One-pot synthesis of carbon nanotube/graphene composite phase change materials based on Pickering emulsion for solar thermal storage

The world is constantly searching for better ways to capture and hold the energy that comes from the sun. One promising approach involves materials that can soak up heat, store it, and release it later when needed. These substances, known as phase change materials, work by absorbing a large amount of energy as they melt from a solid to a liquid, and then giving that energy back as they freeze again. This process allows them to hold vast amounts of thermal energy in a very small space, making them useful for everything from regulating the temperature of buildings to storing solar power for later use. However, these materials face two significant hurdles that have kept them from being widely adopted. First, they are often poor conductors of heat, meaning they take too long to warm up or cool down. Second, when they melt, they turn into a liquid that can easily leak out of their containers, creating a mess and rendering the system useless.

Researchers at Xi'an Shiyou University have developed a new way to solve both problems at once. They created a composite material that traps a wax-like substance called hexadecanol inside a sturdy, three-dimensional net made of carbon. To build this structure, they used a technique called a Pickering emulsion, which relies on tiny solid particles to stabilize a mixture of oil and water, rather than using traditional chemical soaps. In this specific case, the team used sheets of graphene oxide and tiny tubes of carbon nanotubes to act as the stabilizing particles. They mixed these carbon components with molten hexadecanol and water, creating a stable emulsion where the wax droplets were surrounded by a protective shell of carbon. By heating this mixture in a sealed container, they triggered a chemical reaction that turned the graphene oxide into a solid, porous scaffold, effectively locking the wax inside a carbon cage. This one-step process, which the researchers call a one-pot synthesis, resulted in a material that is solid, leak-proof, and much better at moving heat than the wax alone.

The resulting material, which the team tested in several different formulations, showed remarkable improvements over the pure wax. When the researchers measured how well the composite conducted heat, they found that adding the carbon nanotubes made a clear difference. The sample with the highest amount of nanotubes conducted heat 25.40 percent better than the pure hexadecanol. This improvement is crucial because it means the material can absorb solar energy and release stored heat much faster, making it more practical for real-world applications. The carbon framework also solved the leakage problem completely. When the researchers placed the composite on a hot plate set to 70 degrees Celsius, the pure hexadecanol melted and flowed away within minutes. In contrast, the composite material held its shape perfectly, with no visible leakage, even after prolonged heating. The carbon net acted as a rigid skeleton that supported the wax, keeping it contained even when it turned liquid.

Beyond just holding heat, the material proved to be excellent at capturing sunlight and turning it into thermal energy. The carbon components, particularly the graphene and nanotubes, are very good at absorbing light. When the researchers shone a simulated sun on the samples, the composite materials heated up significantly faster than the pure wax. The sample with the most carbon nanotubes reached a temperature of 75 degrees Celsius in just 600 seconds, while the pure wax only reached 70 degrees in the same amount of time. This suggests that the material can harvest solar energy more efficiently, a key requirement for solar thermal storage systems. The researchers also observed that the material cooled down faster than the pure wax, which could be beneficial for systems that need to release heat quickly.

The study confirmed that the carbon and the wax did not chemically react with each other to form a new substance; instead, they were physically combined. The carbon structure simply provided a container and a highway for heat to travel through. While the total amount of heat the material could store was slightly lower than that of the pure wax—because the carbon itself does not melt and store energy—the amount remained high enough to be effective. The researchers noted that the material maintained its ability to store energy even after repeated heating and cooling cycles, indicating strong stability. By using a simple, scalable method to create a material that is leak-proof, thermally conductive, and efficient at capturing solar energy, the team has produced a candidate that could help improve how we store and manage thermal energy in the future.