Supercapacitor fibers, with short charging times, long cycle lifespans, and high power densities, hold promise for powering flexible fabric‐based electronics. To date, however, only short lengths of functioning fiber supercapacitors have been produced. The primary goal of this study is to introduce a supercapacitor fiber that addresses the remaining challenges of scalability, flexibility, cladding impermeability, and performance at length. This is achieved through a top‐down fabrication method in which a macroscale preform is thermally drawn into a fully functional energy‐storage fiber. The preform consists of five components: thermally reversible porous electrode and electrolyte gels; conductive polymer and copper microwire current collectors; and an encapsulating hermetic cladding. This process produces 100 m of continuous functional supercapacitor fiber, orders of magnitude longer than any previously reported. In addition to flexibility (5 mm radius of curvature), moisture resistance (100 washing cycles), and strength (68 MPa), these fibers have an energy density of 306 μWh cm−2 at 3.0 V and ≈100% capacitance retention over 13 000 cycles at 1.6 V. To demonstrate the utility of this fiber, it is machine‐woven and used as filament for 3D printing.

In 2018 alone, global energy demand grew by 2.3% and will rise by 1.3% each year to 2040 [1] making it the fastest growth rate in the last decade predominantly driven by a robust global economy and increased heating and cooling needs. This tremendous need resulted in using fossil fuels to meet nearly 70% of the growth, but also promoted solar and wind generation to have a double-digit growth pace, with solar alone increasing x TABLE OF CONTENTS