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.