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Macroscopic ensembles of nanocarbons, such as fibres of carbon nanotubes (CNT), are characterised by a complex hierarchical structure combining coherent crystalline regions with a large porosity arising from imperfect packing of the large rigid building blocks. Such structure is at the centre of a wide range of charge storage and transfer processes when CNT fibres are used as electrodes and/or current collectors. This work introduces a method based on wide and small-angle X-ray scattering (WAXS/SAXS) to obtain structural descriptors of CNT fibres and which enables in situ characterisation during electrochemical processes. It enables accurate determination of parameters such as specific surface area, average pore size and average bundle size from SAXS data after correction for scattering from density fluctuations arising from imperfect packing of graphitic planes. In situ and ex situ WAXS/SAXS measurements during electrochemical swelling of CNT fibre electrodes in ionic liquid provide continuous monitoring of the increase in effective surface area caused by electrostatic separation of CNT bundles in remarkable agreement with capacitance changes measured independently. Relative contributions from quantum and Helmholtz capacitance to total capacitance remaining fairly constant. The WAXS/SAXS analysis is demonstrated for fibres of either multi- and single-walled CNTs, and is expected to be generally applicable to operando studies on nanocarbon-based electrodes used in batteries, actuators and other applications

In the quest to increase battery performance, nanostructuring battery electrodes gives access to architectures with electrical conductivity and solid-state diffusion regimes not accessible with traditional electrodes based on aggregated spherical microparticles, while often also contributing to the cyclability of otherwise unstable active materials. This work describes electrodes where active material and current collector are formed as a single nanostructured composite network, consisting of macroscopic fabrics of carbon nanotube fibers covered with conformal MoS2 grown preferentially aligned over the graphitic layers, without metallic current collector or any conductive or polymeric additives. The composite fabric of CNTF/MoS2 retain high toughness and show out-of-plane electrical conductivity as high as 1.2 S/m, above the threshold to avoid electrical transport-limited performance of electrodes (1 S/m), and above control nanocomposite LIB electrodes (0.1 S/m) produced from dispersed nanocarbons. Cycled against Li, they show specific capacity as high as 0.7 Ah/g along with appreciable rate capability and cycling stability in low (108% capacity retention after 50 cycles at 0.1 A/g) as well as high current density (89 % capacity retention after 250 cycles at 1 A/g). The composite fabrics are flexible, with high tensile toughness up to 0.7 J/g, over two orders of magnitude higher than conventional electrodes or regular MoS2 material, and full-electrode capacity above state of the art at different current densities., Comment: 31 pages, 6 figures

In this work, we report on the fabrication of continuous transparent and flexible supercapacitors by depositing a CNT network onto a polymer electrolyte membrane directly from an aerogel of ultra-long CNTs produced floating in the gas phase. The supercapacitors combine record power density of $1370 kW kg^{-1}$ at high transmittance ($ca. 70%$), high electrochemical stability during extended cycling ($>94%$ capacitance retention over $20 000 cycles$) as well as against repeated $180{\deg}$ flexural deformation. They represent a significant enhancement of 1-3 orders of magnitude compared to the prior-art transparent supercapacitors based on graphene, CNTs, and rGO. These features mainly arise from the exceptionally long length of the CNTs, which makes the material behave as a bulk conductor instead of an aspect ratio-limited percolating network, even for electrodes with $>90%$ transparency. The electrical and capacitive figures-of-merit for the transparent conductor are $FoMe = 2.7$, and $FoMc = 0.46 F S^{-1} cm^{-2}$ respectively

