By etching electrodes made of monolithic carbon film into a conducting substrate of titanium carbide, Chmiola and Gogotsi 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 two principal systems today for storing electrical energy are batteries and supercapacitors. Batteries store electrical energy in the form of chemical reactants and generally display even higher energy storage densities than supercapacitors. However, the charging and discharging of a battery exact a physical toll on electrodes that eventually ends the battery's life after several thousand charge-discharge cycles. In supercapacitors, energy is stored as electrical charge, which does not impact electrodes during operation. This allows supercapacitors to be charged and discharged millions of times.

“We have known for some time that supercapacitors are faster and longer-lasting alternatives to conventional batteries,” Gogotsi says, “so we decided to see if it would be possible to incorporate them into microelectronic devices and if there would be any advantage to doing so.”

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.”

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.