Colorimetric Tracking of CO2 Absorption via Self-Reporting PEI Hydrogel Particles

CO2 sorbents play a central role in many carbon capture technologies. Visual monitoring of the sorbent behavior during CO2 capture could provide direct feedback and enable the optimization of both the sorbent and the reactor, which remains largely unexplored. In this work, we develop a self-reporting CO2 sorbent, prove its quantitative indication of CO2 absorption and desorption, and apply this method to evaluate different methods of sorbent regeneration. A pH indicator, thymol blue (TB), was embedded into polyethyleneimine (PEI) hydrogel particles, revealing a clear color transition from blue to green, yellow, and orange during CO2 absorption. This optical response was quantified by extracting Hue, a color coordinate in the Hue, Saturation, Value (HSV) space, from the particle images. Time-resolved analysis of the CO2 absorption revealed that Hue closely follows the measured CO2 uptake. A previously developed diffusion-reaction model was employed to investigate the relationship between the optical response and the internal chemistry of the particles. The measured Hue evolution more closely follows the modeled protonation response averaged over a substantial fraction of the particle volume than that of the shallow near-surface regions, supporting its potential use as an optical indicator for CO2 uptake. The robustness of the optical response was further confirmed by evaluating particles with different water contents, and excellent reversibility of the optical response was demonstrated during steam regeneration. As a final step, microwave regeneration was optically evaluated via this method, and shown to be much faster but less complete than steam regeneration. These results demonstrate that PEI/TB hydrogel particles can provide a timely, direct, and convenient optical signal during CO2 absorption and regeneration, enabling pathways for optimization at particle and potentially reactor scales.