Waste fibres from cannabis crops can be transformed into high-performance low-cost pseudo-graphene energy storage devices. Cannabis is quite possibly the most versatile, yet highly controversial, plant we have on the planet: from a popular recreational drug to a potential medicine for a range of incurable conditions. If Carlsberg made a weed, this would be it…
Actually, the type of cannabis plant that we are talking about here is hemp, not so much the type of marijuana or cannabis that bothers so much the authorities in many countries. There are several different plants from the same family and genus, and with a similar look.
Innovating with the Weed
Scientists “cooked” cannabis bark into carbon nano-sheets and built super-capacitors on a par with or better than graphene – the current industry gold standard.
- Graphene is a form of carbon that exists as a sheet, just one atom thick
- Atoms are arranged into a two-dimensional honeycomb structure
- Discovery of graphene announced in 2004 by the journal Science
- About 100 times stronger than steel, graphene conducts electricity better than copper
- Touted as possible replacement for silicon in electronics
- About 1% of graphene mixed into plastics could make them conductive
- Stronger than diamond, more conductive than copper and more flexible than rubber
- This “miracle material” was the target of a £50m investment by UK Chancellor George Osborne.
While this carbon monolayer is the state-of-the-art material for commercial super-capacitors, graphene is prohibitively expensive to produce. If you consider hemp fibre, on the other hand, its structure is the opposite – it makes sheets with high surface area – and that is very conducive to super-capacitors.
The supercapacitor, or double-layer capacitor, differs from a regular capacitor in that it has very high capacitance. Since its invention in 1957, the supercapacitor has evolved and crosses into battery technology by using special electrodes and electrolyte.
Supercapacitors are like a bridge between normal batteries and rechargeable ones. Their biggest advantage is that they can release a large quantity of energy in small periods of time and also get to recharge back to 100% in just seconds.
When large amounts of current flow into a capacitor, it causes the electrolyte solution to boil and turn into a gas. Once turned into a gas, pressure builds rapidly until, hopefully, the safety vent plug releases that pressure.
This rupture can be very dramatic and very destructive. Not only is the boiling liquid and gas very hot, it is also corrosive and will damage any components covered by the solution. Under controlled laboratory conditions, measurements have been taken during a violent, large capacitor rupture. The equivalent explosive force of half-a-hand grenade has been measured.
See this YouTube video featuring what happens when a capacitor is overloaded.
On absolutely no account, should you try this at home!!!
In 1978, the technology was marketed as “supercapacitor” for computer memory backup. In the 1990s, advances in materials and manufacturing methods led to improved performance and lower cost.
While the basic Electrochemical Double Layer Capacitor (EDLC) depends on electrostatic action, the Asymmetric Electrochemical Double Layer Capacitor (AEDLC) uses battery-like electrodes to gain higher energy density, but this is affected by a shorter cycle life and other burdens shared with the battery.
All capacitors have voltage limits. While the electrostatic capacitor can be made to withstand high volts, the super-capacitor is confined to 2.5–2.7V. Voltages of 2.8V and higher are possible but they reduce the service life. To obtain higher voltages, several supercapacitors may be connected in series.
Serial connection reduces the total capacitance and increases the internal resistance.
Strings of more than three capacitors require voltage balancing to prevent any cell from going into over-voltage. Lithium-ion batteries share a similar protection circuit.
The specific energy of the super-capacitor ranges from 1 to 30Wh/kg, roughly 10-50 time less than Li-ion.
The discharge curve is another disadvantage. Whereas the electro-chemical battery delivers a steady voltage in the usable power band, the voltage of the supercapacitor decreases on a linear scale from full to zero voltage.
The linear discharge of the super-capacitor reduces the usable power spectrum, as much of the energy is left behind.
Several types of electrodes have been tried and most are based on the electrochemical double-layer capacitor concept. It is carbon-based, has an organic electrolyte that is easy to manufacture and is the most common system in use today.
Graphene electrodes promise improvements with supercapacitors and batteries, but such development is highly costly and 15 years away.
But… what if you could just grow “pseudo-graphene” organically?
Green Storage Devices
The first step is to cook hemp cannabis – almost like in a pressure cooker. The process is called hydrothermal synthesis. Once you dissolve the lignin and the semi-cellulose – or ‘hemicellulose‘, hemp leaves carbon nanosheets – a pseudo-graphene structure.
