Usually, the fallen autumn leaves of the phoenix tree in northern China are burnt, contributing to the country’s air pollution problem. But researchers in Shandong have developed a process for converting the dried leaves into extremely porous carbon microspheres (pictured).
These can act as supercapacitors, a highly prized and promising group of materials that can store 10-100 times as much energy as a regular capacitor, and charge up and release it quicker. Supercapacitors have the potential to create smartphones or electric vehicles that can charge up in seconds rather than hours.
The researchers ground the leaves into a powder, then heated them to 220C for 12 hours, creating a powder of tiny microspheres. These were then treated with potassium hydroxide and gradually heated to 800C.
The final product, a black carbon powder, has a very high surface area – with many tiny pores chemically etched onto the surface of the microspheres. This gives it its rare electrical properties.
Tests on the material found that when used in conjunction with an electrolyte it had a capacitance (a measure of how much electrical charge it can hold) of 367 farads per gram. That’s three times higher than graphene, a much-hyped form of carbon where the molecules are arranged in a thin sheet.
“The values they're reporting certainly look very impressive for the materials that they're using,” said Ian Hamerton, a reader in polymers and composite materials at the University of Bristol, who was not involved in the research.
"The potential benefit here is that the material being used is from what would otherwise be a waste product,” he told Professional Engineering. “It's taking biomass and repurposing that and recycling that to produce a material that's converted into a high surface-area porous carbon surface.”
The research team, led by Hongfang Ma of Qilu University of Technology, has been focusing on ways of turning waste biomass into porous carbon. They have also successfully used potato waste, corn straw, pine wood, rice straw and other agricultural materials.
Hamerton stressed that, although the process was ostensibly green, it does use potassium hydroxide and very high temperatures. “The activation process uses a very alkaline material,” he said. “To derive the real benefits from this, or to be able to claim the material is being produced in such a way as to be wholly green, one would have to be convinced that the potassium hyrdroxide was being neutralised in such a way that it wasn't producing very highly alkaline waste water. Ideally the process should be analysed using life cycle assessment to determine its impact on the environment.”
China has led the way in the use of supercapacitors in transport, with a fleet of electric supercapacitor buses operating in Shanghai. These recharge very quickly every few miles. This technology could enable vehicles to run for longer periods between charges. “The benefits of a bus using a supercapacitor based on material of this kind, with a higher power density, would be that you have a much longer period of travel between recharges,” said Hamerton.