Cell phones as thin and flex­ible as a sheet of paper. Energy-​​storing house paint. Roll-​​up touch screen dis­plays. These are the sorts of devices that the engi­neering industry is preparing for and expecting. But if any of them is to work, said North­eastern Uni­ver­sity mechan­ical and indus­trial engi­neering pro­fessor Yung Joon Jung, experts also need to create a thin and flex­ible energy-​​storage system. His lab has devel­oped such a system.

In a recently pub­lished article in the journal Sci­en­tific Reports, Jung and col­leagues from North­eastern and Rice Uni­ver­sity pre­sented their design of a flex­ible and trans­parent super­ca­pac­itor, a device that stores energy as an elec­trical field instead of a chem­ical reac­tion, as bat­teries do. As such, it is a prime energy-​​storage can­di­date for the thin, flex­ible devices of the future.

The tech­nology is based on a nano­ma­te­rial devel­oped in Jung’s lab two years ago, which they call a nanocup. One of the per­ceived advan­tages of nan­otubes, Jung explained, is the poten­tial to fill them with other mate­rials, such as elec­trolyte in the case of a super­ca­pac­itor. The inner capacity of nan­otubes has turned out to be too small to achieve this capa­bility, ”but if you have a cup,” Jung said, pointing to his own coffee mug, “you can put any­thing in it you want.”

The first step to making a nanocup is etching nanoscopic divots into an alu­minum film through oxi­da­tion. By tweaking the voltage and time of this process, researchers can tailor the size of the cups. The second step is to layer carbon atoms onto the alu­minum mold using stan­dard carbon nan­otube technology.

Hyun­y­oung Jung, the first author on the paper and a post­doc­toral researcher in Pro­fessor Jung’s lab, has a back­ground in polymer chem­istry. He empha­sized that the new supercapacitor’s nov­elty derives from the large sur­face area and the open tex­tured sur­face of the nanocups. This mor­phology allows them to come into greater con­tact with the elec­trolyte, which drives the for­ma­tion of an elec­trical field and thus the energy storage functionality.

The super­ca­pac­itor, which has not yet been opti­mized, is able to store energy and pro­vide power at levels com­pa­rable to other devices. The dif­fer­ence, how­ever, is its ability to be incor­po­rated into thin film devices. “If we give up trans­parency and mechan­ical flex­i­bility,” Jung said, “we can easily go to that level of com­mer­cially avail­able devices. But my goal is not to lose these two qual­i­ties and simul­ta­ne­ously develop high-​​performance energy devices.”

The research team has already used a flex­ible and trans­parent pro­to­type to power a light. The group plans to make con­tinued improve­ments in power gen­er­a­tion and energy storage.