Hemp Carbon Makes Supercapacitors Superfast
Jul 31, 2013
by Mark Crawford ASME.org
Most people don’t understand the truly diverse value of hemp (Cannabis sativa). Cultures have depended on this hardy plant for centuries for clothing, fabric, and paper. Today, it is also used for food, fuel, medicine, building materials, and plastics. Now the energy storage industry is starting to take notice, thanks to new Canadian research that shows supercapacitors with electrodes made from hemp-based carbon nanosheets outperform standard supercapacitors by nearly 200%.
Graphene, a carbon nanomaterial, is considered to be one of the best materials for supercapicitor electrodes. Graphene is, however, expensive to manufacture, costing as much as $2,000 per gram. Looking for a less-costly solution, researchers at the University of Alberta/National Institute for Nanotechnology (NINT) NRC, and Alberta Innovates-Technology Futures, led by chemical and materials engineering Professor David Mitlin, developed a process for converting fibrous hemp waste into a unique graphene-like nanomaterial that outperforms graphene. What’s more, it can be manufactured for less than $500 per ton.
“Our work actually opens up a very cheap and mass-producible manufacturing method for graphene-quality material—something that has never been achieved before,” says Mitlin.
Activated carbons, templated carbons, carbon nanofibers, carbon nanotubes, and graphene have all been intensively studied as materials for supercapacitor electrodes. High manufacturing costs is one issue—another is that the power characteristics of many of these carbons are limited. This is a result of high microporosity, which increases ion transport limitations.
“It is becoming well understood that the key to achieving high power in porous electrodes is to reduce the ion transport limitations” says Mitlin. “Nanomaterials based on graphene and their hybrids have emerged as a new class of promising high-rate electrode candidates—they are, however, too expensive to manufacture compared to activated carbons derived from pyrolysis of agricultural wastes, or from the coking operations.”
Biomass, which mainly contains cellulose and lignin by-products, is widely utilized as a feedstock for producing activated carbons. Mitlin decided to test hemp bast fiber’s unique cellular structure to see if it could produce graphene-like carbon nanosheets.
Hemp fiber waste was pressure-cooked (hydrothermal synthesis) at 180 °C for 24 hours. The resulting carbonized material was treated with potassium hydroxide and then heated to temperatures as high as 800 °C, resulting in the formation of uniquely structured nanosheets. Testing of this material revealed that it discharged 49 kW of power per kg of material—nearly triple what standard commercial electrodes supply, 17 kW/kg.
Mitlin and his team successfully synthesized two-dimensional, yet interconnected, carbon nanosheets with superior electrochemical storage properties comparable to those of state-of-the-art graphene-based electrodes. “We were able to achieve this by employing a biomass precursor with a unique structure—hemp bast fiber,” says Mitlin. “The resultant graphene-like nanosheets possess fundamentally different properties—such as pore size distribution, physical interconnectedness, and electrical conductivity—as compared to conventional biomass-derived activated carbons.”
The nanosheets ranged in thickness from 10 to 30 nm with high specific surface area (> 2200 m2 g-1) significant mesoporosity (up to 58 percent), and good electrical conductivity (211-226 S m-1). Mitlin indicates the nanosheets are compatible for various ionic liquid-based supercapacitor applications from about 0-100° C.
“At 0° C and a current density of 10 A g-1, the electrode maintains a remarkable capacitance of 106 F g-1,” notes Mitlin. “At 20, 60, and 100° C and an extreme current density of 100 A g-1, there is excellent capacitance retention (72%-92%). These characteristics favorably place the material among the best power-energy characteristics ever reported for an electrochemical capacitor. At a very high power density of 20 kW kg-1 and 20, 60, and 100° C, the energy densities are 19, 34, and 40 Wh kg-1, respectively.”
Moreover, adds Mitlin, the assembled supercapacitor device yielded a maximum energy density of 12 Wh kg-1—significantly higher than those for commercially available supercapacitors. By taking advantage of the complex multilayered structure of a hemp bast fiber precursor, these high-performing carbons were created by simple hydrothermal carbonization combined with activation.
“We were delighted at how well this material performed as supercapacitor electrodes,” says Mitlin. “This novel precursor-synthesis route presents a great potential for facile large-scale production of high-performance carbons for a variety of diverse applications including energy storage, portable electronics, uninterruptable power sources, medical devices, load leveling, and hybrid electric vehicles.”
Mark Crawford is an independent writer.
These characteristics favorably place the new material among the best power-energy characteristics ever reported for an electrochemical capacitor.Prof. David Mitlin, University of Alberta