3D printed graphene micro-supercapacitors

Research led by Kansas State University has shown potential ways to manufacture graphene-based nano-inks for additive manufacturing of supercapacitors in the form of flexible and printable electronics.

As researchers worldwide study the potential replacement of batteries by supercapacitors, an energy device that can charge and discharge very fast – within tens of seconds – a team led by KSU's Suprem Das, assistant professor of industrial manufacturing systems engineering, has found a different approach.

The team has devised a method of using graphene to produce these supercapacitors, and by experimenting with additive manufacturing, better known as 3D printing, have managed to successfully create working prototypes in the hopes of one day creating even smaller supercapacitors – called micro-supercapacitors – so that they could eventually be used for wafer-scale integration in silicon processing.

It was the meeting between Das and Christopher Sorenson, university distinguished professor of physics, that really enabled the research to come to fruition. When the two met, Das realized he could use Sorensen's expertise in additive manufacturing to transform this research study into useful things, in this case, making tiny energy storage devices.

"I wasn't trying to make graphene," said Sorensen when referring to his accidental discovery of cheaply and efficiently producing graphene.

In the early 2000s, he was studying soot formation in flames and discovered that soot could form a gel. His later research moved away from using flames and instead started using a metal chamber with an automotive spark plug as a source of energy. By mixing acetylene (a compressed gas) and a small amount of oxygen in the chamber, he could use the spark plug to create an explosion.

Instead of opening the chamber and letting the smoke out, he left it alone for a few minutes and discovered the smoke formed a gel that he described as "black angel food cake."

Upon looking at the gel under a microscope, he expected to see little round balls of carbon. Instead, though, he saw flat sheets.

"I realized we made graphene, but I didn't know just how significant my discovery was because I wasn't in this big horde of people studying graphene at the time," Sorensen said. "I had my own agenda."

Lorenzo Mosca, a professor of chemistry at the University of Rhode Island who was not involved in the study, described Sorensen's method as a "definite improvement over other fabrication methods."

In addition, Das described Sorensen's method as environmentally friendly compared to more conventional methods for creating graphene.

"It's very novel in the sense that it does not produce any kind of environmentally harmful products," said Das. "The process is green."

Using Sorensen's production process, the research team combined its expertise and, using the newly developed graphene ink, used a method similar to 3D printing to print tiny graphene electrodes on a flexible surface. For this, they used 20-micrometer-thin polyimide – plastic – substrates with high reliability.

They even created one in the shape of Kansas State's mascot, Willie the Wildcat.

The team continued to develop its graphene micro-supercapacitors and tested them for 10,000 cycles of charging and discharging, a number that is promising to evaluate the reliability of these charging devices.

They found that the electrodes retained 80% of its capacity over those cycles, which Das said was "very stable" and "pretty good."

In comparison, a study from 2019 titled "Data-driven prediction of battery cycle life before capacity degradation" reported that the lifespan of lithium-ion batteries in use today ranges between 150 to 2,300 cycles.

Recognized as a wonder material, graphene has provided many advancements due to its many superlative physical properties. As a result, many graphene manufacturing methods have been developed globally, and graphene has been produced in varying quantities. Technologists, however, are well aware that graphene is not yet in the marketplace because none of these methods have had the right combination of economy, ecology, and product quality to allow graphene to fulfill its potential.

Yet, according to Das and Sorensen, both the methods of producing graphene and nano-inks pursued at KSU are on target to address all these requirements.

"If we can make flexible electronics with our cheap inks, that's going to have all kinds of applications. I mean, we can almost imagine having sensors on our clothing and gathering sunlight as we walked down the street to charge our cell phones, I mean, all kinds of crazy things," said Sorensen.

Ultimately, Das and team have formed a synergistic collaboration with Sorensen because of the energy-efficient, highly scalable, and chemical-free nature of the graphene production process and his own group's graphene ink manufacturing process. Both of these processes are patented, or patent-pending and are highly industrially relevant.