Published on October 11th, 2016 | by Tina Casey0
Energy Storage With “Sponge Bob” Carbon-Free Supercapacitor
October 11th, 2016 by Tina Casey
Researchers at MIT are hot on the trail of a new energy storage concept that deploys a sponge-like material instead of carbon. If the line of research proves successful, it would help accelerate the global transition to renewable energy, by providing more powerful and nimble energy storage options for grid scale applications as well as electric vehicles and other devices.
Energy Storage, MOF Style
The material in question is in the class of metal-organic frameworks (MOFs). Loosely speaking, MOFs are fluffy, crystalline materials. The fluffiness is key because it reflects the extremely large surface area characteristic of MOFs, relative to their size.
The surface area of MOFs is far greater than carbon, which is why researchers have been looking at them for various types of storage.
For some examples, you can check out some work on hydrogen storage under way at Lawrence Berkeley National Laboratory. Northwestern University has been looking into MOFs for solar applications, and Pacific Northwest National Laboratory has also been applying MOFs to clean tech.
One area that has not received much attention is supercapacitors (supercapacitor is fancyspeak for an energy storage device that charges and discharges rapidly). That’s because MOFs generally do not conduct electricity very efficiently.
The new MIT research is one of the first successful attempts to apply MOFs to supercapacitors. If you’re wondering why the team decided to invest time in putting a non-conductive material to conductive use, MIT cites researcher Mircea Dincă:
“One of our long-term goals was to make these materials electrically conductive,” Dincă says, even though doing so “was thought to be extremely difficult, if not impossible.” But the material did exhibit another needed characteristic for such electrodes, which is that it conducts ions (atoms or molecules that carry a net electric charge) very well.
As for how they did it, you can get all the details in the journal Nature Materials under the title, “Conductive MOF electrodes for stable supercapacitors with high areal capacitance.“
What’s So Wrong About Carbon
The new breakthrough is significant because carbon is the basis of today’s supercapacitor technology, and carbon is a very testy material to manufacture.
According to MIT, the carbon-based materials used in supercapacitors require a pre-treatment process that involves “strong” chemicals, and temperatures in excess of 800 degrees Celsius.
Although carbon is very cheap compared to the alternatives, when you take manufacturing costs, performance, and lifecycle factors into consideration, MOFs could potentially claim an overall advantage.
Eliminating or reducing carbon could bring down manufacturing costs while reducing environmental impacts and risks in the energy storage supply chain.
The Age Of MOFs Is Just Beginning
One thing that especially excites the research team is that they have achieved a significant measure of success without trying to tailor or “tune” the material for optimal performance.
For their experiments, they used an MOF called Ni3(hexaiminotriphenylene)2. One main attraction of this material is that it can be fabricated under “much less harsh” conditions than carbon-based materials. It also lends itself to tuning, and it does happen to have relatively high conductivity.
Without any extra tweaking, devices made with the MOF compared favorably to commercial supercapacitors, losing less than 10% of performance after 10,000 cycles.
Here’s Dincă enthusing over the possibilities:
We have a new material to work with, and we haven’t optimized it at all. It’s completely tunable, and that’s what’s exciting.
Don’t just take his word for it. MIT sought an opinion from a researcher not involved in the study and they recorded this take from Alexandru Vlad, a professor of chemistry at the Catholic University of Louvain in Belgium:
…With carbons we know pretty much everything, and the developments over the past years were modest and slow. But the MOF used by Dinca is one of the lowest-surface-area MOFs known, and some of these materials can reach up to three times more [surface area] than carbons. The capacity would then be astonishingly high, probably close to that of batteries, but with the power performance [the ability to deliver high power output] of supercapacitors.
Stay tuned, so to speak.
Photo (cropped) by Melanie Gonick via MIT: “To demonstrate the supercapacitor’s ability to store power, the researchers modified an off-the-shelf hand-crank flashlight (the red parts at each side) by cutting it in half and installing a small supercapacitor in the center, in a conventional button battery case, seen at top. When the crank is turned to provide power to the flashlight, the light continues to glow long after the cranking stops, thanks to the stored energy.”