In 2010 Spectrum reported a new approach for creating chip-scale supercapacitors on silicon wafers, proposed by researchers at Drexel University in Philadelphia and the Université Paul Sabatier in Toulouse, France. In an article published in Science, the researchers described how to make supercapacitor electrodes from porous carbon that could stick to the surface of silicon wafers so that they could be micromachined into electrodes for on-chip supercapacitors.
Now the same team has finally succeeded in doing just that.
In a paper published in this week’s Science, researchers from the two initial teams report creating efficient porous carbon electrodes that really stick to the surface of a silicon wafer. They made layers of porous carbide derived carbon (CDC) that are completely compatible with all treatments used in the semiconductor industry, says Patrice Simon, a researcher at Université Paul Sabatier who has researched porous CDC electrodes over the last ten years and co-authored both the 2010 and this week’s paper in Science.
In an initial experiment circa 2010, the researchers exposed a 5 mm-thick ceramic plate of titanium carbide to chlorine at 500° C. They obtained a porous carbon layer on its surface that would fit the bill for electrode material for a supercapacitor. This layer, called carbide derived carbon contained pores that could store large numbers of ions from the electrolyte of a supercapacitor. Deposited on the surface of a silicon wafer, CDC, the researchers theorized, could be micromachined into electrodes.
But to make their theory reality, the researchers had to resolve several issues, such as creating a CDC layer that would actually stick to the surface of a silicon wafer and that would be capable of absorbing sufficient ions to allow storing energy in a supercapacitor.
Fast-forward to today, when the researchers report that they made mechanically stable porous carbon films by fine-tuning the chlorination process. Using a standard sputtering technique, they deposited a 6.3 micrometer thick layer of titanium carbide on the thin insulating SiO2 layer of a standard silicon wafer. Then they exposed this layer to chlorine at 450ºC. The chlorination reaction progressed from the top of the titanium carbide (TiC) layer to the bottom, while chlorine atoms snatched up the titanium atoms, leaving empty 0.6-nanometer pores in the carbon layer.
"We stopped the chlorination before the whole titanium carbide layer became transformed into CDC," says Simon. To their surprise the researchers found that the thin residual one-micrometer TiC layer caused the CDC film to adhere strongly to the SiO2 layer. As a bonus, the TiC layer, being an electric conductor, allows electrons to polarize the CDC layer, which then attracts ions into its pores to compensate for the charge, says Simon.
Using standard microfabrication methods, the team fabricated 2 by 2 mm supercapacitors with 18 interdigitated CDC electrodes on a 7.62 cm silicon wafer and reported an electric capacity of 170 farads per cm2, which allows an energy storage that outperforms current state-of-the-art micro-supercapacitors.
A second, and welcome, surprise was that if you chlorinated completely the initial titanium carbide layer, changing the entire layer into CDC, you could simply peel this layer off the silicon wafer. “The film doesn’t become brittle and remains mechanically stable,” says Simon. This opens the way to the creation of flexible supercapacitors that could be glued onto flexible materials, such as polyethylene, says Simon.