Kentucky’s bourbon industry produces vast quantities of waste grain that is costly to transport and process. Researchers have now found a way to turn that by-product into high-performance energy-storage materials with potential applications in electric vehicles and large-scale grid storage.

More than 95 percent of all bourbon whiskey is made in Kentucky. For each barrel of bourbon, the industry also produces between six and 10 times as much “stillage”—a slurry of spent grain and water. This is normally sold to farmers as a livestock feed or soil additive, but it needs to be dried out first to reduce the weight and make it easier to process.

This is a major burden on distilleries, says Josiel Barrios Cossio, a graduate student in the University of Kentucky’s chemistry department. It either requires a lot of time and space to dry the stillage out via evaporation, or an expensive heating process. He and his colleagues have demonstrated that they can instead directly convert the wet stillage into useful carbon materials that can be used to make electrodes for batteries and supercapacitors.

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In research presented at the spring meeting of the American Chemical Society in Atlanta today, Barrios Cossio showed that the carbon materials could be used to create supercapacitors that match or exceed the energy density of commercial devices, and hybrid lithium-ion supercapacitors that can store up to 25 times as much energy as conventional designs. While the work is just a proof-of-concept, Barrios Cossio says, it could ultimately allow distilleries to turn a waste stream into a source of profit.

“And it’s a win-win scenario, because we can potentially have a more renewable and abundant biomass source, or feedstock, to produce these materials that are every day more in demand from the car industry and renewable energy applications,” he says.

Innovative Energy-Storage Solutions

Barrios Cossio first conceived of the idea while taking part in a research traineeship run by the U.S. National Science Foundation aimed at finding solutions to problems related to water, energy, and food systems. After visiting several distilleries and seeing the scale of the waste produced, as well as the challenges these businesses face in disposing of it, he began thinking of ways to put the stillage to more productive use.

He discovered a group at the Friedrich Schiller University Jena, in Jena, Germany, that had developed a process for converting waste grain from beer breweries into electrode materials for energy-storage devices. Barrios Cossio then spent a summer internship at the lab to learn about their techniques.

After returning to the United States, Barrios Cossio contacted several distilleries to source some stillage to experiment with and soon got a response from the Wilderness Trail Distillery in Danville, Kentucky. “I asked them, ‘Can I take a gallon of stillage?’” he says. “They replied to me some days later saying, ‘Yeah, you are welcome to take it. I would prefer that you take 10,000 gallons and get rid of the stillage from that day.’”

University of Kentucky researchers developed supercapacitor electrodes using bourbon distillery waste that can store more energy per kilogram than commercial devices.Josiel Barrios Cossio

To turn the stillage into useful materials, the researchers relied on a process called hydrothermal carbonization. This involves heating the wet slurry at high pressure to create a fine black carbon powder called hydrochar. One benefit of the process, says Barrios Cossio, is that the high water content of the stillage helps generate the pressure required to power the conversion.

The resulting hydrochar was then used to create two different high-value carbon materials. In one experiment, the team combined the hydrochar with potassium hydroxide and heated the mixture to around 800 °C, creating a material called activated carbon. This material is extremely porous, which means it can have a surface area higher than 1,000 square meters per gram, says Barrios Cossio. That makes it ideal for creating high-capacity supercapacitors, which store energy as charged ions on the surface of the electrode material.

The team showed that a coin-sized double-layer capacitor built using their hydrochar-derived electrodes could store up to 48 watt hours per kilogram—on par with commercially available supercapacitors.

The team also showed that they could create “hard carbon” by heating their hydrochar in a furnace at 200 °C. This material has a similar structure to graphite, which is made up of orderly stacks of single-atom-thick graphene sheets. Unlike graphite, however, in hard carbon the sheets are arranged more haphazardly. This leads to many small pores and defects, which are ideal for storing alkali metal ions, such as lithium and sodium, commonly used in batteries.

Barrios Cossio used their hydrochar-derived hard carbon to create a batterylike electrode infused with lithium ions, and then combined this with an electrode made of activated carbon to produce a hybrid supercapacitor. The device represents a balance between the high-energy capacity of batteries and the fast discharging speeds of capacitors, which Barrios Cossio says could be particularly useful for applications like electric vehicles and grid stabilization.

At present, the devices are just a proof-of-concept. Barrios Cossio admits that scaling up the process to industrial levels will require considerable refinement. The team is also currently conducting a techno-economic analysis to assess whether the approach is commercially viable. But project supervisor Marcelo Guzman, a professor of chemistry at the University of Kentucky, says it could be a promising and sustainable way to meet the growing demand for energy storage.

