High-Performance Supercapacitors from Niobium Nanowire Yarns

The
large-ion-accessible surface area of carbon nanotubes (CNTs)
and graphene sheets formed as yarns, forests, and films enables miniature
high-performance supercapacitors with power densities exceeding those
of electrolytics while achieving energy densities equaling those of
batteries.− Capacitance and energy density can be enhanced by depositing highly
pseudocapacitive materials such as conductive polymers on them.,− Yarns formed from carbon nanotubes are proposed for use in wearable
supercapacitors., In this work, we show that high
power, energy density, and capacitance in yarn form are not unique
to carbon materials, and we introduce niobium nanowires as an alternative.
These yarns show higher capacitance and energy per volume and are
stronger and 100 times more conductive than similarly spun carbon
multiwalled nanotube (MWNT) and graphene yarns.,− The long niobium nanowires, formed by repeated extrusion and drawing, achieve device volumetric peak power and energy
densities of 55 MW·m–3 (55 W·cm–3) and 25 MJ·m–3 (7 mWh·cm–3), 2 and 5 times higher than that for state-of-the-art CNT yarns,
respectively. The capacitance per volume
of Nb nanowire yarn is lower than the 158 MF·m–3 (158 F·cm–3) reported for carbon-based materials
such as reduced graphene oxide (RGO) and CNT wet-spun yarns, but the peak power and energy densities are 200
and 2 times higher, respectively. Achieving
high power in long yarns is made possible by the high conductivity
of the metal, and achievement of high energy density is possible thanks
to the high internal surface area. No additional metal backing is
needed, unlike for CNT yarns and supercapacitors in general, saving
substantial space. As the yarn is infiltrated with pseudocapacitive
materials such as poly­(3,4-ethylenedioxythiophene) (PEDOT), the energy
density is further increased to 10 MJ·m–3 (2.8
mWh·cm–3). Similar to CNT yarns, niobium nanowire
yarns are highly flexible and show potential for weaving into textiles
and use in wearable devices.