  However, their electrocatalytic activity is still poorly understood. This
work deciphers the origin of the catalytic activity of counter-electrodes
(CEs)/current collectors made of self-standing carbon nanotubes fibers (CNTfs)
using Co$^(+2)$/Co$^(+3)$ redox couple electrolytes. This is based on
comprehensive electrochemical and spectroscopic characterizations of fresh and
used electrodes applied to symmetric electrochemical cells using platinum-based
CEs as a reference. As the most relevant findings, two straight relationships
were established: i) the limiting current and stability increase rapidly with
surface concentration of oxygen-containing functional groups, and ii) the
catalytic potential is inversily related to the amount of residual metallic Fe
catalyst nanoparticles interspersed in the CNTf network. Finally, the fine tune
of the metallic nanoparticle content and the degree of functionalization
enabled fabrication of efficient and stable dye-sensitized solar cells with
cobalt electrolytes and CNTf-CE outperforming those with reference Pt-CEs.