“Kentucky is a state that has been investing since 2019 heavily in trying to develop an industry for batteries for cars,” he says. “There has been billions of dollars going into that sector, so there is going to be a big need for material supply. We think we came on board with that problem at the right time, in the right place, and we could have materials that could be really interesting to the battery industry.”

As electronics demand higher energy density, one component has proved challenging to shrink: the capacitor. Making a smaller capacitor usually requires thinning the dielectric layer or electrode surface area, which has often resulted in a reduction of power. A new polymer material could help change that.

In a study published 18 February in Nature, a Pennsylvania State University–led team reported a capacitor crafted from a polymer blend that can operate at temperatures up to 250 °C while storing roughly four times as much energy as conventional polymer capacitors. Today’s advanced polymer capacitors typically function only up to about 100 °C, meaning engineers often rely on bulky cooling systems in high-power electronics. The research team has filed a patent for the polymer capacitors and plans to bring them to market.

Capacitors deliver rapid bursts of energy and stabilize voltage in circuits, making them essential in applications ranging from electric vehicles and aerospace electronics to power-grid infrastructure and AI data centers. Yet while transistors have steadily shrunk with advances in semiconductor manufacturing, passive components such as capacitors and inductors have not scaled at the same pace.

“Capacitors can account for 30 to 40 percent of the volume in some power electronics systems,” says Qiming Zhang, an electrical engineering researcher at Penn State and study author, explaining why it’s important to make smaller capacitors.

A Plastics Blend More Powerful Than Its Parts

The research team combined two commercially available engineered plastics: polyetherimide (PEI), originally developed by General Electric and widely used in industrial equipment, and PBPDA, known for strong heat resistance and electrical insulation. When processed together under controlled conditions, the polymers self-assemble into nanoscale structures that form thin dielectric films inside capacitors. Those structures help suppress electrical leakage while allowing the material to polarize strongly in an electric field, allowing greater energy storage.

The resulting material exhibits an unusually high dielectric constant—a measure of how much electrical energy a material can store. Most polymer dielectrics have values around four, but the blended polymer dielectric in the new work had a value of 13.5.

“If you look at the literature up to now, no one has reached this level of dielectric constant in this type of polymer system,” Zhang says. “Putting two commonly used polymers together and seeing this kind of performance was a surprise to many people.”

Because the material can remain operational even at elevated temperatures—such as those from extreme environmental heat or hot spots in densely built components—capacitors built from this polymer could potentially store the same amount of energy in a smaller package.

“With this material, you can make the same device using about [one-fourth as much] material,” Zhang says. “Because the polymers themselves are inexpensive, the cost does not increase. At the same time, the component can become smaller and lighter.”

How the Polymer Mix Improves Capacitors

The researchers’ finding is “a big advancement,” says Alamgir Karim, a polymer research director at the University of Houston who was not involved in the Penn State development. “Normally when you mix polymers, you don’t expect the dielectric constant to increase.”

Karim says the effect likely arises from nanoscale interfaces created when the polymers partially separate. “At about a 50–50 mixture, the polymers don’t fully mix and instead create a very large interfacial area,” he says. “Those interfaces may be where the unusual electrical behavior comes from.”

If the material can be produced at scale, it could help address a key bottleneck in high-power electronics. Higher-temperature capacitors could reduce cooling requirements and allow engineers to pack more power into smaller systems—an advantage for aerospace platforms, electric vehicles, the electric grid, and other high-temperature environments.

But translating the concept from laboratory methods to commercial manufacturing may present challenges, says Zongliang Xie, a postdoctoral researcher at the Lawrence Berkeley National Laboratory, in California. The Penn State team is now producing small dielectric films, but industrial capacitor manufacturing typically requires continuous rolls of material that can extend for kilometers.

“Industry generally prefers extrusion-based processing because it’s easier and cheaper to control,” Xie says. “Scaling to produce great lengths of film while maintaining the same structure and performance could complicate matters. There’s potential, but it’s also challenging.”

Still, researchers say the discovery demonstrates that new performance limits may still be unlocked using familiar materials. “Developing the material is only the first step,” Zhang says. “But it shows people that this barrier can be broken.”

What do water bottles, eggs, hemp, and cement have in common? They can be engineered into strange, but functional, energy-storage devices called supercapacitors.

As their name suggests, supercapacitors are like capacitors with greater capacity. Similar to batteries, they can store a lot of energy, but they can also charge or discharge quickly, similar to a capacitor. They’re usually found where a lot of power is needed quickly and for a limited time, like as a nearly instantaneous backup electricity for a factory or data center.

