Ordered construction of multiscale functional biochar: Mechanistic insights into high-capacity CO2 adsorption at ambient pressure

The development of functional biochar with high-capacity and rapid CO2 adsorption/desorption capabilities is pivotal for compressed CO2 energy storage systems, effectively mitigating renewable energy intermittency and advancing carbon-neutral power grid infrastructure. A porous biochar with hierarchically structured pores and deliberately incorporated nitrogen functional groups is fabricated through hydrothermal treatment followed by chemical activation in this study. A multimodal approach combining experimental adsorption analyses with molecular dynamics and density functional theory simulations systematically elucidates the structure-activity relationships governing CO2 adsorption-desorption equilibrium. The UHTCK biochar demonstrates narrowly distributed micropores (centered at 0.7 nm) within an ideal hierarchical architecture, achieving a CO2 adsorption capacity of 6.01 mmol/g at 0 °C with 92.05% regeneration efficiency after 20 cycles. Edge functionalization enhances surface polarity, facilitating electrostatic-driven weak interactions (e.g., hydrogen bonding) that promote monolayer CO2 confinement in micropores. Mesopores exhibit wall-proximal CO2 accumulation, while nitrogen-doped mesopores optimize diffusion kinetics. Mechanistic analysis reveals that high microporosity enables substantial CO2 storage, while nitrogen-functionalized surfaces and mesopores thermodynamically balance the adsorption and heat to meet the requirements of adsorption-compression CO2 energy storage systems. Strategic pore-functionality engineering enables ambient-condition rapid CO2 cycling through regulated gas-solid-thermal interactions. This work provides theoretical insights and design principles for developing atmospheric-pressure CO2 capture materials, offering transformative potential for next-generation energy storage systems.