Emergence of fiber supercapacitors

Abstract

Supercapacitors (SCs) are energy storage devices which have high power density and long cycle life. Conventional SCs have two-dimensional planar structures. As a new family of SCs, fiber SCs utilize one-dimensional cylindrically shaped fibers as electrodes. They have attracted significant interest since 2011 and have shown great application potential either as micro-scale devices to complement or even replace micro-batteries in miniaturized electronics and microelectromechanical systems or as macro-scale devices for wearable electronics or smart textiles. This tutorial review provides an essential introduction to this new field. We first introduce the basics of performance evaluation for fiber SCs as a foundation to understand different research approaches and the diverse performance metrics reported in the literature. Next, we summarize the current state-of-the-art progress in structure design and electrode fabrication of fiber SCs. This is followed by a discussion on the integration of multiple fiber SCs and the combination with other energy harvesting or storage devices. Last, we present our perspectives on the future development of fiber SCs and highlight key technical challenges with the hope of stimulating further research progress.

A General Electrode Design Strategy for Flexible Fiber Micro‐Pseudocapacitors Combining Ultrahigh Energy and Power Delivery

“…The specific areal capacitance of our solid‐state fiber device with PVA/KOH electrolyte at 0.41 mA cm −2 is ≈2–350 times of previously reported solid‐state fiber SCs measured at much lower rates (typically 0.01–0.1 mA cm −2 ) reported so far (see Table S1, Supporting Information) 1, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36. More importantly, both the areal energy and power densities of 18.83 µWh cm −2 and 16.33 mW cm −2 based on the device (75.32 µWh cm −2 and 65.32 mW cm −2 based on one electrode) are substantially higher than those of previous advanced fiber devices using hollow rGO/PEDOT: PSS fiber (6.8 µWh cm −2 , 0.166 mW cm −2 ),30 rGO/MnO 2 /PPy@metal yarn (9.2 µWh cm −2 , 1.5 mW cm −2 ),31 MnO 2 /CNT fiber (8.5 µWh cm −2 ),20 PPy@CNTs@urethane elastic fibers (6.13 µWh cm −2 , 0.133 mW cm −2 ),26 GO/CNT@carboxymethyl cellulose fibers (3.84 µWh cm −2 , 0.19 mW cm −2 ),5 and nanoporous Au wire@MnO 2 //CNT/carbon fibers (5.4 µWh cm −2 , 2.53 mW cm −2 ) 36.…”

“…The rapid advances in portable/wearable electronics have stimulated ever‐increasing demand in flexible miniature power modules 1, 2, 3. Solid‐state flexible fiber micro‐supercapacitors (m‐SCs) have gained a surge of attentions 1, 2, 3, 4, 5, 6.…”

“…However, the major bottleneck for the existing fiber m‐SCs lies in their much lower areal energy density relative to routine planar SCs8 or batteries 2. In this context, considerable efforts were concentrated on exploring proper fiber electrode materials with large capacitance for improving the energy density, while maintaining high power density 1, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17. Various carbonaceous materials like activated carbon,9, 10 carbon nanotubes (CNTs),11, 12, 13 reduced graphene oxide (rGO),14, 15, 16 and our recently developed rGO/CNT hybrids17, 18 were exploited as active materials for fiber m‐SCs, yet their applications are restricted by the low capacitance of <200 mF cm −2 .…”

“…Due to the poor conductivity for pseudocapacitive materials, composite electrode design by depositing active materials on one‐dimensional (1D) conductive scaffolds including carbon‐based fibers19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or metal‐based wires,31, 32, 33, 34, 35, 36 was employed to improve the electron transport. Despite some progresses, the improvements in the areal energy density for these fiber m‐SCs are still too modest to cater for many practical requirements and often come at the expense of sacrificing their rate capability or power density 1, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36. This impedes their broad practical applications where both high power and energy densities are required.…”

“…This undoubtedly restricted allowable active material loading, since thicker active material layer may cause poor adhesion, inferior mechanical stability, and high resistivity. In principle, ideal 1D pseudocapactive fiber electrode should consist of high‐surface‐area, conductive and porous framework and concurrently render: (1) efficient long‐range electronic transport; (2) fast ion transport/diffusion; (3) a large fraction of active materials 1, 2. However, as mentioned above, those existing fiber electrodes using 1D metallic wires/or carbonaceous fibers failed to simultaneously satisfy the above three key requirements, thereby resulting in limited areal energy densities and inferior power output 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36.…”