Innovative supercapacitor material from waste

What is the plan water bottles, eggs, hempAnd cement Is it normal? They can be engineered into strange, but functional, energy storage devices called supercapacitors.

As their name suggests, supercapacitors are capacitors with a larger capacity. Similar to batteries, they can store a lot of energy, but they can also charge or discharge quickly, similar to capacitors. They are typically found where a lot of power is needed immediately and for a limited period of time, such as near-instantaneous backup power for a factory or data center.

Typically, supercapacitors are made of two activated carbon or graphene electrodes, electrolytes to introduce ions into the system, and a porous sheet of polymer or glass fiber to physically separate the electrodes. When a supercapacitor is fully charged, all the positive ions collect on one side of the separator sheet, while all the negative ions collect on the other side. When it is discharged, the ions are randomly distributed, and it can switch between these states much faster than a battery.

Some scientists believe that supercapacitors can be created More Very good. He believes there is potential to make these devices more durable at lower cost, and perhaps even perform better, if they were made from better materials.

And maybe they are right. Last month, a group at Michigan Technological University reported making supercapacitors from plastic water bottles that had a higher capacity than commercial bottles.

Does this discovery mean that recycled plastic supercapacitors will soon be everywhere? The history of similar supercapacitor stability experiments does not suggest so.

About fifteen years ago, it seemed like supercapacitors would be in high demand – then, due to heavy investment in lithium-ion technology, batteries became stiffer competition, explains Yuri GogotsyJoe studies materials for energy storage devices at Drexel University. “They became so cheap and so fast at providing energy that for supercapacitors, the range of application became more limited,” he says. “Basically, the trend went from making them cheap and available to making them perform where lithium-ion batteries can’t.”

Still, some researchers are optimistic that eco-friendly devices will find a place in the market. I am hanging like this“We see a promising path to commercialization (for water bottle-derived supercapacitors) once the collection and processing challenges are resolved,” he says, a materials scientist on the Michigan Technological University team.

Here’s how scientists make supercapacitors from strange, unexpected materials:

water bottles

It turns out that your old Poland Spring bottle could one day store energy instead of water. Last month in the journal energy and fuelA Michigan Technological University team published a new method for converting polyethylene terephthalate (PET), the material that makes up single-use plastic water bottles, into both electrodes and separators.

As strange as it may seem, Hu says, this process is “a practical blueprint for circular energy storage that can ride up the existing PET supply chain.”

To make the electrodes, the researchers first cut the bottles into two-millimeter pieces and then mixed powdered calcium hydroxide with them. They heated the mixture to 700 degrees Celsius in vacuum for three hours and left it with an electrically conductive carbon powder. After removing the residual calcium and activating the carbon (increasing its surface area), the powder can be shaped into a thin layer and used as an electrode.

The process of making the separators was much less intensive – the team cut bottles into squares the size of a US quarter and used hot needles to poke holes in them. They optimized the pattern of holes to allow current to flow using special software. PET is a good material for a separator because of its “excellent mechanical strength, high thermal stability and excellent insulation,” Hu says.

Filled with electrolyte solution, the resulting supercapacitor not only demonstrated the potential for the use of environmentally and finance-friendly materials, but also slightly outperformed conventional materials on one metric. The capacitance of the PET device was 197.2 farads per gram, while an analog device with a glass fiber separator had a capacitance of 190.3 farads per gram.

eggs

Wait, don’t make your breakfast sandwich yet! Instead you can engineer a supercapacitor from any of your own materials. In 2019, a team from the University of Virginia showed The electrodes, electrolytes, and separator can all be made from parts of the same object – an egg.

First, the group purchased chicken eggs from the grocery store and sorted their parts into eggshell, eggshell membrane, and white and yolk.

He ground the peels into powder and mixed them with egg white and yolk. The solution was freeze dried and brought to 950 °C for one hour to decompose. After a cleaning process to remove the calcium, the team used heat and potassium treatment to activate the remaining carbon. They then smoothed the activated carbon obtained from the eggs into a film to use as an electrode. Finally, by mixing egg whites and yolks with potassium hydroxide and letting it dry for several hours, they created a type of gel electrolyte.

To make the separator, the group simply cleaned eggshell membranes. Because membranes naturally consist of micrometer-sized fibers interconnected, their underlying structures allow ions to pass through them just as manufactured separators do.

Interestingly, the resulting entirely egg-based supercapacitor was flexible, with its capacity remaining stable even as the device bent or twisted. After 5,000 cycles, the supercapacitor retained 80 percent of its original capacity – less than commercial supercapacitors, but comparable to others made from natural materials.

hemp

Some people may prefer cannabis for more medicinal purposes, but it also has the potential for energy storage. In 2024, a group from Ondokuz Mays University in Turkiye used pomegranate hemp plants To produce activated carbon for electrodes.

They began by drying the stems of hemp plants in a 110 °C oven for a day, and then grinding the stems into a powder. Next, they added sulfuric acid and heat to create biochar, and finally, activated the char by saturating it with potassium hydroxide and heating it again.

After 2,000 cycles, the supercapacitor with hemp-derived electrodes still retained 98 percent of its original capacity, which, surprisingly, is in the range of capacitances made from non-biological materials. The energy density of carbon was 65 watt-hours per kilogram, which was also in line with commercial supercapacitors.

cement

It may have a hold on the construction industry, but is cement coming for the energy sector too? In 2023, a group from MIT shared How he designed electrodes from water, almost pure carbon and cement. The use of these materials, he says, creates a “synergy” between the hydrophilic cement and hydrophobic carbon that aids the electrode’s ability to hold layers of ions as the supercapacitor charges.

To test the hypothesis, the team created 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.

Impressively, the cement supercapacitors were able to maintain capacitance with little loss even after 10,000 cycles. The researchers also calculated that one of their supercapacitors could store about 10 kilowatt hours – enough to meet about a third of an average American’s daily energy use – although this number is only theoretical.