Diffusion-reaction modeling of CO₂ absorption in core-shell hydrogel particles

Amine-based hydrogel sorbents are an emerging material for efficient CO₂ capture. However, the influence of the sorbent properties on their CO₂ absorption dynamics still remains unknown, blocking their optimization for specific applications. Therefore, in this study, we present a particle-scale model describing the spatio-temporal CO₂ absorption in tailor-made particle sorbents comprising a hydrogel core (crosslinked polyethylenimine) and a coating layer (silica nanoparticles). Incorporating both physical diffusion of CO₂ and amine-CO₂ reactions within the particle, the model also addresses the effect of water on both physical diffusion and the reaction. The model is fitted to experimentally measured CO₂ absorption profiles of the particles over time at different temperatures. Subsequently, we validate the model against measurements of the CO₂ absorption profiles for control parameters that were excluded from the fitting (water content and particle geometry), confirming that the physical mechanisms are correctly captured. Finally, we model the spatiotemporal evolution of the CO₂ absorption and the free amine concentration inside the particle. By altering the shell diffusion coefficient and particle size in the model, we show that: (1) the silica shell hardly hinders the absorption process, and (2) decreasing the particle size strongly accelerates the absorption, albeit with sub-quadratic scaling due to the coupling of diffusion and reaction. Our work provides deeper insight into the CO₂ absorption mechanisms of hydrogel sorbents, enabling rational design and optimization of such materials.