1. By etching electrodes made of monolithic carbon film into a conducting substrate of titanium carbide, Chmiola and Gogotsi of Lawrence Berkeley National Lab were able to create micro-supercapacitors featuring an energy storage density that was at least double that of the best supercapacitors now available. When used in combination with microbatteries, the power densities and rapid-fire cycle times of these micro-supercapacitors should substantially boost the performance and longevity of portable electric energy storage devices.
The electrical charge storage densities of the micro-supercapacitors were measured in two common electrolytes. As promising as the results were, Chmiola notes the impressive figures were achieved without the “decades of optimization” that other electronic devices have undergone. This, he says, “hints at the possibility that the energy density ceiling for microfabricated supercapacitors is, indeed, quite high.”
Adds Gogotsi, “Given their practically infinite cycle life, micro-supercapacitors seem ideal for capturing and storing energy from renewable resources and for on-chip operations.”
The next step of the work is to scale down the size of the electrodes and improve the dry etching procedure for removing metal atoms from metal carbides to make the process even more compatible with commercial microfabrication technology. At Berkeley Lab, Chmiola is working on the development of new electrolytes that can help increase the energy storage densities of his micro-supercapacitors. He is also investigating the factors that control the usable voltage window of different electrolytes at a carbon electrode.
“My ultimate goals are to increase energy stored to levels closer to batteries, and preserve both the million-plus charge-discharge cycles and recharge times of less than five minutes of these devices,” says Chmiola. “I think this is what the end users of portable energy storage devices really desire.”
Microbatteries with dimensions of tens to hundreds of micrometers that are produced by common microfabrication techniques are poised to provide integration of power sources onto electronic devices, but they still suffer from poor cycle lifetime, as well as power and temperature range of operation issues that are alleviated with the use of supercapacitors. There have been a few reports on thin-film and other micro-supercapacitors, but they are either too thin to provide sufficient energy or the technology is not scalable. By etching supercapacitor electrodes into conductive titanium carbide substrates, we demonstrate that monolithic carbon films lead to a volumetric capacity exceeding that of micro- and macroscale supercapacitors reported thus far, by a factor of 2. This study also provides the framework for integration of high-performance micro-supercapacitors onto a variety of devices
Researchers at Drexel University in Philadelphia have demonstrated that it’s possible to use techniques borrowed from the chip-making industry to make thin-film carbon ultracapacitors that store three times as much energy by volume as conventional ultracapacitor materials. While that is not as much as batteries, the thin-film ultracapacitors could operate without ever being replaced. These charge-storage films could be fabricated directly onto RFID chips and the chips used in digital watches, where they would take up less space than a conventional battery. They could also be fabricated on the backside of solar cells in both portable devices and rooftop installations, to store power generated during the day for use after sundown. The materials have been licensed by Pennsylvania startup Y-Carbon.