Yolk-shell-type CaO-based sorbents for CO2 capture: assessing the role of nanostructuring for the stabilization of the cyclic CO2 uptake

Improving the cyclic CO<INF>2</INF> uptake stability of CaO-based solid sorbents can provide a means to lower CO<INF>2</INF> capture costs. Here, we develop nanostructured yolk(CaO)-shell(ZrO<INF>2</INF>) sorbents with a high cyclic CO<INF>2</INF> uptake stability which outperform benchmark CaO nanoparticles after 20 cycles (0.17 g<INF>CO<INF>2</INF></INF> g<INF>Sorbent</INF>-1) by more than 250% (0.61 g<INF>CO<INF>2</INF></INF> g<INF>Sorbent</INF>-1), even under harsh calcination conditions (i.e. 80 vol% CO<INF>2</INF> at 900 degrees C). By comparing the yolk-shell sorbents to core-shell sorbents, i.e. structures with an intimate contact between the stabilizing phase and CaO, we are able to identify the main mechanisms behind the stabilization of the CO<INF>2</INF> uptake. While a yolk-shell architecture stabilizes the morphology of single CaO nanoparticles over repeated cycling and minimizes the contact between the yolk and shell materials, core-shell architectures lead to the formation of a thick CaZrO<INF>3</INF>-shell around CaO particles, which limits CO<INF>2</INF> transport to unreacted CaO. Hence, yolk-shell architectures effectively delay CaZrO<INF>3</INF> formation which in turn increases the theoretically possible CO<INF>2</INF> uptake since CaZrO<INF>3</INF> is CO<INF>2</INF>-capture-inert. In addition, we observe that yolk-shell architectures also improved the carbonation kinetics in both the kinetic- and diffusion-controlled regimes leading to a significantly higher cyclic CO<INF>2</INF> uptake for yolk-shell-type sorbents.