Dr Mitlin and his team found that if they heated the fibres for 24 hours at a little over 350 degrees Fahrenheit, then blasted the resulting material with even more intense heat, it would exfoliate into nanosheets.
By fabricating these carbon nanosheets into electrodes and adding an ionic liquid as the electrolyte, Dr Mitlin and his team built their supercapacitors which operate at a broad range of temperatures and a high energy density. Direct comparisons with rival devices are complicated by the variety of measures for performance.
But Mitlin’s peer-reviewed journal paper ranks the device “on par with or better than commercial graphene-based devices“.
“They work down to 0°C and display some of the best power-energy combinations reported in the literature for any carbon. “For example, at a very high power density of 20 kW/kg (kilowatt per kilo) and temperatures of 20°C, 60°C, and 100°C, the energy densities are 19, 34, and 40 Wh/kg (watt-hours per kilo) respectively.”
Fully assembled, their energy density is 12 Wh/kg, two to three times higher than commercial supercapacitors, which can be achieved at a charge time less than six seconds. They can also operate over an impressive temperature range, from freezing to over 200 degrees Fahrenheit.
Finding cheap, sustainable alternatives is the speciality of Dr Mitlin’s former research group at the University of Alberta. They have experimented with all flavours of biowaste – from peat moss to eggs. Most recently, they have been turning banana peel into batteries.
Yes, banana peels… You can turn banana peels into a dense block of carbon – it’s called pseudo-graphite and works great for sodium ion batteries. Banana peel contains minerals that can serve as an electrolyte. Minerals in the greatest number is potassium ions (K+). Banana skins also contain small amounts of sodium chloride (Cl–) salts, which react with the potassium to form potassium chloride KCl – a solid electrolyte capable of ionising and conducting electricity. You can do really interesting things with bio-waste…
Basically, the trick is to tailor the right plant fibre to the right electrical device – according to their organic structure.
Cannabis “bast” fibre – the outer surface of the hemp stalk – has the longest fibres and is normally used for textiles. It is a waste product looking for a value-added application.
At the moment, growers actually pay to have it removed. But if the technology really takes off – it could help economies.
Cannabis is a robust plant. You can grow it almost everywhere. A lot of farmers would welcome the idea of growing hemp. Special machinery is used to separate the more valuable outer hemp fibres from the inner shorter fibres of the stalk.
The most common products still made from hemp fibre are cigarette paper, bank notes, technical filters and hygiene products. Similar uses include art papers and tea bags. Hemp products also include plastic composites for cars and thermal insulation. Several of these uses take advantage of hemp’s high tear and wet strength.
Well-known European companies, such as Mercedes-Benz and BMW, now use hemp for car interiors, including door panels and dashboards. U.S. auto industry suppliers are following the European example and have started to use hemp to make stronger, lighter and relatively less-expensive composite panels.
In China, the crop is widely cultivated. In Canada, the industry for hemp-based textiles is growing.
Until 2012, in the United States, the Controlled Substance Act made it illegal to raise industrial hemp (Cannabis sativa) commercially without a permit from the Drug Enforcement Agency (DEA). However, numerous state and national initiatives have been working to return industrial hemp production to the United States, where it once was a major crop.
Food and fibre uses for industrial hemp have been growing rapidly and increased over 300 percent, to an estimated 25,000 products, in the past few years. Much of that growth is coming from the increased sales of hemp food products.
Since then, cannabis was decriminalised. And the new industry is booming for medical grade and foodstuff products.
Growing Green “Canna-Capacitors”
Having established a proof of principle, Mitlin’s start-up company Alta Supercaps was hoping to begin small-scale manufacturing last year with plans to market devices aimed at the oil and gas industries – where high-temperature operation is a valuable asset. Alta Supercaps‘s move to the United States coincided with a change in regulatory attitudes – with signs that hemp could be making a comeback.
In the United Kingdom, as things stand now, we have a radically different scenario.
The current trend is to criminalise and ban.
Only six licences were granted for the cultivation of low THC cannabis plants in England and Wales in 2013. And with vested governmental interests in the mix…
Of course, hemp cannot do all the things that graphene can. For energy storage though, it works just as well. And hemp fibre only costs a small fraction of the price of graphene – $500 – $1,000 a tonne.
Sorry, George… Looks like the new age of the “canna-capacitor” may be upon us!