This giant bubble on the island of Sardinia holds 2,000 tonnes of carbon dioxide. But the gas wasn’t captured from factory emissions, nor was it pulled from the air. It came from a gas supplier, and it lives permanently inside the dome’s system to serve an eco-friendly purpose: to store large amounts of excess renewable energy until it’s needed.

Developed by the Milan-based company Energy Dome, the bubble and its surrounding machinery demonstrate a first-of-its-kind “CO2 Battery,” as the company calls it. The facility compresses and expands CO₂ daily in its closed system, turning a turbine that generates 200 megawatt-hours of electricity, or 20 MW over 10 hours. And in 2026, replicas of this plant will start popping up across the globe.

We mean that literally. It takes just half a day to inflate the bubble. The rest of the facility takes less than two years to build and can be done just about anywhere there’s 5 hectares of flat land.

The first to build one outside of Sardinia will be one of India’s largest power companies, NTPC Limited. The company expects to complete its CO2 Battery sometime in 2026 at the Kudgi power plant in Karnataka, in India. In Wisconsin, meanwhile, the public utility Alliant Energy received the all clear from authorities to begin construction of one in 2026 to supply power to 18,000 homes.

And Google likes the concept so much that it plans to rapidly deploy the facilities in all of its key data-center locations in Europe, the United States, and the Asia-Pacific region. The idea is to provide electricity-guzzling data centers with round-the-clock clean energy, even when the sun isn’t shining or the wind isn’t blowing. The partnership with Energy Dome, announced in July, marked Google’s first investment in long-duration energy storage.

“We’ve been scanning the globe seeking different solutions,” says Ainhoa Anda, Google’s senior lead for energy strategy, in Paris. The challenge the tech giant has encountered is not only finding a long-duration storage option, but also one that works with the unique specs of every region. “So standardization is really important, and this is one of the aspects that we really like” about Energy Dome, she says. “They can really plug and play this.”

Google will prioritize placing the Energy Dome facilities where they’ll have the most impact on decarbonization and grid reliability, and where there’s a lot of renewable energy to store, Anda says. The facilities can be placed adjacent to Google’s data centers or elsewhere within the same grid. The companies did not disclose the terms of the deal.

Anda says Google expects to help the technology “reach a massive commercial stage.”

Getting creative with long-duration energy storage

All this excitement is based on Energy Dome’s one full-size, grid-connected plant in Ottana, Sardinia, which was completed in July. It was built to help solve one of the energy transition’s biggest challenges: the need for grid-scale storage that can provide power for more than 8 hours at a time. Called long-duration energy storage, or LDES in industry parlance, the concept is the key to maximizing the value of renewable energy.

When sun and wind are abundant, solar and wind farms tend to produce more electricity than a grid needs. So storing the excess for use when these resources are scarce just makes sense. LDES also makes the grid more reliable by providing backup and supplementary power.

The problem is that even the best new grid-scale storage systems on the market—mainly lithium-ion batteries—provide only about 4 to 8 hours of storage. That’s not long enough to power through a whole night, or multiple cloudy and windless days, or the hottest week of the year, when energy demand hits its peak.

After the CO2 leaves the dome, it is compressed, cooled, reduced to a liquid, and stored in pressure vessels. To release the energy, the process reverses: The liquid is evaporated, heated, expanded, and then fed through a turbine that generates electricity. Luigi Avantaggiato

Lithium-ion battery systems could be increased in size to store more and last longer, but systems of that size usually aren’t economically viable. Other grid-scale battery chemistries and approaches are in development, such as sodium-based, iron-air, and vanadium redox flow batteries. But the energy density, costs, degradation, and funding complications have challenged the developers of those alternatives.

Researchers have also experimented with storing energy by compressing air, heating up blocks or sand, using hydrogen or methanol, pressurizing water deep underground, and even dangling heavy objects in the air and dropping them. (The creativity devoted to LDES is impressive.) But geologic constraints, economic viability, efficiency, and scalability have hindered the commercialization of these strategies.