Fabrics of continuous fibers of carbon nanotubes (CNTFs) are attractive materials for multifunctional energy storage devices, either as current collector, or as active material. Despite a similar chemical composition, lithiation/delithiation in CNTFs is substantially different from traditional graphite electrodes. In CNTFs this process is dominated by surface processes, insertion in the bundles interstices, electrochemical doping and often-overlooked partial degradation of the sp2 lattice upon cycling. Through extensive electrochemical analysis, together with in situ Raman spectroscopy measurements, we analyzed the complex lithiation behavior of highly crystalline fibers of CNTs. CNTF can store lithium reversibly with high specific capacity and rate capability, thanks to a large capacitive contribution. Upon lithiation, they undergo electrochemical doping, with longitudinal conductivity increasing by as much as 100 %, concomitant with large downshifts in Raman spectra. However, CNTF are also affected by high first-cycle irreversible capacity, voltage hysteresis and amorphization upon cycling. Electrochemical analysis confirms that SEI formation is responsible for the first-cycle irreversible capacity. Voltage hysteresis is attributed primarily to the trapping of lithium ions in the interstices between stacked nanotubes. Another dominant feature is pre-existing defects, which promote capacitive storage but lead to progressive amorphization of the CNTFs. Indeed, it is evidenced that undesired amorphization is hindered in ultra-pure CNTF without pre-existing defects., Comment: 27 pages, 9 figures

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., Comment: 23 pages, 9 figures

The assembly of aligned carbon nanotubes (CNT) into fibers (CNTF) 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 their implementation into composites and devices. However, CNTF functionalization is still in its infancy, due to the challenges associated with preservation of CNTF morphology. Here, we report a thorough study of the gas-phase functionalization of CNTF fabrics using ozone that was generated in situ from a UV-source. By contrast with liquid-based oxidation methods, this gas-phase approach preserves CNTF morphology, whilst notably increasing its hydrophilicity. The functionalized material is thoroughly characterized by Raman, XPS, TEM and SEM. 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 electrolyte and ionic liquid we decouple the effects of functionalization on pseudocapacitive reactions and quantum capacitance. The functionalized CNTF fabric is successfully used as active material and current collector in all-solid supercapacitor flexible devices with 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 \epsilon-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 Whkg_(MnO_2)^(-1) (16 Whkg-1 including the weight of CNTf) and real specific power of 17 kWkg_(MnO_2)^(-1) (7.5 kWkg-1) at 3.0 V with excellent cycling stability. Moreover, flexible all solid-state supercapacitors were fabricated using PYR14TFSI-based polymer electrolyte, exhibiting maximum energy density of 21 Whkg-1 and maximum power density of 8 kWkg-1 normalized by total active material.

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 cm2 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{\deg} without degradation of their properties, with a reversible 25% increase in energy density in the bent state. Devices produced using CNT fiber electrodes with a higher degree of orientation and therefore better mechanical properties showed similar electrochemical properties combined with composite specific strength and modulus of 39 MPa/SG and 577 MPa/SG for a fiber mass fraction of 11 wt.%, similar to a structural thermoplastic and with higher specific strength than copper.

Finding proper electrocatalysts capable of efficient catalyzing both ORR and OER is of great importance for metal-air batteries. With increasing inclination towards structural and flexible devices, developing a high-performance self-standing air-cathode is highly demanded and challenging, as most of oxygen catalysts are powder and need to be further processed. Here, we construct highly bifunctional air catalyst from macroscopic CNT fibers (CNTf) through direct CVD spinning followed by hydrothermal method. The electrocatalytic properties of the samples were tuned by altering nitrogen-doping and defect densities readily adjusted at different hydrothermal reaction temperatures. The treated CNTfs showed excellent bifunctional activity ({\Delta}E=Ej=10-E1/2=0.81 V) and demonstrated exceptional performance as carbon-based self-standing air-cathodes in liquid and solid-state rechargeable Zn-air batteries, with high capacity of 698 mAh g-1 and ultrahigh energy density of 838 Wh kg-1. The rechargeable Zn-air batteries exhibit a low discharge-charge overpotential and excellent stability. This work provides novel simply achieved self-standing air electrodes with exceptional performance for structural Zn-air batteries.

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.