Solar energy has a simple but annoying weakness. It disappears when the sun does. Even the most efficient systems struggle with this basic reality\u2014no sunlight means no power. Scientists have long tried to fix this by storing solar energy as heat, but doing it efficiently has proven tricky.\u00a0<\/p>\n
Most designs rely on stacking different materials together\u2014one to absorb sunlight, another to store heat, and then another to protect the system. These layers don\u2019t work seamlessly, wasting energy at every boundary.<\/p>\n
Now, researchers have taken a very different approach<\/a> to overcome this problem. Instead of assembling multiple parts, they\u2019ve turned wood into an all-in-one solar energy system.\u00a0<\/p>\n By redesigning its internal structure at the nanoscale, they\u2019ve created a material that can absorb sunlight, store it as heat, and keep generating electricity even after the light is gone.\u00a0<\/p>\n \u201cOur work presents a scalable and environmentally friendly wood-based platform for advanced solar thermal energy harvesting,\u201d the researchers note<\/a> in their study.<\/p>\n Rebuilding wood from the inside out<\/p>\n The researchers started with balsa wood, not for its strength, but for its internal architecture. Under a microscope, balsa looks like a bundle of aligned microtubes, each about 20\u201350 micrometers wide. These channels can guide heat and hold materials, making them a natural scaffold.<\/p>\n However, raw wood reflects sunlight and absorbs water. So the researchers first stripped the wood of lignin<\/a>, the component that gives it color and rigidity. This delignification step boosted porosity above 93 percent, exposing a dense network of reactive surfaces inside the channels.\u00a0<\/p>\n Think of it as hollowing out the wood and turning it into a highly porous sponge\u2014but one that still retains its directional structure. Next, instead of burning the wood (a common method called carbonization), they chemically engineered<\/a> its inner surfaces.\u00a0<\/p>\n They coated the channel walls with ultrathin sheets of black phosphorene\u2014a material that absorbs sunlight across ultraviolet, visible, and infrared wavelengths and converts it into heat. Unlike carbon, phosphorene also brings flame-retardant properties, but it has a weakness. It degrades quickly in the air.<\/p>\n To solve this, the researchers wrapped each nanosheet in a protective layer made from tannic acid and iron ions. This metal\u2013polyphenol network acts like a molecular shield, preventing oxidation while also improving light absorption through charge-transfer effects. Even after 150 days of solar exposure, the coated material remained stable.<\/p>\n A durable, efficient, and waterproof storage system\u00a0<\/p>\n The team then added silver nanoparticles, which enhance light absorption through plasmonic effects\u2014basically amplifying how the material interacts with sunlight. Finally, they grafted long hydrocarbon chains onto the surface, making it extremely water-repellent<\/a>. The finished structure had a contact angle of 153\u00b0, meaning water simply rolls off.<\/p>\n With the scaffold ready, they filled the channels with stearic acid\u2014a bio-based phase-change material. When heated, this substance melts and stores energy; when cooled, it solidifies and releases that energy.\u00a0<\/p>\n This stability translated directly into strong performance. It stored about 175 kJ of heat per kilogram, converted 91.27 percent of incoming sunlight into usable heat, conducted heat nearly 3.9 times more efficiently along the wood\u2019s natural grain, and, when paired with a thermoelectric generator, produced up to 0.65 volts under standard one-sun illumination.<\/p>\n \u201cAs a proof of concept, stable solar\u2013thermal\u2013electric conversion is demonstrated with an output voltage of up to 0.65 V under one-sun irradiation,\u201d the study authors note.<\/p>\n When sunlight hits the material, it heats up and melts the embedded stearic acid. When the light is removed, the stored heat is released gradually, maintaining a temperature difference across a thermoelectric generator. This allows the system to keep producing electricity even after the light source is gone<\/a>.<\/p>\n Moreover, the material proved to be durable. After 100 heating\u2013cooling cycles, its performance barely changed. It resisted burning by self-extinguishing within two minutes, and its antimicrobial surface prevented bacterial growth that could degrade performance outdoors.<\/p>\n Our design \u201cintegrates flame retardancy, superhydrophobicity, and antimicrobial activity, thereby mitigating dust adhesion and microbial colonization that would otherwise deteriorate the outdoor photothermal performance,\u201d the study authors added.\u00a0<\/p>\n The big potential of wood-enabled solar storage<\/p>\n