The tried-and-true grid-scale storage option—pumped hydro, in which water is pumped between reservoirs at different elevations—lasts for decades and can store thousands of megawatts for days. But these systems require specific topography, a lot of land, and can take up to a decade to build.

CO2 Batteries check a lot of boxes that other approaches don’t. They don’t need special topography like pumped-hydro reservoirs do. They don’t need critical minerals like electrochemical and other batteries do. They use components for which supply chains already exist. Their expected lifetime stretches nearly three times as long as lithium-ion batteries. And adding size and storage capacity to them significantly decreases cost per kilowatt-hour. Energy Dome expects its LDES solution to be 30 percent cheaper than lithium-ion.

China has taken note. China Huadian Corp. and Dongfang Electric Corp. are reportedly building a CO₂-based energy-storage facility in the Xinjiang region of northwest China. Media reports show renderings of domes but give widely varying storage capacities—including 100 MW and 1,000 MW. The Chinese companies did not respond to IEEE Spectrum’s requests for information.

“What I can say is that they are developing something very, very similar [to Energy Dome’s CO2 Battery] but quite large in scale,” says Claudio Spadacini, Energy Dome’s founder and CEO. The Chinese companies “are good, they are super fast, and they have a lot of money,” he says.

Why is Google investing in CO2 Batteries?

When I visited Energy Dome’s Sardinia facility in October, the CO₂ had just been pumped out of the dome, so I was able to peek inside. It was massive, monochromatic, and pretty much empty. The inner membrane, which had been holding the uncompressed CO₂, had collapsed across the entire floor. A few pockets of the gas remained, making the off-white sheet billow up in spots.

Meanwhile, the translucent outer dome allowed some daylight to pass through, creating a creamy glow that enveloped the vast space. With no structural framing, the only thing keeping the dome upright was the small difference in pressure between the inside and outside air.

“This is incredible,” I said to my guide, Mario Torchio, Energy Dome’s global marketing and communications director.

“It is. But it’s physics,” he said.

Outside the dome, a series of machines connected by undulating pipes moves the CO₂ out of the dome for compressing and condensing. First, a compressor pressurizes the gas from 1 bar (100,000 pascals) to about 55 bar (5,500,000 pa). Next, a thermal-energy-storage system cools the CO₂ to an ambient temperature. Then a condenser reduces it into a liquid that is stored in a few dozen pressure vessels, each about the size of a school bus. The whole process takes about 10 hours, and at the end of it, the battery is considered charged.

To discharge the battery, the process reverses. The liquid CO₂ is evaporated and heated. It then enters a gas-expander turbine, which is like a medium-pressure steam turbine. This drives a synchronous generator, which converts mechanical energy into electrical energy for the grid. After that, the gas is exhausted at ambient pressure back into the dome, filling it up to await the next charging phase.

Energy Dome engineers inspect the dryer system, which keeps the gaseous CO₂ in the dome at optimal dryness levels at all times.Luigi Avantaggiato

It’s not rocket science. Still, someone had to be the first to put it together and figure out how to do it cost-effectively, which Spadacini says his company has accomplished and patented. “How we seal the turbo machinery, how we store the heat in the thermal-energy storage, how we store the heat after condensing…can really cut costs and increase the efficiency,” he says.

The company uses pure, purpose-made CO₂ instead of sourcing it from emissions or the air, because those sources come with impurities and moisture that degrade the steel in the machinery.

What happens if the dome is punctured?

On the downside, Energy Dome’s facility takes up about twice as much land as a comparable capacity lithium-ion battery would. And the domes themselves, which are about the height of a sports stadium at their apex, and longer, might stand out on a landscape and draw some NIMBY pushback.

And what if a tornado comes? Spadacini says the dome can withstand wind up to 160 kilometers per hour. If Energy Dome can get half a day’s warning of severe weather, the company can just compress and store the CO₂ in the tanks and then deflate the outer dome, he says.

If the worst happens and the dome is punctured, 2,000 tonnes of CO₂ will enter the atmosphere. That’s equivalent to the emissions of about 15 round-trip flights between New York and London on a Boeing 777. “It’s negligible compared to the emissions of a coal plant,” Spadacini says. People will also need to stay back 70 meters or more until the air clears, he says.

