Rice husk biochar adsorbs 6.01 mmol/g CO₂ at ambient pressure

Researchers at Harbin Institute of Technology have made a porous biochar from rice husks that adsorbs CO₂ at ambient pressure and releases it over repeated cycles, according to a study in The Innovation Energy (2026, vol. 3, issue 1, article 100131) doi.org. The material, named UHTCK, adsorbed 6.01 mmol of CO₂ per gram at 0 °C and retained 92.05% of its capacity after 20 adsorption-desorption cycles, the authors report doi.org.

The work addresses adsorption-compressed CO₂ energy storage, one option for smoothing the intermittency of wind and solar power. Conventional compressed-gas storage depends on high-pressure equipment, which is costly and safety-sensitive. The adsorption variant instead uses materials that bind CO₂ at ambient pressure and release it on demand, so the adsorbent must hold a lot of gas, take it up quickly and give it back easily doi.org.

Pore structure and synthesis

The biochar was produced in two steps: hydrothermal treatment of rice husks, an agricultural waste, followed by chemical activation with potassium hydroxide doi.org. The result is a hierarchical pore system. Narrowly distributed micropores centered at about 0.7 nm serve as storage volume, and a developed network of mesopores works as transport channels. Nitrogen-containing functional groups are deliberately introduced onto the pore surfaces doi.org.

Diffusion modeling showed that the connected pores speed up gas transport during uptake and release doi.org. Nitrogen-doped mesopores were found to optimize diffusion kinetics doi.org.

Mechanism from simulations

The team combined adsorption measurements with molecular dynamics simulations and density functional theory calculations to relate pore structure to adsorption-desorption equilibrium doi.org. The simulations showed CO₂ confined as a monolayer in micropores and accumulating near the walls of mesopores doi.org.

Functional groups at the pore edges raise surface polarity. Nitrogen and oxygen groups form weak, electrostatic, hydrogen-bond-like interactions with CO₂: strong enough to hold the molecules, weak enough to let them leave quickly doi.org. According to the authors, high microporosity provides the storage capacity, while nitrogen-functionalized surfaces and mesopores balance adsorption and heat, which meets the requirements of adsorption-compressed CO₂ storage at ambient conditions doi.org.

The study remains a laboratory materials investigation. Capacity was measured at 0 °C, and real storage systems will impose their own temperature and pressure schedules. The authors present the combination of ordered multiscale pores and controlled surface chemistry as a set of design principles for CO₂ capture materials that operate without high-pressure hardware doi.org.

Harbin team turns rice husks into biochar that adsorbs 6.01 mmol/g CO₂ at 0 °C in lab tests

Pore structure and synthesis

Mechanism from simulations

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