Enlarge /. The durian, widely known and revered in Southeast Asia as the "king of fruits", is characterized by its size, strong smell (often compared to rotten onions, raw sewage or turpentine) and its impressive thorn-covered skin
Danita Delimont / Getty Images
The ubiquity in the modern world of consumer electronics has led to a corresponding demand for better supercapacitors for energy storage, which allows our cell phones, tablets, laptops and electric cars to be charged quickly. However, the best materials for building high-performance supercapacitors are often expensive. Now scientists from the University of Sydney in Australia have successfully created an inexpensive alternative in which electrodes for supercapacitors are built from waste waste from durian and jackfruit. This emerges from a new article in the Journal of Energy Storage.
"Durian waste is a low-cost substance that the community desperately wants to get rid of due to its repulsive, foul-smelling smell, a sustainable source that can convert the waste into a product to significantly reduce the cost of energy storage by our chemical – free, green synthesis protocol, "said co-author Vincent Gomes of the University of Sydney in Australia.
Scientists typically rely on a variety of carbon-based materials as electrodes when building supercapacitors: activated carbon, carbon nanotubes, and graphene layers, for example. It is best to use high porosity materials as they help diffuse electrolytes through the electrodes and maximize the surface area.
A 2010 paper found that airgel-based electrodes are even better than standard carbon materials in terms of maximizing capacity. Aerogels are made up of 99.8 percent air, making them pretty much the lightest solid material known. They were first synthesized in 1931 – the result of a bet between Samuel Kistler and Charles Learned about who could best replace all of the liquid in "jellies" with a gas. The trick is supercritical drying, which retains the structure of the original gel. Carbon-based aerogels appeared in the 1980s and are preferred by NASA, among others, for many applications because they are extremely light and have exceptional thermal insulation properties.
However, many of these advanced materials are also costly and are generating interest in using organic waste as a precursor in the manufacture of electrodes from aerogels such as grapefruit peel, paper pulp and watermelon. The waste can simply be freeze-dried to remove water while maintaining the hierarchical structure that ensures good airgel.
"The structural precision of natural biomass with the hierarchical pores that has been developed over millions of years of biological evolution offers an excellent resource as a template for the synthesis of carbon-based materials," wrote Gomes and his co-authors. This in turn means that organic waste would help achieve high-performance energy storage at a lower cost.
Enlarge /. Schematic process for converting durian fruits into a carbon airgel.
Lee, K. et al.
Enter the Durian, known as the "King of Fruits" in the Southeast Asian regions where it is particularly popular. The most striking feature is the strong smell – so persistent that it can linger for days, which is why many hotels and public transport systems in Asia do not allow durian fruits at all. Natural scientist Alfred Russel Wallace praised the fruit as "a rich pudding with a high almond taste" while recognizing that it smelled like rotten onions at first. Novelist Anthony Burgess claimed the experience was "like eating sweet raspberry pudding in the toilet".
Apart from the smell, the inedible spongy core of the Durian is ideal for the production of aerogels based on biomass. First, Gomes et al. selected pieces of durian and jackfruit, looking for those that were very porous and had a large surface. They picked the jackfruit from a tree in Australia and bought the durian at a local market. They then took core samples from each piece of fruit and rinsed them with deionized water to remove dirt and debris.
Next, they converted the fruit waste into a carbon airgel. The samples were placed in Teflon autoclaves and heated to 180 ° C (356 ° F) for ten hours and then cooled overnight. The samples were then rinsed and freeze-dried. To carbonize the freeze-dried samples, they were heated in an oven at 800 ° C (1472 ° F) for one hour, resulting in "black, highly porous, ultra-light aerogels" according to the authors.
Finally, the Australian team used the fruit-derived aerogels to build electrodes and then tested them to see how well they had stored energy. Both durian and jackfruit wastes produced aerogels with excellent energy storage properties, although those based on durian performed slightly better than those made from jackfruit. This makes sense because the Durian-based carbon aerogels also have a significantly greater porosity and surface area than the jackfruit-based aerogels. However, both offer a comparable (and cheaper) alternative to the activated carbon supercapacitors that are currently used for energy storage.
"We have reached a point where we urgently need to find and produce ways to generate and store energy from sustainably sourced materials that do not contribute to global warming," said Gomes. "In view of this and the rapidly exhausting supply of fossil fuels worldwide, naturally obtained supercapacitors are groundbreaking for the development of highly efficient energy stores."
DOI: Journal of Energy Storage, 2020. 10.1016 / j.est.2019.101152 (About DOIs).