Worth the risk? The companies lining up to build these systems seem to think so.

This article appears in the January 2026 print issue as “Grid-Scale CO₂ Batteries Will Take Off in 2026.”

This is a sponsored article brought to you by The University of Sheffield.

Across global electricity networks, the shift to renewable energy has fundamentally changed the behavior of power systems. Decades of engineering assumptions, predictable inertia, dispatchable baseload generation, and slow, well-characterized system dynamics, are now eroding as wind and solar become dominant sources of electricity. Grid operators face increasingly steep ramp events, larger frequency excursions, faster transients, and prolonged periods where fossil generation is minimal or absent.

In this environment, battery energy storage systems (BESS) have emerged as essential tools for maintaining stability. They can respond in milliseconds, deliver precise power control, and operate flexibly across a range of services. But unlike conventional generation, batteries are sensitive to operational history, thermal environment, state of charge window, system architecture, and degradation mechanisms. Their long-term behavior cannot be described by a single model or simple efficiency curve, it is the product of complex electrochemical, thermal, and control interactions.

Most laboratory tests and simulations attempt to capture these effects, but they rarely reproduce the operational irregularities of the grid. Batteries in real markets are exposed to rapid fluctuations in power demand, partial state of charge cycling, fast recovery intervals, high-rate events, and unpredictable disturbances. As Professor Dan Gladwin, who leads Sheffield’s research into grid-connected energy storage, puts it, “you only understand how storage behaves when you expose it to the conditions it actually sees on the grid.”

This disconnect creates a fundamental challenge for the industry: How can we trust degradation models, lifetime predictions, and operational strategies if they have never been validated against genuine grid behavior?

Few research institutions have access to the infrastructure needed to answer that question. The University of Sheffield is one of them.

Sheffield’s Centre for Research into Electrical Energy Storage and Applications (CREESA) operates one of the UK’s only research-led, grid-connected, multi-megawatt battery energy storage testbeds. The University of Sheffield

Sheffield’s unique facility

The Centre for Research into Electrical Energy Storage and Applications (CREESA) operates one of the UK’s only research-led, grid-connected, multi-megawatt battery energy storage testbeds. This environment enables researchers to test storage technologies not just in simulation or controlled cycling rigs, but under full-scale, live grid conditions. As Professor Gladwin notes, “we aim to bridge the gap between controlled laboratory research and the demands of real grid operation.”

At the heart of the facility is an 11 kV, 4 MW network connection that provides the electrical and operational realism required for advanced diagnostics, fault studies, control algorithm development, techno-economic analysis, and lifetime modeling. Unlike microgrid scale demonstrators or isolated laboratory benches, Sheffield’s environment allows energy storage assets to interact with the same disturbances, market signals, and grid dynamics they would experience in commercial deployment.

“The ability to test at scale, under real operational conditions, is what gives us insights that simulation alone cannot provide.” —Professor Dan Gladwin, The University of Sheffield

The facility includes:

• A 2 MW / 1 MWh lithium titanate system, among the first independent grid-connected BESS of its kind in the UK
• A 100 kW second-life EV battery platform, enabling research into reuse, repurposing, and circular-economy models
• Support for flywheel systems, supercapacitors, hybrid architectures, and fuel-cell technologies
• More than 150 laboratory cell-testing channels, environmental chambers, and impedance spectroscopy equipment
• High-speed data acquisition and integrated control systems for parameter estimation, thermal analysis, and fault response measurement

The infrastructure allows Sheffield to operate storage assets directly on the live grid, where they respond to real market signals, deliver contracted power services, and experience genuine frequency deviations, voltage events, and operational disturbances. When controlled experiments are required, the same platform can replay historical grid and market signals, enabling repeatable full power testing under conditions that faithfully reflect commercial operation. This combination provides empirical data of a quality and realism rarely available outside utility-scale deployments, allowing researchers to analyse system behavior at millisecond timescales and gather data at a granularity rarely achievable in conventional laboratory environments.

