In this work we present a combined electrochemical and mechanical study of mesoporous electrodes based on CNT fibres in the context of electric double layer capacitors. We show that through control of the synthetic and assembly processes of the fibres, it is possible to obtain an active material that combines a surface area of 250 m(2) g(-1), high electrical conductivity (3.5 × 10(5) S m(-1)) and mechanical properties in the high-performance range including toughness (35 J g(-1)) comparable to that of aramid fibre (e.g. Kevlar). These properties are a consequence of the predominant orientation of the CNTs, observed by wide- and small-angle X-ray diffraction, and to the exceptionally long CNT length on the millimetre scale. Cyclic voltammetry measurements in a three-electrode configuration and using 1-butyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14TFSI) ionic liquid electrolyte, show that the CNT fibres have a large quantum capacitance, evidenced by the near linear dependence of geometric capacitance (and conductivity) on potential bias. This reflects the low dimensionality of the CNT building blocks, which were purposely synthesised to have 1-5 layers and a high degree of graphitization. From the charge-discharge measurements of supercapacitor devices with symmetric CNT fibre electrodes we obtain power and energy densities as high as 58 kW kg(-1) and 14 Wh kg(-1), respectively. These record-high values for CNT fibre-based supercapacitors, are a consequence of the low equivalent series resistance due to the high conductivity of the fibres, the large contribution from quantum capacitance, and the wide stability window of the ionic liquid (3.5 V). Cycle life experiments demonstrate stable capacitance and energy retention over 10,000 cycles of charge-discharge at 3.5 V.

This work presents a method to produce structural composites capable of energy storage. They are produced by integrating thin sandwich structures of CNT fiber veils and an ionic liquid-based polymer electrolyte between carbon fiber plies, followed by infusion and curing of an epoxy resin. The resulting structure behaves simultaneously as an electric double-layer capacitor and a structural composite, with flexural modulus of 60 GPa and flexural strength of 153 MPa, combined with 88 mF/g of specific capacitance and the highest power (30 W/kg) and energy (37.5 mWh/kg) densities reported so far for structural supercapacitors. In-situ electrochemical measurements during 4-point bending show that electrochemical performance is retained up to fracture, with minor changes in equivalent series resistance for interleaves under compressive stress. En route to improving interlaminar properties we produce grid-shaped interleaves that enable mechanical interconnection of plies by the stiff epoxy. Synchrotron 3D X-ray tomography analysis of the resulting hierarchical structure confirms the formation of interlaminar epoxy joints. The manuscript discusses encapsulation role of epoxy, demonstrated by charge-discharge measurements of composites immersed in water, a deleterious agent for ionic liquids. Finally, we show different architectures free of current collector and electrical insulators, in which both CNT fiber and CF act as active electrodes.

This work presents a scalable method to produce robust all-solid electric double layer capacitors (EDLCs), compatible with roll-to-roll processes and structural laminate composite fabrication. It consists in sandwiching and pressing an ionic liquid (IL) based polymer electrolyte membrane between two CNT fiber sheet electrodes at room temperature, and laminating with ordinary plastic film. This fabrication method is demonstrated by assembling large area devices of up to 100 cm 2 with electrodes fabricated in-house, as well as with commercial CNT fiber sheets. Free-standing flexible devices operating at 3.5 V exhibited 28 F g -1 of specific capacitance, 11.4 Wh kg -1 of energy density and 46 kW kg -1 of power density. These values are nearly identical to control samples with pure ionic liquid. The solid EDLC could be repeatedly bent and folded 180° without degradation of their properties, with a reversible 25% increase in energy density in the bent state. Devices produced using CNT

The assembly of aligned carbon nanotubes (CNTs) into fibers (CNTFs) is a convenient approach to exploit and apply the unique physico-chemical properties of CNTs in many fields. CNT functionalization has been extensively used for its implementation into composites and devices. However, CNTF functionalization is still in its infancy because of the challenges associated with preservation of CNTF morphology. Here, we report a thorough study of the gas-phase functionalization of CNTF assemblies using ozone which was generated in situ from a UV source. In contrast with liquid-based oxidation methods, this gas-phase approach preserves CNTF morphology, while notably increasing its hydrophilicity. The functionalized material is thoroughly characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy. Its newly acquired hydrophilicity enables CNTF electrochemical characterization in aqueous media, which was not possible for the pristine material. Through comparison of electrochemical measurements in aqueous electrolytes and ionic liquids, we decouple the effects of functionalization on pseudocapacitive reactions and quantum capacitance. The functionalized CNTF assembly is successfully used as an active material and a current collector in all-solid supercapacitor flexible devices with an ionic liquid-based polymer electrolyte.

