multifunctional multiscale composites through an effective infiltration-based vacuum-assisted resin transfer moulding (VARTM) process. Multi-walled carbon nanotubes (MWNTs) were infused through and between glass-fibre tows along the through-thickness direction. Both pristine and functionalized MWNTs were used in fabricating multiscale glass-fibre-reinforced epoxy composites. It was demonstrated that the mechanical properties of multiscale composites were remarkably enhanced, especially in the functionalized MWNT multiscale composites. With only 1 wt% loading of functionalized MWNTs, tensile strength was increased by 14% and Young’s modulus by 20%, in comparison with conventional fibre-reinforced composites. Moreover, the shear strength and short-beam modulus were increased by 5% and 8%, respectively, indicating the improved inter-laminar properties. The strain–stress tests also suggested noticeable enhancement in toughness. Scanning electron microscopy (SEM) characterization confirmed an enhanced interfacial bonding when functionalized MWNTs were integrated into epoxy/glass-fibre composites. The coefficient thermal expansion (CTE) of functionalized nanocomposites indicated a reduction of 25.2% compared with epoxy/glass-fibre composites. The desired improvement of electrical conductivities was also achieved. The multiscale composites indicated a way to leverage the benefits of CNTs and opened up new opportunities for high-performance multifunctional multiscale composites.

Air electrodes, made with a mixture of carbon nanotube (CNT)/carbon nanofiber (CNF) and with/without α-MnO2 nano-rods, were prepared for Li-air cells. The charge capacity and cyclability were found to increase largely for the cells made with the α-MnO2 catalyst; however, the first cycle discharge capacities were no different for the cells made with and without the α-MnO2 catalyst. It was found that the discharge capacity of the Li-air cell was mainly due to oxygen deficiency from the pinch-off of the diffusion channel by the deposition product at the air side of the air electrode. Electrochemical impedance spectra at different cycles demonstrated that the charge transfer resistance was increased and decreased during discharge and charge processes, respectively, due to the change of porosity, oxygen concentration, and rate of coefficient of chemical reaction in the air electrode.

wileyonlinelibrary.comsuch as transistor, [ 5 ] triboelectric, [ 6 ] capacitive, [ 7,8 ] piezoelectric, [9][10][11] and piezoresistive properties.Piezoresistive pressure sensors, which transform an input force into an electrical signal caused by the change in the resistance, have attracted considerable attentions by virtue of its simplicity and low cost in design and implementation. Most fl exible piezoresistive sensors are prepared by loading conductive nanomaterials (e.g., carbon nanotubes (CNTs), [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] graphene, [29][30][31][32] nanowires, [33][34][35] nanoparticles) onto fl exible substrates (e.g., fi bers, [ 12,13 ] fi lms, [14][15][16][17] opencell foams [ 29 ] ) via a number of processing methods, such as blending, [ 19,20 ] coating, [ 21,29 ] and printing. [ 17 ] Among the different conductive nanomaterials, carbon nanotubes have attracted a considerable amount of attention due to their remarkably high piezoresistive sensitivity. [ 36,37 ] In addition to the nanomaterials, which are the active sensing elements, the properties of the substrates also play a key role in determining the overall sensor performance. [ 27,28 ] Most studies on the effects of the substrates focus on the modulus, and it has been suggested that porous substrates with reduced elastic modulus result in increased sensing properties. [ 19 ] Yet from the classical mechanics point of view, the other most fundamental property that dictates the elastic properties is the Poisson ratio, which is defi ned as the ratio of the lateral contractile strain to the longitudinal tensile strain for a material undergoing tension in the longitudinal direction. Collectively, they defi ne the elastic properties and deformation characteristics of the materials in a 3D space. Conceivably, the Poisson ratio would impact the sensing performance of piezoresistive sensors; however, this effect has not been studied.Classical mechanics predicts that for isotropic materials, the Poisson ratio lies between -1 and 0.5, a fairly small range. [ 38 ] With a few exceptions such as α-cristobalite, [ 39 ] certain cubic metal, [ 40 ] and few biological tissues, [ 41 ] the range of Poisson ratio of almost all natural or synthetic materials is even smaller, typically 0.3-0.5. [ 42 ] Research on fabrication of auxetic materials or materials with negative Poisson ratios has progressed steadily since the initial report by Lakes [ 43 ] on the possibility of such materials. [ 44,45 ] The performance of fl exible and stretchable sensors relies on the optimization of both the fl exible substrate and the sensing element, and their synergistic interactions. Herein, a novel strategy is reported for cost-effective and scalable manufacturing of a new class of porous materials as 3D fl exible and stretchable piezoresistive sensors, by assembling carbon nanotubes onto porous substrates of tunable Poisson ratios. It is shown that the piezoresistive sensitivity of the sensors increases as the substrate's Poisson's ratio decrease...

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Preformed carbon nanotube thin films (10-20 microm), or buckypapers (BPs), consist of dense and entangled nanotube networks, which demonstrate high electrical conductivity and provide potential lightweight electromagnetic interference (EMI) solutions for composite structures. Nanocomposite laminates consisting of various proportions of single-walled and multi-walled carbon nanotubes, having different conductivity, and with different stacking structures, were studied. Single-layer BP composites showed shielding effectiveness (SE) of 20-60 dB, depending on the BP conductivity within a 2-18 GHz frequency range. The effects on EMI SE performance of composite laminate structures made with BPs of different conductivity values and epoxy or polyethylene insulating layer stacking sequences were studied. The results were also compared against the predictions from a modified EMI SE model. The predicted trends of SE value and frequency dependence were consistent with the experimental results, revealing that adjusting the number of BP layers and appropriate arrangement of the BP conducting layers and insulators can increase the EMI SE from 45 dB to close to 100 dB owing to the utilization of the double-shielding effect.