According to Professor Gladwin, “the ability to test at scale, under real operational conditions, is what gives us insights that simulation alone cannot provide.”

Dan Gladwin, Professor of Electrical and Control Systems Engineering, leads Sheffield’s research into grid-connected energy storage.The University of Sheffield

Setting the benchmark with grid scale demonstration

One of Sheffield’s earliest breakthroughs came with the installation of a 2 MW / 1 MWh lithium titanate demonstrator, a first-of-a-kind system installed at a time when the UK had no established standards for BESS connection, safety, or control. Professor Gladwin led the engineering, design, installation, and commissioning of the system, establishing one of the country’s first independent megawatt scale storage platforms.

The project provided deep insight into how high-power battery chemistries behave under grid stressors. Researchers observed sub-second response times and measured the system’s capability to deliver synthetic inertia-like behavior. As Gladwin reflects, “that project showed us just how fast and capable storage could be when properly integrated into the grid.”

But the demonstrator’s long-term value has been its continued operation. Over nearly a decade of research, it has served as a platform for:

• Hybridization studies, including battery-flywheel control architectures
• Response time optimization for new grid services
• Operator training and market integration, exposing control rooms and traders to a live asset
• Algorithm development, including dispatch controllers, forecasting tools, and prognostic and health management systems
• Comparative benchmarking, such as evaluation of different lithium-ion chemistries, lead-acid systems, and second-life batteries

A recurring finding is that behavior observed on the live grid often differs significantly from what laboratory tests predict. Subtle electrical, thermal, and balance-of-plant interactions that barely register in controlled experiments can become important at megawatt-scale, especially when systems are exposed to rapid cycling, fluctuating set-points, or tightly coupled control actions. Variations in efficiency, cooling system response, and auxiliary power demand can also amplify these effects under real operating stress. As Professor Gladwin notes, “phenomena that never appear in a lab can dominate behavior at megawatt scale.”

These real-world insights feed directly into improved system design. By understanding how efficiency losses, thermal behavior, auxiliary systems, and control interactions emerge at scale, researchers can refine both the assumptions and architecture of future deployments. This closes the loop between application and design, ensuring that new storage systems can be engineered for the operational conditions they will genuinely encounter rather than idealized laboratory expectations.

Ensuring longevity with advanced diagnostics

Sheffield’s Centre for Research into Electrical Energy Storage and Applications (CREESA) enables researchers to test storage technologies not just in simulation or controlled cycling rigs, but under full-scale, live grid conditions.The University of Sheffield

Ensuring the long-term reliability of storage requires understanding how systems age under the conditions they actually face. Sheffield’s research combines high-resolution laboratory testing with empirical data from full-scale grid-connected assets, building a comprehensive approach to diagnostics and prognostics. In Gladwin’s words, “A model is only as good as the data and conditions that shape it. To predict lifetime with confidence, we need laboratory measurements, full-scale testing, and validation under real-world operating conditions working together.”

A major focus is accurate state estimation during highly dynamic operation. Using advanced observers, Kalman filtering, and hybrid physics-ML approaches, the team has developed methods that deliver reliable SOC, SOH and SOP estimates during rapid power swings, irregular cycling, and noisy conditions where traditional methods break down.

Another key contribution is understanding cell-to-cell divergence in large strings. Sheffield’s data shows how imbalance accelerates near SOC extremes, how thermal gradients drive uneven ageing, and how current distribution causes long-term drift. These insights inform balancing strategies that improve usable capacity and safety.

Sheffield has also strengthened lifetime and degradation modeling by incorporating real grid behavior directly into the framework. By analyzing actual market signals, frequency deviations, and dispatch patterns, the team uncovers ageing mechanisms that do not appear during controlled laboratory cycling and would otherwise remain hidden.

