Extended Data Fig. 4: Effect of unit cell coordination number on CO₂ absorption rate.

a, Images (top) of the hierarchical lattice filled with a solution of Na₂CO₃ and a pH-sensitive colour indicator that changes from blue to yellow upon CO₂ saturation. The rate of absorption in the lattice is significantly faster than a liquid pool of the same sorbent volume. b, Hierarchical structure 750 s after exposure to CO₂, with analysed regions of interest outlined. Scale bar, 5 mm. c, Colorimetric map depicting the reaction rate during the first 500 s. The extent of CO₂ absorption is estimated by the colour change from blue to yellow (the sum of green and red channels minus the blue channel for each pixel). Higher reaction rates occur at cells with a lower coordination number—that is, more gas–liquid interfaces (blue arrows)—compared to cells with a higher coordination number (black arrows). The enlarged images of the outlined regions show high rates of reaction occurring at interfaces in the centre of each unit cell. Although it is expected that the reaction will occur more rapidly at struts and corners, where the liquid layer is the thinnest and has the highest surface-area-to-volume ratio, the thin liquid layer results in high reflectivity and low levels of pH indicator, which limits the measurement accuracy in these regions. The cell at the intersection with a higher coordination number shows a slower reaction rate than adjacent cells. d, The extent of reaction progression for cells selected in b. Cells with a coordination number 5 (black) show lower levels of reaction than cells with coordination number 2 (blue and pink).