Innovative Supercapacitor Materials From Waste

What to do water bottles, eggs, hempAnd cement is there anything in common? They can be turned into strange but functional energy storage devices called supercapacitors.

As the name suggests, supercapacitors are similar to capacitors with larger capacity. Similar to: batteriesthey can store a lot of energy, but can also be charged or discharged quickly, like a capacitor. They are typically found where a lot of power is needed quickly and for a limited time, such as as a near-instant backup power source for a factory or data center.

Typically, supercapacitors consist of two activated carbons or graphene electrodes, electrolytes to introduce ions into the system and a porous sheet of polymer or fiberglass to physically separate the electrodes. When supercapacitor fully charged, all the positive ions are collected on one side of the separating sheet and all the negative ions are collected on the other. When it is discharged, the ions are distributed randomly, and it can switch between these states much faster than batteries Maybe.

Some scientists believe that supercapacitors could become more super. They believe there is potential to make these devices greener, cheaper and perhaps even more efficient if they are made from better materials.

And perhaps they are right. Last month, a team from Michigan Technological University reported creating supercapacitors from plastic water bottles that have a higher capacity than commercial ones.

Does this discovery mean that supercapacitors made from recycled plastic will soon be everywhere? The history of such a supercapacitor sustainability experiments show that it does not.

About 15 years ago this it seemed that supercapacitors would be in great demand. Then, thanks to huge investments in lithium-ion technology, batteries have become fierce competition, explains Yury Gogotsistudying materials for energy storage devices in Drexel Universityin Philadelphia. “They have become so much cheaper and faster at delivering energy that the range of applications for supercapacitors has become more limited,” he says. “Essentially, the trend has gone from making them cheaper and more accessible to making them work where lithium ion batteries I can’t.”

Still, some researchers remain hopeful that eco-friendly devices will find their way into the market. Yun Hang Humaterials scientist on the Michigan Technological University team, sees “a promising path to commercialization [for the water-bottle-derived supercapacitor] once the collection and processing issues are resolved,” he says.

Here's how scientists create supercapacitors from strange, unexpected materials:

Water bottles

It turns out your old one Poland A spring bottle could one day store energy instead of water. Last month in the magazine Energy and fuelA Michigan Technological University team has published a new method for recycling polyethylene terephthalate (PET), the material used to make disposable plastic water bottles, into electrodes and separators.

As strange as it may seem, this process is a “practical project energy storage which can be used in the existing PET supply chain,” says Hu.

To make the electrodes, the researchers first crushed the bottles into 2mm grains and then added powdered calcium hydroxide. They heated the mixture to 700°C in a vacuum for 3 hours and obtained electrically conductive carbon powder. By removing the residual calcium and activating the carbon (increasing its surface area), they were able to form a thin layer of the powder and use it as an electrode.

The process of making the separators was much less intensive: the team cut the bottles into squares about the size of a US quarter or a €1 coin and used hot needles to make holes in them. They optimized the pattern of holes for current flow using specialized software. PET is a good material for a cage because of its “superior mechanical strength, high thermal stability and excellent insulation,” says Hu.

The resulting supercapacitor, filled with an electrolyte solution, not only demonstrated the potential for environmentally friendly and economical use of materials, but also slightly outperformed traditional materials in one metric. The PET device had a capacity of 197.2 farads per gram, and a similar device with a fiberglass separator had a capacity of 190.3 farads per gram.

Eggs

Wait until you make a breakfast sandwich! Instead, you can construct a supercapacitor from one of your ingredients. In 2019, the University of Virginia team showed that electrodes, electrolytes and separators can be made from parts of a single object – an egg.

First, the group bought chicken eggs from a grocery store and separated the egg parts into eggshells, eggshell casings, and whites and yolks.

They ground the shells into powder and mixed them with egg whites and yolks. The slurry was freeze-dried and brought to a temperature of 950°C for one hour to decompose. After a purification process to remove calcium, the team applied heat and potassium treatments to activate the remaining carbon. They then turned the activated carbon obtained from the eggs into a film that could be used as electrodes. Finally, by mixing egg whites and yolks with potassium hydroxide and letting them dry for several hours, they formed a kind of gel electrolyte.

To make the separators, the team simply peeled the membranes of eggshells. Because the membranes are naturally composed of interwoven micron-sized fibers, their internal structure allows ions to move through them in the same way that industrial separators do.

Interestingly, the resulting supercapacitor, made entirely from an egg, was flexible: its capacitance remained constant even when the device was twisted or bent. After 5,000 cycles, the supercapacitor retained 80 percent of its original capacity—low compared to commercial supercapacitors, but on par with others made from natural materials.

Hemp

Some people may like hemp for more medical purposes, but it also has potential for energy storage. In 2024, a group from Ondokuz Mayis University in Turkey used pomegranate and hemp plants for the production of activated carbon for the electrode.

They started by drying the stems of the cannabis plants in an oven at 110°C for a day and then ground the stems into powder. They then added sulfuric acid and heated it to create biochar, and finally activated the carbon by saturating it with potassium hydroxide and heating it again.

After 2,000 cycles, the supercapacitor with hemp electrodes still retained 98 percent of its original capacity, which is surprisingly within the range of capacitance made from non-biological materials. Carbon itself had energy density 65 watt-hours per kilogram, which is also similar to commercial supercapacitors.

Cement

It may have an impact on the construction industry, but is cement reaching the energy sectortoo much? In 2023, a group of WITH general how they developed electrodes from water, almost pure carbon and cement. According to them, the use of these materials creates a “synergy” between hydrophilic cement and hydrophobic carbon, which AIDS the ability of electrodes to retain layers of ions when charging a supercapacitor.

To test the hypothesis, the team built eight electrodes using slightly different proportions of the three ingredients, different types of carbon, and different electrode thicknesses. The electrodes were saturated with potassium chloride – an electrolyte – and capacitance measurements began.

What's impressive is that cement supercapacitors were able to maintain capacity with little loss even after 10,000 cycles. The researchers also estimate that one of their supercapacitors could store about 10 kilowatt-hours—enough to supply about a third of the average American's daily energy use—though that number is only theoretical.