These contributions fall into four core areas:

State Estimation and Parameter Identification

• Robust SOC/SOH estimation
• Online parameter identification for equivalent circuit models
• Power capability prediction using transient excitation
• Data selection strategies under noise and variability

Degradation and Lifetime Modelling

• Degradation models built on real frequency and market data
• Analysis of micro cycling and asymmetric duty cycles
• Hybrid physics-ML forecasting models

Thermal and Imbalance Behavior

• Characterizing thermal gradients in containerized systems
• Understanding cell imbalance in large-scale systems
• Mitigation strategies at the cell and module level
• Coupled thermal-electrical behavior under fast cycling

Hybrid Systems and Multi-Technology Optimization

• Battery-flywheel coordination strategies
• Techno-economic modeling for hybrid assets
• Dispatch optimization using evolutionary algorithms
• Control schemes that extend lifetime and enhance service performance

Beyond grid-connected systems, Sheffield’s diagnostic methods have also proved valuable in off-grid environments. A key example is the collaboration with MOPO, a company deploying pay-per-swap lithium-ion battery packs in low-income communities across Sub-Saharan Africa. These batteries face deep cycling, variable user behavior, and sustained high temperatures, all without active cooling or controlled environments. The team’s techniques in cell characterization, parameter estimation, and in-situ health tracking have helped extend the usable life of MOPO’s battery packs. “By applying our know-how, we can make these battery-swap packs clean, safe, and significantly more affordable than petrol and diesel generators for the communities that rely on them,” says Professor Gladwin.

Beyond grid-connected systems, Sheffield’s diagnostic methods have also proved valuable in off-grid environments. A key example is the collaboration with MOPO, a company deploying pay-per-swap lithium-ion battery packs in low-income communities across Sub-Saharan Africa. MOPO

Collaboration and the global future

A defining strength of Sheffield’s approach is its close integration with industry, system operators, technology developers, and service providers. Over the past decade, its grid-connected testbed has enabled organizations to trial control algorithms, commission their first battery assets, test market participation strategies, and validate performance under real operational constraints.

These partnerships have produced practical engineering outcomes, including improved dispatch strategies, refined control architectures, validated installation and commissioning methods, and a clearer understanding of degradation under real-world market operation. According to Gladwin, “It is a two-way relationship, we bring the analytical and research tools, industry brings the operational context and scale.”

One of Sheffield’s earliest breakthroughs came with the installation of a 2 MW / 1 MWh lithium titanate demonstrator. Professor Gladwin led the engineering, design, installation, and commissioning of the system, establishing one of UK’s first independent megawatt scale storage platforms.The University of Sheffield

This two-way exchange, combining academic insight with operational experience, ensures that Sheffield’s research remains directly relevant to modern power systems. It continues to shape best practice in lifetime modelling, hybrid system control, diagnostics, and operational optimization.

As electricity systems worldwide move toward net zero, the need for validated models, proven control algorithms, and empirical understanding will only grow. Sheffield’s combination of full-scale infrastructure, long-term datasets, and collaborative research culture ensures it will remain at the forefront of developing storage technologies that perform reliably in the environments that matter most, the real world.

For the last 12 weeks, California startup Rondo Energy has been operating what it’s calling the world’s largest thermal battery. Rondo’s system converts cheap renewable electricity into heat that can be discharged on demand into industrial processes.

This differs from most next-generation energy-storage strategies, which provide electricity to grids in the absence of sun or wind. Instead, Rondo’s system aims to help decarbonize emissions-heavy sectors like steelmaking and cement.

The system works like a toaster crossed with a blast furnace. Electricity from solar arrays heat iron wires similar to those in a toaster oven. These warm hundreds of tonnes of refractory bricks to temperatures up to 1,500 °C. After four to six hours of charging a day, the heat can be discharged as air or steam, without combustion or emissions.

To discharge heat, a circulating air blower is turned on, pushing air up through the brick stack and heating it to over 1,000 °C before releasing it through an outlet. The heat-delivery rate can be controlled by adjusting the airflow. The battery can discharge steam instead of heat by injecting water into an attached chamber, which the heated air passes through before leaving the battery through the outlet.

