Tag Archives: David Mitlin

Hemp as a substitute for graphene in supercapacitors

As a member of the Cannabis plant family, hemp has an undeserved reputation due to its cousin’s (marijuana) notoriety and consciousness-altering properties. Hemp is, by contrast, the Puritan in the family, associated by the knowledgeable with virtues of thrift and hard work.

An Aug. 12, 2014 news item on Nanowerk highlights a hemp/supercapacitor presentation at the 248th meeting of the American Chemical Society (ACS),

As hemp makes a comeback in the U.S. after a decades-long ban on its cultivation, scientists are reporting that fibers from the plant can pack as much energy and power as graphene, long-touted as the model material for supercapacitors. They’re presenting their research, which a Canadian start-up company is working on scaling up, at the 248th National Meeting & Exposition of the American Chemical Society (ACS), the world’s largest scientific society.

David Mitlin, Ph.D., explains that supercapacitors are energy storage devices that have huge potential to transform the way future electronics are powered. Unlike today’s rechargeable batteries, which sip up energy over several hours, supercapacitors can charge and discharge within seconds. But they normally can’t store nearly as much energy as batteries, an important property known as energy density. One approach researchers are taking to boost supercapacitors’ energy density is to design better electrodes. Mitlin’s team has figured out how to make them from certain hemp fibers — and they can hold as much energy as the current top contender: graphene.

An Aug. 12, 2014 ACS news release features David Mitlin, formerly of the University of Alberta (Canada) where this research took place,, Mitlin is now with now with Clarkson University in New York,

“Our device’s electrochemical performance is on par with or better than graphene-based devices,” Mitlin says. “The key advantage is that our electrodes are made from biowaste using a simple process, and therefore, are much cheaper than graphene.”

The race toward the ideal supercapacitor has largely focused on graphene — a strong, light material made of atom-thick layers of carbon, which when stacked, can be made into electrodes. Scientists are investigating how they can take advantage of graphene’s unique properties to build better solar cells, water filtration systems, touch-screen technology, as well as batteries and supercapacitors. The problem is it’s expensive.

Mitlin’s group decided to see if they could make graphene-like carbons from hemp bast fibers. The fibers come from the inner bark of the plant and often are discarded from Canada’s fast-growing industries that use hemp for clothing, construction materials and other products. …

His team found that if they heated the fibers for 24 hours at a little over 350 degrees Fahrenheit, and then blasted the resulting material with more intense heat, it would exfoliate into carbon nanosheets.

Mitlin’s team built their supercapacitors using the hemp-derived carbons as electrodes and an ionic liquid as the electrolyte. Fully assembled, the devices performed far better than commercial supercapacitors in both energy density and the range of temperatures over which they can work. The hemp-based devices yielded energy densities as high as 12 Watt-hours per kilogram, two to three times higher than commercial counterparts. They also operate over an impressive temperature range, from freezing to more than 200 degrees Fahrenheit.

“We’re past the proof-of-principle stage for the fully functional supercapacitor,” he says. “Now we’re gearing up for small-scale manufacturing.”

I have not been able to confirm the name for Mitlin’s startup but I think it’s called Alta Supercaps (Alta being an abbreviation for Alberta,, amongst other things, and supercaps for supercapacitors) as per the information about a new startup on the Mitlin Group webspace (scroll down to the July 2, 2013 news item) which can still be found on the University of Alberta website (as of Aug. 12, 2014).

For those who would like more technical details, there is this July 2013 article by Mark Crawford for the ASME (American Society of Mechanical Engineers); Note: A link has been removed.

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.”

This image from Wikimedia was used to illustrate the Crawford article,

Hemp bast fiber is a low-cost graphene-like nanomaterial. Image: Wikimedia Commons

Hemp bast fiber is a low-cost graphene-like nanomaterial. Image: Wikimedia Commons

It seems to me that over the last few months there have been more than the usual number of supercapacitor stories, which makes the race to create the one that will break through in the marketplace fascinating to observe.

Walking on eggshells at the University of Alberta

It’s Friday, April 13, 2012 and I wanted a little fun in my headline so Zhi Li and his colleagues in the Mitlin Group at the University of Alberta are not walking on eggshells. They are, instead, carbonizing them as a means of increasing the amount of electrical energy that can be stored as Michael Berger explains in his April 12, 2012 article, Converting eggshell membranes into a high-performance electrode material for supercapacitors (links have been removed from the following excerpt),

Today’s commercial supercapacitors – which are mostly electric double layer capacitors (EDLC) – store energy in two closely spaced layers with opposing charges and offer fast charge/discharge rates and the ability to sustain millions of cycles. Researchers have come up with various electrode materials to improve the performance of supercapacitors, focussing mostly on porous carbon due to its high surface areas, tunable structures, good conductivities, and low cost. In recent years, this has increasingly included research on various carbon nanomaterials such as carbon nanotubes, carbon nano-onions, or graphene.

“An ideal supercapacitor is one with both high energy density and high power density,” Zhi Li, a post doc researcher in David Mitlin’s group at the University of Alberta, explains to Nanowerk. …

In new work recently published in Advanced Energy Materials (“Carbonized Chicken Eggshell Membranes with 3D Architectures as High-Performance Electrode Materials for Supercapacitors”), first-authored by Li, researchers have demonstrated that a common daily waste – the eggshell membrane – can be converted into a high-performance carbon material for supercapacitors.

“Considering over 1000 billion eggs are consumed per year globally, and that 30–40 mg finished carbon is derivable from one egg, the eggshell membrane is indeed a reliable and sustainable resource for clean energy storage,” says Li.

“The most exciting finding, for me, is that the amazing nature-made architecture of chicken eggshell membrane is critical to its performance as electrode materials after carbonization,” notes Li. “Why? The carbonized eggshell membrane is a real ‘integrated system’ composed of interwoven carbon fibers with diameter from 50 nm to 2 µm where the big fibers and tiny fibers are naturally connected together.”

As a consequence of their architecture,

… carbonized eggshell membrane can work at high current loading. That means capacitors based on it can be charged and discharged faster than capacitor based on traditional carbons.

You can find out more about the Mitlin Group here and you can find the article, “Carbonized Chicken Eggshell Membranes with 3D Architectures as High-Performance Electrode Materials for Supercapacitors,” here but it is behind a paywall. By the way, Dr. David Mitlin, group leader, is also a principal investigator at Canada’s National Institute of Technology.