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

Some scientists believe that supercapacitors could become more super. They think there’s potential to make these devices more sustainably, at lower-cost, and maybe even better performing if they’re built from better materials.

And maybe they’re right. Last month, a group from Michigan Technological University reported making supercapacitors from plastic water bottles that had a higher capacitance than commercial ones.

Does this finding mean recycled plastic supercapacitors will soon be everywhere? The history of similar supercapacitor sustainability experiments suggests not.

About 15 years ago, it seemed like supercapacitors were going to be in high demand. Then, because of huge investments in lithium-ion technology, batteries became tough competition, explains Yury Gogotsi, who studies materials for energy-storage devices at Drexel University, in Philadelphia. “They became so much cheaper and so much faster in delivering energy that for supercapacitors, the range of application became more limited,” he says. “Basically, the trend went from making them cheaper and available to making them perform where lithium-ion batteries cannot.”

Still, some researchers remain hopeful that environmentally friendly devices have a place in the market. Yun Hang Hu, a materials scientist on the Michigan Technological University team, sees “a promising path to commercialization [for the water-bottle-derived supercapacitor] once collection and processing challenges are addressed,” he says.

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

Water Bottles

It turns out your old Poland Spring bottle could one day store energy instead of water. Last month in the journal Energy & Fuels, the 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 odd as it may seem, this process is “a practical blueprint for circular energy storage that can ride the existing PET supply chain,” says Hu.

To make the electrodes, the researchers first shredded bottles into 2-millimeter grains and then added powdered calcium hydroxide. They heated the mixture to 700 °C in a vacuum for 3 hours and were left with an electrically conductive carbon powder. After removing residual calcium and activating the carbon (increasing its surface area), they could shape the powder into a thin layer and use it as an electrode.

The process to produce the separators was much less intensive—the team cut bottles into squares about the size of a U.S. quarter or a 1-euro coin and used hot needles to poke holes in them. They optimized the pattern of the holes for the passage of current using specialized 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 an electrolyte solution, the resulting supercapacitor not only demonstrated potential for eco- and finance-friendly material usage, but also slightly outperformed traditional materials on one metric. The PET device had a capacitance of 197.2 farads per gram, while an analogous device with a glass-fiber separator had a capacitance of 190.3 farads per gram.

Eggs

Wait, don’t make your breakfast sandwich just yet! You could engineer a supercapacitor from one of your ingredients instead. In 2019, a University of Virginia team showed that electrodes, electrolytes, and separators could all be made from parts of a single object—an egg.

First, the group purchased grocery store chicken eggs and sorted their parts into eggshells, eggshell membranes, and the whites and yolks.

They ground the shells into a powder and mixed them with the egg whites and yolks. The slurry was freeze-dried and brought up to 950 °C for an hour to decompose. After a cleaning process to remove calcium, the team performed heat and potassium treatments to activate the remaining carbon. They then smoothed the egg-derived activated carbon into a film to be used as electrodes. Finally, by mixing egg whites and yolks with potassium hydroxide and letting it dry for several hours, they formed a kind of gel electrolyte.

To make separators, the group simply cleaned the eggshell membranes. Because the membranes naturally have interlaced micrometer-size fibers, their inherent structures allow for ions to move across them just as manufactured separators would.

Interestingly, the resulting fully egg-based supercapacitor was flexible, with its capacitance staying steady even when the device was twisted or bent. After 5,000 cycles, the supercapacitor retained 80 percent of its original capacitance—low compared to commercial supercapacitors, but fairly on par for others made from natural materials.

Hemp

Some people may like cannabis for more medicinal purposes, but it has potential in energy storage, too. In 2024, a group from Ondokuz Mayıs University in Türkiye used pomegranate hemp plants to produce activated carbon for an electrode.

They started by drying stems of the hemp plants in a 110 °C oven for a day and then ground the stems into a powder. Next, they added sulfuric acid and heat to create a 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 capacitance, which is, astoundingly, in range of those made from nonbiological materials. The carbon itself had an energy density of 65 watt-hours per kilogram, also in line with commercial supercapacitors.

Cement

It may have a hold over the construction industry, but is cement coming for the energy sector, too? In 2023, a group from MIT shared how they designed electrodes from water, nearly pure carbon, and cement. Using these materials, they say, creates a “synergy” between the hydrophilic cement and hydrophobic carbon that aids the electrodes’ ability to hold layers of ions when the supercapacitor is charged.

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.

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 around 10 kilowatt-hours—enough to serve about one third of an average American’s daily energy use—though the number is only theoretical.