Rice husks become a smart carbon sponge: High-capacity CO₂ capture at ambient pressure with easy release

A porous carbon made from rice husks can grab carbon dioxide at everyday pressure and give it back with little energy cost, researchers report in a study that points toward greener storage for renewable electricity. Wind and solar power fluctuate with weather and daylight, so power grids need large-scale storage to keep supply steady. One appealing option compresses CO₂ and stores it—but traditional compressed-gas systems depend on high-pressure equipment, which is costly and safety-sensitive. An emerging alternative, adsorption-compressed CO₂ energy storage, instead relies on materials that bind CO₂ at ambient pressure and release it on demand. The catch is finding an adsorbent that is simultaneously roomy, fast, and easy to unload. A team at Harbin Institute of Technology turned rice husks, an abundant agricultural waste, into such a material. The recipe has two steps: hydrothermal treatment followed by activation with potassium hydroxide. The resulting biochar, named UHTCK, has a hierarchy of pores—abundant micropores centered at about 0.7 nanometers that serve as storage rooms, connected by a well-developed network of mesopores that act as transport corridors. Nitrogen-containing functional groups stud the pore surfaces. In tests, the material adsorbed 6.01 millimoles of CO₂ per gram at 0 °C and 1 bar. After 20 adsorption-desorption cycles it retained 92.05% of its capacity, and diffusion modeling showed its connected pores speed gas transport for rapid uptake and release. To explain why it works, the team paired the experiments with molecular dynamics simulations and density functional theory calculations. The simulations showed CO₂ molecules packing densely into micropores and accumulating near the walls of mesopores. They also revealed that nitrogen and oxygen groups at pore edges form weak, hydrogen-bond-like interactions with CO₂—strong enough to hold the molecules in place, weak enough to let them go quickly. That balance is the point. An energy-storage adsorbent must not only hold a lot of gas but also release it cheaply, cycle after cycle. The study concludes that combining high microporosity for storage with nitrogen-functionalized surfaces and mesopores for transport and heat management meets both requirements at ambient pressure—what the authors call regulated gas-solid-thermal interactions. The work remains a laboratory-stage materials study: capacity was measured at 0 °C, and real storage systems will impose their own temperature and pressure schedules. Even so, the design logic—ordered multiscale pores plus deliberate surface chemistry, built from agricultural waste—offers a blueprint for CO₂ capture materials that work without high-pressure hardware.