  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.
