Knitting the future of supercapacitors

Do you know your half gauge from your interlock? Or can you tell the difference between jersey and knit? If not, it may be time to learn, because a new paper from researchers at Drexel University suggests that knitting could be the future of energy storage.

Writing in Materials Today [DOI: 10.1016/j.mattod.2020.02.005], the team report on their investigation of supercapacitors made with coated cotton and nylon yarn. The active ingredient used are MXenes, a family of conductive, solution-processable 2D ceramics (Ti₃C₂T_x) first discovered in 2011. In previous studies, the researchers demonstrated the knittability of cotton yarns loaded with high quantities of MXenes, and showed that it was possible to etch the compounds in large batches. In this latest paper, they took a much larger step, developing a method for the production of tens of meters of Ti₃C₂T_x-coated yarn electrodes, and studying the effect of stitch structure and geometry on the performance of working knitted devices.

The coating process involved passing commercial yarns at a fixed rate through successive baths filled with MXene dispersions. The flake size and concentration of each dispersion was carefully controlled to provide the yarns with the optimum combination of conductivity and flexibility. The dispersions were housed in four baths made from vinyl tubing, which were positioned so that a length of yarn would spend 18 s in each bath and 9 min drying between them. Yarns were collected onto a winder, and repeatedly fed through the coating line until they reached the desired loading of MXene. The researchers used this setup to coat ten meters each of cotton yarn and multifilament nylon yarn.

Supercapacitors were knitted from these yarns using a standard, flat-bed, industrial-scale 3D knitting machine. Its two yarn feeders each produced an electrode on either side of an polyester yarn, which prevented electrical shorting. As expected, the complex, interconnected loops synonymous with the craft allowed charges to move through the path of least resistance, aiding in the electrical properties of these devices. By changing the number of knit stitches within and between electrodes (the device’s geometry), the authors found that they could tune the yarn spacing to improve the energy density of the device. They achieved the highest areal capacitance with their smallest knitted device – two vertical stripes of MXene-coated yarn electrodes, each one containing three columns of four stitches, separated by two columns of polyester loops (“3x4x2”).

In addition, the authors experimented with two types of knit structure – jersey, which produces a flat, planar textile, and rib, which creates a more dimensional, curled textile – as well as standard stitch patterns. They found that the denser a textile was, the higher its capacitance and the better its rate performance. They also found some difference between the yarns, with nylon producing more resistive devices than cotton.

And finally, they used their findings to produce a series of wearable, knitted supercapacitors – 2-ply MXene-coated cotton yarns in a PVA-H₃PO₄ gel electrolyte – in series and in parallel. The authors say that by using the processes of automated yarn coating combined with industrial knitting technology, “textile supercapacitors can be rapidly designed, programmed, prototyped, and ultimately, mass-produced.”

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Ariana Levitt, Dylan Hegh, Patrick Phillips, Simge Uzun, Mark Anayee, Joselito M. Razal, Yury Gogotsi, Genevieve Dion. “3D knitted energy storage textiles using MXene-coated yarns”, Materials Today, Article in Press. DOI: 10.1016/j.mattod.2020.02.005