WHY THIS MATTERS IN BRIEF
Breakthroughs in supercapacitor technology will have multiple benefits, from helping to create new renewable energy grid-scale electric storage systems and faster electric vehicle charging systems.
Elon Musk last year went on record to say that if anything was going to accelerate the roll out and adoption of electric vehicles on tomorrow’s roads it was more likely to be a breakthrough in supercapacitors, such as the ones created recently that can charge EV’s in seconds not hours, rather than a breakthrough in the EV batteries themselves.
Now, supercapacitors, which are energy storage devices, are at the center of an energy revolution, in part because they can be charged rapidly and deliver intense bursts of power when you most need it. But, until now, all supercapacitors rely on components made from carbon – which need high temperatures, harsh chemicals and lots of money to make, and thanks to a new breakthrough that could all be about to change.
Now researchers at MIT have, for the first time, created a supercapacitor that uses no conductive carbon at all, and that could potentially produce more power than existing supercapacitors.
The teams breakthrough, which was led by Mircea Dincă, an MIT associate professor of chemistry, is being reported in the journal Nature Materials.
“We’ve found an entirely new class of materials for supercapacitors,” said Dincă.
Dincă and his team have been exploring for years a class of materials called Metal-Organic Frameworks, or MOFs, which are extremely porous, sponge-like structures. These materials have an extraordinarily large surface area for their size, much greater than the carbon materials do and that is an essential characteristic for supercapacitors, whose performance depends on their surface area. But MOFs have a major drawback for such applications – they’re not very electrically conductive, which is also an essential property for a material used in a capacitor.
“One of our long-term goals was to make these materials electrically conductive,” said Dincă, even though doing so “was thought to be extremely difficult, if not impossible.”
But the material did exhibit another needed characteristic and that’s that it conducts ions, small charged particles, very well.
“All double-layer supercapacitors today are made from carbon,” said Dincă, “they use carbon nanotubes, graphene, activated carbon, all shapes and forms, but nothing else besides carbon. So this is the first non-carbon, electrical double-layer supercapacitor.”
One advantage of the material used in these experiments, technically known as Ni3(hexaiminotriphenylene)2, is that it can be made under much less harsh conditions than those needed for the carbon-based materials, which require very high temperatures above 800 degrees Celsius and strong reagent chemicals for pre-treatment.
The team says supercapacitors, with their ability to store relatively large amounts of power, could play an important role in making renewable energy sources practical for widespread deployment. They could provide grid-scale storage, such as the ones slated to power Los Angeles in 2021, or SolarCity’s new solar energy grid that they recently installed on the island of Ta’u. Additionally they could also boost the adoption of Electric Vehicles and accelerate the roll out of America’s 25,000 miles of new electric vehicle charging coridoors – and that’s just for starters.
The new devices produced by the team, even without any optimization of their characteristics, already match or exceed the performance of existing carbon-based versions in key areas, such as their ability to withstand large numbers of charge and discharge cycles, and tests have shown that they lost less than 10 percent of their performance after 10,000 cycles, which is comparable to existing commercial supercapacitors.
But that’s likely just the beginning, said Dincă. MOFs are a large class of materials whose characteristics can be tuned by varying their chemical structure. Work on optimizing their molecular configurations to provide the most desirable attributes for this specific application is likely to lead to variations that could easily outperform any existing materials.
“We have a new material to work with, and we haven’t optimized it at all,” he says, “it’s completely tunable, and that’s what’s exciting.”
While there’s been much research on MOFs, most of it has been directed at uses that take advantage of the materials’ record porosity, such as for storage of gases.
“Our lab’s discovery of highly electrically conductive MOFs opened up a whole new category of applications,” said Dincă.
Besides the new supercapacitor uses, the conductive MOFs could be useful for making electrochromic windows, which can be darkened with the flip of a switch, and chemoresistive sensors, which could be useful for detecting trace amounts of chemicals for medical or security applications.
“While the MOF material has advantages in the simplicity and potentially low cost of manufacturing, the materials used to make it are more expensive than conventional carbon-based materials,” said Dincă, “carbon is dirt cheap. It’s hard to find anything cheaper.”
But even if the material ends up being more expensive, if its performance could be significantly better than that of carbon-based materials and it could have a variety of non-traditional applications.
This discovery is “very significant, from both a scientific and applications point of view,” says Alexandru Vlad, a professor of chemistry at the Catholic University of Louvain in Belgium, who was not involved in this research. He adds that “the supercapacitor field was, but will not be anymore, just dominated by activated carbons,” because of their very high surface area and conductivity. But now, “here is the breakthrough provided by Dinca et al – they could design a MOF with high surface area and high electrical conductivity, and thus completely challenge the supercapacitor value chain! There is essentially no more need for carbon in this high demand technology.”
“This work shows only the tip of the iceberg. With carbons we know pretty much everything, and the developments over the past years were modest and slow. But the MOF used by Dincă 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,” he added.