Supercapacitors capable of providing high voltage, energy and power density but yet light, low volume occupying, flexible and mechanically robust are highly interesting and demanded for portable applications. Herein, freestanding flexible hybrid electrodes based on MnO2 nanoparticles grown on macroscopic carbon nanotube fibers (CNTf-MnO2) were fabricated, without the need of any metallic current collector. The CNTf, a support with excellent electrical conductivity, mechanical stability, and appropriate pore structure, was homogeneously decorated with porous akhtenskite ɛ-MnO2 nanoparticles produced via electrodeposition in an optimized organic-aqueous mixture. Electrochemical properties of these decorated fibers were evaluated in different electrolytes including a neutral aqueous solution and a pure 1-butyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (PYR14TFSI). This comparison helps discriminate the various contributions to the total capacitance: (surface) Faradaic and non-Faradaic processes, improved wetting by aqueous electrolytes. Accordingly, symmetric supercapacitors with PYR14TFSI led to a high specific energy of 36 Wh·kg MnO 2 −1 (16 Wh·kg -1 including the weight of CNTf) and real specific power of 17 kW·kg MnO 2 −1 (7.5 kW·kg -1 ) at 3.0 V with excellent cycling stability. Moreover, flexible all solid-state supercapacitors were fabricated using PYR14TFSIbased polymer electrolyte, exhibiting maximum energy density of 21 Wh·kg -1 and maximum power density of 8 kW·kg -1 normalized by total active material.

Chemical functionalization of nanocarbons is an important strategy to produce electrochemical systems with higher energy/power density by generating surface functional groups with additional faradaic contribution, by increasing their surface area and correspondent capacitive contribution and by improving compatibility with aqueous electrolytes and other active materials, such as pseudocapacitive metal-oxides. Here we present an electrochemical method to simultaneously swell and functionalize large electrodes consisting of fabrics of macroscopic fibers of carbon nanotubes that renders the material hydrophilic and produces a substantial increase of specific capacitance and energy density in aqueous electrolytes. Through in-depth characterization of the carbon nanotube fibres (CNTF) by Raman spectroscopy, transmission electron microscopy, X-ray photoelectrocn spectroscopy (XPS) and small-angle X-ray scattering (SAXS) we identify various contributions to such improvements, including surface oxidation, tubular unzipping, debundling and inter-bundle swelling. Changes in hydrophilicity of functionalized CNTF are determined by analyzing the dynamics of spreading of polar and nonpolar liquids in the electrode. The extracted contact angles and polar and dispersive surface energy components for different treatment conditions are in agreement with changes in dipole-moment obtained by XPS. Finally, functionalized CNTF electrodes were employed in current collector-free solid flexible supercapacitors, which show enhanced electrochemical properties compared to as-produced hydrophobic ones. 1. INTRODUCTION The superlative properties of nanocarbons continue to fuel the interest in macroscopic architectures that efficiently exploit their "molecular" properties. The exceptional stiffness, charge mobility and electrochemical stability of CNTs, for example, makes them attractive for applications ranging from lightweight composites, to electrochemical charge storage/transfer processes to biomedical applications. 123 In this quest, macroscopic CNT fibres, yarns and fabrics have emerged as attractive systems in which the CNTs associate in long coherent bundle domains that favor inter-tube charge and stress transfer, while also leaving large mesoporous gaps between bundles and thus giving rise to a large porosity. Such structure leads to an unusual combination of bulk mechanical toughness, electrical conductivity and electrochemical stability above that of many metals, combined with a large specific surface area above 250 m 2 /g. These property envelop is partly the reason of the increasing use of CNT fibres as electrodes/current collectors in multiple energy storage, transfer and conversion devices, including those requiring light weight and augmented mechanical properties 4 .

The pore structure and surface area of CNT-based electrodes is analysed by SAXS/WAXS, including under eletrochemical measurements.

Macroscopic fibres of carbon nanotubes are hierarchical structures combining long building blocks preferentially oriented along the fibre axis and a large porosity arising from the imperfect packing of bundles. Synchrotron small-angle X-ray scattering SAXS measurements show that such structure is a surface fractal with fractal dimension (Ds) of 2.5 for MWCNT fibres and 2.8 for SWCNT fibres. N2 adsorption measurements give similar values of 2.54 and 2.50, respectively. The fractal dimension and deviation from Porod's law are related to density fluctuations associated with the wide distribution of separations between CNTs. These fluctuations are also evident as diffuse wide-angle X-ray scattering (WAXS) from CNTs at distances above (Juan J. Vilatela)2 turbostratic separation. The structure of CNT fibres produced at different draw ratios is compared in terms of degree of orientation and characteristic lengths parallel and perpendicular to the fibre.Drawing not only increases alignment but also the fraction of graphitic planes forming coherent domains capable of taking part in stress transfer by shear; thus increasing both tensile modulus and strength. The invariant-normalized intensity of the (002) equatorial reflection thus takes the form of a degree of crystallinity closely related to tensile properties.