Supercapacitors wrought from thin films

by R. Colin Johnson
Contributing Editor, MEMS Investor Journal

Supercapacitors are touted as the enablers of MEMS battery technologies, but so far thin film and other supercapacitor topologies have come up short of fulfilling the dream.  Now researchers at Drexel University claim to have discovered a method that doubles the capacity of thin film supercapacitors.  Such MEMS capacitors could enable chips to house their own rechargeable power sources, extending the battery lifetimes of all types of mobile devices from handsets to laptop computers.

On-chip battery technologies measuring just tens to hundreds of microns are being fabricated today, holding the promise of self-contained sensors and other chips that house their own power source.  Unfortunately, today these batteries require external supercapacitors to boost their cycle lifetime and temperature range into usable regimes.

To the rescue comes Drexel professor Yury Gogotsi, his doctoral advisee John Chmiola, and their collaborators in France, who claim to have discovered a technique for creating thin film “super-duper” capacitors that could someday be integrated onto chips alongside the circuitry they power.  The key to their “super-duper” capacitors is harnessing monolithic carbon films atop titanium carbide substrates.

Conventional supercapacitors already outperform conventional electrolytic capacitors by thousands of times by virtue of ditching the two plates separated by an intervening electrolytic storage substance, in favor of dual-plates separated by nanometers that can be packed side-by-side. The result is a much larger surface area for a given size, accounting for their supersized capacities over conventional designs.

A conventional supercapacitor's electrodes harness reversible ion adsorption over their vast surface area, rather than storing chemical energy in a bulk electrolyte, resulting not only in a larger capacity, but also in faster charging and discharging, as well as higher efficiency and a much wider temperature range.

The Drexel researchers improve on this system by etching their carbon electrodes from a titanium carbide (TiC) substrate, thereby forming an extremely porous design with a much higher surface area than conventional carbon electrodes.  The result is a very thin electroactive layer that stores twice as much charge as ordinary supercapacitors.

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