Conducting MOF offers cheap, efficient supercapacitor material

Energy storage devices called supercapacitors have become a hot area of research, in part because they can be charged rapidly and deliver intense bursts of power. However, all current supercapacitors employ components made of carbon, which require high temperatures and harsh chemicals to produce.

Now, researchers at the Massachusetts Institute of Technology (MIT) and elsewhere have for the first time developed a supercapacitor that employs no conductive carbon at all, and that could potentially produce more power than existing versions of this technology. The work is reported in a paper in Nature Materials by Mircea Dinca, associate professor of chemistry, Yang Shao-Horn, professor of energy, and four others.

"We've found an entirely new class of materials for supercapacitors," Dinca says.

Dinca and his team have for years been investigating a class of materials called metal-organic frameworks (MOFs), which are extremely porous, sponge-like structures. These materials have an extraordinarily large surface area for their size, much greater than that of the carbon materials currently used in supercapacitors. Although the performance of supercapacitors depends on their surface area, MOFs have a major drawback for this kind of application: they are not very electrically conductive, an essential property for a material used in a capacitor.

"One of our long-term goals was to make these materials electrically conductive," Dinca says, even though doing so "was thought to be extremely difficult, if not impossible." But MOFs do possess another necessary characteristic for such electrodes: they conduct ions very well.

"All double-layer supercapacitors today are made from carbon," Dinca says. "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."

Producing this non-carbon supercapacitor has required developing a MOF that is highly conducting. Technically known as Ni3(hexaiminotriphenylene)2, the MOF can be made under conditions that are far less harsh than those needed for the carbon-based materials, which require temperatures above 800°C and strong reagent chemicals for pre-treatment.

The team says that 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 to help match usage times with generation times, for example, or be used in electric vehicles and other applications.

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 parameters, such as their ability to withstand large numbers of charge/discharge cycles. Tests showed they lost less than 10% of their performance after 10,000 cycles, which is comparable to existing commercial supercapacitors.

But that's likely just the beginning, Dinca says. MOFs are a large class of materials whose characteristics can be tuned to a great extent 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 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 has been much research on MOFs, most of it has been directed at uses like storing gases that take advantage of the materials' high porosity. "Our lab's discovery of highly electrically conductive MOFs opened up a whole new category of applications," Dinca says. 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 is fairly simple and inexpensive to manufacture, the materials used to make it are more expensive than conventional carbon-based materials, Dinca says. "Carbon is dirt cheap. It's hard to find anything cheaper." But even if the material ends up being more expensive, if its performance is significantly better than that of carbon-based materials, it could find useful applications, he says.

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) 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 of carbons for this highly demanded technology.”

Another key advantage, he adds, is that "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 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."

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.