The real challenge in thermal energy storage is not storing heat; it’s being able to charge rapidly and then deliver heat continuously at the same temperature, says John O’Donnell, Rondo Energy’s chief innovation officer. The structure of Rondo’s heat battery, which O’Donnell describes as “a 3D-checkerboard of brick and open chambers,” keeps temperatures uniform and enables rapid charging. “We can turn charging circuits on and off as fast as you can turn your toaster on and off,” O’Donnell says. “So we can be agile.”

In Rondo’s first project, its 100-megawatt-hour battery is supplying heat for an enhanced oil-recovery facility operated by Holmes Western Oil Corp. in Kern County, Calif. The battery, which is about the size of a small office building, is powered by an off-grid, 20-megawatt solar array built for this purpose. It converts the clean electricity into heat, and then generates steam that is injected into oil wells, heating the oil so that it thins out and flows more easily, increasing production.

Holmes Western Oil previously accomplished this with a gas-fired boiler. Cutting it will save Holmes just under 13,000 tonnes of CO₂ emissions annually while also lowering costs, according to Rondo. “Making steam for oil fields is the second largest portion of industrial heat in the state,” says O’Donnell.

Rondo’s choice to deploy its first commercial-scale, emissions-reducing battery for the extraction of a fossil fuel stirred some controversy. Critics argue that deploying clean tech to improve or prolong fossil fuel production is counterproductive.

Thermal Batteries for Clean Industrial Heat

Several other companies are developing thermal batteries with industrial heat applications. Antora Energy makes modular carbon-block heat batteries that can reach over 1,500 °C and are being deployed at pilot industrial sites. EnergyNest is doing early commercial installations of its concrete-based thermal modules, and is partnering with Siemens Energy to scale across Europe. Calectra’s ultrahigh-temperature systems are in the pilot phase, and EarthEn Energy launched its modular low-temperature heat batteries in July.

These companies are focused on heat because it’s central to producing staples such as steel, cement, food, and chemicals. Many of these manufacturing processes run continuously and maintain high temperatures for weeks or months at a time, ranging from 72 °C for pasteurizing milk to over 1,000 °C for making steel or cement.

The cheapest, most efficient way to produce consistent heat has long been with fossil fuels; nothing burns as slow and hot as coal or natural gas. Their energy density, reliability, and low cost have made them hard to replace. However, industrial heat accounts for about 18 percent of greenhouse gas emissions and more than 20 percent of global energy consumption. So innovators aiming to decarbonize these industrial sectors have their work cut out for them.

But solar power is getting cheaper. In 2024, California’s solar fields generated almost as much electricity as its gas plants. “Because of what the wind and solar industry have done, we now have intermittent grid prices that are cheaper than fuel in a lot of places in the world,” says O’Donnell. Some locations generate so much clean power that the grid can’t absorb it all, forcing negative electricity prices for a few hours a day.

How Can Thermal Batteries Scale?

Thermal batteries supplying heat face several challenges. In order for them to scale up, industrial customers must buy renewable electricity wholesale at times of day when it’s cheap, which requires dynamic real-time pricing. Many states only allow industrial customers to buy power at fixed daily rates. “We are really eager to see the regulatory framework get modernized,” O’Donnell says.

The price of natural gas plays a role, too. It’s relatively inexpensive in the United States, thanks to shale gas from fracking, but if its price increases due to exports or other factors, batteries like Rondo’s could become a cheaper source of heat. This is already the case in European countries such as Germany, where the price of natural gas has skyrocketed in the last three and a half years.

Plus, heat batteries could be difficult to integrate into existing industrial infrastructure. Not every facility has space for a battery the size of an office building and a dedicated solar array. The batteries’ high up-front costs and the fact that they’re still a largely unproven technology will make some would-be customers reluctant to give them a try.

Nonetheless, heat batteries like Rondo’s are a promising solution for decarbonizing the industrial sector. “The thermal-storage market is absolutely capable of accelerating to create meaningful impact,” says Blaine Collison, executive director of the Renewable Thermal Collaborative, a coalition focused on decarbonizing thermal energy. “When I look at some of the fundamental characteristics of the technology—relatively straightforward materials, ability to offtake renewable electricity, modularity—I see scale.”

This article was updated on October 31, 2025.

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.