High-Performance\nSupercapacitors from Niobium Nanowire\nYarns

Abstract

The\nlarge-ion-accessible surface area of carbon nanotubes (CNTs)\nand graphene sheets formed as yarns, forests, and films enables miniature\nhigh-performance supercapacitors with power densities exceeding those\nof electrolytics while achieving energy densities equaling those of\nbatteries.− Capacitance and energy density can be enhanced by depositing highly\npseudocapacitive materials such as conductive polymers on them.,− Yarns formed from carbon nanotubes are proposed for use in wearable\nsupercapacitors., In this work, we show that high\npower, energy density, and capacitance in yarn form are not unique\nto carbon materials, and we introduce niobium nanowires as an alternative.\nThese yarns show higher capacitance and energy per volume and are\nstronger and 100 times more conductive than similarly spun carbon\nmultiwalled nanotube (MWNT) and graphene yarns.,− The long niobium nanowires, formed by repeated extrusion and drawing, achieve device volumetric peak power and energy\ndensities 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,\nrespectively. The capacitance per volume\nof Nb nanowire yarn is lower than the 158 MF·m^–3 (158 F·cm^–3) reported for carbon-based materials\nsuch as reduced graphene oxide (RGO) and CNT wet-spun yarns, but the peak power and energy densities are 200\nand 2 times higher, respectively. Achieving\nhigh power in long yarns is made possible by the high conductivity\nof the metal, and achievement of high energy density is possible thanks\nto the high internal surface area. No additional metal backing is\nneeded, unlike for CNT yarns and supercapacitors in general, saving\nsubstantial space. As the yarn is infiltrated with pseudocapacitive\nmaterials such as poly­(3,4-ethylenedioxythiophene) (PEDOT), the energy\ndensity is further increased to 10 MJ·m^–3 (2.8\nmWh·cm^–3). Similar to CNT yarns, niobium nanowire\nyarns are highly flexible and show potential for weaving into textiles\nand use in wearable devices.