Researchers have identified a “sweet spot” at which the length of a threadlike energy storage technology called a “yarn-shaped supercapacitor” (YSC) yields the highest and most efficient flow of energy per unit length. This development is significant for wearable technology.
THE DEVELOPMENT: When it comes to the length of the YSC, it’s a trade-off between power and energy. It’s not only about how much energy you can store, but also the internal resistance we care about.
- The researchers found that YSCs in the 40–60 cm range provided the best overall energy output.
- Previous research on YSCs has delivered varied and sometimes conflicting results when it comes to length-dependent energy output. The aim of the new study was to provide a consistent, comprehensive model to explain changes in YSC performance across a wide range of lengths.
THE STUDY: The research was conducted at the North Carolina State University (NC State), and the findings have been published in the Journal of Power Sources.
- Wei Gao is the corresponding author of the paper on the work and an associate professor of textile engineering, chemistry and science at NC State.
- The paper is titled ‘Modeling of yarn-shaped supercapacitors – Unraveling its length dependent output’, with the other authors being Nanfei He of NC State University, Xi Zhang of Yancheng Institute of Technology, and Junhua Song and Feng Zhao from Storagenergy Technologies, Inc.
- Funding for the study came from the US Army Research Office.
THE RESEARCH: The researchers first fabricated several YSCs using pairs of activated carbon-incorporated electrode yarns and a gel electrolyte. Nylon threads were wrapped around each yarn to prevent shorting, and then the two electrodes were plied together and coated further with the same gel electrolyte.
- They created these YSCs in segments ranging from 10cm to 300 cm in length, and then ran electrical currents of varying frequencies through them. This allowed them to measure two characteristics; internal resistance, which measures how much electrical current is impeded while trying to move through a battery, and capacitance, which is the ability to store electrical energy.
- The researchers found that capacitance increased linearly with length between 10–60 cm, after which gains in capacitance slowed significantly as length increased. The results were also influenced by the frequency of the electricity—or the rate at which the electrical current oscillates.
- Depending on the electrical frequency of the current, the YSCs would see diminishing gains in capacitance up to the 300 cm in length, though some plateaued at around 150 cm. Mathematical models also showed that YSCs between 40–80 cm exhibited the lowest internal resistance, which led researchers to determine that 40–60 cm was the most efficient length overall.
WHAT THEY SAID:
Imagine you can make a yarn, just a regular textile yarn, that you also make into a battery. You can basically hide it in your clothing. If you can do that, you can add so many more functions to your clothing… The technology is not mature yet, and that’s why there is so much funding and so much interest in developing it. We can make yarn batteries, but can we make them durable, reliable, and safe? Can we make them washable? If you’re going to put it on your body, there are so many other challenges besides its energy-storage functions.
— Wei Gao (Corresponding author)
Associate professor of textile engineering, chemistry and science
North Carolina State University
