The ubiquity in the modern world of consumer electronics has created a corresponding demand for better super-capacitors for energy storage, thereby enabling rapid-charging for our mobile phones, tablets, laptops, and electric cars. But the best materials for building high-performance super-capacitors are often costly. Now, scientists from the University of Sydney in Australia have successfully created a low-cost alternative, building electrodes for super-capacitors out of waste scraps from durian and jackfruit, according to a new paper in the Journal of Energy Storage.

"Durian waste, as a zero-cost substance that the community wants to get rid of urgently due to its repulsive, nauseous smell, is a sustainable source that can transform the waste into a product to substantially reduce the cost of energy storage through our chemical-free, green synthesis protocol," said co-author Vincent Gomes of the University of Sydney in Australia.

Scientists have typically relied on a variety of carbon-based materials as electrodes when building super-capacitors: activated carbon, carbon nanotubes, and graphene sheets, for example. It's best to use materials that boast high porosity, since they help diffuse electrolytes through the electrodes, and to maximize surface area.

A 2010 paper found that electrodes based on aerogels are even better than standard carbon materials in terms of maximizing capacitance. Aerogels are 99.8 percent air, making them pretty much the lightest known solid material. They were first synthesized in 1931—the result of a bet between Samuel Kistler and Charles Learned over who could best replace all the liquid in "jellies" with a gas. The trick is super-critical drying, which retains the structure of the original gel. Carbon-based aerogels appeared in the 1980s and are favored for many applications by NASA, among others, since they are extremely light-weight with exceptional thermal insulation properties.

But many of these advanced materials are also costly, sparking interest in using organic waste as precursor materials when making electrodes out of aerogels, such as pomelo peel, paper pulp, and watermelon. The waste can be simply freeze-dried to eliminate water while still retaining the hierarchical structure that makes for a good aerogel.

"The structural precision of natural biomass with the hierarchical pores, developed over millions of years of biological evolution, affords an outstanding resource as a template for the synthesis of carbon-based materials," Gomes and his co-authors wrote. That, in turn, means organic waste would help achieve high-performance energy storage at lower costs.

Enter the durian, known as the "king of fruits" in the Southeast Asia regions where it is especially popular. Its most distinctive feature is its strong odor—so persistent that it can linger for days, which is why many hotels and public transport systems in Asia don't allow durian fruit at all. Naturalist Alfred Russel Wallace praised the fruit as "a rich custard highly flavored with almonds," while acknowledging it initially smelled like rotten onions. Novelist Anthony Burgess claimed the experience was "like eating sweet raspberry blancmange in the lavatory."

Smell aside, the durian's inedible spongy core turns out to be ideal for making biomass-based aerogels. First, Gomes et al. selected pieces of both durian and jackfruit, looking for those that were very porous and had a large surface area. They picked the jackfruit from a tree in Australia and purchased the durian at a local market, then took core samples from each piece of fruit, rinsing them off with deionized water to remove all the dirt and debris.

Next, they converted the fruit waste into a carbon aerogel. The samples were placed in Teflon autoclaves and heated for ten hours at 180° C (356° F), and then cooled over night. Then the samples were rinsed and freeze-dried. To carbonize the freeze-dried samples, they were heated in a furnace for an hour at 800° C (1,472° F), yielding "black, highly porous, ultra-light aerogels," per the authors.

Finally, the Australian team used the fruit-derived aerogels to build electrodes and then tested them to assess how well they stored energy. Both durian and jackfruit waste produced aerogels with excellent energy storage properties, although the durian-based ones performed a bit better than those derived from jackfruit. That makes sense, since the durian-based carbon aerogels also proved to have significantly greater porosity and surface area than the jackfruit-based aerogels. Both, however, provide a comparable (and cheaper) alternative to the activated carbon super-capacitors currently being used for energy storage.

"We have reached a point where we must urgently discover and produce ways to create and store energy using sustainably-sourced materials that do not contribute to global warming," said Gomes. "Confronted with this and the world's rapidly depleting supplies of fossil fuels, naturally-derived super-capacitors are leading the way for developing high efficiency energy storage devices."

DOI: Journal of Energy Storage, 2020. 10.1016/j.est.2019.101152 (About DOIs).