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

Abstract

Improving the cyclic CO₂ uptake stability of CaO-based solid sorbents can provide a means to lower CO₂ capture costs. Here, we develop nanostructured yolk(CaO)-shell(ZrO₂) sorbents with a high cyclic CO₂ uptake stability which outperform benchmark CaO nanoparticles after 20 cycles (0.17 g_CO2 g_Sorbent^-1) by more than 250% (0.61 g_CO2 g_Sorbent^-1), even under harsh calcination conditions (i.e. 80 vol% CO₂ at 900 °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 CO₂ 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 CaZrO₃-shell around CaO particles, which limits CO₂ transport to unreacted CaO. Hence, yolk-shell architectures effectively delay CaZrO₃ formation which in turn increases the theoretically possible CO₂ uptake since CaZrO₃ is CO₂-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 CO₂ uptake for yolk-shell-type sorbents.