Role of Hierarchical Porosity in Dictating Adsorption Dynamics in Direct Air Capture Systems

The surge in global average temperatures has necessitated a reduction in the released carbon dioxide (CO 2 ) levels from anthropogenic sources. Besides curtailing these emissions, there is a demand to remove the CO 2 already accumulated in the atmosphere. Combined experimental and modeling studies can significantly aid in developing durable and less energy-intensive direct air capture (DAC) systems. Here, we develop a physics-based multiscale framework, systematically integrating a pore-scale Lattice-Boltzmann Method (LBM) model with a microstructure-aware macroscale breakthrough model, simulating coupled mass transfer and adsorption kinetics for a carbon-based CO 2 sorbent. A screening of sorbent morphological parameters such as porosity and ratio of micropores/mesopores pertaining to hierarchical porous carbon nanofibers (CNFs) was conducted to delineate their influence on the adsorption performance. It was observed that a balanced pore network comprising mesopores and micropores yielded high adsorption efficiency owing to conducive utilization of surface sites and lower mass transport losses. Mechanistic interrogation at the macroscale revealed an early onset of saturation and an enhancement in the capacity for sorbent microstructures with a hierarchical porous architecture. Insights derived from this work can help formulate guidelines to inform the experimental design of porous CNFs.