Hemp Carbon Makes Supercapacitors Superfast

Hemp Carbon Makes Supercapacitors Superfast Hemp Carbon Makes Supercapacitors Superfast Most people dont 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. Whats 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 materialsomething that has never been achieved before, says Mitlin. Hemp bast fiber is a low-cost graphene-like nanomaterial. Image: Wikimedia Commons Why Hemp? 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 issueanother 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 candidatesthey 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 fibers 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 materialnearly triple what standard commercial electrodes supply, 17 kW/kg. Promising Results 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 structurehemp bast fiber, says Mitlin. The resultant graphene-like nanosheets possess fundamentally different propertiessuch as pore size distribution, physical interconnectedness, and electrical conductivityas 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-1significantly 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. For Further Discussion 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