Laser-assisted fabrication of flexible monofilament fiber supercapacitors

Abstract

Fiber supercapacitors (SCs) are potential and promising candidates for the development of wearable lightweight energy storage units. Thus, investigations have been conducted on various aspects such as electrode materials and device configuration to enhance their performance. The representative types of fiber SC configurations are characterized by certain limitations, including the imprecise assembling process and increased resistance of the SCs. To address these limitations, herein, we report a laser-assisted fabrication method by which active electrodes, current collectors, electrolyte, and a flexible polymer support can be integrated into a monofilament fiber-type SC. Instead of combining two individual electrode fibers, two separate microscale current collectors and active electrodes were implemented on a polymeric monofilament surface by exploiting the micromachining capability of laser processing. Metallic current collectors were printed on a polyvinylidene fluoride fiber by selective laser sintering of Ag nanoparticles and nanowires. This highly conductive layer enabled reduction in the equivalent series resistance, leading to increased specific capacitance. The current collector was coated with a graphene layer, and a thin layer of pseudocapacitive MnO₂ nanostructures was deposited onto the graphene layer to further improve the specific capacitance of the final fiber SC. The monofilament fiber SC exhibited a specific capacitance of 24.5 mF cm⁻² at 0.1 mA cm⁻² in a PVA–Na₂SO₄ electrolyte, along with excellent mechanical flexibility (94.3% capacitance retention in 3000 bending cycles at a bending radius of 7.5 mm). In particular, this laser-assisted electrode patterning method enabled the fabrication of serially connected SCs within a seamless monofilament unit, without post-fabrication assembly processing. Thus, the operation of a highly flexible fiber SC was demonstrated in a wide voltage window. This research suggests the development of an unconventional structure for fiber SCs, which is expected to be highly beneficial for promising flexible/wearable electronics applications.