Imagine turning fruit waste into technology that stores electricity. This would reduce food waste and promote clean energy storage. Postdoctoral researcher Vianney Ngoyi Kitenge transformed mangosteen peels into specialised carbon materials that he used to make supercapacitor energy storage cells. He came up with a simplified way to do this, hugely reducing the cost. This breakthrough converts agricultural waste into valuable components for energy storage technology. He sets out how it works and what’s needed to make it happen.
What is a supercapacitor?
Supercapacitors are a type of energy storage cell, similar to a battery, but with some key differences. They are standalone devices that store and release energy on their own.
The biggest difference between a supercapacitor and a battery is how quickly a supercapacitor can charge and release energy. While batteries are designed to provide energy steadily over a longer period (like minutes or hours), supercapacitors are built to deliver energy very quickly – within seconds or minutes.
This makes them perfect for applications that need a quick burst of power. You probably use supercapacitors every day without realising it. They help devices like flashes in your smartphone camera, portable jump starters for cars, fitness trackers, and smartwatches that need quick energy boosts to work efficiently.
When making devices, the manufacturers choose whether to use a battery or a supercapacitor. This decision is based on how much power is needed and how fast it’s needed. Most of the time, consumers aren’t even aware of whether there’s a battery or supercapacitor inside their devices.
In energy storage cells where electrical charges are stored, electrodes are key. Supercapacitors’ electrodes can be made with activated carbon. This can be made from biomass waste, such as coconut shells, banana peels, mangosteen peels, and coffee grounds. I used mangosteen peels in my research.
What are supercapacitors used for?
Apart from camera flashes and emergency doors, supercapacitors are useful in renewable energy. They act like super-fast energy sponges that can quickly soak up extra electricity when solar panels or wind turbines produce too much. They can also quickly release this energy when too little is produced. This helps keep the power flow steady even when it’s not sunny or windy.
Supercapacitors are still a small player in the energy storage world, with sales at around US$3 billion to US$4 billion yearly. To put this in perspective, the sales of lithium-ion batteries are US$50 billion to US$60 billion annually. Most are made in China and Japan, with some production in Europe and America too.
Supercapacitors haven’t caught on widely yet. This is why scientists continue to work to increase supercapacitors’ total energy storage while maintaining their speed and longevity.
What role can mangosteen peels play?
There has been little research into using the peels of the mangosteen fruit to create carbon. Yet, mangosteen trees grow abundantly from the east coast of South Africa to Somalia and Guinea, and they can withstand drought and rainstorms.
Their peels naturally contain 35%-45% carbon-rich compounds. When processed through drying, oxygen-free heating and chemical activation, these peels transform into activated carbon.
Through my research, I developed a simplified method to transform mangosteen shells into highly porous activated carbon. By combining the dried shells with potassium carbonate and directly heating to 700°C in a single step, I created valuable activated carbon from agricultural waste.
Usually, creating activated carbon is a longer process of pre-heating fruit peels, and reheating. So my method speeds the process up. This faster method makes activated carbon much cheaper by eliminating the initial five hours heating phase at 400-500°C. This saves on electricity, and reduces both production costs and the amount of time the furnace needs to operate. This then makes it more affordable for widespread commercial use.
Using fruit peels to create activated carbon also prevents them from being dumped on landfill sites and instead uses them to make valuable energy storage devices. Just three to five kilograms of fruit peels are enough to produce hundreds of supercapacitors. The global demand for supercapacitors is projected to grow significantly over the coming decade. This demand will be driven primarily by electric vehicles, renewable energy systems, and consumer electronics markets seeking high-power, rapid-cycling energy storage solutions.
Citrus peels can also be used to make activated carbon. Worldwide, the citrus juice industry generates 15 million tonnes of wasted peels, pulp and seeds every year. This waste could be used to make supercapacitors.
What’s needed to make this happen?
Some companies are already turning food and agricultural waste into activated carbon. For example, Haycarb, based in Sri Lanka, turns coconut shells into activated carbon. Takachar in the US is also developing small-scale technology to turn agricultural waste into useful products like activated carbon.
In Africa, fruit processing plants could set up facilities to turn their waste into activated carbon. They could then sell it to energy storage companies or other industries that need it.
To make this happen, experts are needed to further develop the science. Governments and the private sector will need to fund equipment and facilities. Then, the factories making supercapacitors need to be connected with industries that could buy them (electric vehicle, renewable energy and electronics manufacturers).
Making supercapacitors from fruit peels could create jobs and support Africa’s renewable energy goals.
- is a Postdoctoral Fellow, iThemba LABS
- This article first appeared in The Conversation
