(Invited) Knitted Electrochemical Capacitors Via Natural Fiber Welded Electrode Yarns

This report presents a novel ionic liquid-based method for fabricating flexible double layer supercapacitor electrodes utilizing natural substrates. It also investigates the performance of these yarns knitted as fabrics, and with other cellulose-based fibers welded with carbon particles. Scanning electron microscopy, mechanical properties testing, conductivity and other electrochemical testing are performed to characterize both individual yarns and knitted devices.

     Natural fiber welding (NFW) is a process that uses controlled amounts of ionic liquids (ILs) and molecular co-solvents to selectively swell and mobilize biopolymers in fibrous, natural materials for functional (chemical and physical) modification.^1-4  NFW processes control and minimize the amount of dissolution to fibrous natural substrates.  By carefully regulating the ratio of solvent to substrate, solvent efficacy, time, temperature, pressure, location of solvent exposure, etc., hydrogen bonding networks are reconfigured and extended by only mobilizing the outer shell of individual fibers.  The result is that fibers are essentially ‘welded’ together while the cores maintain their native states. In this work, the fiber welding process has been modified to incorporate carbon materials into the biopolymer matrix, delivering flexible carbon containing yarns for wearable supercapacitor applications.

     To determine whether the material is a viable candidate for wearable supercapacitors, the welded carbon-yarns are coated with polymer electrolyte, twisted with stainless steel yarn (current collector) and electrochemically tested in a symmetric configuration through cyclic voltammetry, galvanostatic cycling, and impedance spectroscopy.  Electrochemical testing performed on samples randomly selected across the entire length of yarn electrodes (typically 10-20 ft in length) yielded 8-12 mF/cm, comparable with previously reported literature. However, when the cotton is twisted with steel prior to coating, the capacitance dramatically increased to 45 mF/cm.  Fabrication can have a dramatic effect on the electrochemical performance.

     Because mechanical properties of the cellulose yarns play a crucial role when knitting these capacitive yarns into full fabrics, tensile testing of the yarns pre- and post- welding was conducted. With each yarn, an improved strength was observed at the expense of elasticity; viscose and linen showed the most notable increases in strength (107%, 78% respectively) and toughness (both 22%).  Though many of the cotton yarn electrodes showed favorable electrochemical performance, none were machine-knittable. The linen, viscose, and bamboo yarn supercapacitors were successfully machine knitted at Drexel’s Shima Seiki Haute Tech Lab.  We currently attribute this to the length of the fibers playing a role in flexibility and mechanical strength within the yarn. More work is currently underway to explain this process, and fabricate a welded cotton yarn that can be knitted.

     To date, we have demonstrated a coating process that is repeatable and scalable (coating hundreds of feet at a time), inexpensive, and delivers competitive performance to similar reports in the literature. Given that composites are constructed from a truly ‘green’ substrate (cotton, bamboo, etc.) and utilizing a nonfluorinated binder (cellulose from cotton), we believe this to be a major breakthrough for the prospects of wearable energy storage.^5-7

Acknowledgements: Portions of this work were funded by the Air Force Office of Scientific Research.  Any opinions, findings, conclusions or recommendations expressed in the material are those of the authors and do not necessarily reflect the views of the U.S Navy or U.S Air Force. K. Jost acknowledges support from the DoD National Defense Science and Engineering Graduate (NDSEG) Fellowship.

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