2, Drexel University, Philadelphia, Pennsylvania, United States
Recent progress in the field of nanotechnology and functional fibers has led to the fabrication of textiles with advanced functions, including sensing and actuating, processing and storing data, and communicating with nearby electronics. Incorporating these functionalities into textiles necessitates the integration of energy storage devices into garments. Supercapacitors are promising candidates for wearable energy storage applications, as they can charge and discharge for thousands of cycles, meaning that their lifetime can surpass that of a traditional garment.
Many material design challenges are presented when developing electrode materials for textile-based energy storage devices. Not unlike traditional supercapacitors used in static environments, such as in laptop computers, achieving high-capacitance and high-power requires electrode materials with high surface area and electrical conductivity. However, for textile-based devices, which are used in dynamic environments, the electrode materials also need to be flexible and mechanically robust, i.e. capable of withstanding stresses during industrial-scale textile manufacturing and throughout wear. While several researchers have developed fiber-based supercapacitors that exhibit impressive capacitance, to our knowledge, few have demonstrated scalability.
Here, we capitalized on the attractive electrochemical performance of Ti3C2 MXene, a two-dimensional titanium carbide, and the high surface area of electrospun nanofibers to develop electrode materials for knitted supercapacitors. We developed a modified electrospinning setup to create and collect meters of twisted bundles of nanofibers, known as nanoyarns. These nanoyarns are composed of over 80,000 fibers, with fiber diameters ranging from 300-800 nm depending on the electrospinning parameters chosen. The nanoyarns are flexible and elastic, reaching a strain-to-failure of 300% for poly(caprolactone) (PCL) yarns. Using this setup, nanoyarns with various architectures, such as core-sheath yarns, can be fabricated from a variety of polymer solutions, including PCL, poly(acrylonitrile), and poly(vinylidene fluoride). After functionalizing the surface of the nanoyarns using oxygen plasma, Ti3C2 MXene is incorporated into these yarns through a dipping and drying process, a method that can easily be integrated into industrial-scale yarn manufacturing. Next, these MXene-coated nanofibers are integrated into knitted energy storage devices using industrial and programmable Shima Seiki knitting machines. These devices have the potential to connect with energy harvesting devices and power wearable electronics.